Microcrystalline Silicon Solar Cells Research Articles

Optimization of LED-Based Solar Simulators for Cadmium Telluride and Microcrystalline Silicon Solar Cells

Solar simulators are instruments used for controllable measurements of the properties of solar cells in indoor environments. The purpose of this paper is to examine the peculiarities of the photoresponses of CdTe/CdSeTe and microcrystalline Si solar cells and to reveal the pathways to reduction of spectrum mismatch effects when using light-emitting diode (LED)-based or hybrid LED and halogen lamp-based solar simulators of an A+-class spectrum with a small number of sources. While only four different LED types are needed to achieve an A+-class spectrum under updated IEC 60904-9:2020 standard requirements, as demonstrated by our results, additional ultraviolet LEDs are necessary to reduce the spectrum mismatch. For hybrid solar simulator configurations, the combination of cool white LED arrays and halogen emitters can serve as a main light source. Optimized for both solar cell types, hybrid simulators have a lower spectral deviation and better spectrum coverage compared to LED-only simulators with the same number of distinct source types. In addition, our results predict lower spectral mismatch errors for optimized simulators when compared with conventional Xe lamp-based simulators.

Simulation of Solar Cell Device Performance Based on Hydrogenated Microcrystal Silicon (µc-Si:H) With Finite Element Method (FEM)

The structure and thickness of the solar cell device layer have a big impact on how well the solar cell works. There for, the aim of this study was to examine how hydrogenated microcrystalline silicon solar cells’ thickness and device structure affected how well they work. Simulation or modeling of the structure in one dimension (1D) is used for the analysis. MATLAB programming was used to analyze the simulation result. The optical band gap changes due to the influence of the structure, therefore the thickness of the p-, I, and n layers are kept constant at 250 Å, 9000 Å, and 250 Å respectively. The results showed that the maximum performance is obtained at the optical band gap, Eci = 1,39 eV, and the resulting power is 0.063465 Watt. Whereas in the simulation of the effect of the thickness on the i-layer, the optical band gap is set to a constant value of Eci = 1,4 eV, while the thickness of the p-layer and n-layer is set to 250 Å. The results also indicate that the maximum performance is at the i-layer thickness of 9000 Å and the power generated is 0.063364 Watt.

Textured solar cell modeling in TCAD

We investigate one of the promising methods for increasing the output characteristics of solar cells - surface texturing. The structures and characteristics of microcrystalline silicon solar cells are simulated. Solar Cells with different surface geometries are compared. The first type is the classic flat structure. The last and the most effective is Solar Cell with the surface in the form of microscopic pyramids separated by flat regions.

Stacked Back Reflector Architecture for Advanced Optical Management in State-of-the-Art Single-Junction μc-Si:H Solar Cells

The present work deals with an advanced technology for efficient light management through back reflection in thin silicon based solar cells, in particular, single-junction microcrystalline silicon (μc-Si:H) solar cells. Semiconductor nanoparticles (Ag2S NPs) have been chosen to design the back reflector layer (BRL) by embedding them within two thin layers of indium tin oxide (ITO). Owing to its nominal parasitic light absorption over a broad wavelength region (300–1100 nm), the Ag2S NPs can reflect back a notable amount of light that were supposed to get transmitted through the relatively less thick (∼2 μm) active microcrystalline Si layer, and hence, can function like nanomirrors. Encapsulation of the Ag2S NPs between the ITO layers provides chemical and physical stability to the nanomirrors. By placing such BRL at the back of this p-i-n based superstrate structure, a significant amount of light (>90%), mainly in the red and near infrared (NIR) region, was found to reflect back to the cell and this resulted in a state-of-the-art photoconversion efficiency of 9.36% for single-junction μc-Si:H solar cells. The reported value is one of the best in the class. As the main light absorption zone for μc-Si:H falls in the red and NIR region (bandgap energy of μc-Si:H is 1.1–1.2 eV), such back reflections become worthy. The experimentally observed facts have also been validated through theoretical simulations.

Efficiency enhancement for microcrystalline silicon solar cells by a lens-like structure to maintain improved photocarriers distribution

Spatial distribution of the photogenerated carriers in a hydrogenated microcrystalline silicon (μc-Si : H) solar cell has a significant impact on its electrical characteristics. Photogeneration distribution can be manipulated using photon management techniques. The electrical specifications of a typical thin-film μc-Si : H solar cell under various distributions of photocarriers are thoroughly investigated for both zero and higher bias voltages. For a thin-film μc-Si : H solar cell with a typically low thickness, we conclude that a photogeneration distribution having a greater rate in the central region of the solar cell performs optimally. This result is in contrast to the previously reported results in the literature for some other thin-film solar cell technologies, where the improved performance of a thin-film solar cell is believed to be when the photogenerated carriers are concentrated near the anode. It is demonstrated that the dominant contribution of dark carriers to recombination in the middle of the cell at higher bias voltages causes this effect. Moreover, we propose a lens-like structure by patterning top layers of a μc-Si : H solar cell, which approximately realizes the desired photogeneration distribution and provides an increase in the density of photocarriers in the middle of the solar cell, while it also acts as an antireflection coating. This proposed configuration has a 1.3% improvement in power conversion efficiency compared with a planar μc-Si : H solar cell with identical constituting layers and similar dimensions.

