1. STUDIES ON PEROVSKITE BASED SOLAR CELLS(Literature Study) P.VINUTHA, CA14M004, NCCR, IIT MADRAS.

2. Solar Energy Solar energy is abundant and clean form of energy resource available to us.. Practical Applications of Solar Energy: World energy requirement in 2012 is roughly 17 TW!!!

3. SOLAR CELL A Solar cell is an Electronic device which directly converts sunlight into electricity. Light shining on the solar cell produces both a current and voltage to generate electric power. Steps involved in the operation of solar cell: Absorption of light,generating electron- hole pairs. Separation of photogenerated carriers. Extraction of these carriers to an external circuit and dissipation of power in the load.

4. Generations of solar cells Wafer Based: Highest Efficiency Mono Crystalline Silicon 25% Poly Crystalline Silicon 20% Thin Films: Amorphous Thin Film Silicon 13% CdTe(Cadmium Telluride) 17% CIGS(Copper Indium Gallium Selenide) 20% Low Cost and Highly Efficient: DSSC(Dye Sensitized Solar Cells) QDSSC(Quantum Dot Sensitized Solar Cells) OPV(Organic PhotoVoltaics) QDs-Polymer Hybrid Solar Cells Pervoskite Solar Cells

5. EFFICIENCY LOSSES IN A SEMICONDUCTOR 1.Thermalization Losses, 2,3 Junction and Contact Voltage Losses, 4.Recombination Losses

6. Solar Cell Parameters Short Circuit Current: It is the current through the solar cell when the voltage across solar cell is zero.(i.e when the cell is short circuited). The short circuit current is due to generation and collection of light generated carriers. For an ideal solar cell, at most moderate resistive loss mechanisms, the short circuit current and light generated current are identical. Short circuit current depends upon: Area of solar cell Number of photons Spectrum of incident light Optical properties of the material Collection probability of solar cell.

7. Open circuit Voltage :Maximum voltage obtained from a solar cell and it occurs at zero current. Fill Factor: it determines the maximum power that can be derived from a solar cell. It is defined as ratio of maximum power from a solar cell to the product of V OC And I SC. Efficiency : It is defined as the fraction of incident power which is converted to electricity and is defined as: Terrestrial solar cells are measured under A.M 1.5 and at 25 ° C and input power of 1000W/m 2 Ref:www.pveducation.org

8. Shockley-Queisser Limit It refers to the maximum theoritical efficiency of a solar cell using a p-n junction to collect power from cell. The limit places maximum solar efficiency around 33.7% assuming a single p-n junction with a band gap of 1.34 e V (using AM 1.5 spectrum). Ref:doi:10.1063/1.1736034.

9. Production of Silicon and Silicon Wafers is Expensive and Wasteful

10. Pervoskites represents most significant breakthrough in solar technology since the 1970s… Photovoltaic cell efficiency records : Increasing photovoltaic efficency is solution for further cost reductions of solar power…. But the efficiency of market dominant silicon and established thin film technologies has plateaued… Pervoskite take solar technology to a whole new level: Theoritical Maximum(>30%) far exceeds Silicon for single junction cells. Extremely fast progress in R&D demonsrates a game changing potential. Uses Abundant,inexpensive Materials with a simple cell structure,Low Wastage and Manufacturing Cost.

11. Increasing Efficiency of Perovskites… Ref:Nam -Gyu Park,J.Phys.Chem.Lett,2013,4,2423-2429.

12. What are Pervoskites? Pervoskites is any material with same type of crystal structure as CaTiO 3 Known as Perovskite structure ABX 3. It is named after Russian Mineralogist L.A.Perovski.

13. Typical Pervoskite Solar Absorber Under ideal Conditions to maintain high symmetry cubic structure,the ionic radii of A,B,X should satisfy the requirement that the tolerance factor should be close to 1,where R A, R B,R X are the ionic radii of A,B,X respectively. To Satisfy t=1,the A ion must be larger than the B ion. In halide pervoskites B site is usually occupied by large Sn or Pb atom,so A must be extremely large. CH 3 NH 3 + satisfies the requirement better than any other Ion. CH 3 NH 3 + =2.37 A ° Pb +2 =1.33 A ° I - = 2.06 A °

14. Organic Inorganic Lead Halide Pervoskites (Using Best of both Worlds) It has been agreed that organic-inorganic nature is what makes it so successful. The organic part renders good solubility to the pervoskite and facilitates self assembly, effectively enabling its precipitation/deposition from solution. The inorganic part produces an extended network by covalent and/or ionic interactions. Such strong interaction allows for precise crystalline structure in the deposited films.

