A bi-continuous network structure of p-DTS(FBTTh2)2/EP-PDI via selective solvent vapor annealing
A critical requirement of small molecule non-fullerene acceptor-based solar cells for efficient charge separation and collection is the formation of interconnected phase-separated domains of 10–20 nm. The phase-separation behavior of small molecule donor 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) and acceptor N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) was regulated by donor selective solvent vapor annealing (D-SVA), poor donor solvent vapor annealing (P-SVA) and thermal annealing (TA). It was found that the power conversion efficiency (PCE) was significantly improved from less than 0.2% up to 3.0% after D-SVA. In contrast, a limited improvement of PCE was obtained for P-SVA (1.9%) and TA (1.8%). The difference in the improvement of PCE values was attributed to the formation of a phase-separated structure and the regulation of crystallite sizes under different post treatments. The fibrous crystals of p-DTS(FBTTh2)2 were formed during D-SVA treatment while the EP-PDI component was in the form of small microcrystals, thus leading to the required interconnected phase-separated structure. As a consequence, the moderate phase-separated morphology accompanied by a pure crystalline phase with a medium size could maximize the carrier transport process without compromising exciton separation efficiency, and thus contributes to the final optimal PCE value of up to 3.0% under D-SVA. For P-SVA or TA treatment, we only observed a significant enhancement of the crystallinity of both p-DTS(FBTTh2)2 and EP-PDI components, leading to remarkable film coarsening, or even undesired large phase separation, which is detrimental to the exciton diffusion as well as exciton separation processes.