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

The compound multi-junction solar cell has achieved a remarkable unit conversion efficiency of over 40% under concentrated light conditions. To push this even further, the Okada Research Group at the University of Tokyo and Takano, a company specializing in testing equipment, have developed an innovative evaluation technique known as the "SR-V Method." This method is designed to support the theoretical goal of achieving up to 50% efficiency when sunlight is concentrated. As a result, the development speed of these advanced photovoltaic cells is expected to accelerate significantly. One of the key reasons why compound multi-junction solar cells are so efficient is their ability to combine materials with different bandgaps. This allows them to capture and utilize a broader spectrum of sunlight. For example, the world’s highest-efficiency cell, developed by Sharp in May 2013, reached 44.4% efficiency. It was a triple-junction cell composed of InGaAs, GaAs, and InGaP layers, each optimized for a specific part of the solar spectrum. However, there remains a challenge in evaluating these cells. Since they are manufactured as continuous thin films in a vacuum, it is difficult to measure the performance of individual sub-cells accurately after the final product is formed. This limitation has been a major hurdle in optimizing their design and performance. In multi-junction cells, the sub-cells are connected in series, meaning that the overall current is limited by the weakest sub-cell. Without accurate measurements of each sub-cell’s characteristics, it's hard to fine-tune the system. Traditionally, researchers estimated sub-cell properties based on overall IV curves and spectral sensitivities, but this approach lacked precision. To address this issue, the University of Tokyo and Takano introduced the SR-V Method, which allows for more precise measurement of each sub-cell. The method involves applying a bias voltage while selectively illuminating one sub-cell at a time, while other sub-cells are continuously illuminated. By repeating this process for each sub-cell, detailed electrical characteristics such as series and shunt resistance can be calculated. The data obtained from these measurements are then used to refine the model by comparing estimated values with actual results. This iterative process continues until the error between the two becomes minimal, after which the Powell mixing method is applied to determine the most accurate characteristic values. With modern computing, calculating the characteristics of a dual-junction cell can take around 10 hours. According to the University of Tokyo, the resulting IV curves closely match experimental data, proving the method's effectiveness. The SR-V Method provides insights that previous techniques, including those used by the U.S. National Renewable Energy Laboratory (NREL), could not offer. By identifying subtle variations in crystallinity and film thickness, this method enables improvements that can boost the efficiency of multi-junction cells by 1 to 2 percentage points. Beyond research and development, the SR-V Method also holds great potential for quality control and product inspection. When applied to wafers from overseas manufacturers, it revealed inconsistencies in sub-cell performance across the wafer surface. These differences, undetectable with traditional methods, could affect the long-term reliability of the cells. By identifying such issues early, the new method helps ensure better consistency and durability in commercial products.

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