Showing papers in "Journal of Photonics for Energy in 2015"
TL;DR: In this article, the underlying physics of outcoupling in white OLEDs and review recent progress toward making light extraction more efficient are discussed, as well as the prospects of using top-emitting metal-metal microcavity designs for white OLED and tuning the average orientation of the emissive molecules within the OLED.
Abstract: Organic light-emitting diodes (OLEDs) have been successfully introduced to the smartphone display market and have geared up to become contenders for applications in general illumination where they promise to combine efficient generation of white light with excellent color quality, glare-free illumination, and highly attractive designs. Device efficiency is the key requirement for such white OLEDs, not only from a sustainability perspective, but also because at the high brightness required for general illumination, losses lead to heating and may, thus, cause rapid device degradation. The efficiency of white OLEDs increased tremendously over the past two decades, and internal charge-to-photon conversion can now be achieved at ∼100% yield. However, the extraction of photons remains rather inefficient (typically <30%). Here, we provide an introduction to the underlying physics of outcoupling in white OLEDs and review recent progress toward making light extraction more efficient. We describe how structures that scatter, refract, or diffract light can be attached to the outside of white OLEDs (external outcoupling) or can be integrated close to the active layers of the device (internal outcoupling). Moreover, the prospects of using top-emitting metal–metal microcavity designs for white OLEDs and of tuning the average orientation of the emissive molecules within the OLED are discussed.
178 citations
TL;DR: In this article, the authors discuss the use of cross-linkable organic materials, metal oxides, and orthogonal solvent systems to deposit various functional layers in an organic light-emitting diodes.
Abstract: Organic light-emitting diodes (OLEDs) have become a promising candidate for lighting and display applications. High efficiency OLEDs require a multilayer device architecture to provide exciton confinement and balance charge transport. Conventional OLEDs are made by vacuum process, and the manufacturing cost can be reduced by solution processing. However, unlike vacuum-deposited OLEDs, solution-processed multilayer OLEDs are more challenging to make. The key for multilayer solution processing is to have the layer structure which can withstand solvents used in subsequent processing. We review the materials’ strategies to make multilayer solution-processed OLEDs. Specifically, we will discuss the use of cross-linkable organic materials, metal oxides, and orthogonal solvent systems to deposit various functional layers in an OLED.
147 citations
TL;DR: In this paper, a planar absorber/emitter is incorporated for controlling the thermal radiation spectrum in a solar thermal-thermophotovoltaic (STPV) system.
Abstract: We demonstrate a high-efficiency solar-thermophotovoltaic system (STPV) using a monolithic, planar, and spectrally selective absorber/emitter. A complete STPV system using gallium antimonide (GaSb) cells was designed and fabricated to conduct power generation tests. To produce a high-efficiency STPV, it is important to match the thermal radiation spectrum with the sensitive region of the GaSb cells. Therefore, to reach high temperatures with low incident power, a planar absorber/emitter is incorporated for controlling the thermal radiation spectrum. This multilayer coating consists of thin-film tungsten sandwiched by yttria-stabilized zirconia. The system efficiency is estimated to be 16% when accounting for the optical properties of the fabricated absorber/emitter. Power generation tests using a high-concentration solar simulator show that the absorber/emitter temperature peaks at 1640 K with an incident power density of 45 W/cm2, which can be easily obtained by low-cost optics such as Fresnel lenses. The conversion efficiency became 23%, exceeding the Shockley–Queisser limit for GaSb, with a bandgap of 0.67 eV. Furthermore, a total system efficiency of 8% was obtained with the view factor between the emitter and the cell assumed to be 1.
72 citations
TL;DR: In this article, the development of polymeric solar cells from its origin to the most efficient devices published to date is reviewed, and the authors separate these achieve- ments into three different categories traditionally followed by the scientific community to push devices over 10% power conversion efficiency: active materials, strategies that can mainly modify the active film morphology, and all the different cell layout/architectures that have been used in order to extract as high a photocurrent as possible from the Sun.
