Dyes extracted from rengas (Gluta spp.) and mengkulang (Heritiera elata) wood were investigated as sensitizers in dye-sensitized solar cells (DSSCs). Three types of sensitizers, including individual sensitizer, mixture sensitizer, and co-sensitizer, exhibited different patterns of absorption properties under UV-Vis spectroscopy. The incident photon-to-current efficiency (IPCE) was analyzed via spectral response to examine the generation of photocurrent. Because mixture sensitized DSSCs obtained broader absorption spectra, they were expected to achieve good light harvesting and hence, enhanced photocurrent and conversion efficiency. The photovoltaic performance was further examined by electrochemical impedance spectroscopy (EIS). The mixture sensitized DSSCs exhibited good conversion efficiency (0.21% and 0.30%) compared with individual sensitized DSSCs (0.16% and 0.11%). The co-sensitized DSSCs also showed increased conversion efficiency with ruthenium (N719) dye as a co-sensitizer. The parameters calculated from EIS analysis were used to determine suitable conditions for the dye to be implemented in DSSC. The behavior of electron transport was determined to be efficient due to the increase of electron diffusion coefficient, electron lifetime, and low recombination rate as achieved by the mixture sensitized DSSCs.
Electron Transport Studies of Dye-Sensitized Solar Cells based on Natural Sensitizer Extracted from Rengas (Gluta spp.) and Mengkulang (Heritiera elata) Wood
Nur Ezyanie Safie,a Norasikin Ahmad Ludin,a,* Norul Hisham Hamid,b,cParidah Md. Tahir,c Mohd Asri Mat Teridi,a Suhaila Sepeai,a Mohd Adib Ibrahim,a and Kamaruzzaman Sopian a
Dyes extracted from rengas (Gluta spp.) and mengkulang (Heritiera elata) wood were investigated as sensitizers in dye-sensitized solar cells (DSSCs). Three types of sensitizers, including individual sensitizer, mixture sensitizer, and co-sensitizer, exhibited different patterns of absorption properties under UV-Vis spectroscopy. The incident photon-to-current efficiency (IPCE) was analyzed viaspectral response to examine the generation of photocurrent. Because mixture sensitized DSSCs obtained broader absorption spectra, they were expected to achieve good light harvesting and hence, enhanced photocurrent and conversion efficiency. The photovoltaic performance was further examined by electrochemical impedance spectroscopy (EIS). The mixture sensitized DSSCs exhibited good conversion efficiency (0.21% and 0.30%) compared with individual sensitized DSSCs (0.16% and 0.11%). The co-sensitized DSSCs also showed increased conversion efficiency with ruthenium (N719) dye as a co-sensitizer. The parameters calculated from EIS analysis were used to determine suitable conditions for the dye to be implemented in DSSC. The behavior of electron transport was determined to be efficient due to the increase of electron diffusion coefficient, electron lifetime, and low recombination rate as achieved by the mixture sensitized DSSCs.
Keywords: DSSC; Natural Dye; Hardwood waste; Impedance; Electron Transport
Contact information: a: Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia; b: Faculty of Forestry, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia; c: Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia: *Corresponding author: firstname.lastname@example.org
The development of the dye-sensitized solar cell (DSSC) as an alternative device in solar energy applications has been extensively investigated. DSSCs have advantages such as lower cost and less toxic manufacture than conventional silicon-based cells (Shalini et al. 2015). Generally, DSSC devices are composed of a photoelectrode including the wide bandgap semiconductor metal oxide with adsorbed dye molecules and a counter-electrode made of a catalyst such as platinum covering the conducting glass. A mesoporous metal oxide such as TiO2, ZnO, SnO2, and Nb205 that is coated on the conducting glass is sintered to allow electronic conduction (Narayan 2012). Among them, anatase TiO2, which has high dielectric constant, is the most common material used, as it provides good electrostatic shielding of the injected electron which then prevents from the recombination effects. The high refractive index of anatase TiO2 also leads to an efficient light scattering effect inside the porous photoelectrode, which is important to enhance light harvesting efficiency (Olea et al. 1999). The electrolyte is injected in between the electrodes to help in electron transport and in restoring reduced sensitizer. The most common electrolytes used are composed of the redox couples iodide/triiodide dissolved in liquid organic solvent (Faccio et al. 2011)
The principle of DSSC was proposed to mimic photosynthesis; the ability of the device to harvest visible light and convert chemical energy into electricity is very important (Koyama et al. 2009). Light is absorbed by a component known as the sensitizer, which is grafted on the TiO2 surface of the photoelectrode. The role of the sensitizer is to supply electrons throughout the cell after being oxidized in the visible light range (380 nm to 800 nm). Generally, the sensitizer is made of metal-organic dyes, organic dyes, or natural derivative dyes.
