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MASTER THESIS
Measuring the efficiency and charge
carrier mobility of organic solar
cells
Master`s Thesis within the Master`s program in Physics
ABASI ABUDULIMU
SUPERVISOR
DAVID BARBERO
EXAMINER
BERTIL SUNDQVIST
Department of Physics
Umeå University
Umeå, Sweden 2012
Abstract
P3HT single layer, P3HT/PCBM bilayer and P3HT/PCBM inverted
bilayer devices were produced by spin coating organic layers onto ITO
patterned glass in air, and clamping it with an Au coated silicon wafer,
as top electrode, at the end (Figure13). Normal and inverted bilayer
devices were also fabricated with and without PEDOT:PSS. All devices
were divided into two groups by changing concentration of P3HT
solution. The first group of devices contained 1.0 wt. % P3HT solution
(P3HT in dichlorobenzene); the second group 0.56wt %. Power
conversion efficiency, short circuit current, open circuit voltage, fill
factor and maximum extracted power were measured on all produced
devices.
In contrast, all devices with 1.0wt % P3HT concentration showed
better result than the devices with 0.56wt %. The highest result was
obtained for P3HT single layer devices in both cases with short circuit
current 56uA/cm2, open circuit voltage 0.94mV, maximum power
11.4uW/cm2 and power conversion efficiency of 0.11%. Inverted
bilayer devices performed better than the non-inverted one. The devices
with PEDOT:PSS got slightly better performance than the nonPEDOT:PSS used one.
Charge carrier mobility measurement was done for all fabricated
devices with charge extraction by linearly increasing voltage (CELIV)
and dark injected space charge limited current (DI-SCLC) methods. All
devices showed same magnitude of charge carrier mobility 10 -5 cm2/V.s,
the highest value still belongs to P3HT single layer device. The charge
carrier mobility in all devices observed by DI-SCLC technique is one
order of magnitude higher than by CELIV technique. This may be due
to DI-SCLC method`s restriction on ohmic contacts between material
and electrode.
Contents
Abstract
Introduction and theory …………………………………………….
1
1.1
I-V characterization ………………………………………. 4
1.1.1
Quantum efficiency ……………………………... 8
1.1.2
Equivalent circuit ……………………………….. 10
1.2
Charge carrier mobility measurement methods …………… 14
1.2.1
Time of Flight (TOF) ……………………………. 14
1.2.2
Charge extraction by linearly increasing voltage
(CELIV) …………………………………………. 17
1.2.3
Dark injection-space charge limited current (DISCLC) …………………………………………… 20
Experimental part …………………………………………………. 22
2.1
Materials ………………………………………………….
2.1.1
ITO …………………………………………......
2.1.2
PEDOT:PSS …………………………………….
2.1.3
P3HT …………………………………………....
2.1.4
PCBM …………………………………………..
23
23
23
24
24
2.2
Device production and equipment ……………………….
2.2.1
ITO cleaning ……………………………………
2.2.2
ITO etching …………………………………......
2.2.3
P3HT device preparation ……………………….
26
26
26
27
2.2.4
2.2.5
2.2.6
Normal bilayer device preparation ……………… 28
Inverted bilayer device preparation …………...... 29
Equipment ………………………………………. 31
Result and discussion ……………………………………………… 35
3.1
I-V measurement result and discussion …………………... 36
3.1.1
3.1.2
3.1.3
3.1.4
3.2
P3HT concentration dependence of I-V and power
curve …………………………………………….. 37
I-V and power curve comparison for all
Devices …………………………………………. 50
Devices
performance
with
and
without
PEDOT:PSS ……………………………………. 56
Summary for I-V measurement ………………... 64
Charge
carrier
mobility measurement result and
discussion ………………………………………………... 73
3.2.1
3.2.2
Charge extraction by linearly increasing voltage
(CELIV) ………………………………………... 73
Dark injection-space charge limited current (DISCLC) …………………………………………. 77
Conclusion ……………………………………………………….
83
References ………………………………………………………..
87
Appendices ……………………………………………………….
91
Data from I-V characteristic measurement ………….
91
Data from charge carrier mobility measurement ….... 102
Chapter 1
Introduction and theory
1
The energy demand of the world has been increasing with the fast
development of the society for decades, and the energy sources (coal,
oil and natural gas) which have been used are limited. Energy scenarios
predict a further increase in energy demand by 55% in 2030 compared
to today (1). They must be replaced with renewable energy sources such
as wind and earth heat, which have been believed the best choices as
they would not cause big problem to the nature and it is possible to
produce them with lower cost as well
(2)
. Another potential renewable
energy source is sun light. Converting sun light into electrical energy Solar Cells - has become a most interesting topic.
So far the highest light conversion efficiency for mono-crystalline
silicon solar cell is 25%
(3) (4)
. Due do inorganic solar cell`s
inconvenient production process and material shortness, the organic
solar cells has become a more attractive topic since last two decades.
Organic solar cell uses conductive organic polymers or small organic
molecules, which are environment friendly, for light absorption and
charge transport. The plastic substrate has low production costs in high
volumes, which made it potentially lucrative for photovoltaic
applications with the flexible organic molecules. Three types of organic
solar cell have been introduced: Single layer
hetero-junction
(7)
(5)
, Bilayer
(6)
and Bulk
photovoltaic organic solar cell.
In single layer photovoltaic organic solar cell, the organic electronic
materials are sandwiched between two conductive metals with different
2
work functions mostly ITO (indium tin oxide: high work function) and
Al (aluminum: low work function). The work function difference
between the two electrodes creates an electric field on the organic layer
which will play the role of separating electrons and holes to different
electrodes which are generated by organic material when it is absorbing
light.
Bilayer photovoltaic organic solar cells have two organic layers with
different material between the two electrodes. The two layer organic
materials have different electron affinity and ionization energy which
causes electrostatic force. The electric field could be strong to separate
the electron-hole pairs if the materials are chosen properly to have large
differences in electron affinity and ionization energy.
Another type of organic solar cell is the bulk hetero-junction solar
cell, in which a polymer blend, normally made by mixing electron
donor and acceptor materials together, is sandwiched between the
electrodes. In this type, most of the generated excitons co uld reach the
interface, if the blend length scale is the same as the exciton diffusion
length; then it could be separated efficiently to the opposite electrodes.
3
Figure1(8). Device structure of three common types of organic photovoltaic
solar cells: Single layer (left), Bilayer (middle) and bulk Hetero-junction
(right).
1.1 I-V characterization
The principle of photovoltaic cells is to convert light into electricity.
I-V measurement is the most popular method for solar cell
characterization. It gives very important information about the solar
cell. From the I-V measurement result, one could know which
parameter of the sample should be changed to optimize the cell to get
higher efficiency.
The elements which affect the I-V characteristic of the cell are
material and interface`s conductivity, traps, recombination and charge
carrier diffusion length. According to some articles
(9)
quantum
efficiency, charge collection at electrodes, light absorption and
recombination affect the short circuit current. Recombination and
leakage current influences the open circuit voltage; it could be
4
determined by the material`s energy level. Solar cell internal resistance,
recombination and poor charge collection also decreases the fill factor.
These are the key elements to be optimized to get highest solar cell
efficiency.
The power conversion efficiency (η), the percentage of incident light
that is converted into electrical power, is the most important parameter
to evaluate the solar cell performance. The device architecture could
change the efficiency, and the higher power conversion efficiency tends
to be obtained with complex structure and more expensive process
steps. The definition of the power conversion efficiency is the ratio of
the maximum power output (electrical power generated by the cell), to
the power input (received power from the light) to the cell:
(1)
The most common way to do the I-V characteristic measurement is
to apply a voltage to the electrodes of the solar cell and measure the
current. To make the result comparison convenient, the obtained data,
should always be divided by the actual active surface area of the
sample and reported as current, J (mA/cm2) vs. bias V (V). Figure 1
illustrates the typical I-V characteristics of a solar cell.
5
Figure2. A typical Current-Voltage Characteristic of a solar cell in the dark
and under illumination (10).
Figure3. A typical Power Curve for solar cell.
6
Figure2 and Figure3 show most of the important parameters of solar
cells` I-V characteristics such as: short circuit current (J sc), open circuit
voltage (Voc) and maximum electrical power point (Pmax). When the cell
is measured in the dark, almost no current will be flowing; it increases
only when the charges are injected into the sample by the applied bias
which is larger than the cells` open circuit voltage. Whereas the I-V
curve will move to the downside in relation with amount of the photo
generated charge carriers under the illumination (see Figure 1). The
maximum photo current could be achieved when the applied voltage is
zero, it is called short circuit current (Jsc). The maximum photo voltage
(open circuit voltage) is seen when the current goes to zero. It means
the solar cell`s internal voltage is equal to the applied voltage.
The product of short circuit current and open circuit voltage is equal
to the maximum power if the solar cell is an ideal diode. In practice the
maximum electrical power point of the cell is always found at one point
on the I-V curve, normally it appears in the fourth quadrant (Figure2
right down side). The current and voltage at that maximum power point
is usually marked as I max and Vmax respectively. In solar cells there is
another important parameter the so called fill factor (FF):
(2)
(3)
7
FF represents a measure of the quality of the IV characteristics` shape.
