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

• Implementation of high mobility and high conductivity ZnO:Al into cell stack
• The poly-Si stacks withstand even high-temperature treatments up to 1000 °C.
• The SiN diffusion barrier yields good absorber material quality.
• Reduction of the ZnO:Al layer thickness further improved the material quality.

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 °C for 20 h, rapid thermal annealing (RTA) at 1000 °C for 60 s and hydrogen passivation at 600 °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.