- Email: [email protected]
Spray deposition of electrohydrodynamically atomized polymer mixture for active layer fabrication in organic photovoltaics
Solar Energy Materials & Solar Cells 95 (2011) 352–356
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Spray deposition of electrohydrodynamically atomized polymer mixture for active layer fabrication in organic photovoltaics Sung-Eun Park, Jun-Young Hwang, Kangmin Kim, Buyoung Jung, Woochul Kim, Jungho Hwang n School of Mechanical Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 5 November 2009 Received in revised form 23 February 2010 Accepted 14 April 2010 Available online 18 May 2010
Printing and spray technologies are the most recent and novel approaches to form organic photovoltaics (OPV) with inexpensive, high speed, and environmentally friendly process. With an electrohydrodynamic atomization (EHDA) approach, the active layer composed of polymer mixture (P3HT:PCBM) was successively fabricated. Operating conditions for obtaining the stable cone jet mode were determined with various applied voltages and liquid feed flow rates. The size distribution of EHDA droplets was characterized by aerodynamic particle sizer (APS) measurement. The mode diameters of the droplets were 580 and 670 nm, respectively, when the liquid flow rates were 1 and 20 ml/min. The maximum power conversion efficiency of 0.48% was obtained under AM 1.5 solar simulation for an OPV device fabricated in air. & 2010 Elsevier B.V. All rights reserved.
Keywords: Electrohydrodynamic atomization Organic photovoltaic Active layer
1. Introduction Recent developments in photovoltaics have been focused on the cost-effective and mass-productive photovoltaics. For the fabrication of organic photovoltaics (OPV), the spin coating process has been widely used. In the spin coating process, however, large material consumption, low production rate, and consequently high production cost are involved. Therefore, various fabrication techniques of organic materials such as doctor blading, roll-to-roll (R2R), spray deposition, and inkjet printing are applied recently [1]. These fabrication techniques are characterized by large areal processing and continuous process, and offer the solution to the problem of high cost for photovoltaic technologies for the next step to real production. The usage of the deposition methods has been focused on the formation of transparent electrode of poly(3,4-ethylenedioxythiophene)/polystyrene sulfo-nate (PEDOT/PSS), which is used as a buffer layer between the active layer and indium tin oxide (ITO) electrode. Eom et al. [2] reported the inkjet-printed PEDOT/PSS layer based polymer solar cell and its device performance. Steirer et al. [3] demonstrated that the PEDOT/PSS electrode was ultrasonically spray deposited or inkjet deposited without significant losses to the film. There were several attempts to fabricate the active layer consisting of organic materials in the organic photovoltaics. Schilinsky et al. [4] reported that the power conversion efficiency was 4% in solar cells of which active layer
n
Corresponding author. E-mail address: [email protected] (J. Hwang).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.04.057
was prepared by doctor blading. Hoth et al. [5] made the bulk heterojunction type organic photovoltaics using spray coating and reported an efficiency of 3.1%. Green et al. [6] used air-brush spray deposition for the preparation of active layers with efficiencies of over 2%. Electrohydrodynamic atomization (EHDA) process, also referred to as electrospray process, is the most recent approach to generate fine particles whose diameters range from micro to nanoscales with a narrow size distribution. In the EHDA process, highly charged, relatively monodisperse droplets of controlled size can be produced from various conditions of liquid solution material [7–10]. To maintain droplet monodispersity, the atomization must be operated in the cone jet mode of electrospray [11–13], where axisymmetric surface wave instabilities dominate the liquid jet break-up, resulting in a constant ratio of 1.89 between the primary droplet diameter and the jet diameter [9]. In the EHDA process, since the diameter of the nozzle ( 4100 mm) is much larger than that used in inkjet printing (about 20 mm), nozzle blockages are prevented. Many researchers studied the particle generation via EHDA and produced the nanoparticles composed of organic materials. The sizes of the particles depended on many parameters including liquid flow rate, ink property, and applied voltage. Hogan et al. [8] demonstrated that water soluble and water insoluble, low dispersity polymer particles could be readily prepared by EHDA with geometric mean diameters in the 0.35–2.71 mm size range. Yao et al. [10] introduced fabrication of 1 mm sized polymeric particles in a modified EHDA system. They used organic liquid polylactide co-glycolic acid (PLGA) and suggested an empirical equation for the droplet size .
S.-E. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 352–356
In this paper, we produced monodispersed polymer droplets using EHDA spray deposition method and fabricated lab-scale OPV. A high-speed camera was employed for monitoring the deposition process and the droplet size distribution was measured using an aerodynamic particle sizer (APS). The morphology of the prepared active layers was investigated by a surface profiler as well as by an optical microscope. The electrical characteristics of the active layers were estimated by a sourcemeter and a solar simulator.
