Solution-Processed Fused-Ring Electron Acceptors

Organic solar cells (OSCs) are a type of organic electronics that produce electricity from sunlight by the photovoltaic effect, which is an efficient approach to harvest solar energy. Relative to inorganic solar cells, OSCs have some advantages such as low cost, light weight, flexibility and large area fabrication. Nowadays, power conversion efficiencies (PCEs) exceeding 10% have been achieved for OSCs based on blends of narrow bandgap polymer donors and fullerene acceptors. Fullerenes and their derivatives have been the dominant electron acceptors in OSCs. However, they have some intrinsic deficiencies such as narrow and weak absorption in the visible region, and limited energy level variability. Developments of novel electron acceptors and electron donors are of the same importance in OSCs but the former lags behind the latter. So, making effort to develop novel nonfullerene acceptors is very necessary.In this thesis, we synthesized several series of solution-processed π-conjugation nonfullerene acceptors based on organic semiconductor units, such as naphthalene monoimide (NMI), indacenodithiophene (IDT) and diketopyrrolopyrrole (DPP), and then investigated their optical and electrochemical properties, charge transport and photovoltaic properties. The main contents and results are summarized as follow:1. We synthesized three nonfullerene acceptors by cracking perylene dimide backbone and inserting conjugation units (single bond, thiophene and IDT). These molecules had good absorption spectra in the visible region and proper energy levels. With increasing conjugation of the bridge between naphthalimide units, the absorption spectra red shifted, the maximum extinction coefficients increased, the optical band gaps decreased and the HOMO energy levels were upshifted whereas the LUMO energy levels were downshifted. When fabricating OSCs using these molecules as acceptors and P3HT as the electron donor, the best PCE of OSCs was 2.36%.2. We synthesized a planar fused-ring electron acceptor (ICC6IDT-IC) based on indacenodithiophene. IC-C6IDT-IC showed strong absorption in 500-800 nm with extinction coefficient of up to 2.4×105 M-1 cm-1 and high electron mobility of 1.1× 10-3 cm2 V-1 s-1. The as-cast polymer solar cells based on IC-C6IDT-IC without additional treatments exhibited power conversion efficiencies of up to 8.71%, which was the highest value for fullerene-free OSCs. The traditional opinion on fused-ring electron acceptors is that they must have twisted backbones to suppress self-aggregation and large phase separation and to promote donor/acceptor miscibility. Our results challenged this traditional opinion and demonstrated that planar electron acceptors could exhibit very promising performance in OSCs.3. We developed an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithiophene. Nonfullerene acceptor ITIC-Th with thienyl side chains exhibited strong absorption in the visible and NIR regions, high electron mobility, and suitable energy levels matched well with narrow-band-gap polymer donor PTB7-Th and wide-band-gap polymer donor PDBT-T1. PTB7-Th or PDBT-T1:ITIC-Th blended films showed balanced charge transport with relatively high hole and electron mobilities. The OSCs based on PTB7-Th:ITIC-Th blended films with 3% CN showed PCEs up to 8.7%. Compared with PTB7-Th, PDBT-T1 owned lower HOMO energy level and more complementary absorption with ITIC-Th. PCEs of PDBT-T1:ITIC-Th-based devices with 1% CN was showed as high as 9.6%, which was the highest value reported on fullerene-free OSCs.4. We synthesized a structurally nonplanar molecule (TPE-DPP4) with tetraphenylethylene core and four diketopyrrolopyrroles at the periphery. This molecule showed intensive light absorption ability in the range 450-650 nm with extinction coefficient of up to 1.4×105 M-1 cm-1. The highly twisted structure of TPE-DPP4 may contribute to weak intermolecular interactions and isotropic charge transport ability. The HOMO and LUMO energy levels of TPE-DPP4 were estimated to be-5.31 and-3.68 eV respectively, which can match some typical polymer donors.