Emission spectra and transient photovoltage in dye-sensitized solar cells under
stress tests
M. Giustini*a,b, D. Angelone a, M. Parente a, D. Dini a, F. Decker* a,
A. Lanuti c, A. Reale c, T. Brown c, A. di Carlo c
Dept. of Chemistry, University of Roma “La Sapienza”, P.le Aldo Moro 5, 00185 Roma (Italy)
a
bCSGI c/o Dept. Of Chemistry, University of Bari, Via Orabona 4, 70126 Bari (Italy)
c
CHOSE–Centre for Hybrid and Organic Solar Energy, Dep. of Electronic Engineering
University of Rome Tor Vergata, viale Politecnico 1, 00133 Rome (Italy)
*corresponding author (email: mauro.giustini@uniroma1.it , fax: +39-06-490324)
*corresponding author (email: franco.decker@uniroma1.it , fax: +39-06-490324)
Electronic Supplementary Information
Phosphorescence measurements
The home made sample holder used to acquire the in situ
emission spectra, allowed the precise and reproducible
positioning of the DSC inside the spectrofluorometer, as can be
seen in Figure ESI_1, where the spectra acquired at a month
distance for the cell A of the tris CD41 (the one not aged in any
way) are practically indistinguishable.
Fig. ESI_ 1 Emission spectra acquired for cell A at a month
distance. [exc= 520nm; slits: 5/5 nm; 1nm resolution]
In order to be sure that the detected signal in intact DSCs
was from the N719 dye, the emission spectrum of an intact
DSC has been compared with those obtained for N719 in
solution and of a N719/LiI/I2 mixed solution. For the first
and last spectrum data were acquired in reflection mode
(Figure ESI_2).
Fig. ESI_ 2 Emission spectra of N719 [exc= 520nm; slits 5/5nm;
1 nm resolution].
Red line: [N719]= 6M in anhydrous ethanol; 1cm x 1cm quartz
Hellma fluorescence cuvette.
Green line: N719+LiI/I2 in EtOH/MPM 1:1 v/v ([N719]= 3M;
[LiI]= 0.25 M; [I2]= 0.025M; 0.1 cm quartz Hellma cuvette;
reflection mode.
Blue line: cell A (Y axis on the right); reflection mode.
Emission and VOC correlation
As stated in the manuscript, the phosphorescence decrease
for the UV aged cell seems to be an early symptom of
incipient DSC instability and failure. As a matter of fact, an
empirical correlation between phosphorescence intensity and
VOC upon UV ageing has been indeed found, as shown in
Figure ESI_3. The valence of such a result is questionable
and it is to be considered just as a suggestion for future
deeper investigation on this topic.
Fig. ESI_3 Semilog plot of the VOC as a function of the
phosphorescence intensity at 730nm. The dashed line is a mere
guide for the eye.
Deconvolution of the photopotential decay curves
In order to gain some quantitative information from the
photopotential decay curves, an attempt has been made to
deconvolve them to a sum of exponentials. The fittimg
procedures started with the minimum number of exponential
components required to obtain a reliable fit (i.e.: a
correlation coefficients higher than 0.9990 and a randomly
distributed percentage residuals comprised between ± 1%).
The results of such an analysis for the DSCs named B and C
are reported in Figure ESI_4. As can be seen, the faster
decay of the photopotential shown by the UV aged cell,
coincide with the disappearance of the exponential
component characterized by the lower kinetic constants,
whose contribution to the total decay passes from 29 to less
than 1%. The other components remain unchanged respect
to the relevant kinetic constants of each exponential
component (at least as order of magnitude). By the way, the
relative weight of each component changes, with the faster
gaining weight respect to the not UV aged DSC (from 8 to
17%) and an even more meaningful increment is detected
for the third component that passes from 27 to 60%. From
these data seems, therefore, that one of the processes
characteristic of the charge recombination in DSC
practically ceased to exist, with the others taking its place.
Fig. ESI_4 Exponential deconvolution of the photopotential
decay curves for DSC B (not UV aged; panel a) and C (UV aged;
panel b). Kinetic constants have been normalized for the volume
of the TiO2 layer (2.5∙10-4 cm3). The best fit procedure made use
of the Levenberg-Marquard algorithm implemented in the kernel
of SigmaPlot 10.0.1 software.