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See detailDeliberate and Accidental Gas-Phase Alkali Doping of Chalcogenide Semiconductors: Cu(In,Ga)Se2
Colombara, Diego UL; Berner, Ulrich; Ciccioli, Andrea et al

in Scientific Reports (2017), 7

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of ... [more ▼]

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of gas phase alkali transport in the kesterite sulfide (Cu2ZnSnS4) system (re)open the way to a novel gas-phase doping strategy. However, the current understanding of gas-phase alkali transport is very limited. This work (i) shows that CIGSe device efficiency can be improved from 2% to 8% by gas-phase sodium incorporation alone, (ii) identifies the most likely routes for gas-phase alkali transport based on mass spectrometric studies, (iii) provides thermochemical computations to rationalize the observations and (iv) critically discusses the subject literature with the aim to better understand the chemical basis of the phenomenon. These results suggest that accidental alkali metal doping occurs all the time, that a controlled vapor pressure of alkali metal could be applied during growth to dope the semiconductor, and that it may have to be accounted for during the currently used solid state doping routes. It is concluded that alkali gas-phase transport occurs through a plurality of routes and cannot be attributed to one single source. [less ▲]

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See detailBetter Cu(In,Ga)Se2 solar cells based on surface treated stoichiometric absorbers
Choubrac, Léo UL; Bertram, Tobias UL; Elanzeery, Hossam UL et al

in Physica Status Solidi A. Applications and Materials Science (2017), 214, No. 1

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See detailWhat is the dopant concentration in polycrystalline thin-film Cu(In,Ga)Se2 ?
Werner, Florian UL; Bertram, Tobias UL; Mengozzi, Jonathan et al

in Thin Solid Films (2017), 633

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See detailDoping, Defects And Solar Cell Performance Of Cu-rich Grown CuInSe2
Bertram, Tobias UL

Doctoral thesis (2016)

Cu-rich grown CuInSe2 thin-film solar cells can be as efficient as Cu-poor ones. However record lab cells and commercial modules are grown exclusively under Cu-poor conditions. While the Cu-rich ... [more ▼]

Cu-rich grown CuInSe2 thin-film solar cells can be as efficient as Cu-poor ones. However record lab cells and commercial modules are grown exclusively under Cu-poor conditions. While the Cu-rich material’s bulk properties show advantages, e.g. higher minority carrier mobilities and quasi-Fermi level splitting - both indicating a superior performance - it also features some inherent problems that led to its widespread dismissal for solar cell use. Two major challenges can be identified that negatively impact the Curich’s performance: a too high doping density and recombination close to the interface. In this work electrical characterisation techniques were employed to investigate the mechanisms that cause the low performance. Capacitance measurements are especially well suited to probe the electrically active defects within the space-charge region. Under a variation of applied DC bias they give insights into the shallow doping density, while frequency and temperature dependent measurements are powerful in revealing deep levels within the bandgap. CuInSe2 samples were produced via a thermal co-evaporation process and subsequently characterized utilizing the aforementioned techniques. The results have been grouped into two partial studies. First the influence of the Se overpressure during growth on the shallow doping and deep defects is investigated and how this impacts solar cell performance. The second study revolves around samples that feature a surface treatment to produce a bilayer structure - a Cu-rich bulk and a Cu-poor interface. It is shown that via a reduction of the Se flux during absorber preparation the doping density can be reduced and while this certainly benefits solar cell efficiency, a high deficit in open-circuit voltage still results in lower performance compared to the Cu-poor devices. Supplementary measurements trace this back to recombination close to the interface. Furthermore a defect signature is identified, that is not present in Cu-poor material. These two results are tied together via the investigation of the surface treated samples, which do not show interface recombination and reach the same high voltage as the Cu-poor samples. The defect signature, normally native to the Cu-rich material, however is not found in the surface treated samples. It is concluded that this deep trap acts as a recombination centre close to the interface. Shifting it towards the bulk via the treatment is then related to the observed increase in voltage. Within this thesis a conclusive picture is derived to unite all measurement results and show the mechanisms that work together and made it possible to produce a high efficient Cu-rich thin-film solar cell. [less ▲]

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See detailElectrical Characterization of Defects in Cu-Rich Grown CuInSe2 Solar Cells
Bertram, Tobias UL; Deprédurand, Valérie; Siebentritt, Susanne UL

in IEEE Journal of Photovoltaics (2016), 6(2), 546-551

We study defects in CuInSe2 (CIS) grown under Cu-excess. Samples with different Cu/In and Se/metals flux ratios were characterized by thermal admittance spectroscopy (TAS), capacitance-voltage ... [more ▼]

We study defects in CuInSe2 (CIS) grown under Cu-excess. Samples with different Cu/In and Se/metals flux ratios were characterized by thermal admittance spectroscopy (TAS), capacitance-voltage measurements (CV) and temperature dependent current voltage measurements (IVT). All samples showed two different capacitance responses, which we attribute to defects with energies around 100 and 220 meV. Plus the beginning of an additional step that we attribute to a freeze-out effect. By application of the Meyer-Neldel rule, the parameters of the two defects can be assigned to two different groups, both lying within the energy region of the so-called ‘N1-defect’ that has been observed for Cu-poor absorbers. [less ▲]

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See detailVapour phase alkali species for Cu(In,Ga)Se2 solar cells
Berner, Ulrich; Colombara, Diego UL; Bertram, Tobias UL et al

Scientific Conference (2015, September)