The higher the FF the higher electric power the solar cell could provide
and the more stable current could be extracted from the cell.
The following equation is also used to find the power conversion
efficiency of organic solar cells:
(4)
where
1.1.1
is power of incident light.
Quantum efficiency
Another parameter of high interest for solar cell characterization is
the external quantum efficiency (EQE), which indicates the actual
number of the incident photons which are converted to electrons in the
external circuit. It is the ratio of charge carriers collected at the external
circuit and the number of the incident photons with certain wavelength:
(5)
The EQE could also be used to find the maximum current which could
be extracted from a solar cell by using the definition of photon energy
and spectral response. The energy of a photon is:
8
(6)
where
is the Planck`s constant,
is the light speed and
is the
wavelength of the light. Then the EQE could be written as:
.
(7)
Here the spectral response is:
,
where
(8)
is the light source`s intensity
short circuit current
and
is the
. Then the upper limit of extracted
current could be derived as follows:
∫
(9)
This is the upper limit, the maximum extracted current decreases at
high light
intensity
because
of
the
recombination process.
Monochromatic, low light intensities are used to measure external
quantum efficiency, whereas high light intensity is used in solar cell
efficiency measurement process.
9
The cell must be maintained at a constant temperature and a radiant
source with a constant intensity and a known spectral distribution must
be used. Solar radiation standards have been defined in terms of the
AM1.5 spectrum, most common at present, to compare solar cell
efficiencies. The solar simulator is the most popular equipment to get a
standard AM1.5 spectrum. Researchers have also been using some
calibration techniques, mostly by using a Reference Silicon Solar cell.
1.1.2
Equivalent Circuit Diagram
Figure4. The circuit consists of the following ideal components: light
generated current source ( ), ideal diode, and two parasitic resistors: one
parallel resistor-shunt resistor (
) and one series resistor ( ).
Figure4 could be an approximation to an equivalent circuit diagram
for an organic solar cell approximately. The current source (
10
)
represents the generated current from the incident light and the diode
accounts for the nonlinearity of the I-V curve. The circuit`s I-V
characteristic equals the ideal diode only when the series resistor ( )
goes to zero, and the shunt resistor (
The shunt resistor (
) to infinity.
) comes from the charge carrier recombination,
mostly at the surface of the donor-acceptor junction, whereas the
conductivity of the material, thickness of the active layer and impurity
concentrations are normally responsible for the series resistor ( ) in a
solar cell.
The values of both series resistor ( ) and shunt resistor (
) could
be derived from the I-V curve. The slope of the I-V curve at the
positive bias gives the series resistor ( ), normally under illumination,
while the shunt resistance (
) could be found at negative or positive
bias around zero volt, in dark.
(10)
(Dark, around 0 volt)
(11)
11
Figure5. Typical I-V characteristic of solar cells in the dark as well as under
illumination, current in log scale, voltage is in linear scale (10).
We could derive the following formula for organic solar cells, if the
equivalent circuit as Figure4 could represent a solar cell:
(
Where
diode and
)
(12)
is the photo generated current,
and
is the current on the
are the current and voltage at the load.
Then we can rewrite it as:
12
(
)
(13)
For the ideal diode, the current is:
⁄
(14)
Combine the equation (14) with (13):
(
)(
)
(
)(
⁄
)
(15)
we can rewrite equation (15) as:
(
)
(
) (
⁄
)
If a solar cell is represented with a replacement circuit, Figure3,
(16)
(11)
,
then the I-V curve of the organic solar cells can be fitted with equation
(16) (12).
So, most of the important information about the solar cells could be
obtained from the I-V measurement as mentioned.
13
1.2 Charge Carrier Mobility Measurement
Methods
Charge carrier mobility has been a most important parameter to
understanding the
organic
charge
transportable
materials
and
(13) (14)
application of the optoelectronic device
. Charge carrier mobility
measurement is also another key to further improve organic solar cell
efficiency.
Many different methods have been used to determine charge carrier
mobility in organic materials such as Hall effect measurement
conductivity/concentration method
(SCLC)
, space-charge-limited-current
(17)
, transient space-charge-limited-current
flight (TOF)
,
(16)
(17)
effect transistors (OFET)
(15)
, organic field
(18)
, Admittance spectroscopy
(20)
(21)
, transient electroluminescence
extraction by linearly increasing voltage (CELIV)
(19)
, Time-of-
and Charge
(22)
.
1.2.1 Time of Flight (TOF)
The time of flight method is the most popular one among them for
charge carrier mobility measurement in organic materials. In the TOF
method a pulse light, mostly laser, is used to generate free charge
carriers in the organic layer at the light transparent side of the organic
solar cell and an electric field is used to drive the generated charge
14
carriers to the other electrode. Then the charge carrier mobility can be
calculated by the following formula:
(17)
where
is the charge carrier mobility,
is the active layer thickness, E
is applied electric field, V is voltage and
is transient time. Figure6
shows the experimental set up of the time of flight method:
Figure6. TOF experimental set up.
Even though this TOF method has been the most popular technique
to measure charge carrier mobility for many organic devices, it still has
many restrictions to use for some applications, like organic solar cell.
The sample should fulfill the following conditions to use the TOF
method:
15
First, the conductivity of the cell should be very small to prevent the
generated free charge carriers from falling down and recombine before
they reach the electrode,
.
Second, the time for the generation of charge carriers should be very
short, compared to the charge carrier transit time,
. It
means one should use a fast light source such as N2 and Nd:YAG
lasers to get over this problem.
The third restriction is the thickness of the organic layer; it should be
up to few micrometers to separate electric transportation process from
optical absorption (23).
In my work, the charge extraction by linearly increasing voltage
(CELIV) and dark injection space-charge-limited current (DI-SCLC)
methods were used to measure charge carrier mobility in organic solar
cells because of their low equipment demand, simplicity and reliability.
16
1.2.2 Charge Extraction by Linearly Increasing Voltage
(CELIV)
As mentioned the CELIV method is simple, the data obtained from it
can be analyzed directly, it is applicable for very thin films, thinner
(13)
than 100nm
materials
and also useful for both low and high conductivity
(24)
. Because of those advantages, the CELIV has become the
most attractive method for studying charge transport properties of
organic thin films.
The CELIV method could be used to measure charge carrier
mobilities of thin films both in equilibrium, in the dark (if the number
of free charges in the film is enough to measure), and under the
illuminated condition (if the sample has so few free charge carriers ). It
is called Photo-CELIV if a pulse laser used (25).
In the CELIV method a linearly increasing triangle voltage pulse with
the slope
will be applied to the negative electrode of
the organic solar cell by an arbitrary wave function generator in the
dark to extract the free charges inside the film. Then a digital
oscilloscope will be used to record the extracted current by using its
50ohm internal resistance. At the beginning, a constant displacement
current (capacitive current step
) appears because of the capacitance
of the cell. Then the extracted charge carriers give additional current.
The current continues to increase as the voltage increases until the
17
charge carriers are extracted from the film and the current drops down
to the capacitive step level. In practice, if the duration of the applied
triangle voltage pulse
is not long enough, then there might be
some carriers left in the film and the extraction current will end at a
higher level than the capacitive step. The extracted current peak ( ) at
the time
and the film thickness ( ) will be used to calculate the
charge carrier mobility ( ) in the following three cases (26):
1) Low conductivity case:
<<
(18)
2) High conductivity case:
>>
(19)
≈
3) Moderate conductivity case:
(20)
where
is the sample thickness, A is the voltage slope,
corresponding time to the maximum current peak,
18
is the
is current density
at the maximum charge carrier extraction and
is the capacitive
current step.
The factor (
corrects the electric field redistribution
(25)
.
The Figure7 shows the typical CELIVE experiment set up:
Figure7. Experimental set up for typical CELIV method
(27)
.
Figure7 is for Photo-CELIV when the device has very small number of
free charge carriers in equilibrium. The set up for dark CELIV is same
except the laser pulse.
19
1.2.3 Dark Injection Space-Charge-Limited
Current (DI-SCLC)
Dark injection space-charge-limited current (DI-SCLC) is another
method used in this work to measure charge carrier mobility in organic
solar cells. This method was used to study charge injection in silicon (28),
germanium (29) and semiconductors
(30)
. This method is similar to TOF
technique except the laser pulse in the TOF is replaced with a function
generator; Figure8 is the common experimental set up for DI-SCLC.
Again the DI-SCLC method is operated under the equilibrium
condition, so it is not going to be influenced by the charge carrier
relaxation phenomena as the case in TOF technique.
In dark injection space-charge-limited-current measurement
(31)
, a
positive rectangular voltage pulse, monitored by the oscilloscope`s
1Mohm channel, will be applied to the positive electrode of the organic
solar cell and the transient current can be measured by the
oscilloscope`s 50 ohm channel. It is possible to see a cusp in the
transient current and it is reliable if the contact injector is ohmic
Then the time,
(23)
.
, at which the transient current reaches to the peak
is used to calculate the charge carrier mobility of the organic solar cell
by formula (17), as it is related to charge carrier transit time,
, (32):
(21)
20
Figure8. DI-SCLC experimental set up
Figure9. Typical DI-SCLC signal
(32)
It is also possible to measure the hole or electron mobility separately by
using electron or hole blocking layers. While the principles of DISCLC are well known, the application of it has encountered limited
success due to the lack of Ohmic injecting electrodes.
