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Triazine-based Polyelectrolyte as an Efficient Cathode Interfacial Material for Polymer Solar Cells Nallan Chakravarthi, Um Kanta Aryal, Kumarasamy Gunasekar, Ho-Yeol Park, YeongSoon Gal, Young Rae Cho, Seong Il Yoo, Myungkwan Song, and Sung-Ho Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03187 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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ACS Applied Materials & Interfaces

Triazine-based Polyelectrolyte as an Efficient Cathode Interfacial Material for Polymer Solar Cells Nallan Chakravarthi,†,§ Um Kanta Aryal,†,§ Kumarasamy Gunasekar,† Ho-Yeol Park,† YeongSoon Gal,⊥ Young-Rae Cho,± Seong Il Yoo,*,≠ Myungkwan Song,*,ǁ and Sung-Ho Jin*,† †

Department of Chemistry Education, Graduate Department of Chemical Materials, Institute for

Plastic Information and Energy Materials, Pusan National University, Busandaehak-ro 63-2, Busan 46241, Republic of Korea ⊥Polymer

Chemistry Laboratory, College of Engineering, Kyungil University, Gyeongsan, 712-

701, Republic of Korea ±

Division of Materials Science and Engineering, Pusan National University, Busandaehak-ro 63-

2, Busan 46241, Republic of Korea ≠

Department of Polymer Engineering, Pukyong National University, Sinseon-ro 365, Busan

608-739, Republic of Korea ǁ

Advanced Functional Thin Films Department, Surface Technology Division, Korea Institute of

Materials Science (KIMS), 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 642-831, Republic of Korea

Corresponding author. Tel.: +82 51 510 2727; Fax: +82 51 581 2348. *E-mail: [email protected] (S.I. Yoo), [email protected] (M. Song), [email protected] (S.H. Jin)

1

ACS Paragon Plus Environment


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KEYWORDS: phosphine oxide, triazine, cathode interfacial layer, alcohol soluble polyelectrolyte, polymer solar cells

ABSTRACT: A novel polyelectrolyte containing triazine (TAZ) and benzodithiophene (BDT) scaffolds containing polar phosphine oxide (P=O) and quaternary ammonium ions as pendant groups, respectively, in the polymer backbone (PBTAZPOBr) was synthesized to use it as a cathode interfacial layer (CIL) for polymer solar cell (PSC) application. Owing to the high electron affinity (EA) of the TAZ unit and P=O group, PBTAZPOBr could behave as an effective electron transport material. Due to the polar quaternary ammonium and P=O groups, the interfacial dipole moment created by PBTAZPOBr substantially reduced the work function (WF) of the metal cathode to afford better energy alignment in the device, thus enabling electron extraction and reducing recombination of excitons at the photoactive layer/cathode interface. Consequently, the PSC devices based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5b′]dithiophene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) system with PBTAZPOBr as CIL displayed simultaneously enhanced open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF), whereas the power conversion efficiency (PCE) increased from 5.42% to 8.04%, compared to that of the pristine Al device. The outstanding performance of PBTAZPOBr is attributed not only to the polar pendant groups of BDT unit but also to the TAZ unit linked with the P=O group of PBTAZPOBr, demonstrating that functionalized TAZ building blocks are very promising cathode interfacial materials (CIMs). The design strategy proposed in this work will be helpful to develop more efficient CIMs for high performance PSCs in the future. 2


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1. INTRODUCTION Both academically and industrially, bulk heterojunction (BHJ) polymer solar cells (PSCs) have attracted intense interest in the past decades due to their special advantages of lightweight, ease of fabrication, flexibility, suitability for large area coverage, and inexpensive solution process.1,2 Recently, power conversion efficiency (PCE) surpassing 10% has been reported for single junction and tandem solar cells by increasing the photovoltaic parameters such as open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) by systematic design of new polymers and small molecules,3-5 along with optimizing device fabrication technologies.6-8 In researching for very high performance PSCs, cathode or anode interfacial engineering has been recognized as an important approach for maximizing the PCEs.9,10 The interfacial barriers can be alleviated and the holes and electrons can be easily extracted by incorporating the anode/cathode interfacial layers (AILs/CILs) adjacent to the photoactive layer. Of particular importance is the development of stable low work function (WF) cathode interfacial materials (CIMs), in comparison to high WF anode interfacial materials, because the low WF metals, such as Ca, Ba, and Mg, are very sensitive to oxygen and moisture.11 With the aim of replacing these air sensitive cathodes, various inorganic metal oxides12,13 (e.g., ZnO, TiOx) and metal salts (LiF,14 CsF,15 Cs2CO3,16etc.) have been used as CIMs to effectively improve the performance of PSCs. Nevertheless, these inorganic materials possess poor interfacial contacts with adjacent organic photoactive layers due to basic incompatibilities between organic and inorganic materials, which may result in comparatively poor electron extraction efficiency.11-16 Moreover, 3



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most inorganic materials are sensitive to various factors such as surface adsorption of oxygen and UV-irradiation.17 Instead, water/alcohol soluble organic conjugated polyelectrolyte materials have attracted substantial researches and attentions in the field of PSCs due to their various advantages such as flexibility, orthogonal solubility and chemical divergence.18-20 The molecular structures of these organic materials can be tailored to control the optoelectronic properties.9,10 Among various conjugated polyelectrolytes, polyfluorene (PF) derivatives have been employed to enhance the photovoltaic performance.21,22 The suggested working mechanism for PF-based conjugated polyelectrolyte compounds mainly depends on the interactions between highly polar pendant groups in the CIL and cathode that generate interface dipole.23,24 This reduces the cathode WF and the interfacial energy barrier, thus raising the built-in potential (Vbi) value for the device meanwhile enhancing the Voc, short-circuit current density (Jsc) and fill factor (FF).25,26 Various reports showed that the polar pendant groups of PF derivatives can improve the interfacial contacts and lower the transport loss by doping the fullerene acceptors with the lonepair electrons/anions present in the polar ammonium or sulfonium groups.27,28 Besides the polar-charged pendant groups, the neutral phosphine oxide (P=O) group can also induce significant interface dipole and reduce the WF of the cathode by the transfer of electrons from the P=O group to the metal cathode via dipole interaction. Despite the potential application of compounds with this P=O group as CILs, they have attracted less research attention.29,30

