2 Experimental details
2.1 Preparation of IHfO:H thin films:
IHfO:H thin films were deposited on glass and polished silicon wafers by RPD to investigate their micro-structure, morphology, opto-electronic properties, and chemical valence states. HfO2-doped In2O3 particles (4 wt % HfO2, 99.99 % purity) were used as the target. High-purity argon with the fixed flow rate of 30 sccm and argon-hydrogen mixture (30 % hydrogen) with the varied flow rate between 0 and 1.5 sccm were used as the working gas. The chamber pressure was controlled to be at 0.4-0.5 Pa and the working current of power was set at 60 A. The thicknesses of all the IHfO:H films are around 100 nm (±5 nm). The post-annealing process was carried out in air with the annealing time of 20 minutes and the annealing temperature of 50-250 °C.
2.2 Fabrication of SHJ solar cells:
The n-type Cz crystalline silicon wafers with the thickness of 170 μm and resistivity of 1 Ω·cm were used as the substrates. Alkaline texturing, chemically polishing and RCA cleaning procedures were performed prior to the fabrication of the SHJ solar cells. The SHJ solar cells adopt a bifacial light incident structure of Ag grids/IHfO:H/μc-Si:H(n+)/a-Si:H(i)/n type c-Si/ a-Si:H(i)/a-Si:H(p+)/ITO/Ag grids, as specifically shown in Figure 1. The intrinsic hydrogenated amorphous silicon (a-Si:H(i)) passivation layers with the thickness of 7 nm were deposited on both sides of the n-type crystalline silicon by plasma enhanced chemical vapor deposition (PECVD). A 10 nm p-type amorphous silicon (a-Si:H(p+)) layer and a 7 nm n-type micro-crystalline silicon µc-Si:H(n+) layer, also deposited by PECVD, serve as the hole transport layer (emitter) and electron transport layer, respectively. A 100 nm IHfO:H film, as both the transparent conductive layer and the antireflection layer, was prepared by RPD. A 70 nm ITO film was prepared by the magnetron sputtering method on the back side of the SHJ solar cells. The silver grids on both sides of the device were prepared by screen printing technology. After electrode printing was completed, the whole solar cells were annealed in air at a temperature of 200 °C for 20 minutes.
2.3 Characterization:
The thickness of the IHfO:H films was measured by a step profiler (Bruker DektakXT). X-ray diffraction (Rigaku Smartlab SE) measurement was carried out to analyze the crystal structure of the IHfO:H thin films using a Cu-Kα radiation source (λ = 1.5405 Å), scanned in 2θ mode, with the diffraction angles ranging from 10 to 80 °. The square resistance of the IHfO:H films was measured by the four-probe method, while the carrier concentration and Hall mobility were obtained from the Hall measurement (PSAICSwin Hall 8686). A UV-Vis-NIR spectrophotometer (Perkin-Elmer Labmda 750) was used to obtain the optical transmission spectra of the films. The chemical state characterization of the IHfO:H films was determined using X-ray photoelectron spectroscopy (XPS, Kratos Axis Supra+). In order to exclude the effect of adsorbed air on the surface, all samples were etched with Ar ion beam for 1 minute before the XPS measurement. The light current-voltage (I-V) curves of the solar cells were obtained using a solar simulator under standard conditions (100 mW/cm2, AM 1.5 G, 25 °C). The external quantum efficiency (EQE, BENTHAM INSTRUMENT PVE300-IVT) was measured to study the spectral response of the solar cells.