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Organic Semiconductors and Organic Photovoltaics


By
Renad A AlJefri

August 30, 2016 - Posted in Discussion
​Organic Semiconductors
Organic semiconductors include small molecules, oligomers (short polymers) and polymers. Examples of polymer organic semiconductors are conjugated polymers such as poly(3-hexylthiophene) (P3HT) and poly(p-phenylene vinylene) (PPV). When blended with fullerenes, these polymers make low efficiency photovoltaic material. In addition to organic semiconductors use in solar cells, they are evolving into flexible and more energy efficient LEDs, detectors, transistors and other electronic and spintronic devices. 

Organic Photovoltaics  (OPV)
Organic solar cells and panels have the potential to make solar to electricity conversion widely available and accessible to humankind. Organic solar cells can be made from inexpensive and abundant materials and can be manufactured into high throughput using scalable production with low cost processing and low energy input (1). The photovoltaic material can be made from inexpensive organic semiconductors that can be processed and recycled more economically than competing crystalline inorganic semiconductors. It is possible that organic solar panels can be completely solution processed. This includes solution-processed electrodes, the substrate and integrated organic diodes and organic power optimizer circuits. Furthermore, the flexibility of chemical tuning and the good solution rheology meet the demand of cheap solar cells. 
Organic photovoltaics is an excitonic solar cell (2), which are characterized by bound excitons that are generated after excitation with light. The Excitons are quasiparticles that consist of a strongly bound state of electrons and holes. Organic based PV can be divided into three different types: dye synthesized solar cells, small molecule organic solar cells and polymer solar cells. They all share similar photovoltaic action physics whereby the photovoltaic action is modeled by donor-acceptor systems. The photoactive layer of organic solar cells consists of two materials: an electron donor material and an electron acceptor material. The donor typically does most of the light absorption; current acceptor materials absorb little light. The photo excitation of the donor material (conjugated polymer) generates photoinduced excitons, or bound electron-hole pairs, which can only be separated into negative and positive charge carriers at the donor-acceptor interface. The following discussion focuses on polymer photovoltaics but it is widely applicable to organic Photovoltaics in general. 

Current Status of Organic Semiconductors Photovoltaics
The efficiencies of single junction polymer photovoltaic are increasing on a yearly basis and have reached over 8% on a single junction polymer solar cell with a small area lab cell ~1cm2 (Konarka 8.3% and Solarmer 8.1%) but with limited stability. The lifetime of commercial glass encapsulated modules ,from Konarka, have been limited to three or four years. Small molecules photovoltaic are much more stable than polymer based material, and tandem junction with similar efficiencies (Heliatek 8.3%, 2010) but longer lifetimes were also demonstrated on small area lab cells. Dye-sensitized solar cell (DSSC), which is a slightly older technology, were able to achieve higher efficiencies (over 13%), but they currently suffer from limited lifetime, and from the use a corrosive liquid electrolytes that makes their encapsulations difficult on a commercial scale.

Works Cited 
1. A. J. MoulÈ, Current Opinion in Solid State and Materials Science 14, 123 (2010).
2. C. Wadia, A. Alivisatos, D. Kammen, Environ. Sci. Technol 43, 2072 (2009).
3. F. C. f. S. E. S. Jan Kosny. (Cambridge 2010).

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