Supporting Information
Influence of Sintering on the Structural and Electronic Properties of
TiO2 Nanoporous Layers Prepared Via a Non-Sol-Gel Approach
Sylvia Schattauer*,**, Beate Reinhold*, Steve Albrecht*, Christoph Fahrenson*, Marcel Schubert*,
Silvia Janietz**, Dieter Neher*
*University of Potsdam, Institute of Physics and Astronomy, Karl-Liebknecht-Str. 24-25, 14476
Potsdam, Germany
**Fraunhofer-Institut für Angewandte Polymerforschung, D-14476 Golm, Germany
*E-mail: neher@uni-potsdam.de
Contents
Effect of Pre-Bias on CELIV Transients ................................................................................................................2
Transient Fluorescence Experiments ...................................................................................................................2
Intensity dependence of photocurrents ..............................................................................................................3
Supporting Information References ......................................................................................................................5
Effect of Pre-Bias on CELIV Transients
As pointed out in the manuscript, a pre-bias is applied prior to the extraction voltage ramp to
accumulate electrons in the titania layer. Figure S1 shows CELIV current signals of a sample dipcoated twice and sintered at 550°C for different pre-bias. Upon increasing the pre-bias, the
overall CELIV signal becomes larger, which is due to a higher amount of charge in the layer.
Additionally, the current peak position shifts to longer times, which at the first glance would
suggest a lower mobility at higher carrier densities. The true reason for this shift is that the
sample is pre-biased negatively and that flat band conditions are reached only after a certain
delay time t1. Mobilities corrected for t1 are, independent of pre-bias (see inset of Figure S1).
Fig. S1: Extraction transients of a sample dip-coated twice and sintered at 550°C. The measurement
was performed at 330 K for different values of the negative pre-bias. The inset shows the
calculated mobility corrected for t1, using a build-in field of +0.5 V.
Transient Fluorescence Experiments
One major disadvantage of determining the PL quenching efficiency by steady-state
photoluminescence experiments the possible influence of the glass/TiO2 and the TiO2/polymer
interface on the propagation of the incident and photogenerated light. Though the refractive
index of our nanocrystalline TMO layer is rather close to that of glass and common conjugated
polymers, the introduction of the porous TiO2 layer might lead to cause additional reflection and
scattering. These effects might contribute to the observed difference between the polymer PL
intensity measured on TiO2/glass and on pure glass. We, therefore, complimented the our
steady-state PL investigations by employing time-resolved photoluminescence measurements
on the same samples [1]. The fluorescence decay was evaluated at 500 nm as determined from
the peak positions measured in steady-state PL experiments. The decay curve (Fig. S2) was fitted
using a triple exponential decay function using the software Fluofit (Picoquant), which yielded
three lifetimes i (I = 1-3). From these data, the relative decrease of the lifetimes i was
calculated as follows:
𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝜏𝑖 [%] =
𝜏𝑖,𝑃𝑜𝑙𝑦𝑚𝑒𝑟 − 𝜏𝑖,𝐵𝑖𝑙𝑎𝑦𝑒𝑟
𝜏𝑖,𝑃𝑜𝑙𝑦𝑚𝑒𝑟
∗ 100
We found that addition of the TMO layer decreases the all three lifetime components but that the
variation of the polymer layer thickness had the largest effect on 2. Fig. S2 plots the reduction of
the lifetime 2 as function of polymer thickness. TiO2 layers that have been sintered at 600°C
generally reduced the polymer PL lifetime by about 75%, independent of the deposited polymer
layer thickness, while TiO2 layers sintered at lower temperatures lead to a gradual decrease of
the PL lifetime with decreasing polymer layer thickness. The results of the lifetime
measurements fully support the conclusions from steady state optical spectroscopy.
# Photons [norm.]
10
300°C 7,5nm
300°C 80nm
500°C 6,5nm
500°C 84nm
600°C 8nm
600°C 85nm
-1
10
-2
10
-3
10
8
10
12
14
16
Time [ns]
Relative Lifetime Reduction [%]
1,00
0
0,75
300°C
500°C
550°C
600°C
0,50
0,25
0,00
0
25
50
75
100
Polymer Layer Thickness [nm]
Fig. S2: Fluorescence decay curves for polymer films of different thickness on TiO2 sintered at
300°C, 500°C or 600°C (left). The right graph shows the relative reduction of the fluorescence decay
time (second lifetime component) due to quenching at the hybrid TiO2/polymer interface, relative
to the neat polymer layer, as a function of polymer layer thickness.
