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Organic solar cells (OSC) based on diketopyrrolopyrrole (DPP) small molecule have achieved relatively high efficiency (7%) in recent times. 2,5-di(2-ethylhexyl)-3,6-bis-(5″-n-hexyl-[2,2′,5′,2″]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione (SMDPPEH) is the workhorse material for small molecule OSC as it has good solution-processing capability. However, there was no previous report on improving the device stability in inverted organic solar cells (IOSC) by using this material. Furthermore, the degradation and light-soaking behavior of this material in IOSC are also not well-addressed. In this work, we have fabricated a stable, light-soaking free, solution-processed and efficient SMDPPEH:PC61BM based IOSC for the first time. Fluorosurfactant modified PEDOT:PSS and fluorinated TiOx(F-TiOx) transport layers were used to circumvent the inherent processability and light-soaking issues. An exclusive study on the device stability and light-soaking characteristics were also carried out for the first time. The final device provides the following merits: (i) comparable material stability to P3HT polymer, suggesting a potential for further development of DPP materials for high-efficiency devices; (ii) F-TiOx can be used universally to fabricate a light-soaking free device with a wide range of photoactive materials from polymers to small molecules and; (iii) higher device efficiency compared to the non-inverted counterpart were obtained when the modified transport layers were used.

This article originally appeared in Solar Energy Materials and Solar Cells 150, 2016, Pages 19-31.

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