Myriam Paire

Microscale solar cells for high concentration on polycrystalline Cu(In,Ga)Se2 thin films

We report high concentration experiments on polycrystalline thin film solar cells. High level regime is reached, thanks to the micrometric scale of the Cu(In,Ga)Se2 cells, which strongly decreases resistive losses. A 4% absolute efficiency increase is obtained at a concentration of ×120, and current densities as high as 100 A/cm2 can be measured. These results show that the use of polycrystalline thin films under high concentration is possible, with important technological consequences.

Toward microscale Cu(In,Ga)Se2 solar cells for efficient conversion and optimized material usage: Theoretical evaluation

We develop a model to predict the performances of microscale Cu(In,Ga)Se2 (CIGS) solar cells under concentrated sunlight, based on the study of the influence of the window spread sheet resistance, which is the first limiting factor for concentration on CIGS solar cells. This model can be used to extract the value of the sheet resistance from simple current-voltage or electroluminescence measurements. The scaling benefits associated with the operation of microscale CIGS solar cells are studied. The optimum concentration ratio, linked to the best efficiency, is calculated for different cell sizes. It is predicted that an increase from 20% efficiency, for current CIGS solar cells under 1 sun illumination, up to 30% efficiency can be expected for microscale cells under concentrated sunlight.

Electrodeposition of ZnO window layer for an all-atmospheric fabrication process of chalcogenide solar cell

This paper presents the low cost electrodeposition of a transparent and conductive chlorine doped ZnO layer with performances comparable to that produced by standard vacuum processes. First, an in-depth study of the defect physics by ab-initio calculation shows that chlorine is one of the best candidates to dope the ZnO. This result is experimentally confirmed by a complete optical analysis of the ZnO layer deposited in a chloride rich solution. We demonstrate that high doping levels (>1020 cm−3) and mobilities (up to 20 cm2 V−1 s−1) can be reached by insertion of chlorine in the lattice. The process developed in this study has been applied on a CdS/Cu(In,Ga)(Se,S)2 p-n junction produced in a pilot line by a non vacuum process, to be tested as solar cell front contact deposition method. As a result efficiency of 14.3% has been reached opening the way of atmospheric production of Cu(In,Ga)(Se,S)2 solar cell.

Measuring sheet resistance of CIGS solar cell's window layer by spatially resolved electroluminescence imaging

Abstract A spatially resolved electroluminescence (EL) imaging experiment is developed to measure the local sheet resistance of the window layer, directly on the completed CIGS cell. Our method can be applied to the EL imaging studies that are made in fundamental studies as well as in process inspection. The EL experiment consists in using solar cell as a light emitting device: a voltage is applied to the cell and its luminescence is detected. We develop an analytical and quantitative model to simulate the behavior of CIGS solar cells based on the spread sheet resistance effect in the window layer. We determine the repartition of the electric potential on the ZnO, for given cells characteristics such as sheet resistance and contact geometries. Knowing the repartition of the potential, the EL intensity is measured and fitted against the model. The procedure allows the determination of the window layer sheet resistance.

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 ...

Resistive and thermal scale effects for Cu(In, Ga)Se2 polycrystalline thin film microcells under concentration

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 to smaller devices, in the 0.5–1mm lateral 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 thin film technology which has been limited up to now to very low concentrations, 1–10 suns, due to excessive resistive losses of the window layer and difficult thermal management of the cells, grown on glass substrates. In order to prove that high injection regime can be implemented on polycrystalline thin film solar cells, we fabricated Cu(In, Ga)Se2 (CIGS) thin film microcells with diameter from 7 μm to 150 μm, and characterized them under concentrated illumination. A 4% absolute efficiency increase is obtained at 120 suns, and current densities as high as 100 A cm−2 can be measured, without affecting the cell performances. The temperature increase under high fluxes is drastically reduced in microcells: less than 20 K at 1000 suns for microcells under 50 μm in diameter. These results show that the use of polycrystalline thin films under high concentration is indeed possible, with important technological consequences.

Self-aligned growth of thin film Cu(In,Ga)Se2 solar cells on various micropatterns

We provide the demonstration of a self-aligned growth of thin film Cu(In,Ga)Se2 solar cells and microcells. We created Cu(In,Ga)Se2 solar cells by direct localized electrodeposition and annealing on two patterns: lines of 1105 μm and 105 μm width and 1 cm long. We obtained up to 7.6% efficiency on the 1105 μm wide lines and 5.3% efficiency on 105 μm wide lines. This work demonstrates the possibility to directly grow efficient solar cells on tunable patterns, with very efficient material usage. This is important in the perspective of thin film micro-concentrators and also semi-transparent photovoltaic windows for building integrated applications.

Micro solar concentrators: Design and fabrication for microcells arrays

In this work we look at a micro-concentrating system adapted to a new type of solar concentrator photovoltaic material, well known for flate-plate applications, Cu(In,Ga)Se2. Cu(In,Ga)Se2 solar cells are polycrystalline thin film devices that can be deposited by a variety of techniques. We proposed to use a microcell architecture [1], [2], with lateral dimensions varying from a few μm to hundreds of μm, to adapt the film cell to concentration conditions. A 5% absolute efficiency increase on Cu(In,Ga)Se2 microcells at 475 suns has been observed for a final efficiency of 21.3% [3]. We study micro-concentrating systems adapted to the low and middle concentration range ([4], [5]), where thin film concentrator cells will lead to substrate fabrication simplification and costs savings. Our study includes optical design, fabrication and experimental tests of prototypes.

Quasi-Fermi level splitting evaluation based on electroluminescence analysis in multiple quantum-well solar cells for investigating cell performance under concentrated light

Insertion of InGaAs/GaAsP strain-balanced multiple quantum wells (MQWs) into i-regions of GaAs p-i-n solar cells show several advantages against GaAs bulk p-i-n solar cells. Particularly under high-concentration sunlight condition, enhancement of the open-circuit voltage with increasing concentration ratio in thin-barrier MQW cells has been reported to be more apparent than that in GaAs bulk cells. However, investigation of the MQW cell mechanisms in terms of I-V characteristics under high-concentration sunlight suffers from the increase in cell temperature and series resistance. In order to investigate the mechanism of the steep enhancement of open-circuit voltage in MQW cells under high-concentration sunlight without affected by temperature, the quasi-Fermi level splitting was evaluated by analyzing electroluminescence (EL) from a cell. Since a cell under current injection with a density Jinjhas similar excess carrier density to a cell under concentrated sunlight with an equivalent short-circuit current Jsc = Jinj, EL measurement with varied Jinj can approximately evaluate a cell performance under a variety of concentration ratio. In addition to the evaluation of quasi-Fermi level splitting, the external luminescence efficiency was also investigated with the EL measurement. The MQW cells showed higher external luminescence efficiency than the GaAs reference cells especially under high-concentration condition. The results suggest that since the MQW region can trap and confine carriers, the localized excess carriers inside the cells make radiative recombination more dominant.

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.