Mathieu Baudrit

A comparative study of BSF layers for GaAs-based single-junction or multijunction concentrator solar cells

An effective back surface field is a key structural element for a high-efficiency GaAs concentrator solar cell, either in a multijunction or in a single-junction device. In this paper, several BSF materials are analysed, namely: (1) p++GaAs(Zn), (2) p+Ga0.5In0.5P(Zn) and (3) p++Al0.2Ga0.8As(C). The results of the comparison demonstrate that the best option is C-doped Al0.2Ga0.8As, which exhibits a low series resistance and behaves as an excellent minority carrier mirror; p++GaAs(Zn) shows reduced minority carrier mirror properties resulting from Zn diffusion and p+Ga0.5In0.5P(Zn) is shown to produce important series resistance problems because of an unfavourable heterojunction with GaAs.

Modeling of GaInP/GaAs Dual-Junction solar cells including Tunnel Junction

This paper presents research efforts conducted at the IES-UPM in the development of an accurate, physically-based solar cell model using the general-purpose ATLAS® device simulator by Silvaco. Unlike solar cell models based on a combination of discrete electrical components, this novel model extracts the electrical characteristics of a solar cell based on virtual fabrication of its physical structure, allowing for direct manipulation of materials, dimensions, and dopings. As single junction solar cells simulation was yet achieved, the next step towards advanced simulations of multi-junction cells (MJC) is the simulation of the tunnel diodes, which interconnect the subcells in a monolithic MJC. The first results simulating a Dual-Junction (DJ) GaInP/GaAs solar cells are shown in this paper including a complete Tunnel Junction (TJ) model and the resonant cavity effect occurring in the bottom cell. Simulation and experimental results were compared in order to test the accuracy of the models employed.

Tunnel Diode Modeling, Including Nonlocal Trap-Assisted Tunneling: A Focus on III–V Multijunction Solar Cell Simulation

Multijunction solar cells (MJCs) based on III-V semiconductors constitute the state-of-the-art approach for high-efficiency solar energy conversion. These devices, consisting of a stack of various solar cells, are interconnected by tunnel diodes. Reliable simulations of the tunnel diode behavior are still a challenge for solar cell applications. In this paper, a complete description of the model implemented in Silvaco ATLAS is shown, demonstrating the importance of local and nonlocal trap-assisted tunneling. We also explain how the measured doping profile and the metalization-induced series resistance influence the behavior of the tunnel diodes. Finally, we detail the different components of the series resistance and show that this can help extract the experimental voltage drop experienced by an MJC due to the tunnel junction. The value of this intrinsic voltage is important for achieving high efficiencies at concentrations near 1000 suns.

III–V multijunction solar cells for ultra-high concentration photovoltaics

In this paper, the benefits of the ultra high concentration (¿ 1000 suns) are shown in terms of cost reduction and efficiency increase. Accordingly, the strategy followed at IES-UPM for more than 15 years is the development of III-V solar cells suitable for operation at 1000 suns or more. Recently, we have developed and manufactured a GaInP/GaAs dual-junction cell which results in an efficiency (measured at the Calibration Laboratory, CalLab, of Fraunhofer Institute in Freiburg) of 32.6% for a concentration range going from 499 to 1026 suns. This efficiency is the world record efficiency for a dual-junction solar cell. Besides, the efficiency is still as high as 31% at 3000 suns. The theoretical optimization of this solar cell (based on an accurate modelling) shows a potential efficiency over 36% at 1000 suns. We have extended this strategy to lattice-matched GaInP/Ga(In)As/Ge triple junction solar cells. First manufactured cells exhibit an efficiency of 31.5% at 1000 suns. The theoretical optimization of this cells show that an efficiency over 43% at 1000 suns is achievable. A roadmap has been established in order to reach this value.

Simulating III–V concentrator solar cells: A comparison of advantages and limitations of lumped analytical models; distributed analytical models and numerical simulation

The simulation of the quantum efficiency and the I-V curves at several concentrations of a high efficiency concentrator dual-junction solar cell is presented using three different approaches: 1) analytic simulation with the one-diode model; 2) analytic simulation with distributed circuit models; and, 3) numerical simulation. The main advantages and limitations of each model are discussed and their performance is compared.

3D Modeling of Concentrator III-V Multi-Junction Solar Cells

Concentration based on III-V solar cells is one of the most promising technologies to reduce cost of PV electricity. To achieve high efficiency making a better use of the solar spectrum and under very high concentration, multijunction solar cells are explored at the IES-UPM. To give a real understanding of all the phenomena occurring inside these devices, the development of a reliable theoretical model is essential. In this paper we present the first results obtained in our laboratory simulating lattice-matched GaInP/GaAs dual junction solar cells. To achieve these results we numerically analyze the complete structure including the tunnel junction

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...

Development of CPV solar receiver based on insulated metal substrate (IMS): Comparison with receiver based on the direct bonded copper substrate (DBC) - A reliability study

Most of the CPV solar receivers are based on III-V multijunction cells die-attached to a direct bonded copper (DBC) substrate. An alternative to DBC resides in the insulated metal substrate (IMS). This paper presents the behavior of IMS and DBC receivers when tested under accelerated aging conditions such as described in the IEC 62108. Characterization tools involved in the monitoring of potential degradation are electroluminescence (EL), dark-I (V) (DIV), spectral response (EQE), diode factor measurements (VIM), RX tomography (RX-T), and, of course, illuminated I (V) under various concentration factors. Based on EL, DIV and EQE first results, IMS and DBC age in a similar way. Study is ongoing.

Effects of lens temperature on irradiance profile and chromatic aberration for CPV optics

Lens-based optical concentrators are currently the most common in concentrator photovoltaic systems. This paper discusses experimental results to quantify the effects of temperature on the primary optical elements of three commercial Fresnel-based designs. The designs are: Silicon on Glass Primary with no secondary, PMMA Primary with a Truncated Inverted Pyramid secondary, and a PMMA 4 quadrant Fresnel – Kohler system. We quantify the effects of temperature on the irradiance profile with the variation in peak to average ratio of from 25 – 50 ° C. Furthermore, the effect of temperature on chromatic aberration is represented with the ratio of the top subcell current to middle subcell for a standard triple junction germanium-based cell. For the system with the lowest peak-to-average ratio, PMMA-based 4 Fresnel-Kohler design, we observed a change of 2% in the subcell ratio when the lens was heated to 50 °C.