Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-06-29
2003-04-01
Le, Thien M. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C235S454000, C235S455000
Reexamination Certificate
active
06541754
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for measuring photoelectric conversion characteristics and, more particularly, to a method and apparatus for measuring the photoelectric conversion characteristics of a photoelectric conversion device such as a solar cell, photodiode, photosensor, or electrophotographic photosensitive body and, especially, a stacked photoelectric conversion device.
BACKGROUND OF THE INVENTION
In a stacked photoelectric conversion device in which a plurality of photoelectric conversion elements with different spectral responses are stacked, long-wavelength light that cannot be completely absorbed by the upper photoelectric conversion element on the light incident side is absorbed by the lower photoelectric conversion element, thereby increasing the output or sensitivity. Hence, such stacked photoelectric conversion devices have been extensively developed.
It is very important to accurately measure the output characteristics of a stacked photoelectric conversion device due to the following reasons.
For example, in manufacturing and delivering stacked photoelectric conversion devices whose maximum power is important, a photoelectric conversion device whose maximum power is less than a rated value is determined as a defective product by inspection. However, the maximum power of a photoelectric conversion device to be delivered cannot be guaranteed unless the output can be accurately measured. In addition, if an output measurement error is large, and the measurement error changes depending on the state of the measuring apparatus, the inspection threshold value varies even for photoelectric conversion devices with the same quality, resulting in unstable manufacturing yield. Furthermore, if the inspection threshold value contains a measurement error value to guarantee the quality of photoelectric conversion devices to be delivered, the manufacturing yield inevitably decreases.
If the output of a stacked photoelectric conversion device cannot be accurately predicted, no predicted system characteristic can be obtained or the system efficiency degrades in building a system using the stacked photoelectric conversion device. When the stacked photoelectric conversion device is a solar cell, it considerably affects, e.g., the guaranteed maximum power of the solar cell, manufacturing yield, power generation prediction of a power generation system, and system efficiency.
However, it is very difficult to accurately measure the output characteristics of a stacked photoelectric conversion device. The main reason for this is that the output characteristics of the stacked photoelectric conversion device largely change depending on the spectrum of irradiation light. For example, a double-type solar cell (to be referred to as a “double cell” hereinafter) in which two semiconductor junctions are stacked and connected in series will be described in detail. The upper semiconductor junction on the light incident side is called a top cell, and the lower semiconductor junction is called a bottom cell. The short-circuit current of each cell changes depending on the spectrum of irradiation light because the cells have different spectral responses. As a result, the short-circuit current, fill factor, and open-circuit voltage of the entire double cell change, and the output characteristics of the double cell largely change.
To the contrary, in a single-type cell (to be referred to as a “single cell” hereinafter) having a single semiconductor junction, only the short-circuit current changes depending on the spectrum of irradiation light, and the fill factor and open-circuit voltage are rarely affected. For this reason, when the spectrum dependence of the short-circuit current is corrected, the output characteristics can be almost accurately measured.
Generally, to accurately measure the output characteristics of a photoelectric conversion device, test conditions such as the intensity and spectrum of irradiation light and the temperature of the photoelectric conversion device must be defined. For, e.g., a solar cell, the test conditions are defined as standard test conditions as follows.
Temperature of solar cell: 25° C.
Spectrum of irradiation light: standard sunlight (The spectrum of standard sunlight is defined by JIS C 8911)
Irradiance of irradiation light: 1,000 W/m
2
However, of these standard test conditions, the spectrum of standard sunlight can hardly be obtained even when outdoor sunlight is used. This is because the standard sunlight is obtained only under limited meteorological conditions. It is impossible to obtain the spectrum of standard sunlight using a pseudo sunlight source indoors.
For a single cell, pseudo sunlight sources (solar simulators) are classified into ranks A, B, and C sequentially from one close to the standard sunlight on the basis of the spectrum, variation (to be referred to as a “positional variation” hereinafter) in irradiance depending on the position, and time variation ratio. This ranking is described by JIS C 8912 and JIS C 8933. Using a solar simulator of rank A or B and a secondary reference solar cell having a spectral response similar to that of a solar cell to be measured, the irradiance of the solar simulator is set, thereby correcting an error due to a shift in spectrum. This measuring method is described by JIS C 8913 and JIS C 8934.
The above measuring method is possible for a single cell for which the spectrum affects almost only the short-circuit current. However, in a stacked solar cell, the spectrum affects not only the short-circuit current but also the fill factor and open-circuit voltage, as described above, and the output characteristics cannot be accurately measured by the above measuring method. Hence, the stacked solar cell is excluded from the above-described JIS.
The following technique has been proposed as a method of accurately measuring the output characteristics of a stacked solar cell.
The spectrum of a solar simulator used to measure a stacked solar cell is made adjustable and adjusted to obtain short-circuit current and fill factor values that the stacked solar cell probably generates under standard sunlight, thereby accurately measuring the output characteristics (this technique will be referred to as a “multi-source method” hereinafter) (T. Glatfelter and J. Burdick, 19
th
IEEE Photovoltaic Specialists Conference, 1987, pp. 1187-1193).
That is, each of a plurality of semiconductor junctions of a stacked solar cell is defined as a component cell. Let In.ref (n is the number of each component cell) be the short-circuit current generated by each component cell in the stacked solar cell under standard sunlight and In.test be the short-circuit current generated under a solar simulator. Then, when the spectrum of the solar simulator is adjusted to satisfy
In.test=In.ref (1)
for each component cell, the short-circuit current and fill factor of the stacked solar cell match the values under the standard sunlight.
The above measurement technique assumes use of a solar simulator having an adjustable spectrum. In the above-described reference, to adjust the short-circuit current of each component cell, light components from three light sources: one xenon (Xe) lamp and two halogen lamps are separated into three wavelength bands and then synthesized. By adjusting the irradiances of the three light sources, the intensities of light components in the three wavelength bands are controlled, thereby adjusting the spectrum of the synthesized light.
The solar simulator with variable spectrum is possible for a small irradiation area of 400 cm
2
or less. However, due to the following reasons, it is very difficult to manufacture a solar simulator having an area more than 400 cm
2
.
(i) Since a plurality of light components having different spectra are synthesized, the positional variations in spectrum of the synthesized light and in irradiance become large. The larger the irradiation area is, the more serious these variations become.
(ii) Since the spectrum of partial light from th
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Le Thien M.
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