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Current single‐junction crystalline silicon (c‐Si) solar cells are approaching their power conversion efficiency (PCE) limit. Tandem solar cells are expected to overcome such efficiency limit, with perovskite on c‐Si tandems being a promising candidate for commercialization over the next years. This work aims atdescribing the conditions that tandem cells and modules need to fulfill to successfully enter the market in 2030.We first estimate that industrial c‐Si photovoltaic modules may reach a price level of about 15 c$/W in 2030 at a PCE of 22-24%, with an expected lifetime of 30 years and an annual degradation of 0.5%. For commercial relevance, we anticipate that tandem module efficiencies need to be increased to reach around 30%, while matching lifetime and degradation rate of c‐Si modules. Provided these conditions, we find that these tandem modules could then have a cost bonus of around 5-10 c$/W compared to c‐Si modules for reaching equal levelized cost of energyvalues.

International audience; We report on the development of highly reflective back contacts (RBCs) made of multilayer stacks for ultrathin CIGS solar cells. Two architectures are compared: they are made of a silver mirror coated either with a single layer of In2O3:Sn (ITO) or with a bilayer of ZnO:Al/ITO. Due to the improvement of CIGS rear reflectance, both back contacts result in a significant external quantum efficiency enhancement, in agreement with optical simulations. However, solar cells fabricated with Ag/ITO back contacts exhibit a strong shunting behavior. The key role of the ZnO:Al layer to control the morphology of the top ITO layer and to avoid silver diffusion through the back contact is highlighted. For a 500-nm-thick CIGS layer, this optimized RBC leads to a best cell with a short-circuit current of 27.8 mA/cm2 (+2.2 mA/cm2 as compared to a Mo back contact) and a 12.2%-efficiency (+2.5% absolute).

This paper focuses on low temperature (∼390 °C) growth of Cu(In,Ga)Se2 (CIGS) for high efficiency solar cells. The process developed in this work allows the growth of high quality CIGS thin films on flexible substrates such as polymer foils with lower impurity out-diffusion from the substrate. We carry out advanced analysis of the microstructure of CIGS absorbers prepared using multi-stage coevaporation on polyimide at low temperature leading to state-of-the-art efficiencies exceeding 18%. We also develop a method that can be used to easily interpret the x-ray diffraction data using compositional analysis, when asymmetric or multi-feature peaks are present. This method proves to be particularly useful to rule out in-plane inhomogeneities and undesired phases when in-depth gradients are present. This analysis is combined with electron microscopy and more advanced x-ray analysis (grazing incidence diffraction and pole plots) to create a global model of the film microstructure. We show that the film is composed of at least three stacked layers with different properties and that the microstructure of the layers has an impact on the cell performances. In particular, the short circuit current Jsc is strongly related to the intensity of the (112) peak.

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Flexible, lightweight, Cu(In,Ga)Se2 photovoltaic technology is becoming more and more popular, thanks to the very high efficiencies already achieved (>20%) and to the possible employment of roll-to-roll deposition techniques, offering new application opportunities. In this paper, we aim to improve the understanding and performances of recently discovered new front surface engineering approaches. We show that the improvement of Voc generally observed using heavy alkali (below Na) can be boosted by adding indium during the alkali postdeposition treatments (PDTs). The obtained modification of the absorber using NaF/KF + In PDT leads to an improvement of Voc by 20 mV with respect to NaF/KF PDT and by almost 50 mV with respect to NaF-PDT only. The electrical performances of solar cells prepared with different PDTs are analyzed and discussed in light of structural and optical characterization data, supported by physical modeling. We show that KF PDTs modify the absorber over a thin region close to the surface, and also deep in the bulk of the film. Our results with KF show a record efficiency of 18.1% without antireflecting coating (ARC), a value that is close to the highest reported value of 20.4% (with ARC).

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.

Cu(In,Ga)Se 2 (CIGS) based solar cells including an ultra-thin absorber layer ( 2 with this structure. 450 and 300 nm thick coevaporated CIGS layer were deposited on ZnO:Al and SnO 2 :F at high temperature with an optimized NaF post-deposition treatment. The cells with the SnO 2 :F back contact coupled with the Cu-reflector reaches an efficiency of 11.4% for the 450 nm thick CIGS and 9.5% for 300 nm thick CIGS which are higher than reference cells with Mo (10.2% and 7.2% respectively). With the ZnO:Al substrate, the efficiency is lower (9.2% for 450 nm thick CIGS). This is caused by a more resistive ZnO:Al/CIGS interface than SnO 2 :F/CIGS. Moreover, cells exhibit evidences of back surface recombination suggesting that a passivation of the back interface is required.

