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Intermediate band solar cells (IBSCs) pursue the increase in efficiency by absorbing below-bandgap energy photons while preserving the output voltage. Experimental IBSCs based on quantum dots have already demonstrated that both below-bandgap photon absorption and the output voltage preservation, are possible. However, the experimental work has also revealed that the below-bandgap absorption of light is weak and insufficient to boost the efficiency of the solar cells. The objective of this work is to contribute to the study of this absorption by manufacturing and characterizing a quantum dot intermediate band solar cell with a single quantum dot layer with and without light trapping elements. Using one-dimensional substrate texturing, our results show a three-fold increase in the absorption of below bandgap energy photons in the lowest energy region of the spectrum, a region not previously explored using this approach. Furthermore, we also measure, at 9K, a distinguished split of quasi-Fermi levels between the conduction and intermediate bands, which is a necessary condition to preserve the output voltage of the cell., Comment: 9 pages, 6 figures

Van der Waals structures made of layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have been proposed for the development of ultra-thin photovoltaic devices. The main limitation of these solar cells up to now has been their low open-circuit voltage (Voc), which is typically below 0.55 V even for high illumination levels. Recently, we have presented a p-n Mos 2 homojunction that exhibits a Voc of 1.02 V under broadband illumination equivalent to 40 suns. The use of substitutionally-doped $p$ and $n$ Mos 2 material instead of a heterojunction is crucial to produce a band alignment that enables high Voc. Another important aspect for the realization of large photovoltages in TMDC solar cells is the optimization of metallic contacts. We demonstrate using a simple circuital model that the presence of Schottky barriers at the semiconductor/metal interfaces does not only introduce a non-ohmic series resistance, but also reduces the Voc because the Schottky diodes are photoactive. We characterize the Schottky barrier produced by different metals in combination with $p$ and $n$ Mos 2 . When p-flakes are deposited directly onto a SiO 2 /Si substrate, we find that they are depleted from carriers by a surface doping effect. This depletion contributes to aggravate the effect of the p-MoS 2 /metal Schottky. We show that inserting a flake of hexagonal boron nitride (h-BN) between the p-material and the SiO 2 surface eliminates this effect. Given the already demonstrated strong light absorption of TMDC ultra-thin devices, the achievement of high Voc is a turning point in the path towards high-efficiency TMDC solar cells.

The conversion efficiency of ultra-thin solar cells based on layered materials has been limited by their open-circuit voltage, which is typically pinned to a value under 0.6 V. Here we report an open-circuit voltage of 1.02 V in a 120 nm-thick vertically stacked homojunction fabricated with substitutionally doped MoS2. This high open-circuit voltage is consistent with the band alignment in the MoS2 homojunction, which is more favourable than in widely-used TMDC heterostructures. It is also attributed to the high performance of the substitutionally doped MoS2, in particular the p-type material doped with Nb, which is demonstrated by the observation of electroluminescence from tunnelling graphene/BN/MoS2 structures in spite of the indirect nature of bulk MoS2. We find that illuminating the TMDC/metal contacts decreases the measured open-circuit voltage in MoS2 van der Waals homojunctions because they are photoactive, which points to the need of developing low-resistance, ohmic contacts to doped MoS2 in order to achieve high efficiency in practical devices. The high open-circuit voltage demonstrated here confirms the potential of layered transition-metal dichalcogenides for the development of highly efficient, ultra-thin solar cells.

The three-terminal heterojunction bipolar transistor solar cell (HBTSC) concept enables the realization of a monolithic double-junction device with individual current extraction. We present an HBTSC realized by a heterojunction of GaInP and GaAs. The one-sun open-circuit voltage (V OC ) of the top and bottom junctions are 1.33 V and 0.99 V, respectively, while fill factors (FF) are above 80%. At one-sun illumination, reducing one junction's bias from V OC to maximum power point degrades the performance of the other junction only slightly (

Monolithic two‐terminal III‐V on Si dual‐junction solar cells, designed for low concentration applications, were fabricated by means of surface‐activated direct wafer bonding. The III‐V top cell is a heterojunction formed by an n‐Ga₀.₅In₀.₅P emitter and a p‐Al₀.₂Ga₀.₈As base. An efficiency of 21.1 ± 1.5% at one sun and 23.7 ± 1.7% at 10 suns is demonstrated, which to our knowledge is the best dual‐junction two‐terminal III‐V on Si tandem cell efficiency reported to date under verified reference conditions. The I‐V characterization of these 1‐cm² tandem cells under concentration required the development of a new method using a single‐source multiflash solar simulator and not perfectly matched component cells, also known as pseudo‐isotypes, formed by Si single‐junction cells and optical filters. In addition, the spectrum of the pulsed solar simulator was measured using a high‐speed CMOS spectrometer, allowing the calculation of the spectral mismatch correction factor. Merging these two techniques results in the hybrid corrected pseudo‐isotype (HCPI) characterization method, which shows a fast and accurate performance with a simplified procedure based on a single‐source solar simulator. Pseudo‐isotypes are easily adaptable to new cell designs by simply using a different filter, hence allowing the characterization of new multijunction solar cell architectures.

