Inorganic perovskite solar cells: an emerging member of the photovoltaic community

Jialong Duan ^a, Hongzhe Xu ^a, W. E. I. Sha ^b, Yuanyuan Zhao ^a, Yudi Wang ^a, Xiya Yang ^a and Qunwei Tang *^a
aInstitute of New Energy Technology, College of Information Science and Technology, Jinan University, Guangzhou 510632, PR China. E-mail: tangqunwei@jnu.edu.cn
^bKey Laboratory of Micro-nano Electronic Devices and Smart Systems of Zhejiang Province, College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, China

First published on 30th August 2019

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1. Introduction

Solar radiation releases enormous solar energy, accounting for almost 99% of the total energy in the earth. Therefore, the harvesting of solar energy by energy-conversion devices (i.e., photovoltaics or solar cells) is regarded as a promising solution to address the energy and environmental problems. It has been always a hot topic to convert solar energy into electricity by complicated photoelectrical or photoelectrochemical principles since the discovery of the first solar panel in 1954.^1,2 To date, photovoltaics can be divided into three categories according to the photovoltaic materials: (i) crystalline silicon solar cells, (ii) thin-film technologies, and (iii) the emerging solar cells. As emerging photovoltaics in recent decades, dye-sensitized solar cells (DSSCs),^3–10 organic solar cells (OSCs),^11–22 quantum dot-sensitized solar cells (QDSCs),^23–30 and perovskite solar cells (PSCs)^31–37 are regarded as promising candidates due to their remarkable power conversion efficiency (PCE) enhancement and low production costs. Since 2009, organic–inorganic hybrid perovskite light-harvesters have been well-studied in theory, experiment and module production, and the corresponding PSCs have seen a rapid rise in PCE from 3.8% to 25.2% within ten years.^38–47 These hybrid PSCs are always highlighted by some superiorities: (1) the bandgap of CH₃NH₃PbI₃ (MAPbI₃, 1.55 eV) or CH(NH₂)₂PbI₃ (FAPbI₃, 1.43 eV) is highly suited for wide-spectral photovoltaics;^48,49 (2) the hybrid perovskites present high extinction coefficient and incident-photon-to-current conversion efficiency (IPCE) in devices;⁴⁰ (3) the carrier diffusion length is long in perovskite films.^50,51Science Daily said “There is now an urgent need to tackle the threat of climate change resulting from humanity's overreliance on fossil fuel, and the rapid development of new solar technologies must be part of the plan.” Large area, improved stability, high efficiency, and good reproducibility are the critical competitive elements to assess the commercial application of PSCs. Although the state-of-the-art organic–inorganic hybrid PSCs have comparable PCEs to silicon solar cells, organic–inorganic hybrid perovskite films generally suffer from compositional degradation and crystallization destruction under persistent attack by heat, moisture, or ultraviolet (UV) light,^52,53 mainly arising from the inherent instability of organic volatile components, such as MA⁺ and FA⁺, in thermal and UV conditions. Furthermore, organic MA⁺ and FA⁺ species are prone to react with water molecules or under sunlight and thermal conditions to form intermediate hydrates, which can lead to serious perovskite degradation and therefore poor device lifetime.^54–57

In recent works, the substitution of organic counterparts with inorganic cesium cations (Cs⁺) is considered an effective approach to improve the environmental tolerance of perovskite films under harsh conditions.^58,59 Full-cesium perovskites have been demonstrated to possess high hole mobility (∼520 cm² V s⁻¹) and electron mobility (∼530 cm² V s⁻¹) when obtained from the cold-pressed pellets of annealed CsSnI₃ polycrystalline material, while an electron lifetime of 2.5 μs and an estimated electron mobility of ∼1000 cm² V s⁻¹ were obtained for CsPbBr₃ halide single-crystals.^60,61 Based on these properties, it can be predicted that inorganic perovskites may be another candidate as a light absorber. Since the initial report for fully inorganic cesium lead halides in 1893 and subsequent systematic studies in this field,^62–69 these perovskite halides have been successfully assembled into inorganic PSCs in 2015. To date, enormous efforts have been made to advance the PCE of inorganic devices, Fig. 1 plots the maximized PCEs of inorganic PSCs with CsPbI₃, CsPbI₂Br, CsPbIBr₂, or CsPbBr₃ as light absorbers, where the initial PCEs are all lower than 5% but the maximized value to date is more than 18%, thus demonstrating the great potential for their application in the future. In this review, we summarize the recent advances of all-inorganic PSCs by emphasizing their structural phase transformation, tunable optoelectronic properties, environmental stability, film-making methods, interfacial engineering, and their limited efficiency, as well as the challenges still faced by scientists and their future outlooks.

2. Fundamental background

2.1 Crystal structures of inorganic perovskites

The chemical formula ABX₃ [A (+1), B (+2), X (−1)] is always designated for perovskite-structured materials with a similar crystal structure to the mineral CaTiO₃. The recently emerging all-inorganic perovskites can also be described by a similar chemical formula CsMX₃ (M = Pb²⁺ or Sn²⁺, X = I⁻ or Br⁻), where Cs⁺ occupies a corner of a unit cell with 12-fold coordination, M is a divalent metal cation in general sitting in a body-centered position with 6-fold coordination, and X is a halogen with a face-centred position, as depicted in Fig. 2.⁷⁰

There is a lower energy barrier for the intercalation of CsX into PbBr₂ frameworks during perovskite formation,⁷¹ arising from the smaller ionic radius of Cs⁺ (1.81 Å for Cs⁺) than 2.70 Å for MA⁺ and 2.79 Å for FA⁺. The crystal structure of the perovskite is adjustable in terms of its composition by controlling the tolerance factor (t) and octahedral factor (μ), as proposed by Goldschmidt, in a rational range. Here, t indicates the state of distortion and stability of perovskite structures, and this value can be obtained according to the following formula:⁷², where r represents the ionic radius of atoms of A, B, and X sites, respectively, μ is defined as a r_B/r_X ratio, which is directly correlated to the formation of a BX₆ octahedron.^25,73,74 Both t and μ are used to predict the formability of a perovskite structure. Typically, stable 3D halide perovskite crystals can be formed under the condition of 0.81 < t < 1.0 and 0.44 < μ < 0.90, an ideal cubic perovskite structure at 0.9 < t < 1.0, and a tetragonal or orthorhombic structure in the range of 0.81 < t < 0.9, respectively.^75,76 When t > 1.0, a NH₄CdI₃-type crystal structure is usually favored. When t < 0.8, a CsNiBr₃-type crystal structure is most likely to form.⁷⁷ Taking the crystal restrictions, such as the t range and ionic radius of the commonly used Pb²⁺ (1.2 Å), Br⁻ (1.96 Å) and I⁻ (2.2 Å) ions into considerations, the ionic radius of the A-site ion should be limited to 2.9 Å. Otherwise, the cubic structure will be distorted and the crystal symmetry will be reduced, leading to forming an unsatisfactory crystal with notorious photovoltaic performance or long-term stability. So the inorganic cations with Cs⁺ or Rb⁺ cations could lead to a more favorable tolerance factor to facilitate the stabilization of the photoactive perovskite phase in a broader temperature range and to improve their thermal and light soaking stability.

2.2 Stability of inorganic perovskites

The degradation mechanisms of organic–inorganic hybrid perovskites have been widely explored during the past several years. Organic cations, such as MA⁺ in MAPbI₃, have acidic protons; therefore, the perovskite degradation will be initiated by the reaction between the superoxide and methylammonium moiety under light and oxygen conditions.⁷⁸ In this fashion, the use of all-inorganic perovskites free of organic species can improve the tolerance of the perovskite crystal to oxygen and light. Unfortunately, such inorganic perovskites have been demonstrated to be not stable enough under external stimuli, such as moisture, light, heat, and oxygen. To the best of our knowledge, CsPbBr₃ has the most superior stability among the different inorganic perovskites, while the I-containing perovskites display inferior long-term stability. Different from hybrid perovskites, the inorganic perovskites are free of hygroscopic organic cations, which are responsible for the hydration-induced degradation. Therefore, polymorphic transition, decomposition, oxidation, or their combinations may be the intrinsic origins of their instability properties.⁷⁷ For the state-of-the-art CsPbI₃ inorganic perovskite, the phase instability is mainly due to transitions between the desirable photoactive perovskite black phase and the undesirable non-perovskite yellow phase, although transition to the yellow phase is reversible, attributed to the undesirable tolerance factor. Although the polymorphic transition can be significantly suppressed via increasing the Br dosage (producing a more stabilized structure with t > 0.8 and μ > 0.4), halide segregation may be another issue for the performance degradation under light or an electron beam, especially for perovskites with an I dosage lower than 40%. Based on the above-mentioned discussion, it can be seen that the phase instability of CsPbX₃ can be mainly attributed to the polymorphic transition. There are also other inorganic perovskite materials, such as CsSnX₃ and CsGeX₃, where the facile oxidation of Sn²⁺/Ge²⁺ to Sn⁴⁺/Ge⁴⁺ in ambient air results in a sluggish efficiency and rapid performance degradation, which will be discussed in the following part.

