Textured Si Research Articles

Research Perspective for Perovskite/Silicon Tandem Solar Cells

Multi‐absorbers, and tandem solar cells (TSCs), have enticed research focus with their competitive advantage of crossing the limits posed by Shockley–Queisser efficiency by single‐junction solar cell (SC). Perovskite solar cells (PSC) with their bandgap tunability and ease of solution processing at low temperatures for securing high efficiency (25.8%) provide an ideal candidate for top cell in TSC. Simultaneously, Si SCs with a 95% market share and their ideal low bandgap (1.12 eV) for capturing high‐energy photons in the near‐infrared (NIR) can assume bottom cell in TSC. This review highlights the research perspective for perovskite/Si TSCs to expedite its emergence as future TSC. The 4T device architecture of perovskite/Si TSC optically couples the subcells whereas the 2T configuration ensures the optical and electrical coupling among the constituent subcells. The semitransparency of the top cell, flat as well as textured Si bottom cell, matching of the current between the subcell through the recombination and tunnel junction layers, and modification of perovskites through‐thickness variation are few worth‐considering factors that contribute toward highly efficient perovskite/Si TSC. Finally, the research insight about perovskite/Si and all‐perovskite TSC is analyzed with reference to the ideal choice for the bottom cell.

Lattice mismatch alleviation in p-CdTe/n-Si heterostructure by surface engineering on Si substrate

The study used magnetron sputtering to investigate the growth of cadmium telluride (CdTe) thin films on surface treated n-type silicon (n-Si) substrates. The n-Si substrates were textured using potassium hydroxide (KOH) before the sputter deposition of CdTe. This was followed by cadmium chloride treatment to reduce the strain at the interface of CdTe and Si, which is caused by the incompatible lattice and thermal expansion mismatch (CTE). X-ray diffraction (XRD) analysis showed that the lowest FWHM and dislocation densities were obtained for CdCl2/CdTe/txt-nSi, which aligns with the scanning electron microscopy (SEM) results. In the SEM images, the interface bonding between the CdTe and Si surfaces was visible in the cross-sections, and the top-view images revealed sputtered CdTe thin films conforming to the patterns of pyramidal textured Si as an engineered surface to capture more light to maximize absorption in the CdTe/Si tandem design. The Energy dispersive X-ray (EDX) results showed that all the CdTe deposited on textured n-Si exhibited more Te atoms than Cd atoms, irrespective of the CdCl2 treatment. The presented results suggest that the texturization and CdCl2 treatment improved the morphology and grain boundary passivation of the sputtered CdTe. The adhesiveness of CdTe on the n-Si substrate was also significantly enhanced. Our findings further demonstrate that proper surface treatment of the Si substrate can greatly improve the quality of CdTe grown on Si by reducing the strain that occurs during the growth process. This study demonstrates a valuable method for enhancing the integration of CdTe with Si for two-junction tandem solar cell applications.

Insights into the Si─H Bonding Configuration at the Amorphous/Crystalline Silicon Interface of Silicon Heterojunction Solar Cells by Raman and FTIR Spectroscopy.

In silicon heterojunction solar cell technology, thin layers of hydrogenated amorphous silicon (a-Si:H) are applied as passivating contacts to the crystalline silicon (c-Si) wafer. Thus, the properties of the a-Si:H is crucial for the performance of the solar cells. One important property of a-Si:H is its microstructure which can be characterized by the microstructure parameter R based on Si─H bond stretching vibrations. A common method to determine R is Fourier transform infrared (FTIR) absorption measurement which, however, is difficult to perform on solar cells for various reasons like the use of textured Si wafers and the presence of conducting oxide contact layers. Here, it is demonstrated that Raman spectroscopy is suitable to determine the microstructure of bulk a-Si:H layers of 10nm or less on textured c-Si underneath indium tin oxide as conducting oxide. A detailed comparison of FTIR and Raman spectra is performed and significant differences in the microstructure parameter are obtained by both methods with decreasing a-Si:H film thickness.

