Search

International audience; The development of new thin films solar cells is highly accelerated by using rigorous optimization approach. A large set of parameters is involved in the solar cell operation: active layer thickness, composition, bandgap, doping, contacts, etc. The study of the solar cell performances with respect to these linked parameters is necessary to better understand the underlying physics and to optimize the final device. We propose in this poster presentation (*) a new open-source solar cell optimizer: SLALOM for SoLAr ceLl multivariate OptiMizer. SLALOM implements, for the first time for solar cells, a rigorous multivariate approach while the standard optimization work used to use the one-by-one parameter procedure. SLALOM is implemented to be easily extended to any simulator, the core code itself does not depend on any particular engine. It can be adapted for any solar cell structure, runs locally or remotely on a calculation server, using the SSH protocol and includes a graphical user interface to real-time monitor the optimization. Two study cases, InGaN and CZTS solar cells, are presented to show the SLALOM ability to become a useful tool for the development of novel solar cells.

International audience; The InGaN ternary alloy has the potentiality to achieve high efficiency solar cells: tunable bandgap in the whole solar spectrum, high absorption coefficient, high stability and radiation tolerance. These very promising characteristics make InGaN potentially ideal for designing and developing next-generation high-efficiency thin films solar cells. However, challenging issues remain to address: (i) the difficulty to elaborate sufficiently thick monocrystalline InGaN layers with a high Indium content; (a) the high defects density and the spontaneous and piezoelectric polarizations; (iii) the p-doping which remains difficult to master. In this report, we use rigorous optimization approach based on state-of-the-art optimization algorithms to investigate the effect of defects and polarization (spontaneous and piezoelectric) on a double junction InGaN solar cell. A better understanding of the mechanisms involved in the heterostructure has a crucial impact on the design and elaboration of high efficiency InGaN thin films solar cells which require, in particular, a precise control of the Tunnel Junction elaboration which is still very challenging.

The electrical/structural characterization and simulation of III-N materials for detectors and solar cells are presented, first in a synthetic way then in detail for two specific aspects. For boron gallium nitride (BGaN), the studies revealed the possibility of significantly reducing non-intentional doping in III-N materials, one of the main issues for applications. The second aspect detailed in the memoir concerns the simulation of indium gallium nitride (InGaN) solar cells using, for the first time in this domain, rigorous mathematical optimization methods. This methodology allowed to propose new structures not requiring P-doping, another issue in III-N materials, and to study multijunction structures taking into account technological constraints. The prospects opened up by this work concern the development of high-efficiency/low-cost solar cells with two major parts. Firstly, the design of optical structures to reduce the thickness of the InGaN active layers while optimizing the efficiency. The second part concerns the development of solar cells containing earth-abundant materials, as alternative to InGaN., La caractérisation structurale/électrique et la simulation numérique de matériaux III-N pour détecteurs et cellules solaires sont présentées, d'abord de manière synthétique puis en détaillant deux aspects spécifiques. Pour le nitrure de gallium et de bore (BGaN), la caractérisation électrique et structurale a mis en évidence la possibilité de réduire considérablement le dopage résiduel des matériaux III-N, l'un des verrous pour les applications. Le deuxième aspect détaillé dans le mémoire concerne la simulation numérique de cellules solaires à base de nitrure de gallium et d'indium (InGaN) en utilisant, pour la première fois dans ce domaine, des méthodes d'optimisation mathématique rigoureuses. Cette méthodologie a permis de proposer des structures ne nécessitant pas de dopage P, un autre verrou des matériaux III-N, et d'étudier des structures à multijonction tenant compte des contraintes technologiques. Les perspectives ouvertes par ce travail concernent le développement de cellules solaires à haut rendement et bas coût avec deux volets majeurs. Le premier volet concerne la conception de dispositifs optiques pour réduire l'épaisseur nécessaire des couches actives d'InGaN tout en optimisant le rendement. Le second volet concerne le développement de cellules solaires à base de matériaux abondants, comme alternative à celles utilisant l'InGaN.

