Al doped zinc magnesium oxide ( Zn1-xMgxO:Al) thin films with tunable optical bandgap (3.48 eV-3.66 eV) and resistivity level of 10(-3) Omega.cm were prepared by sol-gel process. The effect of the Mg incorporation on the structural, electrical and optical properties was studied by analyzing the results of X-ray diffraction diffractograms, transmittance spectra, Hall measurement and X-ray photoelectron spectroscopy. Mg incorporation influences the crystal structure through reducing the c-axis lattice constant. Blueshift of absorption edge with Mg incorporation manifests the bandgap widening effect. Mg incorporation also increases the film resistivity through decreasing Hall mobility and carrier concentration. Employed as the front electrode, these Zn1-xMgxO:Al thin films improved the blue response of hydrogenated amorphous silicon solar cells through reducing the optical loss in ZnO:Al. Zn1-xMgxO:Al with x <= 0.13 enables an open-circuit voltage (V-oc) of about 0.9 V and a fill factor (FF) of about 0.7. The Zno.87Mgo.13O:Al empowers a gain of about 9% in short-circuit current density relative to ZnO:Al via the bandgap widening and the improved refractive-index-matching, while the Zno.80Mgo.20O:Al deteriorates V-oc. and FF through increasing the downward band bending at the Zn1-xMgxO:Al/pa-SiC:H interface. This work paves the way for applying the sol-gel derived Zn1-xMgxO:Al thin films to optoelectronic devices such as ultraviolet light detector, ultraviolet light emitting diodes and solar cells.