Pathways toward high-performance inorganic perovskite solar cells: challenges and strategies

Bo Li , Lin Fu , Shuang Li , Hui Li , Lu Pan , Lian Wang , Bohong Chang and Longwei Yin *
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan 250061, P. R. China. E-mail: yinlw@sdu.edu.cn; Fax: +86 531 88396970; Tel: +86 531 88396970

First published on 2nd September 2019

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Bo Li

Bo Li graduated with a BS in materials science and engineering from Shandong University in 2015. He is currently a PhD candidate in the Department of Materials Science and Engineering at Shandong University under the supervision of Professor Longwei Yin. His research interests focus on the structure and photophysics of semiconductor materials and inorganic perovskite solar cells.

Lin Fu

Lin Fu received her BS in materials science and engineering from Shandong University in 2016. She is currently a PhD candidate at Shandong University. Her research interests mainly focus on the synthesis of inorganic perovskite materials, and the technological improvement of perovskite solar cell devices.

Longwei Yin

Longwei Yin is a distinguished professor and director of the Institute of Materials Physics & Chemistry, Shandong University, China. He received his PhD in 2001 from SDU. He carried out his postdoctoral research from 2003 to 2006 in Prof. Yoshio Bando's group at the National Institute for Materials Science, Japan. He won the National Science Fund for Distinguished Young Scholars in 2011. His research interests mainly focus on energy conversion & storage devices, including perovskite solar cells, photo-electrochemical catalysis, secondary batteries, supercapacitors, etc.

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1. Introduction

Organic–inorganic hybrid halide perovskites have been one of the most-studied photovoltaic materials in recent years.^1–10 Since the initial discovery of their potential photovoltaic applications, the power conversion efficiency of organic–inorganic hybrid halide PSCs has reached over 24%, which is comparable with that of the state-of-the-art copper indium gallium diselenide (CIGS) solar cells and close to that of commercial crystalline silicon solar cells.^11–20 This demonstrable rapid development of perovskite photovoltaic technology is largely due to the simple and reproducible solution/vapor-chemistry fabrication techniques and excellent optoelectronic properties such as suitable direct bandgap, extra-long carrier diffusion length and high absorption coefficient of perovskites.^21–26

Despite the high efficiencies and excellent properties, the poor stability of organic–inorganic hybrid halide perovskite materials is still a critical challenge preventing this photovoltaic technology from being commercialized.^27–35 To be marketable for commercial applications, perovskite photovoltaic devices must be able to work continuously for over 20 years under outdoor conditions.³⁶ Thus extensive attention has recently been focused on the long-term stability of perovskite materials with regard to prolonged exposure to heat, moisture, oxygen and light.^37–43 Inorganic materials normally exhibit better stability compared to organic materials, especially in terms of high temperature. One of the feasible ways forward, therefore, is substituting the organic cations with cesium cations (Cs⁺) due to their inherent inorganic stability.^44–48

Cesium-based inorganic perovskites were first applied as the light absorber in perovskite solar cells with around 6% PCE by Hodes's group in 2015.⁴⁹ In this pioneering report, inorganic CsPbBr₃ was demonstrated to work equally as well as the organic–inorganic one, especially generating a high open circuit voltage (V_oc) which is a prominent feature of these solar cells. Furthermore, the inorganic halide perovskite CsPbBr₃ was found to show better stability compared with the organic–inorganic hybrid halide perovskite MAPbBr₃.⁵⁰ During an illumination period of 5 h, the unencapsulated MAPbBr₃ device exhibited a stronger and faster photocurrent density decay of 55% compared to the maximum value, in contrast to the CsPbBr₃ one with only 13% decay. Aging tests were conducted in ambient relative humidity of 60–70% over 2 weeks. The MAPbBr₃-based solar cell suffered a heavy efficiency loss of 85%, while the CsPbBr₃-based one showed no significant decay. However, the inorganic PSCs did not attract much attention because of their low efficiency levels. In 2016, Snaith's group reported bandgap-tunable cesium lead halide PSCs by adjusting the halide (iodide and bromide) stoichiometric ratio, achieving close to 10% power conversion efficiency.⁵¹ Using an all-vacuum-deposition strategy, the mixed halide inorganic perovskite solar cells exhibited a stabilized efficiency exceeding 11%.⁵² In the meantime, cubic phase CsPbI₃ quantum dot PSCs achieved close to 11% efficiency and worked out the issues of cubic phase instability of inorganic perovskites.⁵³ Since then, inorganic halide perovskites have attracted enormous attention from researchers previously working on organic–inorganic perovskites or other optoelectronic materials, and these research efforts have promoted inorganic perovskite material development and understanding of device principles (Table 1).