Abstract: In spite of the impressive development achieved by organic photovoltaics throughout the last decades, especially in terms of reported power conversion efficiencies, there are still important technological and fundamental obstacles to circumvent before they can be imple- mented into reliable and long-lasting applications Regarding device processing, the synthesis of highly soluble polymeric semiconductors first, and then fullerene derivatives, was initially considered as an important breakthrough that would definitely change the fabrication of photo- voltaics once and for all The potential and the expectation raised by this technology is such that it is very difficult to keep track of the most significant progresses being now published in differ- ent and even monographic journals In this paper, we review the development of polymeric solar cells from its origin to the most efficient devices published to date We separate these achieve- ments into three different categories traditionally followed by the scientific community to push devices over 10% power conversion efficiency: active materials, strategies—fabrication/process- ing procedures—that can mainly modify the active film morphology, and all the different cell layout/architectures that have been used in order to extract as high a photocurrent as possible from the Sun The synthesis of new donors, the use of additives and postprocessing techniques, buffer interlayers, inverted and tandem designs are some of the most important aspects that are reviewed in detail in this paper All have equally contributed to develop this technology and bring it at the doors of commercialization © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 101117/1JPE5057214)
65 citations
TL;DR: Kelvin probe force microscopy images suggest that the incorporation of RGO into TiO2 films can promote the appearance of regions with different charge dissipation capacities.
Abstract: In this study, we investigated inverted hybrid bulk-heterojunction solar cells with the following configuration: fluorine-doped tin oxide (FTO) |TiO2/RGO|P3HT:PC61BM|V2O5 or PEDOT:PSS|Ag. The TiO2/GO dispersions were prepared by sol-gel method, employing titanium isopropoxide and graphene oxide (GO) as starting materials. The GO concentration was varied from 0.1 to 4.0 wt%. The corresponding dispersions were spin-coated onto FTO substrates and a thermal treatment was performed to remove organic materials and to reduce GO to reduced graphene oxide (RGO). The TiO2/RGO films were characterized by x-ray diffraction, Raman spectroscopy, and microscopy techniques. Atomic force microscopy (AFM) images showed that the addition of RGO significantly changes the morphology of the TiO2 films, with loss of uniformity and increase in surface roughness. Independent of the use of V2O5 or PEDOT: PSS films as the hole transport layer, the incorporation of 2.0 wt% of RGO into TiO2 films was the optimal concentration for the best organic photovoltaic performance. The solar cells based on TiO2/RGO (2.0 wt%) electrode exhibited a ∼22.3% and ∼28.9% short circuit current density (Jsc) and a power conversion efficiency enhancement, respectively, if compared with the devices based on pure TiO2 films. Kelvin probe force microscopy images suggest that the incorporation of RGO into TiO2 films can promote the appearance of regions with different charge dissipation capacities.
61 citations
TL;DR: In this paper, a detailed insight of the organometal halide based perovskite structure, its unique stoichiometric design, and its underlying principles for PV applications is discussed.
Abstract: The recent power-packed advent of perovskite solar cells is transforming photovol- taics (PV) with their superior efficiencies, ease of fabrication, and cost. This perovskite solar cell further boasts of many unexplored features that can further enhance its PV properties and lead to it being branded as a successful commercial product. This article provides a detailed insight of the organometal halide based perovskite structure, its unique stoichiometric design, and its underlying principles for PV applications. The compatibility of various PV layers and its fab- rication methods is also discussed. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.JPE.5.057402)
53 citations
TL;DR: In this article, the perovskite absorber is processed via one-step deposition employing the small molecule additive, BmPyPhB, which can be dissolved in dimethylformamide along with precursors.
Abstract: In the current study, the perovskite absorber (CH3NH3PbI3) is processed via one-step deposition employing the small molecule additive, BmPyPhB, which can be dissolved in dimethylformamide along with precursors. Here, 1,3-Bis(3,5-di(pyridin-3-yl)phenyl)benzene (BmPyPhB) functions as the morphology controller to introduce an intermediate phase during perovskite film growth, which allows well-defined and precrystallized domains formed before the annealing treatment. Furthermore, a chloroform solvent wash procedure is applied afterward to remove BmPyPhB from perovskite without damaging the predetermined morphology. Thus, postannealing as low as 100°C for 5 min can achieve the optimal power conversion efficiency of 8% in a planar-structured inverted solar cell. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.JPE.5.057405)
48 citations
TL;DR: In this article, a systematic classification and understanding of the materials that have already been reported can aid the development and study of new light-emitting materials through quantitative and qualitative approaches.
Abstract: Organic light-emitting diodes (OLEDs) have attracted considerable attention in both academic and industrial circles. Certain properties of OLEDs make them especially attractive in the lighting market, including area emission characteristics not found in other existing light sources, environmentally friendly efficient use of energy, large area, ultra-light weight, and ultra-thin shape. Fluorescent and phosphorescent materials that are being applied to white OLEDs have been categorized, and the chemical structures and device performances of the important blue, orange, and red light-emitting materials have been summarized. Such a systematic classification and understanding of the materials that have already been reported can aid the development and study of new light-emitting materials through quantitative and qualitative approaches.