In this work, we have synthetized ultra-thin Cu(In,Ga)Se 2 (CIGS) absorbers with an alternative hybrid co-sputtering/evaporation process. Copper, indium and gallium are sputtered simultaneously with the thermal evaporation of selenium, thus avoiding the use of H 2 Se. Different CIGS absorbers with a thicknesses lower than 550 nm were deposited by a one-step stabilized process on Mo/soda lime glass substrates. Hence, the growth mechanisms of ultra-thin CIGS films when varying the power values during hybrid process has been studied. The temperature of the selenium effusion cell and the deposition temperature have been fixed to 190 °C and 550 °C respectively. Deposition time has also been fixed to 20 min. Ultra-thin CIGS solar cells with conversion efficiencies up to 6.5% have been fabricated with an absorber layer thickness of 470 nm.

One of the key parameters to achieve high efficienc y Cu(In,Ga)Se Se2 (CIGS) solar cells is the proper control of Ga grading. From previous results, by using bulk material characterization, a first assessment of the CIGS growth on polyimide foils at low temperature was performed. Moreover, a maximum efficiency of 17.8% ha has been achieved with steep Ga grading. Here, a first step to the establishment of a growth model of CIGS grown at low temperature is proposed by coupling surface and volume characterization. Then, simulation of the photovoltaic performances of the cell is used to complete experimental observations and to explain the benefic ial effect of steep Ga grading.

Metallic substrates are well suited for the deposition conditions of Cu(Inx,Ga1-x)Se2 (CIGS) solar cells due to their high temperature tolerance. In this work, Molybdenum (Mo) and stainless steel substrates were used in CIGS cell fabrication. Mo foil can be utilized as both the substrate and back contact in the cells. On the other hand, stainless steel despite requiring an impurity barrier layer and back contact deposition, remains economically more interesting. The CIGS absorbers were deposited on the metallic substrates in a 3-stage process at 480 °C. Due to the low deposition temperature, no impurity barrier layer was applied on the stainless steel substrates. For the same deposition conditions, the absorbers on the Mo substrates had a lower [Ga]/[Ga] + [In] ratio (GGI) of 0,24 as compared to 0,28 on the stainless steel substrates. Although the same absorber deposition process was applied to both substrate types, the highest efficiency of 14,0% was recorded on the Mo substrates.

A proper control of Ga concentration profile is mandatory to achieve high efficiency Cu(In,Ga)Se2 (CIGS) solar cells. At low temperature, deep gradients, detrimental for carriers' diffusion, are obtained when CIGS is deposited with a standard three-stage process: an optimization of the process is needed. In this study, we show the impact of a modify three-stage process on the depth of the notch by introducing Ga flux during the second stage from 0 nm/min to 1.1 nm/min. A higher open circuit voltage compensated by a lower short current density is obtained due to higher band gap energy. The surface and the bulk of the CIGS layer was analyzed at the end of the second stage by coupling different characterization techniques: glow discharge optical emission spectroscopy, Raman and X-ray photoelectrons spectroscopy. The presence of binary compounds as well as a Ga enrichment at the end of the second stage are observed when Ga is introduced during the second stage.

Devices based on a metal-semiconductor contact are potential candidates for the fabrication of photovoltaic cells as long as the corresponding junctions exhibit good rectifying properties and a low contact resistance. This article presents a detailed electrical analysis of the aluminum/CIGS system. The values of the representative parameters of the quality of the diode such as the barrier height, the ideality factor and the series resistance are deduced from an analytical study of the intensity-voltage characteristics. It is concluded that this system does fulfill the necessary (but not sufficient) basic conditions for being used in photovoltaic cells.

Cu(In1−x,Gax)Se2 (CIGS) thin films can be deposited with a high versatility of composition by a hybrid one-step co-sputtering/evaporation process. In this paper, plasma analysis is performed with an optical emission spectroscopy non-contact tool, following light emissions from different plasma species: sputtered copper, gallium, indium but also evaporated selenium. The variations of plasma characteristics are correlated with target self-bias voltage. Hence, the selenium flow threshold avoiding target poisoning and the main parameter controlling the CIGS composition are determined. This study allows us to set up a process calibration method by means of correlation between the selenium evaporation temperature and the elemental composition of the deposited thin film.

