Investigation Of The Photo-physical Behaviors In Low Bandgap Polymer Solar Cell

With the increasing demand for energy in various forms，solar energy as a cleanpollution-free and sustainable efficient energy gradually become the hot research topic.Bulk heterojunction (BHJ) solar cells which are based on conjugated polymer blendsin the development of low cost photovoltaic power generation have become apromising technology. In these composite materials, the donors and acceptors aredispersed to form nano-interpenetrating network to provide large interfacial area andpercolation path for efficient charge separation and transport. The yield of free chargein polymer solar cell determines device performance, such as the external quantumefficiency, short current, etc, so understanding the generation mechanism of freecharge is important for increasing the energy conversion efficiency of solar cell. Sincethe generation of free charge is a so complex ultrafast kinetic process, which involvesmany more processes such as exciton dissociation, charge recombination, etc, anddepends on the structure and the nanoscale morphology of material, that people isn’table to clarify its mechanism and control it efficiently, up to now. we choose theheterojunction as investigation object, focus on the free charge generation processesin heterojunction and the relationship between the yield of free charge and the carrierconcentration by using femtosecond transient absorption spectroscopy.The photovoltaic performance and the photo-physical behaviors based onPoly{2,7-9,9-dioctylfluorene-alt-5-diethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione}:PC61BM heterojunction have been studied in detail. Thephotocurrent and voltage of devices obviously improve after increasing the amount ofPC61BM in heterojunction. The transient absorption data exhibit that the dissociationprocesses of excitons is fast, but that of the charge transfer (CT) state is complicateand owns two paths according to the energy of CT state. In the first situation, the freecharge may be directly dissociated from the high energy CT states and its generationrate is too fast to be observed in our detection window. In the low energy CT states,the dissociation process slows down, which is estimated to be20.8ps at roomtemperature. The exciton dissociation in the blend film is so efficient that it could directly take place before exciton-exiton annihilation process at high excitationintensity.