Semiconducting behavior of conjugated polymer

Abstract

Over the last few decades, the bulk-heterojunction based organic photovoltaic devices have gained serious attention due to their potential for being cheap, light weighting, flexible and environmentally friendly. In this thesis work, APFO-3/PCBM bulk-heterojunction based organic photovoltaic devices with inverted layer sequence were investigated systematically. Doctor blade technique as a roll-to-roll compatible and cost efficient method was introduced into fabrication processes.

Initial studies focused on the optimization of the electrodes. The conductive polymer PEDOT:PSS was chosen to be the transparent anode. Different PEDOT:PSS films with respect to the film thickness and the deposition temperature were characterized by four-point probe system and UVVIS measurement. Sufficient conductance and transmittance were obtained in the films deposited with wet film thickness setting of 35 m, while the best-working metal cathode contained a 70 nm thick Al layer covered by a thin protecting Ti layer (10 nm-15 nm).

Then, optimized coating temperature and wet film thickness setting for active layer and PEDOT layer were experimentally determined. The highest efficiency of the APFO-3/PCBM bulk-heterojunction based solar devices fabricated by doctor blading was 0.69%, which exceeded the efficiency of spin-coated cells.

For blade coated cells, higher efficiency (3%) was achieved by adding small amount of polystyrene (Mw~30,000,000) into the active solution. The morphological changes after adding the polystyrene were observed by using AFM, and coating temperature dependent phase separation of APFO-3/PCBM/polystyrene blend was also found.

List of abbreviation

Al Aluminum

APFO Alternating polyfluorene

APFO-3 Poly[(9,9-dioctylfluorenyl-2,7- diyl)-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)]

AFM Atomic force microscope

BHJ Bulk-heterojunction

D-A-D Donor-acceptor-donor

EQE External quantum efficiency

FF Fill factor

HOMO Highest occupied molecular orbital

ITO Indium tin oxide

Jsc Short circuit current density

LUMO Lowest unoccupied molecular orbital

LiF Lithium fluoride

Mw Weight average molecular weight

MDMO-PPV Poly[2-methoxy-5-(3,7- dimethyloctyloxy)]-1,4-phenylenevinylene

OPV Organic photovoltaic

PCBM [6,6]-phenyl-C61-butyric acid methyl ester

PCE Power conversion efficiency

PEDOT:PSS Poly(3,4-ethylenedioxy- thiophene):poly(styrenesulfonate)

PIB Polyisobutylene

PS Polystyrene

R2R Roll-to-roll

Ti Titanium

Voc Open circuit voltage

UCST Upper critical solution temperature

WF Workfunction

WFT Wet film thickness

wt - Weight

Introduction

Energy source, as the material basis of human activities, has been a key factor in the development of human society. However, the increasing use of energy since the 18th century has also caused many serious problems, such as global warming and atmosphere pollution. On the other hand, the storage of current main energy sources (oil, coal, etc.) is limited. According to recent predictions, there were only 40 years of petroleum left in the Earth. All those factors are forcing us to replace most of the currently used non-renewable energy sources by renewable energy sources.

Sunlight is by far the largest energy source on the Earth. The energy from the sunlight strikes the Earth in one hour is more the consumption of energy on the planet in a whole year. Therefore, getting energy directly from the sunlight has been acknowledged as an essential way of future global energy production.

The direct conversion of sunlight into electricity can be achieved by photovoltaic devices or sometimes called solar cells. The solar cell technology has been extensively studied, since the first crystalline silicon based solar cell was developed at Bell Laboratories[1]. So far, the traditional solar cells can harvest more than 20%[2] which is already close to the theoretical upper limit of 30%[3], and the silicon based solar cell is still the most dominant solar device that is used and occupied more the 85% of the market[4]. Although, the photovoltaic technology has been introduced to the market for many years, less than 0.1% of the total world energy production is based on it. The major obstacle is that the production cost of silicon based technology is quite expensive. The production of those cells requires intensive processes at high temperature and high vacuum conditions with a large amount of lithographic steps. Therefore, to ensure a penetration of solar cells into the market, new materials have to be developed.

As a relatively novel technology, organic solar cell has been studied intensively over the last few decades. The present efficiency of bulk-heterojunction (BHJ) based organic solar cells have exceeded 6.5%[5], but it is still much below the value for inorganic solar cell, and their lifetime is much shorter as well. However, it shows great promise for decreasing the cost of solar energy. The semiconducting materials in organic solar cells are polymers or small molecules which are much cheaper than inorganic materials like silicon and the fabrication processes of organic solar cells are much simpler. It has the possibility to be the thin, flexible, lightweight device with the potential to be commercialized by roll-to-roll (R2R) print process. This possibility could strongly reduce the cost of fabrication process, which may lead to a widespread of the photovoltaic devices in the near future.

In this thesis, BHJ based organic photovoltaic (OPV) devices were studied. The aim of this project is to fabricate cost efficient ITO free organic solar cells with inverted layer sequence. Therefore, the R2R compatible technique doctor blading was used as the main fabrication method. The optical, electronic, and morphological properties as well as the performance limiting factors of such OPV devices were also investigated.

Background

The semiconducting behavior of conjugated polymer

Chemically, polymers are formed by repetitive covalent bonding of chemical units, or monomers, with molecular weight over 10,000 gm mol-1. Therefore, they are often named as macromolecules when referring to polymeric materials.

Based on the type and number of carbon atoms along the main chain of polymer, polymers can be divided into two major classes: saturated and unsaturated, and most conductive polymers have unsaturated structure. The semiconducting property of polymer originates from the conjugated p-electron system which is formed by the sp2 hybrid orbitals of carbon atoms in the molecule (See Figure 4.1). The p-bonding is significantly weaker compare to the s-bonds which is forming the backbone of the molecules, therefore, p-p* transitions with an energy gap, or bandgap, are the lowest electronic excitations of semiconducting polymers (See Figure 4.2). The bandgap of the conjugated polymer is within the semiconducting range of 1 to 4 eV and characterized by the electron affinity and ionization potential, which refer to the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO), respectively.

Bulk-heterojunction solar cells

Working principle

Absorption of light is the first step when the solar cell starts to work. Only photons with higher energy than the energy of bandgap can be absorbed in active material. The absorption loss can occur due to the reflection, transmission, or insufficient energy of incident photons. Generally, lower bandgap materials absorb more photons; however, the waste of excess energy from the higher energy photons also limits the device performance, therefore, there are certain limitations on the bandgap[6].

