Synthesis, Optical Properties, and Exciton Dynamics of Organolead

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Synthesis, Optical Properties, and Exciton Dynamics of Organolead Bromide Perovskite Nanocrystals Binbin Luo, Ying-Chih Pu, Yi Yang, Sarah Lindley, Ghada Abdelmageed, Hoda Ashry, Yat Li, Xueming Li, and Jin Zhong Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08537 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on October 29, 2015

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The Journal of Physical Chemistry

Synthesis, Optical Properties, and Exciton Dynamics of Organolead Bromide Perovskite Nanocrystals

Binbin Luoa,b‡, Ying-Chih Pua,d‡, Yi Yanga, Sarah Lindleya, Ghada Abdelmageeda,c, Hoda Ashryc, Yat Lia, Xueming Lib* and Jin Z. Zhanga*

a

Department of Chemistry and Biochemistry, University of California, Santa Cruz,

California 95064, United States b

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing

400044, China c

Department of Radiation Physics, National Center for Radiation Research and

Technology (NCRRT), Atomic Energy Authority (AEA), Nasr City, Cairo, Egypt d

Department of Materials Science, National University of Tainan, Tainan 70005, Taiwan,

ROC

1 ACS Paragon Plus Environment


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Abstract

Organolead bromide CH3NH3PbBr3 perovskite nanocrystals (PNCs) with green photoluminescence (PL) have been synthesized using two different aliphatic ammonium capping ligands, octylammonium bromide (OABr) and octadecylammonium bromide (ODABr), resulting in PNC-OABr and PNC-ODABr, respectively. Structural studies by X-Ray diffraction (XRD) and transmission electron microscopy (TEM) determined that the PNCs exhibit cubic phase crystal structure with average particle size dependent on capping ligand (3.9 ± 1.0 nm for PNC-OABr and 6.5 ± 1.4 nm for PNC-ODABr). The exciton dynamics of PNCs were investigated using femtosecond transient absorption (TA) techniques and singular value decomposition global fitting (SVD-GF), which revealed non-radiative recombination on the picosecond time scale mediated by surface trap states for both types of PNCs. The PL lifetime of the PNCs was measured by time-resolved photoluminescence (TRPL) spectroscopy and fit with integrated SVD-GF to determine the radiative as well as non-radiative lifetimes on the nanosecond time scale. Finally, a simple model is proposed to explain the optical and dynamic properties of the PNCs with emphasis on major exciton relaxation or electron-hole recombination processes. The results indicate that the use of capping ligand OABr resulted in PNCs with high PL quantum yield (QY) of ~20% (vs. fluorescein, 95%), which represent interesting optical properties and are promising for potential applications including photovoltaics, detectors, and light emitting diodes (LEDs).

Keywords: Perovskite, CH3HN3PbBr3, Exciton, Dynamics, Photoluminescence


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1. Introduction Organic-inorganic lead halide perovskites with the general formula APbX3 (A=organic ammonium cation, X= Clˉ, Brˉ, Iˉ) have attracted tremendous attention due to unique optical and electronic properties, such as a bandgap tunable with chemical composition and broad absorption covering the visible to near-infrared spectral range.1-4 Furthermore, long diffusion length (0.1~3 µm) and high mobility (8~33 cm2V-1s-1) charge carriers position these perovskites as useful materials for device applications.3,5-15 In particular, perovskites have shown great promise for photovoltaic solar energy conversion.8,15-23 The power conversion efficiency (PCE) of methylammonium lead halide perovskite (CH3NH3PbX3)-based thin films has skyrocketed from 9.7% to 20.1% in the last three years due to recent intensive progress on improving device fabrication techniques.7,15,24 Although methylammonium lead iodide perovskite (CH3NH3PbI3) is most commonly studied, the bromide version (CH3NH3PbBr3) is particularly promising as it shows a narrowed green photoluminescence (PL) emission band (full width at half maximum (FWHM) ~ 20 nm) and can be solution processed at low temperature for device fabrication, important characteristics for light-emitting diodes (LEDs) and display applications.1,4,25,26 However, the intrinsic defect states and instability of CH3NH3PbBr3 have thus far limited its practical application.1,24,27 Semiconductor nanocrystals have generated great fundamental and technological interest because of the quantum confinement effect that results in novel tunable optical properties and high QY emission that are useful for optoelectronic and biomedical applications.28-31 The quantum confinement effect of the nanosized particles could



