  Interface recombination at the absorber buffer interface impedes the
efficiency of a solar cell with an otherwise excellent absorber. The internal
voltage or the quasi-Fermi level splitting (qFLs) measures the quality of the
absorber. Interface recombination reduces the open circuit voltage (VOC) with
respect to the qFLs. The present work explores a facile sulfur-based
post-deposition treatment (S-PDT) to passivate the interface of CuInS2 thin
films grown under Cu-rich conditions, which show excellent qFLs values, but
much lower VOCs. The CuInS2 absorbers are treated in three different
S-containing solutions at 80 oC. Absolute calibrated photoluminescence and
current-voltage measurements demonstrate a reduction of the deficit between
qFLs and VOC in the best S-PDT device by almost one third compared to the
untreated device. Analysis of temperature dependence of the open-circuit
voltage shows increased activation energy for the dominant recombination path,
indicating less interface recombination. In addition, capacitance transient
measurements reveal the presence of slow metastable defects in the untreated
solar cell. The slow response is considerably reduced by the S-PDT, suggesting
passivation of these slow metastable defects. The results demonstrate the
effectiveness of solution based S-treatment in passivating defects, presenting
a promising strategy to explore and reduce defect states near the interface of
chalcogenide semiconductors.
