- Email: [email protected]
Inhibition of charge recombination for enhanced dye-sensitized solar cells and self-powered UV sensors by surface modification
Accepted Manuscript Title: Inhibition of Charge Recombination for Enhanced Dye-Sensitized Solar Cells and Self-powered UV Sensors by Surface Modification Author: Liang Chu Zhengfei Qin Liu Wei Ma Xin’guo PII: DOI: Reference:
S0169-4332(16)31514-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.078 APSUSC 33646
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-4-2016 28-6-2016 12-7-2016
Please cite this article as: Liang Chu, Zhengfei Qin, Liu Wei, Ma Xin’guo, Inhibition of Charge Recombination for Enhanced Dye-Sensitized Solar Cells and Self-powered UV Sensors by Surface Modification, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.078 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.
Inhibition of Charge Recombination for Enhanced Dye-Sensitized Solar Cells and Self-powered UV Sensors by Surface Modification
Liang Chu a,b,*, Zhengfei Qin c, Liu Wei c, Ma Xin’guo d,*
a
Advanced Energy Technology Center, Nanjing University of Posts and Telecommunications
(NUPT), Nanjing 210046, China b
Wuhan National Laboratory for Optoelectronics (WNLO)-School of Physics, Huazhong
University of Science and Technology (HUST), Wuhan 430074, China
c
School of Materials Science and Engineering (SMSE), Nanjing University of Posts and
Telecommunications (NUPT), Nanjing 210046, China d
Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy,
Hubei University of Technology, Wuhan 430068, China Email: [email protected], [email protected]
1
Graphic Abstract Inhibition of charge recombination was utilized to prolong electrode lifetime in dyesensitized solar cells (DSSCs) and self-powered UV sensors based on TiO2-modified SnO2 photoelectrodes. The electrochemical impedance spectroscopy and open-circuit voltage decay measurements indicated that the electron lifetime was significantly prolonged in DSSCs after TiO2 modification. And in self-powered UV sensors, the sensitivity and response time were enhanced.
Highlights The surface modification to inhibit charge recombination was utilized in photovoltaic devices. Enhanced DSSCs and self-powered UV sensors based on SnO2 photoelectrodes were obtained by TiO2 modification.
2
Abstract The surface modification to inhibit charge recombination was utilized in dye-sensitized solar cells (DSSCs) and self-powered ultraviolet (UV) sensors based on SnO2 hierarchical microspheres by TiO2 modification. For DSSCs with SnO2 photoelectrodes modified by TiO2, the power conversion efficiency (PCE) was improved from 1.40% to 4.15% under standard AM 1.5G illumination (100 mW/cm2). The electrochemical impedance spectroscopy and open-circuit voltage decay measurements indicated that the charge recombination was effectively inhibited, resulting in long electron lifetime. For UV sensors with SnO2 photoelectrodes modified by TiO2 layer, the self-powered property was more obvious, and the sensitivity and response time were enhanced from 91 to 6229 and 0.15 s to 0.055 s, respectively. The surface modification can engineer the interface energy to inhibit charge recombination, which is a desirable approach to improve the performance of photoelectric nanodevice.
Keywords: charge recombination, dye-sensitized solar cells, self-powered UV sensors, SnO2 photoelectrodes
1. Introduction In general, the charge recombination is an inevitable electronic process reducing the performance of photovoltaic devices [1], such as photoconductivity, rectification and transistor behaviors. Therefore, the inhibition of charge recombination is one of the most impactful means to enhance the performance of photovoltaic nanodevices, and some strategies have been adopted, including doping [2], interfacial design [3], mixed sizes with 3
quantum scale effect [4], etc. Those methods are all based on energy engineering to facilitate charge transfer for inhibition of charge recombination. Since the first report by Grätzel [5], dye-sensitized solar cells (DSSCs) have attracted great attention as an alternative to silicon-based solar cells for their low cost, environmental friend and simple fabrication procedure. In DSSCs, photoelectrodes are the scaffolds for loading dye molecules and being the channels for electron injection and transport, which greatly decide the performance of DSSCs [6]. Meanwhile, the charge recombination as negative factor, occurs at the surface of the photoelectrode. Typically, SnO2 has been developed as photoelectrode in DSSCs [7], due to the n-type oxide semiconductors with similar band structure to TiO2 and higher electron mobility (100-200 cm2/V/s) [8] than that of TiO2 (0.1-1 cm2/V/s) [9], but the performance is still poor. The main reason is the high charge recombination at SnO2 surface with electrons backward to the redox electrolyte because of the low conduction band of SnO2 (~400 mV lower than that of TiO2). We infer that TiO2 modification can decorate the surface energy of SnO2 to inhibit charge recombination. Moreover, high conductive band-TiO2 modification can increase the open-circuit voltage of pure SnO2. Therefore, the surface modification to inhibit charger recombination is a desirable approach to improve the performance of DSSCs [10]. Self-powered mico/nano-systems have been broadly studied and pursued since the studies by Wang [11,12]. Photovoltaic devices convert light energy into electric energy via photo-generated electron-hole pairs. Therefore, a photovoltaic device is a effective photo sensor and operate without any external power source. Namely, it is a self-powered photo sensor. Ultraviolet (UV) sensors have been widely investigated in environmental and biological analysis, flame sensing, optical communication, missile detection, etc [13,14]. A UV sensor under UV irradiation generates electron-hole pairs and if the charge recombination is high, a obvious UV response is shown only under an loaded external bias as the driving force [15]. The inhibition of charger recombination can make UV sensor operate under zero 4
bias possibly, becoming a photovoltaic device, and make the self-powered property more obvious. As a consequence, the inhibition of charge recombination can improve charge transfer, resulting in enhancement of responsibility and reduction of response time in selfpowered UV sensors. In this work, SnO2 hierarchical spheres were synthetized by microwave-assisted hydrothermal method involving calcination, which were further applied as photoelectrodes in DSSCs and self-powered UV sensors. After the surface of SnO2 photoelectrode was modified by TiO2 layer, the performance of photovoltaic devices was significantly improved because of the interface energy engineering resulting in the inhibition of charge recombination. In DSSCs, PCE was improved from 1.40% to 4.15%. The inhibition of charger recombination in the TiO2-modified SnO2 photoelectrodes can improve the performance by increasing the open voltage and the inner conductive current in SnO2. In UV sensors, the self-powered property was more obvious, the sensitivity was enhanced from 91 to 6229, and the response time was shortened from 0.15 s to 0.055 s under zero bias. The study indicates that the surface modification to inhibit charge recombination is an effective approach to enhance the performance of photovoltaic devices.
