Photovoltaic panels are a big part of the pressing need to fight climate change by making low-carbon energy. For this clean technology to get better, a lot of money needs to be spent. Photovoltaics (PV) study has led to the improvement and commercialisation of many PV technologies over the years. In some parts of the world, PV technologies are now the best way to make energy. Most photovoltaic technology is still built on silicon-based solar cells, but it is still too expensive to compete with fossil fuels.
Scientists are very interested in perovskite solar cells (PSC), which have been claimed to have an efficiency of 24.2% and are the fastest-growing photovoltaic technology. The conversion efficiency is higher than 22.3%, which is the efficiency of multicrystalline silicon solar cells. Other PV rivals, like cadmium telluride (CdTe), gallium arsenide (GaAs), and copper indium gallium selenide (CIGS), have also reported improvements.
But it is very hard for perovskite technology to go from lab-scale cells to 25-year stable solar modules. This is because it needs to make the manufacturing processes bigger and come up with new effective module systems. The cost of making commercially available solar panels can be cut down to less than half of the total cost of installing solar power at a utility level.
To sum up, raising the amount of power produced per module area makes sense if you want to lower the general cost of making electricity and get past the problem of fighting with fossil fuels.
Multi junction solar cells
For semiconductors to receive light, the energy from the light has to be changed into the excitation of electrons from the valence band to the conduction band. This light’s energy should be the same as or higher than the bandgap (Eg) of a semiconductor. The light that hits the bandgap with more energy than it needs will be lost through thermalisation. There is a little bit more than 30% conversion rate for single junction solar cells that are made in the traditional way.
Other PV devices with multiple junctions have a lot of promise and can achieve as high as 32.8% efficiency for double junctions and 37.9% efficiency for triple junctions III-V multijunctions. The most efficient multiple junction devices have been found to be 46.0% when exposed to intense sunlight. This shows how useful these devices are.
Tandem devices can be made in two main styles: the four-terminal (4T) device has top and bottom cells that are connected to the outer circuit through their own cathode and anode electrodes; or the two-terminal (2T) architecture uses a single piece of tandem cells. For each cell, these gadgets show different amounts of electrical and visual independence.
The four-terminal (4T) device is made up of top and bottom cells that are physically stacked on top of each other. The top cell is made of a material with a high bandgap. In a different method, they are not always stacked on top of each other, and a light filter will be used to control how much light is absorbed. In Silicon/Perovskite cells, a 4T device with this beam splitter design worked 28% of the time.
Two solar cells are joined in series to make a monolithic tandem cell, which is also called the two terminal (2T) design. The conditions for the layers’ manufacturing process and the treatments that go with them must be right for the layers that have already been placed and not damage them. There is a recombination layer (or tunnel junction) between the top and bottom cells, which means that only two external wires are needed to get power from the cells.
Making solar cells with this design is becoming more popular because it has benefits like lower prices and better performance due to lower optical and resistance losses. But it’s still hard to make technological progress in the ways that different layers are made. It can be hard for the lower layers to follow the same steps that were used to make the upper layers if the top layers of perovskite and electrical connections are solution processed.
The perovskite materials used in solar cells can separately turn sunlight into electricity, which makes them perfect for use in tandem. The upper and lower joints of 2T devices are linked together in series. This means that the total voltage of the devices is equal to the sum of the two sub cells. To get the most out of the device, the different layers must be carefully chosen so that the current flows through the whole thing.
It was methylammonium lead(II) iodide (MAPbI3) that made the first perovskite material that worked well in a device, with a bandgap of 1.6 eV. By changing the chemical makeup, it is easy to change the bandgap of methylammonium lead-based perovskite. The band gaps in MAPb(I1-xBrx)3 are between 1.6 eV for pure iodide and 2.2 eV for pure bromide.
