'M-A11b 268 EIC LAIIS INC NEWT014 MA F/G 20/S

LASER SPOT SCANNING OF PHOTOELECTROCHEMICAL CELLS(U)AUG B2 M E LANGMUIA. R H MICHEELS N0OOIA 79-C 0700

,CLASSIFIED C-59 8 NL

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°,H

OFFICE OF NAVAL RESEARCH

Contract No. N00014-79-C-0700

Task No. NR 359-723

ATECHNICAL REPORT NO. 5

LASER SPOT SCANNING OF PHOTOELECTROCHEMICAL CELLS

by

M. E. Langmuir, R. H. Micheels, R. A. Boudreau and R. D. Rauh

Presented at

160th Meeting of The Electrochemical SocietySymposia on Photoelectrochemical Processes and

Measurement Techniques for Photoelectrochemical Solar CellsDenver, Colorado

October 11-16, 1981

EIC Laboratories, Inc.121 Chapel Street ,

Newton, Massachusetts 02158

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Margaret E. Langinuir, Ronald H. Micheels, N00014-79-C-0700Robert A. Boudreaua and R. David Rauh

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Paper Presented at the 160th RCS Meeting in Denver, Colorado; Symposia entitledPhotoelectrochomical Processes and Measurement Techniques for Photoelectro-chemical Solar Cells.

19. KEgY WORDS (Contmma. n reverve aide It necesary and Idmeutlf by Mlock number)

Photoelectrochemietry, Spot Scanning; Laser; Cadmum Selenide; Cell Structure;Surface Etch

21.A ST PACT (Con"h~. anmem ide It necessary and Ientify by blook W

Laser scanning of the photoelectrode in a photoelectrochemical cell in a power-ful diagnostic tool for assessing the effect of surface modifications of thesemiconductor electrode on call performance. Photocurrent scan aps ofphotoelectrodes with external and/or white light bias allow the study of theeffect of iR losses in the cell under operating condition.. A dye Laser addsversatility to the system by permitting wavelength dependent responses to be

DO ~'~147) EDITION or 1 NOV oSlS OSOLKTE NLWPDN

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2 Abstract (Cont.)

mapped. The possibility of time resolved measrments, using a pulsedlaser source, is also discussed pg

Aceession For

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aSu o M M CAIG Or FUO T ROCHEIUAL CELLS

U. 3. Langs.ir, R. N. Nichaels, R. A. oudreau and R. D. Pauh3C Laboratories, Inc., 111 Chapel Street, Nwton, i 02158

ABSmACT

Laser scanning of the photoelectrode in a photoelectrochem-Ical cell is a powerful diagnostic tool for assessing theeffect of surface modifications of the semiconductor electrodeon cell performance. Mhotocurrent scan aps of photoelectrodeswith external and/or white light bias allow the study of theeffect of iR looms in the cell under operating conditions. Adye laser adds versatility to the system by permitting wave-length depedet responses to be apped. The possibility of

,time resolved measurens, using a pulsed laser source, isalso discussed.s

2he method, which Involves the recording of photocurrent or photo-voltage developed In a sami ndactor as a spot of light is rasterscanned over the semiconductor surface, has been referred to in theliterature by a variety of descriptive names including scanning lightspot analysis (SLS) (1), scanning light microscopy (SM) (2) and simply,laser scanning (3). The scanning has been acomplished by translatingthe laser beam In both x and y directions with oscillating mirrors(2,3) or by translating the.stage on which the semlconductor or solarcell is mounted (1). The scanner which we describe here is of theformer type. By simply changing the position and focal length of thefocusing lens, resolution my be varied to study single grain bound-aries in polycrystalline semicondctors, or to uap losses in largearea solar cells. he use of laser scanning to study grain boundarieshas been described elsewhere (2,4) and will not be treated here. ..this paper, examples are given of the utility of the method for thestudy of 1) surface modifications of a thin fils CdSe photoelectrodeand 2) losses in photoelectrochemical cells (PECS) which can beascribed to cell design.

