Low cost, earth-abundant light absorbers are of great interest for use in terrestrial photovoltaics and in artificial photosynthesis. Non-conventional solar absorbers, e.g., zinc phosphide (Zn₃P₂), cuprous oxide (Cu₂O) and pyrite (FeS₂), are promising materials for solar energy conversion and have received renewed interests after early work in 1980s. The Lewis group is interested in studying the interface chemistry and fundamental energy-conversion properties of these non-conventional solar absorbers.

Zinc phosphide is one of the most promising materials for low-cost solar cells. It has a direct bandgap near 1.5 eV, both of its constituent elements are cheap and earth abundant and electron diffusion length in p-type Zn₃P₂ is relatively long (>10 um). We have successfully demonstrated synthesis of quasi-single crystal Zn₃P₂ by physical vapor transport (Figure 1). We are investigating various surface treatment for Zn₃P₂ to decrease the surface recombination velocity (Figure 2). We are also investigating solar cell performances of Mg/Zn₃P₂ diodes.

Fig. 1. Preparation of quasi-single crystal Zn₃P₂ substrates.

Fig. 2. Photoluminescence (PL) data demonstrate the improved quality of chemically-treated Zn₃P₂ surfaces. Time-resolved PL decays show a reduction in surface recombination velocity (SRV) and inset shows enhanced steady-state PL from HF:H₂O₂ treatment.

Cuprous oxide is a native p-type semiconductor, with a bandgap of 2.0 eV and relatively high absorption coefficient in the visible spectrum range. We are developing synthesis techniques for preparing high quality Cu₂O substrates (Figure 3). We are using semiconductor/liquid contacts to investigate the energy-conversion properties of prepared substrates (Figure 4). We are also interested in exploring other hetero-junction partners for cuprous oxide substrates.

Fig. 3. Synthesis of Cu₂O substrates by high temperature thermal oxidation.
Fig. 4. 820 mV open-circuit voltages were observed from Cu₂O photoelectrodes in contact with the decamethylcobaltocene^+/0 (Me₁₀CoCp₂^+/0) redox couple. Inset shows an electron diffraction image of the grain structure in Cu₂O substrates.

1. G. M. Kimball, A. M. Muller, N. S. Lewis and H. A. Atwater, “Photoluminescence-based measurements of the energy gap and diffusion length of Zn₃P₂”,Appl. Phys. Lett. 95, 112103 (2009).
2. C. Xiang, G. M. Kimball, R. L. Grimm, B. S. Brunschwig, H. A. Atwater, and N. S. Lewis, “820 mV open-circuit voltages from Cu₂O/CH₃CN junctions”, Energy Environ. Sci.,4, 1311-1318 (2011).