The transition from bipolaron to triplet-polaron magnetoresistance in a single layer organic semiconductor device

Organic Electronics 15 (2014) 1711–1716

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The transition from bipolaron to triplet-polaron magnetoresistance in a single layer organic semiconductor device Hang Gu a, Theo Kreouzis a,⇑, William P. Gillin a,b a b

Materials Research Institute, School of Physics and Astronomy, Queen Mary University of London, Mile End Road E1 4NS, London, United Kingdom College of Physical Science and Technology, Sichuan University, Chengdu 610064, China

a r t i c l e

i n f o

Article history: Received 20 January 2014 Received in revised form 1 April 2014 Accepted 29 April 2014 Available online 17 May 2014 Keywords: Magnetoresistance P3HT Bipolaron Triplet-polaron Magneto-conductance

a b s t r a c t We measure the current–voltage–luminescence (I–V–L) and Magneto-Conductance (MC) response of a poly(3-hexyl-thiophene) (P3HT) based device (Au/P3HT(300 nm)/Al) in forward and reverse bias. In reverse bias (<1 V), the negative MC is described by a single non-Lorentzian function, consistent with the bipolaron theory. In forward bias, the transition from negative saturation MC (low bias) to positive saturation MC (high bias) occurs when the current density exceeds 102 A cm2 and coincides with electroluminescence. Under these conditions the triplet density (1015 cm3) becomes comparable to the hole density (1016 cm3), consistent with the triplet-polaron interaction theory. From the current density dependence of the MC we conclude that in forward bias both mechanisms must be occurring simultaneously, within a given device, and that the overall sign of the MC results from competition between the two mechanisms. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction During the last 10 years there has been an increasing interest in magneto-conductance (and magnetoresistance) in both polymer and small molecule organic semiconductors (for recent reviews please see [1,2]). Magnetic fields have been demonstrated to affect organic semiconductor devices leading to the changes in current, luminescence, photocurrent and efficiency. There are two major classes of theories that have been used to explain magnetic field effects in organic devices, these are exciton based [3–7] and bipolaron [8] based models. Within the excitonic based model there are a number of different processes which can be affected by the magnetic field and which can influence both light emission and device current. For example, the formation rate of singlets and triplets from polaron pairs ⇑ Corresponding author. Tel.: +44 02078823432. E-mail address: [email protected] (T. Kreouzis). http://dx.doi.org/10.1016/j.orgel.2014.04.036 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

appears to be magnetic field dependent and directly affects the light emission from a device [4]. However, it also has a secondary effect in that the changes in triplet population brought about are believed to affect carrier mobility through triplet-polaron interactions [7,9–16]. Of these interactions, some such as site blocking are not expected to have any additional magnetic field dependence whereas others, such as strongly coupled triplet-polaron interactions or ‘‘trion’’ [7,10,13–16] based interactions are expected to have additional magnetic field dependence. Therefore even within the excitonic models there are a number of competing processes which make modelling difficult. Additionally there is the possibility of bipolaron interactions [8], which are also predicted to be magnetic field dependent, occurring in parallel with excitonic effects. These bipolaron processes only require a single carrier type but this is a condition that is difficult to isolate in planar device structures due to the large electric fields that are normally present and hence the difficulty in producing truly

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2. Experimental The device was constructed with a gold anode followed by a layer of P3HT and an aluminium cathode, using a combination of vacuum deposition for the metals and spin coating of the P3HT. The P3HT was dissolved in 1,2-dicholobenzene at a concentration of 30 mg/ml before successively spin coating the solution at 1000 rpm for 1 min resulting in smooth 300 nm films on the substrate. The P3HT films were annealed at 120 °C for 20 min under a nitrogen atmosphere. Both the anode and cathode were defined by shadow masks during the vacuum deposition process. Typically the deposition was carried out at a pressure of 106 mbar, with deposition rates ranging from 0.5 to 5 Å s1. Immediately after growth, the devices were placed in an evacuated (105 mbar pressure) light-tight sample holder in contact with the photo-detector. A Keithley 236 source-measure unit was used in conjunction with a Newport 1830C optical power meter to take current–voltage– luminescence (I–V–L) measurements. MC measurements were taken with the device operated in constant voltage mode. Magnetic field measurements were made from 0 to 260 mT. Immediately before and after each field measurement, a measurement at null field

was taken. The two null-field measurements were then averaged and used to calculate the change in current with applied magnetic field. This procedure was adopted to remove any effects due to drifting in the device characteristics, which can be noticeable at high drive voltages. The Keithley 236 source-measure unit averaged over 16 readings current measurements and the drive voltage was applied to the device only when it was in a stable field. As soon as an individual measurement was made the drive voltage was set to zero while the field was changed.

