Effects of Growth Conditions on Formation Behavior of the Recombination Center in ZnSnP₂ Bulk Crystals

Zinc tin diphosphide (ZTP) is an alternative material for commercialized thin-film photovoltaics due to of the non-toxic and earth-abundant constituent elements, the suitable bandgap, and the high absorption coefficient^[1,2]. Our group has reported the ZTP bulk crystal-based solar cell with the record conversion efficiency of 3.87% which is still lower than the theoretical limit^[3]. In particular, the short circuit current density of 12.1 mA cm^-2 is lower compared with that of CIGS solar cells, which is probably due to the really short minority carrier lifetime of sub-nanosecond^[4].<br/> We prepared ZTP bulk crystals by solution growth method using Sn as flux. In our previous work, the starting composition was on the line of the Sn–ZnP₂ pseudo–binary system^[5], while it is well known that the formation behavior of intrinsic defects which could be carrier traps depends on the chemical potentials of constituent elements. In this study, we thus attempted to enhance the minority carrier lifetime of ZTP crystals by the control of composition, namely chemical potentials in the system. We actually performed the crystal growth in the compositions with 1.3 and 4 mol% of Zn (Zn-rich and P-rich) in addition to the conventional composition, and the other conditions were the same in the previous report^[5].<br/> We characterized the ZTP crystal grown under the conventional condition by deep-level transient spectroscopy (DLTS) in which Schottky diodes with Ag (Schottky) /ZTP / Cu₃P / Cu (ohmic) were used. DLTS measurement showed that ZTP had two traps for electrons and five traps for holes within the bandgap. Particularly, the nearest electron trap level to conduction band minimum (CBM), E1, had extremely high capture cross section of ~10^-11 cm² compared with other traps. Furthermore the time constant for electron capture from CBM to E1 was estimated to be ~10^-4 ns, which was equivalent to the reported minority carrier lifetime^[4], indicating that the trap E1 might be the recombination center of ZTP.<br/> We also evaluated the influence of growth condition on the properties of recombination. The fluorescence lifetimes of minority carriers were characterized by time-resolved photoluminescence (TRPL) at the photon energy of 1.69 eV under room temperature. The net fluorescence lifetime of samples of Zn-rich, conventional, and P-rich conditions were estimated to be 0.39, 0.066, and 0.065 ns, respectively. The lifetime of Zn-rich sample is higher than others and the results in the literature^[4]. Steady–state PL spectra were also measured at 77K and found to be contained several emissions with energy of around 1.28, 1.52, and 1.77 eV. The emission at 1.77 eV might come from Band-to-Band transition and its intensity in the sample of Zn-rich condition was higher than that of others. This indicates that the concentration of the active recombination center E1 is low in Zn-rich sample, which is consistent with the results of TRPL.<br/> In spite of the change of defect properties by growth conditions, the compositions of all the obtained ZTP were stoichiometric. Therefore, it was implied that the compositional difference of a few mole could affect the formation of intrinsic defects, especially related to E1. We consider two contributions of this phenomena from the viewpoint of thermodynamics: entropy and enthalpy. Both of them depend on chemical potentials of constituent elements in the system during crystal growth, where ZTP and liquid are in thermodynamic equilibrium. Hence, we evaluated the chemical potential of elements in liquid phase using sub-regular solution model, and clarified the precipitation temperature is lower in Zn-rich condition. In the presentation, we will discuss more quantitatively the effects of the entropy and the enthalpy on the formation of defects in ZTP.<br/>[1] S. Nakatsuka and Y. Nose, <i>JPCC</i> (2017). [2] T. Yokoyama et al., <i>APEX</i> (2013). [3] T. Kuwano et al., <i>SOLMAT</i> (2021). [4] S. Nakatsuka et al., <i>PSSa</i> (2017). [5] S. Nakatsuka et al., <i>PSSc</i> (2015).