The photovoltaic performance of some chlorophyll-sensitized DSSCs.
Chlorophyll, being the most abundant pigment that commonly found in plants, bacteria, bryophytes and algae, plays a vital role in photosynthesis. Chlorophylls are natural pigments and therefore safe, environmental friendly, easily available and cheap. Chlorophyll has been experimented to function as a photosensitizer in dye-sensitized solar cells (DSSCs) as DSSCs mimic the photosynthesis process in green plants. DSSC was first developed by Gratzel in 1991 and since then has gained tremendous attention as its fabrication is cheap and easy. A DSSC basically comprises a semiconductor that has been soaked in sensitizing dye (chlorophyll), a counter electrode, and an electrolyte containing a redox mediator. The dye absorbs light, which is transformed into electricity. Chlorophyll can be extracted from the leaves of pomegranate, bougainvillea, papaya, Pandanus amaryllifolius, spinach, green grasses, seaweeds, algae and bryophytes. Chlorophyll from these sources has been studied as possible photosensitizers for DSSCs. Most researches done in chlorophyll DSSC use the extracted natural pigments. The type of solvent and pH of the dye solution will also affect the stability of chlorophyll and subsequently the performance of the DSSCs. This chapter will present an inexhaustive overview on DSSCs using chlorophyll as dye.
- light adsorption
- dye-sensitized solar cells
Over billions of years, Mother Nature has been converting light from the sun into energy via photosynthesis. Sunlight is the most abundant and sustainable energy source that is free. The Earth receives energy from the sun at the rate of ~12 × 1017 J s−1 . This has exceeded the yearly worldwide energy consumption rate of ~1.5 × 1013 J s−1 . Therefore, it is a challenge to devise an approach for the effective capture and storage of solar energy for our consumption since fossil fuels such as oil and gas will be depleted in the years to come. In order to imitate the photosynthesis process, Gratzel and coworkers have developed dye-sensitized solar cells (DSSCs) based on the similar working mechanism . Nevertheless, one main difference between photosynthesis of plants and DSSCs is that the energy can be stored in plants for later use but DSSC is unable to store energy. Ever since the birth of DSSCs, they have become the spotlight of attention among scientists and researchers around the world as they are much cheaper, easier to fabricate, and more environmental friendly when compared with conventional silicon solar cells [3, 4]. A DSSC is an electrochemical device that comprises a transparent-conducting oxide (TCO) glass over which is deposited a semiconductor. The semiconductor will be soaked in a dye solution. An electrolyte with reduction-oxidation (redox) mediator and cathode are the other remaining components. The fluorine-doped tin oxide (FTO)/semiconductor/dye assembly is referred to as photoanode. Indium-doped tin oxide (ITO) and FTO are two TCOs used commonly in DSSCs. Titanium dioxide (TiO2) is one of the popular semiconductors used for DSSC since it is cheap, non-toxic, and possesses a large bandgap . TiO2 is deposited on the TCO substrate in the form of TiO2 nanoporous particle network to increase the coverage area for the sensitizing dye. The cathode is made up of another TCO on top of which platinum is deposited. Carbon and conducting polymers can also be employed as counter electrode. If a gel polymer electrolyte is used, it is sandwiched between the photoanode and cathode. The dye, on the other hand, can be categorized into two groups: synthetic and natural. The most frequently used synthetic dye is the ruthenium (Ru)-based dyes but they are not environmental friendly since Ru is a heavy metal . Such dyes are also very expensive due to the scarcity of Ru. By contrast, natural dyes are readily available and thus cheap besides being non-toxic, environmental friendly, biodegradable, easily extracted as well as can be used without any purification . Since DSSC mimics the photosynthesis of green plants, therefore chlorophyll can also function as photosensitizer for DSSC. In fact, report on chlorophyll as photosensitizer on zinc oxide (ZnO) semiconductor was first published by Tributsch in 1972 .
2. Basic working principle of chlorophyll-sensitized DSSC
In this chapter, discussion is based on the TiO2 semiconductor photoanode. However, occasionally we refer to zinc oxide (ZnO) and tin dioxide (SnO2). The dye is chlorophyll extracted from various sources including leaves, grasses, flowers, seaweeds, and algae. The electrolyte is generally in the form of liquid and quasi-solid state. The commonly used mediator is the
The excited chlorophyll molecules (
The oxidized chlorophyll dye molecules (
The electron in the TiO2 conduction band flows out of the device through the load to reach the counterelectrode and reduced the triiodide ion as follows:
The iodide ion is now restored, the electron circuit is completed, and the whole system is back to its original state to start a new cycle. These processes will continue as long as there is light and current is produced in the external circuit continuously. Under illumination, the voltage generated is given by the energy difference between the photoanode’s Fermi level and the electrolyte’s redox potential. Figure 1 illustrates the schematic diagram of the chlorophyll-sensitized DSSC and its operating principle.
