Enhancement Of External Quantum Efficiency Research Articles

Enhanced outcoupling in flexible organic light-emitting diodes on scattering polyimide substrates

Abstract We demonstrate an upscalable approach to increase outcoupling in organic light-emitting diodes (OLEDs) fabricated on flexible substrates. The outcoupling enhancement is enabled by introducing a thin film of microporous polyimide on the backside of silver nanowire (AgNW) electrodes embedded in neat colorless polyimide. This porous polyimide film, prepared by immersion precipitation, utilizes a large index contrast between the polyimide host and randomly distributed air voids, resulting in broadband haze (>75%). In addition, the composite polyimide/AgNW scattering substrate inherits the high thermal (>360 °C), chemical, and mechanical stability of polyimides. The outcoupling efficiency of the composite scattering substrate is studied via optical characterization of the composite substrate and electron microscopy of the scattering film. The flexible scattering substrates compared to glass/indium tin oxide (ITO) allows for a 74% enhancement in external quantum efficiency (EQE) for a phosphorescent green OLED, and 68% EQE enhancement for a phosphorescent white OLED. The outcoupling enhancement remains unharmed after 5000 bending cycles at a 2 mm bending radius. Moreover, the color uniformity over viewing angles is improved, an important feature for lighting applications.

Enhancing the ultraviolet-visible-near infrared photovoltaic responses of crystalline-silicon solar cell by using aluminum nanoparticles

We report to apply Al nanoparticles (NPs) to enhance the photovoltaic response of crystalline- or c-Si solar cell from the ultraviolet (UV) throughout the visible and near infrared (NIR) regimes. Al NPs were induced by solid thermal annealing and embedded in a SiO2 layer that was to passivate the front side of solar cell. Upon the excitation of surface plasmons (SPs) on the Al NPs under light illumination, an enhancement of broadband absorption of the solar cell was observed. The incorporation of Al NPs led to a relative 13.8% increase in photoelectric conversion efficiency of c-Si solar cell, and an external quantum efficiency enhancement from the UV throughout the visible and NIR regimes. The improvement of c-Si solar cell performance was attributed to both effects of absorption and scattering by SPs.

Influence of precursor uniformity on the performance of Cu2ZnSnSxSe4 − x thin film solar cells prepared by the sputtering method

We report the effects of precursor uniformity on the efficiency of Cu2ZnSnSxSe4−x (CZTSSe) solar cells using sputtered precursors. The reduction of Sn cluster agglomeration in the Cu/Sn/Zn stacked precursor created synthesis of more uniformly distributed CZTSSe absorbers during the annealing process. The secondary phase difference between the 80W and 40W sputtered CZTSSe absorbers was confirmed with Raman analysis at 178cm−1 and 259cm−1. The 40W sputtered CZTSSe cell produced blue region enhancement of external quantum efficiency results due to improvement of the pn-junction properties. The CZTSSe 40W cells achieved a maximum power conversion efficiency of 5.7% and short-circuit current of 32.4mA/cm2.

Enhancement of external quantum efficiency and quality of heterojunction white LEDs by varying the size of ZnO nanorods

The size of ZnO nanorods (NRs) plays an important role in tuning the external quantum efficiency (EQE) and quality of light generated by white light emitting diodes (LEDs). In this work, we report on the enhancement of EQE and the quality of ZnO NR-based hetrojunction white LEDs fabricated on a p-GaN substrate using a low temperature solution method. Cathodoluminescence spectra demonstrate that ultraviolet (UV) emission decreases and visible deep band emission increases with an increase in the length of the ZnO NRs. The UV emission could be internally reabsorbed by the ZnO NR excitation, thus enhancing the emission intensity of the visible deep band. Photocurrent measurements validated the fact that the EQE depends on the size of ZnO NRs, increasing by 87% with an increase in the length of the ZnO NRs. Furthermore, the quality of white light was measured and clearly indicated an increase in the color rendering indices of the LEDs with an increase in the length of the ZnO NRs, confirming that the quality of light generated by LEDs can be tuned by varying the length of the ZnO NRs. These results suggest that the EQE and visible deep band emission from n-ZnONRs/p-GaN heterojunction LEDs can be effectively controlled by adjusting the length of the ZnO NRs, which can be useful for realizing tunable white LEDs.

