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Optically detected magnetic resonance studies of nanostructured PPV-composites
Optical Materials 12 (1999) 369±372
Optically detected magnetic resonance studies of nanostructured PPV-composites E.J.W. List a
a,*
, P. Markart a, W. Graupner a, G. Leising a, J. Partee b, J. Shinar b, R. Smith c, D. Gin c
Institut f ur Festk orperphysik, Technische Universit at Graz, Petergasse 16, A-8010 Graz, Austria b Ames Laboratory, US DOE, ISU, Ames, Iowa 50011, USA c Department of Chemistry, University of California, 94720 Berkeley, USA
Abstract We used the spectroscopic technique of photoluminescence detected magnetic resonance (PLDMR) to monitor the dynamics of singlet excitons as well as polarons of isolated poly(p-phenylenevinylene) (PPV) chains incorporated into a self-assembled matrix, ordered at a nanometer scale, and compare it with the results obtained for bulk PPV. The isolation of the PPV chains in the inorganic matrix which is of hexagonal structure is con®rmed by a blue shift of the photoluminescence (PL) spectrum and the increase of the PL quantum yield. The eect on the photo physical properties due to the isolation of the polymer chains is further proved by the dierence in the PLDMR linewidth and the observation of a strong dierence in the exponent in power-law dependence of DPL/PL on the laser power. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 71.38.+i; 72.80.Le; 78.55.Kz Keywords: Organic semiconductors; Magnetic resonance; Nano composites
1. Introduction In the ®eld of organic light emitting diodes (OLEDs), there is a quest for new materials in order to tune the intrinsic physical properties of the bulk materials since it is well known that control of the separation of the polymer chains [1] is vital for the improvement of these kind of devices. Since nature uses microscopic self-assembly processes to structure on a nanometer scale, which leads to impressive properties of the structured materials [2], a very promising strategy is to apply these processes to conjugated systems. As it was shown * Corresponding author. Tel.: +43-316-873-8475; fax: +43316-873-8478; e-mail: [email protected]
in Ref. [3] the ordering of conjugated polymers can be achieved in an inverse hexagonal matrix formed from lyotropic liquid crystalline monomers and an aqueous solution of a PPV-precursor. With this strategy one obtains hexagonal-packed tubular channels as con®rmed by X-ray diraction investigation [3], 1.5 nm in diameter, ordered on a nanometer scale and ®lled with PPV chains. To study the in¯uence of the isolation of the PPV chains on the spin dependent photophysical properties in contrast to the properties of bulk PPV we use the powerful tool of photoluminescence (PL) detected magnetic resonance PLDMR. In the PLDMR experiment we detect the in¯uence of the magnetic resonance induced change in the population of pair of spin 1/2 polarons [4] occurring at
0925-3467/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 6 7 - 1
370
E.J.W. List et al. / Optical Materials 12 (1999) 369±372
g 2 on the PL which is due to the radiative decay of spin 0 singlet excitons. Depending on the model these recombining pairs form triplet excitons which form singlet excitons by triplet±triplet annihilation [5] or recombine nonradiatively reducing the number of singlet quenching sites enabling more singlet excitons to recombine radiatively [6,7]. 2. Experimental For the preparation of the bulk PPV ®lms for PLDMR measurements, the precursor, tetra-hydro-thiophene PPV, was ®lled into OD quartztubes whereas for the quantum yield measurements the precursor was spin-coated onto quartz substrates. The bulk PPV ®lms were converted at different temperatures in order to investigate the in¯uence on the PLDMR signal (see Table 1). The ODMR tubes were vacuum sealed after the conversion. The ®lms consisting of the nano-assembled PPV (n-PPV) as shown in Fig. 1 were prepared on glass substrates as described in [3]. The photoluminescence quantum yield measurements where performed in an integrating sphere as described elsewhere [8]. For the PLDMR measurements the samples were inserted into a He-gas¯ow cryostat and excited by an Ar laser at 458 nm. The changes in the PL signal, induced by the X-band microwaves in a DC magnetic ®eld, were detected by a photodiode connected to a lock-in ampli®er referenced to the microwave chopping frequency.
Fig. 1. Synthesis route of the nanocomposite (from [3]).
