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Solution-processed annealing-free ZnO nanoparticles for stable inverted organic solar cells
Organic Electronics 15 (2014) 1035–1042
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Solution-processed annealing-free ZnO nanoparticles for stable inverted organic solar cells Salima Alem a, Jianping Lu a,⇑, Raluca Movileanu a, Terho Kololuoma a,b, Afshin Dadvand a, Ye Tao a,⇑ a b
Information and Communications Technologies Portfolio, National Research Council of Canada, 1200 Montreal Road, Ottawa, ON K1A 0R6, Canada Printed Functional Solutions, VTT Technical Research Centre of Finland, Kaitovayla 1, 90570 Oulu, Finland
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 21 February 2014 Accepted 26 February 2014 Available online 11 March 2014 Keywords: Zinc oxide nanoparticles Propylamine Inverted solar cell Bulk heterojunction Stability
a b s t r a c t We report the development and application of high-quality zinc oxide nanoparticles (ZnO NPs) processed in air for stable inverted bulk heterojunction solar cells as an electron extraction layer (EEL). The ZnO NPs (average size 11 nm) were dispersed in chloroform and stabilized by propylamine (PA). We demonstrated that the ZnO NP dispersion with 4 vol.% of PA as stabilizer can be used in air directly and remains clear up to one month after preparation. Our inverted solar cells consisted of a blade-coated poly(N-90 heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (1: 4 by weight) active layer sandwiched between a ZnO electron extraction layer and a MoO3/Ag anode. All solar cells with ZnO films fabricated in air using PA-stabilized ZnO dispersions prepared within a time window of one month exhibited power conversion efficiencies (PCE) above 4%. In contrast, if the ZnO film was prepared in air using regular un-stabilized ZnO NP dispersion, the PCE would drop to 0.2% due to poor film quality. More interestingly, X-ray photoelectron spectroscopy and nuclear magnetic resonance measurements indicated that the PA ligands were not covalently bonded to ZnO NPs and did not exist in the deposited ZnO films. The spin-cast ZnO thin films (without any thermal treatment) are insoluble in organic solvents and can be directly used as an EEL in solar cells. This feature is beneficial for fabricating organic solar cells on flexible polymer substrates. More importantly, our nonencapsulated inverted solar cells are highly stable with their PCEs remaining unchanged after being stored in air for 50 days. Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction Due to the high scientific and economic interest in organic photovoltaic research, the power conversion efficiency of organic solar cells has been pushed up steadily in the last decade, currently reaching 10% [1]. This progress
⇑ Corresponding authors. Tel.: +1 613 990 1651; fax: +1 990 0202 (J. Lu). E-mail addresses: [email protected] (J. Lu), ye.tao@nrc-cnrc. gc.ca (Y. Tao).
was achieved through tremendous efforts in the development of low band gap materials [2], the control of fabrication process to optimize the active layer morphology [3], and the improvement of interface engineering [4]. One of the approaches of interface engineering focuses on the incorporation of electron and hole extraction layers between the active layer and the metal cathode/anode, to align the energy levels at the interface, and consequently to allow an efficient extraction of electrons and holes, respectively. In both conventional and inverted organic solar cells, various materials such as polyelectrolytes [4a],
http://dx.doi.org/10.1016/j.orgel.2014.02.024 1566-1199/Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
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inorganic salts [5], and metal oxides (TiOx, ZnO, MoO3, V2O5, WO3) have been investigated and used as n-type or p-type interlayers to enhance the performance and/or the stability of the devices [6]. Among n-type materials, ZnO NP films, formed by sol– gel process, have been widely used in inverted organic solar cells. However, to form crystalline ZnO networks with good electron mobility, an annealing temperature over 200 °C is usually required, which is incompatible with the flexible polymer substrates used in the fabrication of solar cells. The solution-dispersed ZnO NPs have been shown as a good alternative to reduce the annealing temperature, and can be easily processed into films via various coating methods [7]. However, the ZnO NP dispersions in organic solvents are not stable in air and tend to form aggregates and quickly precipitate out, especially in the presence of moisture [8]. Therefore, organic ligands such as alkyl amine [9], alkyl thiol [10], carboxylic acid [11], and alkoxyacetic acid have been used to protect ZnO NPs and prevent their aggregation [8]. However, most ligands reported in the literature have high boiling points, which again require a high temperature annealing (>150 °C) to ensure complete removal of the ligands from the fabricated ZnO NP films in order to get good electron-transport properties. In this paper, we report the use of a short and low boiling point compound, propylamine (PA, bp 48 °C) as an effective stabilizing ligand for the ZnO NPs. The addition of PA at a concentration of 4% (v/v) keeps the ZnO NP dispersion in chloroform, chlorobenzene or dichlorobenzene stable over one month, even after exposure to ambient air. More importantly, the spin-cast ZnO NP films do not require any thermal annealing or plasma treatment and can be directly used as an electron extraction layer in inverted solar cells. In this work, pyridine and triethylamine have also been tested as the ZnO NP stabilizers. However, they are not as effective as propylamine although triethylamine is a stronger base than propylamine. In order to understand the mechanism involved in the stabilization of ZnO NPs by propylamine, the ZnO NP dispersion and the spin-cast films were characterized by nuclear magnetic resonance (NMR) and X-ray photo-electron spectroscopy (XPS), respectively. It was found that the PA ligands were not chemically bonded to ZnO NPs and did not exist in the deposited ZnO films. The concentration of the PA has been optimized. It is interesting to point out that our non-encapsulated inverted solar cells based on the PCDTBT:PC71BM blend and PA stabilized ZnO NPs are quite stable with their PCEs remaining unchanged after being stored in air for 50 days. Therefore, these air-processed and annealing-free ZnO nanoparticles are very valuable for the technology transfer of organic solar cells from the lab to the industry. 2. Experimental
(ppm) relative to internal chloroform residue (7.25 ppm). The XPS spectra were recorded on a Phi 5500 system, using a monochromatic Al X-ray source with beam energy of 1486 eV and a take-off angle of 45°. PCDTBT was provided by St-Jean Photochemicals Inc. and PC71BM was purchased from Nano-C, and they were used without further purification. The ZnO nanoparticles were synthesized according to our previous procedure [7a]. The ZnO nanoparticles were dispersed in anhydrous chloroform. After filtration through a 0.2 lm filter, a clear colorless dispersion was obtained and stored in a nitrogen-purged glovebox. Prior to device fabrication, it was diluted with anhydrous chloroform in order to obtain an optimal ZnO film thickness by spin-casting, and propylamine was added via a micropipette. 2.2. Device fabrication and characterization The bulk heterojunction solar cells were fabricated on pre-patterned indium tin oxide (ITO) coated glass substrates. The sheet resistance and thickness of the ITO are 12 X/sq and 150 nm, respectively. The ITO substrates were thoroughly cleaned in detergent and DI water, ultrasonicated in acetone and isopropyl alcohol for 15 min, and dried in an oven at 120 °C. UV–ozone treatment was then performed for 15 min. A ZnO NP film (20 nm-thick) was spin-cast on top of the ITO substrates from a freshly prepared ZnO dispersion at 700 rpm. Afterward, an active layer was blade-coated onto the ZnO film from a dichlorobenzene solution of PCDTBT:PC71BM with a weight ratio of 1:4 and dried overnight at room temperature in a nitrogenfilled glovebox. The PCDTBT:PC71BM solution with a PCDTBT concentration of 3 mg/mL was prepared three days before deposition and heated at 100 °C over night. The blade speed was set at 35 mm/s to get 70 nm thick active layers. Finally, to complete the solar cell architecture, a bilayer anode consisting of 100 nm Ag on top of 10 nm molybdenum oxide was thermally evaporated through a shadow mask on the active layer to form cells with an active area of 1 cm2. The thicknesses of the films were measured by a Dektak profilometer. The photovoltaic parameters were extracted from the current–voltage (J–V) characteristics measured in air with a Keithley 2400 Digital SourceMeter and the photocurrent was generated under air mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm2 from a ScienceTech SS 500W solar simulator. The light intensity was adjusted using a calibrated Si photodiode with a KG-5 filter purchased from PV Measurements, Inc. The external quantum efficiency (EQE) was performed using a Jobin-Yvon Triax spectrometer, a Jobin-Yvon xenon light source, a Merlin lock-in amplifier, a calibrated Si UV detector, and an SR570 low noise current amplifier. It is worth pointing out that the short-circuited photocurrent reported in this paper was always calculated from the wavelength integration of the product of the EQE curve and the standard AM 1.5G solar spectrum.
