Current‐matching estimation for multijunction cells within a CPV module by means of component cells

An indoor method is presented for the quantification of the current‐matching ratio of a multijunction cell within a concentrator under arbitrary spectral irradiance conditions. The cell current is measured across a very large spectral sweep to force the relevant subcells into a limiting condition. The light spectrum is monitored using component cells to avoid the need for a spectroradiometer and spectral response measurements. The method also provides an estimation of the current losses beyond the overall current mismatch, for example, losses produced in concentrators with chromatic aberration by the non‐uniformity of the incident spectrum across the cell. The method has been applied to a pair of refractive point‐focus concentrator systems; first, a 300X single‐stage Fresnel lens over a lattice‐matched GaInP/Ga(In)As/Ge triple‐junction cell and second, a 1000X two‐stage system with the same Fresnel lens over a homogenizing secondary lens that encapsulates a triple‐junction cell of the same kind but smaller. The experiment demonstrates that the single‐stage concentrator exhibits a higher sensitivity of the current mismatch to variations in the focal distance. Copyright © 2012 John Wiley & Sons, Ltd.     

Spectral study and classification of worldwide locations considering several multijunction solar cell technologies

Multi‐junction solar cells are widely used in high‐concentration photovoltaic systems (HCPV) attaining the highest efficiencies in photovoltaic energy generation. This technology is more dependent on the spectral variations of the impinging Direct Normal Irradiance (DNI) than conventional photovoltaics based on silicon solar cells and consequently demands a deeper knowledge of the solar resource characteristics. This article explores the capabilities of spectral indexes, namely, spectral matching ratios (SMR), to spectrally characterize the annual irradiation reaching a particular location on the Earth and to provide the necessary information for the spectral optimization of a MJ solar cell in that location as a starting point for CPV module spectral tuning. Additionally, the relationship between such indexes and the atmosphere parameters, such as the aerosol optical depth (AOD), precipitable water (PW), and air mass (AM), is discussed using radiative transfer models such as SMARTS to generate the spectrally resolved DNI. The network of ground‐based sun and sky‐scanning radiometers AErosol RObotic NETwork (AERONET) is exploited to obtain the atmosphere parameters for a selected bunch of 34 sites worldwide. Finally, the SMR indexes are obtained for every location, and a comparative analysis is carried out for four architectures of triple junction solar cells, covering both lattice match and metamorphic technologies. The differences found among cell technologies are much less significant than among locations. Copyright © 2016 John Wiley & Sons, Ltd.     

Lateral spectrum splitting concentrator photovoltaics: direct measurement of component and submodule efficiency

To achieve high energy conversion efficiency, a solar module architecture called lateral spectrum splitting concentrator photovoltaics (LSSCPV) is being developed. LSSCPV can concentrate available sunlight and laterally split a single beam into bands with different spectra for absorption by different solar cells with band gaps matched to the split spectrum. Test assemblies of a sample LSSCPV architecture were constructed, each of which contains four p–n junctions and two optical pieces. Independent experiments or simulations had been implemented on the components but by using optimal assumptions. In order to examine the actual performances of all the components, which are dependent on each other and the light source, direct outdoor measurements were made. A set of self‐consistent efficiency definitions was articulated and a test bed was developed to measure the parameters required by the efficiency calculation. By comparing the component efficiency items derived from the outdoor measurement and the expected values based on independent simulations, the potential opportunities for efficiency improvement are determined. In the outdoor measurement at the University of Delaware, the optical component demonstrated 89·1% efficiency. Additional assemblies were tested at the National Renewable Energy Laboratory. One assembly demonstrated 36·7% submodule efficiency, which compares favorably with the 32·6% previously reported verified submodule efficiency. Copyright © 2011 John Wiley & Sons, Ltd.     

太阳能电池栅电极优化设计

本文通过对聚光电池中常用的两种电极结构进行理论模拟,全面具体地分析了电池尺寸以及与电池上电极有关的各种损耗对栅电极优化设计的影响;并且对电极优化设计在不同聚光因子下的影响也进行了模拟,使太阳电池栅电极的优化设计更加完整。     

Optimization of grid design for solar cells

By theoretical simulation of two grid patterns that are often used in concentrator solar cells, we give a detailed and comprehensive analysis of the influence of the metal grid dimension and various losses directly associated with it during optimization of grid design. Furthermore, we also perform the simulation under different concentrator factors, making the optimization of the front contact grid for solar cells complete.     

