Optimization of ZnO Front Electrodes for High-Efficiency Micromorph Thin-Film Si Solar Cells

The quest for increased performances in thin film silicon Micromorph tandem devices nowadays requires an increase of current density. This can be achieved with thin cells by combining both a robust cell design and efficient light management schemes. In this work we identify three key requirements for the transparent conductive oxide electrodes. Firstly, strong light scattering into large angles is needed on the entire useful wavelength range: a front electrode texture with large enough features is shown to grant a high total current (typically >26 mA/cm2 with a 2.4 μm thick absorber material) while sharp features are reported to allow for high top cell current (>13 mA/cm2) and reduced reflection at the ZnO/Si interface. Secondly, sufficiently smooth substrate features are needed to guarantee a high quality of the silicon active material, ensuring good and stable electrical properties (typically Voc around 1.4 V). Thirdly, conduction and transparency of electrodes must be cleverly balanced, requiring high TCO mobility (~50 cm2/V/s) to maintain the sheet resistance below 30 Omega/sq while keeping absorption as low as possible. Optimization of these three key requirements using ZnO electrodes allowed us to realize high efficiency Micromorph devices with 13.5% initial and 11.5% stabilized efficiency.