Progress in Photovoltaics Research and Applications

Published by Wiley
Online ISSN: 1099-159X
Publications
Article
We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n^2 limit can be potentially exceeded in a wide angular and wavelength range using two-dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure which is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances which can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one-dimensional gratings and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to get more practical values for the absorption enhancement, we consider the effect of parasitic loss which can significantly reduce the enhancement factor.
 
Article
We propose a back-reflecting scheme in order to enhance the maximum achievable current in one micron thick crystalline silicon solar cells. We perform 3-dimensional numerical investigations of the scattering properties of metallic nanostructures located at the back side, and optimize them for enhancing absorption in the silicon layer. We validate our numerical results experimentally and also compare the absorption enhancement in the solar cell structure, both with quasi-periodic and random metallic nanostructures. We have looked at the interplay between the metallic nanostructures and an integrated back-reflector. We show that the combination of metallic nanoparticles and a metallic reflector results in significant parasitic absorption. We compared this to another implementation based on titanium dioxide nanoparticles which act as a lambertian reflector of light. Our simulation and experimental results show that this proposed configuration results in reduced absorption losses and in broadband enhancement of absorption for ultra-thin solar cells, paving the way to an optimal back reflector for thin film photovoltaics.
 
Article
Development of alternative thin film photovoltaic technologies is an important research topic due to the potential for low-cost, large-scale fabrication of high-efficiency solar cells. Despite the large number of promising alternative absorbers and corresponding contacts, the rate of progress is limited by complications that arise during solar cell fabrication. One potential solution to this problem is the high-throughput combinatorial method, which has been extensively used for research and development of individual absorber and contact materials. Here, we demonstrate an accelerated approach to development of thin film photovoltaic device prototypes based on the novel CuSbS2 absorber, using the device architecture employed for CuInxGa(1-x)Se2 (CIGS). The newly developed three-stage, self-regulated CuSbS2 growth process enables the study of PV device performance trends as a function of phase purity, crystallographic orientation, layer thickness of the absorber, and numerous back contacts. This exploration results in initial CuSbS2 device prototypes with ~1% conversion efficiency; currently limited by low short-circuit current due to poor collection of photoexcited electrons, and a small open-circuit voltage due to a cliff-type conduction band offset at the CuSbS2/CdS interface (suggested by first-principles calculations). Overall, these results illustrate the potential of combinatorial methods to accelerate the development of thin film photovoltaic devices with novel absorbers.
 
Article
The XTE was launched December 30, 1995. Shortly after launch, it become apparent that the solar array was not performing as expected. On leaving shadow, the array exhibited many discontinuous drops in current output. The size of each of these drops was consistent with the loss of a part of a sell. The current decreases could not be caused by the loss of an entire circuit. This meant that the array may have had numerous cracked solar cells that opened as array got warmer. Studies performed on the array's qualification panel suggest that the cell cracks may have been cased by extensive tap testing performed on the array and that these cracks were undetectable at room temperature using usual inspection method.
 
Article
The first commercial communications satellite with gallium-arsenide on germanium (GaAs/Ge) solar arrays is scheduled for launch in December 1995. The spacecraft, named MEASAT, was built by Hughes Space and Communications Company. The solar cell assemblies consisted of large area GaAs/Ge cells supplied by Spectrolab Inc. with infrared reflecting (IRR) coverglass supplied by Pilkington Space Technology. A comprehensive characterization program was performed on the GaAs/Ge solar cell assemblies used on the MEASAT array. This program served two functions; first to establish the database needed to accurately predict on-orbit performance under a variety of conditions; and second, to demonstrate the ability of the solar cell assemblies to withstand all mission environments while still providing the required power at end-of-life. Characterization testing included measurement of electrical performance parameters as a function of radiation exposure, temperature, and angle of incident light; reverse bias stability; optical and thermal properties; mechanical strength tests, panel fabrication, humidity and thermal cycling environmental tests. The results provided a complete database enabling the design of the MEASAT solar array, and demonstrated that the GaAs/Ge cells meet the spacecraft requirements at end-of-life.
 
Article
This paper presents test results from SCARLET (Solar Concentrator Array with Refractive Linear Element Technology) experiments performed on several Lewis Research Center Lear jet flights and two JPL balloon flights. The tests were performed in support of the BMDO sponsored SCARLET II program, which is building a 2.6 kW SCARLET solar array to supply the primary power for the JPL New Millennium Deep Space 1 Mission. The experiments involve TECSTAR dual junction GaInP2/GaAs/Ge cells flown bare and under two different types of SCARLET lenses. The two types of lenses tested were a developmental design consisting of monolithic THV fluoroplastic and the current baseline flight design consisting of ceria-doped microsheet and silicone. Measured lens and total module efficiencies are presented and the flight data is compared to various solar simulator test results.
 
