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The reduction of charge recombination and performance enhancement by the surface modification of Si quantum dot-sensitized solar cell
Electrochimica Acta 87 (2013) 213–217
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The reduction of charge recombination and performance enhancement by the surface modification of Si quantum dot-sensitized solar cell Hyunwoong Seo a,b,∗ , Yuting Wang a , Giichiro Uchida a , Kunihiro Kamataki a , Naho Itagaki a,c , Kazunori Koga a , Masaharu Shiratani a,b a
Graduate School of Information Science and Electrical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan Center of Plasma Nano-interface Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan c PRESTO, Japan Science and Technology Agency, 5 Sanbancho, Chiyoda-ku, Tokyo, 102-0075, Japan b
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 20 September 2012 Accepted 20 September 2012 Available online 29 September 2012 Keywords: Si quantum dot Quantum dot-sensitized solar cell Surface modification Barrier layer
a b s t r a c t Multiple exciton generation solar cell based on quantum dots has higher theoretical efficiency than single exciton generation solar cell. In this work, Si quantum dots with the diameter of 10 nm were fabricated by the multi-hollow discharge plasma chemical vapor deposition and applied to the quantum dot-sensitized solar cell. In this cell, there was considerable electron recombination with redox electrolyte in the Si–TiO2 network because of large Si particle size. For the reduction of recombination and the enhancement of performance, a barrier layer was introduced. Zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) and zinc acetate dihydrate (Zn(CH3 COO)2 ·H2 O) were employed as precursors for surface modification. Consequently, short circuit current and open circuit voltage of the cells were increased by the surface modification with both precursors. The improvement was ascribed to the inhibition of electrons back transfer from TiO2 to the electrolyte by the barrier layer. This result clearly demonstrated that the surface modification with ZnO was advantageous for the performance enhancement. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Photochemical solar cells such as dye-sensitized and organic solar cells have much attracted in the research fields because of the low manufacturing cost and simple fabrication process. However, they have obvious limitation in the performance because of their characteristics of single exciton generation (SEG) which means only one electron generation by one incident photon. Therefore, quantum dot-sensitized solar cells (QDSCs) using narrow band gap semiconductors such as CdS, CdSe, PbS and PbSe have attracted considerate attention recently [1–7]. The unique characteristics such as tunable optical property and multiple exciton generation (MEG) are expected to enhance the efficiency over Shockley and Queisser limit of 33% [8]. MEG is able to produce multiple excitons by one incident photon in semiconductor nano-crystals and represents a promising route to increase solar conversion efficiency. Hanna and Nozik already proved the possibility of performance enhancement in MEG solar cell [9]. Its theoretical efficiency is 11% higher than that of SEG solar cell. There are many kinds of QDs developed so far and Cd compounds-sensitized solar cell has the best conversion efficiency
∗ Corresponding author at: Center of Plasma Nano-interface Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan. Tel.: +81 92 802 3723; fax: +81 92 802 3723. E-mail address: woong [email protected] (H. Seo). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.09.087
[10]. However, they have some disadvantages such as toxicity and scarcity. Therefore, we focused on Si QD as the alternative to conventional QDs. Si is one of good QD materials and has abundance and absence of toxicity as a dominant material in the photovoltaics. In addition, Si QD has high stability against light soaking as compared with a-Si:H films and a high optical absorption coefficient due to its quantum size effect as compared with c-Si films. However, there are few reports about Si QDSC despite its unique characteristics because the collection of Si QDs is too difficult contrary to easy film deposition. In our previous researches, this problem was solved by multi-hollow discharge plasma chemical vapor deposition (CVD) and carrier generation was proved in the wavelength range of less than 550 nm in Si QDSCs [11,12]. However, there was considerable electron recombination in Si QDSC because of its large particle size. For its improvement, the surface modification was investigated using a barrier layer in this work. The core–shell structure was introduced because it prevents the charge recombination at the TiO2 /electrolyte interface and increases photocurrent. It consists of a nano-porous TiO2 covered with a shell of ZnO as shown in Fig. 1. ZnO exhibits very similar photoelectrochemical properties as TiO2 . Its band gap and electron injection efficiency are also analogous to those of TiO2 . It has additional advantages: a higher flat-band potential to achieve higher photovoltage and suitability for the fabrication of nano-structures with outstanding transport and optical properties [13,14]. Zinc acetate dihydrate (Zn(CH3 COO)2 ·H2 O)
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H. Seo et al. / Electrochimica Acta 87 (2013) 213–217
Fig. 1. Structure of Si quantum dot-sensitized solar cell with the surface modification.
[15,16] and zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) [17,18] were used for a barrier layer. In order to verify the effect of a barrier layer and the enhancement of the performance, the structural characteristics of deposited film, photovoltaic performance and internal electrochemical impedance of completed Si QDSC with a barrier layer were examined. Consequently, Si QDSC with effective ZnO barrier layer had the improvement on the overall performance.
