Temperature-Dependent Photoluminescence and Energy-Transfer

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C: Energy Conversion and Storage; Energy and Charge Transport

Temperature Dependent Photoluminescence and Energy Transfer Dynamics in Mn Doped (CHNH)PbBr Two Dimensional (2D) Layered Perovskite 2+

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Rangarajan Bakthavatsalam, Anupam Biswas, Madhu Chakali, Prakriti Ranjan Bangal, Bhushan P Kore, and Janardan Kundu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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The Journal of Physical Chemistry

Temperature Dependent Photoluminescence and Energy Transfer Dynamics in Mn2+ Doped (C4H9NH3)2PbBr4 Two Dimensional (2D) Layered Perovskite Rangarajan Bakthavatsalam, † § Anupam Biswas, †§ Madhu Chakali, ‡ Prakriti Ranjan Bangal, ‡* Bhushan P. Kore, ◊ and Janardan Kundu║§* ║

Indian Institute of Science Education and Research, Tirupati, Karakambadi Rd, Rami Reddy

Nagar, Mangalam, Tirupati, Andhra Pradesh 517507. †

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi

Bhabha Road, Pashan, Pune, Maharashtra, India, 411008. §Academy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India ‡

Analytical Chemistry Division, CSIR- Indian Institute of Chemical Technology, Uppal Road,

Tarnaka, Hyderabad, Telengana, India, 500007. ◊

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, CV Raman

Rd, Bengaluru, Karnataka, India, 560012. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

ACS Paragon Plus Environment

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ABSTRACT: Reported here is the low temperature photoluminescence, energy transfer mechanism and exciton dynamics of low dimensional Mn2+ doped 2D perovskites that show interesting differences from their 3D doped counterpart. Dopant emission in 2D system shows increased PL intensity and shortened lifetime with increase of temperature with strong dopant emission even at low temperatures. Transient absorption (TA) spectroscopy reveals the dominant role of ‘hot’ excitons in dictating the fast energy transfer timescale. The operative dynamics of the generated 'hot' excitons include filling up of existing trap states (shallow and deep) and energy transfer channel from 'hot' excitons to dopant states. Global analysis and target modeling of TA data provides an estimate of excitons (‘hot’ and band edge) to dopant energy transfer timescale of ~330 ps, which is much faster than the band edge exciton lifetime (~2 ns). Such fast energy transfer timescale arises due to enhanced carrier exchange interaction resulting from, higher exciton confinement, increased covalency, and involvement of ‘hot’ excitons in the 2D perovskites. In stark contrast to 3D systems, the fast energy transfer rate in 2D system results high dopant emission intensity even at low temperatures. Increased intrinsic vibronic coupling at higher temperatures further supports efficient Mn2+ sensitization that ultimately dictates the observed temperature dependence of the dopant emission (intensity, lifetime).


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Introduction: Semiconducting lead halide organic-inorganic perovskites have emerged as a paradigm changing material with unparalleled photophysical properties and tremendous potential for applications in solar cells and light emitting diodes. Three dimensionally networked lead halide perovskites, that have lead halide octahedra interconnected in three dimensions, continue to be of huge research interest.1-5 Concurrently, research efforts are being focused on low dimensional variants wherein the metal halide octahedra is networked along two, one, and zero dimension(s).6-8 The two dimensional variants, that have the general formula of L2PbX4, has seen a resurgence due to their intriguing fundamental properties and applications.9-11 2D perovskites are promising materials for optoelectronics due to their characteristic strongly bound excitons with high binding energies even at room temperature and fast radiative decay rates.9, 12-13 Structurally, 2D perovskites are self-assembled constructs of organic ammonium ligands (L) and metal halide octahedra, wherein the inorganic plane consisting of corner shared metal halide octahedra, is networked in two dimensions and are separated by dielectric spacer ligand layers.10 The quantum and dielectric confinement in these 2D layered perovskite results high exciton binding energies, high oscillator strengths, and fast radiative decay rates. Analogous to the 3D variants, these 2D perovskites are thought to be defect tolerant and can be utilized as semiconductor host materials for energy transfer to appropriately chosen dopants.14-15 The energy transfer from the host to dopants is generally benefitted through strong exchange coupling between the charge carriers of host and dopants.16-17 Many research efforts have been documented for successful Mn2+ doping in CsPbX3 nanocrystals,14,

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probing the photophysical properties of Mn2+ doped 3D CsPbCl3

perovskites,19and the host to dopant energy transfer dynamics20. Notably, Yuan et al reported19increasing dopant PL intensity and decreasing dopant PL lifetime with increasing


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temperature for Mn2+ doped 3D CsPbCl3 perovskites quantum dots. Moreover, they also reported weak dopant emission at low temperatures. The authors rationalized the decreasing dopant PL lifetime with increasing temperature utilizing thermally activated vibronic coupling that relaxes the selection rule of the forbidden dopant transition (4T1g → 6A1g), thereby decreasing the dopant PL lifetime with increasing temperature. The increase in dopant PL intensity was rationalized by introducing temperature dependent energy transfer rate. This energy transfer rate is believed to be smaller than the band edge recombination rate especially at low temperatures where 3D host excitons do not undergo fission. This naturally leads to weak dopant emission at low temperatures where band edge recombination dominates. However, with the increase in temperature that supports exciton fission, the energy transfer rate becomes competitive to the band edge recombination rate. This increased energy transfer sensitizes dopants that leads to the increasing dopant PL intensity with increasing temperature. It is important to note that in these doped 3D systems, the energy transfer rate is thought to be smaller or comparable to band edge emission rate that necessitates the introduction of temperature dependent/competitive energy transfer rate. Very recently, we demonstrated the first effort on Mn2+ doping in (Butylammonium)2PbBr4 based bulk 2D layered perovskite utilizing simple solid state grinding methodology.21 The 2D doped host clearly shows evidences of efficient energy transfer from the excitons (host) to the dopants (Mn2+ ions) due to higher confinement of excitons in 2D perovskites and higher covalency in Mn-Br bond enhancing sp-d exchange interaction in 2D perovskites (w.r.t. 3D APbCl3). However, there are no reports on probing the low temperature photophysics and charge carrier dynamics for Mn2+ doped bulk 2D layered perovskites. Here, we probe the effect of confinement on enhancing exciton-Mn2+ exchange coupling interaction and provide insight into


