Strain-Induced Type II Band Alignment Control in CdSe Nanoplatelet

May 10, 2017 - When the photovoltaic (PV) performance was measured, the polysulfide electrolyte (2 M Na2S, 2 M S, and 0.2 M KCl in a H2O–methanol ...
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Strain-Induced Type II Band Alignment Control in CdSe Nanoplatelet/ZnS-Sensitized Solar Cells Songping Luo,†,§ Miri Kazes,*,‡,§ Hong Lin,*,† and Dan Oron‡ †

State Key Laboratory of New Ceramics & Fine Processing, School of Material Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel S Supporting Information *

ABSTRACT: Colloidal CdSe nanoplatelets (NPLs) deposited on TiO2 and overcoated by ZnS were explored as light absorbers in semiconductor-sensitized solar cells (SSSCs). Significant red shifts of both absorption and steady-state photoluminescence (PL) along with rapid PL quenching suggest a type II band alignment at the interface of the CdSe NPL and the ZnS barrier layer grown on the NPL layer, as confirmed by energy band measurements. The considerable red shift leads to enhanced spectral absorption coverage. Cell characterization shows an increased open-circuit voltage of 664 mV using a polysulfide electrolyte, which can be attributed to a photoinduced dipole effect created by the spatial charge separation across the nanoplatelet sensitizers. The observed short-circuit current density of 11.14 mA cm−2 approaches the maximal theoretical current density for this choice of absorber, yielding an internal quantum efficiency of close to 100%, a clear signature of excellent charge transport and collection yields. With their steep absorption onset and negligible inhomogeneous broadening, NPL-based SSSCs are intriguing candidates for future high-voltage sensitized cells.

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interfaces of the cell.4,8 Meanwhile, we have shown previously that the open-circuit voltage could be improved by using the photoinduced dipole (PID) effect.9,10 This effect primarily originates from the type II sensitization, enabling efficient spatial separation of photogenerated electrons and holes across the photoanode as best shown for a ZnTe/CdSe core/shell QDSSC.11 Additionally, it has been shown that utilizing elongated CdSe nanorods (NRs) rather than spherical QDs in sensitized solar cells (SSCs) can enable better absorption for the same loading ratio and also improve the charge separation properties due to the buildup of a dipole moment along the length of the NR.12 Colloidal nanoplatelets (NPLs) are a relatively new class of semiconductor nanocrystals which have a well-defined thickness of only a few atomic monolayers and tens of nanometers in lateral dimensions.13 This geometry gives rise to 2D electronic confinement with a continuous density of states, and allows for an inherently high carrier density upon optical excitation and reduced nonradiative Auger recombination rates.14 In addition, NPLs possess significantly increased intrinsic linear absorption per unit cell which also does not considerably drop toward the exciton band edge, making NPLs more efficient light absorbers as compared to quantum dots or nanorods.15 Moreover, they exhibit extremely small inhomogeneous broadening since the number of layers, determining the degree of quantum confinement, is identical within the entire ensemble. This results in a desired sharp absorption band edge.

INTRODUCTION The utilization of colloidal semiconductor nanocrystals as sensitizers in solar cells has long been sought after owing to their low cost and ease of processing. The familiar semiconductor-sensitized solar cells (SSSCs) are based on quantum dots (QDs), which exhibit high molar extinction coefficients, multiple exciton generation (MEG), readily tunable optical and electrical properties by adjusting the sizes and shapes, and large dipole moments for enhanced charge separation.1−3 Significant advances have been made for quantum-dot-based solar cells, both in a quantum dot solid architecture and in sensitized solar cells (QDSSCs), with the highest reported efficiency exceeding 12%.4 Despite the photovoltaic performance improvement of QDSSCs, their efficiencies are still below those of dyesensitized solar cells and the organometal halide perovskite solar cells.5,6 One of the reasons for hindered improvements in efficiency is unwanted recombination at the interfaces. Another is the characteristic band tail in their absorption edge due to inhomogeneous broadening predominately caused by inhomogeneity in size and shape, which can result in an effective lower conduction band (CB) edge, leading to transitions into subband-gap states in the photoanode and a buildup of capacitance in the photoanode.7,8 The major obstacle is, however, the lack of a suitable electrolyte that will allow improvement of the photovoltage. Overcoming these may lead to further improvement in the efficiency of SSSCs. The current best semiconductor-sensitized solar cell is based on a Zn−Cu−In−Se QD-sensitized TiO2 photoanode and polysulfide electrolyte with N-doped mesoporous carbon as the counter electrode that benefits from both the Zn−Cu−In−Se QD near-IR (NIR) absorber and suppression of charge recombination at different © 2017 American Chemical Society

