N-doping of pentacene by decamethylcobaltocene

Transcription

1 Appl Phys A (2009) 95: 7 13 DOI /s x N-doping of pentacene by decamethylcobaltocene Calvin K. Chan Antoine Kahn Received: 14 June 2008 / Accepted: 24 November 2008 / Published online: 23 December 2008 Springer-Verlag 2008 Abstract We demonstrate n-type doping of pentacene with the powerful reducing molecule decamethylcobaltocene ( ). Characterization of pentacene films deposited in a background pressure of by X-ray photoemission spectroscopy and Rutherford backscattering confirm that the concentration of incorporated donor molecules can be controlled to a level as high as 1%. Ultraviolet photoemission spectroscopy show Fermi level (E F ) shifts toward unoccupied pentacene states, indicative of an increase in the electron concentration. A 1% donor incorporation level brings E F to 0.6 ev below the pentacene lowest unoccupied molecular orbital. The corresponding electron density of cm 3 is confirmed by capacitance voltage measurements on a metal pentacene oxide silicon structure. The demonstration of n-doping suggests applications of to pentacene contacts or channel regions of pentacene OTFTs. PACS Ln Fr Ph 1 Introduction Promising applications of organic semiconducting materials include low-cost, large-area, and flexible organic thinfilm transistors (OTFTs) that can be used to drive displays, smartcards, radio-frequency identification tags, and sensors. Although pentacene is one of the most studied materials C.K. Chan A. Kahn ( ) Dept. of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA kahn@princeton.edu Fax: used in OTFTs [1 5], its applicability has so far been limited to p-channel transistors. These devices still often suffer from poor hole mobility and large hole injection barriers. P-doping of pentacene OTFTs by tetrafluorotetracyanoquinodimethane (F 4 -TCNQ) demonstrated improved p-type device performance through the reduction of contact resistances [6, 7] and increases in hole concentration and mobility [8]. N-doping of pentacene would enable the realization of n-channel OTFTs and power-efficient complementary organic circuitry. Among various materials available for n-doping organic films [9 14], low ionization energy (IE) metallocenes have proven to be relatively easy to use as efficient donors [15 17]. Table 1 lists the IE and electron affinity (EA) of two organometallic sandwich complexes and three host materials measured by ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES), respectively. For efficient dopant-to-host electron donation to occur, the host EA should be roughly equal to or larger than the IE of the donor molecule. Cobaltocene (CoCp 2 )was found to have an IE = 4.0 ev and was used to n-dope a tris(thieno)hexaazatriphenylene (THAP) derivative [15, 16]. Decamethylcobaltocene ( ) was found to have an even lower IE = 3.3 ev making it a more powerful reductant capable of n-doping a wider range of host materials including copper phthalocyanine (CuPc) [17]. In both cases, n-doping was found to shift the Fermi level (E F ) of the host towards unoccupied states indicating an increase in electron concentration. These experiments also demonstrated the ambipolar characteristics of organic materials, whereby n-doping at typically hole-injection interfaces could modify the potential barriers such that electron injection becomes favorable. Although the EA of pentacene suggests an energetically less favorable electron transfer from (Table 1), the

2 very low IE of still makes it a promising candidate for n-doping pentacene. In this work, incorporation into pentacene films is quantified using X-ray photoemission spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS). UPS demonstrates an increase in the electron concentration in pentacene, which is correlated with the mobile electron concentration detected in capacitance voltage (C V) measurements. The concentration of incorporated donor molecules and activated dopants are shown to be in agreement with physical models. 2 Experimental Experiments were conducted in a three-chamber ultrahigh vacuum (UHV) system equipped for organic and metal film growth, UPS, XPS, and in situ C V measurements. was loaded as received from Sigma Aldrich into a glass ampoule under N 2 atmosphere and sealed to a UHV leak valve. The dispenser setup was mounted on the UHV sample preparation chamber (p base = Torr) equipped with a thermal evaporation cell containing pentacene, and connected to the growth (p base = Torr) and analysis (p base = Torr) chambers, allowing for sample preparation and characterization without breaking vacuum. The leak valve of the dopant ampoule was opened in situ prior to the first use to evacuate the N 2. A second pentacene source