Research Article www.acsami.org
High-Throughput Screening and Optimization of Binary Quantum Dots Cosensitized Solar Cell Ding Yuan,† Lina Xiao,† Jianheng Luo,‡ Yanhong Luo,‡ Qingbo Meng,‡ Bing-Wei Mao,† and Dongping Zhan*,† †
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡ Key Laboratory for Renewable Energy, Chinese Academy of Sciences, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *
ABSTRACT: Quantum dots (QDs) are considered as the alternative of dye sensitizers for solar cells. However, interfacial construction and evaluation of photocatalytic nanomaterials still remains challenge through the conventional methodology involving demo devices. We propose here a high-throughput screening and optimizing method based on combinatorial chemistry and scanning electrochemical microscopy (SECM). A homogeneous TiO2 catalyst layer is coated on a FTO substrate, which is then covered by a dark mask to expose the photocatalyst array. On each photocatalyst spot, different successive ionic layer adsorption and reaction (SILAR) processes are performed by a programmed solution dispenser to load the binary PbxCd1−xS QDs sensitizers. An optical fiber is employed as the scanning tip of SECM, and the photocatalytic current is recorded during the imaging experiment, through which the optimized technical parameters are figured out. To verify the validity of the combinatorial SECM imaging results, the controlled trials are performed with the corresponding photovoltaic demo devices. The harmonious accordance proved that the methodology based on combinatorial chemistry and SECM is valuable for the interfacial construction, high-throughput screening, and optimization of QDSSCs. Furthermore, the PbxCd1−xS/CdS QDs cosensitized solar cell optimized by SECM achieves a short circuit current density of 24.47 mA/cm2, an open circuit potential of 421 mV, a fill factor of 0.52, and a photovoltaic conversion efficiency of 5.33%. KEYWORDS: scanning electrochemical microscopy, quantum dots sensitized solar cell, high-throughput screening, SECM, QDSSC QDSSCs.17−19 Sometimes it is not the photoelectrode, but the counter electrode that limits the photovoltaic conversion efficiency. To obtain the intrinsic properties of photocatalysts, it is essential to ensure that the counter electrode has sufficient power capacity.20,21 Because of the lack of theoretical guidance, huge workload has to be tried to optimize the technical formula of QDSSCs with the demo photovoltaic devices. Combinatorial chemistry is a subfield of chemistry that aims to combine a small number of chemical reagents, in all combinations defined by a given reaction scheme, to yield a large amount of well-defined products in a form that is easy to screen for properties of interests.22,23 Traced back to the 1960s, it is proposed initially as a chemical synthetic method. With the development of microfabrication techniques, the reaction
1. INTRODUCTION Taking the advantages of tunable absorption spectrum from visible to near IR region, low cost, and easy preparation, QDs have been considered as the alternative of dye sensitizers in the TiO2 nanocatalyst-based solar cell system.1−6 Although the photovoltaic conversion efficiency of QDSSCs was improved rapidly in the past few years, it is still lower than that of the dye sensitized solar cells.7−13 Interfacial design and construction is crucial to improve the performance of QDSSCs.6 For example, the chemical stability of QDs and energy match between the QDs sensitizer and TiO2 nanocatalyst should be fully considered during the preparations of QDSSCs.14,15 On the one hand, the band gap of QDs is tunable in principle by both geometric size and chemical components. However, it is loaded opportunistically in experiments by either the successive ionic layer adsorption and reaction (SILAR) processes or the chemical bath methods.16 On the other hand, the counter electrode plays an important role in the performance of © XXXX American Chemical Society
Received: May 21, 2016 Accepted: June 29, 2016
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DOI: 10.1021/acsami.6b06029 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
procedures, including chemical bath deposition of CdS layer and then SILAR deposition of ZnS passivation layer, were the same as reported before.48 The electrolyte used in QDSSCs was prepared by dissolving 1.0 mol/L Na2S and 1.0 mol/L S in deionized water under the protection of pure nitrogen atmosphere. A Cu/Cu2S foil was adopted as the cathode.18 Preparations of the SECM Substrate. The sintered FTO glass slides with dual layer TiO2 catalyst were prepared as described above. An array of holes was fabricated by penetrating a hot needle (diameter: 500 μm) quickly through a thin but nontransparent black PET film (thickness: 100 μm) with a micromanipulator. The prepared TiO2 photoelectrode was encapsulated by the hole-array PET mask. Thus, only the TiO2 arrays are exposed, and the other parts are isolated from both light and electrolyte. A program-controlled