Functional Thin Films for Perovskite Solar Cells

Solar cells are considered one of the promising renewable energy sources for the rising global energy demand. Metal halide perovskites have shown great promise as photovoltaic materials for unique applications, such as building-integrated technologies and electric vehicles due to their flexibility and tunable bandgaps. More importantly, during the past few years, the solar to electric power conversion efficiency (PCE) of metal-halide perovskite solar cells has rapidly increased from 3.9% to 25.5% through material development, device optimization, and interface engineering [1,2]. Despite such a great achievement, to enter into the photovoltaic market, there are many remaining challenges related to various economic and technical issues, including toxicity, operational stability, and scalable fabrication et al. [3].

Amongst different kinds of approaches that are in use in material fabrication, processing, and devices, interfacial engineering with multifunctional purposes is extremely popular to improve device efficiency [4]. The prime reason for this is due to the fact that surface and interface engineering can reduce the defect density and non-radiative recombination of photogenerated carriers in the perovskite films. Furthermore, proper surface passivation by employing organic molecules with a hydrophobic side chain can improve the device’s stability in many folds. For example, benzylamine-modified formamidinium lead iodide perovskite (FAPbI₃) films based solar cells were found to have not only an improved open-circuit voltage from 1.0 V to 1.12 V but also there was an enhancement to the moisture stability of these perovskite films from three days to more than four months [5]. Additionally, the organic passivation layer potentially provides an effective barrier for ion migration and minimizes damage to materials and devices [6].

Long-term operational stability is an essential parameter to determine the commercialization of perovskite solar cells (PSCs), which can be evaluated by the device lifetime under continuous 1-sun illumination with an electric load, especially the maximum power point (MPP) tracking. It is still a challenge to obtain a satisfactory operation lifetime of PSCs compared to that of silicon solar cells. The reason is due to the relatively poorer intrinsic stability of the perovskites, as the soft ionic lattice feature with weak hydrogen bonds and van der Waals interactions would cause ion migration, phase segregation, and photo degradation et al. [7]. Besides perovskite layers, the air stability of the hole transport layer (HTL) or electron transport layer (ETL) also needs to be considered, particularly for the organic transport layer [8,9]. One typical example is Spiro-OMeTAD-based HTL, which relies on hygroscopic and volatile dopants to improve its conductivity. Although the past decade has witnessed the intensive development of hole transport materials (HTMs) for PSCs, for example, CO₂ gas-forming treatment and hydrophobic fluorinated analogs of Spiro-OMeTAD [10,11], there is still a trade-off between high efficiency and high stability [12].

Besides the stability, the toxicity of Pb in perovskites is another challenge to their practical applications. It would be perfect if we can find a less toxic perovskite to replace Pb-based ones even though the amount of Pb in final devices is not too great. One of the attractive alternative candidates is Tin(Sn)-based perovskites, as they possess the same crystal structure as Pb-based perovskites and excellent optoelectrical properties [13]. Sn-based perovskites have shown a PCE up to over 14.0% [14], much higher than other alternative lead-free perovskites. Despite these promising developments, a key challenge with the state-of-the-art Sn-based perovskites is that they are easily oxidized by O₂ [15], even though the stability has been greatly improved by the use of reducing agents, for example, hypophosphorous acid (H₃PO₂) [16], tin halides (SnF₂ and SnCl₂) [15], and anilinium hypophosphite [17].

Halide double perovskites with the formula of A₂N⁺M³⁺X₆, where toxic bivalent Pb²⁺ cations are replaced by a combination of non-toxic monovalent and trivalent cations, are a new generation of non-toxic and stable perovskites [18]. They also exhibit attractive properties suitable for solar cells, such as low exciton binding energy and long carrier lifetimes [19]. In addition, they offer immense opportunities in terms of combinatorial chemistry: a straightforward multiplication of the numbers of available elements for A⁺, N⁺, M³⁺, and X⁻ yields a stunning 4080 combinations. The efficiency of devices is limited by poor absorption properties, low-dimensional electronic structure, and short carrier diffusion lengths in the films [18]. Very recently, by using a hydrogenation method, the bandgap of Cs₂AgBiBr₆ films could be tuned from 2.18 eV to 1.64 eV. The PCE of hydrogenated Cs₂AgBiBr₆ perovskite solar cells can reach up to 6.37% [20]. Another attractive approach to improve the absorption properties is the formation of metal ions doped or alloyed double perovskites. However, it is still a challenge to dope or alloy metal ions into double perovskite films [21].

Overall, perovskites are promising as the new generation of photovoltaic materials with high efficiency. There is still room for tailoring charge carrier recombination to increase PCE. The major challenge is still the long-term stability, particularly for operational stability. Even though the amount of Pb in final devices is not too much, it would be perfect if a less toxic element can be found to replace the Pb. The main challenge for the current Pb-free materials that remains is the low PCE. This Special Issue aims to provide a forum for researchers to share the latest research findings and to develop new ideas and research solutions for perovskite solar cells.