Plenty of Room at the Top: Breaking through the Sunlight-to-Electricity Conversion Limit

The Science

Solar-excited “hot” electrons are usually wasted as heat in conventional silicon solar cells. In a new type of solar cell, known as a hybrid organic-inorganic perovskite cell, scientists found these “hot” electrons last longer. These hot electrons have lifetimes more than a 1000 times longer than those formed in silicon cells. The rotation of oppositely charged ions plays a key role in protecting “hot” electrons from adverse energy-depleting interactions.

The Impact

This research identified a possible route to dramatically increase the efficiency of solar cells. By slowing the cooling of excited “hot” electrons, scientists could produce more electricity. They could devise cells that function above the predicted efficiency limit, around 33 percent, for conventional solar cells.

Summary

Hybrid organic-inorganic lead halide perovskites (HOIP) are promising new materials for use in low-cost solar cells. HOIPs have already been demonstrated in solar cells with solar-to-electricity conversion efficiency exceeding 20 percent, which is on par with the best crystalline silicon solar cells. Research is ongoing to discover why HOIPs work so well for solar energy harvesting and to determine their efficiency limit. A team led by Columbia University has discovered that electrons in HOIPs acquire protective shields that make them nearly invisible to defects and other electrons, which allows the electrons to avoid losing energy. The mechanism of protection is dynamic screening correlated with liquid-like molecular motions in the crystal structure. Moreover, the researchers discovered that the protection mechanism works for electrons with excess energy (with energy greater than the semiconductor band gap); as a result, these so-called “hot” electrons are very long-lived in HOIPs. In a conventional solar cell, such as the silicon cell widely in use today, only part of the solar spectrum is used, and the energy of the “hot” electrons 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, called the Shockley-Queisser limit. However, the long lifetime of “hot” electrons in HOIPs makes it possible to harvest the “hot” electrons to produce electricity, thus increasing the efficiency of HOIP solar cells beyond the conventional limit.

Funding

U.S. Department of Energy, Office of Science, Basic Energy Sciences (growth of macroscopic single-crystal and microplate samples, and time-resolved optical Kerr effect measurements) and the National Science Foundation (photoluminescence experiments).

Publication

H. Zhu, K. Miyata, Y. Fu, J. Wang, P.P. Joshi, D. Niesner, K.W. Williams, S. Jin, and X.Y. Zhu, “Screening in crystalline liquids protects energetic carriers in hybrid perovskites.” Science 353, 1409 (2016). [DOI: 10.1126/science.aaf9570]