21
(33)
Chapter 2
Experimental Part
22
2.1 Materials
2.1.1 ITO
Indium tin oxide (ITO) is colorless and transparent in thin layers, but
it is yellow in bulk. Because of its electrical conductive and optical
transparent properties, it has become the most popular transpare nt
conductive oxide. Again it can easily to be deposited which is another
reason why it is used widely in thin film technology. ITO is a highly
doped n-type semiconductor; its band gap is around 4eV
(34)
. In my
work the provided ITO coated glass was used as one transparent
electrode as the organic active layer needs to absorb light to generate
free charge carriers. Its resistance is around 300-500 ohm/cm.
2.1.2 PEDOT:PSS
Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) abbreviated
PEDOT:PSS is a polymer mixture of two ionomers. It is a transparent,
conductive polymer and water soluble, easy to spin coat. The function
of the PEDOT:PSS in our device preparation is, first, to smooth up the
energy levels between ITO and the organic layer (regular device), it can
improve charge transport. Secondly it could work as electron blocking
layers, then it is possible to measure hole mobility in organic materials
(35)
.
23
The PEDOT:PSS in our experiment is ordered from Sigma. Its
specification is: conductive grade, PEDOT content 0.5 wt. %, PSS
content 0.8 wt. %, concentration 1.3 wt % dispersion in water, band gap
1.6 eV, conductivity 1S/cm. The work function of the PEDOT:PSS is
around 5.1eV.
2.1.3 P3HT
The conjugated polymer Poly (3-hexylthiophene) (P3HT) has been
widely used in field-effect transistors
diodes
(36)
, solar cells, batteries and
(37)
. The most notable properties of this material are its electrical
conductivity, resulting from the delocalization of electrons along the
polymer backbone, and its optical response to environmental stimuli,
with
dramatic
color
shifts
in
response
in solvent, temperature, applied potential, and
to
changes
binding to other
molecules. Both color changes and conductivity changes are induced
by the same mechanism — twisting of the polymer backbone,
disrupting conjugation (38). The P3HT used in our lab was ordered from
Sigma with 98% regioregularity in its solid grade form, P-type. The
lowest unoccupied and the highest occupied energy levels of P3HT are
around 3.3eV and 5.0eV, respectively (39).
2.1.4 PCBM
24
The fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) has been an attractive electron acceptor material in solar cells
(40)
, mostly used in conjunction with an electron donor material such as
P3HT (35). It is soluble in chlorobenzene and chloromethane, this allows
for solution processable donor/acceptor mixes, a necessary property for
printable solar cells. That is the main reason for PCBM to be a more
attractive electron acceptor material compared to the other fullerenes.
We also ordered the PCBM, 99%, from Sigma. The LUMO of the
PCBM is 3.7eV and the HOMO is 6.1eV (41) (42).
PEDOT:PSS
P3HT
PCBM
Figure10. Molecular structure of: PEDOT:PSS, P3HT, PCBM and energy
level diagram.
25
2.2 Device production and equipment
2.2.1 ITO cleaning
ITO coated glasses with size of 3cm ×3cm and resistance of 300-500
ohm/cm were provided. The experiment was started with cutting them
into small pieces, and most of them are in the size of 1.5cm ×1.0cm.
After that the ITO patterned glass was wiped with acetone and
isopropanol. The second cleaning step was sonication in chloroform,
acetone and isopropanol, 15minutes each, respectively. The ITO slides
were rinsed in the deionized water for a few minutes after finishing
each sonication procedures.
2.2.2 ITO etching
For some samples the ITO was etched out from the edge of the
ITO/Glass to make a contact point which can prevent having short
circuit problems in the sample. For the etching, first the ITO side of the
glass was covered with tape, and a certain surface of the ITO from the
edge which I wanted to remove was left uncovered. Then I placed it
into the HCI and HNO3 mixed solution with the ratio of 1:5. After 3
minutes I measured the conductivity of the etched and covered part of
ITO/Glass. The resistance was in the Gohm range for the etched part,
and there is no change for the covered part. The cleaning procedure for
the etched ITO was the same as above.
26
2.2.3 P3HT device preparation
Figure11. P3HT device structure
P3HT
(poly,
dichlorobenzene (
3-hexylthiophene)
solution
was
made
with
) with concentration of 0.56 wt. %. The
solution was heated at 60 C° for 30 minutes, and then the solution
was filtered with 0.45um, PTFE. The filtered P3HT solution was then
spun onto the newly cleaned ITO substrate with a spin speed of 1000
rpm/s for 90 seconds. The spin speed could be in the range of 800 to
2000 rpm/s to get proper thickness of the P3HT layer. It is easy to dry
up the organic layer just by spinning it, a second time, with lower speed
and longer time. When the P3HT layer dried up, the P3HT coated
ITO/Glass was placed onto the gold (Au) coated silicon wafer and held
with a sample holder. At the last step, the device was heated at 150 C°
for 20 minutes to improve the crystallization of the thin film, so that we
could have better charge transport.
27
2.2.4 P3HT/PCBM normal bilayer device production
Figure12. P3HT/PCBM bilayer device structure
PCBM, ([6,6]-phenyl-C61-butyric acid methyl ester), solution, at the
beginning, was also prepared in dichlorobenzene
changed the solvent to dichloromethane (
, later I
), because I could not
get the bilayer device by using dichlorobenzene for both P3HT and
PCBM, as it could also swell the P3HT which is at the bottom.
PEDOT:PSS received from Sigma has small grains, and to dissolve
them I made sonication for 24 hours, but some small grains still exist.
So I filtered with the 0.45um, Syringe-driven filter steriled* PTFE
membrane. Then it was spun onto the cleaned ITO coated glass with
the range of 1000 - 4000rpm for 10 seconds to get a conductive layer
and annealed for 20minutes at 150 C°. After that the P3HT solution
(the solution preparation was the same for all devices as for the P3HT
device) was spun on to the PEDOT:PSS layer at 1000 rpm for 90
seconds, and subsequently the PCBM + dichloromethane, 10 mg/ml,
solution was spin coated onto the top of the P3HT layer. When the
28
PCBM layer dried, it was placed on to the Au/Si and then held with a
sample holder. At the end the device was annealed for 15 minutes at
150 C°.
2.2.5 P3HT / PCBM inverted bilayer device
Figure13. P3HT/PCBM inverted bilayer device structure
I made many samples with inverted structure as that could be less
influenced by the oxidization problem compared to the other samples if
one could not have the vacuum condition. The device structure of the
inverted organic solar cell is: ITO / PCBM / P3HT / PEDOT:PSS / Au.
The interesting and important thing in this inverted device is that the
PEDOT:PSS can prevent the organic layer from oxidization and it also
could prevent the aluminum or gold particles from diffusing into the
organic layer
(43)
. The material preparation and spin coating parameters
were the same as for the P3HT/PCBM regular device production; the
difference is only the material coating order. First PCBM onto the ITO,
29
then P3HT, third PEDOT:PSS and the last step is to clip it with an Au
coated silicon wafer with sample holder and annealing.
I have also tried once to thermally deposit the top electrode at the
end of the lab work and I got one successful device. It gave a nice I-V
curve. The Figure14 shows one of the actual devices which I made in
our lab.
Figure14. Actual device production
First the gold was removed from the edge and middle of the Au/Si
wafer (left picture), to avoid short circuit problem and to get two
organic solar cells in one piece of ITO/Glass. Then the PEDOT:PSS,
P3HT, PCBM were spin coated one by one (picture in the middle), in
the way mentioned in the sample preparation part. At the last step the
device was kept by the sample holder and the efficiency as well as
charge carrier mobility was measured.
30
2.2.6 Equipment
Equipment used during device production:
Snow Jet
Ultra Sonic Bath
Spin Coater
Hot Plate
Figure15 (a). Equipment used during device production
31
Optical Tensiometer
AFM
Figure15(b). Equipment used during device production
1. Snow jet and ultra-sonic bath were used for cleaning ITO/Glass
after cutting and etching.
2. Optical tensiometer was used to measure contact angle of ITO
surface after cleaning.
3. Spin coater was used to deposit organic materials on to ITO/Glass
surface.
4. Hot plate was used to anneal the device to improve crystallinity of
organic materials.
5. Optical Tensiometer was used to measure the contact angle of the
ITO and ITO/PEDOT:PSS surface.
6. AFM (Atomic Force Microscopy) was used to measure active
layer thickness.
32
Equipment for efficiency measurement
Desktop lamp
Pyranometer
Variable resistor
Two Digital Multimeter
Figure16. Equipment for organic solar cell efficiency measurement
1. A desktop lamp (75watt) was used instead of sun light to produce
excitons in organic layer of the device.
33
2. A Silicon Pyranometer (Kipp & Zonen, QMS 101) was used to
calibrate the light intensity to 100mW/cm2.
3. Two digital multimeters (Agilent 34401A and Agilent 34410A)
were used to measure current and voltage produced by organic
solar cell.
4. A variable resistor (100 Kohm) + other larger resistors were used
to get many different current-voltage points in the I-V curve.
