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MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ---PHAN DINH LONG RESEARCH ON THE SYNTHESIS OF ORGANIC SEMI

MINISTRY OF EDUCATION

AND TRAINING

VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY

-PHAN DINH LONG

RESEARCH ON THE SYNTHESIS OF ORGANIC SEMICONDUCTING METERIALS FOR OPTOELECTRONIC

The thesis was completed at Institute of Chemistry-Vietnam Academy of Science and Technology

Supervisors 1: Dr Hoang Mai Ha

Supervisors 2: Assoc Prof Dr Nguyen Phuong Hoai Nam

The thesis could be found at:

- National Library of Vietnam

- Library of Graduate University of Science and Technology

INTRODUCTION

1 Necessity of the thesis

Organic materials are gradually replacing inorganic materials in all areas of science, technology and life In the field of optics, electricity and electronics, organic materials have showed many superior properties, such as: soft, light, easy to manufacture on a large scale and relatively low cost In particular, the research direction of manufacturing organic optoelectronic devices such as organic light emitting diodes (OLEDs), organic solar cells (OSCs), and organic field effect transistors (OFET) have strongly developed in recent years However, in comparison with inorganic materials, organic semiconductors still exhibit major disadvantages such as low carrier mobility, low power conversion efficiency and low durability Therefore, the study to overcome these disadvantages is an urgent task to apply this material into practice

In recent years, the optoelectronic industry has developed and made a great contribution to the economy of Vietnam Some researches on the fabrication of OPV and OLED components have been done over last years However, so far, there are very few domestic research groups which can synthesize organic semiconducting materials In order to approach a new and potential research

direction, we choose the topic: "Research on synthesis of organic semiconducting

for optoelectronic applications"

2 Research objectives of the thesis

The thesis focus on the synthesis of new semiconducting polymers including wide band-gap copolymers, low band-gap copolymers and terpolymers The optical properties, electrical properties, crystal structure and morphology of these polymers were investigated These polymers were applied to OFET and OPV fabrication

3 Main research contents of the thesis

Overview of organic semiconducting materials and organic optoelectronic devices

Synthesis of copolymers based on diketopyrropyrole group (DPP6T-C4) Synthesis of wide band-gap copolymers T-3MT and 2T-3MT

Synthesis of terpolymers 3MTB and 3MTT

Study on the optical properties, electrochemical properties, semiconducting properties of polymers

Study of crystal structure and morphology of synthesized polymers

Fabrication of optoelectronic devices: Organic solar cells (OPV) and organic field effect transistors (OFET)

Chapter 1 INTRODUCTION 1.1 π-Conjugated Organic Materials

The name organic semiconductor denotes a class of materials based on carbon that

display semiconducting properties, the common characteristics is that the electronic

structure is based on π-conjugated double bonds between carbon atoms The

delocalization of the electrons in the π-molecular orbitals is the key feature, that

allows injection delocalization and charge transport Semiconductivity may be

exhibited by single molecules, oligomers and polymers Semiconducting small

molecules include the polycyclic compounds as pentacene, anthracene, rubrene,

Polymeric organic semiconductors include poly(3-hexylthiophene),

poly(p-phenylene vinylene), polyacetylene, polyfluorenes

1.2 Some key reactions in the synthesis of conjugated structures

1.2.1 Suzuki coupling reaction

1.2.2 Stille coupling reaction

1.2.3 Heck coupling reaction

1.2.4 Sonogashira coupling reaction

1.3 Organic electronic devices

1.3.1 Charge Transport in Organic Semiconductors

The theory of charge transport in organic semiconductors has been reviewed

extensively, and many transport models have been proposed based on the

well-documented behavior of inorganic semiconductors However, the exact mechanisms

of charge injection and transport are still seriously debated The general

mechanisms that are pertinent to the design of new semiconducting materials are

outlined here, but for more detailed discussions, one may refer to the papers cited in

this section In classical inorganic semiconductors such as silicon, atoms are held

together with strong covalent or ionic bonds forming a highly crystalline

three-dimensional solid Therefore, strong interactions of the overlapping atomic orbitals

cause charge transport to occur in highly delocalized bands that are mainly limited