microdispenser (CHI 1550, CHI Instruments Co.) was employed to perform the microSILAR processes: (1) the cations’ precursor solution (0.01 M Pb(NO3)2 + x M CdCl2) was dropped onto the TiO2 spots; (2) after 30 s of standing, the spots are rinsed by deionized water; (3) the equimolar Na2S solution was dropped onto the TiO2 spots; and (4) after 30 s of standing, the TiO2 spots are rinsed by deionized water. Steps 1−4 compose one SILAR circle. Beause the micro-SILAR procedures were performed automatically by a programed microdispenser, the volumes of dispensed microdrops should equal be to each other. Thus, the amount of the precursor can be controlled strictly during the micro-SILAR procedures. To improve the photovoltaic performance, usually a few SILAR circles are needed to ensure both the size and the loading amount of the QDs sensitizer. Finally, the chemical bath for the loading of CdS QDs and the SILAR processes for the loading of ZnS passivation layer were performed as reported before.48 Measurements and Characterizations. A CHI 920c workstation (CH instrument Co.) was employed to perform the SECM imaging experiments.47 An optical fiber (diameter: 200 μm) connected with a xenon light source was adopted as the scanning probe to stimulate the photoelectrocatalytic reaction, and the current flowing through the substrate was collected for SECM imaging. The substrate was well leveled by a screw microcalliper. A force sensor and a video monitor were employed to determine the zero-point where the tip touched right with the isolate film. The optic fiber was withdrawn to a tip−substrate distance of 100 μm. The scanning rate is 300 μm/s during imaging. All of these experimental operations are controlled carefully to ensure the comparability of the imaging results. In the SECM experiments, a platinum (Pt) electrode was employed as both the counter and the reference electrode. The other auxiliary electrochemical experiments were performed on a model 263A potentiostat (Princeton Applied Research Co.). A homemade Solar Cell System with a standard solar simulator (Oriel 91192, intensity: 1.5 AM) was employed in the measurements with demo QDSSC devices. Transmission electron microscopy (TEM, JEM-2100, JEOL Co.), energy dispersive X-ray spectroscopy (EDS, S-4800, Hitachi Co.), Xray diffraction spectra (XRD, X’pert Pro, Panalytical Co.), and X-ray photoelectron spectroscopy (XPS, Quantum 2000, PHI Quantera Co.) were employed to characterize the morphology and components of the quantum dots, which were synthesized and loaded on TiO 2 nanoparticles by SILAR processes.
systems of combinatorial chemistry are miniaturized and built on chip.24,25 Positioning systems are therefore integrated into various analytical instruments to identify what happens in each reaction unit.26−29 Because of its high-throughput screening and optimizing capabilities, combinatorial chemistry is applied rapidly into analytical, material, biological, and pharmic science.23,30−32 In material science, it is employed not only to synthesize new materials, but also to screen the best one with expected properties.23,32 Scanning electrochemical microscopy (SECM) is a special scanning probe technique with the capability to image the local chemical reactivity of substrates.33,34 In 2005, Dr. Bard introduced the methodology of combinatorial chemistry into SECM.35−37 They synthesized metal alloy electrocatalyst array on glassy carbon substrate and imaged their catalytic properties on the oxygen electroreduction in proton exchange membrane fuel cells. To study the photovoltaic conversion system, an array of photoelectrocatalyst spots is prepared with a combinatorial formula on the substrate (e.g., ITO, FTO, Ti slide, etc.). The photocatalytic current feedback is collected by SECM when the substrate is illuminated either by a scanning optic fiber or by a macroscale light source, from which the photocatalytic properties can be judged visually.38−40 By using a redox mediator, the interfacial charge transfer kinetics can also be figured out through the SECM feedback mode.41−46 The advantage of the optic fiber scanning mode lies in that the capacity of counter electrode is in no need of being considered because it is usually much larger in size than the single spots of photocatalyst. To improve the signal/noise ratio, we have proposed a light mask method to isolate the substrate area other than the photocatalyst array.47 In this article, we use this method for the high-throughput screening of the binary QDs, PbxCd1−xS, cosensitized solar cell. The technical parameters are optimized rapidly and also verified by the photovoltaic demo devices. The PbxCd1−xS QDs are actually a solid-state solution, which has a better stability than PbS QDs and a better energy matching to CdS QDs/TiO2 system. The performance of PbxCd1−xS/CdS QDs cosensitized solar cell was promoted with a short circuit current density of 24.47 mA/cm2, an open circuit potential of 421 mV, a fill factor of 0.52, and a photovoltaic conversion efficiency of 5.33%.
