Regioregular and Regioirregular Poly(selenophene-perylene diimide

Aug 28, 2018 - The structural regioregularity impact on the performance of polymer–polymer solar cells (PPSCs) was highlighted. Both the copolymer ...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 32397−32403

Regioregular and Regioirregular Poly(selenophene-perylene diimide) Acceptors for Polymer−Polymer Solar Cells Yuming Liang,†,⊥ Shuqiong Lan,‡,⊥ Ping Deng,*,† Dagang Zhou,§ Zhiyong Guo,*,† Huipeng Chen,*,‡ and Hongbing Zhan†,∥ †

College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China Institute of Optoelectronic Display, National & Local United Engineering Lab of Flat Panel Display Technology, Fuzhou University, Fuzhou 350002, China § College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, China ∥ Key Laboratory of Eco-materials Advanced Techmoligy, Fuzhou University, Fujian Province University, Fuzhou 350108, China

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‡

S Supporting Information *

ABSTRACT: We report two new regioregular and regioirregular model copolymer acceptors based on selenophene and perylenetetracarboxylic diimide moieties, respectively, named RR-P(SePDI) and RI-P(SePDI), which were synthesized to study how regioregularity impacts the properties of resulting polymers. The structural regioregularity impact on the performance of polymer−polymer solar cells (PPSCs) was highlighted. Both the copolymer acceptors displayed similar optoelectronic properties. The regioregular RR-P(SePDI) exhibited better and balance bulk chargetransport capability than regioirregular RI-P(SePDI) in active layer films. The typical PPSCs based on the regioirregular RI-P(SePDI) copolymer acceptor and the PTB7Th polymer donor afforded average power conversion efficiencies (PCEs) of about 5.3%. Importantly, reasonably improved average PCEs of about 6.2% were provided by the blend active layer of new regioregular RR-P(SePDI) and PTB7-Th. These results highlight the significant and efficient strategy of rational control regioregularity of the polymer backbone to gain high PCE values in perylene diimide-based PPSCs. KEYWORDS: regioregularity, copolymer acceptor, poly(selenophene-perylene diimide), polymer semiconductor, polymer−polymer solar cells

1. INTRODUCTION Development of conjugated polymer semiconductors for polymer electronic devices is an important and challenging topic of research.1−3 Optimizing the molecular structures is a core issue in this field.4−8 Recently, researchers have been continually refreshing power conversion efficiency (PCE) values in polymer-based photovoltaic devices3−5 with the breakthrough of nonfullerene blend active layers.9−11 A typical active layer normally contains an established organic donor and a start-up organic acceptor, attributing to the lopsided development of donors and acceptors during the last few decades.12−14 Among them, polymer electron acceptors actively promote the development on polymer−polymer based solar cells (PPSCs),15−33 whose active layer typically contains both polymeric donors and acceptors. Polymer− polymer blends are expected to exhibit better thermal stability than other nonfullerene-based solar cells.7,19,29 However, the performances of PPSCs are still lagging behind those of organic solar cells mentioned above. Perylene diimide (PDI) is a unique building block with many advantages (Figure 1). PDI-based small molecular derivatives are one of the most popular and promising acceptor materials since the introduction of the efficient © 2018 American Chemical Society

Figure 1. Chemical structure and typical advantages of the PDI core.

design strategy (e.g., building twisted and/or three-dimensional molecular structures) to suppress the strong selfReceived: May 31, 2018 Accepted: August 28, 2018 Published: August 28, 2018 32397

DOI: 10.1021/acsami.8b09061 ACS Appl. Mater. Interfaces 2018, 10, 32397−32403

Research Article

ACS Applied Materials & Interfaces aggregation of PDI cores and to tune the compatibility between the PDI acceptor and donor materials.34−41 PDI molecular acceptors have possessed commendable PCEs of over 10%.42 Although lots of PDI polymers have been studied as electron acceptors in the photovoltaic field till now, there are still relatively few PDI-based copolymer acceptors that exhibit PCEs over 6% [see Figure S1], which hinges crucially on how to properly tune their aggregation behaviors. As shown in Figure 2, two typical and alternative methods have been

Scheme 1. Synthetic Routes to RI-P(SePDI) and RRP(SePDI)

