Surface plasmon resonance of layer-by-layer gold nanoparticles induced photoelectric current in environmentally-friendly plasmon-sensitized solar cell

Layer-by-layer gold nanoparticles are used to generate photocurrent in an environmentally-friendly plasmon-sensitized solar cell towing to surface plasmon resonance. The efficiency of the photoelectric conversion of gold nanoparticle layers is increased as the intensity of surface plasmon resonance increases. We also explain the experimental results by modeling the phenomenon of charge separation and photocurrent formation, and the relationship between surface plasmon resonance and photocurrent formation, which has potential application in plasmon-sensitized solar cells and plasmonic solar cells in the future. The efficiency of a plasmon-sensitized solar cell can be improved by depositing gold nanoparticles on the cell’s surface. Achieving a high energy-conversion efficiency—the percentage of incident solar energy converted into electrical energy—is the primary goal for any solar cell technology. Yen-Hsun Su and colleagues at National Dong Hwa University in Hualien have now demonstrated that depositing multiple layers of gold nanoparticles on the surface of a plasmon-sensitized solar cell increases the amount of light scattered across its surface, boosting the amount of light absorbed and thus improving its efficiency. Once optimized, such gold-covered plasmonic solar cells have the potential to replace the more popular dye-sensitized solar cells.


INTRODUCTION
Nobel nanoparticles have many of the usual physical and optical properties, including surface plasmon resonance. 1-6 Surface plasmon resonance produces a stronger electromagnetic field on the surface of gold nanoparticles and has been applied to the enhancement of Raman scattering, 7 photoluminescence, 8 biolight emission devices 9 and solar cells. 10 Recently, surface plasmon resonance of noble metals has been used to separate charges on TiO 2 films to form negative potential changes and anodic currents under visible light radiation, 3,11-14 which will potentially be able to facilitate the promising application of plasmon-sensitized solar cells and plasmonic solar cells in the future. A few studies 3,[11][12][13][14] have reported results concerning the phenomenon of plasmon-induced charge separation at TiO 2 films loaded with gold nanoparticles. In these experiments, photons resulting from incident light couple with a plasmon to serve as an exciton to separate the charge on gold nanoparticles. The electron is then ejected from the gold nanoparticle onto TiO 2 films. The photocurrents of TiO 2 films loaded with gold nanoparticles were measured under visible light radiation. However, surface plasmon resonance is an electromagnetic standing wave. Few studies have reported how an electromagnetic standing wave, a frequency-dependent physical parameter, is able to produce a frequency-independent photocurrent (for frequencies of visible light ranging from 10 14 to 10 15 Hz). By understanding the mechanism of surface plasmon resonance and optimizing the fabrication of a photocell, the efficiency of photovoltaic cells is able to be enhanced.
In this paper, we use gold nanoparticles loaded on TiO 2 film to form photocurrents, as shown in Figure 1. Surface plasmon resonance is used to separate the charges on TiO 2 films and form the photocurrent.

