1. Surface Modification of Indium-Tin Oxide Electrodes With Gold Nanoparticles and Its Effect on Organic Photovoltaic Performance Diogenes Placencia and Neal R. Armstrong -- University of Arizona Overview Oxide Problems: Issues with ITO as an oxide bottom contact arise from its surface and electrical heterogeneity. We have recently shown that this electrical activity at the oxide surface is not uniform, and ITO can be considered a partially blocked electrode. Several investigators have postulated that patches of substoichiometric Sn x O y sites in the ITO surface region could serve as electroactive regions. Impact on Device Performance: Series Resistance (R s ) in an organic solar cell is composed of several resistances, one of which is the charge-transfer resistance ( R s,CT ). Partially blocked electroactive sites contribute to the increase in charge-transfer resistance, leading to an increase in overall series resistance, leading to the degradation of both fill factor (FF) and V oc. V R P R S n J J ph J P J R s = R s,B + R s,CT + R S,E + R s,ITO A New Approach to OPV Optimization: Gold nanoparticles are known to improve the catalytic efficiency of reactions such as CO oxidation, but have not been explored for their potential impact on charge transfer resistance on TCO surfaces. We have optimized the solution and gas-phase deposition of gold nanoparticles on ITO to i) minimize loss in transparency in the oxide; ii) increase the electron- transfer rates of solution probe molecules and iii) improve in planar heterojunction solar cell performance. Methods MOx Substrate HAuCl 4 Basic pH Heating Stirring Rinse Gold-Doped MOx + Al BCP C 60 TiOPc 100nm 10nm 40nm 18nm NN TiOPc C 60 BCP Intensity (Cts/s) Pre-Annealing: mainly Au x O y and Au(OH) z species XPS Analysis A sol-gel type pre-cursor is proposed as the predominate Au species at the ITO surface prior to annealing. Annealing produces reduced gold; a final O 2 -plasma etch produces a mixture of metallic and oxide species. Organic Photovoltaic Molecules Binding Energy (eV) Au/ITO k S = 1.2E-2 cm/sec >> k S = 8.4E-4 cm/sec Optical Characterization Sol-gel Precursor Electrochemical Investigation The optical properties of the ITO showed little decrease in transparency after gold NP deposition. Solution electrochemistry of dimethyl ferrocene as a probe molecule showed that Ks can be varied through concentration, approaching values of gold at high loading and low values at lower loadings. Effects on OPV Performance Gold NP Optimization Results ParametersControlTimeTime + pH V oc 0.62 0.64 J sc (mA/cm 2 )-7.24-7.28-8.72 FF0.490.530.54 Efficiency (%)2.222.393.01 TiOPc/C 60 planar heterojunction OPVs showed a significant improvement in device efficiency as Au-NPs were added to the ITO surface: Both exposure time to the Au-precursor solution, and solution pH during this exposure, were important in this optimization. Annealed: Au° OP-etched Surface Composition; Optical/Electrochemical Properties Research Support: NSF-Chemistry and NSF-STC (CMDITR); Office of Naval Research; Department of Energy/Basic Energy Sciences TiOPc deposited on Au-doped ITO electrodes showed an increase in absorbance in the NIR. Phase I TiOPc (Q-band ca. 750nm) decreases in absorbance while Phase II TiOPc ( Q-band ca. 820nm) increases and is thought to play a role in the improved performance of the OPV device. Future Directions Focus will be placed on obtaining HRTEM images of the NPs and subsequent increase in NP loading due to phase II Pc present on the surface. Further investigation will include photocurrent-action analysis. 1. M. Haruta; S. Tsubota; T. Kobayashi; H. Kageyama; M. Genet; B. Delmon. Journal of Catalysis 144, 175 (1993) 2. J. Guzman; B. C. Gates. Journal of the American Chemical Society 126, 2672, (2004) 1 2 Conclusions Gold NPs were found not to affect series resistance. However, it was discovered that there was a change in the phase of the TiOPc (from phase I to Phase II), which led to the increase in photocurrent and overall increase in device performance.