Investigation of combinatorial coevaporated thin film Cu2ZnSnS4. I. Temperature effect, crystalline phases, morphology, and photoluminescence

Abstract

Cu2ZnSnS4 is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325 °C) and high (475 °C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line in the quasi-ternary Cu2S-ZnS-SnS2 phase system, and this effect is attributed to structural similarity between the Cu2ZnSnS4, Cu2SnS3, and ZnS crystalline phases. At 475 °C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line. Room-temperature photoluminescence imaging reveals a strong correlation for these samples between film composition and photoluminescence intensity, where film regions with Cu/Sn ratios greater than ∼2 show strong photoluminescence intensity, in comparison with much weaker photoluminescence in regions that received excess Sn flux during growth or subsequent processing. The observed photoluminescence quenching in regions that received excess Sn flux is attributed to the effects of Sn-related native point defects in Cu2ZnSnS4 on non-radiative recombination processes. Implications for processing and performance of Cu2ZnSnS4 solar cells are discussed.

Cu–Zn disorder and band gap fluctuations in Cu2ZnSn(S,Se)4: Theoretical and experimental investigations

“…Refs. , and, like the low V oc , it is generally explained by the existence of spatial potential fluctuations in the band structure (band gap or electrostatic potential fluctuations, see e.g., Ref. ).…”

“…It also follows that the energy of the RT‐PL might be used to estimate the state of Cu–Zn disorder in a given sample, all other things being equal. In the literature, this peak usually occurs in the region of 1.3–1.35 eV , which, from Fig. , translates to S values in the range 0–0.4.…”

“…This explanation is consistent with the widespread observation that the band‐to‐band transition in the room temperature photoluminescence (RT‐PL) spectrum occurs at energies several 100 meV below the fundamental band gap, i.e. 1.3–1.35 eV for CZTS . Different causes of fluctuations in the band gap have been proposed, such as inclusions of stannite or the presence of defect complexes .…”

A review on applications of Cu2ZnSnS4 as alternative counter electrodes in dye-sensitized solar cells

“…45,55,64,97,100,[112][113][114][115][116][117][118][119][120][121][122][123][124] Performance of CZTS could be enhanced after treatment with TiCl 4 , making composite with another alternative electro-catalysts such as graphene, carbon or may be variation in the precursor concentration or film deposition mechanism. In situ fabricated CZTS film was shown periodic deposition upon tuning the concentration of CZTS precursor and ethanol during solvothermal synthesis process and designated as CZTS 1,2,3 4, and 5.…”

Raman mapping analysis for removal of surface secondary phases of CZTS films using chemical etching

“…The effective removal of SnS can be also evident from few of semicircular grains on the CZTS surface in SEM images shown in Figure 2C3 . 15,16 These defects are most likely to result in deep-level recombination centers further reducing the performance of the solar cells. FWHMs of three locations vary within 4 nm for this as-grown sample.…”