Projects

Polycrystalline Thin Films

The University of Toledo

Publications

Project Objective:  Explore the opportunities and identify the potential difficulties of II-VI materials (primarily CdZnTe) for top cells in double-junction devices with either two-terminal or four-terminal structures.  Two types of bottom cell materials classes will be utilized and analyzed for their suitability and compatibility with the top cell.  These bottom cells will include copper indium diselenide and HgCdTe.  The goal of this effort is to identify pathways, based on II-VI semiconductor top cells, which can lead to the goal of 25% efficient tandem structures based on polycrystalline thin films.

Approach/Background:  During Year1, Univerity of Toledo (UT) is investigating RF sputter-deposited films of the ternary II-VI alloys with band gaps larger than CdTe for use as a top cell in a double-junction, tandem structure.  UT's lower-tier subcontractor, First Solar is studying CdTe and CdZnTe deposited by vapor transport deposition.  This provides complementary studies involving a moderate temperature deposition process, RF sputtering (typically less than 300° C), and a higher temperature process, VTD (typically above 550° C).  A major effort is being placed on the development of a transparent back contact for CdTe with the initial emphasis on rf sputtered ZnTe doped by reactive sputtering with nitrogen.  The University of Toledo, with assistance of First Solar, is developing and fabricating complete structures which include the bottom cell as well. Efforts on four-terminal devices will be on developing and optimizing transparent back contacts for the top cell and on reducing top-cell window-layer absorption, which are critical for tandem cell performance. 

By the end of Year I, a best-effort four-terminal, laminated device will be fabricated based on a CdZnTe or CdTe top cell (with transparent contact) and a CI(G)S bottom cell.  The bottom cell will be obtained from an independent laboratory or supplier.

During Year II, effort will continue on reducing top-cell window-layer absorption through studies on high-resistivity buffer layers, on optimization of the wide-gap top cell absorber layer, and on fabrication of recombination junctions between a CI(G)S bottom cell and the top cell.  Effort will also be made to evaluate the suitability of sputter deposition of HgCdTe ternary alloys as a bottom cell.  By the end of Year II, UT with assistance of First Solar expect to fabricate a prototype two-terminal, double-junction tandem device.

Status/Accomplishments:  Nitrogen-doped ZnTe produced by reactive sputtering is attractive as a possible component layer of a back contact for CdTe-based solar cells.  It has the advantages of being free of copper, having a close valence band match to CdTe, and can be doped heavily p-type with substitutional nitrogen.  In addition, because it is transparent to photons below 2.2 eV, it is a candidate for a back contact/tunnel junction in tandem cells using CdTe or CdZnTe top cells.  Up to this point we have characterized films of ZnTe:N sputtered with various N₂/Ar ratios.  These films have been studied by x-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy, optical absorption, variable angle spectroscopic ellipsometry (VASE) and the Hall effect.  We also report optical emission spectra of N₂ during reactive sputtering.

The optical emission spectra indicate high concentrations of the first positive band of N₂ (B ³P_g ® A ³S_u).  The strongest vibrational transition is for v'=9 ®v"=5 at 590.6 nm.  This vibrational state lies only 2.8 eV below the 10 eV dissociation energy of N₂.  Since the ³S_u state is metastable, significant density of these metastable nitrogen molecules should impact the growth surface.  Due to its low dissociation energy we expect better incorporation of atomic N from the metastable N₂ than from ground state N₂.  In fact, it is likely that the excited molecular species are incorporated more easily into the growing film than even atomic N since for the atomic species, the binding energy of N in ZnTe can only be released via phonons.  By contrast, the second atom of the molecular species can carry away the binding energy as kinetic energy.

The XRD and AFM show decreasing grain size (from about 130 nm to 10 nm) as the N₂ content increases from 0 to 5% in the sputter gas.  The crystallographic texture changes from predominantly <111> undoped to slightly <220> at high nitrogen concentration.  Electrical conductivity is p-type decreasing from about 10⁵ Ohm-cm to 5 Ohm-cm with increasing N₂ in the sputter gas.  The carrier concentration has been observed up to ~5 x 10¹⁸ cm^-3.  However, mobilities are low in these small-grained films.  Effort is being made to improve the mobility and grain size.  Raman scattering indicates some evidence for an amorphous fraction at the high nitrogen concentrations in the sputter gas.  The optical transmission, without correction for reflection, of these doped films on glass is above 75% for wavelengths above the band gap.

Currently these ZnTe:N films are being used to prepare bilayers with sputtered, n-type ZnO:Al.  These structures are being used to determine suitability as recombination layers between Cd(Zn)Te and CI(G)S junctions.

Planned FY 2002 Activities:   During FY 2002 (second half of Year I and beginning of Year II), major efforts will be placed on RF sputtering of CdZnTe alloy films and the optimization of post-deposition chloride treatments of the films to optimize the optical and electrical properties.  Further efforts will be made to use reduced CdS thickness in the top cell in combination with suitable high resistivity transparent conductors, in processes compatible with vapor transport deposition.  Major efforts will be placed on fabricating functional recombination junctions from polycrystalline ZnTe:N/ZnO:Al bilayers and a start will be made on the incorporation of these into prototype tandem, two-terminal cell structures of CdZnTe and CIGS.  The effort will include theoretical analyses of the role of defects in tunnel junctions or recombination junctions of this type.  A sputter system will be set up for deposition of HgCdTe films with the goal of fabricating prototype HgCdTe bottom cells on wide gap CdTe-based top cells on glass in the normal, inverted structure.

University of Toledo High-Performance PV Publications:
"Optical, structural and transport properties of reactively sputtered ZnTe:N," National Center for Photovoltaics Program Review Meeting, Oct. 14-17, 2001 (Lakewood, CO). (PDF 198 KB) 

J. Drayton, C. Taylor, A. Gupta, R.G. Bohn, A.D. Compaan, B.E. McCandless, and D. Rose

"Properties of RF Sputtered ZnTe:N films for Back Contact to CdS/CdTE Solar Cells," Mat. Res. Soc. Symp. Proc. 668, I-VI Compound Semiconductor Photovoltaic Materials, ed. By. R. Noufi, R.W. Birkmire, D. Lincot, and H.W. Schock (Symposium H, MRS spring meeting Apr. 2001) (to be published). 

J. Drayton, A. Gupta, K. Makhratchev, K.J. Price, R.G. Bohn, and A.D. Compaan