Amorphous Silicon Thin-film Solar Cells Research Articles

Effect of TiO2 nanopatterns on the performance of hydrogenated amorphous silicon thin-film solar cells

We investigate how TiO2 nanopatterns formed onto ZnO:Al (AZO) films affect the performance of hydrogenated amorphous silicon (a-Si:H) solar cells. Scanning electron microscopy results show that the dome-shaped TiO2 nanopatterns (300nm in diameter) having a period of 500nm are formed onto AZO films and vary from 60 to 180nm in height. Haze factor increases with an increase in the height of the nanopatterns in the wavelength region below 530nm. Short circuit current density also increases with an increase in the height of the nanopatterns. As the nanopatterns increases in height, the fill factor of the cells slightly increases, reaches maximum (0.64) at 100nm, and then decreases. Measurements show that a-Si:H solar cells fabricated with 100nm-high TiO2 nanopatterns exhibit the highest conversion efficiency (6.34%) among the solar cells with the nanopatterns and flat AZO sample.

On-line thermal dependence study of the main solar cell electrical photoconversion parameters using low thermal emission lamps

This paper presents a non-conventional methodology and an instrumental system to measure the effect of temperature on the photovoltaic properties of solar cells. The system enables the direct measurement of the evolution of open-circuit voltage and short-circuit current intensity in relation to a continuously decreasing temperature. The system uses a high-intensity white light-emitting diode light source with low emissions of radiation in the infrared region of the electromagnetic spectrum, resulting in a reduced heating of the photovoltaic devices by the irradiation source itself. To check the goodness of the system and the methodology designed, several measurements were performed with monocrystalline silicon solar cells, dye-sensitized solar cells, and thin-film amorphous silicon solar cells, showing similar tendencies to those reported in the literature.

Numerical Analysis of Efficiency Enhancement in Plasmonic Thin-Film Solar Cells by Using the SILVACO TCAD Simulator

A physical model for simulating plasmonic solar cells (SCs) using the SILVACO TCAD simulator is established and the effects of some factors on the efficiency enhancement of the amorphous silicon thin film SCs are simulated. Through this simulation, it is demonstrated that our method can successfully simulate the optical and electrical properties of plasmonic solar cells without the overestimation of the characteristics and without the neglect of parameter change in the device operation process. It is shown that not only the size and kind of metal nanoparticles but also other factors, such as the surrounding medium, the distance from the bottom of particles to the device surface, and the light incident angle, play important roles in the optical and electrical properties of plasmonic SCs.

Study on the fabrication of silicon nanoparticles in an amorphous silicon light absorbing layer for solar cell applications

Hydrogenated amorphous-silicon (a-Si:H) thin-film solar cells have advantages of relatively simple technology, less material consumption, higher absorption ratio compared to crystalline silicon, and low cost due to the use of cheaper substrates rather than silicon wafers. However, together with those advantages, amorphous-silicon thin-film solar cells face several issues such as a relatively lower efficiency, a relatively wider bandgap, and the Staebler-Wronski effect (SWE) compared to other competing materials (i.e., crystalline silicon, CdTe, Cu(InxGa(1−x))Se2 (CIGS), etc.). As a remedy for those drawbacks and a way to enhance the cell conversion efficiency at the same time, the employment of crystalline silicon nanoparticles (Si-NPs) in the a-Si matrix is proposed to organize the quantum-dot (QD) structure as the light-absorbing layer. This structure of the light absorbing layer consists of single-crystal Si-NPs in an a-Si:H thin-film matrix. The single-crystal Si-NPs are synthesized by using SiH4 gas decomposition with CO2 laser pyrolysis, and the sizes of Si-NPs are calibrated to control their bandgaps. The synthesized size-controlled Si-NPs are directly transferred to another chamber to form a QD structure by using co-deposition of the Si-NPs and the a-Si:H matrix. Transmission electron microscopy (TEM) analyses are employed to verify the sizes and the crystalline properties of the Si-NPs alone and of the Si-NPs in the a-Si:H matrix. The TEM results show successful co-deposition of size-controlled Si-NPs in the a-Si:H matrix, which is meaningful because it suggests the possibility of further enhancement of the a-Si:H solar-cell structure and of tandem structure applications by using a single element.