An exciton, which has a pair of electrostatically bounded electron and hole, forms after a photon absorbed in the active material. The bounded electron-hole pair can diffuse, recombine, or dissociate into free electrons and holes. The small diffusion length of primary excitons in the amorphous and disordered organic semiconductors is an important limiting factor. Such excitons, with large exciton binding energy[7-9] (>100 meV) and short diffusion length (4 to 20 nm), created in the bulk materials of organic semiconductors cannot be dissociated into free charger carriers by the thermal excitations[10,11]. However, it is observed that they dissociate at metal-semiconductor interfaces or at donor-acceptor interfaces, where the discontinuous electronic potential exists. An ultrafast, reversible, metastable photoinduced electron transfer from conjugated polymers onto fullerenes in solid films was observed in the mixture of conjugated polymer with fullerene[14, 40]. This phenomenon resulted in the development of BHJ devices. BHJ OPV devices were first introduced by Heegers group[41] in 1995, which ideally could maximize the interfacial area between the donors and acceptors. Typically, BHJ based organic solar cells can be divided into 3 classes

The BHJ based polymer/fullerene OPV device is showed schematically in Figure 4.3. The solid-state composite film with BHJ structure is within a few nanometers of a neighboring donor-acceptor interface. Therefore, efficient exciton dissociation can be found in solar devices with BHJ structure. A simplified working principle of OPV device is shown in Figure 4.4.

The free charges must be able to reach the electrodes in order to contribute to the output power of solar devices. However, there is always a risk of trapping or recombination, which limits the charge transport efficiency. Due to the increasing risk for trapping or recombination with the increase of distance that charge travels, a thin layer is desired, but optical absorption is simultaneously reduced. Therefore, the optimization of active layer thickness is very important.

Current-voltage characteristics

In order to characterize a photovoltaic device, several parameters are usually introduced. They are: open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), power conversion efficiency (PCE), and external quantum efficiency (EQE). A typical current-voltage characteristic of organic cell is illustrated in Figure 4.5.

The open-circuit voltage Voc is defined as a voltage between two terminals of a device when there is no external load. For a solar cell device, it is equal to the applied voltage at which the net current in the cell is zero under illumination.

The short-circuit current density Jsc for a solar device is defined as the current in the external circuit when the external load is a short circuit. It is equal to the extracted photogenerated current when there is no applied voltage.

The fill factor FF is defined as

JmaxVmax indicates the maximum power that can be extracted from the solar cell and it is represented as the filled rectangle in Figure 2.5. In general, a large series resistance and small shunt resistance tend to reduce the FF.

The power conversion efficiency ? is the most important parameter for solar cells as it indicates how efficient the cell can work, and it is defined as the power output divided by the incident light power. It can be can be calculated from

Where Jsc is the short circuit current in mA m-2, Voc is the open circuit voltage in V, FF the fill factor, and Plight the incident solar radiation in W m-2.

External quantum efficiency considers the actual fraction of incident photons that can be converted to charges, as

Where h is the Plancks constant, c is the speed of light and e is an elementary charge. Jsc,i and fin,i are the short-circuit current density at a specific excitation wavelength and the incident monochromatic photon flux, respectively.

Performance factors

As mentioned previously, the most important parameter PCE is dependent on three key factors, namely Jsc, Voc, and FF.

Jsc of organic solar cells is related to several factors such as light intensity, light absorption rate, exciton generation and dissociation rate, and the carrier transport in the active layer etc. The absorption of light or exciton generation rate would depend on the active layer thickness and bandgap of active materials. The exciton dissociation rate mainly depend on the interfacial area between donor and acceptor, and in BHJ films, the dissociation rate is quite efficient. Molecules with high electron affinity, such as fullerene, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), are the most popular materials that used as the acceptor for organic solar cells[12-17]. The carrier transport in the active layer is controlled by the donor-acceptor interpenetrating networks; meanwhile, the morphology of active film would also play a pivotal role.

The Voc of BHJ based organic solar cells is limited by many factors, such as morphology of active layer, electrodes, and film quality, etc[18-20]. Usually, the difference between the HOMO energy of the donor and the LUMO energy of the fullerene acceptor determines the Voc.

The third parameter that determines PCE is the FF. Compare to Voc and Jsc, the FF is much more sensitive. In general, it depends on the mobility-life product of active material, thickness of active layer as well as morphology of active layer. Moreover, big series resistance would also lower the FF in BHJ based organic solar cells.

Device structure

The schematic picture of a typical BHJ based organic solar cell is shown in Figure 4.6 (a). The solar devices are usually spin coated onto substrates coated with transparent indium tin oxide (ITO). A layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), or PEDOT:PSS in short, is deposited between active layer and ITO and acting as the anode which collects the holes. The opaque cathode is usually deposited by thermal evaporation, and commonly used materials are aluminum (Al) and lithium fluoride (LiF)[21-23].

Figure 4.6. It illustrates the schematic diagram of (a) traditional non-inverted structure, (b) inverted structure. No ITO glass in the inverted device and the bottom cathode layer sequence could lower the cost of manufacturing when it faces to the large-scale mass industrial production.

Because of the lower efficiency and shorter lifetime of organic solar cells compare to their inorganic counterpart, it is extremely important to use abundant and cheap materials as well as cost efficient production methods if organic solar cells can be commercialized. Hence, ITO glass which elevates the prize of organic solar devices should be replaced. Meanwhile, R2R printing technique as a cost efficient deposition method should be applied when large scale and mass productions are required. Solar cells with inverted layer sequence are introduced as the ITO free cells. The schematic diagram of inverted organic solar cell studied in this project is shown in Figure 4.6 (b). The metal cathode (Al/Ti) is evaporated on a plastic substrate by thermal evaporator before the deposition of any other layers, which has lower cost compared to those non-inverted solar cells that have their metal electrodes evaporated at the last step. The transparent top anode is high conductive PEDOT:PSS, and active layer is sandwiched between the two electrodes. Therefore, with this inverted structure, cost efficient and flexible organic solar device can be fabricated.

Materials and methods

Materials

Metal cathode

The most commonly used cathode for organic solar cells is Al. In order to lower the workfunction (WF) of cathode and prevent the reaction between the Al and PCBM, a thin layer of LiF is usually deposited between Al and active layer[24]. For the inverted cells, cathode is always prepared before deposition of other layers. However, when the Al is exposing to the air, a highly insulating oxide would be formed at the Al surface, therefore a protecting layer is required. LiF could not be deposited too thick, because thick LiF layer results in a low Jsc and it is found to be detrimental to electron injection as well. So we use titanium (Ti) as the protecting material in this project, since it has desired WF and sufficiently high electron mobility, even if oxidized.

