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(3)
Exciton dissociation: At the donor: acceptor interface, the exciton dissociates
forming free charge carriers.
(4)
Charge transport: Once free charge carriers are generated, they are trans-
ported to their respective electrodes according to the electric field in the
device. The holes and electrons travel through the percolated network of
donor and acceptor, respectively. Once collected by the electrodes, they are
channeled into the external circuit.
Poly(3-hexylthiophene-2,5-diyl (P3HT) and [6,6]-phenyl-C 61 -butyric acid
methyl ester (PCBM) are extensively studied and commonly used donor polymer
and acceptor molecule, respectively. A comprehensive overview of all photoactive
polymers and acceptor molecules could be found elsewhere [ 8 , 9 ]. Similarly,
several reviews could be consulted for deeper understanding of device physics of
PSCs [ 7 , 10 , 11].
2.3 Photovoltaic Characterization
Like inorganic solar cells, the photovoltaic properties of a PSC are described with
four key parameters derived from a current-voltage (IV) curve (Fig. 5 ). These
parameters are open circuit voltage (V OC ), short circuit current density (J SC ), fill
factor (FF), and power conversion efficiency (PCE). PCE is the most common
parameter used as a figure of merit to describe the performance of any solar cell. It
indicates the percentage conversion of power density received by a solar cell from
the incident light into electrical power. From the IV curve, PCE can be deduced as
follows:
PCE ðÞ ¼ P max
P in
100 ¼ I sc V oc FF
area P in
100
ð 1 Þ
FF describes the ''squareness'' of a JV curve and is given by the ratio of (I max 9
V max )/(I SC 9 V OC ). FF denotes the extent of internal loses of generated current in a
solar cell and is affected by high series -and low shunt- resistance. Shunt resistance
is calculated from the inverse of the slope at the J sc point in the IV curve. Low-
shunt resistance or ''shunting'' is a result of manufacturing defects where the
positive and negative electrodes within the device are not well isolated (for
example, intercalation of spikes in the topology of bottom electrode into the top
electrode). Such defects result in current leakage and excessive current leakage
may lead to lowering of V oc and can even cause ''short circuit'' rendering the solar
cell nonfunctional. Series resistance, on the other hand, is characterized by the
inverse slope at the V oc point in the IV curve. Series resistance is a result of
recombination at the material interfaces, defects, poor BHJ morphology, lack of
percolation in the donor: acceptor network, the contact resistance, and the sheet
resistances of the electrodes. Comprehensive information on the device physics of
PSCs can be found elsewhere [ 10 ].
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