Enhancement of the Performance of Dye-Sensitized Solar Cell by

Apr 10, 2008 - The performance enhancement of dye-sensitized solar cells (DSCs) in lithium-free and lithium- ... Subjecting the cells to forward bias ...
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J. Phys. Chem. C 2008, 112, 7084-7092

Enhancement of the Performance of Dye-Sensitized Solar Cell by Formation of Shallow Transport Levels under Visible Light Illumination Qing Wang,† Zhipan Zhang, Shaik M. Zakeeruddin, and Michael Gra1 tzel* Laboratory for Photonics and Interfaces, Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland ReceiVed: January 16, 2008; In Final Form: February 14, 2008

The performance enhancement of dye-sensitized solar cells (DSCs) in lithium-free and lithium-containing electrolytes under visible light-soaking was examined by impedance spectroscopy and photovoltage transient decay measurements. The improvement was found to arise from the formation of electronic transport levels close to the conduction band, resulting most likely from photoinduced proton intercalation in the TiO2 nanoparticles. These shallow trapping states accelerate the charge carrier transport within the nanocrystalline films without deteriorating the open circuit photovoltage. Subjecting the cells to forward bias in the dark produces a similar effect, whereas the introduction of lithium ions in the electrolyte suppresses the phenomena due to prevailing lithium ion intercalation. The redistribution of localized states in the band gap of TiO2 and the resulting conduction band edge movement appears to play a significant role in the performance of the DSC.

1. Introduction After more than one decade of research, dye-sensitized solar cells (DSCs) currently present a credible alternative to conventional solar cells.1-4 At the heart of the DSC is a nanocrystalline semiconductor oxide (typically TiO2) film that is covered by a monolayer of sensitizer. During the illumination of the cell, electrons are injected into the conduction band of the oxide by the photoexcited dye molecules adsorbed on the nanoparticle surface. The sensitizer is regenerated by hole injection into a redox electrolyte or solid p-type conductor. Ruthenium polypyridyl complexes have proved to be efficient and stable sensitizers, the cis-RuL2(SCN)2 (L ) 2,2′-bipyridyl-4,4′-dicarboxylic acid) dye (N3 dye) capturing a large domain of the visible spectrum. Incident photon-to-electron conversion efficiencies (IPCEs) attaining almost unity have been obtained when light losses due to the conducting glass current collector are taken into account.5-9 The validated overall solar to electric power conversion efficiency (η) has now reached a value of 11.1%8 under standard AM 1.5 reporting conditions. It is well-known that the IPCE or short circuit photocurrent (JSC) obtained with a given sensitizer depends strongly on the nature of the electrolyte and the type of mesoscopic TiO2 film employed, whereas the open circuit photo voltage (VOC) is often less affected. If low IPCE and JSC values are observed with cells for which the rate of diffusion of I-/I3- in the electrolyte and the dye-regeneration cannot be questioned, the poor performance must arise from either inefficient electron injection by the sensitizer or incomplete collection of the photogenerated carriers; that is, a low charge extraction yield. Efficient and very rapid electron transfer from the excited dye into the TiO2 conduction band is readily accomplished by matching the excited states level of the sensitizer to the energy of the conduction band edge and anchoring it to the surface via * To whom correspondence should be addressed. E-mail: michael. [email protected]. † Present address: Department of Materials Science & Engineering, National University of Singapore.

a carboxylate or phosphonate group.10-13 The dye must be molecularly engineered to ascertain adequate electronic coupling between the lowest unoccupied orbital (LUMO) and the Ti 3d orbitals.14,15 Although electrolyte additives such as 4-tertbutylpyridine (tBP) and Li+ ions can influence the charge injection rate, the latter remains much faster than the natural excited-state decay time of the sensitizer, assuring a high quantum efficiency of carrier generation.16 At short circuit state, the yield of charge extraction of a DSC is basically indicative of the competition between the transport of the conduction band electron through the mesoscopic TiO2 film and their back-reaction with the oxidized sensitizer or with triiodide ions in the electrolyte, the latter being the dominant loss channel. The electron transport is driven by an electrochemical potential gradient and normally appears to involve traplimited diffusion to the current collector.9,17-28 The charge recombination on the other hand involves surface states presenting at the TiO2/electrolyte interface, the sensitizer itself blocking the heterogeneous electron transfer from the conduction band of the solid to the triiodide in solution.9,27,29-32 Electrolyte additives,33-42 light exposure,43-45 and thermal aging4,46,47 were reported to have significant effects on the photovoltaic performance and stability of DSC. One of the intriguing observations made by several groups is the recovery of DSC performance following thermal aging by subjecting the cells to light-soaking. The present study attempts to elucidate this interesting phenomenon, which has significant practical implications, using voltammetry, impedance spectroscopy, and photovoltage transient measurements. Our results contribute to a deeper understanding of the electronic and ionic processes that govern the operation of DSC, providing important hints for improving the performance of DSC. 2. Experimental Section 2.1. Preparation of Dye-Sensitized Nanocrystalline TiO2 Electrodes and Cell Fabrication. The preparation of mesoscopic TiO2 film has been detailed in ref 9. Briefly, a paste

10.1021/jp800426y CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008

Performance of Dye-Sensitized Solar Cell TABLE 1: Electrolyte Compositions and DSC Cell Codes Used in This Study. cell

electrolyte

1

1.0 M 1-propyl-3-methylimidazolium iodide (PMII), 0.1 M I2 in 3-methoxypropionitrile (MPN) 1.0 M PMII, 0.1 M I2, 0.1 M lithium bis(trifluoromethane sulfone)imide (LiTFSI) in MPN 1.0 M PMII, 0.1 M I2, 0.5 M N-methyl-benzimidazole (NMB) in MPN 1.0 M PMII, 0.1 M I2, 0.5 M NMB, and 0.1 M LiTFSI in MPN

2 3 4

consisting of 20-nm-sized TiO2 colloid and ethyl cellulose in terpineol was first screen-printed on a fluorine-doped SnO2 conducting glass (TEC15, 15Ω/0) to form a transparent layer. Subsequently, a second scattering layer made up of a paste containing 400 nm anatase TiO2 particles (CCIC, Japan) was coated onto the nanocrystals to form a light-scattering layer. The screen-printed films consisted finally of a 10-µm-thick transparent nanoparticle layer and a 4 µm scattering layer. The active area of the electrode is around 0.23 cm2. The layer thickness was determined by using an Alpha-step 200 surface profilometer (Tencor Instruments, USA). A porosity of 0.63 for the transparent layer was measured with a Gemini 2327 nitrogen adsorption apparatus (Micromeretics Instrument Corp., USA). The film was heated to 500 °C in air and calcined for 20 min before dipping it into a 2 × 10-4 M cis-RuLL′(SCN)2 (L ) 2,2′-bipyridyl-4,4′-dicarboxylic acid, L′ ) 4,4′-dinonyl-2,2′bipyridyl) (Z907) dye solution in acetonitrile/tert-butanol (1:1 v/v) solution overnight. Finally, the dye-coated electrodes were rinsed with acetonitrile to remove any weakly adsorbed Z907 molecules. Dye-sensitized solar cells were prepared as outlined previously.9 Four different electrolytes were employed whose compositions are given in Table 1. The cells were subjected to light-soaking at 60 °C in a sun simulator at full solar intensity (1000 W/m2 intensity, 400 nm UV cutoff filter) and to thermal stress in the dark in an oven at 80 °C. 2.2. Photocurrent-Voltage Curve Measurements. A 450 W xenon lamp (Osram XBO 450, USA) equipped with appropriate filters was used as the light source for the I-V measurements. Its output closely matched the AM 1.5 solar spectrum in the region of ∼350-750 nm, reducing any spectral mismatch correction to less than 2%. Light intensities were adjusted with wire mesh attenuators. The current-voltage characteristics were determined by applying an external potential bias to the cell and measuring the photocurrent using a Keithley model 2400 digital source meter (Keithley, USA). 2.3. Electrochemical Measurements in the Dark. Cyclic voltammetry (CV) and impedance measurements were performed with a computer-controlled AutoLab PSTA30 instrument. The scan rate of the CV measurements was 0.1 V/s. Impedance measurements employed a frequency range of ∼0.005-100 kHz, the amplitude of the voltage modulation being 10 mV. Unless otherwise mentioned, all impedance measurements were carried out in the dark under various bias voltages and temperatures. The latter was controlled using a thermostated chamber (Salvis Lab, Rengenli). 2.4. Photo Voltage Transient Measurements. Photovoltage transient measurements used a 200 ms light pulse generated by a ring of red-light-emitting diodes (LED, Lumiled). The pulse had a rectangular profile controlled by a fast solid-state switch, and its intensity was adjusted to keep the increase of the VOC below 5 mV. An LED array was also used to adjust the VOC, providing a white light bias that was incident on the FTO front contact of the device. Usually, transients were measured at

J. Phys. Chem. C, Vol. 112, No. 17, 2008 7085 different white light intensities ranging from 1500 to 1 W/m2 by tuning the electric power supplied to the LEDs. 3. Results and Discussion 3.1. Charge Collection and Charge Extraction in DyeSensitized Solar Cells. Charge collection of a DSC is used to describe the collection of photoinjected electrons from the TiO2 nanoparticle anode at the FTO front contact. It provides information on the competition between the transport of electrons through the mesoscopic TiO2 film and their recapture by the oxidized mediator in the electrolyte. The recombination with oxidized dye can be neglected here because the surface concentration of the oxidized sensitizer is reduced to a very low level due to its rapid reaction with iodide ions. Impedance spectroscopy and other methods have revealed that the interfacial charge recombination involves electron transfer between the TiO2 and I3- in the electrolyte.9,26,32 The dependence of the charge-transfer resistance (Rct) on the potential follows an exponential law similar to the Tafel equation,

