Measuring the efficiency and charge carrier mobility of organic solar …583401/FULLTEXT01.pdf ·...

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

http://www.physics.umu.se/om-institutionen/personal/bertil-sundqvist

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 non-

PEDOT: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 (DI-

SCLC) …………………………………………… 20

Experimental part …………………………………………………. 22

2.1 Materials …………………………………………………. 23

2.1.1 ITO …………………………………………...... 23

2.1.2 PEDOT:PSS ……………………………………. 23

2.1.3 P3HT ………………………………………….... 24

2.1.4 PCBM ………………………………………….. 24

2.2 Device production and equipment ………………………. 26

2.2.1 ITO cleaning …………………………………… 26

2.2.2 ITO etching …………………………………...... 26

2.2.3 P3HT device preparation ………………………. 27

2.2.4 Normal bilayer device preparation ……………… 28

2.2.5 Inverted bilayer device preparation …………...... 29

2.2.6 Equipment ………………………………………. 31

Result and discussion ……………………………………………… 35

3.1 I-V measurement result and discussion …………………... 36

3.1.1 P3HT concentration dependence of I-V and power

curve …………………………………………….. 37

3.1.2 I-V and power curve comparison for all

Devices …………………………………………. 50

3.1.3 Devices performance with and without

PEDOT:PSS ……………………………………. 56

3.1.4 Summary for I-V measurement ………………... 64

3.2 Charge carrier mobility measurement result and

discussion ………………………………………………... 73

3.2.1 Charge extraction by linearly increasing voltage

(CELIV) ………………………………………... 73

3.2.2 Dark injection-space charge limited current (DI-

SCLC) …………………………………………. 77

Conclusion ………………………………………………………. 83

References ……………………………………………………….. 87

Appendices ………………………………………………………. 91

Data from I-V characteristic measurement …………. 91

Data from charge carrier mobility measurement ….... 102

1

Chapter 1

Introduction and theory

2

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 (5), Bilayer (6) and Bulk

hetero-junction (7) photovoltaic organic solar cell.

In single layer photovoltaic organic solar cell, the organic electronic

materials are sandwiched between two conductive metals with different

3

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 could 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.

4

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

5

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.

6

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.

7

Figure2 and Figure3 show most of the important parameters of solar

cells` I-V characteristics such as: short circuit current (Jsc), 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 Imax and Vmax respectively. In solar cells there is

another important parameter the so called fill factor (FF):

(2)

(3)

8

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 is power of incident light.

1.1.1 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:

9

(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:

, (8)

where is the light source`s intensity and is the

short circuit current . 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.

10

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 ( )

11

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 ( ) to infinity.

The shunt resistor ( ) 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)

12

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:

( ) (12)

Where is the photo generated current, is the current on the

diode and and are the current and voltage at the load.

Then we can rewrite it as:

13

(

)

(13)

For the ideal diode, the current is:

⁄ (14)

Combine the equation (14) with (13):

(

) (

) (

) (

⁄ )

(15)

we can rewrite equation (15) as:

(

) (

) (

⁄ ) (16)

If a solar cell is represented with a replacement circuit, Figure3, (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.

14

1.2 Charge Carrier Mobility Measurement Methods

Charge carrier mobility has been a most important parameter to

understanding the organic charge transportable materials and

application of the optoelectronic device (13) (14). 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 (15),

conductivity/concentration method (16), space-charge-limited-current

(SCLC) (17), transient space-charge-limited-current (17), organic field

effect transistors (OFET) (18), Admittance spectroscopy (19), Time-of-

flight (TOF) (20), transient electroluminescence (21) and Charge

extraction by linearly increasing voltage (CELIV) (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

15

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:

16

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.

17

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

than 100nm (13) and also useful for both low and high conductivity

materials (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

18

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, is the

corresponding time to the maximum current peak, is current density

19

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.

20

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 (23).

