Effects of air and light exposure on the opto-electronic properties of polymer:fullerene solar cells
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. Effects of air and light exposure on the opto-electronic properties of polymer:fullerene solar cells Antonietta De Sio1, Enrico Da Como2, Elizabeth von Hauff1, Jürgen Parisi1 1
Energy and Semiconductor Research Laboratory, Institute of Physics, Carl von Ossietzky University of Oldenburg, 26111 Oldenburg, GERMANY
2
Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-MaximiliansUniversity Munich, Munich 80799, Germany
ABSTRACT In this study we investigate how exposure to ambient air and light during device processing affects the opto-electronic properties of poly-3-hexylthiophene (P3HT) : [6,6]-phenyl-C61butyric acid methyl ester (PCBM) bulk heterojunction solar cells (BHJ). The properties of pure P3HT layers prepared in an inert atmosphere, under ambient conditions, and additionally degraded under light in ambient conditions were investigated using photoluminescence (PL) and photoinduced absorption (PIA). It was observed that exposure to air during processing leads to oxygen doping of the polymer. Exposure to air combined with light was found to significantly decrease the PL and PIA signals. The current-voltage (I-V) and external quantum efficiency (EQE) characteristics of solar cells fabricated in an inert atmosphere were compared to solar cells processed under ambient conditions. It was observed that processing in air leads to a reduction in the photocurrent in the devices which is attributed to electron trapping by oxygen in the active layer. INTRODUCTION Polymer:fullerene bulk heterojunction (BHJ) solar cells [1,2] have recently reached power conversion efficiencies of over 7 % [3,4]. In addition to high efficiencies, recently the stability and lifetime of BHJ has also been a focus of intense research. Studies have been performed to gain insights into the physics behind the degradation mechanisms, specifically on the effects of light and oxygen on the opto-electronic properties of P3HT layers and on P3HT:PCBM solar cells [5-8]. In this study we use PL and PIA spectroscopy to investigate how exposure to air and light degrades P3HT layers. The opto-electronic properties of P3HT:PCBM solar cells prepared in ambient atmosphere are compared to those of solar cells prepared in an N2 atmosphere. We observe that the concomitant exposure to air and light can lead to degradation in P3HT. Processing P3HT:PCBM blends in air led to a reduction in the photocurrent which was attributed to electron trapping due to oxygen in the blend.
EXPERIMENT For the PL/PIA investigations, regioregular P3HT (supplied by Merck) was spun on sapphire substrates from solution (1wt% in ortho-Dichlorobenzene (oDCB)) and annealed at 150° for 10 minutes. Samples were measured at 80K. The excitation wavelength was 532 nm and the laser power was 10 mW. The laser was chopped at 80Hz. To fabricate the solar cells, P3HT was blended with PCBM (Solenne BV) in a weight ratio of 1:1 in a 1.5 wt % solution of oDCB. Patterned ITO coated glass substrates were cleaned in acetone and then isopropyl alcohol and subsequently exposed t
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