Abstract: There is an intense interest in utilizing semiconducting quantum dot in photovoltaic application due to their facile solution processability, and tunable light absorption characteristics owing to the effects of quantum confinement. Essentially, PbS quantum dots, with their extended optical absorption into the near infrared, have been found to produce a well-respected photocurrent in solar cells. Here, we show that upright and inverted colloidal PbS quantum dot solar cells show different optical charge generation profile and nongeminate carrier recombination losses. 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.
Presentation type: Oral Research category: hybrid/perovskite materials and devices