Acceptors For Organic Photovoltaics Research Articles

Impact of end‐capped engineering on the optoelectronic characteristics of pyrene‐based non‐fullerene acceptors for organic photovoltaics

AbstractPyrene‐based molecules are being explored as prospective fullerene‐free acceptors for organic solar cells (OSCs), due to their easy accessibility, structural planarity, and excellent electron delocalization. In this work, we successfully designed and analyzed pyrene‐based acceptor materials (QL1–QL8) to investigate their photophysical and electro‐optical parameters. Various geometric parameters were computed at the MPW1PW91/6‐31G(d,p). Advanced quantum chemical approaches were employed to characterize the molecules. All the tailored molecules (QL1–QL8) exhibit a lower bandgap than the reference (R), signifying their superiority. Among these, QL8 was found to have a maximum absorption (λmax) at 791.37 nm and an optical bandgap (ELUMO − EHOMO) minimum of 2.11 eV. Redshifted absorption spectra are observed in both gaseous and solvent phases for all the designed (QL1–QL8) molecules in contrast to R. Among these, QL4 exhibits the highest light harvesting efficiency (0.9826), and open‐circuit voltage. A detailed donor–acceptor investigation of QL8/PBDB‐T revealed the marvelous charge switching at the donor–acceptor interface. The approach used in this study is anticipated to facilitate the manufacturing of highly efficient OSC molecules.

Unveiling the impact of exchange-correlation functionals on the description of key electronic properties of non-fullerene acceptors in organic photovoltaics.

Non-fullerene electron acceptors have emerged as promising alternatives to traditional electron-acceptors in the active layers of organic photovoltaics. This is due to their tunable energy levels, optical response in the visible light spectrum, high electron mobility, and photochemical stability. In this study, the electronic properties of two representative non-fullerene acceptors, ITIC and Y5, have been calculated within the framework of density functional theory using a range of hybrid and non-hybrid density functionals. Screened range-separated hybrid (SRSH) approaches were also tested. The results are analyzed in light of the previously reported experimental outcomes. Specifically, we have calculated the oxidation and reduction potentials, fundamental and optical gaps, the highest occupied molecular orbital and lowest unoccupied molecular orbital energies, and exciton binding energies. Additionally, we have investigated the effects of the medium dielectric constant on these properties employing a universal implicit solvent model. It was found that hybrid functionals generally perform poorly in predicting oxidation potentials, while non-hybrid functionals tend to overestimate reduction potentials. The inclusion of a large Hartree-Fock contribution to the global or long range was identified as the source of inaccuracy for many hybrid functionals in predicting both redox potentials and the fundamental and optical gaps. Corroborating with the available literature, ∼50% of all tested functionals predicted very small exciton binding energies, within the range of ±0.1eV, that become even smaller by increasing the dielectric constant of the material. Finally, the OHSE2PBE and tHCTHhyb functionals and the optimal tuning SRSH approach emerged as the best-performing methods, with good accuracy in the description of the electronic properties of interest.

Design of Furan‐Based Acceptors for Organic Photovoltaics

AbstractWe explore a series of furan‐based non‐fullerene acceptors and report their optoelectronic properties, solid‐state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced band gap and red‐shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels–Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to most non‐fullerene acceptors, and the photostability depends strongly on the molecular structure. Placement of furans next to the acceptor termini leads to better photostability, well‐balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker offers the best photostability and power conversion efficiency (>14 %), outperforming all furan‐based acceptors reported to date and all indacenodithiophene‐based acceptors. Our findings show the possibility of photostable furan‐based alternatives to the currently omnipresent thiophene‐based photovoltaic materials.

Perylene Diimide-Fused Dithiophenepyrroles with Different End Groups as Acceptors for Organic Photovoltaics.

