An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systemsElectronic supplementary information (ESI) available: Detailed-balance calculations for the current-voltage properties of the light absorbers. See DOI: 10.1039/c3ee40453f

• Main Authors: Hu, Shu, Xiang, Chengxiang, Haussener, Sophia, Berger, Alan D, Lewis, Nathan S
• Format: Journal Article
• Language: English
• Published: 18.09.2013
• ISSN: 1754-5692; 1754-5706
• DOI: 10.1039/c3ee40453f

The solar-to-hydrogen (STH) efficiency limits, along with the maximum efficiency values and the corresponding optimal band gap combinations, have been evaluated for various combinations of light absorbers arranged in a tandem configuration in realistic, operational water-splitting prototypes. To perform the evaluation, a current-voltage model was employed, with the light absorbers, electrocatalysts, solution electrolyte, and membranes coupled in series, and with the directions of optical absorption, carrier transport, electron transfer and ionic transport in parallel. The current density vs. voltage characteristics of the light absorbers were determined by detailed-balance calculations that accounted for the Shockley-Queisser limit on the photovoltage of each absorber. The maximum STH efficiency for an integrated photoelectrochemical system was found to be ∼31.1% at 1 Sun (=1 kW m −2 , air mass 1.5), fundamentally limited by a matching photocurrent density of 25.3 mA cm −2 produced by the light absorbers. Choices of electrocatalysts, as well as the fill factors of the light absorbers and the Ohmic resistance of the solution electrolyte also play key roles in determining the maximum STH efficiency and the corresponding optimal tandem band gap combination. Pairing 1.6-1.8 eV band gap semiconductors with Si in a tandem structure produces promising light absorbers for water splitting, with theoretical STH efficiency limits of >25%. Band energy diagram of an integrated water-splitting system with a solar-to-hydrogen efficiency plot.