Millisecond current checks for perovskite-silicon tandems

A tandem cell reaches the tester, and in 7 ms to 12 ms the machine can reveal how much current comes from each of its two subcells. Researchers at Germany’s Fraunhofer Institute for Solar Energy Systems, led by Christoph Messmer, demonstrated the electrical method first on a small in-house cell and then on a full-size industrial device.

The problem is production speed. Conventional tools such as spectral-response and spectrometric analysis can separate the perovskite top cell’s current from the silicon bottom cell’s, but they take more than an hour and require complex optical setups. Fraunhofer ISE’s approach uses the standard current-voltage test already performed on every cell, with a firmware-level change rather than new hardware, light sources or spectral filters.

The trick is inside the silicon cell. A brief forward bias, ideally close to open-circuit voltage, charges its capacitive reservoir. When the tester rapidly switches to a lower voltage, that stored charge briefly discharges and prevents silicon from limiting the tandem current. The first current plateau therefore corresponds to the perovskite subcell; once the reservoir is depleted, the current falls to the silicon-limited level.

On the industrial device, the two plateaus closely matched results from conventional spectrometric characterization. Repeated scans were also highly reproducible, with a relative standard deviation below 0.1%, and the researchers said they detected no measurable device degradation from the fast measurements.

So what changes in practice? Manufacturers could make 100% inline subcell-current sampling feasible, using statistical process-control charts to catch drift in perovskite thickness, composition or morphology, as well as optical-stack variations, before those changes spread into yield loss. The method works only when silicon is the current-limiting subcell; if perovskite limits the tandem first, the silicon current remains hidden. Fraunhofer ISE says the principle could also apply to III–V-silicon tandems and triple-junction devices.