Abstract unavailable.

Abstract unavailable.

We focus upon the role of interfacial energetics and morphology in influencing the separation of CT states into dissociated charge carriers. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors. For the PDI film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). Furthermore, we compare the charge recombination dynamics observed in films comprising P3HT blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from nongeminate to monomolecular (geminate) recombination dynamics. We show that this transition cannot be explained by a difference in interfacial energetics. However, this transition does correlate with nanomorphology data that indicate that both pyrazolinofullerenes yield a much finer phase segregation with correspondingly smaller domain sizes than observed with PCBM. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends.

Novel semi-random poly(3-hexylthiophene) (P3HT) based polymers P3HTT-DTP, P3HTT-BTD-DTP, P3HTT-TP-DTP and P3HTT-DPP-DTP containing the strong donor dithienopyrrole (DTP) as well as different acceptors (benzothiadiazole (BTD), thienopyrazine (TP) or diketopyrrolopyrrole (DPP)) were synthesized by Stille copolymerization and their optical, electrochemical, charge transport, and photovoltaic properties were investigated. All polymers (except for the all donor polymer P3HTT-DTP) show considerably broadened absorption compared to P3HT due to the donor-acceptor effect and their multichromophoric nature. The introduction of the strong donor DTP leads to increased HOMO energies and thus decreased open-circuit voltage (Voc) (compared to previously published semi-random polymers) as well as an amorphous character of P3HTT-DTP, P3HTT-BTD-DTP and P3HTT-TP-BTD resulting in low hole mobilities and moderate solar cell efficiencies (0.18% to 0.36%). The exception is P3HTT-DPP-DTP, which is semi-crystalline and has a high hole mobility of 1.4×10−4  cm2 ?startVend?−1 s−1 comparable to P3HT, as well as increased photocurrent (10.7  mA/cm²) due to broad and uniform photoresponse up to 850 nm leading to a promising non-optimized device efficiency of 2.83%.

A new Förster resonance energy transfer (FRET) concept for a multichromophoric organic sensitizer in a dye-sensitized solar cell is presented based on a phenyl base body that accommodates the separately linked donor and acceptor moieties. The whole assembly is attached to the surface of a ZnO nanorod electrode via a carboxylic anchor group. FRET activity was demonstrated with UV-VIS measurements, and the charge separation dynamics at the inorganic/organic interface were analyzed with fs transient absorption and terahertz pump/probe for the precursors and fully assembled FRET units.

Bulk heterojunction organic thin-film solar cells utilizing soluble phthalocyanine derivatives, 1,4,8,11,15,18,22,25-octaalkylphthalocyanine (CnPcH₂, n = 6, 7, 9, 10), were investigated. Two broad peaks existing in the external quantum efficiency spectra almost correspond to the Q-band and B-band of CnPcH2. The solar cell utilizing C6PcH2 had the best photovoltaic properties as evidenced by open-circuit voltage, short-circuit current density, fill factor, and energy conversion efficiency. Almost the same photovoltaic properties were observed in the solar cells utilizing C9PcH₂ and C10PcH₂. We discuss the photovoltaic properties by taking into consideration the crystal structure and electronic state of CnPcH2 from the results of the absorbance spectra, X-ray diffraction measurement, and polarization microscope observation.

In this work, semi-transparent inverted polymer solar cells with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate; PEDOT:PSS) top electrodes were fabricated by spin-coating process. Poly(3-hexylthiophene; P3HT):[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) was used as a model material combination for a bulk heterojunction solar cell, because this material combination has been frequently studied, and its properties and performance have been well established. For enhancing the wetting of P3HT:PCBM blend film, different plasma etching conditions were tried. In addition, different high boiling point organic additives were tried to enhance the conductivity of PEDOT:PSS. The performance of solar cells with different fabrication conditions for the top electrode was compared. The best performance was obtained for Ar plasma etching to improve wetting of PEDOT:PSS and the addition of ethylene glycol to improve conductivity.

Alkali fluorides, mostly LiF and CsF, are well-known to improve electron injection/extraction in organic light-emitting diodes (OLEDs) and organic solar cells (OSCs). They are also utilized, though to a lesser extent, for hole injection in OLEDs. Here we demonstrate a new role for such fluorides in enhancing OSCs' hole extraction. We show that an ultrathin air-plasma-treated alkali fluoride layer between the indium tin oxide (ITO) anode and the active layer in copper phthalocyanine (CuPc)/C70-based OSCs increases the short circuit current by up to ∼ 17% for cells with LiF and ∼ 7% for cells with NaF or CsF. The effects of the fluoride layer thickness and treatment duration were evaluated, as were OSCs with oxidized and plasma-treated Li and UV-ozone treated LiF. Measurements included current voltage, absorption, external quantum efficiency (EQE), atomic force microscopy, and x-ray photoelectron spectroscopy, which showed the presence of alkali atoms F and O at the treated ITO/fluoride surface. The EQE of optimized devices with LiF increased at wavelengths >560  nm, exceeding the absorption increase. Overall, the results indicate that the improved performance is due largely to enhanced hole extraction, possibly related to improved energy-level alignment at the fluorinated ITO/CuPc interface, reduced OSC series resistance, and in the case of LiF, improved absorption.

Measurement of charge transport and recombination parameters in bulk heterojunction (BHJ) materials is of great interest to better understand the science underlying organic solar cells. We discuss the use of space charge limited current measurements under solar illumination to measure the mobility of lateral structures in which charge transport is parallel to the plane of the substrate. We compare these mobilities with mobilities calculated from field-effect measurements of the same structures. Changes in mobility as a function of device length and electric field are consistent with numerical simulations of these lateral structures. Lateral organic photovoltaic structures are potentially very useful in evaluating a number of basic material properties of BHJs including three-dimensional transport.

