*Corresponding author. E-mail:rpr@pss.fit.edu1Wilberforce University Scientist-in-Residence.

Solar Energy Materials & Solar Cells 57 (1999) 167—178

Electrodeposited CdS on CIS pn junctions

R.P. Raffaelle!,*, H. Forsell!, T. Potdevin!, R. Friedfeld!,J.G. Mantovani!, S.G. Bailey", S.M. Hubbard", E.M. Gordon",1,

A.F. Hepp"

! Department of Physics and Space Sciences, Florida Institute of Technology, 150 W. University Blvd.,Melbourne, FL 32901, USA

" NASA Lewis Research Center, Cleveland, OH 44135, USA

Received 1 July 1998

Abstract

We have been investigating the electrochemical deposition of thin films and junctions ofcadmium sulfide (CdS) and copper indium diselenide (CIS). We show that it is possible tofabricate pn junctions based on n-type CdS and p-type CIS entirely by electrodeposition. CIS isconsidered to be one of the best absorber materials for use in polycrystalline thin-filmphotovoltaic solar cells. CdS provides a closely lattice-matched window layer for CIS. Elec-trodeposition is a simple and inexpensive method for producing thin-film CdS and CIS. Wehave produced both p- and n-type CIS thin films, as well as a CdS on CIS pn junction viaelectrodeposition. Elemental analysis of the CdS and CIS thin films was performed using X-rayphotoelectron spectroscopy and energy dispersive spectroscopy. Optical band gaps weredetermined for these films using optical transmission spectroscopy. Carrier densities of the CISfilms as a function of their deposition voltage were determined from capacitance vs. voltagemeasurements using Al Schottky barriers. Current vs. voltage characteristics were measured forthe Al on CIS Schottky barriers and for the CdS on CIS pn junction. ( 1999 Elsevier ScienceB.V. All rights reserved.

Keywords: CuInSe2; CdS; Schottky barriers; Electrodeposition

0927-0248/99/$ — See front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 7 - 0 2 4 8 ( 9 8 ) 0 0 1 7 2 - X

1. Introduction

Thin-film polycrystalline copper indium diselenide (CIS) has several remarkableproperties which make it an attractive solar cell absorber material [1]. Its structurehas the ability to tolerate a large range of stoichiometries. The native defects asso-ciated with a deviation from stoichiometry allow the material to be doped bycontrolling its composition. The large number of defects that can be obtained in thismaterial remain relatively electrically benign. Therefore, it is possible to producepolycrystalline materials containing non-stoichiometric defects with properties sim-ilar to the single-crystal form. These properties make this an ideal material forinexpensive synthesis techniques where defect densities tend to be high.

Cadmium sulfide (CdS) is an ideal material for making a photovoltaic pn junctionwith p-type CIS. CdS is naturally n-type with a wide optical band gap (2.4 eV) andclose lattice match to CIS [2]. Conversion efficiencies of 17.7% have been achieved forchemical bath deposited CdS on physical vapor deposited CIS-based solar cell [3].Electrochemical deposition is a simple and inexpensive technique for producingthin-film CdS and CIS [4,5]. This method is less toxic and more easily scalable thantraditional vapor deposition methods used to produce these materials.

The stoichiometry of an electrodeposited thin film is controlled by its depositionpotential. The native defects resulting from non-stoichiometry in CIS will determineits minority- and majority-carrier types and their densities. CIS has remarkably stableelectrical properties over a wide range of stoichiometries due to electrically neutraldefect pairs [6]. CIS will change from n to p as the Cu to In ratio changes from lessthan one to greater than one, and the carrier density dramatically increases withdeviation from stoichiometry [7]. CIS thin films with different electrical and opticalproperties, as well as semiconductor type, can be deposited from the same aqueoussolution by simply changing the deposition voltage [8].

The optical band gap of CdS and CIS can be obtained from their optical absorptionspectrum using

a"A

hl(hl!E

')1@2, (1)

where a is the absorption coefficient, A is a constant, hv is the energy of the absorbedlight, and E

'is the band gap [9]. This equation applies to direct energy gap materials.

Schottky barriers on CIS and CdS on CIS pn junctions have rectifying current vs.voltage (I—») characteristics. The barrier height and ideality factor can be determinedfor a Schottky barrier from its I—» characteristics. The I—» characteristics of a pnjunction can be used to investigate surface effects, generation and recombination ofcarriers in the depletion region, and series resistance effects among others.

The theoretical barrier height for a Schottky contact is the difference between themetal work function and the semiconductor electron affinity. The experimentallydetermined barrier height often deviates from this ideal behavior. It is dominated bythe semiconductor surface states and is found to be independent of the metal workfunction [10]. Schottky barrier junctions have previously been produced on both n-and p-type CIS (Au and Al, respectively) [2].

