Deposition of hole-transport materials in solid-state dye-sensitized solar cells by doctor-blading
Introduction
Dye-sensitized solar cells (DSCs) have received international research focus due to their high power conversion efficiency and low cost [1], [2], [3], [4]. However, liquid-based DSCs suffer from potential problems such as solvent evaporation and leakage [5]. Such disadvantages can be solved by replacing the liquid-electrolyte with solid-state hole-transport materials (HTMs), such as spiro-OMeTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) [6], [7], [8], [9]. Solid-state dye-sensitized solar cells (ss-DSCs) made with spiro-OMeTAD have attained efficiencies over 5% [10] and have exhibited good stability under elevated-temperature light-soaking [11]. To date, all efficient ss-DSCs have been fabricated using spin-coating techniques to infiltrate the spiro-OMeTAD. This method wastes material and cannot scale up for larger area devices. Doctor-blading, a roll-to-roll compatible, large-area coating technique, is an attractive alternative to spin coating. It has been used to deposit the active layer of several types of solar cells, such as the mesoporous TiO2 layer in dye-sensitized solar cells [12], the polymer–fullerene blend in bulk heterojunction solar cells [13], [14], and Cu(In,Ga)Se2 (CIGS) thin film solar cells [15]. In this work, doctor-blading is used to deposit spiro-OMeTAD into mesoporous TiO2 films. To the best of our knowledge, this is the first report of efficient ss-DSCs made from doctor-blading.
Section snippets
Pore filling mechanism
Pore filling of TiO2 mesoporous films with spiro-OMeTAD has been proposed as one of parameters that limits the ss-DSC power conversion efficiency [16], [17]. The mechanism of pore filling has been explained in detail in previous publications [18], [19]. Here we provide a brief description of the pore filling mechanism. Pore filling of spiro-OMeTAD in mesoporous TiO2 films is usually accomplished by depositing a concentrated solution of spiro-OMeTAD on top of the TiO2 film. Immediately after
Conclusions
We have demonstrated the use of doctor-blading to deposit spiro-OMeTAD in solid-state dye-sensitized solar cells. After optimizing the solution concentration and temperature, we achieved an equal pore filling fraction with doctor-blading and spin coating for 2.5 and 5-μm thick films. The efficiencies of devices made with these two deposition techniques are comparable, and the materials utilization of spiro-OMeTAD is significantly better for doctor-blading. This demonstrates the potential for
Sample preparation
FTO substrates (Hartford Glass TEC 15) were first patterned through etching with Zinc powder and HCl (12%). Then, the substrates were cleaned by sonication sequentially in 10% Extran solution, acetone and isopropanol [28]. Samples used for solar cell measurements were coated with a compact layer of TiO2 (50 nm) by aerosol spray pyrolysis using air as carrier gas [29]. Mesoporous TiO2 films were then deposited from commercially-available paste (Dyesol 18NR-T) using screen-printing. These sheets
Acknowledgements
This publication was partially based on work supported the Center for Advanced Molecular Photovoltaics (Award No. KUS-C1-015-21), made by King Abdullah University of Science and Technology (KAUST). It was also partially supported by the Office of Naval Research. We thank Brian E. Hardin for valuable discussions. I.-K. Ding is supported by a Chevron Stanford Graduate Fellowship.
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