In the first chapter, I discussed several types of hybrid low-dimensional perovskite and nanocrystal materials, as well as design guidelines and general synthesis techniques. In Chapter 4, we demonstrated tuning the PL properties of water-stable 2D organics.
Unfortunately, compared to the rapid development of all inorganic lead halide perovskites that have archived the highest EQE of 20.3%, the development rate of the low dimensional OI-HP with the general formula A'2A''n-1BX3n+1 , ten despite the strong merits of a large surface area, lagged significantly behind with the EQE of 14.46%. From the XRD patterns, the appearance of peaks at similar positions shows that 2D OI-HP:Mn has inherited its layered structure type. The structure was further confirmed to have the plate/disk-shaped morphology from SEM images (Figure 4.2a-f). Although the thicknesses were much more than a single layer of perovskite, this is likely to be attributed to their self-organization between layers. The constituent elements were confirmed by SEM/EDS analysis (Table 4.1).
In Figure 4.4, the high-resolution XPS spectra of Pb element showed that the binding energy of Pb2+ 4f5/2 and Pb2+ 4f7/2 is almost the same when Mn 2+ ions were introduced into the 2D OH-HP network structure. ] Binding energy peaks of Mn and O1 derivatives were presented in all doped compounds. We conclude that the strong temperature dependence of the exciton PL for 1 and 3 is consistent with a simple model of energy transfer between the self-trapped exciton state and the free exciton. At room temperature, the activation energy barriers separating the free exciton and self-trapped exciton states can be easily overcome, and carriers are thermally removed from the self-trapped exciton states to the free exciton state . The process is illustrated by the KET and KBET processes in Figure 4.5d. Waterproof fluorescent performance. a) Solid luminescence images under 365 nm UV light and b) Fluorescence images after immersion in water for 1 h for 5, 3, 1, c) Schematic of the probable water resistance mechanism and water static contact angles for (PEA) 2PbBr4:Mn (PEAP- Mn) coated on glass and glass.
Due to the large constant of SOC, it is expected that the inter system crossing (ISC) between singlet STE and triplet STE is significantly efficient and the 3P1-1S0 transition rate becomes relatively high.[37,38] The ISC process is further confirmed. by similar PLE spectra monitored at 475 nm and 615 nm.
Introduction to low dimensional perovskites
Introduction to perovskite nanocrystals
Optical properties of low dimensional perovskites
Stability of ow dimensional perovskites
All room temperature PL spectra of all the solid compounds were collected using a Cary Eclipse fluorometer, (Varian) in the solid state. PL microscopic images and single particle PL spectra of all the powder samples were captured in LSM 780 NLO (manufacturer: Carl Zeiss) under one channel from 405 nm to 680 nm.
There is no report on synthesis of fluorescent α-phase stabilized quasi-2D FAPbI3 [FA= CH(NH2)2] perovskite nanocrystals (NCs) in water. We report top-down synthesis of fluorescent quasi-two-dimensional (2D) α-FAPbI3 NCs in aqueous media by controlling the electronic states of lead and NCs size of FAPbI3.
Single particle imaging and photoluminescence (PL) study of α-FAPbI3 NCs bear the signature of dual emission indicating the formation of self-trapped excited states. Here, we report, for the first time, a top-down aqueous synthesis of α-FAPbI3 NCs coated with OCT from bulk δ-FAPbI3 and it is stable in ambient condition for more than six months.
Results and discussion
The spectra in Figure 4.5 show the room temperature PL characteristics and UV-vis absorption characteristics of 2D perovskite 1-6. The obvious difference between confocal fluorescence images and the luminescence images under UV light is considered to be due to the efficient energy transfer emission (red orange color under UV light) and self-reabsorption back energy transfer (blue color with faint red) color in confocal fluorescence images), as shown in figure 4.5d. In Figure 5.2b, we investigate the presence of organic part of NCs and SCs by Fourier transform infrared (FTIR) microscopy.
The SC PL spectra shown in Figure 5.7b show STE emission in metal-doped Cs2SnCl6 [15,27], providing more evidence of the discrepancy between the absorption and PLE spectra. In addition, 3 shows the excitation wavelength-dependent PL and PLE spectra shown in Fig. 5.6, which are attributed to doublet emission centers with a transition of singlet (1P1-1S0) and triplet (3Pn-1S0) STEs, respectively . For better insight, we perform a density of states (DOS) calculation to investigate the emission shift with density functional theory (DFT), shown in Figure 5.7c.
Finally, we produced thin films using the colloidal solution of 5 and 6 shown in Figure 5.8c and d.
Room temperature (R.T.) stabilized α-FAPbI3 perovskite solar cells (PSCs) are widely considered as the most potential candidate to break the efficiency record of silicon-based solar cells. This is the first time that 3D-FAPbI3 has been stabilized in the presence of no capping ligands in an ambient environment. These discoveries, we hope, will pave the way for a new approach to the use of FAPbI3 solar cells.
Solar cells based on MAPbI3 have achieved high efficiency over 23.7% with device stability over a thousand hours. However, the efficiency of σ-FAPbI3-based PSCs reached 25.2%. However, this will increase the band gap or passivate the surface of the FAPbI3 structures, both of which are detrimental to further optimizing the performance of α-FAPbI3-based solar cells. Quasi-2D FAPbI3 composed of 2D and 3D inorganic perovskite layers is obtained with long-term stability under ambient conditions.
