Fig. 7: Chemical identification/passivation of the CuInSe2 metastable defects and impact on PV device performance.
From: Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface
a PEC transients of Cu-rich CIS subject to bold KCN etch (RB) after repeated exposure to air at room temperature (note that illumination starts at time 0 and, for discussion purposes, the curves have been y-shifted such that the steady-state current densities under illumination coincide). b Evolution of the photocurrent density JPh as a function of air-exposure time, revealing an initial regime of photocurrent increase (i) exemplified in the schematics by O2-induced passivation of VSe donors and corresponding removal of the bulk trap recombination pathway, followed by a subsequent decrease (ii), due to the detrimental formation of Cu-Se an In-O at the CIS surface. c JV characteristics of solar cell devices comprising Cu-poor (P, red) and Cu-rich (R, blue) CIS absorbers subject to weak KCN (W, dashed) and bold KCN (B, solid) etching. The Voc improvement obtained with CIS-RB films upon Se-only and Se-KF PDTs is shown by the arrow. The bar chart refers to the corresponding activation energies of the main capacitance step extracted from thermal admittance spectroscopy (see Supplementary Figs. 7 and 8). d Proposed schematic illustration of point defect distribution in Cu-poor and Cu-rich films after bold KCN etching (PB and RB), showing the different extent of Cu and Se depletion leading to the formation of [VCuVSe] divacancy complexes. The divacancy complex occurs near the surface of CIS-PB but extends deeper in the bulk of CIS-RB. The deposition of CdS by chemical bath close to room temperature (solar cell completion) induces S incorporation into VSe, i.e., dissolution of the complex. The S incorporation is sufficient to passivate the corresponding electronic states present at the surface of PB film but is insufficient to reach deeper in the RB film; hence, some of the defect complexes remain. Post-deposition treatments with Se at high temperature prior to cell completion enable a more effective replenishment of the Se vacancies, with a beneficial passivating effect that increases the VOC of the photovoltaic cell.
