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

Tomo Sakanoue* and Henning Sirringhaus*

The mobility µ of solution-processed organic semiconductors has improved markedly1,2 to room-temperature values of 1–5cm2 V−1 s−1. In spite of their growing technological importance3, the fundamental open question remains whether charges are localized onto individual molecules or exhibit extended-state band conduction like those in inorganic semiconductors4. The high bulk mobility of 100cm2 V−1 s−1 at 10K of some molecular single crystals5 provides clear evidence that extended-state conduction is possible in van-der-Waalsbonded solids at low temperatures. However, the nature of conduction at room temperature with mobilities close to the Ioffe–Regel limit remains controversial6. Here we investigate the origin of an apparent ‘band-like’, negative temperature coefficient of the mobility (dµ/dT < 0) in spin-coated films of 6,13-bis(triisopropylsilylethynyl)-pentacene. We use optical spectroscopy of gate-induced charge carriers to show that, at low temperature and small lateral electric field, charges become localized onto individual molecules in shallow trap states,butthatamoderatelateralelectricfieldisabletodetrap them resulting in highly nonlinear, low-temperature transport. The negative temperature coefficient of the mobility at high fields is not due to extended-state conduction but to localized transport limited by thermal lattice fluctuations.

Regarding the nature of charge transport at room temperature, some authors have suggested that the strong coupling to thermally excited intra- and intermolecular vibrations localizes the charge carriers7–9, whereas others have observed evidence of extendedstate conduction even at room temperature10,1. On the surface of thin organic films in field-effect transistor (FET) configurations, a negative temperature coefficient (dµ/dT <0), which is commonly considered as a fingerprint of band-like transport, has not yet been observed12,13. In most organic thin films, the fieldeffect mobility exhibits thermal activation behaviour determined by pronounced energetic disorder at the interface with the gate dielectric14. Recently, several high-mobility, semicrystalline conjugatedpolymershavebeenreportedtoexhibithighlynonlinear transport properties at low temperatures15,16. These have been interpreted as a manifestation of one-dimensional Luttinger liquid physics17,buttheirmicroscopicoriginremainsunderdebate18,19.

To investigate surface transport in highly ordered thin organic films over a wide temperature range we selected spin-coated 6,13- bis(triisopropylsilylethynyl) (TIPS)-pentacene films in contact with a perfluorinated, low-dielectric-constant polymer gate dielectric in a top-gate, bottom-contact FET architecture (inset of Fig. 1a). TIPS-pentacene formed uniform, polycrystalline films with a large domain size of over 100µm (inset of Fig. 1b). The devices exhibited p-channel FET characteristics with high room-temperature saturation and linear mobilities of 1.2cm2 V−1 s−1 and 0.8cm2 V−1 s−1, respectively, and negligible hysteresis in both output (Fig. 1a) and transfer characteristics (Fig. 1b).

Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK. *;

¬9 V ¬6 V b ¬1 V


Cytop Al

PFBT/Au TIPS-pentacene

VG = ¬12 V

200 µm

VD = ¬10 V

Figure 1 | Characteristics of a TIPS-pentacene top-gate transistor. a, Output characteristics of a device with channel length L=5µm, channel width W =100µm and a 120-nm-thick dielectric. The inset shows a schematic of the device structure. PFBT: pentafluorobenzene thiol. b, Transfer characteristics. The inset shows a polarized optical micrograph of a TIPS-pentacene film.

In devices with a relatively short channel length (L = 5µm), the temperature dependence of the transistor current varied strongly with applied electric field. For intermediate drain and gate voltages,

VD and VG, respectively (VD = VG = −15V), the FET current was nearly temperature independent between room temperature and 200K, but then decreased monotonously with decreasing temperature. At higher voltages (VD = VG = −30V), the FET current increased by ∼25% on cooling from room temperature to 140K, before dropping slightly at even lower temperatures. However, even at 4.3K the current remained about the same as


300 K 4.3 K

¬10 V

¬15 V ¬20 V

¬25 V ¬30 V

300 K 280 K

180 K~140 K

80 K 60 K

4.3 K 20 K

300 K 280 K 260 K 240 K 220 K 200 K 180 K 160 K 140 K 120 K 100 K

80 K 60 K 40 K 20 K 4.3 K

300 K 4.3 K

¬25 V

ID (mA)

