Multijunction Solar Cell Structures Research Articles

Strain Dependent Performance Analysis of InGaN Multi-junction Solar Cell

This study reports the strain-dependent efficiency of the InGaN-based multi-junction solar cell (MJSC) for the first time. The route of strain in MJSC is identified to be the results of dissimilar lattice constants between layers of sub-cell grown epitaxially with bandgap stepping. Utilising multi-layered strain model, the state of strain and its magnitude is evaluated for three types of MJSC structures referred to as MJSC-1, MJSC-2, and MJSC-3. It is found that the MJSC position-dependent strain is strongly dependent on the sub-cell thickness as well as on the number of sub-cells. Employing the MJSC position-dependent strain in combination with deformation potentials, strain-induced energy bandgap is calculated when imposed under tensile strain condition. Finally, the strain-dependent efficiencies of different MJSC structures are estimated and obtained to be lower with that of reported with strain effects over sighted. The loss of efficiency is identified to be due to the open circuit voltage which decreases under tensile strain condition. Among the MJSC structures studied here, MJSC-3 with 7-layers is less efficient and its efficiency decreases up to 3.01% when strain effect is taken into consideration.

A Brief Review of High Efficiency III-V Solar Cells for Space Application

The demands for space solar cells are continuously increasing with the rapid development of space technologies and complex space missions. The space solar cells are facing more critical challenges than before: higher conversion efficiency and better radiation resistance. Being the main power supply in spacecrafts, III-V multijunction solar cells are the main focus for space application nowadays due to their high efficiency and super radiation resistance. In multijunction solar cell structure, the key to obtaining high crystal quality and increase cell efficiency is satisfying the lattice matching and bandgap matching conditions. New materials and new structures of high efficiency multijunction solar cell structures are continuously coming out with low-cost, lightweight, flexible, and high power-to-mass ratio features in recent years. In addition to the efficiency and other properties, radiation resistance is another sole criterion for space solar cells, therefore the radiation effects of solar cells and the radiation damage mechanism have both been widely studied fields for space solar cells over the last few decades. This review briefly summarized the research progress of III-V multijunction solar cells in recent years. Different types of cell structures, research results and radiation effects of these solar cell structures under different irradiation conditions are presented. Two main solar cell radiation damage evaluation models—the equivalent fluence method and displacement damage dose method—are introduced.

Point-Defects Assisted Zn-Diffusion in AlGaInP/GaInP Systems During the MOVPE Growth of Inverted Multijunction Solar Cells

We investigate the dynamics of Zn diffusion in MOVPE-grown AlGaInP/GaInP systems by the comparison of different structures that emulate the back-surface field (BSF) and base layers of a GaInP subcell integrated into an inverted multijunction solar cell structure. Through the analysis of secondary ion mass spectroscopy (SIMS), electrochemical capacitance-voltage (ECV) and spectrally resolved cathodoluminescence (CL) measurements, we provide experimental evidence that 1) the Zn diffusion is enhanced by point defects injected during the growth of the tunnel junction cathode layer; 2) the intensity of the process is determined by the cathode doping level and it happens for different cathode materials; 3) the mobile Zn is positively charged and 4) the diffusion mechanism reduces the CuPt ordering in GaInP. We demonstrate that using barrier layers the diffusion of point defects can be mitigated, so that they do not reach Zn-doped layers, preventing its diffusion. Finally, the impact of Zn diffusion on solar cells with different Zn-profiles is evaluated by comparing the electrical I-V curves at different concentrations. The results rule out the introduction of internal barriers in the BSF but illustrate how Zn diffusion under typical growth condition can reach the emitter and dramatically affect the series resistance, among other effects.

