Solution fabrication methods and optimization strategies of CsPbBr₃ perovskite solar cells

Abstract

In recent years, all-inorganic CsPbBr₃ perovskite solar cells (PSCs) have attracted tremendous attention due to their superb stability under thermal and moisture induction. Great efforts have been made to fabricate CsPbBr₃ PSCs with enhanced efficiency and stability. To date, the power conversion efficiency of CsPbBr₃ PSCs has steadily increased from the initial 5.95% to the current 11.08%, demonstrating attractive potential in the photovoltaic field. In this article, we briefly review the development and current status of CsPbBr₃ PSCs, summarize the solution preparation methods of CsPbBr₃ films, and analyze emphatically the optimization strategies of CsPbBr₃ PSCs. Finally, we discuss the potential applications and prospects of CsPbBr₃ PSCs.

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1 Introduction

In recent years, perovskite solar cells (PSCs) have aroused extensive research attention from academia to industry due to their excellent photoelectric properties and low preparation cost.^1–11 To date, the certified power conversion efficiency (PCE) of organic–inorganic hybrid PSCs has reached up to 26.1%,¹² almost comparable to crystalline silicon solar cells. However, the intrinsic instability of organic–inorganic hybrid perovskites hinders their further commercial application because of their volatile organic cations (such as MA⁺ and FA⁺) and low formation energies.^13–15 To enhance their stability, the substitution of organic cations with inorganic cations (Cs⁺, Rb⁺) is one of the effective methods, and a lot of work has been performed.^16,17 In 2015, Park's team used inorganic cation Cs⁺ to partially replace FA⁺ in FAPbI₃, and obtained a PCE of 16.5% as well as significantly enhanced stability based on an FA_0.9Cs_0.1PbI₃ solar cell.¹⁶ Subsequently, by careful processing control and development of a low-temperature phase transition route, Snaith's team innovatively fabricated all-inorganic CsPbI₃ PSCs and achieved a PCE of 2.9% at room temperature. Importantly, these results have confirmed that organic cations are not an essential component of perovskite materials, and pave the way for fabricating PSCs with better thermal stability.¹⁸ Unfortunately, CsPbI₃ perovskite normally exists in a yellow non-perovskite phase at room temperature, unsuitable as a perovskite absorber; it becomes a black cubic perovskite phase (bandgap of ∼1.73 eV) only after being heated above ∼310 °C, which limits its photovoltaic applications.¹⁹

In order to obtain C_S-based PSCs that can exist stably at room temperature, Kulbak et al. used CsPbBr₃ perovskite (bandgap of ∼2.3 eV) as the absorber layer and fabricated all-inorganic PSCs based on the structure of fluorine-doped tin oxide (FTO)/compact-TiO₂ (c-TiO₂)/mesoporous-TiO₂ (m-TiO₂)/CsPbBr₃/hole transfer layer (HTM)/Au, and a PCE of 5.95% was achieved as well as superb thermal stability.²⁰ In the subsequent study, as a contrast, Kulbak et al. fabricated both CsPbBr₃-based PSCs and MAPbBr₃-based PSCs, and found that their PCEs were almost the same, but the former possessed better long-term stability.²¹ Meanwhile, Snaith's team systematically investigated the effect of varying the I⁻/Br⁻ proportion on the performance of CsPbI_xBr_3−x films, and observed that with the increase of Br⁻, the CsPbI_xBr_3−x film demonstrated a wider bandgap and better thermal as well as humidity stability.²² Recently, Fang's team deeply analyzed the impact of halide mixing (I, Br, Cl) on the crystallization and phase evolution in CsPbX₃ perovskite through in situ grazing-incidence wide-angle X-ray scattering, and also found that Br promoted black perovskite phase formation and suppressed the yellow non-perovskite phase transition.²³ In view of its robust stability and wide bandgap, CsPbBr₃ perovskite demonstrates great potential applications in semitransparent and tandem solar cells, thus attracting much attention from researchers.^24,25 So far, the champion PCE of CsPbBr₃ PSCs has reached 11.08%,²⁶ which is still much lower than the Shockley–Queisser (S–Q) limit of 16.37%, leaving large room for improvement.²⁷

For highly efficient CsPbBr₃ PSCs, it is critical to obtain a high-quality CsPbBr₃ film. In general, two main routes are used to fabricate CsPbBr₃ films; one route is solution deposition, and the other is vacuum evaporation. Considering the easy operation and low cost, solution deposition is a more reliable route. And with the continuous improvement of solution deposition, the fabricated film has been greatly improved.^28,29 To date, the highest efficiency of CsPbBr₃ PSCs by solution deposition is 11.08%, which is still slightly higher than that (10.91%) of CsPbBr₃ PSCs by vacuum evaporation, but the latter's fabrication is much more complex.³⁰ Therefore, this article focuses on CsPbBr₃ PSCs based on solution deposition.

In this article, we systematically review the development and current status of CsPbBr₃ PSCs. First, the crystal structure and photoelectric properties of CsPbBr₃ perovskite are briefly introduced. Then, three solution deposition methods for CsPbBr₃ films are summarized. Subsequently, the device structure, performance and optimization strategies of CsPbBr₃ PSCs are analyzed in detail. At the end of this article, the potential development direction and application prospects of CsPbBr₃ PSCs are discussed.

