Applications and functions of rare-earth ions in perovskite solar cells

Lead halide perovskites are a type of semiconductors with cubic crystal structure of ABX₃ (Fig. 1), where A is a monovalent cation in terms of methylammonium (MA⁺), formamidinium (FA⁺) and Cs⁺, B is a divalent metal ion of Pb²⁺ or Sn²⁺, and X is a monovalent anion of I⁻, Br⁻ or Cl⁻. The perovskites present some promising optoelectronic properties, such as high light absorption ability, tunable bandgap, ambipolar charge transport, long electron/hole diffusion lengths, cost-effective solution processability and high charge mobility.^[8] The perovskite solar cells were derived from dye-sensitized solar cells, which was initially developed by Grätzel et al.^[9] In 2009, Kojima et al.^[1] firstly replaced the light absorbing organic dyes with the perovskites of MAPbBr₃ and MAPbI₃ in the dye-sensitized solar cells, resulting in the first solar cells based on perovskites with power conversion efficiency of 3.8%. However, the perovskites are easily corroded by the liquid electrolytes in the perovskite-sensitized solar cells, leading to extremely poor stability. Until 2012, Kim et al. used the solid state hole transport material of spiro-OMeTAD to replace the liquid electrolytes, resulting in the so-called perovskite solar cells with simultaneously improved efficiency and stability.^[10] Since then, the perovskite solar cells have obtained aggressive research attention with yearly thousands of publications, and the highest certified efficiency has recently reach up to 25.5%,^[11] not far from the efficiency of crystalline silicon solar cells.

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In a typical perovskite solar cell, the photosensitive perovskite layer is sandwiched between a transparent front electrode and a reflective back electrode. In general, charge transport layers such as electron transport layer (ETL) and hole transport layer (HTL) are inserted between the cathode/perovskite or perovskite/anode interfaces, respectively, to facilitate the majority charge collection at the respective electrode. In some cases, a mesoporous layer is inserted as a scaffold layer to enlarge the contract area between the perovskite and charge transport layer. According to the layer sequence and morphology structure of the charge transport layer, the perovskite solar cells can be classified into four types such as regular mesoporous structure, regular planar structure, inverted mesoporous structure and inverted planar structure as demonstrated in Fig. 2.

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Under sunlight illumination, the photons with energy over the bandgap of the perovskite can be captured, and the electrons can be excited from the ground state to the conduction band, leaving the positively charged holes retained at the valence band orbits. Owing to the small binding energy, the electrons can overcome the attractive force of the holes and transport to the cathode with the favor of ETL. In the meanwhile, the holes can transport across the HTL and be collected at the anode. The specific energy band structure diagram and charge transport dynamic of the perovskite solar cells are clearly demonstrated in Fig. 3.

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The golden triangle of photovoltaic technologies contains production cost, power conversion efficiency and long-term stability, as shown in Fig. 4. Even though the solar cells based on perovskites have achieved rapid progress in the last decade, the pursuit of highly reproducible, efficient and operationally stable perovskite solar cells is still the main target for their further development towards practical applications. The recent researches have been mainly focused on the development of strategies to fabricate high quality perovskite thin films, synthesis high mobility and energy level matched charge transport materials, design environmentally friendly manufacturing methods,^[8] construct optical architectures to maximize light absorption.^[12–15] However, the perovskite materials are vulnerable to various aging stresses such as oxygen, moisture and ultraviolet (UV) light, seriously restricted their long term stability.^[16–19] The penetration of oxygen and moisture into the perovskite layer can be effectively prevented by introducing extra encapsulating layers,^[20,21] while the UV light illumination is inevitablev for all photovoltaic devices, which is prone to decompose the perovskite materials and cause the device performance degradation. Therefore, strategies to efficiently block the UV light are highly demanded to protect the device from the high energy light damage.

