Abstract
We report on the design and structure principles of 5,5′-6,6′-tetrachloro-1,1′-dioctyl-3,3′-bis-(3-carboxypropyl)-benzimidacarbocyanine (Dye 1). Such metal-free amphiphilic cyanine dyes have many applications in dye-sensitized solar cells. AFM surface topographic investigation of amphiphilic molecules of Dye 1 adsorbed on TiO2 anode reveals the ability of spontaneous self-organization into highly ordered aggregates of fiber-like structure. These aggregates are known to exhibit outstanding optical properties of J-aggregates, namely, efficient exciton coupling and fast exciton energy migration, which are essential for building up artificial light harvesting to the photovoltaic device. A light-to-electricity conversion efficiency of DSSC based on the metal free amphiphilic Dye 1 is , which is about 50% of that based on metal-based N719 Ru-dye (Di-tetrabutylammoniumcis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)). DFT and TD-DFT studies show that large intramolecular charge transfer takes place from the HOMO to LUMO. HOMO is localized on a part of the molecule with almost no contribution from the carboxylic moiety. This clearly indicates that the anchoring carboxylic group plays a minor role.
1. Introduction
The design and synthesis of functional dyes have lately received much attention due to their potential applications as sensitizers in dye-sensitized solar cell (DSSC) [1]. Dyes with wide absorption range, specifically in the visible region, and containing an anchoring group such as carboxylic acids, are ideal candidates [2]. The DSSC process requires light-induced excitation of a HOMO electron. The anchoring group then acts as a bridge and injects this excited electron into the conduction band of TiO2 and the dye gets oxidized. The oxidized dye is then neutralized to ground state by redox system [3]. The efficiency of the cell is affected by three factors: the energy difference between the excited state of the dye and the conduction band of the semiconductor, the efficiency and nature of the dye-semiconductor binding, and the properties of the redox couple in the electrolyte [2]. Additionally, the electron density should be localized near the injecting group in the excited state [4]. It has also been shown that the redox potential of adsorbed dye depends on the pH of the electrolyte and on the potential applied to the semiconductor [5]. By choosing an appropriate sensitizer, the tuning of the photoresponse of the semiconductor can be optimized [6].
The most successful charge transfer metal complexes of organic molecules employed so far in DSSC are cis-dithiocyanatobis-(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (red dye) and trithiocyanato 4,4′,4′′-tricarboxy2,2′,6′,2′′-terpyridine ruthenium(II) (black dye), which yields overall conversion efficiencies of up to 11% under AM1.5 irradiation [7]. However, ruthenium dyes are very expensive. To overcome the high cost, metal free sensitizers such as organic dyes and natural dyes are being investigated as alternative sensitizers for DSSC applications [8]. Recent literature reports indicate achievement of efficiency of 9.8% using organic dyes [9]. Several organic dyes have been reported in the literature [10, 11]. Although the main aim is to fabricate and optimize dyes of high efficiency, it is also of interest to investigate dyes from the viewpoint of fundamental research connected to issues like conjugation, charge transfer, and binding on TiO2 surface.
The self-assembly of amphiphilic molecules into fibrous structures has been studied extensively due to their high potential for many profitable applications. Although very different in their head group chemistry, many natural as well as synthetic amphiphilic compounds derived from carbohydrates, carbocyanine dyes, or amino acids tend to form twisted fibrous structures by molecular self-assembly in water predominantly [12–15]. For example, the spectral properties and photostability of the (Dye 1; Figure 1) J-aggregate have been investigated in solution and upon adsorption on TiO2 nanoparticles [12]. The self-assembly of Dye 1 has been also studied on a glass surface by noncontact atomic force microscopy (NCAFM). The dye molecules form a well-defined fiber-like structure that extends for tens of micrometers. The internal structure of the fibers has been clearly resolved and showed a number of small tubes wrapped around each other to form a helical structure [12].