2. Experimental
353
The active layer of a P3HT and PCBM mixture was deposited onto a substrate (glass slide: 4 cm 4 cm) having two ITO line patterns (each line: 8 mm 4 cm, thickness 100 nm). Before the deposition process, the substrate was cleaned via sonication in detergent, DI water, acetone, and finally isopropyl alcohol, and a conducting polymer of PEDOT/PSS layer was spin coated. After the EHDA deposition process, the substrate was dried at the temperature of 140 1C for 20 min and cooled to room temperature. Finally, the sample was transferred in a thermal evaporator for metal electrode (Al) deposition at the pressure of 2 10 6 ˚ Torr. The deposition rate was 2 A/s. The pattern width, length, and thickness were 8, 4 cm, and around 100 nm, respectively. The active area of the fabricated device was 64 mm2. The surface
2.1. Materials The polymeric solution containing a mixture of regioregular poly(3-hexylthiophene) (P3HT), purchased from Rieke Metals, and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM), purchased from Nanocraft Inc., was prepared for EHDA. Both P3HT and PCBM were dissolved in chlorobenzene, at a ratio of 1:1.
The polymer nanoparticles were generated by the EHDA system, which was similar to those used previously for micropatterning of solutions based on conductive metals and ceramics [14–16]. The experimental setup consisted of a liquid supply system, a moving stage system, an electrical system, and a visualization system. The liquid supply system included a syringe pump, a feeding tube, and a stainless steel nozzle (inner diameter: 150 mm, outer diameter: 300 mm). The moving stage system consisted of an X–Y stage and a motor controller with a driver. The electrical system consisted of a high voltage power supply ( DC 15 kV) and two electrodes. The pin-type nozzle used for the liquid supply system was also used as the anode. The ring-type copper electrode (1 mm in thickness and 11 mm in diameter) located 5 mm below the nozzle and 50 mm above the substrate was used as the ground electrode. High speed images were taken with a visualization system consisting of a high speed camera (Motion Pro HS-4, Redlake Inc.). This camera was capable of capturing 5130 frames/s when the images were fixed at a 512 512 pixel resolution. The exposure time was fixed to 10 ms in all captures. The electrohydrodynamically generated droplets were sampled into an APS measurement system. The APS (Model 3321, TSI Inc.) is used to determine the concentration and size distribution of particles from 0.5 to 20 mm in an aerodynamic diameter at concentrations up to 1000 particles/cm3. This instrument sizes particles aerodynamically by a time-of-flight and/or optically by light scattering intensity. The aerodynamic diameter da is the diameter of the unit density (r0 ¼1 g/cm3) sphere that has the same settling velocity as the particle of diameter dd (Stokes diameter). The relationship between the Stokes diameter and the aerodynamic diameter is
r0 rd
1=2 Cc ðda Þ Cc ðdd Þ
Flow rate 1 µl/min
2.2. Experimental setup
dd ¼ da
Table 1 Operating range of different modes with applied voltage and flow rate.
5 µl /min
20 µl /min
~3 kV
4 kV
Dripping
5 kV
6 kV
Pulsating cone jet
Dripping
Dripping
Pulsating cone jet
Pulsating cone jet
7 kV
Stab le cone jet
8 kV
9 kV
Multi jet
Stable cone jet
Stable cone jet
Multi jet
Multi jet
1=2
ð1Þ
where rd is the droplet density. Cc(dd) and Cc(da) are the slip correction factors for droplet diameter (Stokes diameter) and aerodynamic diameter, respectively. To minimize the penetration of ambient aerosol particles into the experimental setup, all experiments were conducted in a class 100 (above 0.5 mm) clean room. All experiments were repeated three times and the results of measurements were averaged.
Fig. 1. Shapes of meniscus and jet break-up to the atomization: (a) dripping (b) pulsating cone jet (c) stable cone jet, and (d) multi-jet.
354
S.-E. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 352–356
roughness of the photovoltaic layer was measured using a surface profiler (KLA Tencor, Alpha-step IQ) with an accuracy of 71 nm. The performance of fabricated devices was measured using an AM 1.5G solar simulator (XES-301S, SAN-EI Co.) at 100 mW/cm2 light intensity. Current density–voltage (J–V) curves were recorded using a Keithley 2400 SourceMeter.
3. Results and discussion
Fig. 2. Size distribution of electrohydrodynamic atomized droplet at a flow rate of Q¼ 1 ml/min.
In the EHDA for generation of fine aerosols with a narrow size distribution, the seeding solution must be atomized in a stable cone jet mode. With changing the applied voltage and flow rate, various electrohydrodynamic modes were obtained in our experiments: dripping mode, pulsating cone jet mode, stable cone jet mode, and multi-jet mode. At voltages less than 4 kV, the liquid was dripping from the nozzle as extended pendants (dripping mode), regardless of the flow rate. In the dripping
Fig. 3. Optical microscope photograph and surface morphology at a flow rate of (a) Q ¼20 ml/min, (b) 5 ml/min, and (c) 1 ml/min.
