MASTER THESIS
Measuring the efficiency and charge
carrier mobility of organic solar
cells
Master`s Thesis within the Master`s program in Physics
ABASI ABUDULIMUSUPERVISOR
DAVID BARBEROEXAMINER
BERTIL SUNDQVISTDepartment 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 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
2.2.4 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
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 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.
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 infl uences 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).
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 (Pma x). 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 Vma x respectively. In solar cells there is
another important parameter the so called fill factor (FF):
(2)
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 ( )
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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)
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.
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.
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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:
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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.
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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 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
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).
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
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
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 ser ies 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 C urr en t, 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 P ow er, P ( uW/ cm 2 ) 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 C urr en t, 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 P ow er, P ( uW/ cm 2 ) 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 C urr en t, 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 P ow 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. %.
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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 C urr en t, 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 P ow er, P ( uW/ cm 2 ) 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. %.
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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 C urr en t, 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 2.5 P ow er, P ( uW/ cm 2 ) Voltage (v) 1.0wt % P3HT 0.56wt % P3HT
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. %.
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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 C urr en t, I ( uA /cm 2 ) Voltage (v) 1.0wt % P3HT 0.56wt % P3HT
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. %.
48 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 2 4 6 8 10 P ow er, P ( uW/ cm 2 ) Voltage (v) 1.0wt % P3HT 0.56wt % P3HT
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
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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.
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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 C urr en t, 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 P ow er, P ( uW/ cm 2 ) 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.
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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 C urr en t, 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 10 P 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,
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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
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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.
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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 C urr en t, 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 P ow er, P ( uW/ cm 2 ) 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 Device group 2: 0.56wt. % P3HT 0.0 0.2 0.4 0.6 0.8 1.0 0 5 10 15 20 25 C urr en t, I ( uA /cm 2 ) Voltage (v) With PEDOT:PSS No PEDOT:PSS
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.
59 0.0 0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5 P ow er, P ( uW/ cm 2 ) Voltage (v) With PEDOT:PSS No PEDOT:PSS
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,
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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 -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 C urr en t, I ( uA /cm 2 ) Voltage (v) With PEDOT:PSS No PEDOT:PSS
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.
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 2.5 P ow er, P ( uW/ cm 2 ) Voltage (v) With PEDOT:PSS No PEDOT:PSS
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.
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 C urr en t, I ( uA /cm 2 ) Voltage (v) With PEDOT:PSS No PEDOT:PSS
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.
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 1.4 P ow er, P ( uW/ cm 2 ) Voltage(v) With PEDOT:PSS No PEDOT:PSS
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
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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
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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.
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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
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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
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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
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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 whe n 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.
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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
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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.
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Figure45. Organic material shrinking after completing material coating
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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.
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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.
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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:
