1. Study of Catalytic effect of Advanced Counter Electrode materials for Solar Energy Conversion
Nandini Sharma
Teaching cum Research Fellow, Research Lab for Energy Systems, Department of Physics, NSIT (Delhi University),Delhi.

2. Presentation Overview
Introduction & Abstract
Solar cells (DSSC & QDSC ): Components
Counter electrode : CATALYST
Introduction
Properties
Possible materials
Merits & Demerits
Conclusion
Future Perspectives
Research needs

3. ABSTRACT

4. SOLAR CELL??
A solar cell or photovoltaic cell (PV cell) is a solid-state electronic device that converts the energy from sun directly into electricity by photovoltaic phenomenon.

5. SOLAR ENERGY = RENEWABLE RESOURCE
ONE YEAR OF
SOLAR ENERGY
PROVIDED BY SUN
ONE YEAR OF EARTH’S
FOSSIL ENERGY
CONSUMPTION
x 10,000 =
SOLAR ENERGY = RENEWABLE RESOURCE

6. Appealing Characteristics
Consumes no fuel
No pollution
Wide power-handling capabilities
High power-to-weight ratio

7. Solar Energy Spectrum
Power reaching earth 1.37 KW/m2

8. Efficiency Losses in Solar Cell
1 = Thermalization loss 2 and 3 = Junction and contact voltage loss 4 = Recombination loss

9. Solar Cell Characteristics
Circuit model RS IL
dark
VOC RP V
Maximum power rectangle
light
ISC
Critical parameters: VOC, open circuit voltage
ISC, short circuit current
FF, fill factor = area max. power rectangle
VOC . ISC
Operating diode in fourth quadrant generates power

10. What Makes a Good Solar Photovoltaic Material?
High photon capture cross-section in the UV and Visible
Efficient production of photo excited charges
Efficient transfer of photo excited charges to some external circuit (i.e. able to be integrated into a suitable PV device platform)
Cheap, biocompatible (preferably bioavailable), processable, stable (chemically), etc.

11. Improved Efficiencies
A historical perspective regarding photovoltaic development and achieved conversion efficiencies.
Improved Efficiencies

12. Cell components
Photo anode
Electrolyte
Counter electrode

13. DSC & QDSC Device

14. Counter electrode
CE collect electrons from the external circuit and catalyze the redox species in electrolyte.
Ideal CE has 80% optical transparency at 550nm, sheet resistance of 20 ohm sq-1, 2-3 ohm cm2 charge transfer resistance.
Catalytic activity is affected by both the electronic structure of catalyst as well as electronic structure of host metal.
Comparative study is being done for different counter electrode materials for Dye sensitized solar cells (DSCs)

15. CATALYST Increases rate of a chemical reaction
Do not undergo any chemical change itself
A good catalytic activity is the chief property that a cathode must possess  To increase the rate of reaction at electrode electrolyte interface and reduce the over potential.
Lower value of RCT which is the charge transfer resistance between counter electrode and electrolyte interface Ideally, the value of RCT is 1 Ωcm2 corresponding to minimum losses in the system.
CE value play a very crucial factor in system circuitry and catalytic activity  Reduces the redox species and accepts hole from hole transporters.

16. Catalyst Properties Durable Abundant and Easily Available
Cost effective
Flexible
Catalyst Properties
Good Stability towards electrolyte
High Thermal & Electrical Conductivity
Durable
(resistant to corrosion, covalent solids, and high melting temperatures of ionic crystals)

18. PLATINUM HIGH TRANSMITTANCE
LOW CHARGE TRANSFER RESISTANCE TOWARDS ELECTROLYTE
ANTI-CORROSIVE
MORE DENSITY OF CATALYTIC SITES
HIGH ELECTRICAL CONDUCTIVITY
PLATINUM

19. BULK Pt Pt Composites PLATINUM MATERIALS Large surface area
Pt NANOPARTICLES
BULK Pt
Pt Composites
Large surface area
High value of transmittance
Low RCT (charge transfer resistance)
Anti-corrosive property
More density of catalytic sites
High electrical conductivity
Sining Yu et al., Adv. Mater. 2014( 26), 6210.

