| Summary: | 博士 === 國立臺灣大學 === 高分子科學與工程學研究所 === 104 === This thesis consists of five topics concerning polymer photovoltaics. In the first part, in this study, a series of novel soluble fullerene derivatives with different alkyl substitutions, including B-C60、MB-C60、EB-C60、nBB-C60、iPB-C60 and tBB-C60, were synthesized from Bingel-Hirsch reaction and employed as acceptor to fabricate polymer solar cells. The compatibility between these C60 derivatives and poly(3-hexylthiophene) (P3HT) increased with the increase of extent branches number of alkyl substitutions. In these molecules, both MB-C60 and EB-C60 tend to crystallize whereas iPB-C60 and tBB-C60 can homogeneously distribute inside polymer matrix upon the drying of their blend solutions with P3HT. UV-vis, PL, TEM and XRD were used to characterize the blending films after annealing. Besides, the mobility of electrons and holes were measured to analyze the photoelectric properties of the films. The results clearly indicate that both interfacial properties of two phases and mobility of electrons and holes play an important role in the performance of devices. The cells were fabricated with the structure of ITO/ PEDOT:PSS/ P3HT:Alkyl-C60/ Ca/ Al. The device based-on tBB-C60 exhibited the highest current density of 10.03 mA/cm2 and the best energy conversion efficiency of 3.59%.
In the second part, we demonstrates that the bis-adduct of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is an effective inhibitor of the aggregation of PCBM inside the poly(3-hexylthiophene) (P3HT) matrix. Substituting some of the PCBM with bis-PCBM apparently reduces the size of PCBM-rich clusters, enhancing both the short-circuit current density (JSC) and the fill factor (FF), leading to a 17% increment in power conversion efficiency (PCE) for a cell with 8.3 wt% bis-PCBM replacement. More importantly, a tiny amount of bis-PCBM significantly improves the morphological stability of P3HT/PCBM blend against high-temperature aging. All P3HT/PCBM:bis-PCBM devices exhibit extremely stable PCEs, which do not visibly change upon heating at 150 ℃for 15 hours.
In the third part, we simultaneously employed grazing incidence small-angle and wide-angle X-ray scattering (GISAXS and GIWAXS) techniques to quantitatively study the structural evolution and kinetic behavior of poly(3-hexylthiophene) (P3HT) crystallization, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) aggregation and amorphous P3HT/PCBM domains from a bulk heterojunction (BHJ) to a thermally unstable structure. The independent phase separation regimes on the nanoscale (~10 nm), mesoscale (~100 nm) and macroscale (~μm) are revealed for the first time. Bis-PCBM molecules as inhibitors incorporated into the P3HT/PCBM blend films were adopted as a case study of a control strategy for improving the thermal stability of P3HT/PCBM solar cell. The detailed information on the formation, growth, transformation and mutual interaction between different phases during the hierarchical structural evolution of P3HT/PCBM:xbis-PCBM (x＝8~100%) blend films are presented herein. This systematic study proposes the mechanisms of thermal instability for a polymer/fullerene-based solar cell. We demonstrate a new fundamental concept that the structural evolution and thermal stability of mesoscale amorphous P3HT/PCBM domains during heating are the origin of controlling thermal instability rather than those of nanoscale thermally stable BHJ structures. It leads to a low-cost and easyfabrication control strategy for effectively tailoring the hierarchical morphology against thermal instability from molecular to macro scales. The optimum treatment achieving high thermal stability, control of mesoscale domains, can be effectively designed. It is independent of the original BHJ nanostructure design of a polymer/fullerene-based solar cell with high performance. It advances the general knowledge on the thermal instability directly arising from the nanoscale structure.
In the forth part, Organic Photovoltaic (OPV) cells represent a compelling candidate for renewable energy by solar energy conversion. In recent years, versatile light-trapping measures via structures have been intensively explored to optimize photovoltaic performance. In this work, a unique rubbing technique is demonstrated to create nanoscale grooves on the PEDOT:PSS surface and the grating-like features are 500 nm wide and 10 nm deep. The PEDOT:PSS film with grooved surface is used as buffer layers for OPV cell devices based on a P3HT:PCBM bulk heterojunction. The patterned surface has a profound effect on carrier mobility, light trapping, and hole collection efficiency, leading to an increase in the short circuit density, filling factor, and power conversion efficiency. These results indicate the feasibility of the rubbing method can be applicable to high efficiency OPV cells.
In the last part, optimized performances of polymer solar cells has been of magnificent interest in recent years. A variety of approaches have been reported to alter or replace the polymer buffer layers in solar device structures. In this present work, surface modification of indium tin oxide (ITO) coated substrates through the use of self-assembled multilayers by the soft-imprinting method has been applied to adjust the anode work function and device performance in polymer solar cells based on a P3HT:PCBM heterojunction. The efficiency and morphology of the solar device with CF3-terminal group materials as a buffer layer have been measured and investigated. These results demonstrate that the soft-imprinting method is an effective and rapid procedure that enhances the quality of polymer solar cells and indicates potential implications for other organic devices containing an interface between a blended organic active layer and an electrode layer.
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