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Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells
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Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells Chen-Hsueh Lin a, Che-Wei Huang a, Po-Hsin Wang a, Tzung-Fang Guo b, Ten-Chin Wen a,c,∗ a
Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan c Center of Applied Nanomedicine, National Cheng Kung University, Tainan 70101, Taiwan b
a r t i c l e
i n f o
Article history: Received 19 September 2019 Revised 6 November 2019 Accepted 8 November 2019 Available online xxx Keywords: Cationic dye Sol−gel ZnO Interfacial modification Electron transport Polymer solar cells1
a b s t r a c t ZnO layer was modified with the addition of Cationic dyes including Crystal Violet (CV)/Ethyl violet (EV) in sol–gel process for an electron transport layer in inverted polymer solar cells (PSCs). X-ray photoelectron spectra showed the presence of CV/EV at the top of ZnO surface. Besides, oxygen defect was significantly reduced by CV/EV modification due to the chloride occupation. Furthermore, the amount of CV/EV decreased progressively from ZnO surface to bottom, being evidenced by depth profile. With modification, the ZnO surface became smoother and more hydrophobic to improve the contact with active layer. Meanwhile, CV/EV participated in the crystallization which resulted in the larger ZnO crystal grain size. Interface dipole after modification would slightly reduce the work function of ZnO and the energy barrier between ZnO and active layer via Ultraviolet Photoelectron Spectroscopy and External Quantum Efficiency analysis. Accordingly, inverted PSCs possessed better morphology, better electron extraction ability with ZnO modified by CV and EV respectively, rendering the power conversion efficiency up to 8.80% and 9.06% in comparison to the pristine ZnO (7.59%). In conclusion, we demonstrate a facile way to improve morphological and electrical properties of ZnO layer by simply adding CV/EV in sol–gel ZnO to fabricate high performance PSCs. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Among various photovoltaic systems, bulk-heterojunction (BHJ) polymer solar cells (PSCs) with an active layer, which comprises a conjugated polymer donor and a fullerene derivative acceptor, have attracted a lot of attention due to their potential for promising efficiency, flexibility, large scale fabrication and roll to roll process [1–6]. Despite sacrificing performance, inverted architecture of PSCs can significantly improve device stability because it avoid utilizing low work-function metal, which is sensitive to moisture and oxygen [7,8]. Indium tin oxide (ITO) is a suitable cathode for inverted PSCs due to the good transparency, low cost and applicable for various solution process. There are plenty of options for cathode buffer layer on inverted PSCs. Among them ZnO has been widely utilized due to the appropriate energy level and high electron mobility [9–12]. To fabricate high crystallinity and electron mobility ZnO thin film, the annealing temperature of solgel process must surpass 350 °C [13,14]. However, high annealing
∗ Corresponding author at: Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan. E-mail address: [email protected] (T.-C. Wen).
temperature would hinder the potential for flexible solar cell and large-scale fabrication. Therefore, relatively low annealing temperature is necessary for flexible substrates while producing a suitable ZnO thin film [15]. For inverted PSCs, one of the crucial factors for better power conversion efficiency (PCE) is the efficient electron transport at the interface between the active layer and ZnO layer. Unfortunately, the solution-processed ZnO surface is rich with defects such as oxygen vacancy, zinc dangling bonds and unreacted functional groups. These defects at surface could trap the electron and cause recombination. Hence, the efficiency of electron collection would decrease [16,17]. Moreover, the poor interfacial contact of organic active layer and inorganic ZnO layer might reduce the electron extract ability and raise series resistance (Rs ) leading to bad performance and stability of inverted PSCs [18–20]. To improve the interface contact and reduce the defects on the surface, several methods have been applied to achieve the goal such as inserting an interface modification layer between ZnO and active layer. Conjugated polyelectrolyte [21–23], insulating polymer [24–30], organic small molecule [31–33], zwitterion [34,35] and self-assembled monolayer [36–40] have been applied as interlayer on top of ZnO to passivate the surface defects, reduce energy gap, and enhance the electron collection ability of the ZnO layer. Besides interface modification layer, doping is an alternative strategy to enhance
https://doi.org/10.1016/j.jtice.2019.11.010 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Fig. 1. Molecular structures of (a) CV and (b) EV.
Scheme 1. The mechanism illustration of cationic dyes CV/EV modification (i) on the ZnO surface and (ii) in the ZnO layer.
the interface contact. Metals such as aluminum [41–43], gallium [44,45] and indium [46] have been doped into ZnO layer to passivate surface defect and enhance electrical conductivity. Moreover, magnesium doped ZnO layer could not only improve layer property but also exhibit tunable bandgap by controlling doping concentration [47]. Meanwhile, organic small molecules [35] and fullerene derivative [48,49] have been blended into ZnO layer. The ZnO surface which rich in C60 would further promote the compatibility with active layer. Several studies have been reported that molecule with quaternary ammonium group would form interface dipole and reduce work function of ZnO [21,22]. Recently, ionic compound [50] and surfactants [51] with quaternary ammonium group have been reported to add in sol-gel ZnO process to improve ZnO surface. Molecular structures of crystal violet (CV) and ethyl violet (EV) are shown in Fig. 1. In this study, CV/EV containing quaternary ammonium group were added into sol-gel precursor solution of ZnO and fully investigate the role of cationic dyes in ZnO layer. 2. Experimental section 2.1. ZnO thin layer fabrication The ZnO precursor solution was prepared by dissolving zinc acetate dehydrate (Merck, 99.5%, 1 g) and ethanolamine (Acros, 98%, 0.28 g) in 2-methoxyethanol (Aldrich, 98%, 10 mL). After stirring for 8 h for hydrolysis reaction and aging, different weight percent of crystal violet (CV, Sigma-Aldrich, 98%) and ethyl violet (EV, SigmaAldrich, 98%) were added to ZnO precursor solution individually. PTB7-Th and PC71 BM which purchased from Lumtec Corp. The active layer solution was obtained by dissolving PTB7-Th:PC71 BM (1:1.5 wt%) in mixed solvents of chlorobenzene/1,8-diiodooctane (97:3 vol%) with concentration of 20 mg/mL. Indium tin oxide (ITO) substrates (sheet resistance: < 15 /sq) were purchased from
RITEK Corp. The patterned ITO substrates were cleaned ultrasonically with detergent, deionized water, acetone, and isopropyl alcohol in sequence for 15 min each. After that, ITO substrates were treated with oxygen plasma for 25 min. The ZnO precursor solution was spin-coated onto pre-treated ITO substrates at 40 0 0 rpm for 60 s and annealed at 180 °C for 1 h in ambient air to obtain ZnO thin layer (≈30 nm). The active layer solution was filtered through a 0.45 μm polytetrafluoroethylene filter and spin-coated ontop of ZnO layer at 10 0 0 rpm for 60 s. Finally, 10 nm of MoO3 and 80 nm of Ag were thermally evaporated under pressure of ≈10−7 Torr. The area of each device defined by shadow mask was 0.06 cm2 . 2.2. Characterization The surface morphology and properties were examined with Scanning electron microscope (SEM) (HR-FESEM-SU8010, Hitachi), atomic force microscopy (AFM) (Dimension Icon, Bruker, Billerica, MA) operating in tapping mode and water contact angle (FTA10 0 0B, First Ten Angstroms). The crystallinity of samples was analyzed by X-ray diffraction (XRD) (Ultima IV, Rigaku) with wavelength of 1.5406 A˚ at a scanning rate of 0.02°/s. The photoemission spectroscopy was carried out at the 09A2 U5-spectroscopy beamlines and He (21.21 eV) in the National Synchrotron Radiation Research Center in Taiwan for X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), respectively. The depth profile was performed by using PHI 50 0 0 VersaProbe (Scheme 1). 2.3. Methods The current density-voltage measurements of devices were performed in a nitrogen-filled glove box under AM 1.5 G illumination
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Fig. 2. XPS N 1s spectra of (a) doping CV and (b) doping EV. XPS C 1s spectra of (c) doping CV and (d) doping EV.
