Organic-inorganic hybrid perovskites are known as solution-processing semiconductors with highly efficient light emission covering entire visible spectrum region under down-conversion condition
Opto-Electronic Advances, Volume. 5, Issue 2, 200051(2022)
Giant magneto field effect in up-conversion amplified spontaneous emission via spatially extended states in organic-inorganic hybrid perovskites
Up-conversion lasing actions are normally difficult to realize in light-emitting materials due to small multi-photon absorption cross section and fast dephasing of excited states during multi-photon excitation. This paper reports an easily accessible up-conversion amplified spontaneous emission (ASE) in organic-inorganic hybrid perovskites (MAPbBr3) films by optically exciting broad gap states with sub-bandgap laser excitation. The broad absorption was optimized by adjusting the grain sizes in the MAPbBr3 films. At low sub-bandgap pumping intensities, directly exciting the gap states leads to 2-photon, 3-photon, and 4-photon up-conversion spontaneous emission, revealing a large optical cross section of multi-photon excitation occurring in such hybrid perovskite films. At moderate pumping intensity (1.19 mJ/cm2) of 700 nm laser excitation, a significant spectral narrowing phenomenon was observed with the full width at half maximum (FWHM) decreasing from 18 nm to 4 nm at the peak wavelength of 550 nm, simultaneously with a nonlinear increase on spectral peak intensity, showing an up-conversion ASE realized at low threshold pumping fluence. More interestingly, the up-conversion ASE demonstrated a giant magnetic field effect, leading to a magneto-ASE reaching 120%. In contrast, the up-conversion photoluminescence (PL) showed a negligible magnetic field effect (< 1%). This observation provides an evidence to indicate that the light-emitting states responsible for up-conversion ASE are essentially formed as spatially extended states. The angular dependent spectrum results further verify the existence of spatially extended states which are polarized to develop coherent in-phase interaction. Clearly, using broad gap states with spatially extended light-emitting states presents a new approach to develop up-conversion ASE in organic-inorganic hybrid perovskites.
Introduction
Organic-inorganic hybrid perovskites are known as solution-processing semiconductors with highly efficient light emission covering entire visible spectrum region under down-conversion condition
Toward unrevealing the collective emission of up-conversion ASE, magnetic field effect is proposed to be an effective tool. This method has been widely reported in exploring the behavior between light emitting states in organics and halide perovskites
In this work, we prepare the perovskite (MAPbBr3) films with large grains in scale of several micrometers through our previous reported method
Materials and methods
Preparation of perovskite (MAPbBr3) films
The perovskite films were spin-coated with the thickness of 1 micrometer onto the ITO/PEDOT:PSS substrates at N2 atmosphere. Essentially, the perovskite films were prepared with one-step spin-coating method by using the precursor solution (1.5 mmol PbBr2 and 1.58 mmol MABr in 1 mL DMF solution) at the spinning rate of 3000 rpm for 1 minute, followed by annealing at 60oC for 30 minutes. The grain size can be largely changed by partially replacing DMF with DMSO. Prior to the preparation of perovskite films, the PEDOT:PSS films were spin-coated on glass substrates with the thickness of 40 nm, followed with the thermal annealing at 150oC for 0.5 h. With the glass/PEDOT substrates, high-quality perovskite films can be formed through spin coating.
Measurement of absorption spectrum
A thin-film transmittance and reflectance spectrometer (RT1736, Ideaoptics, China) was used to measure the absorption of the sample with an integrating sphere (R7, Ideaoptics, China) to eliminate the influence of scattering, equipped with a visible spectrometer (PG2000-pro, Ideaoptics, China) and a NIR spectrometer (NIR1700, Ideaoptics, China). A halogen lamp and a deuterium lamp were used as visible and infrared light source. The transmittance and reflectance of the sample and the substrate were measured, and then the absorption can be derived by the formula A= 1-T-R. The absorption of substrate was deducted from the whole absorption of the sample to derive the net absorption of the perovskite film.
Measurement of photoluminescence spectrum
All pumping wavelengths in the experiment were selected from a Coherent Legend regenerative amplifier (150 fs, 1 kHz, 800 nm) which is seeded by a Coherent Mira 900 oscillator (100 fs, 80 MHz). The Mira is seeded by a Coherent Verdi Laser Generator (continuous wave, 532 nm). A Coherent Opera Solo was used to convert the 800 nm pulse laser to wavelength suitable for the experiment, without changing other parameters. The excitation beam was focused on the sample with the diameter of about 100 micrometers. The output signals were sent into the spectrometer (Horiba, iHR320)) and detected by a charge coupled device (Horiba, Synapse).
Measurement of angular dependence
An R1 Angular-Resolved System (R1, Ideaoptics, China) equipped with a visible spectrometer (PG2000, Ideaoptics, China) was used to measure the angular dependence of the emission from the perovskite (MAPbBr3) film. The sample was fixed vertically, and a photodetector was fixed on a mechanical arm which can rotate around a horizontal axis in the film plane. The excitation beam focused perpendicularly on the sample intersecting with the axis by a lens. By rotating the mechanical arm to different position, photoluminescence in different emitting angle can be measured.
