Improvement of amorphous silicon/crystalline silicon heterojunction solar cells by light-thermal processing

Abstract

It has been known that amorphous silicon/crystalline silicon heterojunction solar cells (HJT cells) can be improved by light-thermal processes. The present work aims to acquire a further understanding of the effect of illumination intensity and temperatures in a broader range and the roles of light and heat in the improvement. Naked HJT cells without grids were used for more reliable measurement of minority carrier lifetimes. Experiments of light-thermal processes of the naked HJT cells, with lighting from red LED arrays and heating/cooling from a thermal stage/air fan, have been carried out, with the illumination intensity ranging in 1–8 kW/m2 and cell temperature ranging in 160–220 °C. The higher illumination intensity results in longer lifetimes of the cells, the optimum cell temperature appeared to be around 180 °C, with the lower temperature needing a longer processing time to reach peak lifetime increment, while the higher temperature tends to degrade the passivation of the cell. The cell temperature of 180 °C was then chosen for the light-thermal processes of commercial HJT cells of ∼23.8% efficiency on a pilot line, with 12 kW/m2 red illumination. An average efficiency gain of 0.77%abs and a maximum gain of 0.87%abs have been achieved, with a processing time of 80 s. It is encouraging that the light intensity required is much lower than the previously reported level for similar improvement, indicating much lower power consumption for industrial application of the process. Through semi-shading experimentation of the light-thermal process, it is postulated that the effect of illumination does not rely on the direct effect of the injected photons, but possibly resulted from the effect of light-generated carriers.