213 Bulgarian Chemical Communications, Volume 48, Special Issue G (pp. 213-218) 2016 Vacuum co-deposition of organic solar cell structures G. Georgieva 1 , D. Dimov 2 , G. Dobrikov 3 , D. Karashanova 2 , A. Kirilov 4 , F. Markova 4 , E. Bubev 4 , A. Georgiev 4 , R. Yordanov 3 , and I. Zhivkov 1, 2, * 1 Faculty of Chemistry, Centre for Materials Research, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic 2 Institute of Optical Materials and Technologies ”Acad. J. Malinowski”, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. bl. 101/109, 1113 Sofia, Bulgaria 3 Department of Electronics, Faculty of Electronic Engineering andTechnologies, Technical University - Sofia, 8 Kliment Ohridski blvd., 1000 Sofia, Bulgaria 4 Department of Organic and Physical Chemistry, 1756 Sofia, 8 "St. Kliment Ohridski" Blvd, University of Chemical Technology and Metallurgy, Bulgaria. Received October 10, 2016; Revised November 21, 2016 An algorithm for calculation the mass ratio between evaporated components in vacuum co-deposition is presented. The algorithm is demonstrated via preparation of organic solar cells. Zinc phthalocyanine (ZnPc) based solar cell samples of types ITO|PEDOT:PSS|ZnPc:C60|Al were prepared in a clean room conditions. The active bulk heterojunction (BHJ) organic composite ZnPc:C60 film was prepared with mass ratio of 4.2:1. The surface morphology was characterized by SEM. Photoelectrical measurements were carried out on the samples prepared, demonstrating the advantages of the composite material with respect to the single ZnPc layer. The algorithm developed allows further precise optimization of the BHJ cells to be performed. Keywords: vacuum co-deposition, organic electronics, solar cells. INTRODUCTION Thin films of low molecular weight semiconductors are usually prepared by variety of complex techniques, including physical or chemical vapour deposition, organic molecular beam epitaxy or solution-based deposition techniques. The performance of small molecular organic devices is highly sensitive to the film morphology and processing conditions. Often, the solution-deposited active layers of devices (e.g. spin coated films) exhibit a high portion of microcrystallites and aggregates. The vapour deposition techniques provide high-quality crystalline films characterized by improved charge-transport properties compared with those of solution-deposited films. The vacuum technique has the advantage of dry film deposition process excluding the usage of solvents. Moreover this technique provides more opportunities to control important film parameters as film thickness and uniformity. As a consequence the films deposited exhibit more stable and reproducible properties with a comparison to solution deposited films. In the last decades it has been shown that the charge separation on the donor acceptor interface considerable increases the efficiency [1]. The simplest way to provide the charge separation in the organic substances is the formation of bi-layer donor/acceptor structure [2]. In case of insoluble donor and acceptor materials the vacuum deposition suggests possible solution for bi-layer structure formation. Further increase of the efficiency was achieved introducing p-i-n structures. Pfeiffer and co- workers [3, 4] have developed p-i-n technology using high vacuum deposition process. Efficiency of 3.6% has been obtained. The exciton diffusion length in the organic semiconductors is about 10 nm but the optimized light absorption requires film thicknesses of about 100 nm. This discrepancy makes the bi-layer structure less effective. This problem is overcome later developing the BHJ structure. BHJ comprises an active layer with mixed donor and acceptor species to allow for much thicker films to better absorb sunlight [5]. Vacuum co-deposition allows easier way to obtain BHJ composite layer, grown from insoluble © 2016 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria *To whom all correspondence should be sent: E-mail: zhivkov@fch.vutbr.cz