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

strucutre for single and multi senstized film can be see in **Figure 29(d)**. Therefore, the charge density in TiO2 conduction band (*n*s) with multisensitized system was increased by about 124%, 562% for TTAR and YD2-oC8 single dye. The relative competition between electron–hole recombination and electron diffusion is conveniently described by the electron diffusion length, *Ln*, the relation *L*<sup>n</sup> = (*D*eff � *τ*eff)

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

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

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

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

propylphenyl)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

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,

silylpropylphenyl)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

silane chemistry based cross linkable hole transport material 4,4<sup>0</sup>

� based liquid electrolytes is mainly attributed to the

� ion absorbs a significant part of



The effective diffusion length *L*<sup>n</sup> of the conduction band electrons for single- (TTAR, YD2-oC8, YDD6) and multi-senstitized DSSC can be calculated from this equation. The *L***<sup>n</sup>** for all samples is estimated to be �24.6 μm of TTAR, 16.3 μm of YD2-oC8, 9.52 of YDD6 and � 32.1 μm of TTAR/ YD2-oC8/YDD6, respectively. This calculated data suggests that multisenstitized sensitizer leads to enhance the collection of being photogenerated electrons in comparison to those in single

sensitizer. The detailed parameters can be seen in **Table 6**.

*Solar Cells - Theory, Materials and Recent Advances*

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

unique performance of I� /I3

encapsulation of the cells challenging; (3) the I3

state electrolyte system and used by default.

in earlier section.

**230**

cross-linkable organiosiloxane cross-linkable molecule, 4,4<sup>0</sup>

1/2.

charge carriers for use its various optoelectronic devices. Both the performance and the stability of the devices improved dramatically when TPDSi2 was used the interfacial layer in OLED (organic light emitting diode) and OPV (organic photovoltaic) devices. The TPDSi2 molecule has two key components-widely used hole transport moteity triphenyldiamine (TPD) and a trichlorosilyl (SiCl3) pendant. The SiCl3 subgroups can be easily introduced into a charge conducting moiety like TPD via the highly efficient catalytic hydrosilylation reaction. The Si-Cl bonds are very stable during storage under inert atmosphere, but they are readily hydrolyzed by hydroxyl (OH-) groups. Moisture content of ambient atmosphere and surface OH groups that are present on oxides surfaces of ITO (indium tin oxide), SiO2 (silicon dioxide) and TiO2 [160, 161] substrates, along with water molecules that are physiadsorped on the surface, are the relevant sources of such OH groups. Once the SiCl3 groups come in contact with the OH groups, they get hydrozyzed at a rapid pace and inadvertently, Si-Cl groups from two different molecules get hydrozyzed by the two OH groups of the same water molecule. The very strong Si-O-Si bond hence formed, covalently linking two TPDSi2 molecules. Alternatively, Si-O-Si bonds are formed when two neighboring silanol groups undergo condensation reaction. The Si-O-Si bonds will extend over the length and breadth of the film in all directions and hence form a tightly held network of TPD units. While TPDSi2 is a very soluble organic solid, when it is cast onto a film allowed to hydrolyze, it will form a heavily cross-linked network that is rigid, insoluble and rugged. Our strategy in this study was to fill up the TiO2 pores with the soluble small molecule TPDSi2 and then to allow the molecules to cross link and form an extensive network that is in contact with both the deep lying dye molecules and the back electrode and hence shuttle holes from the former to the latter with great efficiency. **Figure 30(a)** shows the schematic image and the chemical structure of DSSCs utilizing P3HT and TPDSi2 organic seminconductor as HTM. A mesoporous TiO2 nanocrystlline film is

#### **Figure 30.**

*(a) Device architecture (top)of ss-DSSC and chemical structures with transmission electron microscope image (bottom) of P3HT and TPDSi2 (b) illustration of p-doping effect on TPDSi2 by additive treatment (top) and complete energy diagram of FTO/TiO2/Z907/HTM/Au ssDSSC (bottom) (c) photovoltaic performance of Z907 dye based ssDSSCs employing P3HT and TPDSi2 as HTMs with and without additive treatment.*

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 large-scale processability of TPDSi2 as an efficient HTM.

**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 HOMO energies.

The overall energy alignment including all DSSC device components, specifically, FTO/TiO2/Z907/HTMs (P3HT + additives or TPDSi2 + additives)/Au is presented as **Figure 30(b)**.

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

**Sample**

**233**

P3HT P3HT + additives

TPDSi2 TPDSi2 + additives

*aDetermined*

*bDetermined*

*cDetermined*

*dELUMO = IP + Eopt,gap.*

**Table 7.** *Summary of* 

*electrochemical,*

 *optical, charge transport and photovoltaic*

 *data for the HTMs used in this study.*

 *at the UV absorption onset.*

 *by UPS.*

 *by CV.*

**EHOMOa (eV)**

? ? 5.31

5.09

5.11

2.90

2.22

NA

2.8 103

0.683

 4.97

 60.3

 2.05

5.37

2.90

2.47

NA

4.6 104

0.485

 2.81

 46.7

 0.64

4.98

1.90

3.08

 1.1 102

4.94

1.91

3.03

 4.3 103

 **IPb (eV)**

 **Eopt,gapc (eV)**

 **ELUMOd (eV)**

**μTFT**

**(cm2/Vs)**

**μSCLC**

1.0 103

2.3 103

0.497

 1.69

 39.4

 0.33

0.166

 0.38

 32.1

 0.02

**(cm2/Vs)**

 **Voc (V)**

 **Jsc**

**(mA/cm2)**

 **FF (%)**

 **PCE (%)**

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


#### **Table 7.**

*Summary of electrochemical, optical, charge transport and photovoltaic data for the HTMs used in this study.*