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increase exciton binding energy and result in enhanced excitonic emission.1 Due to very large surface-to-volume (S/V) ratio, nanocrystals serve as great model systems for studying the effects of surfaces, interfaces, and related issues such as charge carrier dynamics.32 While a significant amount of work has been done on II-VI semiconductor quantum dots (QDs),33-36 limited work has been reported on PNCs. Recently, Schmidt and co-workers reported the synthesis of CH3NH3PbBr3 PNCs, which showed strong PL emission for LED application.6 Similarly, Zhang, et al. demonstrated that suitable surface passivation of CH3NH3PbBr3 perovskite quantum dots (PQDs) by octylamine could improve the PL emission to achieve high PL QY of ~70%, as compared to a thin film of perovskite microsized crystals with low PL yield of < 0.1%.1 The PL lifetime of colloidal CH3NH3PbBr3 PQDs was measured, revealing a short-lived component of 6.6 ns and a long-lived component of 18.0 ns with relative amplitude of 63.6% and 36.4%, respectively. This result suggests that the PL decay mainly takes place through the ultrasfast 6.6 ns component, which may be attributed to excitonic radiative recombination. Although PNCs are promising materials for improved LED performance and potentially useful for solar cells, their photophysical properties are largely unknown at present, especially in terms of fundamental charge carrier dynamics and the effect of materials properties such as size and surface characteristics.1,37 Understanding the exciton dynamics in PNCs and perovskites in general is essential for developing high performance perovskites for various applications.11,38-41 To date, few dynamics studies have been reported on perovskite thin films. For instance, Manser, et al. demonstrated that the bandedge recombination of charge carriers in CH3NH3PbI3 thin films follows second-order kinetics, indicating a two-body recombination mechanism,


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with a rate constant of 2.3 ± 0.6 × 10-9 cm3s-1.41 In the meantime, the interaction between CH3NH3PbI3 and H2O was investigated by studying the changes in exciton dynamics, morphology, and crystal structure of CH3NH3PbI3 when exposed to H2O vapor.42 The results indicated that H2O was able to complex with perovskite and decrease its absorption properties, resulting in decreased photovoltaic efficiency and stability. In contrast, very few studies have reported on complete charge carrier dynamics of CH3NH3PbBr3 PNCs from the ultrafast picosecond to nanosecond time scale. In this work, CH3NH3PbBr3 PNCs were prepared using aliphatic ammonium bromides of different lengths as capping ligands to study their optical and structural properties as well as their exciton dynamics. Characterizations of the structural and optical properties of the PNCs samples were carried out with X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Vis absorption and PL spectroscopy. Exciton dynamics of PNCs samples were studied using femtosecond pump-probe laser spectroscopy as well as time-resolved photoluminescence (TRPL) spectroscopy. The dynamics data were analyzed using singular value decomposition global fitting (SVD-GF) to extract related lifetimes. This study reveals that capping ligand selection plays a critical role in directly affecting the energy level as well as the dynamic processes of charge carriers and improving the PL QYs of PNCs.

2. Experimental Section 2.1 Materials



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All chemicals were used as received without any further purification, including toluene (spectroscopic grade, Fisher Scientific)， acetone (spectroscopic grade, Fisher Scientific), N,N-dimethylformamide (DMF, spectroscopic grade, Fisher Scientific), PbX2 (X=Cl, Br, I, 99.99%, Alfa Aesar), methylamine (40%, TCI), octylamine (99%, Acros), octadecylamine (97%, Sigma-Aldrich), hydrobromic acid (48%, Sigma-Aldrich), 1-octadecene (ODE, 90%, Alfa Aesar), and oleic acid (90%, Alfa Aesar). 2.2 Synthesis of MABr, OABr and ODABr The precursors, including octadecylammonium bromide (ODABr), octylammonium bromide (OABr) and methylammonium bromide (MABr), were synthesized by reacting the corresponding amine with hydrobromic acid, according to a reported procedure.16,43 In the case of MABr, 15 mL hydrobromic acid was mixed with 10 mL methylamine in a 150 mL beaker under thorough stirring at 0 oC until a clear solution was formed. Precipitation was induced by heating the solution at 70 oC for 12 h and the resultant precipitate was washed two times with absolute ethanol to remove any impurities. The final product was collected after drying at 50 ºC for 3 h. OABr and ODABr were prepared by mixing octadecylamine/octylamine with hydrobromic acid in a 1:1 molar ratio. After the solution was cooled down to room temperature, the precipitate was washed three times with diethyl ether and then dried under vacuum. 2.3 Synthesis of PNCs Synthesis of PNCs samples. PNC-OABr was synthesized by a modified nontemplate method that has been reported by Schmidt, et al.6,44 First, a solution of OABr (0.12 mmol in 4 ml ODE) was stirred and heated to 120 oC. After the solution became homogeneous,


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MABr (0.08 mmol in 200 µL DMF) and PbBr2 (0.2 mmol dissolved in 200 µL DMF) solutions were added. The resultant solution was cooled down to room temperature and PNC-OABr was subsequently precipitated by addition of acetone. The PNC-OABr precipitate was washed and collected by centrifugation (5000 rpm, 5 min) and redispersed into toluene at the desired concentration for further characterization. The same procedure was followed for the preparation of PNC-ODABr except that additional added of oleic acid (200 µL in 4 ml ODE) and OABr was replaced by ODABr. 2.4 Preparation of PNC films CH3NH3PbBr3 PNC films were prepared by dropping 50 µL of PNC-OABr or PNC-ODABr solutions (40 mg/mL) on a glass substrate (2 cm × 2 cm), respectively, and spin coating at 2000 rpm for 1 min. 2.5 Characterization Methods X-Ray diffraction (XRD, Rigaku Americas Miniflex Plus powder diffractometer) analysis was used to investigate the crystalline phase and phase change at a voltage of 40 kV and current of 30 mA. The scanning angle range was 10-50o (2θ) with a rate of 3o/min. Transmission electron microscopy (TEM) and High resolution TEM (HRTEM) were carried out to investigate the morphology and interlayer spacing of the PNCs samples. The TEM study was carried out using an FEI UT Tecnai HRTEM microscope operated at 200 kV accelerating voltage, located at the Molecular Foundry National Center for Electron Microscopy (NCEM) at Lawrence Berkeley National Laboratory. The particle size and size distribution of the PNCs were also examined by dynamic light scattering (DLS) with the Malvern Zetasizer (Nano-ZS). UV-Vis spectra (Agilent Technologies,



