Advanced Optimization Strategies for Kesterite Solar Cells: About Pathways and Landscapes?

This was an Invited talk at the 4th Asia-Pacific Kesterite Workshop at the 31st PVSEC in Sydney 2021

Abstract:

The efficiency of kesterite solar cells has been barely rising over the past years. This is being attributed commonly to Sn-related deep defects in the absorber. In this presentation, I will discuss the influence of the elemental composition of the precursor and absorber and show that they can be tuned independently from each other. I will briefly explain the process developed in our labs, which allows for starting at various precursor ratios but eventually reaching a very similar targeted composition region at the end of the process. This is realized by an in-process composition shift during the high temperature annealing step under selenium atmosphere. This composition shift was previously reported only for the Cu/Sn ratio in the layer and was recently extended for additional Ge incorporation and tuning of the Sn/Ge ratio. We find that although the final composition ends up in a similar region, the starting precursor composition has a clear influence on the opto-electronic and structural properties of the kesterite absorber and on the resulting device properties. We believe that this type of process can be used to guide the formation reactions during the kesterite growth along a preferred formation pathway. Following the common agreement in the community that kesterite thin films are usually grown far away from equilibrium, it appears to be a reasonable assumption that the as-grown absorbers may contain residuals of the varied formation pathway and in-process composition shift even if the final composition range is always reached. These lead to noticeable device differences in VOC-deficit, Urbach tailing and an impact on the defect landscape which is probed by low temperature photoluminescence and Raman investigations. Further, it was found that the chosen composition pathway during the growth also influences the long-term stability of devices.