OPTICAL ABSORPTION STUDIES IN SINGLE AND MULTILAYER THIN FILMS OF CuS, PbS,CdS and CuPc

5.1 Introduction

For probing the band structure in semiconductors, one of the most direct

methods is the optical absorption studies. We can determine the optical band gap

of a material as well as determine whether the valence band and conduction band

extrema occur at the same or difFerent points in the k-space, knowing the

frequency dependence of the absorption processes such as fundamental

absorption, free carrier absorption, excitonic absorption and impurity absorption.

Electrons can be excited from the valence band to the conduction band with the

absorption of a photon of energy equal to the band gap of the material. Rapid

drop in the absorption coefficient on the higher energy side of the absorption

band leads to the band edge in semiconductors which can be analysed to get the

optical band gap energy of the material.

The metal phthalocyanines belong to the point group D4,, and the electronic

structure of metal phthalocyanines was described by Gouteman and coworkers.'*2

Charge transfer and electron absorption studies of metal phthalocyanine by

YOS hida' et .a1 suggests the effect of hybridisat ion between intramolecular excitations.

Fundamental information on the molecular structure of solid films can be obtained by

analysis of their molecular spectra. Molecular solid films are assemblies of organic

molecules loosely bound together by Van der Waals type of interaction.

Phthalocyanines exist in various crystal forms that arise from different molecular

stacking arrangements. Their optical properties depend on its crystal structure4.

Salim et.a15 and Lmamendi et.a16 studied and characterized the chemical bath

deposition of PbS thin films. Nishchenko et. a17 studied the laser induced mass hansfer

in rnultilayer thin films by means of Auger electron spectroscopy. Powell et.a18

characterized the copper oxidation and reduction in multilayer films of copper using

spectroscopic ellipsometry.

In this chapter, the investigations on the energy band gap and the optical

absorbance of Copper Phthalocyanine (CuPc), Copper Sulphide (CuS), Lead

Sulphide (PbS), Cadmium Sulphide (CdS), multilayer CuS-CuPc, PbS-CuPc,

CdS-CuPc and PbS-CUS films are presented.

5.2 Theory

Absorption is a phenomenon of fundamental interest because of its

relation to the dynamics of the electrons and ions of the medium under the

influence of electromagnetic radiation. From the dependence of the absorption

band edge on frequency, one can determine the energy gap of the material. When

the energy of the incident photon is greater than that of the band gap (hv>Eg) the

absorption coefficient 'a' is given by.9

( c c + C" )* where B~ = ------------------ Here the constants b, , b, >O and C, , C, 20

4 0 , + b " )

For hv - Eg 5 B~ equation 5.2.1 simplifies to

A a (hv - Eg )3'2

hv 3~~

For hv -Eg >> B~ equation 5.2.2 becomes

where A is a constant.

When the energy of the incident photon exceeds the band gap energy (Eg)

of the material, an electron is excited from the valence band to the conduction

band. There are two types of transitions4irect and indirect. Transition involving

photons only is direct and transition involving photon and phonon is indirect. A

direct vertical optical transition near the fundamental absorption edge in a

semiconductor is shown schematically in figure 5.2.1 . The valence band

maximum and conduction band minimum appear at ihe same point in the

Brillouin zone at k=O. The direct band gap is estimated from equation 5.2.3. The

procedure consists of' plotting a' versus hv and extrapolating the linear region of

the curve to the energy axis; the intercept being identified as the band gap.

When transition requires a change in both energy and momentum, a

double transition process is required because photon cannot provide change in

momentum. For overcoming this, emission or absorption of phonons occurs.

Momentum is conserved through a phonon interaction resulting in an indirect

transition as shown schematically in figure 5.2.2.

band

band

Figure 5.2.1 Direct transition from valence band to conduction band

E

Valence band

Figure 5.2.2 Indirect transition from valence band to conduction band

The indirect band gap is determined from the relation,

a = (hv - E ~ ) ~

There is a shift in the band gap towards higher energy for the film having

higher carrier density. This shift is due to filling of states near the bottom of the

conduction band and is known as Burstein ~ o s s " ~ " shift and is illustrated in

figure 5.2.3.

