Harvesting Triplet Excitons in Lead-Halide Perovskites for Room

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Harvesting Triplet Exciton in Lead-Halide Perovskites for Room-temperature Phosphorescence Hongwei Hu, Daming Zhao, Yang Gao, Xianfeng Qiao, Teddy Salim, Bingbing Chen, Elbert E. M. Chia, Andrew C. Grimsdale, and Yeng Ming Lam Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00315 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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Chemistry of Materials

Harvesting Triplet Exciton in Lead-Halide Perovskites for Room-temperature Phosphorescence Hongwei Hu,† Daming Zhao,‡ Yang Gao,# Xianfeng Qiao, § Teddy Salim, † Bingbing Chen, † Elbert E. M. Chia, ‡ Andrew C. Grimsdale,† and Yeng Ming Lam*,† †School

of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore ‡Division

of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore #School

of Physics and Energy, Shenzhen University, Shenzhen, 518060, China

§State

Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, 510640, China ABSTRACT: Room-temperature phosphorescence (RTP) has a much longer lifetime than fluorescence as it involves triplet excitons. This extended lifetime enables the design of advanced optoelectronics and biological sensing technologies. Despite the omnipresence of triplet states, harnessing these triplet excitons remains challenging for most organic materials due to the forbidden transitions between singlet and triplet states. Here we report novel organic-inorganic hybrid perovskites based on conjugated organic cations with low-lying triplet energy levels to extract triplet excitons from the inorganic component, thereby generating RTP with long lifetime in millisecond range. Dexter type energy transfer was confirmed to occur in this type of hybrid perovskites with transfer efficiency of up to 80%. More impressively, multiplecolored phosphorescence was achieved by facile design of the system using organic cations with different triplet exciton energies. These results are expected to greatly expand the prospects of hybrid perovskites with functional organic cations for versatile display applications.

Room-temperature phosphorescence (RTP) has attracted extensive attention due to its unique photophysical processes via triplet excitons and its important applications in bioimaging, chemical sensing, lightharvesting and light-emitting devices.1 However, both the formation of triplet excitons via intersystem crossing (ISC) and their radiative relaxation to ground states are spin forbidden, making RTP challenging in most organic semiconductors.2 The efficient RTP is usually observed in organometallics containing heavy metals such as iridium (Ir) or platinum (Pt) to promote ISC through metal-ligand charge transfer.3-5 Pure organic materials require sophisticated molecular design and additional heavy atoms (e.g. halogens) to assist the ISC.6,7 To obtain efficient RTP, these organic materials are usually subjected to special conditions such as liquid-nitrogen temperature, inert gas environment and rigid host system.8-10 Organic-inorganic hybrid perovskites are rising to be important candidates for both photovoltaics and lightemitting devices due to their outstanding optoelectronic properties.11-13 High photoluminescence and

electroluminescence efficiency have been realized in this new class of emitter.14-16 Furthermore, the integration of organic and inorganic materials at a molecular scale provides an ideal platform for the study of short-range energy transfer. Very recently, triplet excitons have been observed in both 2-dimensional (2D) and 3-dimensional (3D) perovskites, exhibiting an important role in the light emission processes.17,18 Excitonic energy transfer from 2D perovskite or 3D perovskite nanocrystals to the molecular triplet states in organic cations or ligands has been demonstrated.19-21 However, hybrid perovskite exhibiting efficient RTP remains largely unexplored. In this study, we fabricated hybrid perovskites based on novel organic cations with triplet energy level lying below the inorganic excitons to harvest the triplet excitons. Time-resolved spectroscopy confirms Dexter-type electron transfer (DET) from excitons in the inorganic layers to the molecular triplet states. Efficient phosphorescence at room temperature was achieved with long lifetime of several milliseconds. Furthermore, a wide range of emission color can be achieved and tuned by facile molecular design.

