Perovskite solar cell structure and the principle of perovskite solar cell

The organometallic halide perovskite structure solar cell is a solar cell with an all-solid perovskite structure as a light absorbing material, and has a high energy gap of about 1.5eV, and a film of several hundred nanometers thick can fully absorb below 800 nm. It has an important application prospect in the field of photoelectric conversion. Perovskite solar cells are known as "new hopes in the field of photovoltaics" due to their good absorbance and charge transfer rate, as well as huge development potential. As battery efficiency records continue to be refreshed, more research on perovskite batteries continues to emerge, covering structural design, working mechanisms, and optimization of all aspects of the manufacturing process.

Perovskite solar cell structure

The organometallic halide perovskite structure solar cell is a solar cell having an all-solid perovskite structure as a light absorbing material. The material preparation process is simple and the cost is low. The structure of the perovskite material is ABX3, in which A is an organic cation, B is a metal ion, and X is a halogen group. In this structure, the metal B atom is located at the center of the cubic unit cell, the halogen X atom is located at the face of the cube, and the organic cation A is located at the apex of the cube. Compared with the structure connected by co-edge and coplanar, the perovskite structure is more stable and is beneficial to the diffusion and migration of defects.

In the perovskite structure used for high-efficiency solar cells, the A site is usually an organic cation such as HC(NH2)2+ (abbreviated as FA+) or CH3NH3+ (abbreviated as MA+), and its main function is to maintain charge balance in the crystal lattice, but The size of the A ion can change the size of the energy gap. When the radius of the A ion increases, the lattice expands, resulting in a correspondingly smaller energy gap, and a red shift of the absorption edge, thereby obtaining a larger short-circuit current and a high battery conversion efficiency of about 16%. Metal ion B is usually PB ion, PB has good stability, but it is often replaced by GE, SN, Ti due to toxicity. Taking SN as an example, SN-X-SN bond angle is larger than PB, and the energy gap is narrower. ASnX3 exhibits a high open circuit voltage and good optoelectronic characteristics with little voltage loss. However, in the same family of elements, the smaller the atomic number, the worse the element stability. In order to solve the stability problem, PB and SN are combined in a certain ratio to reduce the instability caused by SN, and at the same time, high conversion efficiency is obtained. The halogen group X is usually iodine, bromine and chlorine. Among them, a perovskite solar cell with an iodine group is inferior in mechanical properties (e.g., elasticity, strength, etc.) to a battery having a bromine group. The electron absorption spectrum is broadened from Cl to I in turn, and the red shift of the energy gap is also increased successively. This is because as the atomic weight increases, the electronegativity of the element becomes weaker, and the covalent interaction with the metal ion B becomes stronger. The organic-inorganic halides of the ABX3 type have different structures at different temperatures.

The basic structure of a perovskite solar cell is usually a substrate material / conductive glass (substrate glass coated with an oxide layer) / electron transport layer (titanium dioxide) / perovskite absorber layer (hole transport layer) / metal cathode.

(a) mesostructured perovskite solar cells;

(b) Planar heterojunction structure perovskite solar cell

After the incident light is incident through the glass, photons with energy greater than the forbidden band width are absorbed to generate excitons, and then the excitons are separated in the perovskite absorption layer, become holes and electrons, and are respectively injected into the transport material. It is from the perovskite material into the hole transport material, and the electron injection is from the perovskite material into the electron transport material (usually the titanium dioxide film). Based on this, perovskites have two types of structures: mesoscopic structures and planar heterojunction structures. Mesoscopic structures Perovskite solar cells are based on dye-sensitized solar cells (DSSCs) and are similar in structure to DSSCs: calcium The titanium ore structure nanocrystals are attached to the mesoporous oxide (such as TiO2) framework material, and the hole transport material is deposited on the surface, and the three together serve as a hole transport layer. In this structure Mesoporous oxide (TiO2) is both a framework material and can also transport electrons. The planar heterojunction structure separates the perovskite structure material and sandwiches it between the hole transport material and the electron transport material excitons are separated in a sandwich of perovskite material that transports both holes and electrons.

