The mathematical framework for this movement is frequently modeled using , which defines the rate of charge transfer ( ketk sub e t end-sub ) between two molecules:
Organic semiconductors (OSCs) differ from their inorganic counterparts due to their , which results in "soft" materials with narrow energy bands. Unlike covalently bonded crystals, OSCs consist of conjugated π-electron systems formed by -orbitals of sp2s p squared -hybridized carbon atoms.
The energy difference between the HOMO and LUMO is the fundamental electronic bandgap ( Egcap E sub g physics of organic semiconductors pdf
When an organic semiconductor absorbs light (a photon), it does not typically create free electrons and holes as in inorganic materials. Instead, the Coulombic attraction between the electron and its positively charged hole is so strong that they remain bound, forming a quasi-particle known as an (specifically, a Frenkel exciton). These excitons have a binding energy on the order of 0.5 eV, making them a distinct and critical player in all optoelectronic devices. The fate of these excitons—whether they will recombine to emit light or dissociate into free charges to generate electricity—is the central drama of OLEDs and organic solar cells, respectively.
The Lowest Unoccupied Molecular Orbital (equivalent to the conduction band). The mathematical framework for this movement is frequently
In a perfect silicon crystal, electrons move as waves. In organic semiconductors, the structural disorder and vibrations mean that charge carriers behave more like particles "hopping" from one site to another.
In inorganic crystals, charges move as delocalized waves through periodic energy bands (band transport). In organic solids, the weak Van der Waals interactions create significant structural disorder, altering how charges move. Instead, the Coulombic attraction between the electron and
, which is significantly higher than in inorganic crystals ( kBTk sub cap B cap T at room temperature).
OLEDs operate on the principle of . Electrons and holes are injected from opposing electrodes.