WO2019072163A1

WO2019072163A1 - Materials and processes for tandem organic solar cells

Info

Publication number: WO2019072163A1
Application number: PCT/CN2018/109437
Authority: WO
Inventors: He Yan; Shangshang CHEN
Original assignee: The Hong Kong University Of Science And Technology
Priority date: 2017-10-10
Filing date: 2018-10-09
Publication date: 2019-04-18

Abstract

Provided herein are tandem organic solar cells having two or more sub-cells, their methods of preparation, and methods of use thereof. Also provided, are formulations including donor materials and acceptor materials useful in the preparation of the organic solar cells described herein.

Description

MATERIALS AND PROCESSES FOR TANDEM ORGANIC SOLAR CELLS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of United States Provisional Application Number 62/606,755, filed on October 10, 2017, the contents of which being hereby incorporated by reference in their entirety for all purposes.

TECHNICAL FIELD

The present disclosure generally relates to tandem organic solar cells (OSCs) , methods for their preparation, and the use of formulations containing organic semiconductors as photoactive layers and other components used in the preparation of the tandem OSCs.

BACKGROUND

In recent years there has been growing interest in the use of organic semiconductors, including conjugated polymers, for various electronic applications. One particular area of importance is the field of organic photovoltaics (OPVs) . Organic semiconductors have found use in OPVs as they allow devices to be manufactured by solution-processing techniques, such as spin casting and printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices.

Over the past two decades, great success has been achieved in the OSC field, and efficient OSCs with power conversion efficiencies (PCEs) over 10 %have been reported. Despite the great success achieved in this field, the PCEs of OSCs still lag behind that of perovskite and other inorganic solar cells. One of the reasons for the low PCE of OPV is the narrow absorption range of organic materials, which only covers a fraction of the solar spectrum. Another major loss mechanism for single junction solar cells is potential loss due to thermalization of hot carriers created when photons of energy greater than the bandgap of the OPV are absorbed. The limitations of single junction photovoltaics can be overcome by using tandem or multiple structure. By stacking two or more photoactive layers with different absorption ranges in a tandem structure, the tandem solar cells can harvest much more sunlight in a potentially broader wavelength range compared to single-junction solar cells. The tandem structure is one of the most promising strategies to boost the PCE of OSCs to over 15 %.

Most of the previously reported tandem OSCs are based on two sub-cells with complementary absorptions, in which poly (3-hexylthiophene) (P3HT) is a commonly used wide-bandgap donor polymer in the front sub-cell. However, P3HT-based cells or sub-cells always tend to suffer from large voltage loss, thus limiting the voltage output of the tandem OSCs. On the other hand, most of the semiconductors used in the rear sub-cells are diketopyrrolopyrrole (DPP) based polymers, whose optical absorptions are limited to 900 nm. As a result, the short-circuit current density of previously reported tandem OSCs are smaller than 12 mA cm ^-2. To achieve more efficient tandem OSCs, it is critical to extend the optical absorption of tandem devices.

SUMMARY

The tandem OSCs of the present disclosure can comprise two or more photoactive layers between two electrodes, wherein each of photoactive layers comprises at least one donor material and one acceptor material. The constituents of the two or more photoactive layers can be selected to provide complementary absorption ranges, which can increase the efficiency of the resulting tandem OSC.

In a first aspect, provided herein is a tandem OSC comprising: a front sub-cell comprising a first donor material and a first acceptor material; a recombination layer comprising a hole transport layer and an electron transport layer; and a back sub-cell comprising a second donor material and a second acceptor material, wherein the recombination layer is disposed between the front sub-cell and the back sub-cell; the first acceptor material is a perylenediimide (PDI) -based semiconductor; and the second acceptor material is a non-fullerene-based semiconductor.

In a first embodiment of the first aspect, provided herein is the tandem OSC of the first aspect, wherein the PDI-based semiconductor is represented by the formula:

wherein

Z is an aromatic unit;

Ar is aryl or heteroaryl;

Y for each instance is independently selected from, -O-, -S-, -Se-, -Te-, and -N (R ₁) -, wherein each R ₁ is independently a straight-chain, branched, or cyclic alkyl group; or Y for each instance independently represents two hydrogen; and

R for each instance is independently selected from the group consisting of straight-chain, branched, and cyclic alkyl with 2-40 C atoms, wherein one or more non-adjacent C atoms are optionally replaced by –O–, –S–, –C (=O) –, –C (=O) O–, –OC (=O) –, –OC (=O) O–, –CR ₂=CR ₃–, or –C≡C–, wherein R ₂ and R ₃ are indepedently selected from hydrogen and alkyl; and wherein one or more H atoms are optionally replaced by F, Cl, Br, I, CN, aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl, or heteroaryloxycarbonyl having 4 to 30 ring atoms unsubstituted or substituted by one or more non-aromatic groups; or R for each instance is indepedently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, araalkyl, heteroaryl, alkenyl, and alkynyl.

In a second embodiment of the first aspect, provided herein is the tandem OSC of the first embodiment of the first aspect, wherein the PDI-based semiconductor is represented by the formula:

In a third embodiment of the first aspect, provided herein is the tandem OSC of the first aspect, wherein the PDI-based semiconductor comprises a subunit represented by the formula:

wherein R ₄ for each instance is independently selected from the group consisting of straight-chain, branched, and cyclic alkyl with 2-40 C atoms, wherein one or more non-adjacent C atoms are optionally replaced by –O–, –S–, –C (=O) –, –C (=O) O–, –OC (=O) –, –OC (=O) O–, –CR ₂=CR ₃–, or –C≡C–, wherein R ₂ and R ₃ are indepedently selected from hydrogen and alkyl; and wherein one or more H atoms are optionally replaced by F, Cl, Br, I, CN, aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl, or heteroaryloxycarbonyl having 4 to 30 ring atoms unsubstituted or substituted by one or more non-aromatic groups; or R ₄ for each instance is indepedently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, araalkyl, heteroaryl, alkenyl, and alkynyl.

In a fourth embodiment of the first aspect, provided herein is the tandem OSC of the third embodiment of thefirst aspect, wherein the PDI-based semiconductor is selected from the group consisting of:

In a fifth embodiment of the first aspect, provided herein is the tandem OSC of the first aspect, wherein the second acceptor matieral is selected from the group consisitng of:

wherein for R ₅ for each instance is independently C ₄-C ₁₂ alkyl; and R ₆ for each instance is independently C ₂-C ₈.

In a sixth embodiment of the first aspect, provided herein is the tandem OSC of the first aspect, wherein the first donor material is a polymer comprising a repeating unit represented by:

wherein R ₇ for each instance is independently selected from C ₄-C ₁₂ alkyl.

In a seventh embodiment of the first aspect, provided herein is the tandem OSC of the first aspect, wherein the second donor material is is a polymer comprising a repeating unit represented by:

wherein R ₈ for each instance is independently selected from C ₁-C ₈ alkyl.

In an eighth embodiment of the first aspect, provided herein is the tandem OSC of the first aspect, wherein the hole transport layer is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , polyaniline (PANI) , vanadium (V) oxide (V ₂O ₅) , molybdenum (VI) oxide (MoO ₃) , and Tungsten (VI) oxide (WO ₃) ; and the electron transport layer is at least one material selected from the group consisting of zinc oxide (ZnO) , tin (IV) oxide (SnO ₂) , lithium fluoride (LiF) , zinc indium tin oxide (ZITO) , poly [ (9, 9-bis (3′- (N, N-dimethylamino) propyl) -2, 7-fluorene) -alt-2, 7- (9, 9–dioctylfluorene) ] (PFN) , poly [ (9, 9-bis (3'- ( (N, N -dimethyl) -N -ethylammonium) -propyl) -2, 7-fluorene) -alt-2, 7- (9, 9-dioctylfluorene) ] (PFN-Br) , and poly [9, 9-bis (6’- (N, N-diethylamino) propyl) -fluorene-alt-9, 9-bis (3-ethyl (oxetane-3-ethyloxy) -hexyl) -fluorene] (PFN-OX) .

In a ninth embodiment of the first aspect, provided herein is the tandem OSC of the eighth embodiment of the first aspect, wherin the PDI-based semiconductor is represented by the formula:

wherein R for each instance is independently selected from C ₆-C ₂₀ alkyl; the second donor material is is a polymer comprising a repeating unit represented by:

wherein R ₈ for each instance is independently selected from C ₁-C ₈ alkyl; and the second accepter material is selected from the group consisitng of:

In a tenth embodiment of the first aspect, provided herein is the tandem OSC of the ninth embodiment of the first aspect, wherein the front sub-cell has an optical absorption onset between 500 and 750 nm and the back sub-cell has an optical absorption onset between 800 to 1100 nm.

In an eleventh embodiment of the first aspect, provided herein is the tandem OSC of the tenth embodiment of the first aspect, wherein the hole transport layer is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , polyaniline (PANI) , silver, and molybdenum (VI) oxide (MoO ₃) ; and the electron transport layer is at least one material selected from the group consisting of zinc oxide (ZnO) and tin (IV) oxide (SnO ₂) .

In a twelfth embodiment of the first aspect, provided herein is the tandem OSC of the ninth embodiment of the first aspect, wherin the PDI-based semiconductor is represented by the formula:

wherein R is –CH (C ₆H ₁₃) ₂; the first donor material is is a polymer comprising a repeating unit represented by:

the second donor material is is a polymer comprising a repeating unit represented by:

the second accepter material is selected from the group consisting of:

the hole transport layer is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , polyaniline (PANI) , silver, and molybdenum (VI) oxide (MoO ₃) ; and the electron transport layer is at least one material selected from the group consisting of zinc oxide (ZnO) and tin (IV) oxide (SnO ₂) .

In a thirteenth embodiment of the first aspect, provided herein is the tandem OSC of the first aspect, wherein the PDI-based semiconductor and the second acceptor material are represented by the formula:

the first donor material and the second donor material are represented by the formula:

the hole transport layer is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , silver, and molybdenum (VI) oxide (MoO ₃) ; and the electron transport layer is zinc oxide (ZnO) .

In a fourteenth embodiment of the first aspect, provided herein is the tandem OSC of the thirteenth embodiment of the first aspect, wherein the front sub-cell has a thickness of 100 nm and the back sub-cell has a thickness of 120 nm.

In a fifteen embodiment of the first aspect, provided herein is the tandem OSC of the eleventh embodiment of the first aspect, wherein the back sub-cell further comprises a third acceptor material selected from from the group consisting of PC61BM and PC71BM.

