Impact of buffer layers on the performance of graded CIGS solar cells: a numerical approach

We present a numerical simulation study based on tri-layer double-graded (notch structure) CIGS solar cell. As the In to Ga ratio is an important component for tuning the bandgap, it is important to vary the bandgap and take the advantage in terms of V-oc and J(sc). In present simulation, initially, we optimized the CIGS dual absorber layer of bandgap 1.42 eV on top, 1.14 eV in the middle and 1.20 eV in the bottom to form a Notch-like structure of the absorber. Conventional buffer layer of CdS is closely optimum for CIGS with smaller bandgap, but shows poor alignment for higher Ga ratio CIGS absorber with higher bandgap. Ga to In ratio change can lead to two different types of conduction band offsets (CBO), positive CBO leads to spike and negative offset results in cliff. The optimum spike offset of 0.3 eV is achievable in case of higher bandgap CIGS absorbers using Zn-based buffer layers like Zn1-zMgZO and ZnS. Zn1-zMgzO is the most promising of the three buffer layers and shows the efficiency up to 25.48% under AM 1.5 G illumination.

Automated summary

This study presents a numerical simulation based on tri-layer double-graded CIGS solar cell, focusing on optimizing bandgap and buffer layer to achieve higher efficiency, with Zn1-zMgZO showing promising results.