Charge Transport through Inhomogeneous Polymeric Materials
Open Access
- Author:
- Vakhshouri, Kiarash
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 18, 2013
- Committee Members:
- Enrique Daniel Gomez, Dissertation Advisor/Co-Advisor
Michael John Janik, Committee Member
Scott Thomas Milner, Committee Member
John B Asbury, Special Member - Keywords:
- Charge mobility
X-ray diffraction
Intracrystalline
Intercrystalline
Crystallinity
Morphology
Structure
Miscibility - Abstract:
- The generation of unique properties through mixing of organic semiconductors has enabled improved performance and novel functionalities in organic electronic devices. In organic light emitting diodes (OLEDs), isolated phases of a second material within the photoactive layer can act as recombination centers, enhancing the overall device performance. Mixing of flexible polymer semiconductors with high-mobility small organic molecules can yield high-performance flexible thin film transistors. Solution-processed, bulk-heterojunction (BHJ), thin-film organic solar cells rely on the self-assembly of polymer/fullerene donor/acceptor mixtures to create the necessary morphology with a high interfacial area for efficient photocurrent generation. Efficient conversion of absorbed photons into photocurrent requires sufficiently intimate mixing of the donor and acceptor phases such that photogenerated excitons can easily find an interface, as well as a sufficiently large thermodynamic driving force for charge separation at the interface. At the same time, efficient transport of separated charges towards the electrodes requires a certain degree of phase segregation between the two materials, to enable ordered molecular packing within each phase and also minimize interfacial recombination. Despite the importance of creating inhomogeneous mixtures of organic semiconductors and the tremendous recent advances in the performance of the aforementioned devices, it remains a challenge to fully describe the optoelectronic properties of organic semiconductor mixtures and understand the effects of structural and morphological parameters on charge transport. Recently, it has been shown that highly regioregular poly(3-hexylthiophene) (RR-P3HT) and poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT) are promising materials for organic electronic applications due to the relatively high charge carrier mobility, high solubility in different organic solvents and acceptable film-forming properties. Charge carrier mobility in polymer semiconductors depends critically on crystallinity of the ordered regions, orientation of the conjugated lamellae, π-π stacking, and also connectivity between ordered regions. By varying the thermal annealing parameters and casting solvents, we have systematically studied charge transport within poly(3-hexylthiophene) (P3HT) and poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT) thin films as a function of the crystallinity and found that there is not a universal relationship between crystallinity and mobility. Using an aggregate model for absorption spectra of P3HT thin films suggests that the conjugation length for P3HT thin films modulates charge mobility. Higher boiling point solvents result in higher conjugation length and consequently higher mobility. Our polarized soft X-ray scattering (P-SoXS) studies revealed anisotropic scattering profiles for P3HT thin films with higher charge mobility suggesting that longer intracrystalline order leads to higher mobilities in P3HT thin films. This is remarkable since anisotropic scattering has not been reported for thin films spun cast from a single component. In the case of PBTTT thin films, our P-SoXS studies confirm that higher orientational correlation lengths leads to higher charge carrier mobilities, but more importantly, through measuring thin-film modulus by buckling phenomena we could correlate mechanical properties and orientational correlation length (OCL) with PBTTT charge mobilities. Our results reveal that interconnectivity among crystallites modulates the charge mobility for PBTTT thin films and higher number of tie chains leads to higher OCL and consequently higher mobility. We also found that the charge mobility depends on the crystallization kinetics; rapid crystallization can affect the microstructure of polythiophenes by increasing the density of tie molecules. As a consequence, there are more pathways connecting ordered regions which results in higher charge carrier mobilities. Our results suggest that controlling the crystallization kinetics might be an important factor for maximizing the charge mobility in semicrystalline polythiophene thin films. In addition to single component semicrystalline thin films, blends of solution-processable organic molecules and conjugated polymers are of interest due to their potential use in low-cost, light-weight, scalable and flexible photovoltaic devices. One challenge lies in that, once mixed, self-assembly often does not lead to the formation of pure phases. We showed that RR P3HT/phenyl-C61-butyric acid methyl ester (PCBM) mixtures separate into two phases due to the crystallization of the polymer. The P3HT rich phase, composed of P3HT crystals, is essentially pure. The PCBM-rich phase, however, is a mixture of amorphous P3HT with PCBM. The role of mixing within nanophases, however, on charge transport of organic semiconductor mixtures is not fully understood. We have examined the electron mobility in amorphous blends of P3HT and PCBM. Our studies revealed that the electron mobility is correlated with the miscibility of amorphous polythiophene/fullerene blends, which is determined by measuring the Flory-Huggins interaction parameter. Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. In fact, our results suggest that donor/acceptor mixtures which are completely miscible would exhibit sharp drop offs in charge transport efficacy with dilution. Strongly immiscible systems, on the other hand, would readily phase separate into large domains and prevent efficient charge separation. Consequently, partial miscibility of donor/acceptor mixtures is critical for efficient device performance. Our findings regarding charge transport in the fullerene-rich phase of polythiophene-fullerene solar cells can also be extended to any semicrystalline polymer-fullerene mixture.