Gold NPs
TiO 2 Figure 1 Scheme of plasmon-induced charge separation on gold nanoparticles in a solar cell. The SPR-induced dipole acts as an energy level. SPR enhances the pumping rate of the photoelectronic formation. The photoelectric current will rise. The chemical potential of electrolyte will change due to the external electric field of SPR. The open-circuit voltage is the difference between the conductive band of TiO 2 and the chemical potential of the electrolyte. The open-circuit voltage will be shifted at the same time. Gold NPs, gold nanoparticles. We then use a theoretical model to confirm the phenomenon of charge separation and photocurrent formation and the relationship between surface plasmon resonance and photocurrent formation. This result is applicable to improve the intensity of photocurrents in the field of plasmon-sensitized solar cells.
The color of the solution turned from yellow to black. Au NPs were modified by tetraoctylammonium bromide. The morphology of the Au NPs in hexane under transmission electron microscopy (TEM) observation and on TiO 2 FTO substrate is presented in Figure 2.
Mercapto-propyl-tri-methoxy-silane (95%; Alfa Aesar) was used as a molecular linker connecting the substrate with the Au NPs through selfassembled monolayers. The TiO 2 /FTO was immersed in the solution (NH 4 OH/H 2 O 2 /H 2 O55:5:1 in volume) for 3 h in order to get modified OH 2 on the surface of the substrate. The TiO 2 /FTO was then immersed in 1% 3-mercaptopropyltrimethoxysilane hexane for 1 day, and then in a Au NPs colloid for 1 day. Au NPs were modified on the substrate. Molecular linkers, including 2 wt-% 2-mercaptoethanol (98%; Alfa Aesar) and 2 wt-% 1,3-propanedithiol (97%; Alfa Aesar) in hexane, were then added to modify the surface of the Au NPs. The Au NPs were immersed with the molecular linkers for 1 day. The sample was immersed in the Au NPs colloid again for 1 day. By repeating the above process five times, layer-by-layer Au NPs were fabricated on the substrate as an anode electrode. Thickness and fraction of layer-by-layer Au NPs on the TiO 2 /FTO substance are presented in Figure 3. Pt was sputtered on the indium-doped tin oxide glass as the cathode electrode.
The morphologies of the Au NPs were observed by TEM. The asgrown product was observed by a Hitachi model HF-2000 transmission electron microscope operating at 200 kV. The absorption of Au NPs was observed by UV-visible spectrometry (Hitachi U-2001 spectrophotometer). The sample was directly injected into a quartz tube for UV-visible light analysis. The optical properties and thickness were determined by ellipsometry (h-VASE; Woollam, Lincoln, NE, USA) in the visible light range. The electric properties, current and voltages, were measured by a high voltage Source-Measure unit (Keithley 237). The power of the simulated solar energy (AM1.5) was 100 mW cm 22 . The J-V curve under solar simulator radiation was recorded.

RESULTS AND DISCUSSION
Energy level of surface plasmon resonance A metal cluster with a diameter greater than 3 nm has bulk-like physical properties and energy levels that are not discrete. 16 In our case, the diameter of gold NPs is ,6 nm as shown in Figure 2a. The Fermi-level of bulk-like gold NPs is the work function. Surface plasmon resonance induces excitons to separate the positive and negative charges. The density of states of the surface plasmon resonance range from 510 to 580 nm in the visible light region (see also Figure S1 in Supplementary  Information). In a solar cell, the Fermi-level of bulk-like gold NPs contacts that of the semiconductor due to the balance of the electronic density on the surface. Energy level, E, referred by the expectation value of the surface plasmon resonance's induced dipole, is under the work function of gold NPs. The difference in energy between E and the work function is E g , as shown in Figure 1. The SPR of gold NPs induces a dipole, which acts as an energy level to support the transition of photoelectrons. Photoelectrons then transfer from the SPR level of the gold NP to the conduction band of the semiconductor. 17 Photoelectrons are injected into the anode electrode and are inhibited from returning due to the Schottky barrier. 18 The charged gold NP is renewed by the donor from the electrolyte, as shown in Figure 1.

SPR enhances photoelectric voltage
Open-circuit voltage (V oc ) is determined by the difference between the chemical potential of the electrolyte and the conductive band (CB) of a semiconductor (V oc~C B{m). For a regular solution case, the Gibbs free energy of an electrolyte is presented as the following: where m is the Gibbs free energy of electrolyte, X Ce 4z is the ratio of Ce 41 , X Ce 3z is the ratio of Ce 31 , G Ce 4z is the Gibbs free energy of Ce 41 , G Ce 3z is the Gibbs free energy of Ce 31 , and V is the interaction relationship between G Ce 4z and G Ce 3z (V is constant for a regular solution.). When an external electrical field is applied to the system, the free energy is represented as G95G2qV, where q is the charge of the system, V is the potential of the external electrical field and G is the Gibbs free energy. For the Ce 41 system, the free energy is G Ce 4z '~G Ce 4z { z4q ð ÞV due to the four electrons of the system. On the other hand, for the Ce 31 system, the free energy is G Ce 3z 'G Ce 3z { z3q ð ÞV , because of the three electrons of the system. The open-circuit voltage (V oc ) is represented as the following: The open-circuit voltage (V oc ) is proportional to the external electrical field.
The SPR of gold NPs applies a strong external electric field in the near field. As the number of gold NPs increases, the intensity of SPR rises, as shown in Figure 4. When the array of gold NPs is random, as in Figure 2b, no special peak of SPR appears due to the geometric effect. The open-circuit voltage (V oc ) is proportional to the external electric field for our experimental results, as shown in Figure 5c, corresponding to the above result. SPR produces a stronger electric field that enhances the open-circuit voltage of a photovoltaic cell.