Then the time, , 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)

21

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 DI-

SCLC are well known, the application of it has encountered limited

success due to the lack of Ohmic injecting electrodes. (33)

22

Chapter 2

Experimental Part

23

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).

24

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 (36), solar cells, batteries and

diodes (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 to changes

in solvent, temperature, applied potential, and 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

25

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.

26

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.

27

2.2.3 P3HT device preparation

Figure11. P3HT device structure

P3HT (poly, 3-hexylthiophene) solution was made with

dichlorobenzene ( ) 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.

28

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 , later I

changed the solvent to dichloromethane ( ), 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

29

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,

30

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.

31

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

32

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.

33

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.

34

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.

35

Chapter 3

Result and discussion

36

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

37

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

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50

60

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

38

0.0 0.2 0.4 0.6 0.8 1.0

0

2

4

6

8

10

12

Po

we

r, P

(u

W/c

m2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

39

Bilayer: GLASS/ITO / PEDOT:PSS / P3HT / PCBM / Au/Si

0.0 0.2 0.4 0.6 0.8

0

5

10

15

20

25

30

35

40

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

40

0.0 0.2 0.4 0.6 0.8

0

1

2

3

4

5

6

7

Po

we

r, P

(u

W/c

m2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

41

Bilayer (without PEDOT:PSS): GLASS/ITO / P3HT / PCBM / Au/Si

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25

30

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

42

0.0 0.2 0.4 0.6 0.8 1.0

0

1

2

3

4

5

6

7

Pow

er,

P (

uW

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

43

Inverted Bilayer: GLASS/ITO / PCBM / P3HT / PEDOT:PSS / Au/Si

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0

2

4

6

8

10

12

14

Curr

ent,

I (

uA

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

44

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.0

0.5

1.0

1.5

2.0

Po

we

r, P

(u

W/c

m2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

45

Inverted Bilayer (without PEDOT:PSS): GLASS/ITO / PCBM / P3HT

/ Au/Si

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0

2

4

6

8

10

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

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. %.

46

0.0 0.2 0.4 0.6 0.8

0.0

0.5

1.0

1.5

2.0

2.5P

ow

er,

P (

uW

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

Figure28. Power curves comparison of two Inverted Bilayer (without


line represents the power curve of the device made with 1.0wt % P3HT


47

Bulk: GLASS/ITO / PCBM : P3HT / Au/Si

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0

5

10

15

20

25

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

Figure29. I-V curves comparison of two Bulk hetero junction (without


line represents the I-V curve of the device made with 1.0wt % P3HT


48

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0

2

4

6

8

10P

ow

er,

P (

uW

/cm

2)

Voltage (v)

1.0wt % P3HT

0.56wt % P3HT

Figure30. Power curves comparison of two Bulk hetero junction (without


line represents the power curve of the device made with 1.0wt % P3HT


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

49

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.

50

3.1.2 I-V and power curve comparison of all devices

Devices group 1: 1.0wt. % P3HT solution was used

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50

60

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

P3HT

Bilayer

Inverted

Bulk

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.

51

0.0 0.2 0.4 0.6 0.8 1.0

0

2

4

6

8

10

12

Po

we

r, P

(u

W/c

m2)

Voltage (v)

P3HT

Bilayer

Inverted

Bulk

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.

52

Devices group 2: 0.56wt. % P3HT solution was used

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

P3HT

Bilayer

Inverted

Bulk

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.

53

0.0 0.2 0.4 0.6 0.8 1.0

0

2

4

6

8

10P

ow

er,

P (

uW

/cm

2)

Voltage (v)

P3HT

Bilayer

Inverted

Bulk

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 hetero-

junction 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,

54

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

55

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.

56

3.1.3 Device performance with and without PEDOT:PSS

Normal Bilayer: with and without PEDOT: PSS

Device group 1: 1.0wt. % P3HT

0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25

30

35

40

Curr

ent,

I (

uA

/cm

2)

Voltage (v)

With PEDOT:PSS

No PEDOT:PSS

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.

