Wide Band Gap Non-Fullerene Small Molecular Acceptors Containing Spirobifluorene and Benzotriazole with Three Different End-Capped Groups for P3HT-Based Organic Solar Cells

Spirofluorene （SF） and benzo[d][1,2,3]triazole （BTA） have been considered as promising building blocks to construct n-type photovoltaic materials. Herein, three new small molecule acceptors （SMAs） named BTA21, BTA23 and BTA27 with the structure of A2=A1-D-AI^A2 have been designed, in which SF and BTA were used as a central unit of D and bridged acceptor unit of A1, respectively. In addition, 3-ethylrhodanine, 2-（3-ethyl-4-oxothiazolidin-2-ylidene）malononitrile and malononitrile were chosen as terminal acceptor units to modulate the properties of the final SMAs. Three SMAs show wide optical band gaps （Eg） of 2.19, 2.15 and 2.22 eV, respectively, with gradually down-shift of the lowest unoccupied molecular orbital {LUMO） levels in the order of BTAZl, BTA23 and BTA27 depending on the electron-withdrawing capability of terminal acceptor units. BTA21 shows great advantages with respect to donor poly（3-hexylthiophene） （P3HT） over BTA23 and BTA27, such as well energy-level matching, complementary absorption and proper morpholgy, Concequently, P3HT：BTA21 shows the best power conversion efficiency （PCE） value of 3.28% with an open-circuit voltage （Voc） of 1.02 V, a short-circuit current （Jsc） of 5.45 mA.cm-2 and a fill factor （FF） of 0.59. These results indicate that the terminal acceptor group end-capped in SMAs plays a significant role in controlling their optical, electronic, and photovoltaic properties.

Recent advances and prospects of asymmetric non-fullerene small molecule acceptors for polymer solar cells

Recently,polymer solar cells developed very fast due to the application of non-fullerence acceptors.Substituting asymmetric small molecules for symmetric small molecule acceptors in the photoactive layer is a strategy to improve the performance of polymer solar cells.The asymmetric design of the molecule is very beneficial for exciton dissociation and charge transport and will also fine-tune the molecular energy level to adjust the open-circuit voltage(Voc)further.The influence on the absorption range and absorption intensity will cause the short-circuit current density(Jsc)to change,resulting in higher device performance.The effect on molecular aggregation and molecular stacking of asymmetric structures can directly change the microscopic morphology,phase separation size,and the active layer's crystallinity.Very recently,thanks to the ingenious design of active layer materials and the optimization of devices,asymmetric non-fullerene polymer solar cells(A-NF-PSCs)have achieved remarkable development.In this review,we have summarized the latest developments in asymmetric small molecule acceptors(A-NF-SMAs)with the acceptor-donor-acceptor(A-D-A)and/or acceptor-donor-acceptor-donor-acceptor(A-D-A-D-A)structures,and the advantages of asymmetric small molecules are explored from the aspects of charge transport,molecular energy level and active layer accumulation morphology.

Molecular design towards two-dimensional electron acceptors for efficient non-fullerene solar cells

Non-fullerene polymer solar cells(NF-PSCs) have gained wide attention recently. Molecular design of non-fullerene electron acceptors effectively promotes the photovoltaic performance of NF-PSCs. However,molecular electron acceptors with 2-dimensional(2 D) configuration and conjugation are seldom reported.Herein, we designed and synthesized a series of novel 2 D electron acceptors for efficient NF-PSCs. With rational optimization on the conjugated moieties in both vertical and horizontal direction, these 2 D electron acceptors showed appealing properties, such as good planarity, full-spectrum absorption, high absorption extinction coefficient, and proper blend morphology with donor polymer. A high PCE of 9.76%was achieved for photovoltaic devices with PBDB-T as the donor and these 2 D electron acceptors. It was also found the charge transfer between the conjugated moieties in two directions of these 2 D molecules contributes to the utilization of absorbed photos, resulting in an exceptional EQE of 87% at 730 nm. This work presents rational design guidelines of 2 D electron acceptors, which showed great promise to achieve high-performance non-fullerene polymer solar cells.

