PROGRESS IN PHOTOVOLTAICS RESEARCH AND APPLICATIONS, VOL. 4, 341-353 (1996)

Research Chemical Reaction Analysis of Copper Indium Selenization Sandeep Vernals2, Nese Orbey'32, Robert W. Birkmire** and T. W. Fraser Russell'92 ' Department of Chemical Engineering, Universify of Delaware, Newark, DE 19716, USA 21nstitute of Energy Conversion, Universify of Delaware, Newark, DE 19716. USA

A chemical reaction analysis of the selenization of copper indium layers to form copper indium diselenide is presented. Time-progressive selenization reactions were carried out in a laminar flow tubular reactor in a dilute HzSe atmosphere at 400°C. Copper, indium and copper-indium thin films were reacted for 1-60 min. The reactedjlms are analyzed by x-ray diflraction and atomic absorption spectrophotometry to identifv the chemical species present in the reacted films. A reaction network for film formation is proposed and data from time-progressive selenizations were analyzed to obtain species composition profiles. Rate expressions are postulated and a mathematical model for the selenization is developed. The behavior of the model is compared with the experimentally determined species compositions to obtain specific reaction rate constants.

INTRODUCTION

opper indium diselenide and related alloy thin-film solar modules have the potential for inexpensive electric power generation. The current challenge is to manufacture such modules at a C low enough cost to produce electricity in the 10-15 cents per kWh range. Impressive gains have

been made in small-area device efficiencies, now between 15 and 17%.14 The CuInSe2 thin films used for these high-efficiency solar cells have been made by multi-source elemental evaporation of Cu, In, Ga and Se onto heated substrates at low pressures in a batch physical vapor deposition (PVD) reactor or by the reaction of Cu-In-Ga metal precursors in an Se-containing atmosphere. However, there are no commercial-scale CuInSe2 manufacturing facilities operating today. Part of the reason for this is an inability to 'scale-up' the cm2 laboratory cell processing to m2 commercial-scale panel manufacturing. It is a non-trivial task to design a commercial-scale reactor that is capable of producing a thin, uniform polycrystalline film which will make a high-efficiency 40-50 W panel. For rational 'scale-up' a reactor and reaction analysis of the laboratory reactors is needed.

In this paper a reaction analysis is presented for the formation of CuInSe2 in a tubular chemical vapor deposition (CVD) reactor where copper-indium bilayers are reacted in flowing H2Se. For the reaction analysis: the pertinent chemical species which take part in the reaction to form CuInSez layers must be identified; reaction rate rate expressions for each reaction must be postulated; the form of the rate expression must be verified; and the reaction rate constants must be obtained by comparing mass balance model equation predictions with species compositions at various times. The reaction analysis of film

* To whom correspondence should be addressed.

CCC 1062-7995/96/0S0341-13 01996 by John Wiley & Sons, Ltd

Received 16 April I996 Revised 10 July 1996

342 S. VERMA ET AL.

growth provides the quantitative information needed to form CuInSe2 and for the design of a commercial-scale process.

In the present work, pure copper, pure indium and copper-indium bilayers are reacted with H2Se in a tubular CVD reactor. Samples are removed from the reactor at different times and analyzed by x-ray diffraction (XRD) to identify the species in the growing film and to obtain the volume fraction of each species. Using these data a reaction pathway for the growing film is proposed and a set of rate expressions consisting of coupled differential equations is developed. Rate constants are estimated by comparing model predictions with composition-time data obtained in the tubular CVD reactor.

EXPERIMENTAL

First, metal precursors to be selenized in the CVD reactor were prepared. Single layers of Cu and In and Cu-In bilayers were deposite$ by electron gun evaporation on a 25.4 x 25.4 mm2 molybdenum-coated soda-lime glass. About 2500 A of Cu was deposited followed by 6140 A of In to give a Cu/In atomic ratio of 0.9 and a final CuInSe2 thickness of about 2 pm. The Cu-In bilayers were annealed prior to selenization for 20 min at 150°C in a vacuum to form a two-phase mixture of CullIn9 and In. The presence of these phases was experimentally verified by XRD measurements, and this is in agreement with the reported phase diagram of this system.' The bilayers were placed in the reactor on graphite substrate holders to ensure a uniform temperature across the substrate.

