This document discusses band gap engineering of hybrid organic-inorganic lead halide perovskites. It describes how the band gap of methylammonium lead iodide bromide perovskites can be tuned between 1.55-2.29 eV by varying the halide composition in the solution during a two-step deposition process. Using a mesoporous scaffold improves the mechanical stability of the perovskite films. Annealing after deposition prevents instant degradation but affects uniform film formation. The goal is to optimize the fabrication process and characterize the resulting perovskite films.

1. Band gap engineering of
hybrid organic inorganic
lead-halide perovskites
Kirill Popov
David Cahen Group
Department of Materials and Interfaces

2. What is a band?

3. Band structure of solids

4. Key band positioning types

5. Key band positioning types

6. The principle of photovoltaics

7. Solar radiation
Maximum in spectrum ∽ semiconductors band gap

8. Energy loss pathways
• Radiative recombination
• Relaxation to band edges
• Blackbody radiation
• Solar spectrum is not uniform
• Other: non-radiative
recombination, finite mobility

9. Energy loss pathways
• Radiative recombination
• Relaxation to band edges
• Blackbody radiation
• Solar spectrum is not uniform
• Other: non-radiative
recombination, finite mobility

10. Energy loss pathways
• Radiative recombination
• Relaxation to band edges
• Blackbody radiation
• Solar spectrum is not uniform
• Other: non-radiative
recombination, finite mobility

11. Shockley-Queisser Limit

12. Shockley-Queisser Limit
33.7% for Egap
of 1.34 eV

13. How to overcome the limit?

14. How to overcome the limit?
Multijunction solar cells:
«stacking»

15. Perovskite
CaTiO3
Lev Perovski
(1792–1856)
• Fairly popular structural type among ABX3 compounds
• May undergo distortions: axial stretch, octahedra twist,..

16. Hybrid lead halide perovskites
•Several easy preparation techniques exist
•Cheap precursors, no rare elements
•Relatively good conductance

17. MAPbX3
Band gap can be tuned by varying halide composition

18. Device efficiency
x

19. Device efficiency
x
Recent reports of 19.3% efficiency!

20. Device architecture
Au
HTM
Absorber
ETM
FTO
Glass
HTM - hole transport material
ETM - electron transport material
FTO - fluorine-doped tin oxide (transparent conductor)

21. Spin-coating

22. Two-step deposition: the procedure
1. Spin-coating PbBr2 and PbI2
2. Dipping the films in MABrxI1-x solutions

23. The project
• Fabrication of MAPb(I,Br)3 films by two-step
deposition
• Characterization of the films compositions and
band gaps by their optical properties
• Optimization of the fabrication procedure

24. First step
• Samples pre-heated to 100 ºC
• 1 mol/l solutions of PbX2 in DMF at 100 ºC used
• Spin-coating parameters: 6500 rpm, 550 rpm/sec
acceleration, 90 sec
• Annealing after spin-coating: 70 ºC, 30 min
• Profilometry: 700-800 nm thickness

25. Second step
• Solution of MABr and MAI in iPrOH
• C (total) = C (MA+) = 0.05 mol/l
• 1h dipping time

26. Deposition on glass
• Adhesion between glass and perovskite is quite low
• Fast rate of film degradation on exposure to air is
observed
0 10 20 30 40 50 60 70 80 90 100 PbBr2
%Br in solution

27. Deposition on mesoporous Al2O3
• Mp-alumina deposited by spin-coating colloidal
Al2O3 and ethylcellulose solution with post-annealing
at 550ºC for 2 hours
• Significantly improved mechanical stability of the
films
PbBr2 0 20 40 60 80 100
%Br in solution
0 20 40 60 80 100
%Br in solution
PbI2

28. Light absorbance
Absorption edge corresponds to band gap value

29. Photoluminescence
via PbBr2
via PbI2

30. Band gap values
• JH Noh et al.: Eg = 1.57 + 0.39x + 0.33x2 (eV) for MAPb(I1-xBrx)3
• Eg = 1.54 + 0.16x + 0.45x2 (eV) for films prepared by dipping PbI2 in MAI1-xBrx solution

31. Adding post-annealing step
• Samples have been annealed at 100 ºC for 20 min
• Visible degradation signs disappear at the cost of
impaired uniformity
PbBr2 0 20 40 60 80 100
%Br in solution
0 20 40 60 80 100
%Br in solution
PbI2

32. Band gaps
• Eg = 0.41x+1.53 (eV) for perovskites prepared by
dipping PbI2 in MAI1-xBrx solution

33. Conclusions
• Methyl ammonium lead iodide bromide band gap may be engineered
between 1.55 and 2.29 eV by changing solution composition in two-step
deposition process
• Perovskite films are significantly less likely to be damaged mechanically if
mesoporous scaffold is used
• Tetragonal MAPbI3 phase formation is found to be preferable at all anion
compositions of dipping solution
• Annealing perovskites after dipping prevents instant degradation but
affects uniform film formation process
• Annealing converts quadratic dependence of band gap value on solution
composition to linear

34. Future directions
• Elemental and phase characterization of the films
• Investigation into film degradation and its effect
perovskite electronic structure
• Unfixing different parameters - total concentration,
time, annealing temperature etc.

35. Thanks
Igal Levine
Professor David Cahen and his group
Professor Gary Hodes and his group
Kupcinet-Getz Summer Program