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Roll-to-roll micro-gravure printed large-area zinc oxide thin film as the electron transport layer for solution-processed polymer solar cells
Accepted Manuscript Roll-to-roll micro-gravure printed large-area zinc oxide thin film as the electron transport layer for solution-processed polymer solar cells Chujun Zhang, Qun Luo, Han Wu, Hengyue Li, Junqi Lai, Guoqi Ji, Linpeng Yan, Xiaofeng Wang, Dou Zhang, Jian Lin, Liwei Chen, Junliang Yang, Changqi Ma PII:
S1566-1199(17)30125-8
DOI:
10.1016/j.orgel.2017.03.015
Reference:
ORGELE 4012
To appear in:
Organic Electronics
Received Date: 10 February 2017 Revised Date:
10 March 2017
Accepted Date: 14 March 2017
Please cite this article as: C. Zhang, Q. Luo, H. Wu, H. Li, J. Lai, G. Ji, L. Yan, X. Wang, D. Zhang, J. Lin, L. Chen, J. Yang, C. Ma, Roll-to-roll micro-gravure printed large-area zinc oxide thin film as the electron transport layer for solution-processed polymer solar cells, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.03.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Abstract Graphic
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R2R Micro-gravure Printed ZnO Thin Film
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500 nm
ACCEPTED MANUSCRIPT Roll-to-Roll Micro-gravure Printed Large-Area Zinc Oxide Thin Film as the Electron Transport Layer for Solution-Processed Polymer Solar Cells Chujun Zhang, Linpeng Yan,b
†, a
Qun Luo,
†, b
Han Wu, †, a Hengyue Li, a Junqi Lai,
Guoqi Ji,
b
Xiaofeng Wang, d Dou Zhang, d Jian Lin, b Liwei Chen, c Junliang
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Yang *, a ,d and Changqi Ma *, b a
c
Hunan Key Laboratory for Super Microstructure and Ultrafast Process, School of
Physics and Electronics, Central South University, Changsha 410083, Hunan, P. R.
b
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China
Printable Electronic Research Center, Suzhou Institute of Nano-Tech and
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Nano-Bionics, Chinese Academy of Sciences (CAS), Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou 215123, P. R. China c
i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech
and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China State Key Laboratory of Powder Metallurgy, Central South University, Changsha
410083, China
†
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d
These authors contributed equally to this work.
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* Corresponding authors: [email protected] (J.L. Yang), +86-731-88660256
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[email protected] (C.Q. Ma), +86-512-62872769
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Abstract Large-scale, roll-to-roll (R2R) micro-gravure printing process was developed to deposit
the
electron
transport
layer
(ETL)
using
low-temperature,
solution-processable zinc oxide (ZnO) nanoparticle ink on flexible substrate for
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fabricating inverted organic solar cells (OSCs). The properties of micro-gravure R2R printed ZnO thin film was optimized via using web tension, substrate pre-treatment and printing speed, leading to high-quality and thickness controllable ZnO thin films. The inverted OSCs using R2R micro-gravure printed ZnO thin film as the ETL
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showed performance parameters comparable to those of spin-coated ZnO thin film ETL on the flexible substrate in both P3HT:PCBM and PTB7-Th:PC71BM based
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devices. The research demonstrated that the potentially commercial large-scale, R2R micro-gravure printing process could be used to produce high-quality ZnO thin film with controllable thickness for efficient inverted OSCs, which would accelerate the development of fully R2R micro-gravure printing OSCs and their commercialization.
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Keywords: Roll-to-roll; micro-gravure printing; ZnO; organic solar cells
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ACCEPTED MANUSCRIPT 1. Introduction Organic solar cells (OSCs) provide a promising alternative for producing electricity from clean and sustainable solar energy radiation, and have attracted great research interest in the last decade due to the advantages of low cost,
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semi-transparency, flexibility and printing-process [1-6]. The recent progress in OSCs development has led to the power conversion efficiency (PCE) over 10 % for small-size devices [7-11] and over 5% for the modules [12-14]. The roll-to-roll (R2R) printing/coating processes are desirable for fabricating large-scale, flexible OSCs with
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low cost and high output, and they are probably the potential techniques for commercializing OSCs [15-17]. However, challenges still remain in producing high
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efficiency, large-area OSCs via R2R printing or coating process due to the more difficult controllability as compared with the spin-coating process, and normally the PCEs of R2R printed OSCs are much lower than that of spin-coated OSCs. Meanwhile, the inverted structure OSCs have become the preferred device configuration because it shows better stability and controllability than the
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conventional configuration [18-20]. Metal oxides such as titanium oxide (TiOx), zinc oxide (ZnO) and stannic oxide (SnO2) have been used as the electron transport layer (ETL) in OSCs for improving the stability, the charge collection and the PCEs
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[21-23].
ZnO has good electron mobility and excellent degree of transparency in the visible wavelength range. Thus it has been widely used in OSCs as the ETL [24-25].
o
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Normally, ZnO nanoparticle thin film requires a sintering temperature higher than 350 C via a sol-gel process, which incomparable with flexible R2R substrate that only
endure low-temperature (< 150 oC) and short-time treatment. Although it is reported that TiO2 nanocrystalline thin film could be prepared by solution-processed and annealing-free method [26], the PCEs of inverted OSCs with R2R printed TiO2 layer is still very low [27-28]. Hence, it is very interesting and important to develop low-temperature, solution-processed ZnO layer for OSCs. Krebs’ group reported that ZnO nanoparticles could be processed via large-scale R2R slot-die coating and used as the ETL in printed inverted OSCs modules [16,29-32]. 3
ACCEPTED MANUSCRIPT Herein, we report the controllable fabrication of ZnO layer via large-scale R2R micro-gravure printing on flexible ITO/PET substrates. Processing parameters including web tension, substrate pre-treatment and printing speed were adjusted to optimize the quality and control the thickness of the resulting ZnO films. The inverted
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OSCs using R2R micro-gravure printed ZnO thin film as the ETL showed the comparable performance parameters to the devices with ZnO thin film fabricated by spin-coating deposition on the flexible substrate. The research shows that large-scale and low-cost R2R micro-gravure printing is capable of producing high-quality ZnO
2. Experimental details 2.1 Materials
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fabricate large-area, highly efficient OSCs.
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thin films with controlled morphology and thickness, and thus can potentially
Poly(3-hexylthiophene)(P3HT), poly[(ethylhexyl-thiophenyl)-benzodithiophene(ethyhexyl)-thienothiophene] (PTB7-Th) and [6,6]-Phenyl-C61-butyric acid methyl
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ester (PC61BM) were purchased from Solarmer Energy Inc. The mlybdenum oxide (MoO3) was purchased from Sigma-Aldrich Inc. The [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) was purchased from American Dye Source Inc.
