Highly Efficient Inverted D:A1:A2 Ternary Blend ... - ACS Publications

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Jun 29, 2017 - Shao-Ling Chang, Fong-Yi Cao, Wen-Chia Huang, Po-Kai Huang, Chain-Shu Hsu, and Yen-Ju Cheng*. Department of Applied Chemistry, ...

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Highly Efficient Inverted D:A1:A2 Ternary Blend Organic Photovoltaics Combining a Ladder-type Non-Fullerene Acceptor and a Fullerene Acceptor Shao-Ling Chang, Fong-Yi Cao, Wen-Chia Huang, Po-Kai Huang, Chain-Shu Hsu, and Yen-Ju Cheng* Department of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsinchu, 30010, Taiwan S Supporting Information *

ABSTRACT: A formylated benzodi(cyclopentadithiophene) (BDCPDT) ladder-type structure with forced coplanarity is coupled with two 1,1-dicyanomethylene-3-indanone (IC) moieties via olefination to form a non-fullerene acceptor, BDCPDT-IC. The BDCPDT-IC, as an acceptor (A1) with broad light-absorbing ability and excellent solution processability, is combined with a second PC71BM acceptor (A2) and a medium band gap polymer, PBDB-T, as the donor (D) to form a ternary blend with gradient HOMO/ LUMO energy alignments and panchromatic absorption. The device with the inverted architecture using the D:A1:A2 ternary blend has achieved a highest efficiency of 9.79% with a superior Jsc of 16.84 mA cm−2. KEYWORDS: ladder-type structure, non-fullerene acceptor, fullerene acceptor, ternary blend, organic photovoltaics

1. INTRODUCTION Solution-processed organic photovoltaic cells (OPVs) using a donor and an acceptor material have been an important research topic for clean and renewable energy.1−3 Although enormous p-type polymers have been developed for OPVs over the past 2 decades, n-type electron acceptors are still dominated by the traditional [6,6]-phenyl-C61(or C71)-butyric acid methyl esters (PC61BM or PC71BM) as a result of their superior electron affinity and electron mobility.4−7 However, fullerene acceptors also have several intrinsic deficiencies that hinder the further breakthrough of OPVs. The weak light-absorbing ability of fullerenes is the biggest obstacle that greatly restricts exciton generation and thus photocurrent. Tuning of the LUMO energy levels of the monoadduct fullerenes (ca. 3.9−4.0 eV) by chemical modification is also not feasible. To circumvent these drawbacks, development of non-fullerene n-type acceptors (NFAs) emerges as a new research avenue that has made significant progress in recent years.8,9 By implementing molecular engineering, the organic-based NFAs can possess broader absorption and higher-lying LUMO energy levels with tunable optical band gaps, which are beneficial for enhancing both current density (Jsc) and open-circuit voltage (Voc).10−13 Perylene diimide (PDI) 14−16 and naphthalene diimide (NDI)17−19 derivatives with twisting 3D structural architectures have been demonstrated as a successful category of NFAs with promising OPV performances. Over the past few years, we have developed a variety of the multifused ladder-type donors (abbreviated as LDs) as building blocks for creating various fascinating donor−acceptor (D−A) conjugated polymers.20−29 The forced coplanarity of the LDs restricts rotational disorder © 2017 American Chemical Society

between adjacent aryl rings to enhance charge carrier mobility.30 These ladder-type conjugated molecules have regained significant attention because a new class of NFAs using an electron-rich LD, such as hexacyclic indacenodithiophene (IDT)31−36 and heptacyclic indacenodithieno[3,2-b]thiophene (IDTT)37−42 end-capped with two electron-deficient acceptors, has successfully achieved remarkable OPV efficiencies.8,9 Such an A−LD−A-type architecture induces efficient intramolecular charge transfer (ICT), thereby extending the absorption window to the longer wavelengths.43 In 2012, we first reported a multifused ladder-type benzodi(cyclopentadithiophene) (BDCPDT) molecule.44 Due to the coplanar and extended conjugated structure, the D−A copolymer incorporating the BDCPDT unit showed much higher OPV efficiency than its corresponding nonfused counterpart.44 It is envisaged that the BDCPDT unit with C2h symmetry could function as a promising LD for the design of new non-fullerene acceptors. To this end, a diformylated BDCPDT as the central LD was condensed with two 1,1dicyanomethylene-3-indanone (IC) moieties via olefination to form an A−LD−A-type molecule denoted as BDCPDT-IC (Figure 1) with good thermal stability and good lightharvesting absorption in the visible region. The four 4hexylphenyl bulky substituents at the two sp3-carbons can reduce intermolecular interactions to provide sufficient solubility. Received: May 11, 2017 Accepted: June 29, 2017 Published: June 29, 2017 24797

DOI: 10.1021/acsami.7b06650 ACS Appl. Mater. Interfaces 2017, 9, 24797−24803

Research Article

ACS Applied Materials & Interfaces

Figure 1. Chemical structures of BDCPDT-IC, PBDB-T, and PC71BM.

