**7. Electrolyte and solid state hole transport material**

The liquid electrolytes possess some important features such as easy preparation, high conductivity, low viscosity, and good interfacial wetting between electrolytes and electrodes and thus high conversion efficiency for the DSSCs [153, 154]. Today, the best working redox-couple known so far is the iodide/triiodide system. The unique performance of I� /I3 � based liquid electrolytes is mainly attributed to the favorable penetration into the nanoporous semiconductor film, very fast dye regeneration, and relatively slow recombination losses through reaction with injected photoelectrons. Cobalt based redox mediators bring the concomitant improvements in the *V*oc, which produced the highest efficiency of 13% for traditional DSSCs [127]. However, there are several negative features limiting industrial application following reason: (1) iodine is extremely corrosive toward metals such as copper or silver, which are used as current collectors in some DSSCs; (2) acetonitrile as a main solvent has a relatively high vapor pressure, which makes proper encapsulation of the cells challenging; (3) the I3 � ion absorbs a significant part of visible light, stealing photons from the sensitizing dye. These drawbacks can be potentially remedied via the use of solid-state hole transport materials (HTM). The premise of the effective solid state DSSC is that viscous HTM materials would penetrate into the all the deep lying empty spaces in the porous TiO2 network, and form a continuous film that connects these filled pores all the way up to the back electrode. Hence, our sphere typed TiO2 electrode give many advantages for solid state electrolyte system and used by default.

In our research, three different classes of solid state electrolyte are investigated (i) iodide based plastic crystal electrolyte prepared by mixing synthesized Nmethyl-N-butylpyrrolidinium iodide (P1,4I), I2, and succinonitrile [51]; (ii) a novel cross-linkable organiosiloxane cross-linkable molecule, 4,4<sup>0</sup> -bis((*p*-trichlorosilylpropylphenyl)phenylamino) biphenyl (TPDSi2) [155]; (iii) perovskite typed inorganic materials, CsSnI3 and Cs2SnI6. (this part did not deal with this book) [104, 156]. P1,4I based electrolyte exhibit the highest value, reaching about 9% using a masked frame measurement technique [51]. The detailed information is explained in earlier section.

As an organic molecule system, several different systems have been proposed including melt processing, [157] in-situ polymerizastion, [158] use of low *Tg* materials, and even use of small molecule/polymeric blends [159]. Among them, silane chemistry based cross linkable hole transport material 4,4<sup>0</sup> -bis((*p*-trichlorosilylpropylphenyl)phenylamino biphenyl) (TPDSi2) is applied for organic HTM. Silane chemistry has not been previously employed in DSSC, but our group and others have exploited their property of forming a robust cross-linked network of
