Delayed Luminescence in Lead Halide Perovskite Nanocrystals - The

Article pubs.acs.org/JPCC

Delayed Luminescence in Lead Halide Perovskite Nanocrystals Vladimir S. Chirvony,*,†,‡ Soranyel González-Carrero,‡ Isaac Suárez,† Raquel E. Galian,‡ Michele Sessolo,‡ Henk J. Bolink,‡ Juan P. Martínez-Pastor,† and Julia Pérez-Prieto‡ †

UMDO (Unidad de Materiales y Dispositivos Optoelectrónicos), Instituto de Ciencia de los Materiales, Universidad de Valencia, Valencia 46071, Spain ‡ Instituto de Ciencia Molecular, Universidad de Valencia, c/Catedrático J. Beltrán, 2, Paterna 46980, Spain

Downloaded via AUSTRALIAN NATL UNIV on August 13, 2018 at 11:13:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The mechanism responsible for the extremely long photoluminescence (PL) lifetimes observed in many lead halide perovskites is still under debate. While the presence of trap states is widely accepted, the process of electron detrapping back to the emissive state has been mostly ignored, especially from deep traps as these are typically associated with nonradiative recombination. Here, we study the photophysics of methylammonium lead bromide perovskite nanocrystals (PNCs) with a photoluminescence quantum yield close to unity. We show that the lifetime of the spontaneous radiative recombination in PNCs is as short as 2 ns, which is expected considering the direct bandgap character of perovskites. All longer (up to microseconds) PL decay components result from the rapid reversible processes of multiple trapping and detrapping of carriers with a slow release of the excitation energy through the spontaneous emission channel. As our modeling shows, the trap (dark) and excitonic states are coupled by the trapping−detrapping processes so that they follow the same population decay kinetics, while a majority of excited carriers are in the dark state. The lifetime of the PNCs delayed luminescence is found to be determined by the depth of the trap states, lying from a few tens to hundreds meV below the emitting excitonic state. The delayed luminescence model proposed in this work can serve as a basis for the interpretation of other photoinduced transient phenomena observed in lead halide perovskites.

1. INTRODUCTION Over the past few years, lead halide perovskites have become the most studied photovoltaic material, resulting in record power conversion efficiency exceeding 22%.1,2 Meanwhile, it has also been shown that perovskites are efficient materials for low-threshold light amplification and lasing,3,4 as well as lightemitting diodes (LEDs).5 Besides the fast development in several different applications, the origin of some unusual properties of perovskites, and in particular the long charge carrier lifetimes6−9 (as well as diffusion lengths10,11), has not yet been disclosed. Several possible explanations have been proposed for the remarkably long-lived excited states in these materials, with the common perspective of a forbidden character of the electronic transitions involved in the photoluminescence (PL). Spatial segregation of electron−hole pairs across the metal−halide bond has been suggested as one possible origin.12 An indirect character of the semiconductor in the emissive state has also been considered.13,14 It was also suggested that the recombination rate in perovskites is reduced due to the spin-forbidden transition,15 originating from a spinsplit indirect-gap.16 The present study proposes a new perspective to interpret the unusually long-lived PL lifetimes in perovskites, through a phenomenological model, which does not use the concept of forbidden optical transitions. For this purpose, we have investigated methylammonium lead bromide © 2017 American Chemical Society

(MAPbBr3) perovskite nanocrystals (PNCs) with PL quantum yield (PLQY) close to unity and extremely long lifetime. Polycrystalline and nanostructured perovskites behave like other direct gap materials, having an absorption coefficient as high as inorganic semiconductors such as CdSe, CdTe, InP, or GaAs.17,18 For direct gap semiconductors with high absorption coefficient α (or the molar extinction coefficient ε, which is more used in chemistry), the characteristic luminescence lifetime is expected to be extremely short, in the order of few nanoseconds.19,20 Interestingly, the PL decay kinetics of II−VI direct gap semiconductor QDs is also characterized by very long-lived (up to microseconds) components, although their relative amplitudes are orders of magnitude lower than those observed for perovskites. These PL decay tails in QDs are ascribed to the population of long-lived nonemitting (dark) traps.21,22 These dark states (surface traps) serve as a reservoir that supplies a population to the higher-lying emissive excitonic levels, and, as a result, the observed kinetics of the PL decay are longer as compared to the spontaneous emission kinetics predicted on the basis of the extinction coefficients.21,22 Because of the analogies between the two classes of materials Received: April 21, 2017 Revised: May 30, 2017 Published: May 30, 2017 13381

DOI: 10.1021/acs.jpcc.7b03771 J. Phys. Chem. C 2017, 121, 13381−13390

Article

The Journal of Physical Chemistry C

samples were prepared by drop casting of concentrated (2 mg/ mL) toluene dispersion and a 50-fold more diluted sample on silicon wafers. These substrates had been previously sonicated for 10 min in a freshly prepared H2O2:NH4OH:H2O (1:1:2) solution, and the process was repeated three times using new solutions. They then were sonicated for 10 min in milli-Q water twice and dried under a N2 stream. The AFM images were analyzed following a reported methodology.27 Optical Measurements. UV−visible spectra of the samples were recorded using the UV−visible spectrophotometer Agilent 8453E. Steady-state luminescence spectra were measured on a Amnico Browman series 2 luminescence spectrometer. The photoluminescence quantum yields were measured with use of an integrating sphere (Hamamatsu C9920-02 absolute PL quantum yield measurement system). Time-resolved photoluminescence experiments were carried out using the fluorescence lifetime spectrometer C11367 Quantaurus-Tau with LED pulse light sources. The measured PL decay kinetics were fitted with a triexponential function of time (t):

(large absorption and longer than predicted PL lifetime), the observed long-lived PL decay components in PNCs might also derive from the presence of low-lying dark states. These dark states, similar to the case of direct gap II−VI semiconductor QDs, efficiently trap carriers after photoexcitation. In the absence of nonradiative deactivation channels (PLQY is close to unity for our PNCs), the only possibility of deactivation for the trapped carriers is detrapping to the higher-lying (and essentially less populated) excitonic state, followed by radiative recombination. Thus, there is a slow “leak” of the excitation energy from the linked pair of excited states (trap and excitonic) through the spontaneous emission channel. This delayed luminescence is analogous to that observed in semiconductor QDs21,22 and, from the point of view of mathematical formalism (but not physical mechanism), to the thermally activated delayed fluorescence (TADF) observed in several molecular systems.19,23,24 We show that the spontaneous radiative recombination of excitons in these PNCs occurs with lifetime as short as about 2 ns, confirming the direct character of the bandgap in perovskites. Following our calculations, the multiexponential luminescence decay kinetics in MAPbBr3 PCNs is formed due to the population of traps located from about 60 to about 180 meV below the emitting excitonic state. Because the fast trapping of carriers is an inherent property of perovskites, the delayed luminescence might be a common mechanism responsible for the long-lived PL in this class of materials.

