- Email: [email protected]
Photoluminescence quenching in a polymer thin film field effect luministor
Synthetic Metals, 55-57 (1993) 4139-4144
4139
PHOTOLUMINESCENCE OUENGHING IN A POLYMER THIN FILM FIELD EFFECT LUMINISTOR P. DYREKLEV and O. INGAN,~.S Laboratory of Applied Physics, Department of Physics (IFM), Link0ping University, S-581 83 Link6ping, Sweden J. PALOHEIMO and H. STUBB Semiconductor Laboratory, Technical Research Centre of Finland Otakaari 7B, SF-02150 Espoo, Finland
ABSTRACT The interaction between photoexcited excitons and field induced charges in poly(3-hexylthiophene) has been studied using photoluminescence. Thin films of poly(3-hexylthiophene) and LangmuirBlodgett films of poly(3-hexylthiophene)/Arachidic acid show a quenching of the photoluminescence intensity upon charge injection in a field effect device. The quenching is discussed in terms of polaronsJbipolarons acting as recombination centra for the excitons, giving non-radiative decay. The rate constant for non-radiative recombination of the excitons is shown to be proportional to the number of injected charges. This field effect luministor device shows the coupling between the optical and electronic properties of excitations in conjugated polymers. INTRODUCTION Electronic and optical properties of conjugated polymers have been extensively studied since the discovery of these materials. Among the interesting topics is the coupling between the electronic and optical properties.This question has become more important recently in connection with the discovery of electroluminescence in e.g. poly(paraphenylene vinylene) (PPV) [ 1]. Studies of the electronic states giving rise to electroluminescence as well as photoluminescence have been performed [2]. The interpretation of luminescence is also a question of increasing theoretical significance. McKenzie et al. [3] have suggested a model lbr the vibrational fine structure of the luminescence in polythiophene, based on a Huang-Rhys model of multiphonon emission during the electronic transition from the excited state geometry to a different ground state geometry. A similar fine structure has been shown to exist also in the absorption spectra of the poly(3-alkythiophenes) (P3AT) [4,5]. One approach to an experimental study of the photoluminescence process is via the interaction with injected charge in the polymer. It has been shown [6] that the photoluminescence from polythiophenc is quenched when doping the polymer electrochemically with C104. Another way of generating charges is photo injection. This has been done in partially converted precursor-route PPV [7]. Light induced quenching of the photoluminescence is observed at high illumination intensitities and low temperatures. In both these cases it is argued that the suppression of PL is due to an increase of non0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
4140
radiative transitions close to a bipolaron/polaron state. The local decrease of the bandgap close to a polaron/bipolaron acts as a trap for excitons, falling down into non-radiative recombination. Using poymer electronic devices we get the opportunity to inject charges into the polymer without simultaneously injecting dopants (as in the electrochemical doping) and without injecting both positive and negative charges in balance (as in photoinjection). Photoluminescence quenching in a metal-insulator-semiconductorstructure based on P3AT has been reported recently [8,9]. In our studies we use a field effect transistor with thin films of poly(3-hexylthiophene) to achieve the charge injection. The polymer films are in the form of spin coated or Langmuir Blodgett (LB) films, in the latter case in a mixture with arachidic acid. This device gives up to 50 % quenching of the photoluminescence upon charge injection in the thinnest LB films. We call our device a polymer field effect luministor. EXPERIMENTAL The field effect device is made on a n-doped silicon wafer acting as the gate electrode. The substrate is covered with a thermally grown oxide layer with the thickness 300 nm. The source and drain electrodes are made of gold (thickness 20 nm) undercoated with chromium (thickness