In a sixteenth embodiment of the first aspect, provided herein is the tandem OSC of the first aspect, wherein the tandem OSC has a power conversion efficiency (PCE) of 12 to 13.2 %.

In a second aspect, provided herein is a method for preparing the tandem OSC of the first aspect, comprising the steps of:

a. depositing a solution comprising the first acceptor material and the first donor material on a substrate thereby forming a first thin film;

b. annealing the first thin film thereby forming the front sub-cell;

c. depositing a solution comprising the hole transport layer on the front sub-cell thereby forming a first composite;

d. annealing the first composite thereby forming an annealed first composite;

e. depositing a solution comprising the electron transport layer or a precursor of the electron transport layer on the annealed first composite thereby forming a second composite;

f. annealing the second composite thereby forming an annealed second composite;

g. depositing a solution comprising the second acceptor material and the second donor material on to the annealed second composite thereby forming a third composite; and

h. annealing the third composite thereby forming the tandem OSC of first aspect.

In a first embodiment of the second aspect, provided herein is the method of the second aspect, wherein step of annealing the first thin film; annealing the first composite; annealing the second composite; and annealing the third composite are all conducted at a temperature at or below 100 ℃.

In a second embodiment of the second aspect, provided herein is the method of the first embodiment of the second aspect, wherein the electron transport layer precursor is is diethyl zinc.

The present subject matter further relates to the use of the formulations as described herein as a coating or printing interlayer, especially for the preparation of optoelectronic (OE) devices and rigid or flexible organic photovoltaic (OPV) cells and devices.

The formulations, methods and devices of the present disclosure provide surprising improvements in the efficiency of the OE devices and the production thereof. Unexpectedly, the performance and the efficiency of the OE devices can be improved, if these devices are prepared using a formulation as described herein. Furthermore, the formulations of the present disclosure provides an astonishingly high level of film forming. Especially, the homogeneity and the quality of the films can be improved. In addition thereto, the present subject matter enables better solution printing of OE devices, especially OPV devices.

BRIEF DESCRIPTION OF THE DRAWINGS

It should be understood that the drawings described herein are for illustration purposes only. The drawings are not necessarily to scale, with emphasis generally being placed upon illustrating the principles of the present teachings. The drawings are not intended to limit the scope of the present teachings in any way.

FIG. 1A depicts a tandem OSC consisting of two sub-cells (referred to as the front sub-cell 110 and the back sub-cell 120) in accordance with certain embodiments described herein. Each sub-cell comprises a donor material and acceptor material heterojunction. The two sub-cells of the tandem device are separated by a recombination layer 130 comprising an electron transport layer 140 and a hole transport layer 150.

FIG. 1B depicts a tandem OSC consisting of two sub-cells with an alternative configuration of the recombination layer 130 in which the position of the electron transport layer 150 and the hole transport layer 140 are interchanged in accordance with certain embodiments described herein.

FIG. 2A depicts a schematic illustrating an inverted tandem OSC according to certain embodiments described herein.

FIG. 2B depicts a schematic illustrating a standard tandem OSC according to certain embodiments described herein.

FIG. 3 depicts the atomic force microscope (AFM) height (left) and cross-section (right) images of glass/ITO/P3TEA: SF-PDI ₂/PEDOT: PSS/ZnO (scan area is 1 μm×1 μm and the vertical data scale is 10 nm) in accordance with certain embodiments described herein.

FIG. 4 depicts the current-voltage and EQE curves of P3TEA: SF-PDI ₂ double-junction solar cell with various thickness of two sub-cells in accordance with certain embodiments described herein.

FIG. 5 EQE curves of a P3TEA: SF-PDI ₂ double-junction solar cell in accordance with certain embodiments described herein.

DETAILED DESCRIPTION

Definitions

Throughout the application, where compositions are described as having, including, or comprising specific components, or where processes are described as having, including, or comprising specific process steps, it is contemplated that compositions of the present teachings can also consist essentially of, or consist of, the recited components, and that the processes of the present teachings can also consist essentially of, or consist of, the recited process steps.

In the application, where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that the element or component can be any one of the recited elements or components, or the element or component can be selected from a group consisting of two or more of the recited elements or components. Further, it should be understood that elements and/or features of a composition, an apparatus, or a method described herein can be combined in a variety of ways without departing from the spirit and scope of the present teachings, whether explicit or implicit herein

The use of the terms "include, " "includes" , "including, " "have, " "has, " or "having" should be generally understood as open-ended and non-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa) unless specifically stated otherwise. In addition, where the use of the term "about" is before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term "about" refers to a ±10%variation from the nominal value unless otherwise indicated or inferred.

It should be understood that the order of steps or order for performing certain actions is immaterial so long as the present teachings remain operable. Moreover, two or more steps or actions may be conducted simultaneously.

As used herein, a "P-type semiconductor material" or a "donor" material refers to a semiconductor material, for example, an organic semiconductor material, having holes as the majority current or charge carriers. In some embodiments, when a p-type semiconductor material is deposited on a substrate, it can provide a hole mobility in excess of about 10 ^-5 cm ²/Vs. In the case of field-effect devices, a p-type semiconductor also can exhibit a current on/off ratio of greater than about 10.

As used herein, an "N-type semiconductor material" or an "acceptor" material refers to a semiconductor material, for example, an organic semiconductor material, having electrons as the majority current or charge carriers. In some embodiments, when an n-type semiconductor material is deposited on a substrate, it can provide an electron mobility in excess of about 10 ^-5 cm ²/Vs. In the case of field-effect devices, an n-type semiconductor also can exhibit a current on/off ratio of greater than about 10.

As used herein, "mobility" refers to a measure of the velocity with which charge carriers, for example, holes (or units of positive charge) in the case of a p-type semiconductor material and electrons (or units of negative charge) in the case of an n-type semiconductor material, move through the material under the influence of an electric field. This parameter, which depends on the device architecture, can be measured using a field-effect device or space-charge limited current measurements.

As used herein, “homo-tandem” refers to the tandem solar cells constructed from the photoactive layers with identical optical absorptions.

As used herein, “hybrid tandem” refers to the tandem solar cells constructed from the photoactive layers with optical absorptions.

As used herein, “sub-cell” refers to the photoactive layers that can convert light into electricity in tandem solar cells.

As used herein, a compound can be considered "ambient stable" or "stable at ambient conditions" when a transistor incorporating the compound as its semiconducting material exhibits a carrier mobility that is maintained at about its initial measurement when the compound is exposed to ambient conditions, for example, air, ambient temperature, and humidity, over a period of time. For example, a compound can be described as ambient stable if a transistor incorporating the compound shows a carrier mobility that does not vary more than 20%or more than 10%from its initial value after exposure to ambient conditions, including, air, humidity and temperature, over a 3 day, 5 day, or 10 day period.

As used herein, fill factor (FF) is the ratio (given as a percentage) of the actual maximum obtainable power, (Pm or Vmp *Jmp) , to the theoretical (not actually obtainable) power, (Jsc *Voc) . Accordingly, FF can be determined using the equation:

FF = (Vmp *Jmp) / (Jsc *Voc)

where Jmp and Vmp represent the current density and voltage at the maximum power point (Pm) , respectively, this point being obtained by varying the resistance in the circuit until J *V is at its greatest value; and Jsc and Voc represent the short circuit current and the open circuit voltage, respectively. Fill factor is a key parameter in evaluating the performance of solar cells. Commercial solar cells typically have a fill factor of about 0.60%or greater.

As used herein, the open-circuit voltage (Voc) is the difference in the electrical potentials between the anode and the cathode of a device when there is no external load connected.

As used herein, the power conversion efficiency (PCE) of a solar cell is the percentage of power converted from absorbed light to electrical energy. The PCE of a solar cell can be calculated by dividing the maximum power point (Pm) by the input light irradiance (E, in W/m ²) under standard test conditions (STC) and the surface area of the solar cell (Ac in m ²) . STC typically refers to a temperature of 25℃ and an irradiance of 1000 W/m2 with an air mass 1.5 (AM 1.5) spectrum.

As used herein, a component (such as a thin film layer) can be considered "photoactive" if it contains one or more compounds that can absorb photons to produce excitons for the generation of a photocurrent.

As used herein, "solution-processable" refers to compounds (e.g., polymers) , materials, or compositions that can be used in various solution-phase processes including spin-coating, printing (e.g., inkjet printing, gravure printing, offset printing and the like) , spray coating, electrospray coating, drop casting, dip coating, blade coating, and the like.

As used herein, a "semicrystalline polymer" refers to a polymer that has an inherent tendency to crystallize at least partially either when cooled from a melted state or deposited from solution, when subjected to kinetically favorable conditions such as slow cooling, or low solvent evaporation rate and so forth. The crystallization or lack thereof can be readily identified by using several analytical methods, for example, differential scanning calorimetry (DSC) and/or X-ray diffraction (XRD) .

As used herein, "annealing" refers to a post-deposition heat treatment to the semicrystalline polymer film in ambient or under reduced/increased pressure for a time duration of more than 100 seconds, and "annealing temperature" refers to the maximum temperature that the polymer film is exposed to for at least 60 seconds during this process of annealing. Without wishing to be bound by any particular theory, it is believed that annealing can result in an increase of crystallinity in the polymer film, where possible, thereby increasing field effect mobility. The increase in crystallinity can be monitored by several methods, for example, by comparing the differential scanning calorimetry (DSC) or X-ray diffraction (XRD) measurements of the as-deposited and the annealed films.

As used herein, a "polymeric compound" (or "polymer" ) refers to a molecule including a plurality of one or more repeating units connected by covalent chemical bonds. A polymeric compound can be represented by General Formula I:

*- (- (Ma) _x- (Mb) _y-) _z*

General Formula I

wherein each Ma and Mb is a repeating unit or monomer. The polymeric compound can have only one type of repeating unit as well as two or more types of different repeating units. When a polymeric compound has only one type of repeating unit, it can be referred to as a homopolymer. When a polymeric compound has two or more types of different repeating units, the term "copolymer" or "copolymeric compound" can be used instead. For example, a copolymeric compound can include repeating units where Ma and Mb represent two different repeating units. Unless specified otherwise, the assembly of the repeating units in the copolymer can be head-to-tail, head-to-head, or tail-to-tail. In addition, unless specified otherwise, the copolymer can be a random copolymer, an alternating copolymer, or a block copolymer. For example, General Formula I can be used to represent a copolymer of Ma and Mb having x mole fraction of Ma and y mole fraction of Mb in the copolymer, where the manner in which comonomers Ma and Mb is repeated can be alternating, random, regiorandom, regioregular, or in blocks, with up to z comonomers present. In addition to its composition, a polymeric compound can be further characterized by its degree of polymerization (n) and molar mass (e.g., number average molecular weight (M) and/or weight average molecular weight (Mw) depending on the measuring technique (s) ) .

