High efficiency thin upgraded metallurgical-grade silicon solar cells on flexible substrates

Management of Light Absorption Based on Sidewall Reflection for Fabricating High-Performance Flexible Nano-Coned Sb2Se3 Thin Film Photovoltaics

High performance and robustness are the key factors to boosting wearable and portable applications. Although the 1D crystal structure makes the Sb2Se3 thin film more tolerant to physical deformation upon bending, the conventional planar structures still cannot undergo repeated mechanical bending due to the induced stress/strain inside devices, which can be well addressed by constructing three-dimensional nanostructures. Besides, the electron diffusion length has two values, 0.3 μm in the [221] direction and 1.7 μm in the [001] direction, in the Sb2Se3 thin film, which limits the absorber thickness, for getting an effective carrier collection; thus, a strong light trapping effect enabling sufficient light harvesting is needed to allow the use of a very thin light absorption layer. Herein, the nanoconed Sb2Se3 solar cells have been designed, and their light absorption behaviors were investigated within a finite-element simulation under the substrate back-reflection, indicating that the reflection of the bottom part always works positively, while the effect of the nanocone sidewall on absorption enhancement largely depends on its geometry, arising from resonant or scattering modes. These results provide a practical guide in designing/establishing an easier/simpler way to fabricate high-performance and mechanically stable flexible nanostructured Sb2Se3 thin film solar cells.

Theoretical Design of Inorganic Flexible Bulk Photovoltaic Materials

Current flexible photovoltaics (PVs) are usually based on low-dimensional structures of inorganic semiconductors and hybrid perovskites, as well as organic materials. Here, we propose a type of inorganic flexible bulk PV material, evaluating its structure flexibility, electronic structure, and PV efficiency in the framework of density functional theory, suggesting α-Ag₂S as the best candidate. It is found that the band structure and effective masses of α-Ag₂S can be significantly modulated by external strain, whereas leaving the high PV efficiency was not affected much. The flexibility of α-Ag₂S can be further enhanced by applying electron doping during stretching or applying hole doping during compression. We further studied the intrinsic defect properties of α-Ag₂S by using the Heyd-Scuseria-Ernzerhof hybrid functional, and the calculation results show that α-Ag₂S is a defect-tolerant semiconductor even when an external strain is applied. Our results open the door for searching inorganic flexible bulk PV materials for robust flexible solar cells.

Nanowires for High-Efficiency, Low-Cost Solar Photovoltaics

Solar energy is abundant, clean, and renewable, making it an ideal energy source. Solar cells are a good option to harvest this energy. However, it is difficult to balance the cost and efficiency of traditional thin-film solar cells, whereas nanowires (NW) are far superior in making high-efficiency low-cost solar cells. Therefore, the NW solar cell has attracted great attention in recent years and is developing rapidly. Here, we review the great advantages, recent breakthroughs, novel designs, and remaining challenges of NW solar cells. Special attention is given to (but not limited to) the popular semiconductor NWs for solar cells, in particular, Si, GaAs(P), and InP.

Silicon Nanocanyon: One-Step Bottom-Up Fabrication of Black Silicon via in-Lasing Hydrophobic Self-Clustering of Silicon Nanocrystals for Sustainable Optoelectronics

We report a novel one-step bottom-up fabrication method for multiscale-structured black Si, which is characterized by randomly distributed microscale Si layers covered with sub-100 nm protrusions with submicron boundary grooves. The unique multiscale structure, suggested as a "nanocanyon," effectively minimizes light reflection over a broad spectrum by diversifying the scattering routes from the nanotextured surface to the wide distributed boundary micronanoscale grooves. This structure was achieved by hydrophobic clustering and local aggregation of instantaneously melted Si nanocrystals on a glass substrate under laser irradiation. This method can replace the complicated conventional silicon processes, such as patterning for selective Si formation, texturing for improved absorption, and doping for modifying the electrical properties, because the proposed method obviates the need for photolithography, chemical etching, vacuum processes, and expensive wafers. Finally, black Si photosensor arrays were successfully demonstrated by a low-cost solution process and a laser growth sintering technique for microchannel fabrication. The results show the great potential of the proposed fabrication method for low-cost and sustainable production of highly sensitive optoelectronics and as an alternative to conventional wafer-based photosensor manufacturing techniques.

