Study On Preparation And Performance Of Black Silicon And Black Silicon Solar Cells

As well known, the crystalline silicon solar cells take up most of photovoltaic market. Along withthe increasing requirement from human in the whole world, higher photoelectricity conversionefficiency of crystalline silicon solar cells is requested all the time. And, among all the ways forincreasing the efficiency, to reduce the reflectance of silicon surface is a much efficient one. Therefore,the concept of black silicon comes out. Black silicon is one kind of materials which can nearly absorball the light from ultraviolet to near-infrared, due to which the black silicon attracts extensive attentionfrom related fields. Also, the preparation methods and properties of black silicon become the hotresearching topics.In this thesis, we first used reactive ion etching (RIE) method with sulfur hexafluoride (SF6) andoxygen (O2) as reactive gases to prepare the black silicon on pyramid-structured single-crystallinesilicon (sc-Si) surface. Comparing with the current RIE processes, the experiments in this study werecarried out at room temperature, without pulse bias voltage and without any mask before the etching.The effects of different etching parameters on the properties of black silicon were studied. Also, weanalyzed the obtained results according to the mechanism of RIE process. The research resultsshowed that the etching power, flow ratio of SF6and O2(F SF6/FO2) as well as the etching timepresented very important effects on the morphology, surface reflectance and minority carrier lifetimeof black silicon. Finally, under the etching power of150W,F SF6/FO2of18sccm/6sccm andetching time of20min, we obtained a black silicon sample with a surface reflectance of4.94%in thewavelength range of400-900nm and this sample showed a minority carrier lifetime of8μs.However, the RIE process needs to be carried out in vacuum environment. Besides, the price of aRIE facility is too expensive and it needs a long preparation period. In consideration of thesedisadvantages of RIE process for fabricating black silicon, it is not suitable for low-costindustrialization. Therefore, we selected the metal-assisted chemical etching (MACE) method toprepare the black silicon due to its simple process, short preparation period and low cost. However,the current used metals are all noble ones, such as platinum (Pt) and gold (Au). Therefore, in order tolower the cost of MACE process for fabricating black silicon, we used nickel (Ni) instead of Pt andAu as assisted-metal. The effects of different process parameters on the properties of black siliconwere studied and the formation mechanism of black silicon by Ni-assisted chemical etching was alsoinvestigated according to the experimental results. The results showed that the etching temperature,H2O2concentration and etching time presented important effects on the morphologies, surface reflectance and PL property of black silicon. After the optimization of process parameters, a blacksc-Si with a reflectance of1.59%from400nm to900nm was obtained. Besides, according to theexperimental results, we found that Ni could be etched off during the assisted etching process, whichindicated that the transfer rate of electrons from Si to Ni is slower than that from Ni to O-. The reasonfor this phenomenon is that the electronegativity of Ni is1.91, which is just a little higher than that ofsilicon (1.90) in contrast to the phenomena happened when using higher electronegativity metal likePt (2.28) or Au (2.54) as assist metals. Additionally, it was found that the etching direction tended to<111>.On the other side, the forementioned Ni nanoparticles (Ni NPs) were fabricated by magnetronsputtering method, which is not a low-cost one. Consequently, we used chemical method to fabricatethe Ni NPs. But only Ni thin film was formed by chemical method instead of nanoparticle shape andthe Ni film fell off easily from the silicon wafer. Therefore, in order to lower the preparation cost,silver (Ag) was chosen as the assisted metal for the fabrication of black silicon on pyramid-structuredsc-Si surface and the effects of different parameters on the properties of black silicon was investigated.The results showed that the morphologies, surface reflectance and photoluminescence (PL) propertyof black silicon were greatly affected by the deposition time of Ag nanoparticles, H2O2concentration,etching temperature and etching time. In addition, the anti-reflective mechanisms of siliconnanostructures with different etching depth and different etching diameter were investigated accordingto the experimental results. Finally, after the optimization of etching parameters, black sc-Si withreflectance of0.98%from400nm to900nm was obtained. In order to lower cost further and simplifythe preparation process, we used one-step etching process and reduced the AgNO3concentration to1/200of the original one. Finally, black sc-Si with reflectance of1.11%from400nm to900nm wasobtained.In the current photovoltaic market, multi-crystalline silicon (mc-Si) solar cell takes up more thansc-Si solar cell, which is due to the lower cost of mc-Si. Besides, in the aspect of surface reflectance,pyramid-structured sc-Si is13%and4%before and after the deposition of silicon nitride (SiNx) from400nm to900nm. However, the reflectance of acid-textured mc-Si wafer is higher than20%and itstill shows10%after the deposition of SiNx. Consequently, the surface reflectance of mc-Si can bereduced much more than that of sc-Si. Therefore, in consideration of the price and reflectance, weused Ag-assisted chemical etching method to fabricate the black mc-Si solar cell. However, there wasmuch non-well covered silicon surface which was not covered with SiNx layer due to the pores withsmall diameter by Ag-assisted etching. In other words, the non-well covered silicon surface was therecombination centers of carriers. As a result, dilute sodium hydroxide solution (NaOH) was used to widen the etched pores for a better passivation of the bottom of pores. The effects of NaOH treatmenttime on the morphologies and optical property were investigated. Finally, black mc-Si solar cells withbest conversion efficiency of18.03%were fabricated by Ag-assisted chemical etching method, ofwhich the Voc，Jsc and FF were632mV,36.08mA/cm2and79.07%, respectively.And, the efficiencyof18.03%is0.64%higher than that of the traditional mc-Si solar cell.