Therefore,

developing

new

polyelectrolytes

and

understanding

their

structure−property relationships have become a hot topic for attaining exceptional CILs. However, to the best of our knowledge, no molecular entity with both P=O and quaternary ammonium ions as pendant polar functionalities present in a single molecular structure has been reported. Therefore, to understand the impact of combining both polar groups on the 4


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photovoltaic performance, we targeted our research on such diversified compounds for CILs with the study aim of developing very finely tuned materials to improve the photovoltaic performance. Additionally, triazine (TAZ) is a versatile framework quite broadly used in organic light-emitting diodes (OLEDs). Such TAZ-based compounds are considered to be effective electron transporting materials for OLEDs, due to the superior thermal stability of the π-skeleton and high electron affinity (EA) of the TAZ. In our previous work, we synthesized an alcohol-soluble small molecule

(2-(3-(diphenylphosphoryl)-2,4-difluorophenyl)-4,6-diphenoxy-1,3,5-triazine,

(PO-TAZ), where the TAZ unit is linked with the P=O group, and applied it as interlayers in PSC, perovskite solar cells and n-channel organic field-effect transistor devices.30 Our research suggested that functionalized TAZ derivatives are potentially good fragments for cathode modification materials in organic electronics. Here, we report the design and synthesis of a new polyelectrolyte based on TAZ, benzodithiophene (BDT) units in the polymer backbone and the P=O group and quaternary ammonium ions as a pendant groups (PBTAZPOBr). The strong withdrawing nature of the P=O group and TAZ moiety provides the polymer with large EA, thereby enhancing the energy level alignment and electron extraction ability. In addition, the P=O and quaternary ammonium ions not only ensure that the polyelectrolyte is easily processable from alcohol solvents, but also afford the cathode WF with versatile tunability. As a result, conventional PSC devices containing poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5b′]dithiophene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7):[6,6]-phenyl-C71-butyric

acid

methyl

ester

(PC71BM)

photoactive

layer

with

PBTAZPOBr exhibited the highest PCE value of 8.04%, which was significantly higher than those obtained for the devices without PBTAZPOBr (PCE of 5.42%), and with methanol 5



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(MeOH)/dimethyl sulfoxide (DMSO; 9:1, v/v) treatment (PCE of 7.04 %) and even with poly[(9,9-bis(3′-(N,N-dimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) with PCE of 7.55%. To the best of our knowledge, this is the first time a polyelectrolyte composed of both the P=O group and quaternary ammonium ions as a pendant groups in the polymer backbone was used as CIL in PSC devices. 2. RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes for the brominated PO-TAZ (monomer 3), PBTAZPO precursor polymer, and PBTAZPOBr polyelectrolyte are shown in Scheme 1, and the detailed procedures are described in the Supporting Information. Initially, 1,3,5trichlorotriazine was coupled with 2,4-difluorobenzeneboronic acid to generate compound 1, which was subsequently reacted with two equivalents of 4-bromophenol in the presence of diisopropylethyl amine to afford intermediate 2. The compound 2 was then treated with lithium diisopropylamide and chlorodiphenylphosphine to generate an intermediate which was further used without purification. Upon further oxidation of the unpurified intermediate using hydrogen peroxide, the required brominated PO-TAZ (monomer 3) was obtained. 4,8-Bis((6bromohexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethylstannane)31 (4) with two hexyl side chains was used to polymerize with 3 to obtain PBTAZPO with good film forming ability. Polymerization of 4 with 3 by using the Pd-catalyzed Stille coupling reaction led to the formation of the PBTAZPO precursor polymer. The weight average molecular weight (Mw) measured by gel permeation chromatography (GPC) was 6240 with a polydispersity index (PDI) of 1.7 for PBTAZPO. Finally, PBTAZPOBr polyelectrolyte was obtained by quarternization of PBTAZPO with trimethylamine in THF for 24 h.32 The Mw of PBTAZPOBr could not be determined by GPC, possibly because the ionic character of the PBTAZPOBr assists the polymer 6


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to form aggregates on the column fillers. However, the Mw value of PBTAZPOBr is expected to be almost similar to that of the analogous neutral precursor polymer PBTAZPO, as the quaternization of PBTAZPO was performed under mild conditions in order to prevent the decomposition of the polymer main chains during the reaction. The solubility behavior of the quaternized polymer PBTAZPOBr differed from that of the precursor neutral polymer PBTAZPO. Specifically, PBTAZPO is freely soluble in common organic solvents such as tetrahydrofuran (THF), chloroform (CHCl3), and chlorobenzene (CB), but cannot be dissolved in general polar solvents such as DMSO and MeOH. In contrast, PBTAZPOBr is readily soluble in MeOH, DMSO and DMF, but completely insoluble in low polar solvents, like THF, CHCl3, and toluene. In addition to the solubility of PBTAZPOBr in orthogonal solvents, it also possess good film formation ability. Therefore, PBTAZPOBr-based CILs can be introduced in single layer/multilayer solar cell devices by using solution-processing techniques without disturbing the adjacent photoactive layers as the light absorbing layers are generally processed from aromatic chlorinated/hydrocarbon solvents. Further, all the intermediates and final products were carefully purified, and the chemical structures of the products were characterized by spectroscopic methods. The 1H-NMR and

19

F-NMR spectra of the monomer 3 complies with the chemical

structure, as shown in Figures S1 and S2. The 1H-NMR spectrum of PBTAZPOBr is shown in Figure S3. The FT-IR spectra of both PBTAZPO and PBTAZPOBr, shown in Figure 1a, clearly reveal a sharply characteristic absorption at 1168 cm−1 for the P=O group. Further, the absorptions occurring at around 1580, 1510, 1430, and 1360 cm−1 indicate the characteristics of the s-triazine groups present in both PBTAZPO and PBTAZPOBr polymer backbones. The thermal properties of PBTAZPOBr were investigated by thermogravimetric analysis (TGA), as shown in Figure 1b. The temperature was increased from 40 to 800 °C at a heating rate of 10 °C 7



