Photovoltaic (PV) cells made from III-V semiconductor materials have the highest demonstrated optical-to-electrical conversion efficiency (around 40%); however, the epitaxial-growth process used to fabricate these cells is expensive and slow, formerly limiting them to space-based and concentrating PV applications. But a new vapor-liquid-solid (VLS) growth process from University of California, Berkeley scientists makes it possible to grow polycrystalline indium phosphide (InP) thin films on molybdenum (Mo) substrates, clearing a path towards the fabrication of large-area, ultra-efficient PV cells for terrestrial applications.
The growth process begins with deposition of indium films (with tunable thickness from 0.2 to 2 μm) on 25-μm-thick Mo foils via either electron-beam evaporation or electroplating, followed by e-beam evaporation of a 50-nm-thick silicon oxide (SiOx) cap. This stack is then heated in hydrogen to a temperature that melts the indium, allowing introduction of a phosphorous vapor that diffuses into the indium and, upon cooling, results in the precipitation of solid InP crystals. Besides planar films, textured films with improved carrier collection can be fabricated by introducing 1-μm-diameter silica beads, for example. After etching away the SiOx cap, luminescence and photogenerated current levels were found to be comparable to or better than copper indium gallium selenide (CIGS) thin-film PV materials. Contact Ali Javey at [email protected].
