1. Dibyajyoti Ghosh
University of Bath
Perovskite Materials for Solar Cells : Atomic-scale Insights from ab initio Modelling

2. Talk Outline Introduction Metal Halide Perovskite A-cation mixing
Energy : Source and Crisis
Photovoltaics and Perovskite Solar Cells
Metal Halide Perovskite
A-cation mixing
Effect on Stability
Change in Dynamics
Lead Free materials
Sn-Ge Mixing: Stability and Optoelectronics

3. Energy
London
Paris
Berlin
New York

4. Energy Crisis
oursolarenergy.com

5. Environmental Effect

6. Renewable and Green Energy

7. Renewable and Green Energy
Energy Share In UK ( )

8. Separate the charge carriers
Solar Cells
Separate the charge carriers
Absorb sunlight
Extract the carriers

9. Materials for Solar Cells

10. Perovskites : Structure
Chemical Formula: ABX3
A = Monovalent cation
B = Divalent metal
X = Monovalent anion

11. Methylammonium lead iodide CH3NH3PbI3
Organic-Inorganic Perovskite
Methylammonium lead iodide CH3NH3PbI3
PbI6 octahedra
CH3NH3+
Easy synthesis
Tune absorption
Long diffusion length
Eames et. al. Nature Comm. 2015, 6,  7497

12. Perovskite Solar Cells
Sunlight
Glass or Plastic
Ti-oxide
Perovskite e- h+
Back Contact

13. Upscaling and Reproducibility
Promising Material: But ….
Lead Toxicity
Imperial College, London
Degradation under
ambient conditions
Upscaling and Reproducibility
Ion migration
Eames et. al. Nature Comm. 2015, 6,  7497

14. Dynamic Lattice Effects : Dynamics: Periodic octahedral tilting
FAPbI3
Dynamics:
Periodic octahedral tilting
Tumbling and libration of organic cations
Weller et. al., J. Phys. Chem. Lett (2015)
Quarti et. al. , Phys. Chem. Chem. Phys. (2015)
Effects :
Dynamic band-gap
PL efficiency, excited state dynamics
Motta et. al., Nat. Commun (2015)
Park et. al., Nano Lett (2017)

15. Stability and Dynamics of Perovskites
Compositional Engineering

16. Cation Mixing: Increased Stability
BX6
MAPbI0.6Br0.4
FA0.83Cs0.17PbI0.6Br0.4
McMeekin et. al., Science (2016)
Possible reasons :
Higher dipole interactions
Increased entropy of mixing
Binek et. al., J. Phys. Chem. Lett. (2015)
Yi et. al., Energy Environ. Sci., (2016)

17. Key Questions : Mixed A-cation Perovskites
Effect on overall structure ? Origin of enhanced stability ?
Effect of the structural dynamics ?
Which System ?
FAPbI3 as the parent
compound

18. Methods Method : ab initio molecular dynamics
On-the-fly calculation of forces from DFT
Solve Newtonian EOM for moving atomic nuclei
Simulation Parameters:
Supercell with 768 atoms for FAPbI3 (~ 26 kAU/ps)
NPT simulations at 300 K using Nose-Hoover thermostat
Time step of 1 fs to capture motion of H-atoms
PBEsol functional with DZVP basis set, GTH pseudopotentials, DFT-D3 for dispersion

19. Mixed Cation Perovskites: Overall Structure and Stability

20. A-site Cation Mixing: Structural Trends
Volume reduction with mixing of small cations

21. Volume reduction with mixing of small cations
Tilting of Octahedra
Tilting of Octahedron θ
FA0.9Cs0.1
FAPI
FA0.8MA0.1(CsRb)0.1
FA0.97MA0.03
FA0.8MA0.1Cs0.1 FA
FA0.97Cs0.03
FA0.9Cs0.1
FA0.97Rb0.03
FA0.9Rb0.1
Volume reduction with mixing of small cations

22. Stronger Hydrogen Bonding : Stability Effect ?
Increased H-bond strength Red-shift in N-H stretching
Matsui et. al., Adv. Mater., (2017)
Structural stability
FA0.9Cs0.1PbI3
Octahedra tilting I---H-N hydrogen bonds

