π-Extended cis-Configured Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells: Panchromatic Response

Abstract

Light absorbing sensitizer is the heart of the third generation photovoltaic technologies such as dye-sensitized solar cells. Despite possessing large extinction coefficients for the metal-free organic dyes, the light-harvesting efficiency is limited with the narrow absorption profile in either visible or NIR regions of the solar spectrum. Though monolayer formation of organic sensitizers on semiconducting metal oxide surface leads to dye–dye interaction that leads to broadening the absorption profile, the contribution of the aggregated state to the power conversion process is found out to be poor. Here, we have engineered far-red active π-extended cis-configured unsymmetrical squaraine dyes with carboxylic acid and cycnoacetic acid as anchoring groups with controlled aggregation and panchromatic light absorption by including the steric and electronic factors. The presence of sp3-C (out-of-plane) and N-alkyl (in-plane) groups played an important role in modulating the assembly of the dyes on the TiO2 electrode; the cis-squaraine unit helps to extend the conjugation in the NIR region besides enhancing the visible transition and the incident photon-to-current conversion efficiency profile that showed the importance of anchoring groups for the panchromatic response. Dye PSQ10 showed solar-to-electric conversion with an onset of 850 nm with the device performance of 7.0% (V oc = 0.581 V, J sc = 17.06 mA/cm2, FF = 70%).

First-principles studies of substituent effects on squaraine dyes

“…These effects are due to the alteration of energy transfer pathways. 53,82,[94][95][96][97] Conformers have also been shown to change transition dipole orientation, further potentially affecting exciton delocalization. 98 Certain applications may benet from different conformers, such as dye-sensitized solar cells in part because of accommodation of different anchoring options on substrates.…”

“…50 They can potentially be structurally tailored for a wide variety of applications. [51][52][53] First synthesized by Treibs and Jacobs, 54 the central feature of SQ dyes includes an electron-decient squaric moiety, combined with electron-rich groups in a symmetric manner by means of a methine bridge. 52 The photophysical features and extensive structural tunability make squaraines well-suited candidates for the investigation of exciton delocalization when assembled.…”

Dye-sensitized solar cells strike back

“…Due to a modest photovoltage (577 mV) – as is common in the NIR region with dye sensitized solar cells – the overall power conversion efficiency was limited to 6.9% PCE. 317 An ethyl cyanoacetate-derived squaraine dye (HSQ4) with dual anchors was shown to have a substantially increased stability relative to mono-anchored squaraine dyes with no change in PCE after 1000 hours. 318 In this same study, the ethyl cyanoacetate group was found to give a dye with a significantly higher excited state oxidation potential than a dicyanomethylene derived dye, which correlated to a higher IPCE peak value (80% versus 70%).…”

The influence of three diphenylpyran isomer co-sensitizers with different sterical structures on N719-based dye sensitized solar cells

“…These redox potentials are well-matched with the conduction band of TiO 2 and the redox potential values of iodide/tri-iodide electrolyte, which is energetically helpful for electron injection and dye-regeneration. 31…”