SPR enhances photoelectric current
The electron excitation process 19,20 in a photovoltaic cell is shown as follows: where g is the generation, t is the lifetime, w n is the energy level for electrons and n is the particle number. When processing for a short time interval, g, w n and n are represented as follows: TiO 2 /FTO substance is at 520 nm from UV-visible spectroscopic observation. SPR intensity is recorded at 520 nm. The count of background contributed by the TiO 2 / FTO substance is 24.6. Volume of layer-by-layer Au NPs is calculated by ellipsometric data, which is multiplied by coverage fraction. The relationship between SPR intensity and volume is a linear one (correlative coefficient is 10.97). As the volume of Au NPs increase, the intensity of SPR also increases. The extinction intensity is referred to as the SPR intensity. Then this can be applied in the electron excitation process: And n~n 0 e Ef {E0 ð Þ =kBT and E f {E 0~{ ew n [r n~{ en k B T w n . Then In the above equation, the chemical effective capacity is C mC ch~e 2 n k B T , the physical effective current is i ph 5eg, and the reaction effective resistance is R rec~e 2 n tk B T {1 . In the above equation, the term, eg, is the special solution for the effective current: J !{eg. SPR can produce a strong electric field that functions as the hot spot to enhance the pumping rate of the electrons between bands. During this process, the generation, g, will increase. As the generation increases, the effective current in a photovoltaic cell will rise. The short-circuit current will rise at the same time. In our experiment, the short-circuit current is proportional to the SPR intensity, as is shown in Figure 5b, which presents the relation:

SPR enhances photoelectric properties
In a photovoltaic cell, the current-voltage curve of a solar cell yields important operational parameters, 21 among which are the short-circuit current J sc , the open-circuit voltage V oc , the current J mp and voltage V mp at the maximum power point P max . The fill factor (FF) is defined as The solar conversion efficiency (g) is given by where P s is the input solar irradiance (in mW cm 22 ). In a p-n junction solar cell model, 22 the V mp , J mp and FF can be determined as: where J s is the saturated current in a photovoltaic cell, and When SPR is applied in a solar cell, V oc 0~V oc zq 4X 2 Ce 4z z À

3X 2
Ce 3z ÞV and J sc !V are put into the above equations, then V mp 9, J mp 9 and FF9 are represented in the SPR solar cell case as: where b~1zeV oc =k B T and a~eq 4X 2 Ce 4z z3X 2 Ce 3z À Á . (b21)1aV/ k B T and 2ln(b1aV/k B T) act as the antagonism for the FF9, which makes the relationship between SPR intensity and the fill factor irregular, as shown in Table 1. The solar conversion efficiency (g9) is given by The solar conversion efficiency (g9) is a second-order equation of the external electric SPR field. In our experiment, the solar conversion efficiency (g9) is a second-order equation of SPR intensity, as shown in Figure 5d.
By modeling the phenomenon of photoelectric conversion efficiency induced by SPR, the mechanism of solar conversion efficiency can be better understood, which could be used to enhance the efficiency of plasmon-sensitized solar cells in the future.

CONCLUSION
Gold nanoaprticles separate charges under visible light radiation. We fabricated gold nanoparticles using the layer-by-layer method on a TiO 2 film to form multilayers in order to increase the intensity of the photocurrent. The efficiency of photoelectric conversion increased as the thickness increased. After optimization, nobel metal nanoparticles are possible candidates to replace dyes in solar cells and to act as plasmon-sensitized solar cells and plasmonic solar cells in the future.