Page 226 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 203

Exciton
Electron
Light
Hole

Anode Cathode
Donor | Acceptor Donor | Acceptor Donor Acceptor
(b)

Donor | Acceptor Donor | Acceptor Light Donor
X
Anode Cathode

(a) Acceptor
(c)
FIGURE 6.6 (a) Energy level diagrams for organic donor and acceptor materials.
Photoexcitation of the donor creates a tightly bound electron-hole pair known as an
exciton. The electron from the exciton can lower its energy by passing to the lower-
lying LUMO level of the acceptor, splitting the exciton; excitons in the acceptor are
split when a hole transfers to the higher-lying HOMO level of the donor. (b) Schematic
of a discrete heterojunction device; excitons created close to the donor/acceptor
interface are split into free electrons and holes that are then transported to the
cathode and anode by the acceptor and donor layers, respectively. (c) Schematic of a
bulk heterojunction device, in which the donor and acceptor materials are blended
together on a nanometer length scale; all excitons are created close to an interface,
resulting in a high yield of free carriers; for effi cient operation, continuous pathways
must exist from the point of generation to the electrodes, otherwise charges become
trapped at dead ends and eventually recombine.

bound in the form of intermolecular charge-transfer excitons even
after partitioning has occurred, leaving them susceptible to eventual
geminate recombination. To achieve complete dissociation, the elec-
trons and holes must (at a pictorial level) gain sufficient kinetic energy
in the charge-transfer process to overcome their residual attraction,
†
which is thought to require an offset of at least 0.5 eV in the energies
of the relevant frontier orbitals. 8

† Unlike solar cell applications where one wishes to avoid excessive energy loss
during the charge-transfer process to maintain high power conversion effciencies,
in photodetector applications it is beneficial to make this offset as large as possible
so as to maximize the free carrier yield.