Textured Silicon Research Articles

Surface Optimization of Random Pyramid Textured Silicon Substrates for Improving Heterojunction Solar Cells

Key steps in the fabrication of high-efficiency a-Si:H/c-Si heterojunction solar cells are the controlled pyramid texturing of the c-Si substrates to minimize reflection losses and the subsequent passivation by deposition of a high-quality a-Si:H layer to reduce recombination losses. This contribution reviews our recent results on the optimization of the wet-chemical texturing of crystalline Si wafers for the preparation of heterojunction solar cells with respect to low reflection losses, low recombination losses and long minority carrier lifetimes. It is demonstrated, that by joint optimization of both saw damage etch and texture etch the optical and electronic properties of the resulting pyramid morphology can be controlled. Effective surface passivation and thus long minority charge carrier lifetimes are achieved by deposition of intrinsic amorphous Si ((i) a-Si:H) layers. It is shown, that optimized (i) a-Si:H deposition parameters for planar Si (111) wafers can be transferred to a-Si:H layer deposition on random pyramid textured Si (100) wafers. Statistical analysis of the pyramid size distribution revealed that a low fraction of small pyramids leads to longer minority charge carrier lifetimes and, thus, a higher Voc potential for solar cells.

Lithography-free wide-angle antireflective self-cleaning silicon nanocones.

Low-cost, wideband, and wide-angle antireflective layers are of prime importance to photovoltaic and other optoelectronic applications. We report a novel fabrication methodology of random textured silicon nanocones (SiNCs) array through metal-assisted chemical etching combined with oxidation. The optical properties of the fabricated structure are studied theoretically and experimentally. The random textured SiNCs array showed very promising broadband antireflective properties through the entire visible wavelength range at different incident angles up to ±60°. In addition, the nanostructures inherently could become self-cleaning due to the high contact angle. This random cheap textured SiNCs array increases the absorption efficiency of photodetectors and reduces its cost.

Plasma-initiated rehydrogenation of amorphous silicon to increase the temperature processing window of silicon heterojunction solar cells

The dehydrogenation of intrinsic hydrogenated amorphous silicon (a-Si:H) at temperatures above approximately 300 °C degrades its ability to passivate silicon wafer surfaces. This limits the temperature of post-passivation processing steps during the fabrication of advanced silicon heterojunction or silicon-based tandem solar cells. We demonstrate that a hydrogen plasma can rehydrogenate intrinsic a-Si:H passivation layers that have been dehydrogenated by annealing. The hydrogen plasma treatment fully restores the effective carrier lifetime to several milliseconds in textured crystalline silicon wafers coated with 8-nm-thick intrinsic a-Si:H layers after annealing at temperatures of up to 450 °C. Plasma-initiated rehydrogenation also translates to complete solar cells: A silicon heterojunction solar cell subjected to annealing at 450 °C (following intrinsic a-Si:H deposition) had an open-circuit voltage of less than 600 mV, but an identical cell that received hydrogen plasma treatment reached a voltage of over 710 mV and an efficiency of over 19%.

PEDOT:PSS as an Alternative Hole Selective Contact for ITO-Free Hybrid Crystalline Silicon Solar Cell

Organic–inorganic hybrid solar cells were fabricated by spin coating of conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole selective contact upon textured n-type crystalline silicon wafers. Two different PEDOT:PSS commercial products (Clevios HTL Solar and PH1000) were compared, enhancing their conductivity and wettability by addition of dimethyl sulfoxide cosolvent and Capstone FS-31 surfactant. Transmission line measurements (TLM) revealed that the sheet resistance of the PEDOT:PSS layer was comparable with that of conventional Indium–Tin oxide (ITO) electrodes, allowing its use as a conductive transparent layer in an ITO-free solar cell. Quasi-steady-state photoconductance measurements revealed an implied open-circuit voltage of 644 mV for HTL Solar, which is equivalent to a saturation current density of 255 fA/cm 2. In general, the performance of HTL Solar was better than PH1000, exhibiting power conversion efficiency (1 × 1 cm2 cell area) of 11.6%, as compared with 8.5%.

Analysis of the Effect of Silicon with Nano-Size Surface Structure on an Electrode Formed Using Screen Printing.

