Photon harvest enhancement by metal-free organic dyes

Abstract

Energy harvesting is the process by which energy is derived from external sources such as solar power, thermal energy, wind energy, captured, and stored for later use. Photovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors. Photovoltaic (PV) energy harvesting technology offers significant advantages over wired or solely battery-powered sensor solutions: virtually inexhaustible sources of power with little or no adverse environmental effects. In recent years new PV technologies have come to the forefront in energy harvesting such as dye sensitized solar cells (DSSC) and organic photovoltaic solar cells. The dye absorbs light much like chlorophyll does in plants. The plastic used in organic solar cells has low production costs in high volumes. Combined with the flexibility of organic molecules, organic solar cells are potentially cost-effective for photovoltaic applications. Molecular engineering (e.g. changing the length and functional group of polymers) can change the energy gap, which allows chemical change in these materials. The optical absorption coefficient of organic molecules is high, so a large amount of light can be absorbed with a small amount of materials. However, the main disadvantages associated with organic photovoltaic cells are low efficiency, low stability and low strength compared to inorganic photovoltaic cells. This study sought for the high performance DSSCs and organic photovoltaic solar cells by synthesizing novel dyes and organic molecules. For dye sensitized solar cells (DSSCs) that can be an economically viable alternative to conventional inorganic solar cells, ruthenium-complexes are the most efficient sensitizers identified thus far. Although Ru-complexes are the most efficient sensitizers known so far, their use in large-scale applications is likely to be limited because ruthenium metal is scarce and expensive, and careful, non-ecofriendly and tricky purification steps are required in their syntheses. Thus, researchers have examined metal-free organic dyes to take advantage of their simple synthesis, tunable absorption spectrum, high molar extinction coefficients, and long-term stability. In this study, metal-free organic dyes that can perform on par with organic-metal complex dyes have been sought to enable the wide spread practical implementation of DSSCs. We have rationally developed a DSSC that yields an unprecedented high photocurrent density of 20.9 mA cm-2, reaching an efficiency of > 9% in full sunlight (AM 1.5G, 100 mWcm-2) without an antireflection layer. This good performance results from the harmonization of the resonant multiple light scattering in the photoelectrode of the hierarchically structured TiO2 aggregates (an average size of 660 nm) with the photon absorption of the novel metal-free organic dye in the visible region. It enabled enhanced light harvesting and charge collection. The device also displayed very good long-term stability when aged without a UV filter. We also investigated the effect of different electrolyte using a cobalt redox electrolyte [Co(III/II)(bpy)3](PF6)3/2 which induce high positive redox potential because the cobalt complexes have several advantages over I3-/I- electrolyte. This was accomplished by adopting molecular design of dye molecules. Novel dye contains two hexyl moiety in -conjugated bridge, which can prevent electron recombination and is suitable for the cobalt polypyridine redox mediators. However, the current density was decreased with the increase of the open circuit voltage because of the trade-off between the open circuit voltage and the current density. When the photoelectrode thickness was 9 m, the DSSC with cobalt compex electrolyte showed the best performance of 6.12 % efficiency while that of the N719 was 4.93 % and the same dye with I3-/I- electrolyte showed photo conversion efficiency of 5.06 %. Thus it enables the possible improvement with thinner device and less amount of the dye. Organic solar cells (OSCs) have also attracted significant attentions due to their potentials of the low-cost process and flexible applications. The performances of solution processed OSCs using conjugated polymer as well as small molecules have dramatically enhanced with power conversion efficiency (PCE) up to ~10.6% and ~9%, respectively. In contrast, vacuum-deposited OSCs show relatively lower efficiency although they have advantages in the reproducibility and the ease of controlling the purity of materials through the sublimation. Among the vacuum-deposited OSCs, metal-phthalocyanine (M-Pc) compounds are widely used as donor materials due to the high absorption coefficient in the visible and near-infrared region and good thermal stability. However, M-Pc compounds show relatively low open circuit voltage (VOC) due to the small difference between the highest occupied molecular orbital (HOMO) level of M-Pc compounds and the lowest unoccupied molecular orbital (LUMO) level of fullerene derivatives as well as low fill factor (FF) due to the difficulty in forming charge transporting path in the co-deposited layer. Therefore, new donor materials have been proposed to overcome the drawbacks of M-Pc compounds. In OSCs, thiazole-based D-A alternative copolymer donors have showed very promising performance, while thiazole-based small molecules for OSCs relatively received less attention. We found that simple thiazole unit had potential for high performance OSCs. In this study, we report novel electron donor molecules based on D-π-conjugated linker-A structure with compact packing and intramolecular charge transfer characteristics for efficient organic solar cells. The donor molecules that have the D-π-A structure featuring an electron-rich triphenyl amine as the electron donor unit, dicyanovinylene as the acceptor unit and π-conjugated linkers of thienothiophene, thiophene, and thiazole units were synthesized. The π-conjugated linkers were carefully designed to have the planar structure, efficient conjugation length, appropriate energy level and to induce compact packing in the solid state. As a result, newly synthesized thiazole based compound led to highly efficient organic solar cell with the PCE of 6.2% under AM 1.5G illumination with the intensity of 100 mWcm-2.