Improving Polymer Based Photovoltaic Devices by Reducing the Voltage Loss at the Donor-Acceptor Interface
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Improving Polymer Based Photovoltaic Devices by Reducing the Voltage Loss at the Donor-Acceptor Interface Date J. D. Moet1, Lenneke H. Slooff1, Jan M. Kroon1,2, Svetlana S. Chevtchenko2,3, Joachim Loos2,3, Marc M. Koetse2,4, Jorgen Sweelssen2,5, and Sjoerd C. Veenstra1,2 1 Solar Energy, ECN, P.O. Box 1, Petten, NL-1755ZG, Netherlands 2 Dutch Polymer Institute, Eindhoven, NL-5600 AX, Netherlands 3 Laboratory of Polymer Technology, Eindhoven University of Technology, P.O. Box 513, Eindhoven, 5600 MB, Netherlands 4 Holst Centre / TNO, Hightech Campus 48, Eindhoven, 5656 AE, Netherlands 5 Science and Industry, TNO, P.O. Box 6235, Eindhoven, 6500 HE, Netherlands
ABSTRACT This paper discusses the characterization and optimization of organic solar cells based on a bulk heterojunction consisting of an alternating copolymer, containing a fluorene and a benzathiadiazole unit with two neighboring thiophene rings, and a fullerene derivative (PCBM). The resulting power conversion efficiency amounts 3.9±0.2 % (AM1.5, 100 mW/cm2) and these polymer solar cells are therefore considered auspicious for further research. INTRODUCTION Important progress has been made in the field of solar energy since the first crystalline silicon solar cell based on a p-n junction was presented in 1954. Extensive research on various inorganic semiconductors has revealed that crystalline silicon is by far the most applicable. With power conversion efficiencies exceeding 15%, present-day crystalline silicon solar cells appear to be suitable replacements for conventional sources of electricity. However, the high production costs of crystalline silicon currently still narrow its potential. Since the discovery of electrical conductivity in doped polyacetylene in 1977 [1], much attention has been drawn to conjugated polymers. It was realized that this class of organic materials offers advantageous properties as compared to inorganic semiconductors. Advantages include the mechanical flexibility of organic materials, the tunability of their opto-electronic properties, easy incorporation in various kinds of devices and, above all, low-cost fabrication of these devices [2]. The high potential of conjugated polymers in solar cell applications has already become visible in polymer:fullerene bulk heterojunction solar cells with power conversion efficiencies approaching 5 %. There remains room for improvement, as device model calculations predict ultimate efficiencies exceeding 10% [3, 4]. In recent years, polyfluorenes have gathered much attention in organic light emitting diode (OLED) research, because of their specifically good transport properties, stability (both thermal and water-/air-stability) and tunability. Polyfluorene-based polymers with lower band gap energies have been synthesized in attempts to harvest more light at higher wavelengths, i.e., beyond the visible part of the sun’s spectrum.
Promising results have been reported for solar cells based on various kinds of polyfluorene derivatives, in particular, the work in the groups of And
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