2 Crystal structure and properties of CsPbBr₃

The superb photoelectric properties of PSCs are due to the unique crystal structure of perovskite materials. The general formula of perovskite is ABX₃, where the A cation coordinates with 12 X anions and the B cation coordinates with 6 X anions, constituting cuboctahedral and octahedral geometry, respectively (Fig. 1). The octahedral factor μ (μ = R_B/R_X) and the tolerance factor τ (τ = (R_A + R_X)/2^1/2(R_B + R_X)) determine the geometric stability of the perovskite structure to some extent (where R_A, R_B and R_X represent the effective ionic radii of the A, B and X sites, respectively). Previous study has confirmed that the perovskite structure can be stabilized when 0.442 < μ < 0.895, and 0.813 < τ < 1.107.³¹ For CsPbBr₃ perovskite, the value of τ is 0.824, showing a stable perovskite structure. In contrast, CsPbI₃ perovskite, possessing a larger anion radius, has a smaller τ value of 0.807,³² thus leading to its instability at room temperature.

Fig. 1 Crystal structure of the perovskite. Reproduced with permission.³¹ Copyright 2014, American Chemical Society.

Generally, depending on the temperature, CsPbBr₃ exists in three different phases including cubic α phase, tetragonal β phase, and orthogonal γ phase. At ambient temperature, CsPbBr₃ perovskite resides in the stable γ phase. When the temperature increases to 88 °C, it transforms into the unstable β phase, and it will further transform into the α phase as the temperature increases above 130 °C. Once the temperature drops to room temperature, the α and β phases return to the γ phase, indicating superior stability under thermal and moisture induction.^33–35

As to the photoelectric characteristics, CsPbBr₃ perovskite possesses a wide bandgap of ∼2.3 eV, and can only absorb spectra with wavelengths less than ∼540 nm, which seriously restricts the improvement of the device efficiency. However, wide bandgap means a high open circuit voltage (V_oc) for the corresponding device, and so far, the champion V_oc of CsPbBr₃ PSCs is up to 1.702 V, which is the highest among CsPbX₃ PSCs.²⁶ In addition, for CsPbBr₃ single crystals, its carrier mobility is up to ∼2000 cm² V⁻¹ s⁻¹, the carrier diffusion length is over 10 μm, and the carrier lifetime is approximately 2.5 μs.^36,37 These superior properties facilitate charge extraction and transport. All of these parameters indicate that CsPbBr₃ perovskite is an excellent light-absorbing material and has vast potential applications in the photovoltaic field.

3 Solution preparation of CsPbBr₃ films

Since PbBr₂ and CsBr are difficult to dissolve in one solvent at the same time, it is hard to prepare CsPbBr₃ films with suitable thickness by the traditional one-step solution method. To solve the challenge of this vast solubility difference, two-step solution methods and multi-step solution methods are widely used to fabricate CsPbBr₃ films. In addition, a novel one-step solution method is also proposed to obtain CsPbBr₃ films with adequate thickness and purity phase.³⁸

3.1 Two-step solution method

Since no solvent could be found to dissolve both CsBr and PbBr₂ at the same time, Kulbak et al. utilized a two-step solution method to prepare CsPbBr₃ films for the first time in 2015. As shown in Fig. 2(a), PbBr₂/DMF solution was spin-coated onto the substrate, and was then dried at 75 °C for 30 minutes. Subsequently, the PbBr₂-coated substrate was dipped in CsBr/methanol solution for 10 minutes and annealed at 250 °C for 10 minutes after being dried. Thus, a CsPbBr₃ film with a thickness of ∼450 nm was prepared. Based on such CsPbBr₃ films, the corresponding PSCs achieved a champion PCE of 5.95% using the architecture of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/PTAA/Au (Fig. 2(b)).²⁰ However, CsPbBr₃ precursor films decompose quickly when being dipped face-up in CsBr/methanol solution, thus leading to poor morphology of the CsPbBr₃ films (Fig. 2(a)). In order to inhibit the decomposition, Teng et al. dipped the CsPbBr₃ precursor films face-down in CsBr/methanol solution and fabricated smooth and uniform perovskite films. Based on the architecture of FTO/c-TiO₂/CsPbBr₃/carbon (Fig. 2(c)), the CsPbBr₃ PSC achieved a PCE of 5.86%.³⁹

Fig. 2 (a) Comparison between conventional dipping and surface-down dipping based on a two-step solution method. Reproduced with permission.³⁹ Copyright 2018, American Chemical Society. Cross-sectional SEM image of photovoltaic devices based on (b) the conventional dipping method and (c) the surface-down dipping method. Reproduced with permission.^20,39 Copyright 2015, American Chemical Society.