In this study, we synthesized four new A-DA'D-A acceptors (where A and D represent acceptor and donor chemical units) incorporating perylene diimide units (A') as their core structures and presenting various modes of halogenation and substitution of the functional groups at their end groups (A). In these acceptors, by fusing dithiophenepyrrole (DTP) moieties (D) to the helical perylene diimide dimer (hPDI) to form fused-hPDI (FhPDI) cores, we could increase the D/A' oscillator strength in the cores and, thus, the intensity of intramolecular charge transfer (ICT), thereby enhancing the intensity of the absorption bands. With four different end group units─IC2F, IC2Cl, IO2F, and IO2Cl─tested, each of these acceptor molecules exhibited different optical characteristics. Among all of these systems, the organic photovoltaic device incorporating the polymer PCE10 blended with the acceptor FhPDI-IC2F (1:1.1 wt %) had the highest power conversion efficiency (PCE) of 9.0%; the optimal PCEs of PCE10:FhPDI-IO2F, PCE10:FhPDI-IO2Cl, and PCE10:FhPDI-IC2Cl (1:1.1 wt %) devices were 5.2, 4.7, and 7.7%, respectively. The relatively high PCE of the PCE10:FhPDI-IC2F device resulted primarily from the higher absorption coefficients of the FhPDI-IC2F acceptor, lower energy loss, and more efficient charge transfer; the FhPDI-IC2F system experienced a lower degree of geminate recombination─as a result of improved delocalization of π-electrons along the acceptor unit─relative to that of the other three acceptors systems. Thus, altering the end groups of multichromophoric PDI units can increase the PCEs of devices incorporating PDI-derived materials and might also be a new pathway for the creation of other valuable fused-ring derivatives.

Strong Intermolecular Interactions Induced by High Quadrupole Moments Enable Excellent Photostability of Non‐Fullerene Acceptors for Organic Photovoltaics

AbstractUnderstanding degradation mechanisms of organic photovoltaics (OPVs) is a critical prerequisite for improving device stability. Herein, the effect of molecular structure on the photostability of non‐fullerene acceptors (NFAs) is studied by changing end‐group substitution of ITIC derivatives: ITIC, ITIC‐2F, and ITIC‐DM. Using an assay of in situ spectroscopy techniques and molecular simulations, the photodegradation product of ITIC and the rate of product formation are identified, which correlates excellently to reported device stability, with ITIC‐2F being the most stable and ITIC‐DM the least. The choice of acceptor is found to affect both the donor polymer (PBDB‐T) photostability and the morphological stability of the bulk heterojunction blend. Molecular simulations reveal that NFA end‐group substitution strongly modulates the electron distribution within the molecule and thus its quadrupole moment. Compared to unsubstituted‐ITIC, end‐group fluorination results in a stronger, and demethylation a weaker, molecular quadrupole moment. This influences the intermolecular interactions between NFAs and between the NFA and the polymer, which in turn affects the photostability and morphological stability. This hypothesis is further tested on two other high quadrupole acceptors, Y6 and IEICO‐4F, which both show impressive photostability. The strong correlation observed between NFA quadrupole moment and photostability opens a new synthetic direction for photostable organic photovoltaic materials.

Molecular tuning of non-fullerene electron acceptors in organic photovoltaics: a theoretical study

On the basis of the famous A–D–A-type non-fullerene acceptor IT-4F, this work investigates the effects of introducing methyl groups and substituting dicyano with O on optoelectronic properties and photovoltaic performances.

Mechanism of the Photodegradation of A‐D‐A Acceptors for Organic Photovoltaics**

AbstractHerein, we elucidate the photodegradation pathway of A‐D‐A‐type non‐fullerene acceptors for organic photovoltaics. Using IT‐4F as a benchmark example, we isolated the photoproducts and proved them isomers of IT‐4F formed by a 6‐e electrocyclic reaction between the dicyanomethylene unit and the thiophene ring, followed by a 1,5‐sigmatropic hydride shift. This photoisomerization was accelerated under inert conditions, as explained by DFT calculations predicting a triplet‐mediated reaction path (quenchable by oxygen). Adding controlled amounts of the photoproduct P1 to PM6:IT‐4F bulk heterojunction cells led to a progressive decrease in photocurrent and fill factor attributed to its poor absorption and charge transport properties. The reaction is a general photodegradation pathway for a series of A‐D‐A molecules with 1,1‐dicyanomethylene‐3‐indanone termini, and its rate varies with the structure of the donor and acceptor moiety.

Mechanism of the Photodegradation of A-D-A Acceptors for Organic Photovoltaics*.

Herein, we elucidate the photodegradation pathway of A-D-A-type non-fullerene acceptors for organic photovoltaics. Using IT-4F as a benchmark example, we isolated the photoproducts and proved them isomers of IT-4F formed by a 6-e electrocyclic reaction between the dicyanomethylene unit and the thiophene ring, followed by a 1,5-sigmatropic hydride shift. This photoisomerization was accelerated under inert conditions, as explained by DFT calculations predicting a triplet-mediated reaction path (quenchable by oxygen). Adding controlled amounts of the photoproduct P1 to PM6:IT-4F bulk heterojunction cells led to a progressive decrease in photocurrent and fill factor attributed to its poor absorption and charge transport properties. The reaction is a general photodegradation pathway for a series of A-D-A molecules with 1,1-dicyanomethylene-3-indanone termini, and its rate varies with the structure of the donor and acceptor moiety.