We discuss fabrication processes for implementing nanometer scale confinement in an organic bulk heterojunction device architecture, including formation and integration of the confining self-assembled template. Such confinement has a beneficial influence on the electrical properties of blended poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester organic solar cell active layers. Crystallization of the blend upon annealing is inhibited by the confining template, which we understand through analysis of x-ray scattering measurements.

We provide a more compact fabrication process than the prior work using [6,6]-phenyl C61-butyric acid methyl ester (PCBM) buffer layer to enhance the performance of organic solar cells. The device with an extra PCBM layer exhibits an improved power conversion efficiency (PCE) of 20% as compared with the devices with conventional structures. Ultraviolet and x-ray photoemission spectroscopy indicate that thermal annealing treatments result in better mixture structures of PCBM layers with poly(3-hexylthiophene) at the blended surfaces. Therefore, the device with inserted PCBM provides more ideal hetero-structures and vertically graded PCBM distribution to optimize the charge extraction efficiency.

We report efficient organic bulk heterojunction solar cells, utilizing spray-patterned films of single-wall carbon nanotubes for the transparent electrode. High power conversion efficiencies of up to 3.6% were obtained using a blend of poly(3-hexylthiophene) and phenyl-C₆₁ butyric acid methyl ester as the active layer, comparable to conventional devices utilizing indium tin oxide as the transparent electrode.

Fabrication and photovoltaic performances for flexible transparent conductive oxide-less (TCO-less) flat and cylinder dye-sensitized solar cells (DSCs) are reported. The cylinder solar cell consists of a porous silicone tube, a protected stainless steel metal mesh (protected SUS mesh) working as a counter electrode, a gel electrolyte sheet, a dye/porous titania layer fabricated on a protected SUS mesh working as a anode, and a thermally shrinkable plastic tube, from the inside to the outside. The thermally shrinkable tube was used to reduce the gap between a cathode and an anode. In addition, a porous silicone tube was used for injecting electrolytes smoothly into the gel electrolyte layer. 5.08% efficiency (FF: 0.68; Voc: 0.68 V; Jsc: 11.07  mA/cm²) was observed. A flexible TCO-less flat DSCs with 6.1% efficiency which was improved by narrowing a gap between two electrodes is also reported.

We review recent work on photonic-crystal fabrication using soft-lithography techniques. We consider applications of the resulting structures in energy-related areas such as lighting and solar-energy harvesting. In general, our aim is to introduce the reader to the concepts of photonic crystals, describe their history, development, and fabrication techniques and discuss a selection of energy-related applications.

Stability of organic photovoltaic cells is the key issue for their commercialization. Despite intensive investigations to clarify the causes of failure of devices, the process of degradation is not yet well understood. In this work, we made use of the trap measurements by the charge-based deep-level transient spectroscopy to study devices that have been aged by continuous exposure to artificial sunlight for 20 h, and we compared the trap parameters to those obtained in freshly prepared samples. With regard to poly (3-hexylthiophene) (P3HT)-based devices, the P3HT:phenyl C61 butyric acid methyl ester blend cells showed an additional deep trap level and a higher trap density. In the degraded devices, all the existing traps in the fresh sample were found, and there was no creation of additional defect levels. The density of several trap levels in the polymer was strongly reduced after aging. Analysis of the results suggests that a phase separation in the photoactive blend has occurred, leading to a better organization of the polymer domains to lower defect states.

An introduction to the special section by the guest editors.

Vibronic coupling constants (VCC) in aromatic diboranes are calculated. For a carrier-transporting molecule, vibronic couplings should be small. Vibronic couplings, or reorganization energy, can be controlled by applying the vibronic coupling density (VCD) analysis. To suppress vibronic couplings, electron density difference should be localized not on bonds but on atoms. Aromatic diboranes as electron-transporting molecules are designed based on this design principle. Introducing the protecting groups that prevent the boron atoms in the diborane from being attacked suppresses the vibronic couplings. Based on the nonequilibrium Green's function (NEGF) theory, energy dissipations through a single molecule are also calculated.

In this work we report exceptional efficacy, lifetime and color stability for all-phosphorescent white stacked organic light-emitting devices (SOLED®s). We report data for all-phosphorescent white SOLED pixels with two emissive units connected in series by a charge generation layer (CGL). At 3,000  cd/m², efficacy = 54 to 56  lm/W and lifetime to 70% of initial luminance LT70 ≈ 20,000  h, with color rendering index (CRI) = 82 to 83 and chromaticity meeting Energy Star criteria. We further report data for a 15  cm×15  cm white SOLED panel that operates at 3,000  cd/m² with 48  lm/W efficacy, CRI = 86 and chromaticity meeting Energy Star criteria. The panel has extremely low operating temperature that is only 6.4°C above ambient, and exceptional lifetime of LT70 ≈ 13,000  h when operated at 3,000  cd/m².

White organic light-emitting devices (WOLEDs) can be fabricated using a simple, low-cost device structure with a single uniformly doped emissive layer. The Pt-17 emitter used in these devices obtains excellent color rendering (CRI = 80) as well as bright white electrophosphoresence (CIE x = 0.37, y = 0.40) by combining efficient monomer and efficient excimer emission as demonstrated by excellent external quantum efficiency (ηext = 15.9%). The Pt-17 based WOLED is also compatible with state-of-the-art charge injection and blocking materials as well as high out-coupling device structures. Application of these existing technologies is expected to extend luminance efficiencies of Pt-17 devices to world-class values (46  lm/?startWend? and 100  lm/?startWend? respectively). In addition to avoiding the difficulty and cost of fabricating more complex device structures, the color of a single-doped device also is uniquely independent of voltage, current density, and age. Molecules like fluorine-free Pt-17 are uniquely positioned to utilize excimer emissions in order to reduce manufacturing costs and provide solutions to satisfy many of the requirements for the next generation of organic solid-state lighting.