57

0.0 0.2 0.4 0.6 0.8 1.0-1

0

1

2

3

4

5

6

7

Po

we

r, P

(u

W/c

m2)

Voltage (v)

With PEDOT:PSS

No PEDOT:PSS

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.

58


0.0 0.2 0.4 0.6 0.8 1.0

0

5

10

15

20

25

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

With PEDOT:PSS

No PEDOT:PSS

Figure37. I-V curves comparison for normal bilayer devices made with and


line represents the bilayer device which includes PEDOT:PSS, whereas the

red dashed line is for a device without PEDOT:PSS.

59

0.0 0.2 0.4 0.6 0.8 1.0

0

1

2

3

4

5P

ow

er,

P (

uW

/cm

2)

Voltage (v)

With PEDOT:PSS

No PEDOT:PSS

Figure38. Power curves comparison for normal bilayer devices made with


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,

60

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


-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0

2

4

6

8

10

12

14

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

With PEDOT:PSS

No PEDOT:PSS

Figure39. I-V curves comparison for inverted bilayer devices made with and


line represents the device which includes PEDOT:PSS, whereas the red

dashed line is for a device without PEDOT:PSS.

61

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.0

0.5

1.0

1.5

2.0

2.5P

ow

er,

P (

uW

/cm

2)

Voltage (v)

With PEDOT:PSS

No PEDOT:PSS

Figure40. Power curves comparison for inverted bilayer devices made with


solid line represents the device which includes PEDOT:PSS, whereas the red


62

Device group2: 0.56wt. % P3HT

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0

2

4

6

8

10

12

Cu

rre

nt,

I (

uA

/cm

2)

Voltage (v)

With PEDOT:PSS

No PEDOT:PSS

Figure41. I-V curves comparison for inverted bilayer devices made with and


line represents the device which includes PEDOT:PSS, whereas the red


63

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4P

ow

er,

P (

uW

/cm

2)

Voltage(v)

With PEDOT:PSS

No PEDOT:PSS

Figure42. Power curves comparison for inverted bilayer devices made with


solid line represents the device which includes PEDOT:PSS, whereas the red


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

64

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

65

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.

66

Overall device performance comparison

1% P3HT

⁄

⁄

(%)

η

(%)

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

67

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 ⁄ and a maximum power of

2.1 ⁄ . 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

68

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

69

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 that 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

70

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

71

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 non-

uniformed 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

72

Figure45. Organic material shrinking after completing material coating

Figure46. Organic material at the edge of the device surface

73

Figure47. Organic material at the center of the device surface

74

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.

75

1.0wt% P3HT µ

⁄

(cm)

P3HT 8.64E-05 2.22E-06 0.24 1.07 1.35E-05

BL 6.78 E-05 3.47E-06 0.12 0.68 1.87E-05

BL NO PED 5.77E-05 3.60E-06 0.09 0.59 1.79E-05

Inverted 3.74E-05 4.75E-06 0.07 0.58 1.90E-05

Invert No PED 2.10E-05 5.84E-06 0.06 0.47 1.75E-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 4.10E-06 0.06 0.51 1.45E-05

Invert No PED 1.78E-05

5.00E-06 0.049 0.38 1.38E-05

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.

76

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.

77

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:

78

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 9.29E-08 5.71 1.87E-05

BL NO PED 5.71E-04 7.72E-08 9.82E-08 5.71 1.79E-05

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

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 9.57E-08 5.71 1.35E-05

BL NO PED 2.97E-04 7.60E-08 9.67E-08 5.71 1.28E-05

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

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

79

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:

80

Figure48. An example of Organic film (material in this picture was P3HT)

thickness measurement by AFM

81

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.

82

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

83

Chapter 4

Conclusion

84

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 , maximum extracted power of

11.4mW/ 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 spin-

coating 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

85

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 DI-

SCLC 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.