Variance-resistant PTB7 and axially-substituted silicon phthalocyanines as active materials for high-Voc organic photovoltaics

While the efficiency of organic photovoltaics (OPVs) has improved drastically in the past decade, such devices rely on exorbitantly expensive materials that are unfeasible for commercial applications. Moreover, examples of high voltage single-junction devices, which are necessary for several applications, particularly low-power electronics and rechargeable batteries, are lacking in literature. Alternatively, silicon phthalocyanines (R2-SiPc) are inexpensive, industrially scalable organic semiconductors, having a minimal synthetic complexity (SC) index, and are capable of producing high voltages when used as acceptors in OPVs. In the present work, we have developed high voltage OPVs composed of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno [3,4 b]thiophenediyl}) (PTB7) and an SiPc derivative ((3BS)2-SiPc). While changes to the solvent system had a strong effect on performance, interestingly, the PTB7:(3BS)2-SiPc active layer were robust to spin speed, annealing and components ratio. This invariance is a desirable characteristic for industrial production. All PTB7:(3BS)2-SiPc devices produced high open circuit voltages between 1.0 and 1.07 V, while maintaining 80% of the overall efficiency, when compared to their fullerene-based counterpart.

Effects of Different Ring‐Expanded Strategies for Nonfullerene Acceptors in Organic Photovoltaics under Donor and Acceptor Excitation

The ring‐expanded strategy in nonfullerene acceptors (NFAs) with the acceptor–donor–acceptor backbone has been reported to be an effective method to improve the fill factor (FF), open circuit voltage (VOC), and short circuit current (JSC) simultaneously in organic photovoltaics. However, design control is still missing in the ring‐expanded strategy, and is urgently needed to further develop the origins and rules. To give insight into this strategy, a detailed theoretical study of the ring‐expanded mechanism is performed on the systems comprising different 9,9′‐bifluorenylidene‐based cores and 1,1‐dicyanomethylene‐3‐indanone group. Some main parameters involved in photoelectric conversion mechanism under the donor excitation (DE) and/or acceptor excitation (AE) are assessed by changing the position and size of ring‐expanded modes. The results show that the external ring‐expanded modes can not only maintain the original advantage as much as possible, variations in sizes and positions also offer them an opportunity to regulate the aforementioned parameters systematically, leading to better improvement regardless of AE or DE. Thus, the steady improvement in performance mentioned previously is the key to overcoming the negative correlation among FF, VOC, and JSC. This insight and discovery of the ring‐expanded strategy provides new design approaches for the next generation of NFAs.

Efficient Exciton Diffusion in Micrometer-Sized Domains of Nanographene-Based Nonfullerene Acceptors with Long Exciton Lifetimes in Blend Films with Conjugated Polymer.

Phase-separated structures in photoactive layers composed of electron donors and acceptors in organic photovoltaics (OPVs) generally exert a profound impact on the device performance. In this study, nonfullerene acceptors (NFAs) where a heteronanographene central core was furnished with branched alkoxy chains of different lengths, TACIC-EH, TACIC-BO, and TACIC-HD, were prepared to adjust the aggregation tendency and systematically probe the relationships of film structures with photophysical and photovoltaic properties. The side-chain length showed negligible effects on the absorption properties and energy levels of TACICs. In addition, regardless of the chain length, all TACIC films exhibited characteristically long singlet exciton lifetimes (1330-2330 ps) compared to those in solution (≤220 ps). Using a conjugated polymer donor, PBDB-T, the best OPV performance was achieved with TACIC-BO that contained medium-length chains, exhibiting a power conversion efficiency (PCE) of 9.92%. TACIC-HD with the longest chains showed deteriorated electron mobility due to the long insulating alkoxy groups. Therefore, the PBDB-T:TACIC-HD-based device revealed a low charge collection efficiency and PCE (8.21%) relative to the PBDB-T:TACIC-BO-based device, but their film morphologies were analogous. Meanwhile, TACIC-EH with the shortest chains showed low solubility and formed micrometer-sized large aggregates in the blend film with PBDB-T. Although the charge collection efficiency of PBDB-T:TACIC-EH was lower than that of PBDB-T:TACIC-BO, the efficiencies of exciton diffusion to the donor-acceptor interface were sufficiently high (>98%) owing to the elongated singlet exciton lifetime of TACIC-EH. The PCE of the PBDB-T:TACIC-EH-based device remained moderate (7.10%). Therefore, TACICs with the long singlet exciton lifetimes in the films provide a clear guideline for NFAs with low sensitivity of OPV device performance to the blend film structures, which is advantageous for large-scale OPV production with high reproducibility.

