Abstract
Keywords
1. Introduction
Self-assembled two-dimensional (2D) Ruddlesden–Popper phase organic–inorganic halide perovskites with quantum-well structures have attracted much research attention in optoelectronic devices, lasers, and light-emitting diodes (LEDs) by virtue of their superior features such as enhanced stability, high absorption coefficient, tunable light-emitting wavelength, high color purity, and high exciton binding energy[
In this work, we have fabricated and [A: n-butylammonium ] using the same method, which has different defect densities and different exciton-phonon scattering intensities. Taking advantage of the temperature-dependent photoluminescence (PL) spectra, transient absorption (TA) spectra, time-dependent PL relaxation kinetics, and PLQY experiments, we also revealed that 2D Sn-based perovskites have stronger exciton-phonon scattering intensity and higher defect-state density relative to 2D Pb-based perovskites, which lead to a significant broadening of the emission linewidth and accelerate the exciton relaxation process of 2D Sn-based perovskites, and these effects reduce the PLQY of 2D Sn-based perovskites. These results can guide further improvement in the emission performance of 2D Sn-based perovskites, in which we should select new structures for organic cation layers with relatively high rigidity to reduce the exciton-phonon scattering intensity and use antioxidants to reduce the defect-state density and thus the energy non-radiative loss.
2. Results and Discussion
To explain the physical mechanism affecting the optical properties of 2D Sn-based perovskites, we prepared and films by the spin-coating method. The schematic of the hybrid quantum-well structure of and crystal structures [Fig. 1(a)] shows the perovskite octahedra sandwiched between organic spacer molecules (). Figure 1 shows the ultraviolet-visible (UV-Vis) absorption, steady-state PL spectroscopy, and PLQY of and . The inset shows that the optical bandgap of and was obtained as 1.97 eV and 2.37 eV by the Tauc-plot method, respectively. The PLQY of is lower than that of , indicating that the defect density of is higher than that of . Such defect states might arise from undesirable and uncontrolled conversion of to , as the oxidation potential of , during the process of sample preparation in a glove box with almost the nitrogen environment, in which there is still a trace amount of oxygen that allows the to oxidize to [
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Figure 1.Spectral characteristics of (BA)2BI4 (B: Sn/Pb) perovskites. (a) Schematic of the hybrid quantum-well structure of the crystal structure, showing the perovskite octahedra sandwiched between organic spacer molecules (BA+). The UV-visible (UV-Vis) absorption, steady-state photoluminescence (PL) spectra of (b) (BA)2SnI4 and (c) (BA)2PbI4. (d) PLQY of (BA)2SnI4 and (BA)2PbI4.
Figure 2.Analysis of PL characteristics. (a) Multi-peak fit analysis of (BA)2PbI4, (b) multi-peak fit analysis of (BA)2SnI4, (c) percentage of subpeaks of (BA)2PbI4 and (BA)2SnI4, (d) normalized PL decay kinetics.
Figure 3.Temperature dependence of the steady-state PL spectra. Contour map of the temperature-dependent steady-state PL spectra of (a) (BA)2SnI4 and (b) (BA)2PbI4. Fitting of the temperature-dependent FWHM of (c) (BA)2SnI4 and (d) (BA)2PbI4. The red fitting line of the data (blue point) is obtained by Eq. (
The variation of PL FWHM with temperature involves different physical processes of scattering, and the analysis of temperature-dependent PL FWHM is the main means to evaluate the electron-phonon coupling mechanism in various semiconductors[
Sample | ||||
---|---|---|---|---|
Table 1. Best-Fitting Parameters of the (BA)2SnI4 and (BA)2PbI4 Perovskites
Femtosecond TA spectroscopy was utilized to further investigate the photophysical processes of the nonequilibrium interactions of photogenerated carriers[
Figure 4 shows the false-color 2D TA mappings of the and thin films. For the TA spectra of , there are photoinduced ground-state bleaching signals approximately at 610 nm and 518 nm, which are caused by the band filling effect induced by pump light[
Figure 4.Carrier relaxation dynamics. False-color 2D TA mappings of (a) (BA)2SnI4 and (b) (BA)2PbI4. Evolution-associated spectra (EAS) obtained upon global analysis of the TA data of (c) (BA)2SnI4 and (d) (BA)2PbI4. Decay-associated spectra (DAS) obtained by performing global analysis on the TA spectra of (e) (BA)2SnI4 and (f) (BA)2PbI4. The inset shows the results of fitting the band-edge exciton relaxation dynamics.
The relaxation process of photogenerated carriers at low excitation fluence of is analyzed by global fitting based on three relaxation decay components with different lifetimes[
3. Conclusion
In summary, we have fabricated samples and [A: n-butylammonium ] using the same method and have revealed that 2D Sn-based perovskites have stronger exciton-phonon scattering intensity and higher defect density of states relative to 2D Pb-based perovskites by the temperature-dependent PL spectra, TA spectra, time-dependent PL relaxation kinetics, and PLQY experiments. These factors lead to a significant broadening of the emission linewidth and accelerate the exciton relaxation process, which reduces the PLQY of 2D Sn-based perovskites. Our results can be a guide for further improving the emission performance of 2D Sn-based perovskites by selecting new structures of organic cation layers with relatively high rigidity to reduce the exciton-phonon scattering intensity and by using antioxidants to reduce the defect-state density in the material and thus the energy loss of non-radiative transitions.
4. Experimental Section
4.1. Syntheses of the (BA)2SnI4 and (BA)2PbI4 perovskites polycrystalline thin films
The glass substrate was cleaned sequentially with detergent, deionized water, ethanol, and isopropanol. Then, the substrate was treated with oxygen plasma for 10 min and dried in an argon flow. For the synthesis of perovskite film, 0.1 mmol and 0.2 mmol BAI were dissolved in 1 mL dimethyl formamidine (DMF) : dimethyl sulfoxide (DMSO) () to form the perovskite precursor solution, which was heated and stirred at 70°C for a few hours before use. Subsequently, the above-mentioned precursor solution was deposited on top of the glass substrate via a spin-coating process at 2500 r/min for 60 s in the argon-filled atmosphere. Then, the perovskite film was obtained after thermal annealing at 70°C for 5 min. The fabrication procedure of perovskite thin films is identical to that of .
4.2. Temperature-dependent PL measurement
For temperature-dependent PL measurement, polycrystalline thin films prepared on silica substrates were mounted in a cryostat (Janis ST-100) and cooled by liquid nitrogen. The samples were excited by the continuous wave (CW) laser excitation at a wavelength of 473 nm, power density of 2 µJ/cm2, and 25 K intervals. Fluorescence is separated by the 150 g/mm grating in the Monochromator SP2500 of Princeton Instruments. Then, the spectral information was collected by the PIXIS-100BX CCD at .
4.3. Photoluminescence quantum yield
PLQY of polycrystalline thin films prepared on silica substrates was measured using the Edinburgh FLS1000 instrument with an excitation wavelength of 520 nm.
4.4. UV/visible absorption
UV-Vis absorption spectra of polycrystalline thin films prepared on glass substrates were collected by a Lambda 950 UV-Vis spectrometer.
4.5. Time-resolved PL
The TRPL kinetics was detected by a HORIBA DeltaFlex ultrafast time-resolved fluorescence spectrometer, where the excitation wavelength is 405 nm at .
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