Patent Application
20130312828
This application claims priority under 35 U.S.C. §119 from European Patent Application No. 1209080.9 filed May 24, 2012, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention is related to the field of thin-film photovoltaic devices, which are devices including amorphous semiconductor materials as light absorbing materials. More particularly, it is directed to thin-film PV cells including a band-stop filter to reduce photo degradation effects.
2. Description of the Related Art
Provided are some basic definitions of the related art. Photovoltaics (PV) generate electrical power by converting solar radiation into direct current electricity through materials exhibiting the photovoltaic effect. A photovoltaic cell, or PV cell, also solar cell or photoelectric cell is a solid state device, semiconductor PV cells, or a polymer device (e.g., organic PV cells) that converts energy of light directly into electricity by virtue of the photovoltaic effect.
A thin-film photovoltaic cell, or thin-film solar cell, is a solar cell made of one or more thin layers (or thin films) of photovoltaic material, deposited on a substrate. The thickness of a deposited layer ranges from nanometers to tens of micrometers or more. Thin-film PV cells can be categorized according to the PV material used, for example: Amorphous silicon (a-Si), Other thin-film silicon (TF-Si), Cadmium Telluride (CdTe), Copper indium gallium selenide (CIS or CIGS), and Dye-sensitized solar cell (DSC) or other polymer/organic solar cells, etc. A photovoltaic module, also solar module, solar panel or photovoltaic panel, is an assembly of connected photovoltaic cells.
Amorphous silicon (a-Si) refers to a non-crystalline phase of silicon. It can be deposited in thin films at relatively low temperatures, typically in the range of 200-300° C., onto a variety of substrates, and offers capabilities for many applications including photovoltaics at a moderate cost. A-Si material can be passivated by hydrogen. Hydrogen atoms bond to dangling bonds. Hydrogen passivation can reduce dangling bond density by orders of magnitude, so that most Si atoms are 4-fold coordinated, just as in single-crystalline Si (c-Si) or SiH4 molecule. Hydrogenated amorphous silicon (a-Si:H) has a sufficiently low amount of defects to be used within devices. Without hydrogen, a-Si would have a high density of defects due to unpassivated dangling bonds, which notably alters photoconductivity. In the literature, the presence of hydrogen in a-Si:H-based devices is believed to be the main factor in leading to the light induced degradation effects of the material, termed the Staebler-Wronski Effect.
Plasma-enhanced chemical vapor deposition (PECVD) is a dominant technique used for the growth of a-Si:H-based amorphous materials. Hydrogenated amorphous silicon thin-film cells typically use a p-i-n structure. Usual panel structures include front side glass, transparent conducting oxides, thin film silicon, back contact, polyvinyl butyral (PVB), and back side glass.
One aspect of the present invention provides a photovoltaic device. The photovoltaic device includes an amorphous photovoltaic material and a band-stop filter structure having a stopband extending from a lower limiting angular frequency ωmin≧0 to an upper limiting angular frequency ωmax where ωmax>ωmin. The band-stop filter structure is arranged in the photovoltaic device relative to the photovoltaic material in order to attenuate electromagnetic radiations reaching the photovoltaic material with angular frequencies of ω* in the stopband, so that ωmin<ω*<ωmax. The angular frequencies ω* correspond to electronic excitations ω* from valence band tail (VBT) states of the amorphous photovoltaic material to conduction band tail (CBT) states of the amorphous photovoltaic material.
Another aspect of the present invention provides method of operating a photovoltaic device. The method includes exposing the photovoltaic device to radiation. The method further includes filtering, via a band-stop filter structure, out electromagnetic radiation reaching a light absorbing material with angular frequencies of ω* within the stopband. The band-stop filter has a stopband extending from a lower limiting angular frequency ωmin≧0 to an upper limiting angular frequency ωmax, where ωmax>ωmin. The angular frequencies ω* correspond to electronic excitations ω* from valence band tail (VBT) states of the amorphous photovoltaic material to conduction band tail (CBT) states of the amorphous photovoltaic material. The method further includes producing an electrical power using the photovoltaic device.