Highly Efficient Organic Light‐Emitting Diode Using A Low Refractive Index Electron Transport Layer

A low refractive index electron transport layer (ETL) can be very effective in enhancing the out‐coupling efficiency of an organic light‐emitting diode (OLED). However, most organic films show a refractive index close to 1.8. In this work, it has been discovered that tris‐[3‐(3‐pyridyl)mesityl]borane (3TPYMB) has a low refractive index of 1.65 (at 550 nm), which is the lowest refractive index ETL among the commonly used ETLs up to date. Using 3TPYMB as an ETL, a solution processed OLED is demonstrated with nearly a 76% enhancement in external quantum efficiency (EQE). Optical simulation results of this study show that 59% of the enhancement comes from the low refractive index 3TPYMB, and the remaining 17% from the change in charge balance due to the 3TPYMB ETL in the OLED devices.

C70/C70:pentacene/pentacene organic heterojunction as the connecting layer for high performance tandem organic light-emitting diodes: Mechanism investigation of electron injection and transport

A high performance tandem organic light-emitting diode (OLED) is realized by employing a C70/C70:pentacene/pentacene organic heterojunction as the efficient charge generation layer (CGL). Not only more than two time enhancement of external quantum efficiency but also significant improvement in both power efficiency and lifetime are well achieved. The mechanism investigations find that the electron injection from the CGL to the adjacent electron transport layer (ETL) in tandem devices is injection rate-limited due to the high interface energy barrier between the CGL and the ETL. By the capacitance-frequency (C-F) and low temperature current density-voltage (J-V) characteristic analysis, we confirm that the electron transport is a space-charge-limited current process with exponential trap distribution. These traps are localized states below the lowest unoccupied molecular orbital edge inside the gap and would be filled with the upward shift of the Fermi level during the n-doping process. Furthermore, both the trap density (Ht) and the activation energy (Ea) could be carefully worked out through low temperature J-V measurements, which is very important for developing high performance tandem OLEDs.

Integrating nanostructured electrodes in organic photovoltaic devices for enhancing near-infrared photoresponse

We introduce a simple methodology to integrate prefabricated nanostructured-electrodes in solution-processed organic photovoltaic (OPV) devices. The tailored “photonic electrode” nanostructure is used for light management in the device and for hole collection. This approach opens up new possibilities for designing photonically active structures that can enhance the absorption of sub-bandgap photons in the active layer. We discuss the design, fabrication and characterization of photonic electrodes, and the methodology for integrating them to OPV devices using a simple lamination technique. We demonstrate theoretically and experimentally that OPV devices using photonic electrodes show a factor of ca. 5 enhancement in external quantum efficiency (EQE) in the near infrared region. We use simulations to trace this observed efficiency enhancement to surface plasmon polariton modes in the nanostructure.

Ag nanoparticles-embedded surface plasmonic InGaN-based solar cells via scattering and localized field enhancement.

Ag nanoparticles are embedded in intentionally etched micro-circle p-GaN holes by means of a thermal agglomeration process to enhance the light absorption efficiency in InGaN/GaN multi-quantum-well (MQW) solar cells. The Ag nanoparticles are theoretically and experimentally verified to generate the plasmon light scattering and the localized field enhancement near the MQW absorption layer. The external quantum efficiency enhancement at a target wavelength region is demonstrated by matching the plasmon resonance of Ag nanoparticles, resulting in a Jsc improvement of 9.1%. Furthermore, the Ag-nanoparticle-embedded InGaN solar cell is effectively fabricated considering the carrier extraction that more than 70% of F.F. and 2.2 V of high Voc are simultaneously attained.

N-acetyle cysteine assisted synthesis of core⿿shell Ag2S with enhanced light transmission and diminished reflectance: Surface modifier for c-SiNx solar cells

We report the synthesis of an amino acid viz. N-acetyle cysteine (NAC) assisted silver sulphide (Ag2S) nanoparticles by in situ reduction technique of silver nitrate (AgNO3) with sodium borohydride (NaBH4) in presence of NAC, where NAC acted as the sulphur donor as well as encapsulating agent. The prepared nanoparticles were characterized by transmission electron microscopy, scanning electron microscopy, atomic force microscopy, X-ray diffraction technique and dynamic light scattering technique. An enhancement was noticed in the current⿿voltage characteristics, with major reduction in reflectance when c-SiNx solar cells were coated with such nanoparticles. Broadband enhancement of external quantum efficiency (EQE) was also observed.