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Cary 60) and fluorescence spectra (FluoroMax-3) were measured by using a quartz cuvette (1 cm × 1 cm) as a sample container at 25 oC. The PL QY for each PNC sample was determined by Eq. 1:45

QY = QY

(1)

QYst is the quantum yield of a standard substance, fluorescein, which is known to be 95% in a 0.1 M NaOH solution with excitation at 470 nm.45 Ast and As are the absorbance values of the fluorescein and PNC samples at wavelength of 470 and 365 nm, respectively. Ast was adjusted to be less than 0.1 and close to As. Ds and Dst are the corresponding integrated intensities for the PNC samples and fluorescein with excitation wavelength of 365 nm and 470 nm, respectively. ns and nst are the refractive indices of the solvents used (ns = 1.49 for toluene and nst = 1.33 for H2O).46 2.6 Femtosecond Transient Absorption (TA) Spectroscopy Ultrafast transient differential absorption measurements were conducted using an amplified femtosecond Ti-sapphire laser system described previously.47-49 In all experiments, a pump wavelength of 380 nm was generated from an optical parametric amplifier (OPA) and used to excite the perovskite film samples. In the wavelength-dependent study, < 180 fs output pulses with energy of 500, 230, 90, and 40 nJ/pulse were used. A white light continuum (WLC) spanning 450 – 700 nm was used as the probe and was detected using a charge-coupled device (CCD) detector. The TA data after pumping with 40 nJ/pulse was analyzed using Matlab and singular value decomposition (SVD) procedures in which 3 basis vectors were kept from the U


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(time-independent) and V (wavelength-independent) matrices to extract the B-spectrum (the time-independent initial amplitudes associated with each probe wavelength for a given decay time constant). The resulting time dependence in the V matrix was fit with a double or triple exponential shown in Eq. 2, which was used to describe the exponential decay of the TA signal at each wavelength, where I(t) is the dA amplitude at time (t), and Ai is the initial intensity of the component of the decay corresponding to the lifetimeτi. These global fitting procedures were developed in house and reported previously. 49,50 = ∑

(2)

2.7 Time-Resolved Photoluminescence (TRPL) Spectroscopy Time-resolved PL spectra were measured using a home-built single photon counting system. GaN diode laser (λ = 375 nm) with 50 ps pulse duration was used as the excitation source.46,51,52 The signals collected at the excitonic emission of PNCs were dispersed with a grating spectrometer, detected by a high-speed photomultiplier tube, and then correlated using a single photon counting card. The PL was collected at a 90° angle and passed through a magic angle polarizer and into a monochromator. The decay spectrum was recorded at 10 nm steps from 450 to 550 nm, which encompassed the majority of the PL peaks from the perovskite samples. Each decay trace was normalized to collection time and was used to reconstruct the PL decay spectrum. This spectrum was subsequently deconvolved with SVD and fit globally using a double exponential decay function to obtain the B-spectra, as shown in Eq. 2. The b-spectra as well as the extracted lifetimes were integrated using Eq. 3 to report the photon flux, ΦP: 33,50,53



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Φ = !

"

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(3)

3. Results and Discussion 3.1 Structural Determination of PNCs by XRD and TEM The PNC structure and the influence of capping ligand (OABr vs. ODABr) was addressed via XRD and TEM. The XRD patterns of PNC-OABr and PNC-ODABr shown in Figure 1a are used to determine their crystal structures. The peaks centered at 14.80°, 21.03°, 29.98°, 33.55°, 36.93°, 42.83° and 45.69° could be assigned to organic-inorganic CH3NH3PbBr3 perovskite with cubic phase structure (Pm-3m space group).6 On the contrary, the peaks centered at 10.91°, 14.30°, 19.00°, and 28.7° could be assigned to ODABr for PNC-ODABr, and 14.80° could be assigned to OABr for PNC-OABr. Obviously, the XRD patterns of PNC-OABr and PNC-ODABr are similar, which indicates that the same crystal structure is obtained regardless of which alkyl ammonium bromide is used in the synthesis. However, as compared to that of OABr in the PNC-OABr XRD pattern, the XRD pattern of PNC-ODABr possesses a higher intensity ODABr peak at 10.91°, which could be attributed to the lower solubility of ODABr than that of OABr in ODE solvent.


