Halide perovskite and nanocrystal optoelectronics

6 problems

• Why lead-halide perovskites keep long carrier lifetimes despite defects

  Contested

  Solution-made perovskite films have many defects, yet light-generated charges in them survive for millionths of a second, much longer than in comparably imperfect ordinary semiconductors. Proposed reasons include charges wrapped in lattice distortions (polarons), a constantly fluctuating lattice, and defects whose energy levels lie harmlessly near the band edges.

• Mechanism of light-induced halide segregation in mixed-halide perovskites

  Contested

  Under light, mixed iodide-bromide perovskites un-mix into iodine-rich and bromine-rich patches, which lowers solar-cell voltage; in the dark they mix again. The driving force is debated.

• Mechanism of slow hot-carrier cooling in lead-halide perovskites

  Partially resolved

  Light with extra energy creates hot charges that normally lose that excess to the lattice within a fraction of a trillionth of a second. In perovskites they stay hot much longer at high light intensity, and why is debated.

• Which ions migrate in operating perovskite devices and why

  Contested

  Charged atoms (ions) and empty atomic sites in perovskites drift under electric fields and light, causing current-voltage hysteresis and device degradation. Which species move, how fast, and why light speeds them up is not settled.

• Origin of power-law fluorescence blinking in semiconductor nanocrystals

  Open

  A single tiny crystal of CdSe or perovskite glows, then goes dark, then glows again, with on and off periods spread over every timescale from microseconds to minutes. Why the durations follow a power law, instead of having one typical time, is unknown.

• Does dynamic Rashba splitting slow radiative recombination in perovskites

  Contested

  Heavy lead atoms and a fluctuating lattice could split the energy bands by spin, making light emission slightly indirect and slower. Whether this effect is large enough to matter is disputed.