3 and of the nanoassembled PPV (n-PPV). The PL spectra of the bulk PPV samples are nearly identical. They show a pronounced maximum at 2.25 eV and shoulders at 2.07 eV and at 2.45 eV. The energetic spacings between the vibronic replica and the 0±0 transition located at 2.45 eV are
3. Results and discussion 3.1. The photophysical properties Fig. 2 presents the PL, absorption and the PL excitation spectra of the bulk PPV samples PPV1-
Fig. 2. (a) PL and absorption spectrum of PPV1, (b) PL and absorption spectrum of PPV2, (c) PL, absorption spectrum (solid line) and PL excitation spectrum detected at (dashed line) of PPV3, and PL and PL excitation spectrum of n-PPV (detected at 500 nm) (d).
Table 1 Conversion parameters for the dierent bulk PPV samples and the nano assembled PPV. Photoluminescence quantum yield of all samples gPL , the full width at half maximum FWHM and the PLDMR signal intensities DPL/PL of the PLDMR spectra of Fig. 2 Conversion time (h) Conversion temperature (°C) gPL (%) DPL/PL FWHM (G)
PPV1
PPV2
PPV3
n-PPV
3 350 5 1.2 ´ 10ÿ3 28
8 250 17 6.0 ´ 10ÿ4 27
17 150 20 <2 ´ 10ÿ6 ±
4 250 30 4.0 ´ 10ÿ4 16
E.J.W. List et al. / Optical Materials 12 (1999) 369±372
found to be 0.19 eV. The absorption spectra of the bulk PPV samples are characterized by a onset at 2.4 eV and a broad maximum located at 2.75 eV. The PL excitation spectrum (dashed line in (c)) of PPV3, a rather thick ®lm, shows a maximum at 2.4 eV. The PL emission spectrum of n-PPV is blue shifted by 0.3 eV with respect to the PL emission spectra of the bulk PPV. Due to the low concentration of PPV in the n-PPV sample and the strong scattering of the inorganic matrix we use an excitation spectrum to characterize the absorption properties of the n-PPV sample. However the excitation spectrum shows a broad maximum at 3.25 eV. Although we observed PL spectra similar in their spectral position from the bulk PPV samples we obtained rather dierent absolute PL quantum yields (gPL ) which are summarized in Table 1. 4. The polaron resonance In order to show the eect of separation of the PPV chains we compare the nanostructured PPV sample chains with bulk PPV and analyze the signal intensity and the linewidth of the polaron pair PLDMR signal at g 2. The normalized polaron resonance spectra (g 2) of the nano assembled n-PPV ®lm (a) and the bulk PPV samples all recorded at 20 K are shown in Fig. 3. The PLDMR intensities are summarized in Table 1. For the bulk PPV sample with the lowest gPL (PPV1) we obtain the strongest polaron pair PLDMR signal (DPL/PL 1.2 ´ 10ÿ3 ). The ®t to a single Lorentzian line yields a full linewidth at half maximum (FWHM) of 28 G. The lineshape analysis of the polaron pair spectrum of PPV2 (DPL/PL 6.0 ´ 10ÿ4 ) revealed a FWHM of 27 G. The sample PPV3 did not exhibit any PLDMR signal within the detection limit of the spectrometer (>2 ´ 10ÿ6 ) although the PL was bright. The FWHM of the n-PPV sample is dramatically lowered to 16 G compared to the widths of the bulk sample. The PLDMR signal intensity with DPL/PL 4.0 ´ 10ÿ4 is found to be comparable to the one of PPV2. Comparing the obtained results for the bulk PPV samples PPV1, PPV2 and PPV3 one ®nds an
371
Fig. 3. Polaron pair resonance at g 2 of (a) PPV1, (b) PPV2, (c) PPV3 and (d) n-PPV recorded at 20 K with a microwave power of 810 mW and a laser power of 25 mW.
