Organic Solar Cells Based on Evaporated Planar and Bulk Heterojunctions of a PPV- pentamer and C 60 W. Geens 1 , T. Aernouts 1 , J. Poortmans 1 and G. Hadziioannou 2 1 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 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 C 60 act as respectively donor and acceptor materials in planar heterojunction (MEH-OPV5/C 60 ) and bulk heterojunction (MEH-OPV5:C 60 ) 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/C 60 cells exhibit a higher open-circuit voltage (~ 1.00 V) than the MEH-OPV5:C 60 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:C 60 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 Mat. Res. Soc. Symp. Proc. Vol. 708 © 2002 Materials Research Society BB9.3.1