Isoindigo derivatives as promising hole transport materials for perovskite solar cells

The role of hole transport material (HTM) layer is critical to obtain high performances for perovskite solar cells (PSCs). The development of novel HTMs with good stability, low cost, and high hole mobility for PSCs is an essential research direction. Herein, new isoindigo-based HTMs were designed and investigated using density functional theory (DFT) computations combined with Einstein relation and Marcus theory. It was found that the energies of HOMO and LUMO levels were increased with an enhancement in the electron-donating character of substituents located on the diphenylamine moieties. The molecules with X = N(Me)(2), NHMe, NH2, OMe, OH, and Me substituents exhibited appropriate energy level alignments with respect to MAPbI(3), which indicated the hole could be injected from the MAPbI(3) to the designed HTMs. Based on the solvation energies, the designed HTMs would be expected to be soluble in the dichloromethane solvent. The results of the absolute hardness and electrostatic surface potential calculations revealed that the stability of such HTMs was decreased with an increase in the electron-donating character of the substituents positioned onto the diphenylamine groups. All of the designed HTMs, except for the X = OH molecule, illustrated considerably greater hole mobility than that of the Spiro-OMeTAD. In view of the matched HOMO level, good hole mobility, plus acceptable solubility and stability, the designed isoindigo based HTMs would have the potential for replacing the Spiro-OMeTAD in PSCs.