Nano-crystalline thin films fabricated by Si-Al co-sputtering and metal induced crystallization for photovoltaic applications

Fabrication of nano-crystalline thin film solar cell by co-sputtering of silicon and aluminum, and metal induced crystallization is reported in this paper. After a brief introduction on magnetron sputtering and aluminum induced crystallization techniques, it discusses the drawbacks of "aluminum induced layer exchange" concept as the common method for fabricating poly-Si thin films; and proposes Si-Al co-sputtering as a simple alternative fabrication method. Then a three-steps process is introduced for the fabrication of "Al electrode/n-Si wafer/p-Si thin film/Al electrode" solar cell. Material and electrical characterizations of the fabricated p-Si thin films are done by Raman spectroscopy, X-ray diffraction, optical microscopy, scanning electron microscopy and Hall Effect measurement to study the crystallization ratio, grain size, morphology, carrier density and mobility of the prepared films, respectively. The best fabricated samples show a crystallization ratio of 88% and the largest grain size of 68 nm. The hole concentration in the films is in the order of 10(18) to 10(19) cm(-1) with a maximum mobility value of 17 cm(2)/V s. Then, photovoltaic properties of the fabricated solar cells are determined by under dark and illumination J-V curves, and external quantum efficiency measurements. Dark J-V curve of the best sample represents an ideality factor of 1.39, rectification ratio of 1993 at 1 V and reverse saturation current density of 62.1 nA/cm(2) which confirms the formation of a good p-n junction. The illuminated J-V curve under AM1.5G conditions exhibits a photovoltaic conversion efficiency of 6.02% for the best sample, with a short circuit current density of 24.9 mA/cm(2) and open circuit voltage of 456 mV. The external quantum efficiency of this sample reaches to more than 59% at the wavelength of 700 nm. Finally, a simple model is proposed to describe the formation of nano-crystalline Si thin film, deposited by Si-Al co-sputtering, during the thermal annealing.