Effects of Inserting Highly Polar Salts Between the Cathode and Active Layer of Bulk Heterojunction Photovoltaic Devices
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Effects of Inserting Highly Polar Salts Between the Cathode and Active Layer of Bulk Heterojunction Photovoltaic Devices Sean E. Shaheen, Christoph J. Brabec1, N. Serdar Sariciftci1, and Ghassan E. Jabbour Optical Sciences Center, University of Arizona, Tucson, AZ 85721, USA 1 Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University of Linz, A-4040 Linz, Austria ABSTRACT Thermal deposition of small amounts of various salts at the interface between the active layer and the aluminum cathode was shown to alter the performance of bulk heterojunction photovoltaic devices. LiF and LiBr were found to enhance the power conversion efficiency as compared to devices with no interfacial salt, but Cs and K compounds were found to severely diminish the device performance. It is suggested that the Li compounds preferentially align to produce a bulk dipole moment at the interface, whereas the Cs and K compounds do not.
INTRODUCTION Since the discovery of ultrafast photoinduced electron transfer between a conjugated polymer and a fullerene, photovoltaic devices based on blends of these materials have become promising candidates for inexpensive, large scale solar power conversion [1,2]. When blended together, the conjugated polymer and fullerene species form a bulk heterojunction of interpenetrating donor-acceptor networks. Upon absorption of a photon anywhere within the blend, mobile charges are created by electron transfer from donor to acceptor with a timescale of less than 50 fs. The resulting free hole and electron are then driven toward the anode and cathode, respectively, under the influence the built-in electric field that is established by the difference in the work-functions of the electrodes. Recent improvements in the morphology of the conjugated polymer : fullerene blend have resulted in a power conversion efficiency of 2.5% for a device under simulated solar (AM1.5) illumination [3]. This device also contained a small amount of LiF that was thermally deposited onto the active layer surface prior to deposition of the aluminum cathode. This technique was previously demonstrated to improve the electron injection efficiency in organic light emitting diodes (OLEDs). Here we investigate the use of a variety of similar materials at the active layer / cathode interface of the bulk heterojunction solar cell in order to better understand the mechanism by which this enhancement occurs. Materials that were investigated included LiF, CsF, LiBr, CsBr, KBr, and Cs formate.
EXPERIMENT The structure of the bulk heterojunction solar cell is shown in figure 1. The fabrication of all the devices was performed in an identical manner except for the choice of salt deposited onto the active layer. First, a 150 nm thick film of poly(ethylene dioxythiophene) doped with polystyrene sulphonic acid (PEDOT:PSS, Bayer AG) was first spin-cast from a water solution C5.51.1
Salt
Aluminum
Active layer
PEDOT:PSS ITO Glass
Light
Figure 1. Schematic diagram of the bulk heterojunction photovoltaic device geometry
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