WO2019137354A1

WO2019137354A1 - Thiophene-based fused aromatic systems

Info

Publication number: WO2019137354A1
Application number: PCT/CN2019/070780
Authority: WO
Inventors: He Yan; Yuzhong Chen
Original assignee: Hong Kong University of Science and Technology
Current assignee: Hong Kong University of Science and Technology
Priority date: 2018-01-10
Filing date: 2019-01-08
Publication date: 2019-07-18
Anticipated expiration: 2020-07-10
Also published as: CN111418079B; CN111418079A

Abstract

Relates to thiophene-based small molecule acceptors, their methods of preparation, and formulations thereof useful for preparing photoactive layers in organic solar cells (OSCs).

Description

Thiophene-based Fused Aromatic Systems

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of United States Provisional Application Number 62/709,172, filed on January 10, 2018, the contents of which being hereby incorporated by reference in their entirety for all purposes.

TECHNICAL FIELD

The present disclosure generally relates to organic semiconductors, their methods of preparation, and formulations thereof useful for preparing photoactive layers in organic solar cells (OSCs) .

BACKGROUND

Organic solar cells (OSC) are considered to be a promising, low-cost, and environmentally friendly solar technology, because OSCs can be produced using economical printing methods and generally do not need any toxic materials.

A typical OSC device consists of a pair of materials that function as electron donor and electron acceptor. One of the most commonly used class of electron acceptors are fullerene-based electron acceptors. Conventional fullerene-based OSCs have achieved great successes with power conversion efficiencies (PCEs) reaching beyond 10%. However, the use of fullerene derivatives as the electron-accepting material suffers have several drawbacks, which include low absorbance in the visible region, costly production and purification processes and morphological instability.

To address some of the aforementioned shortcomings, a great deal of research has focused on developing non-fullerene based OSCs, which are expected to be the next generation of OSCs that will be more efficient and stable and lower in cost than conventional fullerene-based OSCs. There are several material options to construct non-fullerene-based OSCs. Among them, OSCs based on a polymer donor and a small molecular acceptor (SMA) have experienced rapid development in the past three years. To develop efficient polymer: SMA OSCs, intensive research efforts have been devoted to the design and synthesis of novel SMA materials.

As nearly half of solar energy is located in the near-infrared region, it is important to develop low-bandgap (1.20–1.35 eV) SMAs to maximize the absorption in near-infrared region and achieve high short-circuit current density (Jsc) and power conversion efficiencies (PCEs) . However, minimizing voltage loss (e.g., below 0.7 V) in such low-bandgap SMAs is an ongoing challenge. Accordingly, there is a need to develop improved SMA compounds that overcome at least some of the aforementioned problems.

SUMMARY OF THE INVENTION

Provided herein are SMAs based on a fused terthieno [3, 2-b] thiophene core. The SMAs can exhibit an ultralow-band gap and can show a wide adsorption range extending to the near-infrared region.

In a first aspect, provided herein is a small molecular acceptor (SMA) having the Formula I:

wherein each A is independently selected from the group consisting of:

each B is absent; or each B is independently selected from the group consisting of:

each V is independently selected from the group consisting of hydrogen, alkyl, Cl, Br, CN, OR ₆, and NHR ₆;

each of X and Y is independently hydrogen, F, Cl, Br, CN, OR ₆, or NHR ₆;

each of W is independently O, S, Se, or Te;

each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of alkyl, cycloalkyl, alkyl phenyl, alkyl thienyl and alkyl aryl with 2-40 C atoms, wherein one or more non-adjacent C atoms is optionally replaced by –O–, –S–, – (C=O) –, –C(=O) O–, –OC (=O) –, –O (C=O) O–, –CR ₇=CR ₈–, or –C≡C–, and one or more hydrogen 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;

R ₅ is alkyl or cycloalkyl;

R ₆ is alkyl or cycloalkyl; and

each of R ₇ and R ₈ is independently hydrogen or alkyl.

In a first embodiment of the first aspect, provided herein is the SMA of the first aspect, wherein each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of C ₂-C ₂₀ alkyl, C ₂-C ₂₀ cycloalkyl, C ₂-C ₂₀ alkyl phenyl, C ₂-C ₂₀ alkyl aryl, and C ₂-C ₂₀ alkyl thienyl.

In a second embodiment of the first aspect, provided herein is the SMA of the first aspect, wherein each B is absent; and each A is independently selected from the group consisting of:

In a third embodiment of the first aspect, provided herein is the SMA of the second embodiment of the first aspect, wherein each of X and Y is independently hydrogen, Cl, or F.

In a fourth embodiment of the first aspect, provided herein is the SMA of the third embodiment of the first aspect, wherein each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of C ₂-C ₂₀ alkyl, C ₂-C ₂₀ cycloalkyl, C ₂-C ₂₀ alkyl phenyl, C ₂-C ₂₀ alkyl aryl, and C ₂-C ₂₀ alkyl thienyl.

In a fifth embodiment of the first aspect, provided herein is the SMA of the third embodiment of the first aspect, wherein each of R ₁, R ₂, R ₃, and R ₄ is a para-substituted C ₃-C ₁₂ alkyl phenyl.

In a sixth embodiment of the first aspect, provided herein is the SMA of the first aspect, wherein each B is independently selected from the group consisting of:

each A is independently selected from the group consisting of:

and each W is independently O or S.

In a seventh embodiment of the first aspect, provided herein is the SMA of the sixth embodiment of the first aspect, wherein each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of C ₂-C ₂₀ alkyl, C ₂-C ₂₀ cycloalkyl, C ₂-C ₂₀ alkyl phenyl, C ₂-C ₂₀ alkyl aryl, and C ₂-C ₂₀ alkyl thienyl.

In an eighth embodiment of the first aspect, provided herein is the SMA of the first aspect, wherein the compound has the Formula II:

wherein A is:

V is hydrogen or alkyl;

each of X and Y is independently hydrogen, F, Cl, or CN; and

R ₉ is C ₂-C ₂₀ alkyl.

In a ninth embodiment of the first aspect, provided herein is the SMA of the first aspect, wherein the compound is selected from the group consisting of:

In a second aspect, provided herein is a photoactive layer comprising at least one donor material and at least one SMA of the first aspect.

In a first embodiment of the second aspect, provided herein is the photoactive layer of the second aspect, wherein the at least one donor material is a polymer comprising a repeat unit having the Formula III:

a polymer comprising a repeating unit having the Formula IV:

wherein each R ₁₀ is independently selected from the group consisting of C ₂-C ₂₀ alkyl.

In a second embodiment of the second aspect, provided herein is the photoactive layer of the first embodiment of the second aspect, wherein the at least one donor material is a polymer comprising a repeat unit having Formula III; and the at least one SMA has the Formula II:

wherein A is:

each of X and Y is independently hydrogen, F, Cl, or CN;

V is hydrogen or alkyl; and

R ₉ is C ₂-C ₂₀ alkyl.

In a third embodiment of the second aspect, provided herein is the photoactive layer of the second embodiment of the second aspect, wherein A is:

wherein each of X and Y is independently hydrogen or Cl; and R ₉ is C ₆-C ₁₂ alkyl.

In a fourth embodiment of the second aspect, provided herein is the photoactive layer of the third embodiment of the second aspect, wherein the at least one donor material is poly [ [4, 8-bis [5- (2-ethylhexyl) -2-thienyl] benzo [1, 2-b: 4, 5-b′] dithiophene-2, 6-diyl] -2, 5-thiophenediyl [5, 7-bis (2-ethylhexyl) -4, 8-dioxo-4H, 8H-benzo [1, 2-c: 4, 5-c′] dithiophene-1, 3-diyl] ] (PBDB-T) .

