Dictionary of Terms in Organic Electronics. Specific Terms from F to H.

Fill factor

Fill factor is a property of a completely assembled photovoltaic cell. It determines a maximum power and power conversion efficiency that a solar cell with a certain short circuit current density I_sc (or J_sc) and open circuit voltage V_oc can produce.

Where:
I_mpp = current that a solar cell produces at the maximum power point (mpp) of I/V curve in the 4th quadrant.
V_mpp = voltage that a solar cell produces at the maximum power point (mpp) of I/V curve in the 4th quadrant.
I_sc = short circuit current
V_oc = open circuit voltage

Maximum power point: is a point on I/V curve, where I x V represents a maximum value (maximum power).

Maximum power is always lower than a product of I_sc and V_oc due to many factors, among which series and shunt resistances of a device are critical.

Hole transport

Hole is a positive charge (+1) that can move across a semiconductor. Hole conductivity occurs when positive charges can move, whereas both electrons and negatively charged ions (anions) cannot move easily across a semiconductor. Correspondingly, the negative charges contribute minimally to overall conductivity of the material.

Holes can form near the interface of a semiconductor (silicon on the scheme below) and positively charged bias (usually metallic anode). That occurs when an electron from a silicon atom transfers to the metal. Subsequent movement of the hole, thus generated, to the cathode occurs through electron pair transfer form a neighboring bond to the positively charged atom. As a result, the charge appears on the neighboring atom whereas the previously charged atom becomes neutral. The process continues until the hole reaches the cathode and takes an electron from it. As a result - negative and positive charges from both biases are neutralized and all Si-Si bonds on the hole's pathway are regenerated.

Holes may preexist also in a semiconductor that is not connected to the charged electrodes. This type of semiconductors is called p-conductors or p-transporters. Holes can roam in these materials whereas their negatively charged counterparts (negative ions) cannot move easily and stay on permanent positions. Concentration of holes may be increased by p-doping, addition of a component with high electron affinity that is called p-dopant. This component takes electrons from semiconductor atoms and traps them in a form of static negative ions. Positive charges, holes, roam around unless electrical field is applied. Once it is applied, they start moving in one direction - to the negatively charged cathode.

Hole conductivity in organic semiconductors occurs when:
1. Molecules of an organic compound/polymer contain long chains of conjugated unsaturated bonds.
2. Molecules of an organic compound/polymer possess electron donating properties, e.g. can 'surrender' one of their electrons easily. Many types of organic conducting polymers are p-type semiconductors. The most widely known are: polyacetylene, polypyrroles, polythiophenes, polyphenylvinylenes (PPVs).

Mechanism of the hole conductance in organic semiconductors is somewhat more complex than that of inorganic materials. Hole conductance may occur along the chain of conjugated unsaturated bonds (simplified scheme see below) as well as between the chains or planes (hopping transport). In any case, the polymers that possess higher order of the chains organization possess also better hole conductivity and, correspondingly, hole mobility. Holes in organic materials may exist and move also together with an unpaired electron, e.g. cation-radical.

In general, hole mobility should be lower than electron mobility, because mechanism of the hole transport requires breakage and formation of covalent chemical bonds, which takes both energy and time to occur. This is always true for inorganic semiconductors, whereas organic semiconductors most often exhibit higher hole mobility than electron mobility. This phenomenon is apparently due to generally higher stability and lower energy of organic cations (carbenium ions) over organic anions (carbanions).