Baodan Zhao

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.

High Open-Circuit Voltages in Tin-Rich Low-Bandgap Perovskite-Based Planar Heterojunction Photovoltaics

Low-bandgap CH3 NH3 (Pbx Sn1-x )I3 (0 ≤ x ≤ 1) hybrid perovskites (e.g., ≈1.5-1.1 eV) demonstrating high surface coverage and superior optoelectronic properties are fabricated. State-of-the-art photovoltaic (PV) performance is reported with power conversion efficiencies approaching 10% in planar heterojunction architecture with small (<450 meV) energy loss compared to the bandgap and high (>100 cm2 V-1 s-1 ) intrinsic carrier mobilities.

Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy

In band-like semiconductors, charge carriers form a thermal energy distribution rapidly after optical excitation. In hybrid perovskites, the cooling of such thermal carrier distributions occurs on timescales of about 300 fs via carrier-phonon scattering. However, the initial build-up of the thermal distribution proved difficult to resolve with pump–probe techniques due to the requirement of high resolution, both in time and pump energy. Here, we use two-dimensional electronic spectroscopy with sub-10 fs resolution to directly observe the carrier interactions that lead to a thermal carrier distribution. We find that thermalization occurs dominantly via carrier-carrier scattering under the investigated fluences and report the dependence of carrier scattering rates on excess energy and carrier density. We extract characteristic carrier thermalization times from below 10 to 85 fs. These values allow for mobilities of 500 cm2 V−1 s−1 at carrier densities lower than 2 × 1019 cm−3 and limit the time for carrier extraction in hot carrier solar cells.Carrier-carrier scattering rates determine the fundamental limits of carrier transport and electronic coherence. Using two-dimensional electronic spectroscopy with sub-10 fs resolution, Richter and Branchi et al. extract carrier thermalization times of 10 to 85 fs in hybrid perovskites.

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.

Growth of Nanosized Single Crystals for Efficient Perovskite Light-Emitting Diodes

Organic-inorganic hybrid perovskites are emerging as promising emitting materials due to their narrow full-width at half-maximum emissions, color tunability, and high photoluminescence quantum yields (PLQYs). However, the thermal generation of free charges at room temperature results in a low radiative recombination rate and an excitation-intensity-dependent PLQY, which is associated with the trap density. Here, we report perovskite films composed of uniform nanosized single crystals (average diameter = 31.7 nm) produced by introducing bulky amine ligands and performing the growth at a lower temperature. By effectively controlling the crystal growth, we maximized the radiative bimolecular recombination yield by reducing the trap density and spatially confining the charges. Finally, highly bright and efficient green emissive perovskite light-emitting diodes that do not suffer from electroluminescence blinking were achieved with a luminance of up to 55 400 cd m-2, current efficiency of 55.2 cd A-1, and external quantum efficiency of 12.1%.

Research data supporting "High Circular Polarization of Electroluminescence Achieved via Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films"

Left and right-handed circularly polarized components of Electro-Luminescence and corresponding dissymmetry factors.

Control of Interface Defects for Efficient and Stable Quasi-2D Perovskite Light-Emitting Diodes Using Nickel Oxide Hole Injection Layer

Abstract 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 (NiOx) and poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate hole injection interfaces are investigated. The better interface formed between the NiOx/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.

Ultrafast carrier cooling and thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy

Hybrid perovskite semiconductors have attracted strong scientific interest for optoelectronic applications. Despite the current drive for higher optoelectronic device efficiencies and stabilities, little information exists about the fundamental non-equilibrium interactions of photo-excited charge carriers in these materials. Time-resolved experiments with ultrashort laser pulses have been used to study the carrier dynamics in hybrid perovskites with transient absorption techniques. The cooling process was found to occur on 100s of femtosecond timescales [1-3], with a strong contribution from a hot phonon effect. The carrier thermalization process is expected to occur on much faster timescales [4-6] with seminal studies of thermalization dynamics in GaAs reported timescales ranging from 100 fs to 4 ps [5,7,8]. These results provided insights into dephasing times [7], band structure [5], and carrier-carrier scattering processes [9]. However, the initial build-up of the thermal distribution is typically difficult to resolve due to the requirement of high resolution, both in time and in energy.