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Figure 1．(a) XRD pattern of PNC-OABr and PNC-ODABr. TEM images of PNC-OABr (b) and PNC-ODABr (c). Inset: High magnification HRTEM images of PNC-OABr (b) and PNC-ODABr (c).

The morphology of the PNCs was determined using HRTEM, as shown in Figure 1b, which indicates particle size distribution and local crystallinity. PNCs with an average particle size of 3.9 ± 1.0 nm were obtained when OABr was used as a capping ligand (Figure S1a). In addition, the well-resolved lattice fringes of the fully crystalline PNCs can be clearly seen in the inset of Figure 1b. An interlayer spacing of 2.99 Å (Figure 1b inset) was obtained for the PNC-OABr sample, which agrees well with the (002) plane of CH3NH3PbBr3 perovskite reported in a previous study.6 On the other hand, when the capping ligand OABr was replaced by ODABr in the synthesis, the average particle size of the resultant PNC-ODABr increased to 6.5 ± 1.4 nm (Figure S1b). The broader particle size distribution for PNC-ODABr may be due to the poor solubility of ODABr in ODE solvent as compared to that of OABr. Similar to PNC-OABr, PNC-ODABr is fully crystalline and has an interlayer spacing of 2.99 Å (Figure 1c, inset). Based on a previous



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report, the Bohr radius of CH3NH3PbBr3 perovskite is around 2.0 nm,54 which indicates that the as-prepared PNCs may exhibit some quantum confinement effect since their physical size is close to the Bohr exciton diameter (4 nm). In addition, the particle size distribution of the PNC samples was consistent with the results of DLS measurement as shown in Figure S2. Note that we also observed 2D plate-like nanostructures by TEM characterization (not shown) in addition to our PNCs, as reported in Tyagi’s study.37 However, the PNC samples used in the present study were further purified by centrifugation to separate the nanoplates from the as-prepared PNC solution to reduce the influence of different crystal geometries on resultant optical properties. 3.2 Optical Properties of PNCs UV-Vis electronic absorption and PL emission spectra of PNC-OABr and PNC-ODABr are shown in Figure 2. The UV-Vis spectrum of PNC-OABr shows a sharp rise at 525nm with saturation peak at 507 nm. In addition, a weak, second peak at 470 nm is also visible. The PL spectrum of PNC-OABr after 365 nm excitation shows a symmetric band peaked at 513 nm (~2.42 eV), which may be attributed to charge carrier recombination of conduction band (CB) electrons with valence band (VB) holes or trapped electrons with holes in surface states.32 Based on the measurements of UV-Vis and PL spectra as well as QY calculation, the room temperature PL QY for PNC-OABr was around 20% (vs. fluorescein, 95%). The high PL QY of PNC-OABr results from the stronger attraction between electron-hole pairs due to quantum confinement and resultant enhancement of the excitonic emission. More importantly, there is ~20 nm blue-shift of PL emission as compared to that of CH3NH3PbBr3 pervoskite bulk material, which has been reported in recent literature to be ~535 nm.25,26,54,55 The PL blue-shift and high QY


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confirm that the quantum confinement effect is present in the PNC-OABr sample, which is also consistent with the particle size as characterized by TEM.

Figure 2. UV-Vis electronic absorption (dashed) and PL emission (solid, λex =365 nm) spectra of PNC-OABr (blue) and PNC-ODABr (red) in toluene. Inset: photographs of the PNC-OABr and PNC-ODABr samples with normal room light and irradiation with a UV lamp.

The UV-Vis and PL spectra of PNC-ODABr exhibit features similar to those of PNC-OABr but different in certain aspects. The first excitonic absorption band of PNC-ODABr is peaked at 527 nm, with a tail at longer wavelengths that is likely due to scattering as a result of poor particle suspension. Similarly, a weak, second peak is observed at 470 nm. The PL spectrum of PNC-ODABr shows a symmetric band with a maximum at 529 nm. As compared to that of PNC-OABr, the PL peak of PNC-ODABr is



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significantly red shifted, which can be attributed to the larger particle size and therefore less quantum confinement. The PL QY of the PNC-ODABr sample was estimated to be ~10 %, lower than that of PNC-OABr, which may be due to the weaker attraction between electron-hole pairs because of less quantum confinement and the existence of a larger number of surface defects caused by the poorer solubility and weaker capping ability of ODABr as compared to OABr. Although the PNC-ODABr show lower PL QY and poorer suspension as compared to PNC-OABr, both samples exhibit relatively narrow PL bands, with full width at half maximum (FWHM) of 28 nm and 24 nm for PNC-OABr and PNC-ODABr, respectively. The narrow PL bands are indicative of high uniformity in particle size and morphology. The high PL QYs of PNC-OABr and PNC-ODABr suggest that the PNC have high optical quality and crystallinity. 3.3 Exciton Dynamics via Transient Absorption Spectroscopy The femtosecond transient absorption (TA) spectra of PNC-OABr and PNC-ODABr films were recorded using a 380 nm pump (with pulse energy of 500, 230, 90, and 40 nJ/pulse) and a white-light probe (450-700 nm) as a function of time delay between the pump and probe pulses. The wavelength-dependent TA (excited-state absorption) and transient bleach (TB, ground-state depletion) profiles of PNC-OABr and PNC-ODABr are shown as 3D and 2D plots in Figure 3a,b. The transient profiles of PNC-OABr are dominated by a sharp and symmetric TB recovery feature at 470 nm and a broad TB recovery feature from 475 to 510 nm. The time profile of the sharp 470 nm TB band can be fit with a triple exponential with time constants of 0.5 ps, 6 ps, and 50 ps. The broad TB from 475 to 510 nm can be fit by a triple exponential with time constants of 1.5 ps, 90 ps, and 500 ps. The multiple TB features reflect the dynamics of holes in the VB of


