C60 double layer

Solid State Communications, Vol. 99, No. 9, pp. 623-626, 1996 Coqyri$t 0 ‘“6 Elsevi~r Science Ltd Printed m rest Bntam. All nghts reserved 0038-1098/96 S12.00+ .OO

Pergamon

PII soo38-1098(96)0019&6

ELECTRICALLY

DETECTED ELECTRON SPIN RESONANCE (EDESR) IN THIOPHENE FILMS AND A THIOPHENE& DOUBLE LAYER

THIN

A. Maier, A. Grupp and M. Mehring Physikalisches Institut, Universitat Stuttgart, 70550 Stuttgart, Germany (Received 20 February 1996; accepted 25 March 1996 by M. Cardona)

We report on the investigation of EDESR signals under light illumination in thin films of an end-capped oligothiophen with four thiophene rings (EC4T) and a molecular photodiode made of a double layer of EC4T& between gold electrodes. The different EDESR signals are interpreted in terms of photogenerated positive (P+) and negative (P-) polarons which undergo spin-dependent recombination. Copyright 0 1996 Elsevier Science Ltd Keywords: A. ferroelectrics, A. fullerenes, A. thin films, D. photoconductivity and photovoltaics, E. electron paramagnetic resonance.

1. INTRODUCTION Conventional electron spin resonance (ESR) is a widely applied spectroscopic technique which leads to detailed information on localized and delocalized electron spins in solids and liquids. In the solid state the ESR lines are usually broad (several mT) and a spin concentration of 10’3-10’4 is usually required in order to achieve reasonable signal-to-noise ration. It was observed, however, some time ago that ESR cannot only be detected in the conventional way by measuring the microwave absorption, but also by observing the change in conductivity under microwave irradiation while sweeping the magnetic field through resonance [l-7]. This socalled electrically detected ESR (EDESR) is based on the spin dependent recombination of spin carrying charge carriers. The sensitivity of this technique seems to be much higher than the conventional technique. In semiconductors [6] and organic solids [7] this technique allows us to observe fine details which are not accessible by conventional ESR. Different models have been proposed in the past in order to account for the change in conductivity when electron spin transitions are induced by microwave irradiation [l-6]. The physics involved is basically connected with the fact that recombination of charge carriers can only be accomplished when both carriers form a singlet state. The recombination rate and therefore the concentration of charge carriers depends on the singlet character of the recombining

pair. In general, however, there are four different spinstates in such a pair, which can be expressed as a mixture of three triplet states and one singlet state. Microwave irradiation causes transitions between these states and therefore leads to a change in the population of the singlet state resulting in a change of charge carrier concentration. In this contribution we will demonstrate EDESR under light illumination in two different samples, namely thin films of a four ring oligothiophene (EC4T) and a double layer consisting of an oligothiophene and a Cm layer between two gold electrodes. We observe in both films a reduction of the conductivity under ESR resonance conditions. This is similar to what has been observed in semiconductors and molecular films. The thiophene-Cm combination seems to be particularly attractive because of the donor properties of the oligothiophenes (depending on their chain length) and the acceptor properties of Cso. It has been demonstrated recently that electron transfer occurs between oligothiophene and Cm under light illumination [8].

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2. EXPERIMENTAL

The experimental layout is sketched in Fig. 1 where only the double layer is shown. A gold film of 1Onm thickness was evaporated under high vacuum conditions on a quartz substrate, followed by the thiophene and Cm film each of about 100 nm

