Determination of the energy levels is typically performed by electrochemistry,
and cyclic voltammetry (CV) has been established as method of choice. The
polymer is either dissolved in the supporting electrolyte or deposited on the
working electrode. The measurements are usually performed with a three-electrode
set-up that includes a working electrode, a reference electrode (for example
Ag/AgCl), and a counter or auxiliary electrode. As electrolytes, acetonitrile
(MeCN) or dichloromethane in the presence of conducting salts such as tetrabutylammonium hexafluorophosphate (TBAPF 6 ) are well-established. It is
recommended to use ferrocene/ferrocenium (Fc/Fc
+
) as external reference for
each measurement to make electrochemical potentials comparable [31]. Detailed
electrochemical characterization of P3HT films has been performed, for example,
by Trznadel et al. [30] and Skompska et al. [32].
A typical reversible anodic oxidation cycle of a P3HT film is shown in Fig. 2b, in
which the oxidation onset potential E
ox
onset is marked. It is assumed that the onset
potential values in cyclic voltammograms correspond to the oxidation/reduction of
the polymer chains with the largest conjugated π-system. Because the oxidation
corresponds to the removal of electrons from the highest occupied molecular orbital
(HOMO), and the reduction to adding electrons to the lowest occupied molecular
orbital (LUMO), the onset oxidation and reduction potentials are closely related to
the HOMO and LUMO energies. From the graph in Fig. 2c, an E
ox
onset ¼ 0.02 V
(versus Fc/Fc
+
) and an E
red
onset ¼ À 2.26 V (versus Fc/Fc
+ ) can be determined for a
P3HT film with a weight-averaged molecular weight (M w ) of 112 kg/mol and a
polydispersity index (PDI) of 2.4. The graph also contains data on the in-situ
conductance behavior of the P3HT film, which was measured in an electrolytegated transistor configuration (for the method of CV with in-situ conductance
measurements see, for example, [33]). One can see that only upon doping (oxidation or reduction) does the otherwise nonconducting P3HT film become
conducting.
From the onset potential values of the oxidation and reduction, the HOMO and
LUMO levels, respectively, of the polymer can be calculated according to the
following equations [34]:
E HOMO
ð
Þ¼ À E
ox
onset vs: Fc=Fc
þ
ð
Þþ5:1
À
Á
eV
½
ð1Þ
E LUMO
ð
Þ¼ À E
red
onset vs: Fc=Fc
þ
ð
Þþ5:1
À
Á
eV
½
ð2Þ
For the P3HT film in Fig. 2c, this leads to a HOMO level of À5.12 eV and a
LUMO level of À2.84 eV. In the literature, a factor of 4.8 eV instead of 5.1 eV is
often used, which dates back to a 1995 publication by Pommerehne and coworkers
[35]. For a discussion of the validity of the approach of using onset potentials for
HOMO/LUMO determination we refer to [33, 34].
From the HOMO and LUMO level, Eq. (3) can be used to estimate the electrochemical band gap, which is a critical quantity for polymer-based organic
photovoltaics:
44
K. Tremel and S. Ludwigs
and cyclic voltammetry (CV) has been established as method of choice. The
polymer is either dissolved in the supporting electrolyte or deposited on the
working electrode. The measurements are usually performed with a three-electrode
set-up that includes a working electrode, a reference electrode (for example
Ag/AgCl), and a counter or auxiliary electrode. As electrolytes, acetonitrile
(MeCN) or dichloromethane in the presence of conducting salts such as tetrabutylammonium hexafluorophosphate (TBAPF 6 ) are well-established. It is
recommended to use ferrocene/ferrocenium (Fc/Fc
+
) as external reference for
each measurement to make electrochemical potentials comparable [31]. Detailed
electrochemical characterization of P3HT films has been performed, for example,
by Trznadel et al. [30] and Skompska et al. [32].
A typical reversible anodic oxidation cycle of a P3HT film is shown in Fig. 2b, in
which the oxidation onset potential E
ox
onset is marked. It is assumed that the onset
potential values in cyclic voltammograms correspond to the oxidation/reduction of
the polymer chains with the largest conjugated π-system. Because the oxidation
corresponds to the removal of electrons from the highest occupied molecular orbital
(HOMO), and the reduction to adding electrons to the lowest occupied molecular
orbital (LUMO), the onset oxidation and reduction potentials are closely related to
the HOMO and LUMO energies. From the graph in Fig. 2c, an E
ox
onset ¼ 0.02 V
(versus Fc/Fc
+
) and an E
red
onset ¼ À 2.26 V (versus Fc/Fc
+ ) can be determined for a
P3HT film with a weight-averaged molecular weight (M w ) of 112 kg/mol and a
polydispersity index (PDI) of 2.4. The graph also contains data on the in-situ
conductance behavior of the P3HT film, which was measured in an electrolytegated transistor configuration (for the method of CV with in-situ conductance
measurements see, for example, [33]). One can see that only upon doping (oxidation or reduction) does the otherwise nonconducting P3HT film become
conducting.
From the onset potential values of the oxidation and reduction, the HOMO and
LUMO levels, respectively, of the polymer can be calculated according to the
following equations [34]:
E HOMO
ð
Þ¼ À E
ox
onset vs: Fc=Fc
þ
ð
Þþ5:1
À
Á
eV
½
ð1Þ
E LUMO
ð
Þ¼ À E
red
onset vs: Fc=Fc
þ
ð
Þþ5:1
À
Á
eV
½
ð2Þ
For the P3HT film in Fig. 2c, this leads to a HOMO level of À5.12 eV and a
LUMO level of À2.84 eV. In the literature, a factor of 4.8 eV instead of 5.1 eV is
often used, which dates back to a 1995 publication by Pommerehne and coworkers
[35]. For a discussion of the validity of the approach of using onset potentials for
HOMO/LUMO determination we refer to [33, 34].
From the HOMO and LUMO level, Eq. (3) can be used to estimate the electrochemical band gap, which is a critical quantity for polymer-based organic
photovoltaics:
44
K. Tremel and S. Ludwigs