Basic Research Needs for Solar Energy Utilization

INORGANIC PV CELLS

Inorganic PV cells are based on solid-state semiconductors, for example Si or GaAs. Very small amounts of impurities (called dopants)

are added to the semiconductors to endow them with either extra free and mobile electrons (an n-type semiconductor) or extra free and

mobile holes (a p-type semiconductor). When the n-type and p-type semiconductors are joined together, they form what is called a p-n

junction, and an electric field is created between the two regions. When the cell is illuminated, and electrons and holes are thus created,

this electric field helps to efficiently separate the negative electrons from the positive holes. Two electrical contacts to the cell then

provide a path for the electrons to leave the cell; pass through the external circuit to deliver electric power; and then, finally, return to the

cell to recombine with the holes to neutralize them and complete the circuit. The semiconductors do not absorb solar photons that have

energies lower than the semiconductor band gap; photons with energies higher than the band gap are absorbed, but the extra energy

above the band gap is converted to heat rather than to electrical energy. This loss of photon energy to heat is one of the reasons why

the calculated maximum efficiency of conventional solar cells is limited to about 32%.

The figure shows a conventional single p-n junction inorganic PV cell.

The basic principle of operation in an organic

PV cell is that illumination of a mixture of

electron-rich (donor) and electron-poor

(acceptor) species leads to separation of (+) and

(-) charge carriers. Collection of these carriers at

electrodes produces an electrical current that can

do work on a load in an external circuit.

Although the basic principles behind the

operation of organic PV cells and conventional

p-n junction cells are similar, the details of key

events, such as charge carrier generation and

charge transport, are very different in the organic

cells. Consequently, the basic understanding of

the physics and chemistry of organic PV systems

lags behind that of conventional p-n junction

Figure 7 Flexible, plastic PV cell