Ultrathin Crystalline Silicon Solar Cells Research Articles

Effects of frequency upconversion layer and two-dimensional arrays of inverted nanocone hole on the performance of ultrathin crystalline silicon solar cells

We report the design and optimization of photonic crystal (PhC) structures within the crystalline silicon absorption layer in ultrathin solar cells with a frequency upconversion layer. In the PhC structure, inverted nanocone hole (ICH) arrays with different diameters and inclination angles are used, and the light-trapping performances of air and Si3N4 filled ICH arrays are systematically studied by COMSOL Multiphysics, respectively. The simulation results indicate that the average optical absorption of the solar cells and the local electric field intensity in the upconversion layer can be greatly improved by using PhC structures within the absorption layer. For ICH in height of 1.6 μm, the maximum absorptivity of the solar cells with air filled ICH arrays is 90.75%, while the maximum absorptivity of solar cells with Si3N4 filled ICH arrays is 85.62%. In addition, the upconversion effects of β-NaYF4:Er3+/Yb3+ nanomaterials on the power conversion efficiency of the solar cells were also investigated. The presented model will be the basis for further preparations of high-efficiency ultrathin crystalline silicon solar cells.

Optical optimization of ultra-thin crystalline silicon solar cells by a co-simulation approach of FEM and GA

Optical optimization of ultra-thin crystalline silicon solar cells by a co-simulation approach of FEM and GA

Enhanced light absorption of ultrathin crystalline silicon solar cells via the design of front nanostructured silicon nitride

Enhanced light absorption of ultrathin crystalline silicon solar cells via the design of front nanostructured silicon nitride

Moth-eye nanostructure PDMS films for reducing reflection and retaining flexibility in ultra-thin c-Si solar cells

Ultra-thin crystalline silicon (c-Si) solar cells have the advantages of flexibility and light transmittance. The surface etched texture structure for reducing reflectivity usually sacrifices the flexibility of the solar cells. In this work, a kind of polydimethylsiloxane (PDMS) film with Moth-eye nanostructure was introduced to the surface of flexible ultra-thin c-Si solar cells, which was proved to enhance the flexibility of the solar cells besides reducing reflectivity and improving conversion efficiency. The finite element method was used to study the photoelectric and mechanical properties of the solar cells, the results show that the PDMS film has good flexibility to protect the ultra-thin c-Si surface, resulting in a 33.4% reduction in surface stress. At the same time, the refractive index of PDMS film with Moth-eye nanostructure has gradient change in the direction of incident light and is between that of air and c-Si. The PDMS film with Moth-eye nanostructure, as the antireflective layer on c-Si surface, reduces the reflectivity by 46.4% and improves conversion efficiency of ultra-thin c-Si solar cells by 42.3%. This work provides a method for other flexible solar cells to improve flexibility and reduce reflectivity.

Corrigendum to “Free-standing 2.7 μm thick ultrathin crystalline silicon solar cell with efficiency above 12.0%” Nano Energy, volume 70, April 2020, 104466

Corrigendum to “Free-standing 2.7 μm thick ultrathin crystalline silicon solar cell with efficiency above 12.0%” Nano Energy, volume 70, April 2020, 104466

Free-standing 2.7 μm thick ultrathin crystalline silicon solar cell with efficiency above 12.0%

In this paper, a record-breaking efficiency of 12.3% is experimentally demonstrated for a flexible free-standing, 2.7-μm-thick ultrathin crystalline silicon (c-Si) solar cell, which is the highest ever-reported sub 3 μm c-Si solar cells. The first breakthrough of this study is achieving an exceptionally small exposure feature size of 350 nm using photolithography stepper without any additional stringent requirements on wafer flatness or depth of focus (DOF) of the projection system. This small exposure feature size enables the fabrication of a light-trapping scheme: a design of close-packed 700-nm-period touching inverted-pyramid nanostructures for enhanced light absorption. Furthermore, to reduce contact recombination, this work is also first to report the integration of nickel oxide (NiOx) as hole-selective contacts in conjunction with the nano-structure light trapping scheme on a free-standing ultrathin cell. The efficacy of these two mechanisms to enhance ultrathin c-Si cell efficiency is characterized in terms of the short circuit current density and open circuit voltage. The free-standing cell also shows good mechanical flexibility and compliance. In addition to demonstrating silicon-compatible fabrication, this work shows the potential of ultrathin c-Si technology to not only reduce material cost, but also maintain competitive device performance.

