Fine-Tuning the Quasi-3D Geometry: Enabling Efficient Nonfullerene

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Finely Tuning the Quasi-3D Geometry: Enabling Efficient Nonfullerene Organic Solar Cells Based on Perylene Diimides Zhitian Liu, Linhua Zhang, Ming Shao, Yao Wu, Di Zeng, Xiang Cai, Jiashun Duan, Xiaolu Zhang, and Xiang Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16406 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Applied Materials & Interfaces

Finely Tuning the Quasi-3D Geometry: Enabling Efficient Non-fullerene Organic Solar Cells Based on Perylene Diimides Zhitian Liu,a Linhua Zhang,a Ming Shao,b,* Yao Wu,a,c Di Zeng,a Xiang Cai,b Jiashun Duan,b Xiaolu Zhang,a Xiang Gaoa,*

a.

School of Material Science & Engineering, Wuhan Institute of Technology,

Wuhan, 430073, China. b.

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science

and Technology, Wuhan 430074, China. c.

CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and

Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China.

corresponding authors: Prof. Ming Shao ([email protected] or [email protected]) and Dr. Xiang Gao ([email protected])

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ABSTRACT The geometries of perylene diimides(PDI) based acceptors is important for improving the phase separation and charge transport in organic solar cells. In order to finely tune the geometry, diphenyl, spiro-fluorene and benzene were used as the core moiety to construct quasi-3D nonfullerene acceptors based on PDI building blocks. The molecular geometries, energy levels, optical properties, photovoltaic properties and exciton kinetics were systematically studied. The structure-performance relationship was discussed as well. Owing to the finest phase separation, the highest charge mobility and smallest non-geminate recombination, the power conversion efficiency of nonfullerene solar cells using PDI derivatives with diphenyl core (BP-PDI4) as acceptor reached 7.3% as high performance wide bandgap donor material PBDB-T was blended.

Keywords: organic solar cells; nonfullerene acceptors; perylene diimides; quasi-3D structure; core unit

1. Introduction Bulk-heterojunction (BHJ) organic solar cells (OSCs) have shown great potential to be one promising candidate for the next-generation energy sources with distinct properties such as low cost, light weight, convenience for solution processing and potential of producing large-area flexible devices.1-4 Compared to traditional fullerene

derivatives based electron acceptors such as


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[6,6]-phenyl-C61 (or C71)-butyric acid methyl ester (PC61BM or PC71BM), non-fullerene acceptors own many advantages, including easy chemical modification, tunable absorption and energy levels, intensive absorption in the visible range, cheap production on a large scale via readily accessible organic synthesis methods and reduced energy losses, thus have become a strong competitor to fullerene derivatives.5, 6 Among existing non-fullerene acceptors, perylene diimides (PDIs) derivatives have attracted much attention due to its high electron affinities, simple synthesis and functionalization, and intensive absorption in the visible range.7-9 High power conversion efficiencies (PCEs) over 9.5% have been reported.10 The large π-conjugated plane of PDI molecules readily leads to strong tendency of π-π stacking and molecular aggregation. On one hand, strong electronic coupling between PDI molecules facilitate charge transport. Nevertheless, serious PDI molecule aggregation in the absence of control can form microscale aggregates. The oversized PDI phase domain will inhibit photo-generated excitons arriving at the donor/acceptor interface followed by charge dissociation, which is detrimental to the performance of OSCs.5,

13

Additionally, this specific molecular configuration facilitates the formation of excimers which traps excitons undergo transport, consequently results in low photocurrent generation efficiencies.11,

12

Therefore, the fine control of

aggregation level of PDI molecules to avoid forming oversized domain is critical to realize high efficient nonfullerene solar cells. Diversified molecule



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design, such as functionalization at the N-,14, 15 bay-16-18 or ortho-positions of PDIs19, 20 and constructing a fused non-plannar configuration21, 22, all have been proved effective to alleviate strong tendency of aggregation.12,

23

The N- or

ortho-functionalization avoids the distortion of PDI cores,12, 19 and the fusion enhances the solid ordering. Although these methods are believed to enhance the charge mobility,12,