inverse relation between the polaron PLDMR amplitude DPL/PL and gPL , i.e., the lower gPL the more intense the polaron PLDMR. This result is consistent with the scenario in which defects promote the formation and stabilization of Coulomb bound polaron pairs while reducing gPL . The magnetic resonance enhanced recombination of these defect stabilized polaron pairs [9] and the in¯uence on the PL is therefore more pronounced in the sample with the higher defect content. We emphasize that this correlation and the above conclusion are independent of the process, which is still debated [5,6], by which polaron recombination enhances the PL. The rather small dierence in PLDMR signal intensity comparing PPV2 and nPPV, although there is a large dierence in the gPL , is assigned to fact that the high gPL in n-PPV is due to the isolation of the polymer chain in the nano tubes [1] and not due to a decrease of the defect content in this sample. The isolation of the chains hinders the formation of interchain defect stabilized polaron pairs. Furthermore one has to take into account the in¯uence of the surrounding polar matrix which might also enable the formation of defects. The analysis of the total linewidth depending on the sample yielded that the FWHM of n-PPV was smaller by a factor of 1.75 compared to the bulk PPV samples PPV1 and PPV2. Since in the nano structured PPV we observe the PLDMR signals
372
E.J.W. List et al. / Optical Materials 12 (1999) 369±372
from isolated PPV chains this eect has to be compared to PLDMR spectra of polymers dissolved in a solvent or dispersed in an inert matrix [7]. The foregoing analysis suggests a scenario where, on the isolated polymer chains in n-PPV a polaron pair forms as an intra-chain interconjugation segment pair in which the two polarons are rather distant, separated by a chemical defect. In bulk PPV the formation of inter-chain polaron pairs is much more pronounced. If inter-chain polaron pairs are much closer, their PLDMR lineshape will be dipolarly broadened relative to the intrachain pairs. 5. Excitation power dependence The laser power dependence of the polaron resonance for the n-PPV sample and the bulk PPV sample PPV2 are depicted in Fig. 4(a) and (b), respectively. The experimentally observed powerlaw dependence of DPL/PL on the laser power revealed an exponent of 0.56 for the resonance of the bulk PPV2 sample and 0.27 for the n-PPV sample. The PL was observed to be linear with the
laser power (2±50 mW), for bulk PPV and sublinear for n-PPV. No signi®cant linewidth broadening of the PLDMR signal was observed within the experimental range of the laser power. In conclusion, the blue shift of the PL spectrum and the increase of gPL in the n-PPV sample is assigned to the insulation of the polymer chains in the nano tubes. The larger polaron PLDMR linewidth in the bulk PPV compared to n-PPV is attributed to the increase of interchain polaron pair formation over intrachain-interconjugation-segment polaron pairs. The observation of a strong dierence in the exponent in the power-law dependence of DPL/PL on the laser power observed in PPV2 and n-PPV indicates a drastically dierent recombination mechanism leading the change in the PL in this experiment. Acknowledgements E.J.W.L. gratefully acknowledges the partial ®nancial support of the BMWV for the work at the Ames Lab and of the FWF P. Nr. 12806-PHY for the work in Graz. Ames Lab is operated by the also thank M. Graupner and A. Hermetter. References
Fig. 4. Laser power dependence of (b) PPV1 and (d) n-PPV recorded at the maximum of the polaron pair resonance at g 2 at 20 K with a microwave power of 810 mW and a laser power from 2 to 45 mW.
[1] J. Cornil, A.J. Heeger, J.L. Bredas, Chem Phys. Lett. 272 (1997) 463. [2] A.H. Heuer et al., Science 255 (1992) 1098. [3] R.C. Smith, W.M. Fischer, D.L. Gin, J. Am. Chem. Soc. 119 (1997) 4092. [4] E.L. Frankevich, G.E. Zoriniants, A.N. Chaban, M.M. Triebel, S. Blumstengel, V.M. Kobryanskii, Chem. Phys. Lett. 261 (1996) 545. [5] V. Dyakonov, E.L. Frankevich, Chem. Phys. Lett. 227 (1998) 203. [6] J. Shinar, Synth. Met. 78 (1996) 277. [7] W. Graupner, J. Partee, J. Shinar, G. Leising, U. Scherf, Phys. Rev. Lett. 77 (1996) 2033. [8] Y.S. Liu, P. de Marco, W.R. Ware, J. Phys. Chem. 97 (1993) 5995. [9] W. Graupner, M. Sacher, M. Graupner, C. Zenz, G. Grampp, A. Hermetter, G. Leising, Mat. Res. Soc. Symp. Proc. 488 (1998) 789.
