  Heterostructures based on atomically thin semiconductors are considered a
promising emerging technology for the realization of ultrathin and ultralight
photovoltaic solar cells on flexible substrates. Much progress has been made in
recent years on a technological level, but a clear picture of the physical
processes that govern the photovoltaic response remains elusive. Here, we
present a device model that is able to fully reproduce the current-voltage
characteristics of type-II van der Waals heterojunctions under optical
illumination, including some peculiar behaviors such as exceedingly high
ideality factors or bias-dependent photocurrents. While we find the spatial
charge transfer across the junction to be very efficient, we also find a
considerable accumulation of photogenerated carriers in the active device
region due to poor electrical transport properties, giving rise to significant
carrier recombination losses. Our results are important to optimize future
device architectures and increase power conversion efficiencies of atomically
thin solar cells.
