Shining light on excited-state dynamics in perovskite materials

July 20, 2018, Los Alamos National Laboratory
Theoretically predicted structures of MAPbI3 hybrid perovskite: a neutral cluster. b negatively charged (polaron) cluster. c Various perspective comparisons of the neutral (red sphere) and polaron (blue sphere) PbI64- octahedral structures centered in the clusters. The structural comparisons show that the polaron state geometry is more bent and elongated than the neutral, indicated by displacement vectors (green arrows). Credit: Los Alamos National Laboratory

Through a close collaboration between experimentalists at University of California Berkeley and theorists in Los Alamos's Theoretical Division group T-1, the Center for Nonlinear Studies (CNLS), and Center for Integrated Nanotechnology (CINT), researchers have performed femtosecond transient absorption and density functional theory simulations to probe the excited-state dynamics of hybrid perovskite MAPbI3 and found evidence for coherent skeletal vibration dynamics that lead to formation of a polaron state underscoring charge separation. The low-frequency wave packet modes are observed principally for the Pb-I bending and stretching vibrations resulting from the different geometry of the polaron state compared to the neutral state. The high efficiency of MAPbI3 perovskite solar cells may be yet another example of the importance of vibrational coherence in efficient photochemical dynamics.

Elucidating electron-phonon coupling in hybrid organic-inorganic perovskites will help to understand the high photovoltaic efficiency. The team observed low-frequency Raman modes and related nuclear displacements of the Pb-I framework, indicating how these vibrational motions lead to formation as charge carriers in hybrid perovskites.

Hybrid organic-inorganic perovskites are materials with attractive optoelectronic properties including exceptional solar cell performance. Its improved properties have been attributed to polaronic effects involving stabilization of localized charge character by structural deformations and polarizations. Herein, we examine the Pb-I structural dynamics leading to polaron formation in methylammonium lead iodide by transient absorption, time-domain Raman spectroscopy, and . Methylammonium lead iodide MAPbI3 perovskite exhibits excited-state coherent nuclear wave packets oscillating at ~20, ~43, and ~75 cm-1 which involve skeletal bending, in-plane bending, and c-axis stretching of the I-Pb-I bonds, respectively. The amplitudes of these wave packet motions report on the magnitude of the excited-state structural changes, in particular, the formation of a bent and elongated octahedral PbI64- geometry. In addition, we have determined the theoretical excited-state geometry and structural changes between the neutral and polaron states using a normal-mode projection method, which supports and rationalizes the experimental results. Thus, this study reveals the polaron formation via nuclear dynamics in perovskite that may be important for efficient and collection.

Explore further: Why perovskite solar cells are so efficient

More information: Amanda J. Neukirch et al. Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials, Nano Letters (2016). DOI: 10.1021/acs.nanolett.6b01218

Polaron Signatures in Excited-State Dynamics of Methylammonium Lead Iodide Perovskites" Nature Communications (2018)

Related Stories

Why perovskite solar cells are so efficient

April 25, 2018

Solar cells with efficiencies above 20% and produced at low costs – perovskites make this possible. Now, researchers of Karlsruhe Institute of Technology (KIT) have gained fundamental insight into the function of perovskite ...

Twisting molecule wrings more power from solar cells

November 14, 2017

Inside a solar cell, sunlight excites electrons. But these electrons often don't last long enough to go on to power cell phones or warm homes. In a promising new type of solar cell, the solar-excited electrons have better ...

Stability challenge in perovskite solar cell technology

December 23, 2016

While solar cell technology is currently being used by many industrial and government entities, it remains prohibitively expensive to many individuals who would like to utilize it.. There is a need for cheaper, more efficient ...

Lead-free, efficient perovskite for photovoltaic cells

June 8, 2018

A KAIST research team has proposed a perovskite material, Cs2Au2I6 that serves as a potential active material for highly efficient lead-free thin-film photovoltaic devices. This material is expected to lay the foundation ...

Recommended for you

Producing defectless metal crystals of unprecedented size

October 19, 2018

A research group at the Center for Multidimensional Carbon Materials, within the Institute for Basic Science (IBS), has published an article in Science describing a new method to convert inexpensive polycrystalline metal ...

Nanodiamonds as photocatalysts

October 19, 2018

Climate change is in full swing and will continue unabated as long as CO2 emissions continue. One possible solution is to return CO2 to the energy cycle: CO2 could be processed with water into methanol, a fuel that can be ...

Shining light on the separation of rare earth metals

October 18, 2018

Inside smartphones and computer displays are metals known as the rare earths. Mining and purifying these metals involves waste- and energy-intense processes. Better processes are needed. Previous work has shown that specific ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

holoman
not rated yet Jul 20, 2018
photon induced electric field poling of perovskites :

https://drive.goo...T90/view

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.