Microstructure Controlling And Electrical Properties Of Silicon Based Thin Films Used For Solar Cells

From the perspective of mateirals,it is seen that the conversion efficiency of monocrystalline silicon solar cell with most perfect structure is biggest,the conversion efficiency of polycrystalline silicon solar cell with more complete structure is bigger,while the conversion efficiency of amorphous silicon thin film solar cell with disorder structure is least.So the decrease of disorder degree is very important for amorphous silicon thin film solar cell.In recent years,there are many research works to increase order degree of amorphous silicon thin film and prepare microcrystalline silicon thin film,but the influence mechanism of deposition parameters on the microstructure of the film has been not studied deeply and the cognition in growth process deciding directly the microstructure and properties of the film still be unclear.In this work,a seris of silicon thin films have been prepared by adjusting elemental composition,thermodynamics and dynamics conditions,and their microstructure and properties have been characterized.The controling mechanism of the microstructure of silicon thin film is summaried to provide experimental support and theoretical basis for the preparation of the film with more order and uniform structure.The research contents are as follows.Boron and phosphorus doped hydrogenated amorphous silicon thin films have been prepared by adjusting the gas flow ratio of [B₂H₆]/[SiH₄] and [PH₃]/[SiH₄].And their microstructure and optical and electrical properties have been measured.The results indicate that the best flow ratios of [B₂H₆]/[SiH₄] and [PH₃]/[SiH₄] are obtained to be 1.0 % and 1.5 %respectively.And then the optimized samples are annealed in vacuum to study the influence of microstructure on the optical and electrical properties.After annealing,the dark conductivity and carrier concentration increase by 2-3 orders of magnitude for both doped film series.By comparison,the improvement of disorder network structure has more significant effect on the electrical properties of doped silicon thin film than the doping concentration.Based on these results,the object of study in the following section is simplified to be intrinsic silicon thin film.RF power is the dynamic source of the excitation and ionization of reactant gas,so the value of RF power decides the energy of the ions and particles in the plasma and affects directly the adsorption and diffusion process of the reactive group on the film surface.A seriesof amorphous silicon thin films with different atomatic arrangement have been obtained through adjusting RF power and then annealed in same thermodynamics condition.The effect rule of electrical field on atomatic arrangement of amorphous silicon thin films has been revealed.And the dynamic condition benefiting to prepare silicon thin film with more compact and order microstructure has been obtained.In order to explore the growth rule of microcrystalline silicon thin film with uniform two-phase structure,hydrogenated silicon thin films have been deposited at different deposition times.The evolution process of cross-section morphology of the films from substrate to film surface has been investigated intuitively by HRTEM.And the deposition rates of the films in different growth stages have been calculated and the surface states of the films at different depodition times have been observed by AFM.The growth mechanism of microcrystalline silicon thin film has been analyzed.The results indicate that the variation of surface state upon the deposition time may be the major factor for the self-crystallization of microcrystalline silicon thin film.And the deposition rate of the film in crystalline growth mode is bigger than its value in amorphous growth mode.Hydrogen in the deposition process of silicon thin film can compensate effectively the dangling bonds and change the microstructure of the film.Hydrogenated silicon thin films have been prepared at different hydrogen dilution ratios to study the influence of hydrogen dilution ratios on the growth process and two-phase structure of the films.Varied hydrogen dilution ratios are obtained by increasing the gas flow of hydrogen while remaining the gas flow of silane unchanged.The results indicate that as hydrogen dilution ratio increases,the thickness of the amorphous transition layer tends to decrease and the transformation to crystalline from amorphous starts earlier.However,excessive gas flow of hydrogen has an adverse effect on the diffusion of reactive group on the film surface.As a concequence,average grain size of the film increases firstly and then decreases with the increase of hydrogen dilution ratio.Substrate temperature has an influence on the microstructure and electrical properties of silicon thin films by affecting the transportation of deposited particles and ions on the film surface.Hydrogenated silicon thin films have been deposited at different substrate temperatures.According to HRTEM pictures,it is obtained that phase transformation from amorphous to crystalline starts earlier as substrate temperature increases.But according toXRD results,the crystallinity and average grain size of the films increase firstly and then decrease with the increase of substrate temperatures.This is attributed to the decrease of hydrogen content in the chamber at over-high substrate temperature.At the same time,the change rules of minority carrier lifetime and dark conductivity agree with the change rules of crystallinity and average grain size,which showes a close positive relationship between electrical properties and the microstructure.The films with more ordered structure exhibit superior electrical properties.