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the photophysical properties and dynamics of the Mn2+ doped bulk 2D layered perovskites. In lieu

with

our

earlier

report,

Mn2+

doped

bulk

2D

layered

perovskite

[Mn2+:(Butylammonium)2PbBr4] is utilized as our system of interest. Low temperature photoluminescence of the doped host shows strong temperature dependence of the dopant emission (intensity, lifetime). Interestingly, strong dopant emission is observed for doped 2D host even at low temperatures. Femtosecond transient absorption (fsTA) spectroscopic measurements suggests that ‘hot’ excitons play a prominent role in dictating the energy transfer timescale in these reduced dimensional 2D perovskite host systems. The operative dynamics of the generated 'hot' excitons involve filling up the existing trap states (shallow and deep) and fast energy transfer channel from 'hot' excitons to dopant states. Global analysis and target modeling of TA data provides an estimate of excitons ('hot' and band edge) to dopant energy transfer timescale of ~330 ps.

This fast energy transfer timescale, possibly due to higher exciton

confinement, increased covalency, ‘hot’ exciton contribution, outcompetes band edge radiative recombination even at low temperatures and leads to the observed strong dopant emission at low temperatures. Enhanced carrier exchange interaction and increased intrinsic vibronic coupling at higher temperatures further supports efficient Mn2+ sensitization that ultimately dictates the observed overall temperature dependence of the dopant emission (intensity, lifetime). For brevity, 2D doped perovskite-Mn2+:(butylammonium)2PbBr4 is annotated as Mn2+:(BA)2PbBr4 throughout this report. Experimental: Materials:


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Butylammonium Bromide (BABr) was purchased from Greatcell solar, HBr from TCI chemicals, Lead Bromide (PbBr2), DMF, Manganese Bromide (MnBr2) were purchased from Sigma Aldrich. All the chemicals were utilized as such without any purification. Synthesis of Butylammonium Lead Bromide (BA2PbBr4): PbBr2 was dissolved in 15ml HBr was mixed with BABr dissolved in 15 Ml HBr. The resultant mixture was heated at 80oC for two hours and cooled to get crystals of BA2PbBr4. The crystals were separated by filtration and washed with acetone and dried in vacuum overnight Synthesis of BA2PbBr4 and Mn-Doped BA2PbBr4 Thin films: BA2PbBr4 crystals were dissolved in DMF (800mg in 10ml) and the solution was heated at 70 o

C. The hot solution was spin coated (3500 RPM, 40 seconds) on quartz plates preheated to 90

o

C. The films were then annealed at 100 oC for 30 mins.

For Mn-Doped BA2PbBr4, different amount of MnBr2was added to BA2PbBr4-DMF solution depending on the % of Mn with respect to lead. The resulting solution was spin coated and annealed as mentioned previously. Methods: UV-Vis absorption spectroscopy was performed on Shimadzu UV-3600 plus spectrophotometer. All Steady state photoluminescence (PL) measurements were performed using Edinburg FLS 980, spectrometer. PL decay dynamics of Mn2+ emissions were measured using microsecond flash lamp of 100 W power. PL decay measurements for shorter lifetimes were carried out on a TCSPC setup from Horiba Jobin Yvon using 375 nm diode laser (IBH, UK, NanoLED-375L) and analysis done with DAS v6.5 software. Low Temperature steady state PL measurements


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were performed using an Edinburgh FLS-900 spectrofluorometer with a closed cycle He cryostat (Janis). A 405 nm pulsed diode laser was used for time resolved measured measurement. For performing temperature dependent measurements, the samples were spin coated onto a sapphire substrate. The excitation beam was guided towards the sample through an optical fiber, which has wavelength limitation below 400 nm. The measurements were carried out with approx. 25 min temperature stabilization time at each temperature. Powder Xray diffraction measurements were carried with a PANalytic X’Pert Pro using Copper Kα radiation (λ=1.5406 A0). Ultrafast TA measurements were performed in ambient atmosphere at room temperature. Detail experimental femtosecond laser setup is described elsewhere.22 In brief, 100 fs pulse of Ti:sapphire oscillator(Maitai, Spectra Physics) at 800 nm with ~80 MHz repetition rate was coupled to Ti:sapphire femtosecond regenerative amplifier (Spitefire Ace, Spectra Physics) to generate amplified 100 fs pulse at 800 nm wavelength and 1 kHz repetition rate. This amplified pulse served as the pump for both an optical parametric amplifier (TOPAS, Spectra Physics) and the white-light supercontinuum. The tunable laser (UV to near IR) from the optical parametric amplifier was used as the narrowband pump source. The white-light supercontinuum light was generated using CaF2 crystal and was used as broadband probe. We lightly focused the 335 nm pump beam to 0.5-0.7 mm2 and the probe beam to 0.08-0.1 mm2 at the front sample surface of the rotating sample cell. After passing through the rotating sample cell probe and references beams are collected by two optical fibers, which were connected to the entrance slit of the imaging spectrometer (CDP2022i implemented in CDP System Corporation, Exci Pro) which is equipped with UV−Vis photodiode (Si linear photodiode) arrays and IR photodiode (GaAs linear photodiode) arrays with spectral response ranges 200−1000 and 900−1700 nm, respectively. The IRF was estimated to be ≤125 fs. To minimize the photo bleaching and multi-exciton generation


