Fine-Tuning the Dipole Moment of Asymmetric Non-Fullerene Acceptors Enabling Efficient and Stable Organic Solar Cells

Isomerization-Controlled Aggregation in Photoactive Layer: An Additive Strategy for Organic Solar Cells with Over 19.5 % Efficiency

Morphology control of the photoactive layer is crucial for achieving high-performance organic solar cells (OSCs), yet it remains a significant challenge in this field. One effective approach is the additive strategy, which fine-tunes the morphology of the photoactive layer. However, the underlying mechanisms governing the impact of different types of additives from liquid, solid, to volatile solid, on the bulk heterojunction morphology and device performance are not fully understood. Herein, we present an aggregation regulation strategy for acceptor molecules by incorporating three novel isomeric additives: 4-bromo-1,2-dichlorobenzene (LCB), 1-bromo-2,4-dichlorobenzene (SCB), and 2-bromo-1,4-dichlorobenzene (VCB) into the blend active layer. This approach optimizes the bulk heterojunction morphology and enhances the photovoltaic performance of OSCs. Our results reveal that these additives induce stepwise regulation of acceptor molecule aggregation during film formation. The liquid additive LCB primarily extends solvent evaporation time, effectively preventing excessive aggregation, while the solid additive SCB significantly shortens the aggregation period during the film evolution, resulting in the most compact molecular π-π stacking. Furthermore, the volatile solid additive VCB fine-tunes the intermolecular interactions and crystallization within the active layer, promoting optimal molecular self-assembly and aggregation for ideal molecular stacking. Consequently, the power conversion efficiencies of 19.33 % and 19.51 % were achieved for the VCB-processed D18 : L8-BO- and PM6 : L8-BO-based OSCs, respectively, outperforming the LCB-processed and SCB-processed devices.

Oligomerized Acceptors with High Efficiency and Superior Stability in Organic Solar Cells

Oligomerized small molecular acceptors (OSMAs) have rapidly become a research hotspot in the field of organic solar cells due to their advantages of effective combination with definite structure of small molecules and high viscosity and glass transition temperature of polymers. From this perspective, we classify and summarize the representative OSMAs from the three binding sites of end, core, and wing, and we summarize the exploration of new synthesis methods to improve the plights, such as low synthesis yield and difficult purification. Finally, the existing challenges and future research directions are concluded and prospected.

A Three-in-One Hybrid Strategy for High-Performance Semiconducting Polymers Processed from Anisole

The development of semiconducting polymers with good processability in green solvents and competitive electrical performance is essential for realizing sustainable large-scale manufacturing and commercialization of organic electronics. A major obstacle is the processability-performance dichotomy that is dictated by the lack of ideal building blocks with balanced polarity, solubility, electronic structures, and molecular conformation. Herein, through the integration of donor, quinoid and acceptor units, an unprecedented building block, namely TQBT, is introduced for constructing a serial of conjugated polymers. The TQBT, distinct in non-symmetric structure and high dipole moment, imparts enhanced solubility in anisole-a green solvent-to the polymer TQBT-T. Furthermore, PTQBT-T possess a highly rigid and planar backbone owing to the nearly coplanar geometry and quinoidal nature of TQBT, resulting in strong aggregation in solution and localized aggregates in film. Remarkably, PTQBT-T films spuncast from anisole exhibit a hole mobility of 2.30 cm2 V-1 s-1, which is record high for green solvent-processable semiconducting polymers via spin-coating, together with commendable operational and storage stability. The hybrid building block emerges as a pioneering electroactive unit, shedding light on future design strategies in high-performance semiconducting polymers compatible with green processing and marking a significant stride towards ecofriendly organic electronics.

Optimizing Molecular Crystallinity and Suppressing Electron-Phonon Coupling in Completely Non-Fused Ring Electron Acceptors for Organic Solar Cells

High open-circuit voltage (Voc) organic solar cells (OSCs) have received increasing attention because of their promising application in tandem devices and indoor photovoltaics. However, the lack of a precise correlation between molecular structure and stacking behaviors of wide band gap electron acceptors has greatly limited its development. Here, we adopted an asymmetric halogenation strategy (AHS) and synthesized two completely non-fused ring electron acceptors (NFREAs), HF-BTA33 and HCl-BTA33. The results show that AHS significantly enhances the molecular dipoles and suppresses electron-phonon coupling, resulting in enhanced intramolecular/intermolecular interactions and decreased nonradiative decay. As a result, PTQ10 : HF-BTA33 realizes a power conversion efficiency (PCE) of 11.42 % with a Voc of 1.232 V, higher than that of symmetric analogue F-BTA33 (PCE=10.02 %, Voc=1.197 V). Notably, PTQ10 : HCl-BTA33 achieves the highest PCE of 12.54 % with a Voc of 1.201 V due to the long-range ordered π-π packing and enhanced surface electrostatic interactions thereby facilitating exciton dissociation and charge transport. This work not only proves that asymmetric halogenation of completely NFREAs is a simple and effective strategy for achieving both high PCE and Voc, but also provides deeper insights for the precise molecular design of low cost completely NFREAs.

