METHOD AND APPARATUS FOR MEASURMENT OF OHMIC SHUNTS IN THIN FILM MODULES WITH THE Voc-ILIT TECHNIQUE

Technical Field

[0001] The invention refers to a method for the detection of ohmic shunts in a thin film solar cell, especially thin film silicon photovoltaic modules, wherein the solar cell is light stimulated and thermal effects caused by the shunts are detected using Lock-in Thermography as well as an apparatus to detect ohmic shunts in thin-film solar cells.

Background Art

[0002] The Lock-in Thermography (LIT) is a known technique to detect shunts in solar cells. Its realization relies on a thermocamera, a source of excitation and the Lock-in principle. The Lock-in principle is a method used to extract a signal from a statistical noise. The principle of Lock-in Thermography consist of introducing periodically modulated heat into an object and monitoring only the periodic surface temperature modulation phase- referred to the modulated heat supply. Hence, if the surface temperature is measured via an IR-thermocamera, Lock-in Thermography means that the information of each pixel of the image is processed as if it were fed into a Lock-in amplifier. As a result, the noise coming from other heat sources than the one introduced at the Lock-in frequency is masked or at least significantly reduced.

[0003] Commonly, the generation of free charge carriers in a solar cell will generate heat. As the produced heat results from different heating mechanisms, local heating sources, inhomogeneities and cold (dark) areas can be detected by Lock-in Thermography.

[0004] These heating mechanisms can e.g. be:

- Peltier effect;

- recombination;

- tunnel recombination; - thermalisation; and

- collision with the atom network.

[0005] In the Peltier effect, free carriers generated in a solar cell (exhibiting e.g. a p-i-n structure) are drifting from the p to the n layers. Thus they are getting closer to the Fermi level which means that they are losing energy, the energy lost is emitted as heat.

[0006] Recombination occurs when a free carrier on the conduction band

(electron) recombines with a free carrier in the valence band (hole). To reach the valence band, the electron has to lose energy equivalent to the band gap between conduction band and valence band. This energy lost is emitted as heat.

[0007] Tunnel recombination occurs at the n-p interface of tandem cells, where two pin stacks are being used in a serial connection: p-i-n-p-i-n. As the n-p barrier cannot be crossed, free carriers have to tunnel into the interface and recombine in an intermediate state. The energy lost which is emitted as heat is lower than the band gap, but all carriers have to recombine in order to be drifted again.

[0008] Thermalisation occurs when a free carrier has too much energy, e.g. an electron can absorb a photon having a higher energy than the band gap. The energy in excess will be released as heat emission.

[0009] The collision with atom network is also called Joule effect. Collisions of drifting free carriers with atoms from the material produce heat.

[0010] Shunts are heating sources in solar cells which are induced by fabrication process steps. They are caused by, e.g. growth defects, missing pattern, patterning failures etc. As the process steps differ from thin film to crystalline solar cell technology, the root cause of the shunts are different.

[0011] In a "normal" case, when all process steps are well achieved, a thin film solar cell works as described in the following. A photovoltaic thin-film layer is sandwiched between a back contact (BC) and a front contact (FC). A 3- step laser structuring process during manufacturing results in an overlapping roof tile-like structure. This structure again affects a serial connection of individual cells resulting in an increase of module voltage. [0012] A process failure like a localized disturbed material growth during silicon deposition of the photovoltaic layer can lead to a direct shortcut (shunt) between BC and FC.

[0013] The locally missing photovoltaic layer provides a low resistive path to the surrounding current. Therefore, where the layer is missing there will be a higher current density. As the current density is higher than in a "normal" area, the heat generated by the Joule effect in this region is also higher. This results in a localized heat source which can easily be detected by a thermo camera.

[0014] A further origin of shunts can be localized hindered laser patterning. In the case of a missing groove, e.g. because locally laser isolation cannot be performed, a direct connection between two segments of the solar module can appear. This also provides a low resistive path to the current. The high current density in this region results in high localized heat source caused by the Joule effect.

