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The benefits of reducing the thickness of the Cu(In,Ga)Se2 absorber (CIGS) in thin film solar cells outweighs the reduced material costs and production time. It also lowers the minimum quality requirements for the CIGS layer since shorter pathways for electron extraction allow shorter electron diffusion length. However, the design of ultrathin solar cells raises specific issues in terms of absorber quality, light management, and electronic transport. The aim of this paper is to give an overview of the state of the art on ultrathin CIGS solar cells and to present some results and perspectives for the improvement of the opto-electronic properties of ultra-thin solar cells. The impact of the deposition techniques for CIGS such as coevaporation, reactive sputtering and electrodeposition on the properties of ultrathin absorbers will be discussed. Then the potential of replacing the molybdenum back contact by a Transparent Conducting Oxide or a metallic or nanostructured reflectors will be analysed.

Passivating contacts are becoming a mainstream option in current photovoltaic industry due to their ability to provide an outstanding surface passivation along with a good conductivity for carrier collection. However, their integration usually requires long annealing steps which are not desirable in industry. In this work we study PECVD as a way to carry out all deposition steps: silicon oxide (SiOx), doped polycrystalline silicon (poly-Si) and silicon nitride (SiNx:H), followed by a single firing step. Blistering of the poly-Si layer has been avoided by depositing (p+) microcrystalline silicon (μc-Si:H). We report on the impact of this deposition step on the SiOx layer deposited by PECVD, and on the passivation properties by comparing PECVD and wet-chemical oxide in this hole-selective passivating contact stack. We have reached iVoc > 690 mV on p-type FZ wafers for wet-chemical SiOx\(p+) μc-Si\SiNx:H with no annealing step.

International audience; For crystalline silicon (c-Si) solar cells, it is useful to measure accurately the thickness of silicon oxide (SiOx) layer presents on textured c-Si surface to further adapt the fluoride-based etching treatment. Common techniques used to characterize thin films thicknesses, such as ellipsometry or profilometry, are however not suitable for highly textured surfaces. In this work, a methodology based on Electrochemical Impedance Spectroscopy (EIS) has been developed to determine the thickness of anodic SiOx on n-type textured c-Si surface. EIS measurements have been carried out on bare c-Si surface as well as on c-Si surface with various anodic SiOx thicknesses grown by photo-oxidation. The as-obtained Nyquist and Bode diagrams enabled to plot the related Mott-Schottky curves and determine the corresponding flatband potentials (Vfb). A reference standard graph giving the anodic SiOx thickness according to measured Vfb has been therefore established. A shift of Mott-Schottky curves towards higher potential values with increased anodic SiOx thickness has been shown and explained. Mott-Schottky curves of photo-oxidized silicon surfaces have demonstrated a particular shape related to the different behaviors of Si/SiOx/electrolyte device depending on the applied overpotential. These results have been used to study the etching rate of anodic SiOx in NaHF2 fluoride media.

Laser contact opening is a critical step for solar cells manufacturing and needs to be optimized to achieve high efficiencies. In this paper, laser contact opening using a picosecond laser (wavelength 355 nm, pulse duration 10 ps) has been carried out on n-PERT precursors composed of a SiOx/SiOxNy stack on the rear polished side and a SiOxNy layer on the front textured side. By varying peak fluence from 0.130 J/cm2 to 2.159 J/cm2 and spot overlapping, ninety parameters combinations have been tested to open these dielectric layers. Surface morphology characterization, before and after laser ablation, has been realized using Confocal Laser Scanning Microscopy and Scanning Electron Microscopy. Bulk and surface compositions have also been investigated by Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy analysis, respectively. Results have shown the existence of four separated laser impacted areas on the polished side and a related ablation mechanism is suggested. Also, electrical characterization using four probe measurements and calibrated lifetime photoluminescence revealed that electrical properties of the silicon underlying increased when post laser annealing was performed associated with no spot overlapping. Then, nickel electroless deposition has been performed and first characterizations indicate adherence issues and inhomogeneous metallization. Characterization of metallized samples revealed that these observations were closely linked to the non-homogenous surface morphology and composition after laser ablation.

Light management of a solar cell can be enhanced by improving Jsc with proper texturing. Topographic images obtained by confocal laser microscopy or AFM are used to study the roughness of textured surfaces. Fast Fourier Transform is employed to separate and select spatial frequencies, which will be analyzed through MountainsMap®. Various kind of samples can be studied such as textured c-Si solar cells or even tandem devices made of perovskite grains evaporated on Si pyramids. The structural, morphological and geometrical results can be extracted in a fast, automatic and repeatable way with a good throughput to enable statistical analysis.

Cu2ZnSnSe4 solar cell absorbers are synthesized by large-area electrodeposition of metal stack precursors followed by selenization. A champion solar cell exhibits 8.2% power conversion efficiency, a new record for Cu2ZnSnSe4 solar cells prepared from electrodeposited metallic precursors. Significant improvements of device performance are achieved by the application of two etching procedures and buffer layer optimization. These results validate electrodeposition as a credible alternative to vacuum processes (sputtering, co-evaporation) for earth-abundant thin-film solar cell fabrication at low cost. Copyright (C) 2015 John Wiley & Sons, Ltd.

