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

Solar cells using bulk crystals of rare metal-free compound semiconductor ZnSnP₂

We report on current density–voltage (J–V) characteristics of solar cells using bulk crystals of ZnSnP₂ obtained by solution growth, where Sn was used as a solvent. The minority carrier lifetimes of fast and slow components in bulk crystals of ZnSnP₂ were 0.442 and 37.8 ns, respectively, which were obtained by analysis using double exponential function in time‐resolved photoluminescence (TRPL) under the excitation power of 5.05 mW with the beam area of 0.5 mm². The lifetime is close to that of CIGS, which is as high as to achieve the conversion efficiency of over 16%. TRPL also revealed that the recombination at the surface was dominant since the intensity of fast component was much larger than that of slow component. The well‐known structure Al/Al‐doped ZnO/ZnO/CdS/ZnSnP₂/Mo was adopted for solar cells. The short‐circuit current density and the open‐circuit voltage are 1.99 mA cm⁻² and 0.172 V, respectively. The wavelength at the absorption edge in external quantum efficiency is consistent with the bandgap of ZnSnP₂. However, the conversion efficiency is 0.087%. The J–V curve suggests that the reduction of series resistance is required because it is higher than the value expected from the resistivity of bulk ZnSnP₂. The improvement of conduction band offset is also necessary considering from our previous works

20% Efficient Zn_0.9Mg_0.1O:Al/Zn_0.8Mg_0.2O/Cu(In,Ga)(S,Se)₂ Solar Cell Prepared by All-Dry Process through a Combination of Heat-Light-Soaking and Light-Soaking Processes

Development of Cd-free Cu­(In,Ga)­(S,Se)₂ (CIGSSe)-based thin-film solar cells fabricated by an all-dry process is intriguing to minimize optical loss at a wavelength shorter than 520 nm owing to absorption of the CdS buffer layer and to be easily integrated into an in-line process for cost reduction. Cd-free CIGSSe solar cells are therefore prepared by the all-dry process with a structure of Zn_0.9Mg_0.1O:Al/Zn_0.8Mg_0.2O/CIGSSe/Mo/glass. It is demonstrated that Zn_0.8Mg_0.2O and Zn_0.9Mg_0.1O:Al are appropriate as buffer and transparent conductive oxide layers with large optical band gap energy values of 3.75 and 3.80 eV, respectively. The conversion efficiency (η) of the Cd-free CIGSSe solar cell without K-treatment is consequently increased to 18.1%. To further increase the η, the Cd-free CIGSSe solar cell with K-treatment is next fabricated and followed by posttreatment called the heat-light-soaking (HLS) + light-soaking (LS) process, including HLS at 110 °C followed by LS under AM 1.5G illumination. It is disclosed that the HLS + LS process gives rise to not only the enhancement of carrier density but also the decrease in the carrier recombination rate at the buffer/absorber interface. Ultimately, the η of the Cd-free CIGSSe solar cell with K-treatment prepared by the all-dry process is enhanced to the level of 20.0%

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, <i>J</i>_SC, open circuit voltage, <i>V</i>_OC, and fill factor, FF, were 8.2 mA cm^–2, 0.452 V, and 0.533, respectively. However, the value of <i>V</i>_OC 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