Energy is important for our daily lives, but because of inefficient burning technology, modern society rests heavily on fossil fuels, which have very bad warming effects on Earth. Since fossil fuels can’t be made again, they will run out soon, which will cause a major energy problem. To solve these problems, people are looking for clean energy exchange systems that use natural sources like wind and sun energy and work very well. Solar energy is the most common green energy source. It gives off about 10,000 times more energy each day than the whole world needs. Photovoltaic (PV) systems, which use sunlight to make power, are getting more attention as important ways to deal with the huge demand for energy.
Silicon-based solar cells, dye-sensitized solar cells (DSSCs), and perovskite solar cells (PSCs) are some of the different types of solar cells that have been made and used. Researchers have been working on making new types of solar cells that are more efficient and cost less. This is because silicon-based solar cells are expensive and hard to make. DSSCs have a bright future because they have a high PCE, are cheap, easy to make, and can be used in many ways. A new substance with a perovskite structure is very important in solar cells. It acts as photoelectrodes in DSSCs and light collectors in PSCs.
Organic–Inorganic Solar Cells
A Brief Introduction of PSCs
Because they could be used in solar energy-based projects, hybrid organic-inorganic perovskites have become a hot topic in chemistry and materials science. It has been shown that hybrid organic-inorganic perovskite can change the game in photovoltaics (Solar cells), with Photocatalytic Efficiency (PCEs) that are on par with silicon solar cells, cadmium telluride solar cells, and copper indium gallium selenide/sulfide solar cells. Over 6,000 papers have been written about this changing area in the last six years.
The first time that organic-inorganic perovskite was used in solar cells was by Miyasaka et al. They made them on a DSSC design with a thin perovskite layer made of CH3NH3PbI3 (MAPbI3) and CH3NH3PbBr3 (MAPbBr3) on mesoporous TiO2. It was hard to keep these solar cells stable, though, because the liquid electrolyte used in this setup was very acidic. Park and his colleagues made a solar cell with the same dye-sensitized design and got a better PCE of 6.5%. It was shown that spiro-OMeTAD, a solid-state electrolyte, can improve cell stability by acting as a hole transporting material (HTM).
A lot of people are interested in PSCs because their PCE has been rising quickly over the past few years. This is because they have good optoelectrical properties, like a direct band gap, a high sunlight absorption coefficient, and better electron/hole mobility. The fast improvement of PCEs in PSCs can get very close to ideal efficiency levels by designing and creating perovskite materials with different chemical makeup, forming crystalline films, building interfaces, optimising them, and coming up with a new way to build devices.
Light Absorbers
The study looks at the progress made in perovskite-based light absorbers in polystyrene composites (PSCs) in three areas: film processing, chemical engineering, and device stability. This helps with the creation of new materials and our understanding of these light absorbers.
Processing of Perovskite Films
To make the Photovoltaic Cell (PSC) work better, the film quality of the perovskite layers needs to be better. These layers should be smooth, regular, and free of holes. Liu et al. used a vapour deposition method for planar PSCs and got a high PCE of 15%. This method can also be used for multi-stack thin films with bigger cell sizes.
Solution-based manufacturing methods are commonly used in solar cells because they require little energy to make a lot of them. But it’s hard to make thin, even, and flat films out of halide perovskites because they strongly prefer to crystallise in an uneven way when the solvent is removed. To make the crystallisation rate of halide perovskites faster, we need new and better ways to make them.
Kinematic control of the formation and growth of crystal grains is a good way to make halide perovskite layers that work better in electronics and optics. In order to get good PSC performance, Jeon et al. used a liquid engineering method to make uniform perovskite plates. Park and his colleagues showed a quick way to make crystals by spin coating a MAPbI3/dimethylformamide (DMF) solution and then exposing it to chlorobenzene to help make uniform perovskite films. Huang et al. showed a gas-assisted solution-processing method that created uniform perovskite layers made up of single crystal grains that are closely packed together.
There were also two-step ways used to make uniform perovskite layers that worked very well. Burschka et al. created a new two-step process in which PbI2 was first put on a porous TiO2 film and then exposed to an MAI solution, which changed it into MAPbI3 perovskite. Mesoporous PSCs built on a perovskite film with better film quality were able to get a PCE of 15%.
