Characterization of three-dimensional structures in silicon solar cells by spatially-resolved illuminated lock-in thermography

The continuous increase of solar cell efficiencies is related to a large extent to a number of device structure improvements such as the introduction of selective emitters, selective front surface fields (FSF) or local back surface fields (BSF). These structures allow even more complex solar cell designs like passivated emitter and rear contacts/locally diffused (PERC/PERL), metal/emitter wrap through (MWT/EWT) or interdigitated back contact (IBC) cells. All these designs exhibit three-dimensional (3D) doping profiles that are mostly processed with diffusion and are thus not directly visible. In this paper we present a method to characterize these non-visible emitters, FSF and BSF structures in a two-dimensional (2D) camera based spatially-resolved technique, which is based on the illuminated lock-in thermography (ILIT) and free carrier absorption/emission (FCA/FCE). While this technique, also referred to as carrier density imaging (CDI) [1] or infrared lifetime mapping (ILM) [2] and usually used to investigate the wafer material quality, has already been used to investigate phosphorus diffused selective emitter structures [5],[6], here we present to the best of our knowledge the first results of aluminum (Al) alloyed emitters, selective FSF and local Al-BSF structures measured with this method. All spatially-resolved structures were matched with their doping profiles measured with electrochemical capacitance voltage (ECV) measurements. The approach appears very suitable for an in-situ process control of structured emitters, FSFs and BSFs. In principle, patterns with dimensions down to 5 μm can be resolved due to the detected wavelengths (λ) of 3 - 5 μm. We present clearly visible structures with dimensions down to 120 μm on monocrystalline Czochralski silicon (Cz-Si) with a lateral resolution of 30 μm per pixel due to our camera setup. This is suitable for an automatic alignment during contact formation and offers the possib- lity of quantitative measurements.