Preparation Of ZnO/Si Heterojunction Solar Cells

Among various types of solar cells,the p-n junction solar cells of single crystalline silicon are undoubtedly preferred because of their high conversion efficiencies of above 20%.But the high cost of Si solar cells becomes the most vital obstacle to commercialize.Therefore,lowcost solar cells have gained more attention recently because of their lower manufacturing cost, in spite of their relatively low conversion efficiencies.One of the good candidates is heterojunction solar cells formed simply by depositing transparent semiconductor films on Si substrates.The heterojunction solar cells have a number of potential advantages such as an excellent photovoltaic effect and simple processing steps. Relative high conversion efficiencies in 12～15%range have been obtained by employing spray-deposited ITO films on Si substrates.Because of the limited resource of indium on the earth,use of abundant materials is required.Zinc oxide film has good electrical and optical properties in combination with low cost,non-toxicity,and relatively low deposition temperature.All of these virtues make the ZnO/Si heterojunction solar cells to be possible.In this paper,ZnO/n-Si heterojunction solar cells were fabricated on n-Si substrates by the Spray Pyrolysis process.At the beginning,P-Si<100> substrates were used to make textured structure in alkaline and IPA solution,ZnO:Al thin films were deposited on textured Si substrates by Ultrasonic spray pyrolysis techniques.Front and back electrodes were prepared by techniques of lithography,metal sputtering and high vacuum evaporation,and subsequently annealed at a temperature of 400℃under nitrogen environments.Atom Force Microscope,Hall Effect Measurement,X-Ray Diffractomer and Current-Voltage characterizations were employed to characterize solar cell performance.As a result,the conversion efficiencies of the fabricated solar cells of about 5.2%were obtained,with V_oc=355mV,I_sc=36mA,FF=0.41.Textured heterojunction structure effectively reduced light reflects,increased effective p-n junction area,thus improved solar cell performance.The efficiencies were found to decrease rather quickly with time.Silicon oxide film due to oxygen diffusion at the ZnO/Si interface through the porous structure of the ZnO films were regarded as the main effect of the solar cell.After we deposited a thin layer of SiO₂ at the top of the sample,efficiencies decrease were significantly controlled.