ChalmersUniversity of Technology

Organic Semiconductors

Semiconductor Materials Physics

Keke Zhang 810322

Gildardo Morga 830723

Ingrid Åslund 810618

Qingfu Su790519

Yichen Zhang 820817

Introduction

Ever since the semiconductor was discovered, the area of semiconductor material is mainly dominatedby inorganic materials, such as silicon, gallium arsenide, germanium in a way, and so on. But the mechanical properties of these inorganic materials are not always fully satisfied and the fabrication process is very complex involvinghigh costs.

In the 1970s, researchers on organic semiconductors started their investigation and developed a lot of progress very fast in the recent years.Ideal organic semiconductors overcome the disadvantageof traditional semiconductors, while possessexcellent electrical and optical properties.They can be vacuum deposited at low temperatures, drop cast or spin coated from solution, which are low-cost fabrication techniques. But organic semiconductors have a problem of low mobility.Nowadays, scientists are trying to solve this problem.

Background

The behavior of the molecular adsorption on the substrate depends on the molecular-substrate and molecule-molecule interaction, which may result in an ordered molecular monolayer.

If the horizontal molecule-molecule forces dominate the growth process, ordered molecular film can be obtained; if the binding force between substrate and molecule is small, disordered molecules will be formed.

Ionization energy of the molecules can be used to estimate the character of the molecule as a donor or acceptor. High ionization energies make the molecule as an insulator. Molecules with high ionization energy serve as an acceptor while lower values make them a donor. Band gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) plays an important role in the devices manufactured.

If the band gap between them lies in the visible range, the molecular layers are interesting for solar cell and light emitting devices. With the size of the molecule and the corresponding wavelength band gap will decrease.

Methodology

Band structure of organic semiconductors plays a key role in their applications. Quantum calculation and experimental measurement can do good to clarify it. Quantum calculation includes ab-initio, Density Functional Theory (DFT) and semi-empirical. Ab-initio adopts Exact Hamiltonian with approximate wave-functions, DFT uses Approximate Hamiltonian with exact wave-functions, semi-empirical makes use of the same conceptual framework as ab-initio, but neglect many smaller integrals, empirical parameters introduced.

Various characterization methods can be used: ultraviolet photoemission spectra (UPS), soft x-ray photoelectron spectroscopy (SXPS), inverse photoemission spectroscopy (IPES) and near edge x-ray absorption fine structure (NEXAFS) and so on.

Why organic semiconductors are semiconductor

The main difference between organic and inorganic semiconductors is where the charge transport takes place. For inorganic semiconductors it takes place in well-defined bands, while for organic semiconductors it takes place as a stochastic process of hopping between localized states. The later, contributes to the low carrier mobility of organic semiconductors.

Taking polymers for example, they are commonly known as flexible and electrically insulating materials, but conjugated polymers have the potential to be semiconductor or conducting materials.

Firstly, we take a look at polyethylene, why is it an insulating polymer?

Fig. 1 Polyethylene

Taken from

The backbone of polyethylene is formed by saturated sp3 hybridized σ C-C bonds. The covalent bonds are strong and the electronic excitations from the bonding orbital to anti-bonding orbital require a large amount of energy, this leads to the large band gap and the transparence of polyethylene. So polyethylene is an insulator.

Secondly, let’s study polyacetylene, what fascinate properties do they have?

Fig. 2 Polyacetylene

Taken from

Hideki Shirakawa receiving his Nobel Prize at 2000 for his excellent study in conducting Polyacetylene thin film.

The backbone of polyacetylene is formed bysp2 hybridizedσ C-C bonds. It contains three σ-orbital 2s, 2px, 2py and a 2pz orbital, which can overlap along the backbone forming π bonds where electrons are delocalized. That is to say, electrons in π bonds can move around the material.So, conjugated polymers can be considered as semiconductors with a band gap typically in the range of 1-3 eV.

The above is discussed polymer semiconductors. For short molecules, a minimum level of intermolecular interaction is needed to permit local charge transfer to neighboring molecular.Usually in two ways, while one is hopping, the other is delocalized spreading out across several molecules.

In summary, short molecular organic semiconductors should contain π orbital and a suitable intermolecular distance. Suitable means the energy of π orbital should below certain threshold to guarantee the overlap of their wavefunctions.

Whyorganic semiconductors emit lights

The figure below illustrate the principle of how OLED works.

Fig. 3 Principle of OLED

Taken from

The hole and electron injection layers have energy levels that fit the position of the HOMO and LUMO levels respectively. The recombination and thus light generation takes place in the middle, active layer. Usually holes are injected from ITO and the electrons from a metal with low work function.

In addition, the organic solar cell works just inversely of OLED, photons hit the active layer and generate electrons and holes, thus forming a current.

How organic field effect transistors work

OFETs contain either a molecular or polymeric channel connecting the source and drain contacts. The gate electrode controlsthe current between source and drain.

Fig. 4 Structure of OFET

Taken from

Organic Devices

Mainly there are two types of organic devices that one can easily find information about, those are organic field effect transistors and organic light emitting diodes. Both of these two types have been developed heavily during the last decades and there is still development going on. No real commercial devices have been produced yet but it is probably within reach in a couple of years.

