Crystallization Optimization And V-Shape Band Gap Structure Design Of Cu?In,Ga?Se₂ Absorber Layer

As exhausting stage of fossil energy,solar energy has draw more and more public attention as a new type of renewable energy.Solar cell is a photovoltaic device which convert solar energy into electric energy.At present,more and more types of solar cells have been used in light generating.As the third generation solar cells,Cu?In,Ga?Se₂?CIGSe?thin film solar cell,it has several advantages such as adjustable band gap,high absorption coefficient,low cost,high efficiency,etc.The highest efficiency is22.6%.Among numerous CIGSe preparation methods,molecular precursor solution method has many advantages such as simple operation,low cost,etc.Qian's group got the highest conversion efficiency17.3%of solution-process approach by using hydrazine solution in 2016.However,hydrazine is highly toxic,explosive and is not suitable to experimental research and industrial production.In non-hydrazine solution method,the solvent mixture of 1,2-ethylenediamine?en?and 1,2-ethanedithiol?edtH₂?show similar dissolving properties as hydrazine,diamine–dithiol solution is an ideal alternative to hydrazine.However,the power conversion efficiency?PCE?of CIGSe solar cell achieved through this method is still low?only 9.5%?.Thus shortening the efficiency gap between hydrazine and non-hydrazine solution based CIGSe solar cells is the key study point in this paper.The CIGSe devices made by edtH₂/en solvent mixture have two disadvantages of low open circuit voltage(V_oc)and short circuit current(J_sc)compared with CIGSe devices with hydrazine solvent system.The main difference lies in the CIGSe absorption layer.By the analysis of 1,2-ethylenediamine and1,2-ethanedithiol solvent system,here mainly exist the following problems:?1?the cross section of the film has a very thick fine-grained layer which exists excess grain boundary,which will lead to serious carrier recombination;?2?the band gap structure of thin film is uniform,photo-generated electrons can easily recombine with holes at Mo back electrode.In order to solve the problems above,we first adopt alkali-doping to eliminate the fine-grained layers and increase V_oc.Then,we build a V-shaped bandgap gradient in CIGSe film with improved crystalline.The formation of a back-gradient results in an internal drift field??_A?which can help the transport of electrons and enhance J_sc.The main work of this paper are as follows:1.We use soda-lime glass?SLG?as Na source,at the same time we introduce Mo-coated quartz substrates?SiO₂?for comparison.In this part,we study how SLG impacts on thin film crystalline when SLG as substrates and coverages.When SLG as substrates and coverages at the same time,we successfully eliminate the fine-grained layers and get large grains throughout the film,and can eliminate some secondary phase such as Cu_2-xSe,CuInSe₂.We eventually increase V_oc from 528 mV to 583 mV,an improved power conversion efficiency?PCE?of 11.53%was achieved.2.The band gap structure of the thin films is uniform,photoproduction electrons and holes can easily recombine at the Mo back electrode.However,the high efficiency of CIGSe solar cells almost adopt a V-shaped gradient band gap structure,this structure can provide an internal drift field,it is beneficial for the separation and transport of electrons and holes.Here we provided four CIGSe precursor solutions with different Ga content,seprately spin coating on Mo-coated glass.SIMS results show that we successfully built the bouble-Ga gradient.At the same time,we make a study of the location of the notch,we found when?L_f=200 nm,the recombination was minimized.With the incorpration of Ga-gradient in CIGSe solar cells J_sc increased about 17%,PCE increase from 11.40%to 13.12%.3.Because of Ga gradients in CIGSe thin films was made through molecular precursor solution method,the diffusion speed of the Ga is still fast in the process of spin coating and annealing treatment.The slope of Ga back gradient are not clear enough.ACIGSe absorber layer can form(Ag_1-xCu_x)In₂ phase in the process of additional heat treatment,as a result the diffusion speed of Ga slow down.feature of Ag,we spin coating pure Ag solution on the interface of CIGSe and Mo back electrode.The high Ag concentration makes Ga move toward the back of films,the position of the notch drop because of the band gap fall.EDS line scan also showed Ga accumulate to the back of CIGSe film and a stronger Ga back gradient.When the spin-coating/sintering of silver is repeated 2 times,PCE increased from 11.76%to12.37%at the same condition of gradient CIGSe thin film solar cells.