Amorphous Silicon Cells Research Articles

Comparison of the indoor performance of 12 commercial PV products by a simple model

AbstractThis article presents a simple comparative model which has been developed for the estimation of the performance of photovoltaic (PV) products' cells in indoor environments. The model predicts the performance of PV solar cells, as a function of the distance from a spectrum of artificial (fluorescent light, halogen light, and light‐emitting diodes) and natural light. It intends to support designers, while creating PV‐integrated products for indoor use. For the model's validation, PV cells of 12 commercially available PV‐powered products with power ranging from 0.8 to 4 mWp were tested indoors under artificial illumination and natural light. The model is based on the physical measurements of natural and artificial irradiance indoors, along with literature data of PV technologies under low irradiance conditions. The input data of the model are the surface of the solar cell (in m2), the wavelength‐dependent spectral response (SR) of the PV cell, the spectral irradiance indoors, and solar cell's distance from light sources. The model calculates solar cells' efficiency and power produced under the specific indoor conditions. If using the measured SR of a PV cell and the irradiance as measured indoors, the model can predict the performance of a PV product under mixed indoor light with a typical inaccuracy of around 25%, which is sufficient for a design process. Measurements revealed that under mixed indoor lighting of around 20 W/m2, the efficiency of solar cells in 12 commercially available PV products ranges between 5% and 6% for amorphous silicon (a‐Si) cells, 4–6% for multicrystalline silicon (mc‐Si) cells, and 5–7% for the monocrystalline silicon (c‐Si) cells.

Flexible Nanotexture Structures for Thin Film PV Cells Using Wavelet Functions

Light trapping is an important technique in increasing the efficiency of solar cells. Inverse optimization is a systematic numerical approach that allows us to find the limits of light trapping more efficiently. It is an alternative to exhaustive search simulations or experimental measurements. In this study, we use inverse optimization to study light trapping in thin film amorphous silicon cells textured by periodic patterns of metallic surface grating. We use a finite set of Haar wavelets to describe a general form of grating structure composed of multiple rectangular nanostrips. We use a well-known global multiparameter optimization technique called simulated annealing to find the coefficients of the wavelets basis for optimal absorptivity enhancement in thin film silicon. The motivation for choosing wavelet basis (vis-a-vis other orthonormal bases such as Fourier) is the feasibility of fabricating the resulting nanostructures. The resulting improvement in the number of absorbed photons is around 130% for wavelength range of 300–700 nm, which is significantly better than the previous results using simple front surface nanostrips. In addition, we use statistical tools to evaluate the sensitivity of the characteristics of the resulting structure to numerical uncertainties.

Design of 4-terminal Solar Modules Combining Thin-film Wide-Bandgap Top Cells and c-Si Bottom Cells

Optical simulations of 4-terminal hybrid tandem modulescombining high-efficiency crystalline silicon(c-Si) cells with three different thin-film top cells respectively are presented. We considered three types of thin film PV cells: 1. Enlarged-bandgap oxygenated amorphous silicon (a-SiO:H) cells, 2. Wide-bandgap chalcopyrite (CuGaSe2) cells, and 3. Perovskite (CH3NH3PbI3) cells. A methodology for evaluating the efficiency gain of the 4-terminal hybrid tandem module is proposed and used to show how the efficiency of aninterdigitated back contact (IBC)c-Si module with an efficiency of about 19.5% can significantly be increased through this 4-terminal tandem concept. In our model we also included the change in electrical output parameters by the color-filtering effect of the top cells.

Characterization of photovoltaic devices for indoor light harvesting and customization of flexible dye solar cells to deliver superior efficiency under artificial lighting

The field of energy harvesting holds the promise of making our buildings “smart” if effective energy sources can be developed for use in ambient indoor conditions. Photovoltaics (PV), especially in its thin flexible form for easy integration, become a prime candidate for the aim, if tailored for low-density artificial light.We designed a test system which enabled us to measure the performance of PV devices under compact fluorescent lamp (CFL) and light-emitting diode (LED) illumination at different illuminance levels and compared polycrystalline and amorphous silicon cells with our own flexible dye solar cells (DSCs). Whereas poly-Si cells, with 15% outdoor efficiency, delivered at 200lux under CFL only 2.8μW/cm2 power density (and an efficiency of 4.4%), a-Si specifically designed for indoors, gave 5.9μW/cm2 and 9.2% efficiency under the same CFL conditions (and 7.5% efficiency under LED).However, we show that the customization of flexible DSCs, by simply formulating ad-hoc less-concentrated, more transparent electrolytes, enabled these devices to outperform all others, providing average power densities of 8.0μW/cm2 and 12.4% efficiencies under 200lux CFL (more than quadruple compared to those measured at 1sun), and 6.6μW/cm2 and 10% efficiency under 200lux LED illumination.