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2.2 The synthesis of ZnO nanoparticles
ZnO nanoparticles were synthesized through the reaction between KOH and
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Zn(OAc)2 in methanol solvent. The details are described in our previous paper [33]. After 12hrs’ standing, the precipitates were washed two times with methanol and centrifuged at 4000 rpm for 10 min. Finally the ZnO nanoparticles were dispersed in ethanol with a concentration of 15-20 mg/ml through ultrasonic stir. 2.3 R2R printing process and ZnO thin-film characterization ZnO thin film was fabricated by R2R micro-gravure printing using a self-developed R2R multi-function printer (Fig. 1a) on top of ITO-coated PET substrate with a sheet resistance of 25~35 Ωsq-1. The thickness of ITO/PET is 175 µm, of which the thickness of ITO is 0.150 µm. The detailed printing process will be 4
ACCEPTED MANUSCRIPT discussed below. For the reference, ZnO thin film was spin coated onto ITO-coated PET at 1000 rmp for 60s. The quality and morphology of ZnO thin films were characterized using optical microscope (DMM-200C, Caikang, Shanghai), scanning electron microscope (SEM,
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FEI Helios Nanolab 600i, USA), and atomic force microscopy (Agilent Technologies 5500 AFM/SPM System, USA). The thickness of ZnO thin film on flexible ITO/PET substrate was examined by imaging the cross-sectional samples fabricated by focused ion beam, in which two Pt Dep (Pt E Dep:100nm; Pt I Dep:1µm) as the surface
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protection layer was deposited by electron beam and ion beam on the surface of ZnO layer. The thickness of spin-coated ZnO layer was measured to be about 30 nm for the
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reference. 2.4 The fabrication and measurements of OSCs
Flexible OSCs were fabricated using R2R micro-gravure printed ZnO thin films, which were UV-ozone treated for 6 min and annealed at 100 oC in N2 for 10 min. The P3HT:PCBM layer was fabricated through spin-coating the mixture solution of P3HT
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and PCBM in 1,2-dichlorobenzene (O-DCB) with a weight ratio of 1:1 and a concentration of 27 mg/ml at 1000 rpm for 30s. Subsequently, the P3HT:PCBM thin films were dried in vacuum for 2 hours and then thermal annealed at150 oC for 10 min.
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For the fabrication of PTB7-Th:PC71BM devices, the PTB7-Th and PC71BM were first dissolved in chlorobenzene with a weight ratio of 1:1.5 (the polymer
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concentration of 7 mg/mL) at 50 oC for 3 hrs, and a 3% volume ratio of 1,8-diiodooctane (DIO) was added as the additive. The active thin films were fabricated through spin-coating at 1300 rpm for 1 min. On the top of photoactive layers, 20 nm MoO3 and 150 nm Al electrode were deposited through thermal evaporation with a mask, resulting in an active area of 0.09 cm2. The J-V measurement was carried out in nitrogen glove box with a Keithley 2400 source meter under simulated AM 1.5 G solar illumination (100 mW/cm2). External quantum efficiencies (EQE) were measured under simulated one sun operation conditions using bias light from a 532 nm solid state laser (Changchun New 5
ACCEPTED MANUSCRIPT Industries, MGL-III-532). Light from a 150 W tungsten halogen lamp (Osram 64642) was used as probe light and modulated with a mechanical chopper before passing the monochromator (Zolix, Omni-λ300) to select the wavelength [33].
3.1 Roll-to-Roll micro-gravure printing process
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3. Results and discussion
The large-area ZnO thin film was processed using a self-developed R2R multi-function printer, which includes an unwinder, a corona unit, a micro-gravure
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printing system, a heating unit and a re-winder, as shown in Fig. 1a. Two types of rollers, i.e., the transfer roller and the tension monitor roller, are integrated in the
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printer. The five-strip patterned micro-gravure roller was optimized with a line density of 80 lines/cm, a spiral rib angle of 45 o and a depth of 45 µm (Insets in Figure 1a). Based on the specified solution concentration, the printing roller speed and the web speed are the two parameters to adjust the morphology and the thickness of processed ZnO thin film. The ratio of printing roller speed and web speed (R = printing roller
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speed/web speed) is used as a parameter to describe the printing process, in which the web speed was normally fixed at 0.2 m/min, and the prescribed R could be controlled by the printing roller speed. The web tension, corona power and R were used to
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optimize the quality of R2R micro-gravure printed ZnO thin film. Fig. 1b shows the illustration of micro-gravure printing process.
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(a)
(b) web Micro-gravure printing roller flat scraper
Printing roller
Patterned spiral ribs
ink bath 80 µm
Fig. 1. (a) Photo of self-developed R2R multi-function printer with a length of 2.5 m and a height 6
ACCEPTED MANUSCRIPT of 1.6 m. Insets are a photo of micro-gravure printing roller composed of a five-stripe pattern (a width of 13 mm and a gap of 2 mm) and an optical microscope image of engraved micro-gravure printing roller surface with a line density of 80 lines/cm, a spiral rib angle of 45 o and a depth of 45
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µm. (b) Schematic of micro-gravure printing process.
3.2 Morphological and thickness control forR2R micro-gravure printed ZnO thin film Micro-gravure printing is a contact mode for processing thin film (Fig. 1b). The web tension can greatly influence the quality of printed thin film. There are three
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rollers for monitoring the web tension. They are unwinder roller tension (T1), transfer roller tension (T2) and re-winder roller tension (T3), of which T1 = T3. Initially, T1
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and T3 are set at 23 N, and T2 = 30 N. It is obvious that the surface of R2R printed ZnO on ITO/PET substrate is seriously scraped (Fig. 2a). Hence, the web tension is decreased to T1 = T3 = 18 N, and T2 = 20 N. The surface doesn’t show the scraps any more (Fig. 2b). If further decreasing the web tension, the web would be too loose to properly transfer the ink from the printing roller to the web substrate. Thus the web
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tension is optimized as T1 = T3 = 18 N, and T2 = 20 N.
The ink transfer and adhesion are very important during the printing process. Normally, the corona is used to treat the substrate surface for improving its surface
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property and enhance the wetting of ink on the substrate. Fig. 3c-f show the SEM morphology of ITO/PET substrate and R2R micro-gravure printed ZnO thin film on the ITO/PET substrates, which were treated with the corona power at 0 kW, 0.2 kW,
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0.8 kW, respectively. It is obvious that ZnO ink almost doesn’t transfer to the ITO/PET substrate and only little quantity of ZnO nanoparticles covers on the surface if without the corona treatment before printing process (Fig. 2d). Its surface morphology is very similar to that of ITO/PET substrate (Fig. 2c). As the ITO/PET substrate is treated with the corona at a power of 0.2 kW, there are some aggregations of ZnO nanoparticles on the ITO/PET surface but it still can not form a continuous ZnO thin film (Fig. 2e). It suggests that the corona treatment at 0.2 kW just can partially improve the surface property, and some ZnO ink can transfer to the substrate and partially adhesion on it. The surface energy is still too high to let ZnO ink 7
ACCEPTED MANUSCRIPT completely wet the substrate surface. Thus, increasing the corona power to 0.8 kW, ZnO ink can efficiently transfer from printing roller to ITO/PET substrate and wet the substrate surface very well, resulting in the formation of continuous and compact ZnO thin film (Fig. 2f). The corona treatment with suitable power is very important to form
(b)
T1 = 23 N T2 = 30 N T3 = 23 N
T1 = 18 N T2 = 20 N T3 = 18 N
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(a)
ITO/PET
10 µm
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10 µm
(c)
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high-quality ZnO thin film via R2R micro-gravure printing process.
(d)
ZnO on ITO/PET without corona
T1: un-wind roller tension T2: transfer roller tension T3: re-wind roller tension
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1 µm
ZnO on ITO/PET with corona at 0.2 KW
ZnO on ITO/PET with corona at 0.8 KW
(f)
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(e)
1 µm
1 µm
1 µm
Fig. 2. Optical microscopy images of printed ZnO Films on ITO/PET substrate with the web tension at (a) T1=18, T2=20,T3=18 and (b) T1=23 T2=30,T3=23. (c-f) SEM images of ITO/PET substrate and printed ZnO thin films on ITO/PET substrates with the corona treatment at a power of (d) 0 kW, (e) 0.2 kW and (f)0.8 kW, respectively.