Being able to predict the output power of solar cells by analysis of only the first semiconductor layer – commonly called the absorber layer – i.e. before a number of further deposition steps are carried out – would be a clear advantage both at academic research level and for process monitoring in industry. Since the absorber layer is normally the most difficult to fabricate, the hypothesis is that the power conversion efficiency of the solid-state device is dominated by its optoelectronic properties. One option that allows extracting information on the absorber optoelectronic properties is by formation and interrogation of virtually reversible electrolyte (Schottky) junctions 1. We have recently shown that the short circuit current density and thus efficiency of Cu(In,Ga)Se2 solid-state devices can be predicted by measuring the electrochemical photocurrent density of the respective absorber layers in the presence of Eu3+ under illumination 2. In such a system the Eu3+acts as a scavenger for the electrons generated on the p-type semiconductor upon irradiation with photons with energy greater than the semiconductor band-gap. However, this correlation was found complicated by surface non-ideality, leading in some cases to pronounced charge-carrier recombination. Therefore, here we explore an alternative relationship. Similarly to p-n junctions, the electrical behavior vs bias of p-semiconductor/electrolyte junctions is a direct consequence of the energy-barriers for hole transfer 3. As such, the current-voltage dependence can be expressed by the diode equation 4, including the contribution of parasitic shunt paths (Eq. 1). Eq. 1 JD,L = JPh + J0(exp(-eΔV/kT)-1) + ΔV/RSh where JD,L are the dark and illuminated electrical current-density, JPh is the photocurrent density ( = 0 in the dark and < 0 under illumination), J0 is the reverse saturation current, e is the elementary charge, ΔV is the applied voltage, k is the Boltzmann constant, T is the absolute temperature and RSh is the shunt resistance of the solar cell. The dark-current in reverse bias is related to the parasitic terms RSh and J0, with RSh normally dominating. For good solid-state devices RSh should be as large as possible and J0 as small as possible. From inspection of literature solid-state device data the J0 term is more important in determining the overall efficiency of a device than RSh. However, J0 cannot be distinguished from RShin reverse bias, while in forward bias it is possible. Eu2+ ions are required to interrogate the junction in forward bias. Eu2+ acts as an electron donor in the system, revealing the ability of the semiconductor film to accept electrons when kept under forward bias. Due to the exponential term in Eq. 1, the higher this ability to accept electrons, the higher the expected reverse saturation current from such a semiconductor in a full device whilst the effect of RShis negligible. In this work we demonstrate that the reverse saturation current of CuInSe2 solar cell devices can be predicted by measuring their forward bias characteristics in the presence of Eu2+/3+, allowing the prediction of final solid-state device efficiency. To this end a series of CuInSe2 absorber layers were prepared by intentional deviation from a physical vapour deposition routine and they were split into two. Half were completed into solar cell devices and the other half were tested electrochemically. The chosen absorber layers gave solid-state device power conversion efficiencies between 6 and 12.5% 5. The electrochemical experiments consisted of voltammetric analyses in the dark with Eu2+/3+species. The extraction of a term electrochemically-equivalent to the reverse saturation current density is obtained by linear extrapolation of the forward dark current density to voltages close to the open circuit voltage of the semiconductor-electrolyte junction in the dark. Such electrochemically extracted saturation current is compared with the solid-state device saturation current. These findings open the way to a more reliable electrochemically-based estimation of thin film semiconductor properties with a clear link with solid-state device physics. A comparison between this proposed method and the method involving the measurement of the electrochemical photocurrent density will be made. References 1. Peter, L. M., Semiconductor Electrochemistry. Encyclopedia of Life Support Systems: Oxford, 2010. 2. Colombara, D. et al. Electrochemistry Communications 2014, 48, 99-102. 3. Tan, M. X. et al. Progress in Inorganic Chemistry, John Wiley & Sons, Inc.: 2007; pp 21-144. 4. Shockley, W. Bell System Technical Journal 1949, 28, 435-489. 5. Depredurand, V. et al. PVSC Proceedings, 2011 37th IEEE, 19-24 June 2011; 2011; pp 000337-000342. Acknowledgements LPV, LEM, Nexcis team members and Prof. Laurence M. Peter are acknowledged for help and discussion. Funding was provided by the European Commission through the "Scalenano" program (grant agreement nº 284486)/

Solution processing of inorganic thin films has become an important thrust in material research community because it offers low-cost and high-throughput deposition of various functional coatings and devices. Especially inorganic thin film solar cells – macroelectronic devices that rely on consecutive deposition of layers on large-area rigid and flexible substrates – could benefit from solution approaches in order to realize their low-cost nature. This article critically reviews existing deposition approaches of functional layers for chalcogenide solar cells with an extension to other thin film technologies. Only true solutions of readily available metal salts in appropriate solvents are considered without the need of pre-fabricated nanoparticles. By combining three promising approaches, an air-stable Cu(In,Ga)Se2 thin film solar cell with efficiency of 13.8% is demonstrated where all constituent layers (except the metal back contact) are processed from solutions. Notably, water is employed as the solvent in all steps, highlighting the potential for safe manufacturing with high utilization rates.

This paper aims at providing an updated overview of the main achievements in the development of solar cells based on Cu2ZnSn(S,Se)4 (CZTS(Se)) kesterite absorbers obtained by electrodeposition. Although undoubtedly challenging, the ultimate goal is to learn from the past works and build a solid framework for future advances in this field. What is the reason for the lower efficiency of electrodeposited CZTS(Se)-based devices (8%) compared to the world record efficiency achieved with a hydrazine-based solution approach (12.6%)? Can this gap be filled, or there are intrinsic limitations for this achievement? The review is divided into the three main electrodeposition approaches: sequential elemental layer, alloy co-deposition, and chalcogenide co-deposition. It is argued that considerable technical challenges must be overcome for the latter approach to be successfully applied. Plot of the record power conversion efficiencies of kesterite sulfide-based solar cells obtained by electrodeposition (hollow dots), and world record efficiency of CZTS(Se)-based devices (full dots). The dashed line shows the 15% minimum efficiency threshold considered relevant for potential industrial application.

