Hydrophilic Conjugated Polymers with Large Bandgaps and Deep-Lying HOMO Levels as an Efficient Cathode Interlayer in Inverted Polymer Solar Cells.

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Hydrophilic Conjugated Polymers with Large Bandgaps and Deep-Lying HOMO Levels as an Efficient Cathode Interlayer in Inverted Polymer Solar Cells Yuanyuan Kan, Yongxiang Zhu, Zhulin Liu, Lianjie Zhang,* Junwu Chen,* Yong Cao

Two hydrophilic conjugated polymers, PmP-NOH and PmP36F-NOH, with polar diethanolamine on the side chains and main chain structures of poly(meta-phenylene) and poly(metaphenylene-alt-3,6-fluorene), respectively, are successfully synthesized. The films of PmP-NOH and PmP36F-NOH show absorption edges at 340 and 343 nm, respectively. The calculated optical bandgaps of the two polymers are 3.65 and 3.62 eV, respectively, the largest ones so far reported for hydrophilic conjugated polymers. PmP-NOH and PmP36F-NOH also possess deep-lying highest occupied molecular orbital levels of −6.19 and −6.15 eV, respectively. Inserting PmP-NOH and PmP36F-NOH as a cathode interlayer in inverted polymer solar cells with a PTB7/PC71BM blend as the active layer, high power conversion efficiencies of 8.58% and 8.33%, respectively, are achieved, demonstrating that the two hydrophilic polymers are excellent interlayers for efficient inverted polymer solar cells.

1. Introduction The single-junction efficiency in bulk-heterojunction (BHJ) polymer solar cells (PSCs) has been broken through again and again in the past 5 years, which entirely encourages the commercial applications integrated the intrinsically polymeric advantages of light weight, low-cost, and mechanical flexibility.[1–4] Additionally, the technological

Y. Kan, Y. Zhu, Z. Liu, Dr. L. Zhang, Prof. J. Chen, Prof. Y. Cao Institute of Polymer Optoelectronic Materials & Devices State Key Laboratory of Luminescent Materials & Devices South China University of Technology Guangzhou 510640, China E-mail: [email protected]; [email protected] Macromol. Rapid Commun. 2015, 36, 1393−1401 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

improvements of solution printing, especially roll-to-roll (R2R) processing, greatly accelerate the realization of the next generation of practical photovoltaic products.[5] Many efforts have been made towards high efficiency during the research of PSCs. First of all, the significant achievement is the development of photoactive polymer donor materials, especially the ones with efficiency beyond 6% for their thick films (more than 300 nm), since thickfilm PSCs are more compatible with the R2R technique for high-throughput production.[6–8] Second, the gradual optimization of device architecture is very promising as well, especially the utilization of the inverted structure to achieve long-term stability.[9] In inverted PSCs, indium tin oxide (ITO) has been widely utilized as a cathode. In order to realize a PSC with a higher efficiency and a longer device lifetime, a cathode interlayer on the ITO cathode is quite

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DOI: 10.1002/marc.201500163

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necessary.[10,11] Generally, cathode interlayer materials can be simply divided into inorganics and organics.[12,13] In the past several years, introducing an organic cathode interlayer for inverted PSCs has drawn much attention due to effective modification of work function of the ITO cathode for electron collection.[9,14–16] Especially, utilizations of hydrophilic conjugated polymers as the cathode interlayer could greatly elevate the power conversion efficiency (PCE) of inverted PSCs, from which record-high PCEs were achieved.[9b] In the device configuration of an inverted PSC, solar light first passes through the interlayer on the ITO cathode and then is absorbed by the polymer-based photoactive layer.[17] In order not to block the absorption of incident solar light by the active layer, it is more ideal that the hydrophilic conjugated polymer interlayer can possess an enough large bandgap.[18] For example, all visible band of incident solar light could pass through a hydrophilic conjugated polymer interlayer if the bandgap of the interlayer polymer was larger than 3.1 eV. However, the bandgaps of the so far reported hydrophilic conjugated polymers are almost lower than 3.1 eV.[15,16] Para-phenylene was chosen as the building block for some early examples of hydrophilic conjugated polymers, but the bandgaps of the resulting polymers were less than 3.0 eV.[19] 2,7-Fluorene had been selected to construct various hydrophilic conjugated polymers for the cathode interlayer application, but these polymers could absorb some of the visible light.[20] In a report by Zhao and co-workers, a 2,7-fluorene-based homopolymer PF-EP with phosphonate groups on the side chains possessed an optical bandgap of about 2.88 eV, whose absorption edge located

