High throughput system for photovoltaic UV degradation testing

High throughput systems for photovoltaic UV degradation testing of solar cells, and methods of testing for UV degradation of solar cell during manufacture, are described herein. In an example, a high throughput solar cell testing apparatus includes a plurality of real time ultra-violet (RTUV) testing modules. Each of the RTUV testing modules includes an ultra-violet (UV) light source, an optics assembly for focusing light from the UV light source on a sample area, and a detector for receiving photoluminescence energy from the sample area. The high throughput solar cell testing apparatus also includes an acquisition and control assembly coupled to the plurality of RTUV testing modules.

TECHNICAL FIELD

Embodiments of the present disclosure are in the field of renewable energy and, in particular, high throughput systems for photovoltaic UV degradation testing of solar cells, and methods of testing for UV degradation of solar cell during manufacture.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well known devices for direct conversion of solar radiation into electrical energy. Generally, solar cells are fabricated on a semiconductor wafer or substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate. Solar radiation impinging on the surface of, and entering into, the substrate creates electron and hole pairs in the bulk of the substrate. The electron and hole pairs migrate to p-doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions. The doped regions are connected to conductive regions on the solar cell to direct an electrical current from the cell to an external circuit coupled thereto.

Efficiency is an important characteristic of a solar cell as it is directly related to the capability of the solar cell to generate power. Likewise, efficiency in producing solar cells is directly related to the cost effectiveness of such solar cells. Accordingly, techniques for increasing the efficiency of solar cells, or techniques for increasing the efficiency in the manufacture of solar cells, are generally desirable. Some embodiments of the present disclosure allow for increased solar cell manufacture efficiency by providing novel processes for monitoring the fabrication of solar cell structures.

DETAILED DESCRIPTION

“Comprising.” This term is open-ended. As used in the appended claims, this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the units/components include structure that performs those task or tasks during operation. As such, the unit/component can be said to be configured to perform the task even when the specified unit/component is not currently operational (e.g., is not on/active). Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. § 112, sixth paragraph, for that unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.). For example, reference to a “first” solar cell does not necessarily imply that this solar cell is the first solar cell in a sequence; instead the term “first” is used to differentiate this solar cell from another solar cell (e.g., a “second” solar cell).

“Coupled” —The following description refers to elements or nodes or features being “coupled” together. As used herein, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/node/feature, and not necessarily mechanically.

High throughput systems for photovoltaic UV degradation testing of solar cells, and methods of testing for UV degradation of solar cell during manufacture, are described herein. In the following description, numerous specific details are set forth, such as specific tooling configurations, in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to one skilled in the art that embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known fabrication techniques, such as emitter region fabrication techniques, are not described in detail in order to not unnecessarily obscure embodiments of the present disclosure. Furthermore, it is to be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

Disclosed herein are solar cell testing apparatuses. In one embodiment, a high throughput solar cell testing apparatus includes a plurality of real time ultra-violet (RTUV) testing modules. Each of the RTUV testing modules includes an ultra-violet (UV) light source, an optics assembly for focusing light from the UV light source on a sample area, and a detector for receiving photoluminescence energy from the sample area. The high throughput solar cell testing apparatus also includes an acquisition and control assembly coupled to the plurality of RTUV testing modules.

In another embodiment, a solar cell testing apparatus includes a first narrowband light source configured to induce photonic degradation to a solar cell. The inducing includes applying light of a first wavelength to the solar cell. A second narrowband light source is included for applying light of a second wavelength to the solar cell. The second wavelength is greater than the first wavelength. A detector is included and is configured to measure photoluminescence induced from the applied light of the first wavelength. A sheet resistance measurement module is included and is configured to measure a local resistance of a region of the solar cell. An electronic system is included and is configured to monitor the photonic degradation of the solar cell from the photoluminescence measurement, and is configured to monitor the local resistance of the region of the solar cell.

Also disclosed herein are methods for testing solar cells. In one embodiment, a method for testing a solar cell involves applying a first light to a sample region of a solar cell or a partially fabricated solar cell to induce a photonic degradation in the sample region. The method also involves applying a second light to the sample region to induce a local resistance in the sample region. The method also involves monitoring the photonic degradation of the sample region based on a photoluminescence measurement. The method also involves monitoring the local resistance of the sample region based on a sheet resistance measurement.

