Exciton Transport in an Organic Semiconductor Exhibiting Thermally

Apr 20, 2016 - Lunt , R. R.; Giebink , N. C.; Belak , A. A.; Benziger , J. B.; Forrest , S. R. Exciton Diffusion Lengths of Organic Semiconductor Thin...
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Exciton Transport in an Organic Semiconductor Exhibiting Thermally Activated Delayed Fluorescence S. Matthew Menke† and Russell J. Holmes* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States ABSTRACT: Organic semiconductors characterized by a small singlet−triplet exciton energy splitting exhibit efficient reverse intersystem crossing and thermally activated delayed fluorescence. Consequently, exciton transport may occur along both the singlet and the triplet excited states, each with unique photophysical behavior and exciton energy transfer mechanisms. Delayed fluorescence systems, therefore, provide a unique test bed for characterizing the role of exciton spin in transport and diffusion. Concentration- and temperature-dependent photophysical characterization combined with measurements of the exciton diffusion length (LD) for 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) elucidate the relative degree and magnitude of transport along the singlet and triplet molecular excited states as well as the role of the local dielectric environment in determining the intersystem balance.

I. INTRODUCTION The synthetic flexibility intrinsic to the design of organic semiconductors has enabled new paradigms for exciton management in organic optoelectronic devices.1,2 Of particular note are emissive dopants used in organic light-emitting devices (OLEDs) which display near-unity exciton-to-photon quantum efficiencies by overcoming spin-dependent, deleterious exciton decay pathways.3 In an OLED, electrical excitation results in the formation of both singlet and triplet excitons with approximately one singlet exciton generated for every three triplet excitons.4 In traditional fluorescent OLEDs, radiative recombination of the triplet is forbidden by spin-selection rules, limiting the quantum efficiency.5 Phosphorescent dopants overcome this limitation by incorporating heavy atoms (e.g., Ir and Pt) that effectively mix the singlet and triplet exciton states, yielding efficient intersystem crossing and enabling radiative triplet decay.6,7 Further, nonradiative triplet decay can be suppressed by diluting phosphorescent dopants in a wide energy gap host.3,6−10 In an organic semiconductor exhibiting thermally activated delayed fluorescence (TADF), the singlet−triplet exciton energy splitting (EST) is small (EST < 100 meV) compared to most organic semiconductors and typical phosphorescent dopants.11,12 A small EST allows excitons to efficiently intersystem and reverse intersystem cross. Remarkably, efficient intersystem crossing can proceed in these materials without the incorporation of heavy atoms, and devices based on TADF dopants can achieve internal quantum efficiencies for fluorescence of 90−100% in an OLED, indicating the efficient utilization of electrically generated triplet excitons via reverse intersystem crossing.12,13 Considerable work has been directed at the design of molecules with a small EST and a low triplet nonradiative recombination rate to further leverage delayed fluorescence for application in OLEDs.13,14 © XXXX American Chemical Society

In the organic photovoltaic cell (OPV) community, longstanding questions remain regarding the ability to engineer exciton spin for enhanced transport.15−25 The long triplet exciton lifetime (τT ≈ 10−6−10−3 s) is attractive because it could be leveraged to achieve long-range exciton transport and long exciton diffusion lengths (LD). The magnitude of LD, in turn, strongly determines OPV design and optimization. To date, only organic single crystals have effectively exploited these long lifetimes for enhanced exciton transport (LD ≈ 1−10 μm).26−28 The LD is substantially reduced for organic thin films (LD ≈ 1−10 nm) owing to a reduced diffusivity (D ≈ 105−108 nm2 s−1) for triplet excitons in spatially and energetically disordered environments.24,29,30 In contrast, singlet excitons exhibit larger diffusivities (D ≈ 1010−1011 nm2 s−1) owing to the possibility of longer range Förster energy transfer.31−33 With the potential role of triplet excitons in exciton harvesting an open question, organic semiconductors exhibiting TADF provide a unique test bed for understanding the consequences of exciton spin in transport and diffusion. In this work, the effects of intermolecular separation and temperature on molecular photophysics and LD in the archetypical TADF molecule 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene12 (4CzIPN) are investigated and modeled. The results highlight the potential for triplet exciton diffusion in organic thin films and suggest an essential role played by the local dielectric environment in determining the rate of reverse intersystem crossing. This paper is organized as follows. Experimental methods for organic thin film fabrication, measurement of LD, and transient photoluminescence are described in section II. Section III Received: February 18, 2016 Revised: March 30, 2016

