Journal ArticleRadiative recombination in polythiophene is studied by cw photoluminescence and photoinduced absorption spectroscopies and by photoluminescence-detected magnetic resonance. The fast-recombination centers are assigned to intrachain excitons in defect-free chains, while the slow recombination is due to intrachain excitons bound to spin- 1/2 defects

Shinar, J.

Vardeny, Zeev Valentine

English

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RAPJ() COMMU~ICATIO"'S PHYSICAL REVIEW B VOLUME 35, NUMBER 5 15 FEBRUARY 1987-1 Photoexcitation spectroscopy of poly thiophene Z. Vardeny* and E. Ehrenfreund Department of Physics and Solid State Institute, Technion-Israel Institute of Technology, Haifa, Israel J. Shinar Ames Laboratory and Department of Physics, Iowa State University, Ames, Iowa 5001 I F. Wudl Institute for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106 (Received 27 October 1986) Radiative recombination in poly thiophene is studied by cw photoluminescence and photoinduced absorption spectroscopies and by photoluminescence-detected magnetic resonance. The fast-recombination centers are assigned to intrachain excitons in defect-free chains, while the slow recombination is due to intrachain excitons bound to spin- t defects. INTRODUCTION Heterocyclic polymers such as polypyrrole and po-lythiophene with five-membered rings containing a N-H group or a sulfur atom, respectively, are of recent consid-erable interest. They represent a class of conjugated poly-mers with nondegenerate ground-state symmetry, leading thus to a finite confinement potential. I It was suggested that photoexcited electron-hole (e -h) pairs in such nonde-generate polymers quickly recombine because of the con-finement potential which prevents their dissociation. 2 The first confirmation of this idea was observed in cis-(CH)x: 2 Photocarrier recombination is very fast (Jess than 9 psec) 3 and no long-lived cis-related charge photoexcitations could be observed. 4 In other polymers with nondegenerate ground states which contain native defects, disorder, or amorphous character, or have a larger interchain coupling, there may exist other recombination channels which are quite different from those predicted by theory.I,2,5 In par-ticular, the recombination slows down under the following circumstances: (j) crossing to a triplet manifold, (ij) pho-togeneration of interchain excitons, and (iij) excitons bound to various native defects. Neutral poly thiophene (PT) which is stabilized in a sin-gle backbone geometry by the sulfur atom is an ideal can-didate to study the recombination channels in a conjugated polymer with nondegenerate ground-state symmetry. Na-tive spin- t defects exist in PT and were measured by ESR 6 and by spin-echo modulation. 7 Also, long-lived ( - 10 - 2 sec) photocarriers were detected in neutral PT by the photoinduced absorption (P A) technique. 8 These re-sults indicate that some of the above-mentioned mecha-nisms responsible for the slow recombination of the pho-toexcited e -h pairs are operative in PT. In this Rapid Communication we report our study of the cw photoluminescence (PO in PT. The PL spectrum and its excitation profile were measured and compared with the excitation profile for charge photocarriers measured by PA. We found that in addition to free intrachain excitons, there are also excitons bound to native defects. We have successfully studied, for what we believe to be the first time in conducting polymers, the PL optically detected magnetic resonance (ODMR) in PT. The PL-related de-fects could thus be identified as the same spin- t defects which show up in ESR and in light-induced ESR measure-ments as well. 6 We also find it unlikely that triplet states are the cause for the slow PL recombination. PHOTOLUMINESCENCE SPECTROSCOPY The cw PL spectrum and its excitation -rrofile were measured with a dye laser pumped by an Ar laser. The emitted light was dispersed by a monochromator and detected by a Si detector and a lock-in amplifier. The PL spectra were corrected for the grating and detector responses but were not corrected for reabsorption in the sample. The PT samples were pressed KBr pellets 6 con-taining approximately 0.25 wt. % PT powder (obtained by condensation polymerization of 2, 5-di-iodothiophen). PL spectra for electrochemically prepared films were previ-ously reported;9,IO in our study, similar PL results were ob-tained for both types of samples. Figure 1 shows the photoluminescence and optical ab-sorption spectra of neutral PT. The absorption spectrum a(w) consists of a temperature-independent broadband which peaks at around 2.6 eV. The PL spectrum, on the other hand, is strongly temperature dependent and consists of several phonon sidebands superimposed on a much broader PL band (full width at half maximum of -0.3 eV). At 20 K, four well-defined equally spaced phonon sidebands at 1.95, 1.77, 1.59, and 1.41 are observed. At 300 K the overtones become less sharply defined and are shifted upward by about 30 meV. This energy shift rules out resonant Raman scattering (RRS) as the underlying mechanism for the phonon sidebands, which is the case in cis-(CH)x. 2 The energy separation of the phonon side-bands in the PL spectrum is 0.18 eV, which is nearly equal to the strongest line observed in RRS. II This line has been identified as the carbon-carbon stretching mode and it is 2498 © 1987 The American Physical Society RAPID COMMU~ICATlO~S PHOTOEXCITA TION SPECTROSCOPY OF POL YTHIOPHENE 2499 '"'j ;:; poly thiophene E -e u ~ 30 I 3Q w U I-Z Z w w u j=- 2 Q en w 20 LJ... Z LJ... ~ w / 0 :::> u -l 0 Z I- 10 I 0 0 ~ J: a.. a.. 0: 0 en ID ~ 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 PHOTON ENERGY(eV) FIG. 1. Photoluminescence spectra at 20 and 300 K. The right-hand scale shows a(w) for comparison. the mode with the strongest electron-phonon coupling in the PT backbone. We note that the ultrafast PL spectrum measured in the picosecond regime by Wong et al. 10 shows only one broad-band in contrast to the clear phonon sideband structure in the cw PL observed here. To further investigate the origin of the PL centers we have studied the PL excitation profile (denoted hereafter EXPO. The EXPL at 20 K measured for the PL emission at 1.83 eV is shown in Fig. 2. The EXPL abruptly increases at 1.95 eV, well below the one-dimensional (JD) gap at 2.2 eV (as deduced from the RRS analysis 11) or the interband transition maximum at 2.6 eV, and remains constant thereafter. Vibrationalover-tones are not observed in the EXPL spectrum, indicating that the phonon sidebands seen in the PL spectrum (Fig. I) are not due to internal vibrations of a point defect with a strong electron-phonon coupling. Moreover, the 1.95-eV onset coincides with the zeroth-order PL emission in Fig. I, thus ruling out any Stokes shift. 9 We therefore identify the 1.95-eV energy as an exciton level. The difference be-tween the 1 D gap at 2.1- 2.3 e V (Ref. 11) and the exciton -; 30 f-~ >-poly thiophene ; 20 K excitation profile t A t:: 20 4== (J) Z W I-Z -10 -...J a.. 1.7 1.9 2.1 2.3 2.5 2.7 EXCITATION PHOTON ENERGY (eV) FIG. 2. Excitation spectrum of the photoluminescence at 20 K. Poly thiophene 4 20 K ;::i 3 .0 '-~ I- 2 ..... I-<J 1 I.B 2.0 2.2 2.4 2.6 EXCITATION ENERGY (eV) FIG. 3. Excitation profile of the long-lived photocarriers as measured by photoinduced absorption. level at 1.95 eV is interpreted as the binding energy of the exciton. We have also studied the excitation profile of the pho-toinduced absorption of charge carriers (denoted hereafter EXP A) by measuring the excitation spectrum of the pho-toinduced ir-active vibrations which are the clearest signa-ture of the charge defects. 5 In Fig. 3 we show the EXPA at 20 K; it consists of a relatively narrow peak at 1.95 eV followed by a steep rise at energies above 2.4 eV where 3D absorption plays an important role, thus indicating that free-carrier absorption is involved in the charge photogen-eration. The photogenerated carriers above 2 eV are ei-ther directly photoexcited on neighboring chains by inter-chain absorption 8 or separated electron and hole polarons on the same chain. The 1.95-eV peak, in the EXPA spec-trum, may therefore originate either from an interchain exciton or an intrachain exciton level. However, the EXPA extends well below the 1.95-eV level8 where 3D transitions are negligibly small. Moreover, it is difficult to reconcile such a strong exciton coupling (Fig. 1) to the carbon-carbon stretching vibration (which is strictly intra-chain) by an interchain exciton. We therefore suggest that the 1.95-eV peak be assigned to an intrachain exciton bound to defects. With this assignment, the large widths of the PL, EXPL, and EXP A spectra can be accounted for by an inhomogeneous broadening. Furthermore, a bound exciton represents the first step towards e -h separation, since the electron-defect (or hole-defect) coupling is strong. It is not surprising therefore that the quantum ef-ficiency for charge photogene ration peaks at the bound in-trachain exciton level (Fig. 3). The following ODMR re-sults strengthen the proposed assignment for the 1.95-eV peak of the PL. ODMR MEASUREMENTS The ODMR was studied by measuring the changes in-duced in the PL intensity of a PT sample placed in an opti-cally accessed X-band microwave cavity under resonance conditions in a magnetic field. The external magnetic field RAPID COMMUNICATIONS 2500 Z. VARDENY, E. EHRENFREUND, J. SHINAR, AND F. WUDL (H) was modulated to allow for lock-in detection giving rise to derivativelike signals. In general, changes in the PL signal can be caused either via the radiative or the nonra-diative recombination channels. As required by the optical selection rules, the rate at which a pair of photoexcited carriers recombines depends on its spin configuration. Thus, if under optical excitation carriers are created with an isotropic spin distribution, the population of the slow-decaying pairs becomes rapidly larger than the population of the fast-decaying pairs (provided that the spin-lattice relaxation time is not exceeded) and a spin correlation ap-pears. Application of microwave power at resonance will therefore restore an isotropic spin distribution and, as has been shown, 12 will result either in an ODMR-enhancing signal when the radiative recombination channel is affect-ed or in an ODMR-quenching signal when the nonradia-tive recombination channel is involved. ODMR data for a PT sample immersed in liquid nitro-gen at 77 K are shown in Fig. 4. The PL was excited with an Ar+ laser (100 mW/cm 2) in a backscattering configu-ration. The emission was collected by lenses and focused onto a fast Si detector. The derivativelike ODMR signal obtained by sweeping the magnetic field through the reso-nance line is a PL-quenching signal; this was checked by comparing it with a low-defect a-Si:H sample. 