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Abstract
Nanostructured semiconductor nanowires (NWs) present a smart solution for broadband absorption solar cells (SCs) with high efficiency and low-cost. In this paper, a novel design of quad crescent-shaped silicon NW SC is introduced and numerically studied. The suggested NW has quad crescent shapes which create a cavity between any adjacent NWs. Such a cavity will permit multiple light scattering with improved absorption. Additionally, new modes will be excited along the NWs, which are highly coupled with the incident light. Further, the surface reflection from the crescent NWs is decreased due to the reduced surface filling ratio. The finite difference time domain method is utilized to analyze the optical characteristics of the reported structure. The proposed NW offers short circuit current density (Jsc) of 27.8 mA/cm2 and ultimate efficiency (ηul) of 34% with an enhancement of 14% and volume reduction of 40% compared to the conventional NWs. The Jsc and ηul are improved to 35.8 mA/cm2 and 43.7% by adding a Si substrate and back reflector to the suggested crescent NWs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photovoltaic is the most promising technology to obtain electrical power via the direct conversion of sunlight. Solar cells (SCs) based on surface-patterned [1,2], and plasmonic nanostructures [3,4] have been reported to improve the light scattering, and harvesting. Nanodome surface-patterned of a-Si:H with periodic modulation has been suggested with a power conversion efficiency (PCE) of 5.9% [1]. Further, thin film SC with nanocone, nanopyramid, and nanoparabolic texture morphologies have been studied in [2]. It has been found that the parabolic-shaped textures could provide better antireflection properties than other nanostructures. This is due to the high diffraction efficiency of the higher-order-modes supported by the parabolic grating. Additionally, the SC absorption has been increased by using metallic nanostructures which excite new plasmonic modes [3,4]. Recently, nanowire (NW) structures have been the subject of intense study for highly absorbing the solar energy. Compared to planar structures, NW geometry provides concentrated light absorption capabilities within a small volume of material. The light concentration effect provides efficient carrier collection with reduced cost compared to the planar solar cell (SC) [5,6]. The absorption mainly depends on the developed nanostructures physical geometry, lattice arrangement, and material absorption coefficient. Additionally, low quality materials can be used with improved energy harvesting at low cost [7–9]. To improve the absorption over the broadband sun light, different strategies have been employed. It has been shown that the light absorption could be readily tuned by controlling the size, geometry and orientation of the NWs [10]. In this context, Hu and Chen [7] have studied and analyzed the geometrical effects of vertically aligned NWs on the light absorption with maximum ultimate efficiency (${\eta _{ul}}$) of 12.5%. Then, Lin et al. [11] have optimized a square lattice of periodic Si NWs with a length of 2330 nm [12]. Further, the optical properties of disordered vertical Si NWs have been analyzed with random diameter, length and position [13]. The studied randomness have reduced the light reflection with improved light absorption. It is also found that the resonance wavelengths can be controlled by the NW radius and length [7,11]. Wu et al. [14] have also introduced elliptical SiNWs array with an ultimate efficiency of 29.1%.
Different geometries have been investigated to improve the optical absorption especially at longer wavelength region such as rectangular NW [15], nano-cone NWs [16,17], nano-pyramid [18,19] and nano-funnel [6]. In this regards, the rectangular NW gives better external quantum efficiency at lower bandgap region compared to the same-sized hexagonal NW [15]. This backs to the resonant modes which are excited in such highly symmetric designs. On the other hand, nano-cone design [16,17] has enhanced the absorption due to antireflection behavior and multiple optical resonances peaks [20]. Recently, modified nano-pyramid with a bottom substrate has been suggested in [18,19]. This combination results in increasing the coupling with the incident light and hence the short circuit current density is improved to 33.6 mA/cm2. The improvement in the nano-pyramid [18,19] is due to the resonance combination between micro-cavity effect, Fabry-Perot, guided modes and Bloch modes. Also, the enhancement of the nano-funnel is attributed to the combination of modes produced by the top cylinder and the bottom tapered cone [21]. The top cylinder reduces the surface reflection and improves the light trapping of higher energy region. The multiple diameters of the bottom cone increase the leaky mode resonances especially at the lower energy region. The maximum short circuit current and ultimate efficiency of the nano-funnel are equal to 34.2 mA/cm2 and 41.8%, respectively. In 2020, a modified asymmetric crescent-nanohole-shaped NW is proposed with a maximum ${\eta _{ul}}$ and filling ratio (FR) of 41.6% and 38%, respectively [22].
