PDI Derivative through Fine-Tuning the Molecular Structure for

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PDI Derivative through Fine-Tuning Molecular Structure for Fullerene-Free Organic Solar Cells Hua Sun, Xin Song, Jian Xie, Po Sun, Peiyang Gu, Chang Mei Liu, Fei Chen, Qi Chun Zhang, Zhi-Kuan Chen, and Wei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08282 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

PDI Derivative through Fine-Tuning Molecular Structure for Fullerene-Free Organic Solar Cells Hua Sun,†,‡ Xin Song,† Jian Xie,‡ Po Sun,† Peiyang Gu,‡ Changmei Liu,† Fei Chen,† Qichun Zhang*,‡,§, Zhi-Kuan Chen*,†, Wei Huang*,† †

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816 (P. R. China) ‡

School of Materials Science and Engineering, Nanyang Technological University, Singapore

639798, Singapore §

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical

Sciences, Nanyang Technological University, 637371, Singapore

Abstract: A perylenediimide (PDI)-based small molecular (SM) acceptor with both an extended π-conjugation and a three-dimensional structure concurrently is critical for achieving high performance PDI-based fullerene-free organic solar cells (OSCs). Herein, a novel PDI-based SM acceptor have been successfully synthesized through fusing PDI units with a spiro core 4,4’spirobi[cyclopenta[2,1-b;3,4-b’]dithiophene (SCPDT) together via the β-position coupling with thiophene bridges. An enhanced absorption from 350 nm to 520 nm has been observed. Moreover, comparing with previously-reported acceptor SCPDT-PDI4, in which the PDI units

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and SCPDT are not fused together, the LUMO energy level of FSP increases. OSCs containing PTB7-Th as a donor and FSP as an acceptor have been demonstrated to show an excellent performance with a power conversion efficiency as high as 8.89%. This result might attribute to the efficient and complementary photo-absorption, balanced carrier mobilites and favorable phase separation in the blend film. This research could offer an effective strategy to design novel high performance PDI-based acceptors.

KEYWORDS: Perylenediimide, Non-fullerene acceptor, Small molecules, 3D structure, Organic solar cells, Power conversion efficiency 1. INTRODUCTION As one of current hottest research focuses, fullerene-free organic photovoltaics (OPVs) have received many attentions.1-4 Compared with conventional fullerene derivatives (e.g., PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester) and PC71BM ([6,6]-phenyl-C71-butyric acid methyl ester)), non-fullerene acceptors have shown unique superiorities such as strong light absorption, easily-tuned energy levels and good stability.5-7 The rapid development in materials and device engineering has made great progress in fullerene-free organic solar cells (OSCs) and their power conversion efficiencies (PCEs) have surpassed 10% with selected donors,8-10 which are comparable with those of PC61BM/PC71BM based devices. To date, the most attractive high performance non-fullerene-based acceptors are constructed of indacenodithieno[3,2-b]thiophene (IT) or indacenodithiophene (IDT) donor building blocks and propanedinitrile,-2-(2,3-dihydro-3oxo-1H-inden-1-ylidene) (INCN) acceptor moiety.11-14 Non-fullerene acceptors consisting of other structures with good performance are quite rare. Thus, it is urgent to design and synthesize non-fullerene derivatives with new backbones to push the efficiency of OSCs.15


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Recently, perylenediimide (PDI)-based small molecule (SM) acceptors have been widely investigated owing to their high electron mobility, excellent chemical/thermal stability, and strong light absorption.16-20 Previous studies have proven that PDI-based SM acceptors with twisted three-dimensional (3D) structures can effectively overcome excessive aggregation of PDI units and facilitate the isotropic charge transport in the film, thus leading to improved device performance.21-23 However, the twisted structures induced by steric repulsion among PDI units will weaken their charge transport abilities and impede the further enhancement of the photovoltaic performance in OSCs.24-27 Thus, to design and synthesize PDI-based SMs with a 3D architecture but good electron delocalization along the backbone to further enhance charge transport is much desired. As

