In recent years, non-fullerene acceptors (NFAs) have attracted enormous interest in the field of organic solar cells (OSCs), they improve power conversion efficiency (PCE) compared to the classical fullerene acceptor. In this work, OSCs based on PBDB-T as the donor material and the very well-known NFA ITIC, along with its fluorinated and chlorinated derivatives (IT-2F, IT-4F, IT-2Cl, IT-4Cl) were fabricated to investigate the effect of the halogenation end group on the photovoltaic parameters. Optical characterization reveals that both chlorination and fluorination are effective in downshifting the molecular energy levels and redshifting the absorption spectra, which results in higher Jsc but lower Voc compared to pristine ITIC. In addition, the halogenated ITIC device exhibited enhanced FF and PCE. Various optoelectronic techniques were also used to investigate the charge recombination dynamics and charge extraction process. It has been found that (IT-2F, IT-2Cl) show suppressed monomolecular recombination compared to di-substituted NFA (IT-4F, IT-4Cl). Furthermore, fluorinated ITIC has a longer charge carrier recombination lifetime but a lower carrier extraction rate.
Lastly, the best-performing device from the preceding component mixtures PBDB-T:IT-2F was exposed to thermal annealing at different stages of the fabrication process to investigate how annealing affects the photovoltaic parameters. According to our findings, both post and 2-stage annealing improve FF and PCE, but the latter is even more beneficial. In further studies, the annealing effect on the HTL layer (MoOx) has also been investigated. Annealing improved the MoOx’s work function, resulting in higher internal electric field that thereby facilitated hole extraction, as demonstrated by TPC where 2-stage annealed devices exhibited a faster carrier extraction rate.
|Date of Award||Jun 18 2023|
|Original language||English (US)|
- Physical Sciences and Engineering
|Supervisor||Frédéric Laquai (Supervisor)|
- Organic Solar Cells
- Non-Fullerene Acceptor
- Photovoltaic Parameters
- Terminal Halogenation
- Thermal Annealing