Fig. 3. Band structure of surface-modified Si quantum dot-sensitized solar cell.
200 W. To avoid their deposition on the wall of the reactor, the temperature of the wall was kept at 250 ◦ C. SiH4 + e− → SiH2 + H2 + e− SiH4 + SiH2 → Si2 H6 Si2 H6 + SiH2 → Si3 H8
2. Experimental
Si3 H8 + SiH2 → Si4 H10
2.1. Synthesis of Si nano-particles
Si4 H10 + e− → Si4 H− (grow) 10
Si nano-particles were synthesized by multi-hollow discharge plasma CVD. Schematic of the deposition method is shown in Fig. 2. Hydrogen (H2 ) diluted silane (SiH4 ) gas was introduced from the bottom of the reactor. It flows through the hollows of the electrodes and pumped out from the top of the reactor. SiH4 was converted to high-ordered silane with SiH2 and ionized. Then, crystalline Si nano-particles were nucleated and grown in the discharge plasma region. Eq. (1) simplified this process. They were transported to the downstream region with gas flow and collected by stainless meshes [11]. The crystallinity and size of Si nano-particles were controllable by adjusting the gas ration and working pressure, respectively. For the fabrication of Si nano-particles with the diameter of 10 nm used in this work, gas flow rates of H2 and SiH4 were 449 and 1.5 sccm, and total pressure was 655 Pa. High frequency voltage of 60 MHz was applied to the powered electrode and the discharge power was
Fig. 2. Schematic of multi-hollow discharge plasma CVD.
(1)
2.2. Fabrication of Si QDSCs Si QDSCs were fabricated as follows. FTO substrates (10 per sq., Asahi Glass Co.) were used as the transparent conductive oxide (TCO) to make the photo and counter electrodes. The substrates were cleaned by sonicating in ethyl alcohol, and dried using a stream of nitrogen. A uniform TiO2 layer was pasted and sintered at 450 ◦ C for 30 min. And nano-porous Si–TiO2 films were pasted on sintered TiO2 layer by the doctor blade method. And then FTO/TiO2 /Si–TiO2 substrates were surface-modified by using Zn(CH3 COO)2 ·H2 O and Zn(NO3 )2 ·6H2 O aqueous solutions. Its energy band diagram is shown in Fig. 3. The photo electrodes were sintered at 200 ◦ C for 30 min again for the formation of ZnO. Deposited films were characterized by their morphological and compositional properties. The scanning electron microscopy (SEM, S-4200, Hitachi) operated at 15 kV was used to characterize the microstructure of the films. And the formation of the deposited films was ascertained by measuring X-ray diffraction (XRD, X’pert PRO MRD, Philips) under optimized operating conditions of 30 mA and 40 kV. The gilding counter electrode with a thickness of about 100 nm was deposited by DC sputter (JEC-550, JEOL) at a power of 1.5 kV A and a working pressure of 2 Pa. Polysulfide electrolyte is suitable as the redox couple in case of QDSC because QDs are degraded by I− /I3 − electrolyte [19]. Then, polysulfide electrolyte allows strong interactions between Pt and S2− . S2− ions adsorb onto the surface of Pt and suppress the conductivity, catalytic activity, and electrochemical surface area of the counter electrode unlike Au counter electrode [20]. Therefore, gilding counter electrode was used in this work. After that, the photo and counter electrodes were sealed using a thermoplastic hot-melt sealant (SX 1170-25, Solaronix) with a thickness of 25 m. The sealed Si QDSCs were completed by injecting a redox electrolyte through a pre-drilled hole into the counter electrode. The redox electrolyte consisted of 0.5 M Na2 S, 2 M S and 0.2 M KCl in water and methanol mixed
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Fig. 6. XRD patterns of photo electrode deposited with Zn(NO3 )2 ·6H2 O (bottom) and Zn(CH3 COO)2 ·H2 O (top). Fig. 4. Absorbed photon to current conversion efficiency of Si QDSC and the photovoltaic performance under various light intensities (inset).