Nanoscale Size Control of Si Pyramid Texture for Perovskite/Si Tandem Solar Cells Enabling Solution‐Based Perovskite Top‐Cell Fabrication and Improved Si Bottom‐Cell Response

AbstractA monolithic perovskite/silicon tandem solar cell architecture is employed to surpass the single‐junction efficiency limit. Recently, there is an increasing need for the double‐sided textures in the Si bottom cell to be compatible with the solution‐processed perovskite top cell from an industrial perspective. Herein, a silver‐assisted alkaline etching method is applied to fabricate nanoscale Si pyramid textures, and the influence of varying pyramid size (400–900 nm) on the interface morphology and the performance of perovskite/Si tandem cells is investigated. It is demonstrated that electrical shunting starts to increase, and the open‐circuit voltage (VOC) decreases when the texture size exceeds the perovskite thickness (~500nm) due to the non‐uniform top‐cell formation on a rough Si surface. However, when the texture size is reduced to 400–500 nm, all spin‐coated perovskite top‐cell component layers exhibit an even form over the nanopyramid Si, resulting in a high VOC and an enhanced Si bottom cell current (≈1.0 mA cm−2) due to the suppressed reflectance at the top/bottom cell interface without using optical couplers. The double‐sided nanopyramid Si texture offers opportunities to increase tandem cell efficiency while reducing its production cost compared with the commonly used single‐sided textured Si.

Differently shaped Ag crystallites and four current transport paths at sintered Ag/Si interface of crystalline silicon solar cells

The comprehensive understanding of the Ohmic contact mechanism on the front metalization is highly desirable for further improvement of crystalline silicon solar cells performance. However, there are still controversial views about current transport paths between silver electrode and silicon emitter. To clarify this significant issue, we applied the selective acid etching (AE) process and the method of mechanical stripping process to prepare proper samples for direct observation of the Ag/Si contact interface. It is revealed that four kinds of Ag crystallites are grown with variant shapes and sizes on the pyramid tips, {111} planes, edges and tiny bumps at valleys of the textured Si surface resulting from anisotropic etching of Si emitter by the flowing molten glass frit during firing and the consequent epitaxially growth of Ag on the etching pits during cooling. All these Ag crystallites are in direct contact with Si emitter, serving as the most important contact points on the Si side. Meanwhile, three kinds of spherical Ag particles are formed in the resolidified glass frit during cooling which are crucial tunneling paths, conducting between glass-covered Ag crystallites on silicon and Ag bulk above. As a whole, four current transport paths are suggested mainly via Ag crystallites, Ag particles or both. In addition, the four electron transport paths are further confirmed by the Kelvin probe force microscopy characterization. The clarification and detailed understanding of all current transport paths are key points to design better silver paste and sintering process, and hence to improve the solar cell performance.

A Grain Orientation-Independent Single-Step Saw Damage Gettering/Wet texturing Process for Efficient Silicon Solar Cells.

Improving electrical and optical properties is important in manufacturing high-efficiency solar cells. Previous studies focused on individual gettering and texturing methods to improve solar cell material quality and reduce reflection loss, respectively. This study presents a novel method called saw damage gettering with texturing that effectively combines both methods for multicrystalline silicon (mc-Si) wafers manufactured using the diamond wire sawing (DWS) method. Although mc-Si is not the Si material currently used in photovoltaic products, the applicability of this method using the mc-Si wafers as it contains all grain orientations is demonstrated. It utilizes saw damage sites on the wafer surfaces for gettering metal impurities during annealing. Additionally, it can crystallize amorphous silicon on wafer surfaces generated during the sawing process to allow conventional acid-based wet texturing. This texturing method and annealing for 10min allow for the removal of metal impurities and effectively forms a textured DWS Si wafer. The results show that the open-circuit voltage (ΔVoc =+29mV), short-circuit current density (ΔJsc =+2.5mA cm-2 ), and efficiency (Δη=+2.1%) improved in the p-type passivated emitter and rear cells (p-PERC) manufactured using this novel method, as compared to those in the reference solar cells.