The conventional cylindrical NW design has no variation along the NW length [7,16]. Therefore, it suffers from high surface reflection at shorter wavelengths and small light absorption at longer wavelengths. In order to minimize surface reflection and increase the structure light trapping, the quad-crescent NW is suggested in this work. In this context, an innovative design of modified symmetric crescent-shaped SiNW is proposed and numerically analyzed for energy harvesting applications. The reported structure can enhance the absorption in the visible and longer wavelength ranges. The suggested NW has quad crescents therefore a cavity is obtained between any two adjacent NWs. The light scattering through such cavities increases the light trapping along the NWs. Additionally; the intensity of the supported low order modes will be increased. Therefore, more resonant absorption peaks are produced within the suggested design. The proposed quad-crescent-shaped (QCr) NW geometry improves the impedance match between air and the cell. Consequently, the reflection is greatly suppressed with enhanced optical absorption. The reported design is optically studied by using the 3D finite-difference time-domain (FDTD) method via Lumerical software package [23]. The crescent-shaped NWs SC has an efficiency of 34% with an enhancement of 14% and volume reduction of 40% compared to the conventional (Con) NW. The short circuit current density and ultimate efficiency of the QCr-NW with bottom Si substrate are increased to 35.8 mA/cm2 and 43.7%, respectively. It is worth noting that the QCr-NW SC surpasses the optical efficiency of the asymmetric crescent nanohole NW in Ref. [22] with a volume saving of 45%. Therefore, the reported SiNW SC opens a new venue to obtain low-cost and highly efficient SCs.
2. Design considerations and situation approach
2.1 Design considerations
The schematic diagram of the proposed crescent (Cr) Si NWs is shown in Fig. 1(a). The QCr-NWs are arranged in a square lattice in x and y directions with periodicity (p). Figure 1(b) shows the top view (x-y plane) of the reported QCr-NW. The geometrical parameters are the cylinder radius (${R_w}$), crescent radius $\; ({R_c})$, and crescent width $\; ({d_{12}})$. The distance between the NW center and the crescent center is defined by $\; D = {R_w} + \; {R_c} - {d_{12}}$. The suggested array has a cavity between any two adjacent NWs, which increases the multiple light scattering between them. Therefore, light absorption improvement occurs through the proposed design. The optical characteristics are studied via the FDTD Lumerical solver [23]. The NW geometrical parameters are tuned to obtain the maximum absorption though the suggested design. In this study, a periodic boundary condition (PBC) is inserted in x-y plane to decrease the computational time. However, along the proposed design (i.e., z-direction) perfectly matched layers (PML) are utilized to minimize the reflections as shown in Fig. 1(c). Two monitors (M1 and M2) based on frequency domain field are added on the top and below of the structure for calculating the reflectance and transmittance power, respectively [23]. Based on Palik model, the optical constants of the active material (Si) and silver (Ag) metal contact are obtained from the Lumerical database. To enhance the absorption of the long wavelength region, Si substrate and bottom Ag back reflector are used with thicknesses of 2 µm and 0.20 µm, respectively as shown in Fig. 1(c).
Fig. 1. (a) 3D Schematic diagram of the square lattice of QCr-NWs, (b) top view in the x-y plan for the unit cell and (c) 3D computational domain of the unit cell with the Si substrate and Ag back reflector.
