Chemistry ETDs

Author

Fei Li

Publication Date

9-12-2014

Abstract

Current state-of-the-art polymer solar cells adopt the bulk-heterojunction (BHJ) morphologies where the electron donors (i.e. conjugated polymer) and electron acceptors (i.e. fullerenes) as active layers are mixed in an intimate way. The phase separation that modulates the exciton diffusion and charge transport in the current BHJ morphology is an uncontrolled process and thus results in random domain sizes of the polymer/fullerene blend. In addition, the polymer and fullerenes in the blend are intrinsically two immiscible materials and they tend to undergo macrophase separation eventually, which leads to deteriorated device performance. One way to address the abovementioned issues is to attach fullerenes onto the polymers covalently or non-covalently, aiming at controlling the phase separations and suppressing the macrophase separation between the polymer and fullerenes. However, either the device performance or the morphology of the active layer is not satisfactory to meet our needs. In my dissertation, I combine block copolymer self-assembly and hydrogen bonding interactions to construct morphologies that are not only thermally stable, but also controllable on the nanoscale. The controllability of the blend morphologies is simply achieved by tuning the fullerene contents in the polymer/fullerene blend. Moreover, solar cells fabricated from such polymer/fullerene blends perform in a comparable way with the benchmark BHJ solar cell however with much enhanced device thermal stabilities. I believe this methodology will shed light on the polymer design and morphology control for the chemists and engineers in this field to obtain high-performing solar cells with better thermal stabilities. Specifically, I started this project by synthesizing the poly(3-hexylthiophene) (P3HT) based all-conjugated block copolymer (BCP) (P4) selectively functionalized with diaminopyrimidine moieties and a thymine tethered fullerene derivative (F1). Strong interactions between P4 and F1 through the 'three-point' complementary hydrogen bonding are studied by 1H NMR spectroscopy, fluorescent spectroscopy, differential scanning calorimetry (DSC) and atomic force microscopy (AFM). Solar cells employing P4 and F1 at different weight ratios as active layers are fabricated and tested. Although the photovoltaic performances of P4/F1 solar cells were not good, the morphology of the blend exhibited tunable nature simply by adjusting the F1 ratios in the blend. Secondly, I modified the synthesis of the BCP and obtained a polythiophene diblock copolymer selectively functionalized with 1-N-hexyl isoorotic acid (IOA) moieties (P8) with a longer P3HT block and a 2, 6-diaminopyridine tethered fullerene derivative (F2). Solar cells employing P8 blended with different weight ratios of F2 and phenyl-C61-butyric acid methyl ester (PCBM) were fabricated and tested. The best power conversion efficiencies (PCEs) were observed in devices made from P8/F2 blends (10/8 by wt.) and ternary blends of P8/F2/PCBM (10/4/4 by wt.) as active layers, which is much better than those from P4/F2 blends. Thermal stabilities of these solar cells were studied in detail by aging tests and corresponding morphological changes were closely monitored by absorption spectroscopy, optical microscopy, AFM and X-ray analyses. The 'three-point' complementary hydrogen bonding interactions between P8 and F2, in cooperation with block polymer self-assembly, were found to not only improve the thermal stability of solar cells significantly but also lead to tunable active layer morphologies. Nanostructures with long-range order were identified in blend films employing P8, which has never been observed before in conventional polymer/fullerene bulk heterojunction (BHJ) films. Thirdly, by employing the P8/PCBM blend, I further developed a novel methodology of constructing stable and controllable conjugated polymer (CP)/fullerene nanostructures. By building in non-covalent interactions between CP nanofibers (NFs) and fullerene derivatives, supramolecular polymer/fullerene composite NFs are obtained in solution for the first time. Specifically, self-assembly of P8 in mixed solvents leads to well-defined NFs decorated with IOA groups on the periphery, onto which PCBM molecules are subsequently attached non-covalently. Formation of such complex structures are studied in detail and confirmed by UV-Vis absorption spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray scattering measurements. Application of these composite NFs (P8/PCBM 10/4, wt/wt) in organic photovoltaic (OPV) devices not only leads to superior performance but also much improved thermal stability and rarely observed long-range ordered morphology, when compared with conventional bulk heterojunction (BHJ) devices. Last but not least, I also investigated the P8/F2 composite nanofibers formation and found out that the width of the composite nanofibers not only depends on the type of the fullerenes added, but also the amount of fullerenes mixed in the blends. Besides, solar cells fabricated from the composite nanofibers blends outperformed their conventional BHJ devices under the same fabricating conditions. Through 1H NMR observations, I also proposed a formation mechanism of the P8 nanofibers that agrees well with our experimental results.

Language

English

Keywords

Block copolymer, Hydrogen bonding, Self-assembly, Polymer solar cells, morphology control

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Advisor

Qin, Yang

First Committee Member (Chair)

Grey, John K.

Second Committee Member

Melancon, Charles

Third Committee Member

Dunlap, David H.

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