2.3 Inorganic PSC architectures

Similar to hybrid PSCs, the state-of-the-art all-inorganic devices are generally constructed in a regular n–i–p or inverted p–i–n architectures, as shown in Fig. 3a and c, in which both perovskite layers are sandwiched between an n-type electron-transporting layer (ETL) and a p-type hole-transporting layer (HTL). A mesoporous scaffold, either semiconducting or insulating, is also generally included, as shown in Fig. 3b. In physics, the light absorption of a perovskite is a light–matter interaction process, in which the electrons in the interacting materials resonate with the electric field of the irradiating optical signal and induce polarization. The working principle of a typical PSC is based on the photovoltaic effect of a p–n junction, and the built-in potential is formed at p–n junctions. Upon sunlight irradiation, the atoms in an inorganic perovskite absorb photons to release electrons and subsequently generate electron–hole pairs. The photo-induced electrons are driven to an n-type region and holes are driven to a p-type region under this barrier electric field, and therefore, a photogenerated electric field opposite to the direction of the barrier electric field is formed near p–n junctions. Part of the photogenerated electric field counteracts the barrier electric field and the rest causes electromotive force in the thin layer between the n-type and p-type regions to realize electric power output through an external circuit. In this fashion, the intrinsic mechanism for regular n–i–p or inverted p–i–n architectures is identical except for the charge-carrier transfer direction.

Considering the morphology of a transparent conductive oxide (TCO)-attached charge-contact layer, a mesoporous or planar architecture can be visibly distinguished. To date, both structured PSCs have achieved a high efficiency over 20% based on a hybrid perovskite layer, demonstrating great potential in real application.^79,80 The choice of whether to fabricate a mesoporous or planar architecture is highly dependent on the perovskite film quality and properties. For mesoporous PSCs derived from dye-sensitized solar cells,⁸¹ the perovskite layer can be infiltrated within the scaffold via the interconnected channels, resulting in a facile and reproducible device fabrication.^82,83a,b The concept of a planar device originates from TiO₂- or Al₂O₃-based mesoporous PSCs.⁴³ In detail, a planar architecture is implemented on a compact charge-transfer layer without pores, on which it is more facile to deposit a perovskite film through vapor-based technologies, which will be discussed in Section 5. Compared to mesoporous devices, the high-quality and smoother perovskite layer plays an important role in obtaining a satisfactory solar-to-electric conversion efficiency. In fact, a much higher photocurrent or photovoltage can be realized based on a planar architecture, but it will suffer from much worse hysteresis, which may arise from insufficient charge extraction rates, therefore leading to charge accumulation.

3. Inorganic perovskites solar cells

Are organic cations essential for high-efficiency PSCs? This urgent question was analyzed by Hodes and Snaith in 2015, although the first report on inorganic CsPbX₃ crystal data can be traced back to 1893.^62,84,85 Hodes et al. demonstrated that the organic cations are not indispensable for high open-circuit voltage (V_oc) PSCs through comparing hybrid organic–inorganic methylammonium lead bromide (MAPbBr₃) and all-inorganic cesium lead bromide (CsPbBr₃) with different HTLs.⁸⁴ Through identifying the possible fundamental differences in their structural, thermal, and electronic characteristics, they found that a large V_oc profits from the presence of a direct optical bandgap of around 2.3 eV. Simultaneously, the CsPbBr₃ halide had higher thermal stability than MAPbBr₃. However, its large bandgap limited the PCE enhancement of this type of device. Snaith and co-workers fabricated a CsPbI₃-based PSC for the first time via a low-temperature phase transition process at room temperature, achieving a PCE of 2.9%.⁸⁵ Those works identify that the organic cation is not necessary and this paved the way for further developments of much more thermally stable inorganic perovskites as light absorbers. In this section, the state-of-the-art advances of all-inorganic PSCs engineered from compositional, doping, interfacial, and spectral aspects are systematically demonstrated, and the photovoltaic parameters of typical devices based on various inorganic perovskites are summarized in Table 1.

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3.1 Pb-based perovskites

CsPbX₃ (X = Br, I, with stoichiometric ratios of 0:3, 1:2, 2:1, and 3:0) materials can generally form a cubic phase perovskite structure with intrinsic Goldschmidt's values for the tolerance factor t = 0.83–0.84. Inorganic perovskites possess ambipolar behaviors, accompanying unique crystal structures, high absorption coefficients, large exciton diffusion lengths, and excellent charge-transporting properties, indicating that they can cumulatively work as light absorbers and carrier conductors.¹⁴⁵ According to the above-mentioned properties, perovskite films with a thickness of several hundreds of nanometers are enough to realize efficient solar-to-electric conversion.^146–148 CsPbBr₃- and CsPbI₃-based solar cell models are two types of devices that were first fabricated in the early stage of all-inorganic PSCs development. However, CsPbBr₃ is limited by its narrow absorption range and CsPbI₃ has thermodynamic phase instability in high-humidity (even in low temperature with zero humidity) and/or high-temperature conditions. To pursue a long-time prosperity for devices, scientists pay great attention and made many innovations to try to overcome these weakness, including tuning the Br:I ratio and doping foreign elements by compositional engineering to modulate the crystal structure, absorption range, and durability.^111,149,150 These findings have led to the further exploration of many new configurations and derivative materials for CsPbX₃.^149,151Fig. 4a shows the absorption spectra evolution of CsPbI_3−xBr_x perovskites with various Br dosages, referring to an absorbance onset changing from 540 nm for CsPbBr₃ to 718 nm for CsPbI₃.¹¹¹ In detail, there is a linear relationship between the absorption onset values and Br concentration in inorganic perovskites, as shown in Fig. 4b. In this fashion, the light-harvesting ability of CsPbI_3−xBr_x perovskite can be precisely predicted, giving a possibility to obtain an ideal inorganic perovskite material with considerable long-term stability and photovoltaic properties. To date, CsPbI₃, CsPbI₂Br, CsPbIBr₂, and CsPbBr₃ films are the main four types of state-of-the-art inorganic perovskites and their Shockley–Queisser (S–Q) limit efficiencies are 28.9%, 24.4%, 22.1%, and 15.9% in photovoltaic devices, respectively. As shown in Fig. 4c, all the perovskites can form a typical perovskite phase with diffraction peaks at around 14.62°, 20.65°, and 28.94°, corresponding to the (100), (110), and (200) planes, respectively. These characteristic peaks shift to higher angles with increasing the Br dosage owing to the contraction of the crystal lattice upon the partial substitution of I atoms with Br atoms. From the external quantum efficiency (EQE) characterizations (Fig. 4d), it can be seen that the light response range can be extended to 620 nm (2.05 eV for CsPbIBr₂), 700 nm (1.9 eV for CsPbI₂Br), and then 740 nm (1.73 eV for CsPbI₃) from 540 nm (2.3 eV for CsPbBr₃), in accordance with the absorption spectra. It should be noted that the high trap-state density from the thermal dynamical instability of the black α-CsPbI₃ perovskite phase to the δ-CsPbI₃ non-perovskite phase delivers the lowest PCE and EQE values in the early stage.¹¹¹

A high PCE and improved stability are equally significant to promote the commercialization of all-inorganic PSCs. However, the light-harvesting ability and their tolerance to the environment of these inorganic perovskites in ambient conditions is dramatically adverse. As shown in Fig. 4e, the normalized photovoltaic performance of the corresponding device degrades significantly following the order of CsPbI₃ > CsPbI₂Br > CsPbIBr₂ > CsPbBr₃, demonstrating the incompatible power out ability and long-term stability. While CsPbBr₃ perovskite is relatively stable without changes in either color or shape in 40% RH and 25 °C over 3 h as shown in Fig. 4f, CsPbIBr₂, CsPbI₂Br, and CsPbI₃ present rapid decomposition by moisture attack. Therefore, how to simultaneously compromise the light-harvesting ability and stability is a great challenge in the field of inorganic PSCs.

To better understand the properties of light absorbers, which is crucial for the fabrication of high-performance inorganic PSCs, we summarize various Pb-based inorganic perovskite materials that have been used as photoactive layers in this section.

3.2 Pb-free perovskites

Lead-based inorganic perovskites are the most studied materials in all-inorganic perovskite devices, which have achieved PCEs of over 10% with the highest value of 18.4%. In this review, we mainly focus on depicting the progress of Pb-based inorganic perovskite solar cells. However, with an aim to better understand the development situation of this emerging class of photovoltaics, Pb-free perovskite assembled devices are also discussed in this part.

Actually, the first all-inorganic perovskite solar cell was based on Sn-containing materials by completely substituting Pb²⁺ with Sn²⁺ in 2012, achieving an efficiency of 0.88%.¹⁸⁶ Unfortunately, compared with Pb-based inorganic perovskite, Sn-based inorganic perovskites are more susceptible to moisture-mediated degradation, oxygen-mediated oxidation of Sn²⁺ to Sn⁴⁺, and even to beam damage, leading to severe decomposition of the original structure.^187,188 Subsequently, Kumar et al. fabricated a common photovoltaic device with the configuration FTO/TiO₂/CsSnI₃/HTM/Au, showing an improved PCE up to 2.02% by introducing SnF₂ into the lattice of CsSnI₃ to reduce the intrinsic defects, such as Sn-cation vacancies.¹⁸⁹ A corresponding conclusion was also demonstrated in CsSnBr₃ PSC.¹⁸⁷ Hodes et al. reported that the addition of SnF₂ could slightly raise the work function and E_VBM of CsSnBr₃ and increase stability against electron beam damage. After carefully controlling the dosage of SnF₂, the PCE could be increased to over 2% from an initial 0.01%. Further, Mai's group introduced SnF₂ and hypophosphorous acid (HPA) additive to improve the phase stability of a Sn-based perovskite during long-term thermal treatment, as shown in Fig. 12a and b.¹⁹⁰ Arising from the inhibition of the formation of Sn⁴⁺ during the formation process of CsSnIBr₂ films, the long-term stability could be significantly enhanced. Finally, PCEs of around 3% were obtained by CsSnIBr₂ with the mesoscopic PSC architecture. All the above-mentioned results demonstrate the promising application of Pb-free perovskites as photoactive layers in corresponding devices.