2.1. Instrumentation and materials 3. Results and discussion NMR spectra were recorded on a 400 MHz Varian Unity Inova spectrometer. The samples were prepared in deuterated CHCl3. Chemical shifts were reported as d values
Fig. 1 shows the evolution of the ZnO NP dispersion with time before and after exposure to air. Within 5 min
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of exposure, the ZnO NPs underwent quick aggregation due to the reaction with moisture, and the dispersion became cloudy (Fig. 1b) or milky. Within 30 min, the large aggregates settled down in the bottom of the vial due to the gravitational force (Fig. 1c). The rate of this aggregation process depends strongly on the size of the nanoparticles, the type of the solvent, and the humidity in air [12]. By adding PA to the ZnO NP dispersion, even with a volume ratio as low as 0.25%, the dispersion remains clear for a few days. The average particle size measured by a particle analyzer (Zetasizer, Nano ZS Malvern) is about 11 nm in diameter, and 15 nm by atomic force microscopy (AFM). Totally, more than 50 devices using ZnO NP interlayer, cast from un-stabilized and stabilized dispersions with different PA ratios, have been fabricated and characterized. The devices were quite reproducible in term of the efficiency. Table 1 shows the typical photovoltaic parameters of the inverted devices made with ZnO NP films spin-cast under different conditions. The devices made in air with a PA concentration (v/v) higher than 1% exhibited a comparable performance to the device made with a fresh unstabilized ZnO dispersion in glove box, within the statistical fluctuations. On the other hand, the devices made with a low PA concentration (such as 0.25%) exhibited a slightly lower performance, because the ZnO NPs aggregation had probably begun. The device with a ZnO NP film spin-cast in air from the un-stabilized dispersion (Fig. 1b, without adding PA) showed a very poor performance (PCE 0.2%). The big ZnO aggregates resulted in poor film quality and bad coverage on the substrate. As a result, the open-circuit voltage was very low, only reached 0.1 V, and the device series resistance was high, leading to a very low fill factor (0.3). On the other hand, we also noticed that the performance (4.3%) of the device made with a blade-coated PCDTBT:PC71BM active layer was lower than the one made with a spin-coated active layer, which exhibited an EQE-calibrated PCE of 5.8% (Jsc = 10.9 mA/ cm2, Voc = 0.88 V, FF = 0.61). The reason is still under investigation; it may be due to the difference in drying process, resulting in different morphologies and/or interfaces. However, this also indicates that there is still a room to improve the efficiency of blade-coated devices. The lifetime of PA stabilized ZnO NP dispersion has been monitored by UV–Vis absorption spectroscopy and AFM inspection on the resulting thin films. Two disper-
Fig. 1. ZnO NP dispersion when freshly prepared (a) after 5 min (b) and after 30 min (c) of air exposure.
Table 1 Comparison of PV parameters of inverted devices, using ZnO layers spin cast from dispersions with different PA concentrations. Jsc is calculated from the EQE curves. PA-concentration (%)
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
0 (in GB) 0 0.25 0.5 1 2 3 4
9.2 5.4 8.3 9.1 9.2 9.4 9.7 9.4
0.88 0.10 0.85 0.83 0.83 0.84 0.84 0.86
52 30 50 52 51 55 51 52
4.2 0.2 3.6 3.9 3.9 4.3 4.2 4.2
sions of ZnO NPs have been prepared and stabilized by 2% and 4% PA, respectively. The absorbance and the surface morphology of the films spin-cast on quartz substrates are depicted in Figs. 2 and 3, respectively. The absorption of the ZnO NP film obtained from a fresh dispersion exhibits a distinguishable first excitonic absorption peak at 330 nm (Fig. 2). After 24 h, this peak started broadening and shifting toward longer wavelengths, indicating the continuing growth of ZnO NPs in solution at room temperature [13]. This red shift seemed more pronounced in the films spin-cast from the solution stabilized with 2% PA than the ones with 4% PA. In an extreme case, the absorption spectrum of ZnO NP film spin-cast from a 37-day-old 2% PA stabilized dispersion exhibited two absorption peaks at 270 and 350 nm, respectively. This implies that some nanoparticles continued to grow at the expense of other nanoparticles. This can also be seen in the AFM images. The AFM images in Fig. 3 show clearly that the sizes of the ZnO particles in the freshly prepared dispersoin with 2% PA are fairly uniform, but there is an obvious increase in particle/aggregate size after 24 h, followed by a gradual broadening in the particle/aggregate size distribution in the first 7 days, and some particles/aggregates are as large as 40 nm, which is consistent with the slight red shift between 1 and 7 days seen in the absorption spectra in Fig. 2. The non-uniformity of ZnO NP size distribution is much more evident in the films prepared with 37-dayold 2% PA stabilized ZnO dispersion as shown in the AFM image. On the other hand, the topography of ZnO NP films spin-cast from 4% PA stabilized dispersion after 30 days looks similar to the films spin-cast from the 7-day-old 2%-PA ZnO dispersion. Despite some broadening in particle/aggregate size distribution, the RMS roughness of the ZnO films measured by AFM falls in a range from 1.7 nm to 4.3 nm. The J–V characteristics of inverted devices made with aged and freshly prepared ZnO NP dispersions stabilized with 2% PA or 4% PA are presented in Fig. 4. A slight drop in performance was recorded in devices with ZnO films prepared using the 2% PA-ZnO NP dispersion aged in air for 37 days. The PCE dropped to 3.5% with a short-circuit current density (Jsc) of 8.4 mA/cm2, an open circuit voltage (Voc) of 0.84 V, and a fill factor of 0.51. This drop was due to the non-uniform distribution of ZnO NPs in the films which increases the series resistance of the devices. The devices using aged 4% PA-ZnO dispersion (at 30 days) showed a comparable performance to the ones using
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after 1h after 1 day after 2 days after 7 days after 37 days
ZnO + 2% PA
Absorbance (a.u.)
0.06 0.05
0.07
0.04 0.03 0.02 0.01 0.00 200
after 1h after 1 day after 30 days
ZnO + 4% PA
0.06
Absorbance (a.u.)
0.07
0.05 0.04 0.03 0.02 0.01
300
400
500
600
700
800
0.00 200
300
Wavelength (nm)
400
500
600
700
800
Wavelength (nm)
Fig. 2. The evolution in time of the absorption spectra of ZnO films cast from aged 2% PA (left) and 4% PA (right)-stabilized ZnO dispersions.