I(t ) = a1 exp( −t /τ1) + a 2 exp(−t /τ2) + a3 exp(−t /τ3) (1)

where ti represents the decay time of the ith component and ai represents the amplitude of the ith component. The average PL lifetimes (τav) were estimated with the τi and ai values from the fitted curves data according to eq 2: τav =

2. METHODS Synthesis of Methylammonium Lead Bromide Nanocrystals (PNCs). All of the reagents used in the synthesis of the perovskites and the alkyl ammonium bromides were purchased from Aldrich and used as received. The organic solvents were of spectroscopic grade (Scharlab). Methylammonium bromide (CH3NH3Br, MABr) and 2-adamantylammonium bromide (C10H18NBr, ADBr) were synthesized by reaction of the corresponding amine in water/HBr, according to the previously reported procedure.25 PNCs were synthesized using the method previously reported.26 First, concentrated DMF solutions of the components ADBr (0.043 M), MABr (0.089 M), and lead bromide (0.027 M) were prepared. Next, a precursor solution was obtained by mixing ADBr (50 μL, 2.18 μmol), MABr (37 μL, 3.24 μmol), and lead bromide (100 μL, 2.7 μmol) in DMF (MABr:ADBr:PbBr2 molar ratio of 1.2:0.8:1), and it was stirred for 15 min. This solution was dropped into toluene (5 mL) and stirred for 1 h. A strong green luminescent dispersion was immediately observed. Next, the dispersion was centrifuged at 7000 rpm (6300g) for 4 min at 20 °C. The solid was dispersed in toluene (1 mL) and maintained as a colloidal dispersion. Morphological Characterization. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction were carried out by using a field emission gun (FEG) TECNAI G2 F20 microscope operated at 200 kV. TEM samples were prepared from a toluene dispersion of the NPs, and a few drops of the resulting suspension were deposited onto a carbon film supported on a copper grid, which was subsequently dried. The atomic-force microscopy (AFM) topography images of MAPbBr3 NCs were recorded in tapping mode using a Di NanoScope IVa Controller (resonance frequency of 300 kHz, force constant of 40 N/m). A tip radius (rtip) of 10 nm and tip-to-face angle (γ) of 19.4° were used to estimate the experimental width. The

∑ aiτi2 ∑ aiτi

(2)

A third harmonic of a Nd:YAG pulsed laser with 1.1 ns pulse duration was used for investigation of the integrated PL intensity and kinetics as a function of the excitation fluence. Low-temperature time-resolved PL (TRPL) kinetics were measured by pumping the sample with a 200 fs pulsed Ti:sapphire (Coherent Mira 900D) at a repetition rate of 76 MHz doubled to 400 nm with a BBO crystal. The backscattered PL signal was dispersed by a double 0.3-m focal length grating spectrograph/spectrometer (1200 g/mm with 750 nm blaze) and detected by a Si micro photon device (MPD) singlephoton avalanche diode (SPAD) photodetector (connected through a multimode optical fiber to the monochromator); the SPAD was attached to a time correlated single photon counting electronic board (TCC900 from Edinburgh Instruments). The sample was held on a closed-cycle cryostat to measure PL and TRPL at different temperatures in the range 20−300 K.

3. RESULTS Synthesis and Morphological Properties. The synthesis of MAPbBr3 PNCs is based on a previously published protocol26,28,29 and uses adamantylammonium instead of the more common 1-octadecene and oleic acid as the ligand, to avoid the formation of quasi-2D nanostructures with thicknessdependent bandgaps30 that would complicate the interpretation of the results. The PNCs with adamantylammonium demonstrated PL decay components as long as about 1 μs and a PLQY of 0.95 ± 0.05. The PNCs morphological properties were investigated by high-resolution transmission electron microscopy (HRTEM, Figure S1) and atomic force microscopy (AFM, Figure S2), and are reported in the Supporting Information. The PNCs synthesized with this method have diameter in the 6−7 nm range, and form large assemblies both in suspension and after deposition on substrates. These 13382


Article


Figure 1. (a) Absorption and steady-state PL spectra of PNCs thin films. (b) Modeling of the band edge absorption spectrum (○) by the sum of an interband continuum (dashed green line) and excitonic absorption (dotted orange line). The fitted absorption spectrum is shown as a solid blue line. (c) PL decay kinetics of PNCs thin films, measured at the maximum of the PL spectrum (522 nm). The excitation pulse is shown as a red line. The inset shows the dependence of the kinetics on the detection wavelength. (d) Normalized transient PL spectra of PNCs reconstructed from the PL decay kinetics measured in the time interval 0−40 ns. The steady-state PL spectrum is shown for comparison as a solid black line. In all cases, the excitation wavelength was kept at 405 nm.

with time constants of ∼20, ∼130, and ∼650 ns and relative amplitudes of 0.83, 0.15, and 0.02, respectively. The corresponding relative integral contributions of the three components are 0.34, 0.40, and 0.26, respectively. Depending on the PNCs batch, the longest component of the PL kinetics was found to extend up to ∼1.2 μs. Interestingly, we observed that the shorter components of the kinetics depend on the PL detection wavelength (inset in Figure 1c), implying that the PL spectrum changes its profile during time. We measured the PL decay kinetics at different wavelengths and reconstructed the corresponding transient PL spectra at different time delays after excitation (Figure 1d and Figure S4). The transient PL spectrum detected at the time delay Δt = 0 after excitation has a maximum at 519 nm (Figure 1d) that coincides with the excitonic absorption band (518.1 nm, Figure 1b), confirming that the emission in PNCs originates from the spontaneous recombination of excitons. This is expected in the case of nanoparticles due to the relatively high exciton binding energy (35 meV) and the charge confinement in a very small volume. The only evidence of some small contribution of the radiative free carrier recombination might be observed in the transient PL spectrum measured at Δt = 0 as a broadening on the high-energy side of the PL band as compared to the perfect Gaussian contours at Δt > 0 (Figure S4a and Table S1). The observation of the excitonic PL only in case of PNCs is not in contradiction with the idea that mainly free carriers are responsible for charge recombination in iodide and iodide−bromide perovskite layers35 because of essentially

