Photo-Induced Absorption of Substituted Poly(Phenylene Vinylene)-Fullerene Composites for Optical Limiting

San-Hui Chi, Joel M. Hales, Matteo Cozzuol, and Joseph W. Perry†

School of Chemistry and Biochemistry – Center for Organic Photonic and Electronics, Georgia Institute of Technology, Atlanta, Georgia †Corresponding author: joe.perry@gatech.edu

Abstract: MEH-PPV:fullerene composites show strong nonlinear absorption in the near infrared and potential as optical limiters. The photophysics and nonlinear optics of the composites are consistent with the formation of absorbing charge carriers. ©2009 Optical Society of America OCIS codes: (190.0190) Nonlinear optics; (190.4400) Nonlinear optics, materials; (190.4710) Optical nonlinearities in organic materials; (070.6020) Signal processing

1. Introduction

With the growth of laser-tracking and all-optical signal processing (AOSP) technologies, the need for effective optical limiting (OL) materials to protect optical sensing devices is increasing. Promising OL systems require low linear transmission loss, low turn-on threshold, high damage threshold, and large suppression in the near infrared regime. Composite of conjugated polymer (e.g. poly(3-octyl thiophene), P3OT) and fullerene derivatives have been found to possess enhanced nonlinear absorption and OL capability in near IR.[1] The observed optical limiting was attributed to photo-induced charge transfer (CT) between the donor polymer and fullerene acceptor, and the nonlinear absorption was enhanced due to fast CT and the long-lived charge recombination.[1,2] However, the reported composite had significant linear transmission loss and the OL performance was limited.[1] In this study, CT composites consisting of poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV) and the fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been investigated as an OL material system, see Figure 1. Studies of the formation of high optical quality, thick film composites of MEH-PPV:PCBM using the plasticizer, dioctylphthalate (DOP) will be described. Optical limiting measurements of nanosecond laser pulses in the near infrared (700-900nm) spectrum on these composites were performed, resulting in a good OL response. Femtosecond pulse transient-absorption measurements were performed on the same thick MEH-PPV:PCBM:DOP composite films to examine the mechanism of the enhanced nonlinear absorption and nanosecond optical pulse suppression.

Figure 1. Chemical structures of MEH-PPV, PCBM, and DOP and an optical microscope image of a 25µm-thick MEH-PPV:PCBM:DOP composite film.

2. MEH-PPV:PCBM:DOP Composites

MEH-PPV:PCBM composites are interesting as potential nonlinear absorbing optical elements because of the linear and nonlinear absorption properties of MEH-PPV and excited state and radical ion absorption characteristics of both MEH-PPV and PCBM. It has been shown the photo-induced CT can occur between MEH-PPV (electron donor) and PCBM (electron acceptor) leading to strongly absorbing radical ions.[3] MEH-PPV has two photon absorption bands between 780 and 850 nm.[4] One-photon or two-photon excitation of the MEH-PPV:PCBM composite in the 700-900 nm range should lead to the generation of the radical ions and/or excited states that are strongly absorbing in the same wavelength range, as needed for effective nonlinear absorption. The MEH-PPV cation shows an absorption band at ~810 nm and the PCBM anion shows absorption bands ranging from 850 to 1070 nm.[5,6] The singlet and triplet excited state absorptions of both MEH-PPV and PCBM are also located in the near IR regime.[7,8] Additionally, MEH-PPV is an attractive conjugated polymer for optical limiting in the 700-900 nm wavelength region, relative to P3OT, because the bandgap (Eg) is larger for MEH-PPV, providing improved linear

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transmission. Thick film composites of MEH-PPV (80% by weight) and PCBM (20% by weight) were found to exhibit poor optical quality and significant optical scattering. This issue was addressed by examining a ternary composite including DOP to form good quality blends. The MEH-PPV:PCBM:DOP (40%t:10%t:50%;, all by weight) composites were readily processed into good optical quality, thick films (see Fig. 1 right) with reduced scattering loss and provided a sizable optical interaction length (25 µm), which is favorable to the OL performance.

