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Abstract
Thin-film silicon solar cells (TSSC) has received great attention due to its advantages of low cost and eco-friendly. However, traditional single-layer patterned solar cells (SPSC) still fall short in light-trapping efficiency. This article presents an all layers patterned (ALP) conical nanostructured TSSC to enhance the low absorption caused by the thin absorption layers. The Finite-Difference Time-Domain result shows that a photocurrent density up to 41.27 mA/cm2 can be obtained for the structure, which is 31.39% higher than that of the SPSC. An electrical optimization simulation of doping concentration was carried out on the parameters of the optically optimal structure of the model. The power conversion efficiency is 17.15%, which is 1.72 times higher than that of the planar structure. These results demonstrate a success for the potential and prospect of the fully patterned nanostructures in thin-film photovoltaic devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Silicon solar cells are currently the most advanced, effective, and technologically mature solar cells available in the market. Compared to other types of solar cells, silicon solar cells exhibit higher power conversion efficiency (PCE) and better stability [1–3]. However, the high cost of silicon and issues concerning carrier losses have limited the widespread adoption of silicon solar cells in the market. To tackle these challenges, the development of thin-film silicon solar cells (TSSC) has emerged as a viable solution [4–7]. TSSC reduce the production cost of silicon solar cells and the migration distance and recombination of carriers. Therefore, it can also improve the collection efficiency of carriers [8–10]. While the thickness of silicon is a crucial factor in optimizing light absorption, it is essential to strike a balance. If the silicon layer becomes too thin, it can result in inadequate light absorption [11–13]. Researchers have explored diverse approaches to enhance light absorption efficiency in silicon-based devices [12,14,15]. One highly popular and effective method to enhance light absorption in solar cells is through surface or substrate patterning [16–18]. This structure involves creating specific patterns on the surface or substrate of the solar cells, which reduces the reflection of sunlight [17,19]. By reducing reflection, more sunlight can incident to the interior of the solar cells, leading to increased light absorption and improved overall efficiency [16,20]. The light scattering and diffraction enhance the interaction between sunlight and nanostructure, and extend the propagation path of light in the device. Additionally, the resonance effect enables the reorientation and precise control of light towards areas with lower PCE, thereby enhancing the light absorption capability of solar cells in those regions [13,21,22]. Various nanostructures have been applied to the surface or substrate of solar cells, which have shown improved PCE of corresponding devices [23–26].
In recent years, there has been growing attention towards the all layers patterned (ALP) TSSC to harness solar energy more efficiently. Compared with the single layer nanostructured TSSC [25,27–29], the light-trapping effect of ALP TSSC is much more effective and obvious [30,31]. Fan et al. achieved a low-cost process for producing regular nano-cone arrays on polyimide (PI) substrates and manufactured flexible fully patterned amorphous silicon solar cells. Compared to devices fabricated on flat substrates using the same solution process, the PCE nearly doubled [32]. Further, they proposed a dual-interface patterned thin-film amorphous silicon solar cell based on sol-gel chemistry and soft thermal nanoimprint lithography on PI substrates. This new design resulted in an 8.17% improvement in PCE performance, achieving a 48.5% increase compared to that of the flat devices [31]. Qiu et al. used n-type ultra-thin nanocrystalline silicon (nc-Si: H) as the contact layer for rear-junction silicon heterojunction (SHJ) solar cells. By incorporating a higher density of nc-Si: H in the SHJ solar cells, they achieved excellent cell performance with a PCE of 23.87% [33]. The light paths in these ALP TSSC are further extended, and the enhancement of light absorption is more complete [30,32]. It effectively solves the problem of low efficiency caused by insufficient light absorption due to the thin absorption layer material of TSSC [33,34]. As far as we know, most of these above literatures focus more on experimental research, little attention was paid to the systematic study of the parameters dependent light-trapping property of the ALP TSSC, which could be of great use for finding the optimal parameters and pave the way to the practical applications.