2% PA-freshly prepared
2% PA – 2 days
2% PA – 37 days
2% PA – 1 day
2% PA – 7 days
4% PA – 30 days
Fig. 3. AFM images of ZnO films cast from freshly prepared and aged ZnO dispersions, stabilized with 2% and 4% PA.
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20
20 7 days
15
1 hr
ZnO + 4% PA
1 hr
Current density (mA/cm2)
Current density (mA/cm2)
ZnO + 2% PA
37 days
10 5 0 -5 -10
30 days
15 10 5 0 -5 -10 -15
-15 -1.0
-0.5
0.0
0.5
-1.0
1.0
-0.5
0.0
Voltage (V)
0.5
1.0
Voltage (V)
Fig. 4. J–V characteristics of inverted PCDTBT:PC70BM devices using ZnO NP films cast from aged 2% and 4% PA stabilized dispersions.
80
35
(a)
in dark
25
under 100mW/cm
(b)
70
2
60
20 50 15
EQE (%)
Current density (mA/cm2)
30
10 5
40 30
0 20 -5 10
-10
0
-15 -0.5
0.0
0.5
1.0
300
400
Voltage (V)
500
600
700
800
Wavelength (nm)
Fig. 5. The J–V curve (a) and the EQE spectrum (b) of the optimized inverted device made with 4% PA stabilized ZnO dispersion, showing a PCE of 5%.
6.0
Power conversion efficiency (%)
freshly prepared dispersion, with a PCE of 4%, Jsc of 9.2 mA/ cm2, Voc of 0.85 V and FF of 0.51. The photovoltaic parameters of the devices prepared using fresh 2%-PA and 4%-PA stabilized ZnO dispersions are shown in Table 1. Very recently, we optimized the fabrication process and achieved a PCE of 5% (Jsc = 9.6 mA/cm2, Voc = 0.85 V, FF = 0.61) for the device made with 4% PA stabilized ZnO NP dispersion. Fig. 5 shows the J–V characteristic and EQE curve of this device. We also monitored the shelf stability of the non-encapsulated inverted device according to a protocol (ISOS-D-1) reported in the literature [14]. It is worth pointing out that all the inverted solar cells made with regular un-stabilized ZnO NP dispersion in nitrogen or with a PA stabilized ZnO dispersion (within 7 days) in air are stable. As an example, the PCEs of the non-encapsulated inverted solar cells fabricated with a 4% PA-stabilized ZnO dispersion as a function of storage time in air are shown in Fig. 6. As can be seen from Fig. 6, both devices fabricated with different processes retain their original PCEs after being stored in air for more than a month.
Device #1 Device #2 (optimized process)
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 0
10
20
30
40
50
Time (day) Fig. 6. PCE of the non-encapsulated inverted solar cells made with 4% PAstabilized ZnO NPs, as a function of storage time in air.
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PA.esp
a 3.10
2.00 4.28
4.0
ZnO-PA.esp
3.5
3.0
2.5 2.0 1.5 Chemical Shift (ppm)
1.0
b
0.5
0
3.06 10.79 7.77 2.00
0.31
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
Chemical Shift (ppm) Fig. 7. 1H NMR spectra of PA (a) and PA + ZnO NPs (b) in CDCl3.
The amine stabilizers for ZnO NPs are mainly primary amines in the literature, such as octylamine and dodecylamine [9]. However, the stabilization mechanism is not clear. Saunders and coworkers reported that propylamine may form covalent coordinate bonds with ZnO [9c]. However, this is contradictory to our finding that the
spin-cast PA stabilized ZnO film is insoluble in chlorinated organic solvents even without any thermal treatment. In addition, DSC measurement did not indicate any reaction corresponding to the decomposition of the PA ligands covalently bonded to the ZnO NPs. There was only one reaction occurring around 280 °C during the DSC scan, and it was attributed to the cleavage of the acetate residues, as reported in the literature [9c]. To better understand the stabilization mechanism, pyridine and triethylamine were also tested as the ZnO NP stabilizers. It was found that pyridine did not work at all. The ZnO NPs precipitated out quickly once the dispersion was exposed to air. Triethylamine retarded the process of large aggregate formation. When 4% triethylamine was added, the ZnO NP dispersion stayed clear in air for about 2 h and then became cloudy. Triethylamine has more alkyl chains and is a stronger base, as compared with propylamine. However, propylamine is much more effective as the ZnO NP stabilizer than triehtylamine. This suggests that the active proton of the amino group may play an important role for the protection of ZnO NPs. NMR study showed that propylamine was not chemically bonded to ZnO NPs because the proton NMR spectrum of propylamine did not change at all in the presence of ZnO NPs, as shown in Fig. 7. Please note that the peak at 3.45 ppm in the NMR spectrum of the PA and ZnO NPs mixture is assigned to methanol, which is the reaction media for the ZnO NP synthesis and cannot completely removed by centrifugation [8]. The small peak at 1.98 ppm is assigned to the acetate residues attached to the surface of the ZnO NPs. Moreover, XPS analysis on the spin-cast ZnO NP film did not detect the presence of any nitrogen atom, as shown in Fig. 8. This indicated that propylamine had gone during spin-coating and did not exist in the spin-cast film. This is extremely beneficial to the electronic devices.
Fig. 8. XPS survey spectrum of the spin-coated ZnO film.
S. Alem et al. / Organic Electronics 15 (2014) 1035–1042
4. Conclusions In summary, we have reported a simple approach to stabilize the ZnO NP dispersion in air for the fabrication of stable inverted solar cells. The active layer consisting of PCDTBT and PC71BM was blade-coated in air. Our results show that 2% PA-stabilized ZnO dispersion has a lifetime of 7 days, and 4% PA-stabilized ZnO dispersion has a lifetime more than a month. Within the time windows, devices fabricated using these dispersions have very similar performance. This is a big step towards the fabrication of large-area and low-cost organic solar cells in industrial setting. Power conversion efficiency around 5% has been obtained on the inverted solar cells with an active area of 1.0 cm2. More importantly, our non-encapsulated inverted solar cells are quite stable with their PCEs remaining unchanged after being stored in air for 50 days.
[4]
[5]
Acknowledgements The authors thank Mr. Oltion Kodra for XPS measurements, Mr. Eric Estwick and Mr. Hiroshi Fukutani for their technical support, and Mr. Gilles Robertson for the NMR data processing. This project was financially supported by National Research Council of Canada and MW Canada.
