Functional Materials

FUNCTIONAL MATERIALS

Professor: Chiara Bertarelli

Academic Year: 2020-2021

Lecture Notes by L.A. & M.B

SOMMARIO

INTRODUCTION.......................................................................................................................................................... 5

CHEMISTRY OF ORGANIC FUNCTIONAL MATERIALS ................................................................................................. 6

Skeleton ................................................................................................................................................................. 6

Substituents ........................................................................................................................................................... 7

Examples .............................................................................................................................................................. 10

Phenol .............................................................................................................................................................. 10

Toluene ............................................................................................................................................................ 10

Bromobenzene ................................................................................................................................................ 10

Methyl Benzoate .............................................................................................................................................. 11

Benzonitrile ...................................................................................................................................................... 11

Trimethylphenylsilane ..................................................................................................................................... 11

Nitrobenzene ................................................................................................................................................... 12

Fluorobenzene ................................................................................................................................................. 12

Benzenethiol .................................................................................................................................................... 12

CONJUGATED MATERIALS ....................................................................................................................................... 13

Linear Combination of Atomic Orbitals (LCAO) ................................................................................................... 13

Electronic Transitions .......................................................................................................................................... 19

UV-VIS SPECTROSCOPY ............................................................................................................................................ 20

Type of Transition ................................................................................................................................................ 21

Rigidity of the Molecules ..................................................................................................................................... 22

Conjugation.......................................................................................................................................................... 22

Aromaticity .......................................................................................................................................................... 23

Effect of the Substituents .................................................................................................................................... 24

Examples .............................................................................................................................................................. 25

Hexane ............................................................................................................................................................. 25

Benzene vs 1,3,5-hexatriene (Differently Ordered Species)............................................................................ 25

Oligothiophenes .............................................................................................................................................. 26

Ladder Polymer ................................................................................................................................................ 26

Branched Oligomers ........................................................................................................................................ 27

Exercises .............................................................................................................................................................. 28

Exercise 1: Wavelength Scale .......................................................................................................................... 28

Exercise 2: Energy Scale ................................................................................................................................... 29

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Exercise 3: Calculate the Energy Gap

............................................................................................................... 29

Exercise 4 ......................................................................................................................................................... 30

Exercise 5 ......................................................................................................................................................... 31

POLYACETYLENE (PA) ............................................................................................................................................... 32

Synthesis Methods .............................................................................................................................................. 34

Catalytic Polymerization of Acetylene Monomers .......................................................................................... 34

Catalytic polymerization of monomers other than acetylene ......................................................................... 36

Indirect Method ............................................................................................................................................... 37

DOPING OF POLYMERS ............................................................................................................................................ 40

Shirakawa Film ..................................................................................................................................................... 40

Exercise ................................................................................................................................................................ 43

Charge Defects ..................................................................................................................................................... 44

Charge Carrier in Organic Semiconductors .......................................................................................................... 48

Applications of Doped Polymers.......................................................................................................................... 53

Antistatic Coatings ........................................................................................................................................... 53

Chromogenic Smart Windows ......................................................................................................................... 54

Exercises .............................................................................................................................................................. 63

Exercise 1 ......................................................................................................................................................... 63

Exercise 2 ......................................................................................................................................................... 64

SEMICONDUCTORS AND p-n JUNCTION .................................................................................................................. 66

Forward Bias Junction .......................................................................................................................................... 69

Reversed Bias Junction ........................................................................................................................................ 70

Examples .............................................................................................................................................................. 71

Exercise 1 ......................................................................................................................................................... 71

Exercise 2 ......................................................................................................................................................... 72

PERRIN-JABLONSKI DIAGRAM ................................................................................................................................. 73

Vibrational Relaxation ......................................................................................................................................... 74

Internal conversion .............................................................................................................................................. 75

Fluorescence ........................................................................................................................................................ 75

Intersystem crossing ............................................................................................................................................ 76

Phosphorescence ................................................................................................................................................. 78

