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10.1016@j.solidstatesciences.2019.05.021.pdf

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  Contents lists available at ScienceDirect Solid State Sciences  journal homepage: www.elsevier.com/locate/ssscie Facile fabrication of MoS 2 -P3HT hybrid microheterostructure withenhanced photovoltaic performance in TiO 2  nanorod array based hybridsolar cell Bao Sun, Ziting Su, Yanzhong Hao ∗ , Juan Pei, Yingpin Li College of Sciences, Hebei University of Science and Technology, Shijiazhuang, 050018, PR China A R T I C L E I N F O  Keywords: MoS 2 -P3HTHybrid microheterostructureLight absorberHole transport materialTiO 2  nanorod array-based solar cell A B S T R A C T MoS 2 -P3HT hybrid microheterostructure was successfully prepared through ultrasonic process and used as lightabsorber and hole transport material in TiO 2  nanorod array-based hybrid solar cell. The charge transport me-chanism in the solar cell was systematically investigated with di ff  erent methods, such as energy level structure,PL spectra, optical absorption property and EIS measurement. The results indicated that the designed MoS 2 -P3HT hybrid microheterostructure led to enhanced optical property and improved charge transport performancewith reasonable energy band alignment. The solar cell based on TiO 2 /MoS 2 -P3HT exhibited an optimized energyconversion e ffi ciency of 1.28%, with an increment of 58% compared to that of the solar cell based on TiO 2 /P3HT. 1. Introduction Nanostructural two-dimensional molybdenum disul 󿬁 de (MoS 2 ), agraphene-like semiconductor with excellent electronic and opticalproperties, has recently attracted increasing research interests for thepotential application in solar cell technologies [1 – 6]. Initially, MoS 2 was mostly used as counter electrode material substituting for the tra-ditional expensive Pt in dye-sensitized solar cells [7 – 13] or quantum-dot sensitized solar cells [14,15], taking the advantages of its activity for triiodide reduction [16] and stability to prevent electrolyte corro-sion [17]. In the last several years, some researches on other applicationof MoS 2  in solar cell were presented. Du et al. [18] reported a novellow-cost inorganic solar cell based on MoS 2 /TiO 2  microheterostructureprepared by depositing MoS 2  in the mesoporous TiO 2  󿬁 lm. Similar tothe dye-sensitized solar cell, the photovoltaic device used I − /I 3 − electrolyte as hole transport material and Pt-coated FTO glasses ascounter electrodes. The optimized power conversion e ffi ciency reached1.08%. Kesavan et al. [19] prepared a ternary blend system of PTB7,PC71BM and MoS 2  as active material for polymer solar cells. The powerconversion e ffi ciency of the optimized ternary blend system showed anincrement of 17% compared to that of the PTB7/PC71BM binary systemdue to the increased light harvesting and improved charge carriermobility by doping with MoS 2 .In addition, high carrier mobility and high work function of MoS 2 indicated its potential application as a hole-transport material (HTM) insolid-state solar cells [20,21]. However, it was di ffi cult for MoS 2  tosuccessfully form uniform thin  󿬁 lm due to the slightly solubility of MoS 2  in most solvents. Currently, some e ff  ective methods, such aschemical vapor deposition [22] and micro-mechanical exfoliation [23], were exploited for MoS 2  thin  󿬁 lm formation, which all required rig-orous processing condition and complex techniques. In comparison,liquid dispersion and deposition of the MoS 2  was more popular [24,25]. Nevertheless, MoS 2  layers tended to be stacked together because of thehigh surface energy of two dimensional structures. In order to overcomethese obstacles, MoS 2  layers were always blended into some well-dis-persed supports, such as graphene, to form a stable composite  󿬁 lm andapplied into solar cell to enhance the device performance [26,27]. In recent decades, inorganic-organic all-solid-state solar cells[28 – 30], especially perovskite solar cells [31 – 33], have received moreand