Fused-ring acceptor with a spiro-bridged ladder-type core for organic solar cells

Accepted Manuscript Fused-ring acceptor with a spiro-bridged ladder-type core for organic solar cells Shouli Ming, Yahui Liu, Shiyu Feng, Pengcheng Jiang, Cai'e Zhang, Miao Li, Jinsheng Song, Zhishan Bo PII:

S0143-7208(18)32145-4

DOI:

https://doi.org/10.1016/j.dyepig.2018.11.040

Reference:

DYPI 7184

To appear in:

Dyes and Pigments

Received Date: 27 September 2018 Revised Date:

15 November 2018

Accepted Date: 20 November 2018

Please cite this article as: Ming S, Liu Y, Feng S, Jiang P, Zhang Cai', Li M, Song J, Bo Z, Fused-ring acceptor with a spiro-bridged ladder-type core for organic solar cells, Dyes and Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.11.040. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Fused-Ring

Acceptor

with

a

Spiro-Bridged

Ladder-Type Core for Organic Solar Cells

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Shouli Ming,a Yahui Liu,a Shiyu Feng,a Pengcheng Jiang,a Cai’e Zhang,a Miao Li,a Jinsheng Song *, b and Zhishan Bo*, a a

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of

Engineering Research Center for Nanomaterials, Henan University, Kaifeng, 475004,

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b

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Chemistry, Beijing Normal University, Beijing 100875, China.

China.

*Corresponding Author, E-mail: [email protected]; [email protected]

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Abstract

Side-chain engineering could tune the crystallinity, solubility and aggregation styles of fused-ring electron acceptors (FREAs), and thus could influence the morphology

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and charge carrier mobility of blend films as well as the final device performances.

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Herein, we developed a novel FREA called spiro-IDT-O-IC, which is composed of a spiro-bridged IDT core and two 1,1-dicyanomethylene-3-indanone (IC) terminal units. Similar acceptor molecule IDT-O-IC with four 4-octyloxyphenyl side chains is also prepared for the control experiment. The absorptions, energy levels, molecular configurations, and electronic wave functions of these two acceptor molecules have been

comparatively studied.

Polymer

solar

cells

(PSCs)

fabricated

with

PBDB-T:spiro-IDT-O-IC as the active layer demonstrated a PCE of 6.23% with a Jsc of 11.76 mA/cm2, a Voc of 0.84 V and an FF of 0.63. To the best of our knowledge,

ACCEPTED MANUSCRIPT this is the first report that a spiro-bridged ladder type core was used for the fabrication of fused ring acceptors.

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1. Introduction

As a promising cost-effective alternative to inorganic solar cells for the utilization of solar energy, polymer solar cells (PSCs) have received

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tremendous attentions due to their inherent advantages including low-cost,

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lightweight, mechanic flexibility and solution processability.[1-6] Very recently, power conversion efficiencies (PCEs) of about 14% have been achieved for PSCs based on fused-ring electron acceptors (FREAs), which are usually of an acceptor-donor-acceptor

(A-D-A)

structure.[7,8]

The

A unit

is

usually

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1,1-dicyanomethylene-3-indanone (IC) or its derivatives, D unit is a planar ladder-type fused-ring aromatic structure bearing lateral side chains.[9-12] Nowadays, intensive interests have been devoted to the molecular design of

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FREAs because of their energy levels, light absorptions and blend film

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morphologies can be finely tuned by flexible molecular design and synthesis. Indaceno[2,1-b:6,5-b’]dithiophene

(IDT)

and

indacenodithieno[3,2-b]thiophene (IDTT) are two representative fused ring building blocks for the D core and they are widely utilized for the construction of high efficiency FREAs.[13] Such fused ring building blocks possess planar π-conjugated skeleton and decreased recombination energy, which could endow the final materials with large π-electron delocalization, good charge transport

ACCEPTED MANUSCRIPT and broad absorption etc. Various chemical modifications have been applied to these ladder type conjugated cores to extending the conjugation length,[14,15] changing the substituted side chains,[16-19] altering the bridging units etc.[20-26]

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After several year development, it has been gradually recognized that the photovoltaic performance is mainly determined by the light-harvesting ability of active layer materials, the energy level and charge mobility compatibility

