Concentrator Solar Cells Research Articles

Implications of Redesigned, High-Radiative-Efficiency GaInP Junctions on III-V Multijunction Concentrator Solar Cells

Nonradiative recombination in inverted GaInP junctions is dramatically reduced using a rear-heterojunction design rather than the more traditional thin-emitter homojunction design. When this GaInP junction design is included in inverted multijunction solar cells, the high radiative efficiency translates into both higher subcell voltage and high luminescence coupling to underlying subcells, both of which contribute to improved performance. Subcell voltages within two and four junction devices are measured by electroluminescence and the internal radiative efficiency is quantified as a function of recombination current using optical modeling. The performance of these concentrator multijunction devices is compared with the Shockley–Queisser detailed-balance radiative limit, as well as an internal radiative limit, which considers the effects of the actual optical environment in which a perfect junction may exist.

Silicon nanocrystals at elevated temperatures: retention of photoluminescence and diamond silicon to β-silicon carbide phase transition.

We report the photoluminescence (PL) properties of colloidal Si nanocrystals (NCs) up to 800 K and observe PL retention on par with core/shell structures of other compositions. These alkane-terminated Si NCs even emit at temperatures well above previously reported melting points for oxide-embedded particles. Using selected area electron diffraction (SAED), powder X-ray diffraction (XRD), liquid drop theory, and molecular dynamics (MD) simulations, we show that melting does not play a role at the temperatures explored experimentally in PL, and we observe a phase change to β-SiC in the presence of an electron beam. Loss of diffraction peaks (melting) with recovery of diamond-phase silicon upon cooling is observed under inert atmosphere by XRD. We further show that surface passivation by covalently bound ligands endures the experimental temperatures. These findings point to covalently bound organic ligands as a route to the development of NCs for use in high temperature applications, including concentrated solar cells and electrical lighting.

Effect of the circle-grid electrodes on concentrated GaAs solar cell efficiency

In this study, we investigate the effect of the shading factor of the front grid pattern on concentrated solar cell efficiency, taking the trade-off between the series resistance of the electrodes and the amount of incident light into consideration. We examine the thermal effect with regard to five different circle-grid electrode patterns of the front contact. The front contacts with different grid patterns affect the characteristics of light-concentrated-type GaAs single-junction solar cells. The device parameters analyzed include the open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF) and conversion efficiency (η). The results of our study show that for a concentration ratio greater than 60x with AM1.5G, the device with a shading factor of 7.1% has the best cell efficiency of 27.05%, due to the smaller current crowding at the center spot. The results indicate that the conversion efficiency of solar cells can be improved by establishing a compromise between the shading effect and the series resistance effect.

Characterization of solar cells by thermal transient testing

The current paper deals with the application of thermal transient testing as a characterization tool for solar modules. Based on the measurement of different samples (concentrator solar cell, single junction silicon solar cell) we prove the applicability of this measurement technique and address some specific issues of the characterization of solar cells by the thermal transient method.From the measurement metrics such as junction-to-base plate thermal resistance and thermal capacitance(s) can be derived and can serve as a basis of a multi domain solar cell model. The used technique also enables us to verify the quality of attachment layers in a solar module allowing fair quality control and reliability analysis of these devices. Finally a method is proposed to regain the data that is covered by the initial electric transient following the power step. This initial electric transient can be high in large surface devices like solar cells, and covers valuable data describing the structure near to the p–n junction. To eliminate this, simulated transients were fitted to the part of the actual measured thermal transient where the electric transient already decayed. This way the part of the thermal transient that was covered by the electric transient can be reconstructed.

