High-efficiency Thin-film Silicon Research Articles

Performance enhancement of thin-film silicon solar cells by development of core component layers

High efficiency thin-film silicon (Si) solar cells were prepared on flexible substrates by the plasma-enhanced chemical vapor deposition method. To improve their performance, the microstructural, electrical, and optical properties of the core component layers including the metal rear reflectors, p- and n-type doped layers, and intrinsic absorber layers were controlled sophistically. To enable the use of flexible substrates with low heat resistance as well as to enhance light-scattering properties, nanotextured rear reflectors with a Ag/Al:Si bilayer structure were developed by dc magnetron sputtering at a low substrate temperature of below 150 °C. Highly crystalline n-doped seed layers (which effectively eliminate a defect-dense amorphous region formed in the initial growth stage of intrinsic nanocrystalline silicon (nc-Si:H) absorber layers) and p-type wide-bandgap nanocrystalline silicon carbide (nc-SiC:H) window layers (which reduce the parasitic absorption loss in the short-wavelength region and increase the open circuit voltage (VOC)) were successfully applied to enhance the performance characteristics of the solar cells. Through the combination of the developed core component layers, high conversion efficiencies of 8.84% (VOC = 0.53 V, JSC = 25.28 mA/cm2, and fill factor (FF) = 0.66, where JSC is the short circuit current) and 7.48% (VOC = 0.50 V, JSC = 21.07 mA/cm2, and FF = 0.71) were obtained for nc-Si:H solar cells fabricated on stainless steel (SUS) and polyimide substrates, respectively, at a low substrate temperature of below 150 °C. Flexible a-Si:H/nc-Si:H double-junction solar cells fabricated on SUS substrates showed a high conversion efficiency of 11.46% (VOC = 1.38 V, JSC = 11.53 mA/cm2, and FF = 0.72) when the nc-Si:H solar cells developed in this study were applied as a bottom cell.

Combining light-harvesting with detachability in high-efficiency thin-film silicon solar cells.

Efforts to realize thin-film solar cells on unconventional substrates face several obstacles in achieving good energy-conversion efficiency and integrating light-management into the solar cell design. In this report a technique to circumvent these obstacles is presented: transferability and an efficient light-harvesting scheme are combined for thin-film silicon solar cells by the incorporation of a NaCl layer. Amorphous silicon solar cells in p-i-n configuration are fabricated on reusable glass substrates coated with an interlayer of NaCl. Subsequently, the solar cells are detached from the substrate by dissolution of the sacrificial NaCl layer in water and then transferred onto a plastic sheet, with a resultant post-transfer efficiency of 9%. The light-trapping effect of the surface nanotextures originating from the NaCl layer on the overlying solar cell is studied theoretically and experimentally. The enhanced light absorption in the solar cells on NaCl-coated substrates leads to significant improvement in the photocurrent and energy-conversion efficiency in solar cells with both 350 and 100 nm thick absorber layers, compared to flat-substrate solar cells. Efficient transferable thin-film solar cells hold a vast potential for widespread deployment of off-grid photovoltaics and cost reduction.

Stabilized 14.0%-efficient triple-junction thin-film silicon solar cell

We report on a high-efficiency triple-junction thin-film silicon solar cell fabricated using the substrate configuration. An undoped hydrogenated amorphous silicon (a-Si:H) solar cell grown using triode plasma-enhanced chemical vapor deposition, which is more stable against light soaking, was applied to the a-Si:H/μc-Si:H/μc-Si:H triple-junction cells with honeycomb-textured substrates. To find the best balance in short circuit density and fill factor, we quantitatively investigated the effect of current mismatch on triple-junction cells. Accordingly, a stabilized efficiency of 14.04% was achieved in an a-Si:H/μc-Si:H/μc-Si:H triple-junction solar cell with a minimum light-induced degradation of 4%, setting a new record in this type of solar cells.

