Materials and Experimental Techniques - Report Example

Chapter 3 Materials and Experimental Techniques 3 Introduction This chapter describes the various experimental methods and techniques used during this work. The nature of the materials and the required properties of the device determined the choice of process used in the work. It is divided into two parts. The first part describes the materials and techniques used to fabricate organic solar cells, all the steps of which were conducted in a class 1000 clean room to minimize atmospheric contamination. The second part outlines the equipment used for studying the electrical properties and morphology of organic solar cell devices. Although the research in this thesis involves the fabrication of different types of devices with different materials, i.e. organic bilayer heterojunction solar cells (OHJ), organic bulk hetrojunction solar cells and organic tandem solar cells, most of the experimental techniques were similar. In this chapter, Section 3.2 describes all the materials used in this research. The main techniques of film deposition that have been used are explained in Section 3.3. The description of the general experimental details for the fabrication of devices can be found in Section 3.4. However, the detailed experimental techniques for fabrication of any device will be described in the main section about that device. The atomic force microscopy (AFM) used to describe the morphology a surface of films is found in Section 3.5. The UV-visible absorption spectra of the various materials used in the construction of the organic solar cells were obtained using a Hitachi Model U-2000 Double Beam Ultra-Violet/Visible (UV/VIS) spectrophotometer found in section 3.6. Finally, the design of Vacuum system setup and the electrical characteristics measurements setups are summarised in Section 3.7. 3.2 Materials Poly(3-hexylthiophene-2,5-diyl) (P3HT) was purchased from Sigma-Aldrich. Titanium oxide (TiOx) and Poly(amidoamine) (PAMAM) dendritic wedges was synthesized by Alaa El-Betany postdoctoral research in School of Chemistry Bangor University. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) solutions (CLEVIOS AL 4083) and pH500 were purchased from Heraeus. IC70BA was purchased from Solaris Chem Inc. 3.2.1 Poly(3-hexylthiophene-2,5-diyl) (P3HT): Regio-regular poly (3-hexylthiophene) (P3HT) is used as an electron donor/hole transporter and light absorber in this study. Regioregular implies that each 3- hexylthiophene unit in the chain in the polymer is oriented in such a way that the C6H13 residue group is either head to tail or head to head (Joshi 2008). Figure 1: Poly(3-hexylthiophene-2,5-diyl) chemical structure (Sigma-Aldrich 2014) It is because of this property that the polymer has the qualities of better ordering and self-organisation during deposition, meaning that the device mobility is increased substantially (Cantatore 2000). Chirvasea et al. (2003) studied the optical and electrical properties of the polymer and estimated its HOMO level at 4.7–5.1 eV and a HOMO–LUMO gap of 2.14 eV from an absorption spectrum. P3HT has favourable processing properties in solution because good films can easily be manufactured through casting, spin coating or printing at low temperatures (Li, Zhu and Yang 2012). P3HT can be used as a hole transporting polymer for improving PCE or blend OPV cells (Li, Zhu and Yang 2012). A blend of OPV cell was fabricated by Gang Li et al. (2005) using PCBM and P3HT soluble C60 derivative. Crystallinity of P3HT resulted in improvements in PCE. After PCBM and PCBM blend solutions were casted on PEDOT:PSS/ITO, evaporation rates were varied to control the morphology of the blend film. An interdigitated and well mixed blend film was formed by PCBM and P3HT during solvent annealing, as P3HT formed a crystalline fibril-like morphology embedded with PCBM aggregates. A schematic diagram of PCBM and P3HT is shown in figure 2. Figure 2: Schematic diagram of PCBM and P3HT phase separated blend (Yang et al. 2005) The hole mobility of the PCBM and P3HT conjugated polymer is increased because of the blended layers’ phase separated morphology. The absorption efficiency of the conjugated polymer is also improved because of the p3HT aggregates that form fibril-like crystalline morphology (Li, Zhu and Yang 2012). The P3HT conjugated polymer has LUMO levels of 2.7 eV and a band gap of band gap of 2.1 eV (Chirvasea et al. 2003). The polymer serves as an electron donor during photoexcitation and has a high home mobility, approximately 10-3 cm2 V-1 s-1 in thin poorly organized films and approximately approximately 2 × 10-1 cm2 V-1 s-1 in well crystallized films (Kalonga et al. 2013). The polymer has a 550 nm wavelength optical absorption peak and a broad absorption spectrum of 400 to 650 nm (Kalonga et al. 2013). 