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Influence of Mn+2 incorporation in CdSe quantum dots for high performance of CdS–CdSe quantum dot sensitized solar cells
Accepted Manuscript Title: Influence of Mn+2 incorporation in CdSe quantum dots for high performance of CdS-CdSe quantum dot sensitized solar cells Author: M. Venkata-Haritha Chandu V.V.M. Gopi Chebrolu Venkata Thulasi-Varma Soo-Kyoung Kim Hee-Je Kim PII: DOI: Reference:
S1010-6030(15)00350-0 http://dx.doi.org/doi:10.1016/j.jphotochem.2015.09.007 JPC 10007
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
9-7-2015 25-8-2015 8-9-2015
Please cite this article as: M.Venkata-Haritha, Chandu V.V.M.Gopi, Chebrolu Venkata Thulasi-Varma, Soo-Kyoung Kim, Hee-Je Kim, Influence of Mn+2 incorporation in CdSe quantum dots for high performance of CdS-CdSe quantum dot sensitized solar cells, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2015.09.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Mn+2 incorporation in CdSe quantum dots for high performance of CdS-CdSe quantum dot sensitized solar cells M. Venkata-Haritha, Chandu V.V.M. Gopi, Chebrolu Venkata Thulasi-Varma, Soo-Kyoung Kim, Hee-Je Kim* School of Electrical Engineering, Pusan National University, Gumjeong-Ku, Jangjeong-Dong, Busan 609-735, South Korea
*Corresponding authors. Tel.: +82 51 510 7334; fax: +82 51 513 0212. E-mail addresses: [email protected] (H.-J. Kim). Graphical abstract Highlights:
Effect of Mn-doping on the photovoltaic performance of QDSSCs was investigated.
22% increase in η is achieved in Mn-doped CdS-CdSe than the bare CdS-CdSe QDSSCs.
The photo-excited electron lifetimes in the photoanode were significantly improved.
The stability of CdS-Mn-CdSe is superior to CdS-CdSe QDSSCs
Abstract: Quantum dot sensitized solar cells (QDSSCs) have attracted considerable attention recently and become promising candidates for realizing a cost-effective and facile fabrication of solar cell with improved photovoltaic performance. QDs were directly grown on the TiO2 mesostructure by the successive ionic layer absorption and reaction (SILAR) technique. QDSSC based on CdSCdSe photoanode achieves a power conversion efficiency of 3.42% under AM 1.5 G one sun illumination. The loading of Mn +2 metal ions was applied to a CdSe (CdS-Mn-CdSe) photoanode to enhance the adsorption in QDSSCs, which greatly improved the power conversion efficiency. Without the passivation layer, the solar cell based on a CdS-Mn-CdSe QD-sensitized TiO2 photoelectrode shows higher J sc (14.67 mA/cm2), Voc (0.590 V) and power conversion efficiency (4.42%) comparing to Mn-undoped CdS-CdSe QD sensitized TiO2 (Jsc: 11.29 mA/cm2, Voc: 0.568 V, and efficiency: 3.42%), which can be ascribed to superior light absorption, faster electron transport and slower charge recombination for the former. The effective electron lifetime of the device with CdS-Mn-CdSe was higher than those with CdS-CdSe, leading to more efficient electron-hole separation and slower electron recombination. The effects of Mn+2 metal ions on the chemical, physical, and photovoltaic properties of the QDSSCs have been investigated have been investigated by X-ray photon spectroscopy (XPS), UV-vis spectra, photocurrent-voltage (J-V) characteristics and electrochemical impedance spectra (EIS). Keywords: Quantum dot sensitized solar cells; Mn-doping; CdSe; Stability; Recombination;
Introduction: Dye-sensitized solar cells (DSCs) have received increasing attention in the past decade because of their simple fabrication process, low cost, and environmental friendliness and potential
to achieve efficient conversion of sunlight into electricity.1-5 The power conversion efficiency (PCE) of DSSCs based on the planar substrate of rigid conducting glass has reached >11% under under full sun illumination.4,5 In addition to DSSCs, as one of the third generation solar cells, replacing the organic dye by semiconductor quantum dots (QDs) has attracted a lot of attention in dye-sensitized solar cells (DSSCs) due to tunable band gap energy by controlling their size and composition, high extinction coefficients, large dipole moment, multiple exciton generation 9 and hot carrier collection.6-10 QDSSC adopts the principle of DSSC and generally consist of QDs, mesoporous oxide including titanium dioxide (TiO2) and zinc oxide (ZnO), polysulfide electrolyte containing Sn2- /S2- redox couples and a counter electrode (CE).11 Till the date various QD sensitizers such as CdS,12 CdSe,13 PbS,14 PbSe,15 CdTe,16 and CuInS2 17 have been reported in the QDSSCs. Among the metal semiconductor QDs, CdS and CdSe QDs are the most efficient and widely used due to high potential in light harvesting have been paid much attention. Recently, by ligand exchanging method CdSexTe1-x alloyed QDSSCs has achived more than 6% efficiency and high stability.18 Kamat et al. reported Mn-doped CdS sensitized solar cells with an efficiency of 5.4%.19 Up to now, the photovoltaic performance of QDSSCs is still lower than that of DSSCs. Because defects on the surface of QDs serve as temporary surface traps for electrons, they hinder charge transfer into TiO2 conduction bands and therefore greatly decrease the power conversion efficiency in QDSSCs.19 On the other hand, many efforts have been concentrated on designing and synthesizing QDs to achieve elevated photovoltaic performance in QDSSCs. In this scenario a doping transition metal ions would lead to new materials showing extraordinary electronic and photo-physical properties of QDs.20,21 So the doping of metal ions into the semiconductor QDs is an effective method to improve the photovoltaic performance of QDSSCs. CdSe QDs are more
attractive owing to its high potential for light harvesting in the visible light region than the CdS and PbS QDs.22,23 Therefore, the introduction of metal ions into CdSe QDs is more useful to achieve superior photovoltaic performance in the QDSSCs. Here, we have successfully synthesized and designed a high efficiency QDSSC based on Mn-CdSe QDs using successive ionic layer absorption and reaction (SILAR). This present study revealed that the doping of Mn2+ into the CdSe QDs can significantly enhance the performances, such as the light-harvesting, charge-transfer and charge-collection capabilities. As a result, the CdS-Mn-CdSe sensitized solar cell exhibited a high photo-conversion efficiency of 4.42% under simulated illumination of AM 1.5, 100 mW cm-2. The improved performance and stability is due to Mn 2+ dopant raises the conduction band of CdSe, accelerates the electron injection kinetics and reduces the charge recombination. The electron lifetime increases with the Mn +2 dopant in the CdSe, demonstrating that the Mn+2 plays an important role in preventing carrier recombination, and improving the charge transfer and collection.
