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History and development of photovoltaics
A.E. Becquerel is probably the first researcher who reported the photovoltaic effect in 1839 [7]. This effect allows the transformation of solar energy directly into electricity. Then Charles Fritts created the first working solar cell in 1883 with a power conversion efficiency of about 1%. In 1941, Russell Ohl developed the first silicon p/n junction and since then the research on solar cells has been developed very rapidly. Up to 1959, the company Hoffman Electronics has achieved a commercial solar cell showing 10% power conversion efficiency (PCE). Up to date, the best solar cell fabricated from research laboratory can reach over 40% in PCE by various techniques and materials (record 44.4%, Figure 1.1).
According to the time of their first appearance, solar cells can be mainly classified into three generations. The first-generation solar cells are based on silicon wafers mainly containing single crystalline or polycrystalline silicon. These types of solar cells dominate the market owning to their excellent and stable industrial performance. However, without government subsidy, the fabrication cost of crystalline silicon solar cells is relatively high. Their performance can be summarized as follows:
Single crystalline silicon solar cells: Today’s best-performing crystalline silicon solar cells from laboratories achieve about 25% in PCE (Figure 1.1). Lower PCE of 15% ~ 18% can be achieved in cells fabricated by industry [8]. The market share of single crystalline solar cells to total worldwide PV production in 2012 was 40.3% [9].
Polycrystalline silicon solar cells: The polycrystalline silicon solar cells have a laboratory efficiency record of 20.4% (Figure 1.1), while the industrial average efficiency is about 13% to 16% [8]. The market share of polycrystalline solar cells in 2012 was 45.0% [9]. This relatively higher market share is due to their cheaper production process comparing to single crystalline solar cells despite of their lower efficiency. The second-generation solar cells are usually called thin-film solar cells. These cells are made from layers of semiconductors of only a-few-micrometer-thick and exhibit a relatively lower efficiency than those of first-generation solar cells. However, the thin film solar cells are much cheaper to produce due to the reduction of material and less expensive manufacturing processes. This generation of solar cells mainly contains three types: amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) solar cells.
a-Si solar cells typically have a thickness of a few microns which is much thinner than that of crystalline silicon solar cells (thickness ~ 180 microns).The efficiency of a-Si solar cells made from laboratories can be as high as 13.4% (Figure 1.1) while 10% of PCE can be achieved for commercial a-Si solar cells. In 2012, their market share was 4.5% [9].
CdTe solar cells have an ideal bandgap (1.44 eV) and thus their absorption show a good match with the solar spectrum. CdTe solar cells have a laboratory efficiency of 20.4% (Figure 1.1) and an industrial efficiency of 11% with a market share of 6.3% in 2012 [9]. Their modest efficiency together with the use of scarce and toxic elements (i.e. tellurium and cadmium) limit their large-scale commercialization.
CIGS solar cells have a relatively high efficiency (laboratory efficiency 20.8%, Figure 1.1) but their mass production turns out to be relatively difficult at competitive prices. This is due to the vacuum processes and the high temperature treatments required in the manufacture as well as the scarcity of indium. Their industrial efficiency is 14% and they took 3.5% market share in 2012 [9].
Development of organic solar cells
Among the third-generation solar cells, there has been intensive interest on the development of organic solar cells. Compared to inorganic solar cells, they can enable low-cost manufacture such as roll-to-roll printing and large-scale applications. The organic solar cells are typically made from conducting and semiconducting organic materials including polymers and small molecules. The first highly conductive organic polymer, doped polyethylene, was discovered and developed by Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid in 1977 for which they won the Nobel Prize in Chemistry in 2000 [11]. In 1986 C. Tang successfully made the first working heterojunction organic solar cell from copper phthalocyanine (CuPc) and a perylene tetracarboxylic derivative [12]. The device showed an efficiency of about 1% with bi-layer architecture. Then Sariciftci [13] reported the first polymer/buckminsterfullerene (C60) heterojunction device in 1993. In 1994, Yu [14] made the first polymer bulk heterojunction solar cell with poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and C60. In such a cell an acceptor (C60) and a donor (MEH-PPV) component were mixed and processed simultaneously to act as the active film. After this work, the bulk heterojunction configuration became an important device configuration in this research. Organic solar cells developed very rapidly during the last decade (Figure 1.2). Up to date, a laboratory efficiency record of 11.1% has been achieved by Mitsubishi Chem. (Figure 1.1 & Figure 1.2). However, compared to other technologies, the relatively low efficiency of organic solar cells, together with their short lifetime, limit their large-scale commercialization.
