Basic Research Needs for Solar Energy Utilization

ORGANIC PV CELLS

Organic solar cells also operate with junctions, but the n-type and p-type semiconductors are organic compounds, and the interfacial

junction between the n- and p-type regions does not produce an electric field and serves a different purpose than the inorganic p-n

junctions. Furthermore, when electrons and holes are produced upon light absorption in organic solar cells, the negative electrons and

positive holes become bound to one another through strong attractive electrical forces and form coupled electron-hole pairs, which have

been labeled excitons. These excitons have no net electrical charge and cannot carry current — they must be broken apart, or

dissociated, in order to produce the free electrons and holes required in the cell to produce electrical power. This is the function of the

junction between the n- and p-type organic compounds — when the excitons diffuse to this region of the cell, they split apart and

produce the required free electrons and holes. Also, organic solar cells have electrical contacts with different electronic properties.

The organic components of the n- and p-type regions of the organic solar cells can be either molecular semiconductors (like an n-type

perylene and a p-type phthalocyanine) or semiconducting polymers (like a p-type polyphenylvinylidene or polythiophene and an n-type

fullerene). The figures show the PV cell with both types of organic components and their corresponding chemical structures.

light energy directly into fuel eliminates the need for external wires and a separate electrolyzer.
The PEC approach to solar energy conversion has achieved high efficiencies for both electrical
power (>15%) and hydrogen generation (>10%). However, photoelectrode lifetime and cost
issues have restricted commercialization efforts to date (Memming 2001; Bard et al. 2002; Nozik
and Memming 1996).

The electric field formed by the junction at the semiconductor-electrolyte interface plays a large
role in efficiently separating the electron-hole pairs created by light absorption. When electrons
or holes cross the interface between the semiconductor and the electrolyte, they can drive
chemical oxidation or reduction half-reactions at the semiconductor surface to produce either
fuels (e.g., hydrogen, reduced carbon, ammonia) or, in a regenerative photovoltaic configuration,
DC electrical power. In the latter case, termed an electrochemical photovoltaic (EPV) cell, the
oxidation and reduction half-reactions at the two cell electrodes are the inverse of one another,
thus producing no net change in the electrolyte. In the former case, the oxidation and reduction
half-reactions at the two electrodes are different, and their sum produces a net chemical change
in the electrolyte (e.g., decomposition of water into H 2 and O 2 ).