zno-cuo backbone-branch heterostructure for high-efficiency...
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ZnO-CuO Backbone-Branch Heterostructure for High-Efficiency Organic-Inorganic Hybrid Perovskite Solar Cells
Kichang Jung1,3, Taehoon Lim2,3, Alfredo A. Martinez-Morales3
1Department of Chemical and Environmental Engineering, 2Materials Science and Engineering Program,
3Southern California Research Initiative for Solar Energy, College of Engineering Center for Environmental Research and Technology University of California, Riverside, California 92521
(alfmart@ece.ucr.edu)
Abstract
Conclusion
Population growth has led to an unsustainable demand in energy consumption. Due to its renewable, sustainable, and clean nature, solar energy remains as one of the most promising sources of energy. Dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) have emerged as an attractive alternate technology to silicon-based devices, due to their ease of fabrication, low material cost, and versatility of application. However, the absorption materials in these solar cells cannot absorb near-infrared (NIR) light, which inherently limits their efficiency. In this work, CuO is used as a secondary absorption layer to utility NIR light for improving device efficiency. ZnO nanorods (NRs) on fluorine-doped tin oxide (FTO) substrate are synthesized by chemical vapor deposition. CuO is synthesized on ZnO NRs by thermal oxidation. Properties of the synthesized materials are characterized by SEM, XRD, and UV-Vis-NIR photospectroscopy.
Acknowledgements
Reference
Experimental Process & Results In this work, we have successfully synthesized ZnO-CuO nanoscale, core-shell structures by using a two-step synthesis process; 1) chemical vapor deposition, and 2) thermal oxidation. From the XRD results, crystallinity of ZnO and Cu-oxide compounds are determined, and the grain size of Cu-oxide compounds are calculated by using Scherrer’s equation. The diameter of ZnO NRs increases with higher oxygen ratio. Cu-oxide compound nanostructures are synthesized when the grain size of the compounds is small. In order to synthesize many nanostructures of CuO, we will research advanced thermal oxidation method (two-step method). In the future, perovskite solar cells (PSCs) using synthesized ZnO-CuO backbone-branch structure, as a photoelectrode, will be fabricated. Performance of the PSCs using various photoelectrodes structure will be compared to understand the effect of CuO material as a branch on ZnO.
[1] http://ce.construction.com/article_print.php?L=68&C=790 [2] Martin A. Green, Anita Ho-Baillie & Henry J. Snaith, Nature 8, 506-514 (2014) [3] Giles E. Eperon, Victor M. Burlakov, Pablo Docampo, Alain Goriely & Henry J. Snaith, Adv. Funct. Mater. 24, 2151-157 (2014) [4] Liang Li, Tianyou Zhai, Yoshio Bando & Dmitri Golberg, Nano Energy 1, 91–106 (2012) [5] Henry J. Snaith, J. Phys. Chemx. Lett. 4, 3623−3630 (2013)
1.0
0.8
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0.4
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0 250 500 2250 2000 1750 1500 120 1000 750
Wavelength (nm)
Nor
mal
ized
Inte
nsit
y
UV Visible Near-infrared
Perovskite Absorption
Range
Potential absorption region
Background q Photovoltaic Principle
Solar cells consist of two electrodes, an absorber layer, an electron and a hole transport layer (ETL and HTL, respectively). In short, electrons are excited at the absorber layer by incoming photons. The excited electrons are collected by the negative electrode. The wavelength range of the absorbed light depends on the band gap of the absorber material being utilized.
Figure 1. Schematic of solar cell device
q Electron-Transfer Processes
ETL
Perovskite
Perovskite
HTL
Sunlight
21
3
645
7ZnO backbone
CuO branches
FTO glass
Backbone-Branch Structure Au
FTO glass
Photoelectrode
Research Objective
N2 and O2 Exhaust
Zn precursor
Figure 5. Experimental process by chemical vapor deposition.