In a fifth embodiment of the second aspect, provided herein is the photoactive layer of the second embodiment of the second aspect, wherein the at least one donor material is a polymer comprising a repeat unit having the Formula IV; and the at least one SMA has the Formula II:

wherein A is:

each of X and Y is independently hydrogen, F, Cl, or CN;

V is hydrogen or alkyl; and

R ₉ is C ₂-C ₂₀ alkyl.

In a sixth embodiment of the second aspect, provided herein is the photoactive layer of the fifth embodiment of the second aspect, wherein A is:

In a seventh embodiment of the second aspect, provided herein is the photoactive layer of the sixth embodiment of the second aspect, wherein the donor material is poly ( [2, 6 ′-4, 8-di (5-ethylhexylthienyl) benzo [1, 2-b; 3, 3-b] dithiophene] {3-fluoro-2 [ (2-ethylhexyl) carbonyl] thieno [3, 4-b] thiophenediyl} ) (PTB7-Th) .

In a third aspect, provided herein is a photovoltaic cell comprising at least one SMA of the first aspect.

In a fourth aspect provided herein is a photovoltaic cell comprising a photoactive layer of the second aspect.

In further embodiments, SMAs with the structures described herein were demonstrated to exhibit small bandgaps suitable for organic solar cell applications.

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

The formulations, methods and devices of the present subject matter provide surprising improvements in the efficiency of the OE devices and the production thereof. Unexpectedly, the performance, the lifetime and the efficiency of the OE devices can be improved, if these devices are achieved by using a formulation of the present subject matter. Furthermore, the formulation of the present subject matter 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.

Figure 1 depicts the chemical structures of exemplary SMAs IXIC, IXIC-2Cl, and IXIC-4Cl and an exemplary donor material PBDB-T in accordance with certain embodiments as described herein.

Figure 2 depicts an energy-band diagram depicting the energy levels of exemplary SMAs: IXIC, IXIC-2Cl, and IXIC-4Cl and exemplary donor material PBDB-T in accordance with certain embodiments as described herein.

Figure 3 is an exemplary schematic of a single junction photovoltaic cell in accordance with certain embodiments as described herein.

Figure 4A depicts a current-density (J-V) curves for photoactive layers comprising PBDB-T: IXIC (annealed at RT) ; PBDB-T: IXIC (annealed at 100 ℃) ; PBDB-T: IXIC-2Cl (annealed at RT) ; PBDB-T: IXIC-2Cl (annealed at 100 ℃) ; PBDB-T: IXIC-4Cl (annealed at RT) ; and PBDB-T: IXIC-4Cl (annealed at 100 ℃) in accordance with certain embodiments as described herein.

Figure 4B depicts external quantum efficiency (EQE) spectra for photoactive layers comprising PBDB-T: IXIC (annealed at RT) ; PBDB-T: IXIC (annealed at 100 ℃) ; PBDB-T: IXIC-2Cl (annealed at RT) ; PBDB-T: IXIC-2Cl (annealed at 100 ℃) ; PBDB-T: IXIC-4Cl (annealed at RT) ; and PBDB-T: IXIC-4Cl (annealed at 100 ℃) in accordance with certain embodiments as described herein.

Figure 4C depicts photoluminescence quenching spectra of exemplary SMAs: IXIC, IXIC-2Cl, and IXIC-4Cl and photoactive layers comprising PBDB-T: IXIC (annealed at RT) ; PBDB-T: IXIC-2Cl (annealed at RT) ; and PBDB-T: IXIC-4Cl (annealed at RT) excited at 690 nm.

Figure 4D depicts photoluminescence quenching spectra of exemplary SMAs: IXIC, IXIC-2Cl, and IXIC-4Cl and photoactive layers comprising PBDB-T: IXIC (annealed at 100 ℃) ; PBDB-T: IXIC-2Cl (annealed at 100 ℃) ; and PBDB-T: IXIC-4Cl (annealed at 100 ℃) excited at 690 nm.

Figure 4E depicts J _ph versus V _eff curves for photoactive layers comprising PBDB-T: IXIC (annealed at RT) ; PBDB-T: IXIC (annealed at 100 ℃) ; PBDB-T: IXIC-2Cl (annealed at RT) ; PBDB-T: IXIC-2Cl (annealed at 100 ℃) ; PBDB-T: IXIC-4Cl (annealed at RT) ; and PBDB-T: IXIC-4Cl (annealed at 100 ℃) in accordance with certain embodiments as described herein.

Figure 4F depicts light intensity dependence of J _sc for photoactive layers comprising PBDB-T: IXIC (annealed at RT) ; PBDB-T: IXIC (annealed at 100 ℃) ; PBDB-T: IXIC-2Cl (annealed at RT) ; PBDB-T: IXIC-2Cl (annealed at 100 ℃) ; PBDB-T: IXIC-4Cl (annealed at RT) ; and PBDB-T: IXIC-4Cl (annealed at 100 ℃) in accordance with certain embodiments as described herein.

Figure 5 depicts the basic properties of exemplary SMAs IXIC, IXIC-2Cl, and IXIC-4Cl according to certain embodiments described herein.

Figure 6 depicts the basic photovoltaic parameters of PBDB-T: IXIC, PBDB-T: IXIC-2Cl, and PBDB-T: IXIC-4Cl according to certain embodiments described herein.

Figure 7 depicts morphological parameters obtained by RSoXS and grazing-incidence wide-angle X-ray scattering (GIWAXS) of PBDB-T: IXIC, PBDB-T: IXIC-2Cl, and PBDB-T: IXIC-4Cl based photoactive layers and IXIC, IXIC-2Cl, and IXIC-4Cl thin films.

DETAILED DESCRIPTION

OSCs comprising the SMAs described herein exhibit a number of advantageous properties including good near infrared adsorption, which enables the construction of semi-transparent optical photovoltaic (OPV) devices, high PCE, low voltage loss, and high fill rates, and can achieve exceptionally low voltage loss, e.g., 0.59 V (calculated as the difference between the bandgap of the SMA to the Voc of the cell) , even when the bandgap of the OSC is as small as 1.2 eV.

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

A small molecular organic compound is defined as an organic molecule with molecular weight lower than 2,000 g/mol.

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/m2) under standard test conditions (STC) and the surface area of the solar cell (Ac in m2) . 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, -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, "cycloalkyl" by itself or as part of another substituent means, unless otherwise stated, a monocyclic hydrocarbon having between 3-12 carbon atoms in the ring system and includes hydrogen, straight chain, branched chain, and/or cyclic substituents. Exemplary cycloalkyls include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and the like.

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., C6-24 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., -C6F5) , 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-0 bonds. However, one or more N or S atoms in a heteroaryl group can be oxidized (e.g., pyridine Noxide 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 compounds described herein may include one or more groups that can exist as stereoisomers. All such stereoisomer isomers are contemplated by the present disclosure. In instances in which stereochemistry is indicated (for example E/Z double bond isomers) , it is understood that for the sake of simplicity that only one stereoisomer is depicted. However, all stereoisomers and mixtures thereof are contemplated by the present disclosure.

The SMAs provided herein can generally be represented by the Formula I:

wherein each A is independently selected from the group consisting of:

each of X and Y is independently hydrogen, F, Cl, Br, CN, OR ₆, or NHR ₆;

each of W is independently O, S, Se, or Te;

each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of alkyl, cycloalkyl, alkyl phenyl, alkyl thienyl and alkyl aryl with 2-40 C atoms, wherein one or more non-adjacent C atoms is optionally replaced by –O–, –S–, – (C=O) –, –C (=O) O–, –OC (=O) –, –O (C=O) O–, –CR ₇=CR ₈–, or –C≡C–, and one or more hydrogen 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;

R ₅ is alkyl or cycloalkyl;

R ₆ is alkyl or cycloalkyl; and

each of R ₇ and R ₈ is independently hydrogen or alkyl.

