Planar Benzofuran Inside-Fused Perylenediimide Dimers for High

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Planar benzofuran-inside-fused perylenediimide dimers for high VOC fullerene-free organic solar cells Jing Yang, Fan Chen, Junyi Hu, Yanfang Geng, Qingdao Zeng, Ailing Tang, Xiaochen Wang, and Erjun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19563 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Planar Benzofuran-Inside-Fused Perylenediimide Dimers for High VOC Fullerene-Free Organic Solar Cells Jing Yang1,2‡, Fan Chen1,2‡, Junyi Hu1,2, Yanfang Geng*1, Qingdao Zeng1, Ailing Tang1, Xiaochen Wang1, Erjun Zhou*1,2,3

1 CAS

Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in

Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China. 2

Center of Materials Science and Optoelectronics Engineering，University of Chinese Academy

of Sciences, Beijing 100049, China. 3 Henan

Institutes of Advanced Technology, Zhengzhou University, 97 Wenhua Road, Zhengzhou

450003, China.

E-mail: [email protected]; ‡ These

[email protected]

two authors contributed equally to this work.

Abstract Bulk heterojunction organic solar cells based on perylenediimide (PDI) derivatives as electron acceptors have afforded high power conversion efficiency (PCE) but still lagged behind fullerene-based analogs. Design of novel molecular structures by adjusting PDI ring and/or connection mode remains the breakthrough point to improve the photovoltaic performance. After introducing benzofuran at inside bay positions and linked with single bond and fluorene unit, mandatory planar PDI dimers were achieved and named as FDI2 and F-FDI2 respectively. Both acceptors show high-lying LUMO energy levels and realize high VOC beyond 1.0 V when using the classic polymer of PBDB-T as electron donor. However, FDI2 and F-FDI2 gave totally different photovoltaic performance with PCEs of 0.15% and 6.33% respectively. The central 1

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fluorene linkage increased the miscibility of materials and ensured much higher shortcircuit current due to the formation of suitable phase separation. Our results demonstrated that utilizing the mandatory planar skeleton of PDI dimers is simple and effective strategy to achieve high-performance non-fullerene electron acceptors, and the modulation of central conjugated units is also vital. Keywords: perylenediimide; benzofuran; planar; high VOC; fullerene-free solar cell 1.

Introduction Bulk heterojunction (BHJ) organic solar cells (OSCs) based on non-fullerene

acceptors (NFAs) have been extensively studied in virtue of their advantages of adjustable energy levels, wide absorption range and molecular structure variety in comparison with traditional fullerene derivatives.1-5 The emergence of NFAs overcame the shortcomings of fullerenes, exhibited excellent photovoltaic properties with enhanced power conversion efficiency (PCE) exceeding 14% for single-junction device.6-9 This value is obviously higher than that of the reported best fullerene OSCs, demonstrating the viability of NFAs in new generation OSCs. Driven by the rapid progress, many kinds of electron-accepting building blocks such as naphthalene diimide (NDI),10-12 perylene diimide (PDI),13 1,1-dicyanomethylene-3indanone (IC),14-16 benzothiadiazole (BT),17-18 benzo[d][1,2,3]triazole (BTA)19-21 and quinoxaline (Qx)22 have been used to construct NFAs. Among them, the application of PDI derivatives in OSCs underwent through a long period of stagnation, though it was used in bilayer OSCs as acceptor as early as in 1986.23 The major reason is that the intense intermolecular interactions between rigid fused PDI molecules induce poor controllability of phase separation size in the blend system, resulting in serious charge recombination.24 Until recent succefull exploration of tailoring methods, various PDI molecules exhibiting good stability and high electron mobility have become an exciting and active research area within the field of fullerene-free OSCs.25-26 The modified position at the bay or imide position of perylene core as well as the 2


