Page 264 - A Comprehensive Guide to Solar Energy Systems

Chapter 12 • Organic Photovoltaics 267

favors crystallization in the condensed phase. A high degree of crystallinity in both donor
and accept phases is beneficial for two reasons: it helps to maximize the free charge carrier
mobility (velocity per unit field), which enables efficient charge carrier extraction to the
external circuit. electrical losses due to recombination are more prevalent in disordered
organic semiconductor films because free charge carriers are delayed as they move toward
the electrodes, which increases the likelihood of electron-hole recombination, thus erod-
ing the J sc and device FF. A high degree of crystallinity in both donor and acceptor phases
also helps to minimize losses in V oc , because it reduces the energy needed to split the exci-
ton. One widely used means of improving the degree of phase separation and crystallinity
in both donor and accept phases in a bulk heterojunction, is to anneal the film at low tem-
perature (<150°C) either during film deposition (in the case of vacuum deposited OPV) or
post-film deposition (for solution processed OPV), which also helps to ensure that the film
morphology has achieved equilibrium.
What is particularly special about organic semiconductors (both small molecule and
polymer) as a class of semiconducting materials for electronic devices, is their amena-
bility to facile engineering of the HOmO and lumO energies to match the needs of the
application. This can be achieved by chemical modification of the conjugated core, or
by attaching electronegative or electropositive groups at the periphery of the conjugated
core, which modify the HOmO and lumO energies via the inductive effect [18,43,44].
like conventional semiconductors, organic semiconductors can be doped using n or p
type dopants to increase their conductivity. In the context of an OPV device, doping is
however only useful for minimizing the barrier to charge carrier extraction by the elec-
trodes, since dopants in the photoactive region operate as electron-hole recombination
centers, degrading the J sc . However, organic semiconductors are sometimes described as
having either p or n type character even when undoped because they conduct one charge
carrier type more efficiency than the other. An intrinsic preference for the conduction of
electrons (holes) reflects a combination of the accessibility of the lumO (HOmO) for in-
jection or extraction of charge carriers to the external circuit, and the high electron (hole)
mobility. Consequently, the character of an organic semiconductor can be switched from
n to p-type without altering the optical properties by tuning the energy of the orbitals
responsible for charge transport to make them more or less accessible for the injection/
extraction of charge carriers by the electrode. To illustrate the extent to which the proper-
ties of organic semiconductors can be engineered by simple chemical modification it is
useful to consider the case of copper phthalocyanine, which was first recognized for its
semiconducting properties by eley and Vartanyan (independently) in 1948 [45] and for
decades was used as the electron donor in small molecule OPVs [26]. By replacing the
hydrogen atoms at the periphery of the macrocycle with electronegative atoms such as
fluorine, the electron affinity and ionization potential can be increased by approximately
the same amount of ∼1 eV, making the lumO more accessible for electron injection/
extraction without significantly changing the optical properties. using this approach yang
et al. [46] demonstrated that fluorinated copper phthalocyanine can serve an electron
acceptor in OPVs.