Composition dependence of defect energies and band alignments in the Cu(In1−xGax)(Se1−ySy)2 alloy system

The composition dependence of defect energies in polycrystalline ${\mathrm{Cu(In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{)(}{\mathrm{Se}}_{\text{1−y}}{\mathrm{S}}_y{)}_2$ chalcopyrite thin films is investigated by admittance spectroscopy of ZnO/CdS/chalcopyrite heterojunctions. We determine the band alignments within the polycrystalline ${\mathrm{Cu(In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{)(}{\mathrm{Se}}_{\text{1−y}}{\mathrm{S}}_y{)}_2$ semiconductor system using the energy position of the dominant acceptor state as a reference level. Upon alloying ${\mathrm{CuInSe}}_2$ with S the activation energy of the acceptor increases from 300 meV to approximately 380 meV in ${\mathrm{CuIn(Se}}_{\text{0.4}}{\mathrm{S}}_{\text{0.6}}{)}_2.$ A similar result holds when using ${\mathrm{Cu(In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{)(}{\mathrm{Se}}_{\text{1−y}}{\mathrm{S}}_y{)}_2$ with $\text{x≈0.3.}$ In contrast, the acceptor activation energy remains essentially unchanged in the ${\mathrm{Cu(In}}_{\text{1−x}}{\mathrm{Ga}}_x){\mathrm{Se}}_2$ alloy system over the whole composition range $\text{0⩽x⩽1.}$ Taking the acceptor energy as reference, we find a valence band offset ${\mathrm{\Delta E}}_V\text{=−0.23}$ eV between $\mathrm{CuIn}{\mathrm{Se}}_2$ and ${\mathrm{CuInS}}_2.$ The same valence band offset is found between ${\mathrm{Cu(In}}_{\text{0.7}}{\mathrm{Ga}}_{\text{0.3}}){\mathrm{Se}}_2$ and ${\mathrm{Cu(In}}_{\text{0.7}}{\mathrm{Ga}}_{\text{0.3}}){\mathrm{S}}_2.$ In contrast, the combination ${\mathrm{CuInSe}}_2/{\mathrm{CuGaSe}}_2$ displays ${\mathrm{\Delta E}}_V$ below 0.04 eV. Our results indicate that a bulk reference level exists in the ${\mathrm{Cu(In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{)(}{\mathrm{Se}}_{\text{1−y}}{\mathrm{S}}_y{)}_2$ semiconductors which sets the band structure on a common energy scale, thus establishing the natural band lineups within the alloy system automatically. This conclusion is sustained by our finding that the position of the Fermi level at the CdS/chalcopyrite interface exhibits a constant energy distance to the acceptor level. The concentration of bulk acceptors is in addition correlated to the open circuit voltage losses of heterojunction solar cells.