a review on photo luminescence and

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Surface Technology, 26 (1985) 93 - 105 93

A REVIEW ON PHOTOLUMINESCENCE ANDELECTROLUMINESCENCE OF SEMICONDUCTORS FOR

PHOTOELECTROCHEMICAL CELLS

L. PE RALDO BICELLI

Departm ent of Applied Phy sical Chemist ry, Mil an Poly technic, Research Centre on

Electrode Processes, Consiglio N az ional e delle Ricerche, Piaz za Leonardo da Vinci 32,

20133 Milan (Italy)

(Received January 22, 1985)

Summary

This review is concerned with the photoluminescent and electrolumi-

nescent properties of semiconductor electrodes used in photoelectrochemi-cal cells for optical-to-electrical energy conversion. Luminescence notonly provides insight into the excited-state deactivation routes which arecrucial to efficient energy conversion, but also allows one to obtain in arelatively simple manner valuable information about bulk and surfaceimperfections that negatively influence photoelectrochemical performance

of the semiconductor.

1. Introduction

Solar energy represents a major source of energy for the Earth and it

can be transformed into different kinds of energy that are useful to human-ity. One of the various possible transformation methods is the conversionof solar energy into electrical energy by utilizing photovoltaic and photo-

electrochemical (PEC) cells. Both devices involve semiconductors, theformer is based on a homo- or hetero-solid-solid junction, while the latteris based on a solid-liquid junction.

There are numerous reviews (e.g., ref. 1 and references cited therein)that provide a general picture of the PEC solar cells that have been studiedand discuss the difficulties connected with photostability and low efficiencyof semiconducting electrodes. In order to obtain materials whose perfor-mance allows interesting applications, it is imperative to control all surfaceand bulk defects that act as recombination centres of the electron-holepairs.

One method of doing this is to characterize the electronic structureof the semiconductor and to examine the excited-state processes as well astheir deactivation routes, i.e. to study the photoluminescent and electrolumi-nescent properties of the semiconductor itself. In this review, luminescence

@ Elsevier Sequoia/Print ed in The Netherlands



94

from semiconductors will be discussed in connection with their PEC behav-

iour, and the basic principles will be considered by which such excited-state

processes occur. We then concentrate on the actual knowledge of and the

problems relating to photoluminescence (PL) and electroluminescence (EL).

Furthermore, PL and EL are examined as useful in situ techniques for thestudy of semiconductor-electrolyte interfaces.

Although research on light emission from semiconductors in contact

with an electrolyte dates back to the 197Os, at present there is a renewed

interest and several research groups are active in the field.

2. Theoretical considerations

2.1. Photogenerative cells

In PEC cells capable of transforming solar energy into electrical en-ergy (so-called regenerative cells), such as for example the n-CdSI S2-, S,

OH-IPt cell, the electrolyte contains a single dissolved redox couple. On

immersion of the semiconductor in the electrolytic solution there is an

exchange of electrons between the two phases until their Fermi levels, i.e.,

their electrochemical potentials, are equal. Owing to the small charge carrier

concentration in the semiconductor, the transfer process modifies the

electron concentration with respect to the bulk value in a small region,

typically 0.1 to 1 pm wide close to the interface, the so-called space-charge

region, thus producing a parallel bending of the semiconductor valence andconduction bands. In the case when the Fermi level of the redox couple

(representing its Nernst potential) is effectively situated within the semi-

conductor forbidden band at flat-band conditions, a depletion layer of the

majority carriers forms in the proximity of the junction. This implies a

simultaneous bending-down or bending-up of the bands for n-type or p-type

semiconductors respectively, whereas the Fermi level remains flat through-

out the whole material. Absorption of a photon whose energy is equal to or

greater than the band gap energy promotes an electron from the valence

band to the conduction band, while the electric field in the depletion layerseparates the electron-hole pairs thus produced, preventing their recombina-

tion. Thus, in an n-type semiconductor that is part of a PEC cell, the minor-

ity carriers (holes) move toward the electrolyte and oxidize the reduced

component of the redox couple, whereas the majority carriers (electrons)moire toward the bulk of the semiconductor and reach the counterelectrode

to produce the reverse reduction reaction. Therefore, the electrolyte com-

position remains (in principle) the same, and solar energy is converted into

electrical energy.

