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High-efficiency cascade CdS/CdSe quantum dot-sensitized solar cells based on hierarchical tetrapod-like ZnO nanoparticlesw

Downloaded by National Chiao Tung University on 23 September 2012 Published on 27 June 2012 on http://pubs.rsc.org | doi:10.1039/C2CP41760J

Hsin-Ming Cheng,*a Kuo-Yen Huang,b Kun-Mu Lee,c Pyng Yu,a Shih-Chin Lin,d Jin-Hua Huang,b Chun-Guey Wu*e and Jau Tang*a Received 29th May 2012, Accepted 26th June 2012 DOI: 10.1039/c2cp41760j Quantum dot-sensitized solar cells (QDSCs) constructed using cascade CdS/CdSe sensitizers and the novel tetrapod-like ZnO nanoparticles have been fabricated. The cascade co-sensitized QDSCs manifested good electron transfer dynamics and overall power conversion efficiency, compared to single CdS- or CdSe-sensitized cells. The preliminary CdS layer is not only energetically favorable to electron transfer but behaves as a passivation layer to diminish the formation of interfacial defects during CdSe synthesis. On the other hand, the anisotropic tetrapod-like ZnO nanoparticles, with a high electron diffusion coefficient, can afford a better carrier transport than traditional ZnO nanoparticles. The resultant solar cell yielded an excellent performance with a solar power conversion efficiency of 4.24% under simulated one sun (AM1.5G, 100 mW cm2) illumination.

I.

Introduction

Semiconductor quantum dot-sensitized solar cells (QDSCs) have recently attracted considerable attention as promising alternatives to dye-sensitized solar cells (DSCs) because of their unique opto-electronic properties.1–4 Compared with conventional photosensitizers employed in DSCs, quantum dots (QDs) are easier to prepare and cost-effective. Additionally, QDs present higher extinction coefficients,5 and their absorption spectra can be significantly spectrally tuned by band gap engineering via quantum confinement.6 Other beneficial phenomena include multiple exciton generation7 and direct hot carrier transfer8 as well as use of energy transfer-based charge collection,9,10 which is predicted to allow the inherent Shockley–Queisser efficiency limit11 to be exceeded. In recent years, a range of semiconductors have been investigated, including PbS,12 CdS,13 CdSe,14 CdTe,15 Sb2S3,16 Cu2xS,17 (CH3NH3)PbI3,18 CuInS2,19 and some multilayer or hybrid a

Research Center for Applied Sciences (RCAS), Academia Sinica, Taipei 11529, Taiwan. E-mail: [email protected], [email protected]; Fax: +886-3-5750881; Tel: +886-3-5744718 b Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan c Research Center for New Generation Photovoltaics, Department of Chemical and Materials Engineering, National Central University, Taoyuan 32001, Taiwan d Mechnical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan e Research Center for New Generation Photovoltaics, Department of Chemistry, National Central University, Taoyuan 32001, Taiwan. E-mail: [email protected]; Tel: +886-3-4227151 ext. 65903 w Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cp41760j

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sensitizer systems using more than one of these materials.20–31 Among these reports, nanocrystalline TiO2 is mostly used in the photoanode. ZnO is a versatile semiconductor having been reported as an alternative photoelectrode for DSCs32 because ZnO offers a large direct band gap of 3.37 eV, which is similar to TiO2. In addition, ZnO has very high electron mobility for its relatively small electron effective mass as compared to TiO2.33 ZnO also can be tailored to various architectures, zero (0-D) and one dimensional (1-D) nanostructures in particular, which can significantly enhance solar cell performance through offering either a large surface area for sensitizer adsorption or direct transport pathways for photogenerated electrons. ZnO nanoarchitectures indeed show promising designs of the photoelectrodes for improving the photovoltaic performances. However, relative fewer papers13e,14c,d,j,23,24,29,31 have reported so far on QDSCs that employ ZnO photoelectrodes. Among them, an efficiency of 4.15% ZnO QDSCs has been achieved with novel ZnO nanowire arrays as a photoanode.23a Stimulated by the ambition to fabricate high-efficiency QDSCs based on the configuration of ZnO nanostructures, in the present work CdS and CdSe QDs are sequentially deposited onto a novel ZnO photoelectrode composed of tetrapod-like ZnO nanoparticles. The cascade co-sensitized QDSCs manifested a superior electron transfer dynamics and consequent photopower-conversion efficiency, compared to single CdS- or CdSe-sensitized ones. The preliminary CdS layer is not only energetically favorable to electron transfer but behaves as a passivation layer to diminish the formation of interfacial defects during the CdSe synthesis. The evidences are further confirmed by spectroscopy of fluorescence decay and transient absorption. On the other hand, the anisotropic Phys. Chem. Chem. Phys., 2012, 14, 13539–13548

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tetrapod-like ZnO nanoparticles can afford an efficiently direct pathway instead of interparticle hops while using conventional nanoparticles. Thus the prolonged electron diffusion length ensures all photogenerated electrons can be collected. An optimum CdS/CdSe quantum dot-sensitized ZnO solar cell with an energy conversion efficiency as high as 4.24% was consequently achieved in this present work.

II.

