If you increase electron density only, your work function will also increase. In terms of electron density parameter rs, where 1/n = (4/3) Pi rs^3 and n is your electron density, you can estimate the work function change from the simple formula WF=6.15 rs^(1/2). This is valid for a thick layer. See e.g. J. Phys. Cond. Matt 10 (1998) 10815.