Transport in Doped Semiconductors

Attempts to dope organic semiconductors have been made very early in the field, motivated by the prospect of possibly reaching metallic conductivities [108, 109]. These synthetic metals, however, have not been realized. While p-type doping could be obtained, for example, with iodine gases for poly-p-phenylene vinylene (PPV) derivatives, and n-type doping was demonstrated with sodium for a cyano-derivative of PPV, the doping levels obtained were not stable with time. The dopant molecules readily diffused into the organic semiconductor, yet also out of it. Due to the lack of stability, these approaches were not suitable for commercial applications. 

Pioneering work on stable doping in organic LEDs has been carried out by the group of K. Leo in Dresden and has been reviewed by Walzer [107]. It is now clear that F4-TCNQ can act as a dopant because its electron affinity is close to 5 eV [110, 111], which is close to the ionization potential of triphenylamine derivatives and to some phthalocyanines (Pc) [107]. It turns out that doping of ZnPc by 2% of F4-TCNQ raises the conductivity to a level of 10 cm When using TCNQ 

The concentration of free holes generated by the p-dopant can be calculated under the assumption that doping does not alter the hole mobility. Doing so, Zhang 

Of course, there is a price to pay for this because the lower the LUMO of the hole acceptor the more likely it is that an impurity acts as a hole trap and vice versa. 

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