The 3D morphological stability of P3HT nanowire-based bulk heterojunction thin films against light irradiation quantitatively resolved by TEM tomography

Although polymer solar cells (PSCs) continue to improve, with advances in power conversion efficiencies of 15% and beyond, intrinsic device stability in air under sunlight remains a challenge for commercialization. As the key parameter determining PSC performance relies on a bicomponent nanostructured active layer morphology, we investigate the stability of bulk heterojunction (BHJ) thin films composed of preassembled poly(3-hexyl-thiophene) (P3HT) nanowires (NWs) and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) under light irradiation. The three-dimensional (3D) morphological changes caused by light-derived chemical oxidation and degradation are visualized and interpreted using 3D transmission electron microscopy combined with photo-physical characterization. Quantitative analysis of the phase separation and percolation pathways for efficient charge transport is performed. Two solution-processed NW-based thin films prepared via crystallization-driven assembly of P3HT in the presence or absence of PC71BM were compared with conventional P3HT/PC71BM blend films in terms of morphological instability against light irradiation. The introduction of pre-assembled NWs with tight packing of P3HT in thin films with PC71BM reduces the morphological degradation of BHJ thin films.