2. Experimental 2.1. Materials. Tin chloride dihydrate (SnCl2·2H2O, 98.0%), trisodium citrate dihydrate (C6H5Na3O7·2H2O, 99.0%), titanium tetrachloride (TiCl4, 99.0%), ethanol (99.7%), acetone (99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Fluorine-doped SnO2 glass (FTO, 7 Ω∙cm-2) was from Nippin Sheet Glass Co., Ltd. Ruthenium 535-bisTBA (N719) was purchased from Solaronix. Guanidinium thiocyanate (GuSCN, 99.0%) was from Amresco. Lithium iodide (LiI, 99.999%), iodine (I2, 99.99%), 1-methyl-3-propylimidazolium iodide (PMII, 98%), 4-tert-butylpyridine (4-TBP, 96%) and tertbutyl alcohol (99.5 %) were obtained
5
from Aladdin. Acetonitrile (99.8%) and valeronitrile (99%) were from Alfa Aesar. All solvents and chemicals were reagent grade and were used as received without further purification. 2.2. Synthesis of SnO2 microspheres. The reaction solution of 70 ml containing 0.075 M SnCl2·2H2O and 0.15 M C6H5Na3O7·2H2O was obtained following the procedure below. First, SnCl2·2H2O and C6H5Na3O7·2H2O were poured into deionized water with magnetic stirring for 10 min, respectively, and then they were mixed with stirring for another 10 min. The resulting solution was transferred into a 100 ml reaction vessel of the microwave instrument (model HP1510, Shanghai Hengping Instrument and Meter Factory). The reaction occurred at 180 ºC for 3 hours. Here, the hydroxyl and carboxyl groups in the citrate ion attract Sn2+ ions and control the growth direction of the Sn6O4(OH)4 structures [16]. After the reaction, the powder was collected, and washed with deionized water and ethanol for several times by centrifugation. Finally, the powder was dried at 80 ºC overnight, and then calcined at 600 ºC for 3 hours in air to obtain SnO2 microspheres. 2.3. Preparation of photoelectrodes. SnO2 powder (1 g), terpineol (3 g), acetic acid (0.2 ml), ethyl cellulose (0.5 g) and some ethanol were mixed evenly with magnetic stirring, milled in a mortar for about 15 min, and then dispersed with ultrasonic for 10 min to prepare SnO2 paste. To prevent short circuit in photovoltaic devices, the clean FTO glasses were first treated to form a compact TiO2 layer by means of immersing into 40 mM TiCl4 solution at 70 ºC for 30 min. The SnO2 paste was coated onto the treated FTO glasses by printing process. Then the printed SnO2 layers were heated under air condition at 125 ºC for 15 min, at 325 ºC for 5 min, at 375 ºC for 5 min, at 450 ºC for 15 min, and then at 500 ºC for 15 min in a muffle furnace, to obtain the SnO2 photoelectrodes. For TiO2-modified SnO2 photoelectrodes, the SnO2 photoelectrodes were immersed into 40 mM TiCl4 solution at 70 ºC for 1 hour, then rinsed with deionized water and ethanol, and annealed under air condition at 520 ºC for 30 min in muffle furnace.
6
2.4. Fabrication of DSSCs and self-powered UV sensors. For application of DSSCs, the SnO2 and TiO2-modified SnO2 photoelectrodes were heated to 80 ºC, immersed into 0.5 mM N719 dye in acetonitrile/tert-butanol solution (volume ratio of 1:1) for 20 hours at room temperature to sensitize, and then washed with acetonitrile to remove the physically-adsorbed dye molecules. Pt counter electrodes were deposited by magnetron sputtering on cleaned FTO substrates. Pt counter electrode was buckled on the dye-sensitized photoelectrode, between which there was a space separated by polypropylene plastic for injecting electrolyte. The electrolyte was composed of 0.6 M PMII, 0.05 M LiI, 0.03 M I2, 0.1 M GuSCN and 0.5 M 4TBP in acetonitrile and valeronitrile (volume ratio of 85:15). The active area of the devices was 0.15 cm2 without mask. The fabrication of self-powered UV sensors was almost the same as that of DSSCs except that the dye was absent. 2.5. Material characterization and electrochemical measurement. X-ray diffraction (XRD, PANalytical B.V. The Netherlands) measurement with Cu-Ka radiation was performed to characterize the crystal structures. Scanning electron microscope (SEM, FEI NOVA NanoSEM 450), and aberration-corrected transmission electron microscope (TEM, FEI Titan G260-300) were used to observe the morphology of the samples. The TEM sample was prepared by dropping casting ethanolic dispersion of the powder onto a carbon coated Cu grid. Absorption spectra of the samples were recorded with a UV-Vis absorption spectrometer (UV-2550, Shimadzu). The dye molecules were desorbed from the sensitized-photoelectrodes into 0.1 M NaOH solution to test the absorption spectra and calculate the dye-adsorbed amounts. The simulated sunlight and UV light were emitted from a solar simulator illumination (Newport, USA) of AM 1.5 G
(100 mW/cm2) intensity and an ultra-high
pressure mercury lamp (Model CHF-XM500, Beijing Changtuo Technology Co., Ltd.) with a filter (365 nm) of 7.5 mW/cm2 intensity, respectively. The current-voltage, voltage-decay and electrochemical impedance spectroscopy (EIS) measurements were recorded using an Autolab electrochemic workstation (model AUT84315, The Netherlands). EIS spectra were measured 7
in dark condition at a bias of 0.60 V with an amplitude of 10 mV in a frequency range from 100 kHz to 0.1 Hz.
3. Results and discussion As shown in Fig. 1a, the TiO2 layer modifies the SnO2 surface and engineers the surface energy, which prevents back-injecting electrons from SnO2 to electrolyte. According to the transition-probability approach of Bardeen [17,18], the tunneling rate from SnO2 to electrolyte follows exponential decay with increasing the thickness of TiO2 layer. Therefore, the TiO2 modification inhibits the charge recombination. For DSSCs, as shown in Fig. 1b, dye molecules capture light to generate electron-hole pairs, then the electrons inject into TiO2 conduction band and transfer into SnO2 conduction band for the type-II structure of SnO2/TiO2 with energy level gradient. Moreover, it is difficult for the electrons in SnO2 to backflow to recombine with holes in redox electrolyte because of the surface modification of SnO2 by TiO2 layer [19]. For UV sensors, the SnO2/TiO2 generates electron-hole pairs under UV irradiation, and the high conduction band TiO2 layer inhibits the back-flowing electrons to recombine, as demonstrated in the schematic Fig.1c. For clearance and comparison, the TiO2modified SnO2 hierarchical microsphere-based DSSCs and UV sensors are illustrated in schematic Fig. 1d and 1e, respectively. The photoelectrodes were sandwiched with platinum counter electrodes, and the internal spaces between the two electrodes were filled with liquid electrolyte. The SnO2 powder was synthesized by using a microwave-assisted hydrothermal method involving the calcination as seen in the experimental section. The crystalline structure of the powder was characterized by XRD. Fig. 2a shows the typical XRD patterns of Sn6O4(OH)4 powder (i, JCPDS, No. 46-1486) prepared by a microwave-assisted hydrothermal method and the rutile-like SnO2 powder (ii, JCPDS, No. 88-0287) after annealing at 600 ºC for 3 hours. The scanning electron microscopy (SEM) image of Fig. 2b indicates that the as-prepared 8
SnO2 sample is porous hierarchical microspheres. The corresponding enlarged SEM image of Fig. 2c shows the aggregation of a number of small nanoparticles and nanorods. Fig. 2d shows the cross-section of the photoelectrode obtained by printing method from the SnO2 paste, where the thickness is about 11.5 μm. The morphology of SnO2 and TiO2-modified SnO2 hierarchical microspheres was further characterized by TEM technology. Fig. 3a, combined with the illustration, clearly shows that the obtained SnO2 sample is porous microspheres with hundreds of nanometers, composed of small nanoparticles and nanorods. In the high resolution TEM image of Fig. 3b, the lattice fringes of 0.338 nm, 0.219 nm and 0.255 nm correspond to (110), (210), (101) planes of rutile-SnO2, respectively, which are further evidenced that the product is SnO2. The corresponding selected area electron diffraction (SAED) pattern in the inset of Fig. 3b indicates that the as-prepared SnO2 sample is polycrystalline. Fig. 3c shows a TiO2-modified SnO2 nanorod, and Fig. 3c of the HRTEM image corresponds to the outstanding one in the lower left corner of the TEM image. The nanorod shell is rough and can be indexed as TiO2. In the enlarged region of Fig. 3c, there is an obvious interface between SnO2 and TiO2. The lattice fringe spacing of 0.246 nm is consistent with the (101) lattice plane of rutile TiO2 (PCPDFWIN, No. 88-1175), and that of 0.255 nm is in accordance with (101) plane of rutile SnO2. X-ray energy dispersion spectrum (EDS) was further used to analyze the nanorod corresponding to Fig. 3c, as shown in Fig. 3d. In addition to the existence of Sn and O peaks, there are obvious Ti peaks. The Cu peak is from the Cu mesh. The above characterization in Fig. 3 demonstrates that SnO2 is well modified by TiO2. For photoelectrodes of DSSCs, the structure of hierarchical microsphere is an excellent morphology to supply efficient light scattering, large specific surface area and long-range electronic connectivity [20-23]. Fig. 4a shows the typical current density-voltage curves of the DSSCs based on SnO2 and TiO2-modified SnO2 hierarchical microspheres under the illumination of 100 mA/cm2. The corresponding photovoltaic parameters from Fig. 4a are 9