In this work, ALP conical nanostructured TSSC is proposed to solve the problem of the low efficiency of TSSC. Firstly, the Finite-Difference Time-Domain (FDTD) method was used to simulate the optical properties of the materials [13,20,35]. The relationship between the parameters of cone period (P), height (H), and indium tin oxide (ITO) thickness (T) with the structure photocurrent density (Jph) was obtained. Because the substrate with Ag reflector has parasitic effects [36–38], to avoid its adverse effects, the substrate material of the structure is simulated using a Perfect Electric Conductor (PEC). Simultaneously, the optimal structure of the model was electrically explored. The effects of doping concentration on key device parameters such as open-circuit voltage (Voc), fill factor (FF), PCE, and short-circuit current density (Jsc) are analyzed. This study demonstrates the remarkable efficiency improvement of ALP conical nanostructured TSSC through simulation analysis, and reveals the potential of this structure in other optoelectronic device applications.
2. Methods
This study proposed the ALP conical nanostructured TSSC structure. The optical analysis of this structure was performed using the Lumerical FDTD software package and the three-dimensional FDTD method. Figure 1(b) represents a simulated diagram illustrating the model. Each layer of the structure is characterized by a conical nanograting structure. The gratings of each layer are arranged in identical patterns, creating a triangular lattice with a well-organized structure. Figure 1(a) and 1(c) depict the cross-sectional views of the structure model in the XY plane and YZ plane, respectively. The top layer is the ITO electrode layer. As a transparent conductive layer, it can play the role of anti-reflection and effectively reduce the reflection of sunlight [39–41]. Since the thickness of the ITO layer may have a certain effect on the absorption of sunlight in the structural model, the thickness of the ITO layer is further investigated. The interlayer consists of a 1 µm thick Si film. The bottom layer is Ag/PEC. Considering the parasitic effect of metal nanostructures, PEC is used as the substrate electrode. The absorption of the Ag back reflector structure was also subjected to parameter scanning in this study. Comparing the two sets of simulation results provided insights into the relationship between parasitic absorption and metal grating P. Additionally, the fully patterned conical grating structure, denoted as H, was simulated and scanned in the study. The period of the arrangement is denoted as P, the height of the conical structure is H, and the thickness of the ITO layer is T. The absorption of light by solar cells under AM1.5 solar radiation can be expressed using the ideal Jph, which can be obtained from the following Eq. (1) and (2) [18,20,42].
Fig. 1. ALP conical nanostructured TSSC simulation model diagram, (a) presents the cross-sectional view of structure XY, (b) illustrates the 3D simulation model of the structure, and (c) displays the cross-sectional view of structure YZ.
In the formula, e represents the electron charge, λ represents the wavelength, h represents Planck's constant, and c represents the speed of light in a vacuum. IAM1.5(λ) represents the incident spectrum AM1.5. A(λ) represents the absorption of solar energy by silicon, R(λ) represents the reflectance of the material, and T(λ) represents the transmittance of the material.
The size of the nanostructure has a significant impact on the Jph of solar cells. When the characteristic size (Fd) of a nanostructure satisfies Fd < λ/n, sub-wavelength structures can be utilized for antireflection research on the surface of nanostructures [43–45]. Here, λ represents the wavelength of light, and n represents the refractive index of the material used. Therefore, in this study, we selected the P of the nanostructure parameter from 0.10 µm to 0.70 µm. To evaluate the capability of ALP conical nanostructured TSSC in capturing sunlight, we utilized a vertically downward-propagating plane wave light source to simulate the average Jph value. The simulations were conducted under both 0-degree and 90-degree polarization within the range of 300 nm to 1100 nm. The studied structure and plane wave light source exhibit X-axis and Y-axis symmetry. For 0-degree polarization, the applied boundary conditions are symmetry and anti-symmetry. Conversely, for 90-degree polarization, the corresponding boundary conditions are anti-symmetry and symmetry. The positive direction of the Z-axis utilizes the boundary condition of the PML, while the negative direction adopts the boundary condition of the metal layer. To ensure convergence and accuracy of the results, the analog mesh accuracy is set to 0.008 µm for both X and Y axes, and 0.005 µm for the Z-axis. The FDTD solution's power monitor, field distribution monitor, and solar generation analysis groups enable the simulation of reflectivity, electric field distribution, and ideal Jph to be obtained. Therefore, the computational domain for solar energy generation extends vertically from the base of the silicon material to the uppermost part, with a height equivalent to the combined thickness of Si and H. Along the X direction, it expands horizontally by a distance of P, whereas in the Y direction, it extends horizontally by a distance of √3P.
