Roll-to-Roll Fabrication of Bulk Heterojunction Plastic Solar Cells using the Reverse Gravure Coating Technique
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Roll-to-Roll Fabrication of Bulk Heterojunction Plastic Solar Cells using the Reverse Gravure Coating Technique Daniel Tobjork1,2, Harri Aarnio1, Tapio Mäkelä1, and Ronald Österbacka1,2 1 Department of Physics and Center of Excellence for Functional Materials, Åbo Akademi University, Porthansgatan 3, Turku, FI-20500, Finland 2 Graduate School of Materials Research, Turku, Finland ABSTRACT The roll-to-roll reverse gravure (RG) coating technique was used to produce thin homogeneous films (~100 nm) for organic bulk heterojunction solar cells. The conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and the active layer regioregular poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) were successfully subsequently RG coated on an ITO covered plastic substrate in ambient air. Working solar cells were achieved after annealing and thermal evaporation of the top contact. The AM1.5 power conversion efficiency (PCE) of the RG coated organic solar cells was determined to 0.74% (at 100 mW/cm2). This was very similar to the results of a reference device that was spin coated on a glass substrate in a nitrogen glove box. INTRODUCTION Organic bulk heterojunction solar cells can be fabricated on flexible substrates from solution processable materials. This is a great advantage compared to the traditional silicon based solar cells, since this offers the possibility of using similar low-cost high-speed large-area roll-toroll fabrication methods as in the printing industry. Recently, the ink-jet and screen printing methods have been used to apply the active layer for organic solar cells [1-3]. However, the active layer for organic solar cell does not need to be printed with high lateral resolution or registration; instead, the thickness, smoothness and homogeneity are much more important factors. Therefore, when aiming for large areas of low-cost flexible organic bulk heterojunction solar cells, roll-to-roll coating techniques are more suitable for applying thin homogeneous PEDOT:PSS and P3HT:PCBM layers. Spin coating is usually used when applying these layers on the laboratory scale, but has the disadvantage that most of the material ends up as waste. Doctor blading is a roll-to-roll compatible coating method that has been used to apply the active layer for organic solar cells [4]. However, none of these printing and coating methods have been used to fabricate organic bulk heterojunction solar cells in a roll-to-roll process on flexible substrates.
In this work we have studied the RG coating method. It is a roll-to-roll process that produces variable thin homogeneous layers over large areas for a wide range of ink viscosities [5,6]. We have fabricated organic bulk heterojunction solar cells [7] with a structure consisting of indium tin oxide (ITO), PEDOT:PSS, P3HT:PCBM, lithium fluoride (LiF) and aluminum (Al). A schematic image of the solar cell structure is shown in Figure 1. The conducting polymer PEDOT:PSS and the active P3HT:PCBM layer were subs
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