Nonfullerene acceptors based on perylene monoimides

Yutong Ji,Helong Bai,Lixiu Zhang,Youdi Zhang,,and Liming Ding,

1College of Chemistry,Changchun Normal University,Changchun 130032,China

2Key Laboratory of Advanced Green Functional Materials,Changchun Normal University,Changchun 130032,China

3Center for Excellence in Nanoscience (CAS),Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS),National Center for Nanoscience and Technology,Beijing 100190,China

Nonfullerene organic solar cells (NF-OSCs) have become a research hotspot,and the device efficiency has been constantly updated[1-14].The efficiency of binary and ternary solar cells has exceeded 18%[15-17]and 19%[18],respectively.In the early study of OSCs,the development of organic acceptors lagged behind that of organic donors.In addition to the common fullerene acceptors PC61BM and PC71BM,there is growing interest in developing new electron acceptors.The two fullerene acceptors were derived from C60and C70,which were chemically modified to improve the solubility.Later,Liet al.developed a C60derivative ICBA[19]and a C70derivative IC70BA[20].Compared with PC61BM and PC71BM,the lowest unoccupied molecular orbital (LUMO) levels of these two acceptors increased by 0.17 and 0.19 eV,respectively.This is conducive to the increase of open-circuit voltage (Voc).At present,the development of fullerene acceptors is limited,the reason are as follows:(1) they show weak absorption in the visible region,which is not conducive to the full use of sunlight;(2) it is difficult to improve the absorption by the chemical modification;(3) difficult chemical synthesis and high cost;(4) it is difficult to control the morphology,and the aggregation easily takes place in thin films.The advantages of fullerene acceptors are also obvious,e.g.,(1) fullerene acceptors can accept and transport electrons in three dimensions due to their delocalized LUMO;(2) high electron mobility.Perylene diimides(PDIs) have been widely used in biological imaging,and they are widely-studied non-fullerene acceptors.PDIs have many advantages,such as high electron mobility and high electron affinity[21].In 1986,C.W.Tang of Kodak prepared two-layer OSCs by depositing copper phthalocyanine (CuPc) as the donor and perylene tetracarboxylic derivative (PV) as the accepter in vacuo,achieving a power conversion efficiency(PCE) of 1%[22].PDI-based devices were made by solution processing,and the aggregation yielded micron-sized crystals.When the acceptors were blended with donors,large domains formed[23].Since exciton diffusion length and life were limited,the domain size should be well controlled.

The solar cell parameters consist ofVoc,short-circuit current density (Jsc) and fill factor (FF).PDIs have two drawbacks:(1) the low LUMO level leads to lowVoc;(2) PDIs with rigid planar structure tend to form excessive aggregations,affecting the formation of uniform films.Thus,lifting LUMO level and constructing non-coplanar perylene monoimides (PMIs)to improveVocand the morphology are effective strategies.PMI-based nonfullerene acceptors and the photovoltaic performance are summarized in Fig.1 and Table 1.In 2015,a nonfullerene acceptor PMI-F-PMI with a fluorene core and two PMI arms was reported.It presented a lift-up LUMO level around -3.54 eV,which matches well with that of P3HT donor to yield highVoc.P3HT:PMI-F-PMI solar cells gave an efficiency of 2.3%,with aVocof 0.98 V,aJscof 5.61 mA/cm2,and an FF of 42.0%[24].Later,Liet al.used a polymer donor PTZ1,obtaining a PCE of 6.0%,with aVocof 1.30 V,aJscof 7.0 mA/cm2,and an FF of 63.5%[25].The favorable morphology,efficient exciton dissociation,balanced carrier mobilities,and reduced charge recombination also contributed to the increase ofVoc.

Table 1. Materials energy levels and the performance for solar cells.

It is important to understand the effect of different aromatic core on the photovoltaic performance.In 2022,Scharberet al.developed a non-planar acceptor PMI-FF-PMI,consisting of two PMI units bridged with a dihydroindeno[1,2-b]fluorene unit.PMI-FF-PMI:D18 solar cells gave a PCE of 5.34%,with aVocof 1.41 V,aJscof 6.09 mA/cm2,and an FF of 60.9%[26].The 1.41 VVocis the highest record for solution-processed OSCs so far.Though producing a highVoc,the cells presented a relatively large nonradiative voltage loss (ΔVocnon-rad) of 0.25 V,which mainly resulted from the enhancement of spontaneous carrier generation and the decrease of charge carrier in CT state process[27].More recently,Trimmelet al.developed three PMI dimers by changing the substitution position (para,meta or ortho) on the benzene ring.Compared with P-pPh-P and P-mPh-P and P3Ph,P-oPh-P showed better solubility and device efficiency.With introducing two alkyl chains or alkoxy chains onto the benzene ring in P-pPh-P,three new PMI dimers were obtained,namely P-MePh-P,P-HexPh-P,P-DeOPh-P.P-HexPh-P and P-DeOPh-P with long chains exhibited higher crystallinity than P-MePh-P,and P-DeOPh-P with alkoxy chains presented a favorable face-on orientation as indicated by GIWAXS,which is beneficial to charge transport.As a result,PBDB-T:P-DeOPh-P cells offered a PCE of 3.17%,with aVocof 1.00 V,aJscof 7.46 mA/cm2,and an FF of 43.0%[28].Tuning the linking units is a simple approach to develop high-performance PMI-based acceptors.

In short,theVocand PCE for NF-OSCs can be enhancedviatailoring the molecular structures of NFAs and donors.In order to regulate the morphology of the blends,different aromatic cores were introduced into PMI-based acceptors.The LUMO energy levels should also be tuned to match that of the donors.

Fig.1.The chemical structures for PMI-based non-planar acceptors.

Acknowledgements

This work was supported by the Scientific Research Foundation of Education Department of Jilin Province(JJKH20220827KJ),Natural Science Foundation of Changchun Normal University,and Scientific Startup Fund of Changchun Normal University.L.Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045,21772030,51922032,and 21961160720) for financial support.