3. Nonlinear and Transient Absorption Spectroscopy

Optical limiting measurements with 5-ns pulses on 25µm-thick MEH-PPV:PCBM:DOP composite films showed a total pulse energy suppression of 15 dB and a linear transmittance of 70% at 800 nm, which is substantially improved performance relative to the previously reported P3OT-based composites and to films containing only MEH-PPV:DOP (50%:50%; by wt.) (Figure 2, left). In order to better understand the mechanisms responsible for the nonlinear absorption properties observed for the MEH-PPV:PCBM:DOP composites, femtosecond pulse transient-absorption studies were performed. The transient absorption spectra of films of MEH-PPV:DOP (50%:50%; by wt.) and poly(methyl methacrylate) (PMMA):PCBM:DOP (35%:15%:50%; by wt.) and that of 2.4 mM PCBM/toluene solution show dramatically different transient absorption bands compared to the composite (Figure 2, right). The MEH-PPV:PCBM:DOP composite showed the significant quenching of singlet-singlet excited state absorption of MEH-PPV at ~1300 nm and the generation of a distinct transient species that shows strong absorption over the range of 700 to 1100 nm, with a peak wavelength of ~860 nm. This is consistent with charge transfer from the MEH-PPV singlet state to PCBM. The location of the transient absorption band of composite film is quite similar to that of the MEH-PPV cation, ~810 nm; for the film, the red-shifted absorption peak (860 nm) may be due to the presence of DOP, which increases the polarity. PCBM, on the other hand, showed comparably weak excited state absorption from 700 to 1100 nm, most likely due to the relatively low concentration compared to MEH-PPV. These results support the nonlinear absorption in the MEH-PPV:PCBM:DOP composite being due to the photogeneration of MEH-PPV cation absorption following pulsed laser excitation.

Figure 2. Optical limiting measurements (left) with 5 ns, 800 nm pulses and femtosecond transient-absorption spectra (right) of MEH-PPV, PCBM, and composites (700 nm pump wavelength and time delay of 2.8 ps). Samples thicknesses were 25µm and the optical geometry was F/5.

4. Conclusion

Thick, optical quality conjugated polymer-fullerene composites have been prepared and showed exceptional optical limiting capability in the near infrared. Femtosecond time-resolved transient absorption provided evidence for photogeneration of radical ions as being responsible for the strong optical limiting performance in the near IR.

This work was funded in part by DARPA/ONR through the MORPH program (Grant No. N00014-04-1-0095) and by NSF through the CMDITR STC (DMR-0120967) [1] M. Cha, N. S. Sariciftci, A. J. Heeger, J. C. Hummelen, and F. Wudl, Appl. Phys. Lett. 67, 3850-3852 (1995) [2] N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Synthetic Metals 59, 333-352 (1993). [3] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 258, 1474-1476 (1992) [4] S.-J. Chung, G. S. Maciel, H. E. Pudavar, T.-C. Lin, G. S. He, J. Swiatkiewicz, D. W. Lee, J.-I. Jin and P. N. Prasad, J. Phys. Chem. A 106,

7512-7520 (2002) [5] P. A. van Hal, M. P. T. Christiaans, M. M. Wienk, J. M. Kroon, and R. A. J. Janssen, J. Phys. Chem. B 103, 4352-4359 (1999) [6] Steffan Cook, Photo-induced Charge Generation and Recombination in Conjugated Polymer-Methanofullerene Blend Films (Ph.D.

Dissertation, Imperial College London, London, 2006) [7] A. Dogariu, D. Vacar, and A. J. Heeger, Phys. Rev. B 58, 10218-10224 (1998) [8] T. W. Ebbesen, K. Tanigaki and S. Kuroshima, Chem. Phys. Lett. 181, 501-504 (1991)

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