Sulfurization induced surface constitution and its correlation to the performance of solution-processed Cu2ZnSn(S,Se)4 solar cells

Abstract

To obtain high photovoltaic performances for the emerging copper zinc tin sulfide/selenide (CZTSSe) thin film solar cells, much effort has deservedly been placed on CZTSSe phase purification and CZTSSe grain size enhancement. Another highly crucial but less explored factor for device performance is the elemental constitution of CZTSSe surface, which is at the heart of p-n junction where major photogenerated carriers generate and separate. In this work we demonstrate that, despite the well-built phase and large grained films are observed by common phases and morphology characterization (XRD, Raman and SEM), prominent device efficiency variations from short circuited to 6.4% are obtained. Insight study highlights that the surface (0-250 nm) compositions variation results in different bulk defect depths and doping densities in the depletion zone. We propose that suitable sulfurization (at ~ 10 kPa sulfur pressure) drives optimization of surface constitution by managing the Cu, Zn and Sn diffusion and surface reaction. Therefore, our study reveals that the balance of elemental diffusion and interface reactions is the key to tuning the surface quality CZTSSe film and thus the performance of as resulted devices.

Figure 1. J-V performances of CZTSSe solar cells annealed with different sulfur contents.

A prominent photoelectric conversion improvement from short circuited to 6.4% was achieved when sulfur addition increased from 1 mg to 6 mg in sulfurization.

Figure 2. Phases and morphology characterizations of CZTSSe absorber with different sulfur additions (S1 to S4).

(a), XRD patterns with inseted bandgap values estimated from the (112) peak position. Well resolved (202) and (211) peaks were obtained for all samples suggesting good crystallinity of CZTSSe phase; (b), Raman curves with denoted CZTSSe phases; (c), SEM image and EDS mapping for sample S3 indicating ZnS secondary phase sit on the top of the film. The correlated Zn and S are denoted in circle.

Figure 3. Cross-sectional SEM morphologies of samples S3 (a) and S2 (b).

The films thicknesses are around 1 µm and the MoSe₂ layers are within 300 nm. Large CZTSSe grains are obtained after sulfurization (~1 µm). The calculated depletion widths for S3 and S2 are 305 nm, 181 nm, respectively.

Figure 4. Electronic characterization for CZTSSe devices S2 and S3.

Admittance spectroscopy (AS) of S2 (a) and S3 (b) with temperature range of 180 K to 300 K; (c) The trap conductance spectra (G_m-G_d)/ω and equivalent circuit model; (d) Arrhenius plots of S3 and S2 derived from AS patterns. The estimated energetic depth of the defect (E_a) for S3 and S2 are 101 meV and 156 meV, respectively.

Figure 5. AES depth analysis and surface composition variation of S1, S2 and S3.

The depth was estimated from the actual thickness of the films and sputtering time. The beginning (0 nm) stands for the position of CdS/CZTS interface. The concentration ratios of surface and the internal average (I.A) of Cu, Zn and Sn were plotted to manifest the surface constitution variations. The depths of AES curves were calculated from the actual thickness divided by sputtering time.

Figure 6. XPS tests for Sn element on the surface of CZTSSe films prepared with different S additions.

(a) precursor before sulfurization, (b) S1 (1 mg), (c) S2 (4 mg) and (d) S3 (6 mg).

Figure 7. Diffusion routes of Cu, Zn, Sn and S elements during sulfurization.

The upward diffusion of Zn and backward diffusion of Cu elements facilitates the forming of Cu-poor surface, given well balanced surface reaction and elemental diffusion.