Regulating Active Layer Thickness and Morphology for High Performance Hot-casted Polymer Solar Cells
Thicker films of active layer (Polymer: PCBM blend) are required to harness maximum amount of sunlight and to achieve higher efficiencies in polymer solar cells. However, realization of thicker films along with desired morphology of the active layer is very challenging. Herein, thicker films with optimum morphologies have been precisely obtained using hot-casting technique. Superior active layer morphology supports better charge transport. The photovoltaic properties of polymer solar cells fabricated with PTB7-Th:PC71BM were thoroughly studied by varying substrate temperature used for hot-casting. A significant increase in power conversion efficiency to >9% was observed for the solar cells fabricated at 40 °C compared to the one fabricated at room temperature, which displayed an efficiency of ~8%. The main reason for this improvement was the increased thickness of the active layer that facilitated better light absorption and therefore resulting in enhanced short circuit current density. In addition, the devices fabricated at 40 °C exhibited very low rms roughness enabling better charge transport in the device. The recombination studies and mobility estimation were also performed through impedance spectroscopy and SCLC technique, which gave a clearer insight of the hot casting method. The outcome of this moderate hot-casting technique to achieve low roughness morphology in thicker active layer were very promising as this would be a possible breakthrough technique in the manufacturing of highly efficient polymer solar cells.