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

Abstract

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 553K. 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 7.57×1019cm−3 to 5.25×1020cm−3 with increasing the H2 flow rate from 0sccm to 4.0sccm. Optical measurements indicated that the optical band-gap (Eg) of HMGZO thin films varies from 3.43eV to 3.66eV with adjusting H2 flow rate from 0sccm to 4.0sccm. The glass/HMGZO thin film deposited at the H2 flow rate of 4.0sccm exhibits the lowest resistivity of 6.24×10−4Ωcm (sheet resistance Rs~8.68Ω) and an average transmittance (Ta) of 78.8% in the wavelength range from 340nm to 1100nm. 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=0.904V, Jsc=12.906mA/cm2 and FF=0.702).