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Moreover, it was realized that some unintentional defect states are prone to be generated during the perovskite growth, which can act as charge recombination centers to cause unnecessary energy loss and accelerate performance degradation. And also, energetic barriers due to the mismatched energy level between the perovskite and charge transport layers can induce charge accumulation at the interface and prohibit the charge transport and extraction. Even though the perovskites exhibited high tolerance on the defect states, the presence of the defect states and high density charge accumulation can cause limited power conversion efficiency, seriously hysteresis and performance degradation.

In addition, the solar spectrum of AM1.5G is composed of ultraviolet (UV) light, visible light and infrared light ranging from 280 nm to 2500 nm. To improve the power conversion efficiency of solar cells, the light harvesting should be maximized in order to take advantages of the full solar spectrum. However, the perovskites can only absorb photons with energy higher than their bandgaps, most of the photons at near infrared (NIR) wavelength region cannot be captured.^[22] Even though the bandgap is tunable by varying the chemical composition of the perovskites, but the most efficient perovskite solar cells can only absorb a small portion of solar spectrum in the ultraviolet and visible light ranges.^[23] In particular, state-of-the-art efficient perovskite solar cells usually employ perovskites in terms of CH₃NH₃PbI₃ (MAPbI₃), CH(NH₂)₂PbI₃ (FAPbI₃) or their mixtures as the photoactive layers.^[8] The absorption of these perovskites can cover the entire visible light spectrum range with a cutoff at ∼ 850 nm, which is recognized as one of the best light absorbing materials. Even though the solar energy in the NIR region occupies more than half of the total solar energy, it cannot be captured by the perovskites. Therefore, the energy loss in NIR light resulted in limited efficiency of perovskite solar cells.^[24]

Therefore, in order to achieve high performance perovskite solar cells, the aforementioned issues such as the high-energy light induced performance degradation, non-radiative recombination and mismatch between the perovskite absorption and solar spectrum are the remaining challenges that need to be urgently addressed.

Metal oxide semiconductors in terms of TiO₂, SnO₂, NiO_x are the most frequently employed ETLs, HTLs or scaffolds in perovskite solar cells. However, the metal oxides suffer from high density of defect states and difficulty to adjust their electronic properties. Among the metal oxides, TiO₂ and SnO₂ are the most frequently used ETLs in perovskite solar cells owing to the relatively high electron affinity and capabilities of electron transporting and holes blocking.^[38,39] But the significant drawbacks of the metal oxides such as high density of electronic defects, environmental instability and mismatched energy level with perovskites lead to strong hysteresis, low efficiency and rapid degradation under ultraviolet illumination of the corresponding solar cells.^[40] In particular, oxygen vacancies in the metal oxide layer are one of the main sources for the formation of defect states, for instance, the Ti (IV) in TiO₂ is prone to be reduced to Ti (III) (Fig. 7(a)), which can not only act as pathways for non-radiative charge recombination, but also induce fast performance decay upon continuous UV irradiation.^[41] It has been revealed that introducing some rare-earth elements in the TiO₂ layer can effectively eliminate the defect states as demonstrated in Fig. 7(b). For examples, the incorporation of Nd³⁺ ions in TiO₂ layer can successfully enhance the efficiency and stability of the corresponding perovskite solar cells. Precisely, the added Nd³⁺ can partially substitute Ti⁴⁺ in the TiO₂ crystal lattice without obviously affecting the crystal structure as confirmed by the x-ray diffraction spectra (Fig. 7(c)), leading to reduced trap states and mitigated electron non-radiative recombination (Fig. 7(d)). Moreover, the trivalent dopants of the rare-earth elements in TiO₂ can effectively reduce the oxygen vacancies to prevent the high energy light induced desorption of O₂, which can significantly enhance their operational stability (Fig. 7(e)).^[42]

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Furthermore, the rare-earth ions can be used to modify the Fermi levels of the metal oxides. Owing to the high binding affinity of the rare-earth ions to oxygen, the doping of La,^[43] Sm,^[44] Eu,^[45] Y^[46,47] ions in the TiO₂ layer illustrated a shift of the quasi-Fermi level by eliminating oxygen atoms on the surface of TiO₂. The well-aligned Fermi level with the corresponding perovskites can reduce the energetic barriers for charge transport, thereby enhancing the open circuit voltage and power conversion efficiency (Fig. 7(f)). In addition, the electron mobility of the metal oxides was generally lower than the hole mobility of their counterpart such as spiro-OMeTAD, leading to unbalanced charge transport and serious hysteresis. The conductivity of the metal oxide layers can be enhanced by the doping of rare-earth ions due to the increased carrier density.^[46,48] For examples, SnO₂ has recently emerged as an promising candidate of TiO₂ owing to the high electron mobility and well aligned energy level with perovskites.^[39,49] Doping with n-type ions such as Y³⁺ can optimize the energy level alignment and increase the electron mobility of SnO₂,^[50] which accelerates charge extraction and reduces non-radiative charge recombination, resulting in significantly enhanced short-circuit current (J_SC) and fill factor (FF).^[51–53] Moreover, it was known that the electron mobility of metal oxides is related with the crystal structures, for examples, the electron mobility of anatase TiO₂ is higher than that of rutile phase,^[54] and mixing the rutile phase and anatase phase TiO₂ can generate a built-in electric field in ETL layer to promote charge transport and eliminate charge recombination.^[55] Ren and co-workers^[56] employed a Er-doped TiO₂ as ETL in perovskite solar cells. As they found that it was difficult to replace Ti⁴⁺ with Er³⁺ in the crystal lattice because the ionic radius of Er³⁺ is much larger than that of Ti⁴⁺. The formation of chemical bond of Er–O–Ti can only occur at high temperature thermal annealing, which is beneficial to restrain the phase transition from anatase to rutile and generate a mixed anatase and rutile phase of TiO₂.

The HTLs are considered to be a crucial layer to achieve high performance inverted perovskite solar cells, which exhibit less hysteresis with respect to regular configurations and are prone to be integrated with silicon solar cells for tandem devices. Compared to organic HTLs in terms of PEDOT:PSS, P3HT, PTAA and Spiro-OMeTAD, the nickel oxide (NiO_x ) is one of the most promising inorganic HTLs for inverted perovskite solar cells due to its high chemical resistance, low production cost and superior light transmission.^[57–59] However, the efficiency of perovskite solar cells based on NiO_x is still lower than that based on organic HTLs because of the low conductivity and difficulty in tuning the electronic property of NiO_x .^[60,61] Many investigations have demonstrated that the electronic structure of the NiO_x layer can be effectively varied through additive engineering, thus improving the performance of inverted perovskite solar cells.^[62,63] Many rare-earth ions such as Ce³⁺, Nd³⁺, Eu³⁺, Tb³⁺, Yb^{3+ [48]} and La^{3+ [64]} have been explored as additives in the preparation of the NiO_x layer. It was found that these rare-earth ions could hardly entered into the crystal lattice of NiO_x due to the big difference of the ionic radius between the rare-earth ions and Ni, and the dopants were mainly found at the interstitial site or the thin film surface to effectively eliminate the defect states.^[65–67] In 2018, Hu and co-workers^[68] firstly introduced yttrium ion (Y³⁺) ions as dopants in the preparation of NiO_x HTL in inverted perovskite solar cells. It was found that Y³⁺ can be successfully incorporated into the crystal lattice of the NiO_x due to the small ionic radius of the Y³⁺.^[68] Because the Y³⁺ ions have an occupied 4d orbital, the electronic configurations and electrical conductivity of the NiO_x can be modified through Y³⁺ doping,^[47,69,70] resulting in significant enhancement of photogenerated current compared to the device based on pristine NiO_x .^[68]