2. EXPERIMENTAL SECTION Chemicals and Materials. Sodium sulfide (Na2S), sulfur (S), ammonium chloride (NH4Cl), hydrochloric acid (HCl), lead nitrate (Pb(NO3)2), hydrate cadmium chloride (CdCl2·2H2O), hydrate zinc acetate (Zn(Ac)·2H2O), and hydrate sodium sulfide (Na2S·9H2O) were of analytical grade and provided by Sinopharm Co., China. FTO conductive glass slides (thickness 2.2 mm, 14 Ω/cm2) were purchased from Jing Ge Solar Tech Co., Wuhan. All aqueous solutions were prepared with deionized water (18.2 MΩ cm, Milli-Q, Millipore Co.). Preparations of the Demo QDSSCs. A dual-layer TiO2 catalyst was deposited on a FTO glass slide by doctor blading technique: the inner layer was 10-μm thick with 15 nm-diameter TiO2 nanoparticles and the outer layer was 15-μm thick with 300 nm-diameter TiO2 nanoparticles (see Figure S1). The raw TiO2 photoelectrode was sintered at 450 °C for 30 min in a muffle furnace. After cooling to ambient temperature, it was immersed in an aqueous solution containing a specified molar ratio of Pb(NO3)2 over CdCl2 and then in a Na2S aqueous solution for 30 s, alternately, to finish one SILAR circle. After each immersion, the TiO2 photoelectrode was washed thoroughly with deionized water. During the SILAR procedure, the total molar ratio of cations (Pb2+ and Cd2+) over inion (S2−) was 1:1 to form PbxCd1−xS QDs on TiO2 nanoparticles. The subsequent
3. RESULTS AND DISCUSSION SILAR is the best empirical approach to control the sizes and loading amount of QDs sensitizer on TiO2 nanocatalyst. Figure 1 shows the HRTEM images of PbS and PbxCd1−xS QDs deposited on the TiO2 photoelectrode. It is observed that the QDs formed on the surface of TiO2 nanoparticles are homogeneous with an average size of 5 nm. Although the sizes of different QDs are very close to each other, it should be noted that the lattice constant decreases from 0.342 to 0.322 nm, which indicates the formation of PbxCd1−xS QDs.49,50 Because the solubility products (Ksp) of PbS and CdS are 8.0 × 10−28 and 8.0 × 10−27, respectively,51 Pb2+ and Cd2+ can B
DOI: 10.1021/acsami.6b06029 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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results (see Figure S2) show that the characteristic peaks (2θ: 31°) and the half-peak width of PbS remain the same if CdS QDs were deposited separately after the deposition of PbS. However, when Pb2+ and Cd2+ are coprecipitated by S2−, the XRD peak mentioned above shifts and the half-peak width becomes wider (see Figure S2). A similar phenomenon was observed for the characteristic peaks at 2θ of 44°. The individual peak and the enhanced intensities at 2θ of both 31° and 44° indicate the crystalline structure should be homogeneous. The results show that both Cd2+ and Pb2+ occupy the lattice sites of the PbxCd1−xS solid-solution QDs.53 The schematic diagram of the SECM imaging is depicted in Figure 2b. Because of the light mask, all of the area is isolated from the electrolyte and illumination light except the PbxCd1−xS/CdS QDs cosensitized TiO2 nanocatalyst array. The background caused by the electric double-layer charging current and the photocatalytic current of FTO itself at the nonspot area are therefore eliminated. Thus, the signal/ratio is pretty high as reported by us previously.47 When the optical fiber is scanning above the spots, the PbxCd1−xS/CdS QDs cosensitized TiO2 nanocatalyst will be illuminated and give out the photocurrent feedback. From the SECM images shown in Figure 2c, it is observed that, because of the light mask, the background of the photocurrent is almost zero, and the signal/ noise ratio is pretty high as reported previously.47 The first spot from left giving a photocurrent of 6.5 μA is actually the PbS/ CdS QDs cosensitized TiO2 nanocatalyst. When the precursor solution contains 0.20−0.50 M CdS, the photocurrent can be increased doubly as an average of 12.5 μA. The results show that the binary PbxCd1−xS QDs is a better sensitizer than PbS
Figure 1. HRTEM images of PbS QDs (a and b) and TiO2/PbxCd1−xS QDs (c and d) loaded on the TiO2 nanoparticles with 5 SILAR cycles. The precursor solution contains 0.01 M Pb(NO3)2 and 0.3 M CdCl2.
