ZnSnP2 thin-film solar cell prepared by phosphidation method under optimized Zn/Sn atomic ratio of its absorbing layer

ZnSnP2 film is a promising absorber for thin-film solar cell due to earth-abandant and nontoxic element. Phosphidation method is utilized for the fabrication of ZnSnP2 films with different Zn/Sn atomic ratios. ZnSnP2 film with the large ZnSnP2 protrusions are demonstrated with Zn/Sn of ∼1 (near stoichiometry), while the film with the relatively smooth surface is presented with Zn/Sn of above 1 (Zn rich). According to grazing incidence X-ray diffraction measurement, Sn4P3 secondary phase is appeared in ZnSnP2 film with Zn/Sn of below 1 (Sn rich), whereas Zn3P2 secondary phase is presented in the film with Zn rich. On the other hand, Sn4P3 and Zn3P2 secondary phases are not observed in ZnSnP2 thin film with near stoichiometry, thus leading to the longest carrier lifetime, implied by time-resolved photoluminescence measurement. Ultimately, the conversion efficiency of 0.021% with short-circuit current density of 5.03 mA/cm2 is reported for ZnSnP2 thin-film solar cell, where its absorbing layer possesses single ZnSnP2 phase and Zn/Sn ratio near stoichiometry. The energy bandgap of ZnSnP2 thin films by phosphidation method is estimated to be 1.38 eV by external quantum efficiency, implying that the ZnSnP2 has sphalerite structure.