was available in the growth chamber for the preparation of undoped films. Both sources were thoroughly outgassed at 180 C prior to the first use. Doped pentacene films were obtained by depositing host molecules in a background partial pressure of (p d) controlled with the leak valve. When the source is heated to 100 C, a maximum p d = 10 6 Torr is obtained when the leak valve aperture is fully opened. The pentacene deposition rate was monitored with a quartz crystal microbalance, assuming a bulk density of 1.5 g cm 3, while the background pressure was monitored with a standard ionization gauge. Dopant adsorption on, and incorporation into, the host film is driven by donor host electron transfer [16]. The sticking coefficient (S c ) of a donor molecule 8 C.K. Chan, A. Kahn Table 1 Ionization energy (IE), electron affinity (EA), and the transport gap (E g,t ) of various organic molecules onto the host surface is given by the probability of electron transfer, Molecule IE (ev) EA (ev) E g,t (ev) References [ ] (E2 E 1 ) p = γg d (E 1 )g h (E 2 ) exp de 1 de 2, (1) CoCp [15] k B T THAP [15] where g d (E) and g h (E) are the energy distributions of [17] dopant donor and host acceptor states, γ is the host dopant CuPc [17] concentration ratio, and the exponential term is the probability Pentacene [24, 25] of thermal activation from energy E 1 to E 2. In gen- eral, given the broad density of states of organic materials [18 20], g(e) can be approximated by a Gaussian distribution with a standard deviation of σ = 0.3 ev. For energetically favorable electron transfer (E 2 <E 1 ), the sticking coefficient of the donor molecules was shown to be close to one [16]. However, the relatively large 0.6 ev energy offset between the IE and the pentacene EA (Table 1) decreases this probability and reduces S c to approximately 0.01 [21]. Therefore, depositing pentacene at 0.1 nm s 1 in a background pressure p d = 10 7 Torr (equivalent to 0.1 ML s nms 1 ) should result in a donor incorporation of 1%, scaling linearly with host deposition rate or dopant partial pressure. UPS experiments were conducted with the He I (hν = ev) photon line of a helium discharge lamp. Spectra were referenced to E F, as determined separately on an Au film. The position of the vacuum level (E vac ) of each surface was determined using the low-energy secondary electron cut-off [22]. Chemical composition of undoped and doped films were determined via XPS using an Al Kα radiation source (hν = ev). Experimental resolutions were 0.1 ev for UPS and 0.9 ev for XPS. C V measurements were conducted in vacuum at room temperature using an HP 4192A-LS low-frequency impedance analyzer. The quasi-static signal was varied around the bias potential by a sinusoidal ±0.15 V at a frequency of 200 Hz. RBS experiments were performed by Evans Analytical Group using a MeV He 2+ ion beam incident with a 75 angle from the sample normal. Backscattered He atoms were collected 20 off the incident beam. The depth resolution was 25 nm and the Co detection limit was 0.02 atomic % (2 mol%). The hydrogen concentration was simultaneously determined by hydrogen forward scattering. Samples were prepared in vacuum at Princeton and transferred to the RBS system with minimal exposure to air. 3 Results and discussion 3.1 Donor incorporation into pentacene incorporation into the host matrix is confirmed by the XPS spectra of 6 nm films of undoped and doped pentacene. The films were deposited at 0.1 nm s 1 onto solventcleaned Au (Au ). Two doped films were grown in

3 N-doping of pentacene by decamethylcobaltocene 9 Fig. 1 Co 2p and C 1s core levels measured by XPS on a pristine pentacene deposited on Au ; and 6 nm of pentacene deposited on Au under partial pressures of b p d = 10 8 Torr and c p d = 10 7 Torr. Insets: chemical structure of pentacene and background pressures p d = 10 7 and p d = 10 8 Torr, for expected donor concentrations of 1 and 0.1%, respectively. Surface contamination of the Au substrates by incidental exposure to residual dopants was avoided by depositing a 2.5 nm amorphous film of undoped α-npd prior to a bulk film growth. The Co 2p and C 1s core levels are shown in Fig. 1 for (a) undoped, (b) lightly doped ( p d = 10 8 Torr), and (c) moderately doped ( p d = 10 7 Torr) pentacene films. As expected, the undoped film shows no discernable Co 2p, consistent with the absence of in or on top of the film, whereas both doped films display a clear Co 2p 3/2 peak at a binding energy (BE) = 782 ev. The position of theco2p 1/2 peak, shown by the dotted line at 796 ev, is extrapolated from the Co 2p 3/2 assuming a 14 ev spin orbit splitting [16]. The Co 2p 1/2 is not distinguishable from the background intensity of the pristine film. The C 