a

new

spiro-molecule,

4,4’-spirobi[cyclopenta[2,1-b;3,4-b’]dithiophene

(SCPDT)

possesses an orthogonal molecular conformation with two planar cyclopenta[2,1-b;3,4b’]dithiophene (CPDT) units connected together through a spiro-sp3 carbon. Due to the wellconjugated structure and electron-rich property of CPDT, SCPDT-based molecules are expected to show strong intramolecular charge transfer (ICT) characteristics, broad low-energy optical transitions and good charge mobilities.28 Our previous work showed that fullerene-free OSCs based on a 3D SM acceptor SCPDT-PDI4 can reach a modest PCE of 7.11%.29 SCPDT-PDI4 is composed of a SCPDT spiro core and four PDI arms, in which two pairs of PDI units are conjugated with CPDT linker. The PDI units take a twisted conformation with a dihedral angle of 59o relative to the CPDT core. This information suggests that better photo absorption and device efficiency could be achieved if the issue of twisted conformation between PDIs and CPDT is solved. It is well-known that ring fusion is a straightforward approach to construct large conjugated systems and tune the energy levels of organic semiconductors.30-33 Following this


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strategy, we designed and synthesized a new SCPDT-based molecule FSP through directly fusing PDI units with thiophene rings of CPDTs to increase the planarity between PDIs and CPDT core, thus extend the effective π-conjugation length and elevate the lowest unoccupied molecular orbital (LUMO) level of the molecule. Such a design would overcome the twisted πconjugation issue between PDIs and CPDT core and optimize the LUMO energy level of FSP. At the meantime, the spiro structure of FSP still retains a rigid 3D molecular conformation. Solar cells with PTB7-Th as an electron donor and FSP as an electron acceptor exhibit an efficiency up to around 9% due to good absorption, balanced electron/hole mobility and optimal energy levels, indicating a promising prospect for non-fullerene acceptors based on PDI related material systems. 2. RESULTS AND DISCUSSION FSP was synthesized through the Stille coupling and oxidative cyclization reactions (Scheme 1). Four PDI units were connected to SCPDT in the first place, and then ring fusion was taken by fusing four PDI units with SCPDT core. FSP was characterized through NMR spectroscopy and mass spectrum (Figure S5-8). FSP exhibited a favorable thermal stability with a decomposition temperature (Td, 5% weight loss) of 373 oC (Figure S1). In addition, FSP can be dissolved in most common polar organic solvents (e.g. chlorobenzene, chloroform, etc.) with good solubility.


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Scheme 1. Synthetic route to FSP: (a) Pd(PPh3)4, toluene, 110 oC; (b) FeCl3, CH3NO2, toluene 110 oC. The optical properties of FSP were characterized in both solution and film states through UVvis absorption spectroscopy, and all related data have been summarized in Table S1. FSP shows strong absorption from 350 nm to 520 nm, with a large molar absorption coefficient of 1.8 × 105 M−1 cm−1 at 493 nm (Figure S2). Notably, FSP also shows strong absorption from 520 nm to 650 nm owing to the ICT between SCPDT core and PDI units. The fully-fused backbones of FSP with more delocalized π-electrons make this phenomenon more obvious.24,25 A broader and similar absorption profile was obtained in film state due to the weak intermolecular interactions (Figure S2). The similar UV profiles (also can be seen in temperature-dependent solution UVvis spectra Figure S2) in film and solution suggest that the aggregation of PDIs were effectively suppressed. The optical bandgap of FSP was determined to be 1.90 eV according to the onset of thin-film absorption spectrum. As shown in Figure 1a, in comparison, PTB7-Th film shows a strong absorption from 550 nm to 770 nm, which complements to that of FSP (300-630 nm). The complementary absorption between donor and acceptor, which favors solar energy harvesting, are critically important for high performance OSCs.34,35