Nanomorphology in A–D–A type small molecular acceptors-based bulk heterojunction polymer solar cells

Recent developments in acceptor–donor–acceptor(A–D–A) type non-fullerene acceptors have led to substantial improvements in bulk-heterojunction polymer solar cells efficiency. The device performance strongly depends on photoactive layer morphology, as the molecular packing, donor–acceptor interface and phase separation significantly affect the charge-transfer states and charge carrier dynamics. In this review, we start with a brief introduction of the techniques most effectively utilized to characterize multiphase morphology. Then, we summarize recent progress in A–D–A type acceptors, with the emphasis on understanding the molecular structure–morphology–performance relationships. Finally, an outlook on correlating morphological characteristics with photovoltage losses is presented for further improving device performance.

Small bandgap non-fullerene acceptor enables efficient PTB7-Th solar cell with near 0 eV HOMO offset

Three small bandgap non-fullerene(SBG NFAs) acceptors,BDTI,BDTI-2 F and BDTI-4 F,based on a carbon-oxygen bridged central core and thieno[3,4-b]thiophene linker,end-capped with varied electronwithdrawing terminal groups,were designed and synthesized.The acceptors exhibit strong absorption from 600 nm to 1000 nm.The optimal device incorporating designed NFA and PTB7-Th polymer donor achieves a power conversion efficiency of 9.11% with near 0 eV HOMO offset.The work presents a case study of efficient non-fullerene solar cells with small HOMO offsets,which is achieved by blending PTB7-Th with fine-tuned SBG acceptor.

Recent developments of di-amide/imide-containing small molecular non-fullerene acceptors for organic solar cells

Non-fullerene organic solar cells have received increasing attentions in these years,and great progresses have been made since 2013.Among them,aromatic di-amide/imide-containing frameworks have shown promising applications.The outstanding properties of them are highly associated with their unique electronic and structural features,such as strong electron-withdrawing nature,broad absorption in UVvisible region,tunable HOMO/LUMO energy levels,easy modifications,and excellent chemical,thermal and photochemical stabilities.In this review,we give an overview of recent developments of aromatic diamide/imide-containing small molecules used as electron acceptors for organic solar cells.

Inner alkyl chain modulation of small molecular acceptors enables molecular packing optimization and efficient organic solar cells

With the generation of Y6,organic solar cells have reached remarkable achievement of over 19%efficiency.Alkyl chain is of importance to modulate intermolecular stacking and possibly enhance optoelectronic properties of small molecule acceptors(SMAs).Three alkyl chains of 2-ethylhexyl,2-butylocyl and 3-ethylheptyl were selected to obtain G6-EH,G6-BO and G6-EHep molecules,respectively.Compared to G6-EH and G6-BO,G6-EHep was found inducing unfavourable large domain size.Furthermore,we discover that 2-butyloctyl effectively inhibits monomolecular and bimolecular recombination,improves molecular packing,generates more balanced carrier mobility and enhances exciton dissociation.The SMA with 2-butyloctyl alkyl chains(G6-BO)shows the best electrical and morphological characteristics,achieving a higher power conversion efficiency(PCE)of 17.06%,with an open circuit voltage of 0.912 V,a short-circuit current of 24.22 m A cm-2and a fill factor of 77.25%.Finally,using the ternary strategy by incorporating the G6-BO acceptor into PM6:BTP-e C9,we achieved a higher PCE of18.13%with enhanced electron transport.

Synthesis and Photovoltaic Properties of Non-fullerene Solution Processable Small Molecule Acceptors

Two non-fullerene small molecules, BT-C6 and BT-C12, based on the vinylene-linked benzothiadiazole-thiophene（BT） moiety flanked with 2-（3,5,5-trimethylcyclohex-2-en-l-ylidene）malononitrile have been synthesized and characterized by solution/thin film UV-Vis absorption, photoluminescence（PL）, and cyclic voltammetry（CV） measurements. The two molecules show intense absorption bands in a wide range from 300 nm to 700 nm and low optical band- gaps for BT-C6（1.60 eV） and for BT-C12（1.67 eV）. The lowest unoccupied molecular orbital（LUMO） levels of both the molecules are relatively higher than that of [6,6]-phenyl C61 butyric acid methyl ester（PCBM）, promising high open circuit voltage（Voc） for photovoltaic application. Bulk heterojunction（BHJ） solar cells with poly（3-hexylthiophene） （P3HT） as the electron donor and the two molecules as the acceptors were fabricated. Under 100 mW/cm2 AM 1.5 G illumination, the devices based on P3HT：BT-C6（1：1, mass ratio） show a power conversion efficiency（PCE） of 0.67%, a short-circuit current（Jsc） of 1.63 mA/cm2, an open circuit voltage（Vow） of 0.74 V, and a fill factor（FF） of 0.56.

