Ambipolar materials

The photoreceptor of electrophotography is a charge depletion device in which replenishing carriers from the electrodes must be completely prohibited. Since the a-Si H is a typical ambipolar material in which both carriers, i.e., electrons and holes, are allowed to move, great care must be paid to the contact with either the conductive electrode or the free surface... 

On the other hand, an ideal ambipolar FET needs balanced carrier transport and similar performance under both p- and -channel operation modes [5, 6]. Nonetheless, only a few of these polymers can be operated in ambient conditions [7], and fewer showed balanced carrier transport [5, 8]. To our knowledge, none offer both balanced carrier transport and transport under ambient conditions—criteria for an ideal ambipolar material that can significantly simplify circuit design and minimize patterning and fabrication process. 

The fundamentals and the various factors affecting the physics of bulk heterojunction solar cells have already been studied and modeled in several original research papers [1-3, 28, 30, 57-59]. However, diffusion processes driven by photoinduced chemical potential gradients appear to play a very important role in D-A solar cells, a fact that, at least from a molecular point of view, must be taken into account when designing intrinsic D-A ambipolar materials. As discussed below, this impUes that a too intimate, overly homogenous D-A distribution is counterproductive for the operation of bulk heterojunction solar cells. 

Finally, double-cable polymers could be ambipolar materials for photomodu-lated p-n type organic field effect transistors (OFETs). 

HOMO of DBTTF and the LUMO of TCNQ, respectively (Fig. 4b). Source and drain electrodes are several organic metals of the TTF TCNQ type having different chemical potentials predicted using Fig. 4c which is the same as Fig. 2a. For the electrodes whose chemical potentials are set within the conduction band of the channel material, FET exhibited n-type behavior (A in Fig. 4d). When the chemical potentials of organic metals are allocated within or near the valence band of the channel, p-type behaviors were observed (E, F in Fig. 4d). When the chemical potentials of the electrodes are within the gap of the channel, FET exhibited ambipolar-type behavior (B-D in Fig. 4d). Since the channel material is the alternating CT solid, the drain current is not excellent and a Mott type insulator of DA type or almost neutral CT solid having segregated stacks is much preferable in this context. 

Similar to the cases in other small molecules and polymers, most materials composed of phthalocyanine compounds are revealed to work as p-type semiconductors for OFET applications with only few phthalocyanine materials as n-type semiconductors and even less as ambipolar ones. Among the p-type tetrapyrrole semiconductors, monomeric phthalocyanine compounds hold all the trumps with only a few double- and triple-deckers together with some porphyrin derivatives having been reported. 

As for the permeability measurements, most techniques based on the analysis of transient behavior of a mixed conducting material [iii, iv, vii, viii] make it possible to determine the ambipolar diffusion coefficients (- ambipolar conductivity). The transient methods analyze the kinetics of weight relaxation (gravimetry), composition (e.g. coulometric -> titration), or electrical response (e.g. conductivity -> relaxation or potential step techniques) after a definite change in the - chemical potential of a component or/and an -> electrical potential difference between electrodes. In selected cases, the use of blocking electrodes is possible, with the limitations similar to steady-state methods. See also - relaxation techniques. 

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