solution at a volume ratio of 3:7. Another Si QDSC was fabricated as a reference under identical conditions except for a barrier layer. Before their characterization, the completed cells were stored in the dark under open-circuit conditions for 24 h to allow the electrolyte to penetrate into the pores. The photovoltaic performance was measured under 1 sun (air mass 1.5, 100 mW/cm2 ) by a source meter (Model 2400, Keithley Instrument, Inc.). And incident photon to current conversion efficiency (IPCE, SM-250-P1, Bunkoukeiki) was measured from 250 nm to 1000 nm. During their irradiance and characterization, the cells were covered with a black mask fitting the active area of the cell. The irradiated cell area was 0.25 cm2 . The I–V characteristic curve and Eq. (2) were used to calculate the short-circuit current (ISC ) and density (JSC ), open-circuit voltage (VOC ), fill factor (FF) and the overall efficiency () [21,22]. The internal impedance of the DSCs was measured by electrochemical impedance spectroscopy (EIS, SP-150, Biologic SAS). The EIS spectra were measured over the frequency range from 10 mHz to 1 MHz at room temperature. The applied bias voltage and AC amplitude were set at VOC of the DSC and 10 mV, respectively. The electrical impedances were characterized using the Nyquist diagram. =
Pmax FFVOC JSC × 100 = × 100(%) Pin Pin
(2)
was impossible by the limitation of equipment, the rapid increase from 2 Eg point confirmed MEG effect. This was also confirmed by the photovoltaic performance under various light intensities in the inset of Fig. 4. The performance of SEG solar cell is generally decreased with stronger light intensity because SEG sensitizer generates only one electron with incident energy more than 1 Eg (band-gap energy). On the other hand, that of MEG solar cell is not decreased with stronger light because it generates multiple electrons with energy more than 2 Eg . The performance of our Si QDSC was higher than the base line (gray) under the incident light over 1 sun (100 mW/cm2 ). This result also showed MEG of QD. The reactions from Zn(CH3 COO)2 ·H2 O and Zn(NO3 )2 ·6H2 O to ZnO are shown in Eqs. (3) and (4), respectively. This surface modification was verified by SEM and XRD results. Fig. 5 shows SEM images of photo electrode surface-modified with (a) Zn(CH3 COO)2 ·H2 O and (b) Zn(NO3 )2 ·6H2 O. Both solutions were well penetrated into pores and covered Si and TiO2 nano-particles. Their nano-porous structure was maintained. The formation of ZnO was confirmed by XRD patterns in Fig. 6. The diffraction peaks of TiO2 , SnO2 of FTO, Si, and ZnO were denoted in the figure. Zn(CH3 COO)2 H2 O → ZnO + 2CH3 COOH
Zn(NO3 )2 6H2 O + H2 O → Zn2+ + 2NO3 − + 7H2 O, Zn2+ + NO3 − + 2e− → ZnO + NO2 −
3. Results and discussion The characteristics of Si QDs were confirmed with the rapid increase of absorbed photon to current conversion efficiency (APCE) in the range of short wavelength. Fig. 4 shows APCE of Si QDSC. In this case, FTO substrate of the photo electrode was substituted by Al sputtered quartz for the penetration of short wavelength. APCE was increased from 400 nm and rapidly climbed at 2 Eg point (280 nm). Although the measurement in the range of below 250 nm
(3)
(4)
Fig. 7 shows IPCE of standard and surface-modified Si QDSCs. After the surface modification, the overall efficiency was enhanced with both modifications in the visible light region from 300 nm to 700 nm including increased peaks around 350 nm. Especially, Si QDSC surface-modified by Zn(NO3 )2 ·6H2 O had wider absorbance than Si QDSC surface-modified by Zn(CH3 COO)2 ·H2 O. The reasons of the enhancement are the reduction of electron recombination by
Fig. 5. SEM images of photo electrode deposited with (a) Zn(CH3 COO)2 ·H2 O and (b) Zn(NO3 )2 ·6H2 O.
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Fig. 7. Incident photon to current conversion efficiencies of standard and surfacemodified Si quantum dot-sensitized solar cells.
a barrier layer and better electron injection from Si QD by a shift of band edge. The enhancement was also verified by EIS analysis. Fig. 8 shows Nyquist diagram of standard and surface-modified Si QDSCs. In general Nyquist diagram for EIS analysis, three semicircles are observed in the frequency range, and the resistance detected over 1 MHz is affected by the sheet resistance of the TCO. The three semicircles are attributed to the charge transportation at the counter electrode, the electron transfer at the TiO2 /Si QD/electrolyte interfaces, and the charge transportation by ions in the electrolyte in the high, middle and low frequency ranges, respectively [21,23–26]. But the results in Fig. 8 looks like one large semi-circle because 2nd semi-circle in the middle frequency range was much larger than 1st and 3rd semicircles. That is, the charge transportation at the counter electrode and in the electrolyte was not influenced on the cell performance because it is relatively enough fast as compared with the electron transfer at the TiO2 /Si QD/electrolyte interfaces. As can be seen, internal impedances of both surface-modified Si QDSCs were much decreased as compared with standard Si QDSC. It means that the internal electron transfer was improved at TiO2 /Si QD/electrolyte interfaces. In other words, the electron recombination at these interfaces was much reduced by the surface modification.Above enhanced IPCE and reduced internal impedance were reflected in the overall performance. As mentioned above, overall performance was enhanced with a barrier layer. The reduction of electron recombination and better electron injection from Si QD made the photocurrent increased. And well covered TiO2 particles by a barrier layer and a shift of band edge increased the photovoltage. The I–V characteristic curves are shown in Fig. 9. The performance of standard Si QDSC was defined by VOC of 0.11 V, JSC of 0.64 mA/cm2 , and efficiency of 0.020%. The performance of Si QDSCs with a
Fig. 8. Electrochemical impedance spectra of standard and surface-modified Si quantum dot-sensitized solar cells.
