Figure 8.1 Schematic energy level of tandem cells on granular NiPI...……...162
Chapter 1 Introduction
Chapter 1 Introduction
1.1 Background
Supply of energy is one of the main concerns of our society. Because of growing economy and modern lifestyle, consumption of energy rises drastically. The world’s fossil energy resources are still ample for the next coming years –yet, the extraction costs for this kind of energy is still under debate. The current oil prize instability reveals the vulnerability of our economy towards higher energy prizes, not mentioning the political and economical unrest predominant in several main oil producing countries. Because our world highly depends on those supplies, there is a risk of slipping into an energy crisis someday soon. Therefore, over the past several decades, there are a lot of countries to invest large amount of capitals and time in developing the renewable sources of energy.
Among the developments of power producing, the direct conversion of solar energy to electricity by photovoltaic (PV) is emerging as a leading contender for next-generation green power production. Conversion into electrical power of even a small fraction of the solar radiation incident on the Earth’s surface has the potential to satisfy the world’s energy demands without generating CO2 emissions. The progress in the efficiency of research-scale PVs over the past several decades is shown in Figure 1.1. [1]
Chapter 1 Introduction
Figure 1.1 Progress of research-scale photovoltaic device efficiencies, under AM1.5 simulated solar illumination for a variety of technologies (as compiled by Larry Kazmerski, National Renewable Energy Laboratory).
Current PV technology is not yet fulfilling this promise, largely due to the high cost of the electricity produced. Although the challenges of storage and distribution should not be underestimated, a major bottleneck lies in the PV devices themselves. Improving efficiency is part of the solution, but diminishing returns in that area mean that reducing the manufacturing cost is absolutely vital, while still retaining good efficiencies and device lifetimes. Although the PV technology platforms of silicon-based PV and thin-film PV are now undergoing a rapid expansion in production and the power conversion efficiency can easily approach 15% [1], their high costs of fabrication and of raw materials have yielded limited commercial applications. Recently, the next generation PV—organic solar cells (OSCs)—could soon be playing a major role with the advantages of low-cost, large-area, lightweight and shatterproof. The challenge here is that absorbing light in an organic material produces a coulombic bound exciton that requires dissociation at a
Chapter 1 Introduction
incident light, but excitons only diffuse a few nanometres before decaying. The problem is therefore intrinsically at the nano-scale: the composite devices with a large area of internal donor–acceptor interface are needed, but where each carrier has a pathway to the respective electrode. Following this is the separation of charges which is mostly induced by the field generated from the difference in work functions of the electrodes. The following scheme shows the important processes in OSCs (Figure 1.2). At each one of these steps, recombination of the electron and hole can occur, preventing their contribution to the current. In addition to the fundamental restrictions of the device, such as how much light can be absorbed, these recombination losses limit the overall maximum efficiency of energy conversion that can be attained. So that the issues on how to overcome the problems form each step in OSCs are widely investigated.
Figure 1.2 Schematic flow chart showing the important processes in organic solar cells.
Recombination of excitons can be radiative or non-radiative.
Flexible electronics are increasingly being used in a number of applications which benefit from their low production cost, light weight, favorable optical and electrical properties. However, despite these advantages, the range of practical, high-volume,
Chapter 1 Introduction
applications for flexible electronics will remain limited in the future unless a number of challenges related to electrical performance and stability on flexible substrates are compared to the standard devices. Although researchers generally investigate the OSCs on rigid ITO/glass substrates, the final developing objective of OSCs is fabricated on flexible substrates to exhibit the advantages of OSCs. In the developing field of production technologies of flexible electronics, the highest productivity at lowest process costs is gained from roll-to-roll processing. The manufacture of the flexible electronics can be integrated several printing techniques (such as inkjet, screen, flexographic and gravure printings) into the roll-to-roll process to enhance the throughput of products [2-5]. Based on the previous considerations, in this thesis, I will develop several techniques of solution processes to produce the roll-to-roll suitable circuits and electrodes and apply those methods in the flexible OSCs.
Chapter 1 Introduction
1.2 Aims and Objectives
It will be demonstrated in this thesis that several investigations of key technologies of circuits and electrodes for applying in flexible electronics, especially in OSCs. In the present work, I will develop an all-solution method for fabricating thin Ni films on PI (NiPI) with high adhesion and conductivity and it will be suitable for roll-to-roll manufacturing processes in flexible printed circuits. Furthermore, I will devise a simple method to enhance the conductivity of PEDOT:PSS films through spin-coating with various surface-modified compounds, and then apply the PEDOT:PSS films as anodes to the preparation of ITO-free polymer solar cells (PSCs). The ultimate goal is to prepare all-solution-processed inverted polymer solar cells (PSCs) incorporating two solution-processed electrodes—NiPI films as cathodes and high-conductivity PEDOT:PSS films as anodes—on flexible substrates.
