Band-like temperature dependence of mobility in a solution-processed organic semiconductor

Band-like temperature dependence of mobility in a solution-processed organic...

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Modulation voltage

Monochromator

Light source

Semitransparent gate electrode

Absorbance (a.u.)

Neutral Cation

Difference of absorbance (a.u.)

Input Ref

Absorbance (a.u.) 0

300 K 250 K 200 K 150 K 100 K a b c d Δ Δ

Figure 3 | Optical spectroscopy of charge carriers in TIPS-pentacene. a, Schematic of the experimental set-up for CMS measurements. b, Chemical doping absorption spectra of isolated, neutral and radical-cation TIPS-pentacene molecules in dilute solution. Spectrum 1 corresponds to the neutral-molecule absorption in undoped solution. Spectra 2–4 correspond to increasing levels of chemical doping. In spectrum 5 all molecules in solution have been doped. The close correspondence between the solution spectrum and the CMS data that are obtained at very low charge concentrations, of the order of 1012 cm−2, makes it likely that the main charged species observed is a radical cation as opposed to a dication. The inset shows an energy diagram with the expected optical transitions for the neutral molecule and the radical cation. c, Temperature-dependent CMS spectra of TIPS-pentacene FETs (L=40µm, 450-nm-thick dielectric). The thin-film, ultraviolet–visible absorption spectrum of a TIPS-pentacene film prepared on a polyimide-coated glass substrate is also shown for comparison (light blue). At room temperature the optical cross-sections of the CMS charge-induced absorption are 1.6×10−16 and 6.7×10−17 cm2 at 1.24 and 2.6eV, respectively. They are of the same order of magnitude as those determined from the chemical doping spectra. d, Comparison of the CMS spectrum at 43K (black) with the difference between the absorption spectra of a chemically doped and an undoped solution (red).

which manifests itself in the room-temperature characteristics as a slight suppression of the current at low drain voltages (|VD|<5V). By comparing four-point and two-point probe measurements, we found that contact-corrected and uncorrected mobilities exhibit very similar temperature dependence with the same activation energy (Supplementary Fig. S4). Below 140K one might argue that the method of extracting effective mobilities from the saturated transfer characteristics at VD = −30V is not reliable because the characteristics no longer exhibit a well-defined saturation regime.

However, even at such low temperature we can still see indications that the device would enter saturation if application of even higher drain voltages was not prevented by dielectric breakdown (Fig. 2a and Supplementary Fig. S1). The effective mobility values extracted in this regime of, for example, 6cm2 V−1 s−1 at 4.3K, are therefore likely to be an underestimate of the real mobility values, because the true saturated current values are higher than the currents reached at VD =−30V. These electrical characteristics, particularly the pronounced nonlinear lateral-field dependence of the low-temperature transport, are reminiscent of those observed recently in polythiophenebased FETs (refs 15–17), which were interpreted as a manifestation of Luttinger-liquid physics17. For our TIPS-pentacene system we propose a different explanation based on optical charge modulation spectroscopy (CMS). Localized, polaronic charges in organic semiconductors have characteristic optical absorptions differing from the neutral pi–pi∗ absorption, such as the transitions between the highest occupied molecular orbital (HOMO) and the singly occupied molecular orbital (SOMO), and between the SOMO and the lowest unoccupied molecular orbital23,24 (LUMO) of the radical cation (see inset of Fig. 3b). CMS detects these by measuring changes 1T of the optical transmission T through the semitransparent FET on modulation of the gate voltage/carrier concentration21,25 (Fig. 3a). CMS provides us with a direct, spectroscopic probe to prove that the electrical characteristics reflect real changes in the molecular nature of the charge carriers in the accumulation layer as a function of temperature and electric field, and are not due to artefacts caused by the method of mobility extraction or by contact effects.

We first carried out spectroscopy of isolated charged molecules in dilute solution (Fig. 3b). When adding successive amounts of the oxidative dopant FeCl3 to a solution of neutral TIPS-pentacene, the characteristicHOMO–LUMOabsorptionataround1.8–2.5eVwas bleached, and characteristic charge-induced absorptions appeared at 0.9–1.7eV with a pronounced vibronic structure. The peak at 1.32eV with a vibronic replica at 1.48eV can be attributed to the HOMO–SOMO transition, whereas the peak at 0.94eV could be due to the SOMO–LUMO transition. We also detect a higherlying charge-induced absorption around 2.8eV. The isolated radical-cation spectrum in dilute solution provides a reference for interpretingtheCMSdataobtainedinthecrystalline,solidstate.