12 We have carried out measurements in which H was swept up to 500 G around the resonant field but no other features were ob-served, indicating that triplet excitons are not responsible for the ODMR. The ODMR signal gradually decreases as the temperature increases. This decrease with tempera-ture is accompanied by a parallel decrease in the spin-lattice relaxation time,7 thus suggesting that unthermal-ized spins are giving rise to the ODMR process. We there-fore conclude that the weakly correlated pair model 13 fits our data and that the ODMR-quenching signal of the PL is caused by a nonradiative spin- t center acting as a shunt in a competing process to the main photoluminescence channel. When resonance ESR conditions are met, spin flip can occur at this center, thereby enhancing the spin-dependent nonradiative recombination channel. The ODMR results may thus be used to identify the nonradia-tive centers. The symmetric quenching signal in Fig. 4 amounts to a relative change of about 10 - 3 of the PL intensity; it is centered at g =2.003 and the derivative peak-to-peak 'Present address: Engineering Department, Brown University, Providence, Rhode Island 02912. IS. A. Brazovskii and N. N. Kirova, Zh. Eksp. Teor. Fiz. 33, 6 (l98}) [JETP Lett. 33, 4 (198 J)]. 2L. Lauchlan, S. Etemad, T.-C. Chung, A. J. Heeger, and A. G. MacDiarmid, Phys. Rev. B 24, 3701 (l98}). 3W. Hayes, C. N. Ironside, J. F. Ryan, R. P. Steele, and R. A. Taylor, J. Phys. C 16, L 729 (1983). 4Z. Vardeny, J. Orenstein, and G. L. Baker, Phys. Rev. Lett. 50, 2032(1983). 5p. L. Danienson and R. C. Ball, J. Phys. (Paris) 46, 1611 (1985). 6M. Kobayashi, J. Chen, T.-C. Chung, F. Moraes, A. J. Heeger, and F. Wudl, Synth. Met. 9, 77 (1984). 7D. Davidov, F. Moraes, A. J. Heeger, F. Wudl, H. Kim, and poly thiophene ; 80 K -l <t Z (!) en lOG a:: ::;;: 0 0 MAGNETIC FIELD FIG. 4. ODMR of the photoluminescence shown in Fig. I. linewidth is t!..H p.p. -11 G. The g shift is the same and the line is only slightly broader than that of the ESR signal in neutral PT6,7 which is due to spin- t intrachain native de-fects. We therefore conclude that the PL-related defects and the spin- t defects in PT are the same. The quenching ODMR is thus related to e-h pairs excited on chains which contain such a defect; the exciton becomes bound via e-defect (or h-defect) interaction. This process may increase the PL lifetime due to the e -h separation and thus enable the ODMR signal to appear. Excitons on defect-free chains are less susceptible to spin-dependent nonradi-ative recombination since the electron and the hole are highly correlated and their recombination is too fast. 10 In summary, the cw photoluminescence and photoin-duced absorption spectra together with the ODMR results suggest that in addition to the fast radiative recombination centers in PT, slow radiative photoexcitations also exist. We identify these centers as intrachain excitons bound to the spin- t intrachain native defects observed in ESR ex-periments. It should, however, be noted that this is only one of the various recombination channels that could be operative in a real polymer. ACKNOWLEDGMENTS The work at the Technion was supported in part by the U.S.-Israel Binational Science Foundation, Jerusalem, Is-rael. We gratefully acknowledge fruitful discussions with Professor A. J. Heeger. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State Univer-sity under Contract No. W-7405-ENG-82. This work was supported by the U.S. Office of Basic Energy Sciences. L. R. Dalton, Solid State Commun. 53, 497 (1985). 8Z. Vardeny, E. Ehrenfreund, O. Brafman, M. Nowak, H. Schaffer, A. J. Heeger, and F. Wudl, Phys. Rev. Lett. 56, 671 (1986). 9K. Kaneto, Y. Kohno, and K. Yoshino, Solid State Commun. 51,267 (1984). 10K. S. Wong, W. Hayes, T. Hattori, R. P. Taylor, J. F. Ryan, K. Kaneto, Y. Yoshino, and D. Bloor, J. Phys. C 18, L843 (1985). liZ. Vardeny, E. Ehrenfreund, O. Brafman, A. J. Heeger, and F. Wudl, Synth. Met. 18,199 (1986). 12R. A. Street, Phys. Rev. B 26,3588 (1982). 13S. P. Depinna, B. C. Cavanett, I. C. Austin, and T. M. Searl, Solid State Commun. 41, 263 (1982). 

Photoexcitation spectroscopy of polythiophene

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Photoexcitation spectroscopy of polythiophene

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