**4. Hot electron injection from plasmonic metal to TiO<sup>2</sup>**

Schottky barrier is an energy barrier for electrons formed at the junction of metal and semiconductor where their fermi levels merge together to achieve thermal equilibrium, leading to the band bending and blocking the electron flow across the junction. The energy band diagram of Schottky contact is illustrated in **Figure 13**. The height of Schottky barrier equals to the subtraction of work function of metal with the bandgap of semiconductor, and it is usually much smaller in value (e.g., ~0.9 eV for Au/TiO<sup>2</sup> [66] and ~0.2 eV for Ag/TiO2 [67]) than the semiconductor bandgap (i.e., 3.2 eV for anatase TiO2 ). Recent studies proposed hot electrons from plasmonic-excited metal cores could easily overcome this energy barrier and be injected into the TiO2 conduction band, resulting in a new mechanism for plasmon enhancement to DSSCs [68]. In general, the injected hot electrons are considered to be either being converted into photocurrent or functioning as charge carriers in the semiconductor matrix [69]. The photocurrent generated by direct hot electron transfer across the Schottky barrier has been

**Figure 13.** Energy band diagram of Schottky barrier formed at metal/n-type semiconductor interface.

and incident photon-to-current efficiency (IPCE) of the plasmonic biophotovoltaic cell were achieved, while the fluorescence emission of excited LHCII was quenched along with shortened lifetime. Obviously, the electrons in the excited LHCII state flow efficiently through the

Cushing et al. [63, 64] elaborated that three mechanisms are involved charge generation in a semiconductor incorporated with plasmonic metal NPs, including light trapping based on scattering, hot electron/hole transfer, and plasmon-induced resonance energy transfer (PIRET) based on near-field. These are also applicable to the LHCII-PNP hybrids, with the possible mechanisms illustrated in **Figure 12**. During the excitation process, electrons are pumped from the ground state to the excited states of Soret band or Q band of LHCII. However, the excited Soret band quickly goes through an ultrafast excitation energy transfer (EET) to the Q band, as verified by a theoretical modeling [65]. Thus, all fluorescence emission from LHCIIs is at 683 nm, corresponding to a radiative relaxation for the excited electrons to return to

charge transfer process occurs, that is, the excited electrons in Q band are injected to the con-

the metallic core, the incident photons by plasmonic absorption generate a strong near-field oscillation with ~10 nm decay length, which can affect all LHCII adsorbed on the surface of

pling between the plasmon and LHCII, leading to enhanced LHCII excitations at Soret and Q

and thus quenches the fluorescence emission though PIRET induces higher LHCII excitation. These effects collectively enhance the photocurrent of the corresponding plasmonic biophotovoltaic cells. In addition, the plasmonic hot electrons excited at the metal core of PNPs may

shell, and therefore the charge collection efficiency. Details about such interfacial activity

Schottky barrier is an energy barrier for electrons formed at the junction of metal and semiconductor where their fermi levels merge together to achieve thermal equilibrium, leading to the band bending and blocking the electron flow across the junction. The energy band diagram of Schottky contact is illustrated in **Figure 13**. The height of Schottky barrier equals to the subtraction of work function of metal with the bandgap of semiconductor, and it is usu-

from plasmonic-excited metal cores could easily overcome this energy barrier and be injected

DSSCs [68]. In general, the injected hot electrons are considered to be either being converted into photocurrent or functioning as charge carriers in the semiconductor matrix [69]. The photocurrent generated by direct hot electron transfer across the Schottky barrier has been

conduction band, resulting in a new mechanism for plasmon enhancement to

, resulting in the reduced fluorescence intensity. In the presence of

shell. A strong PIRET is enabled by the strong dipole-dipole cou-

surface, upon excitation a

is also facilitated by the near-field

[67]) than the

interface, raise the charge carrier density in

[66] and ~0.2 eV for Ag/TiO2

). Recent studies proposed hot electrons

the ground state of Q band. When LHCII is adsorbed on the TiO<sup>2</sup>

bands. More efficient electron injection from LHCII to TiO<sup>2</sup>

**4. Hot electron injection from plasmonic metal to TiO<sup>2</sup>**

overcome the Schottky barrier at the metal-TiO<sup>2</sup>

ally much smaller in value (e.g., ~0.9 eV for Au/TiO<sup>2</sup>

semiconductor bandgap (i.e., 3.2 eV for anatase TiO2

are unravelled in the next section.

TiO2

TiO2

into the TiO2

network.