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PNC-OABr, and also indicates that the PNCs have complex band structure. One possible reason for the multiple TB features is that the small particle size enhances the quantum confinement effect, which results in more localized holes or more discretized energy levels in the VB of the PNCs. In addition, the notable surface states due to the large S/V ratio may be another possible contributing factor.

Figure 3. 3D (upper) and 2D (lower) plots of transient absorption profiles after excitation with a 380 nm pump (500 nJ/pulse) as a function of probe wavelength (450-700 nm) and delay time (0-100 ps) for PNC film samples: (a) PNC-OABr and (b) PNC-ODABr.

For PNC-ODABr, the transient profiles show a broad TB feature with a maximum at 525 nm, which can be fit by a triple exponential with time constants of 1 ps, 8 ps, and 60 ps. There is also a persistent offset as the signal does not decay completely in the time



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window measured (~1 ns). The broad TB feature may be an indication of extensive surface defects present in PNC-ODABr, which has been suggested based on density function theory (DFT) calculations.56 Since the energy levels of various surface defects are close to the VB edge of CH3NH3PbBr3 perovskite, there may be substantial overlap in the energy levels between the CB and surface defect states. The combination of localized holes and surface defect states with similar energy levels may lead to the observed broad TB feature. The high density of surface defect states will also likely contribute to trapping and non-radiative recombination of charge carriers, thereby lowering the PL QY of PNC-ODABr. Compared to perovskite bulk materials, quantum confinement of the PNCs may enhance the exciton binding energy,57 which could have important consequences on the optical properties and exciton dynamics. By monitoring the TB profiles of the corresponding electronic transitions, we can characterize the dynamic processes of photogenerated charge carriers in PNCs. Here, we focus on analysis of the recovery traces of TB at 505 nm and 525 nm for PNC-OABr and PNC-ODABr, respectively. Figure 4a,b shows the TB recovery profiles of PNC-OABr and PNC-ODABr under four different pump powers (40, 90, 230, and 500 nJ/pulse). The pump power dependence study is important for determining possible non-linear processes that can complicate the interpretation of dynamics results.47,48,58,59 The goal is to find a threshold power under which there is no non-linear effect. The TB recovery traces can be fit with a triple exponential function, and the fitting parameters are summarized in Table 1. The results show that the amplitude of the fast component increased with increasing pump power for both PNC-OABr and PNC-ODABr. The fast component, which essentially disappeared at


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the lowest pump power (40 nJ/pulse) for both samples, is clearly due to a non-linear effect. Since the fast component is absent when the pump power was decreased to 40 nJ/pulse, the dynamics results for this power are considered to be in the purely linear regime and therefore used for analyzing the exciton dynamics in detail.



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Figure 4. TB recovery profiles of (a) PNC-OABr and (b) PNC-ODABr with pump power of 40, 90, 230, and 500 nJ/pulse over 0 to 200 ps time delay. Inset: TB profiles over 0 to 25 ps time delay. (c) Normalized TB recovery profiles of PNC-OABr and PNC-ODABr with 40 nJ/pulse over 0 to 1000 ps time delay. Inset: TB profiles over 0 to 50 ps time delay.

Table 1. Summary of Exciton Lifetime from Triple Exponential Fitting of TB Data as Probed at (a) 505 nm for PNC-OABr and (b) 525 nm for PNC-ODABr. (a)

A1 (%)

τ1 (ps)

A2(%)

τ2 (ps)

A3(%)

τ3 (ps)

(ps)

500 nJ

28 ± 7

8 ± 1.4

54 ± 9

45 ± 3

18 ± 8

250 ± 20

173 ± 17

230 nJ

22 ± 8

8 ± 1.6

52 ± 7

45 ± 5

26 ± 9

250 ± 25

192 ± 21

90 nJ

19 ± 5

7 ± 1.5

27 ± 7

45 ± 6

54 ± 5

250 ± 30

230 ± 23

40 nJ

1±1

6±3

23 ± 8

55 ± 8

76 ± 9

250 ± 23

240 ± 19

(b)

A1(%)

τ1 (ps)

A2(%)

τ2 (ps)

A3(%)

τ3 (ps)

(ps)

