Electrochemical N-type Doping In Metal Oxides And Its Application In Photovoltaics

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Electrochemical N-type Doping In Metal Oxides And Its Application In Photovoltaics

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Title: Electrochemical N-type Doping In Metal Oxides And Its Application In Photovoltaics
Author: Han, Xiaofei
Abstract: Electrodeposition has the advantages of low-cost, high-throughput and large-area processing which is particularly suitable for solar cells fabrication. Two common metal oxides prepared by electrodeposition are discussed here, ZnO and Cu₂O. ZnO is largely used in the solar cells as the window and transparent contact material. Cu₂O is potential absorption material for next generation solar cell. n-type doping of these metal oxides was revealed during electrochemical deposition. For naturally n-type ZnO, n-type doping is achieved by substituting the cation (Zn) with a group III element (Al, Ga and Y) and for naturally p-type Cu₂O, n-type doping is realized by substituting the anion (O) with a halogen (Cl). In both cases, the doping mechanism is believed to be co-precipitation of either ZnO with group III metal oxide (Al₂O₃, Ga₂O₃ and Y₂O₃) or Cu₂O with Cu halide (CuCl). Cl doped Cu₂O films were electrodeposited on Cu coated glass substrate. Photocurrent measurement determined its n-type conductivity. The resistivity of Cl doped Cu₂O was calculated through IV relationship and the lowest resistivity of Cl-doped n-type Cu₂O is around 7 Ω-cm which is suitable for solar cell application. Photocurrent measurement confirms the conductivity of this Cl-doped Cu₂O is n-type. XRD shows the Cl-doped Cu₂O is pure Cu₂O and SEM shows the grain of Cu₂O is small, about 100 nm. Once p-type Cu₂O is realized, a p-n homojuction Cu₂O cell with reasonable high efficiency can be achieved. Al, Ga and Y doped ZnO deposited by electrodeposition shows high transmittance, low absorbance and low resistivity. The minimum sheet resistance is 1.5 Ω with Y doped ZnO and the co-responding resistivity is 6.3 x 10-5 Ω-cm calculated by a parallel circuit model from stack sheet resistance. Different annealing conditions were used for lowest resistivity of ZnO realization. A UV-vis spectrum was used for transmittance and absorbance observation. XRD profiles show the ZnO doesn't mix with ITO after annealing and the SEM images show Y doped ZnO is hexagonal shape with sub-micro grain size. Thermal stability test shows this Y doped ZnO can still maintain low resistivity and high reflectance after 500 °C annealing in N2 ambient. This low resistance and high transmittance ZnO can be used as either top transparent contact or back reflector in the thin film solar cell.
Date: 2011-07-14

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