Function Of Active Layer Thickness Research Articles

Thickness Optimization and Photovoltaic Properties of Bulk Heterojunction Solar Cells Based on PFB–PCBM Layer

We report on the fabrication and study of bulk heterojunction (BHJ) solar cells based on a novel combination of a donor–acceptor poly(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N0-diphenyl)-N,N′di(p-butyl-oxy-pheyl)-1,4-diamino-benzene) (PFB) and [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) blend composed of 1:1 by volume. indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate (PEDOT:PSS)/PFB–PCBM/Ag BHJ solar cells are fabricated by a facile cost-effective spin-coating technique. The thickness of the active film (PFB–PCBM) plays an important role in the efficiency of light absorption, exciton creation, and dissociation into free charges that results in higher power conversion efficiency (PCE). In order to optimize the PCE as a function of active layer thickness, a number of solar cells are fabricated with different thicknesses of PFB–PCBM films at 120, 140, 160, 180, and 200 nm, and their photovoltaic characteristics are investigated. It is observed that the device with a 180 nm thick film demonstrates a maximum PCE of 2.9% with a fill factor (FF) of 53% under standard testing conditions (STC) (25 °C, 1.5 AM global, and 100 mW/cm2). The current–voltage (I-V) properties of the ITO/PEDOT:PSS/PFB–PCBM/Ag BHJ devices are also measured in dark conditions to measure and understand different parameters of the heterojunction. Atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) absorption spectroscopy for the PFB–PCBM film of optimal thickness (180 nm) are carried out to understand the effect of surface morphology on the PCE and bandgap of the blend, respectively. The AFM micrographs show a slightly non-uniform and rough surface with an average surface roughness (Ra) of 29.2 nm. The UV-vis measurements of the PFB–PCBM blend exhibit a reduced optical bandgap of ≈2.34 eV as compared to that of pristine PFB (2.88 eV), which results in an improved absorption of light and excitons generation. The obtained results for the ITO/PEDOT:PSS/PFB–PCBM (180 nm)/Ag BHJ device are compared with the ones previously reported for the P3HT–PCBM blend with the same film thickness. It is observed that the PFB–PCBM-based BHJ device has shown two times higher open circuit voltage (Voc) and, hence, enhanced the efficiency.

On the utilization of supercapacitor electrode materials

Representative supercapacitor electrode materials: activated carbon for an electrochemical double layer capacitor, MgCo2O4 showing redox, i.e. non-pseudocapacitive, and MnO2 showing pseudocapacitive behavior have been examined and compared regarding mass utilization as a function of active layer thickness, i.e. mass loading, of rate of electrode potential change and current density. Conclusions regarding degree of utilization and rational design of active material layer thickness are highlighted, suggestions for suitable reporting of data for rational comparison of results are offered.

Inverting the Handedness of Circularly Polarized Luminescence from Light-Emitting Polymers Using Film Thickness.

The emission of circularly polarized light is central to many applications, including data storage, optical quantum computation, biosensing, environmental monitoring, and display technologies. An emerging method to induce (chiral) circularly polarized (CP) electroluminescence from the active layer of polymer light-emitting diodes (polymer OLEDs; PLEDs) involves blending achiral polymers with chiral small-molecule additives, where the handedness/sign of the CP light is controlled by the absolute stereochemistry of the small molecule. Through the in-depth study of such a system we report an interesting chiroptical property: the ability to tune the sign of CP light as a function of active layer thickness for a fixed enantiomer of the chiral additive. We demonstrate that it is possible to achieve both efficient (4.0 cd/A) and bright (8000 cd/m2) CP-PLEDs, with high dissymmetry of emission of both left-handed (LH) and right-handed (RH) light, depending on thickness (thin films, 110 nm: gEL = 0.51, thick films, 160 nm: gEL = -1.05, with the terms "thick" and "thin" representing the upper and lower limits of the thickness regime studied), for the same additive enantiomer. We propose that this arises due to an interplay between localized CP emission originating from molecular chirality and CP light amplification or inversion through a chiral medium. We link morphological, spectroscopic, and electronic characterization in thin films and devices with theoretical studies in an effort to determine the factors that underpin these observations. Through the control of active layer thickness and device architecture, this study provides insights into the mechanisms that result in CP luminescence and high performance from CP-PLEDs, as well as demonstrating new opportunities in CP photonic device design.

