Environment friendly solvent processed, fullerene-free organic solar cells with high efficiency in air

Fullerene-free organic solar cells (OSCs) have attracted significant interest in the research community over the past few years. Their efficiency has risen rapidly, with multiple reports of record power conversion efficiencies (PCEs) breaching 14%. While encouraging, these performance metrics are often achieved with the utilization of toxic halogenated solvents for the fabrication process, which is less attractive for large-scale manufacturing. Dimeric perylene diimide (PDI) electron transport materials are currently considered amongst the key candidates for the realization of low-cost, highefficiency “green-processed” OSCs. The low-cost synthetic versatility of the PDI skeleton allows for a range of chemical “fine-tuning” and the chromophore has excellent photochemical stability and strong light absorption in the visible region. This report will detail our research into OSCs using PDI dimers as the electron acceptors and active layer fabrication using non-halogenated solvents. PTB7-Th was chosen as the donor material, owing to its good solubility in nonhalogenated solvents, complementary light absorption and suitable energy level alignment for pairing with our PDI acceptors. Two different PDI dimers having linear and branched alkyl chains are studied. We have previously shown that PTB7-Th:PDI based solar cells with active layers processed from 2Me-THF, o-xylene, or 1,2,4-trimethylbenzene could reach PCEs from 5-6%. The processing solvent can be extended to toluene with solar cells exhibiting PCEs of 5%. Thus, this work highlights the many processing options for the PTB7-Th / N-annulated PDI dimer active layer combination.