NATUREMATERIALSDOI:10.1038/NMAT2825 LETTERS

¬10 V ¬15 V

0 V, 300 K Energy (eV)

T/T (a.u.) VD = 0 V

Figure 4 | Drain-voltage dependence of the CMS spectra at 100K of a TIPS-pentacene FET with L=5µm and a 250-nm-thick dielectric. For comparison, the corresponding spectrum at 300K and VD =0V is also shown. All spectra were acquired with a gate voltage of −20V and a modulation bias of ±2V at 37Hz.

The CMS spectra of TIPS-pentacene FETs taken at zero lateral electric field exhibit a characteristic bleaching region around 1.8– 2.2eV (1T/T > 0), owing to the reduction of the number of neutral molecules on charge injection (Fig. 3c). Two of the vibronic peaks observed in the thin-film absorption spectrum at 1.8 and 2.11eV are also observed in the CMS spectrum (the third peak at 1.96eV is visible in CMS as a weak shoulder). The shape of the bleaching signal is different because it is superimposed by a broad charge-induced absorption. We detect two chargeinduced absorption regions (1T/T <0) with peaks around 1.2eV and 2.7eV. The latter, which corresponds to the high-energy radical-cation transition in the chemical doping spectra, does not show much temperature dependence. However, the lower-energy feature exhibits an interesting temperature dependence. Near room temperature, the charge-induced absorption between 1.2–1.9eV is broad and featureless without apparent vibronic structure. However, below 150–200K the peak around 1.24eV sharpens considerably and develops a vibronic replica around 1.45eV. At low temperatures, the spectral shape becomes very similar to that of the isolated radical cation in solution although the peak position in the solid state is shifted by 80meV to lower energies (Fig. 3d), possibly owing to differences in polarization between the solution and solid state.

The temperature at which this sharpening of the CMS spectrum is observed corresponds well to the temperature at which the mobility at intermediate lateral electric fields crosses over from a thermally activated to a temperature-independent/band-like regime. We conclude that the thermal activation of the mobility is due to trapping of charges in shallow traps in the organic semiconductor, which localize the charges onto single TIPS- pentacene molecules. We can exclude trapping on chemically modified TIPS-pentacene molecules, other chemical impurities or states in the gate dielectric, because one would then expect the CMS spectrum of the trapped charge to differ significantly from that of the isolated radical cation. The shallow trap states could in principle be associated with misaligned molecules at grain boundaries. However, we consider it more likely that they are associated with structural defects within the grains, such as static molecular sliding defects at the active interface26, because our channel length is significantly shorter than typical crystalline domainsizes(insetofFig. 1b),andtheactivationenergyissmalland similartotheonemeasuredinsingle-grainpentaceneFETs(ref.27).

We can also draw important conclusions about charge transport at room temperature. The mere observation of a pronounced charge-induced absorption resembling that of the radical cation in solution suggests that, in spite of the negative temperature coefficient of the effective mobility, the charges are not fully extended, but remain localized over a certain number N of molecules. N is not necessarily equal to one as for the shallow trap state occupied at low temperatures, because the room-temperature CMS spectrum is much broader than the radical-cation solution spectrum, and lacks vibronic structure. However, N can not be macroscopic either because each individual molecule carrying a fractional charge e/N would then be expected to exhibit an optical absorption similar to that of the neutral molecule, and only a broad Drude-like optical response would be expected in CMS (refs 10, 28). This is clearly inconsistent with the room-temperature CMS spectrum,particularlythepronouncedhigh-energycharge-induced absorption at 2.8eV. For vacuum-sublimed pentacene FETs N was estimatedtobe∼10fromelectron-spinresonanceexperiments29.

The simultaneous observation of a negative temperature coefficient of the effective mobility and a pronounced chargeinduced absorption of the carriers is consistent with the localization of the charge carriers being brought about, not by polaron selflocalization, but by dynamic, intermolecular lattice disorder7. At any time, charges can be considered to be effectively localized in a region bound by sites where unfavourable configurations for intermolecular charge transfer are encountered. As CMS effectively averages over all such charge configurations in the accumulation layer, this could explain the broad, featureless room-temperature charge-induced absorption. In such dynamic-disorder-limited transport the temperature dependence of the mobility arises from freezing of low-energy intermolecular vibrations7.

Finally, we discuss the origin of the nonlinear dependence of the low-temperature transport on the lateral electric field. We have carried out low-temperature CMS experiments with a constant lateral electric field applied along the FET channel (Fig. 4).