Equipment for charge carrier mobility measurement
Wave Function Generator
Oscilloscope
Figure17. Equipment for charge carrier mobility measurement
1. An arbitrary wave function generator (Agilent 33210A) was used
to extract free charge carriers from the organic solar cell.
2. A digital oscilloscope (Agilent DSO6012A) was used to record
extracted current response.
34
Chapter 3
Result and discussion
35
3.1 I-V measurement result
The experiment set up for efficiency measurement was the same for all
produced devices as Figure 18:
Figure18. Circuit for organic solar cell efficiency measurement
Light from the desktop lamp calibrated by the Pyranometer to
100mW/cm2 was directed to the produced organic solar cell. Variable
resistor was changed step by step to get more points. One digital
multimeter was connected in parallel to measure the voltage in the
variable resistor. The other digital multimeter was connected in series
with the device to measure the current.
The P3HT solution was prepared in dichlorobenzene with
concentration of 0.56wt.% and 1.0wt.%, while the PCBM solutions
36
were in dichloromethane for normal and inverted bilayer devices,
whereas in dichlorobenzene for only two bulk hetero junction devices,
in both with 0.6wt.% concentration. The same solution was applied for
all produced devices.
3.1.1
P3HT concentration dependence of I-V and power cure
Single Layer: GLASS/ITO / P3HT / Au/Si
1.0wt % P3HT
0.56wt % P3HT
60
2
Current, I (uA/cm )
50
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure19. I-V curves comparison of two P3HT devices made with different
P3HT concentrations. Dark solid line represents the I-V curve of the device
made with 1.0wt % P3HT solution; whereas the red dashed line is for
0.56wt. %.
37
1.0wt % P3HT
0.56wt % P3HT
12
2
Power, P (uW/cm )
10
8
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure20. Power curves comparison of two P3HT devices made with
different P3HT concentrations. Dark solid line represents the power curve
of the device made with 1.0wt % P3HT solution; whereas the red dashed
line is for 0.56wt. %.
38
Bilayer: GLASS/ITO / PEDOT:PSS / P3HT / PCBM / Au/Si
1.0wt % P3HT
0.56wt % P3HT
40
35
2
Current, I (uA/cm )
30
25
20
15
10
5
0
0.0
0.2
0.4
0.6
0.8
Voltage (v)
Figure21. I-V curves comparison of two Bilayer devices made with different
P3HT concentrations. Dark solid line represents the I-V curve of the device
made with 1.0wt % P3HT solution; whereas the red dashed line is for
0.56wt. %.
39
1.0wt % P3HT
0.56wt % P3HT
7
6
2
Power, P (uW/cm )
5
4
3
2
1
0
0.0
0.2
0.4
0.6
0.8
Voltage (v)
Figure22. Power curves comparison of two Bilayer devices made with
different P3HT concentrations. Dark solid line represents the power curve
of the device made with 1.0wt % P3HT solution; whereas the red dashed
line is for 0.56wt. %.
40
Bilayer (without PEDOT:PSS): GLASS/ITO / P3HT / PCBM / Au/Si
1.0wt % P3HT
0.56wt % P3HT
30
2
Current, I (uA/cm )
25
20
15
10
5
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure23. I-V curves comparison of two Bilayer devices (without PEDOT:
PSS) made with different P3HT concentrations. Dark solid line represents
the I-V curve of the device made with 1.0wt % P3HT solution; whereas the
red dashed line is for 0.56wt. %.
41
1.0wt % P3HT
0.56wt % P3HT
7
2
Power, P (uW/cm )
6
5
4
3
2
1
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure24. Power curves comparison of two Bilayer devices (without PEDOT:
PSS) made with different P3HT concentrations. Dark solid line represents
the power curve of the device made with 1.0wt % P3HT solution; whereas
the red dashed line is for 0.56wt. %.
42
Inverted Bilayer: GLASS/ITO / PCBM / P3HT / PEDOT:PSS / Au/Si
14
1.0wt % P3HT
0.56wt % P3HT
12
2
Current, I (uA/cm )
10
8
6
4
2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (v)
Figure25. I-V curves comparison of two Inverted Bilayer devices made with
different P3HT concentrations. Dark solid line represents the I-V curve of
the device made with 1.0wt % P3HT solution; whereas the red dashed line is
for 0.56wt. %.
43
1.0wt % P3HT
0.56wt % P3HT
1.5
2
Power, P (uW/cm )
2.0
1.0
0.5
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (v)
Figure26. Power curves comparison of two Inverted Bilayer devices made
with different P3HT concentrations. Dark solid line represents the power
curve of the device made with 1.0wt % P3HT solution; whereas the red
dashed line is for 0.56wt. %.
44
Inverted Bilayer (without PEDOT:PSS): GLASS/ITO / PCBM / P3HT
/ Au/Si
10
1.0wt % P3HT
0.56wt % P3HT
2
Current, I (uA/cm )
8
6
4
2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (v)
Figure27. I-V curves comparison of two Inverted Bilayer (without
PEDOT:PSS) devices made with different P3HT concentrations. Dark solid
line represents the I-V curve of the device made with 1.0wt % P3HT
solution; whereas the red dashed line is for 0.56wt. %.
45
2.5
1.0wt % P3HT
0.56wt % P3HT
2
Power, P (uW/cm )
2.0
1.5
1.0
0.5
0.0
0.0
0.2
0.4
0.6
0.8
Voltage (v)
Figure28. Power curves comparison of two Inverted Bilayer (without
PEDOT:PSS) devices made with different P3HT concentrations. Dark solid
line represents the power curve of the device made with 1.0wt % P3HT
solution; whereas the red dashed line is for 0.56wt. %.
46
Bulk: GLASS/ITO / PCBM : P3HT / Au/Si
25
1.0wt % P3HT
0.56wt % P3HT
2
Current, I (uA/cm )
20
15
10
5
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Voltage (v)
Figure29. I-V curves comparison of two Bulk hetero junction (without
PEDOT:PSS) devices made with different P3HT concentrations. Dark solid
line represents the I-V curve of the device made with 1.0wt % P3HT
solution; whereas the red dashed line is for 0.56wt. %.
47
1.0wt % P3HT
0.56wt % P3HT
10
2
Power, P (uW/cm )
8
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Voltage (v)
Figure30. Power curves comparison of two Bulk hetero junction (without
PEDOT:PSS) devices made with different P3HT concentrations. Dark solid
line represents the power curve of the device made with 1.0wt % P3HT
solution; whereas the red dashed line is for 0.56wt. %.
Discussion1: One could see from the above I-V and Power curves
that the devices made with higher P3HT concentration gave better
results for both I-V and Power in this experiment, and this result is in
good agreement for all devices. The reason could be the too thin P3HT
48
layer for the devices which were made by spin-coating the 0.56wt. %
P3HT solution compare to the others which were used 1.0wt. % P3HT
solution. Since the P3HT layer is an electron donor layer, if it is too
thin then it is not so efficient to capture light, which results in
inefficient charge carrier generation in the organic layer.
49
3.1.2
I-V and power curve comparison of all devices
Devices group 1: 1.0wt. % P3HT solution was used
60
P3HT
Bilayer
Inverted
Bulk
2
Current, I (uA/cm )
50
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure31. I-V curves comparison for all devices based on 1.0wt% P3HT
solution. Black solid line represents P3HT single layer device; red dashed
line is for normal bilayer; blue dotted line is for inverted bilayer and pink
dash-dot line stands for hetero-junction device.
50
P3HT
Bilayer
Inverted
Bulk
12
2
Power, P (uW/cm )
10
8
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure32: Power curves comparison for all devices based on 1.0wt% P3HT
solution. Black solid line represents P3HT single layer device; red dashed
line is for normal bilayer; blue dotted line is for inverted bilayer and pink
dash-dot line stands for hetero-junction device.
51
Devices group 2: 0.56wt. % P3HT solution was used
50
P3HT
Bilayer
Inverted
Bulk
2
Current, I (uA/cm )
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure33. I-V curves comparison for all devices based on 0.56wt% P3HT
solution. Black solid line represents P3HT single layer device; red dash line
is for normal bilayer; blue dot line is for inverted bilayer and pink dash-dot
line stands for hetero-junction device.
52
10
P3HT
Bilayer
Inverted
Bulk
2
Power, P (uW/cm )
8
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure34. Power curves comparison for all devices based on 0.56wt% P3HT
solution. Black solid line represents P3HT single layer device; red dash line
is for normal bilayer; blue dot line is for inverted bilayer and pink dash-dot
line stands for hetero-junction device.
Discussion 2: See Figure31, 32, 33 and 34. The best Power and I-V
curves obtained in the order: P3HT single layer, Bilayer and inverted
devices for both devices group. I don`t have enough data, only two, for
bulk hetero junction device, as I was told not to make the heterojunction solar cells after I had made two samples. But I put its data in
just to show, it is not enough to make any conclusion from it. Anyway,
53
the hetero junction did not give the good result I had hoped for. In
theory the photo-generated charge carriers in the organic film could
reach easily to the junction surface of two materials by diffusing, then it
could be separated easier to the opposite electrodes compare to any
other type of devices. The low quality may have resulted from an
imperfect PCBM:P3HT mixture, I remember that I made that mixture
solution in Dichlorobenzene and stirred around 30 minutes, I have also
checked that most other groups stirred the solution for 24 hours, so I
could say that the bulk hetero-junction solar cells was partially
successful.