Although the device is theoretically expected to operate indefinitely,

the desired PEC reaction may be accompanied by other unwanted reactions,e.g. decomposition of the material. In fact, one of the major problems in

making practical PEC cells is to avoid photocorrosion of the semiconducting

electrode.



95

2.2. Luminescence

There are several methods allowing the production of luminescence

from semiconductors. In this review we mainly consider PL and EL where

the population of the emitting excited state is produced by photon absorp-

tion or by an electric current, respectively. Cathodoluminescence andtriboluminescence are also widely examined, excitation resulting from a

beam of electrons in the former case and from mechanical means in the

latter case.

2.2.1. Pho toluminescence

The mechanism by which PL is produced can easily be understood if

reference is made to the previous observations in connection with PEC

cells.

As stated above, ultraband-gap photon absorption by semiconductors

yields an electron-hole pair which according to Ellis [ 21 “can be regardedcrudely as a one-electron picture of the semiconductor’s excited state”.

In PEC cells, we have seen that these electron-hole pairs are separated in

the space-charge region and give rise to a photocurrent. Thus, the excited-

state deactivation mechanism is non-radiative. In contrast, when a radiative

recombination of the two charge carriers takes place PL is observed.

Besides radiative transitions from the bottom of the conduction band

to the top of the valence band whose emitted energy is equal to the band gap

energy, so-called edge emission [ 31, radiative intra-band gap transitions are

also possible. They involve energy levels lying within the semiconductorforbidden band and give rise to sub-band gap emission, i.e. to peaks situated

at the longer wavelength side of the edge emission peaks.

These intra-band gap levels can be distinguished in surface or near-

surface states and in deep trapping levels arising from bulk imperfections

(impurity atoms and intrinsic defects such as interstitial atoms or atom

vacancies).

Of course, non-radiative recombination of the electron-hole pairs is

also possible resulting in lattice vibration, i.e. in heat production.

2.2.2. Electroluminescence

Typical examples are n-type II-VI binary and ternary semiconducting

compounds in a PEC cell containing an aqueous, alkaline peroxydisulphate

solution [ 4 - 61. By polarizing such an electrode in the dark at potentials

cathodic of its flat-band potential, luminescence of the colour emitted

during PL experiments is observed. This is a general result with strongly

oxidizing electrolytes: bands observed in PL spectra are observed even in

EL spectra, but the reverse is not always true (see below).

As to the EL mechanism, an electron in and/or near the conduction

band is able to reduce an SZOs2 - anion in the dark to yield strongly oxi-dizing SO,; radical anions. The latter, in turn, inject holes (i.e. minority

carriers) into the valence band and/or into some intermediate levels thus

producing a cathodic current. Subsequently, radiative recombination of an



96

electron with the injected hole yields emission of band gap or sub-band gap

energy [7 - 91. Thus, the emitted EL spectrum is a consequence of the

charge-transfer process across the interface, and EL continues until the

oxidizing agent in the solution is completely consumed. Usually, to avoid

EL intensity decay from the initial value, the electrode is continuouslypulsed between a potential where no current is observed and a potential

sufficiently cathodic to initiate EL, while the spectrometer is scanned at

constant speed. In addition, the electrolyte is thoroughly stirred. Some evi-

dence for competitive electrode reduction has also been presented [ 9 - 111.

EL has been observed with a variety of oxidized components of redox

couples, e.g. Fe( CN)6 3- for n-Gap [7,12], and, more recently, Fe3+, Ce4+

and again Fe(CN), 3- for n-GaAs [ 13 - 151. Using one-electron redox cou-

ples, the energetic conditions for hole injection into and/or near the valence

band can be analyzed more carefully than in cases where the injecting

species is a highly active intermediate (e.g. SO,-) whose energy level distribu-

tion is poorly defined.

2.3. Comparisons

In agreement with the above, EL takes place when the semiconductor

bands are in light accumulation conditions very close to flat-band conditions

(i.e. small upward bending for n-type materials) and the oxidizing agent

Fermi level is close to the valence band edge. In contrast, to obtain a PEC

response, the semiconductor bands have to be in depletion conditions

(i.e. strong downward bending) and the redox couple level has to be insidethe band gap. This is in agreement with electric current circulating in oppo-

site directions during EL and PEC experiments, i.e., a dark cathodic current

and a photoanodic current, respectively, are produced.

