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The addition of light, water and oxygen molecules leads to substantial defect-healing in metal halide perovskite thin films. [Image: Courtesy of Matthew T. Klug/University of Oxford]

A team led by Samuel D. Stranks at the University of Cambridge, U.K., reports developing a method for using light, oxygen and humidity to fix defects in the polycrystalline perovskite thin films used in solar cells. The technique removes electron traps in the crystal’s molecular structure, minimizing non-radiative losses and greatly improving the film’s performance (Joule, doi: 10.1016/j.joule.2017.08.006). These “healed” thin-films could, the researchers say, speed up the market rollout of perovskite-based solar cells that are more affordable and more efficient than silicon-based solar cells.

Fixing electron traps

Silicon-based solar cells are very successful at converting the sun’s rays into electricity, but they are also expensive and require a lot of energy to manufacture. Polycrystalline perovskite thin films are a likely candidate for replacing silicon-based solar cells because they not only are relatively cheap and easy to manufacture, but they could also, in theory, outperform silicon. But perovskite thin films haven’t yet reached their theoretical efficiency limits—because perovskite crystals are very susceptible to substantial non-radiative decay when exposed to solar radiation.

This non-radiative decay takes the form of electron traps, where electrons stuck before they can harvest the energy of photons from the sun. If these traps can be repaired, the electrons can more easily move around in the solar cell, which translates to a higher efficiency for converting electrons gathered from sunlight into electricity.  

Previously, Stranks’ group developed a method that could fix these traps by treating the surface of perovskite thin films with light. That exposure created iodide ions that acted as mini-brooms, sweeping away structural defects in their path. But the fixes were temporary; after 10 hours, the ions would eventually return, bringing with them the defects they removed.

Water: An unlikely solution

Now, with an expanded team, Stranks and his colleagues claim that they can permanently fix electron traps in perovskite thin films by exposing them to light, oxygen (O2) and humid air before the final layers of the film are deposited onto the perovskite sheet. This approach, Stranks admits, seems counter-intuitive, because water is typically avoided during pre- and post-manufacturing processes.

The researchers say that when they expose an unfinished perovskite thin-film to light and O2, electrons bind with the O2 to form a superoxide species. This superoxide in turn binds to electron traps in the thin film that can prevent photons from being absorbed and converted into electricity. Water molecules in the humid air create a protective shell that keeps the superoxide species bound to the electron traps even after the light source is removed. After several trials, the scientists found that they got the best results with a 30-minute light exposure and humidity levels between 40 and 50 percent.

The team reports achieving internal photoluminescence quantum efficiencies of 89 percent; carrier lifetimes of 32 µs; and diffusion lengths of 77 µm. Combined, these enhancements could translate to perovskite solar-cell efficiencies of 19.2 percent, with a near-instant rise to stable power output—properties that are close to those of the “best crystalline semiconductors reported to date.”

The international collaboration includes scientists from the Universities of Cambridge, Oxford, and Bath in the U.K., Delft University of Technology in the Netherlands, and the Massachusetts Institute of Technology in the United States.