Mapping the Energetics of Defect States in Cu₂ZnSnS₄ films and the Impact of Sb Doping

Abstract

The sub-bandgap levels associated with defect states in Cu 2 ZnSnS 4 (CZTS) thin films are investigated by correlating the temperature dependence of the absorber photoluminescence (PL) with the device admittance spectroscopy. CZTS thin films are prepared by thermolysis of molecular precursors incorporating chloride salts of the cations and thiourea. Na and Sb are introduced as dopants in the precursor layers to assess their impact on Cu/Zn and Sn site disorder, respectively. Systematic analysis of PL spectra as a function of excitation power and temperature show that radiative recombination is dominated by quasi-donor–acceptor pairs (QDAP) with a maximum between 1.03 and 1.18 eV. It is noteworthy that Sb doping leads to a transition from localized to delocalized QDAP. The activation energies obtained associated with QDAP emission closely correlate with the activation energies of the admittance responses in a temperature range between 150 K and room temperature in films with or without added dopants. Admittance data of CZTS films with no added dopants also have a strong contribution from a deeper state associated with Sn disorder. The ensemble of PL and admittance data, in addition to energy-filtered photoemission of electron microscopy (EF-PEEM), shows a detailed picture of the distribution of sub-bandgap states in CZTS and the impact of doping on their energetics and device performance.

Ex situ Ge-doping of CZTS nanocrystals and CZTSSe solar absorber films

“…As in our previous EF-PEEM studies on CZTS and CZTSSe lms derived from a molecular solution, we believe that this can be associated with an increased Cu-Zn disorder in the lms. 12,28 This is also in agreement with the increased asymmetry of Raman modes observed in the lms.…”

Lanthanum-induced synergetic carrier doping of heterojunction to achieve high-efficiency kesterite solar cells

“…4a and b, it can be seen that La-5 sample exhibits a much wider N t – E ω distribution compared to that of the control sample. 35–37 Besides the similar N t distribution in the high defect energy regime (0.08 eV), La-5 sample exhibits a new N t distribution in the low defect energy regime (0.05 eV), implying an additional defect energy level caused by La doping. At temperatures over 200 K, no obvious defect response can be further observed in the capacitance-f spectra, which means that these detected defects are all shallow acceptors and very easy to ionize.…”

“…At temperatures over 200 K, no obvious defect response can be further observed in the capacitance-f spectra, which means that these detected defects are all shallow acceptors and very easy to ionize. 35,36,38 The Arrhenius plots exhibit that these two samples have a similar defect ionization energy ( E t ) of 0.11 and 0.076 eV, respectively, which can be attributed to V Cu or Cu Zn shallow acceptors. 9,39 For the La-5 sample, the new additional shallow acceptor has an E t of about 0.045 eV, which could arise from the La Sn defects (Fig.…”