  Perovskite photovoltaics has witnessed an unprecedented increase in power
conversion efficiency over the last decade. The choice of transport layers,
through which photo-generated electrons and holes are transported to the
electrodes, is a crucial factor for further improving both the device
performance and stability. In this perspective, we critically examine the
application of optical spectroscopy to characterize the quality of the
transport layer-perovskite interface. We highlight the power of complementary
studies that use both continuous wave (cw) and time-resolved photoluminescence
(PL) to understand non-radiative losses, and additional transient
spectroscopies for characterizing the potential for loss-less carrier
extraction at the solar cell interfaces. Based on this discussion, we make
recommendations on how to extrapolate results from optical measurements to
assess the quality of a transport layer, and its impact on solar cell
efficiency.
