Silicon heterojunction solar cell: a new buffer Layer concept with low-temperature epitaxial silicon

Amorphous silicon/crystalline silicon heterojunction solar cells, deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique, have been fabricated using different technologies to passivate defects at the heterointerface: without treatment, the insertion of a thin intrinsic amorphous layer or that of a thin intrinsic epitaxial layer. The open circuit voltage of heterojunction solar cells fabricated including an intrinsic amorphous buffer layer is strangely lower than in devices with no buffer layer. The structure of the amorphous buffer layer is investigated by high resolution transmission electron microscope observations. As an alternative to amorphous silicon, the insertion of a fully epitaxial silicon layer, deposited at low temperature with conventional PECVD technique in a hydrogen-silane gas mixture, was tested. Using the amorphous silicon/crystalline silicon (p a-Si/i epi-Si/n c-Si) heterojunction structure in solar cells, a 13.5% efficiency and a 605-mV open circuit voltage were achieved on flat Czochralski silicon substrates. These results demonstrate that epitaxial silicon can be successfully used to passivate interface defects, allowing for an open circuit voltage gain of more than 50 mV compared to cells with no buffer layer. In this paper, the actual structure of the amorphous silicon buffer layer used in heterojunction solar cells is discussed. We make the hypothesis that this buffer layer, commonly considered amorphous, is actually epitaxial.