Molecular Design of Unsymmetrical Squaraine Dyes for High Efficiency Conversion of Low Energy Photons into Electrons Using TiO₂ Nanocrystalline Films

Abstract

An optimized unsymmetrical squaraine dye 5‐carboxy‐2‐[[3‐[(2,3‐dihydro‐1, 1‐dimethyl‐3‐ethyl‐1H‐benzo[e]indol‐2‐ylidene)methyl]‐2‐hydroxy‐4‐oxo‐2‐cyclobuten‐1‐ylidene]methyl]‐3,3‐dimethyl‐1‐octyl‐3H‐indolium (SQ02) with carboxylic acid as anchoring group is synthesized for dye‐sensitized solar cells (DSCs). Although the π‐framework of SQ02 is insignificantly extended compared to its antecessor squaraine dye SQ01, photophysical measurements show that the new sensitizer has a much higher overall conversion efficiency η of 5.40% which is improved by 20% when compared to SQ01. UV‐vis spectroscopy, cyclic voltammetry and time dependent density functional theory calculations are accomplished to rationalize the higher conversion efficiency of SQ02. A smaller optical band gap including a higher molar absorption coefficient leads to improved light harvesting of the solar cell and a broadened photocurrent spectrum. Furthermore, all excited state orbitals relevant for the π–π* transition in SQ02 are delocalized over the carboxylic acid anchoring group, ensuring a strong electronic coupling to the conduction band of TiO2 and hence a fast electron transfer.

A High‐Efficiency Panchromatic Squaraine Sensitizer for Dye‐Sensitized Solar Cells

“…[5] More recently, a PCE of 6.29 % has been obtained using a more markedly unsymmetrical mixed pyrrole/indoline squaraine that also exhibits a red-shift of its main absorption peak relative to that of SQ1. [6] This previous work has demonstrated the efficacy of an unsymmetrical design for electron injection and of increased p conjugation to bathochromically shift the main low-energy absorption band; [5,6] however, the possibility that higher-energy absorption bands in squaraines can also contribute to the photocurrent in DSCs has only recently been investigated.…”

A High‐Efficiency Panchromatic Squaraine Sensitizer for Dye‐Sensitized Solar Cells

“…[3] A PCE of 4.5 % under AM 1.5 solar conditions has been obtained using the indoline-terminated dye SQ1 (Scheme 1); [4] a bathochromic shift of the main absorption band and an increase of the PCE to 5.4 % has been obtained for a similar dye in the which conjugation is slightly extended. [5] More recently, a PCE of 6.29 % has been obtained using a more markedly unsymmetrical mixed pyrrole/indoline squaraine that also exhibits a red-shift of its main absorption peak relative to that of SQ1. [6] This previous work has demonstrated the efficacy of an unsymmetrical design for electron injection and of increased p conjugation to bathochromically shift the main low-energy absorption band; [5,6] however, the possibility that higher-energy absorption bands in squaraines can also contribute to the photocurrent in DSCs has only recently been investigated.…”

“…[5] More recently, a PCE of 6.29 % has been obtained using a more markedly unsymmetrical mixed pyrrole/indoline squaraine that also exhibits a red-shift of its main absorption peak relative to that of SQ1. [6] This previous work has demonstrated the efficacy of an unsymmetrical design for electron injection and of increased p conjugation to bathochromically shift the main low-energy absorption band; [5,6] however, the possibility that higher-energy absorption bands in squaraines can also contribute to the photocurrent in DSCs has only recently been investigated. [7] Herein we report the synthesis and optical, electronic, and photovoltaic properties of the unsymmetrical bis(indoline) squaraine sensitizer YR6 (Scheme 1), which incorporates more extended conjugation beyond the squaraine core than SQ1, as well as the more strongly p-accepting carboxycyanovinyl group as a surface-anchoring group instead of a directly attached carboxylic acid.…”

Design Principles of FRET‐Based Dye‐Sensitized Solar Cells with Buried Quantum Dot Donors

“…[ 11 , 12 ] This is of particular interest when considering dyes which are photoactive in the deep red or the near-IR spectral regions. [ 13 ] Ideally, the higher energy components of the solar spectrum will be highly absorbed by a donor, and transferred effi ciently to the acceptor. This can be done either via FRET, or by radiative recombination and re-absorption by an acceptor.…”