Energy level alignment of $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){(\mathrm{S},\mathrm{Se})}_2$ absorber compounds with ${\mathrm{In}}_2{\mathrm{S}}_3$, ${\mathrm{NaIn}}_5{\mathrm{S}}_8$, and ${\mathrm{CuIn}}_5{\mathrm{S}}_8$ Cd-free buffer materials

Motivated by environmental reasons, ${\mathrm{In}}_2{\mathrm{S}}_3$ is a promising candidate for a Cd-free buffer layer in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){(\mathrm{S},\mathrm{Se})}_2$ (CIGSSe)-based thin-film solar cells. For an impactful optimization of the ${\mathrm{In}}_2{\mathrm{S}}_3$ alternative buffer layer, however, a comprehensive knowledge of its electronic properties across the absorber-buffer interface is of foremost importance. In this respect, finding a favorable band offset between the absorber and the buffer layers can effectively reduce the carrier recombination at the interface and improve open-circuit voltage and fill factor, leading to higher conversion efficiencies. In this study, we investigate the band alignment between the most common CIGSSe-based absorber compounds and ${\mathrm{In}}_2{\mathrm{S}}_3$. Furthermore, we consider two chemically modified indium sulfide layers, ${\mathrm{NaIn}}_5{\mathrm{S}}_8$ and ${\mathrm{CuIn}}_5{\mathrm{S}}_8$, and we discuss how the formation of these secondary phases influences band discontinuity across the interface. Our analysis is based on density functional theory calculations using hybrid functionals. The results suggest that Ga-based absorbers form a destructive clifflike conduction-band offset (CBO) with both pure and chemically modified buffer systems. For In-based absorbers, however, if the absorber layer is Cu-poor at the surface, a modest favorable spikelike CBO arises with ${\mathrm{NaIn}}_5{\mathrm{S}}_8$ and ${\mathrm{CuIn}}_5{\mathrm{S}}_8$.

Figure 1

Unit cells of (a) defect spinel-like ${\mathrm{In}}_2{\mathrm{S}}_3$, (b) tetragonal $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){(\mathrm{S},\mathrm{Se})}_2$, (c) cubic $\text{Cu}{(\text{In},\text{Ga})}_5{\text{Se}}_8$ ordered vacancy compounds, and (d) cubic $(\text{Na},\text{Cu}){\text{In}}_5{\text{S}}_8$. Red spheres represent Na/Cu, blue spheres represent In/Ga, and yellow spheres represent S/Se atoms.

Figure 2

Electronic band structure of ${\mathrm{CuIn}}_5{\mathrm{S}}_8$ (left), ${\mathrm{In}}_2{\mathrm{S}}_3$ (middle), and ${\mathrm{NaIn}}_5{\mathrm{S}}_8$ (right). Occupied and unoccupied levels are shown as black and red lines, respectively. The zero of energy is set to the bulk VBM of each structure.

Figure 3

Calculated band lineups of bulk materials. For each material, the lower and upper numbers indicate, respectively, the position of the VBM and the CBM with respect to ${\mathrm{CuInSe}}_2$ in eV. The VBM of ${\mathrm{CuInSe}}_2$ lies at a value in energy scale that corresponds to the ionization potential of the (110) surface of ${\mathrm{CuInSe}}_2$ (5.58 eV [37]).

Figure 4

Electronic total and local density of states (DOS) of absorber (top and middle panels) and buffer (bottom panel) compounds. The energy scales are aligned with respect to the VBM of ${\mathrm{CuInSe}}_2$. The gray shaded area shows the total DOS of each compound.