Characteristics of bifacial solar cells under bifacial illumination with various intensity levels

The characteristics of bifacial solar cells with different rear structures were investigated under front, rear and bifacial illumination with an intensity of 0.4–4.2 suns. Five kinds of solar cells, rear flat local‐BSF cells, rear textured local‐BSF cells (textured RLB cells), rear total‐BSF cells, rear floating‐emitter cells, and triode cells with double‐sided junctions, were tested. The I–V characteristics of the cells under bifacial illumination were measured with a newly designed measurement system that simultaneously illuminated both surfaces of the cells. In the short‐circuit current (J_SC) and the saturation current evaluations, the bifacial illumination effect, which means that the power output of the cell is intrinsically improved by adding rear illumination, was not observed. Although the RLB cells showed a nonlinear increase in J_SC and enhanced V_OC, these increases did not make a practical contribution to extra output because of the low levels of these characteristics. When we evaluated the maximum output power, the bifacial illumination effect was only observed in the triode cell. A triode cell can decrease resistive loss by introducing light from both surfaces, compared with a conventional cell with one junction. Copyright © 2001 John Wiley & Sons, Ltd.     

Thermo‐Phototronic Effect Enhanced InP/ZnO Nanorod Heterojunction Solar Cells for Self‐Powered Wearable Electronics

Enhancing interfacial charge transfer by the inner electric field is crucial for improving photovoltaic performance of heterojunction solar cells. Recent studies are focusing on how to utilize piezo‐phototronic effect (strain‐induced inner electric field) to modulate the interfacial charge transfer, whereas the preservation of solar cells from structure damage and performance decline under long‐term strain becomes increasingly challenging. Here, without use of strain, a thermo‐phototronic effect is presented to enhance the interfacial charge transfer in InP/ZnO nanorod heterojunction solar cells. Under a temperature gradient of 3.5 °C across the device, the output current and voltage of the solar cell under weak light illumination are enhanced by 27.3 and 76%, respectively. Moreover, the performance enhancement can be further regulated by applying different temperature gradients. This study serves as proof‐of‐principle for the thermo‐phototronic effect and pushes forward the maximum utilization of solar energy by a one‐circuit‐based photovoltaic‐thermoelectric system.     

A validated SPICE network simulation study on improving tunnel diodes by introducing lateral conduction layers

In this work, network simulations using LTSpice (Linear Technology, Milpitas, CA, USA) for monolithic triple‐junction solar cells have been performed. In order to simulate the internal structure correctly, the integration of the tunnel diode into the network simulation was mandatory. The tunnel‐diode characteristics are modeled by LTSpice's arbitrary behavioral current sources. The integration of tunnel‐diode characteristics into the network model was validated by comparison of simulated and experimental data. Lattice‐matched triple‐junction solar cells were examined under homogenous illumination between 1 and 1900 suns as well as under non‐uniform digital irradiance. The verified model was then used to study the influence of lateral current spreading in layers surrounding the tunnel diodes. It is shown that a lateral current spreading from high to low illumination intensity regions cannot prevent the tunnel diode from switching to thermal diffusion under the used Gaussian illumination profile as it appears in concentrator photovoltaic applications. Furthermore, resistance regimes of the lateral conducting layers were identified, which would enable a current spreading that is high enough to transport all current exclusively by tunneling. It is shown that the presence of at least one additional layer above and one below the tunnel diode is mandatory. Finally, the necessary layer thicknesses using Al_x−1Ga_xAs as lateral conducting layers are calculated for different doping concentrations and mole fractions x. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of Temperature and Illumination on the Electrical Characteristics of Polymer–Fullerene Bulk‐Heterojunction Solar Cells

