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 20Wm (0.02-sun) and appears to be almost identical in strength when under 1-sun (1000Wm); (v) the underlying physical mechanism likely differs from annealing-induced surface passivation.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The authors are grateful to Yoshimi Watabe and Fumiharu Ishimura for the sample preparation of single cell modules. The authors are also grateful to Mathieu Boccard, Gabriel Christmann, Sylvain Nicolay, Philipp Löper, Jan Haschke, Raphaël Monnard, Jean Cattin, Andrea Tomasi, Gizem Nogay, Andrea Ingenito, Philipp Wyss, Josua Stuckelberger, Silvia Martin de Nicolas, Jonathan Champliaud, and Christophe Allebé for fruitful discussions. The simulations were done with the software package ASA of Delft University of Technology. Financial support from the Swiss Federal Office of Energy, EU FP7 program (CHETAAH Project, Contract No. 609788), and King Abdullah University of Science and Technology (KAUST) is acknowledged.