Ultra-fast plasmonic back reflectors production for light trapping in thin Si solar cells

A fast method is presented to fabricate plasmonic light trapping structures in just ten minutes (>5 × faster than the present state of art), with excellent light scattering properties. The structures are composed of silver nanoparticles (Ag NPs) deposited by thermal evaporation and self-assembled using a rapid thermal annealing (RTA) system. The effect of the RTA heating rate on the NPs production reveals to be crucial to the decrease of the annealing process. The Ag NPs are integrated in thin film silicon solar cells to form a plasmonic back reflector (PBR) that causes a diffused light reflectivity in the near-infrared (600–1100 nm wavelength region). In this configuration the thicknesses of the AZO spacer/passivating layers between NPs and rear mirror, and between NPs and silicon layer, play critical roles in the near-field coupling of the reflected light towards the solar cell absorber, which is investigated in this work. The best spacer thicknesses were found to be 100 and 60 nm, respectively, for Ag NPs with preferential sizes of about 200 nm. The microcrystalline silicon (μc-Si:H) solar cells deposited on such improved PBR demonstrate an overall 11% improvement on device efficiency, corresponding to a photocurrent of 24.4 mA/cm2 and an efficiency of 6.78%, against 21.79 mA/cm2 and 6.12%, respectively, obtained on flat structures without NPs.

Porous multi-junction thin-film silicon solar cells for scalable solar water splitting

Monolithic solar water splitting devices implemented in an integrated design approach, i.e. submerged in the electrolyte, pose a significant limitation when it comes to up-scaling. The ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This fact turns out to be a bottleneck for reaching high device efficiency and maintaining optimum performance upon up-scaling. In this paper, we propose a new device design for integrated monolithic solar water splitting based on porous multi-junction silicon solar cells. Simulation results highlight that porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. In particular, we show how micrometer scale pore dimensions could greatly reduce Ohmic losses, thereby minimizing overpotentials. A square array of holes with a diameter of 20 µm and a period of 100 µm was fabricated on single-junction and multi-junction amorphous and microcrystalline silicon solar cells. A small impact on the open circuit voltage (Voc) and short circuit current density (Jsc) was obtained, with porous triple junction solar cells reaching Voc values up to 1.98 V. A novel device design is proposed based on porous triple-junction silicon-based solar cells.

Management of light trapping capability of AZO film for Si thin film solar cells-via tailoring surface texture

In this paper, novel micro- and nano-composite Al-doped ZnO (AZO) films with broadband-gap light-trapping capacity was successfully fabricated by post-chemical-etching AZO film and re-sputtering self-textured H and Al co-doped ZnO (HAZO) film. The influence of pressure and etching time on the optical and electrical properties of HAZO, AZO, and AZO/HAZO films were systematically investigated. The performance of this novel ZnO film was increased remarkably compared to that of the post-etched AZO film or native textured HAZO film for both long and short wavelengths. Microcrystalline silicon solar cells deposited on this new type of AZO/HAZO films exhibited a strong enhancement of 13.4% in the conversion efficiency, demonstrating its wide application prospect.

Thin-film microcrystalline silicon solar cells: 11.9% efficiency and beyond

High-efficiency thin-film hydrogenated microcrystalline silicon solar cells (µc-Si:H) were developed using a periodically textured substrate at a relatively high growth rate of ∼1 nm s−1. A record efficiency of 11.9% was independently confirmed in a µc-Si:H cell with an absorber thickness of approximately 2 µm. An improvement in fill factor contributed largely to realizing the record efficiency. The potential for further efficiency improvements was examined on the basis of suns–VOC measurement, indicating that an efficiency of 12.5% is practically achievable.

