There are a few different approaches to determining the a-Si:H band gap from optical data based on the Tauc-Lorentz approach, often following either Jellison et al. (J. G. E. Jellison and F. A.Modine, Appl. Phys. Lett. 69, 371 (1996)) or the modified Tauc-Lorentz model of Ferlauto et al. (A. S. Ferlauto, G. M. Ferreira, J. M. Pearce, C. R. Wronski, R. W. Collins, X. Deng, and G. Ganguly, J. Appl. Phys. 92, 2424 (2002)). While the former comprises a minimum set of fit parameters, the latter includes some specific aspects of amorphous semiconductors such as exponentially decaying band tails. Additionally, the Jellison model is based on the assumption of a constant momentum matrix element for optical transitions, while Ferlauto assumes a constant dipole matrix element. When presnting your data, you should mention which approach you have followed. Furthermore, keep in mind that care has to be taken in distinguishing the a-Si:H optical band gap Eopt (as measured with optical methods) from the mobility band gap Eμg,a-Si:H (as determined with electrical techniques), as well as the respective band edges. If you are interested,  you might want to have have a look at our publication T. F. Schulze; L. Korte; F. Ruske & B. Rech: Band lineup in amorphous/crystalline silicon heterojunctions and the impact of hydrogen microstructure and topological disorder Phys. Rev. B 83, 165314 (2011). Amongst others, we have compared the Jellison with the Ferlauto approach in this paper.

Mauro Andrea Bettiati and Lars Korte thank you sir, im using only Tauc-Lorentz approach.