The most stable and high conversion efficiency have been achieved with ruthenium (Ru) polypyridal compounds, such as cis-dithiocyanobis(4,4’-di-carboxy-2,2’-bipyridine) ruthenium(II) (N3), ditetrabutylammonium-cis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylatol)ruthenium(II) (N719), and tris-tetrabutylammonium tri-thiocyanato-4,4’,4”-tricarboxy-2,2:6’,2”-terpyridineruthenium(II) (black dye) (Nazeeruddin et al. 2001; Marwa et al. 2016; Wang et al. 2016). Although the conversion efficiency of ruthenium complex dyes is greater than 10%, there are some disadvantages that limit their potential. For instance, ruthenium is a rare, expensive metal that is not environmentally friendly (Adeloye and Ajibadebu 2014).
Organic dyes also are efficient sensitizers because of their satisfactory stability and large molar absorption coefficient (Zeng et al. 2014). Because organic dyes are metal-free, there is no concern about resource limitation, and their stereochemical structure can be modified (Ooyama and Harima 2012). The modification of molecular structures adjusts the absorption properties and rearranges the energy level of the sensitizer, resulting in better conversion efficiency. However, most molecular modifications of organic dyes require complicated synthesis procedures (Hara et al. 2005). Among the organic dyes, porphyrin-based DSSC has achieved a high conversion efficiency of 13% (Mathew et al.2014).
Natural pigments are good alternative sensitizers because of the simple extraction procedure, low cost, and low environmental impact. These abundant natural pigments are present in some fruits, flowers, and leaves of plants; they contain compounds that strongly absorb visible light at different wavelengths, resulting in different visible colors (Lim et al. 2015). Several natural pigments have been extensively investigated, such as chlorophyll (Wang et al. 2010), anthocyanin (Ozuomba et al. 2013), xanthophyll (Kartini et al. 2015), and carotene (Shanmugam et al. 2013). While the conversion efficiencies achieved (< 3%) are much lower than the industrial requirements (Basheer et al. 2014), numerous studies have investigated various plant species as sources of sensitizers. This paper examines the performance of dye-sensitized solar cells based on natural sensitizers extracted from rengas (Gluta spp.) and mengkulang (Heritiera elata) wood. The highlight in this study is the electron transport mechanism inside the cells, which will predict any fault in the devices. The three types of sensitizers examined were an individual sensitizer (extracted from rengas (Gluta spp.) and mengkulang (Heritiera elata) wood), a mixture sensitizer (mixing both of the individual dyes), and a co-sensitizer (individual sensitizer co-sensitized with ruthenium (N719) dye). The electron kinetics of these sensitizers are discussed.
Preparation of Sensitizers
The procedure for the synthesis of mengkulang and rengas dye has been detailed elsewhere (Safie et al. 2017). The crude extract from rengas and mengkulang was prepared by Soxhlet extraction. Methanol obtained from Sigma-Aldrich (St. Louis, MO, USA) was used as an organic solvent in the cold extraction of dye from the sawdust. The mixture was left overnight at room temperature before undergoing Soxhlet extraction and was handled in low light to minimize photooxidation. The crude extract was stored at 4 °C. Various dye solutions were prepared by mixing the individual dyes (mixture sensitizers) and co-sensitized mengkulang and rengas with ruthenium (N719) dye (co-sensitizers). To prepare a 0.3 mM N719 sensitizer, 50 mL acetonitrile (J. T. Baker® Chemicals, Philadelphia, PA, USA) and 50 mL 4 tert-butyl alcohol (99.7%, Sigma-Aldrich) were mixed with 0.036 g B2 (N719) dye (Dyesol, NSW, Queanbeyan, Australia). The mixture was incubated overnight at room temperature before use.