Experimental work: Materials: Manganese(II) acetate tetrahydrate [Mn(CH3COO)2·4H2O], Cadmium acetate dehydrate [Cd(CH3COO)2.2H2O], sodium sulfide (Na2S), sodium sulfite (Na2SO3), sulfur (S), selenium (Se), potassium chloride (KCl), copper(II) sulfate pentahydrate [CuSO4.5H2O], sodium thiosulfate (Na2S2O3), urea (CH4N2O) are purchased from Sigma-Aldrich, and TiO2 paste (Ti-Nanoxide HT/SP) is supplied by Solaronix. All other chemicals were commercially available and of analytical grade. Preparation of CdS-CdSe and CdS-Mn-CdSe photoanode:
TiO2 films with the thickness of about 7.5 μm were prepared by doctor-blade technique using TiO2 paste of particle size 20 nm (Ti-Nanoxide HT/SP, Solaronix) in an active area of 0.27 cm2, followed by annealing at 450 oC for 30 min.24 Then, TiO2 films were immersed in the aqueous solution of 0.1 M Cd(CH3COO)2.2H2O for 5 min, rinsed with DI water, ethanol and dried with an drier. Then, they were then dipped in the aqueous solution of 0.1 M Na2S for 5 min, rinsed with DI water, ethanol and dried with a drier. The process was conducted at room temperature and repeated five times and as prepared samples is named as TiO2/CdS5. The TiO2-CdS5 electrodes were dipped in the aqueous solution of 0.1 M Cd(CH3COO)2.2H2O for 5 min at room temperature and then immersed in aqueous solution of Na2SeSO3 for 5 min at 50 oC, followed by rinsing with DI water, ethanol and drying with an drier. The process was repeated eight times and the electrodes named as TiO2/CdS5/CdSe8. The Na2SeSO3 aqueous solution was prepared by refluxing 0.2 M Se in an aqueous solution of 0.4 M Na2SO3 at 120 oC for 5 h. To incorporate the doping of Mn 2+, the molar percentage of 10% (10 mM) Mn(CH3COO)2·4H2O was mixed with the Cd(CH3COO)2.2H2O in the deposition of CdSe. This enabled the co-adsorption of Mn 2+ and Cd 2+ ions, which in turn promoted the incorporation of Mn 2+ in the CdSe film. The preparation of 5%, 15% and 20% (5 mM, 15 mM and 20 mM) Mndoped CdS-CdSe electrodes is shown in supporting information†. Finally, the prepared counter electrodes25 and photoanodes were sealed using a sealant (SX 1170-60, Solaronix) at 100 oC. The space between the electrodes was filled with polysulfide electrolyte consisting of 1 M Na2S, 2 M S, and 0.1 M KCl in a solution of methanol and water, which were present at a ratio of 7:3. The cells were tested under the illumination of 1 sun (AM 1.5 G, 100 mW cm-2).
Characterization: X-ray diffraction (XRD) analysis was performed on a D8 ADVANCE with a DAVINCI (Bruker AXS) diffractometer using Cu Kα radiation operated at 40 kV and 40 mA. The surface morphology, and elemental compositions of the electrodes were investigated using a field emission scanning electron microscope (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. The UV- visible absorption spectra were recorded using an OPTIZEN 3220UV spectrophotometer. X-ray photon spectroscopy (XPS) was performed using a VG Scientific ESCALAB 250 with monochromatic Al-Kα radiation of 1486.6 eV and with an electron take-off angle of 90°. The surface roughness of the substrates was characterized using an atomic force microscope (JPK NanoWizard II AFM, JPK Instruments, Berlin, Germany) with a scan rate of 0.8 Hz in contact mode. The current-voltage characteristics of QDSSCs were examined under one sun illumination (AM 1.5G, 100 mW cm−2) using an ABET Technologies (USA) solar simulator with an irradiance uniformity of ±3%. The incident photon to current conversion efficiency (IPCE) spectra was measured using Oriel® IQE-200™. Electrochemical impedance spectroscopy (EIS) was conducted on the QDSSCs using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under one sun illumination, and the frequency ranged from 100 mHz to 500 kHz at room temperature. The applied bias voltage and AC amplitude were set to the Voc of the QDSSCs and 10 mV, respectively. The electrical impedance was characterized using the Nyquist and Bode plots. To perform the stability test, the Mn-CdS-CdSe-ZnSe QDSSC was continuously irradiated with an AM 1.5G, 100 mW cm−2 in working conditions, and the J-V curves were tested every 1 h.
Results and Discussion: Fig. 1 shows a scanning electron micrograph (SEM) of CdS-CdSe and CdS-Mn-CdSe QDs on a TiO2 surface. The TiO2 transparent film cannot directly used as the photoanode, because some disadvantages are found not to benefit the photovoltaic performance: (1) the pore distribution is narrow; and the pore size is usually small; (2) long wavelength light cannot be fully used since the TiO2 transparent film has no light scattering property. Therefore, the introduction of large particles into the photoanode has been used to improve the performance of the solar cells. As we can see in Fig. 1a, the nanoparticles are uniformly distributed in the film and the diameter of the particles range is 21-45 nm. When Mn+2 is doped into CdS-CdSe, the surface structures become closer than with the CdS-CdSe QDs (Fig. 1b). The gaps between the nanostructures were reduced with introducing of Mn+2, which decreased the recombination, leading to increased efficiency. Also, the diameter of the particles increased with the loading of Mn+2 in CdS-CdSe (28-56 nm). The improved surface morphology with the Mn+2 doping in QDs can increase the photovoltaic performance of the QDSSCs. EDX analysis was conducted to investigate the elemental compositions of CdS-CdSe and CdS-Mn-CdSe QD sensitizers on the surface of TiO2, and the results are shown in Fig. 2. The purity of the sample is investigated by EDX analysis. There is no impurity observed in the sample, since there are no elements except for Ti, O, Cd, S, Se and Mn in the EDX spectrum. The orbital states of the Ti, O, Cd, S, Se and Mn are seen in the EDX spectrum. Fig. 2a shows an atomic ratio of Cd:S:Se is 12%:6%:5%, demonstrating that the formation of CdS and CdSe on the surface of TiO2. Further, with the deposition of Mn+2 in QDs, upon ion exchange, 2% Mn content emerges in the film, while the Cd content drops from 12% to 11%, demonstrating the occurrence of ion
exchange from Cd to Mn (Fig. 2b). The energy levels of Mn in CdS-Mn-CdSe film is at 0.650 KeV, 5.89 KeV, and 6.536 KeV correspond to LIαβ, Kα, and Kαβ, respectively. The phase structure of TiO2-CdS-CdSe sample was measured by XRD experiments. Fig. 3 shows the XRD pattern of TiO2: the peaks appearing at 37.7, 48.2 and 70.8 o are reveal that the TiO2 nanoparticles have a tetragonal anatase structure (JCPDS: 00-021-1272). The diamond symbols in the XRD graph represent the FTO substrate, which might be due to the very low thickness of the materials or interference from the XRD signals of the high crystallinity FTO substrate. The diffraction peaks at 2θ values of 26.5, 44.0, and 54.6° are correspond to the (1 1 1), (2 2 0), and (2 2 2) planes of a CdS cubic structure (JCPDS card no. 01-080-0019). The other remaining peaks are due to the deposition of CdSe on the CdS surface. 23.7, 25.3, 27.4, 45.7 and 66.4 o can be assigned to the (1 0 0), (0 0 2), (2 0 1), (1 0 3), and (2 1 0) facet of CdSe hexagonal phase (JCPDS: 01-077-2307), respectively. XPS was measured to know the composition and chemical bond configuration of pure and Mg doped CdSe thin films. Fig. 4 shows the Cd 3d, Mn 2p and Se 3d spectra of CdS-CdSe and CdS-Mn-CdSe films, respectively. The binding energy peaks of Cd 3d5/2 observed at 405.1 and 404.6 eV for CdS-CdSe and CdS-Mn-CdSe films respectively, clearly show that Mn2+ is replacing Cd2+. The binding energy of the Cd 3d3/2 peak for CdS-CdSe and CdS-Mn-CdSe film is observed at 411.8 and 411.3 eV, respectively. The peak deconvolution process in CdS-Mn-CdSe (Fig. 4B) clearly shows that the Mn 2p spectrum of the film is composed of two peaks, at 642 eV (Mn 2p3/2) and 652.8 eV (Mn 2p1/2), indicating the state of Mn2+ in QDs. The spectrum for Se 3d was fitted yielding a single peak at the binding energy of 53.6 eV and 53.1 eV in CdS-CdSe and CdS-MnCdSe, respectively. This is also confirmed through the shift of the corresponding Cd and Se peaks