listed in Table 1, where JSC is short-circurt current density, VOC is open-circurt voltage, and FF is fill factor. The TiO2-modified-SnO2 based DSSCs (TS-DSSCs) exhibits higher performance than that of SnO2 based DSSCs (S-DSSCs). In Fig 4 b, the dark current test indicated the TSDSSCs have lower dark current, meaning the inhibition of charge recombination by TiO2 modification. The absorbed-dye amounts in Tabel 1 were calculated using Lambert-Beer's law from Fig. 1S, which practically remain unchanged after TiO2 modification. In DSSCs, PCE is determined by JSC, VOC and FF according to the following equation [5]: PCE
VOC J SC FF Pin
(1)
where Pin is the illumination power intensity. After TiO2 modification, the photovoltaic parameters of VOC, JSC and FF are all improved. JSC can be approximated by the following equation [24]: J SC e1hinjcc I 0
(2)
where e is the elementary charge, ηlh is the light-harvesting efficiency and mainly related to the amount of adsorbed dye, ηinj is the charge-injection efficiency, ηcc is the charge-collection efficiency, and I0 is the light flux. Our experiments show that η1h is nearly the same because of the unchanged amount of adsorbed dye after modification, and hinj is also supposed to be the same value because the time of electron injection from the excited dye molecules into SnO2 is 150 ps, which is the approximate value of that into TiO2 [25]. ηcc is mainly dependent on the competition between charge recombination and collection. Therefore, the charge recombination mainly has an effect on JSC. Meanwhile, theoretical VOC is determined by the difference between the Fermi level of the active photoelectrode and the oxidation potential of the electrolyte. Since the conduction band of TiO2 is higher than that of SnO2, the existence of the TiO2 layer can lead to higher Fermi level of photoelectrode than that of the mere SnO2.
10
Moreover, the charge recombination reduces the value of VOC than the theoretical value [26, 27]. Therefore, the charge recombination brings negative efforts to both JSC and VOC. To evaluate the charge recombination, EIS measurement was carried out at a bias of 0.60 V under dark condition. Fig. 5a shows the Nyquist plot curves, in which there are two semicircles, one in the high frequency region (>1 kHz) and the other in the frequency region (100-0.1 Hz). The equivalent circuit and the corresponding values by Zview software were shown in Fig. 2S and Table 1S (Supporting Information), respectively. According to the EIS model [27, 28], the smaller semicircle in the high-frequency region denotes the redox reaction at the Pt/electrolyte interface, and the larger one represents the charge transfer at the photoelectrode/electrolyte interface. The charge-transfer resistance of Rct accounts for charge recombination, which is 1360 Ω for TS-DSSCs, and much higher than 116.1 Ω for S-DSSCs. Thus, the recombination of electrons in photoelectrode with holes in electrolyte is more difficult for TiO2-modified SnO2 photoelectrode. The behavior of DSSCs under dark condition resembles a diode with an applied bias, hence Rct can be considered as part of a normalized shunt resistance (rs), which relates to the FF according to the formula: FF = FF0(11/rs), where FF0 is the theoretical maximum FF [29]. It can be inferred that the increasing FF of the TS-DSSCs is originated from the inhibition of charge recombination. Fig. 5b shows the Bode plots of the DSSCs. The electron lifetime can be calculated from curve peak of the spectrum using the equation τ = 1/(2πfmax) [30], where fmax is the maximum frequency of the mid-frequency peak, which is 10.33 Hz and 2.90 Hz for ST-DSSCs, respectively. The electron lifetime of TS-DSSCs is 109.8 ms, which is longer than the 30.8 ms electron lifetime of SDSSCs. To further confirm the inhibition of charge recombination by TiO2 modification, the open-voltage decay method was further employed. When the illustration light was turned off, the voltage decayed continuously, as shown in Fig. 5c. Then the electron lifetime τr (Fig. 5d) can be calculated using data in Fig. 5c and the following formula [31]: 11
k T dV r B OC e dt
1
(3)
where kB is the Boltzmann constant and T is the room temperature. It is clear that the electron lifetime significantly increases by TiO2 modification and the lifetime can be calculated as ~100 ms at VOC = 0.6 V for TS-DSSCs, which is consistent with the liftetime in Fig. 5b. Long electron lifetime implies the inhibition of charge recombination. The SnO2 and TiO2-modified SnO2 based UV sensors were characterized using UV light. In Fig. 6, the current was tested with/without UV irradiation from -1 V to 1 V bias. When the devices are irradiated by UV light, the photo-current is enhanced obviously. Under zero bias, the devices show photo-current under UV illumination, which means that the devices are selfpowered UV sensors. Moreover, the self-powered property is more obvious after TiO2 modification. The self-powered UV sensors are a type of electrochemical devices, and there is no electric field. The power of the charge separation comes from the I-/I3- redox couples [32]. As soon as the electron-hole pairs are generated, the holes will be quickly removed by Idescribed as 3I-+2h+→I3-. The photo-electrons transfer across the photoelectrode, from the external circuit and arrive at the counter electrode. The electrolyte are recovered to I- by the electrons from the counter electrode as I3-+2e→3I -. Due to the I-/I3- redox couples, photocurrent can be continuously regenerated. The responsibility and stability are studied under rectangle pulse UV illumination (on/off) under zero bias, as shown in Fig. 7. The photocurrent signal is reproducible and stable from “off” to “on” states. Furthermore, the photocurrent of the TiO2-modified SnO2 based self-powered UV has been improved to 191 μA from 11.5 μA of the SnO2 based devices, which mainly results from the inhibition of charge recombination. The sensitivity S (= Ilight/( Ilight - Idark) [33], where Ilight and Idark are the currents measured under UV light and dark condition, respectively) was calculated as 91 and 6229 for the SnO2 and TiO2-modified SnO2 based self-powered UV sensors, respectively. The response time (the time needed to 12
reach (1-1/e) of the maximum photocurrent [34]) is decreased to 0.055 s of the TiO2-modified SnO2 based self-powered UV sensor from 0.120 s of the SnO2 based self-powered UV sensor. While the values of the corresponding recovery time (the time needed for the photocurrent to drop to 1/e of its maximum value) are 0.05 s and 0.155 s, respectively. In the TiO2-modified SnO2 based self-powered UV sensors, the faster response time is due to the easier electron traveling related to the inhibition of charge recombination when electron-hole pairs generated under UV light, and its slower recovery time was also attributed to the inhibition of charge recombination process. For inhibition of charge recombination, the photo-generated charges are not easy to recombine; in other words, the charges have long lifetime, therefore the TiO2modified SnO2 based self-powered UV sensors exhibit slower recovery time than bare SnO2 based ones when the UV light was turned off.
