We have designed new conjugated copolymers, based on Phenylene-thiophene oligomers coupled with the electron-rich carbazole (Cz) units. Different configurations have been studied via (3,6)-carbazole, (2,7)-carbazole and (9N)-carbazole position linkage. Based on Density Functional Theory (DFT) and using B3LYP method, the functionalisation with carbazole units deeply improve the electronic and optical properties of the resulting compounds. Different characteristics, such as structural parameters, electronic properties, HOMO-LUMO gaps, molecular orbital densities, ionization potentials (IPs), electronic affinities (EAs), reorganization energies (λhole and λelectron) and charge transport rate (khole/electron) are determined. The lowest excitation energies and the maximal absorption and emission wavelength (λmax ABS, λmax EM) are also underlined using the time-dependent density functional theory (TDDFT) method. The correlation of these different results shows that the introduction of the carbazole unit in the chain backbone improves the electron/hole transport proprieties and so enhances the performance of optoelectronic devices based on these compounds

Ionization potentials (IPs) and electron affinities (EAs) are important quantities input into most models for calculating the open-circuit voltage (V_oc) of organic solar cells. We assess the semi-empirical density-functional tight-binding (DFTB) method with the third-order self-consistent charge (SCC) correction and the 3ob parameter set (the third-order DFTB (DFTB3) organic and biochemistry parameter set) against experiments (for smaller molecules) and against first-principles GW (Green’s function, G, times the screened potential, W) calculations (for larger molecules of interest in organic electronics) for the calculation of IPs and EAs. Since GW calculations are relatively new for molecules of this size, we have also taken care to validate these calculations against experiments. As expected, DFTB is found to behave very much like density-functional theory (DFT), but with some loss of accuracy in predicting IPs and EAs. For small molecules, the best results were found with ΔSCF (Δ self-consistent field) SCC-DFTB calculations for first IPs (good to ± 0.649 eV). When considering several IPs of the same molecule, it is convenient to use the negative of the orbital energies (which we refer to as Koopmans’ theorem (KT) IPs) as an indication of trends. Linear regression analysis shows that KT SCC-DFTB IPs are nearly as accurate as ΔSCF SCC-DFTB eigenvalues (± 0.852 eV for first IPs, but ± 0.706 eV for all of the IPs considered here) for small molecules. For larger molecules, SCC-DFTB was also the ideal choice with IP/EA errors of ± 0.489/0.740 eV from ΔSCF calculations and of ± 0.326/0.458 eV from (KT) orbital energies. Interestingly, the linear least squares fit for the KT IPs of the larger molecules also proves to have good predictive value for the lower energy KT IPs of smaller molecules, with significant deviations appearing only for IPs of 15–20 eV or larger. We believe that this quantitative analysis of errors in SCC-DFTB IPs and EAs may be of interest to other researchers interested in DFTB investigation of large and complex problems, such as those encountered in organic electronics.