Opto-electrical and magnetic resonance investigation of spin-dependent processes in Polymer based Organic Photovoltaic Devices

Date: 19 September 2016

Venue: UAntwerpen, Campus Groenenborger, U0.24 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger)

Time: 4:00 PM

PhD candidate: Biniam Zerai Tedlla

Principal investigator: Etienne Goovaerts

Short description: PhD defence Biniam Zerai Tedlla - Faculty of Science, Department of Physics



Abstract

Organic solar cells are promising as environment-friendly energy sources. In order to improve the yet limited power efficiency and long-term stability, a profound knowledge of the photophysics is needed. This work focuses on the photophysical processes that can have a negative impact on the performance of organic solar cells based on the polymer:fullerene bulk heterojunction (BHJ). To this end spin-sensitive EDMR and PLDMR (magnetic resonance detected via electrical conductivity and photoluminescence, respectively).

In BHJ devices based on the polymer super-yellow PPV (SY-PPV) mixed with various fullerenes, the energetically low-lying triplet excitons (TEs) in the fullerene component were detected and the channels through which they formed were investigated: (i) singlet excitons (SEs) undergo intersystem crossing (ISC) towards TEs in the polymer followed by Dexter energy transfer – a relatively inefficient pathway, (ii) polymer  SEs undergo Förster energy transfer towards the fullerenes were ISC happens, and (iii) SEs decay via the charge transfer (CT) states along the interface towards fullerene TEs – the latter only in combination with relatively high-lying CTs

Application of a so-called {em sensitizer} – here a metallated molecule PdTPBP – for energy upconversion based on triplet-triplet annihilation (sUC-TTA) is a promising approach to collect low-energy photons in the solar spectrum. sUC-TTA showed significant upconversion in the liquid phase, but this could not be reproduced in the solid phase. In blends of SY-PPV with PdTPBP, we could correlate the decrease of upconverted photons at higher sensitizer fraction with the TE population in the polymer. Triplet-triplet annihilation (TTA) in the sensitizer was previously put forward as parasitary process. Our investigation however shows that TE-polaron annihilatie plays an important role, involving polarons originating from the polymer component. We recommend a judicious adjustment of the HOMO-HOMO alignment with an energy barrier frustrating this polaron migration.

In highly efficient organic solar cell, the device structures become more complex aactive layers with multiple materials are employed. Methods that can selectively identify excitations are welcome to complement the conventional techniques. In this thesis we offer illustration of the invaluable contributions that EDMR and PLDMR spectroscopies can offer to nravel the complex processes occurring in organic solar cells.



Link: https://www.uantwerpen.be/en/faculties/faculty-of-science/