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High Performance Small-Molecule Cathode Interlayer Materials with D-A-D Conjugated Central Skeletons and Side Flexible Alcohol/Water Soluble Groups for Polymer Solar Cells Jianxiong Han, Youchun Chen, Weiping Chen, Chengzhuo Yu, Xiaoxian Song, Fenghong Li, and Yue Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10900 • Publication Date (Web): 11 Nov 2016 Downloaded from http://pubs.acs.org on November 13, 2016

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ACS Applied Materials & Interfaces

High Performance Small-Molecule Cathode Interlayer Materials with D-A-D Conjugated Central Skeletons and Side Flexible Alcohol/Water Soluble Groups for Polymer Solar Cells Jianxiong Han, Youchun Chen, Weiping Chen, Chengzhuo Yu, Xiaoxian Song, Fenghong Li* and Yue Wang* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China

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Abstract

A new class of organic cathode interfacial layer (CIL) materials based on isoindigo derivatives (IID) incorporated with pyridinium or sulfonate zwitterion groups were designed, synthesized and applied in polymer solar cells (PSCs) with PTB7:PC71BM (PTB7: polythieno[3,4-b]thiophene-co-benzodithiophene and PC71BM: [6,6]-phenyl C71-butyric acidmethyl ester) as active layer. Compared with the control device, the PSCs with IID based CIL show simultaneous enhancement of open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF). Systematic optimizations of central conjugated core and side flexible alcohol soluble groups demonstrated that isoindigo based CIL material with thiophene and sulfonate zwitterions substituted groups can efficiently enhance the PSCs performance. The highest power conversion efficiency (PCE) of 9.12%, which is 1.75 times that of the control device without CIL, was achieved for the PSC with isoindigo based CIL. For the PSCs with isoindigo based CIL the molecule dependent performance property studies revealed that the central conjugated core with D-A-D characteristic and the side chains with sulfonate zwitterions groups is an efficient strategy for constructing high performance CIL. Our study results may open a new avenue towards the high performance PSCs.

Key words: Cathode interlayer, polymer solar cell, donor-acceptor organic molecules, isoindigo derivatives

1. INTRODUCTION


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Bulk-heterojunction organic/polymer solar cells (PSCs) have drawn enormous attention owing to the unique advantages of synthetic variability, light weight, low cost, large-area roll-to-roll fabrication and the lucrative possibility of integration into flexible devices.1-6 Recently, the power conversion efficiencies (PCE) record have been repeatedly refreshed for single-junction PSCs and a milestone PEC value of 11% has been accomplished.7-12 A great number of efforts have been devoted to improve PCE and minimize energy loss by rational donor/acceptor materials design,13-16 active layer morphology optimization17,18 and device engineering.19-21 Beyond active layer, the cathode interfacial layer (CIL) located between the cathode and active layer is also an important factor in the realization of high-efficiency PSCs by improving the charge extraction, minimizing series resistance (Rs) and so on.22-25 Compared with conventional inorganic CIL such as Ca and LiF,26 organic water/alcohol soluble CIL are not only much more stable in the presence of oxygen and moisture, but also have good solution-processing property and better interfacial contact with organic active layer.27-30 Compared with polymer based CIL, the small-molecular CIL have some intrinsic advantages in terms of well-defined structures, high-purity without batch-to-batch variation and easy modification. Therefore, a large amount of alcohol/water soluble small molecule interlayer materials with or without π-conjugated skeleton have been developed. So far, many efficient organic CIL materials with very extensive molecular structures such as fullerene bearing tertiary amine or sulfonate,31-33 perylene diimides with amino or amino N-oxide,34 quinacridone tethered with sodium sulfonate,35,36 triphenylamine-uorene core featuring a phosphonate side chain,37,38 tetra-n-alkyl ammonium bromides,39-41 metallophthalocyanine (MPc) and porphyrin carry with pyridinium salts,42-45 oligo-zwitterions10,46 and so on47-54 have been employed to fabricate highperformance PSCs. However, the relationship between molecular structure and CIL performance


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still remains unresolved issue. Therefore, it is imperative to design and synthesize a series organic molecules, which have a similar central backbone and regularly various substituted functional groups, and characterize their performance as CILs. These researches will provide important information on the relationship between the molecular structure and CIL performance. In this context, we focused on the organic molecules with isoindigo (IID) as central building block to construct CIL materials with different substituted groups. IID with electron-deficient character is an ideal building block to construct donor-acceptor (DA) molecules and n-type semiconductors.55,56 These results indicate that IID may be a good central moiety to develop CIL materials. On the other hand, IID backbone is easily modified with different substituted groups, which can efficiently regulate energy levels, dipole characteristic, mobility and film formation property of IID derivatives. Herein, we report six isoindigo based CIL materials (IID-PyBr, IID-NSB, IIDPh-PyBr, IIDPh-NSB, IIDTh-PyBr, and IIDTh-NSB) (Figure 1), which show orthogonal solubility to those of the photoactive layer, good film formation and electron transport properties. For CIL materials, the effects of water/alcohol soluble groups and conjugated structures on the PSCs performance will be presented.

Figure 1. Molecular structures of IID-PyBr, IID-NSB, IIDPh-PyBr, IIDPh-NSB, IIDTh-PyBr and IIDTh-NSB.