S.-E. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 352–356
Fig. 4. J–V charactristics of an EHDA deposited bulk heterojunction organic photovoltaic.
mode, the meniscus was not transformed into a cone shape and the generated droplets were larger than the nozzle diameter. At 4.5 kV, the cone jet mode was obtained but the jet was pulsating with a uniform frequency (pulsating cone jet), when the flow rate was 1 or 5 ml/min. When the flow rate was 20 ml/min, the pulsating cone jet mode was obtained at 5.5 kV. At about 6.5 kV, the stable cone jet mode started to be formed for any flow rate, but turned into the multi-jet mode at 7–9 kV. The experimental conditions for various electrohydrodynamic jet modes are summarized in Table 1. Increase in the flow rate caused the applied voltage required for a stable cone jet to increase and broaden the window of the stable cone jet. Our results are well agreed with those of the previous studies [13,17,18]. Fig. 1 shows the images of meniscus and various jetting modes generated at the condition of flow rate 10 ml/min. The APS measurement data of droplet size distribution at a stable cone jet mode are shown in Fig. 2. The flow rate was 1 ml/min. The mode (aerodynamic) diameter was 800 nm and the corresponding Stokes diameter was calculated to be 580 nm with Eq. (1). Further APS measurements were carried out for flow rates of 5 and 20 ml/min. The results were that the mode diameter increased as the flow rate increased; the Stokes diameter was 670 nm at the flow rate of 20 ml/min. For any flow rate, the geometric standard deviation of size distribution was below 1.2, which verified the monodispersity of EHDA droplets at the stable cone jet mode. The pattern quality of P3HT:PCBM active layer is shown in Fig. 3 with the optical micrographs and surface morphologies of the deposited layer. The micrographs show that the size distribution of relics was uniform and the relic size was proportional to the flow rate. These trends well agreed with those of the APS data. The RMS roughness of EHDA deposited active layer at the flow rate of 20 ml/min was 29.3 nm, whereas the roughness at the flow rate of 1 ml/min was reduced to 8.9 nm due to smaller droplet sizes. The absorption of the P3HT in the EHDA deposited P3HT:PCBM layer was confirmed using a UV/vis absorption spectrometer. As the deposited thickness increased, the absorption peak of P3HT ( 500 nm) increased. Current density–voltage (J–V) curves of devices fabricated at different flow rate conditions are shown in Fig. 4. The OPV device fabricated at the flow rate of 10 ml/min showed 51 mV of open circuit voltage (Voc), 1.52 mA/cm2 of short circuit current density (Jsc),
355
26% of fill factor (FF), and 0.02% of power conversion efficiency (PCE). At the flow rate of 20 ml/min, the device showed 62 mV of Voc, 5.57 mA/cm2 of Jsc, 25% of FF, and 0.087% of PCE. When the deposition flow rate was increased to 30 ml/min, the device showed the best performance: Voc and Jsc were increased to 205 mV and 9.13 mA/cm2, respectively; FF was 26% and PCE reached 0.48%. The fill factors were relatively low as 25–26% in all the three cases, which can be attributed to the air atmospheric deposition process. Jorgensen et al. [19] reported that the oxidization during the process can affect the fill factor of PCBM:P3HT based devices. For typical PCBM:P3HT based OPV system, the Voc is around 600 mV. The EHDA fabricated OPV exhibited relatively low Voc. It has been known that the Voc in OPV is affected by the morphology and thickness of the active layer [20–22]. It is reported that the excessive active layer thickness obstructs charge transport in OPV resulting in a low Voc performance. On the other hand, the inhomogenity in the roughness of active layer can be a cause of low Voc. The main focus of this study was to check whether the EHDA process could be used to fabricate P3HT:PCBM based OPV. The EHDA method was confirmed to be applicable using the cone jet mode of a solution consisting of P3HT:PCBM polymers. Even though the PCE of produced cell devices were lower than those of devices fabricated with traditional spin coating processes and other alternative processes, the proposed EHDA fabricated OPV would be improved to result in more efficient devices, after further experiments for achieving optimum active layer condition.
4. Conclusions The EHDA technique was used to fabricate the active layer composed of P3HT:PCBM. Monodisperse polymeric droplets were generated from the stable cone jet at various operating conditions. The effect of liquid flow rate on the size distribution of droplets was investigated using APS measurement. The mode diameters of the droplet were 580 and 670 nm, when the liquid flow rates were 1 and 20 ml/min, respectively. The open circuit voltage and short circuit current density were shown to be related with the flow rate. The maximum power conversion efficiency of 0.48% was obtained under AM 1.5 solar simulation.
