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 APPLICATIONS Major : Organic Chemistry Code : 62.44.01.02 SUMMARY OF DOCTOR THESIS Ha Noi -2019 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 Reviewer 1: … Reviewer 2: … Reviewer 3: … This thesis will be defended at the doctoral thesis evaluation council at Graduate University of Science and Technology - Vietnam Academy of Science and Technology Time……… 2019 The thesis could be found at: - National Library of Vietnam - Library of Graduate University of Science and Technology INTRODUCTION 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" 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 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 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(pphenylene 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 welldocumented 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 threedimensional 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 [2methoxy-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 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 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), SigmaAldrich 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 × 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 Catalyst + Monomer + Solvent Polymerization Precipitation Soxhlet extraction Filter & dry Scheme 2.1 Synthetic scheme of conjugated polymers 2.2.2 Synthesis of narrow-band gap polymer based on diketopyrrolopyrrole group (P (DPP6T-C4)) 10 P(DPP6T)-C4 Svheme 2.2 Synthesis of P(DPP6T-C4) 3,6-bis (5'-bromo-2,2'-bithiophen-5-yl) -2,5-bis (5-decylheptadecyl) pyrrolo [3,4-c] pyrrole-1,4 (2H, 5H ) -dione (7) (275.9mg, 0.2mmol) and 5,5'-bis (trimethylstannyl) -2,2'-bithiophene (10) (98.8mg, 0.2mmol) are dissolved in 20mL toluene in the 3-neck flask The solution is stirred with a magnetic stirrer in an argon inert atmosphere After 10 minutes, tetrakis (triphenylphosphine) palladium (0) Pd(PPh3)4 (23mg, 10mol%) is added to the reaction mixture The reaction mixture is stirred in argon medium at 90° C for hours Then, the mixture is cooled to room temperature, 60mL is added to the reaction mixture and the mixture is 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 characteristic peak for conjugated heterocyclic at δ 7.04ppm-7.09ppm Typical peaks for the 5decylheptadecyl 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 H-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); 1,281,33 (m, 24H);0,86(t, 12H) 13 C 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 H-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) 13 C 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 Determination of structure of 3MTT H-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 13 C 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 h, and then treated with UV/ozone for 20 Then, the wafers were immersed in a mmol/L solution of n-octyltrichlorosilane (OTS) in anhydrous toluene for 30 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 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 After thermally annealing the ZnO layer at 160 °C for 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) 10 spectral response was measured using a tungsten halogen light source combined with a monochromator (Spectra Pro 2300, Acton Research) CHAPTER 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 odichlorobenzene as the eluent at 80 °C The resulting Mns and PDIs were 33.2 kDa and 2.01 for T-3MT and 26.7 kDa 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 11 1.0 (a) (i) (ii) Absorbance (a.u.) 0.8 0.6 0.4 0.2 0.0 400 600 800 1000 Wavelength (nm) 1.4 (b) (i) (ii) Absorbance (a.u.) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400 600 800 1000 Wavelength (nm) Figure 3.6 UV-Vis absorption spectrum of P(DPP6T) -C4 (a) and P(DPP6T) C4/PC71BM combination (1/2) (b) in solution form (i) and thin film (ii) 3.2.1.2 Electrochemical properties of polymer P(DPP6T) -C4 Electrochemical properties of polymer P(DPP6T) -C4 in the film shows that this polymer has an energy level of HOMO = -5.10 eV Combined with the energy width value of the forbidden region of the polymer film Eg = 1.47 eV obtained from the UV-Vis absorption spectrum, we calculated the energy level LUMO = -3.63 eV These energy levels are suitable for making OPV components 3.2.1.3 Crystal structure of P(DPP6T) -C4 The structure of P(DPP6T)-C4 is studied by GI-XRD diffraction in membrane form (Figure 3.8) The out-of-plane diagram of this polymer membrane shows intense diffraction peaks, indicating that the polymers have arranged themselves in order to be perpendicular to the base After the polymer film is incubated at 120°C, the intensity of the diffraction peaks strongly indicates that the crystallinity of the polymer film increases after annealing This phenomenon explains the load mobility of polymer increase after annealing 12 105 2.0 (c) (a) 10 Intensity qz (Å-1) 1.5 1.0 P(DPP6T)-C4 (RT) P(DPP6T)-C4 (120oC) 103 0.5 102 -1.5 -1 -0.5 0.5 1.5 qxy (Å-1) 10 12 14 16 18 20 2 (deg) 2.0 P(DPP6T)-C4 (RT) (d) (b) P(DPP6T)-C4 (120oC) Intensity qz (Å-1) 1.5 1.0 102 0.5 -1.5 -1 -0.5 0.5 1.5 qxy (Å-1) 10 12 14 16 18 2 (deg) Figure 3.8 X-ray diffraction scheme of P (DPP6T) -C4 in membrane form: Schematic of 2-dimensional X-ray diffraction at normal temperature (RT) (a) and at 120oC (b), diagram of 1-dimensional X-ray diffraction out-of-plane (c) and in-plane (d) at ambient temperature and at 120oC 3.2.1.4 Semiconductor properties P(DPP6T) -C4 In order to investigate the charge transport of the P(DPP6T)-C4 copolymers, BGTC TFT devices were fabricated via the spin-coating method The output characteristics showed very good saturation behavior and clear saturation currents that were quadratic to the gate bias The carrier mobility (µ) was calculated by using the saturation region transistor equation, IDS = (W/2L)µ C0 (VG-Vt)2, where IDS is the source-drain current, VG the gate voltage, C0 the capacitance per unit area of the dielectric layer, and Vt the threshold voltage The TFTs fabricated with the P(DPP6T)-C4 exhibited carrier mobilities of 0.57 cm2 V-1 s-1 with pristine film and 1.88 cm2 V-1 s-1 with annealed films, respectively, with a high current on/off ratio (>106) and low threshold voltage 3.2.1.5 P-n multi-layer structure OPV device based on P (DPP6T)-C4/PC71BM The structure of OPV device was: ITO/PEDOT: PSS (40nm)/P (DPP6T) C4 (60nm)/PC71BM (30nm)/LiF (0.8 nm)/Al (100nm) Typically, p-n layers are only fabricated by evaporation in vacuum using single molecules In this study, because P (DPP6T) -C4 is not soluble in chloroform solvent while PC71BM is well soluble in chloroform, we can make p-n layers by conventional spin-coating method 13 (a) Normalized Absorbance (a u.) 3.1.6 BHJ-OPV device made from P (DPP6T-C4) and PC71BM The BHJ-OPV device was made using P (DPP6T) -C4 as a donor and PC71BM as an acceptor (Figure 3.7a) The structure of OPV components is: ITO/PEDOT: PSS (40nm)/P combination (DPP6T) -C4: PC71BM (80nm)/LiF (0.8nm)/Al (100nm) The combination of P (DPP6T) -C4 and PC71BM is dissolved in 1,2-diclorobenzene solvent and the solution is spin-coating on the PEDOT layer: PSS 3.2.2 Physical properties and characteristics of optoelectronic devices of T3MT and 2T-3MT 