For both groups of devices the P3HT single layer solar cell got the
best power and I-V curve. This is a big surprise as we expected the
bilayer would be the best, since the single layer device does not have an
electron acceptor layer like the bilayer device. The generated excitons
in the P3HT layer could only be separated by the electric field
introduced by the two electrodes work function difference. The
excitons in the bilayer cell could be separated easier than in the single
layer as the electric field is stronger in the bilayer. The reason why the
P3HT single layer got higher power conversion efficiency and the best
I-V curve is that first of all maybe the bilayer device has a higher
resistant surface at the P3HT:PCBM junction, because these devices
were prepared by spin coating all material in air, which causes
oxidization problems of the organic material. A second reason could be
that the ratio of the P3HT and PCBM layer thicknesses is not
54
appropriate to get higher efficiency. Again may be I got some dust on
my sample during the spin-coating process.
Inverted organic solar cells had the poorest performance among the
devices. I strongly believe that the problem comes from the
PEDOT:PSS. The PEDOT:PSS was ordered from Sigma Aldrich, its
specification is given in the sample preparation part. It was filtered by a
0.45um non-pyrogenic filter as it contains numerous small dark grains.
At the beginning I filtered directly, then I got only water. The second
time I filtered it after 24 hours sonicating and that filtered solution
looked good. It had the same dark blue color as the original. Just to
make sure I spun the filtered PEDOT:PSS onto the glass substrate and
then I checked the conductivity. The result is that I did not see any
conductance. Just to see the PEDOT:PSS role in my sample, I made
many devices with and without PEDOT:PSS.
55
3.1.3
Device performance with and without PEDOT:PSS
Normal Bilayer: with and without PEDOT: PSS
Device group 1: 1.0wt. % P3HT
40
With PEDOT:PSS
No PEDOT:PSS
35
2
Current, I (uA/cm )
30
25
20
15
10
5
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure35. I-V curves comparison for normal bilayer devices made with and
without PEDOT:PSS, in which P3HT solution was 1.0wt. %. Black solid
line represents the bilayer device which includes PEDOT:PSS, whereas the
red dashed line is for a device without PEDOT:PSS.
56
7
With PEDOT:PSS
No PEDOT:PSS
5
2
Power, P (uW/cm )
6
4
3
2
1
0
-1
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure36. Power curves comparison for normal bilayer devices made with
and without PEDOT:PSS, in which P3HT solution was 1.0wt. %. Black
solid line represents the bilayer device which includes PEDOT:PSS, whereas
the red dashed line is for a device without PEDOT:PSS.
57
Device group 2: 0.56wt. % P3HT
25
With PEDOT:PSS
No PEDOT:PSS
2
Current, I (uA/cm )
20
15
10
5
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure37. I-V curves comparison for normal bilayer devices made with and
without PEDOT:PSS, in which P3HT solution was 0.56wt. %. Black solid
line represents the bilayer device which includes PEDOT:PSS, whereas the
red dashed line is for a device without PEDOT:PSS.
58
5
With PEDOT:PSS
No PEDOT:PSS
2
Power, P (uW/cm )
4
3
2
1
0
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (v)
Figure38. Power curves comparison for normal bilayer devices made with
and without PEDOT:PSS, in which P3HT solution was 0.56wt. %. Black
solid line represents the bilayer device which includes PEDOT:PSS, whereas
the red dashed line is for a device without PEDOT:PSS.
Discussion3: It is recognizable from Figure35, 36, 37 and 38 that the
bilayer devices without PEDOT:PSS showed better result than the one
with PEDOT:PSS. That is the opposite to what we expected. The
explanation for that is that a good PEDOT:PSS film was not formed on
the ITO surface, instead most of it flowed away when it was spun. But
somehow it formed a not continuous very thin PEDOT:PSS layer,
59
otherwise the two bilayer devices with PEDOT:PSS should not have
higher short circuit current compared to the other two devices without
PEDOT:PSS.
Inverted Bilayer: with and without PEDOT : PSS
Device group 1: 1.0wt. % P3HT
14
With PEDOT:PSS
No PEDOT:PSS
12
2
Current, I (uA/cm )
10
8
6
4
2
0
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (v)
Figure39. I-V curves comparison for inverted bilayer devices made with and
without PEDOT:PSS, in which P3HT solution was 1.0wt. %. Black solid
line represents the device which includes PEDOT:PSS, whereas the red
dashed line is for a device without PEDOT:PSS.
60
2.5
With PEDOT:PSS
No PEDOT:PSS
2
Power, P (uW/cm )
2.0
1.5
1.0
0.5
0.0
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (v)
Figure40. Power curves comparison for inverted bilayer devices made with
and without PEDOT:PSS, in which P3HT solution was 1.0wt. %. Black
solid line represents the device which includes PEDOT:PSS, whereas the red
dashed line is for a device without PEDOT:PSS.
61
Device group2: 0.56wt. % P3HT
12
With PEDOT:PSS
No PEDOT:PSS
8
2
Current, I (uA/cm )
10
6
4
2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (v)
Figure41. I-V curves comparison for inverted bilayer devices made with and
without PEDOT:PSS, in which P3HT solution was 0.56wt. %. Black solid
line represents the device which includes PEDOT:PSS, whereas the red
dashed line is for a device without PEDOT:PSS.
62
1.4
With PEDOT:PSS
No PEDOT:PSS
1.0
2
Power, P (uW/cm )
1.2
0.8
0.6
0.4
0.2
0.0
-0.2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage(v)
Figure42. Power curves comparison for inverted bilayer devices made with
and without PEDOT:PSS, in which P3HT solution was 0.56wt. %. Black
solid line represents the device which includes PEDOT:PSS, whereas the red
dashed line is for a device without PEDOT:PSS.
Discussion4: In both inverted bilayer devices with different P3HT
concentration, the samples which contained the PEDOT:PSS gave
better power and I-V curves, see Figure39, 40, 41and 42. This is
opposite to the normal bilayer devices` behavior. I think it is related to
the surface characteristics of the PCBM and ITO. I have checked the
contact angle of the cleaned ITO surface by using an Optical
63
Tensiometer and it was 67°. So I believe the PEDOT:PSS formed a
better film when spin-coated onto a PCBM layer surface than on an
ITO surface.
3.1.4
Summary for I-V characteristic measurement
P3HT Concentration dependence
P3HT
⁄
(Wt. %)
⁄
P3HT
1.0
0.936
56
11.35
P3HT
0.56
0.8
44.6
9.17
BL
1.0
0.82
34.8
6.26
BL
0.56
0.76
23.64
3.6
BL NO PED
1.0
0.91
29.1
6.7
BL No PED
0.56
0.93
19.53
4.5
Invert
1.0
0.61
12.27
2.1
Invert
0.56
0.53
11
1.4
Invert No PED
1.0
0.7
9.65
2.15
Invert No PED
0.56
0.6
5.14
1.02
BHJ
1.0
1.4
22.33
10
BHJ
0.56
0.9
14.25
3.325
Table1: Device performance Overview 1
64
Discussion 5: P3HT concentration: From the table above one could
see that the P3HT concentration dependence of all produced devices`
performance is in good agreement with what I expected. The higher
P3HT concentration in all my devices performed better in my
experiment. But it does not mean that one could have always higher
device performance by increasing the concentration. I believe that it
would be true in some degree as if it has higher concentration, then the
P3HT layer will be thick enough to capture light efficiently to generate
as many charge carriers as possible. But if the P3HT layer is too thick
the generated charge carriers will recombine before reaching the
electrode. So there should be some critical value to get higher organic
solar cell performance. The crystallinity of the organic layers is also
one of the important issues to improve the device performance.
65
Overall device performance comparison
η
1% P3HT
⁄
⁄
(%)
(%)
P3HT
0.94
56.
11.4
21.7
0.11
BL
0.82
34.8
6.26
22
0.06
BL NO PED
0.91
29.1
6.7
25.1
0.067
Inverted
0.61
12.27
2.1
27.4
0.021
Invert No PED
0.7
9.65
2.15
32
0.022
BHJ
1.4
22.33
10
32
0.1
Table2: Device performance Overview 2 (1.0wt. % P3HT)
η
⁄
⁄
(%)
(%)
P3HT
0.8
44.6
9.17
25.6
0.09
BL
0.76
23.64
3.6
20
0.036
BL NO PED
0.93
19.53
4.5
24.7
0.045
Invert
0.53
11
1.4
23.4
0.014
Invert No PED
0.6
5.14
1.02
32.9
0.01
BHJ
0.9
14.25
3.325
26
0.033
Table3: Device performance Overview 3 (0.56wt% P3HT). BL=bilayer
device; BL NO PED= bilayer device without PEDOT:PSS; Inverted NO
PEDOT=inverted bilayer device without PEDOT:PSS. BHJ=Bulk-hetero
junction
66
Discussion 6: I could say that the best power conversion efficiency
(0.11%), the highest open circuit voltage (0.94 ), the highest short
circuit current (
⁄
⁄
) and the maximum power of 9.17
were obtained from the P3HT single layer. It is amazing to
get such a high performance in a single P3HT layer device in air
condition even though it has small fill factor. The lowest organic solar
cell efficiency was found for the inverted bilayer device with a power
conversion efficiency of 0.021%, an open circuit voltage of 0.61 , a
short circuit current of
2.1
⁄
⁄
and a maximum power of
. The normal bilayer device stayed in the middle.
