Theoretical Investigation of Hole Transporter Materials for Energy Devices

We present combined MD-DFT calculations to investigate the electronic, optical, and charge-transport properties of six triphenylamine-based hole-transporter materials (HTMs) used in solid-state dye-sensitized solar cells (ssDSSC), including the state-of-the-art material in this field, 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD). We find that all of the studied materials present typical features of a HTM: (1) delocalized highest occupied molecular orbital (HOMO); (2) hole reorganization energies higher than the electronic ones; and (3) transparency in the visible region of the electromagnetic spectrum. Among the investigated compounds, 4-(4-phenyl-4-alpha-naphthylbutadienyl)-N,N-di(4-tolyl)-phenylamine (HTM1) shows the most promising features: a low HOMO energy level that favors high open-circuit voltage in solar cells, high adiabatic ionization potential ensuring great stability in terms of resistance to ionization, and small exciton binding energy. Our results also indicate that the presence of a butadiene moiety in HTM1 is somehow responsible for a higher molecular flexibility, thus favoring the pore filling of the semiconductor in ssDSSC. Finally, its optimal charge delocalization favors the overlap of the atomic orbitals, thus enhancing the electronic couplings, while its low energetic disorder causes a high hole mobility in the amorphous phase. The obtained results are in qualitative and quantitative agreement with the experimental data and suggest that the current computational approach could be further employed to obtain valuable insights for the design of new HTMs aiming at improving the performances of presently available ssDSSC.