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In this review, we focus on the progress of state-of-the-art inorganic perovskite materials and photovoltaic devices, and offer a unique outlook on the path towards the commercialization of inorganic perovskites from the perspective of existing challenges and coping strategies. First, we review the structure–property-synthesis characteristics and inherent material-level issues and challenges of inorganic perovskites. Theoretical investigations and experimental results of the structures and electronic properties along with the underlying issues of inorganic halide perovskites are beneficial for comprehensive understanding of inorganic materials. Second, on the basis of the issues in the inorganic perovskite research field, we discuss the current state-of-the-art engineering strategies from the aspect of phase-structure and composition regulation, surface-interfacial modification, ion-doping engineering and lead-free perovskite materials design as shown in Fig. 1. These represent the key research areas contributing to the rapid development of inorganic halide PSCs within this short number of years. In addition, the current physical understanding of inorganic halide perovskite materials and photovoltaic devices is summarized systematically. Finally, we propose the prospective research directions to further improve the photovoltaic performance of inorganic PSCs which may help to gain insight into inorganic perovskites and future research prospects.

2. Fundamental background and challenges

Different from organic–inorganic hybrid perovskites, inorganic halide perovskites embrace unique crystal structures due to the incorporation of cesium ions. The unique crystal structure characteristics endow inorganic perovskites with distinctive optical properties and photovoltaic performance, meanwhile, causing new issues and challenges in the perovskite photovoltaic field. In particular, the phase transition temperatures vary dramatically in inorganic perovskites compared with the organic–inorganic ones, and are dependent on chemical composition, which poses serious issues of phase-stability. In this section, we briefly overview the current research progress in understanding the fundamental properties of inorganic perovskites in terms of the crystal structure, electronic properties, theoretical investigations and synthesis methods. On the basis of the specific characteristics of inorganic perovskite materials, we summarized comprehensively the issues and challenges for developing high-performance inorganic perovskite solar cells at the present stage.

2.1. Crystal structure

Generally, perovskite materials possess a typical formula structure of ABX₃, where cation A occupies the corner positions (0, 0, 0), cation B is located at the centre positions (1/2, 1/2, 1/2) and anion X is at the centre of the six planes (1/2, 1/2, 0), constituting an ideal cubic unit cell. To maintain a high-symmetry cubic ABX₃ perovskite crystal structure, the tolerance factor should be considered to be within a range from 0.813 to 1.107, where R_A, R_B and R_X are the ionic radii of the corresponding cations and anions.^54,55 For inorganic halide perovskites, the cesium ion (Cs⁺, 1.81 Å) can match the requirement of t and has been identified as the preferred alternative inorganic cation substitute for the organic cation methylamine (MA⁺, 2.70 Å). The Goldschmidt tolerance factors of MAPbI₃ and CsPbI₃ are calculated to be 1.02 and 0.84 respectively, which are both in the range (0.81–1.11) for structurally stable perovskites.