Although CsPbBr₃ films with suitable thickness can be fabricated by the two-step solution method, the uneven reaction between PbBr₂ and CsBr results in the formation of impurity phases, such as CsPb₂Br₅ and Cs₄PbBr₆, which seriously deteriorate the performance of the photovoltaic device. Therefore, the two-step solution method needs to be improved urgently.⁴⁰

3.2 Multi-step solution method

In order to reduce impurity phases and enhance the perovskite film quality, the multi-step solution method was proposed to prepare CsPbBr₃ films by Duan et al.⁴¹ As shown in Fig. 3(a), after the PbBr₂ layer had been deposited, CsBr/methanol solution was continuously spin-coated on the PbBr₂ layer and annealed for several cycles. The reaction process is as follows

2PbBr₂ + CsBr → CsPb₂Br₅ (n ≤ 3) (1)

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

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

where n represents the number of spin coating cycles. When n is less than 4, CsBr is insufficient, as can be seen from eqn (1), and the impurity phase is CsPb₂Br₅; when n equals 4, the molar ratio of PbBr₂ to CsBr reaches 1 : 1, resulting in a pure CsPbBr₃ perovskite phase; with n further increasing to above 4, an impurity phase of Cs₄PbBr₆ emerges due to the excess of CsBr. The CsPbBr₃ films fabricated by this method appear compact and uniform, and the corresponding PSCs have achieved a champion efficiency of 9.72% based on the mesoporous structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/GQDs/carbon.

Fig. 3 (a) Schematic illustration of the traditional multi-step solution method, reproduced with permission.⁴¹ Copyright 2018, Wiley-VCH. (b) Schematic illustration of the spray-assisted multi-step solution method. Reproduced with permission.⁴² Copyright 2018, Elsevier. (c) Schematic illustration of the enhanced multi-step solution method. Reproduced with permission.⁴³ Copyright 2019, Elsevier.

Considering the complicated process of the multi-step solution method, which is not conducive to commercial application, Duan et al. further improved this method; that is, a spray-assisted multi-step solution method was proposed.⁴² Here, PbBr₂/DMF solution was first spin-coated on an m-TiO₂ layer and then dried at 80 °C. Shortly after, CsBr/methanol solution was sprayed on the PbBr₂ layer, followed by annealing at 250 °C, and this process was repeated several times until a high-quality CsPbBr₃ film was prepared, as shown in Fig. 3(b).

Although the spray-assisted method facilitates the fabrication of large-scale photovoltaic devices and simplifies the fabrication process, it is still difficult to avoid the emergence of impurity phases. In particular, the corresponding PSCs have only obtained a PCE of 6.8%, which means that the film quality needs to be further enhanced. After that, based on the respective merits of the two-step solution method and multi-step solution method, Liu et al. fabricated high-quality CsPbBr₃ films, and the process illustration is given in Fig. 3(c).⁴³ Firstly, CsPbBr₃ film with appropriate thickness was prepared by a two-step solution method, then treated with IPA solvent and annealed at 250 °C. Thereafter, the 0.07 M CsBr solution was spin-coated on the CsPbBr₃ film and annealed at 250 °C; this process was repeated until a pure CsPbBr₃ phase is prepared. Using this method, the fabricated CsPbBr₃ film appears compact and uniform, and the grain size is close to 1 μm.

So far, highly efficient CsPbBr₃ PSCs are commonly prepared by the muti-step solution method^44–46 and the reported champion PCE is up to 11.08%. However, a high temperature treatment over 250 °C is necessary for CsPbBr₃ films, which results in a rapid crystallization rate, leading to poor crystallinity with a non-uniform interface and high defect density in the films. In addition, the process of the muti-step solution method is too complicated, thus increasing the fabrication cost. All of these issues seriously restrict the development of CsPbBr₃ PSCs, which are in urgent need of improvement.

3.3 Novel one-step solution method

Although the preparation methods of CsPbBr₃ films are constantly improving, the challenge of uneven reaction between PbBr₂ and CsBr remains unsolved, which leads to the generation of an impurity phase. Considering the fact that purity-phase perovskite films can be prepared by a one-step solution method, researchers have tried to use a novel one-step solution method to prepare high-quality CsPbBr₃ films. Jiang et al. adopted CsAc and HPbBr₃ as precursor pairs, and overcame the limitation of low solubility of CsBr, preparing compact and uniform CsPbBr₃ films by a one-step solution method. The PSCs based on the above CsPbBr₃ films achieved a PCE of 5.96%, which was the highest value of the same type of cells at that time.³⁸ In 2019, Huang et al. adopted a similar one-step solution method to prepare a CsPbBr₃ perovskite film, that is, CsAc and MAAc were introduced into the perovskite precursor to increase the solubility of the CsPbBr₃ precursor, and a CsPbBr₃ film with sufficient thickness was prepared.⁴⁷

4 CsPbBr₃ PSCs

4.1 Device structure

The architectures of CsPbBr₃ PSCs are briefly divided into two categories: a mesoporous structure and planar structure; among them, the planar structure can be subdivided into a formal planar structure (n–i–p) and inverted planar structure (p–i–n). No matter what kind of structure, the CsPbBr₃ PSCs are mainly composed of several parts, including the front electrode, electron transfer layer (ETL), perovskite layer, hole transfer layer (HTL), and rear electrode, as shown in Fig. 4. So far, the most efficient CsPbBr₃ PSCs have adopted a mesoporous structure and formal planar structure.