at 430 nm.[20b] Poly[(9,9-bis(3′-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), a widely utilized cathode interlayer polymer based on the 2,7-fluorene, possessed a bandgap of 2.91 eV.[20c,20d] A series of hydrophilic conjugated polymers containing 2,7-carbazole unit exhibited bandgaps between 2.79 and 2.91 eV.[21] A hydrophilic poly(triphenylamine) showed a bandgap of 2.79 eV.[14a] When 2,5-thiophene was selected as the building block of hydrophilic conjugated polymers, their bandgaps could be decreased to around 2.03 eV.[13b] Very few hydrophilic conjugated polymers with bandgaps larger than 3.1 eV were reported. Guan et al.[22] reported 2,6-pyridinyl- and 3,5-pyridinyl-based poly(2,7-fluorenes) displayed large bandgaps between 3.19 and 3.37 eV. Very recently, a 2,7-fluorene-based metallopolymer PFEN-Hg also exhibited a large bandgap of 3.35 eV.[14c] It is well known that the bandgap of a conjugated polymer is determined by the conjugated backbone. There are several reports that enlarging bandgaps of conjugated polymers have been demonstrated.[23] When meta-phenylene (mP) unit was introduced on the main chain of conjugated polymers, larger bandgaps could be achieved.[23d] If 3,6-fluorene (36F) was utilized as the building block of a conjugated polymer, obviously an increased bandgap was obtained if compared with a poly(2,7-fluorene).[23e] In this work, the meta-phenylene and 3,6-fluorene were selected to construct new hydrophilic conjugated polymers PmP-NOH and PmP36F-NOH (see Scheme 1), with polar diethanolamine (NOH) as the hydrophilic end groups on side chains. PmP-NOH is a simple copolymer of poly(meta-phenylene) while PmP36F-NOH is an alternating copolymer derived from

Scheme 1. Synthetic routes of monomers and polymers.

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Hydrophilic Conjugated Polymers with Large Bandgaps and Deep-Lying HOMO Levels as an Efficient Cathode . . .

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meta-phenylene and 3,6-fluorene. The films of PmPNOH and PmP36F-NOH showed absorption edges at 340 and 343 nm, respectively, corresponding to optical bandgaps of 3.65 and 3.62 eV, respectively. The bandgaps of the two polymers belong to the largest ones so far reported for hydrophilic conjugated polymers. PmPNOH and PmP36F-NOH possessed deep-lying highest occupied molecular orbital (HOMO) levels of −6.19 and −6.15 eV, respectively, which would supply big potential for anti-oxidation. It should be noted that using a hydrophilic mP-based polymer as a cathode interlayer in the optoelectronic devices had not been investigated before. With PmP-NOH or PmP36F-NOH as a cathode interlayer, we fabricated inverted PSCs based on PTB7:PC71BM (=1:1.5 by weight) as the active layer. Delightedly, the resulting devices with PmP-NOH or PmP36F-NOH as a cathode interlayer showed simultaneous improvements in open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) if compared with a bare ITO cathode. High PCEs of 8.58% and 8.33% were achieved for PmP-NOH-based and PmP36F-NOH-based PSCs, respectively.

