Solid-state DSSCs

In principle, interfacial recombination processes can be inhibited by modifying the interface. The use of t-butylpyridine in the DSSC electrolyte solution to increase its photovoltage is one example [2,97]. We wished to explore general methods for passivating interfacial recombination sites in DSSCs that might allow the use of a variety of redox couples and therefore facilitate making a viable solid-state DSSC. 

Several groups [6-8,82] have discussed the reasons for the very slow rates of reactions (4) and (5) when R/R+ = I 2. This couple may be kinetically ideal for current dye cells, but it is far from ideal in many respects [49] and there are ongoing efforts to find a replacement [99,100], especially for solid-state versions of DSSCs. The major effort directed toward solid-state DSSCs [9,13,53,54] is driven primarily by the difficulties associated with the long-term hermetic sealing of a solar cell containing volatile components such as solvent and I2. 

D. Solid-State and Quasi-Solid-State DSSC... 

On the other hand, Tennakone and co-workers utilized a p-type semiconductor material, such as Cul (band gap,-3.1 eV), as a hole conductor and produced a solid-state DSSC [141,145,146]. Acetonitrile solution of Cul was dropped onto the surface of a dye-coated Ti02 film, which was heated up to approximately 60°C and then the solution penetrated into the film. After evaporation of the acetonitrile, Cul was deposited into a nanoporous Ti02 film. The Au-coated TCO substrate as the counterelectrode was pressed onto the surface of the Ti02/dye/ Cul film. In the system using the santalin dye photosensitizer, an efficiency of 1.8% was obtained under irradiation of 80 mW/cm2 [141] and the efficiency reached 4.5% for the Ti02/N3 dye/CuI/Au system. These results suggested that a highly efficient solid-state DSSC could be produced [145]. In these systems,... 

Cul could be partly in contact with Ti02 directly therefore, the efficiency decreased by the recombination of injected electrons with Cul. In order to increase cell performance, direct contact between the Ti02 film and Cul must be minimized. Solid-state DSSCs have been studied using other organic and inorganic hole conductor materials, such as p-type CuSCN [147,148], polypyrrole [149], and polyacrylonitrile [95]. 

However, the presence of liquid electrolyte has the problems of leakage, robust sealing, and device stability, thus results in limited commercialization. Quasi-solid-state and solid-state DSSCs based on nonvolatile ionic liquid or organic hole-conducting material/polymer as the electrolyte are, therefore, developed to circumvent the sealing problem. 

The same approach was adapted in a later study. An efficient solid-state DSSC was fabricated using hybridized ruthenium dye 8. The hole conducting PEDOT was formed in situ via PEP. The thickness of the mesoporous Ti02 layer of the solar cell was varied. The highest efficiency (2.6% under 100 mW/ cm2 illumination) was achieved by using a 5.8- pm-thick Ti02 layer.52... 

QUASI-SOLID-STATE DSSC SYSTEM USING RTILs BASED ON IODIDE... 

Chapter 7. Devices efficiencies for such solid-state DSSCs are as yet limited to 4% (Snaith et al, 2005), in contrast to efficiencies of over 11% achieved for the more widely studied redox electrolyte-based DSSCs (Chiba et ah, 2006). 

Figure 8.11 Cross-sectional view of a solid-state DSSC containing the hole conductor vpiro-OMeTAD, the structure of which is indicated on the right (left drawing courtesy B. O Regan).

Closely related to liquid electrolyte dye-sensitized solar cells (DSSCs, also known as Gratzel cells ) [283,284], the class of soHd-state DSSCs has been developed to improve device stability and reduce complications in the production process [285-288]. Thus, although polymers can be utilized as replacements for sensitizing dyes (as in liquid electrolyte DSSCs) [289-291], the main effort in applying conjugated polymers focuses on soHd-state DSSCs [45,292-298]. With environmentally friendly production of this polymer based solid-state DSSC in mind, a device based on water-soluble polythiophene derivative has been presented as well [299]. 