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between donor and acceptor, and especially the micromorphology of the active

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layer. Besides the conjugated backbone, the flexible side chain also plays an important role. The micromorphology of photoactive layer can be directly tuned by varying the flexible side chains. As reported by Zhan et al., the replacement of widely utilized 4-hexylphenyl group with 5-hexylthienyl,[19] ITIC-Th showed

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downshifted energy levels, enhanced the electron mobility and photovoltaic performances due to the strengthened sulfur-sulfur intermolecular interaction. When the thiophene side chain is replaced by selenophene one, better optical

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absorption could be achieved.[27] As reported by Heeney et al., C8-ITIC with

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octyl side chains instead of 4-hexylphenyl ones displayed an enhanced crystallization propensity and a PCE of 13% was obtained.[28] Very recently, we have demonstrated that the solubility of acceptors, which is closely related to the

film

morphology,

could

be

improved

by

modification

with

4-(alkylthio)phenyl side chains.[29] In short, side chain engineering plays an important role in regulation of energy levels, charge mobilities, molecular interactions and morphologies of blends, which are crucial for the performance

ACCEPTED MANUSCRIPT of devices. 9,9’-Spirobifluorene, which is of a three-dimensional (3D) cruciform configuration with the two planar and rigid fluorene units connected by a sp3

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carbon, has been utilized to construct spiro-bridged ladder-type oligomers and polymers.[30-31] Herein, we developed a novel acceptor with spiro-bridged IDT as the central core and two IC terminal groups (spiro-IDT-O-IC) as shown in

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Chart 1. The central spiro-bridged IDT bears four alkoxy chains, which can

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guarantee the good solubility of the final molecule. For comparison, IDT-O-IC with four freely rotated 4-alkoxylphenyl groups was also synthesized. These two molecules display similar absorptions in the long wavelength region and possess almost identical energy levels. Quantum chemistry calculations

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demonstrated that the lateral fluorene segments do contribute to the electronic wave-function of HOMO. When PBDB-T is utilized as the donor material, spiro-IDT-O-IC based devices exhibit a concurrent improvement in Jsc and FF,

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resulting in a PCE of 6.23%, which is higher than that of the IDT-O-IC (PCE =

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4.74%) and IDT-IC (PCE = 4.69%) based control devices.[32]

Chart 1. Chemical structures of spiro-IDT-O-IC, IDT-O-IC and IDT-IC

ACCEPTED MANUSCRIPT 2. Results and discussion

2.1 Synthesis The synthetic routes of spiro-IDT-O-IC and IDT-O-IC are shown in Scheme

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1. Starting from compound 1,[30] its Suzuki coupling with thiophene-2-boronic acid pinacol afforded the intermediate 2, which was used for the next step

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without purification. The treatment of crude 2 in CH2Cl2 with a small amount of HCl solution (two drops) furnished the ring closure product spiro-IDT

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(Compound 3) in a yield of 77%. The removal of the two α-protons at thiophene unit with n-BuLi afforded the dianion intermediate, which was quenched by DMF to obtain the corresponding aldehyde (Compound 4) in a yield

of

37%.

Finally,

Knoevenagel

(IC),

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1,1-dicyanomethylene-3-indanone

via

the

target

condensation acceptor

with

molecule

spiro-IDT-O-IC was obtained in a yield of 63%. Starting from compound 5, the

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control molecule IDT-O-IC was prepared by following a literature procedure.[15] The synthetic details and characterizations are provided in the supporting

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information (SI).

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Scheme 1. Synthetic routes for spiro-IDT-O-IC and IDT-O-IC; i) Pd2(dba)3,

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P(o-tol)3, NaHCO3, THF/H2O, reflux; ii) HCl, CH2Cl2, 77%; iii) n-BuLi, DMF, THF, 37%; iv) Pyridine, CHCl3, reflux, 63%; v) n-BuLi, THF; vi) BF3·Et2O, CH2Cl2, 42%; vii) n-BuLi, DMF, THF, 69%; viii) Pyridine, CHCl3, reflux, 73%.

