Highly Stable Organic Solar Cells Based on an Ultraviolet-Resistant Cathode Interfacial Layer

Open AccessCCS ChemistryRESEARCH ARTICLE1 Mar 2022Highly Stable Organic Solar Cells Based on an Ultraviolet-Resistant Cathode Interfacial Layer Qing Liao, Qian Kang, Yi Yang, Zhong Zheng, Jinzhao Qin, Bowei Xu and Jianhui Hou Qing Liao State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Qian Kang State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Yi Yang State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Zhong Zheng State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Jinzhao Qin State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Bowei Xu *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 and Jianhui Hou *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.021.202100852 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Although the photovoltaic efficiency of organic solar cells (OSCs) has exceeded 17%, poor lifetime excludes OSCs from practical use. In particular, UV rays in sunlight may cause the decomposition of organic photovoltaic materials, which has been proved to be the main reason for the efficiency decay. At present, there is still no effective approach to substantially improve the device stability. Herein, we fabricate a highly efficient OSC with exceptional stability under sunlight illumination by incorporating a UV-resistant cathode interlayer (CIL), namely (sulfobetaine-N,N-dimethylamino)propyl naphthalene diimide (NDI-B). NDI-B was designed and synthesized based on the naphthalene diimide (NDI) unit, thereby exhibiting excellent capability of electron collection. Moreover, NDI-B shows strong absorption in the UV region and has good UV resistance. Devices using NDI-B as a CIL exhibited a photovoltaic efficiency of 17.2%, representing the state-of-the-art photovoltaic performance of OSCs. Notably, the NDI-B-modified OSC exhibited a T80 of over 1800 h under full-sun AM 1.5 G illumination (100 mW cm−2), which represents the best stability for OSCs. We demonstrate that the unique ability of the NDI-B interlayer to convert UV light to an additional photocurrent can effectively protect photovoltaic materials from UV-induced decomposition, which is the key to obtain high OSC stability under operational conditions. Download figure Download PowerPoint Introduction Photovoltaic efficiency and long-term stability are the two deciding factors in the practical use of organic solar cells (OSCs).1–3 Over the past few years, the power conversion efficiency (PCE) of single-junction OSCs has been dramatically boosted to over 17% due to the rapid advances of organic photovoltaic material and device engineering.4,5 However, the lifetime of OSCs has only been marginally extended over the past two decades.6,7 The large discrepancy between the PCE and lifetime of the OSCs has been the main obstacle in the development of organic photovoltaics in terms of applied technology.8 At present, efficient OSCs (PCE < 10%) that exhibit a T80 (a lifetime parameter defined as the time over which the PCE decays to 80% of its initial value) longer than 200 h under operational ambient conditions are rather rare.9–11 Such short lifetimes certainly affect the stability of OSCs and prevent them from being put into practical use. Furthermore, challenges associated with improving the stability of OSCs have additionally caused research on OSC lifetime to remain scarce. The number of research publications related to the OSC stability accounts for <5% of the total published work on OSCs.12 Thus, efforts to improve the OSC stability are urgently needed for development of organic photovoltaics. Under operational conditions, ingress of moisture and oxygen, and long-term sunlight illumination were proved to be the main factors to cause the efficiency of OSCs to decay.13,14 Encapsulation technologies have been used to protect OSCs from corrosion by moisture and oxygen, which can effectively improve the device stability.15 However, compared with moisture and oxygen erosion, long-term sunlight illumination may lead to a more significant PCE decline.16 Because the key components of OSCs, such as the photoactive layer and interfacial layer, are mainly constructed from conjugated organic compounds, high-energy UV rays from sunlight can cause serious decomposition of these compounds, making OSCs intrinsically unstable under sunlight illumination.17–19 Although the use of a UV filter may help reduce the UV damage, the installation of an additional optical filter not only complicates the structure of the device but also has a negative effect on the light harvesting of OSCs.20 Currently, there is no effective approach to protect OSCs from UV-induced deterioration, making it