Phase transformation pre-treatment of diamond wire-sawn multi-crystalline silicon wafers for metal-assisted chemical etching of solar cells

Phase transformation by the heat treatment, prior to metal-assisted chemical etching (MACE) of solar cells, has been proposed and studied, as a pre-treatment approach for diamond wire sawn (DWS) Si wafers. The results of the study show that the phase transformation pre-treatment can modify the microstructure of amorphous Si surface layers of DWS multi-crystalline (mc) Si wafers. The modified surface layer proved to be more reactive to the chemical etching solution, which was the primary factor for the improvement in the treated solar cells. A significant improvement on the surface uniformity and morphology of the textured Si wafer was achieved, subsequent to the pre-treatment, during the MACE texturization process. These solar cells based on the treated DWS mc-Si wafers displayed a gain of about 0.18% (absolute value), in the overall photoelectric conversion efficiency (η), when compared to the solar cells of the same batch, for the untreated wafers, on the same production line. It is hence, proven that this pre-treatment method, when used as a supplementary technique to the wet-chemical texturization process, has great potential as an effective application technology for solar cells based on DWS mc-Si wafers.

Contact Formation of Silver Paste and Atmospheric Pressure Chemical Vapor Deposition (n) Poly‐Silicon Passivating Contacts on Planar and Textured Surfaces

One of the main challenges for the industrialization of the passivating contact approach for Si solar cells is the metallization with screen‐printed paste while maintaining the low saturation current density. Using a non‐commercial Ag paste to metallize atmospheric pressure chemical vapor deposition (APCVD) (n) poly‐Si, the metal contact formation for passivating contacts on planar and textured substrates is investigated. The paste creates deep imprints caused by silver crystallite formation at the pyramid tips of textured silicon wafers. In contrast, on planar wafers, the silver crystallite growth stops at the interface between poly‐Si and the Si wafer. Similar contact resistivities are determined by comparing textured and planar Si samples. On planar samples, a contact resistivity of 4.6(14) mΩcm2 and a saturation current density of only 141(10) fA cm−2 for the metallized contact area are demonstrated. Textured samples with a contact resistivity of 2.7(17) mΩcm2 show a higher saturation current of 480(40) fA cm−2. This etching behavior is investigated by structural and elemental analyses using scanning electron microscopy.

Enhancement of reflectance reduction of solar cells by a silicon nanoparticle layer on a textured silicon substrate

To reduce the surface reflectance of Si solar cells, a structure has been developed in which a silicon nanoparticle (SiNP) layer is formed on a pyramid-like structure to simultaneously exploit multiple reflections and refractive index changes. The high refractive index of Si leads to the high reflectance of polished Si wafers. A conventional pyramid-like structure is formed on the surface of the Si wafer to reduce reflections using multiple reflections; however, the resultant sample reflectance is high because of the refractive index difference between air and Si. Nevertheless, although the reflectance of a SiNP layer on a polished Si wafer is high, the short-wavelength reflectance is <10 %. Our SiNP layer was then formed on a textured Si wafer by spin coating with a succession of changes to the spin-coating speed. In this manner, the reflectance after each speed change drastically decreased to <10 %, while the short-circuit current increased from 25.2 to 34.1 mA cm−2. The extremely thin SiNP layer and pyramid-like structure are promising as antireflective layers for solar cells.

Study of silicon rich oxide light emitter capacitors using textured substrates by metal assisted chemical etching

Mono and multilayer silicon rich oxide light-emitting capacitors (SRO-LEC) were fabricated using both polished and textured Si substrates. The textured substrate surface was treated by metal-assisted chemical etching (MACE) for 10 s and 20 s forming nanoneedles. Monolayer devices fabricated using textured substrates required lower energy to emit light and present at least 40% higher emission intensity than non-textured substrates. On the other hand, multilayer devices fabricated on textured and polished substrates showed that both LECs required similar energy to start the emission, however, after the bias is increased up to 5.5 MV/cm the textured LECs presented 36% higher emission intensity. The evidence shows that the electric field is screened by the nanocrystals in the SRO with Ro = 10 at electric fields lower than 5.5 MV/cm.

