Elsevier

Applied Surface Science

Volume 452, 15 September 2018, Pages 337-351
Applied Surface Science

Full Length Article
Experimental and DFT insights into molecular structure and optical properties of new chalcones as promising photosensitizers towards solar cell applications

https://doi.org/10.1016/j.apsusc.2018.05.007Get rights and content

Highlights

  • Novel series of azochalcone dyes (5a → e) has been synthesized and characterized.

  • FESEM analysis of 5a → e dyes films showed nanorods structure.

  • The structural-optical properties of 5a → e dyes were studied: Experimentally and theoretically (DFT).

  • Hybrid Au/chalcone-5e/p-Si/Al heterojunction solar cell was fabricated and studied.

Abstract

A new series of (hydroxyl-arylazo)-chalcone derivatives (5a → e) has been prepared by condensation of 4-(4-acetylphenyl)-azo dyes 3 with 4-anisaldehyde in basic medium. The chemical structures were confirmed by elemental analysis, FT-IR, 1H NMR and Mass Spectra. The thin films of chalcone derivatives have been prepared using thermal evaporation technique under vacuum pressure of 10−5 mbar. The structural formation of these pristine films has been investigated using the field emission scanning electron microscopy (FESEM) and showed nanorod-structured chalcone thin films. The optical features of films are studied using spectrophotometric technique in the spectral wavelength range 200–2500 nm, from which the optical constants and the dispersion parameters are calculated. To identify these obtained experimental results, DFT calculations have been carried out at the B3LYP/6-311G (d,p) level of theory. The value of transport energy (Et) computed from DFT can exceed the value of optical energy gap (Egopt) determined from UV-vis spectroscopy by 0.45–1.15 eV due to the exciton binding energy (EB).Unraveling the correlated structural and optical properties of chalcones 5a → e with photosensitizer-type structure open up the avenue towards fabrication organic/inorganic hybrid (chalcone 5e/silicon) heterojunction solar cell. The optoelectronic properties and photovoltaic parameters of the cell have been studied.

Introduction

In the last few decades, great forward movements have been made towards improvement the performance of the organic solar cells as a potential candidate for the next new generation of photovoltaic solar energy sources. Particularly on the efficiency, stability, durability, cost saving and even environment-friendly, by incorporating various new organic materials which have unique and distinctive characteristics in solar cells fabrication [1], [2], [3]. Undoubtedly, the synthesis, development and utilization of organic materials for constructing each part of photovoltaic solar cells is one of the most important contributions to achieve the full success in this field. Most of organic dyes with effective moieties and high π-conjugated bridge/spacer usually possess a wide range of absorption spectrum in the visible light, additionally, having a configuration consisting of an electron donor and an electron acceptor (donor-π-acceptor) [1], [4], [5]. Under sunlight, the photon energy induces the electrons transport through the intramolecular charge transfer in organic dyes. The intramolecular route in an organic dye starts from an electron donor moiety passing through π-conjugated bridge/spacer to electron acceptor moiety until reaching the anchoring terminal end of the ligand, and finally enter another attached layer of organic or inorganic material. This journey of electron is generally called the “electron injection process”, which is the heart pump of solar conversion processes, in which the solar energy is converted to electrical energy in organic solar cells. Hence, Scientists and researchers are working hard in intensive studies and great efforts in the improvement of organic molecular structure to the enhancement the intramolecular charge transfer in organic materials through adopting many strategies [1], [4], [5]; such as donor-π-bridge/spacer-acceptor structure (D-π-A), acceptor-π -bridge/spacer-acceptor structure (A-π-A) and donor-π-bridge/spacer-donor structure (D-π-D). So, introducing the new chalcone series to elongate the π-backbone for enhancement of the intramolecular charge transfer which reflects on the improvement of electron injection process through these compounds.

Chalcones and their related compounds are well-known to be π-conjugated organic dyes, which can be potentially used in different photovoltaic areas including solar cells, photodetectors, light emitting diodes (LEDs) and photoswitching devices. Additionally, they were used before in nonlinear optics, optical sensing, optical computing, optical limiting, optical communication, electrochemical sensing and photoinitiated polymerization [6], [7], [8]. The simplest structure of chalcone compound consists of two benzene rings linked by α, β-conjugated keto-enol functional groups [9]. The conjugated double bond in between two benzene rings are responsible for the nonlinear optical property [9], [10].

