School report of hin film semiconductor developed in IER-UNAM

1. UNAM Instituto de Energ´ıas Renovables
Universidad Nacional
Aut´onoma de M´exico
Instituto de Energ´ıas Renovables
Semiconductores
Optical properties of thin film semiconductors
Autores:
Castro Grespan Enrico
Profesor:
Dr. Karunakaran Nair Padmanabhan
Pankajakshy
Temixco, Morelos, M´exico
18 de abril del 2017
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Contents
1 Introduction 3
2 Characterization 4
3 Results and discussion 5
3.1 CdS 55 min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 CdS 90 min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3 Sb-S-Se 250 oC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4 Sb-S-Se 270 oC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5 Tec 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.6 Tec 15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.7 Glass 2.5 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Conclusions 11
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1 Introduction
Band gap is one of the most important concepts that has to be clear to develop solar cells. In a
nutshell band gap is the minimum amont of energy required for an electron to jump to a state with
higher energy. To excite an electron that is stuck in its bound state into a free one, energy is requiered,
in this new state the electron can participate in conduction. When the energy of a photon is equal
to or greater than the band gap of the material, the photon is absorbed by the material and excites
an electron into the conduction band. A pair hole-electron is formed a photon is absorbed. The
generation of charge carriers by photons is the basis of the photovoltaic production of energy.
A photon is characterized by either a wavelength, denoted by λ or equivalently an energy, denoted
by E. There is an inverse relationship between the energy of a photon (E) and the wavelength of the
light (λ) given by the Eq.(1)
E =
hc
λ
(1)
where h is Planck’s constant and c is the light velocity.
h = 6.626 × 10−34
J s
c = 2.998 × 108
m/s
It can be demostrated by substituting those constants and converting Joules in electronvolts that
Eq.(1) can be written as
E(eV ) =
1240
λ(nm)
(2)
Optical absorption can be defined has the process where the energy of a photon is removed or
stole by matter, more precisely the electrons of an atom. The absorption coefficient determines how
far into a material light of a particular wavelength can penetrate before it is absorbed. Semiconductor
materials have a sharp edge in their absorption coefficient, since light which has energy below the band
gap does not have sufficient energy to excite an electron into the conduction band from the valence
band.
In order to calculate the optical absorption of a certain material, measurements of optical trans-
mittance and reflectance are needed. As we can see in Fig.(1) by measuring the transmittance and the
reflectance, in a sample with ceratin composition, it is possible to calculate the absorptance according
to the following.
A(λ) = 1 − (Tλ + Rλ)
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Figure 1: Diagram of the optical processes in a thin film.
As it is known solar cells are complex devices, the semiconductors that are used need to have a
substrate and elements to collect the free carriers. There are a couple of ways to do that; one way is
using metal contacts as in common silicion solar cells and other is with Transparent Conductive Oxides
(TCO). Since these materials are on top of our semiconductor film, the need to know the transmission,
reflection and absorption is requiered. In this work we analyse Tec7 and Tec15 as well as the normal
substrate glasses. Likewise, with those values and the solar spectral irradiance is possible to estimate
the amount of power per unit area that goes through our film, as well as the amount that gets reflected
in each region.
2 Characterization
In thin films the absorption coefficient (α) can be yield from Eq.(3) obtained by doing the analysis of
the transmittion and considering multiple reflections within the thin film, that obeys Beer-Lambert
law, in a film with thickness d.
T(λ) =
(1 − Rλ)2e−αλd
1 − R2
λe−2αλd
(3)
α =
1
d
ln
(1 − R)2 + (1 − R)2 + 4RT2
2T
(4)
Another important parameter is photon flux; which determines the number of photons that interact
with the material generating free electrons, and hence the current produced by the solar cell. This
parameter does not have the information about the energy (or wavelength) of the photons. The
solar photon flux can be calculated using Eq.(5) in order to estimate the photon flux s−1m−2 by a
semiconductor with bandgap Eg.
Nph(λ) =
Iλ
Eλq
(5)
Furthermore, by knowing the number of photons with enough energy to excites an electron into
the conduction band (those with energy greater than the band gap). By applying Eq.(6) the upper
limit of the shortcircuit current density can be estimated for each semiconductor.
Nph × q (6)
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3 Results and discussion
According to Eq.(4) the thickness of the films is needed to calculate the value of the absorption
coefficient. A profilometer is used, in this work, to obtein those measurements; these instruments
measure surfaces profile. A reference, for example, the substrate, works as the step up or down.
However this devices are not very precise. Nevertheless thickness can also be estimated using Eq.(7)
which needs the wavelength of two adjascent maxima of our transmittance measurements that will be
discussed later on.
d =
λ1 λ2
2 n(λ1 − λ2)
(7)
Thickness (nm)
Sample Measure Calculated
CdS 55 min 80 105
CdS 90 min 182 266
Sb2S − S3 250◦C 188 n.d
Sb2S − S3 270◦C 140 n.d
Tec15 200 n.d
Optical transmittance (T) and specular reflectance (R, measured at an angle of incidence of 5◦)
spectra of the films were measured in the cell structure using 3101PC Shimadzu UV-VIS-NIR spec-
trophotometer. For the CdS samples remove the film from one side is required. With these measure-
ments it is possible, using the solar spectral irradiance, calculate the amount of light that is transmited
or reflected and also; estimate the number of photons that are absorbed in each film. Eq.(??) give us
the power per square meter that is transmitted through the film in the UV region; Similarly for the
reflectance Eq.(??) and for other spectral regions Eq.(?? - ??).