Reactive thermal evaporated amorphous tin oxide fabricated at room temperature and application in perovskite solar cells

AbstractTin oxide (SnOx) as a functional layer, such as electron transport layer (ETL) or buffer layer, is widely used in perovskite solar cells (PSCs). However, SnOx prepared by spin coating is not suitable to be applied to the solar cells with large area or rough substrates. In this paper, SnOx thin films served as the ETL of PSCs were prepared using a simple reactive thermal evaporation (RTE) method, and a convenient ultraviolet ozone treatment (UV‐O3) at room temperature was carried out to replace the common post‐annealing process. The SnOx ETL prepared by RTE is suitable for substrates with arbitrary morphology, and the performance of the PSCs can be improved significantly after the ultraviolet ozone treatment of the SnOx layer. A power conversion efficiency (PCE) up to 20.62% was achieved on the planar PSCs with the RTE SnOx serving as the ETL. Conformal coverage of the RTE SnOx thin film and the perovskite layer on a textured Si surface was achieved, which is helpful for making fully textured monolithic perovskite/silicon tandem solar cells in the future.

Ion-beam-induced structure fragmentation and dewetting in Au/Ti films

We investigate the formation of Au structures by continuing fragmentation of a Au film separated from Si (100) substrate by a Ti adhesion layer. The ion-beam processing uses 2 keV Cs+ projectiles. It induces the formation of a large variety of structures with dimensions from tens of microns down to a few nanometers, and shapes that include flat Au islands and clusters of Au nanoparticles. We identify different fragmentation mechanisms. In one pathway, Au structures are formed as pinch-off droplets detached from large Au islands undergoing solid-state dewetting. In a second pathway, a continuing cracking fragmentation produces Au structures with a wide range of sizes and shows self-similarity, expressed in a power law for volume distribution of fragments Vα with α = −2.03 ± 0.05. The measured α value is consistent with theoretical results in the literature modeling strong impact fragmentation. In the last stage of fragmentation evolution, Au nanostructures are spread over a texturized Si background. They assume shapes close to hemispherical, reflecting the increasing relevance of surface tension as dimensions decrease.

GaN nanorods on V-groove textured Si (111): significant light trapping for photoelectrocatalytic water splitting

Although gallium nitride (GaN) nanostructures are auspicious for photocatalytic activity, geometrical optimization has paid much attention for a significant light trapping in photoelectrochemical applications. To minimize the optical losses, we designed a prototype V-groove textured Si (100) with (111) facets, and GaN nanorods (NRs) were grown over a prototype substrate using plasma-assisted molecular beam epitaxy. The photocurrent density of V-groove textured GaN NRs in the NaOH electrolyte is found to be 801.62 μA/cm2 at 1.14 V vs reversible hydrogen electrode, which was 2.1-fold larger than that of GaN NRs on plain Si (111). Using this prototype V-groove textured Si(100) with (111) facets, a significant light can be trapped and modulated into GaN NRs. Furthermore, the heterostructure between GaN NRs and V-groove textured Si stimulates effective charge separation and transportation. These results represent an important forward step in solar photoelectrolysis.

Ion-Beam-Induced Structure Fragmentation and Dewetting in Au/Ti Films

We investigate the formation of Au structures by continuing fragmentation of a Au film separated from Si (100) substrate by a Ti adhesion layer. The ion-beam processing uses 2 keV Cs+ projectiles. It induces the formation of a large variety of structures with dimensions from tens of microns down to a few nanometers, and shapes that include flat Au islands and clusters of Au nanoparticles. We identify different fragmentation mechanisms. In one pathway, Au structures are formed as pinch-off droplets detached from large Au islands undergoing solid-state dewetting. In a second pathway, a continuing cracking fragmentation produces Au structures with a wide range of sizes and shows self-similarity, expressed in a power law for volume distribution of fragments Vα with α= -2.03±0.05. The measured α value is consistent with theoretical results in the literature modelling strong impact fragmentation. In the last stage of fragmentation evolution, Au nanostructures are spread over a texturized Si background. They assume shapes close to hemispherical, reflecting the increasing relevance of surface tension as dimensions decrease.

