Photoluminescence from Cu2ZnSnS4 thin films with different compositions fabricated by a sputtering-sulfurization method

Photoluminescence (PL) from Cu2ZnSnS4 (CZTS) thin films fabricated by a sputtering-sulfurization method with considerably different Cu/(Zn+Sn) ratios of 0.60-4.21 and Zn/Sn ratios of 0.29-1.68 was investigated with respect to the defect physics. The ranges include the Cu-poor and Zn-rich (Cu/(Zn+Sn)= 0.78 and Zn/Sn=1.55) CZTS composition, which is determined as a suitable composition for application as a solar cell absorber. The origins of the luminescence are attributed to band-to-impurity (BI), band-to-tail (BT), and band-to-band (BB) recombinations at approximately 1.23, 1.35, and 1.48 eV, respectively, which are typically observed from heavily doped and highly compensated semiconductors as a result of temperature dependence, excitation power dependence, and spectral shapes of the PL spectra. Although the BT and BB band emissions had no strong dependence on the film compositions, an intense BI band emission was dependent on the film composition and observed for Cu/Sn≤2.0. The compositional dependence indicates that a deep acceptor level is formed in CZTS with Cu/Sn≤2.0, which is thus considered to be a suitable composition range for the absorber of a highly efficient solar cell.