Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
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these new organic structures, and to maximize energy extraction efficiencies. The research will<br />
strive to achieve new, low-cost, scalable fabrication methods.<br />
RESEARCH DIRECTIONS<br />
Organic Photovoltaic Structures<br />
The current state <strong>of</strong> the art in solar to electrical<br />
conversion efficiency attained with OPV cells<br />
is in the range <strong>of</strong> 3–5% (Padinger et al. 2003;<br />
Peumans and Forrest 2001; Wienk et al. 2003;<br />
Brabec 2004) (see Figures 27 and 28). To<br />
achieve the breakthroughs that will bring OPV<br />
cell technology to the point where it is<br />
competitive with other renewable power<br />
sources, new molecular, polymeric, and<br />
inorganic semiconductor quantum-confined<br />
structures <strong>for</strong> photovoltaic applications are<br />
needed.<br />
The most efficient polymer-based photovoltaic<br />
cells fabricated to date (Wienk et al. 2003;<br />
Brabec 2004) consist <strong>of</strong> bulk heterojunction<br />
structures containing poly(phenylene vinylene)s<br />
(PPVs) or poly(alkylthiophene)s (PATs)<br />
blended with soluble C60 derivatives, such as<br />
PCBM (Figure 29). While these systems clearly<br />
have merit in that there is a body <strong>of</strong> synthetic<br />
chemistry to guide the synthesis and<br />
purification <strong>of</strong> PPVs, PATs, and C60<br />
derivatives, new families <strong>of</strong> strongly lightabsorbing,<br />
electron donor- and acceptor-type<br />
polymers are needed <strong>for</strong> photovoltaic<br />
applications. Key elements that must be<br />
addressed in the development <strong>of</strong> new organics<br />
include broad, tunable absorption throughout<br />
the 400–1300 nm spectral region, the ability to<br />
control highest occupied molecular orbital<br />
(HOMO) and lowest unoccupied molecular<br />
orbital (LUMO) levels, and high hole and<br />
electron carrier mobilities. A number <strong>of</strong> donor-type polymers with good hole mobility are<br />
already available; however, there is a need <strong>for</strong> development <strong>of</strong> new acceptor-type structures that<br />
feature high electron mobility. Synthesis <strong>of</strong> block, graft, or star polymers featuring variable<br />
HOMO-LUMO gap donor and acceptor segments should be pursued, as well as dendrimer<br />
structures. Polymers that are functionalized to facilitate processing—<strong>for</strong> example, to tune surface<br />
102<br />
Figure 28 Current-voltage curves <strong>of</strong> a good<br />
bulk heterojunction device: Short circuit<br />
current, JSC = 4.9 mA/cm 2 ; open circuit<br />
voltage, VOC = 0.84 V; and fill factor, FF =<br />
60%<br />
Figure 29 A typical conjugated polymer<br />
(MDMO-PPV) and a soluble derivative <strong>of</strong> C60<br />
(PCBM) used in a bulk heterojunction device