Assembly of the DSSCs
To prepare the photoelectrodes, fluorine-doped conducting tin oxide (FTO, ~15 Ω sq-1, Solaronix SA, Aubonne, Switzerland) was cleaned and then washed sequentially with acetone and DI water in an ultrasonic cleaner. In this study, two layers of TiO2 were used including porous and scattering layers as the first and second layer, respectively. TiO2 paste (90 T, Dyesol) was coated onto the conducting surface of the FTO as the first layer by the doctor blading technique. The coated glass was heated at 450 °C for 30 min. The step was repeated followed by the second layer which is a TiO2 paste (WER 2, Dyesol). The coated conducting glass was maintained with an active layer 1 cm2 by scrapping the excess TiO2. The TiO2 thin film was immersed in various dye solutions at room temperature for 24 h to allow the dye molecules to adsorb to the TiO2 surface. Before assembly, the TiO2 thin film was rinsed using the organic solvent to remove excess dye that could lead to dye aggregation. The thin film was dried using nitrogen gas.
For the counter electrode, FTO glass (~8 Ω sq−1, Solaronix) was coated with platinum paste (Solaronix) and sintered at 450 °C for 30 min. Both electrodes were separated by 60 μm-thick Surlyn® layer (Meltonix 1170-60, Solaronix) and sealed by heating. The redox electrolyte (iodolyte AN-50, Solaronix) was then injected to fill the gap between the electrodes. Polymer glue was applied around the edges of the cell to prevent the electrolyte from leaking after the injection.
Instrumentation and Measurement
The optical absorption properties of the dyes were obtained from a UV-Vis spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan). Photoconversion properties of the cells were analyzed through the incident photo-to-current efficiency (IPCE) curve that was measured via spectral response measurement system (IVT Solar PVE-300, Bentham Instruments, Ltd., Berkshire, UK). The surface of the DSSC was exposed under an irradiation of 1000 W/m2, and the photovoltaic performance was conducted using a class AAA solar simulator (XES-40S1, San-Ei Electric Co., Ltd., Osaka, Japan). The kinetic process inside the cells was analyzed via electrochemical impedance spectroscopy (EIS) using Metrohm AUTOLAB (Utrecht, Netherlands). The amplitude of the alternating voltage was 10 mV, and the frequency range was 0.1 Hz to 105 Hz. The equivalent circuit and impedance parameters were obtained by fitting the spectra with NOVA 1.10 software (Metrohm AUTOLAB, Utrecht, Netherlands).
RESULTS AND DISCUSSION
Optical Properties of the Dyes and DSSC Device
The UV-Vis and IPCE spectra are related. Both analyses demonstrate the area where the sensitizer absorbs an intense light intensity and, hence, is capable of converting the absorbed photons into electricity. The IPCE can be expressed as follows,
IPCE (% ) = 1240 × Jph / λ × I (1)
IPCE (λ) = LHE × ϕinj × ηcol (2)
where Jph (mA cm-2), λ (nm), and I (cm-2) refer to the short circuit photocurrent density, wavelength, and intensity of the monochromatic light, respectively. ϕinj is the efficiency of electron injection from the excited dye into the TiO2, ηcol is the efficiency of collection of the injected electron at the electrode, and LHE is light harvest efficiency of the electrode. IPCE is indirectly related to the dye absorption observed in UV-Vis spectra (Ooyama and Harima 2012).
Figure 1 shows the absorption spectra of individual sensitizers; mengkulang and rengas dyes adsorbed onto TiO2 thin film along with the IPCE spectra obtained from the spectral response. Mengkulang dye had a broader absorption spectrum with no intense peaks in the tested wavelengths. However, intense peaks were observed at 400 nm and 576 nm for rengas dye. Rengas dye has a lower capability to harvest light because it has a narrow absorption band (Sayama et al. 2003). Subsequently, the IPCE percentage of mengkulang-sensitized DSSC was higher than rengas-sensitized DSSC at the lower wavelength range (400 nm to 500 nm). However, rengas-sensitized DSSC had a good IPCE percentage at the higher range of 523 nm to 600 nm.
