| 1. |
- Alberto, Helena, V, et al.
(author)
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Characterization of the Interfacial Defect Layer in Chalcopyrite Solar Cells by Depth-Resolved Muon Spin Spectroscopy
- 2022
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In: Advanced Materials Interfaces. - : John Wiley & Sons. - 2196-7350. ; 9:19
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Journal article (peer-reviewed)abstract
- As devices become smaller and more complex, the interfaces between adjacent materials become increasingly important and are often critical to device performance. An important research goal is to improve the interface between the absorber and the window layer by inserting buffer layers to adjust the transition. Depth-resolved studies are key for a fundamental understanding of the interface. In the present experiment, the interface between the chalcopyrite Cu(In,Ga)Se-2 absorber and various buffer layers are investigated using low-energy muon spin rotation (mu SR) spectroscopy. Depth resolution in the nm range is achieved by implanting the muons with different energies so that they stop at different depths in the sample. Near the interface, a region about 50 nm wide is detected where the lattice is more distorted than further inside the absorber. The distortion is attributed to the long-range strain field caused by defects. These measurements allow a quantification of the corresponding passivation effect of the buffer layer. Bath-deposited cadmium sulfide provides the best defect passivation in the near interface region, in contrast to the dry-deposited oxides, which have a much smaller effect. The experiment demonstrates the great potential of low energy mu SR spectroscopy for microscopic interfacial studies of multilayer systems.
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| 2. |
- Sousa, M. G., et al.
(author)
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Cu2ZnSnS4 absorber layers obtained through sulphurization of metallic precursors : Graphite box versus sulphur flux
- 2013
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 535, s. 27-30
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Journal article (peer-reviewed)abstract
- In this work we employed a hybrid method, combining RF-magnetron sputtering with evaporation, for the deposition of tailor made metallic precursors, with varying number of Zn/Sn/Cu (ZTC) periods and compared two approaches to sulphurization. Two series of samples with 1x, 2x and 4x ZTC periods have been prepared. One series of precursors was sulphurized in a tubular furnace directly exposed to a sulphur vapour and N-2 + 5% H-2 flux at a pressure of 5.0 x 10(+4) Pa. A second series of identical precursors was sulphurized in the same furnace but inside a graphite box where sulphur pellets have been evaporated again in the presence of N-2 + 5% H-2 and at the same pressure as for the sulphur flux experiments. The morphological and chemical analyses revealed a small grain structure but good average composition for all three films sulphurized in the graphite box. As for the three films sulphurized in sulphur flux grain growth was seen with the increase of the number of ZTC periods whilst, in terms of composition, they were slightly Zn poor. The films' crystal structure showed that Cu2ZnSnS4 is the dominant phase. However, in the case of the sulphur flux films SnS2 was also detected. Photoluminescence spectroscopy studies showed an asymmetric broad band emission which occurs in the range of 1-1.5 eV. Clearly the radiative recombination efficiency is higher in the series of samples sulphurized in sulphur flux. We have found that sulphurization in sulphur flux leads to better film morphology than when the process is carried out in a graphite box in similar thermodynamic conditions. Solar cells have been prepared and characterized showing a correlation between improved film morphology and cell performance. The best cells achieved an efficiency of 2.4%.
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| 3. |
- Fernandes, P. A., et al.
(author)
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Thermodynamic pathway for the formation of SnSe and SnSe2 polycrystalline thin films by selenization of metal precursors
- 2013
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In: CrystEngComm. - : Royal Society of Chemistry (RSC). - 1466-8033. ; 15:47, s. 10278-10286
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Journal article (peer-reviewed)abstract
- In this work, tin selenide thin films (SnSex) were grown on soda lime glass substrates by selenization of dc magnetron sputtered Sn metallic precursors. Selenization was performed at maximum temperatures in the range 300 degrees C to 570 degrees C. The thickness and the composition of the films were analysed using step profilometry and energy dispersive spectroscopy, respectively. The films were structurally and optically investigated by X-ray diffraction, Raman spectroscopy and optical transmittance and reflectance measurements. X-Ray diffraction patterns suggest that for temperatures between 300 degrees C and 470 degrees C, the films are composed of the hexagonal-SnSe2 phase. By increasing the temperature, the films selenized at maximum temperatures of 530 degrees C and 570 degrees C show orthorhombic-SnSe as the dominant phase with a preferential crystal orientation along the (400) crystallographic plane. Raman scattering analysis allowed the assignment of peaks at 119 cm(-1) and 185 cm(-1) to the hexagonal-SnSe2 phase and those at 108 cm(-1), 130 cm(-1) and 150 cm(-1) to the orthorhombic-SnSe phase. All samples presented traces of condensed amorphous Se with a characteristic Raman peak located at 255 cm(-1). From optical measurements, the estimated band gap energies for hexagonal-SnSe2 were close to 0.9 eV and 1.7 eV for indirect forbidden and direct transitions, respectively. The samples with the dominant orthorhombic-SnSe phase presented estimated band gap energies of 0.95 eV and 1.15 eV for indirect allowed and direct allowed transitions, respectively.
