Cu2ZnSnSe4 polymorphs and secondary phases: characterization by Raman spectroscopy and photoluminescence

Doctoral thesis (2014)

Cu2ZnSn(S,Se)4 (CZTS/Se)has the necessary properties to be suitable as a thin film solar cell absorber. The reported record e ciencies for pure CZTSe are measured on CZTSe absorbers of di erent band gaps ... [more ▼]

Cu2ZnSn(S,Se)4 (CZTS/Se)has the necessary properties to be suitable as a thin film solar cell absorber. The reported record e ciencies for pure CZTSe are measured on CZTSe absorbers of di erent band gaps (0.96 eV and 1 eV as extracted from photoluminescence (PL), for example) although they were supposed to be the same material. This di fference in band gaps is usually explained by a diff erent order of the composing elements leading to crystal structures from other space groups (polymorphs) which have di erent band gaps. Various polymorphs and secondary phases are possible. This implies the importance of possessing tools to discernate the phases that tend to form whilst CZTSe is grown. This is one main goal of this thesis. The main measurement techniques employed during this thesis are Photoluminescence (PL) and Raman spectroscopy. Both are optical, contact-less and non-destructive characterization techniques. Following on from the first chapter which provides relevant background information about the di fferent polymorphs, secondary phases and measurement techniques used, the second chapter introduces the setup and the sample preparation techniques. The third chapter reports the fi rst results of this thesis: the discrimination of secondary phases encountered during the growth of CZTSe solar cell absorbers by Raman and PL. Chapter four discusses the Raman spectrum of CZTSe, especially focusing on the asymmetry of the mode of highest intensity, looking at the origins of this broadening. Additionally, a polarization and orientation Raman study is presented assigning the modes to their symmetry, therefore allowing the di erentiation between crystals from various polymorphs. The next chapter examines the luminescences attributed to CZTSe. An intensity dependent PL study assigns the various luminescences encountered to a mixture of band gaps. In the fi nal chapter, the findings reported on in this thesis are briefly summarized and discussed in the context of the ongoing research on the subject matter. [less ▲]