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Regioregular and Regioirregular Poly(SelenophenePerylene Diimide) Acceptors for Polymer-Polymer Solar Cells Yuming Liang, Shuqiong Lan, Ping Deng, Dagang Zhou, Zhiyong Guo, Huipeng Chen, and Hongbing Zhan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09061 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Applied Materials & Interfaces

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‡,*, 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

KEYWORDS: regioregularity, copolymer acceptor, poly(selenophene-perylene diimide), polymer semiconductor, polymer-polymer solar cells

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 their performance in polymer-polymer solar cells (PPSCs) was highlighted. Both copolymer acceptors displayed similar optoelectronic properties. The regioregular RR-P(SePDI) exhibited better and balance bulk charge transport capability than regioirregular RI-P(SePDI) in active layer films. The typical PPSCs based on regioirregular RI-P(SePDI) copolymer acceptor and PTB7-Th 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 1

ACS Paragon Plus Environment

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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 polymer backbone to gain high PCE values in PDI based PPSCs.

1. Introduction Development on conjugated polymer semiconductors for polymer electronic devices is an important and challenging topic of research.1-3 Optimizing molecular structures is a core issue in this field.4-8 Recently, researchers are continually refreshing power conversion efficiency (PCE) values in polymer based photovoltaic devices3-5 with the breakthrough of non-fullerene blend active layers.9-11 A typical active layer normally contains an established organic donor and a start-up organic acceptor, attributing to lopsided development of donors and acceptors during the last few decades.12-14 Among them, polymer electron acceptors are actively promoting the development on polymer-polymer based solar cells (abbreviated as 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 type non-fullerene 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 (Fig. 1). PDI-based small molecular derivatives are one of the most popular and promising acceptor materials since introduction of efficient design strategy (e.g. building twisted and/or three-dimensional molecular structures) to suppress the strong self-aggregation of PDI cores and then tune the compatibility between PDI acceptor and donor materials.34-41 PDI molecular acceptors have possessed commendably PCEs of over 10%.42 Although lots of PDI polymers have been studied as electron acceptors in photovoltaic field till now, there are still relatively few PDI-based copolymer acceptors exhibit PCEs over 6%[See Fig. S1], which hinges crucially on how to properly tune their aggregation behaviors. As shown in Fig. 2, two typical and 2


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alternative methods have been developed to synthesis of PDI-based polymer acceptors. It has also been demonstrated that π-linkers conjugated with PDI core at bay positions (Fig. 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 photoelectric properties of resulting polymer acceptor materials is 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 regioegulartity of conjugated polymer backbones contributes to higher crystalline order, which should facility 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 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 to polymerization. Based on this, we report herein a comparative study of two new PDI copolymers (Scheme 1), RI-P(SePDI) and RR-P(SePDI), as model polymer acceptors for investigating the regioregularity on their optoelectronic properties. Selenophene was incorporated into both polymer chains due to it was proved to be a simple and efficient π-linker in photovoltaic materials.14,17 We investigated the photovoltaic properities of both polymers via PPSCs contain blends of PTB7-Th3 donor material with

RI-P(SePDI)

or

RR-P(SePDI)

acceptor

respectively.

Regioirregular

RI-P(SePDI) based PPSCs displayed promising average PCEs of about 5.3%, whereas regioregular RR-P(SePDI) PPSCs displayed significantly improved average PCEs of up to 6.2%.

3



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Fig. 1 Chemical structure and typical advantages of PDI core.

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

2. Discussion 2.1 Synthesis and Characterization We synthesized RI-P(SePDI) and RR-P(SePDI) via the route of Scheme 1. The monomer M1 consists of two 1,7- and 1,6- regisomers in mole ratios of about ~4:1 estimated according to its 1H NMR data (see Fig. S2), was readily synthesized according to the literature procedure.69 The monomer M2, consisting of pure 1,7dibromo PDI monomer, is commercially available instead of being prepared by means of repetitive column chromatography and crystallization in the laboratory. The target acceptors RI-P(SePDI) and RR-P(SePDI) were obtained by standard Stille polycondensation of M1 or M2 with 2,5-bis(trimethylstannyl)selenophene (see Experimental Section in Supporting Information). Both copolymers have strong solubilities in toluene, chloroform and other chlorinated solvents. Both copolymers also displayed similar molecular weights (Fig. S6 and S7). New copolymer acceptors were characterized by proton nuclear magnetic resonance and elemental analysis. 4


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New copolymers were also thermally stable for applications in organic photovoltaic devices with the decomposition temperature (Td) above 440 oC (Fig. S8).