23. Mixed Cation Perovskites:
Dynamical Property

24. Change in Dynamics : Locked Octahedra Tilting
FAPbI3
FA0.9Cs0.1PbI3
Smaller fluctuation in Pb-I-Pb angle : more monochromatic light absorption
Amat et. al. , Nano Lett., (2014)

25. Change in FA-cation Dynamics
FAPbI3
FA0.9Cs0.1PbI3 N N
Restricted tumbling motion with mixing
Good Vibrations: Locking of Octahedral Tilting in Mixed-Cation Iodide Perovskites for Solar Cells
D. Ghosh, P. W. Atkins, M. S. Islam, A. Walker, C. Eames,
ACS Energy Lett. 2017, 2, 2424–2429
Can affect the carrier life-time
Huang et. al. , J. Mater Chem. A , (2017)
Kubicki et. al., J. Am. Chem. Soc., (2017)

26. Lead Toxicity:
Lead-free hybrid perovskites

27. Lead Toxicity : Other B-metals ?
B = Sn, Ge
BX6
MABI3

28. Lead Free B-site : Sn-Ge Mixing
Stability
Electronic Property
Ge-content
Ge-content
MAGeI3
MASnI3
Lead-Free Perovskite Semiconductors Based on Germanium–Tin Solid Solutions: Structural and Optoelectronic Properties N. Nagane, D. Ghosh, R. L. Z. Hoye, B. Zhao, S. Ahmad, A. B. Walker, M. S. Islam, S. Ogale, and A. Sadhanala J. Phys. Chem. C, 2018, 122, 5940–5947

29. Concluding Remarks

30. Summary Atomic-scale insights into cation and B-metal mixing
Increased H-bonding: enhanced stability
Reduced dynamics: Can affect PV-behaviour
Lead-free Perovskite materials for solar cells: stable and good optoelectronic properties

31. Compositional Engineering (2017) O2/light degradation (2017)
Recent Work at Bath
Compositional Engineering (2017)
O2/light degradation (2017)
Light soaking + O2/H2O (2017)
Ion-migration
( ) 31

32. Thanks to…

33. Group & Collaboration EoCoE for funding All Collaborators
Prof. A. B. Walker
Prof. M. S. Islam
EoCoE for funding
All Collaborators
Archer HPC and Balena HPC

34. Publications
1. “Phase Behaviour and Polymorphism of Formamidinium Lead Iodide”, O. J. Weber, D. Ghosh, S. Gaines, P. F. Henry, A. B. Walker, M. S. Islam and M. T. Weller (Under review in Chem. Mater.)
2. “Lead Free Germanium-Tin Solid Solutions Based Perovskite Semiconductors: Structural and Optoelectronic Properties”, S. Nagane, D. Ghosh, R. Hoye, B. Zhao, S. Ahmad, A. B. Walker, M. S. Islam, S. Ogale and A. Sadhanala, J. Phys. Chem. C 2018, 122, 5940–5947
3. “Formamidinium-Caesium Mixed Perovskites : Structural and Optoelectronic Effects of Chemical Pressure through Cation-Size Mismatch”, D. Ghosh, A. R. Smith, A. B. Walker and M. S. Islam (Submitted)
4. “Good Vibrations: Locking of Octahedral Tilting in Mixed-Cation Iodide Perovskites for Solar Cells”, D. Ghosh, P. W. Atkins, M. S. Islam, A. B. Walker and C. Eames, ACS Energy Lett., 2017, 2, 2424
5. “Azetidinium lead iodide for perovskite solar cells”, S. R. Pering, W. Deng, J. R. Troughton, P. Kubiak, D. Ghosh, R. G Niemann, F. Brivio, F. E. Jeffrey, A. B. Walker, M. S. Islam, T. M. Watson, P. R. Raithby, A. L. Johnson, S. E. Lewis, P. J. Cameron, J. Mater. Chem. A, 2017, 5, 20658

38. No volume contraction of PbI6
PbI6 Octahedra
Volume of Octahedron
Pb-I
No volume contraction of PbI6

39. Solar Cells
Solar cells: converts the energy of light directly into electricity by the photovoltaic effect
wikipedia.org

40. Single-Junction Photovoltaics

41. Why Perovskite Solar Cells?

42. BOMD