22

bay-substituted PDIs still keep the record of the

highest PCE among these PDI-based non-fullerene acceptors by far.11,

12

Additionally, functionalization at the bay-positions of PDIs is a more facile method to tune the optoelectronic properties, which not only inhibits aggregation but also simulates the 3D geometry of fullerene derivatives which facilitates isotropic charge transport.12, 18, 24, 25 These acceptor molecules with 3D core and PDI arms also form interlocking geometry, which prevent excessive rotation among the PDIs and reinforce conformational uniformity. For example, the orthogonal spiro-bifluorene (SF) core connected with four PDI building blocks (SF-PDI4) were synthesized.18 Besides, increasing dimensionality was demonstrated to improve electron transport and suppress non-geminate charge recombination in a comparative study of monomeric PDI, bis-PDI and SF-PDI4.16 Compared to SF-PDI4, the benzene core connected with three PDI building blocks (Ph-PDI3) induces a more congested structure, smaller molecular size and more twisted geometry, thus short-range stacking of PDI molecules is formed.17


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In this work, we finely tuned the congested and interlocking geometry by cautiously choosing the core unit. Our results demonstrated that biphenyl (BP) core can take advantages of quasi-3D configuration, resulting in irregular orientation of PDI units and small aggregation tendency. The Ph-PDI3 and SF-PDI4 with the same PDI building block were also synthesized. The photovoltaic performance of three acceptors blended with a chosen donor material poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophe ne))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]di thiophene-4,8-dione)] (PBDB-T), owing to their complementary absorption spetra, were compared. Moreover, the active layer morphology, charge mobility and non-geminate recombination were systematically studied. Finally, the highest PCE of 7.3% was achieved from PBDB-T:BP-PDI4 blend, highlighting the importance of fine tuning the geometry of PDI-based acceptors.

2. Results and Discussion 2.1. Synthesis and characterization The PDI derivatives are synthesized by palladium-catalyzed Suzuki coupling reaction between boric esters of core building blocks and mono-brominated PDI unit. The synthesis route of SF-PDI4, Ph-PDI3 and BP-PDI4 is shown in Scheme 1. The details can be found in Supporting Information. The PDI derivatives are identified by


1

H NMR data (see


Supporting Information). The thermal properties of three acceptors are investigated by thermogravimetric analysis. The decomposition temperature (Td, 5% weight loss) under a nitrogen atmosphere are 384 ℃ for SF-PDI4, 391 ℃ for Ph-PDI3 and 391 ℃ for BP-PDI4, indicating the good thermal stability of them.

2.2. Optical and electrochemical properties The normalized UV-vis absorption spectra of three acceptors in tetrahydrofuran (THF) solution (10-6 M) and neat films are shown in Fig. 1. The photoluminescence (PL) spectra of neat and blend films are shown in Fig. S2. The optoelectronic properties are listed in Table 1. As seen in Fig. 1, all three acceptors in THF solution show intense absorption in the visible range of 450~575 nm, with well resolved 0-0 absorption peaks in the range of 520~530 nm and 0-1 peaks in the range of 490~500 nm, and 0-2 peaks at ~460 nm.24 From the absorption onsets of the films, the optical band gaps (Egopt) are calculated to be 1.98, 2.04 and 2.11 eV for SF-PDI4, Ph-PDI3 and BP-PDI4, respectively. From solution to films, the maximum absorption peaks red shift, and the profiles of absorption spectra become broader, especially for SF-PDI4, indicating the strongest aggregation of SF-PDI4 in the solid film. The ratio of 0-1/0-0 transitions of BP-PDI4 is smaller than that of Ph-PDI3, which indicates that BP-PDI4 forms weaker aggregation than Ph-PDI3. This is consistent with the optimized geometry of Ph-PDI3 and


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BP-PDI4, which will be discussed in the next chapter. Additionally, BP-PDI4 exhibits higher extinction coefficiencies than that of SF-PDI4 and Ph-PDI3 both in solution and in films, as shown in Fig. S1. This confirmed the trend of the external quantum efficiency (EQE) measurement. Efficient charge transfer between PBDB-T and PDI derivatives has been proved by nearly complete PL quenching phenomena in the D-A blend films, as seen in Fig. S2.26 The cyclic voltammetry (CV) curves are shown in Fig. S3. The HOMO and LUMO energy levels were calculated from the CV measurement against Fc/Fc+ as the internal standard. The HOMO levels were calculated to be -5.80 eV, -5.81 eV and -5.84 eV for SF-PDI4, Ph-PDI3 and BP-PDI4 respectively. The LUMO levels were calculated to be -3.75 eV, -3.84 eV and -3.77 eV for SF-PDI4, Ph-PDI3 and BP-PDI4 respectively. 2.3. Theoretical calculation To get insight into the electronic distribution and geometrical configurations of BP-PDI4, the density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d, p) level to optimize the ground state geometries of the acceptor molecules.27 The long alkyl chains were replaced with methyl groups to simplify the calculations. The dihedral angles and simulated electron density distributions are illustrated in Fig. 2. The dihedral angles between four PDI planes and the core of BP-PDI4 are 58.2o, 52.7o, 54.8o and 61.6o respectively, comparable to that of SF-PDI4 (56o),18 while larger than that of Ph-PDI3 (51o)17. The PDI planes of BP-PDI4 show twisted planes of 16.02~18.82o, close to that



