Composition at the CuInSe2/ZnO interface: Copper depletion induced by diethyl-zinc

1 Introduction One of the basic requirements toobtain highly efficient chalcopyrite solar cells is a bufferlayer between the absorber and the sputtered ZnOwindow [1]. For one, the buffer is necessary to preventshuntsbetweenfrontandbackcontact,butmainlytoprovidea good structural and electronic contact between absorberand window material. Hence, the interface should exhibitminimum recombination and current loss and allow formaximum open-circuit voltage [2, 3].A good structural contact is achieved by using lattice-matched materials and can be further improved byinterdiffusion during contact formation. This also resultsin a good electronic contact since only few structural defectstates are present. In addition, the respective positions ofelectronic bands of the two materials should allow losslesselectronic transport and simultaneously not decrease theopen-circuit voltage. These band offsets are in turninfluenced by interface defects and interface dipoles. Thedetailed progression of valence- and conduction-band edgesis also influenced by interfacial intermixing [4].AsamodeltodescribethelowinterfacerecombinationforchalcopyritesolarcellswithCdSbuffer,theconceptofaburiedhomojunction inside the absorber has been discussed [5, 6].Here,theFermi-levelpositionisshiftedfromthevalenceband(p-type)forthebulkoftheabsorbertowardtheconductionbandattheinterface(n-type). Thiscouldbeachievedeitherduringtheabsorber growth process, with Cu-poor conditions duringthefinalgrowthsequence,orinducedbycontactformationwiththebuffer.Inthiscase,thebuffermaterialshouldfacilitatethetype inversion in the absorber near the interface region andthereby shift the charge neutrality region away from theinterface into the absorber.Apparently,thetypeinversionisassociatedwithacopperdepletionformingan-typeCuInSe