Alkalis are essential in Cu(In,Ga)Se2 absorber layers for efficient solar cells. Current doping methods rely on solid state diffusion of an alkali through to the absorber layer, e.g. a thin NaF layer on ... [more ▼]

Alkalis are essential in Cu(In,Ga)Se2 absorber layers for efficient solar cells. Current doping methods rely on solid state diffusion of an alkali through to the absorber layer, e.g. a thin NaF layer on Mo or NaCl dissolved in a metal precursor ink[1]. The apparent concentration of alkali in the final absorber is determined by the initial alkali dosing and the use of an interfacial barrier to stop alkali diffusion from the substrate. Until now the vapor–absorber interface as a source or sink of alkali doping has been largely ignored. We show that device efficiency improves from 2 to 8% by gas phase Na adsorption alone. Conversely initial results show that Na can also be desorbed to the gas phase. Although these efficiencies are lower than those obtained by including Na directly in the precursor (device efficiency 13.3% [1]), the findings are relevant to all chalcogenide growers as they show that exact doping, and thus control of device efficiency, is only possible when gas phase adsorption/desorption processes are controlled. [less ▲]

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See detailWhat is the band gap of kesterite?
Siebentritt, Susanne UL; Rey, Germain UL; Finger, Ashley UL et al

in Solar Energy Materials & Solar Cells (2015)

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See detailHighly conductive ZnO films with high near infrared transparency
Hala, Matej UL; Fujii, Shohei; Redinger, Alex UL et al

in Progress in Photovoltaics: Research and Applications (2015)

We present an approach for deposition of highly conductive nominally undoped ZnO films that are suitable for the n-type window of low band gap solar cells. We demonstrate that low-voltage radio frequency ... [more ▼]

We present an approach for deposition of highly conductive nominally undoped ZnO films that are suitable for the n-type window of low band gap solar cells. We demonstrate that low-voltage radio frequency (RF) biasing of growing ZnO films during their deposition by non-reactive sputtering makes them as conductive as when doped by aluminium (ρ≤1·10−3Ω cm). The films prepared with additional RF biasing possess lower free-carrier concentration and higher free-carrier mobility than Al-doped ZnO (AZO) films of the same resistivity, which results in a substantially higher transparency in the near infrared region (NIR). Furthermore, these films exhibit good ambient stability and lower high-temperature stability than the AZO films of the same thickness. We also present the characteristics of Cu(InGa)Se2, CuInSe2 and Cu2ZnSnSe4-based solar cells prepared with the transparent window bilayer formed of the isolating and conductive ZnO films and compare them to their counterparts with a standard ZnO/AZO bilayer. We show that the solar cells with nominally undoped ZnO as their transparent conductive oxide layer exhibit an improved quantum efficiency for λ > 900 nm, which leads to a higher short circuit current density JSC. This aspect is specifically beneficial in preparation of the Cu2ZnSnSe4 solar cells with band gap down to 0.85 eV; our champion device reached a JSC of nearly 39 mAcm−2, an open circuit voltage of 378 mV, and a power conversion efficiency of 8.4 %. [less ▲]

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See detailAlternative etchning for improved Cu-rich CuInSe2 solar Cells
Depredurand, Valérie UL; Bertram, Tobias UL; Thevenin, Maxime UL et al

in Materials Research Society Symposia Proceedings. Materials Research Society (2015), 1771

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See detailPrediction of Photovoltaic Cu(In,Ga)Se2 p-n Device Performance by forward Bias Electrochemical Analysis of Only the p-Type Cu(In,Ga)Se2 Films
Colombara, Diego UL; Bertram, Tobias UL; Depredurand, Valérie UL et al

in Electrochemical Society Transactions (2015), 66(6), 19-25

This work is an attempt to rate the quality of Mo/Cu(In,Ga)Se2 films intended for fabrication of photovoltaic devices. The procedure is based on the simple current-voltage electrochemical analysis of the ... [more ▼]

This work is an attempt to rate the quality of Mo/Cu(In,Ga)Se2 films intended for fabrication of photovoltaic devices. The procedure is based on the simple current-voltage electrochemical analysis of the bilayer in a Eu2+/3+-containing electrolyte solution. Two series of bilayer samples were tested electrochemically, while sister samples were completed into Mo/Cu(In,Ga)Se2/CdS/i-ZnO/Al:ZnO/Ni-Al solid state devices and their current-voltage characteristics measured in the dark. A correlation was found between the reverse saturation current density of the solid state devices and an analogous parameter extracted from the electrochemical response in forward bias. While Eu2+ was found to be metastable in water posing restrictions to the application, reproducible measurements were achieved with a methanol-based solution. The intrinsic simplicity of the proposed methodology makes it particularly suitable for the implementation of a low-cost diagnostic tool. [less ▲]

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See detailIn-Se surface treatment of Cu-rich grown CuInSe2
Bertram, Tobias UL; Depredurand, Valérie UL; Siebentritt, Susanne UL

in Proceedings of the IEEE Photovoltaic Specialist Conference 2014 (2014)

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See detailMetastable defect in CuInSe2 probed by modulated photo current experiments above 390K
Luckas, Jennifer Maria UL; Longeaud, Christophe; Bertram, Tobias UL et al

in APPLIED PHYSICS LETTERS (2014), 104

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See detailInfluence of the Se environment on Cu-rich CIS devices
Depredurand, Valérie UL; Bertram, Tobias UL; Siebentritt, Susanne UL

in Physica B: Condensed Matter (2013), B 439

Detailed reference viewed: 126 (15 UL)