introduction
In terms of high efficiency photovoltaic , luminescence and detection, perovskite hybrid metal halide materials have broad application prospects. These successes are mainly due to the excellent optoelectronic properties of these materials, including the high light absorption coefficient (α) of ~105 cm-1 in the visible region, long life over 1 μs and extremely slow hot carrier cooling. Behind these properties is an intrinsic photophysical mechanism that determines the absorption of light, the heating and cooling of the carrier, and the recombination or charge transfer kinetics.
Recent studies have highlighted the shielding effect of thermal phonon bottlenecks and the presence of polarons on hot carrier scattering to account for the excited state carrier dynamics of these materials. After cooling, it has been reported that at high temperatures, free carrier formation dominates, while physical processes such as exciton oscillation, localization, and transfer are observed. The band-edge light absorption of perovskites may be caused by these exciton states. However, other reports indicate that band-edge light absorption properties are contributed by the tail state or indirect band gap. Clearly, the low-energy electronic states under the direct banding of perovskites play a key role in determining their optoelectronic properties. However, research on these electronic states has not yet gained consensus, leading to continued debate among researchers on the most basic optical and material properties of perovskite materials, such as low energy absorption of single crystals, ultra-long carrier cooling and Composite lifetime, free carrier-exciton, and direct and indirect bandgap properties.
Summary of results
Recently, Professor Meng Qingbo from the Institute of Physics of the Chinese Academy of Sciences and Professor Jacek J. Jasieniak of Monash University in Australia collaborated on Energy. Environ. Sci., entitled: "Identification of High-Temperature Exciton States and Their Phase-Dependent Trapping Behaviour in Lead Halide Perovskites". The researchers studied the band edge and subband energy levels of polycrystalline and single crystal perovskite materials to better understand their photophysical origin. Through temperature, excitation intensity, and time-dependent optical measurements, the researchers observed two exciton states in the perovskite material over a wide temperature range of up to 300K (considered as free and bound excitons). ). These exciton states can explain the previously controversial phenomenon of low energy absorption and multimodal radiation. In addition, the capture and complex kinetics of these excitons have been shown to be strongly dependent on the structural phase of the perovskite. The quadrature phase exhibits ultrafast exciton trapping and significant defect-state radiation, while the tetragonal phase gives a very low single-molecule recombination velocity and a very small charge trapping cross section (10-18 cm2). Based on the theory of multi-phonon transition and lattice relaxation, it is found that the inhibition of charge trapping is mainly due to the increase of charge trap activation energy, probably due to the electron-lattice interaction in the inorganic framework after orthogonal-tetragonal phase transition. Weakened. This is a good explanation for the long carrier lifetime in these material systems. These findings led researchers to have a clearer understanding of the origin of the photophysical properties of perovskite materials and to find high temperature stable exciton states. These exciton states will be one of the concerns when designing energy conversion and utilization devices based on perovskite materials.
Graphic guide
Figure 1. Light absorption characteristics of different concentrations of MAPbBr3 dispersion
(a) normalized light absorption spectrum;
(b) the relationship between low energy light absorption and concentration;
(c), (d) light absorption curves and fits for 0.002 M and 0.2 M perovskite concentrations;
(e) The evolution of exciton absorption when increasing the concentration of perovskite. For the sake of clarity, the continuous band absorption term has been subtracted.
Figure 2. Temperature-dependent steady-state PL spectra of MAPbBr3 films
Experimentally measured PL spectra (a) 145K, (b) 170 K, 200 K, 250 K and 300 K;
(c) Temperature-dependent exciton and defect radiation intensity;
(d) Excitation intensity (Iexc)-dependent exciton PL intensity (IPL) at 110 K and 145 K.
Figure 3. PL of perovskite single crystal
Transmittance wavelength and time attenuation dependent MAPbBr3 single crystal transient PL two-dimensional color map (165 K, (a) surface (b) bulk phase); bulk phase material obtained by single crystal cleavage, it is a thin PMMA The layer is protected.
Typical PL spectrum of a perovskite single crystal in a time delay of 2 ns and 5 ns ((c) surface (d) bulk phase);
(e) measuring the density of perovskite traps by monitoring the relationship between pump density and PL intensity at room temperature;
Figure 4. Exciton capture and trap radiation from perovskite at low temperatures
(a) 70K, two-dimensional color map of the transient PL associated with the emission wavelength and delay time (normalized PL intensity);
(b) Early transient PL;
(c) One-dimensional histograms of conduction band (UC), defect state (Ut) and valence band (UV) in semiconductors;
(d) 70K, the steady-state PL spectrum associated with the excitation intensity of the perovskite;
(e) Logarithmic relationship between the peak value of the defect state PL and the intensity at different temperatures;
to sum up
The researchers systematically studied the absorption and emission characteristics of the band edge and sub-band energy states of hybrid lead halide perovskite materials, and obtained the exciton and defect states and their dynamic properties. The coexistence of free and bound state exciton states was found in polycrystals and single crystals (including surface and bulk phases) over a wide temperature range of up to 300K. The study found that the exciton radiation and capture processes are closely related to the trap state and exhibit phase-dependent behavior. For the quadrature phase, the excitons can be captured in the picosecond scale to produce strong defect state radiation. For the tetragonal phase, this process is significantly suppressed, with charge trapping cross sections as low as 10-18 cm2 and carrier lifetimes as long as hundreds of nanoseconds. This behavior is explained in the theory of multi-phonon transitions. The weakening of the electron-lattice interaction after phase transition leads to an increase in the activation energy of exciton capture, which in turn inhibits rapid charge trapping behavior. These findings have helped researchers to understand the unique photophysical properties of perovskite materials and have important guiding significance for perovskite optoelectronic devices that are controlled by energy properties to achieve higher performance.
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