1. NEW MATERIALS FOR DYE-SENSITIZED
SOLAR CELLSVineet Jagadeesan Nair
Mechanical Engineering and Economics
University of California, Berkeley 2018
jvineet9@berkeley.edu
• Designed, manufacture and tested Dye Sensitized Solar Cells (DSSC) using novel Organic, metal-free Dye Sensitizers
• Extensive Literature Review to gain insights for device optimization to improve Efficiency and Long term Stability (Durability)
• Used Co-sensitization techniques, design improvements and careful material selection
 Champion Power Conversion Efficiency of > 9%
• Dye-Sensitized thin film solar cells have become a viable,
low-cost alternative to more traditional crystalline cells (that
use expensive materials like Si and other semiconductors) ever
since high efficiency Ruthenium based dyes (N719) were
developed by Grätzel in 1991, at UC Berkeley!
• However, difficulty of synthesis, high cost of starting
materials, poor stability (mainly due to the liquid electrolyte)
and negative environmental effects of dyes based on transition
metal complexes hinder their widespread application. Thus,
an effort has been made to design metal-free organic
sensitizers that offer several important advantages.
• The peak efficiency of DSSCs has only reached about half of
that of conventional crystalline and perovskite solar cells.
DSSC Fabrication Process:
1.Use glass plates coated with Fluorine doped Tin Oxide
(FTO) as photoanode, cut to desired size with diamond tip
2.Immerse FTO plates in a solution of TiCl4 in
Ethanol/Deionized water for 30 minutes at 70°C
3.Transparent Layer: Cast a layer of homemade mesoporous
Titanium dioxide paste with a thickness of 20-40 nm,
followed by annealing for six hours
4.Scattering layer: Cast homemade colloidal TiO2 paste to a
thickness of 10 nm, followed by annealing for 6 hours
5.Use another FTO plate sputtered with Platinum as counter
electrode (cathode)
6.Seal the two electrodes, separating them by a small gap
using Surlyn film (0.2 µm), and inject electrolyte (I−/I3− redox
couple) between them
1) REN21, Renewables 2015 Global Status Report, June 2015
2) National Renewable Energy Laboratory (NREL), Best Research Cell Efficiencies (http://www.nrel.gov/ncpv/images/efficiency_chart.jpg)
3) Chaurasia, S., et al. Sensitizers with rigidified-aromatics as the conjugated spacers for dye-sensitized solar cells, Journal of Materials Chemistry C 3.38 (2015): 9765-9780
Table 1. Relative Merits and Demerits of Organic Sensitizers
ACKNOWLEDGEMENTS
0 200 400 600 800
0
5
10
15
20
Photocurrentdensity(mAcm-2
)
Photovoltage (mV)
N719-1
N719-3
N719-3
N719-4
I. Co-sensitization (Dye Cocktail Approach) of SQ2 and DCE2
Figure 7. Standardization of DSSC Manufacturing
200 300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
MolarExtinctionCoefficient(10
-4
*M
-1
cm
-1
)
Wavelength (nm)
SQ2
2:8
4:6
6:4
8:2
DCE2
0 200 400 600
0
10
20
Voltage (mV)
PhotocurrentDensity(mAcm-2
)
SQ2
2:8
4:6
6:4
8:2
DCE2
DCE2:SQ2 Ratio
Power Conversion
Efficiency
η (%)
Open Circuit
Voltage
VOC (mV)
Short Circuit
Current JSC
(mA/cm2)
Fill Factor (FF)
Pure SQ2 (0:10) 2.85 556.7 8.314 0.617
2:8 4.23 586.7 11.154 0.647
4:6 5.92 606.7 15.885 0.614
6:4 5.72 611.7 14.944 0.626
8:2 7.24 630.0 18.225 0.630
Pure DCE2 (10:0) 7.43 643.3 18.035 0.641
0 200 400 600 800
0
10
Voltage (mV)
Photocurrentdensity(mAcm-2
)
20:2
20:4
Table 4. Results from Sequential Adsorption
Adsorption time ratio
DCE2 : YD2-o-C8
η (%) VOC (mV) JSC (mA cm-2
) FF
20:2 Hours 5.455 635 14.140 0.608
20:4 Hours 6.025 640 13.763 0.684
II. Co-sensitization of DCE2 and YD2-o-C8
DCE2:YD2-o-C8 Ratio η (%) VOC (mV) JSC (mA cm-2) FF
10:0 (Pure DCE2) 8.41±0.04 684.5±2.44 18.25±0.03 0.67
8:2 8.95±0.08 685.3±1.45 18.75±0.12 0.70
6:4 9.07±0.11 686.1±2.44 18.92±0.21 0.70
4:6 8.19±0.09 658.0±2.56 18.47±0.17 0.67
2:8 6.39±0.02 699.67±4.85 13.03±0.09 0.70
0:10 (Pure YD2-o-C8) 8.81±0.03 704.6±1.48 18.33±0.17 0.68
Table 3. Photovoltaic parameters of the DSSCs co-sensitized by DCE and YD2-o-C8 dyes
with various ratios. Standard deviation data for each kind were obtained using three cells.
Table 2. Photovoltaic parameters of DSSCs co-sensitized with various ratios of DCE2 and SQ2
dyes. The standard deviation data of each kind of DSSC were obtained using three cells.
