Quantum Efficiency Of Devices Research Articles

Out-Coupling Efficiency Improvement in Organic Light-Emitting Diodes with Inverted Pyramid Glass Substrates

The external quantum efficiency of organic light-emitting diodes (OLEDs) was suppressed by the wave guide mode in the glass substrate and organic layers. In this paper, a mechanism is developed to simulate the optical luminous field for OLEDs based on inverted pyramid substrate structure. Monte Carlo method was used to optimize the structural parameters to enhance the external quantum efficiency of device by changing the substrate structure. A considerable enhancement in the extraction efficiency of the OLED is expected theoretically, and near 42% out-coupling efficiency was achieved in experiment without affecting the electroluminescent spectrum.

Waveguide photon-number-resolving detectors for quantum photonic integrated circuits

Quantum photonic integration circuits are a promising approach to scalable quantum processing with photons. Waveguide single-photon-detectors (WSPDs) based on superconducting nanowires have been recently shown to be compatible with single-photon sources for a monolithic integration. While standard WSPDs offer single-photon sensitivity, more complex superconducting nanowire structures can be configured to have photon-number-resolving capability. In this work, we present waveguide photon-number-resolving detectors (WPNRDs) on GaAs/Al0.75Ga0.25As ridge waveguides based on a series connection of nanowires. The detection of 0–4 photons has been demonstrated with a four-wire WPNRD, having a single electrical read-out. A device quantum efficiency of ∼24% is reported at 1310 nm for the transverse electric polarization.

Figure of merit based maximization of the quantum efficiency of (single-wall-carbon-nanotubes/n-type silicon) hybrid photovoltaic devices

We report on a rational approach to optimize the photovoltaic (PV) properties of devices based on the hetero-nanojunctions formed between single wall carbon nanotubes (SWCNTs) films and n-silicon. By qualifying the optoelectronic properties of the SWCNT film through a figure of merit (FoM), we were able to correlate the latter to both the external quantum (EQE) and power conversion (PCE) efficiencies of associated PV devices. The established correlation guided us to achieve EQE values as high as ∼55%. Furthermore, it is found that higher FoM figures (≥3 × 10−6 Ω−1) lead to higher EQE and PCE values (with an increase of 15% and 2% per decade, respectively). Finally, by optimizing the EQE of the SWCNTs based PV devices and further doping them, we have achieved PCE values as high as ∼4%.

Integrated autocorrelator based on superconducting nanowires

We demonstrate an integrated autocorrelator based on two superconducting single-photon detectors patterned on top of a GaAs ridge waveguide. This device enables the on-chip measurement of the second-order intensity correlation function g(2)(τ). A polarization-independent device quantum efficiency in the 1% range is reported, with a timing jitter of 88 ps at 1300 nm. g(2)(τ) measurements of continuous-wave and pulsed laser excitations are demonstrated with no measurable crosstalk within our measurement accuracy.

Progress of efficiency enhancement of organic light-emitting diodes via surface plasmon

The surface plasmon polariton (SPP), exists at the interface of metal and media, where the optical field and the free electron can interact with each other and results in an oscillations of collective electrons. On one hand, a localized surface plasmon (LSP) can be formed on the surface of metal nanoparticles, and then the quantum yield of fluorescent molecules near the surface metal can be enhanced by the LSP. Therefore, many efforts have been paid to add metal nanoparticles into organic light-emitting diode (OLED) to improve its performance. On the other hand, in a conventional device, SPP can’t emit light to free space because the wave vector of it doesn’t match to that of free light, and then dissipates as heat. By changing the morphology of the metal surface, such as grating structure, the energy of SPP can be coupled into free light field, and the external quantum efficiency of device can be enhanced. It has been widely concerned on SPP to enhance the efficiency of OLED. This article reviews the progress on the following two aspects. Firstly, enhancing the rate of radiative transition of fluorescent molecules by using LSP of metal nanoparticles, which can enhance the internal quantum efficiency of organic light-emitting diodes; Secondly, emiting a light via matching the wave vector of SPP to that of free light with a grating structure of periodicity or unperiodicity, which can enhance the external quantum efficiency of OLED.

Dependence of hole and electron current density of mixed host devices on mixed host composition

The hole and electron current densities of mixed host emitting layer were studied according to the composition of the mixed host to investigate the origin of high quantum efficiency of the mixed host devices. Hole only and electron only devices of mixed host emitting layer were fabricated and the current density of single charge devices was studied. The hole current density was greatly affected by the mixed host composition, while the electron current density was not greatly changed by the mixed host composition. Therefore, the change of hole current density by mixed host composition was critical to the device performances of the mixed host devices.

