A Study On Band Alignments At Heterojunction Interface Of Cu₂ZnSnS₄-based Thin Film Solar Cell

With the depletion of non-renewable resources such as fossil fuels becominggradually severe, as well as the environmental pollution caused by the burning offossil fuels, searching for a renewable non-polluting energy is extremely urgent forthe mankind. As a green renewable energy resource, solar energy with no doubt has agreat potential to be developed and to be used in the future. Besides, the solar energycan be directly converted into electrical energy by solar cells, which will reduce amass of pollution if the fossil fuels can be replaced by them. If we can make good useof this feature of sunlight, it will greatly ease the environmental burden of the earth.Thin film solar cells have many fabricating advantages such as less consumption oforiginal materials and they are thin, light devices. In this work, we will research aquaternary compound Cu2ZnSnS4（CZTS）mainly used as the absorption material insome new kinds of solar cells. This semiconductor material has many advantagesbelonging to the old thin film materials, in addition, it is poisonless and abundant inthe surface layer of the earth. It is another promising material since Cu(In,Ga)Se2wasreported.We applied many experiences from those research about CZTS, CdS and ZnOthin film materials into our new research on band alignments at interfaces of the thinfilm solar cell heterojunctions base on CZTS. The traditional physical vapordeposition method-magnetron sputtering was taken to fabricate every layer of thebattery in this research. The experimental conditions varied with the materials withinevery layer. The factor that we changed when we prepare the absorption layer issputtering speed but the substrate temperature when we prepare the buffer layer. Atlast, we find the best conditions for preparing all the layers. In this paper, we use a typical structure of CZTS-based solar cell isMo/CZTS/CdS/ZnO, Recently, a CZTS thin-film solar cell with a conversionefficiency of9.2%was reported, which is a new efficiency record for such solar cells.in which the interface properties, such as CZTS/CdS and CdS/ZnO interface, are veryessential for obtaining high energy.Among the interface properties, the band offset is one of the most importantparameters because it influences the transport and recombination of carriers. In thiswork, the conduction-band offsets (CBOs) of CZTS/CdS and CdS/ZnO interfaces aredetermined by combining both XPS and optical absorption spectroscopy. we researchthe CZTS/CdS hetrojunction was a type II band alignment, means that the CZTSCBM located above of the CdS buffer, that is the band alignment is of type Ⅱ, This iscommonly interpreted as an indication for a recombination at the (buffer/absorber)interface. So the CBO is crucial for the conversion efficiency of solar cells, whichforming a less barrier to electron transport across the interface. At the same time, Wealso studied the heterojunction between CdS and ZnO,and the band alignment also isof type Ⅱ. The results are well supported by first-principles calculations based onhybrid functional methods.