1. INTRODUCTION
In recent years, organic solar cells (OSCs) are evolving rapidly due to the growth of new photovoltaic materials and device engineering. The single-junction OSCs power conversion efficiency (PCE) has surpassed 19%,[1-8] indicating tremendous potential for future new energy applications. The low bandgap electron acceptors play a picotal role in the rapid development of OSCs. Among them, the commonly used high-performance electron acceptors are fused ring electron acceptors (FREA) such as ITIC, Y6 and their derivatives.[9-14] However, the relatively complex synthetic routes and low yields of FREAs still pose challenges for their future commercial application. To simplify the synthetic route of electron acceptors, nonfused ring electron acceptors (NFREAs) have garnered more and more attention.[15-20] However, the photovoltaic performances of NFREAs are still far lagging behind FREAs. To fabricate high-performance NFREAs, the varied side chain is an important factor, which can largely affect the energy levels, aggregation behavior, etc.[21-25]
In recent years, several skeleton design strategies have been explored for NFREAs.[18, 19, 26-30] Hou et al. designed and synthesized A4T-23, A4T-21, and A4T-16 with different side groups,[17] and demonstrated that A4T-16 with large side groups has excellent photovoltaic performance because it can form better stacking and a 3D network structure. Subsequently, NFREAs TTC6, TT-C8T and TT-TC8,[31] were prepared by adjusting the molecular geometry through changing the steric hindrance of the lateral substituents. Among them, TT-TC8 has a better planar molecular skeleton and can form stronger intramolecular charge transfer effect, exhibiting over 13% PCE in OSCs with D18 as the donor polymer.
Based on the aforementioned considerations, we focus on the design and synthesis of novel NFREAs using side chain engineering strategy. More specifically, NFREAsOC4-4Cl-Ph ,OC4-4Cl-Th and OC4-4Cl-C8 have similar molecular skeletons, but different side chains (hexylbenzene, hexylthiophene and octyl). Side chains can effectively regulate the energy levels, absorption spectra, molecular packings, and blend film morphology of acceptors. As a result, OC4-4Cl-C8 displays a maximum exciton diffusion length, and the corresponding devices exhibit weaker bimolecular recombination, more effective exciton transport, as well as higher and better balanced mobility. Most encouragingly, devices based on OC4-4Cl-C8 demonstrated a champion PCE of 16.56% with a good short-circuit current (J sc) (24.40 mA cm-2) and a suitable open-circuit voltage (V oc) (0.90 V) with a high fill factor (FF) (75.10%). Our work shows that side-chain engineering is an efficient approach to enable high-performance NFREAs.
2. RESULTS AND DISCUSSION
2.1. Materials synthesis
Scheme 1. The synthetic route ofOC4-4Cl-Ph ,OC4-4Cl-Th and OC4-4Cl-C8 .
Reaction conditions: (i) Pd2(dba)3,S -Phos, K3PO4, toluene, 110oC; (ii) NBS, DMF, 0 oC; (iii) Pd(PPh3)4, toluene, 110oC; (iv) POCl3, 1,2-dichloroethane, DMF, 85 oC; (v) pyridine, CHCl3, 25oC.
The synthetic route ofOC4-4Cl-Ph ,OC4-4Cl-Th andOC4-4Cl-C8 is outlined in Scheme 1 . Compared to FREA, these acceptors have fewer synthesis steps and higher yields. The detailed synthesis process are described in the Supporting Information. S -Phos and Pd2(dba)3 are used as the catalyst precursors in a Suzuki cross-coupling of 3,6-dibromothieno[3,2-b]thiophene and (2,6-dibutoxyphenyl)boronic acid to obtain compound 1 in a yield of about 85%[32]. Compound 2 is produced by brominating compound 1 with N-bromosuccinimide in a yield of 95%. Still cross-couplings of compound 2 with tributyl(6-(4-hexylphenyl)thieno[3,2-b]thiophen-2-yl)stannane (3a ), tributyl(6-(5-hexylthiophen-2-yl)thieno[3,2-b]thiophen-2-yl)stannane (3b) or tributyl(6-octylthieno[3,2-b]thiophen-2-yl)stannane (3c ) with Pd(PPh3)4 as the catalyst affords the intermediates 4a , 4b or4c , respectively. Compounds 5a , 5b and5c are prepared by Vilsmeier-Haack reaction (yield~89%). The target acceptorsOC4-4Cl-Ph ,OC4-4Cl-Th and OC4-4Cl-C8 are prepared by Knoevenagel condensation in yields of approximately 75%, and their structures were characterization by 1H and 13C NMR spectroscopy and mass spectroscopy.(Supporting Information).
2.2. Optical and electrochemical properties
The absorption spectra and energy levels of OC4-4Cl-Ph ,OC4-4Cl-Th and OC4-4Cl-C8 are displayed in Table 1 and Figure 2, respectively. In dilute chloroform solutions,OC4-4Cl-Ph , OC4-4Cl-Th and OC4-4Cl-C8 exhibit intensely absorption in the range of 580 to 810 nm with the maximum absorption peak (λ max) located at 738, 740 and 728 nm and their molar absorption coefficients being estimated to be 1.50×105, 1.19×105 and 1.52×105 cm-1 M-1, respectively.
Figure 1. Chemical structures of the polymer donor and small molecular acceptors.
From a solution to neat film, the absorption spectra of these three acceptors are all prominently red-shifted. Compared toOC4-4Cl-Ph and OC4-4Cl-Th , OC4-4Cl-C8 neat film displays a red-shifted and widened absorption ranging from 600 to 920 nm with the λ max located at 804 nm. In addition, the optical band gaps (E gopt) were calculated to be 1.39, 1.39 and 1.35 eV for OC4-4Cl-Ph , OC4-4Cl-Th andOC4-4Cl-C8 , respectively. The electrochemical characteristics of these three acceptors were determined by cyclic voltammetry (CV). The HOMO and LUMO energy levels of OC4-4Cl-Ph , OC4-4Cl-Thand OC4-4Cl-C8 are calculated to be -5.53/-3.93, -5.49/-3.89, and -5.50/-3.53 eV, respectively (Figure S3, Figure S4 and Table 1).
Figure 2. UV-vis absorption spectra of OC4-4Cl-Ph ,OC4-4Cl-Th andOC4-4Cl-C8 .
Table 1. Optical and electrochemical properties ofOC4-4Cl-Ph , OC4-4Cl-Th and OC4-4Cl-C8.