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2.2 Considerations for Chemical Structures by Quantum Calculations Firstly, density functional theory (DFT) quantum calculation is carried out at the

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B3LYP/6-31G* level on the Gaussian 09 package to evaluate the optimal geometric configurations for the two FREA molecules as shown in Figure 1, in which the long

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alkyl chains are replaced by methyl groups to simplify the calculation. As depicted in Figure 1, the skeletons of these two molecules are both of planar conformations regardless of their side chain types, but varied configurations of side chains are observed. In IDT-O-IC, the two alkoxyphenyl substituents could freely rotate around the C-C single bond; whereas the aromatic skeleton of spiro-IDT-O-IC is of shape-persistent, the lateral fluorene substituent is perpendicular to the backbone of acceptor molecule due to the spiro-type bridge. Such conformation could effectively

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stacking by terminal IC groups.

Figure 1. Optimized molecular geometries of simplified spiro-IDT-O-IC and

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IDT-O-IC obtained by DFT calculations.

2.3 Photophysical and Electrochemical Properties

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spiro-IDT-O-IC and IDT-O-IC displayed good solubility in common organic

solvents such as chloroform and o-dichlorobenzene. The UV-vis absorption spectra of spiro-IDT-O-IC and IDT-O-IC in chloroform solutions and as thin films are shown in Figure 2. In solutions, similar absorption behaviours were observed for these two molecules with one main absorption band located in the region of 500-700 nm and the absorption maximum located at 658 nm for IDT-O-IC and 666 nm for spiro-IDT-O-IC with extinction coefficients of 1.74 × 105 M-1 cm-1 and 1.70 × 105

ACCEPTED MANUSCRIPT M-1, respectively (Figure S1). With the incorporation of perpendicular side chains, a slight redshift of about 8 nm is observed. In going from solution to thin film, broader and red-shifted absorptions are detected for both acceptors, indicating the formation

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of J-aggregates in films. Based on the absorption onsets, the optical bandgaps (Eg,opt) were determined to be 1.66 eV and 1.69 eV for spiro-IDT-O-IC and IDT-O-IC, respectively. It is worth noting that the red-shift for spiro-IDT-O-IC is smaller than

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that of IDT-O-IC, indicating the perpendicular fluorene side chains are more

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effective than the two alkoxyphenyl chains in suppressing the aggregation of acceptor molecules, which is also confirmed by XRD measurements (vide infra). Different from IDT-O-IC, a relative strong absorption at short wavelength (300-350 nm) was detected for spiro-IDT-O-IC, which is probably due to the absorption of lateral

1.2

spiro-IDT-O-IC IDT-O-IC

(a)

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0.9 0.6 0.3

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Normalized Absorbance

1.5

0.0

300

400

500

1.5

Normalized Absorbance

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fluorene unit.

1.2

spiro-IDT-O-IC IDT-O-IC

(b)

0.9 0.6 0.3 0.0

600

700

Wavelength/nm

800

300

400

500

600

700

800

Wavelength/nm

Figure 2. Absorption spectra of spiro-IDT-O-IC and IDT-O-IC in (a) chloroform solutions and (b) thin films.

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Figure 3. (a) CV curves of spiro-IDT-O-IC and IDT-O-IC; (b) energy level diagram. Table 1. Optical and electrochemical data of spiro-IDT-O-IC and IDT-O-IC.

spiro-IDT-O-IC IDT-O-IC IDT-IC[32]

λmax/nm (solution) 614, 666 607, 658 --, 656

ε/(× 105 M-1 cm-1) 1.70 1.74 1.51

λmax/nm (film) 635, 693 634, 695 --, 683

Eg,opt/ eV 1.66 1.69 1.72

HOMO/ eV -5.68 -5.70 -5.78

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Compound

LUMO/ eV -3.79 -3.79 -3.84

Eg,cv/ eV 1.89 1.91 1.94

To explore the highest occupied molecular orbital (HOMO) and the lowest

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unoccupied molecular orbital (LUMO) energy levels of these two FREAs, electrochemical experiments were employed and the corresponding cyclic voltammograms are shown in Figure 3a. The HOMO energy levels of

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spiro-IDT-O-IC and IDT-O-IC were calculated to be -5.68 eV and -5.70 eV,