For mixture sensitizers, both M:R (50%:50%) and M:R (60%:40%) dyes illustrated the same pattern of absorption, as shown in Fig. 2. A broad shoulder was observed between 500 nm and 550 nm. The M:R (60%:40%)-sensitized DSSC showed a higher IPCE, which indicated that more photons were absorbed and converted to current in the cells (Hara et al. 2000). The broad shoulder of IPCE spectra for mixture sensitizer was observed in the range of 550 nm to 600 nm. As compared to the absorption spectra, the shoulder was shifted to the longer wavelength. This result suggested that mixture-sensitized DSSCs had a better capability in harvesting light and converting electrons than the individual sensitized DSSCs because the absorption and IPCE spectra covered a wider range of wavelengths.
Furthermore, for co-sensitizers, the absorption spectra had broad shoulders between 400 nm to 550 nm with no intense peak in Fig. 3. The M:N (80%:20%) and M:R:N (40%:40%:20%) sensitizers had a similar pattern. The R:N (80%:20%) sensitizer had two obvious shoulders at 420 nm and 505 nm. The absorption spectrum for co-sensitizers had broad absorption bands compared with individual and mixture sensitizers, as the ruthenium (N719) dye acted as a co-sensitizer to graft more dye molecules onto TiO2, leading to a wider range of light absorption (Wang et al. 2004). The inset shows UV-Vis and IPCE curves of ruthenium (N719) dye-sensitized DSSC for comparison. The M:N (80%:20%)-sensitized DSSC had a higher IPCE percentage throughout the visible light. Both of the IPCE curves for M:N (80%:20%) and ruthenium (N719) dye-sensitized DSSCs had an intense peak at 530 nm. This result suggested that ruthenium (N719) dye enhanced the generated photocurrent via the addition of adsorbed dye molecules onto TiO2 inside the M:N (80%:20%)-sensitized DSSC (Choi et al. 2015). In contrast, the R:N (80%:20%)-sensitized DSSC showed a lower IPCE value, suggesting that the rengas sensitizer masked the other sensitizer from efficient light harvesting and photocurrent generation. There was a similar pattern in the IPCE curves for R:N (80:20%) and M:R:N (40%:40%:20%)-sensitized DSSCs, which lacked an intense peak at 530 nm. Different sensitizers exhibit different absorption bands because of the different adsorption abilities of each sensitizer (Shahid et al. 2013) and eventually will lead to different patterns for the IPCE plot (Kushwaha et al. 2013).
DSSC Device Photovoltaic Performances
Figure 4 shows the current-voltage (I–V) curves of individual, mixture, and co-sensitized DSSCs, and Table 1 summarizes the photovoltaic performance parameters of each DSSC. The energy conversion efficiency (η) was calculated by using Eq. 3,
where the open circuit voltage, Voc (V) is the maximum voltage measured at the zero current condition, and the short-circuit current density, Jsc (mA cm-2), is the maximum current density collected by the cell in short circuit condition. Both values are shown in the I–V curves plotted in Fig. 4. Pin is the intensity of the incident light (100 mW cm-2), and the fill factor (FF) is defined as the ratio of optimum photocurrent and voltage with the product of Voc and Jsc.