3.2.2.1 Optical properties The UV-vis absorption spectra of the two copolymers in the solution and thin film states are shown in Figures 3.12 The maximum absorption wavelengths in the solution states and films were respectively 523 and 520 nm for T-3MT and 529 and 519 nm for 2T-3MT The spectra of both polymers were featureless with no wellresolved vibronic bands and strong absorption spectral bands from 400 to 600 nm due to intramolecular charge transfer (ICT) between 3MT (the accepting unit) and IDT/IDTT (the donating unit) Although the two polymers had different donating units, their absorption maxima and spectral profiles were quite similar 1.2 T-3MT 2T-3MT 0.9 0.6 0.3 0.0 300 400 500 600 700 Wavelength (nm) Normalized Absorbance (a.u.) 1.2 (b) T-3MT 2T-3MT 0.9 0.6 0.3 0.0 300 400 500 600 700 Wavelength (nm) Figure 3.12 UV-vis absorption spectra of T-3MT and 2T-3MT in solution [solvent: chloroform] (a) and thin film state (b) 3.2.2.2 Electrochemical properties of T-3MT and 2T-3MT 14 T-3MT 2T-3MT 0.003 (a) Current (a u.) 0.002 0.001 0.000 -0.001 -0.002 -0.003 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 E (V) vs Ag/AgCl -5.32 -5.87 -4.30 PC71BM -5.4 ITIC -4.17 -4.14 IM-IDT 2T-3MT Energy levels (eV) T-3MT -3.33 -3.27 (b) -5.72 -6,10 Figure 3.16 Cyclic voltammograms of thin films of T-3MT and 2T-3MT on Pt electrode (a) Energy diagrams of T-3MT and 2T-3MT as p-type copolymers and IM-IDT, ITIC, and PC71BM as n-type small molecules (b) Cyclic voltammetry (CV) was employed to determine the energy levels of the two synthesized polymers, and the corresponding parameters are displayed in Figure 3.16 The HOMO energy was estimated from the onset oxidation potential of the CV curve The HOMO level (-5.32 eV) of 2T-3MT was slightly higher than that of T-3MT (-5.40 eV), which might be due to the relatively stronger donor ability of IDTT The lowest unoccupied molecular orbital (LUMO) energy calculated from the HOMO and optical bandgap was -3.33 eV for T-3MT and -3.27 eV for 2T3MT As shown in Figure 3.16b, both polymers had a sufficient energy level offset for use as p-type semiconductors with the three acceptors 3.2.2.3 Crystalline structure of T-3MT, 2T-3MT and their blend with different acceptor Grazing incidence wide-angle X-ray diffraction (GIWAXD) measurements were conducted for the wide-bandgap copolymers and the polymer:acceptor blend films to better understand the performance of the PSC devices (Figure 3.17 and 3.18) The profiles of the as-cast films of the two polymers in the out–of–plane and 15 in–plane modes did not show clear crystalline behavior The diffraction profiles of the polymer:IM-IDT blend films displayed multiple ring patterns along all radial directions, which is attributed to the crystalline nature of the IM-IDT acceptors Correspondingly, a bright diffraction spot was observed along the qz axis, corresponding to the (010) diffraction of the polymer chains in the out–of–plane profile This observation indicates that the polymer chains of T-3MT and 2T-3MT were arranged in a face-on orientation on the substrate, which might be driven by crystallization of the IM-IDT acceptors On the other hand, the blend films of two polymers and ITIC did not show clear diffraction pattern, implying much less crystalline morphology than that of IM-IDT based blend film The polymer:PC71BM blend films showed typical diffraction ring pattern of PC71BMs or its cluster along all radial directions The polymer donors did not show any crystalline character From these results, it was found that the polymer blends with ITIC or PC71BM are much less crystalline compared to those with IM-IDT, which