The current–voltage characteristics of ITO/PEDOT:PSS/OC₁C₁₀‐PPV:PCBM/Al solar cells were measured in the temperature range 125–320 K under variable illumination, between 0.03 and 100 mW cm^–2 (white light), with the aim of determining the efficiency‐limiting mechanism(s) in these devices, and the temperature and/or illumination range(s) in which these devices demonstrate optimal performance. (ITO: indium tin oxide; PEDOT:PSS: poly(styrene sulfonate)‐doped poly(ethylene dioxythiophene); OC₁C₁₀‐PPV: poly[2‐methoxy‐5‐(3,7‐dimethyl octyloxy)‐1,4‐phenylene vinylene]; PCBM: phenyl‐C₆₁ butyric acid methyl ester.) The short‐circuit current density and the fill factor grow monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open‐circuit voltage to 0.91 V was observed upon cooling the devices down to 125 K. This fits the picture in which the open‐circuit voltage is not limited by the work‐function difference of electrode materials used. The overall effect of temperature on solar‐cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9 % at T = 320 K and 100 mW cm^–2 (2.5 % at 0.7 mW cm^–2). The almost‐linear variation of the short‐circuit current density with light intensity confirms that the internal recombination losses are predominantly of monomolecular type under short‐circuit conditions. We present evidence that the efficiency of this type of solar cell is limited by a light‐dependent shunt resistance. Furthermore, the electronic transport properties of the absorber materials, e.g., low effective charge‐carrier mobility with a strong temperature dependence, limit the photogenerated current due to a high series resistance, therefore the active layer thickness must be kept low, which results in low absorption for this particular composite absorber.     

Temperature‐dependent electroluminescence and voltages of multi‐junction solar cells

The spectral electroluminescence of a monolithic triple‐junction solar cell reveals the sub‐cell open‐circuit voltages under variation of temperature and carrier concentration. We present an electroluminescence set‐up that is able to acquire a full spectrum in less than 10 s and give insight into the voltage‐extraction process. The sum of the sub‐cell voltages is in excellent agreement to open‐circuit voltages acquired under concentrated illumination. The temperature‐induced voltage losses are investigated and explained by a bandgap dependent increase in intrinsic carrier concentration. Finally, the accurate extraction of the sub‐cell bandgaps helps to identify a temperature rise with increasing current density. Copyright © 2013 John Wiley & Sons, Ltd.     

A high‐efficiency LPE GaAs solar cell at concentrations ranging from 2000 to 4000 suns

III–V solar cells for terrestrial concentration applications are currently becoming of greater and greater interest. From our experience, concentrations higher than 1000 suns are required with these cells to reduce PV electricity cost to such an extent that this alternative could become cost competitive. In this paper, a single‐junction p/n GaAs solar cell, with efficiencies of 23ċ8 and 22ċ5% at concentration ratios of 2700 and 3600 suns respectively, is presented. This GaAs solar cell is well suited for use with non‐imaging optical concentrators, which possess a large aperture angle. Low‐temperature liquid phase epitaxy (LTLPE) has been the growing technique for the semiconductor structure as an attempt to use a simplified, cheap and clean technique, within a renewable energy perspective. The GaAs solar cell presented is compared with the highest efficiency tandem solar cells at concentration levels exceeding 1000 suns. The GaAs solar cell performance maintains high efficiencies up to 4000 suns, while tandem cells seem to drop very quickly after reaching their maximum. Therefore, single‐junction GaAs solar cells are a good candidate for operating at very high concentrations, and LPE is able to supply these high‐quality solar cells to work within terrestrial concentration systems, the main objective of which is the reduction of PV electricity costs. Copyright © 2003 John Wiley & Sons, Ltd.     

Series resistance imaging in solar cells by lock‐in thermography

Two operation modes of lock‐in thermography are introduced to detect regions of high series resistance in solar cells. These are differential techniques, working in the dark and under illumination, where images taken under two different conditions are used to calculate an image, which is especially sensitive to series resistance variations. Though the series resistance cannot be measured quantitatively by these techniques, regions of increased emitter contact resistance can be reliably detected. A realistic electrothermal modelling of a series resistance defect in a solar cell with and without illumination is presented. The new thermographic techniques are compared with established techniques for series resistance imaging. Especially the technique working under illumination gives results that agree very well with those of other methods. Copyright © 2005 John Wiley & Sons, Ltd.     

