Understanding enhanced light harvesting of a conjugated polymer

Michelle Vezie^a, Sheridan Few^a, Jenny Nelson^a, Jarvist Frost^a, Mariano Campoy-Quiles^b, Bernhard Dölring^b, Alejandro Goni^b, Iain Meager^c, Raja Shahid Ashraf^c, Iain McCulloch^c

^aDepartment of Chemistry and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London, United Kingdom

^bDepartment of Chemistry, Imperial College London, South Kensington Campus London, London, United Kingdom

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Michelle Vezie, presentation 252
Publication date: 5th February 2015

Increasing polymer absorptivity is a critical parameter for increasing the power conversion efficiency of polymer-based organic photovoltaics given the very low device thickness that is often required for optimum charge collection. We study a series of low bandgap diketopyrrolopyrrole polymers with systematically varied alkyl side chain and molecular weight. The high molecular weight polymers exhibit higher extinction coefficients at low excitation energies than any polymers we have measured previously. Devices made with blends of these polymers with PC₇₁BM exhibit exceptionally high photocurrent densities of 22 mA cm^-2 and power conversion efficiencies of 8.5%. We investigate the origin of this enhanced absorption through detailed experimental measurements, including ellipsometry and ultraviolet-visible spectroscopy on films and solutions. Furthermore, quantum chemical calculations probe the impact of chemical structure and molecular conformation on oscillator strength, which is proportional to change in dipole moment in the transition and the overlap between the initial and final states. Insights from these calculations allow us to propose design rules to enhance light harvesting efficiency in conjugated polymers.

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