Highly Efficient Solar Cells Using Organic Dyes with Amorphous Moiety
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0974-CC08-03
Highly Efficient Solar Cells Using Organic Dyes with Amorphous Moiety Sanghoon Kim1, Jae Kwan Lee1, Il Jung1, Kyu-Ho Song1, Chul Baik1, Hyunbong Choi1, Duckhyun Kim1, S. Fantacci2, F. De Angelis2, J -H Yum3, Md. K. Nazeeruddin3, M. Graetzel3, and Jaejung Ko1 1 Korea University, Chungcheongnam-do, 339-700, Korea, Republic of 2 Università di Perugia, Perugia, I-06123, Italy 3 Swiss Federal Institute of Technology, Lausanne, CH - 1015, Switzerland
ABSTRACT Novel organic sensitizers comprising donor; electron-conducting, and anchoring groups were engineered at molecular level and synthesized. The functionalized unsymmetrical organic sensitizers 3-{5-[N,N-Bis(9,9-dimethylfluorene-2-yl)phenyl]-thiophene-2-yl}-2-cyano-acrylic acid (JK-1) and 3-{5'-[N,N-Bis(9,9-dimethylfluorene-2-yl)phenyl]-2,2'-bithiophene-5-yl}-2cyano-acrylic acid (JK-2), upon anchoring onto TiO2 film exhibit unprecedented incident photon to current conversion efficiency 91 %. The photovoltaic data revealed a short circuit photocurrent density of 14.0 ± 0.2 mA/cm2, an open circuit voltage of 753 ± 10 mV and a fill factor of 0.76 ± 0.02, corresponding to an overall conversion efficiency of 8.01 % under standard AM 1.5 INTRODUCTION Dye-sensitized solar cells (DSCs) have attracted significant attention as low-cost alternatives to conventional solid-state photovoltaic devices.1-12 These cells employ mostly ruthenium polypyridyl complexes as charge transfer yielding over 11% solar-to-electric power conversion efficiencies in standard global air mass 1.5 (AM 1.5 sun light 13 In order to widen the choice for this critical component of the DSC, several groups have developed metal free sensitizers and obtained efficiencies in the range of 5-8%.14-18 There are several basic requirements guiding the molecular engineering of an efficient sensitizer are. The excited state should match the energy of the conduction band edge of the oxide. Light excitation should be associated with vectorial electron flow from the light harvesting moiety of the dye towards the surface of the semiconductor surface providing for efficient electron transfer from the excited dye to the TiO2 conduction band, a strong conjugation across the donor and anchoring groups, and good electronic coupling between the lowest unoccupied orbital (LUMO) of the dye and the TiO2 conduction band. A major factor for the low conversion efficiency of many organic dyes in the DSC is the formation of dye aggregates on the semiconductor surface. Such an aggregation phenomenon would affect the light absorption by filtering effect.19 Therefore, for obtaining optimal performance, aggregation of organic dyes needs to be avoided through appropriate structural modification.20 Another important issue for organic dyes is their stability, which is generally lower than that of metal complexes, likely bleeding channels being the formation excited triplet states and unstable radicals.21
In order to incorporate these required properties we have designed and synthesized the novel unsymmetrical organic sensitizer
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