## 3 Results and Discussion
### Opto-Electronic Properties of A-A' copolymers in Solution and in the Solid State
These band gaps are very small and quite similar to that of the corresponding D-A copolymers reported in our previous publication with DTS as donor unit (reported in Table 2 for comparison) [47]. In order to investigate further the effects of alternating two acceptor monomers on the electronic levels, HOMO and LUMO levels were determined by cyclic voltammetry. A direct comparison of A-A' copolymers to D-A, with the evolution of band gaps, HOMO and LUMO levels are presented as a summary in the SI, Table S6. Polymers have been drop-casted onto a platinum electrode from chlorobenzene solutions (10 mg mL\({}^{-1}\)), and submitted to cyclic voltammetry in dry acetonitrile with Bu\({}_{4}\)NP\({}_{6}\) as the supporting electrolyte (see voltammograms in SI, Figures S8-S11). The HOMO and LUMO levels (in eV, ref vacuum) of copolymers were deduced from the oxidation and reduction potentials (see Table 2). For comparison, the DPP, DAP, and BTPBF monomers were also analyzed by cyclic voltammetry in the same conditions. Results previously obtained with analogous D-A' copolymers are also reported in Table 2.  
The HOMO-LUMO band gap determined with monomers showed a strong decrease in the order DPP, DAP, and BTBPF (2.2, 1.8, and 1.3 eV, respectively). Both HOMO and LUMO levels were affected by changing the structure. All these energy levels are reported in Figure 5, and presented with poly(3-hexylthiophene) (P3 HT) and PCBM, for comparison. When comparing the A-A' copolymers, the main observation is that LUMO levels were deepened in correlation with the electron affinity of the comonomer combined with DPP, as it was already observed when copolymerized with DTS. HOMO levels were only slightly modified and were about \(-5.0\) eV on average. When comparing the series of P(DTS-A) copolymers to P(DPP-A) copolymers with the same A, replacing DTS by DPP provoked the lowering of both LUMO and HOMO levels in the same manner, leading to unmodified bandgaps, as already observed with optical gaps. In the case of P(DTS-BTPBF) and P(DPP-BTPBF) also, the change of DTS by DPP strongly affected both HOMO and LUMO levels \begin{table}
\begin{tabular}{l c c c c c c c} \hline  & \({}^{a}\)\(\lambda_{\text{max}}\) (nm) & \({}^{a}\)\(\lambda_{\text{max}}\) (nm) & \({}^{a}\)\(\lambda_{\text{edge}}\) (nm) & \({}^{a}\)\(\mathbf{E_{g}}^{\text{opt}}\) & \({}^{b}\)HOMO & \({}^{b}\)LUMO & \({}^{b}\)\(\mathbf{E_{g}}^{\text{cv}}\) \\  & **Solution** & **Solid State** & **Solid State** & **(eV)** & **(eV)** & **(eV)** & **(eV)** \\ \hline \multicolumn{10}{l}{**Monomers**} \\ \hline DPP & 545 & - & 570 (solution) & 2.13 \({}^{c}\) & \(-5.7\) & \(-3.5\) & 2.2 \\ DAP & 565 & - & 640 (solution) & 1.94 \({}^{c}\) & \(-5.4\) & \(-3.6\) & 1.7 \\ BTPBF & 695 & - & 775 (solution) & 1.60 \({}^{c}\) & \(-5.3\) & \(-4.0\) & 1.3 \\ \hline \multicolumn{10}{l}{**Copolymers**} \\ \hline P(DPP-DPPeH) & 706 & 632 & 980 & 1.35 & \(-5.4\) & \(-4.0\) & 1.4 \\ P(DPP-DPPC12) & 701 & 748 & 1030 & 1.3 & \(-5.5\) & \(-4.1\) & 1.3 \\ P(DPP-DAP) & 842 & 839 & 1230 & 1.1 & \(-5.4\) & \(-4.2\) & 1.2 \\ P(DPP-BTPBF) & 1051 & 1047 & 1450 & 0.9 & \(-5.5\) & \(-4.4\) & 0.9 \\ P(DTS-DPPeH) & 795 & 800 & 1000 & 1.3 & \(-5.4\) & \(-3.9\) & 1.5 \\ P(DTS-DAP) & 837 & 867 & 1180 & 1.1 & \(-5.4\) & \(-4.1\) & 1.3 \\ P(DTS-BTPBF) & 1100 & 1081 & 1440 & 0.9 & \(-5.2\) & \(-4.2\) & 1.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Optoelectronic characteristics for monomers and copolymers. by roughly 0.25 eV, leading to deeper levels without perturbating the bandgaps. Thus, the P(DPP-BTPBF) copolymer exhibited very deep HOMO and LUMO levels at \(-\)5.5 and \(-\)4.4 eV, respectively.