In principle the bilayer and hetero junction should perform better
than the single layer. This is opposite to my experiment result for both
bulk hetero junction and single layer devices. Maybe it would be
possible for bulk hetero junction solar cells to get better result than
single layer organic solar cells if I would have made more samples than
those two. Unfortunately the expected result for both normal and
inverted bilayer devices had not been achieved until the whole
experiment was done.
I can say that the P3HT solution in our experiment worked fine
according to the result from the single layer device, so one possibility
for the reason of getting a lower efficiency in the bilayer than the single
layer device is that maybe the PCBM solution did not work properly,
but it is hard to say. If the PCBM is assumed to work fine then the
67
other possibility could be that the devices were prepared in air, not in
vacuum or in nitrogen atmosphere. This means the first spin-coated
organic layer got oxidized before the second layer was deposited as it
takes a few minutes to dry the first layer before starting the second
layer coating. It is true that the P3HT layer oxidized fast. This is clearly
visible from the color change of P3HT, which I have noticed many
times. In this case there would be a resistant layer at the junction of the
two organic layers which could cause the exited charge carriers to
recombine before reaching the electrode. The dust falling also could be
a problem during the spin-coating.
If any of those reasons is not the case for having a lower device
performance in bilayer than the single P3HT device, then the bilayer
device should have given better result than the single layer device. The
photo-generated charge carrier separation in the bilayer device is more
efficient than in the single layer one, because the bilayer organic device
gets an extra electric field from the two different organic materials
different work functions at the junction surface. This means that the
photo-generated electron hole pairs could be separated more easily with
this extra electric field plus with another electric field which comes
from the work function differences of two electrodes.
Since the bilayer device got lower power conversion efficiency than
the single layer one, it is reasonable to get even lower efficiency for the
bilayer without PEDOT:PSS. The aim of using PEDOT:PSS in the
68
normal bilayer for efficiency measurement is to smooth up the energy
level between ITO and P3HT to increase the charge transport. From
table 2 and table 3 we can see that the short circuit current in devices
which do not use PEDOT:PSS is smaller than in those that used it. That
is what I expected, even though the difference is not big enough. The
open circuit voltage, maximum extracted power, fill factor and
efficiency is higher in both normal and inverted bilayer devices, which
is interesting. The reason for that is tha t the PEDOT:PSS did not form
uniform thin film on the ITO surface when I spun it, and it is too thin.
The ITO which I used had a contact angle of 67°. There were many big
dark blue points on the ITO surface after I spun the PEDOT:PSS, see
Figure 43, which also causes problems.
Figure43. PEDOT:PSS after spin-coating onto the ITO
69
I could say that it worked partially, otherwise the samples
containing PEDOT:PSS (table2, table3) should not have given higher
short circuit current as the it influenced by charge transport in the film,
whereas the open circuit voltage mostly depends on the intrinsic band
gap of the material.
In the inverted bilayer device we used PEDOT:PSS to smooth up the
energy level and mostly to avoid the oxidization of the top organic
layer as well as to protect the top organic layer from aluminum or gold
diffusion. PEDOT:PSS can do that if it forms a uniform film. And the
organic layer becomes more stable.
The inverted bilayer devices with and without PEDOT:PSS became
less efficient in both cases. It is reasonable since in the inverted case
the work function difference between the organic layer and the
electrodes are higher than in the normal one, it could cause lower
charge carrier collection at the electrodes. Again if the PEDOT:PSS
does not work then the device become very bad. Although it did not
fully work as I assumed it did in some degree, because I got a little bit
higher open circuit voltage than in the devices not using PEDOT:PSS.
The open circuit voltage, maximum power, fill factor and efficiency
have the same trend in both normal and inverted organic solar cells, and
these values are higher for non-PEDOT:PSS devices. There are many
reasons for that some of them mentioned above; the solvent for the
PEDOT:PSS is water which may change the morphology of the organic
70
layer if it is not spread uniformly onto the organic layer. Even the
PCBM and P3HTcould be part of the problems as I did see nonuniformed PCBM material on the P3HT layer on normal bilayer
devices, an organic material shrink problem on most of the devices and
also an abnormal material deposition at the edge of the organic layer
after I finished material coating, see Figure 44, 45, 46, 47.
Figure44. Bilayer devices after PCBM spin-coating
71
Figure45. Organic material shrinking after completing material coating
Figure46. Organic material at the edge of the device surface
72
Figure47. Organic material at the center of the device surface
73
3.2 Charge carrier measurement result and
discussion
3.2.1
Charge extraction by linearly increasing voltage (CELIV)
The experiment set up for charge carrier mobility measurement by
using the CELIV method was the same for all produced devices
(Figure 7), except for the laser pulse. The measurement was executed
in the dark, so only the equilibrium free charge carriers would be
measureable
(44)
. In this experiment a triangular voltage pulse was
applied to the device`s cathode (the device was covered with a box to
avoid charge carrier photo-generation), then the current response was
measured by one oscilloscope channel, while the applied voltage pulse
was monitored by the oscilloscope`s other channel. For all the devices
the extracted current was smaller than the capacitive current step,
which means the material has poor conductivity; from that it was
decided to use formula (18) to calculate the charge carrier mobility (µ).
From table 4, 5 below one could see most of the important parameters
of all devices.
74
1.0wt% P3HT
µ
⁄
(cm)
P3HT
8.64E-05
BL
6.78 E-05
2.22E-06
3.47E-06
BL NO PED
5.77E-05
Inverted
Invert No PED
0.24
1.07
1.35E-05
0.68
0.59
1.87E-05
3.60E-06
0.12
0.09
3.74E-05
4.75E-06
0.07
0.58
1.90E-05
2.10E-05
5.84E-06
0.06
0.47
1.75E-05
1.79E-05
Table4: Measured data from CELIV technique, (P3HT 1.0wt %).
0.56wt % P3HT
µ
⁄
(cm)
P3HT
4.47E-05
1.92E-06
0.20
0.87
8.40E-06
BL
4.05E-05
3.24E-06
0.10
0.59
1.35E-05
BL NO PED
3.09E-05
3.52E-06
0.08
0.53
1.28E-05
Inverted
2.92E-05
1.78E-05
4.10E-06
0.06
0.51
1.45E-05
5.00E-06
0.049
0.38
1.38E-05
Invert No PED
Table5: Measured data from CELIV technique, (P3HT 0.56wt %).
BL=bilayer device; BL NO PED= bilayer device without PEDOT:PSS;
Inverted NO PEDOT=inverted bilayer device without PEDOT:PSS.
In this experiment the voltage slope used is A=5.71V/20us. The
resistor used is 47ohm.
75
Discussion 1: From the tables above it is recognizable that among all
devices the P3HT device got slightly higher charge carrier mobility
than the others, which is reasonable since the P3HT single layer device
has the smallest thickness and the resistivity of the device seems to be
lower than that of the others. Again the measured charge carrier
mobility is due to holes because, first, the P3HT is a hole transporting
material, and second, the hole mobility is higher than the electron
mobility in organic materials (19)(37). Again we could see that the bilayer
device got the second highest charge carrier mobility and extracted
current density. According to theory the bilayer device should have a
higher extracted current because it has an electron acceptor layer,
which the single layer P3HT device does not. It is then more easy to
have free charge carriers at the P3HT:PCBM junction due to the
internal electric field as mentioned before. But the thing in our
experiment is that the P3HT single layer device gave a higher
efficiency, so the reasons discussed earlier could also be applied to this,
see discussion6. In the bilayer device the hole mobility is also dominant;
it is higher than the electron mobility, and it could be even nice to say
that if the PEDOT:PSS works perfectly it could block the electrons.
However, if the reasons discussed above are correct then the other
results for all other devices are reliable. The table 4, 5 shows an overall
trend of decreasing charge carrier mobility and extracted current in the
order P3HT device, Bilayer, Bilayer without PEDOT:PSS, normal and
Inverted bilayer device with no PEDOT:PSS.
76
The bilayer device with no PEDOT:PSS showed slightly lower
charge carrier mobility and current density than those which used
PEDOT:PSS. It is understandable since the PEDOT:PSS can smooth up
the energy levels between electrode and organic material. Also in the
inverted case the device with PEDOT:PSS gave a higher charge carrier
mobility and it may be close to the result from the normal bilayer
device if PEDOT:PSS would have fully worked.
3.2.2
Dark injection-space charge limited current (DI-SCLC)
The experiment set up, illustrated in the experimental part (see
Figure8), was used for this experiment. A rectangular voltage pulse was
applied from the same arbitrary wave function generator to the anode of
the device and monitored on one oscilloscope channel, then the current
response was checked by the oscilloscope`s other channel. The
measured device was also covered by a box. In this experiment the time
for maximum current was recorded and the charge carrier mobility was
calculated by equations (21) and (17). Here are the measured data from
this experiment:
77
1.0wt % P3HT
µ
⁄
(cm)
P3HT
3.20E-04
7.84E-08
9.97E-08
5.71
1.35E-05
BL
6.59E-04
7.30E-08
5.71E-04
7.72E-08
5.71
5.71
1.87E-05
BL NO PED
9.29E-08
9.82E-08
Inverted
1.01E-03
4.90E-08
6.23E-08
5.71
1.90E-05
Invert No PED
9.37E-04
4.50E-08
5.73E-08
5.71
1.75E-05
1.79E-05
Table 6: Charge carrier mobility from DI-SCLC (P3HT 1.0wt %).