The coverage and thickness of ZnO thin film is strongly dependent on the R2R printing parameter R, of which the web speed is fixed at 0.2 m/min and the printing 8
ACCEPTED MANUSCRIPT roller speed is controlled at specified values [34,35]. The results in Fig. 3 clearly reveal that the R dramatically influences the coverage and thickness of ZnO thin film via R2R micro-gravure printing fabrication. Fig. 3a is a spin-coated ZnO thin film on ITO/PET substrate for the reference, which is very uniform and smooth. As the R is
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set at 0.6, ZnO doesn’t cover the ITO/PET substrate, and the surface of ITO is exposed (Fig. 3b). Increasing the R to 0.9, the coverage of ZnO thin film is obvious improved, but it still doesn’t fully cover the surface of ITO/PET substrate (Fig. 3c). Further increasing the R to the value larger than 1.2, ZnO can completely cover the
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ITO/PET substrate, and the densely packed and uniform ZnO thin film could be formed (Fig. 3d-f). The increased R comes from the enhancement of the printing
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roller speed under a fixed web speed, which would provide more ZnO inks transferred from printing roller onto ITO/PET substrate, resulting in the enhancement of ZnO thin film coverage. As compared with spin-coated ZnO thin film (Fig. 3a), R2R micro-gravure printed ZnO thin film show the similar morphology except for the aggregation of some ZnO nanoparticles. The AFM images in Fig. 3g and h exhibit
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that the root-mean square (RMS) roughness of spin-coated ZnO thin film is about 2.6 nm while the RMS roughness of R2R micro-gravure printed ZnO thin film at R =1.8 is about 4.0 nm. The results suggest that R2R micro-gravure printed ZnO thin film is
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a little rougher than that of spin-coated ZnO thin film due to the aggregation of some ZnO nanoparticles in R2R printed ZnO thin film, which in some degree would influence the interface contact between the ZnO layer and the active layer, resulting in
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the performance decrease accordingly.
9
ACCEPTED MANUSCRIPT Spin-coating
(b)
R = 0.60
(c)
(d)
R = 1.20
(e)
R = 1.80
(f)
R = 0.90
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(a)
500 nm
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R = 2.40
(h)
RMS ~ 2.6 nm
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(g)
RMS ~ 4.0 nm
Fig. 3. SEM images of spin-coated ZnO thin film on ITO/PET substrate (a) and R2R
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micro-gravure printed ZnO thin films after 0.8 kW corona treatment on ITO/PET substrate at (b) R = 0.6, (c) R = 0.9, (d) R = 1.2, (e) R = 1.8, (f) R = 2.4, respectively. All the images have the same
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scale bar. (g-f) AFM images of spin-coated ZnO thin film and R2R micro-gravure printed ZnO thin film at R = 1.8, which show the RMS of about 2.6 nm and 4.0 nm, respectively.
The R2R micro-gravure printed ZnO thin film becomes denser with increasing
the R, resulting in the thickness increase of ZnO thin film as well. Normally, it is not easy to measure the thickness ZnO thin film on flexible ITO/PET substrate. Here the cross section of ZnO thin film prepared by FIB was used to measure the thickness of ZnO thin film at the R at 1.2, 1.8, and 2.4, respectively, as shown in Fig. 4. The thickness of ZnO thin film with the R at 0.6 and 0.9 is too thin to detect by FIB 10
ACCEPTED MANUSCRIPT cross-sectional image. The thickness of ITO layer is measured at about 150 nm as the reference (Fig. 4a), which is matched with the thickness value from the provider. Because it is very difficult to distinguish the ITO layer and ZnO layer in cross-sectional images due to the weak contrast. Thus the total thickness of ITO layer
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and ZnO layer could be measured together, resulting in the calculated thickness of ZnO thin films to be about 15 nm, 35 nm and 50 nm at R = 1.2, R = 1.8, and R = 2.4, respectively (Fig. 4b-d).
(a)
(b)
ITO substrate
ITO layer
~ 165 nm
150 nm
ITO + ZnO at R = 1.8
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165 nm – 150 nm = ~ 15 nm
PET layer
(c)
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Pt layer
ITO + ZnO at R = 1.2
(d)
ITO + ZnO at R = 2.4
250 nm
~ 180 nm
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180 nm – 150 nm = ~ 30 nm
~ 200 nm
200 nm – 150 nm = ~ 50 nm
Fig. 4. Cross-section SEM images of (a) ITO layer and ZnO thin films on ITO layer at (b) R = 1.2,
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(c) R = 1.8 and R =2.4, respectively. The thickness of R2R micro-gravure printed ZnO thin films on ITO layer are about 15 nm, 30 nm and 50 nm for the R = 1.2, R = 1.8 and R =2.4, respectively.
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All the images have the same scale bar as 250nm.
3.3 R2R micro-gravure printed ZnO thin film as the ETL in inverted OSCs Inverted OSCs with a structure of PET/ITO/ZnO/P3HT:PCBM/MoO3/Ag were
fabricated, in which the ZnO ETL was deposited either by spin-coating or R2R micro-gravure printing at the different R. The J-V curves of OSCs with ZnO ETL were shown in Fig. 5. From the J-V curves, the performance parameters of OSCs, including open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and PCEs are summarized in Table 1. The evolution of Voc, Jsc, FF, and PCEs for P3HT:PCBM-based inverted OSCs with the ZnO ETL at the different R are 11
ACCEPTED MANUSCRIPT represented in Fig. b-e. For Ref OSCs with spin-coated ZnO layer on ITO/PET substrate, the typical PCE is 2.85% with a Voc of 0.55 V, a Jsc of 8.53 mA/cm2, and an FF of 61%. When R2R micro-gravure printed ZnO thin films at the R increased from 0.6 to 1.2 are used as the ETL in OSCs, the performance parameters change
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accordingly, in which the PCEs are in range between 0.9 % and 2.4 %. As discussed above, the increased R from 0.6 to 1.8 results in the tremendous enhancement in the coverage of ZnO thin film on ITO/PET substrate, which is helpful to the electron extraction from the active layer to the electrode. Thus, the PCEs are greatly improved
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from 0.92 % to 2.4%, resulting from the improvement of Voc from 0.41V to 0.56 V, Jsc from 7.51 mA/cm2 to 8.40 mA/cm2, FF from 0.31 to 0.51. Further increasing the R to
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2.4, the PCE does not improve any more. In contrary, it decreases from 2.4 % to 1.73 %, mainly resulting from the decrease of FF (both Voc and Jsc are almost the same). The best performance parameters (PCE = 2.4%) could be produced at the R = 1.8.
As the R increases from 0.6 to 1.8, the coverage of R2R micro-gravure printed
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ZnO thin film on ITO/PET substrate is gradually improved, resulting in the formation of uniform and continuous ZnO thin film. The ZnO interlayer mainly acts as an electron collector and transport layer, rather than an electron acceptor forming the
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heterojunction with the donor polymer P3HT. The blends of P3HT and PCBM are intercalated into the ZnO layer which offer large interfacial area acts as a continuous conducting path to the cathode for efficient electron collection and transport [36].
