wettability or post-deposition cross-linking, or to allow layer-by-layer deposition—are also key
objectives.
Considerable success has also been achieved with molecule-based OPV systems that are
deposited onto conducting substrates, predominantly by vapor deposition and in some cases by
wet deposition methods. Deposition methods have been developed to produce nanostructures or
liquid crystalline phases that enhance exciton dissociation and carrier mobility. Although
advances have been made in the development of molecules for OPVs, there is a strong need for
new compounds. The guiding principles behind the development of new small-molecule systems
are similar to those for the polymers (i.e., molecular systems that afford tunable optical
absorption through the 400–1,300 nm range, control of redox levels, and high exciton, hole, and
electron mobility). General synthetic strategies leading to molecular structures that deposit into
ordered phases with strong, long-range inter-chromophore interactions, such as J- or
H-aggregates, are desired.
A third approach that has shown some measure of success involves “hybrid” photovoltaic
structures consisting of blends or composites of organic polymers and inorganic semiconductors
(Huynh et al. 1999; Sun et al. 2005). Success here has been attained primarily by using donor-
type conjugated polymers (PPVs or PATs) as composites with nanocrystalline inorganic
semiconductors (e.g., metal oxides, CdS, CdSe, and CdTe). In these systems the semiconductors
can enhance the visible and near-infrared absorption and also serve as acceptors with good
electron mobility. Continued work in this area is needed. Inorganic semiconductors have a clear
advantage in providing strong near-infrared absorption; they may also provide a significant boost
in carrier generation efficiency due to photon down-conversion processes. Carrier mobility can
be enhanced by controlling the dimensionality of the inorganic semiconductors and the packing
of the organic molecules. The interface between the organic and inorganic phases can be
controlled by using chemical methods to control the functionality of the semiconductor surface.
The reason for the large drop in energy between that of the absorbed photon and the resulting
open-circuit voltage (Voc) is not understood. Extensive theoretical and experimental work is
needed to increase the power conversion efficiency of such photovoltaic devices by an order of
magnitude. The open-circuit voltage is related to the offset between the HOMO level of the
electron donor/hole transport phase and the LUMO level of the electron acceptor/transport phase,
although the exact mathematical relationship is not fully developed. The origin of the large
difference between Voc and the HOMO-LUMO band offset must be understood, as should the
role of the difference in work functions of the contact electrodes (see Figure 30).
Exciton and Carrier Transport
Photoexcitations in organic semiconductors are fundamentally different from those in inorganic
semiconductors. Whereas light absorption in inorganic semiconductors leads to the direct
generation of mobile charged carriers, light absorption in organic semiconductors leads to the
generation of excitons that typically have an associated binding energy in excess of 0.2 eV.
Exciton and charged carrier transport in organic semiconductors is at the heart of the operation of
these devices; their optimization will require a detailed microscopic understanding of the