Perovskite is a great material for tandem uses because it can be used on top of silicon or thin film solar cells, or it can be used by itself as an all-perovskite tandem device. People are more interested in the first choice since the mixed Sn-Pb perovskite with a small bandgap was discovered and can be used as a bottom sub-cell.
PSC-based multijunction solar cells have shown promise for the future, but problems like picking the right bandgaps for the top and bottom cells to get the best light harvesting can be seen. When choosing the different types of makeup for the different layers in a widely stable product, different factors for tandem cells need to be taken into account. The top cell must be able to let light through with an energy level lower than the light absorber’s band gap. The absorber layers must be able to absorb the most light, and there should be the least amount of reflection in all layers or interfaces. The material must also pass a stability test that can be used for 25 years and be made of non-toxic, earth-abundant elements.
Transparent conductive contact
Transparent conductive oxides (TCO) are very important for optoelectronic systems that need to have high transfer and good conductivity. Tandem devices with two terminals only need one clear wire, but devices with four terminals need three. The way that TCO layers are made must be compatible with how high-quality perovskite film is made. To make clear electrodes, different methods have been used, such as blasting, spin coating, or mechanical transfer of Ag nanowires, carbon nanotubes, and graphene networks. But the fact that sparking uses a lot of power has made people worry that it might hurt the perovskite layer.
A number of changes are being made to find the best method for making clear TCOs, including indium tin oxide (ITO), aluminium doped zinc oxide (AZO), and indium zinc oxide (IZO). ITO is used a lot in optical devices because it works well, is clear, and conducts electricity well. The most common way to make the ITO layer is through magnetron sputtering. A dielectric/metal/dielectric stack, like MoOx(10 nm)/Au(8.5 nm)/MoOx (10 nm), is another choice. This choice does have some problems, though, like absorbing a lot of infrared light and iodine in the perovskite reacting with MoOx.
Recombination layer
When two 2T devices are connected, the recombination layer acts as a link between the upper and bottom cells. It makes sure that electrons and holes can combine efficiently while minimising voltage loss and keeping clarity. It is very important to make recombination layers that are suitable, have low losses, and don’t respond with neighbouring layers. ITO has been used in 2T joints with a 23.6% efficiency, showing good conductivity and clarity. We used a nanocrystalline hydrogenated silicon (nc-Si:H) recombination junction as a recombination layer on the front side of double-sided textured SHJ bottom cells. It worked better than ITO because it fit better with the other layers and the textured bottom cells.
Perovskite tandem devices
Because perovskite single junction devices have gotten better and more people know about other PV technologies, perovskite tandem devices have become more efficient faster than perovskite single junction devices. Perovskite/silicon tandem solar cells are the most well-known example of a system that has been made. Combining perovskite with CIGS solar cells is another choice. This is thought to be the fastest way to get perovskite into industrial use. All of the perovskite tandem devices were launched in 2016, though, so more study into the materials and method development is needed to fully use these technologies to their fullest potential. This review gives you an idea of these choices and how they might be used in the real world.
Theoretical calculations on the potential of perovskite tandem
Theoretical work on tandem devices, which try to absorb light more efficiently by connecting different light absorbers, has been going on for decades. A lot of research has been done on perovskite tandem devices, mainly aimed at improving things like optical losses, reflective losses, thickness limits, and film resistance. Figure 11.14 shows the potential efficiency of 2T and 4T devices. The highest efficiencies are shown in red and dark red areas, which are double junction devices. When it comes to bandgap choices, 4T tandem devices are more open than 2T devices because they don’t need to match the current. In 2T and 4T devices, the choice of bandgap for the top and bottom cells determines the highest conversion efficiency. This is based on the assumption that there are no absorption losses.
Perovskite/silicon tandem devices
Over the years, perovskite tandem devices have been made. The first ones used MAPbI3 as the top cell, which has a bandgap of 1.6 eV. This is not the best bandgap for a top cell to match with a bottom cell that has a 1.1 eV band gap, though. Bailie et al. used a silver nanowire semi-transparent electrode for the perovskite top cell and connected it to a multicrystalline silicon device with a 11.4% efficiency. This made the tandem device 17% more efficient overall. It was the same perovskite semitransparent top cell that was used to make a 17% unprocessed CIGS device 18.6% efficient when used together.
