Crystalline Silicon Thin-film Solar Cells Research Articles

Anti-reflection effect of high refractive index polyurethane with different light trapping structures on solar cells

The textured surfaces to reduce light reflectivity by using acid-alkali chemical etching and SiNx films are generally necessary for commercial crystalline silicon solar cells. However, this etching process requires a large amount of environmentally harmful acid-alkali solution and has limited options for texture and size. To overcome these disadvantages, a new anti-reflection strategy is proposed in this study, which is using soft nanoimprint lithography to prepare the textured structures on the outside of the SiNx films. The polyurethane with a high refractive index of 1.64 is selected as the texture material, and different templates are selected to prepare it into different light trapping structures, including positive-inverted pyramids, inverted lace cones, and positive-inverted moth-eye nanostructures allowing for easy customization of the textured structures. The finite element simulation and experiments demonstrate that these light trapping structures have a wide spectrum anti-reflection performance in visible and near-infrared bands. With the back surface of the commercial passivated emitter rear contact (PERC) bi-facial solar cells as the imprint substrates, some light trapping structures can reduce the surface weighted average light reflectivity (Rw) at the band of 300–1200 nm from 18.31% to less than 10% and the optimal structures can reduce Rw to 8.71%. This anti-reflection strategy can also be applied to thin-film solar cells and crystalline silicon solar cells of other structures, such as HIT, Topcon, Perovskite/c-Si tandem, and so forth, which shows great development potential.

Design and Optimization of a Self-Protected Thin Film c-Si Solar Cell against Reverse Bias

Current mismatch due to solar cell failure or partial shading of solar panels may cause a reverse biasing of solar cells inside a photovoltaic (PV) module. The reverse-biased cells consume power instead of generating it, resulting in hot spots. To protect the solar cell against the reverse current, we introduce a novel design of a self-protected thin-film crystalline silicon (c-Si) solar cell using TCAD simulation. The proposed device achieves two distinct functions where it acts as a regular solar cell at forward bias while it performs as a backward diode upon reverse biasing. The ON-state voltage (VON) of the backward equivalent diode is found to be 0.062 V, which is lower than the value for the Schottky diode usually used as a protective element in a string of solar cells. Furthermore, enhancement techniques to improve the electrical and optical characteristics of the self-protected device are investigated. The proposed solar cell is enhanced by optimizing different design parameters, such as the doping concentration and the layers' thicknesses. The enhanced cell structure shows an improvement in the short-circuit current density (JSC) and the open-circuit voltage (VOC), and thus an increased power conversion efficiency (PCE) while the VON is increased due to an increase of the JSC. Moreover, the simulation results depict that, by the introduction of an antireflection coating (ARC) layer, the external quantum efficiency (EQE) is enhanced and the PCE is boosted to 22.43%. Although the inclusion of ARC results in increasing VON, it is still lower than the value of VON for the Schottky diode encountered in current protection technology.

Cutting sinusoidal gratings to enhance light trapping in thin-film silicon solar cells.

A crystalline silicon thin-film solar cell with a three-layer sinusoidal grating structure is studied. The structure has a double-layer antireflection layer, and the three-layer grating is located in the double-layer antireflection layer and the passivation layer, respectively. The related parameters of the grating structure are optimized by scanning using finite-difference time-domain. The optimization results show that cutting the sinusoidal grating structure can significantly improve the light absorption efficiency of the cell for near-infrared light (750-1100nm), and the enhancement effect is mainly in the transverse electric (TE)-polarized light. This is because the localized surface plasmon resonance and optical waveguide mode under TE-polarized light can be fully excited after the sinusoidal structure is cut. The short-circuit current density (J S C ) of the optimized three-layer sinusoidal grating structure is 19.82m A/c m 2, which is 112.43% higher than that of the planar structure with the same parameters and 23.18% higher than that of the uncut sinusoidal grating structure with the same parameters.

Heterostructure Silicon Solar Cells with Enhanced Power Conversion Efficiency Based on Si x /Ni3+ Self-Doped NiO x Passivating Contact.

Developing efficient crystalline silicon/wide-band gap metal-oxide thin-film heterostructure junction-based crystalline silicon (c-Si) solar cells has been an attractive alternative to the silicon thin film-based counterparts. Herein, nickel oxide thin films are introduced as the hole-selective layer for c-Si solar cells and prepared using the reactive sputtering technique with the target of metallic nickel. An optimal Ni3+ self-doped NiOx film is obtained by tuning the reactive oxygen atmosphere to construct the optimized c-Si/NiOx heterostructure band alignment. A thin SiOx interlayer was further introduced to reduce the defect of the c-Si/NiOx interface with the UV–ozone (UVO) treatment. The constructed p-type c-Si/SiOx/NiOx/Ag solar cell exhibits an increase in the open voltage from 586 to 611 mV and achieves a 19.2% conversion efficiency.

