Improved Performance of CZTS Thin Film Solar Cells

Improved performance of CZTS thin film solar cells
by optimized antireflective coatings
Zhongyang Ge, Pravakar Rajbhandari, Amin Emrani, Tara P. Dhakal,
Charles Westgate, and David Klotzkin
Electrical and Computer Engineering Department
Binghamton University
04/01/2014
Published in APPLIED PHYSICS LETTERS 104, 101104 (2014)
Binghamton University Optoelectronics Devices Laboratory
Outlines:
Thin film solar cell vs Crystalline Si solar cell
 Surface reflection and Gradient refractive index (GRIN) coating
Design and deposition of Aluminum doped Zinc-oxide (AZO)SiO2 based GRIN layer
Performance of AZO-SiO2 GRIN coated CZTS solar cell
Binghamton University Optoelectronics Devices Laboratory
Crystalline solar cell panel vs Thin film solar cell
Crystalline
Manufacture
Flexibility
5 steps: Ingot,Wafer,Cell,Circuit and
Module
Fairly thick and Rigid
Bosch Solar Energy crystalline solar cell
Efficiency
~20%
Thin film
2 steps:Circuit and Module
Thin and flexible
Uni-Solar Ovonic’s thin Film Flexible Solar panels
~10%
Binghamton University Optoelectronics Devices Laboratory
Optical loss on Thin film solar cell
Reflection at cell top surface
Thin film solar cell stack structure
Fresnel equation:
R
Solar irradiance spectrum
Rs  Rp
2
n cos i  n2 cos t
Rs  1
n1 cos i  n2 cos t
2
n cos t  n2 cos i
Rp  1
n1 cos t  n2 cos i
2
Gradient index (Grin) coating
Index changes continuously, interface reflection are
largely alleviated.
Advantages over quarter wavelength coating:
Enhanced at Omni-direction and wide wavelength
Binghamton University Optoelectronics Devices Laboratory
Co- sputtering deposited grin layer
Simultaneous sputtering of AZO and SiO2 targets with different powers
Deposition rates at different thickness
Plasma sputtering system
Nanofabrication Laboratory at SUNY Binghamton
Refractive index at different thickness
Binghamton University Optoelectronics Devices Laboratory
Transmittance results of Grin layer on AZO/glass
Air
Grin
AZO
Quartz glass
SEM image of the gradient index layer coated glass
Refractive index profile of stack structure used in simulation
(a) Measured transmittance. (b) FDTD simulated
transmittance
Binghamton University Optoelectronics Devices Laboratory
Optimization of the thickness on AZO solar cell
60 o
45 o
30 o
15 o
0o
Add up reflectance at different angles for each thickness
FDTD simulated transmittance of different Grin layer thicknes
Optimized thickness ~ 100nm
Binghamton University Optoelectronics Devices Laboratory
Reflectance of AZO solar cell after grin coating
(a)
SEM image Gradient index layer on CZTS solar cell
(b)
Reflectance of coated and uncoated AZO solar cell
(a) Normal reflectance (b) diffused reflectance
Binghamton University Optoelectronics Devices Laboratory
External Quantum efficiency performance
EQE 
electrons / sec
photons / sec
+7.4%
@550nm
+11.9%
@ 450nm
Measured EQE of coated and original AZO solar cell at 0o ,20 o ,30 o and 40 o
degrees.
Shows good Omni-directional improvement
Binghamton University Optoelectronics Devices Laboratory
Power efficiency improvement
10.7%
22.3%
20.2%
22.5%
23.61%
23.77%
19.6% 24.2%
I-V curve of original and grin layer coated solar cell at
normal incidence (0o ),40 o ,60 o and 70 o .
Efficiency increase vs. angle of incidence
10% increase at normal, 20% or greater at all other angles
Binghamton University Optoelectronics Devices Laboratory
Conclusion and future works
Achieved Omni-directional power efficiency improvement by GRIN coating
Apply to high intrinsic power efficiency solar cell, and other optoelectronic
devices.
Use lower refractive index material for air end
 Further optimization of index profile with genetic algorithm
Binghamton University Optoelectronics Devices Laboratory
Binghamton University Optoelectronics Devices Laboratory

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