Combining strong interface recombination with bandgap narrowing and short diffusion length in Cu2ZnSnS4 device modeling
In this work we establish a device model in SCAPS, incorporating bandgap narrowing, short minority carrier diffusion length and interface recombination. The model is based on a reference device with standard structure; sputtered Mo on soda lime glass, a reactively sputtered and annealed Cu2ZnSnS4 (C...
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| Published in: | Solar energy materials and solar cells 2016-01, Vol.144, p.364-370 |
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| Main Authors: | , , , , , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | In this work we establish a device model in SCAPS, incorporating bandgap narrowing, short minority carrier diffusion length and interface recombination. The model is based on a reference device with standard structure; sputtered Mo on soda lime glass, a reactively sputtered and annealed Cu2ZnSnS4 (CZTS) absorber layer, chemical bath deposited CdS and sputtered i-ZnO buffer layers, and front contact formed with sputtered ZnO:Al and an evaporated Ni/Al/Ni grid. The efficiency of the reference device is 6.7%. Model parameter values of the absorber layer are based on the analysis of temperature dependent current–voltage (J–V–T) measurements, capacitance–voltage (C–V) and drive-level capacitance profiling (DLCP) measurements, performed on the reference device, and on the comparison of simulated and measured quantum efficiency (QE) and current–voltage (J–V) performance. Additional parameters are taken from literature. The key elements, electron–hole pair generation and recombination in the absorber layer, are the main focus in this study. Reported values of the absorption coefficient of CZTS vary around one order of magnitude when comparing data from reflectance–transmission (R–T) measurements with ellipsometry measurements, and calculations. Therefore, a modified semi-empirical absorption coefficient, extracted from R–T and QE measurements, with the depletion width from CV and DLCP, is presented and used in this study. The dominating recombination path is evaluated with J–V–T analysis and the zero Kelvin activation energy (EA,0) is extracted from both temperature dependent open circuit voltage (VOC) and from modified Arrhenius plots. In each case,EA,0is found to be substantially smaller than the bandgap energy, even when considering bandgap narrowing due to disorder, which is an indication that the VOCdeficit observed in our CZTS device dominated by interface recombination. Finally, a complete device model is established, with J–V and QE simulations in good agreement with corresponding measurements, where the interface has the biggest impact on the VOCdeficit, but with clear contribution from bulk recombination, with minority carrier diffusion length 250nm, and from bandgap narrowing, giving a lower than nominal bandgap energy of 1.35eV.
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•Thorough J–V–T analysis reveals activation energy EA well under bandgap energy Eg.•Extracting the CZTS absorption coefficient tail, based on QE/CV measurements.•Presenting a device model with bandgap narrowin |
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| ISSN: | 0927-0248 1879-3398 |