High-Performance Dye-Sensitized Solar Cells through Graded

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High Performance Dye-Sensitized Solar Cells through Graded Electron Transport in Band-Engineered W-TiO2 Cascade Layer Suresh Thogiti, Ji Young Park, Thuy Thanh Chau Thi, Do Kyung Lee, Bong Ki Min, Hyeong Jin Yun, and Jae Hong Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Sustainable Chemistry & Engineering

High Performance Dye-Sensitized Solar Cells through Graded Electron Transport in BandEngineered W-TiO2 Cascade Layer Suresh Thogiti1, Ji Young Park1, Chau Thi Thanh Thuy1, Do Kyung Lee2, Bong-Ki Min3, Hyeong Jin Yun4,*, and Jae Hong Kim1,* 1

Department of Chemical Engineering and Yeungnam University, Gyeongsan, South Korea,

712-749 2

Department of Advanced Energy Material Science and Engineering, Catholic University of

Daegu, Gyeongsan, South Korea, 712-749 3

Center for Research Facilities, Yeungnam University, Gyeongsan, South Korea, 712-749

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Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

*Corresponding authors: [email protected] (Jae Hong Kim); [email protected] (Hyeong Jin Yun).

ACS Paragon Plus Environment

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ABSTRACT. The efficiency of TiO2-based dye-sensitized solar cells (DSSCs) has been limited because charge transfer is not fully achieved, and the injected electrons always recombine with the acceptor species before the electrode can collect them. The novel device architecture of offering graded bands into the TiO2 photoanode network of DSSCs is presented to enhance the power conversion efficiency (PCE). Mono (x %) as well as graded (w-x-y-z %) W-doped TiO2 films are prepared to form intermediate bands in the bandgap of TiO2. A significant enhancement in the PCE is achieved when mono x % W-TiO2 films are tested as the photoanode of the DSSCs. Furthermore, DSSCs with band-engineered W-TiO2 cascade layer exhibit significant photovoltaic performance with showing graded carrier transport to current collector. In particular, a graded 1-5-7-9 W cascade film exhibits the highest photovoltaic performance of 11.30 % without decreasing the open-circuit voltage, which is 48 % higher than pristine DSSCs without a blocking layer. These graded carrier transport suggests an innovative route toward the harvesting solar energy.

KEYWORDS. Dye sensitized solar cell, W-doped TiO2, blocking layer, conduction band, graded electron transfer.


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INTRODUCTION. Dye-sensitized solar cells (DSSCs) have received significant interest as the next-generation of efficient photovoltaic devices.1-2 In 1991, Gratzel and co-workers revealed their ground-breaking invention just by switching the morphology of the titanium dioxide (TiO2) photoelectrode from a compact film to a mesoporous one.1 On the other hand, the major drawback associated with utilizing mesoporous TiO2 as a photoelectrode is its random charge transfer, which will cause the electron-hole charge recombination and inhibit the overall power conversion efficiency (PCE).3-4 Also, such TiO2 systems generally show high light transmittance and increased trap states due to the nanosize of the TiO2 nanoparticles and this results in poor light-harvest.5-7 To overcome this limitations, TiO2 semiconductor should typically satisfy the following: a relatively high light scattering ability and large surface area for dye loading. Submicrometer-sized TiO2 beads with abundant mesopores are an ideal material for fulfilling these requirements for better light-harvesting.6 However, the internal surface area in submicrometer-sized TiO2 beads certainly leads to many surface states. This inevitably enhance the charge trapping/detrapping during charge transport in the anode TiO2 semiconductor.7 After evolution for a decade, studies of DSSCs are now experiencing an impediment phase in achieving superior PCE.8-9 To produce high performance DSSCs, the charge transfer rate at the photoelectrode interfacial regions should be enhanced by reducing the contact resistance. In addition, the undesirable charge recombination occurring in the photoelectrode should be controlled properly to obtain efficient solar energy conversion. Charge recombination processes include the back transfer of the photo-injected electrons from the conduction band (CB) of the semiconductor film to the oxidized dye molecules or to the redox couples, and direct decay from the excited dye molecules to the oxidized dye molecules.10-12 Among them, the back transfer of electrons is one of the most significant steps in reducing the charge collection efficiency. With