[0015] Other process failures can cause local heat sources in thin-film solar cells, too. However, this demonstrates that the root causes of heat sources in thin-film and crystalline solar cells are different.

[0016] To observe shunts in a solar cell by Thermography, the solar cell can be excited by: electricity; light; or ultrasound.

[0017] Electric excitation is usually performed in a dark room; therefore this technique is called DLIT (Dark Lock-In Thermography). This method is commonly used for crystalline solar cells. The solar cell is electrically contacted in order to force a current to flow into the layers. The excitation can be done by a voltage source controlled in current or vice-versa. An electric load can also be added to the module in order to achieve a more realistic behaviour of the solar cell.

[0018] In 2003, a Lock-in Thermography technique was introduced using light excitation instead of an electrical source. Different names are used to define this method but the most common one is ILIT (Illuminated Lock-In Thermography). This method includes measuring shunts without contacting the cell and using the current flowing through the shunts when the device is under a correct illumination.

[0019] The measure can be performed in open circuit (Voc), short circuit or with a load added to the solar cell.

[0020] As the ILIT method measures a realistic thermal behaviour of a solar cell, many works have been performed in order to better understand the heating mechanisms of a solar cell. Up to now, only crystalline Silicon solar cells have been studied.

[0021] Whereas, in case of crystalline cells the ILIT method has a lower shunt detection level than DLIT.

[0022] Ultrasound Lock-In Thermography has also been proposed to identify cracks in silicon wafers of solar cells. However, this method is limited to the detection of mechanical induced defects, while other defects, like e.g. etched cracks cannot be detected. Furthermore, the method is sensitive to the illumination conditions.

[0023] A problem associated with prior art is, that actual methods and systems for the detection of shunts are limited to small substrate sizes. This makes it impossible to use the methods and systems in an on-line system for the production of large thin film modules. Furthermore, the differentiation of shunts in different layers of thin film solar cells having more than one photoactive layer is not possible by the known system and methods. Even if the wavelength can be changed to excite a crystalline cell more efficiently, nothing has been proposed to excite multiple junctions in thin film solar cells.

[0024] A limit of the excitation by electricity technique is that a contact is required, which means that the solar cell fabrication must be finished. It is also assumed that the current flow behaviour forced by an external source is different from a natural flow drifted by the doping level of the material.

[0025] However, even when the excitation by light method is contactless and a step by step process-induced shunt monitoring can be performed as soon as the p-n junction is formed, yet this method is realized exclusively for crystalline cells.

Disclosure of Invention

[0026] It is therefore an object of the invention to provide an improved method as well as a system for the detection of ohmic shunts in thin film solar cells. Furthermore, it is an object of the invention to provide a method as well as a system for the differentiation of ohmic shunts in thin film solar cells having more than one photovoltaically active layer.

[0027] With respect to the method, this object is solved by the method according to claim 1. With respect to the system, this object is solved by a system according to claim 7.

[0028] Accordingly, a method for the detection of ohmic shunts in a thin film solar cell is proposed, wherein the solar cell is light stimulated and thermal effects caused by the shunts are detected using Lock-in Thermography (LIT), characterized in that the solar cell is stimulated by at least two different light sources of different wavelengths.

[0029] Advantageously, by using light sources of different wavelengths the separate detection of shunts associated with different wavelengths is enabled.

[0030] Two different light sources in the meaning of the invention should be understood as two separated light emitting devices, each emitting light of a specific wavelength, or one light emitting device which emits a broad spectrum of light in combination with at least two filters, each of the filters only transmitting light of a specific wavelength.

[0031] It has been found that the heating behaviour of the layers is different as the thickness of the layers is not the same in case of thin-films. Thin film technology deals with layers up to 3μm thickness deposited on any substrate or superstrate, doped or not, of semiconductor, metal or glass. Layers deals with structures based on p-i-n, n-i-p, or multiple junctions, like for example tandem or triple, with any intermediate layer associated. These layers can be any variation of silicon layer, like for example amorphous, microcrystalline, also called nanocrystalline, silicon oxide, and the like.

