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Properties of Novel Multilayer Solar Cell Films Introduction: Traditional solid-state solar cells possess high photovoltaic conversion efficiencies, but they are usually made from highly pure crystalline silicon materials; their manufacturing processes are complex, and their costs are high compared to conventional power generation methods, making it difficult to achieve large-scale application on the ground. Gratzel et al. proposed a photoelectrochemical cell using a dye-sensitized TiO2 nanoscale electrode as the anode, achieving a photoelectric conversion efficiency of 10%. After the anode of this battery is photosensitized, electrons are injected into the conduction band of nano-TiO2, while holes remain on the surface dyes, which limits charge recombination; as a result, it is possible to use materials with lower purity, **reducing costs. However, since such batteries require a liquid electrolyte to reduce the oxidized dye molecules, this leads to problems such as complex sealing processes, easy desorption of the dye, rapid degradation, and limitations in terms of battery shape and applicable applications. Mcfarland et al. proposed a new type of multi-layer solar cell structure that captures photons using dye molecules on the metal surface, achieving a high quantum efficiency. Compared to silicon-based solar cells, the performance of this type of cell is not sensitive to the purity and defects of the materials. Compared to nanoscale dye-sensitized solar cells, this battery can automatically reduce the dye without the need for an additional electrolyte. Batteries have advantages such as low material costs and simple manufacturing processes, making them highly valuable for research and offering promising applications. Due to various limitations in the materials, although the theoretical maximum efficiency of this type of battery is the same as that of conventional solid-state solar cells, its actual photovoltaic conversion efficiency at present is only 0.01%. The excessively low adsorption amount of dye molecules on the surface of the Au film (1.33×10-9 mol/cm2) is one of the main factors limiting the quantum efficiency and photovoltaic conversion efficiency of the battery. Furthermore, the gold plating method using cyanide adopted by McFarland et al. poses a serious threat to the environment and the health of operators, as the cyanide in the plating solution is highly toxic; this will also limit the future practicality of batteries. In this paper, nanoscale Au films were prepared using a non-toxic sulfite plating process. The formation process of the Au films was investigated, the effects of various experimental factors on the adsorption performance of bromocresol red on the surface of the Au films were studied, and the maximum dye adsorption capacity on the surface of the Au films was calculated. Multilayer solar cells were fabricated using Au films plated with sulfite, and their photoelectric conversion efficiency was tested. Structure and principle of multi-layer solar cells: The cell is a solid multi-layer structure, consisting successively of a dye molecule layer – a 50-nm Au film – a 200-nm TiO2 layer – and a Ti substrate. Au and TiO2 were chosen to prepare the cell so that the height of the Schottky barrier formed at the metal-semiconductor interface is approximately 0.9 V. Mercuryrhodamine was selected as the photosensitive dye because its excited-state donor level is higher than the Schottky barrier. The photoelectric conversion process in a battery involves 4 steps: The first step is that dye molecules on the surface of the battery absorb photons, generating electrons with higher energy ； The second step is that electrons from the excited state of the photosensitive layer are injected into the conduction band of the adjacent conductor, crossing the metal at an energy Ee higher than the Fermi level Ef ； The third step is that the electron energy Ee is much greater than the Schottky barrier, and the average free path of the electrons is long compared to the thickness of the metal; the electrons pass through the metal and enter the conduction band of the semiconductor (internal electron emission). When collected at the back ohmic contact, the energy of the absorbed photons is stored in the remaining electron free energy, providing an increase in voltage ； Step 4 is the reduction of the oxidized dyes through the transfer of thermoelectrons in the adjacent metal. Similar to Gratzel dye-sensitized solar cells, the cell structure spatially separates the photon absorption process from charge separation and the charge transport process, eliminating the drawback of requiring a reducing agent in the electrolyte for intermolecular charge transfer. Experiment 1. Preparation of Au thin films: Gold foil with a purity of 99.999% was cut into pieces, cleaned and dried, and the gold was dissolved using aqua regia. After the gold is completely dissolved, heating is carried out with continuous stirring (the temperature is kept below 100°C to prevent the formation of water-insoluble monovalent gold compounds), and nitrogen dioxide is removed until a blood-red, thick chloroauric acid solution is obtained. Gold chloride was diluted to a concentration of 20%, and then neutralized with potassium hydroxide solution to a pH of 8–10, yielding Solution A with a light soy-colored appearance ； Dissolve sodium sulfite in distilled water at 50–60°C to obtain solution B ； Slowly add solution B to solution A to obtain a pale yellow solution ； A certain amount of potassium citrate, potassium chloride, and EDTA was added to the solution to adjust the pH of the plating bath to 9. A gold electrode was used as the anode, and a conductive glass/sputtered TiO2 electrode was used as the cathode. During the electroplating process, the temperature of the plating solution is maintained at 40–60°C, and the pH value is kept between 8 and 10, with citric acid and sodium hydroxide being used to adjust the pH. Stir the plating solution continuously to prevent localized low concentrations or high temperatures in it. After electroplating, it was rinsed repeatedly with deionized water and then dried. The surface morphology of the Au film was observed using an S4500 scanning electron microscope. 