Article
In this work it is shown that high quality GaAs photovoltaic devices can be produced by Molecular Beam Epitaxy (MBE) with growth rates comparable to metal-organic chemical vapor deposition (MOCVD) through the subsitution of group III solid sources by metal-organic compounds. The influence the III/V flux-ratio and growth temperatures in maintaining a two dimensional layer by layer growth mode and achieving high growth rates with low residual background impurities is investigated. Finally subsequent to the study of the optimization of n- and p doping of such high growth rate epilayers, results from a preliminary attempt in the fabrication of GaAs photovoltaic devices such as tunnel diodes and solar cells using the proposed high growth rate approach are reported.
 
Article
We have achieved a new record efficiency of 17.6%, (AM0) for a p/n InP homo-epitaxy solar cell. In addition, we have eliminated a previously observed photo-degradation of cell performance, which was due to losses in J(sub sc). Cells soaked in AM0 spectrum at one-sun intensity for an hour showed no significant change in cell performance. We have discovered carrier passivation effects when using Zn as the p-type dopant in the OMVPE growth of InP and have found a method to avoid the unexpected effects which result from typical operation of OMVPE cell growth.
 
Article
The molecular beam epitaxy (MBE) and properties of GaAs layers and single junction GaAs cells on Si wafers which utilize compositionally graded GeSi intermediate buffers grown by ultra-high vacuum chemical vapor deposition (UHVCVD) to mitigate the large lattice mismatch between GaAs and Si were investigated. GaAs cell structures were found to incorporate a threading dislocation density of 0.9-1.5×106 cm-2, identical to the underlying relaxed Ge cap of the graded buffer. AlGaAs/GaAs double heterostructures were grown on the GeSi/Si substrates for time-resolved photoluminescence measurements, which revealed a bulk GaAs minority carrier lifetime in excess of 10 ns, the highest lifetime ever reported for GaAs on Si.
 
Article
An analysis embodied in a PC computer program is presented, which quantitatively demonstrates how the availability of radiation hard solar cells can help minimize the cost of a global satellite communications system. An important distinction between the currently proposed systems, such as Iridium, Odyssey and Ellipsat, is the number of satellites employed and their operating altitudes. Analysis of the major costs associated with implementing these systems shows that operation at orbital altitudes within the earth's radiation belts (10(exp 3) to 10(exp 4)km) can reduce the total cost of a system by several hundred percent, so long as radiation hard components including solar cells can be used. A detailed evaluation of the predicted performance of photovoltaic arrays using several different planar solar cell technologies is given, including commercially available Si and GaAs/Ge, and InP/Si which is currently under development. Several examples of applying the program are given, which show that the end of life (EOL) power density of different technologies can vary by a factor of ten for certain missions. Therefore, although a relatively radiation-soft technology can usually provide the required EOL power by simply increasing the size of the array, the impact upon the total system budget could be unacceptable, due to increased launch and hardware costs. In aggregate, these factors can account for more than a 10% increase in the total system cost. Since the estimated total costs of proposed global-coverage systems range from $1B to $9B, the availability of radiation-hard solar cells could make a decisive difference in the selection of a particular constellation architecture.
 
Article
We proposed a structure of a GaAs solar cell with GaSb/GaAs type-II quantum dot (QD) absorber spatially separated from the depletion region. We developed a model and used the detailed balance principle along with Poisson and continuity equations for calculating of the energy band bending along with the photo- and dark currents, and the conversion efficiency of proposed QD solar cell. Our model takes into account both single- and two-photon absorption as well as non-radiative processes in QDs, like intra-band relaxation of holes in QDs and inter-band electron-hole recombination in type-II QDs, both associated with QDs embedded in the p-doped region of otherwise ideal solar cell. We found that concentration of sunlight improves performance of the proposed solar cell, with concentration from 1-sun to 500-sun raising the efficiency from 30% to 50%
 
Article
In this study we have combined a method of calculating radiation induced damage to solar cells using Non-Ionizing Energy Loss (NIEL), with models of Earth orbiting radiation in arbitrary orbits, to assess the lifetime of solar cells. This paper provides a comparison of the NIEL technique to results from the JPL Radiation Handbook, and to actual space experimental damage results. In addition, we discuss ways of extending the calculation to newer solar cell materials, as well as environments outside of the Earth's orbit.
 