The general crystal structures of inorganic halide perovskites can be divided into orthorhombic, tetragonal, and cubic phase crystal structures according to the environmental temperature as shown in Fig. 2a.⁵⁶ The differences between crystal structures result in different optical and electronic properties.⁵⁷ Here, taking a cesium lead halide perovskite (CsPbX₃) as an example, the typical cubic phase of cesium lead halide perovskites generally appears at high temperature.^58–60 As the temperature decreases, a phase transition with lower-symmetry is observed. The typical chlorine-based inorganic perovskite CsPbCl₃ stabilizes in the cubic phase at a temperature over 50 °C, and shows phase transition from the cubic phase to tetragonal phase at around 46 °C, and to the orthorhombic phase around 42 °C. For bromide-based perovskite (CsPbBr₃), the structural phase transition occurs at 130 °C and 88 °C, at which temperature the cubic perovskite structure turns to tetragonal and further to orthorhombic, respectively.⁶¹ However, for an iodide-based perovskite (CsPbI₃), the cubic phase CsPbI₃ can only remain stable at a high temperature of over 300 °C.⁶¹ Below 300 °C, the cubic phase of CsPbI₃ would change to a more thermodynamically favourable orthorhombic phase, which is also known as the “yellow phase” and is unusable for photovoltaic applications due to its broad band gap of 2.82 eV.⁶³ The unfavourable phase transition can be restrained by partial substitution of iodide with bromide (CsPbBr_xI_1−x), which can decrease the cubic phase transition temperature. Comparatively, the incorporation of bromide into CsPbI₃ increases the band gap and decreases the visible light absorption range.⁶⁴

2.2. Electronic properties

In perovskite semiconductors and photovoltaic devices, optical transitions (absorption and photoluminescence) and charge transfer (injection and extraction) greatly determine the optoelectronic properties and performances, and these parameters strongly rely on the nature of the electronic properties, such as the energy level of the valence band and conduction band.⁶⁵ These electronic structures and properties of inorganic halide perovskites with different halide compositions are found to show analogical features.^66–68 For typical 3D structured inorganic perovskites, the valence bands are constituted by the combination of 6s orbitals of lead and np orbitals of the halides (Cl: n = 3, Br: n = 4, and I: n = 5), while the conduction band is composed of the combination of lead (6p) orbitals and the halide np orbitals with primary contributions of the lead (6p) orbitals.^69,70 The electronic states of inorganic perovskite are only slightly affected by the cesium cation, and thus making no direct electronic contribution to bandgap.⁷¹Fig. 2b depicts the calculated electronic structures of cubic phase inorganic halide perovskites CsPbCl₃, CsPbBr₃, and CsPbI₃, respectively, according to density functional theory in consideration of spin–orbit interactions and relativistic corrections.⁷² It is clear that the energy band structures of inorganic halide perovskites are almost independent of the halogen compositions. Apart from the difference of bandgap values, all the inorganic halide perovskites exhibit direct bandgaps.^69,70,72,73 Furthermore, comparable effective masses of electrons and holes can be predicted based on the electronic band dispersion around the band edges, contributing to the high carrier mobilities.^74–76

As the halide varies from iodide to bromide to chloride, the energy levels of the halide np⁶ orbitals decrease, shifting the valence band toward more positive potentials.⁷⁷ The ratio of the valence band and conduction band of inorganic perovskites with compositions CsPbBr_xI_3−x, CsPbBr₃ and CsPbBr_xCl_3−x is calculated to be 6.1/4.3 eV, 6.5/4.15 eV and 6.5/3.8 eV, respectively.⁷⁸ Meanwhile, a consecutive variation of the halide composition from iodide to bromide to chloride results in systematic change of the optical bandgap. Based on the UV-vis absorption spectra, the optical bandgap values of CsPbX₃ with compositions CsPbBr_xI_3−x, CsPbBr₃ and CsPbBr_xCl_3−x are calculated to be 1.8 eV, 2.35 eV and 2.7 eV, respectively.⁷⁹ In addition, for tin-based inorganic perovskites, the calculated bandgaps decrease from 2.8 to 1.3 eV as the composition of cesium tin halides of CsSnX₃ perovskites varies from chloride to bromide to iodide.⁸⁰ The optical spectra of tin-based perovskites show a red shift compared with those of analogous lead perovskites, which can be assigned to the higher electro-negativity of tin in the “B” site in the ABX₃ perovskite lattice.^81,82 The bandgaps of germanium (Ge) based analogues CsGeX₃ are similar to those of the Pb counterparts (3.43, 2.38, and 1.51 eV for Cl, Br and I, respectively).^83,84