3. Results and discussion P3HT is known to be p-doped due to exposure to oxygen and this has been demonstrated to result in Schottky barriers with metal electrodes such as aluminium [28–31] or gold [32,33]. This can result in a contact with a very low injection barrier between the metal and the Highest Occupied Molecular Orbital (HOMO) of P3HT and efficient hole injection from either contact at low operating voltages. Fig. 1 shows the current voltage characteristic of the Au/P3HT/Al diode which demonstrates current injection at low voltages in both forward and reverse bias but with clear rectification, with approximately two orders of magnitude difference in the current density between forward and reverse bias. This rectification is due to the differences in the Schottky barrier heights for the two contacts. The inset shows double logarithmic plots of the forward and reverse bias current density versus voltage. The solid lines indicate space charge limited current regions (SCLC, j / V2) and we note that the majority of the forward bias results follow this behaviour. In reverse bias, however, the characteristic significantly deviates from SCLC behaviour, particularly at larger bias. We attribute this to electron injection from the Au contact. Fig. 2 shows schematic diagrams for our device under short circuit, small reverse bias and small and large forward bias conditions. Due to the presence of Schottky

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unipolar injection. This problem has been overcome to some extent in recent experiments where polymer blends have been used to try to quench exciton formation [17,18] and recent work based on polymers doped into zeolites which have greatly reduced electric fields and are therefore much more likely to be unipolar [19]. In this work we investigate Magneto-Conductance (MC) in a poly(3-hexyl-thiophene) (P3HT) device. The device demonstrated negative saturation MC under both reverse bias and very low forward bias (0.2 V). Under high forward bias (>5 V) a transition to positive saturation MC was seen which correlates with the onset of light emission. The transition from negative MC at low voltages to positive MC at higher voltages has been observed in the literature [9,20–24], mostly in devices where poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) was used as a buffer layer for hole injection. We note that the presence of a PEDOT:PSS layer may have pronounced magnetic interfacial injection effects [25] and have avoided its use in our devices for this reason. Changes from negative to positive MC resulting from varying blend composition [18], geometric factors [9,26] and magnetic field [27] have also been reported. Here we present results from a simple sandwich structure device Au/P3HT/Al under different measurement conditions. The hole only (low reverse bias) behaviour corresponds to a negative magnetic effect on the current (negative saturation MC) fitted by a single non-Lorentzian function whereas the ambipolar behaviour (forward bias) appears to have a contribution from more than one process. The forward bias results depend strongly on the current density through the device showing a transition from negative saturation MC to positive saturation MC. The transition is due to the competition between the two mechanisms and the dominance of one over the other depending on drive conditions.

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barriers at both interfaces, even at zero bias (Fig. 2a), the device would be expected to be in a ‘‘flat-band’’ condition. Therefore the onset of charge injection is not defined by the difference in the effective work functions for the two electrodes, as in traditional ‘‘undoped’’ organic semiconductors. In the presence of a modest reverse bias (<1 V, Fig. 2b) the device will operate in near unipolar conditions. Given the Schottky barrier at the Al/P3HT interface at low reverse bias voltages it is possible to get hole injection from the Al into the P3HT whereas there will be no electron injection from the gold due to its very high work function. We note that as indicated in Fig. 2b, the P3HT LUMO states do not become available for electrons to tunnel into from the gold Fermi level until the applied bias (nominally) exceeds 2.2 V. Under low forward bias (Fig. 2c), the large work function of gold [34] coupled with the Schottky barrier at the interface allows for very efficient hole injection from gold into P3HT [35]. In contrast to this, electron

