Materials for the Optimization of Solar Energy Harvesting.

Carlos F. O. Graeff,
Mirko Congiu, Silvia L. Fernandes
CEPID/CDMF
2013/07296-2
Materials for the Optimization of
Solar Energy Harvesting

The earth’s population consumes ~ 21 trillion kWhrs of electricity, with ~ 2/3
generated using fossil fuels
2018
World Energy Consumption

20182018
World Energy Production Outlook

4
China
United States
Japan
Germany
Italy
India
Spain
France
Australia
South Chorea
Belgium
Other
Greece
United Kingdom
World
World (TWh)
%/total*
*% calculated considering the global electrical energy production
Global Solar generation for each country
Country
South Africa
Canada
WORLD
ENERGY
SOURCES
HYDRO
1 TIMES
TYDAL
2 TIMES
GEOTHERMAL
5 TIMES
BIOMASS
20 TIMES
SOLAR ENERGY
2850 TIMES
WIND
200 TIMES
All renewable energy sources provide 3078x the
global energy demand
Outlook on Global Solar Energy

Overall: 312 MW
Main Industries Energy cost evaluation A sector-specific view..
Maximum installed Power classified by application sector (MW) – 09/10/2017
Trade market Industry Habitational Rural Others
% Power % Units KW per Unit
Country 2020 2050
Australia, part of Central Asia, Chile, India
(Gujarat and Rajasthan), Mexico, Middle East,
North Africa, Peru, South Africa, United States
(southwest)
5% 40%
United States (remaining area) 3% 20%
Europe (import) and Turkey 3% 15%
Africa (remaining area), Argentina, Brazil and
India (remaining area)
1% 15%
Indonesia (import) 0,5% 7%
China, Russia (import) 0,5% 4%
A global view of installed power
Economic Overview of Brazilian Solar Energy
160.041 MW Total Instaled power

oneelectronenergy
space
Generalized picture
• Metastable high and low
energy states
• Absorber transfers charges into
high and low energy state
• Driving force brings charges to
contacts
• Selective contacts
Green, M.A., Photovoltaic principles. Physica E, 14 (2002) 11-17
High
energy
state
Low
energy
state
Absorber
e-
p+
contact
contact
The Photovoltaic (PV) Effect

7
Measuring a solar cell
Voc
Jsc
dark
light
reverse
forward
Fill Factor =
area of the curve

• Silicon
• Dye sensitized
• Organic
• Perovskites
Types of solar cells

Conductive FTO
Porous TiO2
Dye
REDOX
Cathode on FTO
e- h+
• Low production costs
• Low processing temperatures
• Does not contain toxic materials (As, Ga..)
• Efficiency up to 13%
• Transparent (windows)
Dye Sensitized Solar Cell (DSSC)

VB
CB LUMO
HOMO
REDOX
TiO2 Dye
e-
h+
F
T
O
F
T
O
Counter
electrode
LOAD
EF
Dyes:
N3 D5
Electrolytes:
Counter Electrodes:
Iodine/iodide
Co(II)/(III) polypyridines
Ferrocene
Carbon
clustersPt
Cobalt
Sulfide
Charge Separation in DSSC

• Efficiency: More efficient dyes, transparent dyes (NIR)
• Cost: Replacement of expensive component such as Pt and Ru-based dyes
with natural ones
• Stability: Replacement of liquid electrolytes with gels or solid and reduce the
content of iodine (corrosive)
Challenges in DSSCs improvement

ANODE CATHODE ELECTROLYTE
TiO2 : Synthesis (hydrothermal
and sol-gel) paste preparation
and film deposition
CoS counter electrodes:
preparation via print-
compatible techniques
Co(II)/(III) based electrolytes
for high stability DSSCs and
compatibility with CoS/CuS
Fe2O3: for tandem solar cell
applications
CuS counter electrodes:
preparation and
characterization with iodine-
free electrolytes
Our contribution on DSSCs