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2. EXPERMENTAL SECTION Materials and Characterization. 1H NMR spectra were measured from a Varian Mercury 300 MHz spectrometer (USA) with tetramethylsilane (TMS) as the internal standard. Elemental analyses were performed on a flash EA 1112 spectrometer (Germany). MALDI-TOF mass spectra were recorded on the Kratos AXIMA-CFR Kompact MALDI mass spectrometer with anthracene-1,8,9-triol as the matrix. A Shimadzu UV-2550 spectrophotometer was used to record UV-visible absorption spectra. A scan rate of 100 mV s-1 was performed on a BAS 100W instrument to tackle cyclic voltammetry. Dichloromethane (CH2Cl2) and pyridine were distilled over CaH2 and KOH, respectively. The other reagent and solvents were purchased from Aldrich (USA) and Acros (Belgium), and were used without any purifying. PTB7 was purchased from 1Material (Canada). PC71BM was purchased from American Dye Source (USA). All reactions were carried out using Schlenk techniques under a nitrogen atmosphere. IID-PyBr, IID-NSB, IIDPh-PyBr, IIDPh-NSB, IIDTh-PyBr, and IIDTh-NSB were synthesized according the procedures in Scheme 1, where 1,1'-bis(6-bromohexyl)-[3,3'-biindolinylidene]-2,2'-dione (IIDC6H12Br) and 6,6'-dibromo-1,1'-bis(6-bromohexyl)-[3,3'-biindolinylidene]-2,2'-dione (IIDBrC6H12Br) were prepared by the reported method.57,58 Details of synthesis and characterization are presented as follows. Synthesis. IID-PyBr ([(E)-1,1'-((2,2'-dioxo-[3,3'-biindolinylidene]-1,1'-diyl)bis(hexane-6,1diyl))bis(pyridin-1-ium) bromide]). IID-C6H12Br (1.47 g, 2.50 mmol) was dissolved in 100 mL pyridine and the mixture was refluxed overnight. After cooling down to room temperature, the solid was filtered out and washed by 100 mL CH2Cl2. The product was obtained as a dark red solid (1.52 g, 82%). 1H NMR (300MHz, CD3OD) δ 9.13(d, J = 8.4 Hz, 2H), 9.02 (d, J = 6.6 Hz, 4H), 8.60 (t, J = 7.8 Hz, 2H), 8.12 (t, J = 7.2 Hz, 4H), 7.43 (t, J = 7.5 Hz, 2H), 7.06 (t, J = 7.8


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Hz, 2H), 7.01 (d, J = 7.5 Hz, 2H), 4.66 (t, J = 7.5 Hz, 4H), 3.85 (t, J = 6.9 Hz, 4H), 2.10-2.01 (m, 4H), 1.83-1.73 (m, 4H), 1.54-1.45 (m,8H). MALDI-TOF (m/z): [M+H]+ calculated for C38H44Br2N4O2: 747.2, found: 747.8. Anal. Calcd for C38H44Br2N4O2: C 60.97, H 5.92, N 7.48, found: C 60.53, H 5.99, N 7.81. IID-C6H12N To a 250 mL two-neck flask, IID-C6H12Br (1.47 g, 2.50 mmol) and dimethylamine hydrochloride (415 mg, 5.00 mmol) were dissolved in 100 ml dry N,N-Dimethylformamide (DMF). After vigorously stirring for 10 minutes, K2CO3 (2.76 g, 20 mmol) was rapidly added under the nitrogen atmosphere and the mixture was heated at 100℃ for 8 hours. After cooling down to room temperature, solvent was removed by vacuum evaporation and crude product was purified by column chromatography using silica gel with CH2Cl2, methanol and triethylamine as eluent to yield the dark red solid (730 mg, 53%). 1H NMR (300MHz, CDCl3) δ 9.16 (d, J = 8.4 Hz, 2H), 7.34 (t, J = 7.5 Hz, 2H), 7.03 (t, J = 7.8 Hz, 2H), 6.78 (d, J = 7.8 Hz, 2H), 3.77 (t, J = 7.2 Hz, 4H), 2.27 (s, 12H), 1.77-1.66 (m, 4H), 1.52-1.36 (m, 12H), 1.27 (t, J = 7.5 Hz, 4H). MALDI-TOF (m/z): [M+H]+ calculated for C32H46N4O2: 518.3, found: 519.1. Anal. Calcd for C32H46N4O2: C 74.09, H 8.94, N 10.80, found: C 73.52, H 9.15, N 10.68. IID-NSB

([(E)-3,3'-(((2,2'-dioxo-[3,3'-biindolinylidene]-1,1'-diyl)bis(hexane-6,1-

diyl))bis(dimethylammoniumdiyl))bis(propane-1-sulfonate)]). To a 100 mL two-neck flask, IIDC6H12N (771 mg, 1.5 mmol) was dissolved in 50 mL anhydrous THF at nitrogen atmosphere. After stirring for 10 minutes, 1,3-propanesultone (1.82 g, 15.0 mmol) was rapidly added. The mixture was heated to reflux while stirring for 24 hours, then cooled to room temperature. During the course of reaction, the product was precipitated, then isolated by filtration. The crude product was purified by washing with THF and CH2Cl2 to yield a light red solid (970 mg, 83%). 1

H NMR (300MHz, DMSO- d6) δ 9.12 (d, J = 7.8 Hz, 2H), 7.45 (t, J = 7.5 Hz, 2H), 7.03 (t, J =