Fig. 4 The device structure of PSCs.

4.1.1 Front electrode and rear electrode. The front electrode is located at the incident plane of light illumination, which requires high transmittance and good electrical conductivity. The commonly used front electrodes are transparent conductive oxides (TCOs) such as indium-tin oxide (ITO) and FTO. According to previous literature, the surface work function (WF) of ITO is ∼4.8 eV,⁴⁸ and that of FTO is ∼4.6 eV.⁴⁹ In CsPbBr₃ PSCs, appropriate TCO materials are usually selected according to the device structure, band-gap matching and device fabrication temperature.

The rear electrode is usually made of metal materials (such as Au, Ag, Cu, etc.), non-metal materials (carbon electrode), and novel materials (such as Ag nanowires (NWs), Ag/Al alloys, carbon nanotubes, etc.). A superior rear electrode can facilitate carrier collection and enhance device stability and efficiency. For CsPbBr₃ PSCs, because most of them adopt a mesoporous structure or formal planar structure, a carbon electrode (WF ∼5.0 eV) or Au electrode (WF ∼5.1 eV) is the most suitable, as shown in Fig. 5. In practical applications, a carbon electrode is also widely used as the back electrode, because of its low cost and high stability.

Fig. 5 Energy level diagram of commonly used materials for CsPbBr₃ PSCs.

4.1.2 ETL. The ETL plays a role in transporting electrons and blocking holes in PSCs. Commonly used ETL materials include TiO₂, SnO₂, ZnO, C₆₀, etc., as shown in Fig. 5. Among them, nano-TiO₂ is widely used in photovoltaic devices due to its suitable bandgap, super-high transmittance and stable chemical properties.^50,51 It is usually prepared into compact layers (used as the HTL, preventing carrier recombination at the interface of perovskite/TCO) and mesoporous layers (used as a scaffold of the perovskite layer, being conducive to the formation of a high-quality perovskite layer). As one of the important components, the grain size of TiO₂ and its preparation method will significantly affect the performance of photovoltaic devices.⁵²

In the CsPbBr₃ PSCs, TiO₂ is the most commonly used ETL material; its electron mobility ranges from 0.1 to 10 cm² V⁻¹ s⁻¹,⁵³ which is not well matched with that of the CsPbBr₃ layer. Therefore, some N-type metal oxide semiconductors (MOS) with good photoelectric properties (especially high carrier mobility and suitable energy level alignment), such as SnO₂ (conduction-band minimum of ∼−4.3 eV, bulk electron mobility of 250 cm² V⁻¹ s⁻¹),^37,54,55 ZnO (conduction-band minimum of ∼−4.1 eV, bulk electron mobility of 205–300 cm² V⁻¹ s⁻¹),⁵⁶etc., are considered to be excellent alternatives to TiO₂. For instance, the CsPbBr₃ PSCs with a champion efficiency of 11.08% are fabricated with SnO₂ as the ETL.²⁶

4.1.3 HTL. The HTL is deposited between the perovskite layer and electrode, and used to extract holes and prevent electron transport in the photovoltaic device. Commonly used HTL materials include 2,2′,7,7′-tetrakis (N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD),⁵⁷ poly[bis(4-phenyl)(2,4,6-trimethylphenyl) amine] (PTAA),⁵⁸ poly(3-hexylthiophene-2,5-diyl) (P3HT),⁵⁹ nickel oxide (NiOx),⁴⁴ copper phthalocyanine (CuPc),⁶⁰etc. In addition, a suitable HTL can also inhibit the formation of a Schottky junction between the perovskite layer and metal electrode, reduce the contact resistance, promote hole transformation, and decrease the carrier recombination at the interface.

4.2 Efficiency of CsPbBr₃ PSCs

When the all-inorganic CsPbBr₃ PSCs first appeared in 2015, they were prepared by a two-step solution method and achieved a PCE of 5.95% with the mesoporous structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/PTAA/Au.²⁰ Subsequently, based on the same two-step solution method, Liang et al. fabricated HTL-free CsPbBr₃ PSCs with the structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/carbon, and obtained a PCE of 6.7% as well as little hysteresis and high stability, as shown in Fig. 6(a)–(c).⁶¹ Significantly, this simplified structure eliminates the expensive organic component and noble metal electrode, and has become the classic structure of CsPbBr₃ PSCs. However, the CsPbBr₃ films fabricated by the two-step solution method face the challenge of impurity phases and poor crystallinity, making it difficult to prepare highly efficient devices. To obtain high-quality CsPbBr₃ films, a multi-step solution method was proposed by Tang's team and a compact and uniform perovskite layer was obtained when the deposit cycle (n) equals 4. Based on the structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/carbon, a high PCE of 9.72% was achieved at that time, as shown in Fig. 6(d)–(f).⁴¹ At present, the highly-efficient CsPbBr₃ PSCs are generally fabricated by a multi-step solution method.