The sharp, trapped-charge absorption peak at 1.24eV (VD = 0V) becomesbroadenedwithincreasinglateralfield.TheCMSspectrum at 100K and VD =−10V resembles the one taken at 150K without applied drain voltage, and the spectrum at 100K and VD = −15V is very similar to the broad, featureless charge-induced absorption observed at 300K without drain voltage. This provides clear spectroscopic evidence that charges in single-molecule, shallow trap states can be effectively detrapped into more mobile states by application of the source–drain electric field. It is this process that is responsible for the nonlinear transport properties observed in TIPS-pentaceneFETsatlowtemperature.Ourresultsareconsistent with previous models of temperature-independent field-emission tunnelling at low temperature18,19. We have fitted the electric-field dependence of the temperature-independent effective mobility in the range 4–20K (Fig. 2c) to a simple Fowler–Nordheim tunnelling expression through a triangular barrier, and have extracted barrier heights/trap depths of 13–17meV (Supplementary Fig. S5). Such small values are consistent with the trap depth estimated from the activation energy of the low-field mobility.

Methods

To investigate surface transport in highly ordered thin organic films over a wide temperature range we selected TIPS-pentacene in contact with a perfluorinated, low-dielectric-constant Cytop polymer gate dielectric. We use a top-gate, bottom-contact device architecture (inset of Fig. 1a) with spin-coated films of

LETTERS NATUREMATERIALSDOI:10.1038/NMAT2825

TIPS-pentacene and Cytop. A polyimide layer was inserted to control the surface wettability of the glass substrate. On polyimide, TIPS-pentacene formed uniform, polycrystallinefilmswithalargedomainsizeofover100µm(insetofFig. 1b).

A 50-nm-thick precursor of polyimide (PI-2525, HD MicroSystems) was spin-coated onto the cleaned glass substrates and cured in a nitrogen atmosphere at 160 C for 1h, followed by 300 C for 3h. Standard lift-off photolithography was carried out on the polyimide-coated glass to define the 10-nm-thick Au source and drain electrodes. The Au patterned substrates were cleaned by acetone and isopropyl alcohol with ultrasonication, and treated with oxygen plasma at 150W for 1min. The gold contacts were treated with a 10mM solution of pentafluorobenzene thiol in isopropyl alcohol for 2min to reduce contact resistance. The TIPS-pentacene and Cytop layers were prepared by spin-coating in a nitrogen atmosphere. A 10mgml solution of TIPS-pentacene in tetralin was spun at 1,000r.p.m. for 1min, followed by drying on a hotplate at 100 C for 5min. A layer of Cytop (obtained from Asahi Glass Co.) was successively formed on the TIPS-pentacene film and dried at 90 C for 20min. For measurements at high lateral electric field/source–drain voltage in devices with a short, L = 5 µm, channel length we reduced the thickness d of the gate dielectric to 120–250nm to ensure correct device scaling and obtain clean saturation characteristics at room temperature. For longer-channel, L=40µm, devices we used d =450nm. The CMS measurements in Fig. 3 were acquired in accumulation mode with a d.c. gate voltage of −35V and an a.c. modulation bias of ±5V at 37Hz without drain voltage (V =0). The capacitances of the Cytop layers were 28nFcm , 14nFcm and 5.2nFcm for 120-nm-, 250-nm- and 450-nm-thick films, respectively. These were determined by measuring metal–insulator–metal diode structures of Al/Cytop/Al. For the gate electrode, a 6–20-nm-thick aluminium electrode was preparedbyvacuumevaporationthroughashadowmask.

The chemical doping was carried out by adding a concentrated (1×10 M)FeCl chloroformsolutionintothedilute(1×10 M)TIPS-pentacene chloroform solution. FeCl solution (140µl) was added to 2ml of TIPS-pentacene solution to obtain a fully oxidized spectrum (5 in Fig. 3b). Some unreacted FeCl molecules remain in solution.

References

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Acknowledgements

We thank M. Caironi and M. Bird for many useful discussions, and acknowledge financial supportfromtheTechnologyStrategyBoard(TSB)throughthePOSTEDproject.

Author contributions

T.S. carried out the experiments. T.S. and H.S. developed the interpretation of the data and wrote the manuscript.

Additional information

The authors declare no competing financial interests. Supplementary information accompanies this paper on w.nature.com/naturematerials. Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions. CorrespondenceandrequestsformaterialsshouldbeaddressedtoT.S.orH.S.

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