ABSTRACT Since Gratzel and O'Regan reported high solar-cell performances for sensitized solar cells DSSCs based on polypyridyl ruthenium II complex dyes adsorbed on a nanocrystalline n-t
INTRODUCTION
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 36.4%, 23.5% and 27.8%, respectively [1] 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
2 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
3 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]
Dye-sensitized solar cells (DSSCs) is a major of third generation solar cells, it is a photovoltaic devices in which a dye is used as the light absorber and a
Figure 1.1 Best research-cell efficiencies
4 semiconductor electrode (essentially titanium) allows for the charge separation and the transport of the electron to the external circuit This technology offers the following advantages: low production and investment cost, flexible design opportunities and feedstock availability to large scale application
The history of the dye-sensitized concept started in the late 1960s by sensitization of an organic dye, perylene, with an n-type zinc oxide semiconductor, which showed very poor power con-version Since then much effort has been put in to improve the power conversion efficiency Major revolution came in 1991 when Gratzel and O’Regan reported a DSSC device with an efficiency of 7.1% [5]
Thenceforward, DSSCs are being considered to be a prospective alternative to expensive conventional inorganic solar cells Unlike silicon solar cells, electrons and holes in a DSSC are transported in two different phases, TiO2 and electrolyte respectively, and because of which the chances of recombination in the cell become low Besides, DSSCs do not require ultrahigh pure materials unlike inorganic solar cells.
Introduction of dye-sensitized solar cells (DSSCs)
A DSSCs consists of a photoanode, which is made up of a wide band gap semiconductor (TiO2, SnO2, ZnO, etc.) with a monolayer of dye molecule adsorbed on it, an electrolyte (tri-iodide and iodide redox couple) and a conductive substrate coated with a catalyst (Pt, carbon, etc.) as cathode The wide band gap semiconductor, which absorbs ultra-violet light, is sensitized with the dye molecule that absorbs maximum in the visible range of the solar spectrum and hence, makes efficient use of
5 the sunlight The nanoparticles of the semiconductor provide a large surface area for adsorption of the dye on it, leading to absorption of sufficient amount of light by the photoanode However, these nanoparticles of the semiconductor have to be sintered together in order to have electronic contact between the particles and allow electronic conduction through the layer The porous semiconductor (TiO2) layer is made on a conducting glass substrate (F:SnO2/FTO coated glass), which is externally connected to the cathode The cathode is again a conductive glass substrate with a catalyst such as Pt deposited on it The dye molecules commonly used in DSSCs are polypyridyl complexes of ruthenium and osmium Iodide and tri-iodide redox couple in acetonitrile is a popular liquid electrolyte used in DSSCs
1.2.1 Operating principle of dye-sensitized solar cells
Figure 1.2 Schematic representation of operational principles of DSSCs
The generation of photocurrents in the DSSC occurs through the processes shown in the Figure 1.2 Initially the dye sensitizer absorbs a photon (sunlight) to generate the photoexcited state of the dye (1), then the photoexcited dye injects an
6 electron into the conduction band [-0.5 V vs a normal hydrogen electrode (NHE)] of TiO2 [(2); the dye sensitizer absorbs sunlight (hυ), by which an electron is usually excited from the HOMO to the LUMO of the dye and then the photogenerated electron is injected from the LUMO of the dye to the conduction band of TiO2] The resulting oxidized dye is subsequently reduced back to its original neutral state through electron donation from the I - ions in the redox mediator; this process is usually called dye regeneration or re-reduction (3) The injected electrons move through the network of interconnected TiO2 nanoparticles to arrive at the fluorine- doped tin oxide (FTO) and then through the external circuit to the counter electrode (Pt-coated glass) The I - ion is regenerated by the reduction of I3 - at the counter electrode through the donation of electrons from the external circuit (4) and then the circuit is completed
During this electron flow cycle, however, there are undesirable side processes: the electrons injected into the conduction band of the TiO2 electrode may recombine either with oxidized dye [(5); recombination] or with I3 - at the TiO2 surface [(6); dark current], and radiation less relaxation of the photoexcited dye [(7); decay], resulting in lowering of the photovoltaic performances of DSSCs [6]
The fluorine-doped tin oxide (FTO) transparent conducting glasses (Pilkington, 15 U/cm 2 ) were cleaned with methanol, D.I water and acetone The cleaned FTO glasses (Pilkington, 15 U/cm 2 ) were coated with transparent TiO2 pastes (20-30 nm in diameter, Dyesol Ltd.) using the doctor blade technique,
7 followed by sintering at 450 0 C for 30 min The TiO2 particle scattering layer (200 nm in diameter, Dyesol Ltd.) was deposited on the transparent nanoporous TiO2 films, followed by sintering at 450 0 C for 30 min Two layers of TiO2 films were treated with a 40 mM of a TiCl4 aqueous solution at 70 0 C for 30 min and then sintered at 450 0 C for 30 min After cooling to 100 0 C, the TiO2 films were immersed in dye solutions at 25 0 C for 24 h in the dark and the residual dye was rinsed off with acetonitrile to provide the working electrode The platinum paste was deposited on the FTO glasses using the doctor blade technique, followed by sintering at 450 0 C for 30 min to give the counter electrodes The working electrodes and Pt counter electrodes were assembled into a sealed sandwich cell with a 60 mm thick Surlyn film (Dupont), which was then filled with an electrolyte solution through pre-drilled holes on the Pt counter electrode The electrolyte was used for the devices depend on the conditions of the fabrications
Figure 1.3 Fabrication process of DSSCs
Scope of work and research objective
The photovoltaic performances of DSSCs have been progressed by applying new metal free organic dyes, optimizing the device components and carrying out some fundamental studies The most important advantages of using organic dyes as sensitizers in DSSCs include their easily tunable physicochemical properties, through suitable molecular design and well established synthetic procedures, along with their ease of purification and high molar absorption coefficients The objective of the present work is basically to improve the power conversion efficiency of the DSSCs device by discovering novel organic dyes In addition, this research assists to achieve more understanding into structure property relationship of organic dyes, and the photovoltaic performance of DSSCs devices based on these organic sensitizers The ultimate target is to reach high conversion efficiency in DSSCs based on organic dyes, while retaining their stability under standard reporting conditions
CHARACTERIZATION OF DYE-SENSITIZED SOLAR CELLS 9 2.1 Key component of dye-sensitized solar cells
Transparent conducting glass
In the front of the DSSCs there is a layer of glass substrate, on top of which covers a thin layer of transparent conducting layer This layer is crucial since it allows sunlight penetrating into the cell while conducting electron carriers to outer circuit Transparent Conductive Oxide (TCO) substrates are adopted, including F- doped or In-doped tin oxide (FTO or ITO) and Aluminum-doped zinc oxide (AZO), which satisfy both requirements ITO performs best among all TCO substrates
However, because ITO contains rare, toxic and expensive metal materials, some research groups replace ITO with FTO AZO thin films are also widely studied because the materials are cheap, nontoxic and easy to obtain [7].
TiO 2 as the photoelectrode
Many wide bandgap oxide semiconductors (TiO2, ZnO, SnO2 …) have been examined as potential electron acceptors for DSSCs TiO2 turned out to the most versatile, delivering the highest solar-conversion efficiency TiO2 is chemically stable, non-toxic and readily available in vast quantities It is the basic component of white paints [8]
TiO2 photoelectrode properties favors natural pigments as sensitized of DSSCs because the conduction band of TiO2 photoelectrode coincides well with the excited level (LUMO) of natural pigments (specially with anthocyanins) The match in conduction band and LUMO energy levels is an important condition for efficient electrons injection from the dye to the semiconductor photoelectrode to occur [9]
11 In the standard version of DSSCs, typically film thickness is 2-15μm and the films are deposited using nanosized particles of 10-30 nm The highest solar conversion efficiency is obtained in double layer structures, where an under layer of thickness 2-4 μm is first deposited using larger (200-300 nm) size particles
The most common techniques for the preparation of TiO2 electrodes are doctor blade technique, screen printing, electrophoretic deposition and tape casting method [8].