As used herein, "halo" or "halogen" refers to fluoro, chloro, bromo, and iodo.

As used herein, "alkyl" refers to a straight-chain or branched saturated hydrocarbon group. Examples of alkyl groups include methyl (Me) , ethyl (Et) , propyl (e.g., n-propyl and z'-propyl) , butyl (e.g., n-butyl, z'-butyl, sec-butyl, tert-butyl) , pentyl groups (e.g., n-pentyl, z'-pentyl, neo-pentyl) , hexyl groups, and the like. In various embodiments, an alkyl group can have 1 to 40 carbon atoms (i.e., C1-40 alkyl group) , for example, 1-30 carbon atoms (i.e., C1-30 alkyl group) . In some embodiments, an alkyl group can have 1 to 6 carbon atoms, and can be referred to as a "lower alkyl group. " Examples of lower alkyl groups include methyl, ethyl, propyl (e.g., n-propyl and z'-propyl) , and butyl groups (e.g., n-butyl, z'-butyl, sec-butyl, tert-butyl) . In some embodiments, alkyl groups can be substituted as described herein. An alkyl group is generally not substituted with another alkyl group, an alkenyl group, or an alkynyl group.

As used herein, "alkenyl" refers to a straight-chain or branched alkyl group having one or more carbon-carbon double bonds. Examples of alkenyl groups include ethenyl, propenyl, butenyl, pentenyl, hexenyl, butadienyl, pentadienyl, hexadienyl groups, and the like. The one or more carbon-carbon double bonds can be internal (such as in 2-butene) or terminal (such as in 1-butene) . In various embodiments, an alkenyl group can have 2 to 40 carbon atoms (i.e., C2-40 alkenyl group) , for example, 2 to 20 carbon atoms (i.e., C2-20 alkenyl group) . In some embodiments, alkenyl groups can be substituted as described herein. An alkenyl group is generally not substituted with another alkenyl group, an alkyl group, or an alkynyl group.

As used herein, a "fused ring" or a "fused ring moiety" refers to a polycyclic ring system having at least two rings where at least one of the rings is aromatic and such aromatic ring (carbocyclic or heterocyclic) has a bond in common with at least one other ring that can be aromatic or non-aromatic, and carbocyclic or heterocyclic. These polycyclic ring systems can be highly p-conjugated and optionally substituted as described herein.

As used herein, "heteroatom" refers to an atom of any element other than carbon or hydrogen and includes, for example, nitrogen, oxygen, silicon, sulfur, phosphorus, and selenium.

As used herein, "aryl" refers to an aromatic monocyclic hydrocarbon ring system or a polycyclic ring system in which two or more aromatic hydrocarbon rings are fused (i.e., having a bond in common with) together or at least one aromatic monocyclic hydrocarbon ring is fused to one or more cycloalkyl and/or cycloheteroalkyl rings. An aryl group can have 6 to 24 carbon atoms in its ring system (e.g., C ₆-C ₂₄ aryl group) , which can include multiple fused rings. In some embodiments, a polycyclic aryl group can have 8 to 24 carbon atoms. Any suitable ring position of the aryl group can be covalently linked to the defined chemical structure. Examples of aryl groups having only aromatic carbocyclic ring (s) include phenyl, 1-naphthyl (bicyclic) , 2-naphthyl (bicyclic) , anthracenyl (tricyclic) , phenanthrenyl (tricyclic) , pentacenyl (pentacyclic) , and like groups. Examples of polycyclic ring systems in which at least one aromatic carbocyclic ring is fused to one or more cycloalkyl and/or cycloheteroalkyl rings include, among others, benzo derivatives of cyclopentane (i.e., an indanyl group, which is a 5, 6-bicyclic cycloalkyl/aromatic ring system) , cyclohexane (i.e., a tetrahydronaphthyl group, which is a 6, 6-bicyclic cycloalkyl/aromatic ring system) , imidazoline (i.e., a benzimidazolinyl group, which is a 5, 6-bicyclic cycloheteroalkyl/aromatic ring system) , and pyran (i.e., a chromenyl group, which is a 6, 6-bicyclic cycloheteroalkyl/aromatic ring system) . Other examples of aryl groups include benzodioxanyl, benzodioxolyl, chromanyl, indolinyl groups, and the like. In some embodiments, aryl groups can be substituted as described herein. In some embodiments, an aryl group can have one or more halogen substituents, and can be referred to as a "haloaryl" group. Perhaloaryl groups, i.e., aryl groups where all of the hydrogen atoms are replaced with halogen atoms (e.g., -C ₆F ₅) , are included within the definition of "haloaryl. " In certain embodiments, an aryl group is substituted with another aryl group and can be referred to as a biaryl group. Each of the aryl groups in the biaryl group can be substituted as disclosed herein.

As used herein, "heteroaryl" refers to an aromatic monocyclic ring system containing at least one ring heteroatom selected from oxygen (O) , nitrogen (N) , sulfur (S) , silicon (Si) , and selenium (Se) or a polycyclic ring system where at least one of the rings present in the ring system is aromatic and contains at least one ring heteroatom. Polycyclic heteroaryl groups include those having two or more heteroaryl rings fused together, as well as those having at least one monocyclic heteroaryl ring fused to one or more aromatic carbocyclic rings, non-aromatic carbocyclic rings, and/or non-aromatic cycloheteroalkyl rings. A heteroaryl group, as a whole, can have, for example, 5 to 24 ring atoms and contain 1-5 ring heteroatoms (i.e., 5-20 membered heteroaryl group) . The heteroaryl group can be attached to the defined chemical structure at any heteroatom or carbon atom that results in a stable structure. Generally, heteroaryl rings do not contain O-O, S-S, or S-O bonds. However, one or more N or S atoms in a heteroaryl group can be oxidized (e.g., pyridine N-oxide thiophene S-oxide, thiophene S, S-dioxide) . Examples of heteroaryl groups include, for example, the 5-or 6-membered monocyclic and 5-6 bicyclic ring systems shown below: where T is O, S, NH, N-alkyl, N-aryl, N- (arylalkyl) (e.g., N-benzyl) , SiH2, SiH (alkyl) , Si (alkyl) ₂, SiH (arylalkyl) , Si (arylalkyl) ₂, or Si (alkyl) (arylalkyl) . Examples of such heteroaryl rings include pyrrolyl, furyl, thienyl, pyridyl, pyrimidyl, pyridazinyl, pyrazinyl, triazolyl, tetrazolyl, pyrazolyl, imidazolyl, isothiazolyl, thiazolyl, thiadiazolyl, isoxazolyl, oxazolyl, oxadiazolyl, indolyl, isoindolyl, benzofuryl, benzothienyl, quinolyl, 2-methylquinolyl, isoquinolyl, quinoxalyl, quinazolyl, benzotriazolyl, benzimidazolyl, benzothiazolyl, benzisothiazolyl, benzisoxazolyl, benzoxadiazolyl, benzoxazolyl, cinnolinyl, lH-indazolyl, 2H-indazolyl, indolizinyl, isobenzofuyl, naphthyridinyl, phthalazinyl, pteridinyl, purinyl, oxazolopyridinyl, thiazolopyridinyl, imidazopyridinyl, furopyridinyl, thienopyridinyl, pyridopyrimidinyl, pyridopyrazinyl, pyridopyridazinyl, thienothiazolyl, thienoxazolyl, thienoimidazolyl groups, and the like. Further examples of heteroaryl groups include 4, 5, 6, 7-tetrahydroindolyl, tetrahydroquinolinyl, benzothienopyridinyl, benzofuropyridinyl groups, and the like. In some embodiments, heteroaryl groups can be substituted as described herein.

The tandem OSCs described herein can comprise two or more sub-cells. In instances where the tandem OSC comprises two sub-cells, the two sub-cells are refered to as the front sub-cell and the back sub-cell. Referring to FIG. 1A, the sub-cell that light first passes through is referred to as the front sub-cell 110 and the light then passes through the back sub-cell 120. Each sub-cell in the tandem OSC can comprise at least one donor material and at least one acceptor material. In instances in which the tandem OSC comprises two sub-cells, the front sub-cell 110 can comprise a first donor material and a first acceptor material and the back sub-cell 120 can comprise a second donor material and a second acceptor material, wherein the first acceptor materials is a perylenediimide (PDI) -based semiconductor. In certain embodiments, the front sub-cell 110 does not contain a fullerene-based semiconductor; or the back sub-cell 120 does not include a fullerene-based semiconductor. In certain embodiments, the second acceptor material is a non-fullerene-based semiconductor. In certain embodiments, neither the front sub-cell 110 nor the back sub-cell 120 contains a fullerene-based semiconductor.

Each sub-cell in the tandem OSC is separated by a recombination layer 130 comprising a hole transport layer 140 and an electron transport layer 150. The order in which the hole transport layer and the electron transport layer in the tandem OSC can be reversed as needed depending on whether, e.g., the tandem OSC is in an inverted (FIG. 1A) or conventional configuration. FIG. 1B depicts a conventional tandem OSC in which the relative position of the hole transport layer 140 and the electron transport layer 150 are interchanged. Thus, tandem OSCs in which the electron transport layer 150 is positioned on top of the, e.g., front sub-cell 110, followed by the hole transport layer 140 as depicted in FIG. 1B are within the scope of this disclosure. In certain embodiments, the tandem OSCs described herein are inverted tandem OSCs.

In instances in which the tandem OSC is an inverted tandem OSC, the operation of the tandem OSC is as follows: referring to FIG. 2A a photon is absorbed in one of the front sub-cell 110 or the back sub-cell 130 (depending on the wavelength of the photon and the onset of absorption of the each sub-cell) generating an exciton, which can diffuse in the organic films. The donor material -acceptor material bulk heterojunction interface in each sub-cell provides an active site for the exciton dissociation. Upon dissociation of the light-generated excitons at the bulk heterojunction interfaces, the holes generated in the front sub-cell 110 are transported to the hole transport layer 140, while the electrons are transported to the cathode 180. Holes, which are generated in the back sub-cell 120 are collected by the anode 190 and electrons, which are generated in the back sub-cell 120 are transported to the electron transport layer 150. Electrons from the back sub-cell 120 and holes from the front sub-cell 110 are transported towards the recombination layer, where they recombine. The process and flow of electrons and holes is essentially reversed in a conventional tandem OSC due to the interchanged placement of the cathode 190, anode 180, hole transport layer 140, and electron transport layer 150 as shown in FIG. 2B.