of SF-PDI4 (18o)18 and a little smaller than that of Ph-PDI3 (19.9 o).17 Although the difference in the twist angles of PDI plane and the dihedral angles between the PDI planar and the core unit are not significant, the optimized geometries of these acceptors are quite different as the result of different molecule size and rigidity of core units. Compared with SF unit, biphenyl core unit is much smaller, less than the half size of SF unit. Additionally, biphenyl unit is rotational. The dihedral angle between two benzene rings in the optimized BP-PDI4 geometry is 31.9o as shown in Fig. 2. Thus, the geometries of BP-PDI4 are much more compact than that of SF-PDI4.18 As for Ph-PDI3, the phenyl core unit is so small that propeller shape is necessary to arrange three PDI units around.17 The propeller shape of molecular configuration makes it possible thatPh-PDI3 can aggregate as gears. Although biphenyl core unit double the size of phenyl unit, Ph-PDI3 exhibits more serious aggregation than that of BP-PDI4 as proved by the film absorption spectra and atomic force microscope (AFM) results. This result can be attributed to the irregular orientations of PDI units, which will hinder the aggregation of PDI moieties more effectively. The electron distributions on HOMO and LUMO levels can also be seen in Fig. 2. The HOMO locates on half BP-PDI4, mainly distributing on PDI unit 1 and 2 and partly on the connected benzene ring, while the LUMO levels mainly located on PDI unit 2 and partially on 3 which attached to different benzene rings. This kind of electron distribution is believed to benefit charge separation because only one or two PDI can well pack with donor polymers and other PDI


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moieties can form electron pathway.28 The longer distance between donor polymer and acceptor can reduce the binding energy of photo-generated electron-hole pair.28, 29 Moreover, the calculated LUMO and LUMO+1 levels are -3.55 eV and -3.54 eV respectively. This small difference between LUMO and LUMO+1 indicates highly efficient electron acceptors,30 because the low-lying LUMO+1 compared to LUMO may accelerate the charge separation at the interface and induces efficient electron accepting.31

2.4. OPV properties To evaluate the photovoltaic properties of three synthesized PDI derivatives, solar cells were fabricated with a inverted device configuration of ITO/ZnO/Active layer/V2O5/Al. The optimized donor:acceptor ratios in all the OPV devices were found and kept as 1:1. The fabrication process was described in detail in Supporting Information. The J-V characteristics of the optimized devices processing with 1 v% diphenyl ether (DPE) as the additive were presented in Fig. 3A. For each type of PDI acceptors, more than 60 devices were fabricated and tested. The devices show the good reproducibility. The photovoltaic properties were summarized in Table 2. Devices based on PBDB-T:BP-PDI4 blend showed the highest PCE of 7.3% with Voc of 0.903 V, Jsc of 13.6 mA cm−2, and FF of 0.597, while the PCE of 6.7% for PBDB-T:Ph-PDI3 devices were obtained with Voc of 0.868 V, Jsc of 12.4 mA cm−2, and FF of 0.622, and PBDB-T:SF-PDI4 devices showed the PCE of



5.16% with Voc of 0.926 V, Jsc of 10.4 mA cm−2, and FF of 0.561. Compared with SF-PDI4, Ph-PDI3 and BP-PDI4 with benzene core exhibit much higher Jsc and FF, but slightly lower Voc. Since the Voc is mainly determined by the energy offset between HOMOdonor and LUMOacceptor, the lower Voc of the BP-PDI4 and Ph-PDI3 device can be ascribed to their deeper lying of LUMO (-3.77 eV and -3.84 eV) as evidenced from CV measurements. The EQE spectra of PBDB-T with three PDI derivatives were shown in Figure 3B. The Jsc integrated from the EQE spetra confirmed the trend and accuracy of direct I-V measurement. The EQE response cover a full wavelength range from 300 nm to 700 nm, which matches well with the UV-vis absorption spectra of PBDB-T donor blended with PDI derivatives. In order to expand the absorption range, we also blended three PDI derivatives with another well-known low bandgap donor polymer PTB7-Th. However, the PTB7-Th based devices exhibited a much lower efficiency (~3%) than PBDB-T based device. This result may suggest that our synthesized PDI derivatives has appropriate miscibility with PBDB-T donor, but incompatible with PTB7-Th donor. In PBDB-T:BP-PDI4 blended device, the highest EQE of ~83% was achieved at 540 nm, which was higher than that of Ph-PDI3 based device (78%) and SF-PDI4 based device (65%), indicating strong light absorption of non-fullerene acceptor, sufficient charge dissociation and transport. The electron and hole mobilities of the blends were measured by space charge limited current (SCLC) method using the electron-only device with the


