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Inverted Silicon Nanopencil Array Solar Cells with Enhanced Contact Structures

View Article: PubMed Central - PubMed

ABSTRACT

Although three-dimensional nanostructured solar cells have attracted extensive research attention due to their superior broadband and omnidirectional light-harvesting properties, majority of them are still suffered from complicated fabrication processes as well as disappointed photovoltaic performances. Here, we employed our newly-developed, low-cost and simple wet anisotropic etching to fabricate hierarchical silicon nanostructured arrays with different solar cell contact design, followed by systematic investigations of their photovoltaic characteristics. Specifically, nano-arrays with the tapered tips (e.g. inverted nanopencils) are found to enable the more conformal top electrode deposition directly onto the nanostructures for better series and shunt conductance, but its insufficient film coverage at the basal plane would still restrict the charge carrier collection. In contrast, the low-platform contact design facilitates a substantial photovoltaic device performance enhancement of ~24%, as compared to the one of conventional top electrode design, due to the shortened current path and improved lateral conductance for the minimized carrier recombination and series resistance. This enhanced contact structure can not only maintain excellent photon-trapping behaviors of nanostructures, but also help to eliminate adverse impacts of these tapered nano-morphological features on the contact resistance, providing further insight into design consideration in optimizing the contact geometry for high-performance nanostructured photovoltaic devices.

No MeSH data available.


Schematic illustration of silicon nanostructure arrayed solar devices and corresponding 45° angle-view SEM images of the top electrode contact (Ti/Ag) directly deposited on nanopillars, inverted nanopencils, nanocones by evaporator (a1–c1) and by magnetron sputtering (a2–c2), respectively.The schematic is not drawn in scale.
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f2: Schematic illustration of silicon nanostructure arrayed solar devices and corresponding 45° angle-view SEM images of the top electrode contact (Ti/Ag) directly deposited on nanopillars, inverted nanopencils, nanocones by evaporator (a1–c1) and by magnetron sputtering (a2–c2), respectively.The schematic is not drawn in scale.

Mentions: Figure 1 demonstrates the scanning electron microscope (SEM) images of regular Si nanopillar, nanocone and inverted nanopencil arrays fabricated with the controllable structural pitch, diameter, height, aspect ratio (i.e. pillar height-to-base diameter) and material-filling ratio (i.e. base diameter-to-pitch), in which their large-scale fabrication can be reliably obtained by simply manipulating the dimension of polystyrene spheres (PS) during the nanoscale patterning, the concentration of etchant solution and the processing duration as previously reported19. Afterwards, in fabrications of most nanostructured based solar cells, after the high-quality p-n junction is formed, the top electrode is usually deposited directly onto the nanostructures, and the uniformity of this electrode coverage is essential for the improved photovoltaic device performance. In order to qualitatively assess the conformality of the deposited top electrode, various conventional thin film deposition techniques, namely the evaporation and sputtering, are employed to coat ~540 nm thick of the typical metal electrode stack (40 nm Ti/ 500 nm Ag) directly onto the nano-arrays with the pitch of ~1.2 μm and the height of ~2 μm. As shown in the SEM image in Fig. 2, it is anticipated that the evaporation technique would give non-conformal coatings to all nanostructured arrays due to its line-of-sight deposition process. Although the substrate rotation is always utilized to enhance the evaporated film coverage for the outer surface of these complex geometries, it is still not capable to coat the inner surface of such structures, especially the basal plane of the nano-arrays studied in this work (Fig. 2a1–c1). On the contrary, the sputtering approach can yield the more conformal coating on most nano-arrays since the sputtered atoms ejected into the plasma are existed in their thermodynamic non-equilibrium states with enhanced mobility to result in the scattered deposition for the better film coverage (Fig. 2a2–c2). In addition to the influence of different deposition techniques, the geometrical morphology of nano-arrays is also observed to have a great impact on the uniformity of the deposited electrode stack. It is clear that when the material-filling ratio is kept constant along the height of the array (i.e. nanopillars), this limited open space would restrict the movement of metal particles towards the basal plane during the deposition and eventually lead to the preferential accumulation of particles at the tip area to form the mushroom-like feature (Fig. 2a1,a2). For the arrays with tapered tips (i.e. nanocones and inverted nanopencils), there are more capture cross-section areas available at the tip region, allowing the passage of metal particles to reach the basal plane in a more continuous and compact manner. It should also be noted that the typical p-n junction formation processed by spin-on dopants (SODs) or gas diffusion can as well be easily and uniformly achieved in these tapered nano-arrays due to the similar reason (Supporting Information Figure S1). More importantly, the advantage of using these tip-tapered nano-arrays in attaining the conformal coating as well as the continuous junction formation is even more profound for arrays with the smaller structural pitch and higher aspect ratio, indicating the potency of these nanocones and nanopencils for the highly efficient solar cell structure with the conformal front electrodes directly deposited on the top and the formation of high-quality junctions.