Optical confinement can induce enhancement of the resonance energy transfer between fluorescent molecules by influencing the interaction between the different available energy levels. We study the energy transfer between a pair of molecules, tris(2-phenylpyridine) iridium and bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum, which are extensively used in organic light-emitting diode technologies. These molecules have previously shown Förster energy transfer. We present the result of the dipolar coupling of these two molecules embedded in a poly(N-vinylcarbazole) film and inserted in a colloidal photonic crystal. Due to the presence of the photonic band gap, the efficiency of the energy transfer has been improved. A thermal study of the emission under the effect of the photonic band gap has been performed.

We report data for a pair of singlestack all-phosphorescent 15×15  cm² organic light emitting-diode (OLED) light panels with high efficacy, long lifetime, and very low operating temperature: Panel 1 has 62  lm/W efficacy, CRI = 81 and lifetime to LT70 = 18,000  h at 1000  cd/m2, while Panel 2 has 58  lm/W efficacy, CRI = 82 and lifetime to LT70 = 30,000  h at 1000  cd/m². Operating at a higher luminance of 3000  cd/m2, Panel 2 has 49  lm/W efficacy with lifetime to LT70 = 4000  h. Excellent panel lifetime is enabled by a stable light blue phosphorescent materials system. Panel temperatures are within 10°C of ambient temperature at 3000  cd/m². Panel 2 was further used as a building block to demonstrate an all-phosphorescent OLED luminaire for under-cabinet lighting applications. Operating at approximately 3000  cd/m², the luminaire delivers 570 lm with 52  lm/W total system efficacy, CRI = 86 and CCT = 2940  K.

Solution-processed quantum dot light-emitting diodes (QDLEDs) have recently paved the way for low-cost and color-saturated displays. Indeed, quantum dots (QDs) have the appealing property of emitting at a tunable wavelength determined by their chemical composition and diameter. Therefore, QDLED electroluminescent spectra exhibit a narrow bandwidth (FWHM at approximately 30 nm) and QD-based displays offer high color purity and saturation. Moreover, QDs may be deposited by spin coating, inkjet printing, and stamping, methods already implemented for polymer LEDs at the wafer level. We report the optimization steps and performances of our recent developed devices emitting in the visible range. We present pros and cons relative to each method of QD deposition and develop a strategy for realizing wafer-level QDLEDs.

Organic light-emitting diodes (OLEDs) with high power efficiency are desirable for lighting applications. A prerequisite for high power efficiency is the achievement of low driving voltages, which can be done via charge carrier doping of the transport layers in PIN organic light-emitting diodes (PIN- OLEDs). We have looked at how to combine low voltage with high current efficiency and long lifetime and have therefore investigated different ways of changing the charge carrier balance in PIN-OLEDs. The carrier supply to the emitting layer was adjusted by electron and hole-blocking layers to allow for an undisturbed exciton formation and radiative decay while keeping the total voltage low. In further investigations, we developed highly efficient and stable white PIN bottom emission OLED devices using novel evaporation processable outcoupling enhancement materials inside the PIN OLED stack. In white bottom emission OLEDs the use of this outcoupling enhancement material in combination with a standard micro-lens arrays (MLA) outcoupling film can yield an efficiency enhancement of up to 1.8. By combining the well-balanced stack with the internal outcoupling approach we achieved a power efficiency of 51.9  lm/W. With additional flat external outcoupling, the power efficiency has been further increased to 60  lm/W. The color coordinates are 0.470/0.429 with color rendering index (CRI) of 87. The 50% lifetime of the OLED could be estimated to 90,000 h.

Highly efficient organic light-emitting diodes (OLEDs) are strongly demanded for both display and illumination applications. High efficiency would also help prolong the device lifespan. However, many OLEDs encounter significant roll-off problems, leading to undesired low device efficiency at high luminance, which is unfavorable to their commercial realization for lighting. Hence, OLED devices with mild or even little roll-off are highly expected. We have, nevertheless, observed some OLEDs that exhibit roll-up phenomenon, i.e., that their external quantum efficiency (EQE) or current efficiency increases as the applied voltage or brightness is increased. By taking such advantage of device architecture design, OLEDs with an approaching or even above the theoretical limit EQE are obtained at high luminance. In this report, we present how this works for a yellow OLED that exhibits a record-high power efficiency among reported fluorescent yellow OLEDs, a very-low color temperature OLED with a record-breaking efficacy based on the same color-temperature, and a green OLED. Notably, the yellow OLED exhibits an EQE that increases from 5.4 to 6.2% and current efficiency from 16.4 to 18.7  cd/A as the luminance increases from 1000 to 10,000  cd/m². Plausible mechanisms regarding why roll-up occurs in these devices are discussed.

We report high brightness and low operating voltage efficient green organic light-emitting diodes (OLEDs) based on silicon complementary metal-oxide semiconductor (CMOS) backplane which can be used in applications such as microdisplays. The small molecule top-emitting OLEDs are based on a fluorescent green emitter accompanied by blocking, doped charge transport layers, and an anode fabricated with standard CMOS processes of a 200 mm integrated circuit (IC) fab. The devices are designed to maximize the efficiency under low operative bias so as to fit the limited voltage budget of the IC. This was done by making optical simulations of the device structure, optimizing the organic layer thicknesses and charge injection in the n and p transport layers. The devices reach a current efficacy of 21.6  cd/A at a luminance of 20,000  cd/m². The devices exhibit a voltage swing as low as 2.95 V for a contrast ratio of 1000. The optimized devices have a high lifetime of 6000 and 8800 h at 5000  cd/m². Furthermore, aging inside the emission layer is investigated.