[

Rct ) A exp -

]

eβ V kBT

(1)

where kB is the Boltzmann constant and β is the transfer coefficient, which corresponds to the reciprocal value of the diode quality factor. Although the direct transfer of electrons from the conduction band edge to the acceptor in the electrolyte should give β ) 1, experimentally found values are lower, typically in the range 0.5 < β < 1, indicating electron trapping by surface states prior to the interfacial charge-transfer event. Because the effective screening of the photoinjected electrons by the high ionic strength electrolyte in the mesoporous network eliminates the internal electric field in the oxide semiconductor nanoparticles, the propagation of electrons in mesoscopic TiO2 film occurs via trap-limited diffusion driven by their concentration gradient.25,28 Only free electrons in the conduction band are thought to contribute to the diffusion processes. Therefore, electrons move between localized trap states by thermally activated detrapping to the conduction band. For such a case, the transport resistance, Rt, is given by eq 2,

[ (

Rt ) Rt0 exp -

)]

EF,redox - Ec e V+ kBT e

(2)

where Rt0 denotes a preexponent factor and Ec is the energy of the lower conduction band edge. Application of a forward bias voltage, V, moves the Fermi level of the electrons in the film toward the conduction band edge EFn ) eV + EF,redox, the occupancy of a given trap level being described by the FermiDirac distribution. The charge transport resistance decreases exponentially with the potential V. Increasing the bias potential fills the traps to a higher level, rendering the conduction band states more easily accessible to the electrons. Since the photogenerated electrons undergo forward transport in the TiO2 and back-reaction with the triiodide ions, the charge collection rate at the FTO front contact is

1 1 1 ) τcc τt τr

(3)

where τcc, τt, and τr are the time constants for charge collection, transport and recombination, respectively. Accordingly, the charge collection efficiency (ηcc) can be written as

7086 J. Phys. Chem. C, Vol. 112, No. 17, 2008

ηcc )

(

)

τt Rt 1 1 1 / + )1- )1τcc τcc τr τr Rct

Wang et al.

(4)

That is, the charge collection capability of the DSC depends on the ratio of the charge transport and charge recombination resistances. For a good cell, it is essential to have a high ηcc at any bias voltage between short circuit and open circuit. In contrast to the charge collection at the FTO front contact, charge extraction to the external circuit correlates with the I-V response of a DSC at a given bias potential. For reasons of charge conservation in the circuit, the extraction current is

Jext ) Jinj‚ηcc - Jbias

(5)

where Jbias is the current flowing from the FTO front contact to the electrolyte, counter-balancing the collection current. For the charge extraction efficiency one obtains

ηext )

Jext Jbias Rt Jbias ) ηcc )1Jinj Jinj Rct Jinj

(6)

So at certain bias, ηext depends not only on ηcc, but also on Jbias. If the forward bias voltage is applied in the dark, Jbias is just the dark current. However, the situation under illumination is more complex because the concentration enhancement of I3in the pores of TiO2 film resulting from its “in-situ” formation from dye regeneration under light has to be taken into account.47 So, Rct will decrease and Jbias will be significantly higher than the dark current. Hence, due to the I3- accumulation in the pores, both ηext and ηcc will decrease under light. At the open circuit potential, it is apparent that ηext is zero, indicating that the anodic photocurrent due to the injected electrons is counterbalanced by the reverse current flow from the FTO, and no net charge is extracted. If ηcc is close to unity, according to eq 6, Jbias will be equal to Jinj. At short circuit, since Jbias is close to zero, ηext is just ηcc. Basically, at any bias for a working DSC, ηext can be calculated by knowing the values of Rt, Rct, Jbias and Jinj, for which the resistances can be measured by impedance spectroscopy; Jinj approximates to short circuit current because ηcc is close to unity. As mentioned above, the determination of Jbias is more complex. In principle, it can be determined from the dark current by taking into consideration the increase in the I3- concentration under illumination at the same bias potential. 3.2. Dark Current and Impedance Response of DSC in the Dark. The dark current has a dominant influence on the performance of a DSC. Its response to polarization is highly dependent on the distribution and density of local states and, in particular, the conduction band edge position of the TiO2 nanocrystals. Figure 1A shows data for a cell containing electrolyte 1 under forward and reverse bias. The cathodic current arises from the reduction of I3- in the electrolyte by electrons injected from the FTO electrode into the mesoscopic TiO2 film, as shown schematically in Figure 1B. When the forward bias exceeds -1 V, it reaches a plateau value of 28 mA/cm2, corresponding to the limit for I3- diffusion from the counter electrode. Since under these conditions the I3- concentration in the pores is fully depleted, the current is drawn only from the outer boundary of the mesoscopic TiO2 film. Using 20 µm for the electrolyte layer thickness between the working and the counter electrode, we derive from the plateau current an I3- diffusion coefficient of 1.5 × 10-6 cm2/s, which is about half of the literature value for the same solvent. Closer inspection of the reduction waves in Figure 1A reveals that the onset of the dark current for the electrode covered with

Figure 1. (A) Cyclic voltammograms obtained with a DSC using various electrolytes (as marked) within a wide potential range. Line a, electrolyte 1; line b, electrolyte 2; line c, electrolyte 3; line d, electrolyte 4. The black line shows the curve of the FTO-TiO2/FTO-Pt cell (without dye) using electrolyte 1. The inset shows the expansion of the curves between -1.3 and -0.5 V. All the measurements were performed in the dark and at room temperature. The scan rate is 0.1 V/s. (B) Schematic model showing the interfacial charge-transfer processes at the dye-derivatized TiO2 electrode upon forward and reverse bias.

the Z907 sensitizer occurs at more negative potentials than that of a blank TiO2 film, confirming our earlier observation that the sensitizer blocks electron transfer from the conduction band of the oxide to the I3- in the electrolyte.48 The dark current of the Z907-derivatized film itself is sensitive to the composition of the electrolyte, decreasing in the expected order 3 < 1 = 4 < 2. The sensitizer also exhibits a striking effect on the dark current under reverse bias corresponding to the oxidation of Ito I3-. Loading the nanocrystalline film by a monolayer of Z907 shifts the oxidation wave by 0.75 V from a midpoint potential of 1.27 V to 0.52 V vs I-/I3-. The latter value is very close to the oxidation potential of the adsorbed sensitizer, indicating that the Z907 molecule is the cause of the observed electrocatalytic effect. The bare TiO2 electrode is an insulator under reverse bias. Hence, iodide oxidation can occur only at the FTO surface that is left uncovered and exposed to the electrolyte after deposition of the nanocrystals. Iodide oxidation on FTO demands a high overpotential of 1.27 V. However, in the presence of Z907 the sensitizer is oxidized first by electron transfer to the FTO forming Z907+, which in turn oxidizes Ito I3-. As depicted in Figure 1B, this process can occur on the entire dye-covered TiO2 electrode because efficient cross-surface hole percolation is turned on within the monolayer of Z907 once the surface coverage of the sensitizer exceeds the percolation threshold of 50%.49,50 The high current densities observed here confirm the efficient nature of lateral charge percolation process subsequently followed by rapid dye regeneration. The plateau of the anodic current observed under reverse bias corresponds to the limit for diffusion of triiodide ions from the TiO2 film to the counter electrode. Consequently, the anodic current reaches

Performance of Dye-Sensitized Solar Cell

J. Phys. Chem. C, Vol. 112, No. 17, 2008 7087 TABLE 2: Photovoltaic Performance of DSCs Using the Four Electrolytes before and after Light-Soaking cell 1 2 3 4

Figure 2. Typical impedance spectra of cell 1 measured at (A) forward biases of -0.55 and -0.60 V (B) inverse biases of 0.70, 0.75, and 0.80 V at 0 °C in the dark. The inset of A shows the enlarged spectra in the high-frequency region. Curve a, -0.55; b, -0.60; c, 0.80; d, 0.75; and e, 0.70 V.

a limit close to that observed for the cathodic current under forward bias. While the dark current at forward bias is a key indicator of the photovoltaic performance of the cell, which is closely related to the charge extraction process, the dark current at inverse bias may provide information for the dye regeneration process. Impedance spectroscopy has been proved to be a powerful technique of studying the kinetic processes in DSC.9 Figure 2 shows the typical impedance spectra of the cell obtained at different biases. As frequently shown, under forward bias, the electron transport in the mesoscopic TiO2 film appears as a clearly discernible Warburg-like diffusion feature in the highfrequency region, the interfacial charge recombination process extending as a large arc toward the low-frequency region (Figure 2A), which has been well modeled by Bisquert et al.51-54 The electrical elements Rct and Rt and the chemical capacitance (Cµ) can be extracted by fitting the spectra with an appropriate equivalent circuit, which will be mainly resorted to in this study in order to scrutinize the charge extraction process. In contrast to the forward bias, in which electrons are transported through the mesoscopic film and transferred from a distribution of local states of TiO2 via the conduction band toward I3- in the electrolyte, under reverse bias, holes percolate through the dye monolayer and are intercepted from a fixed energy level (the HOMO of the dye) by I- in the electrolyte.

light-soaking before after before after before after before after

VOC (V)

JSC (mA/cm2)

FF

η (%)

0.688 0.716 0.530 0.563 0.764 0.768 0.702 0.717

14.84 15.15 16.60 16.66 13.08 14.11 15.27 15.68

0.695 0.664 0.574 0.526 0.733 0.694 0.673 0.637

7.10 7.20 5.05 4.93 7.32 7.53 7.21 7.16

Figure 2B shows the spectra under reverse bias, where the Nernst-like finite length diffusion of triiodide in the electrolyte shows an expanding semicircle in the low-frequency region with increasing biases, being consistent with others’ results.55,56 The diffusion coefficient of triiodide is determined from the diffusion impedance to be ∼3.3 × 10-6 cm2/s, which is slightly larger than that obtained from CV measurement. Note that the finite length diffusion corresponding to the cross-surface charge percolation process through the dye monolayer and the kinetic process of dye regeneration are not visible in the Nyquist plots. Apparently, the path for cross-surface hole percolation is shortened by hole-injection into the electrolyte by the dye-regeneration process in the presence of iodide in the electrolyte and is no longer the rate-limiting step. This is in contrast to our previous studies employing an inert electrolyte49,50 in which the Warburg element due to cross surface holediffusion was clearly visible. In the present case, dye regeneration by electron donation from triiodide occurs on a faster time scale than surface hole transport, the time constant for the former process being in the microsecond domain according to our transient absorbance measurement (see S1 in the Supporting Information), which is beyond the frequency range of the present impedance measurements. 3.3. Electrolyte Additive Effects on Charge Extraction of DSC. The effects of electrolyte additives on the photovoltaic performance of DSC have been extensively studied in the literature.16,33-42 For example, guanidinium cations can improve both the VOC and JSC when added to the electrolyte5,40 or coadsorbed on the TiO2 surface.39,42 The present study explores the effect of Li+ ions and NMB on the efficiency of the charge extraction process. Li+ is a potential-determining ion for TiO2 causing its conduction band edge to shift to positive potentials, whereas NMB is an organic base, which tends to deprotonate the oxide surface, causing a negative shift of the conduction band-edge. The cyclic voltammetric measurements in Figure 1 confirm that the presence of Li+ in the electrolyte strongly increases the dark current, but the presence of NMB reduces it. Table 2 lists the photovoltaic data obtained with four DSCs containing these different additives in the electrolyte. In keeping with expectations, the presence of Li+ in the electrolyte gives rise to a significant drop in VOC and increase in JSC, whereas NMB produces exactly the opposite effect. The VOC of the cell is determined by the quasi-Fermi level of the electrons in the nanocrystalline TiO2 film, which is related to their steady-state concentration under illumination. This in turn depends on the relative rates of photoinduced charge carrier generation and dark recombination. A simple consideration shows that57

VOC )

(

RT AI ln βF n k [I -] + n k [D+] 0 1 3 0 2

)

(7)

where k1, k2 is the second-order rate constant for the back reaction of injected electrons with triiodide and with oxidized sensitizer, respectively, and n0 is the concentration of accessible

7088 J. Phys. Chem. C, Vol. 112, No. 17, 2008

Wang et al. The effect of these additives on the short circuit photocurrent and fill factor are also remarkable. The JSC increases by 27% from 13.08 to 16.6 mA/cm2 upon exchanging NMB against Li+. This could result from enhanced injection16 and collection of electrons or from faster regeneration of the dye.58 By contrast, the fill factor decreases by ∼20% from 0.73 to 0.57, reflecting the higher JSC and possibly a lower β value for the lithium ioncontaining electrolyte. In the presence of both Li+ and NMB, VOC and JSC increase and ff decreases slightly from the combined action of the two additives. The Cµ-V curve in Figure 3B shows that the above effect is clearly related to the distribution of localized states. The density of states in the presence of Li+/NMB is a combination of those observed with electrolytes 2 and 3 containing Li+ and NMB alone. That is, Li+ creates new trapping states, whereas the effects of the two additives on the position of the conduction band edge nearly compensate each other. As a result, the conduction band edge shows almost no shift with respect to the additive-free electrolyte.

Figure 3. Photovoltage and photocurrent transient results for four DSCs using the four different electrolytes before and after light-soaking treatments. (A) Voltage dependence of the voltage decay rates; (B) Voltage dependence of the capacitances of the cells. The lines show the one-order exponential fitting. The open symbols show the cells before light-soaking; the solid symbols show those after light-soaking.

electronic states in the conduction band. Since the rate of dye regeneration is much faster than that of the recombination, the second term in eq 7 can be omitted. Hence,

VOC )

RT ln(A′Iτr) βF

(8)

implying that the open circuit photovoltage is logarithmically dependent on the electron lifetime. Figure 3 shows the photovoltage decay rate and the chemical capacitance values, Cµ, for the same cells. Cµ is proportional to the density of electronic states (DOS ) Cµ/e, where e is the elementary charge) and was derived by integrating the transient photocurrent resulting from the photodiode light pulse at short circuit. The voltage decay rate of the cell is enhanced in the presence of Li+, but it is retarded by NMB. Correspondingly, the lifetimes of electrons at the same bias potential are much shorter with Li+ and much longer with NMB as compared with the additive-free electrolyte, lowering the VOC for the former and increasing the VOC for the latter formulation. Figure 3B shows that the addition of Li+ creates additional sub-band gap electron trapping states in addition to positively shifting the conduction band edge, whereas the presence of NMB shifts the conduction band edge to a more negative value, increasing the VOC.

3.4. Light-Soaking Effects on Charge Extraction of DSC. Light-soaking has been a standard procedure for the stability tests of the DSC. During light-soaking, electrons undergo continuous injection from an excited dye molecule into the acceptor states in TiO2. At open circuit voltage, all the injected electrons are recaptured by I3- because they cannot be extracted to the external circuit. Meanwhile, the oxidized dye is regenerated by I-. As a result, the absorbed photon energy is converted to heat through the two-coupled redox cycles involving sensitized electron injection, dye regeneration, and electron recapture by I3-. In addition, because of the light-induced charge accumulation in TiO2, the quasi-Fermi level of the electrons in the semiconductor is lifted up to quite a negative value. It is possible that some interfacial reaction will be enabled in the presence of such high concentrations of electrons in TiO2. As will be shown below, the TiO2/dye/electrolyte interface is subject to some noticeable alterations upon light-soaking at higher temperature, affecting the photovoltaic performance of the DSC. Here, we demonstrate for the first time the unique effects of visible light-soaking on the extraction of photogenerated charge carriers. Table 2 lists the photovoltaic performances of four cells before and after visible light irradiation at 60 °C for 1 h. Importantly, for electrolytes 1 and 4, light-soaking improves both the VOC and JSC. For cell 2 and 3, only the VOC and JSC values, respectively, are enhanced. Remarkably, the overall performance of both lithium-free cells is enhanced during lightsoaking, the effect being greatest for the NMB-containing electrolyte. The Cµ-V curves in Figure 3B suggest that for all cases, light-soaking creates localized states below the conduction band edge. For cells 1, 2, and 4, there is a shift in the DOS to higher voltages, indicating a negative shift of the conduction band edge, consistent with the observed increase in VOC. This agrees also with the increased electron lifetimes derived from the photovoltage transient measurements in Figure 3A. Our observations are reminiscent of those made by Gregg et al.,43,44,59 who discovered that with UV irradiation, nanocrystalline TiO2 films produce surface states at the electrolyte interface in the absence of specifically adsorbed cations, for example, Li+ or Na+. These newly formed sub-band gap surface states, together with the existing ones, formed a continuous percolation pathway (or mobility edge), affording enhanced

Performance of Dye-Sensitized Solar Cell

J. Phys. Chem. C, Vol. 112, No. 17, 2008 7089

TABLE 3: Photovoltaic Performance Evolution of Cell 1 after Subsequent Treatments at Various Conditions. treatmenta

VOC (V)

JSC (mA/cm2)

FF

η (%)

A B C D E

0.688 0.822 0.821 0.728 0.768

14.84 10.93 11.44 12.03 12.07

0.695 0.735 0.716 0.715 0.729

7.10 6.60 6.73 6.26 6.76

a A, original; B, aging at room temperature for 6 months; C, lightsoaking at 60 °C and 400 nm cutoff for 1 h; D, thermal aging in dark at 80 °C for 2 days; E, polarization in dark at -2.0 V and 0 °C for 20 min.

charge transport. As a result, the cell performance, especially the photocurrent, was greatly improved. However, no UV exposure was applied in the present case, in contrast to the experiments of Gregg et al., which required UV illumination to induce the formation of surface states. Our light-soaking process employed only visible photons, the cutoff being 400 nm or even 500 nm. In addition, the generation of sub-band gap states under light occurs even in the presence of lithium ions, although no overall improvement of performance was obtained in this case. In order to further examine this effect, a cell employing electrolyte 1 was subjected to light-soaking after aging at room temperature for 6 months in the dark. What followed in sequential order by the treatments is indicated in the footnote of Table 3. As shown in this table, after 6 months’ aging at room temperature, the VOC increased by 134 mV, while the JSC decreased by 3.91mA/cm2, probably due to water ingression into the cell. Subsequent visible light illumination for 1 h at 65 °C increased the photocurrent by 0.51 mA/cm2 while the VOC remained constant at ∼0.82 V. Figure 4A shows cyclic voltammograms (CVs) recorded in the dark before and after the 1 h light-soaking period. Clearly, the illumination produces an additional current peak at around -0.65 V. This peak has frequently been observed in mesoscopic TiO2 electrodes and is attributed to the filling of shallow trapping states whose density is sufficiently high to participate in electron transport.60-63 The dark current of the cell under larger forward bias is hardly affected by the light treatment. Impedance measurements were also performed on the same cell. Figure 5 shows a semilogarithmic plot of the charge transport (Rt) and the recombination impedance (Rct), as well as the chemical capacitance (Cµ) as a function of bias voltage, all three showing an exponential dependence. Note that the recombination resistance is not affected by the light irradiation. In particular, at -0.82 V, the product RctCµ corresponding to the electron lifetime is practically the same before and after light-soaking, explaining why the VOC of this cell remained constant, as shown in Table 3. By contrast, Rt decreases over the whole potential range under visible light exposure, implying that the newly generated electronic states enhance electron transport while the recombination resistance, Rct, remains unchanged. Therefore, from eq 4, light-soaking enhances the charge collection efficiency. At a bias voltage of -0.70 V, ηcc is estimated to increase from 85 to 92%, explaining the observed augmentation in the photocurrent. Figure 5C shows that the slope (R) of Cµ is decreased after light-soaking, indicating a broadening of the DOS distribution. The density of states is increased significantly below -0.80 V, in accordance with the transient and CV measurements, indicating the generation of sub-bandedge trapping levels during lightsoaking that are capable of transporting electrons without enhancing recombination.