Dye Sensitizer
In the DSSCs component, the dye sensitizer is very important factor; its function is light harvesting and electronic transition Nowadays, the dye sensitizer can be divided in to two general classes: the metal complex sensitizer and the metal- free sensitizer The metal complex sensitizers (N3, N719 or black dye…) have been achieved high efficiency, over 13%
Figure 2.2 Molecular design of a D–π–A organic dye sensitizer for DSSCs
12 However, the metal-complex sensitizers have some problem, such as limited resource, low molar extension coefficient (ε) and expensive, which will limit their application in DSSCs of large scale To get rid of these problems, the metal organic sensitizers have been developed for DSSCs Compared with rare and expensive metal complex, organic dye has the advantages of being eco-friendly, having flexible and diverse form of molecular structures, easier preparation and fabrication The organic dye using for DSSCs should be have D-π-A form, the structure show in the Figure 2.2
For the designing of a new sensitizer, it should fulfil the following essential requirements:
The sensitizer should be strong with broad absorption, it cover the whole visible region and even the part of the near IR region
The molar extinction coefficient (ɛ) of the sensitizer should be high to avoid multi-layer adsorption of the sensitizer on the semiconductor surface
The sensitizer should have anchoring groups such as -COOH, -SO3H etc to strongly bind the dye onto the semiconductor surface
The excited state life-time of the sensitizer should be a sufficiently long span (typically in the ns domain), and excited electrons of the sensitizer should be efficiently injected into the conduction band of the semiconductor to avoid the decay of the excited state dye to the ground state
The excited state level sensitizer should be more positive energy than the conduction band edge of the semiconductor (n-type DSSCs) so that an
13 efficient electron transfer process between the excited dye and conduction band of the semiconductor takes place
For dye regeneration, the oxidation state level of the sensitizer must be more positive (by ca 200–300 mV) than the redox potential of the electrolyte
Unfavorable dye aggregation on the semiconductor surface should be avoided through optimization of the molecular structure of the dye or by addition of co-absorbers that prevent aggregation
The sensitizer should be stable electrochemically, photochemically and thermally for longer periods
The sensitizer should have good solubility in a variety of solvents, be less hazardous, and be low cost and abundantly available [10].
Electrolyte
The electrolyte plays a very important role in the DSSCs by facilitating the transport of charger between the working electrode and counter electrode In general, following are the criteria for materials to serve as electrolytes in DSSCs:
The redox potential of electrolyte should be negative as compared to the oxidation potential or HOMO of the dye
The electrolyte should efficiently regenerate the dye after the process of dye excitation and electron injection to conduction band of oxide semiconductor
It should have high conductivity (~10 –3 S.cm –1 )
It should infiltrate the pores of the photoanode and establish contact with both the electrodes
It should not cause desorption of the dye from the photoanode
It should not react with the sealant and degrade it leading to poor stability of the cell
The absorption of light by the electrolyte in the visible range, in which dye molecules absorb, should be minimum
It should not undergo any chemical change leading to loss of its functionality
It should be stable up to ~80 0 C [11]
An organic electrolyte consists of a redox couple, a solvent and additives
Among them, the redox couple is the most important component since it is directly linked to the open circuit voltage (Voc) of DSSCs There are many redox couples such I - /I3 -, Br - /Br3 -, SCN - /(SCN)2, SeCN - /(SeCN) - 3 etc However, the most popular redox couple is I3 -/I - , because it has better solubility, fast dye regeneration process, low light absorption in the visible region, appropriate redox potential, and a very slow recombination rate between the TiO2 injected electrons and I3 - [9]
The solvent is responsible for the diffusion and dissolution of the I - /I3 - ions A number of solvents had been studied in DSSCs, such as acrylonitrile (AcN), ethylenecarbonate (EC), propylene carbonate (PC), 3-methoxypropionitrile (MePN) and N-methylpyrrolidone (NMP) Each particular solvent has its own donor number, and contributes to the Voc and short-circuit photocurrent (Jsc) A solvent with a high donor number can increase the Voc and decrease the Jsc by lowering concentration of I3 - The lower I3 - concentration helps to slow the recombination rate and as a result, it increases the Voc [9]
Counter electrode
The counter electrode is where the redox mediator reduction occurs The oxidized ions in electrolyte diffuse toward the counter electrode and accept electrons from the external circuit Requirements of a material to be used as a counter electrode in DSSCs is to have a low charge transfer resistance, optimum thickness, high surface area, porous nature good adhesiveness to the transparent conducting oxide, high reflectance of transmitted light, good electrochemical stability in the electrolyte and high exchange current density At the same time, it aids to carry the photocurrent over the whole width of the DSSCs Therefore, the counter electrode must be a good conductor and it needs to have a low overvoltage for the reduction of the redox mediator
Platinum (Pt), thus far, is the preferred material for the counter electrode since it is an excellent catalyst for I3 – reduction The Pt counter electrode is ~200 nm in thickness, and it can be fabricated by sputtering, screen printing or pyrolysis of H2PtCl6 solution onto the FTO substrate The platinized TCO substrate exhibits electrocatalytic activity, which improves the reduction of I3 - by facilitating electron exchange, and it has a high light reflection due to the mirror-like effect of Pt
However, Pt is a rare metal, hence not cost effective for large scale production
Besides the high cost Pt corrodes with the redox mediator I3 - which leads to the generation of platinum iodides like PtI4 which is undesirable This means the Pt counter electrode has a durability issue Therefore, other materials such as carbon
16 nanotube,graphite, conductive polymer etc., are being investigated as an alternative to Pt [9].
Key efficiency parameters of dye-sensitized solar cells
The incident photon to current conversion efficiency (IPCE) is a measure of the efficiency of the solar cell to convert the incoming photons to photocurrent at different wavelengths This is done by measuring the resulting photocurrent of the solar cell when illuminated by monochromatic light The IPCE is a measure of the product of different efficiencies such as light harvesting efficiency (LHE), the quantum yield of electron injection from the excited dye into the TiO2 conduction band ɸinj, the efficiency of regeneration ηreg, and the collection efficiency of the photo-generated charge carrier ηcoll
IPCE = LHE × ɸinj × ηreg × ηcoll (2.1)
Figure 2.3 A typical ICPE spectrum of a DSSCs
17 For calculating the IPCE experimentally one use the following equation:
𝑖𝑛 (𝑚𝑊𝑐𝑚 −2 ) (2.2) The IPCE spectrum is very useful for the evaluation of a new dye sensitizer for DSSCs A typical IPCE spectrum is shown in Figure 2.3 [6]
Measurement of the J/V curves under standard 1.5 AM simulated sunlight (100 mWcm -2 ) is an easy and useful method for the evaluation of the photovoltaic performance of a DSSC A typical J/V curve is depicted in Figure 2.4 The performances of DSSCs are universally represented by the following four key factors:
Figure 2.4 A typical J/V curve of a DSSCs
The Voc value is the difference in electrical potential between two terminals of a cell under illumination when the circuit is open The Voc value can be expressed by Equation (2.3):
𝑐𝑏) − 𝐸 𝑟𝑒𝑑𝑜𝑥 (2.3) in which e is the elementary charge, n is the number of the electrons in TiO2, kB is the Boltzmann constant, T is the absolute temperature, Ncb is the effective density of states, an Eredox is the redox potential of the redox couple
2 Short-Circuit Photocurrent Density (J sc )
