Diketopyrrolopyrrole based A2-D-A1-D-A2 type small molecules for organic solar cells: Effects of substitution of benzene with thiophene

• Three novel A2-D-A1-D-A2 type small molecules were synthesized and characterized.
• The numbers of thiophene units have a great effect on the charge carrier mobility and performance of organic solar cells.
• The DPPBI based devices exhibited the highest PCE of 2.36%.
• This work would be instructive in the rational molecule design of small molecule donor materials for OSCs.

A series of small molecules based on acceptor-donor-acceptor-donor-acceptor molecular skeletons, which contained a diketopyrrolopyrrole core as the central acceptor unit, two oligomeric benzene or thiophene as the donor, and two isoindigo or benzothienoisoindigo as the weaker acceptor, were synthesized. The influence of the number of thiophene on optical physical, crystallinity, charge carrier mobility, the morphologies of blend films, electrochemical and photovoltaic properties was investigated. The absorption of the diketopyrrolopyrrole based compounds showed a general trend of red-shift with increased number of thiophene. The highest occupied molecular orbital energy levels of the materials increased as a result of substitution benzene with thiophene as donor. X-ray diffraction measurement indicated that all these materials have strong crystallinity. Characterizations of morphology by atomic force microscopy and carrier mobilities revealed that the number of thiophene units influenced the microstructure of the blend films and the performance of organic solar cells.

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