Increasing the efficiency of silicon heterojunction solar cells and modules by light soaking

•We unravel the light-induced improvement in Si heterojunction cells and modules.•The performance increase is due to improved passivation of the hetero-interface.•This phenomenon requires the presence of doped a-Si:H films.•The performance increase can be observed under as low as 0.02-sun light intensity.•The mechanism likely differs from the known annealing-induced passivation increase.

Silicon heterojunction solar cells use crystalline silicon (c-Si) wafers as optical absorbers and employ bilayers of doped/intrinsic hydrogenated amorphous silicon (a-Si:H) to form passivating contacts. Recently, we demonstrated that such solar cells increase their operating voltages and thus their conversion efficiencies during light exposure. We found that this performance increase is due to improved passivation of the a-Si:H/c-Si interface and is induced by injected charge carriers (either by light soaking or forward-voltage biasing of the device). Here, we discuss this counterintuitive behavior and establish that: (i) the performance increase is observed in solar cells as well as modules; (ii) this phenomenon requires the presence of doped a-Si:H films, but is independent from whether light is incident from the a-Si:H(p) or the a-Si:H(n) side; (iii) UV and blue photons do not play a role in this effect; (iv) the performance increase can be observed under illumination intensities as low as 20 W m−1 (0.02-sun) and appears to be almost identical in strength when under 1-sun (1000 W m−1); (v) the underlying physical mechanism likely differs from annealing-induced surface passivation.