Amorphous Silicon Alloy Solar Cells Research Articles

Energy loss process analysis for radiation degradation and immediate recovery of amorphous silicon alloy solar cells

Performance degradation of a-Si/a-SiGe/a-SiGe triple-junction solar cells due to irradiation of silicon ions, electrons, and protons are investigated using an in-situ current–voltage measurement system. The performance recovery immediately after irradiation is also investigated. Significant recovery is always observed independent of radiation species and temperature. It is shown that the characteristic time, which is obtained by analyzing the short-circuit current annealing behavior, is an important parameter for practical applications in space. In addition, the radiation degradation mechanism is discussed by analyzing the energy loss process of incident particles (ionizing energy loss: IEL, and non-ionizing energy loss: NIEL) and their relative damage factors. It is determined that ionizing dose is the primarily parameter for electron degradation whereas displacement damage dose is the primarily parameter for proton degradation. This is because the ratio of NIEL to IEL in the case of electrons is small enough to be ignored the damage due to NIEL although the defect creation ratio of NIEL is much larger than that of IEL in the cases of both protons and electrons. The impact of “radiation quality effect” has to be considered to understand the degradation due to Si ion irradiation.

Roll-to-roll manufacturing of amorphous silicon alloy solar cells with in situ cell performance diagnostics

In order to meet the price target necessary for widespread use of solar cell products, Energy Conversion Devices, Inc., ECD, has developed and commercialized a continuous roll-to-roll manufacturing technology for the production of a-Si alloy solar cells. Since the early 1980s, we have advanced this technology from a small-scale pilot machine to a large-scale production machine. In 2002, ECD commissioned a 30 MW per year machine for United Solar Systems Corp. in Auburn Hills, Michigan. The RF PECVD a-Si alloy solar cell processor, designed and built by ECD, deposits triple-junction solar cell materials consisting of nine layers of a-Si alloys in a continuous roll-to-roll operation simultaneously on six coils of 130 μm thick, 0.36 m wide, 2.6 km long stainless-steel substrate at 1 cm/s. In order to minimize production losses due to undetected deviations of production conditions and carry on a continuous program of device optimization, we have developed and are incorporating in situ cell performance diagnostic systems.

Correlation of Material Properties and Open-Circuit Voltage of Amorphous Silicon Based Solar Cells

AbstractCorrelation of hydrogenated amorphous silicon (a-Si:H) alloy material properties and solar cell characteristics have been studied experimentally and by computer simulation. Simulation results show that all three solar cell parameters, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), decrease with increased defect density. For a given intrinsic layer thickness, a larger band gap (Eg) results in a higher Voc but a lower Jsc. However, FF does not depend on band gap. This allows us to distinguish the effect of change in band gap from that in defect density on the variation in Voc. For solar cells with good interface characteristics, a linear relation FF = βVoc + γ is obtained by light soaking experiments and simulation with different defect densities. The slope β is in the range from 2 to 3 V-1 depending on cell properties and light soaking condition, and the intersect γ depends mainly on the band gap. Comparing cells made with high H2 dilution to no H2 dilution, we find that a 58 mV enhancement in Voc with H2 dilution is due to both widening of band gap and reduced defect density. Simulation results also show that a narrower valence band tail leads to a higher Voc. We did not include this effect in the analysis due to lack of available data for correlation between H2 dilution and band tail narrowing.

Amorphous Silicon and Silicon Germanium Alloy Solar Cells Deposited by VHF at High Rates

ABSTRACTA modified very high frequency (MVHF) glow discharge technique is used to deposit amorphous silicon (a-Si) and amorphous silicon-germanium (a-SiGe) alloy solar cells at high deposition rates. High quality a-Si alloy solar cells have been obtained by using MVHF at deposition rates up to ∼10 Å/s. The cells show good initial and stabilized efficiencies comparable to those obtained from conventional radio-frequency (RF) glow discharge deposition at low rates (∼1 Å/s). However, high quality a-SiGe alloy solar cells are more difficult to achieve at high deposition rates. In this paper, we present the progress made on a-SiGe alloy solar cells by incorporating bandgap profiling and appropriate buffer layers. Using the improved a-SiGe alloy solar cells, a-Si/a-SiGe tandem configurations are made and results presented.