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pulse energy was kept below 2 µJ and probe pulse energy was from 0.1-0.5µJ at the sample. TA measurements are performed at rotating as well as at fixed cell condition for different positions of the film for many samples and averaged data are used for analysis to minimize the effect of possible film heterogeneity on the observed kinetics if any were present. Kinetics of TA data are analyzed first at selective wavelength using multi-exponential fit and lifetime values are compared to that obtained by global and target analysis. To obtain a model-based description in terms of precisely estimated rate constants and species related spectral signature (spectrotemporal information), the transient data reported in this paper were analyzed using singular value decomposition (SVD) based global and target analysis.23-24 Global analysis is performed with two different approaches based on superposition principle of least number of independent exponential components and it provides a straightforward description of the data at all measured wavelengths at all the time points simultaneously. The number of independent components fitted to all data are determined by gradually increasing the number of exponential components until the residuals were effectively zero. The simplest description in global analysis uses parallel kinetic model where a number of mono-exponentially decaying independent components, each represented by a single rate constant (reciprocal of the lifetime) and an amplitude at each recorded wavelength, yields the decay associated difference spectra (DADS). The DADSs contemplate the rise and decay of the components with their corresponding decay constants, lifetime values. A second sequential kinetic model, unbranched, unidirectional model, consists of successive mono-exponential decays with increasing time constants estimates gross spectral evolution of the data generating evolution associated difference spectra (EADS). For instance, the first EADS represents spectra just after excitation and it decays with first lifetime into second EADS, in turn, second EADS rises with first lifetime


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and decays with second lifetime, which is longer than first lifetime, into third EADS and so forth. Finally, data are fitted to a full kinetic model (compartmental scheme, each compartment stands for individual state), target analysis, by combination of parallel and sequential kinetic model of global analysis which includes all possible branching routes and equilibrium between compartments specifying the microscopic rate constants that describe the decay of the compartment as well as transfer of excitation between the compartments. This analysis estimates the real spectra of each compartment (excited species) and is termed as species associated difference spectra (SADS). The whole analysis was performed with the R package TIMP and its graphical user interface of Glotaran.25-27

Results and Discussion: Single crystals of host (BA)2PbBr4 2D perovskites and Mn2+ doped host perovskites were synthesized following the reported protocol.21 For fsTA measurements and low temperature PL measurements, thin films were prepared on quartz slides by spin coating a DMF solution of host single crystals and MnBr2 in desired ratio followed by annealing at optimized temperature. As prepared thin films of Mn2+:(BA)2PbBr4 system shows orange emission while host perovskite thin films emit blue light under UV illumination (Figure 1a, b). Figure 1c shows the optical absorption and emission characteristics of the undoped (bottom panel) and doped (top panel) systems. The host shows sharp and strong band edge blue emission with photoluminescence excitation (PLE) closely following the absorption profile. This emission is due to free exciton recombination across band edge. Upon Mn2+ doping, the emission profile has a suppressed band edge emission peak and a strong, broad dopant emission peak at ~600 nm due to the spin forbidden 4T1g → 6A1g Mn2+ transition. The collected PLE spectra for both of these emission


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peaks are very similar and closely resemble the absorption profile, suggesting efficient energy transfer from host to dopants in the as prepared thin film samples. Time resolved PL decay at band edge emission of undoped and doped host are shown in Figure 1d. The decay profile for undoped host can be fitted to a bi-exponential function with decay constants (weights) of 1.32 ns (84%) and 4.24 ns (16%) respectively. The doped host has decay constants (weights) of 1.06 ns (87%) and 3.38 ns (13%) respectively. The average decay constants are estimated (Inset to Figure 1d) to be 2.4 ns and 2 ns for undoped and doped host respectively. Clearly, there is an appreciable change in the obtained values of band edge decay constants between undoped and doped perovskite. This is likely due to energy transfer from host to dopants and possible change in intrinsic crystal parameters of host due to successful dopant insertion. Structural, morphological, and chemical composition analysis of these thin film samples, performed utilizing SEM/EDS and PXRD, reveals the 2D nature of the host and Mn2+ doped system (Fig S1; SI).

Figure 1. Thin film of (a) undoped host and (b) doped host under UV illumination; (c) absorbance, photoluminescence (PL), and photoluminescence excitation (PLE) spectra of undoped (bottom panel) and doped host (top panel) 2D perovskite thin films; (d) time resolved PL decay at band edge emission for undoped and doped host with extracted bi-exponential decay constants and average decay constant.


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Figure 2. (a) Low temperature PL spectra of doped host; (b) normalized integrated PL intensity as a function of temperature for the Mn2+ emission; and (c) for band edge emission for doped host; (d) temperature dependent lifetime of Mn2+ emission for doped host.

