B. J. Stanbery

Epitaxial growth and characterization of CuInSe2 crystallographic polytypes

Migration-enhanced epitaxy (MEE) has been successfully employed to grow epitaxial films of the ternary compound CuInSe2 on (001) GaAs that exhibit distinct coexisting domains of both a nonequilibrium crystallographic structure characterized by CuAu (CA) cation ordering, and the compound’s equilibrium chalcopyrite structure. X-ray diffraction, transmission electron diffraction, and Raman scattering data provide evidence for this structural polytype. Distinctive signatures of the CA polytype are found in the data from each of these methods, and their analyses are consistent with assignment of this crystallographic structure to the P4m2 space group. This structure is found to preferentially segregate into domains that constitute a distinct metastable phase, which may be stabilized by surface kinetic effects favored by the MEE growth process.

Study of Cd-free buffer layers using Inx(OH,S)y on CIGS solar cells

Abstract The alternative buffer layer material In x (OH,S) y was deposited on Cu(In,Ga)Se 2 (CIGS) thin films by chemical-bath-deposition (CBD). The impurities in In x (OH,S) y buffer layers and their atomic concentration were characterized by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses. In addition, AES was used to depth profile the samples. The band-gap energy of the deposited In x (OH,S) y was determined from optical absorption data. Both the dark- and photo-current-voltage ( I – V ) characteristics of the CIGS solar cells with In x (OH,S) y buffer layers were measured, and the results were compared to the CIGS cells deposited with CBD CdS buffer layers.

Role of sodium in the control of defect structures in CIS [solar cells]

The authors have grown CIS epitaxial films on single-crystal GaAs substrates under conditions that enhance the influence of surface effects on the resulting structures and their properties. There is a pronounced morphological dichotomy between indium-rich and copper-rich films. In addition, epilayers with nominally identical compositions and morphologies can exhibit fundamentally different ordering of the lattice in either the equilibrium chalcopyrite (CH) or metastable CuAu (CA) structure. The addition of sodium to In-rich CulnSe/sub 2/ epilayers during the initial stages of epitaxy both suppresses the formation of metastable Cu-CuInSe/sub 2/ and dramatically changes the film morphology. They discuss these results in the context of recent theories of island nucleation and their hypothesis that sodium acts as a surfactant during the growth of CuInSe/sub 2/ by destabilizing the (ln/sub Cu/+2V/sub Cu/) defect associate-complex in the near-surface transition layer, rejecting excess indium from the bulk of the growing film.

Reaction engineering and precursor film deposition for CIS synthesis

We present an analysis of alternative reaction pathways for the synthesis of CuInSe2-based films for photovoltaic applications based on our recent and ongoing investigations of the thermochemistry in the Cu-In-Se ternary, Na-Cu-In-Se quaternary, and constituent unary and binary systems. We also describe our efforts to determine the relationship between film growth conditions in our novel rotating-disc system and resultant phase constitution of precursor reactant films intended for subsequent ex-situ rapid thermal processing. A model for the phase chemistry of sodium in CIS films is presented.

Thermodynamic assessment of the Cu-In-Se system and application to thin film photovoltaics

A critical assessment of the thermochemical and phase diagram data for the CuInSe system has been initiated. This report includes assessment of the Se unary, InSe binary and CuSe binary systems. Based on these assessments and a previously published assessment (Kao et. al, 1993) for the CuIn system, a preliminary evaluation of the the Cu/sub 2/SeIn/sub 2/Se/sub 3/ pseudobinary phase diagram is proposed. The results of these analyses were applied to the calculation of the flux from a selenium effusion cell used for depositing CIS films and to the prediction of crystallization pathways during annealing of CIS precursor films.