In instances in which each B is independently selected from the group consisting of:

the compound of Formula I can be represented by:

wherein each D is independently sulfur or N-R ⁵. In such instances, A can independently be selected from the group consisting of:

each W is independently O or S; and R ₅ is C ₁-C ₂₀ alkyl or C ₃-C ₇ cycloalkyl. In certain embodiments, R ₅ is C ₁-C ₂₀ alkyl; C ₁-C ₁₆ alkyl; C ₁-C ₁₂ alkyl; C ₁-C ₁₀ alkyl; C ₁-C ₈ alkyl; or C ₁-C ₆ alkyl.

In certain embodiments, V is independently selected from the group consisting of hydrogen, alkyl, Cl, Br, CN, OR ₆, and NHR ₆, wherein R ₆ is C ₁-C ₁₄ alkyl; C ₁-C ₁₂ alkyl; C ₁-C ₁₄ alkyl; C ₁-C ₁₂ alkyl; C ₁-C ₁₀; C ₁-C ₈ alkyl; C ₁-C ₈ alkyl; C ₁-C ₆ alkyl; C ₁-C ₄ alkyl; C ₃-C ₁₀ cycloalkyl; C ₃-C ₈ cycloalkyl; C ₃-C ₆ cycloalkyl; or C ₅-C ₈ cycloalkyl. In certain embodiments, C ₁-C ₁₄ alkyl; C ₁-C ₁₂ alkyl; C ₁-C ₁₀; C ₁-C ₈ alkyl; C ₁-C ₈ alkyl; C ₁-C ₆ alkyl; C ₁-C ₄ alkyl; C ₂-C ₁₂ alkyl; C ₄-C ₁₂ alkyl; C ₄-C ₁₀ alkyl; C ₄-C ₈ alkyl; C ₃-C ₁₀ cycloalkyl; C ₃-C ₈ cycloalkyl; C ₃-C ₆ cycloalkyl; or C ₅-C ₈ cycloalkyl. In certain embodiments, V is hydrogen, Cl, Br, CN, or alkyl. In certain embodiments, V is hydrogen or alkyl. In certain embodiments, V is hydrogen or n-C ₆H ₁₃.

In certain embodiments, R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of C ₁-C ₂₀ alkyl, C ₃-C ₂₀ cycloalkyl, C ₁-C ₂₀ alkyl phenyl, C ₁-C ₂₀ alkyl aryl, and C ₁-C ₂₀ alkyl thienyl.

In instances in which one or more of R ₁, R ₂, R ₃, and R ₄ is independently C ₁-C ₂₀ alkyl thienyl, the C ₁-C ₂₀ alkyl thienyl can represented by:

wherein m is a whole number selected between 1-3; and each R is independently C ₁-C ₂₀ alkyl. In certain embodiments, m is 1 and R is C ₂-C ₁₄ alkyl. In certain embodiments, the thiophene is a 2, 5-disubstituted thiophene.

In instances in which one or more of R ₁, R ₂, R ₃, and R ₄ is independently C ₁-C ₂₀ alkyl phenyl, the C ₁-C ₂₀ alkyl phenyl can represented by:

wherein n is a whole number selected between 1-5; and each R is independently C ₁-C ₂₀ alkyl. In certain embodiments, n is 1 and R is C ₂-C ₁₄ alkyl. In certain embodiments, the benzene is a 1, 4-disubstituted benzene as shown below:

In certain embodiments, the benzene is a 1, 4-disubstituted benzene and R ₉ is C ₂-C ₂₀ alkyl; C ₂-C ₁₈ alkyl; C ₂-C ₁₆ alkyl; C ₂-C ₁₄ alkyl; C ₃-C ₁₂ alkyl; C ₄-C ₁₄ alkyl; C ₄-C ₁₂ alkyl; C ₄-C ₁₀; C ₄-C ₈ alkyl; or C ₂-C ₈ alkyl.

In instances in which one or more of R ₁, R ₂, R ₃, and R ₄ is independently C ₁-C ₂₀ alkyl, the C ₁-C ₂₀ alkyl can be a C ₄-C ₂₀ moiety as shown below:

wherein each R ₁₁ is independently C ₁-C ₁₆ alkyl. In certain embodiments, each R ₁₁ is independently C ₂-C ₁₄ alkyl; C ₂-C ₁₂ alkyl; C ₂-C ₁₀ alkyl; C ₂-C ₈ alkyl; or C ₂-C ₆ alkyl.

In instances in which one or more of R ₁, R ₂, R ₃, and R ₄ is independently C ₁-C ₂₀ alkyl aryl, the C ₁-C ₂₀ alkyl aryl can be a mono-, di-, tri-, or tetra-substituted C ₁-C ₂₀ alkyl furan, C ₁-C ₂₀ alkyl oxazole, C ₁-C ₂₀ alkyl pyrrole, C ₁-C ₂₀ alkyl imidazole, C ₁-C ₂₀ alkyl isoimidazole, C ₁-C ₂₀ alkyl triazole, C ₁-C ₂₀ alkyl thiazole, C ₁-C ₂₀ pyridine, or C ₁-C ₂₀ alkyl pyrazine (e.g., 1, 2; 1, 3; or 1, 4 pyrazine) . The C ₁-C ₂₀ alkyl aryl can comprise a C ₂-C ₂₀ alkyl; C ₂-C ₁₈ alkyl; C ₂-C ₁₆ alkyl; C ₂-C ₁₄ alkyl; C ₄-C ₁₄ alkyl; C ₄-C ₁₂ alkyl; C ₄-C ₁₀; C ₄-C ₈ alkyl; or C ₂-C ₈ alkyl.

In certain embodiments, each of R ₅ and R ₆ is independently C ₁-C ₁₄ alkyl; C ₁-C ₁₂ alkyl; C ₁-C ₁₄ alkyl; C ₁-C ₁₂ alkyl; C ₁-C ₁₀; C ₁-C ₈ alkyl; C ₁-C ₈ alkyl; C ₁-C ₆ alkyl; C ₁-C ₄ alkyl; C ₃-C ₁₀ cycloalkyl; C ₃-C ₈ cycloalkyl; C ₃-C ₆ cycloalkyl; or C ₅-C ₈ cycloalkyl.

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

wherein X and Y is independently hydrogen, F, Cl, Br, CN, OR ₆, or NHR ₆. In certain embodiments, X and Y is independently hydrogen, F, Cl, Br, CN, O (C ₁-C ₈ alkyl) , or NH (C ₁-C ₈ alkyl) . In certain embodiments, X and Y is independently hydrogen, F, Cl, Br, or CN. In certain embodiments, X is hydrogen and Y is F; X is hydrogen and Y is Cl; X is hydrogen and Y is Br; X is hydrogen and Y is CN; X is F and Y is H; X is Cl and Y is H; X is Br and Y is H; X is CN and Y is H; X and Y are H; X and Y are F; X and Y are Cl; X and Y are Br; or X and Y are CN.

In certain embodiments, each A is independently selected from the group consisting of:

In certain embodiments, each A is the same moiety.

In certain embodiments, each V is hydrogen.

In certain embodiments, the SMA is represented by the Formula II:

wherein A is:

V is hydrogen or alkyl;

each of X and Y is independently hydrogen, F, Cl, or CN; and R ₉ is C ₂-C ₂₀ alkyl. In certain embodiments, each A is the same group.

In certain embodiments of the SMA of Formula II, each A is:

and

X is hydrogen and Y is F; X is hydrogen and Y is Cl; X is hydrogen and Y is Br; X is hydrogen and Y is CN; X is F and Y is H; X is Cl and Y is H; X is Br and Y is H; X is CN and Y is H; X and Y are H; X and Y are F; X and Y are Cl; X and Y are Br; or X and Y are CN.