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min−1 in a nitrogen atmosphere. The onset temperature of decomposition for PBTAZPOBr was 260 °C, and this high thermal stability is beneficial as CILs for various organic electronic applications. Optical and Electrochemical Properties. The UV−vis absorption spectra of PBTAZPOBr in diluted MeOH solution and in film state are shown in Figure 2a and b, while those of PBTAZPO in diluted CHCl3 solution and in film state are shown in Figure S4a. The PBTAZPO and polyelectrolyte PBTAZPOBr solutions displayed similar absorption bands due to their unchanged polymer backbone, as there is only change in the chemical structure of PBTAZPO at the terminal site of the alkyl chains (-CH2Br to -CH2(NMe)3+Br-). The details of the UV-vis properties of PBTAZPOBr are summarized in Table 1. In diluted MeOH solution, PBTAZPOBr exhibited an absorption band at 287 nm, produced by the π−π* transition of the conjugated backbone, and four different absorption peaks centered at 351, 374, 395, and 449 nm, which may be attributed to the charge transfer between the BDT unit and the TAZ units present in the conjugated polymer backbone. In contrary to that obtained in solution, the absorption spectrum of PBTAZPOBr film showed a bathochromic shift due to intermolecular aggregation in the film state and the same red-shifted phenomenon was also observed in PBTAZPO. The optical band gap (Egopt) calculated from the absorption onset of the PBTAZPOBr thin film was 2.30 eV. To verify the orthogonal solvent properties of PBTAZPOBr, the UV-vis absorption spectra of the PBTAZPOBr thin films before and after rinsing with CB were measured, as shown in Figure 2b. The UV-vis absorption spectra shows no difference in the absorption profiles of the film before and after rinsing in CB, suggesting that CB solvent does not damage PBTAZPOBr layer, which is commonly used to deposit the successive photoactive layer.

8


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Cyclic voltammetry (CV) was used to investigate the electrochemical properties of PBTAZPOBr. As shown in Figure S4b and Table 1, PBTAZPOBr showed Eox and Ered values of 1.25 and -0.30 V, respectively. Therefore, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level values were calculated to be −5.64 and −4.09 eV, respectively. The LUMO of PBTAZPOBr was similar to that of PC71BM (in the range between −3.8 and −4.3 eV), signifying that PBTAZPOBr could form good ohmic contacts with fullerene derivatives.33-35 This could reduce the energy barriers of electron transport from the photoactive layers to the cathode and thus improve the electron extraction efficiency. Conversely, the adequate low lying HOMO compared to that of PTB7 is advantageous for blocking holes from donors in photoactive layers to reduce the recombination of holes and electrons in the PSCs.36 Photovoltaic Performance. To investigate the interfacial modification capability of PBTAZPOBr, BHJ PSCs with device architecture of ITO/PEDOT:PSS/PTB7:PC71BM/CIL/Al were initially fabricated and examined. The photoactive components (PBTAZPOBr) used for this study are shown in Figure 3a. The PBTAZPOBr film was prepared by spin casting its MeOH solution on the surface of the photoactive layer. For comparison, PSCs were also fabricated at the same condition with a PFN interlayer. The measured PBTAZPOBr concentration of 0.5 mg mL-1 (thickness: 11 nm) was the best among the various tested concentrations. The corresponding PSCs incorporating pristine Al, MeOH:DMSO (9:1, v/v) solvent treatment above the photoactive layer, and PFN/Al were also fabricated for comparative study. The device configuration and corresponding energy level diagram are shown in Figure 3b and c, respectively. The current density–voltage (J–V) characteristics for typical PSCs were measured under AM 1.5G irradiation (100 mW cm−2) and are shown in Figure 4a. The corresponding 9



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photovoltaic parameters, including Voc, Jsc, FF, and PCE, estimated from the J−V curves are summarized in Table 2. The control PSCs without any CIL film or any post-treatment (pristine Al) exhibited a PCE of 5.42% with a Voc of 0.66 V, a Jsc of 15.01 mA cm−2, and an FF of 54.11%, which is consistent with previous reports.30 MeOH treatment above the photoactive layer is reported to reduce the surface traps and increase the Vbi across the device, and thus simultaneously improve the Voc, Jsc, and FF for the corresponding PSCs.37 Accordingly, when the PTB7:PC71BM film was treated with MeOH, the PCE was increased from 5.40% to 7.04%, which is similar to previous literature reports.38 Conversely, after introducing a thin layer of PBTAZPOBr film between the photoactive layer and Al electrode, the Voc, Jsc and FF were further enhanced simultaneously, delivering much higher PCE of 8.04% with Voc of 0.76 V, Jsc of 15.77 mA cm−2, and FF of 66.88%. Table 1 shows that the PCE increases in the order according to the interfacial modifications: pristine Al ˂ MeOH:DMSO (9:1 v/v) ˂ PFN ˂ PBTAZPOBr. The PCE of PBTAZPOBr-based devices surpassed that of the devices with PFN CIL, which displayed a maximum PCE of 7.55% (Voc of 0.75 V, Jsc of 15.32 mA cm−2, and FF of 65.22%). These PCE results indicate that the designed polyelectrolyte can potentially be applied as a cathode interfacial modifier for effective PSCs. The enhanced photovoltaic performance is mainly ascribed to the increase of Voc, Jsc and FF. The improved performance of PBTAZPOBr probably stemmed from the increased photocurrent, due to the formation of better ohmic contact at the Al cathode and the better charge transport ability of PBTAZPOBr. The external quantum efficiency (EQE) spectra of PSCs with or without PBTAZPOBr are shown in Figure 4b. All the devices exhibited similar EQE spectra over the wavelength range from 300-800 nm. The integrated Jsc values estimated from the EQE curves were 14.70, 14.82, 15.07, and 15.39 mA cm-2 for the pristine Al, MeOH:DMSO-treated, PFN and PBTAZPOBr10