Efficient deep blue-emitting poly[9,9-bis(2-ethylhexyl)fluorene] derivatives were synthesized via incorporating 2,8-diyl-dibenzothiophene-S,S-dioxide (FSO) moiety into polyfluorene backbone. The photoluminescence (PL) spectra exhibit excellent thermal stability and PL quantum efficiencies distinctly improved upon thermal annealing at various temperatures. Spectrally stable light-emitting diodes were obtained based on resulting copolymers at various driving voltages and operation times. The best device performances were achieved based on copolymer PF-FSO10, which shows a maximum external quantum efficiency of 1.78%, and 2.27% upon thermal annealing in the air, and as high as 3.52% by inserting an additional electron injection layer poly(9,9-bis(6′-(bromilamino)hexyl))fluorene (PFNBr) between the polymer-cathode interface. The stable electroluminescence spectra with CIE coordinates of (0.16, 0.08) and superior device performances indicates that the copolymers could be a promising candidate of deep blue emitters in full-color displays.

Thermally deposited molybdenum oxide films are investigated with X-ray photo-emission spectroscopy, ultra-violet photoemission spectroscopy and inverse photoemission spectroscopy. The Interface between MoO_x and copper pthalocynine (CuPc) is studied and the previously reported device performance improvement is explained, with the help of interface energy level alignment. The effect of oxygen and air exposure on MoOx films and growth of gap states with exposures are studied. The surface chemical compositions of MoOx films, of varying thicknesses from 1 nm to 50 nm, have also been investigated. For all the investigated film thicknesses, the thermally evaporated films are found to be oxygen deficient. It is believed, that the oxygen vacancies can be subdued to a great extent by annealing at elevated temperatures. We annealed the MoO_x thin films in air, at 300 °C for 20 h, and investigated the changes induced by the air annealing.

Displacement current measurement (DCM) is a simple but powerful tool for exploring charge carrier dynamics in organic semiconductor devices. In the first section, we review the basic concept of DCM and how it detects the charge injection, extraction, accumulation, and trapping behaviors in organic semiconductor devices within a quasistatic regime. Subsequently, we present applications of this technique to investigate the device properties of tris-(8-hydroxyquinolate) aluminum (Alq₃)-based organic light-emitting diodes. We observed that light irradiation during device fabrication induces additional negative space charges and charge traps in the Alq₃ layer. In addition, the device containing the illuminated Alq₃ film exhibits a lower luminous efficiency and shorter lifetime compared to the device fabricated in dark conditions, possibly because of the additional hole accumulation in the illuminated Alq₃ film. DCM detects the formation of charge traps in the aged devices, decay of the negative space charge, and increase in hole injection voltage with device aging. The origins of these behaviors can be attributed to orientation polarization and charge traps in Alq₃ film.

A flexible transparent electrode (FTE) is one of the most essential parts for the next generation of flexible optoelectronic devices, including solar cells, displays, and solid-state lighting devices. Although a lot of candidate materials for the FTE such as metallic nanowires, carbon nanotube, and graphene have been investigated, each material has fundamental limits as FTE applications, such as low transmittance (70% to 80%), high sheet resistance (>100  ohm/sq ) and rough surface morphology. Dielectric/metal/dielectric (DMD) electrode structure is a promising candidate for next-generation flexible transparent electrodes. Compared with other transparent electrodes, DMD electrodes show best performance in terms of optical transparency, sheet resistance, and mechanical flexibility. In addition, it has been also reported that the device performances can be significantly enhanced by the microcavity effects with the DMD electrodes. We review the relevant principles and discusses recent progress in DMD electrodes.

Abstract unavailable.

A fluorescent solar collector (FSC) is a waveguide device that can concentrate both diffuse and direct sunlight on to a solar cell. The electrical output of the device depends strongly on the photon fluxes that are absorbed, emitted, and trapped inside the FSC plate. For this reason, it is important to study the photon transport losses inside the collector. One of the losses in FSC to be investigated is scattering, which increases the probability of the escape cone losses. We determine the scattering contributions in FSC by using angle dependence of light internally reflected in the FSC. The cause of scattering in spin-coated polymethylmethacrylate on top of the glass collector is identified as roughness from the top surface, rather than bulk losses. This loss can be suppressed to less than 2% using an index-matching planarization layer.

The recent push for cost reductions in solar electricity production deployment has renewed interest in concentrating photovoltaic systems. One strategy in low-concentration systems has been to reduce balance-of-system costs by reducing tracking accuracy requirements and/or eliminating tracking in the azimuth or altitude direction. However, misalignment with the sun, due to a lower-performing tracker or intentional design, hurts the concentrator's performance. The effect of misalignment on the performance of a 8.1x compound parabolic concentrator developed for inclined single-axis tracking is evaluated. Both the ray tracing simulations and measured compound parabolic concentrator results show significant effects from misalignment on the concentrator's performance. Average irradiance decreases significantly as the acceptance angle is approached in the east-west or north-south direction. Additionally, the maximum irradiance value can increase significantly during misalignment and move locations within the exit aperture, having a significant impact on thermal management design. It is important to incorporate these "real world" effects of intentional and unintentional error in sun-tracking, so that the product design is effective and the true cost of less accurate trackers is understood.

Thermal endurance studies were performed on the major polymeric components used in a Low concentration photovoltaic (LCPV) module using a thermal gravimetric analysis (TGA) method. Activation energy of the various polymers was obtained, and lifetimes were predicted based on the assumption of 5% weight loss for 30 years. Oven aging was used to verify the TGA thermal endurance prediction. Reasonable agreement between TGA prediction and oven baking results were reached, with the exception of ethylene-vinyl acetate (EVA) encapsulant that demonstrated a second-order reaction inconsistent with TGA assumptions. As a result, a more LCPV-relevant way to characterize the degradation of EVA was developed and demonstrated a linear relationship between PV device short-circuit current degradation and yellowness index. For the purpose of this EVA degradation analysis, a 20% reduction in PV device short circuit current was selected as the failure criteria.

