Introduction of perovskite solar cell and its pros and cons
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At present, there are three main types of solar cells: The first generation of solar cells (monocrystalline silicon cells, polycrystalline silicon cells), the second generation of thin film cells (amorphous silicon thin film cells, CIGS thin film cells) And the third generation is new concept batteries (such as perovskite batteries, concentrator batteries, dye-sensitized batteries, quantum dot batteries and polymer batteries). A perovskite solar cell (PSC) is a solar cell that uses an all-solid-state perovskite structure as a light-absorbing material (and does not use perovskite-CaTiO3).
Perovskite solar cell structure and working principle
The PSC structure is generally composed of five parts from top to bottom: photoanode (FTO/ITO conductive glass), electron transport layer (ETL), perovskite photoactive layer, hole transport layer (HTL) and metal back electrodes (Au, Ag , Al). PSC is a thin-film battery, which is mainly deposited on glass at present. At the same time, different degrees of transparency can be achieved by controlling the thickness and material of each layer of materials, and of course the efficiency will also be reduced.
Structure and working principle of perovskite solar cell
The working principle of perovskite solar cells: after sunlight irradiates the light absorbing layer (perovskite layer), photons with energy greater than the forbidden band width are absorbed, the energy of the photon excites electrons that were originally bound around the nucleus, producing excitons (electron-hole pairs). Due to the small exciton binding energy of perovskite materials, they can be separated into free carriers (electrons and holes) at room temperature. After the excitons are separated into electrons and holes, the holes enter the hole transport material (HTM) from the perovskite material, electrons enter the electron transport material (ETM) from the perovskite material and flow to the cathode and anode of the battery, respectively.
The main advantages of perovskite solar cell
Low manufacturing cost
The biggest advantage of perovskite solar cell is its low cost (both material cost and production cost), and the cost per square foot (0.09 square meters) is about $0.25, which is about one tenth of that of silicon cells. First of all, the core raw materials of perovskite cells are abundant and easy to obtain (no rare materials), the supply is not limited, and the price is lower than that of crystalline silicon raw materials.
Secondly, the perovskite film has a high light absorption coefficient, which is about 100 times that of traditional solar cell materials, so its cell film layer can be thinned (in theory, only 1/100 of the thickness can be used to produce the same thickness as crystalline silicon) energy output), significantly reduce the amount of battery materials (perovskite film thickness is usually only 0.3~0.5μm, crystalline silicon thickness is usually 160~180μm.
According to statistics, the power generation of 35kg perovskite is the same as that of 7 tons of silicon, so the use of perovskite materials is reduced by a hundred times). Finally, its manufacturing process can use low-cost process technology that does not require expensive vacuum equipment, and the manufacturing cost has obvious advantages over crystalline silicon batteries.
High photoelectric conversion efficiency
Perovskite solar cell’s light absorption layer perovskite film has a wider band gap than crystalline silicon and can absorb short-wavelength visible light. That is, the absorbed photon energy is greater (E=hc/λ, the shorter the wavelength, the greater the energy), which means a greater photoelectric conversion rate.
At the same time, perovskite solar cell has high charge carrier mobility and good light diffusion performance, and its electron and hole diffusion lengths can exceed 1000nm.
The excellent photo-generated carrier transport properties make the energy loss during photoelectric conversion extremely low.
In addition, perovskite can also tune the band gap by changing the material of the perovskite film layer, so that the photoelectric conversion efficiency of the battery can reach the highest;
It is possible to increase the band gap and improve the conversion efficiency by stacking different perovskite film layers, or stacking perovskite and crystalline silicon cells.
Low quality requirements of raw materials
The preparation of perovskite solar cell has low requirements on the purity of raw materials and is not sensitive to impurities. A cell with a purity of about 90% can be produced with an efficiency of more than 20%, while the crystalline silicon cell requires a material purity of more than 99.9999%.
Simple manufacturing process and low energy consumption
The manufacturing process of perovskite solar cell is simple, it can be prepared in solution, and the process temperature is low, about 100 °C, while the maximum process temperature required for the preparation of crystalline silicon cells exceeds 600 °C, and the process temperature required for the preparation of other thin-film cells is also about 250 °C .
