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

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 5x10 6 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 cm2 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 amp; 956;