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Figure 1a shows the molecular structure of 4biphenylmethylamine (BPMA) cation. 2D perovskite with a formula of (BPMA)2PbBr4 was fabricated by mixing BPMA and PbBr2 via a solution process. Thin film and microcrystals were prepared by one-step spin-coating and two-step conversion, respectively. The thin film shows smooth surface as shown in the scanning electron microscopy images in Figure S1. The structure was confirmed by X-ray diffraction (XRD) as shown in Figure 1b. The dominant (00l) peaks originated from diffractions between inorganic layers suggesting the high crystallinity and orientation with respect to the substrate. This was further verified by grazing-incidence small-angle X-ray scattering (GISAXS) as shown in supporting information (Figure S2). The spacing between the adjacent inorganic layers were approximated from the d-spacing of (001). This spacing is defined by the molecular length of the cation and the resulting perovskites possess a large interlayer spacing of around 23 Å.

Figure 1. a, Molecular structure of BPMA and schematic illustration of the hybrid perovskite structure. b, XRD patterns of the perovskite thin film and microcrystals. The organic cations in the perovskite play an important role in the light emission by either confining the inorganic excitons or assisting their extraction depending on their relative energy levels. For example, the triplet energy level (T1) of a single benzene ring (~3.6 eV) typically lies above the conduction band (CB) of inorganic layers, resulting in the confinement of inorganic excitons.22 This confinement was manifested on the phenylethylammonium (PEA) based 2D perovskite, (PEA)2PbBr4, with confined exciton emission and a small Stokes shift of 15 nm (Figure S3). Here the PEA cation was replaced by BPMA cation

consisting of two connected phenyl groups and extended conjugation which results in triplet levels (2.58 eV) lower than the excited states of inorganic layers (3.06 eV). The photophysical properties of the as-prepared perovskite thin film and microcrystals were investigated. Figure 2a shows the photographs of (BPMA)2PbBr4 thin film and microscopy images of the crystals under ambient light and UV irradiation. Both the samples turn from colorless to bright green under UV light, suggesting a large Stokes shift of their optical absorption and emission. The absorption, photoluminescence (PL) and PL excitation spectra (PLE) for (BPMA)2PbBr4 thin film is shown in Figure 2b. The microcrystals share the same properties due to the very similar high crystallinity for both forms of samples (Figure S4). The absorption of (BPMA)2PbBr4 onsets with a distinctive exciton absorption peak at 410 nm, same as other 2D perovskites due to the quantum confinement for inorganic layers.23 The relaxation of these inorganic excitons directly to the ground states leads to strong fluorescence as demonstrated similarly by (PEA)2PbBr4 (Figure S3). However, this fluorescence is largely quenched in (BPMA)2PbBr4 and a much stronger emission rises from 500 nm to 700 nm. The broad emission can be roughly deconvoluted into three peaks located at 520 nm, 560 nm and 610 nm, a characteristic emission of polyaromatic molecules due to anisotropic molecular vibrations. PL spectra with a delayed time of 0.2 ms after turning off the excitation was recorded to compare with the static PL. The fluorescence peak at 415 nm on static PL disappeared for the delayed PL due to the short lifetime of fluorescence which is typically in ns range. The peaks from 500 nm to 700 nm did not shift but showed a lower intensity at longer wavelength. This discrepancy is caused by the slight difference in the lifetimes for different peaks as shown in Figure S5. The long lifetime and multiple peaks are characteristics of phosphorescence from BPMA cations in (BPMA)2PbBr4. There is no evidence of phosphorescence from standalone BPMA at room temperature but a weak emission at the same wavelength with the phosphorescence of (BPMA)2PbBr4 was detected for the BPMA at 77K (Figure S6). To reveal the origin of phosphorescence in (BPMA)2PbBr4, PLE spectra was recorded and found to overlap with the absorption of inorganic layers (Figure 2b). This indicates the presence of energy transfer from inorganic excitons to BPMA, circumventing the forbidden ISC process, contributed the phosphorescence. There are two types of energy transfer, Förster resonance energy transfer (FRET) and Dexter electron transfer (DET). FRET is a mechanism describing energy transfer between two allowable transitions from donor and acceptor respectively. The donor in its excited state may transfer the energy to an acceptor through nonradiative dipole–dipole coupling. Since the transition from S0 to T1 in BPMA is spin-forbidden, there is no


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Chemistry of Materials resonance coupling between the inorganic layers and organic layers in (BPMA)2PbBr4. Dexter electron transfer is a process that the donor and acceptor bilaterally exchange their electrons. Unlike FRET, the DET mechanism does not involve direct transition from ground state to excited state in the acceptor, allowing the triplet transfer with electron exchange process.23-25 The transfer rate constant of DET exponentially decays as the distance between these two parties increases. The intimate contact between the BPMA cations and inorganic layers in (BPMA)2PbBr4 allows direct electron exchange via Dexter mechanism. Previous studies have suggested the formation of triplet excitons in the inorganic layers of 2D perovskites with varies organic cations.26-32 The

