Organic Semiconductors

Organic optoelectronic devices utilize molecules based on a conjugated carbon backbone with other light elements (hydrogen, oxygen, sulfur…) where interactions between light and electrons can take place. For example:

  • Organic photodetectors generate an electrical signal in response to light shining on the molecules
  • Organic solar cells generate electrical power under a flow of light (from sun or artificial sources) on the molecules
  • Organic LEDs generate light when electrical current is injected into the molecules.

Device architecture

Compared to traditional silicon-based optoelectronic devices, organic materials (polymers and small molecules) offer exciting advantages, including:

  • Possibility of harvesting wavelengths where silicon cannot work (infrared >1100nm)
  • Fabrication is cheap and consumes little energy (e.g., by solution processing)
  • Large absorption coefficient: only small amount of material needed in the device, another bonus point towards sustainability
  • ‘Unlimited’ options for tuning the optoelectronic properties of organic materials by chemical design (e.g., adding side-chains, modifying end groups or molecular core…)
  • Functional and esthetic features such as lightweight, flexibility/bendability, transparency, color-tunability
  • Compatibility with biological systems

The diagram above shows how light-to-electricity conversion works in organic semiconductors, where the photoactive layer is typically a blend of an electron donor (D) and an electron acceptor (A). When photons arrive on the device:

  1. Light is absorbed and a bound electron-hole pair (exciton) is created in D and A domains
  2. Excitons diffuse at the D/A interface
  3. Thanks to the difference between D and A energy levels (HOMO and LUMO molecular orbitals) and interfacial charge transfer state is created, which is less bound than the original exciton
  4. Electrons and holes are fully separated and extracted at the contacts

Despite the impressive leaps in organic device performance achieved in recent years, there are several challenges to overcome, which makes organic optoelectronics a highly interesting and stimulating fields for curious new researchers:

  • The low dielectric constant of organic materials results in a high binding energy between electrons and holes generated under light (i.e., excitons). This requires careful optimization of materials and device design to enhance charge dissociation
  • Organic materials are non-crystalline and prone to formation of traps and defects which limit device performance
  • Designing molecules with certain properties can be very complex for organic chemistry (e.g., far infrared absorbers)
  • Organic materials can be unstable and prone to degradation, especially when utilized in devices under electrical field, high temperatures, sunlight
  • It is not trivial to adapt the fabrication process for organic devices when moving to large-scale industrial production while preserving high performance (issues with reproducibility, large-area deposition, need for green solvents…)