more attention. As an excellent HTM, as well as an optical ab-sorbing material, P3HT was widely used in all-solid-state solar cells[34 – 36]. It was reported that the hole transport mobility of poly (3-hexylthiophene) (P3HT) could be signi 󿬁 cantly improved by dopingwith MoS 2  nanosheets (NSs) [37]. Therefore, in this work, MoS 2  NSsprepared through hydrothermal process were dispersed in the chlor-obenzene under ultrasonication and then blended with P3HT as dis-persed support to form a blend of the MoS 2 -P3HT hybrid micro-heterostructure, which was employed as light absorber layer and HTMin TiO 2  nanorod array-based hybrid solar cell for the 󿬁 rst time. Throughcomparative study on the TiO 2 /MoS 2 -P3HT and TiO 2 /P3HT https://doi.org/10.1016/j.solidstatesciences.2019.05.021Received 7 April 2019; Received in revised form 28 May 2019; Accepted 29 May 2019 ∗ Corresponding author.  E-mail address:  yzhao@hebust.edu.cn (Y. Hao). Solid State Sciences 94 (2019) 92–98Available online 30 May 20191293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.    photoactive layer, it was concluded that the photoactive layer of TiO 2 /MoS 2 -P3HT revealed enhanced optical property and improved chargetransport performance with reasonable energy band alignment. Thephotovoltaic performance of the hybrid solar cell based on TiO 2 /MoS 2 -P3HT exhibited obvious enhancement compared to that of the solar cellbased on TiO 2 /P3HT. 2. Experimental  2.1. Preparation of hierarchical microspheres of MoS  2  NSs powder and FTO/MoS  2  󿬁 lm The hierarchical microspheres of MoS 2  NSs powder were preparedby a hydrothermal method according to reference [38]. For a typicalprocess, 0.4mmol of [NH 4 ] 6 Mo 7 O 24 ·4H 2 O and 40mmol of thioureawere dissolved in 10mL of deionized water. Then, the homogeneoussolution was transferred into a 50mL te 󿬂 on-lined autoclave and held at200°C for 24h. The black precipitate was washed with deionized waterfor three times and collected by centrifugation, followed by drying at80°C for 1h to form the hierarchical microspheres of MoS 2  NSs powder.The obtained MoS 2  NSs powder was made into slurry through suf- 󿬁 cient grinding in the mortar using ethanol as solvent and polyethyleneglycol (PEG with mole weight of 2000) as binder. Then, the slurry wascoated onto a FTO glass by doctor blade technique. The obtained MoS 2 󿬁 lm was annealed at 350°C for 30min under nitrogen atmosphere toform the FTO/MoS 2  󿬁 lm electrode.  2.2. Preparation of MoS  2 -P3HT hybrid microheterostructure and fabrication of TiO  2 /MoS  2 -P3HT solar cell For preparing the blend of MoS 2 -P3HT hybrid microheterostructure, 󿬁 rstly, 20mg of MoS 2  powder was dispersed into 2ml chlorobenzeneand ultrasonicated for 10h under condition of circulating water. Thenthe MoS 2  suspension was centrifuged for 30min and 90% supernatantwas taken out cautiously. Finally, the blend of the MoS 2 -P3HT hybridmicroheterostructure was prepared by dispersing 15mg of P3HT into1ml of the supernatant through magnetic stirring for 2hat 50°C.The solar cell of FTO/TiO 2 /MoS 2 -P3HT/PEDOT:PSS/Ag was fabri-cated as follows. First, the FTO coated glass substrates were ultrasoniccleaned by soap solution, deionized water, acetone and ethanol suc-cessively. TiO 2  nanorod array 󿬁 lm was then grown onto the cleaned anddried FTO substrate by hydrothermal process according to reference[39]. The obtained FTO/TiO 2  nanorod array  󿬁 lm was annealed at500°C for 30min to promote the crystallization of the TiO 2  layer.Subsequently, the prepared chlorobenzol solution of the MoS 2 -P3HThybrid microheterostructure and the PEDOT:PSS solution blended withappropriate amount of FS-300 were spin coated onto the FTO/TiO 2  󿬁 lmat 2000rpm for 50s, respectively. The formed active layer of FTO/TiO 2 /MoS 2 -P3HT/PEDOT:PSS was then annealed at 120°C for 30min.Finally, a Ag  󿬁 lm of 80nm was evaporated onto the surface of the ac-tive layer by thermal evaporation to form the solar cell of FTO/TiO 2 /MoS 2 -P3HT/PEDOT:PSS/Ag. For comparison, the pristine P3HT  󿬁 lmwithout MoS 2  was