It was known that the morphology of underneath charge transport layers determines the thin film quality of the upper perovskite layers and the interfacial contact between the CTL and the perovskite, and the rare-earth ions were found to be able to modify the thin film morphology of the metal oxide layers and align the nanostructure of the scaffold layers.^[71,72] For examples, the SnO₂ thin films can be simply deposited via direct spin-coating of SnO₂ nanoparticles, while the spontaneous aggregation of SnO₂ nanoparticles can result in pinholes or island morphology in the as deposited films (Fig. 8(a)), seriously restricting the device performance. It was found that the addition of La³⁺ can mitigate the SnO₂ nanoparticle aggregation to prevent the formation of pinholes (Fig. 8(b)).^[71] And also, the doping of some rare-earth ions such as Eu³⁺, La³⁺ into NiO_x , can obviously reduce the crystal size of NiO_x crystals (Figs. 8(c) and 8(d)) due to the formation of rare-earth–O bonding that restricted the growth of large crystals, enabling the formation of compact and uniform NiO_x thin films.^[64]

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In mesoporous structured perovskite solar cells, the scaffold layer plays an important role to improve the contact area between the perovskite and CTL. However, the scaffold layer is composed of structured nanoparticles with numerous grain boundaries and morphological defects, leading to serious non-radiative charge recombination. Moreover, the pore-filling of perovskite into the randomly featured nanoparticles is the other difficulty. Therefore, strategies need to be developed to regulate the growth of the scaffold layer with reduced defect states and uniform and well-aligned nanostructures.^[73] For examples, Guo and co-workers^[74] have integrated Sc³⁺, Y³⁺and La³⁺ into the SnO₂ nanospheres. The doping of rare-earth ions can affect the growth dynamics and enable the formation of uniform, monodispersed SnO₂ with enlarged pore width and specific surface area, facilitating the perovskite infiltration. And also, the mesoporous SnO₂ with regular structure can improve perovskite coverage condition, allowing for the growth of uniform and compact perovskites with enlarged grain size. Yang and co-workers^[72] synthesized yttrium-doped SnO₂ nanosheet arrays via low-temperature hydrothermal process. It was found that the hydrolysis of the incorporated Y³⁺ can modify the acidity of the precursor solution and hence promote the growth of uniform and homogeneously distributed Y-SnO₂. The well-aligned SnO₂ nanosheet array (Figs. 8(e) and 8(f)) can effectively improve perovskite infiltration. In the meanwhile, the Y³⁺ can be embedded into the crystal lattice of SnO₂ and optimize the energy level of SnO₂. Therefore, the increased contact area and reduced energetic barrier between the perovskite and SnO₂ can improved the efficiency with significantly reduced hysteresis.

The optoelectronic property of the perovskites can be tailored by the doping of rare-earth ions. For examples, the B-site cations of the perovskite can be partially substituted by rare-earth ions such as Eu²⁺ because of the similar ionic radius (Eu²⁺, 117 pm, Pb²⁺, 119 pm). In 2019, Wu and co-workers^[75] introduced EuI₂ as dopants into the MAPbI₃ perovskite. It was found that the incorporated Eu²⁺ can be embedded into the perovskite crystal lattice to form MAPb_1−x Eu_x I₃, which was consistent with some other reports.^[76] The as formed MAPb_1−x Eu_x I₃ can substantially enhance the absorption at the UV-vis wavelength region due to the formation of some impurity energy levels by the EuI₂ doping. Furthermore, partial substitution of Eu²⁺ into the perovskite lattice affected the electronic orbits and improved the charge mobility, resulting in improved short circuit current density and the conversion efficiency, as calculated by Suzuki.^[77] And also, the absorption edge (Fig. 9(a)) and the light emission (Fig. 9(b)) properties can be finely tailored by incorporating different rare-earth ions in the perovskites.^[76] The reason for the variation of the optical property was ascribed to the different electronic orbitals and ionic radius of the rare-earth ions, which can generate intermediate energetic levels and induce lattice vibration in the doped perovskite crystals. Moreover, the rare-earth ions doping can enhance the photoluminescence property of the perovskites. For instance, the doping of Ce³⁺ into CsPbBr₃ perovskite nanocrystal can improve the lattice order and enrich the conduction band edge states, leading to significantly enhanced PL (Fig. 9(c)).^[78] Furthermore, the as formed intermediate energetic states can boost the photoluminescence quantum efficiency above 100% of the rare-earth ions doped perovskites, in which the captured photon can be split into two low energy photons and emit along the dopants as illustrated in Fig. 9(d), to open many novel application windows.^[79]

