Si<sub>3</sub>AlP: A New Promising Material for Solar Cell Absorber

First-principles calculations were performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si<sub>2</sub>)<sub>0.6</sub>(AlP)<sub>0.4</sub> alloy (Watkins, T.; et al. <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 16212). We found that the most stable structure of Si<sub>3</sub>AlP is a superlattice along the ⟨111⟩ direction with separated AlP and Si layers, which has a similar optical absorption spectrum to silicon. The ordered <i>C</i>1<i>c</i>1-Si<sub>3</sub>AlP is found to be the most stable one among all structures with a basic unit of one P atom surrounded by three Si atoms and one Al atom, in agreement with experimental suggestions. We predict that <i>C</i>1<i>c</i>1-Si<sub>3</sub>AlP has good optical properties, i.e., it has a larger fundamental band gap and a smaller direct band gap than Si; thus, it has much higher absorption in the visible light region. The calculated properties of Si<sub>3</sub>AlP suggest that it is a promising candidate for improving the performance of the existing Si-based solar cells. The understanding on the stability and band structure engineering obtained in this study is general and can be applied for future study of other nonisovalent and lattice-matched semiconductor alloys.