Polymerizing small molecular acceptors for efficient all-polymer solar cells

All-polymer solar cells(all-PSCs)have received attention due to their morphological stability under thermal and mechanical stresses.Currently,the highest reported power conversion efficiency of all-PSCs is over 17%,achieved by utilizing polymerized small molecular acceptors(PSMAs).However,the need for higher regiospecificity to avoid forming isomers during polymerization of SMAs still challenges the further applications of all-PSCs.From this perspective,we focus on some recent studies and highlight the importance of controlling the regioregularity of PSMAs.In particular,integrating PSMAs with regioregularity endows the polymer acceptors with good absorption,superior backbone ordering,and optimal blend morphology compared with those obtained from regiorandom one.Moreover,the distinctive features that are derived from these regioregular PSMAs,such as the possibility of repeatable synthesis and reproducible device performance,herald a brighter future for scaling-up and commercializing all-PSCs.We expect this integrated strategy will inspire researchers to devote more efforts to further narrow the efficiency gap between the PSCs based on SMAs and PSMAs.Finally,we discuss the existing challenges and future prospects of PSMAs as new platform for further advancing all-PSCs.

14.46% Efficiency small molecule organic photovoltaics enabled by the well trade-off between phase separation and photon harvesting

Small molecule organic photovoltaics(SMPVs) were prepared by utilizing liquid crystalline donor material BTR-Cl and two similar optical bandgap non-fullerene acceptor materials BTP-BO-4 F and Y6.The BTPBO-4 F and Y6 have the similar optical bandgap and different absorption coefficients.The corresponding binary SMPVs exhibit different short circuit current density(/sc)(20.38 vs.23.24 mA cm^(-2)),and fill factor(FF)(70.77% vs.67.21%).A 14.46% power conversion efficiency(PCE) is acquired in ternary SMPVs with 30 wt% Y6,companied with a JSC of 24.17 mA cm^(-2) a FF of 68.78% and an open circuit voltage(Voc) of 0.87 V.The improvement on PCE of ternary SMPVs should originate from the well trade-off between phase separation and photon harvesting of ternary active layers by incorporating 30 wt% Y6 in acceptors.This work may deliver insight onto the improved performance of SMPVs by superposing the superiorities of binary SMPVs with similar optical bandgap acceptors into one ternary cell.

Highly crystalline acceptor materials based on benzodithiophene with different amount of fluorine substitution on alkoxyphenyl conjugated side chains for organic photovoltaics

Organic solar cells based on narrow bandgap small-molecule acceptors(SMAs)with highly crystalline characteristics have attracted great attentions for their superiority in obtaining high photovoltaic efficiency.Employing highly crystalline SMAs to enhance power conversion efficiencies(PCEs)by regulating and controlling morphology and compatibility of donor and acceptor materials has turned out to be an effective approach.In this study,we synthesized three different crystalline SMAs by using fluorine substitution on alkoxyphenyl conjugated side chains to modulate the relationship of crystallinity and morphologies,namely ZY1(zero F atoms),ZY2(two F atoms),and ZY3(four F atoms).The three SMAs show the broad absorption edges and similar frontier orbital energy levels,generating the analogical(over 0.9 V)open circuit voltage(VOC)of the polymer solar cells(PSCs).As a result,the PM6:ZY2-based PSCs yield a PCE of 10.81%with a VOC of 0.95 V,a short-circuit current density(JSC)of 16.154 mA cm^(-2),and a fill factor(FF)of 0.71,which is higher than that of 9.17%(PM6:ZY1)and 6.37%(PM6:ZY3).And the PCE(17.23%)of the PM6:Y6:ZY2 based ternary PSCs is also higher than that of 16.32%PM6:Y6 based binary device.Obviously,the results demonstrate that adding fluorine atoms on the conjugated side chains to construct high crystalline materials is a positive strategy to effectively increase the efficiencies of binary and ternary PSCs.

