Synthesis And Photovoltaic Performance Of Non-Planar Perylene Diimide Derivatives As Electron Acceptor

Fullerene and their derivatives are the most used electon acceptors in solution-processed bulk-heterojunction(BHJ) organic solar cells(OSCs). Although fullerene derivatives are more conducive to form unique phase and contribute to multidimensional charge transporting capabilities,which played a central role as electron acceptor in BHJ OSCs, the drawbacks including weak absorption of visible light, poor chemical and electronic tunability, tedious purification and high production cost are also not ignorable. Moreover, despite successes of use in laboratory scale devices, fullerenes are not ideal materials for production in industrial scale. Development of non-fullerene electron acceptor materials is therefore highly interesting for achieving high-performance OSCs with good industrialization potential. Among the non-fullerene acceptor materials exploited so far for OSCs, perylene diimide and their derivatives(PDIs) are the most frequently studied ones. PDIs as electron acceptor in OSCs are mainly due to their optical and electronic properties, such as strong absorption in the visible region between 400 nm to 650 nm, low-lying LUMO level(about-4.0 e V), high electron mobility(101-10-3 V s cm-2), simplicity in synthesis and purification, as well as chemically, thermally, and environmentally robust. However, because of the planarity of the perylene core, the PDI derivatives usually show intenstive aggregation tendency in thin film, leading to forming large crystalline domain over 20 nm. This may limit exciton diffusion and dissociation processes, and is therefore harmful in improving the solar cell. Reducing the intermolecular aggregation tendency of PDIs by introducing bulky group on the PDI core has therefore been considered as the effective method to develop high performance PDI-based electron acceptors. Based on this consideration, a series of non-planar PDI derivatives substituted on the bay-position with different groups were synthesized for use in OSCs, the opical and electronic properties of the synthesized PDIs were also studied in detail.1. Three asymmetrical PDI derivatives substituted on the bay-position with para-alkyl-phenyl groups were synthesized, on which the substitution alkyl side chain was n-propyl(4-PP-PDI), n-hexyl(4-HP-PDI), or n-nonyl(4-NP-PDI). The effect of alkyl chain length on the optical and electrochemical properties, as well as the photovoltaic performance of these materials in solution processed polymer solar cells were systematic studied. Results indicated that the para-alkyl side chain length showed negligible influence on the spectroscopy and redox behaviors of the materials, but significant influence on the photovoltaic performance.2. Three asymmetrical PDI derivatives substituted with para-isopropylphenyl, meta-isopropylphenyl or ortho-isopropylphenyl on the bay-position were synthesized and characterized. The optical and electrochemical properties, thin film morphological characteristic, electron mobility, as well as the photovoltaic performance of these materials in solution processed polymer solar cells were systematically studied. Results indicated that, except for the electrochemical redox behavior, substitution position of the bulky isopropyl group has a strong influence on UV-Vis absorption spectra, PL emission spectra, electron mobilities, and photovoltaic performances of the materials. Solution processed fullerene-free polymer solar cells based on the 3-i PP-PDI as the electron acceptor and P3 HT or PTB7 as the electron donor afforded power conversion efficiencies of 1.12% and 1.73%, respectively.3. A PDI dimmer PDI-F-PDI based on a fluorene core unit was synthesized and characterized. In PDI-F-PDI, the dihedral angle between the PDI plane and the fluorene plane is 122.5°, suggesting a twisted structure of this molecule. Such a twisted structure was found to be able to weaken the intermolecular interaction, which is evidenced by thehigh solubility of PDI-F-PDI in common organic solvents. The photophysical, electrochemical, electron mobility and photovoltaic properties of PDI-F-PDI were also measured. A high PCE of 5.41% was obtained for solution-processed OSCs using PTB7-Th:PDI-F-PDI blend.4. A three-dimension structured molecule(SBF-PDI4) based on PDI and 9,9’-spirobi[9H-fluorene](SBF) core was designed, synthesized and characterized. The synthesized SBF-PDI4 shows good solubility in common organic solvents. The photophysical and electrochemical properties of SBF-PDI4 were also measured. The electron mobility was calculated to be 1.33 × 10-4 cm2 V-1 s-1 using electron only devices by SCLC method. When blended with the conjugated polymer donor of PTB7-Th in solution-processed BHJ OSCs, a PCE of 5.34% was obtained. These results demonstrate that fine-tuning of PDI-based materials with a 3D configuration maybe is a promising way in developing high performance non-fullerene electron acceptors for BHJ OSCs.