Scheme 1. Synthetic Route of BDCPDT-IC

Figure 2. (a) UV−vis absorption spectra of BDCPDT-IC in oDCB solution and thin film, (b) cyclic voltammogram of BDCPDT-IC in CH2Cl2 with a scan rate of 100 mV/s, and (c) energy diagram of BDCPDT-IC, PBDB-T, and PC71BM.

A medium band gap polymer, PBDB-T,45,46 was chosen as the donor to combine with the BDCPDT-IC acceptor (Figure 1) in view of their appropriate HOMO/LUMO energy alignments and complementary absorptions. The optimized OPV devices with the inverted configuration using the binary PBDB-T:BDCPDT-IC blend have achieved a superior efficiency of 9.33%. Considering the fact that the sphericalshaped fullerene derivatives are capable of transporting electron isotropically,47 integration of a non-fullerene ladder-type acceptor with the traditional PC71BM acceptor could have a synergistic effect on device characteristics. Indeed, when PC71BM was incorporated as the second acceptor to form a new ternary PBDB-T:BDCPDT-IC:PC71BM blend, the device has accomplished a highest efficiency of 9.73%.

2. RESULTS AND DISCUSSION Synthesis and Characterization of Materials. The synthesis of BDCPDT-IC is depicted in Scheme 1. The synthesis of compound 1 has been described by our previous work.44 Reaction of 4-hexylphenylmagnesium bromide with compound 1 yielded compound 2, which further underwent intramolecular cyclization to afford the BDCPDT (3) in 80% yield. The Vilsmeier−Haack formylation of 3 by using POCl3/ DMF gave compound 4 in 70% yield. The Knoevenagel condensation of compound 4 with 1,1-dicyanomethylene-3indanone afforded the final product BDCPDT-IC in 87% yield. BDCPDT-IC exhibited a high thermal decomposition temperature (Td) of 371 °C in the thermogravimetric analysis (TGA) measurement shown in Figure S1 of the Supporting 24798

DOI: 10.1021/acsami.7b06650 ACS Appl. Mater. Interfaces 2017, 9, 24797−24803

Research Article

ACS Applied Materials & Interfaces

important characteristic to promote π-electron delocalization and enhance charge mobility. Furthermore, the 4-methylphenyl side chains substituted at the sp3-tetrahedron carbons are situated out of the plane of the conjugated backbones. Such a structural configuration not only prevents strong aggregation without destroying the backbone planarity but also ensures sufficient solubility for solution processability. Photovoltaic Characteristics. Inverted bulk heterojunction devices ITO/ZnO/active layer/MoO3/Ag were fabricated to evaluate the BDCPDT-IC material. The J−V characteristics and the external quantum efficiency (EQE) spectra of the optimized devices are shown in Table 2 and Figure 5. The

Information. The differential scanning calorimetry (DSC) of BDCPDT-IC showed neither a melting point nor a crystallization transition, suggesting that BDCPDT-IC has more amorphous character. This might be due to the fact that the 4-hexylphenyl groups sticking out of the conjugated plane of the BDCPDT backbone attenuate the intermolecular interactions and reduce the tendency of crystallization. Optical and Electrochemical Properties. The absorption spectra of BDCPDT-IC measured in oDCB solution and thin film is shown in Figure 2a. The detailed data are summarized in Table 1. As a result of the π−π* and ICT transitions, Table 1. Optical and Electrochemical Properties of BDCPDT-IC

Table 2. Photovoltaic Parameters of the Inverted ITO/ZnO/ PBDB-T:BDCPDT-IC:PC71BM (D:A1:A2)/MoO3/Ag Devicesa

λmax (nm)

a

λonset (nm)

oDCB

film

Egopt (eV)a

HOMO (eV)b

LUMO (eV)b

Egele (eV)b

823

695

720

1.50

−5.41

−3.87

1.54

D:A1:A2 (wt % ratio)a

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

1:1.5:0 1:1:0 1:1:0.67b

0.84 0.86 0.84

15.33 16.56 16.84

67.44 65.52 68.79

8.68 9.33 9.73

Egopt = 1240/λonset. bDetermined by cyclic voltammetry.

