Tandem solar cells lose efficiency when sunlight shifts

A new study in eScience Energy finds that two-terminal perovskite/silicon tandem solar cells can lose output when outdoor sunlight spectra drift away from lab test conditions. Even so, the devices still delivered more energy per land area than single-junction silicon, pointing to a stronger case for tandem deployment in space-constrained solar markets.

Why it matters: - Two-terminal perovskite/silicon tandem solar cells promise higher efficiency than conventional silicon, but real-world sunlight can erode that advantage. - The study shows spectral changes outdoors can reduce energy output per watt, which matters for bankability, project design and performance forecasts. - The results suggest tandems may still win on land use, a key constraint for utility-scale solar and grid planning.

What happened: - Researchers from Southwest Petroleum University, Tongwei Solar (Chengdu) Ltd., the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, and collaborating institutions in China studied 2T perovskite/silicon tandem cells. - The paper was published online May 9, 2026 in eScience Energy. - The team tested the devices in the lab, modeled annual performance with outdoor spectral datasets and ran economic simulations across major PV climate zones. - The study examined how shifting sunlight colors affect current matching between the tandem cell’s top perovskite layer and bottom silicon layer.

The details: - The researchers fabricated 2T tandem cells and used a tunable LED solar simulator to compare standard, blue-rich and red-rich spectra. - Maximum output occurred only when the two sub-cells were current matched. - Current mismatch reached 4.98% under blue-rich conditions and 4.32% under red-rich conditions. - Corrected measurements showed short-circuit current density and efficiency followed the lower-current sub-cell. - The team built an outdoor spectral performance model using spectral response and the spectral mismatch factor. - The researchers analyzed long-term spectral data from Haikou, China; Albuquerque, United States; Yancheng, China; and Daqing, China. - Those sites represented major Köppen–Geiger climate categories for PV deployment. - Because full spectral measurements are often unavailable, the team proposed a faster assessment method based on the ultraviolet ratio from UV irradiance and global horizontal irradiance. - The ultraviolet ratio showed a strong linear relationship with tandem current mismatch. - The DOI is 10.1016/j.esen.2026.100065.

Between the lines: - The study treats tandem photovoltaics as a field-performance problem, not just an efficiency race. - 2T tandems are more sensitive to spectral variation than single-junction silicon because each sub-cell captures a narrower wavelength range and the weaker current limits the whole device. - The modeling framework could help manufacturers and developers judge where tandems offer the best value before large-scale rollout. - The ultraviolet-ratio shortcut could be useful for site screening and future monitoring networks when full spectrum data are missing.

What's next: - The researchers say future tandem commercialization should combine high-efficiency cells with climate-specific spectral evaluation. - The model can be used to guide spectrum-aware device design for different climates and market conditions. - The approach may help developers decide where tandem modules can beat silicon on economics, not just lab efficiency.

The bottom line: - Tandem solar cells still look promising, but outdoor sunlight diversity is a real performance check that could shape where they are deployed first.