Nanocrystalline zinc oxide for surface morphology control in thin-film silicon solar cells

Zinc oxide (ZnO) is a widely used transparent conductive material for thin-film solar cells. Naturally textured ZnO deposited by low pressure chemical vapor deposition (LPCVD) is currently used to prepare high efficiency thin film silicon solar cells. However it is known that a strong roughness can disturb the growth of microcrystalline silicon and induce performance losses. An alternative approach denominated “nanocrystalline ZnO” is presented which allows to control surface roughness and haze during growth independently of other layer properties such as thickness, doping or conductivity for layers thicker than approximately 80nm. Surface morphologies, optical properties and material properties of the nanocrystalline ZnO layers together with solar cells results are presented.

Combining randomly textured surfaces and one-dimensional photonic crystals as efficient light-trapping structures in hydrogenated amorphous silicon solar cells

One of the foremost challenges in achieving high-efficiency thin-film silicon solar cells is in devising an efficient light trapping system because of the short optical path length imposed by the inherent thin absorption layers. In this paper, an efficient light trapping system is proposed using a combination of randomly textured surfaces and a one-dimensional photonic crystal (randomly textured photonic crystal; RTPC). The influence of the texture on the optical performance of RTPCs is discussed using the results of an experiment and a finite-difference time-domain simulation. This RTPC back reflector (BR) can provide high reflectivity and strong light scattering, resulting in an increased photocurrent density of the hydrogenated amorphous silicon (a-Si:H) solar cell. As a result, the highly textured RTPC BR yielded an efficiency of 9.6% for a-Si:H solar cell, which is much higher than the efficiency of 7.6% on flat AZO/Ag BR and 9.0% on textured AZO/Ag BR. This RTPC BR provides a new approach for creating high-efficiency, low-cost thin-film silicon solar cells.

High-efficiency thin-film silicon solar cells realized by integrating stable a-Si:H absorbers into improved device design

We report that thin-film silicon solar cells exhibiting high stabilized efficiencies can be obtained by depositing hydrogenated amorphous silicon (a-Si:H) absorbers using triode-type plasma-enhanced chemical vapor deposition. The improved light-soaking stability and performance of solar cells are also realized by optimizing the device design, such as p and p–i buffer layers. As a result, we attain independently confirmed stabilized efficiencies of 10.1–10.2% for a-Si:H single-junction solar cells (absorber thickness: ti = 220–310 nm) and 12.69% for an a-Si:H (ti = 350 nm)/hydrogenated microcrystalline silicon (µc-Si:H) tandem solar cell fabricated using textured SnO2 and ZnO substrates, respectively. The relative efficiency degradations of these solar cells are ∼10 and 3%, respectively, under 1 sun illumination at 50 °C for 1000 h.

P-Layer bandgap engineering for high efficiency thin film silicon solar cells

Spectroscopic ellipsometry (SE), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and optical transmittance measurements were used to study and establish a correlation between the open-circuit voltage (Voc) of solar cells and the p-layer optical band gap (Ep). It is found that the ellipsometry measurement can be used as an inline non-destructive diagnostic tool for p-layer deposition in commercial operation. The analysis of ellipsometric spectra, together with the optical transmittance data, shows that the best p-layer appears to be very fine nanocrystallites with an Ep 1.95eV. HRTEM measurements reveal that the best p-layer is composed of nanocrystallites ~9nm in size. It is also found that the p-layer exhibits very good transmittance, as high as ~91.6% at ~650nm. These results have guided us to achieve high Voc value 1.03V for thin film silicon based single junction solar cell.

Triple-junction thin-film silicon solar cell fabricated on periodically textured substrate with a stabilized efficiency of 13.6%

We report a high-efficiency triple-junction thin-film silicon solar cell fabricated with the so-called substrate configuration. It was verified whether the design criteria for developing single-junction microcrystalline silicon (μc-Si:H) solar cells are applicable to multijunction solar cells. Furthermore, a notably high short-circuit current density of 32.9 mA/cm2 was achieved in a single-junction μc-Si:H cell fabricated on a periodically textured substrate with a high-mobility front transparent contacting layer. These technologies were also combined into a-Si:H/μc-Si:H/μc-Si:H triple-junction cells, and a world record stabilized efficiency of 13.6% was achieved.

