Controlling the Dominant Length Scale of Liquid–Liquid Phase Separation in Spin‐coated Organic Semiconductor Films

Organic electronic devices are often made by solution processing a multi-component ink. During solution processing, for example, via spin coating, the solvent evaporates and the solid components deposit on the substrate. The morphology of this layer can range from well-mixed to extensively phase separated. To optimize device performance, it is essential to control the degree and dominant length scale of phase separation. Currently, the mechanism of phase separation induced by solvent evaporation is poorly understood. It has been shown that length scales are influenced by spin speed, drying time, final layer thickness and the ratio between the solid components, but a complete experimental dataset and consistent theoretical understanding are lacking. In this contribution, in situ measurements during spin coating and a simple numerical model are used to understand the drying process. In addition, an advanced image analysis of transmission electron micrographs of films processed under a wide range of processing conditions is carried out. A normalized drying rate is proposed as the key parameter that controls the dominant length scale of phase separation.

[1]  Daniel T. W. Toolan.  In situ studies of spin-coated polymer films , 2014 .

[2]  Shijun Jia,et al.  Polymer–Fullerene Bulk‐Heterojunction Solar Cells , 2009, Advanced materials.

[3]  Pierre Soille,et al.  Morphological Image Analysis: Principles and Applications , 2003 .

[4]  R. Atkin,et al.  Oil core-polymer shell microcapsules prepared by internal phase separation from emulsion droplets. I. Characterization and release rates for microcapsules with polystyrene shells. , 2004, Langmuir : the ACS journal of surfaces and colloids.

[5]  G. Gelinck,et al.  Nanoscale Organic Ferroelectric Resistive Switches , 2014 .

[6]  Kamal Asadi,et al.  Organic non-volatile memories from ferroelectric phase-separated blends. , 2008, Nature materials.

[7]  Stephen Z. D. Cheng.  Phase Transitions in Polymers: The Role of Metastable States , 2008 .

[8]  R. L. Thompson,et al.  Quantitative evaluation of evaporation rate during spin-coating of polymer blend films: Control of film structure through defined-atmosphere solvent-casting , 2010, The European physical journal. E, Soft matter.

[9]  P. Topham,et al.  In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating. , 2011, ACS nano.

[10]  Stephen R. Forrest,et al.  White Organic Light‐Emitting Devices for Solid‐State Lighting , 2004 .

[11]  D. R. Lloyd,et al.  Microporous membrane formation via thermally-induced phase separation. II: Liquid-liquid phase separation , 1991 .

[12]  A. G. Emslie,et al.  Flow of a Viscous Liquid on a Rotating Disk , 1958 .

[13]  Daniel T. W. Toolan,et al.  Direct observation of morphological development during the spin‐coating of polystyrene–poly(methyl methacrylate) polymer blends , 2013 .

[14]  E. Yeatman,et al.  Towards Better Control of Sol-Gel Film Processing for Optical Device Applications , 1997 .

[15]  M. Kemerink,et al.  Morphological device model for organic bulk heterojunction solar cells. , 2009, Nano letters.

[16]  Richard A. L. Jones,et al.  Time-resolved light scattering studies of phase separation in thin film semiconducting polymer blends during spin-coating , 2005 .

[17]  Johannes E. Schindelin,et al.  Fiji: an open-source platform for biological-image analysis , 2012, Nature Methods.

[18]  K. Binder.  Collective diffusion, nucleation, and spinodal decomposition in polymer mixtures , 1983 .

[19]  Klaus Meerholz,et al.  Highly efficient solution-processed phosphorescent multilayer organic light-emitting diodes based on small-molecule hosts , 2007 .

[20]  Michael Niggemann,et al.  Efficiency limiting morphological factors of MDMO-PPV:PCBM plastic solar cells , 2006 .

[21]  Jayanta K. Baral,et al.  Relating chemical structure to device performance via morphology control in diketopyrrolopyrrole-based low band gap polymers. , 2013, Journal of the American Chemical Society.

[22]  D. Stoyan,et al.  Statistical Analysis and Modelling of Spatial Point Patterns , 2008 .

[23]  E. Moons,et al.  Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene/PCBM Blends , 2007 .

[24]  T. Hashimoto,et al.  Slow spinodal decomposition in binary liquid mixtures of polymers , 1985 .

[25]  Wilhelm Burger,et al.  Digital Image Processing - An Algorithmic Introduction using Java , 2016, Texts in Computer Science.

[26]  Daniel T. W. Toolan,et al.  Directed phase separation of PFO:PS blends during spin-coating using feedback controlled in situ stroboscopic fluorescence microscopy , 2013 .

[27]  P. Sonar,et al.  Nanoscale phase domain structure and associated device performance of organic solar cells based on a diketopyrrolopyrrole polymer , 2013 .

[28]  James A. Forrest,et al.  Phase separation morphology of spin-coated polymer blend thin films , 1997 .

[29]  C. Macosko,et al.  Spin Coating of a PMMA/Chlorobenzene Solution , 1991 .

[30]  K. Mecke,et al.  Morphological characterization of spinodal decomposition kinetics , 1999 .

[31]  Sandra Kouijzer,et al.  Predicting morphologies of solution processed polymer:fullerene blends. , 2013, Journal of the American Chemical Society.

[32]  Dunbar P. Birnie,et al.  Combined flow and evaporation of fluid on a spinning disk , 1997 .

[33]  Gang Li,et al.  25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research , 2013, Advanced materials.

[34]  A. Baldan,et al.  Review Progress in Ostwald ripening theories and their applications to nickel-base superalloys Part I: Ostwald ripening theories , 2002 .

[35]  M. Turbiez,et al.  Charge transport and recombination in PDPP5T:[70]PCBM organic solar cells: The influence of morphology , 2014 .

[36]  C. Brabec,et al.  2.5% efficient organic plastic solar cells , 2001 .

[37]  D. R. Lloyd,et al.  Kinetics of droplet growth in liquid-liquid phase separation of polymer-diluent systems : experimental results , 1995 .

[38]  Philippe Barois,et al.  Colloidal ordering from phase separation in a liquid- crystalline continuous phase , 2000, Nature.

[39]  Richard A. L. Jones,et al.  An interfacial instability in a transient wetting layer leads to lateral phase separation in thin spin-cast polymer-blend films , 2005, Nature materials.