Nanostructured solar cells

Conversion into electrical power of even a small fraction of the solar radiation
incident on the Earth’s surface has the potential to satisfy the world’s energy demands without generating CO2 emissions. Current photovoltaic technology is
not yet fulfilling this promise, largely due to the high cost of the electricity produced. Although the challenges of storage and distribution should not be
underestimated, a major bottleneck lies in the photovoltaic devices themselves.
Improving efficiency is part of the solution, but diminishing returns in that area
mean that reducing the manufacturing cost is absolutely vital, whilst still retaining good efficiencies and device lifetimes.

Solution-processible materials, e.g. organic molecules, conjugated polymers
and semiconductor nanoparticles, offer new routes to the low-cost production of
solar cells. The challenge here is that absorbing light in an organic material
produces a coulombically bound exciton that requires dissociation at a
donor–acceptor heterojunction. A thickness of at least 100 nm is required to
absorb the incident light, but excitons only diffuse a few nanometres before
decaying. The problem is therefore intrinsically at the nano-scale: we need
composite devices with a large area of internal donor–acceptor interface, but
where each carrier has a pathway to the respective electrode. Dye-sensitized and
bulk heterojunction cells have nanostructures which approach this challenge in
different ways, and leading research in this area is described in many of the
articles in this special issue.

This issue is not restricted to organic or dye-sensitized photovoltaics, since
nanotechnology can also play an important role in devices based on more conventional inorganic materials. In these materials, the electronic properties can be controlled, tuned and in some cases completely changed by nanoscale confinement. Also, the techniques of nanoscience are the natural ones for investigating the localized states, particularly at surfaces and interfaces, which are often the limiting factor in device performance.

This issue provides concrete examples of how the techniques of nanoscience and nanotechnology can be used to understand, control and optimize the performance of novel photovoltaic devices.