Achieving Improved Solar Absorbance of Small Organic Dyes Featuring Quinoidized Five-Membered Heterocycles
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Achieving Improved Solar Absorbance of Small Organic Dyes Featuring Quinoidized FiveMembered Heterocycles Wei Han Tu1, Yi Yin Tan2, and Sergei Manzhos3* 1 Anderson Junior College, 4500 Ang Mo Kio Avenue 6, Singapore 569843, Singapore 2 Raffles Institution, 1 Raffles Institution Lane, Singapore 575954, Singapore. 3 Department of Mechanical Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576, Singapore *Email: [email protected] ABSTRACT We present rational computational design of phenothiazine dyes for dye-sensitized solar cells containing different five-membered rings (thiophene, furan, and selenophene) by a combined strategy of modified conjugation order and functionalization leading to the quinoidization of the ring. We predict that it is possible to lower the excitation energy by 20% vs. the parent dye by the combination of: change in the conjugation order of the methine unit, its functionalization by the CN group, and replacement of the thiophene ring by furan. INTRODUCTION The design of purely organic, environmentally friendly, and easy to synthesize dyes with strong solar absorbance for dye-sensitized solar cells (DSSC) [1] is necessary to achieve widespread use of this technology. It was recently shown that small organic dyes featuring a thiophene unit separated by a methine unit from the cyanoacrylic attachment group (which is a standard dye design) can be made possess strong solar absorbance by a change of their conjugation order [2, 3, 4, 5]. We have identified the quinoidization of the thiophene unit by the electron withdrawing cyanoacrylic group as the cause of the decrease of the excitation energy, Eexc [2]. In a recent work [6], we attempted to achieve a similar effect on the bond length alternation (BLA, defined as BLA=(CαCβ1+CαCβ2)/2-CβCβ, where Cα label the carbon atoms of the thiophene unit bonded to the S atom and Cβ1,2 the H-terminated carbon atoms) and on the absorption spectrum as achieved by the conjugation order change by functionalizing the methine unit of a parent phenothiazine dye (dye 1S in Figure 1) containing the same thiophenecyanoacrylic linkage. We systematically studied the effect of electron- donating and withdrawing functional groups including pseudo- and superhalogens. From all the studied functionalizations, we proposed to use a fumaronitrile unit as the most practical to induce an increase in the BLA and a concurrent red shift in the absorption spectrum vs. the parent dye (dye 71S in Figure 1). The Eexc of the visible absorption peak was predicted to drop by about 12% vs. the parent dye which would correspond to a redshift of about 70 nm in real dyes [7] and a significantly improved overlap with the solar spectrum [6]. While the redshift approached that achieved by the conjugation order change, the BLA did not change nearly as much (from -0.03 to -0.02) as when the conjugation order was changed (from -0.03 to +0.10). Potential therefore remains for inducing even stronger quinoidization and larger redshift in the absorption sp
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