Received: March 15, 2017 Revised: April 25, 2017 Published: May 10, 2017 11136

DOI: 10.1021/acs.jpcc.7b02460 J. Phys. Chem. C 2017, 121, 11136−11143

Article

The Journal of Physical Chemistry C

with a distance of 0.5−1 cm between the electrodes with an ITO counter electrode. ZnS Coating. A ZnS coating was deposited onto the CdSe NPL-sensitized electrode by a method of successive ionic layer adsorption and reaction (SILAR). Specifically, the sensitized electrode was first dipped into an aqueous solution of Zn(NO3)2·6H2O (either 0.1 or 0.2 M) for 1 min followed by washing with deionized water, then dipped again into a methanol solution of Na2S·9H2O (0.1 M) for 1.5 min, and after that rinsed by methanol thoroughly. This cycle was repeated 3−6 times to obtain a photoluminescence emission around 620 nm. Counter Electrode Preparation. A PbS counter electrode was prepared according to the procedure published by Tachan and co-workers.19 Briefly, a lead foil was pretreated by polishing with a sand paper and then washed with deionized water. After that, it was immersed in concentrated H2SO4 solution (water:acid volume ratio 1:1) for 24 h at room temperature. The foil was subsequently immersed into the polysulfide solution (1 M Na2S, 0.1 M sulfur, and 0.1 M NaOH dissolved in deionized water) for 24 h, rinsed with deionized water, and dried in an air stream. Device Fabrication. The cells were fabricated by assembling the PbS counter electrode and respective photoanodes with Scotch tape as spacer. When the photovoltaic (PV) performance was measured, the polysulfide electrolyte (2 M Na2S, 2 M S, and 0.2 M KCl in a H2O−methanol mixture) was injected. The volume ratio of water and methanol was 3:7. Characterization. The thickness of the TiO2 films was measured with a high-resolution surface profiler (XP-1, AMBIOS, United States). Raman spectroscopy (HORIBA Jobin Yvon HR800, France) was employed to investigate the vibrational properties of the electrodes at room temperature with a 488 nm laser. The absorption properties were measured by a UV/vis/NIR spectrophotometer (Lambda 950, PerkinElmer, United States). Steady-state and time-resolved PL spectra were obtained with a fluorescence spectrometer (FLS920, Edinburgh Instruments, United Kingdom). Highresolution transmission electron microscopy (HRTEM) measurements were executed by a JEM-2100 electron microscope (JEOL, Japan). The X-ray photoelectron spectroscopy (XPS) valence spectra were obtained on an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific Inc., United States) whose work function is 4.75 eV. Photovoltaic performances of all cells were measured under a solar simulator (91192, Oriel, United States) equipped with a 450 W xenon lamp (Newport 69920) running AM 1.5 100 mW cm−2 illumination and a Keithley 2400 source meter for data collection. Incident photon-to-current conversion efficiency (IPCE) spectra were tested by the IPCE measurement system (QEX10 PV, PV Measurements Inc., Unites States). The absorption properties in percentage form were acquired by subtracting the scattering spectrum as obtained from a bare TiO2 electrode. When this is done, the scattering spectrum (but not the magnitude) is assumed to be approximately independent of the deposition of CdSe and ZnS. Electrochemical impedance spectroscopy (EIS) ranging from 106 to 0.1 Hz was performed with a CHI650C electrochemical workstation (Shanghai Chenhua, China) in the dark with a magnitude of the modulation signal of 5 mV.

The above-mentioned properties identify NPLs as intriguing candidates for light harvesting in solar cells. However, integration of NPL sensitizers into photovoltaic devices has not yet been reported. In this work, we report on SSSCs consisting of colloidal CdSe NPLs deposited on a porous TiO2 film by electrophoresis and overcoated with a ZnS layer, showing excellent charge transfer and absorption properties. Absorption and photoluminescence (PL) spectra of these electrodes surprisingly bear close resemblance to a comparable colloidal system of CdSe/ ZnS core/shell NPLs, which exhibit a considerable red shift of both absorption and PL spectra of about ∼65−85 nm upon shell growth, about 8 times larger than for CdSe/ZnS QDs and quantum rods (QRs).7,16 Previously, this pronounced spectral shift in NPLs was explained by changes in the dielectric confinement of excitons within the NPLs.13,17,18 However, we observe an increased photovoltage in our sensitized cells, which leads us to suggest that a type II band alignment is created at the CdSe/ZnS interface, leading to the formation of a photoinduced dipole.9,10 This is supported by a direct X-ray photoelectron spectroscopy study of the energy band positions in the hybrid structures. NPLs thus seem to offer additional flexibility in the control of band offsets in sensitized cells via the introduction of the strain degree of freedom. Finally, we demonstrate the construction of an NPL-sensitized solar cell with a short-circuit current exceeding 10 mA/cm2 despite the relatively high band gap of the absorbing structures, showing near-unity internal quantum efficiency in current collection. With an appropriate high-voltage electrolyte, NPLs could be an interesting candidate for high-voltage cells.

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EXPERIMENTAL METHODS Synthesis and Purification of CdSe NPLs. The 550 nm emitting CdSe NPL synthesis was adopted from Ithurria et al.13 In detail, a mixture of 0.3 mmol of cadmium myristate (Cd(Myr)2) and 15 mL of octadecene (ODE) was put in a flask and degassed under vacuum at 100 °C for 1 h. The temperature was then raised to 240 °C under an argon flow. A solution of 0.15 mmol of selenium dispersed in 1 mL of ODE was rapidly injected at 240 °C. After ∼20 s, 0.3 mmol of cadmium acetate (Cd(Ac)2) was dispensed into the flask. After ∼10 min, 4 mL of oleic acid (OA) was injected into the flask, and the growth was quenched by lowering the temperature. The NPLs were then precipitated from a hexane/ethanol mixture and redispersed in hexane. TiO2 Electrode Preparation. TiO2 films (active area of 1 cm2) were prepared by screen-printing TiO2 paste on fluorinedoped tin oxide (FTO) substrates. The TiO2 paste was prepared by mixing TiO2 nanoparticles with terpineol and ethyl cellulose in ethanol. In this work, screen-printing the paste containing 10 wt % TiO2 nanoparticles and 74.5 wt % terpineol once gave a film thickness of ∼1.6 μm, and ∼3.5 μm TiO2 films can be obtained by performing the process again. TiO2 films of 6.0 μm thickness were prepared from printing another paste containing 18 wt % TiO2 nanoparticles and 73 wt % terpineol. After each screen-printing step, the electrodes were allowed to stand for 6 min, heated to 125 °C for 6 min, and then cooled by air. Electrophoretic Deposition (EPD). First, the TiO2 electrodes were pretreated by baking off at 550 °C for 1 h and then immersed in a 10% by volume 3-mercaptopropionic acid (MPA)/acetonitrile solution for at least 24 h. EPD of NPLs in a hexane solution was performed at a voltage of 500 V 11137

DOI: 10.1021/acs.jpcc.7b02460 J. Phys. Chem. C 2017, 121, 11136−11143

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The Journal of Physical Chemistry C

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RESULTS AND DISCUSSION Figure 1a shows the absorption and PL spectra of a bare CdSe NPL electrode (in black) and the ZnS-coated electrode (in

into the ZnS layer and call for further investigation of the band alignment within this system. To study the photodynamics of this system, we first perform time-resolved PL which should reflect on the kinetics of the charge transfer process at the interface of CdSe NPLs with both ZnS and TiO2. The decay curves (Figure 1b) could be fitted by a triexponential function, and the average PL lifetime could be calculated using eqs 1 and 2 as follows:21 A(t ) = A1e−t / τ1 + A 2 e−t / τ2 + A3e−t / τ3

τ=

(1)

A1τ1 + A 2 τ2 + A3τ3 A1 + A 2 + A3

(2)

The fitting results are summarized in Table 1. Table 1. Emission Decay Analysis of CdSe and CdSe/ZnS Deposited on TiO2 sample TiO2/CdSe TiO2/CdSe/ ZnS

A1

τ1 (ns)

A2

τ2 (ns)

A3

τ3 (ns)

τ (ns)

0.13 0.81

0.08 0.07

0.68 0.17

0.50 0.54

0.19 0.02

3.10 4.08

0.93 0.23

The fitting results (Figure 1b, black lines) show that there are three decay processes with similar rate constants in both systems. However, the relative weights are significantly different. In particular, the initial fast component is much more dominant in the ZnS-coated electrode system. This suggests that the three relaxation processes are decoupled from one another. The initial fast component, which is an order of magnitude shorter than the radiative lifetime of CdSe NPLs in solution, is associated with a rapid, direct electron injection into TiO2.22−24 The slower lifetimes can be associated with unwanted radiative recombination through carrier trapping on the surface of the NPLs and with back-recombination of electrons already injected into the TiO2 with holes left in the NPLs. Notably, even the longest component is much faster than radiative decay of CdSe/ZnS NPLs in solution (20 ns, as shown in Figure 1b). For the ZnS-coated electrode, the relative contribution of both of these unwanted processes is dramatically reduced relative to that of the uncoated electrode. The diminishing long-lived component thus suggests not only a better surface passivation of the NPL but also a significant reduction in the probability of back-recombination, suggesting a modification in the NPL band alignment. In addition, the overall average PL lifetime of the TiO2/CdSe/ZnS electrode (0.23 ns), which is much shorter than that of the TiO2/CdSe electrode (0.93 ns), is another indication of improved charge separation efficiency. The consistent results from both steadystate and time-resolved PL indicate improved charge transfer and separation efficiency were obtained in this system. This conclusion is further supported by the energy band alignment analysis below. To fully characterize the band alignment among TiO2, CdSe NPLs, and ZnS, XPS measurements, complemented by optical absorption spectroscopy, were conducted to study the valence band (VB) and CB positions of these samples. Figure 2a displays the XPS valence spectra of the bare 1.6 μm thick TiO2 film, CdSe NPL-sensitized TiO2 electrode, and CdSe NPLsensitized TiO2 electrode with a ZnS coating layer. The position of the valence band maximum (VBM) of an electrode can be determined on the basis of the intersection of the

Figure 1. (a) Extinction and PL spectra of the CdSe NPL-sensitized TiO2 electrode (black) and CdSe NPL-sensitized TiO2 electrode with ZnS deposition (red). (b) Lifetime measurements of the CdSe NPLsensitized TiO2 electrode and CdSe NPL-sensitized TiO2 electrode with ZnS deposition, light green and red symbols, respectively, along with the three-exponential fit (black line). In addition, the lifetimes of CdSe and CdSe/ZnS NPLs in solution are plotted in green and magenta, respectively.

red). The absorption spectrum of the bare CdSe electrode exhibits distinct narrow peaks, which are typical for NPLs with a band edge excitonic peak at 550 nm corresponding to 5 monolayer (ML) thick CdSe NPLs and a PL spectrum showing a small Stokes shift, with a peak at 554 nm.13,20 Notably, there is almost no luminescence red shift upon deposition on TiO2 despite its relatively high dielectric constant (a refractive index of 2.6 at 600 nm). Further deposition of ZnS by SILAR significantly red-shifted the absorption and PL spectrum, resulting in an emission peak at 614 nm for the CdSe/ZnSsensitized TiO2 electrodes. It should be noted that the PL intensity of the TiO2/CdSe/ZnS electrode is significantly quenched and becomes slightly broader as compared with that of the uncoated TiO2/CdSe. The significant red shift of the absorption and PL spectra and PL intensity quenching of the TiO2/CdSe/ZnS electrode compared to TiO2/CdSe raise the possibility of delocalization of the carriers from CdSe NPLs 11138

DOI: 10.1021/acs.jpcc.7b02460 J. Phys. Chem. C 2017, 121, 11136−11143

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Kubelka−Munk function multiplied by the photon energy (hν) as a function of hν. On the basis of the intersection of the baseline with the tangent line of the spectrum, the Eg of CdSe NPLs is estimated to be 2.22 eV. Interestingly, [F(R)hν]2 of the TiO2/CdSe/ZnS electrode exhibits a band gap of 1.95 eV. In view of the above discussion, the energy band diagram could be drawn as shown in Figure 2c. The VBMs and CBMs of TiO2 and CdSe are in good agreement with literature values, displaying the near Fermi level pinning of the CB of CdSe to the CB of the TiO2.25,26 Also, it is interesting to see that the CB offset from zinc-blende CdSe is indeed a bit smaller than with wurzite CdSe, as expected from their respective VB bulk positions. More importantly, the significant shift upward of the VBM upon ZnS growth strongly indicates that a type II band alignment is formed between the CdSe NPLs and the ZnS overcoating layer, where the electrons stay localized at the CdSe while holes get injected into the ZnS layer. This results in a type II band edge electronic transition of about 1.95 eV, i.e., the measured energy level difference between the CBM of CdSe NPLs and VBM of the hybrid CdSe/ZnS system. Evidently, the energy levels measured by XPS agree well with the above-discussed band gap from Kubelka−Munk theory. A type II behavior at the interface of CdSe NPLs and ZnS can also explain the efficient charge transfer of the ZnS-coated cell as shown from the emission relaxation lifetimes (Figure 1b). Possibly, electron injection into the TiO2 is facilitated by the reduction of the binding energy of the electron following the hole transfer to the ZnS. In addition, the emission lifetimes of CdSe and CdSe/ZnS core/shell NPLs in solution are plotted for comparison (Figure 1b, green and magenta, respectively), showing a prolonged monoexponential radiative decay for the core/shell NPLs as expected from a type II system. Similar observations of a type II spatially indirect transition have been displayed in other CdSe/ZnS systems. The study of Dabbousi et al.27 on CdSe/ZnS core/shell QDs suggested a lattice strain due to the lattice mismatch (12%) between CdSe and ZnS, leading to a relaxed epitaxial growth forming possible dislocation defects at the interface that ultimately result in a decrease in quantum yield (QY). In their work, however, they relate the observed red shift to an increased size distribution of the samples. Shin et al.28 indeed identified the strain at the core/shell interface by atomic resolution microscopy and geometrical phase analysis (GPA). Using the same technique, Mahler et al.29 showed that the deformations inside the CdSe/ CdZnS core/shell nanoplatelet revealed a tetragonal deformation of the CdSe NPL core that allows for epitaxial growth of the CdZnS shell. Smith et al.30 were the first to show the effect of lattice strain on the energy band offsets demonstrated for core/shell growth of the CdTe/ZnSe QD system. A relaxed epitaxial growth will result in a standard type I heterojunction. However, in the case of small QDs, the core and shell can synergistically accommodate the strain, leading to a strained epitaxial growth that converts to type II behavior. For a thickshell type II band diagram, the CB of the core shifts upward, while the CB of the shell gradually shifts down. For a thin-shell type II band diagram both the CB and the VB of the shell shift down with respect to the core. The above-proposed type II band alignment between CdSe NPLs and ZnS is expected to facilitate charge transfer and reduce back-recombination of electrons in the TiO2, leading to improved photovoltaic performance. Here different thicknesses of porous TiO2 electrodes varying from 1.6 to 6.0 μm were

Figure 2. (a) XPS valence spectra and the tangent lines for spectra of TiO2, TiO2/CdSe, and TiO2/CdSe/ZnS electrodes. (b) [F(R)hν]2 vs hν spectra of TiO2/CdSe and TiO2/CdSe/ZnS electrodes evolving from the Kubelka−Munk function using linear extrapolation for the direct forbidden transition. (c) Energy band diagram for TiO2, CdSe NPLs, and ZnS in this system.

baseline with the tangent line of the XPS spectrum. The measured XPS signals correspond to band-edge levels of the layer which is located at the outmost of the electrode. Considering the work function for the XPS instrument is 4.75 eV, the VBM for TiO2, TiO2/CdSe, and TiO2/CdSe/ZnS is estimated to be −7.40, −6.29, and −6.02 eV (versus vacuum), respectively. To determine the positions of the conduction band minimum (CBM), the band gaps (Eg) of the electrodes are required. The band gaps of the TiO2 and ZnS are known to be 3.2 and 3.6 eV, respectively. The band gap of CdSe NPLs can be deduced from the diffuse reflectance spectra (measured with a UV/vis/NIR instrument), and the Kubelka−Munk function (F(R)) was used to convert the reflectance into the equivalent absorption coefficient. Figure 2b shows the square of the 11139

DOI: 10.1021/acs.jpcc.7b02460 J. Phys. Chem. C 2017, 121, 11136−11143

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it to the poor electrolyte penetration into the electrode due to the particular quality of the ZnS deposition. Voc, Jsc, and FF statistics over all measured cells are given in the Supporting Information (Figure S3) along with their correlation statistics. A Voc of 664 mV is about 100 mV higher than that of a similar QD-based cell but is comparable to that of type II QDbased cells.11,32−34 We assign this comparably large Voc to a PID effect created by the improved spatial charge separation across the cell, which we already established in our previous work.9,10 Sensitized cells based on CdSe/ZnS QDs and QRs did not show such a pronounced PID effect, which is in accordance with the negligible PL red shift observed for these systems.12,32,35 Figure 3b shows the schematic illustration of the PID effect. Upon photoexcitation, electrons from CdSe NPLs are injected into the conduction band of TiO2, while the holes are being accumulated at the ZnS layer before being scavenged by the electrolyte. As a result, the negative carriers accumulated in the conduction band of TiO2 and then positively shifted the TiO2 energy bands relative to the vacuum, resulting in a higher Voc. It is noted that when a relatively thinner TiO2 film of 1.6 μm is used, a higher Voc could be obtained. For a 3.5 μm thick electrode, there is already a trade-off between current and voltage since we do not gain much in current but there is already a considerable drop in voltage. The drop in photovoltage is expected after a certain thickness due to increased resistance across the photoanode. Moreover, the highest Jsc of 11.14 mA cm−2 is close to the maximal theoretical current density, corresponding to the photoanode’s absorption threshold of about 650 nm (see Figure 1a).36 Figure 4a,b shows the incident monochromatic photon-tocurrent conversion efficiency (IPCE) spectra of CdSe/ZnSsensitized solar cells and the corresponding absorption spectra. Here we only discuss the photon-to-current properties of solar cells based on TiO2/CdSe/ZnS electrodes because of the poor performance of bare CdSe NPL solar cells. As can be seen from Figure 4a, the IPCE spectra have a good response over the visible wavelengths, which is in good agreement with the trend of Jsc values derived from J−V curves and the absorption spectra (Figure 4b). Also noteworthy is the sharp edge of the IPCE spectrum due to the small inhomogeneous broadening of the NPL sensitizers, allowing for an improved electron injection into the TiO2 CB. When the thickness of the TiO2 film was increased from 1.6 to 3.5 μm, the absorption of the electrodes was increased to ∼50% in the long-wavelength region, resulting also in the improvement of the IPCE values. Upon a further increase in the thickness of the TiO2 film to 6 μm, a small increase in the long-wavelength absorption region is observed. The small increase in absorption is probably related to the NPL coverage in the thicker film by EPD. However, it was accompanied by a decrease in IPCE, suggesting a less efficient carrier extraction probably due to the partial ZnS coating (i.e., only on the outer part of the electrode) along with the increase of resistance across the photoanode. Although elemental

employed in the CdSe NPL-sensitized solar cells to study their photovoltaic performances. Solar cells based on CdSe NPLonly sensitized TiO2 electrodes were also fabricated as a reference. Thicker TiO2 films were not studied because full NPL penetration into the electrode by EPD could not be achieved. It is noteworthy that the active area of all the cells in this work is 1 cm × 1 cm. Figure 3a shows the J−V curves of

Figure 3. (a) J−V characteristics of solar cells based on different CdSe NPL-sensitized photoanodes. (b) Schematic illustration of the photoinduced-dipole (PID) effect in our solar cells before and after light illumination.

the various cells. Photovoltaic characteristic parameters of the cells derived from J−V curves, including the short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), and energy conversion efficiency (η), are summarized in Table 2. Clearly, the photovoltaic performances of CdSe NPLs with the ZnS coating layer sensitized solar cells are much better than those of CdSe NPLs alone. This is not surprising since a ZnS layer has been long proved to protect from chemical degradation by the electrolyte.31 Most significantly, sensitized solar cells of CdSe NPLs with the ZnS coating layer achieved excellent Voc and Jsc values as high as 664 mV and 11.14 mA cm−2, respectively, and an overall conversion efficiency as high as 1.94%, despite the fact that the FF values are relatively low. The low FF is not an intrinsic limitation here, and we attribute

Table 2. Photovoltaic Characteristic Parameters of Cells Fabricated with Different Photoelectrodes photoanode

thickness (μm)

Jsc (mA cm−2)

Voc (mV)

FF

η (%)

Cμ (mF cm−2)

(A) TiO2/CdSe (B) TiO2/CdSe (C) TiO2/CdSe/ZnS (D) TiO2/CdSe/ZnS (E) TiO2/CdSe/ZnS

