ALD zinc tin oxide buffers for chalcopyrite solar cells: Electrical barriers and conduction band cliffs

Sulfide chalcopyrite, Cu(In,Ga)S2, having a wide bandgap (>1.5 eV), favorable optoelectronic properties, and high stability, is a promising top-cell absorber for tandem applications. Adapting device structures optimized for 1.0–1.2 eV absorbers to wide bandgap absorbers requires modification of the buffer layer. This study investigates atomic layer deposition of ZnSnOx as an alternative buffer layer to conventional CdS. Critical parameters for buffer performance are the conduction band offsets at both interfaces of the buffer. To investigate these buffers, we electrically characterize solar cells utilizing different compositions of ZnSnOx. The [Sn]/([Sn] + [Zn]) atomic ratio is controlled by the ratio of Zn–O to Sn–O cycles during atomic layer deposition. Solar cells were fabricated utilizing CuInSe2, Cu(In,Ga)Se2, and Cu(In,Ga)S2 absorbers. These absorbers vary in their conduction band minimum energy. Varying buffer composition has two primary effects on cell performance: (1) Low tin buffers decrease the activation energy of interface recombination, reducing open circuit voltage. These observations indicate a cliff, a decrease in the conduction band minimum from absorber to buffer. (2) High tin buffers reduce the fill factor and, in some cases, even reduce the short circuit current. This observation indicates an electron transport barrier, large conduction band offsets, which limit the transport of electrons across the buffer in either direction. Comparing different absorbers, cliffs occur at lower Sn contents, and the effects of barriers are more dramatic for absorbers with lower conduction band minima. We conclude that increasing tin content shifts the conduction band minimum of these buffers upward.