In certain embodiments of the SMA of Formula II, V is C ₂-C ₁₈ alkyl; C ₂-C ₁₆ alkyl; C ₂-C ₁₄ alkyl; C ₃-C ₁₂ alkyl; C ₄-C ₁₄ alkyl; C ₄-C ₁₂ alkyl; C ₄-C ₁₀; C ₄-C ₈ alkyl; or C ₂-C ₈ alkyl. In certain embodiments of the SMA of Formula II, V is hydrogen.

In certain embodiments of the SMA of Formula II, R ₉ is C ₂-C ₁₈ alkyl; C ₂-C ₁₆ alkyl; C ₂-C ₁₄ alkyl; C ₃-C ₁₂ alkyl; C ₄-C ₁₄ alkyl; C ₄-C ₁₂ alkyl; C ₄-C ₁₀; C ₄-C ₈ alkyl; or C ₂-C ₈ alkyl.

In certain embodiments of the SMA of Formula II, X and Y are H; X and Y are F; X and Y are Cl; X is hydrogen and Y is F; or X is F and Y is H; and R ₉ is C ₂-C ₁₈ alkyl; C ₂-C ₁₆ alkyl; C ₂-C ₁₄ alkyl; C ₃-C ₁₂ alkyl; C ₄-C ₁₄ alkyl; C ₄-C ₁₂ alkyl; C ₄-C ₁₀; C ₄-C ₈ alkyl; or C ₂-C ₈ alkyl.

In certain embodiments, the SMA of Formula I is selected from the group consisting of:

wherein -C ₆H ₁₃ is n-hexyl.

In certain embodiments, the SMA comprises the following aromatic core structure:

R ₁, R ₂, R ₃, and R ₄ are independently selected from the group consisting of straight-chain, branched, cyclic alkyl , alkyl phenyl, alkyl thienyl and other alkyl aryl with 2-40 C atoms, wherein one or more non-adjacent C atoms are optionally replaced by –O–, –S–, –C (O) –, –C (O–) –O–, –O–C (O) –, –O–C (O) –O–, –CR ₂=CR ₃–, or –C≡C–, and wherein one or more hydrogen atoms are optionally replaced by F, Cl, Br, I, or CN or denote 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, the SMA has a general structure of:

wherein A is selected from:

and

X and Y are independently selected from hydrogen, Cl, Br, CN, OR ₅, and NHR ₅, wherein R ₅ is independently a straight-chain, branched, or cyclic alkyl group.

In certain embodiments, the SMA has a general structure:

wherein PI is selected from:

X and Y are independently selected from hydrogen, Cl, Br, CN, OR ₅, or NHR ₅, wherein R ₅ is independently a straight-chain, branched, or cyclic alkyl group; R ₆ is independently a straight-chain, branched, or cyclic alkyl group; and

A is selected from

wherein Z and W are independently selected from O, S, Se, or Te; and

R ₇ is independently a straight-chain, branched, or cyclic alkyl group.

Advantageously, it was found that chlorination significantly redshifts the absorption edge, enhance π–π stacking, and reduces domain size, which cause notable improvements of J _sc and FF of corresponding photovoltaic cells comprising the same. The addition of chlorine atoms can also downshift the LUMO level thus reducing the Voc of the devices. In certain embodiments, the SMA is selected from the group consisting of IXIC, IXIC-2Cl, and IXIC-4Cl, which are depicted in Figure 1. The basic properties of IXIC, IXIC-2Cl, and IXIC-4Cl are all presented in Figures 5 and 6.

Also provided herein, is a photoactive layer comprising at least one donor material and at least one SMA as described herein.

The photoactive layer can comprise a bulk heterojunction comprising at least one donor material and at least one SMA as described herein. The bulk heterojunction may be an interpenetrating network of the at least one donor material and at least one SMA. Unlike a substantially flat heterojunction, the absorption of a photon may occur near the donor-acceptor interface, increasing the probability of charge dissociation. To fabricate the bulk heterojunction, a mixed donor-acceptor molecular film can be deposited on a substrate and annealed, to induce phase-separation. Similarly, two polymers may be spin-cast and allowed to phase-segregate, producing an interpenetrating structure.

Suitable donor materials include conducting polymers (e.g., a conjugated organic polymer) , which generally have a conjugated portion. Conjugated polymers are characterized in that they have overlapping π orbitals, which contribute to the conductive properties of the material. Conjugated polymers may also be characterized in that they can assume two or more resonance structures. The conjugated organic polymer may be, e.g., linear or branched, so long as the polymer retains its conjugated nature. The donor material can be any donor material known in the art. The selection of a suitable donor material is well within the skill of a person of ordinary skill in the art.

Examples of suitable donor materials include one or more of polyacetylene, polyaniline, polyphenylene, poly (p-phenylene vinylene) , polythienylvinylene, polythiophene, polyporphyrins, porphyrinic macrocycles, polymetallocenes, polyisothianaphthalene, polyphthalocyanine, a discotic liquid crystal polymer, and a derivative or a combination thereof. Exemplary derivatives of the electron donor materials include derivatives having pendant groups, e.g., a cyclic ether, such as epoxy, oxetane, furan, or cyclohexene oxide. Derivatives of these materials may alternatively or additionally include other substituents. For example, thiophene components of electron donor may include a phenyl group, such as at the 3 position of each thiophene moiety. As another example, alkyl, alkoxy, cyano, amino, and/or hydroxy substituent groups may be present in any of the polyphenylacetylene, polydiphenylacetylene, polythiophene, and poly (p-phenylene vinylene) conjugated polymers.