The front sub-cell 110 can comprise a PDI-based semiconductor comprising a subunit represented by the formula:

wherein R ₄ for each instance is independently selected from the group consisting of straight-chain, branched, and cyclic alkyl with 2-40 C atoms, wherein one or more non-adjacent C atoms are optionally replaced by –O–, –S–, –C (O) –, –C (=O) –O–, –OC (=O) –, –O–C (=O) O–, –CR ₂=CR ₃–, or –C≡C–, wherein R ₂ and R ₃ are indepedently selected from hydrogen and alkyl; and wherein one or more H atoms are optionally replaced by F, Cl, Br, I, CN, aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl, or heteroaryloxycarbonyl having 4 to 30 ring atoms unsubstituted or substituted by one or more non-aromatic groups. In certain embodiments, R ₄ for each instance is indepedently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, araalkyl, heteroaryl, alkenyl, and alkynyl. In certain embodiments, R ₄ for each instance is independently C ₂-C ₄₀ straight chain alkyl, C ₃-C ₄₀ branched chain alkyl, or C ₃-C ₄₀ cyclic alkyl. In certain embodiments, R ₄ for each instance is independently C ₂-C ₁₄ straight chain alkyl, C ₃-C ₂₀ branched chain alkyl, or C ₃-C ₁₀ cyclic alkyl. In certain embodiments, R ₄ is for each instance is independently C ₄-C ₂₀; C ₆-C ₂₀; C ₄-C ₂₀; C ₈-C ₂₀; C ₈-C ₁₈; C ₈-C ₁₆; C ₁₀-C ₁₆; or C ₁₂-C ₁₆ branched chain alkyl.

In certain embodiments, the front sub-cell 110 comprises the PDI-based semiconductor (that comprise the aforementioned PDI subunit) selected from the group consisting of:

wherein R ₄ for each instance is independently selected from the group consisting of straight-chain, branched, and cyclic alkyl with 2-40 C atoms, wherein one or more non-adjacent C atoms are optionally replaced by –O–, –S–, –C (O) –, –C (=O) O–, –OC (=O) –, –O–C (=O) O–, –CR ₂=CR ₃–, or –C≡C–, wherein R ₂ and R ₃ are indepedently selected from hydrogen and alkyl; and wherein one or more H atoms are optionally replaced by F, Cl, Br, I, CN, aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl, or heteroaryloxycarbonyl having 4 to 30 ring atoms unsubstituted or substituted by one or more non-aromatic groups. In certain embodiments, R ₄ for each instance is indepedently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, araalkyl, heteroaryl, alkenyl, and alkynyl. In certain embodiments, R ₄ for each instance is independently C ₂-C ₄₀ straight chain alkyl, C ₃-C ₄₀ branched chain alkyl, or C ₃-C ₄₀ cyclic alkyl. In certain embodiments, R ₄ is independently C ₂-C ₁₄ straight chain alkyl, C ₃-C ₂₀ branched chain alkyl, or C ₃-C ₁₀ cyclic alkyl. In certain embodiments, R ₄ is independently C ₄-C ₂₀; C ₆-C ₂₀; C ₄-C ₂₀; C ₈-C ₂₀; C ₈-C ₁₈; C ₈-C ₁₆; C ₁₀-C ₁₆; or C ₁₂-C ₁₆ branched chain alkyl.

In certain embodiments, the front sub-cell 110 comprises the PDI-based semiconductor represented by the formula:

wherein R ₄ for each instance is independently selected from the group consisting of straight-chain, branched, and cyclic alkyl with 2-40 C atoms, C ₁-C ₂₀ alkyl, C ₃-C ₈ cycloalkyl, aryl, heteroaryl In certain embodiments, R ₄ is independently C ₂-C ₄₀ straight chain alkyl, C ₃-C ₄₀ branched chain alkyl, or C ₃-C ₄₀ cyclic alkyl. In certain embodiments, R ₄ for each instance is independently C ₂-C ₁₄ straight chain alkyl, C ₃-C ₂₀ branched chain alkyl, or C ₃-C ₁₀ cyclic alkyl. In certain embodiments, R ₄ for each instance is independently C ₄-C ₂₀; C ₆-C ₂₀; C ₄-C ₂₀; C ₈-C ₂₀; C ₈-C ₁₈; C ₈-C ₁₆; C ₁₀-C ₁₆; or C ₁₂-C ₁₆ branched chain alkyl.

wherein R is –CH (C ₆H ₁₃) ₂.

wherein Z is an aromatic unit;

Ar is aryl or heteroaryl;

Y is independently selected from O, S, Se, Te, N-R ₁, wherein R ₁ for each instance is independently a straight-chain, branched, or cyclic alkyl group; or Y for each instance independently represents two hydrogen; and

R for each instance is independently selected from the group consisting of straight-chain, branched, and cyclic alkyl with 2-40 C atoms, wherein one or more non-adjacent C atoms are optionally replaced by –O–, –S–, –C (=O) –, –C (=O) O–, –OC (=O) –, –OC (=O) O–, –CR ₂=CR ₃–, or –C≡C–, and wherein one or more H atoms are optionally replaced by F, Cl, Br, I, CN aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl, or heteroaryloxycarbonyl having 4 to 30 ring atoms unsubstituted or substituted by one or more non-aromatic groups. In certain embodiments, R for each instance is indepedently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, araalkyl, heteroaryl, alkenyl, and alkynyl. In certain embodiments, R for each instance is independently C ₂-C ₄₀ straight chain alkyl, C ₃-C ₄₀ branched chain alkyl, or C ₃-C ₄₀ cyclic alkyl. In certain embodiments, R ₄ is independently C ₂-C ₁₄ straight chain alkyl, C ₃-C ₂₀ branched chain alkyl, or C ₃-C ₁₀ cyclic alkyl. In certain embodiments, R is independently C ₄-C ₂₀; C ₆-C ₂₀; C ₄-C ₂₀; C ₈-C ₂₀; C ₈-C ₁₈; C ₈-C ₁₆; C ₁₀-C ₁₆; or C ₁₂-C ₁₆ branched chain alkyl.

In instances in which each Y represent two hydrogens, the PDI-based semiconductor present in the front sub-cell 110 can be represented by the formula:

wherein Z is an aromatic unit;

Ar is aryl or heteroaryl; and

In certain embodiments, Z is selected from the group consisting of:

In certain embodiments, Ar is selected from the group consisting of:

wherein R for each instance is indepedently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, araalkyl, heteroaryl, alkenyl, and alkynyl. In certain embodiments, R for each instance is independently C ₂-C ₄₀ straight chain alkyl, C ₃-C ₄₀ branched chain alkyl, or C ₃-C ₄₀ cyclic alkyl. In certain embodiments, R ₄ is independently C ₂-C ₁₄ straight chain alkyl, C ₃-C ₂₀ branched chain alkyl, or C ₃-C ₁₀ cyclic alkyl. In certain embodiments, R is independently C ₄-C ₂₀; C ₆-C ₂₀; C ₄-C ₂₀; C ₈-C ₂₀; C ₈-C ₁₈; C ₈-C ₁₆; C ₁₀-C ₁₆; or C ₁₂-C ₁₆ branched chain alkyl.

wherein R is –CH (C ₆H ₁₃) ₂.

The front sub-cell 110 can comrpise a first donor material, wherein the first donor material is a polymer comprising a repeating unit represented by:

wherein R ₇ for each instance is independently selected from C ₄-C ₁₂ alkyl; R ₈ is C ₁-C ₆ alkyl; and R ₉ for each instance is independently selected from C ₆-C ₁₆ alkyl. In certain embodiments, the polymer contains between 20 and 100 repeating units. In certain embodiments, the polymer has an average molecular weight of 40,000 to 80,000 amu.

In certain embodiments, the front sub-cell 110 comprises a first donor material, wherein the first donor material is a polymer comprising a repeating united represented by:

In certain embodiments, the front sub-cell 110 absorbs light in the range of 350-750 nm and/or has an onset of absorption of between 500-750 nm.

The mass ratio of the first donor and the first acceptor can be between 1: 10 to 10: 1. In certain embodiments, the mass ratio of the first donor and the first acceptor can be between 1: 8 to 8: 1; 1: 6 to 6: 1; 1: 4 to 4: 1; or 1: 2 to 2: 1. In certain embodiments, the mass ratio of the first donor and the first acceptor can be between 1: 1.1 to 1: 1.9; 1: 1.2 to 1: 1.8; 1: 1.2 to 1: 1.6; or 1: 1.4 to 1: 1.6.

In certain embodiments, the back sub-cell 120 comprises a second donor material, wherein the second donor material is a polymer comprising a repeating unit represented by:

wherein R ₈ for each instance is independently selected from C ₁-C ₈ alkyl. In certain embodiments, the polymer contains between 5 and 100 repeating units. In certain embodiments, the polymer has an average molecular weight of 10,000 to 100,000 amu.

In certain embodiments, the back sub-cell 120 absorbs light in the range of 350-1,050 nm and/or has an onset of absorption of between 800-1200 nm.

In certain embodiments, the back sub-cell 120 comprises a second acceptor material selected from the group consisting of:

In certain embodiments, the back sub-cell 120 comprises a second acceptor material represented by the formula:

wherein R ₄ for each instance is independently selected from the group consisting of C ₁-C ₂₀ alkyl, C ₃-C ₈ cycloalkyl, aryl, heteroaryl In certain embodiments, R ₄ is independently C ₂-C ₄₀ straight chain alkyl, C ₃-C ₄₀ branched chain alkyl, or C ₃-C ₄₀ cyclic alkyl. In certain embodiments, R ₄ for each instance is independently C ₂-C ₁₄ straight chain alkyl, C ₃-C ₂₀ branched chain alkyl, or C ₃-C ₁₀ cyclic alkyl. In certain embodiments, R ₄ for each instance is independently C ₄-C ₂₀; C ₆-C ₂₀; C ₄-C ₂₀; C ₈-C ₂₀; C ₈-C ₁₈; C ₈-C ₁₆; C ₁₀-C ₁₆; or C ₁₂-C ₁₆ branched chain alkyl.

wherein R ₄ is –CH (C ₆H ₁₃) ₂.

The mass ratio of the second donor and the second acceptor can be between 1: 10 to 10: 1. In certain embodiments, the mass ratio of the second donor and the second acceptor can be between 1: 8 to 8: 1; 1: 6 to 6: 1; 1: 4 to 4: 1; or 1: 2 to 2: 1. In certain embodiments, the mass ratio of the first donor and the first acceptor can be between 1: 1.1 to 1: 1.9; 1: 1.2 to 1: 1.8; 1: 1.2 to 1: 1.6; or 1: 1.4 to 1: 1.6.