Experiments


Preparation of tetrapod-like ZnO nanoparticles and screen-printing pastes In this present study, ZnO nanoparticles were obtained by a novel DC plasma reactor operated at 70 kW and atmospheric pressure. The product contains about 65% tetrapod-shape ZnO nanoparticles (broad size distribution) and the rest of rod-shape nanoparticles. The production of the tetrapod-like ZnO nanoparticles can easily reach 1.2 kg per hour via this method. The detailed synthetic procedure and its relative characterizations were discussed in previous studies.34 The ZnO paste for screen-printing was prepared typically by mixing tetrapod-like ZnO nanoparticles, ethyl cellulose (EC) and terpineol (anhydrous, #86480, Fluka), the detailed procedure is as follows. EC (5–15 mPas, #46070, Fluka) and EC (30–70 mPas, #46080, Fluka) were dissolved in ethanol to yield 10 wt% solution, individually, then 12 g EC (5–15) and 12 g EC (30–70) were added to a round bottomed rotavap flask containing 12 g ZnO nanoparticles and 25 g terpineol. The mixture paste was dispersed in an ultrasonic bath and a rotaryevaporator (BUCHI V850) was used to remove the residual ethanol and water in the mixture. The final formulations of the ZnO pastes were made with a three-roll mill (EXAKT E50). Preparation of CdS/CdSe-sensitized ZnO photoelectrodes The ZnO photoelectrodes were prepared by screen-printing the 0.28 cm2 ZnO films with various thicknesses (7, 13, 20, and 25 mm) on fluorine-doped tin oxide (FTO) substrates (Nippon Sheet Glass Co. Ltd., 10 O/&, 3 mm thickness). The photoelectrodes were then gradually heated under an O2 flow at 350 1C for 30 min to remove the organic materials in the paste. After cooling to room temperature, the ZnO photoelectrodes were then sequentially sensitized with CdS and CdSe QDs. The CdS shell layer was deposited on the ZnO photoelectrodes by successive ion layer absorption and reaction (SILAR) as previously reported.14c Typically, the ZnO photoelectrodes were immersed in a solution containing 0.02 M cadmium nitrate tetrahydrate (Cd(NO3)24H2O, Aldrich, 499.0%) in methanol for 1 min, to allow Cd2+ to adsorb onto the ZnO, and then rinsed with methanol for 1 min to remove the excess Cd2+. Electrodes were then dried in a gentle stream of N2 (or Ar) gas for 1 min. The dried electrodes were then dipped into a solution containing 0.02 M sodium sulfide nonahydrate (Na2S, Acros Organics) in a mixture of methanol and deionized water (1 : 1, v/v) for 1 min, where the pre-adsorbed Cd2+ reacts with S2 to form the desired CdS. Electrodes were then rinsed in methanol for 1 min and dried again with N2 gas. The CdS loading could be controlled digitally by tuning the number of reaction cycles. Then, CdSe QDs were deposited 13540

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on the ZnO/CdS structure using chemical bath deposition (CBD).23a For the nucleation and growth of CdSe QDs, ZnO/CdS structural films were immersed in an aqueous solution containing Cd(CH3COO)2 (J. T. Baker) : Na2SeSO3 (Aldrich) : NH4OH = 2.5 mM : 2.5 mM : 75 mM for 1 h at the solution temperature of 95 1C. The electrodes were then rinsed with copious amounts of 18.2 MO deionized water (Milli-Q system, Millipore) for 5 min and dried again with N2 gas. The loading of CdSe QDs can also be controlled by changing the number of reaction cycles. Cell fabrication and characterization The QDSCs were basically sandwiched together with several parts. Platinum-coated FTO counter electrodes were fabricated by electron-beam evaporation. The internal space between the ZnO photoelectrode and the counter electrode was separated by a hot-melting spacer (Surlyn, DuPont) of 60 mm thickness, and was filled through a hole with polysulfide electrolytes which were composed of 2 M Na2S (Acros), 2 M S (Acros) and 0.2 M KCl (Showa) in a mixture of methanol and deionized water (7 : 3, v/v). For the photocurrent–voltage (J–V) characteristics and electrochemical impedance spectroscopy (EIS) measurements, a solar simulator (Yamashita Denso, YSS-100A) was used to irradiate the surface of the QDSCs, and the data were collected by a Keithley 2400 source meter and a potentiostats-electrochemistry workstation (Princeton Applied Research, PGSTAT 2273), respectively. The light power was adjusted with a set of neutral density filters and calibrated by an NREL-certificated Si solar cell (Oriel, 91150 V) with a KG-5 filter to the intensity of 100 mW cm2 (the equivalent of one sun at AM 1.5G). The action spectra of the incident monochromatic photon to current conversion efficiency (IPCE) for the solar cells was measured as a function of wavelength from 300 to 900 nm using a specially designed IPCE system (QE-III, Enlitech Inc.) through the direct current (DC) mode. Instrumentation The morphology and composition of ZnO photoelectrodes were characterized using a JEOL-6500 field emission scanning electron microscope (FE-SEM) equipped with an Oxford X-Max silicon drift detector (SDD) for energy-dispersive spectroscopy (EDS) facility. The advanced sensitized-nanostructures were analyzed using a JEOL JEM-2100F field emission transmission electron microscope (FE-TEM) operated at 200 KV. The phases were characterized using a Bede D1 thin film X-ray diffractometer (TF-XRD). The optical absorbance was carried out with a UV-VIS spectrophotometer (Jasco V-670) with a tungsten halogen lamp. The steady-state and time-resolved photoluminescence (PL) spectroscopy were monitored using a monochromator with an electron multiplying charge-coupled device (proEM 512, Princeton Instruments) and a timecorrelated single photon counting (TCSPC) system (PicoHarp 300, PicoQuant), respectively. For excitation, a pulsed diode laser at the wavelength of 532 nm and a repetition rate of 10 MHz was utilized. For transient absorption measurements, a pumping apparatus was employed by a frequency-doubled mode-locked Ti:Sapphire laser (Tsunami, Spectra-Physics) This journal is

