Multicrystalline Si Solar Cells Research Articles

The rising star in photovoltaics-perovskite solar cells: The past, present and future

Perovskite solar cell (PVSC), after its invention since 2009, has attracted tremendous attention from both academia and industry, because of its low cost, ease of manufacturing features and the sky-rocketing efficiencies being achieved within such a short period of time. Currently, the new efficiency record has reached 21.0%, comparable to that of the commercialized PV technologies developed for decades, such as multi-crystalline Si, CIGS and CdTe thin film solar cells. It is very possible that PVSCs would one day step over the threshold of marketization, share or even overturn the current PV market dominated by crystalline-Si solar cells. However, there are still several obstacles to be overcome on the road towards PVSC industrialization. This paper has reviewed the brief developing history and the current research status of PVSCs, and explained: Why PVSCs are so important in the next-generation solar cells, why organometal halide perovskites work so well as light absorbers, and what the inherent differences are among different cell configurations. The prospects on how to realize scale-up industrialization of PVSCs have also been given in the sequence of efficiency, stability, cost, toxicity, and short-term objectives. Resolutions to the remaining challenges according to their orders of technical difficulty and importance have also been discussed.

3D-Mössbauer spectroscopic microscope for mc-Si solar cell evaluation

A 3D-Mossbauer Spectroscopic Microscope is developed to evaluate Fe impurities in multi-crystalline Si solar cells, which combines the Mossbauer spectroscopic microscope with a scanning electron microscope (SEM), an electron beam induced current (EBIC), an electron backscatter diffraction (EBSD), and an electron energy analyzer (HV-CSA). In addition, a new moving-coil-actuator with a liner encoder of 100 nm-resolution is incorporated for the operations with both a constant velocity and a constant acceleration mode successfully with the same precision as that obtained by the conventional transducers. Furthermore, a new multi-capillary X-ray lens is designed to achieve a γ-ray spot size less than 100 μm in diameter. The new microscope provides us to investigate the space correlation between Fe impurities and the lattice defects such as grain boundaries in multi-crystalline Si solar cells.

Conversion Efficiency Enhancement of Multi-crystalline Si Solar Cells by Using a Micro-structured Junction**Supported by the National Natural Science Foundation of China under Grant Nos U1332205, 11274153, 11204124, and 11404091.

A new approach for enhancing the conversion efficiency of solar cells is proposed. A surface with columnar structures is employed on the fabrication of multi-crystalline silicon solar cells, for increasing the collection probability of photon-generated minority carriers in the cells. For comparison, three types of surfaces (planar surface, surfaces with columns of 12 μm in radius and columns of 9 μm in radius, respectively) are used. It is demonstrated that the cells with columnar structured surfaces have better spectral response and higher efficiencies than the cells with planar surface, while the cells with columns of 9 μm in radius show better spectral response than the cells with columns of 12 μm in radius. However, the cells with columns of 12 μm exhibit higher efficiencies than the cells with columns of 9 μm in radius for their difference in fill factors. Moreover, the effect of the columnar structured surface on the minority carrier collection efficiency is verified with wafers of different minority carrier lifetimes (0.5 μs and 1.0 μs). This work may significantly consider its potential in the manufacturing of high-efficiency solar cells at low cost by using low quality materials.

One-step-MACE nano/microstructures for high-efficient large-size multicrystalline Si solar cells

We have employed a one-step (direct etching) metal-assisted chemical etching (MACE) technique to grow large-area Si nanostructures with smoother surface morphology and much less porous Si (PS) defects than those under the two-step (depositing and etching) MACE. A 17.63%-efficiency of the nano/ microstructures (N/M-Strus) based multicrystalline Si (mc-Si) solar cells has firstly surpassed that (17.45%) of traditional-micro-textured one with a standard solar wafer size of 156×156mm2. The key to success lies in the reduction of electrical loss by removing PS defects and employing shorter one-step-MACE-smoothened N/M-Strus, together with the optical gain from the combined antireflection of mc-Si N/M-Strus and SiNx:H thin films. The present work opens a way to the mass production of high efficient Si nanostructures based solar cells with a less-process-step and lower-cost approach.

