[1] The present application relates to a method and apparatus for assessing photoresponsive elements, particularly but not exclusively solar cells.

Background to the Invention

[2] Photovoltaic panels or similar devices are the subject of extensive research, particularly with a view to improving the efficiency of solar cells for renewable power generation. As part of this development, it is necessary to characterise a cell's response to illumination. The characteristics of interest are the current density voltage (J-V) response, that is the induced current-voltage curve for the device under a known illumination source, the internal quantum efficiency (IQE), spectral response and external quantum efficiency (EQE). The IQE is the ratio of the number of charge carriers generated in the cell which flow into the external circuit to the number of absorbed photons, and the EQE is ratio of the number of charge carriers generated in the cell which flow into the external circuit to the number of photons incident on the cell. A spectral mismatch factor is also measured, corresponding to the differences in characteristics between the test cell and a known reference cell under the specific illumination source, with respect to a standard illumination source. It is beneficial if the time required to characterise solar cells is as short as possible, particularly for cell production quality control and for rapid device optimisation.

[3] Conventionally, to find the J-V curve a cell is tested using a source which illuminates the cell with "simulated sunlight", so that the spectrum and power of the illuminating light corresponds to sunlight under Air Mass (AM) 1.5 conditions, that is replicating sunlight that has passed through a column of air equal to 1.5 time the height of the atmosphere, with a power per area of lkW/m ² . The bias voltage across the cell is varied and the resulting current generated by the illuminated cell is measured.

[4] To provide the spectral response of a cell, a broad band light source is passed through a monochromator to illuminate the cell with light in a narrow wavelength range. The cell's response is measured as the wavelength range is varied. The cell's output is compared to that of a reference cell with known spectral response in order to obtain the absolute EQE.

[5] These methods have a number of disadvantages. Using a monochromator to determine the spectral response is a slow process with a single scan taking 5 to 10 minutes. The light sources used to measure the J-V curve and spectral response may be different. In addition, sources of "simulated sunlight", such as xenon lamps, are inconsistent and the characteristics of a source can change over time, leading to inconsistencies in the measured characteristics of a cell and requiring repeated calibration of the test equipment between and during tests, especially if different sources are used. The time required to perform EQE measurements mean that this important characterisation step is not routinely used.

Summary of the Invention

[6] According to a first aspect of the invention there is provided a method of testing a photoresponsive element comprising the steps of providing a broad spectrum source wherein the broad spectrum light source is a solar simulator source, passing light from the broad spectrum source through an interferometer to generate an interference pattern, superimposed on top of a background simulated solar spectrum, such that the interference pattern is projected as a perturbation on top of the simulated solar spectrum on a surface of the photoresponsive element, varying the interference pattern, and receiving a signal from the photoresponsive element.

[7] The method may comprise the step of mapping the signal to the frequency domain to provide a spectral response characteristic of the photoresponsive element.

[8] The method may comprise measuring an offset of the sample to provide a J-V characteristic of the photoresponsive element.

[9] The method may comprise applying a bias voltage to the photoresponsive element.

[10] The method may comprise varying the bias voltage to provide a J-V curve of the photoresponsive element.

[11] The method may comprise measuring light transmitted through the photoresponsive element.

[12] The method may comprise measuring light reflected by the photoresponsive element.

[13] The method may comprise calculating the external quantum efficiency of the

photoresponsive element.

[14] The method may comprise determining a maximum power point and measuring at least one of the external quantum efficiency and a spectral response characteristic at a bias voltage at or near the maximum power point.

[15] The method may comprise determining the spectrum of the broad spectrum source. [16] The method may comprise providing a reference cell and comparing a spectral response characteristic of the reference cell and a spectral response characteristic of the photoresponsive element to determine a spectral mismatch factor.

[17] The broad spectrum source may be spatially homogeneous, and the method may comprise illuminating a plurality of photoresponsive elements simultaneously and receiving a signal from each of the photoresponsive elements.

[18] The photoresponsive element may be a solar cell.

[19] The method may comprise controlling the intensity of the broad spectrum light source.