toward lower binding energy value in CdS-Mn-CdSe than the CdS-CdSe. The XPS spectra clearly detects and confirms the presence of Mn in the CdS-Mn-CdSe. To investigate the possibility of changes in surface morphology and roughness by the Mndoping, AFM was carried out for the CdS-CdSe and CdS-Mn-CdSe. Fig. 5 shows the twodimensional (2D) and three-dimensional (3D) AFM images of CdS-CdSe and CdS-Mn-CdSe. The root mean square (RMS) surface roughness of the CdS-CdSe and CdS-Mn-CdSe films are 12.64 nm and 9.17 nm, respectively, as shown in Fig. 5(c) and 5(d). AFM studies indicated that the surface roughness of the CdS-Mn-CdSe film was smaller than that of the CdS-CdSe film. The lower surface roughness is mainly due to the large particle sizes of CdS-Mn-CdSe as compared with CdS-CdSe nanostructures, which might be due to the lower radii of Mn than Cd. The power conversion efficiency of the QD solar cell mainly depends on the small roughness of the QD surface, and a reduction of the surface roughness in CdS-Mn-CdSe tends to improve the conversion efficiency of the QDSSCs, which affects the optical properties.21 The optical performance of TiO2, CdS-CdSe and CdS-Mn-doped-CdSe are characterized by absorbance. Fig. 6 displays the UV-visible spectral curves of the films loaded with CdS-CdSe and CdS-Mn-CdSe QDs. The result shows that the absorbance of the CdS-Mn-CdSe QDs is much higher than that of the CdS-CdSe QDs. The high absorbance of the photoelectrode might be attributed to: the effects of Mn+2 doped into QDs and a greater loading amount of QDs. The greater loading amount of QDs is due to the presence of Mn+2 in the reaction solution might promote the deposition of QDs. It is found that in the same wavelength range, the photoanode of CdS-MnCdSe (688 nm) has broader range of light absorption than the undoped QDs (612 nm); this is corresponding to an increase in current density of Mn-doped solar cell. So we can conclude that
the incorporation of Mn+2 ion in CdSe narrowed the band gap of QDs, and extended the absorption wavelength range.
Fig. 7(a) shows the J-V characteristics of the cells characteristics of the solar cells measured under the illumination of one sun (AM 1.5, 100 mW cm-2). Key performance parameters of the cells with CdS-CdSe and CdS-Mn-CdSe are summarized in Table 1. The optimization of the Mndoping in CdS–CdSe is shown in Fig. S1 and Table S1 of the ESI.† Based on CdS-Mn-CdSe electrode, the solar cell gives a Jsc of 14.67 mA/cm2, a Voc of 0.590 V, and a FF of 0.510, yielding a power conversion efficiency of 4.42%. The photovoltaic performance of CdS-CdSe (Voc: 0.568 V, J sc: 11.29 mA/cm2, FF: 0.533, and ƞ:3.42%) is lower than that of CdS-Mn-CdSe electrode. The remarkable improvement of Jsc with Mn-doping is attributed to the excited charge characteristics, such as generation, injection or collection. It is finding that after Mn doping in CdSe, a significant red shift of the absorption profile is observed, which leads to the conduction band and valence band all improve, forming a cascade energy level which is more conducive to charge transport inside the solar cell. That is advantageous to the electron injection and hole-recovery, reducing the recombination of electron-hole with Mn+2 doping, and improving the ability of photoanode to capture light thus improving the photocurrent and ultimately improving the power conversion efficiency. It is well known that for the solar cells, electron generation characteristics can be evaluated by the incident photon-to-current conversion efficiency (IPCE). Fig. 7(b) shows the IPCE spectra of QDSSCs assembled with CdS-CdSe and CdS-Mn-CdSe QDs, respectively. The maximum IPCE of a CdS-Mn-CdSe sensitized solar cell reaches 89% at 480 nm, which is enhanced by 25%
compared to the maximum IPCE of 71% at 414 nm for a CdS-CdSe sensitized one. The IPCE is substantially improved from visible to near infrared radiation region 800 nm. The IPCE depends on both the absorption of light and the collection of charges. However, as it is demonstrated in eqn (1), the IPCE value of QDSSCs are determined by the light harvesting efficiency of the film (ηlh), the injection (ηinj) and charge collection efficiencies (ηcol) of the photoelectrons, which are affected by the structure of the photoelectrode. 25,26 =
×
×
(1)
The higher ηlh for the CdS-Mn-CdSe QDSSC, which can be confirmed by the above UV-vis and IPCE data, contributes greatly to the Jsc. The ηcol is related to both electron transport rate and charge recombination rate in QDSSC. The IPCE results indicate that excited electrons in CdS-MnCdSe QDSSC can be injected into TiO2 and collected by the electrode more efficiently than the CdS-CdSe QDSSC. Overall, these findings indicate that Mn-doped QDs could efficiently improve the performance of CdS-CdSe QDSSC. Fig. 8 shows the Nyquist plots of QDSSCs with CdS-CdSe and CdS-Mn-CdSe photoanodes measured under a simulated light source of 100 mW cm-2. In Fig. 4(a), the three semicircles correspond to the electron injection at the counter electrode–electrolyte interface and transport in the electrolyte at high frequencies (RCE), and the electron transfer at the TiO2/QD/electrolyte interface and transport in the TiO2 film (Rct) at middle frequency, and Zw in the low frequency region is attributed to the Warburg impedance of the redox couple in the electrolyte, respectively.27,28 The R ct is considered as the charge transfer resistance, which is parallel with corresponding chemical capacitance (Cµ) at the photoanode/electrolyte interface. The series resistance (R s) is the nonzero intercept on the real axis of the impedance plot denotes the
sheet resistance of TCO and the contact resistance of FTO/TiO2 at higher frequencies. The fitting results of the impedance spectra using equivalent circuit are summarized in Table 1. The Mn-doped CdSe with CdS (CdS-Mn-CdSe) exhibited the smallest charge transfer resistance (Rct = 10.44 Ω) and highest chemical capacitance (Cμ = 67.3 µF) compared to the undoped CdS-CdSe (R ct = 11.72 Ω and Cμ = 12.6 µF), indicating faster charge transport at the interface of the photoanode/electrolyte and the QDs. A higher charge transfer resistance denotes a reduction of the electron transfer rate and poor efficiency. The Zw of CdS-CdSe is calculated to be 3.37 Ω, much higher than that of CdS-Mn-CdSe (2.41 Ω), indicating the diffusion of S n2- in CdSCdSe is slower than the CdS-Mn-CdSe.29 From the results, the Mn-doping in CdSe also increases the photovoltaic performance of QDSSC than the undoped CdSe. A low value of Rct is favorable for the electron transport through a longer distance with less diffusive hindrance to some extent, and thus probably resulting in the reduced electron recombination and the longer lifetime.30 The lifetime (τe) of injected electrons in the solar cell can be estimated by the position of the peak angular frequency in Bode plots (Fig. 8b) through the equation that τe = 1/(2πfmax)
(2)
where fmax means the frequency of superimposed AC voltage. The τ e value of CdS-MnCdSe (0.40 ms) was much higher than that of CdS-CdSe (0.032 ms). In view of the decrease of Rct, the electrons are difficult to recombine with the holes in the electrolyte, which results in the decrease of charge recombination. Although it is very difficult to find out the intrinsic cause of the increase of Rct, we believe the possible reason is that the Mn2+ dopant may modify the surface or the interface of QDs to decrease the Rct and reduce the charge recombination.[C] The higher electron lifetime indicates that more electrons survive from the back reaction between the conduction band of the TiO2 and electrolyte, resulting to the improvement in the photocurrent.[D]
Also, the better coverage of the QDs on TiO2 nanoparticles can inhibit the surface recombination of electrons at the TiO2/electrolyte interface.[E] It means that the solar cell fabricated with Mndoped CdSe electrode has a reduced charge recombination and longer electron lifetime, which then resulting in the higher Voc than the Mn-undoped CdSe. The recombination mechanism of QDSSCs based on CdS-CdSe and CdS-Mn-CdSe was further investigated by open-circuit voltage decay (OCVD) measurements. Fig. 9 shows the OCVD curves of the solar cells fabricated with the CdS-CdSe and CdS-Mn-CdSe electrode under open-circuit and dark conditions. When the illumination was removed, considering no electron is transported through the external circuit, the photoinduced electrons are all recombine with the electron acceptors in the electrolyte. Therefore, the Voc would follow an exponential decay during the recombination process. Once the light was shut off, the Voc of the CdS-Mn-CdSe based QDSSC decays slower than that of CdS-CdSe film. It implies that CdS-Mn-CdSe can lead to a longer electron lifetime as compared to the CdS-CdSe based solar cell. The slower decay of CdSMn-CdSecurve indicates that many electrons survive from the reverse reaction, which is conducive to elevated performance of QDSSCs. In Fig. 10, the values of Voc, Jsc, FF and ƞ were obtained from J-V curves every 1 h (60 min) in the course of 10 h continuous light illumination. It is clear that the enhancement of photovoltaic parameters in the first 3 h is due to a capillary effect involving slow polysulfide electrolyte solution permeation into the TiO2 pores and enhanced ionic transport due to heating of the electrolyte under light illumination.34 Fig. 10a shows the Voc of CdS-Mn-CdSe is maintain almost constant 0.590 to 0.606 V, whereas a continuous change is observed in CdS-CdSe (0.568 to 0.548 V). Fig. 10b shows that the CdS-Mn-CdSe has maintain constant Jsc values, while Jsc values of CdS-CdSe CE decreased slightly. The η of CdS-CdSe decline rapidly dropping from 3.42 to 3.28 % and the CdS-
Mn-CdSe shows that the slight improvement in the efficiency was found be 4.61% (4.42 to 4.61%) in 10 h. The other parameter such as FF did not show much decrease in the course of 10 h illumination. These results confirm that the Mn+2 ions doped into QDs increase the efficiencies of light-harvesting, charge-transfer and charge collection, which results in the improvement of photovoltaic performance of the solar cell.
Conclusion: The CdS-Mn-CdSe QDs were obtained by facile SILAR and assembled to QDSSCs. The introduction of Mn +2 metal ions into QDs can improve the light harvesting efficiency to produce more excitons. Therefore, light harvesting and electron collection efficiency of a CdS-Mn-CdSe QDSSC are superior to CdS-CdSe QDSSCs. As a result, CdS-Mn-CdSe sensitized solar cells exhibited a high conversion efficiency of 4.42%, which is 22% greater than that of the CdS-CdSe sensitized solar cell (3.42%). In CdS-Mn-CdSe, the increase of the efficiencies of light-harvesting, charge-transfer and charge-collection results in the improvement of the photovoltaic performance of the QDSSC. Electrochemical impedance spectroscopy shows that the electron life time of the device based on CdS-Mn-CdSe is longer than that of a device based on CdS-CdSe, indicating that the charge recombination at the interface is reduced by the presence of the Mn-doping. Furthermore, the CdS-Mn-CdSe electrode shows superior stability than CdS-CdSe in the sulfide/polysulfide electrolyte in a working state for over 10 h. Without the ZnS passivation layer and the enhancement in the photo-electrochemical performance with quantum-dot sensitization of TiO2 is another milestone for achieving higher photo-conversion efficiencies of 4.42%. These results confirm that the method of doping ions into QDs would be considered as an effective approach to prepare high efficiency QDSSCs.
Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0014437).
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[25] J. Bisquert, A. Zaban, M. Greenshtein and I. Mora-Sero, J. Am. Chem. Soc. 126 (2004) 13550. [26] S. S. Shin, J. S. Kim, J. H. Suk, K. D. Lee, D. W. Kim, J. H. Park, I. S. Cho, K. S. Hong and J. Y. Kim, ACS Nano 7 (2013) 1027. [27] M. Adachi, M. Sakamoto, J. Jiu, Y. Ogata and S. Isoda, J. Phys. Chem. B 110 (2006) 13872. [28] C. V.V.M. Gopi, S.S. Rao, S.-K. Kim, D. Punnoose and H.-J. Kim, J. Power Sources 275 (2015) 547. [29] R. Kern, R. Sastrawan, J. Ferber, R. Stangl and J. Luther, Electrochimi. Acta 47 (2002) 4213. [30] Y. Z. Zheng, X. Tao, L. X. Wang, H. Xu, Q. Hou, W. L. Zhou and J. F. Chen, Chem. Mater., 22, (2009) 928. [31] X. Wang, Y. Liu, X. Zhou, B. Li, H. Wang, W. Zhao, H. Huang, C. Liang, X. Yu, Z. Liu and H.Shen, J. Mater. Chem. 22 (2012) 17531. [32] M.H. Jung, M.J. Chu and M.G. Kang, Chem. Commun. 48 (2012) 5016. [33] J. Chen, W. Lei and W. Deng, Nanoscale 3 (2011) 674. [34] H. McDaniel, N. Fuke, N.S. Makarov, J.M. Pietryga and V.I. Klimov, Nat. Commun. 4 (2013) 2887.
Figure captions: Fig. 1 scanning electron microscopy (SEM) images of the (a) CdS-CdSe and (b) CdS-Mn-CdSe quantum dot sensitized electrodes on the surface of TiO2 Fig. 2 Energy dispersive X-ray spectroscopy (EDX) of (a) CdS-CdSe, (b) CdS-Mn-CdSe QDs on the surface of TiO2 Fig. 3 X-ray diffraction (XRD) pattern of CdS-CdSe sensitized electrode on the surface of TiO2. Fig. 4 XPS spectrum of (A) Cd, (B) Mn, and (C) Se in CdS-CdSe and CdS-Mn-CdSe electrodes. Fig. 5 2D (a, b) and 3D (c, d) AFM images. (a) and (c) are the images of the CdS-CdSe electrode. (b) and (d) are the images of CdS-Mn-CdSe electrode. Fig. 6 UV-vis absorption spectra of (a) bare TiO2, (b) five SILAR cycles of CdS with eight cycles of CdSe (CdS-CdSe), and (c) five SILAR cycles of CdS with Mn-doped eight cycles of CdSe (CdS-Mn-CdSe) Fig. 7 (a) J–V and (b) IPCE spectra of CdS-CdSe and CdS-Mn-CdSe electrodes based QDSSC devices assembled with CuS CE and filled with a polysulfide electrolyte solution (1 M Na2S, 2 M S, and 0.1 M KCl). Fig. 8 (a) Nyquist plot and (b) Bode plot curves of the QDSSCs under one sun illumination, and the frequency ranged from 100 mHz to 500 kHz at room temperature. Fig. 9 Open-circuit voltage-decay (OCVD) measurements of the CdS-CdSe and CdS-Mn-CdSe electrodes after switching off the illumination of QDSSCs. Fig. 10 Stability test. Evolution of photovoltaic parameter values, (a) Voc, (b) Jsc, (c) FF, and (d) η, for the QDSSCs based on CdS-CdSe and CdS-Mn-CdSe electrodes.
Table 1 Performance and ESI results of QDSSC based CdS-CdSe and CdS-Mn-CdSe electrodes under one sun illumination Cell
Voc (V)
Jsc(mA/cm2)
CdS-CdSe
0.568
CdS-Mn-CdSe
0.590
ƞ%
Rs (Ω)
11.29
0.533 3.42
12.02
2.22
11.72
14.67
0.510 4.42
8.17
2.21
10.44
FF
Figures:
Fig. 1
R CE (Ω) Rct (Ω) Cμ (μF)
Zw (Ω)
τe (ms)
12.6
3.37
0.032
67.3
2.41
0.40
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
S1010-6030(15)00350-0 http://dx.doi.org/doi:10.1016/j.jphotochem.2015.09.007 JPC 10007
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
9-7-2015 25-8-2015 8-9-2015
Please cite this article as: M.Venkata-Haritha, Chandu V.V.M.Gopi, Chebrolu Venkata Thulasi-Varma, Soo-Kyoung Kim, Hee-Je Kim, Influence of Mn+2 incorporation in CdSe quantum dots for high performance of CdS-CdSe quantum dot sensitized solar cells, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2015.09.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Mn+2 incorporation in CdSe quantum dots for high performance of CdS-CdSe quantum dot sensitized solar cells M. Venkata-Haritha, Chandu V.V.M. Gopi, Chebrolu Venkata Thulasi-Varma, Soo-Kyoung Kim, Hee-Je Kim* School of Electrical Engineering, Pusan National University, Gumjeong-Ku, Jangjeong-Dong, Busan 609-735, South Korea
*Corresponding authors. Tel.: +82 51 510 7334; fax: +82 51 513 0212. E-mail addresses: [email protected] (H.-J. Kim). Graphical abstract Highlights:
Effect of Mn-doping on the photovoltaic performance of QDSSCs was investigated.
22% increase in η is achieved in Mn-doped CdS-CdSe than the bare CdS-CdSe QDSSCs.
The photo-excited electron lifetimes in the photoanode were significantly improved.
The stability of CdS-Mn-CdSe is superior to CdS-CdSe QDSSCs
Abstract: Quantum dot sensitized solar cells (QDSSCs) have attracted considerable attention recently and become promising candidates for realizing a cost-effective and facile fabrication of solar cell with improved photovoltaic performance. QDs were directly grown on the TiO2 mesostructure by the successive ionic layer absorption and reaction (SILAR) technique. QDSSC based on CdSCdSe photoanode achieves a power conversion efficiency of 3.42% under AM 1.5 G one sun illumination. The loading of Mn +2 metal ions was applied to a CdSe (CdS-Mn-CdSe) photoanode to enhance the adsorption in QDSSCs, which greatly improved the power conversion efficiency. Without the passivation layer, the solar cell based on a CdS-Mn-CdSe QD-sensitized TiO2 photoelectrode shows higher J sc (14.67 mA/cm2), Voc (0.590 V) and power conversion efficiency (4.42%) comparing to Mn-undoped CdS-CdSe QD sensitized TiO2 (Jsc: 11.29 mA/cm2, Voc: 0.568 V, and efficiency: 3.42%), which can be ascribed to superior light absorption, faster electron transport and slower charge recombination for the former. The effective electron lifetime of the device with CdS-Mn-CdSe was higher than those with CdS-CdSe, leading to more efficient electron-hole separation and slower electron recombination. The effects of Mn+2 metal ions on the chemical, physical, and photovoltaic properties of the QDSSCs have been investigated have been investigated by X-ray photon spectroscopy (XPS), UV-vis spectra, photocurrent-voltage (J-V) characteristics and electrochemical impedance spectra (EIS). Keywords: Quantum dot sensitized solar cells; Mn-doping; CdSe; Stability; Recombination;
Introduction: Dye-sensitized solar cells (DSCs) have received increasing attention in the past decade because of their simple fabrication process, low cost, and environmental friendliness and potential
to achieve efficient conversion of sunlight into electricity.1-5 The power conversion efficiency (PCE) of DSSCs based on the planar substrate of rigid conducting glass has reached >11% under under full sun illumination.4,5 In addition to DSSCs, as one of the third generation solar cells, replacing the organic dye by semiconductor quantum dots (QDs) has attracted a lot of attention in dye-sensitized solar cells (DSSCs) due to tunable band gap energy by controlling their size and composition, high extinction coefficients, large dipole moment, multiple exciton generation 9 and hot carrier collection.6-10 QDSSC adopts the principle of DSSC and generally consist of QDs, mesoporous oxide including titanium dioxide (TiO2) and zinc oxide (ZnO), polysulfide electrolyte containing Sn2- /S2- redox couples and a counter electrode (CE).11 Till the date various QD sensitizers such as CdS,12 CdSe,13 PbS,14 PbSe,15 CdTe,16 and CuInS2 17 have been reported in the QDSSCs. Among the metal semiconductor QDs, CdS and CdSe QDs are the most efficient and widely used due to high potential in light harvesting have been paid much attention. Recently, by ligand exchanging method CdSexTe1-x alloyed QDSSCs has achived more than 6% efficiency and high stability.18 Kamat et al. reported Mn-doped CdS sensitized solar cells with an efficiency of 5.4%.19 Up to now, the photovoltaic performance of QDSSCs is still lower than that of DSSCs. Because defects on the surface of QDs serve as temporary surface traps for electrons, they hinder charge transfer into TiO2 conduction bands and therefore greatly decrease the power conversion efficiency in QDSSCs.19 On the other hand, many efforts have been concentrated on designing and synthesizing QDs to achieve elevated photovoltaic performance in QDSSCs. In this scenario a doping transition metal ions would lead to new materials showing extraordinary electronic and photo-physical properties of QDs.20,21 So the doping of metal ions into the semiconductor QDs is an effective method to improve the photovoltaic performance of QDSSCs. CdSe QDs are more
attractive owing to its high potential for light harvesting in the visible light region than the CdS and PbS QDs.22,23 Therefore, the introduction of metal ions into CdSe QDs is more useful to achieve superior photovoltaic performance in the QDSSCs. Here, we have successfully synthesized and designed a high efficiency QDSSC based on Mn-CdSe QDs using successive ionic layer absorption and reaction (SILAR). This present study revealed that the doping of Mn2+ into the CdSe QDs can significantly enhance the performances, such as the light-harvesting, charge-transfer and charge-collection capabilities. As a result, the CdS-Mn-CdSe sensitized solar cell exhibited a high photo-conversion efficiency of 4.42% under simulated illumination of AM 1.5, 100 mW cm-2. The improved performance and stability is due to Mn 2+ dopant raises the conduction band of CdSe, accelerates the electron injection kinetics and reduces the charge recombination. The electron lifetime increases with the Mn +2 dopant in the CdSe, demonstrating that the Mn+2 plays an important role in preventing carrier recombination, and improving the charge transfer and collection.
Experimental work: Materials: Manganese(II) acetate tetrahydrate [Mn(CH3COO)2·4H2O], Cadmium acetate dehydrate [Cd(CH3COO)2.2H2O], sodium sulfide (Na2S), sodium sulfite (Na2SO3), sulfur (S), selenium (Se), potassium chloride (KCl), copper(II) sulfate pentahydrate [CuSO4.5H2O], sodium thiosulfate (Na2S2O3), urea (CH4N2O) are purchased from Sigma-Aldrich, and TiO2 paste (Ti-Nanoxide HT/SP) is supplied by Solaronix. All other chemicals were commercially available and of analytical grade. Preparation of CdS-CdSe and CdS-Mn-CdSe photoanode:
TiO2 films with the thickness of about 7.5 μm were prepared by doctor-blade technique using TiO2 paste of particle size 20 nm (Ti-Nanoxide HT/SP, Solaronix) in an active area of 0.27 cm2, followed by annealing at 450 oC for 30 min.24 Then, TiO2 films were immersed in the aqueous solution of 0.1 M Cd(CH3COO)2.2H2O for 5 min, rinsed with DI water, ethanol and dried with an drier. Then, they were then dipped in the aqueous solution of 0.1 M Na2S for 5 min, rinsed with DI water, ethanol and dried with a drier. The process was conducted at room temperature and repeated five times and as prepared samples is named as TiO2/CdS5. The TiO2-CdS5 electrodes were dipped in the aqueous solution of 0.1 M Cd(CH3COO)2.2H2O for 5 min at room temperature and then immersed in aqueous solution of Na2SeSO3 for 5 min at 50 oC, followed by rinsing with DI water, ethanol and drying with an drier. The process was repeated eight times and the electrodes named as TiO2/CdS5/CdSe8. The Na2SeSO3 aqueous solution was prepared by refluxing 0.2 M Se in an aqueous solution of 0.4 M Na2SO3 at 120 oC for 5 h. To incorporate the doping of Mn 2+, the molar percentage of 10% (10 mM) Mn(CH3COO)2·4H2O was mixed with the Cd(CH3COO)2.2H2O in the deposition of CdSe. This enabled the co-adsorption of Mn 2+ and Cd 2+ ions, which in turn promoted the incorporation of Mn 2+ in the CdSe film. The preparation of 5%, 15% and 20% (5 mM, 15 mM and 20 mM) Mndoped CdS-CdSe electrodes is shown in supporting information†. Finally, the prepared counter electrodes25 and photoanodes were sealed using a sealant (SX 1170-60, Solaronix) at 100 oC. The space between the electrodes was filled with polysulfide electrolyte consisting of 1 M Na2S, 2 M S, and 0.1 M KCl in a solution of methanol and water, which were present at a ratio of 7:3. The cells were tested under the illumination of 1 sun (AM 1.5 G, 100 mW cm-2).
Characterization: X-ray diffraction (XRD) analysis was performed on a D8 ADVANCE with a DAVINCI (Bruker AXS) diffractometer using Cu Kα radiation operated at 40 kV and 40 mA. The surface morphology, and elemental compositions of the electrodes were investigated using a field emission scanning electron microscope (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. The UV- visible absorption spectra were recorded using an OPTIZEN 3220UV spectrophotometer. X-ray photon spectroscopy (XPS) was performed using a VG Scientific ESCALAB 250 with monochromatic Al-Kα radiation of 1486.6 eV and with an electron take-off angle of 90°. The surface roughness of the substrates was characterized using an atomic force microscope (JPK NanoWizard II AFM, JPK Instruments, Berlin, Germany) with a scan rate of 0.8 Hz in contact mode. The current-voltage characteristics of QDSSCs were examined under one sun illumination (AM 1.5G, 100 mW cm−2) using an ABET Technologies (USA) solar simulator with an irradiance uniformity of ±3%. The incident photon to current conversion efficiency (IPCE) spectra was measured using Oriel® IQE-200™. Electrochemical impedance spectroscopy (EIS) was conducted on the QDSSCs using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) under one sun illumination, and the frequency ranged from 100 mHz to 500 kHz at room temperature. The applied bias voltage and AC amplitude were set to the Voc of the QDSSCs and 10 mV, respectively. The electrical impedance was characterized using the Nyquist and Bode plots. To perform the stability test, the Mn-CdS-CdSe-ZnSe QDSSC was continuously irradiated with an AM 1.5G, 100 mW cm−2 in working conditions, and the J-V curves were tested every 1 h.
Results and Discussion: Fig. 1 shows a scanning electron micrograph (SEM) of CdS-CdSe and CdS-Mn-CdSe QDs on a TiO2 surface. The TiO2 transparent film cannot directly used as the photoanode, because some disadvantages are found not to benefit the photovoltaic performance: (1) the pore distribution is narrow; and the pore size is usually small; (2) long wavelength light cannot be fully used since the TiO2 transparent film has no light scattering property. Therefore, the introduction of large particles into the photoanode has been used to improve the performance of the solar cells. As we can see in Fig. 1a, the nanoparticles are uniformly distributed in the film and the diameter of the particles range is 21-45 nm. When Mn+2 is doped into CdS-CdSe, the surface structures become closer than with the CdS-CdSe QDs (Fig. 1b). The gaps between the nanostructures were reduced with introducing of Mn+2, which decreased the recombination, leading to increased efficiency. Also, the diameter of the particles increased with the loading of Mn+2 in CdS-CdSe (28-56 nm). The improved surface morphology with the Mn+2 doping in QDs can increase the photovoltaic performance of the QDSSCs. EDX analysis was conducted to investigate the elemental compositions of CdS-CdSe and CdS-Mn-CdSe QD sensitizers on the surface of TiO2, and the results are shown in Fig. 2. The purity of the sample is investigated by EDX analysis. There is no impurity observed in the sample, since there are no elements except for Ti, O, Cd, S, Se and Mn in the EDX spectrum. The orbital states of the Ti, O, Cd, S, Se and Mn are seen in the EDX spectrum. Fig. 2a shows an atomic ratio of Cd:S:Se is 12%:6%:5%, demonstrating that the formation of CdS and CdSe on the surface of TiO2. Further, with the deposition of Mn+2 in QDs, upon ion exchange, 2% Mn content emerges in the film, while the Cd content drops from 12% to 11%, demonstrating the occurrence of ion
exchange from Cd to Mn (Fig. 2b). The energy levels of Mn in CdS-Mn-CdSe film is at 0.650 KeV, 5.89 KeV, and 6.536 KeV correspond to LIαβ, Kα, and Kαβ, respectively. The phase structure of TiO2-CdS-CdSe sample was measured by XRD experiments. Fig. 3 shows the XRD pattern of TiO2: the peaks appearing at 37.7, 48.2 and 70.8 o are reveal that the TiO2 nanoparticles have a tetragonal anatase structure (JCPDS: 00-021-1272). The diamond symbols in the XRD graph represent the FTO substrate, which might be due to the very low thickness of the materials or interference from the XRD signals of the high crystallinity FTO substrate. The diffraction peaks at 2θ values of 26.5, 44.0, and 54.6° are correspond to the (1 1 1), (2 2 0), and (2 2 2) planes of a CdS cubic structure (JCPDS card no. 01-080-0019). The other remaining peaks are due to the deposition of CdSe on the CdS surface. 23.7, 25.3, 27.4, 45.7 and 66.4 o can be assigned to the (1 0 0), (0 0 2), (2 0 1), (1 0 3), and (2 1 0) facet of CdSe hexagonal phase (JCPDS: 01-077-2307), respectively. XPS was measured to know the composition and chemical bond configuration of pure and Mg doped CdSe thin films. Fig. 4 shows the Cd 3d, Mn 2p and Se 3d spectra of CdS-CdSe and CdS-Mn-CdSe films, respectively. The binding energy peaks of Cd 3d5/2 observed at 405.1 and 404.6 eV for CdS-CdSe and CdS-Mn-CdSe films respectively, clearly show that Mn2+ is replacing Cd2+. The binding energy of the Cd 3d3/2 peak for CdS-CdSe and CdS-Mn-CdSe film is observed at 411.8 and 411.3 eV, respectively. The peak deconvolution process in CdS-Mn-CdSe (Fig. 4B) clearly shows that the Mn 2p spectrum of the film is composed of two peaks, at 642 eV (Mn 2p3/2) and 652.8 eV (Mn 2p1/2), indicating the state of Mn2+ in QDs. The spectrum for Se 3d was fitted yielding a single peak at the binding energy of 53.6 eV and 53.1 eV in CdS-CdSe and CdS-MnCdSe, respectively. This is also confirmed through the shift of the corresponding Cd and Se peaks
