Polymer solar cells: screen-printing as a novel deposition technique
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Polymer solar cells: screen-printing as a novel deposition technique
Tom Aernouts, Peter Vanlaeke, Jef Poortmans, Paul Heremans Polymer and Molecular Electronics, MCP Division Interuniversity Micro-Electronics Centre (IMEC) Kapeldreef 75, B-3001 Leuven, Belgium ABSTRACT Screen-printing is studied as deposition technique for conjugated material based layers. Photovoltaics based on the principle of bulk donor-acceptor heterojunction are tested using a blend of poly(2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) mixed with the C60-derivative (6,6)-phenyl C61-butyric acid methyl ester (PCBM). First, different solution concentrations of the donor MEH-PPV material and of the blend are subjected to rheology measurements. Addition of the acceptor (PCBM) to a donor material based solution induces a decrease of the solution viscosity. However, the overall flow behaviour of the blend remains similar to that of the MEH-PPV based solution. Secondly, it is shown that specific printer settings have to be used to obtain active layers that are suitable for opto-electronic applications. Finally, devices with an overall energy conversion efficiency of 1.25% under standardized simulated solar illumination (AM1.5G; 100mW/cm2) have been obtained showing that screen-printing can be a suitable technique for the deposition of the active layer of polymer solar cells.
INTRODUCTION Solar cells with conjugated materials as active layer have the potential to compete with standard Si-based photovoltaics when production costs are taken into account. The possible use of low-cost substrates as well as the necessity of only a very thin organic active layer leads to a substantial materials cost reduction. Moreover, chemical modifications of the materials such that they become soluble in common solvents make it possible to exploit solution based processes to deposit the active layer, reducing the production costs even further. A common technique to process such conjugated materials is spin coating. In this way, a thin, homogeneous film is easily applied onto large substrates. However, no direct patterning of the deposited layer can be obtained by this technique. Using printing techniques to process organic materials offers the opportunity to obtain patterned films directly onto the substrate. Inkjet printing has in this way already shown its suitability to prepare multi-color displays [1]. The high resolution and the small features that come in reach makes inkjet printing even suitable for fabrication of full plastic transistors. Nevertheless, processing organic solar cells probably does not require this high precision and may need a process with higher throughput. Therefore, screen-printing as a linear casting technique can be interesting as a novel deposition technique to process organic photovoltaics. Moreover, the patterning of the film in one step will even facilitate the production of solar cell modules and the integration of the photovoltaic device into several application tools like smart cards, mobile phones …
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