Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 12

Fig. 1.8 Operation of a solar cell at different biases: a large reverse bias; b small reverse bias;
c positive bias, zero resultant internal field, and corresponding to open circuit condition;
d positive bias, carrier injection

Short circuit current (J sc ). The short-circuit current is defined as the current at
which the externally applied voltage is 0. J sc represents the number of charge carriers
that are generated and eventually collected at the electrodes at short circuit condition.
Enhanced optical/electrical parameters such as a small band gap, high absorption
coefficient, smaller phase separation, and high carrier mobility improve J sc .
Open circuit voltage (V oc ).The open circuit voltage defined as the voltage at
which the current density output is 0. V oc has been reported to be mainly dependent
on the work function difference of metal contacts. If an ohmic contact is formed at
the electrodes, V oc is dependent on the HOMO-LUMO difference between the
donor and the acceptor.
Fill Factor (FF). The fill factor defines the shape of the J-V curve and is
defined as

FF ¼ J mpp V mpp
J sc V oc

ð 1 : 2 Þ

where J mpp and V mpp are the current density and the voltage at the point of
maximum output power respectively. As shown above, FF is the ratio between the
maximum power output point and the maximum attainable power output, i.e., J sc
times V oc . FF represents dependence of current output on the internal field of the
device and is quantified by the series resistance and shunt resistance. For instance,
low carrier mobility will cause carriers to recombine before reaching a hetero-
junction. In this case, increasing the external bias will sweep the carriers, which
otherwise may recombine at lower field strength, to the heterojunction for disso-
ciation, resulting in an increase in current output. This leads to a strong depen-
dence of current on the applied bias, which is shown by a lower FF.