Absorption enhancement of ultrathin crystalline silicon solar cells with frequency upconversion nanosphere arrays

A design of ultrathin crystalline silicon solar cells patterned with α-NaEr0.2Y0.8F4 upconversion nanosphere (NSs) arrays on the surface was proposed. The light trapping performance of α-NaEr0.2Y0.8F4 NSs with different ratios of sphere diameter to sphere pitch was systematically studied by COMSOL Multiphysics. The influence of different NS diameters and ratio to the average optical absorption of ultrathin crystalline silicon solar cell was calculated, as well as the short circuit current densities. The results show that the average optical absorption of solar cells with 2.33 μm silicon covered by α-NaEr0.2Y0.8F4 NSs of 100 nm in diameter and 5.2 in ratio has improved by 8.5% compared to planar silicon solar cells with the same thickness of silicon. The light trapping performance of different thicknesses of silicon solar cells with the optimized configuration of NSs was also discussed. The results indicate that our structure enhances the light absorption. The presented model will be the basis for further simulations concerning frequency upconversion of α-NaEr0.2Y0.8F4 materials.

Absorption Enhancement of Ultrathin Crystalline Silicon Solar Cells with Dielectric Si3N4 Nanostructures**Project supported by National Science Foundation of China (NSFC) (U1765105, 61604087); The Hebei Provincial Young Top-notch Talent Support Program (BJRC2013); Alexander von Humboldt-Stiftung (AUS-1141939-HFST-E)

A design of ultrathin crystalline silicon solar cell with Si3N4 circular truncated cone holes (CTCs) arrays on the top is proposed. In this article, we perform an optical simulation of the structure. The finite-difference time-domain method is used to calculate the optical absorption of different periods, radius of top and bottom circles and depth of Si3N4 CTCs. The short-circuit current density generated by the optimized cells (30.17 mA/cm2) is 32.44% more than the value gained by control group (with flat Si3N4). Then adding a layer of back silver to allow us to better analyze optical absorption. Later, we simulate the optimization of the same configuration of different silicon thicknesses and find that our structure does enhance the light absorption. This work uses a combined path towards achieving higher photocurrent ultrathin crystalline silicon solar cells by constructing the texture of anti-reflection coating.

Double-sided pyramid texturing design to reduce the light escape of ultrathin crystalline silicon solar cells

Reducing the light absorption loss of ultrathin crystalline silicon (c-Si) solar cells is significant to achieve high photocurrent density and photoelectric conversion efficiency. Here, we designed and simulated ultrathin c-Si cells with front pyramids and double-sided pyramids. By adjusting the shape of pyramids, the maximum photocurrent densities reach 36.23 and 37.71 mA/cm2 for the cells with front pyramids and double-sided pyramids, respectively. The reflectivity spectrum indicates that the double-sided pyramidal architecture remarkably suppresses light escape and then enhances the light absorption in long wavelength range, which makes the absorption approach the Yablonovitch limit. The calculated conversion efficiencies of planar, front and double-sided textured cells are 16.94%, 19.65% and 20.45% respectively. Additionally, the difference between randomly and periodically textured cells was investigated and the results show that although the randomly front pyramid texture has a better light absorption in the range of 900–1200 nm, the periodically double-sided pyramids texture exhibit almost the same light absorption in the whole range as the random one. Besides, the solar cells with double-sided pyramids show extremely small angular dependence of incident light. Thus, the double-sided light trapping structure designed in the present work provides an alternative pathway to improve the performance of ultrathin c-Si cells.

Geometrical optimisation of core-shell nanowire arrays for enhanced absorption in thin crystalline silicon heterojunction solar cells.

Background: Elongated nanostructures, such as nanowires, have attracted significant attention for application in silicon-based solar cells. The high aspect ratio and characteristic radial junction configuration can lead to higher device performance, by increasing light absorption and, at the same time, improving the collection efficiency of photo-generated charge carriers. This work investigates the performance of ultra-thin solar cells characterised by nanowire arrays on a crystalline silicon bulk.Results: Proof-of-concept devices on a p-type mono-crystalline silicon wafer were manufactured and compared to flat references, showing improved absorption of light, while the final 11.8% (best-device) efficiency was hindered by sub-optimal passivation of the nanowire array. A modelling analysis of the optical performance of the proposed solar cell architecture was also carried out. Results showed that nanowires act as resonators, amplifying interference resonances and exciting additional wave-guided modes. The optimisation of the array geometrical dimensions highlighted a strong dependence of absorption on the nanowire cross section, a weaker effect of the nanowire height and good resilience for angles of incidence of light up to 60°.Conclusion: The presence of a nanowire array increases the optical performance of ultra-thin crystalline silicon solar cells in a wide range of illumination conditions, by exciting resonances inside the absorber layer. However, passivation of nanowires is critical to further improve the efficiency of such devices.

