Crack formation and Zn diffusion in high-temperature processed poly-Si/ZnO:Al stacks

In this paper, we investigated the feasibility of integrating aluminum doped zinc oxide (ZnO:Al) films as contacting layers into thin film polycrystalline silicon devices produced by solid phase crystallization at 600 degrees C for 20 h, rapid thermal annealing (RTA) at 1000 degrees C for 60 s and hydrogen passivation at 600 degrees C for 15 min. The RTA treatments were found to cause stress and crack formation in the poly-Si layer stack, which is due to thermal strain during high-temperature processing of the layer stacks. Furthermore, the diffusion of contaminants from the glass and especially from the ZnO:Al into the poly-Si is enhanced by RTA. The diffusion of Zn is accompanied by a degradation of the solar cell performance. Therefore, a SiN barrier was incorporated between the ZnO:Al layer and the poly-Si stack. The barrier is effectively preventing the diffusion of Zn impurities, which were measured by Secondary Ion Mass Spectrometry. Furthermore, optical microscopy images have shown that a decreasing thickness of the ZnO:Al layers results in lower stress and consequently in less crack formation in the layer stack, which can be explained by the interplay between layer tension and layer thickness in combination with different thermal expansion coefficients of the individual layers. In return, there is little diffusion of impurities for samples with a thin ZnO:Al layer, which significantly increases the electrical material quality of the poly-Si stack determined by the quasi-stationary open-circuit voltage method. (C) 2014 Elsevier B.V. All rights reserved.