Laurent Lombez

Contactless mapping of saturation currents of solar cells by photoluminescence

We report in this letter the contactless measurement of spatially resolved photocurrent–photovoltage relationship. The method is based on hyperspectral imaging, from which we record cartography of absolute photoluminescence spectra from solar cells. Using the generalized Planck’s law, it is therefore possible to derive the quantitative value of the quasi-Fermi levels splitting, related to the voltage over the junction. It allows us to directly extract optoelectronics properties of the device with a solely optical method. As a proof of concept, we derive saturation currents of a GaAs solar cell and find a good agreement with the standard electrical measurements.

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.

Thermalisation rate study of GaSb-based heterostructures by continuous wave photoluminescence and their potential as hot carrier solar cell absorbers

GaSb-based heterostructures are tested as candidates for a hot carrier solar cell absorber. Their thermalisation properties are investigated using continuous wave photoluminescence. Non-equilibrium carrier populations are detected at high excitation levels. An empirical expression of the power lost by thermalisation is deduced from the incident power dependent carrier temperature. The experimentally determined thermalisation rate is then used to simulate the potential efficiency of a hot carrier solar cell, showing a significant efficiency improvement compared to a fully thermalised single p–n junction of similar bandgap.

Spin dynamics in dilute nitride semiconductors at room temperature

We report optical studies in undoped GaAsN epilayers and InGaAsN quantum wells, which show that a strong electron spin polarization can persist at room temperature. This is a direct consequence of the long spin relaxation time of electrons in dilute nitride materials. Introducing less than 1% of nitrogen in the binary (GaAs) or ternary (InGaAs) alloy increases the electron spin relaxation time at T=300K by a factor greater than 20 in as-grown material before annealing. A drastic drop in the electron spin relaxation time is observed for annealed samples.

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 ...

Quantification of spatial inhomogeneity in perovskite solar cells by hyperspectral luminescence imaging

Vacuum evaporated perovskite solar cells with a power conversion efficiency of 15% have been characterized using hyperspectral luminescence imaging. Hyperspectral luminescence imaging is a novel technique that offers spectrally resolved photoluminescence and electroluminescence maps (spatial resolution is 2 micrometer) on an absolute scale. This allows, using the generalized Planck’s law, the construction of absolute maps of the depth-averaged quasi-Fermi level splitting (Δμ), which determines the maximum achievable open circuit voltage (Voc) of the solar cells. In a similar way, using the generalized reciprocity relations the charge transfer efficiency of the cells can be obtained from the hyperspectral images. Very strong inhomogeneity, both in quasi-Fermi level splitting (Δμ) and in charge transfer efficiency, are found in these vacuum deposited perovskite solar cells. This implies that these efficient solar cells are still far from perfect as many areas in the device do not or only partially participate in the photon to electron conversion processes.

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.