Facile, Noncyanide Based Etching of Spray Deposited Cu₂ZnSnS₄ Thin Films for Secondary Phase Removal

Abstract

The coexistence of secondary phases in the quaternary compound kesterite Cu2ZnSnS4 (CZTS), a promising photovoltaic absorber, is a major problem while synthesizing under Zn or Cu rich conditions. A large variety of secondary phases, such as CuS, Cu1.8S, Cu2S, Cu2SnS3, and ZnS exist on the surface unless they are not removed by dedicated surface treatment before the annealing step. Under a carefully chosen concentrations of the starting precursors (usually, Zn-poor and Sn-rich) for spray pyrolyzed CZTS, the fraction of ZnS is minimized. However, under such growth conditions, binary Cu-sulfides become dominant. In this work, a selective noncyanide based chemical etching procedure is demonstrated prior to the deposition of the buffer layer. The absorber surface was treated with hydrogen peroxide that is known to remove Cu1.8S, Cu2S, and allied secondary phases to a large extent as compared to conventional KCN based techniques. By this treatment, the optical band gap is changed from 1.8 eV to most suitable 1.5 eV, which ensures improved photon absorption.

Effect of surface treatment by hydrogen peroxide solution on Cu2ZnSnS4 thin films

“…shows the SEM images of the CZTS samples (a) before, (b) after 30 wt.%-H 2 O 2 surface treatment dipped in 150 s at 293 K, (c) after 3 wt.%-H 2 O 2 surface treatment dipped in 150 s at 293 K and (d) after 3 wt.%-H 2 O 2 surface treatment in 150 s at 333 K. The surface morphology of the sample dipped in concentrated H 2 O 2 solution at long time is drastically changed compared with untreated sample surface.This morphology is similar to Ref [9]…”

“…Wet etching is divided into the following three processes; transport or diffusion of reactants to the surface, [9]. However, their CZTS films prepared by spray pyrolysis and H 2 O 2 surface treatment is sprayed on the CZTS surface.…”

“…This result suggested that the oxidation of the surface is enhanced with increasing dipping time. Chavta et al are evaluating the crystallinity of the before and after the H 2 O 2 treatment by using the full width at half maximum (FWHM) [9]. They reported the removal of the secondary phase on CZTS surface and the improvement of the crystallinity of CZTS by H 2 O 2 surface treatment.…”

A Deep Dive into Cu2ZnSnS4 (CZTS) Solar Cells: A Review of Exploring Roadblocks, Breakthroughs, and Shaping the Future

“…The aformentioned discussion strongly validates that KCN-etching is a promising approach for removing unwanted secondary phases in CZTS films, but this treatment can instead lead to unwanted voids in a resulting film. [192,193] Therefore, apart from the KCN etching for the removal of Cu x S, Chavda et al [194] applied H 2 O 2 and HCl etching, and comparatively investigated their impacts. They reported that in contrast to conventional HCl etching, the H 2 O 2 etching proved more effective in the removal of Cu x S and related impurity phases from the CZTS deposited by spray pyrolysis.…”

Interfaces of high-efficiency kesterite Cu2ZnSnS(e)4thin film solar cells

“…[123] In addition, Pinto et al reported the Cu 2−x (S,Se) secondary phase can be removed completely by a safer chemical mixture of ethylenediamine and 2-mercaptoethanol. [124] It is indicated that H 2 O 2 can also effectively remove Cu x S phases following by [125] Cu…”