Silicon Absorber Layer Research Articles

Signal Components and Impedance Spectroscopy of Potential p‐Si/n‐CdS/ALD‐ZnO Solar Cells: EIS and SCAPS‐1D Treatments

AbstractA silicon heterojunction (SHJ) solar cell with the attractive and widely used atomic layer deposited (ALD)‐ZnO/n‐CdS/p‐Si configuration is examined in this work to learn more about its electrical properties. Using EIS and SCAPS‐1D, a comprehensive model of the device is created and then simulated. Theoretical aspects of the cell are examined through the use of similar electrical circuit models, focusing on the transmittance spectrum made possible by the ALD‐ZnO layer's low reflectance and high visible transmittance. In this study, the C–V tool is used to study the trap states in the silicon absorber layer under different lighting conditions and wavelengths. The doping concentration and built‐in potential are determined using the Mott–Schottky technique. In addition, the cell's properties are investigated by measuring its G–V, G–F, C–T, and C–F in different real‐world scenarios. As a means of visualizing the electrochemical impedance data, Nyquist plots—sometimes called Cole–Cole plots—are utilized. By utilizing absolute impedance and phase shifts, Bode plots are employed to examine the system's frequency response. Last, the results of the SHJ cell's spectral response measurements are given, which confirm the results of the Nyquist plots.

Monolith Cs1-xRbxSnI3 perovskite – silicon 2T tandem solar cell using SCAPS-1D

Tandem solar cells (TSCs) have gained notoriety by the use of various absorber layers with different bandgaps. The photovoltaic characteristics of Cs1-xRbxSnI3 perovskite – silicon TSCs were determined through simulation using the SCAPS-1D software in this work by first validating the experimentally obtained efficiency of 2.08% for ITO/Cs0.8Rb0.2SnI3/PCBM/BCP/Al structure. The influence of chlorinated and undoped ITO front contact, variation of Electron Transport Layer (ETL) thickness, doping concentration, CBO and variation of absorber layer thickness, defect density, and doping concentration was studied. Optimum VOC (0.9893 V), JSC (30.04 mA/cm2), fill factor (81.78 %) and efficiency (24.31 %) were determined. The bottom cell was simulated independently with reference to experimental data using the structure Al/c-Si (n)/c-Si (p)/c-Si (p + )/Au resulting in an efficiency of 26.68 %. The monolithic Cs0.8Rb0.2SnI3 perovskite – silicon tandem solar cell of the architecture ITO/Cs0.8Rb0.2SnI3/c-Si (n)/c-Si (p)/c-Si (p + )/Au performance was analyzed by varying the thickness, doping concentration, and defect density of the active layers. Optimized parameters obtained were as follows: top perovskite layer thickness (100 nm), doping concentrations (5 × 1019 cm-3) and defect density (1 × 1013 cm-3), and bottom silicon absorber layer thickness (50 μm), doping concentrations (5 × 1016 cm-3), defect density (1 × 1012 cm-3), and a work function of 5.3 eV with chlorinated ITO as the front contact of the tandem cell. Optimized outcomes of efficiency (29.82 %), VOC (0.7992 V), JSC density (43.39 mA/cm2), and fill factor (85.98 %) were realized for the 2T Cs0.8Rb0.2SnI3 perovskite – silicon tandem solar cell.

Energy-harvesting photovoltaic transparent tandem devices using hydrogenated amorphous and microcrystalline silicon absorber layers for window applications

Transparent photovoltaic (TPV) tandem devices were prepared via plasma-enhanced chemical vapor deposition, using hydrogenated amorphous (a-Si:H) and microcrystalline (mc-Si:H) absorber layers that were sandwiched by transparent conducting oxide (TCO) electrodes. The high-performance TPV tandem devices were designed by controlling the film thickness of the mc-Si:H absorbers. For the TPV tandem devices with 300-nm-thick a-Si:H top cells, increasing the absorber thickness (tbottom) of mc-Si:H bottom cells from 1.0 to 2.1 μm induced an increase in power conversion efficiencies (PCEs) from 6.5% to 8.3% with a minor decrease in the average light transmittance from 16.6% to 14.6% at the 500–800 nm wavelengths. The TPV tandem devices exhibited JSC gains from 4.3 mA cm−2 (tbottom = 1.0 μm) to 0.5 mA cm−2 (tbottom = 2.3 μm) under bifacial illumination (front: 100 mW cm−2 and rear: 30 mW cm−2). This further resulted in additional PCE gains from 2.8% (tbottom = 1.0 μm) to 0.4% (tbottom = 2.3 μm), in comparison to the corresponding samples under front illumination only. The TPV tandem devices demonstrated excellent PV performance even at low-light illumination intensities (∼21 mW cm−2) in comparison to those at standard illumination light intensity (100 mW cm−2), while maintaining PCE values as high as 80%.