Broadband angular color stability of dielectric thin film-coated pyramidal textured Si for photovoltaics

In this work we demonstrate the angular color stability of textured c-Si substrates colored by single layer thin film coatings of SiNx. These coatings show higher angular color stability on substrates with a random upright pyramidal surface texture compared to identical coatings on planar silicon substrates. Angle dependent reflectance measurements, supported by a modeling framework, display that the reflectance peaks originating from thin film interference of coated textured substrates only shift about 15 nm with an increasing angle of incidence from 10° to 80°, while the reflectance peaks of planar substrates with identical coatings shift about 120 nm at these angles. More specifically, reflectance peaks of planar substrates shift to shorter wavelengths, leading to a blue shift of the color appearance. The stable peak position of the textured samples is explained by a 2D representation of their surface texture and the primarily double interference interaction on it. While it is well known that a wide range of colors can be realized exhibiting low optical losses with thin film coatings, angular color stability was often not taken into account. However, for building integrated photovoltaics applications, a high angular color stability is desired, underlining the importance of using these textures. In most installed c-Si photovoltaics, similar substrate surface textures and dielectric thin film layers are already used. Therefore, this work envisions a route to facilitate large scale production of colored solar cells on textured c-Si substrates, colored by thin film SiNx layers, with minimized optical losses and improved angular color stability.

Optimization of four terminal rear heterojunction GaAs on Si interdigitated back contact tandem solar cells

High-efficiency, four-terminal tandem solar cells composed of thin GaAs films mechanically stacked onto interdigitated back contact silicon solar cells with a glass interlayer are demonstrated. The optimal thickness of the absorber layer of a rear heterojunction GaAs subcell for use in four terminal tandem solar cells was studied. GaAs top cells with absorber layer thicknesses of 1.5, 1.9, 2.3, 2.8, and 3.5 μm were fabricated on glass and mechanically stacked onto interdigitated back-contact Si bottom cells. All tandem cells were found to have efficiencies above 30% under the AM1.5 G spectrum demonstrating a relatively weak sensitivity to thickness in the four-terminal configuration. We found the 2.8 μm absorber layer cell to have the highest top cell and tandem cell efficiency at 26.38% and 32.57%, respectively. Optical modeling with transfer matrix method for the planar top cell and Lambertian light trapping in the textured Si subcell, along with drift-diffusion Hovel equations, were used to show photon recycling enhancement to the effective diffusion length and VOC of the top cell as a result of the low-index glass interlayer.

Formation of the nanoporous Si structure of solar cell with using a model representation

In this study, to obtain frontal functional nanolayers of solar cells, an analysis of existing models of PSi was made. In order to achieve a low integral reflection coefficient of a textured Si surface, which can be further used as an efficient anti-reflective coating, a number of experiments were performed to establish the influence of technological conditions on the morphology of porous Si fabricated by electrochemical etching. The results of SEM studies showed that the porous Si surface obtained does not contain noticeable damage or cracking, which indicates the correct choice of technological modes of anodic etching. The obtained porous textures characterized by minimal integral reflection, which is a necessary condition for creating the front surface of Si-based solar cells.

