RF Sputter Deposition of Indium Oxide / Indium Iron Oxide Thin Films for Photoelectrochemical Hydrogen Production
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RF Sputter Deposition of Indium Oxide / Indium Iron Oxide Thin Films for Photoelectrochemical Hydrogen Production William B. Ingler Jr. and Abbasali Naseem University of Toledo, Department of Physics and Astronomy, MS 111, McMaster Hall, Toledo, OH. 43606, U.S.A. ABSTRACT This project focuses on using indium oxide and indium iron oxide as an alloy to make a protective thin film (transparent, conductive, and corrosion resistant or TCCR) for amorphous silicon based solar cells, which are used in immersion-type photoelectrochemical cells for hydrogen production. From the work completed, the results indicate that samples made at 250 °C with 30 Watt of indium and 100 Watt of indium iron oxide, and a sputter deposition time of ninety minutes produced optimal results when deposited directly on single junction amorphous silicon solar cells. At 0.65 Volts, the best sample displays a maximum current density of 21.4 mA/cm2. INTRODUCTION This project focuses on using indium oxide and indium iron oxide to make a protective thin film (transparent, conductive, and corrosion resistant, TCCR) to deposit on top of amorphous silicon based solar cells [1-4]. These TCCR coated amorphous silicon (a-Si) solar cells are used for hydrogen production in immersion-type photoelectrochemical (PEC) cells (Figure 1, Figure 2) The results indicate that samples made at 250 °C with 30 W of indium and 100 W of indium iron oxide sputter coated on to amorphous silicon solar cells are the optimal parameters for a stable TCCR coating alloying these two materials. Indium oxide is an ideal layer in a hybrid photoelectrodes with amorphous silicon triple junction solar cells to function as TCCR layer. For a TCCR layer on a-Si solar cells, the standard conditions are the protective top layer needs to be at least 90% transparent; the layer need to be conductive at 7 mA/cm2 or greater at 1.7 V (maximum power point voltage for a-Si); and it must be corrosion resistant for thousands of hours. Additionally, the top layer material typically needs to be a wide band gap material and should mismatch to the bands of the a-Si triple junction. However, pure In2O3 has rather poor stability in basic electrolyte so it needs to be alloyed to a compound that can stabilize the film, and the films need to be deposited at low temperatures (under 270 °C) so that the a-Si:H bonds will not degrade during In2O3-InFe2O4 deposition. These thin films were created using rf (radio frequency) sputter deposition. Two 2” sputter guns were used with respective targets of indium and indium iron oxide. The two main variables considered were the temperature of the depositions as well as the sputter powers of indium and indium iron oxide used. Characterization was done by current-voltage measurements and atomic force microscopy. Indium oxide and indium iron oxide films were deposited using rf magnetron sputter deposition in argon and oxygen using multiple sputter guns simultaneously. The substrates used for this study included borosilicate glass, ITO glass, and triple junction a-Si
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