Heterojunction solar cells on multi- crystalline silicon: surface treatments

In this work we present the results we obtained by processing 1 Ωcm p and n-type, multi-crystalline silicon wafers when a double hydrogenated amorphous/crystalline silicon heterojunction is applied to sandwich the substrate, at the sunward surface to form the emitter and at the rear side to act as back surface field. We have investigated the role of the multi-crystalline silicon surface conditioning, on the base of illuminated and dark current-voltage characteristic, quantum efficiency measurements, and overall photovoltaic solar cell performance. In particular, a comparison between an acidic isotexturing and a chemical polishing treatment is presented and discussed in detail. We have found that the morphology of the multi-crystalline silicon surface plays a tough role in the a-Si:H properties when used as surface passivating layer. The commercial acidic surface treatment, commonly used in diffused junction mc-Si solar cell fabrication, requires a level of passivation which is not easily achievable using the very thin a-Si:H film, needed in heterojunction technology. The deposition of a uniform thickness over a whole textured silicon surface of a thin amorphous layers still requires further investigation and improvement. Nevertheless the high V_oc obtained on p-type doped mc-Si (625 mV) remarks the effectiveness of a-Si:H/mc-Si technology in achieving high photovoltaic efficiency using low thermal budget manufacturing process. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)