organic semiconductors Henning Sirringhaus

Transcription

1 Charge transport physics of highmobility organic semiconductors Henning irringhaus

2 Organic electronics tatus and opportunities OLED Existing markets Emerging applications Advanced prototypes Next generation applications Demonstrators, but technology challenges Phones, MP3, camera $475 million TV, (2006) Lighting, computer wall-side TV (glass) ource: amsung OTFT - Paper-like flexible displays ource: Plastic Logic ource: PolyIC All-polymer & next gen. displays; RFID circuits sensors OPV - - Mobile Power; building Integrated PV ource: Konarka

3 Geim, Nature Materials 3, 183 (2007)

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5 olution processible, organic semiconductors i 12 Å i ynthesized by controlled organic chemistry emiconductors with band gap of ev olution processible in common organic solvents through side chain attachment.

6 Charge transport in organic semiconductor thin films ource-drain electrodes L Organic semiconductor 3.8 Å W I V W FET Ci ( V g VT ) L sd sd L Gate dielectric

7 Organic field-effect transistors Logic PolyIC OLED displays A. alleo Flexible E-paper displays ony ource-drain electrodes L Organic semiconductor W Plastic Logic Gate dielectric

8 Materials as a source of innovation R * R n * C 16 H 33 C 16 H 33 n Performance enhancement N N resulting from exploration of rich C12H25 C12H25 C12H25 C12H25 C16H33 C16H33 N N organic materials chemistry. * R2 N O R1 ustainability benefits: Low- R1 O N R2 n * C 6 H 13 C 6 H 13 i temperature materials; Only naturally abundant elements

9 Organic field-effect transistors Logic PolyIC OLED displays A. alleo Flexible E-paper displays ony ource-drain electrodes L Organic semiconductor W Plastic Logic Gate dielectric

10 Hopping or band-like, extended state transport? 1 Ψ( r) ΦRC(r-r0 ) 3.8 Å ik r j Ψ(r) ) 1 / N Φ(r-rj ) e j Φ(r ) : molecular orbital wavefunction localized on site j r j

11 Factors governing charge transport in organic semiconductors Bandwidth W Due to overlap between nearest- neighbour molecular orbitals in perfect crystal Energetic disorder Broadening of electronic density of states by disorder 2 kt Reorganisation energy Consequence of strong electronelectron and electron-ion interaction +

12 Bandwidth Bandwidth in tight binding approximation for molecular orbitals: H t e mn m a m m mam m, n r R H r R m e n t mn a n m a n Cornil, Chem. Rev. 107, 926 (2007) Bandwidth on the order of several 100 mev. Louie, et al., PRB 67, (2003) Mobility limited by scattering: e m Increasing with decreasing temperature *

13 Evidence for band-like charge transport in single crystals urface mobility (FET) Bulk mobility (TOF) Warta et al. Phys. Rev. B (1985). Appl. Phys. A: olids urf. A (1985). Podzorov et al. Phys. Rev. Lett. 93, (2004). Mean free path needs to be larger than the molecular distance a: λ μ e 1 * 3m k T 2 * 2 m 2 B 2 Wa With W = 100 mev, a = 4 Å and = 1 cm 2 /Vs: * m 9me 0. 2nm At RT band-descriptiondescription seems borderline even in molecular single crystals.

14 Energetic disorder Density of states broadened by disorder; complete localisation of states if disorder strong Origin of energetic disorder: Coulomb traps; on the order of exciton binding energy (0.5eV) Variation in conjugation length, torsional defects Dynamic lattice fluctuations Dipolar disorder in bulk / at interface Bassler, Phys. tat. olid. B 175, 15 (1993) Thermally activated hopping transport 2 2 (,, E) exp( ( ) )exp( ( ˆ 2 2 ) 1/ 2 0 C E ) 3 kt Richards, J. Chem. Phys. 128, (2008)

15 Reorganisation energy Internal reorganisation energy int reorg k int k int D 2 k A 1 k k k Contribution of similar magnitude from relaxation of surrounding medium Marcus rate for electron transfer: 2 ea k T B 2 t k reorg B T 1 2 exp reorg 4k B T Reorganization energy on the order of mev Bredas, et al., Chem. Rev. 104, 4971 (2004)

16 Transport in conjugated polymers Transport even in semicrystalline polymers limited by energetic disorder Light-emitting polymer FETs Zaumseil et al., Nat. Mat. 5, 69 (2006) (cm 2 /V Vs) mev P PMMA Curren nt (A) * N N n * Clean ambipolar transport 113 mev Chua, Nature 434, 194 (2005) /T (K) Zhao, Adv. Mat 21, 3759 (2009) Hallam PRL 103, (2009) K 200 K 260 K Gate Voltage (V) Mobility thermally activated with typical activation energies of mev for both electron and holes.