Intensity dependence of photocurrents
According to the PL quenching described above, excitons generated on the polymer are
efficiently dissociated at the distributed hybrid interface for samples with high temperature
sintered TiO2. The rather poor performance of the corresponding solar cells, therefore, suggests
that either dissociation does not lead to free carriers or that photogenerated carriers are prone
to bimolecular recombination. A powerful tool to test for bimolecular recombination is the
dependence of photocurrent on light intensity. [2].
Fig. S3 shows intensity dependent J/V measurements on hybrid cells comprising TiO2 layers
sintered at 550°C and 600°C. The illumination intensity was varied from 100 mW/cm2 down to
2.5 mW/cm2 by using neutral density filters. Also plotted are photocurrents (defined as the
current under illumination minus the dark current) at a bias chosen to be ca. 0.25 V below the
open circuit voltage under AM1.5 illumination as a function of illumination intensity. In the
double-logarithmic plot of current density and irradiance, samples sintered at 550°C displayed a
constant slope of slightly less than one that indicates the presence of minor space charge and
recombination effects. Results were similar with other donor polymers and for sintering
temperatures below 550°C. Thus recombination of the generated charge carriers is not the
limiting factor for the TiO2 layers sintered at 550°C. Once electrons and holes are separated, they
can be extracted at the electrode without significant losses.
-0,2
550°C
-0,1
600°C
0,0
2
J [mA/cm ]
2
J [mA/cm ]
0,0
0,2
0,4
0,6
2.5%
5%
10%
25%
50%
100%
0,8
1,0
1,0
0,5
0,0
-0,5
0,1
0,2
2.5%
5%
10%
25%
50%
100%
0,3
0,4
0,5
1,0
-1,0
0,5
U[V]
-0,5
-1,0
U[V]
550°C
600°C
1
0.89*
2
Jphoto [mA/cm ]
0,0
0,1
0.77**
0,01
* -0.5V
** -0.25V
1.01**
1
10
100
2
P [mW/cm ]
Fig. S3: Photocurrent characteristics of hybrid solar cells (with the TiO 2 layer sintered at 550°C and
600°C) for different white light illumination intensities. The intensity was varied from
100 mW/cm2 (100 %) down to 2.5 mW/cm2 (2.5 %) by using neutral density filters. Also shown is
the photocurrent of the cells at -0.5 V (550°C) and -0.25 V (600°C) as a function of the incident
illumination power in double-logarithmic representation. Solid lines show linear fits to the data
and the corresponding slopes are indicated in the plot.
In contrast, samples sintered at 600°C displayed a lower slope at high irradiation intensities.
This is indicative of a significant build-up of space charge in combination with bimolecular
recombination losses. We propose that the higher susceptibility of the device sintered at 600°C
is a consequence of the high interpenetration of the polymer with a porous TiO2 network.
Photogeneration of free holes on polymers located within such pores will cause high local
carrier densities. According to the CELIV measurements described above, electrons exhibit a
fairly low mobility when moving within the nanocrystalline TiO2 network. The combination of
these effects may be the cause of significant bimolecular recombination losses. Interestingly,
samples with TiO2-sintered layers display a smaller current even at the lowest intensity (where
bimolecular recombination effects and the build-up of space charge should be insignificant).
Since cells comprising 600°C sintered TiO2 exhibit a very high efficiency for PL quenching, their
poor performance at even low photon flux must be related to efficient geminate recombination
of electron-hole pairs following exciton dissociation across the hybrid interface. Studies on DSSC
revealed that direct recombination of the electrons on TiO2 and holes in either the electrolyte or
an solid-state hole conductor) might severely limit the cell efficiency [3].
Supporting Information References
[1]
[2]
[3]
J. Piris, T.E. Dykstra, A.A. Bakulin, P.H.M. van Loosdrecht, W. Knulst, M.T. Trinh, J.M.
Schins, L.D.A. Siebbeles, Journal of Physical Chemistry C 113/32 (2009) 14500.
M. Schubert, C. Yin, M. Castellani, S. Bange, T.L. Tam, A. Sellinger, H.-H. Horhold, T.
Kietzke, D. Neher, J Chem Phys 130/9 (2009) 094703.
A. Hagfeldt, G. Boschloo, L.C. Sun, L. Kloo, H. Pettersson, Chemical Reviews 110/11 6595.