very challenging to fabricate devices with high operational stability.21–23 Herein, we fabricated an efficient OSC of exceptional device stability under the operational condition by incorporating a UV-resistant cathode interlayer (CIL), namely (sulfobetaine-N,N-dimethylamino)propyl naphthalene diimide (NDI-B). The NDI-B zwitterion was designed and synthesized based on the naphthalene diimide (NDI) unit that demonstrates excellent electron transport properties and suitable energy levels for electron collection.24 Moreover, the NDI unit shows strong absorption in the UV region and a good UV resistance, so that the NDI-based CIL can reduce the damage to the photoactive layer caused by the UV irradiation when the OSC works under sunlight. The device using the NDI-B zwitterion as a CIL exhibited a PCE of 17.2%, representing the state-of-the-art photovoltaic performance of OSCs. More importantly, the NDI-B-based OSC exhibited a T80 of over 1800 h under full-sun AM 1.5 G illumination, which represents the best device stability in the field of OSCs. The NDI-B CIL can convert the absorbed high-energy UV photons to charge carriers, which not only increases the photocurrent of the device but also protects the photoactive layer from UV-induced decomposition. The results prove that incorporation of an interlayer that can convert UV light to photocurrent is an effective approach to create high stability in OSCs. Furthermore, our research indicates that NDI-B can effectively protect various photoactive layers against UV damage, exhibiting a promising perspective for practical use. Experimental Methods Poly1-(5(4,8-bis(4-fluoro-5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′ dithiophen-2-yl)thiophen-2-yl)-5,7-bis(2-ethylhexyl)-3-(thiophen2-yl)-4H,8H-benzo1,2-c:4,5-c′dithiophene-4,8-dione] (PBDB-TF), BTP-eC9, IT-M, 3,9-bis(5,6-difluoro-2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno2,3-d:2′,3′-d′-s-indaceno1,2-b:5,6-b′dithiophene (IT-4F), 3,9-bis(2-methylene(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno2,3-d:2′,3′-d′-s-indaceno1,2-b:5,6-b′dithiophene (ITIC), and other active layer materials were purchased from Solarmer Materials, Inc. (Beijing, China). Chemicals and solvents were purchased from Alfa Aesar (Shanghai, China), Aldrich (Shanghai, China), or TCI Chemical (Shanghai, China), Co. All chemicals were of reagent grade or better and used without further purification. Solvents for chromatography and work-up purposes were generally of reagent grade. Synthesis details ( N, N -dimethylamino)propyl naphthalene diimide 1,4,5,8-Naphthalenetetracarboxylic dianhydride (2.68 g, 10 mmol) was added to a 250 mL two-necked flask. After being flushed by a gentle stream of dry argon, 100 mL dimethylformamide (DMF) and N,N-dimethyl-1,3-propane diamine (3 mL, 21 mmol) were added successively. To this suspension, triethylamine (1 mL) was added, and the suspension was refluxed for 12 h. After cooling the reaction mixture to ambient temperature, the precipitate was filtered and recrystallized from ethanol. A bright yellow solid crystal was obtained with a yield of 75% (3.27 g). 1H NMR (400 MHz, CDCl3, δ): 8.75 (s, 4H), 4.27 (t, 4H), 2.49 (t, 4H), 2.28 (s, 12H), 1.96 (m, 4H) ( Supporting Information Figure S20). 4,4′-(((1,3,6,8-tetraoxo-1,3,6,8-tetrahydrobenzolmn3,8phenanthroline-2,7-diyl)bis(propane-3,1-diyl))bis(dimethylammoniumdiyl))bis(butane-1-sulfonate) To a 250 mL two-necked flask, ( N, N-dimethylamino)propyl naphthalene diimide (NDI-N; 1.31 g, 3 mmol) was added. After being flushed by a gentle stream of dry argon, methanol (120 mL) was injected into the flask, then 1,4-butane sultone (1.22 g, 9 mmol) was added into this mixture. The mixture was further stirred for 48 h at 50 °C, the formed precipitate was filtered off, washed with acetone, and dried in vacuo. The crude product was recrystallized from a mixture of water and ethanol to obtain colorless crystals (1.28 g, yield 60%). 1H NMR (400 MHz, D2O, δ): 8.61 (s, 4H), 4.25 (t, 4H), 3.52 (t, 4H), 3.39 (t, 4H), 3.12 (s, 12H), 2.89 (t, 4H), 2.27 (m, 4H), 1.93 (m, 4H), 1.79 (m, 4H) ( Supporting Information Figure S21). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) (m/z): calcd for C32H44N4O10S2: 708; found: 709 ( Supporting Information Figure S22). Anal. Calcd for C32H44N4O10S2: C, 54.22; H, 6.26; N, 7.90; O, 22.57; S, 9.05. Found: C, 50.97; H, 6.44; N, 7.43; O, 27.25; S, 7.91. Fabrication and characterization of OSC devices Prepatterned indium tin oxide (ITO)-coated glass with a sheet resistance of 10–15 Ω/□ (Ohm per square) was cleaned with sequential sonication in deionized water, acetone, and ethyl alcohol twice for 15 min. The precleaned ITO-coated glass substrates were UV/ozone-treated for 20 min. For ZnO CIL preparation, the 2-methoxyethanol solution of zinc acetate dihydrate with a concentration of 100 mg mL−1 was made first, and a small amount of triethylamine was needed to enhance the solubility of zinc acetate dihydrate. Then the precursor solution was spin-coated onto the ITO substrate at 3000 rpm for 30 s. The substrates were annealed for 1 h at 200 °C. For NDI-B CIL preparation, the aqueous solution of NDI-B with a concentration of 3.5 mg mL−1 was spin-coated onto the ITO substrate at 3000 rpm for 30 s without annealing. Then, all CIL-coated substrates were transferred to the nitrogen-filled glovebox. The active layer materials were dissolved in chloroform with a concentration of 7.5 mg mL−1 containing the donor/acceptor (1/1.2 weight ratio). The solution was heated at 45 °C until total dissolution. The active layers were spin-coated onto CIL-modified substrates at 3500 rpm for 30 s after addition of 0.5% (volume ratio) of 1,8-diiodooctane. After that, the blend films were annealed at 100 °C for 10 min. Finally, 10 nm MoO3 and 100 nm Al anode were evaporated sequentially under vacuum at 2 × 10−4 Pa. The device area was 0.04 cm2. The current density–voltage (J–V) measurements of the devices were performed inside a nitrogen-filled glovebox by Agilent B2912A Precision Source/Measure unit. The J–V curve was tested using the solar simulator (SS-F5-3A; Enlitech) along with AM 1.5 G spectra whose intensity was calibrated by the certified standard silicon solar cell (SRC-2020; Enlitech) at 100 mW cm−2. External quantum efficiency (EQE) was measured using the solar-cell spectral-response measurement system QE-R3011 (Enli Technology Co. Ltd., Beijing, China) with the standard single-crystal Si photovoltaic cell calibrated at each wavelength. Film thickness was obtained by a surface profilometer (Dektak XT; Bruker, Shanghai, China). 1H NMR spectra were measured in deuterated solvents on a Bruker AVANCE 300 MHz or 400 MHz NMR spectrometer at room temperature. The elemental analysis of C, H, and N was performed on a FLASH EA1112. Atomic force microscopy (AFM) measurement was recorded on a Nanoscope V (Vecco) AFM in tapping mode on the ITO substrate (Bruker, Shanghai, China). UV–vis absorption spectra of molecules in solid thin films were measured on a Hitachi U-3100 UV–vis spectrophotometer Hitachi, High Technologies Co., Tokyo, Japan). The solid thin film samples were spin-coated onto quartz plates (1 × 1 cm2). The electrochemical cyclic voltammetry was measured on a CH1650D electrochemical workstation (CH Instruments, Inc., Shanghai, China) in a three-electrode cell in anhydrous acetonitrile solutions of Bu4NPF6 (0.1 M) with a scan rate of 50 mV/s at room temperature under argon. A Ag/Ag+ wire, platinum electrode, and glassy carbon electrode were used as the reference electrode, counter electrode, and working electrode, respectively. The potential of the Ag/Ag+ reference electrode was internally calibrated by using ferrocene/ferrocenium (Fe/Fe+) as the redox couple. Electrochemical impedance spectroscopy measurement The electrochemical impedance spectroscopy (EIS) measurement was performed using a E4990A Impedance Analyzer with a 20 mV ac signal at frequencies from 5 MHz to 20 Hz under the illumination of AM 1.5G, 100 mW cm−2. A bias voltage equal to Voc was applied to offset the total current. Stability test Devices were sealed by glass and tested in air (70% humidity). An array of white light-emitting diodes (LEDs) was used as the light source with an intensity equivalent to one Sun, which was calibrated by matching the device performance to those measured under AM 1.5 G. The initial exposure time is defined as time 0 s. The devices were kept under open-circuit conditions. The temperature of the cells was 45–55 °C during measurements. For stability under dark, the light was only turned on during measurements of the cells. Results and Discussion The chemical structure of NDI-B is presented in Figure 1a. The NDI-B zwitterion was synthesized by the amine-induced ring-opening reaction of 1,4-butane sultone with an NDI-based precursor, resulting in a good yield of 60 %. The molecular structure of NDI-B was confirmed by 1H NMR, MALDI-TOF-MS, and elemental analysis. NDI-B is easily dissolved in water and is completely insoluble in nonpolar solvents such as chloroform and chlorobenzene. The orthogonal solubility of NDI-B to active layer materials is crucial for a multilayer fabrication of the OSCs. Figure 1b shows the absorption spectra of NDI-B and ZnO as solid films, and the inset shows their transmittance spectra. The absorption maxima at 365 nm are ascribed to the π–π* transition of the NDI unit. The high transmittance (over 98%) of the NDI-B film in the broad