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Proceedings Paper

Theory and assignment of intermolecular charge transfer states in squaraines and their impact on efficiency in bulk heterojunction solar cells (Presentation Recording)
Author(s): Christopher J. Collison; Chenyu Zheng; Nicholas Hestand; Brandon Cona; Anirudh Penmetcha; Susan Spencer; Jeremy Cody; Frank Spano
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Paper Abstract

Squaraines are targeted for organic photovoltaic devices because of their high extinction coefficients over a broad wavelength range from visible to near infra-red (NIR). Moreover, their side groups can be changed with profound effects upon their ability to crystallize, leading to improvements in charge mobility and exciton diffusion. The broadening in squaraine absorption is often qualitatively attributed to H- and J-aggregates based on the exciton model, proposed by Kasha. However, such assignment is misleading considering that spectral shifts can arise from sources other than excitonic coupling. Our group has shown that packing structure influences the rate of charge transfer; thus a complete and accurate reassessment of the excited states must be completed before the true charge transfer mechanism can be confirmed. In this work, we will show how squaraine H-aggregates can pack in complete vertical stacks or slipped vertical stacks depending upon sidegroups and processing conditions. Hence, we uncover the contribution of an intermolecular charge transfer (IMCT) state through essential states modeling validated by spectroscopic and X-Ray diffraction data. We further show external quantum efficiency data that describe the influence of the IMCT state on the efficiency of our devices. This comprehensive understanding of squaraine aggregates drives the development of more efficient organic photovoltaic devices, leading towards a prescription for derivatives that can be tailored for optimized exciton diffusion, charge transfer, higher mobilities and reduced recombination in small molecule OPV devices.

Paper Details

Date Published: 5 October 2015
PDF: 1 pages
Proc. SPIE 9567, Organic Photovoltaics XVI, 95670R (5 October 2015); doi: 10.1117/12.2188850
Show Author Affiliations
Christopher J. Collison, Rochester Institute of Technology (United States)
Chenyu Zheng, Rochester Institute of Technology (United States)
Nicholas Hestand, Temple Univ. (United States)
Brandon Cona, Rochester Institute of Technology (United States)
Anirudh Penmetcha, Rochester Institute of Technology (United States)
Susan Spencer, Rochester Institute of Technology (United States)
Jeremy Cody, Rochester Institute of Technology (United States)
Frank Spano, Temple Univ. (United States)

Published in SPIE Proceedings Vol. 9567:
Organic Photovoltaics XVI
Zakya H. Kafafi; Paul A. Lane; Ifor D. W. Samuel, Editor(s)

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