The Study of Charge Carrier Transports in the Calamitic Liquid Crystals 5, 5 -di-(decyl-pyridin-yl) - 2 Bithiophene.

Transcription

1 The Study of Charge Carrier Transports in the Calamitic Liquid Crystals 5, 5 -di-(decyl-pyridin-yl) - 2 Bithiophene Naresh Shakya * C. Pokhrel* and B. Ellman* Y. Getmanenko** and R. J. Twieg** *Department of Physics and **Department of Chemistry Kent State University, OH Presented at American Physical Society March Meeting, March 15-19, 2010, Portland, OR,USA

2 Why organic semiconductors (OS)? (comparing with Si or Ge) Charge carrier mobilities are comparable to amorphous silicon (~ 1 cm 2 /Vs) Enable one to fabricate uniform large-area thin films devices (e.g., organic LED displays) Often easier to purify than inorganic semiconductors Compatible with flexible substrates and exhibit self healing of defects Applications of OS OLED ( Organic Light Emitting Diode) displays e.g. TV and computer monitors Organic Field Effect Transistor (OFET) Printing and photocopying machine Photovoltaic cells

3 Structure of Molecules ( C10) R= C 10 H 21 [5,5 -Di-(5-n- decyl-pyridin-2-yl)-2,2 -bithiophenes ] K [48.5 C] Sm1 [96.1 C] SmG/K1 [179.0 C] SmC [186.7 C] Isotropic Motivation Pyridine moieties form close packing induce very ordered smectic phases therefore, one might expect large mobilities. The lone electron pairs on sulfur actively participate in the delocalized -electron system close molecular packing large transfer integral one might expect large mobilities.

4 Time of Flight Technique Condition: penetration depth << L light pulse duration t e (10 ns) << E 1 1 E V0 x L d 2 dt L 1 2 Transimpedance Amplifier Current to Voltage x E 1 E 1 L = Charge carrier mobility L= Thickness of the sample E= Electric field across the sample =Time of flight E L E

5 Experimental Set up 1-10 ns /0.7 J or 120 ps Nd:YAG Laser Pulsed Laser Pulsed Rate : 10 Hz 4- Dichroic mirrors 2-Laser rod and optical harmonic generator 5- Pure Harmonic Light 3- SSP-Surelite Separation Package box 8-Fast trigger detector 7- Sampler 9 &10 - Reflectors 6- The half wave plate 15-4 K- 300K Cryostat 24- The beam collimator 11- Lens 13- Prism 12-Stimulated Raman Shifter 17- Polarized Ring Light 22- Voltage connecting wire 23- Signal connecting wire 18- Hot stage 14 & 16- Mirror 19- Sample 20- Polarizing microscope 21- CCD Camera 25-18" Vacuum Chamber

6 vs. T for C10 at E = 30,000 V/cm vs. E for C10 at T = 341 K 6

7 Same results under various conditions Heating and Cooling Processes The experiments were performed in C10 by varying T at constant E on both heating and cooling. Same Results Slow and fast Cooling The larger is expected for slow cooling from the isotropic because it reduces the energetic and spatial disorder making better molecular alignment. Same Results Materials were synthesized by two different mechanisms The experiments were performed on C10 materials prepared by two different methods. Same Results Slightly changing the aliphatic chain length The aliphatic chain. length was changed from C 10 H 21 for C10 to C 9 H 19 for C9. Same Results

8 vs. T for C9 at E = 50,000 V/cm C9 [5,5 -Di-(5-n-nonyl-pyridin-2-yl)-2,2 -bithiophenes ] K [4.2] Sm1 [59.7] SmG/K1 [185.1] SmC [187.1] N [188.3] Isotropic R= C 9 H 19 vs. E for C9 at T = 351 K 8

9 Models of hopping conduction that might be applicable to these materials Ordered System Emin Model (It cannot be applied for our data) (E) can arise due to the carrier s need to absorb/emit phonons as they jump from a site and relax from their field-accelerated states to a new site. A larger E thus leads to a greater number of carrier-phonon vertices and therefore strongly decreased. Indeed, in many cases, Disordered System Bassler s Model exp[-e 2 ] 0 for high E Energetic disorder () If the energy of hopping sites are not equal. Positional disorder() If hopping sites are not in equidistant, or If the molecular orientations on the sites are different. First case: 0 and 0 (It cannot be applied for our data) The transfer integral J is constant and each site has same energy. The hopping rate (R) is E independent and hence, Second case: 0, 0 (It cannot be applied for our data) 1/E 0 for high E E tilts the free-energy surface, reducing the activation energy required to hop. With increasing E, the inter-site jump rate increases and so does.

10 Third case: 0 (It can be applied for our data) Insufficient overlap (J 0) to easily pass to another site sites act as dead ends At small E, the charge can simply hop backwards from dead ends As E is increased, backward hopping rate decreases. so falls with E. E At large E, the effects of dead-ends are overcome, rises with E, i.e., is non-monotonic (or at least almost flat for large fields). Our measured (E) reaches a plateau at around E10 5 V/cm consistent with the beginning of a reversal in sign of dμ/de given by Bassler s Model

11 The application of Bassler s model raises a number of questions First The theory predicts T dependence of µ but we found T independent of µ exp exp 2 o C E 3kBT kbt 1.5 for 1.5 for 1.5 Second Furthermore, the theory depends on substantial energetic and positional disorder. It is not clear how such disorder might arise in our highly-order smectic samples. K [ 47 C ] Sm1 [96.1 C] SmG/K1 [179 C] SmC [188C] Isotropic K [ 320 K ] Sm1 [369 K] SmG/K1 [452 K] SmC [461 K] Isotropic These issues require further study

12 Summary Both h and e were measured and are independent on T and dependent on E The qualitative feature of our results could be tentatively explained by disordered hopping conduction proposed by H Bassler. However, the results require further study for better understanding. Thanks 12