[0032] According to an embodiment of the invention, the thin film solar cell comprises at least two stacked photovoltaically active layers, each active layer is stimulated at a different wavelengths of light, wherein for thermographically distinguishing the active layers one wavelength is associated to the Lock-in frequency and the other wavelength is kept constant and/or the different wavelengths are associated to the Lock-in frequency with different delays. This has been found to be advantageous for many applications, since a profound differentiation of shunts in the different active layers of a multi junction thin film solar cell is enabled.

[0033] According to a further embodiment of the invention, the position of the shunt detected is correlated with the position and/or boundaries of the solar cell to yield information about the source of the detected shunt. This has been found to be advantageous for many applications, since the association of shunts with a specific source of the defect is possible.

[0034] The different effects visible in the LIT image can be ascribed to systematic errors during the structuring of the deposited layers into segments or isolated areas and/or to layer deposition variations. These effects will be accordingly distributed.

[0035] According to an embodiment of the invention it is propose to use an algorithm that correlates the position of the effects to the position and/or boundaries of structures, i.e. the solar cell analysed. This correlation will yield information about the source of the observed effect. If the correlation to the position and/or boundaries of the structure is strong, the effects are related to the structuring process. If the correlation is weak but the effect count is high, it is distributed over the substrate and caused by the deposition process. It is further propose to check whether the distribution is evenly over the module or concentrated in certain regions. This yields information about the possible cause in the deposition process.

[0036] According to another embodiment of the invention, the solar cell is light stimulated at open circuits. This has been found to be advantageous for many applications, since the shunts can be detected as soon as a p-i-n junction is formed. No back contact is necessary as the p-i-n itself can absorb light and photogenerated free carriers can recombine within high recombination centres, i.e. shunts. Therefore, the influence of the patterning and the back contacting or any other process step can be observed by the inventive method.

[0037] According to another embodiment of the invention, the solar cell is additionally excited by ultrasound and/or an external electric current. This enables to improve the differentiation of the shunts and the sources of these defects.

[0038] According to another embodiment of the invention, the thermographically examination is performed before and/or after the structuring step of a solar cell. This enables to improve the association of the shunts to specific production steps in the production of thin film solar cells.

[0039] In general, by the inventive method it is proposed to extend and adapt the use of the Voc-ILIT method known from the crystalline cell dimension to the field of thin film solar modules.

[0040] Any combinations of Lock-ln/constant source can be made in order to observe one or more junctions at the same time. The sources can also follow the Lock-in signal with different delay (phase) in order to give to the user broader possibilities of detection.

[0041] With respect to the system, the object of the invention is solved by a system according to claim 7.

[0042] Accordingly, a system for the detection of ohmic shunts in a thin film solar cell by Lock-in Thermography is proposed, said system comprising at least two different light sources emitting light of different wavelength, an IR detection device and a control device, wherein the light sources are connected to the control device so the at least two different wavelengths can individually be associated to the Lock-in frequency.

[0043] Two different light sources in the meaning of the invention should be understood as two separated light emitting devices, each emitting light of a specific wavelength, or one light emitting device which emits a broad spectrum of light in combination with at least two filters, each of the filters only transmitting light of a specific wavelength. [0044] According to an embodiment of the inventive system, the IR detection device comprises multiple thermo cameras and/or a line-array of CCD IR- detectors. By the use of multiple thermo cameras and/or a line-array of CCD IR-detectors the resolution of can be improved.

[0045] According to a further embodiment of the inventive system, the IR detection device is spanning the width of the solar cell to be analysed, preferably close to the surface of the solar cell. Thus an in-situ information right during scanning of the surface is possible. Due to the proximity of the IR detection one can avoid IR capable optics, simplify the analysis and more easily shield the IR detectors from distracting other heat sources of the vicinity.

[0046] According to a further embodiment of the inventive system, the light source is a light table or a linear light source. Especially the combination of a linear light source, like e.g. a linear LED-array, further improves the possibility to gain in-situ information right during scanning of the surface.

[0047] According to a further embodiment of the inventive system, the system can comprise a transport system to convey a solar cell to be analysed. Alternatively, the inventive system may be conveyed relative to the solar cell to be analysed. This allows the integration of the inventive method and system in a thin film solar cell production line.

[0048] According to a further embodiment of the inventive system, the control device comprises a computer system capable to perform a program by which the shunt detected is correlated to the position and/or boundaries of the solar cell analysed. This enables to yield information about the source of the observed effect.

[0049] According to a further embodiment of the inventive system, the system further comprises means to induce heat emission of a shunt by ultrasound and/or an external electric current. This enables to combine different excitation sources, which in turn further improves to distinguish between different shunts and sources of shunts.

[0050] The proposed method and system helps the user to correlate the shunts and the related heating inhomogeneities to the module layout, i.e. the solar cell layout. On one hand, it allows the user to selectively detect the heating source in junctions. So, the shunts can be associated to a particular layer. On another hand, as it is using light excitation, this is also a contactless measurement.

[0051] The present invention permits to introduce an on-line quality control before the laser structuring step in the production of thin film solar cells, also referred to as P2, and successively measuring of the same module after further processing, which in turn gives an excellent tool to qualify the process in between process steps that before were not possible to qualify.

[0052] The use of a particular light table that enables to "illuminate" the module under study at several wavelengths permits the system to distinguish and localize ohmic shunts present in the several active layers (i.e. localize and distinguish those in the amorphous layer to those present in the microcrystalline).

Brief Description of Drawings

[0053] Fig. 1 shows different kind shunts in thin film solar cells;

[0054] Fig. 2 depicts different excitation mechanism and heat generating effect in thin film solar cells; [0055] Fig. 3 shows a system for the detection of ohmic shunts in thin film solar cells according to an embodiment of the invention.

[0056] In Fig. 1 is a schematic representation of the band gap of two consecutive segments of a Tandem module. The uppermost picture depicts the regular electron flow in a thin film solar cell. No shunt is observed, so the electrons produced by the photovoltaic effect flow in the intended manner throughout the solar cell. In the second picture of Fig. 1 , a shunt caused by a missing lay is shown. The electrons produced by the photovoltaic effect flow through the shunt since this is the path of lowest resistance, thereby inducing heat in the area of the shunt. In the second last picture, a shunt caused by a missing structuring in the first structuring step, commonly referred to as P1 , is depicted. In the last picture, a shunt caused by a missing structuring in the second structuring step, commonly referred to as P3, is depicted.

[0057] In Fig. 2 the different excitation mechanism and heat generating effect in thin film solar cells are shown. Electrons may generate heat within the thin film solar cell by either a Peltier effect, recombination, tunnel recombination, thermalisation, or a collision with the atom network, as described in more detail above.

[0058] In Fig. 3 a system for the detection of ohmic shunts in thin film solar cells according to an embodiment of the invention is shown. A thin film solar cell, referred to as thin film module, comprising at least two different photovoltaically active layers is conveyed to a light table. The light table comprises a LED device as light source. The LED device is capable to emit light of at least two different wavelengths. The wavelengths are adapted to separately stimulate a photovoltaic effect in the different layers of the thin film module. An IR camera is adapted to recognize thermal emission on the surface of the thin film module. The IR camera as well as the LED device are connected to a Lock-in control device, by which the light of a specific wavelength emitted by the LED device is coupled the camera signal in the manner of a Lock-in amplifier. By either conveying the IR camera with respect to the thin film module or conveying the thin film module with respect to the IR camera, the shunts detected by their thermal emission recognized by the IR camera can be correlated to position and/or boundaries of the thin film module.

[0059] The particular combinations of elements and features in the above detailed embodiments are exemplary only; the interchanging and substitution of these teachings with other teachings in this and the patents/applications incorporated by reference are also expressly contemplated. As those skilled in the art will recognize, variations, modifications, and other implementations of what is described herein can occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the foregoing description is by way of example only and is not intended as limiting. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. The invention's scope is defined in the following claims and the equivalents thereto. Furthermore, reference signs used in the description and claims do not limit the scope of the invention as claimed.