Thesis Synthesis and Structural Optimization of Functional Photosensitizers for Dye-sensitized Solar Cells Graduate School of Yeungnam University Department of Chemical Engineering Major in Chemical Engineering LE THI THUY Advisor: Professor Jae Hong Kim February 2018 MLS. Thesis Synthesis and Structural Optimization of Functional Photosensitizers for Dye-sensitized Solar Cells Advisor: Professor Jae Hong Kim Presented as M. Thesis February 2018 Graduate School of Yeungnam University Department of Chemical Engineering Major in Chemical Engineering LE THI THUY Le Thi Thuy’s M. Thesis is approved Committee member Ahn, Kwang-Soon signature Committee member Kim, Jae Hong signature Committee member Han, Yoon Soo signature February 2018 Graduate School of Yeungnam University ACKNOWLEDGEMENT Firstly, I would like to express my deepest gratitude to my advisor, Professor Jae Hong Kim for his invaluable advice, motivation and never-ending encouragement throughout my studies.
His guidance always challenged me knowledgeably and provided a perfect atmosphere that I needed to grow as a researcher. I strongly believe that without the help from Prof. Kim I could not have enough will to live in Korea, not even mentioning about completing my thesis today. Besides my supervisor, I would like to appreciate other members of M.
thesis defense committee, Prof. Kwang-Soon Ahn and Prof. Yoon Soo Han for their contribution of time and understanding comments to improve the quality of this M. Next, I am extremely thankful to Dr.
Suresh for his backing and support me in all time of research. And I also would like to say thank you to members of LOFM for their assistance, support and friendship during the time I stayed in Korea. Without them, I would face a lot of troubles in my experiments. Last but not least, I would like to thank my family for their unconditional love, reassurance and encouragement through all my life especially during the time I live overseas.
February 2018 Le Thi Thuy ABSTRACT Since Gratzel and O'Regan reported high solar-cell performances for dye- sensitized solar cells (DSSCs) based on polypyridyl ruthenium (II) complex dyes adsorbed on a nanocrystalline n-type semiconductor TiO? electrode in 1991, DSSCs have expected extensive consideration as a new generation of maintainable photovoltaic devices because of their high incident-solar-light-to-electricity conversion efficiency, colorful and attractive natures, and low cost of production. A characteristic DSSCs is created with a dye-absorbed wide band gap oxide semiconductor electrode, such as TiO2, ZnO, or NiO; a liquid electrolyte containing I/Iz redox couples; and a platinum-coated counter electrode. In the working electrode of the device, the dye sensitizer is very important factor; its function is light harvesting and electronic transition. Up to now, the sensitizer can be divided into two general classes: the metal complex sensitizers and the metal-free organic sensitizers.
The DSSCs using the metal-complex sensitizer such as N3, N179 or black dye have been achieved high efficiency over 13%. However, the metal-complex sensitizers have some difficulties such as limited resource, low molar extinction coefficient (€) and high cost, which will limit their applications in DSSCs of large-scale. To get rid of these problems, the focus has been shifted to metal-free organic sensitizers owing to compared with metal complexes, metal-free organic dyes have also attracted significant attention caused by the benefits of easier preparation and purification, higher structural flexibility, environmental kindliness and prevention of noble metals. This thesis began with the series of organic sensitizers triphenylamine-based metal-free organic dyes (D1-D3) with different electron acceptors, such as 2- cyanoacetic acid, rhodanine-3-acetic acid or 5-oxo-1-phenyl-2-pyrazolin-3- carboxylic acid, connected through anthracene and thiophene a-spacers were synthesized and applied for DSSCs.
The photophysical and electrochemical properties of these dyes were investigated and their performance as sensitizers in DSSCs was measured. Electrochemical studies showed that the LUMO energy levels can be tuned by changing the anchoring groups with different electron withdrawing ability. The power conversion efficiencies of the DSSCs based on D1-D3 decrease as the electron withdrawing ability of their anchoring groups increase in the order of D1 < D2 < D3 and D1-based device showed the higher power conversion efficiency of 1. In the study on the phenothiazine as an electron donor, three novel dyes with single donor-acceptor (T1), double donor-acceptor (T2) and multiple anchoring group (T3) organic dyes have been synthesized to investigate the influence of the hexyloxy benzene unit between the two chromophores and the different number of anchoring groups on the performance of DSSCs.