3. Results and analysis
During the optical simulation study, the thickness parameter T of the ITO material and the nanostructure parameters H and P were varied to investigate the structural parameters relationship with the ideal Jph. Researching the impact of the conical grating structure and the thickness parameter of ITO material on the light-capturing performance. The results are shown in Fig. 2 below.
Fig. 2. (a)-(f) represent the results of Jph values obtained with T as the vertical axis and H as the horizontal axis for P values of 0.20 µm, 0.30 µm, 0.40 µm, 0.50 µm, 0.60 µm, and 0.70 µm.
Figure 2 shows the variation of Jph in relation to H and T for P values ranging from 0.20 µm to 0.70 µm, with Fig. 2(a)-2(g) representing different P values. It can be observed that changes in parameters have an impact on the absorption of sunlight by solar cells. The Jph values obtained from parameter scanning have been all improved, with some values exceeding 30 mA/cm2. Particularly, at P = 0.60 µm, H = 0.50 µm, and T = 0.30 µm, the maximum value of Jph reached 41.27 mA/cm2. Overall, Jph shows a positive correlation with the increase in P and reaches its maximum value when P is 0.60 µm.
Jph is found to be more sensitive to changes in the nanostructure parameter H compared to changes in the thickness of the ITO material. This sensitivity arises from the significant impact of scattering and diffraction effects caused by the conical nanostructured grating on light absorption. While the ITO material acts as an anti-reflective coating, its influence on light absorption is weaker compared to the function of the grating. The scattering and diffraction effects of the multilayer grating help extend the propagation path of light in the solar cell, leading to a significant enhancement in light absorption. The conical grating satisfies the following Eq. (3) [46,47].
Given the significant impact of parameter P on sunlight capture in Fig. 2, a more in-depth investigation is conducted to examine its influence on Jph. Figure 3(a)-3(c) displays the absorption for the entire range of parameter P under different values of H and T. The absorption exhibits an oscillatory pattern with varying values of P, which can be attributed to the corresponding change in the diffraction angle and series resulting from variations in P. In these absorption plots, two main oscillation peaks observed in different absorption curves, and these peaks are solely related to P. One of these peaks can be observed near P = 0.30 µm, while the other exhibits values within the P range of 0.50 µm ∼ 0.60 µm.
Fig. 3. (a)-(c) shows the change curves of Jph versus P, the ITO thickness T is fixed at 0.06 µm, 0.14 µm, 0.22 µm, and 0.30 µm, and H is 0.20 µm, 0.35 µm, and 0.50 µm, respectively. (d)-(f) shows the variation curve of Jph versus H under the same ITO thickness when P is respectively 0.30 µm, 0.50 µm, 0.70 µm.
For the parameter H, its impact on absorption is also significant. Figure 3(d)-3(f) shows the absorption curves for the entire range of parameter H under different values of P and T. It can be observed from the figure that the absorption oscillates in a similar manner with the change of H. However, simply changing the parameter H results in the oscillation of Jph values without showing a specific regularity. This is because changing H primarily affects light scattering, which subsequently influences the magnitude of absorption values. Additionally, it can be observed that the variation of parameter H has different effects on the results under different parameter P conditions.
By observing Fig. 3, it is evident that T does affect the changes in absorption, although its impact is not as notable as that of parameters H and P on the Jph value. This can be attributed to the fact that, in this structure, the role of ITO as an anti-reflective coating in the solar cell is not as significant as the enhancement effect provided by the nanostructure. This further demonstrates the superior performance of the nanostructure in light-trapping efficiency.
