The field of solution processed organic-inorganic halide perovskite based solar cells has emerged in the last couple of years1-6. Intensive research has led to a rapid rise in power conversion efficiency (PCE) since their original publication in 2009 to values reaching 22.1% in 20167, showing superiority over all other third generation photovoltaic devices. The most studied compound in photovoltaic is the methylammonium lead iodide CH3NH3PbI3 (CH3NH3 = MA) perovskite, due to its strong bandgap absorption of about 1.6 eV8, 9. While the predominant research have since been oriented towards the use of these materials as active layer in light-harvesting devices, more recent studies have demonstrated photoelectronic applications of perovskite materials in light-emitting-devices (LEDs)10-14. In these studies, the compound of the perovskite is MAPbBr3 with bandgap of about 2.3 eV. The above mentioned perovskite has a generic structure of APbX3 (A = cationic organic molecules; X = halogens), which can be made from abundant and low-cost starting compounds.
Nano-structured halide perovskite hold great promise for various optoelectronic applications, especially for electroluminescent devices and lasers15-21. Researchers have been developing synthesis approaches to create a variety of nanostructures of organic-inorganic halide perovskite to expand the property space and to achieve new properties. Pérez-Prieto and co-workers did pioneering works about non-template synthesis of perovskite nanopaticles (NPs)22. Nanoscaled MAPbBr3 NPs were first synthesized at mild temperature. MAPbBr3 NPs can be prepared by fine-tuning the molar ratios of all the componments, which either form part of the framework (MABr and PbBr2) or act as the organic capping (octylammonium bromide and 1-octadecene, ODE)23. Zhong and co-workers demonstrated a strategy to prepare MAPbBr3 NPs by ligand-assissted reprecipitation (LARP) technique in order to solve the poor solubility of perovskite precursors in ODE24. All precursors were dissolved into the DMF and dropped into the poor solvent. Typical products had an average diameter of 3.3 nm with a size deviation of ±0.7 nm and the PLQY up to 70%. Ogale and co-workers have recently reported the preparation of MAPbBr3 NPs by electrospay antisolvent-solvent extraction and intercalation 25.
The photoluminescence (PL) spectrum of halide perovskite also can be modified via controlling the ratio of halide26, 27. However, tuning the band-gap in the blue-green region using solution processed chloride-bromide mixed halide perovskite has been a challenging task, given the low solubility of the chloride containing precursor (PbCl2) in solvent DMF. With the lead acetate to synthesis the perovskite NCs, the growth of perovskite crystal is much faster28. Lead acetate also has a high solubility in DMF. So we here use an organic lead source of lead acetate Pb(Ac)2 to synthesis the perovskite NCs.
In this work, we use an organic lead source of lead acetate as Pb precursor to synthesis the mixed halide perovskites MAPbBr3-xClx (0≤x ≤3) (NCs), especially for the blue-green perovskite MAPbBr3-xClx NCs. The obtained colloidal MAPbBr3 NCs with absolute quantum yield reaches 73% at room temperature, which is comparable to the reported MAPbBr3 QDs24. We conducted surface characterization, optical properties and thermal stability to illustrate the MAPbBr3 colloidal NCs obtained by the lead acetate. Finally, we tuned the color by varying the chloride to bromide ratios in the MAPbBr3-xClx (0 ≤ x ≤ 3) perovskite using lead acetate. The PL emission peak can be tuned from 399 to 527 nm. Their FWHM are about 20 nm, indicating better color purity.
The precursors CH3NH3Br and CH3NH3Cl were synthesized from HBr (48% in water, Aladdin) and HCl (37% in water, Beijing Chemical Works) respectively, by reaction with methylamine solution (27%-32% in ethanol, Sinopharm) as follows. First, acid solution (HBr or HCl) was added dropwise to CH3NH2 solution at 0 ℃ with stirring for 2 h. The mixture was evaporated in a rotary evaporator under vacumme at 60 ℃. The resulting solid was washed with diethyl ether (AR, Beijing Chemical Works) for three times and then recrystallized from ethanol (AR, Beijing Chemical Works). The obtained CH3NH3Br and CH3NH3Cl crystals were dried under vacuum and used without further purification.