used to form the solar cell of FTO/TiO 2 /P3HT/PEDOT:PSS/Ag. For better reliability, a set of 3-cells were prepared foreach kind of hybrid solar cells. Device A, B and C were denoted for thesolar cells of FTO/TiO 2 /MoS 2 -P3HT/PEDOT:PSS/Ag. Meanwhile, De-vice D, E and F were denoted for the solar cells of FTO/TiO 2 /P3HT/PEDOT:PSS/Ag.  2.3. Characterization and photoelectrochemical measurements The morphologies and structures of the as-prepared samples werecharacterized by  󿬁 eld emission scanning electron microscopy (FESEM,Hitachi S4800-I, 10.0kV), transmission electron microscopy (TEM H-600) and powder X-ray di ff  raction (XRD, Rigaku, D/MAX-2500 dif-fractometer with Cu K α  radiation). The UV – visible di ff  use re 󿬂 ectionspectra were recorded on a Hitachi UV – Vis spectrophotometer (U-3900).The energy level of the prepared MoS 2  NSs was determined by theoptical absorption and Mott-Schottky measurement of the MoS 2 /FTOelectrode. The Mott-Schottky measurement was conducted with aZAHNER IM6e Impedance Analyzer (ZAHNER Elektrik GmbH & CoKG,Kronach, Germany) in 0.1molL − 1 acetonitrile solution of tetrabutyl- 󿬂 uorobonate using a three-electrode system where the MoS 2 /FTO  󿬁 lmelectrode, the Pt electrode and the statured calomel electrode (SCE)were employed as work electrode, counter electrode and referenceelectrode, respectively.Photocurrent-voltage measurements for the solar cells were per-formed on a Keithley 2400 SourceMeter using simulated AM 1.5 sun-light with an output power of 100mW/cm 2 produced by a solar si-mulator (SOL300-23A). The electrochemical impedance spectroscopyof the devices was measured with a Model 263 galvanostat and a Model5210 lock in ampli 󿬁 er. The frequency was swept from 0.1Hz to100kHz. The PL measurements of the FTO/TiO 2 /MoS 2 -P3HT  󿬁 lm andthe FTO/TiO 2 /P3HT  󿬁 lm were carried out under excitation wavelengthof 500nm on an F-4600 FL spectrophotometer. 3. Results and discussion 3.1. Characterization of the as prepared MoS  2  NSs As shown in Fig. 1a, the morphology of the as-prepared MoS 2  NSspowder is characterized by SEM. It can be seen that the as-preparedMoS 2  powder reveals a  󿬂 ower-like hierarchical microsphere structure,which comprises a lot of curling-up NSs with thickness of a few nan-ometers. Fig. 1b shows the XRD pattern of the FTO/MoS 2  󿬁 lm. Alldi ff  raction peaks of the sample are identi 󿬁 ed by comparison withstandard cards of MoS 2  and FTO. The di ff  raction peaks located at 13.9°, 10 20 30 40 50 60 70 80 (b)   XRD standard card for FTO MoS 2 FTO XRD standard card for MoS 2       I    n      t    e    n    s      i      t    y      /    a  .    u  . 2 / Fig. 1.  SEM image of the as-prepared hierarchical microsphere MoS 2  NSs powder (a) and XRD pattern (b) of the FTO/MoS 2  󿬁 lm compared with standard cards of MoS 2  and FTO.  B. Sun, et al.  Solid State Sciences 94 (2019) 92–98 93  33.7° and 62.0° are indexed to planes of (002), (101) and (107) forMoS 2 . The rest di ff  raction peaks are assigned to the FTO substrate. Noimpurity phases can be observed in the XRD pattern. The sharp dif-fraction peaks indicate the good crystallization of the sample. 3.2. Characterization of the MoS  2 -P3HT hybrid microheterostructure Fig. 2 presents the TEM (a) and HRTEM (b) images of the MoS 2 -P3HT hybrid microheterostructure. As can be seen from Fig. 2a, the 󿬂 ower-like hierarchical microspheres of the MoS 2  NSs are disperseduniformly in the supported P3HT layer. The transparent feature, in-dicating the ultrathin thickness, is in good agreement with the 2D sheet-like morphology of the as-prepared MoS 2  NSs sample. To further con- 󿬁 rm the presence of MoS 2 , the rectangle area in Fig. 2a is selected tomake a high resolution analysis. The resulted HRTEM image is shown inFig. 