3.2.2 Poly(amidoamine) (PAMAM) dendritic wedges: Dendrimers and dendritic polymers find great applications because of their unique structures and because their biological and physical properties can be precisely controlled (Pearson et al. 2012). This is why they are used to develop multifunctional nano-scale devices. These are regularly hyperbranched, nanometer-sized (2-10 nm diameter), flexible, monodispersed macromolecules that have many peripheral functional groups. Their high degree of branching facilitates facile multifunctionalisation because it is possible to conjugate multiple biological and chemical moieties at their surfaces (Pearson et al. 2012). Dendrimers have two functional domains in addition to the peripheral domains that enable their tuning in order to alter their properties such as surface charge, molecular size and weight, and functionality. They have garnered a great deal of interest in the last few decades because of their applicability as molecular scaffolds, which can be used in the development of catalytic nanoreactors, liquid crystals, light harvesting systems and drug delivery systems (Georgiev, Bojinov and Nikolov 2009). Light harvesting assemblies made of dendrimers have also been greatly explored because of their unique properties and structures. Their globular shape offers a large surface area which can be modified using chromophores that results in an efficient photon capture and large absorption cross-section (Georgiev, Bojinov and Nikolov 2009). The structure of dendrimers is characterized by layers between generations (G) or focal points. PAMAM (polyamidoamines) are a popular class of commercially used dendrimers and the use of their flexible aliphatic bone as a scaffold in developing light harvesting antennae could help the development of new high efficiency energy transfer systems (Alamry et al. 2015). Fluorophores like 1,8-naphthalimide derivatives are some of the PAMAM light harvesting antennae that are particularly useful. These derivates have good photostability and strong fluorescence because of which they are used in various applications such as anticancer agents, laser active media, coloration of polymers, analgesics in medicine, fluorescent markers in biology, potential photosensitive biological units, fluorescence switchers, electroluminescent materials and liquid crystal displays (Alamry et al. 2015). Figure 3: PAMAM Dendrimer generation 2 (Dendritech n.d.) PAMAM generation 2 poly(amido amine) has been reportedly used as an electron-collection interlayer material placed between the ITO and active layer of inverted organic solar cells (OSCs). Under AM1.5G illumination, these OSCs were found to have a power conversion efficiency of 3.53%, a figure greater than that of control inverted PSCs have blank ITO (Murugesan, Kuan and Jianyong 2013). It is also comparable with that of control PSCs having normal architecture. PAMAM dendrimers were originally developed in 1979 by Donald Tomalia and have primary amine groups and tertiary amine groups at each brand end and at each branching point, respectively (Murugesan, Kuan and Jianyong 2013). They also possess an amido amine branching structure and an ethylene diamine core. Those with amine-terminated branches are commercially available as cationic “full” generations (G1, G2, etc.) and those with carboxylic acid terminated branches are available as anionic “half” generations (G1.5, G2.5, etc.). Tomalia’s first dendrimer was a result of the reaction of an ammonia core with three methylacrylate molecules followed by three ethylenediamine molecules addition that resulted in the formation of G0 amidoamine (Tomalia 1995). This two step was continued to produce successive amidoamine generations through which the terminal amine groups were doubled each time. PAMAM dendrimers have been employed in chemical sensor, light harvesting, gene transfer, cross link agent, polymer based drug delivery and imaging contrast