In this paper, we report an investigation into the effects of the texturing size of silicon on the efficiency of a screen-printed Si solar cell. To accomplish this, we produced solar cells with various textured surfaces. The method we used to produce these cells included methods such as anisotropic chemical etching (texturing size of about 4 μm) using a mixed solution of KOH and IPA, reactive ion etching (texturing size of about 0.2 μm), and Ag catalyzed etching (texturing size of about 90 nm). The solar cells with an Ag-catalyzed etching textured structure showed the lowest efficiency of 11.87%, with the highest series resistance of 1.32 Ω. In the case of anisotropic chemical etching, the solar cell had the best efficiency of 17.84%, with the lowest series resistance. This means that the electrodes and silicon surfaces were not well-connected with the nano-sized textured silicon surface. The results revealed that conventional silver paste at an average of 1.6 μm is unsuitable for nano-sized textured Si solar cells.

Influence of SiO2 Spacer Layer Thickness on Performance of Plasmonic Textured Silicon Solar Cell

We investigated the effect of SiO2 spacer layer thickness between the textured silicon surface and silver nanoparticles (Ag NPs) on solar cell performance using quantum efficiency analysis. Separation of Ag NPs from high index silicon with SiO2 layer led to modified absorption and scattering cross-sections due to graded refractive index medium. The forward scattering from Ag NPs is very sensitive to SiO2 layer thickness in plasmonic silicon cell performance due to the evanescent character of generated near-fields around the NPs. With the optimized ∼30–40 nm SiO2 spacer layer, we observed an enhancement of solar cell efficiency from ∼8.7 to ∼10 %, which is due to the photocurrent enhancement in the off-resonance surface plasmon region. We also estimated minority carrier diffusion lengths (Leff) from internal quantum efficiency data, which are also sensitive to SiO2 spacer layer thickness. We observed that the Leff values are enhanced from ∼356 to ∼420 μm after placing Ag NPs on ∼40 nm spacer layer due to improved forward (angular) scattering of light from the Ag NPs into silicon.

Solar cell fabrication using two-step textured quasi-single crystalline silicon by alkaline and RIE texturing processes

After alkaline chemical texturing process, SF6/O2 reactive ion etching (RIE) was used for maskless textured Si surface independently from crystal orientation. Additional RIE texturing on alkaline textured Si surface increased the light receiving areas and decreased the surface reflectance by random pyramidal structures with a width of hundreds of nanometers. The weighted average reflectance (WAR) of the textured Si surface by two-step texturing processes was achieved as low as 9.6% in the 300–1200 nm wavelength range. Compared to the alkaline texturing, the alkaline and RIE texturing of front Si surfaces decreased the WAR by 6%. By means of two-step texturing processes for quasi-single crystalline silicon (QSC-Si) surfaces, performance parameters of the solar cell such as conversion efficiency, short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) were examined in 18.2%, 36.6 mA/cm2, 624 mV and 79.7%, respectively.

Investigation of parasitic absorption and charge carrier recombination losses in plasmonic silicon solar cells using quantum efficiency and impedance spectroscopy

Quantum efficiency and impedance spectroscopy tools are employed for understanding the influence of parasitic absorption losses and partial field effect surface passivation by the silver nanoparticles (Ag NPs) on electrical properties of textured silicon solar cells without and with Si3N4 spacer layer. The parasitic absorption losses from Ag NPs reduced the internal quantum efficiency near the surface plasmon resonance region. The passive components like; series and parallel resistances, chemical capacitance of solar cells without and with Ag NPs are estimated after fitting impedance semicircles, which are further used for estimating effective carrier lifetime (τeff) values. Under AM1.5G illumination, cells with Si3N4 spacer layer showed a large decrease in the τeff due to the strong parasitic absorption losses from the Ag NPs. But, the cells without Si3N4 spacer layer showed a small decrease in the τeff due to the reduced surface recombination after partial field effect passivation from near-fields of Ag NPs’ surface plasmon resonances on the emitter surface.

Comprehensive Study of SF6/O2 Plasma Etching for Mc-Silicon Solar Cells**Supported by the National Natural Science Foundation of China under Grant No 61306076, and the Youth Innovation Promotion of Chinese Academy under Grant No Y510411C31.