Table 1. Current-Voltage (I-V) Performance and Power Conversion Efficiency (η) of DSSCs Sensitized with the Individual, Mixture, and Co-sensitized Dyes
Fig. 4. I-V curves of DSSC device sensitized with individual dyes, mixture dyes, and co-sensitized dyes
For individual sensitized DSSCs, the value of open circuit voltage (Voc) was almost constant (0.50 V to 0.54 V), which means that the potential difference between the electrochemical potential of electrolyte and Fermi level of semiconductor did not have much difference (Nazeeruddin et al. 2003). Mengkulang-sensitized DSSC achieved slightly higher conversion efficiency (η), which was 0.16% with an open circuit voltage (Voc) of 0.53 V, short-circuit current density (Jsc) of 0.40 mA cm-2, and fill factor (ff) of 75.98, compared with the results for rengas-sensitized DSSC (η: 0.11%, Voc: 0.50 V, Jsc: 0.30 mA cm-2, and ff: 72.88). Moreover, mixture-sensitized DSSCs were expected to have better performance than the individual-sensitized DSSCs due to the enhancement of absorption discussed previously. The M:R (60%:40%)-sensitized DSSC achieved better photovoltaic performance (η: 0.30%, Voc: 0.53 V, Jsc: 0.90 mA cm-2, and ff: 62.16). However, the M:R (50%:50%)-sensitized DSSC generated slightly lower photovoltaic performance (η: 0.21%, Voc: 0.54 V, Jsc: 0.60 mA cm-2, and ff: 63.28). A good increase in Jsc was obtained for the mixture-sensitized DSSCs, which indicated that they were capable of generating dense electron distribution at the surface of the TiO2 layer and that electrons were injected effectively through the photoelectrode layer (Ooyama and Harima 2012).
The best performance among the co-sensitized DSSC was observed in the M:N (80%:20%)-sensitized DSSC, which showed the conversion efficiency (η) of 0.58% with an open circuit voltage (Voc) of 0.63 V, short-circuit current density (Jsc) of 2.10 mA cm-2, and fill factor (ff) of 44.00. The R:N (80%:20%) and M:R:N (40%:40%:20%)-sensitized DSSCs had similar conversion efficiencies (η), which were 0.33% (Voc: 0.53 V, Jsc: 1.10 mA cm-2, and ff: 57.07) and 0.39% (Voc: 0.55 V, Jsc: 1.60 mA cm-2, and ff: 45.23), respectively. Their Jsc and Voc values were lower than the M:N (80%:20%)-sensitized DSSC. This condition suggested that the dyes did not adsorb onto the TiO2 layer efficiently, which affected the electron transfer from the lowest unoccupied molecular orbital (LUMO) level to the Fermi level of TiO2 (Lim et al. 2015). The high value of short-circuit current density (Jsc) obtained by the M:N (80%:20%)-sensitized DSSC indicated that the high electron distribution at the surface of TiO2caused the Fermi level to shift towards the conduction band (CB) (Mora-sero and Bisquert 2003). As a result, the value of Voc was higher and led to the better conversion efficiency of the cell and process. Regardless of the high photovoltaic performance shown by co-sensitized DSSCs, the presence of ruthenium (N719) dye impacted the results. Compared with the photovoltaic performance achieved by the ruthenium (N719) dye-sensitized DSSC (Table 1), the co-sensitized DSSCs showed poor performance. The kinetics of the electrochemical and the photoelectrochemical processes in the cell influenced the cell performance, as discussed in the next section.
Electrochemical Impedance Study
The electron transport parameters in the DSSC device were further analyzed by using electrochemical impedance spectroscopy (EIS). In this study, EIS analysis was carried out under one sun (AM 1.5) light intensity at an open circuit condition. The theory, parameters, and calculations of this analysis have been discussed previously (Bisquert 2002; Adachi et al. 2006). The impedance of the equivalent circuit indicating the diffusion-recombination effect under boundary condition is expressed as follows (Bisquert 2002; Kern et al. 2002; Wang et al. 2005),
The two important processes involved in this diffusion-recombination effect are the diffusion of an electron across the semiconductor layer of photoanode (ωd) and the electron back reaction with oxidized redox species (ωrec). These parameters can be defined as (Adachi et al. 2006):
Meanwhile, the diffusion-recombination resistances can be defined as (Adachi et al. 2006),
where the constant (con) is defined by,
and where Z (Ω), Rct (Ω), Rrec (Ω), ωd (Hz), ωrec (Hz), τeff (ms), Cμ, KB (J K-1), T, q, A (m2), and ηsrepresent the impedance, electron transport resistance in TiO2, recombination resistance, characteristic frequency correspond to the electron diffusion in TiO2, characteristic frequency corresponds to the electron back reaction with oxidized redox species in the electrolyte, electron lifetime in TiO2, chemical capacitance of TiO2, Boltzmann constant, absolute temperature, elementary charge, area of electrode, and electron density at conduction band of TiO2, respectively. The relationship between characteristic frequencies and diffusion-recombination resistances can be related as follows Eq. 8 (Bisquert 2002).