2. Experiment on the adsorption properties of dyes: The dye used in this paper is mercurochrome, a green crystalline organic compound that can form a red aqueous solution. The chemical name of merbromin is 2,7-dibromo-hydroxymercuric fluorescein monosodium salt, with the molecular formula C20H8O6Br2HgNa2 and a molecular weight of 750.71. Dye solutions of various concentrations were prepared using dyes, and the Au film samples were immersed in these dye solutions for several hours before being removed. After washing, they were dried using nitrogen gas. An aqueous NaOH solution was used to desorb mercuribromide molecules from the surface of the Au film, and then the absorbance of the dye solution after desorption was measured using a TU1800 ultraviolet and visible spectrophotometer. The concentration of the dye in the solution after desorption was determined from a standard curve of solution concentration versus absorbance, thereby allowing the amount of mercuribromide molecules adsorbed to be calculated. 3. Battery fabrication and performance testing: A 200 nm thick, dense TiO2 film was sputtered on ITO conductive glass and annealed at 350°C for 1 hour. 100 nm porous Au films were prepared on TiO2 thin films using the sulfite electroplating method. The TiO2/Au composite film samples were immersed in a 2.5 g/L merbromin aqueous solution for 8 hours; after removal, the samples were washed and dried. A small amount of conductive silver paste is applied to the surface of the Au film as the lead electrodes for the battery. Battery testing uses a 250W xenon light source with an intensity of 100 mW/cm2. The I-V curve is measured using a voltammeter, while the light intensity is measured with a photometer. Experimental results and analysis: 1. Critical thickness of Au films – Reducing the average thickness of the Au films facilitates the passage of more energy-rich electrons, enabling them to cross the Schottky barrier and thus resulting in a higher current output. However, a continuous and intact film can be formed only when the average thickness of the Au deposit exceeds a certain critical value. This critical thickness was determined by studying the deposition morphology of Au over time during the sulfite electroplating process, providing a basis for further research on optimizing the dye adsorption process for Au thin films. The plating current was set at 0.01 A/cm2, and four Au film samples were prepared using four plating times of 5 s, 10 s, 20 s, and 30 s. When the plating time is 5 s, the Au particles are uneven in size and form islands on the surface of the TiO2 film. By increasing the plating time to 10 s, the area of the TiO2 film covered by Au particles increased, and some of these particles grew into grains of roughly uniform size; however, the TiO2 surface was not fully covered. Meanwhile, smaller Au particles continue to deposit in the gaps between the larger Au particles. By increasing the plating time to 20 s, the area of the TiO2 film covered by Au particles increased further; larger Au particles aggregated and grew to form larger granular masses, while smaller Au particles continued to deposit in the gaps and grow as well. When the plating time was increased to 30 s, the Au grains completely covered the TiO2 surface. The size of the Au grains became uniform, their edges were smooth, and the gaps between the grains were fully filled, resulting in a continuous and intact Au film. The average thickness of this Au deposition layer was measured to be 80 nm, which is the critical thickness for film formation. Studies on the deposition process of Au particles show that by selecting an appropriate plating current, a continuous and intact Au film is formed on the TiO2 surface as the plating time increases. At the beginning of the electroplating process, the surface of the substrate is not covered with a layer of gold grains; instead, small \"crystallization nuclei\" are formed. Over time, the number of these crystals increases, and they connect to each other to form a coating. 2. Prepare the standard curve: Dissolve merbromin in an NaOH aqueous solution to prepare merbromin solutions of different concentrations; perform spectral scans on each of these solutions to obtain the absorption curves corresponding to each concentration. A standard curve of absorbance versus solution concentration was plotted, with the absorbance at the maximum absorption wavelength of 518 nm on the horizontal axis and the concentration of the bromocresol red solution on the vertical axis. 3. Study on the dye adsorption process for Au thin films (1) Concentration of mercuribromide red solution: The mercuribromide red dye was dissolved in deionized water to prepare aqueous solutions of different concentrations. The Au film samples were immersed in the aforementioned solutions for 15 hours each, and then soaked in an NaOH aqueous solution for desorption. The absorbance of the desorbed mercuribromide solution at 518 nm was measured for each group; by comparing this with a standard curve, the relationship between the amount of dye adsorbed on the film surface and the concentration of the mercuribromide solution could be determined. When the concentration of the merbromin aqueous solution reaches 2.5 S/L, the dye adsorption on the surface of the Au film reaches saturation. (2) Film immersion time: The sulfite-plated Au film samples were immersed in a 2.5 g/L mercuribromide aqueous solution for durations of 4 h, 6 h, 8 h, 10 h, and 12 h respectively. Afterwards, they were soaked in an NaOH aqueous solution for desorption. The absorbance of the desorbed mercuribromide solution at 518 nm was measured for each group; by comparing this value with a standard curve, the relationship between the amount of dye adsorbed on the film surface and the immersion time could be determined. As a result, the amount of dye adsorbed on the surface of the film increased over time; by the time of 8 hours of immersion, the dye adsorption on the surface of the Au film had reached saturation. (3) Current density for the preparation of Au films To investigate the effect of the Au film preparation process on the dye adsorption properties, a series of sulfite-electroplated Au film samples with different current densities and a thickness of approximately 100 nm were prepared by controlling the electroplating time. The Au film samples were immersed in a 2.5 g/L solution of bromocresol red, left there for 8 hours, and then soaked in an NaOH solution for desorption. The desorbed solution was scanned spectroscopically using an ultraviolet and visible spectrophotometer; the absorbance at 518 nm was measured. By comparing this value with a standard curve, the number of dye molecules was calculated. As the current density increases, the amount of dye adsorbed on the surfaces of different Au films increases significantly. When the current density reaches 0.06 A/cm2, the maximum amount of dye adsorption on the film surface is achieved, at 3.80×10-9 mol/cm2. According to literature reports, using the cyanide plating method for Au films, the adsorption amount of mercuribromide molecules on the surface is 1.33×10-9 mol/cm2. The adsorption amount of bromomercuric red molecules on the surface of Au films electroplated with sulfites is 1.86 times higher than that on Au films electroplated with cyanides. Current densities of 0.03 A/cm2 and 0 were obtained using a scanning electron microscope. Photograph of the surface morphology of the 06A/cm2 Au thin film sample, with a magnification of 50,000 times. For Au films with an electroplating current of 0.03 A, the grain size is relatively uniform, with an average grain diameter of around 50 nm. In the Au film with an electroplating current of 0.06 A, the grain size of the lower-layer grains is smaller, around 50 nm, and their distribution is relatively uniform ； The upper-layer grains have a larger particle size, around 100 nm, and are distributed in a disordered manner. This phenomenon may arise because as the current density increases, polarization intensifies and the plating time shortens, causing the small grains in the lower layer to fill the gaps before they have time to grow larger; new particles can only continue to deposit on top of these small grains and grow laterally. As the current density used in preparation increases, the roughness of the Au film increases, resulting in a structure similar to that of a sponge, which allows more dye molecules to adsorb on the surface of the Au film. 4. Battery performance test results: The photoelectric conversion efficiency of this battery is lower compared to that of the batteries developed by the McFarland team. There may be various reasons for this outcome: (1) The Au film is too thick; the critical thickness for forming an Au film using the sulfite electroplating method is 80 nm, whereas the thickness of the Au film in McFarland cells is only 10–50 nm ； (2) The Au films deposited by sulfite plating have a high surface roughness, with numerous interfaces between grains, which also increases the paths for electron transmission ； (3) The imperfect preparation process leads to an increase in the interfacial barrier between the Au film and the TiO2 film ； (4) The battery’s packaging and testing processes are not sufficiently refined, resulting in damage to the Au film. All of the above points could be reasons for the low output current. Conclusion 1. The sulfite electroplating method can be used to obtain Au films with good film-forming properties and a high surface roughness on the TiO2 film surface; the critical thickness for film formation is approximately 80 nm ； 2. The concentration of the merbromin solution, the immersion time, and the conditions for preparing the Au film all have a significant impact on the amount of dye adsorbed on the surface of the Au film. Experiments show that Au films prepared at a current density of 0.06 A/cm2, when immersed in a mercuric bromide red solution with a concentration of 2.5 g/L for 8 hours, can achieve the highest dye adsorption amount of 3.80×10-9 mol/cm2, which is 1.86 times higher than that of Au films deposited by cyanide plating ； 3. The multilayer solar cell prepared using sulfite-plated Au films had an open-circuit voltage of 0.627 mV, a short-circuit current of 7.05 uA, a fill factor of 0.322, and a conversion efficiency of 0.004%.