The Jsc value is the photocurrent per unit area (mAcm -2 ) when a DSSCs under irradiation is short-circuited The Voc value corresponds to the difference between Ef of the electron in TiO2 and the redox potential of the electrolyte (I3 -/I - ), whereas Jsc is related to the interaction between TiO2 and the dye sensitizer, as well as the absorption coefficient of the dye sensitizer The Jsc value strongly depends on the photophysical and electrochemical properties and molecular structures of the dye sensitizers The Jsc value can be derived by integrating the IPCE spectra to give
𝐽 𝑠𝑐 = 𝑒 ∫ 𝐼𝐶𝑃𝐸(𝜆)𝐼 𝑠 (𝜆)𝑑(𝜆) (2.4) In which Is(λ) is the photon flux at wavelength λ in AM 1.5(100 mAcm -2 )
The FF is an important parameter for a solar cell and is defined as the maximum power output (JmpVmp) divided by the product of Jsc and Voc Equation (2.5):
𝑠𝑐 𝑉 𝑜𝑐 (2.5) The FF is determined from the J/V curve and is an indication of how much of the area of the rectangle for JscVoc is filled by that described by JmpVmp (Figure 2.4)
Thus, the maximum FF value is unity However, the FF value is attenuated by the series resistance of the cell, which includes the sheet resistances of the substrate and counter electrode, electron transport resistance through the photoanode, ion transport resistance, and the charge transfer resistance at the counter electrode Therefore, careful fabrication of the cell is important for attaining high photovoltaic performance [6]
4 Solar Energy-to-Electricity Conversion Yield (ɳ)
The ɳ value of a DSSC is defined as the ratio of the maximum output electrical power of the DSSC to the energy of incident sunlight (I0) [Equation (4)] and is therefore determined by Voc, Jsc, FF, and I0 (generally AM 1.5, 100 mW cm -2 ) ɳ(%) = 𝐽 𝑠𝑐 (𝑚𝐴𝑐𝑚 𝐼 −2 𝑉 𝑜𝑐 (𝑉)𝐹𝐹
2.2.3 Electrochemical impedance spectroscopy (EIS) of DSSCs
Electrochemical impedance spectroscopy (EIS) is one of the most important tools to explain the charge transfer and transport processes in various electrochemical systems including DSSCs Impedance spectroscopy is a powerful method for characterizing the electrical properties of materials and their interfaces Analysis of EIS spectrum of a DSSC provides information about several important charge transports, transfer, and accumulation processes in the cell These are (i) charge transport due to electron diffusion through TiO2 and ionic diffusion in the electrolyte
20 solution; (ii) charge transfer due to electron back reaction at the FTO/electrolyte interface and recombination at the TiO2/electrolyte interface and the regeneration of the redox species at counter electrode/electrolyte interfaces; and (iii) charging of the capacitive elements in the cells including the interfaces, the conduction band, and surface states of the porous network of TiO2 [12]
Figure 2 5 Electrochemical impedance spectroscopy of DSSCs
The most widely used representation of the results of EIS measurements is a Nyquist plot Figure 2.5 represents a characteristics impedance spectrum (Nyquist plot) along with the electrical transmission line model of equivalent circuit for DSSCs The transmission line model is purely a combination of resistant (R) and capacitance (C).The ohmic serial resistance (Rs) corresponds to the electrolyte and FTO resistance The high frequency region represents the series resistance (Rce), corresponding to the diameter of the first semicircle; the larger semicircle in the mid-
21 frequency region reflects the charge transfer/recombination resistance (RTiO2) at the TiO2/dye/electrolyte interface [13]
EFFECT OF ANCHORING GROUP IN ANTHRACENE/THIOPHENE-BRIDGED TRIPHENYLAMINE BASED
Introduction
The greenhouse gas produced from fossil fuels since the commencement of the industrial revolution has caused the global climate change further seriously than the whole history of humanity In recent times, one of the most critical challenges for researchers is the answer of energy demand In this regard, renewable energy sources have been attracting significant attention Among them, dye-sensitized solar cells (DSSCs) technology is attracted as a possible answer to energy demands because of their high photovoltaic performance, easy fabrication, and low cost production compared with other solar cells [14-16] Among all constituents exist in DSSCs, dye- sensitizer is a key element of these solar devices for high photovoltaic performance
Over years of investigation, vast varieties of dye-sensitizers have been reported, including zinc-based porphyrin sensitizers polypyridyl ligated ruthenium complexes, and organic dyes [17-32] However, the cost of the noble metals, complex and expensive purification processes, and environmental issues are the impediments of ruthenium and zinc-based dye-sensitizers and their conversion efficiencies are greatly less than the ideal dye sensitizer Therefore, the development of dye-
23 sensitizer is still one of the most crucial assignments to endorse the development of DSSCs [33]
It is well known that organic dye-sensitizers comprises of donor, π- bridge/spacer, and acceptor/anchoring units (D-π-A) and this structure is related with the potent intramolecular charge transfer (ICT), thus resulting in the facile charge transfer from excited dye sensitizer (via anchoring unit) into the semiconductor surface [34] For an excellent dye-sensitizer, selecting a proper anchoring unit is exceptionally crucial, which governs the binding strength of the dye-sensitize over the semiconductor surface and charge transfer rate [35-36] In most of the organic dye-sensitizers, 2-cyanoacetic acid (CA) is commonly employed into the D-π-A structure as an anchoring unit Also, there are few reports on the effect of different anchoring units on the D-π-A structure
In this work, we have synthesized three organic dyes which have triphenylamine as donor group with different acceptors in their charge-transfer chromophoric system for DSSCs application Anthracene and thiophene conjugate groups were inserted between the donor and acceptor units as the π-spacers, this extended π-conjugation can definitely enhance the light-harvesting effect The anchoring groups employed were 2-cyanoacetic acid (CA), rhodanine-3-acetic acid (RA) and 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid (OPCA) and Figure 3.1 shows corresponding dye molecular structures The UV-vis, electrochemical, incident photon-to-current conversion efficiency (IPCE), and currentdensity- photovoltage (J-V) curves were studied
Experiment details
All reactions were carried out under nitrogen atmosphere Solvents were distilled from appropriate reagents All starting materials and reagents were purchased from Sigma-Aldrich, TCI, and ACROS Co 1H spectra were recorded on Varian Mercury NMR 300 MHz spectrometer Chemical shifts were reported in parts per million down fields from tetramethylsilane (TMS) as an internal standard in appropriate deuterated solvents The optical spectra of dyes in solution were recorded with Agilent 8453 UV-vis spectrophotometer Electrochemical data were recorded
25 using CV-BAS-Epsilon The cyclic voltammogram curves were obtained from a three electrode cell in 0.1 M Bu4NPF6 chloroform solution at a scan rate of 100 mV s -1 , Pt wire as a counter electrode and an Ag/AgCl reference electrode
Figure 3.2 Scheme of preparation route for dye sensitizers D1-D3
In a three-necked, oven-dried, 150 mL round-bottom flask, triphenylamine (3.000 g, 12.244 mmol) was dissolved in 50 mL of anhydrous chloroform, covered with aluminium foil, and stirred at 25 0 C under inert atmosphere for 15 min N- bromosuccinimide (1.96 g, 11.121 mmol) was then added in small portions, and the resulting solution was stirred at 25 0 C for 5 h The reaction mixture was extracted
26 three times with chloroform The combined organic fractions were washed with brine and dried over anhydrous MgSO4 The solvent was removed under reduced pressure and the residue was recrystallized by using ethanol to give a white powder (3.020 g, 76%) 1H NMR (300 MHz, CDCl3): 7.62-7.21 (m, 7H), 7.19-7.00 (m, 5H), 6.99-6.93 (m, 2H)
2 N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (2)