One or more embodiments are directed to calibrated, self-regulating, and high throughput systems for photovoltaic ultra-violet (UV) degradation testing. Embodiments may include implementation of a Real Time Ultra Violet Tester (RTUV Tester) to establish a direct, quantitative connection between the RTUV Photoluminescence (PL) signal and a local performance value (e.g., a performance value such as local resistance for a small location on a partially completed or completed solar cell). Such direct and quantitative calibration with local performance values can enable an in-process application of the RTUV, such as monitoring surface field UV stability in High Volume Manufacturing (HVM) and developing film stacks.

Processes described herein can be multiplexed and automated to deliver greater throughput as well as more consistent procedure and results. For example, by measuring multiple spots on a single cell or on a group of cells simultaneously, the throughput can be increased proportionately. In addition, the potential at a back surface of the cell can be fixed with an external supply, or using contactless light biasing, to provide more repeatable measurements. A secondary excitation source in the visible to near IR range can also be employed to provide a stronger PL signal. Such modifications may be implemented for in-line or end of line (EOL) monitoring with very high sampling rates for more effective screening and rapid feedback to process.

To provide context, for EOL UV testing, RTUV systems may be implemented to reduce EOL analysis time from 1-2 weeks down to 15 minutes. However, in order to further reduce such analysis time and allow for increased cell sampling rates, it may be necessary to greatly reduce the throughput of the system. In addition, other potential instabilities, such as the UV-LED light-source intensity, back contact potential, and temperature increases at the measurement spot may even lead to a reduced effectiveness of the RTUV measurement technique.

Addressing one of more of the above issues, in accordance with one or more embodiments of the present disclosure, one or more of the following is included in, or in conjunction with, a system for testing solar cell degradation to provide improvements over state pf the art real time UV degradation measurement systems: (1) UV power monitoring, (2) RTUV PL signal to local performance calibration, measured with both transient and steady-state signals for cross-corroboration, (3) modeling to guide and support the above calibration, (4) rapid feedback for in-process monitoring, local temperature monitoring by near IR (e.g., approximately 1100 nm) transmittance or conventional thermal (e.g., 2-10 micron) IR measurements, (5) the use of an additional light source for “reading” local performance such as a local resistance, (6) the use of an additional light source for mitigating or reducing a temperature or injection-level differential, (7) sub-bandgap lighting from rear to fill mid-level states, (8) external heating of a wafer to mitigate effects of localized heating, (9) the measurement of multiple spots on a wafer simultaneously and management of any potential crosstalk, (10) contact of a finished cell to monitor its voltage, and to maintain a constant voltage or current if needed, and/or (110automation and multiplexing.

To provide further context, UV power monitoring is accomplished with the use of a UV-sensitive detector positioned directly across from a source. Some of the UV light will penetrate a dichroic mirror, in a manner proportional to the principle beam. Alternatively, such a monitor can be placed below the mirror, or on the side of a lower column. It is to be appreciated that if the impinging UV light is too strong it may cause this monitor to degrade with time, in which case using a shutter to only measure this intensity periodically would be advantageous. In any case, the resulting signal can be used to feedback to the source to maintain constant power.

In a first aspect, multiple RTUV modules are included in one platform. Such an arrangement enables the measurement of several spots at once. Several such multi-spot RTUV stations can further be combined to increase throughput of a solar cell testing apparatus. Furthermore, automation may be implemented to directly unload, measure, and bin cells. The resulting increased throughput, when applied either at the EOL stage or during the fabrication process, can greatly reduce the time needed to feedback directly to process monitoring or development. As an exemplary system including multiple RTUV modules,FIG. 1illustrates a schematic of a high throughput solar cell testing apparatus, in accordance with an embodiment of the present disclosure.

Referring toFIG. 1, a high throughput solar cell testing apparatus100includes a plurality of real time ultra-violet (RTUV) testing modules102. Each of the RTUV testing modules102includes an ultra-violet (UV) light source, an optics assembly for focusing light from the UV light source on a sample area, and a detector for receiving photoluminescence energy from the sample area. The high throughput solar cell testing apparatus100also includes an acquisition and control assembly104coupled to the plurality of RTUV testing modules102, and possibly further coupled to a software analysis and storage module106. The system100may be suitable for testing either partially or completely fabricated solar cells, or both.

In an embodiment, the UV light source of each of the plurality of RTUV testing modules102includes a near UV light emitting diode (LED) having an output wavelength approximately in the range of 300-400 nanometers and, in a particular embodiment, having an output wavelength of approximately 365 nanometers. In an embodiment, the UV light source of each of the plurality of RTUV testing modules102includes a laser light source.

In an embodiment, the optics assembly of each of the plurality of RTUV testing modules102is arranged to provide a UV light spot on the sample area, the UV light spot having a relatively small dimension as compared to a tested solar cell, so as to not impact the performance of the solar cell. In an embodiment, the optics assembly of each of the plurality of RTUV testing modules102is arranged to provide a UV light spot on the sample area, the UV light spot having a dimension of less than approximately 2 millimeters. In a specific such embodiment, the UV light spot is approximately circular, and the dimension of less than approximately 2 millimeters is a diameter of the UV light spot. In another specific such embodiment, the UV light spot is approximately square, and the dimension of less than approximately 2 millimeters is a side of the UV light spot. In an embodiment, the detector for receiving photoluminescence energy from the sample area is an infra-red (IR) detector.

In an embodiment, each of the plurality of RTUV testing modules102includes an ambient light source, e.g., at approximately 660 nm, and is arranged to make pad contact for cell biasing. In one such embodiment, UV degradation is measured in each of the plurality of RTUV testing modules102under cell-biasing conditions. In on embodiment, an electrical vacuum chuck is used to hold and make contact to the cell. Different bias settings can be applied for creating varying effects on both signal strength and degradation strength.

In an embodiment, the high throughput solar cell testing apparatus100is arranged to provide automated insertion of solar cells from cassettes108through the plurality of RTUV testing modules102by a first conveyor belt110. A second conveyor belt112send a tested solar cell into a pass cassette114or a fail cassette116. It is to be appreciated that several features can be included in the system100to enable the monitoring and control of UV power by the software. For example, transient and steady-state RTUV signals can be measured and modeled to extract local resistance before and after of the UV test. In an embodiment, the system100further includes an electronic system coupled to the acquisition and control assembly. The electronic system is configured to determine whether to pass or fail a solar cell or a partially fabricated solar cell based on the photoluminescence energy detected by the detector of one or more of the plurality of RTUV testing modules. In other embodiments, electronic system is configured to determine how to bin or segregate tested solar cells based on the test results.

As described in greater detail below in association withFIGS. 3 and 4, one or more of the plurality of RTUV testing modules102further includes a sheet resistance measurement module coupled to the sample area. The sheet resistance measurement module is configured to measure a local resistance of a region of a solar cell or a partially fabricated solar cell in the sample area. In one such embodiment, the one or more of the plurality of RTUV testing modules102includes a visible light source separate and distinct from the UV light source. The visible light source is configured to provide visible light on the sample area for use in measuring the local resistance of the region of the solar cell or the partially fabricated solar cell in the sample area.

As described above, a plurality of RTUV modules can be included on a single platform to provide a high throughput testing system. As an example, of a suitable RTUV module,FIG. 2illustrates a plan view of a solar cell testing apparatus, in accordance with another embodiment of the present disclosure.

Referring toFIG. 2, a solar cell testing apparatus200is shown for testing a solar cell210, which may be a partially or completely fabricated solar cell. In an embodiment, the solar cell210has a front side opposite a back side of the solar cell210. The solar cell210is placed on a receiving medium208. In one embodiment, the receiving medium208is an electronic scanning and/or translator stage. The solar cell testing apparatus200includes a light source204, which can include an optical tube212to focus light214from the light source204onto a location215of the solar cell210. A photoluminescence signal216can be induced from the applied light214and received, obtained or collected from the solar cell210.

In an embodiment, the photoluminescence signal216is received from the same location215as that focused on by the light214(e.g., as described in greater detail below in association withFIG. 5). The location215can be on the front side and/or back side of the solar cell210. In one embodiment, the light source204is a laser or a light emitting diode (LED). In a specific embodiment, the light source204is a narrowband light source configured to induce photonic degradation to a solar cell, where the inducing includes applying light to the solar cell. In an embodiment, a detector206can be used to receive the photoluminescence signal216from the solar cell210.

In an embodiment, a detector206is included and is positioned to receive the measured photoluminescence signal216induced from the applied light214. In an embodiment, the detector206includes an optical tube213to collect the photoluminescence signal216. In one embodiment, a filter205is used to remove or reduce noise from the photoluminescence signal216.