Key Points in the Latest Developments of High‐Efficiency Thin‐Film Silicon Solar Cells

Thin‐film silicon has been intensively used in photovoltaic applications. In recent years, most of the record efficiencies in thin‐film silicon solar cells have been updated frequently, particularly in microcrystalline silicon solar cells. In this article, key points in the latest developments of high‐efficiency thin‐film silicon solar cells are reviewed with a short overview of thin‐film silicon technology. The main driver for the efficiency improvement of microcrystalline silicon cells was the development of sophisticated light scattering substrates. The well‐designed honeycomb‐textured substrates enabled a systematic and detailed optimization to realize the growth of high‐quality films and significant light absorption enhancement simultaneously. The knowledge obtained about microcrystalline silicon solar cells and the insights regarding light‐induced degradation of a‐Si:H solar cells were successfully transferred to triple junction cells, which was demonstrated by the achievement of the record stabilized efficiency of 14%.

Improvement of near-infrared diffuse reflectance of silver back reflectors through Ag2O formation by a UV-ozone exposure process

We report on the fabrication and analysis of highly optically reflective textured stacks consisting of silver oxide (Ag2O) coated silver for application as scattering back reflectors in thin-film solar cells. Thin Ag2O layers have been formed on textured silver back reflectors by using a UV-ozone (UVO) exposure technique, and during this formation, the stacks display improvement in their diffuse reflectance, particularly in the near infrared (NIR) spectral region. The silver oxide formed is composed of densely packed small grains with sizes from 20nm to 50nm, and their formation measurably increases the roughness of the silver reflector. The optimized silver reflector showed a ~ 6% increase in diffuse reflectance in the spectral range from 800nm to 1100nm while maintaining excellent total reflectance (over 95%). As a result, a high haze value (~ 98%) in the NIR region was achieved using these stacks. The stacks were tested as the back reflector in microcrystalline silicon solar cells. Due to the improved optical properties of the back reflector, a higher external quantum efficiency in the NIR region was obtained, and thus an increased short current density (from 22.3mA/cm2 to 23.6mA/cm2). It is also worth noting that the open circuit voltage was unchanged (0.529V) while the fill factor also increased (from 66.3% to 72.3%).

Enhanced efficiency of ultrathin (∼500 nm)-film microcrystalline silicon photonic crystal solar cells

Enhancing the absorption of thin-film microcrystalline silicon solar cells at 600–1000 nm wavelengths is very important to the improvement of the energy conversion efficiency. This can be achieved by creating a large number of resonant modes utilizing two-dimensional photonic crystal band edges, which exceeds the Lambertian limit of absorption in random textures. We focus on suppressing the parasitic absorption of back-reflector metal and doped layers in photonic crystal microcrystalline silicon solar cells. We achieve a high active-area current density of 22.6 mA cm−2 for an ultrathin (∼500 nm)-film silicon layer and obtain an active-area efficiency of ∼9.1%, as independently confirmed by the CSMT of AIST.

From randomly self-textured substrates to highly efficient thin film solar cells: Influence of geometric interface engineering on light trapping, plasmonic losses and charge extraction

An approach that models the fabrication and enables the characterization of randomly self-textured silicon thin film solar cells is developed and are compared to experimental results. The optical and electrical properties of the solar cells depend on the morphology of the randomly self-textured substrate and the formation of the individual layers of the solar cell. The influence of the interface morphology on the optical and electrical properties is investigated by 3D morphological algorithms. The calculated interface morphologies are compared to measured surfaces, and used as input parameters to simulate the optical wave propagation and to determine the formation of regions with reduced order (cracks) in the solar cells. The calculations are compared to experimentally realized 1.1µm thick microcrystalline silicon solar cells prepared on randomly self-textured substrates with high energy conversion efficiency of up to 9.4%. Guidelines for the optical and electronic optimization are provided.

Modeling of triangular-shaped substrates for light trapping in microcrystalline silicon solar cells

The influence of triangular grating used as a light trapping structure on the optical wave propagation within thin-film microcrystalline silicon (µc-Si:H) solar cells is investigated. A finite difference time domain (FDTD) approach is used to rigorously solve the Maxwell's equations in three dimensions. We apply two parameters of mean surface roughness (Sa) and slope (k) to define triangular structure and study their influence on the absorption of µc-Si:H. When Sa and k are set to 400nm and 1, respectively, a largest enhancement of absorption is achieved. The optimum short circuit photocurrent (Jsc) of a 1-µm thick µc-Si:H solar cell made on such a textured substrate can reach 27.0mA/cm2. The carrier generation rate in the µc-Si:H material is also rigorously analyzed. Finally, we identify some key optical losses in µc-Si:H solar cells and propose for further optimizing the device design.

Enhanced photon management in silicon thin film solar cells with different front and back interface texture.

Light trapping and photon management of silicon thin film solar cells can be improved by a separate optimization of the front and back contact textures. A separate optimization of the front and back contact textures is investigated by optical simulations taking realistic device geometries into consideration. The optical simulations are confirmed by experimentally realized 1 μm thick microcrystalline silicon solar cells. The different front and back contact textures lead to an enhancement of the short circuit current by 1.2 mA/cm2 resulting in a total short circuit current of 23.65 mA/cm2 and an energy conversion efficiency of 8.35%.