Charge Transfer from Carbon Nanotubes to Silicon in Flexible Carbon Nanotube/Silicon Solar Cells

Mechanical fragility and insufficient light absorption are two major challenges for thin flexible crystalline Si-based solar cells. Flexible hybrid single-walled carbon nanotube (SWNT)/Si solar cells are demonstrated by applying scalable room-temperature processes for the fabrication of solar-cell components (e.g., preparation of SWNT thin films and SWNT/Si p-n junctions). The flexible SWNT/Si solar cells present an intrinsic efficiency ≈7.5% without any additional light-trapping structures. By using these solar cells as model systems, the charge transport mechanisms at the SWNT/Si interface are investigated using femtosecond transient absorption. Although primary photon absorption occurs in Si, transient absorption measurements show that SWNTs also generate and inject excited charge carriers to Si. Such effects can be tuned by controlling the thickness of the SWNTs. Findings from this study could open a new pathway for designing and improving the efficiency of photocarrier generation and absorption for high-performance ultrathin hybrid SWNT/Si solar cells.

Nanostructured Silicon Used for Flexible and Mobile Electricity Generation

The use of nanostructured silicon for the generation of electricity in flexible and mobile devices is reviewed. This field has attracted widespread interest in recent years due to the emergence of plastic electronics. Such developments are likely to alter the nature of power sources in the near future. For example, flexible photovoltaic cells can supply electricity to rugged and collapsible electronics, biomedical devices, and conformable solar panels that are integrated with the curved surfaces of vehicles or buildings. Here, the unique optical and electrical properties of nanostructured silicon are examined, with regard to how they can be exploited in flexible photovoltaics, thermoelectric generators, and piezoelectric devices, which serve as power generators. Particular emphasis is placed on organic-silicon heterojunction photovoltaic devices, silicon-nanowire-based thermoelectric generators, and core-shell silicon/silicon oxide nanowire-based piezoelectric devices, because they are flexible, lightweight, and portable.

Large-area, size-tunable Si nanopillar arrays with enhanced antireflective and plasmonic properties

In this paper, a novel method using the modified Langmuir-Blodgett and float-transfer techniques was introduced to construct the perfect PS monolayer nanosphere template with large area up to cm(2). Based on such templates, the diameter, length, packing density, and the shape of Si nanopillar arrays (Si NPAs) could be precisely controlled and tuned through the modified nanosphere lithography combined with a metal-assisted chemical etching (NSL-MACE) method. Manipulation of the etching time can effectively avoid permanent deformation/clumping to generate size-tunable Si NPAs. The optical properties of the Si NPAs can be controlled by the Si NPA morphologies resulting from the different reactive ion etching (RIE) time and chemical etching time. The enhanced antireflective property and electromagnetic field effect of Au/Si NPAs were proved by the results. The new modified NSL-MACE technique with the capability of scale-up fabrication of Si NPAs would be helpful for potential applications in optoelectronic devices.