Acknowledgement This study was supported by a R&D project from the Korea Energy Management Corp. (KEMCO; Grant 2008-N-PV08-P-06-0000). References [1] C.J. Brabec, J.R. Durrant, Solution-processed organic solar cells, MRS Bulletin 33 (2008) 670–675. [2] S.-H. Eom, S. Senthilarasu, P. Uthirakumnar, S.-C. Yoon, J. Lim, C. Lee, H.-S. Lim, J. Lee, S.-H. Lee, Polymer solar cells based on inkjet-printed PEDOT:PSS layer, Org. Electron. 10 (2009) 536–542. [3] K.X. Steirer, J.J. Berry, M.O. Reese, M.F.A.M. van Hest, A. Miedaner, M.W. Liberatore, R.T. Collins, D.S. Ginley, Ultrasonically sprayed and inkjet printed thin film electrodes for organic solar cells, Thin Sol. Films 517 (2009) 2781–2786. [4] P. Schilinsky, C. Waldauf, C.J. Brabec, Performance analysis of printed bulk heterojunction solar cells, Adv. Funct. Mater. 16 (2006) 1669–1672. [5] C.N. Hoth, R. Steim, P. Schilinsky, S.A. Choulis, S.F. Tedde, O. Hayden, C.J. Brabec, Topographical and morphological aspects of spray coated organic photovoltaics, Org. Electron. 10 (2009) 587–593. [6] R. Green, A. Morfa, A.J. Ferguson, N. Kopidakis, G. Rumbles, S.E. Shaheen, Performance of bulk heterojunction photovoltaic devices prepared by airbrush spray deposition, Appl. Phys. Lett. 92 (2008) 1–3 (033301).
356
S.-E. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 352–356
[7] J. Rosell-Llompart, J. Fernandez de la Mora, Generation of monodisperse droplets 0.3 to 4 mm in diameter from electrified cone-jets of highly conducting and viscous liquids, J. Aerosol Sci. 25 (1994) 1093–1119. [8] C.J. Hogan, K.M. Yun, D.-R. Chen, I.W. Lenggoro, P. Biswas, K. Okuyama, Controlled size polymer particle production via electrohydrodynamic atomization, Colloid Surf. A: Physicochem. Eng. Aspect 311 (2007) 67–76. [9] C.J. Hogan, E.M. Kettleson, B. Ramaswami, D.-R. Chen, P. Biswas, Charge reduced electrospray size spectrometry of mega- and gigadalton complexes: whole viruses and virus fragments, Anal. Chem. 78 (2006) 844–852. [10] J. Yao, L.K. Lim, J. Xie, J. Hua, C.-H. Wang, Characterization of electrospraying process for polymeric particle fabrication, J. Aerosol Sci. 39 (2008) 987–1002. [11] M. Cloupeau, B. Prunet-Foch, Electrostatic spraying of liquids in cone jet mode, J. Electrostat. 22 (1989) 135–159. [12] K.Q. Tang, A. Gomez, Generation of monodisperse water droplets from electrosprays in a corona-assisted cone-jet mode, J. Colloid Interface Sci. 175 (1995) 326–332. [13] K.Q. Tang, A. Gomez, Monodisperse electrosprays of low electric conductivity liquids in the cone-jet mode, J. Colloid Interface Sci. 184 (1996) 500–511. [14] D.Y. Lee, E.S. Hwang, T.U. Yu, Y.J. Kim, J. Hwang, Structuring of micro line conductor using electro-hydrodynamic printing of a silver nanoparticle suspension, Appl. Phys. A 82 (2006) 671–674.
[15] D.Y. Lee, Y.S. Shin, S.E. Park, T.U. Yu, J. Hwang, Electrohydrodynamic printing of silver nanoparticles by using a focused nanocolloid jet, Appl. Phys. Lett. 90 (2007) 1–3 (081905). [16] D.Y. Lee, J.H. Yu, Y.S. Shin, D. Park, T.U. Yu, J. Hwang, Formation of ceramic nanoparticle patterns using electrohydrodynamic jet printing with pin-to-pin electrodes, Jpn. J. Appl. Phys. 47 (2008) 1723–1725. [17] J.P. Borra, Y. Tombette, P. Ehouarn, Influence of electric field profile and polarity on the mode of EHDA related to electric discharge regimes, J. Aerosol Sci. 30 (1999) 913–925. [18] S.N. Jayasinghe, M.J. Edirisinghe, Electrostatic atomisation of a ceramic suspension, J. Euro. Ceram. Soc. 24 (2004) 2203–2213. [19] M. Jorgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686–714. [20] J. Liu, Y. Shi, Y. Yang, Solvation-induced morphology effects on the performance of polymer-based photovoltaic devices, Adv. Funct. Mater. 11 (2001) 420–424. [21] C.M. Ramsdale, J.A. Barker, A.C. Arias, J.D. MacKenzie, R.H. Friend, N.C. Greenham, The origin of the open-circuit voltage in polyfluorene-based photovoltaic devices, J. Appl. Phys. 92 (2002) 4266–4270. [22] P. Kumar, H. Kumar, S.C. Jain, P. Venkatesu, S. Chand, V. Kumar, Effect of active layer thickness on open circuit voltage in organics photovoltaic devices, Jpn. J. Appl. Phys. 48 (2009) 1–4 (121501).
