Fabrication And Research Of Inorganic Chalcogenide Thin Films For Solar Cell Application

As the world population increases continuously,the global issuess of environmental pollution and energy crisis are increasingly rigorous.Compared with the traditional fossil energy involving oil,coal and natural gas,solar energy is a clean and non-polluting energy,the utilization process of solar energy will not release greenhouse gases and particle pollution.Photovoltaic(PV)receive a high expectation as one of the most promising energy utilization way in the future.In recent decades photovoltaic have obtained large development.Due to the reasons of limited raw material and manufacturing techniques,the fabrication of solar cells faces inevitable contradictions between production cost and energy conversion efficiency in nowadays.So far,the global PV market is dominated by crystalline silicon solar cell which had a mature technology and market share of above 90%.However,solar cell based on monocrystal silicon and polysilicon need thick light-absorption layer to completely absorb incident sunlight since crystal silicon is an indirect-band-gap material.What's more,high-efficient silicon solar cell requires high-quality silicon-wafer,this reason leads to the high price of PV modules.In recent years,solar cell devices based on inorganic compounds become the research focus.Quaternary semiconductor Copper Indium Gallium Selenium(CIGS)and binary Cadmium Telluride(CdTe)have become the important component of today's commercialized thin film solar cell due to their suitable band gap and high absorption coefficient.However,the expensive indium and the toxic nature of cadmium hindered its universalization.In order to reduce production costs,searching for low-cost and environmental-friendly alternatives have become the research focus for solar cell community.Quaternary compound semiconductor Copper Zinc Tin Sulphide(CZTS)with kesterite structure,derived from chalcopyrite structure of CuInS2(CIS)by substituting element In with equal number of earth-abundant elements Zn and Sn,has been successfully developed for thin film solar cell.Compared with crystalline silicon,CZTS has more elements and complicated crystal structure,the power conversion efficiency(PCE)of CZTS was commonly decreased by the problem of chemical composition and secondary phases.In this thesis,we will take the fabrication of quaternary CZTS solar cell by combustion method as the starting point.Further,the restraining factor of power conversion efficiency and the corresponding solutions were discussed.Beyond that,ternary semiconductor Copper Germanium Sulphide(CGS)was successfully prepared by combustion method,the PCE of device based on CGS achieved 2.7%.Due to the excellent electrical properties of CGS,we further apply CGS nanoparticle synthesized by hot-injection method to organic-inorganic hybrid perovskite solar cell as a new type of hole transporting material.The experiment results show that the perovskite solar cell gained a high PCE with significantly improved device lifetime.The major content of this thesis will be divided into five relatively parts,the specific contents are described as follows:The first part(chapter two)starts with a description of the structure of CZTS thin film solar cell made in our Lab,then the preparation technique and the corresponding experiment parameters of each functional layer was discussed in detail,including the deposition of Mo back contact by DC magnetron sputtering,CZTS absorption layer by pulsed laser deposition,CdS buffer layer by Chemical bath deposition,ZnO window layer by rf-magnetron sputtering and Al top-contact by thermal evaporation.The second part(chapter three)is to apply combustion method in the fabrication of CZTS thin film solar cell.Multicomponent metal oxides will be synthesized by combustion method,then the absorbing layers were fabricated by non-vacuum doctor method followed by annealing at high temperature under sulfur atmosphere.The probable formation mechanism of CZTS from metal oxides system during sulfurization process is analyzed in detail.The best performance of CZTS thin film solar cell achieved a PCE of 1.6%.To address the frequent problem of undesired pores in films prepared by doctor method,the obtained combustion powder was further pressed and sintered into single ceramic target used for pulsed laser deposition(PLD).The corresponding cell device achieved 5%PCE due to the high-quality precursor film coated on Mo substrate deposited by PLD.The third part(chapter four)is based on the work completed in the first part.We explored and prepared a new type of ternary semiconductor Cu2GeS3(CGS)which is a promising light-absorption material for photovoltaics.It was found that excess citric acid can reduce the oxides to metal which eventually was transformed into metal alloy under high temperature.Compared with oxides precursor,metal oxides were easily reacted with sulfur to generate the target compound as confirmed by thermodynamic calculating.The experiment results revealed that ternary CGS,which consist of simpler structure,has a desirable band gap and absorption coefficient,comparable to that of Cu2SnS3.However,the open circuit voltage of solar cell device based on CGS can achieve up to 600 mV,which proved for the first time that ternary Cu-based solar cell based on cheap components can also obtain high open circuit voltage.In the fourth part(chapter five),the ternary compound CGS was further applied to organic-inorganic hybrid perovskite solar cell.The efficiency of perovskite solar cell has been increased to 22.1%in recent few years.However,many experiment results show that the existence of moisture will induce significant decomposition of perovskite,resulting in a short device lifetime.So in our experiment,we use the inorganic CGS nanoparticle to substitute the traditionally used organic hole transporting material.We found that the lifetime for the device based on CGS HTL was significantly enhanced without sacrifice of power conversion efficiency.This investigation provides a material choice for high-efficiency perovskite solar cells with enhanced moisture stability.In the fifth parth(chapter six),we successfully fabricated a planar heterojunction Sb2S3 solar cell with all-inorganic charge-transporting materials.A facile molecular solution process was adopted to deposit Sb2S3 light-absorbing layer with highly smooth and dense film surface.The NiOx nanoparticles was first synthesized by thermal decomposition method and then dispersed in pure water to form the availuable ink for spin-coating.Then we creatively use inorganic NiOx as hole transporting material in Sb2S3-based solar cell.Further we demonstrated that the device performance can be significantly enhanced upon O2 plasma treatment on NiOx layer.