agent technology (Saboktakin, Maharramov and Ramazanov 2008). These are the most common dendrimer class. Lower generation PAMAM dendrimers have an elliptical or planar shape while the higher generation ones are spherical because of the densely packed branches. They can be prepared through two routes, one is the convergent approach and the other is the divergent approach. The convergent approach involves synthesis at the periphery and proceeds inwards while the divergent approach is vice versa. The structure and properties of Poly(amidoamine) (PAMAM) dendrimers have been of particular interest to scientists because the physical properties of their solutes and their adaptability to studies of droplet evaporation are unique. Their molecular structure is highly controllable and they can be easily synthesised in large amounts (Li 2008). 3.2.3 Indene-C70 bisadduct (IC70BA): Buckminsterfullerenes are aromatic, stable, spherical shaped clusters discovered in 1985, consisting of 60 carbon atoms (C60). Because of their electroactive properties, they have gained significant attention. C60 is readily available and acts an electron acceptor that has high electron mobility and accepts up to six electrons when in solution. It has limited solubility and therefore, its deposition is primarily achieved through vacuum deposition. It is widely applied in bilayer heterojunction solar cells that have high efficiencies of up to with a copper phtalocyanine donor (Kroto et al. 1985). In 1995, Hummelen and coworkers had reported Phenyl-C61-butyric acid methyl ester (PC61BM), a methano-fullerene derivative as a soluble C60 version. Efficient charge separation in a BHJ resulting because of ultrafast photoinduced electron transfer to PCBM from a p-type polymer was also demonstrated that year. These two discoveries propelled organic solar cell development further. PCBM is the most widely used acceptor in OPV. Its spherical shape offers it advantage over other planar semiconductors because 3D electron transport is possible in it. However, its drawback is its weak absorption of visible light. In order to address this, a C70 analogue of it, called the PC71BM was developed to have a higher photocurrent in OPV devices as it had a stronger absorption in the solar spectrum’s blue region. Currently, solar cells are highly optimized and those incorporating the C60 and C70 analogues reach more than 8% PCE in conjunction with suitable low-band gap polymers. Materials with lower LUMO have been implemented for bringing about further improvements. Solubilising groups are also being attached to the fullerene core in order to enable the tuning of their LUMO and HOMO levels and also to allow the OED’s solution processing. Various mono, bis and tris adducts have also been developed to take advantage of the changes in LUMO levels and also to achieve higher VOC. For instance, Indene-C60 bisadduct (IC60BA) that has a LUMO level at 3.74 eV (0.17 eV up-shifted than that of PCBM) shows superior photovoltaic performance at 0.84 V VOC and 6.48% PCE when used P3HT-based PSCs as acceptor (Guerrero et al. 2012). Indene-C70 bisadduct (IC70BA) has also been synthesized at a higher LUMO level of 3.72 eV (0.19 eV higher than that of PCBM). P3HT/IC70BA PSCs with methyl-thiophene additives have been shown to have higher PCEs of up to 6.69% with a 0.86 V Voc upon using pre-thermal annealing for 10 minutes at 1500C (Guerrero et al. 2012). Figure 4: Chemical structures of fullerene derivatives used in OPV devices (Miller et al. 2012) 3.2.4 Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS): PEDOT:PSS is used in organic electronics because of its hole conducting properties. It was earlier used as active layer in transistors, buffer or electrode material between dielectric material and the gate electrode (Cruz-Cruz et al. 2014). It has a number of advantages such as good thermal stability, high transparency, and mechanical flexibility. Because of these properties, it has been used in OSCs as an anode buffer layer. It is one of the best hole-conducting buffers as it has a high ionization potential that is almost equal to ITO work function, while its electron affinity is 2.2 eV, which is adequately low for blocking electrons (Han et al. 2011). It is a single component polymer with two ionomers. The first component (PEDOT) is a polythiophene polymer carrying positive charge while the other