Photoelectrochemical Water Splitting using Adapted Silicon Based Multi-Junction Solar Cell Structures: Development of Solar Cells and Catalysts, Upscaling of Combined Photovoltaic-Electrochemical Devices and Performance Stability

Abstract Thin film silicon based multi-junction solar cells were developed for application in combined photovoltaic electrochemical systems for hydrogen production from water splitting. Going from single, tandem, triple up to quadruple junctions, we cover a range of open circuit voltages from 0.5 V to 2.8 V at photovoltaic cell (PV) efficiencies above 13%. The solar cells were combined with electrochemical (EC) cells in integrated devices from 0.5 cm2 to 64 cm2. Various combinations of catalyst pairs for the oxygen and hydrogen evolution reaction side (OER and HER) were investigated with respect to electrochemical activity, stability, cost and – important for the integrated device – optical quality of the metal catalyst on the HER side as back reflector of the attached solar cell. The combined PV-EC systems were further investigated under varied operation temperatures and illumination conditions for estimation of outdoor performance and annual fuel production yield. For 0.5 cm2 size combined systems a maximum solar-to-hydrogen efficiency ηSTH = 9.5% was achieved under standard test conditions. For device upscaling to 64 cm2 various concepts of contact interconnects for reduced current and fill factor loss when using large size solar cells were investigated. To replace high performance noble metal based catalyst pairs (Pt/RuO2 or Pt/IrOx), more abundant and cheaper NiMo (HER) and NiFeOx (OER) compounds were prepared via electrodeposition. With the NiMo/NiFeOx catalyst pair we obtained ηSTH = 5.1% for a 64 cm2 size solar cell which was even better than the performance of the Pt/IrO2 system (ηSTH = 4.8%). In simulated day-night cycle operation the NiMo/NiFeOx catalyst pair showed excellent stability over several days. The experimental studies were successfully accompanied by simulation of the entire PV-EC device using a series connection model which allowed studies and pre-estimations of device performance by varying individual components such as catalysts, electrolytes, or solar cells. Based on these results we discuss the prospects and challenges of integrated PV-EC devices on large area for hydrogen and solar fuel production in general.

Degradation of Ge subcells by thermal load during the growth of multijunction solar cells

AbstractGermanium solar cells are used as bottom subcells in many multijunction solar cell designs. The question remains whether the thermal load originated by the growth of the upper layers of the multijunction solar cell structure affects the Ge subcell performance. Here, we report and analyze the performance degradation of the Ge subcell due to such thermal load in lattice‐matched GaInP/Ga(In)As/Ge triple‐junction solar cells. Specifically, we have detected a quantum efficiency loss in the wavelength region corresponding to the emitter layer (which accounts for up to 20% loss in equivalent JSC) and up to 55 mV loss in VOC of the Ge subcell as compared with analogous devices grown as single‐junction Ge solar cells on the same type of substrates. We prove experimentally that there is no direct correlation between the loss in VOC and the doping level of the base. Our simulations show that both the JSC and VOC losses are consistent with a degradation of the minority carrier properties at the emitter, in particular at the initial nanometers of the emitter next to the emitter/window heterointerface. In addition, we also rule out the gradual emitter profile shape as the origin of the degradation observed. Our findings underscore the potential to obtain higher efficiencies in Ge‐based multijunction solar cells if strategies to mitigate the impact of the thermal load are taken into consideration.

Modeling lattice-matched InP-based multijunction solar cells

Currently, the multijunction solar cell structure gives the highest efficiency for photovoltaic solar cells. In this study, tandem, triple, and quadruple junction multijunction solar cell structures that include an InP subcell on InP substrate were investigated. A model that can calculate both the voltage-current characteristics and external quantum efficiency was demonstrated. The model gave fitted results with the experimental data for a single junction GaAs solar cell and a tandem solar cell that provides efficiency higher than 30%. The model was used to optimize the lattice-matched AlAsSb/InP tandem, AlAsSb/InP/InGaAsP triple, and AlAsSb/InP/InGaAsP/GaAsSb quadruple junction solar cell designs and the highest efficiencies obtained for these cells were 29.9%, 36.69%, and 42.79%, respectively, under AM1.5 spectrum without any sun concentration. This lattice-matched quadruple junction structure gives the opportunity of having high efficiency without wafer bonding.