Multi-Terminal multi-junction solar cells (MJSC) offer higher efficiency potential than series connected (two-Terminal) ones. In addition, for terrestrial applications, the efficiency of multi-Terminal solar cells is less sensitive to solar spectral variations than the two-Terminal series-connected one. In space, generally, cells are always illuminated with AM0 spectrum and no impact is expected from spectral variations. Still, in space, the multi-Terminal approach offers some advantages in comparison with the series-connected architecture approach derived from a higher end of life (EOL) efficiency. In this work we review the potential of multi-Terminal solar cells for achieving extended EOL efficiencies with emphasis in the potential of the three-Terminal heterojunction bipolar transistor solar cell, a novel multi-Terminal MJSC architecture with a simplified structure not requiring, for example, tunnel junctions.

In common multijunction solar cells the subcells are connected in series. In this way, achieving a high voltage at module level is straightforward. However, calculations have proven that the annual energy efficiency limit is higher for independently connected subcells, because they are more tolerant to spectral variations throughout the year. We have recently proposed a three-terminal heterojunction bipolar transistor solar cell (HBTSC) with the maximum limiting efficiency of a dual-junction solar cell, but without the need for a tunnel junction and with only three crucial semiconductor layers. In this work, we present the implementation of a two-terminal module prototype including five HBTSCs which provides a high-voltage power output.

session poster (I43); International audience; Monolithic two-terminal III-V on Si dual-junction (2J) solar cells were fabricated by means of Surface-Activated direct wafer Bonding (SAB). Al 0.2 Ga 0.8 As single-junction cells are grown on GaAs substrate by Metal-Organic Vapor Phase Epitaxy (MOVPE) and bonded at room temperature to independently fabricated Si solar cells. The n+-GaAs//n+-Si bonding interface is characterized by Transmission Electron Microscopy (TEM) revealing a 2-3 nm thick amorphous interlayer. The performance of the 1 cm 2 tandem cells, designed for low concentration applications, was studied by External Quantum Efficiency (EQE) and J-V measurements showing a power conversion efficiency of 17% under one-sun AM1.5G spectrum. To our knowledge, this is the highest efficiency ever reported for a wafer-bonded 2J III-V on Si solar cell. Limitations to performance have been identified and therefore higher efficiencies are expected.

International audience; We evaluate mirror-based and achromatic optics at different working temperatures and cell-to-primary optical element (POE) distances. The magnitude and the effect of chromatic aberration and non-uniform flux profiles are assessed under controlled testing conditions. Our characterization tool is called METHOD [1]: it is capable of measuring multi-spectral irradiance profiles of concentrator photovoltaic optical systems along with the corresponding current (I)-voltage (V) characteristics produced on a concentrator solar cell receiver. In this study, METHOD is adapted to evaluate a mirror from the APOLLON Project [2], characterized in the framework of the European CPVMatch project.

Implementation of a high-efficiency quantum dot intermediate-band solar cell (QD-IBSC) must accompany a sufficient photocurrent generation via IB states. The demonstration of a QD-IBSC is presently undergoing two stages. The first is to develop a technology to fabricate high-density QD stacks or a superlattice of low defect density placed within the active region of a p-i-n SC, and the second is to realize half-filled IB states to maximize the photocurrent generation by two-step absorption of sub-bandgap photons. For this, we have investigated the effect of light concentration on the characteristics of QDSCs comprised of multi-layer stacks of self-organized InAs/GaNAs QDs grown with and without impurity doping in molecular beam epitaxy.

A test method that measures spectrally resolved irradiance distribution for a concentrator photovoltaic (CPV) optical system is presented. In conjunction with electricalI-Vcurves, it is a means to visualize and characterize the effects of chromatic aberration and nonuniform flux profiles under controllable testing conditions. The indoor characterization test bench, METHOD (Measurement of Electrical, Thermal and Optical Devices), decouples the temperatures of the primary optical element (POE) and the cell allowing their respective effects on optical and electrical performance to be analysed. In varying the temperature of the POE, the effects on electrical efficiency, focal distance, spectral sensitivity, acceptance angle and multi-junction current matching profiles can be quantified. This work presents the calibration procedures to accurately image the spectral irradiance distribution of a CPV system and a study of system behavior over lens temperature.

