量子点敏化太阳能电池的对电极研究 Counter electride in quantum dots sensitized solar cells

Nitrogen-doped hollow carbon nanoparticles as ef?cient counter electrodes in quantum dot sensitized solar cells

Jianhui Dong,ab Suping Jia,*a Jiazang Chen,a Bo Li,a Jianfeng Zheng,a Jianghong Zhao,a Zhijian Wang a and Zhenping Zhu *a

Received 18th January 2012,Accepted 18th March 2012DOI:10.1039/c2jm30366c

The functions of nitrogen-doped hollow carbon nanoparticles (N-HCNPs)as counter electrodes in quantum dot sensitized solar cells (QDSSCs)have been studied in this paper.Electrochemical

impedance spectroscopy (EIS)and Tafel-polarization tests reveal a low charge transfer resistance and a high exchange current density between polysul?de electrolyte and the N-HCNPs electrode.Cyclic voltammetry results indicate that the N-HCNPs electrode shows high electrocatalytic activity and excellent tolerance toward the S 2à/S n 2àelectrolyte.A power conversion ef?ciency of 2.67%is achieved for the QDSSCs based on N-HCNPs counter electrodes,which is clearly higher than those of the QDSSCs based on HCNPs,carbon nanotubes and Pt counter electrodes.The results reveal that the N-HCNPs electrode is a promising counter electrode candidate for QDSSCs.

1.Introduction

Along with recent great achievements in dye-sensitized solar cells (DSSCs),1–3quantum dot sensitized solar cells (QDSSCs)4–8have attracted much attention given its combined advantage of quantum dots (QDs)and DSSCs system.As attractive sensitizer candidates for DSSCs,QDs have a high extinction coef?cient and large intrinsic dipole moment.Their band gaps can be easily tuned by changing their sizes and shapes,providing excellent light absorbing materials in sensitized solar cells.QDs can also stably adsorb on metal oxides and can be produced at a signi?-cantly lower cost than dyes.Thus,QDSSCs have the potential to improve the ef?ciency of sensitized solar cells.

Metal chalcogenide QDs with narrow band gaps are the most widely investigated inorganic light-harvesting materials in QDSSCs.6–8Unfortunately,the well-known and most ef?cient I à/I 3àredox couple in DSSCs is not compatible with these sem-iconducting materials,they lead to a slow hole removal that enhances the recombination ef?ciency and a rapid corrosion of QDs.9Consequently,to maintain the favorable band alignment for charge separation,charge transfer processes and QDs stability,a general substitution of the redox couple in the cell con?guration is required.The replacement must be stable with the photoanodes and counter electrodes over a suf?ciently long time.10The most common optimal redox couples applied in QDSSCs are sul?de/polysul?de (S 2à/S n 2à)ones.These redox

couples can effectively stabilize QDs and improve the perfor-mance of QDSSCs using cadmium chalcogenides (S,Se or Te)as sensitizers.11,12However,the surface activity and conductivity of Pt counter electrodes are suppressed because of the adsorption of the sulfur atoms.13,14Thus,Pt is unable to regenerate the poly-sul?de electrolyte in terms of eqn (1),resulting in high series resistances in cells and low photovoltaic conversion ef?ciencies.The ef?ciencies of QDSSCs have been reported to reach values of around 4–5%,15,16which are still much lower than those of DSSCs.

S n 2à+2e /S n à12à+S 2à

(1)

Ultimately,the performance of the QDSSCs is determined by the electrocatalytic properties and tolerance of the counter elec-trodes toward the S 2à/S n 2àredox couples.Therefore,suitable counter electrodes for polysul?de electrolyte in QDSSCs need to be found.Some new counter electrode materials such as Au,Cu 2S,CoS,PbS,and carbon 13,17,18have been reported.Among them,carbon materials have emerged as potential counter elec-trodes for DSSCs and QDSSCs,mainly due to their excellent corrosion resistance and versatile catalytic properties.Recently,Lee et al.19reported an ef?ciency close to that of Pt counter electrode based DSSCs using nitrogen (N)-doped carbon nano-tubes.They conclude that N-doped carbon structures exhibit excellent electrocatalytic performance due to the additional electrons contributed by the N atom.20N-doping is an attractive approach for effectively tuning the physical and chemical prop-erties of carbon materials to be applied in catalysis.19,20However,to the best of our knowledge,reports on the use of N-doped carbon materials in QDSSCs with polysul?de electrolyte are limited.

a

State Key Laboratory of Coal Conversion,Institute of Coal Chemistry,Chinese Academy of Sciences,Taiyuan 030001,China.E-mail:jiasuping@https://www.360docs.net/doc/2317254489.html,;zpzhu@https://www.360docs.net/doc/2317254489.html,;Fax:+86-351-4048433;Tel:+86-351-4048715b

Graduate University of Chinese Academy of Sciences,Beijing 100049,China

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In our previous studies,N-doped carbon materials have been synthesized and showed excellent catalytic properties in electro-chemical catalyzing oxygen reduction reaction in fuel cells and I 3àreduction in DSSCs.20,21In the present work,the property of N-doped hollow carbon nanoparticles (N-HCNPs)used in QDSSCs as counter electrode material were demonstrated and compared with the performance of HCNPs,carbon nanotubes (CNTs)and Pt electrodes.The low charge transfer resistance (R ct )at the N-HCNPs counter electrode/electrolyte interface was ach-ieved based on electrochemical impedance spectroscopy (EIS),revealing the extraordinary catalytic ability of the N-HCNPs counter electrodes for the reduction of S n 2à.The electrocatalytic property of the N-HCNPs was further investigated by Tafel-polarization measurements and cyclic voltammetry (CV).The cells comprising N-HCNPs as counter electrodes showed higher energy conversion ef?ciencies than those comprising HCNPs,commercial CNTs,and conventional Pt counter electrodes.

2.Experimental

Synthesis of N-HCNPs

N-HCNPs were assembled by detonation-assisted chemical vapor deposition as previously reported.20,21Typically,2.5g of picric acid (2,4,6-trinitrophenol)and 0.7mL of dimethylforma-mide were loaded into a sealed stainless steel pressure vessel (14cm 3)equipped with a pressure valve.Detonation was initi-ated by quick heating (20 C min à1)to 320 C (the ignition temperature of picric acid).Subsequently,a high-temperature and high-pressure environment of about 1000 C and 20–45MPa was created inside the vessel.The vessel was then cooled to ambient temperature,the gaseous product was vented and the solid product of N-HCNPs was collected.Preparation of N-HCNPs counter electrodes

N-HCNPs powder pastes were prepared using TiO 2colloid as the connecting adhesive.22The TiO 2colloid was prepared according to a modi?ed procedure described previously.23Brie?y,about 10mL of titanium(IV )n -butoxide was added dropwise to 60mL of 0.1M HNO 3under vigorous stirring.The obtained slurry was then heated to 80 C and kept for 8h.The TiO 2precipitate was peptized to a white transparent colloid powder.The N-HCNPs paste was obtained by grinding 130mg of N-HCNPs powder,0.2mL of TiO 2colloid,0.2mL of 10%Triton X-100aqueous solution and 0.4mL of water in a mortar.The carbon paste was coated onto ?uorine-doped tin oxide (FTO)glass by the doctor-blade method.The substrate was dried for several minutes at room temperature and then heated in air at 450 C for 30min.HCNPs (obtained from Dr Song as previously reported 24)and commercial CNTs (purchased from Tsinghua University of China)counter electrodes were fabricated by the above described method.Traditional Pt counter electrodes were fabricated by thermal decomposition of H 2PtCl 6(30mM in isopropanol)on FTO glass at 400 C for 30min for comparison.Fabrication of QDSSCs

The photoanodes of the QDSSCs were prepared by coating the P25-TiO 2paste onto FTO glass using the doctor-blade method

according to literature.25The TiO 2-coated FTO was annealed at 450 C for 30min.Chemical bath deposition was used to assemble the CdS and CdSe QDs in sequence on the TiO 2?lm as described previously.26The deposition experiment was carried out at 10 C in the dark.The deposition time for CdS and CdSe QDs was 30min and 5.5h,respectively.Finally,surface passivation with ZnS was conducted twice by alternately dipping into 0.1M Zn(CH 3COO)2and 0.1M Na 2S aqueous solution for 1min alternately.The QDs-adsorbed TiO 2electrodes and various counter electrodes were assembled in a sandwich-type cell.The electrolytes were penetrated into the cell,which was composed of 1M Na 2S and 1M S in H 2O.

For the EIS and Tafel-polarization tests,two identical carbon or Pt electrodes were placed face-to-face to form symmetric dummy cells ?lling the S 2à/S n 2àelectrolyte similar to the one applied in fabricating QDSSCs.Characterization

The morphology of the N-HCNPs was characterized using transmission electron microscopy (TEM)system (JEM-2010)operated at 200kV.Nitrogen adsorption and desorption isotherms were detected at 77K on an ASAP2020M automated gas-sorption system after the sample was degassed at 423K and 20m Torr for 6h.The pore size distribution was estimated from the N 2adsorption branch using the Barrett–Joyner–Halenda (BJH)method.The speci?c surface area was determined from nitrogen adsorption using the Brunauer–Emmett–Teller (BET)equation.Elemental composition was investigated via X-ray photoelectron spectroscopy (XPS),using a Thermo ESCALAB 250spectrometer with an Al-K a X-ray source and a 500m m electron beam spot.

The charge transfer resistance was determined by EIS,per-formed on the symmetric cells using an electrochemical work-station (IM6ex,Zahner).In the EIS experiments,the potential applied across the dummy cell was 0V (open circuit potential,V oc ),the perturbation amplitude was 10mV and the frequency range was between 100KHz and 100mHz.For photovoltaic testing,a solar light simulator (Oriel,91192)was used to provide an illumination of 100mW cm à2(AM 1.5).A digital source meter (2400Source Meter,Keithley Instruments Inc.,USA)was used to record the current–voltage plots.CV was carried out on electrochemical station (CHI 660d Shanghai,China)in a three-electrode system with carbon/FTO or Pt/FTO as the working electrode,a foil as the counter electrode,and an Ag/AgCl elec-trode as the reference electrode at different scan rates.The electrolyte was an aqueous solution containing 10mM Na 2S,10mM S,and 0.1M LiClO 4as supporting electrolytes.The electrochemical station was also employed to measure the Tafel-polarization curve of the symmetric cells at a scan rate of 5mV s à1.

3.Results and discussion

TEM was used to characterize the morphology and microstruc-ture of the N-HCNPs.As shown in Fig.1a and 1b,there is obvious contrast between the dark edge and the pale center of the particles,which reveals the hollow core characteristics.The TEM images reveal that the particles have an average outer diameter

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ranging from10nm to25nm and a shell thickness of about2nm to3nm.The nitrogen adsorption–desorption isotherm of the N-HCNPs is shown in Fig.1c.A typical type IV pro?le with an H3 hysteresis is observed,revealing the existence of mesopores and macropores in the N-HCNPs.The pore size distribution calcu-lated from the adsorption and desorption branches by BJH method is shown in the inset in Fig.1c.The N-HCNPs clearly have two predominant mesopores with sizes of approximately 3.3and30nm corresponding to the mesopores in the shell and hollow mesoporous core,respectively.A pore size distribution at 3.3nm shows reversibility for both the adsorption and desorption branches,meaning that the pores at3.3nm is real but not due to the tensile strength effect(TSE)phenomenon.27Such a meso-pore-micropore-organized structure endows the N-HCNPs with a large BET surface area of454m2gà1.The XPS spectrum of the N-HCNPs shows a dominant narrow C1s peak at284eV,an N 1s peak at400eV,and an O1s peak at531eV(Fig.1d).Clearly, the nitrogen atoms are successfully incorporated into the struc-ture of N-HCNPs.The N-doping,large surface area and highly porous structure of carbon particles may facilitate the electro-catalytic activity of the S2à/S n2àredox reaction in the electrolyte solution.The previously reported structure of HCNPs is very analogous to that of N-HCNPs(data not shown).

Fig.2shows the J–V curves of the QDSSCs using N-HCNPs, HCNPs,CNTs and Pt as counter electrodes.The detailed photovoltaic parameters from the J–V curves are summarized in Table1.When the as-prepared N-HCNPs electrode is used as

a counter electrode,the photovoltaic parameters are as follows:

V oc?0.51V,short-circuit current density(J sc)?13.53mA cmà2,?ll factor(FF)?0.40and h?2.67%.Obviously,an improved

solar cell performance is observed for the N-HCNPs counter

electrode compared with the Pt electrode.The

N-HCNPs Fig.1(a,b)TEM images of the N-HCNPs.(c)Nitrogen adsorption–desorption isotherm and pore size distribution(inset)of the N-HCNPs.(d)XPS spectrum of

N-HCNPs.

Fig.2Current density–voltage curve obtained from QDSSCs with N-

HCNPs,HCNPs,CNTs and Pt counter electrodes.