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Hence, the performance parameters Voc, Jsc and FF are enhanced gradually, leading to the obvious improvement in PCEs. As the R is increased to 2.4, the thickness of ZnO thin film can be enhanced accordingly. The performance parameters Voc and Jsc remain almost the same as that at R = 1.8, but the FF is dramatically declined from 0.51 to 0.38, leading to the PCEs downwards to 1.73 %. One of the few disadvantages of ZnO is the restriction to a rather thin film due to the conductivity [37]. Normally, a thicker ZnO layer suppresses the performance due to its low conductivity, while a thin ZnO layer provides a much better electron selective interface and Ohmic contact with the cathode [38,39]. 12
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(a)
2
-2 -4
Ref R= R= R= R= R=
-6 -8 0.0
0.2
0.4 Voltage (V)
0.6
0.8
8.7 8.4 2
Jsc (mA/cm )
0.48 0.44
7.8 7.5
0.40 0.4
0.8
1.2
1.6
2.0
0.6
(d)
0.4
2.4
Printing parameter R
0.4
0.8
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0.4
0.3
1.2
1.6
2.0
1.6
2.0
2.4
2.4
(e)
1.6 1.2
0.8 0.4
0.8
1.2
1.6
2.0
2.4
Printing parameter R
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(f)
2
Current density (mAcm )
1.2
2.0
Printing parameter R
0
0.8
Printing parameter R
2.4
0.5
FF
8.1
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Voc (V)
0.52
(c)
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(b)
PCE (%)
0.56
0.6 0.9 1.2 1.8 2.4
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Current density (mA/cm )
0
-5
(1) Ref ZnO for PSCs Voc = 0.75 V, J sc = 14.69 mA/cm 2
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FF = 0.66,
-10
PCE = 7.27 %
(2) Printed ZnO for PSCs Voc = 0.75 V, J sc = 15.74 mA/cm 2
FF = 0.56,
PCE = 6.61%
Ref ZnO R2R ZnO at R = 1.8
-15
0.00
0.25
0.50 A
0.75
1.00
Fig. 5. (a) The typical J-V curves of P3HT:PCBM-based inverted OSCs with spin-coated ZnO layer and R2R micro-gravure printed ZnO layer at the different R. (b-e) The performance parameters Voc, Jsc, FF and PCEs of P3HT:PCBM-based OSCs as the function of R2R printing parameter R. (f) The typical J-V curves of PTB7-Th:PC71BM-based inverted OSCs with 13
ACCEPTED MANUSCRIPT spin-coated ZnO layer and R2R micro-gravure printed ZnO layer at the R= 1.8.
Table 1. The performance parameters of typical P3HT:PCBM-based and PTB7-Th:PC71BM-based inverted OSCs with spin-coated ZnO layer and R2R micro-gravure printed ZnO layer at the
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different R. Active Materials
ZnO layer
Voc (V)
Jsc (mA/cm2)
P3HT:PCBM
Spin-coating
0.55
8.50
P3HT:PCBM
R2R at R = 0.6
0.41
7.51
P3HT:PCBM
R2R at R = 0.9
0.54
7.60
P3HT:PCBM
R2R at R = 1.2
0.55
8.40
48
2.21%
P3HT:PCBM
R2R at R = 1.8
0.56
8.40
51
2.40%
P3HT:PCBM
R2R at R = 2.4
0.54
8.45
38
1.73%
PTB7-Th:PC71BM
Spin-coating
0.75
14.69
66
7.27%
PTB7-Th:PC71BM
R2R at R = 1.8
0.75
15.74
56
6.61%
PCE
61
2.85%
30
0.92%
46
1.89%
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FF (%)
One may notice that the best OSCs with a PCE of 2.4 % from R2R
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micro-gravure printed ZnO thin film at R = 1.8 is a little worse than that of spin-coated OSCs (2.85%). As compared in Table 1, the performance parameters Voc and Jsc are similar for OSCs with spin-coated ZnO and R2R printed ZnO at R = 1.8.
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The EQE results in Fig. 6a show that OSCs with R2R printed ZnO and spin-coated ZnO exhibit the Jsc of 8.32 mA/cm2 and 8.42 mA/cm2, respectively, which are
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comparable to the values measured from J-V curves (Table 1). But the FF of the former one is much better than that of the latter one. Normally, the FF is low in printed OSCs because of the surface roughness (Fig. 3g and h) and interface contact which would result in the small shunt resistance and relative poorly conducting contacts [15-17,40]. In addition, PTB7-Th:PC71BM-based inverted OSCs were fabricated as well using R2R micro-gravure printed ZnO thin film at the optimized R = 1.8. The J-V curves of typical OSCs were shown in Fig. 5f. OSCs with spin-coated ZnO layer as the ETL yields a typical PCE of 7.27 % with a Voc of 0.75 V, a Jsc of 14.69 mA/cm2, 14
ACCEPTED MANUSCRIPT and an FF of 66 %. While OSCs with R2R micro-gravure printed ZnO layer as the ETL give a typical PCE of 6.61 % with a Voc of 0.75 V, a Jsc of 15.74 mA/cm2, and an FF of 56%. The PCEs of OSCs with R2R processed ZnO layer just show a little smaller than that of OSCs with spin-coated ZnO layer, mainly resulting from the
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obvious decrease in FF. The EQE results in Fig. 6b show that OSCs with R2R printed ZnO and spin-coated ZnO exhibit the Jsc of 15.09 mA/cm2 and 13.70 mA/cm2, respectively, which are very similar to the values measured from J-V curves (Table 1). The results suggest that R2R micro-gravure printed ZnO thin film would be potential
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to fabricate large-area PSC modules. 75
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600
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Fig. 6. The EQE spectrum and integrated Jsc for (a) P3HT:PCBM-based and (b) PTB7-Th:PC71BM-based inverted OSCs with spin-coated ZnO layer and R2R micro-gravure printed ZnO layer at R = 1.8.
3. Conclusion High-quality ZnO thin film with controllable morphology and thickness was fabricated by large-scale R2R micro-gravure printing on flexible ITO/PET substrates. 15
ACCEPTED MANUSCRIPT The printing parameters, including web tension, corona and printing speed R, were used to adjust the transfer and adhesion of ZnO ink on ITO/PET substrates, leading to the formation of flat and uniform ZnO thin film. The R2R micro-gravure printed high-quality ZnO thin film as the ETL could be used to fabricate P3HT:PCBM-based
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and PTB7-Th:PC71BM-based inverted OSCs, which showed the comparable performance parameters as that with spin-coated ZnO thin film as the ETL. The research provides a large-scale, low cost and industrial compatible technique to produce large-area and high-quality ZnO thin film, exhibiting great application
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prospects in OSCs and other flexible optoelectronic devices.
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Acknowledgment
This work was supported by the National Natural Science Foundation of China (51673214, 51473184, 61306073), the Hunan Provincial Natural Science Foundation of China (2015JJ1015), and the Project of Innovation-driven Plan in Central South
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Highlights (1) Large-scale, R2R micro-gravure printing was developed to process ZnO nanoparticles as the electron transport layer (ETL).
inverted organic solar cells (OSCs) on flexible substrate.
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(2) The R2R printed ZnO ETL with low-temperature treatment was used to fabricate
printed and spin-coated ZnO ETLs.
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(3) The inverted OSCs showed comparable performance parameters for using R2R
(4) The commercial large-scale, R2R micro-gravure printing process could be
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potentially used to produce efficient OSCs.
