Abstract
We report an approach to incorporate Ge into Cu2ZnSnSe4 using GeSe vapor during the selenization step of alloyed metallic precursors. The vapor incorporation slowly begins at T ≈ 480 °C and peaks at 530 °C, resulting in a Ge-based composition shift inside the previously formed kesterite layer. We initially observe the formation of a Ge-rich surface layer that merges into a homogeneous distribution of the incorporated element during the further dwelling stage of the annealing. This approach is very versatile and could be used in many similar fabrication processes for incorporating Ge into CZTSe-absorber layers. Because the vapor-based composition shift in the layer happens after the formation of the absorber film towards the end of the fabrication process, most process parameters and the precursor structure may not need any significant re-optimization. The careful integration of this step could help to reduce Sn-related deep defects and accompanying VOC losses. The best CZTGSe-power-conversion efficiency obtained in this series is 10.4 % (with EG = 1.22 eV, FF = 54%, JSC = 36 mA/cm2, VOC = 540 mV, VOCdef,SQ = 417 mV). These results demonstrate the potential of this approach for Ge incorporation into kesterite absorbers.
Highlights
Academic Editor: Samira KhelifiGe incorporation into the kesterite (Cu2ZnSnSe4) system in order to form Cu2Zn(Sn,Ge)Se4(CZTGSe) material is considered to be a promising approach for obtaining photovoltaic absorbers with a higher and tunable band gap in the range of 1.0 eV to 1.5 eV [1,2]
The effective incorporation of Ge into the kesterite absorber is shown in Figure 2 by the energy-dispersive X-ray spectroscopy (EDX) results along with the EQE spectra and the EG values extracted from them
After 100 h of accelerated aging, the mean efficiency slightly overtook the reference. These results indicate that the long-term stability of the kesterite absorber can be improved by Ge incorporation, which needs more careful attention in future investigations
Summary
Academic Editor: Samira KhelifiGe incorporation into the kesterite (Cu2ZnSnSe4) system in order to form Cu2Zn(Sn,Ge)Se4(CZTGSe) material is considered to be a promising approach for obtaining photovoltaic absorbers with a higher and tunable band gap in the range of 1.0 eV to 1.5 eV [1,2]. As a consequence of adding Ge to the precursor, the entire alloy configuration, distribution of elements, and overall composition ratios in the precursor are changed, altering the reaction pathway for the kesterite-phase formation. In such a case, a well-optimized process for Gefree-kesterite growth may need further optimization steps for Ge incorporation. The precursor structure and further process parameters may need a full re-optimization or even significant modifications to avoid the formation of other secondary phases and further undesirable defects
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