The Mn2+ emission observed in the doped host arises due to energy transfer from host to dopants.21 The PL quantum yield of the dopant emission is naturally dependent on the internal quantum efficiency of radiative decay (4T1g → 6A1g) of the ligand field transition (ΦMn) and on the efficiency of energy transfer from host to dopants (ΦET).19,

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The underlying exciton

dynamics is expected to largely govern ΦET. Notably, the energy transfer from host to dopants can take place via the photogenerated 'hot' excitons and/or the excitons at the conduction band edge. The slow intrinsic radiative decay of the dopant emission is due to spin and parity forbidden nature of the Mn2+ transition (4T1g → 6A1g). This emission channel can lift spin


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forbiddances through spin-orbit coupling especially due to the proximity of heavy bromide anions in the ligand field promoting covalency of the Mn-Br bond. The parity forbidden nature of such emission can further be lifted by coupling into symmetry breaking vibrations that are dynamically present in the metal halide octahedra, i.e.vibronic coupling.19 This coupling is expected to be thermally activated due to increased excitation of such symmetry breaking phonon modes at higher temperature. Hence, dopant emission intensity and lifetime is anticipated to show a strong temperature dependence provided there is sufficient energy transfer at these temperatures.19 In order to probe these effects, low temperature PL studies were performed on host and doped host system. For the undoped host system, the band edge emission is observed to increase as the temperature is lowered (see Fig. S2; SI). The logarithm of the integrated PL intensity varies linearly with inverse temperature. This linear regime can be fitted with an Arrhenius equation that yields the activation energy (~320 meV) that is close to the reported exciton binding energy (~300 meV).29 These observed low temperature PL properties of our 2D perovskite is in consonance with earlier report.29 Importantly, Gong et al recently reported30 on the underlying electron-phonon interaction for BA2PbBr4 2D perovskite single crystals that results in non-radiative rate surpassing the radiative rate above 250 K due to non-rigid nature of the ligand packing/assembly. This leads to the reduction of the PL intensity of the band edge emission as the temperature is increased. This is in accordance to our observation of the temperature dependence of the host band edge emission intensity. Gong et al also estimated the radiative lifetime of the BA2PbBr4 system to be 2-3 ns at room temperature,30 which is in consonance to the decay time measured in our time resolved PL experiments (Fig. 1d). With these insights to the properties of the host material, attention was focused on the doped host. For the doped host, the observed band edge PL decay time constant is smaller than that of the


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undoped host (Fig. 1d) at room temperature. Interestingly, band edge emission intensity was found to decrease while the dopant emission intensity increased as the temperature is increased for the doped host (Figure 2a). The normalized integrated PL intensity of the exciton band edge emission and the dopant emission is shown in Figure 2b-c. The band edge PL intensity variation as a function of temperature for doped host is very similar to the undoped host system. However, the dopant emission shows clear increase of PL intensity as the temperature is raised. Strikingly, there is strong dopant emission even at low temperatures. This clearly suggests that even at low temperatures the energy transfer rate is very fast and efficient when compared to band edge recombination rates. Noteworthy, the lifetime of the dopant emission is seen to decrease as the temperature is increased (Figure 2d). The observed lifetime shortening and PL intensity enhancement of the dopant as the temperature is increased is inconsistent with the usual thermal PL quenching process. The shortened lifetime of the dopant emission at higher temperature can be rationalized in terms of thermally activated enhanced coupling to symmetry breaking phonon modes as proposed by Yuan et al.19 This leads to a net increase in the radiative transition probability (4T1g → 6A1g) with increasing temperature with a concomitant decrease in the dopant radiative decay time as observed here. Fitting the temperature dependence of the dopant emission lifetime in Figure 2d provides an estimate of 259 cm-1 for the effective energy of the symmetry breaking phonon mode (ϑeff, Fig. S3, SI). This energy is consistent with the expected energies of metal-ligand phonon band of the (BA)2PbBr4 lattice.30 It is important to note that the temperature dependence of dopant lifetimes is intrinsic in nature and the trend holds true for centrosymmetric systems. The cation site symmetry in both 3D and 2D perovskites is 'nearly' centrosymmetric and dopant lifetime shortening in both (3D, 2D) systems can be associated with this vibronic selection rule relaxation mechanism demonstrating its generality19.


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This thermally activated relaxation of the parity selection rule would naturally reflect on the temperature dependence of the dopant PL intensity if there is sufficient energy transfer that populates the excited state(s) (4T1g, 4T2g) of the dopant ions. This energy transfer can potentially occur through 'hot' excitons and/or band edge excitons. Figure 2b clearly shows that the dopant PL intensity increases as temperature is increased to room temperature. Moreover, the persistence of strong dopant emission even at low temperatures indicates that energy transfer is very efficient and outcompetes fast radiative band edge recombination. Although the band edge radiative decay rate for 2D host is high (high exciton binding energy with minimal exciton fission 31), but the confined nature (quantum and dielectric) of the exciton in the 2D system with high covalency9 enhances Mn2+ sensitization through enhanced exchange interaction that is operative strongly throughout the temperature range investigated here. In fact, room temperature ultrafast transient absorption measurements and carrier dynamics study of our 2D doped host, as discussed in detail later in this report, estimates the energy transfer timescale to be ~330 ps, much faster than the band edge exciton decay timescale (~2-3 ns). This fast energy transfer rate results in the necessary Mn2+ sensitization that occurs throughout the investigated temperature range and leads to strong dopant emission even at low temperatures. Hence, the observed increase in dopant PL intensity with increased temperature is rationalized here in terms of enhanced exchange interaction leading to fast energy transfer that creates excited dopant ions that undergo stronger emission due to relaxation of parity selection rule through thermally activated dynamic symmetry breaking by the coupled phonon modes in the excited state. Notably, temperature dependent competition19 between the energy transfer rate and band edge exciton recombination rate, as invoked for doped 3D system by Yuan et al19, is not necessarily required here to rationalize the increasing dopant PL intensity with increased temperature as the