The impact of mirror shape on energy production in Skyline Solar's reflective trough medium concentration photovoltaic system is reviewed using a combination of commercial and proprietary modeling tools. For linear concentrators, an important parameter for efficiency optimization is the uniformity of the flux line on the photovoltaic cells. A significant source of nonuniformity is the discontinuity of reflected light due to the gap between mirrors along the length of the trough. Standard concentrating solar power trough mirrors have a typical length of 1.5 m with a gap between mirrors of 10 to 20 mm. To reduce nonuniformity of the flux line due to this mirror to mirror gap, Skyline Solar developed a dual curvature mirror that stretches the flux line along the trough axis. Extensive modeling and experiments have been conducted to analyze the impact of this design. The methodology of optimization is presented for the X14 Skyline system architecture, and benefits of up to 3% of energy can be realized at locations with latitude below 30 deg.

Over the past 15 years, tremendous effort has been put forth into increasing the efficiency of multijunction solar cells used in concentrator photovoltaics (CPV) systems. Because the cell represents a small portion of the total CPV system cost, modest improvements in cell efficiencies have a large impact on reducing the levelized cost of electricity. To address the need for higher efficiency CPV cells, Solar Junction is currently manufacturing a lattice-matched multijunction solar cell that employs a 1 eV GaInNAsSb bottom subcell in lieu of the traditional Ge subcell. Our standard 5.5  mm 2 product has recently achieved a world record in power conversion efficiency. In this article, we report on the design, performance, and integration of this high-performance cell. The importance of overdriving the GaInNAsSb bottom subcell is discussed in addition to the optimization of the anti-reflection coating used for cells under glass secondary optics.

Most cost-effective concentrated photovoltaics (CPV) systems are based on an optical train comprising two stages, the first being a Fresnel lens. Among them, the Fresnel-Köhler (FK) concentrator stands out owing to both performance and practical reasons. We describe the experimental measurements procedure for FK concentrator modules. This procedure includes three main types of measurements: electrical efficiency, acceptance angle, and irradiance uniformity at the solar cell plane. We have collected here the performance features of two different FK prototypes (ranging different f -numbers, concentration ratios, and cell sizes). The electrical efficiencies measured in both prototypes are high and fit well with the models, achieving values up to 32.7% (temperature corrected, and with no antireflective coating on SOE or POE surfaces) in the best case. The measured angular transmission curves show large acceptance angles, again perfectly matching the expected values [measured concentration acceptance product (CAP) values over 0.56]. The irradiance pattern on the cell (obtained with a digital camera) shows an almost perfectly uniform distribution, as predicted by raytrace simulations. All these excellent on-sun results confirm the FK concentrator as a potentially cost-effective solution for the CPV market.

A concentrator photovoltaic system can be built by coupling a lens array to a branched planar waveguide, which has one end with a solar cell attached and the other end divided into multiple branches. A right-angle prism is attached to each branch end, redirecting the focused sunlight into the waveguide via total internal reflection so that the light propagates inside the waveguide. With an appropriate design, the light leakage from the waveguide can be made negligible. Our ray-tracing simulations show that the ratio of the optical power exiting the waveguide to an optical power entering the lens-array is close to 87%, with the loss being mostly due to the Fresnel reflections at the lens (8%) and prism surfaces (3%). We can increase geometric concentration by cascading tapered waveguides. The number of cascades should be limited so that the light leakage from the tapered portions remains insignificant. A secondary optical element on each prism would collimate the light, easing this limitation, as well as making the system thinner. The absorption in the waveguide material imposes an inherent limit on the number of cascades and the concentration factor.

Methods developed to maximize the overall reflectance of the second-surface silvered glass used in concentrating solar power (CSP) and concentrating photovoltaics (CPV) solar systems are reported. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.5% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 3.5% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric enhancement, was measured at National Renewable Energy Laboratory to have 95.4% solar weighted reflectance. For comparison, nonenhanced, wet-silvered reflectors of the same 4-mm thickness show reflectance ranging from 91.6% to 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending. Finally, an experiment capable of determining which junction limits the triple junction cell is demonstrated.

Concentrating photovoltaics (CPV) technology includes a diverse array of module and system designs that vary according to the geometry, concentration ratio, and the types of cells and optics used. Research and development (R&D) in CPV is active and highly inventive, resulting in both the highest efficiency solar cells and modules currently available across the PV industry (Solar Junction's triple junction GaAs-based cell: 43.5%; Semprius' CPV module: 33.9%). Despite extremely challenging market conditions, CPV installations have recently flourished, and researchers are avidly pursuing new applications. This special section in the Journal of Photonics for Energy provides insight into some of the new and exciting areas of CPV R&D and conveys the extraordinary level of innovation underway in today's CPV community.

The significant features of a series of stabilization experiments conducted at the National Renewable Energy Laboratory (NREL) between May 2009 and the present are reported. These experiments evaluated a procedure to stabilize the measured performance of thin-film polycrystalline cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) thin-film photovoltaic (PV) modules. The current-voltage (I-V) characteristics of CdTe and CIGS thin-film PV devices and modules exhibit transitory changes in electrical performance after thermal exposure in the dark and/or bias and light exposures. We present the results of our case studies of module performance versus exposure: light soaked at 65°C; exposed in the dark under forward bias at 65°C; and, finally, longer-term outdoor exposure. We find that stabilization can be achieved to varying degrees using either light-soaking or dark-bias methods and that the existing IEC 61646 light-soaking interval may be appropriate for CdTe and CIGS modules with one caveat: it is likely that at least three exposure intervals are required for stabilization.

The mechanical behavior of a photovoltaic (PV) module induced by aerodynamic loads is analyzed in a wind tunnel for three different wind velocities at varied azimuthal and inclination angles. The deformations and flow-induced vibrations of the PV module have been monitored by a displacement sensor. The frequencies of the monitored vibrations are analyzed by Fourier transformation. Finally, the damage potential at cell level caused by wind loads is investigated by electroluminescence. The aim of this work was to study the mechanical transfer behavior of the coupled fluid-structure interaction between the aerodynamic loads and the deformations and vibrations of the module, which is one aspect of the mechanically induced degradation of PV modules.