47 citations
TL;DR: The use of plasmonic back electrodes can impact device performance in a number of ways because the mechanisms of performance improvements are often a complex combination of optical, electrical, and structural effects as mentioned in this paper.
Abstract: Here, we review recent progress on the integration of plasmonic electrodes into bulk-heterojunction organic photovoltaic devices. Plasmonic electrodes, consisting of thin films of metallic nanostructures, can exhibit a number of optical, electrical, and morphological effects that can be exploited to improve performance parameters of ultrathin photovoltaic active layers. We review the various types of plasmonic electrodes that have been incorporated into organic photovoltaics such as nanohole, nanowire, and nanoparticle arrays and grating electrodes and their impact on various device performance parameters. The use of plasmonic back electrodes can impact device performance in a number of ways because the mechanisms of performance improvements are often a complex combination of optical, electrical, and structural effects. Inverted bulk heterojunction device architectures have been shown to benefit from the multifunctionality of plasmonic back electrodes as they can minimize space-charge effects and reduce hole carrier collection lengths in addition to providing improved light localization in the active layer. The use of semi-transparent plasmonic electrodes can also be beneficial for organic photovoltaics as they can exhibit a variety of optical properties such as light scattering, light localization, extraordinary transmission of light, and absorption-induced transparency, in addition to providing an alternative to metal oxide–based transparent electrodes.
43 citations
TL;DR: A review of laser annealing techniques in the development of chalcogenide-based [CdTe and Cu(In,Ga)(S,Se)2] solar cells can be found in this article.
Abstract: We review prior and on-going works in using laser annealing (LA) techniques in the development of chalcogenide-based [CdTe and Cu(In,Ga)(S,Se)2] solar cells. LA can achieve unique processing regimes as the wavelength and pulse duration can be chosen to selectively heat particular layers of a thin film solar cell or even particular regions within a single layer. Pulsed LA, in particular, can achieve non-steady-state conditions that allow for stoichiometry control by preferential evaporation, which has been utilized in CdTe solar cells to create Ohmic back contacts. Pulsed lasers have also been used with Cu(In,Ga)(S,Se)2 to improve device performance by surface-defect annealing as well as bulk deep-defect annealing. Continuous-wave LA shows promise for use as a replacement for furnace annealing as it almost instantaneously supplies heat to the absorbing film without wasting time or energy to bring the much thicker substrate to temperature. Optimizing and utilizing such a technology would allow production lines to increase throughput and thus manufacturing capacity. Lasers have also been used to create potentially low-cost chalcogenide thin films from precursors, which is also reviewed.
37 citations
TL;DR: In this article, the authors review and discuss progress made in these three technologies for solid-state lighting using flexible organic light-emitting diodes (OLEDs) and discuss the technical challenges associated with the development of high performing flexible substrates, electrode materials compatible with these substrates and good encapsulation techniques.
Abstract: Flexible organic light-emitting diodes (OLEDs) are candidates for next-generation solid-state lighting because they have merits such as low driving voltage, various color tuning, designable form, and large-area light emission. Although OLEDs’ efficiency, luminance, and lifetime have been improved enough to be commercialized, they are still inflexible despite being based on organic materials. To achieve efficient and reliable flexible OLEDs for solid-state lighting, flexible substrates for OLEDs should be developed. For this purpose, progress must be made in developing good flexible substrates, electrode materials, and encapsulation techniques compatible with these flexible substrates. Here, we review and discuss progress made in these three technologies for solid-state lighting using flexible OLEDs. Addressing the technical challenges associated with the development of high performing flexible substrates, electrode materials compatible with these substrates and good encapsulation techniques would lead to efficient and reliable flexible OLEDs and make flexible solid-state lighting commercially feasible.
TL;DR: In this paper, the PbI2 adsorbed on the titania substrate showed that Pb-I reacts with the HO-Ti moieties of titania to form PbO-Ti linkages.
