Absorbance/Transmittance/Reflectance of PCDTBT:PC70BM
Organic Blend Layer
Rashmi Swami*, Rajesh Awasthy, B.P.Chandra and Sanjay Tiwari
Photonics Research Laboratory
School of Studies in Electronics ,
Pt. Ravishankar Shukla University, Raipur(C.G.) 492010 India
Email: rashmi.swami3@gmail.com
ABSTRACT
In this paper we are investigating the optical properties of photoactive layer consisting of blend of PCDTBT (an electron donor) PC70BM (an
electron acceptor). Using MATLAB coding for cumulative and net radiation method of multilayer structure we investigated absorbance,
transmittance, reflectance spectra and their variation with wavelength. Both methods are equivalent ( both will give same result) and they take
into account the effect of multiple reflections. Refractive index, extinction coefficient, absorbance of PCDTBT:PC70BM blend film shows
variations in visible range (350 nm - 650 nm). Refractive index of PCDTBT:PC70BM increases from wavelength 450 nm to 612 nm and
reaches to its maximum value 2.2883 at 610 nm wavelength. Extinction coefficient of PCDTBT:PC70BM has maximum value of 0.4408 at
300 nm wavelength then decreases with increase in wavelength. The PCDTBT:PC 70BM PV film showed strong absorption bands in the visible
range from 350 to 650 nm, extending to an absorption onset at 800 nm, and containing two distinct but broad absorption bands centered at ca.
380-420 nm and ca. 475-570 nm. Reflectance is less than 17% in visible and infrared region. Blend layer shows more than 50% transmittance
at wavelengths longer than 570 nm i.e. transmittance is high in infrared region and sharp fall below 350 nm because of strong absorbance in
this region.
Keywords- PCDTBT:PC70BM, absorbance, transmittance, reflectance, refractive index.
I.
INTRODUCTION
Sun is a huge source of renewable energy that can be converted
into electrical energy using solar cells. Organic materials,
particularly polymers, are a promising alternative to traditional
semiconductors as the active material for solar cell because of
their low cost, low temperature & energy processing, low material
requirement, can be used on flexible substrate, can be shaped to
suit architectural application. The key development of organic
solar cells has been made with the pioneering concept of ‘‘bulk
heterojunction (BHJ)’’ photoactive layers [1][2].The bulk
heterojunction (BHJ) PSC [1][3-7] is of particular interest, due to
the efficient photo-induced generation of charge in its blended
photovoltaic (PV) layer, which is composed of interpenetrating,
channel-like domains of separated polymer and fullerene phases.
Recently, as an alternative, poly[N-9’’-hepta-decanyl-2,7carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’- benzothiadiazole)]
(PCDTBT) and [6,6] -phenyl C70-butyric acid methyl ester
(PC70BM) based organic solar cells have received intensive study
due to higher energy conversion efficiency of more than 6% [8].
This is due to a better alignment of molecular orbital energy
levels between the PCDTBT polymer and the PCBM fullerene.
A. Donor molecule
Poly[[9-(1-octylnonyl)-9H-carbazole-2.7-diyl]-2.5-thiophenediyl2.1.3 benzothiadiazole-4.7-diyl-2.5-thiophenediyl] (PCDTBT)
shown in Fig. 1. is one of the next generation donor materials.
The key properties of PCDTBT over standard organic
photovoltaic materials (such as P3HT) :

lower HOMO and LUMO levels

narrow band gap

Increased open circuit voltage

Longer wavelength absorption



Lower concentration and material usage
Improved stability under ambient conditions
High electron and hole generation rate and high
mobility of electron and hole.
Fig. 1. Molecular structure of PCDTBT.
B. Acceptor molecule
Fullerenes are highly symmetrical cage-shaped molecules
constituted only by carbon atoms, which have been thoroughly
studied during the last two decades [9]. Blending conjugated
polymers with electron acceptors such as fullerenes, is a very
efficient way to break apart photoexcited excitons into free charge
carriers. The fullerene PCBM core readily accepts electrons from
a wide range of organic donor materials and exhibits high electron
mobilities even in composite form. In this study, we used the
soluble PC70BM, the chemical structure of which is shown in
Fig.2.
Fig. 2. Molecular structure of PC70BM.
𝜂 =
In this paper we investigated optical properties of photoactive
blend layer of PCDTBT:PC70BM on glass substrate.
II.