Response to “Comment on ‘Towards high efficiency thin-film crystalline silicon solar cells: The roles of light trapping and non-radiative recombinations’” [J. Appl. Phys. 117, 026101 (2015)

Response to “Comment on ‘Towards high efficiency thin-film crystalline silicon solar cells: The roles of light trapping and non-radiative recombinations’” [J. Appl. Phys. <b>117</b>, 026101 (2015)

Influence of Interface Textures on Light Management in Thin-Film Silicon Solar Cells With Intermediate Reflector

High-efficiency thin-film silicon solar cells require advanced textures at the front contacts for light management. In this contribution, the influence of the texture of various transparent conductive oxides (TCO) on the effectiveness of an intermediate reflector layer (IRL) in a-Si:H/μc-Si:H tandem solar cells is investigated. The employed front side TCOs include several types of sputter-etched ZnO:Al, LPCVD ZnO:B and APCVD SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :F. The topographies after different stages of the deposition process of the tandem solar cell, at the front TCO, after deposition of the amorphous top cell and after the deposition of the microcrystalline bottom cell, were characterized by atomic force microscopy at precisely the same spot. The external quantum efficiency of the fabricated solar cells were measured and successfully reproduced by a finite-difference time-domain method applying the measured topographies at each interface of the solar cell. With these simulations, the impact of structure type and feature size on the effectiveness of the IRL is investigated. The highest IRL effectiveness in a tandem solar cell was found for double-textured ZnO:Al. In this contribution, we study the interplay between interface textures and parasitic losses. Our findings are relevant for the design of topography for optimized IRL performance.

High-Stable-Efficiency Tandem Thin-Film Silicon Solar Cell With Low-Refractive-Index Silicon-Oxide Interlayer

We report the recent advances and key requirements for high-efficiency tandem thin-film silicon solar cells composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell. The impact of inserting a low-refractive-index silicon-oxide (SiOx) film as intermediate reflecting layer (IRL) is highlighted. We show that refractive indexes as low as 1.75 can be obtained for layers still conducting enough to be implemented in solar cells, and without no additional degradation. This allows for high top-cell current densities with thin top cells, enabling low degradation rates. A micromorph cell with a certified efficiency of 12.63% (short-circuit current density of 12.8 mA/cm(2)) is obtained for an optimized stack. Furthermore, short-circuit current densities as high as 15.9 mA/cm(2) are reported in the amorphous silicon top-cell of micromorph devices by combining a 150-nm- thick SiOx-based IRL and a textured antireflecting coating at the air-glass interface.

11.0%-Efficient Thin-Film Microcrystalline Silicon Solar Cells With Honeycomb Textured Substrates

In this paper, we present our latest results toward high-efficiency thin-film silicon solar cells. Owing to the superior light trapping capability of periodic textures combined with other technologies, a new world record was achieved for the efficiency of single-junction microcrystalline silicon solar cells, with a conversion efficiency of 11.0%, independently confirmed by the Advanced Industrial Science and Technology (AIST) Characterization, Standards, and Measurement (CSM) team.

Wide bandgap p-type nanocrystalline silicon oxide as window layer for high performance thin-film silicon multi-junction solar cells

High efficiency thin-film silicon multi-junction solar cells require both high open-circuit voltage (Voc) and high blue spectral response in the top amorphous silicon (a-Si:H) cell. Here we investigated the mixed-phase p-type nanocrystalline silicon oxide (p-SiOx) films and used this material as window layer in high Voc a-Si:H p–i–n solar cells. The introduction of oxygen suppresses the nucleation of Si nanocrystallites. Therefore, p-SiOx film with low oxygen content should be used for the contact layer, to guarantee growth of highly conductive Si nanocrystallites in the initial few nanometers. With p-SiOx as p-layer, the optimal p-SiOx film has high oxygen content and thus high bandgap, resulting higher Voc and better spectral response than the standard p-type amorphous silicon carbide alloys (p-SiC) based window layer. Although the optimal p-SiOx film has very low planar conductivity (in the order of 10–12S/cm), the filament-like Si nanocrystallites which grow perpendicular to the substrate enable the adequate transverse conduction for the solar cells. Consequently, a-Si:H solar cells with Voc>1V and FF>70% have been obtained. Finally, the p-SiOx window layers were successfully applied to thin-film silicon multi-junction solar cells. A high initial efficiency of 14.4% has been achieved in a-Si:H/nc-Si:H tandem solar cells.

