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Using a controlled bromine etching on kesterite absorbers, two major results are obtained. We establish the first defect depth profiling and secondary phases depth profiling of a state of the art Cu2ZnSnSe4 (CZTSe) film by using surface sensitive characterization methods (XPS and Raman spectroscopy) on successively etched samples, obtaining a direct insight on the factors hampering the performance of this class of absorber. In a second step, we demonstrate the possibility of significant improvement to the p-n interface in Cu2ZnGeSe4 (CZGSe)/CdS solar cells when a short bromine etching of the absorber is performed, with the Fill Factor improving by more than 7 points. This method offers a simple improvement pathway for state of the art kesterite devices, with a potentially broader application to thin film solar cells where the p-n interface is limiting.

This work combines experimental results and modelling assessment of high Ga, wide bandgap CIGSe solar cells fabricated on a transparent glass/TCO substrate yielding efficiencies above 10% without AR coating. An alkali pre-deposition treatment is performed, and the material analysis of the devices by GDOES, XRD and Raman spectroscopy reveals a significant improvement of the Ga incorporation to the matrix in the presence of an alkali element. The optoelectronic characterization of the solar cells reveals an absolute increase by more than two efficiency points for the alkali-doped samples, overcoming the 10% threshold and being, at the time of this work, the highest reported efficiency for a wide bandgap CIGSe solar cell on transparent substrate. The use of the device developed in this study in a full chalcogenide tandem configuration is assessed by numerical modeling, and different improvement pathways for tandem devices are proposed. Specifically, the replacement of the MoSe2 back interlayer by a more transparent MoO3 is deemed critical in tandem configuration.

In this work, the effect of different alkali doping strategies in kesterite Cu 2 ZnGeSe 4 absorbers is studied, including the impact on the structural and morphological characteristics of the semiconductor material, and the optoelectronic properties of the solar cell devices. The studied alkali elements include Li, Na, and K, and are introduced by two main approaches: post-deposition treatment (PDT), and pre-absorber synthesis (PAS). This study shows that PDT strategies are more interesting than PAS, and Li as the best alkali candidate for the further improvement of Cu 2 ZnGeSe 4 devices performance.

We report on the modeling of semi-transparent ZnTe-based photovoltaic devices. We propose innovative strategies to markedly improve the performances, so far mostly hindered by a poor band alignment at the p-n interface. Alternative buffer materials are considered, and we investigate on the surface selenization of the ZnTE films as well as the feasibility of a purely ZnTe homojunction. Using a realistic set of state-of-the-art parameters, a semi-transparent solar cell with a potential efficiency exceeding 7.5% is demonstrated possible., This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and the Government of Catalonia's Agency for Business Competitiveness (ACCIÓ).

This work exhibits fascinating phenomena behind the synthesis of pure Ge kesterite revealed by means of XRD, Raman spectroscopy and by SEM. The absorber is prepared by a sequential sputtering and annealing process. Stopping the synthesis at different times we show that in the case of Cu 2 ZnGeSe 4 the reaction proceeds via a vapor-liquid-solid phase reaction, thanks to the formation of Ge-Se liquid phases. Optimizing the synthesis process, we report a solar cell device with power conversion efficiency resulted in this study exceed the 6% (V oc = 556 mV, FF = 59.6%, J sc = 19.6 mA/cm2).

In this work, the addition of CuGa layers at the rear interface of Cu 2 ZnSnSe 4 solar cells is investigated as a promising strategy to improve morphological properties of the interface and to potentially create a back electron reflector through the formation of a thin CuGaSe 2 layer. This study shows a beneficial effect of the addition of thin CuGa layers on the different photovoltaic parameters with a remarkable efficiency increase, and preliminary evidences that they might be either creating a back surface field or having a passivation effect.

Alkali doping (Li, Na, K) has shown to be very relevant for obtaining high efficiency kesterite solar cells. Conversely to absorbers produced by chemical routes, the doping with very low alkaline contents of absorbers prepared by PVD routes is very complex. In this work we present an innovative approach based on the introduction of alkali-citrates during the CBD process of CdS, for the selective doping of Cu 2 ZnSnSe 4 /CdS interface. We demonstrate that this simple strategy is very useful for incorporating the alkalis into the system, reporting efficiencies over 10% for the K doped CdS.

Regarding the study of doping in kesterite solar cells, these strategies have been typically limited to the use alkali elements. In this work, we present a complete study of selective cation-doping/substitution by introducing small amounts of several kesterite closely related elements, like Ag (Cu position), Si, Ge, In, and Pb (Sn position). This study provides valuable results and understanding of the effect of different dopants, which could be of great importance to modify material properties and further improve devices characteristics through their future optimization.

