Heterojunction solar cells: a new insight in the intrinsic buffer layer concept

In the literature, the thin buffer layers, used to passivate the junction interface in amorphous silicon / crystalline silicon heterojunction solar cells, are often referred to as intrinsic amorphous layers. However, the optical measurements and high resolution TEM cross section observations show that the so-called i a-Si:H layers, grown on (100) crystalline silicon under the standard PECVD conditions used to deposit good quality amorphous layers on glass, can partially regrow epitaxially on the c-Si substrate. Moreover, when a pyramidal textured c-Si substrate is used, the epitaxial regrowth is much more pronounced compared with a flat c-Si substrate. In this paper, we demonstrate that a partial regrowth is detrimental for the device performance, leading to a degradation of the Voc of the cell with respect to the case where no buffer layer is used. A fully epitaxial buffer layer is shown to improve the interface passivation, and consequently the Voc of the device. Using flat c-Si (CZ grown, 1 ohm.cm) substrates and an amorphous emitter, a 605 mV Voc and a 13.5 % efficiency on 1cm2 cells are obtained. A new hypothesis about the structural nature of the intrinsic 'amorphous' buffer layer used in amorphous silicon / crystalline silicon heterojunction is proposed.