Morphology Control And Device Performance Optimization Of Non-Fullerene Thin Film Organic Solar Cells

The development of organic solar cells is an important way to solve the current energy shortage problem.Compared with traditional silicon solar cells,organic solar cells have the advantages of molecular design,light weight,flexible devices,translucent properties,solution processing,and low cost.In addition to the nature of the materials itself,the phase separation structure in the active layer of the donor and acceptor also has a critical impact on the ultimate power conversion efficiency of the cells.The regulation of the crystallinity of conjugated molecules and the optimization of phase separation structure is a very necessary subject.The active layer morphology of an ideal organic solar cell requires a moderate phase size,which facilitates the diffusion and charge collection of excitons;a good interpenetrating network structure for charge transport;a certain degree of crystallinity facilitates charge transfer;a certain amount of amorphous mixed domains promotes exciton separation.Non-fullerene organic thin film solar cells are distinguished from systems in which the acceptor is fullerene and its derivatives,which use organic conjugated materials as acceptors.The advantage is that the spectral absorption range is wide,and the molecular structure and energy level can be designed.The replacement of fullerenes and their derivatives by non-fullerene receptors has become a hot topic at this stage.The ordered aggregation of non-fullerene small molecular acceptors(SMAs)is critical in determining the charge transport and bimolecular recombination in polymer/SMAs solar cells.However,due to the asymmetric phase separation,the polymers prefer to form crystalline networks,which inhibit the molecular diffusion of SMAs,resulting in weak crystallinity of SMAs.In PBDB-T/IT-4F blends,the crystallization of IT-4F could be improved by THF solvent vapor annealing,while the crystallization of PBDB-T could be enhanced by thermal annealing at 150 ?.Hence,the prior crystallization of IT-4F could be realized by regulating the sequence of two post annealing methods,which facilitates the diffusion of IT-4F and is benefit for the formation of interpenetrating network.The optimized morphology reduced the bimolecular recombination,resulting in an improved device performance from 5.95% to 7.18%.The ordered aggregation of donor and acceptor,appropriate phase size,and a certain mixed domains is critical for photon absorption and free charge transport.However,as the orderly aggregation is enhanced,the film phase region size increases,and the amorphous mixed domains content decreases.The balance of the ordered aggregation and the mixed domains is not easy to achieve.In DRTB-T/IDIC blends,we inhibit the crystallinity by first adding a certain amount of cinene as an additive and then increase it by steam annealing.It not only ensures a proper degree of orderly aggregation,but also forms more mixed domains under the condition that the phase size tends to be good.This way resulting in an improved device performance from 7.55% to 8.54%...