Fast visible light-induced synthesis of annulated perylene bisimides

Perylene-3, 4, 9, 10-bisimides (PBIs) are a family of classic functional dyes that are active in the fabrication of optic-electronic devices, such as non-fullerene acceptors in organic photovoltaics and n-type organic field effect transistors. Functionalization of PBIs with amidation or annulation not only modulates their intrinsic photochemical and photophysical performances but also brings new products of rylene family (e.g. terrylene diimides, coronenes). In this work, we introduce a new photo-synthetic method to produce annulated perylene bisimides with a 2-methylthiophene pendant at its bay position. Unlike the common photo-annulation that undergoes a 1, 6-2H elimination and forms a fused coronene structure, the proposed photo-synthesis presents a bisthienylethene-like cyclization with the H and methyl group toggling on each reaction carbon site. The 2-methylthiophene modified PBIs (PBI-MTs) exhibit fast photo-annulation (k = 4.3 × 10−1·s−1) with high conversion ratio (84%) under pure visible light irradiation (λ = 530 nm), which proves to be an effective and eco-friendly synthetic method for annulated PBI derivatives compared to the traditional UV-induced photosynthesis.

Ring fusion in tetrathienylethene cored perylene diimide tetramers affords acceptors with strong and broad absorption in the near-UV to visible region

Two PDI tetramers with a central tetrathienylethene core were investigated as non-fullerene acceptors in organic photovoltaics and the dramatic differences in their optical absorption spectra were rationalised on the basis of DFT calculations.

Cover Feature: Original Suzuki–Miyaura Coupling Using Nitro Derivatives for the Synthesis of Perylenediimide‐Based Multimers (Eur. J. Org. Chem. 47/2019)

The Cover Feature shows 1-nitro-perylen-ediimide (1-nitroPDI), an attractive starting material for the preparation of “satellite-shaped” PDI-based multimers. The nitro derivative is involved as an original electrophile into efficient palladium-catalyzed Suzuki–Miyaura couplings with polyboronic ester linkers affording multimers of various geometries. The protocol improves the synthetic access to the reported class of tridimensional PDI based multimers, which are of particular interest as Non-Fullerene Acceptors in organic photovoltaics. More information can be found in the Full Paper by L. Rocard, P. Hudhomme et al.

Synthesis and Optoelectronic Characterization of Perylene Diimide-Quinoline Based Small Molecules

Perylene diimide (PDI) is one of the most studied functional dyes due to their structural versatility and fine tuning of the materials properties. Core substituted PDIs are prominent n-type semiconductor materials that could be used as non-fullerene acceptors in organic photovoltaics. Herein, we develop versatile organic building blocks based on PDI by decorating the PDI core with quinoline groups. Styryl and hydroxy phenyl mono and difunctionalized molecules were prepared using mono-nitro and dibromo bay substituted PDIs by Suzuki coupling with the respective boronic acid derivatives. A novel methodology using nitro-PDI under Suzuki coupling conditions as an electrophile partner was successfully tested. Furthermore, the PDI derivatives were used for the synthesis of soluble, electron accepting small molecules combining PDI with weak electron withdrawing quinoline derivatives. The new molecules presented wide absorbance in the visible spectrum from 450 to almost 700 nm while their LUMO levels and their energy levels are in the range of −3.8 to −4.2 eV.

Synthesis of 2,2′-[2,2′-(arenediyl)bis(anthra[2,3-b]thiophene-5,10-diylidene)]tetrapropanedinitriles and their performance as non-fullerene acceptors in organic photovoltaics

Abstract Three novel molecules based on thiophene-fused 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) units linked through functionalized π-conjugated arene bridges are synthesized by three-step sequence of Sonagashira coupling, double thioannulation and Knoevenagel condensation. The energy of their frontier orbitals shows that these molecules are suitable for replacing fullerenes as electron acceptors in composites with conjugated donor polymers used in organic photovoltaics (OPV). The composite of polymer donor PCDTBT with fluorene-linked thiophene-fused TCAQs demonstrates good solubility in organic solvents and bulk heterojunction morphology with characteristic donor and acceptor domain size of several tens of nanometers, favourable for photoelectric conversion. This composite also exhibits prominent light-induced EPR signal indicating efficient photoinduced free charge generation. OPV devices based on this composite were fabricated by vacuum-free method. They show photoelectric conversion with maximum PCE of 0.71%.

