Band-like transport has been realized down to 20 K in solution-processed single-crystal transistors based on dioctylbenzothienobenzothiophene. The mobility increases from 16 to 52 cm2/V s as the temperature is lowered from 300 to 80 K. An abrupt mobility drop is observed around 80 K, but even below 80 K, gradually increasing mobility is restored again down to 20 K instead of thermally activated transport. From the observation of a shoulder structure in the transfer curve, the mobility drop is attributed to a discrete trap state.
It has been reported that mobility measured by the time-of-flight (TOF) method using a high-quality organic crystal increases down to liquid-helium temperatures to reach several hundred cm2/V s.1 Since thin-film organic transistors contain substantial charge trap states,2–4 intrinsic properties of organic semiconductors have been investigated mainly in single crystals.5–7 Recently, band-like charge transport, which means the mobility μ increases with decreasing the temperature, has been reported in single crystals as well as polycrystalline thin films fabricated from solution process.8–13 Eliminating extrinsic disorders at the semiconductor/dielectric interface is an important issue, so one of the most preferable device structures is vacuum gate dielectric combined with highly pure single crystals grown by physical vapor transport.6 However, band transport realized in organic transistors turns to semiconducting transport typically at a temperature between 150 and 200 K because the extrinsic effects at the crystal surface become gradually important at low temperatures.5 It is therefore necessary to further reduce the extrinsic disorders to maintain the band transport below 100 K. Recently, excellent single-crystal transistors have been fabricated by the combined use of self-organized phase-separation and solvent vapor annealing methods; an organic semiconductor, dioctylbenzothienobenzothiophene (C8-BTBT), is spin coated together with poly(methylmethacrylate) (PMMA), and the crystals of C8-BTBT are grown by room-temperature annealing in chloroform vapor.11,14,15 The resulting transistors have exhibited gradually increasing mobility down to 100 K. This method demonstrates that the use of as-grown crystals is crucial to realize band transport down to low temperatures.
In this letter, we show variable temperature analysis of solution-processed single-crystal transistors based on C8-BTBT. We have found clear band-like transport following the power law of μ ∼ T−n by using solution-processed carbon contacts, where μ continuously increases from 300 to 80 K. An abrupt mobility drop is observed between 80 and 130 K depending on the samples, while increasing μ is further maintained down to 20 K. The sample dependence is discussed in view of the difference of the trap states.2,3
C8-BTBT was synthesized following the reported method,16 and purified by repeated recrystallization in hexane. For fabrication of organic transistors, we adapted the self-organized phase-separation method to achieve high quality interface.11 A 2 wt. % chlorobenzene solution of C8-BTBT and PMMA (Aldrich, MW = 10 000) with the weight ratio of 1:1 was spin-coated (2000 rpm, 40 s) on a highly doped n-type silicon wafer with a thermally grown silicon dioxide layer of 300 nm thickness, and a bilayer structure was formed as depicted in Fig. 1(a). Then, the sample was exposed in chloroform vapor overnight, and single crystals of C8-BTBT with several hundreds of micrometer length grew on the PMMA dielectric layer. Two-terminal organic transistors were fabricated with carbon contacts (Dotite XC-12, Fujikura Kasei Co.) as depicted in Fig. 1(a). Figure 1(b) shows an optical image of the organic transistors. The solution-processed carbon contacts are crucial to achieve high-quality as-grown single-crystal devices. The resulting overall capacitance of the gate dielectric was measured to be C = 10 nF/cm2 by using a LCR meter at 20 Hz for a spin-coated PMMA-only film. We conducted variable temperature measurements of organic single-crystal transistors under the vacuum of 10−4 Pa using a low-temperature micro prober system (Riko International). Transfer characteristics were measured in the saturation regime at comparatively high drain voltage of VD = −40 V in order to minimize contact effect.
The transfer characteristics of representative three samples are shown in Fig. 1(c). The logarithmic plots demonstrate that the onset voltages are close to 0 V except for sample 3. This is much improved in comparison with the previous report,11 which implies the use of carbon contacts is preferable for high-performance organic transistors using as-grown single crystals. The transistors show as high mobility as 10 cm2/V s and on-off ratios more than 105 at room temperature. Although the room-temperature characteristics (Fig. 1(c)) look excellent for all samples, close investigation of the low-temperature characteristics reveals considerable sample dependence (Fig. 2). With lowering the temperature, the mobility evaluated from the square drain current ID increases down to about 100 K, and ID increases typically by 2.4 times. In the best device (sample 1), μ increases from 16 to 52 cm2/V s when the sample is cooled from 300 to 80 K (Fig. 3(a)). The inset of Fig. 3(a) demonstrates that the temperature dependence of μ follows a power law μ ∼ T−n at high temperatures, where n is as large as 1.6. Around 100 K, an abrupt drop of μ is observed, while μ again increases or at least shows nearly flat temperature dependence down to 20 K. For the measurements of twelve samples, we have reproducibly observed the abrupt drop approximately at the same temperature around 80 K as exemplified by samples 1 and 3. The drop is not due to the degradation of the crystals because at the heating run a similar jump is observed (Fig. 3(a)). Several samples start to show the drop at a higher temperature around 130 K as represented by sample 2. In such a case, a semiconducting gradual drop of μ is observed between 130 K and 80 K, while the band transport is restored again at lower temperatures. This observation demonstrates how the band-like transport is replaced by the activated transport when the trap amount increases. The previous single-crystal transistors have shown gradual change to the thermally activated transport between 150 and 200 K.5 In contrast, the present transistors basically maintain the band transport down to 20 K. Except for the abrupt drop, the temperature dependence of sample 1 is reminiscent of the TOF mobility.1
The temperature dependence of VT obtained from the ordinary extrapolation of the square ID plot is depicted in Fig. 3(b). The present transistors show remarkably reduced VT at room temperature, and the VT shift is comparatively small; in particular, sample 3 shows practically no VT shift. In sample 2, VT rapidly increases from 130 K to 80 K where μ shows the semiconducting drop.