(PEA)2PbBr4 shows multiple emission peaks at ~7K (Figure S7) in agreement with the theoretical predication of singlet-triplet splitting at low temperatures.26 These triplet excitons formed in the inorganic layers may be harvested by the low-lying T1 in the organic layers via DET process. To validate this hypothesis, we went on to probe the exciton transfer process in the (BPMA)2PbBr4 film using transient absorption (TA) spectroscopy. As shown in Figure 2c, the TA spectra, recorded at multiple delay time from 1 ps to 1 ns, exhibit a long-lived ground-state bleaching (GB) peak positioned around 400 nm, corresponding to the exciton absorption peak in the steady-state spectrum. The decay

Figure 2. a, Photographs of perovskite thin film and microscope images of the microcrystals under ambient light and UV light (Scale bar, 50 m). b, Absorption, PLE, static PL and delayed PL spectra of the (BPMA)2PbBr4 thin film. Delayed PL was recorded 0.2 ms after excitation turned off. c, TA spectra of (BPMA)2PbBr4 thin film with 330 nm pump, photon fluence of 73 μJ/cm2 d, Decay of the GB peak for (BPMA)2PbBr4 and (PEA)2PbBr4, note the break on x-axis separating time scale into hundreds of ps and ns. e, Schematic illustration of the Dexter electron transfer (DET) process between inorganic layer and BPMA cation in (BPMA)2PbBr4, which result in phosphorescence (Phos). profiles of GB on (BPMA)2PbBr4 and (PEA)2PbBr4 (spectra shown in Figure S8) are displayed in Figure 2d. With excitons confined in the inorganic layers, the (PEA)2PbBr4 exhibits a fast relaxation from the excited state to ground state, as reflected from the short lifetime of GB (   ps, decayed within 100 ps). On the contrary, the GB on (BPMA)2PbBr4, after a short decrease in the first few ps, gradually evolves into a slow decay profile and continues beyond the maximum delay time (5 ns) of the ultrafast system. This long-lived GB confirms the DET process in (BPMA)2PbBr4 where the inorganic excitons

were harvested in triplet states in BPMA, thereby maintaining the excited states in ms time scale. Whereas in FRET process, the GB of donor (inorganic layers) will quickly relax as the energy transfer occurs. Figure 2e illustrates the excited state relaxations for generating RTP in hybrid perovskite based on these results. The energy level alignment was derived from the values obtained from Ultraviolet Photoelectron Spectroscopy and optical absorption spectroscopy (Figure S9 ， S10 and S11). Optical excitation leads to the formation of excitons in the inorganic layers, which relax to the triplet states in BPMA



via DET. After Fermi level alignment, the T1 of BPMA is lower than the energy level of inorganic excitons. This energy offset becomes the driving force for this energy transfer. The triplet excitons then slowly relax to ground states thus generating RTP. There is no evidence of phosphorescence from (BPMA)2PbI4 due to the lower inorganic exciton energy than the organic triplet energy (Figure S12), implying the importance of energy offsets in driving the energy transfer process. To investigate the kinetics, we firstly examined the transient fluorescence of (PEA)2PbBr4 and (BPMA)2PbBr4 as shown in Figure 3a. In the absence of energy transfer process, the PL decay of (PEA)2PbBr4 reflects the recombination of excitons in inorganic quantum wells showing a lifetime of 1.13 ns. By contrast, the lifetime of (BPMA)2PbBr4 significantly diminished to 0.23 ns. The DET efficiency can be evaluated from DET = 1 -  /0, where , 0 are the fluorescence lifetime in the presence and absence of triplet acceptor, respectively.17 The DET reaches 80% for (BPMA)2PbBr4 suggesting that a substantial portion of inorganic excitons has been transferred to the BPMA triplet states. Since the Dexter transfer requires the wavefunction overlap between the inorganic excitons and chromophores (biphenyl, BP), the DET decreases exponentially with the distance between them. A new organic cation, 2-(4-biphenyl)ethylammonium, (BPEA), which possesses longer alkyl chain than BPMA, was synthesized. The perovskite based on BPEA shows a much lower DET (~20%) than the (BPMA)2PbBr4 due to the longer distance between inorganic excitons and chromophores (Figure S13), which further confirms the DET process. The phosphorescence was not observed in bilayer structure composed from perovskite layer and chromophore layer, because of the lack of direct binding resulted in much longer distance. These results suggest that the close interaction between inorganic excitons and chromophores is the prerequisite for efficient DET process.

