| Doping of polyphenylacetylene (PPA) with the electron acceptors iodine (I(,2)) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), in solution or via solid-gas reactions, results in the formation of stable charge transfer complexes, which generally raise its dark conductivity and photoconductivity. Doping with arsenic pentafloride (AsF(,5)) generally enhances dark currents, but not photocurrents. The dark currents of the complexes are space-charge-perturbed or limited and show a strongly super-linear dopant concentration dependence at low doping levels. Dark conductivities as high as 10('-4) (Ohm-cm)('-1) were obtained in PPA-AsF(,5) complexes. Predominantly electronic conductivity was observed in these PPA-dopant complexes. All complexes show broad charge transfer bands which extend into the near-IR. The PPA-I(,2) interaction could be analyzed in part by means of a Benesi-Hildebrand plot.;The photocurrent in the red ((lamda) > 700 nm) is proportional to I('a) and to E('b) , when I is the light intensity, E is the field and a and b are generally 0.32-0.5 and 1.1-1.8, respectively. At a photon flux greater than 10('16) photons cm('-2)s('-1) and a field of 7.5 x 10('4) V/cm, the ratio of photocurrent to dark current at (lamda) > 700 nm is usually > 100. Transit times for electrons were resolvable in DDQ-doped PPA yielding a mobility of 2 x 10('-3) cm('2)/V sec. Carrier lifetimes of 4-6 (mu)sec could be observed in moderately doped PPA and are much shorter in the undoped polymer, with mobilities estimated at about 10('-7) cm('2)/V.sec. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.;Undoped trans-rich PPA shows two regions of photoconductivity, one corresponding to the (pi)-(pi)('*) transition, with a band edge at about 500 nm, the other in the red and near-IR. The intensity and spectral dependence of the long wavelength photoconductivity depends strongly on the microstructure of the polymer, e.g. the cis/trans content, and the nature of the dopant. The concentration of shallow traps for electrons could be determined from the time integral of transient photocurrents. These traps, which allow the injection of charge carriers from the electrodes and also limit bulk transport, exert major control over dark and photocurrents in the undoped polymer. The dark currents in trans-rich PPA films 3-7 (mu)m were interpretable in terms of diffusion-limited Schottky-Richadson field-assisted electron emission from the negative metal electrode. Holes are strongly trapped in undoped PPA. |
