Chemistry and Chemical Biology ETDs

Publication Date

11-1-2016

Abstract

Most conjugated molecules including polymers and small molecules applied in organic solar cells (OSCs) have linear structures containing multiple aromatic groups connected in series. However, unfavorable film forming ability and grain boundaries both originated from high crystallinity of linear small molecules are detrimental to device performances. Thus, among multi-dimensional structures, tetrapodal molecules are especially interesting owing to their unique ability to mutually interlock, which prevents dislodging and provides high structural stabilities. Such molecular design can also increase absorption cross-sections and provide more extensively percolating pathways for charge transport, making such molecular tetrapods promising in OSCs applications. In my dissertation, I will not only include the synthesis and characterization of tetrapodal molecules, but also discussing structure-property relationships of such 3-D small molecules and their applications in multi-component OSCs. Besides, I also include the synthesis and characterization of a series of novel fullerene-borate ionic complexes, and their potential application will be discussed, too. To start with, the tetrapodal molecule SO, containing a tetraphenylsilane core and four cyanoester functionalized terthiophene arms, was firstly discussed. Absorption, X-ray scattering and differential scanning calorimetry (DSC) experiments indicate crystalline nature of SO but very slow crystallization kinetics. Solar cells employing SO and phenyl-C61-butyric acid methyl ester (PCBM) were fabricated and evaluated. Relatively low performance was obtained mainly due to the lack of optimal phase separation under various processing conditions including as-cast, thermal annealing and solvent annealing. Addition of poly(thienylene vinylene) (PTV), a low bandgap highly crystalline conjugated polymer, into the SO/PCBM blend was found to induce device favorable phase separation and the polymer was found to act as the major hole conductor. Such ternary blend devices showed cooperatively improved performances over binary devices employing either SO or PTV alone. Since our previous studies on SO and its model compound MO indicate that the slow crystallization behavior in the tetrapod may intrinsically originate from each of its arms, we conjecture that we may be able to increase the crystallization kinetics of these tetrapods by removing the alkyl side chains in the middle of each arm and by using more rigid and planar electron-accepting moieties such as the fluorinated benzothiadiazole (FBTD) units. Thus, a modified tetrapodal molecule SFBTD was synthesized successfully. However, absorption spectroscopy, DSC and XRD experiments reveal low degree of crystallinity in this compound and slow crystallization kinetics. Bulk heterojunction (BHJ) OSCs employing SFBTD and fullerene derivatives exhibit power conversion efficiencies (PCEs) up to 1.05 % and open-circuit voltage (VOC) values as high as 1.02 V. To the best of our knowledge, this is the highest PCE obtained for OSCs employing molecular tetrapods as donor materials. These devices are relatively thermally stable due to the known ability of breakwater tetrapods to inter-lock, preventing dislodging and sliding. The lack of favorable phase separations and low hole mobilities of the blend films are the major factors limiting the device performance. Ternary blend devices by the addition of three low bandgap PTV derivatives were fabricated and discussed. In the last chapter, a series of fullerene-borate ionic complexes were synthesized successfully. Various chromophores can be introduced into our fullerene-borate ionic complex system through facile Sonogashira coupling reaction, so the optical and electrical properties of complex can be easily tuned. The fluorescence quenching study on FP-Ant indicated the photo-induced charge transfer in our complex system, while the existence of long-lived charge separated states is under exploration. The success of obtaining single crystal of FP-Ph gives us some insight to develop the infinite crystalline structures of fullerene-borate ionic complex through ionic interaction between two counter ions. We expect our fullerene-borate ionic complex will also have promising electrical and magnetic properties as some fullerene complexes reported previously.

Project Sponsors

University of New Mexico

Language

English

Keywords

Organic solar cells, molecular tetrapods, power conversion efficiency, fullerene-borates ionic complex and photo-induced charge transfer

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Yang Qin

Second Committee Member

John K. Grey

Third Committee Member

Wei Wang

Fourth Committee Member

Sang Eon Han

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