Similar to CsPbX₃, as shown in Fig. 12c and d, when increasing the Br doping dosage, the optical bandgap of CsSnX₃ increases from 1.27 eV for CsSnI₃ to 1.37, 1.65, and 1.75 eV for CsSnI₂Br, CsSnIBr₂, and CsSnBr₃, respectively.¹⁹¹ Meanwhile, the photovoltaic performances and stability are highly dependent on the composition. An obvious improvement in V_oc and decrease in J_sc could be detected upon incorporating Br element, as shown in Fig. 12e, which was mainly attributed to the reduced Sn-cation vacancies. By adding SnF₂ to further optimize the film quality, an optimal PCE of 1.76% was obtained for CsSnI_2.9Br_0.1 PSCs. By adding SnCl₂ into the CsSnI₃ perovskite precursor, Hatton's group formed a low pin-hole density and highly stable CsSnI₃ perovskite film, achieving a high PCE of 3.56%.¹⁹²

Apart from the Sn-based Pb-free perovskites, employing Ge element to substitute Pb or forming perovskite derived materials, such as A₂BX₆ (2-1-6), A₃B₂X₉ (3-2-9), and A₂B¹⁺B³⁺X₆ (2-1-1-6) are another two strategies to obtain Pb-free perovskites. The typical compounds contain CsGeCl₃, CsGeBr₃, and CsGeI₃, with bandgaps of 3.67, 2.32, and 1.53 eV, respectively.¹⁹³ To date, their efficiency as well as stability are still much lower compared to Pb-containing devices, mainly attributed to the easily oxidized behavior of Ge²⁺ to Ge⁴⁺. Efficient stabilizing of the lattice of CsGe(Sn)X₃ is a great challenge to realize high-performance Pb-free PSC devices. For perovskite-derived materials, including Cs₂SnI₆, Cs₃Bi₂I₉, Cs₃Sb₂I₉, Cs₂BiAgCl₆, Cs₂BiAgBr₆, and so on, although theoretical calculations have predicted that those derived perovskites have ideal bandgaps to achieve high solar-to-electric conversion efficiency, the PCEs of photovoltaics assembled based on these materials are generally around 3%, even after optimizing the element ratio of X site and when improving the film quality.^194,195 All in all, great breakthroughs may be achieved by developing suitable synthetic routes to fabricate these novel materials in the future.

After comparing various inorganic PSCs, it can be seen that the advantages and deficiencies are typically concomitant. Taking CsPb(I_1−xBr_x)₃ into consideration, generally, when x < 0.2, the I-rich phase is preferred insulating the δ phase at room temperature; when x > 0.4, the light-induced segregation dominates under irradiation. To date, the best recorded efficiency of inorganic PSC device that has been achieved is 18.4% for CsPbI₃ perovskite, and 16.37% for CsPbI₂Br perovskite, and 10.91% for CsPbBr₃ perovskite. For Pb-free devices, the photovoltaic performance is still unsatisfactory. Taking all parameters together, CsPbI₂Br films may be regarded as the best choice for inorganic PSCs by balancing the stability and efficiency.

4. Compositional engineering

Although cesium lead halide perovskite is regarded as an alternative light absorber, the nonuniform film coverage, high trap state density, and phase instability still degrade the photovoltaic performances of the corresponding devices fabricated with this material. Doping inorganic perovskites with a smaller ion leads to a reduction of the lattice constants and thus to the contraction of the cubic volume, which has been proved to be an efficient strategy to further enhance the overall photovoltaic performance of inorganic PSCs. Fig. 13 summarizes the radii of ions that have been used in doping engineering and the inner mechanism of A-site and B-site substitutions.

4.1 A-site substitution

A general recognized mechanism involves adjusting the A-site cations to contract the sizes of the octahedral voids, with an aim to obtain an ideal cubic perovskite phase with a Goldschmidt's tolerance factor (τ) of 1, and thus stabilizing the perovskite phase.^114,196 Therefore, the composition of the A-site is crucial for effective PSC devices. Incorporating alkali metal cations has also been systematically studied in hybrid perovskite system, displaying a positive effect on the final photovoltaic performance. Liu's group systematically demonstrated that A-site substitution with alkali metal ions could dramatically improve the perovskite quality by enlarging the grain sizes, reducing the defect state density, passivating the grain boundaries, and increasing the built-in potential (V_bi).¹⁹⁶ Various alkali metal ions with smaller ionic radii have been discovered to substitute for Cs⁺, as shown in Fig. 13b, with an aim to enhance the photoelectric properties of inorganic perovskite solar cells. The first group to kick off the alkali metal doping way was Park's group in inorganic PSCs by means of incorporating potassium cations (K⁺) into the CsPbI₂Br lattice.¹¹⁴ After optimizing the doping dosage, as shown in Fig. 14a and b, the largest shift of the XRD characteristic peak position to a higher angle was observed upon a closer look for the (100) peak of Cs_0.925K_0.075PbI₂Br, which could be correlated with a significant decrease in the lattice constant from 6.0341 Å to 6.0137 Å, resulting in the contraction of the cubic volume and a significant stabilization of the perovskite phase.¹⁹⁷ Furthermore, the increased XRD peak intensity indicated an increase in the crystallinity, whereby the preferred orientation was arranged. The binding energy for Pb 4f and Br 3d shifted from 137.8 to 138.2 eV and 67.95 to 68.35 eV owing to the changed interaction between the elements (see Fig. 14c–f) after doping with K⁺, respectively. Arising from the smaller ion radius of potassium (1.38 Å) than cesium (1.67 Å), the volumetric ratio between PbX₆ octahedra and A-site cations for Cs_0.925K_0.075PbI₂Br could be well-tuned, leading to an enhancement of the phase stability of perovskites. A significant increase in absorbance intensity and the enhancement of charge extraction could also be viewed from Fig. 14g–i, which were highly dependent on the optimization of the as-prepared perovskite film.

According to the passivation effect of potassium cations, the defect state density could be significantly reduced, such as the halide vacancy in the grain boundaries or in the interior of the perovskite film.¹⁹⁸ The non-radiative losses could be dramatically eliminated owing to the suppressed recombination, thereby enhancing the external photoluminescence quantum yield as well as the corresponding photovoltaic conversion efficiency. Finally, a device with the architecture of FTO/bl-TiO₂/Cs_0.925K_0.075PbI₂Br/Spiro-OMeTAD/Au yielded an enhanced efficiency as high as 10.0%, demonstrating the feasibility to regulate the lattice and defect state density.

More recently, Tang's group comprehensively studied a series of alkali metal ions (Li⁺, Na⁺, K⁺, and Rb⁺) with smaller ionic radii as dopants to modulate the film quality of all-inorganic CsPbBr₃ perovskite films.⁹⁰ As shown in Fig. 15, the characteristic peaks shifted to higher binding energy and the diffraction angle could also be discovered, confirming the contraction in the perovskite cubic volume. By optimizing the doping amount, the grain size could be enlarged to 820 nm from the pristine 360 nm size, as shown in Fig. 15d–i, leading to the reduction of the grain boundaries, which would induce shallow states near the valence band edge and hinder hole diffusion. In this fashion, the recombination in perovskite films could be suppressed.¹⁹⁹ An unprecedented PCE of 9.86% and the long-term stability of the HTL-free Cs_0.91Rb_0.09PbBr₃ solar cell guarantee its further application.

