Ultrasonic spray deposition for production of organic solar cells K. Xerxes Steirer a,b, , Matthew O. Reese b, Benjamin L. Rupert b, Nikos Kopidakis b, Dana C. Olson b, Reuben T. Collins a, David S. Ginley b a Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, USA b National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401, USA a r t i c l e i n f o Article history: Received 12 September 2008 Received in revised form 29 October 2008 Accepted 30 October 2008 Available online 24 December 2008 Keywords: Bulk heterojunction Organic Photovoltaic Spray Deposition Process a b s t r a c t Recent improvements of organic photovoltaic power conversion efficiencies have motivated develop- ment of scalable processing techniques. We compare chlorobenzene and p-xylene, as solvents with similar bulk properties, in a case study of ultrasonic spray depositions of bulk heterojunction layers in photovoltaic devices. Structure and morphology of spray-deposited films are investigated via small- angle X-ray diffraction and optical microscopy. Unique phases are observed in bulk heterostructure films sprayed from p-xylene. Films sprayed from chlorobenzene resulted in higher device efficiencies than p-xylene due to large differences in film morphologies. Carrier loss mechanisms are also investigated. Post-production annealing increases power conversion efficiency to 3.2% when chlorobenzene is used. & 2008 Published by Elsevier B.V. 1. Introduction Organic photovoltaics produced by roll-to-roll manufacturing methods may allow for an ultra-low cost solar energy conversion technology. Since the discoveries of photoactive polymers [1], ultrafast charge transfer from polymer to fullerene [2] and the bulk heterojunction concept [3,4] a concerted effort has been put forth to increase the efficiency of these devices. Current certified efficiencies of organic photovoltaic (OPV) devices are up to 5.15% for large 1cm2 device area [5] and 5.9% for small device areas [6] while reports of stability and lifetimes are gaining attention [7,8]. The workhorse OPV approach in recent years has used blends of polymers and fullerenes to create a bulk heterojunction absorber layer. In this approach, well defined, interspersed polymer-rich and fullerene-rich domains act as dual pathways for conduction of oppositely charged carriers to their respective electrodes. The relative domain size, purity and structure affect many of the device properties and ultimately power conversion efficiency (PCE). The benefits of an organic absorber layer are low manufacturing costs from flexible solution processing and potential increase in efficiency due to synthetically tunable electronic and optical properties. However, spin coating methods conventionally used to deposit laboratory-scale OPV devices are generally not scalable. Given the rapid advance of OPV conversion efficiencies attention has been directed toward the development of large area device fabrication techniques [9���11]. Historically, spray deposition has been used in the coating industry for a myriad of applications typically achieving uniform films at low cost including multilayer paints for automobiles, plastic coating and even some electronic materials such as lead zirconate titanate and barium strontium titanate. Spray technol- ogies have recently been shown as compatible with functional organic thin film depositions. A PCE of 2.83% has been reported for a spray-deposited OPV device [10]. These devices were made by an inexpensive handheld airbrush technique used to deposit the active layer. In this paper we discuss the development of ultrasonic spray deposition of bulk heterojunction OPVs. Ultrasonic spray deposi- tion is a relatively new variant of conventional spray approaches but has many strengths including picoliter drop sizes, directional spray deflection with an inert gas, large area uniform coverage for very thin films and the potential for simultaneous multi- component deposition from ganged heads. The ultrasonic spray deposition system used in this study is shown schematically in Fig. 1a. An ultrasonicating surface incorporated into the spray nozzle is used in conjunction with a tri-phase solution flow pump and an inert carrier gas to deposit solution onto the substrate. Computer control allows reproducible depositions with precise deposition rates. Optimization of film thickness is quickly done through layer-by-layer spray deposition. The individual droplets produced by the ultrasonic nozzle are similar in size to those ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.solmat.2008.10.026 Corresponding author at: National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401, USA. Tel.: +1303384 6484 fax: +1303384 6430. E-mail address: ken_steirer@nrel.gov (K.X. Steirer). Solar Energy Materials & Solar Cells 93 (2009) 447���453