S1566-1199(17)30125-8
DOI:
10.1016/j.orgel.2017.03.015
Reference:
ORGELE 4012
To appear in:
Organic Electronics
Received Date: 10 February 2017 Revised Date:
10 March 2017
Accepted Date: 14 March 2017
Please cite this article as: C. Zhang, Q. Luo, H. Wu, H. Li, J. Lai, G. Ji, L. Yan, X. Wang, D. Zhang, J. Lin, L. Chen, J. Yang, C. Ma, Roll-to-roll micro-gravure printed large-area zinc oxide thin film as the electron transport layer for solution-processed polymer solar cells, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.03.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Abstract Graphic
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R2R Micro-gravure Printed ZnO Thin Film
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500 nm
ACCEPTED MANUSCRIPT Roll-to-Roll Micro-gravure Printed Large-Area Zinc Oxide Thin Film as the Electron Transport Layer for Solution-Processed Polymer Solar Cells Chujun Zhang, Linpeng Yan,b
†, a
Qun Luo,
†, b
Han Wu, †, a Hengyue Li, a Junqi Lai,
Guoqi Ji,
b
Xiaofeng Wang, d Dou Zhang, d Jian Lin, b Liwei Chen, c Junliang
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Yang *, a ,d and Changqi Ma *, b a
c
Hunan Key Laboratory for Super Microstructure and Ultrafast Process, School of
Physics and Electronics, Central South University, Changsha 410083, Hunan, P. R.
b
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China
Printable Electronic Research Center, Suzhou Institute of Nano-Tech and
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Nano-Bionics, Chinese Academy of Sciences (CAS), Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou 215123, P. R. China c
i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech
and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China State Key Laboratory of Powder Metallurgy, Central South University, Changsha
410083, China
†
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d
These authors contributed equally to this work.
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* Corresponding authors: [email protected] (J.L. Yang), +86-731-88660256
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[email protected] (C.Q. Ma), +86-512-62872769
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Abstract Large-scale, roll-to-roll (R2R) micro-gravure printing process was developed to deposit
the
electron
transport
layer
(ETL)
using
low-temperature,
solution-processable zinc oxide (ZnO) nanoparticle ink on flexible substrate for
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fabricating inverted organic solar cells (OSCs). The properties of micro-gravure R2R printed ZnO thin film was optimized via using web tension, substrate pre-treatment and printing speed, leading to high-quality and thickness controllable ZnO thin films. The inverted OSCs using R2R micro-gravure printed ZnO thin film as the ETL
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showed performance parameters comparable to those of spin-coated ZnO thin film ETL on the flexible substrate in both P3HT:PCBM and PTB7-Th:PC71BM based
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devices. The research demonstrated that the potentially commercial large-scale, R2R micro-gravure printing process could be used to produce high-quality ZnO thin film with controllable thickness for efficient inverted OSCs, which would accelerate the development of fully R2R micro-gravure printing OSCs and their commercialization.
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Keywords: Roll-to-roll; micro-gravure printing; ZnO; organic solar cells
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ACCEPTED MANUSCRIPT 1. Introduction Organic solar cells (OSCs) provide a promising alternative for producing electricity from clean and sustainable solar energy radiation, and have attracted great research interest in the last decade due to the advantages of low cost,
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semi-transparency, flexibility and printing-process [1-6]. The recent progress in OSCs development has led to the power conversion efficiency (PCE) over 10 % for small-size devices [7-11] and over 5% for the modules [12-14]. The roll-to-roll (R2R) printing/coating processes are desirable for fabricating large-scale, flexible OSCs with
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low cost and high output, and they are probably the potential techniques for commercializing OSCs [15-17]. However, challenges still remain in producing high
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efficiency, large-area OSCs via R2R printing or coating process due to the more difficult controllability as compared with the spin-coating process, and normally the PCEs of R2R printed OSCs are much lower than that of spin-coated OSCs. Meanwhile, the inverted structure OSCs have become the preferred device configuration because it shows better stability and controllability than the
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conventional configuration [18-20]. Metal oxides such as titanium oxide (TiOx), zinc oxide (ZnO) and stannic oxide (SnO2) have been used as the electron transport layer (ETL) in OSCs for improving the stability, the charge collection and the PCEs
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[21-23].
ZnO has good electron mobility and excellent degree of transparency in the visible wavelength range. Thus it has been widely used in OSCs as the ETL [24-25].
o
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Normally, ZnO nanoparticle thin film requires a sintering temperature higher than 350 C via a sol-gel process, which incomparable with flexible R2R substrate that only
endure low-temperature (< 150 oC) and short-time treatment. Although it is reported that TiO2 nanocrystalline thin film could be prepared by solution-processed and annealing-free method [26], the PCEs of inverted OSCs with R2R printed TiO2 layer is still very low [27-28]. Hence, it is very interesting and important to develop low-temperature, solution-processed ZnO layer for OSCs. Krebs’ group reported that ZnO nanoparticles could be processed via large-scale R2R slot-die coating and used as the ETL in printed inverted OSCs modules [16,29-32]. 3
ACCEPTED MANUSCRIPT Herein, we report the controllable fabrication of ZnO layer via large-scale R2R micro-gravure printing on flexible ITO/PET substrates. Processing parameters including web tension, substrate pre-treatment and printing speed were adjusted to optimize the quality and control the thickness of the resulting ZnO films. The inverted
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OSCs using R2R micro-gravure printed ZnO thin film as the ETL showed the comparable performance parameters to the devices with ZnO thin film fabricated by spin-coating deposition on the flexible substrate. The research shows that large-scale and low-cost R2R micro-gravure printing is capable of producing high-quality ZnO
2. Experimental details 2.1 Materials
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fabricate large-area, highly efficient OSCs.
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thin films with controlled morphology and thickness, and thus can potentially
Poly(3-hexylthiophene)(P3HT), poly[(ethylhexyl-thiophenyl)-benzodithiophene(ethyhexyl)-thienothiophene] (PTB7-Th) and [6,6]-Phenyl-C61-butyric acid methyl
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ester (PC61BM) were purchased from Solarmer Energy Inc. The mlybdenum oxide (MoO3) was purchased from Sigma-Aldrich Inc. The [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) was purchased from American Dye Source Inc.
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2.2 The synthesis of ZnO nanoparticles
ZnO nanoparticles were synthesized through the reaction between KOH and
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Zn(OAc)2 in methanol solvent. The details are described in our previous paper [33]. After 12hrs’ standing, the precipitates were washed two times with methanol and centrifuged at 4000 rpm for 10 min. Finally the ZnO nanoparticles were dispersed in ethanol with a concentration of 15-20 mg/ml through ultrasonic stir. 2.3 R2R printing process and ZnO thin-film characterization ZnO thin film was fabricated by R2R micro-gravure printing using a self-developed R2R multi-function printer (Fig. 1a) on top of ITO-coated PET substrate with a sheet resistance of 25~35 Ωsq-1. The thickness of ITO/PET is 175 µm, of which the thickness of ITO is 0.150 µm. The detailed printing process will be 4
ACCEPTED MANUSCRIPT discussed below. For the reference, ZnO thin film was spin coated onto ITO-coated PET at 1000 rmp for 60s. The quality and morphology of ZnO thin films were characterized using optical microscope (DMM-200C, Caikang, Shanghai), scanning electron microscope (SEM,
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FEI Helios Nanolab 600i, USA), and atomic force microscopy (Agilent Technologies 5500 AFM/SPM System, USA). The thickness of ZnO thin film on flexible ITO/PET substrate was examined by imaging the cross-sectional samples fabricated by focused ion beam, in which two Pt Dep (Pt E Dep:100nm; Pt I Dep:1µm) as the surface
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protection layer was deposited by electron beam and ion beam on the surface of ZnO layer. The thickness of spin-coated ZnO layer was measured to be about 30 nm for the
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reference. 2.4 The fabrication and measurements of OSCs
Flexible OSCs were fabricated using R2R micro-gravure printed ZnO thin films, which were UV-ozone treated for 6 min and annealed at 100 oC in N2 for 10 min. The P3HT:PCBM layer was fabricated through spin-coating the mixture solution of P3HT
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and PCBM in 1,2-dichlorobenzene (O-DCB) with a weight ratio of 1:1 and a concentration of 27 mg/ml at 1000 rpm for 30s. Subsequently, the P3HT:PCBM thin films were dried in vacuum for 2 hours and then thermal annealed at150 oC for 10 min.