Under an inert atmosphere, a degassed solution of 1 (3.000 g, 9.250 mmol), bis(pinacolato)diboron (3.054 g, 12.255 mmol), KOAc (3.018 g, 32.225 mmol), and Pd(dppf)2Cl2 (0.338 g, 0.05 mmol) in dry dimethoxyethane (30 mL) was heated at reflux for 15 h After this period, the mixture was cooled to room temperature, filtered, and diluted with CH2Cl2 (50 mL) The organic solution was washed with H2O (2x30 mL) and brine, then dried (anhydrous MgSO4) and evaporated The residue was separated by column chromatography using hexane/CH2Cl2 (9/1 v/v) to give the compound 2 as white solid product (2.170 g, 63%) 1H NMR (300 MHz, CDCl3): 7.74-7.59 (d, 2H), 7.19-7.08 (d, 4H), 7.07-6.90 (t, 4H), 1.48-1.16 (t, 12H)
A 50 mL of three neck round bottom flask was charged with 10- bromoanthracen-9-boronic acid (0.770 g, 2.000 mmol), Pd(PPh3)4 (0.232 g, 10 mol%), THF (20 mL) and 2 M aqueous K2CO3 (2 mL), then the flask was purged with nitrogen gas with 5 evacuate/refill cycles Then 5-bromo-2- thiophenecarboxaldehyde (0.460 g, 2.400 mmol) was added under inert atmosphere
The tube was sealed and heated at 70 0 C with vigorous stirring for 15 h Upon
27 cooling to ambient temperature, the organics were extracted three times with CH2Cl2 The combined organic fractions were washed with brine and dried over MgSO4 The solvent was removed under reduced pressure and the residue was purified by silicagel column chromatography using hexane/ CH2Cl2 (8/2, v/v) as eluent to give as a yellow powder (0.420 g, 57%) 1H NMR (300 MHz, acetone-d6): 10.14 (s, 1H), 8.7-8.62 (d, 2H),8.25 (d, 2H), 7.88-7.77 (m, 4H), 7.68-7.57 (t, 2H), 7.53-7.46 (d, 1H)
4 5-(10-(4-(diphenylamino)phenyl)anthracen-9-yl)thiophene-2-carbaldehyde (4)
To a mixture of compound 3 (0.918 g, 2.500 mmol), compound 2 (0.900 g, 2.500 mmol) and K2CO3 (1.029 g, 8.75 mmol) in toluene/ethanol (15/5 mL) was added Pd(PPh3)4 (0.347 mg, 0.3 mmol) under inert atmosphere After stirring for 24 h at 110 0 C, water (10 mL) and dichloromethane (30 mL) were added The organic layer was separated, and the aqueous layer was extracted with dichloromethane (2×10 mL) The organic layer and the dichloromethane extracts were combined and dried (anhydrous MgSO4), and then filtered The organic solvent was completely removed by rotary evaporation The solid residue was purified by column chromatography using hexane/ CH2Cl2 (8/2, v/v) as eluent to give a pale yellow solid (0.916 g, 70%) 1H NMR (300 MHz, CDCl3): 10.08 (s, 1H), 8.21-7.97 (d, 1H), 7.90- 7.75 (m, 5H), 7.50-7.40 (t, 5H), 7.39-7.20 (m, 10H), 7.15-7.02 (t, 3H)
5.(Z)-2-cyano-3-(5-(10-(4-(diphenylamino)phenyl)anthracen-9-yl)thiophen 2yl)acrylic acid (D1)
Compound 4, 5-(10-(4-(diphenylamino)phenyl)anthracen-9-yl)thiophene-2- carbaldehyde (0.100 g, 0.188 mmol), dissolved in acetic acid (15 mL) was condensed
28 with cyanoacetic acid (0.032 g, 0.376 mmol) in the presence of ammonium acetate (0.029g, 0.376 mmol) Then, the reaction mixture was refluxed well for 5 h under inert atmosphere After cooling to room temperature, the reaction mixture was poured into a crushed ice and solid obtained was washed thoroughly with water to remove excess of acetic acid and cyanoacetic acid Finally, washed with hexane to afford a yellow colored solid D1 (0.045 g, 40%) 1H NMR (300 MHz, DMSO-d6) 8.27 (s, 1H), 8.00 (s, 1H), 7.89–7.72 (t, 5H), 7.48–7.34(m, 8H), 7.33–7.20 (t, 7H), 7.19–7.10 (m, 3H)
6.(Z)-2-(5-((5-(10-(4-(diphenylamino)phenyl)anthracen-9-yl)thiophen 2yl)methylene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (D2)
The synthesis procedure used for compound D1 was followed Anchoring group rhodanine-3-acetic acid (0.036 g, 0.376 mmol) was used to afford D2 (0.115 g, 86%) as dark yellow solid 1H NMR (300 MHz, DMSO-d6): 8.36 (s, 1H), 8.12 (s, 1H), 7.81 (m, 3H), 7.67–7.55 (m, 3H), 7.47–7.14 (m, 15H), 4.75 (s, 2H)
7.(E)-4-((5-(10-(4-(diphenylamino)phenyl)anthracen-9-yl)thiophen- 2yl)methylene)-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazole-3-carboxylic acid (D3)
The synthesis procedure used for compound D1 was followed Anchoring group 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid (0.076 g, 0.376 mmol) was used to afford D3 (0.073 g, 54%) as red solid 1H NMR (300 MHz, DMSO-d6): 9.39 (s, 1H), 8.42 (s, 1H), 8.10-7–7.90 (m, 7H), 7.89–7.75 (m, 9H), 7.65–7.00 (m, 12H)
3.2.3 Assembly and Characterization of the DSSCs
Transparent conducting glass substrates were cleaned sequentially with ethanol, DI water and acetone with ultrasonication Nanocrystalline TiO2 paste was synthesized using ethyl cellulose (Aldrich), lauric acid (Fluka, City, State, Country) and terpineol (Aldrich) The TiO2 particles used were ca 20-30 nm in diameter A prepared TiO2 paste was doctor-bladed onto the pre-cleaned glass substrates, followed by drying at 70 0 C for 30 min and 30 min calcination at 500 0 C A scattering layer consisting of rutile TiO2 particles (250 nm in a size) was deposited on the mesoporous TiO2 films These layers were dipped into an aqueous solution of TiCl4 (0.04 M) at 70 0 C for 30 min The sensitizers were dissolved in dry chloroform (0.2 mM) at 25 0 C and stirred for 24 h The annealed TiO2 electrodes were dipped in the dye solutions for 18 h
Pt counter electrodes were prepared by thermal reduction of the films dip- coated in H2PtCl6 (7 x 10-3 M) in 2-propanol at 400 0 C for 20 min The dye-adsorbed TiO2 and Pt counter electrodes were sandwiched between a 60 x 10 -6 m thick Surlyn (Dupont 1702) layer, which was used as a bonding agent and spacer A I - /I3 - redox couple composed of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.06 M NaI, 0.04 M I2, and 0.4 M 4-tert-butylpyridine in dried CH3CN was then introduced through a hole on the Pt counter electrode The active area of the dye- adsorbed TiO2 films was estimated using a digital microscope camera with image- analysis software (Moticam 1000) The photovoltaic I–V characteristics of the prepared DSSCs were measured under 1 sunlight intensity (100 mW cm −2 , AM 1.5),
30 which was verified with an AIST-calibrated Sisolar cell (PEC-L11, Peccell Technologies, Inc., City, State, Country) The incident photon-to-current efficiencies (IPCEs) were plotted as a function of the light wavelength using an IPCE measurement instrument (PEC-S20, Peccell Technologies, Inc.).
Results and discussion
In organic dyes, the introduction of a powerful electron pulling moiety near to anchoring unit is required to drag the electron density from the donor part, lowering the energy band gap, which shifts optical spectra to the lower energy regime and is expected to aid electron transport to TiO2 semiconductor This situation is noticed for diverse organic D-π-A dyes [37-39] In most of the organic D-π-A dyes, the presence of cyanoacetic acid near to the anchoring part prone to cis-trans isomerization upon light irradiation, which could result in enhanced recombination currents [40]
Elimination of such anchoring unit could enhance the planarity of the chromophore and might aid in diminishing the recombination Hence, switching the cyanoacetic acid acceptor to a different acceptor for those D-π-A chromophores may provide useful information about the importance of acceptor units close to the anchoring unit in modifying the photophysical, electrochemical, and photovoltaic performance
Herein, three anthracene/triphenylamine-based organic dyes, D1, D2, and D3, having different acceptors, such as 2-cyanoacetic acid (CA), rhodanine-3-acetic acid (RA) and 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid (OPCA), were prepared as sensitizers for TiO2-based DSSCs Scheme 1 shows the preparation route for the new
31 sensitizers Firstly, simple triphenylamine was brominated using N- bromosuccinimide in chloroform solvent followed by Suzuki- Miyaura borylation using bis(pinacalato)diboron to obtain intermediate 2 In another step, we have started with 10-bromoanthracen-9-boronic acid which we subjected to Suzuki coupling using 5-bromo-2-thiophenecarboxaldehyde to obtain intermediate 3 Then the intermediates 2 and 3 were subjected to Suzuki couping to obtain aldehyde intermediate 4 Finally, the intermediate 4 was subjected to Knoevenagel condensatoin using 2-cyanoacetic acid, rhodanine-3-acetic acid and 5-oxo-1-phenyl- 2-pyrazolin-3-carboxylic acid to obtain the final target dye-sensitizers D1, D2, and D3, respectively
We have measured UV-vis absorption spectra of the dyes D1-D3 both in solution and on photoelectrode film (Figure 3.3 and Figure 3.4) since their spectral response overlaid with the solar emission spectra will affect the cell photocurrent to a great extent The corresponding data are listed in Table 1 The three dyes with a strong absorption band in the visible part corresponding to the intramolecular charge transfer between donor units and acceptor groups are noticed
