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