2. Experimental Section

2.2. Synthesis of Monomers and Polymers 3,5-Dibromophenol was purchased from Energy Chemical Company. 3,6-Dibromofluorene was prepared through the similar procedure in the reported literatures.[23e] 1,3-Phenyldiboronic acid bis(neopentylglycol) ester was achieved by the similar procedure for the synthesis of 1,4-benzenediborate ester in the published literature.[24]

2.2.1. 5-(6-Bromohexyloxy)-1,3-dibromobenzene (1) Potassium hydroxide (0.67 g, 12.0 mmol) in 30 mL of methanol was mixed with 3,5-dibromophenol (2.519 g, 10.0 mmol) in 30 mL of methanol under an argon atmosphere. The reaction mixture was stirred for 1 h and then added to 1,6-dibromohexane (7.32 g, 30.0 mmol) in 50 mL of acetone. The reaction mixture was refluxed for 12 h. After cooling to room temperature, the solvent was removed under reduced pressure. The organic phase was extracted with dichloromethane (100 mL × 3), washed successively with water, and then dried over anhydrous MgSO4. After removal of the solvent, the residue was purified by column chromatography (dichloromethane/petroleum ether = 1:4 as the eluent) to give the title compound (2.9 g, 70%) as a white solid. 1H NMR (300 MHz, CDCl3), δ (ppm): 7.23 (s, 1H), 6.98 (s, 2H), 3.92 (t, J = 6.3 Hz, 2H), 3.42 (t, J = 6.7 Hz, 2H), 1.89 (m, 2H), 1.80 (m, 2H), 1.50 (m, 4H). 13C NMR (75 MHz, CDCl3), δ (ppm): 160.28, 126.30, 123.10, 116.96, 68.37, 33.65, 32.62, 28.84, 27.84, 25.18. Anal. Calcd (%) for C12H15Br3O: C 34.73, H 3.64; Found: C 35.07, H 3.514.

2.1. Materials and Instrumentations All reagents and solvents, unless otherwise specified, were obtained from Aldrich, Acros, and TCI Chemical Co. and were used as received. Anhydrous tetrahydrofuran (THF) was distilled over sodium/benzophenone under N2 prior to use. All manipulations involving air-sensitive reagents were performed under an atmosphere of dry argon. 1H NMR spectra were recorded on Bruker AV 300 and 600 spectrometers (Switzerland) with tetramethylsilane (TMS) as the internal reference. Molecular weights of the polymers were obtained on a Waters GPC 2410 (England) using a calibration curve of polystyrene standards, with THF as the eluent. Elemental analyses were performed on a Vario EL elemental analysis instrument (Elementar Co., Germany). UV–vis absorption spectra were recorded on an HP 8453 spectrophotometer (USA). Cyclic voltammetry was done on a CHI660A electrochemical workstation (Shanghai Chenhua, China) with platinum electrodes at a scan rate of 50 mV s−1 against an Ag/Ag+ reference electrode with a nitrogen-saturated solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile. Potentials were referenced to the ferrocenium/ferrocene couple by using ferrocene as an internal standard. The deposition of a copolymer on the electrode was done by the evaporation of a dilute THF solution. Tapping-mode atomic force microscopy (AFM) images were obtained using a NanoScope NS3A system (Digital Instruments, USA) to observe the surface morphology. Transmission electron microscopy (TEM) images were obtained using JEM-2100F. The work function was determined by the scanning Kelvin probe system SKP 5050 (KP Technology, England).


2.2.2. 3,6-Dibromo-9,9-bis(6′-bromohexyl)fluorene (2) A mixture of 3,6-dibromo-fluorene (5.0 g, 15 mmol), 1,6-dibromohexane(30 mL), tetrabutylammonium bromide (0.1 g), and sodium hydroxide (30 mL, 50%, w/w) aqueous solution was stirred at 60 °C for 4 h under nitrogen. After diluting the reaction mixture with dichloromethane, the organic layer was washed with water and brine. The separated organic layer was dried over magnesium sulfate, and dichloromethane was evaporated. The nonreacted 1,6-dibromohexane was distilled in a vacuum, and 3,6-dibromo-9,9-bis(6′-bromohexyl)fluorene (8.3 g, 81%) was obtained as a white crystal by chromatography with petroleum ether as the eluent. 1H NMR (300 MHz, CDCl3), δ (ppm): 7.78 (s, 2H), 7.44 (d, J = 8.0 Hz, 2H), 7.19 (d, J = 8.0 Hz, 2H), 3.29 (t, J = 6.8 Hz, 4H), 2.00–1.87 (m, 4H), 1.72–1.59 (m, 4H), 1.19 (m, 4H), 1.12–0.99 (m, 4H), 0.64–0.47 (m, 4H). 13C NMR (75 MHz, CDCl3), δ (ppm): 149.45, 142.05, 130.82, 124.48, 123.48, 121.13, 55.00, 40.02, 33.95, 32.70, 27.85, 27.45, 23.61. Anal. Calcd (%) for C25H30Br4: C 46.19, H 4.65; Found: C 47.31, H 4.352.