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with a Spitfire Pro amplifier and a TOPAS-C optical parameter amplifier to provide 100 femtosecond laser pulses with wavelength ranging from 300 nm to 2000 nm at a 1 kHz repetition. For this study, the single color pump–probe beams (at wavelength 600 nm) were split with a contrast energy ratio of 100 : 1. A perpendicular polarization between the pump and probe beams was employed to minimize the influence from the pump beam scattered light. The probed beam was detected by a photodiode detector (2001-FC, New Focus Inc.), and the data signal was acquired by a personal computer through a lock-in amplifier (SR830, Stanford Research Inc.).

III.

Results and discussion

Fig. 1(a) shows a TEM image of bare tetrapod-like ZnO nanoparticles. Each of the tetrapod-shaped ZnO nanoparticles consists of four crystalline pods with the diameter and length in the range between 30–80 nm and 100–400 nm, respectively. The inset in Fig. 1(a) displays the corresponding selected area electron diffraction (SAED) pattern peculiar to the single crystal wurtzite structure which confirmed the each pod is preferentially oriented in the c-axis direction. Fig. 1(b) and (c), displays the images of the CdS/CdSe-sensitized ZnO nanoparticles

Fig. 1 (a) TEM images of bare tetrapod-like ZnO nanoparticles. (b) and (c) CdS/CdSe-sensitized ZnO nanoparticles. The inset of (a) is the SAED pattern detected from single arm of a tetrapod ZnO nanoparticles.

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scratched from the photoelectrodes after complete synthesis. The results reveal that the additional nanocrystalline CdS and CdSe QDs were uniformly deposited on the tetrapod-like ZnO nanoparticles. A conformal coating of CdS and CdSe QDs on the tetrapod-like ZnO nanoparticles was also confirmed by XRD (see ESI, Fig. S1w), and the lack of diffraction peaks suggests the as-grown CdS QDs on ZnO are nanocrystalline or even amorphous. The observed broad peaks around 251 and 421, however, correspond to the (111) and (220) planes, respectively, indicates that these CdSe QDs produced via CBD onto the SILAR-deposited CdS layers belong to a cubic phase. An advanced analysis was performed to understand better the coverage of the CdS/CdSe layers. The closer observation of a high resolution TEM image of a low-CdSe covered region (selected on purpose), as shown in Fig. 2(a), disclosed that nanocrystalline CdS shell layers of average thickness B1 nm were deposited over the entire ZnO nanoparticle with moderate uniformity. The zinc blende CdSe QDs with diameters of 4–6 nm were afterwards dispersed onto the outer walls of the ZnO/CdS core/shell. The average thickness of about 6.4 nm for CdS/CdSe sensitizer layers can be obtained by subtracting the dark-field TEM area, the ZnO part (Fig. 2(c)), from the bright-field TEM image (Fig. 2(b)). The result also can be confirmed from the difference between the EDS elemental mapping, as shown in Fig. 2(d)–(g). Nanocrystalline CdS and CdSe QDs are widely used as semiconductor sensitizers for QDSCs because their composite absorption spectrum could cover most of the visible region. The optical absorption spectra of the progressive samples in this present work are shown in Fig. 3(a) in the sequence (from plot a to d) of pure ZnO electrode, CdS-sensitized ZnO electrode, CdS-sensitized ZnO electrode, and CdSe/CdSsensitized ZnO electrode, respectively. The 2 mm-thick ZnO thin films were used in particular to avoid interference in discovering their tendency. The absorption peak at 368 nm mainly results from the intrinsic exciton absorption of ZnO. The absorption onset of CdS-sensitized ZnO electrodes after a five-cycle SILAR process shifts to a wavelength of about 500 nm. The comparative CdSe-sensitized ZnO electrodes obtained after the CBD process extend the absorption onset to even longer wavelengths of about 670 nm. The incorporated amounts of CdS and CdSe are tunable by changing the number of SILAR cycles and the immersion time during CBD, respectively. The excitonic peaks usually observed in colloidal QDs were not detected here, reflecting substantial impurities or a comparatively broad size distribution of QDs in either the SILAR or CBD processes, which is not considered a critical issue for the performance of QDSCs. The corresponding band gaps to above the absorption edges can still can be identified as 2.48 and 1.85 eV for CdS- and CdSesensitized ZnO electrodes, respectively. These values are higher than the band gaps reported for bulk CdS (2.25 eV) and bulk CdSe (1.7 eV),35 indicating that the structures of the two materials are still within the scale of quantum confinement during these solution processes. Interestingly, the light absorption for the complete CdS/CdSe-sensitized ZnO electrodes contributes to the superimposition of the absorption of the CdS- and CdSe-modified ones, but has slightly higher absorbance, especially in the long wavelength at about 450–700 nm. Phys. Chem. Chem. Phys., 2012, 14, 13539–13548