at 100 mW/cm2 using a Keithley 2400 source-measure unit. The AM 1.5 G illumination was simulated with an Oriel 300-W Solar Simulator and calibrated by a silicon photodiode with a protective KG5 filter which standardized by National Renewable Energy Laboratory. The percentage of the carrier generation from the illuminated light at different wavelengths was measured by external quantum efficiency (EQE) (QE-R3011, Enli Technology Co., Ltd). 3. Results and discussion To investigate the existence of cationic dye molecules and its influence on surface of ZnO film, X-ray photoelectron spectroscopy (XPS) measurements were conducted to analyze several elements. Fig. 2(a) and (b) indicate that N 1s signal start to appear after modified with 0.5–3 wt% of CV/EV, Fig. 2(c) and (d) also indicate that C 1s signal would appear after modified with 0.5–3 wt% of CV and EV respectively. There is no chance that N and C would exist in pristine ZnO crystal. Therefore, the organic dye molecules indeed exist on the surface of ZnO. On the other hand, the XPS Zn 2p result is presented in Fig. 3(a) and (b) which show that both characteristic peaks of Zn 2p3/2 and 2p1/2 slightly shift to higher binding energy after blending with CV/EV, and the shifts of the peaks could be contributed to the decrease in electron density around Zn. Moreover, the O 1s characteristic peak of ZnO can be separated to two different peaks at 530.6 eV and 532 eV (Fig. 4), which can be assigned to oxygen of the zinc oxide lattice and hydroxyl groups or oxygen vacancies respectively. The decrease in relative intensity of oxygen defect peaks after CV/EV modification indicates that dye molecules could effectively passivate the defects on ZnO
Fig. 3. XPS Zn 2p spectra of ZnO doping with CV and EV.
surface as shown in Fig. 4(b) and (c). Fig. S1 shows the surface morphology of pristine ZnO layers with and without CV/EV modifications through top-view scanning electron microscopy (SEM). There are several aggregation and irregular grains on pristine ZnO film as shown in Fig. S1(a). After CV/EV modification, the ZnO surface become smoother and more uniform which might be able to
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Fig. 4. XPS O 1s spectra of (a) pristine ZnO, (b) doping CV and (c) doping EV.
reduce possibility of electron trapping. Meanwhile, surface topography is another concerning issue that Fig. S2 shows the topography images of pristine ZnO layers with and without CV/EV modification investigated by tapping mode atomic force microscope (AFM). The root mean square (RMS) roughness slightly reduce from 2.05 nm to 1.62 nm and 1.73 nm after CV/EV molecules been doped to ZnO layer, respectively. Furthermore, Fig. S3 shows the water contact angle measurement of ZnO with and without dye modification that pristine ZnO hydrophilic surface with contact angle of 32.37° become more hydrophobic when cooperates with cationic dye additives which attributed to the dimethylaniline group of CV/EV. As mentioned above, the smoother and more hydrophobic surface renders better interface contact and performance of devices. To understand the distribution of cationic dye molecules in ZnO layer after modification, Fig. 5 showed the depth profile with Cl/Zn ratio versus depth. The examination of the depth profile shows that there is the same trend with Cl/Zn ratio i.e., the ratio decreased sharply until 2.5 nm; afterwards it decreased gradually to 25 nm. Meanwhile, the signal of Zn also disappears around 25 nm, indicating that the dye molecules might distribute in the whole ZnO layer. As for the crystallinity of ZnO, the X-ray diffraction (XRD) spectra of pristine ZnO with and without CV/EV modification is analyzed in Fig. 6. Two characteristic peaks at 30.75° and 35.63°
were assigned to (100) and (101) crystal planes of ZnO respectively. After CV/EV modification, both of the characteristic peaks become slender. With Scherrer equation calculation, the grain sizes are 13.63 nm, 24.07, and 24.10 nm for pristine ZnO, CV, and EV modification, respectively. As a result, the crystal lattice of ZnO become the larger after the modification. Namely, CV/EV participated in crystallization during the annealing process and distribute in the whole ZnO layer. According to the results mentioned above, we inferred that during the formation of ZnO lattice, CV/EV received the upward force from the solvent evaporation and the hydrogen bonding force between CV/EV and Zn(OH)2 . Accordingly, still few dye molecules resided in ZnO layer while the most dye molecules were driven to surface of ZnO. Dye molecules at ZnO surface would occupy the oxygen defect sites by Cl− anion with the quaternary ammonium cation staying close, causing ZnO surface more hydrophobic. On the other hand, dye molecules inside ZnO layer would reside in grain boundary of ZnO and form dipole between lone pairs of N and the unreacted hydroxyl groups. To further evaluate the electrical performance, ultra-violet spectroscopy (UPS) was utilized for analysis of ZnO work function with and without CV/EV modification as shown in Fig. 7(a). The work function value of pristine ZnO which is estimated from the low kinetic-energy cut-off is 4.49 eV, after CV/EV modification the work functions slightly shift to 4.32 and 4.28 eV, respectively, indicating the chloride anion and
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Fig. 5. Depth profile of Cl/Zn ratio in ZnO layer with addition of (a) CV and (b) EV.
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quaternary ammonium cation could induce a dipole moment on the surface of ZnO. In addition, EQE measurement also indicates that cationic dye doped devices could collect more efficient carrier and show good agreement with the higher Jsc data of the devices as shown in Fig. 7(b). Therefore, dye molecules modification could reduce the energy barrier and hence improve the electron extraction ability causing the decrease of work function. Meanwhile, we fabricate the electron only device to verify the electron mobility. After CV/EV modification, the current density which is contributed nearly by electron is higher than pristine ZnO further confirmed the increase in electron extraction and the decrease in recombination as shown in Fig. S4. Finally, we apply ZnO modified with CV/EV to fabricate inverted polymer solar cells based on PTB7-Th: PC71 BM system to testify the electrical performance. The J–V characteristics of best performance devices with CV/EV modification are shown in Fig. 8(a), and the detailed photovoltaic parameters are summarized in Table 1. The devices with CV/EV modification show higher short circuit current density (Jsc ) and fill factor (FF) than pristine ZnO devices which can be attributed to the smoother and more hydrophobic surface of ZnO, resulting in better interface contact. Besides, the dye molecules could passivate the surface defects and form interface dipole, leading to better electron coupling activity and reducing the WF of ZnO to improve the electron extraction ability. In addition, the device with EV modification show better performance than CV due to longer alkyl group. As a result, device with EV modification could provide a suitable ZnO surface for active layer above to increase the PCE of device. Meanwhile, the J–V characteristics in the dark condition reveals the reducing leakage current and increasing current density with forward bias after CV/EV modification as shown in Fig. 8(b). Therefore, the charge recombination in device has been dramatically reduced with cationic dye modification. To understand the influence on device stability, pristine ZnO with and without CV/EV modification devices are fabricated and placed in glove box and ambient air for thirty days. As shown in Fig. 9(a), the stability does not vary much under glove box that they almost keep 95% PCE during thirty days measurement. In contrast, the stability under ambient air varies a lot as shown in Fig. 9(b). The PCE of pristine ZnO devices decreases to 70% on the third day and 59% on thirtieth day. However, the devices with CV/EV modification perform better stability, which are 65% and 75%, respectively, after thirty days exposure. At last, the devices with EV modification show better stability than with CV modification could be attributed to the longer alkyl group that provide a more hydrophobic surface to enhance the device lifetime.