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Spray deposition of electrohydrodynamically atomized polymer mixture for active layer fabrication in organic photovoltaics Sung-Eun Park, Jun-Young Hwang, Kangmin Kim, Buyoung Jung, Woochul Kim, Jungho Hwang n School of Mechanical Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 5 November 2009 Received in revised form 23 February 2010 Accepted 14 April 2010 Available online 18 May 2010
Printing and spray technologies are the most recent and novel approaches to form organic photovoltaics (OPV) with inexpensive, high speed, and environmentally friendly process. With an electrohydrodynamic atomization (EHDA) approach, the active layer composed of polymer mixture (P3HT:PCBM) was successively fabricated. Operating conditions for obtaining the stable cone jet mode were determined with various applied voltages and liquid feed flow rates. The size distribution of EHDA droplets was characterized by aerodynamic particle sizer (APS) measurement. The mode diameters of the droplets were 580 and 670 nm, respectively, when the liquid flow rates were 1 and 20 ml/min. The maximum power conversion efficiency of 0.48% was obtained under AM 1.5 solar simulation for an OPV device fabricated in air. & 2010 Elsevier B.V. All rights reserved.
Keywords: Electrohydrodynamic atomization Organic photovoltaic Active layer
1. Introduction Recent developments in photovoltaics have been focused on the cost-effective and mass-productive photovoltaics. For the fabrication of organic photovoltaics (OPV), the spin coating process has been widely used. In the spin coating process, however, large material consumption, low production rate, and consequently high production cost are involved. Therefore, various fabrication techniques of organic materials such as doctor blading, roll-to-roll (R2R), spray deposition, and inkjet printing are applied recently [1]. These fabrication techniques are characterized by large areal processing and continuous process, and offer the solution to the problem of high cost for photovoltaic technologies for the next step to real production. The usage of the deposition methods has been focused on the formation of transparent electrode of poly(3,4-ethylenedioxythiophene)/polystyrene sulfo-nate (PEDOT/PSS), which is used as a buffer layer between the active layer and indium tin oxide (ITO) electrode. Eom et al. [2] reported the inkjet-printed PEDOT/PSS layer based polymer solar cell and its device performance. Steirer et al. [3] demonstrated that the PEDOT/PSS electrode was ultrasonically spray deposited or inkjet deposited without significant losses to the film. There were several attempts to fabricate the active layer consisting of organic materials in the organic photovoltaics. Schilinsky et al. [4] reported that the power conversion efficiency was 4% in solar cells of which active layer
n
Corresponding author. E-mail address: [email protected] (J. Hwang).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.04.057
was prepared by doctor blading. Hoth et al. [5] made the bulk heterojunction type organic photovoltaics using spray coating and reported an efficiency of 3.1%. Green et al. [6] used air-brush spray deposition for the preparation of active layers with efficiencies of over 2%. Electrohydrodynamic atomization (EHDA) process, also referred to as electrospray process, is the most recent approach to generate fine particles whose diameters range from micro to nanoscales with a narrow size distribution. In the EHDA process, highly charged, relatively monodisperse droplets of controlled size can be produced from various conditions of liquid solution material [7–10]. To maintain droplet monodispersity, the atomization must be operated in the cone jet mode of electrospray [11–13], where axisymmetric surface wave instabilities dominate the liquid jet break-up, resulting in a constant ratio of 1.89 between the primary droplet diameter and the jet diameter [9]. In the EHDA process, since the diameter of the nozzle ( 4100 mm) is much larger than that used in inkjet printing (about 20 mm), nozzle blockages are prevented. Many researchers studied the particle generation via EHDA and produced the nanoparticles composed of organic materials. The sizes of the particles depended on many parameters including liquid flow rate, ink property, and applied voltage. Hogan et al. [8] demonstrated that water soluble and water insoluble, low dispersity polymer particles could be readily prepared by EHDA with geometric mean diameters in the 0.35–2.71 mm size range. Yao et al. [10] introduced fabrication of 1 mm sized polymeric particles in a modified EHDA system. They used organic liquid polylactide co-glycolic acid (PLGA) and suggested an empirical equation for the droplet size .
S.-E. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 352–356
In this paper, we produced monodispersed polymer droplets using EHDA spray deposition method and fabricated lab-scale OPV. A high-speed camera was employed for monitoring the deposition process and the droplet size distribution was measured using an aerodynamic particle sizer (APS). The morphology of the prepared active layers was investigated by a surface profiler as well as by an optical microscope. The electrical characteristics of the active layers were estimated by a sourcemeter and a solar simulator.
