Chemistry and Dynamics of Ge in Kesterite: Toward Band-Gap-Graded Absorbers

Abstract

The selenization of metallic Cu−Zn−Sn−Ge precursors is a promising route for the fabrication of low-cost and efficient kesterite thin-film solar cells. Nowadays, efficiencies of kesterite solar cells are still below 13%. For Cu(In,Ga)Se 2 solar cells, the formation of compositional gradients along the depth of the absorber layer has been demonstrated to be a key requirement for producing thin-film solar cells with conversion efficiencies above the 22% level. No clear understanding has been reached so far about how to produce these gradients in an efficient manner for kesterite compounds, but among the possible candidates, Ge arises as one of the most promising ones. In the present work, we evaluate the potential of incorporating Ge in Cu 2 ZnSnSe 4 to produce compositional gradients in kesterites. Synchrotron-based in situ energydispersive X-ray diffraction and X-ray fluorescence have been used to study the selenization of Cu−Zn−Sn−Ge metallic precursors. We propose a reaction mechanism for the incorporation of Ge atoms into the kesterite lattice after the formation of Cu 2 ZnSnSe 4 . Electron microscopy reveals that the annealing process leads to Cu 2 Zn(Sn,Ge)Se 4 absorber layers with an increase of Ge content toward the back contact with independence of the original location of Ge in the precursor layer. The effect of the Ge gradient on the optoelectronic properties of the absorber layer has been evaluated with room-temperature cathodoluminescence. The implications of the results for the development of kesterite solar cells are discussed, with the aim of encouraging new synthesis routes for compositionally graded absorbers.

Characterization of Surface and Heterointerface of Cu2ZnSn1–xGexSe4 for Solar Cell Applications

“…The in‐depth compositional grading of group‐III elements (Ga, In) is one of the most useful techniques for high‐performance solar cells with CIGSe, and similar to cation grading of group‐IV elements (Sn, Ge), is expected for CZTGSe under the same analogy . Accordingly, we expect that the solar cell performance of CZTGSe can be further improved by the potential grading of the CBM in CZTGSe by controlling x in depth because the electron affinity of CZTGSe continuously varies with x .…”

“…Ge‐incorporated Cu 2 Zn(Sn 1– x Ge x )Se 4 (CZTGSe) with kesterite crystal structure has attracted attention as a material for p‐type light absorbers in heterojunction solar cells . This is because the bandgap energy ( E g ) can be tuned from 1.0 to 1.4 eV by adjusting the cationic mixing ratios {Ge/(Ge + Sn) = x } from 0 to 1 .…”

“…In spite of the importance of developing high‐performance solar cells, the origin of the V OC deficit in kesterite is not yet clear. Especially, although V OC is expected to be affected by the electronic structures at the surface and heterointerface, only a few studies have reported on the band alignment of the CdS/Ge‐incorporated CZTGSe interface and electronic properties of the CZGSe surface . Thus, their systematic accumulation is significantly important for the further improvement of CZTGSe solar cells.…”

Effect of deposition temperature on the growth mechanism of chemically prepared CZTGeS thin films

“…CZTS has a high absorption coefficient ( α > 10 4 cm −1 ), direct band gap ( E g ~ 1.5 eV), and p‐type conductivity, which allows for effective absorption of the most incident photons in absorbers with thicknesses of 1 to 2 μm. However, the current efficiency of CZTS‐based solar cells does not exceed 12.6%, because of the short lifetimes of minority charge carriers in the sub‐nanosecond regime . Despite the fact that the maximum possible efficiency of 32% (Shockley‐Queisser limit) for CZTS‐based solar cells has not been reached yet, the significant advances in understanding the functioning problems of photovoltaic devices were achieved.…”

“…However, the current efficiency of CZTS-based solar cells does not exceed 12.6%, 4 because of the short lifetimes of minority charge carriers in the sub-nanosecond regime. 5 Despite the fact that the maximum possible efficiency of 32% (Shockley-Queisser limit) for CZTS-based solar cells has not been reached yet, the significant advances in understanding the functioning problems of photovoltaic devices were achieved.…”