XPS studies of sodium compound formation and surface segregation in CIGS thin films [solar cells]

The authors report X-ray photoelectron spectroscopy (XPS) studies of device-quality CIGS thin films grown on soda-lime glass substrates. The copper atomic concentration profiles within the near-surface region indicate copper depletion, a previously reported observation attributed to the formation of the ternary defect compound Cu(In,Ga)/sub 3/Se/sub 5/. Preferential surface segregation of sodium is also observed. Analyses of their data indicate that the group [I]/[III] atomic concentration ratio {[Cu]+[Na]}/{[In]+[Ga]} is constant to within the accuracy of the measurement. This result is strong evidence that the composition of CIGS films grown by multisource physical vapor deposition in the presence of sodium is thermochemically constrained to lie on the pseudobinary Na(In,Ga)Se/sub 2/-Cu(In,Ga)Se/sub 2/ tie-line in the ternary Na/sub 2/Se-Cu/sub 2/Se-(In,Ga)/sub 2/Se/sub 3/ composition diagram.

Effects of buffer layer processing on CIGS excess carrier lifetime: application of dual-beam optical modulation to process analysis [of solar cells]

A new nondestructive contactless dual-beam optical modulation (DBOM) technique has been developed for the characterization of effective excess carrier lifetimes in CuInGaSe/sub 2/ thin-film solar cells. Developed originally for lifetime measurement of SOI wafers, the technique represents a novel tool with the capabilities of direct measurement of the lifetime effects of intermediate processing steps during device fabrication without the confounding effects that subsequent processes might have on the final device characteristics. This is demonstrated in a controlled experiment intended to elucidate the mechanisms whereby CBD CdS processes improve device performance and to compare the CBD process with an MOCVD dry process. Areal mapping of excess carrier lifetimes across the samples is used to exhibit the substantial inhomogeneity of lifetime even within very high quality sample, demonstrating another capability of DBOM applicable to quality assessment and process optimization.

Investigation of buffer layer process on CIGS solar cells by dual beam optical modulation technique

A contactless, nondestructive reflection mode dual beam optical modulation (DBOM) technique has been employed to investigate the effects of different CdS buffer layer processings (i.e., CBD, MOCVD, and sputtering) on the properties of CIS/CIGS films. The results show a significant increase in the DBOM signal (/spl Delta/I/I) which is attributed to change of the free carrier absorption and of excess carrier lifetime in the absorber layer, after the buffer layer processing. In order to explain the observed DBOM results, an analytical model for the CIGS P-N junction cell is derived which contains the functional dependence of /spl Delta/I/I on the surface/interface recombination velocities, excess carrier lifetimes, diffusion length and depletion layer width in the CIS or CIGS cells. A detailed comparison of the DBOM data versus different processing is presented in this paper.

Long and Short Range Ordering of CuInSe2

Cu K-edge X-ray Absorption Fine Structure (XAFS) measurements were performed on CuInSe2 samples with different compositions near the AIBIIIC2VI stoichiometry and CuIn5Se8. The Cu-Se bond length was determined to be independent of the Cu content. This result is consistent with first-principle total energy calculations for this system.

Structure Investigations of Several In-rich (Cu 2 Se) x (In 2 Se 3 ) 1−x Compositions: From Local Structure to Long Range Order

The observed junction between α-CuInSe 2 and the In-rich compositions in the β-phase domain (e.g. CuIn 3 Se 5 ) appears to play an important role in the photovoltaic process [1]. There remain, however, inconsistencies and uncertainties about the boundary and structure of this phase. In general the structure of this phase belongs to defect tetrahedral family of structures [2], which can be described as normal tetrahedral structures with a certain fixed number of unoccupied structure sites. In this work, the local structures of various (Cu 2 Se) x (In 2 Se 3 ) 1−x semiconductor alloys in the β-phase domain were studied by Extended X-ray Absorption Fine Structure (EXAFS) and the results were compared to those for the α-CuInSe 2 phase. The long- range order of these compositions was studied by X-Ray powder Diffraction (XRD) and electron diffraction. It was found the local structures of these compounds are well defined. These compounds, however, could not be well described by any long-range order structure model, especially the selenium position. First-principles band structure calculations were performed to assist in assigning crystal structures to CuInSe 2 , CuIn 3 Se 5 and CuIn 5 Se 8 . The calculations indicated that the local environments of these compounds are well defined. Their long-range order might depend sensitively on growth history and the configurational entropies as suggested by the similar formation energies of several possible crystal structures for CuIn 3 Se 5 and CuIn 5 Se 8 .