Organic Electronics 15 (2014) 2173–2177

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Letter

Progress towards fully spray-coated semitransparent inverted organic solar cells with a silver nanowire electrode Yong-Jin Kang a, Do-Geun Kim a, Jong-Kuk Kim a, Won-Yong Jin b, Jae-Wook Kang b,⇑ a b

Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon 641-831, Republic of Korea Department of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea

a r t i c l e

i n f o

Article history: Received 22 April 2014 Received in revised form 11 June 2014 Accepted 12 June 2014 Available online 28 June 2014 Keywords: Fully spray-coating Semitransparent Silver nanowire Inverted organic solar cell

a b s t r a c t We demonstrated a fully spray-coated semitransparent organic solar cell, from the lowermost organic layer to the uppermost top electrode. The fabricated devices based on a poly (3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) are semitransparent (70% transparency at long wavelength beyond 650 nm), fully spray-coated from organic layer to top electrode, highly efficient (80% of that of a device with a conventional metal electrode). Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Organic solar cells (OSCs) are considered a promising solar cell technology, because of the tunability of the electronic and optical properties of organic semiconductors, and the potential for low-cost roll-to-roll manufacturing with solution-based printing process [1–7]. OSCs have been intensely investigated for their potential for making unique advances for broader applications. Several such applications would be enabled by high performance transparent devices, including building-integrated photovoltaic cells (BIPV), and integrated PV chargers for portable electronics. Recently, many research efforts have been made toward demonstrating visibly transparent or semitransparent OSCs [7–13]. Transparent conducting electrodes, such as thin metal films [13], metal oxide films [12], metal nanowire [7–9], and conducting polymers [10,11], have been deposited onto photoactive layer, to demonstrate transparent or semitransparent OSCs. However, these demonstrations often result in low transmittance, or low ⇑ Corresponding author. Tel.: +82 63 219 5338; fax: +82 63 219 2341. E-mail address: [email protected] (J.-W. Kang). http://dx.doi.org/10.1016/j.orgel.2014.06.016 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

device efficiency. Recently, transparent conducting electrodes (TCEs) based on silver nanowires (Ag NWs) have been shown to provide high flexibility; while providing transmittances and sheet resistance at the level of indium tin oxide (ITO) [14,15]. Several groups have demonstrated solution processed Ag NW films in OSCs, using them as a bottom or a top electrode, and achieving comparable power conversation efficiencies (PCEs) to ITO-based devices [16–19]. Virtually all devices reported to date, however, were fabricated by using the conventional spin-coating method. The development of fully solutionprocessed devices, based on roll-to-roll compatible process, would greatly enhance the industrial viability of OSC technology. To date, spray-coating has been used to successfully coat layer of photoactive polymers, metal oxides and metal electrodes, demonstrating that it can be used to fabricate OSCs [11,20–24]. To realize the fully printing processed OSCs, an inverted architecture is normally used to avoid the vacuum process used for the deposition of the low-work-function metal cathode such as Al and Ca. In the inverted organic solar cells (IOSCs) applications, a high work-function anode, such as Ag and Au, is used to collect holes; and an

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electron-selective layer onto ITO is used to collect electrons [25,26]. The fully spray-coating process with a device structure of ITO/electron-selective layer/photoactive layer/ hole-selective layer/top electrode is a very promising technique for fulfilling this requirement, owing to its simplicity and low-cost. One critical issue in the fabrication process of semitransparent IOSCs is how to form a high work function and semitransparent top electrode using a solution process, without damaging the device performance. Although a few attempts to spray-coat metal nanowires [7,8], metal nanoparticles [27-29], and conducting polymers [11] as top electrode have been successful, our approach is to use fully spray-coated IOSCs, from the lowermost electron-selective layer, to the uppermost semitransparent top electrode. Further, to demonstrate semitransparent devices, the see-through transmittance can be adjusted, by controlling the thickness of Ag NWs electrode. In this study, a spray-coating process is presented for the fabrication of semitransparent IOSCs. The fabricated device, based on a poly (3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT: PCBM), is semitransparent (70% transparency at long wavelength beyond 650 nm), fully spray-coated, highly efficient (80% of that of a device with a conventional metal electrode) and air-stable (80% retention of original efficiency after 30 days).