Results and discussion
Interestingly, in Figure 3.1, cubic shape NPLs with a thickness of less than 200 nm are generated after IPA vapor treatment. In Figure 3.3, the components of 1-9 are investigated by fourier-transform infrared (FT-IR) spectroscopy and simultaneous differential scanning calorimetry thermogravimetric analysis (SDT) to investigate the structure and introduction of OCT and H3PO2 functional groups to FAPbI3 perovskite. SDT was used to assess the structural phase transition behavior of (H3PO2+OCT)-coated perovskites and only H3PO2-modified perovskites in Figure 3.3d-i.
In addition, our synthesized 3D σ-FAPbI3 NPL exhibits a narrow band gap with a high absorption coefficient in the spectrum extended to 890 nm. This is the first time we have obtained pure 3D σ-FAPbI3 with an intense absorption spectrum extended to 890 nm at R.T.
In synthesis 4, the amount of OCT is 60 µL, the IPA vapor treatment for one week; In synthesis 5, the amount of OCT is 200 µL, the IPA vapor treatment for one week; In case of synthesis 7, the amount of OCT is 60 µL, the IPA vapor treatment for one month;
In the case of synthesis 6, the amount of OCT is 500 µL, IPA steam treatment for one week;. In the case of synthesis 8, the amount of OCT is 200 µL, IPA vapor treatment for one month;. In the case of synthesis 9, the amount of OCT is 500 µL, IPA vapor treatment for one month;.
Two-band emission of water-stable two-dimensional organic-inorganic halide perovskites with Mn(II) dopant. The dual-band emission process of this Mn-doped water-stable 2D perovskite would help in the development of highly efficient 2D water-stable perovskites for practical applications. We demonstrate for the first time that the high fluorescence of a Mn-doped (PEA)2PbBr4 nanocrystal can be stabilized in water within 45 days.
Results and discussion
The visible light spectrum of the fluorescence microscopy image in Figure 4.6 showed obvious coincident single particle fluorescence for each nanoplate compared to the microscopy images in Figures 4.6. The existence of both luminescent processes is confirmed by the confocal PL spectra (Fig. 4.7d-f), including the band edge emission and the field transition of the Mn2+ 4T1-6A1 ligand. In contrast to PL peaks 1 and 3 at room temperature, the unusual development of the PL peaks at 77 K shows that Mn2+ emission dominates the PL spectrum accompanied by a small band edge emission (Figure 4.7h-l).
Metal doped T2SnCl6 SCs are synthesized by hydrothermal crystallization while T2SnCl6 NCs were synthesized by room temperature LARP process (Figure 5.1a). Due to the smaller B-site cation (Sn4+) and larger A-site component (T+), the [SnCl6]2- is completely isolated and surrounded by T+ cations forming the 0D structure with vacancy-ordered cubic phase (Figure 5.1 d). To obtain the detailed optical properties of SCs of 1‒4, we recorded UV-Vis absorption and PL spectra (Figure 5.7a and Figure 5.6).
This further confirms that Bi/Sb codoping in 4 induces PL redshift compared to 2 (Figure 5.7b). To highlight the highly efficient photoluminescence of NCs, we show the PL imaging as well as the PL spectra in Figure 5.8a-b and Figure 5.5.
Keywords: lead-free perovskite, tin and bitmuth perovskites, highly efficient luminescence, doping engineering, Water and thermal stability. To date, there is no report of water-stable, lead-free zero-dimensional (0D) organic-inorganic hybrid colloidal tin(IV) perovskites of A2SnX6 (A = monocationic organic ion) nanocrystals (NCs) with high photoluminescence (PL) quantum yield ( QY). Our findings open a path for the design of lead-free perovskite materials for thin-film-based optoelectronic devices.
Due to organic cation (T+)-controlled large spin-orbit coupling (SOC), T2Sn1-xSbxCl6 NCs exhibit bright orange emission with an enhanced PL-QY of 41%. These high PL-QY perovskite NCs are worth exploring for commercial applications due to their low cost and ease of solution-oriented device manufacturing process. Due to the T-site cation-controlled spin-orbit coupling (SOC), Sb-doped NCs exhibit efficient transitions of triplet self-trapped excitons (STEs) with an enhanced PL-QY of 41%, as well as surface defect-induced blue emission with PL-QY of 6 .9% for T2SnCl6 NCs (5).
Results and discussion
For Bi and Sb codoping in the host lattice, 4 shows one broad emission peak centered at 490 nm with tail extended to 600 nm and the PLE characteristics similar to 3 (Figure 5.7 and Figure 5.6). To further investigate the thermal stability of the samples, simultaneous differential scanning calorimetry (DSC)/ thermogravimetric analysis (TGA) analysis (SDT) was performed to illustrate the ability of temperature tolerance (Figure 5.9c and Figure 5.10). While below 335°C and above 400°C, there is a slight weight loss (Figure 5.10) in TGA curves, which is considered as the dehydration process and decomposition of oxychloride compounds, respectively.[42,43].
Crystal structures and corresponding product images for δ and α phases of FAPbI 3 …
Powder images, crystal structures and conversion scheme of quasi-2D
To further verify the conclusion drawn from the SEM/EDS and XRD pattern, comparative XPS analyzes were performed between 2D OH-HP:Mn and pristine perovskites (Figure 4.2j-l, Figure 4.4). To further confirm the dual-band PL emission processes in layered perovskites, multi-photon confocal fluorescence microscopy and temperature-dependent fluorescence spectrum measurements were performed under UV light (365 nm) (Figure 4.7, Figure 4.6 ). Meanwhile, the red-orange fluorescence peak shifts to a lower energy and becomes the dominant feature (Figure 4.7h-l).
In Figure 4.9c, static contact angle tests of glass and PEAP-Mn films with deionized water were performed. All samples show bright underwater luminescence even after being immersed in water for 7 days (Figure 5.9a) and the components collected from water are characterized by HP-XRD to check its stability (Figure 5.9b).