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sections, 3D nanostructures have been extensively studied to boost the performance
of thin film devices utilizing various advanced light management schemes [40].
As discussed in the previous section, these nanostructures can serve as an antireflection film to reduce the light reflection, resulting in higher performance.
Additionally, they have superhydrophobicity and self-cleaning properties, which
can increase the stability of devices. In addition to this application, these nanostructures can be used as substrates for the fabrication of photovoltaic devices. For this
purpose, all layers in the device need to be deposited conformally on the surface of
nanostructures with the minimum level of the defects. Otherwise, there is no advantage using nanostructures as substrates for solar cell device with many defects.
There are a variety of 3D nanostructures such as nanocones, nanowires, nanopillars,
nanorods, nanopyramids, nanospikes, nanospheres, and nanowells [41]. Among
these nanostructures, nanocones show promising capabilities such as enhanced sunlight harvesting in different incident angles and improved mechanical properties of
devices which are applicable in flexible solar cell devices [42].
Figure 8a1–a4 demonstrate the fabrication process of a-Si solar cell on polyimide (PI) substrate, where the inverted nanocone AAO template was considered as
a mold and the PI substrate was obtained after casting of the PI solution on top of
the AAO mold. For the fabrication of a-Si solar cell on top of PI nanocone, all layers
were deposited using vacuum techniques. The angular-view SEM images of textured devices with aspect ratios of 0.5 and 1.0 are shown in Fig. 8b, c, indicating a
very uniform deposition of all of the layers [43]. The J-V curves of the corresponding devices indicate that the nanocone substrates increase the current density and fill
factor of a-Si solar cells due to higher absorption and better charge collection
(Fig. 8d). Moreover, the aspect ratio of 0.25 demonstrates the best results, suggesting the most uniform layers with lowest defect levels. As illustrated in Fig. 8e, the
EQE of the corresponding devices are in good agreement with J-V results [43].
Notably the nanocone structures sacrificed the open circuit voltage (V oc ) of the
devices as compared to flat substrates due to having more interface recombination.
This suggests that the nanocone samples are induced surface recombination sites
due to the larger surface area at the interface. The V oc drop is the highest value for
nanocone sample with aspect ratio of 1.0 due to the difficulty in the fabrication
process in order to achieve a very conformal films. As mentioned in the last section,
nanostructures AR film can improve the optical absorption of devices in high incident angle such as 60°. This advantage is also demonstrated in the device fabricated
on nanostructures. For instance, nanocone-based device shows much higher integrated absorption and thus PCE as compared to the flat substrate with relatively
15% enhancement (Fig. 8f, g) [43].
Also, the nanocone device depicts much better performance in all incident angles
as compared to the flat device, due to the gradual change in the effective refractive
index of the entire nanostructures. This omnidirectional enhancement of the absorption and performance for the nanocone device is of significance for industrial applications without applying costly solar tracking system. In addition to the above
discussion, nanostructured devices have better mechanical properties as compared
to flat devices, which make them ideal candidates for flexible devices (Fig. 8h, i)
Efficient Light Harvesting in the Nanotextured Thin Film Solar Cells
sections, 3D nanostructures have been extensively studied to boost the performance
of thin film devices utilizing various advanced light management schemes [40].
As discussed in the previous section, these nanostructures can serve as an antireflection film to reduce the light reflection, resulting in higher performance.
Additionally, they have superhydrophobicity and self-cleaning properties, which
can increase the stability of devices. In addition to this application, these nanostructures can be used as substrates for the fabrication of photovoltaic devices. For this
purpose, all layers in the device need to be deposited conformally on the surface of
nanostructures with the minimum level of the defects. Otherwise, there is no advantage using nanostructures as substrates for solar cell device with many defects.
There are a variety of 3D nanostructures such as nanocones, nanowires, nanopillars,
nanorods, nanopyramids, nanospikes, nanospheres, and nanowells [41]. Among
these nanostructures, nanocones show promising capabilities such as enhanced sunlight harvesting in different incident angles and improved mechanical properties of
devices which are applicable in flexible solar cell devices [42].
Figure 8a1–a4 demonstrate the fabrication process of a-Si solar cell on polyimide (PI) substrate, where the inverted nanocone AAO template was considered as
a mold and the PI substrate was obtained after casting of the PI solution on top of
the AAO mold. For the fabrication of a-Si solar cell on top of PI nanocone, all layers
were deposited using vacuum techniques. The angular-view SEM images of textured devices with aspect ratios of 0.5 and 1.0 are shown in Fig. 8b, c, indicating a
very uniform deposition of all of the layers [43]. The J-V curves of the corresponding devices indicate that the nanocone substrates increase the current density and fill
factor of a-Si solar cells due to higher absorption and better charge collection
(Fig. 8d). Moreover, the aspect ratio of 0.25 demonstrates the best results, suggesting the most uniform layers with lowest defect levels. As illustrated in Fig. 8e, the
EQE of the corresponding devices are in good agreement with J-V results [43].
Notably the nanocone structures sacrificed the open circuit voltage (V oc ) of the
devices as compared to flat substrates due to having more interface recombination.
This suggests that the nanocone samples are induced surface recombination sites
due to the larger surface area at the interface. The V oc drop is the highest value for
nanocone sample with aspect ratio of 1.0 due to the difficulty in the fabrication
process in order to achieve a very conformal films. As mentioned in the last section,
nanostructures AR film can improve the optical absorption of devices in high incident angle such as 60°. This advantage is also demonstrated in the device fabricated
on nanostructures. For instance, nanocone-based device shows much higher integrated absorption and thus PCE as compared to the flat substrate with relatively
15% enhancement (Fig. 8f, g) [43].
Also, the nanocone device depicts much better performance in all incident angles
as compared to the flat device, due to the gradual change in the effective refractive
index of the entire nanostructures. This omnidirectional enhancement of the absorption and performance for the nanocone device is of significance for industrial applications without applying costly solar tracking system. In addition to the above
discussion, nanostructured devices have better mechanical properties as compared
to flat devices, which make them ideal candidates for flexible devices (Fig. 8h, i)
Efficient Light Harvesting in the Nanotextured Thin Film Solar Cells