Pablo García-Linares

Self-organized colloidal quantum dots?and metal nanoparticles for plasmon-enhanced intermediate-band solar cells

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.

Quantum Dot Parameters Determination From Quantum-Efficiency Measurements

The energy spectrum of the confined states of a quantum dot intermediate band (IB) solar cell is calculated with a simplified model. Two peaks are usually visible at the lowest energy side of the subbandgap quantum-efficiency spectrum in these solar cells. They can be attributed to photon absorption between well-defined states. As a consequence, the horizontal size of the quantum dots can be determined, and the conduction (valence) band offset is also determined if the valence (conduction) offset is known.

Effect of the encapsulant temperature on the angular and spectral response of multi-junction solar cells

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 (ISC) 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.

Advances on multijunction solar cell characterization aimed at the optimization of real concentrator performance

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 homogene...

The effect of concentration on the performance of quantum dot intermediate-band solar cells

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.

Characterization of Multijunction Concentrator Solar Cells

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.

Modelling of quantum dot solar cells for concentrator PV applications

An equivalent circuit model is applied in order to describe the operation characteristics of quantum dot intermediate band solar cells (QD-IBSCs), which accounts for the recombination paths of the intermediate band (IB) through conduction band (CB), the valence band (VB) through IB, and the VB-CB transition. In this work, fitting of the measured dark J-V curves for QD-IBSCs (QD region being non-doped or direct Si-doped to n-type) and a reference GaAs p-i-n solar cell (no QDs) were carried out using this model in order to extract the diode parameters. The simulation was then performed using the extracted diode parameters to evaluate solar cell characteristics under concentration. In the case of QDSC with Si-doped (hence partially-filled) QDs, a fast recovery of the open-circuit voltage (Voc) was observed in a range of low concentration due to the IB effect. Further, at around 100X concentration, Si-doped QDSC could outperform the reference GaAs p-i-n solar cell if the current source of IB current source were sixteen times to about 10mA/cm2 compared to our present cell.

Characterization of the influence of temperature on achromatic mirrors by means of METHOD

Our characterization tool, called METHOD which was previously built for refractive optics [1], has been adapted and optimized to evaluate a mirror from the APOLLON Project [2], characterized in the framework of the European CPVMatch project. Influence of the glass box of the thermal chamber, homogeneity of the temperature across the sample, and influence of the chromatic light on the energetic spot’s center are studied. We evaluate the mirror’s abilities at different working temperatures and cell-to-primary optical element (POE) distances.

Developing a highly integrated receiverless low concentration module with III-V multijunction cells

Concentrator photovoltaic (CPV) modules are composed of many components and interfaces and require complex fabrication processes, which in turn may cause lack of reliability. The presented work tackles these considerations, proposing an innovative highly integrated low concentration photovoltaic (LCPV) concept. The purpose is to develop a module with a high level of integration by lowering the number of components and interfaces. At first, the linear parabolic mirror, used as concentrator optics, can be considered as multifunctional, combining thermal, structural and optical functionalities. On this basis, the proposed CPV prototype design features a receiverless configuration, where the cells are directly arranged on the rear side of each mirror. Moreover, such implementation claims to demonstrate the applicability of reliable flat PV fabrication processes (such as lamination and cell interconnection) for the manufacturing of this LCPV module. Geometrical considerations, together with thermal and optical...

METHOD: a tool for mechanical, electrical, thermal, and optical characterization of single lens module design

The optical characterization and electrical performance evaluation are essential in the design and optimization of a concentrator photovoltaic system. The geometry, materials, and size of concentrator optics are diverse and different environmental conditions impact their performance. CEA has developed a new concentrator photovoltaic system characterization bench, METHOD, which enables multi-physics optimization studies. The lens and cell temperatures are controlled independently with the METHOD to study their isolated effects on the electrical and optical performance of the system. These influences can be studied in terms of their effect on optical efficiency, focal distance, spectral sensitivity, electrical efficiency, or cell current matching. Furthermore, the irradiance map of a concentrator optic can be mapped to study its variations versus the focal length or the lens temperature. The present work shows this application to analyze the performance of a Fresnel lens linking temperature to optical and electrical performance.