Journal of ELECTRICAL ENGINEERING, VOL. 65, NO. 7s, 2014, 46–49 EFFECT OF SELF–ORGANIZATION IN 2D MOLECULAR SYSTEMS PERSPECTIVE FOR ORGANIC PHOTOVOLTAICS Tom´ aˇ s V´ ary * — Juraj Chlp´ ık * ** — Katar´ ına Bombarov´ a * — Gabriel ˇ C´ ık *** — J´ ulius Cir´ ak * Properties of monolayer films of the amphiphilic molecules using the Langmuir method were studied. We focused on binary systems of organic molecules to form self-organized structures suitable for application in photovoltaics. The studied materials were fullerene nanoparticles as acceptor and oligothiphene polymer as donor in organic molecular heterojunction structure. Mechanical as well as thermodynamic quantities were evaluated for characterizing the 2D molecular binary system at various molar ratios. The formation of self-assembled domains was visualized by AFM and fluorescence microscopy. Keywords: Langmuir monolayers, fullerene, oligothiophene, organic solar cells, molecular self-organization 1 INTRODUCTION Langmuir monolayers are monomolecular layers formed on the surface of a liquid, mostly water. Due to the method of pouring of amphiphilic material mixture on the water surface these monolayers form structures as a result of a self-organized process. Organic molecules con- stituting the monolayer are amphiphilic and they sponta- neously spread at the air/water interface. The Langmuir monolayer is a very suitable model for the study of self- organization in two dimensions. Structures are primar- ily characterized through isotherm measurements. A pair of thermodynamic quantities, temperature and surface pressure, can be easily controlled; surface pressure by a moving barrier over the surface. In the presence of two constituents in the mixture, the properties of the system depend on the mixture molar ratio. In this communication we focus on monolayer proper- ties of a binary system consisting of oligothiophene and fullerene: their mechanical and thermodynamic proper- ties in a monomolecular layer. This combination of ma- terials is perspective in organic photovoltaic cells. The research involving polymer (donor) and fullerene (accep- tor) heterojunctions became intense in the area of pho- tophysics and device physics [1,2] after the evidence of photoinduced electron transfer from the excited state of a conducting polymer onto fullerene (C60). Because a sin- gle polymer layer device presents low efficiency due to the mechanism of charge generation and transport, the use of a C60 molecule, which has a high electron affinity value, sublimed onto the polymer (donor) in a bilayer hetero- junction or mixed in the polymer film (blend) in a bulk heterojunction, increased dramatically the efficiency of the photovoltaic devices [3]. It was found that, besides tailoring electronic properties, the bulk film morphology of the blend is crucial for the photovoltaic performance. The goal of this paper is to find the conditions under which mixture forms spatialy distributed interfaces be- tween domains of components. Interfaces are necessary for exciton splitting and subsequent charge transport. The intermolecular interactions play essential role in an intimate mixing of the components. These phenomena are studied here in a two-dimensional molecular struc- ture, using the monolayer formed at the air/water inter- face as a model molecular system. The phase separation is visualized by fluorescence microscopy techniques on a monolayer transferred from the water surface onto a solid silicon substrate by the Langmuir-Blodgett technology. 2 MATERIALS AND METHOD Fullerene (PCBO) - [6,6]-Phenyl C 61 butyric acid octyl ester, 99% - was purchased from Sigma-Aldrich Co. Oligothiophene hexamer (OTH) 3,3””-bis-decyl[2,2’;5’ .2”;5”;2”’;5”’,2””,2””]sexitiophene-5,5””-didaroxilic acid was synthetized as described in [4]. Both materials were dissolved in chloroform at a con- centration of 0.5 mmol/l. The subphase used was bidis- tilled deionised water (18 M.cm, ELIX 5, Millipore, USA). OTH-PCBO mixture monolayers were formed by spread- ing of approx. 100l of the solution with mixed materials on the water surface in the Langmuir trough using a mi- crosyringe (Hamilton, USA). Langmuir trough model 611M (NIMA Technology, Coventry, UK) was used in monolayer experiments. Work- ing areas of the trough used were 600 cm 2 (max) and 75 cm 2 (min). The measurement of surface tension was car- ried out using the Wilhelmy balance equipped with a filter paper plate. After spreading material to the subphase/air interface the solvent was left to evaporate for 15 minutes to reach stability of the monolayer. Monolayers were com- pressed at constant speed of 2 cm 2 /min. The trough was Institute of Nuclear and Physical Engineering, Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Ilkoviˇ cova 3, 812 19 Bratislava, Slovakia; ∗∗ International Laser Centre, Ilkoviˇ cova 3, 841 01 Bratislava, Slovakia; ∗∗∗ Institute of Chemical and Environmental Engineering, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinsk´ eho 9, 812 37 Bratislava, Slovakia Print ISSN 1335-3632, c  2014 FEI STU