Random Pyramid Texture Research Articles

Impact of Texture Height and Ozone-Based Rounding on Silicon Heterojunction Solar Cells Performance

In this study, the effect of ozone-based rounding was investigated for three random-pyramid texture height ranges for Silicon Heterojunction (SHJ) at the cell and module levels. In a first experiment, 5 ozone-based rounding process times (0, 2, 4, 6, 8 and 10 minutes) were applied to two texture-height ranges (Small and Medium) to observe variations in the reflection response of the wafers. In a second experiment, 3 ozone-based rounding treatment times (0, 2 and 6 minutes) were applied to three texture-height ranges (Small, Medium and Large) and to observe the results both on the cell and on the module levels. The first experiment has shown that ozone-based rounding increases reflection and has a more pronounced effect for smaller than larger sized texture-heights. An increase in the minimum wavelength of reflection is seen after the Transparent Conductive Oxide (TCO) deposition, indicating that shorter deposition times can be used. The results of the second experiment show that, even though the Short-Circuit Current (Jsc) decreases for the three textures with rounding time, it is compensated by an increase in the pseudo-Fill-Factor (pFF). A maximum is seen at the cell level for 2 minutes of rounding for the small and large texture sizes, but which is leveled at the module. In conclusion, these results indicate that 2 minutes of ozone-based rounding can be beneficial for the cell’s final efficiency as the improvement in passivation still compensates the optical losses resulting in a lower Jsc.

Comparison of random upright pyramids and inverted pyramid photonic crystals in thin crystalline silicon solar cells: An optical and morphological study

The potential of enabling a wide range of applications with lightweight, flexible, thin silicon photovoltaic (PV) devices has led to research interest in various light-trapping technologies for thin silicon absorbers. Herein, using both experiments and photonic modelling, we present a detailed comparison between random pyramid texturing and inverted pyramid photonic crystal patterning in thin silicon vis-à-vis light trapping. In particular, we investigate the potential of uniform single-dimension periodic structures in contrast to conventional multi-dimensional random structures for high broadband absorption efficiency in ultra-thin silicon. We find that inverted pyramid photonic crystals cause a large increase in optical path length especially for long wavelength photons due to strong wave interference producing exceptionally high absorption and so does random pyramids though with slightly lower absorption than that of photonic crystals. On the other hand, the random pyramids, fabricated via a one-step etching process with feature sizes ranging from sub-micron to 4 µm, have higher absorption at short wavelengths due to their multi-dimensional size distribution. Overall, we find that random textured pyramids appropriately fabricated can yield a comparable ideal photocurrent density to that by photonic crystals for 10 µm thick silicon.

Ultrafast Random‐Pyramid Texturing for Efficient Monocrystalline Silicon Solar Cells

Herein, an ultrafast random‐pyramid texturing process is proposed for monocrystalline silicon (mono‐Si) solar cells by combining metal‐catalyzed chemical etching (MCCE) and the standard alkaline texturing process. Namely, large numbers of artificial defects are introduced on the wafer surface in 3 min by MCCE; therefore, the process duration of alkaline texturing is largely shortened from 420 s for the as‐cut wafer to 180 s for the wafer with artificial defects due to its high surface reactivity. Moreover, those tiny artificial defects are apt to form small pyramids, resulting in a better light‐trapping performance. As a demonstration, the passivated emitter rear contact solar cell with ultrafast random pyramid texture achieves a power conversion efficiency of 23.02%. Therefore, such a cost‐effective ultrafast texturing strategy can open a promising new route toward the mass production of high‐efficiency industrial mono‐Si solar cells.

