Study On The Basic Physical Properties Of Sb2se3 And Its Applications In Solar Cells

With the overexploitation of traditional fossil fuel,environmental pollution and energy crisis have become increasingly serious.Photovoltaic（PV）devices that convert clean and renewable solar energy into electric energy have attracted extensive attention.At present,the widely studied PV devices include crystalline silicon,thin-film,organic,quantum dot solar cells and so on.Among them,thin-film solar cells have become a hot research topic owning to the advantages of low preparation cost,simple process,flexibility,portability,and suitability for building integration.Copper indium gallium diselenide and CdTe solar cells have achieved power conversion efficiency（PCE）of>22%in laboratory,but the cost and toxicity of raw materials limit their application in real life.Sb₂Se₃ is cheap,non-toxic and stable emerging inorganic solar cell materials,which has the advantages of appropriate bandgap（¹.1 eV）,large absorption coefficient(10⁵cm^-1),simple phase and low crystallization temperature,therefore holds a great potential for development.Within few years,the PCE of Sb₂Se₃ solar cells developed rapidly from0.66%to 7.60%,however lacking behind its theoretical PCE limit（>30%）.The deep understanding of the basic physical properties of Sb₂Se₃ is the key of systematic performance improvement in Sb₂Se₃ solar cells,because it determines the choice of device structure and the direction of process optimization.However,due to the late start and few studies,no systematic study of optoelectronic properties of Sb₂Se₃ are reported so far.There are three main reasons:（i）the indirect-bandgap feature of Sb₂Se₃ creates obstacles for obtaining defect depth,defect density and carrier lifetime from photoluminescence measurements;（ii）the high resistance makes it difficult to obtain defect density,mobility and diffusion length from common Hall effect tests;（iii）the high anisotropy in Sb₂Se₃crystal complicates the study of Sb₂Se₃ physical properties.This thesis aims to establish the database of basic physical properties of Sb₂Se₃ via material and device characterizations,and provide theoretical basis for the development of Sb₂Se₃ solar cells.The research includes the following four parts:（1）Systematic study of the optical properties of Sb₂Se₃.Firstly,the complex refractive index and dielectric constant of Sb₂Se₃ were obtained by ellipsometry and Swanepoel methods,and the dielectric constant at radio frequency is measured by plate capacitor method.Then,the absorption coefficient of Sb₂Se₃ was acquired by absorption,ellipsometry,photothermal deflection spectroscopy and first-principles calculation,and the high absorption coefficient is explained in detail through partial density of states.In addition,the quasi-direct bandgap feature of Sb₂Se₃ was studied by Tauc fitting based on the transmissivity spectra.Finally,according to the absorption coefficient of low energy photons,we studied the tail states and obtained the Urbach energy of Sb₂Se₃ films.（2）Systematic study of the electrical properties of Sb₂Se₃.The conductivity and contact resistance of Sb₂Se₃ were obtained by combining two-and four-probe methods.The carrier mobility was obtained by means of thin-film field effect transistor,time of flight,space charge limited current（SCLC）and high temperature Hall Effect measurements.Then,the carrier lifetime of Sb₂Se₃ film was obtained based on the transient absorption test.Combined with the above data,we calculated the carrier density and diffusion length,which is consistent with the value obtained from the bias dependent quantum efficiency test.At last,we studied the defect activation energy by the temperature dependent conductivity,and calculated the defect density of Sb₂Se₃ by SCLC method.（3）Figure out the main limiting factor on the PCE of Sb₂Se₃ solar cells via device physical characterizations.Based on the basic optical and electric properties of Sb₂Se₃,we firstly studied the PCE loss mechanism of Sb₂Se₃ solar cells from the losses of open-circuit voltage,short circuit current and fill factor in details.Then the capacitance-voltage,temperature dependent admittance spectroscopy,deep level transient spectroscopy,driver level capacitance profiling,etc.were carried out to characterize the defect type,density,depth and distribution in details.Then the carrier dynamic process was studied by transient photovoltage,transient photocurrent,etc.Finally,the effect of parasitic resistances,doping density and defects on device PCE were evaluated by SCAPS software simulation.（4）Propose a model of grain boundary（GB）inversion to achieve a PCE breakthrough in Sb₂Se₃ solar cells.Deep defect recombination mainly restricted the PCE of Sb₂Se₃ solar cells,but it is very difficult to eliminate deep defects on experimental basis.Utilizing the special 1D crystal structure of Sb₂Se₃,we inversed the GBs through Cu ion treatment.Consequently,a built-in electric field was established between p-type grain interiors and n-type GBs,which spatially separated photogenerated carriers,restrained recombination,and enhanced carrier collection.Finally,the efficiency was improved from5.60%to 7.04%.