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estimated energy transfer rate is much faster than the band edge exciton lifetime in our 2D perovskites. As discussed later in detail, the estimated fast energy transfer timescale (~330 ps) primarily originates from fast dopant sensitization through the involvement of ‘hot’ excitons in addition to the conduction band edge excitons. Moreover, stronger carrier exchange interaction is expected for 2D perovskites due to higher confinement and higher bond covalency in the metal bromide framework. Energy transfer timescale estimation and carrier dynamics in our 2D perovskite system, that provides insight to the process of energy transfer, is further discussed and compared/contrasted to that of the reported 3D perovskites below. Exciton dynamics in 3D APbX3 based host perovskites have been reported using transient absorption and transient PL spectroscopy.20,

32-37

Rossi et al reported the ultrafast carrier

dynamics of the Mn doped CsPbCl3 systems demonstrating energy transfer timescales in the range of 400 ps20, much faster than the band edge lifetime (~3 ns). Reports on ultrafast measurements targeted to unravel the exciton dynamics in 2D perovskites through kinetic modeling are scarce.38-39 In order to gain physical insight to the carrier dynamics of host 2D perovskites and to observe the possible opening of new relaxation channel of the photogenerated exciton to the dopant ions for the Mn2+ doped 2D perovskite (estimating energy transfer timescale), room temperature fsTA experiments were performed on thin films of undoped and doped perovskite samples at 3.7 eV (335 nm), slightly above the 1s exciton transition energy (~3.1 eV/400 nm). Figure 1c shows the representative steady state absorption and emission profile of the thin film samples of the undoped and doped perovskite. Figure 3a,d shows the typical profile pictures of fsTA data for undoped (left panel) and doped (right panel) 2D perovskite thin films respectively, which consist of three dimensional heat map of ∆OD (change of optical density), TA spectra at selective gate delays, and time traces at specific probe


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wavelengths. For both the doped and undoped hosts, TA spectra are dominated by strong negative signal (green to blue in heat map) around 1s absorption and relatively weak positive signals (green to red in heat map) on both sides of the negative signal (see Figure S6 & S7 for other set of TA data profile for undoped and doped host Fig S6 & S7). Note, in the perspective of molecular spectroscopy, where differential reflection is assumed to be negligible, negative signal features are generally considered as ground state bleach (GSB) and positive signal features are considered as photoinduced absorption (PIA). However, for photoexcitation of semiconductors, there are variety of physical processes (viz. band edge broadening, band gap alterations, freecarrier absorption) that can take place wherein differential reflection is not negligible40 and resulting TA feature to have asymmetric derivative like peak shape with prominent GSB at around 1s absorption for both the undoped and doped hosts. Such asymmetric feature of TA around GSB has also been reported for semiconductor materials and other 2D perovskites.38-40 However, for both the doped and undoped hosts, the peak position of ground state bleach (GSB) shows bathochromic shift (red shift) as function of delay time (Figure 3 a2, d2) indicating the state filling progression (low lying trap states) of initially generated 'hot' excitons. A comparison of GSB recovery dynamics for undoped and doped host is shown in Figure S4, SI. A clear enhancement of GSB recovery is observed for doped host in comparison to its pristine undoped host over various probed time windows (viz.10 ps, 100 ps, 6 ns; Fig. S4, SI). The GSB recovery dynamic profiles can be well fitted utilizing four exponential functions for (BA)2PbBr4 and five exponential functions for Mn2+:(BA)2PbBr4 utilizing time constant of the slowest component as constraint of fit as extracted from TRPL measurements (2.4 ns and 2 ns respectively for undoped and doped hosts, see Figure 1d). Note, precise determination of the slowest component with nanosecond time constant in 6-8 ns time window, maximum delay time available in our fsTA set


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up, is unrealistic. Time constants (τi) of the four components of (BA)2PbBr4 are found to be similar to that of Mn2+:(BA)2PbBr4 albeit with different amplitudes (ai) (Table 1). For both the undoped and doped hosts, the sub-picosecond recovery component with τ1 = 0.45 ps time constant is consistent with 'hot' exciton cooling times in polar semiconductors.39, 41 The longtime component utilized from TRPL measurements (τ4 ~ 2.4 ns) with small amplitude (a4= 25%) for undoped host undergoes quenching in the doped host (τ4 ~ 2.0 ns; a4= 13%). This quenching could be attributed to energy transfer process. The other components (τ2, τ3) for the undoped and doped host may arise due to the filling up of the trap states. Such low lying excitonic trap states have been identified for n=1 2D perovskites.38 Admittedly, the nature of these trap states in these 2D perovskites is far from being known. However, the 2D perovskites, like their 3D variants, are known to be defect tolerant in nature and can potentially have point defects in crystal lattice that can give rise to trap states.36 Such trap states (deep/shallow trap) act as non-radiative centers that help in recovery of ground state bleach feature within 100 ps timescales. Very interestingly, for the doped host, an additional component with τ5~326 ps time constant is necessary to satisfactorily fit the GSB recovery data. This is in contrast to the undoped case wherein four components fitting is sufficient in describing the GSB recovery data. This extra channel of GSB recovery that opens up in the doped host is attributed to the sensitization of the dopant Mn2+ions channel via energy transfer pathway. The associated rate of energy transfer (1/ τ5) is estimated to be 3x109 s-1. Notably, the estimated energy transfer timescale is much faster than the band edge decay timescale. Both 'hot' excitons and band edge excitons transfer energy to the dopants that gives rise to the observed energy transfer timescale. However, the following target model analysis of the transient data further suggests that the 'hot' excitons transfer energy to the dopants


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with a characteristic rate that is higher (two orders of magnitude) than energy transfer rate from conduction band edge excitons.