wavelength region from 450 to 900 nm can reduce the negative effect on the light-harvesting of the photoactive layer. Using cyclic voltammetry measurements, the lowest unoccupied molecular orbital (LUMO) level of NDI-B was determined to be −3.74 eV ( Supporting Information Figure S1b). The work function (WF) of ITO modified by NDI-B was reduced from 4.62 to 3.95 eV. As shown in Figure 1c, the WF of ITO covered with NDI-B is located above the LUMO level of the electron acceptor BTP-eC9. According to the integer charge-transfer (ICT) model, such energy level alignments can cause spontaneous charge transfer from the acceptor to the CIL, which is essential for the formation of barrier-free contact for electron extraction.25 The surface morphology of NDI-B on an ITO substrate was investigated by AFM. The ITO electrode modified by NDI-B exhibited a uniform and smooth surface with a reduced mean-square surface roughness (Rq) value of 2.52 nm ( Supporting Information Figure S2a). Figure 1 | (a) Synthetic route to NDI-B. (b) Normalized UV–vis absorption spectra of ZnO and NDI-B as films. The inset shows their transmittance spectra. (c) Energy level diagram of the materials used in OSCs. Download figure Download PowerPoint The performance of NDI-B in serving as a CIL was investigated by fabricating OSCs with an inverted structure of ITO/CIL/PBDB-TF:BTP-eC9/MoO3/Al. Here, a blend of PBDB-TF:BTP-eC9 was selected as the active layer, and the chemical structures of PBDB-TF and BTP-eC9 are presented in Figure 2a. In addition, a control device using ZnO as a CIL was also fabricated. The J−V characteristics of the devices and photovoltaic parameters are shown in Figure 2b and Table 1, and the EQE curves are presented in Figure 2c. The optimal thickness of NDI-B was found to be approximately 10 nm. The OSC with a 10 nm NDI-B exhibited a PCE of 17.2%, along with a short-circuit current density (Jsc) of 26.6 mA cm−2, an open-circuit voltage (Voc) of 0.84 V, and a fill factor (FF) of 0.77, representing the state-of-the-art photovoltaic performance of OSCs. In contrast, the photovoltaic performance of the OSCs with ZnO (PCE = 16.5%) is obviously inferior to that of the NDI-B device. In addition, NDI-B shows great tolerance to thickness variation in providing high photovoltaic performance of the OSCs. For instance, even when the NDI-B thickness increased to 100 nm, the PCE of the OSCs was still maintained at a high value of 14.8% ( Supporting Information Figure S3a and Table S1). The device using the 100 nm NDI-B could exhibit high EQE from 450 to 900 nm, implying the outstanding electron transport property of NDI-B ( Supporting Information Figure S3b). Benefitting from the thickness insensitivity in fabricating OSCs, NDI-B exhibits superior compatibility with available printing methods compared with the ZnO CIL. When a blade-coated NDI-B CIL was used to fabricate an OSC device, a PCE of 16.2% was achieved ( Supporting Information Figure S4 and Table S2). Figure 2 | (a) Device structure of the OSCs used in this work and chemical structures of active layers. (b) J−V characteristics of OSCs with ZnO and NDI-B. (c) EQE spectra of OSCs using ZnO and NDI-B as CILs. J−V curve of (d) ZnO and (e) NDI-B devices in the dark and under the illumination of UV light (365 nm, 5 W). (f) Schematic energy-level diagram and ESPs of PBDB-TF and NDI-B. (g) J−V characteristics of bilayer devices with the structure of ITO/NDI-B/PBDB-TF/MoO3/Al and ITO/ZnO/PBDB-TF/MoO3/Al. (h) Nyquist plots of devices modified by ZnO and NDI-B. Inset: the equivalent-circuit model employed for fitting of EIS data. (i) Photo-CELIV curves of the ZnO and NDI-B devices. Download figure Download PowerPoint Table 1 | The Optimized Photovoltaic Parameters of Devices Based on ZnO and NDI-B under the Illumination of AM 1.5 G, 100 mW cm–2 CILs Voc (V) Jsc (mA cm−2) FF PCE (%)a Jsccal (mA cm−2) Rs (Ω cm2) Rsh (kΩ cm2) ZnO 0.84 26.22 0.75 16.47 (15.68 NDI-B 0.84 in are PCE obtained from at 10 devices. ZnO and conjugated such as and ( Supporting Information Figure were also used as CILs for fabricating OSC devices. As shown in Supporting Information Figure and Table devices based on and exhibited of and respectively. The FF of the NDI-B-based device as compared with the and devices indicates a superior electron ability of the NDI-B CIL. Moreover, the Jsc of the device modified by NDI-B is than those of the devices with and which was ascribed to the conversion efficiency by the incorporation of NDI-B. The high PCE of the NDI-B device was achieved by of Jsc and A photocurrent was in the nm and the nm in the device containing the NDI-B interlayer to the device with Thus, a was obtained by the EQE curves from the NDI-B and the ZnO devices ( Supporting Information Figure The in the wavelength from 300 to 450 nm with the absorption of implying that NDI-B can the conversion efficiency of UV light to To demonstrate the capability of NDI-B to convert UV light to the J−V characteristics of the devices under UV light absorption and the power of UV light used in this work are 365 nm and 5 illumination were measured using a UV As shown in and the NDI-B-based device an photovoltaic effect with a photocurrent of mA cm−2. In contrast, no photocurrent was for the ZnO device under the UV the LUMO level of NDI-B is than that of the electron transfer from PBDB-TF to NDI-B is which the efficiency in the UV region is that the high-energy UV photons absorbed by the photoactive layer may be to which is to the OSC Thus, an conversion efficiency in the UV region not only increases the photocurrent but also the stability of the device. were employed to the surface of PBDB-TF and NDI-B at the As shown in Figure of the NDI-B its Because PBDB-TF exhibits an NDI-B can as an electron acceptor and electron transfer at the The charge-transfer was proved by the of the blend of PBDB-TF and NDI-B. The of PBDB-TF by that and electron transfer in the ( Supporting Information Figure We further the of NDI-B in serving as an electron acceptor by fabricating bilayer devices of or (100 As shown in Figure the NDI-B-based bilayer device exhibited a Jsc of mA cm−2, that NDI-B an additional photocurrent when it is in contact with the of UV NDI-B can the FF of the device by improving the electron The of the intensity of AM 1.5 for the NDI-B-based device is very to = that could be when using NDI-B as a CIL ( Supporting Information Figure In the EIS the were by the equivalent-circuit model from the and the fitting results with the data. The resistance in the could be obtained from the on the at high frequencies in Nyquist and was determined to be and for the devices with NDI-B and The for the NDI-B device a reduced charge at the layer In addition, the of the OSCs with NDI-B and as determined by the by a voltage measurements, were × 10−4 and × 10−4 The high charge excellent electron transport properties of the NDI-B CIL. Based on the above NDI-B shows excellent electron transport which can the interfacial charge and enhance the FF of the device. In addition to of this is the effect of NDI-B on long-term device we investigated the stability of OSCs by the device efficiency of cells under ambient conditions (70% at room temperature of As shown in Supporting Information Figure the device modified by NDI-B over of the initial PCE after 2 In contrast, the PCE of the ZnO device dramatically to of its initial value during the test The operational stability of the OSCs is the in their practical use are to work under sunlight Thus, the stability of the devices under sunlight 1.5 was The photovoltaic parameters of the devices were at 50 °C in ambient As shown in Figure the NDI-B device of its initial PCE after illumination for 1800 that a T80 of over 1800 h can be achieved under the conditions. The T80 of over 1800 h represents the best stability in field of OSCs. In contrast, the PCE to of the initial value for the ZnO device after illumination for h. The T80 of the NDI-B device is 200 than that of the control device modified by We of device and lifetimes T80 lifetimes and other lifetimes at which the devices more than 80% of the initial under the operational conditions in Figure the results in this work demonstrate exceptional and operational stability of the OSCs. The outstanding stability of the NDI-B device is crucial for the practical of the OSCs. Figure 3 | (a) of the ZnO and NDI-B devices. cells were measured in air under the illumination of AM 1.5 G, 100 mW cm−2. The temperature was 45–55 (b) Results of and of the OSCs from and this are in Supporting Information Table Download figure Download PowerPoint The high operational stability obtained in this work can be ascribed to the use of the NDI-B CIL. that the protects devices from moisture and oxygen the only other factor the PCE is sunlight illumination and related The of analysis a decomposition temperature of °C for which is to the high temperature under long-term sunlight illumination ( Supporting Information Figure Moreover, no transition was in the measurements of NDI-B ( Supporting Information Figure As shown in Supporting Information Figure the morphology of the NDI-B film after at °C for 300 h. results that the NDI-B film good chemical and which is for excellent OSC stability by under the working Supporting Information Figure the device stability under conditions. The PCE of the NDI-B-modified OSC was of the initial value after the device at °C for the PCE of the ZnO device to of the