by defects, lattice vibrations, or phonon scattering in the solid In contrast, organic

semiconductors are composed of individual molecules that are only weakly bound

together through van der Waals, hydrogen-bonding, and π-π interactions and

typically produce disordered, polycrystalline films Charge delocalization can only

occur along the conjugated backbone of a single molecule or between the π-orbitals

of adjacent molecules Therefore, charge transport in organic materials is thought to

rely on charge hopping from these localized states

1.3.2 Organic field effect transistor (OFET)

The organic field effect transistor (Organic Field Effect Transistor-OFET)

was first made by A.Tsumara in 1986 Since then a lot of research has been done to

improve material quality and improve manufacturing methods of accessories The

organic field effect transistor is of great interest because the semiconductor layer

can be created at low temperatures on a large and flexible area at low cost

1.3.3 Organic photovoltaic cell (OPV)

The first organic solar cell was C.Engel of Eastman Kodak was successfully built in 1986 The term photovoltaic is derived from a combination of words: light (photo) and electricity (voltaic) in Greek Solar cells are capable of converting light into electricity Compared to inorganic solar cells, organic solar cells have many advantages such as: easier manufacturing technology; flexibility, transparency; highly variable, highly flexible; light and low cost

1.4 Overview of research situation on the synthesis of organic semiconductor materials

1.4.1 Research situation in the world

1.4.1.1 Semiconducting polymers bearing only donor groups

Representative polymers developed for polymer-based solar cells are poly (3-hexylthiophene) (P3HT), poly (1,4-phenylene-vinylene) (PPVs), and poly [2-methoxy-5- (3,7-dimethyloctyloxy) -1,4-phenylene-vinylene] (MDMO-PPV) that have been extensively studied

1.4.1.2 Conjugated donor (D)/acceptor (A) copolymers

In other studies, one of the most effective methods to narrow the band gap

of a major polymer is to combine two types of monomers with different electronic nature, one that is rich in electronics - called the donor parts (such as fluorene, carbazole, dibenzosilole, benzodithiophene) and the other component is acceptor, (such as benzodiathiazole and diketopyrrolopyrrole)

In recent years, many D-A conductive polymers have been synthesized and used to fabricate electronic devicescomponents Especially in the field of application to manufacture PSC, according to published documents The conversion

of photovoltaic energy into electricity has reached 10 to more than 12% When there

is fluorine atoms in the molecular composition of polymers, the performance of PSC components made from this polymer is improved in the direction of increasing After nearly 20 years of development, the tendency to synthesize conjugated polymers for the manufacture of optoelectronic components from polymers only carries the group for electronics to structural copolymers and terpolymers The synthesis of conjugated polymers aims to control the energy levels of HOMO and LUMO, improve solubility, and control the structure so that polymers with crystals

are suitable for different applications

1.4.1.3 Copolymers based on the Diketopyrrolopyrrople group (DPP)

Recently, diketopyrrolopyrrole (DPP) has been used extensively in the synthesis of polymers for manufacturing field-effect transistor components (OFET) and organic solar cells (OPV), as they have a strong electron affinity and symmetrical heterocyclic structure creates a flat structure with very strong foreign molecular interactions The solubility of these polymers can be controlled by

changing the alkyl chain length at 2 N positions of the DPP group

1.4.1.4 Wide ban-gap copolymers

Among the various types of polymer donors, donor−acceptor (D−A)-type conjugated copolymers offer high performance in non-fullerene PSC devices However, the absorption spectra of many kinds of D−A type copolymers overlap with those of non-fullerene acceptors because of the strong intramolecular charge transfer (ICT) between the donating and accepting units in the polymer backbone The design and synthesis of wide-bandgap conjugated polymers is one strategy for enhancing the light-harvesting absorption area The combination of a non-fullerene acceptor and wide bandgap polymer can produce relatively broad complementary absorption behavior, which can improve the external quantum efficiency (EQE) of the final PSC and thus also increase the short-circuit current

