Fig. 1.

Molecular characteristics of organic HTMs. a Schematic representation of four-level photocycle of the ESIPT process. b Energy levels of organic HTMs measured by CV and absorbance; energy levels of the other layers composing perovskite solar cell are also added³². c Chemical structures of the designed ESIPT-active HTMs (ME1, ME2, and ME3) and their orbital diagrams in enol and keto forms calculated by DFT. d Chemical structures of the designed non-ESIPT HTMs (M2 and M3) and their orbital diagrams calculated by DFT. M2 and M3 are prepared as non-proton-transfer model compounds of ME2 and ME3. Beside all orbital diagrams, theoretically calculated energy levels and dipole moments are added