Figure 6 Current record efficiencies (as a function of the number of
junctions) compared with the theoretical limits as predicted by the detailed
balance analysispathway to achieving these higher efficiencies may require identification and exploitation of new
phenomena involving photon manipulation, absorption, charge creation, and separation; new
materials; and novel device structures — or it may build on the evolution of present-day
technologies. The grand challenge is to push solar cell efficiencies toward their theoretical limits
while maintaining low cost; this can only be done through fundamental research that identifies
new photon phenomena, new materials, and improved implementation of the more familiar
materials.
Organic Photovoltaics
Solid-state PV cells based on carbonaceous (organic) matter were first discovered 20 years ago
(Tang 1986). Early work on organic photovoltaics using molecular-based systems demonstrated
the concept; however energy conversion efficiencies were low. Considerable excitement in this
area was generated by a report published in the mid-1990s of 2.9%-efficient cells based on
conducting organic polymers mixed with derivatives of C 60 (fullerene) (Yu et al. 1995). During
the past decade, refinements in the chemical components of the cells, improvements in cell
physics, and device engineering have led to individual demonstration cells that operate at greater
than 5% solar-to-electrical-power conversion efficiency. The opportunities and potential payoff
here are significant: low-cost, lightweight, large-area, flexible, high-efficiency solar cells. The
materials are basically like those used in video display technology, and they offer the possibility
of very significant cost reduction, as well as flexibility in installation, form factor, etc. The basic
research goal is to develop sufficient understanding of such materials and structures to improve
their conversion efficiency by a factor of 5–10, and thus obtain robust, scalable efficiencies of
15–25% in cheap, plastic-type solar cells (Figure 7).