All-solution-processed inverted polymer solar cells on granular surface-nickelized polyimide

In this study, we prepared all-solution-processed inverted polymer solar cells (PSCs) incorporating two solution-processed electrodes – surface-nickelized polyimide films (NiPI films) as cathodes and high-conductivity poly(3,4-ethylenedioxythiophene)/poly(styrene-sulfonate) (PEDOT:PSS) films as anodes – and an active layer with a bulk heterojunction morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-buytyric acid methyl ester (PCBM). The granular Ni thin films, which exhibited good adhesion and high-conductivity (ca. 2778 S cm−1) on the polyimide (PI) substrates and possessed a work function different from that of pure Ni metal (WF, 5.4 eV). Using ultraviolet photoelectron spectroscopy, we determined that the WF of the NiPI films was ca. 3.9 eV. Prior to the coating of the photoactive layer, the surface of the NiPI films were treated with titanium(diisopropoxide)bis(2,4-pentanedionate) (TIPD) solution to facilitate the deposition of high-quality active layer and further as a hole blocking layer. The solution processed anodes (solvent-modified PEDOT:PSS films) were further coated and subjected to mild oxygen plasma treatment on the active layer. Short exposure (5 s) to the plasma improved the quality of the surface of the active layer for PEDOT:PSS deposition. These inverted PSCs on flexible granular NiPI films provided a power conversion efficiency of 2.4% when illuminated under AM 1.5 conditions (100 mW cm−2). The phenomenon of light absorption enhancement in those inverted PSCs was observed as indicated in reflective UV–vis, haze factor and external quantum efficiency (EQE) responses. The resulting fill factor (FF) of 0.43 is still significantly lower than the FF of 0.64 for standard devices. When compared to the planar structure, the improvement of absorbance of light and good haze factors was obtained for granular structure which suggests NiPI as a better back contact electrode through enhancing the light trapping and scattering in inverted PSCs.