Figure 2. Typical polymerization routes of PDI cores: (a) N-linked route and (b) bay position-linked route.

developed to synthesise PDI-based polymer acceptors. It has also been demonstrated that π-linkers conjugated with the PDI core at bay positions (Figure 2b) afford distinctly efficient polymer acceptors.43,44 Researchers are focused largely on employing various π-linkers and/or extended π-conjugated PDI cores to build target acceptor polymers.43−57 However, a vital issue on the regioregularity effect of PDI backbones on the photoelectric properties of the resulting polymer acceptor materials has attracted much less attention.58 In addition, the regioregularity of many PDI polymer acceptors were not clearly been declared,43,44 which may puzzle the readers to some extent. It is well-known that regioregularity will crucially impact the properties of conjugated polymer materials.59−65 Importantly, the regioregularity of polymers strongly influences the conjugation length and crystalline order of the polymers.66,67 Improved regioregularity of conjugated polymer backbones contributes to higher crystalline order, which should facilitate charge transport leading to enhanced PCEs in photovoltaic devices.61,64 Difunctional (e.g., dibrominated) PDIs or extended π-conjugated PDIs are usually mixtures of 1,7- and 1,6-regioisomers, which indeed can be further purified and then obtained as 1,7-difunctional products in isomerically pure form.68 Therefore, both 1,7- and 1,6-regioisomers and 1,7-difunctional PDI monomers are available for polymerization. On this basis, we report herein a comparative study of two new PDI copolymers (Scheme 1), RI-P(SePDI) and RRP(SePDI), as model polymer acceptors for investigating the regioregularity on their optoelectronic properties. Selenophene was incorporated into both polymer chains because it was proved to be a simple and efficient π-linker in photovoltaic materials.14,17 We investigated the photovoltaic properties of both polymers via PPSCs containing blends of PTB7-Th3 donor material with RI-P(SePDI) or RR-P(SePDI) acceptor. Regioirregular RI-P(SePDI)-based PPSCs displayed promising average PCEs of about 5.3%, whereas regioregular RRP(SePDI) PPSCs displayed significantly improved average PCEs of up to 6.2%.

Scheme 1. Monomer M1, consisting of two 1,7- and 1,6regisomers in mole ratios of about ∼4:1 estimated according to its 1H nuclear magnetic resonance (NMR) data (see Figure S2), was readily synthesized according to the literature procedure.69 Monomer M2, consisting of pure 1,7-dibromo PDI monomer, is commercially available instead of being prepared by means of repetitive column chromatography and crystallization in the laboratory. The target acceptors RIP(SePDI) and RR-P(SePDI) were obtained by standard Stille polycondensation of M1 or M2 with 2,5-bis(trimethylstannyl)selenophene (see the Experimental Section in the Supporting Information). Both the copolymers have strong solubilities in toluene, chloroform, and other chlorinated solvents. Both the copolymers also displayed similar molecular weights (Figures S6 and S7). New copolymer acceptors were characterized by proton NMR and elemental analyses. New copolymers were also thermally stable for applications in organic photovoltaic devices with the decomposition temperature (Td) above 440 °C (Figure S8). 2.2. Theoretical Calculations. We elucidated the backbone configurations of copolymers RI-P(SePDI) and RRP(SePDI) via theoretical calculations (Figure 3). Both the models displayed similar frontier molecular orbital energy levels. In addition, the calculated dihedral angles between selenophene and PDI in both the models were similar (55°− 64°), whereas the PDI core displayed some angular distortions. In PDI-conjugated systems, it is a challenging problem to control self-aggregation of PDI cores for improving the photovoltaic performance of PDI acceptor materials.14,34−57 First, the sterically twisted conformation of PDI acceptor materials may discourage strong PDI aggregation for obtaining suitable domain sizes for efficient exciton dissociation.14,35 Second, improving the coplanarity of the conjugated systems

2. DISCUSSION 2.1. Synthesis and Characterization. We synthesized RI-P(SePDI) and RR-P(SePDI) via the route shown in 32398

DOI: 10.1021/acsami.8b09061 ACS Appl. Mater. Interfaces 2018, 10, 32397−32403

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Figure 3. Density functional theory calculations of two models.

Figure 4. Absorption spectra of (a) RI-P(SePDI) and RR-P(SePDI) and (b) PTB7-Th.

