Influences of alcoholic solvents on spray pyrolysis deposition of TiO2 blocking layer films for solid-state dye-sensitized solar cells

doi:10.1016/j.jssc.2012.10.010

Journal of Solid State Chemistry

Volume 198, February 2013, Pages 197-202

https://doi.org/10.1016/j.jssc.2012.10.010 Get rights and content

Abstract

Influences of alcoholic solvents for titanium diisopropoxide bis(acetylacetonate) (TPA) precursor solutions on the spray pyrolysis deposited TiO₂ films and the photovoltaic performance of the solid-state dye-sensitized solar cells (SDSCs) using these TiO₂ films as the blocking layers were investigated. Smooth TiO₂ films were obtained by spray pyrolysis deposition of a TPA solution in isopropanol (IPA) at a relatively low temperature of 260 °C. On the other hand, when ethanol was used as solvent, the TiO₂ films fabricated at the same temperature showed much rougher surfaces with many pinholes. Our results showed that ethanol reacts with TPA to form titanium diethoxide bis(acetylacetonate) (TEA), which requires a higher thermal decomposition temperature than that of TPA. SDSCs with TiO₂ blocking layer films fabricated using a TPA solution in IPA showed higher power conversion efficiencies with smaller variations.

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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Influences of alcoholic solvents on spray pyrolysis deposition of TiO2 blocking layer films for solid-state dye-sensitized solar cells

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgment

J. Mol. Catal.

Thin Solid Films

Electrochim. Acta

Coord. Chem. Rev.

Thin Solid Films

Electrochim. Acta

Nature

J. Am. Chem. Soc.

J. Mater. Chem.

Appl. Phys. Lett.

Chem. Commun.

ACS Nano

Science

Acc. Chem. Res.

Nature

ChemSusChem

Adv. Mater.

Adv. Mater.

Nano Lett.

J. Am. Chem. Soc.

Adv. Mater.

Influences of alcoholic solvents on spray pyrolysis deposition of TiO₂ blocking layer films for solid-state dye-sensitized solar cells