1 Introduction
Among the many new energy sources, solar energy has received extensive attention due to its abundant reserves, advantages of clean and pollution-free, and less geographical restrictions. The use of solar energy mainly includes three forms of photothermal conversion, photoelectric conversion, and photochemical energy conversion. A solar cell is a photoelectric conversion device that converts solar energy into electrical energy. It can directly provide power for small electrical appliances and can also be used for grid-connected power generation, and thus has a very broad application prospect. Silicon-based solar cells were first developed and are currently the most mature solar cells. After decades of hard work. The efficiency of monocrystalline silicon solar cells has exceeded 25%, playing a pivotal role in aerospace. However, in terms of civil use, the current cost performance cannot compete with traditional energy sources. Therefore, various types of new solar cells have emerged.
Among the numerous new types of solar cells, Dye-Sensitized Sollar Cells (DSCs) have developed rapidly in recent years. Its research history can be traced back to the 1960s. Germany's Tributsch discovered that dyes adsorbed on semiconductors can generate currents under certain conditions, laying an important foundation for photoelectrochemistry. In fact, until 1991, most dye-sensitized photoelectric conversion efficiency was relatively low (<1%). In 1991, a team led by Professor Michael Gratzel of the Institute of Advanced Industrial Science in Lausanne, Switzerland, introduced nanocrystalline porous films into dye-sensitized solar cells, resulting in a significant increase in the photoelectric conversion efficiency of such cells. Compared to silicon-based solar cells, dye-sensitized solar cells (DSC) have the characteristics of low cost, simple process, and high photoelectric conversion efficiency.
2 Structure and working principle of dye-sensitized solar cells
2.1 Structure of dye-sensitized solar cells
Figure 1 Structure of a dye-sensitized solar cell
The structure of a typical dye-sensitized solar cell includes a nanoporous Ti02 semiconductor thin film, a transparent conductive glass, a dye photosensitizer, a hole transport medium, and a counter electrode.
The porous nano-TiO2 film is a photoanode of the battery, and its performance is directly related to the efficiency of the solar cell. This kind of film is generally coated with TiO2 nanocrystalline particles on the surface of conductive glass and sintered under high temperature conditions to form a porous electrode.
Transparent conductive glass is generally ITO glass or TCO glass, etc. It plays a role of transmitting and collecting electrons.
The dye photosensitizer is adsorbed on the surface of a porous electrode and requires a wide visible spectrum absorption and long-term stability.
The hole transport medium mainly plays a role of redox and electron transport. The main difference between various dye-sensitized cells is also the difference in the hole transport medium.
The counter electrode generally uses a platinum electrode or a platinum electrode with a single electron layer and is mainly used for collecting electrons.
2.2 The working principle of dye-sensitized solar cells
The basic working principle of the dye-sensitized solar cell is as follows: when the energy is lower than the forbidden band width of the porous nano-TiO2 film, but is equal to the characteristic absorption wavelength of the dye molecule, the incident light is irradiated on the porous electrode and adsorbed on the surface of the porous electrode. The electrons are excited to transition to the excited state, and then injected into the TiO2 conduction band, and the dye molecules themselves become oxidation states. The electrons injected into TiO2 are diffused to the conductive glass substrate and then enter the external circuit. The dye molecules in the oxidation state obtain electrons from the electrolyte solution and are reduced to the ground state, and the oxidized electrons in the electrolyte diffuse to the counter electrode, which completes a photoelectrochemical reaction process. In dye-sensitized solar cells, light energy is directly converted into electricity, and no net chemical changes occur inside the battery.
The working principle of the DSC battery is similar to the photosynthesis in nature, unlike the traditional silicon battery. Its absorption of light is mainly achieved by dyes, and the separation and transport of charge is controlled by the kinetic reaction rate. The transport of charge in TiO2 is done by majority carriers, so the requirements for material purity and preparation process of such batteries are not very harsh, making the production cost drop drastically.
3 Advantages of dye-sensitized solar cells
3.1. Price and Process Advantages
Traditional solar cell light absorption and carrier transport is completed by the same species, in order to prevent the recombination of electrons and holes, the materials used must have a high degree of purity, and no structural defects, so the semiconductor The process requirements are very high, which makes it difficult to reduce costs. However, dye-sensitized photoelectrochemical cells only generate carriers on one band, that is, after photosensitization of the anode, electrons are injected into the nano-Ti02 conduction band, while holes remain on the surface of the dye. Therefore, the charge recombination is limited, so that polycrystalline and low purity materials can be used, the process is simpler, and the cost is greatly reduced. At present, the price of dye-sensitized solar cells is 1/5 to 1/10 of silicon solar cells.
