Defect-assisted nonradiative recombination in Cu 2 ZnSn Se 4 : A comparative study with Cu 2 ZnSn S 4

The efficiencies of ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{\mathrm{Se}}_{4}$ (CZTSe) solar cells with a narrower band gap at 1.0 eV are currently higher than those of ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{\mathrm{S}}_{4}$ (CZTS), with the optimal band gap according to the Shockley-Queisser model. To understand this abnormal observation, we studied the nonradiative recombination rates induced by the deep levels of the dominant defects in CZTSe, i.e., the ${\mathrm{Sn}}_{\mathrm{Zn}}(2+/+)$, ${\mathrm{Sn}}_{\mathrm{Zn}}(+/0)$, and $[{\mathrm{Cu}}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{Sn}}_{\mathrm{Zn}}](+/0)$ levels. We found that the effective recombination centers in CZTS, namely, ${{\mathrm{Sn}}_{\mathrm{Zn}}}^{2+}$ and ${[{\mathrm{Cu}}_{\mathrm{Zn}}\text{\ensuremath{-}}{\mathrm{Sn}}_{\mathrm{Zn}}]}^{+}$, have much smaller carrier capture rates in CZTSe, and are less detrimental to the minority carrier lifetime and energy conversion efficiency. The smaller carrier capture rates for CZTSe can be attributed to the higher electronic transition energies, lower phonon frequencies, and weaker electron-phonon coupling effects in CZTSe compared to those in CZTS, because the large Se cations give rise to larger lattice constants and a softer lattice in CZTSe.