Design of parallel-connected polymer tandem solar cells using efficient low bandgap PTB7-Th:PC71BM blend

Abstract

Parallel-connected tandem cells adopting a highly efficient donor polymer, PTB7-Th, combined with acceptor fullerene PC71BM as the back sub-cell was introduced to further improve the performance of polymer solar cells. Design of the device architecture was investigated using modeling and simulation methods based on the transfer matrix formalism. To optimize the device structure, detailed analysis of the effect of active layer thickness, different device structure, and transparent Ag intermediated electrode on the short-circuit current density has been studied. It was found the long-wavelength absorption in the top-illuminated ITO-free back cell was significantly enhanced due to the resonant microcavity effect, leading to an efficient utilization of the incident light and increased photocurrent. Giving these advantages, the power conversion efficiency of the parallel homo-tandem cell was estimated to be ~ 11%, which was ~ 15% higher than that of a single cell of PTB7-Th. Moreover, the maximum achievable current density and the corresponding optimum active layer thickness of the sub-cells varied a little as the thickness of ultrathin Ag layer was changed, indicating that parallel connection architecture provided more freedom in the design and optimization for high-performance tandem solar cells.