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injection from the Al in these conditions is very inefficient as it is only possible via thermionic emission. Although this thermionic emission is inefficient it may be expected that there may be some electron injection, even at low applied voltages as the energy barrier between the P3HT LUMO and Al is much smaller than for Au. We also note that at such low forward bias, electron tunnelling into the LUMO is not possible as there are no available states at the Al Fermi level. At large forward bias (Fig. 2d) it becomes possible for electrons to tunnel into the LUMO of the P3HT from the Al and this tunnelling injection is expected to dominate over any thermionic injection. We therefore expect to see a significant increase in electron injection as the forward bias is increased and eventually the onset of light emission. We can therefore define three transport regimes within this device structure: Reverse bias (Fig. 2b), essentially unipolar with holes injected from the Al contact; Forward bias

Fig. 2. Energetic schematics of the Au/P3HT(300 nm)/Al device under different conditions (a) short circuit condition with Schottky contacts between P3HT and electrodes, (b) under low reverse bias, holes can be injected by tunnelling and thermionic emission from Al but electron injection is forbidden from Au, (c) under low forward bias, holes can be injected from Au and electrons can only be injected from the Al by thermionic emission and (d) under high forward bias, tunnelling injection of electrons takes place from the Al, leading to ambipolar conduction.

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at low voltages (Fig. 2c), predominantly hole transport but with weak thermionic electron injection; Forward bias at higher voltages (Fig. 2d), essentially ambipolar current transport. Fig. 3a shows the MC of the device in reverse bias at low-voltages (0.2 to 1 V). In all cases the MC curves can be well fitted with the single non-Lorentzian function given in Eq. (1).

DI aB2 ¼ I ðjBj þ B0 Þ2

ð1Þ

The characteristic fields (B0) obtained in the MC response fits are consistent with those reported in the literature for P3HT [36] which are of the order of 5 mT (see Supporting information). The fitting results are consistent with the bipolaron theory [8] in the presence of a single carrier type (hole) transport. Under higher reverse bias (P4 V), the magnitude of the B0 term starts to increase, as does the associated error in the fit; this may be due to the onset of weak electron injection from the gold. The weak electron injection will introduce another process, which will affect the quality of the fit. We note that the inset in Fig. 1 shows a clear deviation from SCLC behaviour at such high bias. The MC plots in Fig. 3b correspond to two different values of forward bias. The negative (low bias) curve corresponds to previously measured OMR in this material [37]. The overall magnitude of this effect at the fields measured is low, reaching a maximum value of 0.3%. As the applied bias is increased we see a switch from negative to positive MC in the high field region with the MC reaching a maximum value of +0.2%. Unlike the MC results at negative bias the MC results in forward bias cannot be fitted with a simple function as there is a transient feature at low magnetic fields. For the negative MC low-bias (0.2 to 5 V) results this corresponds to a positive spike of magnitude 0.05% at a field of 7 mT whereas for the positive MC high bias (from 6 to 10 V) results there is a negative spike of the same magnitude and position. These features are different to the ultra-low field features that have been seen in other experiments [37] and are most likely to be -0.2V -0.4V -0.6V -0.8V -1.0V

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due to the interaction of two different processes, one positive and one negative, such as has been observed in thin Alq3 devices [15,38–39]. In this work, we expect there to be a negative non-Lorentzian component due to the hole bipolarons that were measured in reverse bias but we cannot specify precisely what positive components there may be. In the Supplementary material we have carried out detailed fitting of the MC results using the sum of a Lorentzian and a non-Lorentzian function under low and high forward bias conditions. As the relative sizes and signs of the two functions vary with bias, however, we cannot attribute each mathematical form to a specific process and treat the fits as purely empirical. We note that the data in Fig. 3b approaches saturation at fields of the order of hundreds of mT and that we concentrate on high magnetic field MC in the majority of our analysis. As we explained above, we would expect to see some electron injection in forward bias, even at low voltages. There are a number of possible processes that the electron injection could contribute to the MC, such as the positive pair-level interaction affecting the formation rate of singlets and triplets [4,40]. The change of formation rate of singlets and triplets then affect the device current, via site blocking or weak trapping [7,9–16,41], which would be expected to give a positive Lorentzian MC. Alternatively, exciton dissociation at energetically favourable sites would yield a negative Lorentzian of similar form. Finally there could be a strongly coupled triplet-polaron or ‘‘trion’’ interaction which would be expected to be positive with a larger B0 term. As we are not able to unambiguously model the MC results in the forward bias regime, we have instead taken the MC response at a number of different magnetic fields (7 mT, 30 mT, 155 mT and 258 mT) and used these to monitor the trends in the MC response as a function of operating conditions. Fig. 4 shows the differential current measurements obtained under several magnetic fields plotted against the current density through the diode; the measured electroluminescence is also plotted for forward bias only. The voltage range for negative bias measurements is 0.2 to 10 V and for positive bias measurements is 0.2 V to 10 V. The reverse bias results (black symbols) are limited to low currents