Influence of metal grid spacing on the conversion efficiency of concentration solar cell at different illumination levels

• Grid shadowing ratio dominated the performance of SC at low concentration levels. • Grid shadowing ratio & resistance need optimal balance at high concentration levels. • Pioneering study (theoretical & experimental): the effect of metallic grid patterns on SC. The design of front metal grid spacing of linear grid pattern for III–V multi-junction concentrated solar cells is a critical issue when high density photocurrent is induced under concentrated sunlight and a poor grid spacing results in resistive losses. In the present work we have performed outdoor experiment and investigated both theoretically and experimentally the effect of ten different metal grid spacing on the electrical performance of high efficiency GaInP/GaInAs/Ge concentrated solar cells under various concentrating level of sunlight. The shadowing ratio of metal grids was adjusted from 3.07% to 6.66%. We have observed that the variation of experimentally obtained variation of power conversion efficiency data with grid spacing is consistent with the variation of theoretical estimation of total power loss with grid spacing. Moreover, the total power loss was dominated by grid shadowing effect at lower concentration levels; while at higher concentration levels the lowest total power loss condition was found when a compromise occurred mainly between grid shadowing effect and resistance of metal lines.

Carrier extraction from GaSb quantum rings in GaAs solar cells using direct laser excitation

The authors report on the analysis of the hole escape mechanisms from type-II GaSb quantum rings (QRs) embedded within the active region of a GaAs single junction solar cell. When the solar cell is excited by using a 1064 nm infrared laser with excitation energy lower than the bandgap of the GaAs matrix, photogenerated electron-hole pairs are created directly within the GaSb QRs. The QR photocurrent exhibits a linear dependence on the excitation intensity over several decades. The thermal activation energy was found to be weakly dependent on the incident light level and increased by only a few meV over several orders of excitation intensity. The magnitude of the relative absorption in the author's QRs when directly probed by using a 1064 nm laser with an incident power density of approximate to 2.6 W cm(-2) is found to be approximate to 1.4 x 10(-4) per layer. The thermal escape rate of the holes was calculated and found to be approximate to 10(11) to 10(12) s(-1), which is much faster than the radiative recombination rate 10(9) s(-1). This behaviour is promising for concentrator solar cell development and has the potential to increase solar cell efficiency under a strong solar concentration.

High aspect ratio electrodeposited Ni/Au contacts for GaAs‐based III–V concentrator solar cells

AbstractWe report on a photolithographic and electro‐deposition process that results in an optimized front grid structure for high efficiency multi‐junction III–V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6 µm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10−4 to mid 10−5 Ohm cm2 range along with excellent adhesion without sintering. Copyright © 2014 John Wiley & Sons, Ltd.

Numerical simulation of the temperature distortions in InGaP/GaAs/Ge solar cells working under high concentrating conditions due to voids presence in the solder joint

The presence of voids in solar cell solder joints causes a modification of the heat fluxes inside the device during its operation, which in turn leads to local increases in cell temperature and thermal resistance. These temperature increases at device surface lead to a modification of the photovoltaic cell voltage map which translates in a drop in cell output power. Moreover, for pv cells working under high concentration conditions such as III–V multi-junction solar cells for terrestrial application, very high temperature increases can arise as a consequence of void presence, both above the void volume and around it, leading in extreme cases to irreversible damages of the device. In this paper a matlab script is implemented to assess the temperature increase at the top surface of a InGaP/GaAs/Ge solar cell, in the device regions lying outside the void coverage area, for different void sizes found in concentrator solar cells. The obtained results are compared to those of finite element method (FEM) simulations, which are based on an equivalent 2.5 D thermal representation of the cell. A good agreement between FEM simulations and the developed thermal model is observed for small and medium size voids, while for larger voids the error between FEM simulations and the developed model becomes not-negligible. An analytical expression is obtained to assess the device thermal resistance in presence of a random distribution on not-interacting voids. The developed model can be used as starting point to assess the influence of void presence on the power performances of InGaP/GaAs/Ge multi-junction solar cells working under high concentrating conditions.

Optimization of the Metal Finger of a Silicon Concentrator Solar Cell

The key factors affecting the efficiency of silicon concentrator solar cells with boron implanted emitters were investigated. It is found that, by optimization of the front metal finger, the efficiency of the solar cells can be improved by approximately 10% under 10 suns illumination with the finger spacing of 330 μm.