Abstract: Adsorption of PbI2 onto a titania layer was monitored by a quartz crystal microbalance system in solution. The amount of PbI2 adsorbed on the titania layer increased with an increase in the PbI2 concentration in dimethylformamide (DMF). However, PbI2 remained after being rinsed with DMF, suggesting that PbI2 is rigidly bonded to the surface of the titania. The x-ray photoelectron spectroscopy measurement of PbI2 adsorbed on the titania substrate showed that the Pb compound has a composition of PbI0.33, not PbI2, suggesting that part of the Pb-I reacts with the HO-Ti moieties of titania to form Pb-O-Ti linkages. Trap density as measured by the thermally stimulated current method decreased after PbI2 passivation. Perovskite solar cells consisting of porous titania passivated with PbI2 had a higher efficiency than those without the passivation. It was concluded that PbI2 passivation of porous titania surfaces is one of the crucial approaches for enhancing the efficiency of perovskite solar cells with a scaffold layer of porous titania.
TL;DR: In this article, a review of semi-transparent organic photovoltaic devices with metal cavity light trapping configuration is presented, where the absorber layer includes near-infrared absorption polymers, devices incorporating one-dimensional photonic crystals, and devices with a metal cavity.
Abstract: Over the last three decades, progress in the organic photovoltaic field has resulted in some device features which make organic cells applicable in electricity generation configurations where the standard silicon-based technology is not suitable, for instance, when a semi-transparent photovoltaic panel is needed. When the thin film solar cell performance is evaluated in terms of the device’s visible transparency and power conversion efficiency, organic solar cells offer the most promising solution. During the last three years, research in the field has consolidated several approaches for the fabrication of high performance semi-transparent organic solar cells. We have grouped these approaches under three categories: devices where the absorber layer includes near-infrared absorption polymers, devices incorporating one-dimensional photonic crystals, and devices with a metal cavity light trapping configuration. We herein review these approaches.
TL;DR: In this article, the effects of photon recycling on the lifetime of a single junction GaAs/GaAs tandem device were modeled and measured experimentally as a function of concentration between 3 and 100 suns.
Abstract: Modeling single junction solar cells composed of III–V semiconductors such as GaAs with the effects of photon recycling yields insight into design and material criteria required for high efficiencies. For a thin-film single junction GaAs cell to reach 28.5% efficiency, simulation results using a recently developed model which accounts for photon recycling indicate that Shockley–Read–Hall (SRH) lifetimes of electrons and holes must be longer than 3 and 1 μs, respectively, in a 2-μm thin active region, and that the native substrate must be removed such that the cell is coupled to a highly reflective rear-side mirror. The model is generalized to account for luminescence coupling in tandem devices, which yields direct insight into the top cell’s nonradiative lifetimes. A heavily current mismatched GaAs/GaAs tandem device is simulated and measured experimentally as a function of concentration between 3 and 100 suns. The luminescence coupling increases from 14% to 33% experimentally, whereas the model requires increasing electron and hole SRH lifetimes to explain these results. This could be an indication of the saturating defects which mediate the SRH process. However, intermediate GaAs layers between the two subcells may also contribute to the luminescence coupling as a function of concentration.
TL;DR: In this paper, the effect of processing parameters and morphology on solar cell performance, and charge carrier transport in polymer solar cells is discussed, and the basis for selection of different processing conditions for optimized nanomorphology of active layers and reduced bimolecular recombination to enhance opencircuit voltage, short-circuit current density, and fill factor of BHJ solar cells.
Abstract: Conjugated polymers are potential materials for photovoltaic applications due to their high absorption coefficient, mechanical flexibility, and solution-based processing for low-cost solar cells. A bulk heterojunction (BHJ) structure made of donor–acceptor composite can lead to high charge transfer and power conversion efficiency. Active layer morphology is a key factor for device performance. Film formation processes (e.g., spray-coating, spin-coating, and dip-coating), post-treatment (e.g., annealing and UV ozone treatment), and use of additives are typically used to engineer the morphology, which optimizes physical properties, such as molecular configuration, miscibility, lateral and vertical phase separation. We will review electronic donor–acceptor interactions in conjugated polymer composites, the effect of processing parameters and morphology on solar cell performance, and charge carrier transport in polymer solar cells. This review provides the basis for selection of different processing conditions for optimized nanomorphology of active layers and reduced bimolecular recombination to enhance open-circuit voltage, short-circuit current density, and fill factor of BHJ solar cells.
TL;DR: In this article, density functional theory (DFT) and time-dependent DFT(TD-DFT)-based modeling techniques are used to conduct a computational study of the geometry and electronic structure of oxadiazole-based organic sensitizers.
