Challenges and opportunities of electron beam evaporation in the preparation of poly-Si thin film solar cells

Abstract

Electron-beam (e-beam) evaporation provides both exciting opportunities and challenges for the preparation of poly-crystalline silicon (poly-Si) thin film solar cells. A conversion efficiency of 6.7% was recently achieved for solid phase crystallized poly-Si mini-modules on planar SiN-coated glass deposited at a deposition rate of 600 nm/min, demonstrating the excellent electronic quality of e-beam evaporated silicon. Even at significantly increased background pressures of 5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> mbar, the photovoltaic performance of the mini-modules was considerably high, showing a decline in open circuit voltage of 17 mV per cell. The implementation of light trapping structures into the device led to an efficiency increase of 1.1%, yielding module efficiencies of 7.8%. By systematically studying the implementation of ZnO:Al as a front contact layer into the poly-Si solar cell device structure, we unraveled novel features that prove the supreme suitability of ZnO:Al for poly-Si thin film solar cells. Not only can etched ZnO:Al be utilized as a front side texture, but its electrical properties can also improve during the crystallization process of the Si layer, showing a record charge carrier mobility of 67 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs after thermal annealing. In addition, ZnO:Al drastically modifies the crystallization kinetics of the Si on ZnO:Al, enabling us to control the crystallization process by adjusting the deposition temperature. The nucleation process of Si on ZnO:Al was found to be influenced by a variation of the deposition temperature of the amorphous Si in a critical temperature regime of 200 °C to 300°C. The nucleation rate decreased significantly with decreasing deposition temperature, while the activation energy for nucleation increased from 2.9 eV at a deposition temperature of 300°C to 5.1 eV at 200°C, resulting in poly-Si which comprised grains with features sizes of several μm.