Calculation of band offsets in CIGS-based solar cells
Abstract
This project aims to calculate the band offsets between the alpha (chalcopyrite) and beta (ordered defect compound, or ODC) phases of CIGS (nominally Cu(In,Ga)Se2) and related solar absorber materials using density functional theory (VASP package). Band offsets quantify the difference in the VBM and CBM positions at the interface of two materials. They are important for solar cells because they govern photocarrier transport over the p-n junction. In CIGS-based solar cells, a thin Cu-poor layer of ODC often forms naturally at the interface between the main chalcopyrite CIGS absorber and the CdS buffer layer. The bare CIGS/CdS interface has a spike-type conduction band offset of 0.3 eV, at the edge of the optimum range of 0.1-0.3 eV (see doi.org/10.1016/j.solmat.2015.09.048). Earlier calculations indicate that the presence of the ODC layer increases the spike height because it has the CBM level below that of the chalcopyrite CIGS (see doi.org/10.1063/1.5140736). However, this conclusion is obtained using an unstable (ie unrealistic) interface model, with the two materials stacked along the [001] crystallographic axis. In our earlier projects, while searching for stable ODC structures (see DOI: 10.1039/D2FD00105E), we generated several more stable interface structures than previously known. These structures have band gaps in better agreement with experiments for different CIGS-inspired materials (I-III-VI2, where I=Cu,Ag; III=Al,Ga,In, and VI=S,Se,Te). They are also likely to have different interfacial dipole moments and thus critically different conduction band offsets. In this project, we intend to determine the offsets for different I-III-VI2 using the recently discovered interface models that are more representative of the absorber materials grown in actuality. The same methodology as in the previous work (see doi.org/10.1063/1.5140736) will be applied, to facilitate the comparison. The results will be used to understand the characteristics of CIGS solar cells fabricated in our lab (see doi.org/10.1002/solr.202301018, for example) and thereby modify the composition and/or processing conditions for more efficient photovoltaic energy harvesting.