Fig. 6 (a) The structure, (b) energy level alignment and (c) efficiency of the classic CsPbBr₃ PSCs based on the two-step solution method. Reproduced with permission.⁶¹ Copyright 2016, American Chemical Society. (d) The cross-sectional SEM image, (e) J–V curves for different cycles (n) and (f) champion efficiency of the CsPbBr₃ PSCs based on the multi-step solution method. Reproduced with permission.⁴¹ Copyright 2018, Wiley-VCH.

Based on the above study, in order to further improve the performance of CsPbBr₃ PSCs, various optimization strategies have been reported, such as additive engineering,^28,62 interface engineering,^63–65 carrier transfer layer optimization,^45,66 electrode optimization,^29,67 and so on. Table 1 demonstrates the performance parameters of typical CsPbBr₃ PSCs fabricated by the multi-step solution method reported in the last 5 years.

Table 1 The performance parameters of optimized CsPbBr₃ PSCs fabricated by the multi-step solution method in the last 5 years

Device structure	Optimization strategy	PCE (%)	V oc (V)	J sc (mA cm^−2)	FF (%)	Publication time	Ref.
FTO/c-TiO2/m-TiO2/CQDs/CsPbBr3/carbon (PtNi NWs)	Interface engineering	7.86	1.432	6.78	81	2018.5	68
	Electrode optimization
FTO/c-TiO2/m-TiO2/CQDs/CsPbBr3/PQDs/carbon	Interface engineering	7.93	1.45	7.24	75.6	2018.7	69
FTO/c-TiO2/m-TiO2/CsPbBr3/CsSnBr2I QDs/carbon	Interface engineering	9.13	1.39	8.7	75.5	2018.9	70
FTO/c-TiO2/m-TiO2/CsPb0.97Sm0.03Br3/carbon	Additive engineering	10.14	1.594	7.48	85.1	2018.11	71
FTO/c-TiO2/m-TiO2/CsPbBr3/carbon	Electrode optimization	7.62	1.431	6.84	78	2018.11	72
FTO/c-TiO2/m-TiO2/CsPbBr3/BT-BTH/carbon	Novel HTL	9.32	1.5	7.56	81.9	2018.12	73
FTO/SnO2 QDs/CsPbBr3/CsSnBr3 QDs/carbon	ETL optimization	10.6	1.61	7.8	84.4	2019.3	74
	Interface engineering
FTO/c-TiO2/m-TiO2/CsPb0.97Sr0.03Br3/carbon	Additive engineering	9.63	1.54	7.71	81.1	2019.3	75
FTO/c-TiO2/m-TiO2/CsPbBr3/P1Z1/carbon	HTL optimization	10.03	1.578	7.652	83.1	2019.5	76
FTO/c-TiO2/m-TiO2/CsPb0.995Zn0.005Br3/carbon	Additive engineering	9.18	1.56	7.3	80.6	2019.11	77
FTO/c-TiO2/m-TiO2/CsPb0.97Sm0.03Br3/Cu(Cr,Ba)O2/carbon	Additive engineering	10.79	1.615	7.81	85.5	2019.11	28
	Novel HTL
FTO/c-TiO2/m-TiO2/Cs0.91Rb0.09PbBr3/J61-ITIC/carbon	Additive engineering	10.34	1.58	8.18	80	2019.11	78
	Novel HTL
FTO/c-TiO2/m-TiO2/CsPbBr3/PVAc/GO/carbon	Interface engineering	9.53	1.553	7.41	82.8	2019.12	79
FTO/c-TiO2/m-TiO2/CsPbBr3/CsPbI3 NC/carbon	Interface engineering	9.45	1.49	8.66	73.2	2019.12	80
FTO/c-TiO2/m-TiO2/CsPbBr3/C12-QDs/LPP-carbon	Novel HTL	10.85	1.626	7.73	86.3	2020.1	29
	Electrode optimization
FTO/c-TiO2/m-TiO2/CsPbBr3/[BMMIm]Cl/carbon	Interface engineering	9.92	1.61	7.45	83	2020.1	81
FTO/c-TiO2/m-TiO2/CsPbBr3/carbon-PANi/G	Electrode optimization	8.87	1.59	6.87	81.2	2020.2	82
FTO/c-TiO2/m-TiO2/CsPbBr3/carbon	Additive engineering	9.65	1.584	7.42	82.1	2020.4	83
FTO/c-TiO2/m-TiO2/CsPbBr3/MoO2/NC/carbon	Interface engineering	9.4	1.532	7.2	85.2	2020.6	84
FTO/c-TiO2/m-TiO2/CsPbBr3/carbon	Additive engineering	9.68	1.565	7.64	81	2020.8	85
FTO/c-TiO2/m-TiO2/AC/CsPbBr3/ZnPc/carbon	Interface engineering	10.12	1.606	7.64	82.5	2020.1	86
FTO/c-TiO2/m-TiO2/UV-360/CsPbBr3/carbon	Interface engineering	9.61	1.568	7.69	79.7	2021.1	87
FTO/c-TiO2/m-TiO2/CsPbBr3 + Br-GO/Br-GO/carbon	Additive engineering	10.1	1.602	7.88	80.0	2021.5	88
	HTL optimization
FTO/2D SnO2/GQDs/CsPbBr3/carbon	Interface engineering	10.34	1.585	7.94	82.2	2021.3	89
FTO/L-TiO2/CsPbBr3/carbon	Interface engineering	10.75	1.675	7.58	84.6	2021.6	64
FTO/SnO2–RbCl/CsPbBr3/carbon	ETL optimization	10.04	1.601	7.69	81.6	2021.7	66
FTO/TiO2/CsPbBr3/Pt3Co–carbon	Electrode optimization	9.08	1.574	7.24	79.7	2021.8	67
FTO/c-TiO2/m-TiO2/CsPbBr3/HMTA/carbon	Interface engineering	10.08	1.605	7.51	83.6	2021.6	90
FTO/SnO2/CsPbBr3-Ti3C2Clx/Ti3C2Clx/carbon	Interface engineering	11.08	1.702	7.87	82.7	2021.1	26
	Additive engineering
FTO/c-TiO2/m-TiO2/CsPbBr3/DCC/carbon	Interface engineering	10.16	1.611	7.79	80.9	2022.1	91
FTO/c-TiO2/m-TiO2/CsPbBr3/ReSe2/carbon	Interface engineering	10.67	1.622	7.92	83.1	2022.2	92
FTO/c-TiO2/m-TiO2/ASF/CsPbBr3/carbon	Interface engineering	10.08	1.615	7.47	83.6	2022.3	93
FTO/SnO2–SnS2/CsPbBr3/carbon	ETL optimization	10.72	1.635	7.8	84.0	2022.6	45
FTO/c-TiO2/m-TiO2/CsPbBr3/carbon	Interface engineering	10.71	1.65	7.83	82.9	2022.9	94
FTO/c-TiO2/m-TiO2/CsPbBr3-HAM/carbon	Additive engineering	9.05	1.468	7.54	81.7	2023.2	95
FTO/TiO2/CsPbBr3/PEO/carbon	Interface engineering	8.84	1.495	7.8	75.8	2023.3	96
FTO/c-TiO2/m-TiO2/CsPbBr3/PAMPS/NQDs/carbon	Interface engineering	10.21	1.632	7.59	82.4	2023.4	97