Photovoltaic Characteristics of Low-density Concentration GaAs Solar Cells with/without Anti-reflective Coating

We have studied photovoltaic characteristics of single-junction GaAs solar cells with/without an <TEX>$MgF_2/ZnS$</TEX> anti-reflective coating (ARC) illuminated by low-density concentration (<10 suns). By the ARC deposition, the short-circuit current density (<TEX>$J_{SC}$</TEX>) and the fill factor (FF) are increased by <TEX>$5mA/cm^2$</TEX> and 5% at a standard illumination (1 sun), respectively, and the resulted conversion efficiency is enhanced by 45%. In contrast with the cell with no ARC showing a rapid degradation with increasing concentration power, the efficiency of ARC-deposited cell remains almost constant as (<TEX>$17.7{\pm}0.3$</TEX>)% regardless of the concentration. It informs that ARC treatment is very effective in GaAs concentrator solar cells.

The Finite Volume Method in Photovoltaic - The Cooling System of Concentrator Solar Panels

In concentrator cells, light is concentrated using lens to fall on solar cells to produce maximum energy as possible. By using concentrator cells light intensity is increased by targeting on certain area, which in result increases electricity production. Design and optimization of cooling equipment for heating concentrator solar cells is realized by SolidWorks and Flow Simulation systems. The device is analyzed on the flow and heat transfer to optimize the functional sample.

Wafer bonded four‐junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cells with 44.7% efficiency

ABSTRACTTriple‐junction solar cells from III–V compound semiconductors have thus far delivered the highest solar‐electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four‐junction solar cell architectures with optimum bandgap combination are found for lattice‐mismatched III–V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs‐based top tandem solar cell structure was bonded to an InP‐based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four‐junction solar cell with a new record efficiency of 44.7% at 297‐times concentration of the AM1.5d (ASTM G173‐03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III–V multi‐junction solar cells having four and in the future even more junctions. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Defect Management of High Efficiency Multijunction, Space and Concentrator Solar Cells

ABSTRACTIII-V compound multi-junction solar cells have high efficiency potential of more than 50% due to wide photo response, while limiting efficiencies of single-junction solar cells are 31-32%. In order to realize high efficiency III-V compound multi-junction solar cells, understanding and controlling imperfections (defects) are very important. This paper reviews fundamentals of defects and defect management for III-V compound materials, single-junction, multi-junction, space and concentrator solar cells.

집광형 태양전지를 이용한 난방장치 개발

본 연구는 집광장치와 태양추적장치를 이용함으로서 제한된 면적을 가지는 집광형 태양전지의 발전효율을 증가시키는 연구를 진행하였다. 태양전지에서 발생된 열은 온수나 난방에 사용되도록 흡열기를 이용하여 회수하였다. 이로서 집광형 태양전지를 이용한 시스템은 전기와 열을 동시에 공급할 수 있다. 태양위치에 따른 실시간 추적, 일몰 이후 난방장치의 동쪽방향 원상복귀, 원상복귀후 자동종료는 조도센서와 Simulink 프로그램으로 구현하였다. 흡열기 입구 속도, 핀설치 유무, 입구위치에 따른 흡열기 온도를 해석하였으며 실험결과와 일치하였다.

3-D modeling of perimeter recombination in GaAs diodes and its influence on concentrator solar cells

This paper describes a complete modelling of the perimeter recombination of GaAs diodes which solves most unknowns and suppresses the limitations of previous models. Because of the three dimensional nature of the implemented model, it is able to simulate real devices. GaAs diodes on two epiwafers with different base doping levels, sizes and geometries, namely square and circular are manufactured. The validation of the model is achieved by fitting the experimental measurements of the dark IV curve of the manufactured GaAs diodes. A comprehensive 3-D description of the occurring phenomena affecting the perimeter recombination is supplied with the help of the model. Finally, the model is applied to concentrator GaAs solar cells to assess the impact of their doping level, size and geometry on the perimeter recombination.

