DPP-containing Non-fullerene Acceptor Based On Spirofluorene Core:synthesis And Photovoltaic Application

Fullerene and its derivatives have been dominant acceptor material in organic solar cells due to their high electron mobility, good electron transport in three dimensions and good phase separation with donor materials. However, despite successes in laboratory scale devices, fullerenes are not an ideal material for production on an industrial scale. They are expensive to synthesize, difficult to modify chemically, poor absorption in UV-Vis area and relatively fixed HOMO/LUMO level. Therefore, scientists focused on the non-fullerene molecule acceptor materials with good electron mobility. Small molecule acceptors can achieve flexible regulation of energy level and absorption spectra owing to the diversity of synthetical methods and groups, thereby draw more and more attentions.Many studies confirmed that small molecule acceptor with nonplanar structure can inhibit aggregation and crystallization to form good phase separated structure with donor materials, thereby greatly improve the photovoltaic performance. In recent years, diketopyrrolopyrroledione(DPP) based small molecule acceptor materials have aroused extensive interest owing to the relatively high LUMO level and good absorbance. However, most of them yield power conversion efficiencies(PCEs) below 2%.Based these considerations, we introduced spirofluorene(SF) as a central core into DPP based small molecule acceptors. The SF units were linked four DPP units by C-C single bond or C-C triple bond to acquire two series star-shaped DPP based acceptors, SF-DPP and SF-A-DPP, respectively. In order to investigate the influence of the alkyl chains on crystallization and blend film performance with P3 HT, we use different alkyl chains(2-ehtylhexyl, n-octyl and n-dodecyl) modified DPP in the two series and yield three molecular variants in each series, SF-DPP(SF-DPPEH, SF-DPPC8 and SF-DPPC12)and SF-A-DPP(SF-A-DPPEH, SF-A-DPPC8, SF-A-DPPC12). The research shows that there is slight difference between SF-DPP and SF-A-DPP on absorption and energy levels. Compared to P3HT/SF-A-DPP blends, three P3HT/SF-DPP blends have higher PCE owing to their better crystallization and better phase separating. Moreover, the influence of alkyl chains on performance is significant, SF-DPPC8 and SF-DPPC12 has better crystallization than SF-DPPEH due to the steric hindrance of 2-ehtylhexyl chain. Nevertheless, P3HT/SF-DPPEH blend formed best phase separated structure(10-20 nm) and the PCE is obviously higher than P3HT/SF-DPPC8 and P3HT/SF-DPPC12. The PCE of P3HT/SF-DPPEH is up to 3.63% after thermal annealing at 120℃ for10 min, which is currently among the highest efficiencies for non-fullerene based devices with P3 HT.Therefore, we can conclude that spirofluorene can greatly improve the morphology of DPP based small molecule acceptors with P3 HT blend films, thereby largely enhance the photovoltaic performance of devices.