Perovskites, a category of materials at the cutting edge of scientific discovery and renewable energy development, have captured the imaginations of some of the world’s greatest scientists and engineers. These extraordinary materials have the potential to generate more electricity from the sun than nearly anything else, and at a significantly cheaper cost than regular silicon solar cells. Perovskite solar cell research has come a long way in a short time, but there are still some significant obstacles to overcome.
Some perovskite products may be available in the market within the next year or two as a result of the efforts of many determined researchers, so it’s critical to learn about them now. Although there has been considerable research and development in silicon solar cells over the past decades, their efficiency is incremental. In order to meet the world energy demands rising to 2030 and beyond, researchers are looking for disruptive alternative technology to conventional silicon photovoltaics. Solar cells are, in general, categorized into different generations based on the materials required for synthesis, applications, and commercialization. First-generation silicon technology currently dominates the world photovoltaic market claiming ~90% of the installation. The best laboratory record power conversion efficiency (PCE) for silicon solar cells is ~26.7% for single solar cells and ~22% for modules.
These devices are reliable as well as robust in nature. But they suffer from the drawbacks of the high cost of production as well as implementation into solar modules. Second-generation solar cells involve primarily amorphous and thin-film technology such as amorphous silicon, gallium arsenide (GaAs), cadmium telluride (CdTe), copper indium (di) selenide (CIGS) with performance ~23% (NREL, 2020), and lower production costs. But the availability of materials makes them less attractive for large-scale production purposes. However, second-generation thin-film solar cells have a niche market in certain industries such as flexible electronics applications. Third-generation solar cells which are based on nanostructured materials from low-cost manufacturing techniques have attracted more attention due to the simplified fabrication process, and availability of materials and hence, are cost-competitive.
One of the prime candidates with a vertical increase in efficiency is the recently explored perovskite materials which have emerged as a promising alternative due to their cost-effectiveness and high efficiency. Perovskites hold promise for creating solar panels that could be easily deposited onto most surfaces, including flexible and textured ones. These materials would also be lightweight, cheap to produce, and as efficient as today’s leading photovoltaic materials, which are mainly silicon. Perovskites are a mineral of calcium titanium oxide CaTiO3 named after the Russian mineralogist Lev Perovskite. In practice, all crystals having structures of the form AMX3 are classified as perovskite materials. The ideal perovskite crystal structure is cubic.
General Working Principle of Perovskite Solar Cells:
The perovskite layer absorbs sunlight and the energy in a photon is used to excite an electron. This absorption is manifested as an electron being excited from the valence band edge (or highest occupied molecular orbital, HOMO) of the perovskite sensitizer to its conduction band edge (or lowest unoccupied molecular orbital, LUMO) leaving the perovskite in an oxidized state which is neutralized by an electron moving from the HOMO of the adjacent hole transporting layer.
The electron excited to the LUMO of the perovskite is then injected into the LUMO of the ETL and is transported via diffusion to the front contact. The energy levels are thermodynamically aligned in such a way that when an electron from the valence band edge of the perovskite is excited to the conduction band edge, it leaves behind a hole in the perovskite, then another electron from the HOMO of the HTL can fill up its place. Thus, an electric current is generated by the movement of electrons and holes in a hopping manner. The HTL allows the holes extracted from the perovskite layer to pass through and they are extracted into the external circuit. The HTL also functions as an electron-blocking layer and prevents any electron from passing through. The important function of electron-hole charge separation thus occurs at the interfaces of different layers and the electrons and holes are transported through electron and hole-selective conductor layers respectively.
Structure of Perovskite Solar Cells:
An archetypal PSC comprises an n-type compact layer, a mesoporous oxide layer, a light-harvesting perovskite layer, a hole-transporting layer, and two electrodes. The generic structure of a PSC and the different layers are deposited as indicated stepwise.
Step 1: The Fluorine doped Tin Oxide (FTO)/Indium doped Tin Oxide (ITO) coated glass, acts as the substrate for the photoanode of the perovskite device.
Step 2: Above it, there is a dense layer of semiconducting material, primarily TiO2, which functions as the hole-blocking layer or compact layer, deposited usually by spin-coating or spray-coating on top of the FTO substrate. It prevents the holes extracted by the electron-selective layer above from coming into contact with the FTO/ITO glass and inhibits recombination losses.
Step 3: Next is the ETL which facilitates diffusion of the electrons from the photoexcited perovskite layer into the FTO/ITO glass and thus to the external circuit.
Step 4: The perovskite layer which can act either as a sensitizer or absorber or as an electron or hole transporter, although its primary function is that of a sensitizer, is spin-coated over the electron transporting layer.
Step 5: Adjacent to the perovskite layer is the hole transport layer which allows the holes from the excited perovskite to move toward the metallic cathode for extraction.
Step 6: Finally, there is a metallic contact layer which is usually deposited by thermal vaporization on top of the solar cell to function as the counter electrode, also known as back contact.
Challenges and Problems With Perovskite Solar Cells:
Though perovskite materials show promising results in terms of improvement in efficiency, they do suffer from some drawbacks which have retarded their commercialization. Since they are formed of organic cations, they are susceptible to moisture, temperature, UV radiation, and oxygen thus, deteriorating the performance of solar cells within a short time span. There are reports of maximum stability values of just over 1000 h.
Perovskite solar cells are a type of photovoltaic cell that has shown great promise for use in solar panels. They have a number of advantages over traditional silicon solar cells, including lower cost, higher efficiency, and greater flexibility in their design and production. However, there are also challenges that need to be addressed, such as the stability and durability of perovskite cells over time. Research on perovskite solar cells is ongoing, and it is expected that they will play a significant role in the future of solar energy.