Morphology And Cathode Optimization For Efficient Polymer Bulk Heterojunction Solar Cells
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MORPHOLOGY AND CATHODE OPTIMIZATION FOR EFFICIENT POLYMER BULK HETEROJUNCTION SOLAR CELLS Yi Yang, M.S The University of Texas at Arlington, 2009Supervising Professor: Michael Jin Efficiency of polymer bulk heterojunction solar cell is highly dependent on the active layer morphology and can be controlled by specific experimental conditions. In this work, the influences of P3HT:PCBM blend composition, solution concentration, and the thermal annealing on the cell performance are presented. The maximum of Jsc and FF are reached when the weight percentage of PCBM in the blend is 60 % under which both high density of donor and accepter interfaces expecting the formation of efficient percolated electron transport paths. 36 mg/ml is found to be the optimum solution concentration for P3HT:PCBM blend and it can be explained by the relationship between the active layer thickness and resistance. Finally, it is observed that the Jsc and Voc increase from 2.04 mA and 0.47 V to 10.65 mA and 0.58 V respectively after the devices are annealed at 150 °C for 20 min. The increase in Jsc, in particular, can be attributed to the donor/accepter phase separation and charge carrier mobility increase caused by P3HT crystallization. The light absorption increase in active layer by annealing is also responsible for the Jsc increases.In order to increase the FF, charge accumulation expected at the Al/PCBM interface is prevented by depositing Ca prior to Al, which produces higher electrical field at the interface due to the lower work function of Ca compared to Al. The increased electrical field can eliminate the charge barrier at the interface and facilitate the transport of electrons from PCBM to the cathode. In this work, cells with Al and Ca/Al bilayer as the cathodes are fabricated respectively to study the effect of Ca on the cell performance. Compared to cells with Al cathode, FF of the cells with Ca/Al cathode has increased from 21% to 51%. Also, both Jsc and Voc of the cells increase from 9.81 mA/cm2 to 11.22 mA/cm2 and 0.53 V to 0.65 V, respectively. The highest efficiency of 3.32% is realized.