Perovskite Solar CellsNew Promising Materials for Next Generation Solar Cells

Abhishek RanjanBATCH 11, ISP, CUSAT20-10-2016

Slides available at- www.abhishekranjan.work

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OVERVIEW• Basics of Solar cell• Emergence of Pervoskites .• Features of Solar cell.• Structure of Pervoskites• Preparation of Pervoskites solar cells• Future Challenges• References

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Basic Structure of a Photovoltaic Solar Cell• A photovoltaic solar cell is made of three main part:

• Light Absorber; converting incident photons to electron and holes• Carrier Collector/s; capturing the carriers (electron and holes)• Metal Contacts: transferring the carriers to the circuit

• The heart of a solar cell is the absorber layer

Basic schematic of a photovoltaic solar cell

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Three Generations of Solar Cells• Wafer based:

• Monocrystalline silicon 25%• Polycrystalline silicon 20%• Multi-junction cell (different band-gap materials) 40%

• Thin Films:• Amorphous thin film silicon 13%• CdTe (Cadmium Telluride) 17%• CIGS (Copper Indium Gallium Selenium) 20%

• Low Cost and high Efficient:• DSSC (Dye-sensitized solar cells)• QDSSC (Quantum Dot-sensitized solar cells)• OPV (Organic photovoltics)• QDs-Polymer Hybrid solar cells• Perovskite Solar Cells

Highest efficiency

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Emergence of Perovskite Solar Cells• Efficiency jump in photovoltaics research• From 3.8 % in 2009 to 15.9 % in 2014

Science 18 October 2013: Vol. 342 no. 6156 pp. 317-318

15% perovskite solar cell made in University of Oxford

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Material Properties: Good for Photovoltaics, but with Caution

• Cheap Manufacturing: • Lower manufacturing costs expected: directly deposited from solution• Caution: Encapsulation needed, which may increase cost

• Material Properties for High Efficiency Photovoltaics:• 1. High Optical Absorption Coefficient• 2. Excellent Charge Carrier Transport (crystallinity, diffusion length, carrier mobility)• 3. Promising Device Parameter: High Voc of >1.1 V is reported

• Stability: • Study shows it can maintain more than 80% of its initial efficiency after 500 hours.• Caution: More studies needed. Lifetime of 15 years has not been demonstrated. The

ultimate goal of 15-year-lifetime not demonstrated.

• Other Real World Concerns (equally important but omitted here): • Toxicity from Pb• Scaling Problem Nature. 2013, 12, 1087

Nature Comm. 2013, DOI: 10.1038/ncomms3885Science. 2013, 342, 317

Nature. 2013, 499, 316

High Optical Absorption Coefficient

• Perovskite Absorption Coefficient: 1.5X104 cm-1 at 550 nm, which is one order of magnitude higher than conventional ruthenium-based organometallic N719 dye

Nanoscale, 2011, 3, 4088

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Carrier Diffusion Length

quartzCH3NH3PbI3

PCBM

quartzCH3NH3PbI3

Spiro-OMeTAD

electron-extracting layer hole-extracting layer

Science. 2013, 342, 341Science. 2013, 342, 344

e h

PL quenching originates from the charge-carrier extraction across the interface.

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Crystallinity

Tetragonal Structure

Lattice Parameters:a = 8.825 Å, b = 8.835 Å, c = 11.24 Å.

Science. 2012, 338, 643.

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Perovskite Crystal Structure

• Usually have stoichiometry of AMX3 • X is an oxide or halide anion such as Cl, Br and I. • M refers to a metal cation with a coordination number of 6.• The MX6 octahedra share corners and A is usually a large cation

that fills the cuboctahedral holes with coordination number of 12.

• A can be Ca, K, Na, Pb, Sr, other rare metals.

CrystEngComm, 2010, 12, 2646-2662

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Organic–inorganic Hybrid Perovskites• First three-dimensional organic–inorganic hybrid perovskite,

discovered by replacing caesium in CsPbX3 (X = Cl, Br or I) with methylammonium cations (MA = CH3NH3

+) by Dieter Weber, in 1978.

• CH3NH3PbI3 is most common used materials for making high efficiency perovskite solar cells.

• CH3NH3PbI3 is a semiconducting pigment with a direct bandgap of 1.55 eV with absorption coefficient as high as 104–105 cm−1

JACS, 136, 622, 2014

Methylammonium (MA)

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Band Gap Tuning• This field is also known as molecular enginnering.• Bandgap tuning is required to extend the absorption to longer

wavelengths without sacrificing the absorption coefficient.• Changing in any of A, M and X in AMX3 changes the bandgap.• The bandgap also can be tuned in between 1.55 eV and 1.17 eV

by varying the ratio of lead to tin.

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Device structure• The device structure, related materials, and interfacial

modification are key factors in performance of solar cells.• Two typical structures can be constructed: a) mesoscopic

nanostructure and b) planar structure.• Mesoporous TiO2 layer usually is used to collect the electrons• Organic Hole transporting material (HTM) collects the holes• Planar structure has simpler structure and higher efficiency

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Preparation Method• There are two common methods:

1) one step coating: spin-coating a mixed CH3NH3I and PbI2solution 2) two-step coating: spin-coating CH3NH3I after coating with PbI2

SmallVolume 11, Issue 1, pages 10-25, 30 OCT 2014 DOI: 10.1002/smll.201402767http://onlinelibrary.wiley.com/doi/10.1002/smll.201402767/full#smll201402767-fig-0005

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Preparation Method cont.• All deposition process happens at a low temperature (below 150

°C), which is suitable for the fabrication of flexible solar cells based on PET substrates.

• The concentration of the CH3NH3I solution affects the crystal size from about 90 nm to about 700 nm.

• Photovoltaic performance was strongly influenced by the CH3NH3I concentration, i.e., the crystal size of CH3NH3PbI3

• CH3NH3PbI3 degrades in humid conditions and forms PbI2 at higher temperatures due to the loss of CH3NH3I

• Lead (Pb) compounds are very toxic and harmful to the environment.

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Future Challenges of Perovskite Solar Cells• Improving efficiency to exceed thin film CdTe solar cells

• By understanding their material properties and optimal cell designs

• Increasing air and temperature stability• Replacing toxic Pb with a greener element• Is AMX3 (perovskite structure) the best stoichiometry? Have we

tried other structures?• Life time of more than 15 years have not been seen.

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References1.  Small, Volume 11, Issue 1, pages 10-25, 30 OCT 2014 DOI: 10.1002/smll.201402767

2. Nature Materials 13, 838–842 (2014) doi:10.1038/nmat4065

3. Science 18 October 2013: Vol. 342 no. 6156 pp. 317-318

4. CrystEngComm, 2010, 12, 2646-2662 10.1039/C001929A

5. PHYSICAL REVIEW B 90, 045207 (2014)

6. Nature Photonics 8, 506–514 (2014) doi:10.1038/nphoton.2014.134

7. Angewandte Chemie International Edition Volume 53, Issue 11, pages 2812–2824, March 10, 2014

8. Phys. Rev. B 90, 045207 – Published 24 July 2014

9. Wikipedia

10. Solar Energy International Magazine

11. Phys.org

12. Pervoskite.info

13. Science Channel

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