VG = ¬5 V

Conductivity (S cm

I D1/2 (10

¬3 A

I D1/2 (10

¬3 A

300 K 260 K 220 K 180 K 140 K 100 K 60 K 20 K 4.3 K

300 K 260 K 220 K 180 K 140 K 100 K 60 K 20 K

4.3 KMobility (cm

2 V¬1

Temperature (K)

VD= ¬30 V

¬20 V ¬15 V a b cd e

Figure 2 | Temperature-dependent characteristics of TIPS-pentacene FETs with L=5µm, W =100µm and a 120-nm-thick dielectric. a, Temperature dependence of drain current ID as a function of VD at fixed VG =−30V. b, Temperature dependence of the conductivity defined as the ratio of ID/VD at fixed VD =−30V. Following refs 15–17, we calculated the conductivity by assuming the thickness of the accumulation layer to be 1nm and dividing ID by VD. c,d, Square root of drain current as a function of gate voltage, with drain voltages of VD =−15V (c) and VD =−30V (d). e, Temperature dependence of the effective mobility of TIPS-pentacene FETs with different VD.

at room temperature (Fig. 2a). The conductivity at high VD and

VG is nearly temperature independent (Fig. 2b), which is very similar to the behaviour reported recently in some semiconducting polymers15–17. Down to about 140K the output characteristics retained a near-textbook-like shape, with clearly defined linear and saturation regions. However, below 140K they acquired a distinctly positive curvature at intermediate drain voltages (Fig. 2a). Such nonlinear transport properties were observed only when high drain and gate voltages up to ∼−30V were applied to short-channel devices. When voltages were limited to −15V (Supplementary Fig. S1) or the channel length was long (L = 20µm), the output characteristicsshowedclearsaturationbehaviourevenat4.3K.

The increase of transistor current between room temperature and 140K demonstrates unambiguously that, in contrast to other organic FETs, charge transport in TIPS-pentacene FETs at sufficiently high applied voltages improves with decreasing temperature. It is interesting to interpret this behaviour in terms of the temperature dependence of the field-effect mobility, which can be extracted from the slope of the square root of the drain current in the saturation regime20 (Fig. 2c,d). This standard extraction method can be applied safely to the intermediate- the high-voltage characteristics (|VD/G| < 30V) down to about 140 K as the characteristics retain well-defined saturation regions.

With decreasing temperature the square root of the saturated drain current exhibits a positive curvature at low gate voltages and the threshold voltage shifts to more negative values. This is commonly observed in organic FETs and reflects the gate voltage filling up low-mobility trap states induced by residual disorder, and eventually populating more mobile states higher up in the interfacial density of states. The slope of the square root of the drain current at high gate voltages can be used to estimate the effective mobility of charges in these more mobile states. For an intermediate drain voltage, VD = −15V, we observe a slight increase of this effective mobility with decreasing temperature in the temperature range 200K < T < 300K (Fig. 2d), but for T < 200K the effective mobility becomes thermally activated with an activation energy, Ea = 5.7meV. This small value is consistent with the mobility becoming determined by a shallow trap state, and suggests that the degree of energetic disorder at the TIPS- pentacene/Cytop interface is significantly lower than in other solution-processed organic FETs (refs 21,2). For higher lateral electric fields (VD = −30 V), the slope of the saturated transfer characteristics at high gate voltages increases monotonically with decreasing temperature (Fig. 2e, Supplementary Fig. S2), and the extracted effective mobility reaches a value of 2.5 cm2 V−1 s−1 at 140 K (Fig. 2e). This negative temperature coefficient of the mobility is not an artefact of the extraction method. Clearly, to explain the increase of current down to 140K, the effective mobility hastoincrease.Infact,becausethemobilityisevidentlygate-voltage dependent, the effective mobility of charge states populated at higher gate voltages needs to increase even more strongly with decreasing temperature than the absolute current to compensate for the reduced contribution to the current from charges in lowermobility trap states populated at low gate voltages. We also used an alternative transconductance method for extracting mobilities (Supplementary Fig. S3; ref. 20), and similar behaviour was obtained. We checked the potential influence of contact resistance,

LETTERS NATUREMATERIALSDOI:10.1038/NMAT2825 Lock-in amplifier

Detector Cryostat

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