2.2.3. PmP-Br Compound 1 (0.207 g, 0.5 mmol), 1,3-phenyldiboronic acid bis(neopentylglycol)ester (0.151 g, 0.5 mmol), Pd(PPh3)4 (12 mg), and three drops of Aliquat336 were dissolved in a mixture of 12 mL of toluene and 2 mL of 2 M K2CO3 aqueous solution. The mixture was refluxed with vigorous stirring for 2 d under an argon atmosphere. After the mixture was cooled to room temperature, it was poured into 250 mL of methanol. The


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Table 1. Molecular weight, optical properties, and energy levels of the polymers.

Mwa) [kg mol−1]

Mw/Mna)

PmP-NOH

5.9

PmP36F-NOH

6.7

Polymer

λabsb) [nm] Solution

Film

Egc) [eV]

Eoxd) [eV]

HOMOe) [eV]

LUMOf) [eV]

2.07

250

255

3.65

1.39

−6.19

−2.54

2.01

258

262

3.62

1.35

−6.15

−2.53

a)Estimated

by GPC in THF on the basis of a polystyrene calibration; b)Absorption peak; c)Optical bandgap; d)Onset voltage of oxidation process during cyclic voltammetry; e)Calculated according to HOMO = −e (Eox + 4.8), f)Calculated from HOMO level and the optical bandgap. precipitated material was redissolved and filtrated through a funnel and then precipitated again. The resulting solid material was washed successively with acetone for 24 h and dried in vacuum to afford the title polymer as a white solid (88%). GPC: Mw = 5.9 kg mol−1; Mw/Mn = 2.07 (Table 1). 1H NMR (300 MHz, DMSO), δ (ppm): 8.03 (br, 1H), 7.81–7.35 (br, 4H), 7.25 (br, 2H), 4.1 (m, 2H), 3.49 (m, 2H), 1.75 (m, 4H), 1.44 (m, 4H). Anal. Calcd (%) for (C20H25BrO)n: C 65.26, H 6.39; Found: C 66.37, H 6.94.

2.2.4. PmP36F-Br Compound 2 (0.650 g, 1 mmol), 1,3-phenyldiboronic acid bis(neopentylglycol) ester (0.302 g, 1 mmol), Pd2(PPh3)4 (15 mg), and three drops of Aliquat336 were dissolved in a mixture of 12 mL of toluene and 2 mL of 2 M K2CO3 aqueous solution. The mixture was refluxed with vigorous stirring for 2 d under an argon atmosphere. After the mixture was cooled to room temperature, it was poured into 300 mL of methanol. The precipitated material was redissolved and filtrated through a funnel and then precipitated again. The resulting solid material was washed successively with acetone for 24 h and dried in vacuum to afford the title polymer as a light gray solid (400 mg, 70%). GPC: Mw = 6.7 kg mol−1, Mw/Mn = 2.01 (Table 1). 1H NMR (400 MHz, CDCl3), δ (ppm): 8.08– 7.93 (m, 2H), 7.83–7.29 (m, 8H), 3.43–3.13 (m, 4H), 2.03 (m, 4H), 1.71 (m, 4H), 1.24 (m, 4H), 1.12 (m, 4H), 0.95–0.55 (m,4H). Anal. Calcd (%) for C31H34Br2: C 65.73, H 6.40; Found: C 74.96, H 7.028.

