People think that inorganic-organic metal halide perovskite solar cells (LPSCs) are the best next-generation photovoltaic technology because they are cheap, work well, and can be tuned. In the past ten years, the power conversion efficiency (PCE) of lead perovskite solar cells (LPSCs) has gone from 3.8% to over 25%, which is getting close to the Shockley–Queisser limit. Concerns about the climate arise from the use of the harmful metal lead, which could affect its industrial use.
Because it has a small bandgap and no heavy metals, tin perovskite material is a great choice. Kanatzidis’s and Snaith’s groups were the first to report hybrid tin perovskite solar cells. Tin perovskite, on the other hand, wasn’t very efficient at first and was unstable.
As early as before 2016, tin perovskite solar cells (TPSCs) had efficiencies of less than 6% and were hard to make again and again. TPSCs worked better and were more stable when they used a reversed device design (p–i–n), changed the organic cation methylammonium (MA) to formamidinium (FA), and used SnF2 to control crystal growth and stop oxidation.
In 2017, Liao et al. used big molecules to make low-dimensional TPSCs and changed how crystallised tin perovskite was. This made the device much more stable and easy to replicate. Both Ning’s and Loi’s groups said they were more than 9% efficient. Diau group said that the productivity was more than 9%.
Not long ago, device structural engineering led to a big improvement in how well devices work. Another study by Jiang et al. looked into indene-C60 bisadduct (ICBA) as an electron transporting layer (ETL) to improve band alignment between tin perovskite. This caused the open-circuit voltage (VOC) to jump up to 0.94V, showing that tin perovskite solar cells can produce high voltage. The work in this area raises the efficiency of TPSCs to over 12%. The efficiency was further raised to 14.6% by improving the direction and carrier diffusion length.
Tin Perovskite Properties
Crystal Structure
Perovskite usually has an ABX3 structure, where B and X make 4− octahedra and A fills the space left by the corners sharing 4− octahedral structure. The mathematical tolerance factor (t) should be between 0.8 and 1.0 for a stable 3D perovskite structure. Here, RA, RB, and RX are the ionic radii of the A, B, and X sites, respectively. Because Sn2+ has a radius that is similar to that of Pb2+, it can meet the ion size requirement. It is possible to make a twisted structure with rhombohedral, orthorhombic, or tetragonal symmetry when 0.8
Band Structure and Oxidation
Because of how its group and valence electrons are arranged, tin perovskite has a band structure that is like lead perovskite. Tin perovskite has a bandgap of 1.2 to 1.4 eV, which means it is a straight bandgap semiconductor with high absorption coefficients. Both the conduction band minimum (CBM) and the valence band maximum (VBM) have the same k value. This means that the semiconductor has a straight bandgap.
The VBM is mostly made up of I 5p and Sn 5s orbits, while the CBM is mostly made up of Sn 5p orbits. Tin perovskite has higher energy levels than Pb perovskite because its Sn 5s and 5p circles are bigger. The band structure can be changed by making changes to the A-site. MASnI3 has a bandgap of 1.2 eV and FASnI3 has a bandgap of 1.4 eV. When Cs is used instead of FA, the bandgap gets smaller. This makes CsSnI3 have a bandgap of about 1.3 eV.
Based on the Shockley–Queisser equation, TPSCs are up to 33% more efficient than lead perovskite. When lanthanide contraction doesn’t happen, the energy level of the 5s orbit goes up. This makes the 5s electrons energetic and easy to lose. This makes it easy for tin perovskite to oxidise and turn into Sn4+. Oxygen and tin perovskite combine to make SnO2 and A2SnI6. It is hard for oxidation to happen inside the lattice but easy on surface flaws. It’s possible for oxidation to lead to unwanted self-doping effects and nonradiative recombination, and the active 5s electrons of Sn2+ make it strongly Lewis acidic.
Electrical Properties and Defects
Solar cells need to be able to move carriers quickly in order to work better. The exciton binding energy of tin perovskite is smaller than that of lead perovskite. The predicted data for CsSnI3 is 12.2 meV, while the actual data is 18 meV. The hole mobilities of MASnI3 and CsSnI3 are 322 cm2 V−1 s−1 and 585 cm2 V−1 s−1, respectively. Tin perovskite has a carrier lifetime of less than 50 ns, which is a lot less than lead perovskite. This makes the diffusion length shorter, usually less than 1.2 μm. There are a lot of defects in tin perovskite because of oxidation and Sn vacancies. The recombination constants for one molecule are 109 to 1010 s−1 and for two molecules they are 1.4 × 10−9 cm3 s−1. To put it another way, TPSC’s Auger(k3)recombination is a lot less powerful than LPSC’s. A p-type character film is made when tin perovskite easily oxidises. This can be stopped by lowering oxidation and adding 2D perovskites. Recent computer models show that an environment high in iodine might make it easier for Sn2+ to oxidise and for tin vacancies to form. On the other hand, conditions high in Sn make it harder to form flaws because they have higher formation energies.
