- Email: [email protected]
Depth profiling of the degradation of OC1OC10-PPV monitored by positron beam analysis
Synthetic Metals 138 (2003) 43–47
Depth profiling of the degradation of OC1OC10-PPV monitored by positron beam analysis A. Alba Garcı´a, H. Schut*, L.D.A. Siebbeles, A. van Veen Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629JB Delft, The Netherlands
Abstract The Doppler broadening of annihilation radiation (DBAR) technique and UV-Vis photo-absorption spectroscopy were used to monitor the photo-oxidation of thin films (200 nm) of OC1OC10-PPV. Samples were exposed in air to a 308 nm excimer laser and a commercial UV-Vis lamp. By means of the DBAR technique, the degradation was monitored as a function of depth when illuminating from the polymer side or the substrate side. It showed that the degradation is more pronounced on the side where the light affects, indicating that the photo-oxidation is not limited by oxygen diffusion. DBAR results were compared to photo-absorption measurements showing that in the first stages of the photooxidation positrons were more sensitive to changes in the polymer film. The non-destructive character of the DBAR technique and the possibility to perform depth profiling makes it a promising technique to study multi-layer devices. # 2003 Elsevier Science B.V. All rights reserved. Keywords: PPV; Polymer; Degradation; Positron
1. Introduction Electroluminescent polymers, and in particular poly-paraphenylene-vinylene (PPV) and its derivatives, have shown great promise as the active layer in light-emitting diodes, as they are relatively efficient, easy to manipulate, and produce light across the full visible spectrum. Nevertheless, lightemitting diodes based on these materials are known to gradually decrease their performance when operated under certain environmental conditions [1]. Several groups have shown that exposure to air and light results in the photooxidation of the vinyl bond of the PPV and the formation of carbonyl groups [2,3] which quench the luminescence properties of the polymer. The photo-oxidation of PPV has been monitored by techniques such as UV-Vis photo-absorption spectroscopy, photoluminescence spectroscopy, infrared (IR) photo-absorption spectroscopy and Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy. Nevertheless, IR spectroscopy is only sensitive to the surface and the other mentioned techniques yield information integrated over the total thickness of the polymer. Only a few techniques with depth resolution, such as
* Corresponding author. Tel.: þ31-15-2781961; fax: þ31-15-2786422. E-mail address: [email protected] (H. Schut).
elastic recoil detection analysis (ERDA), have been applied to this problem. Positron Annihilation Spectroscopy (PAS) using 22 Na as fast positron source has shown to be a very useful technique to study different properties of polymers like free volume [4,5], corrosion [6] or degradation upon different types of radiation [7,8]. With the development of tuneable low energy positron beams, PAS has been successfully applied to study properties of (thin) multi-layered systems such as degradation and ageing of coatings [9] and adhesion [10]. In this work, we show that PAS can also be used to monitor the degradation of OC1OC10-PPV as a function of depth after exposure to UV light under atmospheric conditions.
2. Experimental 2.1. Sample preparation and degradation Thin films of OC1OC10-PPV were prepared by spin-coating from a 2 g/l chloroform solution of the polymer on a quartz substrate under atmospheric conditions. The film thickness was estimated from stepping profiling measurements to be between 150 and 200 nm. The films were exposed for periods up to 15 h to either a commercial UV-Vis lamp (Philips PL-S 9W/10) with four
0379-6779/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01263-8
44
A. Alba Garcı´a et al. / Synthetic Metals 138 (2003) 43–47
