Method and apparatus providing inline photoluminescence analysis of a photovoltaic device

A method and apparatus are disclosed which use a photoluminescent light intensity signature to characterize a processed photovoltaic substrate.

FIELD OF THE INVENTION

The invention relates to an in-line method and measurement tool which uses photoluminescence to determine characteristics of a photovoltaic device, such as photovoltaic cells and photovoltaic modules containing a plurality of photovoltaic cells.

BACKGROUND OF THE INVENTION

Photoluminescence (PL) is a process in which a substance absorbs photons and then re-radiates photons. Photoluminescent measurement is a contactless and non-destructive method of probing an electronic structure of materials.

Photoluminescence may be used to determine the quality of semiconductor material deposition on a substrate. For example, in thin-film photovoltaic device fabrication, semiconductor window and absorber layer materials are deposited over a substrate. Following deposition the substrate can then be irradiated by shining light into the substrate and measuring the photoluminescent spectrum which can indicate the quality of the semiconductor material depositions. After semiconductor deposition, photovoltaic devices are further fabricated in subsequent multiple steps. A measurement method and apparatus are needed which can detect deviations from desired processing conditions during such further fabrication of a photovoltaic device.

DETAILED DESCRIPTION OF THE INVENTION

The various embodiments described herein provide a photoluminescent method and apparatus for determining deviations from desired processing conditions which occur subsequent to semiconductor layer deposition on a substrate during the manufacture of photovoltaic device. Measurements of the photoluminescent spectra intensity are taken of the photovoltaic device in-line and at a stage after all processing of the photovoltaic internal layers is complete, for example, following completion of photovoltaic device fabrication.

These measurements can reveal, among other things, deviation in a desired processing condition such as deviation from a desired doping concentration for the absorber layer or deviation from a desired temperature used to heat treat a fabricated metallization pattern formed on the absorber layer.

The manufacture of thin-film photovoltaic devices involves many, often complex, processing steps. These steps include, among others, the deposition and treatment of the various films which are deposited over a substrate.FIG. 1illustrates one example of a partially fabricated thin-film photovoltaic device105. The partially fabricated photovoltaic device105can be used to form one or more photovoltaic cells of a completed photovoltaic device. Device105includes a substrate11through which light, illustrated by the arrows, can pass which can be formed of a glass such as soda lime glass, low iron glass, solar float glass, or other suitable glass. A barrier layer13may be formed over the substrate11which is used to lessen the diffusion of sodium from the substrate into other layers of a completed photovoltaic device. The barrier layer15may include, for example, silicon dioxide (SiO2), silicon aluminum oxide (SiAlO), in tin oxide (SiO) or other suitable material. A transparent conductive oxide (TCO) layer15can be deposited over the barrier layer13and is used as one conductor of a completed photovoltaic device. TCO layer15may be formed, for example, of cadmium stannate (Cd2SnO4), cadmium tin oxide (CdO3Sn), fluorine (F) doped tin oxide (SnO), or other known transparent conductive oxide material. A buffer layer17may also be deposited over the TCO layer15to provide a smooth surface for deposition of semiconductor material. The buffer layer may include, for example, tin oxide (SnO2), zinc tin oxide (ZnSnO3), zinc oxide (ZnO) or zinc magnesium oxide (ZnMgO).

The absorber layer21is typically annealed by depositing a cadmium chloride solution (CdCl2) in liquid form on the absorber layer21after which the absorber layer21is annealed by heat treatment at about 400 degrees C. to about 450 degrees C. for a predetermined period of time, for example, about 10 minutes to about one hour. The CdCl2anneal desirably increases the grain size of the absorber layer21which has been found to enhance photo-conversion efficiency.

Since the deposition of the semiconductor materials forming the window19and absorber21layers is important to the functionality and long term stability of a completed photovoltaic device, methods and apparatuses for monitoring the quality of those depositions have been developed. One such apparatus, which uses photoluminescence, is described in U.S. application Ser. No. 13/195,163, filed Aug. 1, 2011. The entirety of this application is fully incorporated herein by reference. The techniques described in the '163 application can provide useful information on the quality, uniformity and stability of the semiconductor window19and absorber21layer depositions shown inFIG. 1by measuring the overall intensity of the photoluminescence.