component (PSS) is a sodium polystyrene sulfonate polymer carrying negative charge. The mixture PEDOT:PSS is used for improving the ITO-anode contact (Choy 2012). Figure 5: PEDOT, PSS and PEDOT:PSS structures (Choy 2012) 3.2.5 Titaniom oxide (TiOx): TiOx in its amorphous form is an equally good electron transfer layer as crystalline TiO2 ­after optimization. It is also as good as zinc oxide as an electron transfer layer because it is characterized with high transparency in the visible range and high electron mobility (Lattante 2014). Studies have shown that it is possible to create a TiOx electron transfer layer from a solution at room temperature without the need for an annealing process after deposition (Hadipour et al. 2013). It has been demonstrated that such a layer was equally good to calcium in extracting electrons from a heterojunction device, leading to the confirmation that TiOx has the right energy level for being an efficient electron transfer layer in combination with active blends. It has also been shown that the TiOx properties are the same when deposition occurs in a glove box or in air, an important requirement for their application in roll-to-roll manufacturing (Hadipour et al. 2013). TiOx has been used in a variety of electronic devices such as in solar cells as an ETL, in thin film transistors (TFT) as channel material and as energy harvesting device possessing piezoelectric properties. Electron mobility of TiOx films is found to be 1.7 x 10-4 cm2 V-1 s-1, with 4.4 eV LUMO level close to Al work function, making it an efficient in electron transport (Hadipour et al. 2013). Its large band gap blocks excitons and holes efficiently. A device enhancement was reported by Hayakawa et al. for a TiOx layer over the active layer but with a small rise in JSC (cited in Chen et al. 2010). Because of its superior hole blocking properties, increase in rectification ratio and shunt resistance (Rsh) have contributed to improvements in FF as well as VOC, while the layer also acts as a barrier against chemical degradation and physical damage (Chen et al. 2010). The TiOx layer also acts as an optical spacer that optimizes efficiency by redistributing the intensity of light. It has also been demonstrated that TiOx layer improves air stability by almost two orders of magnitude (Heeger and Nguyen 2009). It acts as an electron transport layer and as a hole blocking layer because the top of the TiOx valence band is electronegative, 8.1 eV below vacuum (Heeger and Nguyen 2009). The layer acts as a charge selective collection layer and also breaks the symmetry, because of which it results in open circuit voltage. Figure 6: TiOx structure 3.3 Experimental techniques: 3.3.1 Spin coating: Spin coating technique has been used for deposition of thin films for several decades (Sahu, Parija and Panigrahi 2009). This technique can be used for all kinds of solutions (generally polymers) on flat substrates, e.g. silicon wafers or glass slides. Usually the process involves applying a small amount of solution into the centre of the substrate, which is held by a vacuum chuck during the coating process. Then the substrate starts to rotate at a fixed speed, typically about several thousand rpm. The solution is applied continuously and spreads outwards as the substrate is rotating. Spin coating can produce organic thin films with thicknesses ranging from nanometers to micrometers. Figure 7: Stages of the spin coating process (Sahu, Parija and Panigrahi 2009). The theoretical model for the spin coating process is given by the following equation: = ρ ω2 r Where, d is the thickness of the spun film, η is the viscosity coefficient of the solution and ρ is its density with the angular velocity of the spinning ω at the spinning time t. It is useful to say that the final film thickness depends on a variety of factors, such as the drying rate (temperature per minute), viscosity, surface tension, concentration of solids, etc. Furthermore, the choice of the speed plays a major role in determining the film thickness (as shown in figure 8). Figure 8: Variation of film thickness as a function of speed of spinning (Panigrahi et al. 2004). The quality of coating is proportionate to the substrate being free of contamination. In a clean room environment, the substrate is much less susceptible to get contaminated and therefore a higher quality coating can be expected. More particles in the air and surroundings would mean that the substrate would have a higher chance of contamination and therefore a low level of adhesiveness for coating. 