The Synthesis Of Copper-based Chalcogenide Compound Semconductor Nanocrystals And Their Photovoltaic And Thermoelectric Properties

Energy and environment problems are two major problems restricting the development of human society.Photovoltaic power generators can significantly decrease carbon dioxide emission by directly converting sunlight into electricity,and thermoelectric power generators can increase energy use efficiency by recycling waste heat into electricity.Photovoltaic materials and thermoelectric materials can be assembled into hybrid devices or systems,which can avoid photoelectric materials absorb excessive heat and damage to the photovoltaic device,thereby prolonging the working life of the solar cell,and increasing the operational stability of the photovoltaic device,which can increase the overall utilization of energy.Therefore,the thermoelectric conversion properties of photovoltaic materials can be studied at the same time to fully utilize the infrared wavelengths of sunlight,and a new type of photovoltaic light and heat combined technology can be developed in a pioneering way,which has important strategic significance for the realization of energy-saving emission reduction targets in our country.Therefore,in this paper,the copper-based chalcogenide Cu₂Sn?S,Se?₃?CTSSe?and Cu₂ZnSn?S,Se?₄?CZTSSe?nanocrystals were synthesized by hydrothermal method.The ratio of sulfur and selenium atoms in the nanocrystals was controlled.Then,the bandgap is regulated in this way.High quality CTSSe thin film was prepared by nanocrystal ink process.A typical CTSSe thin film solar cell with power conversion efficiency of 0.89%was acquired in this paper.The thermoelectric properties of CZTSSe nanocrystals under different composition were studied.At the same time,the nanoengineering approaches was used to improve the performance of thermoelectric materials.The main studies and results are listed as follow:?1?CTSSe nanocrystals?NCs?with different composition were successfully synthesised by hydrothermal method.The average grain sizes of the 5 CTSSe NCs are⁶.8 nm,which are well crystalline.The bandgaps of CTSSe NCs were tuned approximately linearly from1.52 to 1.20 eV by aggrandizing the sulfur and selenium atomic ratio by controlling the ratio of sulfur and selenium in solid solution NCs.The nanocrystalline ink-based ink method has been successfully applied to the preparation of the CTSSe thin film.The dense and compact CTSSe photovoltaic absorbers with large grain sizes were obtained.The contents of the five elements Cu,Sn,S,and Se in the thin film,which are 30.53,14.71,22.96 and 25.65%of total contents respectively,which agree well with the stoichiometric ratio of elements in CTSSe.Finally,a typical CTSSe thin film solar cell with power conversion efficiency of 0.89%was acquired according to glass/Mo/CTSSe/CdS/i-ZnO/AZO/Ag the structure of traditional thin film solar cells.The open circuit voltage(V_oc)and short-circuit photocurrent density(J_sc)are0.31 V and 8.20 mA cm^-2 respectively.The fill factor?FF?of the thin film solar cell is27.22%.?2?CZTSSe nanocrystals up to 10 grams per batch were acquired by a large-scale hydrothermal synthetic method.The average grain sizes of the 5 CTSSe NCs are⁶.7 nm,which are well crystalline without impure phase.The bandgap engineering of CZTSSe nanocrystals was achieved by controlling the ratios of sulfur and selenium atoms in the solid solution nanocrystals.The bandgaps of CZTSSe NCs were tuned approximately linearly from1.52,1.44,1.39,1.34 to 1.29 eV with the increase of sulfur and selenium atomic ratios.Finally,the thermoelectric properties of CZTSSe NCs with the various ratios of Se/?S+Se??0,0.25,0.5,0.75,1?from 300⁷00 K are elaborated in detail.Electrical conductivity of CZTS?x=0?decrease from 57.2 S cm^-1 to 44.8 S cm^-1 in the measured temperature range from 300 to 700K,which is higher than the value of the stoichiometric CZTS bulk materials.The Seebeck coefficients of CZTSSe NCs with the various x all increase from 300 K to 700 K.The S of CZTSSe NCs?x=0.25?are between 171 and 316?V/K from 300 to 700 K,which is higher than those of CZTSSe NCs at each point of temperature.The thermal conductivity of the CZTSSe NCs?x=0.75?are 0.99 and 0.74 W m^-1 K^-1 from 300 to 700 K,which is higher than those of CZTSSe NCs at each point of temperature.The thermal conductivity of CZTSSe NCs are all very low comparable to CZTS,CZTSe bulk counterparts and other binary TE bulk materials The ZT of CZTSSe NCs?x=0.25?reaches 0.48 at 700 K,which is higher than those of CZTSSe NCs at each point of the temperature.It is indicated that transfering from bulk materials to the solid solution nanomaterials is an effective way to further enhance the TE performance of CZTS NCs.