assemblies represent a useful approximation to perovskite thin film, and their properties will be discussed further in the text. Steady-State and Transient Optical Properties. The absorption spectrum of a PNCs film prepared by drop-casting a toluene suspension on a quartz substrate is reported in Figure 1a. The presence of the excitonic peak at the band gap edge of the spectrum indicates that excitons already exist in the ground state. To separate the excitonic from the band-to-band absorption, we deconvoluted the spectrum using the Elliot− Toyozawa theory, assuming a Gaussian broadening (due to the exciton−phonon interaction) for the excitonic transition.31,32 The best fitting is achieved for an electronic band gap Eg = 2.428 ± 0.002 eV (510.6 nm) and an exciton binding energy Eb = 35 ± 5 meV. The excitonic absorption band maximum is at 2.393 eV (518.1 nm). It is interesting to note that the Eb value we measured for PNCs is in good agreement with that determined for bulk MAPbBr3 (40 meV33), while it is rather low if compared to what was reported for similar MAPbBr3 nanocrystals (>300 meV).28,34 These differences might arise from the method used to determine Eb, the temperatureinduced PL quenching, as compared to the simple fit of the band edge used in this work. The room-temperature steady-state PL spectrum of PNCs (Figure 1a) consists of a single band with maximum intensity at 522 nm and a full width at half-maximum (fwhm) of 23 nm, with PLQY as high as 0.95 ± 0.05. The PL decay kinetics (Figure 1c and Figure S3) is characterized by an average lifetime of about 150 ns and, on a 1 μs time scale, can be described with a sum of three exponentially decaying functions 13383


Article


Figure 2. (a) Representative decay kinetics of the PL intensity (integrated over all PL spectrum) for PNCs obtained at different excitation fluence with 1 ns pulses at 355 nm. Experimental points are shown as hole circles and modeling curves as the corresponding solid lines. The numbers near the curves indicate the corresponding excitation fluence. The instrument response function (IRF) is also shown as a dashed outline. The PL intensity growth observed 8−10 ns after excitation is due to a parasitic second peak of the excitation pulse. (b) Dependence of the integral PL intensity of PNCs on the excitation fluence. Lifetime components τ2 (c) and τ1 (d) as a function of excitation fluence. The straight solid lines are average values. Vertical lines are error bars.

listed in Table S2, and τi as a function of excitation fluence is shown in Figure 2c,d. At the lowest fluence used (7 μJ/cm2), the PL decay kinetics is monoexponential with characteristic lifetime τ2 = 17 ± 3 ns. The relative amplitude of the τ2 decay component, which is about 20 ns at all fluencies (Figure 2c), monotonically decreases and becomes negligible at fluence higher than 1 mJ/cm2. Increasing the excitation fluence unveils a short-lived component with lifetime τ1 of about 2 ns. The short-lived lifetime τ1 increases its relative amplitude between 10 and 100 μJ/cm2 without a change of its value. Next, τ1 systematically decreases to ∼0.2 ns when the fluence exceeds 1 mJ/cm2 (Figure 2d). The dependence of the integrated PL intensity IPL of PNCs on the excitation fluence Iexc is presented in Figure 2b in a log−log plot. This form of presentation allows one to analyze the data within the power dependence IPL ∝ (Iexc)k.38 The PL intensity dependence with k = 1 observed until 1 mJ/cm2 implies that the PLQY is constant in this regime; hence, the appearance of the component τ1 (with a lifetime of about 2 ns) between 10 and 100 μJ/cm2 must be related to a radiative recombination process (because the PLQY is close to unity). We ascribe the τ1 component, observed for excitation fluence within the range 10−100 μJ/cm2, to the spontaneous radiative exciton recombination. Note that such short spontaneous emission lifetime for PNCs is expected from the high absorption coefficient (i.e., high oscillator strength of the direct optical transition) of perovskites, as mentioned before. The relative amplitude of the long-lived PL decay component τ2 is maximal at the lowest excitation fluence used. At the same time, the PLQY is still very high (k = 1); hence, also the τ2 component can be undoubtedly related to a radiative recombination process, this time mediated by trap states. We suggest that, at these conditions, the concentration of excitations is lower than that of traps so that all photoinduced

lower exciton binding energy and an absence of charge confinement in a small volume in the latter case. The red-shifted transient PL spectrum detected 200 ns after excitation has a maximum near 524.5 nm (Figure S4b), while the steady-state PL spectrum (with a maximum at 522 nm) is the result of averaging all instantaneous PL spectra. A detailed investigation of the origin of the PL spectral shift was not the task of the present study; however, we suggest two possibilities to account for this phenomenon: (i) an exciton migration to neighboring lower-energy NCs in an assembly, or (ii) a photon recycling effect36 due to a sequential photon emission− absorption−emission process. Although the spectral red shift is not very large (about 5 nm), it implies that the PL decay kinetics measured at a single wavelength corresponds not only to the exciton state depopulation, but includes also a contribution from the spectral shift. To exclude this contribution, we followed the dynamics of the entire PL spectrum instead of that at selected wavelengths. To separate the kinetics of the spontaneous radiative recombination of carriers from the long-lived PL decay components, we first have to estimate the rate constant of the exciton spontaneous recombination. In agreement with previous investigations, we postulate that the number of traps in perovskites is limited so that sufficiently powerful excitation can fill all of the traps.37 When all traps are filled, the remaining carriers should recombine with the rate constant determined by the Einstein coefficient A. As expected, the PL decay kinetics shows a strong dependence on the excitation fluence (Figure 2a). To quantitatively analyze this dependence, the measured kinetics were fitted by a sum of two exponentially decaying functions A1 exp(−t/τ1) + A2 exp(−t/τ2) convoluted with the instrument response function. The corresponding fitting curves are shown in Figure 2a, the τi and Ai values for each excitation fluence are 13384


Article


Figure 3. (a) Energy diagram describing the delayed luminescence mechanism. The state 0 is the valence band edge, state 1 is the excitonic state, and T is the trap state. The fast PL corresponds to the spontaneous radiative recombination of the excitons, with its rate constant k10. The delayed PL is a secondary emission via the spontaneous recombination channel consequence of a multiple trapping−detrapping process, resulting in thermal equilibration between the 1 and T states. The rate constants k1T and kT1 describe the circular trapping−detrapping and define the relative population of the states. The nonradiative channel for the trap state deactivation is described by its rate constant kT0. The gray arrows symbolize the cyclic process between the bright excitonic and dark trap states. (b) Dynamics of depopulation of the excitonic (1) and trap (T) states of PNCs calculated by solving eqs 4 and 5 with an energy difference ΔE = 63 meV that results in a PL lifetime of 20 ns. In the inset, the same dynamics on the time scale of 0.2 ns are reported.

