New Generation Transparent LPCVD ZnO Electrodes for Enhanced Photocurrent in Micromorph Solar Cells and Modules
ZnO bilayer films were deposited by low-pressure chemical vapor deposition in a single process step by controlling the differential doping of the nucleation and bulk parts of the layers. The resulting 2-μm-thick films are characterized by low free-carrier absorption and electron mobility over 40 cm2 /Vs. They, therefore, combine high transparency in the infrared region and moderate sheet resistance that can be lowered below 20 Ω/sq. These properties make ZnO bilayers ideal candidates as electrodes for the development of micromorph thin-film solar cells with enhanced photogenerated current. The potential of such bilayer front electrodes for a further power improvement and cost reduction of industrial micromorph tandem modules is currently investigated at Oerlikon Solar. The first experiments already show a promising gain in the bottom μc-Si:H cell photogenerated current compared with the current generated with modules deposited on standard uniformly doped ZnO single-layer front contacts.