Corrosion can seriously affect the service life of components for solar energy conversion. We present results of mapping the potential of atmospheric corrosion in coastal regions with the aid of a Geographical Information System (GIS). Concentration of sea salt aerosols, which are the main atmospheric pollutants in maritime coastal regions, gives an indication of the probability of the atmospheric corrosion leading to PV-module degradation. Two approaches to estimate the distribution of sea salt aerosol across the coastal regions worldwide are investigated. The first approach is a geo-statistical analysis that has been used for interpolating chloride deposition data. The second approach is based on modeling the environmental conditions, which are affecting source and distribution of airborne salinity. The comparison of these two approaches provides high accuracy in the description of the variation in airborne salinity across the coastal regions. The assessment of the atmospheric corrosion in coastal regions is based on the international standard ISO 9223. The corrosivity classification is simply defined by three parameters: SO₂ pollution, airborne salinity, and relative humidity. A combination of the results from the geo-statistical approach and modeling is used; the result is a map of atmospheric corrosion in coastal regions.

In the development of materials for enhanced photovoltaic (PV) performance, it is critical to have quantitative knowledge of both their initial performance and their performance over the required 25-year warranted lifetime of the PV system. Lifetime and degradation science, based on an environmental stress and response framework, is being developed to link the intensity and net stress to which materials, components, and systems are exposed to the responses observed and their subsequent degradation and damage accumulation over the lifetime. Induced absorbance to dose (IAD), a metric developed for solar radiation durability studies of solar and environmentally exposed materials, is defined as the rate of photodarkening or photobleaching of a material as a function of radiation dose. Quantitative degradation rates like IAD, determined over a wide range of stress intensities and net stresses, have the potential to predict degradation, failure, and power loss rates in photovoltaic systems over time caused by damage accumulation. Two grades of poly(methyl methacrylate) were exposed and evaluated in two cases of high-intensity ultraviolet exposures. A three- to six-fold increase in photodarkening was observed for one acrylic formulation when exposed to UVA-340 light when compared with concentrated xenon-arc exposure. The other, more highly stabilized acrylic formulation, showed up to three times more photodarkening in the same exposure.

Current accelerated qualification tests of photovoltaic (PV) modules mostly assist in avoiding premature failures but can neither duplicate changes occurring in the field nor predict useful product lifetime. Therefore, outdoor monitoring of field-deployed thin-film PV modules was undertaken at FSEC with the goal of assessing their performance in hot and humid climate under high system-voltage operation. Significant and comparable degradation rate of −5.13±1.53% and −4.5±1.46% per year was found using PVUSA type regression analysis for the positive and negative strings, respectively of 40W glass-to-glass Cu-In-Ga-Se (CIGS) thin-film PV modules in the hot and humid climate of Florida. Using the current-voltage measurements, it was found that the performance degradation within the PV array was mainly due to a few (8% to 12%) modules that had substantially higher degradation. The remaining modules within the array continued to show reasonable performance (>96% of the rated power after ∼ four years).

Photovoltaic (PV) modules are usually considered safe and reliable. But in case of grid-connected PV systems that are becoming popular, the issue of fire safety of PV modules is becoming increasingly important due to the employed high voltages of 600 to 1000 V. The two main factors, i.e., open circuiting of the dc circuit and of the bypass diodes and ground faults that are responsible for the fire in the PV systems, have been discussed in detail along with numerous real life examples. Recommendations are provided for preventing the fire hazards such as designing the PV array mounting system to minimize the chimney effect, having proper bypass and blocking diodes, and interestingly, having an ungrounded PV system.

The field of photovoltaics has undergone dramatic growth over the past few years. This growth has been accompanied by two significant changes in the commercial photovoltaic (PV) market. 1.The major PV market has shifted from the residential and commercial building markets to central station utility power markets. With this switch has come a new focus on the long-term reliability and durability of PV modules and systems. If you are installing tens or even hundreds of megawatts of PV modules, it is absolutely necessary to ensure that they will continue to perform adequately throughout their expected lifetime of at least 25 years. 2.Selling prices for PV modules have decreased from more than $4 per watt to less than $1 per watt in the last 5 years. This reduction in selling process has been driven by low-cost production in the developing world. This means that most of today's PV modules are being manufactured by companies that did not exist 10 or, in many cases, even 5 years ago. Yet the modules must carry 25-year warranties and are expected to survive at least that long without having the benefit of long-term field experience.

Modern mobile communication devices have user interfaces that are dominated by high-quality displays. Increased multimedia use imposes high demands on the design of display modules, as the content available for mobile use becomes visually richer. Especially the power dissipation of the display can limit the amount of time available for multimedia consumption and interaction. In the mobile liquid-crystal display (LCD), the energy efficiency is determined by the backlight design. State-of-the-art backlights direct white light through a display subpixel array, with high uniformity and up to 90% efficiency in white light output. Therefore, it is difficult to obtain system-level energy savings by improving the backlight design alone. Diffractive backlights have recently been proposed to reduce the power dissipation of the display module, and slanted grating arrays are among the enabling optical features that allow for reduction in power dissipation beyond what is available in the state of the art. By the use of diffractive grating arrays, the required primary color (red, green, or blue) is directed through the LCD subpixel array with geometrical registration, instead of flooding the whole LCD with white light and filtering the primary colors through the subpixel color filter array. We present a study on grating structures based on slanted grating arrays fabricated in high refractive index materials. The grating design principles and grating outcoupling results are provided, and an outline of a new embedded system design is given. Emphasis is on grating array design aspects for future energy-efficient display system design. The results show that savings in power consumption can be expected with advanced display system design based on embedded slanted grating array backlight light guide plates.