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DOI: 10.1021/acs.jpcc.6b01679 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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layer of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN)35 and dividing it by the spectrum of an identical film of 4CzIPN:UGH2 deposited on a glass substrate (Figure 1c). Optical transfer matrix simulations combined with analytical solutions to the exciton diffusion equation are used to simulate predicted PL ratios which are sensitive to the LD.36 The index of refraction and extinction coefficient for 4CzIPN and corresponding mixtures of 4CzIPN and UGH2 are explicitly measured with variable-angle spectroscopic ellipsometry. The interface between the quenching layer and the layers of interest has previously been shown to be smooth with a rootmean-squared (RMS) surface roughness of 0.9 nm over an area of 1.5 μm2.37 The photophysical parameters required to connect energy transfer in 4CzIPN to exciton diffusion were measured as a function of temperature and concentration upon dilution in UGH2. The samples were illuminated with an attenuated N2 gas laser at a wavelength of λ = 337 nm and a pulse width < 1 ns. The prompt and delayed kinetics were recorded with a biased, Si photodiode with a response time of 1 ns coupled to a high-speed oscilloscope. The prompt and delayed lifetimes were extracted using biexponential fits of the resulting kinetics. Transient lifetimes were collected under a pump fluence of 1.3 × 1014 photons/cm2. No change in lifetime was observed for a range of pump fluences from 1013 to 1015 photons/cm2. The total ηPL was measured as a function of concentration using an absolute quantum yield technique.38 The prompt and delayed ηPL were extracted by weighting the total ηPL by the separately integrated prompt and delayed exponential decay components. As a function of temperature, the total ηPL was extrapolated by monitoring the change in photoluminescence intensity and normalizing to the ηPL separately measured at room temperature for both the prompt and the delayed components.

provides the theoretical framework for describing exciton transport in TADF systems. The results in section IV show the concentration- and temperature-dependent characterization of 4CzIPN. The transport measurements are then described by decoupling the contributions from singlet and triplet excitons. The conclusions of the paper are presented in section V.

II. EXPERIMENTAL METHODS The LD for 4CzIPN is measured as a function of concentration in the wide energy gap host p-bis(triphenylsilyl)benzene34 (UGH2) using thickness-dependent photoluminescence quenching.32 The molecular structure for 4CzIPN and molecular orbital energy levels for 4CzIPN and UGH2 are shown in Figure 1. The wide energy gap of UGH2 confines

III. THEORY A simplified state diagram for 4CzIPN is shown schematically in Figure 2. The balance of intersystem and reverse intersystem

Figure 1. (a) Molecular structure for 4CzIPN. (b) Molecular orbital energy levels for UGH2 and 4CzIPN. (c) Experimental schematic for photoluminescence quenching. The photoluminescence (PL) ratio is defined as the PL from a quenched film of 4CzIPN diluted in UGH2 by the PL from an unquenched film deposited directly on the glass substrate. (d) PL ratios, corresponding fits, and fit values for LD are shown for pure 4CzIPN and two representative dilutions of 4CzIPN in UGH2.

Figure 2. Schematic energy state diagram for 4CzIPN. Delayed fluorescence arises from one or more cycles of intersystem and reverse intersystem crossing and subsequent radiative emission from the singlet excited state.

photogenerated excitons to 4CzIPN and can effectively vary the average 4CzIPN intermolecular separation upon mixing. All organic thin films were grown by vacuum thermal evaporation ( 200 K) is fit with an Arrhenius relationship to establish an upper bound on the activation energy for reverse intersystem crossing (EA). We infer that this activation energy corresponds roughly to EST. The corresponding fit value is EA = 33 ± 1 meV, similar to the value EST = 29 meV reported in Niwa et al.41 We note that the deviation from the Arrhenius relationship at low temperatures may also result from temperature-independent disorder in the system where low EA configurations are sampled for all temperatures investigated.

(10)

where K is related to the specific orbital interaction, JD is a spectral overlap integral normalized for the extinction coefficient of the ground state molecule, and β is the attenuation coefficient. Importantly, the rate of singlet, Förster energy transfer can be determined from steady state spectroscopic data, such as the absorption and photoluminescence spectra and the ηPL,P (Figure 4b). As a consequence, the parameters that describe triplet, Dexter energy transfer can be fit using eq 7. This, in turn, enables a decoupling of the various contributions to LD and examination of their concentration dependence. Figure 5a shows the kF that is tabulated from separate measurements of the photoluminescence and associated parameters that determine R0. For reference, the calculated R0 increases from R0 = 1.0 ± 0.1 nm for a neat film of 4CzIPN to R0 = 1.8 ± 0.2 nm for a film of 1 wt % 4CzIPN diluted in UGH2. For kD, the β D

