Organic Photodetectors

Mon, 01 Jan 0001 00:00:00 +0000

Organic photodetectors (OPDs) convert light signals carrying information into an electrical current. Light is typically absorbed in a photoactive layer based on a blend (i.e., bulk heterojunction) of two materials, an electron donor and an electron acceptor. In typical devices, the energy offset between two organic materials blended together provides the driving force to dissociate electrons and holes created under light (see the schematic above). To quantify the performance of OPDs, several figures of merit are utilized, such as:

External quantum efficiency (or responsivity): the ratio between how many electrons can be extracted from the device in response to photons hitting it
Dark current: parasitic electric current generated without a light signal, determining the device noise
Detectivity: combining the information of both quantum efficiency and dark current in a single figure of merit
Linear dynamic range: how many orders of magnitude of varying light intensity can generate an electrical signal linearly proportional to the light signal
Photodetector speed: how fast electrical signal can be extracted after light hits the device
Wavelength selectivity vs broadband: depending on OPD application, only photons of specific wavelengths or a wide spectral coverage can be targeted

In our lab, several aspects of OPDs are explored towards deeper understanding of their working mechanisms and improvement of performance and stability, for example:

How can we fabricate OPDs capable of harvesting infrared light? Can we push the photodetection to wavelengths beyond 1500-2000 nm?
How can we optimize molecular design of photoactive materials and architecture of device stack for maximum performance?
What is the origin of performance losses in OPDs? Especially, what loss mechanisms are involved when we target infrared photodetection?
How does OPD performance evolve when we operate in real-world conditions for long periods? Are OPDs stable?
What disruptive technologies can be demonstrated when we integrate infrared OPDs into more complex electronic circuits?

Organic Semiconductors

Mon, 01 Jan 0001 00:00:00 +0000

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.

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:

Light is absorbed and a bound electron-hole pair (exciton) is created in D and A domains
Excitons diffuse at the D/A interface
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
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…)

Research | LabANTI Research Group

Organic Photodetectors

Organic Semiconductors