Sinusoidal nanotextures for light management in silicon thin-film solar cells

Recent progresses in liquid phase crystallization enabled the fabrication of thin wafer quality crystalline silicon layers on low-cost glass substrates enabling conversion efficiencies up to 12.1%. Because of its indirect band gap, a thin silicon absorber layer demands for efficient measures for light management. However, the combination of high quality crystalline silicon and light trapping structures is still a critical issue. Here, we implement hexagonal 750 nm pitched sinusoidal and pillar shaped nanostructures at the sun-facing glass-silicon interface into 10 μm thin liquid phase crystallized silicon thin-film solar cell devices on glass. Both structures are experimentally studied regarding their optical and optoelectronic properties. Reflection losses are reduced over the entire wavelength range outperforming state of the art anti-reflective planar layer systems. In case of the smooth sinusoidal nanostructures these optical achievements are accompanied by an excellent electronic material quality of the silicon absorber layer enabling open circuit voltages above 600 mV and solar cell device performances comparable to the planar reference device. For wavelengths smaller than 400 nm and higher than 700 nm optical achievements are translated into an enhanced quantum efficiency of the solar cell devices. Therefore, sinusoidal nanotextures are a well-balanced compromise between optical enhancement and maintained high electronic silicon material quality which opens a promising route for future optimizations in solar cell designs for silicon thin-film solar cells on glass.

Biomimetic and plasmonic hybrid light trapping for highly efficient ultrathin crystalline silicon solar cells

Designing effective light-trapping structures for the insufficiently absorbed long-wavelength light in ultrathin silicon solar cells represents a key challenge to achieve low cost and highly efficient solar cells. We propose a hybrid structure based on the biomimetic silicon moth-eye structure combined with Ag nanoparticles to achieve advanced light trapping in 2 μm thick crystalline silicon solar cells approaching the Yablonovitch limit. By synergistically using the Mie resonances of the silicon moth-eye structure and the plasmonic resonances of the Ag nanoparticles, the integrated absorption enhancement achieved across the usable solar spectrum is 69% compared with the cells with the conventional light trapping design. This is significantly larger than both the silicon moth-eye structure (58%) and Ag nanoparticle (41%) individual light trapping. The generated photocurrent in the 2 μm thick silicon layer is as large as 33.4 mA/cm², which is equivalent to that generated by a 30 μm single-pass absorption in the silicon. The research paves the way for designing highly efficient light trapping structures in ultrathin silicon solar cells.

High efficiency silicon solar cell based on asymmetric nanowire

Improving the efficiency of solar cells through novel materials and devices is critical to realize the full potential of solar energy to meet the growing worldwide energy demands. We present here a highly efficient radial p-n junction silicon solar cell using an asymmetric nanowire structure with a shorter bottom core diameter than at the top. A maximum short circuit current density of 27.5 mA/cm² and an efficiency of 7.53% were realized without anti-reflection coating. Changing the silicon nanowire (SiNW) structure from conventional symmetric to asymmetric nature improves the efficiency due to increased short circuit current density. From numerical simulation and measurement of the optical characteristics, the total reflection on the sidewalls is seen to increase the light trapping path and charge carrier generation in the radial junction of the asymmetric SiNW, yielding high external quantum efficiency and short circuit current density. The proposed asymmetric structure has great potential to effectively improve the efficiency of the SiNW solar cells.

15.7% Efficient 10-μm-thick crystalline silicon solar cells using periodic nanostructures

Only ten micrometer thick crystalline silicon solar cells deliver a short-circuit current of 34.5 mA cm(-2) and power conversion efficiency of 15.7%. The record performance for a crystalline silicon solar cell of such thinness is enabled by an advanced light-trapping design incorporating a 2D inverted pyramid photonic crystal and a rear dielectric/reflector stack.

A 12%-efficient upgraded metallurgical grade silicon-organic heterojunction solar cell achieved by a self-purifying process