PEDOT:PSS

The ITO is commonly used as anode for organic solar cells. But due to its high price, it is not a good anode material when commercialization of organic solar devices is taken into account. PEDOT:PSS, as a transparent, conductive and flexible material, is a good substitute.

PEDOT:PSS is an abbreviation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). The chemical structure of the normal PEDOT:PSS is given in Figure 5.1. PEDOT:PSS is a polymer mixture of two ionomers. The sodium polystyrene sulfonate, which is a sulfonated polystyrene, is one component of PEDOT:PSS. Some sulfonyl groups are deprotonated and carry the negative charges. The PEDOT is the second component. It is a conjugated polymer based on polythiophene and carries positive charges.

Two different PEDOT:PSS, CLEVIOS P (PEDOT:PSS Baytron P) and CLEVIOS PH 500 (PEDOT:PSS PH500) are studied in this project. Both of them were purchased from H.C. Starck Group, and their properties are given in Table 5.1.

The optimization of PEDOT:PSS layer thickness is very important when it is used as the anode of organic solar cell, since very thin layer yields larger sheet resistance and more pin holes, while thicker layer leads to larger decrease in the transmittance. The WF, as well as the conductivity and roughness of PEDOT:PSS films are dependent on the annealing temperature as well[25], therefore, the thermal treatment of PEDOT:PSS should also be optimized.

APFO-3

The alternating polyfluorene (APFO) copolymer, poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-5,5- (40,70-di-2-thienyl-20,10,30-benzothiadiazole)], is usually called APFO-3. It is the combination of a fluorine monomer and donor-acceptor-donor (D-A-D) co-monomers, and It has been used to fabricate organic solar cells[13, 26, 27]. APFO-3 belongs to the APFO family, and its chemical structure is shown in Figure 5.2.

APFO-3 is designed to have a low band gap. Therefore, the absorption band of APFO-3 is quite board and extends into infrared solar spectrum. The absorption spectrum of APFO-3 is given in Figure 5.3.

PCBM

PCBM is the common abbreviation for the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester. The chemical structure of PCBM is given in Figure 5.4

PCBM is commonly used as electron acceptor in BHJ based organic solar cells. In this project, the solution of APFO-3 and PCBM blend with a fixed donor-acceptor ratio 1 to 4 that dissolved in toluene is used as the active ink. The absorption spectrum of such blend is given in Figure 5.5.

Toluene

It has been reported that for APFO-3/PCBM based BHJ organic solar cells would have the PCE over 2.5% with a mixed solvent of chloroform and chlorobenzene[38]. However, the toxicity of such solvents will increase the production cost of manufacturing, since specific working environment will be required. So, the solvent used in this project are mainly toluene simply because of its low toxicity. However, it would be difficult to dissolve high molecular weight APFO-3 in this case. The chemical structure of toluene is shown in Figure 5.6.

Polystyrene

Polystyrene (PS) is an aromatic polymer which is synthesized from the aromatic monomer styrene. It is a liquid hydrocarbon which is commercially manufactured from petroleum by the chemical industry. PS is one of the most widely used kinds of plastic[28]. The chemical structure of PS is given in Figure 5.7.

As we know, one major advantage of organic solar cells compare to their inorganic counterpart is that the R2R or other printing techniques have the potential to be applied to manufacture such solar devices. However, for large scale production of organic solar cells, the ink viscosity, device life time as well as the environmental stability will become crucial. Adding polymer additives into active ink is one simple way to achieve those requirements. It has been reported by C. J. Brabec, F. Padinger, and N. S. Sariciftci that by adding small amount of polystyrene into poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and PCBM active ink did not affect the efficiency of such solar devices significantly[29]. Meanwhile, the active polymer could be better encapsulated against environmental influence. By the proper choice of additive the charge transfer between conjugated polymer and PCBM could be advantageously tuned as well, either by changing the intermolecular distances through the morphology control or by the dielectric permittivity of the blend system. At the same time, the ink viscosity can also be increased if higher molecular weight of PS is chosen. In this project, a-PS with weight average molecular weight (Mw) about 30,000,000 has been studied as the additive in APFO-3 and PCBM blend system.

Methods

Coating technique

The spin coating is the most important deposition technique for the development of organic solar cells so far. Highly reproducible as well as very homogenous films can be deposited by spin coating technique. In principle, its operation can be described as: an excess amount of a solution is introduced on the substrate which is going to be accelerated to a chosen rotational speed in order to spread the fluid by centrifugal force, as shown in Figure 5.8. The angular velocity of substrate with the overlying solution gives rise to the ejection of most applied solution where only a thin film can be left on the substrate. The thickness, morphology and surface topography of the final film obtained from a particular material in a given solvent and at a given concentration is highly reproducible[30].

It has been commonly acknowledged that spin coating technique is an excellent experimental technique on laboratory scale. However, when the commercialization of the OPV devices is taking into account, where the mass and large scale production is required, spin coating becomes uncompetitive. Several disadvantages such as: incompatible to R2R printing, hard to deposit film in large scale, ink waste, etc. limited its application in industry.

Doctor blading is a technique that is seldom studied in the context of organic solar cells. The mechanism of doctor blading is quite simple. As shown in Figure 5.9, a sharp blade is placed at a fixed distance from substrate surface. Then the ink is dropped in front of the blade. By moving the blade across the substrate with constant speed, a thin wet film can be deposited on the substrate uniformly. The wet film thickness (WFT) is dependent on the distance between the blade and substrate, as well as surface energy of substrate, the surface tension of ink, and the viscosity of ink etc. To simplify, we define the gap width between blade and substrate as the WFT in this project.

In contrast to spin coating technique, doctor blading is quite material saving, since ideally all the active ink can be fully utilized with this technique. Meanwhile, it is also compatible to R2R print that means it can be easily transferred to R2R coating environment; therefore, it is a preferred technique as the precursor of mass manufacture. However, doctor blading requires much more time to find the proper conditions for coating, which makes doctor blading unpopular in laboratories.

Thermal evaporation

Thermal evaporation is a widely used method to deposit films. The source material is evaporated at high temperature, and then ejected atoms travel directly to the substrate where they condense back to a solid state. A schematic picture of thermal evaporator is given in Figure 5.10.

Vacuum is usually required to remove vapors other than the source material before the evaporation begins, because when evaporated atoms collide with other foreign atom may cause a reaction between them. For instance, if aluminum is deposited in the presence of oxygen, it will form aluminum oxide instead of pure aluminum.

