WO2008107132A2

WO2008107132A2 - Reference solar cell

Info

Publication number: WO2008107132A2
Application number: PCT/EP2008/001634
Authority: WO
Inventors: Ralf Adelhelm
Original assignee: Ralf Adelhelm
Priority date: 2007-03-03
Filing date: 2008-02-29
Publication date: 2008-09-12
Also published as: DE102007010490A1

Abstract

A reference solar cell serves as an optical and electrical reference object in measuring the output of solar cells under natural and artificial radiation (solar simulation) such as, for example, as described in IEC60904. A reference solar cell according to the invention comprises a solar cell (201) contained in a housing (200), the cell being selected with regard to (spectral) radiometric requirements. Moreover, a graduated filter (208) with location-dependent transmission (lines or bands) is attached to the solar cell (200), and a spectral screen (209) is provided that exposes part of the graduated filter (208) for the transmission of rays to the solar cell (200) such that, in the case of location-independent spectral radiation of the reference cell, a selective spectral sensitivity of the reference cell is achieved. This results in the improvement of the spectral adaptation of the reference solar cell to a spectral sensitivity of a test solar cell.

Description

REFERENCE SOLAR CELL

The present invention is concerned with reference solar cells, which are used for the determination of photoelectric characteristics in the field of photovoltaics, in particular in the field of quality assurance in the manufacture of solar cells and solar modules or in the design of such devices.

BACKGROUND OF THE INVENTION The demand for high and highest measuring accuracy of electrical characteristics of solar cells exists on the part of the manufacturers as well as users of photovoltaics. Central difficulty here is the generation and measurement of optical radiation in the solar spectral range of 300nm-1200nm in the strict sense, 300nm-1900nm in the broader sense and 250nm to 4μm in the broadest sense.

STATE OF THE ART

The use of reference solar cells and their calibration is described in the publication by S. Winter, T. Wittchen, J. Metzdorf: "PRIMARY REFERENCE CELL CALIBRATION AT PTB BASED ON AN IMPROVED DSR FACILITY, Phys.-Techn. Federal Institute, Bundesallee 100, D-38116 Braunschweig, Germany, IEEE Conference, Glasgow, 2000 known. The use of the spectral mismatch is well defined according to IEC60904-7. The reference cells are of particular importance for solving the radiometric task (in the sense of the mismatch). In general, the required measurement accuracy of electrical characteristics of solar cells is achieved by adapting the spectral sensitivity of a reference cell to the spectral sensitivity of a solar cell to be tested. As a disadvantage of the known reference solar cells, the inherent invariable spectral sensitivity can be considered. In the sense of the required highest measurement accuracy, the mismatch is used for the correction and / or an optimized mismatch is achieved by selecting a suitable reference cell from a set of reference cells with different spectral sensitivity. The mismatch remaining in virtually all cases can be used as a measure of measurement accuracy, although the discussion must be differentiated.

The specially developed reference cell with a replica of a spectral sensitivity of a single subcell of a multiple solar cell is called a component cell. In the case of multiple solar cells for space application, the special component cells are specially developed for use as a reference cell (MOVPE specially grown MOVPE = metal-organic vapor phase epitaxy) and are extremely complex by independent production of a small number. The assessment of the degradation behavior (by cosmic radiation, electrons, protons, etc.), in particular the decrease of the spectral sensitivity and the change of the spectral sensitivity, is of great interest and is directly related to the accuracy of measurements of corresponding samples (for the prediction) of degradations). Since the spectral sensitivity changes considerably due to degradation, • also with increased corrections by the mismatch and

• concomitant decrease of the measuring accuracy of the samples and

• anticipating increasing uncertainty in the forecasts, which can have a significant overall impact on the design of a satellite and is of high economic interest.

In summary, the costly production of reference cells and especially of component cells and the inherently unchanging spectral sensitivity can be seen as a disadvantage of the prior art.