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configuration of ITO/ZnO/PBDB-T:acceptor/Ca/Al and hole-only device with the configuration of ITO/PEDOT:PSS/PBDB-T:acceptor/MoO3/Ag. The dark current density versus effective voltage characteristics were showed in Fig. S4. As listed in Table. 2, the devices based on PBDB-T:BP-PDI4 show the highest electron mobilities (µe) of 3.45×10-5 cm2 V-1s-1 and highest hole mobilities (µh) of 6.28×10-5 cm2 V-1s-1, which is consistent with the observed highest device efficiency. While the devices based on PBDB-T:SF-PDI4 show the lowest µe of 2.11×10-5 cm2 V-1s-1 and µh of 4.17×10-5 cm2 V-1s-1. The ratio of µh/µe decreases in the sequence of SF-PDI4, BP-PDI4, and Ph-PDI3, also in agreement with the increase of FF values. To better understand the relationship of molecular structure and OPV performance and the reason for the difference in Jsc, the recombination kinetics was studied. After free charge carriers are generated, there is a competition between the charge collection at the electrodes and non-geminate recombination in the bulk film.32, 33 Non-geminate recombination includes bimolecular and monomolecular process such as trap-assisted recombination.34, 35 In Jsc-light intensity measurements, the Jsc should follow a power law relationship as J~Iα.36 If the value of α reaches 1, it implies negligible bimolecular recombination, meaning the free carriers can be swept out and collected by the electrodes efficiently. As shown in Fig. 4A, decreased bimolecular recombination in the solar cell devices was observed in the order of SF-PDI4, Ph-PDI3 and BP-PDI4. Meanwhile, the degree of trap-assisted recombination was evaluated by measuring the dependence of Voc on the incident light intensity (Fig. 4B). In this



measurement, the Voc versus the natural logarithm of the light intensity should follow a linear relationship where the slope of curve s > 1.0 kT/q is indicative of significant trap-assisted recombination.37 The Voc-light intensity measurements exhibit the similar trend as Jsc-light intensity measurements shows. These recombination kinetics measurements prove that the BP-PDI4-based devices exhibit the smallest bimolecular recombination and a negligible trap-assisted recombination. This is in good agreement with the largest Jsc value of BP-PDI4-based devices. On the contrary, SF-PDI4 based device exhibits the most significant bimolecular recombination and trap-assisted recombination, resulting in the smallest Jsc.

2.5. Morphology properties The morphology of PBDB-T:PDI derivatives blend films, which is vital to the device performance, was characterized by atomic force microscopy (AFM). The height and phase images are showed in Fig. 5. The active layer of PBDB-T: BP-PDI4 exhibits the most uniform surface with a smallest root mean square (RMS) roughness of 1.19 nm, and a nanoscale phase separation. The PBDB-T:Ph-PDI3 blend film exhibits a rougher surface with the increased RMS of 2.25 nm, and a larger size aggregation. The PBDB-T: SF-PDI4 blend film exhibits the roughest surface with a RMS of 2.90 nm, and the largest phase domains. Film morphology is in good align with the aggregation tendency of these non-fullerene acceptors as demonstrated by the optimized geometries and film absorption spectra. Because the effective exciton diffusion length is


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typically 5~20 nm, a relatively small domain results in more D-A interface, which is beneficial to efficient exciton diffusion and charge separation. As a result, it can be concluded that the decreasing roughness and finer phase separation are in consistence with the increasing of charge mobility, Jsc and final PCE value.

