Nathaniel J. L. K. Davis

Highly Efficient Perovskite Nanocrystal Light‐Emitting Diodes Enabled by a Universal Crosslinking Method

The preparation of highly efficient perovskite nanocrystal light-emitting diodes is shown. A new trimethylaluminum vapor-based crosslinking method to render the nanocrystal films insoluble is applied. The resulting near-complete nanocrystal film coverage, coupled with the natural confinement of injected charges within the perovskite crystals, facilitates electron-hole capture and give rise to a remarkable electroluminescence yield of 5.7%.

Lead Telluride Quantum Dot Solar Cells Displaying External Quantum Efficiencies Exceeding 120

Multiple exciton generation (MEG) in semiconducting quantum dots is a process that produces multiple charge-carrier pairs from a single excitation. MEG is a possible route to bypass the Shockley-Queisser limit in single-junction solar cells but it remains challenging to harvest charge-carrier pairs generated by MEG in working photovoltaic devices. Initial yields of additional carrier pairs may be reduced due to ultrafast intraband relaxation processes that compete with MEG at early times. Quantum dots of materials that display reduced carrier cooling rates (e.g., PbTe) are therefore promising candidates to increase the impact of MEG in photovoltaic devices. Here we demonstrate PbTe quantum dot-based 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 ultrafast time scale with pump–probe transient absorption and pump–push–photocurrent measurements, we identify a delayed cooling effect above the threshold energy for MEG.

Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120%

Multiple-exciton generation—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 to break the Shockley–Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation.

Size and Energy Level Tuning of Quantum Dot Solids via a Hybrid Ligand Complex

The performance of quantum dots (QDs) in optoelectronic devices suffers as a result of sub-bandgap states induced by the large fraction of atoms on the surface of QDs. Recent progress in passivating these surface states with thiol ligands and halide ions has led to competitive efficiencies. Here, we apply a hybrid ligand mixture to passivate PbSe QD sub-bandgap tail states via a low-temperature, solid-state ligand exchange. We show that this ligand mixture allows tuning of the energy levels and the physical QD size in the solid state during film formation. We hereby present a novel, postsynthetic path to tune the properties of QD films.

Star-shaped fluorene–BODIPY oligomers: versatile donor–acceptor systems for luminescent solar concentrators

Luminescent solar concentrators (LSCs) are waveguides doped with luminescent centers that can spectrally and spatially concentrate sunlight. They can reduce the cost of photovoltaic energy production and are attractive prospects for photobioreactors and building-integrated applications. Reabsorption, caused by non-zero overlap between the absorption and emission spectra of the light-emitting centers, often limits LSC efficiency. Donor–acceptor energy-transfer complexes are one method to mitigate reabsorption by shifting the emission away from the main absorption peak. Here we introduce versatile star-shaped donor–acceptor molecules based on a central BODIPY energy acceptor with oligofluorene donor side units. Varying the oligofluorene chain length alters the relative oscillator strengths of the donor and acceptor, changing the severity of reabsorption for a given donor density, but also changing the luminescence yield and emission spectrum. We performed comprehensive device measurements and Monte Carlo ray tracing simulations of LSCs containing three oligofluorene–BODIPY donor–acceptor systems with different oligofluorene chain lengths, and then extended the simulation to study hypothetical analogs with higher donor–acceptor ratios and different terminal acceptors. We found that the measured structures permit waveguide propagation lengths on a par with state-of-the-art nanocrystalline emitters, while the proposed structures are viable candidates for photobioreactor and energy production roles and should be synthesized.