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The thin film morphology of the perovskite layer is another determining factor on the efficiency of perovskite solar cells. The incorporation of rare-earth ions can affect the perovskite crystallization to modify the thin film morphology and lower the formation energy. For examples, when the Eu³⁺ ions were added into inorganic CsPbI₂Br perovskite precursor,^[80,81] as it was illustrated in Fig. 10(a) that the incorporated Eu³⁺ ions can strongly interact with negatively charged halide plumbates (${\rm{Pb}}{X}_{3}^{-}$, ${\rm{Pb}}{X}_{4}^{2-}$, ${\rm{Pb}}{X}_{6}^{4-}$, X = I, Br) and create complex of Eu–PbX₂, which can be used as nucleus to facilitate the perovskite growth with uniformed surface coverage and homogeneously distributed grain size (Figs. 10(b) and 10(c)),^[82] thereby resulting in improved device performance (Fig. 10(d)).^[83] And also, Duan and co-workers^[84] reported a work on the doping of different rare-earth ions (Sm³⁺, Tb³⁺, Ho³⁺, Er³⁺, and Yb³⁺) into CsPbBr₃ perovskite. As they found that these ions can strongly associate with the halide ions and hence affect the perovskite crystallization, and it was interestingly found that the crystal size was strongly related with the atomic number of the rare-earth ions. Particularly, the grain size is gradually increased and the perovskite thin films become more compact with the decrease of atomic number of the rare-earth ions.

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It was known that the perovskites are usually formed at relatively high temperature, for instance, the CsPbI₃ usually presents a yellow non-perovskite phase at room temperature because the black perovskite phase is only stabilized at temperatures above 335 °C,^[85] increasing the production cost. The interaction between the rare-earth ions and the perovskite precursor in the solution can lower the perovskite formation energy by modifying the one-step conversion from precursor to perovskite with larger formation energy barrier to the two-step conversion from precursor to complex and then to perovskite with reduced formation energy barrier, allowing the perovskite growth at low temperature.^[86] In 2018, Jena and co-workers^[87] tried to dope EuCl₃ into the CsPbI₃ perovskite, and it was found that the black perovskite phase of CsPbI₃ can be stabilized at room temperature and ambient condition. They attributed the low temperature phase stabilization to the reduced grain size and crystal structure distortion (Fig. 10(e)), but the underlying mechanism still needs further investigation.