20. Pt FACETS
Using density functional theory, we have investigated the catalytic reaction processes of triiodide reduction over {100}, {111} and {411} facets, indicating that the activity follows the order of Pt(111) > Pt(411) > Pt(100). 
Pt nanocrystals mainly bounded by {100}, {111} and {411} facets were synthesized and used as counter electrode materials for DSCs.
The highest photovoltaic conversion efficiency of Pt(111) in DSCs confirms the predictions of the theoretical study. These findings have deepened the understanding of the mechanism of triiodide reduction at Pt surfaces and further screened the best facet for DSCs successfully.
Bo Zhang et al., Nature, Scientific reports (2013) 3,1836.

21. Bo Zhang et al., Nature, Scientific reports (2013) 3,1836.

22. Type of nano structure affects electrode properties
Studies on vertically aligned Platinum nano-cups in comparison to the planar nano-cups has been reported.
Observed that nano cups has a larger surface area.
Also, the electrolyte generally interact with both inner and outer faces of the cup which leads to more powerful catalytic effect towards the I-/I3- redox species.
Apart of all these things, DSCs data using nano cups showed an efficiency of 9.75% which is on the higher side than that of planar Pt showed an efficiency of 7.87%.
H. Jeong, Y. Pak, Y. Hwang, H. Song, K. H. Lee, H. C. Ko and G. Y. Jung, Small (2012) 8, 3757

23. Pt nano composites
Sara et al. ,in 2014 showed Composites of Pt nanoparticles with graphene coated on FTO or ITO gave an efficiency of 6.35% as it shows higher electron transport rate in comparison to the 5.47% efficiency of Pt film alone.
Composite of Platinum nanoparticle with multi walled carbon nanotube (Pt-NP-MWCNT) is another form of composite with Pt as a CE showed efficiency of around 8% .
Wang et al. in 2011 introduced paper as substrate for nickel coating in order to make low cost and flexible DSCs with reported efficiency of 2.68% in which Pt metal was used as electro catalyst on counter electrode in DSCs.
Pt doped Carbon nano sheets (CNS) based CE were reported in 2010 with power conversion efficiency of 7.56% and 8.05% depending on doping concentration. So, it has large potential to replace FTO (Fluorine doped Tin Oxide) electrodes as the efficiency recorded for Pt on FTO is just 4.5 % .

24. Due to HIGH COST, LIMITED RESERVES
Not appropriate for the redox species like Co3+/Co2+ and polysulfide electrolytes
New alternative materials / Pt FREE Catalysts need to be explored

25. Carbon compounds and Conducting polymers
Carbon Black
Graphene
Fullerenes
CNTs
Carbon
COST EFFECTIVE
ABUNDANCY
EXCELLENT CATALYST

26. Why Carbon materials??
Carbon black has active sites at edges of the crystal and the corresponding film thickness should be accurately controlled as it affects catalytic property and resistance as shown by Murakami et al.
The thin film coated with carbon materials should be porous with high surface area.
Mesoporous carbon synthesized by Nano casting or template synthesis serves as a very useful and efficient CE as it possess special features like pore volume, tunable pore diameter , interconnectivity ,homogeneous pores and large internal surface area which lead to high values of efficiency ( 7.1%) .

27. Conducting Polymers like PANI (polyaniline), PEDOT (poly (3, 4-ethylenedioxythiophene) are used in combination with graphene sheets.
PEDOT- graphene composite was found better than PANI-graphene composite in solar energy conversion efficiency due to high loading of graphene into the PEDOT and reduced value of RCT.
Composite of graphene with nickel metal to form a CE was reported in 2010 which showed efficiency of 5.7%. , which was attained due to the increased rate of charge transfer at the interface and more number of active sites exposed to electrolyte , hence greater is the catalytic acitivity.
Recently, tungsten carbide metal is found to have good adhesion with graphene nano sheets with overall efficiency of 5.88%
MWCNTs and SWCNTs can also be combined with the graphene to form graphene carbon composite as done by Velten et al. These materials are good for roll to roll production and light in weight.