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this aim, many groups have introduced a blocking layer between the active layer and current collector, such as coating a blocking layer on TiO213-16 or the surface modification of TiO2 with metal/anion doping.17-23 A previous study estimated a large hypothetical limiting efficiency of 63.2 % for intermediate band solar cells through the introduction of intermediate bands (IBs) into the bandgap of the wide gap semiconductor,24 where both high and low energy photons can be utilized.25-26 In this case, TiO2 will cover visible part and even infrared part and then produce charges, when an IB suitably located in the bandgap of TiO2 is established. Therefore, not only the electrons pumped from the excited dye to the CB of TiO2, but the charges can also be moved precisely from the IB to the CB. As a result, the charge density in the CB of TiO2 may be improved markedly, which could improve the photocurrent density (JSC) and PCE of DSSCs. Bandgap engineering has been used to produce back surface fields in crystalline semiconductors27 and charge transfer/blocking layers in organic solar cells28 or dye-sensitized solar cells.29 E. Sargent, et al. proposed that graded charge transfer can enhance the electron transport percentage significantly and prevent the back transfer of electron in colloidal quantum dot (CQDs) solar cells.30-31 These devices are fabricated by layering PbS quantum dots with different sizes layer-by-layer to form graded CQD solar cells, so called ‘quantum funnels’. This graded concept allows CQD solar cells to have high solar energy conversion efficiency. DSSCs based on W-doped TiO2 was first outlined by Ko et al., where they observed that the W-introduction can enhance JSC, but reduce open-circuit voltage (VOC).19 To interprete the system by which the W-doping changes JSC and VOC, Zhang et al., doped different concentration of W into TiO2 and studied their consequences on bandgap change and electron recombination.22 Numerous reports account for the improvements due to the blocking layer presence at the


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photoanodes. In most of the studies, the doping metal/element was mostly single layered in TiO2 photelectrode. This study examined whether an improved device architecture can help overcome the charge transfer and recombination limitations in this otherwise highly promising materials system. This paper reports a band engineering strategy within the light absorbing, charge transporting layer to final performance-limiting photoelectrons towards the charge collection TiO2 electrode as well as suppress the probability of charge recombination caused by the back transfer of electron by employing a W-doped TiO2 graded cascade film. W-doping creates an accepter level in the forbidden region, which apparently shifts the energy level of the conduction band (ECB) downward. Because of the energy differences between TiO2 and W-doped TiO2, the photo-generated electron can be transferred in a step-wise manner to the current collector through the W-doped TiO2. In particular, W-doped TiO2 graded cascade structure with various ECB are deposited by repeated spin-coating technique to allow graded electron transfer (Figure 1). The band structure of the W-TiO2 layer is engineered precisely by tuning the W-doping level (mol %) and characterized by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. A maximum PCE of 11.3 % was obtained upon introducing optimized 1-5-7-9 W cascade layer, corresponding to a 48 % increase in efficiency. As compared with DSSC without blocking layer, JSC improved from 16.44 mA/cm2 to 24.90 mA/cm2 for DSSC with optimized WTiO2 cascade layer. PCE increased from 7.61 % to 11.3 % for the cell without mask. For device with mask, PCE enhances from 6.41 % to 9.62 %. Moreover, the ungraded, graded and antigraded blocking layers are constructed with this engineered W-TiO2 film. Their charge transfer and charge recombination rates are compared by performing Stepped light-induced transient measurements of the photocurrent and voltage (SLIM-PCV) measurements, which is developed to characterize the electron diffusion behavior precisely.