4.2 B-site substitution

Can the partial substitution of Pb²⁺ (B-site) with other metal ions (from an extent of doping to alloying) stabilize the desired phase of CsPbX₃ (X = Cl, Br, I) perovskites without changing their optoelectronic properties? Considering the toxicity of Pb²⁺ in typical perovskite crystals, various bivalent nontoxic metal ions (such as Mn²⁺, Zn²⁺, Cd²⁺, Co²⁺, Sr²⁺, Sn²⁺, and alkaline earth metal ions)^{70,113,116,126,197,200–202} and multi-valent nontoxic metal (such as Bi³⁺, In³⁺, Y³⁺, Nb⁵⁺, and lanthanide rare earth ions)^{97,129,203–207} have been introduced into the perovskite lattice to partially substitute Pb²⁺, realizing the optimization of the finally perovskite films. According to previous reports, the reduction of the bond length between B²⁺ and X⁻ can significantly enhance the stability of CsPbX₃ perovskites as well as the corresponding performance of photovoltaic devices owing to the optimized tolerance factor and enhanced formation energy for α-CsPbI₃ at room temperature and for the orthorhombic CsPbBr₃, respectively, as shown in Fig. 16a.²⁰⁸ Zou et al. proved that Pb²⁺ substitution with Mn²⁺ could result in lattice contraction and enhance the formation energy by means of theoretical calculations, thus fundamentally stabilizing the perovskite lattices.¹⁹⁷ Besides, by inserting Mn²⁺ ions into the interstices of the CsPbI₂Br lattice, the aspect ratio of the CsPbI₂Br crystalline grains could be enhanced as high as 8, as developed by Liu's group (Fig. 16b). Arising from the reduced trap density, when the MnCl₂ concentration was increased to 2%, the PCE reached a maximum of 13.47% (as well as a V_oc of 1.172 V, a J_sc of 14.37 mA cm⁻², and an FF of 80.0%).¹¹⁶ In theory, the valence band maximum is mainly determined by the antibonding hybridization B 6s and X np orbitals with dominant contributions from X np, while the conduction band minimum is mainly determined by the antibonding mixing of B 6p and X np orbitals with the dominant contribution from B 6p.²⁰⁹ Following this line of thought, B²⁺ doping can tune the band structure of all-inorganic perovskites. Indeed, upon introducing Mn²⁺ into the CsPbIBr₂ lattice, the valence band maximum values of perovskite films could be regulated from pristine −5.39 eV to −5.22 eV, reducing the energy barrier for effective photogenerated holes extraction. When assembling into a carbon based device, an enhanced efficiency of up to 7.36% could be achieved, with an increase of 19.9% in PCE compared to a control device.²⁰⁰

Partially substituting Pb with Sr has also proved to be an effective strategy to optimize the perovskite film quality. The PCE of CsPb_0.98Sr_0.02I₂Br solar cells increased from 6.6% (for CsPbI₂Br) to 11.3%, with a V_oc of 1.07 V, a J_sc of 14.9 mA cm⁻², an FF of 0.71, and a stabilized efficiency of 10.8%. Sr-doped CsPbI₂Br showed better thermal stability and a large grain size (Fig. 16c).¹¹³ This was the first demonstration of a low-temperature-processed CsPbI₂Br perovskite solar cell that had comparable efficiency to the high-temperature-processed Cs perovskite cells. Meanwhile, Ca²⁺ displayed a similar effect on enlarging the perovskite grain size and reducing the film roughness.²⁰² According to previous reports, incorporating isovalent small ions to increase the stability and suppress the formation of atomic vacancies has been proved to be an effective strategy in hybrid perovskite systems.²¹⁰ Zn²⁺, which has a stronger interaction to halide ions, can effectively manipulate the crystal growth and enlarge the grain size of CsPbI₂Br with enhanced growth orientation, resulting in reduced grain boundaries and accelerated charge transfer. The mechanism behind this phenomenon is mainly attributed to the controlled nucleation and crystalline growth processes, as shown in Fig. 16d. Arising from the interaction between Zn²⁺ and the adjacent I⁻ and Br⁻, the formed crystals easily coalesce and grow together into bigger domains.¹²⁶ Similarly, Tang's group explored the alkaline earth metal ions, such as Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, as dopants to modulate the CsPbBr₃ crystal structure, and found they could improve the PCE up to 9.68%.²⁰¹ By characterizing the recombination within the device, it was discovered that the defect state density could be significantly reduced, demonstrating that doping B²⁺ ions in CsBX₃ film is effective at enhancing the photovoltaic performance of all-inorganic PSCs.

Many works have reported that doping appropriate ions can further stabilize the perovskite lattice owing to the increased formation energy, which will in turn improve the solar-to-electric conversion ability. For example, in order to avoid the undesirable phase degradation from α-CsPbI₃ to the non-perovskite yellow phase δ-CsPbI₃ at room temperature, Zhang's group incorporated 4 mol% Bi³⁺ ions into a CsPbI₃ precursor solution, obtaining a controlled α-CsPbI₃ film with the typical cubic structure (Pm3m).¹²⁹ Along with the increase in doping dosage, the grain size was gradually decreased, similar to the Eu³⁺-doped CsPbI₂Br system,²⁰⁵ which was beneficial for the enhancement of the long-term stability (Section 3.1.4). After systematically optimizing the doping dosage, a recorded efficiency up to 13.21% with excellent stability was obtained. The mechanism behind this phenomenon was similar to HI addition or IPA treatment. As shown in Fig. 16e, with the increase in the Bi³⁺ component, a small microstrain (distorted cubic structure) will be induced in the crystals owing to the smaller ionic radius, hindering the transition from α-CsPbI₃ to δ-CsPbI₃. Different from Bi³⁺ ions doping, recently, it was found that yttrium ions (Y³⁺), as a dopant, could expand the PbI₂ interlayer, allowing DMOS molecules into the PbI₂ planes as a result of the interaction between Y³⁺ and DMSO, which in turn suppressed DMSO evaporation and tuned the film growth process (Fig. 16f).²⁰⁷ As a result, a much enhanced power conversion efficiency (PCE) of 13.25% could be achieved. Besides the efficiency, the long-term stability of the corresponding device could also be well enhanced owing to the stable crystal lattice. The corresponding stabilities of various ions-doped devices are summarized in Table 3.

---

For trivalent metal ions as dopants, it should be noted that trivalent metal ions are more difficult to substitute for Pb²⁺ owing to their unequal electric valence. Therefore, during the formation of the perovskite film, more highly ionic metal cations are believed to be predominantly expelled to the surface and grain boundaries, such as Al³⁺ and lanthanide ions (Ln³⁺).^97,211 Various Ln³⁺ ions, including La³⁺, Ce³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Ho³⁺, Er³⁺, Yb³⁺, and Lu³⁺, have been successfully incorporated into the CsPbBr₃ perovskite lattice to further increase the performance of the corresponding device. By optimizing the doping amount and Ln³⁺ ion species, a device with the architecture of FTO/c-TiO₂/m-TiO₂/CsPb_0.97Sm_0.03Br₃/carbon yielded a champion efficiency as high as 10.14% with a superior open-circuit voltage of 1.594 V, mainly attributed to the enlarged grain size and reduced recombination within solar cells. However, not all Ln³⁺ ions display a positive effect on enhancing the photovoltaic performance. Among them, Lu³⁺, Gd³⁺, La³⁺, Ce³⁺, Nd³⁺, or Eu³⁺ have a nearly unchanged effect on device performance, even dragging the PCE down. Therefore, there may be other mechanism behind Ln³⁺ doping of perovskite films owing to their unique multi-energy level construction.

To date, employing compositional engineering to dope an impurity into the all-inorganic perovskite lattice is reported to be an effective strategy to relax the lattice strain and passivate defects. However, no matter whether replacing the A-site or B-site, the doping amount should be carefully controlled. Excessive impurities will destroy the lattice construction of the perovskite, leading to a degradation of the final device performance.

5. Film-making methods

Perovskite film is the heart of an efficient PSC device; therefore, how to make a high-quality perovskite layer with a large grain size and high film coverage is a prerequisite to enhance the PCE output and the stability of inorganic PSCs. Considering the radiative and non-radiative recombination in perovskite films, tremendous studies are emerging concerning perovskite-making pathways. One basic principle is to combine two inorganic components, i.e., PbI₂/PbBr₂ and CsI/CsBr, to form CsPbI_3−xBr_x perovskite films. In general, the as-developed methods can be divided into three categories according to the film-formation process: one-step, two-step, and multi-step deposition methods. In this section, we discuss the development and advantages of these methods.

5.1 One-step deposition method

Similar to the organic–inorganic hybrid perovskite films, the inorganic CsPbI_3−xBr_x perovskite films can also be made by a classical one-step method through directly depositing precursor sources, including via the spin-coating technique and physical vapor deposition (co-evaporation) technique.

Among the different film-making techniques, the solution-processable technology represents the mainstream method owing to its low-cost, facile processes, and scalable space in producing high-quality perovskite films and therefore high-performance PSCs. To the best of our knowledge, the one-step solution deposition method was first proposed by Snaith and Grätzel to form a film of an organic–inorganic perovskite.^42,212 Later in 2015, Snaith and co-workers successfully prepared a black phase CsPbI₃ film by spin-coating a CsI:PbI₂ solution (1:1 in a molar ratio) in N,N-dimethylformamide (DMF) at a heating temperature of 335 °C, and the as-prepared inorganic CsPbI₃ film maintained its stability for a period of a few weeks.⁸⁵ However, the heating temperature of 335 °C is relatively high for practical photovoltaic applications. They subsequently proposed an additive method by adding a small amount of hydroiodic acid (HI) to the precursor solution to enhance the solubility of perovskite precursors, allowing for the growth of pinhole-free perovskite layers at only 100 °C for 10 min. However, its ambient instability still remained, limiting the fabrication and application of CsPbI₃ in an air atmosphere. Aiming to resolve the issue of stabilizing black perovskite CsPbI₃, Lu et al. modified the one-step spin-coating method by adding a subsequent solvent engineering process with isopropanol (IPA) treatment, achieving a novel low-temperature phase-transition route from a new intermediate Cs₄PbI₆ to stable α-CsPbI₃ for the first time in an air atmosphere.¹³³ So far, this one-step spin-coating technique has been considered universally applicable to fabricate uniform thin films of Cs-based perovskites with a high iodide content, such as CsPbI₃ and CsPbI₂Br.¹¹¹ Later, Mai et al. modified the one-step spin-coating method by adding a gradient thermal treatment process: first, the spin-coated film was placed on a hotplate at 30 °C to form the stable transition film; second, the temperature was increased to 160 °C for 20 min to remove the surplus solvent and to improve the crystallization quality, as shown in Fig. 17a and b. Though the PCE was inferior than that of other inorganic PSCs, it indeed demonstrated that a post-annealing procedure was essential to reduce the Schottky barrier and interface defect state.¹⁰⁶ Subsequently, they used the same film-making method to fabricate all-inorganic CsPbI₂Br PSCs with an inverted FTO/NiO_x/CsPbI₂Br/ZnO@C₆₀/Ag structure by introducing ZnO@C₆₀ bilayer electron transfer layer, achieving a PCE of over 13.3% and a remarkably stabilized power output of 12% within 1000 s. Importantly, the device free of encapsulation exhibited long-term thermal stability, with only 20% of the PCE quenched after being heated at 85 °C for 360 h.¹²⁰