4. Conclusions In summary, the surface modification for energy engineering was carried out to inhibit charge recombination for DSSCs and self-powered UV sensors based on SnO2 hierarchical spheres. For DSSCs, since the SnO2 surface was modified by TiO2 layer, the efficiency increased from 1.40% to 4.15% because of the inhibition of charge recombination. For selfpowered UV sensors, with TiO2 modification, the self-powered property was more obvious, the sensitivity was improved from 91 to 6229, and the response time was decrease from 0.15 s to 0.055 s. The result shows an effective way of surface modification for inhibiting charge recombination to enhance the performance of photoelectronic devices.
Acknowledgements This work was supported by the Natural Science Foundation of Jiangsu Province (BK20150860), the Seed Project Funded by Introducing Talent of NJUPT (NY215022), the National Natural Science Foundation of China (51102150, 51472081). 13
References (1) I. Vass, Physiol. Plant., 2011, 142, 6-16. (2) J. Moser, M. Grätzel and R. Gallay, Helvetica Chimica Acta, 1987, 70, 1559-1604. (3) S. Hattori, W. W. Mou, P. Rajak, F. Shimojo and A. Nakano, Appl. Phys. Lett.,2013, 10, 093302-1-3. (4) J. Chen, W. Lei and W. Q. Deng, Nanoscale, 2011, 3, 674-677. (5) B. O. Regan and M. Grätzel, Nature, 1991,353, 737-740. (6) J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki, and A. B. Walker, J. Am. Chem. Soc., 2008, 130, 13364-13372. (7) S. Chappel and A. Zaban, Sol. Energy Mater. Sol. Cells, 2002,71, 141-152. (8) M. S. Arnold, P. Avouris, Z. W. Pan and Z. L.Wang, J. Phys. Chem. B, 2003, 107, 659663. (9) R. G. Breckenridge and W. R. Hosler, Phys. Rev.,1953, 91, 793-801. (10) Y. D. Duan, N. Q. Fu, Q. P. Liu, Y. Y. Fang, X. W. Zhou, J. B. Zhang and Y. Lin, J. Phys. Chem. C, 2012, 116, 8888-8893. (11) Z. L. Wang and J. H. Song, Science, 2006, 312, 242-246. (12) Z. L. Wang and W. Wu, Angew. Chem. Int. Edit., 2012, 51, 11700-11721. (13) M. Razeghi and A. Rogalski, J. Appl. Phys., 1996, 79, 7433-7473. (14) Z. H. Lin, G. Cheng, Y. Yang, Y. S. Zhou, S. Lee and Z. L. Wang, Adv. Funct. Mater., 2014, 24, 2810-2816. (15) S. M. Hatch, J. Briscoe and S. Dunn, Adv. Mater., 2013, 25, 867-871. (16) N. T. Khoa, S. W. Kim, D. V. Thuan, D.-H. Yoo, E. J. Kim and S. H. Hahn, Cryst. Eng. Comm., 2014,16, 1344-1350. (17) M. C. Payne, J. Phys. C: Solid State Phys., 1986, 19, 1145-1155. (18) S. L. Chuang and Jr. N. Holonyak, Appl. Phys. Lett., 2002, 80, 1270-1-3.
14
(19) B. E. Hardin, H. J. Snaith, and M. D. McGehee, Nat. Photonics, 2012, 6, 162-169. (20) M. D. Ye, C. Chen, M. Q. Lv, D. J. Zheng, W. X. Guo and C. J. Lin, Nanoscale, 2013, 5, 6577-6583. (21) L. Chu, Z. F. Qin, Q. X. Zhang, W. Chen, J. Yang, J. P. Yang , X. A. Li, App. Surface Sci., 2016, 360, 634-640. (22) J. Y. Liao, B. X. Lei, D. B. Kuang and C. Y. Su, Energy Environ. Sci., 2011, 4, 40794085. (23) J. Y. Liao, J. W. He, H. Y. Xu, D. B. Kuang, and C. Y. Su, J. Mater. Chem., 2012, 22, 7910-7918. (24) K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007, 7, 69-74. (25) A. N. M. Green, E. Palomares, S. A. Haque, J. M. Kroon and J. R. Durrant, J. Phys. Chem. B, 2005, 109, 12525-12533. (26) Z. Lan, J. Wu, D. B. Wang, S. C. Hao, J. M. Lin and Y. F. Huang, Sol. Energy, 2007, 81, 117-122. (27) Q. Wang, J. E. Moser and M. Grätzel, J. Phys. Chem. B, 2005, 109, 14945-14953. (28) H. N. Kim and J. H. Moon, ACS Appl. Mater. Interfaces., 2012, 4, 5821-5825. (29) N. Satoh and L. Y. Han, Phys. Chem. Chem. Phys., 2012, 14, 16014-16022. (30) R. Kern, R. Sastrawan, J. Ferber, R. Stangl and J. Luther, Electrochim. Acta, 2002, 47, 4213-4225. (31) A. Zaban, M. Greenshtein and J. Bisquert, Chem. Phys. Chem., 2003, 4, 859-864. (32) R. S. Mane, B. R. Sankapal and C. D. Lokhande, Mater. Chem. Phys., 1999, 60, 196-203. (33) B. Yuan, X. J. Zheng, Y. Q. Chen, B. Yang and T. Zhang, Solid-State Electronics,,2011, 55, 49-53. (34) J. J. Hassan, M. A. Mahdi, S. J. Kasim, Naser M. Ahmed, H. Abu, Appl. Phys. Lett., 2012, 101, 261108-1-3.
15
Figures
Fig. 1. (a) Schematic illustration for the TiO2 modification for inhibition of electrode tunneling from SnO2 to electrolyte. (b) and (c) Schematic illustration for the band structures and electron transfers in DSSCs and self-powered UV sensors, respectively. (d) and (e) Schematics of the DSSCs and self-powered UV sensers, respectively.
16
Fig. 2. (a) XRD pattern of the Sn6O4(OH)4 sample prepared by a microwave-assisted hydrothermal method (curve i) and SnO2 powder with annealing at 600 ºC for 3 h (curve ii). (b) SEM image of the obtained SnO2 powder, (c) SEM image of the same region in (b) at high magnification. (d) Cross-sectional SEM image of the photoelectrode.
17
Fig. 3. (a) TEM image of the SnO2 powder sample and the blown-up image in the inset. (b) HRTEM image and SAED pattern in the inset. (c) TEM and HRTEM images of the TiO2modified SnO2 sample. (d) Energy dispersive x-ray spectroscopy spectrum corresponding to the nanorod in (c).
18
Fig. 4. The current density-voltage curves of S-DSSCs and ST-DSSCs under 100 m/Wcm2 illumination, (b) the corresponding current density-voltage curves under dark condition.
19
Fig. 5. EIS for S-DSSCs and ST-DSSCs under dark condition at an bias of 0.60 V. (a) Nyquist plots, and (b) Bode plots of phase angle versus frequency. (c) Open-circuit voltage decay curve. (d) Electron lifetime determined from (c).
20
Fig. 6. (a) Current-voltage curves for the SnO2 and TiO2-modified SnO2-based UV sensors with and without a UV illumination (365 nm). (a) SnO2, and (b) TiO2-modified SnO2 photoelectrodes.
21
Fig. 7. (a) The reversible characteristics of self-powered UV sensors under pulse UV illumination (on/off). (a) and (c) SnO2 ; (b) and (d), TiO2-modified SnO2 photoelectrodes.
22
Table 1 Photovoltaic parameters of DSSCs. JSC Photoelectrode
Dye-amount VOC(V)
FF
PCE (%)
(mA/cm2)
(nmol/cm2)
SnO2
9.2
0.458
0.332
1.40
SnO2-TiO2
11.7
0.729
0.487
4.15
23
62.4 64.3
S0169-4332(16)31514-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.078 APSUSC 33646
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-4-2016 28-6-2016 12-7-2016
Please cite this article as: Liang Chu, Zhengfei Qin, Liu Wei, Ma Xin’guo, Inhibition of Charge Recombination for Enhanced Dye-Sensitized Solar Cells and Self-powered UV Sensors by Surface Modification, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.078 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.
Inhibition of Charge Recombination for Enhanced Dye-Sensitized Solar Cells and Self-powered UV Sensors by Surface Modification
Liang Chu a,b,*, Zhengfei Qin c, Liu Wei c, Ma Xin’guo d,*
a
Advanced Energy Technology Center, Nanjing University of Posts and Telecommunications
(NUPT), Nanjing 210046, China b
Wuhan National Laboratory for Optoelectronics (WNLO)-School of Physics, Huazhong
University of Science and Technology (HUST), Wuhan 430074, China
c
School of Materials Science and Engineering (SMSE), Nanjing University of Posts and
Telecommunications (NUPT), Nanjing 210046, China d
Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy,
Hubei University of Technology, Wuhan 430068, China Email: [email protected], [email protected]
1
Graphic Abstract Inhibition of charge recombination was utilized to prolong electrode lifetime in dyesensitized solar cells (DSSCs) and self-powered UV sensors based on TiO2-modified SnO2 photoelectrodes. The electrochemical impedance spectroscopy and open-circuit voltage decay measurements indicated that the electron lifetime was significantly prolonged in DSSCs after TiO2 modification. And in self-powered UV sensors, the sensitivity and response time were enhanced.
Highlights The surface modification to inhibit charge recombination was utilized in photovoltaic devices. Enhanced DSSCs and self-powered UV sensors based on SnO2 photoelectrodes were obtained by TiO2 modification.
2
Abstract The surface modification to inhibit charge recombination was utilized in dye-sensitized solar cells (DSSCs) and self-powered ultraviolet (UV) sensors based on SnO2 hierarchical microspheres by TiO2 modification. For DSSCs with SnO2 photoelectrodes modified by TiO2, the power conversion efficiency (PCE) was improved from 1.40% to 4.15% under standard AM 1.5G illumination (100 mW/cm2). The electrochemical impedance spectroscopy and open-circuit voltage decay measurements indicated that the charge recombination was effectively inhibited, resulting in long electron lifetime. For UV sensors with SnO2 photoelectrodes modified by TiO2 layer, the self-powered property was more obvious, and the sensitivity and response time were enhanced from 91 to 6229 and 0.15 s to 0.055 s, respectively. The surface modification can engineer the interface energy to inhibit charge recombination, which is a desirable approach to improve the performance of photoelectric nanodevice.
Keywords: charge recombination, dye-sensitized solar cells, self-powered UV sensors, SnO2 photoelectrodes
1. Introduction In general, the charge recombination is an inevitable electronic process reducing the performance of photovoltaic devices [1], such as photoconductivity, rectification and transistor behaviors. Therefore, the inhibition of charge recombination is one of the most impactful means to enhance the performance of photovoltaic nanodevices, and some strategies have been adopted, including doping [2], interfacial design [3], mixed sizes with 3
quantum scale effect [4], etc. Those methods are all based on energy engineering to facilitate charge transfer for inhibition of charge recombination. Since the first report by Grätzel [5], dye-sensitized solar cells (DSSCs) have attracted great attention as an alternative to silicon-based solar cells for their low cost, environmental friend and simple fabrication procedure. In DSSCs, photoelectrodes are the scaffolds for loading dye molecules and being the channels for electron injection and transport, which greatly decide the performance of DSSCs [6]. Meanwhile, the charge recombination as negative factor, occurs at the surface of the photoelectrode. Typically, SnO2 has been developed as photoelectrode in DSSCs [7], due to the n-type oxide semiconductors with similar band structure to TiO2 and higher electron mobility (100-200 cm2/V/s) [8] than that of TiO2 (0.1-1 cm2/V/s) [9], but the performance is still poor. The main reason is the high charge recombination at SnO2 surface with electrons backward to the redox electrolyte because of the low conduction band of SnO2 (~400 mV lower than that of TiO2). We infer that TiO2 modification can decorate the surface energy of SnO2 to inhibit charge recombination. Moreover, high conductive band-TiO2 modification can increase the open-circuit voltage of pure SnO2. Therefore, the surface modification to inhibit charger recombination is a desirable approach to improve the performance of DSSCs [10]. Self-powered mico/nano-systems have been broadly studied and pursued since the studies by Wang [11,12]. Photovoltaic devices convert light energy into electric energy via photo-generated electron-hole pairs. Therefore, a photovoltaic device is a effective photo sensor and operate without any external power source. Namely, it is a self-powered photo sensor. Ultraviolet (UV) sensors have been widely investigated in environmental and biological analysis, flame sensing, optical communication, missile detection, etc [13,14]. A UV sensor under UV irradiation generates electron-hole pairs and if the charge recombination is high, a obvious UV response is shown only under an loaded external bias as the driving force [15]. The inhibition of charger recombination can make UV sensor operate under zero 4
bias possibly, becoming a photovoltaic device, and make the self-powered property more obvious. As a consequence, the inhibition of charge recombination can improve charge transfer, resulting in enhancement of responsibility and reduction of response time in selfpowered UV sensors. In this work, SnO2 hierarchical spheres were synthetized by microwave-assisted hydrothermal method involving calcination, which were further applied as photoelectrodes in DSSCs and self-powered UV sensors. After the surface of SnO2 photoelectrode was modified by TiO2 layer, the performance of photovoltaic devices was significantly improved because of the interface energy engineering resulting in the inhibition of charge recombination. In DSSCs, PCE was improved from 1.40% to 4.15%. The inhibition of charger recombination in the TiO2-modified SnO2 photoelectrodes can improve the performance by increasing the open voltage and the inner conductive current in SnO2. In UV sensors, the self-powered property was more obvious, the sensitivity was enhanced from 91 to 6229, and the response time was shortened from 0.15 s to 0.055 s under zero bias. The study indicates that the surface modification to inhibit charge recombination is an effective approach to enhance the performance of photovoltaic devices.
