Synthesis of Zn(II) porphyrin dyes and revealing an influence of their alkyl substituents on performance of dye-sensitized solar cells

Highlights

• Seven Zn porphyrins for dye-sensitized solar cells were obtained by Lindsay method.

• The increase of the alkyl chains length increases photoconversion efficiency.

• Doubling the number of alkyl substituents leads to improvement of the efficiency.

• Zn porphyrin with dodecyl substituents shows the higher efficiency of 1.29 %.

Abstract

In this study, the series of A₃B-type zinc porphyrin dyes have been synthesized for dye-sensitized solar cells (DSSC). All the received zinc-porphyrin complexes bear different alkyl substituents (none, tert-butyl-, methoxy-, hexyloxy-, dodecyloxy-, a pair of hexyloxy-), bulky electron-donating substituent (diphenylamino) in each of three meso-phenyl rings and carboxylic acid as acceptor/anchoring group. The influence of the number and length of alkyl chains on the photophysical properties and DSSCs photovoltaic performance was investigated, as well as quantum-chemical modeling was carried out. Our results reveal that the alkyl chains length and number affect cell performance. We found that DSSC power conversion efficiency increases with an increase in the length and number of alkyl substituents in the porphyrins. A maximum efficiency of 1.29 % was observed in solar cells with dodecyloxy-substituted zinc porphyrin.

Introduction

Every year issues related to the search for renewable energy sources, as well as the transition to environmentally friendly production, are becoming increasingly relevant. The solution to the problem of ever-increasing demand for electricity without significant environmental consequences can be found through the use of alternative energy sources. Today, most renewable energy sources are characterized either by limited potential or are associated with difficulties in their development, which calls into question their feasibility from an economic point of view. Solar radiation is an almost inexhaustible and environmentally friendly source of energy. Betting on solar energy should be considered not only as a win-win but also in the long run as a non-alternative choice for humanity [[1], [2], [3]].

One of the promising devices in this area is dye-sensitized solar cells (DSSCs), extensive studies of which began with the work of the Grätzel scientific group in 1991 [4]. Solar panels of this type are promising in many respects since they can be made from cheap, environmentally friendly materials and do not require complicated equipment in production [5,6]. Besides, they are largely resistant to temperature extremes, effectively absorb radiation at different angles of incidence, are durable and easy to operate. Solar cells of this type can be considered built according to the biomimetic principle, since they simulate natural photosynthetic structures in which photo-induced energy and electron transfer occur [7,8].

There are many works devoted to the study and modification of various constituent parts of the DSSC. The latest works cencerning semioconductor layer describe semiconductor layer preparation method [9], it’s thickness optimization [10], investigation of the effect of different additives to TiO₂ particles [11,12] or ZnO particles [13]. Recent progress on electrolytes development is presented in review [14]. There is a tendency to develop gel electrolytes [15,16] or solid-state DSSC [17], which adresses long-term stability issues and provides additional features such as flexibility [18]. Several new Pt-free counter electrodes were also reported recently [[19], [20], [21]]. However, a significant part of the effort in this field is directed towards the design of the optimal structure of the sensitizer.

Synthetic meso-arylsubstituted porphyrins and their metal complexes are promising candidates as organic sensitizers for light harvesting systems due to a set of unique properties (intense absorption in the visible region, fluorescent properties, suitable HOMO and LUMO levels, high chemical, photo- and thermal stability). Various porphyrins have been designed, and their electronic structure and excitation properties, in addition to their photon-to-current efficiencies, have been explicitly investigated both experimentally and theoretically [22]. Porphyrin sensitizers can provide DSSC power conversion efficiencies of up to 12–13 % [[23], [24], [25]]. Due to the mention set of properies, porphyrins and benzoporphyrins bearing various phenyl groups at their meso-positions have also potential for use as additives for a poly(3-hexylthiophene) (P3HT):phenyl-C₆₁-butyric acid methyl ester blend from which bulk heterojunction solar cells are formed [26]. In favor to this approach, it is found the sensitizing additive of the metal porphyrins to P3HT does not affect the charge carrier mobility in the polymer remarkably [27]. Thus, for the rational design of new dyes, it is important to carry out a comprehensive study of the influence of individual substituents in the structure of the dye on solar cell operation.

Many effective sensitizers contain hydrophobic alkyl or bulky aromatic substituents in their structure [[28], [29], [30]]. Such elements of the structure ensure the alignment of the hydrophobic barrier, minimizing surface recombination losses, and can also provide a more uniform deposition of the dye, preventing aggregation, to which porphyrins are inclined. Several studies on the effect of the length of alkyl substituents in various types of dyes have been reported [[31], [32], [33], [34]]. Moreover, tuning charge carrier mobility by varying the length of alkyl groups in organic semiconducting molecules have been demonstrated as well [35]. Yet, the effect may be different for different classes of compounds. Therefore, a study the effect of alkyl substituents on zinc (II) porphyrin model compounds is actual as such porphyrins are increasingly used in DSSC.

The objective of this work is to compare the DSSC effectiveness of A₃B-type Zn (II) porphyrins with a carboxyl anchoring group and different hydrophobic substituents in the meso-positions (compounds 1-7). We suggest a low-stage approach for A₃B-type porphyrins synthesis with high yields. For these compounds, comprehensive studies, such as photophysical properties, DFT-calculations of frontier molecular orbitals localization, as well as fabrication and photovoltaic parameters measurement of test DSSCs are carried out and the obtained theoretical and experimental data are compared.