1s spectrum of the doped sample shifts by 1 ev towards higher BE with respect to the undoped sample (Fig. 1), consistent with n-doping a shift of filled states away from E F, equivalent to a shift of E F towards unoccupied states. Since the substrate is protected from exposure by the α-npd buffer layer, metal-organic interface modification by a -induced dipole layer is ruled out and the energy level shift is attributed solely to electron transfer from to pentacene. Although the Co spectra may indicate a slight increase in the Co intensity with increased donor concentration, analysis of the Co 2p and C 1s peak intensities reveals a concentration of approximately 8 10 mol% for both doped samples. These values are higher than the previously stated expectations, but are in good agreement with the amount of surface-saturated dopant found for CoCp 2 -doped THAP [16]. Indeed, the growth and transfer process of our films is such that the surface of the completed doped film is exposed to residual resulting in surface dopant concentrations that are likely higher than those of the bulk. Given that incorporated donor concentrations determined from XPS are skewed by surface adsorbed dopants, RBS is used to determine the depth profile of elements in the film and to obtain a precise concentration of.an interface-doped pentacene sample was prepared by depositing 30 nm of moderately doped pentacene ( p d = 10 7 Torr) followed by 90 nm of undoped pentacene in the pristine organic growth chamber. Other donor concentrations were attempted, but pentacene films doped at p d = 10 8 Torr contain Co concentrations below the RBS detection limit, and films doped at p d = 10 6 Torr resulted in phase segregation. The raw RBS spectrum for the interface-doped pentacene sample is plotted in Fig. 2a, and shows the Si (substrate) edge, the C peak, and the Co peak (channels , expanded 200). The Co peak exhibits an asymmetric tail extending towards higher backscattered energies. A similar high energy tail located between channels 280 and 340 is observed for the Si edge. Evidence of tailing features in RBS is indicative of inhomogeneous coverage of the underlying layers resulting from three-dimensional crystallization of the pentacene film. The asymmetric Si and Co tails correspond to backscattered ions from regions ranging from thick

4 10 C.K. Chan, A. Kahn Fig. 2 a RBS spectra of a interface-doped pentacene film deposited on Si(100). The Si tail and the Co signal are expanded 200 for emphasis. b Atomic concentrations vs. depth as determined by RBS Fig. 3 (Top) UPS spectra of a as-loaded Au ; b 6 nm of undoped pentacene grown on Au ; and 6 nm of pentacene doped with at c p d = 10 8 Torr and d p d = 10 7 Torr. (Bottom) Corresponding energy level diagrams pentacene coverage (lower channel number) to essentially no coverage (higher channel number). Atomic force micrographs of undoped and doped pentacene films (not shown) confirm the rough morphology contributing to the inhomogeneous overlayer thickness reflected in the RBS spectrum. Since computational models for fitting RBS data usually assume uniform layers, the RBS spectrum can be simulated by neglecting the Si and Co tails. Iteratively adjusting the depth-dependent elemental concentrations to match the spectra observed in Fig. 2a leads to the calculated depth profiles for the interface-doped pentacene sample shown in Fig. 2b. The 1:1400 atomic ratio of Co:C in the intentionally doped layer corresponds to a molecular donor concentration of 1.6%, which is in good agreement with the expected value (1%). Although the depth profile indicates that is confined to the 30 nm doped region, the Co tail means that dopant diffusion cannot be absolutely excluded. However, the fact that tail features appear equally in both the Si edge and the Co peak suggests that dopant diffusion is not a major issue. 3.2 Impact of N-doping on the energy level structure of pentacene The electronic structure of undoped and -doped pentacene films were investigated using UPS. Au substrates were employed so that the initial E F position of the undoped pentacene film, as determined by the Au /pentacene interface, would be in the lower half of the gap, relatively close to the highest occupied molecular orbital (HOMO) [23]. This way, n-dopant-induced shifts of E F towards the lowest unoccupied molecular orbital (LUMO) could be emphasized. Ultraviolet photoemission spectroscopy (UPS) spectra of (a) the Au substrate, and 6 nm films of (b) undoped, (c) 0.1% doped ( p d = 10 8 Torr), and (d) 1% doped ( p d = 10 8 Torr) pentacene are plotted in Fig. 3 (top panel) and summarized in an energy level diagram (bottom panel). Spectrum (b) shows that the HOMO edge of pristine pentacene/au (work function = 4.51 ev) is 0.63 ev below E F, consistent with previous observations of low holeinjection barriers for intrinsic small molecule films on contaminated Au [23]. The pentacene IE (5.15 ev) is in good