Fig. 6. XRD spectra of pristine ZnO with 1wt% CV and 2wt% EV doping.
Fig. 7. (a) UPS spectra and (b) EQE of pristine ZnO, doping CV, doping EV films.
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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C.-H. Lin, C.-W. Huang and P.-H. Wang et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx Table 1 Detailed photovoltaic parameters of pristine ZnO, doping CV, doping EV devices.
Pristine 1 wt% CV 2 wt% EV
Voc (V)
Js c (mA/cm2 )
FF(%)
PCE(%)
0.78± 0.01(0.78) 0.78± 0.01(0.78) 0.77± 0.01(0.77)
14.66± 0.86(13.40) 15.23± 0.53(14.94) 16.89± 0.34(16.37)
0.68± 0.02(0.68) 0.74± 0.01(0.74) 0.71± 0.01(0.71)
7.59± 0.21(7.32) 8.80± 0.22(8.58) 9.06± 0.17(8.82)
Fig. 8. J–V characteristics of pristine ZnO, doping CV and doping EV devices (a) under AM 1.5 G illumination (b) in the dark condition.
Fig. 9. Stability of pristine ZnO, doping CV and doping EV devices in (a) glove box and (b) ambient air.
4. Conclusions In this study, we demonstrate a facile method to prepare CV/EV modified sol–gel ZnO layer by simply cationic dye molecules modification. CV/EV would distribute in the whole ZnO layer with the highest concentration at the surface of ZnO and decrease gradually downward according to the depth profile. CV/EV on ZnO surface would occupy the oxygen vacancies by Cl− . On the other hands, CV/EV inside ZnO layer would reside in the grain boundary of ZnO and form dipole with unreacted hydroxyl groups, above-mentioned situation could both reduce the obstacle of electron transporting. Moreover, CV/EV on surface could not only make surface smoother and more hydrophobic, but also form interface dipole to reduce WF of ZnO. As a result, CV/EV could provide suitable surface for the active layer above to increase PCE, electron extraction efficiency, stability, and decrease WF. Eventually, the PCE values of CV/EV modification devices were 8.80% and 9.06%, respectively, in comparison to the pristine ZnO (7.59%). Acknowledgments The study is financially supported by Ministry of Science and Technology, Taiwan under grants number 108-2221-E-006-159-MY3
and Center of Applied Nanomedicine (CAN) of National Chen Kung University in Taiwan. The resource assistance from both institutes is gratefully acknowledged. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.11.010. References [1] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science 1995;270:1789–91. [2] Su Y-W, Lan S-C, Wei K-H. Organic photovoltaics. Mater Today 2012;15:554–62. [3] Kaltenbrunner M, Sekitani T, Reeder J, Yokota T, Kuribara K, Tokuhara T, Drack M, Schwödiauer R, Graz I, Bauer-Gogonea S. An ultra-lightweight design for imperceptible plastic electronics. Nature 2013;499:458. [4] You J, Dou L, Yoshimura K, Kato T, Ohya K, Moriarty T, Emery K, Chen C-C, Gao J, Li G. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat Commun 2013;4:1446. [5] Krebs FC, Espinosa N, Hösel M, Søndergaard RR, Jørgensen M. Rise to power–OPV-based solar parks. Adv Mater 2014;26:29–39. [6] Scharber MC. On the efficiency limit of conjugated polymer: fullerene-based bulk heterojunction solar cells. Adv Mater 2016;28:1994–2001. [7] White MS, Olson DC, Shaheen SE, Kopidakis N, Ginley DS. Inverted bulk-heterojunction organic photovoltaic device using a solution-derived ZnO underlayer. Appl Phys Lett 2006;89:3.
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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ARTICLE IN PRESS
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C.-H. Lin, C.-W. Huang and P.-H. Wang et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx [8] Kyaw A, Sun X, Jiang C, Lo G, Zhao D, Kwong D. 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. [9] Beek WJE, Wienk MM, Janssen RAJ. Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer. Adv Mater 20 04;16:10 09–13. [10] Gilot J, Barbu I, Wienk MM, Janssen RAJ. The use of ZnO as optical spacer in polymer solar cells: theoretical and experimental study. Appl Phys Lett 2007;91:3. [11] Liu J, Shao SY, Meng B, Fang G, Xie ZY, Wang LX, Li XL. Electrical characterization of back-gated bi-layer MoS2 field-effect transistors and the effect of ambient on their performances. Appl Phys Lett 2012;100:3. [12] Small CE, Chen S, Subbiah J, Amb CM, Tsang S-W, Lai T-H, Reynolds JR, So F. High-efficiency inverted dithienogermolethienopyrrolodione-based polymer solar cells. Nat Photon 2012;6:115. [13] Ong BS, Li C, Li Y, Wu Y, Loutfy R. Stable, solution-processed, high-mobility ZnO thin-film transistors. J Am Chem Soc 2007;129:2750–1. [14] Morvillo P, Diana R, Mucci A, Bobeico E, Ricciardi R, Minarini C. Influence of annealing treatments on solution-processed ZnO film deposited on ITO substrate as electron transport layer for inverted polymer solar cells. Sol Energy Mater Solar Cells 2015;141:210–17. [15] Sun Y, Seo JH, Takacs CJ, Seifter J, Heeger AJ. Inverted polymer solar cells integrated with a low-temperature-annealed sol-gel-derived ZnO film as an electron transport layer. Adv Mater 2011;23:1679–83. [16] Yang JL, Siempelkamp BD, Mosconi E, De Angelis F, Kelly TL. Origin of the thermal instability in CH3NH3PbI3 thin films deposited on ZnO. Chem Mater 2015;27:4229–36. [17] Yang F, Kang DW, Kim YS. Improved interface of ZnO/CH3 NH3 PbI3 by a dynamic spin-coating process for efficient perovskite solar cells. RSC Adv 2017;7:19030–8. [18] Janotti A, Van de Walle CG. Oxygen vacancies in ZnO. Appl Phys Lett 2005;87:122102. [19] Bai S, Jin Y, Liang X, Ye Z, Wu Z, Sun B, Ma Z, Tang Z, Wang J, Würfel U. Ethanedithiol treatment of solution-processed ZnO thin films: controlling the intragap states of electron transporting interlayers for efficient and stable inverted organic photovoltaics. Adv Energy Mater 2015;5:1401606. [20] MacLeod BA, De Villers BJT, Schulz P, Ndione PF, Kim H, Giordano AJ, Zhu K, Marder SR, Graham S, Berry JJ. Stability of inverted organic solar cells with ZnO contact layers deposited from precursor solutions. Energy Environ Sci 2015;8:592–601. [21] Seo JH, Gutacker A, Sun YM, Wu HB, Huang F, Cao Y, Scherf U, Heeger AJ, Bazan GC. Solution-processed pH-neutral conjugated polyelectrolyte improves interfacial contact in organic solar cells. J Am Chem Soc 2011;133:8416–19. [22] Yang TB, Wang M, Duan CH, Hu XW, Huang L, Peng JB, Huang F, Gong X. Inverted polymer solar cells with 8.4% efficiency by conjugated polyelectrolyte. Energy Environ Sci 2012;5:8208–14. [23] Jo J, Pouliot JR, Wynands D, Collins SD, Kim JY, Nguyen TL, Woo HY, Sun YM, Leclerc M, Heeger AJ. Enhanced Efficiency of Single and Tandem Organic Solar Cells Incorporating a Diketopyrrolopyrrole-Based Low-Bandgap Polymer by Utilizing Combined ZnO/Polyelectrolyte Electron-Transport Layers. Adv Mater 2013;25:4783–8. [24] Zhou Y, Li F, Barrau S, Tian W, Inganäs O, Zhang F. Inverted and transparent polymer solar cells prepared with vacuum-free processing. Sol Energy Mater Solar Cells 2009;93:497–500. [25] Zhou Y, Fuentes-Hernandez C, Shim J, Meyer J, Giordano AJ, Li H, Winget P, Papadopoulos T, Cheun H, Kim J. A universal method to produce low–work function electrodes for organic electronics. Science 2012;336:327–32. [26] Lampande R, Kim GW, Pode R, Kwon JH. Effectiveness of a polyvinylpyrrolidone interlayer on a zinc oxide film for interfacial modification in inverted polymer solar cells. RSC Adv 2014;4:49855–60. [27] Yang X, Wang R, Fan C, Li G, Xiong Z, Jabbour GE. Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes. Org Electron 2014;15:2387–94. [28] Courtright BA, Jenekhe SA. n-Type semiconducting naphthalene diimide-perylene diimide copolymers: controlling crystallinity, blend morphology, and compatibility toward high-performance all-polymer solar cells. ACS Appl Mater Interfaces 2015;7:26167–75. [29] Yu J, Xi Y, Chueh CC, Xu JQ, Zhong H, Lin F, Jen AKY. Boosting performance of inverted organic solar cells by using a planar coronene based electron-transporting layer. Nano energy 2017;39:454–60.
