The promising n-Si-based solar cell is constructed for the purpose of realizing hole- and electron-selective passivating contact, using a textured front indium tin oxide/MoO structure and a planar rear a-SiO/poly(Si(n)) structure severally. The simple MoO /n-Si heterojunction device obtains an efficiency of 16.7%. It is found that the accompanying ternary hybrid SiO(Mo) interlayer (3.5-4.0 nm) is formed at the MoO /n-Si boundary zone without preoxidation and is of amorphous structure, which is determined by a high-resolution transmission electron microscope with energy-dispersive X-ray spectroscopy mapping. The creation of lower-oxidation states in MoO film indicates that the gradient distribution of SiO with Mo element occurs within the interlayer, acting as a passivation of silicon substrate, which is revealed by X-ray photoelectron spectroscopy with depth etching. Specifically, calculations by density functional theory manifest that there are two half-filled levels (localized states) and three unoccupied levels (extended states) relating to Mo component in the ternary hybrid a-SiO(Mo) interlayer, which play the roles of defect-assisted tunneling and direct tunneling for photogenerated holes, respectively. The transport process of photogenerated holes in the MoO /n-Si heterojunction device is well-described by the tunnel-recombination model. Meanwhile, the a-SiO/poly(Si(n)) has been assembled on the rear of the device for direct tunneling of photoinduced electrons and blocking photoinduced holes.
Complete photo-generated minority carrier's quantum tunneling device under AM1.5 illumination is fabricated by depositing tin-doped indium oxide (ITO) on n-type silicon to form a structure of ITO/SiOx/n-Si heterojunction. The work function difference between ITO and n-Si materials essentially acts as the origin of built-in-field. Basing on the measured value of internal potential (Vbi = 0.61 V) and high conversion efficiency (9.27%), we infer that this larger photo-generated holes tunneling occurs when a strong inversion layer at the c-Si surface appears. Also, the mixed electronic states in the ultra-thin intermediate region between ITO and n-Si play a defect-assisted tunneling.
Molybdenum oxide (MoO X , X < 3) has been successfully demonstrated as an efficient passivating hole-selective contact in crystalline Si (c-Si) heterojunction solar cells because of its large bandgap (∼3.2 eV) and work function (∼6.9 eV). However, the severe performance degradation coming from the instability of the MoO X and its interfaces has not been well addressed. In this work, we started with a c-Si(p)/MoO X heterojunction solar cell that yielded a power conversion efficiency (PCE) of 15.86%, in which the MoO X film was synthesized by industry-compatible atomic layer deposition (ALD). The initial PCE dropped to 10.20% after 2 days because of severe migration of O and Ag at the MoO X /Ag interface. We solved this by the insertion of a CrO X layer between the MoO X layer and the Ag electrode. The solar cell was found to be stable for more than 8 months in air because of the suppression of interface degradation. Our work demonstrates an effective way of improving the stability of silicon solar cells with transition metal oxide carrier selective contacts.
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