Cu(In,Ga)Se2 (CIGS) solar cells have achieved the highest efficiencies among thin-film solar technologies. At low temperatures ( V oc and FF with high Ga content at the interface. We show that the insertion of a Ga-rich layer favors the formation of ordered vacancy compound phases, and the absence of Ga during the last stage of the process impacts the overall element diffusion. Optimized Ga gradient leads to 17.8% efficiency solar cells without antireflecting coating or KF postdeposition treatment.

International audience; CIGS based solar cells are still part of the most popular solar cells developed. A chemical study is presented here by using XPS measurements, characterizing the surface chemistry and reactivity, with a focus on deoxidation and further ageing products, among which binary compounds and oxides can be found. Different chemical treatments are also suggested to adjust the surface composition of CIGS materials and to determine the re-oxidation delay. A specific point will be also discussed about the comparative evolution of alkali element distribution during the ageing of conventional CIGS/glass and CIGS/flexible substrate systems, requiring in this case an external alkali source.

International audience; This work explores a strategy to bring together the advantages of co-evaporation and sputtering by developing a hybrid co-sputtering/evaporation process, where copper, indium, and gallium are sputtered with the thermal evaporation of selenium. A 3-stage hybrid co-sputtering/evaporation process for Cu(In,Ga)Se2 (CIGS) thin films solar cells has been developed by controlling the deposition parameters (temperature, sputtering power, and evaporation). (In,Ga)2Se3 layers are deposited in the first stage, followed by Cu2 − xSe and Cu2 − xSe/(In,Ga)2Se3 layers. Material properties at different steps were studied in detail by X-ray fluorescence, energy dispersive X-ray, scanning electron microscopy, glow discharge optical emission spectroscopy, Raman spectroscopy, and X-ray diffraction. Solar cells were completed leading to 9.7% efficiency.

In this work, we show that in order to optimize the efficiency of Cu(In 1 - x ,Ga x )Se 2 (CIGS) solar cells with Cd-free Zn(S,O)-based buffer layers, the Ga concentration in the CIGS absorber layer towards the hetero-interface has to be adapted. We varied the In and Ga deposition rates in the last stage of our 3-stage co-evaporation process, leading to different compositional ratios x f = [Ga] / ([Ga] + [In]) between 0.15 and 0.6 in the top 400 nm of the absorber layer. All absorber layers were then completed with both CdS and Zn(S,O) buffer layers by chemical bath deposition. While cells with our standard grading of x f ≈ 0.4 in the front region result in a best performance of 15% with a CdS buffer, similar efficiencies with a Zn(S,O) buffer layer are only obtained when the Ga content near the hetero-interface is reduced down to x ≈ 0.25. The maximum efficiency for the CdS buffer layer coincides with the maximum open circuit voltage ( V oc ) and fill factor (FF). Interestingly, for the Zn(S,O) buffer layer, this is not the case: the V oc increases steadily for higher Ga ratios, while the FF is fairly constant for 0.25 x

A model is proposed for simulating power loss in utility-scale photovoltaic (PV) systems due to mismatch of module electrical characteristics. The main input to the model is a set of current-voltage (I- V) curves of thin-film Cadmium Telluride (CdTe) modules. The impact of the following effects on power loss due to mismatch are observed: the statistical distribution of module electrical characteristics, spatial temperature variation across an array, shading on an array, and module degradation over time. Additionally, two methods for synthetically generating I- V curves to mimic measured data are described and compared.

In this paper we report an alternative approach to perform in-depth characterisation of Cu(In,Ga)Se2 (CIGS) absorber layers. While usually groups stop their co-evaporation process at different points and analyse the precursor or intermediate phases, we perform in-depth analysis on the finished absorber layer as it will be used in the solar cell. A co-evaporated CIGS layer was cut to several samples, which then were chemically etched to different thicknesses. Compared to sputtering ablation techniques, this avoids the selective abrasion of atoms with different binding energies. The samples were analysed by Raman spectroscopy and X-ray diffraction. In-depth information is obtained by differentiating the signals of samples with different thicknesses after etching and a first order correction for absorption losses was executed. The Ga/(Ga + In) ratio extracted from X-ray diffraction measurements is in good agreement with the double gradient observed by glow discharge optical emission spectroscopy. A slight variation might indicate residual stress in the CIGS layer. A preferred (112) orientation across the whole film together with changing (220), (116) and (312) orientation preferences is reported and explained on the basis of the CIGS crystal structure. Raman signals attributed to ordered vacancy compounds are found throughout the whole sample thickness and not only close to the surface, as often reported in the literature.