Article
InP p(+)/n/n(+) solar cells, fabricated by metal organic chemical vapor deposition, (MOCVD) were irradiated with 0.2 MeV and 10 MeV protons to a fluence of 10(exp 13)/sq cm. The power output degradation, IV behavior, carrier concentration and defect concentration were observed at intermediate points throughout the irradiations. The 0.2 MeV proton-irradiated solar cells suffered much greater and more rapid degradation in power output than those irradiated with 10 MeV protons. The efficiency losses were accompanied by larger increases in the recombination currents in the 0.2 MeV proton-irradiated solar cells. The low energy proton irradiations also had a larger impact on the series resistance of the solar cells. Despite the radiation induced damage, the carrier concentration in the base of the solar cells showed no reduction after 10 MeV or 0.2 MeV proton irradiations and even increased during irradiation with 0.2 MeV protons. In a deep level transient spectroscopy (DLTS) study of the irradiated samples, the minority carrier defects H4 and H5 at E(sub v) + 0.33 and E(sub v) + 0.52 eV and the majority carrier defects E7 and El0 at E(sub c) - 0.39 and E(sub c) - 0.74 eV, were observed. The defect introduction rates for the 0.2 MeV proton irradiations were about 20 times higher than for the 10 MeV proton irradiations. The defect El0, observed here after irradiation, has been shown to act as a donor in irradiated n-type InP and may be responsible for obscuring carrier removal. The results of this study are consistent with the much greater damage produced by low energy protons whose limited range causes them to stop in the active region of the solar cell.
 
Article
The measured degradation of epitaxial shallow homojunction n(+)/p InP solar cells under 1 MeV electron irradiation is correlated with that measured under 3 MeV proton irradiation based on 'displacement damage dose'. The measured data is analyzed as a function of displacement damage dose from which an electron to proton dose equivalency ratio is determined which enables the electron and proton degradation data to be described by a single degradation curve. It is discussed how this single curve can be used to predict the cell degradation under irradiation by any particle energy. The degradation curve is used to compare the radiation response of InP and GaAs/Ge cells on an absolute damage energy scale. The comparison shows InP to be inherently more resistant to displacement damage deposition than the GaAs/Ge.
 
Article
Hydrogen passivation of heteroepitaxial InP solar cells is of recent interest for deactivation of dislocations and other defects caused by the cell/substrate lattice mismatch that currently limit the photovoltaic performance of these devices. In this paper we present strong evidence that, in addition to direct hydrogen-dislocation interactions, hydrogen forms complexes with the high concentration of interstitial Zn defects present within the p(+) Zn-doped emitter of MOCVD-grown heteroepitaxial InP devices, resulting in a dramatic increase of the forward bias turn-on voltage by as much as 280 mV, from ~680 mV to ~960 mV. This shift is reproducible and thermally reversible and no such effect is observed for either n(+)p structures or homoepitaxial p(+)n structures grown under identical conditions. A combination of photoluminescence (PL), electrochemical C-V dopant profiling, SIMS and I-V measurements were performed on a set of samples having undergone a matrix of hydrogenation and post-hydrogenation annealing conditions to investigate the source of this voltage enhancement and confirm the expected role of interstitial Zn and hydrogen. A precise correlation between all measurements is demonstrated which indicates that Zn interstitials within the p(+) emitter and their interaction with hydrogen are indeed responsible for this device behavior.
 
Article
We have been investigating the synthesis of quantum dots of CdSe, CuInS2, and CuInSe2 for use in an intermediate bandgap solar cell. We have prepared a variety of quantum dots using the typical organometallic synthesis routes pioneered by Bawendi, et. al., in the early 1990's. However, unlike previous work in this area we have also utilized single-source precursor molecules in the synthesis process. We will present XRD, TEM, SEM and EDS characterization of our initial attempts at fabricating these quantum dots. Investigation of the size distributions of these nanoparticles via laser light scattering and scanning electron microscopy will be presented. Theoretical estimates on appropriate quantum dot composition, size, and inter-dot spacing along with potential scenarios for solar cell fabrication will be discussed.
 
Article
Results from the first year of operation of the PASP Plus flight experiment are given. The experiment consists of sixteen individual solar cell modules on twelve different panels. Both planar and concentrator technologies are represented as well as several different cell types. The orbit is 363 x 2552 km at an inclination of 70 degrees. There are two main purposes of PASP Plus: (1) to determine the interactions between the space plasma and solar arrays biased to plus or minus 500 volts, and (2) to determine the long term radiation performance of a wide variety of solar cell types.
 