toward lower binding energy value in CdS-Mn-CdSe than the CdS-CdSe. The XPS spectra clearly detects and confirms the presence of Mn in the CdS-Mn-CdSe. To investigate the possibility of changes in surface morphology and roughness by the Mndoping, AFM was carried out for the CdS-CdSe and CdS-Mn-CdSe. Fig. 5 shows the twodimensional (2D) and three-dimensional (3D) AFM images of CdS-CdSe and CdS-Mn-CdSe. The root mean square (RMS) surface roughness of the CdS-CdSe and CdS-Mn-CdSe films are 12.64 nm and 9.17 nm, respectively, as shown in Fig. 5(c) and 5(d). AFM studies indicated that the surface roughness of the CdS-Mn-CdSe film was smaller than that of the CdS-CdSe film. The lower surface roughness is mainly due to the large particle sizes of CdS-Mn-CdSe as compared with CdS-CdSe nanostructures, which might be due to the lower radii of Mn than Cd. The power conversion efficiency of the QD solar cell mainly depends on the small roughness of the QD surface, and a reduction of the surface roughness in CdS-Mn-CdSe tends to improve the conversion efficiency of the QDSSCs, which affects the optical properties.21 The optical performance of TiO2, CdS-CdSe and CdS-Mn-doped-CdSe are characterized by absorbance. Fig. 6 displays the UV-visible spectral curves of the films loaded with CdS-CdSe and CdS-Mn-CdSe QDs. The result shows that the absorbance of the CdS-Mn-CdSe QDs is much higher than that of the CdS-CdSe QDs. The high absorbance of the photoelectrode might be attributed to: the effects of Mn+2 doped into QDs and a greater loading amount of QDs. The greater loading amount of QDs is due to the presence of Mn+2 in the reaction solution might promote the deposition of QDs. It is found that in the same wavelength range, the photoanode of CdS-MnCdSe (688 nm) has broader range of light absorption than the undoped QDs (612 nm); this is corresponding to an increase in current density of Mn-doped solar cell. So we can conclude that
the incorporation of Mn+2 ion in CdSe narrowed the band gap of QDs, and extended the absorption wavelength range.
Fig. 7(a) shows the J-V characteristics of the cells characteristics of the solar cells measured under the illumination of one sun (AM 1.5, 100 mW cm-2). Key performance parameters of the cells with CdS-CdSe and CdS-Mn-CdSe are summarized in Table 1. The optimization of the Mndoping in CdS–CdSe is shown in Fig. S1 and Table S1 of the ESI.† Based on CdS-Mn-CdSe electrode, the solar cell gives a Jsc of 14.67 mA/cm2, a Voc of 0.590 V, and a FF of 0.510, yielding a power conversion efficiency of 4.42%. The photovoltaic performance of CdS-CdSe (Voc: 0.568 V, J sc: 11.29 mA/cm2, FF: 0.533, and ƞ:3.42%) is lower than that of CdS-Mn-CdSe electrode. The remarkable improvement of Jsc with Mn-doping is attributed to the excited charge characteristics, such as generation, injection or collection. It is finding that after Mn doping in CdSe, a significant red shift of the absorption profile is observed, which leads to the conduction band and valence band all improve, forming a cascade energy level which is more conducive to charge transport inside the solar cell. That is advantageous to the electron injection and hole-recovery, reducing the recombination of electron-hole with Mn+2 doping, and improving the ability of photoanode to capture light thus improving the photocurrent and ultimately improving the power conversion efficiency. It is well known that for the solar cells, electron generation characteristics can be evaluated by the incident photon-to-current conversion efficiency (IPCE). Fig. 7(b) shows the IPCE spectra of QDSSCs assembled with CdS-CdSe and CdS-Mn-CdSe QDs, respectively. The maximum IPCE of a CdS-Mn-CdSe sensitized solar cell reaches 89% at 480 nm, which is enhanced by 25%
compared to the maximum IPCE of 71% at 414 nm for a CdS-CdSe sensitized one. The IPCE is substantially improved from visible to near infrared radiation region 800 nm. The IPCE depends on both the absorption of light and the collection of charges. However, as it is demonstrated in eqn (1), the IPCE value of QDSSCs are determined by the light harvesting efficiency of the film (ηlh), the injection (ηinj) and charge collection efficiencies (ηcol) of the photoelectrons, which are affected by the structure of the photoelectrode. 25,26 =
×
×
(1)
The higher ηlh for the CdS-Mn-CdSe QDSSC, which can be confirmed by the above UV-vis and IPCE data, contributes greatly to the Jsc. The ηcol is related to both electron transport rate and charge recombination rate in QDSSC. The IPCE results indicate that excited electrons in CdS-MnCdSe QDSSC can be injected into TiO2 and collected by the electrode more efficiently than the CdS-CdSe QDSSC. Overall, these findings indicate that Mn-doped QDs could efficiently improve the performance of CdS-CdSe QDSSC. Fig. 8 shows the Nyquist plots of QDSSCs with CdS-CdSe and CdS-Mn-CdSe photoanodes measured under a simulated light source of 100 mW cm-2. In Fig. 4(a), the three semicircles correspond to the electron injection at the counter electrode–electrolyte interface and transport in the electrolyte at high frequencies (RCE), and the electron transfer at the TiO2/QD/electrolyte interface and transport in the TiO2 film (Rct) at middle frequency, and Zw in the low frequency region is attributed to the Warburg impedance of the redox couple in the electrolyte, respectively.27,28 The R ct is considered as the charge transfer resistance, which is parallel with corresponding chemical capacitance (Cµ) at the photoanode/electrolyte interface. The series resistance (R s) is the nonzero intercept on the real axis of the impedance plot denotes the
sheet resistance of TCO and the contact resistance of FTO/TiO2 at higher frequencies. The fitting results of the impedance spectra using equivalent circuit are summarized in Table 1. The Mn-doped CdSe with CdS (CdS-Mn-CdSe) exhibited the smallest charge transfer resistance (Rct = 10.44 Ω) and highest chemical capacitance (Cμ = 67.3 µF) compared to the undoped CdS-CdSe (R ct = 11.72 Ω and Cμ = 12.6 µF), indicating faster charge transport at the interface of the photoanode/electrolyte and the QDs. A higher charge transfer resistance denotes a reduction of the electron transfer rate and poor efficiency. The Zw of CdS-CdSe is calculated to be 3.37 Ω, much higher than that of CdS-Mn-CdSe (2.41 Ω), indicating the diffusion of S n2- in CdSCdSe is slower than the CdS-Mn-CdSe.29 From the results, the Mn-doping in CdSe also increases the photovoltaic performance of QDSSC than the undoped CdSe. A low value of Rct is favorable for the electron transport through a longer distance with less diffusive hindrance to some extent, and thus probably resulting in the reduced electron recombination and the longer lifetime.30 The lifetime (τe) of injected electrons in the solar cell can be estimated by the position of the peak angular frequency in Bode plots (Fig. 8b) through the equation that τe = 1/(2πfmax)