Figure 3.3 UV-vis absorption spectra of dye sensitizers D1-D3 in solution
As shown in Figure 3.3, the dye D1 exhibits an absorption maximum at 400 nm with a molar extinction coefficient (ε) of 16455 M -1 cm -1 As compared to D1, the absorption pattern of D2 is bathochromically shifted 14 nm, with absorption maximum showing at 414 nm with a ε of 42675 M -1 cm -1 because of the replacement of cyanoacetic acid with more extended conjugation and strong withdrawing acceptor rhodanine acetic acid Similarly, the replacement of CA in D1 with another acceptor group OPCA, giving D3, causes spectral broadening and an increased ε of 22675 M -1 cm -1 with absorption maximum showing at 404 nm This decrease in energy gap is utterly advantageous for enhancing the light harvesting ability of the sensitizers Particularly, dyes D2 and D3 exhibited broader and higher ε values as compared to dye D1
Figure 3.4 UV-vis absorption spectra of dye sensitizers D1-D3 on TiO 2 film
Figure 3.4 shows absorption spectra of corresponding dyes on TiO2 films All the three dyes showed broadening of absorption spectra compared to the solution spectra (Figure 3.3) because of electronic coupling of the dyes with the TiO2 surface and possibly some dye aggregation This phenomenon was noticed for many organic dyes on TiO2 films From the Figure 3.3, E0-0 values of 2.66, 2.43, and 2.19 eV were calculated for D1-D3 dyes, respectively
Dye ε max (M -1 cm -1 ) λ max (nm) Sol TiO 2
Table 3.1 Photophysical and electrochemical data of dye sensitizers D1-D3
The cyclic voltammograms of the dyes D1-D3 were measured in a 0.1 M n- Bu4NPF6 solution (Figure 3.5) From this we calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to investigate the dye regeneration from redox electrolyte and electron transfer from the excited dye- sensitizer to the conduction band of the TiO2 semiconductor, respectively Generally, the formal oxidation potentials are regarded as the HOMO level of the dye-sensitizer and LUMO values are calculated from the difference between Eox-E0-0 where E0-0 values were calculated from the onset absorption spectra (Figure 3.3) The HOMO, LUMO, and E0-0 values were tabulated in Table 1 The HOMO levels of the dyes were in the range of 1.14 - 1.16 V which were more positive than that of I - /I3 - redox electrolyte (~0.4 V vs NHE) [37] This suggests that oxidized dye can be regenerated via electron donation from the I - ions in the I - /I3 - redox electrolyte Also, the calculated LUMO values (Table 1) are more negative than the conduction band edge of the TiO2 semiconductor (~ -0.5 V vs NHE) [38] suggesting that the excited electrons are efficiently injected into the conduction band of TiO2 The strong electron-pulling acceptors such as RA, OPCA could positively shift the reduction potential values of the dyes D2 and D3 compared to dye D1 with fewer electrons pulling CA acceptor unit This could lead to the lowering of the dye’s LUMO level, resulting in a lower thermodynamic driving for electron injection
Figure 3.5 Cyclic voltammogram of dye sensitizers D1-D3 3.3.4 Photovoltaic properties
Figure 3.6 presents the incident photon-to-current conversion efficiency (IPCE) of the devices based on dyes D1-D3 Dye D1 with acceptor CA showed a stronger solar cell response in the 350-550 nm spectral regions compared to that of other dyes with acceptors RA and OPCA The device with dye D1 showed plateau of more than 40% from 350-470 nm with the highest values at 418 nm and an onset at 550 nm Whereas, the onset of IPCE for D2 and D3 was extended to 600 nm, which corresponds to a bathochromic shift of 50 nm, respectively compared to dye D1 The difference between the IPCE spectra noticed for the studied dyes is in good agreement with their optical absorption spectra (Figure 3.3)
Figure 3.6 IPCE spectra for the DSSCs based on dye sensitizers D1-D3
The relatively higher IPCE values for dye D1 led to higher Jsc values compared with that of dyes D2 and D3 Although the D2 and D3 dyes have the broader and higher absorption in all the regions in comparison with dye D1, the DSSCs based on D2 and D3 show lower IPCE It might be caused by the lower electron injection causing from a lower LUMO levels
Figure 3.7 presents the currentdensity-photovoltage (J-V) characteristics of the devices using the dyes D1-D3 and Table 3.2 lists the corresponding photovoltaic parameters The device based on dye D1 showed better photovoltaic performance with a photo current density (Jsc) of 3.12 mA cm -2 , an open cicuit photovoltage (Voc) of 0.607 V, and a fill factor (FF) of 66.88%, corresponding to power conversion efficiency (η) of 1.27% Upon replacing CA acceptor group with RA acceptor the power conversion efficiency of D2-based device diminished to the 0.91% by the drop of Jsc from 3.12 mA cm -2 to 2.84 mA cm -2
Figure 3.7 J-V curves for the DSSCs based on dye sensitizers D1-D3
Previously, molecular modelling studies showed that the acceptor CA induces vertically oriented dye geometry when it is attached to semiconductor surface, whereas the donor part of the acceptor RA-based dye showed close proximity with the semiconductor surface with a tilted angle Hence, they concluded that the dye cation with the RA group as acceptor unit is prone to meet more inner path recombination with the injected electrons [41-43] The Jsc and efficiency were further decreased to 1.78 mA cm -2 and 0.70%, respectively, for the device with dye D3 where the CA acceptor group was replaced with OPCA acceptor This might be partially attributed to the poor electron injection efficiency as evidenced in the above section On the other hand, compared to dye D1-based device with 0.607 V of Voc, D2 and D3-based devices exhibited a considerably lowered Voc of 0.485 V and 0.541 V, respectively, possibly due to tilted angle orientation on the semiconductor surface, which could lead to the inner path recombination [41-43] Also, the presence strong
38 electron withdrawing group, such as RA or OPCA (compared to CA), near the anchoring group was thought to be detrimental to charge entrapment around withdrawing group and eventually effect electron injection efficiency or recombination As a result, since strong electron withdrawing nature of RA and OPCA groups, dye D2 and D3 showed lowered light-to-electricity energy conversion efficiency compared to dye D1, even though they provide better light harvesting ability
Dye J sc (mA cm 2 ) V oc (V) FF (%) CE (%)
Table 3.2 Photovoltaic data of the DSSCs based on dye sensitizers D1-D3
To further understand the working electrode interface, we performed electrochemical impedance spectroscopy studies (EIS) since this technique has been a useful tool to estimate charge transfer resistance and to know the dye regeneration efficiency Figure 3.8 shows EIS results for DSSCs comprised of D1–D3 under illuminated conditions
Figure 3.8 EIS spectra of the dye sensitizers D1-D3
The radius of the semicircle corresponding to the working electrodes of these dyes are in the order of D3 >D2 > D1 The introduction of RA and OPCA anchoring groups largely increases the charge transfer resistances The increase of charge transfer resistance value would affect the photo regeneration resulting in inferior performance The fitting data of EIS agreed well with photovoltaic performance shown in Figures 3.7, and Table 3.2
Conclusion
Three triphenylamine based metal-free organic dyes (D1-D3) with different anchoring groups (CA, RA, and OPCA) bridged by anthracene and thiophene containing π-spacers were synthesized for the dye-sensitized solar cells application
Specific analyses on the relationship between the dye structure, absorption properties, electrochemical properties, photocurrent-voltage, and incident photon-to-current
40 conversion efficiency curves were studied Despite the broader IPCE, dyes D2 and D3 showed decreased IPCE values, possibly due to decreased LUMO levels and charge entrapment around the strong withdrawing groups The overall power conversion efficiency of the DSSC based on the dye D1 with CA anchoring group (1.27%) was higher than that of all other dyes In a word, it has been showed that the introduction of different anchoring groups with the different withdrawing ability has a significant effect on the power conversion efficiency
SYNTHESIS AND PHOTOVOLTAIC PERFORMANCE OF
Introduction