Annealing at 150 °C for 15 min with CB as the solvent. bDevice with 0.5 vol % DIO as the additive.

a

BDCPDT-IC shows broad and strong absorption from 400 to 800 nm in oDCB solution with a maximum absorption peak at 695 nm. The λmax is further bathochromically shifted to 720 nm in the thin film. Compared to the hexacyclic non-fullerene acceptor ITIC,37−42 the heptacyclic BDCPDT-IC exhibits more red-shifted and broader absorption, which could enhance Jsc in OPV devices. The electrochemical properties of BDCPDT-IC in CH2Cl2 were evaluated using cyclic voltammetry (CV) (Figure 2b). According to the onsets of the oxidation and reduction curves, the HOMO and LUMO energy levels of BDCPDT-IC were estimated to be −5.41/−3.87 eV, which are lower-lying than those of the p-type PBDB-T polymer (−5.33 and −2.92 eV)45,46 but higher-lying than those of PC71BM (−5.96 and −3.98 eV) to guarantee efficient exciton dissociation and transportation (Figure 2c). Note that the energy offset between the LUMO of BDCPDT-IC and the HOMO of PBDB-T is as large as 1.46 eV, which could lead to a high open circuit voltage (Voc). The electrochemical band gap (1.54 eV) of BDCPDT-IC estimated from the CV is fairly consistent with the optical band gap (1.51 eV). Density Functional Theory Calculations. Figure 3 shows the optimal molecular geometry and frontier molecular orbitals of the BDCPDT-IC calculated with the Gaussian 09 suite with the 6-31G(d) basis set. The hexyl groups are simplified by methyl groups for calculations. The electron density in HOMO/LUMO of BDCPDT-IC is spread over the entire πsystem. From the side view of the optimal geometry, BDCPDT-IC adopts a highly coplanar structure, which is an

device using the binary PBDB-T:BDCPDT-IC (1:1.5 in wt %) blend delivered a high efficiency of 8.68% with a Voc of 0.84 V, a high Jsc of 15.33 mA cm−2, and a fill factor (FF) of 67.44%. The high Jsc is mainly attributed to the enhanced light-harvesting ability of BDCPDT-IC acceptor. By adjusting the D/A blending ratio to 1:1 wt %, the device achieved the optimal performance with a Voc of 0.86 V, a Jsc of 16.56 mA cm−2, and an FF of 65.52%, leading to a higher PCE of 9.33%. The external quantum efficiency (EQE) curve (Figure 5b) showed a broad response from 300 to 800 nm with a maximum EQE value of 76.2%, indicating efficient photoharvesting and charge collection. The LUMO level of BDCPDT-IC (−3.87 eV) is lower-lying than that of the widely used acceptor ITIC (−3.83 eV). As a result, the PBDB-T:ITIC-based device showed the slightly higher Voc of 0.90 V than the PBDB-T:BDCPDT-ICbased device (0.86 V).46 The grazing-incidence wide-angle Xray diffraction (GIWAXRD) of the PBDB-T:BDCPDT-IC (1:1 in wt %) blend exhibited a (010) peak at qz = 1.74 Å−1, which corresponds to a π-stacking distance of ca. 3.60 Å, indicating that the polymer predominately adopts a face-on π-stacking orientation that is known to facilitate the vertical charge transport (Figure 4a). Introducing PC71BM as the third component (A2) into the PBDB-T:BDCPDT-IC (D:A1) blend to form a D:A1:A2 ternary blend could be advantageous, considering that PC71BM, having

Figure 3. (a) Calculated HOMO/LUMO frontier molecular orbitals of BDCPDT-IC and (b) top view and side view of the optimized geometry of BDCPDT-IC. 24799

DOI: 10.1021/acsami.7b06650 ACS Appl. Mater. Interfaces 2017, 9, 24797−24803

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Figure 4. Two-dimensional GIWAXRD images of the blend films (a) PBDB-T:BDCPDT-IC (1:1 in wt %) and (b) PBDB-T:BDCPDT-IC:PC71BM (1:1:0.67 in wt %).

Figure 5. (a) J−V curves and (b) IPCE spectra of PBDB-T:BDCPDT-IC (1:1 in wt %) and PBDB-T:BDCPDT-IC:PC71BM (1:1:0.67 in wt %) devices.