Basic knowledge of organic solar cells
To better understand the operational mechanism of organic solar cells, the basic knowledge of organic chemistry and organic solar cells (OSCs) is introduced in this section.
Organic semiconductors and their applications in solar cells
Conjugated semiconducting polymers are long-chain carbon-based large molecules. Some of them show good electrical and optical properties and have many applications in electronic and optoelectronic devices. In 1958, Giulio Natta first synthesized the linear polyethylene, but the conductivity of this crystalline polyethylene was very low. Until 1977, the conductivity of polyethylene was improved remarkably by doping halogen elements [15]. It has been reported that the conductivity can be enhanced to be close to 560 Ω-1∙cm-1 when the polyethylene film is doped with halogen or arsenic pentafluoride ions [16]. Later in 1982, Weinberger et al. [17] investigated polyethylene as an active material in organic solar cells. The initial device performance was poor. Then different polythiophenes [18], poly(p-phenylene vinylene) (PPV) [19] and PPV derivatives (e.g., MEH-PPV (Figure 1. 3) [14]) were introduced into organic PV field. The highly regioregular poly(3-hexylthiophene) (P3HT) (Figure 1. 3) came to become the material of choice starting from 1990s. Since then extensive studies have been carried out on P3HT-based solar cells. Recently, low bandgap polymers, such as Poly[2,6-(4,4 ) (PTB7, Eg = ~ 1.8 eV) (Figure 1. 3), are intensively investigated as active materials in organic solar cells due to their ability to harvest more photons from the solar spectrum.
The electronic structure of conjugated polymers
Conjugated polymers have long chains consisting of carbon atoms. These carbon atoms form alternating single (σ) and double bonds (π). Following the law of linear combination of atomic orbitals and the hybrid orbital theory, due to this conjugated structure, the electrons in double bonds can delocalize over several backbone carbon atoms and form a delocalized π electron system which gives the polymer semiconducting properties.
Figure 1. 4 illustrates the structure, electron cloud distribution and bonding π orbital/anti-bonding π* orbital of ethylene. This is the simplest conjugated molecule. The carbon atom has an electronic configuration of 1s22s22p2. When forming bonds with other atoms, the 1s2 electrons do not participate in bonding since these electrons are strongly bounded to the nucleus. For the 2s2 and 2p2 orbitals, in the case of ethylene, each carbon atom makes covalent σ bonds with two hydrogen atoms, a single and double bond with the other carbon atom. To achieve this goal, the 2s and the in-plane 2px and 2py orbitals overlap with the adjacent carbon atom to form three degenerate sp2 hybridized orbitals (termed sp2 orbitals). These hybridized orbitals form three in-plane σ bonds with the other atoms. These hybridized orbitals form three in-plane σ bonds with the other atoms. These single bonds are strong and rigid. The electrons are localized over the two bonded atoms and they are associated with a large bonding energy.
Table of contents :
List of Tables
General introduction
Chapter I Introduction and Background Knowledge
1.1. Introduction
1.2. State-of-art of photovoltaics
1.2.1. History and development of photovoltaics
1.2.2. Development of organic solar cells
1.3. Basic knowledge of organic solar cells
1.3.1. Organic semiconductors and their applications in solar cells
1.3.2. The electronic structure of conjugated polymers
1.3.3. Excitons and polarons in organic semiconductors
1.3.4. Photo-conversion process
1.3.4.1. Absorption of photons and generation of excitons
1.3.4.2. Diffusion of excitons
1.3.4.3. Dissociation of excitons
1.3.4.4. Transport of charge carriers
1.3.4.5. Extraction of charges
1.3.5. Architectures of organic solar cells
1.3.5.1. General architecture of organic solar cells
1.3.5.2. Single layer cells
1.3.5.3. Planar heterojunction cells
