Presented on American Physical Society March Meeting, March 16-20, 2009, Pittsburgh, PA,USA.

Transcription

1 Presented on American Physical Society March Meeting, March 16-20, 2009, Pittsburgh, PA,USA. A highly-ordered, high mobility organic semiconductor grown from a mesophase: A test of polaron band theory Naresh Shakya, Chandra Pokhrel, Brett Ellman, Shin - Woong Kang, and Satyen Kumar Department of Physics, Kent State University Yulia Getmanenko and Robert J. Twieg Department of Chemistry, Kent State University Acknowledgments This work was supported, in part, by the National Science Foundation and by an the Ohio Board of Regents Research Challenge award. Use of the Advanced Photon Source (APS) was supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Office of Science. The Midwestern Universities Collaborative Access Team s (MUCAT) sector at the APS is supported by the U.S. DOE, BES, Office of Science, through theames Laboratory.

2 Introduction Why organic semiconductors? (comparing with Si or Ge) Often easier to purify Good solubility in organic solvents Low temperature processing Charge carrier mobility is comparable to amorphous silicon thin films Enable one to fabricate uniform large-area thin films Compatible with flexible substrates Parameters effecting mobility Aromaticity Closely packed molecular alignment Large overlap or transfer integral High positional and rotational order

3 Molecular Structure of the sample 1,4-di-(5-n-tridecylthien-2-yl)-benzene A thiophene-benzene-thiophene core with alkyl tails Calamitic liquid crystal Working Formula for the Experiment d E = Charge carrier mobility d= Thickness of the sample E= Electric field across the sample =Time of flight

4 Experimental Set up Figure 2-6: The basic set up for a carrier mobility measurement by the time of flight technique- (22)Voltage wire (23)Signal wire (24)ITO transparent electrode (25)Commercially available ITO coated glass substrate (26)Sample inside the cell (27)Laser light (28)Diodes (29)Current amplifier (30)Lecroy Wave Runner LT364L Digital Oscilloscope (31)SRS (Stanford Research Systems) Model DS345 30MHz Synthesized Function Generator or SRS Model PS325 / 2500 V voltage source (32)Capacitor (33) Voltage gain amplifier.

5 Study of Mesophases Differential scanning calorimetry (DSC) Polarized light microscopy Powder x-ray diffraction DSC diagram The phase sequence for cooling [1] I C SmF C SmG 91.7 C SmH 76.6 C Cr1 [1] - The smectic-f phase is relatively unusual, with 363 entries in the Liq.Crys 2003 database. A very useful review of the various smectics may be found in Gray and Goodby, Smectic Liquid Crystals, p. 153 (2004).

6 Intensity Powder x-ray diffraction [2] C C 44.0 C SmF SmG Cr1 2 Angle (Degree) Note both the high degree of order in the mesophase o In the SmF phase, it shows very large positional order o We measured the following correlation lengths () w.r.t. smectic planes. = orthogonal = nm = parallel = nm Note the similarity of the structures of the smectic and crystalline phases. [2] Powder x-ray studies were performed at the Advanced Photon Source at Argonne National Laboratory using A photons with the sample contained in a Lindeman capillary.

7 The Time of Flight Traces For μ ( hole ) vs. T at constant E For μ ( hole ) vs. E constant T = 129 C

8 T dependence of µ at constant E max > 0.1 cm 2 /Vs at lowest T ~ 6 x 10-3 cm 2 /Vs at highest T E Electric field µ Mobility T Temperature