2.2.5. PmP-NOH Diethanolamine (0.5 g) was added to a solution of PmP-Br (100 mg) in a mixture of 20 mL of THF and 5 mL of dimethylformamide (DMF). The mixture was stirred vigorously for 48 h at 60 °C under an argon atmosphere. After cooling to room temperature, the mixture was poured into 250 mL of H2O. The resulting polymer was then collected and dried in vacuum to afford the final polymer as a white solid (49 mg, 46%). 1H NMR (300 MHz, DMSO), δ (ppm): 8.04 (br, 1H), 7.81–7.32 (br, 4H), 7.29 (br, 2H), 4.01 (m, 2H), 3.37 (m, 6H), 2.44(m, 4H), 1.7 (m, 4H), 1.27(m, 4H). Anal. Calcd (%) for (C24H35NO3)n: C 74.33, H 8.79; Found: C 73.02, H 8.209.

2.2.6. PmP36F-NOH

2.3. Fabrication and Characterization of Solar Cells The device structure was ITO/PmP-NOH or PmP36F-NOH/ PTB7:PC71BM (=1:1.5 by weight) /MoO3/Al. Patterned ITO-coated glass with a sheet resistance of 15–20 ohm square−1 was cleaned by a surfactant scrub and then underwent a wet-cleaning process inside an ultrasonic bath that began with deionized water, followed by acetone and 2-propanol. A thin cathode interlayer PmP-NOH or PmP36F-NOH of 10 nm was spin-cast onto the ITO substrate and then dried by baking in the N2 glove box at 100 °C for 5 min. The active layer based on PTB7:PC71BM, with a thickness of 95 nm, was then deposited on the top of the PmP-NOH or PmP36F-NOH interlayer by casting from a chlorobenzene solution or a mixed solvent of chlorobenzene/1,8-diiodoctane (97/3, by volume) and then kept overnight under vacuum. The thickness of the active layer was verified by a surface profilometer (Tencor Alpha-500, USA). A 10 nm MoO3 layer and a 100 nm Al layer were subsequently evaporated through a shadow mask to define the active area of the devices (≈16 mm2) and form a top anode. All of the fabrication processes were performed inside a controlled atmosphere of nitrogen dry box (Vacuum Atmosphere, USA) that contained less than 10 ppm oxygen and moisture. The power conversion efficiencies of the resulting PSCs were measured under 1 sun, AM 1.5G (air mass 1.5 global) spectrum from a solar simulator (Oriel model 91192, USA) set to 100 mW cm−2. The J–V characteristics were recorded with a Keithley 2410 source unit (USA). The external quantum efficiencies of the conventional solar cells were measured with a commercial photo modulation spectroscopic setup that included a xenon lamp, an optical chopper, a monochromator, and a lock-in amplifier operated by a PC computer. A calibrated Si photodiode was used as a standard. The work function was determined by scanning Kelvin probe system SKP 5050 (KP Technology).

3. Results and Discussions

Diethanolamine (0.5 g) was added to a solution of PmP36F-Br (153 mg) in a mixture of 20 mL of THF and 5 mL of DMF. The mixture was stirred vigorously for 48 h at 60 °C under an argon atmosphere. After cooling to room temperature, the mixture was poured into 250 mL of H2O. The resulting polymer was then

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collected and dried in vacuum to afford the final polymer as a white solid (146 mg, 87.6%). 1H NMR (400 MHz, DMSO, δ): 8.52– 8.26 (m, 2H), 7.75–7.36 (m, 8H), 3.41–3.21 (m, 8H), 2.46–1.8 (m, 12H), 1.17–0.5 (m, 20H). Anal. Calcd (%) for C39H54N2O4: C 76.17, H 9.17, N 4.55; Found: C 77.02, H 9.22, N 4.01.

3.1. Synthesis and Characterization The synthetic routes of the two key intermediates 1 and 2 are shown in Scheme 1. With the commercial available