Tandem GaInP/GaAs//Si(inactive) solar cells were manufactured by direct wafer bonding under vacuum. At this early stage, an inactive silicon substrate was used (i.e. n+ Si substrate instead of an active n-p Si junction). Bonded devices presented an S-shaped J-V curve with a kink close to Voc caused by a built-in potential barrier at the III-V//Si interface that reduces the fill factor and therefore the efficiency of the device by 7% compared to the stand-alone GaInP/GaAs tandem cells. Nevertheless, losses in Jsc and Voc caused by the bonding process, account for less than 10%. AlGaAs single junction cells, designed to be bonded on a silicon cell for low concentrator photovoltaics (LCPV), were also manufactured reaching an efficiency of 15.9% under one sun AM1.5G spectrum for a 2cm2 cell.

Concentrator solar cell front-grid metallizations are designed so that the trade-off between series resistance and shading factor (SF) is optimized for a particular irradiance. High concentrator photovoltaics (CPV) typically requires a metallic electrode pattern that covers up to 10% of the cell surface. The shading effect produced by this front electrode results in a significant reduction in short-circuit current (I SC) and hence, in a significant efficiency loss. In this work we present a cover glass (originally meant to protect the cell surface) that is laser-grooved with a micrometric pattern that redirects the incident solar light towards interfinger regions and away from the metallic electrodes, where they would be wasted in terms of photovoltaic generation. Quantum efficiency (QE) and current (I)-voltage (V) characterization under concentration validate the proof-of-concept, showing great potential for CPV applications

Multijunction solar cells (MJSC) use anti-reflective coatings (ARC) to minimize Fresnel reflection losses for a family of light incidence angles. These coatings adapt the refractive index of the cell to that of the surrounding medium. Patterns with sizes in the range of the light wavelength can be used to further reduce reflections through diffraction. Transparent nanopatterns with a gradual profile, called moth-eye nanostructures, can adapt the refractive index of the optical interfaces (often with n∼1.5) used to encapsulate concentrator solar cells to that of the air (n air∼1). Here we show the effect of a nanometric moth-eye ARC with a round motif deposited on commercial MJSC that achieves short-circuit current (I SC) gains greater than 2% at normal incidence and even higher in the case of tilted illumination. In this work, MJSC with different moth-eye ARC are characterized under quantum efficiency (QE) as well as under concentrated illumination I-V in order to assess their potential. Simulations based on coupled wave analysis (RCWA) are used to fit the experimental results with successful results.

This chapter summarizes the state of the art in instruments and methods for the characterization of concentrator multijunction (MJ) solar cells (MJSC). The current-voltage characteristic (I–V curve) under illumination and the spectral response or quantum efficiency (QE) are the main properties of a solar cell. The measurement of the I–V curve as a function of light concentration provides the most relevant information on cell performance such as peak efficiency or series resistance losses. Concentrator solar simulators should fulfill strict spatial uniformity and spectral requirements to provide accurate measurements. Precise spectral tunability is required to reduce errors in the measurement of latest-generation cell architectures, especially those not based on a germanium bottom cell with an excess of current. The main types of concentrator solar simulators are described, and advice on appropriate reference sensors and measurement precautions is provided. Spectral characterization under controlled simulator conditions is essential to analyze MJ cell behavior under the ever-varying spectral conditions found in real operation. The relevance of the QE is that it allows calculating the photocurrent generated by each subcell under a particular spectral irradiance and diagnosing possible malfunctioning of the different regions of the cell. Cell electroluminescence (EL) is also discussed as a useful tool for assessing defects. Spatially resolved EL can be used to analyze solar cell internal defects, sheet resistivity, or thermal inhomogeneity under steady concentrated light. Typical characterization set-ups, test procedures, and potential instrumentation issues are discussed for QE and EL.

Multi-junction solar cells (MJSC) operating at working conditions under concentration are subjected to temperatures (T) for which the optical coupling provided by their anti-reflective coatings (ARC) has not been optimized. High temperatures and wide ray angles produced by the concentrator on the optical interface of the cell can significantly modify the ARC performance. This effect is especially pronounced for ARCs adapted to silicone encapsulant because the silicone refractive index (n) is significantly sensitive to temperature, modifying the optimal design thickness and material composition. This effect is magnified for tilted rays whose optical path length through the ARC layer is most modified. In this work, an absolute external quantum efficiency (EQE) characterization system is adapted to perform angular and temperature spectral response analysis, allowing to quantify the impact of the optical mismatch caused by the increase of encapsulant temperature for each of their junctions. The intricacies of this upgraded characterization technique are explored, providing insight on important unexpected measurement variables such as finger orientation with respect to the incident ray bundle. A significant spectral mismatch between junctions due to the change in silicone temperature has been observed, leading to short-circuit current (I SC ) losses as high as 6% with respect to the design conditions (T=25°C) for rays impinging the cell with tilt angles of 70° and more realistic operation temperatures of 65°C. The losses arise from current mismatch between subcells produced by variations in the optical coupling. Lessons from this analysis may be taken into account by future CPV system designers.