• Fundamental part of the DSSC
• Reestablishes the redox equilibrium
• Catalytic material deposited on FTO
• Transparency
• Low charge-transfer resistance (Rct)
• Stable
• Cheap
Conductive FTO
REDOX
Catalyst FTO
Characteristics:
Desired features:
Sketch:
• Gold
• Platinum
• NiS and CoS
• other
chalcogenides
• graphite
Available catalysts:
$
The counter electrode (Cathode of n-DSSCs)

Narrow-gap p-type semiconductor, cheap,
abundant and highly catalytic for the redox
process involving I-/I3
-
Cobalt Sulfide:
LOW-ENERGY: S.I.L.A.R., electrodeposition, chemical
bath, hydrothermal
HIGH-ENERGY: sputtering, CVD, sulfurization
Classic fabrication methods:
Based on a single chemical
precursor ink, spread on the
substrate and thermally
activated. We proposed two
inks:
• The first based on organic
solvent (thinner), suitable
for small devices..
• The second based on a
water soluble precursor.
Suitable for large-area
applications
Our strategy:
Our water-based method for the deposition of
CoS thin films

1. The precursor is dissolved into an appropriate solvent
2. The obtained Ink is spread on the substrate to obtain a film
3. Thermal treatment converts the precursor film into a CoS film
deposition
Precursor
film
CoS
thin film
Temp.
Chemical Precursor Method

Targets:
1. Synthesize a cheap chemical precursor
2. Soluble in water
3. Lower the thermal treatment temperature
4. Study the compatibility of CoS with other redox pairs
Thioglycolic acid Ammonium thioglycolate
pKa1= 3.83 Perm. salt
pKa2= 9. 30 Lower toxicity
Toxic
NH3 NH3, Co2+
M. Congiu et al., Solar Energy 122: 87-96-December 2015
Methodology- a water-based ink

1
• 1mL TGA in 10mL H2O
2
• NH4OH 7M dropped
• pH 7,5 - 8,0
3
• 1:1 CoCl2
• Separation by adding ethanol(c.a. 20mL)
Preparation and characterization of CoSCH2CO2

TGA is a bidentate ligand:
Two moieties (SH and COOH)
The characteristic peak of
S-H stretching is missing in
the complex
How TGA is binding the metal ion?

Loss of H2O
Degradation (loss of 39%
of initial mass)
Constant weight
• Molecular weight of CoSCH2CO2 is 149.02 g/mol
• After a loss of 39%  90.88 g/mol
• CoS molecular weight is 90.99 g/mol
So it is very probable that the chemical composition of the residue
is Co:S 1:1
Other gaseous species
Thermal Degradation Study

The deposition process...

CoLE FCE
Co(II)Co(III) shuttles
ACN:MPN
70:30
ferrocene/ferrocenium
PC
• Iodine/iodide is corrosive and, in some conditions, produces bleaching of the dye
• It produces radicals under UV radiation (addition to double bonds)
• Is aggressive also against TiO2 and the sealing materials of the device
Efficiency
world record
Suitable for
natural dyes
Alternative Electrolytes

Electrolyte CE Rs (Ωcm2 ) Rct (Ωcm2) Cdl(μFcm-2) n Ws-R Ws-T Ws-P
HSE CoS 13.34 ± 1.50 1.53 ± 0.25 12.0 ± 0.1 0.81 ± 0.10 2.31 ± 0.50 1.32 ± 0.15 0.38 ± 0.10
HSE Pt 12.33 ± 1.34 2.32 ± 0.15 40.9 ± 3.5 0.96 ± 0.12 2.54 ± 0.43 1.16 ± 0.21 0.51 ± 0.09
CoLE CoS 12.25 ± 1.07 2.40 ± 0.20 18.0 ± 2.2 0.80± 0.15 13.00± 1.35 2.81 ± 0.20 0.44 ±0.10
FCE CoS 12.70 ± 1.20 2.23 ± 0.25 4.1 ± 0.6 0.79 ± 0.19 10.46 ± 2.17 0.83 ± 0.12 0.38 ±0.15
IT WORKS WITH IODINE-FREE ELECTROLYTES !!
Electrochemical analysis of our CoS
electrodes with different electrolytes