To evaluate the growth of CuInSe2, Cu-In layers were reacted in a 3.4% HzSe in an Ar atmosphere for 1-45 min in a tubular CVD reactor. The reaction temperature was 400"C, since CuInSe2 solar cells with efficiencies of -12% were fabricated at this temperature. The reactor system was designed and constructed to safely handle the extremely toxic H2Se gas. A schematic diagram of the reactor set-up is given in Figure 1. The experimental set-up has a quartz, 50.8-mm diameter, 750-mm long reactor with a heating jacket, a gas-handling system and a waste treatment system for H2Se effluent. The tubular reactor was designed with a push-pull feed-through which allowed short reaction time experiments. High-purity (99.999%) HzSe and Ar gases from Matheson Gas Products were used in the experiments.

bar to remove excess moisture and oxygen. A residual gas analyzer (RGA) was used to check the desired vacuum level. The reactor was then filled with argon to atmospheric pressure. The temperature of the reactor was increased to 400°C and the reactant gas flow rates were established and allowed to stabilize for 10 min. Using the push-pull feed- through, the substrates were inserted into the heated zone of the quartz tube. At the end of the experimental run, the substrate was pulled out of the heated zone to the end of the quartz tube and allowed to cool in an Ar atmosphere to room temperature. The experimental set-up and procedure are given in detail by Verma.6 Both Cu-In bilayers and single layers of copper and indium were selenized to obtain the species concentration versus time data reported in this paper.

Prior to each run, the reactor was evacuated to pressures of < 2.6 x

Analysis of data

Each selenized sample was analyzed by XRD to identify the species in the growing film. A Phillips/ Norelco scanning 28 diffractometer modified for thin-film analysis was used. X-ray scans were performed for 28 = 10-90" for a step size of 0.05" and integrated for 2 s per step. To resolve closely spaced peaks, high-resolution scans were carried out at a step size of 0.02" integrated for 5 s per step. A typical scan is given in Figure 2. Phase identification in the XRD patterns was carried out by comparing with the standard d-spacing given in the Joint Council for Powder Diffraction Studies (JCPDS) card files. For the species for which standard JCPDS card files were not available, computer-generated d-spacings were used.738

The XRD spectra were also analyzed to obtain the relative amounts of the identified species. For a homogeneous sample showing no preferred orientation, the peak intensity is directly proportional to the

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344 S. VERMA ET AL.

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Figure 2. A sample XRD spectrum Cu-In bilayer selenized at 400°C for 15 min

volume fraction of the specie^.^ This relation was applied using single-phase thin-film standards to determine the volume fraction of individual species in the reacted Cu-In layers6 Thin films of Cu, In, CullIn9, In2Se and InSe2 with thickness of 150-850 nm were analyzed by XRD to establish a quantitative standard. The texture of the standard films was compared with that of reacted films to ensure the validity of the analysis. For all films analyzed, there were no detectable differences between the orientation of the standards and reacted films. The mass of the growing selenized film as a function of time was measured using an atomic absorption spectrophotometer (2380 Perkin Elmer). The elemental concentration of Cu, In and Se as a function of reaction time was determined from atomic absorption spectroscopy (AAS) measurements. From the elemental composition and the volume fraction of the species, the concentration of the species formed during reaction as a function of time was determined. The estimated accuracy of the concentration of the species is 15%.6

RESULTS

Copper film selenization The reaction of Cu films resulted in single-phase Cu2-,Se for all times from 1 min to 45 min. The following chemical reaction is proposed for the selenization of copper films

2Cu + Se =$ Cu2Se kl (1)

where kl is the rate constant (cm3 mol-' min-') for reaction (1) and CuzSe is written to approximate the Cul.ssSe phase actually observed in the reactions.