Low-quality silicon such as upgraded metallurgical-grade (UMG) silicon promises to reduce the material requirements for high-performance cost-effective photovoltaics. So far, however, UMG silicon currently exhibits the short diffusion length and serious charge recombination associated with high impurity levels, which hinders the performance of solar cells. Here, we used a metal-assisted chemical etching (MACE) method to partially upgrade the UMG silicon surface. The silicon was etched into a nanostructured one by the MACE process, associated with removing impurities on the surface. Meanwhile, nanostructured forms of UMG silicon can benefit improved light harvesting with thin substrates, which can relax the requirement of material purity for high photovoltaic performance. In order to suppress the large surface recombination due to increased surface area of nanostructured UMG silicon, a post chemical treatment was used to decrease the surface area. A solution-processed conjugated polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was deposited on UMG silicon at low temperature (<150 °C) to form a heterojunction to avoid any impurity diffusion in the silicon substrate. By optimizing the thickness of silicon and suppressing the charge recombination at the interface between thin UMG silicon/PEDOT:PSS, we are able to achieve 12.0%-efficient organic-inorganic hybrid solar cells, which are higher than analogous UMG silicon devices. We show that the modified UMG silicon surface can increase the minority carrier lifetime because of reduced impurity and surface area. Our results suggest a design rule for an efficient silicon solar cell with low-quality silicon absorbers.

Incident light adjustable solar cell by periodic nanolens architecture

Could nanostructures act as lenses to focus incident light for efficient utilization of photovoltaics? Is it possible, in order to avoid serious recombination loss, to realize periodic nanostructures in solar cells without direct etching in a light absorbing semiconductor? Here we propose and demonstrate a promising architecture to shape nanolenses on a planar semiconductor. Optically transparent and electrically conductive nanolenses simultaneously provide the optical benefit of modulating the incident light and the electrical advantage of supporting carrier transportation. A transparent indium-tin-oxide (ITO) nanolens was designed to focus the incident light-spectrum in focal lengths overlapping to a strong electric field region for high carrier collection efficiency. The ITO nanolens effectively broadens near-zero reflection and provides high tolerance to the incident light angles. We present a record high light-conversion efficiency of 16.0% for a periodic nanostructured Si solar cell.

Plasmonic effects of au/ag bimetallic multispiked nanoparticles for photovoltaic applications

In recent years, there has been considerable interest in the use of plasmons, that is, free electron oscillations in conductors, to boost the performance of both organic and inorganic thin film solar cells. This has been driven by the possibility of employing thin active layers in solar cells in order to reduce materials costs, and is enabled by significant advances in fabrication technology. The ability of surface plasmons in metallic nanostructures to guide and confine light in the nanometer scale has opened up new design possibilities for solar cell devices. Here, we report the synthesis and characterization of highly monodisperse, reasonably stable, multipode Au/Ag bimetallic nanostructures using an inorganic additive as a ligand for photovoltaic applications. A promising surface enhanced Raman scattering (SERS) effect has been observed for the synthesized bimetallic Au/Ag multispiked nanoparticles, which compare favorably well with their Au and Ag spherical nanoparticle counterparts. The synthesized plasmonic nanostructures were incorporated on the rear surface of an ultrathin planar c-silicon/organic polymer hybrid solar cell, and the overall effect on photovoltaic performance was investigated. A promising enhancement in solar cell performance parameters, including both the open circuit voltage (VOC) and short circuit current density (JSC), has been observed by employing the aforementioned bimetallic multispiked nanoparticles on the rear surface of solar cell devices. A power conversion efficiency (PCE) value as high as 7.70% has been measured in a hybrid device with Au/Ag multispiked nanoparticles on the rear surface of an ultrathin, crystalline silicon (c-Si) membrane (∼ 12 μm). This value compares well to the measured PCE value of 6.72% for a similar device without nanoparticles. The experimental observations support the hope for a sizable PCE increase, due to plasmon effects, in thin-film, c-Si solar cells in the near future.

Materials and structures for stretchable energy storage and conversion devices

Stretchable energy storage and conversion devices (ESCDs) are attracting intensive attention due to their promising and potential applications in realistic consumer products, ranging from portable electronics, bio-integrated devices, space satellites, and electric vehicles to buildings with arbitrarily shaped surfaces. Material synthesis and structural design are core in the development of highly stretchable supercapacitors, batteries, and solar cells for practical applications. This review provides a brief summary of research development on the stretchable ESCDs in the past decade, from structural design strategies to novel materials synthesis. The focuses are on the fundamental insights of mechanical characteristics of materials and structures on the performance of the stretchable ESCDs, as well as challenges for their practical applications. Finally, some of the important directions in the areas of material synthesis and structural design facing the stretchable ESCDs are discussed.