In certain embodiments, the back sub-cell 120 further comprises a third acceptor material, wherein the third acceptor material is a fullerene based-acceptor material, such as PC61BM and PC71BM.

In certain embodiments, the front sub-cell 110 comprises a first donor material, wherein the first donor material is a polymer comprising a repeating unit represented by:

wherein R ₇ for each instance is independently selected from C ₄-C ₁₂ alkyl and a first acceptor material represented by the formula:

wherein R for each instance is independently selected from C ₆-C ₂₀ alkyl; and the back sub-cell 120 comprises a second donor material, wherein the second donor material is is a polymer comprising a repeating unit represented by:

wherein R ₈ for each instance is independently selected from C ₁-C ₈ alkyl and a second accepter material selected from the group consisitng of:

wherein for R ₅ for each instance is independently C ₄-C ₁₂ alkyl and R ₆ for each instance is independently C ₂-C ₈.

The tandem OSCs described herein include a recombination layer located between each sub-cell present (e.g., in between the front sub-cell 110 and the back sub-cell 120) in the tandem OSC. The recombination layer comprises a hole transport layer 140 and an electron transport layer 150. As shown in FIG. 2A and FIG. 2B the positioning of the hole transport layer 140 and the electron transport layer 150 can be interchanged depending on whether the tandem OSC has a conventional configuration or an inverted configuration.

In certain embodiments, the hole transport layer comprises at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , polyanaline (PANI) , vanadium (V) oxide (V ₂O ₅) , Molybdenum oxide (MoO ₃) , and Tungsten oxide (WO ₃) .

In certain embodiments, the electron transport layer comprises at least one material selected from the group consisting of zinc oxide (ZnO) , tin oxide (SnO ₂) , lithium fluoride (LiF) , zinc indium tin oxide (ZITO) , poly [ (9, 9-bis (3′- (N, N-dimethylamino) propyl) -2, 7-fluorene) -alt-2, 7- (9, 9–dioctylfluorene) ] (PFN) , poly [ (9, 9-bis (3'- ( (N, N -dimethyl) -N -ethylammonium) -propyl) -2, 7-fluorene) -alt-2, 7- (9, 9-dioctylfluorene) ] (PFN-Br) , and poly [9, 9-bis (6’- (N, N-diethylamino) propyl) -fluorene-alt-9, 9-bis (3-ethyl (oxetane-3-ethyloxy) -hexyl) -fluorene] (PFN-OX) .

In certain embodiments, the front sub-cell 110 comprises a PDI-based semiconductor represented by the formula:

wherein R is –CH (C ₆H ₁₃) ₂; the first donor material present in the front sub-cell 110 is is a polymer comprising a repeating unit represented by

the second donor material present in the back sub-cell 120 is is a polymer comprising a repeating unit represented by

the second accepter material present in the back sub-cell 120 is selected from the group consisting of:

the hole transport layer 140 is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , polyaniline (PANI) , silver, and molybdenum (VI) oxide (MoO ₃) ; and the electron transport layer 150 is at least one material selected from the group consisting of zinc oxide (ZnO) and tin (IV) oxide (SnO ₂) .

In certain embodiments, the front sub-cell 110 comprises a first acceptor material and the back sub-cell 120 comprises a second acceptor represented by the formula:

the front sub-cell 110 comprises a first donor material and the back sub-cell 120 comprises a second donor material represented by the formula:

the recombination layer 130 comprises a hole transport layer 140 selected from at least one material from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , silver, and molybdenum (VI) oxide (MoO ₃) ; and the electron transport layer 150 is zinc oxide (ZnO) .

The tandem OSCs described herein can further comprise at least one cathode buffer layer 160, an anode buffer layer 170, or both a cathode buffer layer 160 and an anode buffer layer 170.

The cathode buffer layer 160 can function as an electron transfer layer, which allows electrons generated in an adjacent sub-cell to be transferred to the adjacent cathode, while blocking hole generated in the adjacent sub-cell from being injected into the adjacent cathode. Suitable materials useful as a cathode buffer layer 160 include, but are not limited to, LiCoO ₂, LiF, CsF, zinc (II) oxide (ZnO) , Cs ₂CO ₃, titanium (IV) oxide (TiO _x) , zirconium (IV) acetylacetonate, poly [ (9, 9-bis (3- (N, N-dimethylamino) propyl) -2, 7-fluorene) -alt-2, 7- (9, 9-dioctylfluorene) (PFN) , ( (N, N-dimethyl) -N-ethylammonium) -propyl) -2, 7-fluorene) -alt-2, 7- (9, 9-dioctylfluorene) ] dibromide (PFN-Br) and mixtures thereof.

The anode buffer layer 170 can function as an hole transfer layer, which allows holes generated in an adjacent sub-cell to be transferred to the adjacent anode, while blocking electrons generated in the adjacent sub-cell from being transferred into the adjacent anode. Suitable materials useful as an anode buffer layer 170 include, but are not limited to molybdenum (VI) oxide MoO ₃, nickel oxide NiO _x, vanadium (V) oxide (V ₂O ₅) , tungsten (VI) (WO ₃) , PEDOT: PSS and mixtures thereof.

While the tandem OSCs exemplified in the examples below contain only two sub-cells (e.g., a front sub-cell and a back sub-cell) in an inverted configuration, also contemplated within this disclosure are tandem OSCs containing more than two sub-cells and/or tandem OSCs in a standard configuration. For example, the teachings provided herein can be applied in the manufacture of tandem OSCs containing any number of sub-cells, for example, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, or more sub-cells. The preparation of such tandem OSCs is well within the skill of a person of skill in the art.

In certain embodiments, the tandem OSCs described herein can achieve a PCE of over 12 %, along with a short-circuit current density of over 12 mA cm ^-2 under AM 1.5G illumination (100 mW cm ^-2) . In certain embodiments, the tandem OSCs have a PCE between 12%to 13.3%; 12.2%to 13.3%; 12.3%to 13.3%; 12.5%to 13.3%; 12.3%to 13.2%; 12.6%to 13.2%; or 12.9%to 13.2%.

In instances in which the tandem OSC has an inverted configuration, the transparent cathode may generally include any transparent or semi-transparent conductive material. Indium tin oxide (ITO) can be used for this purpose, because it is substantially transparent to light transmission and thus facilitates light transmission through the ITO cathode layer to the sub-cells of the tandem OSC without being significantly attenuated. The term “transparent” means allowing at least 50 percent, commonly at least 80 percent, and more commonly at least 90 percent, of light in the wavelength range between 350-150 nm to be transmitted.

The anode 190 can be any anodic material known to those of skill in the art. In certain embodiments, the anode comprises aluminum, gold, copper, silver, or a combination thereof.

The transparent cathode 180 can comprise a metal oxide transparent electrode, such as an indium tin oxide (ITO) , fluoride-doped tin oxide (FTO) , zinc oxide (ZnO) , indium zinc oxide (IZO) or Al-doped zinc oxide (AZO) , or be formed of a conductive polymer, carbon nanotube or graphene. In certain embodiments, the transparent cathode 180 is ITO/Ag/ITO, aluminum doped ZnO/metal, a thin metal layer, doped or undoped single walled carbon nanotubes (SWNTs) , or patterned metal nanowires comprising gold, silver, or copper.

The cathode 181 may include a metal or alloy having a relative low work function, such as aluminum, lithium, magnesium, calcium, indium, potassium, alloys thereof or combinations thereof.

The tandem OSCs described herein can further comprise a light transmitting substrate on the exterior surface of the transparent anode 191 or transparent cathode 180. The light transmitting substrate can serve to protect the tandem OSC from physical and/or chemical damage. The light transmitting substrate may be a glass substrate or a light transmitting resin substrate having a high light transmittance, such as of polyethylene terephthalate (PET) , polystyrene, polycarbonate, polymethylmethacrylate, polyimide, and the like.

Advantageously, the tandem OSCs described herein can be processed or at least partially processed using solution-processing, such as by dip coating, spray coating, ink-jet printing, spin coating, roller coating, and/or thermal evaporation methods. Moreover, in certain embodiments, the tandem OSCs provided herein can be prepared using methods and reagents as described herein, which advantageously do not require high temperatures for annealing the front sub-cell, the recombination layer, and the back sub-cell. Using lower annealing temperatures, e.g., between 70 and 110 ℃ or between 80 and 100 ℃, can limit damage caused by melting, e.g., the first and second donor materials and the first and second acceptor materials in the front sub-cell and the back sub-cell, which can result in improved efficiency of the formed tandem OSCs.

Conventional methods for forming a zinc (II) oxide layer (e.g., when forming a zinc (II) oxide cathode buffer layer or electron transport layer) typically rely on sol-gel, spray-coating, and nanoparticle depositions. In most instances, the as-deposited zinc (II) oxide film requires annealing at high temperature in order to optimize the crystalline structure of the zinc (II) oxide thin film. However, such high temperature annealing steps can negatively affect the structure of the bulk heterojunction in the front sub-cell, which can lower the PCE of the resulting tandem OSC. Advantageously, the methods for forming the zinc (II) oxide electron transport layer as described herein do not require a high temperature annealing step. Instead, they can be conducted below 100 ℃, which is above the crystallization temperature and/or melting temperature of the first donor material and the second donor material.

The tandem OSCs can be fabricated on a transparent cathode (e.g., ITO) coated glass substrate. The cathode buffer layer 160 can be prepared using any method known in the art, such as by physical vapor deposition, chemical vapor deposition, or sputtering, and other processes. In certain embodiments, the cathode buffer layer 160 can be prepared by the deposition of a solution of an organic zinc compound in an organic solvent followed by annealing the deposited organic zinc compound solution at a temperature of 60 to 120; 70 to 120; 80 to 120; 80 to 110 or 80 to 100 ℃ thereby forming a zinc oxide cathode buffer layer 160. Suitable organic zinc compounds include any aryl, alkyl, cycloalkyl, alkenyl, and alkynyl zinc species. In certain embodiments, the organic zinc compound is a dialkyl zinc compound, such as dimethyl or diethyl zinc. Due to the reactivity of the organic zinc compound, the organic zinc compound is typically present in anhydrous solvent, such as an ether, alkane, and/or aromatic solvent. In the examples below, a solution of diethyl zinc in tetrahydrofuran is deposited on the ITO layer by spin coating. The deposited thin layer of diethyl zinc is then annealed at a temperature of 60 to 120; 70 to 120; 80 to 120; 80 to 110 or 80 to 100 ℃.