Performance Dependence of Organic Solar Cells as a Function of Active Layer Thickness, Luminous Intensity and Temperature of Thermal Annealing

Performance Dependence of Organic Solar Cells as a Function of Active Layer Thickness, Luminous Intensity and Temperature of Thermal Annealing

Thickness-dependent internal quantum efficiency of narrow band-gap polymer-based solar cells

Active layer thickness-dependent internal quantum efficiencies (IQE) of photocurrent within bulk heterojunction organic photovoltaic (OPV) devices were characterized. These active layers consisted of an electron-donating narrow band-gap polymer and an electron-accepting fullerene derivative. In order to calculate IQE spectra as a function of active layer thickness, incident photon-to-current conversion efficiency (IPCE) spectra and internal absorptance spectra of active layers with various thicknesses were estimated in these solar cell devices. The transfer matrix method (TMM) was used to calculate the internal absorptance spectra of active layers by using experimental optical constants of thin layers typical for these types of OPV devices including narrow band-gap polymer blend film. In addition, spatially resolved absorptance spectra were used to analyze obtained IPCE spectra as well as OPV device parameters (e.g., short circuit current density) at various active layer thicknesses. Finally, charge-carrier-collecting probability as a function of active layer thickness was suggested with which the relationship between initial exciton generation and final power conversion efficiency can be more quantitatively described.

A model for studying the performance of P3HT:PCBM organic bulk heterojunction solar cells

In the study of bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM), P3HT/PCBM device performance is strongly depending on the thickness of active region, materials in use and fabrication methods. In such devices, optoelectronic behaviors such as charge carrier generation and recombination, photocurrent generation and charge transport mechanism are different in devices of different fabrication methods. An electrical model accounting for study of the performance of P3HT:PCBM organic bulk heterojunction solar cells is developed to achieve the device parameter for high performance of the polymer-fullerene bulk heterojunction solar cells. In this model, by solving the drift–diffusion equations by considering the boundary condition and uniform potential energy, the effects of the active region thickness, potential energy and photocurrent generation on the performance of P3HT:PCBM bulk heterojunction solar cell have been studied. Simulated current–voltage characteristics as a function of active layer thickness reveal relatively good agreement between the model's predictions and published modeling and experimental reports.

Influence of active region thickness on the performance of bulk heterojunction solar cells: electrical modeling and simulation

In the study of bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM), P3HT/PCBM device performance is strongly depends on the thickness of active region. In such devices, optoelectronic behaviors such as charge carrier generation and recombination, photocurrent generation and charge transport mechanism is different in devices of different thickness. An electrical model accounting for study the influences of active layer thickness on the device performance, is developed to achieve the device parameter for high performance of the polymer–fullerene bulk heterojunction solar cells. In this model, by solving the drift–diffusion equations by considering the boundary condition and uniform potential energy, the effects of the active region thickness on the performance of P3HT/PCBM bulk heterojunction solar cell has been studied. By using this model, we were able to show that the thickness dependence of electrical characteristics is responsible for a reduction of the overall performance of the solar cells. Simulated current–voltage characteristics as a function of active layer thickness reveals relatively good agreement between the model’s predictions and published modeling and experimental reports.