Abstract: Density functional theory (DFT) and time-dependent DFT(TD-DFT) modeling techniques are used to conduct a computational study of the geometry and electronic structure of oxadiazole-based organic sensitizers. A DFT study on the thermodynamic aspects of the charge transport processes associated with dye-sensitized solar cells (DSSCs) suggests that the system with 1,2,4-oxadiazole has a balance among the different crucial factors and may result in the highest incident photon to charge carrier efficiency. The dye/(TiO2)8 anatase clusters were also simulated to illustrate the electron injection efficiency at the interface. This study provides basic understanding of the impact of molecular design on the performance of oxadiazole dyes in DSSCs.
TL;DR: In this article, the principles, impediments, and recent progress in the development of ultrathin flexible Ag electrodes for use in flexible optoelectronic devices are reviewed and discussed.
Abstract: This paper reviews the principles, impediments, and recent progress in the development of ultrathin flexible Ag electrodes for use in flexible optoelectronic devices. Thin Ag-based electrodes are promising candidates for next-generation flexible transparent electrodes. Thin Ag-based electrodes that have a microcavity structure show the best device performance, but have relatively low optical transmittance (OT) due to reflection and absorption of photons by the thin Ag; this trait causes problems such as spectral narrowing and change of emission color with viewing angle in white organic light-emitting diodes. Thinning the Ag electrode to <−10 nm thickness (ultrathin Ag) is an approach to overcome these problems. This ultrathin Ag electrode has a high OT, while providing comparable sheet resistance similar to indium tin oxide. As the OT of the electrode increases, the cavity is weakened, so the spectral width of the emission and the angular color stability are increased.
TL;DR: In this paper, two CdTe and two copper indium gallium (di)selenide (CIGS)-type modules were tested for potential-induced degradation (PID) with positive and negative 1000-V biases applied to the active cell circuit in an 85°C, 85% relative humidity environmental chamber.
Abstract: Two CdTe and two copper indium gallium (di)selenide (CIGS)-type modules were tested for potential-induced degradation (PID) with positive and negative 1000 V biases applied to the active cell circuit in an 85°C, 85% relative humidity environmental chamber. Various degradation mechanisms could be seen with signatures such as shunting, transparent conductive oxide (TCO) corrosion, charge carrier lifetime reduction, and dead active layer at edges along with resulting cell mismatch. All modules tested exhibited degradation by system voltage stress in chamber, but only one module type has degraded in parallel field tests. I−V curve data indicated that one CdTe-type module sequentially exhibited shunting followed by a recovery and then series resistance losses. This module type showed TCO delamination from the glass in the environmental chamber tests and also exhibited power degradation within 5 weeks in field tests. Relative rates of Coulomb transfer from the voltage-biased active cell circuit to ground are compared for the modules in chamber tests to those placed outdoors under system voltage stress to extrapolate the anticipated time to failure in the field. This analysis correctly indicated which module type failed in the field first.
TL;DR: A spectral fluid filter for potential use in hybrid photovoltaic/thermal concentratings solar collectors has been developed, targeting maximum absorption above and transmission below a desired wavelength as mentioned in this paper.
Abstract: University of Tulsa, 800 S. Tucker Drive, Tulsa, Oklahoma 74104, United StatesAbstract. A spectral fluid filter for potential use in hybrid photovoltaic/thermal concentratingsolar collectors has been developed, targeting maximum absorption above and transmissionbelow a desired wavelength. In this application, the temperature-dependent bandgap of thepotential solar cell is used in the optimization of the filter. Dispersing a mix of colloidal nano-particles in a heat transfer fluid is shown to absorb 86% of sub-bandgap insolation while absorb-ing only 18% above bandgap insolation. Transmission above bandgap light would be directlyabsorbed into the photovoltaic (PV) cell while absorbedphotons transfer energydirectly into theheat transfer fluid ultimately reducing the number of heat transfer steps. Placement of a filter infront of the PV cell is shown to decrease losses by converting an additional 2% of the total solarenergy into thermal energy since it allows recollection of light reflected off the receiver.
TL;DR: In this article, a versatile solution-processing method called "Spray+brush" (SB) has been adopted to achieve a power-conversion efficiency of 3.52% for pure organometal halide perovskite devices.
Abstract: Perovskite compounds have the potential to transform photovoltaics technology, as they are easy to fabricate, have better stability, and possess superior power conversion efficiency. In this research, a versatile solution-processing method called “spray+brush” (SB) has been adopted to achieve a power-conversion efficiency of 3.52% for pure organometal halide perovskite devices. It has been observed that this method is more efficient and cost effective than the perovskite devices fabricated by spray (1.95%) and brush (1.17%) methods alone. The SB method of solution processing can be promising for various other organic coatings.
