Research Note
The stability of the CuInSe2 solar mini-module IV characteristics under continuous and light/dark irradiation cycle tests

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Abstract

The stability of a CIS solar mini-module during a light/dark cycle and continuous light irradiation was investigated. Under both test conditions, the maximum power was improved during the early stages, then subsequently deteriorated. The major factors causing this phenomenon were changes in the fill-factor and open circuit voltage. The details of some parameters relevant to these factors are considered herein. In addition, the dependencies of light intensity and temperature in terms of the IV characteristics were measured.

Introduction

We expect to see large-scale applications of the thin-film solar cell based on CuInSe2 (CIS) in the future, and various structures for these cells are currently being developed. At present, a conversion efficiency of over 18% in the mini cell and of 12% or more in practical use of the small-sized module can be achieved [1], [2], and this thin film type solar cell is therefore promising in this regard.

Given that a solar module is exposed to the natural environment and potentially severe weather conditions, an exact reliability evaluation is needed. Thus far, the stability of performance has been excellent. However, cells and modules of this type are still at the developmental stage, and a verification of their stability over the long term has not been fully made. For example, several reports have shown that performance improves at the initial stage, and that most degradation is not caused by outdoor exposure [3], [4]. However, remarkable degradation has been reported to be observed under a damp heat test [5]. Furthermore, an increase in the defective state inside the CIS or at the junction of the CIS and CdS, which affects cell performance, has been observed [6]. In order to facilitate the application of thin-film solar cells, the various factors that influence stability over the long term need to be clarified.

We investigated the behavior of the cell characteristics with time under different light irradiation conditions, using a small size CIS solar cell (1–2 cm2) [7]. Our results showed that the characteristics of this cell became unstable over time. Several problems regarding the stability of the performance were clarified.

We performed cycle tests of the “accelerated light irradiation/dark state” use of the mini-module. The results are reported below, along with the results of our investigation of the behavior of module characteristics over time. In addition, we describe the findings we made by comparing the changes in the module characteristics between the condition of continuous light irradiation and that of a light irradiation/dark cycle.

Section snippets

Characteristics of the module used for tests

We used a commercial 5 W CIS mini-module with an active area of approximately 0.06 m2. The conversion efficiency in the 1-SUN was 9–11%. The light intensity and the temperature dependencies of the cell characteristics are shown below. The temperature characteristic was used for compensation of the test data.

Experimental details

The accelerated light-irradiation tests were performed under the three conditions shown below and under a light intensity of 2-SUN using a xenon lamp, in an environment with a room temperature of 26 °C and ventilation of 6 m/s. The tests were carried out under such intense light irradiation in order to accelerate degradation as much as possible.

  • Test A: 10 cycle tests in the light-induced (5 h)/dark state (19 h) under the open circuit condition.

  • Test B: continuous light-induced test of 50 h under

Conclusion

The cycle test of the light irradiation/dark state was performed using a solar mini module based on CIS. Improvements in the Pmax in the early stages of the test and subsequent degradation were observed. The causes of this phenomenon were investigated by examining the relationship among the changes in the Voc, FF, VPmax, and values of the dark characteristics. The same pattern with the passage of time was obtained in the tests of continuous irradiation. In contrast, it was found that different

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