Selective Annealing of Transparent Conductive Oxides on Silicon Heterojunction Solar Cells Enabled by UV Laser Processing

In this study, we investigate the influence of the wavelength on the laser annealing of transparent conductive oxides (TCO). The main goal is to confine energy deposition to the film thickness of 70 nm to 100 nm, aiming to mitigate degradation of the underlying silicon heterojunction (SHJ) solar cell surface passivation. Addressing industrialization aspects, the experiment encompasses planar and textured surfaces, three different TCOs with distinct structural and optical properties (polycrystalline Sn-doped In2O3, polycrystalline Al-doped ZnO, amorphous F-doped In2O3) and laser wavelengths ranging from 248 nm over 308 nm to 343 nm, the latter being accesible with high-throughput systems. A comparable sheet resistance (Rsh) reduction is achieved for each TCO on planar substrates, despite absorptance ranging from 33% to 75% across the studied wavelength range. Specific parameters yield Rsh lower than the hotplate reference, while the implied open-circuit voltage (iVOC) of the SHJ solar cell remains unaffected, indicating no induced recombination-active defects in the surface passivation. Although textured surfaces limit the parameter space for laser annealing, comparably low Rsh values to the hotplate processes without iVOC reduction is achieved. Our study shows that UV ns laser annealing of TCOs on temperature-sensitive SHJ solar cells is feasible on textured surfaces and with high-throughput, low-cost laser systems, demonstrating the potential for industrial solar cell production.