Kang et al. also prepared poly-31-33 [79]. The heterocycle units of poly-31 can form doubly hydrogen bonded homodimer, while the heterocycle units of poly-32 and poly-33 form triply hydrogen bonded heterodimer. In the solid state, these hydrogen bonding motifs would drive the ditopic molecules to form polymeric strucmres, both of which were utilized to increase the energy conversion efficiency of solid state DSSC up to 4.6 and 4.5 %, respectively, at 1 sun condition. Their better performance than the poly-30 electrolyte was attributed to the slower electron recombination rates and the faster ionic conductivity of the electrolytes formed from them. 

There are two kinds of polymer material that used in quasi-solid/solid state DSSCs. For quasi-solid electrolytes, polyionic liquids have been proposed as solvent and redox couple as solute. They appear in molten salts and present many promising properties, such as, high chemical and thermal stability and high ionic conductivity Their main drawback is related to its high viscosity, which makes the ions diffusion rather slow. As the transport of ions to the counter electrode in an ionic liquid matrix represents a rate-limiting step in DSSC (Bella, 2015), the performance of quasi-solid electrolytes based solar cell is imsatisfled. 

FIGURE 9.1 Flexible solar cells based on different kinds of polymer substrates. (A) Photograph of a flexible DSSC based on ITO-coated PET substrate wrapped on a pen. (B) Schematic illustration of the layer structure of a solid-state DSSC based on PEDOT on Goretex film as a counter electrode. (C) Schematic illustration (left) and photograph (right) of the PSC based on ITO-coated PET substrate. (D) Comparison of PSCs fabricated on conventional FTO/glass and flexible PEDOT PSS/PET substrate. (E) Schematic illustration of the flexible solar cell based on Ag-grid/PET substrate. 

Figure 4.13 Structure of (a) PVA-g-VIC4Br and (b) solid-state DSSC. Reproduced with permission from Ref. [55].

Charge transport in solid-state DSSCs based on nanocrystalline TiOa appears to be electron-limited at short-circuit, meaning that the transport of electrons through the mesoporous Ti02 is slower than the transport of holes through the... 

While maximizing the charge diffusion length should enable much thicker devices to be fabricated, which still exhibit extremely efficient charge collection efficiency, there is another basic issue with the solid-state DSSC which may well prove to impose the most significant... 

Solid-state redox mediators or hole conductor materials would make it possible to construct completely solid-state DSSCs that will probably have considerable added commercial value. One of the main difficulties in substituting liquid electrolytes is the need for an interpenetration of the sensitised metal oxide by the electrolyte, in order to have efficient contact between the sensitiser cation (the hole) and the mediator. Additionally, prospective solid hole collectors should have the following properties the valence band of the hole collector material must be located above the bottom of the sensitiser dye ground state it must be transparent throughout the visible spectrum, where the dye absorbs fight and the deposition of the solid material should be done without degrading the monolayer of sensitiser dye adsorbed on Ti02. 

Up to now, many types of polymer gel electrolytes have been used in quasi-solid-state DSSCs, including polyacrylonitrile (PAN), poly(ethylene oxide) (PEO), polymethylmethacrylate (PMMA), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). PVDF-HFP exhibits high ionic conductivity and stability at room temperature however, the complex preparation technology and the poor mechanical strength of these gel polymer-based DSSCs represent a bottleneck to their introduction to the market. To overcome this problem, the electrospinning of such polymers has been performed with the aim of integrating the resulting fibrous, easy-to-obtain, and low-cost materials as electrolytes [23]. 

Jin et al. [94] embedded the liquid crystal (E7) into the electrospun PVDF-HFP nanoflber. Owing to the high ionic conductivity (2.9 x 10 ) of the E7 embedded on PVDF-HFP polymer gel electrolyte, higher values of current density (14.62 mAcm ) and efficiency (6.82 %) of the quasi-solid-state DSSCs were achieved. The quasi-solid-state DSSC of PVDF-HFP polymer gel electrolyte was found to have a 6.35 % PCE. In addition to that, overall efficiency of the newly prepared DSSCs was 6.82 %, which was nearly equivalent to that of a DSSC made using liquid electrolyte. They believed that DSSCs containing E7 embedded on PVDF-HFP polymer gel electrolyte will find commercial utility. 

Fig. 5.13 (a) Schematic of quasi-solid-state DSSCs in which electrospun nanofiber mats are employed. SEM images of electrospun (b) BPPO and (c) PVDF nanofiber mats (Reprinted from Seo et al. [95]. Copyright 2011, with permission from Elsevier)... 

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