非对称稠环光伏电子受体（英文）_司文钦.pdf

Chem.J.Chinese Universities,2023,44(7),2023014920230149(1/12)CHEMICAL JOURNAL OF CHINESE UNIVERSITIES高 等 学 校 化 学 学 报综合评述非对称稠环光伏电子受体司文钦1，2，李腾飞1，林禹泽1，2（1.中国科学院化学研究所有机固体实验室,北京分子科学国家研究中心,北京 100190；2.中国科学院大学,北京 100049）摘要 近几年,受益于稠环电子受体材料的蓬勃发展,有机太阳能电池的能量转换效率从富勒烯时代的12%迅速提高到非富勒烯时代的20%.其中,非对称结构的分子设计策略发挥了重要作用.本文按照稠环骨架、末端基团和侧链3种非对称分子设计策略,综合评述了非对称稠环电子受体的研究进展,并讨论了其中的结构-性能关系;最后,对非对称稠环电子受体的未来发展进行了展望.关键词 非对称分子结构；稠环电子受体；非富勒烯受体；有机太阳能电池中图分类号 O633.5；TM914.4 文献标志码 A doi：10.7503/cjcu20230149Asymmetric Fused-ring Photovoltaic Electron AcceptorsSI Wenqin1，2，LI Tengfei1*，LIN Yuze1，2*（1.Beijing National Laboratory for Molecular Sciences，CAS Key Laboratory of Organic Solids，Institute of Chemistry，Chinese Academy of Sciences，Beijing 100190，China；2.University of Chinese Academy of Sciences，Beijing 100049，China）Abstract Benefiting from the development of fused-ring electron acceptors（FREAs），power conversion efficiencies of organic solar cells have rapidly increased from 12%for the fullerene era to 20%for the non-fullerene era.The asymmetric molecular design strategy plays an important role in the enhancement of photovoltaic performance.In this paper，we review the research progress of asymmetric FREAs according to the following three kinds of molecular design strategies：the asymmetric fused-ring backbone，asymmetric end groups and asymmetric side chains，discuss the structure-property relationship，and finally provide an outlook on the future development of asymmetric FREAs.Keywords Asymmetric structure；Fused-ring electron acceptor；Non-fullerene acceptor；Organic solar cell1 IntroductionSolution-processable organic solar cells（OSCs）have drawn significant interest due to their benefits such as mechanical flexibility，low weight，and semitransparency16.High-performance OSCs are typically constructed with a bulk heterojunction（BHJ）structure involving the nanoscale phase separation of donor and acceptor materials79.In the initial stage of OSCs research，fullerene acceptors（FAs），and its derivatives were widely used as acceptor materials due to their three-dimensional（3D）structure，high electron mobility，and suitable electron affinity10.However，FAs have drawbacks such as weak visible absorption，high 收稿日期：2023-03-30.网络首发日期：2023-04-25.联系人简介:林禹泽,男,博士,研究员,主要从事有机光伏半导体及应用方面的研究.Email:李腾飞,男,博士,主要从事近红外有机光伏半导体材料及器件方面的研究.Email:基金项目:国家自然科学基金(批准号:22105208,52173189)和中国博士后科学基金(批准号:2021M703263)资助.Supported by the National Natural Science Foundation of China(Nos.22105208,52173189)and the China Postdoctoral Science Foundation(No.2021M703263).CHEMICAL JOURNAL OF CHINESE UNIVERSITIES高 等 学 校 化 学 学 报综合评述Chem.J.Chinese Universities,2023,44(7),2023014920230149(2/12)synthesis cost，and poor morphological stability.The demand for replacing fullerenes in OSCs has led to the rapid development of non-fullerene acceptors，which have tunable structure and energy levels，broad and strong absorption，and easy synthesis and purification11，12.In particular，the power conversion efficiencies（PCEs）of 18%20%have been achieved in OSCs with the emergence of fused-ring electron acceptors（FREAs）represented by ITIC and Y61316.Generally，FREA materials are composed of three components：central electron-donating core unit（D），outstretched side chain，and terminal accepting unit（A）2，which can be classified into symmetric and asymmetric FREAs according to the molecular symmetry.By introducing asymmetric elements into the above three parts，the chemical structures can be more diversified and the photoelectrical properties will be finely adjusted relative to their symmetric counterparts.The asymmetric FREA materials began to emerge in 2017 and developed rapidly in the last several years17，18.Enormous efforts have been dedicated to the design and synthesis of