Superhydrophobic Carbon Nanotube Electrode Produces a Near‐Symmetrical Alternating Current from Photosynthetic Protein‐Based Photoelectrochemical Cells

The construction of protein‐based photoelectrochemical cells that produce a variety of alternating currents in response to discontinuous illumination is reported. The photovoltaic component is a protein complex from the purple photosynthetic bacterium Rhodobacter sphaeroides which catalyses photochemical charge separation with a high quantum yield. Photoelectrochemical cells formed from this protein, a mobile redox mediator and a counter electrode formed from cobalt disilicide, titanium nitride, platinum, or multi‐walled carbon nanotubes (MWCNT) generate a direct current during continuous illumination and an alternating current with different characteristics during discontinuous illumination. In particular, the use of superhydrophobic MWCNT as the back electrode results in a near symmetrical forward and reverse current upon light on and light off, respectively. The symmetry of the AC output of these cells is correlated with the wettability of the counter electrode. Potential applications of a hybrid biological/synthetic solar cell capable of generating an approximately symmetrical alternating current are discussed.     

Electrical performance evaluation of low‐concentrating non‐imaging photovoltaic concentrator

Second generation prototype photovoltaic facades of reduced costs incorporating devices with optically concentrating elements (PRIDE) incorporate 6 mm wide ‘Saturn’ solar cells at the absorber of the dielectric concentrator. The concentrators were made using injection moulding technique with potential to manufacture in large‐scale applications. Four different concentrator panels have been experimentally verified at outdoors to identify the non‐identical current–voltage (I–V) curves. The I–V curve, fill factor and solar to electrical conversion efficiency of four PRIDE concentrator modules have been evaluated from the 24 manufactured in the ‘IDEOCONTE’ project. The maximum solar to electrical conversion efficiency and the fill factor of the PRIDE concentrator were 9·1 and 70%, respectively. The mismatch loss of the ‘unit concentrators’ has been identified that occurred due to the lack of bonding between the concentrator unit and the solar cell and the rear glass. The average power concentration ratio of PRIDE concentrators manufactured by the improved method was 2·10 compared to a similar non‐concentrating panel and the optical efficiency of the PRIDE system was 83%. Copyright © 2008 John Wiley & Sons, Ltd.     

Thin‐film GaAs epitaxial lift‐off solar cells for space applications

In the present work the space compatibility of thin‐film GaAs solar cells is studied. These cells are separated from their GaAs substrate by the epitaxial lift‐off (ELO) technique and mounted behind a CMG cover glass which at the same time serves as a stable carrier for the thin film cells. In the present initial stage of development these cells have an average efficiency of about 15·4% under AM0 illumination due to not yet optimized grid contacts and anti‐reflection coatings. Inspection after irradiation by 1 MeV electrons, thermal vacuum and thermal cycling experiments reveal that degradation of the cells is largely due to delamination and micro‐cracking. Based on these results, glass dehydration and adhesive degassing procedures are implemented in the ELO cell processing. As a consequence, even in this premature phase, newly produced cells show a radiation hardness comparable to or better than that of commercially available GaAs cells on Ge substrates and are virtually unaffected by severe thermal cycling. Copyright © 2005 John Wiley & Sons, Ltd.     

Simulation of the real efficiencies of high-efficiency silicon solar cells

The temperature dependences of the efficiency η of high-efficiency solar cells based on silicon are calculated. It is shown that the temperature coefficient of decreasing η with increasing temperature decreases as the surface recombination rate decreases. The photoconversion efficiency of high-efficiency silicon-based solar cells operating under natural (field) conditions is simulated. Their operating temperature is determined self-consistently by simultaneously solving the photocurrent, photovoltage, and energy-balance equations. Radiative and convective cooling mechanisms are taken into account. It is shown that the operating temperature of solar cells is higher than the ambient temperature even at very high convection coefficients (~300 W/m² K). Accordingly, the photoconversion efficiency in this case is lower than when the temperature of the solar cells is equal to the ambient temperature. The calculated dependences for the open-circuit voltage and the photoconversion efficiency of high-quality silicon solar cells under concentrated illumination are discussed taking into account the actual temperature of the solar cells.     