Morphology Control for Efficient Nonfused Acceptor-Based Organic Photovoltaic Cells

Non-fused electron acceptors have huge advantages in fabricating low-cost organic photovoltaic (OPV) cells. However, morphology control is a challenge as non-fused C─C single bonds bring more molecular conformations. Here, by selecting two typical polymer donors, PBDB-TF and PBQx-TF, the blend morphologies and its impacts on the power conversion efficiencies (PCEs) of non-fused acceptor-based OPV cells are studied. A selenium-containing non-fused acceptor named ASe-5 is designed. The results suggest that PBQx-TF has a lower miscibility with ASe-5 when compared with PBDB-TF. Additionally, the polymer networks may form earlier in the PBQx-TF:ASe-5 blend film due to stronger preaggregation performance, leading to a more obvious phase separation. The PBQx-TF:ASe-5 blend film shows faster charge transfer and suppressed charge recombination. As a result, the PBQx-TF:ASe-5-based device records a good PCE of 14.7% with a higher fill factor (FF) of 0.744, while the PBDB-TF:ASe-5-based device only obtains a moderate PCE of 12.3% with a relatively low FF of 0.662. The work demonstrates that the selection of donors plays a crucial role in controlling the blend morphology and thus improving the PCEs of non-fused acceptor-based OPV cells.

A computational insight into enhancement of photovoltaic properties of non-fullerene acceptors by end-group modulations in the structural framework of INPIC molecule

Improving the open circuit voltage is a major challenge for enhancing the overall efficiency of organic solar cells. Current work has concentrated on improving open-circuit voltage by designing new molecular frameworks from an INPIC molecule having a conjugated fused core. We modulated the structure by changing the terminal groups of the reference molecule (INPIC) with seven strong electron-withdrawing units. We investigated various optoelectronic attributes, charge transfer, and photovoltaic and geometrical parameters by compiling the B3LYP/6-31G(d,p) functional of the DFT approach. The optical absorption for modulated molecules ranges from 748.51 nm to 845.96 nm while showing higher oscillation strength than INPIC. At the same time, their impressive charge transport is attributed to their smaller excitation and exciton binding energy, higher electron/hole mobility, narrower band gap, and a more than 99 % intramolecular charge transfer. The larger dipole moments help in the dense interaction of acceptors with employed donor J61 which, in turn, improves charge transfer at the donor-acceptor interface. One of the triumphs that are difficult to get in organic molecules is success in achieving a higher open circuit voltage (VOC). Our conceptualized molecular frameworks of acceptors are featured with a notable VOC improvement in the range of 1.84-2.05 eV. Thus, the results of the current investigation pave the root for architecting the acceptor molecules with impressive optoelectrical properties that may be capable of providing high photovoltaic output. Thus these acceptors can be utilized for the development of advanced organic solar cells in future.

Exploring the Electronic, Optical, and Charge Transfer Properties of A-D-A-Type IDTV-ThIC-Based Molecules To Enhance Photovoltaic Performance of Organic Solar Cells

Improving the charge mobility and optoelectronic properties of indacenodithiophene-based small molecule acceptors is a key challenge to improving overall efficiency. In this current research, seven newly designed molecules (DT1-DT7) comprising the indacenodithiophene-based core are presented to tune energy levels, enhance charge mobility, and improve the photovoltaic performance of IDTV-ThIC molecules via density functional theory. All the molecules were designed by end-capped modification by substituting terminal acceptors of IDTV-ThIC with strong electron-withdrawing moieties. Among all the examined structures, DT1 has proved itself a superior molecule in multiple aspects, including higher λmax in chloroform (787 nm) and gaseous phase (727 nm), narrow band gap (2.16 eV), higher electron affinity (3.31 eV), least excitation energy (1.57 eV), and improved charge mobility due to low reorganization energy and higher excited state lifetime (2.37 ns) when compared to the reference (IDTV-ThIC) and other molecules. DT5 also showed remarkable improvement in different parameters, such as the lowest exciton binding energy (0.41 eV), leading to easier charge moveability. The improved open-circuit voltage of DT4 and DT5 makes them proficient molecules exhibiting the charge transfer phenomenon. The enlightened outcomes of these molecules can pave a new route to develop efficient organic solar cell devices using these molecules, especially DT1, DT4, and DT5.