Figure 5.2.3 Illustration of Burstein Moss shifi

The shift is given by the relation,

~g = E& + A E ~ ~ ~

where Ego is the intrinsic band gap and A E ~ ~ ~ is the BM shiA. The shift is

related to the carrier density as,

where m* is the reduced effective mass

. The plot of a2 versus hv for direct allowed transition, am versus hv for direct

forbidden transition, a'' versus hv for indirect allowed transition and a'" versus hv for

indirect forbidden transition will lead to straight lines. By extrapolating these lines to

a = 0, one can determine the corresponding band gap of the material. The hctional

dependence of absorption coefficient on photon energy near a transition can tell

whether the transition is allowed or forbidden. The nature of transition can give

information about the electronic states from which the transition takes place and this

in turn helps to get the band structure of the semiconducting material. l 2

5.3 Experiment

CuS, PbS, CdS and multilayer PbS-CuS films have been prepared by chemical

deposit ion technique as described in chapter 3. The copper phthalocyanine (CuPc)

powder used in this study is obtained from Aldrich chemical company Inc: USA. Thin

films of CuPc are deposited at room temperature onto precleaned glass substrates with

pre-evaporated high purity silver electrodes, at a base pressure of 1 u5 Torr using a Hind

Hivac Vacuum coating wit. The evaporation is carried out by resistive heating of the

CuPc powder fiom a molybdenum boat and the rate of sublimation is kept ~onstant. '~

The optimum rate of evaporation is adjusted to be 10- 15 nrn per minute.

For multilayer films, the sulphide films are used as substrates for CuPc

deposition. CuPc is evaporated onto these sulphde films with pre-evaporated lugh

purity silver electrodes, at a base pressure of lom5 Torr by resistive heating fiom a

molybdenum boat as per the procedure described in section 3.6 of chapter 3. The

99

optimum rate of evaporation is adjusted to be 10-1 5 nm per minute. These films are

annealed in air at different temperatures. h e a l i n g is canied out in a fiunace whose

temperature is controlled by a controller cum recorder. The annealing time is two hours.

The optical absorption spectrum in the range 300-900 nm is recorded

using the Shimadzu UV-Visible 160A spectrophotometer. The spectrophotometer

consists of a Spex Minimate monochromator (containing a 50 W quartz halogen

lamp illuminating a diffraction grating of 900 lines/nm) interfaced to a computer,

which can record the spectrum from 300 to 1100 nm. Optical absorption spectra

for as deposited and annealed samples arc analysed using the theory described in

section 5.2 ofthis chapter to get the energy band gap of the material.

5.4 Results and Discussion

Figures 5.4.1, 5.4.2, 5.4.3, 5.4.4, 5.4.5, 5.4.6 and 5.4.7 show the

absorbance versus wavelength spectra for CuPc, CuS, rnultilayer CuS-CuPc,

PbS-CuPc, CdS, CdS-CuPc and multilayer PbS-CuS respectively. The spectrum

of CuPc films shows an absorption edge at 306 nm and an absorption peak at 620

nm. The spectrum,of CuS films shows an absorption peak at 306 nm. The

spectrum of CuS-CuPc films shows an absorption peak at 325 nm. The spectrum

of PbS-CuPc films shows an absorption peak at 310 nm. The spectrum of as

deposited CdS lilms shows an absorption peak at 379 nm and for annealed

samples (523 K and 573K) absorption peaks are at 381 nm and 425 nm respectively.

The spectrum of multilayer CdS-CuPc films shows an absorption peak at 620 nm.

The spectrum of PbS-CUS tilms shows an absorption peak at 300 nm.

CuPc-As dep. I Ann. 523K-2 Ann. 573U-3

Figure 5.4.1 Absorbance versus wavelength spectra of CuPc films of thickness 2180A

4.0 -

3.5 -

3.0 -

2.5 -

u, a Q 2.0 -

1.5 -

7.0-

0.5 -

0.0 -

-0.5 r I r I I I I I I I

300 400 500 600 700 800 900

Wavelength (nm)

2.25 -

2.00-,

1.75 -

1.50 -

2

0.25 -

0.00 1 I 1 I 1 1

350 400 450

Wavelength (nm)

Figure 5.4.2 Absorbance versus wavelength spectra of CuS films of thickness 3 120A

Wavelength (nm)

Figure 5.4.3 Absorbance versus wavelength spectra of rnultilayer CuS-CuPc films of thickness 5340A

PbS-CuPc,As dep-1 Ann- 523K-2 Ann- 573K-3

Figure 5.4.4 Absorbance versus wavelength spectra of multilayer PbS-CuPc films of thickness 6450A

