Nitrogen-Doped Mesoporous Carbons as Counter Electrodes in

Letter pubs.acs.org/JPCL

Nitrogen-Doped Mesoporous Carbons as Counter Electrodes in Quantum Dot Sensitized Solar Cells with a Conversion Efficiency Exceeding 12% Shuang Jiao,† Jun Du,† Zhonglin Du,† Donghui Long,*,§ Wuyou Jiang,§ Zhenxiao Pan,† Yan Li,† and Xinhua Zhong*,†,‡ †

Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ College of Materials and Energy, South China Agricultural University, 483 Wushan Road, Guangzhou 510642, China § School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: The exploration of catalyst materials for counter electrodes (CEs) in quantum dot sensitized solar cells (QDSCs) that have both high electrocatalytic activity and low charge transfer resistance is always significant yet challenging. In this work, we report the incorporation of nitrogen heteroatoms into carbon lattices leading to nitrogen-doped mesoporous carbon (N-MC) materials with superior catalytic activity when used as CEs in Zn−Cu−In−Se QDSCs. A series of N-MC materials with different nitrogen contents were synthesized by a colloidal silica nanocasting method. Electrochemical measurements revealed that the N-MC with a nitrogen content of 8.58 wt % exhibited the strongest activity in catalyzing the reduction of a polysulfide redox couple (Sn2−/S2−), and therefore, the corresponding QDSC device showed the best photovoltaic performance with an average power conversion efficiency (PCE) of 12.23% and a certified PCE of 12.07% under one full sun illumination, which is a new PCE record for quantum dot based solar cells.

A

Recently, benefiting from mesoporous carbon (MC) CEs with high specific surface area and a Zn−Cu−In−Se QD sensitizer with outstanding light-harvesting properties, a certified PCE of 11.6% has been obtained, which is a new record efficiency for QD based solar cells.15 Unfortunately, this record PCE for QDSCs is still inferior to those of traditional dye sensitized solar cells (DSSCs) and the new star perovskite solar cells (PSCs).25,26 To enhance the performance of QDSCs with a carbonaceous material-based CE, it is an effective way to further enhance the catalytic activity of the carbonaceous materials toward electrolyte reduction. It has been demonstrated that doping of nitrogen heteroatoms into a carbon lattice could improve the catalytic activity of carbonaceous materials.27,28 Besides, the electrical conductivity of carbon materials is also improved in this nitrogen-doping process, which favors the reduction of total internal resistance in CEs.27−29 Until now, numerous results have proved that nitrogen-doped carbonaceous materials can serve on high electrocatalytic electrodes for oxygen reduction in fuel cells 29,30 and triiodide reduction in DSSCs.31−34 To our best knowledge, only a few reports focus

s a representative of low-cost third-generation photovoltaic devices, quantum dot sensitized solar cells (QDSCs) have attracted increasing attention due to the prominent advantages of quantum dot (QD) sensitizers and the potential possibility of multiple exciton generation (MEG), which renders their theoretical power conversion efficiency (PCE) up to 44%.1−6 Typically, a QDSC device is composed of three parts: a QD sensitized TiO2 film photoanode, polysulfide electrolyte, and a counter electrode (CE).2 The role of the CE is to collect electrons from the external circuit and then transfer them into the redox electrolyte by catalyzing reduction of oxidized species in electrolyte.7,8 It has a significant impact on the photovoltaic performance of the QDSCs. The ideal CE materials should exhibit high electrocatalytic activity toward electrolyte regeneration, excellent electrical conductivity (i.e., low sheet resistance), good chemical stability, and low production costs.7−11 To date, many CE catalytic materials such as noble metals, conducting polymers, metallic compounds, carbonaceous materials, and heterostructure materials have been extensively studied.7 Among them, carbonaceous materials (including porous carbon, carbon black, activated carbon, carbon nanotubes, carbon nanofibers, carbon foams, graphene, etc.) would be promising candidates for CE materials, mainly due to their good electrocatalytic properties, competitive prices, and high durability in comparison to other types of CE materials.12−24 © XXXX American Chemical Society

Received: December 7, 2016 Accepted: January 11, 2017 Published: January 11, 2017 559

