No abstract

The presence of Ge during the synthesis of thin film kesterite Cu2ZnSnSe4 (CZTSe) solar cell absorbers boosts their power conversion efficiency, especially due to an improved open circuit voltage. The mechanism underlying this beneficial effect of Ge is still under debate. We gained deep insights into the role of Ge by applying advanced synchrotron nanoprobe-based X-ray fluorescence spectroscopy and X-ray absorption near-edge structure spectroscopy to cross-sectional lamellas taken from high efficiency devices. We observe that Ge remains in the CZTSe absorber layer after the synthesis process with a specific heterogeneous distribution. Different grains contain different Ge concentrations. Moreover, Ge depletion exists at random grain boundaries but not at symmetric Σ3-boundaries, leading to different band alignments. The incorporated Ge occupies Sn lattice sites in the CZTSe crystal structure; however, the concentration is only 0.1 to 0.5 at %. Also Ge aggregates in nanoscale inclusions, which we could identify to be GeO2 that likely lessen the beneficial effect of Ge on the photovoltaic performance.

The atomic-scale structure and vibrational properties of semiconductor alloys are determined by the energy required for stretching and bending the individual bonds. Using temperature-dependent extended x-ray absorption fine-structure spectroscopy, we have determined the element-specific In-As and Ga-As effective bond-stretching force constants in (In,Ga)As as a function of the alloy composition. The results reveal a striking inversion of the bond strength where the originally stiffer bond in the parent materials becomes the softer bond in the alloy and vice versa. Our findings clearly demonstrate that changes of both the individual bond length and the surrounding matrix affect the bond-stretching force constants. We thus show that the previously used common assumptions about the element-specific force constants in semiconductor alloys do not reproduce the composition dependence determined experimentally for (In,Ga)As.

Kesterite based materials gain more and more relevance in the pursuit of affordable, efficient and flexible absorber materials for thin film photovoltaics. Alloying Cu2ZnSnSe4 with Ge could allow controlled band gap engineering as already established for Cu(In,Ga)(S,Se)2 based solar cells. This study investigates the local atomic arrangements of Cu2Zn(Sn,Ge)Se4 alloys by means of low temperature Extended x-ray Absorbtion Fine Structure Spectroscopy. The element specific bond lengths are used together with x-ray diffraction data to derive the anion positions of the different local configurations. Ab initio theoretical calculations are performed to predict the influence of structural parameters such as anion position and lattice constants on the band gap energy. Combining the results of the experimental and theoretical studies suggests that the overall influence of the structural changes on the band gap bowing due to alloying is significant yet smaller than the total non-linear change of the band gap energy. Consequently, it is concluded, that band gap bowing stems from both structural and electronic changes.