**5. Conclusion**

The aim of this chapter is to combine experimental analyses and theoretical calculations to present a comprehensive study of the structural and optical properties of organic electronic devices. Based on model compounds, Highest Occupied Molecular Orbital (HOMO), Lowest Unoccupied Molecular Orbital (LUMO) levels, Ionization Potential (IP), Electron Affinity (EA) as well as electronic structures for two samples are examined. The optoelectronic parameters studied here are essential for better understanding of the exchange between polymer and electrodes in PLEDs and PPCs. The experimental and computational results are compared and discussed.

The first part of this chapter shows how important it is to combine thienylene, dialkoxyphenylene and bipyridine fragments to obtain compounds with a strong electronic delocalization. As a result, analysis of the results obtained in the gas phase has allowed us to understand the crucial role played by the intra-molecular S--O and N--H interactions in determining the planarity of the compound. This leads to the formation of a donor–acceptor type of arrangement within the polymer backbone and an intra-molecular charge transfer for the TBT-BIPY copolymer model compound. In addition, we have presented the optical and emission properties of these compounds by studying the ground and first excited states of copolymer models.

In the second part, we have used the density functional theory DFT/B3LYP to investigate the photo-physical properties of some copolymers in alternate donor-acceptor structure. In fact, the modification of chemical structures can greatly modulate and improve the electronic and optical properties of pristine copolymers. Hence, added to benzothiadiazole units, the introduction of carbazole motives in the copolymer backbone results in a better overlap of the absorption spectrum with the solar spectrum. In addition, the hexylthiophene linkage is found not only as a conjugated bridge but also it reduces the steric interaction between aromatic rings and thus enhances the effective charge transfer between donor and acceptor units.

In fact, the obtained theoretical data derived from DFT/B3LYP/3-21G\* method are in good agreement with the available experimental data. The resulting optimized BHJ active layer shows a -stacking configuration governed by a Wander walls interaction. A model energy band diagram is introduced, simulating the energy behaviour of this active layer. Based on this design concept, the PSC using the blend of P3HT2BTCz with fullerene derivatives, exhibit a promising performance with a PCE up to 5%. This approach provides great flexibility in fine-tuning of the absorption spectra and energy levels of the resultant polymers for achieving high device performance.

Finally, these results clearly indicate that these new compounds with alternating donoracceptor structures are promising materials for application in optoelectronic devices. Devices fabrication and characterization are in progress and will be published elsewhere.
