Impact of alkyl chain length of 1,n-diiodoalkanes on PC71BM distribution in both bulk and air surface of PTB7:PC71BM film

• The effect of varied alkyl chain length of additives on the PC71BMs was systematically investigated for the first time.
• The length of alkyl chain of additives influences on the surface morphology of PTB7:PC71BM bulk heterojunction film.
• The electrical properties of PTB7:PC71BM solar cells have been found to closely correlate with chain length of additives.
• A new four-step route is finally proposed to further optimize the performance of additive based polymer solar cell.

The impact of alkyl chain length of different additives, such as 1,4-diiodobutane (DIB), 1,6-diiodohexane (DIH), 1,8-diiodooctane (DIO) and 1,10-diiododecane (DID), on the PC71BM distribution in PTB7:PC71BM-based polymer solar cells, is systematically investigated, for the first time. Among these additives, DIO is found to have the optimum alkyl chain length that maximizes the performance of PTB7:PC71BM based polymer solar cells, attaining a power conversion efficiency as high as 8.84%, which is almost four times higher than that without any additives. For DID additives (longer alkyl chain length than DIO), a drop in efficiency to 7.91% was observed. Experimental investigations show that the microstructure of the bulk and the surface layer as well as the surface morphology of the PTB7:PC71BM polymer film can be controlled simultaneously by varying the alkyl chain length of additives. Results also show that the substantial improvement in performance is attributed to the improved 1) phase segregation, 2) PC71BM distribution uniformity in the bulk of the PTB7:PC71BM film, 3) surface smoothness and 4) high PTB7 content at the interface between the active layer and the top electrode.

Schematic illustration of the morphologies and components of PTB7:PC71BM films without and with DIB, DIH, DIO and DID additives for different efficiency respectively.Figure optionsDownload as PowerPoint slide