3. Conclusion In summary, we systematically studied geometry effects of the core unit in PDI-based quasi-3D non-fullerene acceptors. The orientation of PDI unit can be modified by cautiously choosing the core unit, which will affect the molecule stacking tendency, phase separation of blend film and the photovoltaic performance. When BP-PDI4 is blended with the donor material PBDB-T in the weight ratio of 1:1, the finest phase separation and smallest non-geminate recombination were observed, resulting in the highest PCE of 7.3%. This work highlights the significance of geometries and the orientation of PDI units when design novel PDI-based non-fullerene acceptors.

Supporting Information Measurements and characterization, synthesis process, device fabrication and characterization methods, extinction coefficiencies spectra and photoluminescence spectra, cyclic voltammetry curves and charge carrier mobility



Acknowledgements This work was supported by Wuhan Yellow Crane Program for Excellent Talents, Hubei Technology Innovation Major Project (2016AAA030), the Recruitment Program of Global Youth Experts of China, the Foundation for Outstanding Youth Innovative Research Groups of Higher Education Institution in Hubei Province (T201706), the Foundation for Innovative Research Groups of Hubei Natural Science Foundation of China (2017CFA009), and Youth Project of Hubei provincial department of education (Q20171501).

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Table.1 The optoelectronic properties of three acceptors λabs sol.

λabs film

Materials

LUMO (eV)

HOMO (eV)

(nm)

(nm)

SF-PDI4

530

546

-3.75

-5.80

Ph-PDI3

524

543

-3.84

-5.81

BP-PDI4

524

536

-3.77

-5.84

Table. 2 J-V Characteristics of solar cells and charge carrier motilities of PBDB-T:acceptor blend films

Voc

Jsc

FF

PCEmax

(V)

(mA/cm )

(%)

(%)

µh

(cm2

(cm2

V-1s-1)

V-1s-1)

PCEaverage

Materials 2

µe

µh/µe

(%)

SF-PDI4

0.93

10.4

56.1

5.4

5.0±0.2

2.11×10-5

4.17×10-5

1.98

Ph-PDI3

0.87

12.4

62.2

6.7

6.5±0.2

3.09×10-5

5.39×10-5

1.74

BP-PDI4

0.90

13.6

59.7

7.3

7.0±0.3

3.45×10-5

6.28×10-5

1.82

Scheme 1 Synthetic route to SF-PDI4, Ph-PDI3 and BP-PDI4


1.0

A

SF-PDI4

Ph-PDI3 BP-PDI4

0.8 0.6 0.4 0.2 0.0 350

400

450

500

550

600

650

Wavelength (nm)

700

Nomalized Absorption Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nomalized Absorption Intensity (a.u.)


1.0

B

Page 22 of 25

SF-PDI4 Ph-PDI3

0.8

BP-PDI4 PBDB-T

0.6 0.4 0.2 0.0 400 450 500 550 600 650 700 750

Wavelength (nm)

Fig. 1 The UV-vis absorption spectra of acceptors in THF solution (A) and of acceptors and donors in neat films (B)

Fig. 2 The optimized geometries and calculated electron density distributions of BP-PDI4


Page 23 of 25

100

4

A

B 80

0 -2

SF-PDI4

-4

Ph-PDI3 BP-PDI4

-6

EQE (%)

Current density (mA/cm2)

2

-8 -10

60

40

SF-PDI4 Ph-PDI3

20

BP-PDI4

-12 -14 -16 -0.2

0 0.0

0.2

0.4

0.6

0.8

300

1.0

400

500

600

700

800

Wavelength (nm)

Voltage (V)

Fig. 3 The J-V curves (A) and external quantum efficiency (EQE) (B) of PBDB-T blended with SF-PDI4, Ph-PDI3 and BP-PDI4 in weight ratios of 1:1 with DPE as additive

0.95

10

A

B 0.90

-2

Current density ( mA cm )

0.85

1

Voc (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.75

SF-PDI4 S=0.78

0.1

0.80

Ph-PDI3 S=0.81

SF-PDI4 Ph-PDI3 BP-PDI4

0.70

BP-PDI4 S=0.86

0.01

1.23 KT/q 1.1 KT/q 1.01 KT/q

0.65

1

10

2

Light intensity (mW/cm )

100

1

10

-2

Light intensity (mW cm )

Fig. 4 Jsc (A) and Voc (B) versus the light density of the solar cell devices


100


Fig. 5 The height (upper row) and phase (lower row) images of PBDB-T:SF-PDI4 (A), PBDB-T:Ph-PDI3 (B) and PBDB-T:BP-PDI4 (C).


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Table of Contents


Fine-Tuning the Quasi-3D Geometry: Enabling Efficient Nonfullerene

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