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6.9 Hz, 2H), 7.04 (d, J = 8.1 Hz, 2H), 4.36 (t, J = 5.1 Hz, 4H), 3.81 (t, J = 6.6 Hz, 4H), 3.25-3.20 (m, 4H), 2.98 (s, 12H), 2.45 (t, J = 6.9 Hz, 4H), 2.00-1.91 (m, 4H), 1.77-1.63 (m, 8H), 1.39-1.30 (m, 4H), 1.06 (t, J = 6.9 Hz, 4H). MALDI-TOF (m/z): [M+H]+ calculated for C38H58N4O8S2: 763.0, found: 763.4. Anal. Calcd for C38H58N4O8S2: C 59.82, H 7.66, N 7.34, S: 8.40, found: C 59.75, H 7.92, N 7.01, S 8.46. IIDPh-C6H12Br To a 250 mL two-neck flask, IIDBr-C6H12Br (1.86 g, 2.5 mmol), phenylboronic acid (1.22 g, 10.0 mmol), tris(o-anisyl)phosphine (P(O-tyl)3, 135 mg, 0.376 mmol), 50 mL toluene were added. The mixture was degassed by three times of freeze-pumpthaw operation, and then Pd2(dba)3 (134 mg, 0.14 mmol) was added. The mixture was refluxed for 12 hours, cooled to room temperature, and poured into 200 mL water. After an extraction with 100 mL dichloromethane, the solvent of the organic phase was removed under vacuum and the resulting residue was purified by silica gel column chromatography with CH2Cl2 and petroleum ether (PE) as eluent to afford the target compound as black solid (878.5 mg, 47.5%). 1

H NMR (300MHz, CDCl3) δ 9.27 (d, J = 8.4 Hz, 2H), 7.68 (d, J = 7.5 Hz, 4H), 7.52 (t, J = 7.5

Hz, 4H), 7.44 (t, J = 6.9 Hz, 2H), 7.32 (dd, J1 = 8.4 Hz, J2 = 1.5 Hz, 2H), 7.02 (s, 2H), 3.89 (t, J = 7.2 Hz, 4H), 3.42 (t, J = 7.2 Hz, 4H), 1.92-1.83 (m, 4H), 1.82-1.74 (m, 4H), 1.57-1.43 (m, 8H). MALDI-TOF (m/z): [M+H]+ calculated for C40H42Br2N2O2: 740.1, found: 740.4. Anal. Calcd for C40H42Br2N2O2: C 64.70, H 5.70, N 3.77, found: C 64.75, H 5.47, N 3.93. IIDPh-PyBr ([(E)-1,1'-((2,2'-dioxo-6,6'-diphenyl-[3,3'-biindolinylidene]-1,1'-diyl)bis(hexane6,1-diyl))bis(pyridin-1-ium) bromide]). The synthesis procedure was as same as that of IIDPyBr. The product was obtained as black solid with 78% yield. 1H NMR (300MHz, CD3OD). δ 9.21 (d, J = 8.1 Hz, 2H), 8.99 (dd, J 1= 8.7 Hz, J2 = 2.7 Hz, 4H), 8.58 (t, J = 8.4 Hz, 2H), 8.06 (dd, J 1= 8.4 Hz, J2 = 3.3 Hz, 4H), 7.74 (d, J = 8.1 Hz, 2H), 7.67(t, J = 8.1 Hz, 2H), 7.50 (dd, J 1=


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7.5 Hz, J2 = 2.4 Hz, 4H), 7.44 (d, J = 7.8 Hz, 2H), 7.33 (dd, J1 = 7.2 Hz, J2 = 2.7 Hz, 2H), 7.19 (s, 2H), 4.64 (t, J = 7.5 Hz, 4H), 3.91 (t, J = 7.5 Hz, 4H), 2.09-1.99 (m, 4H), 1.83-1.74 (m, 4H), 1.54-1.42 (m, 8H). MALDI-TOF (m/z): [M+H]+ calculated for C50H52Br2N4O2: 898.2, found: 898.8. Anal. Calcd for C50H52Br2N4O2: C 66.67, H 5.82, N 6.22, found: C 66.93, H 5.77, N 6.31. IIDPh-C6H12N The synthesis procedure was as same as that of IID-C6H12N. The product was obtained as black solid with 63% yield. 1H NMR (300MHz, CDCl3) δ 9.24 (d, J = 8.1 Hz, 2H), 7.64 (d, J = 7.8 Hz, 4H), 7.49 (t, J = 7.2 Hz, 4H), 7.37 (t, J = 7.2 Hz, 2H), 7.25 (dd, J1 = 8.1 Hz, J2 = 1.8 Hz, 2H) 7.00 (s, 2H), 3.85 (t, J = 7.5 Hz, 4H), 3.40 (t, J = 7.2 Hz, 4H), 2.86 (s, 12H), 1.98-1.87 (m, 4H), 1.85-1.72 (m, 4H), 1.67-1.52 (m, 8H). MALDI-TOF (m/z): [M+H]+ calculated for C44H54N4O2: 670.4, found: 670.8. Anal. Calcd for C44H54N4O2: C 78.77, H 8.11, N 8.35, found: C 78.88, H 7.89, N 8.42. IIDPh-NSB

