Research On Si Based ZnO Films And Its Interface For Solar Cells

Solar cells have become one of the most promising renewable and affordable energy sources in the modern world.The p-Si/n-ZnO heterojunction has drawn enormous scientific and technological attention among the many competing technologies for various solar cells due to the developing tendency will be low cost,green and high efficiency.Al doped ZnO（AZO）has been proved to be a promising n-type ZnO semiconductor because of its excellent photoelectric properties such as low resistivity(≤10^-3Ωcm),high mobility（≥36cm²/Vs）,high transmittance（≥90%）in visible wavelength and including all merits of intrinsic ZnO.The resultant p-Si/n-ZnO heterojunction solar cell possesses many fascinating advantages such as simple structure,green,abundant,low cost,stable physiochemical properties and broaden spectra.Furthermore,Si/ZnO heterojunction is also widely used in micro/nano/optoelectronic devices and systems such as photodiode,detectors and sonics.Although the performance of solar cell is mainly affected by pn junction,many problems about interaction and bonding mechanism of Si/ZnO heterojunction,Si/ZnO interfacial modification and properties of（n-）ZnO thin films have not been fully clarified,and the Si/ZnO junction has been studied largely by empirical or semi-empirical methods,which limit the Si/ZnO application in solar cells and other aggressive fields.Therefore,studying on Si-based ZnO films and its interface are the main basic research for improving the performance of Si/ZnO devices.In this dissertation,the Si based ZnO films and its interface have been meticulously studied from such aspects as growth and interaction of Si based ZnO films,properties of ZnO series films,the properties of Si-based ZnO under thermal shock,Si/ZnO heterojunction solar cells and its interfacial modification.The main contents and results are as follows:1.Intrinsic ZnO films were deposited on p-Si（100）substrates by radio frequency（RF）magnetron sputtering,and the growth mechanism of Si based ZnO flims was studied.AFM and XRD results show that Si based ZnO flims belong to island-layer growth mechanism.Here the smallest grains grow and form nucleation gradually,resulting in nucleation being closely connected with each other and the formation of isolated island,in which case the Si based ZnO film exhibits（002）preferential growth.As sustained deposition,these islands are linked with each other and grains recombination and refinement,the ZnO film at this stage shows obvious（002）preferential growth.2.ZnO and AZO films were fabricated onto p-Si（100）wafers by RF magnetron sputtering,then the undoped and Al doped ZnO films coated Si substrates were annealed in oxidation and reduction atmosphere respectively.The interfacial reaction and bonding mechanism of Si based films were investigated by experiments（XRD,FTIR,XPS,Hall and PL）and simulations（first principle and molecular dynamics）.The results show that certain interstitial Si atoms in silicon substrate interact with O atoms belonged to ZnO at Si/ZnO interface and leave oxygen vacancies in ZnO film due to Si robbing O atoms related to Zn–O bonding,moreover,the interaction between Si and ZnO could produce new disordered built-in electric field.The interfacial interaction between Si and ZnO depends on oxygen content coming from ZnO coating layer,the more sufficient oxygen coming from ZnO film are,the smaller width of Si/ZnO interfacial interaction,and the more fully effective interaction of Si/ZnO can be.Furthermore,the fully effective interaction between Si and ZnO on condition that there are two interstitial O atoms exist near Si/ZnO interface,the interstitial Si bonds with four O atoms and SiO₄ phase form,among which,three O atoms come from Zn-O bonding,and the other one is the interstitial O atom.3.According to the physical and chemical characteristics of undoped and Al doped ZnO,different structures of Al-doped ZnO and ZnO/AZO bilayer were constructed and studied.ZnO,AZO and ZnO/AZO films were deposited on p-Si（100）substrates by RF magnetron sputtering and the oxidation annealing was carried out at 400°C,500°C and600°C respectively.Experiments（XRD and FTIR）and simulation（first principle）show that the Al-O bond of AZO film is weaker after 400°C annealing due to partial Al ions not bonding with O ions,it can be inferred that partial Al atoms exist in films as interstitial state at lower annealing temperature（400°C）,moreover,with the annealing temperature increased to 500°C and 600°C,the interstitial Al atoms could change into substitutional state of Al replacing Zn site,which means interstitial Al atoms are greatly affected by annealing temperature.Compared with substitutional Al doped ZnO,interstitial Al doped ZnO and ZnO/AZO bilayer both have poor photoelectric properties such as conductivity,transmittance and photoconductivity.4.It is generally known that Si/ZnO devices suffer from high and low thermal atmosphere during working and they are also subjected to diurnal temperature,therefore,the effect of thermal shock on properties of Si based ZnO and AZO thin films were studied based on life acceleration theory.According to IEC61215 standard for solar cells<Environmental Reliability>testing criterion,the transient thermal shock loading was carried out in the range of-40°C⁺85°C to treat Si based ZnO and AZO films.The results show that the interfacial bonding strength between Si and ZnO is stable,the crystal structure and bonding properties of ZnO are also stable with the shock times increasing.Moreover,the grains of Si based ZnO films are agglomerated and coarsened first,then refined,the transmission and photoluminescence properties of AZO films degenerate significantly.In addition,the Si/ZnO interface interaction has a significant negative impact on the optical properties of ZnO and AZO films under thermal shock.5.Based on the study above,the photovoltaic properties of p-Si/n-ZnO heterojunction solar cells and its interface modification were studied.Using Al as back electrode,the solar cell with Al/p-Si/n-ZnO/OMO structure gave open circuit voltage V_oc of 275.2mV,short circuit current density J_sc of 17.2 mA/cm²,a fill factor F.F of279.8,and conversion effiencyηof 13.25%under standard test conditions.The optimal interfacial modification for Si/ZnO solar cells is to prepare ZnO thin films on p-Si（100）substrates as specific growth stage and being oxidized（ZnO-O for short）,the obtained Al/p-Si/ZnO-O/n-ZnO/OMO solar cell could reach V_oc=393.2mV,J_sc=20.2 mA/cm²,F.F=237.3,andη=18.86%.The thermal shock was performed based on the fourth research content and the photovoltaic performance of Si/ZnO solar cells was studied,the results show the photovoltaic performance decreases significantly after thermal shock.