Progress on the intrinsic a-Si:H films for interface passivation of silicon heterojunction solar cells: A review

Amorphous/crystalline silicon heterojunction (SHJ) solar cells are well known for their inherent high open-circuit voltage (VOC) potential, much better temperature coefficient, higher bifaciality, a simpler fabrication process, and low thermal-budget manufacturing. Near-perfect passivation of the silicon surfaces by a few nanometers thin layers of undoped hydrogenated amorphous silicon (i-a-Si:H), forms the basis of SHJ solar cells resulting in VOC well above 720 mV. The front emitter and the back surface field are formed by low-temperature deposition of slightly thicker doped a-Si:H(p or n) layers, eliminating the necessity of high-temperature dopant-diffusion and complexity of local metal contact formation to the silicon wafer. As the photovoltaic electricity is set for terawatt-scale deployment, new investors are now considering high-volume manufacturing of the high-efficiency SHJ solar cells, which currently offers the highest module efficiency among the c-Si PV technology. It should be noted that despite the seemingly simple cell structure and lean processing, it has been a challenging task for the researchers to realise the formation of exceptionally well-passivated a-Si:H/c-Si interfaces and SHJ cells with high VOC and efficiency. This paper presents a review of the progress in developing i-a-Si:H films, from “transition-zone” to “underdense” layers, and bilayers for interface passivation of SHJ solar cells.