Article
The degradation and failure mechanisms of a stable photovoltaic device comprising a bilayer heterojunction formed between poly(3-carboxythiophene-2,5-diyl-co-thiophene-2,5-diyl) (P3CT) and Buckminsterfullerene (C60) sandwiched between indium tin oxide (ITO) and aluminium (Al) electrodes were elucidated by the time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis in conjunction with isotopic labelling using 18O2 after a total testing time of 13 000 h. This experiment allowed us to understand the chemistry that takes place in three dimensions during degradation and failure of the device under accelerated testing conditions. The cell was subjected to continuous illumination with an incident light intensity of 1000 W m−2 (AM1·5) at 72 ± 2°C under a vacuum of <10−6 mBar. During the illumination period, IV-curves were recorded at regular intervals and the short circuit current of the device was monitored every 10 s for 10 760 h. The total illumination time was 12 200 h. During this period of time, the device performance degraded and the device was finally left in the dark at 25°C in an atmosphere where the oxygen had been replaced with the isotope 18O2. After 800 h in this atmosphere in the dark, the final IV-curves in the dark and under illumination were recorded. The main purpose of this work was the analysis using TOF-SIMS imaging and depth profiling of the degraded cell. The combined analyses correspond to the three-dimensional chemical imaging of the device showing specifically where the oxygen had reacted during exposure. Several general findings were made that are applicable to similar devices. It was found that the oxygen diffuses into the device through the Al electrode in between the Al grains and through microscopic holes in the Al electrode. Once inside the device the oxygen diffuses in the lateral and vertical plane until the counter electrode is reached. C60 was found to be susceptible to the incorporation of 18O but P3CT was not under the conditions in question. The other prominent degradation pathway was found to be the diffusion of electrode materials into the device. Both electrode materials diffuse through the entire device to the counter electrode. Copyright © 2007 John Wiley & Sons, Ltd.
 
Article
A power conversion efficiency record of 10.1% was achieved for kesterite absorbers, using a Cu2ZnSn(Se,S)4 thin-film solar cell made by hydrazine-based solution processing. Key device characteristics were compiled, including light/dark J–V, quantum efficiency, temperature dependence of Voc and series resistance, photoluminescence, and capacitance spectroscopy, providing important insight into how the devices compare with high-performance Cu(In,Ga)Se2. The record kesterite device was shown to be primarily limited by interface recombination, minority carrier lifetime, and series resistance. The new level of device performance points to the significant promise of the kesterites as an emerging and commercially interesting thin-film technology. Copyright © 2011 John Wiley & Sons, Ltd.
 
Article
The use of Ga1−xInxAs instead of GaAs as a bottom solar cell in a GayIn1−yP/Ga1−xInxAs tandem structure increases the flexibility of choosing the optimum bandgap combination of materials for a multijunction solar cell. Higher theoretical efficiencies are calculated and different cell concepts are suggested for space and terrestrial concentrator applications. Various GayIn1−yP/Ga1−xInxAs material combinations have been investigated for the first time and efficiencies up to 24·1% (AM0) and 27·0% (AM1·5 direct) have been reached under one-sun conditions. An efficiency of 30·0–31·3% was measured for a Ga0·35In0·65P/Ga0·83In0·17As tandem concentrator cell with prismatic cover at 300 suns. The top and bottom cell layers of this structure are grown lattice-matched to each other, but a large mismatch is introduced at the interface to the GaAs substrate. This cell structure is well suited for the use in next-generation terrestrial concentrators working at high concentration ratios. For the first time a cell efficiency up to 29–30% has been measured at concentration levels up to 1300 suns. A small prototype concentrator with Fresnel lenses and four tandem solar cells working at C = 120 has been constructed, with an outdoor efficiency of 23%. Copyright © 2001 John Wiley & Sons, Ltd.
 
Article
Monolithic interconnected modules (MIMs) are large area, high voltage PV devices which perform well at very high light intensities. They are therefore well suited for the assembly of dense array receivers. The latter can be employed in solar concentrator systems such as parabolic dishes at a concentration ratio of 1000 Suns or more. This paper reports on progress in the development and testing of GaAs MIMs and of water-cooled dense array receivers assembled from MIMs. The MIMs are electrically protected by integrated bypass diodes and, under indoor laboratory tests, reach an efficiency of 20·0% at 1000 Suns and 22·9% at 200 Suns. Several dense array receivers have been assembled, one of which was tested outdoors at 1-Sun and at concentration ratios of several hundred Suns and up to slightly above 1000 Suns using the PETAL solar dish facility in Sede Boqer, Israel. In addition to I–V curve measurements, the high-concentration tests included measurements that quantified the light intensity distribution over the dense array. Deformations in some of the I–V plots were observed for intensity distributions that departed substantially from perfect uniformity. The shapes of these plots were successfully reproduced by an electronic network simulation of the inhomogeneously illuminated receiver. 1-Sun I–V curve measurements and visual inspections performed before and after exposure of the module to concentrated sunlight revealed no indications of degradation. Copyright © 2007 John Wiley & Sons, Ltd.
 
Article
This paper describes the overall design and experimental results obtained with the PV-FIBRE concentrator system (CPV). This system uses a parabolic dish to collect and concentrate the sunlight which is then further guided by a transparent rod and finally by individual short fibres. Eventually, the individual fibres are connected to single cells which are located indoors. Dual-junction III-V-based solar cells with an efficiency of 30% and operating at 1000× are applied in the PV-FIBRE system. This new system approach allows a suitable optical and electrical interconnection in order to reduce the losses and the indoor operation of the cell receiver. All elements of the CPV system (collector, tracker, transmission rod, cells, fibre bundles, and cooling circuit) have been manufactured according to main design requirements and have been evaluated separately. Finally, the PV-FIBRE CPV system has been installed in Madrid and tested under real operation conditions. The system has demonstrated an optical efficiency of 62% is feasible, providing homogeneous illumination to the cells. Therefore, this concept can lead to overall efficiencies exceeding 20% when combined with MJ solar cells. In this paper we report on the main achievements, identified problems as well as lessons learned and future research lines to improve the system performance. Copyright © 2007 John Wiley & Sons, Ltd.
 
