The Fabrication Of Ordered Micro/Nanostructures And Their Applications In CIGS Thin Film Solar Cells

Nowadays,developing and exploiting new alternative energy sources have become a task of top priority due to the increasing energy crisis and the serious environmental deterioration.As a new energy source,the utilization of solar energy has attracted much attention due to its renewable,clean and pollution-free nature.The incident light can be directly converted to electrical energy without any mechanical rotation or contamination by-products.Among various types of solar cells,thin film solar cells have the advantages of low cost,easy to transport,facilitating building integration,and large-scale production.Undoubtedly,it is a best choice to develop thin film solar cells in the future.As an important member of thin-film solar cells,CIGS thin-film solar cells have received extensive attention since the 1970s.It has a series of excellent characteristics such as high photoelectric conversion efficiency,strong anti-radiation ability,adjustable forbidden band width and high light absorption coefficient,thus,to be an ideal solar photoelectric semiconductor material.Currently,the device structure with soda-lime glass?SLG?/Mo/CIGS/CdS/i-ZnO/AZO/MgF₂ structure is recognized as the best one,after continuous optimization and development of CIGS thin film solar cells.The key functional part on CIGS-based solar cells is the heterojunction which consists of p-type CIGS absorber layer and the n-type CdS buffer layer.Its main working mechanism is as following:under the excitation of external incident photons,electron-hole pairs can be generated in the CIGS absorber layer.Subsequently,the built-in electric field in the p-n junction will promote the separation of electron-hole pairs.Finally,the electrons will be collected by the top electrode,and the holes are accumulated at the back electrode,then creating a constant current through the external circuit.Although intensive research has been performed to the CIGS solar cell technology,there is still a trade off or competation between the light absorption efficiency and carrier collection efficiency due to its thin film nature.How to ensure the high light absorption efficiency and good carrier collection efficiency in single device is still one of the key issues and technologies for further improving the performances of CIGS solar cell devices.As we all know,nanostructures possess many characteristics such as large specific surface area,wide optical band gap,multiple exciton generation effects,and quantum confinement effect.In recent years,studies on silicon-based thin-film solar cells revealed that the introduction of ordered nanostructures is one of the effective methods to solve the contradiction between light absorption efficiency and carrier collection efficiency.In this paper,in order to further improve the performance of CIGS thin film solar cells,the main contradiction and key issues between the light absorption efficiency and the carrier collection efficiency that exist for the thin film structure are discussed.Different micro-nano ordered structures are designed and fabricated using cheap and easy handling colloidal sphere lithography?NSL?.Three different ordered micro/nano structures are designed,fabricated and introduced into different functional layers of CIGS thin film solar cells from the viewpoint of increasing light absorption efficiency and improving the carrier collective efficiency.The main research contents are as follows:?1?The zinc oxide cavity structure is prepared at the top electrode as an anti-reflection layer to increase light absorption:Using the liquid surface self-assembly transfer technology,a hexagonal close-packed PS colloidal sphere template is assembled onto the substrate.AZO was used as the seed layer and PS microspheres as masks.The diameter of PS microspheres was adjusted and hydrothermal growth method was used to construct ordered zinc oxide?ZnO?cavity structures with different periods.The ZnO cavity structure was prepared by using PS microspheres with diameter of 300 nm as a template.The cavity structure was introduced to enhance the light absorption efficiency on the front surface of the CIGS thin film device by the light trapping effect.The results showed that the short circuit current density(J_SC)and photoelectric conversion efficiency were significantly improved compared to those without nanostructure.?2?Preparation of Mo nanomesh structure at back-electrode to increase the carriers collection efficiency:1.3?m PS microspheres were used as template on Mo/soda-lime glass substrate.Subsequently,reactive ion beam etching?RIE?was employed to prepare non-dense packed PS microspheres template.After the Mo sputtering process,the PS microspheres were removed using toluene under soft ultrasonication.Finally,the Mo nanomeshes structures with different depths were obtained.Mo nanomeshes structures with different depths were introduced to CIGS thin film solar cell.The introduction of Mo nanomeshes increased the contact area between the back electrode and the CIGS absorber.The CIGS absorber layer was embedded in the molybdenum nanomeshes to reduce the diffusion distance of carriers and improve the efficiency of carriers collection.The short circuit current density(J_SC)and the photoelectric conversion efficiency are increased compared to those without the Mo mesh structure.However,we also found that the open circuit voltage was significantly reduced by the introduction of Mo nanomeshes.In addition,the molybdenum nanomesh structure based on glass substrate was prepared to construct thin film photovoltaic device with double-side light transmission.A photoelectric conversion efficiency of 3.52%was obtained when light was directly incident on the front side of device,and when light was incident vertically from the back side,the photoelectric conversion efficiency was 1.78%.?3?Solution-etched CIGS thin films with a cone-like nanostructure at the heterojunction to increase the separation efficiency of carriers:A non-close packed PS microsphere monolayer template was assembled on the CIGS absorber film.A certain concentration of Br₂/CH₃OH etching solution was used to etch the CIGS thin film.And then the PS microspheres were removed to obtain cone-like CIGS surface nanostructures.This structure was assembled into a CIGS thin-film solar cell device in order to increase the contact area of the heterojunction,thereby enhancing the carrier separation efficiency.However,the device performance test results showed that the open circuit voltage(V_OC)was reduced,the short-circuit current density(J_SC)and fill factor?FF?were basically unchanged,and the photoelectric conversion efficiency was reduced that compared to the solar cells device,which assembled with the untreated CIGS layer.Further work is still needed to optimize the structure of the tapered nanostructured surface.