Study On Electrodeposition,structure And Property Of CdTe Thin Films

Cadmium telluride(CdTe)thin film solar cells is based on heterojunction consist of p-CdTe and n-CdS.CdTe thin film solar cells have attracted domestic and international attention due to high photoelectric conversion efficiency and simple preparation process.As a light absorption layer in solar cells,CdTe with 1.46 eV band gap and high absorbance is typical II-VI direct bandgap semiconductors.Various techniques,including of close-spaced sublimation(CSS),vapor transport deposition(VTD),magnetron sputtering,electrodepostion and etc.have been used to deposit CdTe films.Among these methods,electrodeposition is much more attractive for its simple,low-cost and large-area production for high-quality CdTe films.As a window layer in CdTe thin film solar cells,the formation of buffer layer is related to the concentration of chlorine in CdS films and the introduction of Cl in CdS films would affect performance of cells.The introduction of Cl in CdTe films is necessary for high-quality CdTe solar cells.The Cl was generally introduced to electrolyte to enhance the CdTe films quality in electrodeposition.However,the effect of Cl on electrodeposition mechanism was unclear.Nowadays,the holes were generally found in CdTe films prepared by electrodeposition after annealing due to small grains and numerous grain boundaries in as-prepared CdTe films.The holes play a role of carrier recombination center in CdTe solar cells,which leads to poor cell performance.Therefore,a suitable annealing method for electrodeposited CdTe films is required to enhance the performance of CdTe solar cells.Meanwhile,the suitable treated process of back contact layer is required to optimize to enhance cells performance due to specific crystal structure and thickness of electrodeposited CdTe films.This thesis focuses on these issues and the main research content is as follows:(1)The effects of residual Cl on structure and property of CdS films and cell performance were investigated.Two types of CdS thin films were synthesized via chemical bath deposition method from solution of acetate and chloride,respectively.All films were found to be hexagonal and n-type.After heat treatment,the crystallite size of CdS thin films prepared with cadmium chloride precursor is larger than those prepared with cadmium acetate precursor and its surface is smoother and more compact.The band gap of all CdS films get narrower through thermal treatment,and the band gap of CdS films from chloride solution is wider than those synthesized from the acetate solution.From the Mott–Schottky plots,the carrier concentration of the annealed films is reduced and introducing more chlorine will increase the carrier concentration.The results suggest that the residual chlorine will lead to the reduction of sulfur vacancies and release of stress.Moreover,the performance of CdTe solar cell prepared by CdS films from Cl-containing solution is superior to that prepared by CdS films from CH3COO-containing solution.(2)The effects of introduction of chlorine into electrolyte solution on structure and performance of electrodeposited CdTe film were investigated.The structure and property of CdTe films from the solution with Cl were improved compared with that from the solution without Cl,which is attributed to the improved conductivity.The Cl residual should contribute to the conductivity and further reduces the potential drop across the film thickness compared to that from electrolyte solution without Cl,which promotes the growth-dominated deposition and produces the well-crystallinity CdTe films.Further,the more negative deposition potential could produce better crystallinity regardless of Cl is included or not.Moreover,Cl residual also improved the device performance after annealing.(3)A two-step annealing process was developed for the electrodeposited CdTe films.An additional pre-annealing step before the Cl-activation annealing reduced the grain boundaries by improving the crystallinity.During the second Cl-activation step,Cl diffusion along the grain boundaries was suppressed.As a results voids were largely reduced at CdS/CdTe interface.The fewer voids at CdS/CdTe interface reduced carrier recombination and results in the higher EQE and photovoltaic efficiency.This two-step annealing process provides possibility for improving cell performance of a low-temperature CdTe film.(4)In order to optimize the treated process of back contact layer,the effects of etched time,Cu layer thickness and annealed temperature on performance of CdTe thin film solar cells were investigated.It is found that the optimal cell performance was obtained when 10 s etched time,8 nm Cu thickness and 200 ? annealed temperature.It is attributed to that the etched treatment for CdTe surface leads to eliminate of high resistant oxide layer and creates a Te-rich layer.The heavily doped layer was fomed and the back contact barrier was reduced due to CuxTe layer formed through depositing a Cu layer on Te-rich layer and annealing,which leads to improved cell performance.