Supplementary Material
Reduced ultraviolet light induced degradation and enhanced light
harvesting using YVO4:Eu3+ down-shifting nano-phosphor layer in
organometal halide perovskite solar cells
Nikhil Chander, A.F. Khan,a) P.S. Chandrasekhar, Eshwar Thouti, Sanjay Kumar Swami, Viresh Dutta and Vamsi
K. Komarala.
Fabrication procedure of perovskite solar cells1:
Fluorine doped tin oxide (FTO) glass substrates were etched using 2M HCl solution and Zn
powder. The etched substrates were cleaned sequentially in acetone, de-ionized water and
propanol. A ~50 nm compact TiO2 blocking layer was deposited on FTO substrates by spray
pyrolysis of a diluted solution of titanium diisopropoxidebis(acetylacetonate) in ethanol at
450C. Commercial TiO2 paste (18-NR-T, Dyesol) in ethanol was used to deposit a mesoporous
TiO2 layer (~400 nm) over the blocking layer. The films were annealed at 500C in air for 30
mins followed by immersion of films in 40 mM aqueous TiCl4 solution at 70C for 20 min and
subsequently annealed at 500C. The prepared TiO2 films were then coated with PbI2 by spin
coating at 6500 rpm for 10 s, followed by heating at 70C for 30 min. After this step, the films
were dipped in a 10 mg/ml solution of methylammonium iodide in propanol for 20 s and heated
at 70C for 30 min. The hole transport material (HTM) composition was 0.170 M 2,2’,7,7’tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9’-spirobifluorene
(spiro-OMeTAD),
0.064
M
bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, Alfa Aesar) and 0.198 M 4-tertbutylpyridine (TBP, 96%, Aldrich) in chlorobenzene. The HTM solution was spin coated at 4000
rpm over the perovskite-sensitized TiO2 films. The counter electrode was formed by sputtering
of ~80 nm thin Au contacts. The devices were tested with a metal mask with active areas ~0.25
cm2.
Incident photon to current conversion efficiency (IPCE) measurements:
IPCE measurements were performed using ReRa SpeQuest (The Netherlands) quantum
efficiency measurement setup under a bias of 0.1 sun. A chopping frequency of 20 Hz was used
to chop the incident light from the xenon and halogen lamps. Due to some hardware limitations,
the chopping frequency could not be set below 10 Hz (for Dye sensitized solar cells (DSSC),
perovskite solar cells (PSC) a chopping frequency of 1-3 Hz is generally employed). This results
in the reduction of IPCE spectrum. The chopping frequency effect on dye solar cells has been
studied in detail by Xue et al.2 Although no such study has been reported for perovskite cells, it
is expected that generalizations from the DSSC study can be used for perovskite cells also. The
use of higher chopping frequency may have resulted in reduction of IPCE spectra and thus a
difference in photocurrent values was observed. For control PSC, Jsc calculated by integrating
the IPCE spectra was 13.31 mA/cm2 while the Jsc measured from J-V curves was 16.7 mA/cm2.
For down-shifting phosphor coated PSC (DS-PSC), Jsc calculated by integrating the IPCE
spectra was 14.38 mA/cm2 while the Jsc measured from J-V curves was 18.1 mA/cm2. The
difference in photocurrent between control and DS-PSC calculated from J-V curves is
1.4mA/cm2 and that from IPCE spectra is ~1.1 mA/cm2. Thus, it seems that higher chopping
frequency is responsible for a lower IPCE obtained in our case. Another difference in these two
measuring techniques is the fact that IPCE spectra were not recorded below 350 nm. AM1.5G
spectrum has contributions from wavelengths below 350 nm also, so this might have also
contributed towards the observed higher photocurrent from J-V measurements using a solar
simulator (Sol3A, Oriel, USA).
Figure S1. Total and diffused transmittance spectra of bare and phosphor coated FTO glass
substrates recorded using an integrating sphere.
Transmittance and reflectance measurements:
The transmittance spectra of bare FTO substrate and spray deposited YVO4:Eu3+ DS NPs layer
on FTO substrate are shown figure S1. The substrate coated with DS NP layer shows a minor
reduction in transmittance from 300-400 nm (average ~1.24%) and a slight increment in
reflectance (figure S2) from 300-400 nm (average ~0.104%). This shows that the light is
primarily absorbed by the phosphor layer in 300-400 nm region, i.e. decrease in transmittance is
because of absorption and not because of (back) scattering.
The phosphor coated film does not show a noticeable reduction in transmittance from 500-800
nm. But the diffused transmittance of phosphor coated film is markedly increased. This means
that the long-wavelength light is being diverged from its path, or in other words, it is being
scattered which also results in a small enhancement observed in IPCE. Diffused transmittance
has also increased for short-wavelength light, but this increase in scattering of short-wavelength
light is not beneficial as the perovskite material already has a high absorption coefficient for
these short wavelengths and the dominant factor in the 300-400 nm region is the absorption of
UV light by the phosphor film.
Figure S2. Total and diffused reflectance spectra of bare and phosphor coated FTO glass
substrates recorded using an integrating sphere.
Separating the contributions from down-shifting and scattering:
By integrating the area under the curve of IPCE spectra of the two PSCs, a difference of ~1.1
mA/cm2 in photocurrent between the two devices was obtained. This value is very close to the
1.4 mA/cm2 difference observed in J-V measurements; the difference in the two values may be
due to the non-accounting of light below 350 nm in IPCE measurements and also because of
chopping frequency effects (see above). Integration of area from 350-450 nm gives a
photocurrent difference of ~0.6 mA/cm2. This rough quantitative estimate shows that the downshifting effect is contributing more than half (~55%) to the total enhancement observed in
photocurrent. This contribution could turn out to be still higher if the wavelengths below 350 nm
are included in the analysis. Nevertheless, this approximate evaluation tells us that about 4.7%
out of the total 8.5% enhancement is occurring due to down-shifting of UV photons to red
photons and the remaining 3.8% is because of scattering of long-wavelength light by the
phosphor film.
Figure S3. Solar spectral irradiance.
Figure S4a. Current density-voltage curves of a perovskite solar cell kept in dark under ambient
conditions.
Figure S4b. Incident photon to current conversion efficiency spectra of a perovskite solar cell
kept in dark under ambient conditions recorded at t = 0 and 360 hr.
Table S1: Time variation of photovoltaic parameters of a perovskite solar cell kept in dark under
ambient conditions.
Device
PSC at 0 hr
Jsc (mA/cm2)
16.5
Voc (mV)
833
FF(%)
53.8
Efficiency(%)
7.39
PSC at 12 hr
15.8
803
51.5
6.54
PSC at 72 hr
11
785
49.1
4.24
PSC at 144 hr
5.92
776
53.1
2.44
PSC at 216 hr
4
625
57.4
1.43
PSC at 288 hr
2.83
581
57.1
0.94
PSC at 360 hr
1.23
483
49.7
0.3
(1) J. Burschka, N. Pellet, S.J. Moon, R.H. Baker, P. Gao, M.K. Nazeeruddin and M. Gratzel,
Nature 499, 316 (2013)
(2) Guogang Xue, X. Yu, T. Yu, C. Bao, J. Zhang, J. Guan, H. Huang, Z. Tang and Z. Zou, J.
Phys. D: Appl. Phys. 45, 425104 (2012).