Novel Nonconjugated Polymer as Cathode Buffer ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24082−24089

www.acsami.org

Novel Nonconjugated Polymer as Cathode Buffer Layer for Efficient Organic Solar Cells Yunhao Cai,† Li Chang,§ Longzhen You,† BingBing Fan,† Hongliang Liu,*,‡ and Yanming Sun*,† †

School of Chemistry, Beihang University, Beijing 100191, P. R. China Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and Department of Chemistry, Lanzhou University, Lanzhou 730000, P. R. China Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on January 29, 2019 at 06:57:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: A novel nonconjugated polymer named poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt) (PAMPS-Na) was designed and synthesized. The PAMPS-Na has good solubility in polar solvents, such as water, methanol, and ethanol, which can be used as the cathode buffer layer in organic solar cells (OSCs) through solution processing without damaging the underlying active layer. Moreover, it was found that PAMPS-Na can significantly decrease the Al work function when it was modified with Al. To reveal its universal application in organic photovoltaic devices, a variety of photovoltaic donor materials, including two medium-band gap polymers, a wide-band gap polymer, and a small molecule donor were employed to fabricate OSCs. Compared with OSCs with Ca/Al electrode, the devices based on PAMPS-Na/Al exhibited higher photovoltaic performance, mainly because of the increased short-circuit current. Additionally, OSCs with PAMPS-Na/Al displayed better ambient stability than devices with Ca/ Al. It is also interesting to find that the performance of the devices can tolerate a wide change of PAMPS-Na’s thickness, enabling the potential for large-scale fabrication of OSCs. The results suggest that PAMPS-Na is a promising candidate as the cathode buffer layer to improve the efficiency and stability of OSCs. KEYWORDS: organic solar cells, cathode buffer layer, nonconjugated polymer, efficiency, stability

■

increase the work function (WF) of anode (usually ITO).22,23 Low WF active metals such as Ba and Ca have been used as the cathode at the expense of sensitivity toward environmental moisture and oxygen. Accordingly, high WF metals including Ag, Al, or Au are adopted to improve the stability.24 On the other hand, insertion of solution-processable and stable cathode buffer layer has been identified to be another effective strategy to realize high performance and stable OSCs.17,25−28 A number of n-type metal oxide semiconductors, such as zinc oxide (ZnO),29,30 titanium dioxide (TiO2),31,32 metal salts including lithium fluoride (LiF),33−35 cesium fluoride (CsF),36 cesium carbonate (Cs2CO3),37,38 and so forth have been used as electron transport layer. Unfortunately, these inorganic cathode buffer layers are not well compatible with the organic active layer, resulting in inferior electron extraction ability. Furthermore, most of them are usually susceptible to degradation by surface adsorption of oxygen and UVirradiation.16

INTRODUCTION The key advantages of bulk-heterojunction (BHJ) organic solar cells (OSCs) including low cost, light weight, flexibility, and ease of fabrication have distinguished this technology as promising alternative to silicon-based solar cells.1−4 Impressive advances in power conversion efficiencies (PCEs) owing to combined improvements in design of novel photovoltaic materials, use of innovative device structure, engineering of interfacial layer, and optimization of BHJ morphology have been recently achieved.5−13 In a standard BHJ solar cell, the active layer is typically sandwiched between a transparent tindoped indium oxide (ITO) and a metal electrode.14 In pursuit of high-performance OSCs, one of the main challenges is to control the interface between the active layer and the electrodes.15−18 The contact barrier at the interfaces must be efficiently reduced to form a good ohmic contact.19 The broad selection of interface materials can not only establish a tunable energy level alignment between the contact interface to favor the charge extraction and transportation but also increase interfacial stability of OSCs eventually.16,20,21 In conventional devices, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is commonly used as the anode buffer layer to © 2018 American Chemical Society

Received: May 10, 2018 Accepted: June 27, 2018 Published: June 27, 2018 24082

DOI: 10.1021/acsami.8b07691 ACS Appl. Mater. Interfaces 2018, 10, 24082−24089

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Chemical structures of the donor materials and PAMPS-Na. (b) Energy level diagram of each component material used in the fabrication of OSCs. (c) UPS structure of Al and Al/PAMPS-Na.

materials, optimum PCEs of 9.09 and 7.94% were obtained, respectively, by PAMPA-Na-based devices, which are greatly improved (13 and 8% enhancement) in comparison with those (8.05 and 7.30%) of the control devices. It should be noted that the performance of the devices using PAMPS-Na is relatively insensitive to the thickness of the cathode buffer layer. In addition, the device with PAMPS-Na as the cathode buffer layer was proved to possess better stability in ambient in relative to device without it. Our work has successfully manifested that PAMPS-Na is a high-efficient and costeffective cathode buffer layer for use in OSCs.