The doping of rare-earth ions can stabilize the perovskite crystal structure and boost the device stability. First of all, the element substitution in the crystal lattice or interstitial doping at grain boundaries by the rare-earth elements can effectively reduce the trap states, where the degradation is likely to occur, via strong interaction with antisite defects in the perovskite crystals, effectively stabilizing the crystal structure. For examples, the incorporated Eu³⁺ in CsPbI₂Br perovskite were found at the interstices of the crystal lattice and at the grain boundaries of the perovskites, and the transition of the perovskite phase to non-perovskite phase can be significantly mitigated or can be partially repaired via thermal treatment after the Eu³⁺ doping, leading to improved thermal stability.^[82] The incorporation of rare-earth ions in the perovskite lattice can modify the parameters of the perovskite crystal unit, for instance, embedding La³⁺ ions into CsPbI₂Br perovskite crystal can cause lattice shrinking^[88] and the incorporation of Sm³⁺ was found to enhance the binding energy between the rare-earth and Br compared to that of Pb–Br,^[84] which contributed to the improvement of moisture stability, enabling air condition processability. Moreover, it was known that Eu³⁺ can be easily reduced to Eu²⁺. On this basis, Wang and co-workers^[89] demonstrated a new concept by introducing redox shuttle in the perovskite thin films using the ionic pair of Eu³⁺–Eu²⁺, which can continuously oxidize Pb and reduce I in a cyclical process to avoid the formation of metallic Pb and iodine, as demonstrated in Fig. 11(a). As a result, excellent light stability and thermal stability were obtained, and 90% of the initial PCE can be retained after 8000 h in the Eu doped perovskite device. On the other hand, Feng and co-workers^[90] used Eu-porphyrin complex as dopants in the preparation of perovskites. As shown in Fig. 11(b), the porphyrin can interact with PbI₂ to form two-dimensional perovskites within the 3D perovskites and passivate the defect states at the grain boundaries.^[91,92,93] The as-formed 2D–3D hybrid perovskite architecture can significantly boost the device stability when exposed to heating and humidity. And also, the incorporation of Eu can significantly boost the light stability of the perovskite device compared to the perovskites using pure porphyrin as dopants. The similar phenomenon can be found by using the Eu–metal–organic framework (MOF) complex.^[94] Therefore, the combination of rare-earth elements with other materials or in the format of complex can be used to bring in synergetic effects for synchronous improvement of the resistance against moisture, heat, and UV light.^[90] In addition, the perovskite phase stability, especially for inorganic perovskites, can be enhanced by tailoring the grain size into the nanometer range owing to the contribution of surface energy.^[95] As aforementioned, the addition of the rare-earth ions into the perovskite precursor can modify the crystallization dynamics and hence the grain size of the as deposited perovskites, to improve the perovskite stability.

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It was well established that some specific rare earth ions can convert near-infrared light into perovskite sensitive light via two-photon or multiphoton up-conversion processes, and numerous investigations have demonstrated their promising application potentials in the mesoporous scaffold layer of perovskite solar cells.^[96,97] For instance, the incorporation of some Er ions into the TiO₂ layer can not only regulate the TiO₂ crystal growth, but also convert near infrared light into green light owing to the instinct f–f transitions as illustrated in Fig. 12(b), which can promote the NIR light excitation of perovskite and result in significant enhancement of the photocurrent density of the corresponding perovskite solar cells (Fig. 12(c)).^[98] To optimize the up-conversion properties, the co-doping by incorporating some additional rear-earth ions such as Yb³⁺, etc. into the Er³⁺ doped TiO₂ can enhance the up-conversion efficiency and further broaden the absorption range of the perovskite layer to generate extra photocurrent in the solar cells.^[99] In the co-doping system, the Yb³⁺ can be excited by the near infrared incidence radiation with long life time, and the energy can be transferred to the Er³⁺ ions and release as high-energy visible light. Therefore, the combination of different rare-earth ions can adjust the optical properties and conversion efficiency of the up-converting materials. In addition, combining some metal ions into the up-conversion layer together with the rare earth ions can further enhance the up-conversion efficiency.^[100,101] For examples, the combinations of the rare earth ions and the metal ions, for examples, Ho³⁺–Yb³⁺–Mg^{2+ [102]} and Er³⁺–Yb³⁺–Li^{+ [103]} were employed as dopants to prepare TiO₂, which can boost the up-conversion luminescence to improve the overall efficiency of perovskite solar cells.