[6]
References [1] (a) M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 41), Prog. Photovoltaics Res. Appl. 21 (2013) 1–11; (b) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, Y. Yang, A polymer tandem solar cell with 10.6% power conversion efficiency, Nat. Commun. 4 (2013) 1446. [2] (a) Y. Liang, Z. Xu, J. Xia, S. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, For the bright future—bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%, Adv. Mater. 22 (2010) E135–E138; (b) H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Polymer solar cells with enhanced open-circuit voltage and efficiency, Nat. Photonics 3 (2009) 649–653; (c) T.-Y. Chu, J. Lu, S. Beaupré, Y. Zhang, J.-R. Pouliot, J. Zhou, A. Najari, M. Leclerc, Y. Tao, Effects of the molecular weight and the side chain lengths on the photovoltaic performance of the dithienosilole/ thienopyrrolodione copolymers, Adv. Funct. Mater. 22 (2012) 2345– 2351; (d) C.E. Small, S. Chen, J. Subbiah, C.M. Amb, S.W. Tsang, T.H. Lai, J.R. Reynolds, F. So, High-efficiency inverted dithienogermole– thienopyrrolodione-based polymer solar cells, Nat. Photonics 6 (2012) 115–120. [3] (a) J. Peet, J. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols, Nat. Mater. 6 (2007) 497–500; (b) J.K. Lee, W.L. Ma, C.J. Brabec, J. Yuen, J.S. Moon, J.Y. Kim, K. Lee, G.C. Bazan, A.J. Heeger, Processing additives for improved efficiency from bulk heterojunction solar cells, J. Am. Chem. Soc. 130 (2008) 3619–3623; (c) C.V. Hoven, X.D. Dang, R.C. Coffin, J. Peet, T.Q. Nguyen, G.C. Bazan, Improved performance of polymer bulk heterojunction solar cells through the reduction of phase separation via solvent additives, Adv. Mater. 22 (2010) E63–E66; (d) W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Adv. Funct. Mater. 15 (2005) 1617–1622; (e) G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868; (f) B.R. Aïch, J. Lu, S. Beaupré, M. Leclerc, Y. Tao, Control of the active layer nanomorphology by using co-additives towards high-
[7]
[8]
[9]
[10]
1041
performance bulk heterojunction solar cells, Org. Electron. 13 (2012) 1736–1741. (a) Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Enhanced powerconversion efficiency in polymer solar cells using an inverted device structure, Nat. Photonics 6 (2012) 591–595; (b) A.K. Kyaw, D.H. Wang, V. Gupta, J. Zhang, S. Chand, G.C. Bazan, A.J. Heeger, Efficient solution-processed small-molecule solar cells with inverted structure, Adv. Mater. 25 (2013) 2397–2402; (c) S.H. Liao, Y. Li, T. Jen, Y. Cheng, S. Chen, Multiple functionalities of polyfluorene grafted with metal ionintercalated crown ether as an electron transport layer for bulkheterojunction polymer solar cells: optical interference, hole blocking, interfacial dipole, and electron conduction, J. Am. Chem. Soc. 134 (2012) 14271–14274; (d) Y. Yuan, T.J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, Y. Yang, J. Huang, Efficiency enhancement in organic solar cells with ferroelectric polymers, Nat. Mater. 10 (2011) 296– 302; (e) F. Zhang, M. Ceder, O. Inganäs, Enhancing the photovoltage of polymer solar cells by using a modified cathode, Adv. Mater. 19 (2007) 1835–1838. (a) H.-H. Liao, L.-M. Chen, Z. Xu, G. Li, Y. Yang, Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer, Appl. Phys. Lett. 92 (2008) 173303; (b) G. Cheng, W.-Y. Tong, K.-H. Low, C.-M. Che, Thermal-annealingfree inverted polymer solar cells using ZnO/Cs2CO3 bilayer as electron-selective layer, Sol. Energy Mater. Sol. Cells 103 (2012) 164–170; (c) C.J. Brabec, S.E. Shaheen, C. Winder, N.S. Sariciftci, P. Denk, Effect of LiF/metal electrodes on the performance of plastic solar cells, Appl. Phys. Lett. 80 (2002) 1288–1290. (a) J.K. Lee, N.E. Coates, S. Cho, N. Sung Cho, D. Moses, G.C. Bazan, K. Lee, A.J. Heeger, Efficacy of TiOx optical spacer in bulkheterojunction solar cells processed with 1,8-octanedithiol, Appl. Phys. Lett. 92 (2008) 243308; (b) Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger, Inverted polymer solar cells integrated with a low-temperature-annealed sol–gel-rerived ZnO film as an electron transport layer, Adv. Mater. 23 (2011) 1679–1683; (c) J. Meyer, R. Khalandovsky, P. Görrn, A. Kahn, MoO3 films spincoated from a nanoparticle suspension for efficient hole-injection in organic electronics, Adv. Mater. 23 (2011) 70–73; (d) T. Stubhan, N. Li, N.A. Luechinger, S.C. Halim, G.J. Matt, C.J. Brabec, High fill factor polymer solar cells incorporating a low temperature solution processed WO3 hole extraction layer, Adv. Energy Mater. 12 (2012) 1433–1438; (e) K. Zilberberg, S. Trost, J. Meyer, A. Kahn, A. Behrendt, D. Lützenkirchen-Hecht, R. Frahm, T. Riedl, Inverted organic solar cells with sol–gel processed high work-function vanadium oxide hole-extraction layers, Adv. Funct. Mater. 21 (2011) 4776–4783. (a) T.-Y. Chu, S.-W. Tsang, J. Zhou, P.G. Verly, J. Lu, S. Beaupré, M. Leclerc, Y. Tao, High-efficiency inverted solar cells based on a low bandgap polymer with excellent air stability, Sol. Energy Mater. Sol. Cells 96 (2012) 155–159; (b) M.J. Tan, S. Zhong, J. Li, Z. Chen, W. Chen, Air-stable efficient inverted polymer solar cells using solution-processed nanocrystalline ZnO interfacial layer, ACS Appl. Mater. Interfaces 5 (2013) 4696–4701; (c) M. AzizIbrahem, H.-Y. Wei, M.-H. Tsai, K.-C. Ho, J.-J. Shyue, C.W. Chu, Solution-processed zinc oxide nanoparticles as interlayer materials for inverted organic solar cells, Sol. Energy Mater. Sol. Cells 108 (2013) 156–163. F.C. Krebs, Y. Thomann, R. Thomann, J.W. Andreasen, A simple nanostructured polymer/ZnO hybrid solar cell-preparation and operation in air, Nanotechnology 19 (2008) 424013. (a) M.L. Kahn, M. Monge, V. Colliére, F. Senocq, A. Maisonnat, B. Chaudret, Size- and shape-control of crystalline zinc oxide nanoparticles: a new organometallic synthetic method, Adv. Funct. Mater. 15 (2005) 458–468; (b) W.J.E. Beek, M.M. Wienk, M. Kemerink, X. Yang, R.A.J. Janssen, Hybrid zinc oxide conjugated polymer bulk heterojunction solar cells, J. Phys. Chem. B 109 (2005) 9505–9516; (c) R. Rhodes, M. Horie, H. Chen, Z. Wang, M.L. Turner, B.R. Saunders, Aggregation of zinc oxide nanoparticles: from non-aqueous dispersions to composites used as photoactive layers in hybrid solar cells, J. Colloid Interface Sci. 344 (2010) 261–271. N.S. Pesika, Z. Hu, K.J. Stebe, P.C. Searson, Quenching of growth of ZnO nanoparticles by adsorption of octanethiol, J. Phys. Chem. B 106 (2002) 6985–6990.
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[11] S. Sakohara, M. Ishida, M.A. Anderson, Visible luminescence and surface properties of nanosized ZnO colloids prepared by hydrolyzing zinc acetate, J. Phys. Chem. B 102 (1998) 10169–10175. [12] Z. Hu, G. Oskam, P.C. Searson, Influence of solvent on the growth of ZnO nanoparticles, J. Colloid Interface Sci. 263 (2003) 454–460.