In recent years, various conjugated polymers such as poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)alt-2,7-(9,9-dioctylfluorene)] (PFN),39,40 ethoxylated polyethylenimine, and so forth41 have emerged as promising interfacial layer materials in OSCs.16,42−45 However, the acquisitions of some of these conjugated polymers require complicated synthesis routes along with a time-consuming purification procedure.46,47 Therefore, it is highly desirable to develop a new buffer layer with low processing cost and simple synthesis route, enabling the potential commercialization of OSCs. In this contribution, we reported high-performance OSCs via employing poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt) (PAMPS-Na) as the cathode buffer layer. This nonconjugated polymer has good solubility in polar solvents such as water, methanol and alcohol. More importantly, PAMPS-Na can be easily synthesized with a high yield via simple radical polymerization. To certify the universality of PAMPS-Na on the photovoltaic performances of OSCs, BHJ devices adopting two medium-band gap materials PTB7, PTB7-Th, a wide-band gap polymer PBDTST1, a small molecule p-DTS(FBTTh2)2 as electron donors, and PC71BM as electron the acceptor were fabricated. The sodium salt solution was simply prepared by spin-coating its methanol solution onto the active layer at room temperature, without any thermal annealing or any other post-treatment. It was interesting to find that the new cathode buffer layer can effectively improve the J sc and further the PCE of corresponding devices. The PCE of PTB7:PC71BM-based device with a PAMPS-Na buffer layer reached 8.24%, which is 13% increased in comparison with traditional Ca/Al electrode. For OSC using PTB7-Th as the electron donor, an improved PCE of 9.16% was achieved after insertion of the new cathode buffer layer, whereas the control device delivers a PCE of 8.32%. When adopting wide-band gap D−A type polymer PBDT-ST1 and small molecule p-DTS(FBTTh2)2 as donor

■

EXPERIMENTAL SECTION

Device Fabrication. OSCs were fabricated with a conventional architecture of ITO/PEDOT:PSS/active layer/PAMPS-Na or Ca/Al. ITO-coated glasses were sequential ultrasonic cleaned for 15 min in soapy water, deionized water, acetone, and isopropyl alcohol. ITO glasses were dried overnight in a vacuum oven and treated with UV ozone for 30 min before use. A 40 nm thick PEDOT:PSS (Heraeus Clevios P VP A 4083) layer was spin-cast on top of the ITO substrates, followed by annealing at 150 °C for 10 min in air. The active layers were deposited by spin coating the dichlorobenzene solution of PTB7:PC71BM (1:1.5 w/w, PTB7 is 10 mg mL−1), the chlorobenzene solution of PTB7-Th:PC71BM (1:1.5 w/w, PTB7-Th is 10 mg mL−1), the chloroform solution of PBDT-ST1:PC71BM (1:1 w/w, PDBT-T1 is 7 mg mL−1), and the chlorobenzene solution of pDTS(FBTTh2)2:PC71BM (1.4:1 w/w, p-DTS(FBTTh2)2 is 20 mg mL−1) on the top of PEDOT:PSS for 40 s, respectively. Subsequently, a thin layer of PAMPS-Na (1.5 mg mL−1 in methanol) was spincoated for 40 s on the top of the active layer and functioned as a cathode buffer layer, without any additional thermal annealing or post-treatment. The PAMPS-Na methanol solution was simply made by adding the PAMPS-Na powder into the methanol solvent and then stirred for 2 hours. Finally, the substrates were transferred to a vacuum chamber and the metal cathode (Al or Ca/Al) was thermally deposited at a vacuum level of 5 × 10−5 Pa. The photoactive area of the devices is 4.8 mm2, calibrating by a shadow mask. 24083


Research Article


Figure 2. (a−d) J−V curves of PTB7/PTB7-Th/PBDT-ST1/p-DTS(FBTTh2)2:PC71BM solar cells with and without PAMPS-Na layer and (e−h) the corresponding EQE curves of these OSCs.

Table 1. Device Parameters of the OSCs with and without PAMPS-Na active layer PTB7:PC71BM PTB7-Th:PC71BM PBDT-ST1:PC71BM p-DTS(FBTTh2)2:PC71BM

electrode Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al

Voc (V) 0.79 0.79 0.80 0.80 0.89 0.89 0.80 0.80

± ± ± ± ± ± ± ±

0.003 0.007 0.004 0.006 0.006 0.005 0.003 0.004

Jsc (mA cm−2) 14.25 15.64 14.57 15.76 12.72 13.62 13.41 14.45

± ± ± ± ± ± ± ±

0.12 0.16 0.26 0.23 0.20 0.18 0.21 0.15

PCE (%)a

FF 0.63 0.64 0.69 0.70 0.68 0.73 0.65 0.67

± ± ± ± ± ± ± ±

0.02 0.02 0.01 0.01 0.02 0.01 0.02 0.01

7.29 8.24 8.32 9.16 8.05 9.09 7.30 7.94

(7.09 (7.93 (8.04 (8.82 (7.70 (8.85 (6.99 (7.75

± ± ± ± ± ± ± ±

0.13) 0.16) 0.18) 0.10) 0.18) 0.11) 0.16) 0.12)

a

The average PCE values were both obtained from 10 devices.

