Design of novel organoboron small-molecule dyes based on dipolar nitrogen-boron-nitrogen unit for solution-processed photovoltaic devices

The nitrogen-boron-nitrogen (N–B–N) structure combined with strong polar B–N and B←N bonds simultaneously, opens a new window for the development of organoboron photovoltaic materials with narrow band gap and low highest occupied molecular orbital (HOMO) energy level. In this paper, two N–B–N-embedded blocks including boron dipyrromethane (BODIPY) and double B–N bridged bipyridyl (BNBP), are exploited to rationally regulate the band gap and energy level of the resulting material by virtue of their strong electron affinities. Thus, the thiophene as a weak electron-donating part is directly attached to strong electron-withdrawing BODIPY and BNBP respectively, giving much improved photoelectric properties for such simple push-pull architectures. To further improve photoelectric and thermal properties of the material, two novel extended D-π-A-π-D-type organoboron molecules namely BDPTPA and BNBPTPA, in which BODIPY and BNBP as strong electron-deficient core (A) connect strong electron-rich triphenylamine (D) via thiophene π-spacer, are synthesized by Stille reaction between BODIPY/BNBP and “π-D” segment. Therefore, BDPTPA exhibits a preferable electrochemical band gap as small as 1.36 eV close to near-infrared (NIR) region, which is one of the lowest values among the BODIPY-based small-molecule donors (SMDs). Furthermore, BNBPTPA shows stronger light absorption of up to 1.16 × 105 M−1 cm−1, deeper HOMO energy level of −5.28 eV and very excellent thermal stability (Td = 428 °C). It is mentioning that, BNBPTPA is first used as SMD material, blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) for solution-processed organic solar cell. Consequently, a reasonable power conversion efficiency (PCE) of 6.78% has been achieved with a satisfactory short-circuit density (Jsc) of 17.27 mA cm−2 and open-circuit voltage (Voc) of 0.87 V, which is the highest efficiency for BNBP-based SMDs so far. Our work demonstrates that a proposed molecular design and synthesis with dipolar N–B–N-containing unit and “π-D” fragment, can greatly broaden the types of new photovoltaic donor materials.