EL is a typical surface phenomenon being initiated at the semicon-

ductor-electrolyte interface, the hole diffusion length L, (we are still

considering an n-type semiconductor) in practice giving the thickness of the

surface layer involved. However, excitation wavelengths used to induce PL

will typically have penetration depths a--’ (where cx is the incident photon

absorption coefficient) exceeding lo-’ cm. Therefore, when CW-’ s muchgreater than L,, PL originates from the bulk of the material, whereas EL

gives a typical picture of the semiconductor surface, i.e., PL and EL have a

different spatial origin. Since hole diffusion lengths generally decrease with

increasing charge carrier density, the above conditions are met better the

higher the semiconductor is doped. Conversely, very pure and lightly doped

materials show similar PL and EL spectra, provided the same emissive

excited state is populated.

3. Experimental results and discussion

Significant experimental PL and EL results of various authors will be

discussed, whereas those regarding photoelectrochemistry (examined in a



91

previous review [ 11) will be considered in connection with luminescence

phenomena only.

3.1. Photoluminescence spectral distribution

Figure l(a), taken from ref. 5, is an example of PL spectra due ton-type CdS,Se, _ X single crystals in 5 M OH- electrolyte. After illumination

with ultra-band gap (457.9 nm) light, green, yellow-green, orange, red and

red luminescences are obtained corresponding to the x-values of 1.00, 0.74,

0.49, 0.11 and 0.00 respectively, i.e. the emitted light is red shifted with

increased selenium content.1 I I I

i

(b) 0I I 1 1 J

500 600 700 800

WAVtLENtTH, nm

Fig. 1. (a) Un corr ected PL spectra of CdS,Se_, sam ples irr adia ted in 5 M OH elec-

tr olyte while th ey were held at -1.50 V (SCE). Th e ar ea of th e sam ples was about 0.25

cm2 a nd t hey were excited with 457.9 nm light of about 1.0 mW power (excita tion spike

sh own at l/100 th e scale of th e PL spectru m). Em ission inten sities are not directly com-

par able because of differen ces in geomet ry. (b) Uncorrected EL spectra of th e sam plesin (a) obtained without cha nging their geometr y. The electr olyte in th e EL experiments

was 5 M OH--O.1 M SzOs2-. Electr odes wer e cont inu ously pulsed between 0.00 V (11 s)

an d -1.50 V (SCE) (1 s) while the emission spectr ometer was scann ed at 12 nm min-‘.

For both PL an d EL, a spectra l resolut ion of 2.0 nm was employed. (From ref. 5.)



98

High resolution PL spectra occasionally show variation in breadths

depending on excitation wavelength owing to self-absorption effects, the

spectral mismatch occurring almost exclusively in the high energy tail of

the emission band where the probability for emitted light absorption is

greatest.As expected, PL spectra often contain more than one peak. An example

is shown in Fig. 2, taken from ref. 16. The PL spectrum of aluminium-

doped n-type ZnSe is dominated by a broad peak, but a weaker and sharper

one is also present. The energetic proximity of the latter to the band gap,

together with its temperature dependence (a marked short-wavelength shift

on cooling at 77 K is observed) allowed the designation as edge emission.

The broader sub-band gap emission was, however, assigned to a transition

involving a deep trapping level arising from a complex (VznAlzn) based on

a zinc vacancy and on a next-neighbour donor impurity, i.e. aluminium

on a zinc site [ 17 - 201.

I L 1 I

500 600 700

WAVELENGTH. nm

Fig. 2. Corrected, front&-face PL spectra of n-ZnSe:Al at 295 K (broken line) and 77 K(full line). Both spectra were taken at the same sensitivity (a loo-fold increase in sensi-

tivity was used in scanning the 410 - 480 nm region). The sample was excited with iden-

tical intensities of 405 nm light without disturbing the experimental geometry. (Fromref. 16.)