I0 nm) vacuum evaporated on the silicon dioxide. The electrodes are finger shaped forming a conduction channel with the length 10 I.tm and width 8 cm. A more complete description of the FET can be found in reference 9 and references therein. Thin films of P3HT and P3HT/AA (60%/40%) were made on the FET structure by spincoating from a chloroform solution, and as Langmuir-Blodgett films, respectively. The thickness of the spincoated films is 70 nm and for the LB-films the thickness is about 3 nm per layer. LB films were made with 3 and 7 layers. The polymer was photoexcited by an Ar+-Laser at the energy 2.54 eV (488 nm). Typically, the intensity was 0.1 mW focused to a spot approximately the size of the electrodes (2 ram2). The laser intensity was chosen to get the best signal to noise ratio but still avoiding effects of high laser intensity. Higher intensities give a decay of the photoluminescence intensity with exposure time. The sample was mounted at about 45 ° to the excitation beam and the emission was collected at 90 ° to the excitation beam by a lens and focused onto the entrance slit of the monochromator. The luminescence was detected using a monochromator, photomultiplier and a lock-in amplifier triggered by a mechanical chopper. The drain and source electrodes were connected together. For the gate voltage both a DC voltage source and a square wave pulse generator were used. When using the pulse generator, this was triggering the lock-in amplifier. All measurements were made at room-temperature in laboratory atmosphere. RESULTS The electrical properties of the transistors were studied by measuring the drain-source currentvoltage (I-V) characteristics. From the I-V-curves we can get the field effect mobility and conductivity. These then give us an estimation of the charge carrier concentration which, however, may underestimate the real active dopant concentration. We calculate mobilities and also notice that all transistors are normally on, both observations are consistent with earlier results [10]. The carder concentrations are (at least) 3-1017 , 8.1017 and 1.1018 cm -3 in spincoated, 7-layer LB and 3-layer LB films, respectively. One can calculate that gate voltages of the order of some ten volts may
4141
change the average carrier concentration by the same amounts. This means that both accumulation and depletion of positive charges can be achieved in these transistors. Luminescence spectra were recorded for the different types of films on the FET-structure. Figure 1 shows the spectrum (I/Imax) for the spin coated film together with the spectra for the LB-films. The emission at 1.91 eV (650 nm) was then recorded with a gate voltage of-100 V switched on and off in equal time intervals. With the gate voltage of-100 V applied, the relative change of the luminescence intensity was =0.04. The quenching was also observed for -75V and -50V, but at -25V no clear decrease in intensity could be seen. \
,'--, \x,/
", \ ,
o.s
/
Figure 1. Photoluminescence spectra from spin coated (solid line), 7 layer LB- (dashed line) and 3 layer LB-films (dotted line).
/
\
~//! / / ./ 0
1.5
2.5
2.0 Energy (eV)
Figure 2. Photoluminescence intensity from a 7 layer LB-film (solid line) and a 3 layer LB-film (dashed line) recorded at 1.9 eV. The gate voltage (VG=-100V) switched on and off in equal time intervals (030s:VG=0V, 30-60s: VG=-100V, and so on). 0
100
200 Time (s)
For the LB-films the same type of measurements were made. Figure 1 shows the photoluminescence spectrum (I/Imax) from a 7 layer LB-film on the FET-structure. The maximum intensity is located at approximately 1.9 eV. As before the monochromator was set to 1.91 eV and the luminescence with and without gate voltage was detected. Figure 2 shows that the quenching from a gate voltage of -100V now is about 25% (solid line). This high value shows the effect of reducing the f'dm thickness for the thin LB films. A square wave gate voltage of -30 V and 50 Hz was also used for the 7 layer film. The relative change in PL-intensity with the AC gate voltage at 1.91eV is =-0.04. Compared to the luminescence decrease from a gate voltage of-25 V DC, approximately -0.16, the relative change is smaller from the square wave gate voltage. The luminescence spectrum from a 3 layer LB-film (I/Imax) is shown in figure 1. The maximum intensity is now at 2.15 eV. We can see that the position of maximum intensity in the PL-spectrum is shifted to higher energies for thinner films and that the absolute intensity is smaller for thinner films. Figure 2 (dashed line) shows that the relative luminescence quenching from a gate voltage of -100 V at 1.91 eV was approximately the same as for