[13] M. Shim, P. Guyot-Sionnest, Organic-capped ZnO nanocrystals: synthesis and n-type character, J. Am. Chem. Soc. 123 (2001) 11651–11654. [14] M.O. Reese, F.C. Krebs, et al., Consensus stability testing protocols for organic photovoltaic materials and devices, Sol. Energy Mater. Sol. Cells 95 (2011) 1253–1267.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Solution-processed annealing-free ZnO nanoparticles for stable inverted organic solar cells Salima Alem a, Jianping Lu a,⇑, Raluca Movileanu a, Terho Kololuoma a,b, Afshin Dadvand a, Ye Tao a,⇑ a b
Information and Communications Technologies Portfolio, National Research Council of Canada, 1200 Montreal Road, Ottawa, ON K1A 0R6, Canada Printed Functional Solutions, VTT Technical Research Centre of Finland, Kaitovayla 1, 90570 Oulu, Finland
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 21 February 2014 Accepted 26 February 2014 Available online 11 March 2014 Keywords: Zinc oxide nanoparticles Propylamine Inverted solar cell Bulk heterojunction Stability
a b s t r a c t We report the development and application of high-quality zinc oxide nanoparticles (ZnO NPs) processed in air for stable inverted bulk heterojunction solar cells as an electron extraction layer (EEL). The ZnO NPs (average size 11 nm) were dispersed in chloroform and stabilized by propylamine (PA). We demonstrated that the ZnO NP dispersion with 4 vol.% of PA as stabilizer can be used in air directly and remains clear up to one month after preparation. Our inverted solar cells consisted of a blade-coated poly(N-90 heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (1: 4 by weight) active layer sandwiched between a ZnO electron extraction layer and a MoO3/Ag anode. All solar cells with ZnO films fabricated in air using PA-stabilized ZnO dispersions prepared within a time window of one month exhibited power conversion efficiencies (PCE) above 4%. In contrast, if the ZnO film was prepared in air using regular un-stabilized ZnO NP dispersion, the PCE would drop to 0.2% due to poor film quality. More interestingly, X-ray photoelectron spectroscopy and nuclear magnetic resonance measurements indicated that the PA ligands were not covalently bonded to ZnO NPs and did not exist in the deposited ZnO films. The spin-cast ZnO thin films (without any thermal treatment) are insoluble in organic solvents and can be directly used as an EEL in solar cells. This feature is beneficial for fabricating organic solar cells on flexible polymer substrates. More importantly, our nonencapsulated inverted solar cells are highly stable with their PCEs remaining unchanged after being stored in air for 50 days. Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction Due to the high scientific and economic interest in organic photovoltaic research, the power conversion efficiency of organic solar cells has been pushed up steadily in the last decade, currently reaching 10% [1]. This progress
⇑ Corresponding authors. Tel.: +1 613 990 1651; fax: +1 990 0202 (J. Lu). E-mail addresses: [email protected] (J. Lu), ye.tao@nrc-cnrc. gc.ca (Y. Tao).
was achieved through tremendous efforts in the development of low band gap materials [2], the control of fabrication process to optimize the active layer morphology [3], and the improvement of interface engineering [4]. One of the approaches of interface engineering focuses on the incorporation of electron and hole extraction layers between the active layer and the metal cathode/anode, to align the energy levels at the interface, and consequently to allow an efficient extraction of electrons and holes, respectively. In both conventional and inverted organic solar cells, various materials such as polyelectrolytes [4a],
http://dx.doi.org/10.1016/j.orgel.2014.02.024 1566-1199/Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
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inorganic salts [5], and metal oxides (TiOx, ZnO, MoO3, V2O5, WO3) have been investigated and used as n-type or p-type interlayers to enhance the performance and/or the stability of the devices [6]. Among n-type materials, ZnO NP films, formed by sol– gel process, have been widely used in inverted organic solar cells. However, to form crystalline ZnO networks with good electron mobility, an annealing temperature over 200 °C is usually required, which is incompatible with the flexible polymer substrates used in the fabrication of solar cells. The solution-dispersed ZnO NPs have been shown as a good alternative to reduce the annealing temperature, and can be easily processed into films via various coating methods [7]. However, the ZnO NP dispersions in organic solvents are not stable in air and tend to form aggregates and quickly precipitate out, especially in the presence of moisture [8]. Therefore, organic ligands such as alkyl amine [9], alkyl thiol [10], carboxylic acid [11], and alkoxyacetic acid have been used to protect ZnO NPs and prevent their aggregation [8]. However, most ligands reported in the literature have high boiling points, which again require a high temperature annealing (>150 °C) to ensure complete removal of the ligands from the fabricated ZnO NP films in order to get good electron-transport properties. In this paper, we report the use of a short and low boiling point compound, propylamine (PA, bp 48 °C) as an effective stabilizing ligand for the ZnO NPs. The addition of PA at a concentration of 4% (v/v) keeps the ZnO NP dispersion in chloroform, chlorobenzene or dichlorobenzene stable over one month, even after exposure to ambient air. More importantly, the spin-cast ZnO NP films do not require any thermal annealing or plasma treatment and can be directly used as an electron extraction layer in inverted solar cells. In this work, pyridine and triethylamine have also been tested as the ZnO NP stabilizers. However, they are not as effective as propylamine although triethylamine is a stronger base than propylamine. In order to understand the mechanism involved in the stabilization of ZnO NPs by propylamine, the ZnO NP dispersion and the spin-cast films were characterized by nuclear magnetic resonance (NMR) and X-ray photo-electron spectroscopy (XPS), respectively. It was found that the PA ligands were not chemically bonded to ZnO NPs and did not exist in the deposited ZnO films. The concentration of the PA has been optimized. It is interesting to point out that our non-encapsulated inverted solar cells based on the PCDTBT:PC71BM blend and PA stabilized ZnO NPs are quite stable with their PCEs remaining unchanged after being stored in air for 50 days. Therefore, these air-processed and annealing-free ZnO nanoparticles are very valuable for the technology transfer of organic solar cells from the lab to the industry. 2. Experimental
(ppm) relative to internal chloroform residue (7.25 ppm). The XPS spectra were recorded on a Phi 5500 system, using a monochromatic Al X-ray source with beam energy of 1486 eV and a take-off angle of 45°. PCDTBT was provided by St-Jean Photochemicals Inc. and PC71BM was purchased from Nano-C, and they were used without further purification. The ZnO nanoparticles were synthesized according to our previous procedure [7a]. The ZnO nanoparticles were dispersed in anhydrous chloroform. After filtration through a 0.2 lm filter, a clear colorless dispersion was obtained and stored in a nitrogen-purged glovebox. Prior to device fabrication, it was diluted with anhydrous chloroform in order to obtain an optimal ZnO film thickness by spin-casting, and propylamine was added via a micropipette. 2.2. Device fabrication and characterization The bulk heterojunction solar cells were fabricated on pre-patterned indium tin oxide (ITO) coated glass substrates. The sheet resistance and thickness of the ITO are 12 X/sq and 150 nm, respectively. The ITO substrates were thoroughly cleaned in detergent and DI water, ultrasonicated in acetone and isopropyl alcohol for 15 min, and dried in an oven at 120 °C. UV–ozone treatment was then performed for 15 min. A ZnO NP film (20 nm-thick) was spin-cast on top of the ITO substrates from a freshly prepared ZnO dispersion at 700 rpm. Afterward, an active layer was blade-coated onto the ZnO film from a dichlorobenzene solution of PCDTBT:PC71BM with a weight ratio of 1:4 and dried overnight at room temperature in a nitrogenfilled glovebox. The PCDTBT:PC71BM solution with a PCDTBT concentration of 3 mg/mL was prepared three days before deposition and heated at 100 °C over night. The blade speed was set at 35 mm/s to get 70 nm thick active layers. Finally, to complete the solar cell architecture, a bilayer anode consisting of 100 nm Ag on top of 10 nm molybdenum oxide was thermally evaporated through a shadow mask on the active layer to form cells with an active area of 1 cm2. The thicknesses of the films were measured by a Dektak profilometer. The photovoltaic parameters were extracted from the current–voltage (J–V) characteristics measured in air with a Keithley 2400 Digital SourceMeter and the photocurrent was generated under air mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm2 from a ScienceTech SS 500W solar simulator. The light intensity was adjusted using a calibrated Si photodiode with a KG-5 filter purchased from PV Measurements, Inc. The external quantum efficiency (EQE) was performed using a Jobin-Yvon Triax spectrometer, a Jobin-Yvon xenon light source, a Merlin lock-in amplifier, a calibrated Si UV detector, and an SR570 low noise current amplifier. It is worth pointing out that the short-circuited photocurrent reported in this paper was always calculated from the wavelength integration of the product of the EQE curve and the standard AM 1.5G solar spectrum.
2.1. Instrumentation and materials 3. Results and discussion NMR spectra were recorded on a 400 MHz Varian Unity Inova spectrometer. The samples were prepared in deuterated CHCl3. Chemical shifts were reported as d values
Fig. 1 shows the evolution of the ZnO NP dispersion with time before and after exposure to air. Within 5 min
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of exposure, the ZnO NPs underwent quick aggregation due to the reaction with moisture, and the dispersion became cloudy (Fig. 1b) or milky. Within 30 min, the large aggregates settled down in the bottom of the vial due to the gravitational force (Fig. 1c). The rate of this aggregation process depends strongly on the size of the nanoparticles, the type of the solvent, and the humidity in air [12]. By adding PA to the ZnO NP dispersion, even with a volume ratio as low as 0.25%, the dispersion remains clear for a few days. The average particle size measured by a particle analyzer (Zetasizer, Nano ZS Malvern) is about 11 nm in diameter, and 15 nm by atomic force microscopy (AFM). Totally, more than 50 devices using ZnO NP interlayer, cast from un-stabilized and stabilized dispersions with different PA ratios, have been fabricated and characterized. The devices were quite reproducible in term of the efficiency. Table 1 shows the typical photovoltaic parameters of the inverted devices made with ZnO NP films spin-cast under different conditions. The devices made in air with a PA concentration (v/v) higher than 1% exhibited a comparable performance to the device made with a fresh unstabilized ZnO dispersion in glove box, within the statistical fluctuations. On the other hand, the devices made with a low PA concentration (such as 0.25%) exhibited a slightly lower performance, because the ZnO NPs aggregation had probably begun. The device with a ZnO NP film spin-cast in air from the un-stabilized dispersion (Fig. 1b, without adding PA) showed a very poor performance (PCE 0.2%). The big ZnO aggregates resulted in poor film quality and bad coverage on the substrate. As a result, the open-circuit voltage was very low, only reached 0.1 V, and the device series resistance was high, leading to a very low fill factor (0.3). On the other hand, we also noticed that the performance (4.3%) of the device made with a blade-coated PCDTBT:PC71BM active layer was lower than the one made with a spin-coated active layer, which exhibited an EQE-calibrated PCE of 5.8% (Jsc = 10.9 mA/ cm2, Voc = 0.88 V, FF = 0.61). The reason is still under investigation; it may be due to the difference in drying process, resulting in different morphologies and/or interfaces. However, this also indicates that there is still a room to improve the efficiency of blade-coated devices. The lifetime of PA stabilized ZnO NP dispersion has been monitored by UV–Vis absorption spectroscopy and AFM inspection on the resulting thin films. Two disper-
Fig. 1. ZnO NP dispersion when freshly prepared (a) after 5 min (b) and after 30 min (c) of air exposure.
Table 1 Comparison of PV parameters of inverted devices, using ZnO layers spin cast from dispersions with different PA concentrations. Jsc is calculated from the EQE curves. PA-concentration (%)
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
0 (in GB) 0 0.25 0.5 1 2 3 4
9.2 5.4 8.3 9.1 9.2 9.4 9.7 9.4
0.88 0.10 0.85 0.83 0.83 0.84 0.84 0.86
52 30 50 52 51 55 51 52
4.2 0.2 3.6 3.9 3.9 4.3 4.2 4.2
sions of ZnO NPs have been prepared and stabilized by 2% and 4% PA, respectively. The absorbance and the surface morphology of the films spin-cast on quartz substrates are depicted in Figs. 2 and 3, respectively. The absorption of the ZnO NP film obtained from a fresh dispersion exhibits a distinguishable first excitonic absorption peak at 330 nm (Fig. 2). After 24 h, this peak started broadening and shifting toward longer wavelengths, indicating the continuing growth of ZnO NPs in solution at room temperature [13]. This red shift seemed more pronounced in the films spin-cast from the solution stabilized with 2% PA than the ones with 4% PA. In an extreme case, the absorption spectrum of ZnO NP film spin-cast from a 37-day-old 2% PA stabilized dispersion exhibited two absorption peaks at 270 and 350 nm, respectively. This implies that some nanoparticles continued to grow at the expense of other nanoparticles. This can also be seen in the AFM images. The AFM images in Fig. 3 show clearly that the sizes of the ZnO particles in the freshly prepared dispersoin with 2% PA are fairly uniform, but there is an obvious increase in particle/aggregate size after 24 h, followed by a gradual broadening in the particle/aggregate size distribution in the first 7 days, and some particles/aggregates are as large as 40 nm, which is consistent with the slight red shift between 1 and 7 days seen in the absorption spectra in Fig. 2. The non-uniformity of ZnO NP size distribution is much more evident in the films prepared with 37-dayold 2% PA stabilized ZnO dispersion as shown in the AFM image. On the other hand, the topography of ZnO NP films spin-cast from 4% PA stabilized dispersion after 30 days looks similar to the films spin-cast from the 7-day-old 2%-PA ZnO dispersion. Despite some broadening in particle/aggregate size distribution, the RMS roughness of the ZnO films measured by AFM falls in a range from 1.7 nm to 4.3 nm. The J–V characteristics of inverted devices made with aged and freshly prepared ZnO NP dispersions stabilized with 2% PA or 4% PA are presented in Fig. 4. A slight drop in performance was recorded in devices with ZnO films prepared using the 2% PA-ZnO NP dispersion aged in air for 37 days. The PCE dropped to 3.5% with a short-circuit current density (Jsc) of 8.4 mA/cm2, an open circuit voltage (Voc) of 0.84 V, and a fill factor of 0.51. This drop was due to the non-uniform distribution of ZnO NPs in the films which increases the series resistance of the devices. The devices using aged 4% PA-ZnO dispersion (at 30 days) showed a comparable performance to the ones using
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after 1h after 1 day after 2 days after 7 days after 37 days
ZnO + 2% PA
Absorbance (a.u.)
0.06 0.05
0.07
0.04 0.03 0.02 0.01 0.00 200
after 1h after 1 day after 30 days
ZnO + 4% PA
0.06
Absorbance (a.u.)
0.07
0.05 0.04 0.03 0.02 0.01
300
400
500
600
700
800
0.00 200
300
Wavelength (nm)
400
500
600
700
800
Wavelength (nm)
Fig. 2. The evolution in time of the absorption spectra of ZnO films cast from aged 2% PA (left) and 4% PA (right)-stabilized ZnO dispersions.
