Morphology Control of Bulk Heterojunction Layer in Polythiophene-based Solar Cells
Date Issued
2008
Date
2008
Author(s)
Chang, Yi-Ming
Abstract
The morphology of bulk-heterojunction layer is an important key in determining the performance of polymer-based solar cells. In order to obtain a nano-scaled bi-continuous interpenetrating network of electron donor/acceptor blend, desirable phase compatibility or annealing treatment is essential procedures for morphology optimization. Therefore, considerable efforts have been devoted to this study on controlling the morphology of bulk-heterojunction layer in the polymer-based solar cells. In the first part, we demonstrated that the light illumination of regioregular poly(3-hexylthiophene) (P3HT) in air resulted in the degradation of polymer, thus potentially reducing the conjugation length, the absorption/emission intensities, the size of the crystal domain and the charge-carrier mobility. These effects lead to undesirable morphology and poor performance of solar cells. Therefore, the development of efficient polymer solar cells depends not only on fabrication in an inert atmosphere, but also on the synthesis and storage of material in a suitable environment to ensure polymer quality. In the second part, a series of poly[3-hexylthiophene-co-3-(6-hydroxyhexyl)thiophene] (P3HT-OH):titania (TiO2) hybrids were synthesized via the in-situ sol-gel reaction of titanium (IV) n-butoxide in the presence of P3HT-OH. Introducing hydroxyl moiety onto the side chain of polymer significantly promotes the polymer-titania interaction, producing a uniform distribution of TiO2 throughout the polymer matrix in the hybrid solutions and hybrid films. Furthermore, this hybrid can be further utilized as an efficient compatibilizer in preparing photoactive P3HT:TiO2:hybrid films with excellent miscibility. The solar cell based on such a hybrid exhibited the 3.4 and 2.4-fold higher value of photocurrent and power conversion efficiency compared to the device based on P3HT:TiO2 blend without added hybrid. In the third part, we successfully demonstrated an novel electric annealing technique for fabricating efficient P3HT:[6,6]-phenyl C61-butyric acid methyl ester (PCBM) solar cells by applying a direct current in a short annealing time for only 20 sec. The solar cells based on this approach exhibited distinctly enhanced device performance. The results from space-charge limited current measurement and electrochemical impedance analysis indicated a significant improvement in the charge-carrier mobility and bulk resistance for thus-prepared P3HT:PCBM bulk-heterojunction films. The power conversion efficiency and stability of this solar cell was comparable to that of conventional thermally annealed devices. This approach provides an operationally simple and rapid process for improving the solar cell efficiency. Finally, an annealing-free solar cell fabrication technique based on the addition of small concentration of 3-hexylthiophene, the monomer of P3HT into the P3HT and PCBM blend solution from which the bulk-heterojunction layer was prepared by spin-cast was demonstrated. The polymer solar cells based on such films exhibited distinctly enhanced photocurrent and fill factor without any thermal annealing or solvent annealing process. The results from UV-vis absorption spectra, X-ray diffraction spectra, and TEM images indicated a significant improvement in the absorbance, crystallization, and nano-scaled morphological optimization for thus-prepared P3HT:PCBM bulk-heterojunction films. Very importantly, the solar cells fabricated from this annealing-free process performed similarly in power conversion efficiency and incident photo-to-electron conversion efficiency as conventionally annealed solar cells, indicating the feasibility of this simple and rapid annealing-free fabrication process.
Subjects
Annealing
Bulk-heterojunction
Conjugated Polymer
Morphology
Photovoltaic Devices
Polymer Solar Cells
SDGs
Type
thesis
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