Enhanced Open-Circuit Voltage by Using 2,7-Pyrene as a Central Donor Unit in A-π-D-π-A-Type Small-Molecule-Based Organic Solar Cells

To get insights into the relationship between molecular structure and performance in photovoltaic cells, two small molecules of T(3TDRCN)₂ and Pr(3TDRCN)₂ were synthesized with an A-π-D-π-A type backbone structure. The T(3TDRCN)₂ and Pr(3TDRCN)₂ were using n-octyl side chains substituted trithiophene (3T) as the π-linker and 2-(1,1-dicyanomethylene)-rhodanine (DRCN) as terminal acceptor (A) group, while varied the central donor (D) unit from thiophene (T) to 2,7-pyrene (Pr). Both SMs were applied as donor materials along with fullerene acceptor (PC₇₁BM) for solution-processed bulk-heterojunction solar cells. The impacts of central donor units on their optical absorption, electrochemical property, hole mobility, and solar cell performance were primarily studied. The T(3TDRCN)₂:PC₇₁BM based devices yielded a power conversion efficiency (PCE) of 2.60 % with an open circuit voltage (V_oc) of 0.89 V. Interestingly, the Pr(3TDRCN)₂ based solar cell exhibited an higher PCE of 4.47 % with an improved V_oc of 0.95 V. Our work indicates that using large π-conjugated aromatic donor with weak-electron donating property as central unit can improve V_oc, as well as high photovoltaic efficiency of the A-π-D-π-A type SMs in organic solar cells.