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based devices, respectively. The integrated Jsc values are very well matched with Jsc obtained from the J−V measurements. The EQE of the device with the PBTAZPOBr layer showed a noticeable enhancement in the wavelength region ranging from 550 to 700 nm over the pristine Al device, which ensued in higher Jsc. As shown in Table 2, the PBTAZPOBr-based device has lower series resistance (Rs) than that of the pristine Al or MeOH/DMSO-treated devices. These results clearly confirmed that the PBTAZPOBr-based device possessed the best ohmic contact at the photoactive layer/cathode interface.38 We fabricated the PSC devices with a broad thickness range of 7 to 50 nm to examine the dependency of PCE on thickness of PBTAZPOBr. Interestingly, when the thickness of PBTAZPOBr was increased to 15 and 20 nm, the PCE of the devices remained at the high values of 7.86% and 7.29%, respectively (Figure 5, Table 3). It is worth stating that energy level matching is also a key factor that governs the varied thickness dependence. Considering the fact that the LUMO level of PFN (-2.14 eV) is higher than PC71BM, it will certainly form a barrier for electron flow that blocks electrons to reach the metal cathode.20 This would result in the significant photocurrent drop for high thickness window of PFN. Whereas, the LUMO of PBTAZPOBr is comparable to that of PC71BM, electrons can be easily injected into the PBTAZPOBr and subsequent transport within the PBTAZPOBr film, so there is no electron barrier formed between the PC71BM/PBTAZPOBr interface. Therefore, even a highly thicker PBTAZPOBr can be applied to achieve high performance PSC devices without disturbing the electron extraction properties at the photoactive layer/cathode interface. Morphology. To understand the effect of PBTAZPOBr on the surface morphology, atomic force microscopy (AFM) was used and the images are shown in Figure 6. The morphology of MeOH:DMSO-treated PTB7:PC71BM films was reported elsewhere, so in the 11



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current work, we ignored the discussion of the morphology of the MeOH:DMSO-treated PTB7:PC71BM films.39 The pristine PTB7:PC71BM blend on ITO displayed a smooth surface root mean square (rms) roughness of 1.02 nm, whereas PTB7:PC71BM/PBTAZPOBr developed a slightly rougher surface with rms roughness of 1.95 nm. The well-separated morphological features from the microphase separation were reduced due to the improved rms value for PTB7:PC71BM/PBTAZPOBr film. The rough surface gives a better interface and enhances the contact area between PBTAZPOBr and the Al cathode, which enhances the electron extraction and collection.40 The water contact angle (θ) measurements were performed on the surfaces of PTB7:PC71BM, PTB7:PC71BM/MeOH:DMSO treatment and PTB7:PC71BM/PBTAZPOBr layers. As shown in Figure 7, the PTB7:PC71BM layer exhibited a θ value of ~112°. After spin casting of PBTAZPOBr and treatment of MeOH:DMSO on top of PTB7:PC71BM layer, the surface became more hydrophilic with θ value of 81° and 99°, respectively. The contact angle results indicated that the hydrophobic nature of the pristine PTB7:PC71BM photoactive film had been converted to hydrophilic nature after spin casting of the PBTAZPOBr layer. This suggest that the ionic components were piled up at the topmost organic surface, which is in good correlation with earlier observations for other water/alcohol soluble conjugated polymer multilayer structures.41 The surface modification implies that the PBTAZPOBr would favor charge injection and collection in the fabricated PSCs. The enhanced hydrophilic properties due to PBTAZPOBr revealed better wetting properties and compatibilities between the photoactive layer and metal electrode, which led to increased FF via the decreased contact resistance.42 Charge Transport Properties. To further investigate the improved device performance after the incorporation of the PBTAZPOBr layer, the J−V characteristics of the PSCs were measured in the dark, as shown in Figure 8a. The devices with pristine Al and MeOH:DMSO12


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treated exhibited a high leakage current in the reverse direction and more series resistance in the forward direction. However, the devices with PBTAZPOBr showed diode characteristics with a lower leakage current and lower series resistance, which supports the better charge transport and extraction abilities with the reduction of hole and electron recombination and thus the better overall PSC performance. Earlier research reports revealed that the reverse dark current depression enhanced Voc when using polyelectrolyte CILs in PSCs.43,44 Therefore, the reduction in reverse dark current may also be partially advantageous for the increase in Voc for PBTAZPOBr-based PSCs. In dark condition, the device with PBTAZPOBr showed a turn-on voltage in the range of 0.6-0.7 V, compared to that of 0.4-0.6 V for the pristine Al device, which indicates that the Vbi across the devices was increased when PBTAZPOBr was spin coated on the surface of the photoactive layer. Thus this increase of Vbi may be account for the improvement in Voc in the devices after the deposition of the PBTAZPOBr layer. It is worth stating that the enhanced Voc of PBTAZPOBr-based devices could also be endorsed to the decreased WF of the Al electrode. Further, it was well supported by the earlier research reports that CILs inserted between the photoactive layer and metal cathode would augment the Vbi of the PSCs via the dipole interaction between the ionic functional groups and the cathode surface, consequently leading to an improved Voc.9,10 Furthermore, the WF of the Al cathode could be reduced by the significant interface dipole induced by the transfer of electrons from the P=O group to the metal cathode via dipole interaction.29 Clearly, the PBTAZPOBr polyelectrolyte containing both pendant polar quaternary ammonium ions and the P=O group exhibits the firm interaction and dipole moment between PBTAZPOBr and Al electrode. In our previous report, we showed that the PO-TAZ molecule reduces the WF of ITO from 4.4 to 2.68 eV and of PTB7:PC71BM film from 4.08 to 3.56 eV. In this scenario, our designed 13



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polyelectrolyte also reduces the WF of ITO and metal electrodes, as PO-TAZ is also an integral part of the chemical structure of PBTAZPOBr. The schematic image of the interfacial dipole formation is illustrated in Figure 3d. This WF lowering contributed to increasing Vbi in the OPV device and thus facilitated the electron extraction. Therefore, the Vbi enhancement due to the interfacial dipole induced by the polar functionalities of PBTAZPOBr is the prime reason behind the increase in Voc.23 Moreover, both the possible reduction of WF of the metal electrode due to the interfacial dipole and the appropriate LUMO of PBTAZPOBr led to a better energy level alignment, which facilitated the electron transport/extraction and decreased the recombination loss, thereby improving Jsc and FF.45,46 To evaluate the effect of the PBTAZPOBr layer on the charge recombination process, the control device with pristine Al, MeOH:DMSO-treated device and the device with PBTAZPOBr/Al were analyzed by electrical impedance spectroscopy (EIS) and Mott-Schottky (M-S) plot, as shown in Figure 8b and Figure S5, respectively. Fitting the impedance spectra in Nyquist plots revealed the highly reduced bulk resistance (decrease in the radius of the Nyquist plot) of the PBTAZPOBr-based device compared to that of the device without PBTAZPOBr and the MeOH:DMSO-treated device. This suggests that the charge recombination was retarded due to the incorporation of the PBTAZPOBr layer. M-S plots of the PSC devices depict the significance in the Vbi due to the incorporation of CIL. To estimate Vbi, the C–V characteristics were examined using the following M-S equation: C-2 = 2(Vbi -V)/A2 e εr εoN. Here C, V, A, e, εr, εO and N indicate the capacitance for each bias voltage, bias voltage, device active area, charge, relative dielectric constant (εr =3), permittivity in vacuum and charge carrier concentration, respectively. The linear region under low forward bias is related to the formation of a Schottky contact, which was fitted to a plot of C-2 versus Voltage (V) and after extrapolation of C-2=0, Vbi 14