International audience; Nitride-based semiconductor materials are promising candidates for the manufacturing of very high efficiency solar cells. The Indium Gallium Nitride (InGaN) ternary alloy has a particular interest, because its energy gap can be varied in a wide spectral range just by changing the indium composition. This can lead to bandgap engineering, which would ease the design of multi-junction solar cells. Within this context, the performance of a double junction solar cell based on InGaN was simulated. In this work, we simulated globally the solar cell structure using realistic physics models and InGaN parameters obtained with experimental measurements.The modeled solar cell is composed of two p-n junctions vertically stacked having decreasing Bandgaps and connected by a specifically designed tunnel junction. This allows the conversion of different parts of the solar spectrum, thus enhancing the solar light absorption efficiency and the photocarriers transport. The device is simulated in the framework of a drift-diffusion model using the ATLAS device simulation framework from the Silvaco company. The optimization is achieved by coupling ATLAS with multivariate mathematical optimization methods based on state-of-the-art optimization algorithms. For that, we used a Python package that we developed in the SAGE software interface.The objective is to optimize the conversion efficiency of the solar cell by simultaneously optimizing several physical and geometrical parameters of the solar cell. It is an unprecedented multivariate optimization for solar cells which takes into account the correlation between these parameters.We have optimized eleven parameters simultaneously and the optimum conversion efficiency obtained is 24:4% with a short-circuit current (JSC) of 12,9 mA/cm2, an open-circuit voltage (VOC) of 2,29 V and a fill factor (FF) of 82,5 %.However, InGaN grown on available substrates have high densities of structural defects. It also has spontaneous and piezoelectric polarization fields which lead to the presence of high densities of electrostatic charges in the interfaces of the InGaN epitaxially grown layer. We thus highlight and discuss the effects of this polarization and the effects of structural defects on the photovoltaic characteristics of the solar cell.

The electrical/structural characterization and simulation of III-N materials for detectors and solar cells are presented, first in a synthetic way then in detail for two specific aspects. For boron gallium nitride (BGaN), the studies revealed the possibility of significantly reducing non-intentional doping in III-N materials, one of the main issues for applications. The second aspect detailed in the memoir concerns the simulation of indium gallium nitride (InGaN) solar cells using, for the first time in this domain, rigorous mathematical optimization methods. This methodology allowed to propose new structures not requiring P-doping, another issue in III-N materials, and to study multijunction structures taking into account technological constraints. The prospects opened up by this work concern the development of high-efficiency/low-cost solar cells with two major parts. Firstly, the design of optical structures to reduce the thickness of the InGaN active layers while optimizing the efficiency. The second part concerns the development of solar cells containing earth-abundant materials, as alternative to InGaN.; La caractérisation structurale/électrique et la simulation numérique de matériaux III-N pour détecteurs et cellules solaires sont présentées, d'abord de manière synthétique puis en détaillant deux aspects spécifiques. Pour le nitrure de gallium et de bore (BGaN), la caractérisation électrique et structurale a mis en évidence la possibilité de réduire considérablement le dopage résiduel des matériaux III-N, l'un des verrous pour les applications. Le deuxième aspect détaillé dans le mémoire concerne la simulation numérique de cellules solaires à base de nitrure de gallium et d'indium (InGaN) en utilisant, pour la première fois dans ce domaine, des méthodes d'optimisation mathématique rigoureuses. Cette méthodologie a permis de proposer des structures ne nécessitant pas de dopage P, un autre verrou des matériaux III-N, et d'étudier des structures à multijonction tenant compte des contraintes technologiques. Les perspectives ouvertes par ce travail concernent le développement de cellules solaires à haut rendement et bas coût avec deux volets majeurs. Le premier volet concerne la conception de dispositifs optiques pour réduire l'épaisseur nécessaire des couches actives d'InGaN tout en optimisant le rendement. Le second volet concerne le développement de cellules solaires à base de matériaux abondants, comme alternative à celles utilisant l'InGaN.

Choosing the Indium Gallium Nitride (InGaN) ternary alloy for thin films solar cells might yield high benefits concerning efficiency and reliability, because its bandgap can be tuned through the Indium composition and radiations have little destructive effect on it. It may also reveal challenges because good quality p-doped InGaN layers are difficult to elaborate. In this letter, a new design for an InGaN thin film solar cell is optimized, where the player of a PIN structure is replaced by a Schottky contact, leading to a Metal-IN (MIN) structure. With a simulated efficiency of 19.8%, the MIN structure performs better than the previously studied Schottky structure, while increasing its fabrication tolerance and thus functional reliability a. Owing to its good tolerance to radiations [1], its high light absorption [2, 3] and its Indium–composition–tuned bandgap [4, 5], the Indium Gallium Nitride (InGaN) ternary alloy is a good candidate for high–efficiency–high–reliability solar cells able to operate in harsh environments. Unfortunately, InGaN p-doping is still a challenge, owing to InGaN residual n-doping [6], the lack of dedicated ac-ceptors [7] and the complex fabrication process itself [8, 9]. To these drawbacks can be added the uneasy fabrication of ohmic contacts [4] and the difficulty to grow the high-quality-high-Indium-content thin films [10] which would be needed to cover the whole solar spectrum. These drawbacks still prevent InGaN solar cells to be competitive with other well established III-V and silicon technologies [11]. In this letter, is proposed a new Metal-IN (MIN) InGaN solar cell structure where the InGaN p-doped layer is removed and replaced by a Schottky contact, lifting one of the above mentioned drawbacks. A set of realistic physical models based on actual measurements is used to simulate and optimize its behavior and performance using mathematically rigorous multi-criteria optimization methods, aiming to show that both efficiency and fabrication tolerances are better than the previously described simple InGaN Schottky solar cell [12].

International audience; no abstract

International audience; A detailed investigation on the performance of an InGaN-based double-junction solar cell was carried out. We have globally simulated the solar cell using empirical InGaN material parameters, to avoid any overestimation in the solar cell performances. In order to take into account the interdependence of the solar cell physical and geometrical parameters, ensuring the absoluteness of the optimized photovoltaic performances, the cell was optimized using a multivariate optimization algorithm that simultaneously optimizes eleven physical and geometrical parameters. We obtained an optimal efficiency of 24.4%, with a short circuit current J SC = 12.92 mA=cm 2 , an open circuit voltage V OC = 2.287 V and a fill factor FF = 82.55%. We then quantitatively investigated the impact on the solar cell performances of the internal polarization and structural defects in InGaN. We have shown that theinternal polarization reduces the performance of the cell by inducing an electric field which does not favor an efficient collection of photo-generated carriers. We have also investigated the impact of structural defects in InGaN, including disorder and deep defects, and correlated their effect to the InGaN doping concentration.

International audience; Because of remarkable properties of InGaN, we simulated and optimized an InGaN-based dual-junction solar cell connected by a specifically designed tunnel junction. The device is simulated in the framework of a drift-diffusion model using the ATLAS device simulation framework from the Silvaco company. The optimization is done by coupling ATLAS with multivariate mathematical optimization methods based on state-of-the-art optimization algorithms. For that, we used a Python package that we developed in the SAGE software interface. The objective is to optimize the conversion efficiency of the solar cell by simultaneously optimizing several physical and geometrical parameters of the solar cell. It is an unprecedented multivariate optimization for solar cells which takes into account the correlation between these parameters. For this solar cell, we optimized simultaneously 11 parameters of the structure. An optimum conversion efficiency of 24% was predicted for this designed solar cell.

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; In thin film polycrystalline Cu(In,Ga)Se2/CdS solar cells, the intrinsic and interface defect properties are of primary importance for their performance. In this paper, we have performed Capacitance Spectroscopy, XPS and Kelvin probe measurements in order to gain a better understanding of the heterointerface properties at different stages of its formation. All investigations were carried out on Cu(In,Ga)Se2 co-evaporated samples elaborated in pilot-line conditions onto which CdS layers with different thickness were deposited by CBD. From the capacitance measurements versus frequency and temperature, the gap states density evolution was derived using the Walter et al. method [1]. A shallow interface defect was identified with a strong evolution of its density of states with the CdS layer thickness. XPS profiles of the heterojunction were obtained by sputter-etch so as to study the composition at the buried interface. Kelvin probe measurements under dark and illumination showed that the interface is not built until ZnO is deposited and that significant charge is trapped in CdS under illumination. The results obtained by these different characterization techniques at different stages of the heterojunction formation were correlated to the evolution of the solar cells electrical performances.

International audience

International audience; no abstract

International audience; no abstract

International audience; no abstract

International audience; no abstract

In this work were elaborated thin films of Zinc Magnesium Oxide (ZnMgO) by ultrasonic spray pyrolysis. Zn(1-x)Mg(x)O is a wide bandgap compound semiconductor with useful properties for optoelectronic devices, mainly ultraviolet “solar-blind” detectors, as an active layer, and solar cells where it can be used as a buffer layer. The main difficulties related to the elaboration of Zn(1-x)Mg(x)O using solution-based and spray pyrolysis techniques are the wide miscibility gap between MgO and ZnO and the use of toxic and unsafe solvents such as methanol. In this study, we were able to elaborate Zn(1-x)Mg(x)O with varying Mg content up to 70% with water-based solutions only. The difficulty using only water-based solutions is related to the inherent unfavorable properties such as strong hydrogen bonds with high boiling point. However, we managed to optimize the precursors solution concentration, the number of scans, the flow rate, the pressure and the temperature, to obtain homogeneous and good quality films. The solid composition in the thin films was measured using X-ray fluorescence to assess the Mg solubility limit in Zn(1-x)Mg(x)O and its incorporation efficiency. High transmittance of about 90 % in the visible region was obtained in all the films with a very small variation with the thickness, suggesting a highly controlled process of the elaboration.The bandgap and the Urbach energy variation with the composition were investigated and linked to the morphology and structural properties. A miscibility limit of 30 % of Mg was obtained, confirmed by the X-ray fluorescence measurements and correlated to the bandgap variation with the Mg composition extracted from the transmittance spectra analysis. The structural properties were investigated using confocal micro-Raman spectroscopy which permitted to identify the wurtzite structure as the only phase in all the films with composition less than 30 %, with a phase separation occurring beyond this limit. The electrical properties were characterized using the van der Pauw and Hall effect technique using indium to form the ohmic contacts. The n-type carrier concentration dramatically decreases from about 2*10^(19) to 5*10^(15) cm^(-3) when increasing Mg composition from 0 % to 7 %. The thin films become semi-insulating for Mg composition higher than 7 %. The correlation of the electrical results with the optical and structural properties suggests that, in addition to the increase of the bandgap, a compensation mechanism related to the defects introduced in the alloy plays a central role in the decrease in the carrier concentration. The mobility slightly increases with the Mg composition suggesting that the films crystal quality was not significantly degraded by the increase of the magnesium composition in the considered range. The modulation of the conductivity, in addition and partially related to the modulation of the bandgap, is of high interest to the detector or the solar cell device.