2.2 Simulation approach
Figure 2 shows the flow chart of the optical simulation strategy. It is aimed from this study to obtain the optimum dimensions to maximize the short circuit current density (Jsc) and hence the ultimate ${\eta _{ul}}$ and power conversion (PCE) efficiencies. The absorption of the NW is calculated by $\; A = 1 - \; R(\lambda )- T(\lambda )$ where R and T are the reflectance and transmittance normalized powers and λ is the incident light wavelength. For ideal operation, charge recombination is ignored where every absorbed photon (i.e. energy higher than material bandgap) generates only one electron-hole (e-h) carrier. The short circuit current density ${J_{sc}}$ corresponding to the number of generated charges is given by [11]:
Further, the power conversion efficiency (PCE) for the proposed QCr-NW and the conventional NW designs is also investigated as follows:
where $\; {V_{oc}}$ is the maximum open circuit voltage that can be obtained by the SC. It is supposed that each absorbed photon gives one electric-hole (eh) pair. The FF is the fill factor which determines the maximum obtained power from the SC to the product of ${J_{sc}}$ and $\; {V_{oc}}$. The material recombination is not considered here with a unity carrier collection efficiency. From the AMG-1.5, the total incident power (Pin) is equal to 100 mW/cm2 [24,25]. For ideal SC, The open circuit voltage is obtained from the short circuit current (${J_{sc}}$) and the dark saturation current density (${J_o}$) [28]:3. Results and discussion
In this study, the NW radius (${R_w}$) and length are taken as 200 nm and 2330 nm [12], respectively. First, the proposed QCr-NW structure is initially tested at ${R_c}$ = 65 nm and ${d_{12}}$=30 nm. Figure 3(a) shows the optical absorption spectra for the initial QCr-NW and the cylindrical Con-NW with radius of 200 nm. It may be seen that the reported design has higher absorption than the cylindrical NW design. Figure 3(b) depicts the ${\eta _{ul}}$ and the ${J_{sc}}$ of the studied designs. The ${J_{sc}}$ and ${\eta _{ul}}$ of the conventional NW are equal to 24.4 mA/cm2 and 29.8%, respectively. However, 26 mA/cm2 and 31.7% are obtained by the QCr-NW as shown in Fig. 3(b) with an enhancement of 6% compared to the conventional one. The physics behind such an enhancement is due to the crescent shape with better matched impedance with the air. Therefore, the reflection is suppressed while the optical absorption is strengthened. Moreover, the periodic nanostructure produces Bloch electromagnetic modes corresponding to the resonant absorption peaks [29]. It may be seen that the proposed QCr-NW induces new Bloch mode resonances compared to the ordinary cylindrical NW as shown in Fig. 3(a).
Fig. 3. (a) Absorption spectra and (b) the optical ${J_{SC}}$ and ${\eta _{ul}}$ of the QCr-shaped NW (${R_w}$ = 200 nm, ${R_c}$ = 65 nm and ${d_{12}}$ = 30 nm) versus the conventional NW radius (${R_w}$ = 200 nm).
Fig. 4. (a) Variation of the ${J_{sc}}$ with the NW crescent-width (${{\boldsymbol d}_{12}}$) and radius (${{\boldsymbol R}_{\boldsymbol c}}$), (b) The absorption spectra, and field profiles at $\lambda $ = 908 nm for the (c) Con-NW and (d) QCr- NW designs.
Figure 4(a) presents the dependence of the Jsc on the crescent radius (${R_c}$) and width (${d_{12}}$). The dash black color indicates the limit of the QCr-NW parameters to preserve the crescent shape. At ${d_{12}}$=30 nm, the proposed NW is close to the conventional cylindrical counterpart. Therefore, the Jsc is nearly constant at any crescent radius (${R_c}$) value. The Jsc of the proposed design at ${d_{12}}$=30 nm is equal to 25.2 mA/cm2 which is near to that of the solid NW of 24.4 mA/cm2. As the crescent width increases, the Jsc is significantly changed. The maximum Jsc is obtained when the crescent width (${d_{12}}$) is equal to 70 nm at ${R_c}$ <60 nm. For ${d_{12}}$< 100 nm, the ${J_{SC}}$ is significantly decreased due to the high volume reduction of the Si QCr-NW. The maximum ${J_{SC}}$ of 27.8 mA/cm2 is obtained at crescent radius and width of 65 nm and 70 nm, respectively, with an enhancement of 14% compared with the Con-NW. The optical absorption spectra of the optimized QCr-NW and conventional NW structures are shown in Fig. 4(b). It is shown that an absorption enhancement is achieved along the studied wavelength range by the suggested design compared to the conventional NW SC. The reported design has an average absorption of 64% in the wavelength range from 300 nm to 1100 nm while 56% is obtained by the Con-NW. Table 1 shows the maximum ${J_{sc}}$ and ${\eta _{ul}}$ of the proposed QCr-NW structure compared to the Con-NW. The maximum ${\eta _{ul}}$ of the proposed QCr-NW and Con-NW designs are equal to 34% and 29.8%, respectively. It may be seen that the maximum absorption enhancement occurs at λ = 908 nm. Figures 4(c) and 4(d) show the corresponding field profiles at λ = 908 nm through 3×3 periodic arrays of both designs in the x-y plane. It may be seen that strong Bloch modes [29,30] are supported across the periodic QCr-NW with optimized inner cavities. Therefore new cavity resonances are induced with new absorption peaks as shown in Fig. 4(b).
