Preparation And Performance Of Flexible Cd Te Solar Cells

Solar energy is considered as the most striking renewable and clean energy,and has attracted extensive attention.In recent years,tremendous progress has been made in utilization of solar energy,especially in developing solar cells.Solar cells can directly convert energy of sunlight into electricity,which has attracted a lot of interest both from scientific research and industrial production because of its convenience and cleanliness.Currently among all the commercialized solar cells,single crystal silicon solar cell has dominated the solar cell market.However,due to the low light absorption,the shortage of silicon feedback and high cost involved in the fabrication of silicon solar cell,the single crystal silicon solar cells have been restricted for application.Therefore,thin film solar cells emerged and have attracted increasing attention due to its stability and growth with easy deposition techniques,abundant availability and so on.Cadmium telluride?CdTe?-based thin film solar cells are considered as the fastest-growing solar cells because of their simple structure and material,low cost,high theoretical conversion efficiency,etc.CdTe solar cells can be fabricated in superstrate or substrate configurations.Generally,solar cells using the substrate configuration can be fabricated on polyimide and metal foils,which is light and flexible,and has a great potential for application in building-integrated photovoltaic technology.In contrast to solar cells using the superstrate configuration,the industrialized production of the solar cells using the substrate configuration is limited by its poor conversion efficiency.Therefore,the method to improve the conversion efficiency of solar cells using the substrate configuration and reduce their costs attracts more and more attention.In this paper,vertically aligned CdTe nanorods arrays and compact CdTe films are successfully electrodeposited on Ni flexible sheets.The effects of deposition parameters on the morphology,structure and composition of CdTe films are investigated in detail.Based on the characterization results,the possible growth mechanism of CdTe film is carefully proposed.Moreover,a novel three-dimensional and planar heterojunction flexible photovoltaic device are fabricated using CdTe nanorods arrays and thin film,respectively.Then the photoelectric conversion properties of these two types of solar cells are further analyzed.This paper studies mainly in the following respects:?1?Verticallly aligned CdTe nanorods arrays are successfully grown on Ni substrates by a simple one-step and template-free electrodeposition method.The morphology,structure and composition of CdTe nanorods arrays are investigated.The effect of deposition potential,reaction time,concentration of reactants,pH of the solution on the morphology,structure and composition of CdTe nanorods arrays is systematically studied.And the possible growth mechanism of CdTe nanorods arrays is proposed.?2?A uniform and compact CdTe thin film is successfully fabricated on Ni substrate via electrodeposition method.The morphology,structure and composition of CdTe thin films are carefully examined.The influence of the concentration of sodium citrate in the growth solution on the morphology,structure and composition of CdTe thin films is investigated in detail.The results show that sodium citrate plays a significant role in directing the morphological evolution of CdTe from rod-shaped array to compact film.The composition of the as-prepared films is changed from Te-rich to Cd-rich,and the conversion from p-type conduction to n-type in CdTe films is also achieved with the increase of sodium citrate concentration.When the concentration of sodium citrate is 0.08 mol L^-1,a uniform and compact CdTe thin film with better crystallinity can be deposited on Ni substrate,and the composition of CdTe film is near stoichiometric.In conclusion,the addition of sodium citrate during electrodeposition plays a significant role in the growth and property of CdTe film.More importantly,this is one of the strengths of electrodeposition technique which shows that by simply changing the concentration of sodium citrate,p-and n-types of CdTe can be obtained.?3?CdS film is deposited on the surface of CdTe nanorods arrays by three different methods,chemical bath deposition?CBD?method,homogeneous precipitation method,and radio frequency?RF?sputtering.The growth and formation of CdS/CdTe heterojunction are investigated in detail.Further investigations reveal that,in comparison with the CBD method,a better coverage of CdS on the surface of CdTe nanorods arrays can be obtained by homogeneous precipitation method due to the slow and even hydrolysis of urea.CdS films with uniform and dense particles prepared by RF sputtering show better crystallinity and formation of CdS/CdTe heterojunction.For the optoelectronic test,the fabricated solar cells based on the CdS films prepared by RF sputtering shows a short-circuit current density of 9.51 mA cm^-2 and an open-circuit voltage of 0.47 V,yielding a better energy conversion efficiency of 2.50%with a fill factor of 0.56,indicating that this novel three-dimensional heterojunction flexible photovoltaic device has a significant potential for other optoelectronic devices,and also suggests a bright future for further material investigation and solar cell optimization.?4?CdS window layer of all solar cells is deposited on CdTe film by RF sputtering.The growth and formation of CdS/CdTe heterojunction are investigated in detail.The CdTe films prepared using 0.07 M,0.08 M,and 0.09 M sodium citrate are used.The maximum energy conversion efficiency of 2.73%is achieved for the device based on the Cd Te film grown using 0.08 mol L^-1 sodium citrate,and its open-circuit voltage,short-circuit current density and fill factor are 0.53 V,10.46 mA cm^-2,49.28%,respectively.This study suggests that the as-prepared CdTe thin film has a great potential for application in thin film solar cells,and the sodium citrate-assisted synthesis of CdTe film suggests a promising technique for large-scale fabrication of flexible solar cells.