278 research outputs found
Charge-Carrier Recombination in Halide Perovskites.
The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.EPSRC DTP Studentshi
Optical tuning of the diamond Fermi level measured by correlated scanning probe microscopy and quantum defect spectroscopy
Quantum technologies based on quantum point defects in crystals require
control over the defect charge state. Here we tune the charge state of shallow
nitrogen-vacancy and silicon-vacancy centers by locally oxidizing a
hydrogenated surface with moderate optical excitation and simultaneous spectral
monitoring. The loss of conductivity and change in work function due to
oxidation are measured in atmosphere using conductive atomic force microscopy
(C-AFM) and Kelvin probe force microscopy (KPFM). We correlate these scanning
probe measurements with optical spectroscopy of the nitrogen-vacancy and
silicon-vacancy centers created via implantation and annealing 15-25 nm beneath
the diamond surface. The observed charge state of the defects as a function of
optical exposure demonstrates that laser oxidation provides a way to precisely
tune the Fermi level over a range of at least 2.00 eV. We also observe a
significantly larger oxidation rate for implanted surfaces compared to
unimplanted surfaces under ambient conditions. Combined with knowledge of the
electron affinity of a surface, these results suggest KPFM is a powerful,
high-spatial resolution technique to advance surface Fermi level engineering
for charge stabilization of quantum defects
(3-Aminopropyl)trimethoxysilane Surface Passivation Improves Perovskite Solar Cell Performance by Reducing Surface Recombination Velocity
We demonstrate reduced surface recombination velocity (SRV) and enhanced
power-conversion efficiency (PCE) in mixed-cation mixed-halide perovskite solar
cells by using (3-aminopropyl)trimethoxysilane (APTMS) as a surface passivator.
We show the APTMS serves to passivate defects at the perovskite surface, while
also decoupling the perovskite from detrimental interactions at the C60
interface. We measure a SRV of ~125 + 14 cm/s, and a concomitant increase of
~100 meV in quasi-Fermi level splitting in passivated devices compared to the
controls. We use time-resolved photoluminescence and excitation-correlation
photoluminescence spectroscopy to show that APTMS passivation effectively
suppresses non-radiative recombination. We show that APTMS improves both the
fill factor and open-circuit voltage (VOC), increasing VOC from 1.03 V for
control devices to 1.09 V for APTMS-passivated devices, which leads to PCE
increasing from 15.90% to 18.03%. We attribute enhanced performance to reduced
defect density or suppressed nonradiative recombination and low SRV at the
perovskite/transporting layers interface.Comment: 22 pages, 6 figure
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The role of spin in the kinetic control of recombination in organic photovoltaics.
In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC)This is the accepted version of the original publication available at: http://www.nature.com/nature/journal/v500/n7463/full/nature12339.html
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