TL;DR: In this paper, light-trapping structures are fabricated on crystalline (100) silicon (c-Si) surfaces by picosecond (ps) laser irradiation followed by chemical etching.
Abstract: Light-trapping structures are fabricated on crystalline (100) silicon (c-Si) surfaces by picosecond (ps) laser irradiation followed by chemical etching. First, 1064-nm ps laser scanning is used to form micropore arrays on c-Si. The ps laser processing causes little reconsolidation of the silicon surface, which is beneficial to achieve precise etching. Control of the laser scanning interval, number of scans, and etching time gives a micro-nano hierarchical structure. In this hierarchical structure, the average diameter of the micropores is 25 to 30 μm, while the size of the finer nanostructures on the micropore inner walls ranges from dozens to hundreds of nanometers. Unlike traditional laser texturing techniques, the whole laser process is carried out without mask and photolithography. The reflectance of the c-Si surface with a micro-nano hierarchical structure is as low as 6% in the wavelength range from 400 to 1000 nm without coating. Moreover, the samples also show good hydrophobicity. This is a potential method to fabricate economical antireflective structures that are ideal for applications in c-Si solar cells and self-cleaning c-Si microelectronic devices.
TL;DR: A chip-scale package with conformal and uniform structures for white light-emitting diodes used in lighting and backlight unit (BLU) applications is presented in this article.
Abstract: This article presents a chip-scale package (CSP) with conformal and uniform structures for white light-emitting diodes used in lighting and backlight unit (BLU) applications. The CSP structures produce higher light extraction efficiency and lower assembly-dependent packaging compared with conventional surface-mounted devices (SMDs). Simulation results show that compared with SMDs, the luminous efficiency of CSP structures is 8.81% higher in lighting applications and 9.43% higher in BLU applications. This is likely due to light loss in the light bowl of the SMDs. Moreover, CSPs with a conformal phosphor structure exhibit low assembly dependence and redundancy, and rb-CSPs with a conformal structure are a more effective light source in both lighting and BLU applications.
TL;DR: In this article, the authors highlight major breakthroughs in photonics for energy conversion and conservation and highlight the role of light in solar light harvesting for electrical and thermal power generation; chemical and fuel generation; as well as photonic sensors for energy applications.
Abstract: In celebration of the 2015 International Year of Light, we highlight major breakthroughs in photonics for energy conversion and conservation. The section on energy conversion discusses the role of light in solar light harvesting for electrical and thermal power generation; chemical energy conversion and fuel generation; as well as photonic sensors for energy applications. The section on energy conservation focuses on solid-state lighting, flat-panel displays, and optical communications and interconnects.
TL;DR: In this article, the authors investigate a modeling approach that allows for the determination of com- plex refractive indices of bulk-heterojunctions by superimposing the optical models of the respective fullerenes and polymers.
Abstract: The knowledge of the complex refractive indices of all thin layers in organic solar cells (OSCs) is a prerequisite for comprehensive optical device simulations that are particularly important for sophisticated device architectures, such as tandem OSCs. Therefore, refractive indices are often determined via spectroscopic ellipsometry and subsequent time-consuming modeling. Here, we investigate a modeling approach that allows for the determination of com- plex refractive indices of bulk-heterojunctions by superimposing the optical models of the respective fullerenes and polymers. The optical constants of neat (6,6)-phenyl C71-butyric acid methyl ester (PC71BM), poly{(4,4'-bis(2-ethylhexyl)dithieno(3,2-b;2',3'-d)silole)- 2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT) and poly(2,6-(4,4-bis(2-ethylhexyl)- 4H-cyclopenta(2,1-b;3,4-b'')dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT) are determined, covering the OSC relevant spectral region from 250 to 1,000 nm. Then the blends PSBTBT∶PC71BM and PCPDTBT∶PC71BM are described within an effective medium approxi- mation. From this approximation, the mass density ratio of polymer and fullerene can be derived. This approach furthermore allows for a uniaxial anisotropic optical description of the polymers and provides insight into thin-film morphology. In contrast to x-ray diffraction experiments, this method also allows for probing amorphous materials. Spectroscopic ellipsometry can be a valuable tool for the investigation of bulk-heterojunction morphologies of the latest high- performance OSC materials that exhibit a low degree of crystallinity. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or repro- duction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JPE.5.057204)
TL;DR: In this paper, the authors use a commercial technology-computer-aided-design (TCAD) software, Silvaco Atlas, to design and model plasmonic gold nanoparticles integrated in optoelectronic device models of thin-film amorphous silicon (a-Si:H) PV cells.
