Photophysics in Organic Semiconductors

What is it about?

Photoinduced charge generation forms the physical basis for Organic Photovoltaics (OPVs), a promising light-to-electricity conversion technology. The fundamental processes involved are light absorption by organic semiconductors (generally π-conjugated polymers) to generate photoexcited states (Frenkel excitons) followed by charge transfer and charge separation in presence of suitable acceptor (fullerene). For maximum device efficiency, the absorbed photon energy must be utilized completely. However progressive relaxation losses of high-energy or hot-excited states form major bottleneck for maximum derivable voltage (Voc). Recently, this efficiency limiting factor is challenged by the role of hot-carriers in efficient charge generation. Therefore if dissociation of hot-exciton is tailored to be temporally faster than all relaxation processes, then energy loss pathways could be minimized. Implementation of this concept of hot-carrier photovoltaics demands critical understanding of molecular parameters that circumvent all energy relaxation processes and favor hot-carrier generation. In my dissertation work, I have examined the fate of excitons in the context of polymer backbone and morphology, and therefore obtain a fundamental structure-function correlation in organic semiconductors.

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Photo by Mariana Proença on Unsplash

Photo by Mariana Proença on Unsplash

Why is it important?

Photoinduced charge transfer (CT) at donor (π-conjugated polymer) and acceptor (fullerene) molecular interfaces is central to the design of efficient organic photovoltaics. The CT reaction is critically dependent on the nature of organic backbone and their molecular packing. However it remains a challenge to track all molecular coordinates that drive the CT process after generation of photoexcited states or excitons. In my PhD thesis, I have probed the fate of excitons in π-conjugated polymers while correlating it to its backbone structure and packing morphology. We built a femtosecond stimulated Raman spectrometer to monitor realtime structural changes in donor-π-acceptor backbone, and enumerate the role of π-bridge torsions during intramolecular CT (Nature Communications 2017). Our results highlighted that synthetically tuning the π-bridge torsion timescale will ensure hot-CT generation while slowing down energy-loss pathways. In addition we found that crystallinity of polymer film tuned via side-chain engineering can be correlated with the generation of hot-carriers. Remarkably tracking CT within solution aggregates of polymer:fullerene, we provided the first experimental evidence for polymer:fullerene:solvent ternary phase that seeds the eventual film morphology (Nanoscale 2016). Therefore optimizing the ternary interaction in solution itself is crucial for designing polymer film morphology for efficient solar cells. Morphological control on packing of an emerging class of organic semiconductors also allowed for the discovery of singlet fission within crystalline nanoaggregates. My thesis therefore opens up new guidelines for increasing the yield of hot-carriers in organic photovoltaics.

Perspectives