5 N-doping of pentacene by decamethylcobaltocene 11 agreement with previously measured values (Table 1 and Refs. [24, 25]). Spectra (c) and (d) show a shift of the occupied states to higher BE (equivalent to the shift of E F towards the LUMO) as the incorporation increases to 0.1 and 1%. At the highest doping concentration tested here, the total E F shift of 1.28 ev results in an electron injection barrier of φ e = 0.6 ev, as determined from the hole-injection barrier and the pentacene transport gap (Table 1). N-doping is therefore achieved despite the energy difference between the donor IE and the acceptor EA. The impact of the surface concentration of dopants on the measured E F position must be carefully considered. UPS measurements are sensitive to the first few monolayers of the film surface and a heavy surface concentration of dopants could skew the results. However, instead of observing an E F position in the gap independent of the intended bulk dopant concentration, which would be consistent with a constant saturated surface concentration of, E F is seen to vary significantly with the bulk doping concentration. This confirms that the measurements are sensitive to the excess carrier concentration in the bulk pentacene film. The equivalence between the surface and the bulk Fermi levels is likely facilitated by a reduction in electron transfer probability between and pentacene at the surface of the film. Indeed, comparing the position of the Co 2p peak in Fig. 1 with those found in Ref. [16] indicates that the surface adsorbed donors are largely unactivated. The lack of charge transfer at the surface is likely attributed to a reduction in electronic polarization, which increases the IE of the dopant and reduces the EA of the host [26], and thereby increases the energetic barrier for dopant activation. With the absence of surface donor host electron transfer, the position of E F is determined by the bulk doping concentration, i.e., the concentration of donor molecules incorporated just below the surface that have been fully polarized by the bulk lattice. A quantitative analysis of the electron concentration is done using Fermi Dirac (FD) statistics in a broadly distributed Gaussian density of states [21]. The carrier concentration is given by n = g(e,σ)f(e,e F )de, (2) where g(e,σ) is the density of states and f(e,e F ) is the FD distribution function. Using a broad distribution (σ = 0.3 ev),thelumo-e F differences of 0.81 and 0.6 ev observed in Figs. 3c and 3d for increasingly doped pentacene films yield electron concentrations of and cm 3, respectively [21]. These values reflect the density of electrons, which may differ from the number of incorporated donor molecules. Assuming a host molecular density of cm 3, 0.1 and 1% donor incorporation levels are equivalent to the inclusion of approximately and cm 3 molecules in the pentacene matrix. The number of activated dopants is therefore only 1% of the incorporated donor molecules, in agreement with the previously mentioned probability of electron transfer from dopant to host. 3.3 Excess carrier concentrations determined by electrical measurements Generally, current voltage measurements using Schottky diodes are used to investigate the impact of doping on the injection and transport properties of organic films [15, 17]. However, the polycrystalline nature of pentacene produces films with intergrain pinholes that short-circuit vertical diode structures. Therefore, the impact of n-doping on the electrical properties of pentacene is investigated here using C V measurements in the planar metal oxide semiconductor capacitor (MOScap) configuration shown in the inset of Fig. 4. These MOScaps also likely contain metallic regions that extend through the organic film, but shorts are buffered by the presence of the SiO 2 layer. The measured capacitance is therefore the area-weighted capacitances of the series pentacene/oxide capacitance with an area A s and the parallel oxide capacitance with a pinhole area A p. Since grain boundaries represent less than 5% of the total device area as observed from atomic force micrographs, A p is small and the MOScap capacitance can be approximated by C = A ( s A s + A p C s ( 1 = + 1 C s C ox C ox ) 1 + A p A s + A p C ox ) 1. (3) The capacitances (C ox,c s ), effective thicknesses (d ox,d s ), and dielectric constants (ε ox,ε s ) are those of the SiO 2 oxide and pentacene, respectively. Fig. 4 C V measurements of a undoped and b -doped pentacene MOS capacitors. Note the different scales for the undoped and doped devices shown on the left and right axes, respectively. The inset shows the schematic structure of the device