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[30] Lin PC, Wong YT, Su YA, Chen WC, Chueh CC. Interlayer modification using eco-friendly glucose-based natural polymers in polymer solar cells. ACS Sustain Chem Eng 2018;6:14621–30. [31] Wu C-H, Chin C-Y, Chen T-Y, Hsieh S-N, Lee C-H, Guo T-F, Jen AK-Y, Wen T-C. Enhanced performance of polymer solar cells using solution-processed tetra-n-alkyl ammonium bromides as electron extraction layers. J Mater Chem A 2013;1:2582–7. [32] Nian L, Zhang WQ, Wu SP, Qin LQ, Liu LL, Xie ZQ, Wu HB, Ma YG. Perylene bisimide as a promising zinc oxide surface modifier: enhanced interfacial combination for highly efficient inverted polymer solar cells. ACS Appl Mater Interfaces 2015;7:25821–7. [33] Yu W, Huang L, Yang D, Fu P, Zhou L, Zhang J, Li C. Efficiency exceeding 10% for inverted polymer solar cells with a ZnO/ionic liquid combined cathode interfacial layer. J. Mater. Chem. A 2015;3:10660–5. [34] Ouyang XH, Peng RX, Ai L, Zhang XY, Ge ZY. Efficient polymer solar cells employing a non-conjugated small-molecule electrolyte. Nat Photon 2015;9:520. [35] Guo BA, Han JX, Qiu J, Yu CZ, Sun YQ, Li FH, Hu ZH, Wang Y. Alcohol-Soluble Isoindigo Derivative IIDTh-NSB as a Novel Modifier of ZnO in Inverted Polymer Solar Cells. ACS Appl Mater Interfaces 2017;9:42969–77. [36] Goh C, Scully SR, McGehee MD. Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cells. J Appl Phys 2007;101:114503. [37] Hau SK, Yip HL, Acton O, Baek NS, Ma H, Jen AKY. Interfacial modification to improve inverted polymer solar cells. J Mater Chem 2008;18:5113–19. [38] Hau SK, Yip HL, Ma H, Jen AKY. High performance ambient processed inverted polymer solar cells through interfacial modification with a fullerene self-assembled monolayer. Appl Phys Lett 2008;93. [39] Yip HL, Hau SK, Baek NS, Ma H, Jen AKY. Polymer solar cells that use self-assembled-monolayer-modified ZnO/metals as cathodes. Adv Mater 2008;20:2376–82. [40] Hau SK, Cheng YJ, Yip HL, Zhang Y, Ma H, Jen AKY. Effect of chemical modification of fullerene-based self-assembled monolayers on the performance of inverted polymer solar cells. ACS Appl Mater Interfaces 2010;2:1892–902. [41] Alstrup J, Jorgensen M, Medford AJ, Krebs FC. Ultra fast and parsimonious materials screening for polymer solar cells using differentially pumped slot-die coating. ACS Appl Mater Interfaces 2010;2:2819–27. [42] Oh H, Krantz J, Litzov I, Stubhan T, Pinna L, Brabec CJ. Comparison of various solgel derived metal oxide layers for inverted organic solar cells. Sol Energy Mater Sol Cells 2011;95:2194–9. [43] Trost S, Zilberberg K, Behrendt A, Polywka A, Gorrn P, Reckers P, Maibach J, Mayer T, Riedl T. Overcoming the “light-soaking” issue in inverted organic solar cells by the use of Al: ZnO electron extraction layers. Adv Energy Mater 2013;3:1437–44. [44] Shin KS, Lee KH, Lee HH, Choi D, Kim SW. Enhanced power conversion efficiency of inverted organic solar cells with a Ga-doped ZnO nanostructured thin film prepared using aqueous solution. J Phys Chem C 2010;114:15782–5. [45] Thambidurai M, Kim JY, Song J, Ko Y, Song HJ, Kang CM, Muthukumarasamy N, Velauthapillai D, Lee C. High performance inverted organic solar cells with solution processed Ga-doped ZnO as an interfacial electron transport layer. J Mater Chem C 2013;1:8161–6. [46] Kyaw AKK, Sun XW, Zhao DW, Tan ST, Divayana Y, Demir HV. Improved inverted organic solar cells with a solgel derived indium-doped zinc oxide buffer layer. IEEE J Select Top Quant Electron 2010;16:1700–6. [47] Yin Z, Zheng Q, Chen SC, Cai D, Zhou L, Zhang J. Bandgap Tunable Zn1-xMgxO Thin Films as Highly Transparent Cathode Buffer Layers for High-Performance Inverted Polymer Solar Cells. Adv Energy Mater 2014;4:1301404. [48] Liao SH, Jhuo HJ, Cheng YS, Chen SA. Fullerene derivative-doped zinc oxide nanofilm as the cathode of inverted polymer solar cells with low-bandgap polymer (PTB7-Th) for high performance. Adv Mater 2013;25:4766–71. [49] Liao SH, Jhuo HJ, Yeh PN, Cheng YS, Li YL, Lee YH, Sharma S, Chen SA. 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. [50] Wu C-K, Sivashanmugan K, Guo T-F, Wen T-C. Enhancement of inverted polymer solar cells performances using cetyltrimethylammonium-bromide modified ZnO. Materials (Basel) 2018;11:378. [51] Chen Y-C, Lin C-H, Guo T-F, Wen T-C. Surfactant-enriched ZnO surface via sol–gel process for the efficient inverted polymer solar cell. ACS Appl Mater Interfaces 2018;10:26805–11.