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The energy levels of FSP was determined by cyclic voltammetry (CV) measurement (Figure 1b). The LUMO and the highest occupied molecular orbital (HOMO) energy levels were estimated to be −3.70 and −5.57 eV, respectively, according to the reduction onset potential (Ered onset ) and oxidation onset potential (E oxonset ) (Table S1) with ferrocene (-4.80 eV) as a standard reference. The energy levels of FSP and PTB7-Th matched well with each other. Notably, the LUMO level of FSP was higher than that of SCPDT-PDI4 by 0.10 eV. Although the LUMO energy offset between the donor and the acceptor was relatively small, the electron transfer from PTB7-Th to FSP appeared to be highly efficient, which was supported by EQE and photoluminescence (PL) quenching measurement as shown later. Meanwhile, higher LUMO level will lead to a higher open-circuit voltage (Voc) in OSCs.36

Figure 1. (a) Normalized UV-visible absorption spectra of FSP and PTB7-Th in the film state. (b) Cyclic voltammograms of FSP with Fc/Fc+ as the reference. (c) Schematic device structure


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of PTB7-Th:FSP based OSCs. (d) Energy level diagrams of the different components in the inverted OSCs. Bulk heterojunction (BHJ) OSCs with an inverted device architecture of ITO/ZnO/PTB7Th:FSP /MoO3/Ag (Figure 1c) were fabricated to characterize the photovoltaic performance of FSP. The energy levels of the different layers of the devices are shown in Figure 1d. The current density-voltage (J-V) curves and corresponding EQE spectra for the optimized devices have been shown in Figure 2a, 2b. The photovoltaic parameters are presented in Table 1 and Figure S3. The OSC with a donor/acceptor (D/A) ratio of 1:1 shows an optimal performance. The resulting PTB7-Th/FSP-based OSC shows a high PCE of 5.28% with a Voc of 0.91 V, a short circuit current density (Jsc) of 12.1 mA cm-2, and a fill factor (FF) of 47.5%. Proven to be an effective way to boost the performance of OSCs, solvent additive of diphenyl ether (DPE) was introduced into the active materials solution. The photovoltaic parameters with different amount of DPE are summarized in Table 1. When 5% DPE (v/v) was added, the PCEs dramatically increased from 5.28% to 8.89% with a Voc of 0.90 V, a Jsc of 16.6 mA cm-2, and a FF of 59.5%. Due to the high boiling point of DPE (259 oC), the thin-film morphology can be optimized and the crystalline ordering in domains can be improved during the film drying.37,38 The current density deduced from integration EQE spectra matched well with Jsc obtained from the corresponding J-V curves. The broad and high spectral response in 350-800 nm region indicates that both PTB7-Th and FSP have made great contribution to photo-current generation. When 5% DPE was added, the EQE was enhanced by 20% to more than 30% in the region of 350 to 750 nm. The bulk charge transport properties of the optimized PTB7-Th:FSP blend films was investigated by space charge-limited current (SCLC) method. Without DPE, both hole and electron mobilities of the as-prepared films were determined to be 2.45 × 10-4 and 1.54 × 10-5


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cm2 V-1 s-1, respectively (Figure 3a, b). After the addition of 5% DPE, the hole and electron mobilities were increased to 2.52 × 10-4 and 7.23 × 10-5 cm2 V-1 s-1. The higher and more balanced mobilities of the PTB7-Th/FSP blend film with 5% DPE are responsible for the improved Jsc and FF in the devices. The correlation of illumination intensity-Jsc and PL quenching experiments were carried out to investigate the exciton recombination and dissociation (Figure 3c, 3d and Figure S4). When the slope of the illumination intensity-Jsc curves reaches 1, it implies a weak bimolecular recombination, and the free carriers can be swept out and collected by the electrodes efficiently.39 As shown in Figure 3c, for the blend films, the slopes without and with 5% DPE are 0.95 and 0.99, respectively. The slopes of the two devices indicate that the bimolecular recombination is very weak and can be further suppressed with DPE additive. The PL quenching efficiencies (∆PL, relative to FSP, Figure 3d) for PTB7Th/FSP blend films without and with 5% DPE are estimated to be 87% and 98% (both 99%, ∆PL, relative to PTB7-Th, Figure S4), respectively. The significant PL quenching indicates that efficient photo-induced charge transfer occurs because of the absence of excessively large aggregates in the blend films.26 Compared to PTB7-Th/FSP blend film without DPE additive, the higher PL quenching efficiency of the blend film with DPE additive suggests better exciton dissociation and more effective charge transfer in the blend film.