17 oluble molecular semiconductors - TIP pentacene i i Anthony et al. J. Am. Chem. oc. 123, 9482 (2001). High solubility and crystallinity Mobility > 1 cm 2 /Vs What is the nature of charge transport in this molecular semiconductor? Temperature and electric field-dependence of field-effect mobility Hall effect measurements Optical spectroscopy of charges as a function of temperature and electric field

18 FET characteristics at room temperature Mobility = 1.5 cm 2 /Vs

19 Temperature dependence of mobility in long channels Weak band-like temperature dependence near room temperature Below 200 K: hallow trap, activation energy of 14 mev

20 Temperature dependence in short channels L = 5 m V g = -15V V d = -15V V = 30V V g = -30V V d = -30V

21 Temperature dependence in short channels L = 5 m V g = -15V V d = -15V V g = -30V V d = -30V Nonlinear charge transport at low temperatures

22 Mobility dependent on lateral electric field L = 5 m akanoue, Nature Materials 9, 736 (2010) Mobility (at high field) determined by band-like, scattering physics. Energetic disorder reduced to < mev. Origin of field-dependence of mobility?

23 What is nature of charge carriers? Charge modulation spectroscopy Accumulated charges Visible or IR light Visibleibl ubstrate emi transparent Gate electrode Dielectric Neutral molecule Charged molecule l emiconductor IR Induced absorptions of charge carriers in accumulation at the interface are a measure of degree of molecular relaxation and degree of carrier localisation.

24 Chemical doping of TIP pentacene in solution TIP pentacene solution (2x10-5 M) + aturated FeCl 3 solution 2 TIP pentacene + 2 FeCl 3 2 TIP pentacene FeCl 2 + 2Cl - Isolated cation Cation Neutral Neutral molecule Charged molecule

25 CM spectra of FETs at room temperature CM (charges in FET) In solution (isolated cation) Bleaching of neutral molecule Charge induced d absorption Absorption of cation Room temperature charge induced absorption in FET is broader than absorption of isolated radical cation in solution.

26 Temperature dependence of CM spectra L=40 m

27 Comparison of low T CM and chemical doping spectrum CM in solid state Radical cation in solution At low temperatures charges in FET are localized on single molecule Existence of shallow trap states (10-15 mev deep)

28 Origin of the nonlinear transport at low temperatures CM spectra with constant source-drain bias akanoue, Nature Materials 9, 736 (2010) With increasing lateral field low T CM spectrum of charges in shallow trap evolves towards room temperature spectrum without lateral field. Mechanism for nonlinearity at low temperatures - Electric field-induced detrapping from shallow traps.

29 The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Electric field dependence of low T mobility Fowler Nordheim tunneling * 3 E 0 2 E 0 0 exp E 4 2m 3 e Trap depth = mev extracted from Fowler-Nordheim fit is consistent with low field activation energy of the mobility.

30 Time scales for molecular reorganisation Time scale for Bloch electron formation due to bandwidth J 0 = 100 mev: 14 J 410 s J 0 Interaction with intramolecular vibrations governed by characteristic vibronic energy E = -1 v cm : v Fast Leads to renormalization of transfer integrals E v s Interaction with intermolecular vibrations governed by characteristic optical and acoustic phonon energy E <50 l 100 cm -1 : l E l s low Leads to dynamic disorder in transfer integrals and reorganisation energy Localisation by dynamic disorder

31 Localisation by dynamic disorder t = t 1 t = t 1 + t(ps) t = t 1 + t*(ps) Electric field

32 Model for dynamic disorder limited transport Band like temperature dependence of mobility Band-like temperature dependence of mobility due to freezing of thermal lattice fluctuations. Troisi et al. PRL. 96, (2006).

33 Outlook Beginning to achieve understanding of charge transport on molecular scale tatic energetic disorder < 10 mev Potential of studying transport phenomena governed by energy scales of several mev. Emerging interest in better understanding of spin as well as thermal transport properties. Realisation of device functions relying on unique charge transport and optoelectronic properties of these molecular materials.

34 Acknowledgements Tomo akanoue (CM) Jui-Fen Chang (Hall) Takafumi Uemura, Jun Takeya, Osaka University Jerome Cornil, Yoann Olivier, University of Mons Alessandro Troisi, University of Warwick Marie-Beatrice Madec, tephen Yeates, University of Manchester Funding: Technology trategy Board (TB), EPRC