at 60 °C for 12 h. The PAMPS-Na powder was obtained by a subsequent vacuum freeze-drying process. As displayed in Figure S1, the PTB7-Th:PC71BM and PTB7Th:PC71BM/PAMPS-Na films show nearly identical UV−vis absorption spectra, which demonstrates that the newly developed cathode buffer layer has no impact on the absorption of the active layer. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of PAMPS-Na were determined by CV, the HOMO and LUMO of PAMPS-Na were calculated to be −6.51 and −3.24 eV, respectively (Figure S2). Ultraviolet photoelectron spectroscopy (UPS) was performed to clarify the WF change of PAMPS-Na-modified Al electrode. As shown in Figure 1c, the bare Al exhibited a typical WF of −4.30 eV, in accordance with the value reported in the literature.48 Evidently, upon a thin layer of PAMPS-Na deposited on the Al substrate, the WF of Al was shifted to −3.51 eV, which is thought to be favorable for electron collection. The effects of PAMPS-Na on the photovoltaic performance of OSCs were investigated with a conventional structure of ITO/PEDOT:PSS/active layer/PAMPS-Na/Al, where PTB7, PTB7-Th, PBDT-ST1, and p-DTS(FBTTh2)2 were used as the donors and PC71BM was used as the acceptor. Figure 2 presents current density−voltage (J−V) curves of the devices with different cathodes under the illumination of AM 1.5G, 100 mW/cm2. The resulted Jsc, Voc, FF, and PCE values, as determined from the J−V curves are summarized in Table 1. OSCs based on PTB7 with traditional Ca/Al cathode showed a PCE of 7.29%, with a Jsc of 14.37 mW/cm2, a Voc of 0.79 V,

Characterization and Measurements. In UV−vis absorption measurement, a Hitachi (model U-3010) UV−vis spectrophotometer was used. Cyclic voltammetry (CV) measurement was performed in a conventional three-electrode cell using a Pt plate as the working electrode, Pt wire as the counter electrode, and Ag/Ag+ electrode as the reference electrode on a Zahner IM6e Electrochemical Workstation in a tetrabutylammonium hexafluorophosphate (Bu4NPF6) (0.1 M) acetonitrile solution at a scan rate of 20 mV s−1. Tapping mode atomic force microscopy (AFM) images of the films were acquired from Digital Instruments Dimension V SPM System with NanoScope V controller. The current density−voltage (J−V) characteristics of the devices were measured with a Keithley 2400 source measurement unit under the illumination of AM 1.5G, 100 mW/cm2 with an Enlitech solar simulator (Taiwan, China). The light intensity was calibrated with a standard silicon solar cell. The external quantum efficiencies (EQEs) of solar cells were analyzed using a solar cell quantum efficiency measurement system PV measurement QEX10. The layer thickness was measured using an Ambios Technology XP-2 surface profilometer.

■

RESULTS AND DISCUSSIONS

PAMPS-Na is synthesized by free-radical polymerization (Scheme S1). The polymerization was performed in distilled water with AMPS as the monomer and APS as the initiator with monomer and initiator weight ratio of 100:1. The AMPS was first converted into the sodium form as obtained by previous titration of AMPS (5 g, 24.2 mmol) in distilled water by dropwise addition of NaOH solution until adjusting the pH at 10. The reaction mixture was purged with the nitrogen gas for 30 min, and polymerization was carried out under stirring 24084


Research Article

ACS Applied Materials & Interfaces Table 2. Hole Mobilities and Electron Mobilities of the Devices with Ca/Al and PAMPS-Na/Al Electrode active layer PTB7:PC71BM PTB7-Th:PC71BM PBDT-ST1:PC71BM p-DTS(FBTTh2)2:PC71BM

electrode

hole mobility (cm2 V−1 s−1)

Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al

8.24 1.23 1.09 1.58 6.51 1.63 7.59 1.16

× × × × × × × ×

−4

10 10−3 10−3 10−3 10−4 10−3 10−4 10−3

electron mobility (cm2 V−1 s−1) 8.36 2.19 7.51 2.50 9.40 3.06 9.70 1.98

× × × × × × × ×

10−4 10−3 10−4 10−3 10−4 10−3 10−4 10−3

Figure 3. Dark currents of the (a) PTB7:PC71BM, (b) PTB7-Th:PC71BM, (c) PBDT-ST1:PC71BM, and (d) p-DTS(FBTTh2)2:PC71BM solar cells with or without PAMPS-Na layer.