86

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.

87

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91

Appendices

Appendix A: Data from I-V characteristic measurement

Single layer (P3HT 1.0wt %) Size=0.3*0.6mm2

V (V) I (uA/cm2) P (mW/cm2 ) FF n (%)

3E-05 56.01 0.0015

0.027 54.70 1.52

0.055 53.30 2.96

0.11 49.20 5.47

0.17 44.10 7.46

0.23 40.02 9.10

0.28 36.70 10.21

0.33 33.30 11.10

0.39 29.01 11.30

0.44 25.60 11.40 0.217 0.11

0.50 22.20 11.10

0.55 19.20 10.61

0.61 16.11 9.82

0.67 12.80 8.60

0.72 10.14 7.31

0.78 7.70 5.96

0.83 5.32 4.26

0.89 3.25 2.72

0.94 0.54 0.47

92

Single Layer (P3HT 0.56wt %)

Size=0.2*0.65mm2


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 0.256 0.09

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

93

Normal Bilayer ( 1.0wt % P3HT) Size=0.3*0.55mm2


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 0.22 0.06

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

94

Normal Bilayer ( 0.56wt % P3HT) Size=0.3*0.55mm2


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 0.201 0.036

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

95

Normal Bilayer (1.0wt % P3HT) No PEDOT Size=0.35 * 0.5mm2


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 0.251 0.067

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

96

Normal Bilayer (0.56wt % P3HT) No PEDOT Size=0.3 * 0.5mm2


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 0.247 0.045

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

97

Inverted Bilayer (1.0wt % P3HT) Size=0.15 * 0.5mm2


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 0.274 0.021

0.53 2.02 1.07 0.61 0.28 0.17

Inverted Bilayer (0.56wt % P3HT) Size=0.4 * 0.5mm2


1.35E-04 11.03 0.0015 0.025 10.01 0.25 0.05 9.60 0.48 0.10 8.50 0.85 0.15 7.41 1.12 0.20 6.45 1.29 0.25 5.51 1.38 0.234 0.014

0.30 4.57 1.37 0.35 3.55 1.26 0.40 2.70 1.08 0.46 1.61 0.74 0.52 0.57 0.30 0.53 0.33 0.17

98

Inverted Bilayer (0.56wt % P3HT) No PEDOT:PSS

Size=0.35 * 0.6mm2


9.52E-06 5.14 4.89E-05 0.071 4.95 0.35 0.095 4.76 0.45 0.12 4.67 0.56 0.14 4.52 0.65 0.17 4.38 0.73 0.19 4.14 0.79 0.21 3.95 0.85 0.24 3.86 0.92 0.26 3.67 0.96 0.29 3.48 0.99 0.31 3.24 1.01 0.33 2.95 0.98 0.36 2.86 1.02 0.33 0.01

0.38 2.62 0.99 0.40 2.38 0.964 0.44 2.05 0.89 0.48 1.71 0.81 0.50 1.33 0.66 0.52 1.14 0.59 0.57 0.62 0.36 0.60 0.33 0.20

99

Inverted Bilayer (1.0wt % P3HT) No PEDOT:PSS

Size=0.25 * 0.8mm2


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 0.318 0.022

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

100

Bulk hetero-junction (1.0wt % P3HT)

Size=0.3 * 0.5mm2


6.67E-06 22.33 0.00015 0.03 22.20 0.74 0.07 22.03 1.49 0.13 21.67 2.92 0.20 20.80 4.16 0.27 20.33 5.45 0.34 19.33 6.48 0.40 18.87 7.62 0.47 18.13 8.53 0.53 17.33 9.24 0.60 16.07 9.64 0.67 14.93 9.99 0.32 0.1