Perovskite Composition Engineering
Three-Dimensional TPSC
Tin Halide Perovskite Solar Cells (TPSCs) were first being made, and most of the work that was done was on creating the makeup of tin perovskite. The 2014 study by Snaith et al. on MASnI3 solar cells showed that they were only 6.4% efficient and were hard to replicate. To make the quality of MASnX3 perovskite films better, new film growing methods were created. For example, Kanatzidis et al. made MASnI3−xBrx solid crystals and changed their method by using SnI2-dimethyl sulfoxide (DMSO) as an intermediate phase. The researchers Wang et al. looked into the low-temperature vapor-assisted solution process (LT-VASP) for film growth through ion exchange/insertion reactions. They got a PCE of 7.78%, which is one of the best values for MASnX3-based perovskite solar cells.
Because it is less likely to oxidise, FASnX3 TPSC has been created in recent years. Koh et al. first made FASnI3 solar cells, but the PCE is only 2.1% because the cells have a rough shape and charges mix. To make the device work better, Seok et al. added pyrazine to the precursor to make a SnF2−pyrazine complex. This changed the shape of the film and slowed down oxidation, which led to a PCE of up to 4.8%. The PCE that Liao et al. got was 6.22%, and the PCE that Diau et al. got was 9.6% after adding ethylenediammonium diiodide (EDAI2) to FASnI3 to passivate the grain boundaries. Han et al. used poly(vinyl alcohol) (PVA) to connect with FASnI3 and create hydrogen bonds that stopped the movement of iodide. Hayase et al. added ethylammonium cation to the FASnI3 perovskite structure. This made the film much more stable and reduced the number of defects, leading to an efficiency of over 13%.
A lot of attention has been paid to inorganic tin perovskite because it has a high formation energy that could make TPSCs more stable against oxidation. Chen and his colleagues made a Schottky solar cell out of CsSnX3 and got a PCE of 0.9%. Moghe and others created a CsSnBr3 layer by adding SnBr2, CsBr, and fluoride passivating dopants one at a time. The layer kept almost all of its absorption after being exposed to air for 24 hours.
Low-Dimensional TPSC
For perovskite solar cells, low-dimensional shapes are often used to make the devices more stable. Simulations of density functions show that tin perovskite with low dimensions has a lower oxidation energy than 3D structures. Large organic cations, such as phenylethylamine (PEA), can separate perovskite slabs from oxygen and water molecules in the air. Quantum wellstructures can also protect Sn 5s-electrons in the valence shell by weakening the antibonding connection between Sn 5s and I 5p.
Usually, big amine molecules are added to the A spot to make low-dimensional perovskites. The 2D structure can be broken down into two groups: the R–P phase (Ruddlesden–Popper) phase and the D–J phase (Dion–Jacobson). Most of the time, molecules with one amine unit make the R–P structure, while molecules with two amine units make the D–J structure possible. PEA is the chemical that is most often used to make the R–P phase structure.
When PEA is added, it creates a low-dimensional R–P perovskite that is less likely to oxidise than 3D peers. This makes the material 5.94% more efficient. Adding big amine molecules can change how quickly crystals grow, which makes the film more crystallin.
The yields of TPSCs were raised to 9.4%, 12.4%, and 14.6%, respectively, with the help of perovskite contact passivation, device engineering, and better crystal quality. A VOC of 0.94V is made when low-dimensional shapes are used, which is close to the possible maximum number.
Using molecules with double amine groups to join the stacked structure in a D–J shape could make the structure even more stable. In the first study, Padture et al. used 4-(aminomethyl)piperidinium to make TPSC, which had a highest PCE of 10.9%. Recently, low-dimensional structure has been added to artificial TPSCs. This improves crystal direction and crystallinity, making the device much more efficient and stable compared to the control film.
Additives Manipulation
The study looks into how useful additives can improve the quality of tin perovskite film. It focusses on three types of defects: changing the way crystals grow and reducing defects.
