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We report on the design, synthesis and characterization of light harvesting small molecules for use in solution-processed small molecule bulk heterojunction (SM-BHJ) solar cell devices. These molecular materials are based upon an acceptor/donor/acceptor (A/D/A) core with donor endcapping units. Utilization of a dithieno (3, 2-b;2′, 3′-d) silole (DTS) donor and pyridal2, 1, 3thiadiazole (PT) acceptor leads to strong charge transfer characteristics, resulting in broad optical absorption spectra extending well beyond 700 nm. SM-BHJ solar cell devices fabricated with the specific example 5, 5′-bis7- (4- (5-hexylthiophen-2-yl) thiophen-2-yl) -1, 2, 5thiadiazolo3, 4-cpyridine-3, 3′-di-2-ethylhexylsilylene-2, 2′-bithiophene (6) as the donor and 6, 6-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor component showed short circuit currents above −10 mA cm−2 and power conversion efficiencies (PCEs) over 3%. Thermal processing is a critical factor in obtaining favorable active layer morphologies and high PCE values. A combination of UV-visible spectroscopy, conductive and photo-conductive atomic force microscopies, dynamic secondary mass ion spectrometry (DSIMS), and grazing incident wide angle X-ray scattering (GIWAXS) experiments were carried out to characterize how thermal treatment influences the active layer structure and organization.
Welch et al. (Sat,) studied this question.