Compact Light-Emitting Diode-Based AAA Class Solar Simulator: Design and Application Peculiarities

In this paper, we report on the efficient design of a light-emitting diode (LED)-based AAA class solar simulator, employing only 19 high-power emitters for a usable illuminated area of at least 5 cm in diameter with at least 1 sun irradiance. Such a low number of emitters was achieved by selectively employing secondary optics for several LED groups and taking advantage of wide emission angle for others. The so-called A class spectrum was also achieved for the larger area of more than 6 cm × 6 cm, covering zones with B and C class irradiance nonuniformity. Five distinct solar cell external quantum efficiency spectra were considered for theoretical evaluation of possible measurement peculiarities related to different solar cell technologies. These computer-generated spectra contained essential features, which is typical for high-efficiency crystalline and amorphous silicon, Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ZnSn(S,Se) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> solar cell technologies. Significant photocurrent distribution nonuniformity change is predicted only for amorphous silicon cells due to a much narrower efficient absorption spectrum.

Tradeoff Evaluation Improves Comparative Life Cycle Assessment: A Photovoltaic Case Study

SummaryCurrent life cycle assessment (LCA) interpretation practices typically emphasize hotspot identification and improvement assessment. However, these interpretation practices fail in the context of a decision‐driven comparative LCA where the goal is to select the best option from a set of dissimilar alternatives. Interpretation of comparative LCA results requires understanding of the trade‐offs between alternatives—instances in which one alternative performs better or worse than another—to identify the environmental implications of a specific decision. In this case, analysis must elucidate relative trade‐offs between decision alternatives, rather than absolute description of the alternatives individually. Here, typical practices fail. This article introduces a probability distribution‐based approach to assess the significance of performance differences among alternatives that allows LCA practitioners to focus analyses on those aspects most influential to the decision, identify the areas that would benefit the most from data refinement given the level of uncertainty, and complement existing hotspot analyses. In a case study of a comparative LCA of five photovoltaic technologies, findings show that thin‐film cadmium telluride and amorphous silicon cell panels are most likely to perform better than other alternatives. Additionally, the impact categories highlighted by the new approach are different than those highlighted by typical external normalization practices, suggesting that a decision‐driven approach to interpretation would redirect environmental research efforts.

Triple-junction hybrid tandem solar cells with amorphous silicon and polymer-fullerene blends.

Organic-inorganic hybrid tandem solar cells attract a considerable amount of attention due to their potential for realizing high efficiency photovoltaic devices at a low cost. Here, highly efficient triple-junction (TJ) hybrid tandem solar cells consisting of a double-junction (DJ) amorphous silicon (a-Si) cell and an organic photovoltaic (OPV) rear cell were developed. In order to design the TJ device in a logical manner, a simulation was carried out based on optical absorption and internal quantum efficiency. In the TJ architecture, the high-energy photons were utilized in a more efficient way than in the previously reported a-Si/OPV DJ devices, leading to a significant improvement in the overall efficiency by means of a voltage gain. The interface engineering such as tin-doped In2O3 deposition as an interlayer and its UV-ozone treatment resulted in the further improvement in the performance of the TJ solar cells. As a result, a power conversion efficiency of 7.81% was achieved with an open-circuit voltage of 2.35 V. The wavelength-resolved absorption profile provides deeper insight into the detailed optical response of the TJ hybrid solar cells.

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.

Analysis of spectral effects on the energy yield of different PV (photovoltaic) technologies: The case of four specific sites

This work is aimed at looking into the impact of the solar spectrum distribution on the energy yield of some PV (photovoltaic) materials on both monthly and annual time scales. The relative spectral responses of eight different PV solar cells – representative of each considered technology – have been selected. Modeling solar spectra in four sites located in the north hemisphere together with a 12-month experimental campaign in which spectral irradiances were periodically recorded in two of these sites located in Spain have provided highly interesting results. Regardless of the site, the considered amorphous silicon and cadmium telluride PV cells experience the most noticeable modeled and experimental variations of their monthly spectral gains, whilst flatter seasonal ones are identified in the remaining considered PV technologies. Thus, the maximum monthly variations of these spectral gains take place in the a-Si PV cell, ranging from −16% (winter) to 4% (summer) in Stuttgart. Anyway, the monthly spread of the spectral effects decreases as the latitude of the site does. Last, the impact of the solar spectrum distribution is remarkably reduced when the period of integration is enlarged up to a year. In fact, annual spectral gains keep below 2.2% for all the studied technologies and sites.