Article
In order to help keep readers up-to-date in the field each issue of Progress in Photovoltaics will contain a list of recently published journal articles most relevant to its aims and scope. This list is drawn from an extremely wide range of journals, including IEEE Transactions on Electron Devices, Journal of Applied Physics, Applied Physics Letters, Progress in Photovoltaics and Solar Energy Materials and Solar Cells. To assist the reader, the list is separated into broad categories, but please note that these classifications are by no means strict. Also note that inclusion in the list is not an endorsement of a paper's quality. Copyright © 2010 John Wiley & Sons, Ltd.
 
Article
Amidst the different silicon thin-film systems, the epitaxial thin-film solar cell represents an approach with interesting potential. Consisting of a thin active c-Si layer grown epitaxially on top of a low-quality c-Si substrate, it can be implemented into solar cell production lines without major changes in the current industrial process sequences. Within this work, ∼30-μm-thick epitaxial layers on non-textured and highly doped monocrystalline Czochralski (Cz) and multicrystalline (mc) Si substrates have been prepared by CVD. Confirmed efficiencies of 13·8% on Cz and 12·3% on mc-Si substrates have been achieved by applying an industrial process scheme based on tube and in-line phosphorus diffusion, as well as screen-printed front and back contacts fired through a SiNx anti-reflection coating. An extensive solar cell characterisation, including infrared lock-in thermography and spectral response measurements is presented. Copyright © 2003 John Wiley & Sons, Ltd.
 
Device structure of the transparent CdTe cell
XRD patterns of two nano-CdS:O/CdTe test samples etched by different etchants: (a) NP etchant; (b) BM etchant
TEM and EDS results of a $ 10 A ˚ Cu on BM-etched Te-rich CdTe sample annealed at 250 C in He for 30 min
Optical properties of a CTO/ZTO/nano-CdS:O/CdTe/Cu x Te cell with or without an ITO/MgF 2 AR coating on the CdTe backside
Article
To fabricate a high-efficiency polycrystalline thin-film tandem cell, the most critical work is to make a high-efficiency top cell ( > 15%) with high bandgap (Eg = 1·5–1·8 eV) and high transmission (T > 70%) in the near-infrared (NIR) wavelength region. The CdTe cell is one of the candidates for the top cell, because CdTe state-of-the-art single-junction devices with efficiencies of more than 16% are available, although its bandgap (1·48 eV) is slightly lower for a top cell in a current-matched dual-junction device. In this paper, we focus on the development of a: (1) thin, low-bandgap CuxTe transparent back-contact; and (2) modified CdTe device structure, including three novel materials: cadmium stannate transparent conducting oxide (TCO), ZnSnOx buffer layer, and nanocrystalline CdS:O window layer developed at NREL, as well as the high-quality CdTe film, to improve transmission in the NIR region while maintaining high device efficiency. We have achieved an NREL-confirmed 13·9%-efficient CdTe transparent solar cell with an infrared transmission of ∼50% and a CdTe/CIS polycrystalline mechanically stacked thin-film tandem cell with an NREL-confirmed efficiency of 15·3%. Copyright © 2005 John Wiley & Sons, Ltd.
 
Article
This communication reports an MgF2/ZnO/CdS/Cu(In,Ga)Se2/Mo/glass polycrystalline solar cell with a confirmed total-area conversion efficiency of 16.4%. the thin-film Cu(In,Ga)Se2 absorber was fabricated by computer-controlled physical vapor deposition (PVD) from the elemental sources. the resulting absorber has a Gal/In compositional grading that we refer to as a notch. Capacitance-voltage (C-V) measurements also reveal a graded doping profile in the region near the electronic p-n junction. the enhanced device performance is characterized by an open-circuit voltage (Voc) of 660 mV and a particularly high fill factor (FF) of 78.7%.
 
Article
Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined and new entries since July 2000 are reviewed. Copyright © 2001 John Wiley & Sons, Ltd.
 
Article
Thin-film silicon solar cells are promising candidates for meeting the requirements of high efficiency combined with low cost. We use liquid-phase epitaxy to grow thin silicon films on highly doped substrates. A solar cell made on this material has been measured independently to have an efficiency of 17.0%. an important step in obtaining these high cell efficiencies is the removal of most of the heavily doped substrate on which the epitaxial layer is grown.
 