Fig. 9. I–V characteristic curves of standard and surface-modified Si quantum dotsensitized solar cells.
barrier layer were increased over 2 times. The surface-modified Si QDSC with Zn(CH3 COO)2 ·H2 O was defined by VOC of 0.18 V, JSC of 0.87 mA/cm2 , and efficiency of 0.046% and the surface-modified Si QDSC with Zn(NO3 )2 ·6H2 O was defined by VOC of 0.15 V, JSC of 1.13 mA/cm2 , and efficiency of 0.051%. 4. Conclusions In this work, Si nano-particles synthesized by multi-hollow discharge plasma CVD with the diameter of 10 nm were used for the fabrication of Si QDSCs. There was considerable electron recombination because of its large particle size. To prevent the recombination in the Si–TiO2 nano-porous network, ZnO barrier layer was applied to the photo electrode as the surface modification. Zn(NO3 )2 ·6H2 O and Zn(CH3 COO)2 ·H2 O) were employed as precursor to surface-modify. As a result, IPCE and internal impedance of Si QDSCs were much increased after the surface modification and this enhancement was reflected in the overall performance. VOC and JSC were much increased with the surface modification and the efficiencies were increased over 2 times. These results clearly demonstrated that the surface modification with ZnO was advantageous for the performance enhancement. Acknowledgment This work was partially supported by TEMCO Memorial Foundation and New Energy and Industrial Technology Development Organization (NEDO). References [1] Y. Lee, B. Huang, H. Chien, Highly efficient cdse-sensitized TiO2 photoelectrode for quantum-dot-sensitized solar cell applications, Chemistry of Materials 20 (2008) 6903. [2] N. Zhao, T. Osedach, L. Chang, S. Geyer, D. Wanger, M. Binda, A. Arango, M. Bawendi, V. Bulovic, Colloidal PbS quantum dot solar cells with high fill factor, ACS Nano 4 (2010) 3743. [3] D. Guimard, R. Morihara, D. Bordel, K. Tanabe, Y. Wakayama, M. Nishioka, Y. Arakawa, Fabrication of InAs/GaAs quantum dot solar cells with enhanced photocurrent and without degradation of open circuit voltage, Applied Physics Letters 96 (2010) 203507. [4] T. Ju, R. Graham, G. Zhai, Y. Rodriguez, A. Breeze, L. Yang, G. Alers, S. Carter, High efficiency mesoporous titanium oxide PbS quantum dot solar cells at low temperature, Applied Physics Letters 97 (2010) 043106. [5] X. Wang, G. Koleilat, J. Tang, H. Liu, I. Kramer, R. Debnath, L. Brzozowski, D. Aaron, R. Barkhouse, L. Levina, S. Hoogland, E. Sargent, Tandem colloidal quantum dot solar cells employing a graded recombination layer, Nature Photonics 5 (2011) 480. [6] Y. Tachibana, H. Akiyama, Y. Ohtsuka, T. Torimoto, S. Kuwabata, CdS quantum dots sensitized TiO2 sandwich type photoelectrochemical solar cells, Chemistry Letters 36 (2007) 88. [7] I. Robel, V. Subramanian, M. Kuno, P. Kamat, Quantum dot solar cells. harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films, Journal of the American Chemical Society 128 (2006) 2385.