Two-dimensional high efficiency thin-film silicon solar cells with a lateral light trapping architecture.

Introducing light trapping structures into thin-film solar cells has the potential to enhance their solar energy harvesting as well as the performance of the cells; however, current strategies have been focused mainly on harvesting photons without considering the light re-escaping from cells in two-dimensional scales. The lateral out-coupled solar energy loss from the marginal areas of cells has reduced the electrical yield indeed. We therefore herein propose a lateral light trapping structure (LLTS) as a means of improving the light-harvesting capacity and performance of cells, achieving a 13.07% initial efficiency and greatly improved current output of a-Si:H single-junction solar cell based on this architecture. Given the unique transparency characteristics of thin-film solar cells, this proposed architecture has great potential for integration into the windows of buildings, microelectronics and other applications requiring transparent components.

Improved light trapping effect for thin-film silicon solar cells fabricated on double-textured white glass substrate

Light trapping effect using rough surface transparent conductive oxide (TCO) is one of the best ways to achieve high efficiency thin-film silicon solar cells. Several types of rough ZnO film fabricated by metal organic chemical vapor deposition technique onto the glass, which are etched by reactive ion etching, have been proposed so far as promising TCO substrates. In this paper, newly developed ZnO substrate with extremely high light scattering property comparing with typical pyramidal texture one was developed. By applying this newly developed ZnO substrate to the solar cell, higher short circuit current of about 2% has been achieved comparing with typical pyramidal texture one without sacrificing other parameters. This result showed that the newly developed substrate is suitable as a front TCO substrate for high performance thin-film silicon solar cell.

Tailoring the surface morphology of zinc oxide films for high-performance micromorph solar cells

Self-textured zinc oxide polycrystalline films prepared by metalorganic low-pressure chemical vapor deposition combine excellent transparency, conductivity and light-scattering ability when used as electrodes for high-efficiency thin-film silicon solar cells. However, the growth of silicon layers with low defect density, which are necessary for high-performance solar cells, often requires the rough surface morphology of as-deposited zinc oxide films to be smoothened. This is usually achieved by a post-deposition argon plasma-etching treatment. We investigate here an alternative method to modify the surface morphology by changing the zinc oxide growth conditions over only the last hundreds of nanometers of the total film thickness. We discuss two types of zinc oxide cap layers, one grown with the addition of ethanol and the other with an enhanced diethylzinc (DEZ) precursor flow. We show that the presence of either type of cap layer does not alter the layer׳s electrical properties, but does slightly diminish the layer׳s light-scattering ability. The cap layer grown with ethanol leads to a more pronounced leveling of the film texture, while the high-DEZ-grown layer better preserves the sharp features of the underlying ZnO. Finally, zinc oxide films with cap layer are used as front electrodes for tandem amorphous/microcrystalline silicon solar cells, and are compared to rough films treated with an argon plasma. With rising thickness of the capping layers, the efficiency of tandem cells increases reaching values over 12%, and approaches that of cells on Ar plasma treated ZnO films.

Quantitative measurement and design of texture morphology for high-efficiency thin-film silicon solar cells

To decouple the trade-off relationship between short-circuit current ISC and open-circuit voltage VOC of thin-film silicon solar cells, which vary with texture morphology, it is necessary to first clarify the relationship between a solar cell’s properties and its texture morphology. We have developed a method for quantitatively measuring the texture morphology, which has enabled us to identify novel indices on the basis of texture width and angle individually correlated to ISC and VOC. A texturing process based on these indices should allow ISC and VOC to be improved independently.