2he laser spot scanner system, pictured in Figure 1, is based ontwo galvanometer scanners (General Scanning G100-PD) and a 3.5 mW Heftlase= source attenuated with neutral density filters and focused witha plano-convex lens. Lenses of varying focal lengths were used fordifferent resolution and man range requirements. For high resolutionstudies, a 50 M focal length lens, Ll, placed after the scannermirrors, ms used which produced a spot with an l1p (lIe) diameter.For large cells a 200 m laos, L 2 , placed before the scanner Mirror,

I • ni I1

-7

was used instead. the scanner mIrrors were driven In a raster patternby two rap generators through two scan control amplifiers (GeneralScanning CC-0l-l). 1btcret was amplified and monitored with apotentiostat (haul 551) for fast scan rates and a picoamaeter CWeIthly480) for slow scan speeds. Sbotavoltage was monitored with a 100 k~rzbandwidth ampl if ier. The two-dimensional raster scanned poournand photavoltage images were either displayed an an oscilloscope, usedIn the x-y mdfor rapid scanning, or an an x-y chart recorder forslow scans. the oscilloscope imageis were produced byj modulating theCRT beam Intensity with the 'Ltcurn or photovoltag. signal. Mienthe x-y recorder uas employed, a "X-io ation" scheme uas used Inwhich the photocurrent or photovoltage signal is superimposed on theX-position input to the recorder with an op-amp adder, allowing quan-titative measurements to be medes. A pen lifting circuit us also usedwith the chart recorder to blank the retrace In the Y-scan direction.

In order to obtain additional Information from spot scanning, aN2 pumped, palsed dye laser (Rolectron ML 11) was employed as ascanner light source. The lamer wavelength uwe tuneable over the)range 360-940. na and the pulse width usx about 5 nsec. 1he pulsedphotovoltage signal warn averaged with either a boaccar integrator or alock-in amplifier depending on the signal decay time.

STUDY OF SURFACE NI~FZCTZQS CF TIM IU0'IOELC

The effect of surface modif ications of electrodeposited MCe anTi foil are shown in Figure 2. A 1 cm2, 4 micron thick film uas sub-jected to the following treatments In successions A, annealing at600OC in air for 15 minutes; 3, a 2-second etch with cowc. MCI: C,anodization for 3 seconds in 0.13 82 904 at 4. SW. The VZO parametersafter each treatment were measured In aqueous polysulfide electrolyteat 45 Wu/cm2 Intensity with an ZR filtered tungsten-halogen lamp(see Table 1). Figure 2 shows the corresponding laser scan mapsa and

a photograph of the electrode surface after trea-en C.

The annealed etlectrode had a low photoresponse with a slow timconstant. After the HMl etch, the response uas rapid and almost afactor of 10 higher, although same baret spots appeared as Indicated

bthe small dips In the photoresponse. The drastic reduction of theINC parameters after anodization were attributable to cracking andflaking off of the film as iow by a comparison of the scan map, andthe corresponding photograph of the electrode surface.

often, the changes In yhotoresponse with surface modification aresubtle and the meaamits at 130 parameters are very sensitive toexperimental conditions such as light Intensity at the photoelectrodeand reproducible positioning. If seve ral surface modif Ications -m

bmade on ems electrode and recrded an one scan map, this difficulty iis bviated. Figure 3 is an atmle of an array of etch sateps" on aIsingle chemtical bath deposited Cef film an titenass. Virtualy so

2

dif feren was observed at short circuit, but %tien the cell was heldat -0.4V# I.*., olose to the open circuit voltage, the etch stepscould be discerned. fths, chan In collection efficiency withsurface modif ication should be measured at small band bending. Ikeuse of fcrward bias has also been shown to be advantageous to detectdefects in solid state photocells (3).

Mnch interest has been show recently in the ZnCl 2 tIMent OfCase and CdS photoelectroems (S-8). 2he treat 1nt apparently 941-

enhances the photocurrent and the open circuit lphotovoltage. ToInvestigate whether it had any effect on subgap states usually presenton polished Cdse crystal murfacesx, a I cm2 single crystal was giventhe localized combination of surface treatments shown In Figure 4%.At a bias voltage of -0.45V vs. a CoB counter electrode In aqueouspolysulfiLde electrolyte, the photocurrent on the polished half of theelectrode was very small compared to that on the etched half. Figure43 shows the scan map of the etched portion of the crystal. An in-crease In photocurrent of MU10% is observed whien the beamt crosses Intothe region of YaiC12 treatment (indicated by an arrow in Figure 45).7

