42 research outputs found
Germanium-lead perovskite light-emitting diodes.
Reducing environmental impact is a key challenge for perovskite optoelectronics, as most high-performance devices are based on potentially toxic lead-halide perovskites. For photovoltaic solar cells, tin-lead (Sn-Pb) perovskite materials provide a promising solution for reducing toxicity. However, Sn-Pb perovskites typically exhibit low luminescence efficiencies, and are not ideal for light-emitting applications. Here we demonstrate highly luminescent germanium-lead (Ge-Pb) perovskite films with photoluminescence quantum efficiencies (PLQEs) of up to ~71%, showing a considerable relative improvement of ~34% over similarly prepared Ge-free, Pb-based perovskite films. In our initial demonstration of Ge-Pb perovskite LEDs, we achieve external quantum efficiencies (EQEs) of up to ~13.1% at high brightness (~1900 cd m-2), a step forward for reduced-toxicity perovskite LEDs. Our findings offer a new solution for developing eco-friendly light-emitting technologies based on perovskite semiconductors
High Circular Polarization of Electroluminescence Achieved via Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films.
We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor-acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices
Control of Interface Defects for Efficient and Stable Quasi-2D Perovskite Light-Emitting Diodes Using Nickel Oxide Hole Injection Layer.
Metal halide perovskites (MHPs) have emerged as promising materials for light-emitting diodes owing to their narrow emission spectrum and wide range of color tunability. However, the low exciton binding energy in MHPs leads to a competition between the trap-mediated nonradiative recombination and the bimolecular radiative recombination. Here, efficient and stable green emissive perovskite light-emitting diodes (PeLEDs) with an external quantum efficiency of 14.6% are demonstrated through compositional, dimensional, and interfacial modulations of MHPs. The interfacial energetics and optoelectronic properties of the perovskite layer grown on a nickel oxide (NiO x ) and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate hole injection interfaces are investigated. The better interface formed between the NiO x /perovskite layers in terms of lower density of traps/defects, as well as more balanced charge carriers in the perovskite layer leading to high recombination yield of carriers are the main reasons for significantly improved device efficiency, photostability of perovskite, and operational stability of PeLEDs
Efficient and ultra-stable perovskite light-emitting diodes
Perovskite light-emitting diodes (PeLEDs) have emerged as a strong contender
for next-generation display and information technologies. However, similar to
perovskite solar cells, the poor operational stability remains the main
obstacle toward commercial applications. Here we demonstrate ultra-stable and
efficient PeLEDs with extraordinary operational lifetimes (T50) of 1.0x10^4 h,
2.8x10^4 h, 5.4x10^5 h, and 1.9x10^6 h at initial radiance (or current
densities) of 3.7 W/sr/m2 (~5 mA/cm2), 2.1 W/sr/m2 (~3.2 mA/cm2), 0.42 W/sr/m2
(~1.1 mA/cm2), and 0.21 W/sr/m2 (~0.7 mA/cm2) respectively, and external
quantum efficiencies of up to 22.8%. Key to this breakthrough is the
introduction of a dipolar molecular stabilizer, which serves two critical roles
simultaneously. First, it prevents the detrimental transformation and
decomposition of the alpha-phase FAPbI3 perovskite, by inhibiting the formation
of lead and iodide intermediates. Secondly, hysteresis-free device operation
and microscopic luminescence imaging experiments reveal substantially
suppressed ion migration in the emissive perovskite. The record-long PeLED
lifespans are encouraging, as they now satisfy the stability requirement for
commercial organic LEDs (OLEDs). These results remove the critical concern that
halide perovskite devices may be intrinsically unstable, paving the path toward
industrial applications.Comment: This is a preprint of the paper prior to peer review. New and updated
results may be available in the final version from the publishe
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The role of photon recycling in perovskite light-emitting diodes
Abstract: Perovskite light-emitting diodes have recently broken the 20% barrier for external quantum efficiency. These values cannot be explained with classical models for optical outcoupling. Here, we analyse the role of photon recycling (PR) in assisting light extraction from perovskite light-emitting diodes. Spatially-resolved photoluminescence and electroluminescence measurements combined with optical modelling show that repetitive re-absorption and re-emission of photons trapped in substrate and waveguide modes significantly enhance light extraction when the radiation efficiency is sufficiently high. In this manner, PR can contribute more than 70% to the overall emission, in agreement with recently-reported high efficiencies. While an outcoupling efficiency of 100% is theoretically possible with PR, parasitic absorption losses due to absorption from the electrodes are shown to limit practical efficiencies in current device architectures. To overcome the present limits, we propose a future configuration with a reduced injection electrode area to drive the efficiency toward 100%
Blue-Green Color Tunable Solution Processable Organolead Chloride-Bromide Mixed Halide Perovskites for Optoelectronic Applications.
Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425-570 nm). This was accomplished by controlling the halide composition of CH3NH3Pb(BrxCl1-x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride-bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH3NH3PbCl3 perovskite LED with a narrow emission FWHM of 5 nm.We acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme (Cambridge) for the Physics of Sustainability. Support from the Deutsche Forschungsgemeinschaft (NIM Excellence Cluster) is gratefully acknowledged. A.S. acknowledges the funding and support from the Indo-UK APEX project. F.D. acknowledges funding and support from a Herchel Smith fellowship. M.D.V. acknowledges funding and support from the ERC-StG 337739-HIENA. A.S. thanks Dr. D. Di for the insightful discussions. P. D. gratefully acknowledges support from the European Union in the form of a Marie Curie Intra-European fellowship.This is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/acs.nanolett.5b0236
Lead-Free Perovskite Semiconductors Based on Germanium-Tin Solid Solutions:Structural and Optoelectronic Properties
Solar
cells and optoelectronics based on lead halide perovskites
are generating considerable interest but face challenges with the
use of toxic lead. In this study, we fabricate and characterize lead-free
perovskites based on germanium and tin solid solutions, CH<sub>3</sub>NH<sub>3</sub>Sn<sub>(1–<i>x</i>)</sub>Ge<sub><i>x</i></sub>I<sub>3</sub> (0 ≤ <i>x</i> ≤
1). We show that these perovskite compounds possess band gaps from
1.3 to 2.0 eV, which are suitable for a range of optoelectronic applications,
from single junction devices and top cells for tandems to light-emitting
layers. Their thermodynamic stability and electronic properties are
calculated for all compositions and agree well with our experimental
measurements. Our findings demonstrate an attractive family of lead-free
perovskite semiconductors with a favorable band-gap range for efficient
single-junction solar cells
High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes
Perovskite-based optoelectronic devices have gained significant attention due
to their remarkable performance and low processing cost, particularly for solar
cells. However, for perovskite light-emitting diodes (LEDs), non-radiative
charge carrier recombination has limited electroluminescence (EL) efficiency.
Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting
record-high external quantum efficiencies (EQEs) exceeding 20%, and an EL
half-life of 46 hours under continuous operation. This performance is achieved
with an emissive layer comprising quasi-2D and 3D perovskites and an insulating
polymer. Transient optical spectroscopy reveals that photogenerated excitations
at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps.
The dominant component of the photoluminescence (PL) is primarily bimolecular
and is characteristic of the 3D regions. From PL quantum efficiency and
transient kinetics of the emissive layer with/without charge-transport
contacts, we find non-radiative recombination pathways to be effectively
eliminated. Light outcoupling from planar LEDs, as used in OLED displays,
generally limits EQE to 20-30%, and we model our reported EL efficiency of over
20% in the forward direction to indicate the internal quantum efficiency (IQE)
to be close to 100%. Together with the low drive voltages needed to achieve
useful photon fluxes (2-3 V for 0.1-1 mA/cm2), these results establish that
perovskite-based LEDs have significant potential for light-emission
applications