Spray deposition of electrohydrodynamically atomized polymer mixture for active layer fabrication in organic photovoltaics

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Abstract

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 μl/min. The maximum power conversion efficiency of 0.48% was obtained under AM 1.5 solar simulation for an OPV device fabricated in air.

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 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], [8], [9], [10]. To maintain droplet monodispersity, the atomization must be operated in the cone jet mode of electrospray [11], [12], [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 (>100 μm) is much larger than that used in inkjet printing (about 20 μm), 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 μm size range. Yao et al. [10] introduced fabrication of 1 μm 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 .

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.

Section snippets

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.

Experimental setup

The polymer nanoparticles were generated by the EHDA system, which was similar to those used previously for micro-patterning of solutions based on conductive metals and ceramics [14], [15], [16].

Results and discussion

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 mode,

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 μl/min, respectively. The open circuit voltage and short circuit current density were shown to be related with

Acknowledgement

This study was supported by a R&D project from the Korea Energy Management Corp. (KEMCO; Grant 2008-N-PV08-P-06-0-000).

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