Table 1. Fitting Parameters of Normalized Transient Absorption Data for GSB dynamics. Sample (BA)2PbBr4 Mn2+:(BA)2PbBr4

τ1/ps (a1) 0.45±0.1 (-19%) 0.40±0.1 (-44%)

τ2/ps (a2) 3.6±1.0 (-29%) 3.3±1.0 (-20%)

τ3/ps (a3) 65±12.0 (-20%) 31± 5.0 (-15%)

τ4/ps (a4) ~2400 (-25%) ~2000 (-13%)

τ5/ps (a5) -----326±25.0 (-8%)

GSB bleach recovery dynamics, although can provide partial information on the kinetics of exciton decay processes, but it fails to advocate the rate of trapping in possible multilevel trapped systems. Guided by the above results of the multi-channel recovery pathways of GSB (Table 1), we tested the three-dimensional data matrices globally with a kinetic target model to have spectro-temporal parameters consisting of spectra of individual excited state (compartment) species (species associated difference spectra, SADS), their lifetimes, their formation and decay rate constants. It is important to note that the global kinetic target model analysis of the TA data, although seldom reported for perovskite samples, is an important and requisite step to gain physical insight to the various pathways through which the generated excitons decay. For undoped host, a minimum of four states kinetic model is necessary to satisfactorily fit the data and the same is presented in Figure 3b. In this model, ‘hot’ exciton, shallow trapped exciton, deep trapped exciton, and band edge exciton states are considered in the relaxation process. Upon photo-excitation, ‘hot’ excitons are generated within the IRF (150 fs) of the instrument which simultaneously decay to shallow trap states (k1), deep trap states (k2) and band edge state


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(k3). Then shallow and deep trapped excitons decay via non-radiative pathways with rate constants k4 and k5 respectively while radiative recombination of band edge exciton occurs with rate constant k6. To note, here we have utilized a kinetic scheme that closely resembles the kinetic scheme reported for 3D perovskites.32-34 Unlike the delayed luminescence model proposed for 3D perovsites,33 here our kinetic scheme does not consider equilibrium between exciton in the band edge state and the shallow trap states. Based on this simplistic model, TA data can be reproduced with excellent agreement (goodness of fit is shown in Figure S5 a, SI). The estimated population time profiles and corresponding SADS are shown in Figure 3c and rate of formation and rate of decay of each SADS are shown in the target scheme (Figure 3b). The lifetime values associated with four states (SADS) are found to be 0.35, 3, 51 and 2400 ps respectively. The first SADS (SADS1) with very short lifetime (350 fs) can be recognized as ‘hot’ exciton.39, 41 SADS1 is found to be relatively broad with strong PIA feature at 412 nm close to the GSB peak feature at 400 nm. SADS2 with a narrow profile, represents shallow trap states of ~3ps lifetime with GSB and PIA peaks at 402 nm and 414 nm respectively. Similarly, SADS3 corresponds to deep trap state of 51 ps lifetime with GSB and PIA peak at 402 nm and 411 nm. Finally, SADS4 can be attributed to the band edge exciton with GSB peak at 403 nm along with a weak band at 395 nm and lifetime of this state was constrained to be of 2.4 ns in accordance with TRPL measured band edge lifetime. Notably, the observed time constants for the shallow and deep trap states in our 2D perovskites is very similar to recent reports on exciton dynamics in 3D perovskites that utilize similar kinetic scheme.34 For modeling the exciton dynamics in 2D perovskites, we have utilized the results of the exciton dynamics in 3D perovskites that reports3237

the lifetimes and plausible nature of such trap states: carrier trapping in shallow and deep traps

with lifetimes of ~6 ps and ~50 ps respectively wherein variety of point defects (interstitial,


19


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vacancy, cation substitution, and anti-site substitution) could potentially highlight the chemical nature of such trap states. Gaining insight into the nature of these trap states (shallow/deep) and their energetic positioning (w.r.t band edge), although highly desirable for understanding the physical aspects of the exciton decay, but is currently beyond the scope of this work.


20

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Figure 3. Profile picture of fsTA data obtained upon 335 nm excitation of undoped (BA)2PbBr4 (a) and doped Mn2+:(BA)2PbBr4 (d) system. ΔOD heat map as a function of probe wavelength (vertical) and probe delay (horizontal) for undoped and doped samples (a1,d1) respectively; Time gated spectra at selected delay times (a2, d2) and time traces at selected probe wavelengths (a3, d3) for undoped and doped samples respectively; Kinetic scheme used for target analysis of the


21


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TA data of undoped and doped samples (b,e) respectively; Species associated difference spectra, SADS, along with their population profiles (inset) of undoped and doped hosts (c,f) respectively. The dashed line in (c,f) is the IRF of the TA instrument. The global lifetimes and the estimated rate constants are indicated in the respective scheme. The time axis of population profiles is linear until 1 ps and logarithmic thereafter.