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was extracted. The pristine PTB7:PC71BM device (Figure S5) exhibited a Vbi of 0.72± 0.02 V, compared to improved values of 0.74± 0.02 V and 0.77±0.02 for MeOH:DMSO-treated and PBTAZPOBr-based devices, respectively. High Vbi plays a key role in effective exciton dissociation, which contributed to increasing Voc. Thus, the improved Vbi is advantageous for better Voc of the devices with PBTAZPOBr. It can be concluded that the charge recombination is suppressed and facilitates charge extraction due to the incorporation of PBTAZPOBr in accordance with the observed improvement of Jsc and FF. To analyze the obvious charge transport properties of the PBTAZPOBr, electron-only devices

with

the

configuration

of

ITO/Al/PBTAZPOBr/Al,

ITO/Al/PC71BM/Al

and

ITO/Al/PC71BM/PBTAZPOBr/Al were fabricated.19 The J−V characteristics of electron-only devices were measured, and the results were fitted by using a space-charge-limited current (SCLC) model, as shown in Figure 9. The respective electron mobility was calculated according to the Mott-Gurney equation, J = (9/8) εrε0µ (V2/L3) where, εr is the dielectric constant (εr = 3), ε0 is permittivity of free space. L is the thickness of the PC71BM and PBTAZPOBr, µ is the charge mobility, and V is the voltage drop across the device.47 The electron mobility of sole PC71BM and PBTAZPOBr was calculated as 1.82 × 10-4 and 9.45 × 10-5 cm2 V−1 s−1 respectively. The electron mobility of PC71BM/PBTAZPOBr (11 nm) was extracted as 3.05 × 10-4 cm2 V−1 s−1. The electron mobility was significantly improved by using PBTAZPOBr compared to that of the pristine films. These results signify that the devices with PBTAZPOBr have higher charge transportation capability than that of other devices, which matches with the higher FF and improved dark current. 3. CONCLUSIONS

15



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In summary, a novel polyelectrolyte, PBTAZPOBr, based on the TAZ moiety with P=O and quaternary ammonium ion as a pendant groups in the polymer backbone, was synthesized and applied as CIL in high performance PSCs. The features of the polymer backbone of the polyelectrolyte strongly influenced the properties. The introduction of a thin PBTAZPOBr layer at the interface between the PTB7:PC71BM photoactive layer and the Al cathode simultaneously improved Voc (from 0.66 to 0.76 V), Jsc (from 15.01 to 15.77 mA cm−2), and FF (from 54.11% to 66.88%), which noticeably enhanced PCE (from 5.42% to 8.04%), as compared to that of the PSC with the pristine Al cathode. The performance of the PSC with PBTAZPOBr was much better than that with the well-known PFN, due to the improved FF and Jsc. On one hand, the TAZ moiety provided good thermal stability and high EA, which improves the energy level alignment, and the electron transport and extraction properties. On the other hand, the polar P=O and quaternary ammonium ions enabled PBTAZPOBr processibility in alcoholic solvents, along with modulation of the electrode WF and better wettability toward the metal electrode. These useful characteristics of solution-processed PBTAZPOBr support the viability of large-area, roll-to-roll manufacturing techniques in the near future. These preliminary results demonstrate that appropriately functionalized TAZ and its analogues with different polar pendant functionalities are very promising materials for cathode interface modification. ■ ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Detailed synthesis procedure and corresponding NMR spectra, UV, CV, and molecular orbital diagrams of the polymer and polyelectrolyte, and the specific device fabrication and materials ■ AUTHOR INFORMATION 16


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Corresponding Author *Tel +82 51 510 2727, Fax +82 51 581 2348, e-mail: [email protected] (S.I. Yoo), [email protected] (M. Song), [email protected] (S.-H. Jin). Author Contributions §

These authors contributed equally to this work.

Notes The authors declare no competing financial interest. ■ ACKNOWLEDGEMENTS This work was supported by a grant fund from the National Research Foundation (NRF) (2016R1E1A1A01942593) by the Ministry of Science, ICT & Future Planning (MSIP) of Korea. ■ REFERENCES (1) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868−5923. (2) Wong, W.-Y.; Ho, C.-L. Organometallic Photovoltaics: A New and Versatile Approach for Harvesting Solar Energy Using Conjugated Polymetallaynes. Acc. Chem. Res. 2010, 43, 1246−1256. (3) Ha, Y.-H.; Lee, J. E.; Hwang M.-C.; Kim, Y. J.; Lee, J.-H.; Park, C. E.; Kim, Y.-H. A New BDT-Based Conjugated Polymer with Donor-Donor Composition for Bulk Heterojunction Solar Cells. Macromol. Res. 2017, 24, 457−462.

(4) Kim, J. Y.; Kim, Y. U.; Kim, H. J.; Um, H. A.; Shin, J.; Cho, M. J.; Choi, D. H. Side-chain Engineering of Diketopyrrolopyrrole-Based Copolymer Using Alkyl Ester Group for Efficient Polymer Solar Cell. Macromol. Res. 2016, 24, 980−985.

17



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

Page 18 of 37

(5) Somasundaram, S.; Jeon, S.; Park, S. Triphenylamine and Benzothiadiazole-Based D-AA’ and A’-A-D-D-A-A’ Type Small Molecules for Solution-Processed Organic Solar Cells.