Exemplary conjugated organic polymer donor materials include poly [ [4, 8-bis [5- (2-ethylhexyl) -2-thienyl] benzo [1, 2-b: 4, 5-b′] dithiophene-2, 6-diyl] -2, 5-thiophenediyl [5, 7-bis (2-ethylhexyl) -4, 8-dioxo-4H, 8H-benzo [1, 2-c: 4, 5-c′] dithiophene-1, 3-diyl] ] (PBDB-T) ; poly [ (5, 6-dihydro-5-octyl-4, 6-dioxo-4H-thieno [3, 4-C] pyrrole-1, 3-diyl) {4, 8-bis [ (2-butyloctyl) oxy] benzo [1, 2-b: 4, 5-b′] dithiophene-2, 6-diyl} ] (PBDTBO- TPDO) ; poly [ (5, 6-dihydro-5-octyl-4, 6-dioxo-4H-thieno [3, 4-c] pyrrole-1, 3-diyl) [4, 8-bis[ (2-ethylhexyl) oxy] benzo [1, 2-b: 4, 5-b′] dithiophene-2, 6-diyl] ] (PBDTTPD) ; poly [ [5- (2-ethylhexyl) -5, 6-dihydro-4, 6-dioxo-4H-thieno [3, 4-c] pyrrole-1, 3-diyl] [4, 8-bis [ (2-ethylhexyl) oxy] benzo [1, 2-b: 4, 5-b′] dithiophene-2, 6-diyl] ] (PBDT-TPD) ; Poly [ [4, 8-bis [5- (2-ethylhexyl) -2-thienyl] benzo [1, 2-b: 4, 5-b′] dithiophene-2, 6-diyl] [2- (2-ethyl-1-oxohexyl) thieno [3, 4-b] thiophenediyl] ] (PBDTTT-C-T) ; poly [1- (6- {4, 8-bis [ (2-ethylhexyl) oxy] -6-methylbenzo [1, 2-b: 4, 5-b′] dithiophen-2-yl} -3-fluoro-4-methylthieno [3, 4-b] thiophen-2-yl) -1-octanone] (PBDTTT-CF) ; poly [ [5- (2-ethylhexyl) -5, 6-dihydro-4, 6-dioxo-4H-thieno [3, 4-c] pyrrole-1, 3-diyl] (4, 4′-didodecyl [2, 2′-bithiophene] -5, 5′-diyl) ] (PBTTPD) ; poly [ [9- (1-octylnonyl) -9H-carbazole-2, 7-diyl] -2, 5-thiophenediyl-2, 1, 3-benzothiadiazole-4, 7-diyl-2, 5-thiophenediyl] (PCDTBT) ; poly [2, 6- (4, 4-bis- (2-ethylhexyl) -4H-cyclopenta [2, 1-b; 3, 4-b′] dithiophene) -alt-4, 7 (2, 1, 3-benzothiadiazole) ] (PCPDTBT) ; poly [ (5, 6-dihydro-5-octyl-4, 6-dioxo-4H-thieno [3, 4-c] pyrrole-1, 3-diyl) [4, 4-bis (2-ethylhexyl) -4H-silolo [3, 2-b: 4, 5-b′; ] dithiophene-2, 6-diyl] ] (PDTSTPD) ; poly [ (5, 6-difluoro-2, 1, 3-benzothiadiazol-4, 7-diyl) -alt- (3, 3”’-di (2-octyldodecyl) -2, 2’, 5’, 2”, 5”, 2”’-quaterthiophen-5, 5”’-diyl) ] (PffBT4T-2OD) ; Poly [ (5, 6-difluoro-2, 1, 3-benzothiadiazole-4, 7-diyl) -alt- (3, 3′′′-di (2-nonyltridecyl) -2, 2′, 5′, 2′′, 5′′, 2′′′-quaterthiophen-5, 5′′′-diyl) ] (PffBT4T-C9C13) ; poly [2, 7- (9, 9-dioctylfluorene) -alt-4, 7-bis (thiophen-2-yl) benzo-2, 1, 3-thiadiazole] (PFO-DBT) ; poly [2, 7- (9, 9-dioctylfluorene) -alt-4, 7-bis (thiophen-2-yl) benzo-2, 1, 3-thiadiazole] (PFO-DBT) ; poly ( [2, 6′-4, 8-di (5-ethylhexylthienyl) benzo [1, 2-b; 3, 3-b] dithiophene] {3-fluoro-2 [ (2-ethylhexyl) carbonyl] thieno [3, 4-b] thiophenediyl} ) (PTB7-Th) ; poly (3-dodecylthiophene-2, 5-diyl) (P3DDT) ; poly (3-octylthiophene-2, 5-diyl) (P3OT) ; and poly [2, 7- (9, 9-dioctyl-dibenzosilole) -alt-4, 7-bis (thiophen-2-yl) benzo-2, 1, 3-thiadiazole] (PSiF-DBT) .

In certain embodiments, the at least one donor material is a polymer comprising a repeat unit having the Formula III:

wherein each R ₁₀ is independently C ₂-C ₂₀ alkyl. In certain embodiments, R ₁₀ is C ₄-C ₂₀ alkyl; C ₄-C ₁₈ alkyl; C ₄-C ₁₆ alkyl; C ₄-C ₁₄ alkyl; C ₄-C ₁₂ alkyl; C ₆-C ₁₂ alkyl; or C ₆-C ₁₀ alkyl. In certain embodiments, R ₁₀ is 2-ethylhexyl.

In certain embodiments, R ₁₀ can be represented by a moiety as shown below:

wherein each R ₁₂ is independently C ₁-C ₁₆ alkyl. In certain embodiments, each R ₁₂ is independently C ₂-C ₁₄ alkyl; C ₂-C ₁₂ alkyl; C ₂-C ₁₀ alkyl; C ₂-C ₈ alkyl; or C ₂-C ₆ alkyl.

In certain embodiments, the at least one donor material is a polymer comprising a repeat unit having the Formula IV:

In certain embodiments, R ₁₀ can be represented by a moiety as shown below:

The conjugated organic polymer donor materials can have an average molecular weight of 5,000-250,000 amu. In certain embodiments, the conjugated organic polymer donor materials can have an average molecular weight of 5,000-10,000; 5,000-20,000; 10,000-50,000; 50,000-100,000; or 100,000-150,000; 150,000-200,000; or 100,000-200,000 amu.

In certain embodiments, the donor material is PBDB-T. The PBDB-T can have an average molecular weight of 40,000 to 100,000; 40,000 to 80,000; 40,000 to 60,000, or greater than 50,000 amu.

In certain embodiments, the donor material is PTB7-TH. The PTB7-TH can have an average molecular weight of 150,000 to 200,000; 170,000 to 200,000; 190,000 to 200,000; or greater than 190,000 amu.

Advantageously, the absorption range of the SMAs described can be in the infrared region, ～800-1,000 nm. Consequently, photoactive layers comprising the SMAs described herein can transmit a large portion of the light in the visible region. By appropriate selection and amount of the at least one donor material, a photoactive layer that is capable of transmitting a substantial portion of light in the visible region can be prepared. Such photoactive layers may be particularly useful in the preparation of semitransparent organic solar cells for window and outer wall applications. In certain embodiments, the photoactive layer transmits up to 30%, 40%, 50%, 60%, 70%, 80%, 85%, or 90%of light in the visible range.

Also provided is a photovoltaic cell comprising the photoactive layer described herein. The photovoltaic cell can be a single junction, double junction, or multi-junction cell.

An exemplary single junction photovoltaic cell is depicted in Figure 3. The photovoltaic cell can comprise a transparent cathode 150, an electron transport layer 140, the photoactive layer described herein 130, an anode interlayer 120, and an anode 110.

The transparent cathode 150 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 photoactive layer 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 350-750 nm to be transmitted.

In certain embodiments, the electron transport layer 150 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 electron transport layer is ZnO.

The anode interlayer 120 can 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 anode interlayer is vanadium (V) oxide (V ₂O ₅) , molybdenum oxide (MoO ₃) .

The anode 110 can comprise 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. In certain embodiments, the anode comprises aluminum.

Depending on the composition of the electron transport layer 150, it can be made using any method known in the art, such as by sequential physical vapor deposition, chemical vapor deposition, sputtering, and the like.

In instances in which the electron transport layer 150 comprises ZnO, it 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 on the surface of the transparent cathode and 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 photoactive layer comprising the at least one SMA and at least one donor material can be prepared by forming a photoactive layer solution comprising the at least one SMA and at least one donor material and depositing the photoactive layer solution onto the electron transport layer 150 and optionally annealing the applied photoactive layer solution thereby forming the photoactive layer.

The solvent used to prepare the photoactive layer solution can be a solvent in which the at least one SMA and at least one donor material are substantially soluble in when solvent is heated above room temperature. The solvent can be 1, 2-dichlorobenzene, 1, 3-dichlorobenzene, 1, 2, 4-trichlorobenzene, chlorobenzene, 1, 2, 4-trimethylbenzene, chloroform and combinations thereof. In certain embodiments, the photoactive layer solution further comprises one or more solvent additives, such as 1-chloronaphthalene and 1, 8-octanedithiol, 1, 8-diiodooctane, and combinations thereof. In certain embodiments, the solvent is at least one of 1, 2-dichlorobenzene and chlorobenzene and optionally contains the solvent additive 1, 8-diiodooctane. In instances where the solvent further comprises a solvent additive, the solvent additive can be present between about 0.1%to about 8% (v/v) ; about 0.1%to about 6% (v/v) ; about 0.1%to about 4% (v/v) ; or about 0.1%to about 2% (v/v) in the solvent.

