Wide-Bandgap Cu(In,Ga)S₂ Photocathodes Integrated on Transparent Conductive F:SnO₂ Substrates for Chalcopyrite-Based Water Splitting Tandem Devices

Abstract

Whole-chalcopyrite-based tandem devices for photoelectrochemical (PEC) water splitting have emerged as a promising route for obtaining ∼20% solar-to-hydrogen efficiencies. Here we pursue this approach by demonstrating integration of the top cell wide-bandgap (E G ) chalcopyrite onto a transparent conductor, which is a critical step in the realization of tandem devices. We report specifically on our efforts to synthesize photoactive Cu(In,Ga)S 2 thin films on transparent conductive F:SnO 2 (FTO), while preserving the optoelectronic properties of the FTO substrate and preventing the formation of a resistive SnS x interfacial layer. We demonstrate that such attributes can be achieved via closespace sulfurization (CSS) of lower E G Cu(In,Ga)Se 2 precursors, coevaporated on FTO at low temperature. Depending on Cu(In,Ga)Se 2 precursors' Ga and In content, the resulting Cu(In,Ga)S 2 solar absorbers have E G energies spanning from 2.05 to 2.45 eV. The CSS process, which includes a low-temperature annealing in sulfur vapor followed by a high-temperature crystallization under inert atmosphere, allowed for up to 95% Se substitution with S in the chalcopyrite lattice, tuning both E G and band edge positions that impact PEC performance. Photoelectrochemical measurements performed under AM1.5 G illumination in 0.5 M H 2 SO 4 on the 2.05 eV CuInGaS 2 photocathode revealed a saturation photocurrent density (J SAT ) of −5.25 mA/cm 2 , a value corresponding to 38% of the absorber's optical limit. We further concluded that such low J SAT originates from subpar optical absorption of Cu(In,Ga)S 2 absorbers. Future improvements of the CSS process are expected to improve material quality toward our end goal of achieving whole-chalcopyrite tandem PEC devices.

Water Splitting: From Electrode to Green Energy System

“…Compared to Si solar cell, the outstanding advantage of CIGS is that the band gap energy can be modulated to effectively absorb the solar spectrum, so it is also widely used to achieve water splitting [129][130][131]. For purpose of overcoming the problem of low energy to drive overall water splitting, connected series into a monolithic device can be Fig.…”

Performance and limits of 2.0 eV bandgap CuInGaS2 solar absorber integrated with CdS buffer on F:SnO2 substrate for multijunction photovoltaic and photoelectrochemical water splitting devices

“…Downloaded on 7/26/2021 2:24:31 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.This present paper builds upon the results reported in Ref [30]. and focuses on the integration of thicker (1200 nm) 2.0 eV CIGS absorbers on FTO and interfaced with CdS buffer layers.…”

“…From calculated natural band alignments in Ref [64]. and the analysis of the CIGS band edges with composition,65 we estimate a cliff-like CBO of ~0.5-0.6 eV for 0.73 GGI CdS/CIGS heterojunction. If we also consider the possible formation of offstoichiometric Cu-deficient phases, or "ordered vacancy" compounds (OVCs) in the sulfide interface, this can also influence the resulting band offsets with CdS.…”

“…In this paper, we report on our efforts to integrate wide-E G chalcopyrite PV on SnO 2 :F (FTO) substrates for CIGSSe-based MJSC and PEC water splitting applications. Our recent published results have demonstrated that bandgap tunable CuGa(S,Se) 2 29 (CGSSe) and Cu(In,Ga)S 2 30 (CIGS) can be successfully integrated on FTO, while preventing the formation of resistive SnS x back interface. With our process, co-evaporated CIGSe precursors are first post-treated with elemental sulfur at 350 C (dosing stage), and then annealed at 500-550 C under inert atmosphere (alloying stage).…”

Identifying Reactive Sites and Surface Traps in Chalcopyrite Photocathodes

“…One class of materials that offers a particular challenge is the chalcopyrites Cu(In,Ga)(S,Se) 2 . These materials are gaining momentum as prospective photocathodes for H 2 production in photoelectrochemical (PEC) water splitting cells [8–12] . Their attractiveness relies on their superb optoelectronic properties, their compatibility with solution‐based manufacturing techniques, their ready‐to‐market photovoltaic performance, and their well‐positioned energy bands to trigger the hydrogen evolution reaction (HER), among others [13–15] .…”