In this work, we present a comparative study of different transition metal oxides, including n-type (TiO2, V2O5, MoO3) and p-type (Co3O4) oxides, as possible candidates for back electron reflector concepts in kesterite thin film photovoltaic technologies. For this purpose, oxides nano-layers deposited on the Mo back contact, with and without a capping Mo layer, are introduced in the Cu2ZnSnSe4 devices, analyzing their impact on the material and devices properties. We demonstrate that n-type oxides, in particular TiO2 can be an effective electron back reflector, showing remarkable improvements in the long-wavelength collection as well as 1-2 mA/cm2 absolute increase of JSC.

Raman spectroscopy has demonstrated to be a powerful tool for Cu 2 ZnSnSe 4 (CZTSe) characterization, allowing the assessment of relevant parameters such as crystal quality, secondary phases and defect presence. In this work a detailed analysis of CZTSe vibrational properties using non-bandgap Raman resonance effects is performed. UV-based Raman spectroscopy is presented as a promising technique for the precise assessment of V Cu and Znsn point defects, associated with 174 and 245 cm−1 Raman regions. Based on these results, correlation between the peak intensity, associated with V cu point defects, with the maximum value of the efficiency of the final devices is presented and discussed.

In the last years, kesterite based solar cells have achieved conversion efficiencies that although relatively low, are very attractive for the photovoltaic industry. The main efficiency limitations are related to the complexity of this material, and into a large extent are linked to the uncontrolled formation of grain boundaries and their nature. In this work we present an innovative approach based on the bi-directional crystallization of Cu 2 ZnSnSe 4 (CZTSe) absorbers by using combined back/front Ge nanolayers. In common CZTSe layers we observe two type of grain boundaries (GBs) with very different composition: horizontal and vertical ones. The use of this innovative bi-directional crystallization strategy allows us to reduce the horizontal-type GBs density, demonstrating a high impact on the solar cell devices parameters. In particular, FF and J sc are largely improved, correlating with the reduction of the series resistance associated to the elimination of the horizontal GBs, and permitting to easily achieve efficiencies exceeding 10%.

Post-deposition soft annealing (PDA) process for kesterites, either at low ( 250 °C) temperatures, on finished devices is known to improve the characteristics of chalcogenide thin film solar cells. Here we report the effect of PDA process for Cu 2 ZnSnSe 4 based solar cells, showing that with the increasing PDA temperature, there is creation of optoelectronically beneficial V Cu and Zn Cu point defects at the surface region. This is explained by the migration of Cu cations from the CZTSe surface to the near-surface region and vice-versa for the Zn cations, which perfectly correlates with the evolution of the devices parameters.

In this work we report the use of CdS bi-layers as possible buffers in Cu 2 ZnSnSe 4 (CZTSe) / CdS heterojunctions for solar cell applications. A first CdS layer with different thicknesses were deposited onto CZTSe and then annealed under air or Ar atmosphere at different temperatures, followed by a second CdS layer deposition. We demonstrate how this strategy contributes to increase the efficiency of the devices, enhancing the VOC and FF of the solar cells due to the improvement of the heterojunction properties. A complete characterization of the bi-layers and devices will be presented.

Cu 2 SnS 3 is an attractive material for photovoltaic applications due to its inherent properties, and because is formed only by earth abundant elements. In this work we present the preparation of Cu 2 SnS 3 layers by a sequential process based in the DC-sputtering of metallic precursors followed by a reactive annealing. We analyze the impact of metallic composition (Cu/Sn ratio) and order (Cu/Sn or Sn/Cu), as well as thermal treatment conditions (one-step vs two-steps annealing, temperature and S partial vapor pressure). In a first optimization we report a device with 2.4% efficiency, discussing the efficiency limitations and challenges to increase this value.

In this study we present beneficial effects on the device performance with a Ge-assisted crystallization of nanocrystalline CZTSe precursors. For low Ge content layers, an increase in doping density is observed, which results in 8.6% efficiency devices and V oc values of above 470 mV, corresponding to Voc deficits of 583 mV, comparable to current record devices. High Ge content layers exhibit enhanced grain growth, however, they are also associated with deterioration in cell performance. Admittance spectroscopy measurements identified the appearance of a deep defect for high Ge doping. These results indicate that an accurate control of group IV (Ge, Sn) elemental composition seems mandatory for high device performance.