DOI: 10.1021/acs.jpclett.6b02864 J. Phys. Chem. Lett. 2017, 8, 559−564

Letter

The Journal of Physical Chemistry Letters on the use of nitrogen-doped carbonaceous materials as CEs in QDSCs.35 Herein, nitrogen-doped mesoporous carbons (N-MCs) with different nitrogen contents were prepared via a traditional silica template method using formaldehyde, phenol, and melamine as precursors.36 Following our previous results, these as-prepared N-MCs were supported on Ti mesh substrates, which possess the properties of superior chemical stability as well as excellent electrical conductivity, and were employed as CEs for QDSCs.14,15 As a demonstration, QDSCs were constructed using a Zn−Cu−In-Se QD sensitized TiO2 photoanode and polysulfide electrolyte (Sn2−/S2−). Compared with the reference plain MC based QDSCs, the achieved N-MC based ones present a remarkable enhancement in fill factor (FF). As a result, the average PCE was increased from 11.44 to 12.23% with the adoption of N-MCs as CE catalytic materials, and the obtained PCE is the best photovoltaic performance for QD based solar cells. N-MCs were prepared using a colloidal silica nanocasting route following a previous report.36 In this reaction, formaldehyde, melamine, and phenol served as carbon precursors, while the melamine was used to offer nitrogen atoms for doping. The molar ratio of melamine to phenol (M/P) was adjusted to obtain N-MCs with variable nitrogen contents. The detailed synthetic procedure is available in the Supporting Information (SI). With increasing M/P molar ratio from 0 to 2, the nitrogen contents in N-MC samples enhanced to 8.58 wt % gradually (Table 1), which were measured by elemental

Figure 1. TEM images of (a) pristine MC and (b) representative NMC-8.58%. (c) XRD patterns and (d) Raman spectra of pristine MC and N-MCs with different nitrogen contents.

introduce more disorder in the graphitic structure. These doped nitrogen heteroatoms in MC would spoil the local symmetry of the graphite lattices and increase the edge-plane defect sites, which act as active sites for electrocatalysis in carbon materials.38 Therefore, the more disordered graphitic structure caused by nitrogen doping might be beneficial to polysulfide reduction in QDSCs. To further confirm the subtle structural variation, Raman spectroscopy for N-MCs and reference MC was measured, with results shown in Figure 1d. It can be found that the central positions of characteristic D and G bands for each samples are located at about 1356 and 1592 cm−1, respectively, which do not depend on nitrogen content in the samples. The intensity ratio of the D and G bands (ID/IG) was used to characterize the amount of structural defects. As shown in Figure 1d, the ratio of ID/IG enhances slightly with an increase of nitrogen content, indicating that the density of the defect state of graphite-like layers in N-MC materials increases with the improvement of nitrogen content. This finding is consistent with that of XRD. High-resolution X-ray photoelectron spectroscopy (XPS) was measured to explore the nitrogen bonding configuration for N-MC samples. Taking N-MC-8.58% as an example, the XPS survey spectrum (Figure 2a) possessed three peaks centered at 284.6, 400.1, and 531.4 eV, attributing to C 1s, N 1s, and O 1s, respectively. Through peak fitting, the C 1s peak (Figure 2b) was divided into a main peak and three weak peaks centered at 284.6, 285.2, 286.2, and 289.1 eV, corresponding to a CC bond, CN bond, C−N bond, and C−O bond, respectively.39 This result confirms that most carbon atoms are graphite carbons (sp2 carbons) and that the nitrogen heteroatoms have doped into the graphite-like layers. The N 1s peak (Figure 2c) can be curve-fitted into three peaks located at 398.7 (38.6% area), 400.3 (51.9%), and 401.4 eV (9.5%), corresponding to pyridinic N, pyrrolic N, and graphitic N, respectively.40 As shown in Figure S2, the pyridinic N and the pyrrolic N atoms are located at the edges of the graphite-like layers, while the graphitic N atoms are doped inside of the graphite carbon plane, bonding with three sp2 carbon atoms.41 The presence of pyridinic N and pyrrolic N atoms would introduce more

Table 1. Nitrogen Content in N-MCs from Elemental Analysis Results samples N content (%)

