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Sulfur-based kesterite Cu2ZnSnS4 (CZTS) nanoparticles (NPs) have been produced in a wet-chemical synthesis route which provides high yields of CZTS-NPs per synthesis cycle. These NPs can be used as raw material for electronic inks, which can be processed to light-harvesting absorber thin films in photovoltaic devices. In the corresponding phase diagram, the desired kesterite phase is found only in a very narrow, off-stoichiometric composition region. Accordingly, kesterite materials are likely to contain unwanted secondary phases such as ZnS, Cu2S, Cu2SnS3, which may limit the photovoltaic device performance in terms of current-blocking domains and formation of low-energy recombination channels. These phases can form already during the synthesis but may also evolve at high temperatures, for example, during annealing of the thin film. To limit the formation of Cu2S and Cu2SnS3 secondary phases Zn-rich and Cu-poor compositions were chosen corresponding to device-grade CZTS. In this paper we report on the phase and composition analysis of CZTS using X-ray diffraction, Raman spectroscopy with different excitation wavelengths, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analysis was performed on annealed CZTS pellets prepared from condensed CZTS NP powders. To extract the phase constitution and the composition from the surfaces and from the sample interior the CZTS pellets were horizontally cleaved after the annealing step. The bulk of the pellet consists of the required CZTS phase with very small quantities of Cu2- xS and ZnS. The utmost amount of these secondary phases was found on the top of the sample whereby copper sulfide forms preferentially isolated domains. As these surface-located secondary phases can be removed chemically, CZTS absorber films in good crystalline quality are to be expected. [ABSTRACT FROM AUTHOR]

The increasing importance of the Cu(In,Ga)Se2 based thin films photovoltaic industry claims for the development of new assessment and monitoring tools to answer the needs existing in the improvement of the control of the processes involved in the production of solar cells modules. In this frame, a strong interest has been given to the development methodologies for the assessment of the CIGS absorber, nevertheless advanced optical tools for the characterization of the other layers in the solar cells are still missing. In this work, we report a non-destructive optical methodology based on resonant Raman concepts that has been developed for the characterization of Al doped ZnO layers (AZO) that are used as window layer in Cu(In,Ga)Se2 solar cells. Doping the ZnO layer with Al leads to the presence of a characteristic defect induced band at 510 cm−1 spectral region. The correlation of the relative intensity of this band with the resistivity of the layers provides a fast and reliable tool for their electrical monitoring. Analysis of solar cells fabricated with layers of different conductivities has allowed demonstration at cell level of the proposed methodology for the determination of efficiency losses related to degradation of the resistivity of the AZO layers. [ABSTRACT FROM AUTHOR]

ZnO nanorod arrays placed within the architecture of the Cu(In,Ga)Se2 solar cell present promising concepts to maximize photoconversion by optical and electrical enhancement. Three different Cu(In,Ga)Se2 solar cell architectures utilizing ZnO nanorods are introduced. New technological insights are given for electrodeposited ZnO nanorod anti-reflection coatings, examining the reflectance of devices before and after encapsulation. For the first time, device results from superstrate and bifacial CIGSe solar cells incorporating ZnO nanorods are presented. The potential benefits as well as technological aspects of fabricating these new architectures are discussed. Scanning electron microscope picture of the cross-section of an industrially produced CIGSe solar cell with a ZnO nanorod anti-reflection coating. [ABSTRACT FROM AUTHOR]

In this work, pneumatic spray pyrolysis was used to deposit Cu2ZnSnS4 kesterite thin film layers onto soda lime glass substrates/Mo coated substrates (substrate configuration). Furthermore, pneumatic spray pyrolysis was also tested as a potential way to grow kesterite thin film layers on ZnO nanorod arrays on SnO2:F coated glass substrates (superstrate cell configuration). In this case semi-transparent samples were obtained. A summary of the process parameters optimization to effectively coat the ZnO nanostructures is presented. In both solar cell configurations, absorbers were selenized at 400-500 °C and compared. For the substrate configuration, we present a device with a power conversion efficiency of 1.86%, Voc = 292 mV, Jsc = 16.2 mA/cm2 and FF = 39.6%. (a) Cross-sectional SEM image of ZnO nanorods covered by Cu2ZnSn(S1- xSe x)4 in a superstrate configuration, (b) layout of the conceptual idea. [ABSTRACT FROM AUTHOR]

In this work, Raman spectroscopy and X-ray diffraction were applied together to evaluate the crystal structure and the phonon modes of photovoltaic grade Cu2ZnSn(SxSe1-x)4 thin films, leading to a complete characterization of their structural and vibrational properties. Vibrational characterization has been based on Raman scattering measurements performed with different excitation wavelengths and polarization configurations. Analysis of the experimental spectra has permitted identification of 19 peaks, which positions are in good accord with theoretical predictions. Besides, the observation of Cu2ZnSnS4-like A symmetry peaks related to S vibrations and Cu2ZnSnSe4-like A symmetry peaks related to Se vibrations, additional Raman peaks, characteristic of the solid solution and previously not reported, are observed, and are attributed to vibrations involving both S and Se anions. [ABSTRACT FROM AUTHOR]