Industrialization of Hot Wire Chemical Vapor Deposition for thin film applications

The consequences of implementing a Hot Wire Chemical Vapor Deposition (HWCVD) chamber into an existing in-line or roll-to-roll reactor are described. The hardware and operation of the HWCVD production reactor is compared to that of existing roll-to-roll reactors based on Plasma Enhanced Chemical Vapor Deposition. The most important consequences are the technical consequences and the economic consequences, which are both discussed. The technical consequences are adaptations needed to the hardware and to the processing sequences due to the different interaction of the HWCVD process with the substrate and already deposited layers. The economic consequences are the reduced investments in radio frequency (RF) supplies and RF components. This is partially offset by investments that have to be made in higher capacity pumping systems. The most mature applications of HWCVD are moisture barrier coatings for thin film flexible devices such as Organic Light Emitting Diodes and Organic Photovoltaics, and passivation layers for multicrystalline Si solar cells, high mobility field effect transistors, and silicon heterojunction cells (also known as heterojunction cells with intrinsic thin film layers). Another example is the use of Si in thin film photovoltaics. The cost perspective per unit of thin film photovoltaic product using HWCVD is estimated at 0.07€/Wp for the Si thin film component.

Honeycomb Structure on Multi-crystalline Silicon Al-BSF Solar Cell With 17.8% Efficiency

In this paper, large-area honeycomb (HC) texturing of multi-crystalline Si substrates with a newly developed process chain based on roller nanoimprint lithography and inline-capable plasma etching is demonstrated. The realized HC-textures show optical properties superior to random pyramids on c-Si, while the surface passivation quality is comparable. First, results on 156 × 156 mm2 multi-crystalline Si solar cells with an aluminum back-surface field show an efficiency of η = 17.8% compared with η = 17.3% on isotextured reference samples. This efficiency gain of 0.5% results from an increase in $J_{{\rm sc}}$ of up to 1.3 mA/cm2, which underlines the high optical and electrical quality of the texture. On a smaller area (50 × 50 mm2), the gain in $J_{{\rm sc}}$ of HC-textured cells compared with isotextured references was even higher with up to 2.5 mA/cm2.

Study on the front contact mechanism of screen-printed multi-crystalline silicon solar cells

The two key issues of front contact of screen-printed multi-crystalline silicon have been studied. Firstly, the effects of doping profiles in the emitter on the contact-resistance have been systematically investigated. Constant source diffusion is used to explain the doped phosphorus profile in the textured emitter by simulation. Secondly, the penetration mechanism of silver into SiNx layer was studied by using SEM and EDX to reveal the microstructure of the Ag/Si contact. A model has been proposed to explain the formation of the ohmic contact. It is found that the effectiveness of the contact is determined by the direct contact area of the silver crystallites to both the emitter and the finger. For a commercial multi-crystalline silicon solar cell of 156×156mm2, the optimal contact-resistance of about 1.2mΩ is suggested, corresponding to the doping sheet resistance of around 80–90Ω/□. Multi-crystalline Si solar cells with excellent results, up to 18.51% in conversion efficiency, have been achieved. Moreover, to further understand the model, effects of the firing temperature and the thickness of SiNx layer on the contact-resistance have been investigated in the solar cells. It is proved that the formation of the silver crystallites is related to the existence of SiNx layer.

Influence of Different Types of Recombination Active Defects on the Integral Electrical Properties of Multicrystalline Silicon Solar Cells

In this contribution the influence of different types of recombination-active defects on the integral electrical properties of multicrystalline Si solar cells is investigated. Based on a previous classification scheme related to the luminescence behavior of crystal defects, Type-A and Type-B defects are locally distinguished. It is shown that Type-A defects, correlated to iron contaminations, are dominating the efficiency by more than 20% relative through their impact on the short circuit current ISC and open circuit voltage VOC in standard Si material (only limited by recombination active crystal defects). Contrarily, Type-B defects show low influence on the efficiency of 3% relative. The impact of the detrimental Type-A defects on the electrical parameters is studied as a function of the block height. A clear correlation between the area fraction of Type-A defects and both the global Isc and the prebreakdown behavior (reverse current) in voltage regime-2 (−11 V) is observed. An outlier having an increased full-area recombination activity is traced back to dense inter- and intragrain nucleation of Fe precipitates. Based on these results it is concluded that Type-A defects are the most detrimental defects in Si solar cells (having efficiencies > 15%) and have to be prevented by optimized Si material quality and solar cell process conditions.

