Influence of Copolymer Interface Orientation on the Optical Emission of Polymeric Semiconductor Heterojunctions

We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier ($\sim 65\mathrm{meV}$) or an attractive interaction which can enhance the charge-transfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.

Figure 1

(a) Normalized PL intensity for the $\mathrm{PFB}:\mathrm{F}8\mathrm{BT}$ blend and homopolymer films integrated from 0–100 ns (excitation: 3.05 eV, $<4\mathrm{nJ}/{\mathrm{cm}}^2$, plotted on the logarithmic scale to show the small feature around 2.8 eV). (b) PL intensity at 300 fs after excitation (excitation: 3.06 eV, $\sim 50\mathrm{nJ}/{\mathrm{cm}}^2$). The homopolymer PFB and F8BT emission are normalized to the respective PFB and F8BT emission band from the blend to display the spectral shift. Inset shows the chemical structures of the PFB and F8BT and the two extreme relative orientations discussed in the text.

Figure 2

(Top) Time-resolved PL of the blend film showing (from the top curve down) the PL dynamics from both PFB and F8BT components measured at 0.1, 0.3, 0.7, 1.1, 3.1, 6, 22, and 50 ps after excitation. The decay dynamics of the blend film at 2.25 eV [2.7] compared with that of the pure F8BT film [pure PFB film] are shown in (a) and (c). The average emission energy from the F8BT and PFB components of the blend film were also extracted and are shown in comparison with their pure-film counterparts in (b) and (d), respectively.

Figure 3

At the top the PFB (2 repeat units) and F8BT (5 repeat units) ground-state structures are shown. The planes containing the two structures were slid parallel to each other with the separation distance of 0.45 nm. The contour plot shows, as a function of the position of the two planes, the Coulombic interaction energy between the PFB charge redistribution upon photoexcitation with the F8BT ground-state charge distribution. Positions marked A and B in the contour plot correspond to (a) and (b) arrangements (A: repulsive, B: attractive interaction). F8BT charge redistribution upon excitation interacting with the PFB ground-state charge gives a slightly more complicated pattern (not shown) with smaller interaction energy in the range of $-20$ to 20 meV.