Photoanode Dye Electrolyte Conter electrode Area (cm2) Jsc(mAcm2) Voc (mV) FF(%) η(%)
SP D5 HSE Pt 0.25 15.7 ± 0.8 705 ± 15 61.4 ± 1.1 6.9 ± 0.5
SP D5 HSE CoS 0.25 16.4 ± 0.9 685 ± 25 61.3 ± 0.8 6.8 ± 0.5
DB D5 HSE Pt 1.00 7.2 ± 1.4 655 ± 18 62.1 ± 1.1 3.1 ± 0.9
DB D5 HSE CoS 1.00 7.2 ± 1.6 665 ± 21 63.4 ± 0.7 3.1 ± 0.9
DB N719 HSE Pt 1.00 9.0 ± 0.8 702 ± 7 56.4 ± 1.2 3.6 ± 0.5
DB N719 HSE CoS 1.00 9.1 ± 0.8 700 ± 11 51.3 ± 1.1 3.5 ± 0.5
DSSC and Stability

• Easy to process
• Water-based
• High efficient CoS electrodes
• Compatible with iodine-free electrolytes
Main Results

Light
CB
VB
HOMO
LUMO
LOAD
I-/I3
-
REDOX
NiO
h+
3 I-  I3
- + 2e-
Very low efficiency from 0.2 to 0.03%
Congiu, M. ; Bonomo, M. ; de marco, M. L.; Dowling, D. P. ; Di Carlo, A. ; Dini, D. ; Graeff’, C. F. O. . Cobalt sulfide as counter electrode in p-type dye-
sensitized solar cells. Chemistryselect, v. 1, p. 2808-2815, 2016
The first application of CoS in p-type DSC

• There are dyes specific for p-type DSSCs: Erythrosine B
and P1
• Photocathode NiO Rapid Discharge Sintering RDS (UCD,
Dublin)
Waveguide
MW
generator
Vacuum
Quartz window
Sample’s holder
Awais et al., 2011
Application in p-type DSSCs

p-DSC configuration VOC / mV JSC /mA cm-2 FF / % η / %
NiO/ERYB/Pt-FTO 80 -1.059 34.8 0.030
NiO/ERYB/CoS 74 -1.051 32.5 0.026
NiO/P1/Pt-FTO 94 -2.650 32.6 0.119
NiO/P1/CoS 94 -2.500 31.6 0.074
Characterization under AM 1.5

• Interfacial Sulfurization
of Cu(II)acetylacetonate
with thiourea
• Water/DCM (interface)
Iodine/Iodide redox couple is corrosive, lead to the formation of free-radicals under Uv-radiation
(halogen)
• Cu2-xS are a class of cheap and highly conductive p-type semiconductors
Bottom-up built of hexagonal
stacked CuS nanoplates
HIGH POROSITY
• Water/methanol
• Isopropyl alcohol
• Powder product
Washing and purification of CuS
• A suspension of
hexagonal stacked
nanoplates of CuS is
prepared in ethanol (1
mg/mL)
Dispersion in Ethanol and
preparation of the ink
In another investigation... We discover an excellent
efficient Cu2-xS with ferrocene electrolyte

• The CuS nanocrystals suspension (ink) is drop-casted on
clean FTO
• After a fast (10 minutes) thermal treatment (200 oC) in air,
CuS is converted into Cu2-xS (partial desulfurization)
• A mechanically adherent rough surface coating is then
obtained
FM imaging
XRD
Annealed (digenite (Cu1.8S, 47–1748) and
djurleite (Cu1,97S, 20–0365). JCPDS)
As deposited (covellite CuS, 79–2321, JCPDS)
FTO substrate
Preparation of Cu2-xS electrodes

• Corrosion tests
.CONGIU, M. ; NUNES-NETO, O. ; DE MARCO, M.L. ; DINI, D. ; Graeff, C.F.O. . Cu2−xS
films as counter-electrodes for dye solar cells with ferrocene-based liquid electrolytes. Thin
Solid Films , v. 612, p. 22-28, 2016.
On the other hand.. Cu2-xS is fully compatible with FCE showing high electrocatalytic
performances
This electrode is not suitable for Iodine-based
electrolytes…