3.2 High theoretical photoelectric conversion efficiency
Current dye-sensitized solar cells are mainly liquid electrolytes, and their theoretical photoelectric conversion rate can be stabilized at more than 10%. Compared with polysilicon solar energy, they are not inferior to those of solid-state organic hole-transport materials. Under monochromatic light, it can even reach 33%.
3.7 Other advantages
Dye-sensitized solar cells have high transparency and can be made into transparent products; on the flexible substrate, the battery can be made into various forms, which greatly expands its application range; can be used under various lighting conditions; The incident angle is not sensitive, can make full use of the refracted light and the reflected light; the working temperature is wide, and the upper limit can reach up to 70°C.
4 Problems and Development Prospects of Dye-Sensitized Solar Cells
4.1 Major issues at the current stage of dye-sensitized solar cells
At present, the photoelectric conversion efficiency of dye-sensitized solar cells (area < 0.5cm2) has reached 11.04%. However, the photoelectric conversion efficiency with a large area and practical significance has been around 5% (up to 5.9%), and cells with an area greater than 100 cm 2 have not been reported. Compared with the conversion efficiency of traditional silicon solar cells, there is still a certain gap, and the photoelectric conversion efficiency of dye-sensitized solar cells remains to be improved.
At present, a wide range of liquid electrolyte dye-sensitized solar cells are used, mainly using liquid organic small molecular compound solvents, which have a low boiling point, are volatile, and have a large fluidity, which can cause problems such as electrode corrosion, electrolyte leakage, and short lifetime. It is difficult to seal and long-term use of the battery.
The main challenges in the development of dye-sensitized solar cells include the following: low-temperature preparation and flexibility of high-efficiency electrodes (photoanode and counter electrode); design and development of inexpensive, stable, full-spectrum dyes; encapsulation of liquid electrolytes and Preparation of high-efficiency solid electrolytes and solutions to related problems.
4.2 Development Prospects of Dye-Sensitized Solar Cells
Due to a series of problems in liquid electrolyte dye-sensitized solar cells, it is an important research direction to develop an all-solid-state dye-sensitized solar cell in search of a suitable solid-state hole-transport material instead of a liquid electrolyte.
Fig.2 Schematic diagram of all solid-state sensitized titanium dioxide solar cells
The all-solid-state sensitized solar cell is mainly composed of a transparent conductive substrate, a dense titanium dioxide layer, a dye-sensitized multi-phase junction, and a metal electrode. Among them, the introduction of a dense titanium dioxide layer is to prevent direct contact between the conductive substrate and the hole transport material and cause a short circuit. Dye-sensitized multiphase junctions mainly contain porous titania films, dyes, hole transport materials, and some additives.
The working principle of the all-solid-state sensitized solar cell is that the electrons of the dye in the multiphase junction are excited by light having a lower energy than the band gap of titanium dioxide to an excited state, and then injected into the conduction band of titanium dioxide, and the dye molecule itself is transformed into Oxidation state. The electrons injected into the titanium dioxide are enriched in the conductive substrate and flow through the external circuit to the metal electrode. The dye molecules in the oxidation state get electrons through the hole transport layer (or the holes in the dye molecules are injected into the hole transport layer and finally reach the metal electrode and are reduced. Like the liquid electrolyte dye-sensitized solar cells, the entire process There is no change in the apparent appearance of various substances, and light energy is converted into electrical energy.
In addition to all-solid-state sensitized solar cells, the future development direction of dye-sensitized solar cells includes the following aspects: low-temperature preparation and flexibility of high-efficiency electrodes (photoanode and counter electrode); inexpensive and stable full-spectrum dyes Design and development; liquid electrolyte encapsulation and preparation of high-efficiency solid electrolytes and solutions to related problems.
5 Summary
At present, dye-sensitized solar cells have been developed to the stage of industrialization. Under the existing technology, further reduction of costs, improvement of efficiency and stability, and promotion of industrialization are inevitable trends. Dye-sensitized solar cells have huge price advantages over other types of solar cells, although there are still some problems. However, we believe that in the near future, with the further development of technology, such solar cells will have a very broad application prospects.
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