500 nJ

38 ± 3

1 ± 0.2

45 ± 3

12 ± 1.5

17 ± 3

102 ± 12

79 ± 11

230 nJ

30 ± 8

2 ± 0.1

36 ± 4

10 ± 2

35 ± 4

100 ± 18

90 ± 16

90 nJ

13 ± 3

2 ± 0.2

38 ± 5

15 ± 1.6

49 ± 7

101 ± 16

92 ± 15

40 nJ

4±5

1 ± 0.9

35 ± 8

10 ± 1.2

65 ± 9

105 ± 13

100 ± 12

Before focusing on the linear dynamics, it is worthwhile to comment further on the non-linear dynamics, especially since this can easily occur in such ultrafast measurements and lead to erroneous interpretation of results. The fast component exhibiting non-linear increase in amplitude with increasing power is due to high-order kinetic processes such as exciton-exciton annihilation or Auger recombination, as observed in other semiconductor QDs.58,59 The generation of multiple excitons or electron-hole pairs per



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QD could first lead to saturation of the trap states. Upon saturation of the trap states, additional excitons generated could accumulate and result in exciton-exciton annihilation.58,60 To focus on the linear charge carrier dynamics, we compare the results using the lowest possible pump power (40 nJ/pulse) in which there is negligible non-linear effect. The normalized TB recovery profiles of PNC-OABr and PNC-ODABr are shown in Figure 4c. The average exciton lifetime of PNC-OABr and PNC-ODABr can be calculated using Eq. 4 and the results of the mathematical fitting of the experimentally measured lifetimes in Table 1:47,61 < $ >= $ & + & $& & + ( $( & / $ + & $& + ( $(

(4)

A longer average exciton lifetime of 240 ± 19 ps was observed for PNC-OABr, as compared to 100 ± 19 ps for PNC-ODABr. The longer exciton lifetime of PNC-OABr indicates a lower density of trap states within the bandgap or better passivation of the PNCs surface by OABr. This result is also consistent with the higher PL QY of PNC-OABr than PNC-ODABr. While single wavelength fitting of the TB recovery trace can provide direct information about the exciton lifetime, we can gain more detailed information about the dynamic processes involved by analyzing the TA/TB results globally, i.e. for multiple wavelengths at the same time. For this purpose, SVD global fitting of the TA signals of the PNC-OABr and PNC-ODABr has been carried out, as has been successfully applied in dynamic analysis of other QDs such as ZnSe:Cu QDs, core/shell/shell/ CdSe/ZnSe/ZnS, and alloyed CdxZn(1-x)Se/ZnSe/ZnS QDs.33,50,53,62


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The plot of the initial intensity of each lifetime component, Ai, as a function of probe wavelength is referred to as the B-spectrum. Figure 5 shows the plots of the B-spectra for the PNC-OABr and PNC-ODABr along with the initial TA/TB spectra and the sum of the B-spectra. For comparison, the raw data is also shown at the related pump probe time delay. For the PNC-OABr (pump with 40 nJ/pulse) sample, the lifetime was found as 6 ± 3 ps, 55 ± 8 ps, and 250 ± 23 ps. The global fitting result of PNC-OABr shows that the 6 ps component was composed of two sharp bands with peaks at λmax = 470 nm and 482 nm and a broad band with a peak at λmax = 496 nm. For the 50 ps component, contribution from the peaks at λmax = 470 nm and 482 nm was decreased while the broad band (λmax = 498 nm) red-shifted. The 300 ps component exhibits only a broad feature at λmax = 501 nm. According to previous studies of the exciton dynamics of the semiconductor nanocrystals or QDs,32,63 the fastest (6 ps) component usually can be attributed to the trapping of electron or hole into shallow trap (ST) states, e.g. interstitial defects of CH3NH3 and Br; cation substitutions (CH3NH3 on Pb or Pb on CH3NH3); Pb, Br and CH3NH3 vacancies; antisite substitution of CH3NH3 on Br.56 As time evolves, the trapped electron or hole can be further trapped into deep trap (DT) states, e.g. interstitial defects of Pb; antisite substitutions (Pb on Br, Br on CH3NH3 and Br on Pb).56 We attribute the medium (50 ps) component trapping to DT states from ST states or the band edges. The slow (300 ps) component can be ascribed to non-radiative recombination of charge carriers in ST states. The long-lived offset (>1 ns) can be attributed to radiative decay of the exciton or radiative recombination of the electron and hole.



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Figure 5. SVD fit results for (a) PNC-OABr and (b) PNC-ODABr with pump power of 40 nJ/pulse, B spectra showing the wavelength dependence of the initial amplitude of the various time constants. The raw data at a number of different time delays is also shown for comparison.