The invention is concerned with the exact measurement of optoelectronic characteristics of solar cells and modules. It is an object of the invention to provide a reference solar cell, which overcomes the known deficiencies of the prior art and a has on a test cell scheduled customizable spectral sensitivity. A further object is the production of a component solar cell, which simulates the spectral sensitivity of a degraded GaAs solar cell in a GalnP / GaAs / Ge triple solar cell (GalnP / GaAs / Ge multi junction solar cell for space application).

DETAILED DESCRIPTION OF THE INVENTION

The inventive solution is based on a selectable spectral sensitivity of a reference solar cell. There is a similar problem in photometry, as in the paper by D Gundlach, W. Hammer, Adaptation of a photofinisher to the standard spectral value function according to the spectral template method, color 27, 1978/79, No. 1/6 (hereinafter [Gundlach]) presented. The change according to the invention and the transmission according to the invention of said method to the new application objective "adaptation of a reference solar cell to any (but fixed) spectral sensitivity of a solar cell to be tested (for example under a solar simulator) leads to a spectral template , as it is part of the reference cell according to the invention.

The functional principle will be explained with reference to FIG. Radiation with sufficient spatial independence in spectral and spatial distribution meets the reference cell. By irradiation of the non-dispersing element of a gradient filter 208, the radiation is spatially resolved with respect to the spectral properties. A solar cell with sufficient homogeneity 201 detects (detects) the spatially resolved radiation mixture with its own inherent spectral sensitivity and produces a localized current that is typically collected across the surface (electrical connector 202). The output signal is to be understood as an integral current via the spatially resolved current generation, caused by the spatially resolved transmittance of the gradient filter 208. Thus, the output signal of the solar cell with graduated filter has a new spectral sensitivity which is determined by the geometry of the active surface of the solar cell compared to the geometry of the solar cell encryption run filters can be adjusted according to plan. A geometrically freely selectable active surface of the solar cell, for example by means of cutting the solar cell, in scheduled positioning in relation to the gradient filter thus leads to a reference cell according to the invention. The technical realization of a geometrically freely selectable active surface is complex and usually difficult to change. Therefore, according to the invention, a spectral template 209 is accessed, which covers a planned portion of the solar cell 201, and thus allows a selectable geometry of the irradiated surface of the solar cell. In some cases, the shadowed part of the solar cell behaves electrically as an insulator. Thus, by means of spectral templates 209 produced mechanically (or photolithographically) (and their simple exchange), a spectrally sensitive spectral sensitivity of the reference cell can be achieved. Prerequisites for the production and use of the reference cell according to the invention are, on the one hand, planar graduated filters, which can be produced inexpensively and over a large area in recent times, and also (large) flat solar cells, which meet the requirements (eg linearity) of a (spectral) radiometric detector and preferably have sufficient spatially resolved homogeneity with respect to their spectral sensitivity. Furthermore, a sufficient homogeneity of the spectral radiance distribution of the solar simulator (the radiation to be assessed) is important.

This gives rise to competing demands on the surface, which gains in resolution for the combination graduated filter / spectral template, but at the same time becomes susceptible to the spatial dependence of the spectral irradiance, which is unavoidable in solar simulators and generally in any radiation source. The latter is also to be considered in the calibration of such a reference cell.

A reference cell according to the invention is similar in appearance and the schematic structure of the conventional reference cell according to the prior art. Exemplary embodiments of the schematic construction of the reference cell according to the invention are shown in the drawings 1-3 in cross-section and in Figures 4-5 shown as an exploded view. The following elements are shown in the drawings:

200 housings

201 solar cell 202 electrical connector of the solar cell 203 temperature sensor

204 electrical connectors (schematically summarized) for the temperature sensor

205 Peltier element (electrical connector is omitted in the drawing)

206 potting compound, adhesive

207 transparent potting compound, transparent adhesive

208 gradient filter

209 Spectral template 210 Separate housing for a resistor and plugs or sockets

211 resistance

212 screws for mounting the separate housing 210

213 Opening of the housing 200 for carrying out the electrical connector in the separate housing 210 to achieve plugs or sockets

214 sockets or plugs for electrical contact to the solar cell, the temperature sensor and the Peltier element