Scheme 1. Synthetic routes to RI-P(SePDI) and RR-P(SePDI). 2.2 Theoretical Calculations We

elucidated

backbone

configurations

of

copolymers

RI-P(SePDI)

and

RR-P(SePDI) via theoretical calculations (Fig. 3). Both models displayed similar frontier molecular orbital energy levels. In addition, those calculated dihedral angles between selenophene and PDI in both models were similar (55o ~ 64o), while PDI core displayed some angular distortions. In PDI conjugated systems, it is a challenging problem that how to control self-aggregation of PDI cores for improving the photovoltaic performance of PDI acceptor materials.14,34-57 For one thing, the sterically twisted conformation of PDI acceptor materials may discourage strong PDI aggregation for obtaining suitable domain sizes in order to efficient exciton dissociation.14,35 For another thing, improving coplanarity of conjugated systems leads to improved molecular order and crystallinity, which are beneficial for charge carrier transport.14,45,50 5



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Fig. 3 DFT calculations of two models. 2.3 Absorption spectra and Electrochemical energy levels Fig. 4 displayed absorption spectra of RI-P(SePDI) and RR-P(SePDI) acceptors and widely-used PTB7-Th donor. Both acceptors showed very similar absorption characteristics, and possess complementary absorption with selected donor. The electrochemical behaviors of both copolymer acceptors were investigated via cyclic voltammentry (CV) and the CV curves were presented in Fig. 5. The 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 RR-P(SePDI) were 1.48 and 1.44 eV respectively. The HOMO levels of RI-P(SePDI) and RR-P(SePDI), as respectively estimated from Eg,opt and ELUMO, were −5.36 and −5.31 eV. Accordingly, both copolymer acceptors displayed very similar and suitable energy levels.

6


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

Fig. 5 (a) Cyclic voltammogram of RI-P(SePDI) and RR-P(SePDI) as thin films; (b) the energy level diagram of RI-P(SePDI), RR-P(SePDI) and PTB7-Th. 2.4 Photovoltaic Performance. PTB7-Th donor was selected to evaluate the capacity of RI-P(SePDI) and RR-P(SePDI) as copolymer acceptors in PPSCs. Firstly, the absorption spectra of acceptor:PTBT-Th (1:1, wt/wt) blends were measured (Fig. S9). Both blends display strong absorption feature in 300~780 nm. Secondly, both blends show high quenching efficiency of over 90% (Fig. S10). They further indicated PTB7-Th was a suitable donor. The photovoltaic performance characteristics of them were further studied. Their

device

configuration

was

expressed

as

glass/ITO/PEDOT:PSS/blend

film/PFN/Al. Fig. 6 and Table 1 respectively displayed the current density-voltage curves and corresponding parameters. It indicates that the PPSCs based on 7



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RR-P(SePDI) typically afford higher PCEs than those devices based on RI-P(SePDI), which can mainly owe to the improved short-circuit current density (Jsc) of the former. 1-chloronaphthalene (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 PTB7-Th:RI-P(SePDI) and PTB7-Th:RR-P(SePDI) devices (1:1, wt/wt; 6 vol% CN) , which were 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 in compared with regioirregular RI-P(SePDI).

Fig. 6 Current density−voltage curves of devices contain active layers of PTB7-Th:RI-P(SePDI) (1:1, wt/wt) or PTB7-Th:RR-P(SePDI) (1:1, wt/wt) processed without or with 6 vol % 1-chloronaphthalene(CN). Table 1 Photovoltaic performances of devices (PTB7-Th:Acceptor = 1:1, wt/wt) Acceptor

JSC (mA/cm2)

VOC (V)

FF (%)

PCEb (%)

RI-P(SePDI)

8.2± 0.3

0.80± 0.03

0.60± 0.03

3.9± 0.3

RR-P(SePDI)