compound 3,5-dibromophenol as the starting material, monomer 1 was achieved in a good yield of 70% by the typical Williamson ether reaction combined with 1,6-dibromohexane. In the presence of the strong base aqueous solution (50% NaOH), monomer 2 was readily obtained by the efficient nucleophilic substitution reaction with an excess amount of 1,6-dibromohexane. The chemical structures of intermediate 1 and 2 were confirmed by 1H NMR, 13C NMR, and elemental analysis (see the Experimental Section for details). Polymers PmP-NOH and PmP36F-NOH were prepared in two steps and their synthetic procedures are also outlined in the Scheme 1. First, under the Pd(PPh3)4-catalyzed polymerization of typical Suzuki coupling reaction, the precursor polymers PmP-Br and PmP36F-Br were successfully synthesized where the intermediates 1 and 2 as the functional monomer and the diborate ester compound as the comonomer, respectively. Second, the target polymers PmP-NOH and PmP36F-NOH were achieved by the post-treatment of PmP-Br and PmP36F-Br with diethenolamine in a mixture solvent (THF/DMF = 4:1). Some variations of characteristic 1H NMR peaks could be found.[25] For instance, in the high field region, the end CH2-Br group in PmP-Br exhibited a hydrogen resonance centered around 3.49 ppm while the signal was shifted to 3.37 ppm in PmP-NOH and a new chemical shift at 2.44 ppm was emerged, confirming the success of transformation. Polymers PmP-NOH and PmP36F-NOH, with the same polar diethanolamine end group tethered on the alky side chain, can be easily dissolved in methanol or ethanol. It was also found that the two polymers could be dissolved in THF, chloroform, and DMF. The weightaveraged molecular weights (Mw) of PmP-Br and PmP36FBr, the two precursors, are 5.9 kg mol−1 and 6.7 kg mol−1, with a polydispersity index (Mw/Mn) of 2.07 and 2.01, respectively. For PmP-NOH and PmP36F-NOH, comparable molecular weights to their precursors would be expected, based on the high yields of the transformation reactions. The chemical structures of PmP-Br, PmP36F-Br, PmP-NOH, and PmP36F-NOH were confirmed by 1H NMR and elemental analysis (see the Experimental Section for details). 3.2. Absorption Spectra UV–vis absorption spectra of solutions and films of PmPNOH and PmP36F-NOH are shown in Figure 1. PmP-Br and PmP36F-Br, the two precursors of PmP-NOH and PmP36F-NOH, respectively, are also included for comparison. In solution, the absorption spectra of PmP-NOH and PmP36F-NOH match those of their corresponding precursors very well. In Figure 1a, the PmP-NOH solution shows an absorption maximum (λabs) at 250 nm and an absorption edge at 335 nm. For PmP36F-NOH solution in Figure 1b, the λabs value and absorption edge are 258 and


Figure 1. Normalized UV–vis absorption spectra of solutions and films of a) PmP-NOH and b) PmP36F-NOH, with their precursors in solution for comparison.

340 nm, respectively. In electromagnetic spectrum of ultraviolet radiation and recommended by the ISO standard (ISO-21348),[25] the solution absorptions of PmP-NOH and PmP36F-NOH are mainly arranged in the middle ultraviolet region with a small portion in the near ultraviolet. In addition, the λabs values for the solid states of PmPNOH and PmP36F-NOH are 255 and 262 nm, respectively. From the solutions to the films, the red shifts are very limited of about 4–5 nm, indicating that there is no obvious increasing of main chain planarity for the two polymers. Moreover, the absorption edges of the films of PmP-NOH and PmP36F-NOH are located at 340 and 343 nm, respectively. Owing to the π-bridge unit of meta-phenylene, PmPNOH and PmP36F-NOH possess less conformation variation from solution to film and exhibit more limitation of agglomeration, which effectively inhibits the extension of effective conjugated length of polymers. It is worthy to point out that, in Figure 1b, the relative absorbance in the range from 300 to 340 nm for PmP36F-NOH, an alternating copolymer based meta-phenylene and 3,6-fluorene, is nearly half of that of poly(3,6-fluorene) homopolymer in the report by Wu and co-workers.[23e] From the absorption edge of polymer films, the optical bandgaps for the PmPNOH and PmP36F-NOH are 3.65 and 3.62 eV, respectively. The near ultraviolet absorptions of the films of PmP-NOH and PmP36F-NOH are beneficial for the light harvest of


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Table 2. Photovoltaic performances with PTB7:PC71BM = 1:1.5 as the active layer and using different cathodes.