Thickness determination for active layer

The thickness of active layer as an important parameter for organic solar cells, strongly affects the device performance. The commonly used equipment that could be used to measure the film thickness was DekTak stylus surface profilometer which required us to scratch off some active layer before measurement. However, since the substrate we used was always the plastic, scratching could not be performed without destroying the substrate. In order to determine the thickness of active layer in this project, we used another method in which the color of active layer which was caused by the inference of light played an important role. We first coated several active layers on the metal electrodes which were deposited on a glass substrate, with different WFT settings. Since reflective electrodes were used here; and due to the inference of light, the colors of films were extremely sensitive to the thickness of active layer. Later on, we measured the thicknesses of those active layers by DekTak stylus surface profilometer, and listed relation between the colors and thicknesses as shown in Figure 5.11. This relation should be also valid for those films that coated on plastic substrate. As a result, the thickness of active layer on plastic substrate can be determined by the color sheet. But one should notice that the color of active layer is material specific, which means different active materials and metal electrode materials as well as the thickness of metal electrodes will give different results.

Figure 5.11. The color sheet is used for determining the thickness of active layer. The color of active layer would depend on active materials, electrode materials, as well as the thickness of active layer and the thickness of metal electrode. In this case, the active material is the mixture of APFO-3 and PCBM with donor/acceptor ratio 1:4 (weight, the concentration of APFO-3 was 3 mg ml-1). The active layer is deposited on the metal electrode which contains a 15 nm thick Ti on 80 nm thick Al.

Simple and efficient are the main advantages of using this method to determine the thickness of active layer of an inverted organic solar cell. We do not need to measure the thickness for every device we fabricated. Meanwhile instead of getting a uniform film, doctor blading usually gives nonuniform films. By using the color sheet, the thickness variations in the thin film and how much the thickness changes from one region to another in this film can be both found out immediately.

Four-point probe

The sheet resistivity of different PEDOT:PSS films were measured by four-point probe method. The four-point probe setup is a system to evaluate the resistance of bulk materials or sheet resistance of thin films. The measurement technique is based on two pairs of electrodes, one supplying the current the other measuring the voltage. The four-point probe setup used in this project consists of four equally placed metal tips. As shown in Figure 5.12, current is injected through the outer two tips; while, the voltage cross the inner tips are measured to determine the sample resistivity.

Ubbelohde viscometer

The viscosity of different solutions was measured by Ubbelohde viscometer in this project. Ubbelohde viscometer is a commonly used gravity type viscometer; and it uses a capillary based method to measure the viscosity. It consists of four reservoir bulbs and a capillary tube as shown in Figure 5.13.

In principle, the time needed for the test ink to flow from the upper graduation mark to lower mark is measured. By multiplying the time taken by the factor of the viscometer, the kinematic viscosity is obtained.

Atomic force microscope

The atomic force microscope (AFM) is one of the most commonly used and powerful tools for determining the surface topography of materials. In principle, it works like our fingers which touch and feel and object when we are in the dark. And the signal got by the fingers can transport to brain which is able to reconstruct the topography of the object. The AFM uses a sharp tip which is attached to a cantilever to scan the sample surface point by point, and the computer works as the brain to visualize sample surface. A schematic illustration of AFM is given is Figure 5.14.

Typically the radius of tip is on the order of nanometers. When it is brought close to sample surface, a deflection of cantilever would occur due to the force between the tip and the sample. The defection can be measured using a laser spot reflected from the top surface of cantilever and into a photodetector; as a result, after the tip scan over a certain region, the surface topography of the sample can be built.

Results and discussions

In this part, detailed results got from this project work will be showed. Solar cells that studied during this project were mainly fabricated manually by the doctor blade technique which was not a stable fabrication method; therefore the results showed here were based on the statistic point of view. Different experiments were performed, and for different cases, more than five repetitions were done in order to get the statistic results. The active materials were mainly APFO-3 and PCBM with the donor/acceptor ratio 1:4 (weight, the concentration of APFO-3 was 3 mg ml-1), and toluene was used as solvent.

Optimization of anode

EDOT:PSS Baytron P vs. PEDOT:PSS PH500

At the beginning of this project, two kinds of PEDOT:PSS, PEDOT:PSS Baytron P and PEDOT:PSS PH500 were studied. Both PEDOT:PSS were blade coated on plastic substrates at room temperature with the same WFT setting and then measured by four-point probe and UVVIS. The PEDOT:PSS film acts as the transparent anode, which means, its transmittance and sheet resistivity would strongly affect the final device performance. Therefore, the PEDOT:PSS films with higher conductivity as well as transmittance are desired.

Table 6.1. It shows the results from resistivity measurements for PEDOT:PSS Baytron P and PH500 films that deposited with the same wet film thickness (WFT) setting (35m) and at the same coating temperature (room temperature). Four samples were deposited for each case, and four regions (Region 1 to Region 4) on each sample were tested.

Table 6.1 shows the resistivity of PEDOT:PSS Baytron P and PH500 films that deposited under the same conditions. According to H.C. Starck, there should exist large differences between those two PEDOT:PSS, but that was not what we get in our experiments. However, PH500 still has a lower resistivity compare to Baytron P, while the UVVIS measurement (Figure 6.1) shows that the higher transmittance is achieved in PH500 films. Therefore, PEDOT:PSS PH500 was chosen as the anode of our solar devices in this project.

Resistivity and transmittance of PEDOT:PSS PH500 film

Obviously, the optical transparency is a very important factor for the inverted organic solar cells since the PEDOT:PSS layer is the first to be passed by the sunlight, and therefore, the Jsc can be directly affected, the sheet resistance on the other hand is also an important parameter as it determines the efficiency of large scale devices. The sheet resistivity and transmittance of PEDOT:PSS PH500 film would both depend on the film thickness. Thicker film gives an enhanced conductance, but a poorer transmittance. In order to study the relation among film thickness, resistivity, and transmittance, different PEDOT:PSS PH500 films were deposited with different WFT settings. Results from four-point probe are shown in Table 6.2.

Table 6.2. Results of resistivity measurements for PEDOT:PSS PH500 films that deposited with the different WFT settings at same coating temperature (room temperature). Four samples were deposited for each WFT setting, and four regions (1 to 4) on each sample were tested. (Unit: ohm square-1)

It has been estimated that in order to minimize the power loss caused by the series resistance, for a 1 cm wide solar cell with this inverted structure, the sheet resistivity of the anode has to be lower than 200 ohm square-1. Therefore, by reading from Table 6.2, we found the films deposited with WFT setting between 25 m and 35 m could give the sufficient conductivity. Transmittance spectrum of PEDOT:PSS PH500 films deposited with different WFT settings are shown in Figure 6.2. Compare to the absorption spectrum of APFO-3 (Figure 5.3) which has two absorption maximum peaks for lights with wavelength around 385 nm and 550 nm, around 80 percent lights can go through the PEDOT:PSS PH500 films that is deposited with WFT of about 30 m, that indicates the PEODT:PSS PH500 can be an efficient transparent anode for solar cells have inverted structure. Therefore, in this project we used PEDOT:PSS PH500 as the transparent anode.