Light scattering from black silicon surfaces and its benefits for encapsulated solar cells

Black silicon (b-Si) has been widely investigated as a potential replacement for more traditional antireflective schemes for silicon solar cells, such as random pyramids, due to its reduced broadband reflectance and improved light-trapping properties. Wavelength and angle resolved scattering (WARS) reflectance measurements provide the means of analysing the amount of light scattered from a textured surface, which can be of interest when considering the amount of light trapped through total internal reflectance (TIR) at various interfaces in an encapsulated photovoltaic module. Here we present and analyse results from WARS measurements on b-Si surfaces fabricated using metal assisted chemical etching (MACE). Large angle scattering is observed for the entire spectrum, increasingly so for shorter incident wavelengths and increasing height of texture features. This is predicted to result in 35–40% of the reflected light being trapped by TIR at the glass-air interface and redirected back onto the sample, when the sample is encapsulated in standard PV module materials. This leads to a calculated additional boost of up to 0.45% in the photogenerated current of an encapsulated black silicon solar cell. This exceeds the calculated 0.21% boost due to TIR predicted for an encapsulated solar cell employing the industry-standard random pyramid texture with a thin film antireflective coating. • Black silicon nanostructures are fabricated for antireflection and light-trapping. • Nanostructures exhibit broadband reduced hemispherical reflectance below 3%. • Custom setup is used to measure wavelength and angular position of reflections. • Portion of reflected light trapped at various solar cell interfaces exceeds 40%. • Photocurrent relative gain of 0.45% is calculated for multiple nanostructure heights.

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.

Application of a Genetic Algorithm in Four-Terminal Perovskite/Crystalline-Silicon Tandem Devices

Perovskite/crystalline-silicon (c-Si) tandem solar cells offer a viable roadmap for reaching power conversion efficiencies beyond 30%. In this configuration, however, the silicon cell now mainly receives an infrared rich illumination spectrum where the absorption coefficient of silicon is poor. To boost the light absorption in this wavelength interval, transmitted through the top cell, a solution is to introduce a dedicated nanoscale texture on the front side of the silicon bottom cell and tweaking the optical elements in its design. These optical elements are manifold, with tunable geometrical dimensions, layer thicknesses, and refractive indices. For optically optimizing nanostructured silicon solar cells, electromagnetic wave solving methods, such as the rigorous coupled-wave analysis (RCWA), are normally used but they become computationally infeasible when several design variables are involved in an optimization problem. In this work, a natural selection algorithm, known as the genetic algorithm, is coupled with RCWA for extracting the optimal values of various design parameters in four-terminal perovskite/c-Si tandem devices in parallel. Both two-side-contacted and interdigitated back-contacted silicon heterojunction cell structures, featuring an inverse nanopyramid grating texture on the front, are optimized for five interdependent variables using a genetic algorithm for the bottom cell application and benchmarked against their random pyramid textured analogs. Our study shows that an optimized inverse nanopyramid grating texture can outperform the standard random pyramid texture when considering incidence angle variations. More importantly, the study illustrates how a genetic algorithm can support modeling complex solar cell structures with numerous degrees of freedom.

Excellent ONO passivation on phosphorus and boron diffusion demonstrating a 25% efficient IBC solar cell

AbstractThis work presents results of a laboratory‐scale interdigitated back contact (IBC) solar cell with an independently measured efficiency of 25.0%, featuring open‐circuit voltage of 716 mV, short‐circuit current of 43.0 mA.cm−2 and fill factor of 81.0%. Notably, the high efficiency was achieved based on significant improvements resulting from the optimised cell structure, excellent SiO2‐SiNx‐SiOx (ONO) surface passivation, detailed bulk lifetime management strategy and improved random pyramid texturing. Experimental details and analysis of the individual improvements over prior work are presented in‐depth.

Infrared Absorption Enhancement Using Periodic Inverse Nanopyramids in Crystalline-Silicon Bottom Cells for Application in Tandem Devices

Carefully tailored periodic nanostructures on the light wavelength scale, such as diffraction gratings, benefit from wave optics for efficiently trapping the weakly absorbing infrared photons in crystalline-silicon (c-Si) absorbers. In contrast with the conventional random pyramid texture, diffraction gratings can be designed to target specific wavelength ranges by the selection of the grating pitch. Absorption enhancement at infrared wavelengths in a silicon solar cell is especially desired when it operates below a perovskite top cell in a tandem device. In this article, inverse nanopyramid gratings of 800 nm pitch are proposed as an alternative front-surface texture to random pyramids in silicon heterojunction devices with interdigitated back contacts that are to be used as bottom cells in four-terminal perovskite/c-Si tandem devices. By doing so, we report a short-circuit current density gain of 0.53 mA/cm2 with respect to the random pyramid texturing for the bottom c-Si cell. The rationale to substitute random pyramids by inverse nanopyramid gratings is, however, not justified in single-junction operation despite achieving the power conversion efficiency of 22.3% since the degraded optical performance at shorter wavelengths offsets the absorption enhancement at longer wavelengths, resulting in similar levels of short-circuit current densities for both texture types.