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respectively according to the equation: EHOMO = - e[(Eox - E (Fc/Fc+) + 4.8] (eV), where Eox is the onset oxidation potential. Similarly, the LUMO energy levels of -3.79 eV were determined for both spiro-IDT-O-IC and IDT-O-IC, following the equation: EHOMO = - e[(Ered - E (Fc/Fc+) + 4.8] (eV), where Ered is the onset reduction potential. For clarity, the energy level diagram is drawn in Figure 3b. It’s obvious that spiro-IDT-O-IC presents similar HOMO/LUMO levels to the control IDT-O-IC, indicating that the perpendicular fluorene side chains have little impact

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the reduced band gaps would strengthen the light harvesting ability. In addition, visualized HOMO and LUMO orbitals of these two molecules are depicted by the DFT calculations to assist understanding the electronic wave-functions as shown in

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Figure 4. Both molecules present very similar electron distributions at LUMO states

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and efficient electron flow could be observed from the electron donating core to the electron deficient moieties. However, clear differences were observed at the HOMO states i.e. the freely rotating benzene rings in IDT-O-IC don’t have electron distribution, but the perpendicular fluorene units do contribute to the HOMO of

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spiro-IDT-O-IC.

Figure 4. Frontier molecular orbitals of the optimized spiro-IDT-O-IC and IDT-O-IC calculated with the DFT B3LYP/6-31G* level.

ACCEPTED MANUSCRIPT To evaluate the crystalline properties of these acceptor molecules, thin films on Si substrates were investigated by X-ray Diffraction (XRD) and the results are depicted in Figure 5. The pure IDT-O-IC film exhibited two obvious

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diffraction peaks located at 4.25o and 25.38o, corresponding to a lamellar distance of 20.77 Å and a π−π stacking distance of 3.51 Å, respectively. The XRD result of IDT-O-IC is in good agreement with that of IDT-IC in our

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previous study,[32] demonstrating that 4-alkoxyphenyl side chain has little

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influence on the molecular packing of acceptors. Different from IDT-O-IC, no sharp diffraction peak is observed for spiro-IDT-O-IC at either small or large angle region, indicating that the perpendicular side chains in spiro-IDT-O-IC could effectively restrict the molecular aggregation, which is also consistent

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with the DFT quantum prediction and UV-vis absorption results.

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Intensity / a.u.

spiro-IDT-O-IC IDT-O-IC

0

10

20

2θ (O)

30

40

Figure 5. XRD curves of spiro-IDT-O-IC and IDT-O-IC.

2.4 Photovoltaic Performance The photovoltaic performances of spiro-IDT-O-IC and IDT-O-IC as acceptors were evaluated with an inverted device structure of ITO/ZnO/active

ACCEPTED MANUSCRIPT layer/MoO3/Ag. The widely used polymer PBDB-T was selected as the donor for the active layer preparation and the device fabrication details are described in the Supporting Information. The J-V curves and the external quantum

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efficiency (EQE) spectra of optimized devices are shown in Figure 6 and the detailed optimization results are listed in Table S1 and Table S2 (Supporting Information). The optimal PBDB-T:acceptor (w/w) ratios for spiro-IDT-O-IC

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and IDT-O-IC based devices are 1:1.8 and 1:1.2, respectively. The as-cast

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PBDB-T:IDT-O-IC active layer exhibited a moderate PCE of 4.74% with a Voc of 0.83 V, a Jsc of 10.32 mA cm-2, and an FF of 0.55. The result is consistent with IDT-IC based PSCs (PCE of 4.69%) [32]. When spiro-IDT-O-IC was used as the acceptor, an improved PCE of 6.23% was obtained due to the elevated Jsc

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and FF as shown in Table 2. Meanwhile, the EQE curves in Figure 6b showed a broad response from 300 to 780 nm, indicating efficient photo harvesting and charge collection, and the absorption in the short wavelength region may

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account for the improved Jsc for spiro-IDT-O-IC. In addition, the calculated

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Jcalc from the integration of EQE spectra for both spiro-IDT-O-IC and IDT-O-IC are well consistent with the observed Jsc values in J–V measurements and the deviations are within 5%.

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Figure 6. (a) J–V and (b) EQE curves of PBDB-T:spiro-IDT-O-IC and

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PBDB-T:IDT-O-IC based devices.