Another important aspect is that the light interference inside the PEDOT:PSS film would also affect the total device performance, since the sunlight contains rays with different wavelengths and intensities, and the active materials used in this project do not equally absorb the sunlight with respect to the wavelength. Therefore, we need to have constructive interference in the PEDOT:PSS film for the light with certain wavelength. This would be dependent on the thickness of PEDOT:PSS as well, and the result will be experimentally showed later.

Reproducibility of PEDOT:PH500 film

Table 6.3 shows the sheet resistivity of PEDOT:PSS PH 500 films that deposited at different coating temperatures, and the statistic results are also plotted in Figure 6.3. The depositions were performed on a hotplate where temperature could be controlled. Only small differences can be observed since all of films were deposited with the same WFT setting.

Table 6.3. The results from resistivity measurements for PEDOT:PSS PH500 films that deposited at different coating temperatures. The WFT setting was of 35 m for all cases. Four samples were deposited at different temperatures and four regions (1 to 4) on each sample were tested. (Unit: ohm square-1)

The resistivity of PEDOT:PSS films that deposited at higher coating temperature varied in a narrower region, compare to those films deposited at room temperature. That means higher reproducibility can be achieved at when coating temperature is increased. Meanwhile, the PEDOT:PSS films coated at elevated temperature gave a slightly smaller resistivity, which indicated the conductivity of PEDOT:PSS was also improved by elevated coating temperature. Similar result has been reported by Youngkyoo et al.[25].

Influences of thermal treatment

Optimization of coating temperature for PEDOT:PSS layer

In previous discussion, we discussed that by elevating the coating temperature for PEDOT:PSS, the variations of sheet resistance of formed PEDOT:PSS film could be controlled in a small region. And 60 oC coating temperature seemed to be high enough for depositing a reproducible PEDOT:PSS film. In this part, we studied the influence of coating temperature for PEDOT:PSS on device performance.

Table 6.4. It shows the representative performances of APFO-3/PCBM (1:4 weight) based BHJ solar cells which had different coating temperatures for their PEDOT:PSS layers. 80 nm thick active layers were coated at 90 oC; WFT setting for PEDOT:PSS layer was 35 m. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

Figure 6.4. JV characteristics of APFO-3/PCBM (1:4 weight) based BHJ solar cells which had different coating temperatures for their PEDOT:PSS layers is plotted by the representative results. 80 nm active layers were coated at 90 oC ; WFT setting for PEDOT:PSS layer was 35 m. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

Four different temperatures were set as the coating temperatures for PEDOT:PSS layer, and clearly change in evaporation rate of the solvent which was the water in PEDOT:PSS wet film could be observed. The devices were left on the hotplate for a while after the deposition of PEDOT:PSS films in order to make the PEDOT:PSS films stable and then moved to another hotplate and annealed at 90 oC for five minus. Different performances of solar cells made from the four cases were obtained as shown in Table 6.4. Reasons for the different device performances are unclear, but our suggestions for this observation will be given. As we found in the experiments, after the coating temperature went up to around 150 oC, wet PEDOT:PSS film dried immediately after the blade went over. Coating speed should have played a very important role for the formation of dry film in this case, manually control of coating speed started to be insufficient. As a result, the PEDOT:PSS film became much rougher compare with those deposited at lower temperatures. This may explain why an unexpected low current was got for such cells made at high PEDOT:PSS deposition temperature.

On the other hand, those cells made from 90 oC and 120 oC PEDOT:PSS deposition temperatures gave a slightly improvement of Jsc compare to those made from 60 oC coating temperature. The reason might be that the faster evaporation of water in the PEDOT:PSS left a smoother interface between the active layer and PEDOT:PSS. In another word, the longer time that water exists, the more chances would exist for water to corrode the active layer. Therefore, higher Jsc were got for the cells made from higher PEDOT:PSS deposition temperature.

Another observation was that Voc of solar cells made at higher PEDOT:PSS coating temperatures were lowered. Youngkyoo et al. have reported that annealing of PEDOT:PSS layer would lower the WF of PEDOT:PSS, which lowers the Voc of solar devices[25]. This may explain why the lower Voc and FF were got for the cells had higher PEDOT:PSS coating temperature.

Optimization of coating temperature for active layer

Figure 6.5. It summarizes the representative performances of APFO-3/PCBM (1:4 weight, the concentration of APFO-3 was 3 mg ml-1) based BHJ solar cells which had different coating temperatures for their active layers. The coating of PEDOT:PSS layers were performed at 60 oC with WFT setting of 35 m; the thickness of active layer was around 80 nm. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

Many groups have reported that for spin coated solar cells, depending on the solvent-removal speed, the morphology of active layer can be controlled[31], and by controlling the evaporation rate of solvent, the molecular ordering of polymer chains can be improved[32]. Most of the studies about the influence from evaporation rate of solvent were focused on polymers which could crystallize[33-36]. However, APFO-3 does not have this property, and the results would be different[42].

Figure 6.6. JV characteristics of APFO-3/PCBM (,1:4 weight, the concentration of APFO-3 was 3 mg ml-1) based BHJ solar cells which had different coating temperatures for their active layers is plotted by the representative results.

In order to study the influence of solvent evaporation rate on device performance, active layers were deposited at different temperatures by using a hotplate. The WFT settings for active layers and PEDOT:PSS layers were 5 m and 35 m, respectively. Coating temperature for PEDOT:PSS layer was fixed to 60 oC. Solar cells were moved to another hotplate and annealed for 5 min after the deposition of PEDOT:PSS. Figure 6.5 summarized the results from solar cells which had different coating temperatures for their active layers. Both Jsc and FF were increased significantly when the coating temperature for active layer was elevated to around 90 oC.

Figure 6.6 shows the JV characteristics of solar cells that had different coating temperatures for their active layers. The shape of JV curve changed from a rather straight line to a typical diode curve when the coating temperature for active layer was elevated from room temperature to 90 and 100 oC. However, when the coating temperature went up to 130 oC, coating then became really difficult, worse performance was obtained at such high temperature.

