13716
J. Phys. Chem. 1996, 100, 13716-13719
Carrier Generation Process in Poly(p-phenylene vinylene) by Fluorescent Quenching and Delayed-Collection-Field Techniques M. Esteghamatian,† Z. D. Popovic,‡ and G. Xu*,† Materials Science & Engineering Department, McMaster UniVersity, Hamilton, Ontario, Canada L8S 4L7, and Xerox Research Centre of Canada, 2660 Speakman DriVe, Mississauga, Ontario, Canada L5K 2L1 ReceiVed: February 2, 1996; In Final Form: April 10, 1996X
The carrier generation process in poly(p-phenylene vinylene) (PPV) has been investigated by using fieldinduced fluorescent quenching and delayed-collection-field techniques under pulsed illumination. Relative photoresponse and fluorescent quenching have been measured at electric fields of up to 300 V/µm. The results demonstrate a linear relation between fluorescent quenching and photoresponse at high electric fields, indicating that almost all field-quenched excited states lead to carrier generation. Experimental results also indicate that the time decay of e-h pairs is highly dispersive and the majority (70%) of them decay nonexponentially to ground state in ∼1 ms after illumination. Fluorescent quenching and carrier generation efficiencies obtained at the highest applied electric field are 34% and 42%, respectively. Results also suggest that carrier generation in PPV is a two-step process. In the first step, excited singlet states dissociate into bound geminate e-h pairs, and in the second step, the geminate pairs are separated into free carriers. Both steps are influenced by the applied electric field.
Introduction The attractive optoelectronic properties of poly(p-phenylene vinylene) (PPV) make it a good candidate for a variety of technological applications. Moreover, the synthesis of this conjugated polymer via the water-soluble precursor route provides the flexibility to easily fabricate electronic devices. Light-emitting diodes based on undoped or doped PPV can now emit light in various parts of the visible spectrum. Other potential applications of PPV are in photoconductors and xerographic devices. Although photoconductivity and the mechanism of carrier generation in PPV have been intensely researched in recent years, the exact nature of the process, free carrier generation versus exciton model, is yet to be resolved. Eckhardt et al.1 showed that in several conjugated polymers including PPV the threshold of photoconductivity (PC) coincided with the onset of photoabsorption (PA). Lee et al.,2 by conducting steady state and picosecond photoconductivity experiments, verified this agreement and concluded that photoexcitation in PPV led to direct generation of free carriers through an interband π-π* transition, a process analogous to interband excitation in inorganic semiconductors. The agreement between PC and PA thresholds, however, cannot be simply taken as the evidence for photogeneration of free carriers (see Bassler et al.3). This coincidence can be merely due to the extrinsic rather than intrinsic charge carriers. By using a delayed-collection-field technique, this study has been conducted to provide more insight into the field-induced fluorescent quenching and carrier generation process. In order to directly evaluate the correlation between fluorescence quenching and photoresponse, simultaneous measurements of the two quantities have been made. Experimental Detail PPV solution was synthesized by mixing R,R′-dichloro-pxylene and tetrahydrothiophene according to the standard precursor route. A slight modification was, however, introduced by using dialysis tubes with molecular weight cutoffs of † ‡ X
McMaster University. Xerox Research Center of Canada. Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00341-3 CCC: $12.00
Figure 1. Schematic diagram of the PPV cell used in the measurements.