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4.2.1 Additive engineering. Additive engineering is widely used in organic–inorganic hybrid PSCs; it can affect perovskite crystallization and defect passivation in the bulk or at the surface,⁶² and it is also suitable for the optimization of CsPbBr₃ PSCs. For example, Guo et al. incorporated SnBr₂ into the CsPbBr₃ layer in order to fabricate a high-quality perovskite layer with better crystallization and more suitable energy-level. The fabricated CsPbBr₃ layer appears smooth and nonporous; meanwhile, the valence-band energy-level of the CsPbBr₃ film moved from −5.6 eV to −5.46 eV, which could better match with the rear-electrode carbon. When the doping amount of SnBr₂ is 0.2%, the PCE is promoted from the initial 6.85% to 8.63% based on the device structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/carbon.⁹⁸

Using the same strategy, Zhao et al. introduced alkaline earth metal cations (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) into CsPbBr₃ perovskite, partially replacing Pb²⁺. After additive passivation (especially Sr²⁺), high-quality CsPbBr₃ films with a smooth surface and big grain size are obtained, as shown in Fig. 7(a) and (b). In addition, based on the structure of FTO/c-TiO₂/m-TiO₂/CsPb_0.97Sr_0.03Br₃/carbon, a PCE of 9.63% was obtained (Fig. 7(c) and (d)), and the device stability was also significantly enhanced. Under the condition of 80% relative humidity, the efficiency of the unencapsulated PSCs remained above 80% of the initial value when the light illumination lasted for 800 hours.⁷⁵ In particular, Tang's team incorporated Ti₃C₂Cl_x MXene into CsPbBr₃ film to realize strain release and defect passivation in the bulk and surface, and finally obtained high-performance CsPbBr₃ PSCs with a PCE of 11.08% and a V_oc of 1.702 V, which were the highest values thus far. The device structure is FTO/SnO₂/CsPbBr₃–Ti₃C₂Cl_x/Ti₃C₂Cl_x/carbon, as shown in Fig. 7(e) and (f).²⁶ Similar additive engineering has also been performed for Lanthanide metal cations, Zn²⁺, Rb⁺, and so on.^71,77,78

Fig. 7 Top-view SEM images of (a) pristine and (b) Sr-doped CsPbBr₃ films fabricated by the multi-step solution method, (c) cross-sectional SEM image of the CsPbBr₃ PSC based on the structure of FTO/c-TiO₂/m-TiO₂/CsPb_0.97Sr_0.03Br₃/carbon, and (d) J–V curves of CsPbBr₃ PSCs doped with different alkaline earth metals. Reproduced with permission.⁷⁵ Copyright 2018, Royal Society of Chemistry. (e) Structure and (f) J–V curves of CsPbBr₃ PSCs passivated by Ti₃C₂Cl_x. Reproduced with permission.²⁶ Copyright 2021, Wiley-VCH.