To make perovskite films with better surface coverage, fewer pinholes, and a well-controlled shape, scientists can use advanced methods like the vapor-assisted solution process, spin coating, surface passivation, and fast deposition crystallisation. MAPbI3 was successfully made using the spin coating method, but it was still hard to get high-quality plates of pure-phase MAPbI3 perovskites with good coverage.
Vacuum evaporation looks like a good way to make MAPbI3 film where the properties of the film can be precisely controlled. When MAI and PbCl2 were evaporated together, they formed a phase called MAPbI3. The perovskite film was much more regular than the one that was made by processing it in a solution. However, the high hoover used in this method made it impossible to make a lot of things. We made perovskite films with full covering and small grains using a vapor-assisted solution method that was based on the good reaction between the PbI2 film and the CH3NH3I vapour. This process worked 100% of the time to change the precursors.
Compositional Engineering
When it comes to structure and chemical freedom, organic-inorganic perovskites are very useful. They also have optical qualities that can be changed by functional doping or replacement. It is possible to fully swap the I in MAPbI3 with either Cl or Br to make MAPbCl3 and MAPbBr3, and large single crystals of these substances have been made.
We saw that the band gaps of halide perovskites got bigger as the ionic size of the halide got smaller. The band gaps of MAPbCl3, MAPbBr3, and MAPbI3 single crystals were 2.97, 2.24, and 1.53 eV, respectively. The band gaps of polycrystalline perovskite films, on the other hand, were bigger than those of single crystals. The PCEs of PSCs were largely determined by the diffusion length of the photo-generated electron-hole pair in the halide perovskite. This length kept the thickness of the perovskite layer to a few hundred nanometres.
A study by Hao et al. looked at how well PSCs with CH3NH3Sn1−xPbxI3 as light absorbers worked. They found that the band gaps weren’t linear (Vegard’s law) between 1.55 and 1.35 eV, but were smaller (less than 1.3 eV), which made the sunlight-harvesting region bigger, going up to 1050 nm. The PSCs that had CH3NH3Sn0.5Pb0.5I3 as the light absorbent had a high short-circuit current density (Jsc) of over 20 mA cm−2 when they were exposed to artificial sunlight.
Krishny et al. used computer screening and experiments to show that germanium (Ge) could also be used instead of lead (Pb) in halide perovskites for PSCs. Scientists made CsGeI3 crystals with stable rhombohedral crystal shapes. These crystals didn’t change phase when they were used in a solar cell. We also made MAGeI3 crystals and compared their structure, band gaps, and temperature stability to those of CsGeI3 crystals. The PSCs that had CsGeI3 and MAGeI3 had photocurrent densities of 5.7 and 4.0 mA cm−2, but the PCEs were very low (less than 0.2%).
In halide perovskites, the A-site cation can also be changed or doped, along with the X-site and B-site doping. MA was added by FA ions that had a slightly larger ionic radius. This made the lattice bigger and changed how the PbI6-octahedra were tilted. Because of this, the band gap goes down from 1.59 eV for MAPbI3 to 1.45–1.52 eV for FAPbI3. Choi and others used adding Cs to MAPbI3 perovskites to improve the PCEs of the PSCs. The PCEs went from 5.51 to 7.68%, which is a clear improvement. The better film shape was blamed for this rise in the PCE.
Device Operational Stability
In the past five years, photovoltaic solar cells (PSCs) have shown that they can convert sunlight into electricity very efficiently, with photocurrent efficiency (PCEs) that are about the same as those of regular solar cells. However, the longevity of PSCs is still not good enough. The next task is to make the halide perovskite as stable in PSCs as other parts of the cells. The strong interaction between halide perovskites and water makes humidity a key factor in how poorly PSCs work. It has been said that encapsulation techniques can make PSCs more stable by keeping them away from outside factors that could break them down.