CHEMICAL REACTION ANALYSIS OF COPPER INDIUM SELENIZATION 345

Table I. Indium layer selenization-summary of XRD results (T = 400°C)

Time (min)

Phases observed

Metals Binary selenides

1.5 3 5 I

10 15 30 45

In InzSe, InSe InSe, In2Se3 InSe, In2Se3 In2Se3 In2Se3 In2Se3 In2Se3 In2Se3

Indium $film selenization A summary of the species present in the indium films after selenization at different times is presented in Table I. Analysis of the indium film selenized for 1.5min indicates the presence of In2Se, InSe and elemental indium. The indium film progressively reacts in the presence of selenium species to form In2Se and InSe. The dominant phase present in the indium film selenized for 3 rnin is InSe. The only other species identified in the film is In2Se3. Species In2Se, which was dominant at 1.5 min, is no longer observable in the film. At 5 min, the species present in the film include In2Se3 and some InSe. The InSe species is present in the film selenized for 5 min, but disappears in the film selenized for 10 min. At reaction times longer than 10 min, In2Se3 is the sole indium selenide present in the reacted film. Thus, the indium selenization is postulated to proceed through successive indium selenides InzSe, InSe and In2Se3.

Based on the species identified, the following reactions are proposed between indium and selenium for pure In layers

21n + Se + In2Se k2 (2)

In2Se + Se 2InSe k3 (3)

2InSe + Se + InzSe3 k4 (4)

we let k2, k3 and k4 denote the rate constants for reactions (2), (3) and (4), respectively.

Copper-indium bilayer selenization These experiments are carried out with films having a Cu-In ratio of 0.9. At the beginning of selenization, the film consists of liquid In with some dissolved Cu and a copper indium CuJn,, species which has been identified in room-temperature XRD measurements as Cul1Ing by comparison with a computer- generated diffraction Other researchers have also observed this Cul Jng species.10711 We believe that the Culling phase consists of small particles which are dispersed in the liquid In. The Cul lIng particle size distribution may well depend upon the time-temperature profile that the Cu-In film was subjected to during the heat-up to the reaction temperature. However, there are no experiments to determine this size distribution to date.

The species present in the films reacted in the selenization reactor for 1-45 rnin are summarized in Table 11. The film, reacted for 1 min, reveals the presence of the Culling alloy and InzSe. The film reacted for 2 min contains Culling, JnzSe, InSe and CuInSe2. The films selenized for times of > 10 min consist of CuInSe2 and InSe. The species InSe is detected in the film selenized for 2 rnin and persists up to 30 min, indicating a progressive selenization of In2Se to InSe, consistent with the reaction of pure indium layers.

346

Table 11. Copper-indium bilayer selenization-summary of XRD results (7' = 400°C)

S. VERMA ET AL.

Phases observed

Time Binary Ternary (min) Metals selenides selenides

1 CUIIIQ, In InZSe 2 cu111n9 InzSe, InSe CuInSez 5 CulJng In&, InSe CuInSez

10 InSe CuInSe2 15 InSe CuInSe2 30 InSe CuInSez 45 CuInSeZ

However, unlike in single-layer In selenization, no In2Se3 was observed. The film selenized for 45 min consists of only single-phase CuInSe2.

As indicated in Table 11, Se reacts very quickly with the liquid In to form In2Se and InSe. We know from the single-phase selenization of copper that Se reacts very rapidly to form Cu2Se. This must also occur with the dissolved Cu in the liquid In in the bilayer experiments. However, no Cu2Se is observed in the bilayer experiments. We believe that this may be due to one or more of the following reasons:

(i) very rapid reaction with indium selenides to form CuInSe2; (ii) its presence may be masked because all the major XRD peaks for Cu2Se and CuInSe2 overlap; (iii) the amount formed is very small and below the detectable limit of the x-ray diffraction instrument.

We have included Cu2Se in the proposed chemical equations for the growing film. The CullIn9 is thought to dissociate as the amount of liquid In changes due to selenization. We are not

sure of the mechanism but a crude representation of the process can be shown by one of the following reactions

2CullIng + 29Se =+ llCu2Se + 18InSe kso

or

2CqlIng + 20Se + 1 lCu2Se + 9In2Se ks

The formation of CuInSe2 is fast. It is observed after 2 min of reaction. This leads us to postulate that it is formed by one of the following reactions

Cu2Se + InzSe + 2Se + 2CuInSe2 k6 (6)

Cu2Se + 2InSe + Se + 2CuInSe2 k7 (7)

These binary metal selenide precursors have been proposed before in the formation of CuInSe2. Number of moles of species versus time data will be used in an attempt to distinguish which of the above chemical equations is the most reasonable.

