In the title compound, C12H7I2N, the tricyclic aromatic ring system is essentially planar, with an r.m.s. deviation of 0.0272 Å. The two I atoms are marginally out of plane, with the C—I bonds angled at 3.9 (2) and 1.1 (2)° with respect to the planes of their respective benzene rings, above and below the plane of the carbazole ring system. No classical hydrogen bonds are observed in the crystal structure.

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Recently, diketopyrrolopyrrole (DPP)-based photovoltaic materials have been continuously modified to improve the power conversion efficiency (PCE). However, it was reported that 2,5-Bis(2-ethyl-hexyl)-3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-pyrrolo [3,4-c]pyrrole-1,4-dione (BFT), composed of benzofuran-connected DPP donor, exhibited a relatively low PCE. Besides, the analogue named 2,5-Diethylhexyl-3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-pyrrolo [3,4-c] pyrrole-1,4-dithione (BFT-SS), even showed a PCE lower than 0.001% when compounded with fullerene derivatives. The extremely low PCE could attribute to the low-lying lowest unoccupied molecular orbital (LUMO) energy levels. Hence, in this article, we proposed that both BFT-SS and its rotamer BFT-SS' with low LUMO energy levels could be converted as acceptors. And comparative analysis was demonstrated with the parent compound BFT and BFT'. The results highlight the effect of sulfur atom(S) in the thioketo groups could twist the molecules and reduce the LUMO energy level, thereby possess low the charge injection barrier and reorganization energy (λ), during the conversion of BFT-SS and BFT-SS' from donor to acceptor. Finally, we sincerely expect the discussion to provide a meaningful idea for the development of acceptors. K E Y W O R D S bulk-heterojunction, conversion of donor to acceptor, density functional theory, diketopyrrolopyrrole, organic solar cells | INTRODUCTIONWith strengthening awareness on sustainable development, the research on organic solar cells (OSCs) have received a rapid increasing in the past decades. [1,2] Among all kinds of OSCs, bulk-heterojunction (BHJ)-type OSCs, in which donor (D) and acceptor (A) are completely mixed, have drawn more interest and more than 13% power conversion efficiency (PCE) has been achieved so far. [3] For BHJ-type OSCs, the entire process of photoelectric conversion work at the heterojunction interface. [4] Consequently, engineering reasonable donor and acceptor materials is particularly important for improving PCE of BHJ-type OSCs. However, most of the acceptors are sensitive to air and moisture, the negative charge carriers of acceptors are easily oxidized, while exposing to air for long periods of time. So it is quite difficult to synthesize novel acceptors. [5][6][7][8][9][10][11] These days, researchers gain insight into devising novel acceptors by slightly changing the structure of high-performance donors. [12,13] At present, the strategy of changing donor into acceptor is mainly introducing electron-withdrawing groups on donors to decrease lowest unoccupied molecular orbital (LUMO) energy level. [14][15][16] Diketopyrrolopyrrole (DPP)-based small molecules generally consist of an adjacent aromatic substituent and π-conjugated segments, of which optical and electronic properties could easily modulate. [17] So there is a reasonable strategy for designing DPP-based small molecules to facilitate PCEs. [18] At present, DPP-based electron donors showed satisfying progress with high PCEs of 8% and 9.4%, respe...