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx
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Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells Chen-Hsueh Lin a, Che-Wei Huang a, Po-Hsin Wang a, Tzung-Fang Guo b, Ten-Chin Wen a,c,∗ a
Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan c Center of Applied Nanomedicine, National Cheng Kung University, Tainan 70101, Taiwan b
a r t i c l e
i n f o
Article history: Received 19 September 2019 Revised 6 November 2019 Accepted 8 November 2019 Available online xxx Keywords: Cationic dye Sol−gel ZnO Interfacial modification Electron transport Polymer solar cells1
a b s t r a c t ZnO layer was modified with the addition of Cationic dyes including Crystal Violet (CV)/Ethyl violet (EV) in sol–gel process for an electron transport layer in inverted polymer solar cells (PSCs). X-ray photoelectron spectra showed the presence of CV/EV at the top of ZnO surface. Besides, oxygen defect was significantly reduced by CV/EV modification due to the chloride occupation. Furthermore, the amount of CV/EV decreased progressively from ZnO surface to bottom, being evidenced by depth profile. With modification, the ZnO surface became smoother and more hydrophobic to improve the contact with active layer. Meanwhile, CV/EV participated in the crystallization which resulted in the larger ZnO crystal grain size. Interface dipole after modification would slightly reduce the work function of ZnO and the energy barrier between ZnO and active layer via Ultraviolet Photoelectron Spectroscopy and External Quantum Efficiency analysis. Accordingly, inverted PSCs possessed better morphology, better electron extraction ability with ZnO modified by CV and EV respectively, rendering the power conversion efficiency up to 8.80% and 9.06% in comparison to the pristine ZnO (7.59%). In conclusion, we demonstrate a facile way to improve morphological and electrical properties of ZnO layer by simply adding CV/EV in sol–gel ZnO to fabricate high performance PSCs. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Among various photovoltaic systems, bulk-heterojunction (BHJ) polymer solar cells (PSCs) with an active layer, which comprises a conjugated polymer donor and a fullerene derivative acceptor, have attracted a lot of attention due to their potential for promising efficiency, flexibility, large scale fabrication and roll to roll process [1–6]. Despite sacrificing performance, inverted architecture of PSCs can significantly improve device stability because it avoid utilizing low work-function metal, which is sensitive to moisture and oxygen [7,8]. Indium tin oxide (ITO) is a suitable cathode for inverted PSCs due to the good transparency, low cost and applicable for various solution process. There are plenty of options for cathode buffer layer on inverted PSCs. Among them ZnO has been widely utilized due to the appropriate energy level and high electron mobility [9–12]. To fabricate high crystallinity and electron mobility ZnO thin film, the annealing temperature of solgel process must surpass 350 °C [13,14]. However, high annealing
∗ Corresponding author at: Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan. E-mail address: [email protected] (T.-C. Wen).
temperature would hinder the potential for flexible solar cell and large-scale fabrication. Therefore, relatively low annealing temperature is necessary for flexible substrates while producing a suitable ZnO thin film [15]. For inverted PSCs, one of the crucial factors for better power conversion efficiency (PCE) is the efficient electron transport at the interface between the active layer and ZnO layer. Unfortunately, the solution-processed ZnO surface is rich with defects such as oxygen vacancy, zinc dangling bonds and unreacted functional groups. These defects at surface could trap the electron and cause recombination. Hence, the efficiency of electron collection would decrease [16,17]. Moreover, the poor interfacial contact of organic active layer and inorganic ZnO layer might reduce the electron extract ability and raise series resistance (Rs ) leading to bad performance and stability of inverted PSCs [18–20]. To improve the interface contact and reduce the defects on the surface, several methods have been applied to achieve the goal such as inserting an interface modification layer between ZnO and active layer. Conjugated polyelectrolyte [21–23], insulating polymer [24–30], organic small molecule [31–33], zwitterion [34,35] and self-assembled monolayer [36–40] have been applied as interlayer on top of ZnO to passivate the surface defects, reduce energy gap, and enhance the electron collection ability of the ZnO layer. Besides interface modification layer, doping is an alternative strategy to enhance
https://doi.org/10.1016/j.jtice.2019.11.010 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Fig. 1. Molecular structures of (a) CV and (b) EV.
Scheme 1. The mechanism illustration of cationic dyes CV/EV modification (i) on the ZnO surface and (ii) in the ZnO layer.
the interface contact. Metals such as aluminum [41–43], gallium [44,45] and indium [46] have been doped into ZnO layer to passivate surface defect and enhance electrical conductivity. Moreover, magnesium doped ZnO layer could not only improve layer property but also exhibit tunable bandgap by controlling doping concentration [47]. Meanwhile, organic small molecules [35] and fullerene derivative [48,49] have been blended into ZnO layer. The ZnO surface which rich in C60 would further promote the compatibility with active layer. Several studies have been reported that molecule with quaternary ammonium group would form interface dipole and reduce work function of ZnO [21,22]. Recently, ionic compound [50] and surfactants [51] with quaternary ammonium group have been reported to add in sol-gel ZnO process to improve ZnO surface. Molecular structures of crystal violet (CV) and ethyl violet (EV) are shown in Fig. 1. In this study, CV/EV containing quaternary ammonium group were added into sol-gel precursor solution of ZnO and fully investigate the role of cationic dyes in ZnO layer. 2. Experimental section 2.1. ZnO thin layer fabrication The ZnO precursor solution was prepared by dissolving zinc acetate dehydrate (Merck, 99.5%, 1 g) and ethanolamine (Acros, 98%, 0.28 g) in 2-methoxyethanol (Aldrich, 98%, 10 mL). After stirring for 8 h for hydrolysis reaction and aging, different weight percent of crystal violet (CV, Sigma-Aldrich, 98%) and ethyl violet (EV, SigmaAldrich, 98%) were added to ZnO precursor solution individually. PTB7-Th and PC71 BM which purchased from Lumtec Corp. The active layer solution was obtained by dissolving PTB7-Th:PC71 BM (1:1.5 wt%) in mixed solvents of chlorobenzene/1,8-diiodooctane (97:3 vol%) with concentration of 20 mg/mL. Indium tin oxide (ITO) substrates (sheet resistance: < 15 /sq) were purchased from
RITEK Corp. The patterned ITO substrates were cleaned ultrasonically with detergent, deionized water, acetone, and isopropyl alcohol in sequence for 15 min each. After that, ITO substrates were treated with oxygen plasma for 25 min. The ZnO precursor solution was spin-coated onto pre-treated ITO substrates at 40 0 0 rpm for 60 s and annealed at 180 °C for 1 h in ambient air to obtain ZnO thin layer (≈30 nm). The active layer solution was filtered through a 0.45 μm polytetrafluoroethylene filter and spin-coated ontop of ZnO layer at 10 0 0 rpm for 60 s. Finally, 10 nm of MoO3 and 80 nm of Ag were thermally evaporated under pressure of ≈10−7 Torr. The area of each device defined by shadow mask was 0.06 cm2 . 2.2. Characterization The surface morphology and properties were examined with Scanning electron microscope (SEM) (HR-FESEM-SU8010, Hitachi), atomic force microscopy (AFM) (Dimension Icon, Bruker, Billerica, MA) operating in tapping mode and water contact angle (FTA10 0 0B, First Ten Angstroms). The crystallinity of samples was analyzed by X-ray diffraction (XRD) (Ultima IV, Rigaku) with wavelength of 1.5406 A˚ at a scanning rate of 0.02°/s. The photoemission spectroscopy was carried out at the 09A2 U5-spectroscopy beamlines and He (21.21 eV) in the National Synchrotron Radiation Research Center in Taiwan for X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), respectively. The depth profile was performed by using PHI 50 0 0 VersaProbe (Scheme 1). 2.3. Methods The current density-voltage measurements of devices were performed in a nitrogen-filled glove box under AM 1.5 G illumination
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Fig. 2. XPS N 1s spectra of (a) doping CV and (b) doping EV. XPS C 1s spectra of (c) doping CV and (d) doping EV.