Highly efficient ITO-free polymer solar cells based on metal resonant microcavity using WO3/Au/WO3 as transparent electrodes

Indium tin oxide (ITO)-free polymer solar cells (PSCs) with the structure of Glass/tungsten trioxide (WO3)/Au/WO3/PCDTBT: PC70BM/LiF/Al was fabricated and studied. The multilayer structure of WO3/Au/WO3 is used as the potential transparent electrode to replace ITO. Metal resonant microcavity, which can enhance light harvesting of active layers, was constructed between Au and Al electrodes. According to the J–V and IPCE characterization with 70nm active layer, power conversion efficiency (PCE) of the ITO-free microcavity device is approaching 4.55%, which is higher than that of the ITO-based device. However, PCE of the ITO-free device is much lower than that of the ITO-based device when the thickness of active layer increases to 130nm. The opposite experimental tendency leads to theoretical research toward the simulation of light absorption and optical electric field and the calculation of maximum short circuit current density (Jsc max) as a function of active layer thickness based on ITO-free and ITO-based devices. The research results show that microcavity effect is closely linked to intrinsic absorption of active layers.

Quantum Efficiency of Organic Solar Cells: Electro-Optical Cavity Considerations

Organic solar cells (OSCs) are composed of one or more layers of order 100 nm thickness sandwiched between metallic and transparent electrodes. As such, they are low finesse, multilayer optical cavities where the optical field distribution is governed by the complex refractive indices and thicknesses of all layers in the “solar cell stack”. Optical interference and parasitic absorbance in nonactive layers can have a dramatic effect on the shape of the measured external quantum efficiency (EQE), the parameter often used to optimize device structure and derive critical insight regarding charge generation and extraction. In this communication, we study a model high efficiency OSC system (PCDTBT/PC70BM) as a function of active layer thickness, blend composition and processing. The spectral shapes of the measured EQEs show strong thickness and blend ratio dependence. However, when correctly determined, the internal quantum efficiencies (IQEs) are spectrally flat. The differences in EQE spectral shape predomina...

Spectral aspects of cavity tuned absorption in organic photovoltaic films

Concentration of light and infrared capture are two favored approaches for increasing the power conversion efficiency (PCE) of photovoltaic devices. Using optical transfer matrix formalism, we model the absorption of organic photovoltaic films as a function of active layer thickness and incident wavelength. In our simulations we consider the absorption in the optical cavity formed by the polymer bulk heterojunction active layer (AL) between the aluminum cathode and indium tin oxide (ITO) anode. We find that optical absorption can be finely tuned by adjusting the ITO thickness within a relatively narrow range, thus eliminating the need for a separate optical spacer. We also observe distinct spectral effects due to frequency pulling which results in enhanced long-wavelength absorption. Spectral sculpting can be carried out by cavity design without affecting the open circuit voltage as the spectral shifts are purely optical effects. We have experimentally verified aspects of our modeling and suggest methods to improve device design.

Light/negative bias stress instabilities in indium gallium zinc oxide thin film transistors explained by creation of a double donor

The analysis of current-voltage (I-V) and capacitance-voltage (C-V) characteristics for amorphous indium gallium zinc oxide Thin film transistors as a function of active layer thickness shows that negative bias under illumination stress (NBIS) is quantitatively explained by creation of a bulk double donor, with a shallow singly ionized state ε(0/+) > EC-0.073 eV and a deep doubly ionized state ε(++/+) < EC-0.3 eV. The gap density of states, extracted from the capacitance-voltage curves, shows a broad peak between EC–E = 0.3 eV and 1.0 eV, which increases in height with NBIS stress time and corresponds to the broadened transition energy between singly and doubly ionized states. We propose that the center responsible is an oxygen vacancy and that the presence of a stable singly ionized state, necessary to explain our experimental results, could be due to the defect environment provided by the amorphous network.

Short-circuit current density and spectral response modelling of bulk-heterojunction solar cells

A general model to calculate short circuit current and spectral response of the bulk-heterojunction solar cell is discussed here. In order to get the short circuit current density at each wavelength the generation rate of electron and holes are calculated by the optical modelling based on transfer matrix approach. The drift–diffusion equation is then solved to get the current density at each wavelength using the calculated generation rate. Spectral response is found to be very similar to the experimental data of (3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methylester (PCBM) based bulk-heterojunction solar cell. The variation of short circuit current as a function of active layer thickness has been calculated using the model which matches with the experimental observations taken from literature. The sub-linear dependence of short circuit current on incident light intensity has also been explained by the model.

