Banyan Root Structured Mg-Doped ZnO Photoanode Dye-Sensitized

Subscriber access provided by University of Sussex Library

Article

Banyan Root Structured Mg Doped ZnO Photoanode Dye Sensitized Solar Cells C. Justin Raj, Kandasamy Prabakar, Karthick S.N, Hemalatha K. V, Min -Kyu Son, and Hee-Je Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp308847g • Publication Date (Web): 21 Jan 2013 Downloaded from http://pubs.acs.org on January 21, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Banyan Root Structured Mg Doped ZnO Photoanode Dye Sensitized Solar Cells C. Justin Raj, Kandasamy Prabakar*, S. N. Karthick, K. V. Hemalatha, Min-Kyu Son and Hee-Je Kim* Pusan National University, School of Electrical Engineering, San 30, Jangjeong-Dong, Gumjeong-Ku, Busan-609 735, South Korea. KEYWORDS: metal-oxide semiconductor, photoanodes, Zinc Oxide, Electron transport, impedance spectroscopy

ABSTRACT: We report a peculiar banyan root like Mg doped ZnO photoanode to result in high electron transport, retardation of interfacial charge recombination, improved light harvesting efficiency and overall enhanced photovoltaic performance of dye sensitized solar cells (DSSC). DSSC based on 5 molar % of Mg doped ZnO electrode of very low thickness ~4 µm gained an improved short-circuit current density of 9.98 mAcm-2, open-circuit photo voltage of 0.71 V, fill factor of 0.58, and overall conversion efficiency of 4.11% under 1 sun illumination.

1. Introduction Semiconducting zinc oxide (ZnO) with wide optical band gap and admirable electron mobility has attracted much attention towards alternative photoanode for dye sensitized solar cells (DSSC) 1, 2. Although, ZnO photoanode possess similar properties that of TiO2 3, some factors have been limiting the efficiency and performance of the ZnO based DSSCs. The competition between the generation and recombination of photoexcited carriers, the unstable ZnO surface to acidic dye have been found to restrict the conversion efficiencies of ZnO DSSC. Tremendous efforts have been made to improve the performance of the ZnO-DSSC by introducing nanoparticles with different morphologies, increasing the thickness of photoanode, fabricating blocking core shell electrodes etc 4-6. Recently morphological dependent ZnO photoanode such as wires, nanotrees, aggregates etc have attracted many researchers for DSSCs application owing to their improved photovoltaic performance. Ko et.al 6 have reported a branched tree like ZnO nanowires by multi-step synthesis technique and achieved a conversion efficiency of 2.63% due to their large surface area and fast electron transfer of the injected electrons. Zhang et.al 5 has reported hierarchically structured ZnO photoanode to improve the performance of DSSC. Since, these photo electrodes posses large surface areas for dye molecule adsorption and the sub micron sized nanoparticles as self light scattering layer increase light harvesting efficiency. McCune et.al 7 have reported 3D caterpillar like structured ZnO photoanode adopting a multi-step fabrication technique for DSSC with high solar to power conversion efficiency. Even though the above reports on DSSCs shows an at-

tracting current density and conversion efficiency but they still posses low open circuit voltage and fill factor, which are the crucial factor for an efficient DSSCs. Also, the overall fabrication of these types of photoanodes involves multi-step and time consuming complicated process. Apart from the morphology of photoanode, the metal doped ZnO also shows an improvement in the performance of DSSCs. It has been reported that the incorporation of metal ions such as Li, I in ZnO can improve the stability of the photoanode surface, increasing the electron harvesting efficiency and enhances the photovoltaic performance of the DSSCs 8, 9. Moreover, ZnO based DSSC especially needs better focused research as wells as understanding the mechanisms determining DSSC performance 10, 11. Considering these facts, the current research was focused to improve the overall performance of ZnO based DSSC with a peculiar surface morphology prepared by simple one step drop casting technique. The conduction band minimum (and Fermi energy) shift upwards 12, 13 when ZnO is doped with Mg will favour more driving force for electron injection from the excited dye. In addition, the incorporation of magnesium into the wurtzite ZnO matrix can be favored by the similarity of the ionic radii of Zn2+ (0.60 Å) and Mg2+ (0.57 Å). The solid solubility of MgO in ZnO matrix have been reported on wurtzite Zn1-xMgxO layers with significant Mg compositions upto 33% and 49% 14,15 . The appropriate amount of Mg content can tune the band gap as well as work function of ZnO for the desired applications 16. In this work, we have made an attempt to incorporate Mg ions in ZnO by simple and single step drop casting technique in order to study the

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

effect of band gap tuned photoanode DSSCs. Apart from the effect of band gap shift, the presence of Mg ions may hinder the interfacial recombination of the photo-excited electrons and could possibly increase the stability of the photoanode surface against the acidic dye.

2. Experimental methods The precursors for the synthesis of Mg doped ZnO thin films were analytical grade zinc acetate dihydrate (Sigma-Aldrich, 99.99%), 2-methoxyethanol (C3H8O2), and monoethanolamine. Mg doping was achieved by the introduction of appropriate amount of magnesium acetate dihydrate. A solution containing 0.5 mol of zinc acetate, magnesium acetate (0–20 Mol. %), 10 ml 2methoxyethanol and monoethanolamine ([MEA]/[Zn2+]=1) was vigorously stirred for 1 hr with a magnetic stirrer, keeping the temperature of the solution at 65 °C. Then, the solution was aged for 5 hr to obtain the required sol. The sol was dropped over a selected area (active area 0.25 cm2) on ultrasonically cleaned fluorine doped tin oxide (FTO) conducting substrate using a polymer mould in a hot plate maintained at a temperature of 125 °C. The drop casting was repeated for five times at the same temperature to get homogeneous film. The obtained film was sintered at 400°C for 1 hr to evaporate the organic impurities. The thicknesses of the films were found to be ~4 µm and were used for the fabrication of DSSCs. The detailed fabrications of DSSC have been reported by the author elsewhere 17. The crystalline nature, particle size and phase purity of the thin films were analyzed by X-ray diffraction (XRD, D/ Max-2400, Rigaku) using a Cu Kα source operated at 40 kV and 30 mA in the 2θ range of 20–80°. Surface morphology and thickness measurements of the films were performed by field emission scanning electron microscope (FE-SEM, S-4200, Hitachi) operated at 15 kV. X-ray photon spectroscopy (XPS) was performed using a VG Scientific ESCALAB 250 with monochromatic Al-Kα radiation of 1486.6 eV and with an electron take off angle of 90°. During the measurements, the pressure of the sample chamber was kept at 10−10 Torr. The survey spectrum was scanned in the binding energy (BE) range of 0.00–1400.00 eV in steps of 1.00 eV. The current–voltage characteristics of the DSSCs were performed under 1 sun illumination (AM 1.5G, 100 mWcm−2) using San Ei Electric (XES 301S, Japan) solar simulator having the irradiance uniformity of ±3%. The IPCE spectrum was measured using PV measurement (Inc., QEX7 series). Electrochemical impedance spectroscopy (EIS) was performed using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under 1 sun illumination. The optical absorption of the Mg doped ZnO films with and without dye were recorded using OPTIZEN 3220UV spectrophotometer.

Page 2 of 9

3. Results and discussion Figure 1 shows the SEM image of the ZnO film which resembles a continuous branch of banyan tree roots and also illustrated schematic of the coated ZnO film photoanodes. This depicted root like networks consist of large number of primary spherical ZnO nanoparticles of ~ 40 nm and form a secondary branches of a banyan root like network. The root branches are of ~ 400 - 800 nm diameter offers a self – scattering layer for the film to enhance the light harvesting ability of the photoelectrode. Figure 2 a-d shows the SEM images of pure and ZnMgO films, which furnishes a clear picture of the morphology and the inset magnified image shows the branches composed of large number of crystallites with sizes ranging from 30 - 40 nm. The SEM images indicate that all the films have a wrinkle network with uniform size distributions. Figure 2 a and b show exact cylindrical root like continuous network formed by large number of spherical ZnO particles for pure and 5 molar % Mg doped ZnO respectively. The branches were broadened and the gaps between branches were diminished by increasing the percentage of magnesium concentration as shown in Figure 2 c and d. Figure 3 shows the SEM image of 5 molar % Mg doped ZnO branch network scratched from the film and demonstrates the porous interior of the branch with large number of nanoparticles.

Figure 1 a) SEM image of the banyan root like ZnO Film b) the schematic representation of the banyan root like ZnO nanoaggregates

The crystallinity and the particle sizes of ZnO and ZnMgO films were analyzed by the XRD method. Figure 4a shows the X-ray diffraction patterns of bare, 5, 10 and 20 molar % of Mg doped ZnO films on FTO substrate. As seen in figure, the films are polycrystalline


Page 3 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry with hexagonal wurtzite structure without any new phase due to the addition of Mg. The lattice parameters of the unit cells were calculated from all diffracted peaks with the help of software program Unitcell (method of TJB Holland & SAT Redfern 1995). From this calculation the a values increase slightly and c values decreases with increasing Mg concentration (c= 5.2108 Å, 5.2067 Å, 5.2007 Å and 5.1933 for ZnO, 5%, 10% and 20 mol% of Mg doped ZnO respectively) and unit volume decreases from 47.685 to 47.669 Å3 which is well agreement with the previous reports 18, 19. The decreases in lattice constants of ZnO with increases in Mg concentration are mainly due to the incorporation of Mg ions in the interstitial sites of wurtzite structure. This also confirm through the shift of corresponding (002) peak towards higher angle (Figure 4b). The sizes of the nanoparticles were estimated using the Scherer relation20 and found to be ~36 - 40 nm from the prominent (101) diffracted peak which is in good agreement with that observed in SEM images. XPS was measured to know the composition and chemical bond configuration of pure and Mg doped ZnO thin films. The energy scale was calibrated with the C1s peak of the carbon at 284.28 eV. Figure 5a (i) and (ii) shows the typical XPS survey spectra of ZnO and highly efficient 5 mol % Mg doped ZnO thin films and the inset shows the Mg 2p and 1s binding energies peaks at 48.2 and 1303.71 eV respectively. The peaks related to C 1s, O 1s, Zn 2p3/2 and Zn 2p1/2 have been observed in both the samples.

and 1043.87 eV respectively. This shows a good symmetry, indicating the presence of Zn in the 2p chemical

Figure 3 SEM image showing the porous interior of banyan root like ZnO film.

Figure 2 SEM images of a) ZnO photoanode, b) 5 mol %, c) 10 mol %, d) 20 mol % Mg doped ZnO photoanodes

Figure 5b (i) and (ii) shows the Zn 2p spectra of ZnO and 5 mol % Mg doped ZnO films respectively. The addition of Mg can either replace Zn2+ or form MgO secondary phase. The binding energy peaks of Zn 2p3/2 observed at 1020.69 eV and 1020.76 eV for ZnO and Mg doped ZnO repectively, clearly shows that Mg2+ is replacing the Zn2+ 21. The binding energy of Zn 2p1/2 peak for ZnO and Mg doped ZnO film is observed at 1043.79



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4 a) XRD pattern of ZnO and ZnMgO films on FTO substrate b) shift of (002) peak position of ZnO with respect to Mg concentration.

Page 4 of 9

sorption in the visible region above 380 nm. This may be due to the submicron (400 - 800 nm) sized cylindrical ZnO root branches and interspacing between branches is comparable to the wavelength of the incident visible light. Therefore, with in the photoanode, the path length of the incident light is increased and the light harvesting efficiency is increased 24, 25.

Figure 5 a) The XPS survey of ZnO and 5 mol % Mg doped ZnO films and inset shows the Mg binding energy peaks; the comparison of XPS spectrum of b) O c) Zn for both (i) ZnO (ii) 5 mol % Mg doped ZnO.

states in ZnO thin film 22. Figure 5c (i) and (ii) shows the O 1s spectra of ZnO and 5 mol % Mg doped ZnO films respectively and the binding energy peak at 529.51 eV is attributed to the O2- ions on the wurtzite structure of ZnO 23. The XPS spectra clearly detects and confirms the presence of Mg where as the Mg peak is not detected in the XRD pattern which indicates that the wurtzite structure of ZnO is not affected by the trace amount of Mg content in the sample. Figure 6a shows the optical absorbance spectrum of ZnO and ZnMgO films. The gradual blue shift of the fundamental absorption edges with increase in Mg concentration has been observed. The exitonic peak of ZnO persists in the absorption spectra even after considerable increase in Mg incorporation. Obviously, the optical band gap of the film (3.22 to 3.5 eV) increased with the Mg concentration in ZnO matrix (Figure 6a inset). Compared to 10 and 20 molar % Mg films, ZnO and 5 molar % Mg doped ZnO film shows little enhanced ab-

Figure 6 (a) The optical absorption spectrum of ZnO and ZnMgO films (b) The I-V behaviour of solar cells consisting of ZnO and ZnMgO photoanodes at 100 mWcm-2.

The branch diameter of 400 - 500 nm is observed for 5 mol % Mg doped ZnO and the interspacing length between branches is about 300-500 nm causes high absorption in the visible region than that of the other three films. But, in the case of highly Mg doped films, the branches have broadened and the spaces between branches were diminished and show low absorption in the visible region. Figure 6b shows the resultant I-V curve of the ZnO and ZnMgO DSSC measured under a stimulated light source of 100 mWcm-2 power density. Table 1 represents


Page 5 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry the performance of ZnO and ZnMgO in terms of efficiency (η), fill factor (FF), Voc and Jsc. The excellent performance with the conversion efficiency (η) of 4.11% obtained for the DSSC fabricated using 5 mol % Mg doped ZnO photoanode with the thickness of ~ 4 µm, is best among our experimental DSSCs. The three different compositions of ZnMgO DSSCs show high and almost similar open circuit voltage (Voc) in the range of ~ 0.71 V and fill factors (FF) of 0.58 - 0.65 compared to ZnO DSSC. But, the current density (Jsc) drastically decreases with increase in Mg concentration over 5 molar %. The high photocurrent density, enhanced Voc and FF lead to higher conversion efficiency for 5 mol % Mg doped ZnO than that of ZnO photoanode. The enhancement in the conversion efficiency is due to the positive influence of Mg ions on the photovoltaic performance by affecting either the morphological structure or the surface chemistry of the ZnO nanoparticles as well as the photoelectrode film. This suggests that the surface network of ZnMgO is greatly reinforced compared to the ZnO photoanode. The preferential replacement of Zn2+ by Mg2+ ions within the wurtzite structure seems to make the incorporation of Mg2+ dopant highly effective to increase the surface stability. Consequently, Zn2+–dye aggregates may be greatly reduced, and thus the dye molecules are adequately adsorbed on the electrode surface. The amount of dye adsorbed on ZnO and ZnMgO electrodes were estimated by desorbing the dye (Figure 7a). Compared to ZnO film, ZnMgO electrodes show decreased dye adsorption with respect to increase in Mg concentration. This strongly implies that Zn2+–dye aggregates are considerably reduced, owing to the increased resistance of electrode surface to acidic dye. On the other hand, the high concentration of Mg ions increases the optical band gap and insulating property of ZnO films which reduces the rate of electron transfer to FTO. This lowers the steady state charge density in the photoanode and affects the overall performance of the cells 26, but in dark I-V measurement (Figure 7b) shows and slight enhancement in the onset potential of DSSCs with higher Mg concentration. To ensure the exact mechanism of improvement and decline in the photovoltaic performance with respect to Mg concentration, the energy band diagram was determined.

Figure 7 (a) The optical UV- absorption spectrum of N719 dye desorbed from photoanodes using 0.1M NaOH solution b) Dark current vs. voltage plot for ZnO and Mg doped ZnO.

According to Yadav et. al, the HOMO-LUMO gap of ZnO increases with increasing the concentration of Mg ions16 and X. Qiu et. al 27 theoretically explained as, the incorporation of Mg ions in Zn site of ZnO structures results in an apparent variation of the conduction band, but the valence band remains almost same as that of ZnO. This is confirmed through the band gap broadening with increases in Mg ion concentration. Using these concepts, the energy band diagram of Mg doped ZnO films was constructed from the calculated band gap, obtained conduction band minimum (CBM) 28, 29 value and compared with energy band diagram of pure MgO30 and is schematically depicted in Figure 8. From this energy band diagram, the enhancement of 5 molar % Mg doped ZnO DSSC was due to small shift in CBM towards the higher energy level. This tends to move CBM little closer to the LUMO level of N719 dye and makes an effortless transfer of photo exited electron to the conduction band of photoanode than that of bare ZnO photoanode. However, in case of 10 and 20 mol % Mg



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 9

doped ZnO photoanode, CBM shifts much closer to LUMO level of the dye. Generally, one can expect higher electron transfer rate and efficiency owing to their effortless transfer of photo-injected electron to the closer CBM from the LUMO of dye, but conflicting to this it shows a decline in current density and affecting overall efficiency of the cells. This was well agreement with the report of X. Qiu et. al for decrease in photo catalytic activity of heavily doped Mg ions in ZnO. The same trend of decrease in overall efficiency was found in our case for 20 mol % Mg doped ZnO. Normally, the phase diagram of ZnO-MgO system shows the thermodynamic solubility limit of Mg ions in the ZnO lattice was ~ 4% 31 . But excess of Mg ions will enter in to the interstitial site of ZnO with a lattice distortion (up to 33%) 15 this was well agreement with the XRD peak shift in our experiment. This excess amount of Mg ions can affect the interstitial site of wurzite ZnO by forming a set of shallow acceptor levels above the Fermi level. These shallow levels due to higher Mg concentration acts as a trapping or recombination center for the photo injected electrons. Even though the CBM moves close to the LUMO level of N719 dye and making large injection of photo electrons into the photoanode, the injected electrons were mostly trapped or recombined by the shallow acceptor level and make less probability of electron diffusion inside the photoanode and reduce the overall current density. Moreover, the Mg doped ZnO photoanode shows an enhancement in the Voc than bare ZnO. This is mainly due to the shifting of conduction band edge towards the vacuum level 13. Figure 9 ZnO and ZnMgO DSSCs (a) IPCE spectrum (b) Nyquist plots with the equivalent circuit.

Figure 8 a) Energy levels of the ZnO and Mg doped ZnO photoanodes. The valence band maximum (VBM; red colour) and the conduction band minimum (CBM; blue colour) represented in eV. The ground (HOMO; red) and excited states (LUMO; blue) of N719 dye is also shown. The energy scale is referenced to the vacuum level.

Figure 9a shows the IPCE spectra of DSSCs based on ZnO and ZnMgO photoanodes. The IPCE value depend on the photon absorption efficiency which is determined by the dye’s ability to absorb a photon, electron injection into the oxide, the collection efficiency of the electron, the dye regeneration efficiency, thickness of the photoanode etc. Apart from the above, regeneration efficiency of the dye is also important; as long as the dye remains in oxidized state, it can be a source of back electron transfer from the supporting oxide. From the figure, IPCE value at 500 nm is 40.2% for 5 mol % Mg doped ZnO DSSC which is comparable to the IPCE reported for high efficient N719 dye based ZnO DSSC 32, 33. The IPCE values of 5 mol % Mg doped ZnO is higher than the ZnO based DSSC which is attributed to the enhancement of light-harvesting efficiency resulted from electron collection, comparable dye adsorption, easy transfer of electron to the conduction band and less recombination of photo excited electrons with the electrolyte. The decreased IPCE in case of 10 and 20 mol % Mg doped DSSC is due to the lower light-harvesting efficiency, poor dye adsorption and limited electron transfer inside the photoanode.


Page 7 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Electrochemical impedance spectroscopy (EIS) was used to characterize the internal resistance and chargetransfer kinetics of ZnO and ZnMgO photoanodes 34. The obtained Nyquist plots (Z*) for the frequency range of 0.1Hz to 1 MHz are shown in Figure 9b. The Z* plot of ZnO and ZnMgO photoanodes measured under light illumination (100 mWcm-2) consists of two semicircle with a nonzero intercept at high frequency end. Impedance analysis software Z VIEW was used to model the impedance spectra based on equivalent circuit consisting of a series of two parallel RC circuits. The saturation in the low frequency range of the second semicircle was fitted with and Warburg diffusion element as shown in the inset of Figure 9b35, 36. A series resistor (Rs) is added to the circuit to account the nonzero intercept on the real axis of the impedance plot which represents the sheet resistance of TCO. The Warburg element can be used to monitor mass transport limitations in the DSSC through the observation of diffusion impedance Zd with the characteristic frequency ωd as given in equation 1 37.

/ /

(1)

/ /

where i = (-1)1/2 and ω is the frequency of the potential perturbation. The electron transfers resistance (Rpt) at counter electrode/electrolyte, charge transfer resistance (Rct) at photoelectrode/electrolyte interface, the diffusion impedance (Zd) of electrolyte and the chemical capacitance (Cµ) were directly obtained from the fit. Electrochemical parameters determined from EIS analysis and lifetime of electrons (τr) are summarized in Table I. The results shows that the DSSC fabricated using 5 mol. % Mg doped ZnO photoanode exhibited the smallest charge transfer resistance Rct and high chemical capacitance (Cµ) indicating faster charge transport at the interface of ZnMgO/electrolyte and the dye. The charge transfer resistance increases with increase in Mg ions leads to higher resistance of the photoanode and at the same time the declined chemical capacitance (Cµ) indicates less number of electron densities with respect to the Mg ions. The high value of charge transfer resistance implies a reduction of electrons transfer rate and poor efficiency 38. Table 1 Performance and ESI results of ZnO and ZnMgO DSSCs Parameters Voc(V)

ZnO Mg 5% 0.67

-2

Jsc (mAcm ) 6.15 FF

0.47

Mg 10%

Mg 20%

0.71

0.72

9.98

6.54

3.98

0.61

0.65

0.58

η (%)

1.97

4.11

Rs

18.6

17.5

2.87 17.7

Rpt

29.2

30.4

Rct

53.2

42.6

0.74

1.91

According to the EIS model, the effective electron lifetime before recombination (τr) in the photoelectrode can be estimated from the minimum angular frequency (ωmin) value of the impedance semicircle at middle frequencies in the Nyquist plot: τr = 1/ ωmin. The electron lifetime (τr) was highest for the 5 and 10 mol % Mg doped ZnO DSSC with 9.62 ms and 7.32 ms respectively. The increase in electron lifetime supports reduction in the recombination of injected electrons with the I3- in the electrolyte 39, 40. The low electron life time of bare ZnO cell is due to the higher recombination rate of the injected electrons with the electrolyte. In case of 20 mol % Mg doped ZnO DSSC, the excess amount of Mg ions increases the resistances of photoanode and large trap level was set by Mg ion in ZnO interstitial site and reduces the probability of electron transfer and which reflects in the reduction of electron life time of photoinjected electro inside the photoanode.

4. Conclusions In summary, Mg doped ZnO based banyan root like structured photoanode DSSC were fabricated by a simple single step drop casting technique. The porous and continuous root like network with the submicron sized branches promote high electrolyte diffusion, large electron transport, increased optical path length and improved light harvesting efficiency of the cells. The 5 mol % Mg doped ZnO DSSC shows very high efficiency (4.11%) and enhanced performance with Voc= 0.71 V, FF = 0.58 and Jsc = 9.98 mAcm-2 and improvement in IPCE compared to the bare ZnO DSSC. These experimental evidences confirmed that the incorporation of 5 mol % Mg in the Zn site strongly influence the properties of photoanode by preventing the spontaneous recombination of the photo exited electrons and moves the conduction band of photoanode near to dye’s LUMO level for fast transfer of electron. But, increases in Mg ions proportionally raise the optical band gap and set up and trap level (shallow acceptor level) for injected electrons and lower the electron transfer rate and affect device efficiency. These confirm that minimal amount Mg ions on ZnO were found very effective to improve the surface properties both in chemical stability and electronic surface state by reinforcing the surface network.

AUTHOR INFORMATION Corresponding Author *Corresponding author. Tel.: +82 51 510 7334; fax: +82 51 513 0212. E-mail address: [email protected] (K. Prabakar), [email protected] (Hee-Je Kim)

18.4

ACKNOWLEDGMENT

32.8

30.2

48.4

64.7

The author C.J.R would like to thank the BRAIN KOREA21 (BK21) for its financial support. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0009749).

Zd

14.2

15.3

15.1

15.4

Cμ (μF)

4.44

7.98

5.44

2.84

τr (ms)

4.43

9.62

7.32

2.94

REFERENCES



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(1)Memarian, M.; Concina, I.; Braga, A.; Rozati, S. M.; Vorniero, A.; Sberveglieri, G. Hierachically assembled ZnO nanocrystallites for high efficiency Dye-Sensitized solar cells. Angew. Chem. Int. Ed. 2011, 50, 12321 – 12325. (2) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nanowire dye-sensitized solar cells. Nature Mater. 2005, 4, 455459. (3) O’Regan, B.; Gratzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737-740. (4) Joo, J.; Kwon, S. G.; Yu, J. H.; Hyeon, T. Synthesis of ZnO Nanocrystals with Cone, Hexagonal Cone, and Rod Shapes via Non-Hydrolytic Ester Elimination Sol–Gel Reactions. Adv. Mater. 2005, 17, 1873-1877. (5) Zhang, Q.; Dandeneau, C. S.; Zhou, X.; Cao, G. ZnO Nanostructures for Dye-Sensitized Solar Cells. Adv. Mater. 2009, 21, 4087–4108. (6) Ko, S. H.; Lee, D.; Kang, H. W.; Nam, K. H.; Yeo, J. Y.; Hong, S. J.; Grigoropoulos, C. P.; Sung, H. J. Nanoforest of Hydrothermally Grown Hierarchical ZnO Nanowires for a High Efficiency Dye-Sensitized Solar Cell. Nano Lett. 2011, 11, 666–671. (7) McCune, M.; Zhang, W.; Deng, Y. High Efficiency DyeSensitized Solar Cells Based on Three- Dimensional Multilayered ZnO Nanowire Arrays with “Caterpillarlike” Structure Nano Lett. 2012, 12, 3656-62 (8) Zhang, Q.; Dandeneau, C. S.; Candelaria, S.; Liu, D.; Garcia, B. B.; Zhou, X.; Jeong, Y. H.; Cao, G. Effects of Lithium Ions on Dye-Sensitized ZnO Aggregate Solar Cells. Chem. Mater. 2010, 22, 2427-2433. (9) Zheng, Y. Z; Tao, X.; Hou, Q.; Wang, D. T.; Zhou, W. L.; Chen, J. F. Iodine-Doped ZnO Nanocrystalline Aggregates for Improved Dye-Sensitized Solar Cells. Chem. Mater. 2011, 23, 3–5. (10) Peter, L. M. The Gratzel cell: Where next? J. Phys. Chem. Lett. 2011, 2, 1861 – 1867. (11) Bisquert, J. Dilemmas of Dye-Sensitized Solar Cells. Chem. Phys. Chem. 2011, 12, 1633−1636. (12) Olson, D. C.; Shaheen, S. E.; William, M. S.; Maikel, J. M.; Hest, F. A. M. V.; Collins, R.T. ; Ginley, D. S. Band-Offset Engineering for Enhanced Open-Circuit Voltage in Polymer–Oxide Hybrid Solar Cells. Adv. Funct. Mater. 2007, 17, 264-269. (13) Duan, Z.; Pasquier, A. D.; Lu, Y.; Xu, Y.; Garfunkel, E. Effects of Mg composition on open circuit voltage of Cu2O–MgxZn1−xO heterojunction solar cells. Sol. Energy Mater. Sol. Cells 2012, 96, 292-297. (14) Ohotomo, A.; Kawasaki, M.; Koida, T.; Masubuchi, K.; Koinuma, H.; Sakurai, Y.; Yoshida, Y.; Yasuda, T.; Segawa, Y. MgxZn1xO as a II–VI widegap semiconductor alloy. Appl. Phys. Lett. 1998, 72, 2466-2468. (15) Park, W. I.; Chul, Y. G.; Jang, H. M. Metalorganic vaporphase epitaxial growth and photoluminescent properties of Zn1−xMgxO (0≤x≤0.49) thin films. Appl. Phys. Lett. 2001, 79, 20222024. (16)Yadav, M. K.; Ghosh, M.; Biswas, R.; Raychudhuri, A. K.; Mookerjee, A. Band-gap variation in Mg and Cd doped Zno nanostructures and molecular clusters. Phys. Rev. B 2007, 76, 195450–195459 (17) Raj, J. C.; Karthick, S. N.; Hemalatha, K. V.; Son, M. K.; Kim, H. J.; Prabakar K. Magnesium doped ZnO nanoparticles embedded ZnO nanorodhybrid electrodes for dye sensitized solar cells. J. Sol-Gel Sci. Tech. 2012, 62, 453-459. (18) Ohtomo, A.; Kawasaki, M.; Koida, T.; Masubuchi, K.; Koinuma, H.; Sakurai, Y.; Yoshida, Y.; Yasuda, T.; Segawa, Y. MgxZn1-xO as a II–VI widegap semiconductor alloy. Appl. Phys. Lett. 1998, 72, 2466-2468. (19) Shan, F. K.; Kim, B. I.; Liu, G. X.; Liu, Z. F.; Sohn, J. Y.; Lee, W. J.; Shin, B. C.; Yu, Y. S. Blueshift of near band edge emission

Page 8 of 9

in Mg doped ZnO thin films and aging. J. Appl. Phys. 2004, 95, 4772-4776. (20) Patterson, A. L. The Scherrer Formula for X-Ray Particle Size Determination. Phys. Rev. 1939, 56, 978–982. (21) Etacheri, V.; Roshan, R.; Kumar, V. Mg doped ZnO nanoparticles for efficient sunlight driven photocatalysis. Appl. Mater. Interfaces 2012, 4, 2717 – 2725. (22) Langer, D. W.; Vesely, C. J. Electronic core levels of Zinc chalcogenides. Phys. Rev. B 1970, 2, 4885 – 4892. (23) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg G. E. Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer, Eden Prairie, 1979. (24) Kim, J. B.; Kim, P.; Pegard, N. C.; Oh, S. J; Kagan, C. R.; Fleischer, J. W.; Stone, H. A.; Loo, Y. L. Wrinkles and deep folds as photonic structure in photovoltaics. Nature Photon. 2012, 6, 327-332. (25) Zhang, Q. F.; Chou, T. P.; Russo, B.; Jenekhe, S. A.; Cao, G. Z. Light scattering enhanced energy conversion efficiency in hierarchically-structured ZnO dye-sensitized solar cells. Adv. Funct. Mater, 2008, 18, 1654-1660. (26) Wu, S.; Han, H.; Tai, Q.; Zhang, J.; Xu, S.; Zhou, C.; Yang, Y.; Hu, H.; Chen, B.; Sebo, B. et al. Enhancement in dyesensitized solar cells based on MgO-coated TiO2 electrodes by reactive DC magnetron sputtering. Nanotech. 2008, 19, 215704– 215709. (27) Qui, X.; Li, L.; Zheng, J.; Liu, J.; Sun, X.; Li, G. Origin of enhanced photocatalytic activities of semiconductor: A case study of ZnO doped with Mg 2+. J. Phys. Chem. C 2008, 112, 12242-12248. (28) Kraut, E. A.; Grant, R. W.; Waldrop, J. R.; Eowalczyk, S. P. Precise Determination of the Valence-Band Edge in X-Ray Photoemission Spectra: Application to Measurement of Semiconductor Inter face Potentials. Phys. Rev. Lett. 1980, 44, 1620-1623 (29) Janotti, A.; Walle, C. G. V. Absolute deformation potentials and band alignment of wurzite ZnO, MgO and CdO. Phys. Rev. B 2007, 75, 121201-121204. (30) Robertson, J.; Falabretti, B. Handbook of Transparent Conductors. Editor, Ginley D.S. Springer, 2010, 38. (31) Segnit, E. R.; Holland, A. E. The system MgO-ZnO-SiO2. J. Am. Ceram. Soc. 1965, 48, 409-413. (32) Ren, Y.; Zheng, Y. Z.; Zhao, J.; Chen, J. F.; Zhou, W.; Tao, X. A comparative study on indoline dye- and ruthenium complexsensitized hierarchically structured ZnO solar cells. Electrochem. Commun. 2012, 16, 57-60. (33) Zhanga, L.; Yanga, Y.; Fana, R.; Wanga, P.; Lia, L. Enhance the performances of dye-sensitized solar cell by a new type of sensitizer to co-sensitize zinc oxide photoelectrode with ruthenium complex. Dyes and Pigments, 2012, 92, 1314-1319. (34) Wang, Q.; Moser, J. E.; Gratzel, M. Electrochemical Impedance Spectroscopic Analysis of Dye-Sensitized solar Cells. J. Phys. Chem. B 2005, 109, 14945-14953. (35) Santiago, F. F.; Bisquert, J.; Belmonte G. G.; Boschloo, G.; Hagfeldt, A. Influence of electrolyte in transport and recombination in dye-sinsitized solar cells studied by impedance spectroscopy. Sol. Energy Mater. Sol. Cells 2005, 87, 117-131. (36) Bisquert, J. Chemical capacitance of nanostructured semiconductors: its origin and significance for nanocomposite solar cells. Phys. Chem. Chem. Phys. 2003, 5, 5360–5364. (37) Bisquert, J.; Cahen, D.; Hodes, G.; Ru1hle, S.; Zaban, A. Physical Chemical Principles of Photovoltaic Conversion with Nanoparticulate, Mesoporous Dye-Sensitized Solar Cells. J. Phys. Chem. B 2004, 108, 8106-8118. (38) Lai, S. C.; Connor, K. N. P.; Ke, L. Degradation Mechanisms and electron kinetics analysis in aging dye sensitized solar cells using electrochemical impedance spectroscopy. J. Electro Chem. Soc. 2011, 158, H1193-H1200. (39) Chen, H. W.; Liang, C. P.; Huang, H. S.; Chen, J.G.; Vittal, R.; Lin, C.Y.; Wu, K. C. W.; Ho K. C. Electrophoretic deposition of


Page 9 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mesoporous TiO2 nanoparticles consisting of primary anatase nanocrystallites on a plastic substrate for flexible dye-sensitized solar cells. Chem. Commun. 2011, 47, 8346–8348. (40) Wang, K. P.; Teng, H. Zinc-doping in TiO2 films to enhance electron transport in dye-sensitized solar cells under lowintensity illumination. Phys. Chem. Chem. Phys. 2009, 11, 94899496.

Banyan root like morphological Mg doped ZnO thin photoanode is shown to result in high electron transport, retardation of interfacial charge recombination, improved light harvesting efficiency and overall enhancement in the photovoltaic performance of dye sensitized solar cells.


Banyan Root Structured Mg-Doped ZnO Photoanode Dye-Sensitized

Recommend Documents