Potential natural breakdown factors are also active when the cell is not doing anything, which could affect the perovskite and other cell parts. For instance, small-molecule HTMs crystallise at high temperatures, which makes it harder for them to make electrical contact with the halide perovskite and the metal current collector. The fast loss of function of PSCs is caused by gold (Au) moving around inside small-molecule HTMs. To make PSCs more stable with small-molecule HTMs, it is important to slow down the movement of Au. Perhaps carbon-based HTMs could be the answer, but the PCEs of PSCs made with carbon-based HTMs were not as high as those made with small-molecule HTMs.
It has been shown that polymeric HTM is a good barrier to stop Au from moving. Adding chromium interlayers and alumina nanoparticle buffer layers also makes PSC more stable by stopping Au from moving. Layers that don’t let water through have been shown to improve the stability of PSC in wet circumstances. A layer of hydrophobic ammonium ions was used to protect the perovskite surface from water. This was followed by a layer of amphiphilic 1, 1, 1-trifluoro-ethyl ammonium iodide. A water-absorbing polymer (PEG) was also used to stop the reaction between water and the halide perovskite. This made the perovskite heal itself.
The main part of PSCs is the perovskite light absorption layer, and how long it lasts is very important for making them commercially viable. Ionic flaws can make the PSC less stable because they move from the bulk to the contact surface with the selective contact, which lowers the efficiency of charge separation. To make PSCs more stable, methods like functional doping/substitution and surface passivation should work.
Electron Transportation Materials
Photovoltaic (PSCs) longevity is affected by things like how unstable halide perovskites are, how they connect to other materials, and charge transfer materials like ETMs and HTMs. It has been shown that TiO2 is not stable when exposed to ultraviolet (UV) light. This has led to many ways to make PSCs last longer. To get long-lasting PSCs without lowering their performance, one way to do it without adding more steps is to create new ETMs that can replace or change TiO2.
The perovskite oxide BaSnO3 (BSO), which has a band gap of 3.2 eV, has been used in many areas, such as as a gas monitor, transistor, and clear conducting oxide. Recently, BSO perovskite oxide has been used as an ETM instead of TiO2 in PSCs because it has the same perovskite structure as halide perovskites and can move electrons around more easily. The PCE of PSCs with BSO as ETM was higher than that of TiO2. This was because BSO and MAPbI3 had better contact. BSO’s high electron mobility made it easy to collect charges, and its wide band gap kept it from competing with MAPbI3 in sunlight absorption.
But this new BSO-based PSC still has some problems that need to be fixed. For instance, BSO had a faster rate of electron–hole exchange when the voltage was high. This problem needs to be fixed in the future so that BSO can be a strong ETM option for use in PSCs. A simple peroxide-precipitation method was used to make well-dispersed BSO nanoparticles. This improved the spread of the BSO film, reduced leaking issues, and found the best thickness for the ETM layer, all of which led to better PCEs of the PSCs.
LBSO perovskite oxide with La was found to have a better electron mobility of 320 cm2 V−1 s−1 and a conductivity of 4000 S cm−1 at room temperature. While LBSO films can be used on glass and bendable substrates, they can’t because the LBSO perovskite phase forms at temperatures above 1000 °C. It is very important to come up with better ways to make the phase-pure LBSO thin film below 500 °C.
Dai and his colleagues looked into a mesoporous ETM that is based on LBSO. They made LBSO nanoparticles and used them as mesoporous ETMs by spin coating at a low temperature of 510 °C. The mesoporous LBSO-based PSC had a high PCE of 15.1% after it was optimised. In spite of this, the making temperature (510 °C) of BSO perovskite was still too high to make high-performance solar cells on a glass base.
People think it’s a big problem that UV light breaks down PCEs in PSCs with TiO2 as the ETM. This makes it hard to use PSCs outside in the sun. Shin et al. described an experiment where they soaked PSCs in light from an AM 1.5G lamp that used a xenon or metal-halide lamp and UV light to find out how stable they were with LBSO and TiO2 as ETMs. The stability of the PSC with LBSO ETM was much better than that of TiO2. The creation of LBSO perovskite as an ETM in PSCs could speed up the commercialisation of PSCs since they don’t need a UV filter like TiO2-based PSCs do.