at 100 mW/cm2 using a Keithley 2400 source-measure unit. The AM 1.5 G illumination was simulated with an Oriel 300-W Solar Simulator and calibrated by a silicon photodiode with a protective KG5 filter which standardized by National Renewable Energy Laboratory. The percentage of the carrier generation from the illuminated light at different wavelengths was measured by external quantum efficiency (EQE) (QE-R3011, Enli Technology Co., Ltd). 3. Results and discussion To investigate the existence of cationic dye molecules and its influence on surface of ZnO film, X-ray photoelectron spectroscopy (XPS) measurements were conducted to analyze several elements. Fig. 2(a) and (b) indicate that N 1s signal start to appear after modified with 0.5–3 wt% of CV/EV, Fig. 2(c) and (d) also indicate that C 1s signal would appear after modified with 0.5–3 wt% of CV and EV respectively. There is no chance that N and C would exist in pristine ZnO crystal. Therefore, the organic dye molecules indeed exist on the surface of ZnO. On the other hand, the XPS Zn 2p result is presented in Fig. 3(a) and (b) which show that both characteristic peaks of Zn 2p3/2 and 2p1/2 slightly shift to higher binding energy after blending with CV/EV, and the shifts of the peaks could be contributed to the decrease in electron density around Zn. Moreover, the O 1s characteristic peak of ZnO can be separated to two different peaks at 530.6 eV and 532 eV (Fig. 4), which can be assigned to oxygen of the zinc oxide lattice and hydroxyl groups or oxygen vacancies respectively. The decrease in relative intensity of oxygen defect peaks after CV/EV modification indicates that dye molecules could effectively passivate the defects on ZnO
Fig. 3. XPS Zn 2p spectra of ZnO doping with CV and EV.
surface as shown in Fig. 4(b) and (c). Fig. S1 shows the surface morphology of pristine ZnO layers with and without CV/EV modifications through top-view scanning electron microscopy (SEM). There are several aggregation and irregular grains on pristine ZnO film as shown in Fig. S1(a). After CV/EV modification, the ZnO surface become smoother and more uniform which might be able to
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Fig. 4. XPS O 1s spectra of (a) pristine ZnO, (b) doping CV and (c) doping EV.
reduce possibility of electron trapping. Meanwhile, surface topography is another concerning issue that Fig. S2 shows the topography images of pristine ZnO layers with and without CV/EV modification investigated by tapping mode atomic force microscope (AFM). The root mean square (RMS) roughness slightly reduce from 2.05 nm to 1.62 nm and 1.73 nm after CV/EV molecules been doped to ZnO layer, respectively. Furthermore, Fig. S3 shows the water contact angle measurement of ZnO with and without dye modification that pristine ZnO hydrophilic surface with contact angle of 32.37° become more hydrophobic when cooperates with cationic dye additives which attributed to the dimethylaniline group of CV/EV. As mentioned above, the smoother and more hydrophobic surface renders better interface contact and performance of devices. To understand the distribution of cationic dye molecules in ZnO layer after modification, Fig. 5 showed the depth profile with Cl/Zn ratio versus depth. The examination of the depth profile shows that there is the same trend with Cl/Zn ratio i.e., the ratio decreased sharply until 2.5 nm; afterwards it decreased gradually to 25 nm. Meanwhile, the signal of Zn also disappears around 25 nm, indicating that the dye molecules might distribute in the whole ZnO layer. As for the crystallinity of ZnO, the X-ray diffraction (XRD) spectra of pristine ZnO with and without CV/EV modification is analyzed in Fig. 6. Two characteristic peaks at 30.75° and 35.63°
were assigned to (100) and (101) crystal planes of ZnO respectively. After CV/EV modification, both of the characteristic peaks become slender. With Scherrer equation calculation, the grain sizes are 13.63 nm, 24.07, and 24.10 nm for pristine ZnO, CV, and EV modification, respectively. As a result, the crystal lattice of ZnO become the larger after the modification. Namely, CV/EV participated in crystallization during the annealing process and distribute in the whole ZnO layer. According to the results mentioned above, we inferred that during the formation of ZnO lattice, CV/EV received the upward force from the solvent evaporation and the hydrogen bonding force between CV/EV and Zn(OH)2 . Accordingly, still few dye molecules resided in ZnO layer while the most dye molecules were driven to surface of ZnO. Dye molecules at ZnO surface would occupy the oxygen defect sites by Cl− anion with the quaternary ammonium cation staying close, causing ZnO surface more hydrophobic. On the other hand, dye molecules inside ZnO layer would reside in grain boundary of ZnO and form dipole between lone pairs of N and the unreacted hydroxyl groups. To further evaluate the electrical performance, ultra-violet spectroscopy (UPS) was utilized for analysis of ZnO work function with and without CV/EV modification as shown in Fig. 7(a). The work function value of pristine ZnO which is estimated from the low kinetic-energy cut-off is 4.49 eV, after CV/EV modification the work functions slightly shift to 4.32 and 4.28 eV, respectively, indicating the chloride anion and
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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Fig. 5. Depth profile of Cl/Zn ratio in ZnO layer with addition of (a) CV and (b) EV.
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quaternary ammonium cation could induce a dipole moment on the surface of ZnO. In addition, EQE measurement also indicates that cationic dye doped devices could collect more efficient carrier and show good agreement with the higher Jsc data of the devices as shown in Fig. 7(b). Therefore, dye molecules modification could reduce the energy barrier and hence improve the electron extraction ability causing the decrease of work function. Meanwhile, we fabricate the electron only device to verify the electron mobility. After CV/EV modification, the current density which is contributed nearly by electron is higher than pristine ZnO further confirmed the increase in electron extraction and the decrease in recombination as shown in Fig. S4. Finally, we apply ZnO modified with CV/EV to fabricate inverted polymer solar cells based on PTB7-Th: PC71 BM system to testify the electrical performance. The J–V characteristics of best performance devices with CV/EV modification are shown in Fig. 8(a), and the detailed photovoltaic parameters are summarized in Table 1. The devices with CV/EV modification show higher short circuit current density (Jsc ) and fill factor (FF) than pristine ZnO devices which can be attributed to the smoother and more hydrophobic surface of ZnO, resulting in better interface contact. Besides, the dye molecules could passivate the surface defects and form interface dipole, leading to better electron coupling activity and reducing the WF of ZnO to improve the electron extraction ability. In addition, the device with EV modification show better performance than CV due to longer alkyl group. As a result, device with EV modification could provide a suitable ZnO surface for active layer above to increase the PCE of device. Meanwhile, the J–V characteristics in the dark condition reveals the reducing leakage current and increasing current density with forward bias after CV/EV modification as shown in Fig. 8(b). Therefore, the charge recombination in device has been dramatically reduced with cationic dye modification. To understand the influence on device stability, pristine ZnO with and without CV/EV modification devices are fabricated and placed in glove box and ambient air for thirty days. As shown in Fig. 9(a), the stability does not vary much under glove box that they almost keep 95% PCE during thirty days measurement. In contrast, the stability under ambient air varies a lot as shown in Fig. 9(b). The PCE of pristine ZnO devices decreases to 70% on the third day and 59% on thirtieth day. However, the devices with CV/EV modification perform better stability, which are 65% and 75%, respectively, after thirty days exposure. At last, the devices with EV modification show better stability than with CV modification could be attributed to the longer alkyl group that provide a more hydrophobic surface to enhance the device lifetime.