4142
the 7 layer film. Some of the above measurements were also made for positive gate voltages. Here an increase in the luminescence intensity could be observed. DISCUSSION The above results clearb; show that the photoluminescence from P3HT can be quenched by the injection of positively charged polaronic states in the polymer. It is also possible to enhance the luminescence intensity by depleting the positive charges originating from an unintentionaldoping. As the area of the transistor consists of equal parts of gold contacts and silicon dioxide it can be assumed that only half the polymer area will be affected by the electric field. This means that the quenching is twice what we have measured. In the case of LB-films the quenching will be approximately 50%. The recombination of the polaron-exciton can follow two main paths, radiative or non-radiative, indicated by a quantum efficiency less than 100% [ 10]. The emitted intensity will be controlled by the lifetime of the excitons; the lifetime will be a combination of the intrinsic lifetime in the pure material and the lifetime determined by the interaction with other charges and impurities. This interaction will depend on the rate of encounter between the species. Assuming that the mechanism for luminescence quenching discussed above holds also for the field injected polarons, then the diffusion of excitons and polarons/bipolarons controls the rate of non-radiative recombination. In the simplest model with no spatial resolution we calculate the number of polaron-excitons from the balance of generated and decayed excitons. When npe,ss is the steady state polaron-exciton density and ¢x(~,)I0the number of absorbed photons per volume, using the rate constants k0 and kl for radiative and non-radiative decay, respectively, we get an expression for the steady state photoluminescence, PL
PL = npe,sskO-
a(k)Ioko ko+kl
Assuming the rate constant for the nonradiative decay to be proportional to the polaron/bipolaron concentration, which can be regarded as proportional to the gate voltage V, we can express the relative PL-intensity PL(V=0) CV PL(V) - 1 + k0+kl(0) Where we have defined a constant C, relating kl to V through the properties of the MOS-capacitance and the polaron exciton interaction. The rate constant kl (0) is determined by the non-radiative decay routes independent of injected charges. This expression is similar to the Stern-Volmer equation describing diffusion controlled luminescence quenching in solutions of organic molecules[ 11]. PL(0)/PL(V) as a function of gate voltage is plotted in figure 3 for a 7 layer LB FET. It shows good agreement with the expression above.
4143 1.6.
14 PL(O)IPL(V) 1.2
1.0-
0.8-
0.6
-200
,
,
f
-100
0
100
200
G a t e Voltage (V)
Figure 3. The ratio betwen zero gate voltage photoluminescence intensity and photoluminescence intensity at various gate voltages plotted varsus gate voltage for the 7 layer LB-film. The photoluminescence is recorded at 1.9 eV.
The quenching from a gate voltage of -30 V/50 Hz is smaller than from -25 V DC. That indicates that a time longer than 10 ms is needed to inject all charges into the polymer film. The time response of the PL signal to a gate voltage will be a function of the diffusion of (bi)polaron species. Assuming that positive charge is injected into the polymer sample from the ohmic gold contacts on applying a negative gate voltage, we can estimate the traversal time of these charges from the dimension and mobility of carriers. In the absence of an electric field (drain and source are shorted in the experiments), we estimate the diffusion time of the injected charges from the contact halfway through the structure (L/2) by tt
tt-
eL 2 4kTIl
(8)
For values of L=10 I.tm and I-t= 10-4 cm2/Vs 15 we find the time tt to be 0.1 s. This is consistent with the fact that we observe a lower amplitude of photoluminescence quenching when applying a 50 Hz gate voltage, than at steady state conditions. The decaying of the change in the luminescence after switching gate gate voltage resembles the decay in the changes in the drain current as a function of time, seen earlier in spin-coated and LB transistors [12]. The reason for this may be related to time constants needed to obtain a thermodynamical equilibrium after injection. It could also be related to field-induced dopant diffusion [ 13] or trapping phenomena at the polymer-oxide interface. The spectral appearance of the luminescence differs between the different types of polymer films and different samples. The maximum intensity shifts from 1.95 eV to 2.14 eV going from the spincoated film to the thinnest LB-film (3 layer). We may interpret this as a consequence of the large variations in conformation of the polymer chain possible in a LB-film. Recent X-ray diffraction studies[14] reveals that the polymer chain in a LB-film is likely to span several layers of amphiphilic molecules. The shift of intensities between the various peaks in different films should be a consequence of the differing geometries in the ground and excited states in these films [15]. These differences may
4144
include major geometrical variations, from (at the extremes) random coil-like to rod-like forms of the polymer. It may also include effects due to packing and local geometry. CONCLUSIONS We have here shown that the photoluminescence from thin films of P3HT can be quenched by the injection of positive charges. The charges are stored in polarons/bipolarons which are mobile and therefore gives a more efficient quenching than charged polarons/bipolarons originating from doping. The rate constant for the non-radiative decay, responsible for the quenching seems to follow a simple linear dependence of the density of injected polarons/bipolarons. This luministor device creates a new coupling between electronic and optical phenomena in polymer electronics. ACKNOWLEDGEMENTS We thank Hard Lipsanen of the Optoelectronics Laboratory, Helsinki University of Technology for the guidance in the use of their luminescence setup. This work is partly supported by the Swedish Natural Science Research Council. REFERENCES 1. J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Bums and A.B. Holmes, Nature 347. 539 (1990) 2. U. Rauscher, H. BEssler, D.D.C. Bradley and M. Hennecke, Phvs. Rev. B 42, 9830 (1990) 3. R.H. McKenzie and J.W. Wilkins, Svnth. Met. 43. 3615 (1991)4. S.D.D.V. Rughooputh, S. Hotta, A.J. Heeger and F. Wudl, J. Polym. Sci. Polvmer Phvs.. 25, 10 (1987) 5. O. Ingan/is, W.R. Salaneck, J.E. 0sterholm and J. Laakso, Svnth. Met. 22. 395 (1988) 6. S. Hayashi, K. Kaneto and K. Yoshino, Solid State Comm. i51. 249 (1987) 7. D.D.C. Bradley and R.H. Friend, J.Phvs.: Condens. Matter 1. 3671 (1989) 8. K.E. Ziemelis, A.T. Hussain, D.D.C. Bradley, R.H. Friend, J. Riihe and G. Wegner, Phvs. Rev. Lett.. 66. 2231 (1991) 9. P.Dyreklev, O.Ingan~, J.Paloheimo and H.Stubb, a. in "Electronic Properties of Polymers", Proceedings of IWEPP 91, Edited by H.Kuzmany, M.Mehring and S.Roth (Springer-Verlag, Heidelberg, 1992) b.J. ADol. Phvs. 71. 2816 (1992) 10. J. Paloheimo~ E. Punkka, H. Stubb and P. Kuivalainen, in the Proceedings of NATO ASI, "Lower Dimensional Systems and Molecular Devices",Spetses, Greece, 12-23 June 1989. Editor: R.M. Metzger. Plenum Press 11. R.H. Friend, D.D.C. Bradley and P.D. Townsend, J. Phvs. D: Anol. Phvs. 20. 1367 (1987) 12. "Luminescence Spectroscopy" Edited by M.D. Lumb, Academic Press, l_~ndon (1978) 13. J. Paloheimo, H. Stubb, P. Yli-Lahti and P. Kuivalainen, Svnth. Met. 41. 563 (1991) 14. O. Ingan~, G. Gustafsson and C. Svensson, Svnth. Met. 41. 1095 (1991) 15. M. F. Rubner and T. A. Skotheim, in "Conjuga/ed Polymers: The Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials", edited by J. L. Br6das and R. Silbey (Kluwer, Dordrecht, 1991) 16. M. Sundberg, O. Ingan~is, S. Stafstr/Sm, G. Gustafsson and B. Sj6gren, Solid State Comm. 71. 435 (1989)
4139
PHOTOLUMINESCENCE OUENGHING IN A POLYMER THIN FILM FIELD EFFECT LUMINISTOR P. DYREKLEV and O. INGAN,~.S Laboratory of Applied Physics, Department of Physics (IFM), Link0ping University, S-581 83 Link6ping, Sweden J. PALOHEIMO and H. STUBB Semiconductor Laboratory, Technical Research Centre of Finland Otakaari 7B, SF-02150 Espoo, Finland
ABSTRACT The interaction between photoexcited excitons and field induced charges in poly(3-hexylthiophene) has been studied using photoluminescence. Thin films of poly(3-hexylthiophene) and LangmuirBlodgett films of poly(3-hexylthiophene)/Arachidic acid show a quenching of the photoluminescence intensity upon charge injection in a field effect device. The quenching is discussed in terms of polaronsJbipolarons acting as recombination centra for the excitons, giving non-radiative decay. The rate constant for non-radiative recombination of the excitons is shown to be proportional to the number of injected charges. This field effect luministor device shows the coupling between the optical and electronic properties of excitations in conjugated polymers. INTRODUCTION Electronic and optical properties of conjugated polymers have been extensively studied since the discovery of these materials. Among the interesting topics is the coupling between the electronic and optical properties.This question has become more important recently in connection with the discovery of electroluminescence in e.g. poly(paraphenylene vinylene) (PPV) [ 1]. Studies of the electronic states giving rise to electroluminescence as well as photoluminescence have been performed [2]. The interpretation of luminescence is also a question of increasing theoretical significance. McKenzie et al. [3] have suggested a model lbr the vibrational fine structure of the luminescence in polythiophene, based on a Huang-Rhys model of multiphonon emission during the electronic transition from the excited state geometry to a different ground state geometry. A similar fine structure has been shown to exist also in the absorption spectra of the poly(3-alkythiophenes) (P3AT) [4,5]. One approach to an experimental study of the photoluminescence process is via the interaction with injected charge in the polymer. It has been shown [6] that the photoluminescence from polythiophenc is quenched when doping the polymer electrochemically with C104. Another way of generating charges is photo injection. This has been done in partially converted precursor-route PPV [7]. Light induced quenching of the photoluminescence is observed at high illumination intensitities and low temperatures. In both these cases it is argued that the suppression of PL is due to an increase of non0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
4140
radiative transitions close to a bipolaron/polaron state. The local decrease of the bandgap close to a polaron/bipolaron acts as a trap for excitons, falling down into non-radiative recombination. Using poymer electronic devices we get the opportunity to inject charges into the polymer without simultaneously injecting dopants (as in the electrochemical doping) and without injecting both positive and negative charges in balance (as in photoinjection). Photoluminescence quenching in a metal-insulator-semiconductorstructure based on P3AT has been reported recently [8,9]. In our studies we use a field effect transistor with thin films of poly(3-hexylthiophene) to achieve the charge injection. The polymer films are in the form of spin coated or Langmuir Blodgett (LB) films, in the latter case in a mixture with arachidic acid. This device gives up to 50 % quenching of the photoluminescence upon charge injection in the thinnest LB films. We call our device a polymer field effect luministor. EXPERIMENTAL The field effect device is made on a n-doped silicon wafer acting as the gate electrode. The substrate is covered with a thermally grown oxide layer with the thickness 300 nm. The source and drain electrodes are made of gold (thickness 20 nm) undercoated with chromium (thickness I0 nm) vacuum evaporated on the silicon dioxide. The electrodes are finger shaped forming a conduction channel with the length 10 I.tm and width 8 cm. A more complete description of the FET can be found in reference 9 and references therein. Thin films of P3HT and P3HT/AA (60%/40%) were made on the FET structure by spincoating from a chloroform solution, and as Langmuir-Blodgett films, respectively. The thickness of the spincoated films is 70 nm and for the LB-films the thickness is about 3 nm per layer. LB films were made with 3 and 7 layers. The polymer was photoexcited by an Ar+-Laser at the energy 2.54 eV (488 nm). Typically, the intensity was 0.1 mW focused to a spot approximately the size of the electrodes (2 ram2). The laser intensity was chosen to get the best signal to noise ratio but still avoiding effects of high laser intensity. Higher intensities give a decay of the photoluminescence intensity with exposure time. The sample was mounted at about 45 ° to the excitation beam and the emission was collected at 90 ° to the excitation beam by a lens and focused onto the entrance slit of the monochromator. The luminescence was detected using a monochromator, photomultiplier and a lock-in amplifier triggered by a mechanical chopper. The drain and source electrodes were connected together. For the gate voltage both a DC voltage source and a square wave pulse generator were used. When using the pulse generator, this was triggering the lock-in amplifier. All measurements were made at room-temperature in laboratory atmosphere. RESULTS The electrical properties of the transistors were studied by measuring the drain-source currentvoltage (I-V) characteristics. From the I-V-curves we can get the field effect mobility and conductivity. These then give us an estimation of the charge carrier concentration which, however, may underestimate the real active dopant concentration. We calculate mobilities and also notice that all transistors are normally on, both observations are consistent with earlier results [10]. The carder concentrations are (at least) 3-1017 , 8.1017 and 1.1018 cm -3 in spincoated, 7-layer LB and 3-layer LB films, respectively. One can calculate that gate voltages of the order of some ten volts may
4141
change the average carrier concentration by the same amounts. This means that both accumulation and depletion of positive charges can be achieved in these transistors. Luminescence spectra were recorded for the different types of films on the FET-structure. Figure 1 shows the spectrum (I/Imax) for the spin coated film together with the spectra for the LB-films. The emission at 1.91 eV (650 nm) was then recorded with a gate voltage of-100 V switched on and off in equal time intervals. With the gate voltage of-100 V applied, the relative change of the luminescence intensity was =0.04. The quenching was also observed for -75V and -50V, but at -25V no clear decrease in intensity could be seen. \
,'--, \x,/
", \ ,
o.s
/
Figure 1. Photoluminescence spectra from spin coated (solid line), 7 layer LB- (dashed line) and 3 layer LB-films (dotted line).
/
\
~//! / / ./ 0
1.5
2.5
2.0 Energy (eV)
Figure 2. Photoluminescence intensity from a 7 layer LB-film (solid line) and a 3 layer LB-film (dashed line) recorded at 1.9 eV. The gate voltage (VG=-100V) switched on and off in equal time intervals (030s:VG=0V, 30-60s: VG=-100V, and so on). 0
100
200 Time (s)
For the LB-films the same type of measurements were made. Figure 1 shows the photoluminescence spectrum (I/Imax) from a 7 layer LB-film on the FET-structure. The maximum intensity is located at approximately 1.9 eV. As before the monochromator was set to 1.91 eV and the luminescence with and without gate voltage was detected. Figure 2 shows that the quenching from a gate voltage of -100V now is about 25% (solid line). This high value shows the effect of reducing the f'dm thickness for the thin LB films. A square wave gate voltage of -30 V and 50 Hz was also used for the 7 layer film. The relative change in PL-intensity with the AC gate voltage at 1.91eV is =-0.04. Compared to the luminescence decrease from a gate voltage of-25 V DC, approximately -0.16, the relative change is smaller from the square wave gate voltage. The luminescence spectrum from a 3 layer LB-film (I/Imax) is shown in figure 1. The maximum intensity is now at 2.15 eV. We can see that the position of maximum intensity in the PL-spectrum is shifted to higher energies for thinner films and that the absolute intensity is smaller for thinner films. Figure 2 (dashed line) shows that the relative luminescence quenching from a gate voltage of -100 V at 1.91 eV was approximately the same as for
4142
the 7 layer film. Some of the above measurements were also made for positive gate voltages. Here an increase in the luminescence intensity could be observed. DISCUSSION The above results clearb; show that the photoluminescence from P3HT can be quenched by the injection of positively charged polaronic states in the polymer. It is also possible to enhance the luminescence intensity by depleting the positive charges originating from an unintentionaldoping. As the area of the transistor consists of equal parts of gold contacts and silicon dioxide it can be assumed that only half the polymer area will be affected by the electric field. This means that the quenching is twice what we have measured. In the case of LB-films the quenching will be approximately 50%. The recombination of the polaron-exciton can follow two main paths, radiative or non-radiative, indicated by a quantum efficiency less than 100% [ 10]. The emitted intensity will be controlled by the lifetime of the excitons; the lifetime will be a combination of the intrinsic lifetime in the pure material and the lifetime determined by the interaction with other charges and impurities. This interaction will depend on the rate of encounter between the species. Assuming that the mechanism for luminescence quenching discussed above holds also for the field injected polarons, then the diffusion of excitons and polarons/bipolarons controls the rate of non-radiative recombination. In the simplest model with no spatial resolution we calculate the number of polaron-excitons from the balance of generated and decayed excitons. When npe,ss is the steady state polaron-exciton density and ¢x(~,)I0the number of absorbed photons per volume, using the rate constants k0 and kl for radiative and non-radiative decay, respectively, we get an expression for the steady state photoluminescence, PL
PL = npe,sskO-
a(k)Ioko ko+kl
Assuming the rate constant for the nonradiative decay to be proportional to the polaron/bipolaron concentration, which can be regarded as proportional to the gate voltage V, we can express the relative PL-intensity PL(V=0) CV PL(V) - 1 + k0+kl(0) Where we have defined a constant C, relating kl to V through the properties of the MOS-capacitance and the polaron exciton interaction. The rate constant kl (0) is determined by the non-radiative decay routes independent of injected charges. This expression is similar to the Stern-Volmer equation describing diffusion controlled luminescence quenching in solutions of organic molecules[ 11]. PL(0)/PL(V) as a function of gate voltage is plotted in figure 3 for a 7 layer LB FET. It shows good agreement with the expression above.
4143 1.6.
14 PL(O)IPL(V) 1.2
1.0-
0.8-
0.6
-200
,
,
f
-100
0
100
200
G a t e Voltage (V)
Figure 3. The ratio betwen zero gate voltage photoluminescence intensity and photoluminescence intensity at various gate voltages plotted varsus gate voltage for the 7 layer LB-film. The photoluminescence is recorded at 1.9 eV.
The quenching from a gate voltage of -30 V/50 Hz is smaller than from -25 V DC. That indicates that a time longer than 10 ms is needed to inject all charges into the polymer film. The time response of the PL signal to a gate voltage will be a function of the diffusion of (bi)polaron species. Assuming that positive charge is injected into the polymer sample from the ohmic gold contacts on applying a negative gate voltage, we can estimate the traversal time of these charges from the dimension and mobility of carriers. In the absence of an electric field (drain and source are shorted in the experiments), we estimate the diffusion time of the injected charges from the contact halfway through the structure (L/2) by tt
tt-
eL 2 4kTIl
(8)
