Effective Passivation and Tunneling Hybrid a-SiOx(In) Layer in ITO/n-Si Heterojunction Photovoltaic Device.

In this article, using controllable magnetron sputtering of indium tin oxide (ITO) materials on single crystal silicon at 100 °C, the optoelectronic heterojunction frame of ITO/a-SiOx(In)/n-Si is simply fabricated for the purpose of realizing passivation contact and hole tunneling. It is found that the gradation profile of indium (In) element together with silicon oxide (SiOx/In) within the ultrathin boundary zone between ITO and n-Si occurs and is characterized by X-ray photoelectron spectroscopy with the ion milling technique. The atomistic morphology and physical phase of the interfacial layer has been observed with a high-resolution transmission electron microscope. X-ray diffraction, Hall effect measurement, and optical transmittance with Tauc plot have been applied to the microstructure and property analyses of ITO thin films, respectively. The polycrystalline and amorphous phases have been verified for ITO films and SiOx(In) hybrid layer, respectively. For the quantum transport, both direct and defect-assisted tunneling of photogenerated holes through the a-SiOx(In) layer is confirmed. Besides, there is a gap state correlative to the indium composition and located at Ev + 4.60 eV in the ternary hybrid a-SiOx(In) layer that is predicted by density functional theory of first-principles calculation, which acts as an "extended delocalized state" for direct tunneling of the photogenerated holes. The reasonable built-in potential (Vbi = 0.66 V) and optimally controlled ternary hybrid a-SiOx(In) layer (about 1.4 nm) result in that the device exhibits excellent PV performance, with an open-circuit voltage of 0.540 V, a short-circuit current density of 30.5 mA/cm2, a high fill factor of 74.2%, and a conversion efficiency of 12.2%, under the AM 1.5 illumination. The work function difference between ITO (5.06 eV) and n-Si (4.31 eV) is determined by ultraviolet photoemission spectroscopy and ascribed to the essence of the built-in-field of the PV device. In addition, the strong inversion layer in the surface of the n-Si substrate is tentatively correlated to the a-SiOx(In) interface layer as well.