9:45 - 10:10InvitedID: 136
OPTICAL STUDIES OF SPONTANEOUSLY FORMED BRIGHTLY EMITTING QUASI-2D PEROVSKITES NANOCRYSTALS IN AMORPHOUS MATRIX
1Department of Physics,The Hong Kong University of Science and Technology (HKUST), Hong Kong S.A.R., China; 2Department of Physics, The University of Hong Kong (HKU), Hong Kong S.A.R., China
Ruddlesden-Popper (RP) organic-inorganic halide perovskites are promising materials for LED applications due to its visible wavelength tunability, improved environmental stability and high efficiency. Although energy funnelling has been shown to give bright emission in the lowest bandgap domains of the 2D film, the controllability of the domain distribution and energy landscape is challenging with low reproducibility. Another method to bright emission is to achieve carrier confinement within nanocrystals. We showed that significant enhancement of the light emission in RP perovskite films by anti-solvent induced spontaneous formation of nanocrystals in an amorphous matrix, which effectively suppressed the defect emission and nonradiative recombination processes.
An astonishing increase in external PLQY (16 times) was observed in nanocrystals in amorphous matrix compared to monocrystalline phase with large grains. Correspondingly, the luminescence lifetime shows a much slower decay rate than the crystalline film. Temperature-dependent studies reveal a reduced luminescence quenching from thermally activated non-radiative pathways in the nanocrystals, alongside suppression of a large defect emission seen in 2D perovskites. This is supported by successful prevention of the thermal luminescence quenching upon further addition of trioctylphosphine oxide (TOPO) passivation layer, raising external PLQY by 2.5 times to a value of ~30%. An internal PLQY of 64% was estimated by calculating the out-coupling efficiency and photon recycling in the film. A green LED with the active layer consisted of the nanocrystals perovskite and TOPO was fabricated and the device exhibited a turn-on voltage of 3.6V, maximum luminance of 1460cd/m2and maximum EQE of 2.25%.
We have demonstrated that the defect emission and non-radiative recombination in RP perovskite films can be effectively suppressed by morphology modification and the use of a passivating agent (TOPO). Consequently raising the quantum yield and resulting bright green emission from LED devices fabricated.
10:10 - 10:25ID: 164
THE REMARKABLE PHOTOPHYSICS OF FORMAMIDINIUM TIN TRIIODIDE – ON HOT STATES AND DARK STATES
University of Groningen, Groningen, The Netherlands
Formamidinium tin triiodide (FASnI3) had been considered as a candidate for halide perovskite (HaP) thin film solar cells already early on when the field started to develop. Its susceptibility to oxidation, however, severely limited the achieved power conversion efficiency for a long time and basic studies of its photophysics have been less attractive compared with those of lead based variants. Recently, though, the extraordinary long lifetime of hot charge carriers along with major improvements in material stability has renewed the relevance of FASnI3 and the need to understand its opto-electrical properties.
We therefore investigated the photophysics of FASnI3 – predominantly through photoluminescence spectroscopy. We show how the behaviour of thin films is strongly dependent on the processing conditions and what effect this has on the cooling of hot charge carriers. The well-reported self-doping of the material not only affects the position of the PL emission and its lifetime, but can also dominate the PL linewidth through carrier scattering.
A central finding of our study is the emergence of a long-lived second state emitting at lower energy with a lifetime up to 10 microseconds. We therefore use extensive characterisations of single crystals, to discuss the origin of this state and decay pathways of excited carriers.
Our study shows that the processing conditions are not only important for the performance of solar cells, but also that the material’s photophysics can be obscured entirely in films of poor quality. Studies on single crystals are thus crucial to unravel the mechanisms behind the long hot carrier lifetime.
10:25 - 10:40ID: 225
CHARGE CARRIER RECOMBINATION DYNAMICS AND CARRIER-PHONON INTERACTIONS IN SOLUTION PROCESSED BISMUTH HALIDE SEMICONDUCTORS
University of Cambridge, Cambridge, United Kingdom
A promising class of lead-free perovskites for photovoltaic applications
include bismuth halides, such as MA3Bi2I9, BiOI, and
Cs2AgBiBr6. Although these materials have been predicted to exhibit
defect tolerance, as seen in lead-halide perovskites, and already display
improved stabilities and long charge carrier lifetimes, the power
conversion efficiencies of the corresponding devices have not reached
the level of lead-based perovskites. We explore potential reasons for this,
including the disconnected nature of the bismuth halide
octahedra in the crystal structure, which limits carrier mobility, and
the lower levels of absorption due to indirect bandgaps. We probe the
behaviour of excited states in many bismuth-halide compounds with
various effective dimensionalities using ultrafast transient absorption,
Raman, and teraherz spectroscopy. Overall, this work indicates that
bismuth-based materials have the potential to be used in efficient optoelectronic
devices, but there is a need to account for the effects of strong
carrier-phonon coupling and localisation of electronic states on carrier
scattering rates. We therefore present the charge carrier - crystal lattice interaction
strength as an important factor to consider in the identification of novel semiconductors for efficient next-generation solar cells.