2. Experimental 2.1. Materials. Tin chloride dihydrate (SnCl2·2H2O, 98.0%), trisodium citrate dihydrate (C6H5Na3O7·2H2O, 99.0%), titanium tetrachloride (TiCl4, 99.0%), ethanol (99.7%), acetone (99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Fluorine-doped SnO2 glass (FTO, 7 Ω∙cm-2) was from Nippin Sheet Glass Co., Ltd. Ruthenium 535-bisTBA (N719) was purchased from Solaronix. Guanidinium thiocyanate (GuSCN, 99.0%) was from Amresco. Lithium iodide (LiI, 99.999%), iodine (I2, 99.99%), 1-methyl-3-propylimidazolium iodide (PMII, 98%), 4-tert-butylpyridine (4-TBP, 96%) and tertbutyl alcohol (99.5 %) were obtained
5
from Aladdin. Acetonitrile (99.8%) and valeronitrile (99%) were from Alfa Aesar. All solvents and chemicals were reagent grade and were used as received without further purification. 2.2. Synthesis of SnO2 microspheres. The reaction solution of 70 ml containing 0.075 M SnCl2·2H2O and 0.15 M C6H5Na3O7·2H2O was obtained following the procedure below. First, SnCl2·2H2O and C6H5Na3O7·2H2O were poured into deionized water with magnetic stirring for 10 min, respectively, and then they were mixed with stirring for another 10 min. The resulting solution was transferred into a 100 ml reaction vessel of the microwave instrument (model HP1510, Shanghai Hengping Instrument and Meter Factory). The reaction occurred at 180 ºC for 3 hours. Here, the hydroxyl and carboxyl groups in the citrate ion attract Sn2+ ions and control the growth direction of the Sn6O4(OH)4 structures [16]. After the reaction, the powder was collected, and washed with deionized water and ethanol for several times by centrifugation. Finally, the powder was dried at 80 ºC overnight, and then calcined at 600 ºC for 3 hours in air to obtain SnO2 microspheres. 2.3. Preparation of photoelectrodes. SnO2 powder (1 g), terpineol (3 g), acetic acid (0.2 ml), ethyl cellulose (0.5 g) and some ethanol were mixed evenly with magnetic stirring, milled in a mortar for about 15 min, and then dispersed with ultrasonic for 10 min to prepare SnO2 paste. To prevent short circuit in photovoltaic devices, the clean FTO glasses were first treated to form a compact TiO2 layer by means of immersing into 40 mM TiCl4 solution at 70 ºC for 30 min. The SnO2 paste was coated onto the treated FTO glasses by printing process. Then the printed SnO2 layers were heated under air condition at 125 ºC for 15 min, at 325 ºC for 5 min, at 375 ºC for 5 min, at 450 ºC for 15 min, and then at 500 ºC for 15 min in a muffle furnace, to obtain the SnO2 photoelectrodes. For TiO2-modified SnO2 photoelectrodes, the SnO2 photoelectrodes were immersed into 40 mM TiCl4 solution at 70 ºC for 1 hour, then rinsed with deionized water and ethanol, and annealed under air condition at 520 ºC for 30 min in muffle furnace.
6
2.4. Fabrication of DSSCs and self-powered UV sensors. For application of DSSCs, the SnO2 and TiO2-modified SnO2 photoelectrodes were heated to 80 ºC, immersed into 0.5 mM N719 dye in acetonitrile/tert-butanol solution (volume ratio of 1:1) for 20 hours at room temperature to sensitize, and then washed with acetonitrile to remove the physically-adsorbed dye molecules. Pt counter electrodes were deposited by magnetron sputtering on cleaned FTO substrates. Pt counter electrode was buckled on the dye-sensitized photoelectrode, between which there was a space separated by polypropylene plastic for injecting electrolyte. The electrolyte was composed of 0.6 M PMII, 0.05 M LiI, 0.03 M I2, 0.1 M GuSCN and 0.5 M 4TBP in acetonitrile and valeronitrile (volume ratio of 85:15). The active area of the devices was 0.15 cm2 without mask. The fabrication of self-powered UV sensors was almost the same as that of DSSCs except that the dye was absent. 2.5. Material characterization and electrochemical measurement. X-ray diffraction (XRD, PANalytical B.V. The Netherlands) measurement with Cu-Ka radiation was performed to characterize the crystal structures. Scanning electron microscope (SEM, FEI NOVA NanoSEM 450), and aberration-corrected transmission electron microscope (TEM, FEI Titan G260-300) were used to observe the morphology of the samples. The TEM sample was prepared by dropping casting ethanolic dispersion of the powder onto a carbon coated Cu grid. Absorption spectra of the samples were recorded with a UV-Vis absorption spectrometer (UV-2550, Shimadzu). The dye molecules were desorbed from the sensitized-photoelectrodes into 0.1 M NaOH solution to test the absorption spectra and calculate the dye-adsorbed amounts. The simulated sunlight and UV light were emitted from a solar simulator illumination (Newport, USA) of AM 1.5 G
(100 mW/cm2) intensity and an ultra-high
pressure mercury lamp (Model CHF-XM500, Beijing Changtuo Technology Co., Ltd.) with a filter (365 nm) of 7.5 mW/cm2 intensity, respectively. The current-voltage, voltage-decay and electrochemical impedance spectroscopy (EIS) measurements were recorded using an Autolab electrochemic workstation (model AUT84315, The Netherlands). EIS spectra were measured 7
in dark condition at a bias of 0.60 V with an amplitude of 10 mV in a frequency range from 100 kHz to 0.1 Hz.
3. Results and discussion As shown in Fig. 1a, the TiO2 layer modifies the SnO2 surface and engineers the surface energy, which prevents back-injecting electrons from SnO2 to electrolyte. According to the transition-probability approach of Bardeen [17,18], the tunneling rate from SnO2 to electrolyte follows exponential decay with increasing the thickness of TiO2 layer. Therefore, the TiO2 modification inhibits the charge recombination. For DSSCs, as shown in Fig. 1b, dye molecules capture light to generate electron-hole pairs, then the electrons inject into TiO2 conduction band and transfer into SnO2 conduction band for the type-II structure of SnO2/TiO2 with energy level gradient. Moreover, it is difficult for the electrons in SnO2 to backflow to recombine with holes in redox electrolyte because of the surface modification of SnO2 by TiO2 layer [19]. For UV sensors, the SnO2/TiO2 generates electron-hole pairs under UV irradiation, and the high conduction band TiO2 layer inhibits the back-flowing electrons to recombine, as demonstrated in the schematic Fig.1c. For clearance and comparison, the TiO2modified SnO2 hierarchical microsphere-based DSSCs and UV sensors are illustrated in schematic Fig. 1d and 1e, respectively. The photoelectrodes were sandwiched with platinum counter electrodes, and the internal spaces between the two electrodes were filled with liquid electrolyte. The SnO2 powder was synthesized by using a microwave-assisted hydrothermal method involving the calcination as seen in the experimental section. The crystalline structure of the powder was characterized by XRD. Fig. 2a shows the typical XRD patterns of Sn6O4(OH)4 powder (i, JCPDS, No. 46-1486) prepared by a microwave-assisted hydrothermal method and the rutile-like SnO2 powder (ii, JCPDS, No. 88-0287) after annealing at 600 ºC for 3 hours. The scanning electron microscopy (SEM) image of Fig. 2b indicates that the as-prepared 8
SnO2 sample is porous hierarchical microspheres. The corresponding enlarged SEM image of Fig. 2c shows the aggregation of a number of small nanoparticles and nanorods. Fig. 2d shows the cross-section of the photoelectrode obtained by printing method from the SnO2 paste, where the thickness is about 11.5 μm. The morphology of SnO2 and TiO2-modified SnO2 hierarchical microspheres was further characterized by TEM technology. Fig. 3a, combined with the illustration, clearly shows that the obtained SnO2 sample is porous microspheres with hundreds of nanometers, composed of small nanoparticles and nanorods. In the high resolution TEM image of Fig. 3b, the lattice fringes of 0.338 nm, 0.219 nm and 0.255 nm correspond to (110), (210), (101) planes of rutile-SnO2, respectively, which are further evidenced that the product is SnO2. The corresponding selected area electron diffraction (SAED) pattern in the inset of Fig. 3b indicates that the as-prepared SnO2 sample is polycrystalline. Fig. 3c shows a TiO2-modified SnO2 nanorod, and Fig. 3c of the HRTEM image corresponds to the outstanding one in the lower left corner of the TEM image. The nanorod shell is rough and can be indexed as TiO2. In the enlarged region of Fig. 3c, there is an obvious interface between SnO2 and TiO2. The lattice fringe spacing of 0.246 nm is consistent with the (101) lattice plane of rutile TiO2 (PCPDFWIN, No. 88-1175), and that of 0.255 nm is in accordance with (101) plane of rutile SnO2. X-ray energy dispersion spectrum (EDS) was further used to analyze the nanorod corresponding to Fig. 3c, as shown in Fig. 3d. In addition to the existence of Sn and O peaks, there are obvious Ti peaks. The Cu peak is from the Cu mesh. The above characterization in Fig. 3 demonstrates that SnO2 is well modified by TiO2. For photoelectrodes of DSSCs, the structure of hierarchical microsphere is an excellent morphology to supply efficient light scattering, large specific surface area and long-range electronic connectivity [20-23]. Fig. 4a shows the typical current density-voltage curves of the DSSCs based on SnO2 and TiO2-modified SnO2 hierarchical microspheres under the illumination of 100 mA/cm2. The corresponding photovoltaic parameters from Fig. 4a are 9