Abstract: Rapid prototyping of photovoltaic (PV) cells requires a method for the simultaneous simulation of the optical and electrical characteristics of the device. The development of nanomaterial enabled PV cells only increases the complexity of such simulations. Here, we use a commercial technology-computer-aided-design (TCAD) software, Silvaco Atlas, to design and model plasmonic gold nanoparticles integrated in optoelectronic device models of thin film amorphous silicon (a-Si:H) PV cells. Upon illumination with incident light, we simulate the optical and electrical properties of the cell simultaneously, and use the simulation to produce current-voltage (J-V) and external quantum efficiency (EQE) plots. Light trapping due to light scattering and localized surface plasmon resonance interactions by the nanoparticles has resulted in the enhancement of both the optical and electrical properties due to the reduction in the recombination rates in the photoactive layer. We show that the device performance of the modeled plasmonic a-Si:H PV cells depends significantly on the position and size of the gold nanoparticles, which leads to improvements either in optical properties only, or in both optical and electrical properties. The model provides a route to optimize the device architecture, by simultaneously optimizing the optical and electrical characteristics, which leads to a detailed understanding of plasmonic PV cells from a design perspective and offers an advanced tool for rapid device prototyping.
TL;DR: In this article, the authors demonstrate how three substituents and structural permutations within an Aa residue widely alter its reduction potential, and demonstrate how a set of Aa residues can serve as building blocks for molecular electrets with widely diverse electronic properties, presenting venues for bottom-up designs.
Abstract: © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE). The diversity of life on Earth is made possible through an immense variety of proteins that stems from less than a couple of dozen native amino acids. Is it possible to achieve similar engineering freedom and precision to design electronic materials? What if a handful of nonnative residues with a wide range of characteristics could be rationally placed in sequences to form organic macromolecules with specifically targeted properties and functionalities? Referred to as molecular electrets, dipolar oligomers and polymers composed of non-native aromatic beta-amino acids, anthranilamides (Aa) provide venues for pursuing such possibilities. The electret molecular dipoles play a crucial role in rectifying charge transfer, e.g., enhancing charge separation and suppressing undesired charge recombination, which is essential for photovoltaics, photocatalysis, and other solar-energy applications. A set of a few Aa residues can serve as building blocks for molecular electrets with widely diverse electronic properties, presenting venues for bottom-up designs. We demonstrate how three substituents and structural permutations within an Aa residue widely alter its reduction potential. Paradigms of diversity in electronic properties, originating from a few changes within a basic molecular structure, illustrate the promising potentials of biological inspiration for energy science and engineering.
TL;DR: In this paper, the effect of a secondary donor on the PDTSTTz/PCBM system was investigated by adding different proportions of poly(3-hexylthiophene-2,5-diyl) (P3HT).
Abstract: In this investigation, photovoltaic (PV) parameters improvements in poly[2,6-(4,4 '-bis( 2-ethylhexyl) dithieno[3,2-b:2',3'-d]silole)-alt-5,5'-(3,6-bis[4-(2-ethylhexyl) thienyl-2-yl]-s-tetrazine] and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PDTSTTz: PCBM) blend were made through solvent additives and secondary donor addition. Short carbon chain solvent additives such as iodomethane (IMe), iodoethane (IEt), iodobutane (IBu), and diiodomethane (DIMe) were used. The results have shown that the short circuit current density (J(sc)), as well as power conversion efficiency (PCE) of PDTSTTz: PCBM blend cell increased with the increase in length of carbon chains of the additives. IBu, with relatively the longest carbon chain, has better performance-improving impact than IMe (with the shortest carbon chain). The same trend was observed for fill factor. The other PV parameter, open circuit voltages (V-oc), did not show significant change following these solvent additives. The effect of a secondary (additional) donor on the PDTSTTz/PCBM system was investigated by adding different proportions of poly(3-hexylthiophene-2,5-diyl) (P3HT). Because P3HT (secondary donor) and PDTSTTz (hosting donor) complement each other in light absorption, the PDTSTTz/PCBM system containing an optimum proportion of P3HT could provide a wider range of light absorption, and as a result it exhibited a higher short circuit current (11.08 mA/cm(2)) and then a PCE of 2.42%. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
TL;DR: In this article, two π-conjugated acceptor-donor-acceptor-conditional acceptor type oligothiophenes were designed and synthesized with thienothiadiazole (TT) as the core and dicyanovinyl (DCV) as terminal acceptor groups for comprehensively investigating and understanding structure-property relationships.