Frontside scattering structures for enhanced performance in flexible ultrathin crystalline silicon solar cells

We demonstrate that frontside scattering structures combining a metal nanomesh transparent electrode with dielectric nanosphere (NS) arrays may improve the performance of ultrathin crystalline silicon (c-Si) solar cells. The increased light scattering as characterized by increased haze from these structures leads to longer path lengths within the c-Si and thus, higher short-circuit current densities and improved power conversion efficiencies. We demonstrate a 69% improvement in power conversion efficiency with metal nanomesh/NS coatings compared to indium tin oxide. Furthermore, we demonstrate the ultrathin film c-Si solar cells are robust under repeated bending.

Photocurrent enhancement for ultrathin crystalline silicon solar cells via a bioinspired polymeric nanofur film with high forward scattering

Ultrathin crystalline silicon (c-Si) solar cells represent an important technology development direction for reducing material usage and thus lowering the cost of solar electricity. Here, we demonstrate a prototype device with high efficiency assisted by a micro-/nano-structured polycarbonate thin nanofur film inspired by water ferns. The ultrathin c-Si solar cells consist of a 17 μm-thick c-Si absorber and exhibit an efficiency of up to 17.3% and short-circuit current density (Jsc) of 35.8 mA cm−2. With the assistance of the thin nanofur, the efficiency is increased to 18.1% due to an increase in Jsc to 37.4 mA cm−2. The photocurrent enhancement is attributed to, firstly, a more favorable refractive index transition due to the polymer addition and, secondly, the high forward scattering of the structured film, which increases the optical pathlength within the ultrathin absorber layer of the devices. To our knowledge, the present prototype device demonstrates the highest Jsc and efficiency in the area of ultrathin c-Si solar cells with an absorber layer of less than 20 µm.

Enhancement of light trapping for ultrathin crystalline silicon solar cells

The discussion of ultrathin crystalline silicon solar cell (UCSC) thickness and light enhancement patterns has long been a practical topic in its field with the objective of achieving optimum efficiency. In this work, we distinguish between the effects of the top and bottom structural designs on the light absorption of ultrathin crystalline-silicon solar cell. Nanopyramid (NP) front surface and distributed Bragg Reflector (DBR) bottom layer as enhancement agent was combined and its relation with thickness of solar cell was studied thoroughly. The relationship of photonic interaction between the top and bottom surfaces is also examined. Based on our simulation results, the optimized cell architecture consists of a 4 μm-thick film combined with nanopyramid of 800 nm period and bottom reflector of 4 pair DBRs. The maximum achievable photo-current density of 36.72 mA/cm2 was obtained, which is among the best results of prior research for UCSC.

Limitation of Optical Enhancement in Ultra-thin Solar Cells Imposed by Contact Selectivity

Ultra-thin crystalline silicon (c-Si) solar cell suffers both from poor light absorption and minority carrier recombination at the contacts resulting in low contact selectivity. Yet most of the research focuses on improving the light absorption by introducing novel light trapping technique. Our work shows that for ultra-thin absorber, the benefit of optical enhancement is limited by low contact selectivity. Using simulation we observe that performance enhancement from light trapping starts to saturate as the absorber scales down because of the increase in probability of the photo-generated carriers to recombine at the metal contact. Therefore, improving the carrier selectivity of the contacts, which reduces the recombination at contacts, is important to improve the performance of the solar cell beyond what is possible by enhancing light absorption only. The impact of improving contact selectivity increases as the absorber thickness scales below 20 micrometer (μm). Light trapping provides better light management and improving contact selectivity provides better photo-generated carrier management. When better light management increases the number of photo-generated carriers, better carrier management is a useful optimization knob to achieve the efficiency close to the thermodynamic limit. Our work explores a design trade-off in detail which is often overlooked by the research community.

Design and analysis of an ultra‐thin crystalline silicon heterostructure solar cell featuring SiGe absorber layer

Here, the authors studied a silicon-germanium (Si 1-x Ge x ) absorber layer for the design and simulation of an ultra-thin crystalline silicon solar cell using Silvaco technology computer-aided design. Seeking ways to design and fabricate solar cells using 100 μm thicker silicon substrates is the subject of intense research efforts among the photovoltaic (PV) community. The aim is to further reduce the substrate thickness to 20 μm without compromising the efficiency of the solar cell. A thin layer of SiGe film with the Ge composition of 15% has been introduced in this work that assists in absorbing the longer wavelength of the sunlight spectrum. The effects of the doping concentration and absorber layer thickness on the conversion efficiency have been examined. The simulated results exhibited significant enhancement in the sunlight absorption as compared to the reference structure based on crystalline silicon. The highest efficiency of 16.8% with an overall solar cell thickness of ~26 μm has been observed. The proposed heterostructure solar cell design will support the industrial development of an efficient, low-cost, shorter energy payback time, and light-weight PV technology for its widespread implementation.

