| Cu?In,Ga?Se2?CIGS?thin film solar cells show a great potential for large-scale photovoltaic applications due to their low cost,high efficiency,high absorption coefficient,tunable band gap,compositional tolerance,good weak light,outstanding stabilities and so on.CIGS thin film solar cells are a multilayer structure,the efficiency improvement depend on the optimizations of all functional layers and the interfaces.Besides the optimization of CIGS absorber,the optical and electrical properties of transparent conductive oxide?TCO?thin films and various post-treatments are also significant to high efficiency CIGS thin film solar cells.The TCO films are applied in CIGS thin film solar cells as window layers,which possess a low resistivity and high light transmission.Aluminum doped zinc oxide?AZO?are widely used in CIGS thin film solar cells due to their rich raw and processed materials,low price,There are many methods for preparation of AZO thin films,including magnetron sputtering,chemical vapor deposition,pulsed laser deposition and sol-gel method and and so on.Magnetron sputtering is a vacuum process,which has a stable process and thus is easy to carry out for mass production of AZO thin films with good material qualities.In this thesis,AZO thin films were prepared by RF magnetron sputtering and DC reactive sputtering.The effects of O2-Ar flow rate on the morphology,crystal structure,optical and electrical properties of AZO thin films applied in CIGS thin film solar cells are investigated.The results show that the dynamic deposition rate,the carrier concentration and the mobility of RF sputtered AZO thin films decrease with the increasing O2-Ar flow rate while the total transmittance increases.In a similar way,the dynamic deposition rate of the DC reactveily sputtered AZO thin films decreases with the increase of oxygen flow rate?oxygen partial pressure?while the transmittance increases.All AZO thin films show a?002?preferred orientation from the XRD diffraction patterns and the?002?peak position shifts towards a small angle of 2?with increasing oxygen flow rate.The surface structure changed from a popcorn shape to a tortious shell.The lower Al content in Zn:Al target leads to the faster dynamic deposition rate of DC reactively sputtered AZO thin films.With the increase of target voltage,the resistivity of AZO film decreases to the minimum value of 3×10-3??cm.When the target voltage increases further,the resistivity increases.When the reactive sputtering reaches to the metal mode,the resistivity of AZO will decrease with the increase of the voltage of the target.CdS thin films site between the absorption layer of the narrow band gap and the ZnO of wide band gap,which are applied in CIGS thin film solar cell as buffer layer.On one hand,CdS thin films can effectively reduce the energy band mismatch and lattice mismatch so as to decrease the interface recombination and improve the efficiency of CIGS thin film solar cells.On the other hand,the buffer layer covered the surface of CIGS can also effectively prevent sputtering damage during the preparation of ZnO thin film.Therefore,it is very important to optimize the properties of CdS materials and the properties of CIGS/CdS heterojunction surface.In this thesis,the air-annealing treatments on Cu?In,Ga?Se2/CdS at different annealing temperatures from 25°C to 150°C have been carried out.The Voc and FF,as well as corresponding conversion efficiency of Cu?In,Ga?Se2 solar cells after the air-annealing treatment,are improved and then decrease after reaching the maximum value at 106°C,besides the Ncvv and the decay lifetime increase and then decrease after reaching the maximum value at106°C.this indicates that proper CdS air-annealing treatment is beneficial to improve the performance of the solar cell,which is related to the passivation of Se vacancies and InCu defects and positive interface discharges etc.the chemical absorbed oxygen could occupy the donor-like Se vacancies VSee as well as antisite InCu defect at the surface?and/or interfaces?and grain boundaries It effectively increases the acceptor defect density(Ncv)which reduces the width of space charge region and the carrier recombination in the space charge region and then increase the Voc.However,the performance of over annealed?high temperature annealing,long time annealing?solar cells decreases.This is mainly caused by the migration of Cu into the bulk,excess passivation of interface defects?reduction of positive interface discharge?as well as the destruction of the interface?an increase of interface states?.CIGS thin film solar cells with conversion efficiency of 17.2%have been obtained by optimizing the air-annealing treatment on CIGS/CdS.Alkali metal doping can obviously improve the open circuit voltage(Voc)and fill factor?FF?of the CIGS thin film solar cells.Howerver,the relatively thicker CdS thin films are the main factor limiting the high short circuit current density.Combined the doping of alkali metal element into CIGS absorber with employing of the thinner CdS thin film,the performance of the CIGS thin film solar cells is improved and the CIGS thin film solar cells with the highest conversion efficiency of 18.8%are obtained. |
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