Silicon Phthalocyanines as Acceptor Candidates in Mixed Solution/Evaporation Processed Planar Heterojunction Organic Photovoltaic Devices

Silicon phthalocyanines (SiPc) are showing promise as both ternary additives and non-fullerene acceptors in organic photovoltaics (OPVs) as a result of their ease of synthesis, chemical stability and strong absorption. In this study, bis(3,4,5-trifluorophenoxy) silicon phthalocyanine ((345F)2-SiPc)) and bis(2,4,6-trifluorophenoxy) silicon phthalocyanine ((246F)2-SiPc)) are employed as acceptors in mixed solution/evaporation planar heterojunction (PHJ) devices. The donor layer, either poly(3-hexylthiophene) (P3HT) or poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), was spin coated followed by the evaporation of the SiPc acceptor thin film. Several different donor/acceptor combinations were investigated in addition to investigations to determine the effect of film thickness on device performance. Finally, the effects of annealing, prior to SiPc deposition, after SiPc deposition, and during SiPc deposition were also investigated. The devices which performed the best were obtained using PCDTBT as the donor, with a 90 nm film of (345F)2-SiPc as the acceptor, followed by thermal annealing at 150 °C for 30 min of the entire mixed solution/evaporation device. An open-circuit voltage (Voc) of 0.88 V and a fill factor (FF) of 0.52 were achieved leading to devices that outperformed corresponding fullerene-based PHJ devices.

Single-Junction Organic Solar Cell with over 15% Efficiency Using Fused-Ring Acceptor with Electron-Deficient Core

Summary Recently, non-fullerene n-type organic semiconductors have attracted significant attention as acceptors in organic photovoltaics (OPVs) due to their great potential to realize high-power conversion efficiencies. The rational design of the central fused ring unit of these acceptor molecules is crucial to maximize device performance. Here, we report a new class of non-fullerene acceptor, Y6, that employs a ladder-type electron-deficient-core-based central fused ring (dithienothiophen[3.2-b]- pyrrolobenzothiadiazole) with a benzothiadiazole (BT) core to fine-tune its absorption and electron affinity. OPVs made from Y6 in conventional and inverted architectures each exhibited a high efficiency of 15.7%, measured in two separate labs. Inverted device structures were certified at Enli Tech Laboratory demonstrated an efficiency of 14.9%. We further observed that the Y6-based devices maintain a high efficiency of 13.6% with an active layer thickness of 300 nm. The electron-deficient-core-based fused ring reported in this work opens a new door in the molecular design of high-performance acceptors for OPVs.

Electronic, optical, and charge transport properties of A-π-A electron acceptors for organic solar cells: Impact of anti-aromatic π structures

Organic solar cells based on acceptor-π-acceptor (A-π-A) electron acceptors have attracted intensive attention due to their increasing and record power conversion efficiencies. To date, almost all of the reported A-π-A electron acceptors are based on aromatic π structures. Here, we have investigated the impact of anti-aromatization of the π-bridges on the optoelectronic properties of A-π-A electron acceptors by (time-dependent) density functional theory. Our calculations show that besides the frontier molecular orbitals corresponding to the aromatic π-bridge based acceptors (“aromatic” acceptors), additional and unique occupied and unoccupied frontier orbitals are found for the acceptors based on the anti-aromatic π-bridges (“anti-aromatic” acceptors). Moreover, by tuning isomeric structures of the π-bridges (e.g., fusion orientations or linking positions of thiophene moieties), the optical excitation energies for the transition between the additional occupied and unoccupied levels turn to be close to or substantially lower with respect to those for the transition between the “aromatic” frontier orbitals. The optical absorption of the “anti-aromatic” acceptors is thus either stronger or broader than the “aromatic” acceptors. Finally, the reorganization energies for electron transport are tunable and dependent on the π-bridge structures. These results indicate a great potential of “anti-aromatic” electron acceptors in organic photovoltaics.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells.

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.