The photoactive layer solution can be deposited on the substrate using any method known to those of skill in the art including, but not limited to, spin coating, printing, print screening, spraying, painting, doctor-blading, slot-die coating, and dip coating.

Once the photoactive layer solution is deposited, the solvent can be removed (e.g., at atmospheric pressure and temperature or under reduced pressure and/or elevated temperature) thereby forming the thin film comprising the donor material and optionally be annealed. The step of annealing can occur at 80 to 150 ℃; 80 to 120 ℃; or 90 to 110 ℃.

In embodiments in which the anode interlayer 140 comprises vanadium (V) oxide (V ₂O ₅) , molybdenum oxide (MoO ₃) , the anode interlayer can be deposited by sequential thermal evaporation of the e.g., vanadium (V) oxide (V ₂O ₅) , molybdenum oxide (MoO ₃) onto photoactive layer 130.

The anode 110 can be deposited on the anode interlayer 140 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.

Photovoltaic cells comprising the photoactive layers described herein exhibit amongst some of the highest PCEs of photovoltaic devices based ultra-low bandgap acceptors. Figure 5 presents photovoltaic properties of exemplary photovoltaic cells comprising IXIC, IXIC-2Cl, and IXIC-4Cl.

The hole and electron mobility of IXIC, IXIC-2Cl, and IXIC-4Cl neat and blend films were measured by space charge limited current (SCLC) method. As shown in Figure 6, the electron mobilities of IXIC, IXIC-2Cl, and IXIC-4Cl neat film are increased gradually with addition of chlorine atoms. The hole and electron mobilities of IXIC, IXIC-2Cl, and IXIC-4Cl blend films are summarized in Figure 6. Annealing not only increases the hole and electron mobility of PBDB-T: IXIC, PBDB-T: IXIC-2Cl, and PBDBT: IXIC-4Cl based blend films but also decreases the ratio between hole mobility and electron mobility, which explains the enhancement of FF after annealing. It should be noticed that after annealing PBDB-T: IXIC-2Cl based blend film has the highest electron mobility and most balanced carrier mobility among all blend films, contributing to high FF of PBDB-T: IXIC-2Cl based photovoltaic cells. Besides, due to highest hole mobility, PBDB-T: IXIC-4Cl based photovoltaic cells can gain highest FF of 71.2%.

To further investigate the morphology, domain size, and domain purity of IXIC, IXIC-2Cl, and IXIC-4Cl based photoactive layers before and after annealing, atomic force microscope (AFM) and resonant soft X-ray scattering (RSoXS) were carried out. The corresponding AFM height sensor images, phase images, and RSoXS profiles are displayed in Figure 4a-i. As shown in Figures 4b, c, e, f, h, and i, the PBDB-T: IXIC, PBDB-T: IXIC-2Cl, and PBDBT: IXIC-4Cl based active layers all show smooth morphology with small root-mean-square (RMS) roughness (0.835–1.14 nm) . Annealing reduced the RMS roughness to achieve a more even surface, which is beneficial for contact between active layer and top electrode. The curve of RSoXS results shown in Figure 4j shows reasonable domain size and domain purity and indicates that annealing treatment not only reduces the domain size of PBDB-T: IXIC, PBDB-T: IXIC-2Cl, and PBDB-T: IXIC-4Cl blend films, but also enhances the domain purity of three blends (Table 2) , which is beneficial to promote J _sc and FF. The largest domain size (31.96 nm) and lowest domain purity (0.93) of PBDB-T: IXIC based photoactive layer among the three blends does not benefit to form nanofiber structure and continuous interpenetrating networks, which lead to relatively low Jsc and FF of the corresponding OSCs. After adding chlorine atoms on the EG, the PBDB-T: IXIC-2Cl and PBDB-T: IXIC-4Cl blend film surprisingly exhibited smaller domain size and higher domain purity. Therefore, the Jsc and FF of PBDB-T: IXIC-2Cl and PBDB-T: IXIC-4Cl based OSCs were improved. The R-SoXS results support the GIWAXS results and prove that chlorination on EGs can reduce domain size and increase domain purity.

EXAMPLES

Example 1 -Synthesis of TTDTT-CHO

To a solution of thieno [3, 2-b] thiophene (3.00 g, 21.40mmol) in THF at -78℃, 2.0 M n-butyl lithium in hexane (11.20 mL, 22.47 mmol) was added dropwise under N ₂. The reaction mixture was stirred for 1h at -78℃, and then TIPSCl (4.54g, 23.53mmol) was added. The mixture was returned to r.t. and stirred overnight. The reaction was quenched with water and extracted with ethyl acetate for three times. The combined organic phase was washed with water followed by brine. Then the solution was dried over Na ₂SO ₄ and concentrated under reduced pressure. The residue was purified by flash column chromatography (eluent: n-hexane) to get the product as white solid (4.24 g, 67%) .

To a solution of compound 1 (4.24 g, 14.28mmol) in THF at -78℃, 2.0 M n-butyl lithium in hexane (7.80 mL, 15.71mmol) was added dropwise under N ₂. The reaction mixture was stirred for 1h at -78℃, and then 1.0M trimethyltin chloride in THF (17.14mL, 17.14mmol) was added. The mixture was returned to r.t. and stirred overnight. The reaction was quenched with an aqueous potassium fluoride and extracted with ethyl acetate for three times. The combined organic phase was washed with water followed by brine. Then the solution was dried over Na ₂SO ₄ and concentrated under reduced pressure. The residue as white solid was used directly without further purification (5.96g, 91%)

To a solution of compound 3 (450.0mg, 1.02mmol) , Pd ₂ (dba) ₃ (46.8 mg, 0.051mmol) and P (o-tol) ₃ (124.2mg, 0.408 mmol) in toluene (10 mL) , compound 2 (1.87 g, 4.08mmol) was added under N ₂. The reaction mixture was stirred for 12 h at 110 ℃. Then, the reaction mixture was cooled to r.t. and poured into an aqueous potassium fluoride. The mixture was extracted with ethyl acetate for three times. The combined organic phase was washed with water followed by brine. Then the solution was dried over Na ₂SO ₄ and concentrated under reduced pressure. The residue was purified by flash column chromatography (eluent: n-hexane: CH ₂Cl ₂= 1: 1) to get the product as light yellow solid (493 mg, 55%) .

To a solution of 4-hexyl-1-bromobenzene (817mg, 3.4mmmol) in THF (10 mL) at -78 ℃, n-butyl lithium (2.0 mL, 1.3 mmol, 1.6 M in hexane) was added under N ₂. The mixture was kept at -78 ℃ for 1 h. Then a solution of compound 2 (370 mg, 0.42 mmol) in THF (10 mL) was added slowly. After the addition, the mixture was stirred at room temperature overnight and then poured into water and extracted for three times with ethyl acetate. The combined organic phase was washed with water followed by brine. Then the solution was dried over Na ₂SO ₄ and concentrated under reduced pressure.

Then the crude was dissolved in CH ₃COOH (30 mL) . The concentrated sulfuric acid (0.1mL) was added dropwise to the solution at 0 ℃. Then the solution was heated to 40℃ and stirred for 5 hours. The reaction was quenched with water and extracted with ethyl acetate for three times. The combined organic phase was washed with water followed by brine. Then the solution was dried over Na ₂SO ₄.

After remove solvent, the crude was dissolved in THF (50mL) . Tetra-n-butyl ammonium fluoride (1.54g, 5.9mmol) was added to the solution and stirred overnight. After poured into water, it was extracted for three times with ethyl acetate. The combined organic phase was washed with water followed by brine. Then the solution was dried over Na ₂SO ₄ and concentrated under reduced pressure. The residue was purified by flash column chromatography (eluent: n-hexane: toluene= 10: 1) to get the product as light yellow solid (267mg, 59%) .