2.3. Theoretical calculations

With the help of advanced experimental characterization methods, the physicochemical properties of inorganic perovskites are obtained to some extent. However, in order to gain deeper insight into the relationships between the crystal structures, electronic structures and power conversion performances of inorganic perovskites, theoretical studies are required urgently. Primarily, theoretical calculations point out that the replacement of organic cations with inorganic Cs⁺ makes no difference in the optical or electronic properties.⁸⁵ Alkali metal lead halides (RbPbI₃ and CsPbI₃) with the cubic perovskite structure were examined using PbI₂ as an initial structural model, suggesting that while the Pb²⁺ 6s lone-pairs were stereochemically inert, the presence of proximal instabilities could have an effect on the functional properties of these materials.⁸⁶ By high-resolution in situ synchrotron XRD measurement, Marronnier et al.⁸⁷ found that undercooled CsPbI₃ can temporarily maintain a metastable perovskite polytype (black γ-phase) structure at room temperature, stabilizing the metastable perovskite polytype (black γ-phase) structure crucial for photovoltaic applications. The competition among low-temperature phases of CsPbI₃ was also inspected through energy and vibrational entropy calculations, indicating that the key to avoiding the transition from the black phase to yellow phase was preventing the order–disorder entropy term arising from double-well instabilities.⁸⁸

In addition to helping in understanding the physicochemical properties, the theoretical calculations have also been made it easy to find new compositions and structures of efficient and stable kinds of perovskites. In addition to the typical 3D perovskite structure of ABX₃, heterovalent elements also have been studied to explore new perovskite systems. With the B-site cation in the +4 oxidation state, Cs₂PdBr₆, with the formula A₂BX₆, was indicated to have dispersive electronic bands by density functional theory calculations.⁸⁹ And this solution-processable Cs₂PdBr₆ was observed to be resistant to water, indicating excellent long-term stability for optoelectronic applications. Composed of a zero-dimensional dimer form and a two-dimensional layered form, it was proposed that Cs₃Sb₂I₉ possessed a nearly direct bandgap with a similar high level of absorption to MAPbI₃, giving guidance to experiments and assistantly demonstrating the experimental results.⁹⁰ The basic physical properties of lead-free Ge-based inorganic perovskites AGeX₃ (A = Cs or Rb; X = I, Br, or Cl), including dielectric constants, photoabsorption coefficients, effective masses of charge carriers, exciton binding energies, and electronic band structures, were predicted using density functional theory calculations, which were proposed to act as light absorbers for solar cells.⁹¹ Besides, chalcogenide perovskites ABX₃ (X = S or Se; A, B = metals with a combined valence of 6) were computed as a new family for photovoltaic applications because of their more environmentally friendly compositions than lead halide perovskites. CaTiS₃, BaZrS₃, CaZrSe₃ and CaHfSe₃ were identified to be promising materials for solar cells since their calculated absorption properties were relatively suitable.⁹²

2.4. Synthesis strategies

For fabricating high-performance inorganic halide perovskite solar cells, the morphologies and qualities of perovskite thin films are largely studied for photovoltaic performance. In terms of polycrystalline films, solution-chemistry routes and vapor deposition processes are predominant due to the facile and reproducible process in fabricating high-quality perovskite thin films. The solution-chemistry techniques for depositing perovskite thin films can be divided into two main strategies: one-step and two-step methods. Fig. 3a shows the typical one-step deposition method of inorganic perovskite films. In this method, the perovskite films are directly deposited from the precursor solution on planar or mesoporous substrates. Because the inorganic halide perovskite CsPbX₃ can be synthesized by the interaction of CsX and PbX₂, a typical two-step method constitutes the first step of PbX₂ deposition, and then the conversion of CsPbX₃ during the second step as shown in Fig. 3b.⁹³ The qualities of the inorganic perovskite film deposited by the one-step method are primarily determined by the film shrinkage during the crystallization of perovskite due to the removal of the solvent.⁹⁴ The main challenge for the two-step method is the volume expansion of the PbX₂ precursor due to the intercalation of CsX. Thus, the key factor for solution-chemistry deposition of perovskite films is to control film shrinkage in the one-step method and film expansion in the two-step method.⁹⁴

