Abstract
The band structure characteristics of a copper indium gallium sulfur selenide (Cu(In1–xGax)SeS, CIGS) solar cell incorporating a cadmium-free zinc sulfide (ZnS) buffer layer were investigated using technology computer-aided design simulations. Considering the optical/electrical properties that depend on the Ga content, we numerically demonstrated that the front gradient bandgap enhanced the electron movement over the band-offset of the ZnS interface barrier, and the back gradient bandgap generated a back side field, improving electron transport in the CIGS layer; in addition, the short circuit current density (JSC) and open circuit voltage (VOC) improved. The simulation demonstrated that the conversion efficiency of a double graded bandgap cell is higher than with uniform or normal/reverse gradient cells, and VOC strongly correlated with the average bandgap in the space charge region (SCR) of CIGS. After selecting VOC from the SCR, we optimized the band structure of the CIGS cell with a Cd-free ZnS buffer by evaluating JSC and the fill factor. We demonstrated that the cell efficiency of the fabricated cell was more than 15%, which agrees well with the simulated results. Our numerical method can be used to design high-conversion efficiency CIGS cells with a gradient band structure and Cd-free buffer layer.
Highlights
Copper indium gallium sulfur selenide (Cu(In1−x Gax )SeS, CIGS) solar cells have been widely studied because of their various advantages
Using a technology computer-aided design simulation (TCAD), we investigated the effect of the gradient bandgap near the interface of the zinc sulfide (ZnS) buffer layer on the conversion efficiency parameters of the CIGS cells, which has not been fully proven yet
In order to apply the Ga gradient effect to the device simulation, we used the results of Wei et al.; bandgap gradient effect of the CIGS layer by the variation of the Ga content mainly appears as an the bandgap gradient effect of the CIGS layer by the variation of the Ga content mainly appears as an increase in the conduction band derived from first-principles bandgap theory [13]
Summary
Copper indium gallium sulfur selenide (Cu(In1−x Gax )SeS, CIGS) solar cells have been widely studied because of their various advantages. We demonstrate a numerical procedure to design a gradient band structure for CIGS solar cells with a Cd-free ZnS buffer layer and demonstrate bandgap-optimization to fabricate the CIGS solar cells using a two-stage process. It is well known in CIGS solar cells the band alignment between absorber layer and buffer layer is so important [8]. Using a technology computer-aided design simulation (TCAD), we investigated the effect of the gradient bandgap near the interface of the ZnS buffer layer on the conversion efficiency parameters of the CIGS cells, which has not been fully proven yet. This work will help us understand the engineering of the bandgap profile and management of the defect density related to the Ga/(Ga + In) ratio, which is key to achieving high efficiencies
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