Efficient hydrogen evolution from water over thin film photocathode composed of solid solutions between ZnSe and Cu(In, Ga)Se2 with composition gradient

Abstract

Solid solutions between ZnSe and Cu(In, Ga)Se2 (ZnSe:CIGS) have promising properties as photocathodes for solar hydrogen production from water, such as a long absorption edge of about 900 nm and a large driving force for reaction, >0.9 V, originated by a deep valence band maximum of 1.0–1.1 V vs normal hydrogen electrode (NHE). However, their performance is limited with an incident photon-to-current conversion efficiency (IPCE) of 65% at 400 nm and a half-cell solar-to-hydrogen energy conversion efficiency (HC-STH) of 3.6% so far. Te addition during ZnSe:CIGS thin film deposition by vacuum co-evaporation clearly decreased the optimal deposition temperature from 450 to 380 °C and resulted in columnar shaped grains of submicrometer size in diameter and with almost no grain boundary between the film surface and the backside electrode of Mo, which is significantly larger than the case of without Te addition. Interestingly, a Ga/In compositional ratio gradient was further introduced to the depth profile, which can facilitate charge separation. Structural characterizations using XRD and cross-sectional transmission electron microscopy revealed that the composition gradient was mainly formed by the diffusion of In through grain boundaries in the Ga-rich layer, and thus, the mixing between the Ga-rich and In-rich layers was more significant at a lower deposition temperature owing to the smaller grain size. The photocathode fabricated from the ZnSe:CIGS thin film with the composition gradient showed a very high IPCE of 89% at 540 nm and 0 V vs the reversible hydrogen electrode, and HC-STH of 3.7%, which is higher than values reported thus far. In conclusion, it is clarified that the potential gradient, which can be introduced by composition gradient, is beneficial for photoelectrodes and photocatalysts to achieve higher performance.

Intercalative and non-intercalative photo-recharge using all-solid-state cells for solar energy conversion and storage

“…Introducing buffer layers and adjustment of band bending and gradient are useful methods to control carrier separation in photoelectrodes. 42,43 Also in the eld of solar cells, eld effect passivation is oen utilized to suppress the recombination between electrons and holes. [44][45][46][47] Therefore, the photorechargeable battery would be also improved by introducing buffer/passivation layers and band engineering.…”

Efficient hydrogen evolution from water over a thin film photocathode composed of solid solutions with a composition gradient of ZnTe and CdTe

“…However, the potential gradient allows the harnessing of electrons outside the depletion layer. 17 In addition, the VBM of CdTe and ZnTe permits hole-selective diffusion to the back side electrode, ITO. These band alignment features could explain the enhanced hydrogen evolution from water at high potential in the case of the TO/Cu/ZnTe/CdTe/CdS/Pt (RTA) photocathode.…”

“…The enhancement of hydrogen evolution by introduction of a functional structure, the composition gradient, is because of the formation of a conduction band minimum (CBM) potential gradient, which allow us to utilise electrons photoexcited outside the depletion layer. 17 It should be noted that at around the flat-band potential, band bending is very small and the depletion layer is also very thin. This CBM potential gradient likely contributes to the increase in photocurrent around OP.…”

Enhancing Photocathodic Performances of Particulate-CuGaS2-Based Photoelectrodes via Conjugation with Conductive Organic Polymers for Efficient Solar-Driven Hydrogen Production and CO2 Reduction

“…Photoelectrochemical water splitting and CO 2 reduction using water as an electron donor have gathered much attention as a promising candidate for methodology to convert solar energy to chemical energy. − Many efforts have been made to investigate photoelectrochemical properties of Cu-contained compounds, such as sulfides, − selenides, − and oxides. − This is because most of the Cu-containing materials exhibit a p -type semiconductor character that is indispensable for employing them as photocathodes in photoelectrochemical reactions. Among them, it has been reported that Cu-contained metal-sulfide-photocatalyst powders are useful as photocathodes to reduce H 2 O and CO 2 to H 2 and CO, respectively, under simulated solar light. − ,− ,, Moreover, their band gaps correspond to visible light and can be flexibly controlled by the formation of solid solutions. − ,,,,,− Nevertheless, photoelectrochemical performances of the particulate-based photocathodes are mostly lower than those of the thin films made by vaper deposition ,,, and precursor coating followed by either sulfurization or selenization. ,,,,, A main reason why the performances of the particulate-based photocathodes are lower is considered to be insufficient contacts between the particulate photocatalysts and a conductive substrate such as FTO (fluorine-doped tin oxide). In other words, there are many cavities between the particulates and the FTO.…”

“… 7 − 10 , 13 − 15 , 17 , 19 Moreover, their band gaps correspond to visible light and can be flexibly controlled by the formation of solid solutions. 7 − 9 , 13 , 15 , 16 , 19 , 36 − 42 Nevertheless, photoelectrochemical performances of the particulate-based photocathodes are mostly lower than those of the thin films made by vaper deposition 16 , 22 , 28 , 30 and precursor coating followed by either sulfurization or selenization. 7 , 14 , 17 , 23 , 27 , 29 A main reason why the performances of the particulate-based photocathodes are lower is considered to be insufficient contacts between the particulate photocatalysts and a conductive substrate such as FTO (fluorine-doped tin oxide).…”