The advantages of these organic, or rather polymeric, devices are that the processing is much easier and have lower cost. This is mostly due to the fact that they can be made in more normal conditions. Less than 100 Celsius degrees for making an organic thin film compared to 600-800 Celsius degrees for the usual inorganic compounds and that one doesn’t need to use ultra high vacuum to make the organic thin films. Another thing that makes the organic semiconductors interesting is that they are so diverse in their properties that far from all their possibilities have been investigated.

The transistors are used to make more flexible and smaller electrical circuits, in for example computers while the light emitting diodes are used to make monitors with higher resolution and which are more flexible then commercial monitors.

The organic field effect transistor consists of a polymeric layer with one source and one drain, made of some kind of metal, at one of its surfaces (with some space in between these two) and the other surface is covered with a gate made of silicon oxide and n-doped silicon.

The light emitting diodes are made by putting a thin plastic film between two electrodes, where the anode should have a large work function, usually indium tin oxide is used, and the cathode should have a small work function, for this calcium is usually used.

This figure shows a typical construction of an organic light emitting diode. (Taken from

This figure shows a typical construction of an organic field effect transistor. (Taken from

Plastic Electronics

Apart to the applications of furniture, toys and plumbing, plastics can be used to create electronic devices. Recently, groups from Cambridge University (UK), Bell Laboratories (US) and Phillips Research Laboratory (Netherlands) have shown progress in development of organic devices such as: integration of an organic transistor to a light-emitting diode, construction of field effect transistors with a polymer, electrical circuitries needed to produce a programmable generator of digital code, etc.

In the figure (taken from Science Daily web page) we can distinguish organic molecules which yield to n-type in the left and p-type in the right semiconductors for organic devices.

Future applications

  • One can replace liquid-crystal displays (in laptops and desktop computers) for organic light-emitting devices
  • Organic integrated circuits
  • Smart cards carrying personal information
  • Battery-powered electronic applications
  • Organic transistor
  • Identifications tags
  • Electronic books
  • Frequency identification for cell phones

Advantages in general for organic semiconductors

  • High performance offered
  • Low costs
  • Many of the organic semiconductors are photoconductive under visible light
  • Due to their low carrier mobility, organic semiconductors can be used in high-frequency (more than 10 MHz) applications
  • Increment of the efficiency (~10% or more for phosphorescence and ~3% for fluorescence)
  • Polymer structures can be simpler than small-molecule structures
  • Due to their high conductivity, polymer-based devices have operating voltages in the 2-5-V range

Disadvantages which can be presented

  • Organic materials have often proved to be unstable
  • Making electrical contacts to organic thin films is really difficult
  • Degradation of their electronic properties when exposed to air, water or ultraviolet light
  • Difficulty of having purified and doped materials
  • It is difficult to manufacture devices which operate at voltages between 5 - 10 V
  • Materials tend to be mechanically fragile and are easily attacked by other chemical substances
  • When stacking numerous organic layers, optical cavities can be presented and the emission spectra is altered
  • Charge conduction in these insulating materials requires very high electric fields
  • The number of molecular building blocks suitable for making semiconductor polymers is limited.

Organic and molecular materials-solar cell part

Fig 1: •Active layer ~100 nm thick, •Low temperature processing enables use of flexible substrates

In this report, I want to show the using part for organic and molecular materials.

The phosphorescent PIN-type OLEDs with high power efficiency even at high brightness, the performance of organic PIN-type heterojunction solar cells where an intrinsic photoactive layer is sandwiched between two highly doped wide-gap layers can be improved.

A general structure and energy level alignment are shown in Fig 2. This architecture has several advantages as compared to cells where the photoactive layer is directly sandwiched between two metals with different work junction:

i) Recombination at the contacts is avoided as neither minority carriers nor excitons can penetrate into the wide-gap layers.

ii) The optimum thickness of the active layer is often very thin for small molecular materials (30-50nm), which leads to shorting problem.

The doping concept is based on coevaporation of two organic compounds. The typical dopant content varies from 0.05 to 4-mol%, increasing the conductivity of the wide-gap whole transport layers (HTL) and electron transport layers (ETL) by more than four orders of magnitude.

For p-type doping of hole transporting materials like phthalocyanines and amorphous triarylamines or diamines, the strong acceptor molecule tetrafluoro-tetracyano-quinodimethane [5,6]. The devices have been fabricated on commercial semi-transparent ITO coated glass substrates with a surface resistance of <50/sq. The typical active area is approximately 3-4mm2

References:

  • Semiconductors and heterostructures compendium 2005,Thorvald Andersson
  • Materials science structure and properties compendium,Electronic properties and structure of solids,Igor Zoric
  • Fabrication and characterization of nanometer scale organic electronic devices: application to field-effect transistors,Celio Enrique Clavijo Cedeño (
  • Organic electronics, IBM Research, J. M. Shaw and P. F. Seidler (
  • The dawn of organic electronics, Stephen Forrest, Princeton University, Paul Burrows, Pacific Northwest National Laboratories & Mark Thompson,University of Southern California (
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