Identifying the efficient inter-conversion between singlet and triplet charge-transfer states by magneto-electroluminescence study

Using the magneto-electroluminescence (MEL) as a tool, we demonstrated the efficient inter-conversion between singlet and triplet charge-transfer (CT) states in exciplex-based organic light-emitting diodes (OLEDs). Results show that the MEL of exciplex-based device is larger than that of exciton-based device by a factor of 3.2. The emission of exciplex-based devices comes from the direct intermolecular electron-hole pair recombination and their spin exchange energy is much smaller, which causes the efficient inter-conversion between singlet and triplet states. This argument was supported by the consistent evolutions of the MEL and EL spectra versus applied bias and donor concentrations. Finally, the bandgap effects on the MEL as well as the external quantum efficiency of exciplex-based devices were discussed. Our findings of MEL may offer a feasible way to unravel underlying mechanisms that limit the EL efficiency in the OLEDs.

Subnanosecond charge photogeneration and recombination in polyfluorene copolymer-fullerene solar cell: Effects of electric field

Influence of electric field on the subnanosecond charge photogeneration dynamics in the polymer solar cell based on polyfluorene copolymer BisDMO-PFDTBT blended with PC(61)BM was examined with transient absorption spectroscopy. The charge dynamics showed no difference under short- or open-circuit conditions and under a forward bias of 0.79 V (1.6 × 10(5) V/cm), implying negligible field effects on the subnanosecond dynamics of charge photogeneration/recombination. However, under the reverse biases of -2 V (4.0 × 10(5) V/cm) and -5 V (1.0 × 10(6) V/cm), significant enhancement of charge photogeneration and apparent suppression of polaron pair recombination were observed, which agrees with the field-assisted enhancement of external quantum efficiency of the solar cell devices.

Optimization of the electroluminescence from SiNx-based light-emitting devices by modulating the size and morphology of silver nanostructures

A maximal enhancement of ~6.5 times of the external quantum efficiency (EQE) for SiNx-based light-emitting devices (LEDs) is achieved by magnetron sputtering a silver nanostructures layer onto the active matrix. The enhancement of EQE is affected by the dimension and morphology of silver nanostructures, which can be controlled by the sputtering time and the post treatment of rapid thermal annealing. The optimal size of silver nanostructures is about 100 nm in diameter by comparing the integrated electroluminescence intensity under the same input power. The optimization of EQE for SiNx-based LEDs is discussed by considering the contributions of the enhancement of light-extraction efficiency induced by the surface roughening of the front electrode, internal quantum efficiency due to the coupling between excitons and localized surface plasmons, and carrier injection efficiency. Our work may provide an alternative approach for the fabrication of Si-based light sources with promising luminescence efficiency.

Indium-tin-oxide free transparent electrodes using a plasmon frequency conversion layer

Transparent electrodes to enhance external quantum efficiency (EQE) in optoelectronic devices are proposed based on the suppression of surface plasmons (SPs) at the metal–dielectric (or metal–organic) interface using a frequency conversion layer. Plasmonic absorption at metal-based electrodes causes severe optical losses in the planar stacks of optoelectronic devices. Even though Ag is suitable for transparent electrodes owing to its lowest absorption coefficient compared to other metals, the surface plasmon resonant frequency (SPRF) of Ag is located in the visible region (i.e., ωSP ∼ 3.9 eV, λSP = 500–550 nm). Thus, incident light is absorbed by surface plasmon resonance (SPR) at the interface between Ag and dielectric materials. These plasmonic resonances could be dramatically suppressed by adding a 2 nm-thick Al interlayer with resonance frequency out of the visible region (i.e., ωSP ∼ 15 eV, λSP = 250–300 nm), which results in an extreme enhancement of the optical transmittance of Ag-based electrodes from 68% to 91% at 470 nm. These approaches for highly transparent and conductive multilayer stacks are applicable to universal optoelectronics because they are straightforward, cost-effective and reliable even in large area fabrication.

