Pronounced thickness effect of permanent charge-introduced branched poly(ethylene imine) surface-modifying layers in polymer:nonfullerene solar cells

Abstract

Modification of electrode surfaces in polymer:nonfullerene solar cells have attracted keen interest because it greatly affects the device performance. In this study, we tried to modify the indium-tin oxide (ITO) electrode surface by introducing a branched poly(ethylene imine) (bPEI) nanolayer, which is doped with 3-hydroxypropane-1-sulfonic acid (HPSA), and investigated the thickness effect of HPSA-doped branched PEI layers on the performance of inverted-type polymer:nonfullerene solar cells. The doping ratio of HPSA to bPEI was set to 20 mol% to secure sufficient room for dipole formation, while the bPEI:HPSA thickness was varied between 0 and 5.5 nm. Results showed that the open circuit voltage of devices was remarkably increased by coating the 1.0–3.1 nm-thick bPEI:HPSA layers on the ITO electrodes. The efficiency of solar cells was strongly dependent on the bPEI:HPSA thickness and exhibited the highest efficiency (12.13 %) at 1.5 nm, which is much higher than ca. 7.7 % in the case of 1.5 nm-thick pristine bPEI layers. The improved performance of solar cells was correlated with surface morphology and wetting characteristics, electrode work function change and coverage effect by X-ray photoelectron spectroscopy, and device stability after 60-day storage.