The T3 with double branches phenothiazine bases device shows a broader and higher IPCE as well as photo- current density (Jsc) with an improved photovoltage (Voc). In contrast, T1 with single branch presents a reasonably low IPCE within the whole spectral region, along with Jsc and Voc of 10.51mA/cm? and 0. The higher Jsc and Voc gained with the device based on T2 dyes and show a highest conversion efficiency of 5. TABLE OF CONTENTS Chapter 1: INTRODUCTIONN.
Introduction of dye-sensitized solar cells (DSSCS). Operating principle of dye-sensitized solar ceÌÌS. Fabrication of DSSCS. Scope of work and researCch Objj€CfÏVG.<-5c<scsscsseeSsSSeeeSeeseseeeesessse 8 Chapter 2: CHARACTERIZATION OF DYE-SENSITIZED SOLAR CELLS.
Key component of dye-sensitized solaar' C€ÌÌS. Transparent Conducting ØÌ24SS .s- << s=< «se se sư se 10 2. TiOz as the photoelectrode 2. Họ HH HH HH.
Key efficiency parameters of dye-sensitized solar cells. Incident photon to current conversion efficiency (IPCE). Current-voltage characteristics (J/V curves) 2. Electrochemical impedance spectroscopy (EIS) of DSSCs.
19 Chapter 3: EFFECT OF ANCHORING GROUP IN ANTHRACENE/THIOPHENE-BRIDGED TRIPHENYLAMINE BASED ORGANIC DYES FOR DYE-SENSYITIZED. Materials and instruments 3. Assembly and Characterization of the DSSCs. Results and discussion 3.
Design and sy'I(Ï€SÏS. Electrochemcal DTOID€TFẨÏ€S. Photovol{aÏC€ DFOID€TEÏ€S.oc- 5< << s3 1 010 90030580030484000038400050404000058010040008 39 Chapter 4: SYNTHESIS AND PHOTOVOLTAIC PERFORMANCE OF NOVEL PHENOTHIAZINE SENSITIZERS CONTAINING HEXYLOXY BENZENE UNIT AND MUTI -ACCEPTOR FOR DYE-SENSITIZED SOLAR 4. Assembly and Characterization of the DSSCs.
Results and đÏSC(ISSỈOTI. Absorption properties in solution and on TiO2 films. Electrochemcal DTOID€TFẨÏ€S. Photovoltaic performances of the DSSCs .o 25-55 HH Hư ưng nu mg 65 TREFER.
5c << HH TH HH 0000040008040010038000504400000000046 67 LIST OF FIGURES Figure 1. Best research-cell efẨÏCÏ€IICÏ€S .-- 5-5 5< 5< 5s sex s9 5e se 3 Figure 1. Schematic representation of operational principles of DSSCs. Fabrication process Of DSSCS.
Typical configuration of a DSSCS .----<ccscseĂeseeseseeeesesesesse 9 Figure 2. Molecular design of a D-2—A organic dye sensitizer for DSSCs. A typical ICPE spectrum of a DSSCS .-<-c55=<seses<<eesese 16 Figure 2. A typical J/V curve Of a DSSCS.
Electrochemical impedance spectroscopy of DSSCs. Chemical structures of dye sensifizers D1-D3. Scheme of preparation route for dye sensifizers D1-D3. UV-vis absorption spectra of dye sensitizers D1-D3 in solution.
UV-vis absorption spectra of dye sensitizers D1-D3 on TiO2 film. Cyclic voltammogram of dye sensitizers D1-D3. IPCE spectra for the DSSCs based on dye sensitizers D1-D3. J-V curves for the DSSCs based on dye sensitizers D1-D3.
EIS spectra of the dye sensitizers D1-ID. Chemical structures of dye sensifizers T'1- T3. Scheme of preparation route for dye sensitizers T1, T2, and T3. UV-vis absorption spectra of dye sensitizers T1-T3 in solution.