Figure 3. a, Transient fluorescence decay of (BPMA)2PbBr4 and (PEA)2PbBr4, measured at the emission wavelength of 420 nm and 415 nm, respectively. Excitation at 375 nm with power density of ~300 mW/cm2 b, Phosphorescence decay curve for (BPMA)2PbBr4. Temperature dependence and excitation intensity dependence of phosphorescence intensity for (BPMA)2PbBr4 (c and d). To investigate the relaxation of molecular triplet excitons, time-resolved measurement was performed on (BPMA)2PbBr4 at the phosphorescence wavelength (527 nm). As shown in Figure 3b, the decay curve exhibits a mono-exponential character with a long lifetime (p) of 4.73 ms, a typical behaviour of molecular triplet excitons. The PL quantum yield (PLQY) for (BPMA)2PbBr4, which accounts mainly for the molecular triplet exciton emission, was 5.6%. This value is substantially lower than the PLQY of (PEA)2PbBr4 (31%), Table 1. Combining the high DET and PLQY of (PEA)2PbBr4, the molecular triplet excitons harvested in (BPMA)2PbBr4 was estimated to around 25% (NT = DET ∙ PLQY). The low PLQY for (BPMA)2PbBr4 suggests that most triplet excitons were quenched by non-radiative recombination. The phosphorescence rate (kp) and non-radiative recombination rate (knr) were then calculated based on the phosphorescence lifetime together with the PLQY, as described in the supporting information. As shown in Table 1, the knr (163.7 s-1) is more than three times larger than that of kp (47.7 s-1), revealing that the large ratio of non-radiative to radiative relaxation of triplet excitons limited the PLQY of (BPMA)2PbBr4. To further investigate the quenching route, we examined the temperature dependent PL intensity as shown in Figure 3c. The PL intensity at 77K is around 4 times higher than that at room temperature and it gradually decreases with rising temperature. This suggests that molecular-vibration-induced thermal quenching is the main reason for the high knr at room temperature.33 Figure 3d shows the dependence of phosphorescence intensity on the excitation power.


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Chemistry of Materials Because of the high absorption coefficient of hybrid perovskite, the high DET and long p, accumulation of triplet excitons occurs during continuous excitation. By fitting the dependence curve, a linearity of 0.91, slightly lower than one, was obtained. This suggests a greater portion of non-radiative recombination at higher excitation intensity, implying the triplet-triplet interaction may contribute to the quenching process.2 In (BPMA)2PbBr4, the close molecular packing of BPMA in organic layers causes high frequency of collisions between two adjacent molecules. Considering the long-lived triplet excitons, these collisions may lead to substantial quenching resulting in a high knr and subsequently decreasing the phosphorescence efficiency. Designing novel organic cations with more rigid structure and less

intermolecular interactions are expected to reduce such non-radiative recombination and achieve higher phosphorescence yield. To achieve multi-color phosphorescence, two additional organic cations, piperonylammonium (PiperA) and 3,4,5-trimethoxybenzylammonium (TOBA), were synthesized and their molecular structure is shown in Figure 4a (top for PiperA, bottom for TOBA). The electrondonating alkoxy groups on benzene ring were designed to fine tune the electron density and shift their triplet levels. The perovskites fabricated with these novel cations and PbBr2 show high crystallinity as confirmed from the XRD in Figure S15. These hybrid perovskites exhibit bright phosphorescence, similar