During the growth of perovskite films, two processes determine the film quality: the first nucleation step and the crystal growth step, which are highly dependent on the solubility of the perovskite precursor and the solvent evaporation rate, respectively. Following this line of thought, the precursor solution temperature is another crucial parameter for suppressing the formation of too many nuclei and for increasing the crystallization rate. Liu's group systematically studied the relationship between crystallization and the fabrication temperature, and found that a precursor solution temperature fixed at 100 °C could dramatically enlarge the grain size of a CsPbI₂Br film.²¹³ When assembling into a planar architecture device, a maximum PCE of up to 14.81% was achieved, demonstrating the importance of temperature in one-step spin-coating technology.

According to the above-mentioned discussion, precise control of the crystal growth plays a crucial role in obtaining high-quality perovskite films. Therefore, gradient thermal annealing (GTA) was further developed by Chen et al., in which the phase conversion was conducted under 50 °C for 1 min, 100 °C for 1 min, and then 160 °C for 10 min, as shown in Fig. 17c.¹²⁴ When further processing the film with anti-solvent (ATS) treatment, such as toluene (Tol) and isopropanol (IPA), a high-quality CsPbI₂Br film with increased average grain sizes and reduced defect density could be obtained, which was mainly attributed to the rapid extraction of the residual solvent by isopropanol. Finally, a record efficiency of up to 16.07% was achieved. Strikingly, You's group further improved the one-step spin-coating method with an intermediate solvent-controlled growth, called SCG, to obtain CsPbI₃-perovskite films free of pinholes and a crystal size above 5 μm.¹³⁹ SCG, in detail, refers to the process whereby the precursor films were stored in a nitrogen glove box for several tens of minutes before annealing on a hotplate (Fig. 17d). This stagnation allowed the residual DMSO to improve the film quality by enhancing the mass transport and diffusion, achieving the champion PCE (15.7%) of CsPbI₃ perovskite solar cells; further, there was no change in the XRD patterns or absorption spectra after 7 days of storage.

In summary, there are dozens of reports about depositing high-quality perovskite films with a large grain size by a one-step method. The growth of perovskite film follows the “Volmer–Weber” growth mode, accompanied with a classic “heterogeneous” nucleation process and grain growth theory. In detail, the number of grains per unit area (Z) is determined by the nucleation formation rate (v_n) and the crystal growth rate (v_c), following the formula: Z ∝ (v_n/v_c), where Z is inversely proportional to the grain size. Therefore, retarding the nucleation process and accelerating the grain growth process are two strategies to increase the film quality. To date, the one-step method has been applicable to most cesium perovskites, such as CsPbI₃, CsPbI₂Br, and even CsPbIBr₂, but not for CsPbBr₃, owing to its inferior solubility.

Although the solution-processable method provides great advantages to establish an ideal perovskite film, the environmental conditions during perovskite film formation, such as temperature, precursor solubility, atmosphere, annealing time, and so on, lead to an inferior repeatability. In order to overcome these issues, the physical co-evaporation technique was further attempted to obtain an inorganic perovskite light-harvesting layer by Ho-Baillie et al.¹⁰⁴ As is well known, a dual source thermal evaporation technique was first employed in a hybrid perovskite system.²¹⁴ In detail, an equal molar of CsX and PbX₂ were simultaneously evaporated onto an FTO glass substrate supported electrode, leading to the successful crystallization of a CsPbX₃ perovskite film. This physical process did not involve any solvents, therefore, there was no solvent residue left in the film and so this technology is applicable to deposit insoluble materials. Troshin and co-workers prepared CsPbI₃ films by thermally co-evaporating CsI and PbI₂ precursors under a vacuum environment for planar heterojunction inorganic PSCs (Fig. 18a).¹³⁰ Different from spin-coated films, the low-temperature brown perovskite phase of CsPbI₃ is a particular feature for vacuum thermal co-evaporation processes, as it is unstable and undergoes an irreversible transformation to a yellow phase under heating at temperatures above 60 °C. On the contrary, when heated up to 320 °C, the yellow phase undergoes a phase transition back to the black phase, which is depicted in Fig. 18b. Film annealing at 320 °C offers the best optimal light absorbance, which is consistent with the phase transition temperature from the orthorhombic yellow phase to the cubic black phase. After annealing, the film quality can be significantly optimized, with an increased grain size up to 1–3 μm. In this fashion, it is obvious that the annealing temperature is still necessary for fabricating a high-quality perovskite film. The perovskite films prepared by the co-evaporation method present few impurity defects and a dense and uniform surface. However, this method requires a high vacuum and high-strength equipment as well as large energy consumption.

Vacuum co-deposition is the third technique to make inorganic perovskite films; for example, Lin et al. prepared CsPbI₃ and CsPbI₂Br films with the aid of the physical vapor behavior of sublimed cesium-based perovskite precursors.¹¹⁵ Since the smooth CsI surface dramatically changes to a rough and diffusive appearance when exposed to air, a vacuum chamber with observation and measurement windows is utilized to avoid air exposure during the formation of the perovskite phase. Through tuning the annealing time, the best CsPbI₂Br solar cell showed a promising PCE of 11.8%, with a V_oc of 1.13 V, J_sc of 15.2 mA cm⁻², and FF of 0.68. This method has advantages of a precise deposition rate and accurate molar ratio of precursor materials, as shown in Fig. 19, which are critical for device performance, leading to small hysteresis and superior stability.

Meanwhile, taking the different solubilities of components into consideration, especially for double perovskites with a complex composition, vapor-deposited technology shows great potential for high-quality perovskite films as well as for fabricating lead-free semiconductors. Liu's group successfully developed a sequential-vapor-deposition method to fabricate an all-inorganic double perovskite Cs₂AgBiBr₆ film, and a maximum PCE of 1.37% was achieved.²¹⁵ Compared to solution-processable methods, physical-vapor-deposition demonstrates universal applications in photovoltaic areas.

5.2 Two-step deposition method

In general, the development of inorganic perovskite films follows the same steps as organic–inorganic hybrid perovskites, in which a two-step spin-coating technique has been widely employed for fabricating a high-quality perovskite layer. To date, the two-step or multi-step deposition method (will be discussed in the following part) has been mainly employed to fabricate Br-rich all-inorganic perovskites owing to the solubility limitations of Br⁻ ions in conventional solvent, which leads to perovskite films with B-rich compositions being difficult to be obtained. Hodes et al. proposed that the two-step spin-coating technique can be used to form cesium lead halide perovskites.⁸⁴ In detail, the first step is to spin-coat lead bromide solution in DMF onto a mesoporous TiO₂ scaffold and to then dry at 70 °C. Subsequently, these lead bromide films are dipped in a heated solution of the cesium halide salt(s) in methanol for 10 min, thereby realizing the successful perovskite formation. Meanwhile, Jin's group also employed this method to assemble a carbon-based device with the architecture of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/carbon, obtaining an efficiency of 6.7% together with excellent long-term stability.⁸⁸ However, the uncontrollable dipping process drags down the reproducibility and high-quality film formation. To address these issues, Chen et al. systematically explored the relationship between the CsPbBr₃ film quality and dipping time as well as the reaction temperature, demonstrating tat the morphology and composition was highly dependent on the two parameters. Finally, the PCE could be increased to 3.9% from the pristine 1.3% after optimizing the reaction time and temperature.⁸⁶ In fact, the phase of the perovskite film obtained by this method is complicated. Therefore, Yu and coworkers modified this two-step method by introducing a face-down dipping process to restrict the decomposition of the unwanted precursor films in solution and to fabricate high-quality CsPbBr₃ perovskite films with an average grain size of 860 nm.²¹⁶Fig. 20 illustrates the differences in depositing perovskite films, and therefore inorganic perovskite film structures, between the face-up dipping and face-down dipping process.