The EQEs of these OSCs with Ca/Al or PAMPS-Na/Al cathode are presented in Figure 2e−h. Similar curves cover a wide spectral range but higher EQEs of devices with a thin PAMPS-Na were observed. OSCs based on PTB7:PC71BM displayed the highest EQE of 64% at 560 nm using traditional Ca/Al cathode, whereas the device with PAMPS-Na showed improved light response in the range of 550−800 nm, showing a maximum EQE of 69%. For the PTB7-Th:PC71BM-based and PBDT-ST1:PC71BM-based devices, the shape of the EQE curves of the device with PAMPS-Na were almost the same with those of the control devices across the entire wavelength range. However, higher light responses in the range of 340− 720 nm and 360−650 nm along with highest EQE values of 71 and 77% were achieved for PTB7-Th- and PBDT-ST1-based devices with a thin PAMPS-Na layer. Besides, in comparison with the devices with Ca/Al cathode, the EQE spectra of OSCs based on p-DTS(FBTTh2)2:PC71BM with a PAMPS-Na exhibits a notable enhancement in wavelength ranges 345− 430 nm and 490−600 nm. These results were well consistent with the current values from the J−V measurements and the higher Jsc achieved in devices inserting a PAMPS-Na layer than the control devices. On account of investigating the charge-transport properties, we fabricated hole-only and electron-only devices with configuration of ITO/MoOx/donor:PC71BM (with or without PAMPS-Na)/MoOx/Al and ITO/Al/donor:PC71BM (with or

and an FF of 65%; the introduction of the PAMPS-Na layer led to obvious increment in Jsc to 15.80 mW/cm2 and PCE to 8.24% with a Voc of 0.79 V and an FF of 66%. In the case of PTB7-Th-based device, a PCE of 8.32% with a Jsc of 14.83 mW/cm2, a Voc of 0.80 V, and an FF of 70% was obtained using a Ca/Al cathode. Devices with a thin PAMPS-Na cathode interlayer showed a higher PCE of 9.16%, with an enhanced Jsc of 15.99 mW/cm2 and a Voc of 0.80 V and an FF of 71%. Similarly, PAMPS-Na displayed a remarkable performance in the OSCs based on wide-band gap D−A copolymer PBDT-ST1. The control device showed a PCE of 8.05%, a Voc of 0.89 V, and an FF of 70%. With PAMPS-Na as the electron transport layer, same Voc of 0.89 V was obtained, whereas both Jsc and FF were improved to 13.80 mW/cm2 and 74%. Finally, a high PCE of 9.09% was achieved. Furthermore, when using a small molecule p-DTS(FBTTh2)2 as the electron donor and PC71BM as the acceptor, the device without the PAMPS-Na cathode buffer layer exhibited a PCE of 7.30%, with a Jsc of 13.62 mW/cm2, a Voc of 0.80 V, and an FF of 67%. Relative to the control device, an improved Jsc of 14.60 mW/cm2, an FF of 68%, and a Voc of 0.80 V were produced, leading to a PCE of 7.94%. Apparently, comparing the PAMPS-Na-based devices with the Ca/Al-based OSCs, the higher PCEs were mainly ascribed to the increased Jsc and FF, which partially resulted from the high and more balanced charge carrier mobility in PAMPS-Na/Al-based devices. 24085


Research Article


Figure 4. AFM height images of the solar cells (a−d) without PAMPS-Na and (e−h) with PAMPS-Na layer.

Figure 5. Degradation of normalized (a) Voc, (b) Jsc, (c) FF, and (d) PCE values of PTB7-Th:PC71BM-based devices with and without PAMPS-Na cathode buffer layer.

Na as cathode interlayer were significantly suppressed in comparison with the bare Ca/Al control device. This result manifests that PAMPS-Na possesses favorable hole-blocking properties and prevents hole current leakage through the cathode interface at reversed bias, giving rise to corresponding devices’ performance.49 While in the region over 1 V, higher injection current was observed from PAMPS-Na-based devices, which is indicative of improved electron extraction from the Al electrode. To gain deeper insights into the improved Jsc, AFM with tapping mode was subsequently carried out to study the surface morphologies of the films with or without a PAMPSNa modifier. As shown in Figure 4, the pristine PTB7:PC71BM, PTB7-Th:PC 71 BM, PBDT-ST1:PC 71 BM, and p-DTS(FBTTh2)2:PC71BM films displayed a roughness of 0.99, 3.33, 1.53, and 2.48 nm, respectively. After deposition of PAMPS-Na upon the active layer, the surfaces became smoother and more homogeneous with a decreased roughness of 0.88, 1.02, 1.07, and 2.20 nm, respectively, indicating that PAMPS-Na has good film forming properties on these blends. In consequence, a better charge transport is assumed from good interfacial adhesion between the Al cathode and the