However, there are also subsequent fabrication steps which must further occur to produce a completed photovoltaic device.FIG. 2illustrates an example of a completed photovoltaic device106fabricated from the partially fabricated device105shown inFIG. 1. The partially fabricated photovoltaic device105has been further processed to include, among other things, a copper doping of the absorber layer21, the formation of a back contact (metallization) layer23over the absorber layer, and the provision of a back cover over the back contact layer23. An interlayer material27may also be provided on the sides of the fabricated layers13through21and optionally also between the back contact layer23and back cover25. The fabrication steps required to copper dope the absorber layer, as well as the process steps required to form the back contact23, which include a heat treatment of the deposited back contact23, can also affect quality and stability of the completed photovoltaic device106. For example, improper copper doping concentration or non-uniform copper doping of the absorber layer21can affect electrical performance of the completed photovoltaic device106, as well as long term stability. The copper doping is used to increase charge mobility in the absorber layer21and reduces that contact resistance between the absorber layer21and the metal contact layer23. Moreover, during the back contact formation process heat is used to drive a deposited metal into the absorber layer21to provide a good contact therewith. The temperature of this heating process may also affect the quality of the contact and thus metal/absorber layer performance and quality and stability of the completed photovoltaic device106.

Embodiments of the invention use a photoluminescence (PL) tool at the back end of a photovoltaic device manufacturing line and after processing of the internal layers of the photovoltaic device is completed to detect and measure the intensity of a photoluminescence spectra which can indicate deviations in process conditions subsequent to the deposition and CdCl2anneal treatment of absorber layer21. Deviations which can be detected include, among others, deviations in a desired copper doping concentration and deviations in the heating temperature for back contact metallization.

FIG. 3schematically illustrates the processing of a partially fabricated device105(FIG. 1) towards a completed photovoltaic device106(FIG. 2). The partially completed PV device105, following a CdCl2deposition and anneal of absorber layer21, is conveyed by conveying mechanism103, for example by driven rollers, to and through the various stages illustrated inFIG. 3. The partially completed photovoltaic device following the CdCl2anneal treatment, is provided to a doping station111at which copper doping is applied to the absorber layer21. Following this, the partially completed photovoltaic device105proceeds to a metallization stage113where the back metal contacts23are applied. An optional ZnTe layer may also be deposited after the CdClztreatment of the absorber layer, with or without copper doping, and before the metallization stage113. Subsequent to the metallization stage113, a heat treatment is applied at heat treatment stage115to drive metal applied in the metallization stage113into the absorber layer, after which a final photovoltaic device assembly is performed at stage117. At the final assembly stage117the interlayer27and back cover25are applied and the substrate11and back cover5of the completed photovoltaic device106are laminated together. The final module assembly stage117also provides an edge seal to the completed photovoltaic device106, and adds a cord plate or junction box over an opening in the back cover25to provide electrical connections to the one or more photovoltaic cells of the completed photovoltaic device106.

Following final assembly at stage117, the completed photovoltaic device106is subject to a biasing operation at bias stage119at which bias voltages are applied to the completed photovoltaic device106to condition the completed photovoltaic device106for use. Embodiments of the invention provide a photoluminescence tool100in-line, after processing of all internal material layers is completed, such as after final assembly of completed photovoltaic devices106. Alternatively, the photoluminescence tool100can be provided in-line before bias stage119. After passing the photoluminescence tool100and biasing stage119, the completed photovoltaic devices106are passed to customer fulfillment.

In many instances sample testing of a few of the completed photovoltaic devices106, using a so-called light soak test may occur at stage123. In such a testing, some, but not all, of the completed photovoltaic devices106are removed from the production line and are tested over periods of days or weeks to determine how well other like completed photovoltaic devices106will perform in the field. While this testing does provide useful information on the quality of the completed photovoltaic devices106, it is done on a sample basis and takes considerable time, and not all completed photovoltaic devices106are subject to the test. By contrast, the photoluminescent (PL) analysis tool100can provide qualitative information for each completed photovoltaic device106which can be used to determine deviations from certain post CdCl2anneal process conditions, to provide an indication of the quality and stability of completed photovoltaic devices106leaving the production line.

The provision of the photoluminescent (PL) tool100in-situ and in-line in particular enables an assessment of deviations from a proper copper doping concentration of the absorber layer21at stage111as well as temperature deviations at heat treatment stage115. Thus, the photoluminescent tool100can provide, information on the quality and stability of each completed photovoltaic device106as a result of the doping111, metallization113, and heat treatment115stages. Such information can be gathered in real-time such that any abnormalities can be detected as a completed PV device106leaves the production line.