From Eqs. 5 and 6, effective electron diffusion coefficient, Deff (cm2 s-1) and electron diffusion length, Ln (μm) can be written as follows (Adachi et al. 2006; Chiu et al. 2009),
where the effective electron lifetime, τeff (ms) and effective recombination rate constant, keff (s-1) can be expressed in the term of peak frequency, fmax (Hz) obtained from Bode phase plot at the range of 1 Hz to 1 kHz (Adachi et al. 2006; Deng et al. 2011).
Table 2 summarizes the electron transport properties of the DSSC device sensitized with the individual, mixture, and co-sensitized dyes. The various electron transport parameters were gained after fitting the EIS curves using NOVA 1.10 software using the equivalent circuit shown in the inset of Fig. 5. The equivalent circuit elements include sheet resistance of the conducting glass (Rs), constant phase element (CPE), and interface charge transfer resistance (Rrec and Rct). Rrec is the electron transfer resistance of the charge recombination process at TiO2/dye/electrolyte interface, while Rct is the transport resistance of electron diffusion at TiO2/dye interface. The CPE is replacing double layer capacitance, Cdl to account for the deviation of the Cdl from the ideal interfacial capacitance, which is related to the roughness of the electrode used (Omar et al. 2013; Sarker et al.2014; Kumara et al. 2015).
Fig. 5. Nyquist plot based on the electrochemical impedance of the DSSC devices sensitized with the individual, mixture, and co-sensitized dyes. The inset shows the equivalent circuit of this study.
The Nyquist plot includes three semicircles present at different ranges of frequency. At high-frequency region between 1 kHz to 100 kHz, the arc exists due to the charge transfer at the counter electrode. In the intermediate region (1 Hz to 1 kHz), the middle arc is according to charge transfer at the TiO2/dye/electrolyte interface meanwhile diffusion in the redox electrolyte produces an arc at low frequency (0.2 Hz to 0.75 Hz) (Omar and Abdullah 2014). However, according to literature (Sarker et al. 2014), DSSCs do not necessarily show three peaks in a Bode phase plot or three distinct arcs in the Nyquist plot. From the Nyquist plot in Fig. 5, the magnitude of Rrec at the TiO2 layer/electrolyte interface was directly proportional to the diameter of the second arc. The device sensitized with M:R (50%:50%) had a longer diameter, followed by rengas, M:R (60%:40%), and Mengkulang, respectively. The Mengkulang-sensitized DSSC had the lowest value of Rrec, which was 150 Ω. The M:R (60%:40%), rengas, and M:R (50%:50%)-sensitized DSSCs had values of 202 Ω, 331 Ω, and 407 Ω, respectively.
The Nyquist plot of co-sensitized DSSC is shown in Fig. 6. The R:N (80%:20%)-sensitized DSSC had the longest diameter of the second arc and the highest value of Rrec (26.2 Ω). M:R:N (40%:40%:20%) and M:N (80%:20%)-sensitized DSSCs had Rrec values of 21 Ω and 18.3 Ω, respectively. From Table 2, the value of Rct obtained was smaller than Rrec (Rrec >> Rct) for the individual- and mixture-sensitized DSSCs. The co-sensitized DSSCs had smaller Rrec values than the Rct (Rrec << Rct) condition. The value of Rrec/Rct for co-sensitized DSSCs were much smaller than 1 at 0.18, 0.17, and 0.19 for M:N (80%:20%), M:R:N (40%:40%:20%), and R:N (80%:20%), respectively. The shape of the central arc deviated, especially at a higher frequency or known as Gerischer impedance (Sarker et al. 2013). Furthermore, only rengas and M:R (50%:50%)-sensitized DSSCs obtained an Rrec/Rct value higher than 10, which was 17.2 and 24.1, respectively. Thus, the shape of the central arc was a true circle as shown in Fig. 5. This finding was similar to the results of Adachi et al. (2006).