EXPERIMENTAL WORK
Vacuum evaporation and solution processing techniques are the
most commonly used thin film preparation methods in the
fabrication of organic solar cells. Since polymers decompose
under excess heat, most photovoltaic polymers are solution
processed films through spin coating. During the process of spincoating, an excess amount of the D/A (Donor/Acceptor) blend
solution is dropped on the substrate, which is then rotated at a
high speed in order to spread the fluid by centrifugal force.
Solutions of D/A blends can be prepared by dissolving donor and
acceptor materials in a common solvent. The blend films in this
study were all prepared by spin-coating method. As shown in
Fig.3. above glass substrate the photoactive layers were spin-cast
at 500–3000 rpm by using a blend solution of PCDTBT (7 mg/ml)
and PC70BM (28 mg/ ml) dissolved in 1,2 dichlorobenzene (oDCB). The BHJ films were dried at 60 degree centigrate (40 min)
to remove the solvent. Using MATLAB we obtained the graph of
absorbance, transmittance, reflectance.
N/ cos 𝜙 for p-polarized irradiance,
N cos 𝜙 for s-polarized irradiance.
Here 𝜂1 is determined using N1 and 𝜙1 and 𝜂2 is determined
using N2 and 𝜙2 . The reflection coefficient depends on the
polarisation state of light. Direct sunlight is generally considered
to be unpolarised [11], i.e. it contains equal amounts of p- and spolarised irradiance. The fraction of energy in the refracted
(transmitted) beam is simply given by
t=1-r
(2)
B. Reflection and refraction at a coated
interface
The reflection coefficient of the interface between medium 1 and
2 can be reduced significantly if a thin coating is added to the
interface. This coating is characterized by a thickness dc and a
refractive index Nc = nc−ikc. The coating is considered to be thin
if its optical thickness (ncdc) is smaller than the coherence length
of the incident light, which is approximately 1 μm for solar
irradiance [11]. If the refractive index of the coating lies in
between the refractive indices of the neighbouring media, then the
reflection coefficient of the combination of the coating and the
underlying medium is given by-
r=|
𝜂1 − 𝑌 2
𝜂1 + 𝑌
|
(3)
where Y can be interpreted as the ‘effective refractive index’ of
this combination [10], given by-
Y=
𝜂2 cos 𝛿+ 𝑖𝜂𝑐 sin 𝛿
𝜂
cos 𝛿+ 𝑖( 2⁄𝜂𝑐 ) sin 𝛿
(4)
Phase difference 𝛿 is given by –
𝛿=
Fig. 3. Structure of PCDTBT:PCBM blend over glass
substrate
Where n1 = refractive index of first medium(medium 0) air = 1.
n2 = refractive index of PCDTBT:PC70BM blend (medium 1).
k2 = extinction coefficient of PCDTBT:PC70BM blend (medium
1) .
2𝜋𝑁𝐶 𝑑𝑐
𝜆 cos 𝜙𝑐
(5)
where 𝜆 is the (vacuum) wavelength of the irradiance and the
angle of refraction in the coating 𝜙𝑐 is given by Snell’s law.
C. A system with two interfaces
As shown in Fig. 4, there are three media involved (labelled 0, 1
and 2), separated by two interfaces (labelled 1 and 2). Each
medium is characterised by a complex refractive index N ( N = n
– ik , where n is the real refractive index and k is the extinction
coefficient). The first and final media are semi-infinite, medium 1
(or layer 1) is characterized by a thickness d1.
n3 = refractive index of glass substrate (medium 2) = 1.5.
III.
CACULATION METHODS
Here we are explaining different methods for the calculation of
absorbance, transmittance, reflectance of layer.
A. Reflection and refraction at an uncoated
interface
First the most simple interface, without coating, is considered.
Using a notation similar to Macleod’s [10], the reflection
coefficient is given by
r=|
η1− η2 2
η1+η2
|
(1)
where 𝜂 is the modified refractive index given by
Fig. 4. A cross-section of a simple multilayer system
containing two interfaces. Left: the internally reflecting
sub-rays considered in the cumulative method are
indicated. Right: the net-radiation fluxes considered in
the net-radiation method are indicated.