ZnO/FTO
Heating Zone 1 Heating Zone 3 Heating Zone 2
N2
O2
Zn vapor
a b c d
e f g h
Sample N2 (sccm) O2 (sccm) Gas ratio (O2/N2) Diameter (nm) a 90 10 0.100 175 b 92 8 0.080 140 c 94 6 0.060 91 d 96 4 0.040 45 e 98 2 0.020 63 f 99 1 0.010 31 g 100 0.5 0.005 36 h 100 0.3 0.003 24
0.00 0.02 0.04 0.06 0.08 0.10 0.120
50
100
150
200
250
Dia
met
er o
f ZnO
(nm
)
Gas ratio (O2/N2)
FTO
Cu seed layer CuO layer
1. E-beam evaporation 2. Thermal oxidation
As-dep 200oC 250oC 300oC
350oC 400oC 500oC 600oC
500 nm
100 nm
0 100 200 300 400 500 6000
10
20
30
40
50 CuO(111) CuO(-111) Cu2O(111)
Ave
rage
gra
in s
ize
(nm
)
Temperature (oC)
0
1
2
3
4
5
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7
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Den
sity
of n
anod
ots
(/µm
2 )
- Scherrer’s Equation
500nm
300nm ZnO NRs
CuO on ZnO NRs
CuO Nanostructure
500nm
500nm
q Thermal oxidation - 500oC - 1 hr - Air
w/ HCl cleaning
w/o HCl cleaning
q HCl cleaning - 1.0 M HCl - 20 sec - Before thermal
oxidation - Room temperature
500nm
Cu on ZnO
Cu 41.9
Zn 34.0
O 24.1
Sn None
CuO on ZnO
Cu 29.4
Zn 18.0
O 48.6
Sn 4.0
CuO
Cu 32.8
Zn None
O 51.1
Sn 16.1
Desirable
Undesirable
q Limitation of Perovskite
For higher performance of solar cell devices, processes 4-7 must be minimized, while 1-3 should be maximized.
Figure 2. Schematic of electron-transfer processes
Non-absorption of near-infrared light limits efficiency of perovskite-based solar cells.
q Overall Goal Improve the conversion efficiency of perovskite solar cells by using novel heterostructure photoelectrode and modified absorber.
CuO
FTO glass
Core-Shell Structure
ZnO NRs Au
Spiro-OMeTAD Perovskite
Current Work Target Structure
Final Device
Perovskite Solar Cell MAPbI3 MAPbI3-xClx MAPbI2Cl MAPbICl2 MAPbCl3 FAPbCl3
Figure 3. Solar spectrum distribution
q ZnO NRs Synthesis on FTO glass q CuO Synthesis on FTO glass q ZnO-CuO Core-Shell Synthesis
500nm
Figure 6. SEM images of ZnO NRs synthesized with various gas ratio.
Table 1. ZnO NRs diameter with respect to process gas ratio
Figure 7. ZnO NRs diameter with respect to process gas ratio
Figure 8. CuO Synthesis by thermal oxidation
Figure 9. SEM images of Cu-oxide compound on FTO glass with various thermal oxidation process temperature.
Figure 10. X-ray diffraction pattern for Cu-oxide compounds of FTO glass.
Figure 11. Average grain size and nanodot density of Cu-oxide compounds.
τ : Average grain size θ: Bragg angle k: Shape factor (0.9) λ: X-ray wavelength β: Full half width at half maximum
Figure 12. SEM characterization of CuO obtained on ZnO NRs with/without HCl pre-treatment before thermal oxidation.
Figure 14. X-ray diffraction pattern for ZnO-Cu, ZnO-CuO and CuO.
Table 2. Energy-dispersive X-ray spectroscopy (EDX) results for ZnO-Cu, ZnO-CuO and CuO.
Figure 13. Transmittance spectra for ZnO-Cu, ZnO-CuO and CuO in the ultra violet, visible, and near-infrared light range.
The transmittance spectra shows CuO on ZnO NRs can absorb near-infrared light with wavelength longer than 800 nm. Therefore, CuO on ZnO NRs can be used as a secondary absorption layer in perovskite solar cells to widen the absorption range towards the near-infrared light.
This research was partially funded by the University of California Advanced Solar Technologies Institute (UC Solar).
Figure 6 shows that the diameter of ZnO NRs i n c r e a s e w i t h h i g h e r oxygen ratio during CVD synthesis (from 24 to 175 nm). This result indicates that the diameter of ZnO NRs can be controlled during growth.
Cu-oxide compounds nanodots are synthesized with small grain size at thermal oxidation temperature ranging from 250 oC to 350 oC
Electron flow
Optimization of Perovskite Layer
Figure 4. Current and proposed research work
7% Ultraviolet (300-400 nm) 47% Visible (400-700 nm) 46% Near-infrared (700-2500nm)
ETL
(n-t
ype)
HTL
(p
-typ
e)
Abs
orbe
r
Electrode
SUNLIGHT
h+
e-
Electrode
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