main lines at 549, 439, 407 and 371 nm, or a 7 ns pulsed excimer laser (Lumonics HE-420) with a power of about 10 mJ/cm2 at 308 nm. The samples were illuminated from the polymer side (front illumination) or from the substrate side (back illumination). 2.2. Monitoring the photo-oxidation The active polymer layer was characterised at different stages of degradation by UV-Vis photo-absorption spectroscopy and the Doppler Broadening of the Annihilation Radiation (DBAR) technique. UV-Vis photo-absorption spectra were recorded using a Uvikron 940 spectrophotometer. DBAR spectra were recorded using the slow positron beam available at the Delft University of Technology. Positrons are implanted into the material with an energy in the range of 0.1–15 keV. The intensity of the beam is about 105 positrons/s and the pressure inside the measurement chamber is 106 Pa. An implanted positron slows down by successive collisions until it reaches thermal energy. Having thermalised, the positron starts to diffuse and because of its positive charge it is repelled by the atom cores. This makes the positron a very sensitive probe to characterise regions with lower local densities. Ultimately it will annihilate with an electron in its vicinity thereby emitting two g quanta in opposite directions each with an energy around 511 keV. In amorphous polymers, a positron may combine with an electron to form positronium (Ps), a hydrogen-like electron–positron bound state. Ps exhibits two spin states which are called para (p-Ps) and ortho (o-Ps) for the singlet and the triplet state, respectively. The vacuum lifetime of p-Ps is about 125 ps and that of o-Ps is 140 ns. However, in amorphous polymers the longer o-Ps lifetime is reduced to typically 1–4 ns due to the annihilation of the positron of the o-Ps with one of the surrounding electrons. This process is known as pick-off annihilation and is exploited in the socalled positron lifetime experiments to obtain information on the size of the open volume in which the Ps is formed. In a typical DBAR experiment, one of the two annihilation g quanta is detected and its energy is measured using a high resolution Ge detector. Due to the momentum of the electron–positron pair at the moment of annihilation, the energy of the detected g quanta is Doppler shifted. Neglecting the positron contribution to the total momentum, the detected g quanta carry the information about the momentum distribution of the annihilating electrons. After accumulating typically 106 annihilation events, the final width of Doppler broadened 511 keV photo-peak is characterised by the S parameter. This parameter is defined as the ratio of annihilation events with low momentum electrons to the total events in the photo-peak. Besides low momentum events, the S parameter is also sensitive to p-Ps decay as the intrinsic momentum of this particle is very low. The S parameter is known to drop when positrons are trapped in the vicinity of elements with deeper core electrons, such as oxygen or fluorine [12].
The Doppler broadening spectra were recorded for selected positron implantation energies (E) after different exposure times. The DBAR results were modelled using the program VEPFIT [13]. It simulates the depth dependent implantation, diffusion and trapping of positrons by solving the time averaged diffusion equation. In VEPFIT, the positron implantation profile P(z,E) is described by the derivative of a Gaussian: " # 2z z 2 Pðz; EÞ ¼ 2 exp z0 z0 with the mean depth hzi of positrons implanted with energy E given by [13] A 3 hzi ¼ E1:6 ¼ z0 G r 2 In this expression r is the density and A is a constant. This relation, adapted to a layered structure with different densities, as is the case here, is used to estimate the energy at which half of the positrons annihilate in the PPV layer and thus will locate also the position of the PPV–quartz interface. This energy (at about 4 keV) is shown in Fig. 2a and b by the dashed line.
3. Results Fig. 1 shows the absorption spectrum (optical density OD ¼ log (I/I0) versus photon energy) of OC1OC10-PPV after different exposure times to the UV-Vis lamp. Similar results were obtained when exposed to the laser. The spectrum shows a progressive bleaching and a blue shift of the broad peak at 2.5 eV, assigned to the p–p transition of the conjugated polymer [11]. This shift is attributed to the loss of conjugation upon photo-oxidation. No effect was observed when the samples were exposed to light or to oxygen only, for periods up to at least 15 h.
Fig. 1. Absorption spectrum of PPV after front-side illumination to the UV-Vis lamp for the times indicated.
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Fig. 2. S parameter of PPV on a quartz substrate as a function of the positron implantation energy after UV-Vis lamp illumination for the times indicated. (a, Left panel) front-side illumination and (b, right panel) back-side illumination. The dashed line at about 4 keV indicates the positron energy at which half of the implanted positrons annihilate in the PPV layer and thus locates the PPV–quartz interface. The drawn lines are obtained by the VEPFIT fitting program.
Fig. 2a shows the measured S parameter (the contribution of low momentum electrons) as a function of the positron energy (or depth) for different exposure times to the UV lamp (front illumination). Looking at the as prepared spectrum, we can distinguish four regions depending on the positron energy. With increasing positron implantation energy firstly the surface is probed (positron energy below 0.5 keV), secondly the PPV bulk (energy between 0.5 and 4 keV), thirdly the region where the S parameter drops from the PPV value to the substrate value (energy between 4 and 6 keV) and finally the quartz substrate (energies above 6 keV). Looking at the PPV region, the photo-oxidation of the active polymer layer results in a significant change in the positron spectrum, as shown in Fig. 2a. Firstly, the overall S parameter of the PPV layer decreases gradually with the increasing exposure time from a value of 0.62–0.53. Secondly, the change in S is not homogeneous over the PPV layer but more pronounced in regions closer to the surface. Similar results were obtained when laser light was used. Furthermore, the S parameter did not change after 12 h of exposure to light or air only. Two factors can be responsible for the faster decrease of S observed at regions closer to the surface. One of them is the high concentration of oxygen at the surface during exposure, which results in a more effective photo-oxidation of the PPV. The second one is the light absorption profile given by the Lambert–Beer law, which is at maximum at the surface and therefore enhances the photo-oxidation in this area. In order to discriminate between these two possibilities, a sample was exposed to the UV lamp from the backside. In this way, the maximum of the absorption profile is near the PPV– quartz interface while the oxygen has to diffuse from the front side to the PPV–quartz interface. Fig. 2b shows the measured S parameter as a function of the positron energy for different exposure times to the UV lamp after illumination from the back side. In this experiment, again an overall decrease of the S parameter is
Fig. 3. S parameter at 2 keV positron energy as a function of the OD at the maximum of lower energy excitation band at 2.5 eV for different exposure times (front side) Data are shown for two samples illuminated with the UV-Vis lamp or laser, respectively. The lines are drawn to guide the eye.
Fig. 4. Schematic of the layered structure used in VEPFIT to model the PPV layer on a quartz substrate. The PPV layer is divided in to two layers with fitting parameters S1 and S2 of which the values are indicated by the horizontal lies. The dashed lines represent the light absorption profile in the PPV layer.
A. Alba Garcı´a et al. / Synthetic Metals 138 (2003) 43–47
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Table 1 Fitted S parameter values for front and back illumination of PPV layers as a function of illumination time using the UV-Vis lamp Front-side illumination
Back-side illumination Ssurface
S
As spin coated
0.577
0.618
Illumination time (min)
Ssurface
S1
15 90 270 810
0.555 0.547 0.540 0.532
0.605 0.578 0.542 0.533
Ssurface
S
As spin coated
0.577
0.618
S2
Illumination time (min)
Ssurface
S1
S2
0.597 0.591 0.561 0.529
30 90 240 450 870
0.560 0.554 0.546 0.538 0.540
0.601 0.594 0.583 0.565 0.534
0.581 0.568 0.542 0.525 0.532
observed as was obtained when illuminating from the front side. However, the degradation is now more pronounced at the interface PPV–quartz, clearly seen by the faster decrease of S at 3 keV. This shows that the PPV layer degrades from the side where the light affects, indicating that the degradation of PPV is not limited by oxygen diffusion on the time scales used in the experiment. Fig. 3 shows the correlation between the S parameter and the OD for a sample illuminated with the UV-Vis lamp and another sample illuminated with the laser. The S parameter at 2 keV is plotted versus the OD taken at the maximum of the broad absorption band at about 2.5 eV for exposure times up to several hours. In both cases, the relative change in the OD for short exposure times is much smaller than the relative changes observed in the S parameter. These results show that due to the depth selectivity PAS is more sensitive as compared to photo-absorption spectroscopy at least in the first stage of the photo-oxidation process. Fig. 4 shows schematically the light absorption profile of the PPV layer for back and front illumination. Assuming the S parameter inside the PPV follows this profile, we model the PPV region by defining two layers of fixed width (100 nm) each, on top of the substrate, as shown in Fig. 4. Fitting parameters were the S parameter of each PPV layer (S1 and S2) and the positron diffusion lengths (Lþ), defined as the distance travelled by the positron between the thermalisation and the final annihilation. For the substrate, the parameters were fixed to S ¼ 0:578 and Lþ ¼ 60 nm as obtained from a independent reference measurement on bare quartz. The fitting results are shown in Fig. 2a and b by the drawn lines. The values of the fitted S parameters are summarised in Table 1. The as spin-coated sample can be fitted with only one layer, indicating that the S parameter is homogeneous over the whole PPV layer. After exposing the PPV, it can be seen that the S parameter of the layer where the light affects decreases faster. When illuminating from the front (light affects on layer 1), S1 decreases faster than S2. When illuminating from the back side (light affects on layer 2) a similar trend is observed, but now S2 decreases faster than S1. After complete degradation, both layers reach the same S value. It is possible to fit the longest exposed curves with one
PPV layer and the substrate, indicating that the S parameter again becomes homogeneous over the whole layer after complete degradation.
4. Discussion We have shown that the S parameter decreases upon photooxidation. This implies that as the exposure time increases the positron annihilates with electrons of higher momentum. An explanation of this phenomenon requires a detailed understanding of the positron and positronium chemistry in these materials, which is still under investigation. A detailed discussion will be presented in a future work. In addition a decrease in the Ps fraction, and thus the p-Ps fraction also leads to a drop in the S parameter. The p-Ps fraction was derived independently from the Doppler broadening measurements following a procedure, which decomposes the measured photo-peak into 3–4 Gaussian profiles [14,15]. The contribution of the Gaussian with a width comparable to the resolution of the detector is then associated with the annihilation of p-Ps with nearly zero momentum. We have observed a decrease from a 70% Ps (or equivalently 17.5% p-Ps) in the as prepared sample to no Ps formation in the completely degraded sample (see Fig. 5). The factors that cause Ps formation to be completely inhibited are still under investigation. One possibility is the
Fig. 5. The fraction of p-Ps in the PPV layer as a function of the illumination dose using the laser.
A. Alba Garcı´a et al. / Synthetic Metals 138 (2003) 43–47
reduction of the space available to form positronium (free volume). Several studies in other polymers show a decrease in the p-Ps fraction when the free volume is reduced by means of, e.g. temperature reduction, ageing or compression [16,17]. Another possibility is that the mechanism of Ps formation in these materials is not via track processes [18] but via direct interaction with the polymer chain, taking an electron directly from the material to form positronium. In this case, the increase in the ionisation energy as a consequence of the shortening of the chains upon photo-oxidation of the polymer would increase the energy necessary to extract an electron from the chain to form Ps, resulting in a decrease of Ps formation.
5. Conclusion Positron Annihilation Spectroscopy and UV-Vis photoabsorption spectroscopy have been used to monitor the photo-oxidation of OC1OC10-PPV upon exposure to a laser and a UV-Vis lamp. The results, which were independent of the light source used, showed that positrons can monitor the degradation of PPV thin films as a function of depth. Exposure of the samples from the front or the back side showed that the degradation is more pronounced on the side were the light affects. This indicates that the degradation is not limited by oxygen diffusion on the time scale of the experiment. Due to the depth selectivity of the DBAR beam technique it has a higher sensitivity for at least the first stages in the degradation process as compared to the photo-absorption technique. The results show the potential of the DBAR beam technique to study systems with depth resolution and sensitivity to chemical changes in a non-destructive way.
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References [1] T.P. Nguyen, M. Spiesser, A. Garnier, M. De Kok, V.H. Tran, Mater. Sci. Eng. B60 (1999) 76–81. [2] B.H. Cumpston, K.F. Jensen, Synth. Met. 73 (1995) 195–199. [3] D.G.J. Sutherland, J.A. Carlisle, P. Elliker et al., Appl. Phys. Lett. 68 (1996) 2046–2048. [4] D. Hofman, M. Heuchel, Y. Yampolski, V. Khotimskii, V. Shantarovich, Macromolecules 35 (2002) 2129–2140. [5] Z.Q. Chen, T. Suzuki, K. Kondo, A. Uedono, Y. Ito, Jpn. J. Appl. Phys., Part 1: Regul. Pap. Short Notes Rev. Pap. 40 (2001) 5036– 5040. [6] M.M. Mandani, R.R. Miron, R.D. Granata, J. Coat. Technol. 69 (872) (1997) 45–54. [7] G. Shariff, P.M. Sathyanarayana, M.C. Thimmegowda et al., Polymer 43 (9) (2002) 2819–2826. [8] T. Suzuki, Y. Oki, M. Numajiri et al., J. Polym. Sci., Part B: Polym. Phys. 30 (1992) 517–525. [9] H. Cao, R. Zhang, C.S. Sundar et al., Macromolecules 31 (1998) 6627–6635. [10] R. Escobar Galindo, A. van Veen, A. Alba Garcı´a, H. Schut, J. Th de Hosson, Acta Mater. 