Fig. 6. XRD spectra of pristine ZnO with 1wt% CV and 2wt% EV doping.
Fig. 7. (a) UPS spectra and (b) EQE of pristine ZnO, doping CV, doping EV films.
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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C.-H. Lin, C.-W. Huang and P.-H. Wang et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx Table 1 Detailed photovoltaic parameters of pristine ZnO, doping CV, doping EV devices.
Pristine 1 wt% CV 2 wt% EV
Voc (V)
Js c (mA/cm2 )
FF(%)
PCE(%)
0.78± 0.01(0.78) 0.78± 0.01(0.78) 0.77± 0.01(0.77)
14.66± 0.86(13.40) 15.23± 0.53(14.94) 16.89± 0.34(16.37)
0.68± 0.02(0.68) 0.74± 0.01(0.74) 0.71± 0.01(0.71)
7.59± 0.21(7.32) 8.80± 0.22(8.58) 9.06± 0.17(8.82)
Fig. 8. J–V characteristics of pristine ZnO, doping CV and doping EV devices (a) under AM 1.5 G illumination (b) in the dark condition.
Fig. 9. Stability of pristine ZnO, doping CV and doping EV devices in (a) glove box and (b) ambient air.
4. Conclusions In this study, we demonstrate a facile method to prepare CV/EV modified sol–gel ZnO layer by simply cationic dye molecules modification. CV/EV would distribute in the whole ZnO layer with the highest concentration at the surface of ZnO and decrease gradually downward according to the depth profile. CV/EV on ZnO surface would occupy the oxygen vacancies by Cl− . On the other hands, CV/EV inside ZnO layer would reside in the grain boundary of ZnO and form dipole with unreacted hydroxyl groups, above-mentioned situation could both reduce the obstacle of electron transporting. Moreover, CV/EV on surface could not only make surface smoother and more hydrophobic, but also form interface dipole to reduce WF of ZnO. As a result, CV/EV could provide suitable surface for the active layer above to increase PCE, electron extraction efficiency, stability, and decrease WF. Eventually, the PCE values of CV/EV modification devices were 8.80% and 9.06%, respectively, in comparison to the pristine ZnO (7.59%). Acknowledgments The study is financially supported by Ministry of Science and Technology, Taiwan under grants number 108-2221-E-006-159-MY3
and Center of Applied Nanomedicine (CAN) of National Chen Kung University in Taiwan. The resource assistance from both institutes is gratefully acknowledged. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.11.010. References [1] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science 1995;270:1789–91. [2] Su Y-W, Lan S-C, Wei K-H. Organic photovoltaics. Mater Today 2012;15:554–62. [3] Kaltenbrunner M, Sekitani T, Reeder J, Yokota T, Kuribara K, Tokuhara T, Drack M, Schwödiauer R, Graz I, Bauer-Gogonea S. An ultra-lightweight design for imperceptible plastic electronics. Nature 2013;499:458. [4] You J, Dou L, Yoshimura K, Kato T, Ohya K, Moriarty T, Emery K, Chen C-C, Gao J, Li G. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat Commun 2013;4:1446. [5] Krebs FC, Espinosa N, Hösel M, Søndergaard RR, Jørgensen M. Rise to power–OPV-based solar parks. Adv Mater 2014;26:29–39. [6] Scharber MC. On the efficiency limit of conjugated polymer: fullerene-based bulk heterojunction solar cells. Adv Mater 2016;28:1994–2001. [7] White MS, Olson DC, Shaheen SE, Kopidakis N, Ginley DS. Inverted bulk-heterojunction organic photovoltaic device using a solution-derived ZnO underlayer. Appl Phys Lett 2006;89:3.
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010
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C.-H. Lin, C.-W. Huang and P.-H. Wang et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx [8] Kyaw A, Sun X, Jiang C, Lo G, Zhao D, Kwong D. 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. [9] Beek WJE, Wienk MM, Janssen RAJ. Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer. Adv Mater 20 04;16:10 09–13. [10] Gilot J, Barbu I, Wienk MM, Janssen RAJ. The use of ZnO as optical spacer in polymer solar cells: theoretical and experimental study. Appl Phys Lett 2007;91:3. [11] Liu J, Shao SY, Meng B, Fang G, Xie ZY, Wang LX, Li XL. Electrical characterization of back-gated bi-layer MoS2 field-effect transistors and the effect of ambient on their performances. Appl Phys Lett 2012;100:3. [12] Small CE, Chen S, Subbiah J, Amb CM, Tsang S-W, Lai T-H, Reynolds JR, So F. High-efficiency inverted dithienogermolethienopyrrolodione-based polymer solar cells. Nat Photon 2012;6:115. [13] Ong BS, Li C, Li Y, Wu Y, Loutfy R. Stable, solution-processed, high-mobility ZnO thin-film transistors. J Am Chem Soc 2007;129:2750–1. [14] Morvillo P, Diana R, Mucci A, Bobeico E, Ricciardi R, Minarini C. Influence of annealing treatments on solution-processed ZnO film deposited on ITO substrate as electron transport layer for inverted polymer solar cells. Sol Energy Mater Solar Cells 2015;141:210–17. [15] Sun Y, Seo JH, Takacs CJ, Seifter J, Heeger AJ. Inverted polymer solar cells integrated with a low-temperature-annealed sol-gel-derived ZnO film as an electron transport layer. Adv Mater 2011;23:1679–83. [16] Yang JL, Siempelkamp BD, Mosconi E, De Angelis F, Kelly TL. Origin of the thermal instability in CH3NH3PbI3 thin films deposited on ZnO. Chem Mater 2015;27:4229–36. [17] Yang F, Kang DW, Kim YS. Improved interface of ZnO/CH3 NH3 PbI3 by a dynamic spin-coating process for efficient perovskite solar cells. RSC Adv 2017;7:19030–8. [18] Janotti A, Van de Walle CG. Oxygen vacancies in ZnO. Appl Phys Lett 2005;87:122102. [19] Bai S, Jin Y, Liang X, Ye Z, Wu Z, Sun B, Ma Z, Tang Z, Wang J, Würfel U. Ethanedithiol treatment of solution-processed ZnO thin films: controlling the intragap states of electron transporting interlayers for efficient and stable inverted organic photovoltaics. Adv Energy Mater 2015;5:1401606. [20] MacLeod BA, De Villers BJT, Schulz P, Ndione PF, Kim H, Giordano AJ, Zhu K, Marder SR, Graham S, Berry JJ. Stability of inverted organic solar cells with ZnO contact layers deposited from precursor solutions. Energy Environ Sci 2015;8:592–601. [21] Seo JH, Gutacker A, Sun YM, Wu HB, Huang F, Cao Y, Scherf U, Heeger AJ, Bazan GC. Solution-processed pH-neutral conjugated polyelectrolyte improves interfacial contact in organic solar cells. J Am Chem Soc 2011;133:8416–19. [22] Yang TB, Wang M, Duan CH, Hu XW, Huang L, Peng JB, Huang F, Gong X. Inverted polymer solar cells with 8.4% efficiency by conjugated polyelectrolyte. Energy Environ Sci 2012;5:8208–14. [23] Jo J, Pouliot JR, Wynands D, Collins SD, Kim JY, Nguyen TL, Woo HY, Sun YM, Leclerc M, Heeger AJ. Enhanced Efficiency of Single and Tandem Organic Solar Cells Incorporating a Diketopyrrolopyrrole-Based Low-Bandgap Polymer by Utilizing Combined ZnO/Polyelectrolyte Electron-Transport Layers. Adv Mater 2013;25:4783–8. [24] Zhou Y, Li F, Barrau S, Tian W, Inganäs O, Zhang F. Inverted and transparent polymer solar cells prepared with vacuum-free processing. Sol Energy Mater Solar Cells 2009;93:497–500. [25] Zhou Y, Fuentes-Hernandez C, Shim J, Meyer J, Giordano AJ, Li H, Winget P, Papadopoulos T, Cheun H, Kim J. A universal method to produce low–work function electrodes for organic electronics. Science 2012;336:327–32. [26] Lampande R, Kim GW, Pode R, Kwon JH. Effectiveness of a polyvinylpyrrolidone interlayer on a zinc oxide film for interfacial modification in inverted polymer solar cells. RSC Adv 2014;4:49855–60. [27] Yang X, Wang R, Fan C, Li G, Xiong Z, Jabbour GE. Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes. Org Electron 2014;15:2387–94. [28] Courtright BA, Jenekhe SA. n-Type semiconducting naphthalene diimide-perylene diimide copolymers: controlling crystallinity, blend morphology, and compatibility toward high-performance all-polymer solar cells. ACS Appl Mater Interfaces 2015;7:26167–75. [29] Yu J, Xi Y, Chueh CC, Xu JQ, Zhong H, Lin F, Jen AKY. Boosting performance of inverted organic solar cells by using a planar coronene based electron-transporting layer. Nano energy 2017;39:454–60.