2b. The observed lattice space about 0.62nm is in accordance withthe characteristic inter layer distance (0.62nm) for (002) plane of MoS 2 , indicating that the MoS 2  is well crystallized and successful dis-persed in P3HT support.The detailed elemental distribution in the MoS 2 -P3HT hybrid mi-croheterostructure is illustrated by EDS in Fig. 3. The correspondingelemental analysis data is displayed in Table 1. In order to form a dense 󿬁 lm for more accurate EDS testing, the blend of MoS 2 -P3HT hybridmicroheterostructure dispersed in chlorobenzene was dropped onto aglass substrate and the MoS 2 -P3HT  󿬁 lm was formed after solvent eva-poration. It can be seen from Fig. 3a that the EDS peaks of C, Mo and Sare detected, further suggesting the presence of MoS 2  in the P3HT layer.The other detected elements, i.e. Si, Na, Ca, Mg, Al, should be identi 󿬁 edas the components of the glass substrate. Particularly noting, the mainpeaks of Mo and S are overlapped. To better distinguish the two ele-ments, we made a careful observation during the EDS scanning process.It was found that two small peaks of Mo were appeared on both sides of the overlapped peak and a peak of S was recorded on the right shoulderof the overlapped peak under automatic detection. So an enlarged EDSspectrum is presented in Fig. 3b to clearly show the small peaks of Moto better distinguish its presence. The detected small peaks of Mo and Sare marked with red circles. As shown in Table 1, a small amount of Moelement also indicates the presence of Mo in the detected  󿬁 lm.Combined the TEM and HRTEM results with the EDS analysis, it canbe con 󿬁 rmed that the MoS 2  NSs are successfully dispersed into theP3HT support through ultrasonic and blended process. 3.3. Fabrication and characterization of the TiO  2 /MoS  2 -P3HT solar cell In this work, a solid-state hybrid solar cell, in which the MoS 2 -P3HThybrid microheterostructure used simultaneously as photoactive layerand HTM was spin-coated onto the TiO 2  nanorod array, was fabricatedand characterized by SEM shown in Fig. 4. Fig. 4a gives the top-view SEM image of the hydrothermal TiO 2  nanorod array prepared on FTO Fig. 2.  TEM (a) and HRTEM (b) images of the MoS 2 -P3HT hybrid microheterostructure. Fig. 3.  EDS spectra of the MoS 2 -P3HT  󿬁 lm on glass substrate: (a) regular spectrum, (b) enlarged spectrum. Table 1 Elemental analysis data for the MoS 2 -P3HT  󿬁 lm on glass substrate. Element Weight/% Atomic/%MoS 2 -P3HT  󿬁 lm C K 63.75 83.71S K 4.76 2.34Mo L 8.95 1.47Glass Substrate Si K 15.95 8.98Na K 2.33 1.60Ca K 3.22 1.27Mg K 0.56 0.36Al K 0.48 0.28Total 100.00  B. Sun, et al.  Solid State Sciences 94 (2019) 92–98 94  substrate. All the nanorods have smooth surface and square shape witha relatively uniform size about 30nm in diameter. The cross-sectionalview SEM image of the TiO 2  nanorod array, as shown in Fig. 4b, revealsa well-aligned and uniform array with a length about 1.2 μ m. The ap-propriate array density is favorable to the subsequent deposition of theMoS 2 -P3HT hybrid microheterostructure. Compared Fig. 4c withFig. 4a, it can be seen that some distinct changes appear in SEM image.A layer of   󿬁 lm is covered onto and even penetrated into the TiO 2  na-norod array. The above observation indicates that the MoS 2 -P3HT hy-brid microheterostructure has been successfully deposited into the TiO 2 nanorod array to form the active layer of FTO/TiO 2 /MoS 2 -P3HT.However, it can be obviously found from Fig. 4c that the surface of theTiO 2  array is not completely covered by MoS 2 -P3HT layer. Some TiO 2 nanorod tips are still exposed. So a layer of PEDOT:PSS is spin-coatedbefore the thermal evaporation of Ag to avoid short circuit between theexposed TiO 2  nanorods and Ag electrode. Fig. 4d shows the cross-sec-tional view SEM image of the whole hybrid solar cell based on the FTO/TiO 2 /MoS 2 -P3HT/PEDOT:PSS/Ag  󿬁 lm, in which the FTO substrate, theTiO 2 /MoS 2 -P3HT active layer and the PEDOT:PSS/Ag  󿬁 lm weremarked with di ff  erent colors. With a thermal evaporation process,about 80nm thick of Ag  󿬁 lm was smoothly deposited onto the activelayer with good contact, which is an important factor for the quick ande ffi cient hole collection to the Ag electrode. 