3.3.2 Thermal evaporation: The physical vapour depositions of metal (Al cathode) were conducted by thermal evaporation using a Kurt. J. Lesker mini-spectros system. The electrical energy in this technique was used to heat a filament and that led to heating a deposition material to evaporation point. This vapour material condensed in the form of a thin film on the cold substrate surface, while a shadow mask was used to create a pattern of the thin film. Very high levels of vacuum, about 10-4 Torr, were required in this process to allow for a long mean free path to reduce film impurities (Harsha 2005). However, low pressures were usually used, about 1 x 10-7 Torr, to prevent reaction between the vaporized materials and the atmosphere. The depositing of two organic sources at the same time was possible in the Kurt. J. Lesker spectres with two individual sensors. Therefore, it was easy to monitor the evaporation rates of these two sources. Furthermore, in this technique, the film thickness was monitored by quartz crystal sensor (QCS) and the processes of film deposition were controlled by the Sigma software. 3.4 Experimental details 3.4.1 Substrate preparation: ITO glass substrates were cut into small pieces with dimensions of 2.5cm x 2cm. Each of these substrate slides had four devices on shadow mask. The substrates were cleaned with soap, warm water and deionised water, then substrates were placed on a cleanroom wipe inside the fume hood. Then, a few drops of acetone were dispensed from the acetone bottle onto the substrates to remove the photo resist. The substrates were wiped gently with a cleanroom wipe to remove the solvent and any remaining photo resists and were placed on a clean tissue. They were arranged in the substrates holder, then the holder was placed in acetone beaker and the acetone was poured into the beaker to a level above the substrates. The beaker was covered with a piece of foil then it was placed in the ultrasonicate for 10 minutes. After that, the beaker was removed from the ultrasonicator and the substrates were removed from the holder with a clean tweezers and blow-dry the substrates with the compressed nitrogen guns. Previous steps were repeated but this time using isopropanol instead of acetone. Followed with UV-ozone treatment for 20 minutes. 3.4.2 Film deposition 3.4.2.1 Spin coating of the hole transfer layer: The EMS spin coater model 4000 was used in this process. Conducting poly (3, 4-ethylenedioxylenethiophene)-polystylene sulfonic acid (PEDOT:PSS,Baytron P) was spin-cast (5000 rpm) for 40 second with thickness ~40 nm from aqueous solution (after passing a 0.45 μm filter). The substrate was subjected to drying for 10 minutes at 140°C in air, and then for spin-casting of the photoactive layer, it was moved into a glove box. 3.4.2.2 Spin coating of active layer: In the bilayer Heterojunction solar cells the donor (P3HT) was spin coated first then the acceptor. A solution of the Poly (3-hexylthiophene) (P3HT) was prepared by dissolving 0.03g of P3HT in 2 ml of chloroform (Sigma Aldrich) in a 20ml glass vial yielding a concentration of 1% w/w. The vial was placed inside an ultrasonic bath at a temperature of 50oC for 30 minutes to increase the solubility of P3HT in the chloroform. Finally, the solution was syringed through a 0.2μm PTFE filter into a second clean vial to remove any un-dissolved particles. Then, the P3HT was spin-cast at 800 rpm for 60 second with thickness ~100 nm on top of the PEDOT layer for the second charge separation layer and then PAMAM dendrimers spin-cast at 500 rpm for 30 second on top of the P3HT with the thickness ~70 nm. 3.4.2.3 Spin coating of electron transfer layer: TiOx precursor solution in methanol was spin-cast at 5000 rpm for 20 second with thickness ~20 nm in air on top of the active layer. After 10 minutes in air at 80°C, the Precursor is converted to TiOx by hydrolysis. 3.4.2.4 Evaporation of top electrodes: Aluminium electrodes were deposited on the top of electron transfer layer through a shadow mask. The deposition thickness of the aluminium electrodes was 100 nm, with evaporation rate of 0.1 nm s-1 using the Kurt. J. Lesker deposition system. 