7
[30] Lin PC, Wong YT, Su YA, Chen WC, Chueh CC. Interlayer modification using eco-friendly glucose-based natural polymers in polymer solar cells. ACS Sustain Chem Eng 2018;6:14621–30. [31] Wu C-H, Chin C-Y, Chen T-Y, Hsieh S-N, Lee C-H, Guo T-F, Jen AK-Y, Wen T-C. Enhanced performance of polymer solar cells using solution-processed tetra-n-alkyl ammonium bromides as electron extraction layers. J Mater Chem A 2013;1:2582–7. [32] Nian L, Zhang WQ, Wu SP, Qin LQ, Liu LL, Xie ZQ, Wu HB, Ma YG. Perylene bisimide as a promising zinc oxide surface modifier: enhanced interfacial combination for highly efficient inverted polymer solar cells. ACS Appl Mater Interfaces 2015;7:25821–7. [33] Yu W, Huang L, Yang D, Fu P, Zhou L, Zhang J, Li C. Efficiency exceeding 10% for inverted polymer solar cells with a ZnO/ionic liquid combined cathode interfacial layer. J. Mater. Chem. A 2015;3:10660–5. [34] Ouyang XH, Peng RX, Ai L, Zhang XY, Ge ZY. Efficient polymer solar cells employing a non-conjugated small-molecule electrolyte. Nat Photon 2015;9:520. [35] Guo BA, Han JX, Qiu J, Yu CZ, Sun YQ, Li FH, Hu ZH, Wang Y. Alcohol-Soluble Isoindigo Derivative IIDTh-NSB as a Novel Modifier of ZnO in Inverted Polymer Solar Cells. ACS Appl Mater Interfaces 2017;9:42969–77. [36] Goh C, Scully SR, McGehee MD. Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cells. J Appl Phys 2007;101:114503. [37] Hau SK, Yip HL, Acton O, Baek NS, Ma H, Jen AKY. Interfacial modification to improve inverted polymer solar cells. J Mater Chem 2008;18:5113–19. [38] Hau SK, Yip HL, Ma H, Jen AKY. High performance ambient processed inverted polymer solar cells through interfacial modification with a fullerene self-assembled monolayer. Appl Phys Lett 2008;93. [39] Yip HL, Hau SK, Baek NS, Ma H, Jen AKY. Polymer solar cells that use self-assembled-monolayer-modified ZnO/metals as cathodes. Adv Mater 2008;20:2376–82. [40] Hau SK, Cheng YJ, Yip HL, Zhang Y, Ma H, Jen AKY. Effect of chemical modification of fullerene-based self-assembled monolayers on the performance of inverted polymer solar cells. ACS Appl Mater Interfaces 2010;2:1892–902. [41] Alstrup J, Jorgensen M, Medford AJ, Krebs FC. Ultra fast and parsimonious materials screening for polymer solar cells using differentially pumped slot-die coating. ACS Appl Mater Interfaces 2010;2:2819–27. [42] Oh H, Krantz J, Litzov I, Stubhan T, Pinna L, Brabec CJ. Comparison of various solgel derived metal oxide layers for inverted organic solar cells. Sol Energy Mater Sol Cells 2011;95:2194–9. [43] Trost S, Zilberberg K, Behrendt A, Polywka A, Gorrn P, Reckers P, Maibach J, Mayer T, Riedl T. Overcoming the “light-soaking” issue in inverted organic solar cells by the use of Al: ZnO electron extraction layers. Adv Energy Mater 2013;3:1437–44. [44] Shin KS, Lee KH, Lee HH, Choi D, Kim SW. Enhanced power conversion efficiency of inverted organic solar cells with a Ga-doped ZnO nanostructured thin film prepared using aqueous solution. J Phys Chem C 2010;114:15782–5. [45] Thambidurai M, Kim JY, Song J, Ko Y, Song HJ, Kang CM, Muthukumarasamy N, Velauthapillai D, Lee C. High performance inverted organic solar cells with solution processed Ga-doped ZnO as an interfacial electron transport layer. J Mater Chem C 2013;1:8161–6. [46] Kyaw AKK, Sun XW, Zhao DW, Tan ST, Divayana Y, Demir HV. Improved inverted organic solar cells with a solgel derived indium-doped zinc oxide buffer layer. IEEE J Select Top Quant Electron 2010;16:1700–6. [47] Yin Z, Zheng Q, Chen SC, Cai D, Zhou L, Zhang J. Bandgap Tunable Zn1-xMgxO Thin Films as Highly Transparent Cathode Buffer Layers for High-Performance Inverted Polymer Solar Cells. Adv Energy Mater 2014;4:1301404. [48] Liao SH, Jhuo HJ, Cheng YS, Chen SA. Fullerene derivative-doped zinc oxide nanofilm as the cathode of inverted polymer solar cells with low-bandgap polymer (PTB7-Th) for high performance. Adv Mater 2013;25:4766–71. [49] Liao SH, Jhuo HJ, Yeh PN, Cheng YS, Li YL, Lee YH, Sharma S, Chen SA. 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. [50] Wu C-K, Sivashanmugan K, Guo T-F, Wen T-C. Enhancement of inverted polymer solar cells performances using cetyltrimethylammonium-bromide modified ZnO. Materials (Basel) 2018;11:378. [51] Chen Y-C, Lin C-H, Guo T-F, Wen T-C. Surfactant-enriched ZnO surface via sol–gel process for the efficient inverted polymer solar cell. ACS Appl Mater Interfaces 2018;10:26805–11.
Please cite this article as: C.-H. Lin, C.-W. Huang and P.-H. Wang et al., Sol–gel ZnO modified by organic dye molecules for efficient inverted polymer solar cells, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.11.010