Technology
Perovskite-Silicon Tandem Solar Cell Efficiency Limits
Quick fact
A perovskite-silicon tandem cell has already achieved over 34% efficiency in the lab, while the best single-junction silicon cell stays around 27%, and the theoretical limit for a tandem can exceed 45%.
Why this is interesting
You know solar cells have a theoretical efficiency ceiling—so how could stacking two different materials break past it and double the power output per panel?
Read the full explanation
Understanding Perovskite-Silicon Tandem Solar Cell Efficiency Limits
Think of sunlight as a rainbow of photons. A single silicon cell can only absorb photons with enough energy to jump over its 1.1 eV bandgap, and it wastes the excess energy of higher-energy (blue) photons as heat. Perovskite-silicon tandem cells split the rainbow: a thin perovskite layer (with a wider bandgap around 1.7 eV) captures the high-energy photons first, then lets the lower-energy (red and infrared) photons pass through to the silicon cell below. By using two materials with complementary bandgaps, the tandem minimizes wasted energy and captures more of the solar spectrum, pushing the efficiency ceiling far beyond what either material can achieve alone.
A deeper explanation
The ultimate efficiency limit stems from the Shockley-Queisser limit for a single p-n junction, which is about 33.7% for a 1.34 eV bandgap, but silicon's 1.1 eV limits it to about 29.4%. In a tandem, the sub-cells are series-connected, so the total current is limited by the lower of the two currents (current matching), and the voltage adds up. The theoretical efficiency limit for a perovskite-silicon tandem (two cells) is about 45%, but practical losses—such as optical reflection, parasitic absorption, imperfect bandgap tuning, and electrical contact resistance—reduce that. To approach the limit, engineers tune the perovskite composition (e.g., cesium and formamidinium mixtures) to adjust its bandgap to the optimal value (~1.7 eV), stack the cells with an interconnecting layer that minimizes resistance, and add anti-reflection coatings. The key insight is that the tandem breaks the single-junction limit by splitting the solar spectrum more effectively, distributing photon energy to the material that can convert it most efficiently.