Fig. 5. The field profiles across the $dd`$ plane for the (a) proposed Cr- NW and (b) Con-NW along the longitudinal z direction at wavelength of 908 nm.
Table 1. The optimum geometrical parameters and the maximum ${{\boldsymbol \eta }_{{\boldsymbol ul}}}$ and ${{\boldsymbol J}_{{\boldsymbol sc}}}$ of the proposed QCr-NW structure compared to the Con-NW.
To further underline the implied mechanism of the absorption enhancement, the electric field profiles along the studied designs are investigated. Figures 5(a) and 5(b) show the field profiles of the QCr-NW and the conventional NW in the longitudinal z direction along the $\textrm{dd}`$ plane at wavelength of 908 nm. It is evident that the light coupling through the suggested NW is better than that of the conventional design. This may back to the lateral effective index matching along the proposed design which improves the scattered light coupling into the proposed design. Further the enhancement may be attributed to the geometry of the NW that guides the scattered light towards the QCr-NW core. Therefore, the resonances peaks and hence the light absorption are increased as shown in Fig. 4(b). The coupled light is absorbed by the Si active material of the proposed NW which increases the e-h pair generation and the power conversion efficiency (PCE).
Figure 6(a) shows the absorption spectra at different crescent widths; ${d_{12}}$ = 30 nm, 70 nm (the optimum) and 120 nm. The normalized fundamental mode profiles at the aforementioned widths are shown in Figs. 6(b), 6(c), and 6(d), respectively, in the x-y plane. In this study, the QCr-NWs are fixed at the optimized crescent radius value (${R_c}$ = 65 nm). When the light is incident on a structure, different modes are excited such as radiative mode, trapping travelling modes and trapped localized modes [29]. At the small width (${d_{12}}$=30 nm), the proposed structure is closer to the solid Con-NW. Therefore, the numbers of the coupled modes are small and weak with reduced absorption. At the larger width (${d_{12}}$=120 nm), the volume of the QCr-NW is deceased. Therefore, the confinement of the fundamental mode will be reduced with increased optical radiation losses as shown in Fig. 6(d). So the absorption spectra along the studied range are decreased as shown in Fig. 6(a). At the optimum width (${d_{12}}$=70 nm), the number of coupled modes will be increased with improved light absorption. Figure 6(c) shows the well confinement and concentration of the fundamental mode at the optimized design. Further the optimized QCr-NW periodic structure has high coupling between the diffracted orders of reradiated energy with the supported modes along the QCr-NW design. Thus, the incident light is highly tapped; and hence the optical absorption is improved as shown in Fig. 4(b).
Fig. 6. (a) The absorption spectra and the fundamental mode profiles at QCr-NWs widths of (b) ${d_{12}}$=30 nm, (c) 70 nm and (d) 120 nm at $\lambda $ = 908 nm. All field profiles are obtained at the optimum crescent radius; ${R_c}$ = 65 nm.
Based on the grating theory [31,32], the absorption improvement of the proposed QCr-NW is clarified by the reflective diffraction order. Figures 7(a) and 7(b) show the diffraction angle and the strength of the reflected orders of the studied designs at $\mathrm{\lambda }$ = 908 nm. It may be seen that the reflective diffraction intensity for the proposed QCr-NW is reduced for the optimized QCr-NW in comparison to the conventional one. This means that the QCr-NW improves the light trapping and hence absorption compared to the conventional counterpart. This is attributed to the textures of the proposed QCr-NW which induces the multiple light scattering between the adjacent NWs.