Simulations of solar cell absorption enhancement using resonant modes of a nanosphere array

We propose an approach for enhancing the absorption of thin-film amorphous silicon solar cells using periodic arrangements of resonant dielectric nanospheres deposited as a continuous film on top of a thin planar cell. We numerically demonstrate this enhancement using three dimensional (3D) full field, finite difference time domain simulations and 3D finite element device physics simulations of a nanosphere array above a thin-film amorphous silicon solar cell structure featuring back reflector and anti-reflection coating. In addition, we use the full field finite difference time domain results as input to finite element device physics simulations to demonstrate that the enhanced absorption contributes to the current extracted from the device. We study the influence of a multi-sized array of spheres, compare spheres and domes, and propose an analytical model based on the temporal coupled mode theory.

Optimal design of light trapping in thin-film solar cells enhanced with graded SiN_x and SiO_xN_y structure

In this paper, a graded SiNx and SiOxNy structure is proposed as antireflection coatings deposited on top of amorphous silicon (α-Si) thin-film solar cell. The structural parameters are optimized by differential evolution in order to enhance the optical absorption of solar cells to the greatest degree. The optimal design result demonstrates that the nonlinear profile of dielectric constant is superior to the linear profile, and discrete multilayer graded antireflection coatings can outperform near continuously graded antireflection coatings. What's more, the electric field intensity distributions clearly demonstrate the proposed graded SiNx and SiOxNy structure can remarkably increase the magnitude of electric field of a-Si:H layer and hence, enhance the light trapping of a-Si:H thin-film solar cells in the whole visible and near-infrared spectrum. Finally, we have compared the optical absorption enhancements of proposed graded SiNx and SiOxNy structure with nanoparticles structure, and demonstrated that it can result in higher enhancements compared to the dielectric SiC and TiO2 nanoparticles. We have shown that the optimal graded SiNx and SiOxNy structure optimized by differential evolution can reach 33.31% enhancement which has exceeded the ideal limit of 32% of nanoparticles structure including plasmonic Ag nanoparticles, dielectric SiC and TiO2 nanoparticles.

Effect of nanotextured back reflectors on light trapping in flexible silicon thin-film solar cells

Nanotextured Ag back reflectors were used to enhance the short-circuit current of flexible silicon thin-film solar cells with an n-i-p configuration by means of optical confinement. A nanotextured topography with a root-mean-square (σrms) surface roughness of 88.0nm was successfully induced by abnormal grain growth of Ag films, which was controlled by varying the deposition temperature and film thickness in a direct current (dc) magnetron sputtering process. Effective light scattering of the long wavelengths over 600nm was achieved on the nano-textured Ag back reflectors, resulting in enhanced absorption of weakly absorbing, long-wavelength light in the hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) thin-film solar cells. Compared with similar reference cells on flat back reflectors, a 34.4% increase in the short-circuit current density (Jsc) for an a-Si:H solar cell and an 11.9% for μc-Si:H were observed in the solar cells on the nanotextured back reflectors, with little reduction in fill factor (FF) and open circuit voltage (Voc). Due to the increase of the Jsc by the improved light absorption at the long wavelengths, the conversion efficiency (η) of the a-Si:H and μc-Si:H solar cells increased from 5.59% to 7.60% and from 4.31% to 4.64%, respectively.

Experimental analysis and computer simulation of a methodology for laser focusing in the solar cell characterization by laser beam induced current

This paper presents a quick methodology for focusing a laser beam on a photoactive surface based on performing a single line scan while simultaneously modifying the laser's position over the surface and the distance between the focusing lens and the active surface. The methodology was tested using the computer simulation technique. Several configurations were computer simulated by programming different experimental situations to discover the situations in which this focusing methodology would provide optimum results. The conclusions obtained from computer simulation methodology were checked by means of experimental tests using several solar cells, such as a thin-film amorphous silicon solar cell, a monocrystalline silicon solar cell, and a polycrystalline silicon solar cell. From the tests performed, we concluded that optimum focusing is achieved in systems in which the laser beam induced current signal generated by the photoactive surface has no large heterogeneities (e.g., fingers or grain boundaries), artefacts, or defects. Thus, the best results are achieved in systems where the surface of the photovoltaic device has a certain degree of homogeneity.

