Efficiency improvement in silicon nanowire/conductive polymer hybrid solar cells based on formic acid treatment

Abstract

We investigated the mechanisms causing the improvement of power conversion efficiency (PCE) in hybrid silicon nanowire/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells by formic acid treatment. After 5 min of formic acid treatment at elevated temperature (140 °C), the average PCE of the device was improved from 8.00% to 9.22%. After the formic acid treatment, the conductivity of the PEDOT:PSS film increased from 683 S cm−1 to 1582 S cm−1 and the average built-in voltage also increased from 0.44 V to 0.50 V. The larger built-in voltage can suppress the recombination of photogenerated charges in the hybrid solar cells and in turn render the silicon nanowire/PEDOT:PSS junction barrier closer to an ideal Mott–Schottky barrier. Through C–V and trap-state density calculations, we confirmed that this improvement was attributable to the decrease in the density of interfacial states in the hybrid solar cells after the formic acid treatment. Our findings indicate the critical roles of the polymer conductivity and the quality of the organic/inorganic interface in the performance of hybrid silicon nanowire/PEDOT:PSS solar cells.