Improving the Morphology of PCDTBT:PC70BM Bulk Heterojunction by Mixed-Solvent Vapor-Assisted Imprinting: Inhibiting Intercalation, Optimizing Vertical Phase Separation, and Enhancing Photon Absorption

Abstract

In this paper, mixed-solvent vapor-assisted imprinting annealing was proposed to improve the lateral phase separation, vertical distribution of fullerene component and the photon absorption in poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C71 butyric acid methyl ester (PC70BM) blend system. After the mixed-solvent vapor-assisted imprinting annealing (one is good for PCDTBT, that is, carbon disulfide (CS2) and the other is good for PC70BM, that is, tetrahydrofuran (THF)) for 20 min at the vapor pressure of P = 0.9 (25 °C), both the crystallinity of PCDTBT and the aggregation of PC70BM were promoted, resulting an interpenetrating network, as confirmed by spectroscopic ellipsometry (SE) and transmission electron microscope (TEM). Furthermore, the fullerene content on the top surface was enhanced due to the reduced energy difference between the top and bottom surface of the film as well, which was demonstrated by X-ray photoelectron spectroscopy (XPS). Besides the morphology transition, surface relief grating structures were also introduced to the film surface, resulting in an increased probability of photon absorption due to light diffraction. As a consequence, the final structure of active layer was effective in enhancing photon absorption, improving the carrier mobility, and reducing the carrier recombination, which lead to a power conversion efficiency (PCE) of 7.20% under AM1.5G illumination, almost 55% higher than PCE of reference device (PCE = 4.66%).