Altering alkyl-chains branching positions for boosting the performance of small-molecule acceptors for highly efficient nonfullerene organic solar cells

The emergence of the latest generation of small-molecule acceptor(SMA)materials,with Y6 as a typical example,accounts for the surge in device performance for organic solar cells(OSCs).This study proposes two new acceptors named Y6-C2 and Y6-C3,from judicious alteration of alkyl-chains branching positions away from the Y6 backbone.Compared to the Y6,the Y6-C2 exhibits similar optical and electrochemical properties,but better molecular packing and enhanced crystallinity.In contrast,the Y6-C3 shows a significant blue-shift absorption in the solid state relative to the Y6 and Y6-C2.The as-cast PM6:Y6-C2-based OSC yields a higher power conversion efficiency(PCE)of 15.89%than those based on the Y6(15.24%)and Y6-C3(13.76%),representing the highest known value for as-cast nonfullerene OSCs.Prominently,the Y6-C2 displays a good compatibility with the PC71BM.Therefore,a ternary OSC device based on PM6:Y6-C2:PC71BM(1.0:1.0:0.2)was produced,and it exhibits an outstanding PCE of 17.06%and an impressive fill factor(FF)of 0.772.Our results improve understanding of the structureproperty relationship for state-of-the-art SMAs and demonstrate that modulating the structure of SMAs via fine-tuning of alkylchains branching positions is an effective method to enhance their performance.

A small molecular electron acceptor based on asymmetric hexacyclic core of thieno[1,2-b]indaceno[5,6-b']thienothiophene for efficient fullerene-free polymer solar cells

A novel A-D-A （acceptor-donor-acceptor） type non-fullerene small molecule, A201, consisting of an asymmetric thieno[1,2-b]indaceno[5,6-b＇lthienothiophene （TITI＇） unit as middle D part and 2-（3-oxo-2, 3-dihydroinden-l-ylidene） malononitrile （IC） groups as end-capped A parts was designed and synthesized. The asymmetric TITT building block showed a higher dipole moment of 0.85 Debye （1 Debye = 3.33564 × 10^-3μcm） compared with the symmetric analogues of indacenodithiophene （IDT） and indacenodithieno[3,2-b]thiophene （IDTr） of 0.098 and 0.13 Debye, respectively. The solution-processed bulk heterojunction solar cells using a benzotriazole （BTA）-based polymer of J71 as donor and A201 as acceptor, showed a power conversion efficiency （PCE） of 9.36% with an open-circuit voltage （Voc） of 0.88 V, a short-circuit current Use） of 13.15 mA cm^-2, and a fill factor （FF） of 0.B7, under the illumination of AM 1.5G at 100 mW cm^-2. The high PCE of this material combination could be attributed to its broad absorption spectrum and the high hole mobility （#h） and electron mobility （μh） of 9.56 × 10^-4 and 5.1× 10^-4 cm^2 V^-1 s^-1, respectively. These results indicate that the asymmetric electron-donating segments are promising to construct A-D-A type small molecular acceptors, which could largely enhance the diversity of building blocks to design photovoltaic materials.

Delicate chemical structure regulation of nonfullerene acceptor for efficient and large thickness organic solar cells

Inspired by the success of CH-series acceptors, a small-molecular acceptor, CH-Tz was reported by adopting a new conjugationextended electron-deficient unit([1,2,5]thiadiazolo[3,4-b]pyrazine) on the central core. Owing to the enhanced inter-/intramolecular interactions, CH-Tz exhibited near-infrared absorption and an effective three-dimensional molecular packing network in its single crystal. When blended with polymer donor PM6, the binary device achieved a high power conversion efficiency(PCE) of 18.54%, with a notable short-circuit current density(J_(sc)) of 27.54 m A cm-2and an excellent fill factor(FF) over 80%,which can be partly ascribed to the balanced charge transport properties in the blend film. After employing D18-Cl as the third component, an enhanced PCE of 18.85% was achieved due to a more obvious fiber network. Impressively, the CH-Tz-based OSC devices show excellent thermal stability and thickness insensitivity. Record-breaking Jscof 28.92 m A cm-2was reached for PM6:D18-Cl:CH-Tz ternary device with a thickness of 560 nm. Besides, CH-Tz shows potential in fabricating multicomponent high-performance organic solar cells, where over 19% efficiency could be realized in the quaternary device. Our work advances the strong influence of electron-deficient central units on molecular photovoltaic properties and guides the design of acceptors for stable and large-thickness organic solar cells.

