High‐Efficiency Digital Inkjet‐Printed Non‐Fullerene Polymer Blends Using Non‐Halogenated Solvents

Abstract

Inkjet printing (IJP) of polymer solar cells is ideal for small‐area off‐grid electronics with low power consumption. However, IJP is quite a complex technique compared with techniques such as spin coating or doctor blading. The IJP of polymer blends is reported based on ITIC derivatives as non‐fullerene acceptors (NFAs) using non‐halogenated solvents. The results show that fluorination of NFA is essential to form highly stable inks in o‐xylene, because ITIC has significantly insufficient solubility compared with ITIC‐4F. The importance of tetralin as a multifunctional co‐solvent for printing highly efficient PM6:ITIC‐4F blends is demonstrated, as even at very low concentrations, tetralin not only improves ink jettability and open nozzle time, but also improves drying behavior of the blend layer, resulting in blends with homogeneous micro‐ and nanoscale morphology. The resulting solar cells using inkjet‐printed polymer blends show a maximum efficiency of 10.1%. Moreover, IJP produces significant changes in the nanoscale and microscale morphology. In particular, the formation of a thin PM6 capping layer on the blend surface along with improved phase separation and crystallinity in both the donor and acceptor greatly reduces the recombination of charge carriers in thick blends, making inkjet‐printed photoactive films very promising for industrial applications.

Non‐Fullerene Acceptors with an Extended π‐Conjugated Core: Third Components in Ternary Blends for High‐Efficiency, Post‐Treatment‐Free Organic Solar Cells

“…All blends show similar surface morphology composed of a mixture of fibril and grain-like patterns that are quite typical for blends prepared from PM6. [25] PM6:BITIC-C8, PM6:BITIC-PhC6, and PM6:BITIC-PhC6F4 show rms values of 2.12, 2.03, and 2.53 nm, respectively, close to PM6:ITIC-4F layers. [25] In contrast, PM6:BITIC-C8F4 blend shows a smoother surface with rms = 0.54 nm.…”

“…[25] PM6:BITIC-C8, PM6:BITIC-PhC6, and PM6:BITIC-PhC6F4 show rms values of 2.12, 2.03, and 2.53 nm, respectively, close to PM6:ITIC-4F layers. [25] In contrast, PM6:BITIC-C8F4 blend shows a smoother surface with rms = 0.54 nm. Solar cells using inverted device structures as glass/indium-doped tin oxide (ITO)/ZnO/activelayer/MoO 3 /Ag were then processed by deposition of the blend solutions on a ZnO electron-transport layer via spin-coating under argon (more details about fabrication in the Experimental Section).…”

“…We also synthesized by comparison with ITIC-4F, the fluorinated derivate leading to improved solubility in o-xylene compared to ITIC. [25] Hwang and co-workers recently explored a similar molecular design and demonstrated an enhanced absorptivity of the blends and a good processability, thereby achieving solar 54 cm 2 modules with efficiencies reaching 9.2 %. [26] Owing to the good performances of NFAs based on 4,9dihydro-s-indaceno[1,2-b : 5,6-b']dithiophene (IDT) units, [5,27] we synthesized a series of four NFAs called BITIC-C8, BITIC-PhC6, BITIC-PhC6F4, and BITIC-C8F4 based on an IDT central core (for structures see Scheme 1).…”

“…[25] The photovoltaic properties of the NFAs were thus evaluated by processing BITIC-C8 and BITIC-PhC6 blended with PM6 from chlorobenzene solution, while BITIC-PhC6F4 and BITIC-C8F4-based blends were processed from o-xylene, identical to PM6:ITIC-4F blend that was used as reference material. [25] Prior to device characterization, we studied the surface morphology of the binary blends obtained from the solutions as mentioned before, using AFM. Topography images together with root mean square roughness (rms) are shown in Support-ing Information.…”

“…For a transfer to industry, they also have to be soluble and processable from non-halogenated solvents such as o-xylene. [24,25] Keeping in mind these criteria, we propose in this work a molecular design, inspired by ITIC, 2,2'-[ [6,6,12,12-Tetrakis(4hexylphenyl)-6,12-dihydrodithieno [2,3-d:2',3'-d']-s-indaceno [1,2b:5,6-b']dithiophene -2,8-diyl]…”