[20] According to a second aspect of the invention there is provided an apparatus for testing a photoresponsive element, comprising a broad spectrum light source wherein the broad spectrum light source is a solar simulator source, an interferometer to receive light from the light source and generate an interference pattern at the surface of a photoresponsive element, and a controller operable to modulate the interferometer to vary the interference pattern, and receive a signal from the photoresponsive element.

[21] The apparatus may further comprise a processing element to map the signal to the frequency domain to provide a spectral response characteristic of the photoresponsive element.

[22] The apparatus may further comprise a processing element operable to measure an offset of the signal to provide a J-V characteristic of the photoresponsive element.

[23] The controller may be operable to apply a bias voltage to the photoresponsive element.

[24] The controller may be operable to vary the bias voltage to provide a J-V curve of the photoresponsive element.

[25] The apparatus may further comprise a first sensor to measure light transmitted by the photoresponsive element.

[26] The apparatus may further comprise a second sensor to measure light reflected by the photoresponsive element.

[27] A processing element may be operable to calculate the external quantum efficiency of the photoresponsive element. [28] The apparatus may further comprise a third sensor to determine the spectrum of the light emitted from the broad spectrum source.

[29] The apparatus may further comprise a reference cell, a processing element being operable to compare a spectral response characteristic of the reference cell and a spectral response characteristic of the photoresponsive element to determine a spectral mismatch factor.

[30] The broad spectrum source may be spatially homogeneous, the apparatus being adapted to test and a plurality of photoresponsive elements simultaneously, the controller being operably to receive a signal from each of the photoresponsive elements.

[31] The photoresponsive element may be a solar cell.

[32] The intensity of the broad spectrum light source may be controllable.

Brief Description of the Drawings

[33] An embodiment of the invention is described by way of example only with reference to the accompanying drawings, wherein;

[34] Figure 1 is a diagrammatic illustration of an apparatus embodying the present invention,

[35] Figure 2a is an example of a signal received from a NIST (National Institute of Standards and technology USA) calibrated silicon solar cell element using the apparatus of Figure 1,

[36] Figure 2b is an example of a signal received from a photoresponsive element (under test) using the apparatus of Figure 1,

[37] Figure 2c is a Fourier transform of the signal of Figure 2a,

[38] Figure 2d is a Fourier transform of the signal of Figure 2b

[39] Figure 2e is an EQE spectrum calculated using the data I of Figures 2c and 2d,

[40] Figure 3 is a flow chart illustrating operation of the apparatus of Figure 1, and

[41] Figure 4 is shows testing a plurality of photoresponsive elements. Detailed Description of the Preferred Embodiments

[42] With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred

embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

[43] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

[44] In the present description, the term 'photoresponsive element' is intended to refer to any electrical or electronic component which is responsive to incident light and where it is desired to test the characteristics of the component. Although the method and apparatus described herein are principally intended for use in testing solar cells, that is components of any type which generate a current in response to absorption of solar radiation, it will be apparent that other components may be tested using the method and apparatus. The photoresponsive element may be for example a Si, InGaAs, or Ge photovoltaic diode.

[45] An example of the apparatus will now described with reference to figure 1. The apparatus is generally shown at 10, and comprises a broad spectrum light source 11, an interferometer 12 and a test part 13 including a photoresponsive element 14 under test, each of which will be described in more detail below.

[46] The light source 11 may comprise any combination of light emitters, filters and other elements to provide a light beam having desired spectral and power values. Where the

photoresponsive element under test is a solar cell, the light source 11 may comprise a xenon lamp with filters to provide a solar simulator source simulating solar illumination with the appropriate intensity and spectral characteristics, such as an AM I.5 or AMO source. It may optionally comprise two or more light sources, such as a xenon lamp and a halogen lamp in order to obtain a spectral output better matched to the desired spectrum. Further alternative solar simulator sources may include LEDs with phosphors, or combinations of the LEDs with tungsten halogen light sources, or just tungsten halogen sources, or in general a flat light source with a uniform intensity across the spectral range. Multiple sources can be combined using dichroic mirrors. The solar simulator source preferably has a suitable intensity such that the intensity of illumination at the

photoresponsive element corresponds to sunlight under the desired AM conditions. Further, it is preferable that change the intensity of the source can be changed to simulate different conditions i.e. low light levels, or conditions under high intensity. The intensity of the source is preferably directly controllable, although the light incident on the photoresponsive element may be varied in other ways, for example by using suitable optical components such as filters or beam splitters.

