Article pubs.acs.org/cm
Ladder-Type Dithienonaphthalene-Based Small-Molecule Acceptors for Efficient Nonfullerene Organic Solar Cells Yunlong Ma,†,‡ Meiqi Zhang,†,‡ Yu Yan,§ Jingming Xin,∥ Tao Wang,§ Wei Ma,∥ Changquan Tang,†,‡ and Qingdong Zheng*,† †
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, Fujian 350002, China ‡ University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China § School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, China ∥ State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China S Supporting Information *
ABSTRACT: Two novel small molecule acceptors (DTNIC6 and DTNIC8) based on a ladder-type dithienonaphthalene (DTN) building block with linear (hexyl) or branched (2ethylhexyl) alkyl substituents are designed and synthesized. Both acceptors exhibit strong and broad absorption in the range from 500 to 720 nm as well as appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Replacing the linear hexyl chains with the branched 2-ethylhexyl chains has a large impact on the film morphology of photoactive layers. In the blend film based on DTNIC8 bearing the branched alkyl chains, morphology with well-defined phase separation was observed. This optimal phase morphology yields efficient exciton dissociation, reduced bimolecular recombination, and enhanced and balanced charge carrier mobilities. Benefited from these factors, organic solar cells (OSCs) based on PBDB-T:DTNIC8 deliver a highest power conversion efficiency (PCE) of 9.03% with a high fill factor (FF) of 72.84%. This unprecedented high FF of 72.84% is one of the highest FF values reported for nonfullerene OSCs. Our work not only affords a promising electron acceptor for nonfullerene solar cells but also provides a side-chain engineering strategy toward high performance OSCs.
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INTRODUCTION Organic solar cells (OSCs) are emerging as promising photovoltaic technology to convert sunlight directly to electricity because of their low-cost, lightweight, mechanical flexibility, and large-area solution processability.1−6 Bulk heterojunction (BHJ) OSCs consist of a pair of energy level matched materials that function as electron donor and electron acceptor, respectively.7,8 Over the past two decades, fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) have been the dominant acceptor materials because of their fast photoinduced electron transfer, high electron mobility, and isotropic charge transport, as well as the easy formation of phase-separated domains.9−11 With the synergistic efforts of donor material innovation and device optimization, significant progresses have been made in fullerene-based OSCs, and power conversion efficiencies (PCEs) over 10% have been achieved in single-junction OSCs.12−19 Despite these encouraging results, it becomes more and more difficult to further improve the PCEs of fullerene-based single-junction OSCs, because the fullerene derivatives have intrinsic drawbacks of poor absorption in the visible wavelength region and relatively fixed energy levels.20 To overcome these drawbacks, non© 2017 American Chemical Society
fullerene acceptors are explored as potential alternatives to the conventional fullerene derivatives for OSCs.21−24 Currently, many nonfullerene acceptors have been explored and used for the fabrication of high performance OSCs.25−40 Among these acceptors, the ones with acceptor−donor− acceptor (A-D-A) backbone architectures attract increasing attention and show great potentials due to their easily tunable energy levels, broad absorption, and facile synthetic procedures.41−46 For the A-D-A-type acceptors, the central core (D) containing a ladder-type fused ring is an essential building block. Such rigidified coplanar ladder-type structures can suppress the rotational disorder around the interannular single bonds and lower the reorganization energy, which in turn enhances the intrinsic charge mobility.8,47,48 In addition, the out-of-plane side chains on the sp3-hybridized bridging carbon atoms of the ladder-type aromatics ensure the resulting acceptor material with good solution processability and prevent the formation of large-sized phase separation in blended films.24 As a typical and successful example, Zhan et al. recently Received: July 10, 2017 Revised: August 24, 2017 Published: August 25, 2017 7942
DOI: 10.1021/acs.chemmater.7b02887 Chem. Mater. 2017, 29, 7942−7952
Article
Chemistry of Materials
Figure 1. (a) UV−vis absorption spectra of DTNIC6 and DTNIC8 in solutions; (b) absorption spectra of DTNIC6, DTNIC8, and PBDB-T in films; (c) cyclic voltammograms of DTNIC6 and DTNIC8; (d) device architecture and chemical structures of PBDB-T, DTNIC6, and DTNIC8; (e) energy level diagram of PBDB-T, DTNIC6, and DTNIC8.
Scheme 1. Synthetic Route of the Nonfullerene Acceptorsa
Reagents and conditions: (i) N2H4·H2O, diethylene glycol, 180 °C, 24 h; (ii) 1-bromohexane or 2-ethylhexyl bromide, sodium tert-butoxide, DMSO, 90 °C, 12 h; (iii) POCl3, DMF, ClCH2CH2Cl, reflux; (iv) INCN, pyridine, CHCl3.