Figures 6 and 7 show the Bode phase plot of the individual, mixture, and co-sensitized DSSCs. The parameter fmax within the range of 1 Hz to 1 kHz was inversely proportional to the τeff as defined in Eq. 12. fmax corresponds to the charge-transfer at the TiO2/dye/electrolyte interface, and τeff is the time constant for how long it takes for an electron to diffuse at TiO2 before the excess electron recombine. For individual and mixture-sensitized DSSCs, M:R (50%:50%)-sensitized DSSC obtained lower frequency position of fmax at an intermediate region of frequency followed by rengas, mengkulang, and M:R (60%:40%)-sensitized DSSCs, respectively.
Because the fmax of M:R (60%:40%)-sensitized DSSC shifted to the high-frequency region, the τeffobtained was the lowest, which was 0.594 ms. Meanwhile, the M:R (50%:50%), rengas, and mengkulang-sensitized DSSCs obtained 7.508 ms, 4.272 ms, and 2.431 ms, respectively. For co-sensitized DSSCs, the position of fmax was at the lowest frequency of intermediate region for M:N (80%:20%)-sensitized DSSCs, thus leading to the highest value of τeff, which was 17.493 ms. However, the R:N (80%:20%) and M:R:N (80%:20%)-sensitized DSSCs obtained the same value of τeff, which was 9.953 ms because the position of fmax obtained was the same. This result suggests that the R:N (80%:20%) and M:R:N (80%:20%)-sensitized DSSCs achieved similar photovoltaic performance, as discussed previously.
The thickness of the TiO2 thin film (L) measured in this study was the controlled variable (17.74 μm). The magnitude of effective electron diffusion length (Ln) was calculated using Eq. 10. Ln for mengkulang, rengas, M:R (50%:50%), and M:R (60%:40%)-sensitized DSSCs was measured to be more than L, with values of 31.97 μm, 73.66 μm, 87.06 μm, and 35.66 μm, respectively. This condition (Ln >> L) was important for the diffusion of electrons through the photoelectrode. By referring to Eq. 7, when Ln >> L and the value of Rrec are finite, Rrec << Rct, where recombination resistance must be higher than that of charge transport resistance. The higher value of Rrec inhibits the recombination process from occurring. This was the condition desired to achieve efficient DSSC device operation at moderate potentials (Adachi et al. 2006; Omar et al. 2013). The values of Rrec and Rct obtained for mengkulang, rengas, M:R (50%:50%), and M:R (60%:40%)-sensitized DSSCs in this study establilshed the relationship for both conditions.
The magnitude of electron density (ηs) that accumulates at the surface of a photoelectrode could be predicted from the value of constant (con) calculated by using Eq. 7. A higher value of calculated conwould lead to lower concentration of injected electrons accumulated at the conduction band of TiO2. From the value of con in Table 2, M:R (50%:50%)-sensitized DSSC was expected to have higher electron density followed by mengkulang, rengas, and M:R (60%:40%)-sensitized DSSCs. For the co-sensitized DSSCs, the M:N (80%:20%)-sensitized DSSC was predicted to have higher electron density, followed by M : R : N (40% : 40% : 20%) and R:N (80%:20%)-sensitized DSSCs. This concentration of electrons would eventually diffuse to the photoelectrode with the effective electron diffusion coefficient (Deff), as calculated by using Eq. 9 and assumed being lost with the recombination rate constant (keff), which was equal to the reciprocal of τeff (Kern et al. 2002). DSSC devices need a higher value of Deff and ηs but a lower value of keff to achieve efficient performance (Lim et al. 2016). For individual and mixture-sensitized DSSCs, M:R (50%:50%)-sensitized DSSC showed the lowest value of con (which means higher value of ηs), and the lowest value of keff (133.2 s-1) and Deff were calculated to be quite low, which was 1.01 × 10-6 cm2 s-1. In contrast, the M:R (60%:40%)-sensitized DSSC was predicted to have a lower value of electron density (ηs), which means that the electrons were lost rapidly. This was demonstrated by a shorter electron lifetime (τeff) and the higher rate of recombination (keff), which was 1684.7 s-1. However, Deff was recorded as 2.14 × 10-6 cm2 s-1, which was much larger than that of the M:R (50%:50%)-sensitized DSSC. Thus, faster electron transport through TiO2 layer was expected where it eventually increased the photocurrent density at the photoelectrode. This explained why the value of Jsc achieved for M:R (60%:40%)-sensitized DSSC was higher than M:R (50%:50%)-sensitized DSSC, thus leading to higher conversion efficiency (η).