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RESULTS AND DISCUSSION. The x % W-TiO2 (x = 0, 1, 3, 5, 7, and 9 mol %) blocking layer was fabricated by adding x mole % tungsten (VI) ethoxide (C12H30O6W) solution (ethanol 40 mL + deionized water 6 mL) into titanium (IV) isopropoxide (TTIP) solution (20 mL ethanol). The number x in x % W-TiO2 indicates the mole % of adding W in solution. As mixed solution was then deposited onto the fluorinated tin oxide (FTO) substrate by spin-coating (3000 rpm for 30 seconds) technique followed by thermal treatment at 500 °C for 30 min. A set of single layer depositions with varied concentration of W (i.e., 0, 1, 3, 5, 7, and 9 mol %) in x % W-TiO2 films were formed on the top of FTO film. The thickness of the W-TiO2 film was tuned by controlling the number of spin-coating steps (Figure S1A). Their thickness was measured by scanning electron microscopy (SEM) and resulting images are shown in Figure S1B-G. Three identical samples were fabricated under each experimental condition, and their average and standard deviation were recorded. The thickness of W-TiO2 was controlled from 0.2 to 0.55 µm showing a narrow thickness distribution. In the case of the 6-times repeated spin-coating sample, the standard deviation was quite small, indicating that a uniform film can be prepared successfully. Six different DSSCs were prepared with the substrate, in which each x % W-TiO2 blocking layer was deposited. Among them, the W-TiO2 film with a thickness of 0.4 µm shows the best photovoltaic performance (Figure S1H). Therefore, the optimal 0.4 µm of the W-TiO2 blocking layer was deposited for further studies. On this W-TiO2 film, the active and scattering TiO2 layer was deposited subsequently using the doctor-blade method. Figure 2A presents a cross-section SEM image of the photoanode. The image shows a triple-layered structure, including the WTiO2 layer, dye sensitized active layer, and scattering layer, which are approximately 0.4, 7.4, and 7.5 µm, respectively. The W-TiO2 layer was deposited uniformly on the FTO glass substrate.


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The thickness and the crystalline structure of the graded W-TiO2 film were examined by high resolution cross sectional transmission electron microscopy (TEM). The thickness of the graded W-TiO2 film grown onto FTO substrate was measured to be 400 nm (Figure 2B), which is consistent with the SEM analysis. In the HRTEM lattice image of graded W-TiO2 (Figure 2C), lattice fringes with d-spacing value of 0.34 nm was observed, which corresponds to the (101) plane of the anatase TiO2. The Figure 2D shows the diffraction pattern taken at a selected area of the graded W-TiO2, showing several circular rings with spots, which proves the polycrystallinity of the graded W-TiO2. The elemental composition of the graded W-TiO2 coated on FTO glass substrate was inspected further by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) equipped with energy-dispersive X-ray spectrometry (EDS) analysis. The STEM-HAADF image and corresponding mapping analysis of Ti, W, and O are depicted in Figure 2E-H confirming that the uniform distribution of W throughout the TiO2 film. The doping effect on the crystal structure of the prepared graded W-TiO2 film was analysed by X-ray diffraction (XRD). Figure S2 shows the XRD pattern of the undoped and graded W-TiO2 films. As shown in Figure S2, all the XRD patterns were indexed to the anatase phase TiO2 with tetragonal system (JCPDS Card No. 21-1272). The XRD patterns clearly show the anatase phase was maintained without a phase change after the W grading. No peaks produced from impurities such as WO3 or other crystalline forms of W are distinguishable. Further, the ionic radii of Ti4+ (0.605 Å) and W6+ (0.60 Å) are comparable, indicating that W+6 was incorporated into TiO2 through lattice replacement.22 The extra signals were attributed to the FTO glass film. To examine the effects of the band-structure on charge transfer, the energy level of the valence (VB) and conduction band (CB) should be examined according to the amount of W-dopant. Xray photoelectron spectroscopy (XPS) was first performed to observe the effect of the W-dopant


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amount on the energy level of the VB (EVB). Figure S3 shows the spectra of 9 % W-doped TiO2 blocking layer. Figure S3A, S3B, and S3C show the Ti 2p, O 1s, and W 4f spectrum in higher resolution, respectively. In Figure S3A, Ti 2p spectra revealing two peaks of Ti 2p1/2 and Ti 2p3/2 at 465.2 eV and 459.5 eV confirm that titanium exists as Ti4+.32 It is documented that the binding energies of Ti 2P1/2 and 2P3/2 of pristine TiO2 sample are located at 464.0 and 458.4 eV, respectively. In our case, the binding energies of Ti 2p1/2 and 2p3/2 of W doped TiO2 samples are observed at 465.2 and 459.5 eV, respectively.32 This slight positive shift is ascribed to the incorporation of W in the lattice of titania as W-O-Ti linkage. So further we confirms that W6+ ions are integrated into the TiO2 lattice by expelling some Ti4+ ions, since the ionic radius of W6+, 0.605 Å, is only slight larger than that of Ti4+, 0.60 Å.22,32 The O 1s spectrum is deconvoluted into four peaks based on the XPS analyses data reported elsewhere; four species of oxygen are present in the film. The peaks at a binding energy of 533.48 and 532.78 eV correspond to O2 molecules and -OH adsorbed on the surface, respectively, whereas the peaks at 531.78 and 530.88 eV indicate oxygen vacancies formed by the W-doping process and the oxygen atom bound with titanium, respectively. Some papers claimed that the peak at 531.78 eV is related to the oxygen bound to the tungsten dopant.32-33 Some other reported that the peak at 531.78 eV is characteristic of -OH adsorbed on the surface.34 Either explanation shows that these peaks appear from the electronic state of oxygen atom binding to metal atoms. The EVB of metal oxide is determined by the electronic state of oxygen atom binding with metal atoms. Therefore, the binding energy of the convoluted peak related to the oxygen binding with metals is strongly related to the EVB.35-36 The position of this peak does not change as different amounts of Wdopant were incorporated into W-TiO2 (Figure 3A). Regardless of the doping level, the peak of binding oxygen appears at 531.08 eV, which indicates that W-doping does not affect the EVB.


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These results are in well consistent with previous results.22 ,37 W 4f spectra presented in Figure S3C reveal two peaks of W 4f5/2 and W 4f7/2 at 38.5 eV and 36.3 eV, indicating that the tungsten element in the W-doped TiO2 exists in the form of W6+.38 The loss of spectra between the double peaks is due to the essential Ti 3p signal at 37.5 eV. On the other hand, the existence of W in XPS, the positive shift of binding energies of Ti 2p1/2 and 2p3/2 and the undetectable Wassociated phase in the XRD pattern further depict that W is indeed doped into the TiO2 lattice. The XPS outcome presented that the sample consist of Ti and W at their highest oxidation state (Ti4+, W6+). Six types of films with different levels of W dopant ranging from 0 to 9 % were prepared by varying the amount of the W precursors. The quantitative study for the W-doped TiO2 thin film is also conducted by XPS. As shown in Figure S4, the peaks related to W 4f tend to increase with increasing concentration of the W precursor in the stock solution. With quantitative study with XPS results, we found that the tungsten dopant level is identical to the concentration of W precursor. Doping TiO2 with metal atoms is commonly used to tailor the energy level of the conduction band (ECB).20 An estimation of flat band potential (Efb) from the Mott-Schottky graph is commonly believed to be the most valid approach.39-41 A semiconductor-electrolyte interfaced with Csc-2 is determined by the following equation:

=

− −

(1)


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From the above equation 1, a plot of Csc-2 vs. E would be predicted to be linear, and the xintercept can be utilized to attain the Efb value. The relationship between Csc-2 and E can be secured by employing the potential dynamic electrochemical impedance spectroscopy (EIS) method. Figure 3B shows Mott-Schottky plots of the x % W-TiO2 layer with different amounts of W, measured at a frequency of 1 kHz. Pure TiO2 thin films without W has an Efb of -0.73 V vs. Ag/AgCl, which is close to the results reported previously.39 The Efb tends to shift toward a positive direction with increasing amount of x % W. W-TiO2 film containing 1, 3, 5, 7, and 9 % of W exhibits an Efb of -0.65, -0.54, -0.41, -0.37, and -0.32 V vs. Ag/AgCl, respectively. The positive shift of the Efb clearly shows the downward shift of of ECB in the band structure. Based on the results from XPS and Mott-Schottky experiments, Figure 3C presents the apparent band structure of the W-TiO2 thin film. The EVB of TiO2 was obtained from the literature (EVB of TiO2 = -7.45 eV)40,42,43 and it is believed that this value is not changed by changing the W-doping level, which is verified by XPS. This clearly shows that Efb tends to shift downward with increasing amount of W. In particular, doping with 9 % W leads to a 0.42 eV decrease in the Efb. Metal ion doping results in the formation of either a donor level above the original VB or an acceptor level below the original CB in the forbidden band. In particular, when a high valence metal ion (e.g. W6+) is incorporated substitutionally into TiO2 crystalline structure, the acceptor level is formed below the conduction band of TiO2. The formation of the acceptor level leads to the apparent downward shift of ECB, as shown in Figure 3B. The estimated ECB (Table S1) values of the x % W-TiO2 film containing 1, 3, 5, 7, and 9 % of W are -3.94, -4.14, -4.22, -4.27, and 4.29 eV, respectively which are enough positive than the ECB of pure-TiO2 (-3.88 eV), ensure that the band engineering is efficient to charge funneling the injected electrons towards transparent conductive oxide (TCO).