2. Experimental
353
The active layer of a P3HT and PCBM mixture was deposited onto a substrate (glass slide: 4 cm 4 cm) having two ITO line patterns (each line: 8 mm 4 cm, thickness 100 nm). Before the deposition process, the substrate was cleaned via sonication in detergent, DI water, acetone, and finally isopropyl alcohol, and a conducting polymer of PEDOT/PSS layer was spin coated. After the EHDA deposition process, the substrate was dried at the temperature of 140 1C for 20 min and cooled to room temperature. Finally, the sample was transferred in a thermal evaporator for metal electrode (Al) deposition at the pressure of 2 10 6 ˚ Torr. The deposition rate was 2 A/s. The pattern width, length, and thickness were 8, 4 cm, and around 100 nm, respectively. The active area of the fabricated device was 64 mm2. The surface
2.1. Materials The polymeric solution containing a mixture of regioregular poly(3-hexylthiophene) (P3HT), purchased from Rieke Metals, and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM), purchased from Nanocraft Inc., was prepared for EHDA. Both P3HT and PCBM were dissolved in chlorobenzene, at a ratio of 1:1.
The polymer nanoparticles were generated by the EHDA system, which was similar to those used previously for micropatterning of solutions based on conductive metals and ceramics [14–16]. The experimental setup consisted of a liquid supply system, a moving stage system, an electrical system, and a visualization system. The liquid supply system included a syringe pump, a feeding tube, and a stainless steel nozzle (inner diameter: 150 mm, outer diameter: 300 mm). The moving stage system consisted of an X–Y stage and a motor controller with a driver. The electrical system consisted of a high voltage power supply ( DC 15 kV) and two electrodes. The pin-type nozzle used for the liquid supply system was also used as the anode. The ring-type copper electrode (1 mm in thickness and 11 mm in diameter) located 5 mm below the nozzle and 50 mm above the substrate was used as the ground electrode. High speed images were taken with a visualization system consisting of a high speed camera (Motion Pro HS-4, Redlake Inc.). This camera was capable of capturing 5130 frames/s when the images were fixed at a 512 512 pixel resolution. The exposure time was fixed to 10 ms in all captures. The electrohydrodynamically generated droplets were sampled into an APS measurement system. The APS (Model 3321, TSI Inc.) is used to determine the concentration and size distribution of particles from 0.5 to 20 mm in an aerodynamic diameter at concentrations up to 1000 particles/cm3. This instrument sizes particles aerodynamically by a time-of-flight and/or optically by light scattering intensity. The aerodynamic diameter da is the diameter of the unit density (r0 ¼1 g/cm3) sphere that has the same settling velocity as the particle of diameter dd (Stokes diameter). The relationship between the Stokes diameter and the aerodynamic diameter is
r0 rd
1=2 Cc ðda Þ Cc ðdd Þ
Flow rate 1 µl/min
2.2. Experimental setup
dd ¼ da
Table 1 Operating range of different modes with applied voltage and flow rate.
5 µl /min
20 µl /min
~3 kV
4 kV
Dripping
5 kV
6 kV
Pulsating cone jet
Dripping
Dripping
Pulsating cone jet
Pulsating cone jet
7 kV
Stab le cone jet
8 kV
9 kV
Multi jet
Stable cone jet
Stable cone jet
Multi jet
Multi jet
1=2
ð1Þ
where rd is the droplet density. Cc(dd) and Cc(da) are the slip correction factors for droplet diameter (Stokes diameter) and aerodynamic diameter, respectively. To minimize the penetration of ambient aerosol particles into the experimental setup, all experiments were conducted in a class 100 (above 0.5 mm) clean room. All experiments were repeated three times and the results of measurements were averaged.
Fig. 1. Shapes of meniscus and jet break-up to the atomization: (a) dripping (b) pulsating cone jet (c) stable cone jet, and (d) multi-jet.
354
S.-E. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 352–356
roughness of the photovoltaic layer was measured using a surface profiler (KLA Tencor, Alpha-step IQ) with an accuracy of 71 nm. The performance of fabricated devices was measured using an AM 1.5G solar simulator (XES-301S, SAN-EI Co.) at 100 mW/cm2 light intensity. Current density–voltage (J–V) curves were recorded using a Keithley 2400 SourceMeter.
3. Results and discussion
Fig. 2. Size distribution of electrohydrodynamic atomized droplet at a flow rate of Q¼ 1 ml/min.
In the EHDA for generation of fine aerosols with a narrow size distribution, the seeding solution must be atomized in a stable cone jet mode. With changing the applied voltage and flow rate, various electrohydrodynamic modes were obtained in our experiments: dripping mode, pulsating cone jet mode, stable cone jet mode, and multi-jet mode. At voltages less than 4 kV, the liquid was dripping from the nozzle as extended pendants (dripping mode), regardless of the flow rate. In the dripping
Fig. 3. Optical microscope photograph and surface morphology at a flow rate of (a) Q ¼20 ml/min, (b) 5 ml/min, and (c) 1 ml/min.