Perovskite oxides (PSCs) are electrode materials that have been used in many different situations, such as high-performance solar cells (PSCs). Researchers have found that SrTiO3 is a good ETM for developing PSCs because it makes them work much better. Adding SrTiO3 created a halide perovskite layer with big grains and good surface coverage, which made the material work better. The safety of PSCs with SrTiO3 as ETM, on the other hand, was not looked into. This is something that should be researched in the future.
It has been shown that making double-layer or hybrid ETMs can improve the performance of PSCs that use single-phase or single-layer perovskite oxide ETMs. For instance, adding a mesoporous double layer (MDL) of BaTiO3 and TiO2 as an ETM raised the solar performance of PSCs from 9.89% to 12.4%. The bigger MAPbI3 crystals and the MDL made it easier for sunshine to pass through and improved Jsc. The OCV of the PSC with BaTiO3/TiO2 MDL was also raised by lowering the recombination of electrons and holes.
One type of PSCs, though, had a high OCV but a smaller average Jsc than those with TiO2. To get around this, a SrTiO3/TiO2 hybrid ETM was used, which had the best PCE of 10.6%. Wang et al. created a new ETM for PSCs out of graphene and SrTiO3. This combination had a better Jsc because graphene is better at moving charges around and conducting electricity. It was still not possible to get a high PCE because the FF of these PSCs was too low. More study is needed to find out how stable PSCs are when SrTiO3 is used as the ETM.
Dye-Sensitized Solar Cells
A Brief Introduction of DSSCs
Because they don’t recombine as often as regular solar cells, DSSCs opened up new areas of PV technology when they were invented in 1991. Many people are interested in DSSCs as one of the most hopeful types of solar cells because they can be made in a wide range of forms, colours, and levels of transparency, and they work better than other solar cells at collecting and using light to make electricity. On the other hand, low PCE, bad security, and high price are problems that come up during commercialisation. A dye-sensitized mesoporous semiconductor photoanode, a photocathode (counter electrode), and a solution with redox partners are the main parts of a DSSC. Some things that have been tried to make the PCE better are nanostructured oxides, thick and packed layers, and panchromatic dyes or multi-dye systems. In the last 20 years, a lot of work has been made in making the PCEs of DSSCs better. The PCE of DSSCs went up from 7.12% in 1991 to 13% in 2014. This means that DSSC technology is now usable if the cost of solar cell parts can go down while the PCE goes up. Researchers are mainly interested in changing the physical and chemical features of the main parts of a cell, like the photoanode (metal oxide semiconductor), the electrolyte (redox couple-based), and the photocathode.
Photoanodes
Creating better photoanodes is important for making the photoinduced electron-hole pairs (PECs) of DSSCs better because they let us use photoinduced electron-hole pairs to test how well DSSCs work. The photoanode of DSSCs is an oxide-based layer with mesopores that is made up of nanoparticles that have been fused onto a conducting base, like FTO glass. TiO2, which is the most common photoanode, has a band gap of 3.2 eV and has been the subject of a lot of study. The choice of other cell parts in DSSCs, like the colour or the sensitiser, is limited, though, because TiO2 is also used.
Some perovskites with an ABO3 structure are better at photovoltaics than TiO2, and it’s easy to change how well they work by adding or taking away functional groups in the A-site, B-site, and O-site. A lot of people use BSO and LBSO as photoanodes in DSSCs because they are better at collecting light and moving electrons around than TiO2. Because perovskite oxides have a flexible structure, it is possible to choose the right chemical elements and make them in a smart way.
To improve the PCE of BSO-based DSSCs, different ways of preparing BSO have been tried. The 43-lm-thick BSO photoanode gave the largest PCE from the DSSC. This was because it was so good at collecting electrons, which suggests that BSO could be a good photoanode for DSSCs. It was shown that a chemical bath deposition (CBD) method could improve the PCE of DSSC with BSO photoanode. The PCE of DSSCs got 21.7% better after CBD treatment.
It was shown that adding Fe improved the performance of the BSO photoanode in DSSCs. A 0.03 mol% Fe-doped BSO photoanode got the highest PCE (7.78%), which is because it created ferromagnetism and changed the nanostructure.
