Effects of thermally cross-linkable polymeric additive in the photoactive layer of polymer solar cells

Performance and stability are the most important issues to be considered to propel and advance the polymer solar cell (PSC) technology to be realized in commercial market. Herein, we propose a new concept of stabilizing polymer-fullerene morphology for PSCs and simultaneously improve device performance by introducing thermally cross-linkable polymer as an additive in the photoactive layer. The cross-linker, poly[9,9-bis(6-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis(3-ethyl(oxetane-3-ethyloxy)hexyl)-fluorene] (PFN-ox), forms a three-dimensional network embedded in the polymer blend after thermal treatment and plays various roles including reducing the carrier recombination at the donor/acceptor interface, improving the electrical property of PSCs, enhancing the hydrophobicity of the photoactive layer, slightly decreasing fullerene domain size, and retarding the crystal growth of fullerenes. Those characteristics lead to the improvement of device performance from the moderate improved fill factor and stable device operation under ambient condition (>85% efficiency retention after 2 months). The stability effects of PFN-ox are not only valid in polymer-fullerene blends, but also in polymer-polymer systems, showing its general characteristics. We believe that our finding can advance the development of stable PSCs and other optoelectronic devices based on organic materials.