band gap engineering of hybrid perovskites for solar cells
TRANSCRIPT
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Band gap engineering of hybrid organic inorganic lead-halide perovskites
Kirill Popov David Cahen Group
Department of Materials and Interfaces
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What is a band?
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Band structure of solids
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Key band positioning types
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Key band positioning types
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The principle of photovoltaics
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Solar radiation
Maximum in spectrum ∽ semiconductors band gap
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Energy loss pathways
• Radiative recombination
• Relaxation to band edges
• Blackbody radiation
• Solar spectrum is not uniform
• Other: non-radiative recombination, finite mobility
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Energy loss pathways
• Radiative recombination
• Relaxation to band edges
• Blackbody radiation
• Solar spectrum is not uniform
• Other: non-radiative recombination, finite mobility
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Energy loss pathways
• Radiative recombination
• Relaxation to band edges
• Blackbody radiation
• Solar spectrum is not uniform
• Other: non-radiative recombination, finite mobility
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Shockley-Queisser Limit
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Shockley-Queisser Limit
33.7% for Egap of 1.34 eV
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How to overcome the limit?
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How to overcome the limit?
Multijunction solar cells: «stacking»
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Perovskite
CaTiO3Lev Perovski(1792–1856)
• Fairly popular structural type among ABX3 compounds • May undergo distortions: axial stretch, octahedra twist,..
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Hybrid lead halide perovskites
•Several easy preparation techniques exist •Cheap precursors, no rare elements •Relatively good conductance
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MAPbX3
Band gap can be tuned by varying halide composition
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Device efficiency
x
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Device efficiency
x
Recent reports of 19.3% efficiency!
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Device architecture
GlassFTOETM
AbsorberHTMAu
HTM - hole transport materialETM - electron transport material FTO - fluorine-doped tin oxide (transparent conductor)
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Spin-coating
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Two-step deposition: the procedure
1. Spin-coating PbBr2 and PbI2
2. Dipping the films in MABrxI1-x solutions
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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
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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
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Second step
• Solution of MABr and MAI in iPrOH
• C (total) = C (MA+) = 0.05 mol/l
• 1h dipping time
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Deposition on glass• Adhesion between glass and perovskite is quite low
• Fast rate of film degradation on exposure to air is observed
PbBr2 3020100 8070605040 10090
%Br in solution
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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 200 806040 100
%Br in solution
200 806040 100
%Br in solution
PbI2
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Light absorbance
Absorption edge corresponds to band gap value
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Photoluminescence
via PbI2
via PbBr2
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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
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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 200 806040 100
%Br in solution
200 806040 100
%Br in solution
PbI2
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Band gaps
• Eg = 0.41x+1.53 (eV) for perovskites prepared by dipping PbI2 in MAI1-xBrx solution
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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
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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.
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ThanksIgal Levine
Professor David Cahen and his group
Professor Gary Hodes and his group
Kupcinet-Getz Summer Program