215 dowel pins in housing 200

216 pass holes in the spectral template

EMBODIMENTS

Central components within the meaning of the invention are the graduated filter 208 and the spectral template 209. The graduated filter preferably has a line or band transmission in a spectral range which covers the required spectral sensitivity. The gradient interference filter VERIL BL 200 (gradient range 400nm-1000nm) is suitable for use as a filter for a silicon solar cell (sensitivity range: 300nm-1150nm) to measure the spectral sensitivity of a GaAs subcell (sensitivity range: 670nm- 900nm) of a GalnP / GaAs / Ge space solar cell (total sensitivity range: 300nm-1900nm). This combination is also suitable for simulating the spectral sensitivity of the lower part cell of a so-called micro-morph silicon solar cell.

Suitable graduated filters with a graduated area, which already start at 300 nm and cover the spectral range up to 1200 nm, are not known from commerce, but can be produced in principle. The same applies to the broadest spectral range of solar radiation of 250nm to 4μm of interest here.

The production of graduated filters in the dipping process, especially SoI-GeI dipping processes, have the technical potential to produce such filters for the said spectral ranges. The characterization of the graduated filter essentially comprises

- the maximum local spectral transmittance τ (I, λ) _max

- the local half width HW (I)

spectral half-width HW (λ)

The characterization treatment known from [Gundlach] must be extended to the said spectral ranges.

It should be noted that it is not exclusively the requirement to have a local spectral transmittance for lines or bands to the graduated filter. In general, a gradient filter with a pass-filter characteristic with a position-dependent edge is also possible. Any other location-dependent transmittance can also be used successfully. The combination with full filters is possible.

The production of the spectral template with respect to the geometry of the aperture is carried out by means of characterization numerically and empirically and iteratively.

As a further embodiment, a reference cell (with a suitable filter and a suitable solar cell) with a set of special spectral templates is mentioned here. It is thus possible to produce a special spectral template which has a band characteristic with constant spectral sensitivity of the reference cell in a planar spectral response. moderate wavelength range, eg 500nm-600nm. Thus, after suitable calibration of the reference cell, the energy content in the wavelength range from 500 nm to 600 nm of a solar simulator (or any radiation source) can be determined. A set of specific spectral templates for corresponding wavelength ranges can determine the energy levels of a spectrum to perform a classification of a solar simulator according to IEC60904-9. Since the edge steepness is limited, the ideal-typical constant spectral sensitivity at the said wavelength range limits can not abruptly disappear. An error estimate for this method of energy content measurement is obtained by measurements using further spectral templates which generate a narrow-band spectral sensitivity of the reference cell precisely at the stated wavelength range limits (in the example by 500 nm and by 600 nm). So has the full set of spectral templates for the classification of a spectrum according to IEC60904-9

- 6 spectral templates for the wavelength ranges 400nm-500nm, 500nm-600nm, 600nm-700nm, 700nm-800nm, 800nm-900nm. 900nm-1100nm for energy content determination and - 7 spectral templates for the range limits around 400nm, 500nm,

600nm, 700nm, 800nm, 900nm, llOOnm for error estimation of the energy content measurement.

A further particular embodiment arises when, instead of a solar cell, a plurality of electrically isolated solar cells are installed in a precisely aligned manner and the electrical measurement of the output signals of the solar cells is made possible separately. Thus, simultaneous measurements are possible, which allows a reduction in the number of spectral templates. Mathematically, there is no difference in the ideal-typical view. However, the division into a plurality of solar cells can compensate for an undesirable effect: The spectral template expediently leads with its aperture effect to unirradiated parts of the solar cell. However, undesired non-linearities can cause the non-irradiated part of the solar cell to act as parasitic resistance. Through several solar cells The ratio of unirradiated area to irradiated area can be improved.