Multijunction solar cell model for translating I–V characteristics as a function of irradiance,spectrum, and cell temperature

We describe here a lumped diode model for concentrator multijunction solar cells, in which the temperature, irradiance and spectrum dependences are explicitly included. Moreover an experimental method based on it for the prediction of the I‐V curve under any irradiance‐spectrum‐temperature conditions from a single input measurement is proposed and applied to a set of commercial triple‐junction solar cells in order to demonstrate its validity. Component ‘isotype’ cells are used as reference cells for intensity and spectrum, sparing the measurement of light spectrum and cell spectral response. Finally, a mean RMS prediction error of 0.85% over a range of 100X‐25°C to 700X‐75°C is reported for the whole set when the model parameters inherent in the cell are assumed to be the same for every sample. If optimum parameters are extracted for every cell, the RMS error is reduced to 0.53%. Copyright © 2010 John Wiley & Sons, Ltd.     

Limiting factors on the semiconductor structure of III–V multijunction solar cells for ultra‐high concentration (1000–5000 suns)

The main limiting factors of multijunction solar cells operating under ultra‐high concentration (>1000 suns) are examined by means of 2D physically based numerical modelling. The validation of the model is carried out by fitting calibrated light concentration measurements. Because the series resistance is the most important constraint in the electrical performance of the solar cell under ultra‐high irradiance, it is analysed and quantified detailing different contributions such as: (i) the electrical properties of the emitter; (ii) window layer of the top cell; and (iii) the band discontinuities formed at heterojunctions. We found the role of window layer to be important at very high concentrations (above 700 suns), while at ultra‐high concentrations, (above 1000 suns) a gain in efficiency (~ 1% absolute) can be obtained by a proper structural design of the window layer. In the case of the heterojunctions included in the multijunction solar cell, the impact of a high‐band offset can be mitigated by increasing the doping level density thus favouring the tunnelling effect. Moreover, the influence of different recombination mechanisms and high‐injection effects at ultra‐high irradiance is discussed. Finally, an optimisation of the complete solar cell taking into account the ohmic contacts to work under ultra‐high irradiances (from 1000 to 5000 suns) is presented as well as the implications on the use of ultra‐high irradiance in different multijunction solar cell architectures. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluation of the reliability of high concentrator GaAs solar cells by means of temperature accelerated aging tests

Evaluating the reliability, warranty period, and power degradation of high concentration solar cells is crucial to introducing this new technology to the market. The reliability of high concentration GaAs solar cells, as measured in temperature accelerated life tests, is described in this paper. GaAs cells were tested under high thermal accelerated conditions that emulated operation under 700 or 1050 suns over a period exceeding 10 000 h. Progressive power degradation was observed, although no catastrophic failures occurred. An Arrhenius activation energy of 1.02 eV was determined from these tests. The solar cell reliability [R(t)] under working conditions of 65°C was evaluated for different failure limits (1–10% power loss). From this reliability function, the mean time to failure and the warranty time were evaluated. Solar cell temperature appeared to be the primary determinant of reliability and warranty period, with concentration being the secondary determinant. A 30‐year warranty for these 1 mm²‐sized GaAs cells (manufactured according to a light emitting diode‐like approach) may be offered for both cell concentrations (700 and 1050 suns) if the solar cell is operated at a working temperature of 65°C. Copyright © 2012 John Wiley & Sons, Ltd.     

Development of concentrator modules with dome‐shaped Fresnel lenses and triple‐junction concentrator cells

The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. We have achieved a 26·6% peak uncorrected efficiency from a 7056 cm² 400 × module with 36 solar cells connected in series, measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1176 cm² area measured by Fraunhofer ISE and NREL are reported as 27·4% and 24·8% respectively. The peak uncorrected efficiency for a 550× and 5445 cm² module with 20 solar cells connected in series was 28·9% in house. The temperature‐corrected efficiency of the 550 × module under optimal solar irradiation condition was 31·5 ± 1·7%. In terms of performance, the annual power generation is discussed based on a side‐by‐side evaluation against a 14% commercial multicrystalline silicon module. For reliability, some new degradation modes inherent to high concentration III‐V solar cell system are discussed and a 20‐year lifetime under concentrated flux exposure proven. The fail‐safe issues concerning the concentrated sunlight are also discussed. Moreover, the overall scenario for the reduction of material cost is discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

Performance comparison of AlGaAs,GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10⁻⁴ Ω cm² followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10⁻⁴ Ω cm² within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Laminated Organic Photovoltaic Modules for Agrivoltaics and Beyond: An Outdoor Stability Study of All-Polymer and Polymer:Small Molecule Blends