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nature of the attached substituents strongly impact their solubility, electronic structure, absorption capability. According to the number of parents PDI units, PDI-based small molecules can be divided into single-PDI, double-PDI and multi-PDI. Single-PDI is easy to form a strong crystal, therefore the performance is relatively poor and the highest PCE is only 4.1%.27 Although multi-PDIs have non-planar molecular architecture which can effectively suppress aggregation and also exhibited encouraging performance in recent several years,28-32 their synthesis and purification procedures are relatively cumbersome, In addition, their electron mobility is relatively low due to the distortion of PDI ring. On the other hand, double-PDIs not only are easily functionalized but also provide a simple backbone to investigate the structureproperties relationship. The simplest double-PDI, di-PBI gave a low PCE of 4.2% due to severe twist of two PDI units.33 A variety of PDI dimers were later obtained by introducing conjugated groups varying from simple thiophene ring to complex fusering units as linkages at bay positions.34-39 A record PCE of 9.5% was obtained by using spirobifluorene as linkage of PDI dimer.40 The results indicate the appropriate twisting and flexible structure is critical aspect to yield high performance. Furthermore, heteroannulation at outside bay positions in PDI dimers is also recognized as an effective strategy to tune molecular configuration and even energy levels.41 The outside-fused PDI dimer based on N-annulation showed a high PCE of 7.55% with P3TEA as donor polymer.42 After replacing of N atom with S, the PCE can be improved up to 8.01%.43-44 Se-annulation PDI dimers largely increased PCE to 8.4%,45 and further enhanced PCE up to 10.2% in a ternary device.46 These results indicate that the modest twist angle by tuning PDI skeleton can balance the exciton separation and charge transport. Recently, it was found that outside-fused PDI dimer showed higher twisted angle than inside-fused PDI dimer possessing relatively planar configuration.47 A thiazole inside-fused PDI dimer gave a PCE of 4.5%.48 We observed that nearly planar thianaphthene inside-fused PDI dimer showed a gratifying PCE up to 5.8%.49 Together with N-annulation PDI,50 there are only three inside-fused PDI dimers currently reported, as shown in Figure 1. In order to further upshift the LUMO energy level to 3



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improve the VOC, we introduced heteroatom oxygen into the PDI skeleton and designed here a new type of benzofuran inside-fused PDI building block, namely FDI. By direct coupling or inserting fluorene unit, two PDI dimers were obtained and named as FDI2 and F-FDI2 respectively. The polymer of PBDB-T was selected as the polymer donor, because PBDB-T has performed well with fullerene and non-fullerene small molecular acceptor.37, 51-52 Compared to thianaphthene inside-fused PDI dimer, FDI2 and F-FDI2 exhibiting higher LUMO energy levels gave improved VOC over 1.0 V. The insertion of fluorene unit induced suitable phase separation at micrometer scale facilitating exciton diffusion, charge separation and transport. As a result, PBDB-T:F-FDI2 exhibited a much higher PCE of 6.33% than that of PBDB-T:FDI2 device (0.15%). Inside-fused PDI dimers

Orignal PDI dimers O

N

O

O

N

O

O

N

O

O

N R

O

O

O

N

O O

Ar

Ar O

N

R

R

R

R

R

R

O

N

O

O

R

N R

O

O

N

O

R N S

O

N R

Ar

O

N

O

O

N

O

O

O

New structure

R

N

S

Figure 1. The chemical structures of original PDI dimers and corresponding insidefused derivatives.

2. Results and Discussion 2.1 Material synthesis and characterization

4


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O

R N

O

O O Br

R N

O

OH B OH

O

O

R N

O

O

N R

O

O

1

N R

O

Br

O

2

N R

O

(C4H9)3Sn

Sn(C4H9)3

R N

O

Pd(PPh3)4

C8H17

C8H17

O B O

O

O

O

O B O

O

O

O

+

R N

O

Toluene

4

N R

4 O

+

O

3

R = -CH(C6H13)2

4

O

CH2Cl2

I2, Benzene O

O

Br2

O

hv

R N

R N

N R

O

FDI2

O

N R

O

Pd(PPh3)4

O

O

O

C8H17

R N

O

C8H17

Toluene 2M K2CO3

O

O

N R

O

F-FDI2

O

N R

O

Figure 2. The synthesis method of two benzofuran inside-fused perylenediimide dimers FDI2 and F-FDI2.

