Figure 5 Transmittance in the wavelength range of 400-2300 nm for the IHfO:H films deposited with H2 concentrations of 0 %, 0.5 %, 0.8 %, 1.2 %, and 1.5 % after post-annealing at 200 ℃ in air.
Figure 5 shows the transmittance of the IHfO:H films with different H2 concentrations in the wavelength range from 400 nm to 2300 nm. The hydrogen concentration does not significantly affect the transmittance of the IHfO:H films in the visible region (400-800 nm) with an average transmittance of ~87 %. In the NIR region (800-1500 nm), the average transmittance is in the range of ~85 % to ~88 %, which is much higher than ~79 % for the ITO films with the same thickness,33 thanks to the weaker NIR free carrier absorption (FCA) effect.34 When the hydrogen concentration is higher than 0.8 %, the average transmittance of the IHfO:H films increases slightly in the range of 1500-2300 nm which, however, is beyond the optical absorption range of crystalline silicon. Considering the application in crystalline silicon solar cells, it can be said that the hydrogen doping concentration (0-1.5 %) hardly effect the optical properties of the IHfO:H films at an annealing temperature of 200 ℃.
According to above experimental results and discussion, adding a decent H2 gas when depositing IHfO:H films significantly increases the carrier mobility and thus reduces its sheet resistance, but hardly influences the film transmittance. Too much hydrogen can lead to a significant degradation of the electrical properties of the IHfO:H films. Comprehensive consideration of the optical and electrical properties, the optimum H2 concentration is determined to be 0.8 %.
3.2 The effect of post-annealing on IHfO:H films
Post-annealing is a conventional treatment to improve the crystallinity28 and to activate the dopants of the TCO films.35 XRD patterns of the IHfO:H films prepared with 0.8 % H2 concentration annealed at 100-250 ℃ are shown in Figure 6. The XRD pattern of an as-deposited sample is included. Four diffraction peaks of (211), (222), (332), and (444) belonging to In2O3 can be determined from the XRD patterns of all the samples. Increasing the annealing temperature does not affect the dominant orientation of the (222) crystal plane in the IHfO:H films, however, the FWHM of the (222) diffraction peak is significantly reduced and the estimated average grain size increases gradually as depicted in Figure 6. As the annealing temperatures is above 200 °C, the average grain size exceeds 50 nm indicating better crystallinity of the IHfO:H films grown by RPD than that by magnetron sputtering.19,36 It is worth noting that the as-deposited IHfO:H film is also crystallized in the presence of hydrogen during the deposition. Nevertheless, the as-deposited sample without hydrogen doping is amorphous (Figure S2). That means both post-annealing and hydrogen doping promote the film crystallization.