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Fig. 2 (a) HRTEM image of CdS/CdSe-sensitized a ZnO nanoparticle. (b) The bright-field TEM image of CdS/CdSe-sensitized ZnO nanoparticle. (c) The dark-field TEM image based on the ZnO(0002) reflection spot. (d) TEM image of CdS/CdSe-sensitized ZnO nanoparticle. (e)–(g) The corresponding EDS elemental mapping of Zn, Cd, and overlay, respectively.

This result also indicates that the deposition rate of CdSe QDs on the ZnO/CdS electrodes is faster than on the bare ZnO electrodes, which is consistent with the same result discovered from our XRD results and previous reports in the TiO2 system.20,21b The insets in Fig. 3(a) also show photographs of the corresponding samples which change color from gray for the bare ZnO electrodes to orange after deposition of a CdS shell via the SILAR process, and then eventually turn to dark brown following deposition of CdSe QDs via CBD. The relatively fast deposition rate for CdSe QDs on the ZnO/CdS electrodes can be examined from these photographs as well. Fig. 3(b) displays the wavelength distribution of an incident monochromatic photon with the current conversion efficiency (IPCE) spectra of various QDSCs. Due to the UV cut-off effect caused by the FTO substrate, the spectra under 380 nm are deteriorated. However, the photocurrents at the peak at approximately 370 nm still can be detected, which is due to direct light harvesting by the ZnO semiconductor. The IPCE obtained for the CdS-sensitized ZnO QDSCs is quite similar to the bare ZnO one, except for a little hump around 410 nm. On the contrary, the CdSesensitized ZnO QDSCs extends its spectrum to the longer wavelengths. A much higher value can be obtained when the CdSe QDs were deposited on a CdS-coated ZnO photoelectrode, compared to both CdS- and CdSe-sensitized ZnO QDSCs. The range of photocurrents approximately agrees with the corresponding UV-Vis spectrum shown in Fig. 3(a). However, the remarkable increment in current density of the CdS/CdSe-sensitized QDSCs can be interpreted that the preliminary CdS layer between CdSe and ZnO would not restrain the transport of photoexcited electrons from CdSe to ZnO but instead enhance the cascade electron transfer through the Fermi level re-alignment. The observation is 13542


similar to the previous reports by Lee et al. concerning sequential CdS and CdSe QDs on TiO2.21 Fig. 4(a) shows the detailed comparison of the photocurrent– voltage (J–V) characteristics for QDSCs constructed using tetrapod-like ZnO photoelectrodes (14 mm-thick) with CdS and CdSe sensitizers under AM 1.5 full sunlight illumination (100 mW cm2). For the cells based on just CdS-sensitized ZnO photoelectrodes, the spectrum reveals a short-circuit photocurrent density (Jsc) of 1.4 mA cm2, an open-circuit photovoltage (Voc) of 229 mV, a fill factor (FF) of 0.24, and a energy conversion efficiency (Z) of 0.078%; the performance was just slightly higher than the cells based on bare ZnO photoelectrodes as a probable result of insufficient light adsorption for these CdS QDs. For CdSe-sensitized QDSCs, the spectrum reveals that Jsc = 8.94 mA cm2, Voc = 287 mV, FF = 0.2, and Z = 0.52%; the higher photocurrent density and efficiency is because the light absorption of CdSe-sensitized ZnO photoelectrodes was broader than that of the CdSsensitized ones. On the other hand, the slightly lower FF indicates the increasing of carrier recombination between the CdSe QDs attached to the ZnO photoelectrode and the electrolyte. The immediate degradation of the CdS-sensitized QDSCs can also be observed for certain photocorrosion reactions. When complete CdS/CdSe-co-sensitized ZnO photoelectrodes were applied, the photovoltaic performances were significantly enhanced as Jsc = 9.43 mA cm2, Voc = 685 mV, FF = 0.4, and Z = 2.56% for the 1 h CBD process. Furthermore, we have made more than 20 nominally identical devices with a longer CBD period (2 h) for which the improved power conversion efficiencies ranged from 3.5–4.2% can be achieved. The best device shows a static performance of Jsc = 13.85 mA cm2, Voc = 722 mV, FF = 0.42, and Z = 4.24%. The Jsc values of the representative devices agree, This journal is

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Fig. 3 (a) Absorption spectra of B2 mm thick films made of a: an as-prepared ZnO electrode, b: a CdS-sensitized ZnO electrode, c: a CdSe-sensitized ZnO electrode, and d: a CdSe/CdS-sensitized ZnO electrode. Insets are digital camera images of the corresponding samples. (b) IPCE spectra of various QDSCs composed of various photoelectrodes (14 mm-thick).

within measurement error, with those obtained from the measured IPCE spectra. This remarkable performance revealed the comparable advantages for the CdS/CdSe quantum dotsensitized solar cells composed of tetrapod-like ZnO nanoparticles with ZnO nanowire arrays.23a The photovoltaic measurement results for various devices are also listed in Table 1. In general, the current densities for QDSCs are determined by the initial number of photogenerated carriers, the electron injection efficiency from QD-sensitizers to photoelectrodes, and the recombination rate between the injected electrons and holes of excited QDs or redox species in the electrolyte. Based on the assumption of the same injection efficiency for the given QDSCs systems, it is reasonable that the photocurrent density may be directly affected by the variation in the electron recombination rate. The preliminary CdS shells from the SILAR process behave as the conformal passivation layers which promote charge collection and prevent the recapture of the photoinjected electrons by the oxidized species, Sn2 (n = 2–5), within the electrolyte. These additional CdSe QDs with broader light adsorption were then deposited onto CdS-decorated ZnO photoelectrodes, consequently resulting This journal is

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Fig. 4 (a) J–V characteristics for QDSCs constructed using various photoelectrodes (14 mm-thick) under AM 1.5 full sunlight illumination (100 mW cm2). (b) Schematic representation of the charge transfer of the CdS/CdSe-sensitized ZnO photoelectrode and possible electron transport pathway across the tetrapod-like ZnO nanoparticles. Table 1 Photovoltaic parameters of the QDSCs determined by J–V characteristics using various photoanodes (14 mm-thick ZnO photoelectrodes for all present work) Photoelectrode a

ZnO/CdS(5) ZnO/CdSe(2)a ZnO/CdS(5)CdSe(1)a ZnO/CdS(5)CdSe(2)a ZnO NWs/CdSCdSeb

Jsc/mA cm2

Voc/V

FF (%)

Z (%)

1.44 8.94 9.43 13.85 17.3

0.229 0.287 0.685 0.722 0.627

23.8 20.3 39.7 42.4 38.3

0.078 0.52 2.56 4.24 4.15

a

Present work. The number after the designated semiconductors indicates the number of repeated corresponding SILAR (for CdS) or CBD (for CdSe) processes; data recorded under 1 Sun condition (AM 1.5, 100 mW cm2) with shading masks, active area: 0.25 cm2. b Ref. 23a. ZnO QDSCs via very similar process to the present cells, but using ZnO photoanodes composed of 16 mm-long ZnO nanowires (NWs).

in better photovoltaic performance. We noted that the present QDSCs were shunt near the Voc, due to the interfacial recombination process, which can be overcome through the surface-modified engineering by using ZnS passivation or Phys. Chem. Chem. Phys., 2012, 14, 13539–13548

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functional dipole molecules.14i,36 For this present work, the best performance has been achieved when the electrode was prepared by using five SILAR cycles for the CdS layers and then a 2 h immersion time for CBD with the CdSe QDs. We found that an excess immersion time (over 4 h) during CBD apparently degrades the QDSCs’ performance. This might be as a consequence of the unnecessary interface energy transfer for overcrowding CdSe QDs or the enlargement of the CdSe QD size which decreases the driving force for the effective electron transfer because the conduction band edge of large size CdSe QDs located below the ZnO. For the purpose of improving the penetration of the electrolyte in the mesoporous photoelectrodes, methanol–water (7 : 3 by volume) co-solvents have been used in the polysulfide electrolyte by Lee et al.21a The co-solvents, which reduce the surface tension but maintain the ion dissociation of the electrolyte solution, were then widely used in the QDSCs21–26,29,30 as well as in this present work. However, Mora-Sero´ et al.2 have suggested that the methanol oxidation at the photoelectrode could cause a current doubling effect by scavenging photogenerated holes and might consequently lead erroneously to high power conversion efficiencies. To examine these suspicions, we thus compared the QDSCs made with polysulfide electrolytes in co-solvent, pure water and pure methanol, respectively (see ESI, Fig. S2w). The solvent effects were investigated via a solvent exchange experiment with the sequential experimental processes (see the supplementary experiments in the ESIw). The same photoelectrode was used serially to construct these comparative devices in order to minimize the inaccuracy of the J–V characteristics. Two cycles of this experiment have been performed to confirm further the validity and repetition. As shown in Fig. S2,w the cells made of electrolytes with co-solvents exhibit better performances as compared with purely aqueous and methanolic ones. The imperceptible influence of current doubling on the performance of QDSCs has been verified as a result of almost the same Jsc when using co-solvent and pure water; the validity was also confirmed with their corresponding IPCE values. The Voc slightly increased when the aqueous electrolyte was used, implying the band edge shift of ZnO to negative potentials or a more positive redox potential. The origin of this is presently unknown and remains for further investigations. Even though the devices with the aqueous electrolytes have lower efficiencies, this is only due to the decrease in FF that might come from the slightly insufficient solubility of the redox couple. On the other hand, the devices with pure-methanol electrolytes have lower photocurrent densities due to the awful solubility of the redox couple in the electrolytes, for which the turbidity can obviously be seen in the inset of Fig. S2(a).w The incorrect ion ratio and excess precipitation of redox species (Na2S, especially) in the electrolyte decrease Jsc and the overall power conversion efficiency. The numerical data set of Fig. S2 is summarized comparatively in Table S2.w Furthermore, the influence of current doubling on the performance of QDSCs has also been confirmed from the stability of the cells. We found that the device made of electrolytes with co-solvents would tend to be more stable even a few weeks after fabrication (ESI, Fig. S3w). The methanol acts as a hole scavenger and will provide a sacrificial donor and consume itself while a 13544


nonregenerative photooxidation happens. The long-term stability of our QDSCs showed that an efficient hole removal from the excited semiconductor to the redox couple might play a crucial role instead of hole capture during methanol oxidation. Carrier regeneration is dominant within the photochemical conversion. We thus conclude that there is no overestimation in the performances of the ZnO QDSCs owing to current doubling by using a methanol–water co-solvent in a polysulfide electrolyte. The co-solvent system provides a good solubility for the redox couple; meanwhile, it maintains the long-term stability and high efficiency for the ZnO QDSCs. In addition, we also compared the variations in the J–V characteristics of QDSCs depending on the thickness of the tetrapod-like ZnO photoelectrode (ESI, Fig. S4w). The optimal energy conversion efficiencies were achieved with 14 mm-thick ZnO films. It is interesting to note that the influence of the thickness effect on the performance of QDSCs using the tetrapod-like ZnO photoelectrode nanoparticles is relatively obvious in comparison with those that use tetrapod-like ZnO nanoparticles in previous DSC reports.37 A possible explanation is ascribed to the high extinction coefficient and sufficient light-harvesting capability of these semiconductor-sensitized ZnO architectures, even when quite thin photoelectrodes are applied. The degraded performances for thicker ZnO photoelectrodes can be interpreted as a result of incomplete infiltration that causes insufficient penetration of CdSe QDs into the deep areas of the ZnO photoelectrodes (as shown in ESI, Fig. S5w) and finally lower the Jsc and Voc. On the other hand, the thicker photoelectrodes also cause an inhomogeneous density of the incident light that could contribute to the lower electron Fermi level of photoanode. This might be another possible cause of poor QDSCs performance. The optimum photoanode is eventually composed of a 14 mm-thick ZnO electrode consisting of 1 nm-thick CdS layers and CdSe QDs with an average size of 4–6 nm. A simplified diagram describing the mechanism of charge transfer within the cascade CdS/ CdSe QD sensitizers and the charge transport within the ZnO photoelectrodes is also presented as Fig. 4(b). Beyond the improvement by structural engineering, to realize the interfacial electron-transfer, we further analyzed the excited QDs’ deactivation by monitoring the fluorescence decay. Even though the photoluminescence quantum efficiency of CdSe QDs from the CBD process is smaller than that obtained from the traditionally organometallic synthesis, the faint emission around wavelengths from 600 to 700 nm can still be investigated. Meanwhile the yield of the fluorescence decreases substantially for QDs on tetrapod-like ZnO photoelectrodes with respect to QDs on glasses, suggesting the occurrence of PL quenching. Fig. 5(a) shows the time-resolved emission decay of CdSe QDs grown on bare glasses and ZnO photoelectrodes with and without coating preliminary CdS layers, recorded at a wavelength of 655 nm with 532 nm diode laser excitation. A biexponential decay kinetics was found to be satisfactory in the determination of emission lifetimes. The lifetimes and corresponding amplitudes are listed in Table 2. The long lifetime was attributed to electron–hole radiative recombination (in the case of QDs coupled to glass) or a combination of radiative recombination and electron transfer from the QDs to the photoelectrodes (in the case of QDs coupled to ZnO), This journal is

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excited state decay is strongly enhanced for CdSe QDs anchored onto ZnO surfaces, indicative of a faster interfacial electron transfer behaving as a new relaxation process. The short lifetimes approach the limitation of instrument response (0.21 ns for our TCSPC system), and will be discussed detail afterwards using another transient technique. The rate constants can be determined numerically for the charge injection process, ket, by comparing the lifetimes of CdSe on glass and ZnO surface (before and after CdS decoration) by the expression:


Ket ¼

Fig. 5 (a) Emission (at 655 nm) decay of CdSe QDs deposited on glasses, bare ZnO photoelectrodes and CdS-sensitized ZnO photoelectrodes. The excitation wavelength is 532 nm. (b) Transient absorption spectra (photon wavelength at 600 nm) of CdSe QDs attached to various photoelectrodes. The 2 mm-thick ZnO films are, in particular, used to prevent unnecessary light scattering. The solid lines represent the kinetic fit using biexponential decay analysis. The green dashed lines are deconvoluted components corresponding to the spectrum of CdSe-sensitized ZnO photoelectrode. Table 2

Kinetic parameters of the CdSe emission decay analysisa

Photoelectrode

a1

t1/ns

a2

t2/ns

Ket (108/s)

SiO2/CdSe ZnO/CdSe ZnO/CdS/CdSe

0.89 0.98 0.95

0.27 0.24 0.24

0.11 0.02 0.05

2.84 1.98 1.53

— 1.52 3.02

a

Time coefficients, including lifetime and corresponding amplitude, can be obtained from fitting the biphasic decay function as a1et/t1 + a2et/t2. The electron-transfer rate constant was estimated from the formula (1).