Measurement of magnetic field effects
An electromagnet was used to generate a constant magnetic field of 1.2 T paralleled to the perovskite (MAPbBr3) film. The constant magnetic field was switched on and off through electrical current during up-conversion ASE measurement to generate magneto-ASE phenomena. Excitation beam was focused on the sample perpendicularly by a lens. The light emission intensity was recorded once per second with 10 average times by a THORLABS PDA36A2 silicon photodetector, with several filters before the detector to eliminate excitation beam. The measurement was started with the electromagnet turned off. After 30 points were collected, the measurement was paused until the electromagnet was switched on and the constant current was kept to generate the magnetic field of 1.2 T. The electromagnet was then switched off to complete the measurement cycle.
Results and discussion
The scanning electron microscopy (SEM) image in
Figure 1.
The multi-photon absorption process can be categorized into two types: one-step process and multi-step process. For the first type, the absorption process is realized on condition that the multiple photons are coherent and have wavefunctions overlap with the initial and final state of the material. Thus, switching the excitation source from coherent one to non-coherent one will cause largely reduced absorption cross section. This process is irrelevant to the gap states. In contrast, the multi-step process in the multi-photon absorption is influenced by the gap states. The electron in valance band is firstly excited into the gap when one below-bandgap photon is absorbed, followed by latter step(s) to excite the electron into conduction band. In this situation, the gap states play an important role in determining the absorption cross section. Specifically, the magnitude is irrelevant to the excitation source, i.e. coherent and non-coherent excitation sources have the same absorption cross section. In our experiment, we compared the ASE spectra at identical pump fluence excited by coherent and noncoherent light sources (
To further explore multi-photon absorption through broad gap states, the infrared excitation wavelength was largely tuned from 900 nm to 1200 nm and 1500 nm to detect up-conversion light emission. The power-law is effective to identify the types of radiative recombination only when we consider the PL intensities at initial time, i.e. PL0 in transient PL measurement
Figure 2.
As up-conversion ASE occurred in the perovskite (MAPbBr3) films, the light-emitting states become strongly coupled with each other, which is supposed to provide large response to magnetic field. Therefore, we explore the magnetic field effects of light-emitting states responsible for up-conversion ASE. As reported, magnetic field effects can be observed when an external magnetic field is able to disturb the conversion between different states
Figure 3.
We should note that the spatially extended states are the states with delocalized wavefunctions, which are supposed to be realized through orbital order interaction. When an orbital order is established between spatially extended states, the light-emitting states can develop a cooperative interaction, providing the necessary condition to realize an ASE. Dynamically, this requires that the orbital order needs to be at least partially conserved during ASE. More importantly, when an external magnetic field suppresses the relaxation of orbital order to enhance the cooperative interaction between light-emitting states, ASE intensity can be largely increased, leading to magnetic field effects with the magnitude larger than 100%. Essentially, the orbital order leads to cooperative light-emitting states towards realizing up-conversion ASE. To verify the spatially extended states with cooperative order, magnetic field effects in down-conversion ASE were measured.
To further understand the characteristics of spatially extended states by directly exciting broad gap states, we investigate the spatial distribution of up-conversion ASE by monitoring the angle-resolved emission intensity. The angle-resolved experimental system was used to measure PL intensity at variable angles ranging from 0° to 90° relative to the film plane, with the excitation beam (700 nm) perpendicularly focused on the film plane, as depicted in
Figure 4.
Conclusion
In summary, up-conversion ASE is realized by directly exciting the broad gap states in hybrid perovskite (MAPbBr3) films prepared with large cubic grains in the order of micrometers. The 2-photon, 3-photon, and 4-photon PL indicates that the broad gap states possess strong multi-photon absorption characteristics, providing the necessary condition to generate up-conversion light emission. By directly exciting broad gap states with NIR pumping fluence exceeding threshold intensity, a spectral narrowing phenomenon in up-conversion light emission was observed, showing as an up-conversion ASE in perovskite (MAPbBr3) films prepared with large grains. Importantly, a static magnetic field of 1.2 T can significantly increase the ASE intensity by more than 150%, leading to giant magnetic field effects in up-conversion ASE regime. The observed giant magnetic field effects provide an evidence to show that the orbital order is established between spatially extended states in the hybrid perovskite (MAPbBr3) films. Essentially, the orbital order between spatially extended states provides the necessary condition to generate a cooperative interaction between light-emitting states towards realizing up-conversion ASE. This is verified by similar phenomenon in down-conversion ASE: giant magnetic field effects (120%) are also observed in down-conversion ASE. Clearly, with broad gap states, establishing the orbital order between spatially extended states presents a new mechanism to realize up-conversion ASE in hybrid halide perovskites. This creates new opportunities to use hybrid halide perovskites in infrared sensing, biological detection, and lasing technologies. Moreover, the giant magnetic field effects observed in ASE regime provides the promise to utilize hybrid halide perovskites in spintronics and biological magnetic probing.
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Tangyao Shen, Jiajun Qin, Yujie Bai, Jia Zhang, Lei Shi, Xiaoyuan Hou, Jian Zi, Bin Hu. Giant magneto field effect in up-conversion amplified spontaneous emission via spatially extended states in organic-inorganic hybrid perovskites[J]. Opto-Electronic Advances, 2022, 5(2): 200051
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Received: Sep. 1, 2020
Accepted: Oct. 30, 2020
Published Online: Mar. 28, 2022
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