Disparity in optical charge generation and recombination processes in upright and inverted PbS quantum-dot solar cells

Abstract

The role of optical charge generation and nongeminate recombination on the photocurrent of upright and inverted colloidal-PbS quantum-dot solar cells is investigated. With a controlled active layer thickness, upright (PbS/fullerene) devices are found to present overall better photovoltaic performance relative to inverted devices, notwithstanding the better NIR photoconversion efficiency in the latter. Through detailed analysis and numerical optoelectronic simulations, we show that beyond incidental differences, these two device architectures have fundamentally dissimilar properties that stem from their particular optical generation characteristics and the nature of the recombination processes at play, with the inverted devices affected only by trap-assisted losses and the upright ones suffering from enhanced bimolecular recombination. This study unveils the role of device geometry and inherent material properties on the carrier generation and collection efficiency of the light-generated photocurrent in colloidal quantum-dot solar cells.