In the illustration of a perovskite structure, a “hot” electron is located at the center of the image. Positive molecules (red and blue dumbbells) surround the “hot” electron. The distortion of the crystal structure and the liquid-like environment of the positive molecules (blurred dumbbells at the periphery of the image) screen (yellow circle, partially shown) the “hot” electron. The “shield” protects the hot electron and allows it to survive 1,000 times longer than it would in conventional silicon solar cells. Source: Xiaoyang Zhu, Columbia University

Low-cost solar cells composed of hybrid organic-inorganic lead halide perovskites (HOIPs) have demonstrated solar-to-electricity conversion efficiency exceeding 20 percent, comparable to that of the best crystalline silicon solar cells. Researchers from Columbia University and University of Wisconsin-Madison have discovered why HOIPs work so well for solar energy harvesting.

Electrons in HOIPs acquire protective shields that render them nearly invisible to defects and other electrons, which allow the electrons to avoid losing energy. This dynamic screening mechanism is correlated with liquid-like molecular motions in the crystal structure. The protection mechanism works for electrons with excess energy — possessing energy greater than the semiconductor band gap. The rotation of oppositely charged ions plays a key role in protecting “hot” electrons from adverse energy-depleting interactions. These hot electrons have lifetimes more than a 1,000 times longer than those formed in silicon cells, in which their energy is wasted.

Excess electron energy generated initially from the absorption of high-energy photons in the solar spectrum is lost as heat before the electron is harvested for electricity production. For conventional solar cells, this loss is partially responsible for the theoretical efficiency limit of around 33 percent.

These electrons are harvested in HOIPs to generate electricity and increase solar cell efficiency beyond the predicted limit.