168 R.P. Raffaelle et al./Solar Energy Materials & Solar Cells 57 (1999) 167—178

The current density vs. voltage for a Schottky barrier can be expressed as

J + JSe(qV@nkT) for »Ak¹/q, (2)

where JSis the saturation current density and n is the ideality factor [10]. The ideality

factor is very close to unity at low dopings and high temperature but can deviate fromthis when doping is increased or the temperature is lowered [11]. The barrier heightcan be determined using

/B/"

k¹

qln A

AH¹2

JSB, (3)

where A* is the effective Richardson constant which can be estimated using

AH"4pqmHk2

"h3

, (4)

where m* is the effective mass [10].The Shockley ideal pn junction diode equation follows the same form as Eq. (2)

above [10]. The ideality factor is determined under forward bias and is normallyfound to range from 1 to 2. It equals 1 when the diffusion current dominates andequals 2 when the recombination current dominates.

The capacitance of an ideal Schottky barrier as a function of reverse bias voltagecan be expressed as

C"ACqe4N

$/2A»"*

!Ak¹

q B!»BD1@2

, (5)

where A is the junction area, e4is the dielectric constant, N

$is the semiconductor

doping density, and »"*

is the built-in voltage [10]. The slope of 1/C2 vs. » can be usedto determine the carrier density. Doping variations in an epitaxial layer would resultin non-linearity of 1/C2 vs. ».

2. Experimental details

A series of approximately 1 lm thick films based on the CuxIn

2~xSe

2system were

electrodeposited from a single aqueous solution. The deposition potentials rangedfrom !1.0 to !1.3 V in 0.05 V increments as measured with respect to a saturatedcalomel electrode (SCE). The deposition potentials used to deposit the CIS thin filmswere based upon our previous work and chosen to give a range of p-type films nearstoichiometry [12]. The thin films were deposited on mechanically polished molyb-denum discs or indium tin oxide (ITO) coated glass. Mo makes a good ohmic contactwith CIS and does not react with the deposition solution used. The ITO coated glassprovides a transparent conductive substrate for optical studies. The deposition volt-ages used with the ITO subrtates were shifted by !1.0 V with respect to those forMo. The deposition solutions consisted of 1 mM CuSO

4, 10 mM In

2(SO

4)3, 5 mM

SeO2, and 25 mM Na-citrate.

R.P. Raffaelle et al. /Solar Energy Materials & Solar Cells 57 (1999) 167—178 169

CdS was deposited from a solution consisting of 2 mM CdSO4, 0.1 M Na

2S2O

3.

This solution is allowed to stir for 30 min, and then dilute sulfuric acid is added untila pH of 2.3 is obtained. The deposition voltages ranged from !0.8 to !1.5 V vs.SCE in 0.05 V increments. The CdS was deposited on Mo, Ni, ITO coated glass, andon as-deposited CIS thin films.

The depositions were generated and monitored by a Keithley 236 Source/Measure Unit interfaced to a personal computer and a EG & G 362 ScanningPotentiostat. The composition of the CdS and CIS films were determined byenergy dispersive spectroscopy (EDS) with ZAF standardless analysis. The com-position of the CdS films was also determined by X-ray photoelectron spectro-scopy (XPS). The XPS was combined with argon sputtering to depth profile thefilms.

The theoretical thickness of the films was determined by

¹"

1

nFAAitm

o B, (6)

where n is the number of electrons transferred, F is Faraday’s number, A is theelectrode area, i is the applied current, t is the deposition time, M is theformula weight, and o is the density [13]. For our calculations, we used the formulaweight (336.28 g/mol) and density (5.77 g/cm3) of intrinsic CIS [14]. This is anapproximation, since the formula weight and density vary with composition. Thenumber of electrons transferred was taken as 13 according to the total electrodereaction:

Cu2`#In3`#2SeO2~3

#12H#13e~PCuInSe2#6H

2O. (7)

For our cadmium sulphide calculations, we used a formula weight (144.46 g/mol)and density (4.82 g/cm3) [14]. The number of electrons transferred was taken as2 according to the following reaction:

Cd2`#S2O2~

3#2e~PCdS#SO2~

3. (8)

The absorption coefficient vs. photon energy of the films deposited on ITO wasdetermined from transmittance measurements in a Perkin-Elmer Lambda 19 spectro-photometer. Linear least-squares analysis was used to determine the optical bandgaps according to Eq. (1).

A CdS on CIS pn junction was attempted by first depositing a 1 lm thick p-typeCIS film using a deposition potential of !1.25 V vs. SCE. This film was thenremoved from the CIS bath, rinsed with distilled deionized water, and placed in theCdS solution, where a 1 lm thick CdS film was deposited at a voltage of !1.2 V vs.SCE.

