Insights On The Limiting Factors Of Cu2zngese4 Based Solar Cells

Germanium-based wide band gap kesterite semiconductor 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 similar to 1.47 eV (Cu2ZnGeSe4-CZGeSe) to similar to 2.2 eV (Cu2ZnGeS4-CZGeS), as well as non-toxic constituents makes this compound a strong candidate for further scientific exploration. However, the record efficiency of Cu2ZnGeSe4 solar cells is still significantly lower than those of its predecessors Cu2ZnSn(SxSe(1-x))(4) (CZTSSe), Cu(In,Ga)Se-2 (CIGS) and CdTe thin-film solar cells. The comprehensive understanding of the factors limiting the performance of Cu2ZnGeSe4 based solar cells is the purpose of this work, by combining a complete characterization of the morphological, structural, compositional and optoelectronic properties of Cu2ZnGeSe4 absorbers and devices. Besides, an in-depth investigation of the main limitations is carried out, specifically focusing on studying the origin of the large V-OC deficit, the main recombination 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%, V-oc of 606 mV, J(SC) of 17.8 mA/cm(2) and FF of 60%. The results presented here demonstrate that the large voltage deficit of CZGeSe solar cells could be mainly ascribed to a Fermi level pinning at the interface, while modifications of the buffer layer to induce a "spike" at the p-n junction could be beneficial. Additionally, low carrier diffusion lengths and 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.