these new organic structures, and to maximize energy extraction efficiencies. The research will
strive to achieve new, low-cost, scalable fabrication methods.
RESEARCH DIRECTIONS
Organic Photovoltaic Structures
The current state of the art in solar to electrical
conversion efficiency attained with OPV cells
is in the range of 3–5% (Padinger et al. 2003;
Peumans and Forrest 2001; Wienk et al. 2003;
Brabec 2004) (see Figures 27 and 28). To
achieve the breakthroughs that will bring OPV
cell technology to the point where it is
competitive with other renewable power
sources, new molecular, polymeric, and
inorganic semiconductor quantum-confined
structures for photovoltaic applications are
needed.
The most efficient polymer-based photovoltaic
cells fabricated to date (Wienk et al. 2003;
Brabec 2004) consist of bulk heterojunction
structures containing poly(phenylene vinylene)s
(PPVs) or poly(alkylthiophene)s (PATs)
blended with soluble C 60 derivatives, such as
PCBM (Figure 29). While these systems clearly
have merit in that there is a body of synthetic
chemistry to guide the synthesis and
purification of PPVs, PATs, and C 60
derivatives, new families of strongly light-
absorbing, electron donor- and acceptor-type
polymers are needed for photovoltaic
applications. Key elements that must be
addressed in the development of new organics
include broad, tunable absorption throughout
the 400–1300 nm spectral region, the ability to
control highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular
orbital (LUMO) levels, and high hole and
electron carrier mobilities. A number of donor-type polymers with good hole mobility are
already available; however, there is a need for development of new acceptor-type structures that
feature high electron mobility. Synthesis of block, graft, or star polymers featuring variable
HOMO-LUMO gap donor and acceptor segments should be pursued, as well as dendrimer
structures. Polymers that are functionalized to facilitate processing—for example, to tune surface
Figure 28 Current-voltage curves of a good
bulk heterojunction device: Short circuit
current, JSC = 4.9 mA/cm^2 ; open circuit
voltage, VOC = 0.84 V; and fill factor, FF =
60%Figure 29 A typical conjugated polymer
(MDMO-PPV) and a soluble derivative of C 60
(PCBM) used in a bulk heterojunction device