The front sub-cell 110 can be fabricated by depositing a thin layer of a solution comprising the first donor material and the first acceptor material onto a substrate, e.g., the cathode buffer layer 160, and annealing the deposited thin film thereby forming the front sub-cell 110. The first donor material and the first acceptor material as described herein are typically deposited from solvents that the first donor material and the first acceptor material are substantially soluble in at the temperature of deposition. Such solvents, include but not limited to alkanes, halolakanes, aromatic, haloaromatic, and combinations thereof. Exemplary solvents that are useful for preparing the solution of the first donor material and first acceptor material include petroleum ether, chloroform, carbon tetrachloride, dichloroethane, trichloroethane, benzene, chlorobenzene, 1, 2-dichlorobenzene, 1, 3-dichlorobenzene, 1, 4-dichlorobenzene, ortho-xylene, meta-xylene, para-xylene, 1, 2, 3-trimethyl benzene, 1, 2, 4-trimethyl benzene, 1, 3, 5-trimethyl benzene, and combinations thereof. In certain embodiments, the solvent comprising the first donor material and the first acceptor material further comprises 1, 8-diiodooctane. The 1, 8-diiodooctane can be presented at a concentration of 2-3%v/v.

The concentration of the first donor and the first acceptor in the solvent can be between 1 mg/mL to 1,000 mg/mL. In certain embodiments, the concentration of the first donor and the first acceptor in the solvent is 1 mg/mL to 100 mg/mL; 1 mg/mL to 50 mg/mL; 1 mg/mL to 40 mg/mL; 1 mg/mL to 30 mg/mL; 1 mg/mL to 20 mg/mL; 5 mg/mL to 20 mg/mL; 5 mg/mL to 15 mg/mL; 7 mg/mL to 15 mg/mL; or 7 mg/mL to 12 mg/mL.

The solution comprising the first donor material and first acceptor material can be heated above the crystallization temperature of the first donor material and the first acceptor material to ensure that during the deposition process that they remain substantially in the solution phase. The temperature of the solution can be heated to between 40 to 140; 50 to 140; 60 to 140; 70 to 140; 70 to 130; 70 to 120; 70 to 110; 80 to 110; 80 to 100; or 85 to 95 ℃. The substrate can also be pre-heated prior to deposition of the solution containing the first donor material and first acceptor material. In such instances, the substrate is preheated to 40 to 140; 50 to 140; 60 to 140; 70 to 140; 70 to 130; 70 to 120; 70 to 110; 80 to 110; 80 to 100; or 85 to 95 ℃ prior to and during deposition. The solution containing the first donor material and first acceptor material can be deposited using any method known in the art, such as by dip coating, spray coating, ink-jet printing, spin coating, and roller coating.

Once deposited on the substrate, the thin film comprising the first acceptor and the first donor can be annealed at a temperature between 70 to 120; 70 to 110; 80 to 110; 80 to 100; or 85 to 95 ℃ thereby forming the front sub-cell 110.

The front sub-cell 110 can have a thickness of 60 to 140 nm. In certain embodiments, the thickness of the front sub-cell 110 is 80 to 120; 90 to 120; 90 to 110; or 90 to 100 nm. In certain embodiments, the front sub-cell 110 is 10-30 nm thinner than the back sub-cell 120.

As discussed in greater detail below, depending on the composition of the recombination layer 130, it can be made using any method known in the art, such as by sequential physical vapor deposition, chemical vapor deposition, sputtering, dip coating, spray coating, ink-jet printing, spin coating, and roller coating of the hole transport layer 140 and the electron transport layer 150. The selection of the method (s) for preparing the recombination layer 130 is well within the skill of a person of skill in the art.

In embodiments in which the hole transport layer 140 comprises MoO ₃/Ag, the hole transport layer can be deposited by sequential thermal evaporation of the e.g., MoO ₃ and Ag onto the front sub-cell 110. Typically, the thus formed hole transport layer 140 has a thickness of 10 to 20 nm; 12 to 20 nm; or 12 to 17 nm.

In embodiments in which the hole transport layer 140 comprises PEDOT: PSS or PANI, the hole transport layer can be prepared by depositing a solution or dispersion PEDOT: PSS. Useful solvents for depositing the PEDOT: PSS of PANI layer include, but are not limited to water, alcohols, such as methanol, ethanol, isopropyl alcohol, and aminoethanol, ethers, acetonitrile, haloalkanes, esters, ketones, aromatics, and combinations thereof. After the solution comprising PEDOT: PSS or PANI is deposited as a thin layer on to the substrate, e.g., the front sub-cell 110, the thin layer comprising the hole transport layer can optionally be annealed by heating it to a temperature between 60 to 90 ℃ prior to deposition of the solution comprising the electron transport layer.

The electron transport layer 150 can be prepared by depositing a solution comprising an electron transport layer precursor. In such embodiments, the electron transport layer is prepared by the deposition of a solution comprising an organic zinc compound in an organic solvent followed by annealing the deposited organic zinc compound solution at a temperature of 60 to 120; 70 to 120; 80 to 120; 80 to 110 or 80 to 100 ℃ thereby forming the electron transport layer 150. Suitable organic zinc compounds include any aryl, alkyl, cycloalkyl, alkenyl, and alkynyl zinc species. In certain embodiments, the organic zinc compound is a dialkyl zinc compound, such as dimethyl or diethyl zinc. Due to the reactivity of the organic zinc compound, it is typically deposited from an anhydrous solvent, such as an ether, alkane, and/or aromatic solvent. In the examples below, a solution of diethyl zinc in tetrahydrofuran is deposited on the ITO layer by spin coating. The deposited thin layer of diethyl zinc is then annealed at a temperature of 60 to 120; 70 to 120; 80 to 120; 80 to 110 or 80 to 100 ℃.

The SnO ₂ layer is deposited according to the following procedure. SnO ₂ (15%in water, colloidal dispersion) was diluted with deionized water (1: 6 v/v) and then spin-coated on the front sub-cell at a spin rate of 3000 to 5000 rpm to achieve a thin layer of 20-50 nm.

The back sub-cell 120 can be fabricated by depositing a thin layer of a solution comprising the second donor material and the second acceptor material onto a substrate, e.g., the recombination layer 130, and annealing the deposited thin film thereby forming the back sub-cell 120. The second donor material and the second acceptor material as described herein are typically deposited from solvents that the second donor material and the second acceptor material are substantially soluble in at the temperature of deposition. Such solvents, include but not limited to alkanes, halolakanes, aromatic, haloaromatic, and combinations thereof. Exemplary solvents that are useful for preparing the solution of the second donor material and second acceptor material include petroleum ether, chloroform, carbon tetrachloride, dichloroethane, trichloroethane, benzene, chlorobenzene, 1, 2-dichlorobenzene, 1, 3-dichlorobenzene, 1, 4-dichlorobenzene, ortho-xylene, meta-xylene, para-xylene, 1, 2, 3-trimethyl benzene, 1, 2, 4-trimethyl benzene, 1, 3, 5-trimethyl benzene, and combinations thereof. In certain embodiments, the solvent comprising the second donor material and the second acceptor material further comprises 1, 8-diiodooctane. The 1, 8-diiodooctane can be presented at a concentration of 2-3%v/v.

The solution comprising the second donor material and the second acceptor material can be heated above the crystallization temperature of the second donor material and the second acceptor material to ensure that during the deposition process that they remain substantially in the solution phase. The temperature of the solution can be heated to between 40 to 140; 50 to 140; 60 to 140; 70 to 140; 70 to 130; 70 to 120; 70 to 110; 80 to 110; 80 to 100; or 85 to 95 ℃. The substrate can also be pre-heated prior to deposition of the solution containing the second donor material and second acceptor material. In such instances, the substrate is preheated to 40 to 140; 50 to 140; 60 to 140; 70 to 140; 70 to 130; 70 to 120; 70 to 110; 80 to 110; 80 to 100; or 85 to 95 ℃ prior to and during deposition. The solution containing the second donor material and second acceptor material can be deposited using any method known in the art, such as by dip coating, spray coating, ink-jet printing, spin coating, and roller coating.

The back sub-cell 120 can have a thickness of 80 to 140 nm. In certain embodiments, the thickness of the back sub-cell 120 is 80 to 130 nm; 90 to 130 nm; 100 to 130 nm; 110 to 120 nm; or 115 to 125 nm. In certain embodiments, the back sub-cell 120 is 10-30 nm thicker than the front sub-cell 110.

In certain embodiments, the front sub-cell 110 has a thickness of 80 to 120 nm and the back sub-cell 120 has a thickness 100 to 130 nm; the front sub-cell 110 has a thickness of 90 to 110 nm and the back sub-cell 120 has a thickness 100 to 120 nm; the front sub-cell 110 has a thickness of 100 to 110 nm and the back sub-cell 120 has a thickness 115 to 125 nm; or the front sub-cell 110 has a thickness of 95 to 105 nm and the back sub-cell 120 has a thickness 115 to 125 nm.

The anode buffer layer 170 buffer layer can be deposited onto the back sub-cell using any method known in the art, such as by physical vapor deposition, chemical vapor deposition, sputtering, dip coating, spray coating, ink-jet printing, spin coating, and roller coating. In the examples below, the anode buffer layer 170 (e.g., V ₂O ₅ or MoO ₃) is deposited using thermal vaporization. The anode buffer layer 170 can have any thickness, the selection of which is well within the skill of a person of skill in the art. In certain embodiments, the thickness of anode buffer layer 170 is 1 to 10 nm; 5 to 10 nm; or 6 to 8 nm.

The anode 190 can be deposited on the anode buffer layer using any method known in the art, such as by physical vapor deposition, chemical vapor deposition, or sputtering. In the examples below, an aluminum anode is deposited using thermal vaporization. The anode 190 can have any thickness, the selection of which is well within the skill of a person of skill in the art. In certain embodiments, the thickness of anode 190 is 50 to 150 nm; 70 to 130 nm; or 80 to 120 nm.

Hereinafter, the tandem OSCs, their methods of preparation, and the use of formulations in their preparation thereof will be described in more detail with reference to the following examples. These examples are provided for illustrative purposes only and are not to be in any way construed as limiting the present disclosure.

EXAMPLES

Example 1 –Fabrication of homo-tandem organic solar cells based a PEDOT: PSS/ZnO recombination layer

Step 1: Preparation of PEDOT: PSS solution.

