The temperature evolution of vibrations of CH3NH3PbI3 (MAPI) is studied by combining first principles and classical molecular dynamics and compared to available experimental data. The work has a fundamental character showing that it is possible to reproduce the key features of the vibrational spectrum by the simple physical quantities included in the classical model, namely the ionic-dispersive hybrid interactions and the mass difference between organic and inorganic components. The dynamics reveals a sizable temperature evolution of the MAPI spectrum along with the orthorhombic-to-tetragonal-to-cubic transformation and a strong dependence on molecular confinement and order. The thermally induced weakening of the H-I interactions and the anharmonic mixing of modes give two vibrational peaks at 200-250 cm(-1) that are not present at zero temperature and are expected to have detectable infrared activity. The infrared inactive vibrational peak at ∼140 cm(-1) due to molecular spinning disappears abruptly at the orthorhombic-to-tetragonal transition and forms a broad molecular band red-shifting progressively with temperature. This trend is correlated to the reduced confinement of the rotating cations due to thermal expansion of the lattice.

The use of fullerene as acceptor limits the thermal stability of organic solar cells at high temperatures as their diffusion inside the donor leads to phase separation via Ostwald ripening. Here it is reported that fullerene diffusion is fully suppressed at temperatures up to 140 °C in bulk heterojunctions based on the benzodithiophene‐based polymer (the poly[[4,8‐bis[(2‐ethylhexyl)oxy]‐benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]‐thieno[3,4‐b]thiophenediyl]], (PTB7) in combination with the fullerene derivative [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC70BM). The blend stability is found independently of the presence of diiodooctane (DIO) used to optimize nanostructuration and in contrast to PTB7 blends using the smaller fullerene derivative PC70BM. The unprecedented thermal stability of PTB7:PC70BM layers is addressed to local minima in the mixing enthalpy of the blend forming stable phases that inhibit fullerene diffusion. Importantly, although the nanoscale morphology of DIO processed blends is thermally stable, corresponding devices show strong performance losses under thermal stress. Only by the use of a high temperature annealing step removing residual DIO from the device, remarkably stable high efficiency solar cells with performance losses less than 10% after a continuous annealing at 140 °C over 3 days are obtained. These results pave the way toward high temperature stable polymer solar cells using fullerene acceptors.

The origin of the dissolution of methylammonium lead trihalide (MAPI) crystals in liquid water is clarified by finite-temperature molecular dynamics by developing a MYP-based force field (MYP1) for water-MAPI systems. A thermally activated process is found with an energy barrier of 0.36 eV consisting of a layer-by-layer degradation with generation of inorganic PbI films and solvation of MA and I ions. We rationalize the effect of water on MAPI by identifying a transition from a reversible absorption and diffusion in the presence of vapor to the irreversible destruction of the crystal lattice in liquid due to a cooperative action of water molecules. A strong water-MAPI interaction is found with a binding energy of 0.41 eV/HO and wetting energy of 0.23 N/m. The water vapor absorption is energetically favored (0.29 eV/HO), and the infiltrated molecules can migrate within the crystal with a diffusion coefficient D = 1.7 × 10 cm/s and activation energy of 0.28 eV.

High carrier mobility is often invoked to justify the exceptionally long diffusion length in CH3NH3PbI3 perovskites. Using a combination of an ab initio band structure and scattering models, we present clear evidence that large electrical and Hall mobilities are crucially related to the low scattering rate of carriers with polar optical phonons, which represents the dominant mobility-limiting mechanism at room temperature. With a charge-injection regime at room temperature, we obtained carrier relaxation times (τrel) of ∼10 fs, which are typical of polar inorganic semiconductors, and electrical mobilities (μ) as high as ∼60 cm(2) V(-1) s(-1) and 40 cm(2) V(-1) s(-1) for electrons and holes, respectively, which were robustly independent on the injected carrier density in the range of n ∼ 10(14) cm(-3) to 10(20) cm(-3). In the absence of a significant concentration of trapping centers, these mobilities foster diffusion lengths of ∼10 μm for the low injection density regime (n ∼ 10(15) cm(-3)), which are in agreement with recent measurements for highly pure single-crystal perovskites.