3.2. PL quenching

The effect on PL properties of the passage of a photocurrent was

examined with different n-type semiconductors in aqueous solutions con-

taining a redox couple, that is in the same conditions of a PEC experiment

[ 5, 21 - 231. In passing from open-circuit to in-circuit conditions the inten-

sities of the PL peaks are quenched by the applied electric field. This is

shown in Fig. 3(a), taken from ref. 2, for an n-CdSe single-crystal electrode

in an alkaline sulphide-polysulphide solution. Since the spectral distribution

is not appreciably altered, changes in PL are monitored at a single wave-

length [ 241.



99

I I

650 700 750 801

(a) WAVELENGTH, nm

J

260 -

::

150- 514.5 nm EXCITATION -.

f40 -

5;;30 -

s .

320 -

5 .

FlO -

4 .

z o-

(C)r:::::‘::;::::’

-1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3

@IPOTENTIAL, VOLTS vs. SCE

Fig. 3. (a) Uncorrected PL spectrum of an n-CdSe single-crystal electrode excited with514.5 nm light in a 1 M OH--l M S2--0.5 M S electrolyte. Curves A, B and C were taken

at -1.50 V (open circuit), -1.45 V and -1.30 V us. SCE, respectively; all spectra wereobtained with an identical sample geometry. Photocurrent (b) and emission intensity (c)monitored at &,a, _ 720 nm us. potential for an n-CdSe single-crystal electrode excitedat 514.5 nm in a 5 M OH--O.11 M Se2--0.004 M Se 22- electrolyte. The photocurrent

quantum efficiency, f#~,, at -0.3 V (SCE) was measured and found to be about 0.9.

Related experiments with n-CdSe electrodes are given in ref. 6. (From ref. 2.)

Simultaneous measurements of photocurrent and PL intensity as a

function of electrode potential allow one to obtain the so-called ILV curves

reported in Figs. 3(b) and 3(c). There is a typical inverse correlation betweenphotocurrent and PL intensity, as the former increases the latter decreaseswhen increasingly anodic potentials are scanned. In fact, as more anodicpotentials are applied to the semiconductor, downward band bending inthe space-charge region is increased. Therefore, electron-hole separation ispromoted and radiative electron-hole recombination inhibited. Conse-quently, the photocurrent is enhanced and photoluminescent intensitydiminis)led.

Quenching of semiconductor PL by applied electric fields has been

quantified by a dead-layer model [ 25 - 271 which has recently been foundto be applicable also to PEC-based systems [ 2, 5, 16, 28 - 301. In its simplestform, the model states that electron-hole pairs formed within some fractionof the depletion width are swept apart by the field and do not contributeto PL.



100

3.3. EL spectral distribution

In Fig. l(b), taken from ref. 5, EL spectra of n-type CdS,Sei_, single

crystals in a 5 M OH--O.1 M S,Os 2- electrolyte are reported. The electrodes

were continuously pulsed between 0 and -1.5 V (SCE), thus avoiding EL

intensity decay.Another example is shown in Fig. 4, taken from ref. 15, where the

relationship between the EL spectrum and the oxidized component of the

redox couple in the electrolyte is considered. For n-GaAs, both edge (at

0.87 ,um) and sub-band gap (at 1.1 pm) emissions are observed. The latter

is related to a deep acceptor level due to gallium vacancies. The remarkable

difference between EL intensity of Ce4+- or Fe3+-containing solutions has

been explained on the basis of the higher redox potential of the Ce3+/Ce4+

redox couple assuring a better overlapping of the electrolyte empty energy

levels and both the valence band and the deep acceptor level. Therefore,

Ce4+ ions easily inject holes into the semiconductor, whereas Fe3+ ions

do not [ 151.

EL spectra are typically recorded under conditions which facilitate

comparisons with PL spectra. As Figs. l(a) and l(b) clearly show, there is

a striking similarity between the PL and the EL spectra, thus indicating

that an identical emissive excited state is involved in both experiments.

In other cases, however, particularly when a high resolution is em-

ployed, spectral differences in the high energy tail of EL and PL spectra

are observed [ 5, 6, 151. This spectral mismatch has been attributed to

greater self-absorption of the emitted light in the PL experiment [ 5, 61.