The integration of organic photovoltaic (OPV) modules on greenhouses is an encouraging practice to offset the energy demands of crop growth and provide extra functionality to dedicated farmland. Nevertheless, such OPV devices must meet certain optical and stability requirements to turn net zero energy greenhouse systems a reality. Here a donor:acceptor polymer blend is optimized for its use in laminated devices while matching the optical needs of crops. Optical modeling is performed and a greenhouse figure-of-merit is introduced to benchmark the trade-off between photovoltaic performance and transparency for both chloroplasts and humans. Balanced donor:acceptor ratios result in better-performing and more thermally stable devices than acceptor-enriched counterparts. The optimized polymer blend and state-of-the-art polymer:small-molecule blends are next transferred to 25 cm² laminated modules processed entirely from solution and in ambient conditions. The modules are mounted on a greenhouse as standalone or 4-terminal tandem configurations and their outdoor stability is tracked for months. The study reveals degradation modes undetectable under laboratory conditions such as module delamination, which accounts for 10–20% loss in active area. Among the active layers tested, polymer:fullerene blends are the most stable and position as robust light harvesters in future building-integrated OPV systems.     

Semi‐quantitative temperature accelerated life test (ALT) for the reliability qualification of concentrator solar cells and cell on carriers

An adequate qualification of concentrator photovoltaic solar cells and cell‐on‐carriers is essential to increase their industrial development. The lack of qualification tests for measuring their reliability together with the fact that conventional accelerated life tests are laborious and time consuming are open issues. Accordingly, in this paper, we propose a semi‐quantitative temperature‐accelerated life test to qualify solar cells and cell‐on‐carriers that can assure a minimum life when failure mechanisms are accelerated by temperature under emulated nominal working conditions with an activation energy >0.9 eV. A properly designed semi‐quantitative accelerated life test should be able to determine if the device under test will satisfy its reliability requirements with an acceptable uncertainty level. The applicability, procedure, and design of the proposed test are detailed in the paper. Copyright © 2015 John Wiley & Sons, Ltd.     

Temperature accelerated life test on commercial concentrator III–V triple‐junction solar cells and reliability analysis as a function of the operating temperature

A temperature accelerated life test on commercial concentrator lattice‐matched GaInP/GaInAs/Ge triple‐junction solar cells has been carried out. The acceleration of the aging has been accomplished by subjecting the solar cells at temperatures markedly higher than the nominal working temperature inside a concentrator, and the nominal photo‐current condition (820 X) has been emulated by injecting current in darkness. Three tests at different temperatures have been carried out. The failure distributions across the three test temperatures have been fitted to an Arrhenius–Weibull model. An Arrhenius activation energy of 1.59 eV was determined from the fit. The reliability functions and parameters of these solar cells at two nominal working conditions (80 and 100 °C) have been obtained. In both cases, the instantaneous failure rate function monotonically increases, that is, the failures are of the wear‐out kind. We have also observed that the reliability data are very sensitive to the nominal temperature condition. In fact, at a nominal working condition of 820 X and 80 °C, assuming that the concentration module works 5 h per day, the warranty time obtained for a failure population of 5% has been 113 years. However, for a nominal working condition of 820 X and 100 °C, the warranty time obtained for a failure population of 5% has been 7 years. Therefore, in order to offer a long‐term warranty, the working temperature could be a key factor in the design of the concentration photovoltaic systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Design,fabrication, and analysis of transparent silicon solar cells for multi‐junction assemblies

Transparent silicon solar cells can lead to an increased efficiency of silicon‐based multi‐junction assemblies by transmitting near and below band gap energy light for conversion in a low band gap solar cell. This analysis shows that the maximum efficiency gain for a low band gap solar cell beneath silicon at a concentration of 50 suns is 5.8%, based on ideal absorption and conversion of the photons. This work analyzes the trade‐offs between increased near band edge absorption in the silicon and silicon solar cell transparency. Application of these results to real cases including a germanium bottom solar cell is analyzed, leading to a range of cases with increased system efficiency. Non‐ideal surfaces and real silicon and germanium solar cell device performance are presented. The range of practical system gains may be as low as 2.2 – 1% absolute when compared with the efficiency of a light‐trapped silicon solar cell for 1‐sun operation, based on this work. Copyright © 2012 John Wiley & Sons, Ltd.     