Owing to the speedy and ceaseless exhaustion of fossil fuel resources, there are now serious concerns over alternative sources of energy sufficient to supply for the world’s increasing energy requirements [44] Solar energy, which is huge and essentially unlimited, has the largest potential to satisfy the future worldwide need for renewable energy without appreciable environmental matters [45] During the past two decades, noteworthy attention has been attracted by dye-sensitized solar cells (DSSCs) as alternatives to traditional solar cells, due to their low cost fabrication combined with high photovoltaic performance [46] After the successful exploration in 1991 by Gratzel's group, DSSCs based on Ru dyes have been extensively studied and achieved solar-to-electrical power conversion efficiency values of over 13% [47-49] However, the metal complex sensitizers such as N3, N719 or black dye have some problems such as restricted resource, low molar extinction coefficient (ε) and expensive, which will limit their applications in DSSCs of large scale [50] As a promising class of sensitizers, the metal-free organic dyes possess several advantages such as low cost, easy synthesis, high ε, as well as environmental friendly in comparison with the metal complex based organic dyes
42 Overall, these results indicate that the commercial application of metal-free organic dyes in DSSCs is promising
Among the several classes of metal-free organic dyes, the phenothiazine- based dyes hold a great percentage and have the exclusive advantages: (i) the non- planar butterfly conformation of phenothiazine in the ground state can block the molecular aggregation and the creation of molecular excimers, which is advantageous for achieving high photovoltage; (ii) the electron-rich sulfur and nitrogen heteroatom render phenothiazine a stronger donor than other amines, even better than triphenylamine, carbazole, tetrahydroquinoline and so on Notably, some researchers discovered that the DSSCs based on phenothiazine dyes showed better photovoltaic performances than commercial N719 under identical fabrication and test conditions To sum up, the possibility of superior structure and electron-rich property of phenothiazine derivatives guarantees their bright future as dyes in DSSCs [50]
In this work, we had reported that organic dyes of double donor-acceptor branched phenothiazine as photosensitizers for DSSC turned out more efficient compared to those of single branch due to the increase of their electron extraction paths from electron donor and the higher molar extinction coefficients Thus, if the TiO2 surface of the DSSCs would adsorb the same number of T2 and T1 molecules, the light-harvesting units might be doubled in DSSCs with T2 compared to that with T1 Even when the TiO2 surface adsorbs somewhat less T2 molecules, the number of light harvesting units can be enhanced, provided that the intramolecular distance of the chromophores is smaller than their average intermolecular distance [51] Besides,
43 we also introduced the effect of alkoxy group in the π-conjunction of dye molecular in the cell efficiency Alkoxy and aromatic groups between electron donating units also reduce aggregation of the dyes on TiO2 films Alkyl and alkoxy chains with different donors have been investigated and these modifications can reduce the recombination rate within DSSCs and thus increase open-circuit voltage [52] The T3 with multiple electron acceptors was synthesized to compare with T2 The structure of these dyes shown in the Figure 4.1
Figure 4.1 Chemical structures of dye sensitizers T1-T3
Experimental
Figure 4.2 Scheme of preparation route for dye sensitizers T1, T2, and T3
A mixture of phenothiazine (6 g, 0.03 mol), Sodium-tert-butoxide (8.65 g, 0.09 mol), and Palladium(II) acetate (0.675 g, 0.003 mol), Tri-tert-butylphosphine (0.6 g, 0.003 mol) in Tetrahydrofuran (60 mL) and the mixture was degassed in 15 min After that 4-bromophenol (6.675 g, 0.039 mol) dissolved in THF was added and the mixture reaction was stirred at 65 0 C for 18 h After cooling the room temperature, inorganic salts were filtered The solvent was removed under reduced pressure The mixture reaction was extracted three times with dichlomethane in the presence of sodium chloride and was dried by anhydrous MgSO4 Purification of the crude product was carried out by column chromatography using silica gel and ethyl acetate/hexane (1/10) to give product as white solid (6.8 g) in 77.8% yield
2 1,6-bis(4-(10H-phenothiazin-10-yl)phenoxy)hexane (2)
A 100 mL 3-neck flask was charged with 4-(10H-phenothiazin-10-yl)phenol (3 g, 0.01 mol), Potassium carbonate (4.15 g, 0.03 mol) and 18-crown-6 (0.8 g, 0.003 mol) in 40 mL of Tetrahydrofuran The mixture was degassed in 20 min After that 1,6-dibromohexane (1.22 g, 0.005 mol) dissolved in THF was added The resulting mixture was stirred for 8 h at 70 0 C The reaction mixture was then quenched with water and extracted three times with dichlomethane and was dried by anhydrous MgSO4 The solvent was removed under reduced pressure and the product was purified by column chromatography on silica gel with acetate/hexane (0.5/10) to give product as white solid in 18.65% yield
3 10, 10'-((hexane-1,6-diylbis(oxy))bis(4,1-phenylene))bis(10H-phenothiazine-3- carbaldehyde) (3)
POCl3 (1.61 g, 10.53 mmol) was added drop by drop into DMF (0.77 g, 10.53 mmol) at 0 0 C under nitrogenous environment with continuous slow stirring A solid glassy material formed which was dissolved in 1,2-dichoroethane (10 mL) and stirred for additional 30 min Then 1,6-bis(4-(10H-phenothiazin-10 yl)phenoxy)hexane (0.7 g, 1.053 mmol) dissolved in 1,2-dichoroethane l (20 mL) was injected into the flask and maintained at 70 0 C for overnight After cooling, the reaction mixture was poured into ice water (100 mL) The product was extracted with CH2Cl2 three times and dried over anhydrous MgSO4 The solvent was removed by rotary evaporator The concentrated mixture was subjected to column chromatography using silica gel and acetate/hexane (1/10) as mobile phase Bright yellow color crystals of product were obtained in 71.05% yield
4 10,10'-(4,4'-(hexane-1,6-diylbis(oxy))bis(4,1-phenylene))bis(10H-phenothiazine- 3,7-dicarbaldehyde) (4)
POCl3 (3.23 g, 21.06 mmol) was added drop by drop into DMF (1.54 g, 21.06 mmol) at 0 0 C under nitrogenous environment with continuous slow stirring A solid glassy material formed which was dissolved in 1,2-Dichoroethane (10 mL) and stirred for additional 30min Then 1,6-bis(4-(10H-phenothiazin-10- yl)phenoxy)hexane (0.7 g, 1.053 mmol) dissolved in 1,2-Dichoroethane l (20 mL) was injected into the flask and maintained at 70 0 C for overnight After cooling, the reaction mixture was poured into ice water (100 mL) The product was extracted
47 with CH2Cl2 three times and dried over anhydrous MgSO4 The solvent was removed by rotary evaporator The concentrated mixture was subjected to column chromatography using silica gel and acetate/hexane (1/10) as mobile phase Bright yellow color crystals of product were obtained (0.32, 39.02%)
5 (2Z,2'E)-3,3'-(10,10'-((hexane-1,6-diylbis(oxy))bis(4,1-phenylene))bis(10H phenothiazine-10,3 diyl))bis(2-cyanoacrylic acid) (5)
Compound 10,10'-((hexane-1,6-diylbis(oxy))bis(4,1-phenylene))bis(10H phenothiazine-3-carbaldehyde) (0.2 g, 0.278 mmol), dissolved in acetic acid (15 mL) was condensed with cyanoacetic acid (0.12 g, 1.39 mmol in the presence of ammonium acetate (0.11 g, 1.39 mmol) Then, the reaction mixture was refluxed well for 5 h under inert atmosphere After cooling to room temperature, the reaction mixture was poured into a crushed ice and solid obtained was washed thoroughly with water to remove excess of acetic acid and cyanoacetic acid Finally, washed with hexane to afford a red colored solid of (2Z,2'E)-3,3'-(10,10'-((hexane-1,6- diylbis(oxy))bis(4,1-phenylene))bis(10H phenothiazine-10,3 diyl))bis(2-cyanoacrylic acid) (0.17 g, 70.83%)
6 (2E,2'E,2''E,2'''E)-3,3',3'',3'''-(10,10'-((hexane-1,6-diylbis(oxy))bis(4,1- phenylene))bis(10H-phenothiazine-10,7,3-triyl))tetrakis(2-cyanoacrylic acid) (6)
Compound 10,10'-(4,4'-(hexane-1,6-diylbis(oxy))bis(4,1phenylene))bis(10H- phenothiazine-3,7-dicarbaldehyde) (0.12 g, 0.15 mmol), dissolved in acetic acid (15 mL) was condensed with cyanoaceticacid (0.13 g, 1.5 mmol) in the presence of ammonium acetate (0.12 g, 1.5 mmol) Then, the reaction mixture was refluxed well