The I—» characteristics of the Schottky barriers and pn junctions were measuredusing a signatone wafer probing station and a computer controlled Keithley 236Source/Measure Unit. The Keithley 236 was replaced by a Keithley 590 CV Analyzerin this setup to measure the capacitance vs. voltage (C—» ) behavior of the Schottkybarriers. The C—» measurements were performed at 1 kHz.

170 R.P. Raffaelle et al./Solar Energy Materials & Solar Cells 57 (1999) 167—178

Fig. 1. SEM micrograph of CIS electrodeposited at !1.1 V vs. SCE on mechanically polished Mo. Thewhite length scale bar corresponds to 1.0 lm.

3. Results

The cyclic voltammetry of the CdS solution with a Mo working electrode revealsa broad peak at !0.85 V vs. SCE. This peak presumably corresponds to the potentialfor the reduction of Cd. This peak did not shift when Ni, ITO coated glass, or CIScoated Mo electrodes were substituted for the Mo working electrode.

EDS on the CdS films was difficult due to the overlapping of the S k-line and theMo L-line of the substrate (i.e., 2.3 and 2.29 eV, respectively). However, qualitativeestimates based on the EDS results showed an increasing Cd/S ratio with morenegative deposition potentials.

XPS was used to determine the surface stoichiometries of the CdS films. XPS was alsoused in conjunction Ar sputtering to perform depth profiling. The XPS results on theCdS films were consistent with our qualitative estimates based on EDS. A one-to-oneCd to S ratio was obtained on a CdS film deposited at !1.2 V vs. SCE on Mo. ExcessCd was found in samples deposited at more negative potentials than !1.2 V vs. SCE.Depth profiling studies showed a slightly higher S content near the surface of the films.

We have previously shown that the cyclic voltammogram of the CIS depositionbath with a Mo working electrode has three peaks which correspond to the atomicconstituents (e.g., Cu at !0.4 V, Se at !0.8 V, In at 1.0 V vs. SCE) [15]. A shift inthese peaks of approximately !1.0 V vs. SCE was measured when an ITO coatedglass was substituted for Mo as the working electrode. Thus, we would expect thefilms deposited at !1.2 V vs. SCE on Mo to correlate with those deposited at!2.2 V vs. SCE on ITO coated glass.

EDS analysis on the CIS films deposited on Mo showed they had excess Se, withatomic percents between 56% and 58% for the range of deposition potentials used.The Cu to In ratio varied linearly over this range and decreased with increasingdeposition potential, which was consistent with our previous results [12].

R.P. Raffaelle et al. /Solar Energy Materials & Solar Cells 57 (1999) 167—178 171

Fig. 2. SEM micrograph of CdS electrodeposited at !0.85 V vs. SCE on mechanically polished Mo. Thewhite length scale bar corresponds to 1.0 lm.

SEM micrographs show that the series of CIS thin films on Mo were polycrystallineand dense with a sub-micron grain size and a uniform thickness (see Fig. 1). However,surface roughness increased with more negative deposition potentials. The CdS filmswere much smoother that their CIS counterparts with a grain size of approximately100 nm (see Fig. 2).

The XRD analysis of the CIS film deposited at !1.1 V vs. SCE on Mo showed thebasic chalcopyrite structure. The films had a predominant (1 1 2) orientation withrelatively weak intensities from other diffraction peaks. The XRD of CdS deposited at— 1.2 V vs. SCE on Ni had a geenockite structure, which is an altered zinc-blendestructure similar to wurtzite [10]. XRD of CdS films deposited at more negativedeposition potentials showed increasing peaks due to elemental Cd.

Analysis of the transmission spectra of the series of films deposited on ITO revealsa linear region of (ahl)2 vs. photon energy for both the CdS and CIS films on ITOcoated glass, indicating a direct energy gap (see Figs. 3 and 4). The band gap of theCdS was found to be 2.4 eV, which is consistent with literature values [2]. The opticalband gap for CIS deposited at !2.2 V vs. SCE on ITO was found to be 1.1 eV, whichalso is in good agreement with previous studies. The non-linear behavior below theenergy gap has been attributed to transitions involving defect states in the bandgap [16].

The I—» behavior of the Al contacts on CIS films deposited at less negativepotentials than !1.25 V vs. SCE showed the anticipated rectifying behavior indicat-ing a p-type film. A few of the Al contacts on the CIS films deposited at !1.3 V vs.SCE showed rectification, however most of these contacts were ohmic. This wouldindicate that the films are starting to become n-type at voltages at or more negativethan !1.3 V vs. SCE. The semilog plots of current density vs. voltage showed theanticipated linear behavior (see Fig. 5). Linear least-squares analysis was used to find

172 R.P. Raffaelle et al./Solar Energy Materials & Solar Cells 57 (1999) 167—178

Fig. 3. (ahl)2 vs. photon energy for CdS electrodeposited at !1.2 V vs. SCE on ITO coated glass. TheX-intercept of the linear least-squares fit to the linear portion of the graph yields an optical band gap of2.4 eV.