([(E)-(6,6'-diphenyl-[3,3'-biindolinylidene]-1,1'-diyl)bis(hexane-6,1-

diyl))bis(dimethylammoniumdiyl))bis(propane-1-sulfonate)]). The synthesis procedure was as same as that of IID-NSB. The product was obtained as black solid with 88% yield. 1H NMR (300MHz, CD3OD) δ 9.21 (d, J = 8.4 Hz, 2H), 7.73 (d, J = 7.5 Hz, 4H), 7.50 (t, J = 7.2 Hz, 4H), 7.43 (t, J = 7.5 Hz, 2H), 7.32 (dd, J1 = 8.5 Hz, J2 = 1.5 Hz, 2H), 7.02 (s, 2H), 4.49 (t, J = 6.9 Hz, 4H), 3.92 (t, J = 7.2 Hz, 4H), 3.03 (t, J = 7.2 Hz, 4H), 2.83 (s, 12H), 2.23-2.09 (m, 4H), 1.851.68 (m, 8H), 1.53-1.42 (m, 8H), 1.36-1.28 (m, 4H). MALDI-TOF (m/z): [M+H]+ calculated for C50H66N4O8S2: 914.3, found: 914.5. Anal. Calcd for C50H66N4O8S2: C 65.62, H 7.27, N 6.12, S, 7.01; found: C 65.82, H 7.13, N 6.00, S, 7.15. IIDTh-C6H12Br To a 250 mL two-neck flask, IIDBr-C6H12Br (1.86 g, 2.5 mmol), thiophen-2ylboronic acid (1.60 g, 12.5 mmol), tetrabutylammonium bromide (TBAB, 417 mg, 1.30 mmol), 50 mL toluene and 50 mL K2CO3 aqueous solution (2 M) were added. The mixture was degassed


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by three times of freeze-pump-thaw operation, and then Pd(PPh3)4 (420 mg, 0.13 mmol) was added. The mixture was refluxed for 12 hours, cooled to room temperature, and poured into 200 mL water. After an extraction with 100 mL ethyl acetate, the solvent of the organic phase was removed under vacuum and the resulting residue was purified by silica gel column chromatography with CH2Cl2 and petroleum ether (PE) as eluent to afford the target compound as black solid (848 mg, 45%). 1H NMR (300MHz, CDCl3) δ 9.18 (d, J = 8.4 Hz, 2H), 7.45 (dd, J1 = 3.6 Hz, J2 = 0.9 Hz, 2H), 7.37 (dd, J1 = 5.1 Hz, J2 = 1.2 Hz, 2H), 7.31 (dd, J1 = 8.4 Hz, J2 = 1.8 Hz, 2H), 7.13 (dd, J1 = 3.9 Hz, J2 = 1.5 Hz, 2H), 6.98 (d, J = 1.5 Hz, 2H), 3.85 (t, J = 6.9 Hz, 4H), 3.41 (t, J = 6.9 Hz, 4H), 1.92-1.73 (m, 8H), 1.58-1.44 (m, 8H). MALDI-TOF (m/z): [M+H]+ calculated for C36H38Br2N2O2S2: 752.9, found: 753.3. Anal. Calcd for C36H38Br2N2O2S2: C 57.30, H 5.08, N 3.71, S 8.50, found: C 57.62, H 5.37, N 3.21, S 8.43. IIDTh-PyBr

([(E)-1,1'-((2,2'-dioxo-6,6'-di(thiophen-2-yl)-[3,3'-biindolinylidene]-1,1'-

diyl)bis(hexane-6,1-diyl))bis(pyridin-1-ium) bromide]). The synthesis procedure was as same as that of IID-PyBr. The product was obtained as black solid with 80% yield. 1H NMR (300MHz, CD3OD) δ 9.04 (d, J = 8.4 Hz, 2H), 8.97 (dd, J1 = 6.9 Hz, J2 = 1.2 Hz, 4H), 8.55 (t, J = 7.8 Hz, 2H), 8.06 (t, J = 7.5 Hz, 4H), 7.55 (dd, J1 = 3.6 Hz, J2 = 1.8 Hz, 2H), 7.49 (dd, J1 = 5.1 Hz, J2 = 1.2 Hz, 2H), 7.24 (dd, J1 = 8.4 Hz, J2 = 1.5 Hz, 2H), 7.14 (dd, J1 = 5.1 Hz, J2 = 3.9 Hz, 2H), 7.05 (d, J = 1.5 Hz, 2H), 4.62 (t, J = 7.5 Hz, 4H), 3.78 (t, J = 7.8 Hz, 4H), 2.06-1.97 (m, 4H), 1.771.68 (m, 4H), 1.50-1.40 (m, 8H). MALDI-TOF (m/z): [M+H]+ calculated for C46H48Br2N4O2S2: 911.2, found: 911.8. Anal. Calcd for C46H48Br2N4O2S2: C 60.53, H 5.30, N 6.14, S 7.02, found: C 60.11, H 5.52, N 6.19, S 7.43. IIDTh-C6H12N The synthesis procedure was as same as that of IID-C6H12N. The product was obtained as black solid with 58% yield. 1H NMR (300MHz, CDCl3) δ 9.19 (d, J = 8.4 Hz, 2H),


