We highlight the recent progress in ultrafast dynamic microscopy that combines ultrafast optical spectroscopy with microscopy approaches, focusing on the application transient absorption microscopy ...(TAM) to directly image energy and charge transport in solar energy harvesting and conversion systems. We discuss the principles, instrumentation, and resolutions of TAM. The simultaneous spatial, temporal, and excited-state-specific resolutions of TAM unraveled exciton and charge transport mechanisms that were previously obscured in conventional ultrafast spectroscopy measurements for systems such as organic solar cells, hybrid perovskite thin films, and molecular aggregates. We also discuss future directions to improve resolutions and to develop other ultrafast imaging contrasts beyond transient absorption.
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.
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.
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.
Oxidative stress is a hallmark of several aging and trauma related neurological disorders, but the precise details of how altered neuronal activity elicits subcellular redox changes have remained ...difficult to resolve. Current redox sensitive dyes and fluorescent proteins can quantify spatially distinct changes in reactive oxygen species levels, but multicolor probes are needed to accurately analyze compartment-specific redox dynamics in single cells that can be masked by population averaging. We previously engineered genetically encoded red-shifted redox-sensitive fluorescent protein sensors using a Förster resonance energy transfer relay strategy. Here, we developed a second-generation excitation ratiometric sensor called rogRFP2 with improved red emission for quantitative live-cell imaging. Using this sensor to measure activity-dependent redox changes in individual cultured neurons, we observed an anticorrelation in which mitochondrial oxidation was accompanied by a concurrent reduction in the cytosol. This behavior was dependent on the activity of Complex I of the mitochondrial electron transport chain and could be modulated by the presence of cocultured astrocytes. We also demonstrated that the red fluorescent rogRFP2 facilitates ratiometric one- and two-photon redox imaging in rat brain slices and Drosophila retinas. Overall, the proof-of-concept studies reported here demonstrate that this new rogRFP2 redox sensor can be a powerful tool for understanding redox biology both in vitro and in vivo across model organisms.
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. In Chapter 4, we report direct visualization
of hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films by ultrafast transient absorption microscopy. This work demonstrates three distinct transport regimes. (i)
Quasiballistic transport, (ii) nonequilibrium transport, and (iii) diffusive transport. Quasiballistic
transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers
of transport distance that could overcome grain boundaries. The nonequilibrium transport
persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport
limit. These results suggest potential applications of hot-carrier devices based on hybrid
perovskites to ultimately overcome the Shockley-Queisser limit.
In the next work, we investigated a way to extend non-equilibrium carrier lifetime, which
ultimately corresponds to an accelerated carrier transport. From the knowledge of the hot carrier
transport work, we showed a proof of concept that the excess kinetic energy corresponds to long
range carrier transport. To further develop the idea of harvesting hot carriers, one must investigate
a way to make the carriers stay hot for a longer period (i.e. cool down slower). In Chapter 5, we
slow down the cooling of hot carriers via a phonon bottleneck, which points toward the potential
to overcome the Shockley-Queisser limit. Open questions remain on whether the high optical
phonon density from the bottleneck impedes the transport of these hot carriers. We show a direct
visualization of hot carrier transport in the phonon bottleneck regime in both single crystalline and
polycrystalline lead halide perovskites, more specifically, a relatively new class of alkali metal
doped perovskites (RbCsMAFA), which has one of the highest power conversion efficiencies.
Remarkably, hot carrier diffusion is enhanced by the presence of a phonon bottleneck, the exact
opposite from what is observed in conventional semiconductors such as GaAs. These results
showcase the unique aspects of hot carrier transport in hybrid perovskites and suggest even larger
potential for hot carrier devices than previously envisioned by the initial results presented in
Chapter 4. The final chapter will be divided into two sections, as we summarize and highlight our
collaborative efforts towards homogenization of carrier dynamics via doping perovskites with
alkali metals and our work on two-dimensional hybrid quantum well perovskites. Further studies
on the champion solar cell (RbCsMAFA) were performed to elucidate the role inorganic cations
play in this material. By employing transient absorption microscopy, we show that alkali metals
Rb+
and Cs+
are responsible for inducing a more homogenous halide (Iand Br-
) distribution,
despite the partial incorporation into the perovskite lattice. This translates into improved electronic
dynamics, including fluorescence lifetimes above 3 µs and homogenous carrier dynamics, which
was visualized by ultrafast microscopy. Additionally, there is an improvement in photovoltaic
device performance. We find that while Cs cations tend to distribute homogenously across the
perovskite grain, Rb and K cations tend to phase segregate at precursor concentrations as low as
1%. These precipitates have a counter-productive effect on the solar cell, acting as recombination
centers in the device, as argued from electron beam-induced current measurements. Remarkably,
the high concentration of Rb and Cs agglomerations do not affect the open-circuit voltage, average
lifetimes, and photoluminescence distribution, further indicating the perovskite’s notorious defect
tolerance.
A new class of high-quality two dimensional organic-inorganic hybrid perovskite quantum
wells with tunable structures and band alignments was studied. By tuning the functionality of the
material, the strong self-aggregation of the conjugated organic molecules can be suppressed, and
2D organic-halide perovskite superlattice crystals and thin films can be easily obtained via onestep solution-processing. We observe energy transfer and charge transfer between adjacent
organic and inorganic layers, which is extremely fast and efficient (as revealed by ultrafast
spectroscopy characterizations). Remarkably, these 2D hybrid perovskite superlattices are stable, due to the protection of the bulky hydrophobic organic groups. This is a huge step towards the
practicality of using perovskites for optoelectronics, since stability is always a huge concern with
water-sensitive materials. The molecularly engineered 2D semiconductors are on par with III-V
quantum wells and are promising for next-generation electronics, optoelectronics, and photonics.
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