Delayed fluorescence .......................................................................................................................................... 79

Thermally Activated Delayed Fluorescence ..................................................................................................... 79

P-type Delayed Fluorescence .......................................................................................................................... 79

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Examples .............................................................................................................................................................. 80

Exercise 1 ......................................................................................................................................................... 80

Exercise 2 ......................................................................................................................................................... 82

Quantum Yield ..................................................................................................................................................... 83

Materials’ Behaviours .......................................................................................................................................... 84

Examples .............................................................................................................................................................. 87

Exercise 1 ......................................................................................................................................................... 87

Exercise 2 ......................................................................................................................................................... 88

LIGHT EMITTING DIODES (LEDs) .............................................................................................................................. 89

Exercise ................................................................................................................................................................ 93

ORGANIC LIGHT EMITTING DIODES (OLEDs) ........................................................................................................... 95

OTHER PHOTONIC DEVICES ................................................................................................................................... 102

Photodetectors .................................................................................................................................................. 102

Photovoltaic Cells .............................................................................................................................................. 104

Exercises ............................................................................................................................................................ 105

Exercise 1 ....................................................................................................................................................... 107

Exercise 2 ....................................................................................................................................................... 107

Exercise 3 ....................................................................................................................................................... 108

HETEROJUNCTION SOLAR CELLS ............................................................................................................................ 109

Material Selection .............................................................................................................................................. 112

Acceptor Materials (n-type) ........................................................................................................................... 113

Donor Materials (p-type) ............................................................................................................................... 114

HYBRID SOLAR CELLS ............................................................................................................................................. 125

Organic/Inorganic Heterojunction Solar Cells ................................................................................................... 125

Dye-Sensitized Solar Cells (DSSCs) or Grätzel Cells ............................................................................................ 126

Perovskite Solar Cells ......................................................................................................................................... 127

SOLAR CELLS FINAL COMPARISON ........................................................................................................................ 128

METAL OXIDE FIELD EFFECT TRANSITIORS (MOSFETs or FETs) ............................................................................. 130

Output characteristics ....................................................................................................................................... 133

FETs Manufacturing ........................................................................................................................................... 134

ORGANIC FIELD EFFECT TRANSITIORS (OFETs) ...................................................................................................... 134

Exercise .............................................................................................................................................................. 135

MATERIALS CHOICE AND PROCESSING FOR OFETs ............................................................................................... 137

Small Molecules ................................................................................................................................................. 138

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Polymers ............................................................................................................................................................ 140

Exercise .............................................................................................................................................................. 141

PHOTOCHROMIC MATERIALS ................................................................................................................................ 143

Trans-Cis Isomerization ..................................................................................................................................... 148

Electrocyclization ............................................................................................................................................... 149

Exercise .............................................................................................................................................................. 152

LIQUID CRYSTALS ................................................................................................................................................... 153

PRIMARY STRUCTURE ........................................................................................................................................ 153

SECONDARY STRUCTURE ................................................................................................................................... 155

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INTRODUCTION

A material is roughly said to be smart or functional when it is able to provide a response to an external input. We

can classify smart materials in different ways but, according to the reaction to a given output, we can divide them

as follows:

• Type 1, in which the stimulus causes a change in some physical properties of the material

• Type 2, whose output is the generation or the transformation of energy

These are the fundamental features (especially the first 2) for a system to be defined as smart

• Reversibility (most fundamental): when the material is exposed to an input, it provides an output but then it

is possible to return to the original state so as to be again subjected to the input and get an output. There are

2 possibilities:

- 1 metastable state, if, by stopping the input, the material spontaneously returns to the initial state

- 2 metastable states, if an input transforms the initial material in another one and another stimulus

must be introduced to return at the initial case

• Graduality of the transformation, which depends on how the input affects the material:

- Transient response: in some materials the strength of the input is proportional to the response

- Response by dose: there is a threshold that, when overcome, causes the transition (no

proportionality relation between the input