Conformal thin film silicon photovoltaic modules

A polymer composite material system and a process for encapsulation of thin film solar cells were developed for profiled roofing elements, in view of building-integrated photovoltaic (PV) applications. Amorphous silicon cells were deposited on a polyethylene naphthalate film and encapsulated with additional polymer layers in the form of a flat laminate using industrial processes. The process technology developed in this work included a pre-forming step of the PV laminate and a moulding step of a glass fibre-reinforced polyester composite. These two steps were optimised using thermo-elastic analyses, with attention paid to laminate designs and process windows compatible with the thermo-mechanical limits of the fragile active PV layers. A demonstrator with standard 1.0 m×1.8 m corrugated roofing profile, a weight of 6.3 kg and 60 W of output power was produced. The long-term endurance of the conformal modules was also validated using humidity-freeze, damp-heat and water immersion tests.

Front contact layer of multiphase silicon-carbon in thin film silicon solar cell

In order to increase the quantum efficiency as well as conversion efficiency, we propose the boron doped hydrogenated multiphase silicon-carbon (called as “multiphase silicon-carbon”) as the front contact layer in thin film silicon solar cells. The multiphase silicon-carbon consists of amorphous carbon, amorphous silicon, and crystalline silicon-like clustering phase. We achieved a high conductivity and a low optical absorptance of multiphase silicon-carbon and compared it with the existing boron doped microcrystalline silicon. Applying this layer between transparent conductive oxide and the p layer, the amorphous silicon and silicon-germanium (a-Si/a-SiGe) double junction cell showed an increase of quantum efficiency in short wavelength and an improvement of the conversion efficiency by about 0.6% in 1 cm2 area.

Enhanced recovery of light-induced degradation on the micromorph solar cells by electric field

The recovery of light-induced degradation of the tandem micromorph solar cell by applying reverse bias is compared with the single-junction amorphous silicon solar cell. The illuminated current density-voltage characteristics and external quantum efficiency show that the degradation of both the micromorph and the amorphous silicon cells can be recovered by applying sufficient reverse bias. The micromorph cell was recovered at smaller reverse bias than amorphous silicon cell. The abundant H in the microcrystalline silicon bottom cell of the micromorph cell can act as a reservoir to repair the defects in the amorphous silicon top cell at the reverse bias. This is responsible for small recovery bias of tandem cells.

Deposition of device quality amorphous silicon and solar cell from argon dilution of silane

In our present study hydrogenated amorphous silicon (a-Si:H) thin films and solar cells have been prepared in a conventional single chamber rf-PECVD unit from silane–argon mixture by varying radio frequency (rf) power densities from 6mW/cm2 to 50mW/cm2. By optimizing the properties of the intrinsic material we have chosen a material which is deposited at 6mW/cm2 rf power density, 0.2Torr pressure, 175oC substrate temperature and by 97% argon dilution. For this material minority carriers (holes) diffusion length (Ld) measured in the as deposited state is 180nm and it degrades by 15% after light soaking. This high Ld value indicates that the material is of device quality. We have fabricated a single junction solar cell having the structure p-a-SiC:H/i-a-Si:H/n-a-Si:H without optimizing the doped layers. This set exhibits a mean open circuit voltage of 0.8V and conversion efficiency of 7.7%. After light soaking conversion efficiency decreases by 15% which demonstrates that it is possible to deposit device grade material and solar cells from silane–argon mixture.

Ab-initio study of silicon and tin as a negative electrode materials for lithium-ion batteries

An investigation of Li-M (M: Si, Sn) components using density functional theory (DFT) is presented. Calculation of total energy, structural optimizations, bulk modulus and elastic constants with Li-Sn, Li-Si are performed through DFT calculations. From the comparable study of Li-Sn and Li-Si, it is found that silicon experience drastic mechanical degradation during lithiation than tin-based Li-Sn components. With increasing lithium net charge transfer to metals, the filling of anti-bonding orbital makes M-M covalent bonding weak ionic bonding in both Li-Si and Li-Sn. However, the difference of change of mechanical degradation during lithiation in Li-Si and Li-Sn results from the sensitivity of transition of covalent bonding. We check this from sharp decreasing of yield stress in Li-Si case. Furthermore, we simply make up amorphous Si cell with an additional Li atom at the center of the largest void to simulate the lithiation of amorphous silicon. Volume expansion of amorphous silicon cell agrees with the experiment observation and theoretical data of Li-Si compounds.

