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

The compound multi-junction solar cell has demonstrated a remarkable unit conversion efficiency of over 40% under concentrated light conditions. To push this boundary even further, the Okada Research Office 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 help achieve the theoretical goal of 50% efficiency under concentrated sunlight, potentially accelerating the development of high-performance compound multi-junction photovoltaic cells. One of the key reasons for the high efficiency of these cells is their layered structure, which combines materials with different bandgaps. This allows them to capture a broader range of the solar spectrum. For instance, Sharp's triple-junction cell, which achieved a record 44.4% efficiency in May 2013, consists of three sub-cells made from InGaAs, GaAs, and InGaP, each optimized for a specific part of the light spectrum. However, a major challenge remains: because these cells are fabricated as continuous thin films in a vacuum, it's difficult to measure the performance of individual sub-cells accurately. This limitation has hindered precise optimization of each layer. Previously, researchers estimated sub-cell characteristics based on overall IV curves and spectral responses, but this approach was not always accurate. To address this issue, the University of Tokyo and Takano introduced the SR-V method. This technique allows for more detailed analysis by measuring the current of each sub-cell individually while applying controlled bias voltages. During the process, light corresponding to other sub-cells is continuously applied, ensuring that the measurement reflects real-world conditions. By repeating this for each sub-unit, the method enables a more accurate assessment of each component’s electrical properties. Using the SR-V method, researchers can calculate parameters like series and shunt resistance for each sub-cell. They compare measured data with theoretical models and refine the values until the results align closely. This process, which takes about 10 hours for a double-junction cell using a computer, leads to more reliable predictions of the overall IV characteristics. Compared to traditional methods like those used by NREL, the SR-V method provides deeper insights into sub-cell performance. It helps identify issues such as poor crystallinity or uneven film thickness, which can then be addressed to improve efficiency by up to 1–2 percentage points. Beyond research, this new measurement technique also shows promise in quality control for commercial products. When applied to wafers from overseas manufacturers, the SR-V method revealed inconsistencies in sub-cell performance across the wafer surface—something that previous methods couldn’t detect. These variations could affect long-term reliability, making the SR-V method a valuable tool for both R&D and production line testing.

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