Most reported CsPbBr₃ films are prepared by a solution process; however, the rapid liquid-phase reaction is hard to control, resulting in an inferior perovskite film with enormous pores and notorious efficiency. Therefore, employing an intermediate phase to modulate the final perovskite quality is an effective strategy. Inspired by the rapid reaction between MAPbI₃ and gaseous Br₂ and HBr,¹⁴⁶ Sun et al. fabricated a CsPbBr₃ perovskite film with a Br₂-vapor-assisted CVD method, as shown in Fig. 21a, in which fast anion exchange from CsPbI₃ to CsPbBr₃ could be realized owing to the effective incorporation of Br⁻ into CsPbI₃ inorganic perovskites in a few of seconds.⁸⁷ First, the CsPbI₃ precursors were spin-coated onto the FTO/c-TiO₂/m-TiO₂ substrates, and then transferred into a quartz tube furnace. After pumping the quartz tube for 5 min, the CsPbI₃ precursors were heated to 150 °C and then Br₂ vapor was injected into the hot quartz tube for reaction. Finally, the perovskite films were taken out and rinsed with isopropanol, and annealed in ambient air to obtain a bright yellow CsPbBr₃ layer. Finally, a relatively high PCE of up to 5.38% (Fig. 21b) was achieved based on this CsPbBr₃ all-inorganic perovskite solar cell with carbon as the back electrode. The perovskite layer made by the vapor-assisted method always features high coverage, a smooth surface, and tiny grain size, thus reducing the surface recombination rate during carrier transportation and thereby achieving a high V_oc output. Moreover, the vapor-assisted technique does not need special requirements, it is economical and environmental compared to the co-evaporation or solution-processable spin-coating methods.

Perovskite-structured quantum dots (QDs) have also been applied to assemble perovskite films for photovoltaic applications, and are beneficial for eliminating the inconclusive self-assembly crystallization processes as determined by different factors including the precursor ratio, solvent, processing additives, substrate roughness and surface energy, atmospheric/environmental conditions, annealing temperature, and treatment time. To the best of our knowledge, CsPbI₃ QDs have recently been utilized to stabilize the crystal phase for the first time, which was discussed in Section 3.1.4.¹²⁸ By spin-coating QDs onto the surface of TiO₂ film, a high V_oc of 1.23 V and efficiency of 10.77% were obtained, demonstrating the great potential for enhancing the overall photovoltaic performance of all-inorganic PSCs. Subsequently, a CsPbBr₃ QDs “ink” was developed to fabricate fully air-processed and stable solar cells, exhibiting an efficiency over 5% with high V_oc values.^157,217,218 This strategy may provide a new path to resolve the dissolution issue of the CsPbBr₃ precursor.

The uniform deposition and reliable anchoring of perovskite precursors in larger sizes often requires specific conditions, but spray-assisted deposition is regarded as a straightforward, fast, and cost-effective way to pattern substrates on a large scale. Ho-Baillie et al. demonstrated the feasibility of spray-assisted deposition to overcome the poor solubility of the bromide ion in one-step and two-step solution methods.¹⁰⁵ In this method, CsI was sprayed onto a spin-coated PbBr₂ layer under ambient conditions (see Fig. 21c). The substrate temperature during the spraying of CsI and the annealing temperature were found to be the most crucial parameters for the quality of the perovskite film. As depicted in Fig. 21d, the main peaks at 14.82°, 21.05°, and 29.94° corresponding to the (100), (110), and (220) planes of the CsPbIBr₂ perovskite orthorhombic phase could be clearly detected. With a much higher substrate temperature, the segregation of CsI (27.5°) became prominent. Fig. 21e demonstrates the crystallization of CsPbIBr₂ perovskite after annealing at 275 °C, 300 °C, 325 °C, or 350 °C for 10 min in atmospheric conditions, suggesting that a good annealing temperature range is from 275 °C to 350 °C. Using the optimized spray-assisted method, the best-performing inorganic FTO/bl-TiO₂/mp-TiO₂/CsPbIBr₂/Spiro-OMeTAD/Au PSC achieved a stabilized PCE of 6.3% with negligible hysteresis.

5.3 Multi-step deposition method

To date, the multi-step technology is generally employed to fabricate all-brominated CsPbBr₃ perovskite films. During the fabrication process of CsPbBr₃ perovskite films, CsPb₂Br₅ or Cs₄PbBr₆ will be formed when employing the traditional two-step solution-processed film-making technique. To effectively resolve this issue, a novel multi-step method to form a high-purity cesium lead bromide film with vertical and monolayer-aligned largest grains were proposed by Tang's group, leading to maximizing the charge transportation kinetics and therefore an increased PCE from 6.7% to 9.72% after incorporating GQDs.⁸⁹

A schematic diagram of multi-step deposition technology is shown in Fig. 22a–d, involving a single PbBr₂ layer and the multi-step spin-coating of a CsBr layer. By tuning the number of deposition cycles with the CsBr solution, the components, crystal structure, and morphological alignment of the as-prepared perovskite films could be well controlled. From the SEM images, it could be seen that the film coverage and grain size were insufficient when the deposition number was low. Along with increasing the deposition number, an ideal perovskite film could be obtained when the deposition number was controlled at 4 times. The mechanism behind this phenomenon can be explained by the gradual phase conversion and crystal growth processes. Through characterizing the phases of as-prepared perovskite films with various deposition numbers of CsBr solution, the crystalline phase conversion during the multi-step process can be summarized into three reactions as follows:

2PbBr2 + CsBr → CsPb2Br5 (n ≤ 3)	(1)

CsPb2Br5 + CsBr → 2CsPbBr3 (n = 4)	(2)

CsPbBr3 + 3CsBr → Cs4PbBr6 (n ≥ 5)	(3)

In this fashion, fewer CsBr coating cycles generally lead to the formation of a PbBr₂-rich CsPb₂Br₅-dominant film, while a higher number of CsBr coating cycles will accelerate the reaction of the CsPbBr₃ phase with CsBr to form a CsBr-rich Cs₄PbBr₆-dominant film. Therefore, by a precise control of the amount of CsBr, a high-purity inorganic perovskite film with a minimized mixed phase can be obtained. The thin film with vertical and monolayer-aligned grains has a lower density of defect states and fewer grain boundaries, and therefore the absorption layer is expected to further improve the device performance upon assembly into a photovoltaic device.

Similarly, spray-coating technology has been widely used in industrial processes due to its advantages in scaling up for large area cella.^219,220 Inspired by the above-mentioned conclusion, a multi-step spraying method was further developed to fabricate large-area solar cell devices by Tang's group.⁹⁸ Their studies revealed an identical phase conversion from PbBr₂-rich CsPb₂Br₅ to perovskite-structured CsPbBr₃ and further to CsBr-rich Cs₄PbBr₆ by spraying CsBr multilayers sequentially, which was in accordance with a previous report.⁸⁹ Following the above-mentioned discussion, an evaporation-assisted solution method was further developed, as shown in Fig. 23a. Based on their conclusion, the corresponding phase conversion from CsPb₂Br₅ to CsPbBr₃ and then to Cs₄PbBr₆ was also confirmed. By optimizing the CsBr deposition time and accelerating the charge transfer, a high efficiency of up to 10.45% was obtained.¹⁰² Obtaining a high-purity CsPbBr₃ film provides a new avenue for making high-performance inorganic PSCs by multi-step spray-assisted method besides the commonly used one-step and two-step techniques, which require a high-temperature annealing process.

To further improve the efficiency of inorganic PSCs, many modifications have been developed recently. Liu et al. developed a facile and modified multistep spin-coating strategy, wherein the PbBr₂ film was first immersed into CsBr solution (two-step deposition method), with a subsequent spin-coating of a multi-layer of CsBr to realize a highly crystalline CsPbBr₃ film. Upon interfacial modification, a remarkable efficiency as high as 8.79% was obtained.²²¹ Meanwhile, Tong et al. constructed a gradient bandgap architecture of CsPbBr₃/CsPbBr₃–CsPb₂Br₅/CsPbBr₃–Cs₄PbBr₆via thermal evaporation and spin-coating technology, achieving an enhanced efficiency of up to 10.17%.¹⁰¹ Furthermore, they developed a strategy to fabricate high-efficiency CsPbBr₃-based PSCs by forming perovskite derivative phases (CsPb₂Br₅ and Cs₄PbBr₆) via a vapor growth method. During the post-annealing process, the derivative phases as nucleation sites are transformed to the pure CsPbBr₃ phase accompanied by crystal rearrangement and a retardation in the rapid recrystallization of perovskite grains. As shown in Fig. 23b, the phase transition-induced (PTI) method is beneficial for eliminating the internal stress and for lowering the surface potential barrier. Owing to the improved film quality, a PCE of 10.91% was achieved for n–i–p structured PSCs with silver electrodes, and a PCE of 9.86% for hole-transport-layer-free devices with carbon electrodes.¹⁰³ To date, it is obvious that a key advantage of halide perovskites among the various different high-performance semiconductors is their processability. As discussed in this section, a wide variety of strategies can be employed to fabricate all-inorganic perovskite films, providing more options on the path of commercialization.

6. Interfacial engineering

6.1 Theoretical analysis (the detailed balance model)

The detailed balance model is an indispensable tool for predicting the efficiency limit of solar cells.^222,223 The model only considers the detailed balance between absorbed photons and emitted photons. The current density of a solar cell is calculated as the difference between the photons radiating and the photons absorbed by the cell:

J = Je(V) − Jph	(2)

where V is the applied voltage, J_e represents the radiative recombination current, and J_ph is the photogenerated current due to the absorption of incident light in inorganic perovskite materials, i.e.,

	(3)

where c₀ is the speed of light in air, Γ is the global AM 1.5G spectrum of the Sun, λ is the wavelength, and q is the elementary charge, and a(λ,L) is the absorptivity of the inorganic perovskite solar cell with an active perovskite layer thickness of L.