A Medium-Bandgap Nonfullerene Acceptor Enabling Organic Photovoltaic Cells with 30% Efficiency under Indoor Artificial Light

The correlation between molecular structure and photovoltaic performance is lagging for constructing high-performance indoor organic photovoltaic (OPV) cells. Herein, this relationship is investigated in depth by employing two medium-bandgap nonfullerene acceptors (NFAs). The newly synthesized NFA of FTCCBr exhibits a similar bandgap and molecular energy level, but a much stronger dipole moment and larger average electrostatic potential (ESP) compared with ITCC. After blending with the polymer donor PB2, the PB2:ITCC and PB2:FTCCBr blends exhibit favorable bulk-heterojunction morphologies and the same driving force, but the PB2:FTCCBr blend exhibits a large ESP difference. In OPV cells, the PB2:ITCC-based device produces a power conversion efficiency (PCE) of 11.0%, whereas the PB2:FTCCBr-based device gives an excellent PCE of 14.8% with an open-circuit voltage (V_OC ) of 1.05 V, which is the highest value among OPV cells with V_OC values above 1.0 V. When both acceptor-based devices work under a 1000 lux of 3000 K light-emitting diode, the PB2:ITCC-based 1 cm² device yields a good PCE of 25.4%; in contrast, the PB2:FTCCBr-based 1 cm² device outputs a record PCE of 30.2%. These results suggest that a large ESP offset in photovoltaic materials is important for achieving high-performance OPV cells.

Benzotriazole-Based Non-Fused Ring Acceptors for Efficient and Thermally Stable Organic Solar Cells

Non-fused ring acceptors (NFRAs) have attracted significant attention for non-fullerene organic solar cells (OSCs) owing to their chemical tunability and facile synthesis. In this study, a benzotriazole-based NFRA with chlorinated end groups (Triazole-4Cl) is developed to realize highly efficient and thermally stable NFRA-based OSCs; an analogous NFRA with non-chlorinated end groups (Triazole-H) is synthesized for comparison. Triazole-4Cl film exhibits the high-order packing structure and the near-infrared absorption capability, which are advantageous in charge transport and light harvesting of the resulting OSCs. In particular, the strong crystalline behavior of Triazole-4Cl results in enhanced self-aggregation, leading to high charge carrier mobility. Owing to these properties, a PBDB-T(polymer donor):Triazole-4Cl OSC demonstrates a high short-circuit current, fill factor, and power conversion efficiency (PCE = 10.46%), outperforming a PBDB-T:Triazole-H OSC (PCE = 7.65%). In addition, the thermal stability of a PBDB-T:Triazole-4Cl OSC at an elevated temperature of 120°C exceeds that of a PBDB-T:Triazole-H OSC. This is mainly attributed to the significantly higher cold crystallization temperature of Triazole-4Cl (205.9°C). This work provides useful guidelines for the design of NFRAs to achieve efficient and thermally stable NFRA-based OSCs. This article is protected by copyright. All rights reserved.

γ-Ester-Functionalized 1,1-Dicyanomethylene-3-indanone End-Capped Nonfullerene Acceptors for High-Performance, Annealing-Free Organic Solar Cells

Modifying the end-capping groups in nonfullerene acceptors (NFAs) is an effective strategy for modulating their properties and that of the entire NFAs. This study reports the synthesis of a novel γ-ester-functionalized IC end-capping group (IC-γe) and its incorporation into the benzothiadiazole-fused central core, yielding isomer-free IC-γe end-capped NFAs, such as Y-IC-γe, Y-FIC-γe, and Y-ClIC-γe. The resultant NFAs exhibited similar absorption profiles but upshifted the lowest unoccupied molecular orbital energy level compared with those of the ester-free analogues, such as Y6 and Y7. Without thermal annealing, an excellent power conversion efficiency (PCE) of 16.4% is realized in the annealing-free OSC based on Y-FIC-γe with the PM6 donor polymer, which outperforms the OSCs based on Y-IC-γe and Y-ClIC-γe. In addition, the OSCs based on asymmetric Y-FIC-γe and Y-ClIC-γe have higher thermal stability with more than 83% PCE retention at an elevated temperature after 456 h than the symmetric Y-IC-γe case. In this study, we not only establish the structure-property relationship regarding the ester functionality and symmetricity tuning on the NFAs but also diagnose the reasons for the best-performing Y-FIC-γe-based OSCs, providing useful information for a novel high-performing NFA design strategy.