Recent patent issues on intermediate reflectors for high efficiency thin-film silicon photovoltaic devices

Abstract The recent surge of unveiled US patents on the intermediate reflectors for thin-film silicon (Si) photovoltaic (PV) devices reflects the paramount importance of light trapping to improve the conversion efficiency. Here, the recent patent issues on the intermediate reflectors of thin-film Si PV devices are reviewed. Highly transparent and conductive metal oxide intermediate reflectors have the advantage of the higher efficiency for the fabricated multi-junction solar cells compared to the Si alloy intermediate reflectors. However, their high lateral electrical conductivity leads to the lateral shunting during the monolithic series integration of segments. To avoid the lateral shunt creations, an additional laser scribe or a coating process that induces a high production cost is necessary. In addition, a low conversion efficiency for hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline Si (μc-Si:H) double-junction PV modules employing a metal oxide intermediate reflector stems from the decrease in the active area as a result of the additional process. Meanwhile, double-junction PV modules employing an n-type hydrogenated microcrystalline Si oxide (n-μc-SiO x :H) intermediate reflector provide a higher conversion efficiency. Since the Si alloy intermediate reflector can avoid the lateral shunting, it may be a promising option for cost-effective mass production of large-area thin-film Si multi-junction PV modules. Although the developed intermediate reflectors have the high potential, the current status is limited at the research and development (R&D) level. Therefore, the up-scaling with the low cost, high throughput, and high yield is a key technological mission for mass production.

Thin-Film Silicon Triple-Junction Solar Cells on Highly Transparent Front Electrodes With Stabilized Efficiencies up to 12.8%

High-efficiency thin-film silicon triple-junction solar cells in p-i-n configuration have been fabricated using amorphous silicon top cell absorber layers, as well as microcrystalline silicon middle and bottom cell absorbers. The triple-junction cells were fabricated on boron doped zinc oxide (ZnO) films with different surface morphologies. To this end, the naturally grown rough ZnO surfaces were flattened using an Ar plasma for three different treatment times. For the shortest time, we achieved a summed current density over 30 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and initial and stabilized conversion efficiencies of 13.5% and 12.5%, respectively. For the medium treatment time, we obtained the highest efficiencies (13.7% initial and 12.8% stable), whereas the longest treatment time led to the highest open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ) of 1.91 V but lower current densities, leading to efficiencies of 12.9% initial and 12.2% stable, respectively. These results were obtained by combining various recently developed features and approaches: first of all, we implemented high-quality μc-Si:H cells with novel buffer layers, leading to very high efficiencies. Second, we applied randomly textured pyramids on the front glass to improve light in-coupling, and finally, we used very thin (~140 nm) top cells that led to a low light-induced degradation (5%-7% relative loss in efficiency).

Towards high efficiency thin-film crystalline silicon solar cells: The roles of light trapping and non-radiative recombinations

Thin-film solar cells based on silicon have emerged as an alternative to standard thick wafers technology, but they are less efficient, because of incomplete absorption of sunlight, and non-radiative recombinations. In this paper, we focus on the case of crystalline silicon (c-Si) devices, and we present a full analytic electro-optical model for p-n junction solar cells with Lambertian light trapping. This model is validated against numerical solutions of the drift-diffusion equations. We use this model to investigate the interplay between light trapping, and bulk and surface recombination. Special attention is paid to surface recombination processes, which become more important in thinner devices. These effects are further amplified due to the textures required for light trapping, which lead to increased surface area. We show that c-Si solar cells with thickness of a few microns can overcome 20% efficiency and outperform bulk ones when light trapping is implemented. The optimal device thickness in presence of light trapping, bulk and surface recombination, is quantified to be in the range of 10–80 μm, depending on the bulk quality. These results hold, provided the effective surface recombination is kept below a critical level of the order of 100 cm/s. We discuss the possibility of meeting this requirement, in the context of state-of-the-art techniques for light trapping and surface passivation. We show that our predictions are within the capability of present day silicon technologies.