Tokyo University Develops SR-V Method to Improve Multi-junction Photovoltaic Cell Efficiency

The compound multi-junction solar cell can achieve a unit conversion efficiency of more than 40% under the condition of spot light irradiation. In order to further increase its conversion efficiency, the Okada Research Office of the University of Tokyo and Takano, which specializes in testing equipment, have jointly developed a new evaluation method called "SR-V Method." This method is based on the theoretical goal of achieving a unit conversion efficiency of 50% when concentrating. The development speed of compound multi-junction photovoltaic cells is expected to be further accelerated.

The efficiency of the compound multi-junction photovoltaic cell is higher because it overlaps materials with different band gaps and can fully utilize sunlight in a wide range of wavelengths. For example, the unit with the highest global cell conversion efficiency of 44.4% developed by Sharp in May 2013 is a triple junction Note 1) cell that overlaps the three subcells InGaAs, GaAs, and InGaP.

However, since the compound multi-junction photovoltaic cell is continuously formed into a thin film in vacuum, even if the characteristics of the entire photovoltaic cell finally produced can be measured, the characteristics of each sub-cell cannot be accurately measured. This is a problem faced by compound multi-junction photovoltaic cells. A subject.

Compound multi-junction photovoltaic cells connect each sub-cell in series, so the sub-cell with the smallest amount of current limits the overall amount of current. If it is impossible to accurately measure the characteristics of each subunit, it is difficult to adjust it. Previously, only the characteristics of each subunit were estimated based on the overall IV characteristics and the spectral sensitivity of each subunit, and then the development policy was determined.

This time, the University of Tokyo and Takano developed a measurement method that can solve this problem. In addition to compound multi-junction photovoltaic cells, products such as organic and thin-film silicon have also been promoted. It is estimated that new measurement methods can contribute to the development of various types of photovoltaic cells.

Calculated from measured values

The SR-V method can measure the current value by changing the bias voltage while discontinuously illuminating the light corresponding to the measured subunit. At the same time, light corresponding to sub-units other than the measured object is also continuously irradiated in advance (Fig. 1). Repeat this operation for each subunit.

Then, based on the bias voltage and current measurement results obtained for each subunit, the series resistance and shunt resistance of each subunit are calculated. The specific method is to compare the relationship between the bias voltage and the current value that is estimated based on the characteristic values ​​such as the series resistance and the actual measurement result, and then modify the characteristic value to reduce the error between the two. This operation is repeated until the error becomes sufficiently small, and then the "Powell mixing method" is used to calculate the characteristic value. If using a computer, the calculation of the double-junction photovoltaic cell can be completed in about 10 hours. According to the University of Tokyo, the overall IV characteristics were calculated based on the data obtained. The results are basically consistent with the experimental values.

Fig. 1 measures the change of the current value by measuring the bias voltage to measure the monochromatic light corresponding to the measured subunit, while changing the bias voltage. Based on the obtained data, the characteristics of each subunit are calculated.

From the characteristics obtained by the SR-V method, many of the information are not available from previous evaluation methods and recently developed evaluation methods such as the American Renewable Energy Laboratory (NREL) (Table 1). According to the newly identified characteristic value, the improvement of the crystallinity and film thickness of the subunits can be found, “can make compounds The conversion efficiency of multi-junction photovoltaic cells is increased by 1 to 2 percentage points."

In addition to R&D use, new measurement methods are also expected for product inspection purposes. When using new methods to measure compound multi-junction photovoltaic cell wafers of other countries' manufacturers, there is a deviation in the characteristics between subunits located at different positions in the wafer plane (Fig. 2), and when the original method for measuring overall IV characteristics was used , you can't see this characteristic difference. According to the University of Tokyo, this uneven quality may have an impact on the long-term reliability of photovoltaic cells.

Figure 2 can also be applied to product inspection applications to check the characteristics of a compound multi-junction photovoltaic cell on a wafer surface of an overseas manufacturer. There is a difference in the characteristics of each unit, and elements such as crystallinity are found to be problematic.

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