Figure 5. (a) Cyclic voltammogram of RI-P(SePDI) and RR-P(SePDI) as thin films; (b) energy level diagram of RI-P(SePDI), RR-P(SePDI), and PTB7-Th.

leads to improved molecular order and crystallinity, which are beneficial for charge-carrier transport.14,45,50 2.3. Absorption Spectra and Electrochemical Energy Levels. Figure 4 displays the absorption spectra of RIP(SePDI) and RR-P(SePDI) acceptors and the widely-used PTB7-Th donor. Both the acceptors showed very similar absorption characteristics and possess complementary absorption with the selected donor. The electrochemical behaviors of both copolymer acceptors were investigated via cyclic voltammetry (CV), and the CV curves are presented in Figure

5. The lowest unoccupied molecular orbital (LUMO) levels (ELUMO) of RI-P(SePDI) and RR-P(SePDI) were about −3.88 and −3.87 eV, respectively. The optical bandgaps (abbreviated as Eg,opt) of acceptors RI-P(SePDI) and RRP(SePDI) were 1.48 and 1.44 eV, respectively. The highest occupied molecular orbital levels of RI-P(SePDI) and RRP(SePDI), as estimated, respectively, from Eg,opt and ELUMO were −5.36 and −5.31 eV. Accordingly, both the copolymer acceptors displayed very similar and suitable energy levels. 32399

DOI: 10.1021/acsami.8b09061 ACS Appl. Mater. Interfaces 2018, 10, 32397−32403

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ACS Applied Materials & Interfaces 2.4. Photovoltaic Performance. PTB7-Th donor was selected to evaluate the capacity of RI-P(SePDI) and RRP(SePDI) as copolymer acceptors in PPSCs. First, the absorption spectra of acceptor/PTBT-Th (1:1, wt/wt) blends were measured (Figure S9). Both blends display strong absorption feature in 300−780 nm. Second, both the blends show a high quenching efficiency of over 90% (Figure S10). They further indicated that PTB7-Th was a suitable donor. Their photovoltaic performance characteristics were further studied. Their device configuration was expressed as glass/ indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/blend film/poly[(9,9-bis(3′-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)]/Al. Figure 6 and Table 1 respectively, displays the

We conducted atomic force microscopy (abbreviated as AFM) to compare the blend film morphology of RI and RR devices for the best photovoltaic performance. As shown in Figure S11, both blend active layers displayed a homogeneous morphology, which could be part of the reason for both RI and RR devices showing relatively high photovoltaic performance. However, it does not explain why RR-P(SePDI) can be a more efficient polymer acceptor compared to RI-P(SePDI) in PTB7-Th PPSCs. Therefore, we further accomplished grazing-incidence wide-angle X-ray scattering (abbreviated as GIWAXS)71 to make clear about the crystallization of polymer acceptors. Figure 7 displays the GIWAXS profile (out of plane)

Figure 7. Out-of-plane GIWAXS profile of neat RI-P(SePDI) and RR-P(SePDI). Figure 6. Current density−voltage curves of devices containing active layers of PTB7-Th:RI-P(SePDI) (1:1, wt/wt) or PTB7-Th:RRP(SePDI) (1:1, wt/wt) processed without or with 6 vol % CN.

of polymer acceptors obtained from the two-dimensional GIWAXS data. Polymer RR-P(SePDI) displayed an obvious peak at q ≈ 3.1 nm−1. It can arise from a typical lamellar distance of RR-P(SePDI) isolated via its chains. None of the peaks was observed in RI-P(SePDI), indicating the amorphous structure for polymer RI-P(SePDI). We also investigated the bulk charge-transport characteristics of blends via spacecharge-limited-current (SCLC) measurement (see Figure 8 and Table 2). Electron mobility (μe) and hole mobility (μh) of the blend of the RI device were calculated to be 3.51 × 10−4 and 7.25 × 10−5 cm2 V−1 s−1, corresponding to a μe/μh value of 4.83. However, the blend of the RR device showed μe of 5.43 × 10−4 and μh of 3.59 × 10−4 cm2 V−1 s−1, corresponding to a favorable μe/μh value of 1.51. The blend of the RR device possesses high and balanced mobilities. It may have contributed to the observed improved photovoltaic performance compared to the blend of RI devices.