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Figure 2. (a) Characteristic J-V curves of the PTB7-Th:FSP-based solar cells without and with DPE (5%, v/v) as solvent additive under AM 1.5G irradiation (100 mW cm−2). (b) EQE curves of the corresponding devices.

Figure 3. Dark current density-voltage characteristics for (a) hole-only and (b)electron-only devices with optimized PTB7-Th/FSP BHJ films. (c) Jsc versus light density of the two devices. (d) Photoluminescence spectra of the neat FSP film and the PTB7-Th:FSP blend films without and with DPE (5%, v/v) as solvent additive, excitation at 520 nm. Table 1. Photovoltaic performances of PTB7-Th:FSP solar cells, prepared with different D:A ratio and DPE contents, under illumination of AM1.5G (100 mW cm−2).


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a)

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D:A (w/w)

DPE

VOC

JSC

FF

PCE

PTB7-Th:FSP

[v/v,%]

[V]

[mA cm−2]

[%]

[%]a)

1.5:1

None

0.89

10.9

40.0

3.92 (3.72)

1:1

None

0.91

12.1

47.5

5.28 (5.13)

1:1.5

None

0.92

10.1

45.4

4.23 (4.19)

1:1

3

0.90

16.8

53.8

8.13 (8.03)

1:1

5

0.90

16.6

59.5

8.89 (8.75)

1:1

7

0.89

14.4

55.3

7.03 (6.88)

The average data obtained from analysis of 10 devices are shown in brackets. The morphology of the BHJ films is known to closely correlate to mobility, Jsc and finally

PCE.40,41 Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were adopted to characterize the morphology of PTB7-Th/FSP active layers. The AFM images (Figure 4a,b,d,e) of the blend films reveal that DPE additive leads to a smoother and more homogeneous film. The root mean square (RMS) roughness of the blend films decreased from 2.73 nm to 0.91 nm when 5% DPE was added. As shown in Figure 4c and 4f, both the blend films without and with DPE show interpenetrating fiber structures. The formation of fiber structures, which is a common characteristic in high performance OSCs,42,43 suggests that FSP presents relatively fine miscibility with PTB7-Th. The fibrillar networks are beneficial to exciton dissociation and charge transport, thus achieving higher Jsc and FF. A more homogeneous morphology with a smaller domain size could also be observed in TEM images of the films with 5% DPE additive, which is consistent with AFM measurement. These features are favorable for charge separation and transport,27 which is also supported by bimolecular recombination


10

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experiment and PL quenching data. The more efficient charge separation and transport could explain the higher Js and FF well when DPE was added.

Figure 4. AFM (a, b) height and (d, e) phase images (scale bar: 1 µm) of PTB7-Th:FSP (1:1, w/w) blend film prepared (a, d) without and (b, e) with DPE (5%, v/v) as solvent additive. TEM images (scale bar: 200 nm) of PTB7-Th:FSP blend films (c) without and (f) DPE (5%, v/v) as solvent additive. To find out the reasons that accounts for the good photovoltaic performance of FSP, the optimized geometries were obtained by the DFT calculations at the B3LYP/6-31G(d) level. As shown in Figure 5a,b, FSP presents a rigid 3D structural conformation with two planar backbones connected through a spiro-sp3 carbon. The ring fusions caused CPDT and PDI units to be planar heteroarenes with extended conjugation. The planar and extended conjugation structure was expected to induce broad absorption and high electron mobility for FSP.32 Due to the spiro structure of SCPDT, the two heteroarenes embedded in each other with a dihedral angle of 90o


