Effect of thienyl units in cyanoacrylic acid derivatives toward dye-sensitized solar cells

Highlights

• A series of cyanoacrylic acid derivatives with thienyl units were synthesized.

• The effect of chemical structure on their properties was demonstrated.

• The selected compounds were tested in dye-sensitized solar cells.

• The highest PCE exhibited cell with symmetrical molecule contains bithiophene core.

• Co-sensitization enhanced efficiency to 6.3% being higher than with a neat N719.

Abstract

A series of heterocyclic donor-acceptor systems were synthesized and well characterized by using ¹H, ¹³C NMR, FT-IR, and elemental analysis. They were designed to investigate the effect of thiophene and cyanoacrylic acid number units on the thermal, optical, electrochemical and finally photovoltaic properties of dye-sensitized solar cells prepared with the selected compounds. The effect of chemical structure on their properties was demonstrated. They showed the beginning of thermal decomposition between 230 and 270 °C. The compounds absorbed the radiation in the range of 300–500 nm or 200–400 nm. They were electrochemically active and varied in energy band gap from 3.40 to 1.58 eV. Additionally, their optimized geometry, HOMO–LUMO levels, ionization potential, and electron affinity were evaluated using density functional theory. The photovoltaic devices based on TiO₂ sensitized with the obtained molecules exhibited low power conversion efficiency, which was the highest for the device containing the symmetrical molecule with bithiophene structure. Under co-sensitization, the cell made of the same compound gave significant enhancement of efficiency of 6.3% being higher to that of the individual device prepared from dye N719 (5.75%). Moreover, the effects of immersion time of TiO₂ electrode in the dye solution and co-sensitization methods were tested. The surface morphology of photoanode was investigated using atomic force microscopy.

Introduction

The growing demand for electric power as well as the promotion of ecological awareness is undoubtedly the main reasons why scientists from around the world focus their research on an alternative to non-renewable energy sources. Among them, photovoltaics [1,2], in particular, dye-sensitized solar cells is a dynamically developing energy segment (DSSCs) [3,4]. The study published by Grätzeland O'Regan in 1991 met a great interest over the years [5]. DSSCs have gained recognition for the simplicity and minimalized costs of production as well as their ability to the utilization of diffused light. The photovoltaic efficiency of the constructed DSSCs was improved to 14% [[6], [7], [8]]. The key component responsible for the performance of DSSCs is the sensitizer, which plays a main role in light-harvesting and the electron injection [9]. In general, dyes in DSSCs can be divided into two groups: ruthenium or zinc based complexes and metal-free sensitizers. However, the use of metal complexes as sensitizers, though high performance, [7] is impractical due to the limited availability of materials, high-costs, and difficulties with the synthesis [9]. The most known type of metal-free dyes is composed by an electron donor (D), an electron acceptor (A), and a π-bridge, conjugated spacer between them, which is called a D-π-A system [[10], [11], [12], [13], [14], [15]]. Due to photoexcitation, the electron from the donor is fast transferred to the acceptor and a negative charge is injected into the semiconductor. The acceptors, such as carboxylic acid, cyanoacrylic acid, rhodanine-3-acetic acid or trialkoxysilyl group, can additionally play a role of dye anchoring groups with the surface of a semiconductor [16]. It was found that the photovoltaic performance of DSSCs is substantially affected by the type as well as the number of anchoring groups [17,18].

Nowadays, an extremely popular building block of donors (D) are electron-rich thiophene rings [7,10,11,13,14,16,[18], [19], [20], [21], [22]]. They can be successfully introduced to the structure of the final dye using many chemical reactions e.g. coupling [11,13,14,18,[23], [24], [25], [26]]. Moreover, thiophene units easily react in the formylation reaction [[27], [28], [29]], thus, that they can be connected with an acceptor group (A) such as cyanoacrylic acid [16,26,[30], [31], [32], [33], [34]] or rhodanine-3-acetic [16,30,32,35,36]. It is also worth emphasizing that compounds containing thiophene rings often present good thermal stability [[37], [38], [39], [40]], low oxidation potentials [[41], [42], [43]] and show low energy band gaps [37,38,41]. It was found that minor changes in the molecular structure may be decided about the possibility of applying them in DSSCs [32,44]. Therefore, the aim of our work was investigating the influence of the number and structure of thiophene rings on the properties of dyes when the anchoring group is cyanoacrylic acid. Additionally, the synthesized mono- and dicyanoacrylic acid derivatives may reveal the impact of the number of acceptor groups on the UV–vis absorption, thermal and electrochemical behavior and finally on utilization in DSSCs. All dyes presented in this work were obtained by Knoevenagel condensation of selected aldehydes containing thiophene substituents with cyanoacrylic acid. The structure of the synthesized compounds was verified by NMR (¹H, ¹³C, H-H COSY, H-C HMQC, H-C HMBC), FTIR spectroscopy and elemental analysis. Next, their thermal, optical and electrochemical properties supported by DFT calculations were discussed. Finally, the selected compounds were applied for construction of dye-sensitized solar cells.