H. Seo et al. / Electrochimica Acta 87 (2013) 213–217 [8] C. Chang, Y. Lee, Chemical bath deposition of CdS quantum dots onto mesoscopic TiO2 films for application in quantum-dot-sensitized solar cells, Applied Physics Letters 91 (2007) 053503. [9] M. Hanna, A. Nozik, Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers, Journal of Applied Physics 100 (2006) 074510. [10] Y. Lee, Y. Lo, Highly efficient quantum-dot-sensitized solar cell based on Cosensitization of CdS/CdSe, Advanced Functional Materials 19 (2009) 604. [11] Y. Kawashima, K. Nakahara, H. Sato, G. Uchida, K. Koga, M. Shiratani, M. Kondo, Quantum dot-sensitized solar cells using Si nanoparticles, Transactions of the Materials Research Society of Japan 35 (2010) 597. [12] G. Uchida, K. Yamamoto, Y. Kawashima, M. Sato, K. Nakahara, K. Kamataki, N. Itagaki, K. Koga, M. Kondo, M. Shiratani, Surface nitridation of silicon nanoparticles using double multi-hollow discharge plasma CVD, Physica Status Solidi C 8 (2011) 3017. [13] C. Hsu, N. Wu, Synthesis and photocatalytic activity of ZnO/ZnO2 composite, Journal of Photochemistry and Photobiology A: Chemistry 172 (2005) 269. [14] G. Marci, V. Augugliaro, M. Lopez-Munoz, C. Martin, L. Palmisano, V. Rives, M. Schiavello, R. Tilley, A. Venezia, Preparation characterization and photocatalytic activity of polycrystalline ZnO/TiO2 systems. 2. Surface, bulk characterization, and 4-nitrophenol photodegradation in liquid-solid regime, Journal of Physical Chemistry B 105 (2001) 1033. [15] H. Seo, M. Son, J. Kim, I. Shin, K. Prabakar, H. Kim, Enhancement in the photovoltaic performance of a dye-sensitized solar cell by an optimized ZnO barrier layer, Physica Scripta T139 (2010) 014029, http://dx.doi.org/10.1088/0031-8949/2010/T139/014029. [16] S. Rani, P. Suri, P. Shishodia, R. Mehra, Synthesis of nanocrystalline ZnO powder via sol-gel route for dye-sensitized solar cells, Solar Energy Materials and Solar Cells 92 (2008) 1639.
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The reduction of charge recombination and performance enhancement by the surface modification of Si quantum dot-sensitized solar cell Hyunwoong Seo a,b,∗ , Yuting Wang a , Giichiro Uchida a , Kunihiro Kamataki a , Naho Itagaki a,c , Kazunori Koga a , Masaharu Shiratani a,b a
Graduate School of Information Science and Electrical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan Center of Plasma Nano-interface Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan c PRESTO, Japan Science and Technology Agency, 5 Sanbancho, Chiyoda-ku, Tokyo, 102-0075, Japan b
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 20 September 2012 Accepted 20 September 2012 Available online 29 September 2012 Keywords: Si quantum dot Quantum dot-sensitized solar cell Surface modification Barrier layer
a b s t r a c t Multiple exciton generation solar cell based on quantum dots has higher theoretical efficiency than single exciton generation solar cell. In this work, Si quantum dots with the diameter of 10 nm were fabricated by the multi-hollow discharge plasma chemical vapor deposition and applied to the quantum dot-sensitized solar cell. In this cell, there was considerable electron recombination with redox electrolyte in the Si–TiO2 network because of large Si particle size. For the reduction of recombination and the enhancement of performance, a barrier layer was introduced. Zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) and zinc acetate dihydrate (Zn(CH3 COO)2 ·H2 O) were employed as precursors for surface modification. Consequently, short circuit current and open circuit voltage of the cells were increased by the surface modification with both precursors. The improvement was ascribed to the inhibition of electrons back transfer from TiO2 to the electrolyte by the barrier layer. This result clearly demonstrated that the surface modification with ZnO was advantageous for the performance enhancement. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Photochemical solar cells such as dye-sensitized and organic solar cells have much attracted in the research fields because of the low manufacturing cost and simple fabrication process. However, they have obvious limitation in the performance because of their characteristics of single exciton generation (SEG) which means only one electron generation by one incident photon. Therefore, quantum dot-sensitized solar cells (QDSCs) using narrow band gap semiconductors such as CdS, CdSe, PbS and PbSe have attracted considerate attention recently [1–7]. The unique characteristics such as tunable optical property and multiple exciton generation (MEG) are expected to enhance the efficiency over Shockley and Queisser limit of 33% [8]. MEG is able to produce multiple excitons by one incident photon in semiconductor nano-crystals and represents a promising route to increase solar conversion efficiency. Hanna and Nozik already proved the possibility of performance enhancement in MEG solar cell [9]. Its theoretical efficiency is 11% higher than that of SEG solar cell. There are many kinds of QDs developed so far and Cd compounds-sensitized solar cell has the best conversion efficiency
∗ Corresponding author at: Center of Plasma Nano-interface Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan. Tel.: +81 92 802 3723; fax: +81 92 802 3723. E-mail address: woong [email protected] (H. Seo). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.09.087
[10]. However, they have some disadvantages such as toxicity and scarcity. Therefore, we focused on Si QD as the alternative to conventional QDs. Si is one of good QD materials and has abundance and absence of toxicity as a dominant material in the photovoltaics. In addition, Si QD has high stability against light soaking as compared with a-Si:H films and a high optical absorption coefficient due to its quantum size effect as compared with c-Si films. However, there are few reports about Si QDSC despite its unique characteristics because the collection of Si QDs is too difficult contrary to easy film deposition. In our previous researches, this problem was solved by multi-hollow discharge plasma chemical vapor deposition (CVD) and carrier generation was proved in the wavelength range of less than 550 nm in Si QDSCs [11,12]. However, there was considerable electron recombination in Si QDSC because of its large particle size. For its improvement, the surface modification was investigated using a barrier layer in this work. The core–shell structure was introduced because it prevents the charge recombination at the TiO2 /electrolyte interface and increases photocurrent. It consists of a nano-porous TiO2 covered with a shell of ZnO as shown in Fig. 1. ZnO exhibits very similar photoelectrochemical properties as TiO2 . Its band gap and electron injection efficiency are also analogous to those of TiO2 . It has additional advantages: a higher flat-band potential to achieve higher photovoltage and suitability for the fabrication of nano-structures with outstanding transport and optical properties [13,14]. Zinc acetate dihydrate (Zn(CH3 COO)2 ·H2 O)
214
H. Seo et al. / Electrochimica Acta 87 (2013) 213–217
Fig. 1. Structure of Si quantum dot-sensitized solar cell with the surface modification.