3.4 - 3.2 - 3.0 -

2.8 -

2.6 -

2.4 - I

2.2 - I

cn 2.0 - P

1,a:

1.6 - 1.4-

1.2-

1.0-

0.8 - 0.6 - 0.4 -

0.2 - 1 I I 1 I I r I I I I

300 400 500 600 700 800 900

Wave Length (nm)

CdS-As dep.1 Ann. 523 K-2 Ann. 573 K-3

0 - ! I I I v I 1 I I I 300 400 500 600 700 800 900

Wave length(nm)

Figure 5.4.5 Absorbance versus wavelength spectra of CdS films of thickness 6 100A

CdS-CuPc,As dep. Ann, 523 K

1

Ann. 573 K 2 -3

I I I I I I I

200 300 400 500 600 700 800 900

Wave length (nm)

Figure 5.4.6 Absorbance versus wavelength spectra of multilayer CdS-CuPc films of thickness 8250A

PbS-CuS,As dep-I Ann. 523 K -2 Ann. 573 K -3

Wave Length (nm)

Figure 5.4.7 Absorbance versus wavelength spectra of multilayer PbS-CuS films of thickness 935OA

The UV-Visible spectrum observed for phthalocyanines originates from

molecular orbitals within the aromatic 18x electron system and from overlapping

orbitals on the central rnetal.I4 The conjugated double bonds within the crystal

structure of the film create electron orbitals which overlap between molecular n

orbitals. These electrons are able to transler energy throughout the structure and

are responsible for the absorption peaks.'5 To obtain information about direct or

indirect interband transitions, the fundamental absorption edge is analysed within

the one electron theory of ~ a r d e e n ' ~ Schmeisser et. all7 show evidence for the

absorption band at low energies. The absorption coefficient 'a' is related to the

photon energy 'hv' by l 8

a = a, (hv -Eg )" 5.4.1

where Eg is the optical band gap. The value of n is !4 for allowed transitions and 312

for forbidden kinsitions. The absorption coetticient is given by the relationI9

where A is the absorbance of the film and t is the thickness. For CuPc, CuS,

PbS, CdS and rnultilayer films, the absorption coefficient a ~1 o3 cm-' and the

fundamental absorption edge is related to direct allowed interband transitions.

Hence the direct allowed band gap is determined by plotting a2 as a function of

photon energy hv which gives a straight line. The intercept of this straight line on

the energy axis (i-e. u2 = 0) gives the direct band gap.

108

Figures 5.4.8, 5.4.9, 5.4.10, 5.4.1 1, 5.4.12, 5.4.13 and 5.4.14. give the plot

of a2 versus hv for CuPc, CuS, CuS-CuPc, PbS-CuPc CdS, CdS-CuPc, and

multilayer PbS-CuS films respectively. The optical band gap and maximum

absorption coefficient of the as deposited and annealed samples are determined.

Optical band gap and absorption coefficient decreases with annealing. This may

be due to the reduction in trap sites. The difference in band gap due to annealing

could be due to the removal of the trapping levels in the forbidden band region

during annealing. Visible radiation is sufficient to excite electrons fioin their ground

state to an excited state.20 it is suggested that annealing of the films results in an

increase in grain size, which causes a decrease in grain boundary areas.2 '

2.4 2.8 3.2 3.6 4.0

hv (eV)

Figure 5.4.8 Plot of a2 versus hu for CuPc films of thickness 21 80A

Figure 5.4.9 PI ot of u2 versus hu for CuS films of thickness 3 120A

Figure 5.4.10 Plot of a2 vcrsus hu for multilayer CuS-CuPc films of thic kness 5340A

PbS-CuPc-Asdep. Ann. 523 K

A Ann. 573K

Figure 5.4.1 1 Plot of a* versus hu for multilayer PbS-CuPc films of thickness 6450A

CdS-As dep. Ann. 523K Ann. 573K

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

hv (eV)

Figure 5.4.12 Plot of a* versus hu for CdS films of thickness 61OOA

Figure 5.4.13 Plot of a2 versus hu for rnultilayer CdS-CuPc films of thickness 8250A

Figure 5.4.14 Plot of a' versus hu fbr multilayer PbS-CuS films of thickness 9350A

