Influence of thermal annealing on microstructure, energetic landscape and device performance of P3HT:PCBM-based organic solar cells

Abstract

Thermal annealing alters the morphology of organic donor-acceptor bulk-heterojunction thin films used in organic solar cells. Here, we studied the influence of thermal annealing on blends of amorphous regio-random (RRa) and semi-crystalline regio-regular (RR) poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester. Since the P3HT:PCBM blend is one of the most studied in the OPV community, the existing research provides a solid foundation for us to compare and benchmark our innovative characterization techniques that have been previously under-utilized to investigate bulk heterojunction organic thin films. Here, we combine advanced novel microscopies and spectroscopies, including polarized light microscopy (PLM), photo-deflection spectroscopy (PDS), hyperspectral photoluminescence spectroscopy (HPLS), and energy resolved-electrochemical impedance spectroscopy (ER-EIS), with structural characterization techniques, including grazing-incidence wide-angle x-ray scattering (GIWAXS), grazing-incidence x-ray diffraction (GIXD), and Raman spectroscopy, in order to reveal the impact of thermal annealing on the microstructural crystallinity and morphology of the photoactive layer in organic solar cells. Coupled transfer matrix and drift-diffusion simulations were used to study the impact of the density of states (DOS) on the solar cells' device parameters, namely the short-circuit current (JSC), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE).