Enhancing Power Conversion Efficiency of Dye Sensitized Solar cell ...

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Enhancing Power Conversion Efficiency of Dye Sensitized Solar cell using TiO2-MWCNT composite Photoanodes Umer Mehmood, Ibnelwaleed A. Hussein, Amir Al-Ahmed, Shakeel Ahmed

 Abstract—The aim of this work is to improve the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs) using composite films consisting of titanium oxide (TiO2) nanoparticles and multiwalled carbon nanotubes (MWNTs). Transmission electron microscope (TEM) was used to confirm the dispersion of carbon nanotubes in TiO2. Composite photoanode based Solar cells were characterized by UV-Visible absorption spectroscopy, photocurrent–voltage (I–V) characteristics and electrochemical impedance spectroscopy (EIS). It was found that the power conversion efficiency (PCE) (ηc) of DSSCs strongly depends on the concentration of CNTs in the nanocomposite films. The solar cell assembled with photoanode containing 0.06 % MWCNTs, shows the highest efficiency of 5.25%, which is 46% greater than unmodified photoanode. The Density functional theory (DFT) quantum modeling technique was used to compute the band gap of TiO2 and CNT-TiO2 clusters. Index Terms-- Power, Composites, Photovoltaics, multi-walled carbon nanotubes, efficiency, density functional theory

I. INTRODUCTION

THE

burning of fossil fuels raises the amount of carbon dioxide in the atmosphere. Therefore, alternative sources of energy are needed so that mankind can survive on the Earth without depending on fossil fuels. Solar energy is one of the renewable energy sources that will contribute to the security of

The authors would like to acknowledge the funding provided by King Abdulaziz City for Science and Technology (KACST) through project # 11ENE1635-04.

Umer Mehmood is with Department of Chemical Engineering and Center of Research Excellence in Renewable Energy, Research Institute King Fahd University of Petroleum & Minerals (KFUPM) Dhahran 31261, Kingdom of Saudi Arabia ([email protected]) Ibnelwaleed A. Hussein belongs to Processing Center, College of Engineering, Qatar University, PO Box 2713, Doha, Qatar ([email protected]) Shakeel Ahmed belongs to Center for Refining & Petrochemicals, RI, KFUPM, KSA ([email protected]) Amir Al-Ahmed is with Center of Research Excellence in Renewable Energy, Research Institute, KFUPM. ([email protected])

future energy supplies. DSSCs have gained global attention in the past fifteen years owing to their easy processing and low production cost as compared to silicon solar cells. Meanwhile, the CNTs are also being extensively explored and employed due to their excellent morphological and electrical properties. These two apparently discrete inventions (CNTs and DSSCs) were fortuitously linked together in 1991. In this year, Grätzel devised dye sensitized solar cell [1] while Iijima discovered CNTs [2]. The overall performance of f DSSC mainly depends on a dye. It absorbs sunlight and produce excitons. It forms strong chemical bond with the porous surface of TiO2. Presently, ruthenium(II)-polypyridyl complexes based DSSCs have got overall power conversion efficiencies (PCE) over 11% under standard (Global Air Mass 1.5) illumination [3]–[6]. However, the efficiency of dye-sensitized solar cells is still very low compared with the silicon solar cells due to recombination of injected electrons with the electrolyte and slow electron transport process [7]. In this work, we employed CNTs-TiO2 based photoanode. The insertion of CNTs in TiO2 films may improve the electron transport in DSSCs [8]–[13], owing to the creation of complex interpenetrating networks and favorable electrical conductivity [14]–[16]. It is supposed that one-dimensional carbon nanostructures allow the photocurrent to flow more efficiently in DSSCs by increasing orientation orders. Here, we use a direct mixing technique to obtain a hybrid photoanode of DSCs, consisting of nanocrystalline TiO2 and MWCNTs. We also employed DFT to investigate the thermodynamic aspects of charge transport processes involved in DSSC. It is an effective tool as compared to other high-level quantum approaches because the computed orbitals are suitable for the typical MO-theoretical analyses and interpretations[17]. Many theoreticians have successfully applied this technique in calculating the electronic structure properties of photosensitizers [18]–[24].

II. COMPUTER SIMULATION Amsterdam Density Functional (ADF) program (2013.01) was used to perform DFT calculations. Tetragonal anatase crystal structure was selected with the single layer (001) surface slab. Then, we created 4×1 supercell from this slab. All atoms were

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mapped within the unit cell. Hybrid function at Becke, parameter Lee-Yang-Parr (BYLP) level and triple-ζ polarization basis function were used to optimize the TiO2 and CNT doped TiO2 clusters in BAND and ADF modes respectively. Relativistic effects were considered in all the calculations by applying zero order regular approximation in its scalar approximation (ZORA) [17], [24], [25].

