The optoelectronic properties of organic materials based on triphenylamine that are relevant to organic solar photovoltaic cells

In this paper, six symmetrical compounds with a donor-acceptor-donor (D-A-D) structure based on triphenylamine as a core and bridged with various spacer fragments with electron-acceptor character, such as quinoxaline, phenazine, benzo[g] quinoxaline, benzo[b] phenazine, thieno[3,4-b] pyrazine and thieno[3,4-b] quinoxaline, have been studied theoretically using two quantum methods, DFT and TD-DFT, together by considering their use in organic BHJ solar cells as potential donors of electron. Intramolecular charge transfer (ICT) interactions have been found for all of the compounds due to the electron withdrawing properties caused by the two imine nitrogen atoms in the pyrazine and the electron-donating properties of the other two amine nitrogen atoms in the two triphenylamines that compose the bridge. The ICT interactions are strengthened when the benzene or thiophene ring are fused on the pyrazine acceptor unit, which leads to a bathochromic shift of the ICT band. Moreover, our quantum calculations show that thiophene is a more appropriate ring than the benzene ring for enlarging the ICT interactions and expanding the absorption spectrum. We remark also that when a thiophene ring is fused on the quinoxaline unit in the D6 compound, a significant shift towards the near-infrared is realized, and it exhibits a maximum absorption wavelength at 690 nm compared to the other studied compounds with a threshold that is over 800 nm. This is attributed to the enhanced charge density on the acceptor spacer and narrow band gap of this compound, as revealed by our theoretical calculations. Finally, these results show that extending the conjugation of a compound based on pyrazine as an acceptor in a D-A-D structured compound can be realized by incorporating the thiophene unit in the bridge and then strengthening the ICT interactions.