S.-E. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 352–356
Fig. 4. J–V charactristics of an EHDA deposited bulk heterojunction organic photovoltaic.
mode, the meniscus was not transformed into a cone shape and the generated droplets were larger than the nozzle diameter. At 4.5 kV, the cone jet mode was obtained but the jet was pulsating with a uniform frequency (pulsating cone jet), when the flow rate was 1 or 5 ml/min. When the flow rate was 20 ml/min, the pulsating cone jet mode was obtained at 5.5 kV. At about 6.5 kV, the stable cone jet mode started to be formed for any flow rate, but turned into the multi-jet mode at 7–9 kV. The experimental conditions for various electrohydrodynamic jet modes are summarized in Table 1. Increase in the flow rate caused the applied voltage required for a stable cone jet to increase and broaden the window of the stable cone jet. Our results are well agreed with those of the previous studies [13,17,18]. Fig. 1 shows the images of meniscus and various jetting modes generated at the condition of flow rate 10 ml/min. The APS measurement data of droplet size distribution at a stable cone jet mode are shown in Fig. 2. The flow rate was 1 ml/min. The mode (aerodynamic) diameter was 800 nm and the corresponding Stokes diameter was calculated to be 580 nm with Eq. (1). Further APS measurements were carried out for flow rates of 5 and 20 ml/min. The results were that the mode diameter increased as the flow rate increased; the Stokes diameter was 670 nm at the flow rate of 20 ml/min. For any flow rate, the geometric standard deviation of size distribution was below 1.2, which verified the monodispersity of EHDA droplets at the stable cone jet mode. The pattern quality of P3HT:PCBM active layer is shown in Fig. 3 with the optical micrographs and surface morphologies of the deposited layer. The micrographs show that the size distribution of relics was uniform and the relic size was proportional to the flow rate. These trends well agreed with those of the APS data. The RMS roughness of EHDA deposited active layer at the flow rate of 20 ml/min was 29.3 nm, whereas the roughness at the flow rate of 1 ml/min was reduced to 8.9 nm due to smaller droplet sizes. The absorption of the P3HT in the EHDA deposited P3HT:PCBM layer was confirmed using a UV/vis absorption spectrometer. As the deposited thickness increased, the absorption peak of P3HT ( 500 nm) increased. Current density–voltage (J–V) curves of devices fabricated at different flow rate conditions are shown in Fig. 4. The OPV device fabricated at the flow rate of 10 ml/min showed 51 mV of open circuit voltage (Voc), 1.52 mA/cm2 of short circuit current density (Jsc),
355
26% of fill factor (FF), and 0.02% of power conversion efficiency (PCE). At the flow rate of 20 ml/min, the device showed 62 mV of Voc, 5.57 mA/cm2 of Jsc, 25% of FF, and 0.087% of PCE. When the deposition flow rate was increased to 30 ml/min, the device showed the best performance: Voc and Jsc were increased to 205 mV and 9.13 mA/cm2, respectively; FF was 26% and PCE reached 0.48%. The fill factors were relatively low as 25–26% in all the three cases, which can be attributed to the air atmospheric deposition process. Jorgensen et al. [19] reported that the oxidization during the process can affect the fill factor of PCBM:P3HT based devices. For typical PCBM:P3HT based OPV system, the Voc is around 600 mV. The EHDA fabricated OPV exhibited relatively low Voc. It has been known that the Voc in OPV is affected by the morphology and thickness of the active layer [20–22]. It is reported that the excessive active layer thickness obstructs charge transport in OPV resulting in a low Voc performance. On the other hand, the inhomogenity in the roughness of active layer can be a cause of low Voc. The main focus of this study was to check whether the EHDA process could be used to fabricate P3HT:PCBM based OPV. The EHDA method was confirmed to be applicable using the cone jet mode of a solution consisting of P3HT:PCBM polymers. Even though the PCE of produced cell devices were lower than those of devices fabricated with traditional spin coating processes and other alternative processes, the proposed EHDA fabricated OPV would be improved to result in more efficient devices, after further experiments for achieving optimum active layer condition.
4. Conclusions The EHDA technique was used to fabricate the active layer composed of P3HT:PCBM. Monodisperse polymeric droplets were generated from the stable cone jet at various operating conditions. The effect of liquid flow rate on the size distribution of droplets was investigated using APS measurement. The mode diameters of the droplet were 580 and 670 nm, when the liquid flow rates were 1 and 20 ml/min, respectively. The open circuit voltage and short circuit current density were shown to be related with the flow rate. The maximum power conversion efficiency of 0.48% was obtained under AM 1.5 solar simulation.