Effective light trapping in c-Si thin-film solar cells with a dual-layer split grating.

We propose a dual-layer split nanograting structure in crystalline silicon thin-film solar cells (TFSCs). The split nanograting is designed by introducing two partitioning factors and split times. By employing the finite-difference time-domain method, the light trapping performance and relevant parameters of TFSCs are analyzed and optimized. Numerical computation of optical and electrical simulation shows that the optimal dual-layer split nanograting structure has demonstrated great enhanced light absorption compared with the planar structure. Enhancement of the light trapping effect is associated with light coupling to waveguide modes. The short-circuit current density is reached at 21.66mA/cm2 with an improvement of 54.6% over the planar structure. All results provide a parting thought for the design of TFSC grating structures.

Highly efficient SiO2 trapezoidal grating-based thin-film solar cell

A crystalline silicon thin-film solar cell (c-Si TFSC) with a trapezoidal grating is newly introduced and analyzed. The three-dimensional (3D) finite-difference time-domain (FDTD) method is employed to optimize the geometrical parameters of the trapezoidal grating and hence maximize the light absorption. The proposed trapezoidal grating TFSC offers an optical ultimate efficiency ( η ) of 32.3%, with an enhancement of 81% relative to the conventional TFSC. The light absorption enhancement within wavelength range (300–1100 nm) is based on the diffraction grating, which supports Bloch modes through the suggested solar cell. The electrical characteristics of the proposed design are also studied using the finite-element method. The cell doping concentration, junction thickness, and recombination process are also investigated to further enhance the power conversion efficiency (PCE). The reported design offers a short circuit current density J s c of 24.8 m A / c m 2 and PCE of 12.5% with an improvement of 83% over the conventional TFSC.

Period-mismatched sine dual-interface grating for optical absorption in silicon thin-film solar cells.

Crystalline silicon thin-film solar cells with period-mismatched sine dual-interface gratings are proposed. Several structural parameters of the front and rear gratings, such as heights, periods, and duty ratios, are optimized using the finite-difference time-domain method. The mechanisms of absorption enhancement are also illustrated by analyzing the optical and electrical performance in thin-film solar cells with different grating arrangements. Numerical results indicate that the period-mismatched sine dual-interface grating structure shows obvious improvement in absorption efficiency and is more suitable for grating structures with small period. The short-circuit current density of the period-mismatched dual-interface sine grating structure is improved to 18.89mA/cm2, an increase of 41.39% as compared with the planar structure. The research findings can be utilized to guide the design of grating structures for thin-film solar cells.

Light absorption enhancement in ultrathin film solar cell with embedded dielectric nanowires

A novel design of thin-film crystalline silicon solar cell (TF C-Si-SC) is proposed and numerically analyzed. The reported SC has 1.0 µm thickness of C-Si with embedded dielectric silicon dioxide nanowires (NWs). The introduced NWs increase the light scattering in the active layer which improves the optical path length and hence the light absorption. The SC geometry has been optimized using particle swarm optimization (PSO) technique to improve the optical and electrical characteristics. The suggested TF C-Si-SC with two embedded NWs offers photocurrent density ({J}_{ph}) of 32.8 mA cm−2 which is higher than 18 mA cm−2 of the conventional thin film SC with an enhancement of 82.2%. Further, a power conversion efficiency of 15.9% is achieved using the reported SC.

Efficient and stable thin-film crystalline silicon solar cell by introducing rotation factor in surface square pillar array grating

To obtain efficient and stable light trapping, angle rotation is introduced to form rotated square pillar array grating (SPAG) solar cells. Compared with the unpatterned stack slab and the optimized uniform SPAG cells, the maximum short-circuit current ( J sc) of the optimized rotated SPAG is increased by 78.54% and 3.21%, respectively. Moreover, besides the fact that the low-incidence angular sensitivity of J sc could be maintained, J sc of the optimized rotated SPAG will always be larger than that of the optimized uniform SPAG at any incident angle. Furthermore, when the structural parameters of the subsquare pillar slightly deviate from the optimum, the absorption only deceases slightly as well, which indicates both a high structural tolerance and a stable absorption performance. In addition, our results show not only that the proposed rotated SPAG is promising to make light trapping efficient and stable but also that introducing rotation disorders is promising for other high-absorption pseudounordered surface structures.

Light-trapping schemes for silicon thin-film solar cells via super-quadratic subwavelength gratings.

