Influence of composition shift and reaction pathway on kesterite device properties and long-term stability

Abstract

The preparation process for CZTSe absorbers at UOL includes an independent tuning of the starting and ending composition of the layer in terms of Cu/Sn ratio. This is realized by a composition shift during the high temperature step of the annealing/selenization by Sn incorporation / Sn loss from the layer. In this work, we present a correlation between the precursor/absorber composition and the resulting device parameters.

In brief, different starting compositions have a fundamental influence on the alloy structure of the precursor and hence can influence the dominating reaction pathway. For the Cu rich case the dominating alloy is Cu₃Sn which moves towards Cu₆Sn₅ + Cu₃Sn for lower Cu/Sn ratios. For high added Sn amounts the fraction of non-alloyed elemental Sn in the precursor layer rises. These differences can influence the reaction pathway and the resulting defect landscape, leading to a significant influence on final device properties.

A series of precursors with different Cu/Sn ratios were processed in identical annealing conditions ending up with an absorber composition in the usual composition region, favourable for high efficiency devices. The resulting device properties such as PCE, E_U, V_OC-def and long-term device stability (after 1000h accelerated aging at 85°C) were found to correlate with the starting composition. Preliminary results from low-temperature PL analysis indicate an influence on activation energies of defects.