Optimization of Organic Solar Cells and Photodetectors Based on Pentacene Thin Films
- PDF / 233,983 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 9 Downloads / 209 Views
I6.30.1
Optimization of organic solar cells and photodetectors based on pentacene thin films S. Yoo, B. Domercq, and B. Kippelen Center for Organic Photonics and Electronics, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA ABSTRACT We report on the photovoltaic properties of efficient multilayer devices based on pentacene thin films as a function of light intensity. Light-induced effects are characterized by the refined equivalent circuit model which incorporates an additional shunt resistance and an additional diode whose properties are function of the light intensity. Relative effects of these intensitydependent parameters as well as the route to further optimization are discussed. INTRODUCTION Thin films of pentacene, a polyacene with five aromatic rings, have been successfully demonstrated as transport layers in organic field-effect transistors (OFET) with mobilities often as high as > 1 cm2/Vsec,[1,2] and have often been used as a model system in understanding the various aspects of organic materials.[3] However, its applications to photovoltaic cells were relatively rare. Photovoltaic (PV) devices [4] or photodetectors [5] based on junction between metal or heavily-doped inorganic semiconductor and pentacene were previously reported, but the power conversion efficiencies of PV were relatively low. Recently, we have demonstrated that efficient solar cells can be achieved when pentacene is paired with C60 in a bilayer structure to form a donor/acceptor heterojunction.[6] [See Fig. 1 (a) for device structure.] Highly efficient photocurrent generation was observed through out the visible spectral range with a peak external quantum efficiency of 58% at the wavelength λ of 670nm, which corresponds to the absorption peak of pentacene films [See Fig. 1 (b)]. Note that such efficient light harvesting was attributed partly to the relatively large exciton diffusion length in the pentacene layer which allows for high photogeneration quantum efficiency. Malliaras and his coworkers have also reported cells based on pentacene and C60 but with a different device structure [7]. Optimizing organic solar cells first requires identification of the loss mechanisms in the cells under investigation. To this end, the photovoltaic properties of pentacene/C60 solar cells were investigated as a function of light-intensity in this work. Similar approaches were used by other authors in small molecule multilayer-,[8,9] polymer/C60 bulk heterojunction-, [10,11] and nanorod/polymer-type solar cells[12] in order to gain insight into the various physical processes which may occur in accordance with the intensity of illumination. In this work, the photovoltaic characteristics are parameterized as a function of the light intensity using the simple equivalent circuit used for inorganic pn-junction type solar cells, with the comprehensive analysis on the relative effects of these parameters on overall photovoltaic performance. Light intensitydependent parameters are identified, and the modifi
Data Loading...