Extreme light absorption by multiple plasmonic layers on upgraded metallurgical grade silicon solar cells

We fabricate high-efficiency, ultrathin (∼12 μm), flexible, upgraded metallurgical-grade polycrystalline silicon solar cells with multiple plasmonic layers precisely positioned on top of the cell to dramatically increase light absorption. This scalable approach increases the optical absorptivity of our solar cells over a broad range of wavelengths, and they achieve efficiencies η ≈ 11%. Detailed studies on the electrical and optical properties of the developed solar cells elucidate the light absorption contribution of each individual plasmonic layer. Finite-difference time-domain simulations were also performed to yield further insights into the obtained results. We anticipate that the findings from this work will provide useful design considerations for fabricating a range of different solar cell systems.

Optical attenuation of plasmonic nanocomposites within photonic devices

Plasmonic nanostructures enable microscopic optical manipulation such as light trapping in photonic devices. However, integration of embedded nanostructures into photonic devices has been limited by tractability of nanoscale and microscale descriptions in device architectures. This work uses a linear algebraic model to distinguish geometric optical responses of nanoparticles integrated into dielectric substrates interacting with macroscopic back-reflectors from absorptive and nonlinear plasmonic effects. Measured transmission, reflection, and attenuation (losses) from ceramic and polymer composites supporting two- and three-dimensional distributions of gold nanoparticles, respectively, are predictable using the model. A unique equilateral display format correlates geometric optical behavior and attenuation to nanoparticle density and back-reflector opacity, allowing intuitive, visual specification of density and opacity necessary to obtain a particular optical performance. The model and display format are useful for facile design and integration of plasmonic nanostructures into photonic devices for light manipulation.

Reporting performance in organic photovoltaic devices

Research into organic photovoltaics (OPVs) is rapidly growing worldwide because it offers a route to low temperature, inexpensive processing of lightweight, flexible solar cells that can be mass manufactured cheaply. Unlike silicon or other inorganic semiconductors (e.g., CdTe, CIGs), OPVs are complicated by the requirement of having multiple materials and layers that must be integrated to enable the cell to function. The enormous number of research hours required to optimize all aspects of OPVs and to integrate them successfully is typically boiled down to one number-the power conversion efficiency (PCE) of the device. The PCE is the value by which comparisons are routinely made when modifications are made to devices; new bulk heterojunction materials, electron- and hole-transport layers, electrodes, plasmonic additives, and many other new advances are incorporated into OPV devices and compared with one, or a series of, control device(s). The concern relates to the statistical significance of this all-important efficiency/PCE value: is the observed change or improvement in performance truly greater than experimental error? If it is not, then the field can and will be misled by improper reporting of efficiencies, and future research in OPVs could be frustrated and, ultimately, irreversibly damaged. In this Perspective, the dangers of, for instance, cherry-picking of data and poor descriptions of experimental procedures, are outlined, followed by a discussion of a real data set of OPV devices, and how a simple and easy statistical treatment can help to distinguish between results that are indistinguishable experimentally, and those that do appear to be different.

Upgraded silicon nanowires by metal-assisted etching of metallurgical silicon: a new route to nanostructured solar-grade silicon

Through metal-assisted chemical etching (MaCE), superior purification of dirty Si is observed, from 99.74 to 99.9884% for metallurgical Si and from 99.999772 to 99.999899% for upgraded metallurgical Si. In addition, large area of silicon nanowires (SiNW) are fabricated. The purification effect induces a ∼35% increase in photocurrent for SiNW based photoelectrochemical cell.