The PEDOT: PSS precursor solution can be prepared by diluting commercially available aqueous dispersions with polar solvents, including water, methanol, ethanol, isopropyl alcohol, aminoethanol and so on, in a weight ratio from 0.1 %to 10 %. Various additives, surfactants or stabilizers, including dimethyl sulfoxide, fluorinated surfactants and so on, can be added into the PEDOT: PSS solution to alter its wettability, mobility or acidity-basicity.

Step 2: Preparation of ZnO precursor.

ZnO precursor can be prepared by dissolving diethyl zinc (DEZ) in various organic solvents, including hexane, toluene, tetrahydrofuran and so on, in a weight ratio from 0.1 %to 2 %.

Step 3: Fabrication of homo-tandem solar cells.

Diethylzinc (15 %wt in toluene) , MoO ₃ and vanadium (V) oxide (V ₂O ₅) were purchased from Sigma-Aldrich and used as received without further treatment. The synthesis of P3TEA and SF-PDI ₂ can be found elsewhere. Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, 15 wt. %in toluene diluted with tetrahydrofuran with a ratio of 1: 7 v/v) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA: SF-PDI ₂ ratio 1: 1.5 w/w) were prepared in 1, 2, 4-trimethylbenzene (polymer concentration: 9 mg mL ^-1, 1, 8-octanedithiol 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The P3TEA (average molecular weight of 40,000 to 80,000 amu) : SF-PDI ₂ blend films were then annealed at 90 ℃ for 5 min, After that, PEDOT: PSS (Clevios HTL SOLAR, PEDOT: PSS dispersion in water having a solids content of 1.5 to 2.5%) was deposited on top of the front sub-cell at a speed of 5000 rpm for 30 s, and the resultant thickness was 55 nm. Subsequently, another thin ZnO layer (25 nm) was spin-coated onto the PEDOT: PSS layer followed by a thermal annealing step at 80 ℃ for 10 min. AFM height (left) and cross-section (right) images of glass/ITO/P3TEA (average molecular weight of 40,000 to 80,000 amu) : SF-PDI ₂/PEDOT: PSS/ZnO can be found in FIG. 3 (scan area is 1 μm×1 μm and the vertical data scale is 10 nm) . Then, the identical active layer solutions were spin-coated on the top of ZnO at various spin conditions. The optimized active layer thickness for the front and rear sub-cells were 100 nm and 120 nm, respectively. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10 ^-4 Pa, a thin layer (7 nm) of V ₂O ₅ was deposited as the anode interlayer, followed by the deposition of 100 nm of Al as the top electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 2: Fabrication of homo-tandem organic solar cells based on a MoO ₃/Ag/ZnO recombination layer

Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA (average molecular weight of 40,000 to 80,000 amu) : SF-PDI2 ratio 1: 1.5 w/w) were prepared in chlorobenzene (polymer concentration: 10 mg mL ^-1, 1, 8-diiodooctane 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The blend films were then annealed at 90 ℃ for 5 min, After that, a thin layer of MoO ₃ (10 m) and a thin layer of silver (Ag, 5 nm) were thermally evaporated on the front active layer. Subsequently, another thin ZnO layer (25 nm) was spin-coated onto the MoO ₃/Ag layer followed by a thermal annealing step at 80 ℃ for 10 min. Then, the identical active layer solutions were spin-coated on the top of ZnO at various spin conditions. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10 ^-4 Pa, a thin layer (7 nm) of V ₂O ₅ or MoO3 was deposited as the anode interlayer, followed by the deposition of 100 nm of Al as the top electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 3 –Fabrication of hybrid tandem organic solar cells based a PEDOT: PSS/ZnO recombination layer

Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 ratio 1: 1.5 w/w) were prepared in 1, 2, 4-trimethylbenzene (polymer concentration: 9 mg mL ^-1, 1, 8-octanedithiol 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 blend films were then annealed at 90 ℃ for 5 min, After that, PEDOT: PSS (Clevios HTL SOLAR, PEDOT: PSS dispersion in toluene having a solids content of 1.5 to 2.5%) was deposited on top of the front sub-cell at a speed of 5000 rpm for 30 s, and the resultant thickness was 55 nm. Subsequently, another thin ZnO layer (25 nm) was spin-coated onto the PEDOT: PSS layer followed by a thermal annealing step at 80 ℃ for 10 min. Then, the PTB7-Th (average molecular weight 10,000 to 100,000 amu) : IEICO-4F solution were spin-coated on the top of ZnO at various spin conditions. The optimized active layer thickness for the front and rear sub-cells were 100 nm and 120 nm, respectively. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10 ^-4 Pa, a thin layer (7 nm) of V ₂O ₅ was deposited as the anode interlayer, followed by the deposition of 100 nm of Al as the top electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 4 –Fabrication of hybrid tandem organic solar cells based a MoO ₃/Ag/ZnO recombination layer

Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 ratio 1: 1.5 w/w) were prepared in chlorobenzene (polymer concentration: 10 mg mL ^-1, 1, 8-diiodooctane 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The blend films were then annealed at 90 ℃ for 5 min, After that, a thin layer of MoO ₃ (10 nm) and a thin layer of silver (Ag, 5 nm) were thermally evaporated on the front active layer. Subsequently, another thin ZnO layer (25 nm) was spin-coated onto the Ag layer followed by a thermal annealing step at 80 ℃ for 10 min. Then, the PTB7-Th (average molecular weight 10,000 to 100,000 amu) : IEICO-4Fsolution were spin-coated on the top of ZnO at various spin conditions. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10 ^-4 Pa, a thin layer (7 nm) of V ₂O ₅ was deposited as the anode interlayer, followed by the deposition of 100 nm of Al as the top electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 5 –Fabrication of hybrid tandem solar cells based a PANI/ZnO recombination layer

Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 ratio 1: 1.5 w/w) were prepared in chlorobenzene (polymer concentration: 10 mg mL ^-1, 1, 8-diiodooctane 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The blend films were then annealed at 90 ℃ for 5 min. After that, a thin layer of PANI was dissolved in xylene and was spin-coated onto the top of the front active layer at a spin rate of 5000 rpm. Subsequently, another thin ZnO layer (25 nm) was spin-coated onto the PANI layer followed by a thermal annealing step at 80 ℃ for 10 min. Then, the PTB7-Th (average molecular weight 10,000 to 100,000 amu) : IEICO-4F solution were spin-coated on the top of ZnO at various spin conditions. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10 ^-4 Pa, a thin layer (7 nm) of V ₂O ₅ was deposited as the anode interlayer, followed by the deposition of 100 nm of Al as the top electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 6 –Fabrication of hybrid tandem solar cells based a PANI/SnO ₂ recombination layer

Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 ratio 1: 1.5 w/w) were prepared in chlorobenzene (polymer concentration: 10 mg mL ^-1, 1, 8-diiodooctane 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The blend films were then annealed at 90 ℃ for 5 min. After that, a thin layer of PANI was spin-coated onto the top of the front active layer. Subsequently, another thin SnO ₂ layer (15%in water, colloidal dispersion) diluted with deionized water (1: 6 v/v) ) was spin-coated onto the PANI layer followed by a thermal annealing step at 80 ℃ for 10 min. Then, the PTB7-Th (average molecular weight 10,000 to 100,000 amu) : IEICO-4F solution were spin-coated on the top of SnO ₂ at various spin conditions. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10 ^-4 Pa, a thin layer (7 nm) of V ₂O ₅ was deposited as the anode interlayer, followed by the deposition of 100 nm of Al as the top electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 7 –Fabrication of hybrid tandem solar cells based a PEDOT: PSS/SnO ₂ recombination layer

Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 ratio 1: 1.5 w/w) were prepared in chlorobenzene (polymer concentration: 10 mg mL ^-1, 1, 8-diiodooctane 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The blend films were then annealed at 90 ℃ for 5 min, After that, a thin layer of PEDOT: PSS was spin-coated onto the top of the front active layer. Subsequently, another thin SnO ₂ layer (25 nm) was spin-coated onto the PEDOT: PSS layer followed by a thermal annealing step at 80 ℃ for 10 min. Then, the PTB7-Th (average molecular weight 10,000 to 100,000 amu) : IEICO-4F solutions were spin-coated on the top of SnO ₂ at various spin conditions. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10 ^-4 Pa, a thin layer (7 nm) of V ₂O ₅ was deposited as the anode interlayer, followed by the deposition of 100 nm of Al as the top electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 8 –Fabrication of hybrid tandem solar cells with ternary sub- cell

Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 ratio 1: 1.5 w/w) were prepared in 1, 2, 4-trimethylbenzene (polymer concentration: 9 mg mL-1, 1, 8-octanedithiol 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 blend films were then annealed at 90 ℃ for 5 min, After that, PEDOT: PSS (Clevios HTL SOLAR, PEDOT: PSS dispersion in toluene having a solids content of 1.5 to 2.5%) was deposited on top of the front sub-cell at a speed of 5000 rpm for 30 s, and the resultant thickness was 55 nm. Subsequently, another thin ZnO layer (25 nm) was spin-coated onto the PEDOT: PSS layer followed by a thermal annealing step at 80 ℃ for 10 min. Then, the PTB7-Th (average molecular weight 10,000 to 100,000 amu) : IEICO-4F: PCBM solutions with various weight ratios were spin-coated on the top of ZnO at various spin conditions. The optimized active layer thickness for the front and rear sub-cells were 100 nm and 120 nm, respectively. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10-4 Pa, a thin layer (7 nm) of V ₂O ₅ was deposited as the anode interlayer, followed by the deposition of 100 nm of Al as the top electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 9 –Fabrication of hybrid semitransparent tandem solar cells

Pre-patterned ITO-coated glass substrates were cleaned by sequential sonication in soap deionized water, deionized water, acetone, and isopropanol for 30 min of each step. After UV/ozone treatment for 60 min, a ZnO electron-transporting layer (～23 nm) was prepared by spin-coating a ZnO precursor solution (diethyl zinc, diluted with tetrahydrofuran) at 5000 rpm, and then annealed at 100 ℃ for 15 min. Active layer solutions (P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4 ratio 1: 1.5 w/w) were prepared in 1, 2, 4-trimethylbenzene (polymer concentration: 9 mg mL-1, 1, 8-octanedithiol 2.5 %v/v as additive) . To completely dissolve the polymer, the active layer solution should be stirred on a hotplate at 90 ℃ for at least 1 hour. Before spin-coating, both the polymer solution and substrates were preheated on a hotplate at ～90 ℃. Active layers were spin-coated from the warm polymer solutions onto the preheated substrates in a N ₂ glovebox at 1200-1500 rpm. The P3TEA (average molecular weight of 40,000 to 80,000 amu) : FTTB-PDI4blend films were then annealed at 90 ℃ for 5 min, After that, PEDOT: PSS (Clevios HTL SOLAR, PEDOT: PSS dispersion in toluene having a solids content of 1.5 to 2.5%) was deposited on top of the front sub-cell at a speed of 5000 rpm for 30 s, and the resultant thickness was 55 nm. Subsequently, another thin ZnO layer (25 nm) was spin-coated onto the PEDOT: PSS layer followed by a thermal annealing step at 80 ℃ for 10 min. Then, the PTB7-Th (average molecular weight 10,000 to 100,000 amu) : IEICO-4F or PTB7-Th (average molecular weight 10,000 to 100,000 amu) : IEICO-4F: PCBM solutions were spin-coated on the top of ZnO at various spin conditions. The optimized active layer thickness for the front and rear sub-cells were 100 nm and 120 nm, respectively. After that, substrates were transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of ～1.5×10-4 Pa, a thin layer (7 nm) of V ₂O ₅ or MoO ₃ was deposited as the anode interlayer, followed by the deposition of thin Ag or Au electron (5 to 30 nm) as the transparent electrode. All devices were encapsulated using epoxy inside the glovebox.