Fig. 4. (ahl)2 vs. photon energy for CIS electrodeposited at !2.2 V vs. SCE on ITO coated glass. TheX-intercept of the linear least-squares fit to the linear portion of the graph yields an optical band gap of1.1 eV.

R.P. Raffaelle et al. /Solar Energy Materials & Solar Cells 57 (1999) 167—178 173

Fig. 5. Natural log of the current density vs. voltage behavior for an Al Schottky barrier on CIS depositedat !1.2 V vs. SCE. The y-intercept of the linear least-squares fit to the linear portion of the graph yieldsa barrier height of 0.5 eV.

Fig. 6. Average Al Schottky barrier heights vs. CIS deposition potential. The error bars represent thestandard deviation of the mean from measurements of several junctions.

the y-intercepts for Ln (J) vs. » data measured from several junctions on each sample(see Fig. 6). These intercepts were used to determine the junction barrier heights.A value for the hole effective mass of 0.71 was used in our calculations [17]. Meanvalues for the measured barrier heights were between 0.45 and 0.62 eV. The ideality

174 R.P. Raffaelle et al./Solar Energy Materials & Solar Cells 57 (1999) 167—178

Fig. 7. Inverse capacitance squared vs. voltage for an Al on CIS Schottky barrier. The CIS was deposited at!1.2 V vs. SCE. The slope of the linear least-squares fit yields a carrier density of 1.1]1020 cm~3.

factors as determined from the slopes of the Ln (J) vs. » data were much greater thanone. The barrier heights are smaller than the theoretical prediction of 1.15 eV basedon an ideal junction and Al work function of 4.28 eV [10] and an electron affinity of4.48 eV for CIS [2]. The measured barrier heights of the Schottky barriers were foundto be insensitive to the CIS stoichiometry over the range of deposition potentials used.

The capacitance vs. voltage measurements of the Schottky barriers demonstratedlinear 1/C2 vs. » behavior for forward bias voltages up to 1.6 V (see Fig. 7). Theaverage carrier densities based on the linear least-square slopes of the 1/C2 vs. » datawere plotted vs. the deposition potential (see Fig. 8). The carrier densities ranged from9]1019 to 9]1020. These carrier densities were quite large and increased withincreasing Cu to In ratio (as found by EDS). These results were consistent withprevious measurements performed on vapor deposited CIS films [18]. A value of 8.1was used for the high-frequency dielectric constant in our calculations [17].

The I—» results of the CdS on CIS pn junction showed the anticipated rectifyingbehavior (see Fig. 9). Plotting the natural log of the current density vs. the appliedvoltage we are able to identify a characteristic generation—recombination regionfollowed by a diffusion current region under increasing forward bias (see Fig. 10).There is a considerable series resistance effect at large forward bias. There is alsoa significant reverse leakage current. This behavior is consistent with the largenumbers of defects indicated by the carrier densities of CIS found via our Schottkybarrier studies.

R.P. Raffaelle et al. /Solar Energy Materials & Solar Cells 57 (1999) 167—178 175

Fig. 8. Average carrier density vs. CIS deposition voltage. Error bars represent the mean standarddeviations of least-square fits to the 1/C2 vs. » data.

Fig. 9. Current density vs. voltage behavior of an electrodeposited CdS on CIS pn junction.

4. Conclusions

The electrodeposited CuxIn

2~xSe

2and Cd

xS1~x

films were shown to havestoichiometries which could be varied with deposition potential. The optical band

176 R.P. Raffaelle et al./Solar Energy Materials & Solar Cells 57 (1999) 167—178

Fig. 10. Natural log of the current density vs. voltage behavior of an electrodeposited CdS on CIS pnjunction: (a) generation—recombination region, (b) diffusion current region, (c) series resistance effect, and(d) reverse leakage current.

gaps for our CIS and CdS were approximately 1.1 and 2.4 eV, respectively, in goodagreement with previous studies. The semiconductor type of CIS as a function ofdeposition potential was verified by the rectifying behavior of Schottky barriers. Theresults were consistent with the type indicated by our EDS results and expected nativedefects in CIS. The carrier densities of the CIS films, as determined by the C—»behavior of these Schottky barriers, were found to increase dramatically with increas-ing Cu to In ratio. Current vs. voltage measurements confirmed the ability to deposita pn junction by electrodepositing a CdS thin film on a previously electrodepositedp-type CIS film.

Acknowledgements

We would like to thank F. Montegani and the Ohio Aerospace Institute (OAI) inconjunction with the NASA-ASEE program for the support of this work. We wouldalso like to thank O. Melendez and J. Hurley at NASA-Kennedy Space Center(NASA-KSC) for assistance in obtaining the XPS and XRD results.

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