The Optimal Design Of CdS Buffer Layer And Its Application In CIGS Thin Film Solar Cells

In recent years,energy demand has been increasing year by year due to the rapid development of social economy.However,conventional energy reserves are limited and the serious environmental pollution caused by their heavy use should be given enough attention.It is urgent to find new energy sources to replace conventional energy sources.The energy that solar energy radiates to the earth in one hour is equivalent to the annual energy consumption of the earth and it is pollution-free.Therefore,solar energy is considered to be one of the most ideal new energy sources.Among all the thin-film solar cells,due to its main advantages such as good stability,strong radiation resistance,easy large-area deposition and easy monolithically integrated interconnected components processing,CIGS solar cells are generally regarded as one of the most promising high-efficiency and low-cost solar photovoltaic technologies.The key structure of a CIGS cell is a P-N heterojunction composed by the CIGS absorber layer and the CdS buffer layer.From the energy band diagram of CIGS/CdS,it can be found that the collection of photo-generated electrons will be hindered,and J_SCC and FF will rapidly decrease,when the conduction band mismatch??35?Ec?between CdS and CIGS is too large.On the other hand,for a sufficiently negative?35?Ec,the CIGS inversion at the interface will be eliminated and the recombination of interface states will have a negative influence effect on the V_OC.Therefore,as a buffer layer in Cu?In,Ga?Se₂?CIGS?thin film solar cells,CdS plays an important role.It is well known that chemical bath deposition is the most common method for depositing CdS thin films.Moreover,the controllable growth of CdS film by CBD has crucial influences on the quality of the CdS films.High-quality cadmium sulfide films play a crucial role in the quality of P-N heterojunction.In order to obtain a mature CdS deposition process,it is imperative to explore and optimize the CBD-CdS process conditions.Although the CdS buffer layer preparation in CIGS thin-film photovoltaic devices is mainly based on the CBD deposition method,a large amount of CdS is precipitated in a homogeneous nucleation manner in the solution,resulting in a large amount of CdS waste and severe Cd metal pollution.Also,the small bandgap of CdS films prevents some near-ultraviolet light from penetrating into the CIGS absorber layer.In order to solve the above problems,we prepared one-dimensional structure of CdS nanowires and CdS/Cd?OH?₂ core-shell nanowires,which were applied as buffer layers of CIGS photovoltaic devices.Such research work provides new research ideas and methods to find new buffer layers with low pollution and high efficiency.The main experimental results are as follows:1.Optimization of chemical bath deposited cadmium sulfide buffer layer for high-efficient CIGS thin film solar cells.According to experimental results,the concentrations of cadmium acetate have crucial influences on the morphology,structure and optical performance of CdS films.The photovoltaic devices were fabricated with the prepared CdS films as buffer layer.A highest efficiency of 11.42%was achieved for the CdS buffer layer deposited at the cadmium acetate concentration of 0.052 M,which mainly benefited from its compact surface,high crystalline quality and good optical transmittance.To further improve the performance of the CIGS solar cells,the CdS films deposited at the cadmium acetate concentration of 0.052 M were annealed at different temperatures.An improved champion efficiency of12.57%is achieved at the annealing temperature of 180?.2.Solvothermal synthesis of CdS nanowires for high performance CIGS photovoltaic application.The CdS nanowires were successfully prepared by a simple hydrothermal synthesis method without using any templates.Compared with other methods,the preparation cost was effectively reduced.The effects of synthesis conditions on the structure,surface morphology and chemical composition of CdS nanowires were systematically investigated.By optimization,we successfully prepared CdS nanowires with a diameter of 30 nm and a length of several micrometers,which has good dispersion and high yield.The result from CdS nanowires revealed that Cd/S atomic ratio of about 1:1,there was no stoichiometry deviation.We applied the prepared CdS nanowires to CIGS photovoltaic devices and studied the effect of spin coating times?buffer layer thickness?on device performances.The results showed that the optimal thickness of CdS nanowires buffer layer was obtained when the number of spin coating reached 16 cycles,and the maximum value of the device efficiency reached 3.30%.The device efficiency is still low at present,it is necessary to further optimize its synthetic methods to achieve high performance solar cells.3.Ultralong CdS/Cd?OH?₂ core-shell nanowires buffer layer material for Cu?In,Ga?Se₂ thin film solar cells were prepared by simple hydrothermal synthesis and subsequent ion-exchange route.The structure,surface morphology and chemical composition of Cd?OH?₂ and CdS/Cd?OH?₂ core-shell nanowires were investigated systematically.In order to explore the influences of nanowire buffer layer thickness on the performances of CIGS thin film solar cells,CdS/Cd?OH?₂ core-shell NWs were spin-coated on the CIGS absorbers with different coating cycles.The best efficiency was observed for the fabricated device with the NWs coating of 28 cycles,which reached 5.51%,mainly because it had the appropriate nanowire buffer layer thickness.In order to eliminate the defects and improve the contact between NWs buffer layer and CIGS absorber,the devices with NWs buffer layer coating of 28 cycles were annealing treated.When the annealing temperature reacheed 140oC,the efficiency of the device was further improved to 7.00%,which was close to the conversion efficiency of the CBD-CdS/CIGS device under the same conditions.It was worth mentioning that the short-circuit current density of CdS/Cd?OH?₂ nanowire device exceeded the CBD-CdS device.It was mainly attributed to the wide and adjustable bandgap of CdS/Cd?OH?₂ nanowires,which reduced the absorption of photons in the buffer layer and was beneficial to the generation of photogenerated carriers in CIGS devices.