In addition to the solution-chemistry route, a dual-source vacuum vapour deposition technique is also introduced to fabricate highly uniform polycrystalline inorganic halide perovskite films. As shown in Fig. 3c, the PbX₂ and CsX vapours obtained by heating elemental sources, are transported to the substrate and crystallize to form perovskites after annealing. The first uniform inorganic perovskite films were prepared by Ma et al., in which CsPbIBr₂ was carefully deposited via a dual source thermal evaporation process.⁹⁵ Large size perovskite crystal grains of 500–1000 nm were acquired by controlling the annealing and substrate temperature of 250 °C and 75 °C, respectively. It is also shown that precise control of precursor co-deposition is essential for the formation of desired perovskite films with an excellent crystal structure. Hutter et al. investigated the charge carrier dynamics of CsPbI₃ thin films via the comparison of vapor and solution deposition strategies.⁹⁶ The time-resolved microwave conductivity technique was applied to detect the charge carrier mobilities, which demonstrates that the mobilities of both vapour and solution deposited films were approximately 25 cm² V⁻¹ s⁻¹. However, an extra-long lifetime of tens of microseconds was observed in the vapour-deposited CsPbI₃ thin film, whereas the perovskite films fabricated by the solution deposited method exhibited a photo-excited decay lifetime of only 200 ns. The trap densities of the vapor deposited perovskite film based on time-resolved microwave conductivity curves were far lower than the value for solution-deposited films, which explained the excellent carrier lifetime of vapour deposited films due to less trapping and recombination.

2.5. Inorganic halide perovskites: issues and challenges

3. Engineering strategies for inorganic halide perovskites

On the basis of the issues and challenges discussed above, in this section, we present and summarize systemically the current state-of-the-art engineering strategies, including phase-structure engineering, composition regulation, surface-interfacial modification, lead-free perovskite materials design, and physical mechanism analyses. Specifically, focusing on the phase structure instability, the studies of cesium lead iodide (CsPbI₃) which possesses the most desired bandgap and highly unstable phase structure are reviewed based on four different strategies. The limited absorption range can be regulated by composition engineering by A-site, B-site and halide-site ion substitution and doping. The issues of interfacial and surface defects can be inhibited by interfacial engineering and rational transport layer design. In addition, systematic investigations of lead-free perovskites based on homovalent elements and heterovalent elements are summarized to address lead toxicity. The in-depth analyses of the photophysical performance of several inorganic PSCs (such as transient-absorption/-photoluminescence) are reviewed.

3.1. Phase-stable cesium lead iodide

Cubic-phase stabilization is extremely important for the application of cesium lead halide inorganic perovskite materials in the fabrication of perovskite solar cells. From a light absorption point of view, cesium lead iodide (CsPbI₃) is the most desirable choice due to its bandgap of about 1.7 eV. However, CsPbI₃ in the black cubic phase (α) generally formed at high temperature (over 300 °C) is inclined to convert into the yellow orthorhombic phase (δ) at room temperature and is sensitive to moisture as shown in Fig. 4a. This non-perovskite structure material is less photoactive with an optical bandgap of more than 2.8 eV.¹⁰⁴ In this section, the strategies on how to stabilize the CsPbI₃ inorganic perovskite cubic-phase have been discussed. By decreasing the perovskite grain size, the perovskite surface energy can be increased for obtaining a stable cubic-phase CsPbI₃ inorganic perovskite. Meanwhile, substituting metal cations with different ions or doping metal ions in the CsPbI₃ crystal lattice can also produce cubic-phase stable CsPbI₃ inorganic perovskites. In addition, surface engineering via various organic additives has also been applied to stabilize cubic-phase CsPbI₃ for photoelectric applications.

3.2. Composition engineering

Similar to organic–inorganic hybrid perovskites, inorganic halide perovskites share the common optical properties of composition-tunable bandgap and emission spectrum.¹¹⁰ By changing the halide elements (Cl, Br, and I), the optical absorption and photoluminescence spectrum can be finely tuned throughout the whole visible light range from 400 to 700 nm (Fig. 6a and b).¹¹¹ Theoretical studies have revealed that the electronic structures of inorganic halide perovskites are related to the p orbitals of halide and the p orbitals of lead, and thus, the optical properties and bandgaps can generally be tuned by rational composition engineering. By adjusting the proportion of precursors during synthesis, their optical absorption and emission colors can be easily tuned.¹¹² In this way, their chemical compositions can be precisely tuned to yield various optical properties.