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counter electrode based QDSSCs yields a h value of 2.67%,which is caused by a considerable increase in the J sc .However,the cell with HCNPs as counter electrode has lowest h because of the decreased J sc .A comparable ef?ciency is observed for the CNTs counter electrode,although the V oc and J sc are slightly lower

than those of the Pt counter electrode.This ?nding can be mainly ascribed to the enhancement in FF resulted from low Nernst diffusion impedance Z w as discussed https://www.360docs.net/doc/2317254489.html,pared with the three carbon materials used in QDSSCs,the N-HCNPs electrode shows better photovoltaic performance than the HCNPs and CNTs electrodes,this ?nding is consistent with a previous report on the excellent electrocatalytic performance of N-doped carbon structures due to additional electrons contributed by the N atom.To evaluate the electrochemical characteristics of N-HCNPs,HCNPs,CNTs and Pt electrodes,EIS was performed on the symmetrical dummy cells comprising two identical electrodes mentioned in the introduction.28,29Fig.3a shows the cross-sectional view of the symmetric cell.The impedance spectra can be interpreted and modeled using the equivalent circuits shown in Fig.3b.The Nyquist plots of the symmetric cells are shown in Fig.3c and 3d.Two semicircles are observed in the measured frequency range of 100mHz to 100KHz for four symmetric cells.By ?tting the plots using Z-VIEW software,R s is de?ned as the ohmic series resistance in the high frequency range around 100KHz,including the sheet resistance of the two identical elec-trodes and the electrolytic resistance.R ct is determined from the semicircles in the frequency region of 1–100KHz (the ?rst cycle),which includes the charge-transfer processes occurring at the counter electrode/electrolyte,correlated with the electrocatalytic activity for the S 2à/S n 2àredox reaction.The low frequency (100mHz to 10Hz,the second cycle)intercepted on the real axis represents the ?nite layer Z w of the S 2à/S n 2àredox couple in the electrolyte.Table 2summarizes the obtained impedance parameters after ?tting for various counter electrodes.

The spectra shown in Fig.3c and data listed in Table 2,reveal that the R s values are the same level for the Pt and the N-HCNPs electrodes.Thus,the sheet resistance of the N-HCNPs electrode is as good as that of the Pt electrode.The R ct values of the N-HCNPs,HCNPs,CNTs and Pt counter electrodes are 1.96,120.96,2.81,69U cm 2,respectively.The R ct of N-HCNPs elec-trode is far lower (about 40times)than that of the Pt electrode,suggesting an acceleration of the electron transfer process at the electrolyte–N-HCNPs counter electrode interface and the extraordinary catalytic ability for the reduction of S n 2à.The better catalytic ability of N-HCNPs is expected to improve

Table 1Photovoltaic parameters of QDSSCs based on various counter electrodes Counter electrode R s /U cm 2R ct /U cm 2Z w /U cm 2N-HCNPs 8.2 1.9651.8HCNPs 8.11120.96246.4CNTs 9.33 2.81 3.3Pt

8.14

69

646.5

Fig.3(a)Symmetric cell for EIS measurements.(b)Equivalent circuit diagram of the symmetric cell.(c,d)Nyquist plots of N-HCNPs,HCNPs,CNTs and Pt electrodes.(d)Expanded range of the ordinate and abscissa from (c).

Table 2EIS parameters of symmetrical cells with different counter electrodes Counter electrode J sc /mA cm à2V oc /V FF h (%)N-HCNPs 13.530.510.40 2.67HCNPs 8.140.490.38 1.45CNTs 8.840.490.47 2.09Pt

9.80

0.50

0.40

2.01

Fig.4Tafel curves of symmetrical cells fabricated with N-HCNPs,HCNPs,CNTs and Pt electrodes.

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the photovoltaic performance in QDSSCs by increasing J sc of the solar cells as shown in J –V curves.The higher R ct of the Pt counter electrode is attributed to the attachment and poisoning of S in the polysul?de solution,increasing the charge transfer resistance.30Z w of N-HCNPs is also much lower than that of the Pt electrode,but the lowest Z w is obtained for the CNTs elec-trode.This result indicates a better electrolyte diffusion capa-bility in the CNTs layer,which facilitates faster mass transport and improves the photovoltaic performance of cells by increasing FF as shown in Table 1.For the HCNPs electrode,the higher R ct and Z w signify that this material is not suitable as an electro-catalytic electrode for polysul?de electrolyte.