Devices and methods embodying the present invention will now be described, by way of non-limiting examples, and in reference to the accompanying drawings.
Present inventors have identified a fundamental mechanism responsible for the photodegradation effect, the Staebler-Wronski effect. Based on this discovery, they have devised new solutions to reduce photodegration effects in PV devices. These solutions take advantage of one or more band-stop filter structures, or closely related filter structures, incorporated in a PV device.
The main idea is to incorporate a band-stop filter structure to block electronic excitations responsible for the photodegradation in the active amorphous material(s). The reason is, as present inventors have discovered, that electronic excitations from valence band tail (VBT) states to conduction band tail (CBT) states of an amorphous photovoltaic material are responsible for photodegradation effects.
In the prior literature, light induced degradation was generally believed to be associated to the presence of hydrogen atoms passivating a-Si dangling bonds. Currently, it is proposed that VBT-to-CBT electronic excitations are causing this photodegradation. Such conclusions were drawn based on large-scale ab initio calculations and interpretations. Thus, the main goal of the PV designs proposed is to filter out that part of the light spectrum that causes the light-induced changes in amorphous PV semiconductor materials.
In reference to
These amorphous semiconductors are the active PV components, which exhibit the desired photovoltaic effects, as known per se. They can consist of one or more layers of absorbing material, which upon exposure to radiation make it possible for producing electrical power. Present designs further include a band-stop filter structure 20, having a given stopband. The band-stop filter structure can include a filter layer 20 on one side of the absorber layer structure as shown in
As known by the ordinary person skilled in the art about band-stop filters in general, a band-stop filter structure can be designed to at least attenuate (i.e., substantially) or possibly completely prevent radiations having frequencies within a given stopband to pass through the filter and reach component(s) it is meant to protect.
In the present invention, and as a result of the above discovery, the stopband corresponds to electronic excitations ω* from valence band tail states to conduction band tail states of the amorphous material. Thus, unlike existing methods which focus on the quality of the amorphous PV material, it is suggested to modify properties of radiations reaching it. When a PV device is exposed to radiation, e.g., sunlight with frequencies distributed over a wide spectrum, the filter structure filters out undesired radiation reaching the absorber. A PV device can be advantageously used to produce electrical power with reduced degradation upon exposure to sunlight.
In
The frequency range that induces the degradation is material-specific. Therefore, the band-stop filter structure has to be adjusted according to the amorphous PV semiconductor material(s) used in the device.
The energy band diagram is complemented by a schematic density of electronic states (DoS) for the same amorphous semiconductor. The DoS exhibits five distinct regions:
Each of the mobility lines separates extended states ES from localized states LS.
The above-mentioned concepts, i.e., LS, ES, ECM, EVM, ωgap, etc., are now discussed. The lack of long-range order in a material such as a-Si:H has consequences on electrical and optical properties. The bonding disorder results in band tails and localization, while point-like defects result in electronic levels in the midgap. In a perfect single-crystalline solid, where a perfect crystalline solid means infinite, defect-free crystalline solid with no impurities, an electron would experience a periodic potential. It then follows from Bloch's theorem that the electronic states are “extended” states, inasmuch as electron wavefunctions extend all over the crystal. The position of an electron with given momentum k can be anywhere in the crystal, as a direct consequence of the uncertainty principle. In Mott's model, which is accepted as a standard model of amorphous materials, localized and extended states are separated by mobility edges. The CB mobility edge is denoted with ECM, in analogy with the conduction band in single crystalline semiconductors. ECM denotes the lowest allowed energy of an “unbound” electron, the lowest extended state energy level which is unoccupied at zero temperature. Similarly, the valence band edge is denoted by EVM.
Thus, extended states ES and localized states LS are separated by the mobility edges at energy ECM in the conduction band CB and energy EVM in the valence band VB. At zero temperature, only carriers above ECM and below EVM are mobile and can contribute to conduction. The energy difference between the two mobility edges is referred to as the mobility gap, which is typically about 1.8 eV. The localized states below ECM and above EVM are referred to as band tail states. They are generally assumed to decrease/increase exponentially, i.e., to be exponentially distributed in energy, notably because of the correlation of the conduction band tail with the Urbach edge. The exponential tail of the optical absorption coefficient in the vicinity of the bandgap energy in amorphous materials. The mobility edge, the VB and CB band tails and similarly the optical gap ωgap, are key descriptors of amorphous semiconductors, well documented in the literature. A standard definition of the optical gap is notably found in J. Tauc, R. Grigorovici, and A. Vancu, Phys. Stat. Sol. 15, 627 (1966). This definition is echoed in many textbooks and is assumed here.
As discussed above, present designs of PV devices filter out radiations having frequencies ω* within the stopband of the filter, which corresponds to excitations ω* from VBT states to CBT states of the amorphous PV material. The filtered frequencies ω* are located between an upper limiting angular frequency ωmax and a lower limiting angular frequency ωmin of the stopband (ωmax>ωmin≧0). The determination of these limiting frequencies is explained later.
The band-stop filter should preferably not eliminate or significantly reduce the useful part of the light spectrum ω>ωM that generates the photocurrent. Thus, ω* should be smaller than or equal (or at least close) to the mobility gap ωM of the amorphous PV material. Indeed, the electronic excitations induced by photons with energies ω* can be related to the sub-gap absorption due to optical transitions from VBT-LS to CBT-LS. Accordingly, the maximal transition energy allowed ωmax, corresponding to an upper limiting angular frequency ωmax of the stopband is preferably equal to or slightly smaller or larger than the mobility gap ωM of the amorphous material. As a result, all, or essentially all, electronic excitations ω* from VBT states to CBT states can be prevented. In the example of
Similarly, one understands from
However, in variants, filters could be used which exhibit a lower limiting frequency ωmin=0. In this case, the band-stop filter reduces to a mere high-pass filter, as per definitions of band-stop vs. high-pass filters. Yet, suitably tuning the upper limiting or cutoff frequency ωmax as described above (e.g., ωmax=ωM), undesired frequencies are anyway prevented. Such variants can for example be contemplated for single-junction PV cells such as depicted in
In this regard, photons with energies sufficiently smaller than the bandgap value create a negligible photocurrent, but these photons can still be harmful for the amorphous material since they can give rise to transitions between valence and conduction band tails that are each exponentially extending towards the midgap. Frequency ωmin can be regarded as an estimate for a cutoff frequency that is a “frontier” between relative harmless and essentially harmful radiations in the low frequency limit. In this respect the ωM can be regarded as the corresponding high frequency “frontier”.
As discussed above, the discovery mentioned earlier, there is a priori no need to filter out radiation frequencies ω for which ω<ωmin, such that a filter structure according to embodiments of the present invention could extend between ωmin≧0 and ωmax>ωmin, especially in single-junction-like devices. However, things are somewhat more complicated for multi-junction cells. Layered structures as in
The cell design of
Referring to
For instance, Δω can be such that Δω˜2EU, where the average Urbach energy EU is defined as the average of its CB and VB components, i.e., EU=(EUC+EUV)/2, or equivalently in terms of angular frequencies, Δω=Δωc+Δωv with Δωc=EUC/ and Δωv=EUV/. The stopband can essentially extend between a lower limiting angular frequency ωmin=ωgap−Δωc−Δωv and an upper limiting angular frequency ωmax=ωgap+Δωc+Δωv, where ωgap corresponds to the optical gap ωgap of the amorphous PV material, and where Δωc and Δωv are the Urbach energy of conduction band tail states and valence band tail states, respectively. Thus, the energy corresponding to the width of the stopband can essentially be 2(EUC+EUV). Generally, the width of the stopband shall typically correspond to twice the sum of the extents of the CBT and VBT.
Of course, better descriptors of the dispersion of the CBT and VBT than the corresponding Urbach energies could be relied upon, e.g., for those materials for which the CBT or VBT is found to deviate from the ideal exponential behavior, etc. Other descriptors could for instance be used, which are still proportional to EUC and EUV, x EUC and x EUV, where 0≦x≦ωgap/(EUC+EUV).The result to be obtained still should remain essentially the same, i.e., filtering excitations from VBT states to CBT states of the amorphous PV material. The considered stopband limiting frequencies could be refined based on knowledge of the density of energy states or other electronic structure data available for the amorphous material used, or still based on filter manufacture constraints. However, optimal stopband limiting frequencies shall likely be located within the ideal theoretical limits specified above, between ωmin=ωgap−Δωc−Δωv and ωmax=ωgap+Δωc+Δωv.
The width of the tail states of the conduction band and of the valence band in amorphous semiconductors can be separately measured, by photomodulated infrared absorption. In addition, the mobility gap can be measured by conductivity experiments. Thus, all quantities needed to specify the stopband characteristics are measurable or even computable quantities, inherent to the amorphous PV materials used.
As discussed above, this amorphous PV material is preferably a-Si:H. Thus, as it can be verified, ωgap is typically between 1.7 and 2.0 eV, in accordance with typical values of the gap in a-Si:H films. Investigating the energy distribution of band tail states at the Urbach edges, it appears that appropriate values for Δω shall be typically less than 0.2 eV (e.g., in the range 0.05 to 0.2 eV). For good quality material, this value shall typically be close to 0.1 eV or even less.
Suited values for the stopband can be chosen according to the band gap ωgap and Urbach energy values of the amorphous PV semiconductor. In that respect, the absorber can include other materials, such as hydrogenated amorphous silicon doped with germanium and/or carbon, e.g., a-Si1-xGex:H or a-Si1-x-yGexCy:H, where x and y are between 0 and 1 (for example 0.1). Introduction of Ge/C atoms adds extra degrees of freedom for control of the properties of the material. In this case, increasing concentrations of carbon widen the electronic gap between conduction and valence bands; germanium results in the opposite. For completeness, the amorphous material can further include Cadmium telluride (CdTe), Copper indium diselenide CIS, an inorganic amorphous material or an organic photovoltaic material. In each of said materials, filtering excitations from VBT states to CBT states is expected to decrease the photodegradation effect.
The band-stop filter structure can be obtained in several manners. In particular, the device can have a layer structure, and the band-stop structure can result from one of more layers of said layer structure, as illustrated in
The simplest way to obtain the desired band-stop filter is to provide a filter layer 20 or 21, on one side of the active PV layer 34, e.g., contiguous with a layer of the layer structure, as depicted in
The device 10a schematically depicted in
The arrangement of the sole layers 32, 34 and 36 is essentially similar to prior devices. Since the mobility of holes moving towards the p-type layer is significantly lower than electrons moving towards the n-type layer, the p-type layer is placed on top, after the filter, i.e., where light intensity remains stronger, so that the collection efficiency of the holes is enhanced because the holes have to travel shorter distances than the electrons to reach the respective layers (n-type for electrons and p-type for holes). Still, layers 32, 34 and 36 are arranged “under” the band-stop filter layer 20, to attenuate or remove undesired radiation frequencies.
In this regard, any material, which can be deposited or placed on top of the solar cells and exhibit suitable filtering characteristics as described above a priori convene, provided that the overall performance of the solar cells is not compromised. Various methods are known in the art which allows for adjusting a band-stop of a band-stop filter.
For instance, the bandstop filter can be designed as a thin film, layer 20, of a material that shows strong absorption within a particular wavelength range (stop band). For example, such a layer can be made with a suitable transparent diluent, often KBr—potassium bromide, doped with an absorbing material with typical concentration ˜1%. The actual concentration of the absorbing material is chosen to tune the upper and lower limiting frequencies of the bandstop filter. For example, band-stop optical filters made of potassium bromide (KBr) doped with tetramethyl ammonium halides or metal chromates can be relied upon. Many other suitable materials can be found in commonly-used databases of published optical spectra, e.g. see the Sadtler Standard Spectra.
A single-layer bandstop filter, layer 20, can also be designed as a frequency selective surface (FSS). The FSS is essentially a grated surface of some suitable dielectric material layer 20 placed on top of layer 42 that acts as a reflector for a particular range of light wavelengths (stop band of the filter), for example, such a filter can be designed as an array of periodic patches. The filter properties, upper and lower limiting frequencies, can be easily tuned by changing the layer thickness, t, between 400-900 nm, width of patches, w, about 500 nm, period of the array (p about 500-1000 nm) and refraction index of the material used for the patches. Possible materials suitable for these purposes would be chalcogenide glasses, e.g. Ge—Se or Ge—Sb—Se. Other types of surface grating as well as dielectric materials can also be used for making a band-stop filter suitable for the present invention.
Using alternating layers of high and low index dielectric materials like Ta2O5/SiO2 (or SiO2/Ta2O5), TiO2/SiO2 (or SiO2/Ta2O5) for a composite filtering film 20 placed on top of glass (layer 42) also allows for designing band-stop filter with desired characteristics. Other types of dielectric materials can also be used for making a band-stop filter suitable for embodiments of the present invention.
In other words, the band-stop filter can be designed as a thin film 20, placed on top, made of a single material, or still including several layers of different materials.
The lateral dimensions of layer filter 20 or 21 can essentially be the same as those of other layers 32, 34, 35, 36, in embodiments shown in
Also a filter can be designed as a multilayered structure. For example, layer 20 can be a dielectric Bragg mirror made of dielectric materials. Typically, the characteristic thickness should be about or larger than λ=2πc/ωM i.e., 600 nm for ωM=2 eV, using standard notations.
Next, the device 10b schematically depicted in
The layer arrangement of the sole layers 42, 44, 32, 34, and 36 is again similar to prior art devices. Namely, Si-based layers are deposited on glass 42, coated with a layer of transparent conducting oxide (ZnO:Al). However, a band-stop filter layer 20 is placed on top, which filter has the desired properties that are discussed above.
Expanding on this,
Integration of subcells such as depicted in
Other variations shall be described later in reference to
To demonstrate advantages of a band-stop filter as described above i.e., coupled to a solar cell to reduce light-induced degradation, it is sufficient to show the dependence of photodegradation on the wavelength of the absorbed light. Present inventors have performed yield measurements for a set of a-Si-based solar cells with narrow bandpass filters that select radiation with particular wavelengths. The solar cells used were a-Si-based solar cells as shown in
This reduction of the overall efficiency degradation leads to an improvement of the solar cell performance on the long-term usage of the solar cells, allowing for more efficient collection of the solar energy, i.e. generating more electrical power from the sun radiation. For example, reducing the overall efficiency degradation by 25%, 50% and 100% will increase stable yield of the currently-manufactured state-of-the-art solar cells up to ˜5%, 10% and 20%, respectively, which corresponds to equivalent increase of the generated electrical energy. Such increases would be considered as substantial in mass production of solar energy. As discussed above, a band-stop filter structure can consist of a simple filter placed on top of the device. In variations discussed below, the band-stop filter structure can result from a more complex design.
For instance, the PV device can have a layer structure that includes a repeating pattern of two contiguous layers (or more) of different materials, e.g., a a-Si:H layer 34 and a a-SiGe:H layer 35, as illustrated in
Layer 35 (a-SiGe:H) could be active as well. For instance, layer 35 is also active if its thickness is comparable to that of layer 34, which is the case for devices in
The filter layer 20 (21) in
Concerning the dimensions: lateral dimensions (aperture area) are macroscopic, and can for instance be of a few hundreds cm2. Therefore, the on-top filter 20 can be supplemented by additional “absorbing” layers 35, as in the device 10c of
According to some embodiments of the present invention, instead of using a filter single layer, a super-lattice is used which acts as a dielectric omnidirectional reflector (dielectric Bragg mirror) for radiations having undesired frequencies, as in
More designs of PV devices are briefly described in reference to
Finally, practical implementations of present photovoltaic device could take the form of a package, notably including thin-film PV cells, electrodes, as well as other components as usual in the art. Such devices can further include a shield to protect some of these components from exposure to radiations. In addition, the shield can be arranged to encapsulate the cell package to protect against dust, humidity, etc. The shield can further include a cover window. In other variations, the filter structure can be placed contiguous with the window or this window could even be used as a filter, to filter undesired frequencies, instead of providing a filter layer as described above. More generally, a filter having characteristics as described earlier could be embedded as part of such a shield.
While the present invention has been described in reference to certain embodiments, it will be understood by those skilled in the art that various changes can be made and equivalents can be substituted without departing from the scope of the present invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention will not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. Thus, many variations are possible. For example, other materials, chemical compositions and layer dimensions than those recited in the above examples could be used to obtain essentially the same results as provided in embodiments of this invention, i.e., where the filter's stopband corresponds to electronic excitations from VBT states to CBT states of the amorphous material used as absorber.
Another embodiment of the present invention provides a photovoltaic device that can include an energy ωmax corresponding to an upper limiting angular frequency ωmax of the stopband is less than or equal to a mobility gap ωM of the amorphous photovoltaic material, and preferably is equal to said mobility gap ωM.
Another embodiment of the present invention provides a photovoltaic device that can include an energy ωmin corresponding to the lower limiting angular frequency of the stopband is less than an optical gap ωgap of the amorphous photovoltaic material.
Another embodiment of the present invention provides a photovoltaic device that can include an energy 2 (Δωc+Δωv) corresponding to a width of the stopband of the amorphous material corresponds to twice the sum of: an energy Δωc corresponding to the extent of a conduction band tail of the amorphous photovoltaic material and an energy Δωv corresponding to the extent of a valence band tail of the amorphous photovoltaic material.
Another embodiment of the present invention provides a photovoltaic device that can include the stopband extends between a lower limiting angular frequency ωmin=ωgap−Δωc−Δωv and an upper limiting angular frequency ωmax=ωgap−Δωc−Δωv, wherein ωgap corresponds to an optical gap ωgap of the light absorbing material, and wherein Δωc and Δωv respectively correspond to energies Δωc and Δωv, respectively corresponding to the extent of a conduction band tail of the amorphous photovoltaic material and the extent of a valence band tail of the amorphous photovoltaic material.
Another embodiment of the present invention provides a photovoltaic device that can include Δωc and Δωv are the Urbach energy of conduction band tail states and the Urbach energy of valence band tail states.
Another embodiment of the present invention provides a photovoltaic device that can include the amorphous photovoltaic material comprises hydrogenated amorphous silicon.
Another embodiment of the present invention provides a photovoltaic device that can include the amorphous photovoltaic material comprises hydrogenated amorphous silicon and: ωgap is between 1.7 and 2.0 eV; and Δωc+Δωv is less than 0.2 eV, and preferably is between 0.1 and 0.2 eV.
Another embodiment of the present invention provides a photovoltaic device that can include the amorphous photovoltaic material includes hydrogenated amorphous silicon doped with germanium or doped with germanium and carbon:
Another embodiment of the present invention provides a photovoltaic device that can include the device has a layer structure, with one of more layers of said layer structure forming the band-stop structure.
Another embodiment of the present invention provides a photovoltaic device that can include the layer structure that further includes a pattern of two or more contiguous layers of different materials, preferably a-Si:H and a-SiGe:H, this pattern being repeated along the layer structure.
Another embodiment of the present invention provides a photovoltaic device that can include one of the contiguous layers is designed to absorb radiation frequencies within the stopband, and another one of said contiguous layers, preferably both contiguous layers, comprises said amorphous photovoltaic semiconductor material.
Another embodiment of the present invention provides a photovoltaic device that can include the layer structure forms a Bragg reflector, the latter configured to reflect radiations with frequencies within the stopband, and preferably configured as an omnidirectional reflector.
Another embodiment of the present invention provides a photovoltaic device that can include the band-stop structure is a band-stop filter layer contiguous with a layer of the layer structure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1209080.9 | May 2012 | GB | national |