Contact Selectivity Engineering in a 2 μm Thick Ultrathin c-Si Solar Cell Using Transition-Metal Oxides Achieving an Efficiency of 10.8.

In this paper, the integration of metal oxides as carrier-selective contacts for ultrathin crystalline silicon (c-Si) solar cells is demonstrated which results in an ∼13% relative improvement in efficiency. The improvement in efficiency originates from the suppression of the contact recombination current due to the band offset asymmetry of these oxides with Si. First, an ultrathin c-Si solar cell having a total thickness of 2 μm is shown to have >10% efficiency without any light-trapping scheme. This is achieved by the integration of nickel oxide (NiOx) as a hole-selective contact interlayer material, which has a low valence band offset and high conduction band offset with Si. Second, we show a champion cell efficiency of 10.8% with the additional integration of titanium oxide (TiOx), a well-known material for an electron-selective contact interlayer. Key parameters including Voc and Jsc also show different degrees of enhancement if single (NiOx only) or double (both NiOx and TiOx) carrier-selective contacts are integrated. The fabrication process for TiOx and NiOx layer integration is scalable and shows good compatibility with the device.

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%.

Photocurrent enhancement of ultrathin front-textured crystalline silicon solar cells by rear-located periodic silver nanoarrays

Ultrathin crystalline silicon solar cells are alternative technology roadmap to achieve more cost-effectiveness. However, when silicon thickness is reduced to less than 50µm, light absorption loss becomes severe, especially in the long wavelength range of 900–1200nm. Therefore, it is significant to investigate the enhancements of photocurrent and efficiency by improving light absorption in the long wavelength range. In this work, we design and fabricate approximately 20µm-thick front-textured monocrystalline silicon solar cells. When periodic silver nanoarrays with precise geometries are applied to the rear of the solar cells, the measured external quantum efficiency clearly reveals an enhancement in the longer wavelengths. For our best sample, the short-circuit current density and the absolute efficiency present the enhancements of ΔJsc=0.8mA/cm2 and Δη=0.6%.

Periodic Upright Nanopyramids for Light Management Applications in Ultrathin Crystalline Silicon Solar Cells

Motivated by the primary benefit of reduced material cost, the thickness of crystalline silicon solar cells has been continuously reduced. Laboratory and industrial studies have explored ultrathin crystalline silicon solar cells below 50 μ m with ambitious endeavors toward thicknesses of only a few micrometers. Ultrathin crystalline silicon solar cells require compatible small-scale surface textures to enhance the optical absorption. For this purpose, a novel submicron periodic nanostructure—periodic upright nanopyramids (PuNPs)—is fabricated by an integrated process of laser interference lithography and anisotropic etching of silicon in an alkaline solution. By simulation and measurements, we demonstrate that PuNPs are able to reduce front surface reflectance more effectively than conventional micron-scale pyramid textures and previously investigated periodic inverted nanopyramids (PiNPs). With a silicon nitride antireflection coating, we predict that PuNPs reduce the front surface reflectance to below 1% at an angle of incidence of 8°, which is comparable to black silicon. The superior antireflective property of PuNPs contributes to an absorbed photocurrent density of 40.8 mA/cm2 for a 40 μ m silicon absorber layer, which is 0.7 mA/cm2 higher than PiNPs, 0.8 mA/cm2 higher than inverted pyramids and 1 mA/cm2 higher than upright pyramids.

Efficiency enhancement in ultrathin crystalline silicon solar cells with composite surface gratings

We investigate the light absorption in an ultrathin crystalline silicon solar cell with composite grating structures using three dimensional rigorous coupled wave analysis. Comparing with conventional surface gratings, the composite grating which is a superposition of two subgratings with different length scales demonstrates efficient light trapping in thin film solar cells in a broad spectral range. The light absorption is found to be close to the Yablonovitch limit in both visual and infrared regions. The optimized structure yields an ideal photocurrent density of 33.9mA/cm2 at an equivalent thickness of 2µm. This is comparable to the performance of the recently proposed double-sided grating design (Wang et al., 2012 [19]). These results suggest that the composite grating structures promise efficiency enhancement in ultrathin solar cells with improved technology compatibility.