The role of chemical design in the performance of organic semiconductors

Abstract

Organic semiconductors are solution-processable, lightweight and flexible, such that they are increasingly being used as the active layer in a wide range of new technologies. The versatility of synthetic organic chemistry enables the materials to be tuned such that they can be incorporated into biological sensors, wearable electronics, semi-transparent photovoltaics and flexible displays. These devices can be improved not only by developing their synthetic chemistry but also by improving the analytical and computational techniques that enable us to understand the factors that govern material properties. Judicious molecular design provides control of the semiconductor frontier molecular orbital energy distribution and guides the hierarchical assembly of organic semiconductors into functional films where we can control the properties and motion of charges and excited states. This Review describes how molecular design plays an integral role in developing organic semiconductors for electronic devices in present and emerging technologies. [H1] Molecular orbital design considerations The energies of frontier molecular orbitals and the distribution of the orbitals in a π-conjugated molecule play critical roles in intra-and intermolecular charge transport, light absorption/emission, charge injection/extraction/trapping and electrochemistry. This is true for organic small molecules and conjugated polymers alike. In each case, the energy of the highest occupied molecular orbital (HOMO) largely depends on the electron density and delocalization of the π electrons throughout a π-conjugated backbone. Substituents that donate electron density mesomerically (for example, lone pair donation from N, O or S heteroatoms) or inductively (for example, alkyl chains) can contribute to raising the HOMO energy EHOMO, decreasing the solid-state ionization potential (IP, Box 1). Conversely electronwithdrawing groups, such as-F,-C(O)R or-C≡N groups, can act to lower both the HOMO and lowest unoccupied molecular orbital (LUMO) energy, leading to an increase in the solid-state electron affinity (EA). BOX 1 | Energy levels in an isolated organic molecule and a molecular crystal or polymer.

Selection of the two enantiotropic polymorphs of diF-TES-ADT in solution sheared thin film transistors

“…This has partly been due to the emphasis placed on the design and synthesis of novel stable and soluble organic semiconductors (OSCs). 1,2 In the past decades, in the category of small molecule OSCs, pentacene and rubrene were extensively studied and considered as benchmarks in the field due to the high performances in single crystal organic fieldeffect transistors (OFETs). 3,4 The search for solution-processable materials has moved the attention to newly synthesized classes of heteroacenes mainly based on pentacene derivatives like di-fluorinated triethylsilylethynyl anthradithiophene (diF-TES-ADT) 5,6 and fused thiophene rings in a ladder-shaped molecular structure, such as [1]benzothieno [3,2-b] [1]benzothiophene (BTBT) and dinaphtho [2,3-b:20,30-f]thieno [3,2-b]thiophene (DNTT) derivatives.…”

Organic Bioelectronics: From Functional Materials to Next‐Generation Devices and Power Sources

“…The strong correlation between functionality and chemical structure suggests that it is practically possible to design a chemical structure uniquely suited to a particular application. [231] This also leads to applications and modalities that may currently be beyond our imagination. At this point, it will be exciting to apply machine learning approaches for the design of new CPs and to see how such computational techniques will replace the currently employed experimental efforts to provide insights into structure-functionality relationships.…”

Combining Surface Templating and Confinement for Controlling Pharmaceutical Crystallization

“…Several areas outside of pharmaceuticals have made use of the advantage of surface chemistry and/or confinement control to engineer crystals of a wide variety of materials including organic semiconductors (OSCs), proteins, inorganic compounds, and even explosives. OSCs are carbon-based conductive materials that are highly sought after due to being light-weight and flexible while offering the properties of inorganic semiconductors [ 119 ]. Gao et al used bisurea-based organogels to obtain single crystals of a variety of OSCs.…”