Modelling of Joule heating based self-alignment method for metal grid line passivation
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Modelling of Joule heating based self-alignment method for metal grid line passivation M. Janka1, P. Raumonen2, S. Tuukkanen1 and D. Lupo 1 1
Department of Electronics and Communications Engineering, Tampere University of Technology, P.O.Box 692, FI-33101 Tampere, Finland 2
Department of Mathematics, Tampere University of Technology, P.O. Box 553, FI-33101 Tampere, Finland ABSTRACT A Joule heating based self-alignment method for solution-processable insulator structures has been modeled for the passivation of metal grid lines, for example for organic light emitting diodes or photovoltaic cells. To minimize overhang of the passivation layer from line edges, we have studied the Joule heating approach using solution-processable, cross-linkable polymer insulator films. Finite element simulations were performed to investigate the heating of the sample using glass and poly(ethylene terephthalate) (PET) substrates. The sample was at room temperature and the current was selected to induce a temperature of 410 K at the conductor. It was found that the selection of substrate material is crucial for the localization of cross-linking. For a PET substrate, the temperature gradient at the edge of the conductor is approximately twice the gradient for glass. As a result, using a glass substrate demands high selectivity from the polymer cross-linking, thus making PET a more suitable substrate material for our application. A flexible PET substrate is, in addition, compatible with roll-to-roll mass-manufacturing processes. INTRODUCTION The performance of organic photovoltaic devices (OPV) and light emitting diodes (OLED) is dependent on the device active area. Since known transparent conductors have a relatively high resistivity, the resistance of the transparent anode limits the power conversion efficiency in large devices for OPV applications and brightness homogeneity of OLEDs, due to the large lateral voltage drop inside the electrode. Integration of a metal grid with the transparent conductor improves the performance of OPVs and OLEDs. [1, 2] Metal grids, however, decrease the device active area and thus they need to have low area coverage and high conductivity; they should be as narrow and thick as possible. This topology makes the anode and cathode prone to shorting. To avoid shorting, an insulating layer is put on top of the grid lines. This layer should cover only the grid lines in order to minimize the nonluminescent and non-illuminated areas; accurate alignment of the passivation layer is thus critical for maximizing the aperture ratio of the device. It is possible to use either photolithography or printing for the definition of the passivation layer. However, printing generally requires large overprinting and photolithography an additional alignment step, which is challenging when aiming at high throughput in roll-to-roll production. A self-alignment method offers better registration than printing without additional alignment steps. Here we present finite element simulations of Joule heating in an aluminum grid line on P
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