Spring 2013 IAB & Research Review
March 6 - 8
III-V Photovoltaics: A Vapor-Liquid-Solid Route Towards High Efficiency Non-Epitaxial Thin Film Cells
Semiconductor growth from a liquid phase, such as liquid phase epitaxy, vapor liquid solid growth and recrystallization, has been widely used in the past for many material systems and geometries, enabling electronic and optical devices such as photovoltaics, transistors, and lasers. Specifically, III-V semiconductors are often used for optical devices due to their large absorption coefficients and ideal bandgap. Additionally, III-V photovoltaics demonstrate the highest efficiencies for both single and multijunction devices. However, growth of high quality III-V semiconductors for photovoltaic applications is limited to metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) on lattice matched substrates. These processes are limited by high precursor and substrate cost, low materials utilization yield, and process scalability. Thus developing a growth technique that enables direct growth of III-V materials on non-epitaxial substrates is a key challenge. Here we extend the vapor liquid solid growth mode to enable growth of polycrystalline InP thin films on Mo foils. This process results in films with grain sizes of ~50-100 microns, mobilities up to ~500 cm^2/V-s, and minority carrier lifetimes of ~2 ns. Furthermore, under 1-sun equivalent illumination, a quasi-Fermi level splitting (Voc) of ~0.93V, is observed, close to similarly doped single crystalline InP wafers. Thus, the process, termed thin-film vapor-liquid-solid (TFVLS) growth, enables near single crystal optoelectronic quality InP (i) without the constraints of an epitaxial substrate, (ii) with lower-cost precursors and high materials utilization yields, and (iii) utilizing scalable growth technologies. While InP is used here as a model system, the TFVLS method is general and can be applied to other material systems.