Development of porous TiO2 nanofibers by solvosonication process for high performance quantum dot sensitized solar cell
Graphical abstract
Introduction
Quantum dots sensitized solar cell (QDSSC) has been identified as a promising area of research for the past one decade due to its tremendous future potential in commercialization after dye sensitized solar cell(DSSC) [1]. Quantum dots such as CdSe [2], CdS [3], PbS [4], CuInSe2 [5] etc, have been used as sensitizers in QDSSCs due to their meritorious properties which includes tunable bandgap, multi-exciton generations, high extinction coefficient (105 cm−1), large dipole moments in addition to their intrinsic photostability and low cost of preparation process involved in synthesis of quantum dots etc [6], [7]. Among various inorganic semiconductor sensitizers, CdSe QDs do satisfy the criteria of being used as effective sensitizer as it is seen to possess wide absorbance range in the visible region like organic dyes with high stability along with inherent conduction band position of ~− 3.6 eV which is responsible for higher charge separation kinetics of electrons that helps to deliver maximum photocurrent density inside the cell [8], [9]. Although, the QDSSC gradually has arrived at the verge of attaining high efficiency still there are few shortcomings that impede them to produce expected power conversion efficiency (PCE). Here, selection of photoanode material holds the major concern of the researcher as it is responsible for the electron transport to the external circuit and provides anchored sites for the QDs. Therefore, to improve the photocurrent density of the cell many studies have been directed towards the development of photoanode material that would comprise of one dimensionality and high specific surface area.
1D- photoanodes are found to be effective in producing high efficiency as electrons are seen to move faster along the length of these structures to the external circuit, which reduces back electron recombinations, while 0D photoanodes are disordered arrangement of nano particles that give enough time for these photoelectrons to back electron recombinations with the electrolyte and thereby producing low photocurrent efficiency in QDSSCs [10]. In this context, 1D-morphologies such as nanorods [11] and nanowires [12] developed by hydrothermal method, where the scope of high surface area found less in compare to 1D-nanofibers produced by electrospinning technique. The one dimensional electrospun TiO2 NFs has been proved to be an efficient photoanode due to its extraordinary properties such as high internal light scattering [13], [14], higher trapping of photons, low transmittance, more adsorption sites for QDs and easy diffusion of electrolyte, higher reproducibility of 1D-structured nanofibers with diameters < 100 nm [15], [16]. However, the efficiency of the electrospun TiO2 NFs based QDSSC was not upto the mark due to the lack of sufficient surface area of the photoanode material essential to adsorb maximum quantity of QDs. Thus, S.Chattopadhyay.et al. [17] group have employed organic polymers/co-block polymers such as PVP and P-127 into the titania precursor solution to introduce pores with high surface area in the TiO2 NFs. Another method was attempted by H.Y.Chen et al. [18] to increase the surface area of TiO2 NFs by adding paraffin oil as pore forming agent into the precursor solution to use as porous photoanode material for DSSC. These methodologies are quite complicated as more parameters are required to control such as viscosity, calcination temperature etc., which may effect the formation of TiO2 NFs.
Therefore, in the present investigation, we tried to adopt a unique solvosonication method for the preparation of porous and high surface area TiO2 NFs rather than going for complicated solution process. In this context, we have conducted a solvosonication process which seems a unique combination of electrospinning and influence of ultrasonication using glycerol as pore forming agent to obtain porous electrospun TiO2 NFs. The effect of solvosonication time duration on the formation of porosity, specific surface area and photovoltaic performances were also investigated in detail.
Section snippets
Materials
Titanium(IV) isopropoxide (TiP, 99%), polyvinyl pyrrolidone (PVP, Mw = 1300,000), cadmium chloride (CdCl2, 99.9%), selenium powder (99.9%), oleylamine (technical grade, 70%), 1-Dodecanethiol (> 98%) were procured from the Sigma-Aldrich. Methanol (> 99.5%), ethanol (> 99%), hexane (> 95%), acetic acid glacial (99–100%) and glycerol (GR 87%) were purchased from the Merck India Ltd. without further purification.
Synthesis of CdSe Quantum dots
CdSe quantum dots were prepared by one pot synthesis method as previously reported [8],
BET Surface area studies
The effect of glycerol on the specific surface area and porosity of TiO2 NFs were confirmed by BET analysis and their values are given in Table 2. Its characteristic loop of adsorption and desorption are shown in Fig. 2. It can be observed that the porosity increases with increase in solvosonication time duration from 30 to 90 min and attained the maximum specific surface area of 42.93 m2 g−1. Further, increase in the solvosonication time duration, does not influence the specific surface area
Conclusion
In summary, the porous TiO2 NFs were prepared successfully with a high surface area of 42.93 m2 g−1 with the maximum porosity of 40.42% by solvosonication process using glycerol as the pore forming medium. These porous TiO2 NFs could enhance the adsorption of CdSe QDs, provide light scattering effect and electrolyte diffusion into the porous TiO2 NFs based photoanode that improved the overall power conversion efficiency to 2.15% with Jsc of 9.21 mA/cm2 in comparison with the conventional TiO2
Acknowledgements
The authors gratefully acknowledge the CSIR, New Delhi for providing the financial support (No. 01/2810/14/EMR-II dated 24-11-2014) and the CIF Pondicherry University for providing the instrumentation facilities. One of the authors, Ms. N. Singh sincerely thanks the UGC, New Delhi for providing Research Fellowship under the RGNF scheme.
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