Phase and film formation pathway for vacuum-deposited $\mathrm{C}{\mathrm{u}}_2\mathrm{BaSn}{\left(\mathrm{S},\mathrm{Se}\right)}_4$ absorber layers

Earth-abundant copper barium thioselenostannate, $\mathrm{C}{\mathrm{u}}_2\mathrm{BaSn}{\left(\mathrm{S},\mathrm{Se}\right)}_4$, absorbers have recently demonstrated promising optoelectronic and defect resistance properties for solar harvesting applications. The highest photovoltaic device efficiencies have been achieved in vacuum-based co-sputter deposited films, yet there is a tendency for a multilayer formation consisting of large, plateletlike surface grains and a smaller grain-sized underlayer (often accompanied by voids). In this work we use a combination of in situ and ex situ x-ray diffraction and scanning electron microscopy to unravel the coupling of phase evolution to film morphology. We find that Cu surface migration and associated Cu-rich phases play a defining role in determining the overall film structure.

Figure 1

(a) XRD patterns of films annealed under S at various temperatures, with standards for CuS [Joint Committee on Powder Diffraction Standards (JCPDS) 6–464] and CBTS (JCPDS 30–124) shown at the bottom and (b) in situ EDXRD intensity of the CuS 102 scattering peak (middle panel), Cu Kα fluorescence (bottom), and temperature profile (top) of a precursor film under S.

Figure 2

Cross-section SEM images of films annealed under S for various temperatures and times (scale bar, 1 µm).

Figure 3

(a) Cross-section SEM image with EDS mapping of (b) Cu (Lα, 0.930 keV), (c) Ba (Lα, 4.465 keV) and Sn (Lα, 3.443 keV), and (d) Mo (Lα, 2.293 keV) for a film annealed at 250 °C for 10 min under S (scale bar, 1 µm). Note that the Mo plot contains contributions from S (Kα, 2.307 keV) due to peak overlap.

Figure 4

SEM plan-view (top) and cross-section (bottom) images of films of (a), (c) CBTS annealed at 500 °C under S for 10 min and (b), (d) CBTSSe annealed at 570 °C under Se for 10 min (scale bar for all panels, 1 µm).

Figure 5

(a), (b) SEM images of two representative locations of the CBTSSe film formed from a BaS/Sn/Cu precursor stack (scale bar, 1 µm). Image (b) shows the presence of blistering.

Figure 6

(a) SEM image showing blister cavity formed between Mo and $\mathrm{Ba}{\mathrm{S}}_3$ layers of BaS/Sn/Cu precursor stack sulfurized at 300 °C (scale bar, 1 µm); (b) XRD patterns of BaS/Sn/Cu precursor stack sulfurized at 200 °C and 300 °C, with standards for BaS (JCPDS 8–454) and $\mathrm{Ba}{\mathrm{S}}_3$ (JCPDS 1-85-1702).

Figure 7

SEM images of films of (a) CBTS annealed at 450 °C under S for 3 h and (c) CBTSSe subsequently annealed at 570 °C under Se for 10 min (scale bar, 1 µm), both showing a reduction in surface grains compared to CBTS and CBTSSe where 10-min sulfurizations were used. (b) XRD patterns of co-sputter-deposited films annealed under S at the indicated temperatures and times showing single-phase CBTS, with CBTS standard (JCPDS 30–124).

Figure 8

SEM image of (a) CBTS and (b) CBTSSe films formed from a $\mathrm{C}{\mathrm{u}}_2\mathrm{Sn}{\mathrm{S}}_3/\mathrm{BaS}$ stack (scale bar, 1 µm).

Figure 9

XPS Al Kα spectra of the Cu $2p$, Ba $3d$, and Sn $3d$ core levels of CBTS formed from co-sputter-deposited (black) and $\mathrm{C}{\mathrm{u}}_2\mathrm{Sn}{\mathrm{S}}_3/\mathrm{BaS}$ stacked (gray) films.