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