1.4.1.5 Terpolymers

Over the past few years, random terpolymers bearing one donor and two different acceptors (or one acceptor and two different donors) having conjugated polymer backbones have emerged as promising materials with many advantages over electron-donating binary copolymers owing to the ability to fine-tune the internal morphology of the active layer, solubility, absorption range, energy levels, and polymer chain orientation of the former in blend films Therefore, highly efficient nonfullerene PSCs can be realized by using specific terpolymer systems and simple device fabrication processes

1.4.1.6 Synthesis of acceptor

From 2000 to 2010, fullurene derivatives were used as an acceptors in PSC The integration of acceptors in recent years has been developed in the following three directions:

Non-fullerene acceptors with wide band gap energy (Eg> 1.9 ev)

Non-fullerene acceptors with medium forbidden region energy (1.9 eV> Eg > 1.5 ev )

Non-fullerene acceptors with narrow-level region energy (Eg< 1.5 eV)

1.4.2 Domestic research situation

Overview the published literatures, nowadays, domestic studies have very little research on the synthesis of conductiing polymer, mainly focusing on the fabriction of of opto electronic devices

The research team of Pham Hong Quang, University of Natural Sciences is also working on making solar cells but based on CIGS thin film structure

Meanwhile, the synthesis of semiconducting polymers in Vietnam is still in its infancy Beside the research group of Hoang Mai Ha at the Institute of Chemistry, only the research group of Nguyen Tran Ha at National University of Ho Chi Minh City has synthesized some polymers with conjugate structure and the first step has been made organic solar cells using these polymers

CHAPTER 2 EXPERIMENT

2.1 Chemicals and equipment

2.1.1 Chemicals and solvents

All chemicals used to synthesize copolymers and receiver elements used in this study were purchased from Tokyo Chemical Industry Co., Ltd (TCI), Sigma-Aldrich and ACROS Co

2.1.2 Instrumentation

The molecular weights of the polymers were determined by gel permeation

chromatography (GPC, Agilent 1200 series GPC system) with o-dichlorobenzene

(ODCB) as the eluent (T58088C) on an Agilent GPC 1200 series instrument, relative to a polystyrene standard C, H, N, and S elemental analysis was performed

on an EA1112 (Thermo Electron Corp., West Chester, PA, USA) elemental analyzer The absorption spectra of the polymers as thin films and solutions (chloroform, conc 1 × 10−5 mol L−1) were obtained using a UV-vis absorption spectrometer (Agilent 8453, photodiode array-type) The oxidation potentials of the two copolymers were measured using cyclic voltammetry (Model: EA161 eDAQ) The employed electrolyte solution contained 0.10 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile Ag/AgCl and Pt wire (0.5 mm in diameter) electrodes were used as the reference and counter electrodes, respectively (scan rate = 20 mV s−1)

Grazing incidence X-ray diffraction (GI-XRD) measurements were carried out at the 3C (SAXS I) beamlines (energy = 11.040 keV, pixel size = 79.6 μm, wavelength

= 1.126 Å, 2 = 0°–20°) at the Pohang Accelerator Laboratory, Korea The

parameters qxy and qz represent the components of the scattering vectors parallel and perpendicular to the film surface, respectively Atomic force microscopy (AFM, Advanced Scanning Probe Microscope, XE-100, PSIA, tapping mode with a silicon cantilever) was used to characterize the surface topographies of the thin-film samples

2.2 Synthesis of polymers

2.2.1 Synthetic scheme

Scheme 2.1 Synthetic scheme of conjugated polymers

2.2.2 Synthesis of narrow-band gap polymer based on diketopyrrolopyrrole group (P (DPP6T-C4))

to room temperature, 60mL is added to the reaction mixture and the mixture is

Catalyst + Monomer + Solvent

stirred for another 10 minutes to precipitate the polymer The precipitated polymer

is filtered and purified by Soxhlet extraction method with acetone, tetrahydrofuran (THF), and chloroform to collect the insoluble polymer Next, this polymer part is dissolved in hot 1.2-diclorobenzene solvent Insoluble particles in 1,2-diclorobenzen are filtered out The polymer solution is then concentrated and precipitated by methanol to obtain 153 mg of dark green products, fusion efficiency of 54%

Determination of structure of P(DPP6T-C4)

1H-NMR spectrum of this polymer has only 1 characteristic peak for conjugated heterocyclic at δ 7.04ppm-7.09ppm Typical peaks for the 5-decylheptadecyl group are shown relatively clearly on 1H and 13C-NMR spectra Anal Calcd for (C86H128N2O2S6)n, Found: C, 73.22%; H, 9.04%; N, 1.97%; O, 2.32% and S, 13.45%

Gel chromatography method is used for mass determination of polymer: Mn = 13292; Mw = 37801; PDI = 2,844

2.2.3 Synthesis of wide band gap copolymers T-3MT and 2T-3MT

2.2.3.1 Synthesis of T-3MT

Scheme 2.3 Synthesis of T-3MT

Pd2(dba)3(0) (9.2 mg, 10.0 μmol) and P(o-tolyl)3 (12.2 mg, 40.0 μmol) were added to a solution of compound 18 (0.247 g, 0.2 mmol) and compound 24 (0.06 g, 0.2 mmol) in toluene:DMF (9:1 v/v, 20 mL) at room temperature under

Ar atmosphere and the reaction mixture was allowed to stir at 100 °C for 24 h The cooled reaction mixture was poured into methanol (200 mL) to precipitate the copolymer The precipitated polymer was purified by Soxhlet extraction with methanol, acetone, and chloroform, successively After reducing the volume of chloroform fraction in vacuo, the copolymer was then precipitated in methanol The final product was obtained in 83% yield

Determination of structure of T-3MT

1H-NMR (500 MHz, CDC13):(ppm) 7,90 (s, 1H); 7.52 (s, 1H); 7,49 (s, 2H), 7,35 (s, 1H);7,18 (d, 8H); 7,11 (d, 8H);3,84 (s, 3H); 2,56(t, 8H);1,52-1,59 (m, 8H); 1,28-1,33 (m, 24H);0,86(t, 12H)

13C NMR (125MHz, CDCl3): (ppm) 163,26; 141,89; 140,02; 128,57; 128,05; 35,61; 31,71; 31,25; 29,18; 22,59; 14,07

Anal Calcd for (C70H76O2S3)n: C, 80.41; H, 7.33; S, 9.20 Found: C, 80.23; H, 7.41; S, 9.18

Gel chromatography method to determine the mass of polymer: Mn= 33,2kDa, PDI = 2.01

2.2.3.2 Synthesis 2T-3MT

Scheme 2.4 Synthesis of 2T-3MT

Pd2(dba)3(0) (9.2 mg, 10.0 μmol) and P(o-tolyl)3 (12.2 mg, 40.0 μmol) were added to a solution of compound 23 (0.269 g, 0.2 mmol) and compound 24 (0.06 g, 0.2 mmol) in toluene:DMF (9:1 v/v, 20 mL) at room temperature under Ar atmosphere and the reaction mixture was allowed to stir at 100 °C for 24 h The cooled reaction mixture was poured into methanol (200 mL) to precipitate the copolymer The precipitated polymer was purified by Soxhlet extraction with methanol, acetone, and chloroform, successively After reducing the volume of chloroform fraction in vacuo, the copolymer was then precipitated in methanol The final product was obtained in 88% yield

Determination of structure of 2T-3MT

1H-NMR(500MHz, CDC13): (ppm)7,91 (s, 1H); 7.52 (s, 1H); 7,50 (s, 2H), 7,36 (s, 1H); 7,18 (d, 8H); 7,10 (d, 8H); 3,83 (s, 3H); 2,56(t, 8H);1,54-1,59 (8H); 1,26-1,34 (m, 24H);0,86(t, 12H)

13C NMR (125 MHz, CDCl3): (ppm) 163,32; 141,96; 139,99; 128,57; 128,04; 35,61; 31,72; 31,25; 29,19; 22,60; 14,08

Anal Calcd for (C74H76O2S5)n: C, 76.77; H, 6.62; S, 13.85 Found: C, 76.51; H, 6.67; S, 13.78

Gel chromatography method to determine the mass of polymer: Mn = 26,7kDa, PDI = 2,21