On the contrary, the lifetime of PNC-ODABr was found to be 10 ± 1.2 ps and 105 ± 13 ps. The global fitting result of PNC-ODABr shows that the 10 ps component was composed by a broad feature with λmax = 525 nm. The 105 ps component represents a red-shifted broader feature with λmax = 541 nm and mixed with a positive peak at shorter wavelength (λmax = 510 nm) as compared to the profile of the 10 ps component. The broad TA/TB feature likely indicates that trapping into a manifold of trap states (including STs and DTs) dominates the early dynamics of the electron and hole, similar to PNC-OABr. Therefore, the fastest recovery component (10 ps) can be attributed to the trapping of the electron and/or hole into ST and DT states. The medium (100 ps) component can be attributed to non-radiative recombination of the trapped electron and


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hole in the ST states. We did not obtain a clear slow component or offset in the global fitting result of PNC-ODABr, due to the low S/N ratio for PNC-ODABr. However, based on the result of single wavelength fitting of PNC-ODABr in Figure 4c, the signal did not fully recover on the time scale over 1 ns, which indicates a long-lived component of PNC-ODABr that can be attributed to radiative recombination of the electron and hole. 3.4 Exciton Dynamics via Time-resolved PL Spectroscopy 3D TRPL decay traces of the suspended PNC-OABr and PNC-ODABr solution samples were measured at 10 nm intervals spanning the steady state PL emission band and are shown in Figure 6a,d. The TRPL profile of the PNC-OABr shows an overall slower decay as compared to that of PNC-ODABr. SVD global fitting was used for both samples to find the best lifetime that can fit the decay traces at each wavelength, and the fitting parameters are shown in Table 2. For the PNC-OABr sample, the decay of its TRPL profile could be fit with two symmetric bands with peaks at λmax = 498 nm and 512 nm and lifetimes of 33 ± 4 ns and 1100 ± 90 ns, respectively, as shown in Figure 6b. In addition, the B-spectra from SVD global fitting of the TRPL profiles show the contribution of each lifetime to the initial PL intensity. Consequently, each component of the double exponential decay at a single wavelength can be used to determine the overall photon flux, as shown in Figure 6b. In addition, the B-spectra are in good agreement with the steady state PL spectra, which indicates the high quality of the reconstructed TRPL data by global fitting. The fast decay (33 ns) component with emission band at λmax = 498 nm contributes 6% to the overall steady state PL of the PNC-OABr sample, which can be attributed to the recombination process of CB electron and VB hole. In addition, the observed emission lifetime (33 ns, τob) of the recombination of CB electron and VB hole 23 ACS Paragon Plus Environment


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can be combined with the PL QY to determine the radiative (τr) and non-radiative lifetimes (τnr) using the following equations:64 QY =

$*+ $,

1 1 1 $*+ = $, + $ ,

(5)

(6)

According to Eq. 5 and 6 the radiative and non-radiative lifetimes of the electron-hole recombination from CB to VB of PNC-OABr can be calculated to be around 165 and 42 ns, respectively. On the other hand, the slow decay (1100 ns) component contributes 94% to the overall steady state PL with a redder emission band at λmax = 512 nm, suggesting radiative recombination of the long-lived electron-hole pairs in the DT states of PNC-OABr. This result is unexpected, but possible. Long-lived charge carriers in deep trap states of QDs have been reported from hundreds of ps to tens of ns or even longer, such as in ZnSe:Cu QDs. 53,62 The TRPL profile of PNC-OABr sample could be fit with two symmetric bands with peaks at λmax = 505 and 528 nm and lifetime of 18 ± 2 ns and 450 ± 30 ns, respectively, as displayed in Figure 6e. The broad fast decay (18 ps) component of the emission profile at λmax = 505 nm contributes 26% to the overall steady state PL of PNC-OABr, which can also be attributed to recombination of the electron and hole from the band edges. Similarly, the radiative and non-radiative lifetimes of the electron-hole recombination from CB to VB of PNC-OABr can be calculated as 180 and 20 ns, respectively. The slow (450 ps) component with the redder emission λmax at 528 nm contributes 74% to the


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overall steady state PL, which can be attributed primarily to radiative recombination of the trapped electron and hole in DT states.

Figure 6. 3D TRPL spectra of (a) PNC-OABr and (d) PNC-ODABr, which were collected at 10 nm intervals spanning the width of the PL emission peaks. SVD fit results



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of TRPL for (b) PNC-OABr and (e) PNC-ODABr. The PL decay at each wavelength was separated into two lifetime components. The integrated PL of (c) PNC-OABr and (f) PNC-ODABr from the two components are plotted vs. wavelength and fit to Gaussian functions. The sum of the two Gaussians is compared to the normalized steady state PL spectrum.

Table 2. Summary of PL Lifetimes for Various Decay Components in PNCs by Global Fitting. A1% of PL

τ1 (ns)

A2 % of PL

τ2 (ns)

τr (ns)

τnr (ns)

PNC-OABr

6±1

33 ± 4

94 ± 6

1100 ± 90

165 ± 15

42 ± 3

PNC-ODABr

26 ± 3

18 ± 2

74 ± 4

450 ± 30

180 ± 16

20 ± 2

3.5 Energy Level and Exciton Dynamics of PNCs The fitting results of the TA/TB and TRPL spectra of PNC-OABr and PNC-ODABr are used to propose a model to explain the exciton or charge carrier dynamics, as shown schematically in Figure 7. For the PNC-OABr, there are five major pathways for the exciton or photoexcited electron-hole pair to relax or recombine. First, following photoexcitation and fast cooling within the CB and VB, the electron and hole in the CB and VB can be trapped into ST states, with a lifetime of ~6 ps. Second, the trapped electrons and holes can be further trapped into DT states, with a time constant of ~50 ps. Meanwhile, the electrons and holes in the CB and VB can also directly relax into DT