48 for 5 h under inert atmosphere After cooling to room temperature, the reaction mixture was poured into a crushed ice and solid obtained was washed thoroughly with water to remove excess of acetic acid and cyanoacetic acid Finally, washed with hexane to afford a red colored solid (2E,2'E,2''E,2'''E)-3,3',3'',3'''-(10,10'- ((hexane-1,6-diylbis(oxy))bis(4,1-phenylene))bis(10H-phenothiazine-10,7,3 triyl))tetrakis(2-cyanoacrylic acid) (0.1 g, 62.5%)
4-bromophenol (4 g, 0.023 mol), 1-bromohexane (4.96 g, 0.03 mol) and anhydrous potassium carbonate 9.54 g, 0.43 mol) were heated in refluxing THF (40 mL) under nitrogen for overnight After cooling to room temperature, the reaction mixture was filtrated, the solvent was removed under reduced pressure and the product was purified by column chromatography on silica gel with dichlomethane/hexane (1/10) to give product as white liquid in (4.8 g) 81.22% yield
A mixture of 1-bromo-4-(hexyloxy)benzene (2 g, 0.008 mol), Sodium-tert- butoxide (2.3 g, 0.024 mol), and Palladium(II) acetate (0.18 g, 10% mol), Tri-tert- butylphosphine (0.16 , 10% mol) in Tetrahydrofuran (30 mL) and the mixture was degassed in 15 min After that phenothiazine (2.07 g; 0.01 mol) dissolved in THF was added and the mixture reaction was stirred at 65 0 C for 18 h After cooling the room temperature, inorganic salts were filtered The solvent was removed under reduced pressure The mixture reaction was extracted three times with dichlomethane in the presence of sodium chloride and was dried by anhydrous MgSO4 Purification
49 of the crude product was carried out by column chromatography using silica gel and ethyl acetate/hexane (1/10) to give product as white solid (2.51 g) in 83.22% yield
POCl3 (2.05 g, 13.35 mmol) was added drop by drop into DMF (0.97 g, 10.53 mmol) at 0 0 C under nitrogenous environment with continuous slow stirring A solid glassy material formed which was dissolved in 1,2-dichoroethane (15 mL) and stirred for additional 30min Then 10-(4-(hexyloxy)phenyl)-10H-phenothiazine (1 g, 2.67 mmol) dissolved in 1,2-dichoroethane l (20 mL) was injected into the flask and maintained at 70 0 C for 6 h After cooling, the reaction mixture was poured into ice water (100 mL) The product was extracted with dichlomethane three times and dried over anhydrous MgSO4 The solvent was removed by rotary evaporator The concentrated mixture was subjected to column chromatography using silica gel and acetate/hexane (1/10) as mobile phase Bright yellow color crystals of product were obtained in (0.82 g) 75.93% yield
10 (Z)-2-cyano-3-(10-(4-(hexyloxy)phenyl)-10H-phenothiazin-3-yl)acrylic acid (10)
Compound 10-(4-(hexyloxy)phenyl)-10H-phenothiazine-3-carbaldehyde (0.2 g, 0.5 mmol), dissolved in acetic acid (15 mL) was condensed with cyanoacetic acid (0.21 g, 2.5 mmol in the presence of ammonium acetate (0.19 g, 2.5 mmol) Then, the reaction mixture was refluxed well for 5 h under inert atmosphere After cooling to room temperature, the reaction mixture was poured into a crushed ice and solid obtained was washed thoroughly with water to remove excess of acetic acid and cyanoacetic acid Finally, washed with hexane to afford a red colored solid of (Z)-2-
50 cyano-3-(10-(4-(hexyloxy)phenyl)-10H-phenothiazin-3-yl)acrylic acid (0.18 g, 62.5%)
All 1 H NMR spectra are recorded on a Varian Mercury NMR 300 Hz spectrometer using CDCl3 and DMSO-d6 purchased from Cambridge Isotope Laboratories, Inc Elemental analyses are performed at the Center for Organic Reactions using an elemental analyzer (EA1112, Thermo Electron Corporation) The optical extinction spectra are recorded using a UV-vis/NIR spectrophotometer (845X, Agilent) The redox properties of the three dyes are examined by cyclic voltammetry (BASi, EC-epsilon) The electrolyte is 0.1 M Bu4NPF6 in DMF solvent, Ag/AgCl and Pt wire (0.5 mm in diameter) electrodes are used as the reference and counter electrodes, respectively The scan rate is 100 mV/s
4.2.3 Assembly and Characterization of the DSSCs
The DSSCs are fabricated as follows The conducting glass substrate (FTO;
TEC8, Pilkington, 8 Ω/cm 2 , thickness of 2.3 mm) is cleaned in ethanol by ultrasonication TiO2 pastes (TiO2 particles size: approximately 20-30 nm) are prepared using ethyl cellulose (Aldrich), lauric acid (Fluka), and terpineol (Aldrich)
The prepared TiO2 paste is coated on a pre-cleaned glass substrate using a doctorblade, and sintered at 450 0 C for 30 min The thickness of the sintered TiO2 layer (8 àm) is measured using a surface profiler (Alpha-step IQ surface profiler, KLA Tencor) The TiO2 paste is recoated over the sintered layer using approximately 250 nm sized TiO2 particles as a scattering layer and sintered again at 450 0 C for 30
Results and discussion
The absorption properties of the dyes in the solvent ethanol and sensitized on nanocrystalline TiO2 films and the normalized absorption and emission spectra of these dyes are shown in Figure 4.3 and Figure 4.4, the corresponding data are summarized in Table 4.1
Figure 4.3 UV-vis absorption spectra of dye sensitizers T1-T3 in solution
In the solution (Figure 4.3), it’s observable that all of the dyes exhibit two absorption bands The first one appearing at 280–340 nm is assigned to the localized π-π* transition, another at the longer wavelengths pointing at 380–530 nm is
53 attributed to intramolecular charge transfer (ICT) between the phenothiazine donor and terminal cyanoacetic acid acceptor The absorption maximum for dyes T2 (424 nm) and T3 (460 nm) exhibit a red-shift compared with that of T1 (422 nm), the red- shifted absorption spectra should be ascribed to the lower energy gaps which result from the introduction of single electron donor and acceptor groups
Moreover, it is interesting to compare the absorption spectra of dyes T2 and T3, the four anchoring groups attached to double branches of phenothiazine in dye T3 finally results in a higher molar extinction coefficient (ɛ = 24944 M -1 cm -1 ) of T3 because of the LUMO is mainly located at the acceptor substructure, which is directly adsorbed to the TiO2 surface, an easy electron transfer is provided
Dye εmax (M -1 cm -1 ) λmax (nm) Sol TiO2
Table 4.1 Photophysical and electrochemical data of dye sensitizers T1-T3
The UV–vis absorption spectra of these dyes adsorbed on TiO2 films are exhibited in Figure 4.4 From Figure 4.4, the tendency of the absorption spectra on TiO2 film is consistent with that in the solution (Figure 4.3) However, the maximal absorption peaks for T1, T2 and T3 are red-shifted by 26, 25 and 43 nm from 422, 424 and 460 nm in the solution to 448, 449 and 503 nm on TiO2 films, respectively
The red-shifted absorption peaks on TiO2 films of T1, T2, and T3 are ascribed to the
54 J-aggregation of the dyes on TiO2 surface, which always occurs with the presence of carboxyl groups in the molecules [53] Apparently, the maximum absorption peaks of T3 present much more red-shifted relatively to that of T1 and T3, indicating that dyes T4 have a stronger tendency to form J-type aggregation on TiO2 surface
Figure 4.4 UV-vis absorption spectra of dye sensitizers T1-T3 on TiO 2 film 4.3.2 Electrochemcal properties
To fabricate efficient DSSCs, besides the light harvesting yield of a dye- adsorbed TiO2, it is also of importance that there dyes are favorable energy offsets of the dye molecules with respect to the TiO2 nanocrystals and redox electrolytes Here, the electrochemical behaviors of these dyes were measured by cyclic voltammetry (Figure 4.5) and energetic data are listed in Table 4.1 The oxidation potentials (Eox) correspond to the highest occupied molecular orbitals (HOMO) The HOMO levels of T1, T2, and T3 are positive than that of the iodide/tri-iodide redox potential value
55 (0.4 V vs NHE), which indicate that the oxidized dyes may be efficiently regenerated by the electrolyte It is worthy to note that the introduction of phenothiazine with single and double branches can large difference between iodide/tri-iodide potential and the HOMO level is, favorable for the dye regeneration
Figure 4 5 Cyclic voltammogram of dye sensitizers T1-T3
Furthermore, based on the band gaps (E0-0) estimated from the onset of the UV visible absorption spectra, the lowest unoccupied molecular orbitals (LUMO) were obtained And the LUMO levels of all dyes are more negative than the conduction band edge (CB) of TiO2 (0.5 V vs NHE), indicating that electron can be energetically injected into the TiO2 conduction band from the excited dyes