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Figure 5. (a) Nearest-neighbor hopping rate for singlet and triplet excitons. The rate of singlet, Förster energy transfer was tabulated from experimental measurements. The pre-exponential rate constant and attenuation coefficient were set as fitting parameters for the rate of triplet, Dexter energy transfer. (b) Experimental and resultant fit values for LD for 4CzIPN as a function of dilution in UGH2. (c) Separated singlet and triplet mean-squared displacements (MSD) for 4CzIPN as a function of dilution in UGH2. Also shown is the contribution to the singlet MSD by excitons that do not undergo intersystem crossing (“No cycles”) and the triplet MSD for excitons that only undergo a single cycle of intersystem and reverse intersystem crossing (“One cycle”).

and pre-exponential rate constant (KJD) are set as fitting parameters defined when fitting the concentration dependence for LD. A simple, nearest-neighbors model can be used to connect the rates for energy transfer and the diffusivity (D = d2k). Figure 5b shows the excellent agreement between the experimental and the fit LD with resulting fit values of β = 0.08 Å−1 and KJD = 1.8 × 108 s−1. The value for β is on par with measurements, for example, of intramolecular energy transfer in ethynyl-bridge molecular complexes,50 though it is much smaller than values reported for intermolecular energy transfer between Ir-centered dendrimers.24,51 Inspection of the separated triplet and singlet MSD (Figure 5c) reveals that both the singlet and the triplet states contribute equally to exciton diffusion for pure films of 4CzIPN. At low concentrations, however, the triplet exciton diffusion is more prominent. This occurs for two reasons. First, kF decreases more rapidly upon dilution (increases to d) than the fit values for kD. In fact, at concentrations less than 10 wt % 4CzIPN, triplet, Dexter energy transfer proceeds at a faster rate. This behavior is the result of a larger value for β, which weakens the attenuation of the exponential decay. Second, the τT increases from τT = 0.3 ± 0.1 μs for a neat film of 4CzIPN to τT = 0.6 ± 0.1 μs for a film of 1 wt % 4CzIPN diluted in UGH2. An increase in dwell time and cycling through the triplet state coupled with the weaker concentration dependence for kD results in longer range triplet diffusion in 4CzIPN at dilutions between 1 and 25 wt % 4CzIPN. To separate these contributions, the dashed line in Figure 5c shows what the triplet MSD would be if only a single cycle of intersystem and reverse intersystem crossing were to take place. Also shown in Figure 5c is the singlet MSD contribution by excitons that do not undergo intersystem crossing. Clearly, the presence of efficient interconversion in TADF increases the overall MSD of both the singlet and the triplet, as would be expected from eqs 5 and 6. It is important to note, however, that the overall singlet

MSD cannot exceed the limit set by the case where kISC = 0 and τS−1 = kR,S + kNR,S. The weak concentration dependence for kD contrasts with the typical notion of Dexter energy transfer as being short range and inefficient for large intermolecular separations greater than 10 Å.45 Correspondingly, the value for β−1 is approximately as large (β−1 ≈ 1.3 nm) as the predicted molecular diameter, suggesting that the triplet excited state wave function may be delocalized along the perimeter of the molecule. It should also be noted that the small value for β may also reflect aggregation of the 4CzIPN in the most dilute films. Similar behavior has been exhibited by phosphorescent dopants diluted in wide energy-gap host materials.52

V. CONCLUSION In 4CzIPN, both the EA for reverse intersystem crossing and the relative magnitude of singlet and triplet exciton transport are found to be concentration dependent. While enhanced triplet exciton transport can be achieved in dilute film, the corresponding LD is still limited by the small diffusivity for triplet excitons. This result highlights the role of concentration in determining the dielectric environment and, subsequently, the balance between intersystem and reverse intersystem crossing. In the broader scope of organic optoelectronics, concentration is a powerful tool for leveraging specific excitonic pathways within a given molecular system. Taken together, the concentration-dependent molecular photophysics and exciton diffusion lengths provide unique insight into exciton transport along multiple spin states in a molecule exhibiting thermally activated delayed fluorescence.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. E

DOI: 10.1021/acs.jpcc.6b01679 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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†

S.M.M.: Department of Physics, Cavendish Laboratory, University of Cambridge, J. J. Thompson Avenue, Cambridge CB3 0HE, United Kingdom.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) under DMR-1307066. S.M.M. acknowledges support from a University of Minnesota Doctoral Dissertation Fellowship. The authors thank Prof. C. Adachi for providing initial supplies of 4CzIPN.

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