To further evaluate the electrocatalytic activity of the N-HCNPs electrode,Tafel-polarization tests were conducted in a dummy cell similar to the one used in the EIS measurements.29The plot of log J versus overpotential for four different counter electrodes is shown in Fig.4.The exchange current density (J 0)can be obtained as the intercept of the extrapolated linear region of anodic and cathodic branches when the overpotential is zero,which is a direct measure of the electron transfer kinetics at the interface under equilibrium conditions.31Fig.4shows that the anodic and cathodic branches of the Tafel curves have the largest slope for the N-HCNPs,indicating a higher J 0on this electrode based on the Tafel equation.Hence,the N-HCNPs show better electrocatalytic activity,in agreement with the EIS results.

CV was also performed to evaluate the electrochemical cata-lytic activities and stabilities of four different materials.The

electrochemical behaviors of N-HCNPs,HCNPs,CNTs and Pt electrodes for the S 2à/S n 2àredox couple was investigated from 0.3V to à0.8V at a scan rate of 0.01V s à1.The reduction (negative currents)and oxidation (positive currents)peaks are assigned to the reduction of S n 2àto S 2àand the oxidation of S 2à,respectively.In the study of counter electrodes in QDSSCs,the reduction peak current was the most relevant,expressing the catalytic ability of counter electrode for S n 2àreduction as shown in eqn (1).Fig.5a shows that the typical pair of redox peaks is observed for the Pt and N-HCNPs electrode.The current density at the reduction peak of the N-HCNPs electrode is conspicuously larger than the Pt electrode,suggesting the larger active surface of the N-HCNPs electrode.The reduction peak of the N-HCNPs electrode is shifted slightly toward the positive side,indicating that the reduction reaction can occur at a lower overpotential on the N-HCNPs electrode and consequently a higher V oc can be obtained in the N-HCNPs counter electrode based QDSSCs.The electrochemical stabilities of the counter electrodes were also studied by monitoring successive CV behavior with the number of cycles.As shown in Fig.5b,the redox current greatly declines and the reduction peak disappears after ?ve cycles for the Pt electrode,which means the strong corrosion between the poly-sul?de electrolyte and Pt electrode.In contrast,with increased number of cycles,the CV curves of the N-HCNPs electrode do not change signi?cantly (Fig.5c)and show stable peak current density (Fig.5d).This ?nding indicates that the N-HCNPs electrode has excellent electrochemical stability in

polysul?de

Fig.5(a)Cyclic voltammograms for N-HCNPs,HCNPs,CNTs and Pt electrodes obtained at a scan rate of 10mV s à1in 10mM Na 2S,10mM S aqueous solution containing 0.1M LiClO 4as supporting electrolyte.(b,c)Cyclic voltammograms of the different cycle times with a Pt electrode and N-HCNPs electrode.(d)The reduction peak current versus cycle times for N-HCNPs electrode.

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electrolyte.In addition,the CNTs electrode also shows good tolerance to polysul?de electrolyte after 15cycles of CV scanning (data not shown),but its reduction peak is not obvious (Fig.5a).This phenomenon may have arisen from the slow electron transfer at the interface of the CNTs electrode and electrolyte,suggesting relatively poor catalytic ability.The reduction and oxidation peaks are not obvious for the HCNPs electrode,indicating very poor catalytic ability for polysul?de electrolyte,similar to the EIS results.Thus,by combining the results of EIS and CV,we can conclude that the N-HCNPs electrode has superior electrocatalytic activity and excellent electrochemical stability in polysul?de electrolyte.

4.Conclusions

N-HCNPs materials have been successfully used as a counter electrode in QDSSCs system.The QDSSCs using this N-doped carbon material achieves a conversion ef?ciency of 2.67%,which is the highest among HCNPs,commercial CNTs and conven-tional Pt electrodes.The R ct of the N-HCNPs electrode toward the polysul?de electrolyte greatly decreased compared with those of HCNPs,CNTs and Pt electrodes.The exchange current density value for the N-HCNPs/polysul?de system is signi?-cantly higher than the other systems.The N-HCNPs electrode also shows better electrochemical stability and tolerance to sulfur poisoning in polysul?de electrolyte.These results demonstrate that the N-HCNPs electrode is a promising material that can replace expensive Pt counter electrode for QDSSCs.The better performance of the N-HCNPs electrode is primarily attributed to the higher electrocatalytic activity of the N-HCNPs material because of N-doping.N-doping may introduce many active sites in the porous structure,but this proposal needs further validation.

Acknowledgements

This work was supported by the Knowledge Innovation Project of the Chinese Academy of Sciences (No.Y1YCA119D1).

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F C O A L o n 07 M a y 2012P u b l i s h e d o n 20 M a r c h 2012 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 2J M 30366C