The Preparation and Transport Properties of Quantum Selective Silicon-Based Heterojunction Photovoltaic Devices
Supervisor:ma zhong quan
Quantum-selective silicon-based heterojunction photovoltaic device generally consists of crystalline silicon,passivation tunneling layer,and carrier selective layer,which has the advantages of low negative temperature coefficient,less parasitic absorption,and high theoretical conversion efficiency.It has attracted a lot of attention from photovoltaic researchers.In this paper,the controllable preparation,band matching and quantum transport mechanism of three kinds of photovoltaic devices(ITO/SiOx(In)/n-Si,MoOx/SiOx(Mo)/n-Si,MoOx/a-Si:H(i)/n-Si)have been studied,based on the semiconductor device physics and principles&technologies of silicon solar cells.We focused on the introduction of new multifunctional materials to improve the interface passivation effect,enhance the hole tunneling probability,and improve the photovoltaic performance of the device.Meanwhile,we analyzed the performance failure mechanism of MoOx/a-Si:H(i)/n-Si structure photovoltaic device,revealing the cause of the formation of the“S”output curve.The main research results of this thesis are as follows:(1)The ITO/SiOx(In)/n-Si heterojunction photovoltaic device was fabricated by using RF magnetron sputtering of ITO films directly on n-Si bulk.The simple structure device had a photoelectric conversion efficiency of 12.2%,an open-circuit voltage of 540 mV,a short-circuit current density of 30.5 mA/cm2 and a fill factor of 74.2%.The ITO film photoelectric characteristics,device photovoltaic characteristics and interface region micro structure were investigated.Combined with the first-principles calculation results of amorphous SiOx(In)materials,we found that the built-in field of the device essentially came from the work function difference between ITO film and n-Si substrate.Larger work function difference induced a sub-micron p-type inversion layer on the n-Si surface,which can form an equivalent built-in electric field of similar p-n junction.An ultra-thin amorphous SiOx layer(1.2-1.7 nm)containing In elements was naturally formed at the ITO/n-Si interface region,which could effectively passivate the n-Si surface.Both direct and defect-assisted tunneling of photogenerated holes through the SiOx(In)layer were present.There was a gap state in the a-SiO2 layer induced by In atom,which reduced the barrier height and contributed to quantum transport.In doping also introduced a shallow acceptor level,and formed p-type contact with the n-Si bulk.These results help to deepen the understanding of the basic principles of ITO/SiOx(In)/n-Si heterojunction photovoltaic devices,and provide theoretical and experimental evidence for improving the performance of such devices.(2)The MoOx/SiOx(Mo)/n-Si/SiOx/pOly-Si(n+)heterojunction photovoltaic device was successfully constructed for the purpose of realizing hole-and electron-selective passivating contact.The device had a maximum photoelectric conversion efficiency of 16.7%,an open-circuit voltage of 600 mV,a short-circuit current density of 38.2 mA/cm2 and a fill factor of 72.9%.The effects of SiOx(Mo)interlayer on the built-in field and quantum transport of devices were studied by using micro structural characterization techniques and first principles calculation methods.The results indicated that the accompanying ternary hybrid a-SiOx(Mo)layer(3.5-4.0 nm)was formed at the MoOx/n-Si boundary zone without pre-oxidation.The high work function difference between the MoOx film and n-Si substrate induced a p-type inversion layer inside the n-Si surface,generating a built-in field.There were two half-filled levels and three unoccupied levels relating to Mo component in the ternary hybrid a-SiOx(Mo)interlayer,which played the roles of defect-assisted tunneling and direct tunneling for photogenerated holes,respectively.Furthermore,the transport process of photogenerated holes in the MoOx/n-Si heterojunction device was well-described by the tunnel-recombination model.The device has good stability and simple preparation process,indicating that the MoOx/SiOx(Mo)/n-Si heterojunction photovoltaic device has potential application value.(3)Based on MoOx thin film work function measurement,MoOx/a-Si:H(i)interface chemical composition characterization and AFORS-HET software simulation,the failure analysis of MoOx/a-Si:H(i)/n-Si heterojunction photovoltaic device was performed.The results showed that the SiOx came into being in the process of vacuum evaporation of MoOx film onto a-Si:H(i)film,blocking the transport of carriers.With increment of the MoOx deposition rate,on the one hand,the decline of holes collecting capabilities is ascribed to the reduction in MoOx work function.On the other hand,an increasing O/Si ratio of a-SiOx leads to a rise of valence band offsets(△Ev),which mainly explain the“S-shape”due to a barrier that impedes thermionic emission of holes.At the same time,the SiOx layer becomes thicker(>4 nm),resulting in a lower tunneling probability of holes.Our research conclusions of the"S" type J-V curve of the MoOx/a-Si:H(i)/n-Si device provides a new idea for further optimizing the performance of the device.