Another developing field of electrodes will be studied to control the morphology of active layers in the small molecular solar cells, and therefore I will investigate the rod-like CuPc structures on indium–tin oxide (ITO), PEDOT:PSS and Au substrates to demonstrate an ideal bulk heterojunction structure using the self-assembly of CuPc as the donor material and fullerene as the acceptor.
The objectives of this thesis are as follows:
To study and review relevant literatures on surface-metalized PI films.
To develop a simpler way to fabricate a metal thin film (as the conducting and diffusion barrier layer) on PI films, and excellent adhesion between the metal and PI phases will be obtained.
To study and review relevant literatures on the solution-processed transparent conducting layers.
Chapter 1 Introduction
To investigate the origin effect of conductivity enhancement of PEDOT:PSS films from the comprehensive analysis results.
To demonstrate the ITO-free PSCs with the high-conductivity PEDOT:PSS films and understand the limits for larger area cells.
To study and review relevant literatures on inverted PSCs and the solution-processed techniques of PSCs.
To investigate the interface problems of fabricating inverted PSCs on NiPI films.
To inspect the optical properties occurring in NiPI-based PSCs and recommend how to improve the performance in the next generation of PSCs.
To study and review relevant literatures on small molecular solar cells.
To investigate the morphological control of CuPc and its application in organic solar cells.
To calculated the surface energy difference of PEDOT and PSS phases, and recommend how to improve the performance in the next generation of small molecular solar cells.
Chapter 1 Introduction
1.3 Brief Structures of this Thesis
This thesis comprises eight chapters. Contents of each chapter are detailed as follows:
Chapter 1 provides the background of this study, stating the reason why the study is conducted. In addition, the aims and objectives of this thesis are outlined in this chapter.
Chapter 2 is literature review of research in the field of circuits, electrodes and organic solar cells. Key area focuses on the fabrication of all-solution-processed electrodes for application of flexible printed circuits and organic solar cells (such as polymer solar cells and small molecular solar cells).
Chapter 3 is a short overview of the materials used and the experimental details that we developed for reproducible device preparation will be presented.
Chapter 4 represents the theory and formation of surface-nickelized polyimide films using a wet chemical process. This investigation will synthesize nickel nanoparticles as seeds (catalysts) and a Ni metal layer as an adhesion-promoting layer on the surface of a polyimide film, and excellent adhesion between the nickel and polyimide phases was observed. It can reduce the cost of the catalyst and simplify the process of coating the adhesion-promoting layer on the PI film.
Chapter 5 describes the goal and experiment methodology to develop a simple method for modifying the electrical conductivity of PEDOT:PSS films intended for use as electrodes in ITO-free solar cells. The high-conductivity PEDOT:PSS films could be obtained simply through spin-coating of a solvent onto pre-coated PEDOT:PSS films. We employed the comprehensive analysis results to determine the origin of the conductivity enhancement.
Chapter 1 Introduction
We found that the performance of the PPVs was related to the surface morphologies, chemical structures, and electrical conductivities of the PEDOT:PSS films. We performed a comprehensive investigation of the effects of alcoholic and ethereal solvents, including methoxyethanol, ethanol, and 1,2-dimethoxyethane, to determine the driving force for the conductivity enhancement of the PEDOT:PSS films.
Chapter 6 reports the fabrication of all-solution-processed inverted PSCs featuring granular NiPI as the cathode material (back contact electrode) on flexible substrates;
combining with the techniques in chapter 4 and chapter 5, these devices have the following configuration: PI/Ni (cathode)/ titanium(diisopropoxide)bis (2,4-pentanedionate (TIPD)/P3HT:PCBM/PEDOT:PSS (anode). Furthermore, when compared to the planar structure, the improvement of absorbance of light and good haze factors was obtained for granular structure which suggests NiPI as a better back contact electrode through enhancing the light trapping and scattering in inverted PSCs.
Chapter 7 reports the organic photovoltaic (OPV) cells possessing an ideal bulk heterojunction (BHJ) structure using the self-assembly of copper phthalocyanine (CuPc) as the donor material and fullerene (C60) as the acceptor. The variable self-assembly behavior of CuPc on a diverse range of substrates (surface energies) allowed us to control the morphology of the interface and the degree of carrier transportation within the active layer.
