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Germanium-based wide band gapkesteritesemiconductor Cu2ZnGe(S,Se)4(CZGeSSe) is considered a very promising absorber compound as top cell in tandem devices. Autonomy to tailor the band gap from ~1.47eV (Cu2ZnGeSe4-CZGeSe) to ~2.2eV (Cu2ZnGeS4-CZGeS), as well as non-toxic constituents makes this compound a strong candidate for further scientific exploration. However, the record efficiency of Cu2ZnGeSe4solar cells is still significantly lower than those of its predecessors Cu2ZnSn(SxSe1-x)4(CZTSSe), Cu(In,Ga)Se2(CIGS) and CdTe thin-film solar cells. The comprehensive understanding of the factors limiting the performance of Cu2ZnGeSe4based solar cells is the purpose of this work, by combining a complete characterization of the morphological, structural, compositional andoptoelectronicproperties of Cu2ZnGeSe4absorbers and devices. Besides, an in-depth investigation of the main limitations is carried out, specifically focusing on studying the origin of the large VOCdeficit, the mainrecombination mechanisms, electric transport properties, band tails and possible Cu2ZnGeSe4/CdS band offset effects. The champion CZGeSe solar cell device reported in this work shows an efficiency of 6.5%, Vocof 606mV, JSCof 17.8mA/cm2and FF of 60%. The results presented here demonstrate that the large voltage deficit of CZGeSe solar cells could be mainly ascribed to aFermi levelpinning at the interface, while modifications of thebuffer layerto induce a “spike” at the p-n junction could be beneficial. Additionally, lowcarrier diffusion lengthsand lifetimes, along with possible back contact recombination, are identified as the main culprits for the limited carrier collection for low-energy photons. Finally, some strategies are proposed to face and overcome most of these issues and to help improving the CZGeSe performance. |