M/P-0.5

M/P-1

M/P-2

2.89

4.76

8.58

analysis. For convenience, N-MCs with various nitrogen contents are denoted as N-MC-x, where x represents the nitrogen content in N-MC samples. The mesoporous structures of the resultant N-MCs and reference MC were investigated by a transmission electron microscope (TEM). As shown in Figure 1a,b, both samples are composed of spherical mesopores with a disordered amorphous carbon structure. Moreover, the mesoporous structure was not obviously changed with the doped nitrogen heteroatoms. This is validated furthermore by the similar BJH pore size distributions and nearly identical BET surface areas (Figure S1, Table S1). The X-ray powder diffraction (XRD) of N-MC and pristine MC samples has been recorded to analyze the subtle structural variation (Figure 1c). As expected, the N-MC samples with different nitrogen contents as well as the MC sample show similar diffraction patterns with wide diffraction peaks located at around 24.5 and 43.7°, which correspond to the reflection of (002) and (100) planes of graphite, respectively.37 This result indicates that these N-MC and MC samples are disordered. With the increase of nitrogen content in N-MC samples from 0 to 8.58 wt %, the positions of the two peaks remain substantially unchanged. However, the (002) diffraction peak broadens gradually. The intensity of the (100) diffraction peak decreases systematically. Because the same carbonization temperature (800 °C) was used in the preparation of these carbonaceous samples, the above changes of XRD pattern suggest that the doping of nitrogen heteroatoms tends to 560

DOI: 10.1021/acs.jpclett.6b02864 J. Phys. Chem. Lett. 2017, 8, 559−564

Letter

The Journal of Physical Chemistry Letters

Figure 3. (a) SEM image of the cross section of the N-MC-8.58%/Ti CE. Photovoltaic properties of Zn−Cu−In−Se QDSCs based on NMC/Ti CEs and reference MC/Ti CE, (b) J−V curves, (c) IPCE spectra of champion QDSCs using different CEs, and (d) certified efficiency of Zn−Cu−In−Se QDSCs based on N-MC-8.58%/Ti CE.

Figure 2. (a) XPS survey spectrum, (b) high-resolution C 1s XPS, (c) high-resolution N 1s XPS of representative N-MC-8.58%, and (d) content distributions of three types of nitrogen atoms in N-MCs with different nitrogen contents.

coefficient, and a high conduction band edge.15,42−45 The current density (J)−photovoltage (V) curves of champion NMC/Ti and reference MC/Ti CE based cells under one full sun illumination are shown in Figure 3b. Average photovoltaic parameters based on five cells were analyzed and are listed in Table 2. The detailed photovoltaic parameters and corresponding J−V curves of individual cells are shown in Table S2 and Figure S3. The QDSCs with the MC/Ti CE showed an average PCE of 11.44% (with Voc = 0.764 V, Jsc = 24.72 mA/cm2, and FF = 0.606, respectively). It is worth noting that this obtained PCE is on the same level as the previously reported best result.15 By replacing the MC/Ti with N-MC/Ti CE, the PCE increased to 12.23% (Jsc = 25.53 mA/cm2, Voc = 0.758 V, FF = 0.632), which is a new photovoltaic record for QD based solar cells. The increased PCE is mainly ascribed to the improvement of FF values with the increase of nitrogen content in the carbonaceous materials. This should be due to the reduction of charge transfer resistance for CEs, as discussed below. The Voc values of the resultant cells are nearly independent of the nitrogen content. As the maximum Voc is determined by the energy difference between the Fermi level of the electron in TiO2 and the redox potential of the redox couple in polysulfide electrolyte; these similar Voc values indicate that nitrogen heteroatoms in N-MCs have no obvious impacts on the Fermi level of TiO2 and the redox potential of the polysulfide electrolyte.46,47 The Jsc values show a slight increase (24.72, 25.44, 25.46, and 25.53, respectively) with the variation of nitrogen content from 0 to 8.58%. To further verify this growth trend of Jsc values, the incident photon conversion efficiency (IPCE) spectra of the cells based on different CEs were tested, and corresponding results are shown in Figure 3c. As expected, similar IPCE curves for all cells are observed. By integrating IPCE curves, the calculated Jsc for pristine MC and N-MC2.89%, N-MC-4.76%, and N-MC-8.58% CE based solar cells are 23.96, 24.59, 25.02, and 25.11 mA cm−2, respectively, which are close to the measured values from J−V results, as shown in Table 2. It is highlighted that the photovoltaic performance of a

disorder to the carbonaceous materials and broaden their Raman peaks. However, the doped graphitic N heteroatoms would shift the Raman peaks. In this work, owing to the same pyrolysis temperature of 800 °C, the distribution of these three N species in each N-MC sample with various nitrogen contents is nearly identical (Figure 2d). Specially, a majority of nitrogen heteroatoms (>90%) are located at the edges of graphite-like layers, corresponding to broadening D bands for N-MCs (Figure 1d). Furthermore, because of the similar distribution of N species in each N-MC sample, the nitrogen content should be the decisive factor affecting their catalytic properties and applications. Following our previous procedure, the prepared N-MCs as well as pristine MC were applied to construct CEs supported on a Ti mesh substrate (referred to a N-MC/Ti and MC/Ti CEs henceforth).14,15 The carbon pastes obtained by dispersing these carbonaceous materials into an ethyl cellulose−terpineol mixture were filled into the Ti mesh substrates via the screen printing process for preparing the CEs. From the cross section image of the representative N-MC-8.58%/Ti CE (Figure 3a), the thickness of the carbon film is about 292 μm, which is very close to the optimal thickness in our previous report.14 In QDSCs, the thickness of the carbon film is a significant factor affecting the catalytic activity of CEs. Generally, the catalytic activity of CEs toward polysulfide reduction is improved with increasing thickness of the carbon film, resulting in low charge transfer resistance and a high FF, short-circuit current density (Jsc), and PCE of the resulting devices. In order to focus on the carbonaceous materials themselves and ensure the reliability of the comparison in different carbonaceous CEs, an identical thickness of carbon films was adopted in the preparation of NMCs and pristine MC based CEs. The obtained N-MC/Ti and reference MC/Ti electrodes were used as CEs for constructing sensitized solar cells using polysulfide/sulfide as the electrolyte and Zn−Cu−In−Se QDs as sensitizers. Here, the Zn−Cu−In−Se QD sensitizer could provide a wide light-harvesting range, a high absorption 561

DOI: 10.1021/acs.jpclett.6b02864 J. Phys. Chem. Lett. 2017, 8, 559−564

Letter

The Journal of Physical Chemistry Letters Table 2. Parameters from J−V Curves and EIS for Different CEsa CE MC N-MC-2.89% N-MC-4.76% N-MC-8.58% certified cell

Jsc (mA cm−2) 24.72 25.44 25.46 25.53 25.21

(24.82) (25.29) (25.56) (25.67)

Voc (V) 0.764 0.761 0.763 0.758 0.765

(0.769) (0.764) (0.768) (0.759)

FF 0.606 0.617 0.626 0.632 0.626

PCE (%)

(0.603) (0.622) (0.633) (0.639)

11.44 11.94 12.16 12.23 12.07

± ± ± ±

0.05 0.07 0.17 0.13

(11.51) (12.02) (12.43) (12.45)

Rs (Ω)

Rct (Ω)

1.23 1.08 1.11 1.04

2.210 1.295 0.953 0.939

a Values outside of the parentheses are average photovoltaic performances and standard deviations for five Zn−Cu−In−Se QDSCs with different CEs in parallel, and the inside are values for champion cells in each group.

Figure 4. Electrochemical properties of different CEs. (a) Nyquist plots, (b) Tafel polarization curves, and (c) cyclic voltammograms.

To further verify the improvement of catalytic activity, Tafel measurements were performed in a symmetric cell. The Tafel curves of N-MC/Ti and pristine MC/Ti CEs are shown in Figure 4b. A continuous increase of exchange current density (J0) is observed with an increase of the nitrogen content in the carbonaceous samples. The relationship between the J0 and Rct can be expressed following eq 17

representative Zn−Cu−In−Se cell based on N-MC-8.58%/Ti CE was certified by the National Center of Supervision and Inspection on Solar Photovoltaic Products Quality of China (CPVT) and the certified PCE was 12.07% (Figure 3d, Table 2). The detailed information is available in the SI. Electrochemical impedance spectroscopy (EIS) is often adopted to explore the origin of the different catalytic activity of CEs in sensitized solar cells.7,9 In this work, EIS measurements of symmetric dummy cells consisting of two identical CEs were carried out at 0 V bias potential with a frequency range of 1 MHz to 0.1 Hz. The resultant Nyquist plots of four different CEs are shown in Figure 4a. Two semicircles were observed in the Nyquist plot for each CE, being related to the resistance and capacitance (R1 and C1) of the solid−solid contact between carbonaceous materials and the Ti mesh substrate and charge transfer resistance and capacitance (Rct and Cce) at the CE/electrolyte interface, respectively.16,47,48 The equivalent circuit shown in Figure S4 was used to fit the obtained EIS data.16,49,50 The key parameters including series resistance (Rs) and charge transfer resistance (Rct) were obtained and are listed in Table 2. Owing to the outstanding conductivity of the Ti mesh, the Rs values of all CEs are small and at a similar level. Rct of the CE decreased with the increase of nitrogen content in carbonaceous materials. Particularly, with the nitrogen content increasing from 0 to 8.58%, the corresponding Rct decreases from 2.210 to 0.939 Ω cm2. Generally, Rct can significantly affect the cell performance from two aspects. First, the internal series resistance (Rseries) of the CE includes both the Rs and Rct at the CE/electrolyte interface (i.e., Rseries = Rs + Rct).22,51 Because the Rs of each CE is similar, the lower Rct can lead to lower Rseries, which is beneficial for the FF. Second, Rct represents the reaction barrier in the polysulfide reduction, which is catalyzed by CE materials at the interface of CE/electrolyte.7,51 Therefore, the lower Rct indicates the effective reduction of Sn2− to S2−, which favors the improvement of photovoltaic performance, especially the Jsc value.

J0 = RT /nFR ct

(1)

where R stands for the universal gas content, T represents the temperature, n stands for the number of electrons contributing to the charge transfer at the interface, and F is Faraday’s constant. Therefore, J0 is inversely proportional to Rct. In this work, the decreasing trend of J0 obtained in Tafel polarization measurements is in agreement with the variation trend of Rct in EIS. This gives further support to the increased FF, Jsc, and PCE of the resultant QDSCs with use of nitrogen-doped carbonaceous CEs.7,8,14 Cyclic voltammogram (CV) measurements were used to further study the electrocatalytic activity of four different CEs toward the reduction of polysulfide electrolyte. The electrochemical behaviors of CEs for the S2−/Sn2− redox couple were tested from 0.5 to −1.5 V with a scan rate of 10 mV/s. As shown in Figure 4c, the CV curve for each CE displays two peaks. The left one (negative current) corresponds to the reduction reaction of Sn2− to S2−, while the other one (positive current) is assigned to the oxidation of S2− in electrolyte.7,35 Because the CE in QDSCs is mainly related to catalyze the reduction of Sn2− in polysulfide electrolyte, it is worth paying special attention to the reduction peak in this work. The current densities at the reduction peaks of the N-MC/Ti CEs are obviously larger than that of the pristine MC/Ti CE, indicating that the introduction of nitrogen heteroatoms improves the catalytic activity of the CEs. In addition, the current density of reduction peaks for different CEs decreased monotonously with increasing nitrogen content in the CEs. This result is consist with the variation tendency of Rct obtained in the EIS measurements. 562

DOI: 10.1021/acs.jpclett.6b02864 J. Phys. Chem. Lett. 2017, 8, 559−564

Letter

The Journal of Physical Chemistry Letters

(8) Meng, K.; Chen, G.; Thampi, K. R. Metal Chalcogenides as Counter Electrode Materials in Quantum Dot Sensitized Solar Cells: a Perspective. J. Mater. Chem. A 2015, 3, 23074−23089. (9) Wu, M.; Lin, X.; Wang, Y.; Ma, T. Counter Electrode Materials Combined with Redox Couples in Dye- and Quantum Dot-Sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 19638−19656. (10) Wu, M.; Lin, X.; Wang, T.; Qiu, J.; Ma, T. Low-Cost DyeSensitized Solar Cell Based on Nine Kinds of Carbon Counter Electrodes. Energy Environ. Sci. 2011, 4, 2308−2315. (11) Wu, M.; Ma, T. Recent Progress of Counter Electrode Catalysts in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 16727− 16742. (12) Fan, S.; Fang, B.; Kim, J. H.; Kim, J.; Yu, J.; Ko, J. Hierarchical Nanostructured Spherical Carbon with Hollow Core/Mesoporous Shell as a Highly Efficient Counter Electrode in CdSe Quantum-DotSensitized Solar Cells. Appl. Phys. Lett. 2010, 96, 063501. (13) Fan, S.; Fang, B.; Kim, J. H.; Jeong, B.; Kim, C.; Yu, J.; Ko, J. Ordered Multimodal Porous Carbon as Highly Efficient Counter Electrodes in Dye-Sensitized and Quantum-Dot Solar Cells. Langmuir 2010, 26, 13644−13649. (14) Du, Z.; Pan, Z.; Fabregat-Santiago, F.; Zhao, K.; Long, D.; Zhang, H.; Zhao, Y.; Zhong, X. H.; Yu, J.; Bisquert, J. Carbon CounterElectrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11%. J. Phys. Chem. Lett. 2016, 7, 3103−3111. (15) Du, J.; Du, Z.; Hu, J.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong, X. H.; et al. Zn−Cu−In−Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201−4209. (16) Yang, Y.; Zhu, L.; Sun, H.; Huang, X.; Luo, Y.; Li, D.; Meng, Q. Composite Counter Electrode Based on Nanoparticulate PbS and Carbon Black: Towards Quantum Dot-Sensitized Solar Cells with Both High Efficiency and Stability. ACS Appl. Mater. Interfaces 2012, 4, 6162−6168. (17) Zhang, X.; Huang, X.; Yang, Y.; Wang, S.; Gong, Y.; Luo, Y.; Li, D.; Meng, Q. Investigation on New CuInS2/Carbon Composite Counter Electrodes for CdS/CdSe Cosensitized Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 5954−5960. (18) Parand, P.; Samadpour, M.; Esfandiar, A.; Iraji Zad, A. Graphene/PbS as a Novel Counter Electrode for Quantum Dot Sensitized Solar Cells. ACS Photonics 2014, 1, 323−330. (19) Zeng, X.; Xiong, D.; Zhang, W.; Ming, L.; Xu, Z.; Huang, Z.; Wang, M.; Chen, W.; Cheng, Y.-B. Spray Deposition of Water-Soluble Multiwall Carbon Nanotube and Cu2ZnSnSe4 Nanoparticle Composites as Highly Efficient Counter Electrodes in a Quantum DotSensitized Solar Cell System. Nanoscale 2013, 5, 6992−6998. (20) Hao, F.; Dong, P.; Zhang, J.; Zhang, Y.; Loya, P. E.; Hauge, R. H.; Li, J.; Lou, J.; Lin, H. High Electrocatalytic Activity of Vertically Aligned Single-Walled Carbon Nanotubes Towards Sulfide Redox Shuttles. Sci. Rep. 2012, 2, 368. (21) Seol, M.; Ramasamy, E.; Lee, J.; Yong, K. Highly Efficient and Durable Quantum Dot Sensitized ZnO Nanowire Solar Cell Using Noble-Metal-Free Counter Electrode. J. Phys. Chem. C 2011, 115, 22018−22024. (22) Sudhagar, P.; Ramasamy, E.; Cho, W.; Lee, J.; Kang, Y. Robust Mesocellular Carbon Foam Counter Electrode for Quantum-Dot Sensitized Solar Cells. Electrochem. Commun. 2011, 13, 34−37. (23) Ganapathy, V.; Kong, E.; Park, Y.; Jang, H. M.; Rhee, S. Cauliflower-Like SnO2 Hollow Microspheres as Anode and Carbon Fiber as Cathode for High Performance Quantum Dot and DyeSensitized Solar Cells. Nanoscale 2014, 6, 3296−3301. (24) Radich, J. G.; Dwyer, R.; Kamat, P. V. Cu2S Reduced Graphene Oxide Composite for High-Efficiency Quantum Dot Solar Cells. Overcoming the Redox Limitations of S2−/Sn2− at the Counter Electrode. J. Phys. Chem. Lett. 2011, 2, 2453−2460. (25) Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.; Hanaya, M. Highly-Efficient Dye-Sensitized Solar Cells with Collaborative Sensitization by Silyl-Anchor and Carboxy-Anchor Dyes. Chem. Commun. 2015, 51, 15894−15897.

In conclusion, N-MCs were prepared via a colloidal silica template approach. The obtained N-MC materials were supported on Ti mesh substrates and used as CEs for QD sensitized solar cells. In comparison to the pristine MC/Ti CE based solar cells, the doped nitrogen heteroatoms in MC can improve the values of Jsc, the FF, and the overall PCE of the resultant QDSCs. By optimizing the nitrogen content in NMCs, the fabricated cell devices achieved an average PCE of 12.23% (Jsc = 25.53 mA/cm2, Voc = 0.758 V, FF = 0.632), which is a new photovoltaic performance record for QD based solar cells. The experimental data of EIS, Tafel polarization, and CV measurements reveal that the better performance of N-MC/Ti CE based QDSCs is mainly ascribed to the higher electrocatalytic activity of the N-MC materials toward polysulfide reduction.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02864. Experimental procedures and additional photovoltaic measurement results (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected]. Tel/Fax: (+86) 21 6425 0281 (D.L.). ORCID

Donghui Long: 0000-0002-3179-4822 Xinhua Zhong: 0000-0002-2062-8773 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research is supported by the Natural Science Foundation of China (no. 91433106, 21573249), the Program of Introducing Talents of Discipline to Universities (B16017), and the Fundamental Research Funds for the Central Universities in China.

■

REFERENCES

(1) Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389−458. (2) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell. Science 2011, 334, 1530−1533. (3) Kamat, P. V. Boosting the Efficiency of Quantum Dot Sensitized Solar Cells Through Modulation of Interfacial Charge Transfer. Acc. Chem. Res. 2012, 45, 1906−1915. (4) Beard, M. C.; Luther, J. M.; Semonin, O. E.; Nozik, A. J. Third Generation Photovoltaics Based on Multiple Exciton Generation in Quantum Confined Semiconductors. Acc. Chem. Res. 2013, 46, 1252− 1260. (5) Carey, G. H.; Abdelhady, A. L.; Ning, Z. J.; Thon, S. M.; Bakr, O. M.; Sargent, E. H. Colloidal Quantum Dot Solar Cells. Chem. Rev. 2015, 115, 12732−12763. (6) Kamat, P. V. Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 908−918. (7) Hwang, I.; Yong, K. Counter Electrodes for Quantum-DotSensitized Solar Cells. ChemElectroChem 2015, 2, 634−653. 563

