Natural and nature-inspired materials in organic electronics

The emerging field of organic electronics is motivated by the notion of mass-producing cheap and sustainable electronic devices for displays, photovoltaic cells, integrated circuits, and sensors. Future large-scale application of sustainable organic electronics based on biodegradable materials would have a positive impact on the current problem of electronic waste. We have focused our work recently on exploiting natural and nature-inspired materials in organic electronics applications.^1–3 We have shown that devices performing at the state-of-the-art level can be fabricated entirely from inexpensive and biodegradable materials (see Figure 1).⁴

Our studies of natural dyes also revealed properties with interesting potential consequences for organic semiconductors, such as hydrogen bonding. That type of bonding is a relatively strong dipolar interaction present in many natural chemical systems, being responsible for the unique properties of water and the forces holding together DNA and RNA strands. Hydrogen bonding of so-called pi-conjugated molecules, where pi-pi interactions are strengthened by hydrogen bonds, yields highly ordered, high var epsilon (dielectric constant) organic films. The strong intermolecular interactions influence the dynamics of photogenerated excited states in such materials, which allow for many possibilities compared to disordered van der Waals bonded solids used in organic electronics today.

Figure 1. An example of materials set for a biomaterials-based organic field effect transistor. This device shows well-balanced electron and hole mobilities of 1×10⁻²cm²/Vs and is resistant to degradation in air. Shellac is a natural polyester resin produced by beetles and harvested in Southeast Asia and India for use as a wood varnish. Tetratetracontane is an oligoethylene present in some medicinal plants. Indigo is a dye known since antiquity and is currently the most highly produced dyestuff in the world, primarily for coloring denim.

Indigoid dyes represent an interesting class of organic semiconducting materials. Indigoids are among the very few known blue-colored, natural-origin chromophores. Indigo and 6,6'-dibromoindigo (Tyrian Purple) have been exploited for thousands of years as valuable dyestuffs. We found that vacuum-evaporated indigo films show high ordering with a single-crystalline texture and exceptionally high dielectric constants (in the range of 5–6). These properties translate into high carrier mobilities in indigo and Tyrian Purple. The latter, for example, demonstrates field effect mobility of 0.4cm²/Vs for holes and 0.3cm²/Vs for electrons. Both materials show ambipolar transport, attributed to the electrochemistry of reversible oxidation of amine groups and reversible reduction of keto groups. Tyrian Purple was also remarkable for air-stable transport of both electrons and holes. Transistor and diode devices with this material showed no degradation during operation over several weeks in air.

Figure 2. (a) The crystal packing diagram of indigo. The pi-stacking of indigo molecules is also reinforced by hydrogen bonding, with each molecule bonded to four of its neighbors. (b) X-ray diffraction of an indigo thin film on a glass substrate shows a single diffraction peak in the growth direction. The wide amorphous peak is exclusively from the glass substrate. a.u.: Arbitrary units. 2Θ: Angle of diffraction.

The crystal packing of indigo is shown in Figure 2(a) along with the x-ray diffraction plot for a thin film: see Figure 2(b). The larger strength of hydrogen bonding relative to van der Waals interactions produced ordered crystalline films with a higher dielectric constant than typical organic materials. We demonstrated this in hydrogen-bonded indigoids and acridones. With the knowledge that the exciton binding energy is proportional to var epsilon⁻², we sought to create nonexcitonic single-material solar cells. We obtained encouraging preliminary results with the dye quinacridone, a common pigment used in paints and cosmetics that features intermolecular hydrogen bonding similar to indigo. Contrary to indigo, which is well known for having extremely efficient internal conversion, making it a poor candidate for such a solar cell, quinacridone is highly fluorescent and has a long excited state lifetime. Single-layer metal-insulator-metal structures recorded short-circuit photocurrents in the milliamp per square centimeter range and external quantum efficiencies of 10% at the absorption peak of the dye. Temperature dependence of photocurrent studies showed excitation binding energies below 100meV. Single-layer polymer cells, in contrast, featuring poly(thiophene) or poly(phenylene vinylene), showed short circuit currents in the microamp range and quantum efficiencies of less than 1%. We believe that highly ordered organic materials with strong intermolecular interactions could be used to produce highly efficient nonexcitonic organic solar cells.

Our recent research shows that natural and nature-inspired materials not only can be used to create organic devices with state-of-the-art performance but also provide interesting conceptual clues, such as hydrogen bonding, about the molecular design of organic semiconductors. Understanding and using the strong intermolecular interactions in these materials is the focus of our future research. We are working to develop better absorbers to make competitive single-layer solar cells and to control and optimize film growth to maximize mobilities and performance in transistor-based devices.