In an embodiment, an electronic system220is connected207to the light source204, detector206and the receiving medium208. In one embodiment the electronic system220is used to modulate the light214from the light source204. In one embodiment, the electronic system220is used to monitor the photoluminescence signal216received at the detector206and record the photoluminescence signal216. In one embodiment, the electronic system220is configured to monitor photonic degradation of a solar cell from the photoluminescence signal216. In one embodiment, the electronic system220is configured to determine whether to pass or fail a solar cell based on the monitoring. In one embodiment, the electronic system220is used to control the movement of the receiving medium208(e.g., a scanning stage). In some embodiments, the light214is scanned along the surface of the solar cell210. In one such embodiment, the light214is scanned along the front side and/or back side of the solar cell210.

In an exemplary embodiment, the receiving medium208(e.g., a scanning stage) is used to move the location215from one location to another location on the solar cell210. In one embodiment, the light214is scanned from one location on the solar cell to another location using galvanometric scanners. In an embodiment, a plurality of photoluminescence measurements received from scanning from one location to another location on the solar cell is used to generate a map, (e.g., a photonic degradation map) or other indicator of the degradation of the solar cell.

In an embodiment, the electronic system220includes an analog to digital converter (ADC), a current amplifier or pre-amplifier to boost, or a pico-ammeter to read the signal from the photoluminescence measurement. In an embodiment, the electronic system220includes an electronic control system to control the light from the light source204and/or to control the movement of the receiving medium208(e.g., a scanning stage). In some embodiments, additional electronics and/or software are incorporated into the electronic system220.

Various components of the electronic system220and/or one or more portions of the disclosed techniques can be implemented by a processor unit executing program instructions stored on a memory. In various embodiments, the processor unit can include one or more processors or cores. The processor unit can contain a cache or other form of on-board memory. The memory is usable by the processor unit (e.g., to store instructions executable by and data used by the processor unit). The memory can be implemented by any suitable type of physical memory media, including hard disk storage, floppy disk storage, removable disk storage, flash memory, random access memory (RAM-SRAM, EDO RAM, SDRAM, DDR SDRAM, Rambus® RAM, etc.), ROM (PROM, EEPROM, etc.), and so on. The memory can consist solely of volatile memory in one embodiment. The circuitry can include an I/O interface configured to couple to and communicate with other devices (e.g., to receive a value representing the threshold voltage), according to various embodiments.

Articles of manufacture that store instructions (and, optionally, data) executable by a computer system to implement various techniques disclosed herein are also contemplated. These articles of manufacture include tangible computer-readable memory media. The contemplated tangible computer-readable memory media include portions of the memory subsystem of a computer system (without limitation SDRAM, DDR SDRAM, RDRAM, SRAM, flash memory, and various types of ROM, etc.), as well as storage media or memory media such as magnetic (e.g., disk) or optical media (e.g., CD, DVD, and related technologies, etc.). The tangible computer-readable memory media may be either volatile or nonvolatile memory.

In another aspect, calibration to resistance of both the RTUV steady state and transient signal is performed. Such testing can allow for a direct correlation of the measured signal to cell performance. For example, local temperature measurement (e.g., utilizing IR transmittance at approximately 1100 nm) or a conventional thermal IR measurement can be obtained to test for the effects of a local temperature increase on the PL signal. Use of additional lighting and/or higher base temperature of the cell can be included in an RTUV module to mitigate any effects from such temperature change. In an embodiment, in order to increase signal strength, a secondary excitation beam in the visible to near IR is further included in the RTUV module. In addition, by making electrical contact to the cell and holding it in forward bias, the PL signal may be increased by several orders of magnitude. In another embodiment, an external light source of a relatively longer wavelength is included. In another embodiment, the cell is illuminated from either the front or back with a sub-bandgap light to reduce noise.

As described above, a plurality of RTUV modules can be included on a single platform to provide a high throughput testing system. As an example, of a suitable RTUV module which has a capability measuring the local resistance of the region of the solar cell,FIG. 3illustrates a cross-sectional view of a solar cell testing apparatus, in accordance with another embodiment of the present disclosure.