Annealing Kinetics of Amorphous Silicon Alloy Solar Cells Made at Various Deposition Rates

ABSTRACTA systematic study has been made of the annealing kinetics of amorphous silicon (a-Si) alloy solar cells. The cells were deposited at various rates using H2 dilution with radio frequency (RF) and modified very high frequency (MVHF) glow discharge. In order to minimize the effect of annealing during light soaking, the solar cells were degraded under 30 suns at room temperature to quickly reach their saturated states. The samples were then annealed at an elevated temperature. The J-V characteristics were recorded as a function of annealing time. The correlation of solar cell performance and defect density in the intrinsic layer was obtained by computer simulation. Finally, the annealing activation energy distribution (Ea) was deduced by fitting the experimental data to a theoretical model. The results show that the RF low rate solar cell with high H2 dilution has the lowest Ea and the narrowest distribution, while the RF cell with no H2 dilution has the highest Ea and the broadest distribution. The MVHF cell made at 8Å/s withhigh H2 dilution shows a lower Ea and a narrower distribution than the RF cell made at 3 Å/s, despite the higher rate. We conclude that different annealing kinetics plays an important role in determining the stabilized performance of a-Si alloy solar cells.

Amorphous Silicon Alloy Solar Cells Near the Threshold of Amorphous-to-Microcrystalline Transition

ABSTRACTA systematic study has been made of amorphous silicon (a-Si) alloy solar cells using various hydrogen dilutions during the growth of the intrinsic (i) layer. We find that the open-circuit voltage (Voc) of the cells increases as the dilution increases; it then reaches a maximum before it decreases dramatically. This sudden drop in Voc is attributed to the transition from amorphous silicon to microcrystalline inclusions in the i layer. We study i-layer thicknesses ranging from 1000 Å to 5000 Å and find that the transition occurs in all thicknesses investigated. Based on this study, a-Si alloy p i n solar cells suitable for use in the top cell of a high efficiency triple-junction structure are made. By selecting an appropriate dilution, cells with Voc greater than 1 V can be achieved readily. Solar cells made near the threshold not only exhibit higher initial characteristics but also better stability against light soaking. We have compared top cells made near the threshold with our previous best data, and found that both the initial and stable efficiencies are superior for the near-threshold cells. For an a-Si/a-Si double-junction device, a Voc value exceeding 2 V has been obtained using thin component cells. Thicker component cells give rise to an initial active-area efficiency of 11.9% for this tandem structure.

Science and technology of amorphous silicon alloy photovoltaics

Significant advances have been made in our understanding of amorphous silicon alloy materials and solar cells that have resulted in the achievement of high cell and module efficiency. The R&D results have also been translated to production, and several manufacturers have expanded their production capacity, raising the worldwide annual capacity exceeding 25 MW. In this paper, we shall review the progress in the science and technology of amorphous silicon alloy photovoltaics and discuss the future directions.

Effect of Excitation Frequency on the Performance of Amorphous Silicon Alloy Solar Cells

ABSTRACTWe have studied amorphous silicon alloy solar cells made by using a modified-very-highfrequency glow discharge at 75 MHz with a deposition rate of ∼6 Å/s. The solar cell performance is compared with those made from conventional glow discharge at 13.56 MHz with lower deposition rates. Cells made at ∼6 Å/s with 75 MHz showed comparable stabilized efficiency to those made at ∼3 Å/s with 13.56 MHz. The best performance, however, was obtained with ∼1 Å/s, including a stabilized 9.3% a-Si alloy single-junction cell employing conventional glow discharge technique. Using 75 MHz, we have achieved 11.1% and 10.0% initial active-area efficiencies for a-Si alloy and a-SiGe alloy n i p cells, respectively. An initial efficiency of 11.0% has also been obtained in a dual bandgap double-junction structure.