Optically detected magnetic resonance studies of nanostructured PPV-composites E.J.W. List a
a,*
, P. Markart a, W. Graupner a, G. Leising a, J. Partee b, J. Shinar b, R. Smith c, D. Gin c
Institut f ur Festk orperphysik, Technische Universit at Graz, Petergasse 16, A-8010 Graz, Austria b Ames Laboratory, US DOE, ISU, Ames, Iowa 50011, USA c Department of Chemistry, University of California, 94720 Berkeley, USA
Abstract We used the spectroscopic technique of photoluminescence detected magnetic resonance (PLDMR) to monitor the dynamics of singlet excitons as well as polarons of isolated poly(p-phenylenevinylene) (PPV) chains incorporated into a self-assembled matrix, ordered at a nanometer scale, and compare it with the results obtained for bulk PPV. The isolation of the PPV chains in the inorganic matrix which is of hexagonal structure is con®rmed by a blue shift of the photoluminescence (PL) spectrum and the increase of the PL quantum yield. The eect on the photo physical properties due to the isolation of the polymer chains is further proved by the dierence in the PLDMR linewidth and the observation of a strong dierence in the exponent in power-law dependence of DPL/PL on the laser power. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 71.38.+i; 72.80.Le; 78.55.Kz Keywords: Organic semiconductors; Magnetic resonance; Nano composites
1. Introduction In the ®eld of organic light emitting diodes (OLEDs), there is a quest for new materials in order to tune the intrinsic physical properties of the bulk materials since it is well known that control of the separation of the polymer chains [1] is vital for the improvement of these kind of devices. Since nature uses microscopic self-assembly processes to structure on a nanometer scale, which leads to impressive properties of the structured materials [2], a very promising strategy is to apply these processes to conjugated systems. As it was shown * Corresponding author. Tel.: +43-316-873-8475; fax: +43316-873-8478; e-mail: [email protected]
in Ref. [3] the ordering of conjugated polymers can be achieved in an inverse hexagonal matrix formed from lyotropic liquid crystalline monomers and an aqueous solution of a PPV-precursor. With this strategy one obtains hexagonal-packed tubular channels as con®rmed by X-ray diraction investigation [3], 1.5 nm in diameter, ordered on a nanometer scale and ®lled with PPV chains. To study the in¯uence of the isolation of the PPV chains on the spin dependent photophysical properties in contrast to the properties of bulk PPV we use the powerful tool of photoluminescence (PL) detected magnetic resonance PLDMR. In the PLDMR experiment we detect the in¯uence of the magnetic resonance induced change in the population of pair of spin 1/2 polarons [4] occurring at
0925-3467/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 6 7 - 1
370
E.J.W. List et al. / Optical Materials 12 (1999) 369±372
g 2 on the PL which is due to the radiative decay of spin 0 singlet excitons. Depending on the model these recombining pairs form triplet excitons which form singlet excitons by triplet±triplet annihilation [5] or recombine nonradiatively reducing the number of singlet quenching sites enabling more singlet excitons to recombine radiatively [6,7]. 2. Experimental For the preparation of the bulk PPV ®lms for PLDMR measurements, the precursor, tetra-hydro-thiophene PPV, was ®lled into OD quartztubes whereas for the quantum yield measurements the precursor was spin-coated onto quartz substrates. The bulk PPV ®lms were converted at different temperatures in order to investigate the in¯uence on the PLDMR signal (see Table 1). The ODMR tubes were vacuum sealed after the conversion. The ®lms consisting of the nano-assembled PPV (n-PPV) as shown in Fig. 1 were prepared on glass substrates as described in [3]. The photoluminescence quantum yield measurements where performed in an integrating sphere as described elsewhere [8]. For the PLDMR measurements the samples were inserted into a He-gas¯ow cryostat and excited by an Ar laser at 458 nm. The changes in the PL signal, induced by the X-band microwaves in a DC magnetic ®eld, were detected by a photodiode connected to a lock-in ampli®er referenced to the microwave chopping frequency.
Fig. 1. Synthesis route of the nanocomposite (from [3]).