5.4.1. CuPc films

For CuPc, a band gap energy of 2.88 ev2* is obtained for as deposited

films. For annealed samples (523 K and 573 K) the band gaps obtained are 2.85

eV and 2.8 eV respectively and are shown in figure 5.4.8. The band gap and

maximum absorption coefficient obtained for as deposited and annealed CuPc

films are given in table 5.4.1. Optical band gap and maximum absorption

coefficient decreases with annealing temperature. Schrneisser et. a1 " have

predicted the optical band gap of CuPc to be higher than 2.42 2 0.02 eV, which

is in agreement with the present value of 2.88 e ~ . ~ ~ Ambily et. all3 obtained the

optical band gap oSCuPc as 2.71 eV.

Table 5.4.1. Optical band gap energy and maximum absorption coefficient of CuPc thin films of thickness 2180A

Samples

1 .CuPc (As deposited)

2.CuPc (Annealed at 523K)

3.CuPc (Annealed at 573K)

Optical band gap (eV)

2.88

2.85

2.8

Absorption coefficient x 10" cm-'

0.40

0.39

0.26

5-4.2. CuS films

For CuS films, a band gap energy of 2.1 eV is obtained for as deposited

films23. But for annealed samples (523 K and 573 K), we have obtained the band

gaps as 1.95eV and 1.9 eV respectively and are shown in figure 5.4.9. The band

gap and maximum absorption coefficient obtained fbr as deposited and annealed

CuS films are given in table 5.4.2. Optical band gap and maximum absorption

coefficient decreases with anneal ing temperature.

Table 5.4.2. Optical band gap energy and maximum absorption coefficient of CuS thin films of thickness 3120 A

Absorption coefficient x 1010 cem-l

0.15

0.09

0.09

Samples

I .CuS (As deposited)

2.CuS(Annealed 523K)

3 .CuS(Annealed 573 K)

Optical band gap (ev)

2.1

1.95

1.9

5.4.3 Multilayer CuS-CuYc films

For multilayer CuS-CuPc films, a band gap energy of 2.93 eV is obtained for

as deposited films. For annealed samples (523 K and 573 K) we have obtained the

band gaps as 2.36 eV and 2.25 cV respectively and are shown in figure 5.4.10. The

optical band gap and maximum absorption coefficient of the as deposited and

annealed multilayer CuS-CuPc films are calculated and are given in table 5.4.3.

Optical band gap and maximum absorption coefficient decreases with annealing

temperature. This may be due to the reduction in trap sites. The difference in

band gap due to annealing could bc due to the removal of the trapping levels in

the forbidden band region during annealing.24

Table 5.4.3. Optical band gap energy and maximum absorption coefficient of multilayer CuS-CuPc thin films of thickness 5340A

Samples

1. Multilayer CuS-CuPc (As deposited)

2. Mu1 ti layer CuS-CuPc (Annealed 523K)

3. Multilayer CuS-CuPc (Annealed 573K)

Optical band gap (eV)

2.93

2.36

2.25

Absorption coefficient x 10" cm-'

0.2 1

0.20

0.15

5.4.4 PbS films

The band gap energy of PbS was reported to be 0.4 eV by ~ z a r o f f * ~ in

"Introduction to Solids". Also Hideyuki Kanazawa et.alZ6 reprted the band gap of

PbS as 0.41 eV. Because of its narrow band gap energy, PbS is useful for long-

wavelength detectors and diode lasers.

5.4.5 Multilayer PbS-CuPc films

For multilayer PbS-CuPc films, a band gap energy of 3.6 eV is obtained

for as deposited films. For annealed samples (523 K and 573 K), we have

obtained the band gaps as 3.4 eV and 3.375 eV respectively and are shown in

figure 5.4.1 1. The band gap and absorption coefficient obtained for as deposited

and annealed multilayer PbS-CuPc samples are given in table 5.4.5. Optical band

gap and maximum absorption coefficient decreases with annealing temperature.

This may be due to the reduction in trap sites.