For MAPbBr3, 36 mmol·L-1 of Pb(Ac)2∙3H2O (Ac = CH3COO, AR, Sinopharm) and 108 mmol·L-1 of MABr were dissolved in 5 mL DMF (AR, Beijing Chemical Works) with 20 μL of n-octylamine (99%, Aladdin) and 0.5 mL of oleic acid (85%, Aladdin) to form a precursor solution. 0.2 mL of precursor solution was dropped into 5 mL of toluene (AR, Beijing Chemical Works) with vigorous stirring. Along with the mixing, strong green PL emission was observed. The MAPbCl3-based NCs were fabricated using the same strategy with 36 mmol·L-1 of Pb(Ac)2∙3H2O and 108 mmol·L-1 of MACl as the precursor. For the synthesis of mixed-halid-based perovskite NCs, separate precursor solutions of MAPbBr3 and MAPbCl3 were mixed with different volume ratios.
Transmission electron microscopy (TEM) images were captured using HITACHI HT7700 (Japan). X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2500 X-ray diffractometer (Cu Kα radiation, λ= 0.15402 nm, Japan). X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VG Scientific ESCALab 220i-XL (England) spectrometer with standard Al Kα radiation. The UV-Visible spectra (UV-Vis) were recorded using HITACHI U3010 UV-Vis spectrophotometer (Japan). The steady-state PL spectra were performed at room temperature on a HITACHI F-7000 fluorescence spectrophotometer (Japan). The PL quantum yields of colloidal perovskites were measured by using a Edinburgh FLS980 (England) absolute PL quantum yield measurement system with monochromatic light source (Xe lamp, 150 W) and integrating sphere. The lifetime was measured using a Compact fluorescence lifetime spectrometer C11367 (Japan), Quantaurus-Tau, with LED excitation wavelength of 365 nm.
Colloidal perovskite NCs were prepared by the modified ligand-assisted reprecipitation method as reported previously24. The scheme of the synthesis process is shown in Fig. 1. In this work, Pb(Ac)2 and MABr for the synthesis of MAPbBr3, were dissolved together with oleic acid (OA) and octylamine (OLA) in the good solvent dimethylformamide (DMF), resulting in a transparent solution (Fig. 1a), which has no light emitting under 365 nm UV-lamp. The precursor solution was subsequently dropwise to the poor solvent toluene under vigorous stirring at room temperature (Fig. 1a). Poor solvent means that in which both perovskite precursors are completely insoluble. Initially, the sample fluoresces blue, but with each addition of further precursor solution, the color shifts to green (Fig. 1b). Finally, the semitransparent colloidal with green emitting under UV-lamp was formed. Smaller MAPbBr3 NCs were formed when tiny amounts of the precursors (7 μL) were added into the toluene. The smaller MAPbBr3 NCs exhibited blue-shifted emission due to quantum confinement analogous to conventional semiconductors. The change in photoluminescence with increasing amount of precursor indicates that the formation takes place via seed-mediated growth15, 17.
Fig. 2a shows a typical TEM image of MAPbBr3 NCs, it is observed that typical MAPbBr3 NCs have an average diameter of 11 nm with a size deviation of ±3 nm (Fig. 2b). The particle size distribution of MAPbBr3 NCs is uniform without large crystals. In order to analyze the phase structure, XRD (Fig. 2b) patterns were applied to characterize the obtained samples. The diffraction peaks of MAPbBr3 NC at 15.07°, 21.26°, 30.28°, 33.90°, 43.31°, 46.00° can be index to the (100), (110), (200), (210), (220) and (300) planes, respectively, corresponding to a cubic phase group29, 30. In order to understand the chemical composition and the surface properties of the MAPbBr3 NCs, the samples were subjected to XPS analyses and the results are shown in Fig. 2(d-f). The XPS data in Fig. 2d show two symmetric peaks of Pb 4f7/2 and Pb 4f5/2 at binding energy value 138.4 and 143.2 eV, respectively. No sign of metallic Pb was observed in the NCs. The Br 3d peak can be fitted into two peaks with binding energies of 68.5 and 69.5 eV, respectively. The N 1s XPS data are plotted in the Fig. 2e. In the MAPbBr3 NCs, N 1s peak can be fitted to two peaks at 399.3 and 402.1 eV, indicating existence two chemical states. The peak at 399.3 eV can be assigned to the presence of -NH2 from OLA, while the peak at binding 402.1 eV originates from methylamine. The -NH2 group as a ligand intercalate with the MAPbBr3 NCs and control the size of the MAPbBr3 NCs15-17.