380 nm
λ=300 nm
Iλ Tλ ∆λ (8)
380 nm
λ=300 nm
Iλ Rλ ∆λ (9)
780 nm
λ=380 nm
Iλ Tλ ∆λ (10)
780 nm
λ=380 nm
Iλ Rλ ∆λ (11)
4000 nm
λ=780 nm
Iλ Tλ ∆λ (12)
4000 nm
λ=780 nm
Iλ Rλ ∆λ (13)
Weighing those values with the solar radiation, as shown in Eq.(14 and 15), the percentage of
radiation transmitted and reflected are calculated.
Tvis = 100 ×
780 nm
λ=380 nm
Iλ Tλ ∆λ
780
λ=380
Iλ ∆λ
(14)
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Rvis = 100 ×
780 nm
λ=380 nm
Iλ Rλ ∆λ
780 nm
λ=380 nm
Iλ ∆λ
(15)
3.1 CdS 55 min
Region % Transmitted % Reflected % Absorbed
UV 14.17 5.23 80.6
Vis 69.85 11.94 18.21
NIR 72.60 21.43 5.97
Photon flux s−1m−2 for CdS 55 min, with a Eg = 2.84 eV , is 1.984 × 1020 which is 3.83% of the
available resource. Short circuit current density equal to 3.178 mA/cm2
R
T
500 1000 1500 2000
20
40
60
80
CdS55min
wavelength (nm)
%
Figure 2: Optical transmittance (T) in blue and
reflectance (R) in red versus wavelength plots for
CdS deposited at 80◦C during 55 min from a
chemical bath containing cadmium-citrate com-
plex.
A
T+R
500 1000 1500 2000
20
40
60
80
100
CdS55min
wavelength (nm)
%
Figure 3: Optical transmittance (T) plus re-
flectance (R) in pink and absorption (A) in green
versus wavelength plots for CdS deposited at 80◦C
during 55 min from a chemical bath containing
cadmium-citrate complex.
3.2 CdS 90 min
Region % Transmitted % Reflected % Absorbed
UV 0.43 5.20 94.37
Vis 52.26 14.75 32.99
NIR 76.40 14.21 9.39
Photon flux s−1m−2 for CdS 90 min, with a Eg = 2.47 eV , is 4.596 × 1020 which is 8.87% of the
available resource. Short circuit current density equal to 7.363 mA/cm2
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2.0 2.5 3.0 3.5 4.0 4.5 5.0
Energy (eV)
200000
400000
600000
800000
1×106
α/cm-1
CdS55min
Figure 4: Absorption coefficient (α) corrected by a T + R = 0.93 versus wavelength plots for CdS
deposited at 80oC during 55 min from a chemical bath containing cadmium-citrate complex.
R
T
500 1000 1500 2000
20
40
60
80
100
CdS90min
wavelength (nm)
%
Figure 5: Optical transmittance (T) in blue and
reflectance (R) in red versus wavelength plots for
CdS deposited at 80oC during 90 min from a chem-
ical bath containing cadmium-citrate complex.
T+R
A
0 500 1000 1500 2000
20
40
60
80
100
CdS90min
wavelength (nm)
%
Figure 6: Optical transmittance (T) plus re-
flectance (R) in pink and absorption (A) in green
versus wavelength plots for CdS deposited at 80oC
during 90 min from a chemical bath containing
cadmium-citrate complex.
3.3 Sb-S-Se 250 o
C
Region % Transmitted % Reflected % Absorbed
UV 0.00 49.94 50.06
Vis 0.77 51.80 47.43
NIR 41.74 46.50 11.76
Photon flux s−1m−2 for SbSSe3 at 250oC, with a Eg = 1.33 eV , is 9.461 × 1020 which is 18.25%
of the available resource. Short circuit current density equal to 15.156 mA/cm2
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2.0 2.5 3.0 3.5 4.0
Energy (eV)
50000
100000
150000
200000
α/cm-1
CdS90min
Figure 7: Absorption coefficient (α) corrected by a T + R = 0.93 versus wavelength plots for CdS
deposited at 80oC during 90 min from a chemical bath containing cadmium-citrate complex.
RT
0 500 1000 1500 2000
20
40
60
80
100
Sb-S-Se 250ºC
wavelength (nm)
%
Figure 8: Optical transmittance (T) in blue and
reflectance (R) in red versus wavelength plots for
Sb-S-Se film deposited at 250oC by thermal evap-
oration.