15. Evolution to Revolution (No longer a dye sensitized solar cell)

16. Production of Pervoskite Solar Cell: (simple,Lower cost,lower embodied energy,greatly reduced environmental Impact)

17. Why Pervoskite solar cells have Enormous potential?

18. Electronic Structure of CH 3 NH 3 PbI 3 The band structure,total and partial DOS of CH 3 NH 3 PbI 3. The partial DOS has been enlarged 5 times for clarity. Ref: 10.1002/adma.201306281

19. The Conduction Band Minimum comes mainly from non bonding p orbitals of lead. While the valence band maximum from antibonding hybridized lead 6s and iodine 5p orbitals. The orbitals of organic cation are located atleast 5 e V below the VBM and 2.5 e V above CBM thus CH 3 NH 3 + thus functioned as electrostatic charge compensation. The effective mass of electrons(holes) around bottom of conduction band(top of valence band) can be approximately fitted by where (k) are energy dispersion relation functions. Intuitively,the more dispersive(flat) the band near the edges is lighter(heavier) the effective mass is. The dispersed VBM and CBM resulted in reduction of effective electron and hole mass from 0.32m o and 0.36 m o to 0.23 m o and 0.29 m o where m o is free electron mass. Such small effective mass of both electrons and holes is responsible for ambipolar conductivity in Pervoskites. Large Dielectric Constant strongly screens the charged defects and impurities to increase the diffusion lengths.

20. Properties of Methyl Ammonium Lead Halide (that made it Emerging Solar Technology) Material Properties for High Efficiency PhotoVoltaics : 1.High Optical Absorption Coefficient:reduces both the thickness and challenges in collecting photogenerated carriers. The Egde Transition for CH 3 NH 3 PbI 3 comes from mixed-(Pb s,I p) to Pb p orbitals. Ref:Wan Jian Yin,J.Mater.Chem. A,DOI: 10.1039/c4ta05033a

21. The intra atomic Pb s to Pb p transition probability is high, therfore halides Pervoskites show stronger Absorption. absorption(cm -1 ) It is seen that the optical absorption of CH 3 NH 3 PbI 3 is one order of magnitude higher than that of GaAs in the visible range. Because visible light accounts for major portion of full solar spectrum high absorption in its range is critical for achieving high efficiency cells.

22. 2.Excellent Charge Carrier Transport (Crystallinity,diffusion length,carrier mobility) The achievable chemical potentials for thermodynamic growth of CH 3 NH 3 PbI 3 and the formation energies of intrinsic point defects at chemical potentials A,B,C.Defects with high formation energies are shown by dashed lines. Ref:Wan Jian Yin,J.Mater.Chem. DOI: 10.1039/c4ta05033a A,

23. Schottky defects such as PbI 2 and CH 3 NH 3 I vacancies,were found not to generate defect states having energies within perovskite bandgap.this was attributed to ionic bonding in Perovskite. Elemental Defects such as Pb,I and CH 3 NH 3, vacancies associated with Frenkel defects were found to form shallow levels near band edges. Luckily the Formation energies of Deep level defects are higher compared to shallow levels.Deep energy levels traps the charge carriers. Shallow Defect Levels are responsible for High Diffusion Lengths in Perovskite Solar Cells High Diffusion length is important because light generated electrons and holes can travel enough distances to be extracted as current,instead of losing energy in the cell as heat..

24. Transition energy levels of a) intrinsic acceptors and b) intrinsic donars in CH 3 NH 3 PbI 3.Zero is the energy referred to VBM. Ref:Wan Jian Yin,J.Mater.Chem. A,DOI: 10.1039/c4ta05033a

25. 3.High Open circuit Voltage(V oc ): Low non radative recombination rates compared to other thin film polycrystalline semiconductors. This property manifests itself in relatively small difference between V oc of experimental cells and their effective bandgap potential(E g /q). The Maximum Voltage of a Semiconductor absorber is approximately its Band Gap(E g ) subtracted by 0.25 e V. The ratio of V oc /E g indicates indicates how well the material will perform as solar cell absorber. Ref:M. Green et al. Solar cell efficiency tables (version 42) July 2013

26. Ferroelectric Effect Hybrid Pervoskites exhibit spontaneous electric polarization,and the presence of ferroelectric domains will result in internal junctions that may aid electron-hole Seperation and transportation. The spontaneous dipole moment is due to the alignment of dipole moments of Organic cations. Schematic diagram of feeroelecric multidomains as hypothesized in halide pervoskite solar cells.