The optical properties of MAPbBr3 NCs were investigated by steady-state absorption, photoluminescence (PL), and recombination lifetime. Fig. 3a shows the PL behavior and absorption spectra MAPbBr3 NCs. The abrupt absorption onsets and emission peaks at 527 nm correspond well with the band-to-band transition of bromide perovskite. Its FWHM is ~20 nm. This is comparable to previous colloidal MAPbBr3 quantum dots (QDs) solution results23, 24, indicating better color purity. The absolute PLQY of colloidal solutions was 73%. The high PLQY indicated the reduction of nonradiative decay in high-quality MAPbBr3 NCs. The recombination lifetime of MAPbBr3 NCs was determined by measuring PL decay at the emission peak wavelength (λpeak). The PL decay curve of colloidal MAPbBr3 NCs was shown in the Fig. 3b. The curve is fitted with a triexponential function of time F(t), where τi is the decay time and ai is a prefactor.
The average recombination lifetime (τave) was obtained from the triexponential decays according to the equation (2):
The PL decay fitting result is shown in the table of Fig. 3b. The τave of MAPbBr3 NCs is 97.4 ns, indicating a low recombination rate compared to the MAPbBr3 QDs. The triexponential decay suggests that there are three components in the colloidal solution. The fast decay is related to trap-assisted recombination between the NCs, whereas the slow decay is related to exciton recombination inside the NCs13.
We used circulating water controller system to test the thermal stability (Fig. 4). The experimental temperature ranged from 20 to 80 ℃. The PL intensity decrease when the temperature increased (Fig. 4a). Along with the quenching of MAPbBr3 NCs emission, the peak wavelength has a little blue shift and there is no obvious change about the FWHM. The intensity of the NCs emission increased with the cooling of the temperature (Fig. 4b). After the temperature cooling to the original temperature, the peak wavelength and the FWHM can fully recover, but the PL intensity cannot recover. The relative intensity of MAPbBr3 NCs is shown in Fig. 4c. From Fig. 4c, we can see that the PL intensity was decreased to about 75% after heat treatment. It is possibly due to some surface breakage that may promote the recombination which is similar to the conventional inorganic QDs33.
We fabricated a series of colloidal MAPbBr3-xClx NCs with Pb(Ac)2. Fig. 5a shows the photo of NCs with compositions varying from pure Br to pure Cl colloidal NCs. The colloidal samples show different emission colors from green to blue when illuminated with UV-lamp. We use XRD pattern to study the structural properties of the MAPbBr3-xClx NCs, and the result is shown in the Fig. 5b. Multiple reflections demonstrate that all the MAPbBr3-xClx NCs associated with the cubic Pm3m space group. Most remarkably, we find that all Bragg peaks slightly shift towards a high angle along with the increase in the smaller Cl substitution ratio. The inset of Fig. 5b shows a cubic lattice constant a as a function of x. The cubic lattice constant ais extracted from the angular position of the (100) Bragg reflection 2θ using λ/2a = sinθ. The monotonic trend in the lattice constants from MAPbBr3 to MAPbCl3 indicates an expansion of the perovskite cage. The normalized PL peak wavelength of the MAPbBr3-xClx NCs is shown in Fig. 5c. The change in emission indicates the increase in the bandgap as Br is replaced with Cl. The PLQYs of the MAPbBr3-xClx NCs were measured using a fluorescence spectrometer equipped with an integrating sphere and excitation at a wavelength of 365 nm and the data were summarized in Table 1. It is surprising that such a slight variation in the MAPbBr3-xClx can lead to such a remarkable difference in their PL emission intensity. The exact reason is still not clear, it might be associated with the orientation and vibration restraint of the MA cation in the MAPbBr3-xClx NCs 30. In addition, Pb(Ac)2 also can be used to synthesis the MAPbBr3-xIx NCs and the PL emission spectra are shown in the inset of Fig. 5c. The PL decay curves of MAPbBr3-xClx NCs are shown in the Fig. 5d. The fitting results are shown in Table 1. We can see that the corresponding τave of MAPbBr3-xClx NCs decreases with the increase of Cl substitution ratio. This trend is the same as that of PLQY, indicating the reduction of nonradiative decay in high-quality MAPbBr3 NCs.
In summary, mixed halide perovskites MAPbBr3-xClx NCs are successfully synthesized by using lead acetate. We also tune the color by varying the chloride to bromide ratios in the MAPbBr3-xClx (0 ≤x ≤ 3) perovskite using lead acetate. The PL emission peak can be tuned from 399 to 527 nm. Their FWHM are about 20 nm, indicating better color purity. The NCs are uniformly dispersed in toluene with an average size of (11 ± 3) nm. The photoluminescence quantum yield of MAPbBr3 NCs reaches ~73%. The τave of MAPbBr3 NCs is 97.4 ns, indicating a low recombination rate. Above all, Pb(Ac)2 can be used to prepared MAPbBr3-xClx NCs, so it may be a promising lead source to fabricate the perovskite optoelectronic devices.