Light Trapping in Solar Cells: Can Periodic Beat Random?

Theory predicts that periodic photonic nanostructures should outperform their random counterparts in trapping light in solar cells. However, the current certified world-record conversion efficiency for amorphous silicon thin-film solar cells, which strongly rely on light trapping, was achieved on the random pyramidal morphology of transparent zinc oxide electrodes. Based on insights from waveguide theory, we develop tailored periodic arrays of nanocavities on glass fabricated by nanosphere lithography, which enable a cell with a remarkable short-circuit current density of 17.1 mA/cm(2) and a high initial efficiency of 10.9%. A direct comparison with a cell deposited on the random pyramidal morphology of state-of-the-art zinc oxide electrodes, replicated onto glass using nanoimprint lithography, demonstrates unambiguously that periodic structures rival random textures.

Enhanced spectral response by silicon nitride index matching layer in amorphous silicon thin-film solar cells

We employed the low temperature hydrogenated amorphous silicon nitride (a-SiNx:H) prepared by plasma-enhanced chemical vapor deposition as a refractive index (n) matching layers in a silicon-based thin-film solar cell between glass (n=1.5) and the transparent conducting oxide (n=2). By varying the stoichiometry, refractive index and thickness of the a-SiNx:H layers, we enhanced the spectral response and efficiency of the hydrogenated amorphous silicon thin-film solar cells. The refractive index of a-SiNx:H was reduced from 2.32 to 1.78. Optimizing the a-SiNx:H thickness to 80nm increased the JSC from 8.3 to 9.8mA/cm2 and the corresponding cell efficiency increased from 4.5 to 5.3%, as compared to the cell without the a-SiNx:H index-matching layer on planar substrate. The a-SiNx:H layers with graded refractive indices were effective for enhancing the cell performance.

Broadband Enhancement in Thin-Film Amorphous Silicon Solar Cells Enabled by Nucleated Silver Nanoparticles

Recently plasmonic effects have gained tremendous interest in solar cell research because they are deemed to be able to dramatically boost the efficiency of thin-film solar cells. However, despite of the intensive efforts, the desired broadband enhancement, which is critical for real device performance improvement, has yet been achieved with simple fabrication and integration methods appreciated by the solar industry. We propose in this paper a novel idea of using nucleated silver nanoparticles to effectively scatter light in a broadband wavelength range to realize pronounced absorption enhancement in the silicon absorbing layer. Since it does not require critical patterning, experimentally these tailored nanoparticles were achieved by the simple, low-cost and upscalable wet chemical synthesis method and integrated before the back contact layer of the amorphous silicon thin-film solar cells. The solar cells incorporated with 200 nm nucleated silver nanoparticles at 10% coverage density clearly demonstrate a broadband absorption enhancement and significant superior performance including a 14.3% enhancement in the short-circuit photocurrent density and a 23% enhancement in the energy conversion efficiency, compared with the randomly textured reference cells without nanoparticles. Among the measured plasmonic solar cells the highest efficiency achieved was 8.1%. The significant enhancement is mainly attributed to the broadband light scattering arising from the integration of the tailored nucleated silver nanoparticles.

Properties of thin-film silicon solar cells at very high irradiance

The focussed beam of a low-power helium–neon laser is used to study accelerated light-induced degradation (Staebler–Wronski effect) and high steady-state photocarrier generation rates in amorphous and microcrystalline silicon thin-film solar cells, at up to 13MWm−2 irradiance. Even at these high power densities, COMSOL® simulations indicate that heat diffusion into the substrate, aided by spreading conduction via the Ag back-contact, restricts the temperature rise to less than 14°C. Short-circuit current may be measured directly, and the I–V characteristic estimated by taking into account shunting by the inactive part of the cell. The improved resistance to degradation of microcrystalline silicon cells is shown to persist to high irradiance. Computer simulations of an amorphous silicon solar cell are presented that are consistent with measured un-degraded and degraded properties, and offer insight into prevailing defect creation processes and carrier recombination mechanisms.

