Photovoltaic thin film solar cells based on kesterite Cu2ZnSn(Sx,Se1–x)4 compounds (CZTSSe) have reached >12% sunlight‐to‐electricity conversion efficiency. This is still far from the >20% record ...devices known in Cu(In1–y,Gay)Se2 and CdTe parent technologies. A selection of >9% CZTSSe devices reported in the literature is examined to review the progress achieved over the past few years. These devices suffer from a low open‐circuit voltage (Voc) never better than 60% of the Voc max, which is expected from the Shockley‐Queisser radiative limit (S‐Q limit). The possible role of anionic (S/Se) distribution and of cationic (Cu/Zn) disorder on the Voc deficit and on the ultimate photovoltaic performance of kesterite devices, are clarified here. While the S/Se anionic distribution is expected to be homogeneous for any ratio x, some grain‐to‐grain and other non‐uniformity over larger area can be found, as quantified on our CZTSSe films. Nevertheless, these anionic distributions can be considered to have a negligible impact on the Voc deficit. On the Cu/Zn order side, even though significant bandgap changes (>10%) can be observed, a similar conclusion is brought from experimental devices and from calculations, still within the radiative S‐Q limit. The implications and future ways for improvement are discussed.
The possible role of the cationic (Cu/Zn) disorder and anionic (S/Se) distribution on the severe Voc deficit generally reported in CZTSSe‐based solar cells is investigated. It is concluded that none of these disorders is likely to be the direct main culprit for the Voc deficit, but that the partially‐ordered state is preferred as it promotes radiative recombination.
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Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth ...gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se
films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe
grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se
and Cu(In,Ga)
Se
phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells.
The existence of disorder is one possible reason for the limited performance of kesterite solar cells. Therefore further knowledge of the order-disorder phase transition, of factors which influence ...the degree of order and of methods to determine this material property is still required. In this study we investigated the order-disorder transition in the kesterite material Cu2ZnSnSe4 by in-situ optical transmission spectroscopy during heat treatments. We show in-situ results for the temperature dependence of the band gap and its tailing properties. The influence of cooling rates on the phase transition was analyzed as well as the ordering kinetics during annealing at a constant temperature. The critical temperature of the phase transition was determined and the existence of a control temperature range is shown, which allows for controlling the degree of order by the cooling rate within this range. Additionally we performed Raman analysis to link Raman spectra to the degree of order in Cu2ZnSnSe4. A correlation between the intensity ratio of A-modes as well as B-/ E- Raman modes and the degree of order was found.
Kesterite‐based Cu2ZnSn(S,Se)4 semiconductors are emerging as promising materials for low‐cost, environment‐benign, and high‐efficiency thin‐film photovoltaics. However, the current state‐of‐the‐art ...Cu2ZnSn(S,Se)4 devices suffer from cation‐disordering defects and defect clusters, which generally result in severe potential fluctuation, low minority carrier lifetime, and ultimately unsatisfactory performance. Herein, critical growth conditions are reported for obtaining high‐quality Cu2ZnSnSe4 absorber layers with the formation of detrimental intrinsic defects largely suppressed. By controlling the oxidation states of cations and modifying the local chemical composition, the local chemical environment is essentially modified during the synthesis of kesterite phase, thereby effectively suppressing detrimental intrinsic defects and activating desirable shallow acceptor Cu vacancies. Consequently, a confirmed 12.5% efficiency is demonstrated with a high VOC of 491 mV, which is the new record efficiency of pure‐selenide Cu2ZnSnSe4 cells with lowest VOC deficit in the kesterite family by Eg/q‐Voc. These encouraging results demonstrate an essential route to overcome the long‐standing challenge of defect control in kesterite semiconductors, which may also be generally applicable to other multinary compound semiconductors.
Kesterite Cu2ZnSnSe4 (CZTSe) thin‐film solar cells with independently confirmed 12.5% total area efficiency are demonstrated using a novel strategy to effectively control the formation of intrinsic defects and defect clusters in CZTSe by carefully engineering the local chemical environment (e.g., suitable local chemical composition, oxidation states of cations) during film growth.
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Photovoltaic (PV) electricity generation is the fastest growing energy source globally and also humanity's best opportunity to achieve the urgently required deep decarbonisation of the energy sector. ...As PV enters the terawatt scale, with millions of modules in a single PV power plant, quality testing of installed PV modules becomes indispensable to guarantee PV as a reliable long‐term source of electricity. We present a method that extends the use of photoluminescence (PL) imaging to field‐deployed solar modules in full sunlight. The method takes advantage of sunlight absorption in the Earth's atmosphere in a narrow spectral range around 1,135‐nm wavelength and recent developments in ultranarrow bandpass optical filter technology. The technical principles and experimental data are provided. This method lays the foundations for PL imaging, a powerful inspection method for the PV industry and research, to be applied to routine high‐volume inspection of fielded PV modules on large‐scale solar power plants.
We present a method that extends the use of photoluminescence (PL) imaging to field‐deployed solar modules in full sunlight. The method takes advantage of sunlight absorption in the Earth's atmosphere in a narrow spectral range around 1,135‐nm wavelength and recent developments in ultranarrow bandpass optical filter technology. This method lays the foundations for PL imaging, a powerful inspection method for the PV industry and research, to be applied to routine high‐volume inspection of fielded PV modules on large‐scale solar power plants.
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8.
Cover Image Rey, Germain; Kunz, Oliver; Green, Martin ...
Progress in photovoltaics,
09/2022, Volume:
30, Issue:
9
Journal Article
Peer reviewed
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9.
Cover Image Rey, Germain; Kunz, Oliver; Green, Martin ...
Progress in photovoltaics,
September 2022, Volume:
30, Issue:
9
Journal Article
Peer reviewed
The cover image is based on the Research Article Luminescence imaging of solar modules in full sunlight using ultranarrow bandpass filters by Germain Rey et al., https://doi.org/10.1002/pip.3563.
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10.
What is the bandgap of kesterite? Siebentritt, Susanne; Rey, Germain; Finger, Ashley ...
Solar energy materials and solar cells,
December 2016, 2016-12-00, Volume:
158
Journal Article
Peer reviewed
There are different ways to determine the bandgap of a semiconductor. In the case of strong tailing they lead to different results. Various versions of Tauc’s plot give the gap of extended states, ...whereas the photoluminescence and the quantum efficiency extend into the tail states. The absorption edge in kesterite is determined by tail states therefore different methods to determine the band gap lead to different results. To decide whether the main recombination path is in the bulk or at the interface, the activation energy of the recombination rate should be compared to the energy of the radiative recombination in the bulk. This is the energy of the photoluminescence maximum and can be approximated by the linear extrapolation of the low energy edge of the quantum efficiency spectrum.
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•Determines role of tail states for optical characterization of semiconductors, in particular kesterite.•Discusses influence of tail states on transport and quantum efficiency.•Demonstrates role of tails states in recombination process.•Clarifies interpretation of VOC extrapolation in the presence of tail states.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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