Article
Development of a Cu(In,Ga)Se2 thin film solar cell on a polyimide film with a conversion efficiency of 17.1%, measured under standard test conditions at the European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) of the European Commission, Ispra, is reported. The drastic improvement from the previous record of 14.1% efficiency is attributed to a more optimized compositional grading, better structural and electronic properties of the absorber layer as well as reduced reflection losses. Basic film and device properties, which led to the improvement in the efficiency record of flexible solar cells are presented for the new process and compared to the old process. Copyright © 2011 John Wiley & Sons, Ltd.
 
Article
We obtained 17.9% cell efficiency on thin and large mc-Si REC wafers using ECN's metal-wrap-through (MWT) concept. Optimization of several cell processing steps led to an increase of more than 2% absolute in cell efficiency. With these cells 36-cell modules were manufactured at 100% yield in our industry scale module pilot line. The highest module efficiency obtained (as independently confirmed by JRC-ESTI) was 17%. In this module the average cell efficiency was 17.8%; this shows a small difference between cell and module efficiency. Copyright © 2011 John Wiley & Sons, Ltd.
 
Article
We report a world-record, total-area efficiency of 17.1% for a polycrystalline thin-film Cu(In,Ga)Se2-based photovoltaic solar cell. the incorporation of Ga to raise the absorber bandgap has been accomplished successfully and in such a manner that an open-circuit voltage of 654 mV and a fill factor of greater than 77% have been achieved. We describe briefly the deposition process, the device structure, and the device performance characteristics.
 
Article
Up to now solar cells fabricated on tricrystalline Czochralski-grown silicon (tri-Si) have shown relatively low short-circuit current densities of about 31–33 mA/cm2 because the three {110}-oriented grains cannot effectively be textured by commonly used anisotropic etching solutions. In this work, we have optimised a novel chemical texturing step for tri-Si and integrated it successfully into our solar cell process. Metal/insulator/semiconductor-contacted phosphorus-diffused n+p junction silicon solar cells with a silicon-dioxide-passivated rear surface and evaporated aluminium contacts were manufactured, featuring a spatially uniform surface texture over all three grains on both cell sides. Despite the simple processing sequence and cell structure, an independently confirmed record efficiency of 17.6% has been achieved. This excellent efficiency is mainly due to an increased short-circuit current density of 37 mA/cm2 obtained by substantially reduced reflection and enhanced light trapping. Copyright © 2003 John Wiley & Sons, Ltd.
 
Article
A solar cell process designed to utilise low-temperature plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiNx) films as front and rear surface passivation was applied to fabricate multicrystalline silicon (mc-Si) solar cells. Despite the simple photolithography-free processing sequence, an independently confirmed efficiency of 18.1% (cell area 2 × 2 cm2) was achieved. This excellent efficiency can be predominantly attributed to the superior quality of the rear surface passivation scheme consisting of an SiNx film in combination with a local aluminium back-surface field (LBSF). Thus, it is demonstrated that low-temperature PECVD SiNx films are well suited to achieve excellent rear surface passivation on mc-Si. Copyright © 2002 John Wiley & Sons, Ltd.
 
Article
Hot-wire chemical vapor deposition (HWCVD) is a promising technique for very fast deposition of high quality thin films. We developed processing conditions for device- quality silicon nitride (a-SiNx:H) anti-reflection coating (ARC) at high deposition rates of 3 nm/s. The HWCVD SiNx layers were deposited on multicrystalline silicon (mc-Si) solar cells provided by IMEC and ECN Solar Energy. Reference cells were provided with optimized parallel plate PECVD SiNx and microwave PECVD SiNx respectively. The application of HWCVD SiNx on IMEC mc-Si solar cells led to effective passivation, evidenced by a Voc of 606 mV and consistent IQE curves. For further optimization, series were made with HW SiNx (with different x) on mc-Si solar cells from ECN Solar Energy. The best cell efficiencies were obtained for samples with a N/Si ratio of 1·2 and a high mass density of >2·9 g/cm3. The best solar cells reached an efficiency of 15·7%, which is similar to the best reference cell, made from neighboring wafers, with microwave PECVD SiNx. The IQE measurements and high Voc values for these cells with HW SiNx demonstrate good bulk passivation. PC1D simulations confirm the excellent bulk- and surface-passivation for HW SiNx coatings. Interesting is the significantly higher blue response for the cells with HWCVD SiNx when compared to the PECVD SiNx reference cells. This difference in blue response is caused by lower light absorption of the HWCVD layers (compared to microwave CVD; ECN) and better surface passivation (compared to parallel plate PECVD; IMEC). The application of HW SiNx as a passivating antireflection layer on mc-Si solar cells leads to efficiencies comparable to those with optimized PECVD SiNx coatings, although HWCVD is performed at a much higher deposition rate. Copyright © 2007 John Wiley & Sons, Ltd.
 