Example 10 -Device characterizations

Device J-V characteristics were measured under AM 1.5G (100 mW cm ^-2) using a Newport solar simulator. The light intensity was calibrated using a standard Si diode (with KG5 filter, purchased from PV Measurement) to bring spectral mismatch to unity. J-V characteristics were recorded using a Keithley 2400 source meter unit. Typical cells have devices area of 5.9 mm ², defined by a metal mask with an aperture aligned with the device area. EQEs were measured using an Enlitech QE-SEQE system equipped with a standard Si diode. Monochromatic light was generated from a Newport 300W lamp source. These test protocols are exactly the same as that we used in previously certified OPVs. All thicknesses of the layers involved were determined by variable angle spectroscopic ellipsometry (J. A. Woollam Co. α-SE) in the transparent wavelength range of the films. AFM measurements were performed by using a Scanning Probe Microscope-Dimension 3100 in tapping mode. UV-Vis absorption spectra were acquired on a Perkin Elmer Lambda 20 UV/VIS Spectrophotometer. All film samples were spincast on ITO/ZnO substrates. The J-V and EQE curves are shown in FIG. 4 and FIG. 5, respectively.

Table 1. Photovoltaic performances of P3TEA: SF-PDI ₂-based double-junction tandem solar cells based on a PEDOT: PSS/ZnO recombination layer.

Table 2. Photovoltaic performances of P3TEA: SF-PDI ₂ homo-tandem solar cells based on a MoO3/Ag/ZnO recombination layer.

Table 3. Photovoltaic performances of P3TEA: FTTB-PDI4/PTB7-Th: IEICO-4F hybrid tandem solar cells based on different recombination layers.

Table 4. Photovoltaic performances of P3TEA: SF-PDI2/PTB7-Th: IEICO-4F hybrid tandem solar cell.

Table 5. Photovoltaic performances of semitransparent tandem OSCs.

Claims

A tandem organic solar cell (OSC) comprising:

a front sub-cell comprising a first donor material and a first acceptor material;

a recombination layer comprising a hole transport layer and an electron transport layer; and

a back sub-cell comprising a second donor material and a second acceptor material, wherein the recombination layer is disposed between the front sub-cell and the back sub-cell; the first acceptor material is a perylenediimide (PDI) -based semiconductor; and the second acceptor material is a non-fullerene-based semiconductor.
The tandem OSC of claim 1, wherein the PDI-based semiconductor is represented by the formula:

wherein

Z is an aromatic unit;

Ar is aryl or heteroaryl;

Y for each instance is independently selected from, -O-, -S-, -Se-, -Te-, and -N (R ₁) -, wherein each R ₁ is independently a straight-chain, branched, or cyclic alkyl group; or Y for each instance independently represents two hydrogen; and

R for each instance is independently selected from the group consisting of straight-chain, branched, and cyclic alkyl with 2-40 C atoms, wherein one or more non-adjacent C atoms are optionally replaced by –O–, –S–, –C (=O) –, –C (=O) O–, –OC (=O) –, –OC (=O) O–, –CR ₂=CR ₃–, or –C≡C–, wherein R ₂ and R ₃ are indepedently selected from hydrogen and alkyl; and wherein one or more H atoms are optionally replaced by F, Cl, Br, I, CN, aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl, or heteroaryloxycarbonyl having 4 to 30 ring atoms unsubstituted or substituted by one or more non-aromatic groups; or R for each instance is indepedently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, araalkyl, heteroaryl, alkenyl, and alkynyl.
The tandem OSC fo claim 2, wherein the PDI-based semiconductor is represented by the formula:
The tandem OSC of claim 1, wherein the PDI-based semiconductor comprises a subunit represented by the formula:

wherein R ₄ for each instance is independently selected from the group consisting of straight-chain, branched, and cyclic alkyl with 2-40 C atoms, wherein one or more non-adjacent C atoms are optionally replaced by –O–, –S–, –C (=O) –, –C (=O) O–, –OC (=O) –, –OC (=O) O–, –CR ₂=CR ₃–, or –C≡C–, wherein R ₂ and R ₃ are indepedently selected from hydrogen and alkyl; and wherein one or more H atoms are optionally replaced by F, Cl, Br, I, CN, aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl, or heteroaryloxycarbonyl having 4 to 30 ring atoms unsubstituted or substituted by one or more non-aromatic groups; or R ₄ for each instance is indepedently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, araalkyl, heteroaryl, alkenyl, and alkynyl.
The tandem OSC of claim 4, wherein the PDI-based semiconductor is selected from the group consisting of:
The tandem OSC of claim 1, wherein the second acceptor matieral is selected from the group consisitng of:

wherein for R ₅ for each instance is independently C ₄-C ₁₂ alkyl; and R ₆ for each instance is independently C ₂-C ₈.
The tandem OSC of claim 1, wherein the first donor material is a polymer comprising a repeating unit represented by:

wherein R ₇ for each instance is independently selected from C ₄-C ₁₂ alkyl.
The tandem OSC of claim 1, wherein the second donor material is is a polymer comprising a repeating unit represented by:

wherein R ₈ for each instance is independently selected from C ₁-C ₈ alkyl.
The tandem OSC of claim 1, wherein the hole transport layer is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , polyaniline (PANI) , vanadium (V) oxide (V ₂O ₅) , molybdenum (VI) oxide (MoO ₃) , and Tungsten (VI) oxide (WO ₃) ; and the electron transport layer is at least one material selected from the group consisting of zinc oxide (ZnO) , tin (IV) oxide (SnO ₂) , lithium fluoride (LiF) , zinc indium tin oxide (ZITO) , poly [ (9, 9-bis (3′- (N, N-dimethylamino) propyl) -2, 7-fluorene) -alt-2, 7- (9, 9–dioctylfluorene) ] (PFN) , poly [ (9, 9-bis (3'- ( (N, N -dimethyl) -N -ethylammonium) -propyl) -2, 7-fluorene) -alt-2, 7- (9, 9-dioctylfluorene) ] (PFN-Br) , and poly [9, 9-bis (6’- (N, N-diethylamino) propyl) -fluorene-alt-9, 9-bis (3-ethyl (oxetane-3-ethyloxy) -hexyl) -fluorene] (PFN-OX) .
The tandem OSC of claim 7, wherin the PDI-based semiconductor is represented by the formula:

wherein R for each instance is independently selected from C ₆-C ₂₀ alkyl; the second donor material is is a polymer comprising a repeating unit represented by:

wherein R ₈ for each instance is independently selected from C ₁-C ₈ alkyl; and the second accepter material is selected from the group consisitng of:

wherein for R ₅ for each instance is independently C ₄-C ₁₂ alkyl; and R ₆ for each instance is independently C ₂-C ₈.
The tandem OSC of claim 10, wherein the front sub-cell has an optical absorption onset between 500 and 750 nm and the back sub-cell has an optical absorption onset between 800 to 1100 nm.
The tandem OSC of claim 10, wherein the hole transport layer is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , polyaniline (PANI) , silver, and molybdenum (VI) oxide (MoO ₃) ; and the electron transport layer is at least one material selected from the group consisting of zinc oxide (ZnO) and tin (IV) oxide (SnO ₂) .
The tandem OSC of claim 10, wherein wherin the PDI-based semiconductor is represented by the formula:

wherein R is –CH (C ₆H ₁₃) ₂; the first donor material is is a polymer comprising a repeating unit represented by:

the second donor material is is a polymer comprising a repeating unit represented by:

the second accepter material is selected from the group consisting of:

the hole transport layer is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , polyaniline (PANI) , silver, and molybdenum (VI) oxide (MoO ₃) ; and the electron transport layer is at least one material selected from the group consisting of zinc oxide (ZnO) and tin (IV) oxide (SnO ₂) .
The tandem OSC of claim 1, wherein the PDI-based semiconductor and the second acceptor material are represented by the formula:

the first donor material and the second donor material are represented by the formula:

the hole transport layer is at least one material selected from the group consisting of poly (3, 4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) , silver, and molybdenum (VI) oxide (MoO ₃) ; and the electron transport layer is zinc oxide (ZnO) .
The tandem OSC of claim 14, wherein the front sub-cell has a thickness of 100 nm and the back sub-cell has a thickness of 120 nm.
The tandem OSC of claim 10, wherein the back sub-cell further comprises a third acceptor material selected from from the group consisting of PC61BM and PC71BM.
The tandem OSC of claim 1, wherein the tandem OSC has a power conversion efficiency (PCE) of 12 to 13.2 %.
A method for preparing the tandem OSC of claim 1 comprising the steps of:

a. depositing a solution comprising the first acceptor material and the first donor material on a substrate thereby forming a first thin film;

b. annealing the first thin film thereby forming the front sub-cell;

c. depositing a solution comprising the hole transport layer on the front sub-cell thereby forming a first composite;

d. annealing the first composite thereby forming an annealed first composite;

e. depositing a solution comprising the electron transport layer or a precursor of the electron transport layer on the annealed first composite thereby forming a second composite;

f. annealing the second composite thereby forming an annealed second composite;

g. depositing a solution comprising the second acceptor material and the second donor material on to the annealed second composite thereby forming a third composite; and

h. annealing the third composite thereby forming the tandem OSC of claim 1.
The method of claim 18, wherein step of annealing the first thin film; annealing the first composite; annealing the second composite; and annealing the third composite are all conducted at a temperature at or below 100 ℃.
The method of claim 19, wherein the electron transport layer precursor is is diethyl zinc.