Page 258 - A Comprehensive Guide to Solar Energy Systems

Chapter 12 • Organic Photovoltaics 261

FIGURE 12.4 Simplified energy level diagram depicting a donor-acceptor heterojunction and the four key processes for
photocurrent generation: (1) photon absorption to form an exciton; (2) exciton diffusion to the organic heterojunction;
(3) exciton splitting; and (4) charge carrier extraction.

they are not influenced by the built-in electric field that exists across the semiconductor
layers in an OPV as a result of the difference in work function between the two electrodes.
excitons do however diffuse randomly, visiting hundreds of individual molecules before
relaxation to the ground state (Fig. 12.4 process 2) with the emission of light, or nonra-
diatively by dissipating their energy as heat (i.e. quanta of thermal energy called phonons
[3]). In an OPV excitons are split at the junction between two dissimilar molecules having
offset HOmO and lumO orbitals as illustrated in Fig. 12.4 (process 3). The heterojunction
provides the thermodynamic driver for the spontaneous splitting of the exciton, as there is
both a favorable enthalpy change and entropy change when a single exciton is dissociated
to form two charge carriers.
It is important that the heterojunction in an OPV is carefully engineered to ensure that
the potential energy step is just enough to dissociate the exciton but no more, since the
maximum potential difference across an OPV is determined by the difference in energy
between a hole in the HOmO of the molecule that has been oxidized (i.e., the electron do-
nor) and the electron in the lumO of the molecule that has been reduced (i.e., the electron
acceptor). In practice, the minimum energy offset required at the heterojunction is 0.2–
0.3 eV. Once the Frenkel exciton has been split the electron and hole in adjacent molecules
are still coulombically bound to one another, although much less strongly than when on
the same molecule, so a moderate electric field in conjunction with the chemical potential
gradient that results from the concentration gradient of charge carriers at the heterojunc-
tion, is sufficient to ensure efficient extraction of the charge carriers to the external circuit.
Indeed, the efficiency with which photons can be converted to electrons in the external
circuit in an OPV can approach 100% provided the excitons are formed within less than
one exciton diffusion length of the heterojunction in both the donor and acceptor phases
[27,28]. The donor-acceptor heterojunction in an OPV is equivalent to the p-n junction in
a conventional inorganic PV, in that it is the part of the device that generates free charge
carriers (Fig. 12.5).