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between the two heteroarenes. The standard cruciform X-shape of FSP has effectively attenuated the aggregation in its solid state.44 In addition, the rigid 3D conformation of FSP has also led to good miscibility with the polymer donor PTB7-Th, which has been confirmed by AFM and TEM images. The LUMO and HOMO orbitals of SCPDT-PDI4 and FSP are presented in Figure 5c,d. For SCPDT-PDI4, the HOMO electron density only localizes at SCPDT core, while the LUMO orbital just distributes at a PDI unit. The frontier orbitals of FSP are delocalized on PDIs and CPDTs, especially for LUMO orbital. According to the calculations, FSP exhibits large effective π-conjugation, which is consistent with the strong absorption induced by the intramolecular charge transfer.


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Figure 5. (a) Front, and (b) top views of the optimized geometry of FSP. LUMO and HOMO orbitals of (c) SCPDT-PDI4 and (d) FSP (simulated with Gaussian B3LYP/6-31G (d)). 3. Conclusion In summary, a novel SM acceptor FSP, consisting of SCPDT core and PDIs, has been successfully synthesized and characterized. FSP maintains a long effective conjugation and a 3D structure concurrently through fusing PDI units with thiophene rings of CPDTs to increase the planarity between PDIs and CPDT core. The 3D rigid structure effectively prevents over aggregation of PDI chromophores and improves the miscibility with the polymer donor, contributing to a favorable interpenetrating fiberillar structure with an appropriate phase separation and exciton dissociation; meanwhile, the ring fusion raises the LUMO energy level and results in the good absorption of FSP. BHJ OSCs based on PTB7-Th and FSP achieved a high PCE of up to 8.89% with an impressive Voc of 0.90 V, a Jsc of 16.6 mA cm-2 and a FF of 59.5% by introducing a small amount of DPE as an additive. The results successfully demonstrate that building 3D structures with an extended effective π-conjugation could be a promising strategy to pursue high performance PDI-based acceptors. 4. EXPERIMENTAL SECTION Materials and Characterization: All chemicals and reagents were purchased from J&K, 1Materials, Sigma-Aldrich and TCI and used without further purification unless stated otherwise. 4Br-SCPDT45 and PDI-Br46 were synthesized according to the procedures reported previously. The detailed measurement procedures of 1H and

13

C NMR spectra, Mass spectrum (MALDI-

TOF MS), Differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA), Cyclic voltammograms measurements, UV-visible absorption spectra, Photoluminescence (PL) spectra,


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Atomic Force Microscope (AFM), J-V curves and EQE spectra can be found in our previous papers.28,29 Transmission Electron Microscopy (TEM) images were operated at 200 KeV (bright field mode, Tecnai Bio twin, FEI). Synthesis of 4Sn-SCPDT and SCPDT-PDI4: 4Sn-SCPDT and SCPDT-PDI4 were synthesized following the procedure in our previous work.29 4Sn-SCPDT (yield: 80%): 1H NMR (400 MHz, CDCl3, δ): 6.54 (s, 4H), 0.31 (s, 36H); SCPDT-PDI4 (yield, 60%); 1H NMR (300 MHz, CDCl3, δ, 50 °C): 8.81 (s, 4H), 8.69 (d, J = 7.9 Hz,4H), 8.62–8.55 (m, 12H), 8.50 (d, J = 8.1 Hz, 4H), 8.09 (d, J = 8.1 Hz, 4H), 7.25 (s, 4H), 5.26–4.77 (m, 8H), 2.35–1.54 (m, 32H), 1.44–0.93 (m, 128H), 0.89–0.66 (m, 48H);