Transfer characteristics of samples 1 and 2 below the μ drop are plotted logarithmically (Figs. 4(a) and 4(b)). The transfer curves are deviated from the ideal transistor characteristics and show clear shoulder structures. In sample 1, the shoulder abruptly appears at 60 K and persists below this temperature; whereas in sample 2, the shoulder gradually grows up from 130 K to 80 K but the structure is obscure below 80 K. Thus, a jump of VT is observed more or less around 80 K coming from the suppression of ID in the subthreshold region. A similar shoulder structure has been previously ascribed to the discrete trap states.3 As an attempt to investigate the origin of the mobility drop, transfer characteristics with a shoulder are reproduced by numerical simulation assuming a discrete localized state (Figs. 4(a) and 4(b)). The current drops in the subthreshold region appear because carriers are not fully activated into the valence band until the discrete level is filled. As we have discussed before,3 the discrete localized level is too shallow to be observed in the transfer curve at high temperatures, so the shoulder structure emerges only at the sufficiently low temperatures. Instead of the shoulder structure, sample 3 shows relatively large VON = −10 V associated with deeper trap levels, which suppress ID even at high temperatures. Therefore, the observed shoulder structure, together with the abrupt mobility drop and the VT anomaly, is attributable to the discrete trap level. From the behavior of sample 2, we can imagine how the gradual crossover from the band-like to thermally activated transport observed in the typical organic single-crystal transistors is induced by the accumulation of trap levels. It is noteworthy that, when we fabricate a very clean transistor, the transfer characteristics shows clear fine structures together with an abrupt drop of mobility at low temperatures instead of the gradual change to the activation process.
As another possibility, the shoulder structure is attributable to nonlinear contact resistance. Bias and temperature dependent contact resistance has been reported in organic transistors.17–21 The observed shoulder structure is explained if the contact resistance drops above a certain VG. In other words, there is a possibility that the 80 K drop is due to an anomalous increase of the contact resistance below this temperature, which gives significant influence particularly in the subthreshold region.
In order to further analyze the trap states, the activation energy EA is phenomenologically estimated in analogy with Lang's trap analysis from the Arrhenius plot of ID (Fig. 4(c)) in the high temperature range (290–220 K).22 We have used the limited temperature range to accurately estimate EA. For sample 1, EA is negative in all the VG range, whereas for samples 2 and 3, EA turns from positive to negative at −VG ∼ 3 V and 21, respectively. On the basis of the interface approximation and the exponential trap distribution, N(E) = NG exp[(E − EC)/kTG], we have previously predicted that EA becomes negative above a certain VG related to the total number of the trap states, VG = qQT/C = qNGkTG/C, where q is the elemental charge.2,3 This is because with increasing VG the activated transport is replaced by the band transport when all traps are filled. From the critical VG, we can regard sample 1 as almost trap free, while sample 2 still has a small number of traps corresponding to qQT/C = 3 V (1.9 × 1011/cm2). In sample 3, −VON = 10 V is subtracted from the crossing point at 21 V to give a relatively large number of traps qQT/C = 11 V (6.8 × 1011/cm2), though the global characteristics still follow the band-like behavior at large VG. Although the behavior around 80 K does not seem to be directly related to the total trap amount, the magnitude of the mobility, which decreases in the order of sample 1 > 2 > 3 (Fig. 4(c)), is closely related to the total trap number. The observed trap numbers, corresponding to 0 V, 3 V, and 11 V, are remarkably smaller than those of the ordinary organic thin-film transistors, in which qQT/C is typically several hundred volts (3 × 1013/cm2).2,3 Thus, the subthreshold temperature gradient is a good indicator of the trap amount.
In conclusion, in the solution-processed organic single-crystal transistors based on C8-BTBT, band transport is realized practically down to 20 K by using carbon contacts. In the best device, the mobility increases from 16 to 52 cm2/V s by following a power law μ ∼ T−n (n ∼ 1.6) as the temperature is the lowered from 300 to 80 K. However, an abrupt drop of μ is reproducibly observed around 80 K, where a shoulder appears in the transfer curves. Although it seems ironical, when we fabricate very clean transistors, the transfer characteristics show fine structures reflecting the trap states. The 80 K anomaly is probably not due to the intrinsic transition of the C8-BTBT crystal, but we could not exclude the possibility that the anomaly is associated with the extrinsic effect depending on the fabrication conditions, for example, coming from the PMMA layer or the contact effect. Since such influence is not too serious, μ increases again below 80 K. Then, the activated process is mostly eliminated, and the finally remaining instability is the 80 K anomaly.