138
Figure 4g shows the light concentration around the surface of metallic nanoparticles due to resonant plasmon excitation, which significantly increases the local
field around the nanoparticle and thereby the light absorption. Therefore, the metallic nanoparticles work as an effective antenna for the sunlight and enhance the localized surface plasmon mode [29, 32].
In another example, the role of nanocone arrays as an anti-reflection layer is
studied in a CdS/CdTe solar cell. Figure 5a shows the angular view of the nanocone
array made of PDMS. In order to fabricate these nanostructures, the electropolished
aluminum foil is imprinted by a silicon mold containing nanopillars. Then, several
steps of anodization and etching are necessary to obtain an ordered inverted nanocone structure of AAO, as reported elsewhere [33]. Afterward, the AAO template is
used as a mold and the PDMS solution is casted on this template, followed by heating and peeling of the nanocone PDMS AR layer. After preparation of PDMS AR
layer, it can be attached to the glass side of device without any glue, as shown in
Fig. 5b. The PDMS film has a strong Van der Waals interaction with glass substrate
and thus it can be easily attached or detached from the devices. This self- assembly
property leads to convenient mounting and replacement of these AR films which is
promising for industrial applications [34]. Interestingly, these nanostructures not
only increase the optical absorption but also have superhydrophobicity properties as
illustrated in Fig. 5c. As it can be observed, nanocone can increase the contact angle
of water droplet from 98° to 152°. This characteristic of the nanocone array can
increase the stability of a solar cell device in a humid environment, which is beneficial for practical application [33].
This nanocone AR film can be attached to different types of solar cell devices.
For instance, the simulated reflectance spectra of solar cells without and with AR
film from 400 to 900 nm are shown in Fig. 5d. In this measurement, both pitch and
height of the nanocone were 1 μm. By applying AR film, the reflectance of film is
decreased by 4% as compared to device without AR film, which is due to the tapered
nanocone geometry inducing a gradual change in refractive index from air to PDMS
[33]. Moreover, the difference between refractive indexes of glass and PDMS is
negligible. After applying AR film, there is still a bit reflectance which comes from
the interface of FTO layer and electron transport layer (ETL). This can be addressed
by roughening the surface of glass before FTO coating. The cross-sectional electric
field intensity (E
2
) distribution of electromagnetic wave at 600 nm calculated by
FDTD simulation is shown in the inset images of Fig. 5d. Notable, the electromagnetic plane wave propagates downward. As seen, in the presence of nanocone, the
interference and reflected wave are weaker and the electric field distribution inside
the nanocone structure is stronger [35]. As mentioned earlier, the geometry of nanocone (pitch and height) can be largely tuned based on the fabrication method. The
optimization of nanocone geometry is shown in Fig. 5e. For this purpose, the total
integrated reflectance was estimated and plotted versus pitch and height of nanocone structure. The results show that PDMS nanocone with 1 μm pitch and 1 μm
height is the optimum structure to reduce the reflectance significantly. As a result,
the external quantum efficiency (EQE) spectra of CdS/CdTe solar cells without and
with AR film, shown in Fig. 5f, clearly demonstrates lower reflectance for device
M. M. Tavakoli
Figure 4g shows the light concentration around the surface of metallic nanoparticles due to resonant plasmon excitation, which significantly increases the local
field around the nanoparticle and thereby the light absorption. Therefore, the metallic nanoparticles work as an effective antenna for the sunlight and enhance the localized surface plasmon mode [29, 32].
In another example, the role of nanocone arrays as an anti-reflection layer is
studied in a CdS/CdTe solar cell. Figure 5a shows the angular view of the nanocone
array made of PDMS. In order to fabricate these nanostructures, the electropolished
aluminum foil is imprinted by a silicon mold containing nanopillars. Then, several
steps of anodization and etching are necessary to obtain an ordered inverted nanocone structure of AAO, as reported elsewhere [33]. Afterward, the AAO template is
used as a mold and the PDMS solution is casted on this template, followed by heating and peeling of the nanocone PDMS AR layer. After preparation of PDMS AR
layer, it can be attached to the glass side of device without any glue, as shown in
Fig. 5b. The PDMS film has a strong Van der Waals interaction with glass substrate
and thus it can be easily attached or detached from the devices. This self- assembly
property leads to convenient mounting and replacement of these AR films which is
promising for industrial applications [34]. Interestingly, these nanostructures not
only increase the optical absorption but also have superhydrophobicity properties as
illustrated in Fig. 5c. As it can be observed, nanocone can increase the contact angle
of water droplet from 98° to 152°. This characteristic of the nanocone array can
increase the stability of a solar cell device in a humid environment, which is beneficial for practical application [33].
This nanocone AR film can be attached to different types of solar cell devices.
For instance, the simulated reflectance spectra of solar cells without and with AR
film from 400 to 900 nm are shown in Fig. 5d. In this measurement, both pitch and
height of the nanocone were 1 μm. By applying AR film, the reflectance of film is
decreased by 4% as compared to device without AR film, which is due to the tapered
nanocone geometry inducing a gradual change in refractive index from air to PDMS
[33]. Moreover, the difference between refractive indexes of glass and PDMS is
negligible. After applying AR film, there is still a bit reflectance which comes from
the interface of FTO layer and electron transport layer (ETL). This can be addressed
by roughening the surface of glass before FTO coating. The cross-sectional electric
field intensity (E
2
) distribution of electromagnetic wave at 600 nm calculated by
FDTD simulation is shown in the inset images of Fig. 5d. Notable, the electromagnetic plane wave propagates downward. As seen, in the presence of nanocone, the
interference and reflected wave are weaker and the electric field distribution inside
the nanocone structure is stronger [35]. As mentioned earlier, the geometry of nanocone (pitch and height) can be largely tuned based on the fabrication method. The
optimization of nanocone geometry is shown in Fig. 5e. For this purpose, the total
integrated reflectance was estimated and plotted versus pitch and height of nanocone structure. The results show that PDMS nanocone with 1 μm pitch and 1 μm
height is the optimum structure to reduce the reflectance significantly. As a result,
the external quantum efficiency (EQE) spectra of CdS/CdTe solar cells without and
with AR film, shown in Fig. 5f, clearly demonstrates lower reflectance for device
M. M. Tavakoli