Table 5.4.5. Optical band gap energy and maximum absorption coefficient of multilayer PbS-CuPc thin films of thickness 6450A

Samples

1. Multilayer PbS-CuPc (As deposited)

2. Multilayer PbS-CuPc (Annealed 523K)

3. Multilayer PbS-CuPc (Annealed 573K)

Optical band gap (eV)

3 -6

3.4

3.37

Absorption coefficient x 10' rm-I

0.12

0.1 1

0.1 1

5.4.6 CdS films

For CdS films, a band gap energy of 2.4 e ~ ' ~ is obtained for as deposited

films. For annealed samples (523 K and 573 K) the band gaps are 2.3 eV and

2.22 eV respectively and are shown in figure 5.4.12. The band gap and maximum

absorption coefficient obtained for as deposited and annealed CdS samples are

given in table 5.4.6. Optical band gap and maximum absorption coefficient

decreases with annealing temperature.

Table 5.4.6. Optical band gap energy and maximum absorption coefficient of CdS thin films of thickness 6100A

Absorption coefficient x 1010 em-l

0.19

0.18

0.16

Samples

1 .CdS (As deposited)

2.CdS (Annealed at 523K)

3.CdS (Annealed at 573K)

Optical band gap (eV)

2.4

2.3

2.22

5.4.7 Multilayer CdS-CuPc films

For rnultilayer CdS-CuPc films, a band gap energy of 3.75 is obtained

for as deposited films. For annealed samples (523 K and 573 K) the band gaps are

obtained as 3.3 eV and 3.25 eV respectively and are shown in figure 5.4.13. The

bmd gaps and mavjmum absorptjon coeftjcieent obtained for a depsited and

annealed multilayer CdS-CuPc samples are given in table 5.4.7. Optical band gap

and absorption coefficient decreases with annealing temperature. This may be due to

the reduction in trap sites. The difference in band gap due to annealing could be due

to the removal of the trapping levels in the forbidden band region during annealing.

CuPc film can serve as a protective layer for thin semiconducting CdS films against

photocorrosion in photoelectrochcmical cells, photoelectrocatalysis and

electrocatal ysis in he1 cells. Hybrid organic-inorganic photovoltaic junctions already

yield higher efficiencies than all organic junctions, which are also actively

in~esti~ated.~' Valkonen et. a12%tudied CdS - ZnS multilayer thin films gown by

Silar technique.

Table 5.4.7. Optical band gap energy and maximum absorption coefficient of multilayer CdS-CuPc thin films of thickness 8250A

2. Multilayer CdS-CuPc (Annealed at 523K)

Samples

1. Multilayer CdS-CuPc (As deposited)

3. Mu lti layer Cd S-CuPc (Annealed at 5 73 K)

Optical band gap (ev)

3 -75

Absorption coefficient ~ 1 0 " cm-'

0.12

5.4.8. Mul tilayer PbS-CuS films

A band gap energy of 3.75 eV is obtained for as deposited multilayer

PbS-CuS For annealed samples (523 K and 573 K) the energy band gaps

obtained are 3.625 eV and 3.5 eV respectively and are shown in figure 5.4.14. The

optical band gap and maximum absorption coefficient obtained for as deposited and

annealed multilayer PbS-CuS samples are calculated and is given in table 5.4.8.

Optical band gap and maximum absorption coefficient decreases with annealing

temperature.

Table 5.4.8. Optical band gap energy and maximum absorption coefficient of multilayer PbS- CUS thin films of thickness 9350A

Samples

1. Multilayer PbS-CuS (As deposited)

2. Multilayer PbS-CuS (Annealed 523K)

3. Multilayer PbS-CuS (Annealed 573K)

Optical band gap (ev)

3.75

3.62

3.5

Absorption coefficient x lo8 cm-I

0.10

0.07

0.04

5.5 Conclusion

Optical characterisation of the as deposited and annealed CuPc, CuS, PbS,

CdS, multilayer CuS-CuPc, PbS-CuPc, CdS-CuPc and PbS-CuS thin films have

been studied. Optical studies are done to determine the band gap, maximum

absorption coefficient and the effect of annealing on band gap. The optical

absorption spectrum in the range 300nm-900nrn is recorded using the Shimadzu

UV-Visible spectrophotometer. The band gap energy is found to decrease with

annealing temperature. The estimated accuracy in the measurement of the energy

gap is 2 0.02eV. The energy band gap measurements provide a measure of the

trapping levels. It has been reported that annealing causes a redistribution of traps

and hence a drop in band gap energy.29

For all samples, absorption coefficient also decreases with annealing

temperature. This may be due to the reduction in trap sites. The difference in

band gap due to annealing could be due to the removal of the trapping levels in

the forbidden band region during annealing.

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