In this work, a low cost passivated back-contact for ultra-thin Cu(In, Ga)Se 2 -based (CIGS) solar cells to improve the carrier collection is developed. The current loss due to rear-interface recombination was first estimated with an accurate opto-electrical model. We compared the use of a sol-gel TiO2 and an ALD-Al 2 O 3 layer for the back-contact passivation. 400–420 nm CIGS cells were fabricated on the oxide/Mo substrate with point-contacts patterned by nanoimprint lithography. The use of a patterned-Mo/Al 2 O 3 back-contact leads to an increase of the cell performance compared to the standard Mo back-contact. The passivation effect is discussed and is characterized by photoluminescence.

We investigate ultrathin CIGS solar cells with a nanostructured back mirror. Numerical calculations are used to optimize the optical design based on multi-resonant absorption. The impact of the different materials (CdS/ZnS, metal of the back contact: Mo/Au/Ag) is studied, and short circuit current densities above 36 mA/cm2 are predicted for CIGS absorbers as thin as 200 nm. We have developed a fabrication process based on the transfer of the CIGS solar cells, and nanoimprint lithography for the nanostructured back mirror. Light-trapping effects and Jsc improvement are evidenced in our first experimental results.

Photovoltaic cells based on chalcogenides CIGS (Cu(In, Ga)Se 2 ) thin films are a very promising technology. To improve cells performances, a fine optimization of the CIGS absorber properties is needed. Hence, we developed a cross strategy method combining the surface, volume and specific interfaces of the final device characterizations. These features deal with a large panel of physico-chemical techniques for the chemical composition (XPS, EDS, ICP-OES, GD-OES, AES), the morphology (SEM, AFM) and the optical parameters (spectroscopic ellipsometry) determination. This article demonstrates the crucial interest of this cross strategy on CIGS absorbers and focus on the accuracy and complementarities of each technique.

It is currently possible to prepare Cd-free Cu(In,Ga)Se2-based solar cells with efficiencies similar or higher than their CdS references. In these cells, higher efficiencies are generally obtained from soft chemical-based techniques giving conformal depositions such as chemical bath deposition (CBD), ion layer gas reaction (ILGAR) or atomic layer deposition (ALD). However most of these devices are characterized by their pronounced transient behaviour. The aim of this paper is to compare these different chemical-based methods (CBD, ALD, ILGAR…) and to try to provide evidence for the dominant influence of the interface between the Cd-free buffer layer and the window layer on the performance and on the metastable electronic behaviour of these solar cells.

Molybdenum thin films are widely used as back contact for CIGS-based solar cells. In this paper, the properties of Mo layers deposited by DC and RF sputtering are investigated in view of a specific optimization of electrodeposited CIGS solar cells. In the first part of the paper RF and DC films are grown on soda lime glass, in a pressure range from 2 to 20 mTorr, and for various RF power and DC current. They are then characterized by optical, electrical and structural methods. It appears that the films deposited by RF mode sputtering are more reflective, conductive and adherent than those obtained by DC mode. Structurally, they present different behaviors with respect to nucleation and growth of CIGS precursor layers by electrodeposition. A large difference is observed in the photovoltaic properties of completed cells, with much better performances obtained with DC Mo layers.

International audience; In this work, we have developed a hybrid one-step co-sputtering/evaporation Cu(In,Ga)Se2 (CIGS) process, where Cu, In and Ga are sputtered simultaneously with the thermal evaporation of selenium, thus avoiding the H2Se use. An appropriate control of the selenium flux is very important to prevent the target poisoning and hence some material flux variations. Indeed, the control of the CIGS composition must be rigorous to ensure reproducible solar cell properties. In this regard, a study of the correlations between plasma species and thin film composition, structure and morphology has been performed by varying power values and Se evaporation temperature in the 170 to 230 °C range. We started by studying the plasma with a powerful technique: optical emission spectroscopy, following light emissions from different plasma species: sputtered Cu, Ga, In but also evaporated Se. Hence, we determined the Se flow threshold avoiding target poisoning and the main parameter controlling the CIGS composition. Obviously, we also focused our interest on the material. We measured film composition and thickness of the samples with X-ray fluorescence and by energy dispersive X-ray. Different phases formed during the process were identified by Raman spectroscopy and X-ray diffraction. The optoelectronic cell properties showed promising efficiency of 10.3% for an absorber with composition ratios of [Cu]/([In] + [Ga]) = 1.02 and [Ga]/([In] + [Ga]) = 0.44. Finally, this work shows that we are able to control this hybrid process and thus the structure and composition of CIGS thin film for industrial transfer in the photovoltaic field.