higher than that of the binary blend (16.56 mA cm−2), indicating that introduction of PC71BM does not affect the charge mobility. The incorporation of PC71BM might slightly disrupt the polymer stacking. Nevertheless, by diffusing PC71BM into the polymer domain, the formation of additional PBDB-T:PC71BM and BDCPDT-IC:PC71BM interfaces will facilitate the charge generation and transport. It is noted that the ternary device showed the higher FF value than the binary device. The improvement might be also associated with the morphological change.48 It should be emphasized that the current high-performance NFA-based OPVs were mainly fabricated using the conventional device configuration, which requires the use of PDEOT:PSS as a hole-conducting layer and N,N′-bis(dimethylaminopropyl-N′′′-oxide)-perylene-3,4:9,10-tetracarboxidiimide (PDINO) as a cathode interlayer.31,41,42,51 In this research, the devices are based on the inverted structure without using aqueous PEDOT:PSS for better device stability. Coincidently, during the preparation of this paper, Chen et al. reported the same non-fullerene material, which has been only used for the binary conventional devices to achieve high efficiencies.52 However, using the strategy of a ternary blend with two acceptors and the inverted configuration OPV devices

the lowest-lying HOMO/LUMO energy levels, can further provide a cascade energy gradient among the three components to facilitate the electron/hole transport and reduce charge recombination.48−50 To test this concept, we formulated a ternary PBDB-T:BDCPDT-IC:PC71BM (1:1:0.67 in wt %) blend where the weight ratio of PBDB-T:BDCPDT-IC is still kept as 1:1 with the addition of 67 wt % extra PC71BM. Encouragingly, the device using the D:A1:A2 ternary blend outperformed the binary-based device, accomplishing a highest efficiency of 9.73% with the improved Jsc of 16.84 mA cm−2 and FF of 68.79%. The device parameters using other ternary blend ratios can be found in the Supporting Information (Figure S2 and Table S1). The strengthening of absorption at the shorter wavelengths upon adding PC71BM accounts for the enhancement of Jsc. Consistently, the ternary device shows higher EQE values than the binary device in the 400−500 nm region in the IPCE spectra. Compared to the binary blend, the GIWAXRD of the PBDB-T:BDCPDT-IC:PC71BM (1:1:0.67 in wt %) blend exhibited a weaker (010) peak at qz = 1.73 Å−1, indicating that PBDB-T still maintains the face-on π stacking orientation with a slightly longer distance (dπ) of ca. 3.63 Å after the incorporation of PC71BM (Figure 4b). However, the current density of the ternary blend device (16.84 mA cm−2) is actually 24800

DOI: 10.1021/acsami.7b06650 ACS Appl. Mater. Interfaces 2017, 9, 24797−24803

ACS Applied Materials & Interfaces



has not been attempted and demonstrated. Our work found that introducing PC71BM to the PBDB-T:BDCPDT-IC blend accomplishes the highest efficiency for the inverted devices.

4. EXPERIMENTAL SECTION Fabrication of the Devices. The preparation of ZnO/ITO subtracts can be found in the previous report.53 The chlorobenzene solutions of binary PBDB-T:BDCPDT-IC or ternary PBDDT:BDCPDT-IC:PC71BM in an optimal weight ratio of 1:1 and 1:1:0.67 with 0.5 wt % DIO as additive were heated at 65 °C and spincoated (3000 rpm for 60 s) on top of the ZnO/ITO substrate followed by thermally heating at 150 °C for 15 min. The MoO3 layer (7 nm) and silver anode (100 nm) were deposited by vacuum evaporation. The devices were measured under ambient conditions without encapsulation.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06650.



REFERENCES

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3. CONCLUSIONS In summary, a rigid and coplanar heptacyclic benzodi(cyclopentadithiophene) (BDCPDT) ladder-type structure is formylated to end-cap with two 1,1-dicyanomethylene-3indanone (IC) moieties via olefination to form an A−LD−Atype BDCPDT-IC acceptor. The BDCPDT-IC, as an acceptor (A1) with broad light-absorbing ability and excellent solution processability, is blended with a medium band gap polymer PBDB-T as the donor (D) and PC71BM (A2) as the second acceptor to form the matchable HOMO/LUMO energy alignments and complementary absorption. The device with an inverted architecture using the D:A1:A2 ternary blend has achieved a highest efficiency of 9.79%. We envision that employing a ternary blend to simultaneously take advantage of a ladder-type non-fullerene acceptor and a fullerene acceptor is a promising and feasible approach for achieving high-efficiency solar cells.



Research Article

General measuremnets and characterization, synthetic procedures, computational details, more device details, and NMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Yen-Ju Cheng: 0000-0003-0780-4557 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We thank the Ministry of Science and Technology and the Ministry of Education in Taiwan, for financial support. We thank the National Center of High-Performance Computing (NCHC) in Taiwan for computer time and facilities, and Dr. U-Ser Jeng and Dr. Chun-Jen Su at BL23A1 station of the National Synchrotron Radiation Research Center (NSRRC) for the GIWAXS experiments. 24801

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Research Article

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DOI: 10.1021/acsami.7b06650 ACS Appl. Mater. Interfaces 2017, 9, 24797−24803

Research Article

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DOI: 10.1021/acsami.7b06650 ACS Appl. Mater. Interfaces 2017, 9, 24797−24803

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