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To test their photovoltaic performance, the current density-photovoltage (J-V) curves are studied for each N719 dye adsorbed solar cell under standard global AM 1.5 solar light conditions (Figure S5A and B). Fig. S5B and C shows standard deviation obtained from five identically prepared devices. Table S1 lists the JSC, VOC, fill factor (FF), and overall efficiency of power conversion (η). The error bars in Table S1 were calculated from the J–V curves of five DSSCs for each condition. In the absence of a blocking layer, the solar cell shows low photovoltaic performance with JSC, VOC and FF values of 15.48 mA/cm2, 0.70 V, and 61.50 %, respectively, giving a η value of 6.62 %. Employing a pure TiO2 blocking layer leads to an enhancement in the PCE to 7.19 %. The x % W-doped TiO2 ungraded blocking layer increases the photovoltaic performance more significantly up to 3 % doping. The JSC and η tends to increase with increasing x % W up to 3 %, and decreased when larger amounts (> 5 %) of W dopant are incorporated. In particular, the value of η is improved remarkably to 7.76 % when 3 % W is incorporated in TiO2, which is 12 % higher than the DSSCs without a blocking layer. A heavily doped TiO2 ungraded blocking layer higher than 5 % did not show better photovoltaic performance than the 3 % W-doped one because of the high probability of charge recombination in the W-doped TiO2 ungraded blocking layer. The results showed a monotonic decline in VOC with rising concentrations of W. The decrease in VOC is caused by the high probability of charge recombination in the layer. The metal dopant in TiO2 can be the center of charge recombination in the lattice.20,42-43 In addition, the incorporation of a hetero-atom in the TiO2 lattice causes various crystalline defects, which are regarded as electron trapping sites. Therefore, an increase in the amount of W produced more centers of charge recombination by showing a decrease in VOC. In the competition between charge transfer and recombination, the doping level is optimized to 3 %, showing the best PCE among the DSSCs examined. This likelihood is also noticed in the


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EIS analysis (Figure S6). The overall charge transport impedance in the TiO2 can be approximated by performing EIS. This technique is convenient for evaluating the interfacial electrochemical behaviors of QDSSCs,44 DSSCs,45 and photoelectrochemical cells.46 Figure S6 present Nyquist plots for the photovoltaic cell, where each blocking layer is employed at the photoanode. Table S1 lists the charge transfer resistance of the working electrode (Rct,a). This value is obtained by fitting the experimental results with the theoretically simulated curve based on the electrochemical equivalent circuit. It is observed that there are two semicircles in the Nyquist plots. The larger (at lower frequency) and smaller (at higher frequency) semicircles in the Nyquist plot were attributed to charge transfer resistances at the working electrode/dye/electrolyte interface and counter electrode/electrolyte interface, respectively. The charge transfer resistance at the counter electrode interface is almost similar in all of the devices due to the use of the same couner electrode and electrolyte. The photoanode without the blocking layer shows the highest charge transfer resistance (10.91 Ω). Employing a pure TiO2 blocking layer leads to a decrease in the charge transfer resistance (10.00 Ω). This tends to decrease with increasing amount of W dopant up to a 3 % doping level and increase as heavier W dopant is incorporated. This shows that charge transfer occurs most actively at the interfacial of the photoanode when 3 % W-doped TiO2 is applied to the blocking layer. This is in good agreement with the JSC and overall power conversion efficiency. Stepped light-induced transient measurements of the photocurrent and voltage (SLIM-PCV) calculations47-51 were performed to verify which parameters affect the device performance (w.r.t. amount of W), (Figure S7, Table S2). Figure S7A shows an example of photocurrent decay and data fitted with a following equation: 47-51


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=

× " #$/&'

(2)

where JSC (t), β, and τc are the photo-current decay, weight parameter and the time constant. From the above fitting, De can be derived with the formula: 47-51

( =

)

(3)

.++&'

where De is the diffusion coefficient and L is the electrode thickness. As shown in Figure S8A, the De increased with increasing doping level up to 3 %, and decrease at the heavily doping region (> 5 %). 3 % W-doped TiO2 shows the highest De (2.67 x 10-5 cm2/s) among the DSSCs tested, which means that a larger photo-generated charge is diffused in the photoanode through this blocking layer. This high diffusion coefficient is induced by stepwise charge transfer to the current collector TCO through the W-doped blocking layer. In particular, the 3 % W-doped TiO2 blocking layer stimulates electron movement from TiO2 to the TCO. The electron lifetime (τe) of photo-generated electrons were calculated from stepped lightinduced transient of photo-voltage measurements. Figure S7B shows the resulting voltage transients for each DSSCs and the τe of a photo-generated electron can be obtained from the following equation: 47-51


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,- = . × " #$/&/

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(4)

where VOC (t) and α are the photo-voltage decay and weight parameter, respectively. As shown in Figure S8B, the τe does not change significantly according to the amount of W dopant. All the DSSCs tested shows approximately τe of 0.21 s using the neutral density (ND) filter with an O.D of 2.0. The linearly fitted results between τe and Jsc are not remarkably different from each other, as shown in Figure S8B, which means that τe is not considered a critical factor determining the photovoltaic performance. Finally, the diffusion length, Le can be estimated as follows: 47-51

0 = 1( × 2

(5)

Figure S8C shows the estimated results and akin to the case of diffusion coefficient, the 3 % W-doped sample displays the longest Le among the x % W-TiO2 devices. The longest Le of the 3 % W-doped blocking layer can be ascribed mostly to the high De values. Thus, higher photoinjected electrons can move to the TCO with a higher De, ending in notable improvement of the device performance. Based on the band structure presented in Figure 3, this study attempts to realize graded electron transport in the band-engineered W-TiO2 graded structure and achieve high performance dye-sensitized solar cells. To make band-engineered cascade film, the substrates were dropped


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successively into the four kinds of stock solution with different concentrations of tungsten precursor at each step. The DSSCs with the W-TiO2 graded structure of successive w, x, y, and z mol % of W from top to bottom is labelled to w-x-y-z W. Figure 4 and Table 1 present the photovoltaic performance and parameters, respectively. Figure S9A shows corresponding incident photontoelectron conversion efficiency (IPCE) spectra and standard deviation obtained from four identically prepared devices. The error bars in Table 1 were calculated from the J–V curves of four DSSCs for each condition. Most of the DSSCs show much better photovoltaic performance than the DSSCs with the 3 % W-doped TiO2 blocking layer. In particular, 1-5-7-9 W exhibits the best performance with JSC, VOC, and FF values of 20.18 mA/cm2, 0.71 V, and 61.81 %, respectively, to give a η value of 8.56 %, which is 30 % higher than the DSSCs without a blocking layer (6.62 %). Interestingly, both JSC and VOC are improved compared to the DSSCs with a 3 % W-doped TiO2 blocking layer. The increase in JSC was interpreted by funneling energy through the band-engineered graded structure. This is confirmed by the IPCE curves as presented in Figure S9A. As shown in Figure 1, the graded structure of w-x-y-z W introduction resulted in a down-shift of the CB. Thus, as it is well known, the driving force for charge transfer was increased. Corresponding to the enlarged driving force, JSC and IPCE were increased with all graded films (w-x-y-z W). This designates that the remarkable improvement of JSC or IPCE induced by the introduction of graded structure is at least partially ascribed to the increased energy difference between the CB of TiO2 and excited state of the dye-sensitizer. Besides the increased driving force for efficient charge transfer, inhibition of charge recombination can improve charge collection efficiency and thus enlarge JSC or IPCE, which will be discussed later. However, the change in JSC with different graded structure might be attributed to the fact that


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doping may induces the weak or strong crystallization of the sample (i.e., varied defects within the W doped TiO2 particles), resulting in increased or decreased recombination at the defects.52 EIS analysis (Figure S9D) showed that noticeable differences in the charge transport resistance were in the order of 1-5-7-9 W (9.17 Ω) > 1-3-5-9 W (9.70 Ω) > 3-5-7-9 W (9.80 Ω) > 1-3-7-9 W (9.97 Ω) > 1-3-5-7 W (10.12 Ω). The charge transfer resistance value for the DSSCs using 15-7-9 W was smaller than those using the other w-x-y-z W films due to the enhanced charge transfer ability. The change in photocurrent is partially attributed to the difference in charge transfer ability. Graded electron transfer enhances the charge transfer rate from TiO2 to the current collector. This band-engineered graded film prevents the unwanted recombination of electrons from the TCO to the redox couple. Although the total amount of W in 1-5-7-9 W is larger than that of 3 % W-doped TiO2, VOC of 1-5-7-9 W is slightly higher than the DSSCs with the 3 % W-doped TiO2 blocking layer. The photoanode in DSSCs experiences various charge recombination processes, including charge recombination in the electrode semiconductor and back transfer of photo-injected electrons from the TCO to the redox couple. Charge recombination is accelerated more in the 1-5-7-9 W, but blocks the back transfer of electrons more efficiently to compensate for the former charge recombination. As a result, 1-5-7-9 W can exhibit large improvement in JSC without a decrease in VOC, to yield high photovoltaic performance. Figure 5 shows the SLIM-PCV analysis results of the DSSCs ungraded (pure TiO2), graded (15-7-9 W), and anti-graded (9-7-5-1 W) layer, which is calculated by fitting the photo-current and voltage decay with a different ND filter. Table S3 lists corresponding parameters and Figure S10 shows their SLIM-PCV curve, which is obtained using a setup as described above. As shown in Figure 5A, De of the DSSCs with the graded film is much higher than the ungraded and anti-


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graded one. This means that a larger amount of photo-injected electrons can be transported to the TCO through the graded structure with a graded electron transfer regime. The graded conduction bands can drive the photogenerated electron to be transferred to the current collector more rapidly. On the other hand, the anti-graded layer prevents the photogenerated electrons from being transferred rapidly, but accelerates the back transfer of collected electrons to cause the reduction of I3- in the electrolyte. As shown in Figure 5B, the anti-graded device exhibits the shortest electron life time among the samples tested due to the enhanced back transfer reaction. The graded one shows both a higher diffusion coefficient and longer electron life time to yield a longer diffusion length (Figure 5C). In other words, the graded charge transfer regime enhances the amount of charge transfer significantly by reducing the probability of charge recombination. A direct consequence of this recombination suppression outcome is that the Fermi level (EF) can be negatively shifted under light conditions with a particular illumination intensity (I). This effect is complementary to the enhancement of VOC, since the calculated photovoltage (Vphoto) is derived from the following equation:22

, 34-$- 5 =

67 8 # 69/: ;

(6)

99 %) in deionized water. All solutions were purged with Ar before experiments. The potentiodynamic electrochemical impedance spectroscopy (EIS) experiments were conducted at a frequency of 1 kHz employing a computercontrolled potentiostat (Iviumstat, Ivium). The DSSCs fabrication and its device characterization


21


Page 22 of 41

techniques were followed according to our earlier reports39-42 and were provided in the Supporting Information. The stepped light-induced transient photocurrent-voltage decay measurements (SLIM-PCV) were analyzed by a similar to our previous reports39-42 and as detailed elsewhere43 using a red-light (HeNe) pulsed diodes (Thorlabs HNL210L system).


22

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FIGURES

Figure 1. Schematic diagram of charge transfer from the photosensitizer (N719) to the current collector (FTO) through a graded W-TiO2 blocking layer.


23


Page 24 of 41

Figure 2. (A) SEM image of a cross section of prepared photoanode; Cross sectional TEM images of (B) graded W-TiO2 layer/FTO, (C) HRTEM, (D) SAED pattern, (E) STEM-HAADF image and (F-H) the corresponding EDAX elemental mapping images of titanium, tungsten, and oxygen, respectively.


24

Page 25 of 41

X 109

O1s C-2 (µ µF-2cm4)

Intensity (A.U.)

(A)

9% 7% 5% 3% 1% 0% 535.9

533.6

531.3

529.0

Binding Energy (eV)

(C)

2 9% (B) 0 2 7% 0 2 5% 0 2 3% 0 2 1% 0 2 0% 0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

Potential vs. Ag/AgCl (V)

Efb

-4

Potential (eV)

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


-3.8

ECB -5

-4.0

-6

-4.2

-7

-4.4

Efb 0 2 4 6 8 10

EVB

-8 0

2

4

6

8

10

Amount of W Dopant (atomic %) Figure 3. (A) XPS spectra of O 1s electron and (B) Mott-Schottky plot for each W-TiO2 blocking layer. (C) Estimation of the band-structure for each W-TiO2 blocking layer, derived by combining the XPS results and Efb from the Mott-Schottky plots. EVB was obtained from the literature.


