Recombination in SnO2-Based Quantum Dots Sensitized Solar Cells

Article pubs.acs.org/JPCC

Recombination in SnO2‑Based Quantum Dots Sensitized Solar Cells: The Role of Surface States Qingli Huang,† Fan Li,† Yun Gong,† Jianheng Luo,† Shize Yang,‡ Yanhong Luo,† Dongmei Li,† Xuedong Bai,§ and Qingbo Meng*,†,§ †

Key Laboratory for Renewable Energy, Chinese Academy of Sciences; Beijing Key Laboratory for New Energy Materials and Devices; Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China ‡ International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China § Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Tin oxide (SnO2) is one of the most promising electron transporters to further enhance the performance of quantum dots sensitized solar cells (QDSCs). Unfortunately, the performance of SnO2-based QDSCs is still poor. It was observed that surface modification toward a SnO2 photoelectrode such as a TiCl4 treatment is crucial to dramatically increase the performance of the devices. However, the mechanism of the TiCl4 treatment remains poorly understood. Here, systematic studies on the photoelectrochemical properties of SnO2-based QDSCs were performed in order to clarify the mechanism by which the TiCl4 treatment improves the performance of solar cells. Impendence spectroscopy results reveal that the photogenerated electrons transport in the porous SnO2 network rather than the TiO2 coating. Furthermore, a physical model considering the existence of monoenergetic surface states at the SnO2 surface was used to simulate the behavior of chemical capacitance at various forward biases. The accordance between the decrease of the surface states and the recombination reduction clearly indicates that the surface states act as the recombination centers to influence the device performance, which can be well described by Marcus-Gerischer theory. These combined findings provide new understanding of the recombination mechanism of SnO2-based sensitized solar cells and guidelines for further improving the performance of this system.

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INTRODUCTION Quantum dot sensitized solar cells (QDSCs) have received increasing attention since quantum dots (QDs) exhibit attractive properties over conventional sensitizers (i.e., organic dyes, ruthenium-polypyridine complexes, etc.).1−7 The bandgap of QDs can be tuned by changing the particle size due to its quantum confinement effect, so it permits one to design the device simply by using QDs with different sizes as light absorbing materials in order to fully utilize the solar spectrum.1,8,9 Additionally, QDs have very high extinction coefficients,10 and are quite cost-effective due to adopting facile and less expensive synthetic methods.11−14 Especially, considering the hot electron injection and multiple exciton generation (MEG), beyond ∼44% of light-to-electricity conversion efficiency of quantum dot solar cells could be expected theoretically. 15−19 Although, at present, the maximum efficiencies of QDSCs are still low,20−22 QDSCs still have great potential as candidates for next-generation solar cells. The energy level alignment between QDs and electron transporter influences the photogenerated charge injection efficiency.23,24 At present, TiO2 as the electron transporter is commonly employed in QDSCs. However, it is found that the conduction band of some narrow bandgap QDs (such as PbS, PbSe) with large sizes is deeper than that of TiO2, leading to © XXXX American Chemical Society

inefficient injection of photogenerated electrons from QDs into the conduction band of TiO2.25,26 Therefore, one strategy to significantly improve the efficiency of QDSCs is replacing TiO2 with an electron transporter that has a more positive conduction band.27,28 SnO2 is supposed to be a good candidate with a conduction band edge around −4.66 eV, 300−500 meV lower than that of the TiO2, which can exhibit more efficient charge transfer from QDs to SnO2.24,26,27,29−31 Moreover, the bulk SnO2 has an electron mobility of 240 cm2/(v·s), 2−3 orders of magnitude higher than that of TiO2.31,32 As a result, it is expected to achieve higher electron mobility in the corresponding nanostructured SnO2 electrodes and thus benefit electron collection. To date, some work related to SnO2-based QDSCs has already been reported.27−29,33,34 For example, Wang et al. reported the PbS-sensitized SnO2 QDSCs, which can present a conversion efficiency of 2.23% and an extremely high photocurrent density of 17.38 mA·cm−2.27 Snaith et al. fabricated infrared active hybrid solar cells, SnO2/PbS/spiroOMeTAD or P3HT, which can generate 4 times the photocurrent density under simulated sunlight in comparison Received: March 14, 2013 Revised: May 7, 2013

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dx.doi.org/10.1021/jp402601v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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was deposited on PbS sensitized films by the CBD method at 10 °C for 1 h. Finally, the photoelectrode was passivated with ZnS by twice dipping into 0.1 M Zn(CH3COO)2 and 0.1 M Na2S aqueous solutions for 1 min alternately. Preparation of Counter Electrodes. Cu2S counter electrodes were fabricated by following the reported method.38,39 Brass foil was first washed with ethanol in an ultrasonic bath, subsequently etched in 37% HCl at 70 °C for 10 min to remove the zinc from the copper−zinc alloy and the color of the brass foil changed from yellow to copper red. Then the brass foil was rinsed with deionized water. The etched brass foil was masked by 3 M tape and etched by several drops of aqueous polysulfide solution again, leading to Cu2S counter electrodes. Morphology, Optical Measurement, and Photoelectrochemical Characterization. The morphology and structure of the SnO2 photoelectrodes were characterized by transmission electron microscopy (JEOL 2010 FEG TEM). Samples for TEM investigations were prepared by detaching the SnO2 film mechanically from the substrate and dispersing in ethanol, followed by transferring one drop of the suspension onto a carbon-coated copper grid. The optical properties were obtained by ultraviolet−visible−near-infrared (UV−vis−NIR) spectroscopy (Shimadzu, UV2550). The PbS/CdS-coated SnO2 film, polysulfide electrolyte (1 M Na2S and 1 M S) and Cu2S electrode were assembled into a sandwich-type cell. A mask with a window of 0.15 cm2 was clipped on the photoelectrode side to define the active illumination area of the cell. The cells were irradiated by an Oriel solar simulator 91192 under AM 1.5 illumination (100 mW·cm−2), and the photocurrent−photovoltage (J−V) characteristics of the cells were recorded on a Princeton Applied Research, Model 263A. IS measurements were carried out ona ZahnerIM6e Electrochemistry Workstation in the dark over different bias voltage in the frequency range from 1 × 105 to 0.1 Hz. The obtained impedance spectra were fitted with the Zview software in terms of appropriate equivalent circuit.

with the corresponding TiO2 solar cell.28 However, the performance of SnO2-based QDSCs is still unsatisfactory. It is supposed that strong recombination occurs at the SnO2/QDs/ electrolyte interfaces, thus leading to lower device efficiency. Therefore, surface modification toward the SnO2 electrode such as a TiCl4 treatment is crucial to improve the cell performance.27−29,33,34 The TiO2 coating formed on the SnO2 surface is usually considered to be a barrier layer to inhibit the recombination between the electrons from SnO2 and the holes from electrolyte and QDs. In recent works, the variation of surface states are speculated to play an important role in inhibition of recombination processes.27,29,33 However, the recombination mechanism of SnO2 electrodes is still poorly understood. It is believed that systematic investigation on the kinetic processes (i.e., charge recombination and charge transport) is urgent, which can supply guidelines for seeking effective surface modification to further improve the performance of SnO2-based QDSCs. In this work, TiCl4 treatment can bring ∼40 times higher conversion efficiency of the SnO2-based QDSCs than that without TiCl4 treatment. Impedance spectroscopy (IS) has been employed to investigate the effect of TiCl4 treatment on the cell performance. First, extremely small transport resistance is found in the photoelectrode, indicating that the photogenerated electrons transport in the porous SnO2 network rather than the TiO2 coating. Furthermore, a physical model considering the existence of surface states at the SnO2 surface was used to simulate the behavior of chemical capacitance at various forward biases. It is found that high density of monoenergetic surface states exists at the SnO2 surface without the TiCl4 treatment, and most surface states can be passivated by the TiCl4 treatment, which is in accordance with the increase of the charge transfer resistance and electron lifetime. This can be well explained by the Marcus−Gerischer model for electron transfer. Our work provides a new insight into the SnO2-based sensitized solar cells, and points out that eliminating the surface states effectively is of great importance for further improving SnO2-based sensitized solar cells.

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RESULTS AND DISCUSSION Photovoltaic Performance of PbS/CdS QDSCs with and without TiCl4 Treatment. The J−V curves for QDSCs incorporating bared SnO2 and SnO2 with the TiCl4 treatment are shown in Figure 1, and the corresponding photovoltaic performances and parameters are summarized in Table 1.

EXPERIMENTAL DETAILS Preparation of Mesoscopic SnO2 Electrodes. Tin(IV) oxide (SnO2) nanoparticles (Alfa Aesar, Nanoarc) were used to synthesize the SnO2 paste by following the same method for TiO2 paste.29,35 SnO2 electrodes (10 μm-thick) were deposited on fluorine-doped tin oxide conducting glass (FTO, thickness: 2.2 mm, Pilkington, 14 Ω/square) by the doctor blade method. The films were then sintered at 500 °C for 30 min in air. The resulting mesoporous SnO2 electrodes were semitransparent. The SnO2 electrodes were treated by 40 mM aqueous TiCl4 solution at 70 °C for 40 min and then washed with deionized water and ethanol. For comparison, the 10 μm-thickness TiO2 electrodes (P25, Degussa) were also prepared and treated with the same method. Preparation of PbS/CdS-Sensitized Mesoscopic Electrodes. The successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD) techniques were employed to assemble PbS and CdS on the photoelectrode, respectively.7,36,37 Briefly, the photoelectrode film was dipped into 0.02 M Na2S aqueous solution for 30 s, thoroughly rinsed with Milli-Q ultrapure water, then dipped into 0.02 M Pb(NO3)2 solution for another 30 s, and rinsed with Milli-Q water again, which was defined as one SILAR cycle. Here, the SILAR cycles for PbS deposition were three. Subsequently, CdS

Figure 1. The J−V curves for QDSCs measured under AM 1.5 illumination of 100 mW·cm−2, incorporating SnO2 photoelectrodes with and without TiCl4 treatment, respectively. B



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Table 1. Photovoltaic Performance and Parameters of the PbS/CdS Sensitized Nanostructured SnO2 Photoelectrode with and without TiCl4 Treatmenta

a

photoelectrode

Jsc (mA·cm−2)

Voc (mV)

FF

η (%)

bare SnO2 SnO2 with TiCl4

1.76 19.12

106 300

0.21 0.28

0.04 1.60

Measured under AM 1.5 illumination of 100 mW·cm−2.

According to Figure 1, the QDSCs incorporating bare SnO2 photoelectrode can only present a short-circuit current density (Jsc) of 1.76 mA/cm2 and an open-circuit voltage (Voc) of 106 mV, resulting in poor power conversion efficiency of 0.04%. After the TiCl4 treatment, the Jsc significantly increases to 19.12 mA/cm2, and the Voc increases to 300 mV, thus leading to an efficiency of 1.60%. Obviously, the TiCl4 treatment is indispensable for the SnO2-based PbS/CdS QDSCs. UV−vis−NIR Absorption Spectra of PbS/CdS Cosensitized SnO2 Films. To investigate the influence of TiCl4 treatment on the optical property of the PbS/CdS cosensitized SnO2 film, the UV−vis−NIR absorption spectra of photoelectrodes with the same thickness and the same deposition procedures are shown in Figure 2. No matter whether the TiCl4

Figure 3. HRTEM images of SnO2 nanoparticles (a) without TiCl4 treatment and (b) with TiCl4 (40 mM) treatment. All the films were sintered at 500 °C prior to and after the surface treatment.

nanoparticles with TiCl4 treatment. The morphology of SnO2 nanoparticles is altered significantly after the TiCl4 treatment. Crystalline islands and terraces rather than continuous uniform shells were formed on the SnO2 surface, which is consistent with the result of Snaithet al.32 Several atomic layers of TiO2 close to the SnO2 surface are ascribe to the rutile structure, which can well match the SnO2 lattice and minimize interfacial strain.32 However, as the thickness of TiO2 increases, the crystalline phase of TiO2 turns into an anatase structure, which is different from the literature.32 Dynamic Measurement of SnO2-Based QDSCs. IS was used to analyze the effect of the TiCl4 surface treatment on the dramatic improvement of the performance of the SnO2-based QDSCs. As we know, the nanocrystalline TiO2 is widely used as an electron transporter in QDSCs. The dramatic improvement of the performance is possibly attributed to the influence of the TiO2 coating. In other words, the injected electrons may transport in the TiO2 coating. In order to clarify the transport route of photogenerated electrons, the TiO2-based QDSCs were also fabricated. Figure 4a (4b, zoom of graph 4a) shows representative IS spectra (Nyquist plot) for TiO2-based and SnO2-based (with and without TiCl4 treatment) QDSCs at low applied forward bias (200 mV) under dark conditions. For the TiO2-based QDSCs, the obtained IS spectra (the blue symbols in Figure 4a,b) present the obviously Warburg behavior

Figure 2. UV−vis−NIR absorption spectra for SnO2/PbS/CdS/ZnS and SnO2/TiO2/PbS/CdS/ZnS films.

treatment was carried out, the light absorption of both photoelectrodes can extend to 1400 nm in the measured wavelength range. The absorption in long wavelength range is mainly attributed to the absorption of PbS QDs. For the photoelectrode with the TiCl4 treatment, the absorbance intensity of the spectra is relatively higher than that without the TiCl4 treatment. It indicates that the SnO2 surface treated by TiCl4 is more favorable for quantum dots deposition, which contributes to the total absorption. However, the light absorption is apparently not the main reason for the dramatic difference in the photocurrent between the photoelectrodes with and without TiCl4 treatment. Therefore, more investigations on the effect of TiCl4 treatment on cell performance are necessary. Microstructure Examination. Figure 3a shows a highresolution transmission electron microscopy (HRTEM) image of the SnO2 nanoparticles without TiCl4 treatment. The size of SnO2 nanoparticles is approximately 20 nm, which shows clear lattice fringes with d-spacing of 0.264 nm corresponding to a crystal plane of (101) indexed to the cassiterite structure. Figure 3b shows the corresponding HRTEM image of the SnO2 C



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Figure 4. (a) Impedance experimental (symbols) and fitting (solid lines) results of SnO2-based (with and without TiCl4 treatment) and TiO2-based QDSCs at applied forward bias (200 mV) under dark conditions. (b) Zoom of graph a. An obvious straight line, which represents the transport resistance feature, can be observed for the TiO2-based QDSCs in the high frequency region. (c) Charge transport resistance, Rtr, obtained from IS results for TiO2-based QDSCs as a function of the voltage drop at the sensitized photoelectrode, VF. (d) The transmission line equivalent circuit model used to fit the experimental data of TiO2-based QDSCs. (e) Simplified equivalent circuit model used to fit the experimental data of SnO2based QDSCs.

based photoelectrodes. The fitting results were obtained using the equivalent circuit shown in Figure 4e. We find an exponential rise of Cμ versus VF for the SnO2-based QDSCs without the TiCl4 treatment at forward voltage region and an obvious plateau feature at the intermediate voltage region. After the TiCl4 treatment, the plateau feature almost disappears, and a good exponential rise of Cμ occurs at the entire measurement voltage region. A similar feature, which is considered to be related to the monoenergetic surface states, has been reported in TiO2-based QDSCs with surface modification (such as ZnS, molecular dipoles).43,44 On the basis of the concept that the capacitance, Cμ, of the photoelectrode describes the change of electron density under a small variation potential drop at the photoelectrode,45,46 a reasonable speculation is that the presence of TiO2 coating changes the surface states of SnO2. To verify this speculation, we used a simplified physical model of photoelectrode based on the multiple-trapping (MT) model considering the existence of surface states at the SnO2 surface, which was first developed by Bisquert, to simulate the behavior of Cμ at various forward biases.44 Figure 6 shows the simplified photoelectrode model used in the simulation. The nanostructured SnO2 photoelectrode is extended to a monolayer SnO2 film and covered by a dielectric layer (DL) with thickness of d. The dielectric layer includes the semiconductor layers (TiO2, PbS, CdS, ZnS) which do not participate in electron transport process. Based on the existing theoretical models for TiO2 nanostructured photoelectrode,46−48 the distribution of the electronic states in SnO2 is supposed to divided into three classes: (a) the conduction band states, (b) the bulk trap states, and (c) the surface states, as indicated in Figure 6. (a) Conduction Band States. The concentration of free electrons in the conduction band (nc) is determined by the quasi-Fermi level of electrons according to46