Thin films of Zn(O,S) were deposited by atomic layer deposition from diethylzinc, water ( H2O), and hydrogen sulfide ( H2S). First, a study on the influence of the H2S/(H2O+H2S) pulse ratio from pure ZnO to pure ZnS was performed at deposition temperature Tdep=120 and 200 °C. Zn(O,S) films had higher S content than expected, and this effect was stronger at Tdep=200 °C. Then, Zn(O,S) films have been synthesized over the range of temperature 120–220 °C at the constant H2S/(H2O+H2S) pulse ratio of 9%. For Tdep 180 °C confirmed that exchange reactions occurred between the Zn(O,S) growing films and H2S. The grazing incidence x-ray diffraction patterns showed Zn(O,S) films with hexagonal wurtzite structures and with an optimum crystallization for temperatures Tdep=160–180 °C. Indeed, in this temperature range, well crystallized and large grains were obtained which was in good ...

In this work, we have developed a hybrid one-step co-sputtering/evaporation Cu(In,Ga)Se 2 process, where Cu, In and Ga are sputtered simultaneously with the thermal evaporation of selenium, thus avoiding the use of H 2 Se. An appropriate control of the selenium flux is very important to prevent the target poisoning and hence some material flux variations. Indeed, the control of the CIGS composition must be rigorous to ensure reproducible solar cell properties. In this regard, a study of the correlations between plasma species, thin film composition and morphology has been performed by varying Se evaporation temperature in the 170 to 230 °C range.

The control of the in-depth compositional gradient in Cu(In,Ga)Se 2 (CIGS) absorber layers is known to be crucial to achieve very high solar to electric energy conversion efficiencies in CIGS-based solar cells. In this work we varied the Ga-gradient extrinsically, i.e. by changing the ratio of In and Ga deposition rates throughout the deposition of CIGS. We show that the double Ga gradient intrinsically achieved by the 3-stage co-evaporation process can be further optimized. The challenge thereby is to lower the optical band gap while maintaining a high open circuit voltage and fill-factor.

International audience; Inkjet printing has a high potential for cost reduction in solar cell and thermoelectric industry. This study demonstrates that silicon thin films can be produced by inkjet-printing of a silicon ink followed by consequent drying and annealing steps. Ink formulation is a crucial parameter for sintering of the silicon nanoparticles and control of the microstructure at low temperature. Evolution of the sintering steps is monitored by scanning electron microscopy and by Raman spectroscopy which offers a fast and precise characterization of the microstructure and chemical composition of the thin films. While denser and more crystalline layers are obtained, cracks appear within thin film and substrate because of stress provoked by oxidation of the surface. Electrical conductivity is improved with higher annealing temperature until a threshold where both physical degradation and oxidation of the layers limits strongly the carrier transport phenomenon. In opposition transmission of the thin films is altered with increasing annealing temperature. Evolution of the thermal conductivity is performed by Raman spectroscopy and can be tailor in a large range between ~1 to ~100 W/mK. Therefore control of the microstructure evolution with applied annealing process allows tailoring of both microstructure and thermal conductivity of the thin films.

In order to develop photovoltaic devices with increased efficiency using less rare semiconductor materials, the concentrating approach was applied on Cu(In,Ga)Se 2 thin film devices. Microscale solar cells down to a few micrometers wide were fabricated. They show, at around x475, an efficiency of 21.3%, thanks to concentrated illumination (532 nm laser), compared to 16% efficiency under non-concentrated illumination. Due to the miniaturization, ultrahigh fluxes can be studied (< ×1000), without damaging the device. We analyse the high concentration regime of these micro-devices. Under ultrahigh light fluxes the collection efficiency decreases on certain devices. We attribute this effect to the screening of the electric field at the junction under high illumination. Numerical simulations of p-n junctions under intense fluxes corroborate this hypothesis. We built a homemade finite element method program, solving Poisson and continuity equations without resorting to the minority carrier approximation. We study the electric field at a p-n junction as a function of illumination intensity, and highlight the screening phenomena. Cu(In,Ga)Se2 thin films prove to be appropriate for a use under concentration, leading to significant gains in terms of efficiency and material usage. On these particular devices, ultrahigh illuminations can be used and the electric regime studied.