We propose an approach for enhancing the absorption of thin-film amorphous silicon solar cells using periodic arrangements of resonant dielectric nanospheres deposited as a continuous film on top of a thin planar cell. We numerically demonstrate this enhancement using three dimensional (3D) full field, finite difference time domain simulations and 3D finite element device physics simulations of a nanosphere array above a thin-film amorphous silicon solar cell structure featuring back reflector and anti-reflection coating. In addition, we use the full field finite difference time domain results as input to finite element device physics simulations to demonstrate that the enhanced absorption contributes to the current extracted from the device. We study the influence of a multi-sized array of spheres, compare spheres and domes, and propose an analytical model based on the temporal coupled mode theory.

Nonimaging Optics is the theory of thermodynamically efficient optics and as such depends more on thermodynamics than on optics. Hence, I propose a condition for the best design based on purely thermodynamic arguments, which I believe has profound consequences for design of thermal and even photovoltaic systems. This new way of looking at the problem of efficient concentration depends on probabilities, the ingredients of entropy, and information theory while optics in the conventional sense recedes into the background. The paper takes a pedagogical and retrospective approach, as to place the new approach in perspective, it helps to first appreciate what has succeeded in nonimaging optics, and turned some devices [e.g. the compound parabolic concentrator (CPC) cone] into a commodity. I will conclude with some speculative directions on where the new ideas might lead.

The influence of the cluster-core unit in cluster-decorated p-Si on photo-electrochemical (PEC) hydrogen evolution has been investigated using a homologous series of cubane-like heterobimetallic sulfide compounds. These compounds stem from the generic cluster structure A₃S₄ or A₃B?startSend?₄ (A = W, Mo; B = Co, Cu). We find that the Mo-based (A = Mo) cluster-decorated Si photoelectrodes show higher PEC performance than otherwise equivalent W-based (A = W) cluster-decorated ones. This is consistent with higher electrocatalytic activity of the Mo-based clusters supported on n-Si when measured in the dark. The result of stability tests is that photoelectrodes decorated with clusters without Co (B ≠ Co) can exhibit promising stability, whereas clusters of the structure A₃CoS₄  (A = W, Mo) yield photoelectrodes that are highly unstable upon illumination. X-ray photoelectron spectroscopy (XPS) results suggest that both oxidation and material loss play a role in deactivation of the A₃CoS₄ materials. Additionally, we observe that the photocurrent depends linearly on the light intensity in the limiting current region, and the corresponding incident photon to current efficiency (IPCE) may reach approximately 80%. Density functional theory (DFT) calculations of the clusters adsorbed on the hydrogen-terminated Si surface are used to estimate and compare cluster adsorption energies on the surface as well as the H-binding energies, which is a descriptor for electrocatalytic activity.

Unlike the conventional light-emitting diode (LED) luminaire with a planar substrate and only the forward emission, the proposed LED luminaire with a curved ceramic substrate can perform both the forward and the backward emissions and inherits the merits of good heat-dissipation and low cost from the ceramic substrate. Assembled with the proper primary optics, an illustrated LED bulb has been designed, fabricated and measured. The measured luminous intensity of the LED bulb has shown the backward emission and designed distribution with the beam-angle of 133 deg. To broaden the application areas, such a LED bulb on a curved substrate has been modularized as a streetlight. The measured results of the proposed streetlight have shown that the beam angle of the luminous intensity and the luminaire efficiency are 132 deg and 86%, respectively. Meanwhile, its luminous characteristics also fit the Chinese standard for lighting design of urban roads.

We present an analysis of the degradation of the optical and electrical properties of high-power light-emitting diodes (LEDs) used for general lighting applications. The study was conducted by submitting the LEDs to different current and temperature stress conditions. Those conditions are based on typical operating conditions that can be encountered on LED-based luminaires for general lighting. LEDs were stressed under four different operating currents, and two of those were stressed at two junction temperatures, controlled with a thermoelectric cooler. Results described in this paper indicate that the lumen droop due to an increase in nonradiative recombination is correlated with the stress conditions in accordance with the literature. However, the LED samples showed a forward-voltage droop which seems to be independent of the stresses. For all the stress conditions, the LED forward voltages decreased by about 1% after 1000 h of stress time. A link between forward voltage and lumen output was made through LED efficiency. Also, the yellow peak/blue peak ratio was measured and showed an increase after 1000 to 1200 h. This is attributed to LED chip degradation. These observations suggest the use of both current and voltage control to optimize the use of LEDs in general lighting.

A study on dye-sensitized solar cells (DSSCs) with extracts of Syzygium guineense as sensitizer is reported for the first time. DSSCs were assembled using natural dye extracted from Syzygium guineense as sensitizer. The photoelectrochemical performance of the quasi-solid state DSSCs based on the ethanol extract dye showed V_OC of 0.506 V and J_SC of 2.03  mAcm−2; and a power conversion efficiency of 0.51%. UV-vis spectroscopy studies of light absorption of the natural dye were done. Furthermore, the ethanol extract obtained from Syzygium guineense was further purified stepwise through solvent-solvent extraction. The photoelectrochemical performance for the extracts with different solvents indicated that the individual components have synergistic effect in the performance of the DSSC.

Light trapping is a key issue for high efficiency thin-film silicon solar cells. The authors present three-dimensional electromagnetic simulations of an n-i-p substrate-type microcrystalline silicon solar cell applying a plasmonic reflection grating back contact as a novel light-trapping structure. The plasmonic reflection grating back contact consists of half-ellipsoidal silver nanostructures arranged in square lattice at the back contact of thin-film silicon solar cells. Experimental results of prototypes of microcrystalline silicon thin-film solar cells showed significantly enhanced short-circuit current densities in comparison to flat solar cells and, for an optimized period of the plasmonic reflection grating back contact, even a small enhancement of the short-circuit current density in comparison to the reference cells applying the conventional random texture light-trapping structure. The authors demonstrate a very good agreement between the simulated and experimental spectral response data when taking experimental variations into account. This agreement forms an excellent basis for future simulation based optimizations of the light-trapping by plasmonic reflection grating back contacts. Furthermore, from the simulated three-dimensional electromagnetic field distributions detailed absorption profiles were calculated allowing a spatially resolved evaluation of parasitic losses inside the solar cell.