Werner et al. wrote about a perovskite/silicon tandem device that used a texture on the back of silicon hetro-junction bottom cells to improve the NIR response. It was a low-temperature planar near-infrared (NIR) clear perovskite solar cell with a 0.25 cm2 active area that converted 16.4% of the light it saw. It was possible to make very efficient devices with a manually stacked 4-terminal perovskite/SHJ tandem setup, and these devices had an efficiency of up to 25.2%.
You should have B1.8 eV for the top cell and 1.1–1.2 eV for the bottom cell for a double junction cell to work well. Caesium and formamidinium mixed perovskite FA0.83Cs0.17Pb(I0.6Br0.4)3 was suggested as a great way to get the right bandgap and solve the main problem of methylammonium loss. These top-notch materials have been used in the newest pair devices.
A top cell made of MAPbI3/TiO2 was used in a 2T design on top of a silicon cell in 2015 to get 13.7% efficiency for 1 cm2 devices. Bush et al. showed a monolithic (2T) that was 23.6% efficient and had a surface of 1 cm2 in February 2017. A mixed formamidinium/cesium lead halide perovskite [Cs0.17FA0.83Pb(Br0.17I0.83)3] was used to make the top perovskite cell. It has a bandgap of 1.63 eV and is more stable than the usual MAPbI3 perovskite. To make it more stable, a nickel oxide layer was used as a hole-transport material.
Sahli et al. showed another way to make highly crystalline perovskite. They used a rough crystalline silicon cell as the bottom cell and a flipped structure to make a very good perovskite. The conformal perovskite layer was made in two steps: first, porous lead iodide and caesium bromide were evaporated together; then, the cation solution (formamidinium iodide, FAI), and formamidinium bromide, (FABr) were spun on top. The last layer of CsxFA12xPb(I, Br)3 perovskite absorber was heated to 150 C in room air.
After all of these changes, a totally detailed monolithic/SHJ tandem device was made that has a current density of 19.5 mA cm22 and a confirmed conversion efficiency of 25.2%.
Perovskite/CIGS tandem devices
When compared to silicon solar cells, using CIGS solar cells as bottom cells has benefits like being able to tune the bandgap and having a smaller area. Ag and MoOx were used by Guchhait et al. as buffer layers for sputtering ITO into semitransparent perovskite solar cells that were going to be connected to CIGS bottom cells. 20.7% of the power was used by semitransparent cells with Ag/ITO and a 4T tandem design. Fu et al. showed a good way to set up perovskite solar cells using a glass base, In2O3:H, PTAA, CH3NH3PbI3, PCBM, ZnO nanoparticles, and a ZnO:Al/Ni-Al grid. This solar cell had a high VOC of 1.116 V and a very high efficiency of 16.1%.
The perovskite top cells that were made showed a high transparency of 80.4% in the 800-1200 nm wavelength range. A 4T tandem device with this perovskite top cell and Cu(In, Ga)Se2 and CuInSe2 bottom cells worked 22.1% of the time and 20.9% of the time. This is a great finding when you realise that the perovskite used was pure MAPbI3 with a bandgap of 1.56 eV. By making the perovskite sheet better, the same group at EMPA reported in late 2017 that the 4T perovskite/CIGS combo device was now 22.7% more efficient.
At first, making the 2T perovskite tandem device was hard because it was hard to make the different layers that went on top of each other. The first paper to talk about a 2T perovskite-based tandem device used MAPbI3 on top of a kesterite Cu2ZnSn(S, Se)4 bottom cell. In 2015, the same group wrote about a successful attempt to make the 2T monolithic tandem gadget work better. A CIGS structure that didn’t contain ZnO could handle being heated to 120 C for several hours without any damage, which was needed to make the top layers.