1.6 3.5 1.6 3.5 6.0

0.36 0.35 8.30 11.14 4.30

289 264 664 636 608

0.38 0.39 0.35 0.25 0.34

0.039 0.036 1.94 1.76 0.88

8.09

11140

2.03 5.18 6.54

DOI: 10.1021/acs.jpcc.7b02460 J. Phys. Chem. C 2017, 121, 11136−11143

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100% in the full response range. This result implies excellent charge transport and collection yields in the cells. To verify that the increase in Voc in the PID cell indeed is a result of the TiO2 CB being pushed up by the PID effect and not due to charge build up in the anode, we calculate the chemical capacitance (Cμ), which reflects the charge-storing capacity of materials with a high density of sub-band-gap states. The chemical capacitance (Cμ) was obtained from the EIS analysis (Figure S4, Supporting Information) using the expression37 Cμ =

⎤ ⎡ α NLe 2 exp⎢ (E F − ECB)⎥ kBT ⎦ ⎣ kBT

(3)

where NL is the total density of the localized states, α is a constant that determines the distribution of electronic states below the conduction band (ECB), and EF is the electron Fermi level. Cμ of the ZnS-coated electrode decreased in comparison with that of the uncoated one, indicating a shift of the CB of the TiO2. Cμ increased progressively with increased TiO2 film thickness as it depends on the volume of the TiO2. As can be seen from Table 2, the trend in Cμ values is in inverse correlation with the Voc for the different cells, suggesting that indeed the increased Voc results from the formation of a PID across the cell.

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CONCLUSIONS Interesting optical and electronic properties were obtained for the TiO2/CdSe NPLs/ZnS electrode, including a red shift of absorption and PL together with a reduced PL lifetime compared with those of the electrode without ZnS. This should be attributed to the type II energy level alignment between CdSe NPLs and ZnS, enabling efficient charge transfer and separation. SSSCs based on CdSe NPL/ZnS-sensitized relatively thinner TiO2 films exhibit better performances, achieving the highest Voc of 664 mV and the highest Jsc of 11.14 mA cm−2, respectively, and eventually an overall conversion efficiency as high as 1.94%. The high Voc is explained by the PID effect and is comparable to the best records for cells of this type. The high Jsc is close to the theoretical limits, corresponding to an IQE approaching 100% as measured. Although we have already made great progress in fabricating an NPL-sensitized solar cell with good performance, there is still space for improvement of this constructed type II system. For example, an increase of the NPL amount on porous TiO2 could be achieved by modification of the TiO2 film or an improved deposition procedure of NPLs, which will improve the absorption properties and suppress recombination in the cell. On the whole, this work allows to further envisioning the design of improved SSSCs, particularly in conjunction with better electrolytes supporting high-voltage cells.

Figure 4. (a) IPCE spectra of CdSe/ZnS-sensitized solar cells based on different TiO2 thicknesses. (b) Corresponding absorption spectra of different photoanodes for the discussed solar cells. (c) IQE curves of CdSe/ZnS-sensitized solar cells based on different TiO2 thicknesses calculated from IPCE and absorption spectra.

analysis carried out on an focused ion beam (FIB) lamella cross section of a 6 μm electrode showed that Cd and Zn are present across the whole depth of the electrode in a relativity constant ratio, because of large variations in the deposition quality, a partial coating cannot be excluded. The internal quantum efficiency (IQE; Figure 4c) represents the ratio of the number of extracted charge carriers and the number of photons absorbed in the active layer, which can be obtained via dividing the IPCE by the absorption. Notably, solar cells based on relatively thinner electrodes achieved remarkably high IQE values, with most values above 80% and even approaching

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02460. Electrode characterization, band-gap energy calculations using the Kubelka−Munk function, statistics of cell performances, and EIS analysis (PDF) 11141

DOI: 10.1021/acs.jpcc.7b02460 J. Phys. Chem. C 2017, 121, 11136−11143

Article

The Journal of Physical Chemistry C

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Miri Kazes: 0000-0003-0796-3945 Hong Lin: 0000-0002-3382-5960 Author Contributions §

S.L. and M.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express our gratitude for the support provided by the Ministry of Science & Technology, Israel, and the Ministry of Science & Technology, People’s Republic of China: the ChinaIsrael Cooperative Scientific Research Fund (Grant 2015DFG52690) and the Joint NSFC-ISF Research Program (Grant 51561145007), jointly funded by the National Natural Science Foundation (NSFC) of China and the Israel Science Foundation (ISF). M.K. acknowledges Dr. Ronit Povitz-Biro for STEM and EDX measurements, Katya Rechav for FIB cross section measurements, and Dr. Hagai Cohen for XPS measurements.

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