may explain the lower Jsc and FF In addition, the enhanced Jsc for the PSC employing the polymer and IM-IDT can be ascribed to the predominant face-on orientation of the polymer chains, which might facilitate charge transport via intermolecular hopping in the vertical direction 0 Out-of-plane 0.5 1.0 Out-of-plane 0.5 1.0 1.5 0.5 2.0 1.0 1.5 q (Å2-1) 0 Out-of-plane 0.5 Intensity (a.u.) Intensity (a.u.) 1.0 1.5 In-plane 0.5 2.0 q qxy (Å-1) 1.0 1.5 q(xy) (Å-1) q (Å-1) Intensity (a.u.) Intensity (a.u.) qz (Å-1) 1.5 In-plane qq(xy) (Å-1) qxy (Å-1) qz (Å-1) 1.0 q2 (Å-1) Intensity (a.u.) Intensity (a.u.) qz (Å-1) (d) 0.5 2.0 (c) 1.5 In-plane q(xy) q (Å-1) qxy (Å-1) (b) Intensity (a.u.) Intensity (a.u.) qz (Å-1) (a) Out-of-plane In-plane 0 0.5 1.0 qxy (Å-1) 1.5 q(xy) q (Å-1) 2.0 0.5 1.0 q2 (Å-1) 1.5 , Figure 3.17 2D GIWAXD patterns of the as-cast films of (a) T-3MT, (b) T3MT:IM-IDT, (c) T-3MT:ITIC, (d) T-3MT:PC71BM 16 0 Out-of-plane 0.5 1.0 Out-of-plane 0.5 1.0 1.5 0.5 2.0 1.0 1.5 q2 (Å-1) 0 Out-of-plane 0.5 Intensity (a.u.) Intensity (a.u.) 1.0 1.5 In-plane 0.5 2.0 1.0 1.5 q2 (Å-1) q(xy) q (Å-1) qxy (Å-1) Intensity (a.u.) 0 Intensity (a.u.) qz (Å-1) 1.5 In-plane q(xy) q (Å-1) qxy (Å-1) qz (Å-1) 1.0 q2 (Å-1) Intensity (a.u.) Intensity (a.u.) qz (Å-1) (d) 0.5 2.0 (c) 1.5 In-plane q(xy) q (Å-1) qxy (Å-1) (b) Intensity (a.u.) Intensity (a.u.) qz (Å-1) (a) Out-of-plane 0.5 1.0 1.5 q(xy) q (Å-1) qxy (Å-1) 2.0 In-plane 0.5 1.0 1.5 q2 (Å-1) Figure 3.18 2D GIWAXD patterns of the as-cast films of (a) 2T-3MT, (bf) 2T3MT:IM-IDT, (c) 2T-3MT:ITIC, and (d) 2T-3MT:PC71BM Cấu trúc tinh thể T-3MT, 2T-3MT tổ hợp polyme: 3.2.2.4 Morphological studies of the blend films The surface topography of the as-cast active layer with no solvent additive was evaluated by atomic force microscopy (AFM) 3.2.2.5 Non-fullerene polymer solar cells To investigate the performance of non-fullerene PSCs based on the two polymers used in this study, inverted BHJ PSCs were fabricated with the ITO/ZnO/polymer:n-type small molecule/MoO3/Ag configuration.35 Two kinds of non-fullerene small molecules (IM-IDT and ITIC) and PC71BM as a fullerene derivative were selected for evaluation of the optimized device performance in BHJ-type PSCs Simple device fabrication was achieved by using a solventadditive-free as-cast blend film of the polymers and acceptors in different blend ratios Among the three kinds of PSCs, the polymer:IM-IDT blends showed higher 17 PCE values than the polymer:ITIC blend, and the best PCE of 3.96% was obtained with the 2T-3MT:IM-IDT(1:1 wt ratio)-based PSC (b) T-3MT:IM-IDT(1:1) T-3MT:ITIC(1:2) T-3MT:PC71BM(1:1) -3 -6 -9 -0.2 0.0 0.2 0.4 0.6 2T-3MT:IM-IDT(1:1) 2T-3MT:ITIC(1:1.5) 2T-3MT:PC71BM(1:2) Current density (mA/cm ) Current density (mA/cm ) (a) 0.8 1.0 -2 -4 -6 -8 -0.2 1.2 0.0 Voltage (V) (c) (d) 60 T-3MT:IM-IDT (1:1) T-3MT:ITIC (1:2) T-3MT:PC71BM (1:1) 0.8 1.0 1.2 40 EQE (%) EQE (%) 0.6 2T-3MT:IM-IDT (1:1) 2T-3MT:ITIC (1:1.5) 2T-3MT:PC71BM (1:2) 50 40 30 20 30 20 10 10 0.4 Voltage (V) 60 50 0.2 400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm) Figure 3.20- 3.21 J-V curves (a, b) and EQE spectra (c, d) of PSCs employing the as-cast blend films of T-3MT and 2T-3MT with IM-IDT, ITIC and PC71BM No solvent additive was used for fabricating the blend films 3.2.3 Physical properties and characteristics of optoelectronic devices of 3MTB and 3MTT 3.2.3.1 Optical properties of polymers 3MTB and 3MTT The UV-vis absorption spectra of the two terpolymers in the solution and thin film states are shown in Figure 3.22 The strong intramolecular charge transfer absorption spectral bands (π= 400~700 nm) of these