Transparent conductive Mg and Ga co-doped ZnO thin films for solar cells grown by magnetron sputtering: H2 induced changes

•Hydrogenated Mg and Ga co-doped ZnO (HMGZO) films were deposited by magnetron sputtering.•Electrical properties of ZnO films were improved with H2 introduction.•Optical band gap of HMGZO films varied from 3.43 eV to 3.66 eV.•Electron mobility of HMGZO films was enhanced after vacuum annealing.•Single junction a-Si:H solar cell on HMGZO layer with an efficiency of 8.2% was obtained.

Transparent conductive hydrogenated Mg and Ga co-doped ZnO (HMGZO) thin films for solar cells were deposited via pulsed direct current (DC) magnetron sputtering on glass substrates at a substrate temperature of 553 K. The structural, morphological, electrical, and optical properties of HMGZO thin films were investigated with various H2 flow rates. The experiment results showed that the HMGZO thin films are polycrystalline with a hexagonal wurtzite structure exhibiting a preferred (002) crystal plane orientation and typical HMGZO thin films present cone-like texture surface. The carrier concentration rapidly increases from 7.57×1019 cm−3 to 5.25×1020 cm−3 with increasing the H2 flow rate from 0 sccm to 4.0 sccm. Optical measurements indicated that the optical band-gap (Eg) of HMGZO thin films varies from 3.43 eV to 3.66 eV with adjusting H2 flow rate from 0 sccm to 4.0 sccm. The glass/HMGZO thin film deposited at the H2 flow rate of 4.0 sccm exhibits the lowest resistivity of 6.24×10−4 Ω cm (sheet resistance Rs~8.68 Ω) and an average transmittance (Ta) of 78.8% in the wavelength range from 340 nm to 1100 nm. Burstein–Moss band-filling, band gap renormalization effects determined by carrier concentrations and the incorporation of Mg atoms together contribute to an Eg widening phenomenon. The electron mobility of HMGZO thin films can be effectively improved through thermal annealing process. Finally, the HMGZO thin film was preliminarily applied in pin a-Si:H thin film solar cell with an efficiency of 8.20% (Voc=0.904 V, Jsc=12.906 mA/cm2 and FF=0.702).