1.3.5.4. Bulk heterojunction cells
1.3.5.5. Tandem solar cells
1.4. Organic bulk heterojunction solar cells using polymers and small molecules
1.4.1. Hole extraction layer: PEDOT:PSS
1.4.2. Bulk heterojunction
1.4.2.1. Donor material: P3HT
1.4.2.2. Acceptor material: PCBM
1.4.2.3. P3HT:PCBM film
1.4.3. Photovoltaic Characteristics of organic solar cells
1.4.3.1. Current-voltage response and efficiency
1.4.3.2. Performance-limiting factors
1.5. Plasmonic organic solar cells
1.5.1. Plasmons
1.5.1.1. Bulk plasmons
1.5.1.2. Surface plasmons on planar metal-dielectric interfaces
1.5.1.3. Localized surface plasmons in metallic nanoparticles
1.5.2. Organic solar cells utilizing localized surface plasmons
1.5.2.1. Mechanisms of light absorption enhanced by localized surface plasmons
1.5.2.2. Plasmonic organic solar cells
1.6. Conclusions
Chapter II Experimental methods and techniques
2.1. Materials
2.2. Film and photovoltaic device preparation
2.2.1. General preparation technique for thin films: Spin-coating
2.2.2. Preparation of films and photovoltaic device samples
2.2.2.1. General preparation procedure for films on glass substrate or ITO-coated substrate and for photovoltaic devices
2.2.2.2. Preparation of sample PEDOT:PSS films and OSCs for optimization of PEDOT:PSS layer by post-deposition thermal annealing
2.2.2.3. Preparation of sample PEDOT:PSS films and OSCs for optimization of PEDOT:PSS layer by adding glycerol
2.2.2.4. Sample preparation for optimization of photoactive layer
2.2.2.5. Sample preparation for plasmonic OSCs using Ag NPSMs in PEDOT:PSS layer
2.2.2.6. Sample preparation for plasmonic OSCs using Ag NPSMs and glycerol in PEDOT:PSS layer
2.3. Characterization Methods
2.3.1. Characterization technique for Ag nanoparticles and solutions
2.3.1.1. UV-visible absorption for solutions
2.3.1.2. X-ray Diffraction
2.3.1.3. Transmission electron microscopy
2.3.2. Characterization techniques for films
2.3.2.1. Thickness determination
2.3.2.2. Integrating sphere photometer for films
2.3.2.3. Goniophotometry
2.3.2.4. Atomic Force Microscopy and conductive Atomic Force Microscopy
2.3.2.5. Four point probe measurement
2.3.3. Characterization techniques for solar cells
2.3.3.1. Current-Voltage Characterization
2.3.3.2. External quantum efficiency
2.4. Conclusion
Chapter III Structural, optical and electrical properties of PEDOT:PSS thin films doped with silver nanoprisms
3.1. Introduction
3.2. Ag NPSM synthesis and characterizations
3.2.1. Ag NPSMs synthesis
3.2.2. Characterizations of Ag NPSMs
3.3. Hybrid PEDOT:PSS-Ag NPSM solutions and films
3.3.1. Preparation of hybrid PEDOT:PSS-Ag NPSM solutions and films
3.3.2. Characterizations of hybrid PEDOT:PSS films
3.3.2.1. Absorptance
3.3.2.2. Bidirectional Reflectance Distribution Function (BRDF)
3.3.2.3. Surface profile and electrical conductivity
3.4. Conclusions
Chapter IV Plasmonic organic solar cells using silver nanoprisms
4.1. Optimization of regular P3HT:PCBM solar cells
4.1.1. Introduction
4.1.2. Optimization of PEDOT:PSS layer
4.1.2.1. Thermal annealing of PEDOT:PSS films
4.1.2.2. Glycerol modified PEDOT:PSS
4.1.3. Optimization of photoactive layer
4.1.3.1. Thermal annealing
4.1.3.2. Solvent annealing and additive for photoactive layer
4.2. Studies of plasmonic solar cells composing silver nanoprisms in PEDOT:PSS.
4.2.1. Introduction
4.2.2. plasmonic solar cells composing silver nanoprisms in PEDOT:PSS .
4.2.2.1. Introduction
4.2.2.2. Characterization and discussion
4.2.3. Plasmonic OSCs composing Ag NPSMs and glycerol in PEDOT:PSS layer
4.2.3.1. Introduction
4.2.3.2. Characterization and discussion
4.3. Conclusions
Appendix I Ag Nanospheres Synthesis
Appendix II Ag nanoprisms synthesis by one-step thermal reduction
Appendix III Photovoltaic characteristics of PTB7:PC70BM organic solar cell
Appendix IV Phase transfer of Ag nanoprisms from aqueous solution into organic solvents
Appendix V Plasmonic organic solar cells using Ag nanoprisms in active layer .
References