Figure 5. DSSC Construction Schematic
Figure 6. DSSC Testing
Figure 9. (a) Molecular Structures of the DCE series sensitizers and (b) I3– reduction
behaviors at TiO2 / DCE2 / electrolyte and TiO2 / DCE4 electrolyte interfaces: These are a
new series of organic dyes developed in our lab. DCE2 in particular implements a Di-
anchor, Crown Ether structure to trap Lithium ions which in turn capture I3
- ions from
the electrolyte. This prevents them from reducing oxidized dye molecules before electron
injection, and the decrease in charge recombination rate improves DSSC performance.
RESULTS
Figure 13. Graphs showing (a) Photocurrent per unit area (J) vs Photovoltage (V) and (b) Incident photon-to-current conversion efficiency (IPCE)
Figure 10. UV-vis spectra and J-V curved for DCE2-SQ2 cosensitized devices
Pros Cons
Cheaper and easier synthesis (large
scale production) and hazard-free
disposal
Absence of metal-ligand charge transfer (MLCT) effect
Flexibility in molecular design Narrow, sharp light absorption bands
Higher molar extinction coefficient
(usually greater than 3*10-4 M-1 cm-1)
No single dye with strong absorption over broad range
of wavelengths in visible region (400-750 nm)
Shorter exciton lifetimes and chemical stability (many
being volatile organic compounds)
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
MolarExtinctionCoefficient(10
-4
*M
-1
cm
-1
)
Wavelength (nm)
YD2-o-C8
2:8
4:6
6:4
8:2
DCE2
MATERIALS AND METHODS
INTRODUCTION
Working Principle and Mechanism of DSSCs:
Figure 1. Growth of Solar PV capacity:
Solar PV is projected to projected to
meet > 30% of global energy demand
by 2050
CONCLUSIONS
REFERENCES
Figure 11. Unsymmetrical Squaraine Dye (SQ2)
which strongly absorbs wavelengths in 500-700 nm
(NIR) region. Planar structure makes it prone to dye
aggregation, but has high photolytic stability.
Figure 12. High efficiency Zinc Porphyrin dye with intense
absorption peaks near 450, 550 and 650 nm. Long bulky
alkoxy chains prevent electron recombination between
TiO2 nanoparticles. However, still has large valleys (weak
absorption) in the 350-410 nm and 500-600 nm ranges.
MOTIVATION
Supervised by Prof. Jiann T’suen Lin, Dr. Chun-Ting Li, Dr. Chia-Jung Liang
Institute of Chemistry, Academia Sinica, Taipei, Taiwan
Figure 14. UV-vis absorption spectrum
1) Dip in pure DCE2 solution for
longer time (20 h) as primary dye
2) Then soak in pure YD2-o-C8 for
shorter time intervals (2 or 4 h)
as accessory dye
Possible causes for inferior performance
(i) Poor dye loading of YD2-o-C8
(ii) Adsorption replacement between
the 2 sensitizers i.e. YD2-o-C8 may
cause desorption of some DCE2
from the TiO2 surface
III. Stepwise Co-sensitization of SQ2 and YD2-o-C8
Figure 4. Structure of Standard Ru-based N719 dye
Co-sensitization thus generally results in:
• Increased JSC (higher photon absorption) and either unchanged or lower VOC (changes in HOMO/LUMO and Fermi levels) and FF
• In this case, DCE2 is able to compensate for the absorption valleys of YD2-o-C8 especially in the 500-600 nm range
• “Co-sensitization of a simple organic dye with an NIR-absorbing porphyrin dye can greatly retard back reaction and enhance spectral response of solar cells.”
• Obtained PCE of 9.07 % with 6:4 molar (or volumetric) ratio of DCE2:YD2-o-C8 dyes, which is higher than the efficiencies of either of the two pure dyes when used alone
• Can further boost efficiency by replacing classical Iodide/Triiodide redox couple with Co3+/Co2+ complexes
However, conventional metal-based dyes have several
disadvantages including:
1) Expensive and less abundant starting materials
2) Negative environmental effects of transition metal complexes
3) Difficulty of synthesis and modifying molecular design
Goal: optimize both Molar extinction coefficient
(absorption strength) and overlap of the absorption with
the solar spectrum (i.e., the absorption spectral width).
Figure 8. Thin films co-adsorbed
with DCE2:YD2-o-C8
STRATEGIES TO IMPROVE DSSC PERFORMANCE:
• Dark current suppression (mitigate back reactions)
• Retard charge recombination (de-excitation) and formation of aggregates using
either coadsorbate (CDCA) or secondary dye
• Enhance spectral response through molecular design and cosensitization
• Adjust cell parameters like (1) thicknesses of scattering and transparent layers to
adjust dye loading, (2) annealing times and temperatures etc.
• Substitute liquid electrolyte with more stable, solid state electrolyte
Figure 3. Growth of Solar cell efficiencies
Figure 15. J-V curve for Sequential Adsorption
I would like to thank the Cal Energy Program (Berkeley Energy and Climate Institute) and its sponsors, Academia Sinica (AC), the Ministry of Science and
Technology (MOST) of Taiwan, National Taiwan University (NTU), and the Instrumental Center of the Institute of Chemistry, Academia Sinica.
Figure 2. Working principle and mechanism of DSSCs