  Solar cells employing quantum wells can enhance the light absorption but
suffer from the difficulty in photogenerated carrier extraction. Here, we
analyzed the spectral response and the photocarrier collection mechanism of
p-i-n multiple quantum well (MQW) solar cells using the effective-mobility
model. Both the simulation and experiment results imply that the spatial
profiles of electron and hole densities in MQWs play an important role in the
carrier collection process. By considering the recombination increment under
illumination, our findings suggest that the concept of the majority/minority
carriers is important even in the intrinsic region: photogenerated electrons
and holes only experience significant recombination when passing through the
hole-rich and electron-rich regions, respectively. This can accurately explain
the photocurrent behavior in cells with high background doping, background
illumination, and different MQW positions. Based on the experimental findings,
we derived analytical formulae for carrier collection efficiency, which
directly show the impact of each cell parameter and can be used for the
systematic cell design.