The mask-free SF6/O2 plasma etching technique is used to produce surface texturization of mc-silicon solar cells for efficient light trapping in this work. The SEM images and mc-silicon etching rate show the influence of plasma power, SF6/O2 flow ratios and etching time on textured surface. With the acidic-texturing samples as a reference, the reflection and IQE spectra are obtained under different experimental conditions. The IQE spectrum measurement shows an evident increase in the visible and infrared responses. By using the optimized plasma power, SF6/O2 flow ratios and etching time, the optimal efficiency of 15.7% on 50 × 50 mm2 reactive ion etching textured mc-silicon silicon solar cells is achieved, mostly due to the improvement in the short-circuit current density. The corresponding open-circuit voltage, short-circuit current density and fill factor are 611 mV, 33.6 mA/cm2, 76.5%, respectively. It is believed that such a low-cost and high-performance texturization process is promising for large-scale industrial silicon solar cell manufacturing.

Wettability investigating on the wet etching textured multicrystalline silicon wafer

In order to investigate the wettability properties of multicrystalline silicon (mc-Si), the different surface structures were fabricated on the as-cut p-type multi-wire slurry sawn mc-Si wafers, such as as-cut, polished and etched in various acid solutions. The contact angles and the XRD spectra of these samples were measured. It was noted that both the surface structures and the use of surfactant, such as Tween 80, made a stronger effect on wettability of the Si wafer. Due to the lipophilic groups of Tween 80 combined with the Si atoms while the hydrophilic groups of it were outward, a lipophilic surface of Si changed into a hydrophilic one and the rougher the surface, the stronger the hydrophily. Thus, it is feasible to add an appropriate surfactant into the etching solution during black-Si wafer fabrication for solar cells. In addition, different crystal plains of Si had different dangling bond density, so that their surface energies were different. A surface with higher surface energy could attract more water atoms and its wettability was better. However, the effect of crystal plain on the surface wettability was much weaker than surface morphology.

3D optical simulation formalism OPTOS for textured silicon solar cells.

In this paper we introduce the three-dimensional formulation of the OPTOS formalism, a matrix-based method that allows for the efficient simulation of non-coherent light propagation and absorption in thick textured sheets. As application examples, we calculate the absorptance of solar cells featuring textures on front and rear side with different feature sizes operating in different optical regimes. A discretization of polar and azimuth angle enables a three-dimensional description of systems with arbitrary surface textures. We present redistribution matrices for 3D surface textures, including pyramidal textures, binary crossed gratings and a Lambertian scatterer. The results of the OPTOS simulations for silicon sheets with different combinations of these surfaces are in accordance with both optical measurements and results based on established simulation methods like ray tracing. Using OPTOS, we show that the integration of a diffractive grating at the rear side of a silicon solar cell featuring a pyramidal front side results in absorption close to the Yablonovitch Limit enhancing the photocurrent density by 0.6 mA/cm2 for a 200 µm thick cell.

High efficiency textured silicon solar cells based on an ITO/TiO2/Si MOS structure and biasing effects

In this study, the high conversion efficiency of textured silicon solar cells based on an ITO/TiO2/Si metal–oxide–semiconductor (MOS) structure and biasing effects was experimentally demonstrated. An impressive conversion efficiency of >20% was obtained through an increase in the absorption volume and through the efficient collection of the generated photo-carriers when the biasing voltage was applied on the ITO-transparent electrode. The optical reflectance, dark current–voltage (I–V), external quantum efficiency (EQE) and photovoltaic current density–voltage (J–V) under one-sun AM 1.5G illumination of the MOS structure solar cells without voltage biasing were characterized firstly. Then the photovoltaic J–V curves of the MOS structure solar cells with biasing voltages from 0 to 3V were measured and compared. Significant increases in short-circuit current and conversion efficiency were observed when the biasing voltage was increased from 0 to 3V using an external voltage power supply. In addition, the integration of a photovoltaic biasing source using a series of connected p–n junction solar cells (instead of an external voltage power supply) with a MOS structure solar cell on a ceramic platform to enhance the MOS structure solar cell efficiency was demonstrated for the first time in textured silicon solar cells.