9 E Electric field µ Mobility T Temperature E dependence of µ at constant T

10 Theory We report here studies of a material that exhibits polaron band behavior Basic Theory by T.Holstein [3] Modified by D.Emin for non-adiabatic transport o deals with the hopping conduction with activated like temperature dependence [4] Modified by R. Silbey and R. Munn for adiabatic transport o deals tunneling-dominated or band-like conduction [5] A general result of polaron band theories is that there is a narrow crossover temperature,t a, between non-adiabatic and adiabatic conduction, with the latter taking over at low T. ( T) H T µ H - Mobility due to hopping conduction µ T - Mobility due to band like conduction Both µ H and µ T are the functions of T, B,,, and g. B - electronic bandwidth - phonon bandwidth - optical charactersitic phonon frequency g - electron-phonon coupling Condition we consider : B, g 1, and For our results: above T a, μ(t) has positive slope below T a, μ(t) typically has negative slope and can exhibit qualitatively exponential behavior. [3] T. Holstein, Ann. of Phys. 8, 343 (1959).[4] D. Emin, Adv. Phys. 24, 305 (1975). [5] R. Silbey and R. Munn, J. Chem. Phys. 72, 2763 (1980).

11 Theory (Continue) ( T) H T T H ea 1 2 B 4 B [ I0( y) 1] exp h 2B 2B ea B 1 2 hi ( ) 1 0 y B = y n = 1/[exp( ) 1] =1/ k B T y = 4 g 2 [n(n+1)] 1/2 k B - Boltzman s constant a - intermolecular distance - modified phonon bandwidth n - phonon distribution function B B exp g 2 tanh 2 The hyperbolic Bessel function I 0 (y) is a sharply increasing function of its argument and therefore of T (y n and n increases with T). 1

12 Results 1. At constant E, decreases with increasing T. 2. At the highest T mesophase (SmF), was 6 x 10-3 cm 2 /Vs, and changed very slowly with T. However, the T - range available was small. 3. As T decreases, there is a sudden increase in at the transition from the SmF to SmG. 4. max was greater than 0.1 cm 2 /Vs at the lowest T [6]. Similar value has been found by K.Oikawa and his group for the related material. 5. The powder diffraction results suggest that the highly ordered high temperature SmF phase templates the formation of ordered crystalline SmG and then SmH [7]. 6. At the transition from SmG to SmH phases, there is no sudden change in the T dependence of. 7. is independent of E over a wide range of field at all accessible T. 8. Electron TOF traces were too dispersive to allow for mobility measurement. [6]K. Oikawa, H. Monobe, J. Takahashi, K. Tsuchiya, B. Heinrich, D. Guillon, and Y. Shimizu, Chem. Commun. p (2005). [7] H.Iino and J.Hanna, Japanese Journal of Applied Physics Vol. 45, No. 33, 2006, pp. L867 L870

13 Conclusions 1. Since the SmF phase templates the formation of crystalline SmG and then SmH, thin samples of a high-mobility crystalline organic may be grown with large-grained structure, which may have implications for device development. 2. Since there is not a distinct change in μ (T) for the transition from SmG to SmH phases, which differ only in the freedom of molecular rotation, the alignment of adjacent molecular cores does not seem to play an important role in the transport. 3. The sizable jump in mobility (a factor of 3.4) at the transition from the SmF to the much more ordered SmG phase, - may be due to decreased dynamic disorder due to the loss of the fluctuations in the SmF phase on cooling. The main difference between SmF and SmG phases is that SmG has stronger correlation between smectic layers than SmF.

14 Conclusions (Continue) 4. This material is of real interest, e.g., thin-film transistor fabrication because of its high mobility at low T. 5. To our knowledge, this is the first SmH material studied, and the first very high mobility smectic with polaron band-like transport. 6. We report here studies of a material that appears to exhibit polaron band behavior. However, while qualitatively consistent with transport via polaron bands, we find that it is quantitatively difficult to explain the data with physically realistic parameters. In particular, the data demand either quite large typical optical phonon frequencies and/or phonon bandwidths. Letting the phonon bandwidth take on a maximal value, say = 0.5, we find that the data may be fit, albeit with = T k B / with T 400 K. h h

15 Thanks Colleague-Mr. Chandra Pokharel Prof. Robert J. Twieg Prof. Satyan Kumar Dr. Shin-Woong Kang Dr. Yulia Getmanenko