high-performance asymmetric FREAs and PCEs approaching 19%have been realized19.Generally，the asymmetric FREAs exhibit a larger dipole moment and stronger intermolecular interaction relative to their symmetric counterparts，leading to better molecular packing and enhanced electron mobility.Meanwhile，the introduction of asymmetric elements can subtly tune the absorption，energy levels，molecular configuration，miscibility，and crystallinity，which is beneficial to finely optimize the photovoltaic properties of FREAs.In addition，some studies also demonstrated that positive effect of enhancing dielectric constant，suppressing non-radiative recombination，and others can be made by using asymmetric strategies，which will promote the exciton dissociation and reduce the energy loss and hence acquire a higher performance.This review will focus on reviewing recent research advances in three types of asymmetric FREAs（Fig.1），which are based on the engineering of asymmetric molecular designs incorporating core backbone，terminal groups and side chains.The structure-property relationships of these asymmetric FREAs will be discussed in detail and their future development will be prospected.2 Asymmetric FREA MaterialsIn 2015，Zhan et al.13 invented the star molecule ITIC，which has a centrosymmetric fused core of indacenodithieno 3，2-b thiophene as the donor unit with four hexylphenyl side chains and 1，1-dicyanomethylene-3-indanone（IC）as the terminal group on both sides.The device based on ITIC exhibited PCEs up to 6.8%，which was a new record for fullerene-free OSCs at that time.Since then，the OSC field was transformed from the fullerene era to non-fullerene era.In 2019，another star molecule Y6 was designed and synthesized，the authors employed an axisymmetric ladder-type electron-deficient-core-based central Fig.1Illustration of three types of asymmetric FREAs along with the example molecules and their key synthetic routes（1）and（2）represent VilsmeierHaack reaction and Knoevenagel condensation.CHEMICAL JOURNAL OF CHINESE UNIVERSITIES高 等 学 校 化 学 学 报综合评述Chem.J.Chinese Universities,2023,44(7),2023014920230149(3/12)fused ring（dithienothiophen3，2-b-pyrrolobenzothiadiazole）with a benzothiadiazole core14.The OSCs made from Y6 showed a high efficiency of 15.7%，which further boosts the development of OSC field.In the last several years，significant efforts have been devoted to developing high-performance asymmetric FREAs based on ITIC and Y6.2.1FREAs with Asymmetric Fused-ring CoresAsymmetric strategies on the fused-ring core can subtly adjust the absorption region and energy levels via regulating the intensity of intramolecular charge transfer.More importantly，the dipole moment of the molecule can be enhanced obviously and the molecular packing and crystallinity will be further tuned，which is beneficial to improve the charge transport property and acquire a high short-circuit current density（JSC）and fill factor（FF）of the device.Sun et al.20 reported a novel planar and asymmetric acceptor TPTT-IC（Fig.2），which was derived from the symmetric acceptor ITIC by replacing one of the thieno3，2-b thiophene units with a thiophene unit.This modification resulted in a slight blueshift of the film absorption from 703 nm for ITIC to 692 nm for TPTT-IC.Compared with the PBT1-C（Fig.3）ITIC device，the PBT1-C TPTT-IC device showed enhanced and more balanced charge transport properties.The superior charge mobilities of the PBT1-C TPTT-IC blend contributed to the higher Jsc and FF values，leading to an outstanding PCE of 10.3%（Table 12028），a little higher than that of the ITIC-based devices（10.0%）under the same conditions.Following the same strategy of introducing asymmetry into the ladder-type core，Yang et al.21 reported a novel acceptor MeIC1.Compared