非晶硅薄膜太阳能电池的紫外激光刻蚀工艺

非晶硅薄膜太阳能电池的紫外激光刻蚀工艺

Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion

Single-threshold solar cells are fundamentally limited by their ability to harvest only those photons above a certain energy. Harvesting below-threshold photons and re-radiating this energy at a shorter wavelength would thus boost the efficiency of such devices. We report an increase in light harvesting efficiency of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell due to a rear upconvertor based on sensitized triplet–triplet-annihilation in organic molecules. Low energy light in the range 600–750 nm is converted to 550–600 nm light due to the incoherent photochemical process. A peak efficiency enhancement of (1.0 ± 0.2)% at 720 nm is measured under irradiation equivalent to (48 ± 3) suns (AM1.5). We discuss the pathways to be explored in adapting photochemical UC for application in various single threshold devices.

Power effect of ZnO:Al film as back reflector on the performance of thin-film solar cells

Aluminum-doped zinc oxide (AZO) is attracting interest as a potential transparent conducting oxide material for use in amorphous silicon (a-Si) thin-film solar cells. The absorption loss in the n-a-Si:H/Ag interface of the p-i-n thin film solar is high because the extinction coefficient of the Ag that is used as a back reflector is high. In this work, transparent conducting AZO films with a power in the range 500 W to 900 W prepared under Ar-ambient at a substrate temperature of 25 °C by RF-magnetron in-line sputtering. To minimize the absorption loss at longer wavelengths, an AZO layer we inserted at the n-a-Si:H/Ag interface of a solar cell with a glass/SnO2:F/p-a-SiC:H/buffer-layer/i-a-Si:H/n-a-Si:H/Ag structure and the performance of the cell with AZO/Ag deposited instead of Ag on the back contact, was investigated. The effects of the RF-magnetron sputtering deposition parameters on the optical, electrical and structural properties of the AZO films were analyzed. Optimized AZO films with high transmittance (> 80%) and low resistivity (1.47 × 10−3 Ω-cm) in the wavelength range 400–800 nm were obtained under the sputtering conditions. The p-i-n a-Si solar cell with the AZO/Ag back contact had a conversion efficiency of 9.42%, which was higher than that (7.36%) of the cell without the AZO/Ag back contact. Light-soaking of these cells for 1000 h at 60 °C led to stabilization at 8.32% for the best cell. The relative typical efficiency degradation of about 10% for single-junction cells demonstrates that this treatment does not negatively influence stability.

Highly reflective nanotextured sputtered silver back reflector for flexible high-efficiency n–i–p thin-film silicon solar cells

High reflectivity is essential when a metal is used as back contact and reflector in thin-film silicon solar cells. We show that thermal annealing at 150 °C improves the reflectivity of silver films deposited by sputtering at room temperature on nanotextured substrates. The annealing provokes two interlinked effects: rearrangement of the silver layer with a modification of its morphology and an increase of up to 42% in the grain size of the polycrystalline film for the preferential orientation as measured by X-ray diffraction. The main consequence of these two mechanisms is a large increase in the reflectivity of silver when measured in air. This reflectivity increase is also noticeable in devices: amorphous silicon thin-film solar cells grown on annealed silver films yield higher internal and external quantum efficiencies compared to cells grown on as-deposited silver. The morphology modification smoothes down the substrate, which is revealed by a clear increase of the open-circuit voltage and fill factor of the cells grown on top. An amorphous silicon cell with a 200 nm nominally thick i-layer fabricated on a flexible plastic substrate yielded an initial efficiency close to 10% with 15.9 mA/cm 2 of short-circuit current using highly reflective annealed textured silver. We also propose, for industrial purpose, the sputtering of thin silver layer (120 nm) under moderate substrate temperature (∼150 °C) to increase the layer reflectivity, which avoids lengthening of the back reflector fabrication.