To a solution of TTDTT (60mg, 0.060mmol) in THF (10mL) , 1.6 M n-butyl lithium in hexane (0.1 mL, 0.16 mmol) was added dropwise slowly at -78 ℃ under N ₂. The mixture was stirred at -78 ℃ for 1 h, and then anhydrous DMF (0.4 mL) was added. The mixture was stirred overnight at room temperature. Brine was added and the mixture was extracted with ethyl acetate for three times. The combined organic phase was washed with water followed by brine. Then the solution was dried over Na ₂SO ₄ and concentrated under reduced pressure. The residue was purified by flash column chromatography (eluent: n-hexane: CH ₂Cl ₂= 1: 1) to get the product as orange solid (55 mg, 87%) .

Example 2 -Synthesis of IXIC

To a solution of TTDTT-CHO (60 mg, 0.053 mmol) and 1, 1-dicyanomethylene-3-indanone (102 mg, 0.53 mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (50 mg, 64%) . ¹H NMR (400 MHz, CDCl ₃, ppm) : δ =8.837 (s, 2H) , 8.664-8.644 (m, 2H) , 8.107 (s, 2H) , 7.912-7.891 (m, 2H) , 7.762-7.703 (m, 4H) , 7.212-7.155 (m, 16H) , 2.581 (t, 8H, J=7.8 Hz) , 1.604-1.526 (m, 8H) , 1.342-1.247 (m, 24H) , 0.862 (t, 12H, J=6.8 Hz) ; ¹³C NMR (100 MHz, CDCl ₃, ppm) : δ=188.502, 160.525, 152.739, 149.361, 148.531, 147.357, 143.165, 142.844, 140.264, 139.752, 139.204, 138.083, 137.952, 137.099, 137.041, 135.276, 134.539, 129.375, 127.962, 125.418, 123.836, 122.261, 115.014, 114.977, 68.954, 62.698, 35.839, 31.885, 31.405, 29.383, 22.777, 14.281; MALDI-TOF MS: calcd for C ₉₄H ₈₀N ₄O ₂S ₆ (M ⁺) , 1488.4606; found, 1488.4626.

Example 3 -Synthesis of IXIC-2F

To a solution of TTDTT-CHO (54 mg, 0.047 mmol) , 2- (5-fluoro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile and 2- (6-fluoro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile (100 mg, 0.47 mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (41 mg, 57%) .

Example 4 -Synthesis of IXIC-4F

To a solution of TTDTT-CHO (70 mg, 0.062 mmol) , 2- (5, 6-difluoro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile (142 mg, 0.62mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (52 mg, 54%) .

Example 5 -Synthesis of IXIC-4Cl

To a solution of TTDTT-CHO (23 mg, 0.020 mmol) , 2- (5, 6-dichloro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile (52 mg, 0.12mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (18mg, 55%) . ¹H NMR (400 MHz, CDCl ₃, ppm) : δ =8.825 (s, 2H) , 8.721 (s, 2H) , 8.091 (s, 2H) , 7.926 (s, 2H) , 7.184 (m, 16H) , 2.586 (t, 8H, J=7.8 Hz) , 1.602-1.560 (m, 8H) , 1.342-1.247 (m, 24H) , 0.864 (t, 12H, J=6.6 Hz) ; ¹³C NMR (100 MHz, CDCl ₃, ppm) : δ=186.141, 158.139, 153.276, 150.535, 148.681, 148.171, 143.339, 140.118, 139.850, 139.496, 139.387, 138.870, 138.689, 138.360, 137.857, 136.847, 136.115, 129.455, 127.927, 127.020, 125.257, 122.160, 121.239, 114.585, 69.434, 62.725, 35.835, 31.887, 31.409, 29.918, 29.371, 22.780, 14.278; MALDI-TOF MS: calcd for C ₉₄H ₇₆N ₄O ₂S ₆Cl ₄ (M ⁺) , 1624.3047; found, 1624.3020.

Example 6 -Synthesis of IXIC-2Cl

To a solution of TTDTT-CHO (89 mg, 0.078 mmol) , 2- (5-chloro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile and 2- (6-chloro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile (178 mg, 0.78mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (55mg, 45%) . ¹H NMR (400 MHz, CDCl ₃, ppm) : δ =8.818-8.803 (m, 2H) , 8.576-8.555 (m, 2H) , 8.080 (s, 1.6H) , 7.981 (s, 0.4H) , 7.981-7.807 (m, 2H) , 7.672-7.646 (m, 2H) , 7.214-7.164 (m, 16H) , 2.587 (t, 8H, J=7.8Hz) , 1.586-1.526 (m, 8H) , 1.326-1.247 (m, 24H) , 0.862 (t, 12H, J=6.2Hz) ; ¹³C NMR (100MHz, CDCl ₃, ppm) : δ=186.988, 159.233, 152.974, 149.973, 148.526, 147.693, 143.224, 143.076, 141.232, 139.903, 139.221, 138.421, 138.187, 138.135, 137.506, 136.905, 134.931, 129.388, 127.911, 126.454, 123.885, 121.625, 114.865, 114.733, 68.945, 62.666, 35.795, 31.850, 31.381, 29.875, 29.337, 22.740, 14.234; MALDI-TOF MS: calcd for C ₉₄H ₇₈N ₄O ₂S ₆Cl ₂ (M ⁺) , 1556.3826; found, 1556.3867. ¹H NMR (400 MHz, CDCl ₃, ppm) : δ =8.818-8.803 (m, 2H) , 8.576-8.555 (m, 2H) , 8.080 (s, 1.6H) , 7.981 (s, 0.4H) , 7.981-7.807 (m, 2H) , 7.672-7.646 (m, 2H) , 7.214-7.164 (m, 16H) , 2.587 (t, 8H, J=7.8 Hz) , 1.586-1.526 (m, 8H) , 1.326-1.247 (m, 24H) , 0.862 (t, 12H, J=6.2 Hz); ¹³C NMR (100 MHz, CDCl ₃, ppm) : δ=186.988, 159.233, 152.974, 149.973, 148.526, 147.693, 143.224, 143.076, 141.232, 139.903, 139.221, 138.421, 138.187, 138.135, 137.506, 136.905, 134.931, 129.388, 127.911, 126.454, 123.885, 121.625, 114.865, 114.733, 68.945, 62.666, 35.795, 31.850, 31.381, 29.875, 29.337, 22.740, 14.234; MALDI-TOF MS: calcd for C ₉₄H ₇₈N ₄O ₂S ₆Cl ₂ (M ⁺) , 1556.3826; found, 1556.3867.

Example 7 -Synthesis of IXIC-M

To a solution of TTDTT-CHO (50 mg, 0.044 mmol) , 2- (5-methyl-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile and 2- (6-methyl-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile (92 mg, 0.44mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (55mg, 83%) .

Example 8 -Synthesis of IXTC

To a solution of TTDTT-CHO (62 mg, 0.059 mmol) , 2- (6-oxo-5, 6-dihydro-4H-cyclopenta [c] thiophen-4-ylidene) malononitrile (120 mg, 0.59mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (60mg, 67%) .

Example 9 -Synthesis of IXIC-2F-C6

To a solution of TTDTT-CHO-C6 (43 mg, 0.033 mmol) , 2- (5-fluoro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile and 2- (6-fluoro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile (70 mg, 0.33 mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (27mg, 48%) .

Example 10 -Synthesis of IXIC-4F-C6

To a solution of TTDTT-CHO-C6 (76mg, 0.058 mmol) , 2- (5, 6-difluoro-3-oxo-2, 3-dihydro-1H-inden-1-ylidene) malononitrile (134 mg, 0.58mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (69mg, 69%) .

