Metal-free dye sensitized solar cells emerged as an important source of a renewable energy resource because of low production, simple fabrication process and a wide availablility of organic dyes. This review mainly focuses on metal-free sensitizers having rigidifed-aromatics as the conjugated spacer for applications in DSSCs. Based on their structures dyes have been divided into three major subsegments: (i) ladder type polyphenylenes with or without heteroatoms; (ii) rigidfied-aromatics as a donor segments; (iii) fused polyaromatics. Planarization of organic molecules through rigidification of aromatics allows effective communication and their smaller reorganization energy is beneficial to photoinduced charge transfer. Dyes having rigidified aromatics as a conjugated spacer emerged as promising candidates and a maximum efficiency of 12.5 % has been achieved with DSSCs without coadsorbate or co-sensitizer.
From studies on chlorocyclohexane, 1-chloro-1-methylcyclohexane, cisand trans-4-t-butyl-1-chlorocyclchexane, cisand trans-4-t-butyl-l-chloro-l-methylcyclohexane1 and l-chloro-1,4,4-trimethylcyclohexane, the conformational energy of chlorine was determined by a study of the molar absorbance of the C-C1 stretching frequency in the infrared, supplemented by variable-temperature infrared spectroscopy and equilibrium measurements, to be 0.4 kcal/mole, favoring the equatorial conformation when the chlorine was secondary, and it was found that, when the chlorine and methyl were geminal, the conformational energy was 1.1 kcallmole, favoring the chlorine axial and methyl equatorial.
A series of phenothiazine based dyes (OMS1-3), comprising different conjugation lengths and numbers of electron deficient (cyanovinyl) moieties with cyanoacrylic acid as an anchor, have been synthesized. The dyes display broad UV-visible absorption, from 389 nm to 484 nm. The higher molar extinction coefficient and longer absorption peak are achieved as the conjugation length and numbers of electron deficient units increase. The cell performance based on these dyes exhibits efficiencies ranging from 0.68-4.00%, compared to a standard N719-based device (PCE ¼ 7.49%) fabricated under similar conditions. Although the OMS3 dye has two electron deficient units between phenothiazine units, an insignificant electron trapping effect is observed. From the results, the OMS3 based cell exhibits the highest short circuit current (J SC ) at 8.72 mA cm À2 and the highest open-circuit voltage (V OC ) at 0.66 V, together with the best cell performance at 4.00%. Fig. 6 Schematic division and dihedral angles of OMS2 and OMS3 dyes. 9788 | RSC Adv., 2018, 8, 9783-9789 This journal is
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