Uncovered - Tips and tricks for solution-processed film deposition in organic electronics

Jaime Ortiz-López et al. are runners up in the 2012 Materials Today cover competition.

Twenty years ago it was unthinkable that organic materials could have an active role in the electronics industry, other than being used as passive electric insulators in polymeric wire sheaths. Today, the picture has changed dramatically thanks to the advent of what is generically called “organic electronics”. This new branch of electronics is based on the use of polymers and small organic molecules that can be electrically conductive or semiconductive. This technology is called ‘organic’ because the main chemical component of these materials is carbon. Pioneering research work in this field started at Cambridge University in the 1990s1.

Big technological steps have been advanced since the preparation and discovery of conductive polyacetylene in 1977 for which Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa were awarded the 2000 Nobel Prize in Chemistry. Conductive polymers are normally semiconducting but acquire metallic-like conductivity through the addition of chemical dopants such as iodine or bromine. Their high conductivity results from the delocalization of electrons along the polymer backbone acting analogous to free electrons in metals, from which derives the term “synthetic metals”. This property is intimately related to their structure which consists of alternating single and double carbon-carbon bonds along the polymer back bone, hence these polymers are generally known as “conjugated”. The first applications of conductive polymers ranged from shield covers against electromagnetic radiation, to anti-static coatings, and smart windows for sunlight screening, and beyond. In semiconductor form, these polymers are now applied as the active material in light-emitting diodes, solar cells and mobile telephone displays. These materials respond to the application of an electric potential with the emission of light (electroluminescence) or generating a current in response to the absorption of light (photovoltaic effect). These applications in electronic gadgets originated the term “plastic electronics”, meaning that conventional electronics based on metallic conduction and inorganic semiconductors can be substituted by conducting and semiconducting polymeric plastics.

Besides conjugated polymers, the range of materials used in organic electronics nowadays includes small molecules with molecular weights much lower than those of polymer chains. These might be all-carbon molecules such as fullerene C60 or C70; polycyclic aromatic hydrocarbons such as pentacene; complex arrangements of phenyl rings intermingled with nitrogen, oxygen or hydrogen atoms like N,N'-di-naphthalene-1-yl-N,N'diphenyl-benzidine (abbreviated NPB), or like 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (abbreviated PTCBI) or like bathocuproine (abbreviated BCP). Another class of molecules is nicely symmetric, with a metal atom at the center of their structure as is the case of tris-8-hydroxyquinolinato-aluminum (abbreviated Alq3); metal phthalocyanines (abbreviated MePc, with Me a transition metal), or tris-bipyridine-ruthenium dichloride (abbreviated RuBpy).

In solid state, RuBpy typically forms a hexahydrated red-colored salt. In solution, the [Ru(Bpy)3]2 cation is easily detached from the chlorine ions and holds interesting optical and electronic properties2. The structure of Ru(Bpy)3 molecular cation resembles a three-leafed clover with a ruthenium atom at the center. Each clover leaf consists of two contiguous pyridine rings linked to the Ru atom through each of the two nitrogen atoms of the pyridine pair. RuBpy has maximum absorbance around 450 nm and photoluminescent emission occurs at 620 nm in acetonitrile solution with the presence of non-coordinating hexafluorophosphate PF6- anions. RuBpy thin films in organic light emitting diodes (OLED) show blue-shifted electroluminescence emission around 583 nm.

Owing to its luminescent properties, RuBpy attracted our attention as a convenient material for the construction of OLEDs3. Six years ago at the Solid State Physics Group of the Escuela Superior de Física y Matemáticas4 of the Instituto Politécnico Nacional in Mexico City we started the study and applications of organic semiconductors (both polymeric and small molecules) as a new line of research extending our initial work on inorganic semiconductors as well as on the synthesis and applications of fullerenes and carbon nanotubes. Last year we ran into difficulties in the construction of OLEDs from solution processed deposition of RuBpy films on flexible substrates. Ramón, a doctorate student and coauthor of this manuscript, was in charge of the preparation of these devices. For this purpose he designed and constructed a simple low-cost spin coating system5 that he later improved with the addition of a robotic arm. With his spin coating system he prepared a batch of RuBpy films for his devices but after many attempts he was not able to obtain sustained emission from his OLEDs. Gabriela, a colleague and also coauthor of this manuscript, analyzed Ramón’s films via atomic force microscopy. The result was the AFM image that turned out to be one of the two runners up in Materials Today 2012 Cover Competition. This image reveals the reason of Ramón’s unsuccessful preparation of his OLED devices. The surface of the RuBpy film is quite rough, looking like a lunar landscape due to burst air bubbles introduced by Ramón’s plan to stir the solution by briskly shaking the vial just before deposition of the RubPy film. The heights of the topographical features are rainbow color coded giving an additional aesthetically pleasing look to the image. After viewing the image, Ramón learned his lesson, and he successfully obtained the desired electroluminescence in his OLEDs.

Further Reading

1. J. M. Shaw and P. F. Seidler, IBM J Res & Dev (2001) 45, 3.
2. K. W. Lee, J. D. Slinker, A. A. Gorodetsky, S. Flores-Torres, H. D. Abruña, P. L. Houston and G. G. Malliaras, Phys Chem Chem Phys (2003) 5, 2706.
3. H. Rudmann, S. Shimada, and M. F. Rubner, J Am Chem Soc (2002) 124, 4918.
4. Visit our site http://www.sepi.esfm.ipn.mx
5. Ramón Gómez Aguilar, Jaime Ortiz López, Lat Am J Phys Educ (2011) 5, 368.