Shigeru Nakatsuka

Band offset at the heterojunction interfaces of CdS/ZnSnP2, ZnS/ZnSnP2, and In2S3/ZnSnP2

Heterojunctions were formed between ZnSnP2 and buffer materials, CdS, ZnS, and In2S3, using chemical bath deposition. The band offset was investigated by X-ray photoelectron spectroscopy based on Kraut method. The conduction band offset, ΔEC, between ZnSnP2 and CdS was estimated to be −1.2 eV, which significantly limits the open circuit voltage, VOC. Conversely, ΔEC at the heterojunction between ZnSnP2 and ZnS was +0.3 eV, which is within the optimal offset range. In the case of In2S3, ΔEC was a relatively small value, −0.2 eV, and In2S3 is potentially useful as a buffer layer in ZnSnP2 solar cells. The J−V characteristics of heterojunction diodes with an Al/sulfides/ZnSnP2 bulk/Mo structure also suggested that ZnS and In2S3 are promising candidates for buffer layers in ZnSnP2 thin film solar cells, and the band alignment is a key factor for the higher efficiency of solar cells with heterojunctions.

Impact of Heterointerfaces in Solar Cells Using ZnSnP2 Bulk Crystals

We report on the optimization of interface structure in ZnSnP2 solar cells. The effects of back electrode materials and related interface on photovoltaic performance were investigated. It was clarified that a conventional structure Mo/ZnSnP2 showed a Schottky-behavior, while an ohmic-behavior was observed in the Cu/ZnSnP2 structure annealed at 300 °C. STEM-EDX analysis suggested that Cu-Sn-P ternary compound was formed at the interface. This compound is considered to play an important role to obtain the ohmic contact between ZnSnP2 and Cu. In addition, it was clarified that the aqua regia etching of ZnSnP2 bulk crystals before chemical bath deposition process for the preparation of buffer layer was effective to remove the layer including lattice defects introduced by mechanical-polishing, which was supported by TEM observations and photoluminescence measurements. This means that the carrier transport across the interface was improved because of the reduced defect at the interface. Consequently, the conversion efficiency of approximately 2% was achieved with the structure of Al/ZnO;Al/ZnO/CdS/ZnSnP2/Cu, where the values of short circuit current density, JSC, open circuit voltage, VOC, and fill factor, FF, were 8.2 mA cm-2, 0.452 V, and 0.533, respectively. However, the value of VOC was largely low considering the bandgap value of ZnSnP2. To improve the conversion efficiency, the optimization of buffer layer material is considered to be essential in the viewpoint of band alignment.

Bandgap Control of ZnSnP2 via phase transition between chalcopyrite and sphalerite

ZnSnP2 is a promising candidate for solar absorber materials from the viewpoint of high absorption and earth-abundant constitution elements. In this paper, the crystal growth experiments with various cooling rates were carried out and we successfully obtained ZnSnP2 crystals under every condition. The structure of grown crystals studied by XRD was indicated to be chalcopyrite-type 2, while the decrease of the degree of order was observed with the increase of cooling rate. The bandgaps of ZnSnP2 crystals decreases with increasing the cooling rate due to the decrease of the degree of order.