Challenges in tin perovskite solar cells

Abstract

Perovskite solar cells are the rising star of third-generation photovoltaic technology. With a power conversion efficiency of 25.5%, the record efficiency is close to the theoretical maximum efficiency of a single-junction solar cell. However, lead toxicity threatens commercialization efforts and market accessibility. In this context, Sn-based perovskites are a safe alternative. Nevertheless, the efficiency of Sn-based devices falls far behind the efficiency of Pb-based counterparts. This concise review sheds light on the challenges that the field faces toward making Sn-based perovskites the perovskite photovoltaic benchmark. We identified four key challenges: materials and solvents, film formation, Sn(II) oxidation, and energy band alignment. We illustrate every single challenge and highlight the most successful attempts to overcome them. Finally, we provide our opinion on the most promising trends of this field in the future.

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Mahmoud Aldamasy

Mahmoud Aldamasy received his MPhil degree in Physical Chemistry from Zagazig University, Egypt. He has been working at Egyptian petroleum research institute (EPRI) on petroleum applications. He is now a DAAD PhD scholar at Helmholtz Center Berlin for Energy and Materials and working on finding new solvents for tin perovskites at the department of novel materials and interfaces for photovoltaic solar cells.

Zafar Iqbal

Zafar Iqbal is a DAAD PhD Scholar at Helmholtz-Zentrum Berlin für Materialien und Energie (HZB). He has earned his MPhil degree in Physical Chemistry from Quaid-i-Azam University, Islamabad. He has been working as a researcher at Department of Chemistry and Chemical Engineering, LUMS, Lahore. He is working on Inorganic tin perovskite halides for energy applications.

Guixiang Li

Guixiang Li is currently working as a PhD student at Helmholtz-Zentrum Berlin für Materialien und Energie GmbH. Previously, he received his master degree from South China University of Technology in 2019. Guixiang's current research includes solution-engineered lead-free tin-based perovskite materials and their optoelectronic applications.

Jorge Pascual

Jorge Pascual Mielgo is a post-doc in Helmholtz-Zentrum Berlin under the supervision of Antonio Abate. His current research focuses on the development of efficient and stable lead-free tin halide perovskite devices. He graduated in Chemistry from the University of the Basque Country and specialised in the field of Organic Chemistry. In 2019, he obtained his PhD degree in Polymat (Donostia-San Sebastian) on the study of fullerenes as additives in metal halide perovskite thin films.

Fahad Alharthi

Fahad A. Alharthi is an associate professor at King Saud University in Saudi Arabia. He got his PhD from Hull university, UK in 2016. His research interests are focussed on inorganic nanomaterials for renewable energy and environmental remediation.

Antonio Abate

Antonio Abate is team leader at Helmholtz-Centrum Berlin and a tenure track professor at University of Naples Federico II. He is researching solar energy conversion with perovskite solar cells. Before the current position, Antonio was leading the solar cell research at the University of Fribourg in Switzerland. He was a Marie SkłodowskaCurie Fellow at École Polytechnique Fédérale de Lausanne. He worked for four years as a postdoctoral researcher at the University of Oxford and the University of Cambridge. Antonio graduated from University of Naples Federico II in 2006, and he got his PhD at Politecnico di Milano in 2011.

Meng Li

Meng Li received his PhD degree in materials science and engineering under the supervision of Prof. Liang-Sheng Liao and Prof. Zhao-Kui Wang from Soochow University, P. R. China, in 2018. Currently, he is a postdoctoral research fellow in the laboratory of Prof. Antonio Abate at the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Germany. His primary research interest is focused on organic and inorganic/organic hybrid materials for application in solar cells. His main research topic is non-toxic photovoltaic materials and devices for outdoor and indoor applications.