48 (2000) 4743–4747. [11] J. Cornil, D. Beljone, R.H. Friend, J.L. Bre´ das, Chem. Phys. Lett. 223 (1994) 82–88. [12] G. Dlubek, K. Saarinen, H.M. Fretwell, Nucl. Instrum. Methods B 142 (1998) 139–155. [13] A. van Veen, H. Schut, J. de Vries, R.A. Hakvoort, M.R. IJpma, Positron beams for solids and surfaces, in: P.J. Schultz, G.R. Massoumi, P.J. Simpson (Eds.), Proceedings of the AIP Conference, vol. 218, American Institute of Physics, New York, 1990, pp. 171–196. [14] G. Dlubek, H.M. Fretwell, M.A. Alam, Macromolecules 33 (2000) 187–192. [15] A. Alba Garcı´a, L.D.A. Siebbeles, A. Rivera et al., Mater. Sci. Forum 363 (2001) 287–289. [16] J. Bholen, R. Kirchheim, Macromolecules 34 (2001) 4210–4215. [17] D. Cangialosi, H. Schut, A. van Veen, S.J. Picken, Macromolecules 36 (2003) 142–147. [18] A. Alba Garcı´a, S.M. Pimblott, H. Schut, A. van Veen, L.D.A. Siebbeles, J. Phys. Chem. B 106 (5) (2002) 1124–1125.
Depth profiling of the degradation of OC1OC10-PPV monitored by positron beam analysis A. Alba Garcı´a, H. Schut*, L.D.A. Siebbeles, A. van Veen Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629JB Delft, The Netherlands
Abstract The Doppler broadening of annihilation radiation (DBAR) technique and UV-Vis photo-absorption spectroscopy were used to monitor the photo-oxidation of thin films (200 nm) of OC1OC10-PPV. Samples were exposed in air to a 308 nm excimer laser and a commercial UV-Vis lamp. By means of the DBAR technique, the degradation was monitored as a function of depth when illuminating from the polymer side or the substrate side. It showed that the degradation is more pronounced on the side where the light affects, indicating that the photo-oxidation is not limited by oxygen diffusion. DBAR results were compared to photo-absorption measurements showing that in the first stages of the photooxidation positrons were more sensitive to changes in the polymer film. The non-destructive character of the DBAR technique and the possibility to perform depth profiling makes it a promising technique to study multi-layer devices. # 2003 Elsevier Science B.V. All rights reserved. Keywords: PPV; Polymer; Degradation; Positron
1. Introduction Electroluminescent polymers, and in particular poly-paraphenylene-vinylene (PPV) and its derivatives, have shown great promise as the active layer in light-emitting diodes, as they are relatively efficient, easy to manipulate, and produce light across the full visible spectrum. Nevertheless, lightemitting diodes based on these materials are known to gradually decrease their performance when operated under certain environmental conditions [1]. Several groups have shown that exposure to air and light results in the photooxidation of the vinyl bond of the PPV and the formation of carbonyl groups [2,3] which quench the luminescence properties of the polymer. The photo-oxidation of PPV has been monitored by techniques such as UV-Vis photo-absorption spectroscopy, photoluminescence spectroscopy, infrared (IR) photo-absorption spectroscopy and Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy. Nevertheless, IR spectroscopy is only sensitive to the surface and the other mentioned techniques yield information integrated over the total thickness of the polymer. Only a few techniques with depth resolution, such as
* Corresponding author. Tel.: þ31-15-2781961; fax: þ31-15-2786422. E-mail address: [email protected] (H. Schut).
elastic recoil detection analysis (ERDA), have been applied to this problem. Positron Annihilation Spectroscopy (PAS) using 22 Na as fast positron source has shown to be a very useful technique to study different properties of polymers like free volume [4,5], corrosion [6] or degradation upon different types of radiation [7,8]. With the development of tuneable low energy positron beams, PAS has been successfully applied to study properties of (thin) multi-layered systems such as degradation and ageing of coatings [9] and adhesion [10]. In this work, we show that PAS can also be used to monitor the degradation of OC1OC10-PPV as a function of depth after exposure to UV light under atmospheric conditions.
