Low-Cost Synthetic Routes for Fabricating Tandem/Multi-Junction Photoelectrochemical Devices

Using sunlight to convert widely available and inexpensive feed stocks such as water, CO₂, and industrial wastes (including halogenides) reliably to fuels and value added chemicals, efficiently and cost-effectively, has been, and continues to be a major goal. Most such reactions require a minimum of 1eV or more photon energies. For example, the minimum free energy required to split water reversibly is 1.23 eV. However, kinetic limitations and other sources of inefficiencies (for example, water oxidation on photoanodes is kinetically sluggish, and CO₂ reduction at photocathodes needs high overpotentials) make the practical energy needed to carry such reactions even considerably higher. A single-junction photosynthetic device would therefore need to base on a semiconductor with a band gap (Eg) > 2.5 eV for it to carry out water splitting or CO₂ reduction, precluding the exploitation of a substantial portion of the solar spectrum. Using tandem/multi-junction photovoltaic architectures is an attractive strategy to overcome these limitations. Such strategies have been estimated to be capable of achieving ~18% solar-to-hydrogen conversion efficiencies.  John Turner and his colleagues [1] demonstrated a solar-to-H₂ conversion efficiency of 12.4% using multi-junction III-V semiconductors in 1990’s. However, the high cost and complexity associated with device fabrication using Si  and III-V semiconductors to produce triple junction devices have impeded commercialization. Here, we report a novel and low-cost wet chemical synthesis route to fabricate a tandem junction PEC device through direct deposition of inexpensive and efficient metal oxide/sulfide based photoanodes on single junction Si solar cells which act as photocathodes. The above structures were able to generate sufficiently high cell voltages to drive valuable, and, at times, challenging photoelectrochemical processes sustainably.

Reference:

1. Khaselev, O. & Turner, J. A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting. Science (New York, N.Y.) 280, 425–7 (1998).