[15,16] and zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) [17,18] were used for a barrier layer. In order to verify the effect of a barrier layer and the enhancement of the performance, the structural characteristics of deposited film, photovoltaic performance and internal electrochemical impedance of completed Si QDSC with a barrier layer were examined. Consequently, Si QDSC with effective ZnO barrier layer had the improvement on the overall performance.
Fig. 3. Band structure of surface-modified Si quantum dot-sensitized solar cell.
200 W. To avoid their deposition on the wall of the reactor, the temperature of the wall was kept at 250 ◦ C. SiH4 + e− → SiH2 + H2 + e− SiH4 + SiH2 → Si2 H6 Si2 H6 + SiH2 → Si3 H8
2. Experimental
Si3 H8 + SiH2 → Si4 H10
2.1. Synthesis of Si nano-particles
Si4 H10 + e− → Si4 H− (grow) 10
Si nano-particles were synthesized by multi-hollow discharge plasma CVD. Schematic of the deposition method is shown in Fig. 2. Hydrogen (H2 ) diluted silane (SiH4 ) gas was introduced from the bottom of the reactor. It flows through the hollows of the electrodes and pumped out from the top of the reactor. SiH4 was converted to high-ordered silane with SiH2 and ionized. Then, crystalline Si nano-particles were nucleated and grown in the discharge plasma region. Eq. (1) simplified this process. They were transported to the downstream region with gas flow and collected by stainless meshes [11]. The crystallinity and size of Si nano-particles were controllable by adjusting the gas ration and working pressure, respectively. For the fabrication of Si nano-particles with the diameter of 10 nm used in this work, gas flow rates of H2 and SiH4 were 449 and 1.5 sccm, and total pressure was 655 Pa. High frequency voltage of 60 MHz was applied to the powered electrode and the discharge power was
Fig. 2. Schematic of multi-hollow discharge plasma CVD.
(1)
2.2. Fabrication of Si QDSCs Si QDSCs were fabricated as follows. FTO substrates (10 per sq., Asahi Glass Co.) were used as the transparent conductive oxide (TCO) to make the photo and counter electrodes. The substrates were cleaned by sonicating in ethyl alcohol, and dried using a stream of nitrogen. A uniform TiO2 layer was pasted and sintered at 450 ◦ C for 30 min. And nano-porous Si–TiO2 films were pasted on sintered TiO2 layer by the doctor blade method. And then FTO/TiO2 /Si–TiO2 substrates were surface-modified by using Zn(CH3 COO)2 ·H2 O and Zn(NO3 )2 ·6H2 O aqueous solutions. Its energy band diagram is shown in Fig. 3. The photo electrodes were sintered at 200 ◦ C for 30 min again for the formation of ZnO. Deposited films were characterized by their morphological and compositional properties. The scanning electron microscopy (SEM, S-4200, Hitachi) operated at 15 kV was used to characterize the microstructure of the films. And the formation of the deposited films was ascertained by measuring X-ray diffraction (XRD, X’pert PRO MRD, Philips) under optimized operating conditions of 30 mA and 40 kV. The gilding counter electrode with a thickness of about 100 nm was deposited by DC sputter (JEC-550, JEOL) at a power of 1.5 kV A and a working pressure of 2 Pa. Polysulfide electrolyte is suitable as the redox couple in case of QDSC because QDs are degraded by I− /I3 − electrolyte [19]. Then, polysulfide electrolyte allows strong interactions between Pt and S2− . S2− ions adsorb onto the surface of Pt and suppress the conductivity, catalytic activity, and electrochemical surface area of the counter electrode unlike Au counter electrode [20]. Therefore, gilding counter electrode was used in this work. After that, the photo and counter electrodes were sealed using a thermoplastic hot-melt sealant (SX 1170-25, Solaronix) with a thickness of 25 m. The sealed Si QDSCs were completed by injecting a redox electrolyte through a pre-drilled hole into the counter electrode. The redox electrolyte consisted of 0.5 M Na2 S, 2 M S and 0.2 M KCl in water and methanol mixed
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Fig. 6. XRD patterns of photo electrode deposited with Zn(NO3 )2 ·6H2 O (bottom) and Zn(CH3 COO)2 ·H2 O (top). Fig. 4. Absorbed photon to current conversion efficiency of Si QDSC and the photovoltaic performance under various light intensities (inset).