Performance of solar cells fabricated on black multicrystalline Si by nanowire decoration

Vertically aligned Si nanowire (NW) arrays fabricated by metal-assisted etching technique were applied to industrial sized (156mm×156mm) multicrystalline Si cells as an anti-reflective (AR) medium. The NW lengths (between 0.15 and 2.2μm) were controlled by etch duration from 5 to 50min. A completely black surface could be observed, demonstrating excellent AR properties in the entire range of the solar spectrum, even without additional anti-reflective coating layer (e.g., SiNx:H). Standard Si solar cell fabrication protocols were followed for both samples with NW arrays and to reference samples textured in standard stain etch solution. Cell parameters have been studied as a function of NW length. Results show that Si NW arrays can be used on multicrystalline Si solar cells as an AR coating. Without applying a superior passivation technique, cell conversion efficiencies are observed to normally degrade with increasing lengths of NW's, such that the highest efficiency in NW samples was resulted from the shortest NW's. It is clear that an effective passivation eliminating recombination along the NW's and optimized doping could further improve the performance of the cell. Structuring the surface of the multi-crystalline wafers with metal assisted etching is shown to be a promising alternative to presently used acid-based texturing processes.

Random-Texturing of Phosphorus-Doped Layers for Multi-Crystalline Si Solar Cells by Plasmaless Dry Etching

We investigated a texturing process for crystalline Si solar cells by dry etching with chlorine trifluoride (ClF3) gas without plasma excitation. Recently our research group demonstrated improved electrical characteristics of single-crystalline Si solar cells textured by dry etching of the phosphorus-doped layers. In this report, we attempted to improve the electrical properties of multi-crystalline Si solar cells by modifying the experimental procedure and optimizing the process conditions. The reflectance of the treated surfaces was around 10% at 600 nm without an anti-reflection film. We demonstrated the characteristics of multi-crystalline solar cells by random-texturing by plasmaless etching. This is the first report to prove the validity of plasmaless dry texturing for multi-crystalline Si solar cells.

Performance Improvements of Selective Emitters by Laser Openings on Large-Area Multicrystalline Si Solar Cells

This study focuses on the laser opening technique used to form a selective emitter (SE) structure on multicrystalline silicon (mc-Si). This technique can be used in the large-area (156 × 156 mm2) solar cells. SE process of this investigation was performed using 3 samples SE1–SE3. Laser fluences can vary in range of 2–5 J/cm2. The optimal conversion efficiency of 15.95% is obtained with the SE3 (2 J/cm2fluence) after laser opening with optimization of heavy and light dopant, which yields a gain of0.48%abscompared with that of a reference cell (without fluence). In addition, this optimal SE3 cell displays improved characteristics compared with other cells with a higher average value of external quantum efficiency (EQEavg = 68.6%) and a lower average value of power loss (Ploss = 2.33 mW/cm2). For the fabrication of solar cells, the laser opening process comprises fewer steps than traditional photolithography does. Furthermore, the laser opening process decreases consumption of chemical materials; therefore, the laser opening process decreases both time and cost. Therefore, SE process is simple, cheap, and suitable for commercialization. Moreover, the prominent features of the process render it effective means to promote overall performance in the photovoltaic industry.

A simple criterion for predicting multicrystalline Si solar cell performance from lifetime images of wafers prior to cell production

The aim of this paper is to clarify which mean value can be used to predict multicrystalline Si (mc-Si) solar cell performance from lifetime distributions on wafers prior to production. Therefore, a numerical device simulation model is presented that predicts cell performance very precisely from lifetime distributions. This model is used to derive a simple lifetime averaging procedure, which can be used as a criterion for excluding low-quality wafers from production. Compared with standard mean values such as the arithmetic, harmonic, or geometric mean, the criterion derived here significantly reduces the number of mistakenly rejected wafers prior to cell production. In an exemplary case study where 1000 different lifetime distributions were analyzed, our new procedure misjudged only 13 wafers, in contrast with 158 wafers when using the arithmetic mean, 109 wafers when using the geometric mean, and 78 when using the harmonic mean. Further, the influence of lower lifetime regions on mc-cell performance is quantified, showing that low lifetime regions cannot be overcompensated with higher lifetimes in general.