Acknowledgement This study was supported by a R&D project from the Korea Energy Management Corp. (KEMCO; Grant 2008-N-PV08-P-06-0000). References [1] C.J. Brabec, J.R. Durrant, Solution-processed organic solar cells, MRS Bulletin 33 (2008) 670–675. [2] S.-H. Eom, S. Senthilarasu, P. Uthirakumnar, S.-C. Yoon, J. Lim, C. Lee, H.-S. Lim, J. Lee, S.-H. Lee, Polymer solar cells based on inkjet-printed PEDOT:PSS layer, Org. Electron. 10 (2009) 536–542. [3] K.X. Steirer, J.J. Berry, M.O. Reese, M.F.A.M. van Hest, A. Miedaner, M.W. Liberatore, R.T. Collins, D.S. Ginley, Ultrasonically sprayed and inkjet printed thin film electrodes for organic solar cells, Thin Sol. Films 517 (2009) 2781–2786. [4] P. Schilinsky, C. Waldauf, C.J. Brabec, Performance analysis of printed bulk heterojunction solar cells, Adv. Funct. Mater. 16 (2006) 1669–1672. [5] C.N. Hoth, R. Steim, P. Schilinsky, S.A. Choulis, S.F. Tedde, O. Hayden, C.J. Brabec, Topographical and morphological aspects of spray coated organic photovoltaics, Org. Electron. 10 (2009) 587–593. [6] R. Green, A. Morfa, A.J. Ferguson, N. Kopidakis, G. Rumbles, S.E. Shaheen, Performance of bulk heterojunction photovoltaic devices prepared by airbrush spray deposition, Appl. Phys. Lett. 92 (2008) 1–3 (033301).
356
S.-E. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 352–356
[7] J. Rosell-Llompart, J. Fernandez de la Mora, Generation of monodisperse droplets 0.3 to 4 mm in diameter from electrified cone-jets of highly conducting and viscous liquids, J. Aerosol Sci. 25 (1994) 1093–1119. [8] C.J. Hogan, K.M. Yun, D.-R. Chen, I.W. Lenggoro, P. Biswas, K. Okuyama, Controlled size polymer particle production via electrohydrodynamic atomization, Colloid Surf. A: Physicochem. Eng. Aspect 311 (2007) 67–76. [9] C.J. Hogan, E.M. Kettleson, B. Ramaswami, D.-R. Chen, P. Biswas, Charge reduced electrospray size spectrometry of mega- and gigadalton complexes: whole viruses and virus fragments, Anal. Chem. 78 (2006) 844–852. [10] J. Yao, L.K. Lim, J. Xie, J. Hua, C.-H. Wang, Characterization of electrospraying process for polymeric particle fabrication, J. Aerosol Sci. 39 (2008) 987–1002. [11] M. Cloupeau, B. Prunet-Foch, Electrostatic spraying of liquids in cone jet mode, J. Electrostat. 22 (1989) 135–159. [12] K.Q. Tang, A. Gomez, Generation of monodisperse water droplets from electrosprays in a corona-assisted cone-jet mode, J. Colloid Interface Sci. 175 (1995) 326–332. [13] K.Q. Tang, A. Gomez, Monodisperse electrosprays of low electric conductivity liquids in the cone-jet mode, J. Colloid Interface Sci. 184 (1996) 500–511. [14] D.Y. Lee, E.S. Hwang, T.U. Yu, Y.J. Kim, J. Hwang, Structuring of micro line conductor using electro-hydrodynamic printing of a silver nanoparticle suspension, Appl. Phys. A 82 (2006) 671–674.
[15] D.Y. Lee, Y.S. Shin, S.E. Park, T.U. Yu, J. Hwang, Electrohydrodynamic printing of silver nanoparticles by using a focused nanocolloid jet, Appl. Phys. Lett. 90 (2007) 1–3 (081905). [16] D.Y. Lee, J.H. Yu, Y.S. Shin, D. Park, T.U. Yu, J. Hwang, Formation of ceramic nanoparticle patterns using electrohydrodynamic jet printing with pin-to-pin electrodes, Jpn. J. Appl. Phys. 47 (2008) 1723–1725. [17] J.P. Borra, Y. Tombette, P. Ehouarn, Influence of electric field profile and polarity on the mode of EHDA related to electric discharge regimes, J. Aerosol Sci. 30 (1999) 913–925. [18] S.N. Jayasinghe, M.J. Edirisinghe, Electrostatic atomisation of a ceramic suspension, J. Euro. Ceram. Soc. 24 (2004) 2203–2213. [19] M. Jorgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686–714. [20] J. Liu, Y. Shi, Y. Yang, Solvation-induced morphology effects on the performance of polymer-based photovoltaic devices, Adv. Funct. Mater. 11 (2001) 420–424. [21] C.M. Ramsdale, J.A. Barker, A.C. Arias, J.D. MacKenzie, R.H. Friend, N.C. Greenham, The origin of the open-circuit voltage in polyfluorene-based photovoltaic devices, J. Appl. Phys. 92 (2002) 4266–4270. [22] P. Kumar, H. Kumar, S.C. Jain, P. Venkatesu, S. Chand, V. Kumar, Effect of active layer thickness on open circuit voltage in organics photovoltaic devices, Jpn. J. Appl. Phys. 48 (2009) 1–4 (121501).