Large open-circuit voltage improvement by rapid thermal annealing of evaporated solid-phase-crystallized thin-film silicon solar cells on glass

In this letter, we investigate the impact of rapid thermal annealing (RTA) on thin-film polycrystalline silicon (pc-Si) solar cells on glass made by evaporation of amorphous silicon (a-Si) and subsequent solid-phase crystallization (SPC). These devices have the potential to deliver low-cost photovoltaic electricity and are named EVA cells (SPC of EVAporated a-Si). The RTA is used to perform a high-temperature (>700°C) process for point defect annealing and dopant activation. RTA processes have predominantly been developed for wafer-based devices yet also have great potential for low-temperature devices such as thin-film pc-Si on glass solar cells. Parameter variations are performed on EVA solar cells to determine optimum values for point defect removal and dopant activation while minimizing dopant diffusion causing junction smearing. The 1-Sun open-circuit voltage, Voc, of the as-crystallized pc-Si devices is rather modest (135mV). However, after RTA and subsequent hydrogen passivation in a rf PECVD plasma, a Voc of 454mV is realized, representing a large improvement by a factor of 3.4. With an optimized passivation and dopant profile, a Voc greater than 500mV is well within the reach of the EVA technology.