Multijunction solar cells (MJSC) are usually developed to maximize efficiency under test conditions and not under real operation. This is the case of anti-reflective coatings (ARC), which are meant to minimize Fresnel reflection losses for a family of incident rays at room temperature. In order to understand and quantify the discrepancies between test and operation conditions, we have experimentally analyzed the spectral response of MJSC for a variety of incidence angles that are in practice received by a concentrator cell in high-concentration photovoltaic (HCPV) receiver designs. Moreover, we characterize this angular dependence as a function of temperature in order to reproduce real operation conditions. As the refractive index of the silicone is dependent on temperature, an optical mismatch is expected. Regarding other characterization techniques, a method called Relative EL Homogeneity Analysis (RELHA) is applied to processed wafers prior to dicing, allowing to diagnose the wafer crystalline homogeneity for each junction. Finally, current (I)-voltage (V) characterization under strongly unbalanced light spectra has also been carried out for a number of low-level irradiances, providing insight on each junction shunt resistance and corresponding radiative coupling.

The Intermediate Band Solar Cell (IBSC) is a novel photovoltaic device with the potential of surpassing the efficiency limit of conventional solar cells. It is based on a new class of materials characterized by the insertion of a collection of energy levels within the material bandgap. These levels act as the so-called Intermediate Band (IB) and cause a larger portion of the solar spectrum to be useful for photovoltaic conversion. Sub-bandgap photons can ideally be collected via two-step absorption mechanisms through the IB and thus enhance the photogenerated current without a significant voltage degradation. In this chapter, the authors show the state of the art of the modeling and simulation within the IBSC research field.

A colloidal deposition technique is presented to construct long-range ordered hybrid arrays of self-assembled quantum dots and metal nanoparticles. Quantum dots are promising for novel opto-electronic devices but, in most cases, their optical transitions of interest lack sufficient light absorption to provide a significant impact in their implementation. A potential solution is to couple the dots with localized plasmons in metal nanoparticles. The extreme confinement of light in the near-field produced by the nanoparticles can potentially boost the absorption in the quantum dots by up to two orders of magnitude. In this work, light extinction measurements are employed to probe the plasmon resonance of spherical gold nanoparticles in lead sulfide colloidal quantum dots and amorphous silicon thin-films. Mie theory computations are used to analyze the experimental results and determine the absorption enhancement that can be generated by the highly intense near-field produced in the vicinity of the gold nanoparticles at their surface plasmon resonance. The results presented here are of interest for the development of plasmon-enhanced colloidal nanostructured photovoltaic materials, such as colloidal quantum dot intermediate-band solar cells.

The behavior of quantum dot, quantum wire, and quantum well InAs/GaAs solar cells is studied with a very simplified model based on experimental results in order to assess their performance as a function of the low bandgap material volume fraction fLOW. The efficiency of structured devices is found to exceed the efficiency of a non-structured GaAs cell, in particular under concentration, when fLOW is high; this condition is easier to achieve with quantum wells. If three different quasi Fermi levels appear with quantum dots the efficiency can be much higher.

The particular opto-electronic properties of chemically synthesized colloidal nanoparticles can be promising for functional materials, as those required for high efficient photovoltaic (PV) devices. In particular, appropriately-designed semiconductor colloids (quantum dots, QDs) can potentially allow sub-bandgap current generation in intermediate-band solar cells; while metal nanoparticles (MNPs) sustaining surface plasmons can provide both near and far-field light trapping to further boost the generated power. However, the incorporation of colloidal particles in inorganic PV materials is not trivial, therefore their implementation has so far been restricted to organic/polymeric based solar cells. In this work, PbS colloidal QDs have been incorporated in the intrinsic a-Si:H layer of HIT (substrate/a-Si:H hetero-junction) test structures. Both c-Si and GaAs substrates have been used, and in some cases colloidal Au NPs have also been included. The obtained devices are meant as probes to verify the feasibility of incorporating foreign nanoparticles in a cell structure and not as potentially efficient solar cells. Despite the radical novelties incorporated, the devices behaved similarly to the references, thus proving the compatibility of the proposed materials and processes.