This study presents a detailed modelling and numerical simulation of Cu (In, Ga) Se2 (CIGS) solar cells using SCAPS-1D software. The research primarily focuses on exploring the effects of variations in the absorber layer’s bandgap and donor concentrations. Additionally, the study examines the influence of high defect state density on the overall device performance.
In order to improve the VOC and achieve optimal band alignment between the buffer and absorber layers, a sizable bandgap in the buffer layer is required. Thus, the bandgap of absorber has been varying from (0.9 to 1.5) eV. The conversion efficiency (PCE) rises to a higher level as the bandgap becomes wider, but once it reaches a certain point, PEC starts to fall regardless the extent that the bandgap increases. This may be linked to the influence that variations in bandgap and electron affinity had on the cell, which then led to fluctuations in the cell’s performance.
Another parameter has been considering is the carrier concentration. Low concentrations reduce carrier collection at the front contact, lowering JSC. However, a buffer layer with more carriers in the same CBO region might maintain a high JSC value. In effective solar cells, boosting JSC necessitates increasing buffer/absorber interface carrier concentration and bandgap.
The existence of defect states, whether occurring at the interface between CdS and CIGS or inside the absorber layer, consistently affects the overall efficiency of the solar cell. Defects may cause charge paths to get trapped or recombine, which would lead to their loss and failure to contribute to the current transport towards the external load. In this work, a significant defect value was used to emphasize the importance of both bandgap and carrier concentration by explaining the fall in solar cell performance values under synchronous conditions.
Quantum efficiency (QE%) which refers to the relationship between the number of incident charge carriers and the amount of current flowing into an external circuit, also has been obtained. When the objective is to set a standard for maximum efficiency, it’s crucial to maximize the QE% of the CIGS solar cells as much as possible. QE% was evaluated at a doping level of ND=1013 and ND=1019 cm-3 in order to probe its highest values. The results showed that, the highest QE% at ND =1013 is belong to CBO=0, where at the case of ND =1019 the highest QE% when CBO > 0, between (0.3 to 0.5). This has been supported by an enhance in carrier concentration, photo-generated minority carrier current density, and depletion layer width.
Overall, it can be concluded that, the specific characteristics are crucial for optimizing CIGS solar cells. The results reveal that enhancing both the bandgap and donor concentration (ND) significantly improves device efficiency, as it can be achievable to design an ultrathin solar cell that exhibits a power conversion efficiency of 17.88%. This enhancement could be due to higher quasi-Fermi energy-level splitting and an appropriate band offset between the defect layer and the buffer layer, which ultimately resulted in an increase in VOC, FF, and overall conversion efficiency. The research underscores the critical role of optical and electrical properties in the growth and performance optimization of CIGS solar cells.
DOI: https://doi.org/10.1557/s43580-024-00912-2
Author: Tofan Agung Eka Prasetya, S.Kep., M.KKK., Ph.D.
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