Crystallization Regulators
In low-dimensional TPSC, it is important for carriers to move around and for the flaw density to go down that the crystals are highly ordered, have big grains, and have few boundaries. Crystal growth that is orientated vertically to the material can improve performance and carrier movement. To change how quickly crystals grow, additives can be added to the precursor. Wang et al. used NH4SCN to make a 2D-quasi-2D–3D hierarchy structure, which also made the film more crystallin. Kim et al. added FASCN to PEAI-based 2D/3D tin perovskite, which made the crystals more stable and allowed them to be orientated out of plane. NH4Cl was used to make tin perovskite more stable and improve its shape. Large Lewis base molecules can slow down the crystallisation process, and π-conjugated Lewis base molecules with a lot of electrons can control the crystallisation process. PVA was added to the preparation to help FASnI3 form hydrogen bonds with each other and slow down the growth of the crystals.
Deoxidizers
Oxidation is very easy to happen with tin perovskite, so reducing additives like Sn chemicals are needed to stop it. Sn4+ and Sn0 react in a way that changes the proportions of oxidised species in solution, like Sn powder and Sn(0) nanoparticles. Jiang et al. created a one-step process for making SnI2-solution, which lowers the cost of TPSCs and oxidation. Fluorine is often used because it has a high electronegativity, but no one knows how it works yet. To slow down FASnI3 oxidation, volatile reducing agents like Formic acid and N2H4 are being looked into. However, these agents are dangerous to handle because they can explode. Yan et al. improved the stability of the FASnI3 film by adding hydroxybenzene sulfonic acid or salt to the perovskite precursor solution. Wang et al. added phenylhydrazine hydrochloride to TPSC, which made it more stable for keeping and improved the device’s performance even after being exposed to air.
Interfaces Passivating Materials
Surface flaws and rust can have a big effect on how well and how long TPSCs last. Chen et al. added an extremely thin PEABr layer to protect the connection between the perovskite and substrate, which decreased the amount of background carriers. An enclosing layer can stop the surface hanging bond from reacting with oxygen, protecting the perovskite. Some other methods are a 2D structure, an amorphous-polycrystalline structure, a thin layer of poly(methyl methacrylate), and a self-sealing polymer made of poly(ethylene-co-vinyl acetate). Surface passivation is a general way to make an item work better.
Device Architecture Engineering
The structure of the device is very important for carrier movement in TPSCs, which started out with a regular structure. However, the lead perovskite-based structure and layers that move carriers don’t fit well with TPSCs, so more research needs to be done on the device’s structure.
Normal and Inverted Structures
Tin perovskite solar cells (TPSCs) normally have organic molecules as hole transporting layers (HTLs). These molecules need to be oxidised in air to make them more conductor. However, this process makes tin perovskite oxidise and can lead to surface flaws because metal salts are added. As a result, TPSCs with a normal shape tend to be inefficient and hard to make again and again. Inverted structures with NiOx or PEDOT:PSS as HTLs make carrier transfer work well and don’t need any extra steps or chemicals, which makes it easier to make high-quality tin perovskites. These days, the reversed device construction is what most high-efficiency TPSCs are based on.
Band Alignment
In solar cells, carrier movement depends on the interface bands being lined up correctly. Its CBM and VBM are not as deep as those of lead perovskite because the Sn 5s lone-pair has more energy. The CBM of FASnI3 is 0.2 eV higher than that of FAPbI3, which is in line with calculations using density functional theory. Voltage loss is caused by the deep energy level of the layers that move electrons, and the VOC of TPSCs made from PCBM or C60 is usually less than 0.7V. To fix the energy level shift, TPSCs use ICBA, which has a lower lowest empty molecular orbital level of -3.74 eV. This leads to a high VOC of 0.94V. This high LUMO level makes quasi-Fermi level splitting bigger and nonradiative recombination smaller.
Conclusion
This chapter discusses the properties of tin perovskite and the development of tin perovskite solar cells (TPSCs). Improvements have been made to tin perovskite devices through composition engineering, additives, and structural engineering. Composition engineering reduces oxidation and stability, while additives manipulate film growth and defect density. Device structural engineering, including architecture modification and band alignment, enhances carrier transport and device voltage. Despite progress, the PCE of tin perovskite solar cells is significantly lower than that of lead perovskite devices. Challenges include short carrier diffusion length, large defect density, and easy oxidation. To address these issues, further research is needed on crystal growth and degradation, precise control of tin perovskite film structure, creation of a protective layer without compromising carrier transfer, and the design of new carriers transporting layers.
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