Analysis of the Physical Properties and Photoelectrochemical Behavior of c-Si/a-SiC:H(p) Photocathodes for Solar Water Splitting

摘要

Photoelectrochemical (PEC) processes have become an efficient and viable method for solar energy conversion into zero-emission fuels by harnessing and storage the power of the sun, which offers an environment-friendly approach. Hydrogen is considered as a sustainable and suitable energy alternative in comparison to energy systems based on the consumption of conventional hydrocarbon fuels. In particular, hydrogen production by PEC water splitting represents an attractive alternative to enhance the efficiency of water splitting process using sun light. Current research in this field is devoted to the fabrication and evaluation of new photoactive materials that upon sunlight absorption enable water-splitting PEC reactions with high efficiency and durability.Silicon based compounds offer a good opportunity as photoactive material due to its high abundance and current scalable technologies. Silicon is the eighth most abundant element in the universe and constitutes up to 27.2% of the earth's crust weight [1]. Particularly, hydrogenated amorphous silicon carbide (a-SiC:H) thin films have been tested as photocathode material for photo-assisted electrolysis, exhibiting solar to hydrogen conversion efficiencies up to 7.5 %. Due to the capability to tailor the bandgap (1.8 - 3.3 eV) by the incorporation of carbon in a-Si:H, this material has turned out to be a promising candidate for PEC cells, fulfilling the optical bandgap (Egap) primary requirement, i.e. Egap > 2.2 eV. Additionally, the carbon incorporation leads to an enhancement of the corrosion resistance properties in aqueous media. It has been theoretically estimated that a PEC device with an a-Si0.9C0.1:H absorption layer exhibiting 2.0 eV of energy bandgap, can generate a photocurrent density Jph of 15 mA/cm2 (solar-to-hydrogen conversion efficiency ～18 %) when submitted to an Air mass (AM) 1.5 solar spectrum [2]. The specific value of 1.5 for the Air mass is selected for standardization purposes, based on the analysis of solar irradiance data in the United States, and corresponds to a power of 1000 W/cm2 [3]. In the present work, a similar material a-Si0.5C0.5:H exhibiting 2.76 eV of energy bandgap generates a Jph of 17 mA/cm2 when submitted to the same light spectrum. This reflects an improvement in the use of a-SiC:H in PEC water splitting. Research on single thin film photoactive materials does not typically consider the role of the silicon substrate in the photoelectrochemical performance. In this sense, the photoelectrode is a system formed by a p-p, n-n or n-p structure, which depending on the depth of the space charge region may have an important impact on the photoelectrode performance. In this work, this substrate effect has been considered and studied.PEC performance of a-SiC:H is often limited by its non-ideal energy band-edge alignment to the H2O/O2 redox potential, thus limiting the oxygen evolution reaction (OER) and the whole water splitting process [4]. To overcome this downside, an external bias needs to be applied, contributing to counteract the overpotential required to trigger the direct water splitting reaction. The external bias also contributes to reduce overpotentials due to the presence of a surface SiO2 barrier layer and compensates interface charge carriers recombination as well.