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.