Thiophene-based molecules as hole transport materials for efficient perovskite solar cells or as donors for organic solar cells

This study focuses on optoelectronic features of a series of D-A-D molecules composed of thiophene, ethylenedioxythiophene, or 3,4-dimethoxythiophene acceptor groups substituted in several positions using different end-capped electron-donating groups formed from triphenylamine or OMe-triphenylamine. The influence of π-extension on the charge transfer process is probed by inserting two azomethine groups between acceptor and donor (D-π-A-π-D) groups to make them economically profitable like solar cells. The experimental results available in the literature are combined with our detailed B3LYP/6-311G(d,p) study to find relationships between the structural, and optoelectronic properties of molecules and their performance as HTMs or as donors in organic photovoltaic cells. All investigated molecules were preferential competitors for HTMs in perovskite solar cells because of their excellent HOMO delocalization, lower hole reorganization energies, and high light-harvesting efficiency. They all show a spontaneous solvation process within dichloromethane and have larger Stokes shifts with respect to that of spiro-OMeTAD, which is advantageous for the pore-filling of HTMs for PSCs. The bound electron-hole pairs of the studied molecules can easily escape coulomb attraction by dissociating into negative and positive charges, thus facilitating hole transport, and improving the short-circuit current density. D-A-D type molecules have a blue-shifted absorption maximum in the visible light region and consequently have no overlap with the light-harvesting of the perovskite layer. The π-bridges lengthen the electronic conjugation path of the D-π-A-π-D compounds and introduce more flexibility to the molecule's backbone. Their higher radiative lifetimes make them good candidates for their usage in organic solar cell applications.