Assuming that the reverse electron transfer into the excitonic state is phonon-assisted and therefore the population distribution between the trap state T and the excitonic state 1 obeys Maxwell−Boltzmann distribution, the relation between the rate constants kT1 and k1T is described by

carriers are effectively trapped. On the contrary, the component τ1 = 2.2 ns appears when all traps are dynamically filled and there is a possibility for newly generated electron−hole pairs to form excitons that recombine radiatively. When the excitation fluence exceeds 1 mJ/cm2, the PLQY systematically decreases. We believe the decrease in PLQY accompanied by the shortened τ1 to be caused by Auger recombination processes.39 Phenomenological Model. With the parameters extracted from the photophysical measurements, we propose a phenomenological model explaining the formation of the long-lived PL kinetics in PNCs (Figure 3a). As anticipated before, the model is qualitatively analogous to that used for the description of lifetime lengthening in semiconductor QDs and of TADF. It considers two distinct states for electrons, the excitonic state 1 and the trap state T (the scheme considers only electron trapping, but is equally applicable for hole trapping). The key feature of the model is the process of carrier detrapping back to the excitonic state (with rate constant kT1), which has not been previously proposed or quantified.37 It is widely described in the literature that the carrier traps participate in the formation of long-lived PL kinetics, yet inevitably leading to nonradiative deactivation.37 In the case of our PNCs, however, the PLQY is close to unity, meaning that all trapped carriers must return back to the excitonic state and recombine radiatively. The condition for the formation of longlived PL decay kinetics within the proposed scheme is k1T ≫ k10, kT0, where k10 is the rate of the spontaneous radiative recombination of excitons and kT0 corresponds to the trapmediated nonradiative recombination. Because the latter (kT0) is negligible, k1T ≫ k10 = (2.2 ns)−1. The rate constant for charge trapping k1T can be qualitatively evaluated on the basis of the PL decay kinetics measured at low fluencies (Figure 2), when traps are not yet filled and quench the excitonic state. The fact that we do not observe any fast quenching kinetics implies that the characteristic time of trapping is shorter than the time resolution of the setup (∼50 ps). Therefore, we assume k1T ≥ (50 ps)−1 = 2 × 1010 s−1, which agrees with other trapping kinetics reported in the literature.40,41 The coupling between the two excited states 1 and T is described by two additional rates k1T and kT1 that correspond to the trapping and detrapping of electrons, respectively.

k T1 = k1T exp( −ΔE /kBT )

(3)

The excitonic and trap state populations, N1(t) and NT(t), satisfy the following rate equations: dN1 = −k10N1 − k1TN1 + k T1NT dt

(4)

dNT = k1TN1 − k T1NT − k T0NT dt

(5) −1

k10 was experimentally determined (k10 = (2.2 ns) = 4.55 × 108 s−1), and because the PLQY of the system is close to 1, we can reasonably postulate kT0 = 0. Concerning k1T, practically identical PL decay kinetics are calculated for values ranging between 1010 and 1013 s−1, which corresponds to trapping time τtr = (k1T)−1 between 100 fs and 100 ps. We used for our calculations presented in Figure 3b the value k1T = 2.5 × 1011 s−1 (τtr = 4 ps; see explanation below). The system of differential equations was solved numerically for the initial conditions N1(t = 0) = 1 and NT(t = 0) = 0, which means that at t = 0 only the excitonic state 1 is populated. Equations 4 and 5 describe the material at sufficiently low level of excitation when the traps are not yet completely filled. It implies that all excitons, as soon as they are formed, will be promptly dissociated due to electron trapping. To model the experimental PL decay kinetics observed at higher fluencies (when all traps become filled within each individual excitation pulses), we can simply add the spontaneous emission kinetics A0 exp(−t/τ0) (τ0 = 2.2 ns) to the delayed PL kinetics obtained by solving eqs 4 and 5. Numerical solution of eqs 4 and 5 shows that photoexcitation (the population of the excitonic state 1) results in fast equilibration of the 1 and T states so that population of T state rapidly prevails. We suggest that, on the basis of the calculations for PNCs with high PLQY, the experimentally observed kinetics ascribed to the electron trapping correspond instead 13385


Article

The Journal of Physical Chemistry C to a longer process involving equilibration between the emissive and trap state. This equilibration occurs on the time scale of about 50 ps, as shown in the inset of Figure 3b, whereas the time for the elementary trapping process should be much shorter (4 ps is used to solve eqs 4 and 5). More interestingly, after establishing the equilibrium between the 1 and T states, their population decays exponentially with the same time constant that only depends on their energy difference ΔE. Figure 3b shows the dynamics of the excitonic and trap states of PNCs calculated by solving eqs 4 and 5 with ΔE = 63 meV. The relative populations of the T and 1 states are 0.88 and 0.12, respectively, and with the same time constant of 20 ns. We found by solving eqs 4 and 5 that the delayed PL decay components of 128.5 and 628.5 ns, measured for our PNCs (Figure 1c), are obtained for trap states located 125 and 177.5 meV below the excitonic state, respectively (the relative equilibrium amplitudes of the T and 1 states are, respectively, 0.98 and 0.02 for the first case and 0.996 and 0.004 for the second). Therefore, the proposed model enables one to reconstruct the experimentally observed PL decay kinetics by assuming the presence of a set of three trap states separated 63, 125, and 177.5 meV from the excitonic state. Equations 4 and 5 allow one also to calculate the dependence of the delayed PL decay time τPL on the depth of the corresponding trap state ΔE (Figure 4a). The correlation is striking, and when ΔE is higher than about 50 meV, the dependence is exponential. Because of the high PLQY, we have excluded the contribution of nonradiative recombination into this dependence (kT0 = 0). However, this is not the case for most perovskite materials; hence, we analyzed a more general dependence of the delayed PL lifetime τPL on the nonradiative lifetime τNR = (kTO)−1 (Figure 4b) for the most long-lived PL decay component in the kinetics presented in Figure 1c (628 ns at kT0 = 0). We found that the PL lifetime τPL is not sensitive to τNR if the latter is between 100 and 10 μs, that is, when radiative recombination prevails and the PLQY is high. However, when τNR is shorter than 1 μs, the τPL reduces to finally match τNR when this is ≤100 ns. As a consequence, in the latter regime, the overall PLQY of the system is strongly reduced. In the available literature, most time-resolved PL and absorption studies are interpreted without considering the detrapping of electrons (kT1 channel in Figure 3a), which cannot account for the observed delayed luminescence.37 We found only two previous works in which a possibility for charge detrapping was considered. Yamada et al. studied the photophysics of MAPbI3 perovskite, including in the rate equations a member responsible for charge carrier detrapping.41 However, the model was used only for the specific case of very low-intensity excitation (between pJ/cm2 and nJ/cm2 per pulse) and in a limited time domain of a few nanoseconds, to explain a short-lived PL decay component of 50 ps, which was ascribed to the electron trapping kinetics. Tilchin et al. suggested that charge detrapping can be responsible for longlived (tens of ns) components of the PL decay, although no quantitative model was presented.42 A qualitative model of electron localization in the band tail states, which explains long PL decay kinetics in perovskites, has been recently proposed,43 but, also, it does not take into account exciton dissociation− association due to the trapping−detrapping process.

