Patent Application
20070256734
Referring now to
In some instances, p-i-n type photovoltaic devices may be comprised of a plurality of triads 12 stacked in an optical and electrical series relationship. These devices are referred to in the art as tandem devices. In some instances, the materials comprising the various triads of a tandem device may be selected so that the wavelength response of the device may be adjusted to address a broad portion of the optical spectrum.
There are a variety of photovoltaic materials which may be utilized in the fabrication of devices of this type. In one particular group of instances, photovoltaic devices are fabricated to include semiconductor layers comprised of thin film alloys of group IV semiconductor materials. For example, in particular types of photovoltaic devices, the intrinsic layer of the device is fabricated from a hydrogenated alloy of silicon, germanium, or silicon/germanium. The p-doped 16 and n-doped 18 layers may likewise be fabricated from thin film group IV alloy materials, or they may be fabricated from other materials. All of such device configurations are known in the art, and may be used in the practice of this invention.
It has been found that a p-i-n type photovoltaic device will have enhanced resistance to light-induced degradation when the intrinsic layer is fabricated from a nanocrystalline semiconductor material configured so that the crystalline volume in the intrinsic layer decreases as the thickness of the layer increases from its interface with the n-doped layer to its interface with the p-doped layer. As will be explained in detail hereinbelow, control of crystalline volume may be controlled by controlling the deposition parameters used in the fabrication of the layer.
It has also been found that photovoltaic device performance and quality is increased when the substantially intrinsic layer is configured so that the crystalline volume in that layer is greater in the region proximate its interface with the p-doped layer, as compared to the crystalline volume in the bulk of the material. In certain aspects of the invention, this region of higher crystalline volume comprises 10-50% of the thickness of the intrinsic layer.
It has also been found that device performance is enhanced when the nanocrystalline intrinsic layer is configured so that the intermediate range order of that layer increases as the thickness of the layer increases from its interface with the n-doped layer to its interface with the p-doped layer. This increase in intermediate range order may be continuous throughout the thickness of the intrinsic layer, or it may occur in a stepwise manner so that a portion of the layer proximate the interface with the p-doped layer has the highest proportion of material with intermediate range order. This portion may comprise 10-50% of the thickness of the layer.
Semiconductor layers of the type utilized in the devices disclosed herein may be prepared by a plasma-enhanced chemical vapor deposition process wherein electromagnetic energy excites a process gas, which process gas includes precursors of the semiconductor materials, and decomposes these precursors so as to create a plasma, containing deposition species which species deposit as a layer of semiconductor material onto a substrate maintained in, or in proximity to, the plasma. By control of the various parameters of the deposition process, including process gas composition, gas pressure, the frequency of the electromagnetic energy, the intensity of the electromagnetic energy, and others, the nature and quality of the deposited semiconductor material may be controlled.
In a first experimental series, a number of single junction photovoltaic devices were prepared in accord with the foregoing deposition process. The nanostructure of the nanocrystalline intrinsic layer was controlled by controlling the profile of a hydrogen diluent in the process gas, in either a continuous or stepwise manner, and as is known in the art, the degree of crystallinity in the material is correlatable with process gas dilution. The stability of the devices to photodegradation was evaluated by light soaking the devices with a white light illumination of 100 mW/cm2 at 50° C. The current density versus voltage (J-V) characteristics of the devices were measured under AM1.5 illumination in a solar simulator at 25° C. Quantum efficiency (QE) of the devices was measured from 300 nm to 1100 nm. The material structure of the intrinsic layer was directly measured on the solar cells using Raman spectroscopy with different excitation wavelengths.
Data from six devices made and evaluated in accord with the foregoing is summarized in Table I hereinbelow.
As is shown in the table, the intrinsic layer of the devices was fabricated, in some instances by utilizing radiofrequency (RF) energy to create and excite the deposition plasma; while in other instances, a modified very high frequency (MVHF) technique was used for fabricating the intrinsic layers. The hydrogen dilution of the process gas was variously controlled. In some instances, the dilution was maintained at a constant throughout the deposition of the thickness of the layer of intrinsic material. In other instances, the hydrogen dilution was varied, on a continuous basis, throughout the deposition, and this profile is referred to as “dynamic profiling.” In another instance, the profile was varied in a stepwise manner. Parameters of the devices in terms of efficiency, short circuit current, open circuit voltage, and fill factor, were measured both before and after light soaking.
As will be seen from the table, the first two cells, using radiofrequency deposition of the intrinsic layer and a constant hydrogen dilution, show a very large light induced degradation, approximately 14-15%, mainly due to reductions in open circuit voltage and fill factor. The third cell with an optimized hydrogen dilution profiling shows only a 3.5% light induced degradation. Similarly, in the MVHF cells, the cell produced with stepwise hydrogen dilution profiling showed an 8.5% light induced degradation, which is somewhat lower than that for the RF cells with constant hydrogen dilution, but larger than that for the dynamically profiled cells in the MVHF process. The open circuit voltage and fill factor in the 13348 MVHF cell did not degrade after prolonged light soaking; in fact, the fill factor of this cell slightly improved.