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Fig. 3. Typical room temperature results obtained from the Au/P3HT(300 nm)/Al device (a) variation in device current with magnetic field under reverse bias conditions. The solid lines are fits obtained using Eq. (1) and (b) variation in device current with magnetic field under two different forward bias conditions. The solid lines are included as guides to the eye.

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(<104 A/cm2) and always display negative MC (we note that the 7 mT high bias result is too noisy and small to be considered as evidence of any positive MC). This data can always be fitted with a single non-Lorentzian function as is attributed to pure bipolaron effects in this unipolar regime. The forward bias MC results (red symbols) clearly show a change of sign as the current density increases. The data under low magnetic fields (7 mT) correspond to the ‘‘spike’’ seen in Fig. 3b and make a transition from positive MC to negative MC. The saturation MC, on the other hand, makes a transition from negative to positive with increasing bias. As previously discussed, the ‘‘spike’’ region cannot be unambiguously attributed to distinct processes and we shall limit our further discussion to high magnetic field results (155 mT and 258 mT) i.e. to fields approaching the saturation region. Under high magnetic fields (155 mT and 258 mT), low current densities (<102 A/cm2) correspond to low forward bias (<6 V) and display negative MC whereas high current densities (>102 A/cm2) correspond to large forward bias (>6 V) and display positive MC. The onset for measurable electroluminescence in this device is at 2  102 A/cm2, corresponding to positive differential current and large forward bias conditions (>6 V). The positive bias results in Fig. 4 can be split into three distinct regions: Low bias, below 1 V (104 A/cm2), where the saturation MC response is negative and has a magnitude approximately the same as the saturation MC under negative bias. Under medium bias, from 1 to 5 V (104 A/cm2 to 2  102 A/cm2), the saturation MC is negative and decreasing in magnitude as the bias increases. Under large bias, greater than 5 V (>2  102 A/cm2) the

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j (A/cm ) Fig. 4. Differential current obtained under several magnetic fields (7 mT squares, 30 mT circles, 155 mT up triangles and 258 mT down triangles) versus current density for the Au/P3HT(300 nm)/Al diode in forward (Au bottom electrode positive, red symbols) and reverse (Al top electrode positive, black symbols) bias at room temperature (left hand axis). The corresponding electroluminescence (blue diamonds) versus current density in forward bias is plotted against the right hand axis. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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saturation MC becomes positive. In the first region, <1 V, the electron injection is expected to be negligible and hence the high field MC is dominated by the bipolaron process. Despite this there is some evidence of the positive ‘‘spike’’ at low fields which increases in magnitude as the bias is increased and this suggests that there is another process occurring, probably related to the fact that the electron injection is not zero. As the applied bias is further increased, the magnitude of the saturation MC decreases until at a bias of 5 V the high field MC response becomes positive. This voltage corresponds to the point where we are able to detect light emission from the sample in our system and hence where excitonic effects are expected to dominate. As we have mentioned previously, this does not mean that excitons are not present at lower bias values, just that our photodetector is not sufficiently sensitive to measure them. In forward bias the negative saturation MC measured at low current densities (<4  104 A/cm2) is in good agreement, both qualitatively and quantitatively, with previously published results [37] at room temperature. The high current (>2  102 A/cm2) positive saturation MC agrees qualitatively with the reduced hole mobility due to the presence of excited states in P3HT reported by Song et al. [11]. It is worth noting that positive MC has been previously reported in P3HT at high bias [37], but at significantly reduced temperature compared to our results (200 K). Given that the samples reported by Nguyen et al. [25] contained the hole injection layer PEDOT:PSS and that this material displays significant interfacial effects, the discrepancy may be due to the magnetic behaviour of the injection layer. We note that in our device the transition from negative saturation MC to positive saturation MC occurs when the current density exceeds 102 A cm2. The bipolaron theory [8] identifies two competing effects: (i) transport blocking through bipolaron states (negative MC), and (ii) an increase in polaron population at the expense of bipolarons with increasing magnetic field (positive MC). When the long-range Coulombic repulsion between polarons is sufficiently large, (ii) will start to dominate and positive MC will occur. In our experiment, however, the current density increase will not affect this long-range Coulombic repulsion between polarons, so the positive MC cannot be attributed to bipolaron theory. In contrast, the tripletpolaron theory predicts positive MC due to a decrease in triplet states resulting from a magnetic field dependent polaron pair interactions. This decrease in triplet states results in a decrease in site blocking or polaron trapping which in turn increases the current (positive MC). In the regime where the MC is positive (i.e. at current densities exceeding 102 A cm2) and assuming that the current is still hole dominated, using literature values for the hole mobility in P3HT, we can calculate the hole density in our device from the measured current density. At a current density of 102 A cm2 we determine the hole density, nh to be P2.1  1016 cm3 (see Supporting information). Using the luminescence measurement lower limit for detection (1010 W in our set up) and the photon energy and triplet lifetime for P3HT [42] we can calculate the triplet density, nt, at the onset of measurable luminescence to