2. Experimental 2.1. Sample preparation and degradation Thin films of OC1OC10-PPV were prepared by spin-coating from a 2 g/l chloroform solution of the polymer on a quartz substrate under atmospheric conditions. The film thickness was estimated from stepping profiling measurements to be between 150 and 200 nm. The films were exposed for periods up to 15 h to either a commercial UV-Vis lamp (Philips PL-S 9W/10) with four
0379-6779/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01263-8
44
A. Alba Garcı´a et al. / Synthetic Metals 138 (2003) 43–47
main lines at 549, 439, 407 and 371 nm, or a 7 ns pulsed excimer laser (Lumonics HE-420) with a power of about 10 mJ/cm2 at 308 nm. The samples were illuminated from the polymer side (front illumination) or from the substrate side (back illumination). 2.2. Monitoring the photo-oxidation The active polymer layer was characterised at different stages of degradation by UV-Vis photo-absorption spectroscopy and the Doppler Broadening of the Annihilation Radiation (DBAR) technique. UV-Vis photo-absorption spectra were recorded using a Uvikron 940 spectrophotometer. DBAR spectra were recorded using the slow positron beam available at the Delft University of Technology. Positrons are implanted into the material with an energy in the range of 0.1–15 keV. The intensity of the beam is about 105 positrons/s and the pressure inside the measurement chamber is 106 Pa. An implanted positron slows down by successive collisions until it reaches thermal energy. Having thermalised, the positron starts to diffuse and because of its positive charge it is repelled by the atom cores. This makes the positron a very sensitive probe to characterise regions with lower local densities. Ultimately it will annihilate with an electron in its vicinity thereby emitting two g quanta in opposite directions each with an energy around 511 keV. In amorphous polymers, a positron may combine with an electron to form positronium (Ps), a hydrogen-like electron–positron bound state. Ps exhibits two spin states which are called para (p-Ps) and ortho (o-Ps) for the singlet and the triplet state, respectively. The vacuum lifetime of p-Ps is about 125 ps and that of o-Ps is 140 ns. However, in amorphous polymers the longer o-Ps lifetime is reduced to typically 1–4 ns due to the annihilation of the positron of the o-Ps with one of the surrounding electrons. This process is known as pick-off annihilation and is exploited in the socalled positron lifetime experiments to obtain information on the size of the open volume in which the Ps is formed. In a typical DBAR experiment, one of the two annihilation g quanta is detected and its energy is measured using a high resolution Ge detector. Due to the momentum of the electron–positron pair at the moment of annihilation, the energy of the detected g quanta is Doppler shifted. Neglecting the positron contribution to the total momentum, the detected g quanta carry the information about the momentum distribution of the annihilating electrons. After accumulating typically 106 annihilation events, the final width of Doppler broadened 511 keV photo-peak is characterised by the S parameter. This parameter is defined as the ratio of annihilation events with low momentum electrons to the total events in the photo-peak. Besides low momentum events, the S parameter is also sensitive to p-Ps decay as the intrinsic momentum of this particle is very low. The S parameter is known to drop when positrons are trapped in the vicinity of elements with deeper core electrons, such as oxygen or fluorine [12].
The Doppler broadening spectra were recorded for selected positron implantation energies (E) after different exposure times. The DBAR results were modelled using the program VEPFIT [13]. It simulates the depth dependent implantation, diffusion and trapping of positrons by solving the time averaged diffusion equation. In VEPFIT, the positron implantation profile P(z,E) is described by the derivative of a Gaussian: " # 2z z 2 Pðz; EÞ ¼ 2 exp z0 z0 with the mean depth hzi of positrons implanted with energy E given by [13] A 3 hzi ¼ E1:6 ¼ z0 G r 2 In this expression r is the density and A is a constant. This relation, adapted to a layered structure with different densities, as is the case here, is used to estimate the energy at which half of the positrons annihilate in the PPV layer and thus will locate also the position of the PPV–quartz interface. This energy (at about 4 keV) is shown in Fig. 2a and b by the dashed line.