To determine whether hnCl 2 had any effect an mubgap states, theHR*e CIO laser was replaced with a 32-pumpod dye laser and a lok-inampl if ier . The normalized action spectra of a polished and etchedportion of the crystal are shown In Figure St. 2he response, for thepolished area is at least a factor of 10. smaller than that of theetched area and shows a subband gap absorption axizmu at about 760un in agresement with similar spectra reported by Beller et &1. (9).Using 760 un pulsed light fromi the dye laser we recorded two singlephotovoltage means to cover each of the 4 quadrants of the crystaland repeated the scans in the sawe positions with 588 n pulsed light.The average powe at the two wavelengths uas approximately 20 UNi at760 un and 100 jaW at 568 u, and the beam diameter uas about 50ImsIn these experiments. The seans at 760 arn do Indicate that the sob-band gap response is decreased by etching and the 588 un response isIncreased by etching. Ilie MaaC12 also appears to diminish slightlythe muband gap response in both polished and etched regions.

To further characterize the different regions of the crystal, thetime resolved rise and decay of the photovoltage signals caused byband gap and subband gap excitation were, recorded on an oscilloscopetriggered by the laser pulse. The mabband gap excited photovoltagerise and decay times, are not affected by etching but the rise timeof the band gap response Insowah faster In the etched than In thePolished area. 3h principle, it should also be possible to map thefast and slow response areas using a baccar integrator to record thesignal.

Although we cannot yet intezret with any certainty the observedIspectral and kinetic differences In ipbotovoltage response, the

3

sensitivity to surface treamet Indicates the participation of chargetraps, at or near the solid-liquid. Interface, which act an reccobla-tics sites for holes and electrons. bocitation of charge fro the"etraps to the conduction band, for exateple, could result In the wall-defilned subband gap response. The different mechanisms of filling andemptying these traps under su3b- and supraband gap excitation mayex~plain the observed differences In photovoltage transients.

31T1D C CELL 3S11

with additional modification of the basic scanner to provide forwhite light illumination of the photoelectrode while scanning with thechopped Ha~e beam, it ws possible to detect areas of ii dro In PIsas well as surface defects in the jphotoelectrode. This method hasused to analyse two possibl, practical frontwall cell formats for thinf ilm CdSe electrodes shown in Figure 7.

Cell X is a structure with holes In the planar photoelectrods, toenable ion flow to the rear-placed counter electrode. The photoelec-trade is of 4,l5 c=2 nominal area, produced by chemical bath depositionof CdSe on Ti foil. The scan map of a portion of this call, shown InFigure 9, was obtained using a secondary white light bias to enanceiR effects. in addition to revealing the dead areas resulting fromthe perforations, the map shows pinholes and several areas of enhanced

phoocuret scratches In the wjbstzatet, where the film is perhapsthicker or of greater rougbness. Cell polarization and As effectsshow up as slightly enhanced photocurrents immediately adjacent to theholes, and as a general negative curvature of the scans in areas betwethe holes.

Cell 11 contains a 100 cm2 CdSe photoelectrode, prepared b~y elecrdeposition on a louvered Ti substrate. In this case, the slots beneatlthe louvers provide the path for Ion flow. Figure S show a scan ampof the Illuminated portion of the electrode scanned vertically across3 of the louvers. At the right side of Figure 8 is a cross-sectionalview of the louvers. Hex izu photocurrent ocurs where the distancebetween the photoelectrode and the counter electrode is a miniam,i.e., at the flat edge. Areas of maxism loss are, of course, theslits between the louvers, indicated by A. This suggests that 1) thelouvers should be narrower to produce more edges, 2) the angle of bendshould he smaller, and 3) a different design In which the louversovrlap slightly would decrease'the loss area. other areas of loss smarkeda and C are due to bare spots on the Ti substrate.

