磁控溅射法制备NiO_(x)及单面晶硅异质结太阳电池

RF magnetron sputtered NiO_(x) and NiO_(x)/c-Si single-side heterojunction solar cells

NiO_(x)为宽带隙p型半导体材料,其能带结构更适于用作c-Si(n)异质结空穴传输层.为了简化问题,本文研究了NiO_(x)与Si构成的单面异质结无背场太阳电池,其结构为Al栅/ITO/NiO_(x)/Si O_(x)/c-Si(n)/Si O_(x)/Al.通过研究不同溅射参数下NiO_(x)材料的光学、电学及能带结构,分析NiO_(x)/c-Si异质结的载流子输运及界面复合机制.研究表明异质结价带失调值ΔE_(V)的势垒高度及界面态是影响电池性能的关键因素.结合实验与AFORS-HET软件仿真结果,本研究提出提高器件性能的两个途径:一是降低NiO_(x)/c-Si价带失调值ΔE_(V)及界面态密度;二是提高发射区NiO_(x)受主浓度,增强内建电场.本文为研究新型高效NiO_(x)/c-Si异质结太阳电池提供参考并指出了方向.

NiO_(x) is a p-type semiconductor with wide-band gap (3.6–4.0 e V),and the energy band structure at NiO_(x) /c-Si(n) interface is suitable for c-Si(n) heterojunction solar cell.On the one hand,the big conduction band offset forms a reflection barrier of electron that reduces the interface electron concentration and interface recombination.On the other hand,the valance band offset will change with the deposition conditions,which lower the hole transport barrier and make the hole transport smoothly.Therefore,NiO_(x) is a promising hole-selective contact layer for c-Si(n) heterojunction solar cell.In this work,in order to simplify the study,we fabricate single-side heterojunction solar cell with Al-grid/ITO/NiO_(x) /SiO_(x) /c-Si(n)/SiO_(x) /Al-electrode structure.The n-type c-Si(100) wafers with resistivity of 1–10Ωcm and thickness of 200μm were firstly dipped in KOH solution (25wt%) at 80°C for 4 min to remove the surface sawing damage,then standardized RCA1 procedure and DHF (6wt%) dip were adopted to remove the contaminants and native oxide presented on the wafer surface.The SiO_(x) layer was grown in 5wt%H_(2)O_(2)at 80°C for 20 min.Afterward,a NiO_(x) thin film with thickness of 10 nm was deposited on the wafer front side using RF magnetron sputtering,and a 100 nm thick ITO layer was deposited on NiO_(x) by DC magnetron sputtering.Then Al-grid on the front and Al-electrode on the back were deposited by electron-beam evaporation.Finally,completed NiO_(x) /c-Si(n) single-side heterojunction solar cells were characterized by light/dark J-V(SAN EI XEC 500M2 solar simulator,KEYTHLEY 2400 source-meter).We investigate the optical,electronical properties and energy band of NiO_(x) thin films by changing the sputtering conditions,analyze the carrier transport mechanisms and the interface recombination mechanisms of NiO_(x) /c-Si heterojunction.And we find that the valance band offset is the key factor that affects cell performance.The increase of the valance band offset leads to the increase of the series resistance (R_(s)) of the cell,thus reducing the fill factor (FF) and power conversion efficiency (PCE) of the cell (cell Sample B,E and A).When the valance band offset is high and almost the same,the lower the resistivity of NiO_(x) thin film is,the lower the series resistance (R_(s)) of the cell becomes,which contributes to the higher FF and the higher PCE of the cell.This is because the lower resistivity of NiO_(x) thin film narrows the width of the blocking barrier of the hole and increases the tunneling current (cell Sample A and B).In addition,the experiment and the simulation results (AFORS-HET) show that there are two ways to enhance the cell performance.One is to reduce the valence band offset and interface defects of NiO_(x) /c-Si heterojunction so as to lower the interface recombination.The other one is to increase the doping density in NiO_(x) that increase the built-in electric field.Therefore,the passivation of NiO_(x) /c-Si(n) interface,the improvement of acceptor concentration of NiO_(x) emitter,the optimization of back surface field and ITO/NiO_(x) contact are the main directions for further research to achieve high efficiency NiO_(x) /c-Si heterojunction solar cells.