146
Another example is the application of inverted nanocone substrates for the fabrication of photovoltaic devices with different aspect ratios. In this regard, inverted
nanocone AAO was employed as a mold and the PDMS nanocone film was peeled
off from the template after casting to obtain another mold. Then, the epoxy solution
was casted on top of the PDMS nanocone, followed by UV curing. The obtained
inverted nanocone epoxy was used as a substrate for solar cell fabrication (Fig. 10a 1 –
a 2 ). The top and angular view SEM images of PDMS nanocone mold and inverted
nanocone epoxy substrate are depicted in Fig. 10b, c, indicating very uniform and
ordered nanostructures. Figure 10d 1 –d 4 demonstrate the fabrication process of
perovskite solar cell using a two-step evaporation technique for perovskite deposition. The generation rate of electron–hole pairs in the perovskite devices fabricated
on inverted nanocone substrates with different aspect ratios was studied using
FDTD simulation as illustrated in Fig. 10c 1 –c 4 . As seen, the aspect ratio of 1.0 indicates the highest generation rate specially in the proximity of the inverted nanocone
tip. This can be related to the lowest reflection in the sample with high aspect ratio
as compared to the planar device (Fig. 10f). The J-V results for the corresponding
devices indicate that the aspect ratio of 0.5 is the best substrate for device fabrication in terms of both uniformity and optical absorption. As shown in Fig. 10g, the J sc
and PCE enhancements for device with aspect ratio of 0.5 are 37% and 38% as
compared to the planar device, respectively [45].
These improvements are lower for the aspect ratio of 1.0 due to ununiform layers
deposited on inverted nanocone structures. It is noteworthy to mention that the textured devices demonstrate lower V oc as compared to the planar one. This could be
related to larger surface area in nanostructure samples, inducing more recombination sites [45].
As discussed earlier, the nanostructures can improve the mechanical properties
of solar cell devices, which is beneficial for flexible photovoltaic devices. The
mechanical properties of perovskite solar cells fabricated on inverted nanocone
were tested using a bending setup (Fig. 11a). The textured device after 200 bending
cycles maintained 95% of its initial PCE value which is higher than the planar
device with 30% PCE loss, as observed in Fig. 11b. The top-view SEM images of
the corresponding devices suggest that after 200 bending cycles, many crack lines
existed in the planar samples due to the difference between Young’s modulus (E) for
different layers of the device. However, the inverted nanocone device showed no
crack line indicating the role of nanostructure in improvement of the mechanical
properties for the nanotextured devices (Fig. 11c, d). Moreover, the finite element
modeling of the corresponding device can help to have a better understanding of the
cross-sectional stress distribution through the devices. Figure 11e, f depict the contour images of stress distribution for planar and textured devices upon applying
10 N force at the edge of samples, calculated by COMSOL Multiphysics software.
As seen, the concentrated stresses for planar device existed between different layers
of the device. Interestingly, the nanotextured device annihilates these concentrated
stresses at the interfaces and makes a uniform distribution of stress within all the
layers, resulting in better mechanical properties and flexibility for the devices
[43, 45].
M. M. Tavakoli
Another example is the application of inverted nanocone substrates for the fabrication of photovoltaic devices with different aspect ratios. In this regard, inverted
nanocone AAO was employed as a mold and the PDMS nanocone film was peeled
off from the template after casting to obtain another mold. Then, the epoxy solution
was casted on top of the PDMS nanocone, followed by UV curing. The obtained
inverted nanocone epoxy was used as a substrate for solar cell fabrication (Fig. 10a 1 –
a 2 ). The top and angular view SEM images of PDMS nanocone mold and inverted
nanocone epoxy substrate are depicted in Fig. 10b, c, indicating very uniform and
ordered nanostructures. Figure 10d 1 –d 4 demonstrate the fabrication process of
perovskite solar cell using a two-step evaporation technique for perovskite deposition. The generation rate of electron–hole pairs in the perovskite devices fabricated
on inverted nanocone substrates with different aspect ratios was studied using
FDTD simulation as illustrated in Fig. 10c 1 –c 4 . As seen, the aspect ratio of 1.0 indicates the highest generation rate specially in the proximity of the inverted nanocone
tip. This can be related to the lowest reflection in the sample with high aspect ratio
as compared to the planar device (Fig. 10f). The J-V results for the corresponding
devices indicate that the aspect ratio of 0.5 is the best substrate for device fabrication in terms of both uniformity and optical absorption. As shown in Fig. 10g, the J sc
and PCE enhancements for device with aspect ratio of 0.5 are 37% and 38% as
compared to the planar device, respectively [45].
These improvements are lower for the aspect ratio of 1.0 due to ununiform layers
deposited on inverted nanocone structures. It is noteworthy to mention that the textured devices demonstrate lower V oc as compared to the planar one. This could be
related to larger surface area in nanostructure samples, inducing more recombination sites [45].
As discussed earlier, the nanostructures can improve the mechanical properties
of solar cell devices, which is beneficial for flexible photovoltaic devices. The
mechanical properties of perovskite solar cells fabricated on inverted nanocone
were tested using a bending setup (Fig. 11a). The textured device after 200 bending
cycles maintained 95% of its initial PCE value which is higher than the planar
device with 30% PCE loss, as observed in Fig. 11b. The top-view SEM images of
the corresponding devices suggest that after 200 bending cycles, many crack lines
existed in the planar samples due to the difference between Young’s modulus (E) for
different layers of the device. However, the inverted nanocone device showed no
crack line indicating the role of nanostructure in improvement of the mechanical
properties for the nanotextured devices (Fig. 11c, d). Moreover, the finite element
modeling of the corresponding device can help to have a better understanding of the
cross-sectional stress distribution through the devices. Figure 11e, f depict the contour images of stress distribution for planar and textured devices upon applying
10 N force at the edge of samples, calculated by COMSOL Multiphysics software.
As seen, the concentrated stresses for planar device existed between different layers
of the device. Interestingly, the nanotextured device annihilates these concentrated
stresses at the interfaces and makes a uniform distribution of stress within all the
layers, resulting in better mechanical properties and flexibility for the devices
[43, 45].
M. M. Tavakoli