NEW MATERIALS FOR DYE-SENSITIZED SOLAR CELLSVineet Jagadeesan Nair

Mechanical Engineering and EconomicsUniversity of California, Berkeley 2018jvineet9@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 (thatuse expensive materials like Si and other semiconductors) eversince high efficiency Ruthenium based dyes (N719) weredeveloped by Grätzel in 1991, at UC Berkeley!

• However, difficulty of synthesis, high cost of startingmaterials, poor stability (mainly due to the liquid electrolyte)and negative environmental effects of dyes based on transitionmetal complexes hinder their widespread application. Thus,an effort has been made to design metal-free organicsensitizers that offer several important advantages.

• The peak efficiency of DSSCs has only reached about half ofthat 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 inEthanol/Deionized water for 30 minutes at 70°C

3.Transparent Layer: Cast a layer of homemade mesoporousTitanium dioxide paste with a thickness of 20-40 nm,followed by annealing for six hours

4.Scattering layer: Cast homemade colloidal TiO2 paste to athickness of 10 nm, followed by annealing for 6 hours

5.Use another FTO plate sputtered with Platinum as counterelectrode (cathode)

6.Seal the two electrodes, separating them by a small gapusing Surlyn film (0.2 µm), and inject electrolyte (I−/I3− redoxcouple) 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 8000

5

10

15

20

Phot

ocur

rent

den

sity

(mA

cm-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 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Mol

ar E

xtin

ctio

n Co

effic

ient (

10-4

* M

-1cm

-1)

Wavelength (nm)

SQ2 2:8 4:6 6:4 8:2 DCE2

0 200 400 6000

10

20

Voltage (mV)

Phot

ocur

rent

Den

sity

( mA

cm-2

)

SQ2 2:8 4:6 6:4 8:2 DCE2

DCE2:SQ2 RatioPower Conversion

Efficiencyη (%)

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 8000

10

Voltage (mV)

Phot

ocur

rent

den

sity

(mA

cm-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) FF10:0 (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 0.706:4 9.07±0.11 686.1±2.44 18.92±0.21 0.704:6 8.19±0.09 658.0±2.56 18.47±0.17 0.672:8 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 SchematicFigure 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 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Mol

ar E

xtin

ctio

n Co

effic

ient (

10-4

* M

-1cm

-1)

Wavelength (nm)

YD2-o-C8 2:8 4:6 6:4 8:2 DCE2

MATERIALS AND METHODSINTRODUCTION

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 LiangInstitute of Chemistry, Academia Sinica, Taipei, Taiwan

Figure 14. UV-vis absorption spectrum

1) Dip in pure DCE2 solution forlonger time (20 h) as primary dye

2) Then soak in pure YD2-o-C8 forshorter 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 betweenthe 2 sensitizers i.e. YD2-o-C8 maycause desorption of some DCE2from 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 severaldisadvantages 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 usingeither coadsorbate (CDCA) or secondary dye

• Enhance spectral response through molecular design and cosensitization

• Adjust cell parameters like (1) thicknesses of scattering and transparent layers toadjust dye loading, (2) annealing times and temperatures etc.

• Substitute liquid electrolyte with more stable, solid state electrolyteFigure 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 andTechnology (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