Marcelo Alatzatianou Rodrigues
Version of Record 9 September 2024 | Max de Ensino e Pesquisa, Rua Francisco de Vitória, 135, São Paulo 04116-180, SP, Brasil
Abstract
Mesoporous TiO2 films were sensitized with dyes extracted from plant leaves of agronomic interest: sugarcane and Arabica coffee, for the fabrication of dye-sensitized solar cells and photovoltaic performance comparison. Characterization was done using TiO2 UV–vis absorption spectroscopy of the extracts, reflectance spectroscopy to measure film thickness, and J-V curves in solar simulator for photoelectric response. The extracts’ absorption spectroscopy indicated an unexpected but significant concentration of anthocyanin in the coffee leaf extracts. Further, Arabica coffee based-cells achieved the greatest light conversion efficiency of 0.43 %, which was more than 40 % higher than sugarcane leaves (at 0.30 %). It was concluded that the anthocyanin spectral complementarity to chlorophyll boosted the photoelectric conversion efficiency. Based on these findings, future studies may consider the use of multi-pigment plants and, in particular, artificially inducing anthocyanin production in leaves to improve solar cell performance.
Keywords: Natural dyes; Dye-sensitized solar cells (DSSC); Anthocyanin; Chlorophyll; Co-sensitization; Arabica coffee
Introduction
Dye-sensitized solar cells (DSSC) are an emerging photovoltaic technology that has attracted interest due to important characteristics such as low cost, light weight, simple fabrication, semi transparency and favorable efficiency under low-light conditions [1], [2]. Power conversion efficiency (PCE), used herein, although still below competing technologies like crystalline silicon, has improved over the years and recently achieved the impressive mark of 15.2 % (standard AM1.5G) and even up to 30.2 % depending on the ambient light intensity [3].
Nowadays, DSSCs use organic (natural or synthetic) or inorganic dyes as sensitizers. In most cases, because of the superior efficiency and higher stability [4], Ruthenium-based dyes are chosen for DSSC fabrication, despite the disadvantages of high cost, fabrication complexity, scarce natural sources and significative environmental impact. Thus, the use of natural dyes extracted from plants is a very attractive alternative, particularly considering the easy and low cost production, renewable and abundant supply and environmental safety [5]. Natural dye-sensitized solar cells (NDSSC) efficiency varies depending on the plant selection, fabrication materials and utilized protocol, achieving up to 1 % in most cases, although more than 8 % using co-sensitizing and more complex solutions have been reported [1].
Fig. 4. Measured UV–vis absorption spectra of each dye and subtraction between the spectra of coffee leaves and sugarcane.
Fig. 4 shows the obtained absorption spectra of each plant. As expected, leaves from sugarcane and Arabica coffee show very similar shapes, following the typical action spectrum expected from green leaves [17], [18], [19], [20], and also very little intensity differences in the blue and red ranges. Coffee leaves show peaks at 493 nm and 611 nm while sugarcane leaves show peaks at 486 nm, 616 nm and 650 nm, whereas the range 500–600 nm shows lower absorption in both spectra.
Fig. 4
Fig. 5
The curve “coffee minus sugarcane” has great similarities with the anthocyanin reference spectrum (blackberry) considering the high absorption in the 500–600 nm range and the lower level on the red and blue parts of the spectrum, where coffee and sugarcane absorptions are very similar, therefore indicating the strong presence of anthocyanin in the coffee leaves… Firstly by exclusion, since the significant pigments present in higher plants leaves (except for anthocyanin) do not contribute to absorption in the 500–600 nm range [17], [20], as illustrated by Fig. 5 [25]. Secondly, anthocyanin presence in mature leaves has already been reported [26], [27], [28].
2.3.1. Reflectance spectroscopy
Given the great influence of the TiO2 mesoporous layer thickness on the performance of the cells, measurements were conducted by reflectance spectroscopy [14] using Stellarnet® Black-Comet-C-SR 200-1080 nm spectrometer with Stellarnet® R600-8-Vis-NIR reflectance probe, RPH3 support and TF-STD1 thickness standard for measuring methodology confirmation.
2.3.2. UV–vis absorption spectroscopy
Optical properties of each dye were observed by the UV–vis absorption spectra, which were obtained using Stellarnet® Black-Comet-C-SR 200–1080 nm spectrometer with 5 W Tungsten Krypton 2800K light source. The dye absorption spectra were measured right after cell fabrication.
2.3.3. I-V characteristics measurement
Photoelectric conversion characteristics were measured by obtaining I-V curves under illumination using an Ossila® Source Measure Unit (X200) in Solar Simulator with Super Vision® HID Xenon 1B15 6000 K 35 W lamp, calibrated using a Stellarnet® CR2 UV–vis-NIR cosine receptor and Stellarnet® Black-Comet-C-SR spectrometer for standard AM1.5G (100 mW/cm2). From the I-V curves electrical parameters were calculated considering the active area of 0.2827 cm2 (VOC,JSC,PMPP,VMPP,JMPP,FF and η). Electric current readings were also converted to current density (J-V).
