Ambipolar device

By exchanging one of the electrodes, such a diode can be altered from a unipolar hole device into an ambipolar device. Figure 5.10 shows the I/V characteristics of an ITO/PEDOT/MDMO-PPV/LiF-Al device. Here, the LiF-Al electrode should guarantee electron injection under forward bias. 

Figure 5.11 summarizes the temperature dependent transport behavior of unipolar and ambipolar diodes based on MDMO-PPV. Below 190 K, the hole-controlled device (ITO/PEDOT and Au contact) and the ambipolar device (ITO/PEDOT and LiF-Al contact) behave identically. Trap-free SCLC transport is observed and the mobility at this temperature is estimated to be around 10-8 cm2/Vs. For the ambipolar device, a diode-like turn-on is... 

Figure 10.12. Device characteristics of PbSe nanocrystal FETs activated with hydrazine. (a) Plots of 7d and 7 2 versus VG at constant VDS = 40 V for an n-channel FET assembled from 8.1-nm PbSe nanocrystals. L = 8 xm, W = 2300 tm. (b) ID versus VG plot at constant VDS = IV for an ambipolar FET assembled from 8.1-nm PbSe nanocrystals. L = 8 xm, W = 2300 xm. (c) Plots of 7D and 7 2 versus VG at constant Fps = -40V for a p-channel FET assembled from 8.2-nm PbSe nanocrystals. L = 10 tm, W = 3000 xm. (d) Plot of 7D versus FDS, as a function of VG for a p-channel FET assembled from 8.4-nm PbSe nanocrystals. L = 8 xm, W = 2300 xm. The changes in the transistor polarity were induced by controllable adsorption/desorption of hydrazine molecules from the nanocrystal surface. Reproduced from Ref.68, Copyright 2005, with permission from the AAAS.

M Suzuki, S Tokito, F Sato, T Igarashi, K Kondo, T Koyama, and T Yamaguchi, Highly efficient polymer light-emitting devices using ambipolar phosphorescent polymers, Appl. Phys. Lett., 86 103507, 2005. 

As an alternative, top gated devices contacted with TiC show ambipolar behavior and better stability [167, 168]. Ambipolar transistors, however, cannot be used to reproduce the current CMOS technology where both p-type and n-type transistors are used because the combination of the two has superior performance and lower power consumption than devices built with only one type of transistor. An alternative solution was recently presented and consists of selectively applying negative or positive gate voltages to CNT ambipolar transistors, which make them behave as p-type or n-type respectively [169]. 

Fig. 19 (a) The device schematic for a pseudo Y-junction transistor, (b) SEM micrograph of the overall circuit arrangement used in the measurement of the electrical characteristics, with Au contact pads and an FIB-patterned Pt wire contacting the Au pads and the Y-junction. (c) The ambipolar I-V curves resemble that of an n-type semiconductor at a positive gate potential, and a p-type semiconductor at a negative gate potential top), and the equivalent circuit for a pseudo Y-junction SWNT device bottom). (Reprinted with permission from [170, 171])... 

Saudari SR, Frail PR, Kagan CR (2009) Ambipolar transport in solution-deposited pentacene transistors enhanced by molecular engineering of device contacts. Appl Phys Lett 95 023301... 

Saudari SR, Lin Y-J, Lai Y, Kagan CR (2010) Device configurations for ambipolar transport in flexible pentacene transistors. Adv Mater 44 5063-5068... 

Langmuir-Blodgett (LB) technique has been also used for the preparation of Pc-based OFET, as it allows the fine control of both the structure and the thickness of the film at the molecular level [226,227], OFET devices based on amphiphilic tris(phthalocyaninato) rare earth, triple-decker complexes have been prepared by LB technique, showing good OFET performances [228], More recently, ambipolar transport has also been realized in OFET devices through a combination of holeconducting CuPc and n-conducting Cgo fullerene, in which the asymmetry of the... 

Due to the great potential applications and the widespread interests of OFETs, several review articles with emphasis on different aspects of OFETs have been published [8-13], However, despite the extensive studies and increasing research interests in phthalocyanine as well as porphyrin semiconductors, there is still no review article that systematically generalizes the research achievements in the field of tetrapyrrole organic semiconductors for OFETs. Thus, the present contribution should interest scientists in both industrial and theoretical fields. The main contents of this chapter is as follows Firstly, some basic introduction of the structure and characteristics of OFET is provided then theoretical factors that influence the performance of OFET devices is discussed finally, the progress in phthalocyanine-based OFETs in the order of p-type, n-type, and ambipolar semiconductors is summarized. 

Fig. 3 Schematic diagram of current-voltage characteristics of (a) p-type, (b) n-type, and (c) ambipolar OFET device...

Table 6 Performance of some pthalocyanine-based ambipolar OFET devices (mEn means m x 10 )... 

Fig. 5.6. Schematic drawing of a bulk heterojunction device. Charge generation occurs throughout the bulk, but the quality of the two transport networks (p-and n-type channels) is essential for the functionality of the blend as an intrinsic, ambipolar semiconductor. Light emission occurs at the semi-transparent ITO electrode. Electron transport on the fullerenes is marked by full arrows and hole transport along the polymer by dotted arrows...

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