Improved multicrystalline Si solar cells by light trapping from Al nanoparticle enhanced antireflection coating

Significant photocurrent enhancement of 0.4 mA/cm2 has been achieved for industrial textured multicrystalline silicon solar cells, due to the light trapping provided by aluminium nanoparticle enhanced antireflection coating. Aluminium nanoparticles support surface plasmon resonances, which can effectively scatter the light into the solar cells. By blue shifting the detrimental Fano resonances away from the important silicon absorption spectrum, aluminium nanoparticles can provide a broadband light absorption enhancement without a reduction at the short wavelengths. Combining the discovery with 75 nm silicon nitride antireflection coating, which can significantly enhance the absorption at the peak solar spectrum, we have achieved the strong broadband light absorption enhancement.

Performance of nanowire decorated mono- and multi-crystalline Si solarcells

Light trapping ability of a solar cell can be improved by the application of various texturing schemes to the surface of the solar cells. For monocrystalline (mono-c) silicon solar cells, this is practically accomplished through pyramid texturing in a hot alkaline solution. For multicrystalline (multi-c) silicon solar cells, texturing is done by continuous oxidation and random etching of silicon from the surface. In this study, we have used metal assisted etching (MAE) technique to form silicon nanowires on the surface of pyramid textured mono-c, flat mono-c and multi-c silicon solar cells with an area of 156mm×156mm using standard solar cell processing protocols. It was shown that nanowire texturing causes a dramatic decrease in the reflectivity of the cell surface. Current–voltage (I–V) characteristics revealed that the solar cells with nanowires on top of pyramid texturing exhibit the best performance among all investigated cell types. This may be attributed to combined effect of nanowires and pyramids on the surface.

Enhanced photocurrent in crystalline silicon solar cells by hybrid plasmonic antireflection coatings

Photocurrent enhancement induced by plasmonic light trapping is of great interest for photovoltaics. We design and demonstrate hybrid plasmonic antireflection coatings as an efficient light trapping strategy for broadband absorption and photocurrent enhancement in crystalline silicon solar cells. Gold nanoparticles of size ranging from 15 to 150 nm are embedded in standard SiNx antireflection coatings with a thickness of 90 nm. Through optimizing the location of tailored nanoparticles within the SiNx layer, both light scattering enhancement and near-field light concentration can be harnessed. A maximum increase of 6.3% in photocurrent is achieved for textured multi-crystalline Si solar cells with the optimum configuration.

Testing of two different types of photovoltaic–thermal (PVT) modules with heat flow pattern under tropical climatic conditions

In this paper, the testing of two different photovoltaic–thermal (PVT) modules under the tropical climatic conditions of Singapore, is evaluated. For this evaluation, two different types (Type A and Type B) of commercially available PVT modules have been installed and tested at the National University of Singapore (NUS), Singapore. In Type A, the PV module is encapsulated with mono-crystalline Si solar cells and integrated with a tube-and-sheet type thermal collector, whereas, in Type B, the PV module is encapsulated with multi-crystalline Si solar cells and integrated with a parallel-plate type thermal collector. The performance of the PVT modules has been evaluated based on thermal and PV efficiencies.The experiments have been performed at different flow rates (0.03kg/s and 0.06kg/s) under typical day climatic conditions. The thermal performance of the modules has been validated using basic energy balance equations and design parameters. The temperature across the different layers of the PVT modules has been measured to study the heat flow pattern across the modules. It has been found that the average thermal efficiency and PV efficiency for Type A PVT module are 40.7% and 11.8%, respectively, and for Type B are 39.4% and 11.5%, respectively. The electrical efficiency of the PV modules was also compared with and without the thermal collector, and it was found that the average PV efficiency of the PVT modules is about 0.4% higher than the normal PV module.