3 and of the nanoassembled PPV (n-PPV). The PL spectra of the bulk PPV samples are nearly identical. They show a pronounced maximum at 2.25 eV and shoulders at 2.07 eV and at 2.45 eV. The energetic spacings between the vibronic replica and the 0±0 transition located at 2.45 eV are
3. Results and discussion 3.1. The photophysical properties Fig. 2 presents the PL, absorption and the PL excitation spectra of the bulk PPV samples PPV1-
Fig. 2. (a) PL and absorption spectrum of PPV1, (b) PL and absorption spectrum of PPV2, (c) PL, absorption spectrum (solid line) and PL excitation spectrum detected at (dashed line) of PPV3, and PL and PL excitation spectrum of n-PPV (detected at 500 nm) (d).
Table 1 Conversion parameters for the dierent bulk PPV samples and the nano assembled PPV. Photoluminescence quantum yield of all samples gPL , the full width at half maximum FWHM and the PLDMR signal intensities DPL/PL of the PLDMR spectra of Fig. 2 Conversion time (h) Conversion temperature (°C) gPL (%) DPL/PL FWHM (G)
PPV1
PPV2
PPV3
n-PPV
3 350 5 1.2 ´ 10ÿ3 28
8 250 17 6.0 ´ 10ÿ4 27
17 150 20 <2 ´ 10ÿ6 ±
4 250 30 4.0 ´ 10ÿ4 16
E.J.W. List et al. / Optical Materials 12 (1999) 369±372
found to be 0.19 eV. The absorption spectra of the bulk PPV samples are characterized by a onset at 2.4 eV and a broad maximum located at 2.75 eV. The PL excitation spectrum (dashed line in (c)) of PPV3, a rather thick ®lm, shows a maximum at 2.4 eV. The PL emission spectrum of n-PPV is blue shifted by 0.3 eV with respect to the PL emission spectra of the bulk PPV. Due to the low concentration of PPV in the n-PPV sample and the strong scattering of the inorganic matrix we use an excitation spectrum to characterize the absorption properties of the n-PPV sample. However the excitation spectrum shows a broad maximum at 3.25 eV. Although we observed PL spectra similar in their spectral position from the bulk PPV samples we obtained rather dierent absolute PL quantum yields (gPL ) which are summarized in Table 1. 4. The polaron resonance In order to show the eect of separation of the PPV chains we compare the nanostructured PPV sample chains with bulk PPV and analyze the signal intensity and the linewidth of the polaron pair PLDMR signal at g 2. The normalized polaron resonance spectra (g 2) of the nano assembled n-PPV ®lm (a) and the bulk PPV samples all recorded at 20 K are shown in Fig. 3. The PLDMR intensities are summarized in Table 1. For the bulk PPV sample with the lowest gPL (PPV1) we obtain the strongest polaron pair PLDMR signal (DPL/PL 1.2 ´ 10ÿ3 ). The ®t to a single Lorentzian line yields a full linewidth at half maximum (FWHM) of 28 G. The lineshape analysis of the polaron pair spectrum of PPV2 (DPL/PL 6.0 ´ 10ÿ4 ) revealed a FWHM of 27 G. The sample PPV3 did not exhibit any PLDMR signal within the detection limit of the spectrometer (>2 ´ 10ÿ6 ) although the PL was bright. The FWHM of the n-PPV sample is dramatically lowered to 16 G compared to the widths of the bulk sample. The PLDMR signal intensity with DPL/PL 4.0 ´ 10ÿ4 is found to be comparable to the one of PPV2. Comparing the obtained results for the bulk PPV samples PPV1, PPV2 and PPV3 one ®nds an
371
Fig. 3. Polaron pair resonance at g 2 of (a) PPV1, (b) PPV2, (c) PPV3 and (d) n-PPV recorded at 20 K with a microwave power of 810 mW and a laser power of 25 mW.