characteristic of a transmission line,40 where a straight line can be observed at the high frequencies. A good fitting result (the blue line in Figure 4a,b) was obtained using the transmission line model developed by Bisquert (Figure 4d), where we consider a short circuit in the electrolyte.41,42 For the SnO2based QDSCs, the obtained IS includes two semiarcs (the red and orange symbols in Figure 4a,b) whether or not the TiCl4 treatment was carried out. The high-frequency semiarc represents the charge transfer behavior at the counterelectrode. The low-frequency semiarc includes the chemical capacitance (Cμ) of nanostructured photoelectrode and the recombination resistance (Rct) between the photoelectrode and the polysulfide electrolyte.42,43 An excellent fitting result (the red and orange lines in Figure 4a,b) was obtained using a simplified equivalent circuit (Figure 4e).29 The equivalent circuit of the transmission line model (Figure 4d) can be simplified to that of Figure 4e when the transport resistance (rtr) of the electron transporter becomes negligible. Obviously, compared with the TiO2-based QDSCs, the absence of the transmission line in the IS of SnO2based QDSCs indicates a negligible rtr in the SnO2-based photoelectrode. The fitting results of Rtr (= rtr·L, where L is the photoelectrode layer thickness of TiO2-based QDSCs) as a function of the voltage drop at the sensitized photoelectrodes (VF) are shown in Figure 4c. The Rtr for SnO2-based QDSCs is too small to be fitted. The difference of Rtr between the TiO2based and SnO2-based QDSCs supplies clear evidence that the electrons transport in the porous SnO2 network rather than the TiO2 coating, which is consistent with the fact that the bulk SnO2 has a mobility up to 240 cm2/(V·s), which is 100 times faster than that of TiO2.32 As the TiO2 do not participate in the electrons transport process of SnO2-based QDSCs, it can be seen as a kind of sensitizer. It seems that the surface of the SnO2 nanoparticles plays an important role in the recombination dynamics of the carriers. The carrier recombination through the surface of SnO2 in the transport process is changed after the TiCl4 treatment. Figure 5 illustrates capacitance (Cμ), recombination resistance (Rct) and electron lifetime (τn) of the nanostructured SnO2-

nc = NC exp[(E Fn − EC)/kBT ]

(1)

Here, NC is the volume effective density of the conduction band minimum, EFn is the quasi-Fermi level of electrons, EC is the D



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Figure 6. Energy diagram displaying the distribution of localized states in a SnO2-based photoelectrode mainly including conduction band states, exponential distribution of bulk traps, and monoenergetic surface states. EC is the conduction band energy, EFn is the quasi-Fermi energy of the electrons at applied voltage (VF), EF0 is the Fermi energy of the electrons at equilibrium, and ES is the energy of the monoenergetic surface states. VS is the voltage drop at the SnO2, and Vd is the voltage drop at the dielectric layer (DL).

nanostructured SnO2.9 These surface states are energetically located in the bandgap. Assuming that the density of surface states distribution (u(E)) is concentrated in a narrow energy level range and can be represented by a monoenergetic level (ES) as shown in Figure 6, it has the form ⎧ 0 E ≠ ES u(E) = ⎨ ⎩ NS E = ES ⎪ ⎪

(3)

where NS is the total monoenergetic surface states density per unit surface. f(E − EFn) is defined as the electron occupation probability in the electronic states of SnO2 and obeys the Fermi−Dirac distribution function; it has the form46,47 1 f (E − E Fn) = 1 + exp[(E − E Fn)/kBT ] (4) The relationship of electron occupation probability of the monoenergetic surface states f(ES − EFn) with the Fermi level of electrons EFn is shown in Figure 7a. Almost no surface states are occupied by electrons when EFn is low. The surface states are occupied quickly when EFn rises to ES. A step of f(E − EFn) with EFn occurs when EFn crosses ES. The capacitance (Cμ) measured by IS reflects the change of electrons on a small variation of potential drop at the photoelectrode.51 The capacitance (CS) induced by the conduction band states and trap states can be described by the expression46,51

Figure 5. (a) Capacitance, (b) recombination resistance, and (c) lifetime obtained from IS for the SnO2-based QDSCs without TiCl4 treatment (blue symbols) and with TiCl4 treatment (red symbols).

conduction band minimum of SnO2, T is the temperature, and kB is the Boltzmann constant. (b) Bulk Trap States. Assume that an exponentially density distribution of the bulk trap states (g(E)) in the band gap of nanostructured SnO2 similar to a common finding in nanostructured TiO2,46,47,49−51 it has the form g (E ) =

NL exp[(E − EC)/kBT0] kBT0

EC ∂ [ f (E − E Fn)g (E) dE + nC] ∂E E V ⎡ n ⎤ ≈ e 2⎢g (E Fn) + C ⎥ kBT ⎦ ⎣

CS = e 2

∫

(5)

Here, e is the elementary charge. As the surface states are very close to the surface of the dielectric layer, the capacitance induced by surface states relates to an electrostatic geometric capacitance;44,45 it has the form

(2)

where NL is the total volume density of the bulk trap states, and T0 is a parameter with temperature units that determines the depth of the distribution below the lower edge of the conduction band (EC).46 (c) The Surface States. The surface states originate from the extrinsic defects and dangling bonds on the surface of

Cd = εε0 /d

(6)

Here, ε is the relative dielectric constant of the dielectric layer, and ε0 is the vacuum permittivity. E



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VF = VS +

deNS f (ES − E F0 + eVS) εε0

(9)

Figure 7b shows VS as a function of VF. An obvious plateau feature occurs at the intermediate voltage of VF. The cause of this feature can be explained as follows: at the forward voltage region of VF, the surface states are not charged by electrons, and the voltage drop at the sensitized photoelectrode is mainly dropped at the SnO2 to raise the Fermi level of electrons. As the monoenergetic surface states are charged rapidly when the Fermi level of electrons rises close to the energy level of surface states, the increment of VF mainly drops at the dielectric layer at this voltage region, resulting in a plateau of the VS. As a result, the capacitance measured by IS (Cμ) is also the sum of the capacitances contributed by the conduction band states and bulk trap states of SnO2 (CS) and by the surface states (Cd), where CS and Cd are in series; it has the form 1 1 1 = + CμS CSLS CdRS

Here, S, L, and R are the area, thickness, and roughness factors of the SnO2-based photoelectrode film, respectively. The simulation results of the capacitance are shown in Figure 7c. The capacitance (Cμ(i)), which neglects the existence of surface states, is a standard exponential rise as a function of VF. The capacitance (Cμ(s)) contributed by the SnO2 (CS) and by the entire sensitized photoelectrodes (Cμ) shows a plateau feature at intermediate voltage (VF) when one considers the existence of high density of monoenergetic surface states on SnO2 surface. The good agreement between the theoretical predictions (Figure 7c) and the experimental results (Figure 5a) indicates (1) the existence of high density of monoenergetic surface states on the surface of SnO2 nanostructured photoelectrodes and (2) the TiCl4 treatment can eliminate the surface states effectively. The charge recombination resistance, Rct, (Figure 5b) and the electron lifetime, τn, (Figure 5c) of photoelectrode are key parameters in understanding the recombination of the SnO2based QDSCs. Compared with a SnO2-based photoelectrode without TiCl4 treatment, it is observed that a SnO2-based photoelectrode with TiCl4 treatment exhibits significantly higher Rct and longer τn, which indicates lower recombination between the SnO2-based photoelectrode and the surroundings. The accordance between the recombination reduction and the decrease of monoenergetic surface states indicates that the recombination leading to the loss of the photogenerated electrons takes place predominantly from monoenergetic surface states. In other words, the electron recombination process is governed by the monoenergetic surface states localized in the bandgap of electron transporter (SnO2). Figure 8a schematically shows the SnO2-based QDSCs without the TiCl4 treatment. The corresponding transport and recombination processes are shown in energy band diagrams in Figure 8c: (1) the transport of photogenerated electrons in the conduction band states of SnO2, (2) the electron trapping and detrapping process between the internal exponential distribution of bulk trap states and the electronic states in the conduction band, and monoenergetic surface states as recombination centers to trap electrons from conduction band and holes from (3) QDs and (4) electrolyte. The recombination from the surface states to the oxidized form of the QDs and electrolyte can be described by the Marcus− Gerischer model for electron transfer. The corresponding

