Volker Sittinger¹, Daniel Stoll¹, Sebastian Jung¹, André Kaiser¹, Özde Ş. Kabakli², Christian Schwarz², Patricia S. C. Schulze², Juliane Borchert^2
1Fraunhofer Institute for Surface Engineering and Thin Film IST, Braunschweig, Germany
²Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany
This work focuses on the development and optimization of indium zinc oxide (IZO) as top transparent conductive oxide (TCO) for use in perovskite-silicon tandem solar cells. Due to its amorphous structure, IZO enables higher damp-heat stability compared to the crystalline indium tin oxide (ITO) structure which was used as reference material. Also, the charge carrier mobility is larger than with ITO, which leads to improved optical properties. The indium zinc oxide films were produced by means of a reactive sputtering process with indium zinc alloy rotatable targets with 96/4 weight and 92/8 percent. With respect to costs and quality the reactive process is an interesting alternative. Changing from ceramic target to reactive magnetron sputtering allows for a decrease of target costs. The coatings were produced on a large area in-line coater (1 m x 0.6 m) to demonstrate the process capability for industrial application. The indium/zinc rotatable targets were sputtered with argon as sputter gas, and oxygen as reactive gas. The layer preparation was done with a lambda sensor to keep the oxygen partial pressure constant during the process. The IZO films were optimized for conductivity, carrier mobility and absorption. Deposition was carried out at various operating points of oxygen partial pressure p(O₂). Optical properties were determined by measuring transmittance and reflectance in the spectral range from 250 to 2500 nm using a spectrophotometer. Sheet resistance was determined with a four-point probe as well as Hall measurements. The film thickness was determined by fitting of the optical spectra. The measured absorption of the films was weighted with global standard spectral data (AM1.5G) in order to more closely mimic the effect of absorption on the resulting solar cell efficiency. At the optimum operating point, it was possible to achieve resistivities less than 400 µΩcm and absorptance less than 2 % with layer thicknesses of 20 nm. The optimized IZO films were then implemented as top electrode in monolithic perovskite-silicon tandem cells. Device performance was characterized by external quantum yield and current-voltage measurements and compared to a reference ITO as top TCO.