28. Semiconducting Materials
Transition metals sulfides
Oxides,
Nitrides
Carbides

29. 2. VC/MC 3. M/Ppy/C (M=Co, Fe, Ni) 1. WO2/MC
PCE= 7.76 % >> Pt PCE= 7.3 %
REASON:
Higher Catalytic activity to regenerate electrolyte.
High electrical conductivity
1. WO2/MC
PCE= 7.73% towards Iodide and Sulfie Redox couple based electrolytes.
2. VC/MC
Co, PCE= 7.64%
Fe, PCE = 5.07%
Ni, PCE =7.44%
Bare C as CE, PCE=6.26%
3. M/Ppy/C
(M=Co, Fe, Ni)
TUNGSTEN , VANADIUM
1. Wu et al., J.Phys. Chem. C, 2011 (115),
2. Wu et al., J.Am. Chem. Soc., 2012(134), 3419.
3. Liu et al. J.Mter. Chem. A, 2013(1), 1475

30. 4. MoS2/ Graphene 5. MS/Graphene (M=Ni, Co) PCE=6.07%
Bare Pt, PCE= 6.41%
95% OF LEVEL ACHIEVED
4. MoS2/ Graphene
NiS/ Graphene, PCE= 5.25%
CoS/ Graphene, PCE= 5.04%
5. MS/Graphene
(M=Ni, Co)
4. Lin et al., Chem. Commun.,2013(49), 1440.
5. Bi H.et al., Carbon, 2013(61), 116

31. Quantum Dots: Graphene quantum dots (GQDs)
GQDs are edge-bound nanometer- size graphene pieces with lateral dimensions less than 100 nm.
With the quantum confinement and edge effects, they exhibit unique electronic and optical properties
Used for cell imaging, electrocatalysts, and light absorbers in photovoltaic devices.
Compared with carbon nanotubes and graphene, GQDs have much smaller sizes while retaining the graphitic nature.
Moldable
Facilitate collection and transport of carriers
t means that the wavelength at which they will absorb or emit radiation can be adjusted at will: the larger the size, the longer the wavelength of light absorbed and emitted [2].  The greater the bandgap of a solar cell semiconductor, the more energetic the photons absorbed, and the greater the output voltage. On the other hand, a lower bandgap results in the capture of more photons including those in the red end of the solar spectrum, resulting in a higher output of current but at a lower output voltage. Thus, there is an optimum bandgap that corresponds to the highest possible solar-electric energy conversion, and this can also be achieved by using a mixture of quantum dots of different sizes for harvesting the maximum proportion of the incident light.
They can easily be combined with organic polymers, dyes, or made into porous films (“Organic solar power”, this series). In the colloidal form suspended in solution, they can be processed to create junctions on inexpensive substrates such as plastics, glass or metal sheets.

32. Graphene QM dots doped polypyrrole film prepared by electrochemical synthesis (GQDs-PPy) shows synergetic catalytic effect, higher current density and reduced charge transfer resistance than the pure PPys towards electrolyte reduction. [1.pdf]
They also considered the effect of GQD doping amount on photovoltaics efficiency. Doping with 10% concentration gave good efficiency (5.27%) and fill factor values in comparison to plain PPys.
So, appropriate amount of doping is important to improve the performance of PVs. So, it has good potential to replace Pt CE in near future.
Lijia Chen et al., Applied Materials and Interfaces, (2013), 5, 2047.

33. Conclusion
The examples presented throughout this review show that recent progress and important worldwide contributions, which will be instrumental to overcoming Pt dependence in the catalytic fields by optimizing Pt-free CE catalysts and CE design.
More catalytic systems will likely be developed as alternatives to Pt electrodes in DSSCs by introducing diversity and compositional complexity, and may also be useful in other catalytic fields.

34. FUTURE Challenges & Perspectives
Understand electronic and optical properties (especially band structure and charge transport)
Learn how to develop multiple compounds and new composite catalysts.
Explore fundamental reasons for the higher catalytic activity of the composites , figure out role of each part of composite in future research.
Produce good quality, suitable materials – powders, composites, molecularly continuous thin films (of controlled composition & MW)
Understand and test Pt-free catalysts stability in harsh conditions

35. Research Needs Lower Costs
We’ll need to replace Pt with lower cost materials such as mentioned here like carbon-based materials , polymers etc.
Efficiency
Efficiency is a major driver in reducing costs and meeting energy requirements in limited areas
Eco-friendly
Research on new materials and devices should define all potential hazards before commercialization
Sustainability
Need to use materials that are plentiful.
Reliability
Possible failure mechanisms must be identified for new PV materials and devices

36. THANK YOU