Longer lifetime and higher efficiency of CZTS thin-film solar cells

Progress of environmentally-friendly solar cell (photovoltaics; PV) technologies is a worldwide research priorities. To meet the creteria of a sustainable technology, the PV materials shall be affordable, abundant, and non-toxic; here, the total cost includes material, fabrication, handling, and life time, recycling, etc. Today, solar cells generate roughly 1% of total energy consumption and an increase to 10-20% before 2050 implies a huge capacity of manufacturing PV modules. To reduce the material cost and the over-usage of natural resources, thin-film solar cells are of significant interest; the thickness is ~150 micrometers for crystalline Si while ~2 micrometers for thin-film PV. Thinner films implies however more efficient photon-absorbing material and higher requirement of low degradation of the material. Increasing the cell efficiency and device life time, reducing the material cost, and preventing degradation have to go hand in hand when developing solar cell devices. In this project we investigate CZTSSe =Cu2ZnSn(S,Se)4 and CZYS = Cu2Zn(Sn,Ge,Si)S4 alloys. The objectives are to understand the long-term stability, efficiency of PV performance, and the degradation mechanisms. Se-S alloying and Si/Ge-Sn alloying can be utilized to for graded absorber layer that have optimized the performance. Availability of Na can promote beneficial grain growth and also appears to enhance efficiency. In addition, we also theoretically analyze complementary environmental-friendly Cu-X-(S,Se) materials. CZTS precursor thin films are made by reactive sputtering, followed by selenization process. Secondary ion mass spectrometry (SIMS) has been utilized to study the Se content (and Na content). A non-uniform spatial distribution of Se and Na with depth has been detected with SIMS imaging depth profile, which correlates well with results from energy dispersive x-ray spectroscopy. This confirms that SIMS image depth profiling is a method that can be used to gain additional information about the spatial composition in CZTSSe. We have found that even the large CZTSSe grains of pre-sulfurized absorbers did not materially arrest the diffusion of selenium through the thickness of the films. The selenide grains seem grow at the expense of the sulfide grains, rather than by slow diffusion of selenium into existing sulfide grains. During the last year a focus have been on understanding the behavior of Se and Na in CZTSSe. SIMS image depth profiling shows a non-uniform spatial distribution of selenium and supports a mechanism where selenization is accompanied by grain growth rather than substitution of selenium for sulfur. The distribution of Na have been studied during grain growth and in post annealed samples, where it is shown that Na is highly mobile even at temperatures as low as 100 degrees C, and play an important role in the overall efficiency. Solar cell devices have been fabricated and characterized by current-voltage measurements and external quantum efficiency measurements. The incompletely selenized absorbers displayed poorer carrier collection and absorption. Therefore, S-Se grading attempts were performed with gradients in the precursor films were deposited. However, since high chalcogen partial pressure is needed in the annealing step, these gradients were lost, and the S/Se ratio in the final film mainly determined by the annealing conditions. Still, relatively high device efficiencies of up to 9.3% (without antireflective coating) were obtained. Initial experiments with Sn-Ge exchange have been performed by addition of a thin Ge layer on top of CZTS precursors. A promising efficiency of 9.7% was obtained already in one of the first attempts. Our first-principles calculations reveal that CZTYS compounds have similar band structure as CZTSSe, though larger band gap energies which is suitable for a graded material structure with the Cu2Zn(Sn,Ge,Si)S4 alloy. The analyses indicate that the Ge-based alloys will be more stable than the corresponding Si-based alloys despite Si, Ge, and Sn belong to the same column in the periodic system. Intriguingly, incorporating Ge or Si into Cu2ZnSnS4 has rather different impact on the material. The Ge- and Sn-like states are much more hybridized than the corresponding Si- and Sn-like states, and Ge seems to be a more suitable substituting element for Sn; this limits the possibility to grow Si-rich alloys. Furthermore, modeling of Cu2XSnS4 (X = Zn, Ni, Mn, Ca, Mg, Fe, and Be) reveals that all these CZTS-like materials have high concentration of anti-site defect pair. For CZTS this defect is responsible for unwanted disorder of the material. As alternative materials, we show that Cu(Sb/Bi)(S,Se,Te)2 alloys and Cu2SnS3 have properties that can enable ultrathin (< 50 nm) PV devices with sufficiently high efficiencies. We collaborate with Techn Univ of Denmark, Luxembourg Univ, HU-Berlin, Barcelona Univ, NIMS Japan, Arizona State Univ, and Nanyang Techn Univ Singapore.

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Thin-film solar cells based on Cu2ZnSn(S,Se)4 and Cu2Zn(Sn,Ge,Si)S4 are developed for affordable sunlight harvesting with sufficiently high efficiency, long lifetime, and low exploitation of material resources. We optimize the solar cell efficiency with the profile of S/Se, Sn/Ge and Sn/Si gradients in depth, and we investigate the degradation processes, as well as the positive or detrimental impacts on the device performance due to structural disordering and local defect formations. Four scientifically intertwined work packages comprise the project objectives that combine synthesis and device prototyping with structural, electrical and optical characterization together with supporting calculations/simulations. This involves temperature dependent IV coupled to device modeling, with characterization of films by transmittance, reflectance, ellipsometry, and PL. Compositional and impurity profiling obtained by RBS and SIMS, phase analysis by XRD and Raman scattering, and analysis of open volume defects by positron annihilation spectroscopy. Defect calculations and diffusion modeling are performed by means of density functional and molecular dynamics. The project outcome will be a fundamental understanding of material/defects physics, degradation mechanisms, and stability of solar cell performance. The project is carried out in two research groups at the Dept of Physics, Univ of Oslo, linked to the Centre for Materials Science and Nanotechnology, and also at the Ångström Solar Center, Uppsala Univ. We utilize clean room facilities at MiNaLab and Ångström Lab, as well as national high-performance computing centers through NOTUR. The project trains one PhD, one guest-PhD, and two postdoctors in the field. Strong international collaboration serves for networking, exchange of knowledge, and scientific visibility. Long-term international research visits by the PhD student and postdoctors strengthen these contacts.