The crystallographic orientation of the perovskite structural material also affects cell efficiency. Docampo et al. found that when the soaking temperature of the solution is increased, or after the subsequent heat treatment of CH3NH3I and PbCl2, the short-circuit current of the battery is larger and the conversion efficiency is higher. The change in this process is that the long axis of the perovskite structure tends to be parallel to the base, forming anisotropy. The more obvious this anisotropy is, the better the battery performance will be.

The development direction of perovskite solar cells

Improve battery conversion efficiency

Conversion efficiency is the most important measure of solar cell performance. The highest battery conversion efficiency currently certified has reached 20.1% (Figure 3). The bottleneck that limits the conversion efficiency of solar cells is that most of the energy of the incident light is reflected or transmitted, and only the light close to the energy gap of the material of the light absorbing layer can be absorbed and converted into electrical energy. Therefore, the key to improve the conversion efficiency of the battery is to improve the energy band structure of the battery. In addition to the above-mentioned regulation of the energy gap by regulating the ionic groups in the perovskite material, the preparation of multi-junction solar cells with different energy gaps is also one of the important research directions in this field.

In addition, reducing the recombination of electrons and holes in the transmission process to increase the transmission rate is also an important way to improve the conversion efficiency.

(1) Interface regulation. It can be seen from the working mechanism of the perovskite battery that the improvement of the conversion efficiency of the perovskite solar cell depends not only on the absorption capacity of the light but also on the transmission rate of the carrier in the perovskite structure.

(2) Improving the preparation process of the perovskite battery. As a new type of thin film solar cell, perovskite solar cell is similar to other thin film cells, such as spin coating (solution spin coating), vacuum evaporation (gas phase method), etc. No matter what preparation method The purpose of preparing high-purity, low-defect, high-coverage, dense perovskite film and transport layer film. Its essence is to improve the electrical contact between different layers, reduce the defect density, reduce the carrier loss in the transmission process, so as to achieve high battery conversion efficiency.

(3) Attempts to new materials and new battery structures. At present, the most commonly used material for perovskite solar cells is CH3NH3PbI3 as the light absorbing layer, TiO2 as the electron transport layer, and spiro-OmetaD as the solid hole transport layer, and the initial conversion efficiency reaches 8.3%. In order to further improve the conversion efficiency of solar cells and highlight the advantages of perovskite materials, people began to use new materials on different structures of solar cells, or design new battery structures, and hope to achieve breakthroughs.

In general, whether it is the use of new materials or the improvement of the structure of new devices, although various methods have achieved better battery conversion efficiency, they are still slightly lower than the traditional structure of perovskite solar cells, but from the perspectives of cost, stability, and environmental friendliness, they all have high research value.

Improve solar cell stability

Organometallic halide perovskite materials have poor stability under humid and light conditions, and are prone to decomposition and cause battery efficiency to decrease or even fail. Therefore, in addition to continuously improving conversion efficiency, many studies are currently working to improve the stability of solar cells. The stability of perovskite batteries is limited by various environmental factors such as temperature and humidity. There are two ways to improve the stability of perovskite batteries: one is to improve the stability of the perovskite material itself, and the other is to find a suitable transport layer material to isolate the battery from the environment and inhibit the decomposition of the perovskite material.

In the former method, Smith et al. used a two-dimensional hybrid perovskite material (PEA) 2(MA) 2 [Pb3I10] (PEA=C6H5(CH2)2NH3+, MA=CH3NH3+) as the absorbent material (structure such as As shown in Figure 4, the structure can be formed by spin-on deposition without the need for high temperature annealing. Compared with the ordinary three-dimensional perovskite material (MA) [PbI3], the two-dimensional perovskite battery was placed in a humid environment at room temperature for 46 days without causing a significant drop in performance, and had good stability. However, the choice of atoms/atoms that can replace the various components in ABX3 is limited, and related research reports are relatively few. More research in recent years has focused on the latter, looking for suitable transport layer materials.