without PAMPS-Na)/Ca/Al, respectively. As plotted in Figures S3 and S4, devices with traditional Ca/Al cathode exhibited relatively low hole mobilities of 8.24 × 10−4, 1.09 × 10−3, 6.51 × 10−4, and 7.59 × 10−4 cm2 V−1 s−1 and electron mobilities of 1.63 × 10−3, 1.46 × 10−3, 2.39 × 10−3, and 1.90 × 10−3 cm2 V−1 s−1 for PTB7:PC71BM, PTB7-Th:PC71BM, PBDT-ST1:PC71BM, and p-DTS(FBTTh2)2:PC71BM blends, respectively. After incorporation of PAMPS-Na as the cathode buffer layer, all electron mobilities were remarkably enhanced. The electron mobilities of these devices were increased to 2.19 × 10−3, 2.50 × 10−3, 3.06 × 10−3, and 2.49 × 10−3 cm2 V−1 s−1, respectively. Besides, improved hole mobilities of 1.23 × 10−3, 1.58 × 10−3, 1.63 × 10−3, and 1.16 × 10−3 cm2 V−1 s−1 were also obtained in devices with a PAMPS-Na thin layer. The higher carrier mobilities and more balanced carrier transport in the active layers help explain the better Jsc and PCEs achieved in OSCs using PAMPS-Na as a cathode buffer layer. Detailed data are concluded in Table 2. To further reveal the origin of the improvement in Jsc and FF, the J−V characteristics of all devices were also measured under dark conditions (Figure 3). Obviously, in the region −1.5 to 0 V, the reverse currents of the devices using PAMPS24086



■

photoactive layers, which certainly accounts for the observed superior device performance.50,51 We also investigated the effect of cathode buffer layer thickness on the photovoltaic performance of OSCs based on PTB7-Th:PC71BM with PAMPS-Na thickness ranging from 9 to 24 nm. Figure S4 displays the J−V characters, and corresponding photovoltaic parameters of the devices are listed in Table S1. It was found that the PCE remains 8.63% when the thickness of PAMPS-Na is 19 nm. On further increase of the thickness of the new cathode buffer layer, the performance of the device decreases gradually. However, when the thickness of PAMPS-Na is 24 nm, a PCE of 7.51% was still achieved. More importantly, it is generally accepted that interfacial buffer layers play a key role in both the photovoltaic performance and stability of OSCs.52 The stability of devices based on PTB7-Th:PC71BM in the ambient were measured. OSCs were stored and periodically measured in air for 30 days. It was found that OSCs based on PAMPS-Na showed better stability than devices without it. The device without PAMPSNa experienced about 64% degradation in PCE after being exposed to ambient in 30 days, whereas OSCs based on PAMPS-Na can still maintain 77% of its initial efficiency after 30 days. This result testifies the great promise of PAMPS-Na’s application in OSCs (Figure 5).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.L.). *E-mail: [email protected] (Y.S.). ORCID

Hongliang Liu: 0000-0002-6781-3546 Yanming Sun: 0000-0001-7839-3199 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (NSFC) (nos. 51473009, 21674007, and 21734001), the International Science & Technology Cooperation Program of China (2014DFA52820), the Academic Excellence Foundation of BUAA for PhD Students, and the Youth Innovation Promotion Association, CAS (2016026).

