TiO2 photoanode for dye sensitized solar cells

Renewable and Sustainable Energy Reviews 61 (2016) 97–107

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Electron transfer properties of organic dye sensitized ZnO and ZnO/TiO2 photoanode for dye sensitized solar cells Mamta Rani a,b, S.K. Tripathi a,n a b

Centre of Advanced Study in Physics, Department of Physics, Panjab University, Chandigarh 160014, India DAV University, Sarmastpur, Jalandhar, Punjab 144001, India

art ic l e i nf o

a b s t r a c t

Article history: Received 15 April 2015 Received in revised form 13 January 2016 Accepted 3 March 2016

The effect of using bilayer film on the efficiency of dye sensitized solar cells is investigated. ZnO, TiO2 and bilayer ZnO/TiO2 (ZTO) based cells are developed and sensitized with five organic dyes and one cocktail dye composed of five dyes. The electrical measurements such as steady state conductivity and transient photoconductivity are studied at different temperatures for understanding the electron transport and charge recombination mechanism in dye sensitized films. The photosensitivity of the dye sensitized film is higher than bare oxide film, while the carrier life time of dye sensitized metal oxide is smaller as compared to pure metal oxide films because of hole passivation effect. Photovoltaic performance of ZnO and TiO2 based solar cell sensitized with six dyes is compared to that of bilayer ZTO based cells. The current voltage characteristics of the cells show that ZnO modification improves open circuit voltage (Voc), but lowers short circuit current (Jsc), It is explained with help of charge trapping/detrapping and recombination mechanism. Among the six dyes, Rhodamine B (RhB) gives the best performance as sensitizer with TiO2 and ZTO. While Eosin Y (EY) dye gives highest efficiency with ZnO based solar cells. The comparison of conductivity measurements of the samples has emphasized the major role of surface states in recombination mechanism. & 2016 Elsevier Ltd. All rights reserved.

Keywords: ZTO Photoconductivity Dye-sensitized solar cell Conversion efficiency Organic dye

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Experiment details. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3.1. Structure and morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3.2. Steady state photoconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3.3. Transient photoconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 3.4. Photovoltaic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 4.1. Steady state photoconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 4.2. Transient photoconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 4.3. Photovoltaic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

1. Introduction

n

Corresponding author. Tel.: þ 91 172 2534462; fax: þ 91 172 2783336 E-mail addresses: [email protected], [email protected] (S.K. Tripathi).

http://dx.doi.org/10.1016/j.rser.2016.03.012 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

Solar electricity is a steadily growing energy technology today and solar cells have found markets in variety of applications ranging from small-scale electronic devices to large scale power

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plants. Dye-sensitized solar cells (DSSC) show great promise as an inexpensive alternative to conventional p-n junction solar cells [1–5]. DSSCs have undergone considerable development in their fabrication methods, as well as in the analysis of the carrier mechanism therein [6–9]. However, the achieving an effective DSSCs performance is still facing several critical problems, such as short life time, insufficient utilization of light, photo-carrier trapping and recombination, lacking of detailed understanding of interfacial carrier behaviours and so forth [10,11]. Also research efforts are required not only to quantify the trap states in mesoporous metal oxides, but also new mesoporous architectures to increase the conversion efficiency of metal oxide-based photovoltaic's. Some researchers attempted to improve the properties of the TiO2 electrode [12–14] or used a bilayer system [15–18]. The modification of the electronic states of the bilayer system allows the transfer of photogenerated carries between the two semiconductors and explains the observed enhancement of electron– hole pair separation and the increased carrier lifetime, which in turn can improve the efficiency of DSSC [19–21]. As a result of their low cost and versatility high porosity factor and wide band gap, TiO2 is widely used in various application such as photocatalyst, dye sensitized solar cell, gas sensor, optical devices [22–24]. ZnO is an attractive material and has been investigated widely for applications in piezoelectric devices, light emitting diode, in UV sensor and in solar cell [25–28]. The electron mobility is much higher in ZnO than in TiO2 and band gap is similar to TiO2 [29,30]. Therefore, ZnO is considered to be a suitable candidate to be coupled with TiO2 [31,32]. In present work, to get benefits of both constituent properties i.e. high porosity of TiO2 and high electrical conductivity of ZnO, we prepared mesoporous composite ZTO film using ZnO on TiO2 as coating agent and compared the results with individual oxides. ZTO may suppress the recombination and promote the separation of electron hole pairs, improving the DSSC performance [15]. The picture of the electron transport in nanostructured electrode of DSSC is presently incomplete and a lot of research has to be done to quantify the trap states in mesoporous metal oxides [33]. Photoconductivity is a valuable tool to probe charge separation, electron transfer mechanism, charge trapping and carrier recombination mechanism of dye sensitized metal oxide film, as electron transport in these devices plays a decisive role for the electron collection efficiency and therefore for the overall efficiency [34,35]. Two types of photoconduction are defined in solids. One is primary photoconduction, which is a time of flight (TOF) technique [36,37]. In this paper, we adopt the second type of photoconduction in which we use current voltage measurements to study conduction in the dark and photoconductivity as a probe of electron conduction under illumination [38,39]. Through these measurements, it is found that the dark conductivity and photoconductivity of materials vary with temperature and photon flux, photosensitivity discrepancy among different temperatures, recombination dynamics [38]. In literature, most studies have been reported on the photoconductivity behaviour of TiO2 and ZnO nanoparticles individually [40–47], the study of the photoconductivity response of the bilayer ZTO is relatively limited [47–53]. The focus of the current study is to probe charge separation, electron transfer mechanism, charge trapping and carrier recombination mechanism of dye sensitized ZTO films as well as of TiO2 and ZnO and the effect of using bilayer ZTO film on efficiency of dye sensitized solar cell. All metal oxide films are with the low-cost six organic dyes, which are commonly used dyes with TiO2 [54,55] and ZnO [56–58]. So, we present here a comparative study of conductivity, transient photoconductivity and injection dynamics in mesoporous films of TiO2, ZnO, and ZTO nanoparticles sensitized with six organic dyes in the absence of a supporting electrolyte or barrier-like contacts to understand the trapping/detrapping mechanism. which to our knowledge has not

been reported yet. From the data, we have obtained some mechanisms to analyse the performance of DSSC. Photovoltaic mechanism is relevant to phenomena involving light-material interaction, including photon absorption, charge-carrier creation and dynamics, and surface trapping, finally improving material parameters through a correlated technique for photoconductive devices.

2. Experiment details For the present work, porous nanosize ZnO powder and TiO2 nanorods prepared by sol gel method were used to create colloidal paste [59,60]. The porous ZnO powder was deposited on Fluorine doped tin oxide (FTO) by doctor blade technique. The substrates were cleaned using acetone, methanol and distilled water in sequence in an ultrasonic bath. In the paste preparation, the aggregated TiO2, ZnO particles of prepared powder were dispersed by grinding in mortar with particle stabilizers such as PEG (Polyethylene glycol) to prevent re-aggregation of the particles separately. Doctor blade method was used for preparation of films. Two pieces of tape were attached to the edges of the conducting glass so as to obtain a constant thickness film. The TiO2 paste was spread on the surface of conducting substrate with a glass rod, using adhesive tape as spacers. After drying in the air, the films were sintered for 30 min at 450 °C in the air. The sintering is needed to burn out organic binders and surfactants. ZTO film was prepared by coating ZnO nanospheres on prepared sintered TiO2 nanorod film and sintering again at 400 °C. The oxide paste was spread on by using doctor blade method. For this, Polyethylene glycol (PEG) was added to TiO2 and ZnO nanopowder separately to prevent the aggregation of the particles and to improve porosity of films. Six organic dyes EY, Fast Green (FGF), Rose Bengal (Rose), RhB, Acridine orange (AO) and Cocktail dye (C) were used for sensitizing metal oxide films. The dyes were dissolved in ethanol having concentration of 3.0  10  4 M. C dye solution was prepared by mixing different dyes at specific concentration as reported by Rani et al. [61]. Dye sensitized photoelectrodes were prepared by dipping the film in dye solution for 48 h in different dye solutions. Films were then rinsed with ethanol. To carry out the electrical characterizations on these films, a specially designed metallic (stainless steel) sample holder, fabricated in the laboratory, is used. The sample holder is connected to a rotary vacuum pump [British Thomson Houston Ltd., Type BC2410] to maintain a vacuum E10  3 mbar throughout the measurements. The photo and dark conductivities of these films are measured at different temperatures by using heaters which are connected to a variac externally through the sample holder to vary the rate of heating. The heating rate is monitored through the display of the digital panel which is connected to the copperconstantan thermocouple. For the steady state photoconductivity measurements (σph), light is shone on films for 30 s through the quartz window by using a tungsten lamp of 200 W as a photoexcitation source to carry out the photoconductivity measurements. Before measuring σph of the sample, it is first kept in dark till it attains equilibrium. σph is obtained by subtracting measured σ in dark from the measured σ in presence of light. To cut off the IR part of light, water in transparent petridis is kept above the quartz window, while taking readings in the presence of light. For rise and decay measurements, light is shone for 180 s and decay is recorded for the time period of 2500 s after illumination. A sandwich-type DSSC was fabricated with the dye sensitized oxide electrode and platinum as counter electrode. The electrolyte solution was a mixture of 0.5 M tetrapropylammonium iodide (TPAI) and 0.05 M iodine in ethylene carbonate and acetonotrile mixed solvent (60:40 by volume). Solar cell performance was

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studied in standard solar cell laboratory of Solar Energy Centre (SEC), Ministry of New and Renewable Energy, Gurgaon using class AAA solar simulator model SP1000-4966 based Solar Cell Tester of M.S Oriel, USA to study under standard test conditions (STC). Photoelectrochemical data was measured using a 1600 W Xenon light source that was focused to give 1000 W m  2 equivalent to 1 sun at AM 1.5 at the surface of the test cell. Working distance of lamp was 12.00 70.5 cm. I–V characteristics of the cell were recorded using a four point probe Keithley source metre (Model 2400). The active area was typically 1 cm2.

3. Results 3.1. Structure and morphology Fig. 1 shows the SEM images of TiO2, ZnO and bilayer ZTO films. From the micrographs, it is observed that the nanocrystallites interconnect and they form mesoporous inside the aggregates, providing the films with high porosity. Anatase TiO2 at 400 °C consists of well dispersed nanorods with an average aspect ratio 7:1 and ZnO nanocrystals are spherical in shape having diameter in the range 7–17 nm as reported in [59,60].

Fig. 2. Variation of (a) dark conductivity (σd) in low temperature region, (b) dark conductivity (σd) in high temperature region, and (c) photoconductivity (σph) with temperature for ZnO dye sensitized sample.

3.2. Steady state photoconductivity The dc dark conductivity (σd) and photoconductivity (σph) has been measured as a function of temperature for dye sensitized ZnO and ZTO films, which are plotted in Fig. 2(a) and (b) and Fig. 3(a). The electrical results show an exponential increase of σd with T. From dark conductivity curves, it is determined that ZnO and ZTO films formed by sol gel method are highly resistive at room temperature. The σd can be expressed by using Arrhenius equation
 σ dc ¼ σ exp ΔEdc =kT ð1Þ where ΔE is the activation energy and k is Boltzmann's constant. Figs. 2 and 3 show that ln σd versus 1000/T curves has two temperature regions (two different activation energies) for ZnO and one temperature region for ZTO sample, respectively. The dark activation energy ΔEdc of conduction for each region are calculated for assynthesized metal oxide and dye-sensitized metal films and tabulated in Table 1. After attaching dye molecules, the σd of films decreased noticeably as shown in Fig. 2(a) and (b), Fig. 3(a) and Table 1. For the steady state measurements, light is shone on films for 30 s through transparent window of sample holder. Photoconductivity

Fig. 3. Variation of (a) dark conductivity (σd) (b) photoconductivity (σph) with temperature for ZTO dye sensitized sample.

Fig. 1. SEM image of mesoporous (a) TiO2, (b) ZnO and (c) ZTO film.

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Table 1 Dark conductivity (σd), photo-conductivity (σph), activation energy parameters, photo-sensitivity (S), and carrier life time (τ) of metal oxide and dye sensitized metal oxide samples at 300 K Samples

σd (Sm  1)

σph (Sm  1)

S

ΔEdc1, ΔEdc2 (meV)

ΔEph (meV)

τ (s)

ZnO ZnO/EY ZnO/FGF ZnO/Rose ZnO/RhB ZnO/AO ZnO/C ZTO ZTO/EY ZTO/FGF ZTO/Rose ZTO/RhB ZTO/AO ZTO/C

1.46  10  9 1.26  10  12 3.65  10  10 8.01  10  11 5.75  10  10 9.09  10  11 1.81  10  10 4.96  10  12 2.00  10  14 4.00  10  14 6.00  10  14 1.20  10  13 8.00  10  14 2.00  10  14

1.65  10  11 2.07  10  12 5.50  10  11 1.53  10  11 1.31  10  10 1.77  10  12 3.50  10  11 2.70  10  11 2.56  10  12 4.00  10  13 7.60  10  13 5.84  10  12 8.00  10  14 7.80  10  13

0.01 1.64 0.15 0.19 0.23 0.02 0.19 5.44 128 10 12.7 48.8 1 39

3447 6, 499 77 540 7 6, 1221 710 502 7 8, 834 7 7 423 7 7, 6247 10 4007 7, 553 7 8 424 77, 10007 10 417 78, 8377 6 1042 77 8737 6 5667 5 5787 6 594 7 5 458 7 7 656 7 7

630 77 424 78 699 77 460 77 586 78 696 78 453 76 393 77 261 76 421 77 378 76 331 75 421 77 375 76

4.28 3.02 4.26 2.75 3.11 2.66 2.82 4.42 3.20 3.24 3.21 2.81 2.80 3.12

(σph) is obtained by subtracting measured σ in the dark from the measured σ in the presence of light. The temperature dependence of σph is plotted in Figs. 2(c) and 3(b) along with σd, which shows that σph increases exponentially with temperature. The results of photoconductivity measurements indicate that the conduction in ZnO and ZTO samples is through an activated process having single photoactivation energy in the temperature range 306–393 K and nature of the variation in σph with temperature of ZTO is similar as that of ZnO. Photoconductivity of as deposited and dye sensitized oxide film increases with increase in temperature. Photoactivation energy ΔEph value is also calculated using slopes of temperature dependent photoconductivity plots (Figs. 2 (c) and 3(b)) and given in Table 1. Temperature dependent conductivity studies performed on the samples reveal the variation in activation energy upon dye sensitization. Photosensitivity (S) of metal oxide/dye film is higher than the pure oxide film.

Fig. 4. Time dependence of σ during the rise and decay for ZnO dye sensitized electrodes.

3.3. Transient photoconductivity Transient photoconductivity is a very useful method to determine the energy distribution of various species of gap states which influences the carrier mobility and life time of carriers in oxide materials, assuming the response to be controlled by multitrapping processes. These measurements provide valuable information about the material quality for the various photoconductive applications. Photoconduction mechanism of all prepared samples is investigated by performing transient photoconductivity measurements on ZnO and ZTO oxides and all dye sensitized metal oxide films keeping intensity of light constant. Some researchers have reported these measurements for ZnO and TiO2. But no one has discussed how the adsorption of dye on oxides affects the trap levels. To measure the rise and decay of σ with time, metal oxides and dye sensitized metal oxide films are exposed to light. After a certain time of exposure, the light is turned off and the decay of photocurrent is measured as a function of time. The photo-response of these films to illumination in vacuum is shown in Figs. 4 and 5 as a function of time. Beginning with the onset of illumination, traps start to fill up and density of the photon generated carrier increases. The increase in the number of filled traps and mobile carriers continue until generation rate approaches recombination rate and equilibrium is reached in carrier production steady. Solid-state process in photoconduction of conventional semiconductors is expected to be very quick and photo-generated electrons and holes rapidly vanish as soon as the illumination is off. However, oxide materials exhibit slow photoresponse. Photoconductivity decay is convenient to investigate

Fig. 5. Time dependence of σ during the rise and decay for ZTO dye sensitized electrodes.

recombination process. During decay process. photocurrent does not reach to zero for a long time after the incident light is switched off. Persistent photocurrent is observed in all samples. Therefore, persistent photocurrent is subtracted from the measured σph for further analysis of photoconductivity decay to understand the trapping effects. After subtraction, ln σph is plotted against time at different temperatures in Figs. 6 and 7 for metal oxide and dye sensitized metal oxide electrodes. Decay curves for all three samples are non linear. As the light is switched off, the decay is initially abrupt. Decay curve for all the three samples are non linear, which shows that the samples show non-exponential decay

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similar to Eq. (3), where single valued τd is replaced by a multivalued τd. As τd is multi-valued function, its value changes with time during decay and thus will result in a non-exponential decay curve. By calculating the slope at any time t, one can calculate the value of τd at time t by using the equation [62]

τd ¼



   1 1 d Δp Δpo dt

ð4Þ

Fuhs and Stuke [63] have defined this decay time constant as differential life time. To analyse the decay rate for all samples, the concept of differential life time (τd) has been used. According to them

τd ¼ Fig. 6. Time dependence of σ during the decay for ZnO dye sensitized electrodes on logarithmic scale.



   1 dσ ph σ ph dt 1

ð5Þ

The value of τd at different time has been calculated using Eq. (5) for all samples from the slopes σph versus time curve at various times of decay curve of Figs. 6 and 7. The differential lifetime is equal to carrier life time for exponential decay. However, for non-exponential decay, τd increases with time during the decay. Then only the value at t¼0 has a physical meaning, it is equal to the carrier lifetime, which is useful for understanding charge transfer mechanism through dye sensitized metal oxide film. The value of carrier life time can be found from the extrapolation of curve at t¼ 0 from a plot of ln t versus ln τd shown in Fig. 8 and Fig. 9. These figures also show that the rate of rise and decay σph (dσph/dt) is slow in case of ZnO after sensitized it with dye except the highly photosensitized EY/ZnO. While after sensitized ZTO with dye, rate of rise and decay is fast. Carrier life time of all samples is given in Table 1. Carrier life time of dye sensitized metal oxide is smaller as compared to pure metal oxide films. Carrier life time of pure ZTO film is larger than pure ZnO. 3.4. Photovoltaic measurements

Fig. 7. Time dependence of σ during the decay for ZTO dye sensitized electrodes on logarithmic scale.

behaviour indicating the presence of continuous distribution of defect states in all three samples. In case of single trap level, the curve should be a straight line. However, in the present case for all the samples, curve is not straight line having single slope, but slope goes on decreasing continuously as the time of decay increases. This shows the presence of traps at all energies in band gap with different time constant, which indicates the nonexponential decay of σph of all samples. Decay rate appears to be faster with minimum value of persistent current for ZTO. Presence of traps (or gap states) plays a significant role in the recombination mechanism. When the material is exposed to light, a certain proportion of generated free carriers are captured by these traps. These filled traps will be emptied after the exciting light is switched off, at a rate depending upon their cross-section and ionization energy. If it assumed that the traps, which are contributing to σph, are of same kind and are located close to Fermi Level, then the decay time constant (τd) will have a single value and the equation of decay can be written as [62] dp dðpo þ ΔpÞ dΔp Δp ¼ ¼ ¼ dt dt dt τ

ð2Þ

Δp ¼ Δpo e  t=τd

ð3Þ

Here Δpo is the density of carriers at t¼0. However, if different kinds of traps with different energetic depth in the band gap are present, then the decay time constant will be a multi-valued function. The situation can still be represented by the relation

The I–V curves for all dye sensitized TiO2 (5 μm thick), ZnO (5 μm thick), and ZTO (10 μm thick) solar cells under illumination are shown in Fig. 10, Fig. 11 and Fig. 12 respectively. The figures of merit (Voc, Jsc, FF, Jmax, Pmax, η, Rs, Rsh) for these solar cells are listed in Table 2. The addition of ZnO to TiO2 layer increases the Voc as compared to pure TiO2 and ZnO based solar cells. ZTO based cells have higher efficiency with all organic dyes as compared to TiO2 cells, but lower efficiency as compared to ZnO.

Fig. 8. Differential life time versus time for ZnO dye sensitized samples on ln–ln scale.

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Fig. 9. Differential life time versus time for ZTO dye sensitized samples on ln–ln scale.

Fig. 11. Light I–V curve of the ZnO based dye sensitized solar cells.

Fig. 10. Light I–V curve of the TiO2 based dye sensitized solar cells. Fig. 12. Light I–V curve of the ZTO based dye sensitized solar cells.

4. Discussion 4.1. Steady state photoconductivity The positive sign of the ΔEdc indicates the exponential behaviour of temperature dependent current confirming the semiconducting nature of the samples that is expected from stoichiometric ZnO and ZTO. In low temperature regime, conduction can be explained by hopping conduction mechanism (VRH) proposed by Mott [64] and in high temperature regime; conduction can be explained on the basis of grain boundary model (GB) as the investigated semiconducting samples are nanocrystalline. According to GB model, the interface between grains gives rise to a large concentration of trapping states that are capable of trapping carriers. ZnO and ZTO films exhibits high dark current before dye attachment, which may be attributed to large number of dangling bonds between crystallites and hole traps. As, sol–gel synthesized nanocrystalline film is known to be highly disordered with enhanced effective surface areas up to 780 times the geometric area [1]. It reveals that a high density of surface states is dispersed in energy between conduction band (CB) and valence band (VB) of oxide. Point defects can behave as hole traps or electron traps. Due to n type behaviour of ZnO and TiO2, electron traps will be nearly filled and will not contribute to conductivity mechanism. Hole traps are mostly associated with oxygen vacancies, which readily occur on the surface as well as at the grain boundaries within the

Table 2 The figures of merit for all prepared DSSCs. Sample

Voc mV

ZnO/EY ZnO/C ZnO/Rose ZnO/RhB ZnO/FGF ZnO/AO TiO2/EY TiO2/C TiO2/RhB TiO2/Rose TiO2/FGF TiO2/AO ZTO/EY ZTO/C ZTO/RhB ZTO/Rose ZTO/FGF ZTO/AO

521 445 481 447 240 293 484 457 488 508 479 381 667 453 498 509 388 177

Jsc Jmax Pmax FF mA/cm2 (mA/cm2) (mW/cm2) % 5.470 1.611 2.93 4.178 0.142 0.169 0.151 0.192 2.174 0.274 0.183 0.084 0.449 0.198 0.283 0.308 0.194 0.055

4.614 1.268 2.46 3.508 0.081 0.119 0.099 0.109 1.72 0.188 0.125 0.053 0.351 0.153 0.244 0.213 0.147 0.024

1.599 0.410 0.838 1.100 0.011 0.024 0.032 0.032 0.601 0.663 0.041 0.013 0.175 0.050 0.089 0.074 0.040 0.021

η %

Rs Ω

Rsh Ω

56 1.60 26 13369 57 0.41 52 1902 59 0.84 37 2399 59 1.10 25 1468 33 0.011 1010 2957 49 0.024 401 6915 43 0.03 783 9900 38 0.03 712 9738 57 0.60 49 2512 48 0.07 380 7248 47 0.04 521 10573 40 0.013 1696 11484 56 0.18 167 9738 56 0.05 456 16350 63 0.89 308 216359 47 0.08 398 6442 53 0.04 414 14830 21 0.002 3333 2137

amorphous/nanocrystalline oxide. The high concentration of oxygen vacancy may act as electron donors, which can lead to significant dark current. It has been suggested that non-equilibrium holes

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rapidly trapped in deep traps, which may be intensify in particular regions due to inhomogeneity in the lattice such as grain boundaries. Thus, concentrated traps produce a local potential barrier that separates charge carriers by prohibiting electrons from readily combining with the holes and improve conductivity [65]. The decrease in the σd after attaching dye molecules is due to the passivation of some hole traps during the dye attachment [66]. The dyes are expected to attach to the oxide surface via some anchoring group. While the sample is soaked in solution, the protons, previously resided on the negatively charged anchoring group, may now passivate some of the hole traps near the surface. With increase in temperature, the mobility of charge carriers increases, it is possible that they adopt a hopping mechanism to cross the potential barrier and hence produce enhanced current [67–70]. Owing to the wide band gap of oxides, they are capable of absorbing light below 400 nm wavelengths only. The presence of photoconduction upon sub band gap illumination indicates the presence of defect-induced band gap states in oxides [70,71]. Further, the sub band transitions, which are due to localized defect induced band gap states and are commonly seen in nanostructure, can be the reason for the insignificant rise in photocurrent upon illumination with visible light using tungsten lamp [72]. The conductivity increases because of the intrinsic defects such as oxygen vacancies and interstitial zinc atoms. It is reported that the oxygen vacancies are the main contributors for the photoconduction in TiO2 and ZnO [73,74]. The well defined activation energies involved in the temperature dependence of σph plot suggests that the recombination centres are located at relatively discrete levels of localized states. Studenikin et al reported same value of activation energy for ZnO [75]. The activation energy (Eph) decreases in ZTO. This could be attributed to the increased density of shallow traps with increase in porosity and the large surface area as a result of enhanced film thickness [76]. High value of Photosensitivity of metal oxide/dye film than the pure oxide film is related to much lowered dark current for dye sensitized metal oxide film than pure oxide film in vacuum. Reemts et al. [77] has explained the reason of lower dark conductivity on the basis of surface state relaxation process. The higher photosensitivity is due to light absorption by dye and injection of photogenerated electrons into the oxide conduction band. The photon absorbed by a dye molecule may give rise to electron injection into the CB of the semiconductor which may be the one reason for increase in electron density. But dye regeneration may not occur in this case due to absence of electrolyte, so this mechanism cannot effectively contribute to increase in photocurrent. Second reason could be the presence of the dye molecules, which may modify the number of the hole traps, as explained earlier. Thirdly, as adsorbed molecules decreases the free electron density in the ZnO. The presence of the dye molecules may block some of molecular oxygen to form O2 sites and removing conduction electrons resulting in increase in free electron density [78]. 4.2. Transient photoconductivity Photoconductivity of ZnO and TZO is due to sub-band gap illumination in case of photoconductivity transients. The electron and hole pairs are generated as a result of sub band gap illumination corresponding to the optical transition between defect states and band edge. Transient photoconductivity measurements are governed by trapping effects due to the presence of inherent defect states in oxide materials. In case of oxides, under illumination, photo-induced holes discharge the negatively charged

adsorbed oxygen ions

n

h þ þ O2  -O2 ðgÞ

103

o

leading to the photo-

desorption of the oxygen. In the absence of light, every chemisorbed oxygen molecule captures an electron from the conduction  band O2 ðgÞ þ e  -O2 ðadÞ creating a negatively charged top surface and leaving behind a near-surface depletion layer lowering the conductivity of the film. This surface related phenomenon is considered to have a larger characteristic time scale, which means that by turning on/off the light, the process that makes the surface more conductive/resistive is significantly slower than the direct photoconduction response by electron–hole pair generation. An additional process, the so-called bulk phenomenon, is of great importance to conductivity, which depends on the grain boundary structure of the near-surface region. Rapid decay of σph is often attributed to localized-localized recombination mechanism according to which the trapped carriers combine directly with the carriers of opposite sign. Slow photoconductivity decay suggests the presence of carrier traps in band gap of oxides. Trapping centres increase the decay time as they slowly release trapped carriers after removal of the excitation source. Decay response of ZTO is consistent with ZnO indicating that hole trapping effect plays an important role [79]. Initial fast decay [Figs. 4 and 5] suggesting that recombination rate becomes important, Slow photoconductivity decay observed may be due to capture of holes in deep trap levels and subsequently hole emission and carrier recombination [71]. Smaller Carrier life time of dye sensitized metal oxide as compared to pure metal oxide films may be due to decrease in density of trapped states in the dye loaded sample because of hole passivation effect as explained earlier. Carrier life time of pure ZTO film is larger than pure ZnO show recombinations are depressed in ZTO. 4.3. Photovoltaic measurements The Voc is related to the interfacial charge recombination process between the dye-sensitized heterojunction and electrolyte. At open circuit conditions, all photo-generated carriers recombine within the solar cell diode [80]. Thus, if recombination can be minimized, Voc can approach more closely to the maximum values. By comparative electrical study, we found that the conductivity of ZnO is large as compared to ZTO, showing fast electron transfer rate compared to ZTO film; however on the other hand, transient measurements also indicate those recombinations are depressed in ZTO film, which increases the attainable open circuit potential of the ZTO based device. Other explanation is that the use of nanocrystalline ZnO permits the formation of an energy barrier at the TiO2 electrode/electrolyte interface. This energy barrier reduces the back electron transfer from CB of TiO2 to I3  in the electrolyte and to dye, thus reducing the recombination rate and improving the cell performance. There are several mechanisms by which ZnO modification can cause an higher open-circuit voltage such as surface dipole effect, tunnel barrier effect and reducing the surface states as explained in [81]. Series resistance of ZTO films is greater than ZnO but less than TiO2 based solar cell. From a lower charge transfer resistance, a large electron injection driving force and, consequence, a larger Jsc can be obtained. Low efficiency of TiO2 and ZTO based cell is because xanthene dyes such as Rose or EY have poor performance when used to sensitize TiO2 cells [82]. From electrical and photovoltaic measurements, it is found that EY dye is most photosensitive dye for ZnO. Theoretical analyses confirm that low efficiency of TiO2/EY sensitized solar cell is because only a small fraction of the electrons generated in the highest occupied molecular orbital (HOMO) of the dye has sufficient energy to transfer in the lowest unoccupied molecular orbital (LUMO) [83], while energy levels of ZnO are properly matched

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Table 3 The figures of merit for DSSCs from literature. Sample

Voc (mV)

Jsc (mA/cm2)

FF (%)

η (%)

Electrolyte

Counter electrode

Ref.

TiO2/EY

570 579 422 595 890 510 350 250 535 500 725 540 480 550 480 449 530 490 640 490

0.87 1.57 3.44 4.69 3.22 0.90 2.8 0.98 2.36 2.23 0.49 2.12 1.50 0.04 2.10 4.61 0.47 2.02 0.96 1.1

46 50 63 64 53 59   53 21 62 59 58 55 61 61 57 64 64 64

0.026 0.015 0.910 0.017 0.020 0.390 0.130 0.200 0.670 0.240 0.020 0.680 0.400 0.700 0.870 1.260  0.780 0.044 0.490

0.5 M LiI þ 0.05 M I2 þ0.05 M C6H4(OH)2 in C2H3N 0.1 M LiI þ0.05 M I2 in C2H3N 0.5 M LiI þ 0.05 M I2 þC4H7NO 0.5 M TBAIþ0.05 M I2 þ C3H4O3/C4H6O3 (3:1 w/w) 0.5 M KI þ0.05 M I2 þ0.14 M PEG in (C2H3N) 0.3 M KI þ0.03 M I2 in C4H6O3/EG 0.5 M KI þ0.05 M I2 in C4H6O3/C2H4(OH)2 (80:20) 0.5 M CH3CH2CH2)4NI þ0.03 M I2 in (C2H3N) þ(C3H4O3) 4 ml (SiC8H20O4) þ 0.4 ml (C2H4O2) þ 0.5 ml H2O 0.7 M LiI þ 0.07 M I2 in C4H6O3 Iodide/triiodide in (C2H3N) 0.3 M LiI þ 0.03 M I2 in (C4H6O3) 0.5 M PEG þ 0.5 M NaIþ0.05 M I2 þ 0.16 M CuI in C3H4O3/C4H6O3 0.5 mM I2 þ 0.5 M KI in (CH3OH) 0.3 M KI þ 0.03 M I2 in C3H4O3/C4H6O3 0.1 M I2 þ0.5 M TPAI in C3H4O3/C2H3N (20:80) 0.5 M LiI þ 0.05 M I2 in C4H6O3 0.3 M KI þ0.03 M I2 in C4H6O3 0.5 M LiI þ 0.05 M I2 þ0.05 M (C6H4(OH)2 in C2H3N 0.3 M KI þ0.03 M I2 in C4H6O3/C2H4(OH)2

Pt H2PtCl6 Pt Pt Graphite H2PtCl6 H2PtCl6 Pt Pt Pt Carbon H2PtCl6 Pt Pt H2PtCl6 Pt Pt H2PtCl6 Pt H2PtCl6

[85] [86] [87] [87] [88] [89] [90] [91] [92] [93] [94] [57] [61] [95] [89] [96] [97] [98] [85] [89]

TiO2/Rose TiO2/RhB TiO2/AO ZnO/EY

ZnO/FGF ZnO/Rose ZnO/RhB ZnO/AO ZnO/Rose/RhB TiO2 þZnO/EY TiO2 þZnO/Rose

Fig. 13. Schematic representation of components of DSSC and energy levels of FTO/ZnO/dye/electrolyte/Pt for significant. (1) Photo-excitation of electrons into surface states, (2) photo-excitation of electron from HOMO to LUMO of dye, (3) injection from LUMO of dye to ZnO CB, (4) transport of electron through electron traps to HOMO of dye, (5) subsequent electron transfer between distributed surface states and recombination with holes in the valence band, (6) hole capture and thermal detrapping process, (7) injection carrier from surface states of metal oxide to electrolyte and (8) electron injection from FTO to trap states of ZnO.

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with EY dye. RhB dye sensitized TiO2 and ZTO based solar cells has highest efficiency compared to other five dyes. Photocurrent of FGF and AO sensitized oxide is very small due to insufficient bonding between oxide and AO and FGF due to absence of any anchoring group [84]. Table 3, shows the various parameters of ZnO, TiO2 and their composite based solar cell reported literature. The parameters affecting the overall performance of a cell are grain size, porosity, and surface area, amount of dye adsorbed, the electrolyte type and the process for dye revival. A model for band structure of DSSC illustrating recombination process is sketched in Fig. 13. Shape of TiO2 and ZnO nanoparticles is shown pictorially. When dye absorbs photon equal to or more than highest occupied molecular level (HOMO)–lowest unoccupied molecular level (LUMO) gap energy, some of electrons undergoes recombination with holes emitted from hole traps in the valence band. Only the remaining electrons in the HOMO (or ground state) are excited to the LUMO (or excited state) and photo-excited electrons are injected into the CB of metal oxide, which leads to lower photoconductivity of metal oxide/dye as compared to pure oxide film. From CB, there is a random walk of electrons through the oxide film until they are sufficiently close to an oxidized dye molecule and can transfer to HOMO of dye. Photoinjected electrons from HOMO can be lost by recombination process through surface states of oxide. Under sub band gap illumination, electrons situated in the vicinity of surface are photoexcited to VB of oxide to surface states. The hole in the VB undergoes recombination with electron from surface states. With the passage of time, recombination becomes slow due to equilibrium and photoconductivity will change according to excess electron concentration which will deplete upon recombination with holes emitted from hole traps in the valence band. Decay in dye sensitized metal oxide is comparably fast than oxide as charge transfer from dye to ZnO by accessible states resulting in comparably smaller value of τ in case of ZnO/EY. Such charge transfer and subsequent recombination process reduce dark conductivity in oxide/dye film. DSSCs have a 103 to 104 times larger surface area depending on the metal oxide nanoparticle size in the mesoporous network, compared to planar junction solar cells. The large surface area enhances dye loading which leads to enhanced photoabsorption. However, a large density of surface defects acts as carrier recombination centres at the metal oxide/dye/electrolyte interface. Electrons can be injected from excited dye molecule to conduction band of metal oxide and from the FTO to trap states of metal oxide. Recombination at the interface mainly occurs through surface defects in two ways. Firstly, transition of an electron from conduction band of oxide to highest occupied molecular orbitals (HOMO) state of an oxidized dye molecule through diffusion and surface states as discussed above. Secondly, surface states of metal oxide material have played large role in charge transport process. So, we believe that the injection of carriers from the conduction band of metal oxide into the electrolyte happens through surface states at metal oxide/dye/electrolyte interface. an electron is trapped at oxide surface states, which then react with I3  ions and form I  ions (reduction process in electrolyte). In both cases, carrier loss is due to surface states. So, surface traps can significantly affect the overall efficiency of DSSC. So an interesting step forward can be to minimize the surface traps of metal oxide with the help of material engineering to improve the efficiency of solar cells.

5. Conclusions Bilayer ZTO film was prepared in order to improve the performance of solar cells sensitized with simple organic dyes.

105

Conductivity and transient measurements were performed on organic dye sensitized ZnO and ZTO film to understand the charge transport and recombination mechanism through dye sensitized films. Analysis of these measurements indicates the presence of traps at all energies in band gap decrease in dark conductivity, carrier life time and increase in photosensitivity after dye attachment is due to hole passivation effect. The ZTO cells has higher Voc as compared to TiO2 and ZnO based cells due to barrier layer effect of ZnO, which can depress the recombination process in DSSCs. Steady state conductivity have shown that conductivity of ZnO is large as compared to ZTO, showing fast electron transfer rate compared to ZTO film; however on the other hand, transient measurements also indicate those recombinations are depressed in ZTO film, which increases the attainable open circuit potential of the ZTO based device. ZTO cells sensitized with simple organic dyes yield higher efficiencies than corresponding TiO2 cells. Lower value of photocurrent in TiO2 and ZTO cells is related to dye, because xanthene dyes such as Rose or EY give better performance when used with ZnO rather than TiO2. EY dye gave the best performance as sensitizer with both ZnO, while RhB dye is good photosensitizer for TiO2 and ZTO. A model of band structure of DSSC is invoked to explain the effect of surface states of metal oxide material in charge recombination mechanism of photogenerated carriers at metal oxide/dye/electrolyte interface.

Acknowledgement This work is financially supported by U.G.C.N. Delhi (Major Research Project). Mamta Rani would like to acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi, for providing fellowship. Authors are thankful to Hemant Singh, Indraprastha University, New Delhi for sharing his research experience, fruitful discussions and support for solar cell measurements. Authors are also thankful to Solar Energy Centre, MNRE, Government of India, for providing the facility for Photovoltaic measurements.

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