[47] The interferometer 12 comprises a Michelson interferometer, in which an incident light beam 11a hits beam splitter 20 and is directed along two arms 20a, 20b of the interferometer. The light in each arm 20a, 20b strikes a respective mirror 21, 22 and is reflected back towards beam splitter 20, where the light from each path is recombined in output beam 23. At least one of the mirrors, in this case mirror 22, is moveable by modulator 22a in a direction towards and away from the beam splitter 20 as shown by arrow A to vary the relative lengths of arms 20a, 20b. Preferably the mirrors 21, 22 are metal mirrors, so that the reflectivity of the mirrors is independent of the wavelength of the incident light. By controlling the position of the mirror 22, the interference pattern on the photoresponsive element 14 can be varied.

[48] The output beam is directed to test part 13. In this example, the output beam 23 strikes mirror 30 and is directed to photoresponsive element 14. Providing the arms 20a, 20b are sufficiently balanced, a white-light interference pattern is formed at a surface 14a of

photoresponsive element 14. The interference pattern is superimposed on top of the background simulated solar spectrum, such that the interference pattern, as a perturbation on top of the simulated solar irradiance, is projected on the surface of the photoresponsive element 14. A first detector 31 is located behind photoresponsive element 14 to detect transmitted light that is light that passes through the photoresponsive element without being absorbed. If mirror 30 is suitable, for example a half-silvered mirror, light reflected from the photoresponsive element 14 can pass back through mirror 30 and be received at second detector 32 which permits measurement of light reflected by the photoresponsive element 14. A further detector 33 can be placed to receive light from the output beam 23 which passes through the mirror 30 without being reflected. This detector can be used as a separate measure of the light's spectral or power characteristics, to determine the spectrum of the light source for example, or alternatively can be a reference cell, that is a solar cell with known characteristics. In addition, where the third detector 33 is used to measure the light transmitted from the light source 11 and comparison with a test cell is required, the

photoresponsive element 14 may be a test cell, and then replaced by a photoresponsive cell to be tested (or vice versa). In this example, the light hitting the photoresponsive element under test is simulated sunlight at lkW/m ² AMI.5 (1 sun) but the intensity may be varied by controlling the source.

[49] From figure 1, it will be apparent that the provision of first and second detectors allows the calculation of the proportion of the incident light on the photoresponsive element 14 which is absorbed, so allowing the calculation of the IQE and EQE as discussed above, in this example under AM1.5 lkW/m ² (1 sun).

[50] A controller is generally shown at 34, operable in this example to control modulator 22a to move mirror 22, control the bias voltage of photoresponsive element 14, and receive signals from the photoresponsive element 14 and detectors 31, 32, 33. In this example, the signals are transmitted to a processing element 35, such as a general-purpose personal computer, operable to perform the calculation steps discussed below. The controller 34 and processing element 35 are purely illustrative, and may in practice comprise any suitable combination of hardware, in single or separate modules, and may be connected or integrated as desired

[51] By controlling the position of the mirror 22, the interference pattern on the photoresponsive element 14 can be varied, in effect causing the interference fringes to sweep across the surface of the photoresponsive element. The current in the photoresponsive element will vary as shown in the examples of figures 2a and 2b, from a NIST (National Institute of Standards and technology USA) calibrated silicon solar cell and a Perovskite solar cell (AV49) respectively, with periodic modulation within a Gaussian envelope. The spectral response of the photoresponsive element 14 can be determined by transforming the signal of figures 2a, 2b into the frequency domain, conveniently by performing a Fourier transform and particularly by using a fast Fourier transform ('FFT'), thus giving a spectral response curve as shown in figures 2c and 2d respectively. In addition, the offset or pedestal 40 in figures 2a and 2b is the current generated by the photoresponsive device at for the given incident illumination and bias voltage. This dc offset provides the J-V characteristic of the photoresponsive element 14. Before carrying out a Fourier transform, the offset is removed. An example of the EQE spectrum for the Perovskite cell used to produce the spectrum in figure 2d is shown in figure 2e, calculated from the spectra in figures 2c and 2d and the EQE calibration file for the NIST reference solar cell.