In Figure 6.7, AFM results obtained from APFO-3/PCBM blend blade coated from toluene solution are shown. Both films that were coated under different conditions were quite smooth, and no clear phase separation could be observed, but rougher surfaces were got at elevated coating temperature indicated the fast evaporation of solvent has affected the film morphology.

Another interesting phenomenon observed in this part was that the dry film thickness of active layer would become quite sensitive to the coating speed when coating temperature for active layer went above 80 oC. That means even with the same WFT setting for active layer, by decreasing the coating speed, thickness of dry film could be easily increased. This could be directly seen from the change of the colors. The phenomenon did not show up when the active layer was coated at room temperature. And since it would not change the dry film thickness too much, by adjusting the coating speed, dry film thickness could be controlled into the better desired region which was around 80 nm.

Post annealing

Studies that focused on influence of annealing for organic solar cells have been reported intensively[15,37]. In order to study the influence of annealing temperature for the inverted cells, several cells followed the same processes were made. After fabrication process, some cells were measured without annealing while some cells were annealed at different temperatures before measurements.

Table 6.5. It summarized the representative performances of APFO-3/PCBM (1:4 weight, the concentration of APFO-3 was 3 mg ml-1) based BHJ solar cells which were post-annealed at different temperature for five minus. 80 nm thick active layers were deposited at 90 oC with the WFT setting of 5 m, the PEDOT:PSS PH500 layers were coated at 90 oC with the WFT setting of 35 m.

Firstly, for solar cells were made without post annealing, only uncertain results could be obtained. In general, depending on how much time we exposed the cells to air before the measurement, the Jsc and Voc could be quite different. Shorter expose time would give a comparably higher Jsc (~2.3 mA cm-2), but Voc sometimes would be extremely low (0.2V~0.3V). On the contrary, longer expose time would result in a much higher Voc (~0.6 V) but lower Jsc (1 mA cm-2~2 mA cm-2). Results became more stable after the annealing temperature was elevated over 90 oC. And the performances of solar cells annealed at different temperature were given in Table 6.5.

Figure 6.8. JV characteristics of APFO-3/PCBM (1:4 weight, the concentration of APFO-3 was 3 mg ml-1) based BHJ solar cells which were post-annealed at different temperature for five minus is plotted by the representative results. 80 nm thick active layers were deposited at 90 oC with the WFT setting of 5 m, the PEDOT:PSS PH500 layers were coated at 90 oC with the WFT setting of 35 m.

Worse performance was got at 150 oC annealing temperature for the inverted cells. Influence on device performance from annealing could be due to the affected active layer or the PEDOT:PSS layer or the interface between PEDOT:PSS and active layer, which makes it difficult to find out how the device performance is affected. But we know that the WF of PEDOT:PSS would be lowered after annealing which would give rise to lower Voc and FF. On the other hand, the elevated temperature could increase the kinetic energy of molecules in materials and then led to more molecular diffusion between active layer and PEDOT:PSS layer, which might be the reason lower current was got at higher annealing temperature.

Compare to the result got from the previous part, solar cells that were made with 120 oC PEDOT:PSS coating temperature and were annealed at 90 oC for five minus gave the Voc 0.52 V and the FF 0.42; but for cells made with 90 oC PEDOT:PSS coating temperature and annealed at 120 oC for five minus gave the Voc 0.6 V and FF 0.32. It seems that coating temperature, which is corresponding to film formation, would affect Voc strongly, but annealing temperature, would firstly affect the FF. To understand this phenomenon, further studies will be needed.

Metal electrode

As mentioned in previous part, that for inverted cells a thin layer of Ti should be deposited on the Al cathode in order to protect Al from oxidation. However, since the reflectance of Ti is not good, at the same time, its conductivity is also quite poor; the thickness of Ti should be as thin as possible.

Figure 6.9. JV characteristics for APFO-3/PCBM (1:4 weight, the concentration of APFO-3 was 3 mg ml-1) based BHJ solar cells that have different thicknesses of Ti layer is plotted by the representative results. The coating temperatures for PEDOT:PSS layer and active layer were 60 oC and room temperature respectively, the WFT settings were 20 m for the active layer and 35 m for the PEDOT:PSS layer. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

In order to find out the minimum thickness for Ti to be sufficient to protect the Al, four metal electrodes with different Ti layer thickness were deposited on plastic substrates. Active layers and PEDOT:PSS layers were then deposited on the electrodes. All active layers were deposited at room temperature and the WFT setting for them was 20 m. For PEDOT:PSS films, they were coated at 60 oC and with the WFT setting of 35 m. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

The JV curves of solar devices with different Ti layers are illustrated in Figure 6.9. The flat curve for solar cells with 5 nm thick Ti layers indicates the large series resistance exists in the device compare to other cases. And this problem can be overcome by depositing thicker Ti layer.

Table 6.6. The performances APFO-3/PCBM (1:4 weight, the concentration of APFO-3 was 3 mg ml-1) based BHJ solar cells have different thicknesses of Ti layer is summarized from the representative results.

Table 6.6 gives the key parameters for these four kinds of devices. Solar cells with 10 nm and 15 nm thick Ti layer showed a similar performance. But the devices which had 60 nm thick Ti on Al electrode gave a comparably lower current. That might be due to the lower reflectance of thicker Ti layer, which caused a lower photo absorption rate in the active layer; hence lower current could be generated in the solar device. The reflectance of Ti with different thicknesses was also measured by UVVIS and is showed in Figure 6.10. Therefore, future studies in this project were based on the solar cell has 10 to 15 nm thick Ti layer.

Thickness of active layer and PEDOT:PSS layer

Computer simulation

To achieve high performance for solar devices, solar radiation need to be sufficiently absorbed in active layer and the charges must be collected. Therefore, the thicknesses of active layer and PEDOT:PSS layer are both important parameters and they are related to each other due to the interference of light. The computer simulation (See Figure 6.11) shows that at fixed PEDOT:PSS layer thickness, absorption in the active layer oscillates with the increase of active layer thickness. On the other hand, at fixed active layer thickness, the absorption also fluctuates with the variation of PEDOT:PSS layer thickness.

Optimization for active layer

In this part, thickness of active layer dependent device performance will be discussed. The coating of active layers were performed at room temperature, the PEDOT:PSS PH500 layers were coated at 60 oC with WFT setting of 35 m. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

As shown in Table 6.7, solar cells that fabricated by different WFT settings gave different device performances. Five nanometers WFT setting, which was the minimum available setting, resulted in blue active layers with the highest Jsc. Further increase of the WFT setting, decreased Jsc. Compare with the color sheet that introduced in previous part (See Figure 5.11), the dry film thickness of active layer deposited with WFT setting of 5 m was around 70 nm. This result agrees well with the computer simulation, that APFO-3 has an absorption maximum peak for 70 nm thick active layer, as shown in Figure 6.11 (a).