2% PA-freshly prepared
2% PA – 2 days
2% PA – 37 days
2% PA – 1 day
2% PA – 7 days
4% PA – 30 days
Fig. 3. AFM images of ZnO films cast from freshly prepared and aged ZnO dispersions, stabilized with 2% and 4% PA.
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20
20 7 days
15
1 hr
ZnO + 4% PA
1 hr
Current density (mA/cm2)
Current density (mA/cm2)
ZnO + 2% PA
37 days
10 5 0 -5 -10
30 days
15 10 5 0 -5 -10 -15
-15 -1.0
-0.5
0.0
0.5
-1.0
1.0
-0.5
0.0
Voltage (V)
0.5
1.0
Voltage (V)
Fig. 4. J–V characteristics of inverted PCDTBT:PC70BM devices using ZnO NP films cast from aged 2% and 4% PA stabilized dispersions.
80
35
(a)
in dark
25
under 100mW/cm
(b)
70
2
60
20 50 15
EQE (%)
Current density (mA/cm2)
30
10 5
40 30
0 20 -5 10
-10
0
-15 -0.5
0.0
0.5
1.0
300
400
Voltage (V)
500
600
700
800
Wavelength (nm)
Fig. 5. The J–V curve (a) and the EQE spectrum (b) of the optimized inverted device made with 4% PA stabilized ZnO dispersion, showing a PCE of 5%.
6.0
Power conversion efficiency (%)
freshly prepared dispersion, with a PCE of 4%, Jsc of 9.2 mA/ cm2, Voc of 0.85 V and FF of 0.51. The photovoltaic parameters of the devices prepared using fresh 2%-PA and 4%-PA stabilized ZnO dispersions are shown in Table 1. Very recently, we optimized the fabrication process and achieved a PCE of 5% (Jsc = 9.6 mA/cm2, Voc = 0.85 V, FF = 0.61) for the device made with 4% PA stabilized ZnO NP dispersion. Fig. 5 shows the J–V characteristic and EQE curve of this device. We also monitored the shelf stability of the non-encapsulated inverted device according to a protocol (ISOS-D-1) reported in the literature [14]. It is worth pointing out that all the inverted solar cells made with regular un-stabilized ZnO NP dispersion in nitrogen or with a PA stabilized ZnO dispersion (within 7 days) in air are stable. As an example, the PCEs of the non-encapsulated inverted solar cells fabricated with a 4% PA-stabilized ZnO dispersion as a function of storage time in air are shown in Fig. 6. As can be seen from Fig. 6, both devices fabricated with different processes retain their original PCEs after being stored in air for more than a month.
Device #1 Device #2 (optimized process)
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 0
10
20
30
40
50
Time (day) Fig. 6. PCE of the non-encapsulated inverted solar cells made with 4% PAstabilized ZnO NPs, as a function of storage time in air.
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PA.esp
a 3.10
2.00 4.28
4.0
ZnO-PA.esp
3.5
3.0
2.5 2.0 1.5 Chemical Shift (ppm)
1.0
b
0.5
0
3.06 10.79 7.77 2.00
0.31
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
Chemical Shift (ppm) Fig. 7. 1H NMR spectra of PA (a) and PA + ZnO NPs (b) in CDCl3.
The amine stabilizers for ZnO NPs are mainly primary amines in the literature, such as octylamine and dodecylamine [9]. However, the stabilization mechanism is not clear. Saunders and coworkers reported that propylamine may form covalent coordinate bonds with ZnO [9c]. However, this is contradictory to our finding that the
spin-cast PA stabilized ZnO film is insoluble in chlorinated organic solvents even without any thermal treatment. In addition, DSC measurement did not indicate any reaction corresponding to the decomposition of the PA ligands covalently bonded to the ZnO NPs. There was only one reaction occurring around 280 °C during the DSC scan, and it was attributed to the cleavage of the acetate residues, as reported in the literature [9c]. To better understand the stabilization mechanism, pyridine and triethylamine were also tested as the ZnO NP stabilizers. It was found that pyridine did not work at all. The ZnO NPs precipitated out quickly once the dispersion was exposed to air. Triethylamine retarded the process of large aggregate formation. When 4% triethylamine was added, the ZnO NP dispersion stayed clear in air for about 2 h and then became cloudy. Triethylamine has more alkyl chains and is a stronger base, as compared with propylamine. However, propylamine is much more effective as the ZnO NP stabilizer than triehtylamine. This suggests that the active proton of the amino group may play an important role for the protection of ZnO NPs. NMR study showed that propylamine was not chemically bonded to ZnO NPs because the proton NMR spectrum of propylamine did not change at all in the presence of ZnO NPs, as shown in Fig. 7. Please note that the peak at 3.45 ppm in the NMR spectrum of the PA and ZnO NPs mixture is assigned to methanol, which is the reaction media for the ZnO NP synthesis and cannot completely removed by centrifugation [8]. The small peak at 1.98 ppm is assigned to the acetate residues attached to the surface of the ZnO NPs. Moreover, XPS analysis on the spin-cast ZnO NP film did not detect the presence of any nitrogen atom, as shown in Fig. 8. This indicated that propylamine had gone during spin-coating and did not exist in the spin-cast film. This is extremely beneficial to the electronic devices.
Fig. 8. XPS survey spectrum of the spin-coated ZnO film.
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4. Conclusions In summary, we have reported a simple approach to stabilize the ZnO NP dispersion in air for the fabrication of stable inverted solar cells. The active layer consisting of PCDTBT and PC71BM was blade-coated in air. Our results show that 2% PA-stabilized ZnO dispersion has a lifetime of 7 days, and 4% PA-stabilized ZnO dispersion has a lifetime more than a month. Within the time windows, devices fabricated using these dispersions have very similar performance. This is a big step towards the fabrication of large-area and low-cost organic solar cells in industrial setting. Power conversion efficiency around 5% has been obtained on the inverted solar cells with an active area of 1.0 cm2. More importantly, our non-encapsulated inverted solar cells are quite stable with their PCEs remaining unchanged after being stored in air for 50 days.
[4]
[5]
Acknowledgements The authors thank Mr. Oltion Kodra for XPS measurements, Mr. Eric Estwick and Mr. Hiroshi Fukutani for their technical support, and Mr. Gilles Robertson for the NMR data processing. This project was financially supported by National Research Council of Canada and MW Canada.