Referring toFIG. 3, a solar cell testing apparatus300is shown for testing a solar cell310, which may be a partially or completely fabricated solar cell. The solar cell testing apparatus300includes a light source304. The light source304includes an optical tube312to focus light314from the light source304onto a location315of the solar cell310. In an embodiment, a photoluminescence signal316induced from the applied light314is received from the solar cell310. In one such embodiment, the photoluminescence signal316is received from the same location315as that focused on by the light314. The location315can be on the front side and/or back side of the solar cell310. In an embodiment, the light source304is a laser or a light emitting diode (LED). A detector306is used to receive the photoluminescence signal316from the solar cell310.

In an embodiment, the light314is co-axial, e.g., the light314in the same optical axis as that of the photoluminescence signal316, as depicted inFIG. 3. In one embodiment, light from a narrowband light source and the measured photoluminescence316are at least partially co-axial. In an embodiment, a dichroic mirror317is used to separate the light314and the photoluminescence signal316. In one embodiment, the dichroic mirror317is used to separate light from a narrowband light source and the measured photoluminescence316. In one embodiment, the detector306further includes an optical tube313to collect the photoluminescence signal316. In one embodiment, a filter305is used to remove or reduce the source illumination (e.g., light314) and/or background noise from the photoluminescence signal316.

In an embodiment, the solar cell testing apparatus300includes a second light source330. In one embodiment, the second light source330provides a light334. In one embodiment, the second light source330is a narrowband light source. In a specific such embodiment, the second light source330is a visible light source, e.g., a red light source provides light334at approximately 660 nm. In one embodiment, the second light source330is a flooding light LED array.

In an embodiment, the second light source330is used together with or as part of a sheet resistance measurement module. In one such embodiment, the sheet resistance measurement module further includes a source meter332. In an embodiment, the sheet resistance measurement module does not include additional optics and is based on a flooding second light source330, as depicted. In another embodiment, however, the second light source330is further coupled to an optics assembly for focusing light from the narrowband light source330on a sample area of a solar cell or a partially fabricated solar cell. In either case, in an embodiment, the second light source330provides light334and is used together with the source meter332to provide capability for local resistance measurements.

In an embodiment, an electronic system (not shown), similar to the electronic system described in association withFIG. 2, is connected307to the light source304, detector306and receiving medium308. In one embodiment, the electronic system is used to modulate the light314from the light source304. In one embodiment, the electronic system is used to monitor the photoluminescence signal316received at the detector306and record the photoluminescence signal316. In one embodiment, the electronic system is configured to monitor photonic degradation of a solar cell from the photoluminescence signal316. In one embodiment, the electronic system is configured to determine whether to pass or fail a solar cell based on the monitoring. In one embodiment, the photoluminescence signal316is used to determine the induced degradation to solar cell310. In one embodiment, the electronic system is used to control the movement of the receiving medium (e.g., a scanning stage)308. In an embodiment, the receiving medium308is an electronic scanning and/or translator stage. In one embodiment, the receiving medium308further includes a chuck309, where the solar cell310is placed on the chuck309. In an embodiment, the front and/or back side of the solar cell310is in contact with the chuck309. In an embodiment, a mount302is used to support the solar cell testing apparatus300. In some embodiments, the light314is scanned along the surface of the solar cell310. In an embodiment, the light314is scanned along the front side and/or back side of the solar cell310.

In an exemplary embodiment, the receiving medium308(e.g., a scanning stage) is used to move the location315from one location to another location on the solar cell310. In one embodiment, the light314is scanned from one location on the solar cell to another location using galvanometric scanners. In an embodiment, a plurality of photoluminescence measurements received from scanning from one location to another location on the solar cell is used to generate a map, (e.g., a photonic degradation map) or other indicator of the degradation of the solar cell.

Thus, referring again toFIG. 3, in accordance with an embodiment of the present disclosure, a solar cell testing apparatus300includes a first narrowband light source304configured to induce photonic degradation to a solar cell310. The inducing includes applying light314of a first wavelength to the solar cell310. A second narrowband light source330is included for applying light334of a second wavelength to the solar cell310. The second wavelength is greater than the first wavelength. A detector306is included and is configured to measure photoluminescence induced from the applied light314of the first wavelength. A sheet resistance measurement module332is included and is configured to measure a local resistance of a region of the solar cell310. In one embodiment, an electronic system (, such as or similar to electronic system220) is included and is configured to monitor the photonic degradation of the solar cell310from the photoluminescence measurement, and is further configured to monitor the local resistance of the region of the solar cell310.

