Quantitatively Resolved 3D Morphological Stability of P3HT Nanowire-Based BHJ Films against Light Irradiation by TEM Tomography

Abstract

The long-term stability of excellent power conversion efficiency (PCE) in polymer solar cells (PSCs) is recognized as an important issue for their commercialization. When exposed to sunlight in air, the photo-chemical degradation of active layer by penetration of oxygen and water can reduce the optimized donor/acceptor interfaces, resulting in the morphological change of bicontinuous interpenetrating network structure for efficient charge generation, separation and transport. However, it has not been directly proven due to the lack of appropriate three-dimensional (3D) analysis tools. Herein, for the first time, we visualize the morphological change 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 exposure using TEM tomography (TEMT). In particular, the 3D phase separation and percolation pathways for efficient charge transport could be quantitatively analyzed by extraction of sub-tomogram in TEMT. Two solution-processed NW-based thin films prepared via growth of P3HT in the presence or absence of PC71BM were compared with the conventional blend films in terms of morphological instability against light irradiation. The introduction of preassembled NWs with the robust π-π stacking offer a high tolerance to photo-oxidation affecting a morphological degradation. This study offers a useful strategy for developing PSCs with long-term stability using previously reported CPs with high PCE. The 3D TEM analysis provides a deeper understanding of multi-length-scale thin film morphology that can improve electro-optical device performance and stability.