polymer donors. It should be noted that it is the first time for the hydrophilic conjugated polymers to possess large optical bandgap more than 3.6 eV. 3.3. Energy Levels The HOMO levels of copolymers PmP-NOH and PmP36FNOH (Table 1) were obtained from the onsets of the oxidation potentials during the cyclic voltammetry measurement. The HOMO level for PmP-NOH is located at −6.19 eV, suggesting the polymer with highly antioxidation property. Polymer PmP36F-NOH exhibits a slightly high-lying HOMO level of −6.15 eV. The HOMO levels of the two polymers are much deeper than most of the reported hydrophilic conjugated polymers. The lowest unoccupied molecular orbital (LUMO) levels of the two copolymers are given from the corresponding optical bandgaps and HOMO levels. Due to comparable optical bandgaps and HOMO levels for PmP-NOH and PmP36F-NOH, their calculated LUMO levels of −2.54 and −2.53 eV, respectively, are very close.

Voc [V]

Jsc [mA cm−2]

FF [%]

PCE Rsa) [%] [Ω cm2]

None

0.40

14.7

49.9

2.97

56.4

PmP-NOH

0.76

15.4

73.2

8.58

24.0

PmP36F-NOH

0.75

16.3

68.0

8.33

41.4

a)

Series resistance.

We fabricated inverted PSCs with a device configuration of ITO/PmP-NOH or PmP36F-NOH /PTB7:PC71BM (95 nm)/ MoO3 (10 nm)/Al (100 nm), to investigate cathode interlayer properties of the two hydrophilic meta-phenylene-based polymers. For the PTB7-based active layer, a blend ratio of PTB7:PC71BM = 1:1.5 and 1,8-diiodoctane (DIO) of 3% as a solvent additive was utilized, which were the optimized conditions as demonstrated in literatures.[9b] As revealed in the previous reports,[9a,14a] an ideal cathode interlayer needs a good adhesion capability to both ITO and the subsequently deposited active layer. Also the erosion of the cathode interlayer by the solvent

for deposition of the active layer should be greatly suppressed. A cathode interlayer should realize ohmic contact between the ITO cathode and the active layer. We examined the adhesion properties of PmP-NOH and PmP36FNOH against the washing of chlorobenzene, a solvent for the active layer. It was found that spin-coatings of chlorobenzene on the two kinds of interlayers showed slight decreases of the absorbances of the polymer films. Thus the interlayers of PmP-NOH and PmP36F-NOH are fairly robust to endure solvent erosion during further spin-coating of the active layer. The diethanolamino end groups would endow PmP-NOH and PmP36F-NOH with strong interaction with metals or oxides because of highly polarity. Moreover, the surface property of ITO coated with PmP-NOH or PmP36F-NOH can be entirely modified as well. As measured by a Kelvin probe, the work function of ITO could be shifted from −4.7 to −4.359 eV and −4.358 eV for PmP-NOH and PmP36F-NOH, respectively. Under the modification of these two new interlayer materials, the decreased work function of ITO could match well with the LUMO level of PC71BM and improve the electron extraction from the active layer. An inverted solar cell without interlayer on ITO was also fabricated for comparison, in which the processing conditions for PTB7-based blend film are the same as

Figure 2. J–V characteristics of the inverted solar cells by using different cathode interlayers.

Figure 3. EQE curves of the inverted solar cells by using different cathode interlayers.

3.4. Photovoltaic Performance

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Interlayer





Figure 4. AFM images of the active layer of PTB7:PC71BM based on a) PmP-NOH and b) PmP36F-NOH. TEM images of the active layer of PTB7:PC71BM based on c) PmP-NOH and d) PmP36F-NOH.

those for the devices with interlayer. The measurements of the photovoltaic performances of the inverted solar cells were carried out under illumination of AM 1.5G simulated solar light at 100 mW cm−2. The current density– voltage (J–V) characteristics of the BHJ PSCs are shown in Figure 2. Their solar cell parameters are listed in Table 2. For the inverted solar cells with bare ITO cathode, the device showed Voc, Jsc, and FF of 0.40 V, 14.7 mA cm−2, and 49.9%, respectively. The calculated PCE was 2.97%, comparable to the reported PCE values for PTB7-based PSCs with bare ITO cathode.[14c,26] Introducing PmP-NOH and PmP36F-NOH as the cathode interlayer could largely enhance the photovoltaic performances. The PmP-NOHbased inverted PSC exhibited a Voc of 0.76 V and a FF of 73.2%, showing remarkable increases of 90% and 46%, respectively. Relatively, the increasing of Jsc was limited, and a value of 15.4 mA cm−2 was achieved. The improvements of the three photovoltaic parameters resulted in a high PCE of 8.58%. When PmP36F-NOH was utilized as the cathode interlayer, the PSC displayed a Voc of 0.75 V and a FF of 68.0%. Its Jsc value further increased to 16.3 mA cm−2. A good PCE of 8.33% was obtained. The external quantum efficiency (EQE) curves of the inverted solar cells are shown in Figure 3. In the range of 380 to