In an embodiment, the first narrowband light source304is a UV light source, and the second narrowband light source330is a visible light source. In one such embodiment, the UV light source304includes a near UV light emitting diode (LED) having an output wavelength of approximately 365 nanometers, and the visible light source330is a red light source, e.g., having a wavelength of approximately 660 nm.

In an embodiment, the solar cell testing apparatus300further includes an optics assembly for focusing light from one of the first narrowband light source or the second narrowband light source on a sample area of a solar cell or a partially fabricated solar cell. In one such embodiment, the optics assembly is configured to provide a light spot on the sample area, the light spot having a dimension of less than approximately 2 millimeters. In a specific such embodiment, the light spot is approximately circular, and the dimension of less than approximately 2 millimeters is a diameter of the light spot. In another specific such embodiment, the light spot is approximately square, and the dimension of less than approximately 2 millimeters is a side of the light spot.

In another aspect, methods are described for monitoring both a photoluminescence measurement and a sheet resistance measurement. As an example,FIG. 4is a flowchart400listing operations in a method of testing a solar cell, in accordance with an embodiment of the present disclosure.

Referring to operation402of flowchart400, a method for testing a solar cell involves applying a first light to a sample region of a solar cell or a partially fabricated solar cell to induce a photonic degradation in the sample region. Referring to operation404of flowchart400, a second light is applied to the sample region to induce a local resistance in the sample region. Referring to operation406of flowchart400, the photonic degradation of the sample region is monitored based on a photoluminescence measurement. Referring to operation408of flowchart400, the local resistance of the sample region is monitored based on a sheet resistance measurement.

In an embodiment, monitoring the photonic degradation of the sample region is performed at substantially the same time as monitoring the local resistance of the sample region. In an embodiment, applying the first light involves applying UV light. Applying the second light involves applying visible light.

In an embodiment, the above described method is performed at multiple locations of a solar cell to induce degradation at multiple locations of the solar cell and receive multiple corresponding photoluminescence measurements. In an example, the light can be applied to a plurality of locations on a front side of a solar cell opposite to a plurality of contact pads formed on a back side of the solar cell. Corresponding induced photoluminescence measurements are obtained. In an embodiment, a plurality of photoluminescence measurements is to generate a map (e.g., a photonic degradation map) or other indicator of ultraviolet (UV) induced degradation of the solar cell. In one such embodiment, the induced photonic degradation of the solar cell is monitored during fabrication or after fabrication based on the photonic degradation map.

To provide further context, the capability to measure degradation in an accelerated manner can be crucial to improving a solar cell performance and reliability of the solar cell in the field. For example, photonic induced degradation (e.g., degradation from specific wavelengths of light) can deteriorate the performance of a solar cell over time. In a specific example, ultraviolet (UV) induced degradation can deteriorate the performance of a solar cell out in the field. Thus, specific test methods may be desirable to determine if solar cells are susceptible to ultraviolet (UV) induced degradation during manufacture or prior to product shipment to prevent product which is susceptible to ultraviolet (UV) induced degradation from reaching the customer or installation in the field. The longevity of a solar cell or solar cell module can directly affect the value of the product to a customer and the product's competitiveness in the marketplace. Also, the rapid pace of solar cell process development and qualification can require a high acceleration factor (AF), e.g., to have test results available in a timely manner for use and/or feedback to the solar cell manufacturing process.

Using a broadband source of light, such as a mercury lamp, may have the disadvantage of exposing the solar cell to a broad spectrum of light. In an example, a mercury lamp can emit irradiance in the ultraviolet (UV), visible, and infrared (IR) spectral range. Also, the amount of ultraviolet (UV) light emitted by the mercury lamp can be limited. In one example, long exposure times from a mercury lamp are required to induce similar ultraviolet (UV) degradation a solar cell would undergo from ultraviolet (UV) light exposure in the field. In addition, the spectrum of the mercury lamp contains many spikes and the intensities of these spikes are known to vary, either from lamp to lamp or over time. Such variations can be a source of inconsistency and/or uncertainty for use in ultraviolet (UV) testing.