Macromol. Res. 2016, 24, 226−234. (6) He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y. Single-Junction Polymer Solar Cells with High Efficiency and Photovoltage. Nat. Photonics 2015, 9, 174−179. (7) Zhao, J.-B.; Li, Y.-K.; Yang, G.-F.; Jiang, K.; Lin, H.-R.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (8) Chen, J.-D.; Cui, C.; Li, Y.-Q.; Zhou, L.; Ou, Q.-D.; Li, C.; Li, Y.; Tang, J.-X. SingleJunction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035−1041. (9) Duan, C. H.; Zhang, K.; Zhong, C.; Huang, F.; Cao, Y. Recent Advances in Water/Alcohol Soluble π-Conjugated Materials: New Materials and Growing Applications in Solar Cells. Chem. Soc. Rev. 2013, 42, 9071−9104. (10) Chueh, C.-C.; Li, C. Z.; Jen, A. K.-Y. Recent Progress and Perspective in SolutionProcessed Interfacial Materials for Efficient and Stable Polymer and Organometal Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 1160−1189. (11) Gupta, V.; Kyaw, A. K. K.; Wang, D. H.; Chand, S.; Bazan, G. C.; Heeger, A. J. Barium: An Efficient Cathode Layer for Bulk-Heterojunction Solar Cells. Sci. Rep. 2013, 3, 1965. (12) Kyaw, A. K.; Wang, D. H.; Gupta, V.; Zhang, J.; Chand, S.; Bazan, G. C.; Heeger, A. J. Efficient Solution-Processed Small Molecule Solar Cells With Inverted Structure. Adv. Mater. 2013, 25, 2397−2402.

18


Page 19 of 37

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


(13) You, J. B.; Chen , C. C.; Dou L. T.; Murase , S.; Duan, H. S.; Hawks, S. A.; Xu, T.; Son, H. J.; Yu , L.; Li, G.; Yang, Y. Metal Oxide Nanoparticles as an Electron-Transport Layer in HighPerformance and Stable Inverted Polymer Solar Cells. Adv. Mater. 2012, 24, 5267−5272. (14) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariiftci, N. S.; Denk, P. Effect of LiF/metal Electrodes on the Performance of Plastic Solar Cells. Appl. Phys. Lett. 2002, 80, 1288−1290. (15) Jiang, X.; Xu, H.; Yang, L.; Shi, M.; Wang, M.; Chen, H. Effect of CsF Interlayer on the Performance of Polymer Bulk Heterojunction Solar Cells. Sol. Energy Mater. Sol. Cells 2009, 93, 650−653. (16) Chen, F. C.; Wu, J. L.; Yang, S. S.; Hsieh, K. H.; Chen, W. C. Cesium Carbonate as a Functional Interlayer for Polymer Photovoltaic Devices. J. Appl. Phys. 2008, 103, 103721 (1) − 103721 (5). (17) Cho, N.; Yip, H. L.; Hau, S. K.; Chen, K. S.; Kim, T. W.; Davies, J. A.; Zeigler, D. F.; Jen, A. K.-Y. N-Doping of Thermally Polymerizable Fullerenes as an Electron Transporting Layer for Inverted Polymer Solar Cells. J. Mater. Chem. 2011, 21, 6956−6961. (18) Zhao, Y.; Xie, Z.-Y.; Qin, C.-J.; Qu, Y.; Geng, Y.-H.; Wang, L.-X. Enhanced Charge Collection in Polymer Photovoltaic Cells by Using an Ethanol-Soluble Conjugated Polyfluorene as Cathode Buffer Layer. Sol. Energy Mater. Sol. Cells 2009, 93, 604−608. (19) Wu, Z.; Sun, C.; Dong, S.; Jiang, X.-F.; Wu, S.; Wu, H.; Yip, H.-L.; Huang, F.; Cao, Y. n-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conjugated Polymers for HighPerformance Polymer Solar Cells. J. Am. Chem. Soc. 2016, 138, 2004−2013. (20) Sun, C.; Wu, Z.; Hu, Z.; Xiao, J.; Zhao, W.; Li, H.-W.; Li, Q.-Y.; Tsang, S.-W.; Xu, Y.X.; Zhang, K.; Yip, H.-L.; Hou, J.; Huang, F.; Cao, Y. Interface Design for High-Efficiency NonFullerene Polymer Solar Cells. Energy Environ. Sci. 2017, DOI: 10.1039/C7EE00601B. 19



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

Page 20 of 37

(21) Seo, J.-H.; Gutacker, A.; Sun, Y.-M.; Wu, H.-B.; Huang, F.; Cao, Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. Improved High-Efficiency Organic Solar Cells via Incorporation of a Conjugated Polyelectrolyte Interlayer. J. Am. Chem. Soc. 2011, 133, 8416−8419. (22) He, Z.-C.; Zhang, C.; Xu, X.-F.; Zhang, L.-J.; Chen, J.-W.; Wu, H.-B.; Cao, Y. Largely Enhanced Efficiency with a PFN/Al Bilayer Cathode in High Efficiency Bulk Heterojunction Photovoltaic Cells with a Low Bandgap Polycarbazole Donor. Adv. Mater. 2011, 23, 3086−3089. (23) Duan, C.-H.; Zhang, K.; Guan, X.; Zhong, C.-M.; Xie, H.-M.; Huang, F.; Chen, J.-W.; Peng, J.-B.; Cao, Y. Conjugated Zwitterionic Polyelectrolyte-Based Interface Modification Materials for High Performance Polymer Optoelectronic Devices. Chem. Sci. 2013, 4, 1298−1307. (24) Hoven, C. V.; Garcia, A.; Bazan, G. C.; Nguyen, T.-Q. Recent Applications of Conjugated Polyelectrolytes in Optoelectronic Devices. Adv. Mater. 2008, 20, 3793−3810. (25) Kang, H.; Hong, S.; Lee, J.; Lee, K. Electrostatically Self-Assembled Nonconjugated Polyelectrolytes as an Ideal Interfacial Layer for Inverted Polymer Solar Cells. Adv. Mater. 2012, 24, 3005−3009. (26) Li, C.-Z.; Chang, C.-Y.; Zang, Y.; Ju, H.-X.; Chueh, C.-C.; Liang, P.-W.; Cho, N.; Ginger, D. S.; Jen, A. K.-Y. Suppressed Charge Recombination in Inverted Organic Photovoltaics via Enhanced Charge Extraction by Using a Conductive Fullerene Electron Transport Layer. Adv. Mater. 2014, 26, 6262−6267. (27) Li, C.-Z.; Chueh, C.-C.; Ding, F.-Z.; Yip, H.-L.; Liang, P.-W.; Li, X.-S.; Jen, A. K.-Y. Doping of Fullerenes via Anion-Induced Electron Transfer and Its Implication for Surfactant Facilitated High Performance Polymer Solar Cells. Adv. Mater. 2013, 25, 4425−4430. 20


