Performance Enhancement by Secondary Doping in PEDOT:PSS/Planar-Si Hybrid Solar Cells

Solar cells depend on effectively absorbing light and converting it into electrical current. It is therefore essential to increase conductivity and to limit both reflection and parasitic absorbance to achieve higher photoconversion efficiency. Here, we examine the effect of post-treatment on the absorbance and conductivity of hybrid solar cells comprised of p-type poly(3,4-ethylene-dioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) on an n-type silicon substrate. Three sets of cells based on pristine PEDOT:PSS film, cosolvent enhanced PEDOT:PSS film using ethylene glycol as a cosolvent, and post-treated PEDOT:PSS film using a novel 1:1 binary mixture of ethylene glycol and methanol have been studied. Markedly different film conductivities have been found for the pristine (similar to 0.8 S/cm), cosolvent added (637 S/cm), and post-treated films (1334 S/cm). The photoconversion efficiency obtained over a large set of samples (72 cells) was used to evaluate the cosolvent addition and posttreatment. Post-treatment is found to reproducibly provide films with not only the highest conductivities but also the highest efficiencies along with higher open-circuit voltage and fill factor but lower short-circuit current density when compared to those of the cosolvent added films. The decrease in the latter is attributed to the increase in absorbance in the PEDOT:PSS film. The present work illustrates the delicate challenge in improving the conductivity and carrier collection efficiency of the cells not at the expense of other properties such as absorption.