27. a) Energetic Losses and possible ways for performance improvements in pervoskite based solar cells. A Maximum current density of 28mA/cm 2 is possible for a pervoskite CH 3 NH 3 PbI 3 with band gap 1.5 e V. When considering 20% reflection at TCO glass substrate,about 22mA/cm 2 is the realistic photocutrrent density from a 1.5 e V band gap material. Therefore 17% is realistic efficiency because photocurrent density of 22mA/cm 2,photovoltage of 1.1 e V and fill factor of 0.7 are achievable.

28. BAND GAP TUNING Halide pervoskites can easily achieve band gap tuning by varying the combination of all of three cationic and anionic components. Influence of A cation: The A cation does not play a major role in determining band structure but acts to make charge compensation within the lattice. Nevertheless,it can influence the optical properties by deforming the MX 6 - octahedron network due to variation of its size. A larger or smaller cation can cause the whole lattice to expand or contract and lead to change of B-X bond length which is important in the determination of band gap.

29. Atomic structure of three A site cations and absorption spectra of different pervoskites prepared by solution method. Ref:Peng Gao,Energy Environ.Sci.,2014,7,2448-2463.

30. Influence of Halide anion: The Nature of CBM and VBM was studied by a theoretical method and people found that the red shift of the band gap upon proceeding down the group(Cl to I) is due to the increase in the covalent character of halogen bonding with the lead. Interestingly a pervoskite structure which incorporates two halides allows for continuous tuning of band gap and optical absorption to cover almost entire visible spectrum.

31. Ref:Guiming Peng,Journal of Nano materials,Article id:241853 UV-Vis Absorption spectra of FTO/C-TIO 2 /m-TIO 2 /MAPbI 1-x Br x Au meaured using an integral sphere.

32. Role of Transparent Conducting Film Transparent Conducting films acts as window for light to pass through the active material,as an ohmic contact for carrier transport out of photovoltaic. Transparent materials possess bandgaps with energies corresponding to wavelengths shorter than that of visible light. Transparent conductive oxides are doped metal oxides having >80% of transmittance of incident light as well as conductivities higher than 10 3 S/cm for efficient carrier transport. To date industry standard in TCO is ITO(indium doped Tin Oxide) because of its electric properties and ease of fabrication. Carbon nanotubes are being considered as potential alternative. UV-Vis Absorption spectra of FTO/C-TIO 2 /m-TIO 2 /MAPb

33. Role of Hole Transport Layer It is deposited prior to the metal electrode,it avoids direct contact of metal electrode with the mesoporous-pervoskite layer and therefore increases the selectivity of contact. This reduces recombination as evident from increase in open circuit voltage and a higher luminescence efficiency. Increases the internal Quantum Efficiency independent of applied voltage and illumination wavelength by reducing(diffusion) losses of charges.

34. Ref: wolfgang tress,IEEE,978-1·4799-4398-2/14,2014

35. Role of Electron Transport Layer Electron Transport Layer helps in the extraction of electrons from C.B of semiconductor. Where d 2 is the mean square displacement necessary for electron to reach the pervoskite/ ETL interface from the point where it is photogenerated and L d is diffusion length. The mesoporous architecture typically reduces d to less than 10 nm,that means diffusion length of 100nm is enough to achieve collection efficiency of 99%.

36. Fabrication of Pervoskite Solar cells

37. References: F.Di Giacomo et al,Journal of Power Sources 251(2014) 152-156. Zhou Yang et al.,Chinese Journal of Catalysis35(2014) 983-988. Guiming Peng,Journal of Nano materials Article ID 241853. Peng Gao,Energy Environ.Sci.,(2014)7,2448-2463. Wan Jian Yin,J.Phys.Chem.Lett.,(2014),5,3625-3631. Nam Joong Jeon,Nature,vol 517,(2015). Andrei Buin,Nano Lett.(2014),14,6281-6286. Michael Gratzel,Nature materials,vol13,sep (2014). Pablo P.Boix,Materials Today,Volume 17,(2014). Nam-Gyu Park,J.Phys.Chem.Lett..(2013),4,2423-2429. Nam-Gyu Park,Materials Today,Volume 00,August (2014). Fang Hao,J.Am.Chem.Soc.(2014),136,8094-8099. www.pveducation.org, www.pveducation.org

38. THANK YOU