For Cu-In bilayer selenization, XRD analysis was used to estimate the mole fractions of various species in the film as a function of time.6 To compute the mass (or moles) of a species in the film at any time, the total mass of growing film must also be known. The mass of the film Mf, as determined by AAS, is given in Table III(a). The experimental data for moles of InSe, In2Se, CullIns and CuInSe2 as a function of time as a function of time are presented in Figure 4.

CHEMICAL REACTION ANALYSIS OF COPPER INDIUM SELENIZATION 347

Table III(a). The total mass of the film Mf, as determined by AAS

0 4.33 1.2 5.35 1.8 6.24 4.8 6.69

10.2 1.45 15 1.53 30 7.57 45 1.2

Table III(b). Estimated utilization of Se ~~~ ~~ ~~

Total moles of Se Moles of Se Moles of Se into reactor in the effluent in the film

8 x lo-' 1.2 10-~ 4 x 10-6

REACTION ANALYSIS

Model equations for film growth

A complete analysis of a CVD tubular reactor requires the gas phase, the surface phenomena and the growing film to be quantitatively studied. We have previously completed both an experimental and theoretical analysis of the gas p h a ~ e . ' ~ ? ' ~ The analysis shows that the gas phase dissociation of H2Se into various Se species is 1-10% at the reactor operating temperature and pressure.

The amount of Se in the growing film is measured experimentally with AAS for the entire reaction time; these values are presented in Table III(b). The molar flow rate of H2Se in our experiments is approximately 2 x mol min-'. About a thousand times more Se is supplied to the reactor than is needed for complete reaction in the film. This indicates that mass transfer from the gas phase to the growing film is not rate controlling.

At 400"C, the Cu-In film is a two-phase structure consisting of a solid CullIn9 and a liquid Cu-In phase. The liquid phase provides a pathway for Se to diffuse rapidly through the depth of the film. After the formation of Cu and In selenides, selenium diffusion in the growing film occurs through the bulk (or lattice), along the grain boundaries and through dislocations. Diffusion along the grain boundaries and dislocations is much faster than lattice diffusion and provides short-circuit paths for diffusion in polycrystalline thin films. Auger electron spectroscopy l4 results also show no significant concentration gradient in selenium through the depth of the samples in the bilayers selenized for 2, 5 and 30 min. We therefore assume that the Se diffusion in the growing film is fast enough to assume a constant Se concentration throughout the film.

For the growing film control volume (Figure 3), the mass balance for any ith species may be expressed as

(8) dni dt - = r d ( i ) - r , ( i ) + rmn(i)Vf

where r d ( i ) is the rate of delivery of species i to the film (mol min-'), r , ( i ) is the rate of removal of species i (mol min-'), ni is the number of moles, mmn(i) is the rate of reaction of the ith species (mol min-' ~ m - ~ ) , t is time of reaction and Vf is film volume (cm3). For all the species except selenium, the rate of delivery is zero. The rate of removal is also zero for all species except InzSe, which may have sufficient vapor pressure at typical selenization temperatures to evaporate. We have considered the amount of evaporated In2Se to

348 S. VERMA ET AL.

rate of removal rate of delivery

IdO)

rate of reaclion

I SUBSTRATE sodalime glass with 1p Mo

Figure 3. Control volume definition in fdm growth

be negligible, but this may not be the case at different reaction conditions. Thus, for In, InzSe, InSe, Cul 11119, Cu2Se and CuInSe2, Equation (8) reduces to

Equation (9) must be combined with the reaction rate expressions for each species. As a first approximation we consider the reactions to be first order with respect to the reactants involved. For example, for the depletion of In by reaction 2 the following rate expression is assumed

rrxn(In) = -2kz[Se][In] (10) Gas-phase analysis has established that the reacting selenium gas-phase species could be either H2Se or

one of the Sex species.” Both HzSe and Sex adsorbed from the gas phase dissociate on the film surface to form selenium species. In postulating the chemical equations, the reactant selenium species in the growing film is designated as ‘Se’ to represent any selenium species which may be present. For selenium, Equation (8) becomes

