Marie Jubault

Cu(In, Ga)Se2 microcells: High efficiency and low material consumption

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 ...

Ga gradients in Cu(In,Ga)Se2: Formation, characterization, and consequences

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...

Thin-film microcells: a new generation of photovoltaic devices

One way to increase the efficiency of a solar cell is to increase the power density of the light incident upon it. Indeed, if light is concentrated on a solar cell (e.g., via a lens), its voltage, and thus its efficiency, increases logarithmically. There are, however, limits to this efficiency increase; for example, the temperature of a device under intense light can become high enough to decrease the efficiency and eventually destroy it. Additionally, the output power for large current densities is limited by series resistance. Detrimental effects such as these are manageable on commercial concentrator cells based on high-quality crystalline materials, but it has not been possible to use high light concentrations of >50 suns (where one sun is 1kW/m2/ efficiently on thin-film solar cells. Indeed, most such cells are grown on glass substrates, which are poor thermal conductors, while thin-film semiconductive layers—especially the top, window, layers—are not sufficiently conductive for high-current-density operation. Overcoming these limitations is a particularly attractive prospect because thin-film solar cells can be rapidly deposited on large areas, through the self-assembly of microstructures, at a cost lower than for the current concentrator cells (which are based on epitaxial growth of single crystals). However, thin film solar cells such as copper indium gallium diselenide (Cu(In,Ga)Se2), cadmium telluride (CdTe), and gallium arsenide (GaAs) are based on elements that are not abundant on earth (rare earths). Due to their scarcity, a reduction in the required quantity of these materials could lead to cheaper cells. We have designed a new solar cell architecture that fulfills both the requirements. Figure 1. In this schematic of a photovoltaic device, light passing through a microlens array is concentrated and focused onto miniaturized solar cells.

Cu(In,Ga)Se2 photovoltaic microcells for high efficiency with reduced material usage

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.

Comparative study of patterned TiO 2 and Al 2 O 3 layers as passivated back-contact for ultra-thin Cu(In, Ga)Se 2 solar cells

In this work, a low cost passivated back-contact for ultra-thin Cu(In,Ga)Se2-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-Al2O3 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/Al2O3 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.

In-Situ Cu(In,Ga)Se 2 composition control by Optical Emission Spectroscopy during hybrid co-sputtering/evaporation process

In this work, we have developed a hybrid one-step co-sputtering/evaporation Cu(In,Ga)Se2 process, where Cu, In and Ga are sputtered simultaneously with the thermal evaporation of selenium, thus avoiding the use of H2Se. 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.

Characterization of Cu(In,Ga)Se

We present a new Cu(In,Ga)Se2 characterization tool: Cu(In,Ga)Se2 microcells. By creating pixels on a Cu(In,Ga)Se2 substrate, we are able to test electrically different locations. Moreover, because of the reduced size of the cells, (5-to 500-μm wide), heat and spreading resistance losses are made negligible, which make high flux characterizations available. We analyze current-voltage curves under high concentration to gain insight in the physical properties of Cu(In,Ga)Se2 cells. From our analysis, Cu(In,Ga)Se2 electrodeposited absorbers present resistivity fluctuations that are much more important than co-evaporated ones. These absorbers, as they present more electronic defects, are also more affected by the Voc increase under intense fluxes, and the efficiency gains can be very significant: up to 6% absolute efficiency points at less than 50 suns.

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In order to develop photovoltaic devices with increased efficiency using less rare semiconductor materials, the concentrating approach was applied on Cu(In,Ga)Se2 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.

Electrodeposited and Co-Evaporated Devices by Means of Concentrated Illumination

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.

Physics of Cu(In,Ga)Se2 microcells under ultrahigh illumination intensities

Photovoltaic cells based on chalcogenides CIGS (Cu(In, Ga)Se2) 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.