Solid solutions of CuInSe 2 -ZnSe have recently shown promising potential as a photovoltaic material. In this work we investigate the CuInSe 2 -ZnSe and CuGaSe 2 -ZnSe systems for high V oc photovoltaic applications. It was found that the highest V oc (∼740 mV) was achieved when the In/(In+Ga) ratio in the precursor was 0.13, i.e. very Ga-rich composition. Selenization conditions of CuZnGaIn metallic precursors were studied by changing reactive annealing parameters: temperature, dwelling time, temperature ramp and quantity of elemental Se. The optimal selenization parameters obtained in this study were: 550 °C, 30min, 20 °C/min and 50 mg of Se.

Hybrid buffer layers configurations are investigated for their application in Cu 2 ZnSnSe 4 based solar cells. For this purpose, In 2 S 3 /CdS, ZnS/CdS, CdS/In 2 S 3 and CdS/ZnS were deposited by chemical bath deposition and fully characterized. Additionally, the impact of CdS grown with different precursors (CdSO 4 and Cd(NO 3 ) 2 ) were also tested. Solar cells with 7.3% efficiency were obtained using In 2 S 3 /CdS and 8.9% using ZnS/CdS hybrid configurations, with efficiencies comparable or higher than the reference devices. These promising results point towards the need of exploring alternative hybrid buffers for CZTSe-based solar cells not only to maintain high efficiencies, but also to reduce the Cd content.

Cu2ZnSnSe4 (CZTSe) solar cells were prepared at high temperatures on Si/SiO2 sodium free substrate. Si/SiO2 was used as substrate to study the formation of CZTSe absorber without the possible influence of Na and for analyzing its compatibility with Si microelectronic technology. This substrate allowed reaching higher temperatures than the ones commonly used with soda lime glass (SLG). An efficiency of 5.1 % was obtained on Si/SiO2 substrate with our standard process conditions. The first results of the processing at temperatures higher than the usual ones are also reported and discussed.

In this work, the concept of bifacial solar cells is experimentally demonstrated for the first time for pure-selenium kesterite using several transparent back contacts (TBC). This opens the way to new possible applications for kesterite based thin films solar cells like building integration and/or multi-junction devices for better solar spectrum utilization. The absorbers were produced by using a two step synthesis process (sputtering + reactive annealing) onto transparent conductive oxides. Devices with power conversion efficiencies ranging from 1.8% to 3.3% are here presented.

We report a compositional paradigm in kesterite compounds, where V OC and J SC must be optimized independently from point of view of composition. Device-grade Cu 2 ZnSnSe 4 thin films in solar cells are characterized by non-stoichiometric compositions. The reason for such a broad range is not entirely understood, and a precise “ideal” composition is not known. In this work compositionally graded Cu 2 ZnSnSe 4 films are prepared and composition is correlated to Raman spectral features and optoelectronic properties of a device based on the film. In the very Cu-poor and Zn-rich range a V OC maximum is obtained, while in the less Cu-poor and Zn-rich range a J SC maximum is obtained. In the ideal compositional range most cells are above 5% efficiency, with a maximum of 6.9% at Cu/(Zn+Sn) = 0.76 and Zn/Sn = 1.34, between the two points with maxima in V OC and J SC .

In this work we report the Cu doping of chemical bath deposited CdS for the preparation of Cu 2 ZnSnSe 4 /CdS:Cu hetero-junction. We demonstrate that increasing the Cu concentration in the reaction bath several fundamental properties of CdS are changed (optical, crystalline, morphological), positively impacting in the resulting devices. The improvement of the efficiency from 1.6% to 6.1% is mainly explained by the large increases on V OC , obtaining the higher voltage value reported to now in the literature for this hetero-junction. We propose the formation of a metal-insulator-semiconductor (MIS) type device to explain our experimental results, which opens the possibility to use the MIS structure to solve, at least in part, the voltage deficit problems of kesterites.

Optimized conditions for conventional thermal processing of Cu 2 ZnSnSe 4 are not readily transferable to the rapid thermal processing, though it is of high importance to test the industrial viability of this material. Here the effect of layer order has been investigated for Cu-Zn(O)-Sn precursor stacks selenized by rapid thermal processing to form Cu 2 ZnSnSe 4 thin films. The ordering is shown to have significant effects on the film properties, including composition and elemental loss, morphology, and secondary phase formation. Optoelectronic properties of devices based on these films also show a dependence on precursor stack order. The best performing device has a conversion efficiency of 4.3%, and uses a stack order of Sn/Zn/Cu.

As the interest in Cu 2 ZnSn(S x Se 1−x ) 4 (0 ≤ x ≤ 1) kesterite based thin film solar cells increases, the development of new processes and characterization methodologies are required to satisfy its own peculiarities. In order to achieve better device performance and higher efficiencies a detailed systematic study of the fundamental properties of these materials is required. In this study, Raman spectroscopy and X-ray diffraction (XRD) were applied together to evaluate the crystal structure and the phonon modes of the Cu 2 ZnSn(S x Se 1−x ) 4 solid solutions leading to the complete characterization of the structural and vibrational properties.

