722 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
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Excitonic Properties of Low-Band-Gap Lead-Tin Halide Perovskites
The MAPb1–xSnxI3 (x = 0–1) (MA = methylammonium)
perovskite family comprises a range of ideal absorber band gaps for
single- and multijunction perovskite solar cells. Here, we use spectroscopic
measurements to reveal a range of hitherto unknown fundamental properties
of this materials family. Temperature-dependent transmission results
show that the temperature of the tetragonal to orthorhombic structural
transition decreases with increasing tin content. Through low-temperature
magnetospectroscopy, we show that the exciton binding energy is lower
than 16 meV, revealing that the dominant photogenerated species at
typical operational conditions of optoelectronic devices are free
charges rather than excitons. The reduced mass increases approximately
proportionally to the band gap, and the mass values (0.075–0.090me) can be described with a two-band k·p
perturbation model extended across the broad band gap range of 1.2–2.4
eV. Our findings can be generalized to predict values for the effective
mass and binding energy for other members of this family of materials
Visualizing Buried Local Carrier Diffusion in Halide Perovskite Crystals via Two-Photon Microscopy.
Halide perovskites have shown great potential for light emission and photovoltaic applications due to their remarkable electronic properties. Although the device performances are promising, they are still limited by microscale heterogeneities in their photophysical properties. Here, we study the impact of these heterogeneities on the diffusion of charge carriers, which are processes crucial for efficient collection of charges in light-harvesting devices. A photoluminescence tomography technique is developed in a confocal microscope using one- and two-photon excitation to distinguish between local surface and bulk diffusion of charge carriers in methylammonium lead bromide single crystals. We observe a large dispersion of local diffusion coefficients with values between 0.3 and 2 cm2·s-1 depending on the trap density and the morphological environment-a distribution that would be missed from analogous macroscopic or surface measurements. This work reveals a new framework to understand diffusion pathways, which are extremely sensitive to local properties and buried defects
Influence of Grain Size on Phase Transitions in Halide Perovskite Films
Grain size in polycrystalline halide perovskite films is known to have an impact on the optoelectronic properties of the films, but its influence on their soft structural properties and phase transitions is unclear. Here, we use temperature-dependent X-ray diffraction, absorption, and macro- and micro-photoluminescence measurements to investigate the tetragonal to orthorhombic phase transition in thin methylammonium lead iodide films with grain sizes ranging from the micron scale down to the tens of nanometre scale. We show that the phase transition nominally at ~150 K is increasingly suppressed with decreasing grain size and, in the smallest grains, we only see the first evidence of a phase transition at temperatures as low as ~80 K. With decreasing grain size, we also see an increasing magnitude of the hysteresis in the structural and optoelectronic properties when cooling to, and then upon heating from, 100K. Our work reveals the remarkable sensitivity of the optoelectronic, physical and phase properties to the local environment of the perovskite structure, which will have large ramifications for phase and defect engineering in operating devices.EPSRC NanoDTC
Royal Society
ERC Starting Gran
Static and Dynamic Disorder in Triple-Cation Hybrid Perovskites
A detailed understanding of the carrier dynamics and emission characteristics
of organic-inorganic lead halide perovskites is critical for their
optoelectronic and energy harvesting applications. In this work, we reveal the
impact of the crystal lattice disorder on the photo-generated electron-hole
pairs through low-temperature photoluminescence measurements. We provide strong
evidence that the intrinsic disorder forms a sub-bandgap tail density of
states, which determines the emission properties at low temperature. The PL
spectra indicate that the disorder evolves with increasing temperature,
changing its character from static to dynamic. This change is accompanied by a
rapid drop of the PL efficiency, originating from the increased mobility of
excitons/polarons, which enables them to reach deep non-radiative recombination
centers more easily
Graphene-perovskite fibre photodetectors
The integration of optoelectronic devices, such as transistors and
photodetectors (PDs), into wearables and textiles is of great interest for
applications such as healthcare and physiological monitoring. These require
flexible/wearable systems adaptable to body motions, thus materials conformable
to non-planar surfaces, and able to maintain performance under mechanical
distortions. Here, we prepare fibre PDs combining rolled graphene layers and
photoactive perovskites. Conductive fibres (500/cm) are made by
rolling single layer graphene (SLG) around silica fibres, followed by
deposition of a dielectric layer (AlO and parylene C), another
rolled SLG as channel, and perovskite as photoactive component. The resulting
gate-tunable PDs have response time5ms, with an external
responsivity22kA/W at 488nm for 1V bias. The external responsivity is two
orders of magnitude higher and the response time one order of magnitude faster
than state-of-the-art wearable fibre based PDs. Under bending at 4mm radius, up
to80\% photocurrent is maintained. Washability tests show72\% of
initial photocurrent after 30 cycles, promising for wearable applications
Photo-induced halide redistribution in organic-inorganic perovskite films.
Organic-inorganic perovskites such as CH3NH3PbI3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH3PbI3 films under illumination. We demonstrate that the photo-induced 'brightening' of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance
Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence
© 2019, The Author(s), under exclusive licence to Springer Nature Limited. Metal halide perovskites have emerged as exceptional semiconductors for optoelectronic applications. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. Here, we control the cation alloying to enhance optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photoexcited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale mapping of the optical bandgap, electrically gated transport measurements and first-principles calculations, we demonstrate that spatially varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes.S.F. acknowledges funding from the Studienstiftung des deutschen Volkes and EPSRC, as well as support from the Winton Programme for the Physics of Sustainability. S.M. acknowledges funding from an EPSRC studentship. M.A.-J. thanks Nava Technology Limited, Cambridge Materials Limited and EPSRC (grant number: EP/M005143/1) for their funding and technical support. S.P.S. acknowledges funding from the Royal Society Newton Fellowship and EPSRC through a program grant (EP/M005143/1). T.A.S.D. acknowledges the National University of Ireland (NUI) for a Travelling Studentship and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (HYPERION, grant agreement number 756962). K.F. acknowledges funding from a George and Lilian Schiff Foundation Studentship, an EPSRC studentship and a scholarship from the Winton Programme for the Physics of Sustainability. E.R. acknowledges funding from an ERC starting grant (no. 804523). R.H.F. acknowledges support from the Simons Foundation (grant 601946). Research work in Mons was supported by the Fonds de la Recherche Scientifique de Belgique - Fund for Scientific Research (F.R.S.-FNRS) and the EU Marie-Curie IEF project ‘DAEMON’. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI). D.B. is an FNRS Research Director. S.D.S. acknowledges the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (HYPERION, grant agreement number 756962), the Royal Society and Tata Group (UF150033). F.D. acknowledges funding from the Winton Programme for the Physics of Sustainability
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