• Energy Research

Two new perylenediimides (PDIs) have been developed for use as electron acceptors in solution-processed bulk heterojunction solar cells. The compounds were designed to exhibit maximal solubility in organic solvents, and reduced aggregation in the solid state. In order to achieve this, diphenylphenoxy groups were used to functionalize a monomeric PDI core, and two PDI dimers were bridged with either one or two thiophene units. In photovoltaic devices prepared using PDI dimers and a monomer in conjunction with PTB7, it was found that the formation of crystalline domains in either the acceptor or donor was completely suppressed. Atomic force microscopy, X-ray diffraction, charge carrier mobility measurements and recombination kinetics studies all suggest that the lack of crystallinity in the active layer induces a significant drop in electron mobility. Significant surface recombination losses associated with a lack of segregation in the material were also identified as a significant loss mechanism. Finally, the monomeric PDI was found to have sub-optimum LUMO energy matching the cathode contact, thus limiting charge carrier extraction. Despite these setbacks, all PDIs produced high open circuit voltages, reaching almost 1 V in one particular case.

AbstractSequential photoinduced energy transfer followed by electron transfer and the formation of charge‐separated states, which are primary events of natural photosynthesis, have been demonstrated in a newly synthesized multichromophoric covalently linked triad, PDI‐SiPc‐C60. The triad comprises a perylenediimide (PDI), which primarily fulfils antenna and electron‐acceptor functionalities, silicon phthalocyanine (SiPc) as an electron donor, and fulleropyrrolidine (C60) as a second electron acceptor. The multi‐step convergent synthetic procedure developed here produced good yields of the triad and control dyads, PDI‐SiPc and SiPc‐C60. The structures and geometries of the newly synthesized donor–acceptor systems have been established from spectral, computational, and electrochemical studies with reference to appropriate control compounds. Ultrafast energy transfer from 1PDI* to SiPc in the case of PDI‐SiPc and PDI‐SiPc‐C60 was witnessed. An energy‐level diagram established from spectral and electrochemical data suggested the formation of two types of charge‐separated states, that is, PDI‐SiPc.+‐C60.− and PDI.−‐SiPc.+‐C60 from the 1SiPc* in the triad, with generation of the latter being energetically more favorable. However, photochemical studies involving femtosecond transient spectroscopy revealed the formation of PDI‐SiPc.+‐C60.− as a major charge‐separated product. This observation may be rationalized in terms of the closer spatial proximity to SiPc of C60 compared to PDI in the triad. The charge‐separated state persisted for a few nanoseconds prior to populating the 3SiPc* state during charge recombination.