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7.47 (d, J = 3.6 Hz, 2H), 7.40 (d, J = 5.1 Hz, 2H), 7.33(dd, J1 = 8.4 Hz, J2 = 1.5 Hz, 2H), 7.16 (t, J = 4.5 Hz, 2H), 7.01 (s, 2H), 3.86 (t, J = 7.5 Hz, 4H), 2.30 (s, 12H), 1.81-1.74 (m, 4H), 1.581.39 (m, 12H), 1.38-1.26 (m, 4H). MALDI-TOF (m/z): [M+H]+ calculated for C40H50N4O2S2: 683.2, found: 683.6. Anal. Calcd for C40H50N4O2S2: C 70.34, H 7.38, N 8.20, S 9.39, found: C 70.60, H 7.59, N 8.19, S 9.13. IIDTh-NSB

([(E)-3,3'-(((2,2'-dioxo-6,6'-di(thiophen-2-yl)-[3,3'-biindolinylidene]-1,1'-

diyl)bis(hexane-6,1-diyl))bis(dimethylammoniumdiyl))bis(propane-1-sulfonate)]). The synthesis procedure was as same as that of IID-NSB. The product was obtained as black solid with 88% yield. 1H NMR (300MHz, CD3OD) δ 9.24 (d, J = 8.1 Hz, 2H), 7.50 (d, J = 3.9 Hz, 2H), 7.35 (d, J = 5.1 Hz, 2H), 7.28 (dd, J1 = 7.8 Hz, J2 = 1.2 Hz, 2H), 7.11 (t, J = 4.5 Hz, 2H), 6.99 (s, 2H), 4.35 (t, J = 7.5 Hz, 4H), 3.76(t, J = 7.2 Hz, 4H), 3.18-3.10 (m, 4H), 2.80 (s, 12H), 2.50 (t, J = 6.9 Hz, 4H), 2.08-1.96 (m, 4H), 1.67-1.58 (m, 8H), 1.34-1.26 (m, 4H), 1.09 (t, J = 6.9 Hz, 4H). MALDI-TOF (m/z): [M+H]+ calculated for C46H62N4O8S4: 927.2, found: 927.6. Anal. Calcd for C46H62N4O8S4: C 59.58, H 6.74, N 6.04, S 13.83, found: C 60.04, H 6.59, N 6.33, S 13.47.


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Scheme 1. Synthesis procedures of a) IID-PyBr, IID-NSB, b) IIDPh-PyBr, IIDPh-NSB, c) IIDTh-PyBr and IIDTh-NSB. Device fabrication. (ITO)-coated glass with a sheet resistance of 10 Ω per square was used as the substrate. The substrates were carefully cleaned by acetone, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol under ultrasonic bath. Then, ITO was treated with oxygen plasma for 7 minutes. A thin layer of PEDOT:PSS aqueous solution (Baytron PVP Al 4083) was deposited on the ITO glass through spin-coating at 2000 rpm and annealed in air at 120℃ for 10 minutes, then the device was transferred to a nitrogen gloves box, where the active layer based on polymer and fullerene was spin-coated onto PEDOT:PSS layer. For PTB7:PC71BM PSCs, the active layer was formed by spin-coating from the chlorobenzene:1,8-diiodoctane (97:3, V:V) solution consisting of 10 mg mL-1 PTB7 and 15 mg mL-1 PC71BM, stirred overnight and then dried in vacuum to remove the residual DIO. To optimize the interlayer thickness, methanol solution of IID-interlayer at different concentrations (from 0.5 mg mL-1 to 5 mg mL-1) was spincoated onto the active layer at 2000 rpm for 60 s and then 100 nm Al was evaporated as a cathode. Current density-voltage (J-V) characteristics of the devices were measured under N2 atmosphere in the glove box by using a Keithley 2400 under illumination and in the dark.


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Photovoltaic performance of the device was measured under 1 sun, AM 1.5-G full-spectrum solar simulator (Photo Emission Tech., model #SS50AAA-GB) with optical power intensity of 100 mW cm-2 calibrated with a standard silicon photovoltaic traced to National Institute of Metrology, China. A Q Test station (Crowntech Inc. USA) was used to measure external quantum efficiency (EQE) spectra under ambient condition. AFM images and RMS values were carried out with an S II Nanonaviprobe station 300Hz (Seiko, Japan). The thickness (>30 nm) of IID-interlayer films were determined by DEKTAK 150 platform with average 5 measurements for each sample. The other thickness films were estimated by an absorbance-thickness curve when assumed a linear dependence of the maximum absorbance. XPS and UPS experiments were carried out using a XPS/UPS system equipped with VG Scienta R3000 analyzer in ultrahigh vacuum with a base pressure of 1×10-10 mbar. A monochromatic Al (Ka) X-ray source provides photons with 1486.6 eV for XPS. A monochromatized He Ia irradiation from discharged lamp supplies photons with 21.22 eV for UPS.