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Fig. 1. Scheme of the EC4T/C& double layer sample. thickness. On top of the molecular film another gold electrode was evaporated also of about 1Onm thickness. The C, material was purchased from Hoechst company (gold grade), whereas the EC4T material was donated by P. Bauerle. The sample was handled in a glove box under dry Nz gas. Light illumination was performed with a standard halogen lamp by focussing on the sample in a flow cryostat. I/V characteristics and in addition open circuit voltages in the case of the double layer were measured with standard equipment. The voltage or voltage drop across the sample was fed into a high impedance amplifier and connected to a lock-in amplifier. Magnetic field modulation and microwave irradiation was applied in the usual ESR manner using a commercial X-band ESR spectrometer (Bruker ESP 300). A dielectric resonator was used. Because of the rather slow response of the electrical signal on the modulation and microwave irradiation (low mobility) we used low modulation frequencies of the order of 100 Hz and below. 3. RESULTS AND DISCUSSION Electrically and chemically oxidized end-capped oligothiophenes have been investigated in detail in the past [9, lo]. Also the photogeneration of charged polarons in poly- and oligothiophenes has been established [ 111.Their electronic and optical properties are well characterized and in particular they are known to be good electron donors under light excitation. We therefore constructed the thin film device as described in the experimental section in order to investigate the electrically detected ESR signal of photogenerated polarons. Fig. 2 (top) shows the EDESR signal (full line) of the oligothiophene (EC4T) film together with a simulation (dots) based on the superposition of two different Iines as shown in Fig. 2 (bottom). The change in conductivity under resonance conditions is rather

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Magnetic Field (rnll Fig. 2. EDESR signal (top) of an EC4T thin film sample at room temperature. The experimental curve is represented by a superposition of a Lorentzian and a Gaussian line (bottom). large and amounts to about 1%. We note that the calculated line is hardly distinguishable from the measured line. It should be noted that the simulated line consists of two lines with different shape, g-factor and width. The corresponding values are quoted in Fig. 2 (bottom) and Table 1. The linewidth ABijZ is defined here as the full width at half height (FWHH) of the integrated ESR signal. When lowering the temperature the Lorentzian line begins to broaden and reaches 1.9 mT at about 100K. Below this temperature the simulation with a Lorentzian lineshape is no longer possible. In the same temperature range the resistivity of the sample increases monotonically down to 1OOK and dramatically below 100K. We also note that a Am = 2 signal becomes visible below 100K. Table 1. ESR parameters of the EDESR signal in EC4T at room temperature ESR parameter

Gaussian line

Lorentzian line

g-factor FWHH ABi,*

2.0038 1.7mT

2.0023 1.4mT

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Our current understanding of these observations is based on the polaron model, where positive (P+) and negative (P-) polarons are created under light excitation. The mobility of these polarons leads to electrical conduction and their encounter to spin dependent recombination. Under microwave irradiation at resonance the singlet-triplet admixture of the wavefunction of the bound polaron pair is driven away from singlet character leading to a reduction in free charge carriers (separated polarons) and therefore to an increase in resistivity [12]. It is expected that the mobility of the positive and negative polarons might be quite different. We note that our thin films are polycrystalline and polaron transport must succeed across grain boundaries and crystalline clusters. The observed temperature dependence of the conductivity is therefore similar to variable range hopping. The observation of two different lineshapes, namely a Gaussian and a Lorentzian with two different g-factors is interpreted here in terms of localized (Gaussian) and delocalized (Lorentzian) polarons. This is consistent with the observation that at low temperatures where the resistivity rises steeply, the Lorentzian line converts into a Gaussian lineshape. We conclude from the different g-factors that the Gaussian line at room temperature corresponds to the negative polaron (P-), the Lorentzian line to the positive polaron (P+), which is obviously the mobile charge carrier. Before proceeding to the double layer structure we briefly quote some of our observations on EDESR signals on CbO thin films which partially agree with observations made by others [4, 131. The observed lineshape is more or less Lorentzian and rather broad (AB1lZ = 0.4mT, g = 2.0022). Based on the polaron model we propose the same scenario as outlined for EC4T, with the difference that in Cso both polarons (P+ and P-) seem to be mobile. Their different gfactors cannot be determined uniquely, but based on the known g-factor for C&g_ = 1.9998) and for C&(g+ = 2.0026) including some exchange coupling the EDESR signal can be simulated. Details will be published elsewhere. The electron transfer between thiophenes and Cm has been established by different methods [8, 141. It was therefore tempting to fabricate a double layer of EC4T and Cm with the goal to utilize the electron transfer properties of the two constituents in a molecular photoactive device. The I/V curve of the double layer structure under light illumination, consisting of a EC4T/Cm double layer between gold electrodes, is shown in Fig. 3 (top). Three different regions are noted: 1) reverse bias, 2) intermediate regime and 3) forward bias. The device therefore acts as a