2. Experimental Semitransparent IOSCs were fabricated on patterned ITO-coated glass substrates (sheet resistance 10 ohm/ sqr.) of 2.5  2.5 cm2 size, which were first cleaned in an ultrasonic bath containing acetone, and then boiled in isopropyl alcohol. The substrates were then dried in an oven, and treated with UV–ozone for 5 min. The spray-coating system uses two nozzles, as the core and cladding. The core nozzle was connected to an injection pump for the coating solution, and the cladding nozzle was linked to compressed N2 gas. The spray-coating conditions in air atmosphere were optimized to minimize the surface roughness by varying the solution injection rate, N2 gas flow rate, and printing speed [11,30]. First, a thin film of zinc oxide (ZnO) was spray-coated onto the ITO glass substrate from a ZnO sol–gel solution, and annealed at 300 °C for 20 min in air, resulting in a thickness of 40 nm. The ZnO sol–gel solution was prepared using zinc acetate (16.40 mg, Aldrich) dissolved in 2-methoxyethanol (100 ml, Aldrich) using a magnetic stirrer. Ethanolamine (5 ml, Aldrich) was then added and the resulting solution was kept at 60 °C for 1 h under ambient conditions with vigorous stirring. Second, a P3HT:PCBM blended solution prepared at 1:1 mass ratio in 1,2-dichlorobenzene (10 mg/ml P3HT and 10 mg/ml PCBM) was spray-coated onto the ZnO layer with a thickness of 270 nm, and annealed at 150 °C for 20 min in a glove box. A buffer layer of PEDOT:PSS (AI4083, H.C. Stark): isopropyl alcohol (IPA) (PEDOT:PSS:IPA = 1:6) was prepared using spray-coating onto the P3HT:PCBM layer with a thickness of 40 nm at the temperature of 80 °C. The coated PEDOT:PSS film was annealed at 150 °C for 1 min in a glove box. Finally,

an as-received dispersion containing Ag NW (Cambrios ClearOhm Ink) was spray-coated on the PEDOT:PSS layer, and annealed at 120 °C for 5 min, in a glove box. In the spray-coating of the Ag NWs electrode, a shadow mask was used to cover the PEDOT:PSS layer, to form cell areas of 0.36 cm2. Detailed information of the optimized spraycoating conditions for ZnO, P3HT:PCBM, and PEDOT:PSS layer can be found elsewhere [30]. In an opaque control device, the 40-nm-thick ZnO, 270-nm-thick P3HT:PCBM and 40-nm-thick PEDOT:PSS layers were fully spraycoated. Finally, a 120-nm-thick Ag electrode was evaporated at 3  106 Torr. The fabricated IOSCs was characterized using restricted illumination by inserting a shadow mask to eliminate excess photocurrent from conductive PEDOT:PSS layer [30]. The current–voltage (J–V) characteristics were measured under AM 1.5 simulated illumination, with an intensity of 100 mW/cm2 (Pecell Technologies Inc., PEC-L11 model). The intensity of sunlight illumination was calibrated using a standard Si photodiode detector with a KG-5 filter. The J–V curves were recorded automatically with a Keithley SMU 2400 source meter, by illuminating the IOSCs. The quantum efficiency measurement system (Oriel IQE-200) used to determine the incident-photonto-charge-carrier efficiency (IPCE) comprised a 250 W quartz-tungsten-halogen (QTH) lamp as the light source, a monochromator, an optical chopper, a lock-in amplifier, and a calibrated silicon photodetector. The layer thickness was measured using a surface profiler (KLA-Tencor, P-11). The specular and diffusive transmittance spectra were measured over the wavelengths between 300 and 800 nm by UV–visible spectrophotometry (Varian, Cary 5000). Specular transmittance is measured by detecting only light that comes out of the sample parallel to the incident light. The diffusive transmittance only differs from specular transmittance in that diffusive transmittance includes all forward scattered light measured using an integrating sphere. The film thicknesses, sheet resistances, and optical transmittances were measured using a surface profiler (Alpha Step P-11, Tencor Instruments), a four-point probe system (Mitsubishi Chemical Corporation), and a UV–vis spectrophotometer (Cary 5000, Varian), respectively.