On the potential of light trapping in multiscale textured thin film solar cells

The light propagation in multiscale textured thin film silicon solar cells is studied experimentally and numerically. The short circuit current and energy conversion efficiency of multiscale textured amorphous silicon thin film solar cells are increased compared to nanoscale textured substrates. A gain of the short circuit current of 1.3mA/cm2 is achieved for the multiscale textured solar cell, resulting in short circuit current densities of 16.8mA/cm2 and energy conversion efficiency of 10.7%. The light propagation in the solar cells is determined by Finite Difference Time Domain simulations in three dimensions using realistic interface morphologies. The realistic interface morphologies of solar cells are calculated by 3D algorithms. The optical simulations reveal that the interface morphology of the back reflector of the multiscale textured solar cell has a distinct influence on the short circuit current and quantum efficiency. By tuning the optical losses of the metal back reflector, the short circuit current can be increased beyond 18mA/cm2.

The In Vivo Pericapsular Tissue Response to Modern Polyurethane Breast Implants.

Polyurethane breast implants were first introduced by Ashley (Plast Reconstr Surg 45:421-424, 1970), with the intention of trying to reduce the high incidence of capsular contracture associated with smooth shelled, high gel bleed, silicone breast implants. The sterilization of the polyurethane foam in the early days was questionable. More recently, ethylene oxide (ETO)-sterilized polyurethane has been used in the manufacturing process and this has been shown to reduce the incidence of biofilm. The improved method of attachment of polyurethane onto the underlying high cohesive gel, barrier shell layered, silicone breast implants also encourages bio-integration. Polyurethane covered, cohesive gel, silicone implants have also been shown to reduce the incidence of other problems commonly associated with smooth or textured silicone implants, especially with reference to displacement, capsular contracture, seroma, reoperation, biofilm and implant rupture. Since the introduction of the conical polyurethane implant (Silimed, Brazil) into the United Kingdom in 2009 (Eurosurgical, UK), we have had the opportunity to review histology taken from the capsules of polyurethane implants in three women ranging from a few months to over 3 years after implantation. All implants had been inserted into virgin subfascial, extra-pectoral planes. The results add to the important previously described histological findings of Bassetto et al. (Aesthet Plast Surg 34:481-485, 2010). Five distinct layers are identified and reasons for the development of each layer are discussed. Breast capsule around polyurethane implants, in situ for fifteen and 20 years, has recently been obtained and analysed in Brazil, and the histology has been incorporated into this study. After 20 years, the polyurethane is almost undetectable and capsular contracture may appear. These findings contribute to our understanding of polyurethane implant safety, and give reasoning for a significant reduction in clinical capsular contracture rate, up to 10 years after implantation, compared to contemporary silicone implants. A more permanent matrix equivalent to polyurethane may be the solution for reducing long-term capsular contracture. This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Efficiency of pyramid textured silicon solar cell with aqueous solution deposited ZnO nanotip array

The efficiency of pyramid textured silicon solar cell with ZnO nanotip array was investigated. ZnO seed layer was first prepared on the top side of solar cell by radio frequency sputtering. Zinc nitrate and ammonium were used as precursors for the growth of ZnO nanotip array on ZnO seed layer by aqueous solution deposition at 70 °C. The efficiency of solar cell with ZnO nanotip array increases with the ZnO nanotip length, and the efficiency can be enhanced from 11.74% to 14.38%.

Morphology based statistical analysis of nanosecond pulsed laser texturing of the multicrystalline silicon

In this paper, we investigate the surface morphology of the textured multicrystalline silicon with nanosecond (ns) Nd: YVO4 laser (wavelength of 1064 nm, repetition rate of 10 KHz, and pulse duration of 14 ns). Various surface topographies have been achieved with different laser as well as irradiation parameters. The textured area average roughness and depth have been statistically analyzed through ANOVA test, which could evaluate the significance and effectiveness of the adopted design of experiment. This research work is based on three control factors: Laser fluence, laser pulse overlap percentage, and number of irradiations. The statistical assessments were conducted based on roughness and depth values measured by optical interferometry. The effect of roughness and depth on solar weighted reflectance (SWR) was analyzed and significant reduction in SWR with increase in Ra was observed. In addition, time and energy consumption, which are highly significant in the industrial applications, have been investigated.

