Initial transport of photogenerated charge carriers in $\pi$-conjugated polymers

We have clarified the initial transport of photogenerated charge carriers in two $\pi$-conjugated polymers, namely regiorandom poly(3-hexylthiophene) and copolymer poly(9,9-dioctylfluorene- $\mathrm{co}$-bis-${N,N}^{\prime }\mathrm{}(4-\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{y}\mathrm{l}\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l})\mathrm{}$- bis-${N,N}^{\prime }$-phenyl-1,4-phenylenediamine), using integral mode time of flight. We show how to deconvolute the factors that contribute to the fast initial (pretrapping) transport and eliminate the normally important role of the photogeneration efficiency. We can then determine the dependence of the initial transport distance on applied electric field and temperature, $l_f\mathrm{}(F,T).$ We analyze $l_f\mathrm{}(F,T)\mathrm{}$ using a model where only the charge carriers which thermalize down to an energy not less than $\mathrm{kT}$ from the Coulomb potential maximum in a given electric field, immediately after photoexcitation, participate in the drift. We obtain excellent agreement between our experimental results and the model, showing that the initial fast carriers move a distance $l_f\sim 20-100\mathrm{nm}$ with high mobility before being trapped. The mobility-lifetime product $\left({\mu }_0{\tau }_f\right)$ of the fast carriers is of the order $(1\mathrm{}–2)\times {10}^{-11}{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ in these two $\pi$-conjugated polymers despite significant differences in their detailed chemical structure.