Crystallization/Phase Separation Control And Photostability In Organic Solar Cells Active Layer

Organic solar cells have drawn much attention due to its advantages of solution-processed,light weight,low-cost and flexibility.Power conversion efficiencie(PCE)over 12% have been achieved for single junction based organic solar cells,there are many issues for realizing commercialization of organic solar cells,such as low efficiency,weak stability and difficult to make large-scale roll-to-roll fabrication.Active layer is the most important component in organic solar cells,its nanoscale morphology has vital impact on the performance of devices.Donor and acceptor have appropriate aggregate could form channels for the transport of both holes and electrons,however excessive aggregate will affects the charge separation of exciton.Thus,nanoscale phase separation and bicontinuous interpenetrating network in the active layer play a crucial role in organic solar cells.Morphology control can realize high performance and stable organic solar cells.First,Methanol-chloronaphthalene binary solvents(MeOH-CN)treated the surface of the active layer,not only induced the p-DTS(FBTTh2)2 donor to form nanofibers,optimize phase separation as well.Appropriate aggregation and phase separation have advantage on light absorption,provide passage for charge transfer,increase hole mobility and improve the photoelectric performance.Control the volume content of CN can gain the optimal domain size of donor/acceptor and nanofiber size,realize the best efficiency of 6.5 %.Second,the device photoelectric performance benefit from the improved crystallinity of p-DTS(FBTTh2)2 donor,therefore we employed temperate solvent annealing to treated the active layer.Solvent annealing increased the short-circuit density,fill factor and efficiency,but sacrificed 246 mV on open-circuit voltage(Voc),and the voltage loss can be recovered by post-annealing.Joint the solvent annealing and post-annealing,an optimal efficiency of 7.2% was obtained.By contrast,post-annealing is more effective in recovering Voc,whereas the pre-annealing on active layer cannot completely recover Voc.Analyze the dark current-voltage curve and fitting curve,we found post-annealing have the same ideal factor with pre-annealing,but the reverse saturation dark current density(J0)show one order lower than pre-annealing.The decrease in J0 may originate from the well contact and fewer defect between the active layer and back electrode by post-annealing.Finally,the photostability in high performance PTB7-Th:PC71BM polymer solar cells were researched,we found the fullerene will degrade the excited polymer in the presence of oxygen.What's more,the addition of the traditional photosensitive solvent additive 1,8-diiodooctane(DIO)accelerate the degradation rate of polymer donor.Halogen-free 1,4-butanedithiol(BT)not only can avoid the DIO reaction of excited polymer with the photolysis fragment of DIO,but also suppress the fullerenes catalyze the degradation of PTB7-Th.According to the analysis of macro/micro morphology,light absorption and device performance,the solar cells prepared with BT show higher stability than that prepared without any additives and much better than that with DIO.Most likely,the solvent additives BT can extracting the free and trapped PC71 BM from the donor phase,enhanced the relative purity.