As a new promising sensitizer for DSSC, here we are reporting studies on Dye 1, which exhibits interesting characteristics of self-organization and better charge transportation. It would be of interest to carry out a detailed DFT study of such a dye, to understand its donation capacity, and to shed light on the nature of interaction with TiO2 anode. The light-to-electricity conversion efficiency () of DSSC based on that dye (Dye 1) as a metal-free dye sensitizer has been also tested. Reasonably good efficiency compared to the standard DSSC based on N719 dye sensitizer has been obtained with the solar device based on Dye 1 that indicated a good charge communication between the dye sensitizer and the TiO2 conduction band electrode.
2. Experimental
2.1. Materials and Device Fabrication
The J-aggregating 5,5′-6,6′-tetrachloro-1,1′-dioctyl-3,3′-bis-(3-carboxypropyl)-benzimidacarbocyanine (Dye 1) was obtained from FEW Chemicals GmbH, Wolfen, Germany, and was used as received. The absorption spectra of the free dye solution and the adsorbed dye on the TiO2 film were already published in our previous work [12]. Exactly similar findings were obtained in this work.
The DSSCs were fabricated using the N719 dye and TiO2 film electrodes included in a sample kit donated by Dyesol Company, Australia. The TiO2 coated anodes with approximate thickness of μm and effective area of = 0.16 cm2 were sintered at 450°C for 4 hours and air-cooled before being immersed overnight in acetonitrile solutions of 2.8 × 10−4 M of Dye 1 or N719. After the substrate was adequately washed with anhydrous alcohol and dried in moisture free air, the dye-sensitized TiO2 electrode was obtained. A DSSC was assembled by filling an electrolyte solution (0.6 M tetrapropylammonium iodide, 0.1 M iodine, 0.1 M lithium iodide, and 0.5 M 4-tertbutylpyridine (TBP) in acetonitrile) between the dye-sensitized TiO2 electrode and a platinized conducting glass electrode. The two electrodes were clipped together, and a sealing sheet was used as sealant to prevent the electrolyte solution from leaking.
2.2. Photocurrent-Voltage Curves Measurements
The photovoltaic testing of Dye 1 and N719 DSSCs was carried out by measuring the - character curves using Keithley 2400 source meter and homemade software, under simulated AM 1.5 solar illumination at 100 mW cm−2 from a halogen lamp in ambient atmosphere.
The fill factor (FF) and overall light-to-electrical energy conversion efficiency () of DSSC were calculated according to the following well-known equations: where the is the maximum power output point in the --curve yielding maximum product of current (mA·cm−2) and voltage (V), is the radiation power incident on the cell (mW·cm−2), is the short-circuit current density (mA·cm−2), and is the open-circuit voltage (V), respectively. Profit 6 (Quantum Soft, Switzerland) was used for data analysis and handling.
2.3. AFM Imaging
Noncontact AFM measurements of Dye 1 J-aggregates adsorbed on the TiO2 anode were performed. One type of aggregates of well-defined fiber-like structure is clearly evident, Figure 2(a). Figure 2(b) shows the topographic profile along the line indicated in the topography image in (a), length = 286 nm and height = 25.9 nm. Pico Image Basic software was used to build a basic surface analysis report on multilayer measurement data that is input from Agilent AFM 5500 Atomic Force Microscope (AFM) (N9410S), Agilent Technologies. Measurements were carried out via Dr. M. Ghobashy at National Center of Radiation Research, Atomic Energy Authority, Nasr City, Cairo, Egypt.
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2.4. Computational Details
The geometry optimization of the positively charged molecule in its doublet ground state was carried out in vacuum as implemented in the Gaussian 09 package [16]. The geometry of Dye 1 was optimized using the hybrid exchange-correlation B3LYP [16] functional with the 6-31G(d,p) basis set. The same computational method was applied to single TiO2 molecule.
Gausview 5, Gaussian Inc. (USA), was used for graphs visualization.
