The power conversion efficiencies of Bulk-heterojunctions organic solar cells using a blend of low band-gap polymers and fullerene derivatives have gradually improved up to 11% due to intensive developments such as, the synthesizing of efficient semiconducting donor polymers, the controlling of the morphology of the active layers, the introducing of additional interfacial layers, and the designing of the devices architectures [1]. However, most of these optimizations were done using chlorinated solvents such as chloroform, dichlorobenzene or even trichlorobenzene [2]. These chlorine containing solvents are highly toxic and environmentally hazardous, which limit their usage in the industry. Recently, some research teams started to work on a new processing strategy for the inks formulation by using more benign solvents which meet the industrial standard. Unfortunately, most of the existing low band-gap polymers exhibited a very poor solubility in these non-toxic solvents. The inability to solubilize these polymers in these solvents complicates their use in making inks for large scale organic solar cells. The second limitation for the industrialization of the majority of the champion cells reported so far is mainly due to most films deposition techniques used are based on spin coating [3] instead of more scalable techniques for the industry like blade coating, slot die coating, and roll-to-roll printing. Thus, today the challenge is more on how to prepare efficient inks using non-toxic solvent and adapt these inks to the existing industrial process.

In this work, we report a process for the preparation of a new environment-friendly ink formulation. We first selected a high performance donor polymer the Poly[(5,6-dihydro-5-octyl-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl)[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′;]dithiophene-2,6-diyl]] PDTSTPD [4] in order to prepare the active layer ink. This polymer has already showed a very good compatibility with several non-toxic solvents [5]. In this work, 1,2,4-trimethylbenzene (TMB) instead chlorobenzene (CB) was selected and used as new host solvent in the ink preparation. The films prepared using this new ink was deposited using blade coating method. The solar cells realized provide performances up to 4% using inverted solar cell structure.

[1] T. H. Lee et al RSC. Adv, 7 (2017) 7476.

[2] P-T. Tsai et al, Org. Electon, 15 (2014) 893.

[3] G. Yu, et al, Science, 270 (1995) 178.

[4] T. Chu et al, J Am Chem Soc. 133 (2011) 4250.

[5] B. R. Aïch et al, Org. Electon 15 , (2014) 543.