Article
We report a new record total-area efficiency of 19·9% for CuInGaSe2-based thin-film solar cells. Improved performance is due to higher fill factor. The device was made by three-stage co-evaporation with a modified surface termination. Growth conditions, device analysis, and basic film characterization are presented. Published in 2008 by John Wiley & Sons, Ltd.
 
Article
High-efficiency 4 cm2 screen-printed (SP) textured cells were fabricated on 100 Ω/sq emitters using a rapid single-step belt furnace firing process. The high contact quality resulted in a low series resistance of 0·79 Ωcm2, high shunt resistance of 48 836 Ωcm2, a low junction leakage current of 18·5 nA/cm2 (n2 = 2) yielding a high fill factor (FF) of 0·784 on 100 Ω/sq emitter. A low resistivity (0·6 Ωcm) FZ Si was used for the base to enhance the contribution of the high sheet-resistance emitter without appreciably sacrificing the bulk lifetime. This resulted in a 19% efficient (confirmed at NREL) SP 4 cm2 cell on textured FZ silicon with SP contacts and single-layer antireflection coating. This is apparently higher in performance than any other previously reported cell using standard screen-printing approaches (i.e., single-step firing and grid metallization). Detailed cell characterization and device modeling were performed to extract all the important device parameters of this 19% SP Si cell and provide guidelines for achieving 20% SP Si cells. Copyright © 2005 John Wiley & Sons, Ltd.
 
Article
We present a both-sides-contacted thin-film crystalline silicon (c-Si) solar cell with a confirmed AM1.5 efficiency of 19.1% using the porous silicon layer transfer process. The aperture area of the cell is 3.98 cm2. This is the highest efficiency ever reported for transferred Si cells. The efficiency improvement over the prior state of the art (16.9%) is achieved by implementing recent developments for Si wafer cells such as surface passivation with aluminum oxide and laser ablation for contacting. The cell has a short-circuit current density of 37.8 mA cm−2, an open-circuit voltage of 650 mV, and a fill factor of 77.6%. Copyright © 2011 John Wiley & Sons, Ltd.
 
Article
Thermal oxides are commonly used for the surface passivation of high-efficiency silicon solar cells from mono- and multicrystalline silicon and have led to the highest conversion efficiencies reported so far. In order to improve the cost-effectiveness of the oxidation process, a wet oxidation in steam ambience is applied and experimentally compared to a standard dry oxidation. The processes yield identical physical properties of the oxide. The front contact is created using a screen-printing process of a hotmelt silver paste in combination with light-induced silver plating. The contact formation on the front requires a short high-temperature firing process, therefore the thermal stability of the rear surface passivation is very important. The surface recombination velocity of the fired oxide is experimentally determined to be below S ≤ 38 cm/s after annealing with a thin layer of evaporated aluminium on top. Monocrystalline solar cells are produced and 19·3% efficiency is obtained as best value on 4 cm2 cell area. Simulations show the potential of the developed process to approach 20% efficiency. Copyright © 2008 John Wiley & Sons, Ltd.
 
Article
High and stable lifetimes recently reported for n-type silicon materials are an important and promising prerequisite for innovative solar cells. To exploit the advantages of the excellent electrical properties of n-type Si wafers for manufacturing simple and industrially feasible high-efficiency solar cells, we focus on back junction n+np+ solar cells featuring an easy-to-fabricate full-area screen-printed aluminium-alloyed rear p+ emitter. Independently confirmed record-high efficiencies have been achieved on n-type phosphorus-doped Czochralski-grown silicon material: 18·9% for laboratory-type n+np+ solar cells (4 cm2) with shadow-mask evaporated front contact grid and 17·0% for front and rear screen-printed industrial-type cells (100 cm2). The electrical cell parameters were found to be perfectly stable under illumination. Copyright © 2006 John Wiley & Sons, Ltd.
 
Article
We report the growth and characterization of record-efficiency ZnO/CdS/CuInGaSe2 thin-film solar cells. Conversion efficiencies exceeding 19% have been achieved for the first time, and this result indicates that the 20% goal is within reach. Details of the experimental procedures are provided, and material and device characterization data are presented. Published in 2003 by John Wiley & Sons, Ltd.
 
Article
The combination of a rising demand for reliable, low-cost energy and an increased awareness of environmental quality issues has created new business opportunities for photovoltaics, especially in applications without an installed electric utility grid line.
 
Article
In November 1993, over 50 solar cars left Darwin for the tortuous 3000 kilometre journey south across Australia to Adelaide. the top cars set a pace difficult for conventional vehicles to maintain. the winner, the Honda Dream, averaged 85 kilometres per hour (53 mph), with all top five place getters shattering the previous race record set by the General Motors (GM) Sunraycer in 1987. the improved performance was due to better aerodynamics, motor and power train efficiency and improved output power from the photovoltaic arrays on these cars. This report describes the race with emphasis on the solar cell and module technology used by the leading cars.
 
