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Mapping Nanoscale Charge Separation at Heterojunctions with Ultrafast Electroabsorption Microscopy

Periodic Reporting for period 1 - NanoEAscopy (Mapping Nanoscale Charge Separation at Heterojunctions with Ultrafast Electroabsorption Microscopy)

Reporting period: 2017-09-01 to 2019-08-31

The sun is the most abundant and generously available source of renewable energy to the earth. Current consumption of solar energy is far from its enormous potential. Natural photosynthetic systems, found in plants, bacteria, and algae, use this potential with near unity photo-conversion efficiency via photoinduced charge separation (electron-hole separation). Light absorption creates electron-hole pairs which eventually undergo electron-hole separation and create free electrons and holes. Molecular level understanding of natural photosynthetic systems has paved the way for artificial photosynthetic systems. These systems are based on donor-acceptor assembly where photoinduced charge separation occurs at the donor-acceptor heterojunctions. Understanding of charge separation has gathered significant amount of interest. Despite substantial efforts being directed towards understanding the nature of charge separation, a direct visualization of charge separation at the heterojunction has never been realized.
Charge-separation is the key process in photosynthesis as well as in organic semiconductor. Organic photovoltaic devices are flexible and transparent, and showing increasing efficiency. Understanding charge-separation and charge transport shall provide us with proficient schemes for molecular design and device architecture to achieve unprecedented efficiencies. This work will directly impact society since it has the potential to deliver step-increases in the efficiency of light-harvesting devices that will reduce the cost of optoelectronic devices remarkably.
Overall objective of this project is to successfully optimize the ultrafast pump-probe microscope, a very fast camera, as a platform to directly image electron-hole separation at the in-plane organic heterojunctions.
The project started with the development of an ultrafast transient absorption microscope (TAM) capable of performing pump-probe microscopy with sub-10 femtoseconds (fs) time resolution and simultaneously providing sub-10 (fs) nanometres localization precision. Which means we can monitor movements of carriers that occur slower than 10 fs and longer than 10 nm.
Organic compounds, Perylene diimide (PDI) derivative and Pentacene (Pc), were strategically evaporated thermally on a glass cover slide such that in-plane PDI-Pc (donor-acceptor) heterojunction is materialized.
TAM clicks pictures of the charge-separation at the heterojunction at different times starting right from the beginning of the process till the end. Pump laser pulse excites the carriers and creates the charge carrier distribution at the heterojunction and probe pulse monitors the distribution as a function of time, defined by the delay between pump and probe pulses. By analysing the shape and size of the distribution, we calculate how fast and how far the charges (electrons and holes) move. Wavelike transport of charges is observed where charges can move up to more than 10 nanometres in less than 100 femtoseconds.
The findings were disseminated via several invited seminars and also in international conferences as poster presentations. An article on the technique development is recently published in Journal of Physical Chemistry Letters. A manuscript on direct visualization of charge separation at organic heterojunction is under preparation.
Apart from the success of this project, this platform shall provide intricate details about material properties, charge transport and its correlation with spatial inhomogeneity in variety of material that are promising for optoelectronics applications. As optoelectronic devices are becoming an integral part of our daily life their demand is rapidly increasing. Increase in the efficiency of optoelectronic devices will significantly reduce their cost and make them affordable to economical weaker section of the society. The production needs to match the increasing demands which require increase in the required workforce, this will create more jobs and contribute to the economy.
This image summarizes the work done under this fellowship