Being at the origin of an ohmic contact, the MoSe2 interfacial layer at the Mo/Cu(In,Ga)Se2 interface in CIGS (Cu(In,Ga)Se2 and related compounds) based solar cells has allowed for very high light-to-electricity conversion efficiencies up to 22.3%. This article gives new insights into the formation and the structural properties of this interfacial layer. Different selenization-steps of a Mo covered glass substrate prior to the CIGS deposition by co-evaporation led to MoSe2 interfacial layers with varying thickness and orientation, as observed by x-ray diffraction and atomic resolution transmission electron microscopy. A novel model based on the anisotropy of the Se diffusion coefficient in MoSe2 is proposed to explain the results. While the series resistance of finished CIGS solar cells is found to correlate with the MoSe2 orientation, the adhesion forces between the CIGS absorber layer and the Mo substrate stay constant. Their counter-intuitive non-correlation with the configuration of the MoSe2 interfacial layer is discussed and related to work from the literature.

International audience

Cu(In,Ga)Se2 microcells are photovoltaic devices of increased efficiency and low semiconductor consumption. They show an increase in efficiency due to concentrated illumination up to more than ×100, which is a breakthrough as thin films were previously limited to low concentration applications (about 10 suns). New measurements, made under concentrated natural solar illumination are presented, which confirm the conclusions of laser experiments. We also extend our approach to an other direction, that of using thin Cu(In,Ga)Se2 layers. This reduces further the volume of the solar cells and gives an insight in the effect of thickness as a key parameter controlling the performances of thin film microcells. On thinner microcells, optimum efficiencies are reached at illumination intensities over ×400. Due to their favorable architecture, microcells present efficient resistive and thermal management, leading to gains in efficiency and material usage.

International audience; SnO2:F thin films were deposited on polycarbonate and glass substrates by RF capacitively coupled plasma enhanced chemical vapor deposition (PECVD) using a mixture of tetramethyltin (TMT) [Sn(CH3)4], argon, oxygen as precursors. The effects of the substrate temperature, deposition time and doping on the resistivity and the morphology of the films have been studied. The as-deposited films appear to have higher carrier mobilities than amorphous ones, in the range of 5 and 7.5 cm2 · V−1 · s−1, which could be explained by the presence of nanocrystallites. In order to understand the nanostructure of the films, we studied the formation of nanoparticles and dust particles in the discharge. Finally, we have shown that the incorporation of less than 3% of F in the tin oxide layer could decrease the resistivity down to 3 · 10−3 Ω · cm.

Implementing photovoltaic devices based on high efficiency thin-film technologies on cheap, light-weight and flexible polymeric substrates is highly appealing to cut down costs in industrial production and to accelerate very large scale deployment of photovoltaics in the upcoming years. Lift-off processes, which allow separating active layers from primary substrates and subsequent transfer onto an alternative substrate without modifying the upstream production process and without performance losses, are an emerging alternative to direct growth on polymeric substrates. This study concerns the feasibily of direct mechanical lift-off process for high efficiency Cu(In,Ga)Se2 (CIGS) thin film solar cells grown by coevaporation on glass/molybdenum substrates without performance losses. The study presents an in depth characterization (SEM,AFM,GIXRD,XPS) of samples leading to excellent lift-off properties. They are explained by a specific gallium rich CIGS graded interface structure according to the interfacial sequence glass/Mo/MoSe2/GaxSey/Ga-rich-CIGS. The interfacial layer, attributed to GaSe, has a layered structure and out performs the molybdenum diselenide layered layer which forms spontaneously at the interface Mo/CIGS. It allows a very easy lift-off process at the interface GaSe/CIGS thanks to Van-der-Waals adhesion mechanism in GaSe. Key physical-chemical parameters are identified and analyzed. After lift-off, an efficiency of 14.3%, higher than the initial reference CIGS solar cell efficiency (13.8%) is measured.

