The widespread adoption of solution-processed halide perovskites LEDs demands to surpass the luminous efficacies of conventional LEDs while having full control over the emission properties.
In this talk, we report directional and enhanced emission in a green perovskite LED through hybrid photonic-plasmonic modes. We employ advanced optical modelling powered by genetic algorithms to find the best combination of materials and structures compatible with the fabrication of efficient LEDs. The experimental realisation of the optimum designs allows us to show low-temperature processed devices with enhanced emission and fine control over the emission angle. This methodology is versatile and easily integrable in cost-effective LEDs across the whole visible spectrum, opening avenues for their application in displays and light sources where the angular dispersion of light is crucial.
Despite extensive research on near-infrared organic light-emitting diodes (OLEDs), the external quantum efficiency (EQE) of these devices are far lower than devices with visible light emission. Recently, doublet fluorescent emission from organic radicals has emerged as a new route to more efficient light-emitting devices than those using established non-radical organic emitters. Charge recombination in radical devices results in doublet excitons with nanosecond emission and avoids the efficiency limit usually associated with singlets and triplets. For the application of the organic radicals to near-infrared electroluminescence, the novel near-infrared radical emitter showing around 800 nm emission was designed. Using the organic radical, not only higher than 5% EQE was attained but also the efficiency roll-off and operational lifetime were substantially improved in addition to decreasing turn-on and driving voltage significantly.
Tandem solar cells (TSCs) based on solution-processable semiconductors, including metal-halide perovskites and organic materials, show great promise for overcoming the Shockley-Queisser efficiency limit at low cost. However, difficulty in obtaining low-bandgap (<1.1 eV) perovskite and organic absorbers restricts the spectral range of solarenergy conversion, limiting the possibility of reaching ultrahigh efficiencies. Here we carry out detailed balance limit computations for a wide range of solution-processable materials in combination with a standard perovskite top-cell. Theoretical efficiency of 43% has been calculated for a tandem cell with a bandgap combination of 1.55 eV (perovskite) and 1.0 eV (bottom-cell material) under 1-sun illumination. We find that radiative coupling between the subcells contributes substantially (>11% absolute gain) to the ultimate efficiency via photon recycling. We emphasize the significance of using materials with high luminescence quantum efficiencies to benefit from this important effect. Initial laboratory demonstration of monolithic TSCs operating in the radiative-coupling regime is currently underway.
Multiple exciton generation (MEG) - a process in which multiple charge-carrier pairs are generated from a single optical excitation – is a promising way to improve the photocurrent in photovoltaic devices and offers the potential of breaking the Shockley-Queisser limit. It remains, however, challenging to harvest charge-carrier pairs generated by MEG in working solar cells. Initial yields of additional carrier pairs may be reduced due to ultra-fast intraband relaxation processes, which compete with MEG at early times. Quantum dots of materials, which display reduced carrier cooling rates (e.g. PbTe)[1] or one-dimensional nanostructures (e.g. nano rods)[2] which accelerate the carrier multiplication process are therefore promising candidates to increase the impact of MEG in photovoltaic devices. Here we show that both theorised strategies can lead to solar cells, which produce extractable charge carrier pairs with an external quantum efficiency above 120%, and we estimate an internal quantum efficiency exceeding 150%. Resolving the charge carrier kinetics on the ultra-fast timescale with pump-probe transient absorption and pump-push-photocurrent measurements, we identify a delayed cooling effect above the experimentally- determined threshold energy for MEG[1].
Polymer blends allow control of microstructure in donor-acceptor photovoltaic devices. Here we present measurements of devices containing polyfluorene blend layers of different thicknesses, and we are able to extract characteristic transport lengths for electrons and holes. We also present analytical and numerical modeling of single-layer and bilayer photovoltaic devices, which demonstrates the importance of bound polaron pairs formed after the initial electron transfer from donor to acceptor. Field-assisted dissociation of these polaron pairs is a critical process in determining device performance.