listed in Table 1, where JSC is short-circurt current density, VOC is open-circurt voltage, and FF is fill factor. The TiO2-modified-SnO2 based DSSCs (TS-DSSCs) exhibits higher performance than that of SnO2 based DSSCs (S-DSSCs). In Fig 4 b, the dark current test indicated the TSDSSCs have lower dark current, meaning the inhibition of charge recombination by TiO2 modification. The absorbed-dye amounts in Tabel 1 were calculated using Lambert-Beer's law from Fig. 1S, which practically remain unchanged after TiO2 modification. In DSSCs, PCE is determined by JSC, VOC and FF according to the following equation [5]: PCE
VOC J SC FF Pin
(1)
where Pin is the illumination power intensity. After TiO2 modification, the photovoltaic parameters of VOC, JSC and FF are all improved. JSC can be approximated by the following equation [24]: J SC e1hinjcc I 0
(2)
where e is the elementary charge, ηlh is the light-harvesting efficiency and mainly related to the amount of adsorbed dye, ηinj is the charge-injection efficiency, ηcc is the charge-collection efficiency, and I0 is the light flux. Our experiments show that η1h is nearly the same because of the unchanged amount of adsorbed dye after modification, and hinj is also supposed to be the same value because the time of electron injection from the excited dye molecules into SnO2 is 150 ps, which is the approximate value of that into TiO2 [25]. ηcc is mainly dependent on the competition between charge recombination and collection. Therefore, the charge recombination mainly has an effect on JSC. Meanwhile, theoretical VOC is determined by the difference between the Fermi level of the active photoelectrode and the oxidation potential of the electrolyte. Since the conduction band of TiO2 is higher than that of SnO2, the existence of the TiO2 layer can lead to higher Fermi level of photoelectrode than that of the mere SnO2.
10
Moreover, the charge recombination reduces the value of VOC than the theoretical value [26, 27]. Therefore, the charge recombination brings negative efforts to both JSC and VOC. To evaluate the charge recombination, EIS measurement was carried out at a bias of 0.60 V under dark condition. Fig. 5a shows the Nyquist plot curves, in which there are two semicircles, one in the high frequency region (>1 kHz) and the other in the frequency region (100-0.1 Hz). The equivalent circuit and the corresponding values by Zview software were shown in Fig. 2S and Table 1S (Supporting Information), respectively. According to the EIS model [27, 28], the smaller semicircle in the high-frequency region denotes the redox reaction at the Pt/electrolyte interface, and the larger one represents the charge transfer at the photoelectrode/electrolyte interface. The charge-transfer resistance of Rct accounts for charge recombination, which is 1360 Ω for TS-DSSCs, and much higher than 116.1 Ω for S-DSSCs. Thus, the recombination of electrons in photoelectrode with holes in electrolyte is more difficult for TiO2-modified SnO2 photoelectrode. The behavior of DSSCs under dark condition resembles a diode with an applied bias, hence Rct can be considered as part of a normalized shunt resistance (rs), which relates to the FF according to the formula: FF = FF0(11/rs), where FF0 is the theoretical maximum FF [29]. It can be inferred that the increasing FF of the TS-DSSCs is originated from the inhibition of charge recombination. Fig. 5b shows the Bode plots of the DSSCs. The electron lifetime can be calculated from curve peak of the spectrum using the equation τ = 1/(2πfmax) [30], where fmax is the maximum frequency of the mid-frequency peak, which is 10.33 Hz and 2.90 Hz for ST-DSSCs, respectively. The electron lifetime of TS-DSSCs is 109.8 ms, which is longer than the 30.8 ms electron lifetime of SDSSCs. To further confirm the inhibition of charge recombination by TiO2 modification, the open-voltage decay method was further employed. When the illustration light was turned off, the voltage decayed continuously, as shown in Fig. 5c. Then the electron lifetime τr (Fig. 5d) can be calculated using data in Fig. 5c and the following formula [31]: 11
k T dV r B OC e dt
1
(3)
where kB is the Boltzmann constant and T is the room temperature. It is clear that the electron lifetime significantly increases by TiO2 modification and the lifetime can be calculated as ~100 ms at VOC = 0.6 V for TS-DSSCs, which is consistent with the liftetime in Fig. 5b. Long electron lifetime implies the inhibition of charge recombination. The SnO2 and TiO2-modified SnO2 based UV sensors were characterized using UV light. In Fig. 6, the current was tested with/without UV irradiation from -1 V to 1 V bias. When the devices are irradiated by UV light, the photo-current is enhanced obviously. Under zero bias, the devices show photo-current under UV illumination, which means that the devices are selfpowered UV sensors. Moreover, the self-powered property is more obvious after TiO2 modification. The self-powered UV sensors are a type of electrochemical devices, and there is no electric field. The power of the charge separation comes from the I-/I3- redox couples [32]. As soon as the electron-hole pairs are generated, the holes will be quickly removed by Idescribed as 3I-+2h+→I3-. The photo-electrons transfer across the photoelectrode, from the external circuit and arrive at the counter electrode. The electrolyte are recovered to I- by the electrons from the counter electrode as I3-+2e→3I -. Due to the I-/I3- redox couples, photocurrent can be continuously regenerated. The responsibility and stability are studied under rectangle pulse UV illumination (on/off) under zero bias, as shown in Fig. 7. The photocurrent signal is reproducible and stable from “off” to “on” states. Furthermore, the photocurrent of the TiO2-modified SnO2 based self-powered UV has been improved to 191 μA from 11.5 μA of the SnO2 based devices, which mainly results from the inhibition of charge recombination. The sensitivity S (= Ilight/( Ilight - Idark) [33], where Ilight and Idark are the currents measured under UV light and dark condition, respectively) was calculated as 91 and 6229 for the SnO2 and TiO2-modified SnO2 based self-powered UV sensors, respectively. The response time (the time needed to 12
reach (1-1/e) of the maximum photocurrent [34]) is decreased to 0.055 s of the TiO2-modified SnO2 based self-powered UV sensor from 0.120 s of the SnO2 based self-powered UV sensor. While the values of the corresponding recovery time (the time needed for the photocurrent to drop to 1/e of its maximum value) are 0.05 s and 0.155 s, respectively. In the TiO2-modified SnO2 based self-powered UV sensors, the faster response time is due to the easier electron traveling related to the inhibition of charge recombination when electron-hole pairs generated under UV light, and its slower recovery time was also attributed to the inhibition of charge recombination process. For inhibition of charge recombination, the photo-generated charges are not easy to recombine; in other words, the charges have long lifetime, therefore the TiO2modified SnO2 based self-powered UV sensors exhibit slower recovery time than bare SnO2 based ones when the UV light was turned off.