12 000-14 000. The solution was then cast onto substrates and converted in vacuum at 260 °C for 24 h. Figure 1 represents the cell configuration in which PPV is sandwiched between two insulating layers. These layers, SiOx and Goodyear OMS pliolite (dissolved in cyclohexane), were used to block the carrier injection from the electrodes into the sample, thus enabling us to study the carrier generation process in the PPV film only. In the delayed-collection-field measurements of the photoresponse, a bias voltage (Vappl) is applied to the sample and the sample is illuminated by a pulsed light source. Immediately after light excitation (i.e., after a 0.1 µs or 0.1 ms delay in the current experiments), a constant-collection voltage (Vcoll) is applied and the light-induced component of the voltage drop at the electrodes is measured. Furthermore, fluorescent signals are simultaneously detected. Finally, by varying the bias voltage, photoresponse and fluorescent quenching are measured as a function of the applied field. All equipment required for this experiment (power supplies, pulsed light sources, data acquisition electronics, etc.) were controlled by a personal computer. The waveform of the applied electric field, together with the timing of the pulsed light source, as well as the description of the circuit diagram used in the aforementioned technique, has been described in the appendix of ref 4. The sample was illuminated from the NESA (tin oxide) side by either a white light from a pulsed xenon lamp or the third harmonic of a Nd/YAG laser (λ ) 355 nm). In both cases the results were consistent. As was mentioned before, in measuring fluorescence quenching and photoresponse, the collection delay time (time delay between pulsed excitation and application of the collection field) was set to 0.1 µs for laser excitation and to 0.1 ms for white light illumination. For measuring zero-field © 1996 American Chemical Society
Carrier Generation Process in PPV
J. Phys. Chem., Vol. 100, No. 32, 1996 13717
photoresponse as a function of the collection delay time, pulsed laser excitation was used. The UV/visible absorption and photoluminescent spectra of each component were separately measured to ensure that light absorption and fluorescent emission from the insulating layers and NESA were negligible compared to those of PPV, as illustrated in Figure 2. The sample thickness (d) was calculated from sample capacitance (CS ) 0A/d) to be ∼2 µm. The dielectric constant of PPV was taken from the literature5 as ) 3, and the sample area and capacitance were measured as A ) 40.3 mm2 and CS ) 495 pF, respectively. All measurements were performed in the dark and at room temperature under a gentle flow of nitrogen over the cell surface. The use of nitrogen significantly increases the cell breakdown voltage. Quantities measured were the relative photoresponse (photoinduced voltage drop 5 ms after pulsed excitation normalized to the incident light intensity),
R ) ∆V/Ilight
(1)
and fluorescent quenching, defined as,
φ(E) )
If(O) - If(E) If(O)
(2)
where If(E) is the fluorescent signal normalized to the incident light intensity at the electric field E applied to the sample. In our experiments we were operating in a small signal regime where ∆V was proportional to light intensity, Ilight. Typical ∆V values were 0.1-1 V, much smaller than Vcoll. This assured that space charge effects were not important. If space charge limitations existed, signal saturation with increasing light intensity would be observed. It should be mentioned that the relative photoresponse R, and carrier generation efficiency η, are related through a constant as,
η(E) ) CR(E)
(3)
Assuming that geminate electron-hole (e-h) pairs originate from the first excited singlet state, a simple kinetic model,4,6 which we will refer to as the internal conversion model, gives the correlation between φ(E) and η(E) as,
φ(E) )
ηG(E) - ηG(O) I - ηG(O)
Figure 2. Relative absorption and fluorescent spectra of PPV, Goodyear OMS pliolite, and NESA glass.
(4)
where ηG(E) is the quantum yield for the geminate e-h pairs. The quantum yield of the free carriers is given by,
η(E) ) Ω(E)ηG(E)
(5)
where Ω(E) is the dissociation probability of geminate e-h pairs. At high fields Ω(E) ) 1 and η(E) ) ηG(E), and thus a linear relationship between φ(E) and η(E) ) CR(E) is expected. By using this linear correlation (eq 4) at high fields, the constant C can be determined and relative photoresponse data can be scaled to attain true carrier generation quantum efficiency. It is important to note that linear correlation between fluorescence quenching and carrier generation also holds for the Onsager model11 in which the geminate electron-hole pairs are generated by light excitation and can recombine to produce the first excited singlet state.6 In that case a linear relationship between carrier generation and fluorescence quenching is expected to hold at all fields, not only in the high-field limit which is the case for the internal conversion model discussed previously. In the Onsager model at low fields, carrier generation efficiency is a linear function of the applied field. Consequently, linear dependence between fluorescence quenching and the electric field should also be observed. This will be of importance in our later discussion.
Figure 3. Eappl ) 0.