■

REFERENCES

(1) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666−12731. (2) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153−161. (3) Heeger, A. J. 25th anniversary article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26, 10−28. (4) Thompson, B. C.; Fréchet, J. M. J. Polymer-Fullerene Composite Solar Cells. Angew. Chem., Int. Ed. Engl. 2008, 47, 58−77. (5) Yan, Y.; Liu, X.; Wang, T. Conjugated-Polymer Blends for Organic Photovoltaics: Rational Control of Vertical Stratification for High Performance. Adv. Mater. 2017, 29, 1601674. (6) Li, W.; Ye, L.; Li, S.; Yao, H.; Ade, H.; Hou, J. A High-Efficiency Organic Solar Cell Enabled by the Strong Intramolecular Electron Push-Pull Effect of the Nonfullerene Acceptor. Adv. Mater. 2018, 30, 1707170. (7) Sun, C.; Pan, F.; Bin, H.; Zhang, J.; Xue, L.; Qiu, B.; Wei, Z.; Zhang, Z.-G.; Li, Y. A Low Cost and High Performance Polymer Donor Material for Polymer Solar Cells. Nat. Commun. 2018, 9, 743. (8) Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.; Huang, F.; Qu, Y.; Ma, W.; Yan, H. Terthiophene-Based D-A Polymer with an Asymmetric Arrangement of Alkyl Chains that Enables Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 14149−14157. (9) Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng, H.; Chen, Y. Solution-Processed Organic Solar Cells Based on Dialkylthiol-Substituted Benzodithiophene Unit with Efficiency near 10%. J. Am. Chem. Soc. 2014, 136, 15529−15532. (10) Liao, S.-H.; Jhuo, H.-J.; Yeh, P.-N.; Cheng, Y.-S.; Li, Y.-L.; Lee, Y.-H.; Sharma, S.; Chen, S.-A. Single Junction Inverted Polymer Solar Cell Reaching Power Conversion Efficiency 10.31% by Employing Dual-Doped Zinc Oxide Nano-Film as Cathode Interlayer. Sci. Rep. 2014, 4, 6813. (11) Lin, Y.; Wang, J.; Zhang, Z.-G.; Bai, H.; Li, Y.; Zhu, D.; Zhan, X. An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170−1174. (12) Huo, L.; Liu, T.; Sun, X.; Cai, Y.; Heeger, A. J.; Sun, Y. SingleJunction Organic Solar Cells Based on a Novel Wide-Bandgap Polymer with Efficiency of 9.7%. Adv. Mater. 2015, 27, 2938−2944. (13) Liu, T.; Pan, X.; Meng, X.; Liu, Y.; Wei, D.; Ma, W.; Huo, L.; Sun, X.; Lee, T. H.; Huang, M.; Choi, H.; Kim, J. Y.; Choy, W. C. H.; Sun, Y. Alkyl Side-Chain Engineering in Wide-Bandgap Copolymers Leading to Power Conversion Efficiencies over 10%. Adv. Mater. 2017, 29, 1604251. (14) Cai, Y.; Huo, L.; Sun, Y. Recent Advances in Wide-Bandgap Photovoltaic Polymers. Adv. Mater. 2017, 29, 1605437.

■

CONCLUSION In conclusion, a nonconjugated polymer PAMPS-Na was synthesized and developed as the cathode buffer layer for boosting the electron transport and collection character of OSCs. In comparison with OSCs using traditional Ca/Al electrode, an improved photovoltaic performance was obtained in the devices employing PAMPS-Na as the electron transport layer. The better performance is mainly ascribed to the improved short-circuit current, benefiting from the enhanced carrier transport. It was also observed that the thickness of PAMPS-Na has no significant influence on the device performance, making it possible for its application in largearea device fabrication. Moreover, PAMPA-Na affords an even more appealing feature in its ability in performing as a good moisture/oxygen scavenger to protect device from degradation in air during a long period. As a consequence, it is expected that PAMPS-Na can serve as a stable cathode buffer layer for high performance OSCs.

■

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b07691. Synthetic route of PAMPS-Na, the absorption spectra of PTB7-Th:PC71BM (with or without PAMPS-Na films), cyclic voltammograms of PAMPS-Na, J−V curves of hole-only devices/electron-only devices with and without PAMPS-Na layer, J−V characteristics of different PAMPS-Na thickness under AM 1.5G irradiation (100 mW/cm 2 ), photovoltaic parameters of PTB7Th:PC71BM solar cells without and with PAMPS-Na layer, and photovoltaic parameters of PTB7-Th:PC71BM solar cells with different PAMPS-Na thickness (PDF) 24087


Research Article


ZnO-TiOX composite as cathode buffer layer. Appl. Phys. Lett. 2012, 100, 213906. (33) Ryu, S. O. Effect of Thickness Variation and LiF Interlayer of the Conjugated Polymer/Fullerene Photoactive Layers on the Optical and Electrical Performance of Solar Cell Devices. Mol. Cryst. Liq. Cryst. 2011, 551, 154−162. (34) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P. Effect of LiF/Metal Electrodes on the Performance of Plastic Solar Cells. Appl. Phys. Lett. 2002, 80, 1288−1290. (35) Hung, L. S.; Tang, C. W.; Mason, M. G. Enhanced Electron Injection in Organic Electroluminescence Devices Using an Al/LiF Electrode. Appl. Phys. Lett. 1997, 70, 152−154. (36) Reinhard, M.; Hanisch, J.; Zhang, Z.; Ahlswede, E.; Colsmann, A.; Lemmer, U. Inverted Organic Solar Cells Comprising a SolutionProcessed Cesium Fluoride Interlayer. Appl. Phys. Lett. 2011, 98, 053303. (37) Li, G.; Chu, C.-W.; Shrotriya, V.; Huang, J.; Yang, Y. Efficient Inverted Polymer Solar Cells. Appl. Phys. Lett. 2006, 88, 253503. (38) Liao, H.-H.; Chen, L.-M.; Xu, Z.; Li, G.; Yang, Y. Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer. Appl. Phys. Lett. 2008, 92, 173303. (39) Zhang, L.; He, C.; Chen, J.; Yuan, P.; Huang, L.; Zhang, C.; Cai, W.; Liu, Z.; Cao, Y. Bulk-Heterojunction Solar Cells with Benzotriazole-Based Copolymers as Electron Donors: Largely Improved Photovoltaic Parameters by Using PFN/Al Bilayer Cathode. Macromolecules 2010, 43, 9771−9778. (40) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, 4636−4643. (41) Lu, S.; Guan, X.; Li, X.; Sha, W. E. I.; Xie, F.; Liu, H.; Wang, J.; Huang, F.; Choy, W. C. H. A New Interconnecting Layer of Metal Oxide/Dipole Layer/Metal Oxide for Efficient Tandem Organic Solar Cells. Adv. Energy Mater. 2015, 5, 1500631. (42) Wang, Z.; Li, Z.; Xu, X.; Li, Y.; Li, K.; Peng, Q. Polymer Solar Cells Exceeding 10% Efficiency Enabled via a Facile Star-Shaped Molecular Cathode Interlayer with Variable Counterions. Adv. Funct. Mater. 2016, 26, 4643−4652. (43) Yao, K.; Chen, L.; Chen, Y.; Li, F.; Wang, P. Influence of Water-Soluble Polythiophene as an Interfacial Layer on the P3HT/ PCBM Bulk Heterojunction Organic Photovoltaics. J. Mater. Chem. 2011, 21, 13780. (44) Seo, J. H.; Gutacker, A.; Sun, Y.; Wu, H.; 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. (45) Duan, C.; 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. (46) Zhou, P.; Fang, Z.; Zhou, W.; Qiao, Q.; Wang, M.; Chen, T.; Yang, S. Nonconjugated Polymer Poly(vinylpyrrolidone) as an Efficient Interlayer Promoting Electron Transport for Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 32957−32964. (47) Li, Z.; Chen, Q.; Liu, Y.; Ding, L.; Zhang, K.; Zhu, K.; Yuan, L.; Dong, B.; Zhou, Y.; Song, B. A Nonconjugated Zwitterionic Polymer: Cathode Interfacial Layer Comparable with pfn for Narrow-Bandgap Polymer Solar Cells. Macromol. Rapid Commun. 2018, 1700828. (48) Li, Y.-L.; Cheng, Y.-S.; Yeh, P.-N.; Liao, S.-H.; Chen, S.-A. Structure Tuning of Crown Ether Grafted Conjugated Polymers as the Electron Transport Layer in Bulk-Heterojunction Polymer Solar Cells for High Performance. Adv. Funct. Mater. 2014, 24, 6811−6817. (49) Liu, S.; Zhang, K.; Lu, J.; Zhang, J.; Yip, H.-L.; Huang, F.; Cao, Y. High-Efficiency Polymer Solar Cells via the Incorporation of an Amino-Functionalized Conjugated Metallopolymer as a Cathode Interlayer. J. Am. Chem. Soc. 2013, 135, 15326−15329. (50) Cheng, P.; Yan, C.; Li, Y.; Ma, W.; Zhan, X. Diluting Concentrated Solution: a General, Simple and Effective Approach to

