Reduced Charge Transfer Exciton Recombination in Organic Semiconductor Heterojunctions by Molecular Doping

We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer–fullerene interfaces. We demonstrate that a low concentration of $p$-type dopant molecules ($<4\%$ weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by $\sim 70\%$. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states.

Figure 1

(a)–(c) Photoluminescence spectra of PCBM, PCPDTBT, and PCPDTBT/PCBM ($1:1$ weight ratio) blend, respectively. The insets show the chemical structure of PCBM and PCPDTBT. (d) CT PL spectra as a function of F4-TCNQ concentration in PCPDTBT/PCBM blends. The inset shows the chemical structure of F4-TCNQ. Excitation was performed at 3.1 eV. Note that the spectra and all other experiments were corrected by considering the differences in the number of absorbed photons.

Figure 2

Transient decays of photoluminescence recorded at 1.17 eV for PCPDTBT/PCBM blends with different concentrations of F4-TCNQ (excitation: 3.1 eV). The inset shows the normalized decays plotted on a longer time scale. Solid lines are fits according to the model presented in the text.

Figure 3

(a) Energy scheme for the highest occupied molecular orbital and lowest unoccupied molecular orbital levels of PCPDTBT and PCBM together with the most significant transitions. (b)–(e) PA transients of PCPDTBT/PCBM blends with different concentrations of F4-TCNQ (0%–5%, respectively). Pump (3.1 eV) and probe (0.4 eV) beams had a cross correlation of $\sim 150\mathrm{fs}$. Solid lines are fits according to the model presented in the text.

Figure 4

Photoluminescence spectra of PCPDTBT with different concentrations of F4-TCNQ. The top inset shows the relative shift in the PL peak position as a function of F4-TCNQ concentration expressed in weight% with respect to the polymer.