3. RESULTS AND DISCCUSION The synthetic procedures of the compounds are shown in Scheme 1, Suzuki couplings between IID-H and phenylboronic acid or 2-thiopheneboronic acid were carried out to get IIDPh-C6Br and IIDTh-C6Br. The ionization of IID-C6Br, IIDPh- C6Br and IIDTh-C6Br with pyridine were then performed to afford IID-PyBr, IIDPh-PyBr and IIDTh-PyBr in high yeilds, respectively. The zwitterions substituted products, IID-NSB, IIDPh-NSB and IIDTh-NSB were obtained by ionization of IID-C6N, IIDPh-C6N and IIDTh-C6N with sulfonate. They were fully characterized by NMR, mass spectra, and element analyses. These compounds can be well dissolved in methanol and ethanol while they are insoluble in other common organic solvents such as


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chloroform, chlorobenzene, tetrahydrofuran, and toluene, which can avoid intermixing between active layer and interlayer. Figure 2a presents the UV-vis absorption spectra in dilute solution. IDTh-PyBr and IIDTh-NSB show obviously red-shifted absorptions and smaller optical bandgaps due to the intramolecular charge transfer characteristic from thiophene to IID moiety. Cyclic voltammetry curves (Figure 2b) reveal that the compounds have a deep LUMO energy level from -3.59 to -3.68 eV (Table 1). Table 1. Parameters derived from UV-vis absorption spectra and cyclic voltammograms of the IID derivatives. Solution absorption (nm) Interlayer

a

LUMOc

HOMOd

λmax

λedga

Band gapb (eV)

(eV)

(eV)

IID-PyBr

391

615

2.02

-3.59

-5.61

IID-NSB

391

616

2.02

-3.61

-5.63

IIDPh-PyBr

415

632

1.95

-3.65

-5.60

IIDPh-NSB

415

633

1.95

-3.67

-5.62

IIDTh-PyBr

433

658

1.88

-3.63

-5.51

IIDTh-NSB

434

661

1.88

-3.66

-5.54

The lowest-energy absorption edge of the absorption spectra.

edge.

c

b

Optical band gap estimated from the lowest-energy absorption

Deduced from the onset reduction potentials, assuming that the energy level of ferrocene lies 4.8 eV below the vacuum

level. ELUMO = -(Ereonset + 4.80) eV.

d

Calculated from LUMO and Egopt . EHOMO = - (Egopt -ELUMO) eV.

To check the film formation properties of IID-interlayer on the PTB7:PC71BM layer, atomic force microscopy (AFM) characterizations were performed. The PTB7:PC71BM film is relatively smooth with root-mean-square (RMS) roughness of 2.79 nm.45 The IID-PyBr, IID-NSB, IIDPhPyBr and IIDPh-NSB films onto the PTB7:PC71BM active layers show slightly smoother surfaces with RMS roughness of 2.63, 2.23, 2.10 and 1.81 nm, respectively (Figure 3a-d). The


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IIDTh-PyBr and IIDTh-NSB layers have even smaller RMS roughness of 1.72 and 1.16 nm (Figure 3e,f), indicating the less extent of self-aggregation of IIDTh-PyBr and IIDTh-NSB in films on the active layer. Therefore, the introduction of thiophene in CIL can improve the compatibility between CIL and active layer, which may enhance the PSCs devices performance. Contact angle (θ) measurements (Figure S1) were performed in order to investigate wettability of IID based CIL materials on the PTB7:PC71BM films. The contact angles of methanol solution droplets of six IID molecules varied from 12.2° to 18.0°. In particular IIDTh-NSB drop displayed the smallest contact angle (12.2°), suggesting that IIDTh-NSB has the best wettability on the active layer.

Figure 2. (a) Absorption spectra of the IID derivatives in dilute solutions. (b) Cyclic voltammograms of IID-interlayers measured in DMF.


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Figure 3. Atomic force microscopy images (2×2 µm) of IID-interlayers atop of the PTB7:PC71BM layer. (a) IID-PyBr; (b) IID-NSB; (c) IIDPh-PyBr; (d) IIDPh-NSB, (e) IIDThPyBr; (f) IIDTh-NSB. To study the CIL dependent PSCs performance, the PTB7:PC71BM based devices with a configuration of [ITO/PEDOT:PSS/PTB7:PC71BM/IID-interlayer/Al] were fabricated (Figure 4). For comparison, two control devices (devices 1 and 2) without CIL and with methanol treatment between the PTB7:PC71BM and Al were fabricated, respectively. Figure 5a presents current density–voltage (J–V) characteristics of the two control devices and the PSCs with ca. 8 nm IID derivatives as CILs (devices 3, 4, 5, 6, 7, 8) under the 100 mW cm-2 AM. 1.5G irradiation. The device performance parameters are summarized in Table 2. Intriguingly, the IIDTh-NSB-based device 8 exhibited the highest PCE value of 9.12%, which is nearly 1.8 times of that (5.19%) of the Al-only device 1. The remarkable improvement in PCE is attributed to a simultaneous


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enhancement of open circuit voltage (Voc), short-circuit current (Jsc) and most notably fill factor (FF). The device 7 also exhibited a noticeable PCE of 8.23%. Moreover, the PCE values of devices 3-6 are still much higher than the two control devices. In order to ensure the validity and repeatability of data, we measured at least 30 pixels for all device configurations. Compared with IID- and IIDph-based devices, the improvements in the performance of the IIDTh-based devices mainly arise from higher Jsc and FF, which may be attributed to the D-A-D structure induced the self-doping.59 Meanwhile, not only the sulfonate-based devices show better Jsc and FF than pyridinium-based devices, but also the IIDTh core with two thiophene units exhibit better performance than IID and IIDph-based devices. Combination of the two advantages can lead to the best device performance. To avoid the wildly inaccurate Jsc which leads to an overvalued PCE, external quantum efficiency (EQE) spectra of devices 1-8 from 350 to 800 nm were measured and shown in Figure 5b. The Jsc values integrated from the EQE spectra of devices 1-8 are well agreed with the tested values, which demonstrated our presented Jsc in this manuscript are reliable (Table 2).

