Materials purity, growth, and patterning

This chapter describes the purification and growth of organic materials, device patterning, and coating of large substrate areas using volume manufacturing processes. Organic semiconductors are no different from other electronic materials—purity is of the utmost importance. The chapter begins by describing several purification techniques for achieving and assessing the quality of electronic-grade materials. Next is a discussion of bulk crystal growth, and growth by more technologically important methods for achieving thin films using solution or vapor phase growth processes. Post-growth annealing for achieving desired film morphologies is also discussed. Patterning of device structures by photolithography, stamping and nanoimprint lithography on both planar and curved surfaces are described. This is followed by consideration of manufacturing processes such as roll-to-roll production, and the constraints that such processes place on the choice of substrate. Last, packaging and device encapsulation that maintain substrate flexibility while protecting devices from attack by adverse environments is discussed.

Organic thin film transistors

This chapter lays the foundations of operation of both unipolar and ambipolar organic thin film transistors (OTFTs). Thin film transistors are used in display back planes, digital circuits, memory addressing elements, and photodetection. The discussion describes basic transistor principles and the limitations to their performance. Transistor noise and circuit noise margin, cutoff frequency, transconductance, and gain are discussed. Several different lateral architectures including top and bottom gate, and split gate configurations, as well as vertical transistors are described. High performance OTFT materials and channel morphologies and how they are achieved along with contact patterning are discussed. Phototransistors are introduced, and their characteristics are compared with other photodetectors discussed in Chapter 7. Transistor stability and its implication for circuit performance are detailed. Finally, several circuit applications with particular focus on chemical and medical sensing, and communications are described.

Organic light detectors

This chapter introduces the major concepts governing the operation of organic photoconductors, photodiodes, and solar cells. Quantum efficiency, gain, noise, bandwidth, and the trade-offs between these parameters are discussed. Organic light detectors are used in sensing and communications, although the predominant interest is in solar cells. The unique properties of organics, including flexibility and conformability, also make them useful in applications such as position-sensitive detection and in imaging, as considered in this chapter. Methods for quantifying and measuring solar cell and detector efficiency are described, leading to a derivation of the thermodynamic efficiency limits for solar power generation. Materials and device architectures for minimizing energy loss include single and multijunction cells, singlet fission, and semitransparent cells. Quantifying and achieving very high device reliability is considered, along with criteria for acceptable practical device lifetime. Finally, we discuss processes developed for large-scale and low-cost manufacturing of organic solar cells.

Charge transport in organic semiconductors

In this chapter, the basic principles of the origins of transport levels and bands, charge conduction in disordered materials, and injection from contacts are introduced. Charge transport in organics is fundamentally different than in inorganic semiconductors due to narrow transport bands that, in general, lead to charge transport via hopping, resulting in carrier mobilities that are at most only a few cm2/V s. Processes of charge injection leading to space charge limited transport that defines the current vs. voltage characteristics of the materials are discussed. Methods of measuring mobility, background charge densities, and quantifying charge recombination are described. Doping of organics using both molecular and atomic species to modify their conductivity is also considered. The theory of transport in energetically and structurally disordered films is developed. The chapter closes by describing, from first principles, the theory of conduction over organic and organic/inorganic semiconductor heterojunctions that are used in almost all organic photonic devices.

Bulk and thin film organic crystal structures

Organic electronic devices are comprised of materials that are amorphous, nanocrystalline, microcrystalline, or single crystalline solids bonded by weak van der Waals forces. This chapter presents the terminology and concepts used in defining condensed matter structures, including the real and reciprocal lattices, the unit cell, and crystal symmetry. Bonding forces that form the solid, with emphasis on van der Waals bonds, are quantitatively discussed. In particular, the electrostatic potentials leading to both fixed and induced dipole interactions between molecules are described. Since almost all devices are found in thin film form, growth modes of thin films, from epitaxy, van der Waals epitaxy, quasiepitaxy, to self-assembly are discussed, along with the energetic driving forces that lead to particular thin film structures.

Expect the unexpected: more possibilities for organic electronics

There are a plethora of devices and phenomena that have been explored beyond the more conventional topics covered in the previous eight chapters. In this chapter, several of these interesting and promising topics in organic electronics are introduced. These include electrochemical light emitting devices, strong coupling between excitons and photons in optical microcavities forming so-called exciton polaritons and their associated devices, organic memory devices, and organics in limited dimensional systems. This latter topic includes discussions of energy transfer between organics and two-dimensional electronic materials, and single molecule, one-dimensional devices and phenomena. In each case, the discussions begin with an introduction to the subject, followed by laying the theoretical foundations of device operation, and finally by presenting several exemplary results that illuminate critical concepts. The purpose of this choice of topics is to show the endless horizons that are open to further investigation in the field of organic electronics.

Organic light emitters

This chapter introduces the design and fundamental concepts of organic light emitting diodes (OLEDs) and organic semiconductor lasers (OSLs). The chapter begins by describing device architectures used in fluorescent, phosphorescent, and thermally assisted delayed fluorescent (TADF) devices. Device characterization for both white and monochromatic OLEDs with application to displays and white illumination sources is discussed. OLED displays are compared with liquid crystal displays, and quantification of perceived color and luminosity is described. Materials and layer structures leading to high efficiency even at high brightness where exciton annihilation dominates, is followed by a discussion of methods used to efficiently outcouple light from substrate, waveguide, and surface plasmon modes. A discussion of reliability, with particular emphasis of the relatively short operational lifetime of blue phosphorescent and TADF devices, is provided. The last section is devoted to optically pumped OSLs, along with limitations encountered in achieving electrically pumped organic laser diodes.

Optical properties of organic semiconductors

Organic semiconductors are often called excitonic materials since their optical properties derive from the photogeneration of excitons, that is, bound electron–hole pairs. Organic excitons are either Frenkel or charge-transfer-like with binding energies of 0.5–1.0 eV, making them stable at room temperature. This chapter describes the fundamental optical properties of organics, starting with those of individual molecules, and then building the solid from pairs of molecules (dimers) and oligomers. Theoretical approaches to describe optical properties start by introducing the Born–Oppenheimer approximation and the Franck–Condon principle. Calculational approaches to understanding optical characteristics based on the linear combination of atomic orbitals are described. Both theory and experimental observation of optical phenomena are discussed in detail. Also, electron spin, spin–orbit coupling, fluorescence, and phosphorescence are quantitatively described. Finally, long and short range energy transfer, exciton diffusion, and annihilation processes ae described.

Introduction to organic electronics

Basic concepts and opportunities that define the field of organic electronics and organic electronic materials are presented. This is followed by a brief history of the field that is marked by major advances over the last 70 years in our understanding of carbon-rich, disordered, soft organic semiconductors, and their widespread application to devices. A discussion is provided of common language and the introduction of important classes of both polymeric and small molecular weight organic materials to be used throughout the book. Finally, a list of common myths that have surrounded the field of organic electronics is provided, along with a brief discussion of the differences between these myths and the reality that is the subject of the text.