Example 11 -Synthesis of IXTN-C6

To a solution of TTDTT-CHO-C6 (30 mg, 0.023 mmol) , 2- (3-oxo-2, 3-dihydro-1H-cyclopenta [b] naphthalen-1-ylidene) malononitrile (56 mg, 0.23mmol) in dry CHCl ₃ (10 mL) was added pyridine (0.1 mL) under N ₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, then the mixture was poured into CH ₃OH (100 mL) and filtered, the residue left in filter paper was dissolved by CHCl ₃. After removing the solvent, the residue was purified using column chromatography on silica gel employing petroleum ether/CH ₂Cl ₂ (1: 1, v/v) as an eluent, yielding a dark green solid (17mg, 42%) .

Example 12 -Device Fabrication

Pre-patterned ITO-coated glass with a sheet resistance of ～15 Ω/square was used as the substrate. It was cleaned by sequential sonications in soap DI water, DI water, acetone, and isopropanol. After UV/ozone treatment for 60 min, a ZnO electron transport layer was prepared by spin-coating at 5000 rpm from a ZnO precursor solution (diethyl zinc) . Active layer solutions were prepared in CB/DCB or CB/DCB/DIO with various ratios (polymer concentration: 7-12 mg/mL) . To completely dissolve the polymer, the active layer solution should be stirred on hotplate at 100-120℃ for at least 3 hours. Active layers were spin-coated from warm solutions in a N ₂ glovebox at 600-850 rpm to obtain thicknesses of ～100 nm. The polymer/small molecular acceptor films were then annealed at 100 ℃ for 5 min before being transferred to the vacuum chamber of a thermal evaporator inside the same glovebox. At a vacuum level of 3×10 ^-6 Torr, a thin layer (20 nm) of MoO ₃ or V ₂O ₅ was deposited as the anode interlayer, followed by deposition of 100 nm of Al as the top electrode. All cells were encapsulated using epoxy inside the glovebox. Device J-V characteristics was measured under AM1.5G (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 236 source meter unit. Typical cells have devices area of about 5.9 mm ², which is defined by a metal mask with an aperture aligned with the device area. EQEs were characterized using a Newport EQE system equipped with a standard Si diode. Monochromatic light was generated from a Newport 300W lamp source. The EQE of the device in the present teaching are shown in Figure 3. The V _OC, J _SC, FF and PCE of OPV devices in the present teaching are summarized in the following table.

Example 12b: Photovoltaic parameters of solar cell devices

^a) No annealing. ^b) Annealing at 100 ℃. ^c) The values in bracket are integrated J _sc from EQE spectra. ^d) Measured by space charge limited current (SCLC) method.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present disclosure as shown in the specific embodiments without departing from the spirit or scope of the disclosure as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

A small molecular acceptor (SMA) having the Formula I:

wherein each A is independently selected from the group consisting of:

each B is absent; or each B is independently selected from the group consisting of:

each V is independently selected from the group consisting of hydrogen, alkyl, Cl, Br, CN, OR ₆, and NHR ₆;

each of X and Y is independently hydrogen, F, Cl, Br, CN, OR ₆, or NHR ₆;

each of W is independently O, S, Se, or Te;

each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of alkyl, cycloalkyl, alkyl phenyl, alkyl thienyl and alkyl aryl with 2-40 C atoms, wherein one or more non-adjacent C atoms is optionally replaced by –O–, –S–, – (C=O) –, –C (=O) O–, –OC (=O) –, –O (C=O) O–, –CR ₇=CR ₈–, or –C≡C–, and one or more hydrogen 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;

R ₅ is alkyl or cycloalkyl;

R ₆ is alkyl or cycloalkyl; and

each of R ₇ and R ₈ is independently hydrogen or alkyl.
The SMA of claim 1, wherein each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of C ₂-C ₂₀ alkyl, C ₂-C ₂₀ cycloalkyl, C ₂-C ₂₀ alkyl phenyl, C ₂-C ₂₀ alkyl aryl, and C ₂-C ₂₀ alkyl thienyl.
The SMA of claim 1, wherein each B is absent; and each A is independently selected from the group consisting of:
The SMA of claim 3, wherein each of X and Y is independently hydrogen, Cl, or F.
The SMA of claim 4, wherein each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of C ₂-C ₂₀ alkyl, C ₂-C ₂₀ cycloalkyl, C ₂-C ₂₀ alkyl phenyl, C ₂-C ₂₀ alkyl aryl, and C ₂-C ₂₀ alkyl thienyl.
The SMA of claim 4, wherein each of R ₁, R ₂, R ₃, and R ₄ is a para-substituted C ₃-C ₁₂ alkyl phenyl.
The SMA of claim 1, wherein each B is independently selected from the group consisting of:

each A is independently selected from the group consisting of:

and each W is independently O or S.
The SMA of claim 7, wherein each of R ₁, R ₂, R ₃, and R ₄ is independently selected from the group consisting of C ₂-C ₂₀ alkyl, C ₂-C ₂₀ cycloalkyl, C ₂-C ₂₀ alkyl phenyl, C ₂-C ₂₀ alkyl aryl, and C ₂-C ₂₀ alkyl thienyl.
The SMA of claim 1, wherein the compound has the Formula II:

wherein A is:

V is hydrogen or alkyl;

each of X and Y is independently hydrogen, F, Cl, or CN; and

R ₉ is C ₂-C ₂₀ alkyl.
The SMA of claim 1, wherein the compound is selected from the group consisting of:
A photoactive layer comprising at least one donor material and at least one SMA of claim 1.
The photoactive layer of claim 11, wherein the at least one donor material is a polymer comprising a repeat unit having the Formula III:

a polymer comprising a repeating unit having the Formula IV:

wherein each R ₁₀ is independently selected from the group consisting of C ₂-C ₂₀ alkyl.
The photoactive layer of claim 12, wherein the at least one donor material is a polymer comprising a repeat unit having Formula III; and the at least one SMA has the Formula II:

wherein A is:

each of X and Y is independently hydrogen, F, Cl, or CN;

V is hydrogen or alkyl; and

R ₉ is C ₂-C ₂₀ alkyl.
The photoactive layer of claim 13, wherein A is:

wherein each of X and Y is independently hydrogen or Cl; and R ₉ is C ₆-C ₁₂ alkyl.
The photoactive layer of claim 14, wherein the at least one donor material is poly [ [4, 8-bis [5- (2-ethylhexyl) -2-thienyl] benzo [1, 2-b: 4, 5-b′] dithiophene-2, 6-diyl] -2, 5-thiophenediyl [5, 7-bis (2-ethylhexyl) -4, 8-dioxo-4H, 8H-benzo [1, 2-c: 4, 5-c′] dithiophene-1, 3-diyl] ] (PBDB-T) .
The photoactive layer of claim 12, wherein the at least one donor material is a polymer comprising a repeat unit having the Formula IV; and the at least one SMA has the Formula II:

wherein A is:

each of X and Y is independently hydrogen, F, Cl, or CN;

V is hydrogen or alkyl; and

R ₉ is C ₂-C ₂₀ alkyl.
The photoactive layer of claim 16, wherein A is:

wherein each of X and Y is independently hydrogen or Cl; and R ₉ is C ₆-C ₁₂ alkyl.
The photoactive layer of claim 17, wherein the donor material is poly ( [2, 6′-4, 8-di (5-ethylhexylthienyl) benzo [1, 2-b; 3, 3-b] dithiophene] {3-fluoro-2 [ (2-ethylhexyl) carbonyl] thieno [3, 4-b] thiophenediyl} ) (PTB7-Th) .
A photovoltaic cell comprising at least one SMA of claim 1.
A photovoltaic cell comprising a photoactive layer of claim 11.