Electroabsorption Spectroscopy to Probe Charge Transfer Character in Donor/Acceptor Materials for Organic Photovoltaics

We perform electroabsorption (EA) spectroscopy to understand charge transfer in polymer donor/acceptor blend systems comprised of three different acceptors: (i) fullerene acceptor, (ii) non-fullerene acceptor (IT-series), and (iii) non-fullerene acceptor (Y-series). We observe that the EA spectrum of polymer/fullerene blends and the blend based on 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’, 3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-4F) exhibit features which primarily correspond to first derivative of absorbance at the first optical transition, suggesting localized exciton formation upon photoexcitation. In contrast, the EA spectrum from the blend based on 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2",3’':4’,5']thieno [2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis (5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6) is dominated by second derivative of absorbance spectrum, indicating formation of excited state with charge transfer (CT) characteristics upon photoexcitation. We carried out EA of neat Y6 and observed that the signal in blend originates primarily from Y6. Among the studied polymer donors and acceptors, Y6 exhibits the highest dipole moment change of 7.5±2.5 Debye, consistent with a high degree of CT character, and a relatively large polarization volume of 361±70 ų, suggesting better electron delocalization than other NFA acceptors such as IT4F. Further, we have also developed a new method to resolve CT state in the donor/acceptor blend by observing an additional feature in the EA of blend film at low energy, which is not present in the neat materials. These results provide evidence of EA spectroscopy in resolving the CT state across multiple polymer/non-fullerene acceptor blends and understanding the uniqueness of the Y6 acceptor due its large dipole moment change upon photoexcitation.