Fig. 7. The direction and strength of reflective diffraction order at $\lambda $ = 908 nm for (a) the proposed QCr-NW and (b) the Con-NW.
For strong light-matter coupling, the geometrical parameters of the proposed QCr-NW with Si substrate and Ag back reflector are studied. In this investigation, the thickness Si and Ag-metal substrates are taken by 2000nm and 200 nm, respectively [22]. First, the crescent radius is fixed at ${R_c}$ = 65 nm, while the crescent width (${d_{12}}$) is varied from 10 to 240 nm. It can be shown from Fig. 8(a) that a maximum ${J_{sc}}$ value of 35.2 mA/cm2 is realized at $\; {d_{12}}$=100 nm. Figure 8(b) shows the variation of the crescent radius at the optimum width value. The maximum ${J_{sc}}$ is equal to 35.8 mA/cm2 at crescent radius and width of 80 nm and 100 nm, respectively, with a volume reduction of 40% compared to the Con-NW design. The NW core radius ${R_w}$ is also studied to maximize the absorption of the suggested NW. Figure 8(c) shows the dependence of the ${J_{sc}}$ on the NW radius $({{R_w}} )$ at ${d_{12}}$ = 100 nm. It is evident that the ${J_{sc}}$ is strongly affected by the NW core radius from NW core radius of 150 nm to 300 nm. This may back to the cavity geometry becomes away from its optimum dimension. The optimum NW core radius is still at ${R_w}$=200 nm. This indicates that the optimum NW of conventional design is not influenced by the removed curved area.
Fig. 8. Variation of the ${J_{sc}}$ with (a) the crescent width (${d_{12}}$) at ${R_c}$=65 nm, (b) the crescent radius ($\; {R_c}$) at optimum ${d_{12}}$=100 nm and (c) the NW core radius (${R_w}$) at the optimum crescent geometry with and without holding the optimum Cr-NW width.
Figure 9(a) shows the absorption spectra at different NW lengths at ${R_w}$ = 200 nm, ${R_c}$ = 80 nm and ${d_{12}}$ = 100 nm. The L=2330 nm is considered to be comparable with the thickness of TF SC [7,12]. In this study, three NW lengths of 1160 nm, 2330 nm and 4660 nm are investigated. It is evident that the ${J_{sc}}$ increases by increasing the NW length as shown in Fig. 9(b). It may be also seen that the length has a slight effect on the absorption spectra at short wavelengths where most of the visible light is absorbed by the top part of the proposed QCr-NW. Therefore, the absorption (A) is nearly constant with the length variation at short wavelengths. It is worth noting that the Si material has small absorption coefficient at longer wavelength. Consequently, the absorption is noticeably increased by increasing the NW length at long wavelength as shown in Fig. 9(a). The ${J_{sc}}$ of the proposed design at lengths of 1160 nm, 2330 nm and 4660 nm are equal to 33.8 mA/cm2, 35.8 mA/cm2, and 36.3 mA/cm2, respectively
Fig. 9. (a) The absorption spectra of the QCr-NW at length of 1160 nm, 2330 nm, and 4660 nm, and (b) the corresponding $\; {J_{sc}}$.
Fig. 10. (a) Absorption spectra for the optimized QCr-NW and complementary structures with the substrate and Ag back reflector compared to the Lambertian limit at the equivalent thickness of bulk c-Si, the Jsc and ${\eta _{ul}}\; $ are inset in this figure. (b) The ultimate efficiency versus the incident light angle.
