Fig. 6: Electrical identification of the metastable defects induced by cyanide etching on CuInSe2 absorbers.
From: Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface
a PEC transients of Cu-poor CIS (P, red), stoichiometric CIS (S, black) and Cu-rich CIS (R, blue) films immediately after bold KCN etching (b) (hence: PB, SB and RB). Inset: evolution of the surface recombination kinetic constant (ks.r.) of CIS-P (red) and CIS-R (blue) as a function of air-exposure time after weak KCN etching (W) (hence, PW and RW). Schematics: band diagram of the CIS/electrolyte junction with an additional recombination pathway and corresponding bulk trap kinetic constant (kb.t.). b TR-SPV results on CIS- P (red, shifted by 100 mV) and CIS-R (blue) with bold KCN etching (B). The relaxation curve of CIS-PB acquired in real time after excitation with a continuous-wave light source with 1–2 s duration (Region I) shows a fast decay time, which cannot be resolved by KPFM. To resolve it, modulated illumination was used to obtain frequency spectra (during which a partial SPV decay is observed, region II). Unlike CIS-PB, where the SPV drops sharply to the initial in-dark value after the modulation sequence, CIS-RB does not recover completely (region III). The light modulation sequence has a 50% duty cycle for a total duration of 5.2 s, as shown by the schematic square-wave in the yellow inset. c Normalized average SPV spectra as a function of the illumination modulation frequency (corresponding to region II). Decay times (τ) are determined by fitting (solid thick lines) the data (open circles) with an exponential behaviour yielding: τPB ~ (6.2 ± 1.0) µs and τRB ~ (0.75 ± 0.2) µs (cf. “Methods”). d PL yield of CIS-P (red) and CIS-R (blue) films without etching (U, dotted), and after weak KCN etching (W, dashed) and bold KCN etching (B, solid line). Inset: activation energies of the main capacitance step extracted from the slope of the Arrhenius plots of the thermal admittance spectra (320–50 K) of Schottky junctions with CIS-P (red) and CIS-R (blue). Bold KCN etching yields an activation energy of 200 ± 20 meV on both CIS-P and CIS-R, whereas CIS-PW shows an activation energy of 100 ± 10 meV.