3.4.3 Thin film characterization Atomic Force Microscopy (AFM) Atomic force microscopy (AFM) is a high-resolution scanning probe microscopic technique that helps researchers study surface morphology of thin films. The AFM technique was developed by Binnig, Quate and Gerber in 1985 so they could study materials and take high resolution images of sample surfaces. The technique is employed in analysing van der Waals forces, magnetic forces, repulsive forces and lateral friction forces (Giessibl 2005). In this study, digital instrument nanoscope AFM was used to analyse the surface properties of active layer thin films. Images in the AFM technique are obtained by using a sharp probe to scan the sample surface while analysing the interactions of the sample with the tip. The scanning process involves the placement of a sharp microscope cantilever tip close to the sample’s surface (Giessibl 2005). Distance between the tip and the surface is adjusted in order to keep the cantilever deflection constant and small deflections will cause it to bend upwards (Steinhauser 2014). Measurement of the bending is carried out using a laser spot reflected to sensor following which an image of the surface can be obtained at atomic level resolution if ideal conditions are maintained and the sensor collects adequate information. Depending on the interaction between tip and sample surface, AFM is of two types, one is the contact mode and the other is the non-contact mode. In the contact mode, the tip touches the sample surface while in the non-contact mode, the tip only stays near the surface without contact. Vibrations are induced on the tip and these are monitored using a laser (Giessibl 2005). 3.4.4 Electrical characterisation 3.4.4.1 Vacuum system setup A vacuum system was used in organic solar cells electric characterisation. Low Vacuum system (10 -2 torr) contained a steel chamber connected to a rotary pump. Inside the chamber the substrate holder mounted in copper stage opposite to the light window where the substrate holder dimensions will be 2cm x 2cm where the light source is a tungsten halogen lamp (50W) focused through a window onto the substrate (anode). Although a slight difference in the spectrum exists between the tungsten lamp and the AM 1.5 solar simulator, a tungsten lamp has been used by many researchers in the solar cell field and provides a reasonable light for comparison. The intensity of the light falling on the device could be varied from ~1 to 200 mW/cm2 by moving the light source toward or away from the test sample. The light intensity was measured using a light intensity meter. A light intensity equivalent to AM1.5 radiation from the halogen lamp was set using an AM1.5 calibrated Si photodiode (Thorlabs SM1PD2A). 3.4.4.2 DC measurements The J-V measurements were performed under dark and light by applying the voltage to the Al electrode using computer driven LCR meter HP 4284A running a custom-made analysis program by MATLAB. The software measures J-V curves where FF, Voc, Jsc, and PCE are calculated using applicable equations. 3.4.4.3 Ultra-violet/visible Spectroscopy The UV-visible absorption spectra of the various materials used in the construction of the solar cells were obtained using a Hitachi Model U-2000 Double Beam Ultra-Violet/Visible (UV/VIS) spectrophotometer. The primary light beam was emitted from a deuterium discharge lamp. This beam passed through a half-silvered mirror beam splitter with one beam passing through the sample while the other acted as a reference. The wavelength range was ~190 nm to 1100 nm with a wavelength resolution of 1nm. UV/Vis spectroscopy is a widely used technique for the quantitative determination of solutions of highly conjugated organic compounds, biological macromolecules and transition metal ions (McMahon 2008). Organic compounds, particularly those that are highly conjugated, absorb light in the visible or UV regions of the electromagnetic spectrum. Solvents used for this technique are usually water or ethanol. However, not all solvents are suitable for UV spectrophotometry. The pH and polarity of solvent can also influence the absorption spectrum of the compound under study. For instance, the molar extinction coefficient and absorption maxima of Tyrosine increase with an increase in pH from 6 to 13 or with a decrease in the polarity of the solvent (West et al. 2013). 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