3 research outputs found
Molecular Origins of Defects in Organohalide Perovskites and Their Influence on Charge Carrier Dynamics
The
chemical origins of charge recombination centers in lead-based
organohalide perovskites were investigated using a combination of
quantitative solution chemistry, X-ray diffraction, and time-resolved
photoluminescence spectroscopy. We explored the complex, concentration-dependent
solution equilibria among iodoplumbate coordination complexes that
have been implicated as potential midgap states in organohalide perovskites.
High concentrations of PbI<sub>2</sub>, PbI<sub>3</sub><sup>–</sup>, and PbI<sub>4</sub><sup>2–</sup> were found in precursor
solutions that match those used to deposit perovskite films for solar
cell applications. We found that the concentration of tetraiodoplumbate
PbI<sub>4</sub><sup>2–</sup> is uniquely correlated with the
density of charge recombination centers found in the final perovskite
films regardless of the lead precursor used to cast the films. However,
mixed-halide perovskites commonly referred to as CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> suppressed the formation of PbI<sub>4</sub><sup>2–</sup> in comparison to perovskites that included only iodide, which is
consistent with the longer charge carrier lifetimes reported in mixed-halide
perovskites. These findings bring a molecular-level view to the chemical
origins of charge recombination centers that provides a fundamental
basis from which to understand the reported improvement in uniformity
of perovskite films and devices deposited using sequential methods.
These findings also suggest new approaches to control the formation
of defect precursors during the deposition of organohalide perovskite
absorbers
Electron–Phonon Coupling and Resonant Relaxation from 1D and 1P States in PbS Quantum Dots
Observations
of the hot-phonon bottleneck, which is predicted to
slow the rate of hot carrier cooling in quantum confined nanocrystals,
have been limited to date for reasons that are not fully understood.
We used time-resolved infrared spectroscopy to directly measure higher
energy intraband transitions in PbS colloidal quantum dots. Direct
measurements of these intraband transitions permitted detailed analysis
of the electronic overlap of the quantum confined states that may
influence their relaxation processes. In smaller PbS nanocrystals,
where the hot-phonon bottleneck is expected to be most pronounced,
we found that relaxation of parity selection rules combined with stronger
electron–phonon coupling led to greater spectral overlap of
transitions among the quantum confined states. This created pathways
for fast energy transfer and relaxation that may bypass the predicted
hot-phonon bottleneck. In contrast, larger, but still quantum confined
nanocrystals did not exhibit such relaxation of the parity selection
rules and possessed narrower intraband states. These observations
were consistent with slower relaxation dynamics that have been measured
in larger quantum confined systems. These findings indicated that,
at small radii, electron–phonon interactions overcome the advantageous
increase in energetic separation of the electronic states for PbS
quantum dots. Selection of appropriately sized quantum dots, which
minimize spectral broadening due to electron–phonon interactions
while maximizing electronic state separation, is necessary to observe
the hot-phonon bottleneck. Such optimization may provide a framework
for achieving efficient hot carrier collection and multiple exciton
generation
Dynamic Exchange During Triplet Transport in Nanocrystalline TIPS-Pentacene Films
The multiplication of excitons in
organic semiconductors via singlet
fission offers the potential for photovoltaic cells that exceed the
Shockley–Quiesser limit for single-junction devices. To fully
utilize the potential of singlet fission sensitizers in devices, it
is necessary to understand and control the diffusion of the resultant
triplet excitons. In this work, a new processing method is reported
to systematically tune the intermolecular order and crystalline structure
in films of a model singlet fission chromophore, 6,13-bisÂ(triisopropylsilylethynyl)
pentacene (TIPS-Pn), without the need for chemical modifications.
A combination of transient absorption spectroscopy and quantitative
materials characterization enabled a detailed examination of the distance-
and time-dependence of triplet exciton diffusion following singlet
fission in these nanocrystalline TIPS-Pn films. Triplet–triplet
annihilation rate constants were found to be representative of the
weighted average of crystalline and amorphous phases in TIPS-Pn films
comprising a mixture of phases. Adopting a diffusion model used to
describe triplet–triplet annihilation, the triplet diffusion
lengths for nanocrystalline and amorphous films of TIPS-Pn were estimated
to be ∼75 and ∼14 nm, respectively. Importantly, the
presence of even a small fraction (<10%) of the amorphous phase
in the TIPS-Pn films greatly decreased the ultimate triplet diffusion
length, suggesting that pure crystalline materials may be essential
to efficiently harvest multiplied triplets even when singlet fission
occurs on ultrafast time scales