Preparation and Characterization of Cu 2 Zn(Ge x Sn 1-x )Se 4 Thin-films from Sputtered Elemental Precursors
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Preparation and Characterization of Cu2Zn(GexSn1-x)Se4 Thin-films from Sputtered Elemental Precursors Antony Jan, Yesheng Yee, Bruce M. Clemens Stanford University, 476 Lomita Mall Stanford, CA 94305, U.S.A. ABSTRACT Thin-film absorber layers for photovoltaics have attracted much attention for their potential for low cost per unit power generation, due both to reduced material consumption and to higher tolerance for defects such as grain boundaries. Cu2ZnGeSe4 (CZGSe) comprises one such material system which has a near-optimal direct band gap of 1.6 eV for absorption of the solar spectrum, and is made primarily from earth-abundant elements. CZGSe metallic precursor films were sputtered from Cu, Zn, and Ge onto Mo-coated soda lime glass substrates. These were then selenized in a two-zone close-space sublimation furnace using elemental Se as the source, with temperatures in the range of 400 to 500 C, and at a variety of background pressures. Films approximately 1-1.5 µm thick were obtained with the expected stannite crystal structure. Next, Cu2ZnSnSe4 (CZTSe), which has a direct band gap of 1.0 eV, was prepared in a similar manner and combined with CZGSe as either compositionally homogeneous or layered absorbers. The compositional uniformity of selenide absorbers made by selenizing compositionally homogeneous Cu-Zn-Ge-Sn precursor layers was determined and the band gap as a function of composition was investigated in order to demonstrate that the band gap is tuneable for a range of compositions. For layered Cu-Zn-Ge/Cu-Zn-Sn precursor films, the composition profile was measured before and after selenization to assess the stability of the layered structure, and its applicability for forming a band-gap-graded device for improved current collection. INTRODUCTION In the solar industry the primary figure of merit is energy generated per unit of cost for a solar panel. There continues to be growing interest in thin film absorber layers for photovoltaics, due to their several inherent advantages over conventional PV films. Since silicon has an indirect band gap it doesn’t absorb light strongly, requiring thicker films typically on the order of hundreds of microns; on the other hand, materials with a direct band gap such as GaAs, CdTe, or CIGS need only be just 1-2 µm thick, reducing cost. Furthermore, a thinner film allows for reduced material usage, deposition on flexible substrates, and potentially lower installation costs, all leading to cost savings in module production. There has also been a drive to move away from expensive, rare-earth elemental components such as Te and In, in CdTe and CIGS respectively, towards more earth-abundant materials. CZT(S,Se), a kesterite with a tetragonal crystal structure, is one such direct band gap material utilizing the relatively common elements Cu, Zn, Sn, S, and Se. [1] CZT(S,Se) has already shown great promise, reaching record efficiencies of 12.6% in a relatively short time. [2] However, to really improve the energy/cost figure of merit, the efficiency of the device needs to be imp
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