Authors: S. Giraldo, E. Saucedo, M. Neuschitzer, F. Oliva, M. Placidi, X. Alcobe´, V. Izquierdo-Roca, S. Kim, H. Tampo, H. Shibata, A. Pérez-Rodríguez and P. Pistor
Link: Energy Environ. Sci., 2018, 11, 582-593
The inclusion of Ge into the synthesis of Cu2ZnSn(S,Se)4 absorbers for kesterite solar cells has been proven to be a very efficient way to boost the device efficiency in a couple of recent publications. This highlights the importance to elucidate the mechanisms by which Ge improves the kesterite solar cells properties to such a large extent. In this contribution, we first show how controlling the position and thickness of a very thin (10–15 nm) layer of Ge greatly influences the crystallization of kesterite thin films prepared in a sequential process. Typically, Cu2ZnSnSe4 (CZTSe) films form in a bi-layer structure with large grains in the upper region and small grains at the back. By introducing Ge nanolayers below our precursors, we observe that large CZTSe grains extending over the whole absorber thickness are formed. Additionally, we observe that Ge induces fundamental changes in the formation mechanism of the kesterite absorber. In a detailed analysis of the phase evolution with and without Ge, we combine the results of X-ray fluorescence, X-ray diffraction, and Raman spectroscopy to demonstrate how the Ge influences the preferred reaction scheme during the selenization. We reveal that the presence of Ge causes a large change in the in-depth elemental distribution, induces a stabilizing Cu–Sn intermixing, and thus prevents drastic compositional fluctuations during the annealing process. This finally leads to a change from a tri-molecular towards, mainly, a bi-molecular CZTSe formation mechanism. Kesterite thin films with surprisingly large crystals of several microns in diameter can be fabricated using this approach. The results are related to the increase in device performance, where power conversion efficiencies of up to 11.8% were obtained. Finally, the consequences of the disclosed crystallization pathways and the extension to other chalcogenide technologies are discussed
- Kesterite solar cell record efficiency of 12.6% (2018) 
- Advantages like earth abundant and non-toxic materials of CZTSe will success if the technology reaches 20% efficiency and be ready for industrial manufacturing.
- Disadvantages: Low Voc (Open circuit voltage is an indirect measurement of the recombination process of the solar cell, following Shockley design).
- Potential fluctuations
- Band tailing
- Disorder defects
- Interface recombination
- Secondary phases
- Compositional inhomogeneities
- The presence of Ge drastically modifies the reaction pathway in which the kesterite is formed
- The beneficial effects of Ge incorporation are not limited to some surface modifications it affects the whole bulk of the absorber.
- The observed improvement should be located on the absorber bulk due to an increase in the charge carrier lifetime.
- SEM (5 keV)
- Thickness by SEM (2µm of the absorber layer)
- EDX (20 kV)
- XRS (Brag-Brentano configuration, 4-145º, step 0.017º, )
- Raman (excitation wavelengths: 633 nm, 532 nm, 488 nm )
- XRF (X-ray fluorescence) to determine overall composition and thickness
- Solar cell
- JV curve (Standard parameters)
- EQE (Increase due optimization at the bulk of the absorber)
- Voc vs T (Activation Energy of recombination process)
- Heat treatment to induce grain growth is crucial for a better solar cell. Then Ge assisted crystallization process affect the whole bulk absorber.
- Goal: Increase Voc on the device is the challenge to increase efficiency > 12%.
- Goal: Detect the dominant recombination mechanism.
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