Electrochemical trials in dummy cells EIS determination of charge transfer resistance (Rct)
• In annealed electrodes Rct stabilizes after 100 hours (plateau)
Characterization with FCE electrolyte
Diffusion of the electrolyte

CV stress characterization
THE
ELECTROCATALYTIC
ACTIVITY IS
MANTAINED OVER
HUNDREDS OF
REPEATED CV
SCANS
We obtained a reliable material for application in
FCE-based DSSCs

A tandem solar cell is a PV device with a photoanode and a photocathode (instead of a CE)
• The use of a photoactive cathode increases the Voc
of the device (series)
• This effect should lead to a device with higher
efficiency.
HOWEVER
The mismatch in the photocurrents results in a lower
performance of the tandem device.. For this reason,
nowadays does not exist a tandem solar cell more
efficient than the most efficient n-DSSC (TiO2) (~13%)
While the most efficient p-DSSC remains around 5%
This is due to the high mismatch in anodic and
cathodic photocurrents
Pristine and Al-doped Hematite as an
interesting photoanode of tandem
DSSCs

In order to soften the photocurrent mismatch, a low efficiency anode could be applied such as
Fe2O3
Advantages of hematite:
1. Is a cheap and abundant material
2. Low toxicity
3. Ease of preparation
4. Normally exhibits n-type semiconducting behavior
Main applications:
1. Water-splitting solar devices (Al-doped)
2. Magnetic nanoparticles (pollutants
scavenger-water)
3. Just few application in DSSCs due to the
lower performance
Aims of our investigation:
• Provide a reliable method to fabricate porous pastes
• Improve the efficiency of Fe2O3 anodes using different dyes
• Study the effect of Al-doping on the cells parameters
Use of a lower efficiency anode

Spherical nanoparticles have been fabricated by thermal decomposition of Fe(III) tris-acetylacetonate
PASTE PREPARATION
PROCEDURE
Hematite powder
mix
Terpineol
acetylacetone
Ethylcellulose
Ethanol
Water
milling
Viscous spreadable
paste
3
Notice!
Al(C5H8O2)3 can
be easily mixed
with the
precursor prior to
the thermal
degradation
Fabrication of the paste

25 30 35 40 45 50 55 60 65
0.5
1.0
1.5
2.0
2.5
3.0
B
D
A
Intensity(arb.u.)
(deg.)
C
XRD Al %
0 %
1 %
5 %
10 %
* AlFe2O3
hercynite
α-Fe2O3
1% Al 10 % Al
Smaller particles (coarsening)
CONFOCAL MICROSCOPY
AFM TOPOGRAPHY
Pristine and Al doped hematite porous layers

Table of PV parameters of Al doped hematite photoanodes with
different Al concentrations
** the transmittance at 500 nm was considered as a
transparency index
Two different dyes, the first (N3) is a Ru-complex well known in
DSSCs
While the second is an organic dye (D5).
I vs V under 1 sun
N3
D5
D5 dyes has shown
a higher efficiency
with Fe2O3
.CONGIU, MIRKO ; DE MARCO, MARIA L. ; BONOMO, MATTEO ; NUNES-NETO, OSWALDO ;
DINI, DANILO ; Graeff, Carlos F.O. . Pristine and Al-doped hematite printed films as photoanodes of
p-type dye-sensitized solar cells. Journal of Nanoparticle Research (Online) , v. 19, p. 1-10, 2016.
PV parameters close to those obtained with NiO in p-
type DSSCs (better photocurrent matching)
Sensitization with D5 and N3 dyes and DSSCs
results

1. n-type DSSCs based on porous Fe2O3
shown better PV performance using D5
instead of N3
2. The charge recombination parameter,
calculated from EIS, are similar to those
of a typical p-type DSSC based on NiO
sensitized with erythrosine
All considered
Hematite should be considered as photoanode
For a tandem DSSCs which could really shown an
Efficiency higher than the respective p an n cells
working alone
Conclusions