To eliminate the parasitic absorption effects of Ag, the Ag back reflector grating of the model is replaced with PEC. Figure 4 presents the Jph values of the substrate corresponding to Ag and PEC at different parameter P values. The optimal Jph value for P within the range of 0.10 µm ∼ 0.70 µm is achieved by optimizing the conical nanostructure parameter H and the ITO thickness parameter T. From the graph, it is evident that the Jph values exhibit more pronounced differences in smaller cycles. The reason behind this phenomenon lies in the ability of smaller metallic nanoparticle sizes to induce stronger plasmonic modes, thereby causing a notable enhancement in parasitic absorption [18]. However, as the nanoparticle array period increases, the intensity of these plasmonic modes gradually weakens, ultimately leading to a diminishing effect on the impact of parasitic absorption. At P = 0.60 µm, the optimal Jph value of PEC differs by only 0.91 mA/cm2 concerning Ag's value. This indicates that parasitic absorption from the Ag metal grating at this point is already negligible. Therefore, it can be concluded that within the simulated period range, by increasing the nanoscale structural parameter P, the absorption range of the solar cell material can be matched with the solar spectrum. This enhances the absorption of solar radiation in the photoelectric conversion region and improves the PCE of the solar cell.
Fig. 4. Changes of the optimal Jph value of PEC and Ag back reflector ALP conical nanostructured TSSC when nanostructure parameter P is 0.10 µm ∼ 0.70 µm.
To demonstrate the superior performance of ALP nanostructures, we list the Jph values of three different structures and compare their enhancement efficiencies in Table 1. As can be seen from Table 1, compared with the planar structure, the ALP grating structure added to the PEC back reflector solar cell greatly increases the Jph. The Jph value at the optimal point of the structure is 41.27 mA/cm2, which is about 2.65 times higher than that of the planar structure. Compared to the Jph value of 31.41 mA/cm2 achieved by adding nanostructures only on the Si surface as indicated in Table 2, there is a further improvement of 31.39% [42].
Table 1. Jph comparison of various solar cell structures
Table 2. The photovoltaic characteristics of Si solar cells with various structures.
Table 2 provides an overview of the photovoltaic characteristics exhibited by Si solar cells of various structures. Notably, it is observed that optimizing the structure yields tangible enhancements in optical Jph. Different structures have their unique advantages. Optimized nanocone or nanopyramid gratings can significantly enhance the light-trapping effect. By incorporating surface gratings onto a 200µm-thick Si wafer, one can significantly enhance light absorption. Adding metallic plasmonic nanoparticles on the surface of a solar cell grating, or simultaneously adding grating structures on the surface and substrate, also achieves enhancement of the light-trapping effect.
The conical nanostructure improves the light absorption of solar cells by increasing light-trapping efficiency. Compared with planar structure solar cells, the optimized parameters of the nanostructure show an improvement in the entire wavelength range. Figure 5 illustrates the absorption of the PEC back reflector structure, the Ag back reflector structure, and the planar ITO layer structure listed in Table 1. Additionally, they compare the absorption with the planar structure without an ITO enhancement film.
Fig. 5. Absorption of PEC back reflector ALP conical nanostructured TSSC, Ag back reflector ALP conical nanostructured TSSC, ITO layer planar structure, and Si layer planar structure solar cells in the wavelength range of 300 nm ∼ 1100 nm.
From the Fig. 5, it is evident that as the wavelength changes, the interference conditions generated by a multilayer planar medium lead to the presence of Fabry-Perot resonance modes [49]. Therefore, the planar structure exhibits periodic oscillations in absorption. The planar structure solar cell utilizing the ITO layer material demonstrated a Jph value of 15.55 mA/cm2, which was 32.23% higher than the Jph value of the structure without an ITO layer (11.76 mA/cm2). This can be attributed to the higher light transmittance of the ITO film and its ability to facilitate multiple reflections, thereby enhancing light absorption. The structure with ALP conical nano-grating exhibits significantly higher absorption across the entire spectrum compared to the planar structure due to scattering and diffraction effects. In the short wavelength range, the absorption curve of the PEC back reflector structure overlaps with that of the Ag back reflector. However, beyond approximately 585 nm, the absorption of the Ag back reflector structure surpasses that of the substrate with PEC reflector. This indicates that parasitic absorption mainly occurs in the long wavelength range.