Figure 4. (a) Calculated dependence (○) of the delayed PL decay time τPL on the trap state depth ΔE and fitting of the dependence by an exponential function τ = a eΔE/b with a = 2.61 ns and b = 0.032 eV (red line). (b) Calculated dependence (red line) of the delayed PL decay time τPL on the nonradiative recombination time τNR = (kTO)−1 under initial condition τPL = 628 ns when kTO = 0. The black straight line corresponds to the case when τPL = τNR. The regions marked by green and blue colors correspond to the cases of high and low PLQY, respectively.

4. DISCUSSION The PNCs synthesized for this study exhibit a very high (close to unity) PLQY that makes them very attractive for photonic applications. Despite the growing number of works on photophysics of PNCs,34,43−48 there is not yet a noncontradictory model describing the excitation deactivation in such systems and enabling one to interpret results of different groups in the framework of one same scheme. The delayed luminescence model can explain many puzzling properties of photophysics of PNCs. In particular, the model qualitatively describes a variability of the long-lived PL decay components found for PNCs. We found that the relative contribution of these components (and hence the average PL lifetime) depends on the degree of the particle aggregation/assembly. Indeed, we found that, in the case of strong aggregation, exciton and carrier migration is possible between PNCs, causing the red-shift of the transient PL spectra (Figure 1d). A similar effect was earlier observed for the case of semiconductor QDs.49,50 We suggest that such migration should result in the accumulation of electrons in deepest traps (because the persistence time for trapped carriers increases with the trap depth), leading to long PL lifetimes (see Figure 1c, where the average PL lifetime is about 150 ns). On the other hand, in isolated PNCs, carrier migration should be hindered, and the relative population of the deepest traps should be smaller due to the statistical 13386


Article


PNCs, but further studies are needed to understand the physical mechanisms responsible for the temperature-independence of the delayed PL. The role of traps and importance of delayed luminescence would have large implications also for the charge carrier transport. A similar phenomenon has in fact been observed in TiO2 NCs, where the charge-carrier multiple trapping− detrapping resulted in lengthening the PL kinetics up to milliseconds.53 Those findings had profound implications in the understanding of the multiple trapping model largely applied to nanostructured metal oxide for dye-sensitized solar cells.54,55 One of the consequences of the delayed luminescence is that the common expressions for the PL lifetime (or “average” lifetime, eq 6) and PLQY (eq 7)

presence of those traps in the as-synthesized PNCs. In this case, one would expect a shorter average lifetime because the overall contribution of the deep trap states on the decay kinetics is smaller. To verify this hypothesis, we disassembled the PNCs by depositing them on NaYF4 nanoparticles immobilized on a glass substrate and functionalized with curcubit[7]uril as a linker (see Figure S5 for details).26 In this case, the average PL decay time of the PNCs decreased from ca. 150 to 16.5 ns (Figure S5b), in agreement with the delayed luminescence model. Note that a qualitatively similar, although not so strong, effect of the PL average lifetime shortening was observed under strong (50-fold and more) diluting of PNCs solutions as compared to those used for implementing our standard PL measurements (Figure 1). We have measured the PNCs PL decay kinetics also at low temperature (300, 200, 100, and 20 K) and found that they are substantially identical (see Figure 5 and Figure S6). This

τPL = 1/(k r + kNR )

(6)

ΦPL = k r /(k r + kNR )

(7)

are not adequate to calculate the rate constants kr and kNR of radiative and nonradiative recombination, respectively. In the framework of the scheme shown in Figure 3a, the observed PL lifetime τPL is not related to the radiative lifetime τr = 1/kr and is usually much longer than the latter. For PNCs with high PLQY, τPL is of the order of tens to hundreds nanoseconds, whereas the radiative lifetime is much shorter (τr = 2.2 ns). In case of perovskite systems with low PLQY, τPL = 1/τNR (Figure 4b), as mentioned before in the text. This view might explain the large spreading of reported values for kr and kNR, which were obtained applying the formal expressions 6 and 7 to perovskites. In case of delayed luminescence, expressions 6 and 7 can be rewritten and used so that Figure 5. PL decay kinetics of PNCs measured at 300, 200, 100, and 20 K. Excitation was with 200 fs pulses at 400 nm, and detection was at 530 nm. Excitation fluence was less than 1 μJ/cm2.

τPL = 1/(kDPL + kNR )

(8)

ΦDPL = kDPL /(kDPL + kNR )

(9)

where ΦDPL and τDPL are the quantum yield and the average (effective) lifetime of the delayed PL, kDPL is the effective rate constant of the delayed PL, and kNR is the effective rate constant of nonradiative deactivation of the trap state. At low and moderate excitation fluencies, the quantum yield of the fast PL ΦFPL is negligible, so the full PL quantum yield Φ ≈ ΦDPL. Thus, on the basis of eqs 8 and 9, we can deduce the expressions kDPL = Φ/τDPL and kNR = (1 − Φ)/τDPL. In this formalism, the first-order radiative rate constant kr is not included, and it should be experimentally measured at elevated excitation levels, when all traps are filled, yet in the absence of any nonlinear deactivation channels (such as Auger recombination). We believe that the concept of delayed luminescence is valid not only for well-passivated PNCs, but for perovskite layers too. The main difference between these two cases is in negligible nonradiative losses (kTO = 0) in the former case and essential ones in the latter. This can cause difficulties in the analysis of the delayed PL in perovskite layers because (i) kT0 is not exactly known and should be rather high, and (ii) PL intensity is very inhomogeneous over the perovskite layer so that bright spots demonstrate much longer PL decay kinetics than the low light regions.56 While potentially providing a phenomenological description for several phenomena observed in hybrid perovskites, the delayed PL formalism does not give information about the physical origin of the trap states. In principle, it is not in contrast with the concept of forbidden optical transitions

observation might be counterintuitive as a decrease in temperature should result in a decrease of the detrapping rate kT1 as compared to the trapping rate k1T. Indeed, lengthening of the PL kinetics and decrease of its intensity with temperature is observed in TADF molecular systems, which is used to estimate ΔE, the triplet-to-singlet state energy difference.19,23,24 We suggest that the temperature independence observed for PNCs can be accounted for considering that not a single trap state, but a continuous distribution of trap states (with energies from units to hundreds meV), is available.51 Within our model, the factor responsible for the delayed PL kinetics is the ratio between the trapping and detrapping rate constants, k1T and kT1, which are related by the Boltzmann factor (eq 1). The ratio k1T/kT1 will remain unchanged if the temperature decrease is accompanied by a corresponding decrease of ΔE. It means that, at low temperatures, the shallow traps would provide the same delayed PL kinetics as those originating from deeper traps at room temperature. Concerning deep traps (100 meV and deeper), at low temperature they would lead to much longer kinetics or virtually be prohibited, not contributing to the signal on the nanosecond time scale. It is worth noting that a similar temperature independence of the long-living delayed PL has been recently reported for inorganic QDs, where it was explained on the basis of a tunneling mechanism of the electron detrapping from the metastable surface-trap states.52 We cannot exclude that a similar phenomenon is also taking place in 13387


