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Final Report Summary - ELECTPROPOIPC (Atomically thin layers of Organic-Inorganic Perovskite Crystals – Electronic Properties and Application in Solar Cells)

Lead-halide hybrid perovskite crystals have emerged as promising materials for inexpensive and efficient solar cells. The rapid increase in solar cell power conversion efficiency (>20%) provides impetus to gain fundamental understanding of the interplay between their electronic and structural properties. Within the framework of this project we studied the unique structural dynamics and electronic properties of both three dimensional (3D) and two dimensional (2D), ultrathin organic inorganic perovskite crystals (OIPCs). As the study progressed, it became clear that the special motion of the atoms is a generic property that applies to other organic semiconductors such as conducting polymers and small molecule organic crystals. Thus, in the final part of the project we utilized the tools gained in the outgoing phase (i.e. low-frequency Raman scattering) to commence a study on conducting polymers.

The original project goals were to introduce OIPC as a new family of materials for Van der Waals heterostructures and study their electronic and excitonic properties. As the research progressed, we realized that unconventional structural dynamics (i.e. motion of atoms in the solid material) are responsible for many of the favorable electronic properties of the material and we commenced an ultra-low Raman scattering study to understand atomic motion.
In the first period, OIPCs were synthesized, exfoliated and there excitonic and structural properties were studied in detail and electrical contacts were deposited on a single layer using e beam lithography. Two full articles were published on the subject in Physical Reviews B (DOI: 10.1103/PhysRevB.92.045414) and the Journal of American Chemical Society (DOI: 10.1021/ja512833n). As work progressed, it became increasingly clear that the mechanical stability and chemical homogeneity of the n=2 and 3 OIPC exfoliated layers is low and the our goal was revised to study the structural dynamic of these crystals using, primarily, terahertz (THz) range, Raman spectroscopy. A first manuscript on the subject is now accepted to physical review letters.

From career development standpoint, I gained experience in a wide range of spectroscopic methods including state-of-the-art THz range Raman microspectroscopy that allowed me to obtain a permanent position as a tenure track senior researcher in the Weizmann institute which will begin immediately after the completion of the project.
Main results include the demonstration, for the first time, of exfoliation and isolation of perovskite single layers as well as a detailed study of the layers excitonic properties. The results also include a comprehensive study of the unique structural dynamics of these materials using, THz domain Raman scattering.

Following the successful application of THz-range Raman spectroscopy, the goal of the second period was revised to take full advantage of my newly acquired expertise. We identified an excellent opportunity to combine newly gained skills with the expertise of the Technion group (the host) and study the interplay between structural dynamic and electronic properties in conjugated polymeric films. This study was include the assembly of THz-domain Raman micro-spectroscopy setup and characterization of the low frequency structural modes of the polymeric film in a field effect transistor.

Conjugated, semiconducting polymers have a substantial technological appeal in the field of printed electronics and organic photovoltaic cells (OPVs). They are macromolecules with numerous degrees of conformational freedom, resulting in microstructures varying from amorphous to semi-crystalline. The interplay between microstructure and electrical properties in conjugated polymers is not well understood. Specifically, it is not clear why some conjugated polymers exhibit relatively high electron mobility and some do not. This knowledge gap arises, partially, from the structural complexity of the polymer films, that substantially varies across the order-disorder scale. The primary experimental techniques to study structural confirmation in polymer films are XRD and electron microscopy. Little work has been reported using spectroscopic, vibrational techniques that offer many advantages with respect to spatial resolution and characterization of the instantaneous structure in contrast to the averaged structure probed by XRD.

During the second period we initiated a study on the low-frequency structural dynamics of conjugated polymeric films. Our results, indicate that there is substantial low-frequency motion that can potentially dominate the charge through the films. Unfortunately, photoluminescence from the film surface limited our ability to measure the Raman scattering with sufficient signal to noise ratio. Therefore, I started building a measurement system with near infrared laser that should reduce substantially the photoluminescence.

The expected implications of these studies is to obtain fundamental understanding of the correlation between the electronic properties and structural dynamics of soft semiconductors. This may result in set of design rules for organic-based photovoltaic and light emitting diode application. The development of inexpensive photovoltaic cell based on earth abundant materials is imperative for global reduction of CO2 emission caused by energy consumption.

Reported by

TECHNION ISRAEL INSTITUTE OF TECHNOLOGY
Israel

Subjects

Life Sciences
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