The Effects Of Alkali Metal Li Doping On CIGS Solar Cells Prepared By Low-temperature Process

Copper indium gallium selenide（CIGS）solar cell is one of the most promising solar cells which have the advantages ofhigh light absorption coefficient,and stability in low light etc.At present,the CIGS devices have reached an efficiency of up to23.35%.In order to accelerate the development of industrialization of CIGS device,reducing the production cost is an urgent issue for current research.Low-temperature deposition technology can effectively reduce energy consumption to achieve the purpose of reducing cost.However,lowering the deposition temperature will lead to poor material properties of absorber,which will deteriorate the device efficiency.Previous studies have shown that the introduction of alkali metals into CIGS films contribute to the enhencemt of device efficiency.Recent efficiency breakthroughs of CIGS cell are also achieved by the application of alkali-metal.However,compared with other alkali metals such as Na,K,the doping mechanisms of the alkali metal Li in CIGS cells are rarely involved.In this paper,we have studied the alkali metal Li doping in the different stages of CIGS growth during low-temperature process to improve the efficiency of CIGS thin film cells.We also explored the effect and the mechanism of alkali metal Li doping on the optical and electrical properties of CIGS thinfilms and cells.First,we report our investigation of the effects of Li atom doping on the growth of CIGS when Li is supplied from a precursor of Li Cl or co-evaporation of Li Cl at each step of CIGS depositon.It was concluded that Li atom will participate in the formation of the absorber layer when Li atom droped before the deposition of CIGS layer or at various steps of co-evaporation.The presence of Li can then play a decisive role for suppressing the growth of CIGS.It will affect the diffusion of In and Ga and then cause the separation of high-Ga phase and low-Ga phase.In addition,the diffusion of Cuis also suppressed by Li and lead to deteriorate the crystal of the absorption layer.The main reason is that it will form a layer of quasi-surfactant on CIGS surface when Li participates in the growth of CIGS film.In the Li-free case,the barrier height is low enough to enable the element Cu,In,and Ga to diffuse into the grain of CIGS,while the presence of Li on CIGS surface increases the barrier height,such that element diffusion is impeded.More energy is needed for Cu,In,and Ga to diffuse into the grain.In order to avoid the negative effect of Li on the crystal quality of CIGS film,we introduce the Li into CIGS by Li Cl-PDT（Post-deposition treatment）process.When Li Cl-PDT was carried out,the performance of the device will be significantly improved.In addition,the secondary phases of Alk-In-Se can not be observd after Li treatment.Li elements maybe exist as alloy phase of Li _XCu_1-X（In,Ga）Se₂ on the surface due to its stability.The reason of Li can improve the performance of the device is that it can occupy the copper vacancies(V_Cu),promote the formation of Li _Cu neutral defects and reduce the donor defect density of In _Cu.Finally,the net doping concentration and the electrical properties of the CIGS film will be enhancement.This paper studies the effect of alkali metal Li doping on CIGS thin film and devices prepared by low-temperature process systematically.It explores the mechanism of Li doping on CIGS film solar cells.Finally,the efficiency of 14.1%is achieved by Li Cl-PDT for the CIGS cell deposited under low temperature of 450℃.