Pyramid texturing on silicon solar cells traps light more efficiently

A comparison of idealized and real pyramid textures in silicon solar cells demonstrates that a truly random pyramid texture improves light trapping efficiency.

Periodic inverse nanopyramid gratings for light management in silicon heterojunction devices and comparison with random pyramid texturing

Downscaling the front-side texture from the micron-scale to the nanoscale introduces new possibilities for light management in solar cells and is considered as an opportunity for further reducing g/Wp without compromising on power conversion efficiency. We report on the fabrication and characterisation of periodic inverse nanopyramid textured silicon heterojunction devices on a range of substrate thicknesses for identifying the constraints that could limit the performance of nanostructured cells. The devices are benchmarked against the industry standard random pyramid texture. While the open-circuit voltage is comparable and demonstrates that effective surface passivation can be achieved with inverse nanopyramid textured devices, the device efficiencies are hampered by considerable reflection losses in the 400–800 nm wavelength range. We deduce that these reflection losses arise due to constraints that can be both processing-related or intrinsic to the nanotexture and amount to a short-circuit current density (JSC) loss of 2.2 mA/cm2 on 125 μm thick substrates. Experimental constraints include the imperfect nanopatterning on rough surfaces, the non-conformal deposition of sputtered ITO antireflection coating (ARC) on nanopyramids and the non-ideal area filling fraction of the pattern. The intrinsic limitation comes from the diffraction effect of periodic nanopyramids, whose pitch determines the targeted wavelength for reflectance minima. Moreover, parasitic absorption in ARC significantly lowers the JSC of our cells for both texturing schemes. Therefore, despite being able to effectively passivate, the identified processing-related constraints regarding nanotextured cells will need to be overcome for attaining performance that is equivalent with the random pyramid textured cells.

20%-efficient epitaxial GaAsP/Si tandem solar cells

We present epitaxial 1.7 eV/1.1 eV GaAs0.75P0.25/Si tandem cells with an NREL-certified efficiency of 20.0%, enabled by a thermally stable tunnel junction interconnect along with a hydrogenated amorphous Si (a-Si:H) carrier-selective contact for the Si bottom cell. Building on these promising tandem results, we also demonstrate a 16.5%-efficient GaAs0.75P0.25 single-junction top cell on Si and a 7.78%-efficient GaAs0.75P0.25-filtered Si bottom cell (both NREL-certified) with improved short-circuit current densities. The quantum efficiency of the GaAs0.75P0.25 single-junction top cell on Si is boosted across the whole wavelength range due to the use of a higher growth temperature, indicating an improved minority-carrier diffusion length. The implementation of random pyramid texturing at the Si back surface enables improved quantum efficiency at wavelengths of 900–1200 nm, corresponding to an increase of 1.42 mA/cm2 in short-circuit current density. The improved short-circuit current densities of the sub-cells together show a pathway to >23% efficiency in a two-terminal tandem configuration.

Optical properties of a random inverted pyramid textured silicon surface studied by the ray tracing method

The random inverted pyramid texture has been extensively studied experimentally in high-efficiency monocrystalline silicon solar cells due to its superior optical properties. In this paper, a random inverted pyramid structure model was established, and the optical performance was studied by the ray tracing method. It has five common light paths and can achieve lower reflectance than random upright pyramids. To further analyze the optical properties of random inverted pyramids, a simplified random inverted pyramid model, which consists of two overlapping inverted pyramids, was studied and analyzed in detail. The proportions of the light path related to different overlapping regions, and the reflectance ranging from 10.11% to 10.64%, can be obtained, which is lower than the reflectivity of random upright pyramids. We believe that the random inverted pyramid texture can have a wide range of applications in high-efficiency monocrystalline silicon solar cells.