current density−voltage curves and the corresponding parameters. It indicates that the PPSCs based on RRP(SePDI) typically afford higher PCEs than those devices based on RI-P(SePDI), which is mainly due to the improved short-circuit current density (J SC ) of the former. 1Chloronaphthalene (CN) had been proved as a very effective processing additive in our previous studies,70 and it was used to enhance photovoltaic performance. The optimized PTB7Th:RI-P(SePDI) and PTB7-Th:RR-P(SePDI) devices (1:1, wt/wt; 6 vol % CN) are, respectively, abbreviated as “RI devices” and “RR devices.” The RI and RR devices, respectively, showed average PCEs of 5.3 and 6.2%. Notably, the regioregular RR-P(SePDI) exhibits its structural advantage compared with regioirregular RI-P(SePDI).

Table 1. Photovoltaic Performances of Devices (PTB7-Th:Acceptor = 1:1, wt/wt) acceptor RI-P(SePDI) RR-P(SePDI) RI-P(SePDI)a RR-P(SePDI)a

JSC (mA/cm2) 8.2 9.6 10.6 11.8

± ± ± ±

0.3 0.2 0.3 0.2

VOC (V) 0.80 0.81 0.80 0.81

± ± ± ±

0.03 0.03 0.03 0.03

FF (%) 0.60 0.62 0.63 0.65

± ± ± ±

0.03 0.03 0.03 0.03

PCEb (%) 3.9 4.8 5.3 6.2

± ± ± ±

0.3 0.2 0.3 0.2

a

With 6 vol % of CN in chlorobenzene solution. bAverage data over 10 cells. 32400

DOI: 10.1021/acsami.8b09061 ACS Appl. Mater. Interfaces 2018, 10, 32397−32403

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Figure 8. Typical J−V characteristics of RI and RR devices: (a) hole-only and (b) electron-only.

Notes

Table 2. Charge-Carrier Mobilities of Blends of RI and RR Devices (Tested via SCLC Method) blend (1:1, wt/wt)

μe (cm V

RI-P(SePDI):PTB7-Th RR-P(SePDI):PTB7-Th

2

−1

−1

s )

3.51 × 10−4 5.43 × 10−4

μh (cm V 2

−1

−1

s )

7.25 × 10−5 3.59 × 10−4

The authors declare no competing financial interest.

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μe/μh

ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (NSFC grant nos. 21704015, 51703031, and 51503039), the Education & Scientific Research Project for Young Teachers (JAT170094), the Testing Foundation of Valuable Equipments of Fuzhou University (2017T003), and the Scientific Research Foundation of Fuzhou University (XRC-1660).

4.83 1.51

3. CONCLUSIONS In summary, two new regioirregular and regioregular selenophene-PDI copolymer acceptors, RI-P(SePDI) and RR-P(SePDI), have been developed for PPSCs. The two acceptors showed similar solubilities, molecular weights, thermal stabilities, and optical and electrochemical properties. The active layer films of RI-P(SePDI) and RR-P(SePDI) with PTB7-Th both displayed excellent film homogeneity. The effect of regioregularity on bulk charge-transport and crystallization may be attributed to enhanced photovoltaic performance of the regioregular copolymer acceptor RRP(SePDI). This study highlighted the significance of regioregularity in PDI polymer acceptor materials and provided a feasible strategy to enhance the photovoltaic performance in PDI-based PPSCs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b09061. Detailed experimental section on materials and characterization, device fabrication, and literature overview of high-performance PDI-based polymer acceptors, synthesis of RI-P(SePDI) and RR-P(SePDI), 1H NMR spectra, thermogravimetric analysis plots, GPC test results of copolymer acceptors, absorption spectra, photoluminescence spectra, and AFM height images of blends (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.D.). *E-mail: [email protected] (Z.G.). *E-mail: [email protected] (H.C.). ORCID

Ping Deng: 0000-0001-9567-7951 Huipeng Chen: 0000-0003-1706-3174 Author Contributions ⊥

Y.M.L. and S.Q.L. contributed equally to the work. 32401

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DOI: 10.1021/acsami.8b09061 ACS Appl. Mater. Interfaces 2018, 10, 32397−32403