Experimental study on the elimination of over-plating problems in industrial manufacturing of large-area acidic-textured laser-doped multi-crystalline solar cells

Light-induced plating (LIP) techniques, which are self-aligned and are used in the fabrication of well-shaped metal grids, have been widely studied and used in solar cells, particularly in laser-doped selective emitter (LDSE) ones. However, the application of LIP techniques in standard acidic-textured multi-crystalline silicon wafers with SiNx-coated surfaces can be challenging because of the presence of some unwanted metals on the SiNx surface during the plating process. This phenomenon is called over-plating, a problem in industrial LDSE multi-crystalline Si solar cell manufacturing that this paper aims to eliminate without the addition of any new process steps. The feasibility of changing the texturing process as well as the pretreatment before plating and the deposition of SiNx to reduce over-plating was investigated. The effects of over-plating on solar cell performance were also investigated to determine the mechanism of performance degradation. The cells reached a maximum efficiency of 17.2% and an average efficiency of 17.1% on large-area, commercial-grade, p-type multi-crystalline Si substrates when the over-plating problem was eliminated.

Enhancing the Efficiency of Multicrystalline Silicon Solar Cells by the Inkjet Printing of Silicon-Quantum-Dot Ink

Among all types of solar cells, multicrystalline silicon (Si) solar cells are the most widely produced. The enhancement of the efficiency of multicrystalline Si solar cells may help broaden the deployment of solar cells worldwide. Here we show that the efficiency of state-of-the-art commercially produced multicrystalline Si solar cells can be enhanced by a simple inkjet printing of Si-quantum-dot (Si-QD) ink at the solar cell surface. It is found that the efficiency enhancement results from both the down-shifting of Si QDs and the antireflection of porous Si-QD films at the solar cell surface. The current results demonstrate that Si-based nanotechnology can facilitate the continuous development of traditional Si solar cells.

Statistical theory and applications of lock-in carrierographic image pixel brightness dependence on multi-crystalline Si solar cell efficiency and photovoltage

A solar cell lock-in carrierographic image generation theory based on the concept of non-equilibrium radiation chemical potential was developed. An optoelectronic diode expression was derived linking the emitted radiative recombination photon flux (current density), the solar conversion efficiency, and the external load resistance via the closed- and/or open-circuit photovoltage. The expression was shown to be of a structure similar to the conventional electrical photovoltaic I-V equation, thereby allowing the carrierographic image to be used in a quantitative statistical pixel brightness distribution analysis with outcome being the non-contacting measurement of mean values of these important parameters averaged over the entire illuminated solar cell surface. This is the optoelectronic equivalent of the electrical (contacting) measurement method using an external resistor circuit and the outputs of the solar cell electrode grid, the latter acting as an averaging distribution network over the surface. The statistical theory was confirmed using multi-crystalline Si solar cells.

Wafer-Scale Growth of Silicon Microwire Arrays for Photovoltaics and Solar Fuel Generation

Silicon microwire arrays have recently demonstrated their potential for low-cost, high-efficiency photovoltaics and photoelectrochemical fuel generation. A remaining challenge to making this technology commercially viable is scaling up of microwire-array growth. We discuss here a technique for vapor–liquid–solid growth of microwire arrays on the scale of six-inch wafers using a cold-wall radio-frequency heated chemical vapor deposition furnace, enabling fairly uniform growth over large areas with rapid cycle time and improved run-to-run reproducibility. We have also developed a technique to embed these large-area wire arrays in polymer and to peel them intact from the growth substrate, which could enable lightweight, flexible solar cells with efficiencies as high as multicrystalline Si solar cells. We characterize these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structure and fidelity, and we test their energy-conversion properties using a methyl viologen (MV $^{2+/+}$ ) liquid junction contact in a photoelectrochemical cell. Initial photoelectrochemical conversion efficiencies suggest that the material quality of these microwire arrays is similar to smaller (∼1 cm $^2$ ) wire arrays that we have grown in the past, indicating that this technique is a viable way to scale up microwire-array devices.