(2)
where fmax means the frequency of superimposed AC voltage. The τ e value of CdS-MnCdSe (0.40 ms) was much higher than that of CdS-CdSe (0.032 ms). In view of the decrease of Rct, the electrons are difficult to recombine with the holes in the electrolyte, which results in the decrease of charge recombination. Although it is very difficult to find out the intrinsic cause of the increase of Rct, we believe the possible reason is that the Mn2+ dopant may modify the surface or the interface of QDs to decrease the Rct and reduce the charge recombination.[C] The higher electron lifetime indicates that more electrons survive from the back reaction between the conduction band of the TiO2 and electrolyte, resulting to the improvement in the photocurrent.[D]
Also, the better coverage of the QDs on TiO2 nanoparticles can inhibit the surface recombination of electrons at the TiO2/electrolyte interface.[E] It means that the solar cell fabricated with Mndoped CdSe electrode has a reduced charge recombination and longer electron lifetime, which then resulting in the higher Voc than the Mn-undoped CdSe. The recombination mechanism of QDSSCs based on CdS-CdSe and CdS-Mn-CdSe was further investigated by open-circuit voltage decay (OCVD) measurements. Fig. 9 shows the OCVD curves of the solar cells fabricated with the CdS-CdSe and CdS-Mn-CdSe electrode under open-circuit and dark conditions. When the illumination was removed, considering no electron is transported through the external circuit, the photoinduced electrons are all recombine with the electron acceptors in the electrolyte. Therefore, the Voc would follow an exponential decay during the recombination process. Once the light was shut off, the Voc of the CdS-Mn-CdSe based QDSSC decays slower than that of CdS-CdSe film. It implies that CdS-Mn-CdSe can lead to a longer electron lifetime as compared to the CdS-CdSe based solar cell. The slower decay of CdSMn-CdSecurve indicates that many electrons survive from the reverse reaction, which is conducive to elevated performance of QDSSCs. In Fig. 10, the values of Voc, Jsc, FF and ƞ were obtained from J-V curves every 1 h (60 min) in the course of 10 h continuous light illumination. It is clear that the enhancement of photovoltaic parameters in the first 3 h is due to a capillary effect involving slow polysulfide electrolyte solution permeation into the TiO2 pores and enhanced ionic transport due to heating of the electrolyte under light illumination.34 Fig. 10a shows the Voc of CdS-Mn-CdSe is maintain almost constant 0.590 to 0.606 V, whereas a continuous change is observed in CdS-CdSe (0.568 to 0.548 V). Fig. 10b shows that the CdS-Mn-CdSe has maintain constant Jsc values, while Jsc values of CdS-CdSe CE decreased slightly. The η of CdS-CdSe decline rapidly dropping from 3.42 to 3.28 % and the CdS-
Mn-CdSe shows that the slight improvement in the efficiency was found be 4.61% (4.42 to 4.61%) in 10 h. The other parameter such as FF did not show much decrease in the course of 10 h illumination. These results confirm that the Mn+2 ions doped into QDs increase the efficiencies of light-harvesting, charge-transfer and charge collection, which results in the improvement of photovoltaic performance of the solar cell.
Conclusion: The CdS-Mn-CdSe QDs were obtained by facile SILAR and assembled to QDSSCs. The introduction of Mn +2 metal ions into QDs can improve the light harvesting efficiency to produce more excitons. Therefore, light harvesting and electron collection efficiency of a CdS-Mn-CdSe QDSSC are superior to CdS-CdSe QDSSCs. As a result, CdS-Mn-CdSe sensitized solar cells exhibited a high conversion efficiency of 4.42%, which is 22% greater than that of the CdS-CdSe sensitized solar cell (3.42%). In CdS-Mn-CdSe, the increase of the efficiencies of light-harvesting, charge-transfer and charge-collection results in the improvement of the photovoltaic performance of the QDSSC. Electrochemical impedance spectroscopy shows that the electron life time of the device based on CdS-Mn-CdSe is longer than that of a device based on CdS-CdSe, indicating that the charge recombination at the interface is reduced by the presence of the Mn-doping. Furthermore, the CdS-Mn-CdSe electrode shows superior stability than CdS-CdSe in the sulfide/polysulfide electrolyte in a working state for over 10 h. Without the ZnS passivation layer and the enhancement in the photo-electrochemical performance with quantum-dot sensitization of TiO2 is another milestone for achieving higher photo-conversion efficiencies of 4.42%. These results confirm that the method of doping ions into QDs would be considered as an effective approach to prepare high efficiency QDSSCs.
Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0014437).
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Figure captions: Fig. 1 scanning electron microscopy (SEM) images of the (a) CdS-CdSe and (b) CdS-Mn-CdSe quantum dot sensitized electrodes on the surface of TiO2 Fig. 2 Energy dispersive X-ray spectroscopy (EDX) of (a) CdS-CdSe, (b) CdS-Mn-CdSe QDs on the surface of TiO2 Fig. 3 X-ray diffraction (XRD) pattern of CdS-CdSe sensitized electrode on the surface of TiO2. Fig. 4 XPS spectrum of (A) Cd, (B) Mn, and (C) Se in CdS-CdSe and CdS-Mn-CdSe electrodes. Fig. 5 2D (a, b) and 3D (c, d) AFM images. (a) and (c) are the images of the CdS-CdSe electrode. (b) and (d) are the images of CdS-Mn-CdSe electrode. Fig. 6 UV-vis absorption spectra of (a) bare TiO2, (b) five SILAR cycles of CdS with eight cycles of CdSe (CdS-CdSe), and (c) five SILAR cycles of CdS with Mn-doped eight cycles of CdSe (CdS-Mn-CdSe) Fig. 7 (a) J–V and (b) IPCE spectra of CdS-CdSe and CdS-Mn-CdSe electrodes based QDSSC devices assembled with CuS CE and filled with a polysulfide electrolyte solution (1 M Na2S, 2 M S, and 0.1 M KCl). Fig. 8 (a) Nyquist plot and (b) Bode plot curves of the QDSSCs under one sun illumination, and the frequency ranged from 100 mHz to 500 kHz at room temperature. Fig. 9 Open-circuit voltage-decay (OCVD) measurements of the CdS-CdSe and CdS-Mn-CdSe electrodes after switching off the illumination of QDSSCs. Fig. 10 Stability test. Evolution of photovoltaic parameter values, (a) Voc, (b) Jsc, (c) FF, and (d) η, for the QDSSCs based on CdS-CdSe and CdS-Mn-CdSe electrodes.
Table 1 Performance and ESI results of QDSSC based CdS-CdSe and CdS-Mn-CdSe electrodes under one sun illumination Cell
Voc (V)
Jsc(mA/cm2)
CdS-CdSe
0.568
CdS-Mn-CdSe
0.590
ƞ%
Rs (Ω)
11.29
0.533 3.42
12.02
2.22
11.72
14.67
0.510 4.42
8.17
2.21
10.44
FF
Figures:
Fig. 1
R CE (Ω) Rct (Ω) Cμ (μF)
Zw (Ω)
τe (ms)
12.6
3.37
0.032
67.3
2.41
0.40
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10