Figure 4. Device architecture of PSCs and molecular structures of PTB7 and PC71BM.


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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


Figure 5. Effect of IID-interlayers on the performance of devices based on PTB7:PC71BM. (a) Current density-voltage (J-V) characteristics of PTB7:PC71BM incorporated with IID-interlayers. (b) The corresponding external quantum efficiency (EQE) spectra. Table 2. Photovoltaic parameters of the PSCs based on PTB7:PC71BM with different CIL under the 100 mW cm-2 AM. 1.5G irradiation. Voc

Jsc (JscEQE)

FF

(V)

(mA cm-2)

(%)

Max.

Aver.

(Ω cm2)

1

0.679

14.18 (14.05)

54.0

5.19

5.10

13.7

2

0.715

15.34 (15.19)

60.1

6.59

6.51

8.4

3

0.750

16.18 (15.77)

60.3

7.32

7.24

6.3

4

0.749

16.48 (15.90)

63.5

7.84

7.75

5.7

5

0.748

16.20 (15.92)

61.4

7.44

7.36

6.0

6

0.746

16.51 (16.08)

65.1

8.02

7.95

4.8

7

0.755

16.76 (16.12)

65.0

8.23

8.17

4.2

8

0.753

16.91 (16.22)

71.6

9.12

9.04

3.2

Device

PCE (%)

Rs

Device1:[ITO/PEDOT:PSS/PTB7:PC71BM/Al]; Device2:[ITO/PEDOT:PSS/PTB7:PC71BM/methanol/Al]; Device3:[ITO/PEDOT:PSS/PTB7:PC71BM/IID-PyBr/Al]; Device4:[ITO/PEDOT:PSS/PTB7:PC71BM/IID-NSB/Al]; Device5:[ITO/PEDOT:PSS/PTB7:PC71BM/IIDPh-PyBr/Al]; Device6:[ITO/PEDOT:PSS/PTB7:PC71BM/IIDPh-NSB/Al]; Device7:[ITO/PEDOT:PSS/PTB7:PC71BM/IIDTh-PyBr/Al]; Device8:[ITO/PEDOT:PSS/PTB7:PC71BM/ IIDTh-NSB/Al].


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The ultraviolet photoemission spectroscopic (UPS) measurements (Figure 6a) revealed that the WF values are 4.18 eV for bare Al and 3.53 eV for methanol-treated Al. After deposition of an thin (5 nm) CIL on the Al surface, the much lower WF values 3.34 eV for IID-PyBr, 3.42 eV for IID-NSB, 3.36 eV for IIDPh-PyBr, 3.42 eV for IIDPh-NSB, 3.34 eV for IIDTh-PyBr and 3.37 eV for IIDTh-NSB, respectively. Thereby the six CILs give rise to a decrease of WF of Al about 0.8 eV, which is in agreements with the change tendency of Voc from control devices to the devices with CIL (Table 2). Further proof in CIL improving the built in potential (upper limit of Voc) can be deduced from the dark J-V curves (Figure 6b). Turn-on voltages in the dark are 0.712 V and 0.752 V for control devices 1 and 2, respectively. The turn-on voltages values of devices 3-8 are around 0.790 V. Series resistance (Rs) calculated from the slopes of J-V curves at Voc are shown in Table 2. Compared with the two control devices, apparently, decreases of Rs were harvested from devices 3-8, which contributed to improving the Jsc values. In particular device 8 shows an extremely low Rs value of 3.2 Ω cm2.

Figure 6. (a) Ultraviolet photoelectron spectra of bare Al, methanol treated Al and Al covered by 5 nm IID-PyBr, IID-NSB, IIDPh-PyBr, IIDPh-NSB, IIDTh-PyBr and IIDTh-NSB, respectively. (b) J-V curves of the IID-interlayers incorporated, methanol-treated and Al-only devices under dark.


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Figure 7 presents the CIL substitute dependent PCE diagram. Firstly, IID-PyBr and IID-NSB result in remarkable improvement of PCE and then IIDTh-PyBr and IIDTh-NSB lead to further increase of PCE. These results clearly demonstrate that introduction of PyBr (pyridinium), NSB (zwitterions) and Th (thiophene) groups on IID core can significantly promote the PCE. However, the additions of Ph (phenyl) only result in very slight increase of PCE suggesting that simple extension of conjugation for IID core is insignificant for the enhancement of PCE. NSB is a more efficient functional group for improving PCE than PyBr. The dissociation of positive and negative ions may induce the counterion migration towards active layer, which can depress the PCE of the PSC.10 The introduction of zwitterions on IID backbones can avoid counterion migration and effectively optimize the device performance. Furthermore, the contact property between electrode Al and sulfonate group was confirmed by the XPS measurements (Figure S2) for Al/IID-NSB, Al/IIDPh-NSB and Al/IIDTh-NSB. We infer that the three IID derivatives with NSB were strongly adsorbed on the Al surface due to the contact between the Al and sulfonate group in the IID derivatives. As a result, the sulfonate group with negative charge is close to the Al surface and ammonium group with positive charge is far away from the Al surface as shown in Figure S3. Such a direction of interface dipole is favourable to form the dipole at the surface of metal cathode and improve the electron extraction. In addition, the XPS results clearly demonstrated that the IID derivatives could be n-doped upon contacting with the Al due to electron transfer from Al to sulfonate group as shown in Al2p and S2p XPS spectra for Al/IIDNSB in Figure S2b, for Al/IIDPh-NSB in Figure S2d and for Al/IIDTh-NSB in Figure S2f. The substitution of Th groups on the two terminals of IID core resulted in the formation of a conjugated molecule with D-A-D structure feature,60,61 which could enhance fill factor and improve the device performance.