to the symmetric acceptor MeIC，MeIC1 exhibited a higher LUMO energy level of 0.05 eV and enhanced stacking interaction，which were beneficial for higher charge mobility.Consequently，the PCE of the MeIC1-based devices reached 12.58%，which was higher than that of the MeIC-based devices（12.03%）.Later，Sun et al.22 developed two asymmetric acceptors TPTT-2F and TPTTT-2F based on TPT-2F with a symmetric structure.As the conjugation of the core was extended from TPT-2F to TPTT-2F and TPTTT-2F，the following properties were improved：red-shifted absorption spectra，enhanced Fig.2Chemical structures of ITIC,Y6,FREAs with asymmetric cores and the symmetric counterpartsCHEMICAL JOURNAL OF CHINESE UNIVERSITIES高 等 学 校 化 学 学 报综合评述Chem.J.Chinese Universities,2023,44(7),2023014920230149(4/12)electron mobilities，and intensified intermolecular-stacking.The highest PCE of 12.03%was obtained by using TPTTT-2F matched with PBT1-C.Likewise，Zhan et al.23 synthesized three FREAs with different fused-ring cores and compared their performance in OSCs.AOIC with an asymmetric fused-8-ring core had the best PCE of 13.3%，while F5IC with a symmetric fused-5-ring core and IUIC2 with a symmetric fused-11-ring core showed lower PCEs of 5.31%and 4.06%，respectively，which indicated that unidirectional extension of the fused-ring core is a better strategy than bidirectional extension to optimize the electronic properties and device performance of FREAs.The high performance of PTB7-Th AOIC devices can be attributed to the optimal crystal size and the close-stacking distance achieved Fig.4（A）and（B）.In addition，Yang et al.24 synthesized two asymmetric acceptors（IDT6CN-M and IDT8CN-M）with different numbers of flanking thiophene rings and large dipole moments.Relative to IDT6CN-M，IDT8CN-M shows red-shifted absorption，similar LUMO energy levels and higher electron mobility.PBDB-T IDT8CN-M exhibits slightly better Fig.3Chemical structures of mentioned donor materialsTable 1Optical and electronic properties,and photovoltaics performance of FREAsAcceptorITICTPTTICMeIC1TPT2FTPTT2FTPTTT2FF5ICAOICIUIC2IDT6CNMIDT8CNMABP4T4FBP5T4FBP4T4FTPTINSeTPINBS3TSe4FSN*max/nm703692714698717724694811865693699782813826727741851862EHOMO/ELUMO（eV）-5.71/-3.97-5.78/-3.95-5.59/-5.39-5.84/-4.06-5.75/-4.04-5.69/-4.01-5.82/-4.05-5.50/-3.93-5.32/-3.86-5.62/-3.90-5.54/-3.91-5.65/-3.85-5.63/-3.88-5.71/-3.91-5.80/-3.97-5.77/-4.00-5.60/-3.86-5.51/-3.82DonorPBT1CPBT1CPBDBTPBT1CPBT1CPBT1CPTB7ThPTB7ThPTB7ThPBDBTPBDBTPM6PM6PM6PBT1CPBT1CD18PM6JSC/（mAcm-2）14.915.5 18.32 13.89 15.82 17.63 13.43 24.32 10.77 15.97 17.1121.223.825.4 13.92 16.37 29.40 25.14VOC/V0.970.960.930.870.880.920.640.740.760.920.920.920.890.840.880.850.830.82FF（%）69.369.474.168.673.274.562.174.351.576.178.975.176.377.772.973.375.968.9PCE（%）10.010.3 12.58 8.3310.1712.03 5.3113.3 4.0611.2312.4315.216.917.1 8.9110.2018.4814.30Ref.202021222222232323242425252526262728*max：Maximum absorption of thin film.CHEMICAL JOURNAL OF CHINESE UNIVERSITIES高 等 学 校 化 学 学 报综合评述Chem.J.Chinese Universities,2023,44(7),2023014920230149(5/12)performance in all these aspects.IDT6CN-M and IDT8CN-M achieved high PCEs of 11.23%and 12.43%along with high FF values of 76.1%and 78.9%，respectively.The FF values were among the highest values for OSCs reported at that time.Moreover，asymmetric design strategies also affect the molecular conformation，which has been verified to mainly influence the micromorphology of active layers and energy loss（Eloss）of the devices.By tuning the lateral thiophene orientation of the fused core，Jen et al.25 synthesized three Y6 derivatives with different conformations and ring numbers：BP4T-4F（eight rings），BP5T-4F（eight rings），and ABP4T-4F（nine rings）.The larger dipole moments of BP5T-4F and ABP4T-4F were found