3. Results and discussion To increase Ag NW thickness for application in device top electrodes, a multi-layer coating process was investigated to further lower the sheet resistance of the films, resulting in thickness that increased from 35 to 400 nm with the number of sprayings. As shown in Fig. 1(a), the thickness of Ag NW film was linearly proportional to the number of spray-coatings. The inset of Fig. 1(a) shows that the Ag NWs in the network were uniformly distributed and randomly oriented over the entire coating area with an average diameter of approximately 25 nm and an average length of a few tens of micrometers. Fig. 1(b) and (c) shows the behavior of transmittance and sheet resistance as a function of the thickness of the spray-coated Ag NW electrode. We determined the diffusive (Tdiff) and specular (Tspec) transmittance spectra of Ag NW electrodes coated on PEDOT:PSS/glass substrates over

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Fig. 1. (a) The thickness of Ag NW electrode dependent on the number of spray-coatings. The inset is SEM images of spray-coated Ag NW electrodes. (b) Sheet resistance, diffusive transmittance (Tdiff), and specular transmittance (Tspec) of spray-coated Ag NW electrodes, as a function of thickness. (c) Diffusive transmittance spectra of Ag NW electrodes coated on PEDOT:PSS/glass. For comparison, the transmittance of ITO/glass is also shown. (d) Transmittance (at a wavelength of 550 nm) versus sheet resistance of the Ag NW electrodes.

the visible light region. When evaluating transparent Ag NW electrodes for solar cell applications, the diffusive transmittance, Tdiff, is a more important parameter, than the specular transmittance, Tspec. For all the electrodes tested, Tdiff was greater than Tspec, as shown in Fig. 1(b); and this result was in accordance with those reported previously [19]. The Ag NW electrodes with thickness in the range of 35–400 nm exhibit promising sheet resistance, and Tdiff values in the ranges of 300–4 ohm/sqr. and 95–51%, respectively, showing that Ag NW film are an alternative to conventional metal, as an anode electrode for semitransparent IOSCs. Fig. 1(d) shows a plot the sheet resistance values of the Ag NW electrodes versus their Tdiff values at a wavelength of 550 nm. In general, the transmittance of a transparent nanostructured metallic thin film is calculated using the following relationship [19]:

 188:5 TðkÞ ¼ 1 þ Rsheet

rOp ðkÞ rDC

(thickness) for the Ag NW electrodes. The transmittance of the IOSCs can be adjusted, by controlling the thickness of the Ag NW electrodes. The effects of the transmittance of the Ag NWs anode on the device characteristics of the fully spray-coated semitransparent IOSCs have been studied. Fig. 3 shows the J–V characteristics of the semitransparent IOSCs illuminated from the ITO/glass side (bottom illumination), and the Ag NW side (top illumination). With increasing thickness of Ag NW electrode, the PCE of the semitransparent IOSC increased from 0.87 to 2.35% for the bottom illumination. The series resistance (Rs) of the IOSCs from the inverse slope of the J–V curve at J = 0 decreased significantly from 45.4 to 17.2 X cm2

2

where rOp(k) is the optical conductivity and rDC is the direct current (DC) conductivity of the film. Here, the ratio rDC/rOp can be regarded a figure of merit (FoM), with its high values resulting in the TCE exhibiting desirable properties. This expression has been fitted to the curve shown in Fig. 1d and provides a reasonable fit for FoM in the range of 30–150. A value of FoM  150, such as in the case of the Spray #4 film, is large for transparent nanostructured metallic thin films [19]. The transparency of the fully spray-coated devices is shown in Fig. 2, with different numbers of spray-coating

Fig. 2. Transmittance spectra of semitransparent IOSCs fabricated fully spray-coated process with different thickness of Ag NW anodes, and absorbance spectrum of spray-coated P3HT:PCBM thin film.

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Fig. 3. Comparison of the photovoltaic response of fully spray-coated semitransparent IOSCs illuminated from the ITO/glass side (bottom) and the Ag NW side (top), as a function of the number of spray-coatings of Ag NW anode. Control devices were opaque, and a vacuum-deposited Ag top electrode.

