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Pi-Pi* Semiconducting Bandgaps and Narrow Absorption Bandwidths

Renad AlJefri

October 26, 2016 - Posted in Discussion
Pi-Pi* Semiconducting Bandgaps

Organic semiconductors are made from conjugated organic materials, which have an alternating single bond (sigma bond) and double bonds (sigma and pi bond). The semiconductors’ nature arises from the delocalized and weakly held pi electrons. The pi to pi* energy transition controls the electronic and optical properties of the materials. The band gap of current organic semiconductors ranges from 1.4 eV to 2.5 eV, which enables the fabrication of multijunction solar cells and offers low band gap materials that are close to optimal energy gaps relative to the solar spectrum (3). The narrow absorption bandwidth limits the efficiency of organic single layer solar cells to much less than the S-Q theoretical limit.  

Narrow Absorption Bandwidths 

Organic semiconductors are held by weak van der Walls forces, which make their processing relatively inexpensive but reduce their mobility. The weak intermolecular interactions and the strongly localized electronic wavefunctions lead to a narrow absorption bandwidth as opposed to the larger absorption bandwidth in inorganic semiconductors.
Nevertheless, this narrow spectrum absorption property could be seen as an advantage in some niche applications that require cells coloring, such as building integrated photovoltaics. Furthermore, this property could be overcome by using tandem (multijunction) cells to allow for a wider absorption of the light spectrum.

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