Dependence of photoresponse on collection field,
Results and Discussion In order to carry out valid measurements, the relative photoresponse obtained at a certain bias voltage should be independent of the collection field. In Figure 3, the relative photoresponse of PPV is plotted against the collection field
13718 J. Phys. Chem., Vol. 100, No. 32, 1996
Esteghamatian et al. theory11,6), linear dependence on the electric field would be expected, which is not observed. The quadratic dependence of fluorescent quenching on the electric field is consistent with the direct dissociation of the excited states into geminate pairs. This is a consequence of symmetry. At zero-field dissociation the probability of excited states into e-h pairs oriented in opposite directions will be the same. When an external field is applied, excited state dissociation rate should not depend on the direction of the electric field. Consequently, fluorescent quenching should be an even function of the electric field, i.e., quadratic in the lowest order. Therefore, our results support the interpretation of direct excited state dissociation into geminate e-h pairs rather than generation of excited states by recombination of geminate e-h pairs. At the highest applied field of 300 V/µm, ∼32% fluorescence quenching is achieved and a positive slope with no sign of saturation is observed. It is noteworthy to mention that the zerofield carrier generation (nonzero value of the photoresponse at Eappl ) 0) shown in Figure 4b is mainly due to the collection field (Ecoll ) 150 V/µm) applied to the sample after illumination, which causes the geminate e-h pairs generated at zero field to dissociate into free carriers. The presence of impurities has also been reported to increase zero-field carrier generation in other organic photoconductors, such as perylene,7 which was explained as an extrinsic process inducing exciton dissociation by interaction with impurities. In Figure 4c, fluorescent quenching is plotted against photoresponse; a linear correlation exists at high Eappl between the two quantities. From the slope (m ) 0.966) and the φ intercept (i ) -0.1412) of the best linear fit through the data obtained at high fields, the correlation factor C can be determined by rewriting eq 4 as,
φ(E) )
ηG(O) C R(E) 1 - ηG(O) 1 - ηG(O)
(6)
where ηG(0) is the geminate e-h pair yield at Eappl ) 0. Using the slope and the intercept mentioned above, the proportionality constant is calculated as C ) 0.846 and the φ-η relationship can be expressed as, Figure 4. (a) Fluorescent quenching versus applied electric field. (b) Photoresponse and normalized quantum efficiency versus applied field; the scale for η was calculated by multiplying photoresponse by the proportionality constant C ) 0.846. (c) Fluorescent quenching versus photoresponse.
(Ecoll) at Eappl ) 0. As expected, with an increase in the collection field, photoresponse increases and then tends to reach saturation. However, at collection fields exceeding 170 V/µm, it sharply increases. This behavior, which was observed for both broad-band and laser illumination, can be attributed to field detrapping of deeply trapped carriers and/or change in sample properties at high electric fields. In the current experiments, where applicable, Ecoll was chosen as 150 V/µm. The electric-field-induced fluorescent quenching and photoresponse obtained upon laser excitation and at Ecoll ) 150 V/µm are plotted versus the applied electric field in Figure 4a and b, respectively. It should be mentioned that fluorescent quenching measurements obtained under laser and broad-band illumination were identical, indicating that the results are independent of the peak power of the light source, which is consistent with operating in the linear response regime. For the sake of brevity, the results obtained under broad-band illumination are not presented. As is seen, fluorescent quenching and photoresponse are both strongly field dependent. Furthermore, the quadratic dependence of φ on Eappl is clearly illustrated. As discussed previously, if fluorescent quenching resulted from fieldmodulated recombination of geminate e-h pairs (Onsager
φ(E) ) 1.141η(E) - 0.141
(7)
To obtain true quantum efficiency η, the photoresponse (R) plotted in Figure 4b has been rescaled by incorporating the parameter C determined above, (see also eq 3). As is seen, carrier generation efficiency of ∼42% is achieved at Eappl ) 300 V/µm. The fluorescent quenching efficiency and quantum efficiency for carrier generation obtained here will be compared to the values reported in the literature later in the paper. The experimental data depicted in Figure 4c clearly demonstrate that at high electric fields almost all field-quenched excitations lead to production of free carriers. The departure from linear response at low fields, which is attributed to Ω(E) < 1, shows that at low fields only a fraction of geminate pairs results in the formation of free carriers. This indicates that high electric fields are required for full dissociation of all geminate pairs. Therefore, it is concluded that the carrier generation process in PPV occurs via two steps. In the first step, excited states produce geminate e-h pairs and then the bound e-h pairs dissociate into free carriers under high applied fields. The experimental data presented in Figure 4c also show that the collected charge at Eappl ) 0 is about half of the value expected had the linear fit held for all applied electric fields. This observation, which has also been noted for other materials,8 signifies that only half of the geminate electron-hole pairs generated at zero applied field dissociate into free carriers by the collection field. This could be due to energetics and
Carrier Generation Process in PPV
J. Phys. Chem., Vol. 100, No. 32, 1996 13719 side, most of the e-h pairs are created in the PPV layer adjacent to the NESA electrode. Bearing in mind that electron mobility in PPV is less than hole mobility by 2-3 orders of magnitude, when NESA is negatively biased, electrons will cross shorter distances (have shorter ranges) than holes when NESA is positively biased. Therefore, a larger response is obtained when NESA is positive and holes are moving into the sample bulk. Time-evolution data also indicate that ∼42% of the total (averaged) photoresponse evolves in ∼100 ns and the rest emerges in 1 ms (note that in Figure 6 the y-axis starts at 0.05). The fast-rising part of this graph is attributed to the fast-moving carriers (holes) and the slow-rising part is believed to be due to the sluggish mobility of electrons. Conclusion
Figure 5. Variation of relative photoresponse with the collection delay time.
