Abstract
Earth-abundant Cu2ZnSn(S,Se)4 (CZTSSe) thin film photovoltaic absorber layers are fabricated by annealing Cu2ZnSnS4 (CZTS) nanoparticle thin films in a selenium rich atmosphere. Systematic variation of the selenization time (5, 10, 20 and 40min) and temperature (450, 500, 550 and 600°C) provides insight into the kinetics of the selenization process and in particular recrystallization and grain growth. Se–S anion exchange is found to follow Avrami׳s model in which the CZTS selenization is controlled by an irregular one-dimensional process limited by metal cation re-ordering and grain boundary migration. CZTSSe grain growth is observed to follow a normal relation with a grain growth exponent close to the ideal case of equiaxed grains and the grain boundary migration energy is calculated to be 85.38kJ/mol. These selenization variables have a fundamental influence on the quality of the resulting CZTSSe thin film and consequently the device performance. A peak device solar energy conversion efficiency of 5.4% was obtained for selenization at 500°C for 20min. The device efficiency was found to be highly sensitive to these variables and it is critical to obtain an appropriate balance between grain growth and thin film quality.
Highlights
With the advantage of outstanding optoelectronic properties and earth-abundant constituents, Cu2ZnSn(S,Se)4 (CZTSSe) is a promising alternative thin film photovoltaic (PV) material experiencing rapid progress in recent years [1,2,3,4,5,6]
In order to optimize this part of the fabrication process it is important to understand the grain growth kinetics and in this work we report experiments in which the selenization conditions are systematically controlled
The results indicate that the PV absorber layer morphology and composition are strongly influenced by the selenization conditions and play important roles in determining the performance of solar cells made from these absorbers
Summary
With the advantage of outstanding optoelectronic properties and earth-abundant constituents, Cu2ZnSn(S,Se) (CZTSSe) is a promising alternative thin film photovoltaic (PV) material experiencing rapid progress in recent years [1,2,3,4,5,6]. Cu2ZnSnS4 (CZTS) nanoparticle inks and subsequent selenization offers a nontoxic route to fabricate high quality PV absorber layers and can yield cell efficiencies as high as 9.0% [8]. We have previously shown that by tuning the chemical reaction conditions, CZTS nanoparticle properties such as crystal structure and bandgap energy can be controlled and that the performance of thin film solar cells fabricated using this technique depends critically on the nanoparticle inks [9,10]. The results indicate that the PV absorber layer morphology and composition are strongly influenced by the selenization conditions and play important roles in determining the performance of solar cells made from these absorbers
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