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1. Motivation

Exploring the photoactive properties of halide perovskites started twenty years ago as a scientific curiosity, and it resulted in a novel type of photovoltaics and optoelectronics.¹ Perovskite solar cells (PSCs) grabbed the attention from the photovoltaic community due to their ease of fabrication, optimum optoelectronic properties, low cost, material abundance, and increasing power conversion efficiencies (PCEs) that quickly increased from 3.8% in 2009 to 25.5% in 2021.² These advantageous characteristics encouraged an unprecedented number of researchers to work on developing PSCs. With the inherited knowledge and techniques from previously developed technologies, especially silicon and organic photovoltaics, notable progress has been made in the last ten years to overcome the hurdles that exist in industrialization and commercialization of PSCs.³ It is the first time an emerging technology has quickly overtaken older technologies like dye-sensitized solar cells (DSSC) and compete with other commercially available technologies such as crystalline silicon solar cells, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) in terms of power conversion efficiency (PCE).^4–6 However, long-term stability^7,8 and lead toxicity^9,10 still hinder PSCs from accessing the market, at least on the short term.¹¹

The toxicological nature of lead increases fears of environmental implications that could hinder PSCs' investments and market adoption. This perspective made the replacement or reduction of lead in PSCs a core activity of several research groups. Nevertheless, the current record efficiencies rely on lead as the metallic component inside the perovskite crystal.¹²

According to the US Environmental Protection Agency (EPA), the maximum allowed lead concentrations are 0.15 μg m⁻³ and 0.15 μg L⁻¹ in air and water, respectively. The directives adopted by the European Union against the use of lead limit its concentration in electronic devices to less than 0.1% of the weight of each homogeneous material inside the device. However, current state-of-the-art lead-based perovskite materials exceed this threshold by at least tenfold taking into consideration the high water solubility of lead compounds used in PSCs.¹³ As a backup strategy, the photovoltaic community started looking for a lead-free perovskite technology^14–16 in order to lower or even eliminate lead from PSCs completely.¹¹⁵

Applying the perovskite dimensionality rules, we realize that a very limited number of metal ions could form a high-performing and environmentally friendly perovskite structure,^17,18 such as Bi/Sb-based halides, a semi-perovskite structure, double perovskites, and Sn/Ge-based perovskites.¹⁸ Sn attracted the most attention among these candidates as it fulfills the charge balance, coordination, and ionic size prerequisites.¹⁹ Tin perovskites carry many environmental advantages over lead perovskites. Tin could be more toxic to living organisms than lead when leaked to the environment. However, it is less bioavailable due to the lower water solubility of oxygenated Sn⁴⁺ compounds.^20,21

Tin perovskites show exciting electrical and optoelectronic properties enabling them to excel lead perovskites in terms of photovoltaic performance. The outer electronic configuration of tin is very similar to that of lead (ns², np²), and the atomic size of both elements is comparable (1.49 Å for Pb(II) and 1.35 Å for Sn(II)). However, the atomic number of lead is higher than that of tin, thanks to the relativistic effect.²² Compared to lead perovskites, tin perovskites show a very sharp absorption edge with a direct bandgap ranging from 1.35 to 1.4 eV, close to the Shockley–Queisser limit with an estimated theoretical efficiency of above 30% (e.g., 33.7% at 1.34 eV). Tin perovskites also show higher charge mobilities and a higher external photoluminescence quantum yield (PLQY).^23,24 These properties would enable Sn-based perovskites to overcome their Pb analogs in terms of efficiency while already showing very promising applications for quantum dots, sensors, detectors, and transistors.^25,26

Currently, tin perovskites still lie in the back behind Pb-based counterparts in terms of efficiency due to several causes. First, Sn(II) oxidation to Sn(IV), which causes self-p-doping and devastates the performance of the devices. Second, fast crystallization dynamics, Sn-based perovskites crystallize very fast compared to Pb-based perovskites, which results in nonuniform crystallization, smaller perovskite domains, pinholes, and a considerable amount of defects. Third, energy band alignment, most of the charge transport layers are designed primarily for Pb-based perovskites, but when used blindly for Sn-based perovskites, we find energy band mismatch, which is very harmful to the overall performance of Sn-perovskite devices (Fig. 1).²²

Fig. 1 Efficiency progress of Sn- and Pb-based PSCs over time.

