heterojunction based hybrid si nanowire solar cell assets/pages/20th...obstacles on the road...
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Muhammad Y. BashoutiAlexandre Yersin Department of Solar Energy and Environmental Physics
Sde Boqer Campus, BGU
Material Growth Surface Engineering Devices prototype
M. Y. Bashouti , BGU, 26.09.2016
Characterization
Heterojunction Based Hybrid Si Nanowire Solar Cell
Table of Contents
IntroductionImportant Features of NanowiresObstaclesMolecules
First Chapter: Growth of Silicon NanowireTop Down
Second Chapter: Surface Treatment & CharacterizationTermination with Molecules: Grignard ReactionSurface Transfer Doping
Third Chapter: DevicesSolar Cell
M. Y. Bashouti , BGU, 26.09.2016
Introduction
Introduction
Important features of NanowiresObstaclesMolecules
M. Y. Bashouti , BGU, 26.09.2016
Integration & Devices
Important features of Nanowires
They represent the smallest efficient transport
nm
µm
A Nanowire
Orthogonal Junction
Increased surface scattering
Increased surface area
Reflection ~<10%
λ<1000nm
na
no
wir
e
M. Y. Bashouti , BGU, 26.09.2016
Obstacles on the Road
Obstacles for nanowire growth and surface modifications:
Native oxides
Gold can behave as an catalyst. However:(i) has a high diffusion coefficient 10-6 cm-2sec (10-9-10-14) at 1100°C (ii) Induces deep states inside the band gap
Gold diffusion
SiO2
Lowest surface densities ever reported with thermal oxides! ~1010
cm-2 (Si surface density ~1014 cm-2). However:
Amorphous oxides: (i) Dangling bonds
(ii) Induces trap states at the SiO2/Si interface
(iii) Limits the effect of gate voltage
(iv) No direct chemistry with Si
100
100
100
100
N. Lewis at. el. JACS, (2006): 128, 8990-8991.
H. Haick, at. el. ACS Appl. Mater. Interf. (2012): 4, 4251.
M. Y. Bashouti , BGU, 26.09.2016
Obstacles on the Road
Obstacles for nanowire growth and surface modifications:
SiO2
Native oxides
Gold diffusion Ec
Ev0.33ev
0.54ev
Au-
Au+
Eg
Lowest surface densities ever reported with thermal oxides! ~1010
cm-2 (Si surface density ~1014 cm-2). However:
Amorphous oxides: (i) Dangling bonds
(ii) Induces trap states at the SiO2/Si interface
(iii) Limits the effect of gate voltage
(iv) No direct chemistry with Si
100
100
100
100
N. Lewis at. el. JACS, (2006): 128, 8990-8991
M. Y. Bashouti , BGU, 26.09.2016
Advantages of using Molecules
A promising approach to overcome the aforementionedobstacles is to use molecules. Main advantages are asfollows:
http://www.dailygalaxy.com/
Positive dipoles
+
- +
-
Si
Negative dipoles
Stabilizing the surface (superior oxidation resistance)Molecular surface transfer doping: Controlling the doping level and type (p or n) through organic molecules.
Controlling the molecular density
Controlling cross-linking
Terminating dangling bonds low surface state densitySystematic dipoles (work function design, surface dipoles)
M. Y. Bashouti , BGU, 26.09.2016
Chapter I
Realizing Silicon Nanowires
Chapter 1
Realizing Si Nanowires (Si NWs)
Top Down (Wet and dry etching)
M. Y. Bashouti , BGU, 26.09.2016
Realizing Si Nanowire: Top down
Top down
BottomupChemical Vapor Growth
Self Assembly
Etching (dry or wet)
Main Advantages wet chemistryNo need for special equipment's Fabricating Si NWs in a few minutesLarge areas (4 inch)High aspect ratioDiameter of sub-micro
Bashouti, Y. M.; at. el. (2013): Progress in Surface
Science, 88, 39-60. (Invited Review)
Top down approach: Wet etching
M. Y. Bashouti , BGU, 26.09.2016
Bottom Up: CVD process
Realizing Si Nanowire
Top down
BottomupChemical Vapor Growth
Self Assembly
Etching (dry or wet)Top down approach: Wet etching(two step process)
“Si NW”
Si Wafer(100)
Si Wafer(100)
Solution 1 (masking)AgNO3/HF
Step 1(5-60)s
Solution 2 (etching)
H2O2/HFStep 2
(0.5-60)min
Si Wafer (100)
HNO3
Ag layer
Si Wafer (100)
Main AdvantagesNo need for special equipment's Fabricating Si NWs in a few minutesLarge areas (4 inch)High aspect ratio
M. Y. Bashouti , BGU, 26.09.2016
Top down approach(wet etching) Cont.