Corrugated Organic Light Emitting Diodes Using Low Tg Electron Transporting Materials

A corrugated organic light emitting diode (OLED) with enhanced light extraction is realized by incorporating a corrugated composite electron transport layer (ETL) consisting of two ETLs with different glass transition temperatures. The morphology of the corrugated structure is characterized with atomic force microscopy. The results show that the corrugation can be controlled by the layer thicknesses and annealing temperature. Compared with the control planar device, the corrugated OLED shows a more than 35% enhancement in current efficiency from 31 cd/A to 43 cd/A and a 20% enhancement in external quantum efficiency from 10% to 12% at 100 cd/m(2). In addition, the corrugated OLED also has a greatly improved operational stability. The LT90 lifetime of a device operated at 1000 cd/m(2) is improved greater than 100-fold in the corrugated OLED.

P-205L:Late-News Poster: Improvement of Viewing Angle Dependence and Out-Coupling Efficiency of OLEDs with Strong Microcavity Structure by Introduction of Nanoporous Polymer Films

To improve a viewing angle characteristic as well as a light extraction effect, we fabricated nanoporous polymer film (NPF) as a scattering medium as well as a light extraction component. We designed two types of organic light emitting diodes (OLEDs) with strong microcavity effect containing two different hole transport layer (HTL) thickness (e.g. 30 nm and 60 nm) to investigate each optical behavior of NPF. Very interestingly, we could observe a significant enhancement of external quantum efficiency (EQE) for each device [30 nm thick HTL : 18.0 %, 60nm thick HTL : 31.6 %] when we attached 125 um thick PET film coated with NPF. Furthermore, NPF successfully suppress the viewing angle dependence to realize ideal angular emission even though it's somehow different from that of Lambertian distribution.

Improvement of external quantum efficiency of silicide Schottky-barrier detectors in the 3 to 5 μ m waveband with subwavelength-grating incident plane

Silicide Schottky-barrier midwave infrared detectors for the 3 to 5 μ m waveband typically use a 2 to 8 nm metal-silicide layer grown on silicon substrates as an absorption layer. We demonstrate the use of an SiO 2 -film-coated subwavelength grating as an antireflection incident plane in such detectors to enhance absorption at the 3 to 5 μ m waveband in the metal-silicide layer for improving the external quantum efficiency (EQE). Taking a PtSi/ p -Si structure as an example, we fabricate samples and build a test platform to characterize the EQE of a PtSi/ p -Si Schottky-barrier detector. Simulation results show that low reflection efficiency ( for the subwavelength-grating incident plane and factors of 1.5 to 1.8 enhancement in absorption of the PtSi layer are achieved at normal incidence across the 3 to 5 μ m waveband. Measurement results show that this increase translates into factors of 1.3 to 1.7 enhancement in EQE. This improvement in EQE results from absorption enhancement due to antireflection effects and the forward scattering of incident infrared radiation in the subwavelength grating, which increases the effective optical path in the detector. The results also suggest that fabricating a subwavelength-grating incident plane is a general, low-cost method to enhance EQE in various infrared material platforms used for back-illuminated detectors.

Electrical-optical analysis of photonic crystals GaN-based high power light emitting diodes

Although photonic crystals (PhCs) have been used to enhance the light extraction efficiency (LEE) of light emitting diodes (LEDs) in a lot of researches, the effects of PhCs on the electrical and optical characteristics of GaN-based high power LEDs have been rarely analyzed simultaneously. Herein, the 3D finite-element method (FEM) was employed to investigate the current density distribution in the active layer of LEDs while the LEE was calculated based on the finite-difference time-domain method. Then the influences of several parameters, including the radius, the filling factor, and the etching depth of PhCs on the current density distribution and external quantum efficiency (EQE) of LEDs were examined. It was found that the configurations of PhCs had a significant influence on the electrical and optical performances of LEDs. With the optimized parameters, a 62 % enhancement of EQE and more uniform current density distribution have been obtained comparing to those of LEDs with planar ITO. These analysis results can facilitate the identification of the design rules for high power, high efficiency LEDs.

