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.
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7699679 |
| Date | 15 May 2019 |
| Creators | Jordan M Snaider (6318551) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/CARRIER_TRANSPORT_IN_HYBRID_LEAD_HALIDE_PEROVSKITES_STUDIED_BY_ULTRAFAST_PUMP-PROBE_MICROSCOPY/7699679 |
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