Chapter 8 draws conclusions from the results and discussions. In addition, some prospects are made for future research.
Chapter 2 Literature Review
Chapter 2
Literature Review
2.1 Polymer Substrates
Polymer has attracted a great deal of concern in the past few years because polymers can be applied to the manufacturing of various electronic and display devices. There have been extensive research activities on flexible electronics based on polymer materials.
Flexible printed circuit board (FPCB), packaging, and flexible organic solar cells (OSCs) based on all polymeric materials or partial employment of polymeric materials have been developed due to low cost and ease of fabrication. It is expected that micro-electro-mechanical systems (MEMS) and semiconductor devices as well as flexible displays can be fabricated on flexible substrate for many applications. It is because that flexible electronics offer substantial coupled rewards in terms of being able to develop electronics that are thinner, lighter, robust and can be rolled away when not required. In addition, plastic-based substrates coupled with the recent developments in solution deposition and inkjet printing for laying down electronic materials open up the possibility of cost-effective processing in high volumes using roll-to-roll processing. For instance, polymer solar cells can be created using roll-to-roll manufacturing process. The roll-to-roll process is the process in which transparent electrode, printed active material, primary electrode and substrate are printed onto transparent packaging to make a solar panel. This manufacturing process is inexpensive, environmental friendly, and simple. The schematic diagram of roll-to-roll process of OSC modules is shown in Figure 2.1.
Flexible electronics can be built on metal foil, very thin glass coated with a polymer
Chapter 2 Literature Review
and a variety of plastics. Therefore, to replace glass, a plastic substrate needs to be able to offer the properties of glass, i.e. clarity, dimensional stability, thermal stability, barrier, solvent resistance, low coefficient of thermal expansion (CTE) coupled with a smooth surface. This section will focus only on plastic films that have been given serious consideration as flexible substrates for flexible electronics. Based on different process temperatures and product demands, the suitable polymer substrate can be selected in further applications. The main candidates are shown in Table 2.1 [6].
Figure 2.1 Schematic diagram of roll-to-roll process of OSC modules (from Konarka Technologies, Inc.).
Table 2.1 Basic properties of polymers used for base substrate. (Note: PET: Polyethylene terephthalate; PEN: Polyethylene naphthalate; PC: Polycarbonate; PES: Polyethersulphone;
PAR: Polyarylate; PCO: Polycyclic olefin; PI: Polyimide.)
Chapter 2 Literature Review
2.2 Flexible Printed Circuit Boards
Recently, polyimide (PI) films have become generally used components for flexible electronic devices because they exhibit high glass transition temperatures, low surface roughness, low coefficients of thermal expansion (CTE), and high chemical resistance under typical fabrication conditions. Therefore, metallization of polyimide for metal lines has therefore been the subject of intense study for FPCB, [7-11] with various trials having been conducted in incorporating metal wiring layers onto dielectric polyimide, with the aim of developing high performance microchips. The base materials of FPCB and other soft electronics are copper on polyimide with or without adhesive. The former is called the three-layer mode (3L-FPCB: metal/adhesive/PI), while the latter is the two-layer mode (2L-FPCB: metal/PI). The development of FPCB with smaller line widths and increased wire density is based on the two-layer (Figure 2.2). For example, one common approach is photolithography utilizing a photoresist combined with metal coating on a polyimide substrate in an additive (area-selective deposition of metals by physical and chemical means) and/or subtractive (etching of preformed metal films) manner [7-11]. Another approach is electroless deposition combined with simultaneous or alternating laser irradiation [12, 13]. However, this process sometimes has undesired defects in the pattern due to laser-induced decomposition of the polyimide substrate. The development of a facile, direct metallization process with which metallic patterns could be directly formed onto polyimide substrates is challenging. In addition, device fabrication processes require high reliability in terms of the adhesive strength between the thin metal films and the underlying substrate. To achieve sufficient adhesion between the metal film and polyimide, most conventional metallization processes employ etching of the polyimide surface (typically several micrometers in surface roughness), leading to an increase in contact area to provide good adhesion through mechanical interlocking (anchoring). [7-11] However,
Chapter 2 Literature Review
as the dimensions of pattern details drop to the several micrometer and submicrometer scales, micrometer scale anchoring is not suitable for reliable adhesion. Thus, a new adhesion method is required for realizing future generations of electronic devices.
Figure 2.2 Development of flexible printed circuit boards.