0.74 13.33 9.80 0.80 12.47 9.97 0.87 11.33 9.89 0.93 10.04 9.34 1.01 9.33 9.33 1.07 7.47 8.01 1.13 6.13 6.95 1.20 4.87 5.84 1.27 3.41 4.31 1.34 1.47 1.97 1.39 0.80 1.11 1.40 0.13 0.18

101

Bulk hetero-junction (0.56wt % P3HT)

Size=0.3 * 0.5mm2


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 0.26 0.033

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

102

Appendix B: Data from charge carrier mobility measurement

CELIV technique:

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)

Single Layer (1.0wt% P3HT) 8.64E-05 2.00E-05 2.22E-06 0.24 1.06 1.35E-05 5.71 11.25 50.05 47 Single Layer (0.56wt %P3HT) 4.47E-05 2.00E-05 1.92E-06 0.20 0.87 8.40E-06 5.71 9.5 41.04 47

Normal Bilayer (1.0wt% P3HT) 6.78E-05 2.00E-05 3.47E-06 0.12 0.68 1.87E-05 5.71 5.8 31.86 47 Normal Bilayer (1.0wt% P3HT) 4.05E-05 2.00E-05 3.24E-06 0.10 0.59 1.35E-05 5.71 4.8 27.95 47

Normal Bilayer (1.0wt% P3HT) No PEDOT:PSS 5.77E-05 2.00E-05 3.60E-06 0.09 0.59 1.79E-05 5.71 4.12 27.6 47 Normal Bilayer (0.56wt% P3HT) No PEDOT:PSS 3.09E-05 2.00E-05 3.52E-06 0.08 0.54 1.28E-05 5.71 3.8 25.2 47

Invert Bilayer (1.0wt% P3HT) 3.74E-05 2.00E-05 4.75E-06 0.07 0.58 1.90E-05 5.71 3.2 27.4 47

Invert Bilayer (0.56wt% P3HT) 2.92E-05 2.00E-05 4.10E-06 0.06 0.51 1.45E-05 5.71 3 24.2 47 Invert Bilayer (1.0wt% P3HT) No PEDOT:PSS 2.10E-05 2.00E-05 5.84E-06 0.06 0.47 1.75E-05 5.71 2.7 22.3 47

Invert Bilayer (0.56wt% P3HT) No PEDOT:PSS 1.78E-05 2.00E-05 5.00E-06 0.05 0.39 1.38E-05 5.71 2.3 18.1 47

103

DI-SCLC technique

Mobility, u, (cm2/V.s) tDI (s) ttr (s) U (V)

Thickness, d (cm)

Single Layer (1.0wt% P3HT) 3.20E-04 7.84E-08 9.98E-08 5.71 1.35E-05 Single Layer (0.56wt% P3HT) 1.28E-04 7.60E-08 9.67E-08 5.71 8.40E-06 Normal Bilayer (1.0wt% P3HT) 6.59E-04 7.30E-08 9.29E-08 5.71 1.87E-05

Normal Bilayer (0.56wt% P3HT) 3.34E-04 7.52E-08 9.57E-08 5.71 1.35E-05

Normal Bilayer (1.0wt% P3HT) No PEDOT:PSS 5.71E-04 7.72E-08 9.82E-08 5.71 1.79E-05

Normal Bilayer (0.56wt% P3HT) No PEDOT:PSS 2.97E-04 7.60E-08 9.67E-08 5.71 1.28E-05

Invert Bilayer (1.0wt% P3HT) 1.01E-03 4.90E-08 6.23E-08 5.71 1.90E-05 Invert Bilayer (0.56wt% P3HT) 6.52E-04 4.44E-08 5.65E-08 5.71 1.45E-05 Invert Bilayer (1.0wt% P3HT) No PEDOT:PSS 9.37E-04 4.50E-08 5.73E-08 5.71 1.75E-05 Invert Bilayer (0.56wt% P3HT) No PEDOT:PSS 4.77E-04 5.50E-08 6.99E-08 5.71 1.38E-05

Date post:	21-Mar-2021
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