A
T+R
0 500 1000 1500 2000
20
40
60
80
100
Sb-S-Se 250ºC
wavelength (nm)
%
Figure 9: Optical transmittance (T) plus re-
flectance (R) in pink and absorption (A) in green
versus wavelength plots for Sb-S-Se film deposited
at 250◦C by thermal evaporation.
3.4 Sb-S-Se 270 o
C
Region % Transmitted % Reflected % Absorbed
UV 0.00 49.98 50.02
Vis 0.38 51.44 48.18
NIR 32.62 48.49 18.89
Photon flux s−1m−2 for SbSSe3 at 270◦C, with a Eg = 1.4 eV ,is 1.034 × 1021 which is 19.95% of
the available resource. Short circuit current density equal to 24.23 mA/cm2
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1.0 1.5 2.0 2.5 3.0
Energy (eV)
100000
200000
300000
400000
500000
600000
α/cm-1
Sb-S-Se 250ºC
Figure 10: Absorption coefficient (α) versus wavelength plot for Sb-S-Se film deposited at 250◦C by
thermal evaporation.
T
R
500 1000 1500 2000
20
40
60
80
100
Sb-S-Se 270ºC
wavelength (nm)
%
Figure 11: Optical transmittance (T) in blue and
reflectance (R) in red versus wavelength plot for
Sb-S-Se film deposited at 270◦C by thermal evap-
oration.
A
T+R
500 1000 1500 2000
20
40
60
80
100
Sb-S-Se 270ºC
wavelength (nm)
%
Figure 12: Optical transmittance (T) plus re-
flectance (R) in pink and absorption (A) in green
versus wavelength plot for Sb-S-Se film deposited
at 270◦C by thermal evaporation.
3.5 Tec 7
The table show the result of the analyzed data to see which light is able to surpass Tec 7 film and
reach the semiconductor material.
Region % Transmitted % Reflected % Absorbed
UV 3.01 0.65 96.34
Vis 61.46 4.98 33.56
NIR 62.19 6.76 31.05
For the conductive films we measure optical tranittance and reflectance to also calculate absorption.
Absorption coefficient in this cases is not needed.
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1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
Energy (eV)
5.0×106
1.0×107
1.5×107
2.0×107
2.5×107
3.0×107
α/cm-1
Sb-S-Se 250ºC
Figure 13: Absorption coefficient (α) versus wavelength plot for Sb-S-Se film deposited at 270◦C by
thermal evaporation.
R
T
500 1000 1500 2000
20
40
60
80
100
Tec 7
wavelength (nm)
%
Figure 14: Optical transmittance (T) in blue and
reflectance (R) in red versus wavelength plot for
Tec 7 film.
A
T+R
500 1000 1500 2000
20
40
60
80
100
Tec 7
wavelength (nm)
%
Figure 15: Optical transmittance (T) plus re-
flectance (R) in pink and absorption (A) in green
versus wavelength plot for Tec 7 film.
3.6 Tec 15
Region % Transmitted % Reflected % Absorbed
UV 2.34 9.40 88.26
Vis 74.79 12.48 12.73
NIR 61.76 11.55 26.69
With the data shown in the table for Tec 15 and the previous one for Tec 7, we can conclude that
UV light can not reach the absorber film but this tecnologies present good transmission mostly in the
visible spectrum.
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T
R
500 1000 1500 2000
20
40
60
80
100
Tec 15
wavelength (nm)
%
Figure 16: Optical transmittance (T) in blue and
reflectance (R) in red versus wavelength plot for
Tec 15 film.
T+R
A
0 500 1000 1500 2000
20
40
60
80
100
Tec 15
wavelength (nm)
%
Figure 17: Optical transmittance (T) plus re-
flectance (R) in pink and absorption (A) in green
versus wavelength plot for Tec 15 film.
3.7 Glass 2.5 mm
Region % Transmitted % Reflected % Absorbed
UV 8.59 6.32 85.09
Vis 89.07 9.47 1.46
NIR 85.24 8.21 6.55
For the Corning Glass, the values are close to the TCO wich is precise for solar applications.
R
T
500 1000 1500 2000
20
40
60
80
100
Vidrio 2.5 mm
wavelength (nm)
%
Figure 18: Optical transmittance (T) in blue and
reflectance (T) in red versus wavelength plot for
2.5 mm glass.
A
T+R
500 1000 1500 2000
20
40
60
80
100
Vidrio 2.5 mm
wavelength (nm)
%
Figure 19: Optical transmittance (T) plus re-
flectance (R) in pink and absortion (A) in green
versus wavelength plot for 2.5 mm glass.
4 Conclusions
We analyzed the optical response of the elements that compound a antimony sulfide-selenide solar
cell. Through simple calculations we arrived at shortcircuit current values by optical measurements
and applying the concepts reviewed in lectures. As we can observed not all the photons form the sun
create free electron and not only because of their energy, also as a result of the layer on top of our
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absorber material as it is the conductive glass and the CdS film. There is a balance to be made to
yield the best results becuase thickness control band gap of the CdS but also result in a change of
the absorption. Further analyses need to be done in order to see why different temperatures in the
Sb-S-Se films give us different band gaps.
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