Study On The Electronic Structure And Function Of The Ultra-thin Silica Layer In Silicon Heterojunction Photovoltaic Device

For silicon-based semiconductor-insulator-semiconductor（SIS）photovoltaic device,ultrathin interface amorphous SiO_x layer（a-SiO_x,<2 nm）has been recognized to be an important material.It can be used as a multi-functional passivation layer as well as tunneling pass-through media in SIS device.Therefore,deep understanding of the ultra-thin interface layer is the focus of science.By utilizing first-principles and molecular dynamics approaches,the chemical configuration and electronic states of a-SiO_x（In）（<2 nm）layers as well as its effects on carrier transport mechanism in ITO-SiO_x（In）/n-Si heterojunction solar cells have been investigated.The main research contents and major results are summarized as follows:（1）The interfacial region of ITO/Si materials system was studied by molecular dynamics（MD）,which was integrated in VASP codes.It was found that the SiO_x layers were the result of the inter-diffusion of the In,Sn,O,Si element.Moreover,hybrid bonding structures of In-O-Si and Sn-O-Si were found in the SiO_x layers.The In-O-Si ternary compound may exist in the SiO_x layers.（2）By means of the energy band calculations,In and Sn doping induced gap states（E_v+4.60 eV for In,E_v+4.0 eV for Sn）within a-SiO₂ band gap were found.Bader Charge Analysis indicated that the doping of In into a-SiO_x may form a negative charge center.The total density of states（TDOS）and the partial density of states（PDOS）offered a gross support that an In-O/Sn-O bonding within the background medium of a-SiO_x resulted in the formation of GSII/GSIS states.（3）From the result of formation energy calculations,it was shown that the highest formation energy of such In-O-Si configuration was less than 5.38 eV,which was lower than the energy（10 eV）provided in our experiment environment.It meant that In-O-Si configuration was energetically favorable.In view of our ITO-SiO_x（In）/n-Si solar cells,the work function difference between ITO and n-Si was0.69 eV,so that the shallow acceptor levels（the transition levels of E_v+0.3 eV）could be ionized in order to construct build-in-potential at a-SiO_x（In）/n-Si intermediate region.（4）These gap states（GSII）,induced by In-O-Si electronic structure,could lower the tunneling barrier height and increase the probability of tunneling so as to facilitate the transport of photo-generated carriers,strengthen the short circuit current,and/or create negatively charged defects to repel electrons,thus suppressing carrier recombination at the p-type inversion layer and promoting the establishment of the effective built-in-potential,increasing the open-circuit voltage and fill factor（>72%）.The defect-assisted tunneling was enhanced by the introduction of gap states（E_v+0.30eV and GSII（E_v+7.07 eV））,besides the direct tunneling through the ultra-thin barrier.