a
resulting acceptor material by which OSCs with high fill factors (FFs) can be achieved. At the same time, the decreased electron donating ability of DTN in comparison with IDTT may lead to nonfullerene acceptors with enlarged optical bandgaps. It should be noted that nonfullerene acceptors with various bandgaps are greatly needed to match the donor materials (hole transporting) with different bandgaps.10 However, no attempt has been made so far to use this unique building block to construct nonfullerene acceptors. Besides the central core unit, another issue that might need a specific attention is the selection of side chains. On the one hand, side chains function as solubilizing groups to improve the solubility of the resulting material which is essential during material purification and device fabrication.51 On the other hand, the shape, length, and branch position of side chains have a significant impact on the solid state core interactions of the photoactive layer and, thereby, affect their exciton diffusion,
reported an A-D-A-type small molecule acceptor, 3,9-bis(2methylene(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11tetrakis(4-hexylphenyl)dithieno-[2,3-d:2′,3′-d′]-s-indaceno[1,2b:5,6-b′]dithiophene (ITIC), using ladder-type indacenodithieno[3,2-b]thiophene (IDTT) as the electron donating core unit.24 With suitable donor materials and systematic device optimization, PCEs over 11% have been achieved for the devices based on ITIC.49 Ladder-type dithienonaphthalene (DTN) was first developed by our group as a donor unit for the construction of p-type copolymers (PaDTNBTO). BHJ solar cells using PaDTNBTO as an electron donor and PC71BM as an electron acceptor showed a PCE of 6.44% with an open circuit voltage (Voc) of 0.92 V.50 It is expected that the ladder-type DTN unit could also be used for building nonfullerene acceptors when it is flanked by strong electron-withdrawing groups. The extended π-conjugation of DTN may benefit the electron transport of the 7943
DOI: 10.1021/acs.chemmater.7b02887 Chem. Mater. 2017, 29, 7942−7952
Article
Chemistry of Materials Table 1. Thermal, Optical, and Electrochemical Properties of DTNIC6 and DTNIC8
a
molecules
ε [105 M−1 cm−1]
λmaxsolution [nm]
λmaxfilm [nm]
Egopt [eV]a
HOMO [eV]b
LUMO [eV]b
Td [°C]c
DTNIC6 DTNIC8
2.04 2.13
638 634
670 660
1.70 1.73
−5.87 −5.91
−3.92 −3.93
335 335
Estimated from the onset of the absorption spectrum of thin film. bEnergy levels evaluated by CV. c5% weight loss temperature.
charge dissociation, and charge transport properties.2,52 For fullerene-based OSCs, it is known that small alkyl chains can promote intermolecular π−π interaction and larger alkyl chains can increase the solubility of the resulting materials by reducing the π−π interaction, which could be deleterious for the device performance.4−6 At the same time, side chains may also affect the domain size of the BHJ layer by influencing the miscibility of the fullerene acceptors with polymer donors.4−6 However, side chain effect of nonfullerene acceptors for OSCs has been rarely reported. On the basis of these considerations, herein, we report on two novel ladder-type DTN-based nonfullerene acceptors with linear (hexyl) or branched (2-ethylhexyl) alkyl chains, namely, DTNIC6 and DTNIC8. 2-(3-Oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile (INCN) was chosen as an electron withdrawing group to lower the lowest unoccupied molecular orbital (LUMO) levels (i.e., electron affinities) of the resulting molecules via its carbonyl and cyano groups. After the introduction of two INCN units, an A-D-A-type backbone is constructed which helps to reduce the optical bandgaps and extend the absorption band of the resulting acceptors. The effect of the alkyl substituents on the optical, electrochemical, morphological, carrier transport, and photovoltaic properties of the acceptors are investigated in-depth. By incorporating a benchmark polymer donor (PBDB-T as shown in Figure 1d),53 inverted OSCs with ZnO as an electron transport layer (ETL) were fabricated. The devices based on DTNIC8 showed a superior PCE up to 8.72%, while devices based on DTNIC6 exhibited a moderate PCE of 3.22%. When the ZnO buffer layer was replaced by a bis(2,4-pentanedionate) (TOPD)modified TiO2, the PCE of the best performance DTNIC8based device increased to 9.03%. This performace outperforms that of the PC71BM-based device with the identical polymer donor, indicating DTNIC8 has a great potential to substitute the fullerene derivatives for OSCs.
Both nonfullerene acceptors have good solubility in common organic solvents, such as dichloromethane, chlorobenzene, and chloroform at room temperature, which affords good solution processability of the resulting OSCs. Thermogravimetric analysis (Figure S1) under nitrogen atmosphere demonstrates that both acceptors exhibited excellent thermal stability with a decomposition temperature (5% weight loss) up to 335 °C. Optical and Electrochemical Properties. The UV−vis absorption spectra of DTNIC6 and DTNIC8 in chloroform solution or in thin film are shown in Figure 1, and the detailed optical parameters are summarized in Table 1. The two acceptors have very similar absorption profiles in solution with strong absorption in the range of 500−700 nm. With the 2ethylhexyl substitutions, DTNIC8 in solution displays slightly improved light harvesting properties with a larger extinction coefficient of 2.13 × 105 M−1 cm−1 at 634 nm than that of DTNIC6 (2.04 × 105 M−1 cm−1 at 638 nm). Both acceptors exhibit stronger absorption in the visible region relative to the fullerene derivatives (i.e., PC61BM, extinction coefficient