International audience; The challenge of switching for photovoltaic as a major source of electricity can only be fulfilled by using solar cells based on low toxicity and earth abundant materials combined with a low cost process. One of the most promising solution to achieve this goal is to use a metal oxide absorber such as cuprous oxide (Cu2O). In 2016, a Cu2O/ZnGeO solar cell was demonstrated by the Minani group and reached a record efficiency of 8.1%. A recent modeling of Cu2O solar cells by Rizi et al. confirms that ZnGeO buffer layer outperforms other buffer layers reported experimentally due to a better band alignement. However the experimental and simulated results were obtained by taking into account a solar cell based on a high demanding energy process (T>1000°C). Cuprous oxide thin films have already been developed by low cost and large area compatible processes, but currently in detriment of material quality with grain size in the range of tenth of nanometers and mobility two order of magnitude lower. In order to evaluate the impact of low cost processes on solar cell performances, we analyse the modelisation results of Cu2O/ZnGeO solar cell by carefully taking into account experimental material properties, defects and grain size of Cu2O grown by low cost processes. The conclusion of this analysis will serve as a guideline for solar cells elaboration by spray pyrolysis in our laboratory and measurements of device performances will be compared with our simulated results.

Conducting polymer composites, which are simply physical mixtures of an insulating polymer matrix and intrinsically conducting polymers, are promising new materials for various applications such as antistatic coatings, shielding and electromagnetic absorbers. In this context, a novel conducting composite was successfully produced. It is based on epoxy resin and polyaniline (PANI) doped in benzene sulfonic acid, which was used as conducting filler. The effect of PANI loading levels on the structural, morphological, thermal, electrical and dielectric properties was explored using techniques, such as Raman, Fourier transform infrared and ultraviolet-visible-NIR spectroscopy, X-ray diffraction, atomic force microscopy, scanning electron microscopy, thermogravimetric analysis and differential scanning calorimetry. The electrical behavior and morphology of the synthesized PANI revealed disorder and defects. In addition, as the applied voltage is increased, the current through the sample increases rapidly. The conductivity and dielectric properties were also investigated at room temperature in the frequency range of 1 MHz–1 GHz. The dielectric constant was shown to increase with increased loading at low frequencies and then decreases rapidly, reaching a constant value at higher frequencies. In addition, the AC conductivity of the prepared composites increases with the addition of PANI and is higher at high frequencies. Furthermore, the stability of the composites was found to be influenced by increasing the amount of PANI filler in the matrix. The defects and heterogeneous system of polyaniline (PANI) doped in benzene sulfonic acid (BSA) with a crystalline region dispersed in an amorphous low-conducting region as shown in the scanning electron microscopy micrograph reveal the current–voltage characteristic, which shows a nonohmic variation. It increases in current with applied voltage owing to the conduction mechanisms of PANI due to the formation of polarons and bipolarons. As the applied voltage increases, the formation of polarons and bipolarons increases rapidly, contributing to higher values of current through sample. This characteristic affects the dielectric behaviors of the elaborate composites.

International audience; no abstract

Thin films of GaN with the V/III≈10 ratio were grown by low-pressure metal organic vapour phase epitaxy (LP-MOVPE) using N2 and Dimethylhydrazine (DMHy) as a carrier gas and nitrogen precursor, respectively. For the growth temperatures in the range from 550 to 690 ∘C the GaN layers exhibited good surface morphology. In the temperature range from approximately 550 to 610 ∘C, the growth rate increases with increasing temperature, characteristic of the process limited by surface kinetics with the activation energy of approximately 36 kcal/mol. For the temperatures between 620 and 690 ∘C, the growth rate was nearly independent of temperature, which is indicative of a mass transport limited growth. The activation energy was about 4.6 kcal/mol. Micro Raman spectroscopy revealed a significant relaxation of the selection rules for the scattering by the optical phonons in the films grown at lower temperatures. Variation of the intensity ratio for E 2 H and E 1 phonon modes has been attributed to the changes in the structural quality of the films grown at different temperatures.