Charge carrier collection at buried Cu(In,Ga)S 2 interfaces with opaque front contact for photoelectrochemical hydrogen generation Valentina Corsetti 1 , Léo Choubrac 2 , Nicolas Barreau 2 , David Fermin 1 1 School of Chemistry, University of Bristol, Cantock’s Close, BS8 1TS Bristol, United Kingdom, 2 Institut des Matériaux Jean Rouxel (IMN), 2 rue de la Houssinière, 44300 Nantes, France Hydrogen fuel is a sustainable, clean alternative energy source to fossil fuels, with photoelectrochemical (PEC) production methods allowing for hydrogen generation directly from solar energy. Photocathodes for the hydrogen evolution reaction (HER) require the use of a p-type semiconductor with a high absorption coefficient, appropriate band edge alignment, and suitable band gap. Utilising absorber materials which have proven successful in photovoltaic (PV) applications due to similar advantages is an effective starting point towards PEC HER. In this work, a co-evaporated Cu(In,Ga)S 2 (CIGS) absorber deposited on a transparent Sn:InO 2 (ITO) substrate is investigated for PEC hydrogen evolution under rear illumination. 1 Rear illumination has the advantage of reducing concerns of parasitic light absorption or low light transmission from buried junctions and enables a higher co-catalyst surface coverage. With this come the benefits of requiring thinner absorber layers, decreasing the use of rare and expensive elements, as well as allowing for greater flexibility in terms of electrode architectures and surface compositions towards higher HER efficiencies. Firstly, the CIGS absorber was studied by photoelectrochemical techniques both in the presence and absence of an electron scavenger. This enables a deeper understanding of the absorber material under PEC conditions, and comparison between the surface and bulk charge transfer efficiencies. Due to the low intrinsic activity of the absorber towards HER, the addition of a co-catalyst was necessary to overcome the persistent surface recombination. Therefore, the device architecture utilised for PV applications (ITO/CIGS/CdS/ZnO/NiAlNi) was kept as the photocathode, where the front contact Ni surface was utilised as an earth-abundant co-catalyst for HER in basic conditions. 1,2 This provided the additional benefit of facile comparison between parallel PEC and PV characterisations, such as quantum efficiencies (QE) and fill factor (FF). The HER activity of the Ni-decorated photocathode improved significantly, displaying a highly linear relationship between photon flux and photocurrent as well as evidence of Fermi level pinning by the Ni surface. The relationship between PEC and the electrochemical activity of this Ni surface was studied and deconvoluted by transient photocurrent analysis and voltammetry, revealing the complex surface electrochemistry of this photocathode architecture under PEC HER conditions. References 1. Choubrac, F. Pineau, E. Bertin, L. Arzel and N. Barreau, J. Phys. Energy, 2025, 7 , 02LT01 2. Li, J. Shi and C. Li, Small, 2018, 14 , 1704179
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