Mesoscopic kinetic Monte Carlo modeling of organic photovoltaic device characteristics

Measured mobility and current-voltage characteristics of single layer and photovoltaic (PV) devices composed of poly${$9,9-dioctylfluorene-co-bis[$N,{N}^{\ensuremath{'}}$-(4-butylphenyl)]bis($N,{N}^{\ensuremath{'}}$-phenyl-1,4-phenylene)diamine$}$ (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) have been reproduced by a mesoscopic model employing the kinetic Monte Carlo (KMC) approach. Our aim is to show how to avoid the uncertainties common in electrical transport models arising from the need to fit a large number of parameters when little information is available, for example, a single current-voltage curve. Here, simulation parameters are derived from a series of measurements using a self-consistent ``building-blocks'' approach, starting from data on the simplest systems. We found that site energies show disorder and that correlations in the site energies and a distribution of deep traps must be included in order to reproduce measured charge mobility-field curves at low charge densities in bulk PFB and F8BT. The parameter set from the mobility-field curves reproduces the unipolar current in single layers of PFB and F8BT and allows us to deduce charge injection barriers. Finally, by combining these disorder descriptions and injection barriers with an optical model, the external quantum efficiency and current densities of blend and bilayer organic PV devices can be successfully reproduced across a voltage range encompassing reverse and forward bias, with the recombination rate the only parameter to be fitted, found to be $1\ifmmode\times\else\texttimes\fi{}{10}^{7}$ s${}^{\ensuremath{-}1}$. These findings demonstrate an approach that removes some of the arbitrariness present in transport models of organic devices, which validates the KMC as an accurate description of organic optoelectronic systems, and provides information on the microscopic origins of the device behavior.

Quantum chemical characterization and design of host materials based on phosphine oxide-substituted (triphenylamine) fluorene for (deep) blue phosphors in OLEDs

We studied the electronic structures of a series of fluorene derivatives (p/mPODPFs and p/mPOAPFs) using density functional theory calculations and investigated their performances as host materials in organic light-emitting diodes from three aspects, i.e. triplet energy, ability of charge injection from neighboring organic layer or electrode, and match of the hosts and the reference guests (FIrpic and FCNIr) for efficient energy transfer (EF). Especially for the last aspect, the singlet/triplet (S(1)/T(1)) energies as well as the simulated host emission and guest absorption spectra are investigated to predict the possible emission mechanisms in the host-guest system and therefore to pursue the most suitable host for (deep) blue guest. From the investigated results, we deduced that pPODPF and pPOAPF are suitable for sky-blue FIrpic due to feasible Förster/Dexter energy transfers from pPODPF/pPOAPF to FIrpic, which agrees well with the experimental results. Furthermore, the higher external quantum efficiency (20.6%) of the pPOAPF-based device than that of the pPODPF-based device (13.2%) in experiments was inferred to be attributed to the matching S(1) energies between pPOAPF and FIrpic as well as good hole/electron injection abilities of pPOAPF in spite of a smaller overlap between the pPOAPF emission and FIrpic absorption spectra. By contrast, mPOAPF and mPODPF, designed in the work, may match with deep-blue FCNIr. In particular, mPOAPF may exhibit good performance as a host material for deep blue FCNIr as a consequence of its own balanced hole/electron injection ability and the matching S(1)/T(1) energies between mPOAPF and FCNIr.

Towards the development of a virtual organic solar cell: An experimental and dynamic Monte Carlo study of the role of charge blocking layers and active layer thickness

Monte Carlo (MC) simulations have been used to fully model organic solar cells. The quantum efficiency and short-circuit current of these virtual devices are in excellent agreement with experimental measurements. Simulations show that, contrary to expectation, indium tin oxide/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)/poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methylester (PCBM)/aluminium devices lack effective charge blocking layers at the electrode interfaces. X-ray photoelectron spectroscopy depth profiling shows that despite a PCBM-rich region near the cathode, interface intermixing at the electrodes combined with incomplete PCBM coverage leads to significant interface recombination. This work highlights the effectiveness of MC simulations as a predictive tool and emphasizes the need to control electrode interface processes.

The Importance of Fullerene Percolation in the Mixed Regions of Polymer–Fullerene Bulk Heterojunction Solar Cells

AbstractMost optimized donor‐acceptor (D‐A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ∼80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) with 7.3% PCE and find that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge‐carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.

Flexible Top-Emitting Organic Light-Emitting Diodes Using Semi-Transparent Multi-Metal Layers

In this paper, the improved device performance of top-emitting organic light-emitting diodes (TEOLEDs) with a thin multi-metal layer stack of nickel/silver/nickel (Ni/Ag/Ni) and aluminum/silver/aluminum (Al/Ag/Al) that were used as the anode and cathode on a flexible substrate is discussed. In particular, Indium-Tin-Oxide (ITO) as an anode electrode has been used recently even though it has some problems for flexible devices. Therefore we suggested that a thin multi-metal layer electrode as a new anode is fabricated instead of ITO anode. It was verified that the ITO-free TEOLEDs showed an enhanced probability of the recombination of the electrons and holes through an improved electron/hole charge balance. We also analyzed the optical and electrical characteristics using the current density, luminance, luminance efficiency, external quantum efficiency (EQE), CIE x, y coordinates, and EL spectra of flexible TEOLED devices were characterized. ITO-free, flexible, green-emitting OLEDs with a low cost and a simple process were demonstrated.