D-A-containing terpolymers (3MTB and 3MTT) were broadened and red-shifted upon incorporation of BTz or BiTPD as strong accepting units, compared to the bands of 3MT-Th The absorption spectra of the thin films were red-shifted relative to those of the solutions, indicating strong intermolecular interaction between the polymer backbones of the terpolymers, even though the repeating units of the terpolymers were rather disordered owing to the regio-random 3MT unit The optical bandgaps of 3MTB, 3MTT, and 3MT-Th were 1.78, 1.87, and 1.98 eV, respectively, as measured from the absorption edges in the absorption spectra This indicates that the presence of the BTz of BiTPD units in the polymer backbones resulted in a 18 (a) Normalized Absorbance (a u.) pronounced decrease in the optical bandgap owing to the strong electron withdrawing ability of these units 1.2 0.9 3MTB 3MTT 3MT-Th ITIC 0.6 0.3 0.0 300 400 500 600 700 800 Wavelength (nm) (b) Normalized Absorbance (a.u.) 1.2 0.9 3MTB 3MTT 3MT-Th ITIC 0.6 0.3 0.0 300 400 500 600 700 800 900 Wavelength (nm) Figure 3.22 UV-vis absorption spectra of polymers in chloroform solutions (a) and in films (b) 3.2.3.2 Electrochemical properties of polymers 3MTB and 3MTT The highest occupied molecular orbital (HOMO) energy levels of these polymers were measured by cyclic voltammetry and the lowest unoccupied molecular orbital (LUMO) energies were calculated from the optical bandgaps and HOMO values The cyclic voltammograms and energy level alignment of the synthesized polymers are displayed in Figures 3.25 The onset oxidation potentials (Eoxs) of 3MTB, 3MTT, and 3MT-Th were 1.02, 1.07, and 0.99 V, respectively, and the corresponding HOMO levels were calculated to be -5.45, -5.50, and -5.42, respectively The calculated LUMO levels were -3.67, -3.63, and -3.44 eV for 3MTB, 3MTT, and 3MT-Th, respectively The energy level offsets of the terpolymers compared to those of ITIC, as a nonfullerene acceptor, could guarantee strong photoinduced charge transfer between the polymer and ITIC 19 Ferrocene 3MTB 3MTT 3MT-Th Intensity (a) -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 -3.67 -3.63 -3.44 -5.50 ITIC -5.45 3MT-Th 3MTT -4.14 3MTB (b) Energy levels (eV) E (V vs Ag/Ag+) -5.42 -5.72 Figure 3.25 Cyclic voltammograms of the polymer films (a); energy levels of the three polymers as donors and ITIC as the acceptor in PSCs (b) 3.2.3.3 Crystal structure of 3MTB ,3MTT Grazing incidence wide-angle X-ray diffraction (GIWAXD) measurements were performed for the neat films of the individual terpolymers and the as-cast terpolymer:ITIC blend films to better understand the high performance of the terpolymer-based PSCs (Figure 3.26) For the neat films, the terpolymers obviously exhibited a face-on orientation, with the (010) π-π-stacking peak (d(010) = 3.94 Å for 3MTB, d(010) = 3.99 Å for 3MTT) in the out-of-plane direction The 3MTB and 3MTT:ITIC blends also displayed a strong (010) diffraction of the π–π stacking in the out-of-plane profiles The data indicate that the terpolymer chains in the blend films predominantly adopted the face-on orientation on the substrate, which is similar to the diffraction data for the 3MT-Th:ITIC blend film Such a polymer chain arrangement can lead to excellent charge transport phenomena in a vertically stacked PSC 20 Intensity (a.u.) qz (Å-1) Intensity (a.u.) (a) Out-of-plane In-plane 0 0.5 qxy (Å-1) 1.5 0.5 2.0 1.0 1.5 q2 (Å-1) qq(xy) (Å-1) Intensity (a.u.) Intensity (a.u.) qz (Å-1) (b) 1.0 Out-of-plane In-plane 0 0.5 qxy (Å-1) 1.0 1.5 q2 (Å-1) Out-of-plane 0.5 qxy (Å-1) Intensity (a.u.) 1.0 1.5 In-plane 0.5 2.0 1.0 1.5 q2 (Å-1) qq(xy) (Å-1) Intensity (a.u.) Intensity (a.u.) qz (Å-1) 0.5 2.0 (d) 1.5 qq(xy) (Å-1) Intensity (a.u.) qz (Å-1) (c) 1.0 Out-of-plane In-plane 0 qxy (Å-1) 0.5 1.0 1.5 qq(xy) (Å-1) 2.0 0.5 1.0 1.5 q2 (Å-1) Figure 3.26 Two-dimensional GIWAXD patterns of the as-cast films of (a) 3MTB, (b) 3MTT, (c) 3MTB:ITIC, and (d) 3MTT:ITIC (e) Out-of-plane and in-plane XRD patterns of pristine terpolymers and blend films 3.2.3.4 Morphological studies of the blend films The surface topography of the as-cast active layer with no solvent additive was evaluated by atomic force microscopy (AFM) As shown in Figure 3c−e, the active layers composed of the 3MTB and 3MTT:ITIC blends comprised very fine domains with a small root-mean-square roughness (Rq) of 0.30 and 0.32 nm, respectively These values are slightly smaller than that of the 3MT-Th:ITIC blend film (0.36 nm) 21 (b) 100 3MTB:ITIC (1:1.5) 3MTT:ITIC (1.2:1) 3MT-Th:ITIC (1:1) 3MTB:ITIC(1:1.5) 3MTT:ITIC(1.2:1) 3MT-Th:ITIC (1:1) 80 -3 -6 EQE (%) Current density (mA/cm2) (a) -9 -12 0.8 nm -0.8 nm 40 20 -15 -18 -0.2 0.0 60 0.2 0.4 0.6 Voltage (V) 0.8 1.0 400 Rq = 0.30 nm 600 700 800 Wavelength (nm) (d) (c) 500 (e) Rq = 0.32 nm Rq = 0.36 nm Figure 3.27 (a) Current density-voltage (J-V) curves and (b) EQE spectra of PSCs employing the as-cast polymer:ITIC blend film without solvent additives (c−e) AFM height images of the as-cast polymer:ITIC films 3MTB:ITIC (a), 3MTT:ITIC (b), 3MT-Th:ITIC (c) No solvent additive was used in the active layer 3.2.3.5 Nonfullerene polymer solar cells Nonfullerene BHJ PSCs were fabricated by using the synthesized terpolymers 3MTB and 3MTT; a 3MT-Th-based PSC was also fabricated with the same device configuration and used as a control device An inverted type PSC structure with the ITO/ZnO/polymer:ITIC/MoO3/Ag configuration was selected for consistency with the literature.25 Simple device fabrication was achieved by using a solvent-additive-free as-cast blend film of the polymer and ITIC in different blend ratios The performance and stability (e.g., shelf-life and operational stability) of the PSC devices were investigated 3.2.3.6 Shelf-life and operational stability of the PSCs The shelf-life of the PSC devices was investigated by keeping them out of the glove box without encapsulation under ambient conditions for over 1000 h Interestingly, the devices fabricated with the as-cast films of the terpolymer:ITIC blend displayed higher PCE values, even after 1056 h The operational stability of the PSC devices was also investigated by continuous irradiation (AM 1.5 G illumination at 100 mW cm−2) The device performance was assessed periodically to observe the effect of the illumination time on degradation of the device efficiency The performance of all the PSC devices deteriorated with increasing illumination /operation time Nevertheless, the 3MTB and 3MTT-based PSC devices displayed much better stability than the 3MT-Th-based PSCs The 22 presence of the BTz and BiTPD units not only improved the PCE but also improved the shelf-life and operational stability of the PSCs CONCLUSION AND RECOMMENDATION Conclusion The research results of the dissertation have new contributions, ensuring the science and practice: + Have synthesized a new conjugated copolymers based on the Diketopyrrolopyrrol P group (DPP6T-C4) In particular, the use of decylheptadecyl group at N sites in DPP group improves the solubility of copolymers + Have synthesized new wide band gap polymers: T-3MT and 2T-3MT These copolymers are combined with different acceptors in order to expand the UV-Vis absorption that lead to an improvement of power conversion efficiency of