Charge-Transfer Oligomers and Polymers for Organic Photovoltaics: Structure, dynamics, and their implications for solar devices.

摘要

Charge-transfer oligomers and polymers, whose adjacent building blocks differ in their electron affinities, are promising materials for organic photovoltaic (OPV) devices, achieving higher power conversion efficiencies (PCE) than benchmark homopolymers. However, the effects of this intramolecular charge transfer on OPV device function are not well understood, making optimization of these materials inefficient. In this thesis, the aggregation characteristics and dynamics of these systems are investigated to build a model for how the charge transfer character between these conjugated building blocks affect OPV device function.;The first system, the M series, is a thiophene:thienothiophene oligomer series that exhibits distinct self-assembly properties depending on their length. For oligomers of moderate length, long crystalline fibers are formed upon spin-coating. However, for shorter or longer oligomers, no discernible pattern forms. Using grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM), the relationship between molecular aggregation and macroscopic structure self-assembly in this system is obtained.;The second system, the PTB series, is an alternating thienothiophene: benzodithiophene copolymer library, exhibiting one-time record-breaking power conversion efficiencies in OPV devices. However, these polymers have a poorly understood variation of 2–8% PCE as a function of pendant moieties along the backbone. Therefore, the molecular aggregation is characterized by grazing incidence X­ray diffraction and referenced to the bulk characteristics of corresponding OPV devices.;Ultrafast optical transient absorption measurements were performed on PTB polymers in solution, neat films, and bulk heterojunction (BHJ) films, in order to understand their intramolecular, donor:donor, and donor:acceptor dynamics, respectively. In these studies, exciton dissociation is observed even without an electron acceptor; and the extent of this intramolecular charge pair dissociation is correlated to corresponding device PCE. This finding is not explained in existing OPV models.;The BHJ studies reveal that the system has a memory of the intramolecular polymer dynamics long after electron transfer to the PCBM acceptors. Based on these studies, a model is proposed to explain how ultrafast information can be remembered by the system hundreds of picoseconds later, and how this process relates to the side moieties adorning the polymer fragments. This understanding has the potential to guide future OPV design.