Amaury Delamarre

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.

Characterization of solar cells using electroluminescence and photoluminescence hyperspectral images

We analyze photoluminescence (PL) and electroluminescence (EL) of a GaAs solar cell using a hyperspectral imager that records spectrally resolved images. Thanks to the absolute calibration of the setup, we first investigate the reciprocity relations between solar cells and light-emitting diode and determine the external quantum efficiency from EL images. Spatial variations are observed due to series resistance effect that we can evaluate. Second, the PL experiment allows us to plot the recombination current at a given spatial location versus the quasi-Fermi level splitting at the same location. Under reasonable assumptions, this can be linked to the classical measurement of the short circuit current versus the open circuit voltage. Therefore we perform a contactless mapping of optoelectronic properties such as the saturation currents. The assumptions made in these experiments are discussed in order to correctly investigate polycrystalline solar cells in the future where strong lateral variations exist.

Revisiting the interpretation of biased luminescence: Effects on Cu(In,Ga)Se2 photovoltaic heterostructures

We analyzed the luminescence signal under electrical bias (Lum-V) for several Cu(In,Ga)Se2 solar cells having different absorber growth processes and different buffer layers such as CdS and ZnS. A numerical model is developed taking into account optical and electrical properties of the complete heterostructures. It appears that the absorber-buffer interface has a crucial role in explaining the different behaviors. Our interpretation is based on the quasi Fermi level splitting (QFL) linked to both the applied voltage and the luminescence intensity. Lum-V experiments and its dependence on illumination intensity are discussed and could be used to access transport properties when looking at the depth variation of the QFL and offer a classification of the possible cases.

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.

Material challenges for solar cells in the twenty-first century: directions in emerging technologies

Abstract Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan–French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

Experimental Demonstration of Optically Determined Solar Cell Current Transport Efficiency Map

A recently suggested reciprocity relation states that the current transport efficiency from the junction to the cell terminal can be determined by differentiating luminescence images with respect to the terminal voltage. The validity of this relation is shown experimentally in this paper, by comparison with simultaneously measured electrical currents and simulations. Moreover, we verify that the method is applicable under various light concentrations and applied voltages, which allows us to investigate the cell in relevant conditions. Results evidence several kinds of series resistances affecting the current transport efficiencies. We show that the relative contribution of those different resistances to the loss in current collection is a function of the illumination intensity.

Quantitative optical measurement of chemical potentials in intermediate band solar cells

Abstract. Having shown some demonstration of two photon absorption and multiband emission processes in quantum dots (QD), multiquantum wells (MQW), and highly mismatched alloys, intermediate band solar cells are currently the subject of numerous studies. To better understand the underlying mechanisms, our objective is to experimentally probe the multiband operation of this device. We used photoluminescence recorded with a calibrated hyperspectral imager which provides spectrally resolved images with a spatial resolution of 2  μm and spectral resolution of 2 nm on proof of concept QD and MQW solar cells samples. Device emission can be described with the generalized Planck’s law from which the quasi-Fermi level splitting of the three bands can be determined. The advantage of the technique is that it can be used to investigate the intermediate band material without the need to make contacts or a full device structure. We also discuss the usefulness of a dual-beam method.

InP-based nano solar cells

Light trapping enhancement is a major research field in photovoltaics. Scarce and expensive resources for semiconductor material drive the research on light management in thin absorber layer. This paper reviews some of the known techniques, from back reflector to nanophotonic technologies such as nanowires or plasmonic-enhanced photovoltaic devices. Light trapping enhancement can reach ~100 fold and experimental demonstrations of device exceeding the ray optics limits have been reported.

Effect of low-V/III-ratio metalorganic vapor-phase epitaxy on GaAs solar cells

Single-junction GaAs solar cells were grown by metalorganic vapor-phase epitaxy (MOVPE) at various input V/III ratios. All growth parameters other than V/III ratio were carefully controlled for an accurate comparison. Nearly identical cell performance characteristics including short-circuit current density (J sc) and open-circuit voltage (V oc) indicate that cell performance is independent of V/III ratio. To determine the relationship between the electrically measured V oc and V/III ratio in a more precise manner, photoluminescence (PL) was applied as a potent optical measurement tool, which does not depend on device processing and contacting issues. We also evaluated the projected cell performance under low-concentration sunlight by electroluminescence (EL) analysis. Similarly to electrical measurement, optical measurement showed no obvious degradation owing to a low V/III ratio. This study strongly demonstrates that low-cost high-efficiency GaAs solar cells can be realized by MOVPE using a low V/III ratio.

200nm-Thick GaAs solar cells with a nanostructured silver mirror

We have designed and fabricated ultrathin (200nm) GaAs solar cells. Multi-resonant light trapping is achieved with a nanostructured TiO2/Ag back mirror and the short-circuit current predicted by numerical calculations is Jsc= 25 mA/cm2. We have investigated the physical origin of the resonances using analytical models that can be used to optimize the geometry of nanostructured mirrors suited for ultrathin solar cells. For the fabricated solar cell, the nanostructured TiO2/Ag mirror is combined with localized ohmic contacts. The solar cells are patterned by Soft Nanoimprint Lithography and transferred on a glass substrate. A similar solar cell with Ag flat mirror was also fabricated. We have measured above 60% enhancement on short-circuit current compared to as-grown reference cell.