Synthesis, theoretical and experimental characterisation of thin film Cu2Sn1-xGexS3 ternary alloys (x = 0 to 1): Homogeneous intermixing of Sn and Ge

Cu2Sn1-xGexS3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin
film solar cells. The aim of this study is to investigate the properties of this alloy in thin film form in order
to establish relationships between group IV composition and structural, vibrational and opto-electronic
properties. Seven single phase Cu2Sn1-xGexS3 films are prepared from x ¼ 0 to 1, showing a uniform
distribution of Ge and Sn laterally and in depth. The films all show a monoclinic crystal structure. The
lattice parameters are extracted using Le Bail refinement and show a linear decrease with increasing Ge
content. Using density-functional theory with hybrid functionals, we calculate the Raman active phonon
frequencies of Cu2SnS3 and Cu2GeS3. For the alloyed compounds, we use a virtual atom approximation.
The shift of the main Raman peak from x ¼ 0 to x ¼ 1 can be explained as being half due to the change in
atomic masses and half being due to the different bond strength. The bandgaps of the alloys are extracted
from photoluminescence measurements and increase linearly from about 0.90 to 1.56 eV with increasing
Ge. The net acceptor density of all films is around 1018 cm 3. These analyses have established that the
alloy forms a solid solution over the entire composition range meaning that intentional band gap grading
should be possible for future absorber layers. The linear variation of the unit cell parameters and the
band gap with group IV content allows composition determination by scattering or optical measurements.
Further research is required to reduce the doping density by two orders of magnitude in order to
improve the current collection within a solar cell device structure.