Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Fig. 2. Structure of a) Poly(3-hexyltiophene) and b) Phenyl-C 60
-butyric
acid methyl ester
Absorbance, a.u.
1
0.8
0.6
0.4
0.2
0
300 400 500 600 700 800 900
Wavelength, nm
Fig. 3. Solar radiation spectrum relative absorbance for conjugated
polymer Poly(indenefluorene) (PIF). First absorption peak is localized
for ca. 300 nm (100% of maximum absorbance value) and second
one for 783 nm which is about 40% of previous one
Experiment
Experiment was carried out during my internship in Laboratory
of Photophysics of Organic Nanomaterials of Physics Department
in Moscow State University of Russia. Experiment was
divided into two parts. In first part we prepared two cells based
on PIF and PCBM at two different weight ratio i.e. 1:1 and 2:1.
In second part we added 7,7,8,8- tetracyanoquinodimethane to
new blend of PIF:PCBM (1:1). P3HT:PCBM junctions was made
for comparison purposes. Poly(indenefluorene) was synthesized
for us by U. Scherf from University of Wuppertal, Germany
according to [2]. PCBM, TCNQ and P3HT were purchased from
Sigma-Aldrich. The first part of experiment was to create the
base cells. PIF:PCBM (1:1) and PIF:PCBM (2:1) were dissolved
in 10 microliters of chlorobenzene and spin-coated with
angular speed of 1000 rpm on indium-tin oxide substrates of
surface resistivity less than 10 Ω per square. Aluminum cathode
in first part was evaporated in Maxtek commercial evaporator
(Fig. 5) under pressure of 4.9×10 -6 mbar and ytterbium
cathode in second part under pressure of 5.2×10 -6 (Fig. 5).
Subsequently cells were characterized by self made spectrometer
and density current-voltage characteristic generator
under STC conditions. In part one of experiment we obtained
following characteristics of external quantum efficiency versus
wavelength and photoinduced density of current versus voltage
(Fig. 6, 7, 8). Multiple current-voltage curves for each type of
cell were collected for different points of cathodes (8 pixels) as
presented in Fig. 5.
In order to improve the charge transfer process in relation to
the junction of PIF:PCBM we introduced 5 mg of TCNQ as a good
electron-acceptor into new liquid blend of PIF:PCBM (1:1) in chlorobenzene.
We obtained following results (Fig. 9, 10):
n
N
N
a)
Fig. 4. Chemical formula of a) poly(indenefluorene), b) 7,7,8,8-tetracyanoquinodimethane
b)
N
N
Fig. 5. Left: Prepared organic solar cells with deposited layer of shaped
aluminum, right: metal cathode deposition chamber with structural
mask for selective deposition
EQE (%)
100
10
1
0,1
0,01
1E-3
PIF:PCBM 2:1
PIF:PCBM 1:1
P3HT:PCBM
Current density (mA/cm 2 )
10
5
0
-5
P3HT:PCBM (1:1)
1E-4
400 500 600 700 800 900
Wavelength (nm)
-10
0,00 0,25 0,50 0,75 1,00
Voltage (V)
Fig. 6. Left: Absorption spectra of junctions in visible and near infrared part of spectrum: PIF:PCBM (1:1), PIF:PCBM (2:1) and for comparison
purposes P3HT:PCBM (1:1), Right: Current-voltage characteristic of loaded junction of P3HT:PCBM (1:1)
Elektronika 6/2012 123