Interface Engineering by Intermediate Hydrogen Plasma Treatment Using Dc-PECVD for Silicon Heterojunction Solar Cells
Anishkumar Soman, Ujjwal Das, Steven Hegedus
Abstract
The high open-circuit voltage ( V OC ) in silicon heterojunction solar cells is attributed to the superior amorphous–crystalline silicon interface passivation. In this work, we investigated a method called intermediate hydrogen plasma treatment (I-HPT) in which the intrinsic amorphous silicon is exposed to hydrogen plasma in between the deposition using direct current (d.c.) plasma-enhanced chemical vapor deposition. This method can not only enhance the surface passivation but also find industrial application due to its ease of integration in the production line. Fourier transform infrared spectroscopy reveals that I-HPT makes the bulk of the amorphous matrix more disordered. We anticipate that the improvement in passivation comes from the diffusion of hydrogen from the bulk to the interface and shifting of the film closer to the “amorphous-to-microcrystalline” transition regime. Our optimized I-HPT method can obtain an implied V OC of 746 mV without annealing, which corresponds to a low surface recombination velocity of 3.2 cm/s. Therefore, we propose the I-HPT method as an alternative to high-temperature annealing which can reduce a fabrication step and processing time. The I-HPT films characterized by Raman spectroscopy do not show any hydrogen-induced crystallization. We have also demonstrated the proof of concept by applying I-HPT to a silicon heterojunction solar cell which shows ∼15 mV increase in V OC in the device and an absolute increase in efficiency by 0.3% as compared to a cell with no HPT.