whereas the fast decay normally results from either nonradiative energy transfer or defect trapping. The reduction in the radiative decay times from 2.84 ns to 1.98 ns and 1.53 ns for CdSe on bare ZnO and ZnO/CdS photoelectrodes, respectively, suggests PL quenching as mentioned previously from the decreasing fluorescence yields, particularly for ZnO/CdS photoelectrodes. It should be noted that the dynamics of This journal is

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1 1 tðCdSeþZnOÞ tðCdSeþSiO2 Þ

ð1Þ

The electron-transfer rates for CdSe QDs anchor onto bare ZnO and CdS-sensitized ZnO are 3.02 108 and 1.52 108 s1, respectively. The values are similar to the previous report which demonstrated the electron injection of excited CdSe of systematic QD size but on TiO2 photoelectrodes.38 Besides, it is important to note that the ZnO photoelectrode covered with extra CdS SILAR-layers certainly provides more efficient electron injection dynamics, which is twice as high as the rate constant value of the bare ZnO. To obtain further information on the faster carrier dynamics from QDs on ZnO photoelectrode which was indiscernibly obtained from the aforementioned TCSPC method, femtosecond transient absorption spectra (photon wavelength at 600 nm) were collected with a pump–probe spectrometer in combination with an ultrafast amplified Ti:Sapphire laser system. As shown in Fig. 5(b), additional ultra-rapid relaxation processes in timescales of picoseconds can be observed. The amplitude of the transient signal is proportional to the number of excitons in CdSe QD during excitation. This signal then decays when the QD undergoes electron–hole recombination or charge transfer to an accepting species. For CdSe QDs grown on bare glasses, we obtained two rate constants from the transient absorption kinetic decay spectra by fitting with a biexponential decay function. The detailed results are listed in the ESI (Table S2w). For comparison, the decay times have slightly changed for CdSe QDs grown on CdS-sensitized ZnO photoelectrodes, but dramatically decreased for CdSe QDs synthesized directly on bare ZnO. Typically, the timescales on the order of picoseconds for QD systems are widely described as electron transfer, Auger decay, or electron trapping at surface defects.39 In this present study, we collected transient absorption spectra under low pump fluence to maintain small carrier densities per QD to prevent contribution from Auger decay. Moreover, rapid relaxation processes can still be observed with no added electron-injection pathways while using glass substrates as the counterparts. We thus make the rational assumption that these transient absorption kinetic decays attribute to the various trapping processes. Because of the inherently large surface area of metal-oxide nanoparticles and semiconductor QD sensitizers, a large density of defect states at the surface is expected, potentially affecting interfacial charge transfer. Electron transfer dynamics in dye- or semiconductor-sensitized metal oxide systems has been evaluated in terms of Marcus theory.40 ZnO, an intrinsic n-type semiconductor, usually has a high density of intraband defects Phys. Chem. Chem. Phys., 2012, 14, 13539–13548

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(surface states of oxygen vacancy in general) lying below the conduction band edge, which may speed up the electron injection as a higher reaction driving force. The resultant fast decay in transient absorption responds to the relatively higher defect density for CdSe-sensitized ZnO photoelectrodes as compared with CdSe QDs grown on preliminary CdS-sensitized ZnO photoelectrodes. A possible explanation for this is ascribed to the formation of additional nonstoichiometric defects as the interfaces between the ZnO and CdSe while an alkaline environment was applied during the CBD process due to the chemical instability of ZnO to acidic and basic solutions. These interfacial defects also affect the electronic contact between the different nanoparticles, trap the free electrons when they transport through the interconnected network of particles to the outer electric circuit, and even dramatically increase the photocorrosion reactions when cells are operated. On the contrary, with the pre-coated CdS as the passivation layers, the defect density of CdS/CdSe-sensitized ZnO photoelectrodes could be effectively reduced. The important roles of trap states in the charge transfer between various metal-oxide nanoparticles and sensitizers, such as organic molecules and inorganic semiconductors, have been extensively studied.41,42 However, their effect on the injection process still remains a matter of controversy. Accordingly, systematic studies of realizing defect-relative carrier dynamics in QDSCs are underway. From the mutually complementary conclusion of fluorescence decay and transient absorption, we presently interpret the two probabilities for the performance enhancement of QDSCs with preliminary CdS-sensitized ZnO photoelectrodes: (1) A stepwise type-II band structure can be achieved for ZnO/CdS/CdSe QDSCs after band gap alignment as a result of fast charge separation and cascade electron transfer.21,22 (2) The preliminary CdS layers can behave as passivation layers to diminish the formation of interfacial defects during the CBD process and finally reduce the electron trapping of QDSCs. Recently, electrochemical impedance spectroscopy (EIS) measurements have been widely performed in DSCs43 and progressively in QDSCs.28,44,45 Adequate physical models and equivalent circuits have been proposed and widely applied to analyze the electron transport in the photoelectrode and recombination between the photoelectrode and the electrolyte interface. The Nyquist plots of the impedance data for QDSCs composed of tetrapod-like ZnO photoelectrodes were performed by applying a 10 mV ac signal over the frequency range of 102–105 Hz under illumination at the applied bias of Voc. The comparison between QDSCs from the same fabrication procedure but composed of photoelectrodes with commercial ZnO nanoparticles (Aldrich Ltd; spherical shape, average size B30 nm) was also demonstrated as a contrast. The interior parameters of the devices can be further derived by fitting the impedance data of the Nyquist plots to the expressions based on the equivalent circuit of QDSCs as shown in Fig. 6(a). rw = (Rw/LF) is the transport resistance of the electrons in the ZnO electrode; rk = (RkLF) is the charge-transfer resistance of the charge recombination between electrons in the ZnO electrode and Sn2 in the electrolyte. The thickness LF of all photoelectrodes are about 14 mm; Cm = (cmLF) is the chemical capacitance of the ZnO electrode; 13546


Fig. 6 (a) Equivalent circuit model of QDSCs for EIS simulation. (b) Nyquist plots of CdS/CdSe-sensitized QDSCs composed of 14 mm-thick photoelectrodes with commercial ZnO (denoted as n-ZnO) and tetrapod-like ZnO nanoparticles (denoted as T-ZnO), respectively. The solid lines are the fitting results. The inset shows their corresponding Bode phase plots.

Rs is a series resistance for the transport resistance of FTO and all resistances out of the cell; ZN is the impedance of diffusion of Sn2 in the electrolyte; RPt and CPt are the charge-transfer resistance and the interfacial capacitance at the counter electrode (platinized FTO glass)/electrolyte interface, respectively; RFTO and CFTO are the charge-transfer resistance and the interfacial capacitance at the exposed FTO/electrolyte interface, respectively; RFZ and CFZ are the resistance and the capacitance at the FTO/ZnO contact, respectively. The fitted results are also listed in ESI (Table S3w) which include the firstorder reaction rate for the loss of electrons (keff), the effective electron lifetime (teff = 1/keff), the electron transport resistance (Rw), and the charge transfer resistance related to recombination of electrons at the ZnO/electrolyte interface (Rk). The features for Pt counter electrodes are almost imperceptible in the high frequency range as a result of their relatively small resistance. From the Bode phase plots in the inset of Fig. 6(b), the mid-frequency peak slightly shifts to a higher frequency, corresponding to a decrease of the electron lifetime for QDSCs composed of tetrapod-like ZnO nanoparticles. The origin is mainly attributed to a shrinking of the number of interconnections between each tetrapod-like ZnO particle, which means the electron lifetime could be decreased by diminishing the transport distance from ZnO to a conductive electrode (FTO). Moreover, the larger Rk and smaller Rw values for the tetrapod-like ZnO photoelectrodes indicate the lesser interfacial recombination between the ZnO and electrolyte interface, and the superior electron transport, respectively. The effective electron diffusion coefficient (Deff = (Rk/Rw)LF2keff) also is enhanced by an order of magnitude with utilization of tetrapod-like ZnO. The corresponding diffusion length This journal is

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(Ln = (teffDeff)1/2) (26 mm) significantly exceeds LF in this study, indicating that the prepared QDSCs with tetrapod ZnO photoelectrodes are not transport-limited as all photogenerated electrons can be collected. These anisotropic tetrapod nanostructures can afford a direct conduction pathway instead of interparticle hops while using nanoparticles. Hence, the enhancement in the electron transport properties could be regarded as a cause of the improvement in the QDSCs performance. Comparative experiments reported on ZnO photoelectrodes emphasize the importance of improving the photovoltaic performance by suitable structural engineering. Although the efficiency of QDSCs presently cannot compete with DSC systems, further improvements are in progress through association with distinct QD sensitizers; changing different electrolytes and developing more suitable counter electrodes. There is no consensus on a standard configuration for QDSCs among present investigations. We hope the new insights obtained from this work could shed light on the development of inorganic sensitizers and pave the way for improving the design of ZnO nanostructures for the proposed solar cell applications.

IV.

Conclusion

CdS and CdSe QDs are sequentially deposited onto a novel ZnO photoelectrode composed of tetrapod-like ZnO nanoparticles. The cascade co-sensitized QDSCs manifested a good electron transfer dynamics and consequent photopower-conversion efficiency, compared to single CdS- or CdSe-sensitized ones. The preliminary CdS layer is not only energetically favorable to electron transfer but behaves as a passivation layer to diminish the formation of interfacial defects during CdSe synthesis. On the other hand, the anisotropic tetrapod-like ZnO nanoparticles can afford an efficiently direct pathway and hence can facilitate excellent electron transport without undergoing losses at grain boundaries. The unambiguously enhancement of photopowerconversion efficiency was obtained by suitably controlling the amount of CdS/CdSe sensitizers as well as the optimum thickness of ZnO photoelectrodes. Herein, we consequently achieved a new architecture of novel photoelectrodes for QDSCs with energy conversion efficiency more than 4%.

Acknowledgements Authors acknowledge financial support from Academia Sinica and National Science Council (NSC) of Taiwan through project No. NSC-99-2113-M-001-023-MY3 and NSC-99-2221-E-001002-MY3. H. M. Cheng thanks Peter Chen for useful discussions and S. R. Lee for her assistance with TEM. K. M. Lee and C. G. Wu specially thank the NSC of Taiwan to fund the AROPV Lab. The electron microscopy facilities from MCL/ITRI and NTRC/ITRI are also appreciated.

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