ABSTRACT Zn-rich Cu2ZnSnSe4 (CZTSe) films were prepared by a two-step process consisting in the DC-magnetron sputtering deposition of a metallic stack precursor followed by a reactive anneal under a Se + Sn containing atmosphere. Precursor composition and annealing temperature were varied in order to analyze their effects on the morphological, structural, and optoelectronic properties of the films and solar cell devices. Raman scattering measurements show the presence of ZnSe as the main secondary phase in the films, as well as the presence of SnSe at the back absorber region of the films processed with lower Zn-excess values and annealing temperatures. The ZnSe phase is found to accumulate more towards the surface of the absorber in samples with lower Zn-excess and lower temperature annealing, while increasing Zn-excess and annealing temperature promote its aggregation towards the back absorber region of the devices. These measurements indicate a strong dependence of these process variables in secondary phase formation and accumulation. In a preliminary optimization of both the composition and reactive annealing process, a solar cell with 4.8% efficiency has been fabricated, and potential mechanisms limiting device efficiency in these devices are discussed. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT Cu2ZnSnS4 (CZTS) is a promising thin-film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se2. We introduce a two-step process for fabrication of CZTS films, involving reactive sputtering of a Cu-Zn-Sn-S precursor followed by rapid annealing. X-ray diffraction and Raman measurements of the sputtered precursor suggest that it is in a disordered, metastable CZTS phase, similar to the high-temperature cubic modification reported for CZTS. A few minutes of annealing at 550 °C are sufficient to produce crystalline CZTS films with grain sizes in the micrometer range. The first reported device using this approach has an AM1.5 efficiency of 4.6%, with Jsc and Voc both appearing to be limited by interface recombination. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Cu2ZnSnSe4 kesterite compounds are some of the most promising materials for low-cost thin-film photovoltaics. However, the synthesis of absorbers for high-performing devices is still a complex issue. So far, the best devices rely on absorbers grown in a Zn-rich and Cu-poor environment. These off-stoichiometric conditions favor the presence of a ZnSe secondary phase, which has been proved to be highly detrimental for device performance. Therefore, an effective method for the selective removal of this phase is important. Previous attempts to remove this phase by using acidic etching or highly toxic organic compounds have been reported but so far with moderate impact on device performance. Herein, a new oxidizing route to ensure efficient removal of ZnSe is presented based on treatment with a mixture of an oxidizing agent and a mineral acid followed by treatment in an aqueous Na2S solution. Three different oxidizing agents were tested: H2O2, KMnO4, and K2Cr2O7, combined with different concentrations of H2SO4. With all of these agents Se2− from the ZnSe surface phase is selectively oxidized to Se0, forming an elemental Se phase, which is removed with the subsequent etching in Na2S. Using KMnO4 in a H2SO4-based medium, a large improvement on the conversion efficiency of the devices is observed, related to an improvement of all the optoelectronic parameters of the cells. Improvement of short-circuit current density ( Jsc) and series resistance is directly related to the selective etching of the ZnSe surface phase, which has a demonstrated current-blocking effect. In addition, a significant improvement of open-circuit voltage ( Voc), shunt resistance ( Rsh), and fill factor (FF) are attributed to a passivation effect of the kesterite absorber surface resulting from the chemical processes, an effect that likely leads to a reduction of nonradiative-recombination states density and a subsequent improvement of the p-n junction. [ABSTRACT FROM AUTHOR]

This work reports a process based on the use of an ultrathin (10 nm) ZnO intermediate layer for the improvement of the absorber/back contact interface region in Cu2ZnSnSe4 (CZTSe) kesterite solar cells. Raman microprobe measurements performed directly on the substrate surface after mechanical removal of the absorber layer indicate the occurrence of a decomposition reaction of Cu2ZnSnSe4 in contact with the Mo substrate. This leads to a significant degradation of the quality of the absorber/back contact interface, with the formation of a high density of voids. The presence of an intermediate ZnO layer on the Mo coated substrates inhibits the decomposition reaction, because it prevents interaction between the CZTSe and Mo layers during the annealing process. This leads to a significant improvement in the interface morphology as observed by detailed cross-section scanning electron microscopy. It also correlates with the observed increase of the device conversion efficiency from 2.5% up to 6.0%. The improvement in the optoelectronic characteristics of the cells could be related to a significant decrease of the device series resistance due to the formation of a smoother interface with low density of voids, resulting from the effective inhibition of the CZTSe decomposition reaction at the Mo back contact layer. [ABSTRACT FROM AUTHOR]