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| 4. |
- Salome, Pedro M. P., et al.
(author)
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A comparison between thin film solar cells made from co-evaporated CuIn1-xGaxSe2 using a one-stage process versus a three-stage process
- 2015
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In: Progress in Photovoltaics. - : Wiley. - 1062-7995 .- 1099-159X. ; 23:4, s. 470-478
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Journal article (peer-reviewed)abstract
- Until this day, the most efficient Cu(In,Ga)Se-2 thin film solar cells have been prepared using a rather complex growth process often referred to as three-stage or multistage. This family of processes is mainly characterized by a first step deposited with only In, Ga and Se flux to form a first layer. Cu is added in a second step until the film becomes slightly Cu-rich, where-after the film is converted to its final Cu-poor composition by a third stage, again with no or very little addition of Cu. In this paper, a comparison between solar cells prepared with the three-stage process and a one-stage/in-line process with the same composition, thickness, and solar cell stack is made. The one-stage process is easier to be used in an industrial scale and do not have Cu-rich transitions. The samples were analyzed using glow discharge optical emission spectroscopy, scanning electron microscopy, X-ray diffraction, current-voltage-temperature, capacitance-voltage, external quantum efficiency, transmission/reflection, and photoluminescence. It was concluded that in spite of differences in the texturing, morphology and Ga gradient, the electrical performance of the two types of samples is quite similar as demonstrated by the similar J-V behavior, quantum spectral response, and the estimated recombination losses.
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| 5. |
- Salome, Pedro M. P., et al.
(author)
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Passivation of Interfaces in Thin Film Solar Cells : Understanding the Effects of a Nanostructured Rear Point Contact Layer
- 2018
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In: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; 5:2
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Journal article (peer-reviewed)abstract
- Thin film solar cells based in Cu(In,Ga)Se-2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%.
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| 6. |
- Teixeira, Jennifer P., et al.
(author)
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Recombination Channels in Cu(In,Ga)Se2 Thin Films : Impact of the Ga-Profile
- 2020
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:23, s. 12295-12304
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Journal article (peer-reviewed)abstract
- Depth bandgap profiles via a [Ga]/([Ga]+[In]) variation in the Cu(In,Ga)Se-2 (CIGS) absorber layer have been implemented as a strategy to enhance the performance of CIGS solar cells. Since the [Ga]/([Ga]+[In]) determines to a large extent the position of the conduction band minimum, different Ga-profiles lead to different electronic energy levels structures throughout the CIGS layer. In this paper, from the investigation of the dependence of the photoluminescence (PL) on excitation power and temperature, we critically analyze the impact of a notch or a linear Ga-profile on the CIGS electronic energy levels structure and subsequent dominant recombination channels. Notwithstanding, two radiative transitions involving fluctuating potentials were observed for each sample, and significant differences in the luminescence resulting from the two Ga-profiles were identified. For the CIGS absorber with a notch Ga-profile, two tail-impurity radiative transitions involving equivalent donor clusters and the same deep acceptor level were ascribed to the CIGS/CdS interface region and to the notch region. The probability of radiative recombination in these two regions is discussed. For the CIGS absorber with a linear Ga-profile, two band-impurity radiative transitions involving an acceptor, with an ionization energy compatible with the V-Cu defect were ascribed to the CIGS/CdS interface region. Our results show that the dominant acceptor defects are dependent on the Ga-profile, and they also highlight the complexity of the radiative and nonradiative recombination channels revealed by the tight control of the parameters in the experiment.
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| 7. |
- Cunha, Jose M., V, et al.
(author)
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Decoupling of Optical and Electrical Properties of Rear Contact CIGS Solar Cells
- 2019
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In: IEEE Journal of Photovoltaics. - : Institute of Electrical and Electronics Engineers (IEEE). - 2156-3381 .- 2156-3403. ; 9:6, s. 1857-1862
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Journal article (peer-reviewed)abstract
- A novel architecture that comprises rear interface passivation and increased rear optical reflection is presented with the following advantages: i) enhanced optical reflection is achieved by the deposition of a metallic layer over the Mo rear contact; ii) improved interface qualitywithCIGS by adding a sputteredAl 2O 3 layer over the metallic layer; and, iii) optimal ohmic electrical contact ensured by rear-openings refilling with a second layer of Mo as generally observed from the growth of CIGS on Mo. Hence, a decoupling between the electrical function and the optical purpose of the rear substrate is achieved. We present in detail the manufacturing procedure of such type of architecture together with its benefits and caveats. A preliminary analysis showing an architecture proof-of-concept is presented and discussed.