Assuming specular reflection and a given angle of incidence ø 0,
the reflection coefficients r1 and r2 of interfaces 1 and 2,
respectively, can be determined. Normal incidence is considered
here. The transmission coefficient 𝜏1 of layer 1 is defined as the
fraction of irradiance remaining after a single passage through this
layer
𝜏1 = 𝑒 −𝛼1 𝑑1/𝑐𝑜𝑠 ø1
(6)
where d1/ cos ø1 is the distance the ray has traversed in the layer.
d1(150 nm) is thickness of blend layer. Angle ø1 is calculated
using Snell’s law –
n1 sin ø0 = n2 sin ø1
(7)
For a non-absorbing layer 𝜏 = 1 and for an opaque layer 𝜏 = 0.
Two methods for determining the spectral
absorption factor Aλ of the system as a whole will be presented.
Both methods are equivalent and they take into account the effect
of multiple reflections, indicated in Fig. 4.
1) Cumulative method: The most straightforward strategy is
to follow (trace) an incoming ray of light as sketched in the left
panel of Fig. 4. At the interfaces the ray will split up in a reflected
and refracted sub-ray and multiple internal reflections occur
between the two interfaces. In order to determine the factors RC,
AC and TC accurately, the contribution of each sub-ray has to be
considered. The infinitely many contributions can be expressed as
a geometric series –
RC = r1+
𝑡12 𝜏12 𝑟2
(1−𝑟1 𝑟2 𝜏12 )
TC = 𝑡1 𝜏1 𝑡2 ∑∞𝑛=0(𝑟1 𝑟2 𝜏12 )𝑛
TC =
(8)
(1−𝑟1 𝑟2 𝜏12 )
(10)
where r and t are the reflection and transmission coefficients of
the interfaces (equation 1 &2) and 𝜏 is the transmission
coefficient of the layer (equation 6) .
2) Net-radiation method: Another strategy is to group the
infinitely many sub-rays into net-radiation fluxes as indicated in
the right panel of Fig.4. There are four fluxes (labelled a, b, c and
d) per interface. For example flux q1b contains all sub-rays
travelling away from interface 1 in the upward direction. Because
each flux contains the net-radiation, this method is called the netradiation method [12]. It is convenient to consider the fluxes to be
non-dimensional and to normalise the incident flux, i.e. q 1a = 1.
Further it is assumed that no irradiance is incident from below the
multilayer structure, i.e. q2c = 0. It can be checked that the fluxes
are related in the following way –
q1a = 1
q1b = r1q1a + t1q1c
q1c = 𝜏1q2b
q1d = r1q1c + t1q1a
q2a = 𝜏1q1d
q2b = r2q2a + t2q2c
q2c = 0
q2d = r2q2c + t2q2a
(12)
𝑇𝑛 = 𝑞2𝑑
(13)
𝐴𝑛 = 𝑞1𝑑 − 𝑞2𝑎 + 𝑞2𝑏 − 𝑞1𝑐
(14)
Note that this method is equivalent with the cumulative method,
but it has the advantage that the individual sub-rays do not need to
be considered.
IV.
RESULTS AND DISCUSSION
The complex indices of refraction
were measured by
spectroscopic ellipsometry and used to describe optical
transmission, reflection, absorption characteristic of photoactive
blend layer. Fig.5 shows the variation of refractive index and
extinction coefficient of PCDTBT:PC70BM with wavelength.
Refractive index of PCDTBT:PC70BM increases from wavelength
450 nm to 612 nm and reaches to its maximum value 2.2883 at
610 nm wavelength. Extinction coefficient of PCDTBT:PC70BM
has maximum value of 0.4408 at 300 nm wavelength then
decreases with increase in wavelength.
(9)
𝑡1 𝜏1 𝑡2
AC = 1-RC-TC
𝑅𝑛 = 𝑞1𝑏
(11)
refractive index and extinction coefficient of PCDTBT:PCBM
RC = r1+t12 𝜏12 r2 ∑∞𝑛=0(𝑟1 𝑟2 𝜏12 )𝑛
By solving this set of linear equations, the fluxes are found and
the spectral reflection, absorption and transmission factors can be
found directly
graph of refractive index and extinction coefficient of PCDTBT:PCBM
2.5
n2
k2
2
1.5
1
0.5
0
-0.5
300
400
500
600
700
800
Lambda (nm)
900
1000
1100
1200
Fig. 5. Refractive index and extinction coefficient of
PCDTBT:PC70BM.
