Thin film solar cells based on CdS/CdTe hetero-
structure has shown a drastic improvement changing
from 16.5 to 22.1% efficiency during a short period of time
from ~2013 to ~2016. This has happened in the industrial
environment and the open research in this field has stagnated
over a period of two decades prior to ~2013. Most
of the issues of this hetero-structure were not clear to the
photovoltaic (PV) community and research efforts should
be directed to unravel its complex nature. Issues related to
materials, post-growth treatment, chemical etching prior to
metallisation and associated device physics are the main
areas needing deeper understanding in order to further
develop this device. After a comprehensive research programme
in both academia and in industry on these materials,
surfaces and interfaces and fully fabricated devices
over a period of over three decades by the main author,
the current knowledge as understood today, on all above
mentioned complex issues are presented in this paper. Full
understanding of this structure will enable PV developers
to further improve the conversion efficiency beyond 22.1%
for CdS/CdTe based solar cells
