On the Germanium Incorporation in Cu₂ZnSnSe₄ Kesterite Solar Cells Boosting Their Efficiency

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.

Insights into interface and bulk defects in a high efficiency kesterite-based device

“…Nonetheless, the clear and defined spotty patterns indicate that, despite twinning, the grains in the bulk region of the absorber display a good crystalline quality. Although evidences of the formation of twin boundaries in CZTSe have been recently reported, 43 to the best of our knowledge, this is the first time that twinning defects are directly observed for this material.…”

“…Detecting the emitted X-ray fluorescence radiation for each position by two 3-element silicon drift detectors, and fitting the respective spectra, provides compositional information for each individual spot. 43 Finally, electron energy-loss spectroscopy (EELS) (GIF Quantum EF spectrometer attached to JEOL-ARM-F) was used to study interlayer element diffusion at the front interface.…”

“…5d) show a quite homogeneous composition with no noticeable intra-grain or inter-grain differences or variations at the grain boundaries. Note that despite the size of the beam spot of roughly 50 nm, nanoXRF is well capable of resolving compositional variations of nanosized features like grain boundaries as demonstrated for both CZTSe 43 and CIGS. 45 In order to go in depth into the homogeneity analysis in the nanoXRF mappings, several scanning lines parallel and perpendicular to the absorber surface were selected.…”

Interplay of Performance‐Limiting Nanoscale Features in Cu2ZnSn(S,Se)4 Solar Cells

“…Figure shows the SEM image of the cross section of the same lamella (as shown in Figure 1) but from a slightly different area together with the nano‐XRF maps. The use of a hard X‐ray nanobeam for compositional analysis not only improves the elemental sensitivity at higher energies but also the detection limits, allowing the determination of much lower compositional variations at nanometer scale, such as variations located at grain boundaries [ 30–32 ] or nanoscale secondary phases. [ 13,30 ] The only drawback is that S cannot be detected and quantified, because the experimental setup was under ambient conditions with a significant low‐Z absorption by the air and the detector Be window.…”

“…The use of a hard X‐ray nanobeam for compositional analysis not only improves the elemental sensitivity at higher energies but also the detection limits, allowing the determination of much lower compositional variations at nanometer scale, such as variations located at grain boundaries [ 30–32 ] or nanoscale secondary phases. [ 13,30 ] The only drawback is that S cannot be detected and quantified, because the experimental setup was under ambient conditions with a significant low‐Z absorption by the air and the detector Be window. In a pixel‐by‐pixel basis, the fitting of individual nano‐XRF spectra using the PyMCA code [ 33 ] allows the generation of elemental maps.…”