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For the fabrication of PTB7-Th:PC71BM devices, the PTB7-Th and PC71BM were first dissolved in chlorobenzene with a weight ratio of 1:1.5 (the polymer
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concentration of 7 mg/mL) at 50 oC for 3 hrs, and a 3% volume ratio of 1,8-diiodooctane (DIO) was added as the additive. The active thin films were fabricated through spin-coating at 1300 rpm for 1 min. On the top of photoactive layers, 20 nm MoO3 and 150 nm Al electrode were deposited through thermal evaporation with a mask, resulting in an active area of 0.09 cm2. The J-V measurement was carried out in nitrogen glove box with a Keithley 2400 source meter under simulated AM 1.5 G solar illumination (100 mW/cm2). External quantum efficiencies (EQE) were measured under simulated one sun operation conditions using bias light from a 532 nm solid state laser (Changchun New 5
ACCEPTED MANUSCRIPT Industries, MGL-III-532). Light from a 150 W tungsten halogen lamp (Osram 64642) was used as probe light and modulated with a mechanical chopper before passing the monochromator (Zolix, Omni-λ300) to select the wavelength [33].
3.1 Roll-to-Roll micro-gravure printing process
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3. Results and discussion
The large-area ZnO thin film was processed using a self-developed R2R multi-function printer, which includes an unwinder, a corona unit, a micro-gravure
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printing system, a heating unit and a re-winder, as shown in Fig. 1a. Two types of rollers, i.e., the transfer roller and the tension monitor roller, are integrated in the
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printer. The five-strip patterned micro-gravure roller was optimized with a line density of 80 lines/cm, a spiral rib angle of 45 o and a depth of 45 µm (Insets in Figure 1a). Based on the specified solution concentration, the printing roller speed and the web speed are the two parameters to adjust the morphology and the thickness of processed ZnO thin film. The ratio of printing roller speed and web speed (R = printing roller
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speed/web speed) is used as a parameter to describe the printing process, in which the web speed was normally fixed at 0.2 m/min, and the prescribed R could be controlled by the printing roller speed. The web tension, corona power and R were used to
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optimize the quality of R2R micro-gravure printed ZnO thin film. Fig. 1b shows the illustration of micro-gravure printing process.
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(a)
(b) web Micro-gravure printing roller flat scraper
Printing roller
Patterned spiral ribs
ink bath 80 µm
Fig. 1. (a) Photo of self-developed R2R multi-function printer with a length of 2.5 m and a height 6
ACCEPTED MANUSCRIPT of 1.6 m. Insets are a photo of micro-gravure printing roller composed of a five-stripe pattern (a width of 13 mm and a gap of 2 mm) and an optical microscope image of engraved micro-gravure printing roller surface with a line density of 80 lines/cm, a spiral rib angle of 45 o and a depth of 45
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µm. (b) Schematic of micro-gravure printing process.
3.2 Morphological and thickness control forR2R micro-gravure printed ZnO thin film Micro-gravure printing is a contact mode for processing thin film (Fig. 1b). The web tension can greatly influence the quality of printed thin film. There are three
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rollers for monitoring the web tension. They are unwinder roller tension (T1), transfer roller tension (T2) and re-winder roller tension (T3), of which T1 = T3. Initially, T1
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and T3 are set at 23 N, and T2 = 30 N. It is obvious that the surface of R2R printed ZnO on ITO/PET substrate is seriously scraped (Fig. 2a). Hence, the web tension is decreased to T1 = T3 = 18 N, and T2 = 20 N. The surface doesn’t show the scraps any more (Fig. 2b). If further decreasing the web tension, the web would be too loose to properly transfer the ink from the printing roller to the web substrate. Thus the web
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tension is optimized as T1 = T3 = 18 N, and T2 = 20 N.
The ink transfer and adhesion are very important during the printing process. Normally, the corona is used to treat the substrate surface for improving its surface
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property and enhance the wetting of ink on the substrate. Fig. 3c-f show the SEM morphology of ITO/PET substrate and R2R micro-gravure printed ZnO thin film on the ITO/PET substrates, which were treated with the corona power at 0 kW, 0.2 kW,
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0.8 kW, respectively. It is obvious that ZnO ink almost doesn’t transfer to the ITO/PET substrate and only little quantity of ZnO nanoparticles covers on the surface if without the corona treatment before printing process (Fig. 2d). Its surface morphology is very similar to that of ITO/PET substrate (Fig. 2c). As the ITO/PET substrate is treated with the corona at a power of 0.2 kW, there are some aggregations of ZnO nanoparticles on the ITO/PET surface but it still can not form a continuous ZnO thin film (Fig. 2e). It suggests that the corona treatment at 0.2 kW just can partially improve the surface property, and some ZnO ink can transfer to the substrate and partially adhesion on it. The surface energy is still too high to let ZnO ink 7
ACCEPTED MANUSCRIPT completely wet the substrate surface. Thus, increasing the corona power to 0.8 kW, ZnO ink can efficiently transfer from printing roller to ITO/PET substrate and wet the substrate surface very well, resulting in the formation of continuous and compact ZnO thin film (Fig. 2f). The corona treatment with suitable power is very important to form
(b)
T1 = 23 N T2 = 30 N T3 = 23 N
T1 = 18 N T2 = 20 N T3 = 18 N
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(a)
ITO/PET
10 µm
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10 µm
(c)
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high-quality ZnO thin film via R2R micro-gravure printing process.
(d)
ZnO on ITO/PET without corona
T1: un-wind roller tension T2: transfer roller tension T3: re-wind roller tension
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ZnO on ITO/PET with corona at 0.2 KW
ZnO on ITO/PET with corona at 0.8 KW
(f)
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1 µm
1 µm
1 µm
Fig. 2. Optical microscopy images of printed ZnO Films on ITO/PET substrate with the web tension at (a) T1=18, T2=20,T3=18 and (b) T1=23 T2=30,T3=23. (c-f) SEM images of ITO/PET substrate and printed ZnO thin films on ITO/PET substrates with the corona treatment at a power of (d) 0 kW, (e) 0.2 kW and (f)0.8 kW, respectively.