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Furthermore, the rare-earth containing nanomaterials have also exhibited up-conversion effect and they can be embedded in different layers of perovskite solar cells.^[104–108] For examples, β-NaYF₄:Yb³⁺, Er³⁺ nanoparticles^[109] or nanoprism^[110] were introduced into the TiO₂ as up-conversion layers. Based on the up-conversion effect, part of the near-infrared light was converted into natural visible light, which can then be captured by the photoactive perovskites, thereby improving the utilization rate of solar spectrum and increasing the photocurrent density of the perovskite solar cells. In 2019, Deng and co-workers^[111] added an octahedral Li(Gd, Y)F₄:Yb³⁺, Er³⁺ nanocrystal into the hole transport layer of Spiro-OMeTAD in the perovskite solar cells to broaden the light harvesting. And also, Meng and co-workers^[112] directly incorporated β-NaYF₄:Yb, Er nanocrystals into the CH₃NH₃PbI₃ perovskite layer. It was found that a uniform, dense, high-quality and pinhole-free perovskite film can be obtained in the doped perovskite, which was beneficial for the charge transfer in the photovoltaic device. Moreover, recent investigations have revealed that surface modification is an effective strategy to improve the photovoltaic performance, and the up-conversion materials can also be used as the interface layer in the perovskite solar cells.^[113] For examples, Qiu and co-workers^[114] applied a thin layer of hydrophobic β-NaYF₄:Yb, Er nanoparticles on the top of the perovskite layer, which can fulfill the pinholes of the perovskite films and prevent the penetration of environmental moisture to enhance the device stability. The current density of the optimized perovskite device was obviously improved due to the up-conversion effect of the rare-earth based materials. In 2018, Wu and co-workers^[115] successfully incorporated NaYF₄:Yb³⁺, Er³⁺ nanoparticles on top of TiO₂ as a mesoporous layer. The facilitated electron transport from NaYF₄:Yb³⁺, Er³⁺ to TiO₂ and the upconversion properties of NaYF₄:Yb³⁺, Er³⁺ enabled a significant performance improvement of photoelectric conversion efficiency.

However, some parasitically charge recombination can occur when the NaYF₄ crystals were directly doped into the perovskite device because of the presence of defects at the NaYF₄ surface. To avoid the unintentional defect states, Qi and co-workers^[116] designed a core-shell structure of NaYF₄:Yb, Er@SiO₂ crystals by using a thin insulating layer of SiO₂ to prevent the direct exposure of NaYF₄. The newly optimized up-conversion materials were deposited on the top of the TiO₂ layer, which can not only suppress interfacial non-radiative carrier recombination, but also convert lower-energy NIR photons into visible region and thus enhance the photogenerated current density in the perovskite solar cells. In addition, the up-conversion materials can be used as an independent layer to avoid their contact with the functional layers in perovskite solar cells, for examples, Chen and co-workers^[117] introduced a transparent LiYF₄:Yb³⁺, Er³⁺ single crystal as an independent luminescent upconverter in perovskite solar cells to improve light absorption of the solar cells. This transparent single crystal can continue to exert its up-conversion characteristics and convert near-infrared light into visible light.

Although the above up-conversion materials demonstrated promising application potentials to extend the perovskite responsive range, the weak and narrowband absorption of the rare earth ions (Yb³⁺/Er³⁺) with spectral window of 10–20 nm in the near-infrared range restricted the overall improvement in device efficiency. Lai and co-workers^[118] utilized a hydrophilic IR806 dye-sensitized up-conversion nanocrystal IR806-β-NaYF₄:Yb, Er as an interface layer in the perovskite solar cells, which presented a broadband up-conversion of NIR light from 800 nm to 1000 nm into perovskite responsive light emissions (Fig. 12(d)). Therefore, the light sensitive spectra can be broadened by taking advantages of the high extinction coefficient of the organic ligands to effectively sensitize the long-lived f–f transition.^[119]

In order to unveil the origin of the photocurrent enhancement, some investigations have revealed that the scattering effect and localized surface plasmon resonance of the incorporated nanoparticles and the optimized light electrical field distribution can contribute to the performance enhancement besides the up-conversion effects.^[120–123] Nevertheless, the utilization of near-infrared light in the range 800–1100 nm, which is out of the perovskite absorbing range, can be visualized from the external quantum efficiency spectrum and a power conversion efficiency of 0.37% can be obtained when measured under the excitation of 980 nm laser, indicating that the up-conversion effect cannot be ignored and the 980 nm near-infrared light was successfully converted into visible light and be captured by the perovskite.