The intermediate PDI-Br (4) was synthesized according to similar molecular methods in the literature, and the detailed reaction procedure is described in supporting information.53 FDI2 and F-FDI2 dimers were synthesized by using the commonly Stille and Suzuki coupling reactions, respectively. The synthesis routes of these two dimers are shown in Figure 2. 1H NMR was applied to identify intermediates and target molecules, which can be obtained in Figure S1. These two PDI dimers exhibit good thermal stability as proved by thermogravimetric analysis (TGA), as shown in Figure S2. Their frontier molecular orbitals were assessed through density function theory (DFT) calculations as illuminated in Figure 3. The optimized configurations confirmed our expectation that benzofuran inside-fusion assumed planarity of PDI unit. The directly connected or fluorene-connected two PDI yielded two planar FDI2 and F-FDI2 dimers, which are distinctively different from the reported highly twisted PDI dimers.37 The 5



HOMO and LUMO levels of FDI2 were located on the entire skeleton with little contribution from single-bond, which can be considered as a very weak and small electron-donating unit. For F-FDI2, the LUMO energy level distributes on PDI unit at both sides because of the strong electron-donating of fluorene relative to single-bond, and the HOMO mainly delocalizes on both the benzofuran and fluorene parts with higher density at the fluorene unit.

Figure 3. Top view and side view of optimized geometries as well as the calculated electron density distributions of LUMO and HOMO by DFT at the B3LYP/6-31G (d) level for FDI2 and F-FDI2.

Their light harvesting aptitudes and optical band gap were obtained by UV-vis absorption record, as plotted in Figure 4 and summarized in Table 1. FDI2 and F-FDI2 exhibit similar absorption band in solution, yet F-FDI2 possesses a broader absorption from 340 nm to 630 nm. The maximum molar extinction coefficients ( ε max) are estimated to 1.05 × 105 and 1.56 × 105 M-1·cm-1 for FDI2 and F-FDI2, respectively. The film absorption bands of FDI2 and F-FDI2 are different varying from 300 nm to 750 nm. F-FDI2 exhibits slightly broader absorption than that of FDI2, indicating stronger 6


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dipolar interactions between fluorene units. F-FDI2 shows a significant band at 625 nm as a shoulder, revealing strong intermolecular interaction leading to π-π stacking in solid film. Therefore, the crystalline of F-FDI2 leads to a more preferential organization for charge transport in active layer over FDI2 devices. Compared with absorption in solution, both PDI dimers show broader absorptions in film, suggesting the formation of much stronger aggregation.

0.8 0.6 0.4 0.2 0.0 300

(c)

400 500 600 Wavelength (nm)

C4H9

C2H5 C2H5

C2H5 C4H9

S

S S

O S

S n

S

C2H5

0.4

PBDB-T

In film FDI2

F-FDI2

0.2 0.0 300

400

500 600 Wavelength (nm)

-3.40

C4H9

S O

S

700

0.6

700

-3.61 -3.56

F-FDI2

1.0

0.8

FDI2

F-FDI2

PBDB-T

1.2

1.0 (b)

In solution FDI2

Normalized absorption

-1

1.4

Energy Level [eV]

-1

1.6 (a)

5

Absorption coefficient (10 ·M ·cm )

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


-5.23

C4H9

-5.98

-5.86

Figure 4. UV-vis absorption spectra of two FDI derivetives (a) in CHCl3 solution and (b) in blend films on quartz plates. (c) the chemical structure of PBDB-T and the energy level diagram of active layer materials.

