8 Open-Circuit Voltage in Organic Solar Cells
199
Fig. 8.3 Plot of measured
V OC vs. energy of the CT
state. Reprinted with
permission from [19].
Copyright 2014, American
Chemical Society
E CT = E DA −
e
2
4πε 0 ε r d
(8.2)
where E DA is the energy-level difference between the HOMO of the donor and the
LUMO of the acceptor, ε 0 is the vacuum dielectric constant, ε r is the relative dielectric constant, and d is the distance between electron and hole at the D/A interface.
Street et al. reported a linear correlation between the energy of the CT state and
the measured V OC of the organic photovoltaic devices (Fig. 8.3) [19]. The empirical energy difference between the CT state and eV OC was 0.55 eV. A strategy to
minimize this energy loss should be devised in order to enhance the PCE of OSCs.
8.2.3 Equivalent Circuit Model
An equivalent circuit model of SCs has been developed in inorganic semiconductor
theory. The model in Fig. 8.4 gives the relations between the current density and
voltage in SCs as follows [21]:
J =
1
1 + R S /R P
J 0
exp
V − J R S A
nkT /e
− 1
+
V
R P A
− J ph
(8.3)