13

C NMR (75 MHz, CDCl3, δ): 164.71, 163.81, 152.49, 147.02,

146.25, 144.94, 141.09, 136.73, 135.12, 134.98, 134.48, 133.56, 133.31, 133.09, 131.53, 129.60, 129.18, 128.52, 127.85, 127.74, 123.99, 123.91, 122.99, 121.80, 121.57, 121.40, 120.89, 55.07, 54.83, 32.64, 32.44, 32.23, 31.96, 29.43, 29.33, 27.11, 27.04, 22.77, 14.21; MS (MALDI-TOF, m/z): calcd. for C217H248N8O16S4, 3351.78; found, 3351.13; calcd. for [M+Na]+, 3374.78; found, 3374.13. Synthesis of FSP: FSP was prepared as reported previously.30 Briefly, under N2 atmosphere, SCPDT-PDI4 (335 mg, 0.1 mmol) was dissolved in toluene (20 mL) with 1mL FeCl3 (1 mL, 10 M in CH3NO2) solution. Then heated at 110 oC and kept stirring for 5 h. After cooling down, the reaction mixture was extracted by chloroform (50×2 mL), washed by brine, dried over anhydrous Na2SO4 and filtered. After evaporating the solvent, the residue was purified using column chromatography (eluent: n-hexane/dichloromethane = 1/2) to obtain a black red solid (217 mg, 65%) as the product. 1H NMR (500 MHz, CDCl3, δ): 9.76 (s, 4H), 9.53 (s, 4H), 9.01 (dd, J = 15.2, 7.8 Hz, 8H), 8.84 (dd, J = 16.4, 3.1 Hz, 8H), 5.74–5.24 (m, 8H), 2.95–1.84 (m, 32H), 1.75– 1.20 (m, 128H), 0.85–0.60 (m, 48H);

13

C NMR (126 MHz, CDCl3, δ): 207.04, 164.98, 163.97,


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163.16, 149.76, 145.34, 143.47, 139.30, 132.79, 131.72, 129.36, 127.02, 126.84, 126.54, 124.16, 123.88, 123.12, 122.67, 122.24, 114.07, 55.03, 32.48, 31.77, 30.96, 29.71, 29.26, 27.01, 14.06; MS (MALDI-TOF, m/z): calcd. for C217H240N8O16S4, 3343.71; found, 3343.13; calcd. for [M+Na]+, 3366.71; found, 3366.75. Devices fabrication and characterization: The PTB7-Th:FSP blends with different weight ratios was dissolved in chlorobenzene (CB) (PTB7-Th: 10 mg/mL), being stirred at 60 oC on a hot plate for 8 hours. After the additive DPE was added into the solution for at least 1 hour, the solution can be used for spin coating. The ITO-coated glass substrates were washed and pretreated.28,29 The precursor solution of ZnO was spin-coated onto the ITO glass and annealed (200 °C, 60 min). PTB7-Th:FSP blend solution was spin-coated on the ZnO layer to form the active layer (ca.150 nm) without any thermal or solvent annealing. The optimal condition for blend was 20 mg/ml CB solution with 5% DPE additive. MoOx (ca. 14 nm) and Ag top electrode (ca. 100 nm) was subsequently evaporated (vacuum: ca. 10−4 Pa), forming a 6 mm2 active area. The carrier mobilities of optimized films with PTB7-Th:FSP were determined using spacecharge-limited

current

(SCLC)

Glass/ITO/PEDOT:PSS/Active

measurement.

layer/MoO3/Ag;

Hole-only

diode

configuration:

Electron-only

diode

configuration:

Glass/ITO/ZnO/PTB7-Th:FSP/Ca/Al; The electric-field dependent SCLC mobility was estimated using Equation 147:

(1)


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TGA and DSC curves, UV-visible absorption spectra, PL spectra, J-V curves, optical and electrochemical data. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (Z-K.C.). *E-mail: [email protected] (W.H.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. X.S. is now a PhD candidate of King Abdullah University of Science and Technology. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the National Basic Research Program of China (Fundamental Studies of Perovskite Solar Cells 2015CB932200), National Natural Science Foundation of China (Nos. 51373076, 61605075, 91433118), and SICAM Scholarship by Jiangsu National Synergetic


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Innovation Center for Advanced Materials for financial support. Q.Z. acknowledges financial support from AcRF Tier 1 (RG8/16, RG133/14 and RG 13/15), Singapore. REFERENCES (1)

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PDI Derivative through Fine-Tuning the Molecular Structure for

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