Where: Room TD2202AA Escola Superior de Tecnologia i Ciències Experimentals
Presented by: Prof. Sixto Gimenez Juliá
The photovoltage accessible in semiconducting photoabsorbers typically falls 0.5-1 V below theoretically achievable values predicted by the Shockley-Queisser limit [1]. While the exact reason for this discrepancy remains unclear, surface and interface states within the energy band gap of the photoabsorbers are known to play a crucial role, often inducing Fermi level pinning and charge recombination [2]. Within the photoelectrochemical (PEC) community, two key elements have been identified for maximizing the photovoltage in photoelectrodes for water splitting: (i) the passivation of surface defects to avoid Fermi level pinning, and (ii) the increase of minority carrier concentration at the interface to improve contact selectivity and optimize carrier extraction [3,4].
To understand the role of surface defects in Fermi level pinning, detailed information on the chemical nature and electronic properties of surface states is required. Such information is typically obtained using ultra-high vacuum (UHV) surface science techniques such as X-ray photoelectron spectroscopy (XPS). However, the photoelectrode surface under UHV conditions may not accurately reflect the electrified surface when immersed in the electrolyte. Furthermore, recent studies have demonstrated that semiconductor surfaces undergo extensive structural and chemical transformations during PEC device operation [5,6].
In this seminar, we will showcase our recent development of state-of-the-art techniques to study the structure and dynamics of semiconductor/water interfaces under practical conditions [7]. We have explored the chemical nature of electronic states for selected semiconductors prepared at the Institute for Solar Fuels, namely BiVO4 and α-SnWO4, using synchrotron-based resonant and ambient pressure soft X-ray photoelectron spectroscopy (AP-XPS) [6,8-11]. Specifically, we will demonstrate the feasibility of investigating chemical changes at solid/liquid interfaces using AP-XPS [6,8], and correlate them with the formation of electronic in-gap and surface states.
We will conclude by discussing future perspectives in the field of PEC, particularly regarding the design and development of heterojunctions encompassing back and front selective contacts.
[1] J. Garcia-Navarro et al. Global Challenges 2023, 2300073.
[2] A. J. Bard et al. J. Am. Chem. Soc. 1980, 102, 3671.
[3] A.G. Scheuermann et al., Nat. Mater. 2016, 15, 99.
[4] M. Schleuning et al., Sustainable Energy Fuels, 2022, 6, 3701.
[5] F.M. Toma et al., Nat. Commun. 2016, 7, 12012.
[6] M. Favaro et al., J. Phys. Chem. B 2018, 122, 801.
[7] M. Favaro et al., Surf. Sci. 2021, 713, 121903.
[8] M. Favaro et al., J. Phys. D: Appl. Phys. 2021, 54, 164001.
[9] M. Favaro et al., J. Phys. Chem. C 2019, 123, 8347.
[10] W. Wang et al., J. Am. Chem. Soc. 2022, 144, 17173.
[11] P. Schnell et al., Sol. RRL 2023, 2201104.