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states, likely on a similar time scale. Third, the trapped electrons and holes in ST states can recombine through a primarily non-radiative pathway on the time scale of 300 ps. Fourth, the relaxed electrons and holes in the CB and VB edges can recombine through radiative and non-radiative processes on the time scale of 165 ns and 42 ns, respectively. At last, the long-lived, trapped electrons and holes in DT states can undergo primarily radiative recombination with a time constant of ~1100 ns. In comparison, the PNC-ODABr shows similar dynamic processes but differs at a quantitative level. Following cooling to the band edges, the photoexcited electron and hole in the CB and VB may be trapped into a manifold of trap states (both ST and DT) with 10 ps lifetime. The trapped electron and hole in the ST states can undergo none-radiative recombination with a time constant of 100 ps. Then, the relaxed electrons and holes in the CB and VB can recombine radiatively and non-radiatively with lifetime of 180 ns and 20 ns, respectively. At last, the long-lived, trapped electron and hole in the DT states recombine radiatively with a 450 ns lifetime. While the proposed model is clearly not unique, it does provide some further insight, beyond simple mathematical fittings, into the major dynamics pathways for the photogenerated electron-hole pair in these materials.

In particular, it highlights the importance of capping ligand in

determining the energy levels as well as dynamic processes of the charge carriers.



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Figure 7. Schematic illustration of the energy band structure and proposed assignment of lifetimes of various excitonic processes for (a) PNC-OABr and (b) PNC-ODABr based on the TA and TRPL results.

4. Conclusions We have demonstrated the synthesis of PNCs that exhibit monodisperse particle size around 3.9 nm, narrowed green PL emission, high PL QY ~20%, when OABr as the capping ligand. As the longer aliphatic ammonium, ODABr, was used as the capping ligand, relatively larger (6.5 nm) PNCs with lower PL QY (10%) and poorer particle suspension were obtained. The exciton and charge carrier dynamics in PNCs were studied by ultrafast TA/TB pump-probe and TRPL spectroscopies. The TB features obtained using the lowest pump power (40 nJ/pulse) at 505 and 525 nm probe wavelength for PNC-OABr and PNC-ODABr, respectively, were dominated by exciton relaxation.


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The TB recovery traces could be fit to an exponential function with the average lifetime of 240 ± 19 ps for PNC-OABr and 100 ± 12 ps for PNC-ODABr. The exciton power dependence investigation revealed significant non-linear behavior of PNC samples under high pump power, indicating high-order exciton-exciton annihilation or Auger recombination upon trap-state saturation. SVD global fitting of the TA and TRPL signals provided further insight into the mechanisms of the exciton recombination pathways associated with each lifetime component. The fitting results of PNC-OABr illustrated that the trapping process of the photoexcited electrons and holes from CB into ST states occurs primarily on the 6 ps time scale, and then the trapped electrons and holes can be further trapped into the DT states in 50 ps. In addition, the electron and hole in CB can also relax into ST states directly within 50 ps. The trapped electron and hole in ST states can undergo non-radiative recombination within 300 ps. The localized electron and hole in the CB and VB can recombine through radiative and non-radiative processes with time constants of 165 ns and 42 ns, respectively. The long-lived, trapped electron-hole pairs in the DT states recombine radiatively on the time scale of 1100 ns. On the other hand, the fitting results of PNC-ODABr suggested that more defect states resulted in greater trapping of photoexcited electrons and holes in ~10 ps from the CB and VB. Non-radiative recombination of the trapped electrons and holes was dominant on the 100 ps time scale. The radiative and non-radiative recombination of the relaxed electrons and holes through CB to VB can be estimated as 180 ns and 20 ns, respectively. The long-lived trapped electron and holes recombine radiatively on the time scale of 450 ns. The exciton dynamics study demonstrated that the capping ligand, OABr,



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plays a critical role in successfully passivating surface trap states, generating long-lived excitons and thereby improving the PL QY of PNCs. These results indicate that appropriately-capped PNC films have great potential for practical application in the photovoltaic and LED fields as well as other photoelectrical conversion applications.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (X. M. L.); [email protected] (J. Z. Z.)

Author Contributions The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Acknowledgement This project was supported by the BES Division of the US DOE, Delta Dental Health Associates, UCSC Senate Special Research Fund, and UC MEUXS/CONACYT. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences,

of the U.S. Department of Energy under Contract No.

DE-AC02-05CH11231. Binbin Luo thanks the financial support from the program of China Scholarship Council (CSC). We are also

grateful

for

support

from

UC-MEXUS/CONACYT and UCSC Special Research Fund. We thank Prof. Yung-Jung Hsu for providing the TRPL facility. References (1) Zhang, F.; Zhong, H.; Cheng, C.; Wu, X.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong,


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Band Edge States and Deep Trap States of CdS Nanoparticles Determined by Time-Correlated Single Photon Counting. Chem. Phys. Lett. 2000, 330, 237-242

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Synthesis, Optical Properties, and Exciton Dynamics of Organolead

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