The coverage and thickness of ZnO thin film is strongly dependent on the R2R printing parameter R, of which the web speed is fixed at 0.2 m/min and the printing 8
ACCEPTED MANUSCRIPT roller speed is controlled at specified values [34,35]. The results in Fig. 3 clearly reveal that the R dramatically influences the coverage and thickness of ZnO thin film via R2R micro-gravure printing fabrication. Fig. 3a is a spin-coated ZnO thin film on ITO/PET substrate for the reference, which is very uniform and smooth. As the R is
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set at 0.6, ZnO doesn’t cover the ITO/PET substrate, and the surface of ITO is exposed (Fig. 3b). Increasing the R to 0.9, the coverage of ZnO thin film is obvious improved, but it still doesn’t fully cover the surface of ITO/PET substrate (Fig. 3c). Further increasing the R to the value larger than 1.2, ZnO can completely cover the
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ITO/PET substrate, and the densely packed and uniform ZnO thin film could be formed (Fig. 3d-f). The increased R comes from the enhancement of the printing
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roller speed under a fixed web speed, which would provide more ZnO inks transferred from printing roller onto ITO/PET substrate, resulting in the enhancement of ZnO thin film coverage. As compared with spin-coated ZnO thin film (Fig. 3a), R2R micro-gravure printed ZnO thin film show the similar morphology except for the aggregation of some ZnO nanoparticles. The AFM images in Fig. 3g and h exhibit
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that the root-mean square (RMS) roughness of spin-coated ZnO thin film is about 2.6 nm while the RMS roughness of R2R micro-gravure printed ZnO thin film at R =1.8 is about 4.0 nm. The results suggest that R2R micro-gravure printed ZnO thin film is
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a little rougher than that of spin-coated ZnO thin film due to the aggregation of some ZnO nanoparticles in R2R printed ZnO thin film, which in some degree would influence the interface contact between the ZnO layer and the active layer, resulting in
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the performance decrease accordingly.
9
ACCEPTED MANUSCRIPT Spin-coating
(b)
R = 0.60
(c)
(d)
R = 1.20
(e)
R = 1.80
(f)
R = 0.90
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(a)
500 nm
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R = 2.40
(h)
RMS ~ 2.6 nm
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(g)
RMS ~ 4.0 nm
Fig. 3. SEM images of spin-coated ZnO thin film on ITO/PET substrate (a) and R2R
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micro-gravure printed ZnO thin films after 0.8 kW corona treatment on ITO/PET substrate at (b) R = 0.6, (c) R = 0.9, (d) R = 1.2, (e) R = 1.8, (f) R = 2.4, respectively. All the images have the same
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scale bar. (g-f) AFM images of spin-coated ZnO thin film and R2R micro-gravure printed ZnO thin film at R = 1.8, which show the RMS of about 2.6 nm and 4.0 nm, respectively.
The R2R micro-gravure printed ZnO thin film becomes denser with increasing
the R, resulting in the thickness increase of ZnO thin film as well. Normally, it is not easy to measure the thickness ZnO thin film on flexible ITO/PET substrate. Here the cross section of ZnO thin film prepared by FIB was used to measure the thickness of ZnO thin film at the R at 1.2, 1.8, and 2.4, respectively, as shown in Fig. 4. The thickness of ZnO thin film with the R at 0.6 and 0.9 is too thin to detect by FIB 10
ACCEPTED MANUSCRIPT cross-sectional image. The thickness of ITO layer is measured at about 150 nm as the reference (Fig. 4a), which is matched with the thickness value from the provider. Because it is very difficult to distinguish the ITO layer and ZnO layer in cross-sectional images due to the weak contrast. Thus the total thickness of ITO layer
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and ZnO layer could be measured together, resulting in the calculated thickness of ZnO thin films to be about 15 nm, 35 nm and 50 nm at R = 1.2, R = 1.8, and R = 2.4, respectively (Fig. 4b-d).
(a)
(b)
ITO substrate
ITO layer
~ 165 nm
150 nm
ITO + ZnO at R = 1.8
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165 nm – 150 nm = ~ 15 nm
PET layer
(c)
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Pt layer
ITO + ZnO at R = 1.2
(d)
ITO + ZnO at R = 2.4
250 nm
~ 180 nm
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180 nm – 150 nm = ~ 30 nm
~ 200 nm
200 nm – 150 nm = ~ 50 nm
Fig. 4. Cross-section SEM images of (a) ITO layer and ZnO thin films on ITO layer at (b) R = 1.2,
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(c) R = 1.8 and R =2.4, respectively. The thickness of R2R micro-gravure printed ZnO thin films on ITO layer are about 15 nm, 30 nm and 50 nm for the R = 1.2, R = 1.8 and R =2.4, respectively.
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All the images have the same scale bar as 250nm.
3.3 R2R micro-gravure printed ZnO thin film as the ETL in inverted OSCs Inverted OSCs with a structure of PET/ITO/ZnO/P3HT:PCBM/MoO3/Ag were
fabricated, in which the ZnO ETL was deposited either by spin-coating or R2R micro-gravure printing at the different R. The J-V curves of OSCs with ZnO ETL were shown in Fig. 5. From the J-V curves, the performance parameters of OSCs, including open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and PCEs are summarized in Table 1. The evolution of Voc, Jsc, FF, and PCEs for P3HT:PCBM-based inverted OSCs with the ZnO ETL at the different R are 11
ACCEPTED MANUSCRIPT represented in Fig. b-e. For Ref OSCs with spin-coated ZnO layer on ITO/PET substrate, the typical PCE is 2.85% with a Voc of 0.55 V, a Jsc of 8.53 mA/cm2, and an FF of 61%. When R2R micro-gravure printed ZnO thin films at the R increased from 0.6 to 1.2 are used as the ETL in OSCs, the performance parameters change
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accordingly, in which the PCEs are in range between 0.9 % and 2.4 %. As discussed above, the increased R from 0.6 to 1.8 results in the tremendous enhancement in the coverage of ZnO thin film on ITO/PET substrate, which is helpful to the electron extraction from the active layer to the electrode. Thus, the PCEs are greatly improved
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from 0.92 % to 2.4%, resulting from the improvement of Voc from 0.41V to 0.56 V, Jsc from 7.51 mA/cm2 to 8.40 mA/cm2, FF from 0.31 to 0.51. Further increasing the R to
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2.4, the PCE does not improve any more. In contrary, it decreases from 2.4 % to 1.73 %, mainly resulting from the decrease of FF (both Voc and Jsc are almost the same). The best performance parameters (PCE = 2.4%) could be produced at the R = 1.8.
As the R increases from 0.6 to 1.8, the coverage of R2R micro-gravure printed
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ZnO thin film on ITO/PET substrate is gradually improved, resulting in the formation of uniform and continuous ZnO thin film. The ZnO interlayer mainly acts as an electron collector and transport layer, rather than an electron acceptor forming the
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heterojunction with the donor polymer P3HT. The blends of P3HT and PCBM are intercalated into the ZnO layer which offer large interfacial area acts as a continuous conducting path to the cathode for efficient electron collection and transport [36].