Evolution of silicon bulk lifetime during III–V‐on‐Si multijunction solar cell epitaxial growth

AbstractThe evolution of Si bulk minority carrier lifetime during the heteroepitaxial growth of III–V on Si multijunction solar cell structures via metal‐organic chemical vapor deposition (MOCVD) has been analyzed. In particular, the impact on Si lifetime resulting from the four distinct phases within the overall MOCVD‐based III–V/Si growth process were studied: (1) the Si homoepitaxial emitter/cap layer; (2) GaP heteroepitaxial nucleation; (3) bulk GaP film growth; and (4) thick GaAsyP1‐y compositionally graded metamorphic buffer growth. During Phase 1 (Si homoepitaxy), an approximately two order of magnitude reduction in the Si minority carrier lifetime was observed, from about 450 to ≤1 µs. However, following the GaP nucleation (Phase 2) and thicker film (Phase 3) growths, the lifetime was found to increase by about an order of magnitude. The thick GaAsyP1‐y graded buffer was then found to provide further recovery back to around the initial starting value. The most likely general mechanism behind the observed lifetime evolution is as follows: lifetime degradation during Si homoepitaxy because of the formation of thermally induced defects within the Si bulk, with subsequent lifetime recovery due to passivation by fast‐diffusing atomic hydrogen coming from precursor pyrolysis, especially the group‐V hydrides (PH3, AsH3), during the III–V growth. These results indicate that the MOCVD growth methodology used to create these target III–V/Si solar cell structures has a substantial and dynamic impact on the minority carrier lifetime within the Si substrate. Copyright © 2015 John Wiley & Sons, Ltd.

(Invited) Advanced Photon Management in Printed High-Efficiency Multijunction Solar Cells

High efficiency photovoltaic (PV) cells represent a clean and efficient method to utilize the abundant solar energy in both space and terrestrial applications. Advances in solar cell technology and associated light management systems are the primary determinants of improvements in efficiency. Single junction (SJ) solar cells are already near theoretical efficiency limits defined by thermalization losses and sub-bandgap transparency. Devices that incorporate multiple junctions (i.e. sub-cells) in monolithic stacks, known as multijunction (MJ) cells, provide one of the most attractive routes to ultrahigh efficiency. Over the last decade, increases in the efficiency of MJ cells correspond to nearly 1% (absolute) per year, reaching values that are presently ~44%. Further improvements, however, will require solutions to daunting challenges in achieving lattice-matched or metamorphic epitaxial growth in complex stacks and in maintaining current matched outputs from each of the sub-cells. Here we present materials and strategies to make printed multijunction solar cell structures, to bypass the challenges in conventional multijunction cell design. In the first part, I will talk about printing-based assembly of microscale, quadruple junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1000 suns, and modules with efficiencies of 36.5%. Secondly, I will introduce an alternative device architecture, which uses low refractive index interface materials for enhanced photon recycling in printed multijunction solar cell stacks. These results establish routes to ultrahigh efficiency cells and modules, with potential to approach thermodynamic efficiency limits and realize large-scale photovoltaic energy production.

Novel approaches to MOVPE material deposition for high efficiency Multijunction Solar Cells

Starting from a brief survey on the most important III‐V material engineering approaches which brought multijunction solar cells reaching an efficiency value of 44.7% to realization, new approaches to MOVPE material deposition are presented to further incrementing the solar cell performances and reduce the technology cost. A new MOVPE temperature profile tuning capability has been developed in order to maintain high thermal homogeneity at the wafer surface, also in the case of the deposition of strained structures, as well as to get a fast temperature control at the interfaces between arsenide and phosphide materials. Preliminary results on the possibility to combine group III‐V with group IV elements in the same MOVPE growth chamber in order to expand the band gap engineering possibilities are also presented and demonstrated at device level. As a proof of the concept, SiGe layers have been grown in the same MOVPE reactor used to grow III‐V compounds and InGaP/InGaAs/Ge Multijunction solar cell structures have been realized and characterized after SiGe deposition.

Numerical Investigation of High-Efficiency InGaN-Based Multijunction Solar Cell

A four-junction InGaN-based multijunction solar cell structure is proposed theoretically. The simulation results show that, with the use of appropriately designed compositional grading layers, the performance of InGaN-based multijunction solar cell can be maintained without the cost in performance degradation caused by the polarization-induced electric field and the potential barriers resulting from the heterointerfaces. After the optimization in thicknesses for current matching, a high conversion efficiency of 46.45% can be achieved under 1000-sun AM1.5D illumination, in which the short-circuit current density, open-circuit voltage, and fill factor are 12.2×103 mA/cm2, 4.18 V, and 0.77, respectively. The simulation results suggest that, in addition to the detrimental effects caused by the built-in electric polarization and potential barriers, the issue of crystalline quality is another critical factor influencing the performance of multijunction solar cells.