FIG. 4shows an embodiment of a photoluminescent (PL) tool100which can be used. The PL tool100is provided beneath a conveyed completed photovoltaic device106and includes a monochromatic light source50which can be a light emitting diode, a diode laser or a solid state laser. Light source50can also comprise a white light source placed ahead of a monochromator. The wavelength of the light source50can be chosen depending on the band gap of one or more semiconductor materials contained within photovoltaic device106. For example, light source50can be one of a red, blue or green color wavelength and can be chosen based on the band gap of one or more of the window layer19, absorber layer21, or interface between these layers. It is well known that a CdS window layer19and a CdTe absorber layer21, during deposition and CdCl2absorber layer21annealing, can create a CdSxTe1-xintermediate interface layer between window layer19and absorber layer21.

The tool100further includes an optical system59employing various lenses and filters and which are used to supply a focus and/or collimated beam of light to the completed photovoltaic device106. The optical system59can also reduce variations of the wave-length distribution and can consist, for example, of at least one plano-convex lens and a band-pass filter. The band-pass filter can be positioned between light source50and the optical system59, or it can be integrated into the optical system59or provided at any other suitable position. The band-pass filter or filters can be optional and the decision to include them or not can be based on the wavelength of light from the light source50and/or a particular desired photoluminescence activation wavelength. The measurement tool100can further include an optical mirror58, such as a dichroic mirror, which is used to redirect the light beam from source50and optical system59toward the completed photovoltaic device106. The measurement tool100can further include a lens57chosen and positioned so that the desired focus position of the light beam irradiates one of the window layer19, absorber layer21or interface between them, as desired. In some embodiments, slight over or under focus of the light beam can be acceptable so that more than one of the window19absorber21or intermediate layers are irradiated. As explained below, the intensity and wavelength of light can also be used to select which of the layers is irradiated. Lens57can also be optional depending on the beam collimation and distance to the semiconductor material layers within the completed photovoltaic device106.

Light source50can emit light of any suitable wavelength. For example, light source50can emit red light having a wavelength between 600 nm and 690 nm, for example, at about 660 nm. Light source50can also emit blue light with a wavelength in the range of about 425 nm to about 490 nm, such as about 445 nm, as an example. Light source50can also emit green light having a wavelength in the range of about 500 nm to about 580 nm, for example, at about 532 nm.

The red light can pass through the CdS window layer19and CdS and CdSxTe1-xintermediate interface layer such that photoluminescence spectra is primarily generated by the CdTe absorber layer21. Blue light and green light can be absorbed by all three layers, but since the window and intermediate layers are first irradiated by incident light the resultant photoluminescence spectra is primarily from those two layers.

Semiconductor materials, such as at the window layer19, absorber layer21, and the interface between them, can be excited by the light beam impinging upon one or more of these layers provided by lens57. The process of light emission following excitation of the semiconductor material with light (photons) of energy greater than its band gap is a result of recombination of photo generated electron and hole carriers produced by the photons from light source50. Light emission depends on internal and external quantum efficiencies of each semiconductor layer. If the excited device consists of a bi-layer of material, for example, of a p-type semiconductor absorber layer21deposited on an n-type semiconductor window layer19which also has an interface layer, then the recombination can occur at various locations depending on excitation light intensity, wavelength, and resulting penetration into the layered semiconductor structure. Thus, by selecting the wavelength of the light source50and the focus characteristics of lens57one or more of the window layer19, e.g., CdS, absorber layer21, e.g., CdTe, and the interface, CdSxTe1-x, between them can be irradiated and photoluminescent intensity values obtained.

In addition, the wavelength of light incident on the completed photovoltaic device106can be changed by suitable light source50selection, band-pass filter selection and/or changes in the optic system59and57to select different excitation wavelengths, and focal points which can yield different degrees of penetration into the completed photovoltaic device106. For example, the excitation light wavelength, intensity and/or focal point can be changed to examine one or more of the window layer19, absorber layer21or interface layer between them, as the photovoltaic device106passes across the focal point of lens57during its movement by conveying mechanism103.

The measurement tool100further includes a sensor56for sensing photons produced by the photoluminescence from the completed photovoltaic device106and an optical system61and band-pass filter63which are provided in front of the sensor56. The optical system61can be provided by any suitable combination of lenses and band-pass filter63can be provided by one or more band-pass filter which combination can allow a specific region of the wavelength distribution of the photoluminescent spectra to be detected by the sensor56. For example, optical system61can include a convex lens. The band-pass filter63can be arranged as selectable filters which allow different areas of a photoluminescence spectra to be analyzed. The optical system61and band-pass filter63can also supply focused and/or collimated beams of photoluminescent radiation to be measured by the sensor56.

As noted, the measurement tool100includes an optical mirror58such as a dichoric mirror, which is used to redirect incident illumination from light source50to the optical system57and which also allows photoluminescent radiation received at the optical system57to pass through the optical mirror58to the sensor56through the optical system61and band-pass filter63. The tool100can also include a non-reflective lid18which is used to protect users of a tool100from optical radiation. The radiating beam supplied by light source50, which is collimated, can have a spot diameter of less than about 5 millimeters, for example, a spot diameter of about 1 millimeter. This would be particularly suitable for irradiating a cadmium telluride semiconductor material absorber layer21. Light from the light source50could also be focused towards the cadmium sulfide semiconductor window material layer19in which case a spot diameter in the range of about 100 um to about 500 um can be used.

FIG. 5illustrates an embodiment employing two tools100which can be used to obtain photoluminescence spectra for different ones of the semiconductor window layer19e.g., CdS, absorber layer21e.g., CdTe and interface e.g., CdSxTe1-xbetween them by employing different excitation wavelengths and/or intensity at the respective light sources50and/or by choosing different focal positions for the optical system57. In order to maintain separation between the two tools100a light barrier14can be provided. With the arrangement shown inFIG. 5, each of the respective tools100can be used to excite a different layer or combination of layers of semiconductor layer, e.g., CdS, CdTe or CdSxTe1-x, in the completed photovoltaic device106with a respective excitation wavelength and/or filtered photoluminescence spectra and with a corresponding different photoluminescent spectra being sensed by the respective sensors56. The two tools100can also be used to excite the same semiconductor layer with different light wavelengths and with the same or different reception band-pass filter63, if desired.

One manner in which the excitation light source50and resulting photoluminescence spectra intensity from a completed photovoltaic device106can be used to analyze processing conditions such as Cu doping concentration at stage111, and the temperature of the metallization heat treatment at stage115is now explained in connection withFIGS. 6, 7 and 8.FIG. 6shows light source50excitation and wavelength with blue (B), green (G) and red (R) excitation wavelengths and the resulting BLUE, GREEN and RED photoluminescent spectra. The wavelengths of both the excitation signal and the photolumination spectra are illustrated along the x-axis while the y-axis illustrates the photoluminescence intensity in terms of photoluminescence photon counts detected by sensor56.

FIG. 6Aillustrates an enlarged portion of the RED spectra tail from wavelengths 880 nm to 1040 nm and shows the different spectra which have been absorbed depending on variations to Cu doping concentration at stage111(FIG. 3) or temperature changes at stage115(FIG. 3). Similar changes occur in the BLUE spectra with changes in processing temperature at heat treatment stage115have also been observed. The correlation in the spectra with Cu doping concentration at stage111and temperature changes at stage115are now explained with reference toFIGS. 7-9.

FIG. 7illustrates with a box diagram a correlation between a tail portion of a red photoluminescent spectra and a copper doping concentration. The wavelength range for the tail spectra is about 900 nm to about 1100 nm. The box represents the majority of count values as a light intensity signature taken over a plurality of sample locations of the completed photovoltaic device106. The upper and lower horizontal surfaces of each box represent the range of 25% to 75% of the collected data. The horizontal lines through each box represents the median of the collected data. The horizontal lines above and below the boxes indicate the highest and lowest observed count values. As shown, as the copper used in the doping stage111increases from Cu to Cu+0.3 (parts per million (ppm)) to Cu+0.6 (ppm), the photoluminescent count at the tail portion of red photoluminescence spectra likewise increases. As a result, the tool100illustrated inFIG. 4can be used to determine the level of Cu doping concentration in doping stage111and any deviations of the Cu doping concentration from a desired reference doping concentration.

FIG. 8shows how the tail portion of a red photoluminescence spectra PL in the wavelength range of about 900 nm to about 1100 nm can be used as an indicator of the temperature (in degrees centrigrade) used in heat treatment stage115. Here, an incurring light intensity signature in the form of an increasing count value represented by the sampled locations of a completed photovoltaic device106corresponds to an increase in temperature from a desired temperature T.

Similarly, different spectra is observed in the (FIG. 6) BLUE spectra with temperature changes at heat treatment stage115.FIG. 9illustrates area under blue photoluminescence spectra in the range of about 650 nm to about 800 nm in terms of a box diagram which illustrates where the majority (25% to 75%) of detected photons accrue. As illustrated, the area of the blue photoluminescent spectra between about 650 to about 800 nm shows a light intensity signature having a decrease in count value corresponding to an increase in the temperature used in the heat treatment stage115, with T representing a desired temperature. As can be seen, a higher photon count distribution is obtained when the temperature of the heat treatment stage115is operating at a desired temperature of T, as compared with the count value obtained when heat treatment stage115is operating at the temperature of T+20 degrees centigrade. This correlation between the area of the photoluminescent spectra of the blue excitation, and the temperature of the heat treatment stage115provides an indication of the temperature at which the heat treatment stage115is operating as well as any deviations from a desired temperature T.

FIG. 10illustrates a photoluminescence count analysis module125which receives a photon count value from a tool100over the wavelength set by tool100and from a plurality of sample locations on the completed photovoltaic device106. The analysis tool accumulates the photon count values over the wavelength of interest to develop light intensity signature data such as illustrated inFIGS. 7, 8 and 9and can determine median values. A reference count value PL which represents a count value from a plurality of like locations and which is expected for proper Cu doping at Cu doping stage111, or for proper heat treatment temperature at heat treatment115, is also received by module125. A deviation in the detected and reference median photon count values is indicated by an output of analysis module125. The output indication can be audibly or visually indicated to an operator for action, or could be used as a control signal to a control device127to adjust the operating parameter of Cu doping at stage111or temperature of heat treatment at stage115.

FIG. 11illustrates a variation of theFIG. 5embodiment in which the plurality of tools100are spaced from one another in both the lengthwise and widthwise direction of a completed photovoltaic panel106to gather information from a plurality of widthwise and lengthwise sample locations133as the completed photovoltaic device106passes over the tools100. The plurality of sampling points133are used to gather photon counts of the photoluminescent spectra cover a set wavelength range. The photon count values from the various tools100can be summed to provide the output spectra illustrated inFIG. 6for different wavelengths of excitation signals and for a plurality of locations of a passing completed device106.

As shown inFIG. 12, in many instances, the completed photovoltaic device106will have a plurality of photovoltaic cells145therein which are separated by scribe lines150which pass through one or more semiconductor material layers and which may therefore interfere with the collection of photons. Accordingly, counting photons at such locations should be avoided.FIG. 12illustrates an embodiment in which the scribe lines are eliminated from the sampling locations133illustrated inFIG. 11. The scribe lines are visible through a glass back panel25(FIG. 2). InFIG. 12an imaging camera160is used to take an image of a completed photovoltaic device106showing the location of the scribe lines150. The image is fed to an image analysis device162which identifies the locations of the scribe lines150on the passing device106. This information is then passed to the analysis module125so that any photon collection taken at locations corresponding to the scribe lines are removed from the collected count results before they are used for analysis, for example, compared to the reference photoluminescence spectra (FIG. 10). Alternatively, the light source50of one or more tools100can be controlled so that it is not irradiating the completed photovoltaic device106when a scribe line150passes by the focal point of the irradiating light as the completed photovoltaic device106moves past a tool100. As another alternative, the location of the scribe lines can be fed to a tool100to control the gating of photons to sensor56such that no photons are collected from the location of the scribe lines150.

FIG. 13illustrates another example of an embodiment which can eliminate photon collection at the location of the scribe lines150. In this embodiment, an edge detector139is used to detect the leading edge of the completed photovoltaic device106. Information on the detected leading edge is fed to a timing circuit155along with information on conveyer speed which provides an output signal to the analysis module125indicating specific locations of the scribe lines150as they pass over a tool100which can again be fed to analysis module125or a gating control on a tool100such that any photon data from the scribe locations is eliminated from the accumulating count value, or the light source50does not emit light when a scribe line passes by a tool100.

Various embodiments of the invention have been described which can use photoluminescence spectra information obtained from a completed photovoltaic device106to determine whether a process operating parameter is within prescribed operating conditions or deviates therefrom. The specific operating parameters of copper doping concentration and temperature variations in the stages111and115illustrated inFIG. 3have been described. However, the invention can also be used to measure a photoluminescence signal from a completed photovoltaic device106for other operating parameters to determine whether such other operating parameters are within desired values or deviate from desired values. Also, although embodiments have been disclosed where tool100is located after a bias stage119(FIG. 3), tool100can also be located at any point following the processing of the internal material layers of a completed photovoltaic device106, including before the back cover25is applied or after final assembly stage117, but before the bias stage119. Accordingly, the invention is not limited by the foregoing description and is only limited by the scope of the appended claims.