We systematically investigate the light-trapping schemes of crystalline silicon thin-film solar cells (TFSCs) for three common grating layouts via one-dimensional super-quadratic subwavelength gratings. The effects of antireflective coating, absorber layer thickness, and grating geometry on the light-trapping performance of TFSCs are numerically studied using the finite-difference time-domain method. The results suggest that the conformal aluminum-doped zinc oxide (AZO) coatings have better optical properties than the plane AZO coatings. For the case of only top Si gratings, the grating geometry of degree $n={4}$n=4 can achieve a good trade-off between the shape-dependent light-trapping and antireflection properties, showing the best light-trapping effect; for the case of only bottom Ag gratings, the optical performance of TFSCs is significantly degraded as the degree $n$n increases from $n={1}$n=1 to $n\to\infty$n→∞. The above findings are analyzed and demonstrated in detail from the optical and electrical perspectives, and they can be utilized to guide the design of light-trapping structures for TFSCs.

Light management in crystalline silicon thin-film solar cells with imprint-textured glass superstrate

The implementation of light management textures in thin-film solar cells often simultaneously causes an undesired deterioration of electronic performance. Here, we introduce a simple yet effective technique for improved light management in liquid phase crystallized silicon thin-film solar cells on glass. By imprinting pyramidal textures on the sun-facing side of the glass superstrate, absorber and functional layers of the device remain unaffected while light in-coupling is significantly increased. An increase of short-circuit current density by 2.5mA cm2 was observed by texturing the glass in this way, corresponding to an enhanced power conversion efficiency from 12.9% to 13.8%. Optical simulations allow to attribute the increase in equal shares to an anti-reflective effect at the air-glass interface as well as light scattering and multiple passes through the glass. The technology allows for independent optimization of optical performance without compromising on electronic material issues and is therefore useable for any other solar cell technology using a glass superstrate.

Performance Improvement of Flexible Thin Film Si Solar Cells using Graphite Substrate

We investigated the characteristics of nano crystalline silicon(nc-Si) thin-film solar cells on graphite substrates. Amorphous silicon(a-Si) thin-film solar cells on graphite plates show low conversion efficiency due to high surface roughness, and many recombination by dangling bonds. In previous studies, we deposited barrier films by plasma enhanced chemical vapor deposition(PECVD) on graphite plate to reduce surface roughness and achieved ~7.8 % cell efficiency. In this study, we fabricated nc-Si thin film solar cell on graphite in order to increase the efficiency of solar cells. We achieved 8.45 % efficiency on graphite plate and applied this to nc-Si on graphite sheet for flexible solar cell applications. The characterization of the cell is performed with external quantum efficiency(EQE) and current density-voltage measurements(J-V). As a result, we obtain ~8.42 % cell efficiency in a flexible solar cell fabricated on a graphite sheet, which performance is similar to that of cells fabricated on graphite plates.

Attaining High Photovoltaic Efficiency and Stability with Multidimensional Perovskites.

The power conversion efficiency of organic-inorganic hybrid perovskite solar cells has soared over the past ten years and currently rivals those of crystalline silicon and other thin-film solar cells. Most of the research effort so far has been focused on three-dimensional (3 D) perovskite crystals, producing devices with very high efficiency but poor operational and environmental stability. Two-dimensional (2 D) Ruddlesden-Popper perovskite has recently shown its potential as a highly stable light absorber, albeit with low efficiency. This work reviews the current progress in attaining both high efficiency and stability in solar cells by using 2 D perovskite. In particular, the focus is on multidimensional perovskite as a way to combine the best characteristics of 3 D and 2 D perovskites. Future challenges and potential methods to boost the performance of multidimensional perovskite solar cells further are briefly presented.

Investigation of effective near-infrared light-trapping structure with submicron diameter for crystalline silicon thin film solar cells

To obtain high efficiency in crystalline silicon thin-film solar cells, it is necessary to develop a novel light-trapping structure that not only has high light absorptance and can be fabricated with a small etching margin but also has a small surface area for a small effect of surface recombination. In this study, optical properties of silicon nanowires (SiNWs) with submicron diameters were investigated by the finite-difference time-domain (FDTD) method in the infrared wavelength region, in which the absorption coefficient of crystalline Si is quite low and the increase in optical path length is very important. To verify the simulation results, SiNWs with submicron diameters were successfully fabricated by the metal-assisted chemical etching method using two types of etching masks. The optical properties and minority carrier lifetime of the obtained structures were measured, and the results suggest that SiNWs with submicron diameters such as ∼700 nm are effective for both light trapping and the suppression of surface recombination.

Enhanced light trapping in thin-film silicon solar cells with concave quadratic bottom gratings.