UV-vis absorption spectra of dye sensitizers T1-T3 on TiO: film. Cyclic voltammogram of dye sensitizers T'1- T3. J-V curves for the DSSCs based on dye sensitizers T1-T3. IPCE spectra for the DSSCs based on dye sensitizers T1-T3.
Electrochemical impedance spectra measured under illuminated CONGILION. Electrochemical impedance spectra measured in the dark for DSSCs S€InSÏfiZ€d Dạy “T Í- TÉ. Voc decay curves of DSSCs with TI- T3 based organic DOẨOS€TNSÏÏZ/©TS. The electron life-time derived as a function Of Voc.
63 LIST OF TABLES Table 3. Photophysical and electrochemical data of dye sensitizers D1-D3. Photovoltaic data of the DSSCs based on dye sensitizers D1-D3. Photophysical and electrochemical data of dye sensitizers T1-T3.
Photovoltaic data of the DSSCs based on dye sensitizers T1-T3. EIS analysis of the DSSCs under illumination condition. EIS analysis of the DSSCs under dark condifion. Background As the world is converting more progressive in economy and technology, more energy is being consumed to keep up with the growth and demand on energy boomed over past decades.
Currently, the energy demands are still highly dependent on fossil fuels, natural gases and coal with percentages of 36. Nevertheless, the world will shortly come to an end of fossil fuels due to its non-renewable. Meanwhile, the extravagant use of fossil fuels actually causes irreparable environmental destruction, geopolitical pressures, and disastrously weather changes [2]. The Sun is a winner among all energy sources, and the Earth obtains 174 petawatts (PW) of incoming solar radiation at the upper atmosphere in a year.
The total solar energy absorbed by the Earth’s surface is approximately 3850 zettajoules (ZJ) per year, which is more energy in one hour than what the world used in one year. The amount of solar energy reaching the surface of the planet is so enormous that in one year it is about twice as much as what will ever be obtained from all of the Earth’s non-renewable resources of coal, oil, natural gas, and mined uranium combined. A solar cell, or photovoltaic cell (PV), is a device that converts sunlight directly into electricity by taking advantage of the photoelectric effect. Among all the renewable energy technologies, photovoltaic technology is considered as the most promising one.
Solar PV has been turned into a multi-billion, fast- growing industry, and the most potential of any renewable technologies. The abundant, clean, safe, and affordable photovoltaic technology has been considered to be the most promising one among all the novel energy technologies [3]. Solar cell technologies are traditionally divided into three generations. First generation solar cells are mainly based on silicon wafers and typically demonstrate a performance about 15-20 %.
These types of solar cells dominate the market and are mainly those seen on rooftops. The benefits of this solar cell technology lie in their good performance, as well as their high stability. However, they are rigid and require a lot of energy in production. The second generation solar cells are based on amorphous silicon, CIGS and CdTe, where the typical performance is 10-15%.
Since the second generation solar cells avoid use of silicon wafers and have a lower material consumption it has been possible to reduce production costs of these types of solar cells compared to the first generation. The second generation solar cells can also be produced so they are flexible to some degree. However, as the production of second generation solar cells still include vacuum processes and high temperature treatments, there is still a large energy consumption associated with the production of these solar cells. Further, the second generation solar cells are based on scarce elements and this is a limiting factor in the price.
Third generation solar cells use organic materials such as small molecules or polymers. Thus, polymer solar cells are a sub category of organic solar cells. The third generation also covers expensive high performance experimental multi-junction solar cells which hold the world record in solar cell performance. This type has only to some extent a commercial application because of the very high production price.
A new class of thin film solar cells currently under investigation is perovskite solar cells and show huge potential with record efficiencies beyond 20% on very small area. Polymer solar cells or plastic solar cells, on the other hand, offer several advantages such as a simple, quick and inexpensive large-scale production and use of materials that are readily available and potentially inexpensive. Polymer solar cells can be fabricated with well-known industrial roll-to-roll technologies that can be compared to the printing of newspapers. Although the performance and stability of third generation solar cells is still limited compared to first and second generation solar cells, they have great potential and are already commercialized.
Research interest in polymer solar cells has increased significantly in recent years and it is now possible to produce them at a price that enables projects such as the free OPV initiative [4].