Study On New-type CdTe Thin Film Solar Cells

Cadmium telluride(CdTe)thin film solar cell is one of the most popular thin film photovoltaic cells in the market.It has many unique advantages,such as low cost,low temperature coefficient,good weak light performance,and has important research and commercial value.CdTe is a direct band gap semiconductor material.The band gap of CdTe thin films is 1.45 eV,which is close to the optimal bandgap for solar cells.In addition,the absorption coefficient of CdTe is as high as 105 cm-1 in the visible light range,and a CdTe thin film with the thickness of 1 micron is enough to absorb more than 90%of the photons whose energy is higher than the band gap.Therefore,CdTe is an ideal material for solar cell.Recently,the highest laboratory efficiency and commercial module efficiency of the CdTe solar cell have reached 22.1%and 18.6%,respectively,and the global installed capacity has exceeded 20 GW.However,there are still some fundamental issues for CdTe solar cells to be studied.For example,the traditional CdS window layer will limit the use of photons with wavelengths less than 510 nm.The scarcity of raw materials Te may limit the further development of CdTe solar cells.This thesis focuses on the new window layer material and the thinner absorber layer of CdTe solar cells.In Chapter I,we have got a glimpse of the background and introduced the working principle and output characteristics of solar cells.The advantages,structure and current status of CdTe thin film solar cells have also been summarized.In Chapter II,we have discussed a new type of CdTe solar cell with Mg-doped zinc oxide(MZO)window layer.The performance of CdTe solar cells with MZO window layers was numerically simulated by SCAPS software.The results showed that the performance of the devices with the MZO film band gap of 3.6-3.7 eV was excellent.MZO films with different Mg contents and band gaps between 3.24 and 3.72 eV were fabricated and used as window layers for devices.Experimental results showed that with the increase of Mg content in MZO films,the efficiency had an increasing trend,which was consistent with the simulation results,but the overall efficiency was much lower than the simulated value.The gap between the Voc and the FF accounted for this phenomenon.CdTe solar cells with different thicknesses of MZO window layers were prepared,and the results showed that the performance of the device with 60 nm-MZO was the best.The effects of annealing temperature on the performance of MZO films were investigated and the results show that annealing treatment can significantly enhance the performance of the devices.To increase the carrier concentration,improve the crystal quality of the MZO film,reduce the defect density,and optimize the band alignment at the interface were the key to improving device performance.In Chapter III,we have discussed ultrathin CdTe solar cells.CdTe solar cells with absorber thickness from 50 to 200 nm were fabricated.Compared with the traditional CdS/CdTe devices,even if the thickness of absorption layer was reduced to?1%of the normal one,the Jsc can still maintain about 1/3 of the original,which was 8.2 mA/cm2.The Jsc and the Voc were found to decrease significantly with the thickness of the absorber layer decreasing.The decrease of the Jsc was mainly because of the insufficient light absorption.The reasons,which caused a decrease of Voc,,were also discussed.The leakage mechanism was analyzed and the contribution of ohmic shunting current to the total leakage current was found to increase with the thickness of the absorber layer decreasing.The device characteristics under weak light irradiance and at high temperatures were also investigated.In Chapter IV,we have discussed the fire-through A1 paste for PERC crystalline silicon solar cells.Four kinds of high-activity Al pastes which can fire through the anti-reflection layer and the passivation layer were prepared by a company.The experimental results showed that the specific contact resistivity decreased significantly,the FF was increased,and the Voc was decreased with the increase of the degree of firing through.Studies have shown that the ideal fire-through Al paste needs to avoid the reaction of the Al powder and the glass frit during the sintering process,and to ensure the uniformity of ablation.In Chapter V,the main research contents and conclusions of this thesis were summarized,and the future development trend of CdTe thin film solar cell was predicted.