  While perovskite solar cells (PSCs) are now reaching high power conversion
efficiencies (PCEs), further performance improvement requires a fine management
and an optimization of the light pathway and harvesting in the cells. These go
through an accurate understanding, characterization and modelling of the
optical processes occurring in these complex, often textured, multi-layered
systems. In the present work, we have considered a typical methylammonium lead
iodide (MAPI) solar cell built on a fluorine-doped tin oxide (FTO) electrode of
high roughness (43 nm RMS). By variable-angle spectroscopic ellipsometry (VASE)
of the full PSC device, we have been able to determine the optical constants of
all the device layers. We have designed a one-dimensional (1D) optical model of
the stacked layers where the rough texture is described as layers of
effective-medium index. We have supported the model using data extracted from
scanning electron microscopy, diffuse spectroscopy and photovoltaic efficiency
measurements. We show that the 1D model, while insufficient to describe
scattering by the FTO plate alone, gives an accurate description of the full
device optical properties. By comparison with the experimental external quantum
efficiency (EQE), we estimate the internal quantum efficiency (IQE) and the
effect of the losses related to electron transfer. Based on this work, we
finally discuss the optical losses mechanisms and the possible strategies that
can be implemented to improve light management within PSC devices and further
increase their performances.