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Hence, the performance parameters Voc, Jsc and FF are enhanced gradually, leading to the obvious improvement in PCEs. As the R is increased to 2.4, the thickness of ZnO thin film can be enhanced accordingly. The performance parameters Voc and Jsc remain almost the same as that at R = 1.8, but the FF is dramatically declined from 0.51 to 0.38, leading to the PCEs downwards to 1.73 %. One of the few disadvantages of ZnO is the restriction to a rather thin film due to the conductivity [37]. Normally, a thicker ZnO layer suppresses the performance due to its low conductivity, while a thin ZnO layer provides a much better electron selective interface and Ohmic contact with the cathode [38,39]. 12
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(a)
2
-2 -4
Ref R= R= R= R= R=
-6 -8 0.0
0.2
0.4 Voltage (V)
0.6
0.8
8.7 8.4 2
Jsc (mA/cm )
0.48 0.44
7.8 7.5
0.40 0.4
0.8
1.2
1.6
2.0
0.6
(d)
0.4
2.4
Printing parameter R
0.4
0.8
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0.4
0.3
1.2
1.6
2.0
1.6
2.0
2.4
2.4
(e)
1.6 1.2
0.8 0.4
0.8
1.2
1.6
2.0
2.4
Printing parameter R
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(f)
2
Current density (mAcm )
1.2
2.0
Printing parameter R
0
0.8
Printing parameter R
2.4
0.5
FF
8.1
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Voc (V)
0.52
(c)
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(b)
PCE (%)
0.56
0.6 0.9 1.2 1.8 2.4
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Current density (mA/cm )
0
-5
(1) Ref ZnO for PSCs Voc = 0.75 V, J sc = 14.69 mA/cm 2
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FF = 0.66,
-10
PCE = 7.27 %
(2) Printed ZnO for PSCs Voc = 0.75 V, J sc = 15.74 mA/cm 2
FF = 0.56,
PCE = 6.61%
Ref ZnO R2R ZnO at R = 1.8
-15
0.00
0.25
0.50 A
0.75
1.00
Fig. 5. (a) The typical J-V curves of P3HT:PCBM-based inverted OSCs with spin-coated ZnO layer and R2R micro-gravure printed ZnO layer at the different R. (b-e) The performance parameters Voc, Jsc, FF and PCEs of P3HT:PCBM-based OSCs as the function of R2R printing parameter R. (f) The typical J-V curves of PTB7-Th:PC71BM-based inverted OSCs with 13
ACCEPTED MANUSCRIPT spin-coated ZnO layer and R2R micro-gravure printed ZnO layer at the R= 1.8.
Table 1. The performance parameters of typical P3HT:PCBM-based and PTB7-Th:PC71BM-based inverted OSCs with spin-coated ZnO layer and R2R micro-gravure printed ZnO layer at the
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different R. Active Materials
ZnO layer
Voc (V)
Jsc (mA/cm2)
P3HT:PCBM
Spin-coating
0.55
8.50
P3HT:PCBM
R2R at R = 0.6
0.41
7.51
P3HT:PCBM
R2R at R = 0.9
0.54
7.60
P3HT:PCBM
R2R at R = 1.2
0.55
8.40
48
2.21%
P3HT:PCBM
R2R at R = 1.8
0.56
8.40
51
2.40%
P3HT:PCBM
R2R at R = 2.4
0.54
8.45
38
1.73%
PTB7-Th:PC71BM
Spin-coating
0.75
14.69
66
7.27%
PTB7-Th:PC71BM
R2R at R = 1.8
0.75
15.74
56
6.61%
PCE
61
2.85%
30
0.92%
46
1.89%
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FF (%)
One may notice that the best OSCs with a PCE of 2.4 % from R2R
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micro-gravure printed ZnO thin film at R = 1.8 is a little worse than that of spin-coated OSCs (2.85%). As compared in Table 1, the performance parameters Voc and Jsc are similar for OSCs with spin-coated ZnO and R2R printed ZnO at R = 1.8.
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The EQE results in Fig. 6a show that OSCs with R2R printed ZnO and spin-coated ZnO exhibit the Jsc of 8.32 mA/cm2 and 8.42 mA/cm2, respectively, which are
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comparable to the values measured from J-V curves (Table 1). But the FF of the former one is much better than that of the latter one. Normally, the FF is low in printed OSCs because of the surface roughness (Fig. 3g and h) and interface contact which would result in the small shunt resistance and relative poorly conducting contacts [15-17,40]. In addition, PTB7-Th:PC71BM-based inverted OSCs were fabricated as well using R2R micro-gravure printed ZnO thin film at the optimized R = 1.8. The J-V curves of typical OSCs were shown in Fig. 5f. OSCs with spin-coated ZnO layer as the ETL yields a typical PCE of 7.27 % with a Voc of 0.75 V, a Jsc of 14.69 mA/cm2, 14
ACCEPTED MANUSCRIPT and an FF of 66 %. While OSCs with R2R micro-gravure printed ZnO layer as the ETL give a typical PCE of 6.61 % with a Voc of 0.75 V, a Jsc of 15.74 mA/cm2, and an FF of 56%. The PCEs of OSCs with R2R processed ZnO layer just show a little smaller than that of OSCs with spin-coated ZnO layer, mainly resulting from the
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obvious decrease in FF. The EQE results in Fig. 6b show that OSCs with R2R printed ZnO and spin-coated ZnO exhibit the Jsc of 15.09 mA/cm2 and 13.70 mA/cm2, respectively, which are very similar to the values measured from J-V curves (Table 1). The results suggest that R2R micro-gravure printed ZnO thin film would be potential
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to fabricate large-area PSC modules. 75
8
Ref ZnO R2R ZnO
45
6
4
30
2
15
P3HT:PCBM
0 300
400
500
600
2
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EQE (%)
(a)
Integrated J (mA/cm )
60
0 700
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Wavelength (nm) 16
Ref ZnO R2R ZnO
(b)
12
8
40
EP
EQE (%)
60
300
400
4
2
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20
0
Integrated J (mA/cm )
80
PTB7-Th:PC71BM 500
600
700
0 800
Wavelength (nm)
Fig. 6. The EQE spectrum and integrated Jsc for (a) P3HT:PCBM-based and (b) PTB7-Th:PC71BM-based inverted OSCs with spin-coated ZnO layer and R2R micro-gravure printed ZnO layer at R = 1.8.
3. Conclusion High-quality ZnO thin film with controllable morphology and thickness was fabricated by large-scale R2R micro-gravure printing on flexible ITO/PET substrates. 15
ACCEPTED MANUSCRIPT The printing parameters, including web tension, corona and printing speed R, were used to adjust the transfer and adhesion of ZnO ink on ITO/PET substrates, leading to the formation of flat and uniform ZnO thin film. The R2R micro-gravure printed high-quality ZnO thin film as the ETL could be used to fabricate P3HT:PCBM-based
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and PTB7-Th:PC71BM-based inverted OSCs, which showed the comparable performance parameters as that with spin-coated ZnO thin film as the ETL. The research provides a large-scale, low cost and industrial compatible technique to produce large-area and high-quality ZnO thin film, exhibiting great application
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prospects in OSCs and other flexible optoelectronic devices.
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Acknowledgment
This work was supported by the National Natural Science Foundation of China (51673214, 51473184, 61306073), the Hunan Provincial Natural Science Foundation of China (2015JJ1015), and the Project of Innovation-driven Plan in Central South
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Highlights (1) Large-scale, R2R micro-gravure printing was developed to process ZnO nanoparticles as the electron transport layer (ETL).
inverted organic solar cells (OSCs) on flexible substrate.
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(2) The R2R printed ZnO ETL with low-temperature treatment was used to fabricate
printed and spin-coated ZnO ETLs.
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(3) The inverted OSCs showed comparable performance parameters for using R2R
(4) The commercial large-scale, R2R micro-gravure printing process could be
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potentially used to produce efficient OSCs.