DOI: 10.1021/acs.jpclett.6b02864 J. Phys. Chem. Lett. 2017, 8, 559−564

Letter

The Journal of Physical Chemistry Letters (26) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-Performance Photovoltaic Perovskite Layers Fabricated Through Intramolecular Exchange. Science 2015, 348, 1234−1237. (27) Yu, D.; Nagelli, E.; Du, F.; Dai, L. Metal-Free Carbon Nanomaterials Become More Active than Metal Catalysts and Last Longer. J. Phys. Chem. Lett. 2010, 1, 2165−2173. (28) Wang, H. B.; Maiyalagan, T.; Wang, X. Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications. ACS Catal. 2012, 2, 781−794. (29) Yang, S.; Feng, X.; Wang, X.; Müllen, K. Graphene-Based Carbon Nitride Nanosheets as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reactions. Angew. Chem., Int. Ed. 2011, 50, 5339− 5343. (30) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Jin, Y.; Qiao, S. Nanostructured Metal-Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction. Small 2012, 8, 3550−3566. (31) Xue, Y.; Liu, J.; Chen, H.; Wang, R.; Li, D.; Qu, J.; Dai, L. Nitrogen-Doped Graphene Foams as Metal-Free Counter Electrodes in High-Performance Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2012, 51, 12124−12127. (32) Hou, S.; Cai, X.; Wu, H.; Yu, X.; Peng, M.; Yan, K.; Zou, D. Nitrogen-Doped Graphene for Dye-Sensitized Solar Cells and the Role of Nitrogen States in Triiodide Reduction. Energy Environ. Sci. 2013, 6, 3356−3362. (33) Wang, G. Q.; Kuang, S.; Wang, D. L.; Zhuo, S. P. NitrogenDoped Mesoporous Carbon as Low-Cost Counter Electrode for HighEfficiency Dye-Sensitized Solar Cells. Electrochim. Acta 2013, 113, 346−353. (34) Kouhnavard, M.; Ludin, N.; Ghaffari, B.; Sopian, K.; Ikeda, S. Carbonaceous Materials and Their Advances as a Counter Electrode in Dye-Sensitized Solar Cells: Challenges and Prospects. ChemSusChem 2015, 8, 1510−1533. (35) Dong, J.; Jia, S.; Chen, J.; Li, B.; Zheng, J.; Zhao, J.; Wang, Z.; Zhu, Z. Nitrogen-Doped Hollow Carbon Nanoparticles as Efficient Counter Electrodes in Quantum Dot Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 9745−9750. (36) Chen, H.; Sun, F.; Wang, J.; Li, W.; Qiao, W.; Ling, L.; Long, D. Nitrogen Doping Effects on the Physical and Chemical Properties of Mesoporous Carbons. J. Phys. Chem. C 2013, 117, 8318−8328. (37) Iwashita, N.; Park, C.; Fujimoto, H.; Shiraishi, M.; Inagaki, M. Specification for a Standard Procedure of X-Ray Diffraction Measurements on Carbon Materials. Carbon 2004, 42, 701−714. (38) Banks, C.; Davies, T.; Wildgoose, G.; Compton, R. Electrocatalysis at Graphite and Carbon Nanotube Modified Electrodes: Edge-Plane Sites and Tube Ends are the Reactive Sites. Chem. Commun. 2005, 41, 829−841. (39) Mao, Y.; Duan, H.; Xu, B.; Zhang, L.; Hu, Y.; Zhao, C.; Wang, Z.; Chen, L.; Yang, Y. Lithium Storage in Nitrogen-Rich Mesoporous Carbon Materials. Energy Environ. Sci. 2012, 5, 7950−7955. (40) Park, K.; Jang, J.; Hong, J.; Kwon, Y. Mesoporous Thin Films of Nitrogen-Doped Carbon with Electrocatalytic Properties. J. Phys. Chem. C 2012, 116, 16848−16853. (41) Kapteijn, F.; Moulijn, J.; Matzner, S.; Boehm, H. The Development of Nitrogen Functionality in Model Chars During Gasification in CO2 and O2. Carbon 1999, 37, 1143−1150. (42) Ye, C.; Regulacio, M. D.; Lim, S. H.; Xu, Q. H.; Han, M. Y. Alloyed (ZnS)x(CuInS2)1‑x Semiconductor Nanorods: Synthesis, Bandgap Tuning and Photocatalytic Properties. Chem. - Eur. J. 2012, 18, 11258−11263. (43) Regulacio, M. D.; Win, K. Y.; Lo, S. L.; Zhang, S. Y.; Zhang, X. H.; Wang, S.; Han, M. Y.; Zheng, Y. G. Aqueous Synthesis of Highly Luminescent AgInS2−ZnS Quantum Dots and Their Biological Applications. Nanoscale 2013, 5, 2322−2327. (44) Fan, C. M.; Regulacio, M. D.; Ye, C.; Lim, S. H.; Lua, S. K.; Xu, Q. H.; Dong, Z. L.; Xu, A. W.; Han, M. Y. Colloidal Nanocrystals of Orthorhombic Cu2ZnGeS4: Phase-Controlled Synthesis, Formation Mechanism and Photocatalytic Behavior. Nanoscale 2015, 7, 3247− 3253.

(45) Regulacio, M. D.; Han, M. Y. Multinary I-III-VI2 and I2-II-IV-VI4 Semiconductor Nanostructures for Photocatalytic Applications. Acc. Chem. Res. 2016, 49, 511−519. (46) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (47) Bisquert, J.; Cahen, D.; Hodes, G.; Rühle, S.; Zaban, A. Physical Chemical Principles of Photovoltaic Conversion with Nanoparticulate, Mesoporous Dye-Sensitized Solar Cells. J. Phys. Chem. B 2004, 108, 8106−8118. (48) Murakami, T. N.; Ito, S.; Wang, Q.; Nazeeruddin, M. K.; Bessho, T.; Cesar, I.; Liska, P.; Humphry-Baker, R.; Comte, P.; Péchy, P.; et al. Highly Efficient Dye-Sensitized Solar Cells Based on Carbon Black Counter Electrodes. J. Electrochem. Soc. 2006, 153, A2255− A2261. (49) Kwon, W.; Kim, J. M.; Rhee, S. W. Electrocatalytic Carbonaceous Materials for Counter Electrodes in Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 3202−3215. (50) Choi, H. M.; Ji, I. A.; Bang, J. H. Metal Selenides as a New Class of Electrocatalysts for Quantum Dot-Sensitized Solar Cells: a Tale of Cu1.8Se and PbSe. ACS Appl. Mater. Interfaces 2014, 6, 2335−2343. (51) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Mora-Seró, I.; Bisquert, J. Characterization of Nanostructured Hybrid and Organic Solar Cells by Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2011, 13, 9083−9118.

564

DOI: 10.1021/acs.jpclett.6b02864 J. Phys. Chem. Lett. 2017, 8, 559−564