Broadband absorption enhancement in plasmonic nanoshells-based ultrathin microcrystalline-Si solar cells.

With the objective to conceive a plasmonic solar cell with enhanced photocurrent, we investigate the role of plasmonic nanoshells, embedded within a ultrathin microcrystalline silicon solar cell, in enhancing broadband light trapping capability of the cell and, at the same time, to reduce the parasitic loss. The thickness of the considered microcrystalline silicon (μc-Si) layer is only ~1/6 of conventional μc-Si based solar cells while the plasmonic nanoshells are formed by a combination of silica and gold, respectively core and shell. We analyze the cell optical response by varying both the geometrical and optical parameters of the overall device. In particular, the nanoshells core radius and metal thickness, the periodicity, the incident angle of the solar radiation and its wavelength are varied in the widest meaningful ranges. We further explain the reason for the absorption enhancement by calculating the electric field distribution associated to resonances of the device. We argue that both Fabry-Pérot-like and localized plasmon modes play an important role in this regard.

Aluminium induced texturing of glass substrates with improved light management for thin film solar cells

Aluminium induced texturing (AIT) method has been used to texture glass substrates to enhance photon absorption in microcrystalline thin film Si solar cells. In this process, a thin Al film is deposited on a glass substrate and a non-uniform redox reaction between the glass and the Al film occurs when they are annealed at high temperature. After etching the reaction products, the resultant glass surface presents a uniform and rough morphology. In this work, three different textures (σrms~85, ~95, ~125nm) have been achieved by tuning the dc sputtering power and over them and over smooth glass, pin microcrystalline silicon solar cells have been fabricated. The cells deposited over the textured substrates showed an efficiency improvement in comparison to the cells deposited over the smooth glass. The best result was given for the glass texture σrms~125nm that led to an average efficiency 2.1% higher than that given by the cell deposited on smooth glass.

Simulation of light-induced degradation of μc-Si in a-Si/μc-Si tandem solar cells by the diode equivalent circuit

Silicon-based thin film tandem solar cells consist of one amorphous (a-Si) and one microcrystalline (μc-Si) silicon solar cell. The Staebler - Wronski effect describes the light- induced degradation and temperature-dependent healing of defects of silicon-based solar thin film cells. The solar cell degradation depends strongly on operation temperature. Until now, only the light-induced degradation (LID) of the amorphous layer was examined in a-Si/μc-Si solar cells. The LID is also observed in pc-Si single function solar cells. In our work we show the influence of the light-induced degradation of the μc-Si layer on the diode equivalent circuit. The current-voltage-curves (I-V-curves) for the initial state of a-Si/pc-Si modules are measured. Afterwards the cells are degraded under controlled conditions at constant temperature and constant irradiation. At fixed times the modules are measured at standard test conditions (STC) (AM1.5, 25°C cell temperature, 1000 W/m2) for controlling the status of LID. After the degradation the modules are annealed at dark conditions for several hours at 120°C. After the annealing the dangling bonds in the amorphous layer are healed, while the degradation of the pc-Si is still present, because the healing of defects in pc-Si solar cells needs longer time or higher temperatures. The solar cells are measured again at STC. With this laboratory measured I-V-curves we are able to separate the values of the diode model: series Rs and parallel resistance Rp, saturation current Is and diode factor n.

On the interplay of cell thickness and optimum period of silicon thin‐film solar cells: light trapping and plasmonic losses

AbstractLight trapping and photon management in honeycomb‐textured microcrystalline silicon solar cells are investigated experimentally and by modeling of the manufacturing process and the optical wave propagation. The solar cells on honeycomb‐textured substrates exhibit short circuit current densities exceeding 30 mA/cm2 and energy conversion efficiencies of up to 11.0%. By controlling the fabrication process, the period and height of the honeycomb‐textured substrates are varied. The influence of the honeycomb substrate morphology on the interfaces of the individual solar cell layers and the quantum efficiency is determined. The optical wave propagation is calculated using 3D finite difference time domain simulations. A very good agreement between the optical simulation and experimental results is obtained. Strategies are discussed on how to increase the short circuit current density beyond 30 mA/cm2. In particular, the influence of plasmonic losses of the textured silver (Ag) reflector on the short circuit current and quantum efficiency of the solar cell is discussed. Finally, solar cell structures with reduced plasmonic losses are proposed. Copyright © 2015 John Wiley & Sons, Ltd.

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (µc-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated µc-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.