4. Conclusions In summary, the surface modification for energy engineering was carried out to inhibit charge recombination for DSSCs and self-powered UV sensors based on SnO2 hierarchical spheres. For DSSCs, since the SnO2 surface was modified by TiO2 layer, the efficiency increased from 1.40% to 4.15% because of the inhibition of charge recombination. For selfpowered UV sensors, with TiO2 modification, the self-powered property was more obvious, the sensitivity was improved from 91 to 6229, and the response time was decrease from 0.15 s to 0.055 s. The result shows an effective way of surface modification for inhibiting charge recombination to enhance the performance of photoelectronic devices.
Acknowledgements This work was supported by the Natural Science Foundation of Jiangsu Province (BK20150860), the Seed Project Funded by Introducing Talent of NJUPT (NY215022), the National Natural Science Foundation of China (51102150, 51472081). 13
References (1) I. Vass, Physiol. Plant., 2011, 142, 6-16. (2) J. Moser, M. Grätzel and R. Gallay, Helvetica Chimica Acta, 1987, 70, 1559-1604. (3) S. Hattori, W. W. Mou, P. Rajak, F. Shimojo and A. Nakano, Appl. Phys. Lett.,2013, 10, 093302-1-3. (4) J. Chen, W. Lei and W. Q. Deng, Nanoscale, 2011, 3, 674-677. (5) B. O. Regan and M. Grätzel, Nature, 1991,353, 737-740. (6) J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki, and A. B. Walker, J. Am. Chem. Soc., 2008, 130, 13364-13372. (7) S. Chappel and A. Zaban, Sol. Energy Mater. Sol. Cells, 2002,71, 141-152. (8) M. S. Arnold, P. Avouris, Z. W. Pan and Z. L.Wang, J. Phys. Chem. B, 2003, 107, 659663. (9) R. G. Breckenridge and W. R. Hosler, Phys. Rev.,1953, 91, 793-801. (10) Y. D. Duan, N. Q. Fu, Q. P. Liu, Y. Y. Fang, X. W. Zhou, J. B. Zhang and Y. Lin, J. Phys. Chem. C, 2012, 116, 8888-8893. (11) Z. L. Wang and J. H. Song, Science, 2006, 312, 242-246. (12) Z. L. Wang and W. Wu, Angew. Chem. Int. Edit., 2012, 51, 11700-11721. (13) M. Razeghi and A. Rogalski, J. Appl. Phys., 1996, 79, 7433-7473. (14) Z. H. Lin, G. Cheng, Y. Yang, Y. S. Zhou, S. Lee and Z. L. Wang, Adv. Funct. Mater., 2014, 24, 2810-2816. (15) S. M. Hatch, J. Briscoe and S. Dunn, Adv. Mater., 2013, 25, 867-871. (16) N. T. Khoa, S. W. Kim, D. V. Thuan, D.-H. Yoo, E. J. Kim and S. H. Hahn, Cryst. Eng. Comm., 2014,16, 1344-1350. (17) M. C. Payne, J. Phys. C: Solid State Phys., 1986, 19, 1145-1155. (18) S. L. Chuang and Jr. N. Holonyak, Appl. Phys. Lett., 2002, 80, 1270-1-3.
14
(19) B. E. Hardin, H. J. Snaith, and M. D. McGehee, Nat. Photonics, 2012, 6, 162-169. (20) M. D. Ye, C. Chen, M. Q. Lv, D. J. Zheng, W. X. Guo and C. J. Lin, Nanoscale, 2013, 5, 6577-6583. (21) L. Chu, Z. F. Qin, Q. X. Zhang, W. Chen, J. Yang, J. P. Yang , X. A. Li, App. Surface Sci., 2016, 360, 634-640. (22) J. Y. Liao, B. X. Lei, D. B. Kuang and C. Y. Su, Energy Environ. Sci., 2011, 4, 40794085. (23) J. Y. Liao, J. W. He, H. Y. Xu, D. B. Kuang, and C. Y. Su, J. Mater. Chem., 2012, 22, 7910-7918. (24) K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007, 7, 69-74. (25) A. N. M. Green, E. Palomares, S. A. Haque, J. M. Kroon and J. R. Durrant, J. Phys. Chem. B, 2005, 109, 12525-12533. (26) Z. Lan, J. Wu, D. B. Wang, S. C. Hao, J. M. Lin and Y. F. Huang, Sol. Energy, 2007, 81, 117-122. (27) Q. Wang, J. E. Moser and M. Grätzel, J. Phys. Chem. B, 2005, 109, 14945-14953. (28) H. N. Kim and J. H. Moon, ACS Appl. Mater. Interfaces., 2012, 4, 5821-5825. (29) N. Satoh and L. Y. Han, Phys. Chem. Chem. Phys., 2012, 14, 16014-16022. (30) R. Kern, R. Sastrawan, J. Ferber, R. Stangl and J. Luther, Electrochim. Acta, 2002, 47, 4213-4225. (31) A. Zaban, M. Greenshtein and J. Bisquert, Chem. Phys. Chem., 2003, 4, 859-864. (32) R. S. Mane, B. R. Sankapal and C. D. Lokhande, Mater. Chem. Phys., 1999, 60, 196-203. (33) B. Yuan, X. J. Zheng, Y. Q. Chen, B. Yang and T. Zhang, Solid-State Electronics,,2011, 55, 49-53. (34) J. J. Hassan, M. A. Mahdi, S. J. Kasim, Naser M. Ahmed, H. Abu, Appl. Phys. Lett., 2012, 101, 261108-1-3.
15
Figures
Fig. 1. (a) Schematic illustration for the TiO2 modification for inhibition of electrode tunneling from SnO2 to electrolyte. (b) and (c) Schematic illustration for the band structures and electron transfers in DSSCs and self-powered UV sensors, respectively. (d) and (e) Schematics of the DSSCs and self-powered UV sensers, respectively.
16
Fig. 2. (a) XRD pattern of the Sn6O4(OH)4 sample prepared by a microwave-assisted hydrothermal method (curve i) and SnO2 powder with annealing at 600 ºC for 3 h (curve ii). (b) SEM image of the obtained SnO2 powder, (c) SEM image of the same region in (b) at high magnification. (d) Cross-sectional SEM image of the photoelectrode.
17
Fig. 3. (a) TEM image of the SnO2 powder sample and the blown-up image in the inset. (b) HRTEM image and SAED pattern in the inset. (c) TEM and HRTEM images of the TiO2modified SnO2 sample. (d) Energy dispersive x-ray spectroscopy spectrum corresponding to the nanorod in (c).
18
Fig. 4. The current density-voltage curves of S-DSSCs and ST-DSSCs under 100 m/Wcm2 illumination, (b) the corresponding current density-voltage curves under dark condition.
19
Fig. 5. EIS for S-DSSCs and ST-DSSCs under dark condition at an bias of 0.60 V. (a) Nyquist plots, and (b) Bode plots of phase angle versus frequency. (c) Open-circuit voltage decay curve. (d) Electron lifetime determined from (c).
20
Fig. 6. (a) Current-voltage curves for the SnO2 and TiO2-modified SnO2-based UV sensors with and without a UV illumination (365 nm). (a) SnO2, and (b) TiO2-modified SnO2 photoelectrodes.
21
Fig. 7. (a) The reversible characteristics of self-powered UV sensors under pulse UV illumination (on/off). (a) and (c) SnO2 ; (b) and (d), TiO2-modified SnO2 photoelectrodes.
22
Table 1 Photovoltaic parameters of DSSCs. JSC Photoelectrode
Dye-amount VOC(V)
FF
PCE (%)
(mA/cm2)
(nmol/cm2)
SnO2
9.2
0.458
0.332
1.40
SnO2-TiO2
11.7
0.729
0.487
4.15
23
62.4 64.3