Inverted organic solar cells using a solution-processed TiO2/CdSe electron transport layer to improve performance

In the present work, cadmium selenide (CdSe) nanoparticles are deposited directly on TiO2 film to fabricate the TiO2/CdSe interlayer by a chemical bath deposition method. The inverted organic solar cells using poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction as an active layer and TiO2/CdSe interlayer as an electron transport layer (ETL) are fabricated in air. A series of microstructural, photo-electronic, and electrochemical characterizations on these cells are performed. The TiO2/CdSe structure with respect to either the TiO2 layer or the CdSe layer as the ETL exhibits significantly enhanced external quantum efficiency (EQE) in the visible region. The photoluminescence (PL) measurement shows that the exciton dissociation in the TiO2/CdSe structure is more effective than that in either the TiO2 or CdSe structure. The Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) implies that the charge recombination in the TiO2/CdSe structure can be suppressed with respect to that in either the CdSe or TiO2 structure. The photovoltaic performances of the cells with the TiO2/CdSe ETL are clearly improved compared with the reference cells only with the TiO2 layer or CdSe layer as the ETL.

Coupled optical and electrical study of thin-film InGaAs photodetector integrated with surface InP Mie resonators

High-index dielectric and semiconductor nanostructures with characteristics of low absorption loss and artificially controlled scattering properties have grasped an increasing attention for improving the performance of thin-film photovoltaic devices. In this work, combined optical and electrical simulations were performed for thin-film InP/In0.53Ga0.47As/InP hetero-junction photodetector with periodically arranged InP nano-cylinders in the in-coupling configuration. It is found that the carefully designed InP nano-cylinders possess strongly substrate-coupled Mie resonances and can effectively couple incident light into the guided mode, both of which significantly increase optical absorption. Further study from the electrical aspects shows that enhancement of external quantum efficiency is as high as 82% and 83% in the configurations with the optimized nano-cylinders and the optimized period, respectively. Moreover, we demonstrate that the integration of InP nano-cylinders does not degrade the electrical performance, since the surface recombination is effectively suppressed by separating the absorber layer where carriers generate and the air/semiconductor interface. The comprehensive modeling including optical and electrical perspectives provides a more practical description for device performance than the optical-only simulation and is expected to advance the design of thin-film absorber layer based optoelectronic devices for fast response and high efficiency.

Control of charge dynamics by blending ZnO nanoparticles with poly(3-hexylthiophene) for efficient hybrid ZnO nanorods/polymer solar cells

Photovoltaic performances of hybrid ZnO nanorods/polymer solar cells have been improved by controlling their charge dynamics through addition of ZnO nanoparticles into poly(3-hexylthiophene) (P3HT) photoactive layer. The inter-rod space of ZnO nanorod substrates is completely filled with the solution-processed ZnO nanoparticles/P3HT blends, forming homogeneous junction among the components. The optimum PCE of 1.020 % has been achieved from the device with 13 vol % ZnO nanoparticles loaded. The enhancement in external quantum efficiency has been also observed, indicating the improved excitons separation at the ZnO/P3HT interface. The information on charge dynamics in the system has been investigated by electrochemical impedance spectroscopy. It has been found that the additional space-charge layer formed at the ZnO nanoparticles–contact electrode interface is a reason behind the improvement of open-circuit voltage. Moreover, the formation of ZnO nanoparticles domain extending across the active layer and the percolation path for charge carriers promotes charge transport by reducing transit time of the carriers, extending charge carrier lifetime and enhancing the charge transfer at the ZnO/P3HT interface. Interestingly, it has been found that charge transport in the devices does not limit the device performances, even for the 400-nm-thick active layer.

Opto-Electrical Performance Improvement of Mono c-Si Solar Cells Using Dielectric–Metal–Dielectric (D-M-D) Sandwiched Structure-Based Plasmonic Anti-Reflector