Recently, Kensuke and Hidemi et al. reported a novel surface-modification based method for the metallization of polyimide surfaces. [14-18] The method relies on a simple alkali treatment of the bare polyimide films to introduce the active components of an ion exchange reaction (carboxylic acid groups) [19, 20] and subsequent loading of metal ions into the modified layers by ion exchange reactions. Fabrication of the metallic thin films or patterns was previously achieved through chemical reduction using NaBH4 aqueous solution [14] and ultraviolet (UV)-light-induced photochemical reduction of the adsorbed metal ions using preadsorbed TiO2 nanocrystals as a photocatalyst on the polyimide surface. [16] Although this process allows the polyimide surface to be metallized directly, the formation of metallic thin films is only achieved through the use of a photoresist or
Chapter 2 Literature Review
polyimide is difficult to control. It is suggested that the diffusion of metal ions during the reduction process plays a key role in determining the metal/polyimide interfacial structures.
The following schematic diagram showed that mechanism for direct metallization of a polyimide film surface using an ion-doped precursor layer (Figure 2.3). [18] Furthermore, when using this surface metalized Cu/PI films to apply in FPCB, the copper film side and the polyimide substrate side of the samples obtained can achieve an average adhesive strength of 1.00 kg fcm–1 (Figure 2.4).
Chapter 2 Literature Review
Figure 2.3 Schematic diagram of the mechanism for direct metallization of a polyimide film surface using an ion-doped precursor layer. (A) Reduction of copper ions via self-oxidative decomposition of DMAB in aqueous solution. (B) Formation of copper atoms followed by protonation of the remaining carboxylate anion groups. (C) Ion-exchange reaction between protons and copper ions through the generation of a concentration gradient of these ions in the precursor layer, and further reduction of copper ions at the film surface.
Figure 2.4 Effect of DMAB reduction temperature on the peel strength of the resulting
Chapter 2 Literature Review
2.3 Electrode Materials
In the electronics, the work function (WF) of the electrode materials is very important since it determines together with the LUMO/HOMO and Fermi-level of the semiconductor whether the electrode forms an ohmic or a blocking contact for the respective charge carrier (holes in valance band, electrons in conducting band). In Table 2.2, the periodic table of the elements is listed with its values of WF [21]. The values in Table 2.2 are valid only for poly-crystalline materials. However, many numbers for single crystals which depend on its crystallographic orientation have also been reported [21]. The general electrode materials can be categorized as metal, inorganic semiconductor, conducting polymer and carbon nanotube (CNT) materials. Common metal electrode materials for the electron collecting contact (low WF required) of organic solar cells are Al, Ca, Ag whereas for the hole collecting contact high WF materials like Au are preferred. If the metal materials are used to apply in flexible printed circuits (FPC), the conductivity and adhesion properties of circuits are the major issues. The metals for FPC are usually selected Cu, Ag, Au (high conductivity) as the electron conduction layers and Ni, Cr, Ti as the adhesion-promoting layers [22, 23]. While indium tin oxide (ITO) materials are usually used as transparent conducting layer, which is a degenerated semiconductor comprising a mixture of In2O3 (90%) and SnO2 (10%) with a band gap of 3.7eV and a Fermi-level between 4.5 and 4.9eV is widely used. The large band gap allows no absorption of wavelengths longer than about 350nm. Unfortunately, the high cost of high-quality ITO and its lack of flexibility can limit the applications of electronics incorporating it as an anode material; the limited supply of indium and the transparency of ITO toward visible light are additional problems. Recently the ITO-free conducting materials, such as PEDOT:PSS and CNT films, are widely investigated to replace ITO in the future. An additional advantage of PEDOT:PSS is that it can be manufactured through
Chapter 2 Literature Review
the solution processes. CNT network films appear to be a suitable alternative: they can be prepared through solution processing, and they exhibited high conductivity and flexibility [24, 25]. Figure 2.5 shows the transparency and conductivity of single-walled carbon nanotube network films [24]. Furthermore, the stability of devices fabricated on the SWNT/PET films is much greater than devices on ITO/PET during simple bending tests.
The PSCs with SWNT network films as electrode could be folded over (inducing compressive or tensile strain) down to radii of curvature of ~5 mm with no degradation in power efficiency and radii of ~1 mm with a 20-25 % loss in efficiency. Such CNT-based devices do, however, have their problems. For example, the nature of the transparent
The PSCs with SWNT network films as electrode could be folded over (inducing compressive or tensile strain) down to radii of curvature of ~5 mm with no degradation in power efficiency and radii of ~1 mm with a 20-25 % loss in efficiency. Such CNT-based devices do, however, have their problems. For example, the nature of the transparent