coprecipitate in the presence of S2−, and, in principle, the content of Pb2+ should be higher than that of Cd2+ (see Tables S1−S3). Moreover, due to the lattice size of PbS crystal (5.91 Å) being very close to that of CdS crystal (5.82 Å), a solid solution will be formed during the SILAR processes.52 XRD
Figure 2. (a) Schematic diagram of the TiO2/PbxCd1−xS/CdS/ZnS photoelectrode; (b) schematic diagram of the SECM imaging mode with an optical fiber as the scanning tip; and (c) SECM images of local photocurrent of the TiO2/PbxCd1−xS/CdS/ZnS catalyst array. Substrate potential, 0.2 V vs Pt electrode; illumination intensity, 50 mW/cm2; tip−substrate distance, 100 μm; and scanning rate, 300 μm/s; the electrolyte is an aqueous solution with 0.1 M Na2S and 0.1 M S. C
DOI: 10.1021/acsami.6b06029 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. SECM images of the TiO2/PbxCd1−xS/CdS/ZnS nanocatalyst array prepared with different SILAR circles at fixed Cd2+ concentration in the precursor solutions: (a) 0.2 M Cd2+, (b) 0.3 M Cd2+, (c) 0.4 M Cd2+. Substrate potential, 0.2 V vs Pt electrode; illumination intensity, 50 mW/ cm2; tip−substrate distance, 100 μm; and scanning rate, 300 μm/s; the electrolyte is an aqueous solution with 0.1 M Na2S and 0.1 M S.
Figure 4. SECM images of the TiO2/PbxCd1−xS/CdS/ZnS nanocatalyst array prepared with different Cd2+ concentrations in the precursor solutions at fixed SILAR circles: (a) 3 cycles; (b) 5 cycles; (c) 7 cycles. Substrate potential, 0.2 V vs Pt electrode; illumination intensity, 50 mW/cm2; tip− substrate distance, 100 μm; and scanning rate, 300 μm/s; the electrolyte is an aqueous solution with 0.1 M Na2S and 0.1 M S.
concentration of Cd2+ in the precursor solutions. The photocurrents of the spots with 5 SILAR circles are higher than those with 3 and 7 SILAR circles. Thus, the combinatorial screening is helpful to optimize the technical parameters: 0.3 M Cd2+ concentration in the precursor solution and 5 SILAR circles. To verify the results of the combinatorial screening by SECM imaging mode, the controlled trials are performed with the corresponding photovoltaic demo devices. The photovoltaic conversion efficiencies of the demo devices are shown in Figure 5, which is in harmonious accordance with the combinatorial SECM imaging results shown in Figure 4. At the fixed precursor solution, the demo devices with 5 SILAR circles
QDs, and the SECM imaging mode is proved effective in the screening of the QDs sensitizer for QDSSCs. Combinatorial chemistry methodology is employed now to scheme and optimize the technical processes for the interfacial construction of the QDs sensitized TiO2 nanocatalyst.47 Figures 3 and 4 show the SECM images of the TiO2 nanocatalyst spots with different precursor solutions and different SILAR circles. As observed in Figure 3, at a fixed precursor solution, the photocurrent increases first and then decreases with the SILAR circles. With 0.3 M Cd2+ in the precursor solution, the photocurrent is improved obviously with 4−7 SILAR circles. As observed in Figure 4, at the fixed SILAR circles, the photocurrent increases first and then decreases with the D
DOI: 10.1021/acsami.6b06029 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. Photovoltaic conversion efficiencies of the binary PbxCd1−xS/CdS/ZnS QDSSCs demo devices with different precursor solutions and different SILAR circles.
Figure 6. J−V curves of (a) TiO2/PbS/CdS/ZnS and (b) TiO2/PbxCd1−xS/CdS/ZnS QDSSCs demo devices with different SILAR cycles. The counter electrode is a Cu/Cu2S electrode, and the aqueous electrolyte contains 1 M Na2S and 1 M S.
Figure 7. (a) Absorbance spectra of TiO2/PbS, TiO2/CdS, and TiO2/PbxCd1−xS nanocatalysts. The inset shows the estimation of Eg values of the QDs. (b) Schematic diagram of electron transfer at the boundaries between the QDs cosensitizer, CdS QDs, and TiO2 nanocatalyst.
present the best photovoltaic conversion efficiency. At the fixed SILAR circles, the photovoltaic conversion efficiency increases first and then decreases with the increased Cd2+ concentration in the precursor solution. With 0.3 M Cd2+ precursor solution, the demo devices give the best performance. Because it is a huge workload for the optimization of multicomponent QDs sensitized solar cells with demo devices, the combinatorial SECM screening provides a facile method with high throughput.
Further experiments are performed (Figure 6) to compare the device performance sensitized by PbS/CdS QDs (i.e., 0 M Cd2+ in the precursor solution) and PbxCd1−xS/CdS QDs (0.3 M Cd2+ in the precursor solution) with 3, 5, and 7 SILAR circles. It is observed that the binary PbxCd1−xS QDs can promote the device performance more dramatically than PbS QDs. The short circuit current density is improved from 14.66 mA/cm2 for PbS/CdS QDs to 24.47 mA/cm2 for PbxCd1−xS/ CdS QDs, and the photovoltaic conversion efficiency is improved from 2.66% to 5.33% (see Table S4). Figure 7 may E
DOI: 10.1021/acsami.6b06029 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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be helpful to understand why PbxCd1−xS QDs are better than PbS QDs. This is actually caused by the energy matching between the cosensitizers, CdS QDs, and TiO2 nanocatalyst. Comparing with the previous report in which PbxCd1−xS QDs is also adopted as the cosencitizer, the energy band of the binary QDs is much closer to that of CdS QDs because the Cd content is found much higher than Pb content.51 In our experiment, the Cd content is found much lower than Pb content (see Table S1). As shown in Figure 7, the property of our binary PbxCd1−xS QDs is closer to that of the PbS QDs. Nevertheless, the performances of QDSSCs cosensitized by PbxCd1−xS QDs are better than those cosensitized by PbS QDs. Further discussion should be addressed on the empirical SILAR processes in the interfacial construction of the photoelectrodes. The size of the QDs sensitizer as well as the loading amount will increase with the increasing SILAR circles, which determines their energy band gap and the enhancement of photovoltaic conversion. There should be an optimal size that matches well with both the CdS QDs and the TiO2 photocatalyst. On the other hand, if the loading amount is too small, the charge transfer efficiency is limited by the insufficient amount of photogenerated charges; if the loading amount is too high, the mass transfer should be blocked by the loaded QDs. That is why the photocurrent increases first and then decreases with the increased SILAR circles. The size as well as the loading amount of the QDs cosensitizer are the key issues to the performance of QDSSCs. Future study should be emphasized on the theoretical predictions on the size and loading amount, and how to realize it in the interfacial construction of the photoelectrode. Recently, the QDs cosensitizers are becoming more multicomponent.9 Therefore, the proposed highthroughput screening method based on SECM imaging mode and combinatorial chemistry methodology has a prospective potential in optimizing the technical protocols of QDSSCs.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support of the National Basic Research Program of China (2012CB932902) and the National Natural Science Foundation of China (91323303, 21327002, 21573054 and 21321062) is appreciated.
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REFERENCES
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4. CONCLUSIONS Combining SECM imaging mode with combinatorial chemistry methodology, we proposed a high-throughput and high signal/ noise ratio screening method for the optimization of the binary QDs cosensitized solar cells. Meanwhile, we designed an intermediate layer, the PbxCd1−xS QDs, as the cosensitizer between the CdS sensitizer and TiO2 nanocatalyst. On the basis of the optimized technical protocols screened by SECM, the PbxCd1−xS/CdS QDs cosensitized TiO2 soalr cell achieves a short circuit current density of 24.47 mA/cm2, an open circuit potential of 421 mV, a fill factor of 0.52, and a photovoltaic conversion efficiency of 5.33%. We consider that the size and the loading amount of QDs cosensitizer are the key issues in the interfacial construction of the photoelectrode. Because the QDs cosensitizers are becoming more multicomponent, the proposed combinatorial SECM screening method is a good choice due to its high throughput, high efficiency, and high signal/noise ratio.
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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/acsami.6b06029. SEM images and XRD spectra of the TiO2 nanocatalyst and photoelectrodes, EDS data of the PbxCd1−xS QDs, and detailed data about the performance of demo devices (PDF) F
DOI: 10.1021/acsami.6b06029 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.6b06029 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