As well as making new perovskite photoanodes, cations like Ca2+, Sr2+, and Ba2+ were added to TiO2 to change its molecular structure and create a perovskite oxide that can be used as a photoanode for DSSCs. We made nanomaterials of undoped and Mn-doped BaTiO3 using a co-precipitation method so that they could be used as photoanodes in DSSCs. Adding a TiO2 scattering layer helped electrons move around, and the DSSC efficiency went up 7–17 times compared to solar cells that didn’t have any changes made to the scattering layers.
Adding other useful materials to perovskite oxide has been shown to improve the photocatalytic efficiency (PCE) of DSSCs in the field of photovoltaics. We made SrTiO3:Sm3+@SiO2 (STS@SiO2) core-shell nanoparticles using the Stöber method and a chemical process. These were then used to change the TiO2 photoanode in DSSCs. When UV light hit STS@SiO2 core-shell nanoparticles, they turned it into visible light, which was a good match for the light range that N719 dye absorbs. The DSSC with an STS@SiO2-doped TiO2 photoanode had a better PCE than the pure TiO2 and the STS-doped TiO2 photoanode. It worked better because STS@SiO2-doped TiO2 photoanode stopped electron–hole recombination and had better light scattering and down-conversion skills.
BSO is shown to be an important ETM in PSCs because it has better electron movement than TiO2 and the same level of electrical conductivity. BSO was also treated with TiCl4 and/or a BSO/scattering layer was formed in the photoanodes of DSSCs to improve cell performance by making it easier for photoinduced charge carriers to be collected. Rajamanickam et al. formed BSO nanocuboids or nanoparticles that work as a photoanode for DSSC and had a PCE of 0.71%. Two scattering layer materials (TiO2 and ZnO) were put on the pure BSO nanocuboids/nanoparticles for different TiCl4 treatment times (1, 3, and 5 min). These were used as photoanodes in DSSCs. The DSSCs with the BSO/TiO2 scattering layer, the BSO/ZnO scattering layer, and the BSO photoanodes that had been treated with TiCl4 for one minute had PCEs of 1.14, 1.25, and 3.88%, respectively.
Kim et al. described a SrTiO3–TiO2 composite that was used as a photoanode in DSSCs. This composite had better electron transfer and electron–hole separation. Tang and Yin showed a new Sr-doped TiO2/SrTiO3 nanorod array heterostructure that could be used as a photoanode in DSSCs. This structure had a microstructure that could be changed and could convert light into electricity very well. It changed the way sunlight worked from UV light to visible light when Sr was self-doped into TiO2 and a heterostructure called SrTiO3/Sr-doped TiO2 was made by treating TiO2 nanorod arrays in a Sr(NO3)2 solution.
Not only did perovskite oxide cover the surface of TiO2, but TiO2 nanoparticles were also used to decorate perovskite oxides to make the PCE of DSSCs better when they are used as photoanodes. Small (5 nm) TiO2 particles were added to BaTiO3 using a TiCl4 process, and these were then used as photoanodes for DSSCs. The best photoanode, called BaTiO3/TiO2(4), was made by treating it four times with TiCl4. Most PCE (9.40%) was found in a DSSC that used a compound with an 85:15 weight ratio of TiO2 to BaTiO3. This increase in PCE was due to photo-generated electrons recombining less, charge collection happening faster, and dye sensitisation being better.
Photocathodes
A lot of work has gone into making photoanode materials for DSSCs. Many dyes that are very good at absorbing sunlight are being studied. The most studied sensitisers in DSSCs are those that contain Ru(II). The temperature and treatment time for dye sensitiser binding are very important in determining the Photochemical Characteristics (PCE) of DSSCs. The electrolyte is another important part of DSSCs because electron transfer must happen quickly and continuously for dye generation to happen. The triiodide/iodide (I3 /I−) redox couple in acetonitrile is the most common solution because it dissolves easily and lets ions move around easily.