Table 2. Photovoltaic parameters of optimized devices. Active layer PBDB-T: spiro-IDT-O-IC PBDB-T: IDT-O-IC

Voc (V)

Jsc ( mA/cm−2)

Jcalc ( mA/cm−2)a

FF

PCE (%)

0.84

11.76

11.75

0.63

6.23 (5.98)b

0.83

10.32

9.90

0.55

4.74 (4.60)b

Jcalc values are calculated from the EQE spectra. bAverage PCE of ten independent devices.

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2.5 SCLC Mobilities

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a

Charge transport property plays a vital role in the photovoltaic performances of

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PSCs. High and balanced hole (µh)/electron (µe) mobilities are in favor of reducing charge recombination and increasing the Jsc and FF. Herein, the hole and electron transport properties of blend films were investigated in typical device structures of ITO/PEDOT:PSS/active layer/Au and ITO/ZnO/active layer/Al, respectively, by the space charge limited current (SCLC) model, and the results are shown in Figure 7. The µe/µh ratio of PBTB-T:spiro-IDT-O-IC blend is measured to be 1.43 (5.24×10-5 cm2 V-1 s-1/3.66×10-5 cm2 V-1 s-1); whereas a larger µe/µh ratio of 1.59 (3.63×10-5 cm2

ACCEPTED MANUSCRIPT V-1 s-1/2.28×10-5 cm2 V-1 s-1) was calculated for PBTB-T:IDT-O-IC based devices. The above results could well explain the relative high Jsc and FF for

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spiro-IDT-O-IC based devices.

Figure 7. J1/2-V curves of PBDB-T:acceptor blend films: (a) hole-only devices and (b) electron-only devices.

2.6 Morphologies

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It is well known that the performance of PSC devices is closely related to the morphology of blend films. Therefore, atomic force microscope (AFM) and

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transmission electron microscope (TEM) were used to investigate the top and in-depth morphology of blend films. As shown in Figure 8a, a uniform film surface was

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observed for the PBDB-T:spiro-IDT-O-IC blend with a root mean square (RMS) roughness value of 2.62 nm. As for the PBDB-T:IDT-O-IC blend film, slightly larger domains could be detected with a RMS of 3.23 nm. Such phenomena are well consistent with the XRD results (vide supra), and the uniform morphology for spiro-IDT-O-IC based blend film might be attributed to the bulky perpendicular side chains, which could reduce the aggregation of acceptor molecules. From TEM images, fibrillar structures could be clearly observed in both blend systems, but the fibers are

ACCEPTED MANUSCRIPT thicker in the PBDB-T:IDT-O-IC blend films. As for the PBDB-T:spiro-IDT-O-IC blend films, ordered and thinner nanofibers were presented, which is beneficial to achieving efficient exciton dissociation and charge transport.[33] In all, the appropriate

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morphology might contribute to the high Jsc and FF of PBDB-T:spiro-IDT-O-IC

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based devices.

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Figure 8. AFM height images of (a) PBDB-T:spiro-IDT-O-IC and (b) PBDB-T:IDT-O-IC blend films; and TEM images of (c) PBDB-T:spiro-IDT-O-IC and (d) PBDB-T:IDT-O-IC blend films.

3. Conclusions

In conclusion, we designed and synthesized two fused-ring acceptors (spiro-IDT-O-IC and IDT-O-IC) based on IDT core. Their absorptions, energy levels,

ACCEPTED MANUSCRIPT packing styles and photovoltaic performances were studied. These two acceptors are highly soluble in commonly used organic solvents and their LUMO energy levels are up-shifted relative to IDT-IC. The DFT quantum calculation for spiro-IDT-O-IC

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indicates that the two fluorene segments are perpendicular to the IDT backbone and also have contribution to the electronic wave-function of HOMO. As expected, the perpendicular side chains of spiro-IDT-O-IC could regulate the aggregation of

of

blend

films.

With

a

suitable

phase

separation,

the

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characterization

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acceptor molecules, which has been confirmed by XRD study and the morphology

PBDB-T:spiro-IDT-O-IC blend films display a more balanced µh/µe ratio in SCLC study, enhanced Jsc of 11.76 mA/cm2 and FF of 0.63 in photovoltaic characterization. Finally, a PCE of 6.23% is achieved for spiro-IDT-O-IC based devices, which is of

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about 30% enhancement in relative to the control IDT-O-IC. To the best of our knowledge, this is the first report that a spiro-bridged ladder type core was used for

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the fabrication of fused ring acceptors.