In order to accommodate the energy transfer process in the kinetic scheme for the doped host, two additional channels of deactivation process are introduced into the model of the undoped host: one from the 'hot' excitons (HotkET) and another one from band edge excitons (BEkET) to 4T1g of Mn2+ dopants as shown in Figure 3e. Note, deactivation of 4T1g state of Mn2+ occurs in micro second timescale and spectral contribution due to 4T1g → 6A1g transition does not fall in the detected TA spectral window (370-500 nm). Although 4T1g → 6A1g transition does not dictate the spectral shape pertaining the exciton relaxation dynamics, introduction of two additional ET channels in the kinetic scheme enhances the GSB recovery kinetics. Therefore, introduction of two ET channels in the kinetic scheme effectively introduces one virtual state and inverse of respective rates of energy transfer is assigned as the lifetime of this virtual state. Based on this model, TA data matrices of doped host are globally analyzed that shows excellent fit (Figure S5 b, SI). The estimated population profiles, corresponding SADSs are shown in Figure 3f while the respective rate constants are shown in Figure 3e. Like the undoped host, the first 4 states SADS1, SADS2, SADS3, and SADS4 of the doped host are identified as ‘hot’ exciton, shallow trap, deep trap, and band edge states with lifetime values of 0.34, 2.5, 30 and ~2000 ps respectively. Notably, the spectral signature of these SADSs are very similar to those of undoped host in terms of shape, relative position of GSB and PIA peaks. Interestingly, the spectral signature of fifth SADS (i. e. SADS5) in the doped host is distinctly different than that of all other SADSs with relatively broad GSB and sharp PIA peaks at 395 nm and 408 nm


22

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respectively. Here, SADS5 can be identified as energy transfer state, with lifetime values of ~330 ps [1/(HotkET + BEkET)] and total rate of energy transfer ~3x109 s-1 for which

Hot

kET and

BE

kET are

found to be 2.94x109 and 8.5x107 s-1 respectively. This clearly suggests that a small fraction of 'hot' excitons transfer energy at a very fast rate that dominates the energy transfer timescale while a relatively large fraction of band edge excitons undergo energy transfer to dopants. Combined ('hot' and band edge) exciton population localized at the dopant 4T1g state determine the dopant emission intensity. The undertaken comparative analysis of the fsTA dynamics for the undoped and doped host gives us an idea of the ensuing dynamics of the excitons upon photoexcitation, their trapping in different trap states and importantly, for the doped host, opening up of two new channels for GSB recovery. These new channels become operative due to dopant sensitization by 'hot' excitons and band edge excitons with a combined characteristic timescale of ~330 ps, which is much faster than the exciton lifetime (~2-3ns). The global and kinetic target model based analysis of the fsTA data for the undoped and doped perovskites, reported here for the first time, not only provides useful microscopic rate constants (k1, k2, kET......etc) involved in the relaxation process but also generates the species associated spectra (SADS) whose spectral signatures were not reported earlier for perovskite samples. Here, the estimated kinetic parameters of the various states involved in exciton dynamics in the host 2D perovskites are found to be similar to the results reported for exciton dynamics in 3D perovskites32-37. By no means, the proposed kinetic scheme is unique and it needs further attention/correction as the photogenerated species participate in numerous complex processes (diffusion, bulk trapping and detrapping, radiative and non-radiative bulk recombination, surface recombination) occurring at similar time scales. Moreover, since 'hot' excitons do contribute to the energy transfer process, higher excited states of dopants (4T2g) should be incorporated in our further modeling. Unraveling the exciton


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dynamics in the 2D perovskites utilizing TA and transient PL will be further necessary to gain physical understanding of the various aspects of exciton decay in these 2D low dimensional perovskites. Low temperature PL and fsTA spectroscopic results and their analysis provides us insight to the characteristics of the energy transfer process in doped 2D bulk perovskites. It is very interesting to compare and contrast these characteristics with that of the Mn2+ doped 3D CsPbCl3 perovskite that has been reported19 recently. For both the doped 3D and 2D system, the dopant PL intensity increases while the dopant PL lifetime decreases with increase of temperature. Vibronic coupling based relaxation mechanism19 can rationalize the observed decrease of dopant PL lifetime for both the 3D and 2D system. This intrinsic behavior, derived from nearly centrosymmetric metal halide octahedral geometry in both 3D and 2D system, accounts for the observed similarity of the decrease of dopant lifetime as temperature is increased.19 However, the increase of dopant PL intensity with the increase in temperature has origin that has a subtle dependence on the dimensionality of the perovskite. For the 3D doped host, that undergoes exciton fission, there is a temperature dependent competition between the exciton recombination and energy transfer process: at low temperature exciton recombination dominates while at increased temperature Mn2+ sensitization becomes competitive. This leads to weak dopant emission at low temperatures.19 For the 3D system, the timescales of energy transfer and exciton recombination are thought to be comparable with energy transfer occurring from both charge separated state and excitons at band edge.

19

However, for the doped 2D system, the energy

transfer timescale (~330 ps) is found to be much faster than the band edge decay times (~2-3 ns). For doped 2D perovskites, both band edge excitons and 'hot' excitons transfer energy to dopants. Further, the increased vibronic coupling at elevated temperatures supports efficient energy


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transfer that enhances dopant PL intensity as the temperature is increased to room temperature. To note that the increased covalency of Mn-Br bond in 2D perovskite (in comparison to Mn-Cl bond in 3D system19) and higher confinement in strongly bound 2D systems play a significant role in enhancing the carrier exchange interaction and energy transfer rate. Further, for the doped 2D perovskites, the observed strong dopant emission at low temperatures arises due to fast energy transfer rate that dominates the band edge recombination rate. These factors (fast energy transfer timescale, high covalency, higher confinement, ‘hot’ excitons, vibronic coupling) play a symbiotic role to make Mn2+ sensitization as the dominant channel for 2D doped perovskite within the investigated range of temperatures. In short, the reported doped 2D system demonstrates efficient energy transfer than that of in a doped 3D system. This manifests as strong dopant emission for doped 2D system even at low temperatures where band edge recombination is fast (band edge lifetimes: 2-4 ns and ≥10 ns for 2D and 3D systems respectively). The existence of stronger exchange interaction in 2D system (over the 3D system) can also be appreciated when the band edge lifetime of the 2D system is compared to that of 3D system relative to the energy transfer rate specially at low temperatures where dopant emission for 2D system is stronger than that of in 3D system.