Optimization of electrical properties for performance analysis of p-Si/n-CdS/ITO heterojunction photovoltaic cell

There is always a photogenerated current in solar photovoltaics because of the movement of charge carriers in the photovoltaic device. Therefore, it is necessary and exciting to explore its electrical properties. Thus, in the present communication, the electrical properties (as electron/hole mobilities, the effect of electron affinity, series/shunt resistance, etc.) have been optimized for p-Si photovoltaic cells with thin n-CdS emitter and ITO window layer. In this context, the signature of charge carrier mobilities on the performance of the proposed configuration, i.e., the hole mobility of the p-Si (p-type Crystalline Silicon) absorber layer and the electron mobility of the n-CdS (n-type Cadmium Sulphide) emitter/buffer layer has been optimized. Under the variation in hole mobility from 50 cm2/Vs to 500 cm2/Vs and the electron mobility from 10 cm2/Vs to 100 cm2/Vs simultaneously, the observed 2D contour plots reveal the effective change in power conversion efficiency from 17.72 % to 18.25 %. Moreover, the effect of electrical parameters, i.e., series and shunt resistance, on the characteristic performance parameters such as power conversion efficiency (η), open-circuit voltage (Voc), short circuit current density (Jsc), and fill factor (FF). An increase in series resistance (0 Ωcm2 to 10 Ωcm2) deteriorates the performance of the proposed solar cell considerably (19.66% to 8.87%). Further, the effect of the electron affinity of the p-Si absorber layer on fill factor and open circuit voltage is observed.

Thin silicon interference solar cells for targeted or broadband wavelength absorption enhancement.

We present the concept of interference solar cells reliant on spectrum filtering or splitting to enhance absorption in thin (<13 µm) silicon absorber layers, both for targeted wavelengths and broadband absorption. Absorption enhancement in the long wavelength regime is achieved by fine-tuning of device layer thicknesses to provide destructive interference between reflected and escaped waves. We suggest this concept is also suitable for broadband absorption enhancement when combined with spectrum splitting optics through gradual thickness changes laterally across the device. Using the example of silicon heterojunction solar cells, we have computationally demonstrated a short circuit current density enhancement of 19% (from 25.8 mA/cm2 to 30.7 mA/cm2) compared to a silicon heterojunction cell of the same absorber layer thickness.

Tailored Nanostructures for Light Management in Silicon Heterojunction Solar Cells

Efficient light management is key for optimal performance of silicon solar cells. For monocrystalline single‐junction devices, there is an established industrially viable technology using pyramidal micro‐structured silicon wafers. As the efficiencies of market‐dominating silicon single‐junction solar cells are approaching their physical limit, innovative cell concepts are required to further reduce the levelized cost of electricity. Tandem solar cells combining silicon and perovskite absorber layers are regarded as a promising technology to overcome this limit at low costs. However, the combination of micro‐structured silicon and solution‐processed sub‐micrometer perovskite layers is a challenge, calling for alternative texturing methods. Herein, a technology to implement tailored nanostructures into silicon heterojunction solar cells by combining nanoimprint lithography, reactive ion etching, and wet chemical etching is introduced. These structures have a height below 400 nm and are thus compatible with perovskite spin‐coating. Efficiencies above 20% and an equivalent optical performance at near infrared wavelengths compared with pyramidal micro‐structures are demonstrated. This technology not only enables a variety of tailored structures in both mono‐ and multicrystalline silicon solar cell devices, but also paves the way for optically improved solution‐processed monolithic perovskite–silicon tandem solar cells.

Development of silicon nanowires with optimized characteristics and fabrication of radial junction solar cells with < 100 nm amorphous silicon absorber layer

We report the growth and characterization of Silicon nanowires (SiNWs) via the VLS mechanism with two different tin (Sn) catalyst thicknesses of 2 and 4 nm at 400 and 500 °C by PECVD. The effect of catalyst thickness, plasma power density and growth temperature on the morphology of nanowires is investigated. The structural morphology of the grown NWs is analyzed using FESEM and HRTEM. It has been inferred the density, diameter and length of NWs can be effectively controlled by PECVD process parameters. HRTEM analysis demonstrates a highly crystalline silicon core with (111) plane as preferred orientation. Grazing incidence X-ray diffraction spectra of the samples prepared at two different temperatures confirmed the (111), (220) and (311) plane orientation of the grown NWs. Raman spectra of the SiNWs grown on glass substrate shown broadened spectrum centering at 508 cm-1 confirming the crystalline nature of NWs core surrounded with dense layer of a-Si. Optimized parameters are used to fabricate radial junction solar cells and the effect of catalyst thickness on the performance is explored. Short circuit current density of 13.53 mA/cm2 and open circuit voltage of 0.82 V for 0.25 cm2 solar cell having power conversion efficiency of 6.66% is obtained.

Multi crystalline silicon thin films grown directly on low cost soda-lime glass substrates

Liquid phase crystallization of silicon is a promising technology platform to grow multi crystalline silicon thin films on foreign substrates. For solar cell application it has already been demonstrated that open circuit voltages of up to 661 mV [1] and efficiencies of up to 15.9% [2] can be achieved on a silicon layer of a few microns only. However, while the quality of the material has been continuously improved, the cost factor of the utilized substrate has been given little attention. The present work focuses on the technology transfer from technical glass substrates to low cost soda-lime glass substrates to become more attractive for commercial applications. We demonstrate that despite a large difference in the expansion coefficient between soda lime glass and silicon absorber layer, adhesive silicon thin films can be produced under certain conditions. On these first layers, we were able to fabricate solar cells, which in their cell and material properties were only ~30% below the values of their technical glass counterparts.

Minimization of Reflection-Loss from Etched Nano-structures of Silicon Crystal Wafers

Abstract Significant reduction in reflection loss from crystalline silicon absorber layer is an urgent requirement for increasing the efficiency of solar cells. Present research targets on producing almost black-Si by H2-plasma etching of the c-Si wafers, using planar inductively-coupled plasma-CVD. It is demonstrated that lower gas pressure in the plasma promotes efficient etching of c-Si surface. Plasma etching at 30 mTorr and 200 °C produces, high-density, homogeneous and uniform pyramid-like Si-nanostructures with dimension ∼40–200 nm which closely satisfies the anti-reflection coating (ARC) requirement. An extremely low reflection (∼0.13%) from such optimally etched silicon nanostructures appears most useful for photovoltaic applications

Optical Study and Experimental Realization of Nanostructured Back Reflectors with Reduced Parasitic Losses for Silicon Thin Film Solar Cells.

We study light trapping and parasitic losses in hydrogenated amorphous silicon thin film solar cells fabricated by plasma-enhanced chemical vapor deposition on nanostructured back reflectors. The back reflectors are patterned using polystyrene assisted lithography. By using O2 plasma etching of the polystyrene spheres, we managed to fabricate hexagonal nanostructured back reflectors. With the help of rigorous modeling, we study the parasitic losses in different back reflectors, non-active layers, and last but not least the light enhancement effect in the silicon absorber layer. Moreover, simulation results have been checked against experimental data. We have demonstrated hexagonal nanostructured amorphous silicon thin film solar cells with a power conversion efficiency of 7.7% and around 34.7% enhancement of the short-circuit current density, compared with planar amorphous silicon thin film solar cells.

Advanced light management techniques for two-terminal hybrid tandem solar cells

Multi-junction solar cells are considered for various applications, as they tackle various loss mechanisms for single junction solar cells. These losses include thermalization and non-absorption below the band gap. In this work, a tandem configuration comprising copper-indium-gallium-di-selenide (CIGS) and hydrogenated amorphous silicon (a-Si:H) absorber layers is studied. Two main challenges are addressed in this work. Firstly, the natural roughness of CIGS is unfavorable for monolithically growing a high quality a-Si:H top cell. Some sharp textures in the CIGS induce shunts in the a-Si:H top junction, limiting the electrical performance of such a configuration. To smoothen this interface, the possibility of mechanically polishing the intermediate i-ZnO layer has been explored. The second challenge that is addressed, is the significant current mismatch in these tandem architectures. To enhance absorption in the current-limiting top cell, the ZnO:Al front electrode was textured by means of wet-etching the entire tandem stack. We demonstrated that one can manipulate the morphology of the random textures by varying the growth conditions of the ZnO:Al, leading to better light management in these devices.

Improved Efficiency of Microcrystalline Silicon Thin-Film Solar Cells With Wide Bandgap CdS Buffer Layer

In this paper, we have reported a new structure based upon an optical simulation of maximum light trapping and management in microcrystalline silicon thin-film solar cells by using multitexture schemes and introducing an n-type cadmium sulphide (CdS) buffer layer with the goal of extreme light coupling and absorption in silicon absorber layer. Photon absorption was improved by optimizing the front and back texturing of transparent conductive oxide layers and variation in buffer layer thickness. We have demonstrated that light trapping can be improved with the proposed geometry of 1- μm-thick crystalline silicon absorber layer below a thin layer of wide bandgap material. We have improved the short-circuit current densities by 1.35 mA/cm2 resulting in a total short-circuit current of 25 mA/cm2 and conversion efficiency of 9 % with the addition of CdS buffer layer and multitextures, under global AM1.5 conditions. In this study, we have used a two-dimensional full-vectorial finite element to design and optimize the proposed light propagation in solar cell structure configuration. Our simulation results show that interface morphology of CdS layer thickness and textures with different aspect and ratios have the most prominent influence on solar cell performance in terms of both short-circuit current and quantum efficiency.

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.

Tailoring Nano-Textures for Optimized Light In-Coupling in Liquid Phase Crystallized Silicon Thin-Film Solar Cells

Thin-film solar cells based on liquid phase crystallized silicon (LPC Si) with 8–20 μm thick absorber layers demand for advanced light management to achieve high photocurrent densities. Open-circuit voltages (Voc) >600 mV underline the high silicon material quality of LPC silicon thin-films on nano-textured glass superstrates. We present a 500 nm-pitched sinusoidal nano-texture which outperforms larger pitched gratings with respect to light in-coupling at the buried glass-silicon interface. In the wavelength range of interest reflection of incident light is minimized to values close to 4%, which is the reflection at the sun-facing air-glass interface. Further, the electronic material quality of sinusoidally textured devices is analyzed on basis of a comparison of maximum achieved open-circuit voltages on different texture types. The Voc on sinusoidally textured glass superstrates could be raised to 630 mV by changing the interlayer deposition method from a PVD to a PECVD process. Thus, we are able to unify high optical and electronic properties of silicon absorber layers on sinusoidaly textured glass substrates. These results constitute a crucial step toward fully exploiting the optical potential of LPC silicon thin-film solar cells.

Combining tailor-made textures for light in-coupling and light trapping in liquid phase crystallized silicon thin-film solar cells.

We present tailor-made imprinted nanostructures for light management in liquid phase crystallized silicon thin-film solar cells providing both, increased jsc by enhanced absorption and excellent electronic material-quality with Voc-values >640mV. All superstrate textures successfully enhance light in-coupling in 10-20µm thick liquid phase crystallized silicon thin-films. Moreover, the effect of combining imprinted textures at the front side with individually optimized light trapping schemes at the rear side of the absorber layers on the optical properties is analyzed. With a silicon absorber layer thickness of 17µm maximum achievable short-circuit current density of 37.0mA/cm2 is obtained, an increase by + 1.8mA/cm2 (or 5.1%) compared to the optimized planar reference.

Periodic Upright Nanopyramids for Light Management Applications in Ultrathin Crystalline Silicon Solar Cells

Motivated by the primary benefit of reduced material cost, the thickness of crystalline silicon solar cells has been continuously reduced. Laboratory and industrial studies have explored ultrathin crystalline silicon solar cells below 50 μ m with ambitious endeavors toward thicknesses of only a few micrometers. Ultrathin crystalline silicon solar cells require compatible small-scale surface textures to enhance the optical absorption. For this purpose, a novel submicron periodic nanostructure—periodic upright nanopyramids (PuNPs)—is fabricated by an integrated process of laser interference lithography and anisotropic etching of silicon in an alkaline solution. By simulation and measurements, we demonstrate that PuNPs are able to reduce front surface reflectance more effectively than conventional micron-scale pyramid textures and previously investigated periodic inverted nanopyramids (PiNPs). With a silicon nitride antireflection coating, we predict that PuNPs reduce the front surface reflectance to below 1% at an angle of incidence of 8°, which is comparable to black silicon. The superior antireflective property of PuNPs contributes to an absorbed photocurrent density of 40.8 mA/cm2 for a 40 μ m silicon absorber layer, which is 0.7 mA/cm2 higher than PiNPs, 0.8 mA/cm2 higher than inverted pyramids and 1 mA/cm2 higher than upright pyramids.

Nano-Photonic Structures for Light Trapping in Ultra-Thin Crystalline Silicon Solar Cells.

Thick wafer-silicon is the dominant solar cell technology. It is of great interest to develop ultra-thin solar cells that can reduce materials usage, but still achieve acceptable performance and high solar absorption. Accordingly, we developed a highly absorbing ultra-thin crystalline Si based solar cell architecture using periodically patterned front and rear dielectric nanocone arrays which provide enhanced light trapping. The rear nanocones are embedded in a silver back reflector. In contrast to previous approaches, we utilize dielectric photonic crystals with a completely flat silicon absorber layer, providing expected high electronic quality and low carrier recombination. This architecture creates a dense mesh of wave-guided modes at near-infrared wavelengths in the absorber layer, generating enhanced absorption. For thin silicon (<2 μm) and 750 nm pitch arrays, scattering matrix simulations predict enhancements exceeding 90%. Absorption approaches the Lambertian limit at small thicknesses (<10 μm) and is slightly lower (by ~5%) at wafer-scale thicknesses. Parasitic losses are ~25% for ultra-thin (2 μm) silicon and just 1%–2% for thicker (>100 μm) cells. There is potential for 20 μm thick cells to provide 30 mA/cm2 photo-current and >20% efficiency. This architecture has great promise for ultra-thin silicon solar panels with reduced material utilization and enhanced light-trapping.

Optical Properties of Smooth Anti-reflective Three-dimensional Textures for Silicon Thin-film Solar Cells

Abstract In recent years, thin-film silicon solar cells on glass prepared by liquid-phase crystallization have made progress towards high efficiency solar cells. Current record cells reach wafer-equivalent material quality and morphology using thin-film technologies. However, short-circuit current densities and hence, efficiencies, are still limited. The reflection at the interface between glass superstrate and silicon absorber layer has been identified as one major loss mechanism. These optical losses can be reduced by nanostructuring of the interface. It is important, however, that this nanostructured interface does not lead to a deterioration of silicon material quality simultaneously. Recently, we introduced SMooth Anti-Reflective Three-dimensional (SMART) textures, which consist of temperature-stable SiO x sol-gel nanostructures and a smoothing layer of spin-coated TiO x . These SMART textures on glass superstrates exhibit a smooth interface morphology and hence allow growing high-quality silicon absorber layers by liquid phase crystallization. Here, we investigate the optical properties of the SMART textures with and without an additional SiN x layer in experiment and by 1-dimensional optical simulations. It is shown that both, the SMART textures with and without additional SiN x layer, can outperform the optimized planar interlayer system of current record solar cell devices. Very low mean reflectance values of 11.2% are found in the wavelength regime from 400 nm to 600 nm for an optimized texture consisting of a 50 nm thick SMART texture with additional 15 nm SiN x layer.

Effective Light Trapping in Thin Film Silicon Solar Cells with Nano- and Microscale Structures on Glass Substrate.

For thin film silicon-based solar cells, effective light trapping at a broad range of wavelengths (400-1100 nm) is necessary. Normally, etching is only carried out with TCOs, such as SnO2:F and impurity doped ZnO, to form nano-sized craters in the surface morphology to confer a light trapping effect. However, in this study, prior to ZnO:Al etching, periodic structures on the glass substrates were made by photolithography and wet etching to increase the light scattering and internal reflection. The use of periodic structures on the glass substrate resulted in higher haze ratios in the range from 550 nm to 1100 nm, which is the optical absorption wavelength region for thin film silicon solar cells, than obtained by simple ZnO:Al etching. The periodically textured glass with micro-sized structures compensates for the low haze ratio at the middle and long wavelengths of wet etched ZnO:Al. ZnO:Al was deposited on the periodically textured glass, after which the ZnO:Al surface was also etched randomly using a mixed acid solution to form nano-sized craters. The thin film silicon solar cells with 350-nm-thick amorphous silicon absorber layer deposited on the periodic structured glass and etched ZnO:Al generated up to 10.68% more photocurrent, with 11.2% increase of the conversion efficiency compared to the cell deposited on flat glass and etched ZnO:Al.

Optimized emitter contacting on multicrystalline silicon thin film solar cells

We present an optimized contacting scheme for multicrystalline silicon thin film solar cells on glass based on epitaxially crystallized emitters with a thin Al2O3 layer and a silver back reflector. In a first step a 6.5 µm thick amorphous silicon absorber layer is crystallized by a diode laser. In a second step a thin silicon emitter layer is epitaxially crystallized by an excimer laser. The emitter is covered by an Al2O3 layer with a thickness ranging from 1.0 nm to 2.5 nm, which passivates the surface and acts as a tunnel barrier. On top of the Al2O3 layer a 90–100 nm thick silver back reflector is deposited. The Al2O3 layer was found to have an optimal thickness of 1.5 nm resulting in solar cells with back reflector that achieve a maximum open‐circuit voltage of 567 mV, a short‐circuit current density of 27.9 mA/cm2, and an efficiency of 10.9%. (© 2015 WILEY‐VCH Verlag GmbH &amp;Co. KGaA, Weinheim)