The Yang Yang group reported that CIGS and perovskite worked well together in a 2T tandem device, resulting in a 22.4% yield. The top layers of the CIGS perovskite cell have been kept (CdS/iZnO/BZO), and an indium tin oxide (ITO) layer that has been smoothed has been used as a buffer. A perovskite cell made of Cs0.09FA0.77MA0.14Pb(I0.86Br0.14)3 and a bandgap of 1.59 eV was used as the best one. It had an average transmittance of 80% between 770 and 1300 nm and a conversion efficiency of 14.83 percent.
Shen et al. reported a huge improvement in the efficiency of the 4T CIGS/perovskite tandem device, which led to a total conversion efficiency of 23.9%. A four-cation perovskite was used, along with a molybdenum oxide/indium doped zinc oxide/metal grid (gold) as the clear front contact.
Perovskite-perovskite tandem devices
An novel option is an all-perovskite tandem device, which offers low-cost ways to make all of the absorber layers. The idea was first put forward in 2014 through a computer study that looked at how to get a high open circuit voltage. In the beginning, this idea was worked on by connecting methylammonium lead bromide and methylammonium lead iodide as the top and bottom layers, respectively. The gadget was only 10.8% efficient, though, because it wasn’t very good at collecting light.
So that they could be used as a bottom cell, tin-based perovskites were picked because they have low bandgap and the same structure as lead perovskites. It is possible to make these Sn-based perovskites using the same steps as for lead-based perovskites, which makes them good options for solar cells. When used in solar cells, methylammonium tin iodide has not worked very well and has not been stable.
In 2014, bandgap setting for mixed lead-tin perovskites was shown to make a more stable low bandgap result than pure tin perovskite. Several studies have shown that using optimised solvent mixes and anti-solvent techniques can help make it easier to make high-quality Sn-Pb perovskite films. Low bandgap Sn-Pb perovskite (1.2–1.3 eV) has been processed to have the same stability and better efficiency as Pb perovskite. This makes them good options to replace silicon or CIGS bottom cells in tandem devices.
One of the hardest parts of making all-perovskite 2T devices is getting the top layer ready using solution processing methods. Most of the time, the solvents used to make solutions break down the perovskite bottom layer or make the film quality worse. Several effective methods have been talked about, such as creating buffer layers made of indium tin oxide (ITO), aluminium doped zinc oxide (AZO), or indium zinc oxide (IZO). Because it is highly conductive, clear, and has the right work function, ITO is used in many optoelectronic devices.
The basic process of making 4T all perovskite tandem devices is now easy, but making the low bandgap bottom cell is still hard. The first 2T all-perovskite devices were made with a 1.6 eV MAPI3 bottom cell, which is much wider than the best bandgap. Using mixed Sn-Pb perovskite layers with a bandgap of 1.2 to 1.4 eV has been necessary to make devices that work very well.
In another groundbreaking study, Forga¬cs et al. used a top layer made of solution-processed Cs0.15FA0.85Pb(I0.3Br0.7)3 with a band gap of 2 eV and a back cell made of vacuum-vapor-deposited MAPbI3. Together, these two layers gave the battery a total efficiency of 18.1%. This study also showed that vapour deposition methods could be a good alternative way to make perovskite films in layered devices in the future.
Outlook
This part talks about the possibilities of perovskite tandem devices, which are better than double junction devices in terms of their Shockley-Queisser efficiency limit. Triple junction perovskite devices have a slightly higher efficiency limit (46.7%) than double junction devices (46.0%). This means that the extra cost of adding an extra junction is not as interesting from a business point of view. Because silicon has a 1.1 eV bandgap, putting a double junction perovskite on top of a silicon hetero junction can get up to 35.3% efficiency, which is more than double junction perovskite.