Improved Ray Tracing on Random Pyramid Texture via Application of Phong Scattering

This paper presents an experimental investigation into the light trapping within silicon wafers textured by upright random pyramids. In prior studies, the conventional approach to simulating random pyramids agrees poorly with experimental data at wavelengths greater than 1000 nm in which internal light trapping becomes significant. In this work, we show that expanding the optical simulation to include Phong scattering (with an exponent of 25) improved the agreement between simulation and measurement in regard to hemispherical reflectance, transmittance, and absorptance over the range of 1000–1200 nm. The best-fit Phong exponent was not significantly altered by substrate waviness, structure symmetry, or deposition of a dielectric layer. Thus, the value of the Phong exponent determined in this study is applicable to a variety of different samples textured with upright random pyramids. In addition, the simulated angular reflectance at 532 nm shows a closer agreement with the experimental results in literature when Phong scattering is included.

How cell textures impact angular cell-to-module ratios and the annual yield of crystalline solar modules

Two emerging trends in multicrystalline silicon cell texturing are plasma texturing, and metal catalyzed chemical etching. Both processes roughen silicon surfaces in order to increase light absorption. These processes are attractive as they are applicable to diamond-wire sawn wafers. This work investigates the optical properties of these surfaces, and other conventionally textured surfaces like isotropic acidic and random pyramid textures, are investigated for cells in air and after encapsulation for a large range of angles of incidence. We find that the angular optical performance in air varies strongly with cell texture, but when embedded in a module structure these variations are significantly mitigated: the advantages of a high angular absorption of solar cells are not fully transferred to the module level. This is especially notable for plasma etched, black silicon cell structures which suffer comparatively from poorer index matching and light recycling inside a module structure. The losses caused explicitly by the module embedding (described in the cell to module ratio) are in the range of 1–5% for perpendicular incoming light, and increase to 6–15% at an angle of incidence of 70°. Based on these angular performances, we calculate the annual yield of the modules and find that it varies by less than 2% for the cell textures. Nevertheless, the annual optical yield for the black silicon cell structures are the highest, whereas the metal catalyzed chemical etching cell structures show the lowest performance. Further, we find that different annual distributions of the incoming light at the two investigated locations (Melbourne and Alice Springs) only impact the relative performance of the cell textures at high module tilt angles.

Honeycomb micro-textures for light trapping in multi-crystalline silicon thin-film solar cells.

The liquid phase crystallization (LPC) of silicon is an emerging technology for fabricating 10 - 20 µm thin multi-crystalline silicon layers on glass. LPC silicon solar cells exhibit similar electronic performance to multi-crystalline wafer-based devices. Due to the reduced absorber thickness, however, effective measures for light trapping have to be taken. We present tailor-made micro-structures for light trapping at the LPC silicon back-side, whereby a nano-imprinted resist layer serves as a three-dimensional etching mask in subsequent reactive ion etching. Contrary to state-of-the-art random pyramid textures produced by wet-chemical etching, this method allows to produce tailor-made textures independent of grain orientation. Differently shaped micro-textures were replicated in LPC silicon. Absorptance and external quantum efficiency of periodic honeycomb patterns and random pyramids were found to be equivalent. Thus, the method enables the potential to further optimize light trapping in LPC silicon solar cells.

Al+Si Interface Optical Properties Obtained in the Si Solar Cell Configuration

Al is a commonly used material for rear side metallization in commercial silicon (Si) wafer solar cells. In this study, through‐the‐silicon spectroscopic ellipsometry is used in a test sample to measure Al+Si interface optical properties like those in Si wafer solar cells. Two different spectroscopic ellipsometers are used for measurement of Al+Si interface optical properties over the 1128–2500 nm wavelength range. For validation, the measured interface optical properties are used in a ray tracing simulation over the 300–2500 nm wavelength range for an encapsulated Si solar cell having random pyramidal texture. The ray tracing model matches well with the measured total reflectance at normal incidence of a commercially available Si module. The Al+Si optical properties presented here enable quantitative assessment of major irradiance/current flux losses arising from reflection and parasitic absorption in encapsulated Si solar cells.