Top down approach: Wet etching(two step process: step II – Etching time)
0 20 40 60 80 100 120
0
10
20
30
40
R6
00
nm
(%
)Etching Time (min)
Lowest R<2%
0 20 40 60 80 100 120
0
5
10
15
20
25
30
35
Si N
W a
rray
s le
ngt
h (
m)
Time (min)
Length Vs Time
0 10 20 30 40 50
30
40
50
60
70
Po
rosit
y (
a.u
.)Time (min)
Porosity Vs Time
4 inch Si wafer
300 400 500 600 700 800 900 1000 11000
10
20
30
40
50
60
45 min15 min5 min
SiNW for 2 min
Polished
R
nm)
M. Y. Bashouti , BGU, 26.09.2016
Top down approach(wet etching) Cont.
Top down approach: Wet etching(the mechanism)
(i) 15sec
(ii) 30sec
4Ag+ 4Ag(s) + 4h+
Cathode Reaction:
Anode Reaction:
Si(s) + 2H2O + 4h+ SiO2 + 4H+
H2SiF6 + 2H2O
6HF
Total Reaction
Si(s) + 4Ag+ + 6HF 4Ag(s) + H2SiF6 + 4H+
Nano local electrochemical reaction
M. Y. Bashouti , BGU, 26.09.2016
Top down approach(wet etching) Cont.
Top down approach: Wet etching(the mechanism)
(i)
(ii)
(iii)
4Ag+ 4Ag(s) + 4h+
Cathode Reaction:
Anode Reaction:
Si(s) + 2H2O + 4h+ SiO2 + 4H+
H2SiF6 + 2H2O
6HF
Total Reaction
Si(s) + 4Ag+ + 6HF 4Ag(s) + H2SiF6 + 4H+
Nano local electrochemical reaction
M. Y. Bashouti , BGU, 26.09.2016
Surface Treatment & Characterization
Chapter 2
Surface Treatment & Characterization
M. Y. Bashouti , BGU, 26.09.2016
Surface Modification
Nan
op
arti
cles
Direct Laser Writing
Molecules
Phase Transfer(From Cubic to Wurtzite)
Molecules on Surfaces(Grignard Reaction, Electrografting)
Phase Transfer
Bashouti, Y. M. at el. (2015): Lab-on-Chip, 15, 1742-1747. DOI: 10.1039/c4lc01376j
Bashouti, Y. M. at el. (2016): Sci. Rep. Accepted
Si NW
M. Y. Bashouti , BGU, 26.09.2016
Molecules on Surfaces: Grignard Reaction
Molecules on Surfaces: Grignard Reaction
1) Characterization? 2) Replacing oxide shell?3) Si-C bonds?4) Maximum Coverage?5) Stability? 6) Oxidation Mechanism
Two step chlorination/alkylation process
Etching : Removes the native oxide layer (makes H-Si)
Chlorination: Replace the H by Cl
Alkylation: Terminate the Si-Cl to Si-R
Two Steps Reaction
M. Y. Bashouti, at. el. (2008): JPCC, 112, 19168-19172.
M. Y. Bashouti, at. el. (2009): JPCC, 113, 14823-14828.
M. Y. Bashouti , BGU, 26.09.2016
Alkylation
Si2p & C1s of Terminated Si NW
5 n m2 0 n m 2 0 n m
5 n m
[112]
SiO2
SiO2
Bicrystalline N
SiO2
M. Y. Bashouti, at el. (2012): Phys. Chem.
Chem. Phys., 14, 11877-11881.