The dye-sensitized nanocrystalline solar cell (DSC) offers potential opportunities in the area of renewable energy sources mainly due to its simple fabrication procedure and use of low cost materials. A theoretical model based on the thermionic emission theory was developed to determine the interfacial effect of ZnO/TCO on the performance of DSC. It was found that under conditions where thermionic emission is valid, photoelectric outputs are affected by temperature and Schottky barrier height (ϕb). The model can be used to facilitate better selection of suitable TCO material.

We analyze photoluminescence (PL) and electroluminescence (EL) of a GaAs solar cell using a hyperspectral imager that records spectrally resolved images. Thanks to the absolute calibration of the setup, we first investigate the reciprocity relations between solar cells and light-emitting diode and determine the external quantum efficiency from EL images. Spatial variations are observed due to series resistance effect that we can evaluate. Second, the PL experiment allows us to plot the recombination current at a given spatial location versus the quasi-Fermi level splitting at the same location. Under reasonable assumptions, this can be linked to the classical measurement of the short circuit current versus the open circuit voltage. Therefore we perform a contactless mapping of optoelectronic properties such as the saturation currents. The assumptions made in these experiments are discussed in order to correctly investigate polycrystalline solar cells in the future where strong lateral variations exist.

Absorption peak maxima of two organic dyes differing by the position of the methine unit differ by 61 nm in dioxane and by up to 139 nm in polar solvents. It was previously reported that the difference is not reproduced by time-dependent density functional theory (TDDFT) using ab initio or hybrid functionals. TDDFT errors are different between the molecules, leading to a qualitative failure of TDDFT to predict relative energetics of the dyes. We focus on the effect of polar solvents (acetonitrile, DMSO, methanol, and 2-propanol) on the absorption spectrum, specifically, on the different between the two molecules sign of the solvatochromic shift versus dioxane. Using the correction due to Peach et al., the absolute TDDFT errors can be brought within acceptable ranges of 0.2 to 0.3 eV, and the blue shift versus dioxane is reproduced, although both dyes are predicted to exhibit positive solvatochromism. The inclusion of explicit solvent molecules did not appreciably change either TDDFT energies or the correction term. These results show that in dye design by changing the conjugation order, computational errors are expected to be more important than in the case of an extension of the size of conjugation, especially when polar solvents are used.

In this work, microscopic three-dimensional simulations were performed on nanowire array solar cells to study the impact of surface recombination (SR) on the photovoltaic performance. Both axially and radially arranged p-n junction in III-V-based structures were taken into consideration. From the cases with SR velocity varying from 1e3 cm/s to 1e6 cm/s , the radial p-n-junction nanowire was found to provide better tolerance for SR. The SR difference within the axial and radial p-n-junction structures is explained by analyzing the relevant minority carrier density, followed by a discussion on the impact of SR on the diffusive nature of minority carriers.

We report on the structural, morphological, and optical qualities of thick InxGa1−xN heteroepitaxial layers grown by metalorganic chemical vapor deposition with various growth conditions for applications in wide-band gap solar cells. The indium incorporation depending on the growth temperature and indium precursor flow rate and the crystalline and optical qualities of InGaN layers depending on indium mole fraction were investigated. The InGaN layers with high structural and optical qualities were obtained for indium mole fractions, xIn < 0.18, whereas significant degradation of material qualities was observed for xIn < 0.18, which is associateWe report on the structural, morphological, and optical qualities of thick InxGa1−xN heteroepitaxial layers grown by metalorganic chemical vapor deposition with various growth conditions for applications in wide-bandgap solar cells. The indium incorporation depending on the growth temperature and indium precursor flow rate and the crystalline and optical qualities of InGaN layers depending on indium mole fraction were investigated. The InGaN layers with high structural and optical qualities were obtained for indium mole fractions, xIn < 0.18, whereas significant degradation of material qualities was observed for xIn < 0.18, which is associated with the change of growth mode induced by reduced growth temperature. Stokes shift and microscopic and macroscopic phase separations were also studied. Two types of additional phases besides InGaN matrix, i.e., indium-rich InGaN microstructures and macroscopic InGaN domains, were demonstrated to be suppressed by controlling surface adatom mobility and growth rates.d with the change of growth mode induced by reduced growth temperature. Stokes shift and microscopic and macroscopic phase separations were also studied. Two types of additional phases besides InGaN matrix, i.e., indium-rich InGaN microstructures and macroscopic InGaN domains, were demonstrated to be suppressed by controlling surface adatom mobility and growth rates.

Intrinsic ZnO (i-ZnO) thin films were deposited on glass substrates by radio-frequency magnetron sputtering for exploring the effects of the deposition conditions. These films were optically, electrically, and structurally characterized. Results showed that the properties of i-ZnO thin films changed with the changing of deposition conditions. These i-ZnO films were also integrated into CuIn_1-xGa_xSe₂ (CIGS) solar cells. It was found that the incorporation of an i-ZnO layer in CIGS solar cells led to a significant improvement of homogeneity and efficiency of CIGS solar cells. An increment of 1.71% was gained in the average cell efficiency through adjusting the deposition conditions of i-ZnO layers. Detailed analysis showed that there was no improvement in cell efficiency under the deposition conditions favorable for growing the i-ZnO films. These results indicate that there should be a balance among the optimized performance of each layer deposited for high quality multi-layer devices.

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