The absorptivity a is the ratio of power absorbed by the active layer over the power of incident sunlight, which depends on the thickness of the perovskite layer L, the refractive indices of the materials adopted, and the light-trapping structures. For solar cells having a planar front surface with a perfectly reflecting mirror on the rear, we have:

a(λ,L) = 1 − exp[−2α(λ)L]	(4)

Here, we ignore the reflection from the front surface, and α is the absorption coefficient of the active material. For solar cells having a randomly textured front surface with a perfectly reflecting mirror on the rear, the absorptivity can be written as:^224,225

	(5)

where n_r is the real part of complex refractive index of the active material, and θ_m is the maximum angle of emission (with respect to the normal of the front surface), i.e., light escapes out of the solar cell within a cone with a solid angle of θ_m. For complex nanostructures, the absorptivity can be obtained by numerically solving Maxwell's equations. For the limiting efficiency (Shockley–Queisser limit) case,

	(6)

where λ_g is the bandgap wavelength of the inorganic perovskite material. An ideal cell will absorb all the photons with energy above the bandgap energy.

According to the detailed balance theory and Boltzmann statistics, the radiative recombination current J_e can be expressed as:

	(7)

where J_0e is the radiative saturation current, which is the minimum of the (reverse) saturation current. J_0e can be calculated by the blackbody radiation law:

	(8)

where Γ₀(λ,T) is the blackbody emission spectrum of the solar cell operated at the temperate T, and S(λ,T) is the corresponding thermal radiance of the cell.

For a solar cell system, the dominated thermodynamic loss is caused by the non-radiative recombination, including the Shockley–Read–Hall (SRH) recombination (monomolecular recombination) and Auger recombination. The defects, impurities or deep-level traps at the bulk and interface will induce the SRH recombination, while the high carrier density by injection or generation will induce the Auger recombination, which always occurs in heavily doped inorganic semiconductors. Consequently, to describe the electrical response of a practical inorganic perovskite solar cell, the non-radiative recombination current should be introduced. Thus, eqn (2) should be modified as:

	(9)

where J_n,srh and J_n,aug are the non-radiative recombination currents for the SRH and Auger recombination, respectively, C_srh and C_aug are the SRH and Auger recombination rates, respectively, and J_0n,srh and J_0n,aug are the non-radiative saturation currents for the SRH and Auger recombination, respectively.

One may approximate eqn (9) as a more compact form given by:

	(10)

where J is the total recombination current including the radiative and nonradiative ones and m is the ideality factor, and J₀ is the dark current or saturation current, which should be the superposition of the radiative saturation current J_0e and the non-radiative saturation currents J_0n,srh and J_0n,aug. From eqn (10), we can obtain the expression of V_oc for the inorganic perovskite solar cells:

	(11)

Thus, we can extract the ideality factor m to understand the recombination mechanism of the inorganic perovskite solar cells by investigating the light-intensity-dependent V_oc curve. Fig. 24 shows the V_oc–log(J_ph) curve for a CsPbBr₃ system, and m is approximate to 2. Therefore, the dominant recombination in inorganic PSCs is the SRH recombination.

Also, we can split the saturation current into radiative and non-radiative ones, i.e.,

	(12)

where η_ELQE is the external luminescence quantum efficiency (ELQE).²²⁶ The ELQE is the number of photons emitted into free space over the number of electron–hole pairs generated by light absorption, which is a figure of merit for the photon recycling of inorganic perovskite solar cells.²²⁷ Finally, we consider two circuit quantities involving the series resistance R_s, describing the ohmic loss from the interfaces and electrodes, and the shunt resistance R_sh, describing the shunt current resulting from the voids and defects. The two circuit parameters are essential to the fill factor of the inorganic perovskite solar cells. Incorporating the series and shunt resistances into the detailed balance model, the modified model can be written as:²²⁸

	(13)

here, eqn (13) is the modified detailed balance model to capture the current density–voltage characteristics of the inorganic perovskite solar cells.

From the detailed balance theory and eqn (2), it is easy to predict the limiting efficiency of inorganic PSCs. Table 4 lists the calculated results for the commonly-used inorganic perovskite materials, with organic–inorganic halide perovskite materials also presented as references. Regarding the worst CsSnBr₃ and CsSnI₃ systems, they show giant J_sc losses and V_oc losses due to their low internal quantum efficiency and nonradiative recombination, respectively.^187,192,229 The most promising system is CsPbI₂Br, which shows the lowest PCE loss (40.6%) in all the inorganic perovskite systems. Therefore, accelerating the charge extraction at the interface is crucial for high-performance all-inorganic perovskite solar cells.

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6.2 Interfacial modification

Inorganic perovskites have advantages in terms of their ambient phase stability compared to organic–inorganic perovskites; however, the serious energy barrier at the interface between the perovskite and HTL and/or ETL limits the charge extraction and therefore drags down the device performance. Thus, interfacial engineering is a prerequisite to eliminate interfacial structural and electronic mismatches, lowing the charge transfer barrier for maximized charge transfer dynamics, including ETL/perovskite and HTL/perovskite interfaces.

In a typical perovskite solar cell, free electrons (photogenerated electrons) are usually injected into the TiO₂ layer and then in to the external circuit under an electric field. Although the transfer rate can be up to 10⁹ s⁻¹, substantial charge is still lost when crossing the interface owing to the presence of the energy barrier as well as defects.²³⁰ To better address this issue, enormous efforts have been conducted to boost the charge extraction. Highly conductive materials with a matchable energy structure to the ETL and perovskite (such as graphene QDs, carbon QDs, or C₆₀) have been incorporated in an attempt to accelerate charge transfer.^89,93,120 Upon introducing graphene QDs into the TiO₂/CsPbBr₃ interface, the charge-extraction time could be shortened from 116.2 ns to 99.3 ns.⁸⁹ Furthermore, by combining the C₆₀ with ZnO to form a ZnO@C₆₀ bilayer as an electron-transporting layer, the recombination reaction could be suppressed, owing to the higher driving force of 0.34 eV for C60 than 0.04 eV for ZnO layer, as shown in Fig. 25a and b. Consequently, the all-inorganic CsPbI₂Br perovskite solar cell yielded a PCE as high as 13.3% with excellent long-term stability.¹²⁰ Besides, TiO₂/SnO₂ and SnO₂/ZnO bilayered electron-transporting layers have also been developed to provide a buffer area for charge transfer.^117,221 As shown in Fig. 25c and d, ZnO displayed a desirable cascade energy level alignment between the perovskite and SnO₂ ETL, resulting in suppressed interfacial non-radiative recombination and a high PCE of 14.6%. In this work, the effects induced by the different interfacial ETLs were determined by steady-state photoluminescence (PL) measurements and time-resolved PL decay, demonstrating the beneficial effect on modulating the electron-extraction processes.

Another strategy to realize the rapid transfer of electrons to the ETL layer and to reduce recombination is to tune the electronic state or to increase the conduction band minimum (CBM) of the window layer. A simple interfacial engineering process by passivating SnO₂ with SnCl₂ could significantly reduce the energy loss for a high V_oc device, which may be associated with the interaction between Cl⁻ and SnO₂ according to previous reports in hybrid perovskite solar cells.^231,232 It should be noted the interfacial engineering is universal in all types of photovoltaics. Therefore, enormous works have employed this strategy in organic–inorganic perovskite solar cells, such as with lead-doped mesoporous TiO₂ and nanofibers,²³³ Sb₂S₃,²³⁴ MgO,²³⁵ or Y₂O₃ (ref. 236) as interfacial modification materials, to increase the performance of all-inorganic perovskite solar cells.