4.2.2 Interface engineering. Due to the existence of defects and carrier transport barriers at interfaces, especially at the perovskite layer/ETL interface and perovskite layer/HTL interface, carrier recombination and charge accumulation usually occur at the interfaces, which would obviously deteriorate the performance of the photovoltaic devices.^99,100 Therefore, interface engineering is an effective strategy to improve the performance of CsPbBr₃ PSCs. For example, Yuan et al. introduced carbon quantum dots (CQDs) and phosphorus quantum dots (PQDs) as an intermediate energy level to TiO₂/CsPbBr₃ and CsPbBr₃/carbon interfaces, respectively (Fig. 8(a) and (b)), so as to enhance the electron and hole extraction capability of the device, and thus improve the PCE of CsPbBr₃ PSCs. The PCE of the modified PSCs increased to 7.93%, which is much higher than the 6.05% of the non-modified PSCs, as shown in Fig. 8(c).⁶⁹ Similarly, Zhu et al. deposited N,N′-dicyclohexylcarbodiimide (DCC) at the CsPbBr₃ layer/carbon interface to passivate perovskite surface defects and inhibit non-radiative recombination. Based on the structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/DCC/carbon (Fig. 8(d) and (e)), the DCC-modified PSCs yielded a high PCE of 10.16% (Fig. 8(f)).⁹¹

Fig. 8 (a) The energy level alignment, (b) cross-sectional SEM image and (c) efficiency of CQD/PQD-modified CsPbBr₃ PSCs. Reproduced with permission.⁶⁹ Copyright 2018, Elsevier. (d) The energy level alignment, (e) cross-sectional SEM image and (f) efficiency of DCC-modified CsPbBr₃ PSCs. Reproduced with permission.⁹¹ Copyright 2021, Copyright 2018, Elsevier.

4.2.3 Optimization of the ETL and HTL. In PSCs, the carrier transfer layer (including ETL and HTL) not only plays the role of extraction and transport of photo-generated carriers, but also affects the nucleation and crystallization of the perovskite layer. The optimization of the ETL and HTL is conducive to the enhancement of CsPbBr₃ PSCs. Liu et al. introduced a P3HT/ZnPc composite with a tunable energy level as the HTL in carbon-based CsPbBr₃ PSCs. When the molar ratio of P3HT : ZnPc = 1 : 1 (P1Z1), a champion PCE as high as 10.3% was achieved based on the structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/P1Z1/carbon.⁷⁶ Also based on ETL optimization, Zhao et al. utilized optimized SnO₂ QDs as the ETL, and the efficiency of the device with the architecture of FTO/SnO₂ QDs/CsPbBr₃/carbon reached 9.15%. When further introducing a CsSnBr₃ QD layer as a bridge energy level to reduce the interface energy barrier, the device based on the FTO/SnO₂ QDs/CsPbBr₃/CsSnBr₃ QDs/carbon structure achieved a high PCE of 10.6% with a V_oc of 1.610 V.⁷⁴

4.2.4 Optimization of the electrode. Electrode selection affects the carrier collection efficiency and the long-term stability of the device, and thus electrode optimization is also an effective means of improving device performance. This strategy is particularly effective for carbon electrodes with a tunable energy level. In May 2018, Tang's team incorporated alloyed PtNi NWs into the carbon ink to tune the WF of the rear electrode, so as to reduce the energy level difference at the CsPbBr₃/carbon interface and improve the collection of hole carriers. Supplemented by carbon quantum dots (CQDs) as intermediate energy levels, the CsPbBr₃ PSC with the structure of FTO/c-TiO₂/m-TiO₂/CQDs/CsPbBr₃/carbon (PtNi NWs) yielded a PCE of 7.86%.⁶⁸ After that, by adjusting the ratio of multi-walled carbon nanotubes and carbon black (CB) in the carbon electrode, Tang's team achieved the optimal efficiency of 7.62% when the proportion of CB is 25 wt%.⁷²

4.3 Stability of CsPbBr₃ PSCs

Device stability is one of the important indicators for the commercialization of perovskite PSCs. Although the organic–inorganic hybrid PSCs have achieved high efficiency, the device instability hinders their further development.^101–104 In contrast, CsPbBr₃ perovskite is the most stable CsPbI_xBr_3−x perovskite so far, possessing robust stability under the induction of heat, humidity and light. Coupled with the high stability of the functional layer and electrode materials, CsPbBr₃ PSCs generally reveal superb long-term stability.^105,106 Kulbak et al. initially utilized inorganic cation Cs⁺ instead of organic cation MA⁺ to fabricate all-inorganic CsPbBr₃ PSCs, and direct comparisons in terms of photovoltaic performance and stability were performed between CsPbBr₃ PSCs and MAPbBr₃ PSCs. Further studies demonstrated that both types of device achieved similar photovoltaic performance, while CsPbBr₃ PSCs revealed much better thermal stability compared with MAPbBr₃ PSCs. All the performance parameters of CsPbBr₃ PSCs including V_oc, J_sc, FF and PCE showed no remarkable decay under continuous light illumination for 14 days.²¹ This result also confirms that the substitution of volatile organic cations with inorganic cations is an effective means to improve device stability.

