Influences of alcoholic solvents on spray pyrolysis deposition of TiO2 blocking layer films for solid-state dye-sensitized solar cells
Graphical abstract
Alcoholic solvents used for the TiO2 precursor play a critical role in determining the surface morphology of blocking layers and thus the photovoltaic performance of the SDSCs.
Highlights
► Solvent influences morphology of spray pyrolysis deposited TiO2 blocking layer. ► Ethanol reacts with TPA, resulting poor quality of blocking layer. ► Isopropanol is better than ethanol for obtaining smooth blocking layer. ► SDSC with blocking layer made with isopropanol showed better performance.
Introduction
Dye-sensitized solar cells (DSCs) have the potential to generate electricity from sunlight at low costs compared with silicon technology [1], [2], [3], [4], [5], [6]. DSCs hold the highest power conversion efficiencies (PCE) up to 12.3% [7] among current organic solar cell technologies, but the shortening of their lifetimes when liquid electrolytes are used is one of the major challenges for the widespread commercialization of DSCs [8]. Studies investigating the use of quasi-solids, ionic liquids and solid-state hole transporting materials to replace liquid electrolytes have been reported. Among the various types of solid-state DSCs (SDSCs), those using solid-state hole transporting materials have drawn much attention in the past few years due to (i) elimination of the use of reactive and volatile redox components such as I−/I3−, (ii) simplification of the sealing/packaging of devices and (iii) ability to fabricate SDSCs on flexible substrates and compatibility with roll-to-roll manufacturing [9], [10], [11], [12]. To date, SDSCs using organic hole transporting materials such as 2,2′, 7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spiro-bifluorene (Spiro-OMeTAD) [13], [14] and poly(3,4-ethylenedioxythiophene) (PEDOT) [15] have shown PCEs as high as 7.2%. On the other hand, a high PCE of 10.2% for a SDSC using an inorganic hole transporting material, CsSnI3, was recently reported [16]. In a typical SDSC device with an organic hole transporting material, a hole blocking layer, usually a compact TiO2 thin film, is inserted between a transparent fluorine-doped tin oxide (FTO) electrode and a mesoporous TiO2 layer to avoid direct contact of FTO with the hole transporting material [12], [13], [14], [15]. Without such a TiO2 blocking layer, a loss of current through charge recombination or even current leakage would occur due to an ohmic contact of the FTO electrode with the hole transporting material [9], [17]. The TiO2 blocking layer films can be prepared by several methods such as chemical vapor deposition [18], [19], electron beam evaporation [20], sputtering [21], [22], [23], [24], atomic layer deposition (ALD) [25], [26], [27] and spray pyrolysis deposition [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. Spray pyrolysis deposition of the TiO2 blocking layer films is simple and cost-effective. The most frequently used precursor for spray pyrolysis deposition of TiO2 films is a solution of titanium diisopropoxide bis(acetylacetonate) (TPA) dissolved in ethanol [34], [35], [36], [37], [38], [39]. In this study, we found that the use of isopropanol (IPA) instead of ethanol (EtOH) can improve the morphology of the TiO2 films and the performance of the resulting SDSCs. Our results clearly indicate that EtOH reacts with TPA to produce titanium diethoxide bis(acetylacetonate) (TEA). The resulting TEA requires a higher temperature for decomposition. At a low pyrolysis temperature in the range of 220–260 °C, the use of EtOH as a solvent yields poor TiO2 films, while the use of IPA results in more complete thermal decomposition and provides smooth TiO2 films. SDSCs with the TiO2 blocking layer films fabricated using a TPA solution in IPA show improved photovoltaic performance compared to cells with TiO2 blocking layer films formed using a TPA solution (or more precisely a TEA solution) in EtOH.
Section snippets
Experimental
Spray pyrolysis was carried out using a home-made setup comprising an airbrush with a ϕ 0.3-mm nozzle, a hotplate and a movement control system, as shown in Fig. 1. The spray pyrolysis deposition procedure was similar to the one reported previously [30]. FTO-coated glass substrates (Solaronix, sheet resistance: 15 Ω/square) were immersed sequentially in detergent, de-ionized water, acetone and IPA in an ultrasonic bath for 10 min at a time and then placed on a pre-heated hotplate at a specified
Results and discussion
A commercially available 75% (v/v) TPA solution in IPA was diluted with EtOH, IPA and EtOH-IPA mixtures to form 0.2 M precursor solutions, which were then used to fabricate TiO2 films by spray pyrolysis deposition on FTO glass substrates. Fig. 2 shows the microscopic images of the TiO2 films fabricated at 260 °C using precursor solutions made from the following solvents: IPA/EtOH with volumetric ratios of 3/7, 5/5 and 7/3, pure EtOH (10/0) and pure IPA (0/10). As shown in Fig. 2a, the TiO2 film
Conclusions
We have found that the alcoholic solvent used for the precursor TPA plays a critical role in determining the quality of the TiO2 blocking layer films fabricated by spray pyrolysis deposition, which in turn influences the photovoltaic performance of the resulting SDSC devices. When IPA was used as solvent, smoother TiO2 films with less dark spots were obtained as compared with EtOH as solvent. The difference in the surface morphologies of the TiO2 films obtained using the different solvents most
Acknowledgment
The authors thank the Institute of Materials Research and Engineering (IMRE) of Agency for Science, Technology, and Research (A*STAR), Singapore, for financial support of this work. WH and YL thank the National Sciences Engineering and Research Council of Canada (NSERC) for providing NSERC Discovery Grants.
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