The large interfacial energy differences at perovskite/HTL interfaces is another reason to depress the charge-extraction kinetics. One promising solution to this impasse is to narrow the energy differences by tuning the energy band edge of cell layers or by setting intermediate energy levels at interfaces. Quantum dots (QDs) with tunable bandgaps, high absorption coefficients, and small size have attracted great interest as interfacial modifiers in PSCs to increase electron–hole separation.²³⁷ Aiming to realize the successful fabrication of this physical-of-proof concept graded bandgap construction, perovskite films and QDs films should be exhibited using orthogonal solvents, allowing the subsequent coating of upper multilayers. Especially for perovskite QDs, upon perovskite QDs covered onto the surface of perovskite active layer, anion exchange is inevitable at the interface, resulting in a component-graded heterojunction, as shown in Fig. 26. Liu's group integrated bulk-nanosheet-quantum dots to optimize the energy alignment between CsPbBrI₂ and the hole-transport layers, achieving a PCE of 12.39%. This was the first designation of a 3D–2D–0D multiple graded interface based on CsPbBrI₂ material via a solution process.¹¹⁹ Later on, optical and energy-band manipulation were further employed to enhance the CsPbI₂Br device performance. A stable and high-mobility CsPbI₃ QDs film was obtained through Mn²⁺ substitution, SCN capping, and [(NH₂)₂CH]⁺ treatment, achieving a PCE of 10.97% based on this CsPbI₃ QDs film. Subsequently, a halide-ion-profiled heterojunction was designed at the CsPbBrI₂/CsPbI₃ QD interface to improve the carrier collection. Arising from the extended light absorbance range and minimal charge recombination loss, as a result, the device achieved a PCE of 14.45%.¹²³

For CsPbBr₃ perovskite solar cells, the hole-extraction barrier is a more serious issue due to their larger bandgap, which achieves a barrier value of 0.6 eV in carbon-electrode-based devices. Therefore, modulating the interfaces of CsPbBr₃/carbon provides huge room for efficiency improvement. Tang et al. synthesized a series of p-type and bandgap-tunable QDs, such as CuInS₂/ZnS,⁹² CdZnSe@ZnSe,²³⁸ CsSnBr_3−xI_x (ref. 95) and red phosphorus QDs,^93,94 to bridge the large gap at the perovskite/carbon interface, setting an intermediate energy level between the perovskite and back electrode for the sake of hole transportation from the perovskite, as shown in Fig. 27. By optimizing the species and energy levels of QDs, the efficiency of all-inorganic CsPbBr₃ perovskite solar cells could be enhanced to around 10%, which is much higher than that of the control device. Recently, organic materials, including P3HT,²³⁹ P3HT/zinc phthalocyanine composition,²⁴⁰ polythiophene, polypyrrole, and polyaniline, as well as organic small molecule BT-BTH²⁴¹ were also employed to modify the CsPbBr₃/carbon interface to enhance the overall performance of corresponding devices, demonstrating the importance of reducing the energy loss and recombination by interfacial engineering.

Lowering the work function (WF) of a carbon electrode is another path to accelerate charge transfer. Tang's group developed a novel method to tune the WF of a carbon electrode by doping alloyed PtNi nanowires to accelerate hole extraction from CsPbBr₃ halides (Fig. 28).⁹¹ The WFs of metallic Pt and Ni are −5.65 eV and −4.6 eV, respectively. After alloying, PtNi nanowires with a WF of −5.5 eV could be obtained. Upon combination with carbon paste, the WF of carbon/PtNi composition electrode could be well-tuned ranging from −5.1 to −5.5 eV through controlling the PtNi dosage (1–7 wt%). The preliminary results demonstrate a maximized PCE of 7.17% (J_sc = 6.54 mA cm⁻², V_oc = 1.431 V, FF = 76.6%) at 3 wt% PtNi nanowires. Although the WF of −5.5 eV was more matchable at 7 wt% PtNi to −5.6 eV (the VB value of CsPbBr₃ film), the poor adhesion of carbon/PtNi to the perovskite layer caused serious charge transfer resistance. This research indicated that the matching WF and good adhesion of the carbon back electrode is beneficial to hole extraction and electron–hole separation. Meanwhile, carbon nanotubes were also introduced to adjust the WF of the carbon electrode, whereby the performance of solar cells could also be improved.²⁴²

As discussed above, inserting an intermediate energy level and lowering the WF of the charge-contact layer are effective ways to improve charge separation and extraction. Aiming to further improve the efficiency, a dual interfacial design for efficient CsPbI₂Br perovskite solar was developed by applying an amino-functionalized polymer (PN4N) as a cathode interlayer and a dopant-free hole-transporting polymer poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT) as the anode interlayer.²⁴³ Apart from providing a better energy-level matching, both theoretical and experimental results revealed that the organic materials could couple with perovskite crystals, passivating the surface-region defects and suppressing the photo-induced halide segregation of CsPbI₂Br films. With these positive effects, the optimal device achieved an enhanced efficiency of up to 16.2%, mainly attributed to the reduced trap-assisted nonirradiative recombination and increased charge-extraction ability.

Considering “devices are interfaces,” fully understanding the interfacial atomic and electronic structures is essential for interface design. After introducing functional materials, the lattice stability and charge transfer are two crucial parameters to evaluate the feasibility of interfacial engineering. Overall, new interface modifiers and heterojunction device designs still need to be developed in the future.

7. Challenges and outlook

The birth of PSCs in 2009 undoubtedly heralded a new frontier for the third-generation photovoltaics. PSCs have experienced a great change from liquid to solid junctions and from spatial three-dimensional to planar two-dimensional structures, and the maximized PCE has increased from an initial 3.8% to now 25.2% within these past ten years. As a promising candidate, inorganic PSCs, due to their much more stable phase, have inevitably attracted more and more attention on the basis of the existing technology. However, several challenges still need to be resolved, such as reducing the costs, improving the efficiency and stability, to push their industrialization. Here, some brief aspects are enumerated in the following:

(1) Research on a deep insight into the photophysics mechanism:

At present, although much research is focused on improving the film synthesis methods or material properties, the physical mechanism of inorganic perovskite solar cells is still lacking. A deeper understanding and in-depth study on the mechanism of interfacial charge recombination and regarding the stability of materials would be useful for finding new structures and for further improving the perovskite solar cell performance.

(2) Broadening the absorption spectrum:

Commercial Si solar cells can absorb light from 300 nm to 1200 nm. However, the inorganic CsPbI₃ perovskite film can only absorb light to nearly 700 nm, not to mention the maximized 540 nm for CsPbBr₃ perovskite; such a narrow light response causes serious solar energy loss. Therefore, how to broaden the light absorption of inorganic PSCs is an inevitable course to achieve champion PCE outputs. Compared to hybrid perovskites, the inorganic perovskite is a promising material for tandem solar cells owing to their relatively larger bandgap. Therefore, developing tandem solar cells with inorganic PSCs and Si or CIGS solar cells may be a focus in the future.

(3) Enhanced stability:

Inorganic CsPbBr₃ perovskite is regarded as having high stability under UV light, water, and/or heat attacks; however, other I-containing inorganic PSCs still suffer from light- and steam-induced phase degradation, even with encapsulation technology. So, manufacturing a long-time working PSC device with a high PCE is still an urgent issue to be implemented. Doping CsPbX₃ with smaller radius ions has recently been highlighted as a promising way to achieve phase stability and reduce the trap density of the perovskite layer. Other strategies, such as solvent controlled film growth, alloying a 2D perovskite component into host perovskite, polymer-induced surface passivation, and interfacial engineering, all pave the way to stabilize CsPbI₃.

(4) Environmentally friendly issues:

Lead is a widely-used heavy metal element but is extremely harmful to the human body, especially to children. A lead amount more than 10 μg dl⁻¹ in blood can cause irreversible damage to the mental development of children, so avoiding the usage of lead in perovskite solar cells is a significant work. However, to date, highly efficient perovskite solar cells are all based on toxic metal Pb, or on the partial substitution of Pb with other elements. In this regard, developing lead-free perovskite light-absorbing layers is an important direction in the future. Although some researchers have made great efforts with lead-free perovskite solar cells, their photoelectric conversion efficiency is still low compared to Pb-based devices. Thus, further in-depth systematic research on materials and structures and the preparation methods of new lead-free perovskite solar cells is essential.

(5) Research on large-area devices:

Scale-up and large-area processing technology has become a tricky problem that must be resolved before commercialization. Although some researchers have focused on large-area devices, their film uniformity is still poor as is their efficiency. So how to get high-efficiency large-area devices is still a great challenge. Tremendous progress has already been made in enhancing the performance of small-area cells (typically ≤ 0.1 cm²). However, many of the film-making techniques (such as spin-coating and drop-casting) used in the laboratory do not suit large-scale production. Spin-coating is currently the most common film-making strategy for inorganic perovskite layers but not for large-area processing (≥1 cm²). Besides, much material may be wasted during the process. Spraying and printing methods seem to be promising methods for scale-up. However, clearly, scaling up the size of perovskite solar cells without significant efficiency loss is a remaining challenge to be resolved.

(6) Flexible perovskite solar cells:

With an aim to achieve the building of integrated photovoltaics, there have been many reports on organic–inorganic flexible perovskite solar cells. However, insufficient studies on all-inorganic perovskite solar cells have been undertaken. Recently, Liu's group improved the quality of the perovskite absorber layer by adding dimethyl sulfide, enhancing the efficiency of the flexible perovskite solar cell.²⁴⁴ Zhao's group demonstrated a facile method to fabricate a high-performance all-inorganic CsPbI₂Br perovskite solar cell through a one-step method under 100–130 °C low-temperature annealing process, which was beneficial for assembly on flexible substrate.¹⁷⁶ Therefore, more research should be done to promote the development of flexible inorganic perovskite solar cells.

8. Conclusions

The tremendous progress made in the development of inorganic perovskite solar cells convinces that inorganic perovskites is a promising alternative to organic–inorganic perovskites, due to their low-cost facile fabrication process and long-time stability. This review systematically summarizes the developments of inorganic halide perovskite solar cells in respect of compositional engineering, film-making methods, and interfacial engineering methods. It also covered predicting their limited efficiency using the modified detailed balance model and discussed the recombination mechanism. Finally, by identifying new challenges, several outlooks are provided for further research and potential development in this area.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (61774139, 21503202, 61604143), the Fundamental Research Funds for the Central Universities (11618409, 21619311), and China Postdoctoral Science Foundation (55350315).

Notes and references

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