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Triplet Exciton Diffusion in Platinum Poly-yne Films Hsien-Yi Hsu, Jarrett H. Vella, Jason D. Myers, Jiangeng Xue, and Kirk S. Schanze J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507403m • Publication Date (Web): 29 Sep 2014 Downloaded from http://pubs.acs.org on October 4, 2014

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The Journal of Physical Chemistry

Triplet Exciton Diffusion in Platinum Poly-yne Films Hsien-Yi Hsu,§ Jarrett H. Vella,§ Jason D. Myers,‡ Jiangeng Xue,‡ and Kirk S. Schanze†* †

Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, FL 32611

‡

Department of Materials Science and Engineering, University of Florida, P.O. Box 116400,

Gainesville, FL 32611 *Corresponding author. E-mail: [email protected], Telephone: 352-392-9133. §

Authors contributed equally

Abstract

A time-resolved photoluminescence quenching approach is developed for determining the triplet exciton diffusion coefficient and diffusion length (D and LD, respectively) of phosphorescent conjugated polymers. This method is applied to a solid state organometallic conjugated polymer with the structure [-Pt(PBu3)2-CC-C6H4-CC-]n (where Bu = n-butyl and -C6H4- is 1,4-phenylene). The approach relies on analysis of the lifetime quenching of the polymer’s phosphorescence by a series of three different quenchers that are dispersed into the polymer phase at varying concentration. The lifetime quenching data is analyzed by using a modified Stern-Volmer quenching expression to determine a diffusion controlled quenching rate constant, kq, which is then related to the exciton diffusivity, D, and diffusion length, LD. The diffusion coefficients that are determined using the three quenchers are consistent, D ~ 4 x 10-6 cm2-s-1, and combined with the triplet exciton lifetime of the pristine polymer (τ = 1.05 µs) give

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an exciton diffusion length, LD ~ 22 nm. The results are compared to literature studies of singlet exciton diffusion in conjugated polymers and triplet exciton diffusion in molecular materials. Keywords: exciton diffusion; homogeneous quencher; triplet; platinum poly-yne; time of flight; thin film; length of diffusion; exciton diffusion coefficient Introduction Exciton diffusion plays a significant role in the design and performance of optoelectronic devices including organic light-emitting devices (OLEDs) and organic photovoltaic cells (OPVs). The luminescence efficiency of OLEDs decreases when the excited state is quenched by intermolecular interactions,1-2 exciton interactions,2-3 and excited state annihilation.4-5 These quenching processes have an important influence on phosphorescent OLEDs since the triplet excitons have a longer lifetime than singlet excitons. Nevertheless, internal quantum efﬁciencies close to unity are reached for OLEDs containing phosphorescent emitter materials because the light is generated from both triplet and singlet excitons due to strong spin-orbit interaction.6-8 In OPVs, the singlet excited state is produced by photoexcitation, but triplet states can be produced by intersystem crossing. Several processes can be influenced by the spin state of the exciton, including the diffusion length and charge recombination efficiency. Samiullah et al. found that the presence of heavy metal in the polymer chain results in enhanced photovoltaic cell efficiency due to the formation of triplet excitons.9-11 Singlet excitons diffuse via a Förster mechanism, which involves the coupling of electronic transition dipoles occurring over distances > 5 nm, allowing them to sample a relatively large volume before they decay. Unfortunately, the lifetime of a singlet exciton lies in the ps-ns range, limiting the exciton diffusion length. Triplet exciton diffusion occurs through Dexter transfer, a process that formally involves electron exchange interactions that occur at distances less than 1 nm, limiting the diffusion coefficient.12 However, triplet excitons have lifetimes in excess of 1 µs and this factor increases the exciton diffusion length.13 At present, several methods have been used to determine the 2


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diffusion coefficient and diffusion length of singlet excitons, including photocurrent measurements,14-17 photoluminescence quenching,18-22 and singlet-singlet annihilation.23 Despite these efforts, relatively little is known regarding convenient methods for measuring the exciton diffusion parameters, especially when they involve long-lived triplet excitons. Therefore there is interest in the development of a straight-forward method for studying triplet exciton diffusion in solid-state polymeric materials. Here, we develop a time-resolved photoluminescence quenching approach for determining the triplet exciton diffusion coefficient and diffusion length (D and LD, respectively), and apply it to a solid state organometallic conjugated polymer. This method is based on an analysis of the lifetime quenching of the polymer’s phosphorescence by a series of three different quenchers that are dispersed into the polymer phase at varying concentration. The lifetime quenching data is analyzed by using a modified Stern-Volmer quenching expression to determine a diffusion controlled quenching rate constant, kq, which is then related to the exciton diffusivity, D, and diffusion length, LD. This method is applied to a platinum poly-yne polymer (Ph100, Chart 1), which has been well characterized in previous investigations.24-26 Chart 1