Perovskite solar cells (PSCs)
PSCs- Cheap and easily processable material
Combines distinct merits of several PV technologies

Perovskite material
A + cations
methylammonium (MA +); formamidinium (FA+);
cesium (Cs+), rubidium (Rb+); ethylammonium
(EA+); guanidinium (GA+)
X− anions (I−; Br−, Cl−)
B2+ cations (Pb+2; Sn+2; Ge+2)
Most used material : CH3NH3PbI3

Efficiency of perovskite cells

HTM
Nature Communications 5, Article number: 3834 doi:10.1038/ncomms4834
Work Principle of perovskite solar cells

48
Degradation
•Intrinsic defect of perovskite material
(volatile organic cations, ions diffusion)
• Interfaces,
• Stable conductive hole material,
• Encapsulation
Hysteresis
• Charge traps- charge accumulation
• Mobile ions (I-, Cl-, methylamonium+)
• Ferroeletric dipoles
Challenges in perovskite solar cells
degraded

Nb2O5 vs TiO2
Nb2O5 is similar to TiO2, with
• better chemical stability
• higher electronegativity than TiO2
• band gap allows higher Voc
Brazil - the largest mineral reserves of niobium

FAPESP bulletin
Our contribution

Main results
• Study of Nb2O5 as hole transport layer
• Origin of hysteresis in perovskite solar cells
•Use of different niobium oxide films and its influence on the performance of the solar
cells

NO HYSTERESIS : BETTER ELECTRON EXTRACTION DUE THE BAND GAP
ENGINEERING !
Nb2O5 –
better stability
and no J-V
hysteresis
Comparing TiO2 x Nb2O5

-7.3
-4.0
S.L. Fernandes, A.C. Véron, N.F.A. Neto, F.A. Nüesch, et al., Nb2O5 hole blocking layer for hysteresis-free perovskite solar cells., Materials Letter, 181 (2016) 103–107. doi:
band gap
engineering
glass
FTO
TiO2
- + - +
- +
- +
- +
- +
glass
FTO
Nb2O5
- + - +
- +
- +
- +
- +-- +
No hysteresis: Better electron extraction

Efficient interfacial charge extraction is crucial for mitigating the impact of oxygen-induced degradation.
Combination of non extracted electrons and molecular oxygen decomposed the perovskite materials
High stability: Better electron extraction
RSC Adv., 2016, 6, 38079–38091 | 38079

Comparing different thickness of Nb2O5
Nb2O5 50 nm
no J-V hysteresis
better charge extraction

Recent results
Exploring the properties of niobium oxide films for perovskite solar cells
X-Ray Diffraction UV-Vis
110 nm thick films

57
Current-voltage and conductivity of the niobium oxides films.
Films deposited at 3.5sccm - higher conductivity

58
High performance of 3.5NbO
based solar cells

59
High conductivity 3.5NbO films,
high Jsc cell,
improved efficiency

60
In addition to high conductivity of
Nb-O films, we found a better
charge transfer between 3.5Nb-O
and perovskite films
Photoluminescence

Nb2O5 is an excellent material to be use as hole blocking layer in
perovskite solar improving the stability of the devices, and resulting
in hysteresis free devices
The conductivity of niobium oxide films were improved by changing
the oxygen content improving the efficiency of the cells.
In addition, we found a better charge transfer between 3.5Nb-O and
perovskite films
Conclusions

Prof. Franco Decker (University of Rome “La Sapienza”, Italy)
Claudia Barollo (University of Turim- Italy)
Danilo Dini (University of Rome “La Sapienza”, Italy)
Alessandro Lanuti (CHOSE, Italy)
Aldo di Carlo (CHOSE, Italy)
Anna C.Véron (EMPA, Switzerland)
Frank A. Nüesch (EMPA, Switzerland)
Thank you

Materials for the Optimization of Solar Energy Harvesting.

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