Control of Vibronic Coupling in Hybrid Perovskites and its Impact on Charge Transport

Project Information

PeroVIB

Grant agreement ID: 101018002

DOI

10.3030/101018002

Project closed

EC signature date 17 March 2021

Start date 1 July 2021

End date 30 June 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 224 933,76

EU contribution

€ 224 933,76

Coordinated by

IMPERIAL COLLEGE OF SCIENCE TECHNOLOGY AND MEDICINE
United Kingdom

Periodic Reporting for period 1 - PeroVIB (Control of Vibronic Coupling in Hybrid Perovskites and its Impact on Charge Transport)

Reporting period: 2021-07-01 to 2023-06-30

Summary of the context and overall objectives of the project

To achieve the net-zero emission, we must consider sustainable forms to convert and store energy. Solar cells involving perovskite materials, either alone or as perovskite/Si-tandem devices, emerging as potential alternatives considering high efficiency (~ 29-30%) and cost-effectiveness could become leading renewable-energy technologies. Despite enormous performance progress, their structural instability limited commercialization. Structural fluctuations in the broadly studied ‘soft’ semiconductors like perovskites were shown to play an important role in driving their exceptional optoelectronic properties, while this also introduces structural instability. Intense research interests are now geared towards improving structural dynamics of perovskites as well as electronic properties/dynamics separately, while owing to the soft lattice character, electronic and structural dynamics are highly entangled and time dependent. This entanglement leads to the formation of mixed ‘hybrid’ electronic-vibrational states (i.e. polarons). Characterization of those states calls for a development of an experimental toolkit which would rely on simultaneous probing of electronic response under vibrational stimulation with ultrafast time-resolution.
Keeping this in mind, we develop action spectroscopy in which the interaction of light with the system under study is evaluated based on measuring the ‘outcome’ of the interaction i.e. either photocurrent or photoluminescence in this case. Interaction of the light with the system was implemented through double-resonant excitation (infrared pre-excitation followed by a visible laser) based vibrationally promoted electronic resonance (VIPER) spectroscopy. Development of this new research methodology is anticipated to become a versatile experimental tool in future to study the carrier-phonon coupling in soft semiconductor class.
The main goal of the project is to capture and control this electronic-vibration coupling of hybrid perovskites such that we can build a design principle towards structure-function relationship in perovskites. Finding from this project reveals vibrational mode-selective coupling of the organic cations with the inorganic lattice for a short time-period and this coupling disappears when the organic cation reorients and breaks the H-bonding interaction with the halides. In a larger picture, this coupling is responsible for the intrinsic non-radiative loss channels in the hybrid perovskites. Therefore, our findings demonstrate for the first time that apart from extrinsic loss-factors (like grain boundaries, various trap-centers), controlling the intrinsic vibronic coupling strength through manipulation of the organic cation structure one could be able to increase the efficiency of perovskite solar cells.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Overall results from this PeroVIB project can be divided into two sections-

Development of VIPER experimental set-up and key findings from other IR-optical control set-up: One of the key ingredients for this project to be successful is the development and proper implementation of the proposed spectroscopic approach; VIPER. I spent significant time at first to understand and explore the existing version of IR-optical control spectroscopic approaches (pump-push) available with the ultrafast optoelectronics group of my host. As this would formulate the basis to design and optimize the VIPER set-up based on our requirement. This initial exploration of pump-push spectroscopy enabled me to understand hot-carrier cooling processes in perovskite nanosystems to traditional monolayer based 2D semiconductors. Key findings from these experiments were how the presence of electronic defects could influence/compete the hot-phonon bottleneck behavior in those systems. I further conclude hot-carriers defect tolerance behavior in composition space and found that CsPbI3 systems are highly defect-tolerant compared to other halide counterparts. This understanding of role of phonons density on hot-carrier cooling is very intriguing but not sufficient enough to resolve the individual role of organic and inorganic sub-lattices. This is another driving force to launch this vibrational mode-selective VIPER experiments.

Employment of the VIPER set-up in realizing role of vibrations of organic-cations on electronic properties: This was achieved through employment of VIPER experiments with IR-selective excitation of N-H scissoring and N-C=N stretching of formamidinium lead bromide perovskites followed by sub band-gap visible photons. Combined interactions from these pulses produce photocurrent and photoluminescence when photovoltaic device and nanocrystal film was studied, respectively. Analysis of frequency- and time-domain data reveals specific coupling of the N-C=N stretching with the electronic state for a short time period (~ 300 fs). This finding is expected to be published soon as a full paper. While this coupling dynamics persists for a longer time (tend to vibrational dephasing time) when the experiments were performed at cryogenic temperature (80K) indicating much stronger H-bonding network with the halides. Similarly, my extended efforts show that this coupling could be modified in composition space by mixing some guanidinium cations with them. Furthermore, to develop a broader picture we have been exploring the role of organic cations on some low-dimensional perovskites as well.

Exploitation and dissemination:

All the experimental results were analyzed/discussed with my supervisor and PhD students involved in the group and thereafter phenomenological model or theoretical simulations were undertaken with the help of collaborators to enhance our understanding of the topic. The results were disseminated in presenting these research contents in various reputed conferences from ultrafast spectroscopic community to the interface of material science. Some of these results are already published and two of them will be published soon and rest of the 3-4 manuscripts are in ready to be communicated or in a process to develop a larger picture.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project implementation brought following progress beyond the state-of-the-art:

1. Development of this versatile experimental platform to rationalize the vibronic coupling (electron-phonon coupling) in wide variety of soft semiconductor systems. As there is no direct approach to capture this vibronic coupling, this technique could be a game changer in future.

2. Discovery of the direct influence of organic cations (despite their no contribution in frontier electronic orbitals) on modulating electronic properties of perovskites would open up a new direction of studies to engineer the optoelectronic properties by manipulating the structural deformations (vibrations).

3. One of the broader impacts of this project is we reveal for the first time how one could control intrinsic non-radiative losses in perovskite photovoltaic devices. This would be a key guideline for the device developers and material engineers to design suitable materials with suppressed intrinsic loss pathways through manipulating the organic cation functionality/sizes in hybrid perovskite systems. This research finding might replace the existing trial and error nonintuitive approach relied towards prospective materials development.

Summary for publications page 2