3. Results Fig. 1 shows the absorption spectrum (optical density OD ¼ log (I/I0) versus photon energy) of OC1OC10-PPV after different exposure times to the UV-Vis lamp. Similar results were obtained when exposed to the laser. The spectrum shows a progressive bleaching and a blue shift of the broad peak at 2.5 eV, assigned to the p–p transition of the conjugated polymer [11]. This shift is attributed to the loss of conjugation upon photo-oxidation. No effect was observed when the samples were exposed to light or to oxygen only, for periods up to at least 15 h.
Fig. 1. Absorption spectrum of PPV after front-side illumination to the UV-Vis lamp for the times indicated.
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Fig. 2. S parameter of PPV on a quartz substrate as a function of the positron implantation energy after UV-Vis lamp illumination for the times indicated. (a, Left panel) front-side illumination and (b, right panel) back-side illumination. The dashed line at about 4 keV indicates the positron energy at which half of the implanted positrons annihilate in the PPV layer and thus locates the PPV–quartz interface. The drawn lines are obtained by the VEPFIT fitting program.
Fig. 2a shows the measured S parameter (the contribution of low momentum electrons) as a function of the positron energy (or depth) for different exposure times to the UV lamp (front illumination). Looking at the as prepared spectrum, we can distinguish four regions depending on the positron energy. With increasing positron implantation energy firstly the surface is probed (positron energy below 0.5 keV), secondly the PPV bulk (energy between 0.5 and 4 keV), thirdly the region where the S parameter drops from the PPV value to the substrate value (energy between 4 and 6 keV) and finally the quartz substrate (energies above 6 keV). Looking at the PPV region, the photo-oxidation of the active polymer layer results in a significant change in the positron spectrum, as shown in Fig. 2a. Firstly, the overall S parameter of the PPV layer decreases gradually with the increasing exposure time from a value of 0.62–0.53. Secondly, the change in S is not homogeneous over the PPV layer but more pronounced in regions closer to the surface. Similar results were obtained when laser light was used. Furthermore, the S parameter did not change after 12 h of exposure to light or air only. Two factors can be responsible for the faster decrease of S observed at regions closer to the surface. One of them is the high concentration of oxygen at the surface during exposure, which results in a more effective photo-oxidation of the PPV. The second one is the light absorption profile given by the Lambert–Beer law, which is at maximum at the surface and therefore enhances the photo-oxidation in this area. In order to discriminate between these two possibilities, a sample was exposed to the UV lamp from the backside. In this way, the maximum of the absorption profile is near the PPV– quartz interface while the oxygen has to diffuse from the front side to the PPV–quartz interface. Fig. 2b shows the measured S parameter as a function of the positron energy for different exposure times to the UV lamp after illumination from the back side. In this experiment, again an overall decrease of the S parameter is
Fig. 3. S parameter at 2 keV positron energy as a function of the OD at the maximum of lower energy excitation band at 2.5 eV for different exposure times (front side) Data are shown for two samples illuminated with the UV-Vis lamp or laser, respectively. The lines are drawn to guide the eye.
Fig. 4. Schematic of the layered structure used in VEPFIT to model the PPV layer on a quartz substrate. The PPV layer is divided in to two layers with fitting parameters S1 and S2 of which the values are indicated by the horizontal lies. The dashed lines represent the light absorption profile in the PPV layer.
