Perovskites, these remarkable materials at the forefront of scientific discovery and renewable energy innovation, have truly captivated the minds of some of the world’s top scientists and engineers. These materials possess an incredible potential to harness more solar energy than almost anything else, and at a considerably lower cost compared to conventional silicon solar cells. While we’ve made impressive strides in perovskite solar cell research, there remain significant challenges on our path.
In the coming year or two, we may see perovskite products entering the market, thanks to the relentless efforts of numerous researchers. Thus, it’s imperative to get acquainted with them now. Despite decades of research and development in silicon solar cells, their improvements in efficiency have been rather gradual. To meet the surging global energy demands, projected up to 2030 and beyond, scientists are actively seeking alternative technologies to traditional silicon photovoltaics including cutting-edge solar PV technology.
Solar cells are generally categorised into different generations based on the materials they employ for synthesis, applications, and commercial viability.
Presently, first-generation silicon technology dominates the global photovoltaic landscape, constituting around 90% of installations. The highest recorded laboratory efficiency for silicon solar cells stands at approximately 26.7% for individual cells and about 22% for modules.
While these silicon-based devices are known for their reliability and robustness, they grapple with the drawbacks of high production costs and integration into solar modules.
Second-generation solar cells primarily involve amorphous and thin-film technologies, including amorphous silicon, gallium arsenide (GaAs), cadmium telluride (CdTe), and copper indium (di) selenide (CIGS), achieving a performance level of roughly 23% (NREL, 2020) and lower production costs. However, their limited availability of materials restricts their scalability, although they do find niche applications in industries such as flexible electronics.
Third-generation solar cells, relying on nanostructured materials fabricated through cost-effective methods, have garnered considerable attention due to simplified production processes and the ready availability of materials, rendering them cost-competitive.
One standout candidate that promises a substantial leap in efficiency is the relatively recent exploration of perovskite materials. They are emerging as a compelling alternative due to their cost-effectiveness and impressive efficiency levels. Perovskites hold the potential to revolutionise solar panels, making them easily deployable on various surfaces, including flexible and textured ones. These materials are expected to be lightweight, economical to produce, and as efficient as today’s leading photovoltaic materials, primarily silicon.
Perovskites derive their name from the Russian mineralogist Lev Perovskite and belong to the mineral category of calcium titanium oxide (CaTiO3). In practice, any crystals with structures of the form AMX3 are classified as perovskite materials, and the ideal perovskite crystal structure is cubic.
Perovskite solar cells function by absorbing sunlight, utilising the energy from photons to excite electrons. This absorption process results in the elevation of an electron from the valence band edge (or highest occupied molecular orbital, HOMO) of the perovskite sensitiser to its conduction band edge (or lowest unoccupied molecular orbital, LUMO). This leaves the perovskite in an oxidised state, which is neutralised by an electron moving from the HOMO of the adjacent hole transporting layer.
The excited electron in the LUMO of the perovskite is then injected into the LUMO of the electron transporting layer (ETL) and transported through diffusion to the front contact. The energy levels are strategically aligned to enable an electron from the valence band edge of the perovskite to be excited to the conduction band edge, leaving a hole in the perovskite. Another electron from the HOMO of the hole transport layer (HTL) can fill this hole, generating an electric current through the movement of electrons and holes in a hopping fashion. The HTL facilitates the extraction of holes from the perovskite layer into the external circuit and also serves as an electron-blocking layer to prevent electron passage.
The critical process of separating electron-hole charges occurs at the interfaces of different layers, with electrons and holes transported through electron and hole-selective conductor layers, respectively.
A typical perovskite solar cell comprises several layers: an n-type compact layer, a mesoporous oxide layer, a light-harvesting perovskite layer, a hole-transporting layer, and two electrodes.
Despite the promising strides made in enhancing efficiency, perovskite materials do face certain limitations that have slowed down their commercialisation. Because they contain organic cations, they are susceptible to moisture, temperature fluctuations, UV radiation, and exposure to oxygen. These factors can lead to a degradation of solar cell performance over a relatively short period. Some reports indicate maximum stability values of just over 1000 hours.
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