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| 8. |
- Cunha, Jose M. V., et al.
(author)
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High-Performance and Industrially Viable Nanostructured SiOx Layers for Interface Passivation in Thin Film Solar Cells
- 2021
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In: Solar RRL. - : John Wiley & Sons. - 2367-198X. ; 5:3
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Journal article (peer-reviewed)abstract
- Herein, it is demonstrated, by using industrial techniques, that a passivation layer with nanocontacts based on silicon oxide (SiOx) leads to significant improvements in the optoelectronical performance of ultrathin Cu(In,Ga)Se-2 (CIGS) solar cells. Two approaches are applied for contact patterning of the passivation layer: point contacts and line contacts. For two CIGS growth conditions, 550 and 500 degrees C, the SiOx passivation layer demonstrates positive passivation properties, which are supported by electrical simulations. Such positive effects lead to an increase in the light to power conversion efficiency value of 2.6% (absolute value) for passivated devices compared with a nonpassivated reference device. Strikingly, both passivation architectures present similar efficiency values. However, there is a trade-off between passivation effect and charge extraction, as demonstrated by the trade-off between open-circuit voltage (V-oc) and short-circuit current density (J(sc)) compared with fill factor (FF). For the first time, a fully industrial upscalable process combining SiOx as rear passivation layer deposited by chemical vapor deposition, with photolithography for line contacts, yields promising results toward high-performance and low-cost ultrathin CIGS solar cells with champion devices reaching efficiency values of 12%, demonstrating the potential of SiOx as a passivation material for energy conversion devices.
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| 9. |
- Edoff, Marika, 1965-, et al.
(author)
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Ultrathin CIGS Solar Cells with Passivated and Highly Reflective Back Contacts – : Results from the ARCIGS-M Consortium
- 2019
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In: Proceedings of 36th European Photovoltaic Solar Energy Conference and Exhibition. ; , s. 597-600
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Conference paper (other academic/artistic)abstract
- In this work, we report results from the EU-funded project ARCIGS-M. The project started in 2016 and aims to reduce the use of indium and gallium by enabling the use of very thin Cu(In,Ga)Se2 (CIGS) layers while retaining high efficiency and developing innovative low-cost steel substrates as alternatives to glass. In the project, reflective layers containing TCO´s and silver have successfully been used to enhance the reflective properties of the rear contact. In addition, passivation layers based on alumina (Al2O3) deposited by atomic layer deposition (ALD) have been found to yield good passivation of the rear contact. Since the alumina layers are dielectric, perforation of these layers is necessary to provide adequate contacting. The design of the perforation patterns has been investigated by a combination of modeling and experimental verification by electron beam lithography. In parallel a nano-imprint lithography (NIL) process is further developed for scale-up and application in prototype modules. Advanced optoelectrical characterization supported by modeling is used to fill in the missing gaps in optical and electrical properties, regarding CIGS, interfaces and back contact materials.
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| 10. |
- Salome, Pedro M. P., et al.
(author)
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Influence of CdS and ZnSnO Buffer Layers on the Photoluminescence of Cu(In,Ga)Se-2 Thin Films
- 2017
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In: IEEE Journal of Photovoltaics. - : Institute of Electrical and Electronics Engineers (IEEE). - 2156-3381 .- 2156-3403. ; 7:2, s. 670-675
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Journal article (peer-reviewed)abstract
- The search for alternatives to the CdS buffer layer in Cu(In,Ga)Se-2 (CIGS) solar cells has turned out to be quite promising in terms of power conversion efficiency. In this paper, the typically used chemical-bath-deposited CdS layer is compared with an atomic-layer-deposited Zn1-xSnxOy (ZnSnO). An optical study by external quantum efficiency and photoluminescence on the influence of different buffer layers on the defect properties of CIGS is presented. For both buffer layers, the CIGS bulk and CIGS/buffer interface are strongly influenced by electrostatic fluctuating potentials, which are less pronounced for the sample with the ZnSnO buffer layer. This is associated with a lower concentration of donor defects at the CIGS near-interface layer. A change in the bandgap of the CIGS as a consequence of the buffer layer deposition is observed. This study expands the knowledge of defects in the complex quaternary semiconductor CIGS, which, as discussed, can be affected even by the choice of buffer layer and its deposition process.
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