Subsequently, Liang et al. fabricated all-inorganic PSCs without any volatile materials or expensive electrode. Based on the classical HTL-free structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/carbon, the fabricated CsPbBr₃ PSCs yielded a PCE of 6.7% and demonstrated superb stability under the induction of high temperature (100 °C) and high humidity (90–95%RH), as depicted in Fig. 9(a)–(c).⁶¹

Fig. 9 Normalized PCEs of CsPbBr₃-based and MAPbI₃-based PSCs as a function of storage time without encapsulation (a) in humid air (90–95% RH, 25 °C) and (b) at high temperature (100 °C). (c) Normalized PCEs of CsPbBr₃-based PSCs as a function of storage time during temperature cycling (between −22 °C and 100 °C) in a high-humidity ambient environment. Reproduced with permission.⁶¹ Copyright 2016, American Chemical Society. Normalized PCEs of the TiO₂-based and TiO₂/SnO₂-based PSCs when stored in ambient air with a relative humidity of ∼40% at (d) room temperature (25 °C) for over 1000 h and (e) 60 °C for 720 h, respectively. Reproduced with permission.⁴³ Copyright 2019, Elsevier. Normalized PCEs of the encapsulation-free devices with and without Ti₃C₂Cl_x in air under (f) 25 °C, 80%RH and (g) 85 °C, 40% RH conditions. Reproduced with permission.²⁶ Copyright 2021, Wiley-VCH.

In 2019, based on the structure of FTO/c-TiO₂/SnO₂/CsPbBr₃/CuPc/carbon, Liu et al. fabricated bilayered ETL CsPbBr₃ PSCs with all-inorganic functional layers and obtained a PCE of 8.79%.⁴³ The device stability was extremely excellent, as shown in Fig. 7(b). When stored under ambient conditions (∼40%RH, 25 °C) for over 1000 hours, both devices (including TiO₂-based and TiO₂/SnO₂-based devices) exhibited superior stability with no efficiency decay. If the temperature increased to 60 °C for 720 hours, the efficiency of the TiO₂/SnO₂-based device remains almost unchanged, while only 97.4% efficiency remained for TiO₂-based devices (Fig. 9(d) and (e)). The results means that the introduction of a bilayer ETL facilitates the enhancement of the device efficiency and stability.

After that, based on the structure of FTO/SnO₂/CsPbBr₃–Ti₃C₂Cl_x/Ti₃C₂Cl_x/carbon, Zhou et al. prepared a CsPbBr₃ PSC with an efficiency of 11.08%, adopting the hydrophobic material Ti₃C₂Cl_x as a passivator and intermediate energy-level layer. The results of stability measurement are shown in Fig. 9(f) and (g). Under high humidity conditions (80%RH), an unencapsulated PSC with Ti₃C₂Cl_x showed little degradation after 100 days of storage, which is much superior to CsPbBr₃ PSCs without Ti₃C₂Cl_x. Under high temperature conditions (85 °C), with or without Ti₃C₂Cl_x, the efficiency of the unencapsulated PSCs remained unchanged after 30 days of storage, indicating that the hydrophobic material as a functional layer can effectively improve the device humidity stability.

5 Conclusion and prospects

In this review, the material properties, film preparation, device structure and performance of CsPbBr₃ PSCs are systematically introduced. Attributed to its high stability and wide bandgap, CsPbBr₃ perovskite exhibits huge potential applications in semitransparent and tandem solar cells.^107–109 However, the low efficiency of CsPbBr₃ PSCs hinders its commercialization progress due to the poor crystallinity of the absorber layer. At present, great efforts are focused on the optimization strategies of CsPbBr₃ PSCs, including additive engineering, interface engineering, optimization of the HTL and ETL, optimization of electrode, and so on. Based on the aforementioned strategies, the PCE of the CsPbBr₃ PSCs quickly increased from the initial 5.95% to the current 11.08%, and demonstrated further upward momentum. In addition, the device stability can also be strengthened by the introduction and optimization of a high stability functional layer material.

Unfortunately, although the current efficiency of CsPbBr₃ PSCs has reached 11.08%, it is still far behind the S–Q limit value of 16.37%, and thus there is large room for improvement. To further enhance the performance of CsPbBr₃ PSCs, the next efforts should focus on the following aspects: (1) simplifying the preparation process of CsPbBr₃ films, transferring from a multi-step solution method to a novel one-step solution method, and avoiding the appearance of an impurity phase; (2) developing novel function layer materials with more suitable bandgaps, or regulating the bandgap of existing materials, so as to achieve energy level matching between the CsPbBr₃ layer and carrier transfer layer, and reduce the energy loss caused by the excessive energy level difference; (3) adding a modification layer at the interfaces of perovskite/HTL or perovskite/ETL to smooth the bandgap, and reduce the non-radiative recombination of carriers, thus improving the efficiency of CsPbBr₃ PSCs.

Author contributions

All authors contributed to the writing of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support from the Scientific Research Programs of Educational Department of Hebei Province of China (B2021213), and the National Natural Science Foundation of China (12134010, 12174290).

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Footnote

† Chuanliang Chen and Xiaoman Lu contributed equally to this work.

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