Optimization of thickness and morphology of active layer for high performance of bulk-heterojunction organic solar cells

A two-dimensional model accounting for the effect of both the morphology of active layer and the layer configuration of multilayer-structured device on the device performance, is developed to optimize the device parameter for high performance of the bulk-heterojunction organic solar cells. When the photovoltaic properties of poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester solar cells are calculated as functions of device parameters by using the model, it is found that the optimum thickness of active layer is 80 nm and that the domain size in active layer is about 6 nm. Comparison of simulation with experiment reveals that the simulated short-circuit current as a function of active layer thickness is very consistent with experimental one.

Design of light-trapping microscale-textured surfaces for efficient organic solar cells

Organic photovoltaic (OPV) cells suffer from low charge carrier mobilities of polymers, which renders it important to achieve complete optical absorption in active layers thinner than optical absorption length. Active layers conformally deposited on light-trapping, microscale textured, grating-type surfaces is one possible approach to achieve this objective. In this report, we analyze the design of such grating-type OPV cells using finite element method simulations. The energy dissipation of electromagnetic field in the active layer is studied as a function of active layer thickness, and pitch and height of the underlying textures. The superiority of textured geometry in terms of light trapping is clearly demonstrated by the simulation results. We observe 40% increase in photonic absorption in 150 nm thick active layer, for textures with 2 microm pitch and 1.5 microm height.

Interplay between Energetic and Kinetic Factors on the Ambient Stability of n-Channel Organic Transistors Based on Perylene Diimide Derivatives

The effects of the interplay between energetic and kinetic factors on the air stability of n-channel organic thin-film transistors (OTFTs) were studied using two perylene diimide (PDI) compounds with distinctly different lowest unoccupied molecular orbital (LUMO) levels. On the basis of the empirical energy level windows, one compound (N,N′-bis(2,2,3,3,4,4,5,5,5-nonafluoropentyl)-3,4:9,10-tetracarboxylic acid diimide (PDI-F): −3.84 eV) is at the onset region for air stability, whereas the other (N,N′-bis(cyclohexyl)-1,7-dicyano-perylene-3,4:9,10-tetracarboxylic acid diimide (PDI-CN2): −4.33 eV) is in the air-stable region. Charge-transport behaviors under an inert atmosphere and in air were investigated as a function of active layer thickness. Charge transport in air was greatly affected by the active layer thickness for both compounds, an effect that has been overlooked so far. The ambient stability of the air-unstable PDI-F TFTs increased significantly for thicknesses over ∼10 monolayers (ML). Surprisingly, the previously considered “air-stable” PDI-CN2 TFTs were not stable in air if the active layer thickness was less than ∼4 ML. The molecular packing and orientation of the PDI thin films were investigated using grazing incidence X-ray diffraction (GIXD) and near-edge X-ray absorption fine structure (NEXAFS). We found that the minimum thickness required for air stability is closely related to the LUMO level, film morphology, and film growth mode.

Design of 3-dimensional Textured Organic Solar Cells

AbstractOrganic materials are attractive for photovoltaics primarily because of the prospect of high throughput solution-processible manufacturing using roll-to-roll or spray deposition. In the formation of polymer based photovoltaic devices, the aspect that limits the power conversion efficiency is the bottleneck between short diffusion lengths of the excited states (excitons) in polymers, in the range of 10-20 nm. Optical absorption length, which is in the range of 50-200 nm and much larger than the exciton diffusion length, poses the limit on charge generation and collection. It is important to achieve complete optical absorption in active layers much thinner than optical absorption length to minimize losses due to recombination of charge carriers. Previously, light trapping techniques have been coupled with organic solar cell but without significant success. In this paper, three-dimensional sub micron grating structure is analyzed using finite element method (FEM) simulations for finding the optical absorption in different layers of solar cell to optimize the photonic concentrator effect of the grating structure. The energy dissipation of electromagnetic field in the active layer is studied as a function of active layer thickness, grating pitch and height. The superiority of grating structure in terms of light trapping feature as compared to planar geometry is clearly demonstrated by simulation results.