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Acknowledgement

This research was supported by NSFC (U1704137, 21574013 and 21404031),

Program for Changjiang Scholars and Innovative Research Team in University, Program for Young Scholar sponsored by Henan Province (2016GGJS-021), Program from Henan University (yqpy20140058).

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[26] Zhou Y, Li M, Song J, Liu Y, Zhang J, Yang L, et al. High efficiency small molecular acceptors based on novel O-functionalized ladder-type dipyran building block. Nano Energy 2018;45:10-20.

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small molecular acceptors for high-performance fullerene-free polymer solar cells: thiophene being superior to selenophene. Adv Funct Mater 2017;27(34):1702194. [28] Fei Z, Eisner FD, Jiao X, Azzouzi M, Röhr JA, Han Y, et al. An alkylated indacenodithieno[3,2-b]thiophene-based

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crystallinity exhibiting single junction solar cell efficiencies greater than 13% with low voltage losses. Adv Mater 2018;30(8):1705209. [29] Zhang C, Feng S, Liu Y, Hou R, Zhang Z, Xu X, et al. Effect of non-fullerene

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acceptors' side chains on the morphology and photovoltaic performance of organic

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solar cells. ACS Appl Mater Inter 2017;9(39):33906-12. [30] Wu Y, Zhang J, Fei Z, Bo Z. Spiro-bridged ladder-type poly(p-phenylene)s: towards structurally perfect light-emitting materials. J

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[33] Liu T, Huo L, Chandrabose S, Chen K, Han G, Qi F, et al. Optimized fibril to

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ACCEPTED MANUSCRIPT Captions

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Scheme 1. Synthetic routes for spiro-IDT-O-IC and IDT-O-IC; i) Pd2(dba)3, P(o-tol)3, NaHCO3, THF/H2O, reflux; ii) HCl, CH2Cl2, 77%; iii) n-BuLi, DMF, THF, 37%; iv) Pyridine, CHCl3, reflux, 63%; v) n-BuLi, THF; vi) BF3·Et2O, CH2Cl2, 42%; vii) n-BuLi, DMF, THF, 69%; viii) Pyridine, CHCl3, reflux, 73%. Chart 1. Chemical structures of spiro-IDT-O-IC, IDT-O-IC and IDT-IC

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IDT-O-IC obtained by DFT calculations.

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Figure 1. Optimized molecular geometries of simplified spiro-IDT-O-IC and

Figure 2. Absorption spectra of spiro-IDT-O-IC and IDT-O-IC in (a) chloroform solutions and (b) thin films.

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Figure 3. (a) CV curves of spiro-IDT-O-IC and IDT-O-IC; (b) energy level diagram.

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Table 1. Optical and electrochemical data of spiro-IDT-O-IC and IDT-O-IC.

Figure 4. Frontier molecular orbitals of the optimized spiro-IDT-O-IC and

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IDT-O-IC calculated with the DFT B3LYP/6-31G* level.

Figure 5. XRD curves of spiro-IDT-O-IC and IDT-O-IC.

Figure 6. (a) J–V and (b) EQE curves of PBDB-T:spiro-IDT-O-IC and PBDB-T:IDT-O-IC based devices. Table 2. Photovoltaic parameters of optimized devices.

ACCEPTED MANUSCRIPT Figure 7. J1/2-V curves of PBDB-T:acceptor blend films: (a) hole-only devices and (b) electron-only devices.

Figure 8. AFM height images of (a) PBDB-T:spiro-IDT-O-IC and (b)

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and (d) PBDB-T:IDT-O-IC blend films.

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PBDB-T:IDT-O-IC blend films; and TEM images of (c) PBDB-T:spiro-IDT-O-IC

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Lateral fluorene introduced into the molecular skeleton in a perpendicular way Bulky side chains of spiro-IDT-O-IC to regulate its aggregation in blend films First report of a spiro-bridged ladder core used for A-D-A fused ring acceptors