Page 21 of 37

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


(28) Cho, N.; Li, C.-Z.; Yip, H.-L.; Jen, A. K.-Y. In Situ Doping and Crosslinking of Fullerenes to Form Efficient and Robust Electron- Transporting Layers for Polymer Solar Cells. Energy Environ. Sci. 2014, 7, 638−643. (29) Jeon, S. O.; Kim, J.-H.; Kim, J. W.; Park, Y.; Lee, J. Y. Effect of Polarity of Small Molecule Interlayer Materials on the Open Circuit Voltage and Power Conversion Efficiency of Polymer Solar Cells. J. Phys. Chem. C 2011, 115, 18789–18794. (30) Chakravarthi, N.; Gunasekar, K.; Cho, W.; Long, D. X.; Kim, Y.-H.; Song, C. E.; Facchetti, A.; Song, M.; Noh, Y.-Y.; Jin, S.-H. A Simple Structured and Efficient Triazine-Based Molecule as an Interfacial Layer for High Performance Organic Electronics. Energy Environ. Sci. 2016, 9, 2595–2602. (31) Chen, Y.; Zhang, L.; Li, F.; Shen, L. Crosslinkable Fluorophenyl-Terminated Conjugated Polymer Based on Benzodithiophene and Dithienyl-Substituted Difluorobenzothiadiazole, and its Application in Solar Cells. Faming Zhuanli Shenqing CN 102504212 A, June 20, 2012. (32) Pu, K.-Y.; Pan, S .Y.-H.; Liu, B. Optimization of Interactions between a Cationic Conjugated Polymer and Chromophore-Labeled DNA for Optical Amplification of Fluorescent Sensors. J. Phys. Chem. B 2008, 112, 9295–9300. (33) Zhao, K.; Ye, L.; Zhao, W.-C.; Zhang, S.-Q.; Yao, H.-F.; Xu, B.-W.; Sun, M.-L.; Hou, J.-H. Enhanced Efficiency of Polymer Photovoltaic Cells via the Incorporation of a Water-Soluble Naphthalene Diimide Derivative as a Cathode Interlayer. J. Mater. Chem. C 2015, 3, 9565−9571. (34) Li, C.-Z.; Chueh, C.-C.; Yip, H.-L.; O’Malley, K. M.; Chen, W.- C.; Jen, A. K.-Y. Effective Interfacial Layer to Enhance Efficiency of Polymer Solar Cells via Solution-Processed Fullerene-Surfactants. J. Mater. Chem. 2012, 22, 8574−8578.

21



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

(35) Chen, L.; Xie, C.; Chen, Y.-W. Self-Assembled Conjugated Polyelectrolyte-Ionic Liquid Crystal Complex as an Interlayer for Polymer Solar Cells: Achieving Performance Enhancement via Rapid Liquid Crystal-Induced Dipole Orientation. Macromolecules 2014, 47, 1623−1632. (36) Liu, Y.; Page, Z. A.; Russell, T. P.; Emrick, T. Finely Tuned Polymer Interlayers Enhance Solar Cell Efficiency. Angew. Chem., Int. Ed. 2015, 54, 11485−11489. (37) Zhou, H.; Zhang, Y.; Seifter, J.; Collins, S. D.; Luo, C.; Bazan, G. C.; Nguyen, T.-Q.; Heeger, A. J. High-Efficiency Polymer Solar Cells Enhanced by Solvent Treatment. Adv. Mater. 2013, 25, 1646–1652. (38) Xue, Q.-F.; Hu, Z.-C.; Liu, J.; Lin, J.-H.; Sun, C.; Chen, Z.-M.; Duan, C.-H.; Wang, J.; Liao, C.; Lau, W.-M.; Huang, F.; Yip, H.-L.; Cao, Y. Highly Efficient Fullerene/Perovskite Planar Heterojunction Solar Cells via Cathode Modification with an Amino-Functionalized Polymer Interlayer. J. Mater. Chem. A 2014, 2, 19598−19603. (39) Kong, T.; Wang, H.; Liu, X.; Yu, J.; Wang, C. Improving the Efficiency of Bulk Heterojunction Polymer Solar Cells Via Binary-Solvent Treatment. IEEE J. Photovolt. 2017, 7, 214−220. (40) Wang, G.; Jiu, T.; Sun, C.; Li, J.; Li, P.; Lu, F.; Fang, J. Highly Efficient Organic Photovoltaics via Incorporation of Solution- Processed Cesium Stearate as the Cathode Interfacial Layer. ACS Appl. Mater. Interfaces 2014, 6, 833−838. (41) Park, J.; Yang, R.; Hoven, C. V.; Garcia, A.; Fischer, D. A.; Nguyen, T.-Q.; Bazan, G. C.; DeLongchamp, D. M. Structural Characterization of Conjugated Polyelectrolyte Electron Transport Layers by NEXAFS Spectroscopy. Adv. Mater. 2008, 20, 2491−2496.

22


Page 22 of 37

Page 23 of 37

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


(42) Li, M.-G.; Lv, J.-H.; Wang, L.; Liu, J.-G.; Yu, X.-H.; Xing, R.-B.; Wang, L.-X.; Geng, Y.H.; Han, Y.-C. An Alcohol-Soluble Perylene Diimide Derivative as Cathode Interfacial Layer for PDI-Based Nonfullerene Organic Solar Cells. Colloids Surf., A 2015, 469, 326−332. (43) He, C.; Zhong, C.; Wu, H.; Yang, R.; Yang, W.; Huang, F.; Bazan, G. C.; Cao, Y. Origin of the Enhanced Open-Circuit Voltage in Polymer Solar Cells via Interfacial Modification Using Conjugated Polyelectrolytes. J. Mater. Chem. 2010, 20, 2617−2622. (44) Yang, T.-B.; Wang, M.; Duan, C.-H.; Hu, X.-W.; Huang, L.; Peng, J.-B.; Huang, F.; Gong, X. Inverted Polymer Solar Cells with 8.4% Efficiency by Conjugated Polyelectrolyte. Energy Environ. Sci. 2012, 5, 8208−8214. (45) Mihailetchi, V. D.; Koster, L. J. A.; Blom, P. W. M. Effect of Metal Electrodes on the Performance of Polymer:Fullerene Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2004, 85, 970−972. (46) Hu, L.; Wu, F.; Li, C.; Hu, A.; Hu, X.; Zhang, Y.; Chen, L.; Chen, Y. Alcohol-Soluble nType Conjugated Polyelectrolyte as Electron Transport Layer for Polymer Solar Cells. Macromolecules 2015, 48, 5578−5586. (47) Murgatroyd, P. N. Theory of Space-Charge-Limited Current Enhanced by Frenkel Effect. J. Phys. D: Appl. Phys. 1970, 3, 151−156.