The overall mass balance equation for the growing film is

where p is film density (g ~ m - ~ ) and M Wse is molecular weight of selenium. The rate of delivery of selenium, rd(Se), can be estimated using Equation (12) and the experimentally

measured total mass of the film data (Table III(a)). The volume of the film as a function of time can be calculated through the knowledge of the mass and density of each species and assuming that the solid species form an ideal mixture. The selenium species concentration in the film cannot be measured with any available technique but the total amount of selenium available in the growing film is estimated to be in excess of the amount required for complete reaction of the film.6 The model predictions are done using Equation (9) for each species and the volume of the film and are compared with species concentration versus time data.

Analysis of jilm growth Combining Equations (2), (3) and (5)47), three sets of chemical equations can be considered to describe the reaction chemistry; these are presented in Table IV. Also presented in Table IV are the thermodynamic data available in the literatureI6 for the proposed reactions. The difference between Set 1 and Set 2 is in the reaction between CullIn9 and selenium: the indium selenide formed is assumed to be

CHEMICAL REACTION ANALYSIS OF COPPER INDIUM SELENIZATION 349

Table IV. Postulated chemical equations for CuInSez growth

AH, AGr

at 673 K (kJ mol-')

Set 1 2111 + Se + In2Se In2Se + Se =$ 2InSe 2Cui1 In9 -k 20Se * 1 lCu2Se + 91n2Se Cu$e + 2InSe + Se + 2CuInSe2 2111 + Se 3 IqSe InzSe + Se 2InSe 2Cui,Ing + 29Se + 11Cu2Se+ l8InSe Cu2Se + 2InSe + Se + 2CuInSe2 21x1 + Se + In2Se In2Se + Se =+ 2InSe 2CullIn9 + 20Se * 11CuZSe + 91n2Se Cu2Se + InzSe + Se + 2CuInSez Cu2Se + 2InSe + Se =$ 2CuInSel

Set 2

Set 3

-369 -340 -

-348 -369 -340

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-261 -246

-244 -261 -246

-244 -261 -246

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In2Se (Equation (5b)) in Set 1 and InSe (Equation (5a)) in Set 2. The difference between Set 1 and Set 3 is that in Set 3 an extra reaction between CuzSe and In2Se (Equation (6)) is considered for the final formation of CuInSe2 in addition to the reaction between CuzSe and InSe, both in the presence of selenium. The chemical equation set which best describes experimental composition profiles can be assumed to best represent the film growth.

For each proposed set of chemical equations, species mass balance equations (Equation (9)) were written and solved simultaneously to obtain the rate constants. The differential equations were solved numerically whenever an analytical solution was not possible. An optimization program which uses the simplex algorithm was employed to estimate the rate constants. The simplex algorithm is a multidimensional minimization for finding the minimum of an objective function of more than one independent variable. The objective function was defined to be the sum of squares of the difference between the experimental and predicted species concentrations for all data points. The program required the input of lower and higher limits for rate constants.

The mass balance equations for Set 1 are given in Table V. In this table, k! represents a modified rate constant (cm3 mol-I min-') which is the product of kj and the assumed constant selenium concentration. The mass balance equations for In, Cul ,In9 and In2Se (Table V) have analytical solutions. The required upper and lower limits for rate constants are obtained from these solutions. The values of the rate constants were optimized with the simplex algorithm to get the best fit for all species compositions. The values of the rate constants obtained for Set 1 are presented in Table VI. Figure 4 shows a comparison of the model predictions using these rate constants with the experimental data for Culling, InzSe, InSe and CuInSe2. The good agreement between the data and the predictions for all species is an indication that the proposed set of chemical reactions describes the system well.

Similarly, species conservation equations were written for Set 2 and Set 3 and the resulting differential equations were solved to optimize the values of the rate constants. In Set 2 for In, CullIn9 and In2Se, analytical solutions for mass balance equations are possible (see Appendix). Calculations showed that this set of equations cannot predict the experimental InzSe molar composition profile. This indicates that Equation (5b) rather than Equation (Sa) represents the reaction chemistry of Cul 11x19 and selenium.