菲涅耳透镜下多结电池表面局部高辐射功率对短路电流的影响

为探索以菲涅耳透镜为聚光器的聚光光伏模组中,多结电池中心局部高辐射功率对短路电流的影响,测量菲涅耳透镜的高亮度光斑直径,并据此分别测试室内不同局部光照面积下和户外不同尺寸透镜下的GaInP/GaInAs/Ge三结电池的短路电流,利用电路网络模型分析实验结果。结果表明,短路电流与局部聚光的面积无关;小尺寸菲涅耳透镜聚焦下,沿光轴电流与辐射功率同步变化;透镜尺寸增大到一定程度,电池中心局部承受过高辐射功率,电流受峰值隧穿电流限制,宏观体现为焦平面处短路电流下降。电池放置在焦平面两侧,均可缓解局部高辐射功率,短路电流最高提升 8.0%。     

连续激光辐照三结GaAs太阳电池温度场仿真

为了研究真空环境下1070nm连续激光辐照对三结GaAs太阳电池输出性能的影响,利用COMSOL软件构建了相应物理模型,通过数值仿真研究了激光功率密度、光斑半径、减反膜和热辐射热对流对温度场的影响。结果表明,吸收系数、热导率和光电转换效率是温度演变的3个主要因素;温升幅度随激光功率密度增大而增大;光斑半径越小使得电池表面温差越大;拥有减反膜结构可有效地提高太阳电池转换效率,但也使电池温度较高;热对流散热在电池较低温度（300K~400K）情况下占据主导作用;当入射功率密度为16.7W/cm2、光斑半径与电池半径相同时,经20s后,电池中心温度达到501.521K,导致光电转换效率为0。该数值模拟结果与实验结果基本相符,对激光损伤太阳电池机理研究提供一定的理论依据。     

Spatially resolved evaluation of power losses in industrial solar cells by illuminated lock‐in thermography

The principles of a recently introduced measurement technique for power losses in solar cells, illuminated lock‐in thermography (ILT), are reviewed. The main advantage of ILT over dark lock‐in Thermography (DLT) is measurement under realistic operational conditions of solar cells. The main focus of this paper is to demonstrate the wide range of applications of ILT in identifying the causes of power losses in solar cells. For this purpose different evaluation methods are presented. A method for the evaluation of improvement potentials within a given cell technology is demonstrated. It is shown that different types of series resistance may be localized. Small areas of recombination losses (e.g., grain boundaries) can routinely be detected, which is not possible in dark lock‐in thermography. Good correspondence with light‐beam‐induced current images is found. A realistic evaluation of the impact of recombination losses on solar cell performance is demonstrated on two examples. Finally, process‐ or treatment‐induced recombination losses are investigated. In summary ILT is shown to be an extremely powerful tool in localizing, identifying and quantifying power losses of solar cells under realistic illumination conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

Open‐circuit voltage recovery in type II GaSb/GaAs quantum ring solar cells under high concentration

We report on the open‐circuit voltage recovery in GaSb quantum ring (QR) solar cells under high solar concentration up to 2500 suns. The detailed behaviour of type II GaSb/GaAs QR solar cells under solar concentration, using different temperatures and light illumination conditions, is analysed through optical and electrical measurements. Although enhancement of the short‐circuit current was observed because of sub‐bandgap photon absorption in the QR, the thermionic emission rate of holes was found to be insufficient for ideal operation. The direct excitation of electron–hole pairs into QRs has revealed that the accumulation of holes is one of the causes of the open‐circuit voltage (V_OC) degradation. However, using concentrated light up to 2500 suns, the GaSb QR cell showed much quicker V_OC recovery rate than a GaAs control cell. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of dissociation of iron–boron pairs in crystalline silicon on solar cell properties

