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International Journal of Photoenergy
Volume 2013, Article ID 585196, 7 pages
Research Article

Improvement in the Power Conversion Efficiency of Bulk Heterojunction Photovoltaic Device via Thermal Postannealing of Subphthalocyanine:C70 Active Layer

1Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
2Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan
3Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan

Received 4 January 2013; Accepted 4 June 2013

Academic Editor: Liang-Sheng Liao

Copyright © 2013 Chih-Chien Lee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The authors report an efficient organic photovoltaic device based on subphthalocyanine (SubPc):C70 bulk heterojunction (BHJ) via the postannealing treatment. The power conversion efficiency is improved from 4.5% to 5.5% due to the increase in short-circuit current density () from 8.8 to 12.7 mA/cm2 with the expense of decreased fill factor from 52% to 42%. From external quantum efficiency measurements, the spectral shape-independent enhancement over the entire spectrum suggests that the increased mainly originates from improved charge collection efficiency. To confirm this inference, the hole and electron mobilities in the BHJ are estimated from the space-charge limited current, showing improved transport properties at the optimum temperature. Moreover, the morphologic change is also studied as a function of annealing temperature. A larger grain size is observed with increasing temperature due to the phase separation of SubPc and C70. However, at higher temperatures the strong aggregation of C70 molecules may interrupt the pathway of SubPc, resulting in hindered charge transport and, hence, reduced .