The effect of the Si substrate and Ag back reflector for the optimized QCr-NW and complementary designs are investigated as shown in Fig. 10. The average absorption for the proposed QCr-NW is increased to 81%, while 63% is achieved for the complementary one. The maximum ${J_{sc}}$ and ${\eta _{ul}}$ of complementary QCr-NW design are equal to 29.9 mA/cm2 and 36.5% and, respectively. However, 35.8 mA/cm2 and 43.7% are obtained for the optimized QCr-NW design, with an enhancement of 19.7% compared with the complementary counterpart. Adding Si and Ag-metal substrates enhance the ${\eta _{ul}}$ and ${J_{sc}}$ by 28.5% compared to the suggested crescent NW alone. The reported structure offers a much higher ${J_{sc}}$ and ${\eta _{ul}}$ with an enhancement of 5% compared to the previously studied asymmetric NW with crescent nanohole [22] with a volume reduction of 45%. This enhancement backs to the optimum coupling between QCr-NW and the underlying substrate. Moreover, the enhancement is also related to the smaller filling ratio of the suggested structure compared to the complementary one (FR ∼ 0.79). Thus, the reflection of the complementary is increased and hence absorption is decreased. From 900 nm to 1100 nm, the light absorption of the QCr-NW with substrates is small relative to the absorption at high photon energies. However the proposed complementary design shows a slightly higher absorption enhancement along the longer wavelengths range. This backs to increased path length of the complementary design. The maximum ${\eta _{ul}}$ and ${J_{sc}}$ of the studied QCr-NWs with and without substrates are shown in the inset of Fig. 10(a). Table 2 shows the maximum ${\eta _{ul}}$ and the ${J_{sc}}$ in addition to the optimum geometrical parameters of the proposed design with substrate and back reflector. Figure 10(b) shows the variation of the ${\eta _{ul}}$ with the incident angle (θ) for the reported Cr-NW and its complementary design mounted on the bottom substrate. It may be seen from this figure that the ultimate efficiency is maximum for both structures at θ equals to 0. As θ is increased from zero to 70 °, the ultimate ${\eta _{ul}}$ of the reported QCr-NW and complementary structures are decreased to 23.5% and 21.5%, respectively.
The Lambertian-based light trapping determines the thermodynamic limit of ultimate light absorption in a bulk material with a given thickness. The Lambertian limit is achieved when the structure surface is ideally rough where the incident light is scattered randomly. For an equivalent thickness of c-Si bulk active material (L), the absorption spectra in the Lambertian limit is given by [33,34]
Table 2. The optimum geometrical parameters and ultimate efficiencies for the QCr-NW structure with substrate and back reflector.
Fig. 11. The generation rates distribution across the $\textrm{dd}`$ plane from $\lambda $ = 300 nm to 1100 nm for the (a) conventional and (b) proposed optimum QCr-NW designs.
Fig. 12. The PCE for each of the proposed Cr-NW and the conventional NW at cell temperature of 300 K (a) and the dependence of PCE on the temperature (T)
Table 3. The short current density (${{\boldsymbol J}_{{\boldsymbol sc}}}$), ultimate efficiency (${\eta _{ul}}$), power conversion efficiency (PCE), and NW filling ratio (FR) of the QCr-NW in comparison to the studied solid NW, nanohole- crescent NW and nano-funnel NW designs.
Figures 11(a) and 10(b) show the total generation rate profiles along the proposed QCr-NW and Con-NW with bottom substrates, respectively, across the $\textrm{dd}`$ plane over the studied wavelength range. It is evident that the incident light has further penetration and confinement via the proposed QCr-NW design. The intensity of the e–h generated carriers in the QCr-NW design is much higher than the Con-NW. This is back to the trapping behavior of the QCr-NW not only in the core center but also in the Quad axial teeth as shown in Fig. 11(a). Figure 12(a) shows the power conversion efficiency (PCE) of the reported NW and conventional design with and without bottom substrate at a temperature of 300 K. It can be seen that the proposed design obtains a higher PCE compared to the conventional NW which confirms the effectiveness of the proposed QCr-NW design and the strong effect of its optimized cavities. The PCE of the conventional NW design without substrate is equal to 13.4%. However, 15.4% is obtained by the proposed design with an enhancement of 14.7% compared to the solid NW. The PCE of the QCr-NW is increased to 20.1% with an enhancement of 30% by adding the Ag metal and Si substrate. This confirms the effectiveness of including the bottom substrate during the optimization of the geometric NW parameters. Figure 12(b) shows the effect of the cell temperature (Tc) from 293 K to 318 K on the performance of the studied designs with the bottom substrate. It may be seen that the temperature is inversely proportional with the temperature of the SC. The PCE of the proposed QCr-NW and conventional NW designs are equal to 20.5% and 18.8% at a temperature of 293 K. (20 °C), respectively. When the temperature is increased to 318 K (45 °C), the PCE is decreased to 18.9% and 17.4%, respectively. Table 3 shows the ideal short circuit current (${J_{sc}}$), ultimate efficiency (${\eta _{ul}}$), PCE and NW filling factor (FF) for the proposed QCr-NW design compared to the conventional NW. Also, the previous NW designs of asymmetrical-nanohole-crescent [22], star shaped NW [37] and nanofunnel [21] NWs are shown in this table. It is evident that the reported design reduces the filling ratio of the NW to 21% with a reduction of 40% compared to the conventional NW. Further, an enhancement of 5% is achieved relative to the previously studied nanohole-crescent NW and inverted funnel, respectively with NW volume reduction of 45% and 58%. Therefore, the quad crescent NW offers better absorption, short circuit current, PCE, and lower material consumption relative to other NWs.