The light to electricity conversion efficiency (
Absorb light in the visible region.
Good attachment at the surface of photoelectrode to ensure fast electron transfer.
Good interfacial properties and high stability to enable good absorption to TiO2.
Easily accepting replacement electron from electrolyte.
Excited state of dye must be slightly above the TiO2 conduction band and its ground-state level below the redox potential of the electrolyte.
Lifetime of the dye must be consistent with device life.
Stable enough to sustain about 20 years exposure to natural light.
3. Performance of chlorophyll-sensitized DSSCs
Table 1 summarizes the performance of some DSSCs employing chlorophyll as photosensitizer reported by researchers worldwide. Herein, the illumination of the chlorophyll-sensitized DSSCs was carried out under intensity of 100 mW cm−2 unless stated otherwise.
|Arugula leaves (fresh)||TiO2/FTO||I−/I3− LE||Pt/FTO||0.788||0.599||0.420||0.200|||
|Arugula leaves (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||0.713||0.594||0.430||0.180|||
|Parsley leaves (fresh)||TiO2/FTO||I−/I3− LE||Pt/FTO||0.535||0.445||0.340||0.070|||
|Parsley leaves (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||0.448||0.553||0.400||0.090|||
|TiO2||Not stated||Not stated||1.300||0.616||0.602||0.500|||
|Pawpaw leaves||TiO2/FTO||LE||Not stated||0.649||0.504||0.605||0.200|||
|Pomegranate leaves||TiO2/ITO||I−/I3− LE||Pt/FTO||2.050||0.560||0.520||0.597|||
|Bougainvillea leaves||Au/TiO2/FTO||I−/I3− LE||Pt/FTO||3.230||0.500||0.410||0.618|||
|Spinach oleracea (fresh)||TiO2/FTO||I−/I3− LE||Pt/FTO||0.332||0.590||0.420||0.080|||
|Spinach oleracea (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||1.110||0.583||0.460||0.290|||
|Red spinach leaves||TiO2/ITO||I−/I3− LE||C/FTO||1.000||0.505||0.578||0.583|||
|Green spinach leaves||ZnO/ITO||I−/I3− LE||C/ITO||0.052||0.590||0.530||0.016|||
|Papaya leaves||TiO2/FTO||I−/I3− LE||Pt/FTO||0.360||0.325||0.560||0.070|||
|TiO2/ITO||I−/I3− LE||C/FTO||0.060 mA||0.394||0.250||-|||
|Jatropha leaves||TiO2/ITO||I−/I3− LE||C/FTO||0.042 mA||0.350||0.250||-|||
|Ipomoea leaves extract||TiO2/ITO||I−/I3− LE||Pt/ITO||0.850||0.495||0.536||0.233|||
|Basil leaves (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||1.398||0.581||0.499||0.409|||
|Basil flower||TiO2/FTO||I−/I3− LE||Pt/FTO||1.120||0.600||0.400||0.270|||
|Mint flower||TiO2/FTO||I−/I3− LE||Pt/FTO||0.450||0.560||0.380||0.090|||
|Mint leaves (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||0.980||0.579||0.400||0.227|||
|Lemon leavesa||TiO2/FTO||I−/I3− LE||C/FTO||1.080||0.592||0.100||0.036|||
|Morula leavesa||TiO2/FTO||I−/I3− LE||C/FTO||0.059||0.472||0.050||0.001|||
|Fig leaves (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||2.091||0.596||0.515||0.642|||
|Berry leaves (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||3.573||0.595||0.441||0.939|||
|Banana leaves (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||1.770||0.596||0.492||0.522|||
|Peach leaves (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||2.555||0.611||0.422||0.659|||
|Black tea leaves||TiO2/FTO||I−/I3− LE||Pt/FTO||0.390||0.550||0.400||0.080|||
|SnO2/FTO||I−/I3− LE||Not stated||0.700||0.540||0.610||0.260|||
|La-SnO2/FTO||I−/I3− LE||Not stated||0.820||0.540||0.540||0.290|||
|La-Cu-SnO2/FTO||I−/I3− LE||Not stated||1.010||0.560||0.510||0.310|||
|Eggplant pulp||TiO2/FTO||I−/I3− LE||Pt/FTO||0.350||0.630||0.390||0.090|||
|Moss bryophyte ||TiO2/FTO||GPE||Pt/FTO||5.780||0.600||0.570||1.970|||
|Kelp (brown algae)||TiO2/TCO||I−/I3− LE||Pt/TCO||0.433||0.441||0.620||-|||
|Green algae (fresh)||TiO2/FTO||I−/I3− LE||Pt/FTO||0.134||0.416||0.210||0.010|||
|Green algae (dried)||TiO2/FTO||I−/I3− LE||Pt/FTO||0.397||0.559||0.440||0.100|||
It is evident that the condition of leaves whether fresh or dried affects the adsorption of chlorophyll onto the photoanode surface and consequently the performance. Taya et al.  observed that DSSCs having chlorophyll extracted from fresh leaves of
In the case of chlorophyll extract from ipomoea leaves (leaves of morning glory flower), 50°C is the optimum temperature for TiO2 immersion with efficiency of 0.278% . Lower efficiencies of 0.233 and 0.259% were obtained when the TiO2-soaking temperature in ipomoea leaves extract solution were at 30 and 80°C, respectively for 24 h . Other than temperature, pH of the dye solution is another factor influencing the efficiency. Maintaining the soaking temperature at 50°C, the pH of ipomoea dye solutions was adjusted to pH 1, 2, and 3 . However, there was no mention on the type of acid used. Thus, it is not known whether the anion of acid had any influence on the DSSC performance. Nevertheless, improvement in efficiency can be seen when the acidity of the dye solution was adjusted to pH 1 and 2 with efficiencies of 0.318 and 0.292%, respectively. However, further increasing the pH to 3 decreased the efficiency (