[6]
References [1] (a) M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 41), Prog. Photovoltaics Res. Appl. 21 (2013) 1–11; (b) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, Y. Yang, A polymer tandem solar cell with 10.6% power conversion efficiency, Nat. Commun. 4 (2013) 1446. [2] (a) Y. Liang, Z. Xu, J. Xia, S. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, For the bright future—bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%, Adv. Mater. 22 (2010) E135–E138; (b) H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Polymer solar cells with enhanced open-circuit voltage and efficiency, Nat. Photonics 3 (2009) 649–653; (c) T.-Y. Chu, J. Lu, S. Beaupré, Y. Zhang, J.-R. Pouliot, J. Zhou, A. Najari, M. Leclerc, Y. Tao, Effects of the molecular weight and the side chain lengths on the photovoltaic performance of the dithienosilole/ thienopyrrolodione copolymers, Adv. Funct. Mater. 22 (2012) 2345– 2351; (d) C.E. Small, S. Chen, J. Subbiah, C.M. Amb, S.W. Tsang, T.H. Lai, J.R. Reynolds, F. So, High-efficiency inverted dithienogermole– thienopyrrolodione-based polymer solar cells, Nat. Photonics 6 (2012) 115–120. [3] (a) J. Peet, J. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols, Nat. Mater. 6 (2007) 497–500; (b) J.K. Lee, W.L. Ma, C.J. Brabec, J. Yuen, J.S. Moon, J.Y. Kim, K. Lee, G.C. Bazan, A.J. Heeger, Processing additives for improved efficiency from bulk heterojunction solar cells, J. Am. Chem. Soc. 130 (2008) 3619–3623; (c) C.V. Hoven, X.D. Dang, R.C. Coffin, J. Peet, T.Q. Nguyen, G.C. Bazan, Improved performance of polymer bulk heterojunction solar cells through the reduction of phase separation via solvent additives, Adv. Mater. 22 (2010) E63–E66; (d) W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Adv. Funct. Mater. 15 (2005) 1617–1622; (e) G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868; (f) B.R. Aïch, J. Lu, S. Beaupré, M. Leclerc, Y. Tao, Control of the active layer nanomorphology by using co-additives towards high-
[7]
[8]
[9]
[10]
1041
performance bulk heterojunction solar cells, Org. Electron. 13 (2012) 1736–1741. (a) Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Enhanced powerconversion efficiency in polymer solar cells using an inverted device structure, Nat. Photonics 6 (2012) 591–595; (b) A.K. Kyaw, D.H. Wang, V. Gupta, J. Zhang, S. Chand, G.C. Bazan, A.J. Heeger, Efficient solution-processed small-molecule solar cells with inverted structure, Adv. Mater. 25 (2013) 2397–2402; (c) S.H. Liao, Y. Li, T. Jen, Y. Cheng, S. Chen, Multiple functionalities of polyfluorene grafted with metal ionintercalated crown ether as an electron transport layer for bulkheterojunction polymer solar cells: optical interference, hole blocking, interfacial dipole, and electron conduction, J. Am. Chem. Soc. 134 (2012) 14271–14274; (d) Y. Yuan, T.J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, Y. Yang, J. Huang, Efficiency enhancement in organic solar cells with ferroelectric polymers, Nat. Mater. 10 (2011) 296– 302; (e) F. Zhang, M. Ceder, O. Inganäs, Enhancing the photovoltage of polymer solar cells by using a modified cathode, Adv. Mater. 19 (2007) 1835–1838. (a) H.-H. Liao, L.-M. Chen, Z. Xu, G. Li, Y. Yang, Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer, Appl. Phys. Lett. 92 (2008) 173303; (b) G. Cheng, W.-Y. Tong, K.-H. Low, C.-M. Che, Thermal-annealingfree inverted polymer solar cells using ZnO/Cs2CO3 bilayer as electron-selective layer, Sol. Energy Mater. Sol. Cells 103 (2012) 164–170; (c) C.J. Brabec, S.E. Shaheen, C. Winder, N.S. Sariciftci, P. Denk, Effect of LiF/metal electrodes on the performance of plastic solar cells, Appl. Phys. Lett. 80 (2002) 1288–1290. (a) J.K. Lee, N.E. Coates, S. Cho, N. Sung Cho, D. Moses, G.C. Bazan, K. Lee, A.J. Heeger, Efficacy of TiOx optical spacer in bulkheterojunction solar cells processed with 1,8-octanedithiol, Appl. Phys. Lett. 92 (2008) 243308; (b) Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger, Inverted polymer solar cells integrated with a low-temperature-annealed sol–gel-rerived ZnO film as an electron transport layer, Adv. Mater. 23 (2011) 1679–1683; (c) J. Meyer, R. Khalandovsky, P. Görrn, A. Kahn, MoO3 films spincoated from a nanoparticle suspension for efficient hole-injection in organic electronics, Adv. Mater. 23 (2011) 70–73; (d) T. Stubhan, N. Li, N.A. Luechinger, S.C. Halim, G.J. Matt, C.J. Brabec, High fill factor polymer solar cells incorporating a low temperature solution processed WO3 hole extraction layer, Adv. Energy Mater. 12 (2012) 1433–1438; (e) K. Zilberberg, S. Trost, J. Meyer, A. Kahn, A. Behrendt, D. Lützenkirchen-Hecht, R. Frahm, T. Riedl, Inverted organic solar cells with sol–gel processed high work-function vanadium oxide hole-extraction layers, Adv. Funct. Mater. 21 (2011) 4776–4783. (a) T.-Y. Chu, S.-W. Tsang, J. Zhou, P.G. Verly, J. Lu, S. Beaupré, M. Leclerc, Y. Tao, High-efficiency inverted solar cells based on a low bandgap polymer with excellent air stability, Sol. Energy Mater. Sol. Cells 96 (2012) 155–159; (b) M.J. Tan, S. Zhong, J. Li, Z. Chen, W. Chen, Air-stable efficient inverted polymer solar cells using solution-processed nanocrystalline ZnO interfacial layer, ACS Appl. Mater. Interfaces 5 (2013) 4696–4701; (c) M. AzizIbrahem, H.-Y. Wei, M.-H. Tsai, K.-C. Ho, J.-J. Shyue, C.W. Chu, Solution-processed zinc oxide nanoparticles as interlayer materials for inverted organic solar cells, Sol. Energy Mater. Sol. Cells 108 (2013) 156–163. F.C. Krebs, Y. Thomann, R. Thomann, J.W. Andreasen, A simple nanostructured polymer/ZnO hybrid solar cell-preparation and operation in air, Nanotechnology 19 (2008) 424013. (a) M.L. Kahn, M. Monge, V. Colliére, F. Senocq, A. Maisonnat, B. Chaudret, Size- and shape-control of crystalline zinc oxide nanoparticles: a new organometallic synthetic method, Adv. Funct. Mater. 15 (2005) 458–468; (b) W.J.E. Beek, M.M. Wienk, M. Kemerink, X. Yang, R.A.J. Janssen, Hybrid zinc oxide conjugated polymer bulk heterojunction solar cells, J. Phys. Chem. B 109 (2005) 9505–9516; (c) R. Rhodes, M. Horie, H. Chen, Z. Wang, M.L. Turner, B.R. Saunders, Aggregation of zinc oxide nanoparticles: from non-aqueous dispersions to composites used as photoactive layers in hybrid solar cells, J. Colloid Interface Sci. 344 (2010) 261–271. N.S. Pesika, Z. Hu, K.J. Stebe, P.C. Searson, Quenching of growth of ZnO nanoparticles by adsorption of octanethiol, J. Phys. Chem. B 106 (2002) 6985–6990.
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S. Alem et al. / Organic Electronics 15 (2014) 1035–1042
[11] S. Sakohara, M. Ishida, M.A. Anderson, Visible luminescence and surface properties of nanosized ZnO colloids prepared by hydrolyzing zinc acetate, J. Phys. Chem. B 102 (1998) 10169–10175. [12] Z. Hu, G. Oskam, P.C. Searson, Influence of solvent on the growth of ZnO nanoparticles, J. Colloid Interface Sci. 263 (2003) 454–460.
[13] M. Shim, P. Guyot-Sionnest, Organic-capped ZnO nanocrystals: synthesis and n-type character, J. Am. Chem. Soc. 123 (2001) 11651–11654. [14] M.O. Reese, F.C. Krebs, et al., Consensus stability testing protocols for organic photovoltaic materials and devices, Sol. Energy Mater. Sol. Cells 95 (2011) 1253–1267.