The model predictions using Set 3 chemical equations are about the same as model predictions using Set 1 chemical equations. The values of rate constants for both sets are reported in Table VI.

The model curve for InSe species predicts higher values than the experimental data at long times. In our time-progressive experiments, the starting Cu/In ratio was 0.9, i.e. the film was indium rich. As the film reacted completely to form CuInSez, it absorbed the excess indium into a 7-phase which possesses the same crystal structure as CuInSez but is more accurately decribed as CuInl-,Sez+,. The ?-phase can

350

Table V. Reaction analysis model equations

S. VERMA ET AL.

toler

~ ~ ~ _ _ _ _ ~ ~ _ _ _ ~

-- d([lnl ’f) - - ~ c ~ [ ~ e ] [ ~ n ] Vf = - ~ c ; [ ~ n ] Vf dt

d([1n2se1Vf) = (kz[Se][In] - k3[Se][InzSe] + 9ks[Se][CullIqf)Vf =(k$[In] - k&Se] + 9k;[CullIq])Vf

d(‘lnselVr) = (Uc,[Se][InzSe] - 2k7[Se][InSe][Cu~Se])Vf = (*[InzSe] - 2k‘’[InSe][Cu2Se])Vr

d( [Cul l l~ lvf ) = = - ~ ~ ~ [ ~ u ~ l ~ n g l ~ f

-- d([clsl “I - ( k7 [ Se] [ InSe] [ Cu2 Se] ) Vf = (W7 [ InSe] [ C U ~ Se] ) Vr

dt

dt

dt

dt d([Cu Se V

dt = (llk~[Se][CullIq] - k7[Se][InSe][Cu2Se])Vf = ( l lk;[Cull I~] - k;[InSe][Cu2Se])Vf

some exc

Table VI. Rate constants for Cu-In selenization for chemical equation Sets 1 and 3 (T = 4 W C )

Rate constants Set 1 Set 3

k;(min-’) 1.5 1.5 k;(min-’) 0.3 0.3 k; (min- ‘ ) 0.2 0.2 kb(cm3 mol-’ min-’1 - 10-20 K7(cm’ mol-’ min-’) 26 2&26

9 indium and still exist as a single-phase chalcopyrite.” Our analysis d account the incorporation of excess In into the CuInSez but accounts for In as InSe.

e not take into

The reaction analysis can be used to generate moles of Cu2Se versus time profile (Figure 5) , which is useful because although we cannot measure the amount of Cu2Se experimentally, its presence in the film may affect electronic properties.

Solar cells of CuInSez Solar cells were made on Cu/In layers that were reacted for 60-90 min by depositing a thin, -50-nm CdS window layer by the chemical bath method’* followed by a sputtered ZnO layer. The best device had an active area efficiency of 12.0% with V, = 0.50V, FF = 72% and Jsc = 33.5 mA cm-2.

CONCLUSIONS

The reaction analysis of the time-progressive experiments has produced:

(i) an identification of the chemical species present in a growing CuInSez film as a function of time; (ii) the amount of each species in a growing CuInSe2 film as a function of time;

(iii) an experimentally verified mathematical model of CuInSe2 film growth; (iv) an estimate of the specific reaction rate constants for reaction of copper indium layers at 400°C using

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352 S. VERMA ET AL.

77me (mlnutes) Figure 5. Moles of CqSe as a function of time from model calculations (Set 1)

prediction with the experimental data suggests that in HzSe selenizations the assumption of constant selenium concentration in the growing film is sound.

Acknowledgements

The authors are indebted to Brian McCandless and the professional and technical staff at the Institute of Energy Conversion for their expertise in a number of areas crucial to the analysis described in this paper. This work has been supported in part by the National Renewable Energy Laboratory under subcontract no. XN-0-1023-1, Siemens Solar Industries and the Delaware Research Partnership Program.

APPENDZX

Analytical solutions to mass balance equations for In, CullIn9 and In2Se using Set 2 chemical reactions

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CHEMICAL REACTION ANALYSIS OF COPPER INDIUM SELENIZATION 353

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