Conclusions: Low temperature photoluminescence and energy transfer dynamics in Mn2+ doped (BA)2PbBr4 2D perovskites is reported here. Mn2+ doped 2D perovskite demonstrate strong temperature dependence of the dopant emission (intensity, lifetime): intensity increases while lifetime decreases with the increase of temperature. In contrast to doped 3D systems (Mn2+: CsPbCl3), strong dopant emission is observed even at low temperatures for the 2D systems. Comparative


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analysis of the transient absorption (TA) measurements of undoped and doped 2D perovskite reveals the operative dynamics of the generated 'hot' excitons: filling up of the existing trap states (shallow and deep), transferring energy to dopant states, and relaxing into the conduction band minimum. Global analysis and target modeling of TA data provides an estimate of energy transfer timescale (~330 ps) that is much faster than the band edge exciton decay constant (~2 ns). Energy transfer to dopants occur from both 'hot' and band edge excitons wherein dynamics of ‘hot’ excitons largely dictate the energy transfer rate. This fast energy transfer timescale supports efficient Mn2+ dopant sensitization. The observed temperature dependence of the dopant emission lifetime is an intrinsic property that arises due to enhanced vibronic coupling at higher temperatures. In contrast to 3D case, fast energy transfer timescale in 2D system effectively outcompetes band edge decay even at low temperature and leads to strong dopant emission at low temperatures. Comparative analysis of these observations for doped 3D and 2D systems highlights the role of host dimensionality and ‘hot’ excitons in enhancing the energy transfer rate. The following marked differences between the Mn2+doped 3D and 2D system are noted: for 2D system i) ‘hot’ excitons play an important role in determining the energy transfer rate, ii) energy transfer rate is much faster than band edge recombination rate at all the investigated temperatures, and iii) consequently, the energy transfer is very efficient; while for the 3D system iv) band edge excitons transfer energy to the dopants, v) energy transfer rate is at the best comparable/competitive to the band edge decay rate at high temperatures and is slower at lower temperatures, and vi) consequently, the energy transfer is only modest. Chemical composition based higher covalency and higher confinement in 2D perovskites (in comparison to 3D system) enhances dopant-host carrier exchange interactions in 2D system. With these results, it is anticipated that magnetically doped lower dimensional perovskite (0D, 1D) systems, that can


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now be synthetically accessed, may be a suitable material to further enhance the dopant-exciton exchange coupling interactions.

ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. Experimental details, structural, morphological analysis of thin films, Low T PL of host, fitting of temperature dependent dopant lifetime, GSB recovery dynamics; goodness of global & target analysis of TA data matrices of doped and undoped host; TA profile data for doped and undoped system for different sample. (PDF)

AUTHOR INFORMATION Notes Authors declare no competing financial interests. ACKNOWLEDGMENT The authors would like to thank Dr. S. B. Sukumaran, Dr. P. Hazra, and Dr. S. Ogale for insightful discussions. This work was supported by DST (Grant Nos. SB/S2/RJN-61/2013 and EMR/2014/000478) and CSIR-NCL start-up (Grant No. MLP030326).

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TOC Graphics:


31


Figure 1. Thin film of (a) undoped host and (b) doped host under UV illumination; (c) absorbance, photoluminescence (PL), and photoluminescence excitation (PLE) spectra of undoped (bottom panel) and doped host (top panel) 2D perovskite thin films; (d) time resolved PL decay at band edge emission for undoped and doped host with extracted bi-exponential decay constants and average decay constant. 252x85mm (150 x 150 DPI)


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Figure 2. (a) Low temperature PL spectra of doped host; (b) normalized integrated PL intensity as a function of temperature for the Mn2+ emission; and (c) for band edge emission for doped host; (d) temperature dependent lifetime of Mn2+ emission for doped host. 212x168mm (600 x 600 DPI)



Figure 3. Profile picture of fsTA data obtained upon 335 nm excitation of undoped (BA)2PbBr4 (a) and doped Mn2+:(BA)2PbBr4 (d) system. ΔOD heat map as a function of probe wavelength (vertical) and probe delay (horizontal) for undoped and doped samples (a1,d1) respectively; Time gated spectra at selected delay times (a2, d2) and time traces at selected probe wavelengths (a3, d3) for undoped and doped samples respectively; Kinetic scheme used for target analysis of the TA data of undoped and doped samples (b,e) respectively; Species associated difference spectra, SADS, along with their population profiles (inset) of undoped and doped hosts (c,f) respectively. The dashed line in (c,f) is the IRF of the TA instrument. The global lifetimes and the estimated rate constants are indicated in the respective scheme. The time axis of population profiles is linear until 1 ps and logarithmic thereafter. 220x262mm (119 x 119 DPI)


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Temperature-Dependent Photoluminescence and Energy-Transfer

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