The Effect of Potassium Fluoride Postdeposition Treatments on the Optoelectronic Properties of Cu(In,Ga)Se₂ Single Crystals

Abstract

The power conversion efficiency boost of Cu(In,Ga)Se2 in the past years has been possible due to the incorporation of heavy alkali atoms. Their addition through postdeposition treatments results in an improvement of the open‐circuit voltage, the origin of which has been associated with grain boundaries. Herein, the effect of potassium fluoride postdeposition treatments on the optoelectronic properties of a series of sodium‐free Cu(In,Ga)Se2 single crystals with varying Cu and Ga content is discussed. Results suggest that improvement of the quasi‐Fermi level splitting can be achieved in the absence of grain boundaries, being greater in low‐gallium Cu‐poor absorbers. Secondary ion mass spectrometry reveals the presence of potassium inside the bulk of the films, suggesting that transport of potassium can occur through grain interiors. In addition, a type inversion from n to p in potassium fluoride‐treated low‐gallium Cu(In,Ga)Se2 is observed, which along with study of the carrier lifetime demonstrates that potassium can act as a dopant. The fact that potassium on its own can alter the optoelectronic properties of Cu(In,Ga)Se2 single crystals demonstrates that the effect of postdeposition treatments goes beyond grain boundary passivation.

Rubidium Fluoride Absorber Treatment for Wide‐Gap (Ag,Cu)(In,Ga)Se2 Solar Cells

“…[3,[40][41][42][43] The reason for the gain in V OC is, however, still under debate. Proposed explanations are 1) surface passivation by the formation of an alkali-containing wide-gap (Alkali,Cu)-(In,Ga)-Se surface layer (presumably with openings) [44][45][46][47][48][49] ; 2) mitigation/suppression of GB recombination via reduction or passivation of charged defects in GBs [50][51][52] ; 3) increased absorber doping [53][54][55] (lower doping was observed as well [56] ); 4) reduced concentration of deep defects [57,58] (deemed unlikely by others [50] ); and 5) lower potential fluctuations. [59] Obviously, the situation remains complex and a comprehensive understanding is still lacking.…”

Extrinsic Doping of Ink‐Based Cu(In,Ga)(S,Se)2‐Absorbers for Photovoltaic Applications

“…Importantly, film stoichiometry (Cu/(In+Ga), Ga/(In+Ga), and Se/S ratios), morphology, and compositional gradients play an important role for the resulting effect of dopants: samples with different stoichiometries yield different results when subjected to the same dopant treatment. [ 283 ] The recent successes with vacuum PDTs are inherently challenging to reproduce for non‐vacuum CIGSSe as inert atmosphere is broken after selenization, affecting, or contaminating the surface of the ink‐based CIGSSe absorber. Alternatively, in situ vapor phase deposition processes taking advantage of alkali compound volatility should be further studied and explored.…”

“…Importantly, film stoichiometry (Cu/(In+Ga), Ga/(In+Ga), and Se/S ratios), morphology, and compositional gradients play an important role for the resulting effect of dopants: samples with different stoichiometries yield different results when subjected to the same dopant treatment. [ 283 ]…”

“…[282] However, the authors did not find any evidence of potassium diffusing into the absorber layer, which the authors attribute to the lack of GBs. In 2021, for single crystal CIGSe absorbers, Omar et al [283] (UNI) reported that the effectiveness of a KF-PDT treatment strongly depended on the Cu and Ga content, with the treatment being more effective in Cu-poor samples (causing a type of inversion from n to p), and having no effect in Cu-rich, Ga-poor absorbers. Importantly, film stoichiometry (Cu/(In+Ga), Ga/(In+Ga), and Se/S ratios), morphology, and compositional gradients play an important role for the resulting effect of dopants: samples with different stoichiometries yield different results when subjected to the same dopant treatment.…”

The impact of Kelvin probe force microscopy operation modes and environment on grain boundary band bending in perovskite and Cu(In,Ga)Se2 solar cells

“…Epitaxial Cu-rich CuInSe 2 (CISe) films were grown by metal-organic vapor phase epitaxy (MOVPE) on (100)-oriented semi-insulating GaAs wafers at 530 • C and 50 mbar. Details of the process can be found in [15,42]. The absorbers were approximately 500 nm thick and the samples were transferred with the same suitcase directly into the SPM chamber without air exposure.…”