Inspired by the luminescent down converting ability of the rare earth materials for UV blocking and conversion, the incorporation of the down-conversion materials into perovskite solar cells can reduce the spectral mismatch losses and avoid the high energy light damage to simultaneously enhance the power conversion efficiency and photostability of the corresponding solar cells. There are two main approaches for the application of down-conversion materials in the perovskite solar cells: one is to introduce the down-conversion materials directly into the functional layers of the perovskite solar cells, and the other one is to deposit an extra layer on the backside of the glass substrates.^[124–127]

For examples, Jiang and co-workers^[128] used light-emitting materials of Eu³⁺ doped TiO₂, Meng and co-workers^[129] doped CeO_x into ZnO to successfully construct a down-converting ETL in perovskite solar cells, respectively. It was shown that, when exposed to solar irradiation, the incident visible light can be directly absorbed by the perovskite layer, and part of the high-energy ultraviolet light is converted into low-energy photons by the down-conversion materials and then absorbed by the perovskite layer, thus improving the photogenerated current density and protecting the perovskite layer from UV light damage in the corresponding solar cells, as illustrated in Fig. 13(a). Similar with the up-conversion materials, combing different rare earth ions can enhance the down-conversion efficiency. For instance, Zhang and co-workers^[130] used some additional rare-earth ions in terms of Sm³⁺ and Eu³⁺ to co-dop TiO₂ ETLs in planar CH₃NH₃PbI₃ perovskite solar cells, in which the Sm³⁺ is functioned as a sensitizer to capture the high-energy UV photons and transfer the energy to Eu³⁺, and the Eu³⁺ ions can convert the captured energy into visible light through down-conversion process. The combination of different rare-earth ions can significantly improve the down-conversion efficiency. Moreover, some rare earth complexes were also used as a functional layer in perovskite solar cells.^[131] For instance, the effective luminescent down-conversion materials, such as Eu(TTA)₂(Phen)MAA,^[132] CeO₂:Eu³⁺ nanophosphors,^[125] Sr₂CeO₄:Eu³⁺ nanophosphor^[126] were inserted between the ETL and perovskite layer as an interfacial modifier. They demonstrated that the down-conversion of the ultraviolet light can promote light absorption and inhibit the degradation of the device under UV light irradiation. On the other hand, the introduction of the interfacial layer can promote the interface contact and facilitates the transport and collection of electrons, thereby improving the device performance (Fig. 13(b)).

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And also, recent investigations have demonstrated that directly coating down-conversion materials on the glass substrates can effectively block and convert the UV light without affecting the intrinsic charge transport and recombination.^[99,133,134] For examples, Jiang and co-workers^[127] employed a layer of Eu–4,7-diphenyl-1,10-phenanthroline (Eu-complex) on the backside of the fluorine-doped tin oxide (FTO) glass as down-converting layer (DCL) (Fig. 13(c)). Compared with the pristine perovskite solar cells, the application of such DCL displayed an obvious enhancement of 11.8% in short-circuit current density (J_SC) and 15.3% in efficiency because of the conversion of UV light (300–380 nm) into the perovskite responsive wavelength range. Moreover, the photostability of the optimized device under UV light was significantly enhanced owing to the UV blocking effect (Fig. 13(d)). Similarly, Jia and co-workers^[135] deposited an extra layer of NaYF₄:Eu³⁺ down-conversion nanocrystals on the back of the FTO coated glass substrates. Zhang and co-workers^[136] successfully prepared Sm³⁺ and Ce³⁺ ions co-doped P₂O₅-Li₂O-ZnO-Sm₂O₃-CeO₂ glass-ceramic waveguide as the glass substrates for perovskite solar cells. In such case, under an operational condition, the UV light is firstly filtered by the down-conversion layer and converted into low-energy photons, which can not only expand the spectral response range but also avoid the photocatalytic activity of the TiO₂, thereby improving the photogenerated current and operational stability of the perovskite solar cells. More interestingly, according to the promising up-converting and down-converting properties, full spectrum responsive perovskite solar cells are desirable by convert UV and NIR light to visible light in one device, as illustrated in Figs. 14(a) and 14(b).^[137]

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