We calculated reflectance, transmittance and absorbance using
cumulative method from equations (8,9,10) and using net
radiation method from equations (12,13,14). From both the
methods we got the same graph for absorbance, transmittance and
reflectance with respect to wavelength. Absorbance graph shown
in Fig.6 using cumulative method and in Fig.7 using net radiation
method show that the PCDTBT:PC70BM PV film showed strong
absorption bands in the visible range from 350 to 650 nm,
extending to an absorption onset at 800 nm, and containing two
distinct but broad absorption bands centered at ca. 380-420 nm
and ca. 475-570 nm. The two broad absorption bands with peaks
at 398 and 576 nm were caused by the PCDTBT, and the
absorption near 450 nm was caused by the PC70BM. The
Absorbance graph of PCDTBT:PCBM using cumulative method
0.9
Ac (absorbance for 150nm thickness)
0.8
0.7
Absorbance (Ac)
0.6
Transmittance (Tc) and Reflectance (Rc)
two broad absorption bands with peaks at 398 and 576 nm were
caused by the PCDTBT, and the absorption near 450 nm was
caused by the PC70BM.
Transmittance and Reflectance graph of PCDTBT:PCBM using cumulative method
0.9
0.8
0.7
0.6
0.5
0.4
Tc(transmittance for 150nm thickness)
Rc(Reflectance for 150nm thickness)
0.3
0.2
0.1
0.5
0
300
0.4
400
500
600
0.3
700
800
Lambda (nm)
900
1000
1100
1200
0.2
Fig.8. Transmittance and Reflectance versus wavelength of
incident radiation using cumulative method .
0.1
0
400
500
600
700
800
Lambda (nm)
900
1000
1100
1200
Transmittance and Reflectance graph of PCDTBT:PCBM using net radiation method
0.9
Transmittance (Tn) and Reflectance (Rn)
-0.1
300
Fig. 6. Absorbance versus wavelength of incident radiation
using cumulative method.
Absorbance graph of PCDTBT:PCBM using net radiation method
0.9
An (absorbance for 150nm thickness)
0.8
0.7
Absorbance (An)
0.6
0.5
0.8
0.7
0.6
0.5
0.4
0.3
Tn(transmittance for 150nm thickness)
Rn(Reflectance for 150nm thickness)
0.2
0.1
0.4
0
300
0.3
400
500
600
700
800
Lambda (nm)
900
1000
1100
1200
0.2
0.1
Lambda
0
-0.1
300
400
500
600
700
800
Lambda (nm)
900
1000
1100
1200
Lambda (nm)
Fig. 7. Absorbance versus wavelength of incident radiation
using net radiation method.
Fig.8 (using cumulative method) and Fig.9 ( using net radiation
method) show variation of transmittance and reflectance of
PCDTBT:PC70BM with wavelength. Blend layer shows more
than 50% transmittance at wavelengths longer than 570 nm i.e.
transmittance is high in infrared region and sharp fall below 350
nm because of strong absorbance in this region. Reflectance is
less than 17% in visible and infrared region.
Fig.9. Transmittance and Reflectance versus wavelength of
incident radiation using net radiation method .
V.
CONCLUSION
We investigated the optical properties of photoactive layer
consisting of blend of PCDTBT (an electron donor) PC70BM(an
electron acceptor) using cumulative and net radiation method.
Photoactive blend layer properties investigated include their
refractive
index,
extinction
coefficient,
absorbance/transmittance/reflectance spectra and their variation
with wavelength. Graph of absorbance/transmittance/reflectance
of PCDTBT:PC70BM photoactive blend layer with wavelength is
obtained using MATLAB coding for cumulative and net radiation
method. Refractive index, extinction coefficient, absorbance of
PCDTBT:PC70BM blend film shows variations in visible range
(350 nm - 650 nm). Refractive index of PCDTBT:PC70BM
increases from wavelength 450 nm to 612 nm and reaches to its
maximum value 2.2883 at 610 nm wavelength. Extinction
coefficient of PCDTBT:PC70BM has maximum value of 0.4408
at 300 nm wavelength then decreases with increase in
wavelength. The PCDTBT:PC70BM PV film showed strong
absorption bands in the visible range from 350 to 650 nm,
extending to an absorption and conductivity onset at 800 nm, and
containing two distinct but broad absorption bands and
conductivity band centered at ca. 380-420 nm and ca. 475-570
nm. Blend layer shows more than 50% transmittance at
wavelengths longer than 570 nm i.e. transmittance is high in
infrared region and sharp fall below 350 nm because of strong
absorbance in this region. Reflectance is less than 17% in visible
and infrared region.
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