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Fig. 3. I/V diagram of the EC4T/Ch0 double layer structure at room temperature under light illumination (top) and EDESR spectra recorded at different bias regimes (bottom). molecular photodiode. An open circuit voltage of 220mV is observed. From earlier investigations on electron transfer between EC4T and Cm it is known than an electron can be transferred from EC4T to &,, under light illumination [8] leading to the photogeneration of EC4Tf and C, polarons. The ESR signals and optical absorption spectra of these polarons have been characterized [lo, 81. EDESR signals were recorded in all three regimes at room temperature (Fig. 3 (bottom)). The EDESR signal in the forward bias regime (3) can be simulated by a combination of the EC4T and the Cm EDESR signals observed in the single layer structures. This observation also holds to some extent in the reversed bias regime (1) although there already the additional line feature appears which is most pronounced in the intermediate regime (2). The EDESR signal in the intermediate regime (2) is quite unusual. We propose that it arises from the interface region of EC4T and Cm. There the recombination of EC4T+ polarons with C$ polarons must

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occur. The expected lineshape is therefore due to a combination of Lorentzian lines of EC4Tf and C, polarons with different g-factors. Moreover, an exchange coupling J and a dipolar coupling D are expected in the bound polaron pair leading to a more complicated lineshape than just the overlay of two Lorentzians [12]. Investigations along these lines are in progress.

REFERENCES ::

3. 4. 5.

4. SUMMARY We have investigated electrically detected ESR signals under light illumination in thiophene thin fihns and a molecular photodiode consisting of a double layer of an end-capped oligothiophene (n = 4) and Cm between gold electrodes. The observed signals are interpreted in terms of photogenerated P+ and P- polarons which undergo spindependent recombination which is influenced by microwave irradiation in resonance with the ESR transitions. Mobile (P+) and immobile (P-) polarons were identified in EC4T thin films, whereas in the EC4T/Ca double layer the recombination of mobile EC4T+ polarons with mobile C& polarons has been proposed in the intermediate regime.

Acknowledgements - We are grateful to Professor BZiuerle for providing the endcapped oligothiophene material. The Deutsche Forschungsgemeinschaft (DFG-SFB 329) has given financial support.

6. 1. 8. 9. 10. 11. 12. 13. 14.

Lepine, D.J., Phys. Rev., B6, 1972,436. Kaplan, D., Salomon, I. and Mott, N.F., J. de Physique Lettres, 39, 1978, L-51. Rong, F.C., Buchwald, W.R., Pointdexter, E.H., Warren, W.L. and Keeble, D.J., Sol. State Electrons, 34, 1991, 835. Brandt, S.M., doctoral thesis University Stuttgart 1992. Lannoo, M., Stievenard, D., Deresmes, D. and Vuillaume, D., Mat.&. Forums, 143-147,1994, 1359. Stich, B., Greulich-Weber, S. and Spaeth, J.M., J. Appl. Phys., 77, 1995, 1546. Dyakonov, V., Gauss, N., Rossler, G., Karg, S., RieD, W. and Schwoerer, M., Chem. Phys., 189, 1994,687. Bennati, M., Grupp, A., Bluerle, P. and Mehring, M., (a) Mol. Cryst. Liq. Cryst., 256, 1994, 751, (b) Chem. Phys., 185,1994,221. Bauerle, P., G&z, G., Segelbacher, U., Huttenlecher, D. and Mehring, M. Synth. Met., 55-57, 1993,4786. Segelbacher, U., Sariciftci, N.S., Grupp, A., Bluerle, P. and Mehring, M., Synth. Met., 5557, 1993,4728. Smilowitz, L., Sariciftci, N.S., Wu, R., Gettinger, G., Heeger, A.J. and Wudl, F., Phys. Rev., B47, 1993, 13835. Mehring, M. (to be published). Eickelkamp, T., doctoral thesis University Stuttgart (to be published). Janssen, R.A.J., Moses, D., Sariciftci, N.S. and Heeger, A.J., Mol. Cryst. Liq. Cryst., 256, 1994, 921 ibid. p. 839.