Article
We have developed a crystalline silicon solar cell with amorphous silicon (a-Si:H) rear-surface passivation based on a simple process. The a-Si:H layer is deposited at 225°C by plasma-enhanced chemical vapor deposition. An aluminum grid is evaporated onto the a-Si:H-passivated rear. The base contacts are formed by COSIMA (contact formation to a-Si:H passivated wafers by means of annealing) when subsequently depositing the front silicon nitride layer at 325°C. The a-Si:H underneath the aluminum fingers dissolves completely within the aluminum and an ohmic contact to the base is formed. This contacting scheme results in a very low contact resistance of 3.5 ±0.2 mΩ cm2 on low-resistivity (0.5 Ω cm) p-type silicon, which is below that obtained for conventional Al/Si contacts. We achieve an independently confirmed energy conversion efficiency of 20.1% under one-sun standard testing conditions for a 4 cm2 large cell. Measurements of the internal quantum efficiency show an improved rear surface passivation compared with reference cells with a silicon nitride rear passivation. Copyright © 2005 John Wiley & Sons, Ltd.
 
Article
The paper presents a rear side structure for crystalline silicon solar cells, which is processed at a maximum temperature of 220°C. Using two different material compositions for electrical and optical needs, the layer system has excellent passivation properties, enhances light trapping and allows for a good ohmic contact. With this structure we achieve an independently confirmed conversion efficiency η=20·5% on a 250 μm thick silicon solar cell. Due to the fact that the maximum process temperature is 220°C, this layer system enables new solar cell concepts. Copyright © 2006 John Wiley & Sons, Ltd.
 
Article
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
 
Article
The claim is often made that efficiency is a key factor in determining the marketability of photovoltaic products. If this is the case, a strong correlation between the price of modules and their efficiency might be expected. This relationship is investigated using module pricing data that have recently become available. Conclusions are that there is little correlation between module price and efficiency in this data set and that some thin-film modules currently appear to attract a pricing premium. Copyright © 2004 John Wiley & Sons, Ltd.
 
Article
Cu(In,Ga)Se2 (CIGS) solar cells have been designed for operation under mildly concentrated sunlight. The absorber was deposited via a three-stage evaporation process that has consistently yielded high-performance one-sun devices. The device structure reported here was modified by reducing the thickness of the CdS window/buffer layer to enhance the short-circuit current at the expense of the open-circuit voltage. Operation of the devices under optical enhancement leads to significant increases in the voltage and fill factor. At 14 suns, the open-circuit voltage for this device was 736 mV, the fill factor was 80.5%, and the efficiency was 21.5%. This result represents the first report of a polycrystalline thin-film solar cell with an efficiency in excess of 20%. Published in 2002 by John Wiley & Sons, Ltd.
 
Article
This paper presents the results of 40 silicon-based photovoltaic solar modules originating from six different manufactures which were tested and characterised originally at the European Solar Test Installation, (ESTI), in the period 1982–1984. These same modules have been re-measured in 2004 after 20–22 years of continuous outdoor weathering. These modules are a subset ‘Rack C’ of the JRC Ispra outdoor test field where examples of diverse generations of PV technologies have been subjected to long-term climatic exposure. We have compared the results obtained from these samples with the typical guarantees which are given by module manufacturers and we can see that in general the manufacturers are conservative with their power guarantees. Most modules exceed the minimum power levels given for 10 years exposure, even after 22 years in the field. The results presented in this paper indicate that the degradation observed in the field is comparable to that observed in Type Approval Testing. This result not only validates the testing and stress levels applied in the Type Approval Tests, but also would indicate that (with the majority of modules exceeding the level of 92% of Pmax after 20 years) the actual lifetime of these products is significantly more than 20 years. Copyright © 2005 John Wiley & Sons, Ltd.
 
Article
Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined and new entries since July 2003 are reviewed. Copyright © 2004 John Wiley & Sons, Ltd.
 
Article
The passivated emitter and rear locally diffused cell structure has been redesigned and has yielded independently confirmed one-sun eficiencies of up to 23.5%, the highest ever for a silicon cell. A dotted front emitter contact design, PBr3 liquid-source phosphorus diffusions and 400-μm thick 10-cm diameter wafers have contributed to this improvement. Some of the new cells have demonstrated open-circuit voltages of 709 mV with an ‘inverted’ pyramid surface structure, the highest ever for a 23%-efficiency silicon cell. Further improvement is expected in the near future.
 
Top-cited authors
Martin Green
  • UNSW Sydney
Yoshihiro Hishikawa
  • National Institute of Advanced Industrial Science and Technology
Wilhelm Warta
  • Fraunhofer Institute for Solar Energy Systems ISE
Keith A. Emery
  • National Renewable Energy Laboratory
Ewan Dunlop
  • European Commission