Constructing bulk heterojunction with componential gradient for enhancing the efficiency of polymer solar cells

• PC71BM concentration gradient has been realized by alcohol treatment.
• Exciton separation and charge transfer were promoted for conventional devices.
• For inverted devices, the gradient deteriorated the performance.
• The function of the alcohol treatment was firstly clarified.

Herein, high-efficient PTB7:PC71BM solar cells with bulk heterojunction being optimized by componential distribution have been realized by solvent treating the active layer with a series of alcohols. Subsequent characterizations including X-ray photoelectron spectroscopy (XPS) and Kelvin probe force microscopy (KPFM) reveal that such treatment adjusts the distribution of PC71BM in the bulk heterojunction by making the concentration of PC71BM higher at the solvent treated surface in comparison with that close to the bottom electrode. Such morphological transformation enables the conventional structured devices with great advantages in exciton separation and charge transfer. Therefore, the power conversion efficiency could be remarkably improved from 6.57% to 7.74%. However, for the inverted structured polymer solar cells, the morphology evolution deteriorates the relevant performance, particularly in exciton separation and charge transfer. We attribute these contrary observations to the matching degree of charge transfer direction in the active layer with the charge collection direction in the entire device. Not only providing a designing principle for optimizing the structure of polymer solar cells according to the morphology of active layer, this paper also offers a comprehensive understanding about the influence of solvent treatment on the performance of polymer solar cells.

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