Chronological overview

The first successful attempt to synthesise Sn-based perovskites dates back to 1974 by Scaife and co-workers, where they synthesized and characterized all-inorganic CsSnX₃, where X₃ is Cl₃, BrCl₂, Br₃, Br₂I, BrI₂, and I₃.²⁷ Five years later, Parry and co-workers prepared and studied the optoelectronic properties of the same all-inorganic Sn-based perovskite materials.^28,29 However, hybrid Sn perovskites were never reported until 1988 when Yamada and co-workers prepared and investigated the phase transition properties of MASnBr₃ (MA = CH₃NH₃⁺).^30–32 From the mid-90s till 2001, Mitzi and co-workers studied the dimensionality and structural engineering of different structures of layered perovskites.³³ In addition, they reported more detailed studies on the thermal, optical, magnetic, and electrical properties of layered Sn-based perovskites.^19,34 It was also reported that the conductivity of such layered perovskites varies with the thickness or the number of perovskite layers; in other words, its electric behavior changes from semiconducting to metallic by increasing the number of perovskite layers.^35,36 Furthermore, they exploited the optoelectronic properties, low cost, and low-temperature processability of Sn-based perovskites by demonstrating them as a semiconducting channel in thin-film field-effect transistors; this was the first reported electronic application of a hybrid lead-free tin-based halide perovskite material.^37,38

Sn-Based perovskite materials started to attract attention from the photovoltaic community, especially after 2009 when Chung and co-workers reported the first solar cell based on MAPbI₃ and MAPbBr₃ perovskite photoactive materials.¹ The first report about Sn perovskites was published in 2012 by Chung and co-workers, where they employed CsSnI₃ doped with SnF₂ as a hole transporting material (HTM) in a DSSC using dye N719 as the solar absorber, and they achieved a PCE of 10.2% (8.51% with a mask).³⁹ Later on, in the same year, Shum and co-workers employed CsSnI₃ with a bandgap of 1.3 eV as an independent light absorber in a Schottky solar cell and achieved a PCE of 0.9%.⁴⁰ In 2013, the PV community started to reinvestigate and exploit the optoelectronic properties of Sn-based perovskites as a potential candidate for solar light harvesting.^41–44 The first promising efficiencies were reported in 2014 when Snaith and co-workers reported a PCE of 6.4% based on MASnI₃ as a light absorber.¹⁵ Four days later, Hao and co-workers reported an efficiency of around 5.7% utilizing MASnI₃ and MASnI_3−xBr_x as photoactive absorber materials.¹⁶ These initial results gave the solar cell community a sense of trust in Sn-based materials as potential nontoxic contenders for the rocketing Pb-based perovskites.^45–47