When the WFT setting was increased to 20 m or 35 m, the color of dried active layer could not be predicted any more, and the reproducibility for such cells is poor. The Voc of those solar devices seems to be irrelative to the active film thickness; all of them had the Voc in the region from 0.57 to 0.59 V. Future studies in this project have chosen 5 m as the WFT setting for the deposition of active layers.

Figure 6.11. Computer simulation for thickness of active layer dependent absorption spectrum, (a) indicates the absorption at APFO-3/PCBM (1:4 weight, the concentration of APFO-3 was 3 mg ml-1) based BHJ solar cells with different active layer thicknesses and fixed PEDOT:PSS layer (150 nm), (b)indicates the absorption of solar cells with different PEDOT:PSS layer thicknesses and fixed active layer (100 nm), (c) shows the absorption of solar cell varies with the change of active layer and PEDOT:PSS layer

Figure 6.12. JV characteristics for APFO-3/PCBM based BHJ solar cells that made from different WFT settings for active layers is plotted by the representative results.

Optimization for PEDOT:PSS layer

In order optimize the WFT setting for PEDOT:PSS layer, similar experiments were conducted. All other conditions for the fabrication processes were the same as before, but only the WFT settings for PEDOT:PSS layers were different. As shown in Figure 6.14, the increase of WFT settings firstly enhanced the Jsc to 2.23 mA cm-2, and corresponding PCE was 0.41%. But further increase of PEDOT:PSS WFT setting led to lower currents.

Figure 6.13. JV characteristics for APFO-3/PCBM (1:4 weight) based BHJ solar cells that made from different WFT settings for PEDOT:PSS layers is plotted by the representative results. 70 nm thick active layers were deposited at room temperature, the PEDOT:PSS PH500 layers were coated at 60 oC. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

Figure 6.14. It shows the representative performances of APFO-3/PCBM (1:4 weight, the concentration of APFO-3 was 3 mg ml-1) based BHJ solar cells made from different WFT settings for active layers. 70 nm thick active layers were deposited at room temperature with the WFT setting of 5 m, the PEDOT:PSS PH500 layers were coated at 60 oC. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

As already mentioned before, the interference of lights would be dependent on the thickness of PEDOT:PSS film. The color of final cells made from 35 to 40 m thick wet PEDOT:PSS film were purple, but it changed to bright yellow when 25 or 45 m thick WFT setting was used. It indicated that the interference of lights caused more lights with the wavelength close to the absorption maximum of APFO-3/PCBM blend were reflected away from the device surface when 25 nm or 45 nm WFT setting was used to coat the PEDOT:PSS. Therefore, for such cells lower currents ere got. The lower currents ot from the cells made from 45 m thick wet PEDOT:PSS film could also be due to the poorer transmittance of thicker PEDOT:PSS film, because more lights were absorbed in the PEDOT:PSS layer instead of reaching the active layer. Those deductions matched well with the computer simulation, as shown in Figure 6.11 (b). Two peaks appear at different PEDOT:PSS film thicknesses, and at each peak point, further increase or decrease of PEDOT:PSS film thickness would both lower the absorption in solar cells.

Additives

Viscosity

To make the organic solar cells mass producible, ink viscosity is one of the most important key factors. Polystyrene (PS) (Mw~30,000,000) and polyisobutylene (PIB) (Mw~4,200,000) were used as the additives to increase the ink viscosity in this project. The viscosities of PS in toluene and PIB in chloroform were measured by the Ubbelohde viscometer, and the results are plotted in Figure 6.15. Apparently, PS would give a much higher ink viscosity due to its high molecular weight. Therefore, PS was used as the additive for the future studies.

AFM studies

The surfaces of the APFO-3/PCBM/PS (3:12:1 weight, the concentration of APFO-3 was 3 mg ml-1) blend films deposited at room temperature and 90 oC are quite different as illustrated in Figure 6.16. The isolation of bead-like bright yellow regions with the radius about 25 m and the formation of gray networks which were sit on the film deposited at room temperature indicated the phase separation between PS and APFO-3/PCBM blend, while for the films coated at 90 oC, much smoother surface were obtained and no clear phase separation was observed, although there were some blobs randomly distributed on the film surface. The bead-like bright regions on the films deposited at room temperature and the blobs on the films deposited at 90 oC should both be the PS phase, since the AFM images of pure active films (only APFO-3 and PCBM, no PS) did not show such features.

Further examination of the film deposited at room temperature tells us the bead-like regions and gray regions are basins and ridges, respectively, and the height of the ridge is around 70 nm (See Figure 6.17). The height of blob on the film deposited at 90 oC is also about 70 nm which confirmed that both ridge and blob are separated PS phase.

Figure 6.16. It shows the AFM images of APFO-3/PCBM/PS (3:12:1 weight, the concentration of APFO-3 was 3 mg ml-1) blend films blade coated at (a) room temperature, (b) 90 oC. For the film deposited at room temperature, strong phase separation can be observed. The bright yellow region indicates the APFO-3/PCBM phase while dark gray network indicates the PS phase. For the film deposited at elevated temperature, smoother surface were obtained with some blobs randomly sit on the film surface.

Figure. 6.17. It shows the AFM topography scans of APFO-3/PCBM/PS (3:12:1 weight, the concentration of APFO-3 was 3 mg ml-1) blend films blade coated at (a) room temperature, (The image focused on the boundary between basin and ridge. The bright region is the top of the ridge while darker region is the basin. The height of the ridge is about 70 nm.) (b) 90 oC. (The bright spot is the blob sit on the film surface, the height of the blob is about 70 nm, and the radius of the blob is about 7 m.)

The pictures that focused on the blob and smooth regions on the film deposited at 90 oC as well as the ridge and basin on the film deposited at room temperature are illustrated in Figure 6.18. And all of them appear quite smooth.

Figure 6.18. It shows the AFM topography scans of APFO-3/PCBM/PS (3:12:1 weight, the concentration of APFO-3 was 3 mg ml-1) blend films blade coated at different temperatures. The scans of blob region and smooth region on the film deposited at 90 oC are shown in (a) and (b), respectively. Picture (c) and (d) were taken from the film deposited at room temperature, and (c) shows the top of a ridge, (d) shows the bottom of a basin.