Pt is the photocathode in DSSCs that has been studied the most because it has a high electrocatalytic activity and can move electrons very well. Pt, on the other hand, is very expensive and hard to find, which makes it very limited in how widely it can be used as a photocathode in DSSCs. A lot of research has been done on a number of low-cost options to photocathodes, including carbons, conductive plastics, and metal oxides, sulphides, and selenides.
Carbon nanotubes (CNTs), graphene, and carbon black (CB) have all been looked at as photocathodes because they have a lot of surface area and can carry electricity well. To improve the electrocatalytic performance of carbon-based photocathodes, functionalisation, doping, and shape control are all good ideas. Large-scale uses were limited, though, by the complicated ways they had to be made and how unstable the functional groups were.
More people are interested in using metal oxides as photocathodes because they are easy to find, have better activity and stability, and are cheap. It has been tried to use complex oxides like spinel and delafossite as photocathodes, but they haven’t worked well because they don’t have enough active sites. In the last few decades, perovskite oxides have gotten a lot of attention as catalysts in a wide range of electrocatalysis-based uses. This is because they are very flexible in terms of their structure and makeup.
Pure-phase perovskite oxides, like La0.67Sr0.33MnO3 (LSM), were used as cheap photocathodes for DSSCs instead of Pt, but they didn’t work as well as Pt at IRR. The goal of this work is to create electrocatalysts based on perovskite oxide that can be used as possible photocathodes. This will lower the cost of DSSCs and speed up the process of making them available to the public. Perovskite oxides don’t have a lot of IRR action because they don’t carry electricity well. Adding carbon materials can fix this problem. Some metal oxide/carbon mixtures, like NiO@reduced graphene oxide (RGO) and SnO2@RGO, were also used as photocathodes in DSSCs. There was an idea to use a solution combustion method to make an LSM@RGO hybrid photocathode for DSSCs. This had a high PCE of 6.57%, which was higher than 92% of the Pt photocathode (7.13%).
The study is mostly about making solar performance (PCE) better in DSSCs by using BTO/graphene alloys as photocathodes. The BTO/graphene photocathode worked much better than the pure BTO photocathode because it had better IRR activity, a bigger specific surface area, and lower charge transfer resistance. The DSSC with BTO/graphene photocathode had a higher PCE (9.70%) after the conditions were optimised than the pure BTO photocathode (0.81%).
We made La0.5Sr0.5CoO2.91 perovskite oxides using a sol-gel method and used them as photocathodes for DSSCs. The LSC/RGO nanocomposites were made using an easy physical mixing method. They had good IRR activity because LSC and RGO work together. The PCE of the DSSC with the LSC/RGO photocathode was 6.32%, which was much higher than that of the pure LSC photocathode (3.24%) or RGO photocathode (4.54%). What’s more, this PCE reached 88% of the Pt photocathode (7.18%) under the same conditions.
Perovskite oxides of strontium ruthenate (SrRuO3, SRO) are often used instead of expensive platinum catalysts to speed up the oxygen evolution process and electro-oxidation in fuel cells. In 2016, Cao and his colleagues reported using sputtering SRO perovskite film as a Pt-free photocathode in DSSCs. This film showed promising electrocatalytic activity for IRR and a PCE that was similar to that of a traditional Pt photocathode. In order to make the electrocatalytic activity fit IRR, the writers used epitaxial strain through lattice mismatch.
As a hybrid photocathode (SRO-GQD), graphene quantum dots (GQDs) were used to cover SRO. The highly porous SRO photocathode made using the hydrothermal method had a larger surface area, better ability to diffuse electrolytes, and better IRR activity compared to the pure SRO photocathode. A high PCE of 8.05% was achieved by the DSSC with an SRO-GQD photocathode, which was much higher than those of SRO (7.16%) and Pt (7.44%) photocathodes.
Wang et al. reported a mechanochemical way to make photocathodes out of perovskite/carbon mixtures in another paper. The PCE of the LaNiO3/CB photocathode that was put together by the LaNiO3/CB photocathode was similar to that of Pt (8.11%), which was much better than when LaNiO3 or CB were used as photocathodes alone. Making a highly linked LaNiO3/CB hybrid greatly enhanced the ability to move charges, which added to the good IRR activity of LaNiO3/CB.
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