CCCcwSIC

We have deonstrated that laser spot scanning is a useful toot forstuding the preparation of surfaces of semiconductors employjd In

RMooeectrochomical cells. Spot scanning is also an aid In designingpdactical gecmtrIes for Much devices. Clearly, however, the spot

4 _

scanning technique using liquid-semiconductor junctions goes beyondthe parochial concerns of solar cell studies. The technique can beemployed in the rapid evaluation of bulk and surface properties ofsemiconductors without the need of prior processing. The potentialexists for Interpreting localized responses to light in terms ofdiffusio length, surface recombination velocity, minority carrierlifetim and surface state density, for example, so that surfaceprofile of these properties my be constructed on as-grown aemicon-ductor materials. Hence, this measuremant technique can be used toevaluate semaconductors for a variety of device applications wherelateral homogeneity is Important. Spatial resolution is limited bythe diffraction limit of light, and has not been tested to date Inlaser spot scanning studies, but remains a fertile area for furtherstudy.

2his work was supported by the Depateant of Energy - Solar EnergyResearch Institute and by the Office of Naval Research.

1. T. E. Furtak, D. C. Canfield and S. A. Parkinson, 3. Appl. Phys.,51 6018 (1980).

2. R. M. Fletcher, D. K. Wagner and J. M. Sallentyne, Solar Cells,1, 263 (19 0).

3. D. E. Sawyer and H. K. Kessler, IME Transactions on ElectronDevices, Vol. ED-27(4), 864 (1980).

4. J. R. Szedon, T. A. Twmofonte, T. W. O'Keefe, Solar Cells, 1,251 (1980).

5. D. A. Pratt, M. Z. Langmuir, R. A. o reau and R. D. Rauh, J.Electrochem. Soc., 128, 1627 (1981).

6. G. Bodes, D. Cahen, J. Ianassen and M. David, J. Electrochem. So.127, 2252 (1960).

7. R. Tenne, Appl. Phys., 25, 13 (1981).

8. N. Rassak, J. Reichman and J. Dmarlo, Paper No. 488, 160th Meet-Ing of The Blectrochemical Society, Denver, Colorado, Oct. 1961.

9. A. eller, K. C. Chang and B. Killer, J. Electrochem. Soc., 124,697 (1977).

-5

'I

PUC PARAIIUZS AS A YVUCTIO O WACZTPAMMT OF A CdSe FIM

Treatsnt lbotovoltaag im Pm.x(volts) fjNAlcmz) 7&Nc=Z) ( m

A - nnealing -0.39 2.45 0.25 0.7a - HC1 Etch -0.45 12.5 2.1 4.6C - Anodization -0.43 7.3 1.4 1.0

*W-I lamlp G-2 filter, 45 =/c= 2

Z2ectrolyte: IN Na2S, IN S, IN NaCf.

w NNW

t &

Fig. 1. Pictorlal represntation of laser

scan6r

C M. M. S ..

Fig. 2. e-14e laser scan maps (623.8 rn) of CdSe thin film after A,* ~annealingi S, W-1 etch, C, anodization. Ream diameter n-2Ou, bea

power at electrode surface '1O liW.

zka-I Fig. 3. Zffect of etch time In SK HMl an chemically deposited CdS.thin film, Cell biased at -O.4V. Diagram at the left indicatesetch times for scan map on right. See Fig. 2 for laser parameters.

7

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760 rin 588 rutEtched Etched

Fig. 6. Time resolved photovoltage of CdSe single crystal.

Lead forNear Pleced

Counter

FBI Hos lcrd ill "s,

Channels forkals -xIon Flow

PihatoeCtro& - * * PlexiglassBeer Paced CasingCoun~ter Electrode *

FBI "doBent Louver

1. Perforated Flet Plate 11. Bent Louver

Fig. 7. Prototype frontvall thin f ilm photoelectrochemical cells.

9

Phcrrn

Scratch anSubstrate

Hole In _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Substrate -OW

Fig. 8. Short circuit photocurrent scan map of Chem-ical bath deposited CdSe film on a perforated sub-strate. Beam diameter 'I'5OU; laser itensity koO.*5 ow.

OWrection of Scan

4hy

% Coll WindowElectrode CdIS.

Louvers

Fig. 9. Short circuit photocurrent scan of louvered cell, wehitelight bias. Potentiostatted at O.OV, chopped Hello laser; lock-i~n amplifier. Electrodeposited film.

10

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