Organic/inorganic metal halide perovskites attract substantial attention as key materials for next-generation photovoltaic technologies due to their potential for low cost, high performance, and solution processability. The unique properties of perovskites and the rapid advances that have been made in solar cell performance have facilitated their i. Over the past decade, metal halide perovskites with the chemical structure ABX3 (A = methylammonium (MA), formamidinium (FA), or cesium (Cs); B = Pb, Sn; and X = I−, Br−, or Cl−, or combinations thereof) have emerged as promising photovoltaic (PV) materials due to their extraordinary optical and electrical properties such as high absorption coefficients, low exciton binding energy, bandgap tunability, ambipolar transport characteristics, excellent charge-carrier mobilities, long charge-carrier lifetimes, long carrier diffusion lengths and high defect tolerance1,2,3. These remarkable properties have underpinned the rapid development of PV devices based on perovskite absorbers, which is illustrated by the improvement in power conversion efficiencies (PCEs) from 3.8% to 25.7%4. This significant advance in PV performance has placed perovskite solar cells (PSCs) in the front-of-line for realizing next-generation low-cost PV and integrated technologies. PSCs are slated to hold several advantages over established and emerging PV technologies. For instance, silicon solar cells require pure silicon, produced by heating sand at elevated temperatures (>1000 °C), have complicated manufacturing processes (e.g., texturing, anti-reflective coatings) that are usually carried out using special facilities, and greenhouse gases in their fabrication, all of which add to the fabrication cost. In contrast, perovskite materials can be solution processed, enabling low-embedded energy manufac. The PCEs of single-junction PSCs are approaching the maximum of 25.7% under one sun illumination. Further enhancing the PCE to the theoretical Shockley–Queisser limit (~33%), requires the thermalization of high-energy carriers and photon transmission losses to be reduced8. In order to minimize these energy losses and overcome the Shockley–Queisser limit for a single junction device, designing multiple junctions (tandem or greater solar cells) composed of a wide-bandgap absorber (top layer) and a low-bandgap absorber (bottom layer) have been proposed and implemented9. Such a device configuration allows absorption of the fraction of incident photons with energy higher than the wide-bandgap absorber, while the low energy photons pass through to the bottom subcell where they are harvested by the low-bandgap active layer10. There are two general structures for tandem devices—two-terminal (2 T, also called monolithic) and four-terminal (4 T) tandem solar cells (see Fig. 2). In the former, a single substrate is used to construct both subcells (stacked together with an interconnection layer) with a transparent front electrode and a non-transparent back electrode. In the latter case (4 T), two separate cells are fabricated individually and then physically connected together to form a full device. Due to the lower fabrication cost of the 2 T architecture (i.e., only two electrodes are involved and no extra external circuit is required) and the absence of a physical gap between the two connected subcells, whic. Stability of perovskite solar cellsThe long-term stability of PSCs represents a key obstacle for their commercial deployment. Perovskite materials typically used in solar cells have been shown to be unstable when exposed to oxygen, water, heat, and light. In addition to these external factors, some studies have also shown that the inherent properties of perovskite materials, such as ion migration and low defect formation energy, play a significant role in the rapid decomposition of perovskite films. With 25 years of an outdoor operational lifetime required by the marketplace, PSCs currently lag behind this target. To overcome the stability issues, many strategies have been developed, such as compositional engineering, interface engineering, and surface/bulk defect passivation67,68,69,70,71,72. For instance, by fully or partially substituting the highly volatile A site MA cation with formamidinium (FA) and/or cesium (Cs) in the perovskite, the device stability was found to be enhanced73. Another strategy involves the incorporation of various materials, such as 1D and 2D materials1,74, polymers75, and fullerene derivatives76 into the perovskite film to passivate its defects and hence improve the overall device stability. The replacement of doped HTMs with undoped ones is also a promising route to prevent ion migration and interaction between the dopants and perovskite, bu. Electricity-generating solar panels are generally mounted on the building rooftops. However, PV systems can be building-integrated (BIPV) and are increasingly employed in new ways during the construction of buildings. BIPV includes inclusion of panels on or as parts of the building envelope such as the windows, skylights, exterior walls, or facades. The requirements for BIPV are therefore different for rooftop-mounted systems with a greater emphasis on the optical properties, such as color and transparency, weight and form factor. In addition to color tunability, PSCs can be fabricated on transparent, conductive and flexible substrates, making them attractive for BIPV applications. In the context of perovskite-based BIPV, three major categories have been developed and explored. The first category is semitransparent PSCs, which have been explored for use in building windows and glass roofs. The second category of perovskite-based BIPVs are colorful PSCs, which can be applied in building fences, walls and car park roofs. The third category is smart PV windows (SPWs), which are dual-functional BIPVs created by combining the solar harvesting function with electrochromic/thermochromic functionalities. SPWs are interesting in that they can harvest sunlight to produce electricity while blocking sunlight entering the building on a hot summer day (Fig. 5a). However, before BIPV becomes widespread there are manufacturing and performance related challenges to be addressed84,85,86,87. In the following sub-sections,.