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Scheme 1. Synthetic scheme of monomer and polymers

24


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Page 25 of 37

(a)

PBTAZPO PBTAZPOBr

Transmittance (%)

(b)

100

PBTAZPOBr Weight (%)

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


80 60 40 o

Residual Weight (at 800 C) = 36.1%

1400

1200 1000 Wavenumber (cm-1)

20

200

400 600 o Temperature ( C)

800

Figure 1. (a) Fourier transform infrared spectrum of PBTAZPOBr, and (b) TGA of PBTAZPOBr.

25



0.25

(a) Solution

1.5 1.0 0.5 0.0

Absorbance (a.u.)

2.0 Absorbance (a.u.)

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

Page 26 of 37

Film

(b)

As prepared After rinsing

0.20 0.15 0.10 0.05 0.00

300

400

300

500

Wavelength (nm)

400 500 600 Wavelength (nm)

700

Figure 2. The UV−vis spectra of PBTAZPOBr (a) in solution (MeOH) state, and (b) PBTAZPOBr as a film on quartz plate before and after chlorobenzene rinsing.

26


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Figure 3. (a) Materials used for this study, (b) device configuration of conventional PSC, (c) energy level diagram of each component in conventional PSC, and (d) schematic diagram of the proposed formation of interfacial dipole at the photoactive layer/Al interface in conventional PSC.

27



0

80

(b)

(a) -4 -8

60 Pristine Al MeOH:DMSO/Al PBTAZPOBr/Al PFN/Al

EQE (%)

Current density (mA cm 2)

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

Page 28 of 37

40

Pristine Al MeOH:DMSO/Al PBTAZPOBr/Al PFN/ Al

-12 20

-16 0.0

0.2

0.4

0.6

0.8

0

400

500

600

700

800

Wavelength (nm)

Voltage (V)

Figure 4. (a) J−V and (b) EQE spectra of PTB7:PC71BM-based PSCs treated with MeOH/DMSO, with or without PBTAZPOBr, and with PFN under AM 1.5 G irradiation (100 mW cm−2).

28


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0

80

(a)

(b)

7 nm 11 nm 15 nm 20 nm 34 nm 50 nm

-4

-8

60 EQE (%)

Current density (mA cm-2)

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


-12

7 nm 11 nm 15 nm 20 nm 34 nm 50 nm

40

20 -16 0.0

0.2

0.4 Voltage (V)

0.6

0.8

0

400

500 600 700 Wavelength (nm)

800

Figure 5. J−V (a) and EQE (b) spectra of solar cells under AM 1.5G irradiation at 100 mW cm-2 with various PBTAZPOBr thicknesses.

29



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Figure 6. AFM images of PTB7:PC71BM (a) and PTB7:PC71BM/PBTAZPOBr (b) films.

30


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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 7. Static water contact angle images of (a) PTB7:PC71BM, (b) PTB7:PC71BM treated with MeOH/DMSO, and (c) PTB7:PC71BM after deposition of PBTAZPOBr on top of the photoactive layer.

31


100

(a)

10 1

(b)

2500

Pristine Al MeOH:DMSO/Al PBTAZPOBr/Al

Page 32 of 37

Pristine Al MeOH:DMSO/Al PBTAZPOBr/Al

2000 -ImZ (Ω )

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

Current density (mA cm-2)


0.1

1500

0.01

1000

1E-3

500

1E-4 1E-5 -1.2

-0.8

-0.4

0.0

0.4

0.8

0

1.2

0

1000

Voltage (V)

2000

3000

4000

ReZ (Ω)

Figure 8. (a) J−V characteristics of PTB7:PC71BM-based PSCs treated with MeOH/DMSO, with or without PBTAZPOBr in the dark and (b) Nyquist plots of devices with/without CILs under the AM 1.5 illumination.

32


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Figure 9. (a) Structure of electron only devices and (b) J−V curves of electron-only devices.

33



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Table 1. Optical and Electrochemical Properties of PBTAZPOBr CIL PBTAZPOBr

Mw (Da)

PDI

6240

1.7

Solution λmax (nm)a

Film λmax (nm)B

352

363

Egopt (eV)c

HOMO (eV)

LUMO (eV)

2.30

-5.64

-4.09

a

Absorption maxima measured from UV-visible absorption spectrum in MeOH solution.

b

Absorption maxima measured from UV-visible absorption spectrum in thin film state.

c

Estimated from the onset of the absorption in thin films (Egopt = 1240/λonset).

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Table 2. Photovoltaic Performance of PTB7:PC71BM-based PSCs With or Without CIL under AM 1.5G Irradiation (100 mW cm−2)

a

Voc (V)

Jsc (mA cm-2)

Pristine Al

0.66

15.01

54.11

3.52

5.42a (5.35)b

14.70

MeOH:DMSO/Al

0.74

15.20

62.03

2.32

7.04a (6.98)b

14.82

PFN/Al

0.75

15.32

65.22

2.05

7.55a (7.49)b

15.07

PO-TAZ/Alc

0.74

15.04

68.81

2.43

7.72a (7.64)b

14.60

PBTAZPOBr/Al

0.76

15.77

66.88

1.88

8.04a (8.00)b

15.39

Rs (Ω⋅cm2)

FF (%)

PCEa (%)b

Integrated Jsc (mA cm-2)

Device

Best device. bAverage values calculated over ten devices. cData taken from reference 30.

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Page 36 of 37

Table 3. Photovoltaic Performance of PTB7:PC71BM-based PSCs PBTAZPOBr of Various Thicknesses under AM 1.5G Irradiation (100 mW cm−2) PBTAZPOBr thickness (nm)

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

0

0.66

15.01

54.11

5.42

7

0.74

15.91

66.02

7.86

11

0.76

15.77

66.88

8.04

15

0.76

15.29

65.39

7.62

20

0.75

14.80

64.95

7.29

34

0.75

14.05

63.84

6.77

50

0.75

13.56

61.48

6.29

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GRAPHICAL ABSTRACT

37


Triazine-based Polyelectrolyte as an Efficient ... - ACS Publications

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