Polymer perovskite solar cells are becoming very popular with the
ability to be used in
flexible substrates. Improving the stability of the
polymer perovskite solar cells are very important in terms of its com-
mercial use. Solar cell e
fficiency is improved from 17% to 19% when
the perovskite cells are doped with polymers. These doped polymers act
as bridge between the grains which helps in improving the charge
transport. The use of the polymers is not limited, hence di
fferent
polymer doping with perovskite can be tried for improved e
fficiency of
the cell
[111]
.
The perovskite-based substances having di
fferent properties, in-
cluding piezoelectric, conducting, superconducting and insulating
properties. And also, conventional perovskite compounds manufactured
from high temperature involving method like solid state synthesis
technique
[112,113]
. Halide perovskite-based substances is recently
discovered as a desired substance for high performance with low price
photovoltaic. The perovskite based photovoltaic was highly mature and
then its e
fficiency enhanced rapidly, from 3.8% in the year 2009–22.1%
in the year 2016
[109]
,
[114
–116]
. Nitrogen doping enhances the
conductivity of the device which can further improve the overall e
ffi-
ciency. This can be achieved by doping nitrogen into graphene by N2
treatment on the edge sites. Fabricated cells have shown very good
e
fficiency with better electrocatalytic activity and electrochemical sta-
bility
[117]
. Device performance can be further improved by nanomesh
by CVD technique. 3D Nanomesh can improve the conductivity of the
cell when fabricated over the perovskite layer
[118]
.
Fig. 9
shows the modi
fied CZTS/perovskite device structure with Au
and FTO as the top and bottom layers of the cell. Due to the better
optical absorption of the CZTS and perovskite layers, the power con-
version e
fficiency of the device will be better.
5. Proposed structure
Proposed device structure with the CZTS as top layer is shown in the
Fig. 10
. The band structure alignment of the device is shown in
Fig. 10
(b). It is understood from the band structure that the band o
ffset
between the top layer and the middle layers has e
ffective electron
blocking alignment which will make the top electrode as cathode al-
lowing maximum number of electron to be collected at the top contact.
Similarly, the holes will be collected at the bottom contact of the device
allowing maximum number of carriers generated by the incident light
Fig. 8. perovskite based solar cell device structure and its characteristics, a) device structure, b) SEM image of device, c) band alignment and d) XRD results. (source -
https://phys.org/news/2014-09-liquid-inks-solar-cells.html).
Fig. 9. - Perovskite based CZTS absorbing solar cell, a) device structure, b) conductivity and c) absorbance. (source - https://www.sparrho.com/item/kesterite-
cu2znsns4-as-a-low-cost-inorganic-hole-transporting-material-for-high-e
fficiency-perovskite-solar-cells/2903/).
M. Ravindiran, C. Praveenkumar
Renewable and Sustainable Energy Reviews 94 (2018) 317–329
326
to be converted into electrical energy resulting in very good power
conversion e
fficiency.
Fig. 11
shows the inverted solar cell structure with CZTS layer at the
middle of the solar cell structure with the P type ZnO on the top. Band
alignment of the proposed device is shown in the
Fig. 11
(b). The band
alignment shows an o
ffset between the top layer and the middle layer
resulting in an electron blocking setup. The electrons generated from
the incident light make the electrons to move towards the bottom
electrode and blocking the electrons to travel towards the top layer.
Since the carrier collection is inverted, the top layer is anode and the
bottom layer are cathode. Due to the electron blocking layer, there will
be an improved carrier collection on the top electrode and the bottom
electrode. Hence the power conversion e
fficiency of the proposed solar
cell be much higher when compared to the other CZTS based solar cells.
6. Proposed composites for energy application
Density Functional Theory (DFT) calculations has been performed
with various composites in order to identify better absorber layer for
heterojunction solar cell fabrication. Considerable preliminary works
has been carried out to identify the absorber layer with better electronic
and optoelectronic property. The proposed work will make a new ad-
vancement in the solar cell technology with better power conversion
e
fficiency.
The
Table 2
illustrates the various composites and its respective
electron density, absorption coe
fficient and optical conductivity.
Composites made with the X and Y represents the metal and ferro-
magnetic metal with similar composition of the CZTS. It is understood
from the above table that the composites made with (X)ZnSnSe and (X)
Zn(Y)Se have improved electron density and absorption coe
fficient.
Further investigation of the proposed composite will make a better
composite for energy application. Optimization of the composites can
be done with the di
fferent compositions of the elements in the com-
posite. Hence with the optimized composite property, a better compo-
site material than CZTS can be identi
fied for energy application.
7. Conclusion
Power conversion e
fficiency of solar cells can be improved with the
approaches adopted based on the materials used and the band structure
alignment. CZTS has been a promising material of choice over the re-
cent years. This manuscript has extensively analyzed the various CZTS
based solar cell structures and synthesis methods. To make an improved
device performance with the CZTS based device structure an alternative
approach of the device structure is proposed in the present work. To
further improve the device performance, an extensive analysis of the
composite material is done with the DFT calculations and the compo-
sites with better electron density (37 J/m
3
), absorption coe
fficient
(280000Au) and optical conductivity is analyzed (8 mS).
Fig. 10. Proposed device and band structure with CZTS as top layer.
Fig. 11. Proposed device and band structure with CZTS as middle layer.
Table 2
Comparison of various composites for energy application.
Sl. no
Composite
Electron
density
Absorption
doe
fficient
Optical
conductivity
1
CuZnSnS
35 J/m
3
270000Au
4 mS
2
CuZnSnSe
35 J/m
3
220000Au
4 mS
3
CuZnSnSsS
28 J/m
3
270000Au
4 mS
4
(X)ZnSnS
35 J/m
3
270000Au
8 mS
5
(X)ZnSnSe
37 J/m
3
250000Au
6 mS
6
(X)ZnSnSeS
28 J/m
3
280000Au
8 mS
7
(X)Zn(Y)S
35 J/m
3
250000Au
6 mS
8
(X)Zn(Y)Se
38 J/m
3
270000Au
6 mS
9
(X)Zn(Y)SeS
30 J/m
3
270000Au
6 mS
(X)
– Metal, (Y)- Ferromagnetic Metal.
M. Ravindiran, C. Praveenkumar
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