The Journal of Physical Chemistry C discussed in the Introduction. Charge trapping and detrapping from the emissive state to a state with different multiplicity would result in similar recombination kinetics. On the other hand, and in particular for PNCs, the presence of surface trap states is expected and has been experimentally observed.57 On the basis of EPR studies under illumination, Shkrob et al. found that Pb2+ ions rapidly trap electrons after photoexcitation, in analogy with the process observed in Ag halide microcrystals widely applied in photography.58 In the same work as well as in other studies,59,60 metallic lead was identified as responsible for quenching of the luminescence, by promoting exciton dissociation and charge transfer states. All of these possibilities might be at the origin of the unusual efficiency of photoinduced charge separation in lead halide perovskites. Following our modeling, practically at all time after excitation (up to microseconds), the halide perovskites are in the metastable trap state with separated charges that suppress fast charge recombination, favoring photovoltaic activity.

ACKNOWLEDGMENTS

■

REFERENCES

(1) Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jacobsson, T. J.; Grätzel, M.; Hagfeldt, A. The rapid evolution of highly efficient perovskite solar cells. Energy Environ. Sci. 2017, 10, 710. (2) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D.; Levi, D. H.; Ho-Baillie, A. W. Y. Solar cell efficiency tables (version 49). Prog. Photovoltaics 2017, 25, 3. (3) Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-temperature solutionprocessed wavelength-tunable perovskites for lasing. Nat. Mater. 2014, 13, 476−480. (4) Suárez, I.; Juárez-Pérez, E. J.; Bisquert, J.; Mora-Seró, I.; Martínez-Pastor, J. P. Polymer/perovskite amplifying waveguides for active hybrid silicon photonics. Adv. Mater. 2015, 27, 6157−6162. (5) Kim, Y.-H.; Cho, H.; Lee, T.-W. Metal halide perovskite light emitters. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 11694−11702. (6) Zhumekenov, A. A.; Saidaminov, M. I.; Haque, M. A.; Alarousu, E.; Sarmah, S. P.; Murali, B.; Dursun, I.; Miao, X.-H.; Abdelhady, A. L.; Wu, T.; et al. Formamidinium lead halide perovskite crystals with unprecedented long carrier dynamics and diffusion length. ACS Energy Lett. 2016, 1, 32−37. (7) Bi, Y.; Hutter, E. M.; Fang, Y.; Dong, Q.; Huang, J.; Savenije, T. J. Charge carrier lifetimes exceeding 15 μs in methylammonium lead iodide single crystals. J. Phys. Chem. Lett. 2016, 7, 923−928. (8) Christians, J. A.; Manser, J. S.; Kamat, P. V. Multifaceted excited state of CH3NH3PbI3. Charge separation, recombination, and trapping. J. Phys. Chem. Lett. 2015, 6, 2086−2095. (9) Tian, Y.; Peter, M.; Unger, E.; Abdellah, M.; Zheng, K.; Pullerits, T.; Yartsev, A.; Sundström, V.; Scheblykin, I. G. Mechanistic insights into perovskite photoluminescence enhancement: light curing with oxygen can boost yield thousandfold. Phys. Chem. Chem. Phys. 2015, 17, 24978−24987. (10) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342, 341− 343. (11) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; et al. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347, 519−522. (12) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. High charge carrier mobilities and lifetimes in organolead trihalide perovskites. Adv. Mater. 2014, 26, 1584−1589. (13) Motta, C.; El-Mellouhi, F.; Kais, S.; Tabet, N.; Alharbi, F.; Sanvito, S. Revealing the role of organic cations in hybrid halide perovskite CH3NH3PbI3. Nat. Commun. 2015, 6, 7026. (14) Hutter, E. M.; Gélvez-Rueda, M. C.; Osherov, A.; Bulović, V.; Grozema, F. C.; Stranks, S. D.; Savenije, T. J. Direct-indirect character of the bandgap in methylammonium lead iodide perovskite. Nat. Mater. 2017, 16, 115−120.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03771. AFM images, results of transient photoluminescence measurements, and a brief description of immobilization of PNCs by deposition on NaYF4 NPs (PDF)

■

■

We thank the Spanish Ministry of Economy and Competitiveness (MINECO) through projects MAT2014-55200, PCIN2015-255, CTQ2014-60174-P, partially cofinanced with FEDER funds, TEC2014-53727-C2-1-R, Maria de Maeztu: MDM-2015-0538, and the grant to S.G.-C., Fundación Generalitat Valenciana-Universidad de Valencia (R.E.G.), and Generalitat Valenciana (projects PROMETEOII/2014/059, PROMETEO/2012/053, and PROMETEO/2016/135). We acknowledge Laura Frances-Soriano for providing NaYF4 nanoparticles. V.S.C. gratefully acknowledges Drs. S. M. ́ Bachilo, V. A. Galievsky, and P. J. Rodriguez-Cantó for helpful discussions. M.S. acknowledges the MINECO for his postdoctoral (JdC) grant.

5. CONCLUSIONS It is commonly accepted in the literature that the electronic trap states are responsible for excitation energy losses in perovskite solar cells,8 for low PLQY in perovskite emitters,61 and consequently for the rather low efficiency of perovskitebased LEDs.62 In this work, we show that the role of traps is dual. Besides being responsible for the charge carrier nonradiative deactivation, traps act also as a carrier storage during very long times, up to microseconds in case of passivated perovskite systems. During this time, the dark states (traps) and bright excitonic states are in dynamic equilibrium that results in long-lived emission, which we interpret as a delayed luminescence. By studying perovskite nanocrystals, our model predicts that the traps responsible for the formation of the delayed luminescence are localized from about 60 to about 180 meV below the emissive excitonic state. This energy range is in good agreement with the recently published experimental values of energies of the electron and hole traps in perovskites.63 The model proposed in this work can serve as a basis for interpretation of time-resolved photoluminescence and other types of transient spectroscopies and foster the development of charge transport models accounting for the trapping−detrapping phenomena observed in perovskites.