(a) Schematic diagram of two crystal structures, wherein A and B are structures of three-dimensional material (MA) [PbI3] and two-dimensional material (PEA) 2 (MA) 2 [Pb3I10], respectively;

(b) XRD spectra of different films in the humid environment after the same time, in which 1, 2a, 2b are respectively a two-dimensional material film, a three-dimensional material film with poor spin coating quality and a three-dimensional material film with good spin coating quality. Among the two methods, researchers are looking for better hole transport materials to improve the stability of perovskite solar cells. Good hole transport materials enable excitons to have longer lifetimes and quantum yields, extending battery life. The hole transporting material commonly used in perovskite batteries is p-type doped spiro-OmetaD. There are two ways to improve the material stability by changing the hole transport material: the first is to replace the original hole material with other materials; The other is to add an additive to the hole material or replace the original p-type additive.

(a) Comparison of the stability of two batteries using a tetrathiafulvalene derivative (TTF-1) and a spiro-OmetaD as a hole transporting material;

(b) Adding a PDPPDBTE battery to the stability of the raw material battery;

(c) Stability of the battery after using different dopants;

(d) Battery efficiency changes of different XTHSI after 3 months (where X represents metal elements (such as Li, Co, Ir), and THIS represents diacyltrifluoromethane).

In the second type of method, the introduction of the p-type additive can increase the carrier concentration, thereby reducing the series resistance and the charge transport impedance at the interface. The currently preferred dopant is LiTFSI (lithium bis(acyl trifluoromethane) )imide). However, in an oxygen-containing environment, oxygen consumes lithium ions on the hole transport layer and TiO2 surface, which reduces photocurrent, increases resistance, and reduces battery stability. Therefore, finding better additives can not only improve efficiency. It can further improve the stability. It is one of the hotspots of current research to replace metal Li with other elements.

Achieve environmental friendliness of perovskite solar cells

Due to the environmentally unfriendly nature of lead-containing materials, researchers are striving to achieve lead-free, but the corresponding reduction in battery conversion efficiency. The most direct method is to use the same elements (such as SN) instead of PB. In the MAXI3 material, the energy gap of CH3NH3SnI3 is only 1.3eV, which is much lower than the 1.55eV of CH3NH3PbI3, which can make the absorption spectrum red-shift. The use of CsSnI3 as a light absorbing material and the addition of SnF2 as an additive also reduce the defect density, increase the carrier concentration, and thereby improve the cell efficiency. The absorption spectra of these two alternative absorbing materials undergo a significant red shift and absorb a wider range of incident light.

From the perspective of solving environmental pollution without sacrificing battery conversion efficiency, Chen et al. proposed another idea to recycle the car battery to provide a lead source. Since the lead source in the automotive battery has the same material properties (such as crystal structure, morphology, absorbance and photo-electricity) and photoelectric properties, it not only provides the lead source required for the preparation of the perovskite material, but also solves the waste inclusion. The lead battery cannot be properly handled, so it has certain practical application value.

Conclusion

Perovskite solar cells also have some problems that need to be solved. First of all, people mostly focus on improving materials and preparation methods from different angles to improve the conversion efficiency of batteries. However, they have not established a complete theoretical model to explain the reasons for the improvement of battery conversion efficiency. It is difficult to obtain an accurate and reliable theory of conversion efficiency. Secondly, how to balance stability and conversion efficiency is currently a difficult point. Perovskite solar cells are very sensitive to water vapor and oxygen. Although batteries with stability for up to 4 months have been produced, the efficiency is only 12%. Compared with traditional crystalline silicon cells (lifetime up to 25 years), there are still larger gap. Thirdly, how to realize large-area continuous preparation of perovskite solar cells is also an important issue now. The size of the devices made in the laboratory is only a few centimeters in size, and there is still a distance from meeting industrial needs. Finally, how to avoid the use of environmentally unfriendly heavy metals such as lead while taking into account high conversion efficiencies is also a major challenge. At present, replacing lead with other elements usually requires a more reasonable way to solve the environmental problems caused by lead, so that the perovskite solar cells can be recycled and regenerated, which is equally important for practical industrialization. By improving the interfacial properties between the perovskite layer and other conductive layers, and looking for more efficient electron/hole transport materials, the battery conversion efficiency still has a very large room for improvement, and the stability of the solar cell can be improved. The realization of lead-free perovskite materials has become one of the key factors for the ultimate acceptance of perovskite solar cells in the public.

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