Dye Aggregation, Photostructural Reorganization and Multiple Concurrent Dye···TiO₂ Binding Modes in Dye-Sensitized Solar Cell Working Electrodes Containing Benzothiadiazole-Based Dye RK-1

Abstract

Dye-sensitized solar cells (DSSCs) are widely considered to be promising contenders for the next generation of photovoltaic devices. The light-harvesting component in the DSSCs, that is, the sensitizers, are adsorbed onto the semiconductor layer to form the photoelectrode that absorbs solar energy and converts it into electrical power. The choice of dye has a profound impact on the DSSC performance. Thus, substantial research efforts have been devoted to the development and testing of a large variety of sensitizers during the past decades. We, herein, focus our attention on the metal-free benzothiadiazole-based dye, 2-cyano-3-(4-(7-(5-(4-(diphenylamino)phenyl)-4-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)phenyl) acrylic acid (RK-1), which exhibits excellent power conversion efficiency and high stability. Simulations and experiments were combined to conduct a systematic study of its optical and structural properties. RK-1 shows a broad absorption band over the visible region and the formation of H aggregates in solution. Deprotonation of the cyanoacrylic acid group in RK-1 occurs upon dye adsorption onto TiO 2 . This induces a hypsochromic shift of the absorption band. The emission spectrum of RK-1 shows a broad band and a large Stokes shift (6949 cm −1 ), indicating the presence of several different structural reorganization processes during excitation. Time-dependent density functional theory (TD-DFT) calculations reveal that intramolecular charge transfer in RK-1 molecules should be predominantly attributed to the HOMO− LUMO transition. DFT modeling also reveals the involvement of the cyano group of RK-1 in anchoring to TiO 2 , which makes the dye•••TiO 2 binding structure more stable. Fourier transform infrared spectroscopy data prove that bridging bidentate (BB) and COO/CN (A2) binding modes coexist in the RK-1•••TiO 2 interfacial structure.

Dye-sensitized solar cells strike back

“…In the last decade, the availability of more and more powerful computing facilities allowed the study of the dye/electrode interface at the full atomistic scale. From cluster-size electrodes with few atoms, 184,185 computational tools now have the capability of simulating the full electrode surfaces with periodic boundary conditions, including the attached dyes 186 and, in some cases, also the explicit solvent medium. 137 The characterization of dye/electrode interfaces has provided great advancement in the understanding of the complex interfacial electronic processes.…”

Dye-Anchoring Modes at the Dye···TiO2 Interface of N3- and N749-Sensitized Solar Cells Revealed by Glancing-Angle Pair Distribution Function Analysis

“…This is especially pertinent in light of the recent STM/STS studies 29,30 which show that dye molecules can exist in multiple dye conformations on certain dye•••TiO 2 interfacial structures; yet, their atomic resolution could not be established by STM imaging. Several recent theoretical studies 18,37 have also corroborated this notion that more than one type of dye••• TiO 2 binding configuration may coexist. This evidence of dyeconformation multiplicity is an important finding since it presents fundamental implications for much of the modeling and experimental materials characterization work done to date on dye•••TiO 2 interfaces, whose analysis implicitly assumes one binding mode.…”

“…For example, vibrational spectroscopy (infrared and Raman) has been used to identify changes in dye···TiO 2 binding strengths or anchoring configurations via analysis of associated bond-stretching or bending-mode modulations. − X-ray photoelectron spectroscopy can detect if a dye is anchoring to a TiO 2 surface; if so, it can also quantify the corresponding dye···TiO 2 binding strengths. − X-ray absorption spectroscopy can identify the oxidation states in dyes adsorbed onto TiO 2 surfaces . Optical metrology, such as UV–vis absorption and emission spectroscopy, can be used to survey the propensity for a dye to aggregate on TiO 2 surfaces and determine if the dyes form H- or J-aggregates. , X-ray reflectometry can enumerate the thickness of the dye on a TiO 2 surface. − Neutron reflectometry can determine dye-layer thicknesses of a dye···TiO 2 interfacial structure while this DSSC working electrode is assembled within the device environment . Concerted scanning tunneling microscopy and scanning tunneling spectroscopy (STM/STS) techniques have been used to demonstrate that multiple dye-anchoring configurations can coexist on a TiO 2 surface. , Atomic force microscopy (AFM) can be used to elucidate the spatial distribution of dye molecules on a TiO 2 surface.…”

“…21 Optical metrology, such as UV−vis absorption and emission spectroscopy, can be used to survey the propensity for a dye to aggregate on TiO 2 surfaces and determine if the dyes form H-or J-aggregates. 18,22 X-ray reflectometry can enumerate the thickness of the dye on a TiO 2 surface. 23−27 Neutron reflectometry can determine dyelayer thicknesses of a dye•••TiO 2 interfacial structure while this DSSC working electrode is assembled within the device environment.…”

Cost‐Effective Dyes Based on Low‐Cost Donors and Pd‐Free Synthesis for Dye‐Sensitized Solar Cells

“…The larger Stokes shift and small shoulder of PTZC may imply that PTZC experiences different processes of structural reorganization during the electronic transition. [ 28 ] As shown in Figure 2A, the intersection point of the absorption and emission spectra can be used to estimate the optical gaps ( E 0‐0 ) of BzC and PTZC , which are 2.23 and 2.10 eV for BzC and PTZC , respectively.…”