Study On The Control Of Copper Electroplating By Self-assembled Monolayers

Copper electrode technology for crystalline silicon solar cells can greatly reduce the battery shadow loss,series resistance and material costs,compared to the traditional silver screen printing technique,which is the next generation of crystalline silicon solar cell electrode technology.The previous work of crystalline silicon solar cells without laser grooving in our laboratory is a technic based on the directly plating copper on the silicon solar cells,which simplifies the plateing process and reduces the damage caused by the laser and lithography process.However,for this without-opening pattern technology of Cu front electrodes,to achieve high aspect ration of the Cu front electrode is one of the key to reduce shadow loss of electrodes.In general,with regard to controlling the growth rate and orientation of Cu plating,the organic polymer additives are noramlly added to the electroplating solution to control the growth and surface roughness during the growth process.Nevertheless,controlling the lateral growth of Cu by directly using additives is a challenge.Self Assembled Monolayers(SAMs)are formed by the surface-active molecules,with specific affinity between the head group and the substrate,spontaneously adsorbed on the surface of the substrate.SAMs formation on the different substrates requires different organic molecules.For metal surface,to form SAMs,the thiols group are normally used mainly through the metal-S bond.For silicon oxide substrate,The organosilane molecules are normally used to form SAMs through a stable Si-O-Si bond.SAMs are widely applied in so ft-lithography and surface treatment.It has been reported that octadecanethiol-SAMs on Au substrates,after patterning by soft-lithography,can reduce the lateral growth of Co and achieve 1 μm non-lateral growth,suggesting that it is possible to using SAMs to control the lateral growth of Cu during electroplating.In this paper,the lateral controlling growth of copper electrode of crystalline silicon solar cells was investigated by introducing octadecyltrichlorosilane(OTS)SAMs.In the first part of thesis,the effects of different treatment methods of substrate and the aging time of SAMs were studied.The film formation of OTS-SAMs on Si substrate was analyzed by contact angle,infrared spectroscopy and atomic force microscopy.It was found that the OTS-SAMs assembled for 15 min both on the untreated native SiO2 and the SiO2 surface treated by H2O2 + H2SO4 can form uniform layer.The presence of OTS-SAMs based silicon substrate was investigated farther by cyclic voltammetry(CV)and electrochemical impedance spectroscopy(EIS).It was found that OTS-SAMs had a charge transfer barrier effect.In the second part of thesis,as demonstration,the Cu seed layer was formed by magnetron sputtering with mask patterning;then OTS-SAMs was formed on the silicon substrate and finally the plating of Cu electrode was conducted by both constant voltage and constant current modes to study the OTS-SAMs effect on the Cu growth.OTS-SAMs were used to control electroplating the Cu on silicon,and the morphology and width of copper electrode before and after OTS-SAMs were compared.The results indicate that OTS-SAMs affected the nucleation of Cu for constant voltage electroplating process,under the condition of high voltage and current,the lateral epitaxial growth of Cu is suppressed by OTS-SAMs.In this paper,OTS-SAMs were introduced to regulate the lateral growth of copper electrode of crystalline silicon solar cells.The influence of OTS-SAMs upon the growth of Cuon silicon was investigated.This study provides a method for increasing the aspect ratio of electroplated copper front electrodes.