0.56wt % P3HT
µ
⁄
(cm)
P3HT
1.28E-04
7.60E-08
9.67E-08
5.71
8.40E-06
BL
3.34E-04
7.52E-08
2.97E-04
7.60E-08
5.71
5.71
1.35E-05
BL NO PED
9.57E-08
9.67E-08
Inverted
6.52E-04
4.44E-08
5.65E-08
5.71
1.45E-05
Invert No PED
4.77E-04
5.50E-08
6.99E-08
5.71
1.38E-05
1.28E-05
Table 7: Charge carrier mobility from DI-SCLC (P3HT 0.56wt %).
Measurement results from this DI-SCLC technique do not match
with the results from the CELIV technique, because DI-SCLC is
strongly limited to Ohmic contact materials, in other words, the anode
of the device must be in ohmic contact with the organic material. In my
experiment the PEDOT:PSS is supposed to make quasi- Ohmic contact
78
at the anode
(26)
, so that the PEDOT:PSS is again a problem for this
method.
However, according to the results from the I-V and CELIV
experiments I could again assume that the PEDOT:PSS worked weakly
in the normal and the inverted bilayer devices, so I am only going to
discuss the results for these two devices. If this assumption is valid then
it is evident that the inverted bilayer device got a higher charge carrier
mobility than the normal one, because the morphology of the P3HT
layer helps more PEDOT:PSS molecules to stay on the P3HT layer
than on the ITO surface when the PEDOT:PSS spin coating takes place.
We could see this by comparing the active layer thickness (d) from the
table6, 7.
The measurement of organic layer thickness on the device was done
by Atomic Force Microscopy. Figure34 is an example. A few scratches
were made on the surface of ITO/glass spin-coated by organic material,
and then the height from the bottom to the surface was checked by
using the tapping mode, see figure below:
79
Figure48. An example of Organic film (material in this picture was P3HT)
thickness measurement by AFM
80
Figure49. The scratch made on the surface of the organic layer
Some Pictures taken by optical microscope
Figure50. After removing organic
Figure51. Organic material at the top
material from the surface to make
side of the ITO surface
ITO contact point.
81
Figure52. Material at the edge
Figure53. Material at the edge
Figure54. Material at the side
Figure55. Material at the center
Figure56. Material at the center
Figure57. Material at the center
82
Chapter 4
Conclusion
83
In both the power conversion efficiency measurement and the charge
carrier mobility measurement experiment, the P3HT single layer device
performed better than the other devices, such as normal bilayer,
inverted bilayer and hetero junction solar cells, with a power
conversion efficiency of o.11%, short circuit current of 56uA/
open circuit voltage of 0.94
11.4mW/
,
, maximum extracted power of
and charge carrier mobility of 8.64E-05
⁄
. Here
the hetero-junction device is not going to be considered much as there
is not enough data to make a conclusion.
However the higher short circuit current and higher power
conversion efficiency in the P3HT single layer device than in the
bilayer device does not fit the theory. In principle the bilayer should
have higher power conversion efficiency than the single layer since the
charge carrier separation is more efficient in a bilayer device. The
problem perhaps came from the deposition of organic material by spincoating in air, because then the organic layer oxidization problem takes
place at the material junction surface which probably could form a
resistant layer. This means that the charge carrier separation is not
efficient, or maybe the dip oxidization decreased the conductivity of the
film; that could be a reason for having a lower charge carrier mobility
in bilayer devices than in single layer devices. The measured charge
carrier mobility and extracted current is always higher in a normal
bilayer device than in an inverted device, and again it is always higher
in both normal and inverted device with PEDOT:PSS. This is probably
84
caused by the smoother energy level structure in the normal bilayer
device and the improper work of PEDOT:PSS.
The same PCBM solution, with a concentration of 0.6wt %, was used
for all bilayer devices, but two P3HT solutions with different
concentrations were used to compare the result. It is nice to see that all
devices made with the higher P3HT concentration revealed consistently
good results. But it does not mean that the higher the P3HT
concentration the better the device performance. There should be some
optimum value for that.
The CELIV charge carrier mobility measurement method showed the
highest mobility, hole mobility, for the P3HT single layer device. This
is understandable since P3HT is a hole transport material. The other
devices got smaller charge carrier mobility and extracted current, it
could explain that the other devices` have lower conductivity. Anyway
the differences are not big, and the mobility values for all devices are of
the same magnitude. I could not draw any big conclusion from the DISCLC measurement, because it requires the samples to have at least
one ohmic contact on one side. PEDOT:PSS was supposed to play that
role, but unfortunately it did not work as expected, but somehow it
worked to some degree which could be seen from the I-V character of
the devices. The poor PEDOT:PSS layer in inverted and normal bilayer
devices could be the reason for obtaining one order of magnitude
higher charge carrier mobility by the DI-SCLC measurement method.
85
Overall, all measurement methods worked fine. The measured charge
carrier mobility by CELIV is one order, and all short circuit current is
two orders smaller in comparison with some published paper, and the
reason is mostly the sample production procedure. The devices in this
thesis were prepared by spin coating PEDOT:PSS and other organic
materials one by one on to ITO coated glass in air, and at the end the
top electrode (Au coated silicon wafer) was clipped on to the with
organic material spun ITO/Glass with some pressure. This could give
big problems. In most of the published papers, the devices are produced
in vacuum, the top electrode is thermally evaporated in vacuum, they
tested the efficiency with source measure unit, and the resistance of the
ITO which they used is 15 Ohm/cm. We used two digital multimeters
and a variable resistor to measure the efficiency and the ITO we have
used has a resistance of 300-500 Ohm/cm. These could also be reasons
to get lower power conversion efficiency.
86
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90
Appendices
Appendix A: Data from I-V characteristic measurement