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Figure 7. PCE value plots of devices based on IID-PyBr, IID-NSB, IIDPh-PyBr, IIDPh-NSB, IIDTh-PyBr and IIDTh-NSB CIL. In order to confirm that introducing IIDTh-NSB film not only improves electron extraction but also facilitates a balance of electron and hole currents in the PSCs, single carrier devices with configuration

of

[ITO/Al/PTB7:PC71BM/CIL/Al]

for

the

electron-only

devices

and

[ITO/PEDOT:PSS/PTB7:PC71BM/CIL/MoO3/Al] for the hole-only devices were fabricated and characterized. The J1/2-V curves of electron and hole-only devices are showed in Figure 8. The electron current densities of electron only devices with CIL are much larger than that of the control devices (Figure 8a). Especially, IIDTh-NSB could obviously increase the electron current density, which was in agreement with the increasing trend of FF. However, incorporating interlayer only resulted in a slight change of hole current densities (Figure 8b). These results indicate that the D-A-D structure in CIL molecular skeleton led to increased electron extraction at cathode and more balanced hole and electron current density.


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According to the fundamental organic chemistry and semiconductor concepts, the IIDPh-NSB molecule composed of IID and Ph moieties should have stronger electron-deficient character and more powerful electron transport ability compared with IIDTh-NSB. However, as CIL materials of PSCs the IIDTh-NSB molecule with electron-rich Th groups and D-A-D conjugated structure displayed much stronger ability for the enhancement of electron current density compared with IIDPh-NSB. These results demonstrated that for a very thin CIL the conjugated molecule dipole should be more dominant factor compared with its intrinsic n-type semiconductor characteristic. Therefore, this study provide a unique approach to develop high performance CIL materials based on D-A-D conjugated molecules.

Figure 8. (a) J1/2-V characteristics of the electron-only devices. (b) J1/2-V characteristics of the hole-only devices. The effect of IIDTh-NSB thickness on performance of the device based on PTB7:PC71BM was evaluated by varying IID-interlayer concentration as shown in Figure 9a. Table 3 presents a summary of photovoltaic parameters for the PSCs with different IIDTh-NSB thickness. When the IIDTh-NSB thickness increased from 4 to 25 nm (Figure 9b), the PCE just exhibited slightly drop from 9.12% to 7.40% due to a decrease of Jsc from 16.91 mA cm-2 to 16.08 mA cm-2 and a drop of FF from 71.6% to 61.3%, which accomplished by increase of Rs from 3.2 Ω to 6.6 Ω


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Page 22 of 34

cm2. It appears that IIDTh-NSB is tolerant in various interlayer thicknesses, with Jsc values exceeding 16 mA cm-2 across the entire thickness range from 4 nm to 25 nm.

Table 3. Device parameters of PSCs based on PTB7:PC71BM with various IIDTh-NSB CIL thickness under 100 mW cm-2 AM. 1.5G irradiation

CIL

Thickness

Voc

Jsc

FF

PCE

Rs

(nm)

(V)

(mA cm-2)

(%)

(%)

(Ω cm2)

4

0.753

16.85

69.2

8.78

4.5

8

0.753

16.91

71.6

9.12

3.2

14

0.753

16.58

65.0

8.11

5.1

25

0.752

16.08

61.2

7.40

6.6

IIDTh-NSB

Figure 9. (a) J-V characteristics of various IIDTh-NSB thickness incoporated to the PSCs devices based on PTB7:PC71BM active layer. (b) The absorption spectra of IIDTh-NSB films with different thickness on the quartz substrates. The thickness of 25 nm IIDTh-NSB film was determined by the profilometer. 4. CONCLUSION


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In summary, a series of IID based CIL materials were designed and synthesized. The CIL material molecular structures were optimized by varying the central conjugated core and side water/alcohol soluble groups. The detail characterizations of PSCs with different IID based CIL demonstrated that the central core with D-A-D structural feature and the zwitterions groups can efficiently enhance the PCE of PSCs, respectively. The combination between D-A-D structure and zwitterions side groups resulted in significant PCE value of 9.12% for the devices based on PTB7:PC71BM as active layer. The introduction of thiophene into IID backbone led to the formation of D-A-D conjugated structure, which can efficiently improve the electron current density. As a result, high PCE was achieved. Our experimental results revealed that constructing D-A-D central conjugated core substituted by zwitterions groups should be an efficient strategy to achieve high performance CIL.

ASSOCIATED CONTENT Supporting Information. Contact angle images of IID-interlayer on PTB7:PC71BM BHJ films; Al2p, Br3d and S2p core-level XPS spectra of Al, Al covered by 8 nm IID derivatives; Selfassembly diagram of IIDTh-NSB molecule as cathode interlayer between Al and active layer.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]


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High Performance Small-Molecule Cathode ... - ACS Publications

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