For values of L=10 I.tm and I-t= 10-4 cm2/Vs 15 we find the time tt to be 0.1 s. This is consistent with the fact that we observe a lower amplitude of photoluminescence quenching when applying a 50 Hz gate voltage, than at steady state conditions. The decaying of the change in the luminescence after switching gate gate voltage resembles the decay in the changes in the drain current as a function of time, seen earlier in spin-coated and LB transistors [12]. The reason for this may be related to time constants needed to obtain a thermodynamical equilibrium after injection. It could also be related to field-induced dopant diffusion [ 13] or trapping phenomena at the polymer-oxide interface. The spectral appearance of the luminescence differs between the different types of polymer films and different samples. The maximum intensity shifts from 1.95 eV to 2.14 eV going from the spincoated film to the thinnest LB-film (3 layer). We may interpret this as a consequence of the large variations in conformation of the polymer chain possible in a LB-film. Recent X-ray diffraction studies[14] reveals that the polymer chain in a LB-film is likely to span several layers of amphiphilic molecules. The shift of intensities between the various peaks in different films should be a consequence of the differing geometries in the ground and excited states in these films [15]. These differences may
4144
include major geometrical variations, from (at the extremes) random coil-like to rod-like forms of the polymer. It may also include effects due to packing and local geometry. CONCLUSIONS We have here shown that the photoluminescence from thin films of P3HT can be quenched by the injection of positive charges. The charges are stored in polarons/bipolarons which are mobile and therefore gives a more efficient quenching than charged polarons/bipolarons originating from doping. The rate constant for the non-radiative decay, responsible for the quenching seems to follow a simple linear dependence of the density of injected polarons/bipolarons. This luministor device creates a new coupling between electronic and optical phenomena in polymer electronics. ACKNOWLEDGEMENTS We thank Hard Lipsanen of the Optoelectronics Laboratory, Helsinki University of Technology for the guidance in the use of their luminescence setup. This work is partly supported by the Swedish Natural Science Research Council. REFERENCES 1. J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Bums and A.B. Holmes, Nature 347. 539 (1990) 2. U. Rauscher, H. BEssler, D.D.C. Bradley and M. Hennecke, Phvs. Rev. B 42, 9830 (1990) 3. R.H. McKenzie and J.W. Wilkins, Svnth. Met. 43. 3615 (1991)4. S.D.D.V. Rughooputh, S. Hotta, A.J. Heeger and F. Wudl, J. Polym. Sci. Polvmer Phvs.. 25, 10 (1987) 5. O. Ingan/is, W.R. Salaneck, J.E. 0sterholm and J. Laakso, Svnth. Met. 22. 395 (1988) 6. S. Hayashi, K. Kaneto and K. Yoshino, Solid State Comm. i51. 249 (1987) 7. D.D.C. Bradley and R.H. Friend, J.Phvs.: Condens. Matter 1. 3671 (1989) 8. K.E. Ziemelis, A.T. Hussain, D.D.C. Bradley, R.H. Friend, J. Riihe and G. Wegner, Phvs. Rev. Lett.. 66. 2231 (1991) 9. P.Dyreklev, O.Ingan~, J.Paloheimo and H.Stubb, a. in "Electronic Properties of Polymers", Proceedings of IWEPP 91, Edited by H.Kuzmany, M.Mehring and S.Roth (Springer-Verlag, Heidelberg, 1992) b.J. ADol. Phvs. 71. 2816 (1992) 10. J. Paloheimo~ E. Punkka, H. Stubb and P. Kuivalainen, in the Proceedings of NATO ASI, "Lower Dimensional Systems and Molecular Devices",Spetses, Greece, 12-23 June 1989. Editor: R.M. Metzger. Plenum Press 11. R.H. Friend, D.D.C. Bradley and P.D. Townsend, J. Phvs. D: Anol. Phvs. 20. 1367 (1987) 12. "Luminescence Spectroscopy" Edited by M.D. Lumb, Academic Press, l_~ndon (1978) 13. J. Paloheimo, H. Stubb, P. Yli-Lahti and P. Kuivalainen, Svnth. Met. 41. 563 (1991) 14. O. Ingan~, G. Gustafsson and C. Svensson, Svnth. Met. 41. 1095 (1991) 15. M. F. Rubner and T. A. Skotheim, in "Conjuga/ed Polymers: The Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials", edited by J. L. Br6das and R. Silbey (Kluwer, Dordrecht, 1991) 16. M. Sundberg, O. Ingan~is, S. Stafstr/Sm, G. Gustafsson and B. Sj6gren, Solid State Comm. 71. 435 (1989)