The low energy consumption of the perovskite solar cell manufacturing process is another significant advantage. The energy consumption per watt of monocrystalline modules is about 1.52KWh, while the energy consumption of perovskite modules is 0.12KWh, and the energy consumption per watt is only 1/10 of that of crystalline silicon.
Transparent flexible components can be prepared
The perovskite battery can prepare translucent flexible battery components by using a transparent flexible substrate (perovskite is easier to form a film on a flexible substrate than silicon) and thinning the perovskite film layer. It is used in unconventional new application fields such as glass curtain walls, wearable products, and portable products.
The main disadvantage of perovskite solar cell
At present, the main factors restricting the large-scale commercialization of perovskite solar cells are:
Poor structural stability
The instability of perovskite solar cell is mainly caused by two mechanisms, one is the instability of the perovskite material itself, and the other is related to the interface instability. Perovskite battery materials are extremely sensitive to water, heat, and oxygen environments: commonly used organic hole transport materials decompose rapidly when they meet water;
TiO2 in the commonly used structure has photocatalytic properties, which can catalyze the decomposition reaction of perovskite materials under ultraviolet irradiation.
Therefore, conventional perovskite cells have a very short lifespan in an environment with high water and oxygen content. Although the structural changes that occur are reversible, they will affect the performance, especially moisture is prone to irreversible degradation.
In addition, the low formation energy of lead ions (divalent) in perovskite cells is one of the main reasons for the formation of defects on the surface and grain boundaries of perovskite, uncoordinated lead ions (divalent) can induce photogenerated carrier recombination, and its ion migration pathway is also unfavorable for cell performance, ultimately leading to degradation of photoelectric conversion efficiency.
There are two main ways to improve the stability of perovskite cells. One is to use composite perovskite materials to improve its own stability; the other is to find suitable additive substances to inhibit the decomposition of perovskite materials.
At present, the feasible way to quickly solve the stability of perovskite and achieve commercial mass production is perovskite-crystalline silicon stack (HJT crystalline silicon cell and perovskite superimpose better), while improving the conversion efficiency, it can also seamlessly connect with the existing industry.
Difficulty in large area preparation
At present, it is difficult to prepare large-area high-efficiency perovskite cells. It is difficult to achieve an efficiency of more than 20% for a single-junction cell exceeding 1m², and the conversion efficiency decreases greatly as the cell area increases. The reasons for the difficulty of large-scale preparation: First, it is difficult to produce a large, continuous and uniform perovskite coating;
Second, the photoanode TCO film has a small resistance to collect current, and its resistivity will become more obvious when the area is larger. For example, a 6.25cm² perovskite cell has an efficiency of 20.6%, but when 35 cells are combined into a 412cm² module (close to the area of a 12-inch crystalline silicon cell), the efficiency drops to 12.6%. At present, the world’s largest 1241.16cm² module produced by GCL Nano has an efficiency of 15.31%.
There are environmental problems
Perovskite solar cell is an emerging technology that can accelerate the transition of future energy to sustainable energy. High-efficiency perovskite solar cell materials basically contain toxic metal lead (lead is a neurotoxin that is easily oxidized and volatilized, and is also soluble in water. ), posing a risk to the environment and health.
However, according to the analysis, a small amount of lead leakage does not have much impact on the environmental toxicity, and it is not as good as the overall impact of the production process of crystalline silicon cells on the environment (the silicon material purification process produces HCl waste gas, SiCl4/HCl/HNO3/HF and other waste liquids, and the battery manufacturing process Nitrogen oxide waste gas and acid-base waste liquid with pungent odor can easily lead to total nitrogen exceeding the standard). Currently available methods for mitigating potential lead leakage are physical encapsulation and chemical absorption, and Northern Illinois University has developed DMDP organic coatings, a material that absorbs lead.
As a third-generation solar cell, perovskite solar cell has the fastest development speed. The photoelectric conversion efficiency (PCE) has increased from the initial 3.8% (prepared by Yokohama University, Japan in 2009) to 29.52% (prepared by Oxford Photovoltaic Company in 2021) within 12 years. , showing great commercialization potential.
At present, the average conversion efficiency of industrially mass-produced silicon-based solar cells is generally 20% to 23%. The highest mass production efficiencies of mainstream cells are 26.3% for LONGi’s HJT cells, 23.6% for PERC cells, and 25.4% for one of top 10 solar battery manufacturers JinkoSolar’s TOPCon cells.