(a) (a)

III. EXPERIMENTATION A. Composite anodes preparation First a suspension was prepared by sonicating 10 mg of MWCNTs in 25 mL of ethanol for 4 h. Then an exact mass of MWCNTs suspension was mixed with a known amount of anatase TiO2 paste (T/SP 14451, solaronix) to obtain a composite paste. Five composite samples of TiO2-MWCNTs with the following concentration of MWCNTs were prepared; 0, 0.03. 0.06, 0.09, 0.15 wt %. Each TiO 2-MWCNTs composite paste was then tape casted on FTO glass substrate (2 mm, 7Ω/seq, solaronix) and annealed at 450 oC for 30 min. The thickness of each photoanode was determined using the cross-sectional scanning electron microscopy (SEM) images (JEOL, 6610LV). The average thickness of each film is 10 µm.

(b)

(c)

B. characterization JASCO-670 UV/VIS spectrophotometer was used to record the spectra of SQ2 in methanol and anchored to TiO2 and CNT/TiO2 films. I−V characteristics of the DSSCs were found out by using Keithley 2400 Source Meter and IV-5 solar simulator (Sr #83, PVmeasurement, Inc) at AM1.5G (100 mWcm− 2). The electrochemical impedance spectroscopy (EIS) measurements were performed in the dark using a potentiostat (Bio-Logic SAS (VMP3, s/n:0373) under a −0.7 V forward bias and a 10 mV AC amplitude, and the frequency range was 10 Hz–1M Hz. IV. RESULTS AND DISCUSSION A. Morphological properties of hybrid anode The dispersion of MWCNTs in TiO2 was observed by TEM (JEOL, JEM-2100F) analysis. Fig 1 (c) shows the good dispersion of CNTs, although a few tangles can be observed due to the length of the MWCNTs. The interface connection between MWCNTs and TiO2 can clearly be observed, indicating that TiO2 possesses a good affinity with MWCNTs. The inner core is hardly visible because the surface of MWCNT is well decorated with TiO2 nanoparticles as shown in Fig 1 (d).

(d)

Fig 1: TEM images of a 0.15 % sample (a) Pure TiO2 b) Multi walls of CNT c) Dispersion of MWCNTs in TiO2 and d) Dispersion of single MWCNT in TiO2

B. Electrochemical properties The simulated structures of TiO2 and CNT-TiO2 are shown in Fig 2. Table-1 shows the conduction band, valence band and a band gap of TiO2 and CNT-TiO2 clusters. The computed results show that incorporation of MWCNTs in TiO2 significantly reduces the band gap of TiO2 cluster. As CNTs possess a lower value of the ECB (∼0 EV vs. NHE) than that of TiO2 (−0.5 eV vs. NHE) [26], the charge equilibrium between CNTs and TiO2 would cause a shift of apparent Fermi level to more positive potential as shown in Fig 3 (a). Furthermore, the positive shift owing to mixing of CNTs provides significant driving force to accelerate the electron transport from dye to the conduction band of composite.

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.

(a)

(b) Fig 3: incorporation of MWCNTs in TiO2 (a) causes a shift of energy level and (b) enhance the electrical conductivity [12], [26]

Fig 2. Optimized geometries of a) TiO2 and b) graphene doped TiO2

C. Optical properties The UV-Vis absorption spectra of SQ2 in methanol and anchored to TiO2 and TiO2-MWCNTs are shown in Fig 4. SQ2 dye in methanol solution gives two distinct absorption bands: one relatively weak bond is in the region (583-620 nm) corresponding to the π-π* electron transitions of the conjugated molecules and the other is around 626-686 nm that can be assigned to an intramolecular charge transfer (ICT) between electron-donor and electron acceptor anchoring moieties. However, the absorption shifts to lower energy values when anchored to TiO2 and TiO2-MWCNTs. This is due to the fact that on the electrode the carboxylate groups bind to the TiO2 surface in which Ti 4+ acts as a proton. The interaction between the carboxylate group and the surface Ti4+ ions may lead to increased delocalization of the π* orbital. The energy of the π* level is decreased by this delocalization, which explains the red shift for the absorption spectra. But TiO2-MWCNTs based photoanode has a greater red shift value (668 nm) as compared to pure TiO2. This is because the CNTs may exhibit photosensitizing properties, thus extending photovoltaic properties in the visible spectrum [9]. TABLE I

Valence band, conduction band and band gap of TiO2 and CNT/TiO2 System

ECB (eV)

VB (eV)

Band gap (eV)