3.3. Interfacial engineering and transport layer design

It is well known that there are considerable coordination sites on the surface of perovskite films, causing negative effects like the hysteresis phenomenon and trap centers. For example, the inhomogeneous interface in the perovskite bulk layer may result in a long exciton diffusion length and substantial current leakage,^127,128 while the structural defects, metallic lead atoms and halogen vacancies may reduce V_oc by accelerating the nonradiative charge recombination.¹²⁹ Therefore, interfacial functionalization and transport layer design are important strategies to boost the photovoltaic performance and durability against the environment like humidity, and sometimes to stabilize the cubic phase of Cs-based inorganic perovskites in the meantime.

3.4. Lead-free perovskites

Despite exciting progress of excellent thermal stability and photovoltaic performance for inorganic perovskites, the concern of lead toxicity is still the biggest obstacle for the commercialization of this emerging technology. In recent years, significant efforts have been made to develop light active lead-free halide perovskite materials, especially inorganic lead-free perovskites. In general, there are two categories of lead-free halide perovskite materials: homovalent element (Sn and Ge) based perovskites (such as CsSnI₃ or CsGeI₃) and heterovalent element (Bi and Sb) based perovskites (such as Cs₂AgBiI₆ or Cs₂AgSbCl₆). However, these inorganic lead-free perovskite solar cells show significantly inferior photovoltaic performances to the lead based perovskite counterparts. In this section, we provide reviews on the fundamental understanding of the material properties and photovoltaic performance of lead-free inorganic perovskite materials based on homovalent and heterovalent elements.

3.5. Physical mechanism discussion

Inorganic perovskite materials exhibit a series of distinctive characteristics in their optoelectronic response which have a crucial influence on the photovoltaic performance, particularly for long-time response. In this section, a survey of recent advances concerning physical mechanisms both in inorganic perovskite materials and optoelectronic devices is provided, with the aim of comprehensively covering the generation, transport and recombination dynamics of excitons and charge carriers.

4. Conclusions and outlook

In summary, a survey of the literature reveals the merits and demerits of inorganic halide perovskite materials and photovoltaic devices. During the early stage of perovskite development, composition tuning was investigated to pursue the widest light absorption range of the cubic phase. With the development of various chemical strategies, stable cubic phase inorganic halide perovskites have been successfully produced. Furthermore, a variety of engineering strategies for the aspects of composition, structure, interface, and dimensions have been proposed to enrich inorganic perovskite material systems, optimize optoelectronic device configurations and improve the optoelectronic performance of inorganic PSCs. These studies have led to unprecedented progress in fabricating high-performance inorganic perovskite photovoltaics with over 17% efficiency by various research groups via different material and configuration modulation strategies.

Very recently, all-inorganic PSCs using a carbon counter electrode or novel inorganic transport materials have been developed based on several studies, resulting in lower cost and longer lifetime. For the commercialization of these novel photovoltaic techniques, it is still of primary importance to fabricate high-stability and low-cost PSCs. In addition, despite the exciting progress in stability and efficiency, the presence of toxic lead in their composition is still regarded as one of the major limiting factors hindering their commercialization. Thus, addressing the toxicity issues in these compounds by developing lead-free inorganic perovskites represents a promising direction in the perovskite research field. Furthermore, inorganic halide perovskites with a bandgap of around 1.7 eV can be the most competitive top cells for tandem photovoltaic devices matched with silicon solar cells. For this purpose, cesium lead iodide (CsPbI₃) perovskites have become a promising candidate due to their desirable bandgap and excellent thermal stability. Finally, in addition to the advances in inorganic perovskite device performance, further investigations including theoretical calculations and photophysical characterization toward bulk and polycrystalline materials will help to obtain a better understanding towards the fundamental properties of inorganic perovskites.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Bo Li and Lin Fu contributed equally to this work. We acknowledge support from a project supported by the State Key Program of National Natural Science of China (no. 51532005), the National Natural Science Foundation of China (no. 51872171, 51272137), and the Tai Shan Scholar Foundation of Shandong Province.

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