TCAD studies of novel nanoplate amorphous silicon alloy thin-film solar cells

A novel nanoplate-structured thin-film solar cell was investigated that could solve the conflict between light absorption and carrier transport in a p-type amorphous silicon carbide (a-SiC)/i-type amorphous silicon germanide (a-SiGe)/n-type amorphous silicon (a-Si) thin-film solar cell. This structure has an n-type a-Si nanoplate array on the substrate, a-SiC p-layer, and an a-SiGe i-layer which are sequentially grown along the surface of each n-type a-Si nanoplate. Under illumination by sunlight, light is absorbed along the vertical direction of the nanoplate, while the carrier transport is along the horizontal direction. The nanoplate structure may absorb most of the sunlight and provide a thinner film for the effective transport of photon-generated carriers as compared to the conventional planar structure.

The Effect of Carrier Injection Stress on Boron-Doped Amorphous Silicon Suboxide Layers Investigated by X-ray Photoelectron Spectroscopy

In amorphous silicon solar cells, reducing degradation is one of the key issues in improving cell performance. The degradation of the p-layer can play an important role since it is directly related to the open circuit voltage (Voc) and fill factor (FF) in the cells. In this study, we investigated the changes in boron-doped p-type silicon suboxide (SiOx) layers after carrier injection stress. The boron doping level was varied by controlling B2H6 gas flow rate. When these layers were degraded, the dark conductivity decay decreased from 53% to less than 5%, and the increase in activation energy decreased from 11 to 0.5% depending on the B2H6 gas flow rate increase. Our improvements are explained in conjunction with the three- and four-fold coordinated boron atoms by the shift of the B 1s X-ray photoelectron spectrum. In this paper we present how to improve the stability of hydrogenated amorphous silicon (a-Si:H) thin-film solar cells.

Morphological and Electrical Properties of Self-Textured Aluminum-Doped Zinc Oxide Films Prepared by Direct Current Magnetron Sputtering for Application to Amorphous Silicon Solar Cells

Self-textured Al-doped zinc oxide (AZO) transparent conducting oxide (TCO) layers were grown by DC magnetron sputtering, and their morphological and electrical properties were investigated. The self-textured AZO layers were fabricated by controlling the deposition conditions, without employing wet or dry etching for surface texturing. The polycrystalline AZO layers with a high surface roughness (42 nm) were grown on a glass substrate at various DC plasma power. The lowest sheet resistance was 3.7×10-4 Ω cm and the highest carrier mobility obtained was 30 cm2 V-1 s-1, which are comparable to those of chemically textured AZO layers. The electrical and optical characteristics of the self-textured AZO films are comparable to those of wet-etched AZO films that were etched using the conventional HCl solution. Our study suggests a possible way to inexpensively fabricate amorphous silicon thin-film solar cells.

Morphological and Electrical Properties of Self-Textured Aluminum-Doped Zinc Oxide Films Prepared by Direct Current Magnetron Sputtering for Application to Amorphous Silicon Solar Cells

Self-textured Al-doped zinc oxide (AZO) transparent conducting oxide (TCO) layers were grown by DC magnetron sputtering, and their morphological and electrical properties were investigated. The self-textured AZO layers were fabricated by controlling the deposition conditions, without employing wet or dry etching for surface texturing. The polycrystalline AZO layers with a high surface roughness (42 nm) were grown on a glass substrate at various DC plasma power. The lowest sheet resistance was 3.7×10-4 Ω cm and the highest carrier mobility obtained was 30 cm2 V-1 s-1, which are comparable to those of chemically textured AZO layers. The electrical and optical characteristics of the self-textured AZO films are comparable to those of wet-etched AZO films that were etched using the conventional HCl solution. Our study suggests a possible way to inexpensively fabricate amorphous silicon thin-film solar cells.