The Role of Solvent Additive Processing in High Performance Small Molecule Solar Cells

The use of small volumes of a high boiling point liquid as a “solvent additive” is a deposition processing method that has been implemented in most high/record performing polymer:fullerene-based bulk heterojunction (BHJ) solar cell devices. Recently, solvent additive processing has been employed in a solution processable small molecule (SPSM) BHJ system, viz., 5,5′-bis­{(4-(7-hexylthiophen-2-yl)­thiophen-2-yl)-[1,2,5]­thiadiazolo­[3,4-c]­pyridine}­3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene:[6,6]-phenyl C₇₁ butyric acid methyl ester (p-DTS­(PTTh₂)₂:PC₇₁BM), when a small amount, 0.25 v/v %, diiodooctane (DIO) was added to the casting solution, several key device metrics increased, leading to a high power conversion efficiency (PCE) of 6.7%. X-ray diffraction experiments show that the amount of additive added to the casting solution to make p-DTS­(PTTh₂)₂:PC₇₁BM thin films has several effects on the structure at multiple length scales: for example, the number and orientation of p-DTS­(PTTh₂)₂ crystallites, different π–π stacking distances, and the nanoscale domain size. Additionally, we utilize energy filtered transmission electron microscopy (EFTEM), a technique that significantly enhances the contrast between p-DTS­(PTTh₂)₂ and PC₇₁BM in real space, to further verify the effect of increasing domain size as the additive concentration is increased. Tomographic reconstruction of the TEM micrographs provides a 3D representation of the BHJ structure. These studies show how domain size and tortuosity in all dimensions change due to solvent additive processing, and the overall finding is that the nanostructures of p-DTS­(PTTh₂)₂ have enhanced connectivity when 0.25 v/v % DIO was used. Finally, we show evidence of solvent additive retention in p-DTS­(PTTh₂)₂:PC₇₁BM films when 1 v/v % DIO is used (but absent for 0.25%). This finding, in conjunction with the appearance of two populations of π–π stacking distances when 1 v/v % DIO is used, leads to the identification of one of the specific points of interaction between DIO and p-DTS­(PTTh₂)₂.