As shown in Figure S3, cyclic voltammetry (CV) measurement provided different energy level infromation of F-FDI2 compared to FDI2, suggesting that the central conjugated fluorene unit substantailly pertubed energy level relative to single-bond bridge. F-FDI2 possessed somewhat high-lying both HOMO and LUMO, ensuring 7



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adequate energy difference between HOMO of donor and LUMO of acceptor for high VOC of solar cells. The result further proves that the inside-fusion is an effective method to tune LUMO enegy level.

Table 1. Optoelectronic properties of FDI2 and F-FDI2. UV–vis absorption Material

CV

𝑓𝑖𝑙𝑚 𝑜𝑝𝑡 𝜆𝑠𝑜𝑙 𝑚𝑎𝑥[nm] 𝜆𝑚𝑎𝑥 [nm] 𝐸𝑔 [eV]

HOMO [eV]

LUMO [eV] 𝐸𝑒𝑙𝑒𝑐 [eV] 𝑔

FDI2

481

391

1.97

-5.98

-3.61

2.37

F-FDI2

482

395

1.99

-5.86

-3.56

2.30

2.2 Photovoltaic results These two PDI dimers were subsquently applied for OSC characterization in combination with PBDB-T as donor. A cell configuration of ITO/PEDOT:PSS/active layer/Ca/Al is employed. The photovoltaic performance of the devices based on PBDBT: FDI2 (or F-FDI2) were optimized by using different donor/ acceptor (D/A) weight ratios, annealing temperatures and additives, and the detailed photovoltaic parameters were shown in Table S1 and S2. The current density-voltage (J-V) characteristics of the best cells under illumination are shown in Figure 5. Under identical conditions, FFDI2 afforded dramatically larger cell performance compared to the analogue FDI2, which is contributed from the greatly enhanced JSC (9.88 mA·cm-2 vs 0.55 mA·cm-2) and FF (0.62 vs 0.26) values. With a comparable VOC value, F-FDI2 conferred a desirable PCE of 6.33% considerably larger than that of FDI2 deivce. The pronounced difference in JSC suggests disparate charge separation and transportion properties between FDI2 and F-FDI2 cells. Additionaly, the reduced VOC of PBDB-T:F-FDI2 is inconsistent with the featured larger HOMO(D)-LUMO(A) energy offset, which can be caused by the decreased domain size properly affecting the energy levels of donor and acceptor.54 In order to further gain the origin of significantly different performance, systematic analyses were conducted by the aid of other charaterizations. 8


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Figure 5. (a) J–V curves and (b) EQE spectra of OSC devices based on FDI2 and FFDI2.

Table 2. Characterzation parameters of organic solar cells based on PBDB-T:FDI2 and PBDB-T:F-FDI2 under the illumination of AM 1.5 G,100 mW·cm−2.a VOC

JSC

FF

PCE

Hole mobility

Electron mobility

[V]

[mA·cm-2]

[%]

[%]

[cm2·V-1·s-1]

[cm2·V-1·s-1]

FDI2

1.06 (0.00)

0.55 (0.06)

25.65 (0.21)

0.15 (0.02)

1.58×10-6

9.47×10-7

F-FDI2

1.04 (0.00)

9.88 (0.18)

61.63 (0.59)

6.33 (0.14)

7.81×10-3

1.29×10-6

Compound s

a

The standard deviation of each parameter obtained from 12 cells.