3.4. Photophysical properties of MoS  2  in the TiO  2 /MoS  2 -P3HT solar cell In order to investigate the energy level matching property betweenthe as-prepared MoS 2  and the other materials (i.e. TiO 2  and P3HT) inthe hybrid solar cell discussed in this work, the energy level of the MoS 2 are con 󿬁 rmed through the optical absorption (shown in Fig. 5a) andMott-Schottky measurement (shown in Fig. 5b) tested with the FTO/MoS 2  electrode.It can be seen from Fig. 5a that the optical absorption range of theFTO/MoS 2  electrode covers the visible light region and even reachesthe infrared region. The band gap (  E  g ) of the as-prepared MoS 2  is about1.45eV, which is calculated using the starting wavelength (  λ  abs ) of 853nm by equation (1): =  E λ /eV 1240/( /nm) g abs  (1)As shown in Fig. 5b, the positive slope of the  󿬁 tting line for theMott-Schottky plot indicates that the as prepared MoS 2  is an n-typesemiconductor. Meanwhile, an  E  fb-ref   value (refer to the energy level of the as prepared MoS 2  relative to that of reference electrode) of  − 0.78Vcan be acquired from the intersection of the  󿬁 tted line and the hor-izontal axis in the Mott-Schottky plot. The  E  fb  of the as prepared MoS 2 relative to vacuum energy level can be calculated according to thefollowing equation (2): = − +  −  E E E  (e e ) fb ref fb ref   (2)here in,  E  ref   refers to the energy level of reference electrode relative tothat of vacuum. As a result,  E  ref   and  E  fb-ref   in this measurement arecon 󿬁 rmed as 4.74V and − 0.78V, so the  E  fb  of the as-prepared MoS 2 can be calculated to be − 3.96eV. For an n-type semiconductor, the  E  fb is close to the conduction band (  E  c ) and there is an experienced dif-ference of 0.1 – 0.3eV between the two energy levels. Therefore, the  E  c of the as-prepared MoS 2  can be located at a value of  − 3.66 ∼− 3.86eV. Based on the band gap of 1.45eV, the valenceband of the as-prepared MoS 2  can be con 󿬁 rmed to be − 5.11 ∼− 5.31eV.Based on the calculated energy level of the as-prepared MoS 2 , aschematic diagram of the energy band alignment and charge-transferprocess in the designed solar cell in this work is depicted in Fig. 6. It iscon 󿬁 rmed that the energy level of the as-prepared MoS 2  is wellmatching with that of TiO 2  and P3HT and a favorable energetic ar-rangement is formed in the designed solar cell. Besides, by doping withMoS 2 , the charge separation interfaces can be de 󿬁 nitely increased andthen the charge carrier mobility of the P3HT layer should be improved[37]. Therefore, it is expected to improve the charge separation andtransportation e ffi ciency based on the MoS 2 -P3HT hybrid micro-heterostructure.To further con 󿬁 rm the role of MoS 2  in promoting charge separationand transportation, PL spectra for the electrodes of FTO/TiO 2 /P3HTand FTO/TiO 2 /MoS 2 -P3HT were conducted to further demonstratetheir charge transfer and recombination properties, as shown in Fig. 7.The PL spectra of both electrodes are derived from the same light ab-sorbing material of P3HT and reveal the same spectral characteristics. Itis generally believed that the  󿬂 uorescence emission peak in the PLspectrum indicates the occurrence of photogenerated charge re-combination, and lower  󿬂 uorescence emission peak, i.e. the  󿬂 uores-cence quenching, implies the improved charge trapping and transfer-ring. As compared to the FTO/TiO 2 /P3HT electrode, the FTO/TiO 2 / Fig. 4.  SEM images of the FTO/TiO 2  (a, b), FTO/TiO 2 /MoS 2 -P3HT (c) and FTO/TiO 2 /MoS 2 -P3HT/PEDOT:PSS/Ag (d). 300 400 500 600 700 800 9000.840.880.920.961.00      A     b    s    o    r     b    a    n    c    e      /    a .    u . Wavelength / nm 853nm (a) -3 -2 -1 0 1 201x10 9 2x10 9 3x10 9 4x10 9 (b) Potential/V       C   -      2      /      F   -      2 -0.78V Fig. 5.  Optical absorption characterization (a) and Mott-Schottky measurement (b) of the FTO/MoS 2  electrode. (The measuring conditions for Mott-Schottky test: thepotential window was − 3.0~2.0V, the step width was 0.2V and the delay time was 10 s.)  B. Sun, et al.  Solid State Sciences 94 (2019) 92–98 95
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