6 12 C.K. Chan, A. Kahn Changes in the measured capacitance occur according to the bias regime of the pentacene layer. Of particular interest is when the pentacene film is driven into depletion, since the capacitance is a function of the carrier concentrationdependent depletion width. Assuming a δ-distribution of induced charges, i.e., carriers are induced at the metal insulator interface on the gate side and at the edge of the depletion region in pentacene, the semiconductor capacitance can be expressed as a parallel plate capacitance, C s = ε sε 0 W, (4) where ( ) 2εs ε 1/2 0 W = qn φ s (5) is the width of the depletion region in the semiconductor determined by the semiconductor surface potential, φ s, and the mobile carrier density, n [27]. The surface potential is related to the gate voltage by V G = φ s + ε s 2qn d ox φ s, (6) ε ox ε s ε 0 where d ox is the thickness of the SiO 2 layer. Solving (6) for φ s and substituting into (3) and (5) yields a closed form expression for the MOScap capacitance under a depletion gate bias of V G, C = C ox 1 + 2C2 ox V G ε s ε 0 qn, (7) which can be expressed linearly in the form of its inversesquare, 1 C 2 = 1 Cox 2 + 2V G ε s ε 0 qn, where the slope 2/ε s ε 0 qn is a function of the mobile carrier concentration. To determine the intrinsic and excess carrier concentrations in undoped and -doped pentacene, respectively, MOScaps were fabricated using degenerately doped p + -Si(100) as the metal gate contact, 250 nm of thermally grown dry SiO 2 as the gate oxide, and 100 nm thick layers of pentacene as the semiconductor. N-doped samples were deposited at 0.1 nm s 1 in a p d = 10 7 Torr for expected incorporated and activated concentrations of approximately 1 and 0.01%, respectively. The organic semiconductor layer was contacted via top Au pads thermally evaporated through a shadow mask. Results of the C V measurements in the depletion regime are shown in Fig. 4 as a function of 1/C 2 for (a) the undoped pentacene device at negative gate bias plotted on the left axis, and (b) the -doped MOScap at positive bias plotted on the right axis. The approximately linear curve at intermediate bias confirms a depletion-like behavior of the devices, with deviations at lower bias resulting from carrier trapping at the oxide interface and at higher bias due to the saturation of the depleted region. These deviations result in an error of less than a few percents in the linear fitting of the slope. The most striking feature from these C V responses is the switch in polarity between the undoped and doped devices. The change in the sign of the slope for opposite bias regimes signals a change in majority carrier type from holes in undoped pentacene to electrons in n-doped pentacene. In the undoped film where holes are the predominant carriers [1, 3], an increasingly positive gate bias repels holes away from the organic-oxide interface and creates a depletion region. The increase in the depletion width decreases the depletion capacitance (Fig. 4a). In the case of n-doped pentacene, the organic-oxide interface is depleted of mobile excess electrons with the application of an increasingly negative gate bias. Assuming a dielectric constant of ε s = 3 for pentacene, the slopes of the linear depletion regime indicate a hole concentration of cm 3 in the undoped film and an electron concentration of cm 3 in the doped film. Therefore, not only has the dominant carrier type changed from holes in the undoped film to electrons in the n-doped film, but the excess mobile carrier concentration has also increased by more than an order of magnitude. The experimentally determined electron concentration in the n-doped MOScap configuration is in good agreement with the values obtained from spectroscopy measurements, which confirms the expected 0.01 probability of dopant activation. 4 Summary N-type doping of pentacene by decamethylcobaltocene was demonstrated. X-ray photoemission spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS) measurements showed the presence of donor molecules in the host film in concentrations that can be controlled by the host deposition rate and the background pressure of the dopant. Ultraviolet photoemission spectroscopy (UPS) and C V measurements confirmed higher electron concentrations due to the charge donation from to pentacene, which can be quantified using simple physical models. This successful demonstration of n-doped pentacene suggests that the application of to the contacts or channel regions of pentacene OTFTs should result in the realization of n-type organic thin-film transistors.