800 nm, PmP36F-NOH-based device showed slightly larger EQE. Generally, the Jsc values achieved by the PmPNOH- and PmP36F-NOH-based PSCs could match their EQE curves. In terms of the cathode modifications by the PmP-NOH and PmP36F-NOH interlayers, the elevations of Jsc and FF can prove the enhancement of the electron extraction from the acceptor phase to the ITO cathode and the decreasing of hole-electron recombinations in the active layer. Owing to similar tunings of ITO work function by PmP-NOH and PmP36F-NOH, comparable Voc values are achieved. As deduced from the inverse slope near Voc in the J–V curve, series resistances (Rs) of the inverted solar cells are also summarized in Table 2. Inserting an interfacial layer could obviously decrease the Rs value if compared with the bare ITO cathode. The surface morphologies of the PTB7:PC71BM blend films on the top of the PmP-NOH and PmP36F-NOH cathode interlayers were measured by atomic force microscope. The two blend films (Figures 4a,b) do not show obvious difference. Transmission electron microscope (TEM) also indicates that nanophase separations of the PTB7:PC71BM blend films are comparable (Figures 4c,d).


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Figure 5. The static water contact angle of a) bare ITO, b) ITO coated with PmP-NOH, c) ITO coated with PmP36F-NOH.

Moreover, we carried out measurements of contact angles for static water on a bare ITO substrate as well as PmP-NOH and PmP36F-NOH interlayers on ITO substrates (Figure 5), so as to compare their wetting properties. The contact angles for PmP-NOH and PmP36F-NOH are quite close of around 70°, remarkably higher than the 42.2° for the bare ITO surface. The results indicate that introducing a PmP-NOH or PmP36F-NOH interlayer can largely increase hydrophobicity. A PTB7:PC71BM blend film should be hydrophobic. Thus poor interfacial contact between the ITO substrate and the PTB7:PC71BM blend film could be modified if introducing such a cathode interlayer.

4. Conclusion In summary, using meta-phenylene and 3,6-fluorene as the building blocks for constructions of hydrophilic conjugated polymers with large bandgaps were demonstrated. The films of PmP-NOH and PmP36F-NOH exhibit absorption edges at 340 and 343 nm, respectively, corresponding to large optical bandgaps of 3.65 and 3.62 eV, respectively. A cathode interlayer with a large band is beneficial for incident solar light to reach the photoactive layer in an inverted PSC. The HOMO levels for PmP-NOH and PmP36FNOH are −6.19 and −6.15 eV, respectively. Introducing PmP-NOH and PmP36F-NOH as the cathode interlayer in PTB7-based inverted PSCs, high PCEs of 8.58% and 8.33%, respectively, were achieved, demonstrating that the two polymers are good interlayers for efficient PSCs. Acknowledgements: The authors gratefully acknowledge the financial support of the National Basic Research Program of China (973 program 2014CB643505), National Natural Science Foundation of China (21225418, 51173048, and 51403064), the Fundamental Research Funds for the Central Universities (2014ZB0001), China Postdoctoral Science Foundation (2014M552194), Doctoral Fund of Ministry of Education of China (20120172110008), and GDUPS (2013). Received: March 16, 2015; Revised: April 15, 2015; Published online: May 12, 2015; DOI: 10.1002/marc.201500163 Keywords: 3,6-fluorene; hydrophilic polymers; inverted polymer solar cells; meta-phenylene

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Wide band gap diketopyrrolopyrrole-based conjugated polymers incorporating biphenyl units applied in polymer solar cells.

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