TiO2 CNT-TiO2

-4.10 -4.40

- 7.20 -6.45

3.1 2.05

Fig 4: UV-Vis spectra of SQ2, TiO2/ SQ2 and CNTs-TiO2/ SQ2

D. Photovoltaic performance of DSSCs based composite anode The I–V curves of the DSSCs based on hybrid photoanodes are shown in Fig 5, while the device parameters, i.e., Jsc, Voc and FF, Rsh (shunt resistance) and Rs (series resistance) are shown in Table 2. The DSSC with the highest efficiency is achieved in the case of 0.06%MWCNTs-TiO2, which is about 46% greater than the unmodified TiO2. The high efficiency can be attributed to the incorporation of MWCNTs; a) increases the surface area of hybrid anode and thus more dye loading, b) improve the electron injection efficiency of the electrons due to increase positive potential and c) also increase the electrical conductivity, as shown in Fig 3b. The FF values of DSSCs in Table-2 are also very low due to high Rs and low Rsh. The RSh represents the loss due to surface leakage along the edge of the cell or due to crystal defects. The low shunt resistance causes power losses in solar cells by providing an alternate pathway for the light-generated current, which lowers FF[12]. Table-2 shows that the incorporation of CNTs also increases the Rsh by reducing shorts or leaks in the device. The high thickness of counter electrode or electrolyte causes high Rs [27].While poor fabrication of DSSCs causes low Rsh of devices [28]. However, the decline in Voc at increasing CNTs concentration could be ascribed to the downshift of the potential band edge of the TiO2 conduction band. It is also found that an increase in MWCNTs concentration from an optimum level (0.06%) negatively affects the photovoltaic performance of DSSCs. This reason is that the film transparency decreases owing to high MWCNTs contents. Another possibility of low efficiency

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at a high MWCNTs concentration could be attributed to the formation of CNT agglomerates inside the TiO2 matrix acting as trapping sites that obstruct the fast charge collection at the electrodes. The less charge collection together with light loss owing to CNTs direct absorption strongly declines the efficiency of DSSCs at high CNTs contents.

lead to the smaller values of R2 due to a shorter electron lifetime of the order of few tens of mili seconds [34].

Fig 6: EIS investigation of the modified and unmodified DSSCs

V. CONCLUSION

Fig 5: Current – voltage (J-V) characteristics of DSSCs based on composite photoanodes (TiO2-MWCNTs) TABLE II

Photovoltaic properties of DSSCs Jsc Voc FF(%) η Rsha Rbs (%) (mA/cm2) (mV) (Ω) (Ω) Pure TiO2 13.80 750.3 34 3.59 687 169 0.03%CNT+TiO2 13.90 726.6 43 4.28 988 182 0.06%CNT+TiO2 18.02 727.4 40 5.25 954 136 0.09%CNT+TiO2 16.38 751.2 37 4.47 668 147 0.12%CNT+TiO2 16.10 747.2 36 4.36 712 144 0.15%CNT+TiO2 13.61 723.3 38 3.70 868 164 a: Rs can be found from th eresprocal of the slop of I-V characteristics near to open circuit voltage b: Rsh can be found directly from PV measurement system along with other parameters DSSCs

E. Electrochemical impedance spectroscopy (EIS) analysis EIS, which measures the current response at different frequencies of the applied AC voltage, was used to study the charge transfer resistance of the cells. Fig-6 shows the Nyquist plot of DSSCs which were assembled with TiO2-SQ2 and 0.06%MWCNTS-TiO2-SQ2. Generally, a normal impedance spectrum of DSSCs is represented by three arcs (semicircles). The first semicircle represents the charge transport resistance at counter electrode/electrolyte (R1), second signifies the charge transport resistance at the photoanode / electrolyte interface (R2), and third indicates the diffusion process of I−/I3− redox couple in an electrolyte (Zw) [29]–[32]. The R2 is related to the charge recombination rate, e.g., a smaller R2 indicates a faster charge recombination. The R2 value for DSSC assembled with pure TiO2 is smaller than that of 0.06%MWCNTs-TiO2 DSSC, proposing charge recombination is greatly reduced owing to the incorporation of CNTs, which increase the electron lifetime and diffusion length [33]. However, the CNTs concentration greater than 0.06 wt.% will

The mixing of small amounts of MWCNTs in TiO2 can expressively increase the power conversion efficiency of DSSCs. Optimum concentration (0.06%) of CNTs does not affect the transparency of the TiO2 layer while significantly increasing the charge collection at the photoanode. The enhancement of electron lifetime, dye loading, and the reduction of the recombination phenomena lead to an increase in PCE of the DSSCs, with a maximum value of 5.25% (corresponding to the addition of 0.060 wt % MWCNTs). This is a fast, cheap, and efficient way of increasing the power conversion efficiency of DSSCs. VI. ACKNOWLEDGMENT The authors would like to acknowledge the Center of Research Excellence for Renewable Energy at KFUPM for providing the lab facility. References [1]

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