WAVELENGTH (pm)

Fig. 4. Uncorrected EL spectra of n-GaAs in 0.01 M Fe,(SO& + 1.0 M HzS04 and in0.03 M Ce(SO& + 1.0 M HzSO 4. The electrode potential was continuously pulsed be-

tween 0 (20 s) and -1.75 V (2 s), while the emission spectrum was scanned at 5 w 6-l.

Note the tenfold increase in sensitivity in order to record the spectrum in the Fe,(SO&

electrolyte. (From ref. 15.)



101

As on average EL is significantly more surface sensitive than PL,

several semiconductors show some EL emission peaks below the band-gapenergy which do not appear in the PL spectrum (compare curves a and c ofFig. 5, taken from ref. 31). It has been suggested that these additional peaks

originate from transitions involving hole-accepting, i.e. filled, surface states[4, 5, 7, 9, 31, 321.

The rather exceptional case of a PL peak having no EL counterpart(compare curve c with curve b of Fig. 5) indicates that the empty en-ergy levels of the redox couple in the electrolyte (i.e. the energy levels of

Fe(CN),3-) and the valence band do not overlap. However, such emptylevels overlap the hole-accepting surface states and, therefore, can directlyinject holes into them, thus producing the EL peak around 900 nm shownin curve b of Fig. 5 [31].

1 0 0 0 800 600 (nm)

Fig. 5. CdS PL and EL spectra in the following electrolytes: curve a, EL in i M KOH-0.1

M KzSzOs; curve b, EL in 1 M KOH-0.1 M K3Fe(CN)s; curve c, PL in 0.2 M NazS03-0.2

M NazS04. (From ref. 31.)

3.4. Efficiencies

3.4.1. PL efficiencyThe quantum efficiency of PL q& (defined as photons emitted per

photon absorbed (or per populated excited state)) is very difficult toestimate with accuracy. Therefore, an upper limit can be obtained by findingother experimental conditions (e.g. cooling the material) which markedlyenhance the emission [ 61. Another technique developed by Wrighton et al.

[33] compares light reflected from a non-absorbing material with lightreflected and emitted from the sample.

Sometimes, 4, was observed to decrease with the excitation wavelengthand such behaviour has been attributed to the presence of a near-surfacenon-emissive region which absorbs larger fractions of incident light, thusreducing 9, [ 161.

By considering the different excited-state decay routes, one mayobtain a very simple relationship that connects 4, with the photocurrent



102

quantum efficiency 4, defined as electrons circulating in the external circuit

of the PEC cell per absorbed photon.

Assuming that the only decay routes of the photoproduced electron-

hole pairs are radiative emission (PL), photocurrent and non-radiative

emission, whose quantum efficiencies are $,, 4, and @,,, respectively, 4, +@J,+ @Xl, 1 [ W.

For a cell operating in open-circuit conditions

whereas, when operating in in-circuit conditions

~, = Eli and ~, = ~xi,

hence [22, 341

4

z=1

’ - @Xi

provided the ratio &/#,, of the electron-hole pairs radiatively recombining

to those non-radiatively recombining is unaffected by potential.

The above equation allows the calculation of the degree of PL quench-

ing by potential, but discrepancies from the theoretical predictions have

been observed [ 221. It also allows determination of the & value which may

be compared with the experimental value from PEC measurements. Rea-

sonable agreement was obtained for PEC cells based on n-CdS,Se, - X (0 S

x < 1) and on Te-doped n-CdS [5,22,34,35].

3.4.2. EL efficiency

EL efficiency can be determined in both steady state and pulse experi-

ments. In the first case, the instantaneous efficiency $xL is obtained (photon

emitted per hole injected), in the second case, the integrated efficiency 4xL

(total photons emitted per total holes injected). The measurements are

performed by placing a radiometer next to the emitting surface. Since not

all of the emitted photons are collected, EL efficiencies are lower-limit

estimates [ 351.Taking the definition of EL and PL efficiencies into account, it is

possible to factor I& into 4, (photons emitted per populated excited state)

and $xS (efficiency for excited-state population, i.e. populated excited states

per holes injected) [ 161:

However, conclusions regarding r$Es based on the direct comparison

of EL and PL efficiencies must be drawn carefully. In many systems (e.g.

n-CdSe [6] and n-CdS,Se, _-x [5]) @Es is large, approaching its maximum

value of unity. However, in other cases (e.g. Al-doped n-ZnSe [16]), since

@EL g 4,) @ES is very small, but more likely this reflects the different spatial

origin of EL and PL. Moreover, reduction of the electrode surface during EL

experiments may also contribute to the above discrepancy [ 51.



103

4. Concluding remarks

PL and EL provide a sensitive in situ tool for the characterization of

deep level traps in semiconductors and of intra-band gap states at the semi-

conductor-electrolyte interface, respectively. Moreover, since PL and ELspectra are typical for each semiconducting compound and since by changing

the wavelength of the exciting light it is possible to vary the light penetration

depth, PL and EL also allow one to monitor chemical and electronic changes

occurring in the region close to the interface.

Therefore, in performing PL and EL measurements, it is imperative

to control the electrode potential to avoid too drastic chemical changes at

the semiconductor interface.

Regarding this point, Etcheberry et al. [30] carried out a PL study on

the compositional changes of an n-InP surface in a pH 1 solution of perchlo-

ric acid by polarizing the electrode at more positive and more negative

potentials than the rest potential. From the shape of the PL intensity uersus

potential curve, they concluded that the electrode surface (like that of Ge

and GaAs [ 361) evolves from a stable oxidized state (i.e. oxygen covered)

to a reduced one (i.e. mainly hydrogen covered) when the bias is changed

from positive to negative. In the latter case, adsorbed species which appear

during hydrogen evolution, e.g. atomic hydrogen, are able to enhance PL.

They are assumed to passivate surface defects, thus diminishing non-radiative

recombination.

Electrode potential control is particularly stringent for EL experimentswhere the semiconductor has to be cathodically polarized in order to achieve

light emission.

Therefore, Gerischer et al. [37] have recently questioned whether

observed dissimilarities between PL and EL spectra are “caused by the dif-

ferent distribution of minority carriers in the two experiments or by chemi-

cal changes, at the semiconductor-electrolyte interface, as a direct result

of the different techniques”.

Taking the potentialities of PL and EL techniques into account, several

problems connected with semiconductor surfaces have been studied. Somesignificant examples will suffice to illustrate the situation.

Surface adsorption phenomena resulting from the interaction between

a semiconductor and a solution were investigated. PL intensity of n-InP was

found to decrease as soon as the semiconductor was immersed in an aqueous

electrolyte mainly as a consequence of adsorption of ionic or dipolar species

belonging to the solution [ 301.

Other examples are surface modification due to corrosion and aging

both in the dark and under illumination of photoelectrodes in liquid junc-

tion configuration. Recently, the evolution of the PL spectrum of n-CdSe

in sulphide-polysulphide solutions has been monitored as a function oftime and light intensity in order to elucidate the aging process mechanism

of one of the most promising materials for regenerative PEC cells [38]. The

predominant aging process which occurred under open-circuit conditions



104

was well correlated with the formation of an insulating CdS,Se,_, film atthe surface. By penetrating deeper into the electrode, the mixed compoundseparated into CdS and CdS,Se, _ Y which was considerably poorer in sul-phur [38].

Surface modifications due to treatments such as etching, PEC etchingas well as adsorption of metal ions to which semiconductors are submittedin order to increase their PEC behaviour have also been examined through

their effect on luminescence properties [ 391.Although surface treatments obviously affect EL spectra more than PL

spectra, evidence was found that PL and its spatial distribution may beinfluenced, too.

Both etching and photoetching of CdSe electrodes resulted in anincrease in the emitted light (up to 50 times after photoetching) and in abroadening of the high energy portion of the emission band [6]. Such an

enhancement of PL intensity suggested a substantial increase in PL efficiencyand the concomitant spectral mismatch indicated a greater contribution toPL from regions on average nearer the semiconductor-electrolyte interface.At present, there are uncertainties as to the origin of these effects and

research is still in progress [ 61.It may thus be concluded that PL and EL spectroscopies are very

important tools for surface properties investigation, as even these few

examples have shown. Therefore, it is very easy to predict an increasingdevelopment of these techniques in the near future.

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