External quantum efficiency (EQE) were detected to further analysize and confirm the accuracy of J-V measurements. In particular, the integration of EQE curve can offer integrated current density. FDI2 devices give a very low EQE, which is consistent with the measured low JSC values. By contrast, donor PBDB-T and acceptor F-FDI2 jointly contribute to a broad EQE band varying from 300 nm to 750 nm with the peak value approaching ca. 58%. From UV-vis characterization above, FDI2 shows similar absorption band and range with F-FDI2, therefore, the loss of JSC is not contributed to the light absorption, but possibly to the charge transport procedure. Furthermore, the JSC values calculated from the EQE spectra matched well with the JSC measured under 9



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AM 1.5, 100 mW cm-1. 2.3 charge carrier mobilities, photoluminescence spectra

crystallinity,

film

morphology

and

Hole and electron mobilities of devices were tested by using the space charge limited current (SCLC) method with device structures of ITO/PEDOT:PSS/active layer/Au (hole-only) and ITO/TiOx/active layer/Al (electron-only). The typical J–V curves based on the hole-only and electron-only devices are shown in Figure S4 and S5, respectively. As listed in Table 2, the hole and electron mobilities of PBDB-T:FDI2 blend film are estimated as 1.58×10-6 cm2·V-1·s-1 and 9.47×10-7 cm2·V-1·s-1, respectively. For the blend of PBDB-T:F-FDI2, the hole mobility increases to 7.81×10-3 cm2·V-1·s-1, which is three orders of magnitude than that of FDI2, and the electron mobility also increases by one order of magnitude to 1.29×10-6 cm2·V-1·s-1. Therefore, PBDB-T:F-FDI2 possessed significantly higher charge mobility hole and electron mobility than that of PBDB-T:FDI2. Although the measured electron mobilities through SCLC method are lower than other planar PDI derivatives,55-56 the charge transport property is in parallel with the high performance of PBDB-T:F-FDI2, especailly JSC and FF. Note that the increased hole mobility of PBDB-T in blend of PBDB-T:F-FDI2 is much higher than other PBDB-T:PDI systems,37, 52 but in the same order of magnitude as other PBDBT:polymer blend.57-59 Such an evident difference of charge mobility is related to the molecular crystalline, phase separation as well as domain size. For PDI derivatives, a fatal issue is their problematic self-aggregation, which may induce excessive phase separation mostly limiting performance. Morphology studies can offer direct evidence of phase separation to unravel auxiliarily how the molecular structure affect the performance. The surface morphology of active layer was investigated by tapping atomic force microscopy (AFM). It has been observed that the surface morphology of PDI dimer blends can be largely modulated by the bridging groups between two PDI units, which is originated from the impact of molecular configuration. As shown in Figure 6, the blend PBDB-T:FDI2 film showed much larger 10


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crystalline grains than that of PBDB-T:F-FDI2 film. Although the similar planar structure were optimized above for both FDI2 and F-FDI2, the self-aggregation of FFDI2 could partly be attenuated by the branched side chain attached on the fluorene unit. The solubilizing side chains on the other hand enhance the misibility with donor PBDB-T, leading to a suitable phase separation in active layer PBDB-T:F-FDI2. PBDBT reduced the formation of large crystals of F-FDI2 as a compatibilizer, leading to sufficient charge separation and transportation affording much higher JSC and FF values. In addition, the larger fluorene unit compared to the small single bond in FDI2 can increase the charge separation distance at donor/accptor interface, resuliting in higher JSC.60 The reason why FDI2 afforded fairly small JSC can be attributed to the excessive phase separation from the donor PBDB-T because of crystallization of both PBDB-T and FDI2 due to the weak misibility between PBDB-T and FDI2. The crystallization and large phase separation occurred with FDI2 but not with F-FDI2 was further confirmed below by 2D grazing incidence wide-angle X-ray scattering (GIWAXS) characterizations. 4.7 nm

PBDB-T:FDI2

7.6 nm (b)

PBDB-T:F-FDI2

Out of plane

(e)

Intensity

(a)

PBDB-T:FDI2

PBDB-T:F-FDI2

0.5

-7.2 nm

-5.0 nm (c)

PBDB-T:FDI2

(d)

PBDB-T:F-FDI2

1.0

1.5

2.0

-1

q z (Å ) In plane

(f)

Intensity

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


PBDB-T:FDI2 PBDB-T:F-FDI2

0.5

1.0

1.5

2.0

-1

qxy (Å )