The effect of dissociation of interstitial iron‐substitutional boron (Fe_iB_s) pairs, as it occurs under illumination in iron‐contaminated silicon solar cells, on the solar cell properties has been studied on the basis of numerical device simulations using reported recombination parameters for Fe_i and Fe_iB_s. Most cell parameters are found to degrade during Fe_iB_s dissociation. However, the open‐circuit voltage can also increase within certain ranges of the iron concentration. Critical iron concentrations are determined, giving the threshold contamination level above which a significant degradation in the corresponding cell parameter can be observed. The threshold iron contamination level of the open‐circuit voltage degradation is found to be up to two orders of magnitude larger than the threshold iron level of the short‐circuit current degradation. As the behaviour of the cell parameters under illumination is specific to the dissociation of Fe_iB_s pairs, the characteristic changes in the cell parameters due to illumination may be used as a simple way of identifying iron contamination problems in silicon solar cells. Copyright © 2005 John Wiley & Sons, Ltd.     

Bioinspired Temperature Regulation in Interfacial Evaporation

Human skin shows self‐adaptive temperature regulation through both enhanced heat dissipation in high temperature environments and depressed heat dissipation in cold environments. Inspired by such thermal regulation processes, an interfacial material system with self‐adaptive temperature regulation in the solar‐driven interfacial evaporation system, which can exhibit automatic temperature oscillation to enable pyroelectricity generation while producing water vapor, is reported. The bioinspired interface system is designed with the combination of a thermochromism‐based temperature regulator consisting of tungsten‐doped vanadium dioxide nanoparticles and a polymeric pyroelectric thin film of polyvinylidene fluoride. Under the simulated solar illumination with power density of 1.1 kW m^?2, the bioinspired interfacial evaporation system achieves a self‐adaptive temperature oscillation with the maximum temperature difference of ≈7 °C and this system can simultaneously generate water vapor as well as electricity with an evaporation efficiency of 71.43% and a maximum output electrical power density of 104 µW m^?2, respectively. The study demonstrates a design of thermal management at the interface of solar‐driven evaporation system to exhibit a self‐adaptive temperature oscillation and offers an alternative approach for the multifunctional harvesting of solar energy.     

Effects of ultra‐high flux and intensity distribution in multi‐junction solar cells

We report results of high‐flux experiments on tandem solar cells, with a real‐sun probe predicated on mini‐dish fiber‐optic concentrators. Experimental results and their interpretation focus on: (a) a striking insensitivity of cell efficiency to flux map; (b) the predictability of the flux values at which cell efficiency peaks; and (c) performance of the same cell architecture at markedly smaller cell area. Copyright © 2006 John Wiley & Sons, Ltd.     

非对称CPC—PV系统性能数值模拟计算

在CPC的基础上,对非对称CPC－PV的性能进行数值模拟计算,得到一种 既能增大聚光比,又能有效平衡太阳能电池冬夏季输出功率的聚光器。     

A new method to characterize bifacial solar cells

We present a new method to characterize bifacial solar cells under standard test conditions (STC). The method considers the bifacial operation of the cell and provides the characteristics for simultaneous front and rear side illumination rather than providing the front and the rear side characteristics separately. The method involves measurements of front side electrical parameters (efficiency, open‐circuit voltage, short‐circuit current and fill factor) and rear side short‐circuit current under STC. Two new parameters are introduced, namely bifacial 1.x efficiency (effective efficiency) and gain‐efficiency product, which are calculated from the measured STC parameters. The former provides information related to the cell design considering the bifacial operation, whereas the latter provides the end‐use benefits from the modules with bifacial cells for a particular installation. To calculate the bifacial 1.x efficiency and the gain‐efficiency product, a one‐diode solar cell equivalent circuit is used. Characteristic plots are shown for the newly introduced parameters as a function of rear‐side illumination for various example solar cells. A sensitivity analysis is performed to understand the influence of each single‐sided STC solar cell parameter on the newly introduced parameters. This sensitivity analysis shows that the fill factor and the rear‐to‐front current ratio are the most critical parameters for bifacial solar cells. Copyright © 2012 John Wiley & Sons, Ltd.