25

Page 26 of 41

2

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

Current Density (mA/cm )


20 15 10 5 0 0.0

1-3-5-7 W 1-3-5-9 W 1-3-7-9 W 1-5-7-9 W 3-5-7-9 W

0.2

0.4 0.6 Potential (V)

0.8

Figure 4. Current density-voltage (J-V) spectra of each DSSC containing a graded W-TiO2 blocking layer under simulated solar light illumination (AM 1.5, 100 mW/cm2).


26

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Figure 5. (A) De, (B) τe, and (C) Le for the DSSCs containing ungraded, graded, and anti-graded blocking layer, which were estimated from the SLIM-PCV measurements.


27

2

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

Current Density (mA/cm )


Page 28 of 41

25 20 15 10 5 0 0.0

w/o BL TiO2 Ungraded Graded Anti-graded

0.2

0.4 0.6 Potential (V)

0.8

Figure 6. Current density-voltage (J-V) spectra of each DSSC with soldering at contact under standard global AM 1.5 solar light conditions.


28

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TABLES. Table 1. Photoelectrochemical parameters of the DSSCs containing a graded W-TiO2 blocking layer under standard global AM 1.5 solar light conditions.

JSC

VOC

FF

η

(mA/cm2)

(V)

(%)

(%)

1357

17.29 ± 0.56

0.71 ± 0.008

62.53 ± 3.49

7.64 ± 0.06

1359

18.90 ± 0.20

0.70 ± 0.004

61.20 ± 0.46

8.15 ± 0.02

1379

17.85 ± 0.35

0.70 ± 0.002

62.73 ± 0.32

7.85 ± 0.02

1579

20.18 ± 0.42

0.71 ± 0.001

61.81 ± 7.40

8.56 ± 0.18

3579

18.47 ± 0.43

0.70 ± 0.002

61.24 ± 1.36

7.89 ± 0.05

Sample


29


Page 30 of 41

Table 2. Photoelectrochemical parameters of the DSSCs (with soldering at contact) without a blocking layer, with a TiO2 blocking layer, ungraded, graded, and anti-graded W-TiO2 blocking layer.

Sample

JSC (mA/cm2)

VOC (V)

FF (%)

η (%)

w/o BL

16.44 ± 0.31

0.70 ± 0.002

65.28 ± 3.63

7.61 ± 0.09

TiO2

17.75 ± 0.1

0.70 ± 0.002

69.25 ± 0.08

8.66 ± 0.04

Un-graded

19.91 ± 0.47

0.70 ± 0.001

68.37 ± 0.73

9.48 ± 0.14

Graded

24.90 ± 0.41

0.70 ± 0.002

64.17 ± 3.32

11.30 ± 0.24

Anti-graded

17.53 ± 0.25

0.69 ± 0.008

68.63 ± 0.80

8.30 ± 0.32

ASSOCIATED CONTENT Supporting Information. The thickness of the W-TiO2 blocking layer vs. spin coating times, JV spectra of each DSSC with different spin coating times, XRD spectra of pure and graded WTiO2, X-ray photoelectron spectra of 9 % W-doped TiO2 blocking layer, X-ray photoelectron spectra of W 4f containing different amount of W-dopant, J-V characteristics, error bar, electrochemical impedance and time resolved (A) current density and (B) photovoltage decay results of each DSSC containing different amount of W-dopant, diffusion coefficient, electron lifetime and diffusion length comparison for each DSSC, Standard error analysis of photovoltaic parameters of DSSCs containing different amount of W-dopant, Time resolved (A) current density and (B) photovoltage decay of each DSSC using ungraded, graded and antigraded structure, photoelectrochemical parameters of the DSSCs without a blocking layer, with TiO2


30

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and W-TiO2 blocking layers containing different amounts of W-dopant, Standard error analysis of photovoltaic parameters of DSSCs without a blocking layer, with TiO2, ungraded, graded, and anti-graded W-TiO2 blocking layer. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Jae Hong Kim); [email protected] (Hyeong Jin Yun). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by a grant from the Fundamental R&D program for Core Technology of Materials (10050966) funded by the Ministry of Knowledge Economy, Republic of Korea. and This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy.


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Page 32 of 41

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Table of content

The novel cascade photoanode network of dye-sensitized solar cell is presented to enhance the photovoltaic performance.


41

High-Performance Dye-Sensitized Solar Cells through Graded

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