to reduce the reorganization energy and non-radiative recombination loss relative to the BP4T-4F-based devices Fig.4（C）and（D）.The photoelectric properties and molecular packing of BP5T-4F are similar to those of BP4T-4F，while ABP4T-4F with a completely opposite conformation exhibits unfavorable properties such as blue-shifted absorption and edge-on packing in films.Finally，the three FREAs-based OSCs exhibited relatively high PCEs of 15.2%17.1%with respect to their bandgaps.Fig.42D grazingincidence wideangle Xray scattering(GIWAXS)patterns(A)and corresponding outofplane and inplane GIWAXS cutline profiles of PTB7Th F5IC,PTB7Th AOIC,and PTB7Th IUIC2 blend films(B)23,normalized absorption and electroluminescence(EL)spectra of BP4T4F,BP5T4F,and ABP4T4F neat films(C),Eloss analysis of OSCs for PM6 BP4T4F,PM6 BP5T 4F and PM6 ABP4T 4Fblue line for measured external quantum efficiency(EQE),red line for Fourier transform photoelectron spectroscopyEQE,green line for EL,and black line for EQE of OSCs(D)25（A，B）Copyright 2019，American Chemical Society；（C，D）Copyright 2020，WileyVCH.CHEMICAL JOURNAL OF CHINESE UNIVERSITIES高 等 学 校 化 学 学 报综合评述Chem.J.Chinese Universities,2023,44(7),2023014920230149(6/12)In addition to the asymmetric extension of the fused core，the asymmetric substitution of heteroatoms is also an important strategy to enhance the photovoltaic performance.The large covalent radius of selenium（Se）can decrease the-orbital overlap，generally resulting in improved quinoidal resonance character and further a small optical band gap of corresponding FREAs.Meanwhile，the larger and looser outermost electron cloud of Se atom relative to sulfur enables better polarizability and enhanced intermolecular SeSe interaction，leading to higher charge carrier mobility of relevant FREAs in the aggregation state.For example，by introducing a Se atom into the indacenodithiophene unit and creating an asymmetric FREA SePT-IN，Sun et al.26 achieved a significant improvement in the performance of OSCs.Compared with the symmetric acceptor TPT-IN，SePT-IN showed a broader absorption spectrum，faster electron transport，and stronger intermolecular-stacking interaction.The resulting OSCs blended with the donor PBT1-C exhibited a PCE of 10.20%，which was 1.29%higher than that of the TPT-IN-based devices.Moreover，Jen et al.27 also developed a FREA BS3TSe-4F by asymmetric selenium substitution，which had a higher dielectric constant of 2.5 than the symmetric S9TBO-F（2.36），which helps exciton dissociation for generating free charge carriers.The devices based on D18/BS3TSe-4F achieved an impressive PCE of 18.48%with a high JSC of 29.4 mA/cm2.In addition the nitrogen（N）atoms can also be asymmetrically employed to induce redshift in the absorption of relevant FREA and realize high-performance semitransparent OSCs（ST-OSCs）.An N-substituted electron acceptor SN with a 40 nm red-shifted absorption onset compared to Y6 was designed and synthesized by Zhu et al.28 The PM6 SN-based device achieved a PCE of 14.3%with an ultra-low non-radiative voltage loss of 0.15 V.Benefiting from the extended near infrared absorption and low Eloss，ST-OSCs based on PM6 SN Y6 showed a PCE of 14.0%at an average visible transmittance of 20.2%.2.2FREAs with Asymmetric Terminal GroupsTerminal groups play a crucial role in determining optical bandgap，electron transport property，crystal morphology，and energy level alignment with donor materials.Similar to asymmetric core，asymmetric end groups can also enhance intermolecular interactions and promote tense molecular stacking.In 2019，He et al.29 synthesized two new acceptors by incorporating two chlorine（Cl）atoms into end groups of ITIC：a symmetric acceptor named ITIC-2Cl-and an asymmetric one named-ITIC-2Cl（Fig.5）.The