J Solid State Electrochem DOI 10.1007/s10008-016-3473-3
ORIGINAL PAPER
Synthesis, characterization, and photovoltaic properties of TiO2/CdTe core-shell heterostructure for semiconductor-sensitized solar cells (SSSCs) Azam Mayabadi 1,2 & Amit Pawbake 1 & Sachin Rondiya 1 & Avinash Rokade 1 & Ravindra Waykar 1 & Ashok Jadhavar 1 & Abhijit Date 3 & Vidhika Sharma 4 & Mohit Prasad 4 & Habib Pathan 4 & Sandesh Jadkar 4
Received: 19 October 2016 / Revised: 7 November 2016 / Accepted: 10 November 2016 # Springer-Verlag Berlin Heidelberg 2016
Abstract The present work represents successful synthesis of CdTe-sensitized TiO2 nanoarray thin films by two simple chemical routes. Here, we report the sensitization of TiO2 nanoarrays with CdTe nanoparticles through the electrodeposition technique at ambient condition. The electrodeposition of 30 min leads to the formation of shell of CdTe nanoparticles over TiO2 nanoarrays. This core-shell formation structure was further confirmed by TEM and HR-TEM analysis. Optical absorption and photoelectrochemical study revealed that the TiO2/CdTe core-shell heterostructure extend the absorption edge of titania in the visible region of solar spectrum; it also facilitates the efficient charge transport and reduces the recombination losses. The maximum photoconversion efficiency of 0.32% is obtained for TiO2/CdTe core-shell semiconductorsensitized solar cells (SSSCs).
Keywords Core-shell nanostructure . Semiconductor sensitized solar cell . Chemical synthesis . Grazing incidence X-ray diffraction . Raman spectroscopy and scattering
* Sandesh Jadkar
[email protected] 1
School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India
2
Present address: Department of Energy Engineering and Physics, Amirkabir University of Technology, Tehran, Iran
3
School of Aerospace Mechanical and Manufacturing Engineering, RMIT University, Plenty Road, Bundoora, Melbourne, VIC 3083, Australia
4
Department of Physics, Savitribai Phule Pune University, Pune 411 007, India
Introduction In recent years, there have been growing interest in the application of semiconductor sensitizer nanoparticles (SNPs) such as PbS, CdS [1, 2], CdTe [3], and CdSe [4, 5] due to their diverse applications in various fields, such as electroluminescent devices [6], photovoltaic [7], bio-labeling [8, 9], and antibiofilm agent [10]. These SNPs possess several attractive characteristics like high absorption coefficient, tunable band gap, excellent photostability, and multiple electron generation possibilities, which make them potential absorbers in solar cell devices [11–13]. The SNPs have been used to sensitize wide band gap semiconductors like TiO2, ZnO, and SnO2 in semiconductor-sensitized solar cells (SSSCs) for harvesting the visible and infrared regions of solar spectra. Conversion efficiencies up to 5% have been reported for SSSCs [5, 14, 15]. Depending on the band gap and the electronic affinity of the SNPs, semiconductor heterostructures can be divided into three different cases: type-I, type-II, and type-III band alignment. In a type-II band alignment which is known as coreshell, the position of valance and conduction bands of semiconductor 2 is higher than that of semiconductor 1 and the steps in the conduction and valance bands go in the same direction. Importantly, the difference of chemical potential between semiconductors 1 and 2 causes band bending at the interface of junction. The band bending induces a built-in field, which drives the photogenerated electrons and holes to move in opposite directions, leading to a spatial separation of the electrons and holes on different sides of heterojunction [16]. Thus, the formation of type-II heterostructures is an effective approach to enhance charge separation efficiency for improved photovoltaic activity. Moreover, an ultraviolet excited semiconductor such as TiO2, ZnO, or ZnWO4 coupled with a visible light-excited semiconductor such as CdS, CdSe,
J Solid State Electrochem
CdSSe, C3N4, or CdTe can effectively improve its solar energy utilization efficiency because the synergic absorption of two semiconductors with different band gaps extends the light response range to the whole solar spectrum. Meanwhile, typeII core-shell structure has both valence and conduction bands in the core lower (or higher) than in the shell. As a result, one carrier is mostly confined to the core while the other is mostly confined to the shell. Type-II structures can allow access to wavelengths that would otherwise not be available with a single material. In addition, the separation of charges in the lowest excited states of type-II nanocrystals should make these materials more suitable in photovoltaic or photoconduction applications [17, 18]. Type-II core-shell structures with different band alignments, such as TiO2/ZnO, ZnO/Er2O3, ZnO/ZnS, V2O5/ZnO, ZnSe/CdS, ZnO/CdTe, etc. have been reported [19–24]. These core-shell structures show enhanced light emission or enhanced efficiency of SSSC devices. Among wide band gap semiconductors, TiO2 is a typical transition metal oxide with high chemical stability, strong photocatalytic activity, and high photoelectric performance, which render it an excellent material for solar energy conversion in photocatalysis [25] and solar cells [26]. Specifically, one-dimensional (1D) nanostructured TiO2 have been widely investigated for their growth, properties, functionalization, and applications [27, 28], benefiting from their strong light scattering effects and improved charge collection [29]. Among third-generation solar cell devices, quantum dot solar cells (QDSCs) have exhibited great promise in developing clean and sustainable energy [30]. As the light harvester of these devices, semiconductor QDs display a characteristic tunable band gap, high extinction coefficient, large intrinsic dipole moments, possible multiple exciton generation (MEG) effect, hot electron effect, etc. These virtues make QDSCs appear very attractive in the next generation of solar cells [31]. Despite all these advantages, the performance of QDSCs lags significantly behind their dye-sensitized solar cell (DSSC) counterparts. The performance limitation of QDSCs may be attributed to a number of factors, such as a high charge recombination with a redox couple at the electrode/electrolyte interface, slow hole transfer kinetics, and low catalytic activity of the counter electrode [32]. Moreover, the partial coverage of QDs on the surface of TiO2 means a small junction area, leading to a low current density and high charge recombination at the naked interface of the electrode and the electrolyte which is an important energy loss pathway that deteriorates the performance of the resultant photovoltaic devices [33]. These shortcomings could be overcome by using thin film of semiconductor nanoparticles in lieu of QDs to cover the entire TiO2 surface. In such cases, the junction area between the semiconductor sensitizer and the TiO2 electrode could be enlarged significantly, and the direct contact between the TiO2 and the electrolyte could also be avoided. Consequently, the charge recombination between the TiO2 electrode and hole species in the
electrolyte could be blocked more efficiently by SNPs covering the surface of wide band gap semiconductor rather than QDs. Solar cells with a superior performance could thus be expected by using semiconductor nanoparticles as the light harvester [34]. As reported in the literature, CdTe is a widely used semiconductor in thin film solar cells capable of a significant light to electricity conversion efficiency owing to its nearly ideal band gap (1.5 eV), high absorption coefficient (~105 cm−1) [35], and excellent photostability. These characteristics make CdTe SNPs an ideal material for SSSCs either as sole absorbing material or in combination with dyes, QDs, or SNPs in cosensitized solar cells [5, 36–38]. Among various researches which have been done on SSSCs, there are only few reports on TiO2/CdTe SSSC devices. Most of the research group has been focused on CdS [39, 40], PbS [41], and CdSe semiconductor nanoparticles. In comparison to PbS and CdSe, CdTe nanoparticles have a higher absorption coefficient and a band gap value favorably placed in the visible solar spectrum. It is also demonstrated to possess more energetically favorable interaction with TiO2 in comparison to CdSe nanoparticles [42]. Moreover, the CdTe SSSCs show low photoconversion efficiency in comparison to CdSe SSSCs. Thus, a better understanding of charge transfer at the CdTe/TiO2 interface is important to further improve the performance of CdTe SSSCs. Furthermore, despite the successful employment of CdTe nanocrystals on CdTe/CdSe heterojunction solar cell [43], the utilization of CdTe nanoparticle for solar energy conversion has been limited especially in photoelectrochemical cells [44, 45]. To exploit CdTe nanoparticles in the SSSCs, we have undertaken a systematic study to probe their photophysical and photoelectrochemical behavior by linking them to TiO2 nanoarrays for the first time. In present paper, core-shell heterogeneous CdTe/TiO2 nanorod arrays have been prepared by a two-step chemical synthesis route through combined chemical bath deposition (CBD) and three-electrode electrodeposition technique. Randomly oriented TiO2 nanoarrays, fabricated by CBD, were employed as model substrates to be sensitized by CdTe semiconductor thin films to construct type-II core-shell TiO 2 /CdTe heterostructure. Moreover, the band gaps of the CdTe were systematically tuned by varying electrodeposition time. The SSSC devices were finally fabricated in a liquid-electrolyte sandwiched configuration.
Experimental Film preparation Chemicals and materials Unless otherwise specified, all reagents were used as received without further purification. For the synthesis of TiO2 thin
J Solid State Electrochem
films, titanium (III) chloride (TiCl3) and ammonium hydroxide (NH4OH) were used and purchased from Alfa Aeser (Great Britain). Fluorine-doped SnO2-coated (FTO) glass substrates were purchased from Vin Karola Instruments, USA. For the synthesis of CdTe thin films, cadmium sulfate (CdSO 4), sodium tellurite (Na 2TeO 3), and tartaric acid (C4H6O6) were used and purchased from Thomas Baker (India) and Alfa Aeser (Great Britain), respectively. Doubledistilled water was used for all experiments and obtained using double-distilled water plant procured from Borosil Glass Pvt. Ltd. (India).
counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. The prepared electrochemical bath contains 0.7 M tartaric acid, 10 mM CdSO4, and 6 mM Na2TeO3 with the solution pH of 2. The electrochemical synthesis was carried out at ambient condition for the applied potential of −0.45 V and deposition time of 10, 20, 30, and 40 min. After synthesis, the samples were rinsed extensively with doubledistilled water in order to remove the excess reactants from the substrate. Therefore, four photoelectrodes have been constructed which are denoted as FTC(10), FTC(20), FTC(30), and FTC(40). The number in bracket indicates the time interval for electrodeposition of CdTe nanoparticle thin films.
Construction of TiO2 compact layer Film characterization Firstly, compact or blocking layer of titania (TiO2) was deposited on a transparent conductive fluorine-doped SnO2-coated (FTO) glass substrate using the CBD method. The detailed preparation with optimized parameters has been elaborated in our previous work [46]. Briefly, 5 ml of TiCl3 (aqueous, 15 %) was added to 20 ml of double-distilled water. The pH of the solution was increased from 0.5 to 1.5 ± 0.1 by drop-wise addition of diluted NH4OH with constant stirring. A precipitate rapidly formed without film formation at pH >2.5. By contrast, at pH