[52] Advantageously, it will be apparent that the spectral response and J-V curve may be measured very quickly and simply for a wide range of bias voltages using the method of figure 3. As shown at 50, the sample is illuminated. The bias voltage of the photoresponsive device is set at step 51, and at step 52 the mirror 22 is modulated and the signal of figure 2a is received. At step 53, the offset 40 in figure 2a is measured. At step 54 the spectral response of figure 2b is obtained by performing an FFT, and at step 55 other measurements or calculations, including determining the IQE, EQE and spectral mismatch (if using a reference cell) made. As illustrated at 56 and loop 57, the measurements are repeated over a range of bias voltages. As illustrated at 58 and loop 59, optionally the method may be repeated for different illumination conditions, where the source is controllable. The resolution of the spectral response measurement is in effect constrained by the step size possible in moving mirror 22. In the present example, a resolution of 2nm is possible, and this may be improved by using suitable adaptive optics. Where appropriate, the source spectrum can be characterised before performing the method, and the spectral response of the photoresponsive element 14 under solar illumination conditions can be derived from its spectral response to the known source. This allows some flexibility in selecting a suitable solar simulator source from the source types described above, while still permitting proper characterisation of the photoresponsive element.

[53] The instrument also allows for a new type of solar cell characterisation method, where the photocurrent spectrum is measured at the "maximum power point" or at any desired point of the J- V curve. The instrument may automatically determine the maximum power point and adjust the load to match. This is a more relevant characterisation figure of merit for a solar cell, as it simulates the working conditions of the solar cell by operating with a load resistance in the circuit, in contrast to standard photocurrent EQE measurements that are performed under short-circuit conditions.

[54] In a further embodiment, where the light from the broad spectrum source is spatially homogeneous as illustrated in figure 4, the apparatus may be arranged so that a plurality of photoresponsive elements 14', 14" are simultaneously tested. The optical components may be configured so that the output light from the from the interferometer homogeneously illuminates a sufficient area, here illustrated by circle 60, so that that two or more photoresponsive elements can be equally illuminated at the same time, and a suitable multichannel measurement apparatus, whether part of the controller or otherwise, can measure the signal from each of the

photoresponsive elements and calculate the characteristics of the photoresponsive element as discussed above. This will enable even faster testing of solar cells and other photoresponsive elements.

[55] The method and apparatus described herein are thus advantageous, in that they provide improved accuracy and speed of simultaneously measuring the J-V curve and spectral response of a photoactive element and the spectral mismatch factor for the test apparatus and reference and test cells. The J-V curves and spectral response are measured simultaneously and rapidly, using the same light source. The EQE, for example, may be determined in less than 1 second. This allows characterisation of a photoresponsive device much more quickly and in more detail than present methods, and also removes calibration errors caused by using different sources to perform different parts of the characterisation. Where a light source varies, it will be apparent that the apparatus permits easy recalibration of the source between measurements, for example by checking the response of the reference cell to the illumination source, which hence determines the illumination spectrum enabling regular calculations and application of the spectral mismatch factor. The spectral response is measured under conditions of full illumination, more accurately matching operation in sunlight, rather than only illuminated by light in a narrow wavelength band as would be the case when using a monochromator without background bias light.

[56] Advantageously the method and apparatus described herein can be used to test solar cells under conditions of low intensity e.g. O.lSun and high intensity » lSun, e.g. 2Sun or lOSun. The lower intensities are useful for testing low solar cells optimised for low light conditions and the high intensities are important for devices to be used in solar concentrator applications

[57] In the above description, an embodiment is an example or implementation of the invention. The various appearances of "one embodiment", "an embodiment" or "some embodiments" do not necessarily all refer to the same embodiments.

[58] Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination.

Conversely, although the invention may be described herein in the context of separate

embodiments for clarity, the invention may also be implemented in a single embodiment.

[59] Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above. [60] Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined.