Bulk heterojunction (BHJ) organic photovoltaic (OPV) devices have been extensively studied to achieve power conversion efficiencies well above 10%. New research into alternative materials that allow for cheaper and more industrially viable fabrication of flexible solar cells has been at the forefront of the energy field for the past few years. With these new materials have come new problems in the form of uncontrollable morphologies and reduced performance. Consequently, my thesis has focused on methods for understanding and controlling morphology in BHJs consisting of conventional and unconventional OPV materials. In Chapter 2, the effects of polymer molecular weight (Mn) on the aggregation and crystalline properties of the polymer material are explored. Two polymers are studied, one amorphous and one semi-crystalline polymer. Three perylenediimide (PDI) materials with increasing degrees of intermolecular aggregation are paired with the two polymers to compose polymer:PDI BHJs. The maximum performance point and the corresponding morphology are located through a combination of transmission electron microscopy and grazing incidence wide-angle X-ray scattering analysis. The concept of morphology templating is introduced and extensively developed to understand how morphology is determined in the film. Guidelines for pairing PDIs with appropriate polymer molecular weights are developed based on these results. In Chapter 3, this work is expanded in the form of polymer:polymer BHJ analysis using polymer Mn, crystallinity, and aggregation as variables for optimization. The pairing of an amorphous donor polymer with a semi-crystalline acceptor polymer while varying Mn of both polymers and observing the shifting performance had yet been reported. Clear control of the morphology is demonstrated with this method. The impact on morphology and the resulting performance through tunable aggregation yields a map for determining optimal polymer:polymer BHJ composition. The concept of morphology templating is further expanded to encompass polymer:polymer BHJs. In Chapter 4, the impact of fluorination on the resulting morphology and performance of small-molecule donors when paired with fullerene acceptors is explored. Ternary devices, containing two unique donor materials and one acceptor material, are formed using a non-fluorinated and fluorinated donor small-molecule to achieve a highly tunable open-circuit voltage (Voc) and enhanced short-circuit current density (Jsc). Improved absorption is determined as the source of the Jsc enhancement, resulting from increased crystallinity in the BHJ upon a small addition of the ternary material. Finally, in Chapter 5 an exceptionally high performing non-fullerene acceptor is implemented with three polymeric materials with decreasing highest occupied molecular orbital (HOMO) levels to observe the impact on Jsc, fill factor, and overall photovoltaic performance. Distinct differences in morphology resulting from changes in aggregation are observed between the three polymers, exhibiting differences in domain sizes and correlation lengths. The slightly shifting HOMO levels appear to have a large impact on the overall device performance, with a sharp increase in the rate of hole transfer from the non-fullerene acceptor to the polymer donor with a slightly lower HOMO level, resulting in over 8% efficiency.

Last modified

• 01/29/2019

Creator

• Nicholas Dalton Eastham

DOI

• https://doi.org/10.21985/N27X68

Subject

• Chemistry

Keyword

• organic solar cells
• molecular weight
• nonfullerenes
• aggregation
• crystallinity

Date created

• 2017-01-01

Resource type

• Dissertation

Rights statement

• In Copyright