The chemistry and device physics of organic solar cells based on non-fullerene acceptors

"This project falls within the EPSRC Solar technology, Optoelectronic Devices and Circuits, and Materials for Energy Applications research areas. Organic solar cells (OSCs) have the potential to be next-generation renewable energy harvesters due to their lightweight, solution processability, flexibility and semi-transparency. Recent inventions of record high performance non-fullerene fused ring electron acceptors (FREAs) have increased power conversion efficiencies (PCEs) to over 19% in a single cell device.

Conventionally in organic photovoltaics (OPV), excitons (bound electrons and holes) are intrinsically photogenerated due to the photoactive layer having a low dielectric constant causing excitons with high binding energies and therefore leading to poorer device performance. Thus, separating excitons into free charge carriers requires a heterojunction between donor and acceptor molecules.

However, this heterojunction has been reported to cause instabilities at the interface and limits the PCE, therefore this work will solely focus on single-component homojunction OSCs.

The FREAs that will be investigated throughout the PhD project is Y6, COTIC-4F and COTIC-4Cl. The Y6 molecule is among the common FREAs that has demonstrated an acceleration in PCE. It has an acceptor-donor-acceptor (A-D-A) structure consisting of a core, two electron accepting terminal moieties and solubilising alkyl substituents.

Y6 has proven to intrinsically generate free charge carriers (rather than excitons) without a heterojunction giving hope to the possibility of efficient homojunction devices. COTIC-4F/4Cl are novel narrow bandgap non-fullerene acceptors containing an A-D-D-D-A structure that can enhance intramolecular charge transfer and also lower the optical bandgap to 1.10 eV.

As 50% of solar radiation intensity lies in the near infrared region, possessing a low optical bandgap is therefore desirable to harvest solar radiation. As of yet, it has not been reported whether these narrow bandgap acceptors can also intrinsically generate free charge carriers in neat films. Thus, this work will research the charge dynamics in neat films and if successful single component homojunction devices will be fabricated.

Another active area of research will be the introduction of dopants to the COTIC-4F/4Cl photoactive layer to improve the charge transport properties of OSCs. Along with a significant number of free charge carriers generated by the doping process, device performance-enhancing morphological impacts such as optimised crystallinity and reduced trap density can occur concurrently.

By simultaneously performing both p and n-type doping to the active layer, the aim is to form a p-i-n junction that will enable an efficient transport of charge carriers towards the metal contacts.

FREAs have permitted the current growth in PCE, but only for solution-processed systems. Vacuum processed OSCs were found to have a higher morphological stability than solution processed OSCs which could be due to susceptibility of side chain degradation, and molecules finding near equilibrium structures during film growth. The key advantages of vacuum coating processes are that they are inexpensive and fast to coat large surface areas.

Coupling this with minimal material consumption, low temperature processing and compatibility with flexible substrates, this could potentially make OSCs the cheapest source of electricity in the world. Y6 is too large to be vacuum processed thus, by synthetically removing the bulky alkyl side chains should make it small enough to be vacuum processed."