Highly Efficient Inverted D:A1:A2 Ternary Blend Organic Photovoltaics

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06650 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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ACS Applied Materials & Interfaces

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 Taiwan 30010. E-mail: [email protected] Keywords : Ladder-Type Structure, Non-Fullerene Acceptor, Fullerene Acceptor, Ternary blend, Organic Photovoltaics 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 an 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 bandgap polymer PBDB-T as the donor (D) to form a ternary blend with gradient HOMO/LUMO energy alignments and panchromatic absorption. The

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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.

1. Introduction Solution-processed organic photovoltaic cells (OPVs) using a donor and an acceptor materials have been an important research topic for clean and renewable energy.1-3 Although enormous ptype polymers have been developed for OPVs over the past two decades, n-type electron acceptors are still dominated by the traditional [6,6]-phenyl-C61(or C71)-butyric acid methyl ester (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 to greatly restrict exciton generation and thus photocurrent. Tuning of the LUMO energy levels of the mono-adduct 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 bandgaps, 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 between adjacent


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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 non-fused 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 a A-LD-A-type molecule denoted as BDCPDT-IC (Figure 1) with good thermal stability and good light-harvesting absorption in the visible region. The four 4-hexylphenyl bulky substituents at the two sp3carbons can reduce intermolecular interactions to provide sufficient solubility.

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


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A medium bandgap polymer PBDB-T45,46 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 absorption. 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 spherical shaped 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:BDCPDTIC: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-hexylphenyl magnesium bromide with compound 1 yielded compound 2 which further underwent intramolecular cyclization to afford the BDCPDT (3) in 80% yield. The VilsmeierHaack formylation of 3 by using POCl3/DMF gave compound 4 in 70% yield. The Knoevenagel condensation of compound 4 with 1,1-dicyanomethylene-3-indanone affiorded the final product BDCPDT-IC in 87% yield.


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Scheme 1. Synthetic Route of BDCPDT-IC

BDCPDT-IC exhibited a high thermal decomposition temperature (Td) of 371 oC in the thermogravimetric analysis (TGA) measurement shown in Figure S1. The differential scanning calorimetry (DSC) of BDCPDT-IC showed neither melting point nor 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 2(a). The detailed data are summarized in Table 1. As a result of the π–π* and ICT transitions, BDCPDT-IC shows broad and strong absorption from 400 nm 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 nonfullerene acceptor ITIC37-42, 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 2(b)). According


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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 eV and -2.92 eV) 45,46 but higher-lying than those of PC71BM (-5.96 eV and -3.98 eV) to guarantee efficient exciton dissociation and transportation (Figure 2(c)). 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 bandgap (1.54 eV) of BDCPDT-IC estimated from the CV is fairly consistent with the optical bandgap (1.51 eV).

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 (c) energy diagram of BDCPDT-IC, PBDB-T, and PC71BM.

λonset (nm) 823

λmax (nm) oDCB

Film

695

720

Egopt (eV)a

HOMO (eV)b

LUMO (eV)b

Egele (eV)b

1.50

-5.41

-3.87

1.54

Table1. Optical and electrochemical properties of BDCPDT-IC a

Egopt = 1240/λonset. bdetermined by cyclic voltammetry.


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Density Functional Theory Calculations. Figure 3 shows the optimal molecular geometry and frontier molecular orbitals of the BDCPDT-IC calculated with the Gaussian09 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 π-system. From the side view of the optimal geometry, BDCPDT-IC adopts a highly coplanar structure which is an 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.

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

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 Figure 5 and Table 2. The device using the binary PBDB-T:BDCPDT-IC (1:1.5 in wt%) blend


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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 (Fig. 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 the result, the PBDB-T:ITIC-based device showed the slightly higher Voc of 0.90 V than the PBDB-T:BDCPDT-IC-based device (0.86 V).46 The grazing-incidence wide-angle X-ray diffraction (GIWAXRD) of the PBDBT: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 faceon π-stacking orientation which is known to facilitate the vertical charge transport (Figure 4a).

π-

Figure 4. 2-Dimensional GIWAXRD images of the blend films (a) PBDB-T:BDCPDT-IC (1:1 in wt%) (b) PBDB-T:BDCPDT-IC:PC71BM (1:1:0.67 in wt%).


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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 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.4850

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 PBDBT: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 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 PBDBT: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


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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(propylenedimethylamine)-3,4:9,10-perylenediimide (PDIN) 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 manuscript, 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 ternary blend with two acceptors and the inverted configuration OPV devices has not been attempted and demonstrated. Our work found that introducing PC71BM to the PBDBT:BDCPDT-IC blend accomplishes the highest efficiency for the inverted devices.

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.

Table 2. Photovoltaic parameters of the inverted ITO/ZnO/PBDB-T:BDCPDT-IC:PC71BM (D:A1:A2)/MoO3/Ag devicesa


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D:A1:A2 (wt% ratio)a 1: 1.5:0 1:1:0 1:1:0.67b a

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

0.84 0.86 0.84

15.33 16.56 16.84

67.44 65.52 68.79

8.68 9.33 9.73

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

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-3-indanone (IC) moieties via olefination to form an A-LD-A-type BDCPDT-IC acceptor. The BDCPDT-IC as an acceptor (A1) with broad light-absorbing ability and excellent solution processability is blended with a medium bandgap polymer PBDB-T as the donor (D) and PC71BM (A2) as the second acceptor to form the matchable HOMO/LUMO energy alignments and complimentary absorption. The device with the 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.

4. Experimental Section Fabrication of the Devices. The preparation of ZnO/ITO subtracts can be found in the previous report.53 The chlorobenzene solution of binary PBDD-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 spin-coated (3000 rpm for 60 s) on top of the ZnO/ITO substrate followed by thermally heated 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.


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Supporting Information The Supporting Information including synthetic procedures, computational details, more device details and NMR spectra is available free of charge on the ACS Publications.

ACKNOWLEDGMENT 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 in National Synchrotron Radiation Research Center (NSRRC) for the GIWAXS experiments.

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Graphical abstract


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Highly Efficient Inverted D:A1:A2 Ternary Blend Organic Photovoltaics

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