Organic electronics–new physical chemistry insight

---

Abstract

---

Wenping Hu

Yu-Tai Tao

Henning Sirringhaus

---

Organic electronics have attracted much attention as potential candidates for next generation electronics due to their great advantages compared to conventional inorganic solid state electronics. First, the structural versatility of organic semiconductors allows for the incorporation of a wide range of functionalities by molecular design, which brings a new era of designing electronic devices. Second, the solution processability of organic semiconductors allows for unconventional deposition methods such as inkjet printing to fabricate large area devices at low cost. Third, the mechanical properties of organic semiconductors also allow for flexible electronics. As a highly interdisciplinary science, organic electronics is closely related to chemistry, physics, materials science and electronics/microelectronics. The advancements of these sciences will bring new concepts, models, theories and applications to organic electronics, and further accelerate its development. In this themed issue, organic electronics–new physical chemistry insight will be the focus.

The weak van der Waals bonding in organic semiconductors, which is fundamentally different from the covalent bonds in their inorganic counterparts, results in a much weaker delocalization of electronic wavefunctions among neighbouring molecules, giving unique implications for optical properties and charge carrier transport of organic semiconductors. For example, the existence of well-defined spin states (singlet and triplet) has important consequences for the photophysics of these materials, e.g., the weak intersystem crossing process sets an upper limit for the electroluminescence quantum efficiency in organic light-emitting diodes (OLED). The optical excitations (“excitons”) are usually localized on one molecule and therefore have a considerable binding energy of typically 0.5 to 1 eV, which has to be overcome before a pair of independent positive and negative charge carriers is generated in an organic solar cell (OSC). Finally, one has to bear in mind that charge transport in organic field-effect transistors (OFET) involving ionic molecular states depends on the degree of order of molecules in organic semiconductors. Of today's three major applications in organic electronics, OLEDs, OFETs and OSCs (Fig. 1), physical chemistry plays an indispensable role.

Fig. 1 Applications related to organic electronics in OLEDs, OFETs and OSCs.

OLEDs make it possible to develop a superior flat-panel display technology, which is now commercialized for cellular phone applications and will soon be implemented in large-area high-definition television screens. This success is due to the numerous advantages of OLEDs, such as ease of fabrication process, high luminance, fast response time, low operating voltage, wide viewing angle, and full colour visible emission spectrum. Considering the emission mechanism of OLEDs, the design of hole-transport materials, emitting materials, electron-transport and hole-blocking materials, are closely related with material chemistry. Yet the tuning of energy levels and gaps, the energy level alignment for efficient electron- or hole-injection, the optimization of film thickness for balanced charge transport and recombination, all point to physical chemistry as crucial to the efficiency of OLEDs. Besides, some architectural approaches have been found to be effective in fabricating high-efficiency fluorescent and phosphorescent OLEDs, and may facilitate the commercialization prospects of OLED technology.

The use of flexible organic solar cells is believed to provide a clean, sustainable source of energy. The photovoltaic effect in OSCs can be regarded as the reverse process of OLEDs, i.e., the conversion of light into electrical power, based upon light absorption by molecules to generate relatively localized excited states. Photoactive materials, including polymeric and small molecular semiconductors, play a key role in influencing physical processes involved in energy conversion, which in turn determines the electrical characteristics of the solar cell, such as power conversion efficiency (PCE), short-circuit current density (J_SC), open-circuit voltage (V_OC), and fill factor (FF). The basic requirements include: (a) a low optical bandgap for a broad absorption range matching with the solar spectrum and high extinction coefficient for harvesting more solar energy; (b) long exciton diffusion lengths for effective migration of excitons to D/A interface; (c) high hole or electron mobility for efficient charge transport, which in turn allows a thicker active layer required for increased light harvesting, as well as reduces charge recombination and series resistance; (d) suitable energy levels to ensure a large V_OC and a downhill energy offset for exciton dissociation; (e) excellent thermal stability for vacuum deposition or sufficient solubility to guarantee solution processability. Certainly, the optimization of microscale morphology and the control of phase separation in the active layer are important interface engineering issues for OSCs.

OFET, a device that uses an organic semiconductor instead of inorganic semiconductor in its channel, offers not only an essential building block for the next generation of cheap and flexible organic circuits, but also provides important insight into the theory of charge transport of π-conjugated systems, such as parameters of field-effect mobility, current on/off ratio and threshold voltage. Recent research includes the discovery, design and synthesis of organic/polymeric conjugated systems for OFETs, device optimization, development of applications in radio-frequency identification tags, flexible displays, electronic papers, sensors and so forth. Physical chemistry insight in OFETs includes chemists’ ability to modulate physical properties of organic molecules by fine-tuning their chemical structures. Certainly, the factors dominating the performance of the transistors such as the grain boundaries, morphology, interfaces and crystallization, the way of ordering organic semiconductor molecules and the alignment of the energy levels of organic semiconductors and electrodes for the fabrication of high performance transistors, all relate to the physical chemistry of OFETs.

The aim of this current themed issue focuses on physical chemistry of organic electronics. Besides the above mentioned content, quantitatively understanding the mechanism of the electronic and optical processes of organic materials and devices is also included. As a highly interdisciplinary science, the research and development of organic electronics continually derives ideas, methods, and technologies from other research fields. Therefore, knowledge, results and techniques derived from chemistry, physics, materials science, semiconductors, electronics, nanotechnology and biology have been adopted in organic electronics. In brief, this themed issue covers the most recent developments of this field including OLED, OFET, OSC, new methodology and applications, and much more. Its breadth is evident from the diversity of the 22 papers contributed by the leading scientists in this field. It is our great pleasure to cooperate with such outstanding scientists. Finally, we want to express our gratitude to all editorial members of PCCP, especially Philip Earis, Lois Bradnam, and Alisa Becker for their great contribution during the publication of the themed issue.

---