Enhanced Charge and Energy Transfer in All-Small-Molecule Ternary Organic Solar Cells: Transient Photocurrent and Photovoltage and Transient Photoluminescence Measurements.

A donor-acceptor-donor (D-A-D) molecule, denoted as RC18, consisting of two nickel-porphyrin  terminal donor units (D) and a selenophene-flanked diketopyrrolopyrrole central core, connected via  an ethynylene linker has been synthesized. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels were measured showing values of -5.49 eV and -3.75 eV, respectively. We have utilized RC18 as donor along with two acceptors, DICTF and Y6, for OSCs and found that power conversion efficiencies were 12.10% and 12.59% for RC18:DICTF and RC18:Y6, respectively. The complementary absorption profiles of RC18, DICTF and Y6, along with the intermediate LUMO level of DICTF between RC18 and Y6, led to the fabrication of ternary organic solar cells. RC18:DICTF:Y6 based ternary attained  power conversion efficiency of 16.06%. The observed enhancement in the PCE is attributed to efficient exciton utilization through energy transfer from DICTF to Y6, increased donor-acceptor interfacial area, suppressed charge carrier recombination and improved molecular ordering. These all factors contribute  to improvements in short-circuit current density (JSC) and fill factor (FF). Additionally, the open-circuit voltage (VOC) of the ternary OSC lies between those of the two binary OSCs indicating the formation of an alloy between the two acceptors.