Investigation Of Molybdenum Back Contact And Doped Cu?In,Ga?Se₂ Absorber Layers For Application Ins Cigs Thin Film Solar Cells

Cu(In,Ga)Se2(CIGS)thin film solar cells have attracted the attention of both academy and industry due to their high conversion efficiency,low light intensity,adjustable band gap,etc.The structure of CIGS cells is relatively complex,which in general consists of the back contact,absorption layer,buffer layer,window layers,metal electrod grids and anti-reflection layer.The performances of CIGS thin film cells(such as conversion efficiency,stability)are directly affected by the qualities of all functional layers.In this dissertation,molybdenum(Mo)back contact and CIGS absorber are optimized to obtain high conversion efficiency CIGS thin film cells on soda-lime glass(SLG)substrates and polyimide(PI)substates.The details are summarized as follows:(1)Bottom Mo back electrodes were prepared by magnetron sputtering with different working pressuresfor application in CIGS thin film solar cells.In addition,the effect of thickness of bottom Mo thin film was studied as well.It is found that the high working pressure of 7.9 Pa leads to high resistivity and a low reflection of Mo thin films.Low working pressure and high thickness result in deuse and compact Mo thin films with high crystallinity.A high efficiency of 14.4%was achieved for a CIGS thin film solar cell on Mo back contact prepared at a working pressure of 1.2 Pa.(2)CIGS thin film solar cells were grown on PI and SLG substrates at low substrate temperatures.The properties of prepared CIGS thin films and photovoltaic performances of CIGS solar cells on different substrates were investigated.(112),(220)/(204)and(116)/(312)peaks from x-ray diffraction(XRD)patterns of CIGS thin films show double-peak patterns as the substrate temperature decreases.The CIGS thin films grown on both PI and SLG substrates show layered structures.The bottom and surficial layers of CIGS thin films display small size polycrystalline grains while the middle layer shows large size polycrystalline grains.Both types of CIGS thin film solar cells exhibit similar efficiencies while CIGS thin film solar cells grown on PI substrates show lower open circuit voltage and fill factor but higher short circuit current density.The highest efficiency of 6.14%has been achieved for the CIGS thin film solar cells on PI substrates.(3)It is known that Alkali metal doping in CIGS absorber can effectively improve the performances of solar cells.The potassium(K)was incorporated into the CIGS absorbers by thermal evaporation and post-annealing and the properties of CIGS as well as the performances of thin film solar cells were investigated.It was found that the incorporation of K did not affect the structure and grain size of the CIGS thin films.The open circuit voltage of the CIGS solar cell on SLG substrate was enhanced after K doping.However,the effect of K doping on PI substrate solar cells is not significant.