The electric field distribution of the optimized ALP conical grating on the PEC back reflector at 400 nm, 693 nm, and 1100 nm was further studied in this article, as shown in Fig. 6. The planar structure demonstrates Fabry-Perot mode resonance in the perpendicular direction. Due to the periodic nature of the conical grating, it satisfies the Bloch diffraction mode, thereby inducing diffraction effects [49]. This leads to a modification in the propagation direction of the incident light, resulting in a longer optical path. Consequently, this phenomenon significantly enhances the light-trapping efficiency. It can be seen from the Fig. 6 that the conical grating has a good localized effect on the electric field compared to a planar silicon absorber layer. Moreover, grating structures demonstrate a higher light-trapping effect. More energy is concentrated at the top of the conical structure, which significantly enhancing light absorption over a wide range of wavelengths [18,42]. Hence, the Si absorption layer with a grating structure exhibits higher energy absorption, leading to a reduced internal field energy in contrast to a planar structure. Under comparable structural conditions, the electric field distribution exhibits similarity. However, at longer wavelengths, the difference in electric field energy between Ag grating and PEC grating becomes more pronounced. This is because Ag grating exhibits parasitic absorption of light, leading to a decrease in electric field energy distribution. This explains why there is no overlap in absorption between the PEC back reflector structure and the Ag back reflector structure in Fig. 4.
Fig. 6. Shows the electric field distribution of the planar TSSC with an ITO thickness of 0.30 µm, the optimized ALP conical nanostructured TSSC with Ag back reflector, and the optimized ALP conical nanostructured TSSC with PEC back reflector at λ = 400 nm, λ = 693 nm, and λ = 1100 nm.
In the optical analysis section, it was assumed that the absorption of one photon can result in the generation of an electron-hole pair. Spatially resolved absorption was utilized to calculate the generation rate. However, this approach overlooks all the loss mechanisms of charge carriers during transport and recombination processes, leading to an overestimation of the calculated Jsc [52]. Therefore, it is necessary to study the electrical properties of the optimal design structure to quantify the overall device performance enhancement.
In this study, the optimal structure for optical analysis was electrically simulated using the 3D finite element method in the Lumerical Device software package. A P-N junction with an axially doped structure was employed, where the absorbed photons can generate electron-hole pairs. These carriers are separated by the built-in electric field within the PN junction and driven to the electrodes [53]. As shown in Fig. 7, the red dotted region is the actual simulation region, the donor concentration (ND) of the N++ doped region is 1 × 1020 cm-3, the accepter concentration (NA) of the P-doped region is 6 × 1016 cm-3, and the substrate doping concentration (NS) of the P++ region is 2 × 1018 cm-3. In the device model, an ideal ohmic contact is assumed at the interface between the absorption layer and the electrode. The surface recombination rate was set to 107 cm/s. During the simulation process, carrier loss mechanisms, including radiative, non-radiative SRH, and Auger recombination, were considered.
PCE can be calculated according to the following Eq. (4) [54].
