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Organic semiconductors are synthetically organic molecules that with unique optoelectronic characteristics. Typically building blocks are π bonded (conjugated) molecules made by carbon and hydrogen atoms, at times, heteroatoms such as nitrogen, sulfur, and oxygen. Organic semiconductors generally form of semi-crystalline or amorphous thin films, it could be a semiconducting or conducting form when charges are either injected, upon doping, or by photoexcitation.
The biggest advantage of organic semiconductors is their processability. By introducing appropriate functional groups into the molecules, organic semiconductors can be film-forming by simply coating and printing technologies through their formulation inks, enabling a cost-effective and large area deposition under low temperature and ambient atmosphere. Besides, organic semiconductors not only perform very well in terms of processability, but they also exhibit excellent tunability to adjust the energy levels, film morphology, and optoelectronic properties via molecular design.
Today organic semiconductors are mainly used as active elements in organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic solar cells (OSCs, also known as organic photovoltaics, OPVs) and organic photodetectors (OPDs).
Organic Photovoltaics (OPVs)
OPV is a type of photovoltaic technologies that use organic semiconductor as photoactive element, for light harvesting and charge transport to generate electricity from sunlight or indoor light by photovoltaic effect. Most OPV products are solution-processed, which can be produced by coating/printing technologies via a roll-to-roll continuous manufacturing, resulting in low production costs to fabricate a large volume.
Combined with the diversity of organic semiconductors, molecular design can change the optical bandgap, allowing for optoelectronic tunability. Importantly, the optical absorption coefficient of organic semiconductor is quite high, therefore a tiny of incident light can be absorbed easily with a very thin photoactive layer, performing an outstanding power conversion efficiency under low light condition, which can be a valuable technology for energy harvesting in the indoor environment.
Compared to conventional solar cell technologies, OPVs are lightweight, potentially inexpensive to fabricate, and have less adverse environmental impact. OPVs also have the benefit to exhibit semi-transparent, variable shapes, and tunable colors, suggesting applications in window, walls, and curtain, and portable electronics. In short, OPV is not just a photovoltaic technology, it is a groundbreaking energy harvesting technology with unique features, offering unlimited freedom of design.
Organic Photovoltaics (OPV) – The Emerging Solar Technology
Flexibility – Adaptable to various sizes and shapes with excellent portability
Flexibility including light weight, versatility in various form factors are the attributes that OPVs offer. All these are made possible as the general processing temperature requirement is below 150C
Printability – Roll to roll Manufacturing
Solution processable OPVs offers possibility of using Reel to reel printing methodology to coat the layer by layer, resulting in high production speeds and lower capital costs.
High Indoor Performance
One of the key characteristics of OPVs is their absorption characteristics is most suited to emission spectrum of fluorescent lamps and possibly for white LED as well. Furthermore, they perform better at absorbing low intensity light with high incidence angle.
OPV could be made semitransparent offer possibility to be use as a coating or laminates on windows in the buildings that enables power generation and transmitting light.
Small CO² Footprint
A truly green sustainable energy with shorter energy payback time
Organic Photodetectors (OPDs)
OPDs is an emerging optical sonsor compoent which using organic semiconductors as photoactive layer. OPD have attracted substantial interest for photodetection and imaging technologies because of their advantages of high responsivity, sensitive dynamic range, and ease of large area fabrication; such devices have all the necessary ingredients to be used in high-end image applications.
The spectral response of organic semiconductors can be designed and tuned by adjusting the chemical structure, and the application of organic image sensors can be easily extended by changing the photoactive materials with various light responses. Among, infrared sensor technologies are essential to many applications, including vital sign monitoring, biometric sensors, and machine vision. Silicon is still the mainstream by far for optical sensor manufacturing. However, the spectral response of silicon photodetectors is intrinsically limited when the wavelength is longer than 1000 nm, and that the alternatively semiconducting materials require high-temperature growth and complex bonding processes, and at a cost that remains prohibitive for large-area manufacturing.
Nowadays, organic semiconductors have reached an spectral response beyond near-infrared. Monolithic integration of an OPD and readout integrated circuit also enables several advantages, e.g. larger fill factor, thinner sensor structure, and better detectivity, leading to organic image sensor that are more sensitive to light. The overall tendency indicates that OPD technology has greatly improved and has moved forward to a new landscape.
Future Vision of
Organic Image Sensors
How Organic Photodiode Works?
Both OPV and OPD are based on an organic photodiode architecture. As with inorganic photodiodes, the function of an organic photodiode is to generate current and voltage from light absorption. When the organic photoactive layer (made with organic semiconductors) absorbed the light, leading to excitation of an electron from the HOMO level to the LUMO level. The excited electron will leave behind a positively-charged space also known as a ‘hole’. Due to the opposite charges of the hole and electron, they become attracted and form an electron-hole pair, also known as an ‘exciton’. Due to organic semiconductors have larger exciton binding energy, theoretically the exciton cannot be dissociated directly by thermal energy alone in organic photoactive layer. At least two different organic semiconductors are needed within a photoactive layer. The energy levels between the two different organic semiconductors are offset, also known as electron donor and electron acceptor, with the difference being greater than exciton binding energy, allowing exciton dissociation to occur at the heterojunction.
Here, the concept of bulk heterojunction (BHJ) which the donor and acceptor materials are intimately mixed at the nanoscale level is wildly used in photoactive layer formation. BHJ type photoactive layer exhibits an obvious enhancement in photocurrent generation by optimizing the morphology to induce the formation of a bi-continuous interpenetrating network to increase the exciton dissociation efficiency.
Eventually, the dissociated electron and hole will then diffuse to the appropriate electrodes (cathode and anode) through the relevant interlayers, the photo-induced current or electrical signal are collected