Disparity
in Optical Charge Generation and Recombination Processes in Upright
and Inverted PbS Quantum-Dot Solar Cells
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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