The quality control of individual semiconductor thin films during fabrication of multiple layers is important for industry and academia. The ultimate aim of this research is to predict the efficiency of p–n junction solar cells by photoelectrochemical analysis of the bare p-type semiconductor. A linear correlation between the photocurrent measured electrochemically on Cu(In,Ga)Se2 absorber layers through a Eu3+ electrolyte junction and short circuit current and efficiency of the corresponding solid state devices is found. However, the correlation is complicated by pronounced recombination at the semiconductor/electrolyte interface, while the solid state interface behaves more ideally. Keywords: Photocurrent spectroscopy, Cu(In,Ga)Se2, Opto-electronic quality, Photoluminescence, Short-circuit-current

Electrodeposition followed by rapid thermal annealing is a favorable process for industrial solar thin film fabrication. In this regard, we attempt to develop a process for earth-abundant pure sulfide kesterite solar cells fabrication of Cu2ZnSnS4 (CZTS) at pre-industrial scale (15 × 15 cm2). Synthesis at a large scale is a challenging issue that we attempt to address discussing the choice of annealing parameters. We found out that in our system a low background pressure is needed to ensure a good vapors distribution. However, S, SnS, Zn volatilities are enhanced, making difficult the control of the composition. Annealing temperature profile has a strong influence on the final absorber composition. The introduction of an intermediate reactive pre-alloying step previously to the high temperature annealing is shown to help to obtain more compact absorbers, leading to a maximum power conversion efficiency of 2.4% for a 0.435 cm2 CZTS device.

Cu2ZnSnSe4 solar cells were synthesized by electrodeposition of metal stack precursors followed by selenization, a high potential process for industry, leading to conversion efficiencies above 5%. An additional selenium-capping layer deposited on the precursor before annealing showed improved uniformity and morphology of CZTSe layers compared to other selenization routes. Two different atmospheric annealing systems were used: a closed graphite box in a tubular furnace and a three-chamber dynamic rapid thermal processing furnace. The RTP system gave larger grains and more compact layers, whereas CZTSe selenized in tube furnace had smaller grains and a higher series resistance. Both annealing systems gave best cells power conversion efficiencies over 5%. We will discuss the device photoelectric properties and their relation to material structures and processing.

To meet future energy demand and limit global warming, researchers and industrials aim to make solar technologies more efficient and cost effective. Si-wafer based photovoltaic technology accounted for about 95 % of the total production in 2017 [1] and one of the key processing step allowing to reduce cost and increase efficiency of these solar cells is the metallization step [2]. Nowadays, screen printing is the leading metallization technology as it is simple, fast and suitable for mass production. However, this method relies on silver containing pastes, an expensive metal with a fluctuating cost [3]. Moreover, screen-printing has almost reached its limitations in term of contact quality and different ways to apply metallic contacts are required for high efficiency cell architectures and advanced cells concepts. In this context, nickel copper (Ni/Cu) electrochemical metallization, also called “plating”, is under investigation as it not only gives a solution to metal cost reduction by using cheaper metals but also has the potential to overcome screen-printing limitations [4]. Indeed, Ni/Cu plating is suitable for advanced silicon solar cell architectures and, among others interests, form very thin and non-porous metallic contacts with low contact resistivity and better aspect ratio. Despite this, Ni/Cu plating requires more improvements, especially in terms of reliability and adhesion before being implemented in mass production. Plating process optimization is only achievable through a complete understanding of electrochemical reactions mechanisms involved which is the purpose of this work. We have investigated the Ni/Cu electrochemical metallization for n-PERT (Passivated Emitter Rear Totally diffused) bifacial silicon solar cells having a front textured side and rear polished side. The different steps of the Ni/Cu plating process are the following: 1) localized laser contact opening of dielectric layers using a UV picosecond laser, 2) silicon surface deoxydation to remove silicon oxide, 3) palladium activation of the silicon surface through galvanic displacement, 4) electroless nickel deposition, 5) electrolytic copper deposition, 6) final annealing to form NiSi metallic contact. Firstly, we have demonstrated that the silicon surface morphology and composition were non-homogenous after localized laser ablation over all the laser ablation conditions. Indeed, after laser ablation, silicon oxide is still present on the silicon surface with different thicknesses over the laser opened area geometry. Consequently, palladium activation appears to be non-homogeneous with a higher number of palladium particles on the border of laser opened area. Since nickel deposition rate depends on the number of palladium particles deposited, border effects of the nickel layer have been observed. Adherence and quality of the metallic contacts are thus adversely affected. Accordingly, we have investigated in details the roles of the different parameters involved in surface deoxydation and palladium galvanic displacement reactions in order to obtain a more homogeneous activated silicon surface before nickel deposition. This allowed us to control palladium deposition process by promoting palladium nucleation over growth, enabling to have more homogeneous silicon activated surface. A changer: “”is currently being done for nickel electroless deposition and copper electrolytic deposition to increase the quality of metallic contact and silicon metal interface through a better control of deposition conditions. Moreover, a direct plating route without palladium activation, which is a challenge for metallization of bifacial solar cells, will be presented. References: [1] Photovoltaics report, Fraunhofer Institute for Solar Energy Systems, ISE, Freiburg, 2018. www.ise.fraunhofer.de. [2] V.K. Bajpai, P. Pant, C.S. Solanki, Thin uniform nickel seed layer formation and its impact on Ni-Cu contact adhesion for c-Si solar cell applications, Solar Energy. 155 (2017) 62–74. doi:10.1016/j.solener.2017.06.002. [3] ITRPV Roadmap 2018, itrpv.net/Reports/Downloads/2018/ (accessed February 7, 2019). [4] M.C. Raval, C.S. Solanki, Review of Ni-Cu Based Front Side Metallization for c-Si Solar Cells, Journal of Solar Energy. (2013). doi:10.1155/2013/183812. Figure 1

The development of low cost industrial processes is one of the key issues to make Cu(In,Ga)Se 2 based solar cells reach grid-parity. Such a process is found by using a two-step technology based on the sequential electro-deposition of a metallic precursor followed by a rapid annealing. Three types of metallic precursors (two-compound systems as copper–indium, copper–gallium and three-compound system as copper–indium–gallium) have been electrodeposited on a molybdenum sputtered soda lime glass and alloyed through a low annealing temperature. Then a selenium film has been evaporated and the stack has been annealed at high temperature in a rapid thermal processing furnace. A one-step heating profile has been used from room temperature to 550 °C in less than 1 min. Samples for which the heating was stopped after different annealing times have been characterized using several techniques: X-ray fluorescence spectrometry for elemental composition, X-ray diffraction and Raman spectroscopy for phase composition, scanning electron microscopy for structural analysis and glow discharge optical emission spectroscopy for diffusion study. Preferential formation reactions of the two-compound based metallic precursors have been studied and compared with the copper–indium–gallium metallic precursor used in a two step process. A gallium free system reacts faster than a gallium-based system and presents well-formed ternary compound after a standard selenization. However, the incorporation of gallium can be improved through a longer annealing time or a higher annealing temperature.

Photovoltaic Technical Conference, Aix en Provence, FRANCE, 2015; International audience; Earth-abundant kesterite Cu2ZnSnSe4 material is a promising candidate for the mass production of low-cost thin film solar cells. However, the synthesis of single-phase kesterite films is especially challenging, since the kesterite single-phase region in the equilibrium phase diagram is very narrow. In this study, the metal composition was varied within the Cu-poor composition range in order to study the presence of Sn-Se secondary phases. Both SnSe and SnSe2 are found in copper-poor CZTSe absorbers where Zn/Sn1 because the studied composition range is actually copper-poor zinc-rich and tin-rich. The Sn-Se secondary phases can be detected using X-ray diffraction, a bulk detection method. They are also detected at the absorber's surface by SEM and Raman spectroscopy. Therefore, when the Sn-Se phases are present, at least a part of them is located at the absorber's surface, which is highly detrimental to device performance. Acting as shunting paths, they reduce the device power conversion efficiency and demonstrate an apparent quantum efficiency effect under reverse bias. Removal of these phases from the surface by chemical etching greatly reduces their detrimental influence. Published by AIP Publishing.

Using structural analyses means of ex-situ Raman spectroscopy and X-ray diffraction combined with electrical measurements, we study the phase evolution in the growth by electrodeposition technique of CuInSe 2 on polycrystalline Mo. For this purpose the growth was stopped at different stages, and then the different layers were analysed. First growth steps seem to be controlled by the deposition of secondary phases, like elemental Se and Cu 2 Se binary. After the deposition of approximately 300 nm of material, CuInSe 2 ternary and ordered vacancy compounds start to adequately form. At a thickness close to 2000 nm, the formation of binary Cu x Se is observed, remaining up to the final growth process (4350 nm). All these results are compared with the kinetic model of the system under the consideration of the experimental composition evolution.

Photovoltaic (PV), with more than 300 GW of cumulative capacity installed by the end of 2016, represents about 1.5% of the world electricity production. It is now obvious that the growth of PV will not only continue but even accelerate, as a key player for the energy transition, reaching up about 5 TW by 2050, as projected by the International Energy Association. The main reason for this development is due to a reduction by 10 of PV electricity cost in ten years. This trend makes PV electricity competitive with classical fossil or nuclear electricity in many regions of the world. The decrease in cost has resulted from improvement in materials and methods, an increase in solar energy conversion efficiency and in volume production. Research and Development plays a major role in this process proposing innovation and scientific breakthroughs in existing PV technologies and introduction of new technologies. Scaling effects have played a key role in decreasing the production costs but further progress along with innovative technologies will be needed to lower costs. The comparison between thin film technologies and wafer based technologies is typical of this paradigm. Thin film technologies are basically coating technologies which are inherently better adapted for large area processing (thousands of km2) compared to slicing individual crystalline wafers and assembling them as a mosaic. However, any defect on a part of a large area process induces a huge impact on the whole panel where the slices of Silicon are sorted to produce module. Moreover, the cost reduction of the wafer technology with scaling and the industrial maturity of silicon technology is so rapid that this technology is cost leading. As a consequence, the PV industrial sector is dominated at more than 90% by Silicon solar cells. The remaining 10% is composed of vacuum based thin films. Electrodeposition, despite intrinsic advantages, with real success in industry with, among numerous other examples, damascene copper in semiconductor or permalloys in thin film head, is almost absent of this PV industrial landscape. However, introduction of plating will be easier and more probable for new cell technologies, where new investment has to be done and for which stringer requirements emerge. In this context, we will discuss and make some prospective about two case studies involving electrodeposition in PV: plating contacts on Silicon and Cu-In-Ga based plating for thin films solar cells. On Silicon PV market, Copper represents an alternative to Silver screen printing. Copper has a much lower price than Silver with an equivalent bulk resistivity. Interdigited Back Contact (IBC) and Heterojonction (HJT) solar cells with Cu plating already exists for years. Nonetheless, Cu being a lifetime killer in c-Si, a Ni diffusion barrier is usually used. This approach has been demonstrated for front side metallization, based on Light Induced Plating. The case of emerging bifacial solar cells will be specifically discussed with the use of Ni/Cu or Ni/Ag plating. The remaining challenges will be discussed, including specificity of plating on both sides simultaneously, surface roughness issue and his impact on adhesion, ghost plating linked to surface passivation defects. For producing thin film solar cells, an alternative to vacuum based deposition methods (co evaporation, sputtering), which are presently used, is to use electrodeposition or printing. We will make a comprehensive description of the scale-up of the electrodeposited CIGS precursor layers from the laboratory scale of a rotating disk electrode to a full-size 60 cm x 120 cm compatible with 1 m2/min production of solar panels. More specifically, we will discuss the different fabrication approaches for electrodeposited precursor CIGS materials, development and recycling of solution chemistries for Copper, Indium and Gallium. Resulting fabrication of solar cells will be described. We will also present applications and devices which specifically take advantage from electrodeposition approach that is localized deposition on patterned substrate. The specific mechanism of electrodeposition in microelectrode array configuration and the challenge of upscaling will be discussed. References L. Tous et al, Energy Procedia 124, 922–929 (2017). Q. Huang, et al, Journal of The Electrochemical Society, vol. 158, issue 2, 2011, D57-D61 P.-P. Grand et al, WO 2012/052657 C. Broussillou et al, IEEE, New Orleans (2015) A. Duchatelet et al, Applied Physics Letters, 109 (2016), 253901

This work reports the Raman scattering characterisation of CuInSe 2 precursors grown by single step electrodeposition and the corresponding layers recrystallised under sulphurising conditions for solar cell devices. The analysis of the spectra measured on the as-grown precursors has allowed identifying the main secondary phases in these layers with elemental Se, Cu―Se phases and chalcopyrite Cu-poor ordered vacancy domains. To deepen in the identification of the Cu―Se phases, these measurements have been correlated with the analysis of binary Cu―Se layers. The experimental data indicate that formation of both Se and Cu-Se phases is likely controlled by the Se content in the layers. For values stoichiometry below 1.15, excess Cu in the layers is accommodated in a phase with very low Raman efficiency (as Cu 2 Se). Increasing the content of Se leads to an increase in the spectral contribution from both Se and Cu 2―x Se, being the formation of these phases likely favoured under high excess Se conditions. The characterisation of the corresponding recrystallised layers has allowed analysing the impact of the presence of the secondary phases in the as-grown absorbers on the performance of the final solar cells.

Laboratory studies were conducted in a cube-shaped reactor with the aim of generating and controlling homogeneous turbulence near the wall, with zero mean primary flow and minimal mean secondary flow motions, to achieve a homogeneous electrodeposition process. Turbulence was generated by a jet array, divided into 4 sectors consisting of 16 jets each. Two-dimensional Laser Doppler Velocimetry measurements showed that a high turbulence level can be achieved but that the flow is dominated by steady mean motions on the scale of the tank size. Random activation of the four sectors using an appropriate timing can strongly improve the turbulence conditions compared with continuous activation of the four jet sectors. Random activation leads to suppression of steady mean motions and RMS values increase of 100%.

Single step electrodeposition (ED) of Se-rich CuInSe2 precursors, followed by RTP annealing under sulphurising conditions leading to S-rich CuIn(S,Se)2 films, constitutes a promising technology for low cost high efficiency solar cells. In this work, a Raman scattering (RS) analysis of Se rich precursors grown under ED conditions leading to different chemical compositions is reported. RS has allowed identification of the main secondary phases in these layers with elemental Se, Cu-Se binary and ordered vacancy compound (OVC) phases. The experimental data show a strong dependence of the spectral contributions related to Se and Cu-Se with the layer molecularity, and the formation of these phases is mainly determined by the content of excess Se in the layers. The correlation of these data with the characteristics of the solar cells fabricated with these precursors, shows the strong impact of the presence of the Cu-Se phase on the performance of the final devices. These results point out the key role played by this binary phase on the formation of secondary phases after the sulphurising step.

Inorganic semiconductors have properties that are notoriously difficult to control due to the deleterious impact of crystalline imperfections, and this is especially so in solar cells. In this work, it is demonstrated that materials grown using wet chemistry processes for the preparation of nanocristalline precursors can achieve the same performance as the best state of the art, namely conversion efficiencies above 11% with CuInS2. Interestingly, due to the growth process, the active material inherit a porous morphology that is shown to play a part in the performance and functionality of the active material. The new device morphology leads to a device operation closer to that of nanoscale organic interpenetrated solar cells or dye sensitized solar cells than to those of standard polycrystalline ones.

The electrodeposition of CuInSe2, one of the most appropriate absorber for thin layer photovoltaic solar cells, is investigated with a rotating disc electrode by voltametry, steady-state polarization and electrochemical impedance spectroscopy in dilute acidic solutions. Three main potential domains are distinguished. At low cathodic polarizations, binary Cu-Se compounds are deposited. Indium starts to incorporate beyond a potential threshold which is associated with a current peak similar to a passivation peak and with impedance diagrams showing a negative polarisation resistance. For more negative potentials the impedance diagrams exhibit mainly characteristic features corresponding to charge transfer under diffusional control. A reaction path is proposed. These results are discussed in terms of general behavior of multinary compound deposition. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Raman scattering analysis of S-rich CuIn(S,Se) 2 layers produced by electrodeposition of CuInSe 2 precursors and annealing under sulphurising conditions shows the strong dependence of their crystalline quality on the annealing parameters. Worsening of the crystalline quality is reflected by the presence of an increasing contribution at the higher frequency side of the main Raman band. The comparison of these data with the solar cell parameters points out the existence of a correlation between the efficiency of the devices and the width of the mode. The dependence of the Raman spectra on differences in the process parameters with significant impact in the microcrystalline quality of the layers give interest to the use of Raman scattering as process assessment technique for device quality control.

A detailed Raman scattering analysis of the electrodeposition process of nanocrystalline CulnSe 2 layers for solar cell devices is reported. The correlation of the Raman spectra measured after the growth of the layers with different times has allowed investigating the chemical phases involved in the film formation process and their resulting nanocrystalline structure. The experimental data indicate the presence of elemental Se and Cu-Se binary compounds as the main secondary phases in the layers, which are directly related to the Se and Cu excess conditions used in the electrodeposition growth. These data show the existence of a high content of elemental Se at the region close to the interface of the layer with the Mo-coated glass substrate, this being likely related to a lack of incorporation of In at the initial growth stages. Nanoscopic Cu-deficient domains are also revealed by the presence of a band at the low frequency side of the main CulnSe 2 peak.

CuInSe2 thin films one-step electrodeposited under different conditions were studied by MicroRaman spectroscopy to identify and quantify the individual phases present in the films. From the analysis of the Raman spectra, the main ternary phase (CuInSe2) and elementary selenium Se0 were clearly identified. Specific chemical etches confirm the presence of elementary selenium Se0 and copper selenide binary phases CuxSe in selenium rich film. The amounts of these two phases were evaluated from X-ray Fluorescence measurements and confirmed using phase selective chemical treatments.

The electrodeposition of multinary compounds is of key importance for future technologies. Cu-In-Se system represents a case example, with a great potential in the field of photovoltaics. At IRDEP we achieved efficiencies up to 11.5% based on electrodeposited Cu-In-Se material. In the present paper, nucleation and growth steps of CuInSe2 are studied by polarization, impedance spectroscopy and SEM. The bulk composition is measured by x-ray fluorescence. The chemical environment of Cu, Se and In and the surface composition are analysed by XPS. A quasi-instantaneous 3D nucleation is observed. Binary Cu-Se nuclei are first obtained, which induce the incorporation of indium leading to CuInSe2. With increasing growth, secondary phases and Se(0) are obtained. At the threshold of potential corresponding to indium incorporation, a blocking adsorbate is formed, which is reduced at higher cathodic overvoltage into Cu-In-Se compound. These results are discussed in terms of general behavior of multinary compound deposition.

The company NEXCIS has developed a 2-step process based on the electrodeposition (ED) of a metallic precursor subsequently annealed to produce Cu(In,Ga)(S,Se)2 absorbers (CIGS). The metallic precursor is electrodeposited on a highly conductive Mo-based back-contact on glass. It is subsequently reacted in an atmospheric pressure thermal treatment unit under chalcogens atmosphere to produce CIGS cells up to 17.3% externally certified and 60×120cm2 modules up to 14.0% aperture area efficiency. These figures demonstrate that a low-cost electrodeposition-based process can successfully produce modules with efficiencies close to the ones obtained from vacuum-based processes such as coevaporation or 2-step Sputtering followed by annealing in hydride gases. Still, to be proven acceptable for production, the developed process must also demonstrate that it is robust and capable of repeatedly generating the same output. In this work, statistical process control techniques are used on more than one hundred 60×120cm2 plates to calculate the process capability and show that the ED-based process is well suited for a CIGS production line.

The CIS by electrodeposition (CISEL) project between Electricite de France (EDF), Centre National de la Recherche Scientifique (CNRS)/Ecole Nationale Superieure de Chimie de Paris (ENSCP) and Saint-Gobain Recherche (SGR) aims at developing a low-cost electrodeposition process for Cu(In,Ga)(S,Se) 2 (CIGS) solar cells. The process is characterized by two main steps: (i) deposition of the precursor film and (ii) thermal annealing. This process enables the preparation of a large range of sulfur containing absorbers, with S/(S+Se) atomic ratio from 0% to more than 90%. The films are single phase over the whole composition range. The influence of Sulfur content on the microstructure has been shown with grain sizes decreasing with increasing sulfur content. Efficient solar cells can be obtained from all the different precursor compositions, with efficiencies of over 10% on lab cells on sulfur-rich absorbers, and 6–7% on 30×30 cm 2 devices. The homogeneity of 15×15 cm 2 substrates is also discussed.

This paper reviews the state of the art in using electrodeposition to prepare chalcopyrite absorber layers in thin film solar cells. Most of the studies deal with the direct preparation of Cu(In,Ga)Se 2 films, and show that the introduction of gallium in the films is now becoming possible from single bath containing all the elements. Electrodeposition can also be used to form precursor films with stacked layer structures, of pure elements or of combinations with binary or even ternary films. Thermal annealing treatments are of dramatic importance to provide suitable electronic quality to the layers. They are often done in the presence of a chalcogen (selenium or sulfur) over pressure and there is a tendency to use rapid thermal processes. Less studies are devoted to complete solar cell formation. Significant progresses have been made in the recent period with several groups achieving cell efficiencies around 8–10% on different substrates. A record efficiency of 11.3% is reported for a cell with an absorber presenting a band gap of 1.47 eV. First results on the manufacturability of the corresponding process to large areas are presented.

Photovoltaics is entering a new age. With more than 300 GW of cumulative capacity by the end of 2016, while it was almost zero ten years ago, which represents about 1.5% of the world electricity production, photovoltaics is becoming a key component in the energy transition. The situation is even more evident when considering the contribution of photovoltaics country by country, reaching about 8% in Germany, Italy, Greece. it is also emerging very rapidly in USA, China, India, Chile for instance. It is anticipated that the level of 500 GW will be reached by 2020 and the symbolic one of one TW in the next decade. The reason is that photovoltaics is becoming economically competitive with the traditional energy sector with negociated prices as low as 3 cents per kWh in some regions. However to go futher in this direction will need an disruptive evolution of all the production chain to lower the costs, increase the efficiency and increase the throughput. Up to now the photovoltaic industrial sector is mostly based on wafer based silicon technology and on vaccuum based thin film solar cells of cadmium telluride or copper indium gallium diselenide (CIGS). Electrodeposition is almost absent in this industrial lanscape, excepted for the emerging copper plating technology for the contact in silicon solar cells, and the other experiences for implementing the electrodeposition in thin film technologies failed. Our vision is that in the new context there will be a unique opportunity to demonstrate that electrodeposition can be implemented as a disruptive technology for larger scale PV depoyment. We will recall the results obtained for the electrodeposition of CIGS solar cells from the laboratory to the industrial pilot line in Nexcis ( a spin off of our laboratory). Using a two stage approach from the electrodeposition of metallic precursors followed by thermal selenization, a record cell efficiency of 17.3 % was reached, and modules with full size (120 x60 cm2) at 14% efficiency in 2015, which was equivalent to the state of the art for CIGS modules prepared by vaccuum methods. Unfortunately for economic reasons, as many CIGS companies at that time, the company was stopped in 2015. However the unique features of the CIGS electrodeposition technology for large area and high throughput and robustness was successfully demonstrated. This research line has been maintained in our laboratory and is presently reactivated taking the lessons of the Nexcis experience. We will discuss a new process for the electrodeposition of CIGS absorbers, via an intermediate step consisting in single bath electrodeposition of mixed oxide hydroxyde copper indium gallium layers followed by thermal hydrogen reduction to form the metallic layer before selenization. The deposition of transparent conductive oxide layers by photoelectrodeposition has been successfully developped too and may become a new standard to replace more costly sputtering methods [2-4. We will also present applications and devices which specifically take benefit from a unique advantage electrodeposition approach, that is localized deposition on structured substrates. Two examples will be given: one for the electrodeposition for photovoltaic semitransparent windows and the second one for the deposition of microcells and microlines for a new concept of PV modules working under concentration. In that case we will also discuss of the mechanisms of electrodeposition of multicomponent CIGS films in conditions of microelectrode array configurations, and the challenges for upscaling. We will conclude with some prospective views at longer term as for instance for using electrodeposition in the next generation of PV modules with higher efficiencies (>30%) based on tandem devives, in that case electrodeposition of thin film solar cells could be combined with classical silicon bottom cells. References F. Tsin, et al. Electrodeposition of ZnO window layer for an all-atmospheric fabrication process of chalcogenide solar cell, Scientific Reports 5(2015)8961 A. Duchatelet, G. Savidand, N. Loones, E. Chassaing, D. Lincot, Mechanism of Electrochemical Deposition of Cu-In-Ga Mixed Oxide/Hydroxide Thin Films for Cu(In,Ga)Se-2 Solar Cells, J. Electrochem. Soc. 161(2014)D3120 F. Tsin, D. Hariskos, D. Lincot, J. Rousset, Photo-assisted electrodeposition of a ZnO front contact on a p/n junction, Electrochimica Acta 220(2016)176 A. Duchatelet, K. Nguyen, PP Grand, D. Lincot, M. Paire, Self-aligned growth of thin film Cu(In, Ga)Se-2 solar cells on various micropatterns, Applied Physics Letters, 109 (2016) 253901 J. Rousset, F. Tsin, M. Guc, J. Vidal, A. Lebris, A. Thomere, V. Izquerdo Roca, D. Lincot, Perchlorate-Induced Doping of Electrodeposited ZnO Films for Optoelectronic Applications J. Physical Chem. C 120(2016)18953 F. Tsin, J. Rousset, A. Lebris, D. Lincot, Electrodeposited zinc grid as low-cost solar cell front contact, Progress in Photovoltaics 24(2016)1123

Using micro-Raman spectroscopy, we demonstrate that the formation of the CuS binary phase in CISEL™ cells absorbers is highly determined by the presence of a Cu–Se binary phase in the precursors and depends on the Cu/In ratio. A selective sulphurization mechanism of the Cu–Se phase is proposed as the origin of CuS platelets. For low Cu/In ratio both binaries are associated to the surface and do not affect the device performance. Conversely, when the binary phase in the precursor is present close to the back contact (for high Cu/In ratio), the CuS binary phase is also formed at this region after sulphurization. Strongly associated to this Cu-rich binary phase, a spinel-type phase is also observed in the sulphurized samples. This phase has n-type conductivity, which has a strong impact on the characteristics of the solar cells. The relationship between the presence of these phases and the electrical properties of the final devices is described, showing that the reduction of the Voc and the increase of the Rs are related to the formation of a back diode by the spinel-type layer, as consequence of an undesirable n-p-n structure.

International audience; Abstract The use of binary copper–nickel (Cu–Ni) codeposition from a complexing citrate electrolyte is proposed as a convenient model system for simple, rapid and inexpensive characterization of local mass-transfer limitations arising in the production of ternary copper-indium-selenium (CIS) thin films. Both the Cu–Ni and the CIS systems have been investigated in a small pilot cell and deposit thickness and composition distributions on a 5 × 5 cm2 cathode have been compared. The experimental comparison confirms that the mass-transfer characteristics measured for copper deposition in the binary Cu–Ni codeposition system offer an excellent representation of the mass-transfer-limited deposition of copper and selenium in the ternary CIS system. The binary Cu–Ni system presents a number of advantages for process development, among which the possibility of operating at neutral pH and being much easier to handle, less expensive and less toxic than the CIS system. The results of the study presented here, although targeted to CIS production, may also be of use for the development of other electrodeposition processes in which one or more electro-active species are reduced under mass-transfer control.

Single step electrodeposition (ED) of CuInSe 2 precursors followed by RTP annealing under sulphurising conditions constitutes a promising technology for low cost high efficiency solar cell technologies, obtaining efficiencies up to 11%. Further development of these technologies requires for a deeper understanding on the structure of the absorbers as function of the ED parameters, as well as their impact on the recrystallised layers. Some issues related with the nature of the semiconductor have to be analyzed before obtaining high efficiency devices. In particular, electronic state defects in CuIn(S,Se) 2 prepared by electrodeposition have not been widely studied. In this work we report on this electronic state defects and their modification with different the variation of the Cu/In ratio on the absorber layer.

This article reports a detailed Raman scattering and microstructural characterization of S-rich CuIn(S,Se)2 absorbers produced by electrodeposition of nanocrystalline CuInSe2 precursors and subsequent reactive annealing under sulfurizing conditions. Surface and in-depth resolved Raman microprobe measurements have been correlated with the analysis of the layers by optical and scanning electron microscopy, x-ray diffraction, and in-depth Auger electron spectroscopy. This has allowed corroboration of the high crystalline quality of the sulfurized layers. The sulfurizing conditions used also lead to the formation of a relatively thick MoS2 intermediate layer between the absorber and the Mo back contact. The analysis of the absorbers has also allowed identification of the presence of In-rich secondary phases, which are likely related to the coexistence in the electrodeposited precursors of ordered vacancy compound domains with the main chalcopyrite phase, in spite of the Cu-rich conditions used in the growth. This points out the higher complexity of the electrodeposition and sulfurization processes in relation to those based in vacuum deposition techniques.

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.

CuInSe2 and Cu(In, Ga)Se2 precursor layers have been prepared by electrodeposition, with morphologies suitable for device completion. These precursor films were transformed into photovoltaic quality films after thermal annealing without any post-additional vacuum deposition process. Depending on the preparation parameters annealed films with different band gaps between 1eV and 1.5 eV have been prepared. The dependence of resulting solar cell parameters has been investigated. The best efficiency achieved is about 10,2 % for a band gap of 1.45 eV. This device presents an open circuit voltage value of 740 mV, in agreement with the higher band gap value. Device characterisations (current-voltage, capacitance-voltage and spectral response analysis) have been performed. Admittance spectroscopy at room temperature indicates the presence of two acceptor traps at 0.3 and 0.43 eV from the valance band with density of the order of 2. 1017 cm-3 eV-1.

The electrodeposition of indium on copper substrates is studied in an acid sulfate solution for applications in CuInS 2 (CIS) and Cu(InGa)Se 2 (CIGS) solar cells. The study is focused on the film morphology and thickness uniformity on the nanometer scale. A two-step film growth behavior was observed, a conformal smooth film growth followed by a three-dimensional island growth. A hypothesis involving the Cu-In alloy formation and the Cu-In interdiffusion is proposed. While the room temperature alloy formation promotes the conformal deposition of a few monolayers of alloy, the fast interdiffusion between Cu and In further extends this alloy formation to a thicker layer and delays the typical island formation-growth phenomenon.

The kinetics of the electrodeposition of Cu-In-Se compounds, which are promising materials for photovoltaic devices, is still a matter of discussion. In the present paper, the codeposition mechanism is investigated by in-situ polarization and impedance spectroscopy and by ex-situ chemical and XPS analysis. A reaction path, which accounts for the incorporation of indium, is developed. This path emphasizes the role of surface sites: passivating adsorption of elemental selenium and catalytic sites made of binary CuSe2 phase. The reaction path accounts for the main experimental observations: potential dependence of the current density, composition and impedance.

The growth process of Cu-In-Se compounds was investigated by in-situ electrochemical techniques and ex-situ XPS and Raman spectroscopy. The growth can be divided in three well separated steps. In a first stage, the Cu concentration is high, leading to the formation of a Cu-rich binary phase, Cu2Se, which acts as a precursor for Se(IV) reduction into elemental Se(0). When the surface concentration of elemental Se(0) is high enough, a new surface phase is formed with a stoichiometry close to CuSe2. It acts as a catalytic site for indium incorporation. This results in the formation of CuInSe2 and Cu-poor chalcopyrite phases. The proportions of the different phases evolve during the growth. This complex mechanism is in agreement with the compositional, structural and electrical properties of the samples as a function of the thickness.

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The first stages of the electrochemical deposition of a Cu-In-Se compound on molybdenum-coated glass were investigated. The surface morphology was examined after various deposition times by a field emission gun-scanning electron microscope. The surface composition and chemical environment analysis were characterized by X-ray photoelectron spectroscopy (XPS). Raman spectroscopy, X-ray diffraction, and X-ray fluorescence were also performed. This enabled the growth steps to be evidenced. A quasi-instantaneous three-dimensional nucleation occurs. The first nuclei are made of a copper-rich Cu-Se phase without indium. Codeposition of indium starts when the Se/Cu surface ratio reaches a value close to 1, which confirms that the formation of a Cu-Se phase is a prerequisite for indium incorporation. With increasing the deposition time, the In/Cu surface ratio increases and tends to a constant value close to 0.5, smaller than the bulk value. The Se/Cu ratio increases first rapidly and then more slowly, when coalescence of the nuclei has occurred. XPS analysis shows that this two-step behavior corresponds to an evolution of the chemical environment of the Se species, indicating the presence of several Se compounds in the film. X-ray diffraction and Raman spectroscopy confirm the presence of a Cu-Se phase and elemental Se, in addition to the predominant chalcopyrite phase.

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Surface reactions of molybdenum-covered glass plates, used as back contact for thin Cu(In,Ga)(S,Se) 2 solar cells, are examined at open circuit in acidic electrolytes used for plating CuInSe 2 layers. A multinary element cementation reaction occurs, leading to the corrosion of molybdenum and the formation of CuSe x electroless deposit (x from 0.2 to 0.8), with well-defined compositions. Immediately after immersion, small nuclei appear on the surface, which then grow to form a quasi-continuous layer. The time evolution of the open-circuit potential of Mo and the Se/Cu surface atomic ratio are correlated and are in agreement with thermodynamic data predictions.

The electrochemical behavior of polycrystalline molybdenum thin films in contact with acidic aqueous solutions containing Cu(II), In(III), and Se(IV) species was investigated. The substrate and solutions are used for the electrodeposition of CuInSe 2 films in the field of photovoltaics. The chemical interaction between Mo electrode and the electrolyte at the initial steps of immersion is studied by in situ electrochemical measurements of the time evolution of the open-circuit potential. Ex situ field emission gun-scanning electron microscope observations for morphological investigations, X-ray photoelectron spectroscopy for surface composition, and chemical environment analysis was carried out. Raman spectroscopy, X-ray diffraction, and X-ray fluorescence were also performed. It is shown that molybdenum undergoes electrochemical cementation reactions associated with a characteristic potential increase with immersion time. Immediately after immersion, small nuclei of Cu-Se deposits appear on the surface, which then grow to form a quasi-continuous layer after 400 s. The chemical composition of the layer evolves with immersion time. No indium is incorporated. The global composition changes from a Se/Cu atomic ratio close to 0.3 to a ratio close to 0.7. The final layer contains at least two phases, i.e., umangite Cu 3 Se 2 and CuSe. These complex evolutions are discussed in terms of competing electrochemical reactions and thermodynamic considerations.