9,9′-联芴烯衍生物在有机光伏电池中的应用

由于具有独特的14π电子芳香结构和扭曲的非平面构型,9,9′-联芴烯衍生物成为近年来被广泛研究的一类新型有机光伏材料。9,9′-联芴烯衍生物含有两个刚性平面内的联苯单元,因此具有良好的热稳定性和化学稳定性;且其具有优良的光电特性,很容易接受电子,从而提升自身的最低未占分子轨道(LUMO)能级,使得其有机光伏器件的开路电压增大。此外,9,9′-联芴烯有12个不同的取代位,与富勒烯衍生物相比,其结构更具灵活性。但是,该类化合物的电子迁移率较低,使得其光伏电池的光电转化效率过低。如何通过提高该类材料的电子迁移率、拓展其光谱吸收范围和吸收强度来提高光伏器件的光电转换效率,受到越来越多的关注和研究。阐述了近年来联芴烯衍生物在有机光伏电池中的研究进展,并对其结构、性能进行了简要的分析。最后,对联芴烯类材料的发展前景进行了展望。

有机太阳能电池中基于苝二酰亚胺结构小分子受体进展

最近几年,有机太阳能电池中的非富勒烯小分子受体研究引起了人们的兴趣。其中,苝二酰亚胺(PDI)类分子因具有良好的电子传输能力,较强的电子亲和力,稳定的光、热、化学性能以及化学结构的可设计性带来的性能可调控性而得到广泛的关注。本文总结了近三年来在体异质结有机太阳能电池应用方面PDI小分子受体的研究进展,重点关注了PDI分子结构对其性能的影响,希望为以后PDI类受体分子的设计思路起到一定的启发作用。

基于非稠环核心的小分子受体材料的合成与光伏应用

近年来,基于非稠环核心的电子受体因具有结构简单的优势,成为有机光伏材料领域的研究热点。本研究通过将2个环戊二烯并二噻吩单元和1个氟代苯并[c]-[1,2,5]噻二唑单元通过Stille偶联反应合成非稠环核心单元,随后与末端吸电子单元3-已基罗丹宁通过Knoevenagel缩合反应合成小分子受体材料(5Z,5'Z)-5,5'-((6,6'-(5,6-二氟苯并[c][1,2,5]噻二唑-4,7-二基)双(4,4-双(2-乙基己基)-4H-环戊二烯[1,2-b:5,4-b']二噻吩-6,2-二基))双(甲亚基))双(3-己基-2-硫代噻唑啉酮)(MAZ-4)。该受体材料在350~710 nm范围内具有强的吸收,在给体材料聚(3-己基噻吩)和受体材料MAZ-4质量比为1∶1.2的条件下,有机太阳能电池的最佳能量转换效率为0.76%。

The recent progress of wide bandgap donor polymers towards non-fullerene organic solar cells

This review summarized the recent progress of highly efficient wide bandgap(WBG) donor polymers and their applications in non-fullerene polymer solar cells(NF-PSCs). A brief introduction of the background of WBG donor polymer developments was given. Then the research progress of the reported WBG donor polymers by classification of D-type and DààA type molecular backbones was reviewed. The resulting structure-property correlations of the WBG donor polymers were also discussed to highlight the importance of chemical modifications, which have promoted the great progress of NF-PSC field. Finally,an outlook for future innovations of WBG donor polymers and their NF-PSCs was provided.

Chlorinated polymerized small molecule acceptor enabling ternary all-polymer solar cells with over 16.6% efficiency

Recently,all-polymer solar cells(all-PSCs) based on polymerized small molecule acceptors(PSMAs) have achieved significant progress.Ternary blending has proven to be an effective strategy to further boost the power conversion efficiency(PCE) of the all-PSCs.Herein,a new A-DA′D-A small-molecule acceptor-based PSMA(named as PYCl-T) was designed and synthesized,which possesses similar polymer backbone with the widely used PY-IT,but with chlorine substitution on the A-end groups in the A-DA′D-A structure.PYCl-T was then employed as the third component into the PM6:PY-IT system and the ternary all-PSCs based on PM6:PY-IT:PYCl-T demonstrated a high PCE of 16.62%(certified value of 16.3%).Moreover,the PCE of 15.52% was realized in the enlarged ternary all-PSCs with effective area of 1 cm^(2),indicating the great potential in large-scale applications.Moreover,the optimized ternary blend films of PM6:PY-IT:PYCl-T show excellent thermal stability at 150 ℃.This work demonstrates that the utilization of a ternary blend system involving two well-compatible PSMA polymer acceptors is an effective strategy to boost the performance of the all-PSCs.

有机光伏电池受体材料研究

为提高有机光伏电池的能量转化效率,需开发新型受体材料。重点介绍了富勒烯、有机小分子、有机高分子3类受体材料以及相关的研究成果,并阐述了这些受体材料的演变规律,以及发展过程中面临的挑战和未来发展的方向,以期实现有机光伏电池的商业化应用。