In early 2016, Lee and co-workers reported FASnI₃-based PSCs (FA = NH₂CH = NH₂⁺) utilizing SnF₂–pyrazine complexes to guarantee a uniform dispersion of SnF₂ to avoid perovskite phase separation; an efficiency of 4.8% was achieved, and the encapsulated cells maintained 98% of its initial efficiency for 100 days.^48,49 Later on, Liao and co-workers achieved a PCE of 6.22% based on inverted planar FASnI₃ utilizing the solvent engineering method.⁵⁰ In 2017, the PSC community witnessed remarkable achievements in terms of both efficiency and stability of Sn-based PSCs, most of which were based on inspired techniques from Pb-based PSCs such as cation mixing,⁵¹ incorporating low dimensional perovskites,⁵² and utilizing hollow^53,54 and mixed 2D–3D perovskites.⁵⁵ Such practices led to a jump in the PCE from 6% in 2016 to around 9%,⁵⁵ and significantly enhanced stability.⁵³ Zhao and co-workers adopted the cation mixing strategy to improve cell efficiency; by optimizing the ratio of the FA⁺ and MA⁺ cation mixture, a PCE of 8.12% was obtained along with an improved perovskite film morphology.⁵¹ Shao and co-workers mixed 92% of highly crystalline 3D FASnI₃ with 8% of 2D Sn perovskites in a p–i–n planar device; this increased the PCE to 9% with negligible hysteresis, low shunt losses, and very low trap-assisted recombination.⁵⁵ In early 2018, Ran and co-workers reported a 2D–3D heterojunction Sn-based PSC using phenylethylammoniumiodide (PEAI) as a 2D and FASnI₃ as a 3D perovskite material and a PCE of 6.98% was obtained.⁵⁶ At the same time, Liu and co-workers improved the morphology of perovskite layers by using hot antisolvent and annealing under a low partial pressure of DMSO.⁵⁷ Both groups reported a PCE of around 7%. In 2019, PCEs of Sn-based PSCs continued to surge, approaching the 10% threshold through two different approaches. Jokar and co-workers reported the first approach by using bulk organic molecules such as ethylenediammonium diiodide (EDAI₂) and butylammonium iodide (BAI) to improve film morphology and improve crystallinity in addition to surface passivation, and they achieved a PCE of 8.9%.⁵⁸ Later on, the same group used guanidinium cation GA⁺ as a nonpolar organic cation mixed with an FA⁺ cation in the ratio of 20 : 80 respectively in addition to 1% ethylenediammonium diiodide (EDAI₂) as an additive to suppress Sn(II) oxidation to Sn(IV) and passivate surface defects; 9.6% was achieved after storage in a glovebox environment for 2000 h; this gradual increase in efficiency was attributed to the slow surface passivation and crystal relaxation.⁵⁹ The second approach was reported by Wang and co-workers. They grew parallel-orientated 2D PEA₂SnI₄ on the surface of 3D FASnI₃, using pseudohalogen NH₄SCN as a structure regulator. This hierarchical 2D-quasi-2D–3D Sn-based perovskite structure delivered a PCE of 9.41% with improved air stability and high reproducibility.⁶⁰ In 2020, Nishimura and co-workers achieved a PCE of 13.24% by partially substituting FA⁺ with ethylammonium (EA⁺), which enhanced the film morphology and charge extraction.⁶¹ Recently, two high efficiencies were reported, 13.4%⁶² using phenylhydrazine with FA as mixed cations and a mixture of halogens Cl⁻ and Br⁻ as mixed anions by Wang and co-workers; however, the record efficiency in tin perovskites is 14.63% using one-step synthesis of SnI₂-solvent adducts.⁶³ In terms of stability, Liu and co-workers reported FASnI₃ layers prepared via templated crystal growth with a PCE of 11.22% and decent stability of over 1000 h of light soaking while retaining 95% of the initial efficiency.⁶⁴ These improvements and efficiencies approaching 15% were possible through the big effort of the community. And, despite all the strategies implemented, Sn-based PSCs must still face important challenges if they are to reach the expected performance. The photovoltaic parameters of the best performing tin-based perovskite devices in recent years is summarized in Table 1.

Table 1 Summary of the evolution of tin-based perovskite solar cells

Tin perovskite	Year	Photovoltaic parameters				Ref.
		V oc [V]	J sc [mA cm^−2]	FF	PCE [%]
MASnI3	2014	0.88	16.8	42	6.4	15
FASnI3	2016	0.46	22	61	6.2	50
FASnI3	2017	0.53	24.1	71	9.0	55
FASnI3	2018	0.58	21.3	72	8.9	58
FASnI3	2018	0.61	22	70	9.4	60
FASnI3	2019	0.62	21.2	73	9.6	59
FASnI3	2020	0.84	20.3	78	13.2	61
FASnI2Br	2020	0.81	23	72	13.4	62
FASnI3	2021	0.91	20.6	77	14.6	63

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In this review, we discuss the bottlenecks that hinder Sn-based PSCs from achieving efficiencies comparable to those of Pb-based PSCs.⁶⁵ First, we discuss the effect of solvents and materials on the performance and stability of the devices and the recent efforts to find new solvent systems for FASnI₃. Second, we discuss the film formation and crystallization dynamics in Sn-based perovskites and talk about the most effective tries to make oriented and highly crystalline films. In particular, we discuss the use of 2D/3D mixed perovskites to make highly efficient Sn-based PSCs. After that, we discuss the oxidation of Sn(II) to Sn(IV) and the most successful attempts to quench it. Finally, we discuss the energy band alignment and its effect on charge extraction and overall device efficiency.