New chalcones were prepared using the reaction of 3-acetyl-2,5-dimethyl thiophene with aldehyde in ethanolic NaOH. Optical properties of these compounds based on the polarity of solvent were studied [11]. These compounds are utilized in the antibacterial activity and showed that; pyrazol-chalcone compounds have better antibacterial activity than the reference drug tetracycline [11]. The novel chalcone derivative, (2E)-3-(4-fluorophenyl)-1-(4-{[(1E)-(4- fluorophenyl) methylene]amino} phenyl)prop-2-en-1-one, has been synthesized. The structural analysis was achieved by single-crystal X-ray diffraction, FT-IR and 1H NMR techniques. Comprehensive experimental and theoretical linear and nonlinear optical studies were carried out on the material using spectroscopic measurements, Z-scan technique and density functional theory (DFT) [12]. Gu et al. [13] studied the two photon absorptions and third-order optical nonlinear properties of 2, 4, 5-Trimethoxy-4-nitrochalcone. 4-[(2E)-3-(3-fluorophenyl) prop-2-enoyl] benzonitrile (3FPB), belonging to chalcone family was synthesized by Shetty et al. [14]. The study of absorption spectroscopy showed that the spectrum for 3FPB crystals exhibited the optical window and a lower cutoff wavelength of absorption at 343 nm. Additionally, the nonlinear optical measurements were carried out using Z- scan experiment with Nd:YAG laser nanosecond pulses at a wavelength of 532 nm. Teo et al. [15] synthesized six bis-chalcone compounds using Claisen-Schmidt condensation method. These dyes were used in dye sensitized solar cells (DSSCs). The highest efficiency was recorded from 0.022 to 0.054%. Chambon and co-workers [16] prepared a series of 2’-hydroxychalcones coordinated to borondifluoride which contain a donor-acceptor (D-A) groups. The boron-chalcones complexes were used as donor materials, while [6], [6] phenyl-C61-chalcones were used as the acceptor materials. The solar cells based on these chalcones achieved an efficiency reach to 1.13%. Rajakumar et al. [17] have synthesized novel dendrimers with chalcone as the surface group, thiadiazole as the core units and triazole as the branching. The optical properties showed that as the dendrimer generation increases the absorbance and PL emission increases. DSSC studies revealed that the dendrimers with chalcone show the higher power conversion efficiency reached to 4.5% with open circuit voltage of 0.69 V.

Density functional theory (DFT) is a well-known theoretical method to understand the structural, optical and electronic properties of any molecular system [18]. DFT is used as an excellent tool to identify the structure, functional groups, vibrational modes and molecular interactions of materials [19]. Several studies were reported on the structural analysis, vibrational, optical spectroscopy, and structural formation correlation of the substituted chalcones and their derivatives [20], [21], [22], [23].

Hence, based upon continuation of search for newer organic materials with better structural and optical characterizations, an attempt has been made to synthesize a new (hydroxyl-arylazo)-chalcone derivatives (5a → e) and screening for their molecular structure and optical features. Herein, we have decided to prepare the thin films of chalcone derivatives (5a → e) using the thermal evaporation technique. The morphological, optical and opto-electronic properties of chalcones thin films have been studied. As a support to these experimental results, DFT computational framework had been carried out at the B3LYP/6-311G (d,p) level of theory. Therefore, authors decided to fabricate the hybrid heterojunction solar cell based on chalcone 5e/p-silicon (Si). The electrical and photovoltaic parameters of this device are calculated and evaluated based on previous works.

Section snippets

Synthesis of the intermediate azobenzene derivatives 3a-e

A solution of NaNO2 (0.01 mol) was added drop by drop to a cold solution of 4-aminoacetophenone (0.01 mol) in conc. HCl. The freshly prepared diazonium salt that obtained was added with continuous stirring to cold solution of phenol derivatives (0.01 mol) in NaOH. The mixture was stirred at 0–5 °C for 2 h, diluted with water, and then neutralized by dil. HCl. The dark precipitates were filtered, washed thoroughly with water then dried well. Upon recrystallization from EtOH, intermediate azodyes

Chemical characterization

The syntheses of azo organic materials have attracting interest over the last two decades due to their applications in several scientific and industrial fields. There are many methods and procedures were reported for the preparation of these components [39], [40], [41]. Our strategy to prepare the target arylazo-chalcone derivatives 5a → e is for constructing of photovoltaic solar cells. The scheme 1 shows the synthetic route of 5a → e compounds. Starting by diazotization of 4-aminoacetophenone

Conclusion

The novel series of (hydroxyl-arylazo)-chalcone derivatives (5a → e) has been successfully synthesized and characterized. The homogenous thin films of 5a → e dyes have been successfully prepared using thermal evaporation technique. The structural analysis of the films has been investigated and showed the nanorods-shaped structures are ordered in crosslinking with each other to form network nanostructured.

Theoretically, a Comprehensive study of structural, vibrational analysis and optical

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