2.2.4 Synthesis of terpolyme 3MTB and 3MTT

2.2.4.1 Synthesis of 3MTB

Scheme 2.5 Synthesis of 3MTB

Monomer 29 (181 mg, 0.2 mmol), monomer 24 (11.7 mg, 0.04 mmol), and monomer 30 (47.7 mg, 0.16 mmol) were dissolved into dried toluene (10 mL) and dimethylformamide (DMF; 1 mL) and the solution was then degassed by bubbling with nitrogen for 10 min Subsequently, the catalyst (Pd2(dba)3(0); 9.2 mg, 10.0 μmol) and P(o-tolyl)3 (12.2 mg, 40.0 μmol) were added to the solution and the reaction mixture was stirred at 100 oC for 24 h The solution was cooled to room temperature and poured into 300 mL of methanol to obtain the precipitated polymer The crude product was purified by successive Soxhlet extraction with acetone, hexane, and chloroform The chloroform fraction was concentrated to the minimum volume and the solution was then precipitated in methanol, filtered, and dried to yield the target 3MTB terpolymer The final product was obtained in 89%

Determination of structure of 3MTB

1H-NMR (500MHz, CDC13): (ppm) 8,02; 7,56-7,67; 7,26-7,34; 6,93; 3,84; 2,90-2,98;1,44-1,55; 0,98

13C NMR (125MHz, CDCl3): (ppm) 163,56; 162,90; 137,30; 132,88; 128,04; 124,36; 41,49; 34,38; 32,74; 28,97; 25,86; 23,18; 23,07; 14,28; 10,98 Elemental Anal Calcd for (C67H44O2S5)0.8(C40H42N2S5)0.2: C, 67.11; H, 6.13; N, 0.79; S, 22.40 Found: C, 67.02; H, 6.18; N, 0.81; S, 22.35

Gel chromatography method to determine the mass of polymer: Mn=17.1 kDa, PDI = 2.46

mg, 40.0 μmol) were added to the solution and the reaction mixture was stirred at

100 oC for 24 h The solution was cooled to room temperature and poured into 300

mL of methanol to obtain the precipitated polymer The crude product was purified

by Soxhlet extraction with acetone, hexane, and chloroform, successively The chloroform fraction was evaporated properly and the polymer solution was then precipitated in methanol, filtered, and dried to give the target 3MTT terpolymer The final product was obtained with 83% yield (Mn = 12.1 kDa, PDI = 3.07) Elemental Anal Calcd for (C67H44O2S5)0.8(C62H74N2O4S6)0.2: C, 67.09; H, 6.30; N, 0.51; S, 21.37 Found: C, 66.97; H, 6.34; N, 0.53; S, 21.26

Determination of structure of 3MTT

1H-NMR (500MHz, CDC13): (ppm)8,02; 7,65-7,72; 7,26-7,35; 6,93; 3,81; 2,90-2,96; 1,73-1,79;1,43-1,53; 0,97-1,04

13C NMR (125MHz, CDCl3): (ppm) 154,63; 137,24; 41,50; 34,47; 32,78; 29,00; 25,75; 23,35; 23,15; 14,13; 10,96

Elemental Anal Calcd for (C67H44O2S5)0.8(C62H74N2O4S6)0.2: C, 67.09; H, 6.30; N, 0.51; S, 21.37 Found: C, 66.97; H, 6.34; N, 0.53; S, 21.26

Gel chromatography method to determine the mass of polymer: Mn=17.1 kDa, PDI = 2.46

2.3 Organic thin film transistors (OTFT) Fabrication

To study charge transport properties of the synthesized polymers, BGTC TFT device structure were employed The gate electrode was n-type doped 〈100〉 silicon wafer and the SiO2 gate insulator has a thickness of 300 nm The substrate was cleaned with acetone, cleaning agent, deionized water, and isopropanol in an ultrasonic bath The cleaned substrates were dried under vacuum at 120 oC for 1 h, and then treated with UV/ozone for 20 min Then, the wafers were immersed in a 8 mmol/L solution of n-octyltrichlorosilane (OTS) in anhydrous toluene for 30 min to generate an hydrophobic insulator surface The polymer layer was deposited on the OTS-treated substrates by spin-coating polymer solutions (4 mg mL-1) at 1500 rpm for 40 s For annealing the TFTs, the samples were further placed on a hotplate in air at 180oC for 10 min Finally, the source and drain electrodes were prepared using thermal evaporation of gold (100 nm) through a shadow mask with a channel width

of 1500 µm and a channel length of 100 µm Field-effect current-voltage characteristics of the devices were determined in air using a Keithley 4200 SCS semiconductor parameter analyzer The field-effect mobility upon saturation (µ) is

calculated from the equation: IDS = (W/2L)Ciµ(VG - VTH)2, where W/L is the channel width/length, Ci is the gate insulator capacitance per unit area, and VG and VTH are the gate voltage and threshold voltage, respectively