Single layer (P3HT 1.0wt %)
V (V)
3E-05
0.027
0.055
0.11
0.17
0.23
0.28
0.33
0.39
0.44
0.50
0.55
0.61
0.67
0.72
0.78
0.83
0.89
0.94
I (uA/cm2)
56.01
54.70
53.30
49.20
44.10
40.02
36.70
33.30
29.01
25.60
22.20
19.20
16.11
12.80
10.14
7.70
5.32
3.25
0.54
Size=0.3*0.6mm2
P (mW/cm2 )
0.0015
1.52
2.96
5.47
7.46
9.10
10.21
11.10
11.30
11.40
11.10
10.61
9.82
8.60
7.31
5.96
4.26
2.72
0.47
91
FF
n (%)
0.217
0.11
Single Layer (P3HT 0.56wt %)
Size=0.2*0.65mm2
V (V)
I (uA/cm2)
P (mW/cm2 )
6E-05
44.61
0.003
0.04
42.30
1.63
0.08
40.80
3.14
0.15
36.15
5.56
0.24
31.54
7.52
0.31
27.40
8.43
0.38
23.81
9.17
0.46
20.02
9.10
0.54
15.40
8.30
0.61
11.54
7.11
0.69
7.21
5.02
0.78
1.71
1.31
0.81
0.46
0.37
92
FF
n (%)
0.256
0.09
Normal Bilayer ( 1.0wt % P3HT)
Size=0.3*0.55mm2
V (V)
I (uA/cm2)
P (mW/cm2 )
1.2E-05
34.81
0.0004
0.03
33.33
0.95
0.06
31.50
2.06
0.12
28.72
3.47
0.18
25.61
4.65
0.24
22.70
5.51
0.30
20.02
6.06
0.36
17.21
6.26
0.42
14.50
6.17
0.48
12.11
5.88
0.54
9.70
5.29
0.61
7.45
4.52
0.67
5.41
3.59
0.73
3.30
2.42
0.80
0.87
0.69
0.82
0.48
0.41
93
FF
n (%)
0.22
0.06
Normal Bilayer ( 0.56wt % P3HT)
Size=0.3*0.55mm2
V (V)
I (uA/cm2)
P (mW/cm2 )
1E-04
23.64
0.002
0.03
22.24
0.67
0.06
20.91
1.27
0.12
18.18
2.20
0.18
16.06
2.92
0.24
13.80
3.35
0.31
11.82
3.59
0.36
9.70
3.56
0.42
8.06
3.42
0.48
6.06
2.94
0.55
4.79
2.61
0.61
3.33
2.02
0.72
0.77
0.55
0.76
0.07
0.05
94
FF
n (%)
0.201
0.036
Normal Bilayer (1.0wt % P3HT)
No PEDOT
Size=0.35 * 0.5mm2
V (V)
I (uA/cm2)
P (mW/cm2 )
1.7E-05
29.14
0.0005
0.03
28.30
0.81
0.06
27.43
1.57
0.11
25.26
2.89
0.17
23.11
4.01
0.23
21.16
4.83
0.29
19.21
5.47
0.34
17.14
5.88
0.40
15.42
6.17
0.46
14.60
6.69
0.52
12.01
6.19
0.57
10.30
5.88
0.63
8.61
5.39
0.69
6.96
4.78
0.74
5.93
4.40
0.79
4.92
3.91
0.84
3.86
3.24
0.89
2.50
2.22
0.91
1.24
1.14
95
FF
n (%)
0.251
0.067
Normal Bilayer (0.56wt % P3HT)
No PEDOT
Size=0.3 * 0.5mm2
V (V)
I (uA/cm2)
P (mW/cm2 )
1.20E-04
19.53
0.0023
0.03
18.93
0.63
0.07
18.27
1.28
0.13
16.87
2.25
0.20
15.01
3.01
0.27
13.93
3.72
0.33
12.67
4.22
0.40
11.40
4.56
0.47
10.13
4.73
0.53
8.43
4.50
0.60
7.80
4.70
0.67
6.66
4.44
0.73
5.68
4.16
0.80
4.64
3.73
0.87
3.73
3.24
0.91
2.53
2.29
0.93
1.13
1.06
96
FF
n (%)
0.247
0.045
Inverted Bilayer
(1.0wt % P3HT)
Size=0.15 * 0.5mm2
V (V)
I (uA/cm2)
P (mW/cm2 )
2.80E-04
12.27
0.0034
0.07
11.20
0.75
0.13
10.01
1.33
0.27
7.60
2.03
0.41
5.07
2.06
0.53
2.02
1.07
0.61
0.28
0.17
FF
n (%)
0.274
0.021
Inverted Bilayer
(0.56wt % P3HT)
V (V)
I (uA/cm2)
Size=0.4 * 0.5mm2
P (mW/cm2 )
FF
1.35E-04
0.025
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.46
0.52
0.53
11.03
10.01
9.60
8.50
7.41
6.45
5.51
4.57
3.55
2.70
1.61
0.57
0.33
0.0015
0.25
0.48
0.85
1.12
1.29
1.38
1.37
1.26
1.08
0.74
0.30
0.17
97
0.234
n (%)
0.014
Inverted Bilayer (0.56wt % P3HT)
No PEDOT:PSS
Size=0.35 * 0.6mm2
V (V)
I (uA/cm2)
P (mW/cm2 )
9.52E-06
0.071
0.095
0.12
0.14
0.17
0.19
0.21
0.24
0.26
0.29
0.31
0.33
0.36
0.38
0.40
0.44
0.48
0.50
0.52
0.57
0.60
5.14
4.95
4.76
4.67
4.52
4.38
4.14
3.95
3.86
3.67
3.48
3.24
2.95
2.86
2.62
2.38
2.05
1.71
1.33
1.14
0.62
0.33
4.89E-05
0.35
0.45
0.56
0.65
0.73
0.79
0.85
0.92
0.96
0.99
1.01
0.98
1.02
0.99
0.964
0.89
0.81
0.66
0.59
0.36
0.20
98
FF
n (%)
0.33
0.01
Inverted Bilayer (1.0wt % P3HT)
No PEDOT:PSS
Size=0.25 * 0.8mm2
V (V)
I (uA/cm2)
P (mW/cm2 )
9.00E-05
9.65
0.00087
0.075
9.50
0.71
0.10
9.30
0.93
0.13
9.05
1.13
0.15
8.50
1.28
0.18
8.03
1.41
0.20
7.50
1.50
0.23
7.11
1.59
0.25
6.70
1.68
0.30
6.11
1.83
0.33
5.85
1.90
0.35
5.50
1.93
0.40
5.05
2.02
0.45
4.65
2.09
0.50
4.30
2.15
0.55
3.25
1.79
0.61
2.25
1.35
0.65
1.41
0.91
0.68
0.75
0.51
0.71
0.40
0.28
99
FF
n (%)
0.318
0.022
Bulk hetero-junction
(1.0wt % P3HT)
V (V)
I (uA/cm2)
6.67E-06
22.33
0.03
22.20
0.07
22.03
0.13
21.67
0.20
20.80
0.27
20.33
0.34
19.33
0.40
18.87
0.47
18.13
0.53
17.33
0.60
16.07
0.67
14.93
0.74
13.33
0.80
12.47
0.87
11.33
0.93
10.04
1.01
9.33
1.07
7.47
1.13
6.13
1.20
4.87
1.27
3.41
1.34
1.47
1.39
0.80
1.40
0.13
Size=0.3 * 0.5mm2
P (mW/cm2 )
0.00015
0.74
1.49
2.92
4.16
5.45
6.48
7.62
8.53
9.24
9.64
9.99
9.80
9.97
9.89
9.34
9.33
8.01
6.95
5.84
4.31
1.97
1.11
0.18
100
FF
n (%)
0.32
0.1
Size=0.3 * 0.5mm2
Bulk hetero-junction
(0.56wt % P3HT)
V (V)
I (uA/cm2)
P (mW/cm2 )
3.00E-05
14.25
0.00043
0.025
13.70
0.34
0.05
13.01
0.65
0.10
12.15
1.22
0.15
11.35
1.71
0.20
10.65
2.14
0.25
9.81
2.47
0.31
9.25
2.84
0.35
8.72
3.07
0.40
7.90
3.18
0.45
7.21
3.26
0.50
6.65
3.33
0.55
5.75
3.16
0.60
5.50
3.30
0.65
4.51
2.93
0.70
3.65
2.56
0.75
3.02
2.25
0.81
2.10
1.70
0.87
0.95
0.83
0.90
0.52
0.47
101
FF
n (%)
0.26
0.033
Appendix B: Data from charge carrier mobility measurement
CELIV technique:
Single Layer
(1.0wt% P3HT)
Single Layer
(0.56wt %P3HT)
Normal Bilayer
(1.0wt% P3HT)
Normal Bilayer
(1.0wt% P3HT)
Normal Bilayer
(1.0wt% P3HT)
No PEDOT:PSS
Normal Bilayer
(0.56wt% P3HT)
No PEDOT:PSS
Invert Bilayer
(1.0wt% P3HT)
Invert Bilayer
(0.56wt% P3HT)
Invert Bilayer
(1.0wt% P3HT)
No PEDOT:PSS
Invert Bilayer
(0.56wt% P3HT)
No PEDOT:PSS
Mobility,
(cm2 /V.s)
tpulse(s)
tmax (s)
jdel (mA)
j(0) (mA)
Thikness,
d, (cm)
U (V)
ΔV
(mV)
V(0)
(mV)
R
(ohm)
8.64E-05
2.00E-05
2.22E-06
0.24
1.06
1.35E-05
5.71
11.25
50.05
47
4.47E-05
2.00E-05
1.92E-06
0.20
0.87
8.40E-06
5.71
9.5
41.04
47
6.78E-05
2.00E-05
3.47E-06
0.12
0.68
1.87E-05
5.71
5.8
31.86
47
4.05E-05
2.00E-05
3.24E-06
0.10
0.59
1.35E-05
5.71
4.8
27.95
47
5.77E-05
2.00E-05
3.60E-06
0.09
0.59
1.79E-05
5.71
4.12
27.6
47
3.09E-05
2.00E-05
3.52E-06
0.08
0.54
1.28E-05
5.71
3.8
25.2
47
3.74E-05
2.00E-05
4.75E-06
0.07
0.58
1.90E-05
5.71
3.2
27.4
47
2.92E-05
2.00E-05
4.10E-06
0.06
0.51
1.45E-05
5.71
3
24.2
47
2.10E-05
2.00E-05
5.84E-06
0.06
0.47
1.75E-05
5.71
2.7
22.3
47
1.78E-05
2.00E-05
5.00E-06
0.05
0.39
1.38E-05
5.71
2.3
18.1
47
102
DI-SCLC technique
Mobility,
(cm2/V.s)
Single Layer
(1.0wt% P3HT)
Single Layer
(0.56wt% P3HT)
Normal Bilayer
(1.0wt% P3HT)
Normal Bilayer
(0.56wt% P3HT)
Normal Bilayer
(1.0wt% P3HT)
No PEDOT:PSS
Normal Bilayer
(0.56wt% P3HT)
No PEDOT:PSS
Invert Bilayer
(1.0wt% P3HT)
Invert Bilayer
(0.56wt% P3HT)
Invert Bilayer
(1.0wt% P3HT)
No PEDOT:PSS
Invert Bilayer
(0.56wt% P3HT)
No PEDOT:PSS
u,
tDI (s)
ttr (s)
U (V)
Thickness,
(cm)
3.20E-04
7.84E-08
9.98E-08
5.71
1.35E-05
1.28E-04
7.60E-08
9.67E-08
5.71
8.40E-06
6.59E-04
7.30E-08
9.29E-08
5.71
1.87E-05
3.34E-04
7.52E-08
9.57E-08
5.71
1.35E-05
5.71E-04
7.72E-08
9.82E-08
5.71
1.79E-05
2.97E-04
7.60E-08
9.67E-08
5.71
1.28E-05
1.01E-03
4.90E-08
6.23E-08
5.71
1.90E-05
6.52E-04
4.44E-08
5.65E-08
5.71
1.45E-05
9.37E-04
4.50E-08
5.73E-08
5.71
1.75E-05
4.77E-04
5.50E-08
6.99E-08
5.71
1.38E-05
103
d