*A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

deposited on top of FTO-coated glass substrate. Z907 dye molecules are attached to the surface of TiO2 to form a bicontinuous nanocomposite active layer, which is then subsequently infiltrated by the organic HTM. Finally, a Au counter electrode is deposited to form contact with HTM and encapsulates the device. In this system, the HTM extracts hole from photo-oxidized sensitizer molecules and transfers the holes to the Au counter-electrode, allowing complete dye regeneration. TEM specimens are prepared by transferring spin-coated thin films from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-coated Si substrates onto lacey carbon TEM grids. P3HT exhibits typical semicrystalline polymer features. Such relatively large, continuous domains as a result of polymer aggregation are indicative of poor HTM infiltration into pores of TiO2/sensitizer. On the contrary, TPDSi2 molecular HTM shows significantly smaller features, while forming homogenous and organized networks. Such features are could be advantageous for

**Figure 30(b)** shows a scheme on the possible change of electronic structure by additive treatment by the UPS spectra of P3HT before and after additive treatment. The detailed information can be seen in **Table 7**. In HOMO emission regions, the HOMO band onsets also show no significant difference, corresponding to IPs of 4.94 and 4.98 eV for P3HT before and after additive treatment, respectively. This result is in perfect agreement with CV data (**Table 7**). However, for TPDSi2 films, the additive treatment result in a clear shift both in high binding energy cutoff (HBEC) and HOMO emission regions. In particular, the HBEC position shifts to lower binding energy after additive treatment, whereas the HOMO cutoff shifts close to 0 eV. This effect could indicate a strong p-doping effect of additives on TPDSi2. Scholin et al. [162] observed a similar effect of Fermi level shift to move closer to the HOMO level on Spiro-OMeTAD by Li-TFSI doping. Here, by using the same substrates and a common vacuum level, the shift of Fermi energy to the move closer to the HOMO position is confirmed. In addition, IPs also shift from 5.36 to 5.12 eV after additive treatment, which is in excellent agreement with CV-derived

The overall energy alignment including all DSSC device components, specifically, FTO/TiO2/Z907/HTMs (P3HT + additives or TPDSi2 + additives)/Au is

The exact energetic alignment of HTMs are carefully derived from optical absorption, cyclic voltammetry and ultraviolet photoemission spectroscopy and is discussed below. To carefully investigate the charge transport properties of HTMs, both TFT and space charge limited current (SCLC) measurements are performed. For TFT mobility measurements, bottom-gate/top-contact configurations were employed for all devices fabricated by spin-coating of a 5 mg/mL chlorobenzene solution onto Si/SiO2 substrates using Au source and drain contacts. All TFT and SCLC mobilies are summarized in **Table 7**. For P3HT, typical p-type TFT behavior was observed both in pristine films and films treated with additives. A noticeable increase in p-type mobility from 4.3 <sup>10</sup><sup>3</sup> to 1.1 <sup>10</sup><sup>2</sup> is observed. However, TPDSi2 exhibit relatively low TFT performance, as a result of disconnected domains in the lateral direction. In real DSSC condition, hole transport through the HTMs to the counter electrode can significantly influence charge recombination and device performance. Therefore, SCLC method which directly measures the charge transport in the direction perpendicular to substrate can be employed as a good indicator mimicking real device conditions [163]. For the cross-linked TPDSi2 HTM, it show about 2.8 time higher p-type hole mobility than that of pristine, which is indicative of favorable charge transport properties in DSSCs. Therefore, power conversion efficiency (PCE) of DSSCs of >2% is achieved when using standard amphiphilic dye

large-scale processability of TPDSi2 as an efficient HTM.

*Solar Cells - Theory, Materials and Recent Advances*

HOMO energies.

presented as **Figure 30(b)**.

and TPDSi2 as HTM (see **Figure 30(c)**).

**232**