In this paper, we present a non destructive optical methodology based on the use of pre-resonant UV-Raman scattering measurements, which allows for the on-line fast characterization of ZnO:Al (AZO) transparent conductive oxide (TCO). The experimental data corroborate the possibility to obtain by this methodology reliable information about the electrical properties of the TCO layers. The UV-Raman spectra of AZO are characterized by the presence of a defect-induced band. The relative intensity of this band in the spectra shows a clear correlation with the TCO resistivity. This methodology has been tested in real case devices, showing its potential for its implementation as on-line control tool for TCO's in the photovoltaic industry.

As the interest in Cu2ZnSn(S, Se)4 (CZTSSe) kesterite based solar cells increases, the development of new processes and characterization methodologies are required to satisfy the peculiarities of these emerging and complex materials. In this case, the formation of secondary phases is largely expected affecting the properties of the devices. In this report, we present a highly selective methodology to detect the most probable secondary phases in the CZTSSe system, based in the application of pre-resonant Raman concepts, by choosing the excitation wavelength best suited for each particular case.

One of the limitations of online quality control processes is the inability of detecting electronic defects in semiconductor materials. This is particularly important in low cost processes such as electrodeposited Cu(In,Ga)(S,Se) 2 solar cells, due to the large number of inhomogeneities that can be expected. Standard techniques for characterizing electronic defects are not suitable for online control. However, optical techniques such as Raman scattering can be specially suited for online process monitoring applications. In this paper we analyze the correlation between the Raman spectral features and the presence of electronic defects. Our results confirm the potential of Raman spectroscopy for the indirect detection of point defects which are important for device performance.

Zn-rich Cu 2 ZnSnSe 4 (CZTSe) films were prepared by a two step process. We have varied the Zn/Sn ratio from 1.24 to 1.73 and analyzed the effects of precursor composition and annealing temperature (between 425° C and 550° C) on the morphological, structural, and optoelectronic properties of the films. Raman scattering measurements show the presence of ZnSe as the main secondary phase in the films, as well as SnSe at the back region of the films processed with lower Zn-excess values and annealed at lower temperatures. The effect of the different secondary phases on the optoelectronic properties is discussed. In a first optimization we obtain as a preliminary result a 4.8% efficiency solar cell.

The sodium supply via thermal evaporation of NaF during different stages of a three-stage Cu(In,Ga)Se 2 (CIGS) evaporation process has been investigated. Solar cells were processed on soda lime glass with Si 3 N 4 diffusion barrier and on polyimide foils at low substrate temperature of 475°C compatible with the stability of the polyimide foil. Secondary electron micrographs (SEM) of CIGS layers show inhomogeneous microstructure containing regions of small grains near the back contact when sodium is evaporated during the 1st and the 2nd CIGS growth stage, respectively. The CIGS layer structure is affected only to minor extent if sodium is incorporated in the 3rd stage. In order to correlate the layer inhomogeneities with the composition profiles, the CIGS layers were investigated with depth resolved Raman scattering and sputtered neutral mass spectroscopy (SNMS). Both analyzing techniques reveal a strongly graded composition across the CIGS absorber, with an intermediate Ga-poor region and Ga-rich surface and back regions. The performance of resulting solar cells was characterized by means of current-voltage (J-V) and external quantum efficiency (EQE) measurements. It is found that the photovoltaic performance of the cells depends significantly on the NaF incorporation method. Cells developed with a low temperature growth process yielded high efficiencies of up to 16.4% without antireflection coating when NaF was supplied during the 3rd stage of the CIGS growth process.

This paper describes the detailed microstructral characterisation of S-rich CuIn(S,Se)2 absorbers fabricated by single step electrodeposition (ED) of nanocrystalline CuInSe2 precursors followed by rapid thermal annealing under sulphurising conditions. This has allowed identifying the main secondary phases in the precursors as elemental Se, CuSe and Cu2Se. The Raman spectra from these precursors also show the presence of Cu-poor OVC domains, which contrasts with the Cu-rich conditions used in the ED growth. This has been related to the presence in the annealed layers of In rich secondary phases, which lead to a slightly Cu poor composition. Moreover, the used sulphurising conditions also lead to the formation of a relatively thick MoS2 layer between the absorber and the Mo back contact. The high microcrystalline quality of the layers is likely responsible of the relatively high efficiency value of 11% of the solar cells fabricated with these absorbers.