Tunnel oxide passivated carrier-selective contacts based on ultra-thin SiO2 layers

Carrier-selective contacts are one of the key enabling technologies to approach very high conversion efficiencies close to the theoretical limit of silicon solar cells. The tunnel oxide passivated contact (TOPCon) approach is an alternative to classical heterojunction solar cells enabling efficiencies up to 24.4%. The tunnel oxide is a core element of this contact as it has to reduce the minority carrier recombination but simultaneously must not hamper the majority carrier flow. This paper focuses on ozone-based oxidation techniques, which can potentially be cost effective and industrially feasible methods for the realization of ultra-thin tunnel oxide layers as an alternative to the oxidation in nitric acid (HNO3) reference process. All investigated oxides were applied to the electron-selective contact (n-TOPCon) on planar and textured surfaces. It will be shown that both ozone based oxidation techniques (UV/O3 photo-oxidation and wet-chemical oxidation in ozonized DI-H2O) enable high implied open circuit voltage (iVOC) values exceeding 720 mV on planar and 710 mV on textured surfaces, respectively. Further oxide properties as stoichiometry and layer thickness were analyzed by means of X-ray photoelectron spectroscopy (XPS), spectral ellipsometry (SE) and transmission electron microscopy (TEM). In compliance with earlier results it was found that a minimum oxide layer thickness (approximately 1.3 nm) and a high amount of oxygen-rich sub oxide species are required to obtain a good surface passivation. Using such oxides, a wider range of temperatures can be used during the TOPCon annealing. Applying the ozone-based oxide layers to n-TOPCon solar cells resulted in a high VOC of up to 719 mV and a peak efficiency of 24.9%. Similar results were obtained with the HNO3 reference process (VOC=716 mV, η=24.8%).