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Band gap engineering of hybrid perovskites for solar cells

The research was conducted in summer 2014 under supervision of professor David Cahen at Optoelectronics Materials Group in Department of Materials and Interfaces at Weizmann Institute of Science (Rehovot, Israel).

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Band gap engineering of hybrid perovskites for solar cells

  1. 1. Band gap engineering of hybrid organic inorganic lead-halide perovskites Kirill Popov David Cahen Group Department of Materials and Interfaces
  2. 2. What is a band?
  3. 3. Band structure of solids
  4. 4. Key band positioning types
  5. 5. Key band positioning types
  6. 6. The principle of photovoltaics
  7. 7. Solar radiation Maximum in spectrum ∽ semiconductors band gap
  8. 8. Energy loss pathways • Radiative recombination • Relaxation to band edges • Blackbody radiation • Solar spectrum is not uniform • Other: non-radiative recombination, finite mobility
  9. 9. Energy loss pathways • Radiative recombination • Relaxation to band edges • Blackbody radiation • Solar spectrum is not uniform • Other: non-radiative recombination, finite mobility
  10. 10. Energy loss pathways • Radiative recombination • Relaxation to band edges • Blackbody radiation • Solar spectrum is not uniform • Other: non-radiative recombination, finite mobility
  11. 11. Shockley-Queisser Limit
  12. 12. Shockley-Queisser Limit 33.7% for Egap of 1.34 eV
  13. 13. How to overcome the limit?
  14. 14. How to overcome the limit? Multijunction solar cells: «stacking»
  15. 15. Perovskite CaTiO3 Lev Perovski (1792–1856) • Fairly popular structural type among ABX3 compounds • May undergo distortions: axial stretch, octahedra twist,..
  16. 16. Hybrid lead halide perovskites •Several easy preparation techniques exist •Cheap precursors, no rare elements •Relatively good conductance
  17. 17. MAPbX3 Band gap can be tuned by varying halide composition
  18. 18. Device efficiency x
  19. 19. Device efficiency x Recent reports of 19.3% efficiency!
  20. 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. 21. Spin-coating
  22. 22. Two-step deposition: the procedure 1. Spin-coating PbBr2 and PbI2 2. Dipping the films in MABrxI1-x solutions
  23. 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. 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. 25. Second step • Solution of MABr and MAI in iPrOH • C (total) = C (MA+) = 0.05 mol/l • 1h dipping time
  26. 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. 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. 28. Light absorbance Absorption edge corresponds to band gap value
  29. 29. Photoluminescence via PbBr2 via PbI2
  30. 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. 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. 32. Band gaps • Eg = 0.41x+1.53 (eV) for perovskites prepared by dipping PbI2 in MAI1-xBrx solution
  33. 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. 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. 35. Thanks Igal Levine Professor David Cahen and his group Professor Gary Hodes and his group Kupcinet-Getz Summer Program

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