Organic solar cells based on non-fullerene acceptors of nine fused-ring by modifying end groups

Highlights

• A detailed method to synthesize nine fused-ring SMAs based on benzodithiophene and benzothiadiazole.

• Systematic investigation of SMAs by CV, TGA, UV–vis spectra and so on.

• Performance comparison of SMAs based on the same fused-ring modified with different end-groups.

• SMAs based on nine fused-ring showed an excellent photo-response up to 1000 nm.

Abstract

A series of small molecule acceptors (SMAs) based on a benzodithiophene-pyrrolobenzothiadiazole-based nine fused-ring core and different end groups of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2FIC), 3-(dicyanomethylene)indian-1-one (IC) and 2-(3-ethyl-4-oxo-thiazolidin-2-ylidene) malononitrile (RCN) have been designed and synthesized. The SMAs X94FIC, X9IC, X9Rd and X9T4FIC were chosen as electron acceptors with blending the donor polymer PBDB-T to prepare the organic solar cells (OSCs) and investigate photoelectric performance. Surprisingly, X94FIC showed an excellent photo-response up to 1000 nm, while X9IC and X9T4FIC also possess a photo-response to 900 nm. A power conversion efficiency (PCE) of 7.08% was obtained for the active layers PBDB-T:X94FIC. This result demonstrates that small molecule acceptor containing the nine fused-ring core and the end group with di-fluorine atoms is promising candidate for the development of high performance non-fullerene OSCs.

Introduction

In the past few years, organic solar cells have developed rapidly, and bulk hetero-junction (BHJ) [1] organic solar cells (OSCs) are one of the most commonly used device structures based on p-type organic semiconductor electron donors blended with n-type organic semiconductor electron acceptors as active layer and have been achieved great progress [[2], [3], [4], [5], [6], [7]]. It is attribute to the development of high performance donors [[8], [9], [10], [11], [12]], acceptors [[13], [14], [15], [16], [17], [18]] and electrode buffer layer materials [19,20]. For acceptor materials, they are classified into fullerene acceptors such as PCBM [[21], [22], [23], [24]] and non-fullerene acceptors. Non-fullerene acceptors (NFAs) are mainly small molecule acceptor (SMAs) [[25], [26], [27]] and polymer acceptors. Compared with polymer acceptors, the SMAs have the advantages of strong absorption, adjustable energy level, high carrier mobility, excellent phase separation morphology, and easy modification. Therefore, the development of polymer acceptor materials relative to SMAs are jog along. At the same time, n-type donors also play an important role in organic solar cells as hole transport carriers for photovoltaic devices, while P3HT is the most representative donor photovoltaic material [28,29]. To produce a high performance OPVs, the most important are the match of the HOMO level of donors and the LUMO level of acceptors, high and balanced charge-carrier mobility, and complementary absorption bands in the Vis-NIR range of donors and acceptors. In addition, both the material composition ratio of the blend films and the morphology of the active layer affect the power conversion efficiencies (PCEs) of the OPVs.

In the last ten years, the A-D-A type fused-ring SMAs have received widespread attention from researchers due to its advantages such as adjustable energy levels and strong absorption, its rapid development has greatly promoted the development of OSCs and achieved high efficiency. For example, zhan’ group reported a series of A-D-A type SMAs [[30], [31], [32], [33]], such as IDIC, ITIC etc. For most reported SMAs, the absorption wavelength of the electron acceptors were lower than 800 nm, while the PCEs of the OSCs after matching the appropriate polymer donor materials could exceed 13%. Recently, the non-fullerene small molecule acceptor Y6 that synthesized by Zou and coworkers has an absorption peak exceeding 800 nm [34]. At the same time, Y6 has a HOMO level of −5.65 eV and a LUMO level of −4.10 eV, after matching with a polymer donor PM6, a high PCE of 15.7% was obtained, which is one of the most efficient acceptor materials so far.

In this paper, we plan to synthesize SMAs with near-infrared absorption based on benzodithiophene (BDT) and benzothiadiazole (BT) to improve the efficiency of OSCs. BDT has good symmetry, the molecular conformation presents a planar conjugated structure and small steric hindrance, which is a proven excellent donor unit and it could improve the electron mobility of SMAs [[35], [36], [37], [38], [39], [40], [41]]. Based on the above considerations, we designed and synthesized four SMAs based on a new benzodithiophene-pyrrolobenzothiadiazole central nine fused-ring unit. Firstly, 4,8-bis(2-ethylhexyloxy)benzo [1,2-b:4,5-b'] dithiophene (BDT-OEH) contains four alkoxy side chains, which can effectively improve the solubility of SMAs. Then, three different end groups of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2FIC), 3-(dicyanomethylene)indian-1-one (IC) and 2-(3-ethyl-4-oxo-thiazolidin-2-ylidene) malononitrile (RCN) were used to adjust the light absorption and energy levels of SMAs. In addition, we also added a π-bridge between the central core and the end group to adjust the performance. Finally, we obtained four SMAs, X94FIC, X9IC, X9Rd and X9T4FIC (Scheme 1). For X94FIC, X9IC and X9T4FIC, they were found to have narrow band gaps of 1.41 eV, 1.47 eV, and 1.47 eV, respectively, and X9Rd showed a wide band gap of 1.71 eV. In particular, the light absorption band of X94FIC reached an amazing 1000 nm, while X9IC and X9T4FIC also have a photo-response to 900 nm, which is beneficial for improving the PCE of OCSs.