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H. Gu et al. / Organic Electronics 15 (2014) 1711–1716

be 1015 cm3 (see Supporting information). Therefore, the transition from negative to positive MC occurs when the triplet density becomes comparable to the hole density. We note that both of them are lower than the theoretical upper limit of the hopping carrier site density which we take to be less than or equal to the density of monomer units, nsite 6 nmonomer = 3.8  1021 cm3, calculated using literature values [43] (see Supporting information). 4. Conclusions Our results, based on a Au/P3HT/Al device, show that under low reverse bias, where the device is operating under hole only conditions, the negative MC can only be explained by the bipolaron mechanism. This is confirmed by fitting using a single non-Lorentzian function. Under high magnetic fields (P100 mT) the negative MC in low forward bias is also attributed to the bipolaron mechanism although we suggest there may be some small contribution from electron injection. The positive MC under high forward bias and high field conditions is explained using the triplet-polaron interaction mechanism and this is confirmed by measured electroluminescence. The high magnetic field MC transition from negative to positive occurs at a current density (102 A cm2) when the triplet density (1015 cm3) becomes comparable to the hole density (1016 cm3), making site blocking and other interactions probable. Under appropriate drive conditions (104102 A cm2) both mechanisms occur simultaneously within our device and the overall sign of the MC results from competition between them. Acknowledgement The work was supported by the China Scholarship Council. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.04.036. References [1] W. Wagemans, P. Janssen, A.J. Schellekens, F.L. Bloom, P.A. Bobbert, B. Koopmans, SPIN 1 (1) (2011) 93. [2] H. Gu, X. Zhang, H. Wei, Y. Huang, S. Wei, Z. Guo, Chem. Soc. Rev. 42 (13) (2013) 5907. [3] R.C. Johnson, R.E. Merrifield, Phys. Rev. B 1 (2) (1970) 896. [4] J. Kalinowski, M. Cocchi, D. Virgili, P. Di Marco, V. Fattori, Chem. Phys. Lett. 380 (5–6) (2003) 710. [5] V.N. Prigodin, J.D. Bergeson, D.M. Lincoln, A.J. Epstein, Synth. Met. 156 (9–10) (2006) 757. [6] B. Hu, Y. Wu, Nat. Mater. 6 (12) (2007) 985. [7] P. Desai, P. Shakya, T. Kreouzis, W.P. Gillin, N.A. Morley, M.R.J. Gibbs, Phys. Rev. B 75 (9) (2007) 094423. [8] P.A. Bobbert, T.D. Nguyen, F.W.A. van Oost, B. Koopmans, M. Wohlgenannt, Phys. Rev. Lett. 99 (21) (2007) 216801.

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