Abstract: Two π-conjugated acceptor-donor-acceptor-donor-acceptor-type (A-D-A-D-A) oligothiophenes, TT-(2T-DCV-Hex)2 and BT-(2T-DCV-Hex)2 were designed and synthesized with thienothiadiazole (TT) or benzothiadiazole (BT) as the core and dicyanovinyl (DCV) as the terminal acceptor groups for comprehensively investigating and understanding structure–property relationships. The resulting oligomers were first characterized by thermal analysis, UV-Vis spectroscopy, and cyclic voltammetry. By simply changing the BT to TT core in these two oligothiophenes, the highest occupied molecular orbital levels were varied from −5.55 eV for BT-(2T-DCV-Hex)2 to −5.11 eV for TT-(2T-DCV-Hex)2, and the optical band gaps were varied from 1.72 eV for BT-(2T-DCV-Hex)2 to 1.25 eV for TT-(2T-DCV-Hex)2, ascribed to the stronger electron accepting character of the TT core. However, the power conversion efficiency of bulk heterojunction organic solar cells (OSCs) with TT-(2T-DCV-Hex)2 as donor and [6,6]-phenyl C70-butyric acid methyl ester (PC71BM) as acceptor was measured to be 0.04% only, which is much lower than that of BT-(2T-DCV-Hex)2:PC71BM (1.54%). Compared to the TT-(2T-DCV-Hex)2 system, the BT-(2T-DCV-Hex)2 based device shows smoother film surface morphology, and superior charge generation and charge carrier mobilities. Therefore, the results clearly demonstrate that in addition to modifying the alkyl side chains and π-bridge lengths, the design of new small molecules for high-performance OSCs should also aim to choose suitable acceptor units.
TL;DR: In this paper, the effect of metal nanoparticles on the reflectance of silicon wafers was investigated and it was shown that a dielectric spacer layer between the NPs and the semiconductor is required to achieve a net reduction of reflection.
Abstract: Metal nanoparticles (NPs) can increase the absorption of light within semiconductors and hence improve the efficiency of solar cells. We experimentally investigate the effect that gold and silver NPs have on the reflectance of silicon wafers. The NPs are fabricated using the low cost, large area technique of thermal dewetting. We show that a dielectric spacer layer between the NPs and the semiconductor is required to achieve a net reduction of reflection. Furthermore, the optimum thickness of the spacer layer is found to be independent of NP size and metal type.
TL;DR: In this paper, a solution-grown ZnO nanowire film is characterized using scanning electron microscopy, electron dispersive x-ray, and optical absorption studies, and the electrical properties of these structures are analyzed from currentvoltage (I−V) characteristics.
Abstract: Preparation of n-ZnO/p-Si heterostructures using solution-synthesized ZnO nanowire films and their photovoltaic characterization is reported. The solution-grown ZnO nanowire film is characterized using scanning electron microscope, electron dispersive x-ray, and optical absorption studies. Electrical and photovoltaic properties of the fabricated heterostructures are studied using e-beam-evaporated aluminum as metal contacts. In order to use transparent contact and to simultaneously collect the photogenerated carriers, sandwich-type solar cells were fabricated using ZnO nanorod films grown on p-silicon and indium tin oxide (ITO) coated glass as ITO/n-ZnO NR/p-Si. The electrical properties of these structures are analyzed from current-voltage (I−V) characteristics. ZnO nanowire film thickness-dependent photovoltaic properties are also studied. Indium metal was also deposited over the ZnO nanowires and its effects on the photovoltaic response of the devices were studied. The results demonstrated that all the samples exhibit a strong rectifying behavior indicating the diode nature of the devices. The sandwich-type ITO/n-ZnO NR/p-Si solar cells exhibit improved photovoltaic performance over the Al-metal-coated n-ZnO/p-Si structures. The indium deposition is found to show enhancement in photovoltaic behavior with a maximum open-circuit voltage (Voc) of 0.3 V and short-circuit current (Isc) of 70×10−6 A under ultraviolet light excitation.