Studies Of The Passivation Of HIT Solar Cells And ZnO Transparent Conductive Oxide

The shortage of silicon raw material becomes more and more serious with the development of PV industry. Low-cost, high-efficiency and high-stability solar cells turn out to be the dominating research direction. a-Si/c-Si heterojunction solar cells have been attracting more attention because of low processing temperature, high conversion efficiency and low temperature coefficient. In this thesis, something, especially the surface passivation and ZnO transparent conductive oxide on HIT(Heterojunction with Intrinsic Thin Layer) solar cells are studied.The deposition of a-Si:H thin film is the core in the fabrication of HIT solar cells. The factors, such as deposition pressure, RF power, the dilution of hydrogen and gas-in mode of electrode, of affecting depositon rate of amorhous thin films are studied. With the increase of deposition pressure, the deposition rate of thin films increases. But the rate decreases after the pressure reaches a high value. The dependence of deposition rate on RF power is similar with that on deposition pressure. The higher the dilution of hydrogen is, the lower of the film deposition rate is. The depositon rate and the SiH4 decomposition in showerhead electrode are stronger than those in unilateral gas-in electrodes.The research of minority carrier lifetime after passivation is developed on the basis of the studies on the amorphous deposition rate. The thickness of intrinsic layer must be moderate. With the increase of deposition pressure the lifetime first increases and then decreases. The effect of RF power on lifetime is also in this trend. The minority carrier lifetime can be improved by hydrogen dilution. But if the dilution is too strong, the lifetime becomes worse. The minority carrier lifetime has great changes with the increase of hydrogen pre-treatment time. And the best treatment time is about 2 minutes. The lifetime can be improved a lot by the use of HF/O₃ cleaning system. The lifetime of polished wafer is larger than that of textured wafer.The performances of HIT solar cells are impacted directly by ZnO window layer. In this work, ZnO thin-films are prepared by radio-frequency magnetron sputtering technique at different sputtering Ar pressures and sputtering powers. The growth rates, electrical and optical properties of these films are analyzed. The result shows that the film deposited at sputtering Ar pressure of 4mTorr and sputtering power of 200W has a good compromise between the conductivity and optical transmittance. The V_oc of HIT solar cells becomes higher with these fabrication parameters of ZnO.The overall research of HIT solar cells based on the structure of ZnO/a-Si:H(p)/ a-Si:H(i)/c-Si(n)/ a-Si:H(i)/ a-Si:H(n)/ZnO/Al are investigated. The dependence of performances of solar cells on intrinsic layer, emitter layer and ZnO layer are researched by the measurement of QE and SunsV_oc of HIT solar cells. For the type of 125 polished wafers, the V_oc of all the HIT solar cells are above 700mV with the best deposition parameters. For the 125 textured wafers, the measurement of QE and SunsV_oc are analyzed. The results show that the QE of HIT solar cells are sensitive on the thickness of intrinsic and emitter layers because of the strong absorption of the amorphous for short-wavelength photons. The thickness of amorphous layer should not be too thick. With the introduction of back ZnO, the whole performances of HIT solar cells are enhanced. That indicates that the roll of the back ZnO is positive function.In conclusion, the pseudo efficiency of our best HIT solar cell is 12.79%.