7 N-doping of pentacene by decamethylcobaltocene 13 Acknowledgements The authors gratefully acknowledge the contributions of S. Barlow and S. Marder to the work on n-doping using metallocenes, as well as the funding provided by the National Science Foundation (DMR ) and the Princeton MRSEC of the National Science Foundation (DMR ). References 1. D.J. Gundlach, Y.Y. Lin, T.N. Jackson, S.F. Nelson, D.G. Schlom, IEEE Electron Device Lett. 18, (1997) 2. Y.-Y. Lin, D.J. Gundlach, S.F. Nelson, T.N. Jackson, IEEE Trans. Electron Devices 44, (1997) 3. C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14, (2002) 4. N. Koch, J. Ghijsen, R. Ruiz, J. Pflaum, R.L. Johnson, J.-J. Pireaux, J. Schwartz, A. Kahn, Mat. Res, Soc. Symp. Proc. 708 (2002) 5. N.J. Watkins, Y. Gao, J. Appl. Phys. 94, 5782 (2003) 6. C. Vanoni, S. Tsujino, T.A. Jung, Appl. Phys. Lett. 90, (2007) 7. C.K. Chan, C.D. Dimitrakopoulos, IEEE Electron Device Lett. (in prep.) 8. Y. Abe, T. Hasegawa, Y. Takahashi, T. Yamada, Y. Tokura, Appl. Phys. Lett. 87, (2005) 9. J. Kido, T. Matsumoto, Appl. Phys. Lett. 73, (1998) 10. Q.T. Le, L. Yan, Y. Gao, M.G. Mason, D.J. Giesenand, C.W. Tang, J. Appl. Phys. 87, (2000) 11. A. Nollau, M. Pfeiffer, T. Fritz, K. Leo, J. Appl. Phys. 87, (2000) 12. S. Tanaka, K. Kanai, E. Kawabe, T. Iwahashi, T. Nishi, Y. Ouchi, K. Seki, Jpn. J. Appl. Phys. 44, (2005) 13. K. Harada, A.G. Werner, M. Pfeiffer, C.J. Bloom, C.M. Elliott, K. Leo, Phys. Rev. Lett. 94, (2005) 14. C.K. Chan, E.-G. Kim, J.-L. Bredas, A. Kahn, Adv. Funct. Mater. 16, (2005) 15. C.K. Chan, F. Amy, Q. Zhang, S. Barlow, S. Marder, A. Kahn, Chem. Phys. Lett. 431, (2006) 16. C.K. Chan, A. Kahn, Q. Zhang, S. Barlow, S. Marder, J. Appl. Phys. 102, (2007) 17. C.K. Chan, W. Zhao, S. Barlow, S. Marder, A. Kahn, Org. Electron (2008). doi: /j.orgel I.N. Hulea, H.B. Brom, A.J. Houtepen, D. Vanmaekelbergh, J.J. Kelly, E.A. Meulenkamp, Phys. Rev. Lett. 92, (2004) 19. Y. Shen, K. Diest, M.H. Wong, B.R. Hsieh, D.H. Dunlap, G.G. Malliaras, Phys. Rev. B Rapid Commun. 68, (R) (2003) 20. O. Tal, Y. Rosenwaks, Y. Preezant, N. Tessler, C.K. Chan, A. Kahn, Phys. Rev. Lett. 95, (2005) 21. C.K.-F. Chan, Ph.D. Thesis, Princeton University, D. Cahen, A. Kahn, Adv. Mater. 15, (2003) 23. A. Wan, J. Hwang, F. Amy, A. Kahn, Org. Electron 6, (2005) 24. N. Koch, J. Ghijsen, R.L. Johnson, J. Schwartz, J.-J. Pireaux, A. Kahn, J. Phys. Chem. B 106, 4192 (2002) 25. P.G. Schroder, C.B. France, J.B. Park, B.A. Parkinson, J. Appl. Phys. 91, (2002) 26. F. Amy, C. Chan, A. Kahn, Org. Electron 6, 85 (2005) 27. R.F. Pierret, Semiconductor Device Fundamentals (Addison- Wesley, Reading, 1996)