3 Results and discussion
3.1 Effect of hydrogen concentration on the IHfO:H thin films
Hydrogen doping has been proved to be an effective strategy to modify the performance of the TCO films, such as reducing the square resistance and improving the mobility, by influencing the crystallinity, forming shallow donor states, and passivating the structural defects.25-27 We investigated the effects of hydrogen doping on the IHfO:H films by varying the H2concentrations (gas flow ratio of hydrogen to argon: 0 %, 0.5 %, 0.8 %, 1.2 % and 1.5 %) during the deposition process. Referring to the fabrication process of traditional SHJ solar cells, the series of IHfO:H films were all annealed at 200 °C in air for 20 minutes.
Figure 2 shows the XRD patterns of the IHfO:H films with different hydrogen concentrations after annealing at 200 °C. According to the standard comparison card (JCPDS No.06-0416), the diffraction peaks of (211), (222), (332), and (444) belonging to the cubic bixbyite polycrystalline structure of In2O3 are observed. The absence of diffraction peaks for HfO2indicates that substitutional doping (Hf4+ replacing In3+) forms a solid solution in which HfO2 was dissolved into the In2O3 lattice during the film growth process.28 The intensity of the diffraction peak (222) in all samples is much higher than that of the other peaks, which indicates that (222) is the preferential crystallization direction. We calculated the average grain size D of the IHfO:H films corresponding to the full width at half maximum (FWHM) of the (222) diffraction peak according to the Scherrer formula,19 as included in Figure 2.
It can be seen that the average grain sizes of all the samples are in the range of 50±5 nm, which implies that the H2concentration hardly affects the crystallinity of the films. Annealing can greatly improve the crystallinity of the IHfO:H films, thus weakening the effect of hydrogen doping on the crystallinity of the films, as will be discussed below.