Modeling optical effects and thickness dependent current in polymer bulk-heterojunction solar cells

Device characteristics of polymer based bulk-heterojunction photovoltaic cells incorporating poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] and methanofullerene ([6,6]-phenyl C61-butyric acid methyl ester) as the active materials are examined as a function of active layer thickness. The dependence of short circuit current on optical effects and its oscillatory variation on the polymer layer thickness is explained by solving the short circuit current using the drift-diffusion equations, where the light intensity calculated from the optical transfer matrix theory is used as the input for optical carrier generation. Furthermore, the effects of polymer layer thickness on other device operation parameters such as open-circuit voltage, fill factor, and series resistivity are measured. Considering the variation of above mentioned parameters, an optimized power conversion efficiency as high as 1.8% (under simulated air mass 1.5 global conditions) was achieved for a device with a polymer layer thickness of 55nm.

The effect of active layer thickness and composition on the performance of bulk-heterojunction solar cells

At present, bulk heterojunction polymer solar cells are typically fabricated with an active layer thickness of between 80 and 100nm. This active layer thickness has traditionally been chosen based on convenience and empirical results. However, a detailed study of the effects that active layer thickness has on the short circuit current and efficiency has not been performed for bulk heterojunction polymer solar cells so far. We demonstrate that the performance of these devices is highly dependent on the active layer thickness and, using a well established model for optical interference, we show that such effects are responsible for the variations in performance as a function of active layer thickness. We show that the ideal composition ratio of the donor and acceptor materials is not static, but depends on the active layer thickness in a predictable manner. A comparison is made between solar cells comprised of the donor materials regioregular poly(3-hexylthiophene) and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene) with the acceptor [6, 6]-phenyl C61-butyric acid methyl ester to show that our results are not material specific and that high efficiency solar cells can be fabricated with active layer thickness greater than 100nm for both material mixtures. Finally, a device with an active layer thickness of 225nm is fabricated with a power efficiency of 3.7% under AM1.5 illumination at an intensity of 100mW∕cm2.

High magnetization exchange-couple double-layer TbFeCo for magnetic flux reading optical recording

High magnetization (Mr) and enhanced coercivity magneto-optical (MO) recording media TbFeCo (TFC) were investigated for hybrid magnetic flux reading/optical recording due to their large magnetic flux. To increase the stability of high Mr TFC media, dual-layer disks composed of TM-rich readout layer with saturation magnetization (MS)=370 emu/cm/sup 3/ and RE-rich memory layer were fabricated. Coercivity and interface energy between double layers as a function of active layer thickness were examined. By simulating thermo-magnetic writing, the dual-layer media were shown to be applicable for optical writing/magnetic flux reading recording.

Dielectric Properties of Fine‐Grained Barium Titanate Based X7R Materials

The next generations of multilayer ceramic capacitors are expected to have active ceramic layer thicknesses of 3 μm or less. To retain a reliable ceramic device, it is advantageous to engineer a uniform and fine‐grained microstructure. In this study, a dense ceramic (>96% of theoretical) with fine‐grained size (<0.2 μm), and permittivity of 2800, meeting the EIA X7R temperature characteristic has been obtained using hydrothermally synthesized BaTiO3 powder with precipitation‐coated additives. The temperature coefficient of capacitance (TCC) and the dissipation factor (DF) are investigated as a function of active layer thickness and compared with that of a conventional coarse‐grained dielectric of similar composition and fired density. The fine‐grained ceramic maintained X7R characteristic in active layers as thin as 3 μm under 1 VAC and 1 kHz test conditions. For equivalent active thickness, the DF of the fine‐grained version was consistently lower than the coarse‐grained material. These results indicate that an improved voltage coefficient is possible for finer‐grained materials under otherwise equivalent conditions. The possible causes for this behavior are also discussed.