Inverted Silicon Nanopencil Array Solar Cells with Enhanced Contact Structures
Schematic illustration of silicon nanostructure arrayed solar devices and corresponding 45° angle-view SEM images of the top electrode contact (Ti/Ag) directly deposited on nanopillars, inverted nanopencils, nanocones by evaporator (a1–c1) and by magnetron sputtering (a2–c2), respectively.The schematic is not drawn in scale.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5037459&req=5

f2: Schematic illustration of silicon nanostructure arrayed solar devices and corresponding 45° angle-view SEM images of the top electrode contact (Ti/Ag) directly deposited on nanopillars, inverted nanopencils, nanocones by evaporator (a1–c1) and by magnetron sputtering (a2–c2), respectively.The schematic is not drawn in scale.
Mentions: Figure 1 demonstrates the scanning electron microscope (SEM) images of regular Si nanopillar, nanocone and inverted nanopencil arrays fabricated with the controllable structural pitch, diameter, height, aspect ratio (i.e. pillar height-to-base diameter) and material-filling ratio (i.e. base diameter-to-pitch), in which their large-scale fabrication can be reliably obtained by simply manipulating the dimension of polystyrene spheres (PS) during the nanoscale patterning, the concentration of etchant solution and the processing duration as previously reported19. Afterwards, in fabrications of most nanostructured based solar cells, after the high-quality p-n junction is formed, the top electrode is usually deposited directly onto the nanostructures, and the uniformity of this electrode coverage is essential for the improved photovoltaic device performance. In order to qualitatively assess the conformality of the deposited top electrode, various conventional thin film deposition techniques, namely the evaporation and sputtering, are employed to coat ~540 nm thick of the typical metal electrode stack (40 nm Ti/ 500 nm Ag) directly onto the nano-arrays with the pitch of ~1.2 μm and the height of ~2 μm. As shown in the SEM image in Fig. 2, it is anticipated that the evaporation technique would give non-conformal coatings to all nanostructured arrays due to its line-of-sight deposition process. Although the substrate rotation is always utilized to enhance the evaporated film coverage for the outer surface of these complex geometries, it is still not capable to coat the inner surface of such structures, especially the basal plane of the nano-arrays studied in this work (Fig. 2a1–c1). On the contrary, the sputtering approach can yield the more conformal coating on most nano-arrays since the sputtered atoms ejected into the plasma are existed in their thermodynamic non-equilibrium states with enhanced mobility to result in the scattered deposition for the better film coverage (Fig. 2a2–c2). In addition to the influence of different deposition techniques, the geometrical morphology of nano-arrays is also observed to have a great impact on the uniformity of the deposited electrode stack. It is clear that when the material-filling ratio is kept constant along the height of the array (i.e. nanopillars), this limited open space would restrict the movement of metal particles towards the basal plane during the deposition and eventually lead to the preferential accumulation of particles at the tip area to form the mushroom-like feature (Fig. 2a1,a2). For the arrays with tapered tips (i.e. nanocones and inverted nanopencils), there are more capture cross-section areas available at the tip region, allowing the passage of metal particles to reach the basal plane in a more continuous and compact manner. It should also be noted that the typical p-n junction formation processed by spin-on dopants (SODs) or gas diffusion can as well be easily and uniformly achieved in these tapered nano-arrays due to the similar reason (Supporting Information Figure S1). More importantly, the advantage of using these tip-tapered nano-arrays in attaining the conformal coating as well as the continuous junction formation is even more profound for arrays with the smaller structural pitch and higher aspect ratio, indicating the potency of these nanocones and nanopencils for the highly efficient solar cell structure with the conformal front electrodes directly deposited on the top and the formation of high-quality junctions.

View Article: PubMed Central - PubMed

ABSTRACT

Although three-dimensional nanostructured solar cells have attracted extensive research attention due to their superior broadband and omnidirectional light-harvesting properties, majority of them are still suffered from complicated fabrication processes as well as disappointed photovoltaic performances. Here, we employed our newly-developed, low-cost and simple wet anisotropic etching to fabricate hierarchical silicon nanostructured arrays with different solar cell contact design, followed by systematic investigations of their photovoltaic characteristics. Specifically, nano-arrays with the tapered tips (e.g. inverted nanopencils) are found to enable the more conformal top electrode deposition directly onto the nanostructures for better series and shunt conductance, but its insufficient film coverage at the basal plane would still restrict the charge carrier collection. In contrast, the low-platform contact design facilitates a substantial photovoltaic device performance enhancement of ~24%, as compared to the one of conventional top electrode design, due to the shortened current path and improved lateral conductance for the minimized carrier recombination and series resistance. This enhanced contact structure can not only maintain excellent photon-trapping behaviors of nanostructures, but also help to eliminate adverse impacts of these tapered nano-morphological features on the contact resistance, providing further insight into design consideration in optimizing the contact geometry for high-performance nanostructured photovoltaic devices.

No MeSH data available.