Where Pin is the incident power density at AM1.5, FF is the filling factor, and is defined as (FF = Pmax/JscVoc), Voc is the open-circuit voltage, which can be defined by the following Eq. (5) [54,55]. Where KB is the Boltzmann constant and J0 is the reverse saturation current of the solar cell.The generation rate results from the device's optical numerical simulation were used as input for the electrical numerical simulation, ensuring the relevance of the model. In the electrical analysis section, the doping concentration is identified as the primary factor controlling the performance of solar cells at room temperature. Hence, the impact of doping concentration on device performance parameters is analyzed in this paper. Specifically, the influence of NA on device performance was studied in the range of 1 × 1016 to 1 × 1017cm-3. During this process, ND and NS were fixed at 1 × 1020 cm-3 and 2 × 1018 cm-3, respectively. Figure 8(a)-8(c) show plots of each performance parameter of the optimized structure of the device as a function of NA, respectively. As shown in Fig. 8(a), the PCE increases as NA increases in the range of 1 × 1016 to 6 × 1016 cm-3. After the concentration reaches 6 × 1016cm-3, the PCE decreases slightly, but the decreasing trend is much smaller than the increasing trend before. According to Eq. (4) and a comprehensive comparison of Fig. 8(b) and 8(c), it can be concluded that the change in PCE is mainly affected by Voc during the variation of NA. The optimal value was achieved when NA was 6 × 1016.
The NA was set to the optimal value of 6 × 1016 cm-3, NS was fixed to 2 × 1018 cm-3, and the influence of ND on device performance was studied in the range of 5 × 1019 to 5 × 1020 cm-3. Figure 8(d)-8(f) present the plots of the performance parameters of the optimized device structure as a function of ND. According to Eq. (4), the PCE of the device is mainly affected by its Voc and Jsc. From Fig. 8(d)-8(f), it can be observed that as the ND concentration increases, the PCE initially increases and then decreases. The optimal value is achieved when the ND concentration is 1 × 1020 cm-3. The trend of PCE is completely consistent with the trend of Voc. Therefore, a comprehensive analysis of the effect of NA concentration on PCE can be obtained. The trend of the PCE of the optimal structure in this study with doping concentration mainly depends on Voc, which varies directly proportional to it.
Figure 9(a) and 9(b) respectively show the comparison of the current-voltage (I-V) and power-voltage (P-V) characteristic curves between the optimized design structure proposed in this study and the traditional planar structure when NA = 6 × 1016 cm-3, ND = 1 × 1020 cm-3 and NS = 2 × 1018 cm-3 in the doping region. It can be seen from these two graphs that the optimal nanostructure of the device proposed in this study achieved a performance of PCE = 17.15%, FF = 83%, Voc = 0.65 V, and Jsc = 31.8 mA/cm2. The planar structure achieved a performance of PCE = 6.30%, FF = 81%, Voc = 0.57 V and Jsc = 13.62 mA/cm2. Compared with the planar structure results, the individual performance parameters are significantly improved.
Fig. 9. (a) power-voltage characteristic curve and (b) current-voltage characteristic curve of the studied structure and planar structure.
4. Conclusions
In this study, the ALP nanostructure is applied to each layer of the TSSC materials to obtain excellent light absorption performance. The research findings reveal that the light-trapping efficiency of solar cells can be significantly improved by utilizing a well-optimized ALP nano-grating, as compared to a single-layer grating. Within this structure, a substantial proportion of Jph exceeds 30 mA/cm2, with a peak value of 41.27 mA/cm2. A meticulous investigation into the electrical characteristics of the optimal structure revealed that fine-tuning the doping levels of NA, ND, and NS yields remark results, including a PCE of 17.15%, FF of 83%, Voc of 0.65 V, and Jsc of 31.8 mA/cm2. However, despite achieving high Jph values, the resulting PCE remains relatively low, highlighting the necessity for additional improvements in effective electron and hole transport, reduction of light losses, and other related factors. Recent studies suggest that appropriately designed conical gratings have the potential to significantly enhance the manufacturing rate of solar cells in comparison to planar devices. This offers greater flexibility in the actual manufacturing process. These findings have a positive impact on the research on improving the efficiency of TSSC with cone-shaped nanostructures and provide valuable insights for enhancing the PCE of solar cells.
Funding
National Natural Science Foundation of China (52061009, 62174041, No. 62065004); Major Science and Technology Projects in Yunnan Province (No. 202102AB080008-2); Science and Technology Major Project of Guangxi (No. AD21220056); National level/ Guilin University of Electronic Technology Student Innovation and Entrepreneurship Training Program Project Funding (No. 202110595015).
Disclosures
The authors declare no competing financial interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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