Recent developments in research concerning organic photovoltaics (OPVs) have overseen massive increases in device performance and the ascension of electron acceptor materials that outclass the preeminent acceptor compounds, buckminsterfullerene (C60) derivatives. New design strategies in the molecular structure of perylenediimides (PDIs) and fused-ring electron acceptors (FREAs) have increased single-junction photovoltaic efficiencies to values greater than 17%. This dissertation investigates the excited-state dynamics of various molecular electron acceptors and how chemical structure modifications influence these. Recent advances in the design of perylenediimide acceptors has demonstrated high efficiencies in molecular structures incorporating several PDI chromophores with highly twisted geometries and extended π-conjugation. Headland substitution on the PDI core gives consistently higher device performance than does equivalent bay-substitution. Model compounds with this substitution pattern (o-PDI2, m-PDI2, p-PDI2) show enhanced intersystem crossing and in the case of o-PDI2, symmetry-breaking charge separation. PDIs are also capable of enhanced charge delocalization. In a model triad with two PDIs oriented cofacially to one another, photoinduced electron transfer is increased by a factor of 50% compared to a single PDI unit. This effect is limited to two PDIs, however, as no additional rate increased is observed in the three-PDI compound ZnTPP2-PDI3, owing to weak electronic coupling across all three PDIs. Regardless, PDIs can efficiently delocalize electrons in a conjugated motif that exhibits high solar cell performance, as demonstrated by the blending of Ph(PDI)3 with two donor polymers. The highly fluorinated polymer PBDTTF-FTTE paired with this molecule boasts a power conversion efficiency (PCE) of 9.1% through increased phase separation and reduced bimolecular recombination compared to blends using the fluorine-poor polymer PBDTT-FTTE. More recently designed chromophores of the ITIC and Y6 families of molecules exhibit strong photovoltaic responses exceeding 17%. π-extension and fluorination of both sets of molecules enhance their efficiency in devices. ITN-F4 and ITzN-F4 undergo efficient charge separation despite the formation of an excimer state, while BT-BO-L4F and BT-LIC have enhanced charge transport properties that manifest in accelerated recombination when an external bias is not present. The same is true when blends containing Y6 are thermally annealed, despite this process increasing overall PCE.

Creator

• Alzola, Joaquin Miguel

DOI

• https://doi.org/10.21985/n2-d8gt-pv20

Subject

• Organic chemistry
• Physical chemistry

Language

• en

Alternate Identifier

• etdadmin_upload_842131
• http://dissertations.umi.com/northwestern:15706

Date created

• 2021-01-01

Resource type

• Dissertation

Rights statement

• In Copyright