Photon Reabsorption in Mixed CsPbCl3:CsPbI3 Perovskite Nanocrystal Films for Light-Emitting Diodes

Cesium lead halide nanocrystals, CsPbX3 (X = Cl, Br, I), exhibit photoluminescence quantum efficiencies approaching 100% without the core–shell structures usually used in conventional semiconductor nanocrystals. These high photoluminescence efficiencies make these crystals ideal candidates for light-emitting diodes (LEDs). However, because of the large surface area to volume ratio, halogen exchange between perovskite nanocrystals of different compositions occurs rapidly, which is one of the limiting factors for white-light applications requiring a mixture of different crystal compositions to achieve a broad emission spectrum. Here, we use mixtures of chloride and iodide CsPbX3 (X = Cl, I) perovskite nanocrystals where anion exchange is significantly reduced. We investigate samples containing mixtures of perovskite nanocrystals with different compositions and study the resulting optical and electrical interactions. We report excitation transfer from CsPbCl3 to CsPbI3 in solution and within a poly(methyl methacrylate) matrix via photon reabsorption, which also occurs in electrically excited crystals in bulk heterojunction LEDs.

Formation of Long-Lived Color Centers for Broadband Visible Light Emission in Low-Dimensional Layered Perovskites

We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study ⟨001⟩ oriented thin films of hexylammonium (HA) lead iodide, (C6H16N)2PbI4, and dodecylammonium (DA) lead iodide, (C12H28N)2PbI4, by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half-maximum of about 0.4 eV), emission band below 200 K, similar to emission from ⟨110⟩ oriented bromide 2D perovskites at room temperature. The lifetime of this sub-band-gap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DA2PbI4. DFT simulations of the HA2PbI4 crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to ⟨110⟩ oriented bromide 2D perovskites but a general property of this class of system, and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters.

Energy transfer in pendant perylene diimide copolymers

We report the synthesis, characterisation and polymerisation of two novel asymmetric perylene diimide acrylate monomers. The novel monomers form a sensitiser–acceptor pair capable of undergoing Forster resonance energy transfer, and were incorporated as copolymers with tert-butyl acrylate. The tert-butyl acrylate units act as spacers along the polymer chain allowing high concentrations of dye while mitigating aggregate quenching, leading to persistent fluorescence in the solid state at high concentrations of up to 0.3 M. Analysis of fluorescence kinetics showed efficient energy transfer between the optically dense sensitiser and the lower concentration acceptor luminophores within the polymer. This reduced reabsorption within the material demonstrates that the copolymer-scaffold energy transfer system has potential for use in luminescent solar concentrators.

Multiple exciton generation in quantum dot-based solar cells

Abstract Multiple exciton generation (MEG) in quantum-confined semiconductors is the process by which multiple bound charge-carrier pairs are generated after absorption of a single high-energy photon. Such charge-carrier multiplication effects have been highlighted as particularly beneficial for solar cells where they have the potential to increase the photocurrent significantly. Indeed, recent research efforts have proved that more than one charge-carrier pair per incident solar photon can be extracted in photovoltaic devices incorporating quantum-confined semiconductors. While these proof-of-concept applications underline the potential of MEG in solar cells, the impact of the carrier multiplication effect on the device performance remains rather low. This review covers recent advancements in the understanding and application of MEG as a photocurrent-enhancing mechanism in quantum dot-based photovoltaics.

Singlet Fission and Triplet Transfer to PbS Quantum Dots in TIPS-Tetracene Carboxylic Acid Ligands.

Singlet exciton fission allows for the generation of two triplet excitons for each photon absorbed within an organic semiconductor. Efficient harvesting of these triplets could allow for the Shockley-Queisser limit on the power conversion efficiency of single-junction photovoltaics to be broken. Here, we show that singlet fission molecules bound directly to PbS quantum dots as ligands can undergo singlet fission with near unity efficiency and can transfer triplets sequentially into the PbS with near unity efficiency. Within the PbS, the excitations recombine, giving rise of the emission of photons. This allows for the doubling of the quantum dot photoluminescence quantum efficiency when photons are absorbed by the singlet fission ligand, as compared to when directly absorbed in the quantum dot. Our approach demonstrates that it is possible to convert the exciton multiplication process of singlet fission into a photon multiplication process and provides a new path to harness singlet fission with photovoltaics.