produced using an inkjet nozzle. The nozzle is designed to operate clog-free over a wide range of solution concentrations from the dilute to concentrated. This system was used to deposit the bulk heterojunction active layer in a series of OPV devices. Fabrication began with a conventional OPV process using an indium tin oxide (ITO) -coated glass substrate which was spin coated with a hole blocking poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PED- OT:PSS) layer. The spray-deposited active layer which followed was comprised of a blend of poly(3-hexylthiophene) (P3HT) and [6,6] phenyl C61 butyric methyl ester (PCBM). A Ca/Al back contact was then thermally deposited. The device architecture is depicted in Fig. 1b. This device design when an optimized active layer is deposited by spin coating, has regularly been reported to achieve certified efficiencies near 4% [12,13]. In the present study we compare active layer films ultrasonically sprayed from chloro- benzene and p-xylene as solvents. Previous reports on P3HT:PCBM solutions utilizing p-xylene as a solvent have shown the formation of nanofibrillar aggregates due to the relative nonpolar nature of the solvent. Multiple processing steps have yielded high-efficiency devices made with p-xylene [14,15]. BHJ films spin coated from chlorobenzene have also resulted in favorable morphology when additives or post-processing is utilized [16]. These solvents have largely different dipole moments but, similar boiling points, vapor pressures and surface tensions, which makes them a good case study for spray depositions. It has been shown that boiling point and vapor pressure of the solvent affect the structure and phase segregation properties of BHJ films [11,13,17,18]. We show here that the solvent���solute interaction plays an important role when the BHJ solution is sprayed. We have structurally examined BHJ films ultrasonically sprayed from chlorobenzene and p-xylene and have found unique phases in the p-xylene system. Absorption spectra for each system show enhanced polymer ordering and aggregation combined with significant absorption loss of high energy photons for the p-xylene system, presumably due to the formation of very large PCBM grains that may not fully interconnect with the polymer phase. We have fabricated devices from each system and show greater performance from the films sprayed from chlorobenzene. We analyze loss mechanisms via illumination intensity-dependent short circuit current and large reverse bias current measurements for each system. Finally, we demonstrate a PCE of 3.2% when device active layers are sprayed from chlorobenzene and a flood layer is imposed to reduce surface roughness. 2. Experiment 2.1. Materials Baytron P VP AI 4083, PEDOT:PSS was obtained from HC Starck and was filtered through a 0.45 mm filter prior to use. P3HT was obtained from Rieke and PCBM from Nano-C. Active layer materials were stored in an inert atmosphere and used as received. Anhydrous p-xylene and chlorobenzene from Aldrich were purged with nitrogen to remove any residual oxygen and stored in an inert atmosphere prior to use. 2.2. Device fabrication OPV devices were fabricated with pre-patterned ITO on glass substrates (Colorado Concept Coatings) to define the bottom electrodes. Patterned substrates were scrubbed with an ultrasonic brush using a liquinox/DI water solution and then rinsed in DI water. Following the DI water rinse, they were further cleaned in ultrasonic baths of acetone and then isopropanol. Finally, an oxygen plasma surface treatment of 155 W for 5 min was applied. Thin films of PEDOT:PSS were spin coated onto the prepared ITO substrates at 4000 rpm. PEDOT:PSS-coated substrates were heated at 120 1C for 1h in air. The BHJ solution was prepared in a nitrogen atmosphere and utilized a 1:1 ratio of P3HT:PCBM diluted to 2mg/mL in chlorobenzene or p-xylene. Solutions were stirred on a hotplate held at 60 1C for several hours. After mixing, the solutions were allowed to cool and set for 424 h. The active layer was then ultrasonically spray deposited in a nitrogen glove box with the concentration of H2Oo0.1ppm and of O2 5.0 ppm. For deposi- tion of the ultrasonically spray-coated BHJ films we used a Sonotek ultrasonic spray nozzle #8700-120, Omega Engineering mass flow regulator and a Fluid Metering Inc. VMP Tri reversible flow pump. Active layers were composed of 50 layers, each sprayed at 0.33 mL/min having a carrier gas flow of 7 L/min. The nozzle was 5 cm from the substrate held at 25 1C. Individual layers sprayed from the chlorobenzene solution had average thicknesses of 1171nm. Two additional flood layers where the solution flow rate was doubled were added to some chlorobenzene-sprayed films to investigate the smoothing effect. Some p-xylene-sprayed films were deposited at 78 1C to eliminate large crystallite formations in these films. The optimized parameters were chosen solely dependent upon measured PCE values of chlorobenzene- deposited devices. Electrodes of Ca/Al (20 nm/100 nm) were then thermally evaporated through shadow masks in an integrated dry box system resulting in 0.11cm2 device areas. Post-production annealing was performed on a digitally controlled hotplate at 110 1C for 10 min. 2.3. Measurement X-ray diffraction (XRD) measurements were performed with a Scintag PTS goniometer (Bragg���Brentano geometry) using Cu Ka radiation of wavelength 0.154nm detected with a liquid nitrogen ARTICLE IN PRESS Carrier gas in Out Tri-phase pump Solution Power supply 2.6 Watts XY positioner and heater stage Substrate Ultrasonicating Nozzle Spray Deflector PEDOT:PSS P3HT:PCBM Ca/Al Patterned ITO Fig.1. (a) Ultrasonic sprayer schematic showing carrier gas directing ultrasonically formed droplets onto the substrate. (b) Device architecture for OPV devices with active layers ultrasonically sprayed using chlorobenzene or p-xylene as the solvent. Sandwich structure is ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al. K.X. Steirer et al. / Solar Energy Materials & Solar Cells 93 (2009) 447���453 448