Extraction of photo-generated charge carriers from polymer-fullerene bulk heterojunction solar cells

Abstract

Two models describing charge extraction from insulators have been used to interpret the experimental photocurrent data of 20:80 wt% blends of poly(2-methoxy-5-(3`,7`-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. When only drift of charge carriers is taken into account, a square root dependence on voltage of the photocurrent is expected, governed by the difference between electron and hole mobility. It is demonstrated that both the magnitude and functional dependence of the predicted current are in disagreement with experimental data. However, when both drift and diffusion of charges are taken into account, the predicted photocurrent shows a different behaviour: At low electric fields a linear behaviour is predicted, which results from the diffusion of charges, folllowed by saturation at high fields. The agreement between the theoretical result and the experimental data obtained from MDMO-PPV:PCBM cells is satisfactory when a generation rate of G=1.46 × 10²⁷ electron-hole pairs/m3s is used, showing the importance of diffusion at low fields, i.e., near the open-circuit voltage.