Integration of Fe3O4 nanospheres and micropyramidal textured silicon wafer with improved photoelectrochemical performance

Silicon (Si)-based composites have attracted extensive attention for photoelectrochemical (PEC) application. Herein, micropyramidal textured Si wafer was constructed by wet chemical etching method. Hydrothermally synthesized Fe3O4 nanospheres were further deposited on the micropyramids by a simple dip-coating approach. The integrated Fe3O4 nanospheres and Si micropyramids (denoted as Fe3O4@SiMPs) revealed 20 times of higher PEC efficiency than planar Si wafer, without obvious photocurrent decay and crystalline structure change during chronoamperometry test. The greatly enhanced PEC performance of Fe3O4@SiMPs is largely attributed to the seamless integration of Si micropyramids and Fe3O4 nanospheres. The micropyramidal textured structure can enhance light absorption by reducing the surface reflectance and enhancing the light trapping effect. The micropyramids and Fe3O4 nanospheres on the surface of planar Si wafer offer more specific surface area for the contact of electrode with electrolyte. The Fe3O4 nanospheres not only protect the micropyramids from corrosion, but also accelerate the PEC kinetics by promoting charge transfer between the electrode and the electrolyte. This study can inspire the optimal design of Si wafer-based nanocomposites as efficient PEC catalysts for overall water splitting.

Inter-facet composition modulation of III-nitride nanowires over pyramid textured Si substrates by stationary molecular beam epitaxy

InGaN nanowires (NWs) are grown on pyramid textured Si substrates by stationary plasma-assisted molecular beam epitaxy (PA-MBE). The incidence angles of the highly directional source beams vary for different pyramid facets, inducing a distinct inter-facet modulation of the In content of the InGaN NWs, which is verified by spatial element distribution analysis. The resulting multi-wavelength emission is confirmed by photoluminescence (PL) and cathodoluminescence (CL). Pure GaN phase formation dominates on certain facets, which is attributed to extreme local growth conditions, such as low active N flux. On the same facets, InGaN NWs exhibit a morphology change close to the pyramid ridge, indicating inter-facet atom migration. This cross-talk effect due to inter-facet atom migration is verified by a decrease of the inter-facet In content modulation amplitude with shrinking pyramid size. A detailed analysis of the In content variation across individual pyramid facets and element distribution line profiles reveals that the cross-talk effect originates mainly from the inter-facet atom migration over the convex pyramid ridge facet boundaries rather than the concave base line facet boundaries. This is understood by first-principles calculations showing that the pyramid baseline facet boundary acts as an energy barrier for atom migration, which is much higher than that of the ridge facet boundary. The influence of the growth temperature on the inter-facet In content modulation is also presented. This work gives deep insight into the composition modulation for the realization of multi-color light-emitting devices based on the monolithic growth of InGaN NWs on pyramid textured Si substrates.

Comparison of passivation properties of plasma-assisted ALD and APCVD deposited Al2O3 with SiNx capping

In this paper, we report on the direct comparison of Al2O3 films deposited by plasma-assisted atomic layer deposition (ALD) and atmospheric pressure chemical vapor deposition (APCVD) techniques on the passivation properties of both bare and boron diffused Si surfaces. It is found that ALD and APCVD deposited Al2O3 layers with SiNx capping have very similar chemical composition, hydrogen concentration at the Al2O3/Si interface, and interface defect density (Dit), but the ALD film has slightly higher negative fixed charge (Qf) after a high temperature (~750 °C) contact firing cycle typically used for solar cell fabrication. Both films showed excellent surface recombination velocities of <5 cm/s on planar and textured un-diffused Si wafers, but the ALD films gave 1 to 8 fA/cm2 lower emitter saturation current densities (J0e) on implanted boron emitters with sheet resistance in the range of 90–180 Ω/□. This is attributed to the observed slightly higher negative charge-induced field-effect passivation in the ALD films. However, SiNx/Al2O3/p+/n/n+ passivated emitter rear totally diffused (PERT) cell structures fabricated with 110 and 150 Ω/□ emitter in this study showed no appreciable difference in cell efficiency. Device analysis and simulations showed that even for >23% high-performance solar cells design, with very low bulk and rear contact recombination, the observed small difference in J0e has negligible effect on cell efficiency. Thus, APCVD Al2O3 passivation of B emitter can produce comparable efficiencies to their counterpart ALD Al2O3 passivated n-type Si solar cells.