Ruben Lenaerts invites you to the public defense of his doctoral thesis entitled:

"Development of novel donor and acceptor materials for fullerene-free organic photovoltaics".

Promoter: Prof. Dr. Wouter Maes

Copromoter: Prof. Dr. MSc Koen Vandewal and Dr. Laurence Lutsen

Abstract:

In the state-of-the-art lab-scale organic photovoltaic devices, quite complex donor and acceptor materials are used. Additionally, device stability studies on these high-efficiency active blend donor/acceptor combinations are scarce. In this thesis, some novel electron-accepting materials were developed in conjunction with suitable donor materials, with a particular eye on low synthetic complexity and stability.

Four acceptor-type polymers based on the bis(perylene diimide) electron-deficient monomer were synthesized. These acceptor polymers were chosen for their low synthetic complexity. In polymer solar cells in combination with PTB7-Th as the donor polymer, average power conversion efficiencies (PCEs) ranging from 3.2 to 4.7% were achieved. The photostability in air was also studied. It was found to be significantly higher than for two standard non-fullerene acceptors (ITIC and FBR).

Favorable properties are often offered by the fluorination of the active materials in organic solar cells. Chlorination, which is a lot more attractive than fluorination from a synthetic point of view, has been shown to also yield these advantages, but is significantly less applied. Three donor copolymers based on 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene were synthesized with a varying degree of halogenation of the thienyl substituents. Photovoltaic devices were fabricated with the small molecule acceptor ITIC and two polymer acceptors, the well-known PNDI(2OD)2T and its chlorinated variant PNDI(2OD)2T2Cl. The chlorinated donor polymer afforded the highest average PCE of 8.2% in the ITIC series. Although the highest Voc values were reached for blends of the fluorinated donor polymer, up to 0.98 V with ITIC, and these blends also possessed the lowest Voc losses, the fluorinated donor polymer afforded the lowest device efficiencies with all three acceptors. As the PBDTClT-TQxT donor polymer clearly gave the most promising results combined with an easier synthesis protocol, we can conclude that chlorination is an undervalued approach to afford high-performance OPV materials and blends.

The degradation of P3HT:FBR solar cells, both materials with a low synthetic complexity, was shown to be very fast. Illumination of the P3HT:FBR solar cell devices in air resulted in a 60% PCE loss after two hours and even 95% after 20 hours. It was clear from this study that non-encapsulated P3HT:FBR solar cell devices are not stable in air and even under nitrogen atmosphere low device stabilities were observed. In collaboration with Antwerp University, the exact degradation mechanisms were found.