We report on the influence of the substrate temperature during the 2nd and 3rd stage of the Cu(In,Ga)Se2 3-stage co-evaporation process on the in-depth Ga and In concentrations and correlate these with the solar cell parameters and external quantum efficiency of soda-lime glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al devices. An increased homogenization of the [Ga]/[III] fraction ([III] refers to the total concentration of the group 3 elements Ga and In) with temperature is found. In the investigated temperature range, the highest efficiency was measured for the lowest temperature and the steepest Ga-profile. The tendency of the short-circuit current density matches well with the notch-deepness. Surprisingly, the open-circuit voltage decreases for higher substrate temperatures, even though the Ga-concentration in the space-charge region increases. We propose back-grading variations and reduced back-interface recombination to explain this observation. For the highest of the tested temperatures of 540 °C, a homogenization...

Using solar cells under concentrated illumination is known to improve the conversion efficiency while diminishing the active area and thus material consumption. Recent concentrator cell designs tend to go miniaturized devices, in the 0.5–1 mm range, enabling a better thermal evacuation due to higher surface to volume ratio. If the cell size is further reduced to the micrometric range, spreading resistance losses can be made vanishingly small. This is particularly interesting for the thin film technology which has been limited up to now to very low concentration systems, from ×1 to ×10, due to excessive resistive losses in the window layer and difficult thermal management of the cells, grown on glass substrates. A new solar cell architecture, based on polycrystalline Cu(In,Ga)Se2 (CIGS) absorber, is studied: microscale thin film solar cells. Due to the reduced lateral dimension of the microcells (5 to 500 μm in diameter), the resistive and thermal losses are drastically decreased, enabling the use of high concentration (>×100). This results in a breakthrough for concentration on this type of devices, which were previously limited to the low concentration range (about ×10). Due to light concentration, the open circuit voltage increases up to several thousand suns equivalent, to reach over 900 mV. The temperature increase is limited to less than 20 °C over the ambient at concentration around ×1000. A 5% absolute efficiency increase on microcells at ×475 is observed and a 21.3% ± 0.2% equivalent efficient microcell of 50 μm of diameter is measured.

Silicon nanoparticles-based inks were investigated in respect of their suitability for photovoltaic and thermoelectric applications. Nanoparticles with a diameter ranging between 20 to 150 nm were functionalized in order to avoid oxidation as well as having a good stability in suspension. After inkjet-printing and drying, they were annealed up to 1000 °C under nitrogen atmosphere by both rapid thermal and microwave annealing. The influence of the annealing treatment on the structural, electrical, optical and thermal properties was investigated by Raman, SEM, electrical and optical measurements. SEM and Raman demonstrate evolution of the microstructure at temperature as low as 600 °C. Optical, electrical and thermal properties depend strongly on the annealing temperature and tend to exhibit a modification of physical properties above 800 °C when the smallest nanoparticles begin to melt. The annealing method has been identified to be of primary importance on the layer microstructure and its thermal behavior.

In the past three years, record efficiency of Cu(In,Ga)Se2 (CIGS) based solar cells has improved from 20% up to 22.6%. These results show that CIGS absorber is ideal for thin-film solar cells, even if this technology could be more competitive with a lower manufacture cost. The fabrication of devices with thinner CIGS absorbers is a way to increase the throughput of a factory and to reduce material consumption. This PhD thesis aims to develop cells with a CIGS thickness below 500 nm instead of the conventional 2.0-2.5 µm. However, as reported in the literature, we observed a decrease in cell performance. We carefully analyzed this effect by the comparison between simulations and sample characterizations: it is attributed, on one hand, to a lack of light absorption in the CIGS layer and, on the other hand, to an increased impact of the back-contact (high recombination and low reflectivity). To resolve these problems, we demonstrated theoretically and experimentally that the use of an alternative back-contact, other than molybdenum, such as a transparent conducting oxide coupled with a light reflector, improves the cell efficiency. To achieve this result, an optimization of the CIGS deposition was necessary. Moreover, we proved that a porous oxide layer inserted between the CIGS and the back-contact limits the charge-carrier recombination and removes some parasitic resistance. Finally, an efficiency of 10.7% was achieved for a 480-nm-thick CIGS solar cell with a SnO2:F back-contact passivated with a porous Al2O3 layer.; En quelques années, l'efficacité des cellules solaires à base de Cu(In,Ga)Se2 (CIGS) est passée de 20% à 22.6%. La rapidité de ce développement montre que le CIGS est un matériaux idéal pour les technologies solaires en couches minces. Pourtant, le coût de production cette technologie doit encore être abaissé pour une meilleure compétitivité. La fabrication d'un module avec une couche CIGS plus fine permettrait d'augmenter la production d'une usine et de réduire sa consommation en métaux. Ce travail de thèse vise à réduire l'épaisseur du CIGS d'un standard de 2.0-2.5 µm à une épaisseur inférieure à 500 nm sans altérer les performances des cellules. Cependant, comme rapporté dans la littérature, nous avons observé une diminution des rendements, ce que nous avons analysé en détail en comparant simulations et caractérisations d'échantillons. Celle-ci est causée à la fois par une faible absorption de la lumière dans la couche de CIGS et par un impact important du contact arrière (fortes recombinaisons et faible réflectivité). Pour dépasser ces limites, nous démontrons à la fois théoriquement et expérimentalement que le contact arrière en molybdène peut être remplacé par un oxyde transparent conducteur couplé à un miroir métallique. Nous obtenons de cette manière de meilleurs rendements de cellules. Pour atteindre ce résultat, une optimisation du dépôt de CIGS a été nécessaire. De plus, nous prouvons qu'une couche d'oxyde perforée, insérée entre le CIGS et le contact arrière, limite les recombinaisons des porteurs de charges et réduit l'influence des courants parallèles. Au final, nous avons fabriqué une cellule avec un rendement de 10.7% sur SnO2:F passivé par Al2O3.

