  The presence of interface recombination in a complex multilayered thin-film
solar structure causes a disparity between the internal open-circuit voltage
(VOC,in), measured by photoluminescence, and the external open-circuit voltage
(VOC,ex) i.e. an additional VOC deficit. Higher VOC,ex value aim require a
comprehensive understanding of connection between VOC deficit and interface
recombination. Here, a deep near-surface defect model at the absorber/buffer
interface is developed for copper indium di-selenide solar cells grown under Cu
excess conditions to explain the disparity between VOC,in and VOC,ex.. The
model is based on experimental analysis of admittance spectroscopy and
deep-level transient spectroscopy, which show the signature of deep acceptor
defect. Further, temperature-dependent current-voltage measurements confirm the
presence of near surface defects as the cause of interface recombination. The
numerical simulations show strong decrease in the local VOC,in near the
absorber/buffer interface leading to a VOC deficit in the device. This loss
mechanism leads to interface recombination without a reduced interface bandgap
or Fermi level pinning. Further, these findings demonstrate that the VOC,in
measurements alone can be inconclusive and might conceal the information on
interface recombination pathways, establishing the need for complementary
techniques like temperature dependent current voltage measurements to identify
the cause of interface recombination in the devices.