Effect of hydrogen dilution on the structure of amorphous silicon alloys

We investigate why high levels of hydrogen dilution of the process gas lead to enhanced light soaking stability of amorphous silicon (a-Si) alloy solar cells by studying the microstructural properties of the material using high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. The TEM results show that a-Si alloy (with or without hydrogen dilution) is a heterogeneous mixture of amorphous network and linear-like objects that show evidence of order along their length. The volume fraction of these ordered regions increases with increasing hydrogen dilution.

Materials aspects of amorphous silicon solar cells

Significant progress has recently been made in improving the stable efficiency of amorphous silicon alloy solar cells. This has been achieved through a better understanding of the effect of plasma chemistry and growth kinetics on material characteristics and incorporation of the materials into an optimum device configuration.

Triple-junction amorphous silicon alloy solar cell with 14.6% initial and 13.0% stable conversion efficiencies

We have achieved 14.6% initial and 13.0% stable conversion efficiencies using an amorphous silicon-based alloy in a spectrum-splitting, triple-junction structure. These efficiencies have been confirmed independently by the National Renewable Energy Laboratory. Key factors leading to this major advance include improvements made in the low band-gap amorphous silicon–germanium alloy cell, the pn tunnel junction between the component cells, and the top conducting oxide.

Outdoor measurement of multi-junction solar cells and modules based on the reference sunlight method

To evaluate the indoor measurement accuracy of multi-junction solar cells and modules experimentally, we have successfully used natural sunlight whose spectral irradiance is close to the reference sunlight. The reference sunlight for evaluating the performance of solar cells can be realized by choosing natural sunlight incident on a sun-facing 37° tilted surface at air mass of 1.5, when the atmosphere has turbidity of 0.27, precipitable water of 1.42 cm and so on. The measurement of atmospheric parameters was performed based mainly on the measurement of direct solar spectral irradiance and additionally on the measurements of irradiance and illuminance. In addition, measured values were compared with aerological observation data. The comparison of I–V characteristics measured outdoors and indoors shows that our pulsed-light solar simulator is capable of measuring multi-junction amorphous silicon alloy solar cells and modules accurately enough when its irradiance is set by an amorphous silicon reference cell. This fact is also confirmed by the examination of spectral irradiance on the test plane of the solar simulator. Moreover, we showed that irradiance (or illuminance) can be used instead of atmospheric turbidity and also water vapor content on a surface of the Earth can be used as a rough estimate of precipitable water for finding the reference sunlight conditions.

Improved stability against light exposure in amorphous deuterated silicon alloy solar cell

We have studied light-induced degradation in hydrogenated and deuterated amorphous silicon alloy solar cells. Replacing hydrogen with deuterium in the intrinsic layer of the cell improves stability against light exposure. Possible explanations for the improved stability are discussed.

Improved Stability Against Light-Exposure in Deuterated Amorphous Silicon Alloy Solar Cells

ABSTRACTWe have studied light-induced degradation in hydrogenated and deuterated amorphous silicon alloy solar cells in which intrinsic layers were deposited by using SiH4+H2 and SiD4+D2 gas mixtures respectively. Replacing hydrogen with deuterium in the intrinsic layer of the cell improves stability against light exposure. On the other hand, cells in which intrinsic layers were deposited from SiD4+H2 and SiH4+D2 do not show any improvement in stability. This result shows that improved stability in deuterated cell does not originate from simple replacement of hydrogen with deuterium. From deuterium/hydrogen effusion measurements, we found similar effusion at low temperature (400 °C) in both deuterated film and hydrogenated film prepared with heavy dilution. The latter film was shown to have oriented microstructure which was correlated with higher stability. This correlation strongly indicates that microstructure of the material plays a key role in improving the stability.