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vladimir S. Chirvony: 0000-0003-4121-9773 Isaac Suárez: 0000-0002-2773-8801 Henk J. Bolink: 0000-0001-9784-6253 Julia Pérez-Prieto: 0000-0002-5833-341X Notes

The authors declare no competing financial interest. 13388


Article

The Journal of Physical Chemistry C (15) Zheng, F.; Tan, L. Z.; Liu, S.; Rappe, A. M. Rashba spin-orbit coupling enhanced carrier lifetime in CH3NH3PbI3. Nano Lett. 2015, 15, 7794−8000. (16) Azarhoosh, P.; McKechnie, S.; Frost, J. M.; Walsh, A.; van Schilfgaarde, M. Research update: Relativistic origin of slow electronhole recombination in hybrid halide perovskite solar cells. APL Mater. 2016, 4, 091501. (17) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat. Photonics 2014, 8, 506−514. (18) Ravi, V. K.; Swarnkar, A.; Chakraborty, R.; Nag, A. Excellent green but less impressive blue luminescence from CsPbBr3 perovskite nanocubes and nanoplatelets. Nanotechnology 2016, 27, 325708. (19) Kelley, A. M. Condensed-Phase Molecular Spectroscopy and Photophysics; Wiley: Hoboken, NJ, 2013. (20) Gong, K.; Zeng, Y.; Kelley, D. F. Extinction coefficients, oscillator strengths, and radiative lifetimes of CdSe, CdTe, and CdTe/ CdSe nanocrystals. J. Phys. Chem. C 2013, 117, 20268−20279. (21) Whitham, P. J.; Marchioro, A.; Knowles, K. E.; Kilburn, T. B.; Reid, P. J.; Gamelin, D. R. Single-particle photoluminescence spectra, blinking, and delayed luminescence of colloidal CuInS2 nanocrystals. J. Phys. Chem. C 2016, 120, 17136−17142. (22) Rabouw, F. T.; van der Bok, J. C.; Spinicelli, P.; Mahler, B.; Nasilowski, M.; Pedetti, S.; Dubertret, B.; Vanmaekelbergh, D. Temporary charge carrier separation dominates the photoluminescence decay dynamics of colloidal CdSe nanoplatelets. Nano Lett. 2016, 16, 2047−2053. (23) Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C. Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence. Nat. Photonics 2014, 8, 326−332. (24) Bachilo, S. M.; Benedetto, A. F.; Weisman, R. B.; Nossal, J. R.; Billups, W. E. Time-resolved thermally activated delayed fluorescence in C70 and 1,2-C70H2. J. Phys. Chem. A 2000, 104, 11265−11269. (25) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I): Novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 2015, 15, 3692−3696. (26) Gonzalez-Carrero, S.; Francés-Soriano, L.; González-Béjar, M.; Galian, R. E.; Pérez-Prieto, J. The luminescence of CH3NH3PbBr3 perovskite nanoparticles crests the summit and their photostability under wet conditions is enhanced. Small 2016, 12, 5245−5250. (27) Canet-Ferrer, J.; Coronado, E.; Forment-Aliaga, A.; PinillaCienfuegos, E. Correction of the tip convolution effects in the imaging of nanostructures studied through scanning force microscopy. Nanotechnology 2014, 25, 395703. (28) Zhang, F.; Zhong, H.; Chen, C.; Wu, X.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly luminescent and color-tunable colloidal CH3NH3PbX3 (X = Br, I, Cl) quantum dots: Potential alternatives for display technology. ACS Nano 2015, 9, 4533−4542. (29) Huang, H.; Susha, A. S.; Kershaw, S. V.; Hung, T. F.; Rogach, A. L. Control of emission color of high quantum yield CH3NH3PbBr3 perovskite quantum dots by precipitation temperature. Adv. Sci. 2015, 2, 1500194. (30) Sichert, J. A.; Tong, Y.; Mutz, N.; Vollmer, M.; Fischer, S.; Milowska, K. Z.; García Cortadella, R.; Nickel, B.; Cardenas-Daw, C.; Stolarczyk, J. K.; et al. Quantum size effect in organometal halide perovskite nanoplatelets. Nano Lett. 2015, 15, 6521−6527. (31) Elliott, R. J. Intensity of optical absorption by excitons. Phys. Rev. 1957, 108, 1384−1389. (32) Toyozawa, Y. Theory of line-shapes of the exciton absorption bands. Prog. Theor. Phys. 1958, 20, 53−81. (33) Yang, Y.; Yang, M.; Crisp, R.; Zhu, K.; Beard, M. C. Comparison of recombination dynamics in CH3NH3PbBr3 and CH3NH3PbI3 perovskite films: Influence of exciton binding energy. J. Phys. Chem. Lett. 2015, 6, 4688−4692. (34) Zheng, K.; Zhu, Q.; Abdellah, M.; Messing, M. E.; Zhang, W.; Generalov, A.; Niu, Y.; Ribaud, L.; Canton, S. E.; Pullerits, T. Exciton binding energy and the nature of emissive states in organometal halide perovskites. J. Phys. Chem. Lett. 2015, 6, 2969−2975.