Infrared Absorption of Femtosecond Laser Textured Silicon Under Vacuum

Chalcogen-doped microstructural silicon irradiated by femtosecond laser has high near-uniform absorption on a broad spectrum, but the factors leading to infrared absorption are complex and remain an open problem. To clarify the origin of infrared absorption besides hyperdoped Chalcogen atoms, microstructural silicon is fabricated by femtosecond laser under vacuum condition. The relationship between infrared absorption and as-formed new phases (amorphous silicon: $\alpha $ -Si and nanocrystal silicon) is established. It indicates that the infrared absorption is caused by defects related to Urbach states from $\alpha $ -Si or nanocrystal Si, and these metastable defects disappeared after a thermal annealing process. From the absorption spectrum of microstructural silicon after etching at different times, it could be figured out that the Urbach states exist in both the surface and subsurface regions of black silicon at a depth of about 2.4 $\mu \text{m}$ .

Recurrent Systemic Anaplastic Lymphoma Kinase–Negative Anaplastic Large Cell Lymphoma Presenting as a Breast Implant–Associated Lesion

A woman aged 48 years presented with fevers, chills, weight loss, and night sweats. She had significant lymphadenopathy of the left neck as well as the left axilla. Her history was significant for bilateral breast augmentation with textured silicone implants more than 25 years ago. Excisional biopsy of a cervical lymph node revealed large, atypical cells positive for CD4 and CD30 and negative for Epstein-Barr virus-encoded ribonucleic acid, CD2, CD3, CD5, CD7, CD8, CD15, CD20, pan-keratin, S100, anaplastic lymphoma kinase (ALK), and paired box 5. These findings were consistent with Ann Arbor stage IIIB ALK-anaplastic large cell lymphoma (ALCL). The patient was started on 6 cycles of cyclophosphamide, doxorubicin, vincristine, and prednisone. She initially had no signs or symptoms of breast involvement; however, after developing seroma during the clinical course, the patient underwent capsulectomy and removal of the intact, textured silicone implants. Pathological evaluation demonstrated ALK-ALCL in the left breast capsule with cells displaying a significant degree of pleomorphism with binucleated forms and numerous mitoses. Fluorescence in situ hybridization confirmed the tumor was negative for t(2;5). She presented 8 weeks later showing evidence of recurrent systemic disease.

Impact of Texture Roughness on the Front-Side Metallization of Stencil-Printed Silicon Solar Cells

Realizing narrow contact fingers with low lateral resistance is a major goal for the front-side metallization of silicon solar cells. The formation of screen- or stencil-printed contact fingers is governed by a variety of influencing factors. One of these factors is the surface roughness of the textured silicon wafer. However, only a few investigations have been carried out to investigate this impact in detail. In this study, the influence of arithmetical mean roughness $R_{{\rm a}}$ of four differently textured wafer surfaces on contact finger geometry and lateral finger resistance, as well as optical and electrical losses, has been investigated. It will be shown that texture roughness has a considerable impact on the properties of the front-side grid. Narrower contact fingers could be realized on the smoothest texture, leading to a current density gain of $\Delta j_{{\rm sc}} = +0.27$ mA/cm2. On the other hand, increasing texture roughness has affected the amount of transferred paste and, thus, has led to a lower lateral finger resistance R L. Thus, contact fingers on the roughest texture have benefited from a fill factor gain of Δ FF = +0.24 %abs. A sensitivity analysis of both impacts has shown that the current density gain has overcompensated the fill factor loss. Thus, textures with a small roughness are beneficial with respect to the formation and electrical properties of stencil-printed front-side grids.

Antireflection effect of SiO2 thin film on the pyramidal textured surface of monocrystalline silicon

Surface antireflection structure is important for high-efficiency silicon solar cells by improving the light trapping. This paper demonstrated the effect of the surface SiO2 thin film on the antireflection properties of the pyramidal textured monocrystalline silicon wafer. The pyramidal texture was etched by the mixture solution of Na2SiO3 and Na3PO4, and the SiO2 film was generated by thermal oxidation. After the pyramidal texture was produced on the polished silicon wafer the average reflectance within the wavelength range of 400–900nm reduced from 30% to 15.6%, while it could be lowered further by surface SiO2 thin film. After oxidized in oxygen at 1000°C for 30min, the sample with 111.9nm-thicked SiO2 film exhibited an average reflectance of 3.8% in the visible spectrum.