Figure 4. Cyclic voltammograms of cell 1 (A) before and after lightsoaking and (B) before and after polarization at -2 V. All the curves were measured in the dark and at 0 °C. The scan rate is 0.1 V/s.

According to the well-accepted continuous-time random walk (CTRW) model, the charge transport rate in TiO2 is limited by the waiting time of the electron in the trap and the total density of the trap states at the Fermi level.23,28,64,65 Following this model, charge recombination in DSC is correlated to the rate of charge transport, which has been repeatedly observed in DSC, where the transport-limiting traps are localized predominantly on the surface of the TiO2 particle.32,66 However, in contrast to these predictions, charge transport of cell 1 becomes faster after light-soaking, even though more traps are generated, without accelerating the charge recombination, suggesting a different transport mechanism. The same conclusion can also be drawn from the increased density of localized states, as shown in Figure 5C. These newly formed sub-band edge states appear to be sufficiently dense to pass the charge percolation threshold, coalescing into a mobility edge that is below the conduction band edge of the anatase nanoparticles. Strikingly, these trapping states enhance electron transport but do not contribute to the charge recombination, which is hard to reconcile with the simple CTRW model. The above transient and impedance measurements have shown that the improvement of the photovoltaic performance after light-soaking stems from the generation of electron trapping states below the conduction band edge of TiO2. That is, the increased photovoltage is ascribed to an upward shift of the conduction band edge, while the enhanced photocurrent is associated with the formation of electronic levels below the conduction band capable of transporting electrons. According to the transport-limited recombination process, where the transport-limiting traps are localized mainly on the surface of

7090 J. Phys. Chem. C, Vol. 112, No. 17, 2008

Wang et al. where NL is the total density of the surface states and R is a coefficient that denotes the distribution of surface states below the conduction band edge. In contrast to the transport-limited recombination, the light-soaking effect we observed here produces states that afford faster electron transport without affecting charge recombination. In order to interpret the difference, we assume the presence of two types of localized states in the band gap of TiO2 after light-soaking. Spatially, one is located on the surface of TiO2, following the transport-limited recombination process; the other is located in the bulk (or subsurface). The latter sites contribute only to the charge transport, not to the recombination process. The total density of states can be expressed as

g(E) ) gs(E) + gb(E) )

[

]

R′NL R′(E - Ec) exp kBT kBT

(10)

Here, gb(E) is the bulk density of states. From the slope of the chemical capacitance in Figure 5C, R′ is 0.26, which is smaller than the R value of 0.34 obtained before light-soaking. In the schematic model in Figure 6, the injected electrons can follow two different transport pathways. Transport may occur via the surface states following the multiple trapping model or via the bulk states once their local density is high enough to reach the percolation threshold. Depending on the transport barrier energy, charges may be exchanged between these two types of trapping sites during the transport process. It appears that the more the electrons move through the bulk states instead of the surface states, the more likely they can be extracted. Finally, what could be the physical origin of these transport levels? In the DSC, protons are introduced by the sensitizer or traces of water presented in the electrolyte. Upon light-soaking, electrons are injected from the excited dye molecules into the conduction band of TiO2, raising its Fermi level to quite a negative potential versus the NHE. It is very plausible from thermodynamic considerations that this brings along the intercalation of the surface-adsorbed proton into the lattice of TiO2. This has been confirmed by Betz et al. in their study of lightinduced proton intercalation in TiO2 (B).67 They found that electrochemical intercalation of protons in TiO2 (B) generates new electronic states in the forbidden energy gap. During proton intercalation, TiIV is reduced to TiIII as a result of charge neutralization, which forms new occupied transport levels below the conduction band states in the bulk.

TiO2 + H+ + e- S ΤiΟ(ΟΗ)

Figure 5. Impedance results of cell 1 after various treatments. (A) Electron transport resistance, Rt; (B) electron recombination resistance, Rct; (C) chemical capacitance, Cµ. As indicated in the figure, the cell has been treated under various conditions. These parameters were obtained by fitting the impedance spectra of the cell measured in the dark and at 0 °C. IR drop corrections have been done for potentials lower than -0.7 V.

TiO2 particle, the density of surface states is given by the expression

gs(E) )

[

]

RNsL R(E - Ec) exp kBT kBT

(9)

(11)

It should be noted that the protons are depleted from the electrolyte and the TiO2 surface by the intercalation process, resulting in a local pH increase60,61 unless there is a buffer with sufficient capacity to replenish them. As a consequence, the conduction band edge is negatively shifted, producing the observed increase in photovoltage. From the increased photocurrent, we conclude that the newly formed intraband states afford charge transport without promoting recombination, as shown in Figure 6C. This is easy to understand, since these bulk states are spatially located inside TiO2, being further away from the electrolyte than the surface states. Hence, the interfacial recombination between the injected electron and I3- in the electrolyte is unlikely to occur via the bulk states. In order to prove the above model, the same cell after having a series of treatments (A to D in Table 3) was subjected to electrochemical polarization at -2.0 V and 0 °C for 20 min, with which it is assumed that protons will be doped into the sensitized TiO2 electrode. As the photovoltaic performance

Performance of Dye-Sensitized Solar Cell

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Figure 6. Schematic model showing the energetics and kinetic processes (A) and the electron transport pathway of a DSC before (B) and after (C) visible light-soaking. The transport of electrons is via not only the surface states but also the bulk states, as indicated.

shows in Table 3, both the photovoltage and photocurrent were increased after negative polarization, the tendency being similar to the light-soaking treatment. The CV in Figure 4B shows that the current was increased at lower voltage, showing a peak at around -0.58 V, which is practically identical to the one produced under light-soaking. Upon cathodic polarization, surface-bonded protons are believed to undergo an intercalation reaction as the reaction shown in eq 11. The intercalated protons generate new intraband states.68,69 Meanwhile, the surface of TiO2 becomes depleted of protons, moving the conduction band edge upward. Under the aggressive conditions (-2.0 V and 0 °C), the dark current was greatly reduced due to the proton depletion process. Impedance measurements in Figure 5 show that, similar to light-soaking, the slope R of Cµ with bias voltage becomes smaller, the density of somewhat deeper traps increasing at the expense of very shallow traps. In addition, the charge recombination impedance was significantly increased after negative polarization, in accordance with the increased photovoltage. 4. Conclusions The performance enhancement of dye-sensitized solar cells in lithium-free and lithium containing electrolytes under visible light-soaking was examined by impedance spectroscopy and photovoltage transient decay measurements. The improvement was found to arise from the formation of electronic transport levels close to the conduction band, resulting most likely from photoinduced proton intercalation in the TiO2 nanoparticles. These shallow trapping states accelerate the charge carrier transport within the nanocrystalline films without deteriorating the open circuit photovoltage. Subjecting the cells to forward bias in the dark produces a similar effect, whereas the introduction of lithium ions in the electrolyte suppresses the phenomena due to prevailing lithium ion intercalation. The redistribution of localized states in the band gap of TiO2 and the resulting conduction band edge movement appear to play a significant role in the performance of the DSC.

We believe that the study of charge extraction of DSC under various treatments will help us to better understand the working principle of the cell. These findings here may provide important hints and will open up a way to further improve the photovoltaic performance of DSCs. Acknowledgment. We acknowledge financial support of this work by the Swiss National Science Foundation. We thank Mr. P. Comte for providing the mesoscopic TiO2 films. Q. Wang thanks Dr. Kai Zhu at National Renewable Energy Laboratory (NREL) for the fruitful discussion. Supporting Information Available: Transient absorbance of sensitizers with different electrolytes. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Oregan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (4) Kuang, D. B.; Ito, S.; Wenger, B.; Klein, C.; Moser, J. E.; HumphryBaker, R.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 4146. (5) Gra¨tzel, M. J. Photochem. Photobiol., A 2004, 164, 3. (6) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (7) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. G. J. Am. Chem. Soc. 2005, 127, 16835. (8) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (9) Wang, Q.; Ito, S.; Gra¨tzel, M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J.; Bessho, T.; Imai, H. J. Phys. Chem. B 2006, 110, 25210. (10) Asbury, J. B.; Hao, E.; Wang, Y. Q.; Ghosh, H. N.; Lian, T. Q. J. Phys. Chem. B 2001, 105, 4545. (11) Kallioinen, J.; Benko, G.; Sundstrom, V.; Korppi-Tommola, J. E. I.; Yartsev, A. P. J. Phys. Chem. B 2002, 106, 4396. (12) Persson, P.; Lundqvist, M. J. J. Phys. Chem. B 2005, 109, 11918.

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