Theoretical performance of multi-junction solar cells combining III-V and Si materials.

A route to improving the overall efficiency of multi-junction solar cells employing conventional III-V and Si photovoltaic junctions is presented here. A simulation model was developed to consider the performance of several multi-junction solar cell structures in various multi-terminal configurations. For series connected, 2-terminal triple-junction solar cells, incorporating an AlGaAs top junction, a GaAs middle junction and either a Si or InGaAs bottom junction, it was found that the configuration with a Si bottom junction yielded a marginally higher one sun efficiency of 41.5% versus 41.3% for an InGaAs bottom junction. A significant efficiency gain of 1.8% over the two-terminal device can be achieved by providing an additional terminal to the Si bottom junction in a 3-junction mechanically stacked configuration. It is shown that the optimum performance can be achieved by employing a four-junction series-connected mechanically stacked device incorporating a Si subcell between top AlGaAs/GaAs and bottom In0.53Ga0.47As cells.

Determination of the subcell photovoltage in multijunction solar cells via voltage-dependent capacitance analysis

The letter describes a method for determining the photovoltage, i.e., the quasifermi level splitting at open-circuit conditions, associated with each subcell in a series connected multijunction solar cell structure by voltage-dependent capacitance analysis. Experimental verification and accuracy analysis of subcell photovoltage determination is provided for a 6-tuple AlGaInP/GaInP/AlGaInAs/GaInAs/GaInNAs/Ge solar cell device.

Optimization of annealing conditions of (GaIn)(NAs) for solar cell applications

Abstract (GaIn)(NAs) lattice matched to GaAs and Ge and having a 1 eV bandgap is a promising candidate for future space and terrestrial multi-junction solar cell structures. The present paper summarizes results of a detailed annealing study of the metastable quaternary alloy having an In content of 8% and a N content of 2.8%. It is shown that photoluminescence (PL) intensity can be taken as a measure of improving minority carrier characteristics in solar cell devices. A direct correlation between PL intensity and quantum efficiency in the (GaIn)(NAs) material system is observed. The annealing conditions, e.g. anneal temperature, time and As-stabilization, have to be adapted to the particular In content of the material in order to initiate the site change of the nitrogen atom from a Ga-rich environment upon growth to an In-rich one after annealing. In addition, the dissolution of chain-like N-ordering in [0 0 1] direction is detected. The greatly enhanced optical performance leads to an improved quantum efficiencies of the (GaIn)(NAs) solar cell material.

Growth aspects of AlGaAs/GaAs tunnel diodes for multijunction solar cells

AbstractGrowth aspects related to AlGaAs/GaAs tunnel diodes for multijunction solar cell structures are presented. Test structures have been grown in order to show the correct way for checking the thermal resistance of tunnel diodes. AlGaAs:C lattice contraction as a function of carbon concentration and growth temperature has been studied by using XRD, Hall and Room temperature photoluminescence characterisation. The XRD experimental results have been successfully compared with the lattice contraction model reported by W.E Hoke et al. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Refilling Silicon Grooves by Liquid Phase Epitaxy

Planar epitaxial refilling of grooves etched in (110) oriented silicon wafers has been achieved by using a liquid phase epitaxial growth process. This process has the advantages of requiring lower processing temperatures, maintaining high minority carrier lifetime, and of not producing the undesirable polycrystalline deposits invariably observed on the oxide‐coated surfaces between the grooves during conventional vapor phase epitaxial growth. The effect of epitaxial growth conditions on the planar refill has been examined. It has been found that planar refill can be achieved at slow cooling rates on the order of 0.1–0.2°C/min in the presence of an oxide layer between the grooves to be refilled. At high cooling rates and in the absence of the oxide layer between the grooves, nonplanar surfaces are observed. Typical applications for this planar epitaxial refill process would be in the fabrication of vertical multijunction solar cell structures and in the fabrication of vertically walled gate regions for power junction field effect transistors and field controlled thyristors.