Figure 6. Time evolution of the photoinduced voltage at Eappl ) Ecoll ) 150 V/µm.
orientation of the geminate e-h pairs. At low applied fields geminate e-h pairs are generated in all directions, and the e-h pairs oriented in the opposite direction of the field are forced to recombine, while only the e-h pairs in the direction of the field dissociate into free carriers. The dependence of relative photoresponse on collection delay time is demonstrated in Figure 5. With an increase in the delay time between excitation and charge collection, the photogenerated geminate e-h pairs have more time to recombine, and therefore, carrier generation efficiency decreases. As shown in this figure, the recombination process is highly dispersive and e-h pairs begin to decay after ∼0.3 µs. A majority of the carriers (∼70%) decay nonexponentially to ground state about 1 ms after illumination. The nonexponential decay (extending to such long times) may be attributed to the presence of impurities and their interaction with charge carriers. Time evolution of the photoinduced voltage at Eappl ) Ecoll ) 150 V/µm is depicted in Figure 6. It is evident that the charge collected during the first half-cycle (NESA positive) is higher than the ones measured during the second half-cycle (Au/Pd positive). This asymmetry, which is observed in many materials, can be explained in term of low mobility and/or trapping of electrons. Since the sample is illuminated from the NESA
Strong electric field dependence of fluorescent quenching and photoresponse has been observed in PPV. The data demonstrate a linear correlation between fluorescent quenching and carrier generation efficiency at high applied fields, indicating that all of the field-quenched excitations lead to production of free carriers. It is also concluded that the carrier generation process in PPV follows two steps: bound e-h pairs are generated upon excitation and then dissociate into free carriers under an applied electric field. Carrier generation efficiency of 42% was detected at Eappl ) 300 V/µm. Fluorescent quenching of up to 34% was achieved at Eappl ) 300 V/µm which decreases to 15% at 200 V/µm. This is lower than the value reported by Bassler et al.3 They reported 30% quenching at Eappl ) 200 V/µm, twice the one obtained here. This discrepancy can be due to the differences in sample preparation and thickness measurements. For example, in the current study the sample thickness was determined by taking the dielectric constant as ) 3. However, dielectric constants of 4.3 and even 22 have also been reported9,10 for PPV. Assuming, for example, ) 4, the sample thickness can be recalculated as d ) 2.87 µm, and by readjusting the applied field accordingly, ∼29.7% fluorescent quenching is obtained at Eappl ) 200 V/µm, which agrees well with the value reported by Bassler et al. Fluorescent quenching has also been observed in blends of poly(phenyl-p-phenylene vinylene)polycarbonate by Deussen et al.12 They reported 40% fluorescent quenching at an applied field of ∼270 V/µm for reversebiased ITO/PPPV-PC/Al diodes. Despite the differences in device structure, the results obtained here are in reasonable agreement with those of Duessen et al., suggesting that the fundamental physical processes occurring in these two systems are very similar. References and Notes (1) Eckhardt, H.; Shacklette, L. W.; Jen, K. Y.; Elsenbaumer, R. L. J. Chem. Phys. 1989, 91(2), 1303. (2) Lee, C. H.; Yu, G.; Heeger, A. J. Phys. ReV. B 1993, 47(23), 15543. (3) Bassler, H.; Brandle, V.; Deussen, M.; Gobel, E. O.; Kersting, R.; Kurz, H.; Lemmer, U.; Mahrt, R. F.; Ochse, A. Pure Appl. Chem. 1995, 67(3), 377. (4) Popovic, Z. D. J. Chem. Phys. 1983, 78 (3), 1552. (5) Antoniadis, H.; Hsieh, B. R.; Abkowitz, M. A.; Jenekhe S. A.; Stolka, M. Synth. Met. 1994, 62, 265. (6) Noolandi J.; Hong, K. M. J. Chem. Phys. 1979, 70(7), 3230. (7) Popovic, Z. D.; Loutfy R. O.; Hor, A.-M. Can. J. Chem. 1985, 63, 134. (8) Popovic, Z. D. Chem. Phys. 1984, 86, 311-321. (9) Riess, W.; Karg, S.; Dyakonov, V.; Meier M.; Schwoerer, M. J. Lumin. 1994, 60-61, 906. (10) Karg, S.; Dyakonov, V.; Meier, M.; Riess, W.; Paasch, G. Synth. Met. 1994, 67, 165. (11) Onsager, L. Phys. ReV. 1938, 54, 39. (12) Deussen, M.; Scheidler, M.; Bassler, H, Synth. Met. 1995, 73, 123.
JP960341S
