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Intrinsic and extrinsic photocarriers in polyparaphenylenevinylene

1998; American Physical Society; Volume: 57; Issue: 15 Linguagem: Inglês

10.1103/physrevb.57.9118

1095-3795

B. Dulieu, J. Wéry, S. Lefrant, J. Bullot,

Synthesis and Properties of Aromatic Compounds

We report on photoconductivity in polyparaphenylenevinylene films under continuous wave illumination in the presence of air traces. Near room temperature the photocurrent buildup is extremely slow, steady state not being reached after 1 h illumination. When light is turned off the photocurrent decay kinetics is slower and slower and it takes about 24 h to reach thermal equilibrium. The instantaneous lifetime follows a power-law time dependence ${\ensuremath{\tau}}_{\mathrm{inst}}{(t)=i}_{p}(t)/|{\mathrm{di}}_{p}(t)/dt|\ensuremath{\propto}{t}^{\ensuremath{\alpha}},$ $0\ensuremath{\prec}\ensuremath{\alpha}\ensuremath{\prec}1$ and the decay kinetics can be accurately fitted to a stretched exponential relaxation law ${i}_{p}{(t)=i}_{p}(0)\mathrm{exp}\ensuremath{-}(t/\ensuremath{\tau}{)}^{\ensuremath{\beta}}$ where $\ensuremath{\beta}=1\ensuremath{-}\ensuremath{\alpha}.$ When temperature is decreased down to $\ensuremath{\approx}255\mathrm{K}$ the exponent \ensuremath{\beta} and the relaxation time \ensuremath{\tau} are found to be temperature dependent, \ensuremath{\beta} being linearly dependent upon $T:$ $\ensuremath{\beta}{=T/T}_{0}$ with ${T}_{0}=1175\mathrm{K}.$ The nonlinear decrease of \ensuremath{\beta} between 255 and 190 K is interpreted as a transport mode transition at ${T}_{t}\ensuremath{\approx}225\mathrm{K}.$ When temperature is further decreased in the range 160--77 K the slow component is frozen and leaves room to a fast signal that rapidly reaches steady state and is slightly dependent upon temperature. The magnitude of the slow photocurrent is proportional to the square root of the light intensity whereas that of the fast one is proportional to the light intensity. Modulated photocurrent studies allowed us to show that the fast signal exists in the whole temperature range. Examination of the temperature dependence of the fast photocurrent at low temperature led us to interpret the fast signal as being an intrinsic photocurrent due exciton dissociation. Such an interpretation rests on the recent work of Albrecht and B\"assler on the yield of geminate pair dissociation in an energetically random hopping system with built-in energetic disorder. The slow component is interpreted as being extrinsic due to dissociation of polaron pairs through interaction with oxygenated defects to create positive polarons. Recombination and the functional form of the extrinsic component decay is attributed to the dispersive diffusion of the positive polarons in a random distribution of negatively charged defects.