Figure 7. (a) Occupancy of monoenergetic surface states as a function of the quasi-Fermi level of electrons in SnO2. (b) The voltage at the SnO2 layer (Vs) as a function of the applied voltage in the SnO2-based photoelectrode (VF). (c) Capacitance of the electrode with different density of monoenergetic surface states. The simulation parameters are as follows: T = 300 K, T0 = 1360 K, L = 10 μm, d = 2 × 10−7 cm, ε = 10, R = 5000, EC = −4.66 eV, EF0 = −6.66 eV, Es = −6.46 eV, NC = 1 × 1026 cm−3, Nb = 3 × 1025 cm−3, Ns (Cμ(i))=0, Ns (Cμ(s)) = 6 × 1012 cm−2.

The semiconductor layer coated on the SnO2 surface as a dielectric layer can influence the potential drop between the conduction band of SnO2 and the redox potential of electrolyte as shown in Figure 6. The voltage drop at the sensitized photoelectrode (VF) is mainly the sum of the voltage contributed by the quasi-Fermi level of electrons variation of SnO2 (VS) and by the voltage drop at the dielectric layer (Vd). VF = VS + Vd

(7)

where Vd relates to the electron concentration in the surface states. The negative charge in the surface states is given by Q S = eNSf (ES − E Fn) = CdVd

(10)

(8)

According to eqs 6, 7, and 8, we obtain F



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Figure 8. (a,b) Schematic drawings of the SnO2-based QDSCs before and after TiCl4 treatment. (c,d) The corresponding energy band diagrams of charge transport and recombination processes: (1) the transport process of photogenerated electrons in the conduction band states of SnO2, (2) the electron trapping and detrapping process between an internal exponential distribution of bulk trap states and the electronic states in the conduction band, and recombination of electrons from the high density of monoenergetic surface states of SnO2 nanostructured photoelectrodes with holes from (3) the QDs and (4) the electrolyte.

recombination rate, rs, resulting from electron transfer form surface states at the energy level EFn can be described by the relationship46 rS(E Fn) ∝ nS(E Fn)cSk S

simulate the behavior of chemical capacitance at various forward biases. The good agreement between simulation results and experimental results reveals the influence of the TiCl4 treatment to the monoenergetic surface states at the SnO2 surface. The strong correlation between the decrease of the surface states and the recombination reduction clearly indicates that the surface states act as the recombination centers to influence the device performance, which can be well described by Marcus-Gerischer theory. Although more recombination details remain to be considered, these combined findings provide new understanding of the recombination mechanism of SnO2-based sensitized solar cells and guidelines for further improving the performance of this system.

(11)

where ns is the concentration of electrons in the surface states, cs is the concentration of oxidized states mainly contributed by QDs and polysulfide electrolyte, and ks is the transition probability for tunneling, which is dependent on the energy barrier between the oxidized states and the surface states.52,53 The recombination mechanism of the TiCl4 treatment toward SnO2-based QDSCs can be explained as follows: the formation of TiO2 coating on the SnO2 with the well-matched crystalline interface can minimize defects and dangling bonds, resulting in the reduction of surface states concentration, ns. The oxidized states, cs, remain almost unchanged due to the same QDs and electrolyte used in comparison. The existence of TiO2 between the surface states and the oxidized states as a block layer lowers the transition probability, ks. The reduction of parameters (ns, ks) results in the smaller recombination rate, rs, as shown in Figure 8b,d. As the recombination through the surface states is reduced, the free electrons concentration in SnO2 increases, resulting in the higher short current density (Jsc) and the elevation of quasi-Fermi level, which determines the photovoltage (Voc).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Key Basic Research Program (973 project, No. 2012CB932903), the National Natural Science Foundation of China (Nos. 51072221 and 21173260) and the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-YW-W27).

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CONCLUSION A dramatic increase (∼40 times) of the photovoltaic performance of SnO2-based PbS sensitized solar cells can be achieved by surface modification of TiCl4. Systematic studies indicate that the improved performance is mainly ascribed to the more effective electron collection process. IS was employed as a main characterization method to clarify the mechanism by which the TiCl4 treatment improves the performance of solar cells. IS results reveal that the photogenerated electrons transport in the porous SnO 2 network rather than the TiO 2 coating. Furthermore, a physical model considering the existence of monoenergetic surface states at the SnO2 surface was used to

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