The platinum poly-ynes have been of considerable interest due to their propensity to undergo rapid intersystem crossing following direct excitation, leading to a long-lived and phosphorescent triplet exciton state. Platinum poly-ynes have been the subject of fundamental studies,24,27-28 and they have 3



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been applied as active materials in organic electronic devices, including OLEDs and polymer photovoltaic cells.29-32 The results of the study reveal that the triplet diffusivity in Ph100 is less than that of singlet excitons in organic conjugated polymers, but considerably higher than that of triplet excitons in molecular materials. In addition, it is found that the triplet exciton length is somewhat limited (~22 nm) in Ph100 due to its relatively short lifetime (1 µs). However, the results suggest that the triplet exciton lifetime is reduced by (impurity) trap centers that are present in the polymer at low concentration; if the concentration of the trap centers can be reduced, the triplet exciton diffusion length could be enhanced to > 100 nm in platinum poly-ynes. Experimental Substrate Preparation. Glass slides were cut into 1.2 × 2.5 cm rectangles using a glass scribe. Each slide was hand polished using a Kimwipe until no contaminants were visible to the naked eye. The slides were then placed in a custom made slide holder and sonicated for 10 min each in sodium dodecylsulfate (SDS)/water, water, acetone, then isopropanol.

All water used was purified to a

resistivity of 18.2 MΏ using a Millipore Simplicity water purification system. The slides were dried using filtered, compressed air and then stored in a covered container prior to use.33 Preparation of Blended Polymer/Quencher Films.

Glass vials, approximate volume three

milliliters, were arranged in seven vertical columns, with each column containing three vials. A tetrahydrofuran (THF) solution of Ph100 with a concentration of 20.2 g-L-1 was dispensed into each vial in 20 µL aliquots using a precision glass syringe. A dilute ( 100 nm, which would be sufficient to afford efficient light harvesting and exciton dissociation in a bilayer-type polymer solar cell configuration. In this context, it seems important to answer the following question: Why is the triplet exciton lifetime so short in Ph100 films? Several points of reference are of interest here: In degassed THF solution, the triplet lifetime of Ph100 and its molecular monomer analog are 18 µs – nearly 20-fold larger than that of solid Ph100 (see supporting information Fig. S-4 and ref.40). We believe that the reason for the shorter lifetime in the Ph100 film is 1) the presence of non-emissive (dark) trap centers that quench the triplet exciton, and 2) triplet exciton diffusion within the polymer matrix that efficiently transports excitons to the trap centers. While the chemical nature of these trap centers is not clear, one possibility is that they are associated with regions of the matrix where monomer units in the chain are in close spatial proximity, leading to strong interchain interactions; possibly involving two of the square planar Pt(II) centers forming an excimer-like state that undergoes rapid non-radiative decay.50 The existence of these dark trap centers in platinum poly-ynes has been inferred in previous work by Köhler and co-workers, who used the temperature 17



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dependence of the triplet exciton decay rate to determine the activation energy for exciton transfer in the polymer films.12 These experiments relied on the existence of the trap centers, since their existence leads to a kinetic model where the rate determining step for exciton decay is transport. Regardless, the results here suggest that if it is possible to reduce the trap density in platinum polyynes it should be possible to develop triplet-based semiconducting polymers with exciton diffusion lengths in excess of 100 nm. This could be an important step towards development of high efficiency materials for use in bilayer polymer photovoltaic cells. Summary and Conclusion An approach to measuring triplet exciton diffusion parameters in a phosphorescent platinum polyyne was developed that is based on a Stern-Volmer quenching model.

The approach involves

measurement of the phosphorescence lifetime of the polymer films when blended with varying concentration of a triplet quencher. Assuming the rate determining step for exciton quenching is diffusion, the Stern-Volmer quenching constant, kq, can be related to the exciton diffusion coefficient, D. Quenching studies were carried out with two molecular quenchers, PCBM and PtOEP and a platinum poly-yne co-polymer quencher, and each of these systems gave rise to very similar results, with kq ~ 1.3 x 109 M-1s-1 and a triplet diffusivity of D ~ 4 x 10-6 cm2s-1. The quenching approach was validated through separate study that used the time-of-flight quenching method using polymer films of varying thickness topped with a fullerene quenching layer. The triplet diffusivity in the platinum poly-yne is 102 – 103 less than for singlet excitons in conjugated polymers; however, it is 102 – 103 larger than the triplet diffusivity in many molecular solids. The diffusion length of the triplet exciton is ~22 nm, which is slightly greater than for singlet excitons in typical conjugated polymers. The lifetime of the triplet in the platinum poly-yne is limited to ~1 µs, presumably due to the presence of trap states in the polymer. It is concluded that reduction of the trap 18


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density could substantially increase the exciton lifetime, leading to a material with a greatly enhanced triplet exciton diffusion length. Supporting Information Available: Absorption and emission of Ph100 and Ph95BTD5 films and in THF solution, emission decay kinetics of Ph100 in THF solution, tabular summary of literature summary of exciton diffusion parameters in conjugated polymer films, tabular listing of emission decay kinetics fitting parameters for Ph100/quencher blend films, complete details of synthesis and characterization of Ph100 and Ph95BTD5 and the equation used to compute wt% of Pt-BTD repeat units in Ph100/Ph95BTD5 blend films. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements. This work was supported by the National Science Foundation (Grant No. CHE-1151624). J. X. gratefully acknowledges financial support from the NSF CAREER program and the University of Florida, Office of Research.

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40. Liu, Y.; Jiang, S. J.; Glusac, K.; Powell, D. H.; Anderson, D. F.; Schanze, K. S. Photophysics of Monodisperse Platinum-Acetylide Oligomers: Delocalization in the Singlet and Triplet Excited States. J. Am. Chem. Soc. 2002, 124, 12412-12413. 41. Lapprand, A.; Khiri, N.; Fortin, D.; Jugé, S.; Harvey, P. D. Organometallic Oligomers Based on bis(Arylacetylide)bis(p-Chirogenic Phosphine)Platinum(II) Complexes: Synthesis and Photonic Properties. Inorg. Chem. 2013, 52, 2361-2371. 42. Note that the Pt-Ph phosphorescence emission is not fully quenched for the Ph95BTD5 in THF solution (see Figure S-3b). This is consistent with other studies of energy transfer in conjugated copolymers which demonstrates that energy transfer is typically more efficient in thin films where 3D diffusion can occur. 43. Zhong, H.; Yang, X.; deWith, B.; Loos, J. Quantitative Insight into Morphology Evolution of Thin PPV/PCBM Composite Films Upon Thermal Treatment. Macromolecules 2006, 39, 218-223. 44. Bull, T. A.; Pingree, L. S. C.; Jenekhe, S. A.; Ginger, D. S.; Luscombe, C. K. The Role of Mesoscopic Pcbm Crystallites in Solvent Vapor Annealed Copolymer Solar Cells. ACS Nano 2009, 3, 627-636. 45. Nilsson, S.; Bernasik, A.; Budkowski, A.; Moons, E. Morphology and Phase Segregation of SpinCasted Films of Polyfluorene/PCBM Blends. Macromolecules 2007, 40, 8291-8301. 46. Lakowicz, J. R., Principles of Fluorescence Spectroscopy. Springer: New York, NY, 2009. 47. Morrison, M. E.; Dorfman, R. C.; Clendening, W. D.; Kiserow, D. J.; Rossky, P. J.; Webber, S. E. Quenching Kinetics of Anthracene Covalently Bound to a Polyelectrolyte. 1. Effects of Ionic Strength. J. Phys. Chem. 1994, 98, 5534-5540. 48. Benson, S. W. The Foundations of Chemical Kinetics, McGraw-Hill: New York, 1960.

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49. Mikhnenko, O. V. C., F.; Sieval, A. B.; Hummelen, J. C.; Blom, P. W. M.; Loi, M. A. Temperature Dependence of Exciton Diffusion in Conjugated Polymers. J. Phys. Chem. B 2008, 112, 1160111604. 50. By using the SV model along with the quenching rate found in this study (1.3 x 109 M-1s-1) and a natural lifetime of 18 µs, we compute the trap density in the P100 film of ~40 µM.

This

corresponds to 0.022 traps per 1000 monomer repeat units. If these traps could be eliminated increasing the polymer films' "natural" lifetime to 18 µs, the triplet exciton diffusion length would be ~85 nm.

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Triplet Exciton Diffusion in Platinum Polyyne Films - The Journal of

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