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Table 1 Fitted S parameter values for front and back illumination of PPV layers as a function of illumination time using the UV-Vis lamp Front-side illumination
Back-side illumination Ssurface
S
As spin coated
0.577
0.618
Illumination time (min)
Ssurface
S1
15 90 270 810
0.555 0.547 0.540 0.532
0.605 0.578 0.542 0.533
Ssurface
S
As spin coated
0.577
0.618
S2
Illumination time (min)
Ssurface
S1
S2
0.597 0.591 0.561 0.529
30 90 240 450 870
0.560 0.554 0.546 0.538 0.540
0.601 0.594 0.583 0.565 0.534
0.581 0.568 0.542 0.525 0.532
observed as was obtained when illuminating from the front side. However, the degradation is now more pronounced at the interface PPV–quartz, clearly seen by the faster decrease of S at 3 keV. This shows that the PPV layer degrades from the side where the light affects, indicating that the degradation of PPV is not limited by oxygen diffusion on the time scales used in the experiment. Fig. 3 shows the correlation between the S parameter and the OD for a sample illuminated with the UV-Vis lamp and another sample illuminated with the laser. The S parameter at 2 keV is plotted versus the OD taken at the maximum of the broad absorption band at about 2.5 eV for exposure times up to several hours. In both cases, the relative change in the OD for short exposure times is much smaller than the relative changes observed in the S parameter. These results show that due to the depth selectivity PAS is more sensitive as compared to photo-absorption spectroscopy at least in the first stage of the photo-oxidation process. Fig. 4 shows schematically the light absorption profile of the PPV layer for back and front illumination. Assuming the S parameter inside the PPV follows this profile, we model the PPV region by defining two layers of fixed width (100 nm) each, on top of the substrate, as shown in Fig. 4. Fitting parameters were the S parameter of each PPV layer (S1 and S2) and the positron diffusion lengths (Lþ), defined as the distance travelled by the positron between the thermalisation and the final annihilation. For the substrate, the parameters were fixed to S ¼ 0:578 and Lþ ¼ 60 nm as obtained from a independent reference measurement on bare quartz. The fitting results are shown in Fig. 2a and b by the drawn lines. The values of the fitted S parameters are summarised in Table 1. The as spin-coated sample can be fitted with only one layer, indicating that the S parameter is homogeneous over the whole PPV layer. After exposing the PPV, it can be seen that the S parameter of the layer where the light affects decreases faster. When illuminating from the front (light affects on layer 1), S1 decreases faster than S2. When illuminating from the back side (light affects on layer 2) a similar trend is observed, but now S2 decreases faster than S1. After complete degradation, both layers reach the same S value. It is possible to fit the longest exposed curves with one
PPV layer and the substrate, indicating that the S parameter again becomes homogeneous over the whole layer after complete degradation.
4. Discussion We have shown that the S parameter decreases upon photooxidation. This implies that as the exposure time increases the positron annihilates with electrons of higher momentum. An explanation of this phenomenon requires a detailed understanding of the positron and positronium chemistry in these materials, which is still under investigation. A detailed discussion will be presented in a future work. In addition a decrease in the Ps fraction, and thus the p-Ps fraction also leads to a drop in the S parameter. The p-Ps fraction was derived independently from the Doppler broadening measurements following a procedure, which decomposes the measured photo-peak into 3–4 Gaussian profiles [14,15]. The contribution of the Gaussian with a width comparable to the resolution of the detector is then associated with the annihilation of p-Ps with nearly zero momentum. We have observed a decrease from a 70% Ps (or equivalently 17.5% p-Ps) in the as prepared sample to no Ps formation in the completely degraded sample (see Fig. 5). The factors that cause Ps formation to be completely inhibited are still under investigation. One possibility is the
Fig. 5. The fraction of p-Ps in the PPV layer as a function of the illumination dose using the laser.
A. Alba Garcı´a et al. / Synthetic Metals 138 (2003) 43–47
reduction of the space available to form positronium (free volume). Several studies in other polymers show a decrease in the p-Ps fraction when the free volume is reduced by means of, e.g. temperature reduction, ageing or compression [16,17]. Another possibility is that the mechanism of Ps formation in these materials is not via track processes [18] but via direct interaction with the polymer chain, taking an electron directly from the material to form positronium. In this case, the increase in the ionisation energy as a consequence of the shortening of the chains upon photo-oxidation of the polymer would increase the energy necessary to extract an electron from the chain to form Ps, resulting in a decrease of Ps formation.
5. Conclusion Positron Annihilation Spectroscopy and UV-Vis photoabsorption spectroscopy have been used to monitor the photo-oxidation of OC1OC10-PPV upon exposure to a laser and a UV-Vis lamp. The results, which were independent of the light source used, showed that positrons can monitor the degradation of PPV thin films as a function of depth. Exposure of the samples from the front or the back side showed that the degradation is more pronounced on the side were the light affects. This indicates that the degradation is not limited by oxygen diffusion on the time scale of the experiment. Due to the depth selectivity of the DBAR beam technique it has a higher sensitivity for at least the first stages in the degradation process as compared to the photo-absorption technique. The results show the potential of the DBAR beam technique to study systems with depth resolution and sensitivity to chemical changes in a non-destructive way.
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