solution at a volume ratio of 3:7. Another Si QDSC was fabricated as a reference under identical conditions except for a barrier layer. Before their characterization, the completed cells were stored in the dark under open-circuit conditions for 24 h to allow the electrolyte to penetrate into the pores. The photovoltaic performance was measured under 1 sun (air mass 1.5, 100 mW/cm2 ) by a source meter (Model 2400, Keithley Instrument, Inc.). And incident photon to current conversion efficiency (IPCE, SM-250-P1, Bunkoukeiki) was measured from 250 nm to 1000 nm. During their irradiance and characterization, the cells were covered with a black mask fitting the active area of the cell. The irradiated cell area was 0.25 cm2 . The I–V characteristic curve and Eq. (2) were used to calculate the short-circuit current (ISC ) and density (JSC ), open-circuit voltage (VOC ), fill factor (FF) and the overall efficiency () [21,22]. The internal impedance of the DSCs was measured by electrochemical impedance spectroscopy (EIS, SP-150, Biologic SAS). The EIS spectra were measured over the frequency range from 10 mHz to 1 MHz at room temperature. The applied bias voltage and AC amplitude were set at VOC of the DSC and 10 mV, respectively. The electrical impedances were characterized using the Nyquist diagram. =
Pmax FFVOC JSC × 100 = × 100(%) Pin Pin
(2)
was impossible by the limitation of equipment, the rapid increase from 2 Eg point confirmed MEG effect. This was also confirmed by the photovoltaic performance under various light intensities in the inset of Fig. 4. The performance of SEG solar cell is generally decreased with stronger light intensity because SEG sensitizer generates only one electron with incident energy more than 1 Eg (band-gap energy). On the other hand, that of MEG solar cell is not decreased with stronger light because it generates multiple electrons with energy more than 2 Eg . The performance of our Si QDSC was higher than the base line (gray) under the incident light over 1 sun (100 mW/cm2 ). This result also showed MEG of QD. The reactions from Zn(CH3 COO)2 ·H2 O and Zn(NO3 )2 ·6H2 O to ZnO are shown in Eqs. (3) and (4), respectively. This surface modification was verified by SEM and XRD results. Fig. 5 shows SEM images of photo electrode surface-modified with (a) Zn(CH3 COO)2 ·H2 O and (b) Zn(NO3 )2 ·6H2 O. Both solutions were well penetrated into pores and covered Si and TiO2 nano-particles. Their nano-porous structure was maintained. The formation of ZnO was confirmed by XRD patterns in Fig. 6. The diffraction peaks of TiO2 , SnO2 of FTO, Si, and ZnO were denoted in the figure. Zn(CH3 COO)2 H2 O → ZnO + 2CH3 COOH
Zn(NO3 )2 6H2 O + H2 O → Zn2+ + 2NO3 − + 7H2 O, Zn2+ + NO3 − + 2e− → ZnO + NO2 −
3. Results and discussion The characteristics of Si QDs were confirmed with the rapid increase of absorbed photon to current conversion efficiency (APCE) in the range of short wavelength. Fig. 4 shows APCE of Si QDSC. In this case, FTO substrate of the photo electrode was substituted by Al sputtered quartz for the penetration of short wavelength. APCE was increased from 400 nm and rapidly climbed at 2 Eg point (280 nm). Although the measurement in the range of below 250 nm
(3)
(4)
Fig. 7 shows IPCE of standard and surface-modified Si QDSCs. After the surface modification, the overall efficiency was enhanced with both modifications in the visible light region from 300 nm to 700 nm including increased peaks around 350 nm. Especially, Si QDSC surface-modified by Zn(NO3 )2 ·6H2 O had wider absorbance than Si QDSC surface-modified by Zn(CH3 COO)2 ·H2 O. The reasons of the enhancement are the reduction of electron recombination by
Fig. 5. SEM images of photo electrode deposited with (a) Zn(CH3 COO)2 ·H2 O and (b) Zn(NO3 )2 ·6H2 O.