The reason for the PS phase separated from APFO-3/PCBM could be due the upper critical solution temperature (UCST) type phase behavior existed in the blend. 90 oC coating temperature might be exceed the phase transition temperature of APFO-3/PCBM/PS blend, therefore, the phase separated morphology disappeared and instead we got homogeneous films. Youngmin Lee et al. has studied the P3AT/PS blend and reported that the UCST type phase behavior was also existed[39], but the transition temperature for such blend was much higher, which was about 200 oC. On the other hand, the fast solvent-removal speed could also be the reason that no phase separation occurred at elevated coating temperature. The deposited wet film dried immediately at 90 oC, which left no time for the PS molecules to assemble and to form the network, as a result homogeneous film could be obtained.

Characteristics

JV curves for APFO-3/PCBM/PS based BHJ solar cells made from different inks that had different PS concentrations and deposited at different temperatures for their active layers were given in Figure 6.16 (a) and (b), respectively. And the key parameters for those cells were given in Table 6.8.

Table 6.8. It shows the representative performances of APFO-3/PCBM/PS (the concentration of APFO-3 and PCBM were 3 mg ml-1 and 12 mg ml-1, respectively) based BHJ solar cells with different PS concentrations, 80 nm thick active layers were deposited at 50 oC and 90 oC with the WFT setting of 5 m, the PEDOT:PSS PH500 layers were coated at 90 oC with theWFT setting of 35 m for both cases. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

For the solar cells deposited at 90 oC for their active layers, the ink that had 0.3 mg ml-1 PS concentration resulted in better performed solar cells compare to those without additive. The reason should be that the increased ink viscosity gives rise to more uniform films. However, further increase of additive concentration reduced the device performance. Figure 6.19 (a) showed that for the JV curves got from 0.5 mg ml-1 and 1 mg ml-1 PS had instable noises at the third quadrant and the decrease of FF indicated the existence of PS started to affect the transport property of active layer. As a result, the Jsc decreased with the increase of PS concentration.

However, for those cells which had their active layer deposited at 50 oC, even with 1 mg ml-1 PS concentration, the JV curves for such solar cells still did not show any instable feature, which indicates that 50 oC was below the UCST, and PS phase was separated from the APFO-3/PCBM blend phase in the active layer of such cells. Since the PS did not exist in the APFO-3/PCBM domains, it would not affect the transport property of the active layer (which is contrary to the case we discussed above that at 90 oC coating temperature of active layer, most PS would not separate from the APFO/PCBM blend). In this case, more PS could be added to the active solution to increase the ink viscosity, and the best device was obtained from the solution which has 0.5 mg ml-1 PS.

Figure 6.19. JV characteristics for APFO-3/PCBM/PS (the concentration of APFO-3 and PCBM were 3 mg ml-1 and 12 mg ml-1, respectively) based BHJ solar cells that made with different PS concentrations, 80 nm thick active layers were deposited at 50 oC and 90 oC with the WFT setting of 5 m, the PEDOT:PSS PH500 layers were coated at 90 oC with theWFT setting of 35 m for both cases. Solar cells were moved to a hotplate and annealed for 5 min after the deposition of PEDOT:PSS.

Summary

In this section, we investigated the influence of PS on BHJ based solar cell performance. By adding small amount of PS into the active ink, uniform films could be deposited without detriment to the device performance by doctor blading. However, increased concentration of PS decreased the Isc of solar devices which gave rise to a lower PCE. The concentration of PS that can be added into the active ink without impairing the device performance seems to be dependent on the deposition temperature. At elevated temperature (90 oC), the best worked cells had 0.3 mg ml-1 PS in the active ink, but more PS caused a significant drop in Isc, while for the devices had their active layer deposited at 50 oC, adding 0.5 mg ml-1 of PS gave the best result. Morphological studies showed a clear phase separation for the APFO-3/PCBM/PS blend active layer deposited at 90 oC, but for the active layer deposited at room temperature, the phase separated morphology disappeared.

Spin coating vs. doctor blading

Spin coated cells with about 0.045 cm2 active area were made and measured without adding additive. Compare to blade coated cells, spin coated cells had much smoother films; hence, higher Jsc could be obtained (Table 6.9). However, for spin coated cells, deposition of all layers had to be performed at room temperature; therefore, poorer FF was got.

Although, lower current was got from blade coated cell, the PCE of blade coated cell exceeded the spin coated cells. And by adding 0.3 mg ml-1 PS into active ink, 3 mA cm-2 Jsc could be got, which was just about 5 percent lower than the Jsc got from spin coated cell.

Table 6.9. Performance of solar cells made by spin coating technique and docter blade coating technique is given by the representative results. For spin coated cells, the about 80 nm thick active layer was deposited by spinner with 1000 rpm spin speed, PEDOT:PSS was spin coated with spin speed 800 rpm, both depositions were performed at room temperature. For blade coated cells, 80 nm thick active layer was deposited at 90 oC, the PEDOT:PSS PH500 layers were coated at 90 oC with the WFT setting of 35 m.

Conclusion

In this project, BHJ based organic solar cells with inverted layer sequences were investigated. Active materials used in this project were the APFO-3 and PCBM blend and conductive polymer PEDOT:PSS was chosen to be the transparent anode. The main method used to fabricate the solar devices was doctor blading which was a R2R compatible method.

Initially, different PEDOT:PSS films with respect to the film thickness and the coating temperature were deposited and characterized by four-point probe system and UVVIS measurement. Sufficient conductance and transmittance were obtained in the films deposited with wet film thickness setting of 35 m. And the study of metal cathode showed that the best worked cathode contained a 70 nm thick Al layer covered by a thin protecting Ti layer (10 nm-15 nm).

Then, optimized coating temperature and wet film thickness setting for both active layer and PEDOT:PSS layer were experimentally determined. The most appropriate WFT settings for active layer and PEDOT:PSS layer were found to be 5 m and 35 m, respectively. And with fixed active layer and PEDOT:PSS layer, FF could be increased to 0.44 by increasing the coating temperature for active layer to 90 oC. The highest efficiency of the APFO-3/PCBM BHJ based solar devices fabricated by doctor blading was 0.69%, which exceeded the efficiency of spin-coated cells.

For blade coated cells, higher PCE (0.8 %) was achieved by introducing small amount of PS (Mw~30,000,000) into the active solution, but more PS would lower the device performance. The morphological changes after adding the PS were observed by using AFM, and coating temperature dependent phase separation of APFO-3/PCBM/PS blend was also found.

Future work

Compare to the best APFO-3 based solar cells reported in literature the efficiency is significantly lower, as the open-circuit voltage is much lower for the inverted solar cells.

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