In an embodiment, using light from a narrowband source (e.g., using a laser or a light emitting diode (LED)) allows for reduced exposure times in comparison to using light from a broadband source (e.g., using mercury lamps can take several days or weeks). In an embodiment, a narrowband source can include one or more of the wavelength groups: 101-280 nm, 280-315 nm, or 315-400 nm (corresponding to the ultraviolet (UVA-UVC) wavelength range), 400-500 nm, 500-600 nm, or 600-700 nm (corresponding to the visible range), and 700-800 nm, 800-900 nm, 900-1000 nm (corresponding to the near infrared (IR) wavelength range). In an embodiment, the light is applied for less than a second to induce degradation on the solar cell. In one embodiment, the light is applied to the solar cell up to 1-2 hours or more to induce photonic degradation. In an embodiment, light is applied to the solar cell for a duration in the range of 10 milliseconds-2 hours to induce photonic degradation.

In an embodiment, the light is applied to a passivation region of the solar cell, e.g., an anti-reflective region (AR) of the solar cell. In one such embodiment, the passivation region is on a front side and/or a back side of the solar cell. In a specific embodiment, the light is applied to a passivation region on the front side of the solar cell opposite to a contact region on a back side of the solar cell (e.g., as shown inFIG. 5, described below). In an embodiment, the light is applied to one or more locations of the solar cell.

In an example, the light may be applied to one or more locations on the front side of the solar cell opposite to one or more contact regions on a back side of the solar cell.FIG. 5illustrates a cross-sectional view of an operation involving applying light to a solar cell, in accordance with an embodiment of the present disclosure.

Referring toFIG. 5, light is applied to a solar cell, which may be a partially or completely fabricated solar cell, to induce degradation. In the example, the solar cell210/310has a front side222/322opposite a back side224/324. Light214/314is applied to a location215/315of solar cell210/310to induce a photoluminescence216/316. A photoluminescence signal216/316can be received from the location215/315(e.g., the same location the light214/314is applied). In an embodiment, the location215/315is on a front side222/322of the solar cell210/310. In an embodiment, the location215/315is opposite to a contact region326on a back side224/324of the solar cell210/310. In one embodiment, the contact region326is a contact pad. In one embodiment, the location215/315is a passivation region (e.g., a silicon dioxide layer and/or a silicon nitride layer) of a solar cell.

Other approaches may also induce degradation. As an example,FIG. 6illustrates a cross-sectional view of an operation involving applying light to a solar cell, in accordance with another embodiment of the present disclosure.

Referring toFIG. 6, a solar cell610(which may be a partially or completely fabricated solar cell) has a front side622opposite a back side624. Light614is applied to a location615on the front side622of the solar cell610to induce a photoluminescence616. A photoluminescence signal616can be received from another location628on the back side624of the solar cell610. In one embodiment, the location615is a passivation region (e.g., a silicon dioxide layer and/or a silicon nitride layer) of a solar cell.

In an embodiment, a light source is positioned to face the front side622in order to provide light614to the front side622of the solar cell610. A detector is positioned to face the back side624to receive the photoluminescence signal616from the back side624of the solar cell610.

In an aspect, degradation of a solar cell can be monitored based on at least a photoluminescence measurement. In an embodiment, such monitoring includes receiving a first photoluminescence measurement induced from an applied light and receiving a second photoluminescence measurement induced from the applied light (e.g., light from the same source) after receiving the first photoluminescence measurement. In an example, light illuminating a solar cell can generate electron and hole pairs. At steady state, the density of the generated electron and hole pairs depend on the passivation of the solar cell. Under equal illumination, for a solar cell with good passivation (e.g., low surface recombination), the higher the generated electron and hole density, the higher the photoluminescence intensity. In an embodiment, reduced photoluminescence intensity is used to indicate degradation in a passivation region of the solar cell. Thus, monitoring the measured change of the photoluminescence intensity under constant illumination can be used to determine the change in a passivation (e.g., induced degradation) of a solar cell. In one embodiment, the surface recombination of the solar cell is measured using the photoluminescence intensity.

As exemplary PL information,FIG. 7illustrates exemplary data plotted for photoluminescence as a function of time, in accordance with an embodiment of the present disclosure.