Figure 4. a, Molecular structure of PiperA (top) and TOBA (bottom). PL and PLE spectra of (PiperA)2PbBr4 and (TOBA)2PbBr4 (b and d, respectively). Phosphorescence decay of (PiperA)2PbBr4 and (TOBA)2PbBr4 (c and e, respectively). Inset show the photographs of (PiperA)2PbBr4 (c) and (TOBA)2PbBr4 (e) under UV light. to the (BPMA)2PbBr4, but with different colors defined by their triplet energies. As shown in Figure 4b and d, the phosphorescence maxima of (PiperA)2PbBr4 and (TOBA)2PbBr4 were located at 500 nm and 580 nm, respectively. Despite of their difference in emission spectra, both PLE spectra show their onset wavelength at the inorganic exciton absorption (Figure S15), suggesting the same energy transfer process as (BPMA)2PbBr4. The phosphorescence of (PiperA)2PbBr4 shows a lifetime of 3.2 ms with a PLQY of 7.1%, while the (TOBA)2PbBr4 shows a smaller PLQY of 0.7% with a lifetime of 1.2 ms. These results emphasize the importance of molecular design in achieving high phosphorescence yield in wide color gamut. Furthermore, the principle presented here provides a universal way to study triplet states in various organic materials in hybrid perovskite systems by exploiting the triplet state of the inorganic component,

bypassing the inefficient intersystem crossing in most organic materials. In conclusion, we have demonstrated roomtemperature phosphorescence (RTP) in hybrid perovskites by incorporating functional organic cations. By using transient spectroscopy, we revealed that efficient Dexter type electron transfer enabled an efficient harvesting of triplets in organic molecules that leads to the phosphorescence. A wide range of emission color can be easily tuned by incorporating different organic cations. Further study of the non-radiative recombination of molecular triplet excitons is expected to provide design guidelines of organic cations for hybrid perovskites with high phosphorescence efficiency and long lifetimes. Considering the numerous organic chromophores and the versatile molecular synthetic routes, these results unleash the potential of hybrid perovskites in fully utilizing functional organic materials for versatile optoelectronics.



ASSOCIATED CONTENT Supporting Information. Experimental details, material characterization, XRD, absorption and photoluminescence spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT H.H., and Y.M.L. acknowledge financial support from Institute for Sports Research, Nanyang Technological University. E.E.M.C. acknowledges support from the Singapore Ministry of Education AcRF Tier 2 (MOE2015-T2-2-065). We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation (FACTS), Nanyang Technological University, Singapore, for use of their electron microscopy and XRD facilities.

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(29) Kitazawa, N.; Watanabe, Y. Optical properties of natural quantum-well compounds (C6H5-CnH2n-NH3)2PbBr4 (n=1-4). J Phys Chem Solids. 2010, 71, 797-802. (30) Kitazawa, N.; Aono, M.; Watanabe, Y. Excitons in organic-inorganic hybrid compounds (CnH2n+1NH3)2PbBr4 (n=4, 5, 7 and 12). Thin Solid Films 2010, 518, 3199-3203. (31) Kitazawa, N.; Aono, M.; Watanabe, Y. Synthesis and luminescence properties of lead-halide based organic–inorganic layered perovskite compounds (CnH2n+1NH3)2PbI4 (n=4, 5,7,8 and 9). J Phys Chem Solids. 2011, 72, 1467-1471. (32) Kitazawa, N.; Aono, M.; Watanabe, Y. Temperaturedependent time-resolved photoluminescence of (C6H5C2H4NH3)2PbX4 (X = Br and I). Mater. Chem. Phys. 2012, 134, 875-880. (33) Kwon, M. S.; Yu, Y.; Coburn, C.; Phillips, A. W.; Chung, K.; Shanker, A.; Jung, J.; Kim, G.; Pipe, K.; Forrest, S. R.; Youk, J. H.; Gierschner, J.; Kim, J. Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials. Nat. Commun. 2015, 6, 8947.

Table 1. Photophysical statistics of (PEA)2PbBr4 and (BPMA)2PbBr4 Perovskites

PLQY

τex

ΦDET

τp

kp

knr

(PEA)2PbBr4

31%

1.13 ns

-

-

-

-

(BPMA)2PbBr4

5.6%

0.23 ns

80%

4.73 ms

47.7 s-1

163.7 s-1



Page 8 of 8

True Love Story. Marriage of conjugated organic molecule and perovskite brings in efficient room-temperature phosphorescence. Triplet excitons were produced via electron transfer between inorganic and organic components.


8

Harvesting Triplet Excitons in Lead-Halide Perovskites for Room

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