Before Alkylation Si2p
No oxides
108 104 100 96
No
rma
lize
d C
ou
nts
(a
.u.)
Binding Energy (eV)M. Y. Bashouti , BGU, 26.09.2016
Si2p & C1s of Terminated Si NW
M. Y. Bashouti , BGU, 26.09.2016
290 288 286 284 282 280
No
rma
lize
d C
ou
nts
(a
.u.)
Binding Energy (eV)
Alkylation
H-C-HH
Si NW
C
Si
C1s
C-S
i
C-C
or
C-H
C-O
Impact on Density of State (DOS)
In Suite Measurements
cs = Φ-Eg+ (EF-EV)
d=cs - cB
Band diagram of CH3-SiNW, H-SiNW and SiO2-SiNW surfaces. All the numbers are in eV unit.
We followed the DOS by using the following:
1. Emission of Si2p2. Photo-electron Yield (PYS)3. Work-Function
M. Y. Bashouti at. .el. (2014): Prog. Photovolt: Res. Appl. 2, 1050-1061.
Si2p EF-EVBM F
SiO2 99.77 0.83 4.32
H 99.70 0.98 4.26
CH3 99.55 1.05 4.22
M. Y. Bashouti , BGU, 26.09.2016
d=100nm
hλ=(2-6eV)
Si NW
C H
H
H
Si
e-Si N
W
λ =10nm
Evac
Conduction band
Valance band
EV
EC
e- e- e-
h+h+
h+
Photoemission of Hybrid Si NW
goc= occupied density of statesEVL = Vacuum Levelħw = optical excitation
M. Y. Bashouti , BGU, 26.09.2016
Third Chapter
Chapter 3: Devices
Solar Cells
M. Y. Bashouti , BGU, 26.09.2016
Publications:1. M. Bashouti, et al. (2009): Small, 88:39-60. 5:2761-2769
2. M. Bashouti, et al. (2015): J. Phys. Chem. Lett., 6:3988–3993.
3. M. Bashouti, et al. (2015): ACS Appl. Mater. Interfaces, 7: 21657−21661.4. M. Bashouti, et al. (2016): Scientific Reports. (Accepted).5. M. Bashouti, et al. (2016): J. Mat. Chem. (Under revision).
devices: FET Surface modifications
Devices: FETs, Solar Cells, Diodes &
Transparent electrodes
Projects: Brief Summary
M. Y. Bashouti , BGU, 26.09.2016
Hybrid Solar Cells: PEDOT:PSS/ Si NW
320nm
M. Bashouti, et al. (2014): Prog. Photovolt: Res. Appl. 2, 1050.
M. Bashouti, et al. (2014): JPCC, 117, 9049−9055.
M. Y. Bashouti , BGU, 26.09.2016
Hybrid Solar Cells: PEDOT:PSS/ Si NW
M. Bashouti, et al. (2014): Prog. Photovolt: Res. Appl. 2, 1050.
M. Bashouti, et al. (2014): JPCC, 117, 9049−9055.
M. Y. Bashouti , BGU, 26.09.2016
Hybrid Solar Cells: PEDOT:PSS/ Si NW
M. Bashouti, et al. (2014): Prog. Photovolt: Res. Appl. 2, 1050.
M. Bashouti, et al. (2014): JPCC, 117, 9049−9055.
M. Y. Bashouti , BGU, 26.09.2016
Hybrid Solar Cells: PEDOT:PSS/ Si NW
M. Bashouti, et al. (2015): ACS Appl. Mater. Interfaces, 7: 21657−21661.
M. Bashouti, et al. (2016): Langmuir, (Under revision).
Transparent electrodes
M. Y. Bashouti , BGU, 26.09.2016
Summary
10µm
Wet etching Termination
M. Y. Bashouti , BGU, 26.09.2016
Thank you for your attention!
M. Y. Bashouti , BGU, 26.09.2016