3. Results and Discussion
3.1. Molecular Interactions: Simplified View
To investigate the structural and frontier orbitals of the interacting molecules included in the light to electricity energy conversion device, we performed DFT and time-dependent (TD) DFT calculations on simplified molecules. The results are depicted in Figures 3, 4, and 5. Optimized geometry of Dye 1 molecule studies shows that large intramolecular charge transfer takes place from the HOMO to LUMO, and the donor moiety is not coplanar with the other conjugated part of the molecule (twisted by (~22°)) resulting in unsymmetrical orientation of the carboxylic groups, Figure 3. Furthermore, the anchoring carboxylic group is not contributing to frontier orbitals and thus seems to play a minor role in this type of molecules. This could give an idea about the correlation between the monomer molecular structure and performance in DSSC device, as discussed below. Additionally, based on its contribution to the frontier molecular orbitals (Figures 4 and 5) when interacting with TiO2, the carboxylic groups of Dye 1 should be excluded.
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3.2. Surface Orientation of Dye 1: Topographic Imaging
The self-assembly of amphiphilic molecules into fibrous structures has been the subject of numerous studies over past decades due to various current and promising technical applications [12–14]. Although very different in their head group chemistry many natural as well as synthetic amphiphilic compounds derived from carbohydrates, carbocyanine dyes, or amino acids tend to form fibrous structures by molecular self-assembly in water predominantly twisted ribbons or tubes. Figure 2 shows the NCAFM image of Dye 1 aggregates adsorbed on the surface of the TiO2 anode. One type of aggregates of well-defined fiber-like structure is clearly evident. It is horizontally aligned. This should result in better electronic charge transportation and thus a more efficient photovoltaic device. The following sections will shed more light on this expectation.
3.3. Performance of the Photovoltaic Device
The photocurrent-voltage properties of DSSCs using either Dye 1 or N719 were measured and shown in Figure 6. The fill factors and the current-to-electricity photovoltaic efficiencies were and 0.79 and and 7.6% for Dye 1 and N719 based devices, respectively. An illustrative energy level diagram (Figure 7) describes the electron charge donation from the sensitizer Dye 1 [16]. Moreover, the experimental results could be explained on the basis of the DFT computations and AFM topographic image (Figure 2). The frontier orbitals of the self-assembled Dye 1 (depicted in Figure 3) do not spread over the carboxylic groups and, therefore, it is difficult to expect that these “anchoring” carboxylic groups play a significant role in the process of charge transportation to the TiO2 conduction band. Moreover, Dye 1 molecules appear to be oriented in horizontal tube-like aggregates on the surface of the TiO2 anode. The long-range electron transport to TiO2 anode seems to be occurring through the terminals of the molecular helical wire cable. Although better charge transport within the long helical aggregate wire cable occurs, less effective TiO2 anode molecules are involved in receiving electronic charge. Recent studies on similar J-aggregate molecule favor our explanation [15]. Thus, it could be concluded that the orientation of J-aggregates affects the electron transport capabilities in these artificial light-harvesting device systems.
4. Conclusion
Frontier orbitals of Dye 1—TiO2 molecules have been calculated at DFT-B3LYP/6-31G+(d,p) level of theory, which exclude any significant role of the carboxylic groups in the dye anchor. The AFM topographic imaging of Dye 1 points to horizontal alignment of aggregates of Dye 1 assuring efficient charge transport through the helical wire’s terminals to a limited number of receiving centers on the TiO2 anode. Consequently, such a way of alignment of Dye 1 on the anode of DSSC leads to a lower electron relay. Contrary to our expectation, this is reflected on lower efficient light harvesting DSSC device.
However, a relatively reasonable efficient DSSC device that is based on a metal-free dye sensitizer (Dye 1) has been obtained. The fill factors and the current-to-electricity photovoltaic efficiencies were and 0.79 and and 7.6% for Dye 1 and N719 based devices, respectively. It is concluded that the nature of interaction with the anode and orientation of structural hierarchy dramatically affects the electron transport capabilities in these artificial light-harvesting device systems. Moreover, the anchoring carboxylic groups are not engaged in the electron transport process.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.