FURTHER ADVANTAGES OF THE INVENTION A decisive advantage of a reference cell according to the invention lies in the (ideal typical) identical spectral sensitivity with the solar cell to be measured. This makes spectral corrections in the sense of the mismatch superfluous. An even more advantageous advantage results from the fact that the elaborate development and costly production of special solar cells can be avoided with the reference cell according to the invention. This applies in particular to the component cells which are used in the measurement of multiple solar cells, in particular for space applications. An example of this is the simulation of a spectral sensitivity of a GaAs subcell of a GalnP / GaAs / Ge triple junction solar cell. In the prior art, the component cell is made with only one active pn junction in a special growth program (MOCVD), and the upper GaInP cell merely serves as a passive filter to emulate the desired spectral sensitivity. With the reference cell according to the invention, a silicon solar cell, as already used in the prior art for reference cells, can be used to simulate the GaAs subcell of the multi-junction solar cell. This reduces the production costs considerably. Any "deformed" curves of the spectral sensitivity of multiple solar cells due to degradation can easily be simulated by newly adapted spectral templates.

In general, a multiplicity of spectral templates according to the invention can replace a corresponding number of conventional reference cells.

The reference cell according to the invention with a suitable set of spectral templates can be used for the spectral classification (eg according to IEC60904- 9 or DIS15387) of a solar simulator. In particular, a pulsed irradiation can also be classified spectrally. Thus, the invention solves a hitherto highly problematic metrological task, which is only solved by a few institutions worldwide, but is required in many places (according to IEC60904-9).

The reference cell according to the invention with a suitable set of spectral templates and / or a multiplicity of solar cells under the spectral template permit a multiplex method for determining spectral irradiance. In this way, a cost-effective spectrometer is created from the reference cells together with an evaluation method.

Claims

1. Reference solar cell with essentially one in a housing

(200) composed solar cell (201) which is selected in terms of (spectral) radiometric requirements, in particular linearity and bandwidth and distribution of the spectral sensitivity, characterized in that a history filter (208) with location-dependent transmission (lines or bands) on the solar cell (200) is applied and an additionally applied spectral template (209) releases a part of the gradient filter (208) for the transmission of radiation to the solar cell (200) so that, with spatially independent spectral irradiation of the reference cell, a selectable spectral sensitivity of the reference cell results.

2. Reference cell according to claim 1, characterized in that the solar cell (201), the graduated filter (208) and the spectral template (209) have minimal distance from each other and in particular the solar cell (201) and the filter (208) are glued together.

3. Reference cell according to one or more of claims 1-2, characterized in that the spectral template (209) is either directly between solar cell (201) and gradient filter ter (208) or on a combination of solar cell

(201) and graduated filter (208) is applied.

4. reference cell according to one or more of claims 1-3, characterized in that the graduated filter (208) is an interference gradient filter and in particular in the SoI-GeI- dipping method is produced.

5. reference cell according to one or more of claims 1-4, characterized in that the spectral template (209) is formed as a physical flat planar aperture, one or more free openings (together a location-dependent aperture Opening) in order to allow said radiation to pass through and preferably fully absorb the radiation at the areas to be planatically blanked out.

6. reference cell according to one or more of claims 1-5, characterized in that the spectral template (209) is connected to the housing of the reference cell, on the one hand, a permanently fixed and thus radiometrically calibratable unit of solar cell, graduated filter and SpektralSchablone arises and on the other hand the spectral template (208) can be loosened from the housing of the reference cell in a simple manner and applied as often as desired again with a reproducible position.

7. reference cell according to one or more of claims 1-6, characterized in that the SpektralSchablone (209) has movable diaphragm parts that allow reproducible mechanical changes of the aperture and thus the reference cell has an adjustable spectral sensitivity.

8. reference cell according to one or more of claims 1-7, characterized in that the spectral template (209) is selected so that the resulting spectral sensitivity is constant in a scheduled wavelength interval and thus the resulting reference cell is suitable, the energy content of a wavelength range of To determine spectrum.

9. reference cell according to one or more of claims 1-8, characterized in that a set of scheduled spectral templates is provided, with which the energy contents of a spectrum in certain wavelength ranges are determined, thus allowing a classification of a solar simulator or any radiation source.

10. reference cell according to one or more of claims 1-9, characterized in that instead of a solar cell (201) a large number of electrically separated solar cells are lined up precisely matching each other and thus a simultaneous measurement of the solar cells is made possible.

11. Reference solar cell according to one or more of claims 1-10, characterized in that the reference solar cell is equipped with a temperature sensor.