In the past three years, record efficiency of Cu(In,Ga)Se2 (CIGS) based solar cells has improved from 20% up to 22.6%. These results show that CIGS absorber is ideal for thin-film solar cells, even if this technology could be more competitive with a lower manufacture cost. The fabrication of devices with thinner CIGS absorbers is a way to increase the throughput of a factory and to reduce material consumption. This PhD thesis aims to develop cells with a CIGS thickness below 500 nm instead of the conventional 2.0-2.5 µm. However, as reported in the literature, we observed a decrease in cell performance. We carefully analyzed this effect by the comparison between simulations and sample characterizations: it is attributed, on one hand, to a lack of light absorption in the CIGS layer and, on the other hand, to an increased impact of the back-contact (high recombination and low reflectivity). To resolve these problems, we demonstrated theoretically and experimentally that the use of an alternative back-contact, other than molybdenum, such as a transparent conducting oxide coupled with a light reflector, improves the cell efficiency. To achieve this result, an optimization of the CIGS deposition was necessary. Moreover, we proved that a porous oxide layer inserted between the CIGS and the back-contact limits the charge-carrier recombination and removes some parasitic resistance. Finally, an efficiency of 10.7% was achieved for a 480-nm-thick CIGS solar cell with a SnO2:F back-contact passivated with a porous Al2O3 layer., En quelques années, l'efficacité des cellules solaires à base de Cu(In,Ga)Se2 (CIGS) est passée de 20% à 22.6%. La rapidité de ce développement montre que le CIGS est un matériaux idéal pour les technologies solaires en couches minces. Pourtant, le coût de production cette technologie doit encore être abaissé pour une meilleure compétitivité. La fabrication d'un module avec une couche CIGS plus fine permettrait d'augmenter la production d'une usine et de réduire sa consommation en métaux. Ce travail de thèse vise à réduire l'épaisseur du CIGS d'un standard de 2.0-2.5 µm à une épaisseur inférieure à 500 nm sans altérer les performances des cellules. Cependant, comme rapporté dans la littérature, nous avons observé une diminution des rendements, ce que nous avons analysé en détail en comparant simulations et caractérisations d'échantillons. Celle-ci est causée à la fois par une faible absorption de la lumière dans la couche de CIGS et par un impact important du contact arrière (fortes recombinaisons et faible réflectivité). Pour dépasser ces limites, nous démontrons à la fois théoriquement et expérimentalement que le contact arrière en molybdène peut être remplacé par un oxyde transparent conducteur couplé à un miroir métallique. Nous obtenons de cette manière de meilleurs rendements de cellules. Pour atteindre ce résultat, une optimisation du dépôt de CIGS a été nécessaire. De plus, nous prouvons qu'une couche d'oxyde perforée, insérée entre le CIGS et le contact arrière, limite les recombinaisons des porteurs de charges et réduit l'influence des courants parallèles. Au final, nous avons fabriqué une cellule avec un rendement de 10.7% sur SnO2:F passivé par Al2O3.