There are more layers in tandem devices than in single junctions, which makes visual and electrical losses worse. Reflective optimisation of surfaces and widths of compositional layers can be used to cut down on these losses as much as possible. Electronic losses happen when charge carriers get stuck or join back together with the ground state before they can be extracted. To get rid of these loses, new features are planned for the making process, like structures with different textures.
As perovskite tandem devices have improved quickly, their efficiencies have been said to be higher than those of single junction devices and higher than the silicon record efficiency of 28%. In the next few years, efficiency should rise to 30%, as planned. The development of perovskite solar cells has made the PV market bigger and opened up new ways to improve technologies that are already on the market through tandem devices.
Concluding remarks
Perovskite Solar Cells (PSCs) are a potential new thin film solar technology that aims to meet the growing need for carbon-free energy around the world. Research on perovskite solar cell materials has changed quickly since solid state PSCs were introduced in 2012. This has led to a quick rise in power conversion efficiency (PCE). The approved PCE for solar-to-electricity right now is above 25%, which is high enough for business use. But there are still worries about long-term safety, upscaling, and possible legal problems with chemicals that contain lead.
Small study cells with PCE for PSC are better than those with polycrystalline CdTe, CIGS, and polycrystalline silicon. However, they are still not as good as high quality single crystal silicon (26.1%) and silicon heterostructured solar cells (26.7%). When it comes to future PV technology, there are a number of reasons to look beyond silicon technologies. These include using less material, using less energy during the manufacturing process, and having the benefit of a material class with tuneable band gaps for color-matched architectural integrations or making high-efficiency thin film tandem devices.
A lot of research is being done on compositional engineering using low and high bandgap perovskite materials, compositions with groups that keep moisture away or contact materials that can handle halides, and/or using two-dimensional intermediate thin layers at surfaces. For combining and working with other materials in tandem heterojunctions, it’s important to have a basic understanding of both the bulk properties and the properties at the surfaces.
For solar cell uses, material engineering depends on using the right methods to describe or measure structure and qualities based on theoretical models. The ways that perovskite solar cells are characterised come from dye-sensitized and thin film solar cell technologies, as well as the ways that oxide- and fluoride perovskites are normally characterised. However, not all methods can be used in the same way because of the unique qualities that come from organic counter ions, temperature stability issues, charge buildup, and ionic movement on the nanoscale with time scales that overlap.
The physical properties and characterisation methods used for perovskite solar cell materials and devices are explained in this book. The book focusses on the pros and cons of these methods for studying material properties and figuring out how structure affects function in devices. Lead halide perovskite solar cell materials are soft and have a low energy barrier for defect movement. This means that defects can move around easily when the device is running. The flow of ionic species depends on light and temperature, which causes them to build up at surfaces and break down in the film and at interfaces.
In most lead halide perovskite materials, the number of harmful defects is not very high. However, ionic movement, photo-induced structure changes, and the creation and destruction of defects under light can make voltage decay or photoluminescence decay more difficult. These effects leave unique marks on the way a solar cell works, and the time it takes to happen depends on the chemicals that make up the perovskite material and how much light hits it over time. These marks are called Stark effects, Burstein Mott effects, and hysteresis effects.
To figure out how well perovskite solar cells convert light into electricity, we need accurate measurement methods. Here, we present Stark spectroscopy as a powerful way to study how ions move in perovskite solar cells. It is also important to have protocols for long-term stability tests in order to compare the stability of different perovskite mixtures and the materials that touch them. Spectroscopies that are sensitive to the surface, such as Raman and X-ray photoemission spectroscopies, can tell us a lot about the perovskite device’s surface composition during degradation processes and more complex events like crystalline reformations and interactions with contact materials while the device is functioning.
As progress is made in materials engineering and device optimisation, it will be easier to compare materials made by different groups. This will require a better understanding of basic processes and the creation of standard characterisation procedures. The book is an overview of current work that has been done on studying and characterising perovskite solar cell materials and systems in order to move the field forward for future uses.
Leave a Reply