Mono c-Si solar cells based on plasmonic anti-reflector, an ultra-thin silver (Ag) film sandwiched between silicon nitride (SiNx) layers on non-textured c-Si surface, have been explored. Prior to solar cell fabrication, the plasmonic anti-reflector structure was optimized separately by varying different thicknesses of top and bottom SiNx layers and the sandwiched ultra-thin Ag film. For broad wavelength range (300–1200 nm), minimum weighted reflectance was observed in the range of 8–9 %, for the top and bottom SiNx layers having 70–80-nm thickness. The refractive index of top and bottom SiNx layers was in the range of 1.77–1.80 and 1.92–1.96, respectively. A 180-μm non-textured mono c-Si solar cell is fabricated with plasmonic anti-reflector, having top and bottom SiNx layer thickness of 80-nm and 8–9-nm sandwiched ultra-thin Ag layer, resulted in 17 % solar cell efficiency. The measured efficiency was 1.8 % higher compared to 180-μm mono c-Si solar cell with standard 80 nm SiNx anti-reflector. The short circuit current density (J sc) extracted from external quantum efficiency (EQE) has shown 0.5 mA/cm2 enhancement for the dielectric–metal–dielectric (D-M-D) sandwiched structure (plasmonic anti-reflector)-based solar cell compared to standard SiNx anti-reflection coating (ARC) structure-based solar cell. EQE enhancement was seen for all wavelengths above 700 nm with maximum 25–26 % enhancement at around 1125 nm. Also, in UV wavelength region (300–400 nm), EQE enhancement was seen with maximum 25 % enhancement at around 370 nm. Possible phenomenon for improved anti-reflection and EQE in D-M-D-based plasmonic anti-reflector has been discussed.

Electroluminescence Efficiency Enhancement in Quantum Dot Light‐Emitting Diodes by Embedding a Silver Nanoisland Layer

A colloidal quantum dot light‐emitting diode (QLED) is reported with substantially enhanced electroluminescence by embedding a thin layer of Ag nanoislands into hole transport layer. The maximum external quantum efficiency (EQE) of 7.1% achieved in the present work is the highest efficiency value reported for green‐emitting QLEDs with a similar structure, which corresponds to 46% enhancement compared with the reference device. The relevant mechanisms enabling the EQE enhancement are associated with the near‐field enhancement via an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Ag nano­islands, which are found to lead to good agreement between the simulation results and the experimental data, providing us with a useful insight important for plasmonic QLEDs.

Effective light trapping by hybrid nanostructure for crystalline silicon solar cells

Ag nanoparticles (NPs), as etching catalyst, are applied onto the pyramid textured surface of the semi-finished single crystalline silicon solar cell by spraying a solution containing AgNO3 and sodium citrate. Upon chemical etching, nanopits are formed with the Ag NPs staying inside of the nanopits. Complete solar cells with and without the Ag-assisted etching are fabricated to study the effects of the hybrid nanostructure. It is found that the optical reflection is effectively reduced by the nanostructure and the incident light is harvested more effectively for enhanced external quantum efficiency (EQE) by as much as 17% for the cells before the SiNx anti-reflection coating, and the EQE can be further enhanced by applying an electric bias 1V during the EQE measurement. It is believed that when the solar cell with the Ag NPs is illuminated, the local surface plasmon resonance (LSPR) is induced at the specific wavelength regions around 405 and 810nm. For ultraviolet light, the LSPR effect dominates the increases of the EQE while the effective medium effect is also believed to be responsible for the effective light trapping.

Enhanced energy conversion of up-conversion solar cells by the integration of compound parabolic concentrating optics

Up-conversion (UC) is a promising approach to utilize sub-band-gap photons for solar cells (SCs). Due to the non-linear nature of UC, the optimal excitation power regimes between the solar cell semiconductor and the UC material correspond to a difference in solar concentration of more than an order of magnitude. This difference can be bridged with integrated optics by concentrating the photons transmitted through the solar cell to increase the power density and maximize the intensity of UC luminescence. To realize this, dielectric-filled compound parabolic concentrators (CPCs) were used as integrated optics on the rear side of a planar bifacial silicon solar cell together with a 25% Er3+ doped hexagonal sodium yttrium fluoride (β-NaYF4:Er) UC phosphor. An efficiency increase of 32% from 0.123% to 0.163% under sub-band-gap illumination is quantified by means of the first ever reported I–V characteristics for an up-conversion solar cell (UC-SC) based on c-Si. An enhancement in external quantum efficiency (EQE) is obtained from 1.33% for the non-concentrating reference UC-SC to 1.80% for a solar cell with integrated optics for an excitation at 1523nm with an irradiance of 0.024W/cm2, corresponding to a normalized EQE of 0.75W/cm2. This demonstrates that CPCs are suitable for UC-SC as they increase the concentration in the forwards direction, while maintaining high collection efficiency of the UC emission in the reverse direction. In addition, such an approach enables the optimization of the solar concentration on the UC phosphor independently from the concentration required for the solar cell.