The impact of surface recombination on carrier density for SiHJ solar cells

Mechanical damage on solar cells (SCs) is a critical issue for advanced photovoltaic technologies. Cell cracking (due to handling or hail, for instance) can result in additional surface recombination (SR), which can lower cell efficiency. SR plays a role also in new designs (such as half cut cells), having a perimeter/area ratio larger than conventional cells. For these reasons, studying surface recombination in advanced PV technologies is of paramount importance. In this work we present a detailed analysis of the impact of SR on carrier recombination for c-Si/a-Si:H SCs. In particular, we developed numerical simulations to study the carrier density; the results showed an evident decrease in proximity of the borders (Fig. a). By varying surface recombination rate (S), diffusion length (L) and carrier lifetime (τ) we investigated the impact of the main parameters on device behavior (Fig. b, c, d). To evaluate issues related to recombination and conduction near the edges, a set of SiHJ cells was submitted to EBIC measurements. We performed a cut on an edge of one cell: the comparison of the EBIC decay suggested different dynamics in the recombination (Fig. e) related to slightly visible cracks acting as surface recombination edges. Furthermore, we induced a mechanical damage on the center of the SC; the resulting EBIC scan have shown an evident local damage which affected the recombination rate, as highlighted by the profile EBIC signal comparable to the one measured at the edge (Fig. f, g). In conclusion the data presented provided relevant information for understanding the recombination and transport dynamics near the edges of silicon heterojunction SCs. Moreover, we provide evidence that latent not-visible damage can cause effects leading to surface recombination and, therefore, lower cell efficiency.