Single-Junction Organic Solar Cells with 19.17% Efficiency Enabled by Introducing One Asymmetric Guest Acceptor

The ternary strategy has been widely identified as an effective approach to obtain high-efficiency organic solar cells (OSCs). However, for most ternary OSCs, the nonradiative voltage loss lies between those of the two binary devices, which limits further efficiency improvements. Herein, an asymmetric guest acceptor BTP-2F2Cl is designed and incorporated into a PM1:L8-BO host blend. Compared with the L8-BO neat film, the L8-BO:BTP-2F2Cl blend film shows higher photoluminescence quantum yield and larger exciton diffusion length. Introducing BTP-2F2Cl into the host blend extends its absorption spectrum, improves the molecular packing of host materials, and suppresses the nonradiative charge recombination of the ternary OSCs. Consequently, the power conversion efficiency is improved up to 19.17% (certified value 18.7%), which represents the highest efficiency value reported for single-junction OSCs so far. The results show that improving the exciton behaviors is a promising approach to reducing the nonradiative voltage loss and realizing high-performance OSCs.

Exploiting Novel Unfused-Ring Acceptor for Efficient Organic Solar Cells with Record Open-Circuit Voltage and Fill Factor

Unfused-ring acceptors (UFAs) show bright application prospects in organic solar cells (OSCs) thanks to their easy synthesis, low cost, and good device performance. The selection of central-core building block and suitable side chain are the key factors to achieve high-performance UFAs. Current tremendous endeavors for the development of UFAs mainly concentrate on obtaining higher short-circuit current density (J_sc ), albeit accompanied by low open-circuit voltage (V_oc ) and modest fill factor (FF). Herein, two novel A-D-A'-D-A type UFAs (BTCD-IC and BTCD-2FIC), which have the same new electron-withdrawing central-core dithieno[3',2':3,4;2'',3'':5,6]-benzo[1,2-c][1,2,5]thiadia-zole (DTBT) and cyclopentadithiophene unit (CPDT, substituted by 2-butyl-1-octyl alkyl chain) coupling with different terminals, were designed and synthesized. Two UFAs showed strong and broad light absorption in the wavelength range of 300-850 nm owing to the strong intramolecular charge transfer effect favorable by DTBT core. Compared with BTCD-IC, BTCD-2FIC with F-containing terminal group exhibited higher molar extinction coefficient, lower energy level, higher charge mobility, stronger crystallinity, more ordered molecular stacking, and better film morphology. As a result, when blended with donor polymer PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']-dithiophene-4,8-dione)]), the BTCD-2FIC-based OSC achieved a superior power conversion efficiency (PCE) of 11.32 %, with a high V_oc of 0.85 V, a J_sc of 18.24 mA cm^-2 , and a FF of 73 %, than BTCD-IC-based OSC (PCE=8.96 %). Impressively, the simultaneously enhanced V_oc and FF values of the PBDB-T:BTCD-2FIC device were the highest values of the A-D-A'-D-A-type UFAs. The results demonstrate the application of electron-withdrawing DTBT central-core unit in efficient UFAs provides meaningful molecular design guidance for high-performance OSCs.

Azulene Bridged π-Distorted Chromophores: The Influence of Structural Symmetry on Optoelectrochemical and Photovoltaic Parameters

Conjugated chromophores possessing π-twisted functionality such as tetracyanobutadiene (TCBD) have emerged as promising active layer materials for organic photovoltaics (OPVs). In this study, we disclose the synthesis of two azulenyl chromophores containing one and two TCBD groups. The symmetrical and unsymmetrical structural characteristics of these molecules inflict dissimilar optoelectronic and electrochemical properties. Based on molar absorptivity, aggregation behavior, HOMO-LUMO energies and other quantum chemical parameters, the symmetrical molecule (TATC2) appears to be a better non-fullerene acceptor (NFA) compared to its unsymmetrical counterpart (TATC1). For instance, higher absorptivity and deeper HOMO-LUMO levels for TATC2 (23950 M-1  cm-1 ; -6.01 eV/-3.86 eV) over TATC1 (12200 M1  cm-1 ; -5.46 eV/-3.64 eV) was observed. Validating this structure-property relationship on solar cell prototypes exhibited higher photovoltaic parameters (VOC =0.54 V, FF=0.48, JSC =6.42 mA/cm2 ) for TATC2 than TATC1 (VOC =0.47 V, FF=0.38, JSC =5.77 mA/cm2 ). Though the device parameters are not high, this work uncovers the intrinsic properties of azulene-tethered twisted chromophores as potential π-semiconductor choice for NFA solar cells. In particular, this report explores the utility of azulene-based π-twisted semiconductors as acceptor material for OPVs with cell efficiencies of 1.70 and 1.04 % for TATC2 and TATC1 respectively.