Table 2. Electron Transport Properties of the DSSC Device Sensitized with Individual, Mixture, and Co-Sensitized Dyes Determined by Electrochemical Impedance Spectroscopy
Moreover, for the co-sensitized DSSCs, the M:N (80%:20%)-sensitized DSSC achieved the lowest value of con (which means higher ηs), and lowest keff (57.2 s-1) as compared to R:N (80%:20%)- and M:R:N (80%:20%)-sensitized DSSCs, which both obtained the same value of keff (100.5 s-1). There was a big difference in the Deff values calculated for the M:N (80%:20%)-sensitized DSSC. The conversion efficiency (η) was the highest value because the effective electron lifetime (τeff) of M:N (80%:20%) sensitized DSSC is the longest. Hence, the distribution of electrons at the porous semiconductor had enough time to diffuse across the TiO2 layer before recombination took place. Even though the conversion efficiency of co-sensitized DSSCs was higher than that of individual and mixture-sensitized DSSCs, they were much lower than the ruthenium (N719) dye-sensitized DSSC (Table 1). The Rct value was higher than that of Rrec (Rrec << Rct) for all co-sensitized DSSCs. In this case, the recombination time was much faster than that of electron diffusion. By referring to the results in Table 2, the value obtained for Ln was also lower than the value of L (Ln << L). This was the condition in where the distribution electrons being lost faster and undergo recombination before they could be injected to the conduction band and diffuse across the TiO2 layer. In sum, the conditions of (Rrec << Rct) and (Ln << L) were considered irrelevant and should be avoided in order to achieve the efficient performance of DSSC devices.
- DSSCs were fabricated with natural sensitizers extracted from rengas (Gluta spp.) and mengkulang (Heritiera elata) wood. Different types of sensitizers were introduced, including individual, mixture, and co-sensitized sensitizer. The photovoltaic performance of mixture sensitized DSSCs up to 2-fold increased short-circuit current density (Jsc) compared with individual sensitized DSSCs, indicating improved conversion efficiency.
- From the IPCE curve, a higher photocurrent was achieved for the M:R (60%:40%)-sensitized DSSC, which revealed good light harvesting, charge collection, and dye regeneration efficiency.
- Electron transport behavior in the cells was estimated from the electrochemical impedance spectroscopy analysis. Even though M:R (50%:50%) obtained lower recombination rate, keff, and high estimated electron density, ηs, the conversion efficiency was slightly lower than that of the M:R (60%:40%)-sensitized DSSC because of the electron diffusion coefficient, Deff, was lower.
- Other parameters that were important to the suitability of the sensitizer occurred when Rrec >> Rct and Ln >> L conditions were achieved. In this study, both individual and mixture-sensitized DSSCs followed these conditions. However, co-sensitized DSSCs created the phenomenon in which Rrec << Rct and Ln << L. In this case, the boundary condition was considered irrelevant and should have been avoided.
- An efficient sensitizer needs a high electron diffusion coefficient, Deff, low recombination rate, keff, and high electron density, ηs. The conversion efficiency achieved for M:N (80%:20%), R:N (80%: 0%), and M:R:N (80%:20%)-sensitized DSSCs was higher than individual and mixture-sensitized DSSCs; they were considered not suitable as sensitizers.
- These findings suggested that the mixture sensitizers have great potential to improve the electrical properties and efficiency of sensitizers.
This research was supported by the Ministry of Education (MOE) Malaysia under Look East Policy Grant (LEP 2.0/14/UKM/NT/03/1 and Universiti Kebangsaan Malaysia under Research University Grant (GUP-2015-037).
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Article submitted: June 21, 2017; Peer review completed: September 3, 2017; Revised version received: October 11, 2017; Accepted: October 12, 2017; Published: October 20, 2017.