Researchers in the US have developed a set of guidelines and protocols to assess the performance of three-terminal (3-T) and four-terminal (4-T) tandem solar cells, including those with subcells made of III-V, CdTe, perovskites or silicon materials.
Researchers at the National Laboratory of the Rockies in the US have formulated a comprehensive performance measurement framework for three-terminal (3-T) and four-terminal (4-T) tandem solar cells.
“Our work introduces the first comprehensive performance measurement framework specifically for three-terminal (3-T) and four-terminal (4-T) tandem solar cells, including perovskite-based devices,” Tao Song, corresponding author of the study, told us. pv magazinenoting the need for reliable and standardized performance measurements for emerging multi-terminal tandem 3-T and 4-T tandem designs. “We have demonstrated practical, validated approaches to ensure reported efficiencies are accurate, reproducible and comparable across laboratories and industry,” said Song.
Interest in 3-T and 4-T devices is driven by the potential to reduce current subcell limitations, higher device energy yields and as an alternative path for combining emerging PV materials such as perovskites with established silicon material combinations, such as perovskites and silicon or perovskites and cadmium telluride (CdTe), according to the study.
The researchers illustrated the methods with three different multi-terminal cell configurations, two 3-T devices and one 4-T device. One 3-T was a perovskite/interdigitated-back-contact (IBC) silicon device and the other was a gallium indium phosphide (GaInP)/gallium arsenide (GaAs) device. The 4T device was a GaAs top cell with an IBC silicon bottom cell.
For 3-T tandems, the researchers indicated a simple protocol if both subcells are fast response types. But for devices with slow-response subcells, such as those made with a perovskite absorber, two steady-state methods were described: (i) two-dimensional maximum power point tracking (MPPT) for both subcells, and (ii) a hybrid method that combines MPPT on one subcell with an asymptotically stabilized maximum power (PMAX) scan on the other. “Both approaches produce equivalent results, but differ in measurement time,” the researchers said.
For 4-T tandems, the team described two IV measurement approaches: synchronized scans with spectrum adjustment or sequential scans with individual adjustment of the irradiance for each subcell.
“When dealing with slow-responding subcells, these should be measured using steady-state methods, while fast-responding subcells can be characterized using conventional IV scans,” the researchers explained, adding that in cases where luminescent coupling exists between subcells, the untested subcell should be held at its PMAX “using a good load resistor to ensure accurate determination of the combined output.”
“We expect this framework will help shape future International Electrotechnical Commission (IEC) and American Society for Testing and Materials (ASTM) standards for multi-terminal tandem devices,” Song said.
Song noted that it adapts to both fast and slow responding materials to appropriately account for electrical and optical coupling effects between subcells. He said the framework is “broadly applicable not only to perovskite-based tandems, but also to other multi-terminal solar technologies such as III-V, CdTe and silicon-based tandems.”
In the study’s conclusion, the team noted that not all research labs have access to spectrum tuning hardware and synchronized scanning capabilities. Without such tools, measurement limitations can introduce ‘non-trivial measurable errors’. As a result, it was recommended that third-party testing be performed to establish data and provide accurate baseline measurements.
For other cases, a simpler protocol was provided with the caveat that measurement limitations and potential sources of error must be clearly documented and reported.
“The next step is broader adoption and standardization of these measurement practices. As tandem devices scale toward production, consistent and reliable performance assessments will be essential,” Song said. The group is now extending these methods to industrial-scale wafers and modules, supporting the development of future international standards for tandem PV testing.
The research work is described in detail in “Performance measurement of emerging 3- and 4-terminal tandem solar cells,” published by EES solar energy. A team from Center suisse d’électronique et de microtechnique (CSEM) also took part in the study.
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