An Amorphous Silicon Alloy Triple-Junction Solar Cell with 14.6% Initial and 13.0% Stable Efficiencies

ABSTRACTAn initial conversion efficiency of 14.6% has been achieved using amorphous silicon-based alloy in a spectrum splitting triple-junction structure. After 1000 hours of indoor one-sun light soaking at 50 °C, the stabilized efficiency is 13.0%. Both efficiencies are the highest reported to date for amorphous silicon alloy solar cells and have been independently confirmed by the National Renewable Energy Laboratory. The device was deposited onto a stainless steel substrate coated with textured silver/zinc oxide back reflector. The bottom and middle cells use amorphous silicon-germanium alloys, employing high hydrogen dilution in the gas mixture and bandgap profiling in the cell design. The top cell uses amorphous silicon alloy with high hydrogen dilution. Key factors leading to the achievement include a) improvement of the bottom cell that exhibits an AM1.5 efficiency of 10.4% and quantum efficiency of 45% at 850 nm; b) improvement of the tunnel junctions between the component cells by incorporating a novel multilayered structure with microcrystalline p and n layers; and c) improvement of transparent conductive oxide for enhancing the short wavelength response of the top cell.

Material and device consideration for high efficiency a-Si alloy-based multijunction cells

It is now well-recognized that multi-bandgap triple-junction structures offer the attractive potential of achieving high efficiency with good stability in amorphous silicon alloy solar cells. Although stable cell efficiency exceeding 11% has been demonstrated, significant improvement in our understanding of materials and devices is necessary to reach the goal of 16% stable cell efficiency. Choice of suitable precursors in the plasma and control of growth kinetics will play a key role in improving material property. Innovative device design to aid minority carrier transport will help in obtaining the highest efficiency from a given material. In this paper, the critical issues regarding the optimization of materials and devices to obtain high stable efficiency are discussed.

Effect of ion bombardment during deposition of amorphous silicon and silicon-germanium alloy solar cells

Experiments of electric bias effect on a-Si:H and a-SiGe:H solar cells are carried out in both diode and triode configurations. In triode mode, positive bias on substrate improves the performance of a-Si:H cells, while zero bias the gives best results for a-SiGe:H cells. In diode mode, there is essentially no effect of bias on the cells observed over the voltage range in our experiments.

Characteristics of hydrogenated amorphous silicon alloy solar cells on a Lambertian back reflector

Hydrogenated amorphous silicon (a-Si) alloy n i p solar cells have been prepared on back reflectors exhibiting Lambertian light scattering characteristics. The Lambertian back reflector consists of highly textured Ag/ZnO films deposited on stainless-steel substrates. Contrary to expectation, the current voltage (I-V) characteristics under illumination are marginally poorer than the corresponding characteristics of cells grown on conventional back reflectors. The stabilized efficiency is similar for both cases. The dependence of the cell performance on the texture of the back reflector is discussed.

Deposition of hydrogenated amorphous silicon-carbon films by vacuum ultraviolet photo-CVD

Hydrogenated amorphous silicon-carbon films are deposited using disilane and acetylene with vacuum ultraviolet light (147nm) emitted from a microwave-excited Xe resonance lamp. The film shows a high photoconductivity of 1.2 × 10 −5/cm at an optical bandgap of 2.0 eV which is of the same order as that of device-quality films deposited by a glow discharge method. Excess-hydrogenation of the carbon atoms is avoided, and tetrahedral bondings seem to form rigid random networks. P-type doping properties are examined. A photoconductivity of 3.9 × 10 −6 S/cm at an optical bandgap of 2.0 eV is obtained, which can be applied to the window layer of amorphous silicon-alloy solar cells.

Stability Studies of Hydrogenated Amorphous Silicon Alloy Solar Cells Prepared with Hydrogen Dilution

ABSTRACTWe have fabricated hydrogenated amorphous silicon alloy solar cells using hydrogen dilutions at 175 °C and 300 °C, and obtained improved photovoltaic characteristics in both the initial and degraded states for the highly diluted cells; both the fill factor and the open-circuit voltage exhibit higher values before and after light soaking. Infrared analyses reveal that for a given deposition temperature the amount of bonded hydrogen has similar concentrations between the high and low hydrogen diluted samples. Optical Modelling shows a 20 MeV difference in their optical bandgap. Defect densities obtained from constant photocurrent measurements give similar values for a given deposition temperature both before and after light soaking, inconsistent with solar cell performance.