Synergistic band-tail engineering and efficiency boosting via low-dose Ge incorporation in kesterite solar cells

Tin-related defects and cation disorder are critical factors that limit the performance of Cu ₂ ZnSn(S,Se) ₄ (CZTSSe) thin-film solar cells (TFSCs) Partial substitution of Sn with Ge has been identified as an effective approach to suppressing antisite defects and reducing open-circuit voltage deficit ( V _OC,def ). However, high-concentration Ge doping can have adverse effects on device performance. In this study, we employed an environmentally friendly n-butylammonium butyrate–based solution method to optimize the CZTSSe absorber layers and the resulting solar cells by introducing low-concentration Ge doping, where the atomic ratio of Ge/(Ge+Sn) was varied from 0 to 0.2. The crystal structure, surface morphology, roughness, and elemental chemical states of the films were systematically characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS). Defect state distribution and carrier recombination mechanisms were further investigated using Raman spectroscopy, temperature-dependent admittance spectroscopy (T-AS), and temperature-dependent open-circuit voltage measurements. The optoelectronic performance of the devices was comprehensively evaluated through J – V measurements, electrical impedance spectroscopy (EIS), and external quantum efficiency (EQE) tests. The results demonstrated that low-concentration Ge doping effectively promotes the growth of large-grain layer, reduces surface roughness, widens the bandgap, and significantly suppresses [2Cu _Zn + Sn _Zn ] defect clusters and band-tail states, thereby reducing non-radiative recombination losses in both the bulk and at the interfaces. These synergistic improvements collectively enhance the carrier lifetime (τ _EIS ), open-circuit voltage ( V _OC ), short-circuit current density ( J _SC ), and fill factor (FF). The optimal device, with a Ge doping ratio of 15%, achieved a maximum power conversion efficiency (PCE) of 8.65%. This work confirms that low-concentration Ge doping is an effective strategy for enhancing the performance of CZTSSe solar cells and provides important experimental insights into defect control.