Degradation Mechanism Of Solar Cells Research Articles

AE Characterization of damage in flexible solar cells under torsional loading

Torsional tests of flexible solar cells were conducted to evaluate the electrical performance degradation caused by mechanical loading. To elucidate the degradation mechanism of solar cells, it is indispensable to detect the damage initiation and accumulation due to the applied mechanical loading and to identify which of the damage contributes to the electrical performance degradation. Therefore, the AE technique was used to monitor the damage initiation and accumulation process, and the LT method, which can detect the temperature modulation caused by leakage current generation, was used to identify the damage that contributes to performance degradation. It was shown from AE measurement that damage which is a precursor of electrical performance degradation were detected. It was also found that the damage caused by tensile stress, which was dominant at the specimen ends, contributes the initiation of performance degradation. In addition, damage caused by the principal stress near the center of the specimen, which is inclined at 45° to the specimen length direction, was also suggested to cause significant performance degradation. Consequently, it was demonstrated that the electrical performance degradation processes of flexible solar cells under torsional loading.

The Importance of Quantifying the Composition of the Amorphous Intermixed Phase in Organic Solar Cells

The relation of phase morphology and solid-state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase-pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device-relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk-heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology-function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high-performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.

Potential-Induced Degradation (PID): Introduction of a Novel Test Approach and Explanation of Increased Depletion Region Recombination

In recent years, a detrimental degradation mechanism of solar cells in large photovoltaic fields called potential-induced degradation (PID) has been intensively investigated and discussed. Here, the module efficiency is decreasing down to a fractional part of their original efficiency. In this study, we introduce a PID test at a solar-cell level and for individual module components applicable as a tool for process control in industries and root cause analyses in science departments. Using the proposed method, one example analysis of a solar cell that is degraded by the PID tester is presented. It is shown that PID of the shunting type influences both the parallel resistance (Rp) and the depletion region recombination behavior (J02) of the solar cell. Increased recombination in the depletion region is caused by Na decorated stacking faults crossing the depletion region. This strongly influences recombination behavior in the depletion region, leading to an increased J02 and an ideality factor n2 > 2. However, the defects leave the base of the solar cell primarily unaffected, and hence, J01 recombination remains rather low. Based on these findings, a model for the shunting and the increased depletion region recombination behavior is discussed.