Referring toFIG. 7, photoluminescence data is normalized to simplify presentation of the data. In particular, two photoluminescence measurements,701and703are shown inFIG. 7, as taken from a solar cell which may be a partially or completely fabricated solar cell. In one example, the measurements are taken during exposure to ultraviolet (UV) light. In another example, the measurements are taken after applying ultraviolet (UV) light. As shown, measurement703illustrates an approximately 20% reduction in photoluminescence over time (e.g., over 900 seconds). Measurement701shows less than 5% reduction in the photoluminescence measurement over the same duration. In the example shown, a deterioration, e.g., greater than 5% photoluminescence loss, can be defined as a fail and the ultraviolet (UV) degradation of that solar cell determined to be unacceptable. Thus, in the same example, measurement701can be acceptable (passing solar cell) and measurement703can correspond to a failing solar cell. Although one example is presented herein, other example configurations and/or measurements may be used.

Embodiments described herein may be implemented to enable much faster UV degradation measurements and/or more rapid (if EOL) or nearly instantaneous (if in-line) feedback to process and/or enable greater sampling rates. Although certain modules or materials are described specifically with reference to above described embodiments, some modules or materials may be readily substituted with others with such embodiments remaining within the spirit and scope of embodiments of the present disclosure. Embodiments disclosed herein may be suitable for solar cells based on a silicon substrate or solar cells based on a different material substrate, such as a group III-V material substrate. Embodiments disclosed herein may be suitable for solar cells having back side alternating N+ type and P+ type emitter regions, or for other solar cell arrangements, such as front contact solar cell arrangements. In other embodiments, the above described approaches can be applicable to manufacturing of articles other than solar cells. For example, manufacturing of light emitting diode (LEDs) may benefit from approaches described herein. Furthermore, although applying ultraviolet (UV) light to induce ultraviolet (UV) degradation is described herein, other light sources and degradation modes can be applied and/or induced within the spirit and scope of embodiments contemplated for the present disclosure.

As described above, the local resistance of a sample region may be monitored based on a sheet resistance measurement made at the same time or substantially the same time as a photoluminescence measurement. In one such embodiment, a non-contact measurement of front surface sheet resistance of textured solar wafers is performed using a narrow band light source while simultaneously measuring photoluminescence. It is to be appreciated that, in other embodiments, a standalone sheet resistance measurement may be made and/or a system can include a stand-alone sheet resistance measurement module.

In an embodiment, the monitoring of a front surface sheet resistance is improved by enabling the measurement of textured cells through a non-contact technique. In one such embodiment, actual product wafers (or partially fabricated product wafers) are tested instead of, e.g., polished test wafers. Furthermore, in an embodiment, such an approach enables the testing of larger sample sizes at a speed suitable for inline measurement. Embodiments described herein may be implemented to enable very high detection of bad or faulty cells, reducing field failures.

To provide context, front surface phosphorous doping of solar cells may be critical to ensuring stable performance under ultra-violet (UV) exposure in the field. Insufficient front surface doping is understood to be one cause of field returns related to cell defects. Not to be bound by theory, there may be at least a couple of reasons numerous problematic cells reaching the field: (1) Product cells are typically not measured for sheet resistance. Special polished test wafers are typically measured using a four-point probe. The test wafers do not undergo all processing operations to which product wafers are exposed. Accordingly, test wafers may not be exposed to the very causes of high sheet resistance possibly observed in product wafers. (2) The sample size is small.

Addressing one or more of the above issues, in accordance with an embodiment of the present disclosure, a narrow band light source is used that is absorbed near the surface of silicon (Si) to generate a photo-luminescence signal that can be correlated to the sheet resistance or doping concentration of the surface. It is to be appreciated that, in one embodiment, the wavelength used does not damage the wafer but instead is absorbed close to the surface of the wafer. The intensity of the light may not be critical, however, high intensity light may heat the wafer surface and require more stabilization time. As such, in one embodiment, a relatively low intensity light source is used to provide a high throughput measurement. In one particular embodiment, one large flash is used as an illumination event. In another particular embodiment, small points of light are used as an illumination event to map the surface.

In an exemplary embodiment, a photoluminescence (PL) signal obtained at t=5s from an infra-red (IR) detector correlates to sheet rho (sheet resistance). PL output is measured for multiple spots across the wafer and then averaged for a final resistance value for the cell. In a particular embodiment, a 365 nm LED light source is used. In a particular embodiment, cells are sampled as partially fabricated cells following a front side dopant drive, such as a front-side phosphorous dopant drive.

Thus, high throughput systems for photovoltaic UV degradation testing of solar cells, and methods of testing for UV degradation of solar cell during manufacture, have been disclosed.