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Fig. 7. Incident photon to current conversion efficiencies of standard and surfacemodified Si quantum dot-sensitized solar cells.
a barrier layer and better electron injection from Si QD by a shift of band edge. The enhancement was also verified by EIS analysis. Fig. 8 shows Nyquist diagram of standard and surface-modified Si QDSCs. In general Nyquist diagram for EIS analysis, three semicircles are observed in the frequency range, and the resistance detected over 1 MHz is affected by the sheet resistance of the TCO. The three semicircles are attributed to the charge transportation at the counter electrode, the electron transfer at the TiO2 /Si QD/electrolyte interfaces, and the charge transportation by ions in the electrolyte in the high, middle and low frequency ranges, respectively [21,23–26]. But the results in Fig. 8 looks like one large semi-circle because 2nd semi-circle in the middle frequency range was much larger than 1st and 3rd semicircles. That is, the charge transportation at the counter electrode and in the electrolyte was not influenced on the cell performance because it is relatively enough fast as compared with the electron transfer at the TiO2 /Si QD/electrolyte interfaces. As can be seen, internal impedances of both surface-modified Si QDSCs were much decreased as compared with standard Si QDSC. It means that the internal electron transfer was improved at TiO2 /Si QD/electrolyte interfaces. In other words, the electron recombination at these interfaces was much reduced by the surface modification.Above enhanced IPCE and reduced internal impedance were reflected in the overall performance. As mentioned above, overall performance was enhanced with a barrier layer. The reduction of electron recombination and better electron injection from Si QD made the photocurrent increased. And well covered TiO2 particles by a barrier layer and a shift of band edge increased the photovoltage. The I–V characteristic curves are shown in Fig. 9. The performance of standard Si QDSC was defined by VOC of 0.11 V, JSC of 0.64 mA/cm2 , and efficiency of 0.020%. The performance of Si QDSCs with a
Fig. 8. Electrochemical impedance spectra of standard and surface-modified Si quantum dot-sensitized solar cells.
Fig. 9. I–V characteristic curves of standard and surface-modified Si quantum dotsensitized solar cells.
barrier layer were increased over 2 times. The surface-modified Si QDSC with Zn(CH3 COO)2 ·H2 O was defined by VOC of 0.18 V, JSC of 0.87 mA/cm2 , and efficiency of 0.046% and the surface-modified Si QDSC with Zn(NO3 )2 ·6H2 O was defined by VOC of 0.15 V, JSC of 1.13 mA/cm2 , and efficiency of 0.051%. 4. Conclusions In this work, Si nano-particles synthesized by multi-hollow discharge plasma CVD with the diameter of 10 nm were used for the fabrication of Si QDSCs. There was considerable electron recombination because of its large particle size. To prevent the recombination in the Si–TiO2 nano-porous network, ZnO barrier layer was applied to the photo electrode as the surface modification. Zn(NO3 )2 ·6H2 O and Zn(CH3 COO)2 ·H2 O) were employed as precursor to surface-modify. As a result, IPCE and internal impedance of Si QDSCs were much increased after the surface modification and this enhancement was reflected in the overall performance. VOC and JSC were much increased with the surface modification and the efficiencies were increased over 2 times. These results clearly demonstrated that the surface modification with ZnO was advantageous for the performance enhancement. Acknowledgment This work was partially supported by TEMCO Memorial Foundation and New Energy and Industrial Technology Development Organization (NEDO). References [1] Y. Lee, B. Huang, H. Chien, Highly efficient cdse-sensitized TiO2 photoelectrode for quantum-dot-sensitized solar cell applications, Chemistry of Materials 20 (2008) 6903. [2] N. Zhao, T. Osedach, L. Chang, S. Geyer, D. Wanger, M. Binda, A. Arango, M. Bawendi, V. Bulovic, Colloidal PbS quantum dot solar cells with high fill factor, ACS Nano 4 (2010) 3743. [3] D. Guimard, R. Morihara, D. Bordel, K. Tanabe, Y. Wakayama, M. Nishioka, Y. Arakawa, Fabrication of InAs/GaAs quantum dot solar cells with enhanced photocurrent and without degradation of open circuit voltage, Applied Physics Letters 96 (2010) 203507. [4] T. Ju, R. Graham, G. Zhai, Y. Rodriguez, A. Breeze, L. Yang, G. Alers, S. Carter, High efficiency mesoporous titanium oxide PbS quantum dot solar cells at low temperature, Applied Physics Letters 97 (2010) 043106. [5] X. Wang, G. Koleilat, J. Tang, H. Liu, I. Kramer, R. Debnath, L. Brzozowski, D. Aaron, R. Barkhouse, L. Levina, S. Hoogland, E. Sargent, Tandem colloidal quantum dot solar cells employing a graded recombination layer, Nature Photonics 5 (2011) 480. [6] Y. Tachibana, H. Akiyama, Y. Ohtsuka, T. Torimoto, S. Kuwabata, CdS quantum dots sensitized TiO2 sandwich type photoelectrochemical solar cells, Chemistry Letters 36 (2007) 88. [7] I. Robel, V. Subramanian, M. Kuno, P. Kamat, Quantum dot solar cells. harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films, Journal of the American Chemical Society 128 (2006) 2385.
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