Optical performance of the honeycomb texture – a cell and module level analysis using the OPTOS formalism

A comprehensive analysis of the optical properties of different surface textures for silicon solar cells must include (i) the optical performance at cell level as well as encapsulated to a module stack and (ii) a varying angle of incidence of the solar irradiation, as it influences the optical yield. Within this work, the OPTOS formalism (Optical Properties of Textured Optical Sheets) was used to compare a honeycomb texture featuring a high aspect ratio with state of the art random pyramids and the isotexture. The honeycomb texture exhibits an increased absorbed photocurrent density of 2.3mA/cm2 before and 0.7mA/cm2 after encapsulation compared to the isotexture for a 180µm thick solar cell. The simulated absolute value of 42.3mA/cm2 after encapsulation is even superior to the random pyramid texture with 41.5mA/cm2. Furthermore, a simulation-based optical yield analysis reveals that the angular irradiance of three exemplary terrestrial locations plays a minor role for the relative performance between the textures compared to influence of encapsulation effects.

Improved light management in planar silicon and perovskite solar cells using PDMS scattering layer

Several developing solar cell technologies—including perovskite cells, thin-film cells, epitaxially grown cells, and many tandem cells on silicon—have fabrication constraints that require a planar front surface. However, flat front surfaces result in large reflection losses and poor light trapping within the cell. We investigate scattering layers made from polydimethylsiloxane (PDMS) polymer carrying a random-pyramid texture to reduce such losses. The layers are first tested on a model system consisting of silicon heterojunction solar cells that have zero, one, or two surfaces textured with the same random pyramids (the other surfaces being planar) in order to elucidate the potential and limitations of employing a textured transparent layer instead of a textured absorber. PDMS layers result in short-circuit current density enhancements of 3.0mA/cm2 and 1.7mA/cm2 when applied to the front of a cell with flat front and rear surfaces, and a cell with a flat front surface and textured rear surface, respectively. Optical simulations reveal that the majority of the gain is due to a reduction in front-surface reflection and that the layers contribute only marginally to trapping weakly absorbed infrared light; nevertheless, a cell with a textured rear surface and a PDMS layer at its flat front surface can come to within 0.7mA/cm2 of the performance of a double-side-textured cell. Finally, a PDMS scattering layer is implemented in a planar perovskite solar cell, boosting its short-circuit current density by 1.9mA/cm2 and thus its efficiency by 10.6% relative.

Small pyramidal textured ultrathin crystalline silicon solar cells with double-layer passivation.

Ultrathin crystalline silicon solar cells are a promising technology roadmap to achieve more cost effectiveness. However, experimental reports on ultrathin crystalline silicon cells with thickness less than 20 µm are rare. Here, we experimentally fabricate and investigate ultrathin monocrystalline silicon solar cells consisting of 16 µm-silicon base thickness and low-cost front random pyramidal texture with the feature size of 1-2 µm. The normalized light absorption is calculated to explain the measured external quantum efficiency. The achieved efficiency is 15.1% for the single-layer passivated textured solar cell. In addition, via double-layer passivation of Al2O3/SiNx, the efficiency is further increased to 16.4% for the best textured cell, which significantly improves the absolute efficiency with Δη = 1.3%.

Etching SiC‐slurry and diamond wire‐sawn silicon wafers with HF‐HCl‐Cl2 mixtures: Parameter influences on etch rates and surface structures

Solutions for the wet chemical treatment of silicon wafer surfaces were investigated using mixtures which are based on hydrofluoric acid (HF), hydrochloric acid (HCl), and chlorine (Cl2). We used a DoE‐test plan (Design of Experiments) to evaluate the effects of five selected parameters: concentrations of HF and HCl, gas flow rate of Cl2, stirring, and saw type of the wafer material. High etch rates of up to 0.63 μm min−1 were observed at room temperature, which are comparable to the etch rates of KOH‐IPA solutions. The silicon surface was investigated by reflectivity measurements and scanning electron microscopy (SEM), indicating pyramidal textured and polished surfaces for diamond wire‐ and SiC‐slurry‐sawn wafers. Using an optimized parameter set, random inverted pyramidal surface structures are formed. These random inverted structures show a significant increase in light absorption compared to standard random upright pyramid textures, for example produced by KOH‐IPA solutions.