As will be seen from the foregoing, in this experimental series, control of deposition parameters so as to control the morphology of the intrinsic layers in accord with the foregoing, has significantly improved the performance and stability of the photovoltaic cells.
In a further experimental series, and in order to obtain a better understanding of the mechanism of the light induced degradation of the nanocrystalline cells, and their relation to the deposition process and material structures, Raman measurements were carried out directly on the foregoing six cells.
It is common to determine the crystalline volume fraction from the area under each deconvoluted curve, with a correction factor for the grain size dependence of Raman cross section. For simplicity, only the ratio of areas for each component is set forth. To emphasize the key points,
From the foregoing observation it is apparent that the light-induced degradation in the particular nanocrystalline silicon:hydrogen alloy materials does not increase, with increasing amorphous volume fraction, as was suggested in the prior art. Instead, it decreases. Also, it appears that stable cells have a relatively large and broad intermediate Raman peak. This Raman peak is indicative of intermediate range order, and this order plays a role in the enhanced stability of the devices. While not wishing to be bound by speculation, the regions of intermediate range order may be due to linear like structures formed in high hydrogen dilution plasmas and/or from grain boundaries. The improved stability of the high hydrogen diluted semiconductor material is correlated with intermediate range order.
It appears that in the experimental series, when the nanocrystalline intrinsic layer was deposited under a controlled hydrogen dilution profiling, even though a significant amount of small grains was incorporated into the material, they were not allowed to grow into larger grains. These small grains may not contribute to the sharp crystalline Raman peak, but can contribute to the intermediate peak. From the correlation between the solar cell stability results and the Raman analyses, it is apparent that the presence of a large amount of small grains in intermediate range order, especially near the i-p interface, favors stability.
The increase of intermediate range order along the growth direction of the device is also an important factor. It is known that the i-p interface of p-i-n cells is the dominant junction. The presence of small grains with a reasonable amount of amorphous component in the i-p interface region ensures a good grain boundary passivation and a compact material structure, which reduces defect density and impurity diffusion. As a result, the open circuit voltage of cells thus configured is improved. The high crystalline volume fraction in the bulk of the nanocrystalline intrinsic layer, especially in the n-i region, ensures sufficient long wavelength absorption resulting in a high short circuit density. This also provides high mobility paths for carrier transport resulting in an improved fill factor.
It may be expected that the amorphous component in the i-p region would cause extra light induced degradation. In fact, it is true that the short circuit current in some hydrogen dilution profiled nanocrystalline silicon:hydrogen cells such as numbers 13324 and 13348 of Table I decreases due to the short wavelength response. This reduced short wavelength response is due to recombination in the amorphous phase near the i-p interface, and can be annealed back at high temperature. It is also observed that a loss of fill factor measured under blue light occurred. From the foregoing, it is apparent that a decrease of crystalline volume fraction and grain size along the growth direction of a nanocrystalline cell structure, near the i-p interface is beneficial for cell performance and stability. This feature can be obtained by reducing hydrogen dilution during the deposition of the cell wherein the intrinsic layer is deposited onto an n-doped layer, and can occur naturally when an inversely configured cell is prepared wherein the intrinsic layer is deposited onto the p-doped layer.
Based upon the foregoing principles and observations, a p-i-n type cell was prepared incorporating a nanocrystalline intrinsic layer of a silicon hydrogen alloy. This single junction cell showed an initial active area efficiency of 9.06%. A triple junction cell was prepared in accord with the foregoing principles. The triple junction cell included nanocrystalline intrinsic layers in the middle and bottom cells of the stack. This triple junction cell achieved an initial active area efficiency of 14.1%, and had an efficiency of 13.2% following prolonged light soaking.
Conclusions drawn from the foregoing are that, first of all, the amorphous component is not the determining factor for the light induced degradation of nanocrystalline semiconductor materials; second, smaller grains and intermediate range order and/or better grain boundary passivation improves cell stability; and third, the decrease of crystalline volume fraction along the growth direction of an n-i-p structure, especially near the i-p interface, improves the cell performance and stability. This can be accomplished by an optimized hydrogen dilution profile.
While the foregoing has been described with reference to particular configurations of photovoltaic devices, it is to be understood that these principles may be extended to other configurations of photovoltaic devices, as well as to other photoresponsive devices, and to semiconductor devices in general in which control of photodegradation and/or transport properties is beneficial. In view of the teaching presented herein, numerous modifications and variations of the methods and materials shown herein will be apparent to those of skill in the art. The foregoing is illustrative of specific embodiments and implementations of the invention, but is not meant to be a limitation upon the practice thereof.
This application claims priority of U.S. Provisional Patent Application Ser. No. 60/798,547 filed May 8, 2006, entitled “Stabilized Photovoltaic Device and Methods for Its Manufacture”.
| Number | Date | Country | |
|---|---|---|---|
| 60798547 | May 2006 | US |
