## 3 Results and Discussion
### Opto-Electronic Properties of A-A' copolymers in Solution and in the Solid State  
The optical properties of the different A-A' copolymers P(DPP-DPPeH), P(DPPP-DPPC12), P(DPPP-DAP), and P(DPP-BTBPBF) were studied by UV-vis-NIR absorption spectrophotometry in solution and in thin films (Table 2). As shown in Figure 4, all the copolymers display two characteristic bands. A first weakly absorbing band is located in the short-wavelength region, around 400 nm for P(DPP-DPPeH) and P(DPP-DPPC12) and 470 nm for P(DPP-DAP) and P(DPP-BTBPF). The second band is broad and covers different wavelength ranges depending on the comonomer. Both copolymers exclusively based on DPP, P(DPPP-DPPeH), and P(DPP-DPPC12) absorbed in solution in the same range (500-900 nm) with a maximum absorption at 700 nm. However, a shoulder could be observed at 875 nm for the copolymer with the C12 linear chain, which can be related to the presence of aggregates in solution. As predicted by DFT calculations, copolymers based on DAP and BTPBF presented a large absorption shifted towards the infrared. P(DPPP-DAP) exhibited an absorption from 530 nm up to around 1200 nm while P(DPPP-BTBF) absorbed from 650 to 1400 nm. These results suggest that the electron accepting strength of monomers evolves in the order DPP < DAP < BTPBF. From DPP to DAP, the gain towards the infrared was 140 nm for the maximum of absorption, and 295 nm for the band edge. When comparing DAP and BTPBF, the shift in maximum absorption was 209 nm and 225 nm for the band edge. This confirms that the BTPBF building block is a strong electron acceptor unit, allowing an absorption range between 700 and 1400 nm, with a maximum  
Figure 3: Evolution with chain length of the vertical transition energies of the (A-A\({}^{\prime}\))\({}_{n}\) oligomers, as calculated at the IEFPCM:M06-2X/6-311G(d) level in chlorobenzene. Lines are fits calculated according to equation 1. Optimized values of the Kuhn fit parameters \(E_{0}\) and \(D_{k}\) are 6.63 eV and \(-\)0.92 for A = DPP, 6.10 eV and \(-\)0.93 for A = DAP, and 6.39 eV and \(-\)0.94 for A\({}^{\prime}\) = BTPBF. absorption at 1050 nm. When combined with DTS instead of DPP, the absorption was even more shifted in the NIR by 50 nm. The absorption spectra obtained with copolymers deposited as thin films slightly differ from those measured in solution. For copolymers only based on DPP, a redshift due to \(\pi\)-stacking was only observed with P(DPP-DPPC12), with a bathochromic effect of 47 nm for the maximum absorption and 60 nm for the band edge. It is also worth noting that the charge transfer band became broader with a strong increase of the shoulder at 875 nm, probably coming from the formation of aggregates or a more crystalline material. The change between solution and film was different in the case of P(DPP-DPPeH) that  
Figure 4: Normalized absorption spectra of A-A’ copolymers P(DPP-DPPeH), P(DPP-DPPC12), P(DPP-DAP), and P(DPP-BTPBF); (**a**) in chlorobenzene solutions; (**b**) in thin films (spin coated from chlorobenzene solutions). bears an ethyl hexyl chain on every unit. Indeed, the maximum wavelength was blue shifted, probably due to the branched chains that hinder the molecular packing, while an enlargement of the main absorption band was observed with a shoulder at 850 nm. The charge transfer band was enlarged from solution to solid state, but the maximum absorption was blue shifted of 74 nm, whilst the band edge showed a redshift of 100 nm. For P(DPP-DAP) and P(DPP-BTPBF), the maximum of absorption was not really shifted between solution and solid state; only a redshift of the band edge of around 50 nm was observed, indicating that aggregates can be already formed in solution. All these spectra allowed the calculation of optical bandgaps of 1.3, 1.2, 1.0, and 0.9 eV for P(DPP-DPPEH), P(DPP-DPPC12), P(DPP-DAP), and P(DPP-BTPBF), respectively.