We present a theoretical crystalline silicon thin-film solar cell with concave quadratic bottom gratings using parametric design of the gratings and systematically perform optoelectronic simulations of the proposed grating structures. Numerical results reveal that the concave quadratic grating structure leads to a desirable enhancement of light absorption in the near-IR region (750-1100nm), and this enhancement is dominated by TE-polarized light. The mechanisms of light absorption enhancement under TE-polarized light are based on the hybrid effect of optical waveguide modes and Fabry-Perot modes. Moreover, research results indicate that the concave quadratic grating structure has excellent light-focusing effects, which is conducive to the excitation of optical waveguide modes. The study findings can be utilized to improve the efficiency of thin-film solar cells based on grating structures.

Impact of one-dimensional photonic crystal back reflector in thin-film c-Si solar cells on efficiency

In this work, the effect of one-dimensional photonic crystal on optical absorption, which is implemented at the back side of thin-film crystalline silicon (c-Si) solar cells, is extensively discussed. The proposed structure acts as a Bragg reflector which reflects back light to the active layer as well as nanograting which couples the incident light to enhance optical absorption. To understand the optical mechanisms responsible for the enhancement of optical absorption, quantum efficiency and current density for all structures are calculated and the effect of influential parameters, such as grating period is investigated. The results confirm that our proposed structure have a great deal for substantial efficiency enhancement in a broad range from 400 to 1100 nm.

Biomimetic spiral grating for stable and highly efficient absorption in crystalline silicon thin-film solar cells: erratum.

In the original manuscript, the maximum short-circuit current in Fig. 2 and the description of angular stability for polarization in Fig. 6 are found incorrect owing to negligence. The incorrect maximum integrated absorption also led to errors appeared in Fig. 3 and Fig. 4(d), in which the mistake value was used to plot. In this erratum, all of those mistakes have been corrected. Moreover, both higher Fourier expansion order and the resolution of frequency are adopted in the recalculation to make sure the updated results to be reliable. The updated data presented still support the main conclusions drawn in the previous manuscript.

Biomimetic spiral grating for stable and highly efficient absorption in crystalline silicon thin-film solar cells.

By emulating the phyllotaxis structure of natural plants, which has an efficient and stable light capture capability, a two-dimensional spiral grating is introduced on the surface of crystalline silicon solar cells to obtain both efficient and stable light absorption. Using the rigorous coupled wave analysis method, the absorption performance on structural parameter variations of spiral gratings is investigated firstly. Owing to diffraction resonance and excellent superficies antireflection, the integrated absorption of the optimal spiral grating cell is raised by about 77 percent compared with the conventional slab cell. Moreover, though a 15 percent deviation of structural parameters from the optimal spiral grating is applied, only a 5 percent decrease of the absorption is observed. This reveals that the performance of the proposed grating would tolerate large structural variations. Furthermore, the angular and polarization dependence on the absorption of the optimized cell is studied. For average polarizations, a small decrease of only 11 percent from the maximum absorption is observed within an incident angle ranging from -70 to 70 degrees. The results show promising application potentials of the biomimetic spiral grating in the solar cell.

Laser ablation compatible substoichiometric SiOx/SiNy passivating rear side mirror for passivated emitter and rear thin-film crystalline silicon solar cells

Fabrication of industrial thin-film crystalline silicon solar cells remains challenging because of the high level of light trapping and surface passivation required to achieve a good conversion efficiency, while reducing the process cost. This work proposes a solution of rear side reflector supplying both passivation and light trapping, and guaranteeing compatibility with a laser process for local opening in order to use the passivated emitter and rear cell architecture. The key element is the use of substoichiometric silicon oxide deposited by plasma-enhanced chemical vapor deposition with a higher silicon concentration than the usual nearly stoichiometric oxide. This material is absorbent at usual ultraviolet laser wavelengths, and thus allows laser ablation with limited substrate heat, greatly reducing substrate damage after ablation. A layer of this oxide is incorporated into a SiOx/SiNy dielectric stack, which shows the expected qualities in term of passivation and reflectivity.

Influence of anodic bonding on the surface passivation quality of crystalline silicon

Developing new materials with improved thermal stability is very important as far as the passivation and processing of crystalline silicon solar cells is considered. In this work, we studied two types of hydrogenated silicon based materials (hydrogenated amorphous silicon oxide and hydrogenated microcrystalline silicon oxide) and compared them to the well-established hydrogenated amorphous silicon. We demonstrate the thermal robustness of hydrogenated silicon oxide under anodic bonding conditions (250°C, 1000V). The microstructural and composition evolution of passivation layers under anodic bonding was characterized by Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy and secondary ion mass spectrometry. We found that oxygen can inhibit the degradation of the passivation quality under anodic bonding by suppressing the localized interfacial crystallization and re-passivating the interface. When applied to thin-film crystalline silicon solar cells, the excellent thermal robustness of hydrogenated microcrystalline silicon oxide allows to keep a good passivation to the ultrathin c-Si absorbers (1μm).