Super-intense white upconversion emission of Yb_2O_3 polycrystals and its application on luminescence converter of dye-sensitized solar cells

In this Letter, we present the observation of super-intense upconversion (UC) white emission of Yb2O3 under 980 nm excitation, its evolution on excitation power density, the UC mechanism, and its application on the luminescence converter of dye-sensitized solar cells (DSSCs). It is significant to observe that Yb2O3 demonstrates at least one order more intense UC luminescence than β-phase NaYF4:Yb3+, Er3+ and Yb2O3/DSSCs exhibit much better photovoltaic performance than β-phase NaYF4:Yb3+, Er3+/DSSCs under strong excitation. This indicates that Yb2O3 would become a novel candidate of the solar energy converter, especially in the application of concentrator solar cells.

Photon management with nanostructures on concentrator solar cells

Optimizing the feature sizes of dielectric nanostructures on the top (ZnS) and bottom (SiO2) surfaces of a 1 μm thick GaAs solar cell, we obtain a higher efficiency (34.4%) than a similar cell with a state of the art bilayer antireflection coating and a planar mirror (33.2%). The back side nanostructure increases the photocurrent due to enhanced optical path length inside the semiconductor, while the nanostructure on the front side increases the photocurrent due to lower reflectance losses.

Numerical modeling of loss mechanisms resulting from non‐uniform illumination in multijunction concentrator solar cells

AbstractQuasi‐3D distributed emitter models utilizing a unit cell‐based methodology have been successfully applied to the analysis of lateral current flow in solar cell emitter layers. However, the analysis of the specific loss mechanisms resulting from this flow has not been given adequate attention. In this work, a quasi‐3D model for the simulation of effects related to the lateral flow of current in a solar cell emitter layer, particularly under non‐uniform illumination, is developed. The model is applied to a specific case in which a GaInP/GaAs‐like two‐terminal solar cell is illuminated with a Lorentzian irradiance pattern, which is the expected pattern for parabolic trough concentrator. It is shown that bias‐point loss, which results from the variation in the local operating condition of the cell, is significant under highly non‐uniform illumination at low–moderate optical concentration. Understanding this loss mechanism is useful in considering trade‐offs related to the design of the optical concentrator system, as well as, the design of a grid electrode pattern for maximum power output. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Iron-boron pair dissociation in silicon under strong illumination

The dissociation of iron-boron pairs (FeB) in Czochralski silicon under strong illumination was investigated. It is found that the dissociation process shows a double exponential dependence on time. The first fast process is suggested to be caused by a positive Fe in FeB capturing two electrons and diffusion triggered by the electron-phonon interactions, while the second slow one would involve the capturing of one electron followed by temperature dependent dissociation with an activation energy of (0.21 ± 0.03) eV. The results are important for understanding and controlling the behavior of FeB in concentrator solar cells.

Thermal modeling optimization and experimental validation for a single concentrator solar cell system with a heat sink

A single concentrator solar cell model with a heat sink is established to simulate the thermal performance of the system by varying the number, height, and thickness of fins, the base thickness and thermal resistance of the thermal conductive adhesive. Influence disciplines of those parameters on temperatures of the solar cell and heat sink are obtained. With optimized number, height and thickness of fins, and the thickness values of base of 8, 1.4 cm, 1.5 mm, and 2 mm, the lowest temperatures of the solar cell and heat sink are 41.7 °C and 36.3 °C respectively. A concentrator solar cell prototype with a heat sink fabricated based on the simulation optimized structure is built. Outdoor temperatures of the prototype are tested. Temperatures of the solar cell and heat sink are stabilized with time continuing at about 37 °C–38 °C and 35 °C–36 °C respectively, slightly lower than the simulation results because of effects of the wind and cloud. Thus the simulation model enables to predict the thermal performance of the system, and the simulation results can be a reference for designing heat sinks in the field of single concentrator solar cells.