Stepwise co-sensitization as a useful tool for enhancement of power conversion efficiency of dye-sensitized solar cells: The case of an unsymmetrical porphyrin dyad and a metal-free organic dye

A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H₂[Porph] (ZnP-H₂P), where Zn[Porph] and H₂[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycine moiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO₂ electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H₂P/DC) and D (DC/ZnP-H₂P).The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H₂P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J_sc = 11.72 mA/cm²), open circuit voltage (V_oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J–V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J_sc = 13.45 mA/cm², V_oc = 0.76 V, and FF = 0.75), when a formic acid treated TiO₂ ZnP-H₂P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J_sc value (=13.45 mA/cm²), which resulted from an increased dye loading, and an enhanced V_oc value (=0.76 V), attributed to an upward shift and increased of electron density in the TiO₂ CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO₂ photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (τ_d), and longer electron lifetime (τ_e).     

Optical properties and conductivity of PEDOT:PSS films treated by polyethylenimine solution for organic solar cells

We report on conductivity and optical property of three different types of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films [pristine PH1000 film (PH1000-p), with 5 wt.% ethylene glycol additive (PH1000-EG) and with sulfuric acid post-treatment (PH1000-SA)] before and after polyethylenimine (PEI) treatment. The PEI is found to decrease the conductivity of all the PEDOT:PSS films. The processing solvent of 2-methoxyethanol is found to significantly enhance the conductivity of PH1000-p from 1.1 up to 744 S/cm while the processing solvent of isopropanol or water does not change the conductivity of PH1000-p much. As for the optical properties, the PEI treatment slightly changes the transmittance and reflectance of PH1000-p and PH1000-EG films, while the PEI leads to an substantial increase of the absorptance in the spectral region of 400–1100 nm of the PH1000-SA films. Though the optical property and conductivity of the three different types of PEDOT:PSS films vary with the PEI treatment, the treated PEDOT:PSS films exhibit similar low work function. We demonstrate solar cells with a simple device structure of glass/low-WF PEDOT:PSS/P3HT:ICBA/high-WF PEDOT:PSS cells that exhibit good performance with open-circuit voltage of 0.82 V and fill factor up to 0.62 under 100 mW/cm² white light illumination.     

Non‐destructive optical analysis of band gap profile,crystalline phase,and grain size for Cu(In,Ga)Se2 solar cells deposited by 1‐stage, 2‐stage,and 3‐stage co‐evaporation

Cu(In,Ga)Se₂ (CIGS) thin films co‐evaporated by 1‐stage, 2‐stage, and 3‐stage processes have been studied by spectroscopic ellipsometry (SE). The disappearance of a Cu_2‐xSe optical signature, detected by real time SE during multistage CIGS, has enabled precise endpoint control. Band gap energies determined by SE as depth averages show little process variation for fixed [Ga]/([In] + [Ga]) atomic ratio, whereas their broadening parameters decrease with increasing number of stages, identifying successive grain size enhancements. Refined SE analysis has revealed band gap profiling only for 3‐stage CIGS. Solar cells incorporating these absorbers have yielded increased efficiencies in correlation with phase control, grain size, and band gap profiling. Copyright © 2013 John Wiley & Sons, Ltd.     

Deposition of intrinsic hydrogenated amorphous silicon for thin‐film solar cells – a comparative study for layers grown statically by RF‐PECVD and dynamically by VHF‐PECVD

Hydrogenated amorphous silicon (a‐Si:H) is conventionally deposited using static plasma‐enhanced chemical vapor deposition (PECVD) processes. In this work, a very high frequency (VHF) dynamic deposition technique is presented, on the basis of linear plasma sources. This configuration deploys a simple reactor design and enables continuous deposition processes, leading to a high throughput. Hence, this technique may facilitate the use of flexible substrates. As a result, the production costs of thin‐film silicon solar cells could be reduced significantly. We found a suitable regime for the homogeneous deposition of a‐Si:H layers for growth rates from 0.35–1.1 nm/s. The single layer properties as well as the performance of corresponding a‐Si:H solar cells are investigated and compared with a state‐of‐the‐art radio frequency (RF) PECVD regime. By analyzing the Fourier transform infrared spectroscopy spectra of single layers, we found an increasing hydrogen concentration with deposition rate for both techniques, which is in agreement with earlier findings. At a given growth rate, the hydrogen concentration was at the same level for intrinsic layers deposited by RF‐PECVD and VHF‐PECVD. The initial efficiency of the corresponding p–i–n solar cells ranged from 9.6% at a deposition rate of 0.2 nm/s (RF regime) to 8.9% at 1.1 nm/s (VHF regime). After degradation, the solar cell efficiency stabilized between 7.8% and 5.9%, respectively. The solar cells incorporating intrinsic layers grown dynamically using the linear plasma sources and very high frequencies showed a higher stabilized efficiency and lower degradation loss than solar cells with intrinsic layers grown statically by RF‐PECVD at the same deposition rate. Copyright © 2012 John Wiley & Sons, Ltd.