European materials research society spring meeting

The European materials research society (E-MRS) spring meeting will hold from May 27 to 31 in Nice (France). Angela Fioretti will have a talk on "Low-temperature GaN on Si as a transparent, carrier-selective contact for heterojunction solar cells".
Abstract:
Carrier-selective, passivating contacts have allowed silicon heterojunction (SHJ) cells to reach record-breaking efficiencies >26% when using all-back-contacted designs. However, classical SHJ cell efficiency has been limited to 25.1% due in part to parasitic absorption losses up to 3 mA/cm² in the a-Si layers. Materials with greater transparency could reduce this current loss and increase efficiency while using a simpler design. Gallium nitride (GaN), with a bandgap of 3.4 eV and an advantageous band alignment with crystalline silicon, could be applied as a transparent electron-selective layer. One obstacle to this application is that GaN is typically grown above 800°C; too hot for the SHJ thermal budget. Here, we report on PECVD GaN layers grown at <300°C on silicon. The influence of gas ratio and substrate pretreatment on film crystallinity was investigated by Raman spectroscopy and X-ray diffraction. An over-pressure of nitrogen was required to obtain the GaN phase, and either pre-nitridation or oxidation of the silicon surface led to enhanced crystallinity compared to growth on bare silicon. Full SHJ cells using GaN as electron-selective contact were made, with promising open-circuit voltages >500 mV. Still, low conductivity of the as-yet undoped GaN layers induced series resistance (R_s) losses that limited fill factor and cell efficiency. To address this, we will report ongoing efforts to reduce R_s in GaN-contacted cells via doping of the nitride layer.