Optimizing H-/J-Type Aggregation and Vertical Phase Separation To Improve Photovoltaic Efficiency of Small Molecule Solar Cells by Adding a Macromolecule Additive

Exciton dissociation and charge collection are two key processes for small molecule solar cells. H-type aggregation provides a strong driving force for exciton dissociation due to a higher lowest excited level. Optimal vertical phase separation with the donor and acceptor accumulating on the cathode and anode, respectively, is favorable for charge collection. However, J-type aggregation dominates in 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2), while p-DTS(FBTTh2)2 (donor) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM, acceptor) tend to accumulate on the cathode and anode, respectively, in the bulk heterojunction (BHJ) solar cell. We proposed to add a macromolecule polydimethylsiloxane (PDMS) into the p-DTS(FBTTh2)2/PC71BM chlorobenzene (CB) blend solution to enhance both the H-type aggregation and vertical phase separation synchronously. PDMS can enter the aggregation phase of the alkyl side chain of p-DTS(FBTTh2)2 to increase steric hindrance of the adjacent donor molecules; thus, the ratio of H/J is improved from 0.32 to 0.46. Meanwhile, the network structure PDMS formed impeded PC71BM transferring to the bottom of film, with the weight ratio mPC71BM/mp-DTS(FBTTh2)2 improved from 0.036 to 0.159 on the cathode. Both the H-type aggregation and vertical phase separation are propitious to increase efficiency of exciton separation, from 91.6% to 93.9%. As a result, by adding 2% PDMS into the CB solution with 0.2% 1,8-diiodooctane (DIO), the power conversion efficiency (PCE) increased from 6.04% to 6.59%.