4.3.3 Photovoltaic performances of the DSSCs
The current–voltage (J–V) curves of DSSCs based on these dyes are shown in Figure 4.6 The detailed parameters of short-circuit current density (Jsc), open-circuit
56 voltage (Voc), fill factor (FF) and overall conversion efficiency (CE) are summarized in Table 4.2
Figure 4.6 J-V curves for the DSSCs based on dye sensitizers T1-T3
Among these dyes, the cell based on T2 exhibited a highest efficiency of 5.02%
(Js = 10.81 mA/cm 2 , Voc= 0.68 V, FF= 68.07%) under standard global AM 1.5G solar light irradiation Under the same measuring conditions, the cells sensitized with T1 and T3 gave Jsc of 10.51 and 11.82 mA/cm 2 , Voc of 0.67 and 0.64 V and FF of 67.71 and 64.86 %, corresponding to efficiency of 4.75 and 4.93%, respectively The highest efficiency of the cell based on T3 due to its relatively higher Jsc, which could be ascribed to its broad and higher light harvesting efficiency in the visible region
Sample J sc (mA/cm 2 ) V oc (V) FF(%) CE(%)
Table 4.2 Photovoltaic data of the DSSCs based on dye sensitizers T1-T3
The incident monochromatic photon-to-current conversion efficiency (IPCE) spectra of the DSSCs are plotted in Fig.4.7 The IPCE values of DSSCs based on T2 exceed 70% from 450 nm to 550 nm with a maximum value of 73.5% at 480 nm and tail-off toward 670 nm While maximum IPCE of the dyes T1 and T3 overdoes 71 and 69% at 460 and 510 nm with tail off toward 660 and 700 nm, respectively
Figure 4.7 IPCE spectra for the DSSCs based on dye sensitizers T1-T3
58 Photovoltaic properties of DSSCs were affected by the amount of the dye adsorbed onto the TiO2 surface Generally, the increasing order of efficiency and IPCE of fabricated devices can be explained by the dye loading amount Keeping this in view, to estimate the total amount of dye adsorbed on the TiO2 surface, desorption of the dye from the TiO2 was done using 0.1 M NaOH in DMF/H2O (9:1) mixture The obtained data are summarized in Table 4.2 From the results it is quite evident that amount of dye T1 on TiO2 surface was higher than that of other dyes and the amount of dye T2 on TiO2 surface was lowest of 7.86 mmol/ cm 2
EIS was employed under illumination and dark conditions to evaluate the interfacial charge-transfer/recombination processes in the DSSCs containing these dyes Figure 4.8 shows the Nyquist plots; the high-frequency region represents the series resistance (R1), corresponding to the diameter of the first semicircle; the larger semicircle in the mid-frequency region reflects (R2); the charge transfer/recombination resistance (R3) at the TiO2/dye/electrolyte interface
Figure 4.8 Electrochemical impedance spectra measured under illuminated condition
As shown in Table 4.3, there was little difference in the R2 values because the same counter electrode (Pt) and electrolyte was used On the other hand, there was a substantial difference in the R3 values, which indicates that the charge transfer behavior between TiO2 and the electrolyte is changed significantly, which is due probable to surface modification with different number of donor and acceptor groups of dyes R3 values of 17.94, 17.35, 17.70, 19.96 (Ω) were obtained for the T1, T3, T4 based devices, respectively
Table 4.3 EIS analysis of the DSSCs under illumination condition
Figure 4.9 shows the Nyquist plots for the DSSCs based on the dyes, and Table 4.4 lists the corresponding parameters under dark condition
Figure 4.9 Electrochemical impedance spectra measured in the dark for DSSCs sensitized by T1-T3
Under dark impedance analysis, recombination resistance (R3) values of 37.05, 43.21, 28.11 and 20.55 Ω were obtained for the T1, T2 and T3 based devices, respectively
Table 4.4 EIS analysis of the DSSCs under dark condition
To confirm the electron life-times in the DSSCs, we also measured open- circuit photovoltage decay (OCVD) curves The OCVD technique is a method of monitoring the subsequent decay of photovoltage Voc after turning off the illumination in a steady state Figure 4.10 shows that Voc decay curves of DSSCs with organic phenothiazine based photosensitizers were recorded during relaxation from an illuminated quasi-equilibrium state to the dark state
Figure 4.10 Voc decay curves of DSSCs with T1- T3 based organic photosensitizers
Figure 4.11 shows electron life-times calculated from the OCVD curves, according to the following equation:
Conclusions
Three novel dyes (T1-T3) based on adjustments of phenothiazine with single and double donor-acceptor, introduced of the alkoxy group present in molecular and different number of anchoring groups were designed and synthesized The dye T3 with double branches phenothiazine and multiple acceptor was found higher to the
64 dye T1 with single branch in deference to effective suppression of the charge recombination, increased Voc and electron lifetime Furthermore, the dye T2 with double branches of phenothiazine with a hexyloxy benzene unit prominently displayed the best results in all parameters with Jsc of 10.81 mA/cm 2 , Voc of 0.68 V, FF of 68.07% and ɳ of 5.02% This work provides valuable and essential information for developing and designing new organic dyes for highly efficient DSSCs Further structural modifications of the dye T2 to obtain even better power conversion efficiency of DSSCs performance is in development
CONCLUSION
As one of the crucial parts in DSSCs, the sensitizer is one of the key components of these cells for high power conversion efficiency It is still challenging to search for optimum sensitizers which are capable of absorbing the whole region of visible light to get the high power conversion efficiency Promising strategies to gain higher molecular absorptivity of sensitizers, metal-free organic dyes-based ones in this case The objective of this work is to improve the photovoltaic performance of DSSCs using different types of sensitizer
In the chapter 3, three triphenylamine-based dyes (D1-D3) with different electron anchoring/acceptors, 2-cyanoacetic acid (CA), rhodanine-3-acetic acid (RA) and 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid (OPCA), were designed and synthesized as sensitizers for DSSCs The effects of the different electron acceptors on the photophysical, electrochemical and photovoltaic properties were examined
The dye with the OPCA acceptor unit showed the longest maximum absorption wavelength and the dye with the CA acceptor unit showed highest molar absorption coefficient The overall conversion efficiency of the DSSC based on dye with the CA unit 1.27% was higher than those of the DSSCs based on the dye with the RA unit 0.91% and the dye with OPCA unit 0.70% Cyanoacetic acid proved to be the best electron acceptor in the D-π-A dye for improving the high cell conversion efficiency
The phenothiazine-based dye containing electron-rich nitrogen and sulfur heteroatoms in a heterocyclic structure with high electron-donating ability, and its
66 non-planar butterfly conformation can sufficiently inhibit molecular aggregation and the formation of intermolecular excimers Meanwhile, the N(10)-substituent on can further enhance the charge separation at the oxide solution interface The structural features of phenothiazine-based dye make it a promising type of sensitizers for DSSCs Thus, in the chapter 4, a series of new organic dyes (T1-T3) based on the phenothiazine unit were synthesized, in which a hexyloxy benzene unit as a π-bridge to connect double phenothiazine as a donor, and the multiple anchoring groups was introduced The T2 based device show a bester conversion efficiency compare with T1 and T3 of 5.02% A novel intramolecular sensitization strategy to enhance the optical response and, ultimately, the photovoltaic efficiency of DSSCs was demonstrated In particular, we believe that this new strategy will facilitate the search for new efficient sensitizers
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72 염료감응태양전지는 광전변환 소자기술로, 1991 년 스위스 Gratzel 연구진이 처음 보고한 이후 저렴한 생산비용에 비해 높은 광전변환효율의 장점으로 많은 관심을 받아왔다 염료감응태양전지는 TiO2로 구성된 나노입자, 태양광 흡수율을 높이는 염료, 전해질, 투명전극 등으로 구성된 광합성원리를 이용한 유기태양전지의 한 종류이다 염료감응태양전지는 기존의 일반화 되어 있는 실리콘방식 태양전지와는 달리 자원적인 제약이 적고 대기압하에서, 프린팅 방식 등을 이용하여 제조할 수 있는 장점이 있어 고가의 설비투자를 줄이고 대량 제조가 용이한 것이 장점이다 또한 태양광의 입사각도, 날씨변화 등에 대한 전기발전효율이 우수하고 실내에서의 간접광에 의해서도 발전이 가능한 특징을 가지고 있다 그리고 유기염료가 흡착되므로 칼라를 구현할 수 있어 다양한 칼라 디자인을 접목할 수
염료감응 태양전지용 기능성 광 감응제의 합성과 구조 최적화
지도교수 김재홍 레 티 투이
요약 영남대학교 대학원
화학공학과 - 화학 공학 전공