9.6± 0.2

0.81± 0.03

0.62± 0.03

4.8± 0.2

RI-P(SePDI)a

10.6± 0.3

0.80± 0.03

0.63± 0.03

5.3± 0.3

8


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RR-P(SePDI)a a

11.8± 0.2

0.81± 0.03

0.65± 0.03

6.2± 0.2

with 6 vol% of CN in chlorobenzene solution; b Average data over 10 cells. We conducted atomic force microscopy (abbreviated as AFM) to compare blend film

morphology of RI and RR devices with best photovoltaic performance. As shown in Fig. S11, both blend active layers displayed homogeneous morphology, which could be part of the reason for both RI and RR devices showed relatively high photovoltaic performance. However, it does not explain why RR-P(SePDI) can be 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 crystallization of polymer acceptors. Fig. 7 displayed GIWAXS profile (out of plane) of polymer acceptors obtained from 2D GIWAXS data. Polymer RR-P(SePDI) displayed an obvious peak at q≈3.1 nm-1. It can arise from typical lamellar distance of RR-P(SePDI) isolated via its chains. None of peak was observed in RI-P(SePDI), indicating amorphous structure for polymer RI-P(SePDI). We also investigated the bulk charge transport characteristics of blends via space-charge-limited-current (abbreviated as SCLC) measurement (see Fig. 8 and Table 2). Electron mobility (µe) and hole mobility (µh) of blend of RI device were calculated to be 3.51×10-4 and 7.25 × 10-5 cm2V-1s-1, corresponding to a µe/µh value of 4.83. However, the blend of 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 RR device possesses high and balanced mobilites. It may be contributed to the observed improved photovolataic performance compared to blend of RI devices.

9



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Fig. 7 Out of plane GIWAXS profile of neat RI-P(SePDI) and RR-P(SePDI).

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

Table 2 Charge carrier mobilities of blends of RI and RR devices (tested via SCLC method). µe (cm2V-1s-1)

µh (cm2V-1s-1)

µe/µh

3.51×10-4

7.25 × 10-5

4.83

RR-P(SePDI):PTB7-Th 5.43×10-4

3.59 × 10-4

1.51

Blend (1:1,

wt/wt)

RI-P(SePDI):PTB7-Th

10


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3. Conclusion In summary, two new regioirregular and regioregular selenophene-perylene diimide copolymer acceptors, RI-P(SePDI) and RR-P(SePDI) have been developed for PPSCs. The two acceptors showed similar solubilities, molecular weights, thermal stability, optical and electrochemical properties. The active layer films of RI-P(SePDI) or 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 afford enhanced photovolataic performance for regioregular copolymer acceptor RR-P(SePDI). This study hightlighted the significance of regioregularity in PDI polymer acceptor materials and provided a feasible strategy to enhance the photovolataic performance in PDI based PPSCs.

ASSOCIATED CONTENT Supporting Information It is available free of charge on the ACS Publications website. Detail experimental section on materials, characterization, device fabrication, and literature overview of high performance PDI-based polymer acceptors, and synthesis of RI-P(SePDI) and RR-P(SePDI) (The standard Stille polycondensation method that we previous reported70 was employed in synthesis of both new copolymers). 1H NMR spectra, TGA plots, GPC test results of copolymer acceptors, absorption spectra, PL spectra and AFM height images of blends. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Ping Deng) *E-mail: [email protected] (Zhiyong Guo) 11



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*E-mail: [email protected] (huipeng Chen) ORCID Ping Deng: 0000-0001-9567-7951 Zhiyong Guo: 0000-0002-3181-0327 huipeng Chen: 0000-0003-1706-3174 Author Contributions §

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

Notes The authors declare no competing financial interest.

5. Acknowledgments We acknowledge financial support from National Natural Science Foundation of China (NSFC Grant Nos. 21704015, 51703031, 51503039), Education & Scientific Research Project for Young Teachers (JAT170094), Testing Foundation of Valuable Equipments of Fuzhou University (2017T003) and Scientific Research Foundation of Fuzhou University (XRC-1660). REFERENCES (1) Zhu, X. H.; Peng, J. B.; Cao, Y.; Roncali, J. Solution-Processable Single-Material Molecular Emitters for Organic Light-Emitting Devices. Chem. Soc. Rev. 2011, 40, 3509-3524. (2) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Semiconducting π-Conjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208-2267. (3) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666-12731. 12


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