inverse relation between the polaron PLDMR amplitude DPL/PL and gPL , i.e., the lower gPL the more intense the polaron PLDMR. This result is consistent with the scenario in which defects promote the formation and stabilization of Coulomb bound polaron pairs while reducing gPL . The magnetic resonance enhanced recombination of these defect stabilized polaron pairs [9] and the in¯uence on the PL is therefore more pronounced in the sample with the higher defect content. We emphasize that this correlation and the above conclusion are independent of the process, which is still debated [5,6], by which polaron recombination enhances the PL. The rather small dierence in PLDMR signal intensity comparing PPV2 and nPPV, although there is a large dierence in the gPL , is assigned to fact that the high gPL in n-PPV is due to the isolation of the polymer chain in the nano tubes [1] and not due to a decrease of the defect content in this sample. The isolation of the chains hinders the formation of interchain defect stabilized polaron pairs. Furthermore one has to take into account the in¯uence of the surrounding polar matrix which might also enable the formation of defects. The analysis of the total linewidth depending on the sample yielded that the FWHM of n-PPV was smaller by a factor of 1.75 compared to the bulk PPV samples PPV1 and PPV2. Since in the nano structured PPV we observe the PLDMR signals
372
E.J.W. List et al. / Optical Materials 12 (1999) 369±372
from isolated PPV chains this eect has to be compared to PLDMR spectra of polymers dissolved in a solvent or dispersed in an inert matrix [7]. The foregoing analysis suggests a scenario where, on the isolated polymer chains in n-PPV a polaron pair forms as an intra-chain interconjugation segment pair in which the two polarons are rather distant, separated by a chemical defect. In bulk PPV the formation of inter-chain polaron pairs is much more pronounced. If inter-chain polaron pairs are much closer, their PLDMR lineshape will be dipolarly broadened relative to the intrachain pairs. 5. Excitation power dependence The laser power dependence of the polaron resonance for the n-PPV sample and the bulk PPV sample PPV2 are depicted in Fig. 4(a) and (b), respectively. The experimentally observed powerlaw dependence of DPL/PL on the laser power revealed an exponent of 0.56 for the resonance of the bulk PPV2 sample and 0.27 for the n-PPV sample. The PL was observed to be linear with the
laser power (2±50 mW), for bulk PPV and sublinear for n-PPV. No signi®cant linewidth broadening of the PLDMR signal was observed within the experimental range of the laser power. In conclusion, the blue shift of the PL spectrum and the increase of gPL in the n-PPV sample is assigned to the insulation of the polymer chains in the nano tubes. The larger polaron PLDMR linewidth in the bulk PPV compared to n-PPV is attributed to the increase of interchain polaron pair formation over intrachain-interconjugation-segment polaron pairs. The observation of a strong dierence in the exponent in the power-law dependence of DPL/PL on the laser power observed in PPV2 and n-PPV indicates a drastically dierent recombination mechanism leading the change in the PL in this experiment. Acknowledgements E.J.W.L. gratefully acknowledges the partial ®nancial support of the BMWV for the work at the Ames Lab and of the FWF P. Nr. 12806-PHY for the work in Graz. Ames Lab is operated by the also thank M. Graupner and A. Hermetter. References
Fig. 4. Laser power dependence of (b) PPV1 and (d) n-PPV recorded at the maximum of the polaron pair resonance at g 2 at 20 K with a microwave power of 810 mW and a laser power from 2 to 45 mW.
[1] J. Cornil, A.J. Heeger, J.L. Bredas, Chem Phys. Lett. 272 (1997) 463. [2] A.H. Heuer et al., Science 255 (1992) 1098. [3] R.C. Smith, W.M. Fischer, D.L. Gin, J. Am. Chem. Soc. 119 (1997) 4092. [4] E.L. Frankevich, G.E. Zoriniants, A.N. Chaban, M.M. Triebel, S. Blumstengel, V.M. Kobryanskii, Chem. Phys. Lett. 261 (1996) 545. [5] V. Dyakonov, E.L. Frankevich, Chem. Phys. Lett. 227 (1998) 203. [6] J. Shinar, Synth. Met. 78 (1996) 277. [7] W. Graupner, J. Partee, J. Shinar, G. Leising, U. Scherf, Phys. Rev. Lett. 77 (1996) 2033. [8] Y.S. Liu, P. de Marco, W.R. Ware, J. Phys. Chem. 97 (1993) 5995. [9] W. Graupner, M. Sacher, M. Graupner, C. Zenz, G. Grampp, A. Hermetter, G. Leising, Mat. Res. Soc. Symp. Proc. 488 (1998) 789.