4. Fabriacation technique
Metal-assisted chemical etching (MACE) technique can be introduced to control and fabricate advanced NWs structures as reported in [22,38–40]. The MACE is basically a wet etching method that produces anisotropic high aspect ratio semiconductor nanostructures without causing lattice damage [41]. The proposed quad-crescent design has no variation in z-direction and can be fabricated through a single-step etching process. The metallic particles can be engraved into the semiconductor NW at the specified shape and periodicity [41]. Figure 13 illustrates the suggested fabrication steps to obtain the proposed structure. First, the Si substrate is cleaned by ethanol and acetone. Then, the thermal evaporation technique is employed for depositing the bottom Ag contact on the Si substrate. Glancing angle deposition technique [42,43] may be also utilized to form the proposed crescent shape. Different methods, such as the reactive ion etching and electron beam and holographic lithography can be utilized for precisely control period, shape, and size of crescent design [22]. The Si substrate is etched until the specified length is obtained as shown in step 4. Finally, the mask layer is removed by the etching process using a mixed etchant solution of H2O2 and HF with the catalysis. The periodic array of the optimized QCr-NW structure can be obtained as shown in Fig. 13(e). The fabrication of the proposed design is different than that have been used for the fabrication of nanopyramid, nanocone and nanofunnel shaped NWs. Wang et al. [44] have presented the fabrication of nanopyramid-shaped design using combined process of laser interference lithography and anisotropic KOH etching of silicon in an alkaline solution. Laser interference lithography can be utilized to form the nanomask which defines the nanostructure periodicity before the KOH etching process [44]. However, the funnel-shaped NW design was fabricated using a deep reactive ion etching by Ko et al. [45]. The shape of the conical NW parts was controlled by the flow rates of C4F8 and SF6. For the NW cylinder fabrication, gas mixture of SF6/ C4F8/Ar can be controlled to maintain the symmetry of the solid cylinder [45].
5. Conclusion
Quad-crescent shaped Si NW is presented to enhance the performance of light trapping of nanowire solar cells. The optical characteristics of the proposed structure are numerically studied using finite difference time domain method. The geometrical parameters of the modified QCr-NW is tuned to improve the absorption and hence the optical generation rate. The created cavities can guide the multiple scattered light between the QCr-NWs. This behavior improves the light trapping inside the proposed structure and hence light absorption is improved. Compared to the conventional nanowire, the maximum ${\mathrm{\eta }_{\textrm{ul}}}$ of the crescent NW is increased to 34% with an enhancement of 14%. The optical short circuit current density and ultimate efficiency of the reported design are equal to 35.8 mA/cm2 and 43.7%, respectively. Furthermore, the proposed crescent design has a promising potential to serve as NW SC with high efficiency, less material consumption and hence low cost.
Funding
Science, Technology and Innovation Funding Authority (STIFA) at Egypt under the Institutional Links Grants; Egypt-UK Cooperation: Newton Mosharafa Program (30732) and project (25954).
Acknowledgments
The authors also acknowledge the financial support provided by the Knowledge and Technology Alliance Program at the Academy of Scientific Research and Technology, Egypt.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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