Why polymer solar cells??

Plastic solar cells can be successfuly realized as "bulk heterojunctions" between an organic polymer and organic molecule as electron acceptor, eg fullerene embedded into a conjugated polymer.

Advantages:

• lightweight (which is important for small autonomous sensors)
• disposable
• inexpensive to fabricate
• flexible
• designable on the molecular level
• have little potential for environmental impact.

Challanges:

• low efficiencies (~ 5%) so far
• degradation effects: efficiency decreases due to environment effects: UV rays, moisture etc

An important difference to inorganic solid-state semiconductors lies in the generally poor (orders of magnitudes lower) charge-carrier mobility in these materials, which has a large effect on the design and efficiency of organic semiconductor devices.

However, organic semiconductors have relatively strong absorption coefficients (usually >= 10E5 cm^-1), which partly balances low mobilities, giving high absorption in even <100 nm thin devices.

Another important difference to crystalline, inorganic semiconductors is the relatively small diffusion length of primary photoexcitations (called excitons) in these rather amorphous and disordered organic materials. Exciton binding energies usually exceeding those of inorganic semiconductors. These features of organic semiconducting materials lead generally to devices with very small layer thicknesses of the order <=100 nm.

http://en.wikipedia.org/wiki/Polymer_solar_cell
http://en.wikipedia.org/wiki/Conductive_polymers
Harald Hoppea and Niyazi Serdar Sariciftci: Organic solar cells: An overview, J. Mater. Res., Vol. 19, No. 7, Jul 2004, p1924