Figure 6. (a, b) AFM topographic images (5 μm × 5 μm), (c, d) 2D grazing incidence wide-angle X-ray scattering (GIWAXS) patterns, (e, f) out-of-plane and in-plane profiles of PBDB-T:FDI2 and PBDB-T:F-FDI2 blend films. 11



As previously reported, the prinstine PBDB-T film possesses strong π-π stacking peak at qz ≈ 1.70 Å-1 (d = 3.70 Å) observed in the out-of-plane direction and exhibits a peak at qxy ≈ 0.30 Å-1 in the in-plane direction corresponding to a lamellar distance of 21.0 Å.57 After being blended with FDI2, the strong π-stacking peak of PBDB-T was not largely affected, however this peak is weaken when blending with F-FDI2. Moreover, the peak located at qxy ≈ 0.6 Å-1 (d = 10.48 Å) in regards to backbone repeat direction of PBDB-T is still present, further indicating that the addition of FDI2 did not disarrange the molecular packing of PBDB-T, in other words, large phase separation occurred. In the in-plane direction, the broad peak in the range of 0.2 Å-1 ~ 0.4 Å-1 of PBDB-T:F-FDI2 slightly shifts relative to that of PBDB-T:FDI2, indicating the increased lamellar distance owing to the formation of side-chain interactions. Therefore, the 2D GIWAXS patterns confirmed that PBDB-T exhibits large-scale face-on and edge-on order structure in blend of PBDB-T:FDI2 than PBDB-T:F-FDI2, which is in accordance with the observation in AFM. Photoluminescence (PL) spectra of their blended films are displayed in Figure S6, from which it can be seen that the photoluminescence is fully quenched in PBDB-T: FFDI2, while strong photoluminescence is observed from the blended film of PBDB-T: FDI2. The PL results are consistent with AFM observation.34 FDI2 aggregated large domains in the blend film with PBDB-T, leading to inefficient fluorescene quenching, while F-FDI2 formed much smaller phase domains due to the strikingly curbed aggregation, and thus the quenching became more efficient. Therefore, the excessive phase separation inhibits the exciton diffusion to the donor/acceptor interface, which is consistent with the lower JSC of PBDB-T:FDI2 cell. The phase separation of these two blend films is schematically illuminated as shown in Figure S7.

3. Conclusion In summary, two benzofuran inside-fused PDI dimers (named as FDI2 and F-FDI2) were designed and synthesized. By inside-fused with benzofuran, two molecules 12


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exhibit high-lying LUMO energy levels and totally planar molecular backbone. By varing the bridge group, FDI2 and F-FDI2 exhibited different aggregation ability when blending with the classic p-type polymer of PBDB-T, which result in different film morphology and charge mobilities. As a result, PBDB-T: FDI2 conbination showed inferior photovoltaic performance with a low PCE of only 0.15%, whereas PBDB-T: F-FDI2 realized a largely improved PCE of 6.33%. Futher improvements could probably be achived by tuning the central aromatics and side-chain funtions towards matched energy level and enhanced electron mobility. Our results indicate that mandatory planar benzofuran-inside-fused perylenediimide dimers, together with the choice of suitable bridged unit, is an effective strategy to realize high VOC fullerenefree organic solar cells.

4. Supporting Information The Supporting Information is available free of charge on the ACS publication website at DOI: 10.1021/acsami. Material and method; 1H NMR and 13C NMR spectra; cyclic voltammograms; J–V curves based on the hole-only and electron-only devices; photoluminescence spectra; photovoltaic performance (PDF)

Author information Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements The authors thank the support from the National Key Research and Development Program of China (2017YFA0206600), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences

13



(Grant No. QYZDB-SSW-SLH033), the National Natural Science Foundation of China (NSFC, Nos. 51473040, 51673048, 21875052, 21602040, 21504019, 51773046, 51873044).

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