Influence of OVPD parameters on the performance of organic solar cells utilizing pentacene/PTCDI absorption layers
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Influence of OVPD parameters on the performance of organic solar cells utilizing pentacene/PTCDI absorption layers S. Axmann1*, M. Brast1, N. Wilck2, H. Windgassen2, M. Heuken1,3, H. Kalisch1 and A. Vescan1 1
Device Technology, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany Institute of Semiconductor Electronics, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany 3 AIXTRON SE, Kaiserstr. 98, 52134 Herzogenrath, Germany 2
ABSTRACT As global energy demand is steadily growing, renewable energy generation by solar cells is becoming increasingly important. The use of mono- and polycrystalline silicon solar cells, which nowadays dominate the market, is limited by wafer size, rigidness of substrates and the requirement of large energy amounts for manufacturing. Organic solar cells (OSC) have the potential to overcome these limitations; especially organic vapor phase deposition (OVPD) technology offers the possibility of reproducible, large-scale production at low temperatures and on flexible substrates. We report on planar heterojunction OSC utilizing an active layer of pentacene/N, N'ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI) fabricated by an Aixtron Gen-1 OVPD tool. The influence of substrate temperature was studied using atomic force microscopy (AFM) on single layers and bilayers. In addition electrical characterization with and without illumination of fully processed solar cells which utilize different cathode layers was carried out. AFM images indicate that crystallization of pentacene layers can be widely influenced by substrate temperature, a PTCDI-C13H27 layer atop of these covers the crystallites. Open-circuit voltage was found to be 0.47 V and short-circuit current densities beyond 0.8 mA/cm2 were measured under a spectrum close to AM 1.5 with 100 mW/cm2. Fill factors were determined to be as high as 44 %. INTRODUCTION Renewable energy sources based on photovoltaics have attracted a lot of interest over the last decade. Cell efficiencies increased beyond 20 % [1] and module prices dropped below 2.50 €/Wp [2] encouraging commercialization and research. Current world efficiency records using single-junction inorganic devices are according to [1] 25 % for crystalline Si and 28 % for thin-film GaAs. Over the last 20 years, so called “third-generation” or thin-film solar cells have emerged which offer the possibility of lower manufacturing cost, shorter energy payback time as well as foldable devices and building-integrated photovoltaic (BIPV) applications. As a part of this last generation, organic solar cells have been developed, reaching efficiencies of 8.3±0.3 % for polymer as well as small-molecule-based solar cells demonstrated by Konarka and Heliatek [1]. Pentacene (C22H14) and N, N'-ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI-C13H27, further referenced as PTCDI) are two versatile hole- respectively electronconducting small-molecule materials used in earlier experiments of numerous research groups. Pentacene has shown some of the highest mo
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