2.4 Fabrication of polymer solar cells

Bulk heterojunction (BHJ) PSCs were fabricated with an inverted device configuration (indium-tin-oxide (ITO)/ZnO/polymer:ITIC nm)/MoO3/ Ag) A thin layer of ZnO was fabricated on the surface of ITO-patterned glass, which was treated with UV-ozone for 20 min After thermally annealing the ZnO layer at 160

°C for 1 h, the active layer was prepared on top of the ZnO layer by spin-coating polymer:ITIC blend solutions with various ratios, dissolved in chlorobenzene Subsequently, the electrodes were deposited on the active layers by thermal evaporation to form a 10 nm MoO3 layer and 100 nm Ag layer (0.04 cm2

photoactive area) A Keithley 2400 source meter was used to investigate the current

density–voltage (J–V) characteristics in the dark and under AM 1.5 G illumination

at 100 mW cm-2, as supplied by a solar simulator (Oriel, 1000 W) An AM 1.5 filter (Oriel) and a neutral density filter were employed to adjust the light intensity The incident light intensity was measured with a calibrated broadband optical power meter (Spectra Physics, Model 404) The external quantum efficiency (EQE)

spectral response was measured using a tungsten halogen light source combined with a monochromator (Spectra Pro 2300, Acton Research)

CHAPTER 3 RESULTS AND DISCUSSION 3.1 Results of synthesis of polymers

3.1.1 Results of synthesis of polymer DPP6T-C4

The results of the elemental analysis show that the P synthesis reaction (DPP6T-C4) obtains a high purity product with yield of 54%

The method of determining molecular weight by gel permeation chromatography shows that the polymer P (DPP6T-C4) has Mn = 13292; Mw = 37801; PDI = 2,844 Thus, the product is a polymer with a relatively large molecular weight and a small distribution

3.1.2 Results of synthesis of polymer T-3MT, 2T-3MT

The synthesized T-3MT and 2T-3MT exhibited good solubility in

tetrahydrofuran (THF), chloroform, and monochlorobenzene The number-average

molecular weights (Mns) and polydispersity indices (PDIs) of T-3MT and 2T-3MT

were measured using gel permeation chromatography (GPC) with dichlorobenzene as the eluent at 80 °C The resulting Mns and PDIs were 33.2 kDa

o-and 2.01 for T-3MT o-and 26.7 kDa o-and 2.21 for 2T-3MT, respectively

3.1.3 Results of synthesis of polyme 3MTB and 3MTT

The synthesized 3MTB and 3MTT exhibited good solubility in THF, chloroform,

and monochlorobenzene The number-average molecular weights (Mns) and polydispersity indices (PDIs) of 3MTB and 3MTT were measured using gel

permeation chromatography (GPC) with o-dichlorobenzene as the eluent at 80 °C The resulting Mns and PDIs of 3MTB and 3MTT were 17.1 kDa and 2.46 for 3MTB and 12.1 kDa and 3.07 for 3MTT, respectively

3.2 Characterization of synthesized polymers

3.2.1 Physical properties and characteristics of optoelectronic devices of P (DPP6T) -C4

3.2.1.1 Optical properties of P (DPP6T) -C4 and P(DPP6T) -C4/PC71BM blend

UV-Vis absorption spectra of P(DPP6T)-C4 and P(DPP6T) -C4/PC71BM (1/2) combination in solution form and thin film on quartz substrate are shown in Figure 3.2 Polymer P(DPP6T)-C4 has a weak absorption band at 400nm-550nm and a strong absorption band in the range of 600nm-800nm