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Organic Photodiodes

  • In imaging devices such as cameras or camcorders, image sensors are used to convert the incident light which is reflected from the subject to electrical signal.

  • ​Photodiodes are the unit device of image sensor.

(i.e., Image sensor consists of a large number of photodiodes as a form of array.)

  • Since Si has indirect band gap, Si can't distinguish the color of incident light. Therefore, color-filters are required to distinguish the color for Si-based photodiodes.  

  • However, since the transmittance of color-filter is not a zero, color-filter can absorb incident photons and it not only leads to the decrease of quantum efficiency of photodiode, but also limits the lens design for image sensor application.

  • Moreover, Si has low absorption coefficient (< 10  cm   ) for visible light and therefore, thick active layer (> 1 μm) is needed for sufficient absorption of photons.

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  • Organic semiconductor can be fascinating alternative because of its high absorption coefficient (~ 10   cm   ) and superior processability (spin or spray coating, inkjet printing, etc.).

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  • Moreover, by molecular engineering such as the control of π-conjugation length and/or optimization of molecular orientation, the absorption range of organic semiconductors can be changed to have a selectivity to specific color range (R/G/B).

  • However, to fabricate the conventional organic photodetectors, the absorption spectrums of donor and acceptor materials should coincide and this method can be applied only for green-selection and is practically impossible for blue- and red-selection.

  • To overcome this drawback, we introduced the concept of "Non-absorbing acceptor", which is a compound word of "non-absorbing" and "electron acceptor".

  • Non-absorbing acceptor is the electron acceptor which hardly absorbs the visible light (400 - 700 nm).

  • By using non-absorbing acceptor with color-selective donor, the absorption spectrum of donor material can be fully reflected on the sensing spectrum of the final photodetector.

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