Direct synthesis of high-density lead sulfide nanowires on metal thin films towards efficient infrared light conversion

We report chemical-vapor-deposition (CVD) synthesis of high-density lead sulfide (PbS) nanowire arrays and nano pine trees directly on Ti thin films, and the fabrication of photovoltaic devices based upon the PbS nanowires. The as-grown nanowire arrays are largely vertically aligned to the substrates and are uniformly distributed over a relatively large area. Field effect transistors incorporating single PbS nanowires show p-type conduction and high mobilities. These catalytic metal thin films also serve as photocarrier collection electrodes and greatly facilitate device integration. For the first time, we have fabricated Schottky junction photovoltaic devices incorporating PbS nanowires, which demonstrate the capability of converting near-infrared light to electricity. The PbS nanowire devices are stable in air and their external quantum efficiency shows no significant decrease over a period of 3 months in air. We have also compared the photocurrent direction and quantum efficiencies of photovoltaic devices made with different metal electrodes, and the results are explained by band bending at the Schottky junction. Our research shows that PbS nanowires are promising building blocks for collecting near-infrared solar energy.

P‐116: Efficiency Enhancement in ITO‐free Green Organic Light Emitting Diodes Utilizing Nano‐Composite Scattering Films

AbstractWe demonstrate significant efficiency enhancement of ITO‐free OLEDs by combining both the high‐conductivity conducting polymer PEDOT: PSS as the transparent electrode and the solution‐processed nano‐composite scattering films as optical extraction layers. The external quantum efficiency (EQE) of ITO‐free device can be significantly enhanced by a factor of 1.8 at 1000 cd/m2.

Light Extraction from Organic Light Emitting Diodes Using Chemically Etched Glass Substrates

We have manufactured highly efficient OLED devices fabricated on chemically etched glass substrates. The external quantum efficiency of the OLED devices with the etched glass substrates was increased by 5-27% in comparison with the reference flat glass substrate. Surface morphology, such as indented patterns, significantly affected the external luminance efficiency. A clean surface and the presence of smooth bent edges of indented patterns were found to be important for improving the external luminous efficacy.

Increase in short-wavelength response of encapsulated CIGS devices by doping the encapsulation layer with luminescent material

Photovoltaic devices based on Cu(In,Ga)Se2 (CIGS) absorbers exhibit poor response to short-wavelength (λ) photons, mainly due to the parasitic absorption of the commonly used zinc oxide window and cadmium sulphide (CdS) buffer layers. In this work, it is shown that the energy content of these photons can be better harnessed if the encapsulation layer of modules is doped with luminescent materials. It is shown that the luminescent down-shifting (LDS) of short-λ photons increases the external quantum efficiency (EQE) of CIGS devices by up to 25% absolute in the range 300–380nm and by up to 40% absolute in the region 300–460nm, for devices with 50-nm- and 100-nm-thick CdS layers, respectively. Overall, the short-circuit current density (JSC) of CIGS devices covered with luminescent encapsulation improved by ΔJSC=0.6% for violet-doped EVA and up to ΔJSC=1.8% for a violet–yellow dye mixture. By experimenting with devices of different CdS thicknesses and short-λ EQE profiles, it is suggested that CIGS PV modules equipped with LDS layers must be optimised as a system and all photoactive components of the device be considered in parallel.

Voltage modulated electro-luminescence spectroscopy to understand negative capacitance and the role of sub-bandgap states in light emitting devices

Voltage modulated electroluminescence spectra and low frequency (≤100 kHz) impedance characteristics of red electroluminescent diodes under forward bias are investigated. Light emission under periodic voltage modulation tracks the onset of observed negative capacitance for each modulation frequency. Active participation of sub-bandgap defects including the shallower states in minority carrier recombination dynamics is sought to explain the results. The phenomenon of negative capacitance is understood as a necessary dielectric response to compensate any irreversible transient changes in the injected minority carrier reservoir due to radiative recombinations mediated by slowly responding sub-bandgap defects. Experimentally measured variations of the in-phase component of modulated electroluminescence spectra with forward bias levels and with modulation frequencies support the dynamic influence of these sub-bandgap states in the radiative recombination process. Predominant negative sign of the in-phase component of voltage modulated electroluminescence signal further confirms the bi-molecular nature of light emission. Effect of these states on the net density of minority carriers available for radiative recombination is discussed. These sub-bandgap states can even supress the external quantum efficiency of such devices under high frequency operation commonly used in optical communication.