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Self-doping in high-band gap perovskite filmsMichael Kulbak1, Igal Levine1, Satyajit Gupta1, Einav Barak-Kulbak1, Gary Hodes1, Doron Azulay2, Oded Milo2, Isaac Balberg2* and David Cahen1
1Dept. of Materials & Interfaces, Weizmann Inst. of Science, Rehovot 76100, Israel2The Racah Institute, The Hebrew University, Jerusalem 91904, Israel
Abstract
Acknowledgements
Photoconductivity
We thank the Weizmann Institute’s Sustainability and EnergyResearch Initiative, the Israel Ministry of Science and theIsrael National Nano-Initiative for partial support.
Stand-alone Thin films – Resistivity Measurements
We address the question “HOW DO DIFFERENT CATIONS & ANIONS AFFECT CARRIER’ DIFFUSION LENGTHS and JUNCTION FORMATION in HIGH BANDGAP HALIDEPEROVSKITE (HaP) FILM-based SOLAR CELLS ?” Such HaP films are of interest for solar spectrum splitting and driving photo-electrochemical reactions for fuelproduction. Resolving how the addition of cations and anions change the (unintentional) doping of the HaP is critical for basic understanding of film and devicephysics, as well as for improving device performance.We studied HaP films with the general formula APbX3, where A can be a mixture of formamidinium (FA), methylammonium (MA) and cesium (Cs) and X can be amixture of bromine and chlorine by combining several techniques:- Dark-conductivity and Photo-conductivity, - Electron Beam-Induced Current, EBIC, and - Steady-State Photocarrier-Grating (SSPG).
Conclusions and open question
Junction type and carrier diffusion lengths from EBIC
CsPbBr3
FAMACsPbBr3
MAPbBr3
MAPbBr3-xClx
Solar cell device characteristics
From EBIC Profile to Working Mechanism
n-t
ype
p-type
Mono-exponential decay)(
0
0
nL
xx
Aeyy
n-t
ype
p-t
ype
intrinsic
n-type p-typeSpace Charge
region
Distance
EBIC
curr
en
t
n-type p-typeSpace Charge
region
EBIC
curr
en
t
Distance
FAMACsPbBr3 forms a junctionresembling a p-i-n junction while withthe other films a p-n junction forms
Unlike the single cation devices,FAMACsPbBr3 has a strong hysteresisin the device I-V characteristics
MAPbBr3-xClx MAPbBr3 CsPbBr3 FAMACsPbBr3
Dark Resistivity
[MΩ·cm]0.004±0.002 44±5 0.004±0.002 180±80
Hole Concentration*
[cm-3]3±2 X 1013 2.8±0.3 X 109 3±2 X 1013 7±3 X 108
Perovskite filmAu Au
V
𝜎𝑑𝑎𝑟𝑘 = q ∙ 𝜇 ∙ 𝑝
*Estimated using µ= 50 cm2/Vsec, from our photo-Hall measurements ; assuming that all the dopants are ionized
0.01 0.1 1 10 100
1E-7
1E-6
1E-5
1E-4
±
FAMACsPbBr3
Pho
toco
nd
uctivity [1/c
m]
Light Intensity [mW/cm2]
±
1E18 1E19 1E20 1E21 1E22
Generation Rate [1/cm3 sec]
SSPG
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.50.4
0.5
0.6
0.7
0.8
0.9
1.0
Grating period [m]
Film Type Diffusion Length [nm]
black: darkMAPbBr3-xClx
50±15 (e-) [EBIC]
MAPbBr3
60±15 (e-) [EBIC]
CsPbBr3 77±15 (e-) [EBIC]
red: illuminatedFAMACsPbBr3
640±100 (e-), 990±150 (h+) [SSPG+PC]
MAPbBr3-xClx 360±22 (e-) [EBIC]
L > layer thickness, Unable to extract diffusion length using EBIC
Extracting carrier diffusion lengths for FAMACsPbBr3
Scale bar is 200 nm
Our MAPbBr3-xClx and CsPbBr3 layers are significantly higher doped than MAPbBr3 and FAMACsPbBr3 ones.
• From dark resistivity measurements, MAPbBr3
and FAMACsPbBr3 seem to be much moreintrinsic than MAPbBr3-xClx and CsPbBr3 .
• Although the doping in MAPbBr3 andFAMACsPbBr3 films is similar, the charge carrierdiffusion lengths in FAMACsPbBr3 aresignificantly larger than in MAPbBr3.
• FAMACsPbBr3 films seem to be ambipolar withdiffusion lengths, similar to MAPbI3.
• Could the significant hysteresis observed in cellswith FAMACsPbBr3 layers be due to itsambipolar nature or to the increased numberof cation species present, which enhance ionmigration?