(15) Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K.-Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371−1388. (16) Hu, L.; Wu, F.; Li, C.; Hu, A.; Hu, X.; Zhang, Y.; Chen, L.; Chen, Y. Alcohol-Soluble N-Type Conjugated Polyelectrolyte as Electron Transport Layer for Polymer Solar Cells. Macromolecules 2015, 48, 5578−5586. (17) Yang, L.; Xu, H.; Tian, H.; Yin, S.; Zhang, F. Effect of Cathode Buffer Layer on the Stability of Polymer Bulk Heterojunction Solar Cells. Sol. Energy Mater. Sol. Cells 2010, 94, 1831−1834. (18) Chang, Y.-M.; Leu, C.-Y. Conjugated Polyelectrolyte and Zinc Oxide Stacked Structure as an Interlayer in Highly Efficient and Stable Organic Photovoltaic Cells. J. Mater. Chem. A 2013, 1, 6446. (19) Wang, F.; Tan, Z.; Li, Y. Solution-Processable Metal Oxides/ Chelates as Electrode Buffer Layers for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 1059−1091. (20) Azimi, H.; Ameri, T.; Zhang, H.; Hou, Y.; Quiroz, C. O. R.; Min, J.; Hu, M.; Zhang, Z.-G.; Przybilla, T.; Matt, G. J.; Spiecker, E.; Li, Y.; Brabec, C. J. A Universal Interface Layer Based on an AmineFunctionalized Fullerene Derivative with Dual Functionality for Efficient Solution Processed Organic and Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1401692. (21) You, L.; Liu, B.; Liu, T.; Fan, B.; Cai, Y.; Guo, L.; Sun, Y. Organic Solar Cells Based on WO2.72 Nanowire Anode Buffer Layer with Enhanced Power Conversion Efficiency and Ambient Stability. ACS Appl. Mater. Interfaces 2017, 9, 12629−12636. (22) Kim, Y. H.; Sachse, C.; Machala, M. L.; May, C.; MüllerMeskamp, L.; Leo, K. Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post-Treatment for ITO-Free Organic Solar Cells. Adv. Funct. Mater. 2011, 21, 1076−1081. (23) Wu, Y.; Zhang, W.; Li, X.; Min, C.; Jiu, T.; Zhu, Y.; Dai, N.; Fang, J. Solution-Processed Hybrid Cathode Interlayer for Inverted Organic Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 10428− 10432. (24) Guan, X.; Zhang, K.; Huang, F.; Bazan, G. C.; Cao, Y. Amino N-Oxide Functionalized Conjugated Polymers and their AminoFunctionalized Precursors: New Cathode Interlayers for HighPerformance Optoelectronic Devices. Adv. Funct. Mater. 2012, 22, 2846−2854. (25) Liu, S.; Zhang, K.; Lu, J.; Zhang, J.; Yip, H.-L.; Huang, F.; Cao, Y. High-Efficiency Polymer Solar Cells via the Incorporation of an Amino-Functionalized Conjugated Metallopolymer as a Cathode Interlayer. J. Am. Chem. Soc. 2013, 135, 15326−15329. (26) Zhang, K.; Zhong, C.; Liu, S.; Mu, C.; Li, Z.; Yan, H.; Huang, F.; Cao, Y. Highly Efficient Inverted Polymer Solar Cells Based on a Cross-linkable Water-/Alcohol-Soluble Conjugated Polymer Interlayer. ACS Appl. Mater. Interfaces 2014, 6, 10429−10435. (27) Zhang, Z.-G.; Qi, B.; Jin, Z.; Chi, D.; Qi, Z.; Li, Y.; Wang, J. Perylene Diimides: A Thickness-Insensitive Cathode Interlayer for High Performance Polymer Solar Cells. Energy Environ. Sci. 2014, 7, 1966. (28) Lv, M.; Li, S.; Jasieniak, J. J.; Hou, J.; Zhu, J.; Tan, Z.; Watkins, S. E.; Li, Y.; Chen, X. A Hyperbranched Conjugated Polymer as the Cathode Interlayer for High-Performance Polymer Solar Cells. Adv. Mater. 2013, 25, 6889−6894. (29) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-TemperatureAnnealed Sol-Gel-Derived ZnO film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679−1683. (30) Kyaw, A. K. K.; Sun, X. W.; Jiang, C. Y.; Lo, G. Q.; Zhao, D. W.; Kwong, D. L. An inverted organic solar cell employing a sol-gel derived ZnO electron selective layer and thermal evaporated MoO3 hole selective layer. Appl. Phys. Lett. 2008, 93, 221107. (31) Guerrero, A.; Chambon, S.; Hirsch, L.; Garcia-Belmonte, G. Light-modulated TiOxInterlayer Dipole and Contact Activation in Organic Solar Cell Cathodes. Adv. Funct. Mater. 2014, 24, 6234− 6240. (32) Liu, J.; Shao, S.; Meng, B.; Fang, G.; Xie, Z.; Wang, L.; Li, X. Enhancement of inverted polymer solar cells with solution-processed 24088


Research Article

ACS Applied Materials & Interfaces Enhance Efficiency of Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 2357−2364. (51) Chi, C.-Y.; Chen, M.-C.; Liaw, D.-J.; Wu, H.-Y.; Huang, Y.-C.; Tai, Y. A Bifunctional Copolymer Additive to Utilize Photoenergy Transfer and To Improve Hole Mobility for Organic Ternary BulkHeterojunction Solar Cell. ACS Appl. Mater. Interfaces 2014, 6, 12119−12125. (52) Wang, Y.; Tan, Z.; Li, Y. Solution-processable metal oxides/ chelates as electrode buffer layers for efficient and stable polymer solar cells. Energy Environ. Sci. 2015, 8, 1059−1091.

24089


Novel Nonconjugated Polymer as Cathode Buffer ... - ACS Publications

Recommend Documents