• Julsgaard, Brian
• Mirsafaei, Mina
• Madsen, Morten
• Hansen, John Lundsgaard
• Jensen, Pia Bomholt
• Balling, Peter
• Ram, Sanjay
• Lakhotiya, Harish
• Rubahn, Horst-Günter

Abstract

The use of interfacial layers in organic solar cells has been investigated intensively over the past years, as it has a strong impact on both the power conversion efficiency and stability of the devices. Among the systems investigated are for example alkali salts, ionic liquids, neutral polymers, conjugated polyelectrolytes and various metal-oxides, which have been demonstrated to result in promising device characteristics1. In this context, n-type metal-oxides such as titanium oxide (TiOx) have drawn a lot of attention as electron-transport and exciton-blocking layers, mainly due to its high optical transparency and favorable energy-level alignment with many commonly used electron-acceptor materials. <br/>There are several methods available for fabricating compact TiOx thin-films for use in organic solar cells, including sol-gel solution processing, spray pyrolysis and atomic-layer deposition; however, these methods either require sintering at high temperatures, or result in the existence of trapped sites in the films, which lead to the presence of the well-known s-shape in theJ-V curves of the devices. In order to remove the observed s-shape, and thus improve the fill factor of the cells, light soaking is typically performed, which however brings along several unwanted side-effects in terms of energy losses, material degradation, batch-to-batch variations, etc.2<br/>In this work, we demonstrate the development of TiOx based light-soaking-free inverted polymer solar cells with power conversion efficiency of around 7%, by incorporating sputter deposited TiOx thin-films as electron-transport and exciton-blocking layers. In the work, we report on the effect of different TiOx deposition temperatures and thicknesses on the organic-solar-cell device performance. Besides optical characterization, AFM and XRD analyses are performed to characterize the morphology and crystal structure of the films, and external quantum efficiency measurements are employed to shed further light on the device performance. Our study presents a novel method for implementation of TiOx thin-films as electron-transport layer in organic solar cells that eliminates the need for light soaking and still allows for integration on flexible plastic substrates, which is beneficial for roll-to-roll mass production of flexible organic solar cells. <br/><br/>1. Steim, R.; Kogler, F. R.; Brabec, C. J., Interface materials for organic solar cells. Journal of Materials Chemistry 2010, 20 (13), 2499-2512.<br/>2. Kim, J.; Kim, G.; Choi, Y.; Lee, J.; Park, S. H.; Lee, K., Light-soaking issue in polymer solar cells: Photoinduced energy level alignment at the sol-gel processed metal oxide and indium tin oxide interface. Journal of Applied Physics 2012, 111 (11), 114511.<br/><br/>

Topics

• Deposition
• impedance spectroscopy
• morphology
• polymer
• x-ray diffraction
• atomic force microscopy
• titanium
• interfacial
• tin
• sintering
• power conversion efficiency
• Indium
• solution processing
• spray pyrolysis