with increasing Ag NW thickness, resulting in an improvement of fill factor (FF) from 0.27 to 0.50. However, the topilluminated devices showed that PCEs have maximum value (PCE  1.75%, FF  0.47, Rs  27.8 X cm2) at optimum thickness of Ag NWs electrode. At low thickness of Ag NW film (Spray #1), it shows very poor device performance (PCE0.69%), resulting from the high sheet resistance of Ag NW anode (300 ohm/sqr.). On the other hand, at high thickness of Ag NW electrode (Spray #10), the PCE was slightly reduced from 1.75 to 1.05%, resulting from a greater reduction of Jsc (from 6.40 to 3.78 mA/cm2), than improvement of FF (from 0.47 to 0.50). Besides the solar cells with the semitransparent Ag NWs anode, we investigated opaque control devices with a vacuum-deposited Ag top electrode, resulting in a PCE of 2.93% (FF0.54, Rs  4.3 X cm2), which were comparable to the fully spray-coated semitransparent IOSCs (PCE  2.35%, FF  0.50, Rs  17.2 X cm2). Moreover, the optimized device (Spray #4) displayed 70% transparency at long wavelength beyond 650 nm (Fig. 2), where the P3HT:PCBM active layer is largely absorption-free, opening up potential applications in, e.g., power-generating window and tandem-cell devices. A summary of the respective electronic device properties is given in Table 1. Fig. 4 shows the IPCE spectra of the semitransparent IOSCs illuminated from the Ag NW side (top), and the

Fig. 4. The IPCE spectra of the semitransparent IOSCs illuminated from the Ag NW side (top), and the ITO/glass side (bottom).

Fig. 5. Normalized PCE of the unencapsulated semitransparent IOSCs stored for 30 days in air under ambient conditions. The inset is optical images of semitransparent IOSCs.

ITO/glass side (bottom). The Jsc of the optimized device (Spray #4) obtained by IPCE illumination from the Ag NW and ITO side are 6.29 and 8.26 mA/cm2, respectively. These values are roughly consistent with the results obtained from the J–V characterization as shown in Table 1. Semitransparent OSCs generally showed lower Jsc values due to the absence of reflective back electrodes. In our devices, the Jsc values of bottom-illuminated devices show

Table 1 The performance of the semitransparent IOSCs illuminated from ITO-glass (bottom), and Ag NW (top) surfaces.

a

Illumination side

Devices

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Rs (X cm2)

Top Top Top Top Bottom Bottom Bottom Bottom Bottom

Spray #1 Spray #2 Spray #4 Spray #10 Spray #1 Spray #2 Spray #4 Spray #10 Controla

4.4 6.42 6.40 3.78 5.48 7.69 8.38 8.01 9.18

0.56 0.58 0.58 0.56 0.59 0.58 0.56 0.58 0.58

0.28 0.36 0.47 0.50 0.27 0.34 0.48 0.50 0.54

0.69 1.33 1.75 1.05 0.87 1.53 2.26 2.35 2.93

44.1 34.9 27.8 18.0 45.4 38.1 29.3 17.2 4.3

Control device was opaque and fabricated with a vacuum-deposited Ag top electrode [30].

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0.8 mA/cm2 lower than the control device with Ag-based top electrode. However, top-illuminated device exhibited the 2.7 mA/cm2 lower value than the control, which came from not only the absence of reflective electrodes but also the absorption of PEDOT:PSS layer under the Ag NW electrode. In addition, the reason of the decrease in the Jsc value was ascribed to the decrease in their transmittance for wavelength around 380 nm, which was due to the plasmon absorption of Ag NW film (see Fig. 1c). Fig. 5 shows the stability of the semitransparent IOSCs prepared by the fully spray-coated process. The stability studies were performed in the dark, with ambient conditions tested according to the ISOS-D-1 (shelf) standard [30]. The performance of the semitransparent IOSCs was evaluated for 30 days. The normalized PCE of the unencapsulated semitransparent IOSCs after 30 days showed retention of 80% of the original efficiency, which was similar to opaque control devices with an Ag top electrode reported previously [11]. 4. Conclusions In summary, a fully spray-coated process was developed to take full advantage of the solution process, for making cost-effective printable semitransparent IOSCs. The fully spray-coated semitransparent IOSCs had PCEs of 2.35% and 1.75% under ITO- and Ag NW-side illumination, respectively. They also match the 2.3% efficiency of a recently reported fully solution-processed semitransparent device using the spin-coating method [10]. Further improvements in PCE can be expected with the adoption of lower band gap polymer or tandem-cell architectures, to harvest infrared-photons. Acknowledgments This research was supported in part by the Pioneer Research Center Program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3065528) and in part by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2014R 1A1A1003341). References [1] M. Manceau, D. Angmo, M. Jorgensen, F.C. Krebs, Org. Electron. 12 (2011) 566.

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