(35) Herz, L. M. Charge-carrier dynamics in organic-inorganic metal halide perovskites. Annu. Rev. Phys. Chem. 2016, 67, 65−89. (36) Pazos-Outón, L. M.; Szumilo, M.; Lamboll, R.; Richter, J. M.; Crespo-Quesada, M.; Abdi-Jalebi, M.; Beeson, H. J.; Vrućinić, M.; Alsari, M.; Snaith, H. J.; et al. Photon recycling in lead iodide perovskite solar cells. Science 2016, 351, 1430−1433. (37) Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H. J. Recombination kinetics in organic-inorganic perovskites: Excitons, free charge, and subgap states. Phys. Rev. Appl. 2014, 2, 034007. (38) Sutter-Fella, C. M.; Li, Y.; Amani, M.; Ager, J. W.; Toma, F. M.; Yablonovitch, E.; Sharp, I. D.; Javey, A. High photoluminescence quantum yield in band gap tunable bromide containing mixed halide perovskites. Nano Lett. 2016, 16, 800−806. (39) Saba, M.; Cadelano, M.; Marongiu, D.; Chen, F.; Sarritzu, V.; Sestu, N.; Figus, C.; Aresti, M.; Piras, R.; Geddo Lehmann, A.; et al. Correlated electron-hole plasma in organometal perovskites. Nat. Commun. 2014, 5, 5049. (40) Zheng, K.; Ž ídek, K.; Abdellah, M.; Chen, J. S.; Chábera, P.; Zhang, W.; Al-Marri, M. J.; Pullerits, T. High excitation intensity opens a new trapping channel in organic-inorganic hybrid perovskite nanoparticles. ACS Energy Lett. 2016, 1, 1154−1161. (41) Yamada, Y.; Yamada, T.; Shimazaki, A.; Wakamiya, A.; Kanemitsu, Y. Interfacial charge-carrier trapping in CH3NH3PbI3based heterolayered structures revealed by time-resolved photoluminescence spectroscopy. J. Phys. Chem. Lett. 2016, 7, 1972−1977. (42) Tilchin, J.; Dirin, D. N.; Maikov, G. I.; Sashchiuk, A.; Kovalenko, M. V.; Lifshitz, E. Hydrogen-like Wannier-Mott excitons in single crystal of methylammonium lead bromide perovskite. ACS Nano 2016, 10, 6363−6371. (43) He, H.; Yu, Q.; Li, H.; Li, J.; Si, J.; Jin, Y.; Wang, N.; Wang, J.; He, J.; Wang, X.; et al. Exciton localization in solution-processed organolead trihalide perovskites. Nat. Commun. 2016, 7, 10896. (44) Luo, B.; Pu, Y.-C.; Yang, Y.; Lindley, S. A.; Abdelmageed, G.; Ashry, H.; Li, Y.; Li, X.; Zhang, J. Z. Synthesis, optical properties, and exciton dynamics of organolead bromide perovskite nanocrystals. J. Phys. Chem. C 2015, 119, 26672−26682. (45) Zheng, K.; Ž ídek, K.; Abdellah, M.; Messing, M. E.; Al-Marri, M. J.; Pullerits, T. Trap states and their dynamics in organometal halide perovskite nanoparticles and bulk crystals. J. Phys. Chem. C 2016, 120, 3077−3084. (46) Zhang, Z.-Y.; Wang, H.-Y.; Zhang, Y.-X.; Hao, Y.-W.; Sun, C.; Zhang, Y.; Gao, B.-R.; Chen, Q.-D.; Sun, H.-B. The role of trapassisted recombination in luminescent properties of organometal halide CH3NH3PbBr3 perovskite films and quantum dots. Sci. Rep. 2016, 6, 27286. (47) Huang, H.; Polavarapu, L.; Sichert, J. A.; Susha, A. S.; Urban, A. S.; Rogach, A. L. Colloidal lead halide perovskite nanocrystals: Synthesis, optical properties and applications. NPG Asia Mater. 2016, 8, e328. (48) Wu, X.; Trinh, M. T.; Niesner, D.; Zhu, H.; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B. J.; Zhu, X. Y. Trap states in lead iodide perovskites. J. Am. Chem. Soc. 2015, 137, 2089−2096. (49) Efros, A. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: Dark and bright exciton states. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 4843−4856. (50) Moreels, I.; Raino, G.; Gomes, R.; Hens, Z.; Stoferle, T.; Mahrt, R. F. Band-edge exciton fine structure of small, nearly spherical colloidal CdSe/ZnS quantum dots. ACS Nano 2011, 5, 8033−8039. (51) Leijtens, T.; Eperon, G. E.; Barker, A. J.; Grancini, G.; Zhang, W.; Ball, J. M.; Kandada, A. R. S.; Snaith, H. J.; Petrozza, A. Carrier trapping and recombination: The role of defect physics in enhancing the open circuit voltage of metal halide perovskite solar cells. Energy Environ. Sci. 2016, 9, 3472−3481. (52) Marchioro, A.; Whitham, P. J.; Knowles, K. E.; Kilburn, T. B.; Reid, P. J.; Gamelin, D. R. Tunneling in the delayed luminescence of colloidal CdSe, Cu+-doped CdSe, and CuInS2 semiconductor 13389


Article

The Journal of Physical Chemistry C nanocrystals and relationship to blinking. J. Phys. Chem. C 2016, 120, 27040−27049. (53) Wang, X.; Feng, Z.; Shi, J.; Jia, G.; Shen, S.; Zhou, J.; Li, C. Trap states and carrier dynamics of TiO2 studied by photoluminescence spectroscopy under weak excitation condition. Phys. Chem. Chem. Phys. 2010, 12, 7083−7090. (54) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Trap-limited recombination in dye-sensitized nanocrystalline metal oxide electrodes. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 205321. (55) Barzykin, A. V.; Tachiya, M. Mechanism of charge recombination in dye-sensitized nanocrystalline semiconductors: Random flight model. J. Phys. Chem. B 2002, 106, 4356−4363. (56) de Quilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S. Impact of microstructure on local carrier lifetime in perovskite solar cells. Science 2015, 348, 683−686. (57) Tachikawa, T.; Karimata, I.; Kobori, Y. Surface charge trapping in organolead halide perovskites explored by single-particle photoluminescence imaging. J. Phys. Chem. Lett. 2015, 6, 3195−3201. (58) Shkrob, I. A.; Marin, T. W. Charge trapping in photovoltaically active perovskites and related halogenoplumbate compounds. J. Phys. Chem. Lett. 2014, 5, 1066−1071. (59) Sadoughi, G.; Starr, D. E.; Handick, E.; Stranks, S. D.; Gorgoi, M.; Wilks, R. G.; Bär, M.; Snaith, H. J. Observation and mediation of the presence of metallic lead in organic-inorganic perovskite films. ACS Appl. Mater. Interfaces 2015, 7, 13440−13444. (60) Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; et al. Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes. Science 2015, 350, 1222−1225. (61) Leijtens, T.; Stranks, S. D.; Eperon, G. E.; Lindblad, R.; Johansson, E. M. J.; McPherson, I. J.; Rensmo, H.; Ball, J. M.; Lee, M. M.; Snaith, H. J. Electronic properties of meso-superstructured and planar organometal halide perovskite films: Charge trapping, photodoping, and carrier mobility. ACS Nano 2014, 8, 7147−7155. (62) Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; et al. Perovskite energy funnels for efficient light-emitting diodes. Nat. Nanotechnol. 2016, 11, 872−877. (63) Adinolfi, V.; Yuan, M.; Comin, R.; Thibau, E. S.; Shi, D.; Saidaminov, M. I.; Kanjanaboos, P.; Kopilovic, D.; Hoogland, S.; Lu, Z.-H.; et al. The in-gap electronic state spectrum of methylammonium lead iodide single-crystal perovskites. Adv. Mater. 2016, 28, 3406− 3410.

13390


Delayed Luminescence in Lead Halide Perovskite Nanocrystals - The

Recommend Documents