We study the processes of charge transfer and recombination at the interface between semiconductor nanoparticles and conjugated polymers. These processes are crucial in determining the performance of photovoltaic devices based on these materials. Using femtosecond transient absorption we are able to follow the charge separation on picosecond timescales in blends of spherical CdSe nanocrystals with a poly(p-phenylenevinylene) derivative. Charge separation occurs on timescales of greater than 15 ps, indicating that it is limited by the diffusion of excitons to the nanoparticle interface. We also use time-resolved photoluminescence and quasi-steady-state photoinduced absorption measurements to study the vertical structure in films containing conjugated polymers and semiconductor tetrapods. Finally, we demonstrate that use of slow-evaporating solvents allows the formation of fibrilar structures in poly(3-hexylthiophene) films, and that this is correlated with improved performance in photovoltaic devices containing poly(3-hexylthiophene) and CdSe nanorods.
The photoluminescence (PL)-, electroluminescence (EL)- and conductivity ((sigma) )-detected magnetic resonance of poly(p-phenylene vinylene) (PPV), poly(p-phenylene ethynylene) (PPE), and PPV/CN-PPV LEDs is reviewed and discussed. In the PPV- and PPE-based LEDs the polaron resonance is EL-quenching, but in the PPV/CN-PPV bilayer diodes it contains both EL-quenching and enhancing components. While the (sigma) -detecting polaron resonance is invariably quenching in the PPV devices, in some of the PPE-based LEDs it is (sigma) -enhancing. The PL-enhancing resonance is attributed to nonradiative recombination of trapped polaron pairs, which reduces their population and consequently the rate at which they nonradiatively quench singlet excitons; the EL-enhancing resonance is tentatively assigned to the same mechanism in the CN-PPV layer, but other mechanisms are not ruled out. Interchain coupling, some defects induced by structural disorder, and sites adjacent to dopant molecules (e.g., C60) apparently enhance the generation of these trapped polarons as well as intersystem crossing from the singlet to the triplet manifold. The EL- and (sigma) - quenching resonances are attributed to the fusion of like-charged free polarons to bipolarons, which is also suspected to be induced by disorder and/or impurities. The LEDs also exhibit half-field EL- and (sigma) -detected triplet exciton resonances. Triplet-triplet fusion to singlets and the role of triplets as quenching sites for singlet excitons are discussed as possible mechanisms leading to the triplet resonances.
Simple light emitting diodes can be constructed using fluorescent organic materials. Conjugated polymers can be used both for charge transport and for light emission. It is considered necessary for maximum device efficiency to balance the rates of electron and hole injection. We report the synthesis of a poly(cyanoterephthalylidenene) that was designed to exhibit an increased electron affinity. Electrochemical measurements showed a significant shift in the oxidation and reduction potentials due to the cyano functionality. The use of this polymer in a range of electroluminescent devices is described. Internal quantum efficiencies of up to 4% can be achieved in a bilayer device using stable electrode materials. The route used to synthesize this polymer is amenable to considerable variation in the subunits employed. This allows tuning of both the band-gap and the electron affinity of the resulting polymer.
We have constructed electroluminescent diodes using several layers of conjugated polymers with differing energy gaps; these provide a range of different color light-emitting layers and can be used to control charge injection and transport. Poly(1,4-phenylenevinylene), PPV, and derivatives have been used, with indium tin oxide as hole-injecting electrode and calcium as electron-injecting electrode. For this selection of materials, we show that the sequence of the polymer layers allows control of the color of device emission. Emission from more than one layer can be produced simultaneously. The position and breadth of the light-emitting region of the device provides information about the mechanisms of charge transport and of exciton motion. Various models for multilayer emission are discussed in the paper.
In this paper we discuss the principles of operation of polymer electroluminescent devices, and identify the factors which limit device efficiency. We identify how efficiencies can be improved by careful control of the polymer system, and by the use of multilayer structures to confine holes within the device. Using these techniques we can achieve efficiencies of better than 1% photons per electron in devices based on poly(p-phenylenevinylene). We also describe the use of induced absorption techniques to identify the excited states present within an electroluminescent device and to estimate their concentrations.
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