220 Application of Titanium Dioxide

duction band (CB) of TiO<sup>2</sup>

the ~2–3-nm-thick TiO2

collected and utilized for photodetection and photovoltaics based on well-designed devices with a complete circuit allowing refilling electrons back to the metal [70–73]. However, for the metal@TiO2 NPs embedded in the mesoporous TiO2 film in DSSCs, the sustainability of the photocurrent generation from hot electron injection is under debate considering that the metal core is unaccessible to the electron donors or the external circuit, which is needed for charge regeneration. On the other hand, the initially injected hot electrons may be converted into steady-state charge carriers and sufficiently raise the conductivity of the mesoporous TiO2 frame, as has been indirectly demonstrated by enhanced photoconductivity in metal coupled semiconductors [74, 75]. In addition, a recent study by Cushing et al. reported that metal@TiO2 and metal@SiO<sup>2</sup> @TiO2 NPs can also enhance DSSCs by exciting surrounding TiO<sup>2</sup> matrix and dye molecules with near-field-based plasmon-induced resonance energy transfer (PIRET) beside hot electron injection [63]. In the previous section, we have also discussed such effects on the plasmonic biophotovoltaic cells based on hybrids of natural LHCII and PNP [25]. Actually, these three effects are mixed in most plasmonic photovoltaic cells.

In order to sort out the contributions of hot electron injection, we propose a strategy by comparing the photoconductivity and the photovoltaic properties of the same material, that is, Au@ TiO2 network in two model devices, that is, a micro-gap electrode and a DSSC [76]. The coreshell structure consisting of isolated Au NPs embedded at the nodes of a nanostructured TiO2 network was used as the bridging material in the micro-gap between two Au electrodes and as the mesoporous film on a DSSC anode to measure photoconductance and photocurrent, respectively. Enhancements on the photoconductance and the photocurrent were observed on both devices, with distinct dependence on the illumination wavelength (**Figure 14A** and **B**). This difference was explained with the scheme drawn in **Figure 14C** and **D**. The enhanced photoconductance is ascribed to the hot electron injection from Au NPs to TiO<sup>2</sup> that increase the charge carrier density of the TiO2 network. This interfacial electron injection across the Au/TiO<sup>2</sup>

**Figure 14.** (A and B) Wavelength dependence of photoconductance and incident photon-to-current efficiency (IPCE) studies and (C and D) the schematics of the possible enhancement mechanisms for the Au@TiO<sup>2</sup> network on the microgap electrode and in the DSSC, respectively. Reprinted with permission from Ref. [76].

Schottky barrier (~0.9 eV) can be easily realized under illumination over the whole visible range, allowing extending the enhancement effect to the light in the near-infrared region. The photon energies in wavelength larger than 700 nm are smaller than the energy of semiconductor band gap, Au plasmonic band, and dye absorption band. In the DSSC, the plasmonic generated hot electrons cannot be the source of continuous steady-state photocurrent, since the Au NPs embedded within the TiO2 shell are not accessible by the regenerating agents. The major contribution for the photocurrent enhancement must be the surface plasmonic resonance effect that can only be induced by the illumination in the range where the plasmonic band of Au NPs is resonant with dye absorption band (i.e., band overlap in absorption spectra). However, the injected hot electrons are sufficient to raise the charge carrier density in TiO<sup>2</sup> and reduce the series resistance and charge transfer resistance in the corresponding DSSCs. This facilitates the transport of the photo-induced electrons through the TiO2 network.

## **5. Conclusions**

This chapter reviews the synergistic interplay among TiO2 photoanode, biophotosensitizers (e.g., LHCII) and plasmonic nanoparticles in the photovoltaic devices. The effectiveness of TiO2 as an interfacial photoanode material compatible with photosynthetic proteins and plasmonic nanoparticles was demonstrated. The electron injection from excited LHCII to TiO<sup>2</sup> conduction band was realized due to the perfect match of energy bands, resulting in the photocurrent generation in the LHCII sensitized TiO<sup>2</sup> solar cells. The charge separation at LHCII/ TiO2 interface can be facilitated by incorporation of PNPs. This effect can be ascribed to the near-field-assisted PIRET from PNPs to LHCII across the TiO<sup>2</sup> interfacial layer. The hot electron injection across Schottky barrier from plasmonic core into TiO<sup>2</sup> network can increase the charge carrier density in TiO2 , leading to the increase of photoconductivity and the improved photovoltaic performance of TiO2 -based photoanode. Understanding of these fundamental energy/charge transfer processes and interface properties will inspire future optoelectronic devices with smart designs for outstanding performance.
