The Photovoltaic Properties Study Of Fused Perylene Imide Based Small Molecular Electron Acceptors

In the last decade,non-fullerene electron acceptors（NFAs）as alternatives of traditional fullerene electron acceptors for organic solar cells（OSCs）have become one of the fastest growing areas because of their remarkable advantages such as low-cost,light weight,easy large-area fabrication and feasibility of flexible devices.The soluble fullerene derivatives（PC61BM and PC71BM）have long been the dominant electron acceptors with rapid developments of conjugated polymer donors in the history of OSCs.However,the inherent drawbacks in the low absorption in the solar spectrum and high cost of preparation and purifications as well as the instable morphology of the fullerene derivatives restrict the further improvement in efficiency and have become the strong motivation for researchers to seek potential replacements.Over the past years,the latest power conversion efficiencies（PCEs）of single-junction fullerene-free OSCs based on non-fullerene electron acceptors（NFAs）have reached over 18%,making them viable candidates for commercial applications.With regards to the various types of electron-withdrawing units to construct NFAs,Perylene diimide（PDI）and its derivatives are widely studied by many research groups and become one of the most promising candidates for constructing n-type electron acceptors because of the good absorption,high electron mobility and good chemical and environmental tolerance.However,It is acknowleged that the trade-off in the self-aggregation and charge-transport properties of the PDI-based acceptors is a major obstacle for achieving efficient OSCs,which is mainly attributed to the large,near-planar and rigidπ-conjugated backbone of PDI.To solve this problem,one strategy is to construct a twisted three dimensional（3D）molecular geometry,which can partially undermine the strong intermolecular aggregation and reduce undesirable domain sizes in PDI-based fullerene-free OSCs.However,the excessively twisted 3D PDI molecules will reduce the intermolecularΠ–Πinteractions,and cause low crystallinity and charge transport.Another way is to build the fused perylene diimide（FPDI）unit through the fusion of two PDI units with a two-carbon bridge,which gave an ideal example of balancing the inter-and intra-interactions and the aggregation behavior.The quasi-2D structure of FPDI with adequate twist angle of²5°provides the opportunity to ensure proper aggregation and phase separation in the active layer,which will thus be able to result in an impressive efficiency in NF-OSCs.Our group reported a series of FPDI-based electron acceptors with a quasi-2D FPDI building block.Herein,we first introduced the 10H-spiro[acridan-9,9-fluorene]（SPAc）unit to copolymerize with PDI to prepare the D-A type small molecular acceptor PDI-SPAc,and then we replaced the PDI with a fused PDI moiety to synthesize another D-A type FPDI-SPAc with a quasi-2D structure.The side chain of SPAc unit can effectively prevent the acceptor molecules from forming oversized aggregation as well as increase absorption in the near-UV region.The quasi-2D molecular structural geometries of FPDI units can produce effective pathways in the blend films for the electron transport.Considering the complementary absorption region,matched energy level and suppressed unfavorable aggregation,PTB7-Th:FPDI-SPAc devices achieved a high PCE of 3.92%,a V_oc of 0.72V,a J_sc of 12.4 m A/cm² and an FF of 43.76%;whereas devices based on PTB7-Th:PDI-SPAc give PCE of 0.32%.Our results highlighted the attractive prospect of the quasi-2D FPDI unit for constructing the state-of-the-art electron acceptors for efficient NF-OSCs.In order to further verify the excellent performance of FPDI-based acceptors,we connected two PDI monomers to the bay positions a it through a carbon-carbon single bond,and designed a new type A-A-A FPDI-based small molecule non-fullerene acceptor FPDI-2PDI.PTB7-Th:FPDI-2PDI devices achieved a high PCE of 5.30%,a V_oc of 0.74V,a J_sc of 11.49 m A/cm² and an FF of 62.35%.Compared with PTB7-Th:PDI-SPAc and PTB7-Th:FPDI-SPAc devices,The photoelectric conversion efficiency has been greatly improved.The device based on PBDB-T:FPDI-2PDI achieved a high PCE of 5.09%,a increased V_oc of 0.78 V and FF of 65.31%,but a decreased J_sc of 10.08 m A/cm².The fill factor of the two devices is particularly excellent,which have approached the optimal level of PDI-based NF-OSCs.These results showed that FPDI-based acceptors with quasi-2D molecular structural geometries have significant impact on intermolecular interactions,charge transport,and photovoltaic properties,which need to be further explored and applied.