Optimization of front SiNx/ITO stacks for high-efficiency two-side contacted c-Si solar cells with co-annealed front and rear passivating contacts

In this contribution, we present an electron selective passivating contact metallised with a low temperature process to target front side applications in crystalline silicon (c-Si) solar cells. In addition to an interfacial silicon oxide (SiOx) and an in-situ phosphorous doped micro-crystalline silicon (μc-Si(n)) layer, it comprises an ultra-thin indium tin oxide (ITO) layer of 15 nm for lateral conductivity and a hydrogen rich silicon nitride (SiNx:H) layer which serves as hydrogen (H) reservoir and as anti-reflection coating. We use one single thermal treatment for 30 min at 350 °C to sinter the screen-printed paste, to recover sputtering damage induced during ITO deposition, and to diffuse hydrogen from the SiNx:H layer towards the c-Si/SiOx interface where it passivates interfacial defects. Applied to symmetrically processed textured samples, we find implied open-circuit voltage (iVOC) > 728 mV for optimal ITO thickness of 15 nm and annealing temperatures of 350 °C. The developed stack was applied on the front textured side of co-annealed (800 °C) p-type c-Si solar cells in combination with a tunnel oxide hole selective passivating contact on the rear side. We demonstrate solar cells with fill factor (FF) up to 81.9% and an open-circuit voltage (VOC) up to 719 mV. With a short-circuit current density (JSC) of 38.6 mA/cm2, we obtain a final cell efficiency to 22.8%. We find that the annealing of the SiNx:H/ITO stack strongly increases the ITO free carrier density penalizing the solar cell spectral response at high wavelengths.