Organic Solar Cells Based on Evaporated Planar and Bulk Heterojunctions of a PPVpentamer and C 60
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Organic Solar Cells Based on Evaporated Planar and Bulk Heterojunctions of a PPVpentamer and C60 W. Geens1, T. Aernouts1, J. Poortmans1 and G. Hadziioannou2 IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium 2 Department of Polymer Chemistry and Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands 1
ABSTRACT The technique of vacuum evaporation has been applied to deposit organic photovoltaic active layers. The five-ring PPV-type oligomer 2-methoxy-5-(2'-ethylhexyloxy)-1,4-bis((4’,4”bisstyryl)styrylbenzene) (MEH-OPV5) and C60 act as respectively donor and acceptor materials in planar heterojunction (MEH-OPV5/C60) and bulk heterojunction (MEH-OPV5:C60) devices. These devices were fabricated with ITO/PEDOT:PSS bottom electrodes and Al top contacts. The performance of both solar cell configurations has been compared. It was found that under AM1.5 illumination the MEH-OPV5/C60 cells exhibit a higher open-circuit voltage (~ 1.00 V) than the MEH-OPV5:C60 devices (~ 0.92 V). On the other hand, the limited exciton diffusion length in these materials was reflected in the lower short-circuit current density of the planar heterojunction cells as compared to the bulk heterojunction structures. Overall AM1.5 power conversion efficiencies reaching 2 % are reported. Also the influence of the organic layer thickness and the substrate temperature during deposition on the device performance has been addressed. Thick organic films generally induce a high series resistance that limits both the short-circuit current density and the fill factor. An elevated substrate temperature during deposition of the MEH-OPV5:C60 layers onto ITO/PEDOT:PSS led to the formation of nucleated islands of 100 – 150 nm diameter with holes in between. As a result, no reliable photovoltaic devices could be realized with such organic films. AFM analysis and spectral response measurements supported these findings.
INTRODUCTION The recent progress in the field of spin-cast organic bulk heterojunction solar cells has resulted in devices with reproducible AM1.5 power conversion efficiencies between 2 % and 3 % [1, 2]. With respect to the up-scalability of the fabrication process of these cells, a lot of work still needs to be done. However, promising results using more industrial techniques such as doctor-blade [3] and screen-printing [4] have already been reported. Simultaneously, considerable efforts have focused on the understanding of the working principle of this type of photovoltaic cells. For example, the origin of the open-circuit voltage of spin-cast polymer/fullerene solar cells has been unraveled [5]. Furthermore, temperature dependent analysis of the photovoltaic parameters of such devices pointed out the relation between the mobility of the charge carriers in the donor-acceptor network and the final short-circuit current [6]. The importance of the mobility issue in single as well as in blended layers is also addressed
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elsewhere [7, 8]. The peculiar morphological aspects of the spin-cast photoactiv
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