Nanophotonic light trapping with patterned transparent conductive oxides

Transparent conductive oxides (TCOs) play a crucial role in solar cells by efficiently transmitting sunlight and extracting photo-generated charge. Here, we show how nanophotonics concepts can be used to transform TCO films into effective photon management layers for solar cells. This is accomplished by patterning the TCO layer present on virtually every thin-film solar cell into an array of subwavelength beams that support optical (Mie) resonances. These resonances can be exploited to concentrate randomly polarized sunlight or to effectively couple it to guided and diffracted modes. We first demonstrate these concepts with a model system consisting of a patterned TCO layer on a thin silicon (Si) film and outline a design methodology for high-performance, TCO-based light trapping coatings. We then show that the short circuit current density from a 300 nm thick amorphous silicon (a-Si) cell with an optimized TCO anti-reflection coating can be enhanced from 19.9 mA/cm2 to 21.1 mA/cm2, out of a possible 26.0 mA/cm2, by using an optimized nanobeam array. The key differences and advantages over plasmonic light trapping layers will be discussed.

Sn‐catalyzed silicon nanowire solar cells with 4.9% efficiency grown on glass

ABSTRACTWe present a single pump‐down process to texture hydrogenated amorphous silicon solar cells. Mats of p‐type crystalline silicon nanowires were grown to lengths of 1 µm on glass covered with flat ZnO using a plasma‐assisted Sn‐catalyzed vapor‐liquid‐solid process. The nanowires were covered with conformal layers of intrinsic and n‐type hydrogenated amorphous silicon and a sputtered layer of indium tin oxide. Each cell connects in excess of 107 radial junctions over areas of 0.126 cm². Devices reach open‐circuit voltages of 0.8 V and short‐circuit current densities of 12.4 mA cm−2, matching those of hydrogenated amorphous silicon cells deposited on textured substrates. Copyright © 2012 John Wiley &amp; Sons, Ltd.

High quality amorphous Si solar cells for large area mass production Micromorph tandem cells

This paper reports on the development of an amorphous silicon cell used in the top cell of Micromorph® tandem solar modules produced in the pilot line of Oerlikon Solar in Trübbach — Switzerland. Tuning of the process parameters used for PECVD deposition of the absorber layers such as process pressure, RF power density, SiH4/H2 ratio, and substrate temperature can result in significant improvement in the material quality of the absorber layer and therefore in the performance and light induced degradation of the a-Si cell. We have measured the single layer properties of different absorber layers by infrared spectroscopy and have found a strong correlation between both the microstructure factor R and the H-content bonded to Si and the stabilized efficiency or relative degradation of the a-Si cells containing the corresponding absorber layers. A combination of absorber layers with superior material quality, adapted p-doped and buffer layers and ZnO front and back contacts with enhanced light trapping have achieved record values for the conversion efficiency of industrial thin a-Si single junction cells and modules. Our results show initial efficiencies on test cells prepared on 1.4m2 substrates of over 11%, an active area efficiency of 10.5% for a champion 1.4m2 a-Si single junction module and an 8.7% stabilized conversion efficiency for an industrial 1.4m2 a-Si single junction champion module.

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.

The Simultaneous Effect of the Operating Temperature and Solar Radiation on the Efficiency of Photovoltaic Panels / Jednoczesny Wpływ Temperatury Pracy Oraz Promieniowania Słonecznego Na Wydajnosc Ogniw Fotowoltaicznych

Abstract The proper designing of PV systems requires the use of advanced building energy simulation techniques. It allows to design the best position of the PV array, as well as the right quantity of produced energy in different cases. On the other hand the PV efficiency is not only a constant value but changes according to temperature and solar radiation. This paper is devoted to estimate the simultaneous effect of both weather factors on PV efficiency. The task was achieved by numerical simulation and ESP-r software. Computer simulations have been carried out with the use of the Typical Meteorological Year data for Warsaw (52°N 21°E). The greatest influence of temperature on the efficiency of solar energy conversion was observed for crystalline silicon cells. The influence of the boundary conditions assumed in the study is ignored for amorphous silicon cells in the summer period and regardless of the material type in the winter period.

Tandem Photovoltaic Cells with Amorphous Silicon Cells and Organic Photovoltaic Cells

ABSTRACTWe demonstrate series-connected tandem photovoltaic cells consisting of hydrogenated amorphous silicon (a-Si:H) solar cells and polymer-based organic photovoltaic (OPV) cells. One of the limiting factors of a-Si:H solar cells is their narrow absorption spectrum as compared with that of crystalline silicon solar cells. In order to overcome this limitation, we fabricated a hybrid tandem solar cell by employing a solution-processed OPV subcell based on a low bandgap semiconducting polymer onto the a-Si:H subcell. It was found that the interfacial property of the hole transporting intermediate layer between the subcells strongly affects the photovoltaic property of the tandem cells. By using MoO3 as an efficient hole transporting intermediate layer instead of the conventional conducting polymer, we obtained the power conversion efficiency of 1.84% and the open-circuit voltage (VOC) of 1.50 V which corresponds closely to the sum of the VOCs of the subcells.