Semiconductor quantum-well structures and superlattices are key building blocks in modern optoelectronics, but it is difficult to simultaneously realize defect-free epitaxial growth while fine tuning ...the chemical composition, layer thickness and band structure of each layer to achieve the desired performance. Here we demonstrate the modulation of the electronic structure-and consequently the optical properties-of organic semiconducting building blocks that are incorporated between the layers of perovskites through a facile solution processing step. Self-aggregation of the conjugated organic molecules is suppressed by functionalization with sterically demanding groups and single crystalline organic-perovskite hybrid quantum wells (down to one-unit-cell thick) are obtained. The energy and charge transfers between adjacent organic and inorganic layers are shown to be fast and efficient, owing to the atomically flat interface and ultrasmall interlayer distance of the perovskite materials. The resulting two-dimensional hybrid perovskites are very stable due to protection given by the bulky hydrophobic organic groups.
For optoelectronic devices based on polycrystalline semiconducting thin films, carrier transport across grain boundaries is an important process in defining efficiency. Here we employ transient ...absorption microscopy (TAM) to directly measure carrier transport within and across the boundaries in hybrid organic–inorganic perovskite thin films for solar cell applications with 50 nm spatial precision and 300 fs temporal resolution. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation.
A small amount of alkali metals goes a long way toward improving hybrid perovskites for hot carrier solar cells.
Successful implementation of hot carrier solar cells requires preserving high carrier ...temperature as carriers migrate through the active layer. Here, we demonstrated that addition of alkali cations in hybrid organic-inorganic lead halide perovskites led to substantially elevated carrier temperature, reduced threshold for phonon bottleneck, and enhanced hot carrier transport. The synergetic effects from the Rb, Cs, and K cations result in ~900 K increase in the effective carrier temperature at a carrier density around 10
18
cm
−3
with an excitation 1.45 eV above the bandgap. In the doped thin films, the protected hot carriers migrate 100 s of nanometers longer than the undoped sample as imaged by ultrafast microscopy. We attributed these improvements to the relaxation of lattice strain and passivation of halide vacancies by alkali cations based on x-ray structural characterizations and first principles calculations.
The role of the alkali metal cations in halide perovskite solar cells is not well understood. Using synchrotron-based nano-x-ray fluorescence and complementary measurements, we found that the halide ...distribution becomes homogenized upon addition of cesium iodide, either alone or with rubidium iodide, for substoichiometric, stoichiometric, and overstoichiometric preparations, where the lead halide is varied with respect to organic halide precursors. Halide homogenization coincides with long-lived charge carrier decays, spatially homogeneous carrier dynamics (as visualized by ultrafast microscopy), and improved photovoltaic device performance. We found that rubidium and potassium phase-segregate in highly concentrated clusters. Alkali metals are beneficial at low concentrations, where they homogenize the halide distribution, but at higher concentrations, they form recombination-active second-phase clusters.
Insight into the nanoscale carrier transport in the rapidly developing class of solutionprocessed semiconductors known as metal halide perovskites is the focal point for these studies. Further ...advancement in fundamentally understanding photophysical processes associated with charge carrier transport is needed to realize the true potential of perovskites for photovoltaic applications. In this work, we study photogenerated carrier transport to understand the underlying transport behavior of the material on the 10s to 100s nanometer lengthscales. To study these processes, we employ a temporally-resolved and spatially-resolved technique, known as transient absorption microscopy, to elucidate the charge carrier dynamics and propagation associated with metal halide perovskites. This technique provides a simultaneous high temporal resolution (200 fs) and spatial resolution (50 nm) to allow for direct visualization of charge carrier migration on the nanometer length scale. There are many obstacles these carriers encounter between photogeneration and charge collection such as morphological effects (grain boundaries) and carrier interactions (scattering processes). We investigate carrier transport on the nanoscale to understand how morphological effects influence the materials transport behavior. Morphological defects such as voids and grain boundaries are inherently small and traditionally difficult to study directly. Further, because carrier cooling takes place on an ultrafast time scale (fs to ps), the combined spatial and temporal resolution is necessary for direct probing of hot (non-equilibrium) carrier transport. Here we investigate a variety of ways to enhance carrier transport lengthscales by studying how non-equilibrium carriers propagate throughout the material, as well as, carrier cooling mechanisms to extend the non-equilibrium regime.For optoelectronic devices based on polycrystalline semiconducting thin films, grain boundaries are important to consider since solution-based processing results in the formation of well-defined grains. In Chapter 3, we investigate equilibrium carrier transport in metal halide perovskite thin films that are created via the highly desired solution processing method. Carrier transport across grain boundaries is an important process in defining efficiency due to the literary discrepancies on whether the grains limit carrier transport or not. In this work, we employ transient absorption microscopy to directly measure carrier transport within and across the boundaries. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation. The results of this work provide insight into why this defect tolerant material performs so well. Photovoltaic performance (power conversion efficiency) is governed by the ShockleyQueisser limit which can be overcame if hot carriers can be harvested before they thermalize. To convert sunlight to usable electricity, the photogenerated charge carriers need to migrate long distances and or live long enough to be collected. It is unclear whether these hot carriers can migrate a long enough distance for efficient collection.
Design rules and application spaces for closed-shell conjugated polymers have been well established in the field of organic electronics, but the emerging class of open-shell stable radicals has not ...been evaluated in such detail. Thus, establishing the underlying physical phenomena associated with the interactions between both classes of molecules is imperative for the effective utilization of these soft materials. Here, we establish that Förster Resonance Energy Transfer (FRET) is the dominant mechanism by which energy transfer occurs from a common conjugated polymer to various radical species using a combination of experimental and computational approaches. Specifically, we determined this fact by monitoring the fluorescence quenching of poly(3-hexylthiophene) (P3HT) in the presence of three radical species: (1) the galvinoxyl; (2) the 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO); and (3) the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals. Both in solution and in the solid-state, the galvinoxyl and PTIO radicals showed quenching that was on par with that of a common fullerene electron-accepting derivative, due to the considerable overlap of their absorbance spectrum with the fluorescence spectrum of the P3HT species, which indicated that isoenergetic electronic transitions existed for both species. Conversely, TEMPO showed minimal quenching at similar concentrations due to the lack of such an overlap. Furthermore, computational studies demonstrated that FRET would occur at a significantly faster rate than other competing processes. These findings suggest that long-range energy transfer can be accomplished in applications when radicals that can act as FRET acceptors are utilized, forming a new design paradigm for future applications involving both closed- and open-shell soft materials.
We report the synthesis and molecular junction conductance for a series of oligofluorenes to establish clear structure–property relationships for this electronically important material. We use a ...scanning tunneling microscopy based break-junction method (STM-BJ) to measure single-molecule conductance in oligofluorenes that vary in (a) the number of fluorene repeat units (n = 1–3), (b) bridge carbon substitution (dihydrogen, dimethyl, dihexyl, and didodecyl), and (c) linker-group termination (methyl sulfide versus primary amine). Conductance in oligofluorene molecular junctions is found to occur via tunneling, with a tunneling decay constant, β, of 0.31 per Å, or equivalently, 2.6 per fluorene unit, consistent with other π-conjugated molecular wires. Simple tunnel coupling calculations for model Au2-oligofluorene molecular clusters are reported to validate experimental conductance measurements. Finally, molecular conductance distributions for methyl sulfide terminated oligofluorenes are observed to be extremely broad due to the relatively flat torsional potential energy surface for rotation about the Au–S bond and the strong orientation effect of the conductance through a π-coupled state.
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