OPV devices + terpolymes have been synthesized: 3MTB and 3MT OPV devices made from these terpolymers have good properties with very high shelf-life and operational stability, opening up their potential applications in practice + From the above results, the thesis has contributed to the development of the research direction of synthesis of semiconducting polymer for optoelectronic applications Recommendation Development of the research results of the thesis for the research directions in the same field: + Synthesis of semiconducting polymer for optoelectronic applications + Fabrication of high performance OPVs with high shelf-life and operational stability to develop commercial product 23 LIST OF RELATED SCIENTIFIC WORKS Phan Dinh Long, Nguyen Duc Tuyen, Ngo Trinh Tung, Hoang Mai Ha Effect of the extension of the conjugated structure on the semiconductor properties of Triphenylene molecule, Vietnam Journal of Chemistry, 2016, 54 (6e2) 175-179 Hoang Mai Ha, Phan Dinh Long, Ngo Trinh Tung, Nguyen Duc Nghia Copolymer bearing the diketopyrrolopyrrole group for field effect transistors and organic solar cells, Vietnam Journal of Chemistry, 2016, 54 (6e1) 235-242 Mai Ha Hoang, Gi Eun Park, Dinh Long Phan, Trinh Tung Ngo, Tuyen Van Nguyen, Su Hong Park, Min Ju Cho, and Dong Hoon Choi, Synthesis of Conjugated Wide-bandgapCopolymes Bearing Ladder-type Donating Units and Their Application to Non-fullerene Polyme Solar Cells Macromolecular Research, 2018, Vol 26, pp 844-850 Mai Ha Hoang, GiEun Park, Dinh Long Phan, Trinh Tung Ngo, Tuyen Van Nguyen, Su Hong Park, Min Ju Cho, and Dong Hoon Choi High-Performing Random Terpolyme-Based Nonfullerene Polyme Solar Cells Fabricated Using Solvent-Additive Free As-Cast Blend Films; Journal of Polymer Science, Part A: Polymer chemistry, 2018, 56,1528-1535 Phan Dinh Long, Nguyen Nang Dinh, Nguyen Phuong Hoai Nam; Light absorption and photoluminescence quenchinh properties of bulk heterojunction material based on the blend of poly (N-vinylcarbazole)/ poly(Nhexylthiophene), Communications in Physics, Vol.29, No.1 (2019), pp 57-63 Hoang Mai Ha, Phan Dinh Long , Ngo Trinh Tung , Effect Of Molecular Structure Of Porphyrins On Their Semiconducting Properties, Journal of Science and Technology 54 (3) (2016) 356-365 Hoang Mai Ha, Phan Dinh Long, Nguyen Duc Tuyen, Ngo Trinh Tung, Semiconducting polymer for organic solar cells Vietnam National Conference of Solid Physics and Materials Science – SPMS 2017 Hoang Mai Ha, Phan Dinh Long, Nguyen Duc Tuyen, Tran Phuong Hoa, Ngo Trinh Tung, Nguyen Duc Nghia, Synthesis of Organic Semiconducting Materials for Organic Field Effects Transistor and Organic Solar Cells Vietnam National Conference of Solid Physics and Materials Science - SPMS2015 Mai Ha Hoang, Dinh Long Phan, Hop Tran Thi Thanh, Duc Tuyen Nguyen, Trinh Tung Ngo, Duc Nghia Nguyen, Organic Semiconducting Materials for Field Effect Transistors and Photovoltaic Cells, The 8th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2016), 8-12 November, 2016, Hạ Long, Vietnam 24 ... Figure 3.26 Two-dimensional GIWAXD patterns of the as-cast films of (a) 3MTB, (b) 3MTT, (c) 3MTB:ITIC, and (d) 3MTT:ITIC (e) Out-of-plane and in-plane XRD patterns of pristine terpolymers and blend... the strong electron withdrawing ability of these units 1.2 0.9 3MTB 3MTT 3MT-Th ITIC 0.6 0.3 0.0 300 400 500 600 700 800 Wavelength (nm) (b) Normalized Absorbance (a.u.) 1.2 0.9 3MTB 3MTT 3MT-Th... peak (d(010) = 3.94 Å for 3MTB, d(010) = 3.99 Å for 3MTT) in the out-of-plane direction The 3MTB and 3MTT:ITIC blends also displayed a strong (010) diffraction of the π–π stacking in the out-of-plane