The type of solvent used for pigments extraction can also give different results in the absorption spectrum [45–47]. From the work of Al-Alwani et al. , it has been reported that the UV-vis absorption spectra of chlorophyll extracted from
It should be noted that both betalain and chlorophyll pigments can be extracted from the flowers of
Khan and coworkers  have examined the effect of acid treatment on TiO2 nanoparticles in the making of TiO2 paste to be coated on ITO glass substrate via the doctor blade method. It is found that chlorophyll from red spinach leaves-sensitized DSSC without any acid treatment on TiO2 photoanode exhibited the efficiency of 0.296% which is lower than the TiO2 acid-treated DSSC with chlorophyll extracted from the same source under intensity of 50 mW cm−2 . The presence of acid can prevent agglomeration of TiO2 nanoparticles and results in better TiO2 dispersion and thereby offer more adsorption sites for the dye molecules . Khan et al.  used citric acid (organic acid) and nitric acid (inorganic acid) to prevent TiO2 agglomeration in the DSSC fabricated with chlorophyll from red spinach leaves, and TiO2 treated with citric acid gave higher efficiency of 0.583% compared to that using nitric acid treatment on TiO2 electrode (
It can be noted from Table 1 that SnO2 was employed as photoanode instead of TiO2 in the cell having chlorophyll extracted from
Chang et al.  have investigated the plasmonic effect of gold (Au) nanoparticles with an average size of 27 nm in TiO2 DSSC using chlorophyll from bougainvillea leaves. An efficiency of 0.618% was obtained. The Au nanoparticles showed localized surface plasmon resonance behavior when the frequency of the incident light came close to the surface plasmon frequency of Au and consequently improved light absorption leading to a considerably high efficiency of 0.618% as listed in Table 1. Also, the interface between Au and TiO2 formed a Schottky barrier where electrons will be blocked from re-entering the dye or electrolyte, which decreased electron recombination and improved the DSSC performance. Earlier report on TiO2 loaded with Au nanoparticles prior to chlorophyll sensitization was published by Lai and coworkers . Instead of using
From Table 1, it can be observed that most of the DSSCs employ liquid electrolytes based on
It is worth mentioning from Table 1 that the cell having the efficiency of 0.590% with chlorophyll extracted from shiso leaves used copper iodide (CuI) as hole transport material (HTM) instead of conventional liquid electrolyte . Therefore, the DSSC has the configuration of FTO/TiO2/chlorophyll dye/CuI. CuI, a p-type semiconductor, has bandgap of 3.1 eV and good optical transparency [54, 55]. The p-CuI was coated onto the chlorophyll/TiO2/FTO using dip- and spray-coating technique as this method involves low calcination temperature and thus the degradation of dye will not occur [16, 54]. The p-CuI solid-state DSSC has similar working principle with conventional DSSC except that after photon absorption, the dye molecules will be excited and then inject electrons and holes into TiO2 and p-CuI, respectively. This indicates that the dye at ground state must be positioned below CuI valence band and the dye-excited state should be above the TiO2 conduction band in order to ensure proper functioning of chlorophyll CuI DSSC. With the usage of HTM, there will be no issue on pigment deterioration since natural pigment is unstable against the oxidized species in electrolyte with iodine as redox mediator .
Most of the reports on chlorophyll-sensitized DSSCs summarized in Table 1 do not contain information on the type of chlorophyll used. Among the six chlorophylls, chlorophyll
It has been shown that chlorophyll has good potential to serve as photosensitizer in dye-sensitized solar cells. Moreover, they are cheap, non-toxic, biodegradable, easily found, and easy to use as sensitizer. Although the efficiency is still considerably low with highest efficiency to date being only 4.600% from DSSC with
The authors thank the University of Malaya and Malaysian Ministry of Higher Education (MOHE) for the UMRG grant no. RP024C-14AFR and PRGS grant no. PR001-2014A.