Non-equilibrium and stochastic thermodynamics
Can swapping lead for tin keep perovskite electrons hot longer?
Open access · cc by · source: Europe PMC
Adding more tin to lead perovskite nanocrystals slows how fast excited electrons lose their extra energy, and passivating defects with sodium slows it further.
Study at a glance
- Design
- Other — Femtosecond pump-probe transient absorption on colloidal MA- and Cs-based tin-lead iodide nanocrystals at varied composition, pump energy and fluence.
- N
- No single N; several nanocrystal compositions (MASnI3, MASnxPb1-xI3, CsSn0.4Pb0.6I3 with and without Na doping) measured at a range of carrier densities.
- Population
- Colloidal tin-lead halide perovskite nanocrystals of about 3 nm (MA series) and Cs-based alloys
- Outcome
- Hot-carrier temperature versus time, cooling time constants, band-edge bleach kinetics
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Key findings
Above an injected carrier density of about 10^18 per cubic centimetre a second, slow cooling stage appeared in every composition, the signature of a hot phonon bottleneck. In the methylammonium tin-lead series both the fast and slow cooling times grew as tin content increased, which the authors link to screened carrier-phonon coupling, a wider phonon gap that suppresses LO-phonon decay, and lower thermal conductivity. In caesium tin-lead crystals the bottleneck was weak, but sodium doping removed sub-bandgap trap signals, raised the early carrier temperature and lengthened the slow cooling time by about one-third.
Methodology
The team made perovskite nanocrystals with different tin-to-lead ratios, using either methylammonium or caesium as the A-site cation, plus a caesium tin-lead version lightly doped with sodium. They hit the crystals with femtosecond laser pulses and used transient absorption spectroscopy to track the energy spread of excited carriers over time. Fitting the high-energy tail of the bleach signal with a Boltzmann distribution gave a carrier temperature, whose decay they fitted at low and high excitation densities.
Limitations
This is a spectroscopy study of nanocrystals in solution, not a working hot-carrier solar cell, so no device efficiency gain is demonstrated. Tin incorporation also raises defect densities and shortens carrier lifetimes, so traps compete with the intrinsic physics and the mechanisms proposed (phonon screening, suppressed Klemens decay, reduced thermal conductivity) are inferred rather than separately measured. Carrier temperatures depend on a Boltzmann-tail fitting procedure, and the caesium results rest on one composition with one sodium doping level.
How this study connects
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