2. Precursors & solvents

To find the main reason behind the low efficiency in Sn-based halide perovskites,⁶⁶ we have to take a step backward and follow up the processing steps from the beginning. We should start with the materials we use to prepare the precursor solutions. For example, commercially available reagents may contain a small but effective amount of Sn(IV) (<0.1%), which is enough to convert an ASnX₃ film into a p-type material, with devastating effects on the photovoltaic performance of devices.⁶⁷ Therefore, analyzing the purity of raw materials is necessary to guarantee that Sn(IV) is not present as an initial component, minimizing the incorporation of oxidised species and alleviating self-p-doping. Furthermore, it is also vital for perovskite film formation, as we will explain in a later section.⁶⁸

Several researchers placed great importance on material purification, and different approaches were reported. Ozaki and co-workers followed a chemical purification strategy in which they formed a tin halide-solvent complex [SnX₂(S)_n] where S is a solvent (S = DMF and DMSO).^69,70 In the beginning, they characterized SnI₂ using ¹¹⁹Sn magic-angle spinning (MAS) NMR spectroscopy, thermogravimetric analysis (TGA), and Karl Fischer titration. They found that SnI₂ contains around 10% of SnI₄ and about 10 000 ppm of water, although the bottle was labeled 99.99% purity. The ¹¹⁹Sn-NMR spectra in Fig. 2a show a peak at −1742 ppm, which is attributed to the SnI₄. TGA confirmed the same results as 10.1% weight loss was observed at ∼150 °C, which corresponds to the sublimation of SnI₄ (Fig. 2b), while SnI₂ sublimates at 330 °C. Next, Ozaki and co-workers utilized the difference in the sublimation temperature between SnI₂ and SnI₄ to purify SnI₂ using the vacuum sublimation technique where SnI₄ sublimates and then condensates as an orange powder at ∼150 °C.⁶⁹¹¹⁹Sn-NMR and TGA analyses of the purified SnI₂ using the vacuum sublimation method are shown in Fig. 2c and d.

Fig. 2 Purified SnI₂ reagents, (a and b) ¹¹⁹Sn MAS NMR spectra, and TGA analysis of commercially purchased SnI₂. (c and d) ¹¹⁹Sn MAS NMR spectra and TGA analysis of purified SnI₂ using the vacuum sublimation method. (e and f) Structural illustrations of [SnI₂(DMF)] and [SnI₂(DMSO)].⁶⁹ Reproduced with permission Copyright 2020, American Chemical Society.

They recrystallized SnI₂ in DMF and DMSO to remove any SnO₂ residuals from the vacuum sublimation step for further purity. SnI₂ from the first step was dissolved in DMF and DMSO, and then toluene or dichloromethane was poured gently into the solution, which converted into needle crystals of [SnI₂(DMSO)], [SnI₂(DMSO)₂] and [SnI₂(DMF)] as determined by single-crystal XRD (Fig. 2e and f). The purity of these crystals was confirmed using elemental analysis and ¹¹⁹Sn MAS NMR spectroscopy and found to be stable under an inert atmosphere and could be used conveniently as a pure starting reagent for tin perovskites. The authors merged these purifying techniques with other film formation methods and additives to achieve a PCE of ∼12% for FASnI₃.

Solvents have a very strong influence on perovskite film crystallization and, hence, on the device performance.^71,72 Nevertheless, in Sn-based PSCs, solvents have a much more profound influence due to the sensitivity of Sn(II) towards oxidation.⁷³ In parallel, Sargent's group and our group independently found that DMSO was undergoing a redox reaction with Sn(II), oxidizing it to Sn(IV).^74,75 Solid-state ¹¹⁹Sn-NMR analysis of the received SnI₂ did not show any Sn(IV) peaks, implying no content of it or a concentration lower than the measured detection limit of 1.5 wt%. By exposing the same sample to air, the Sn(IV) signal appeared and increased with exposure to air. After that, fresh precursor solutions of FASnI₃ were prepared and characterized for the presence of Sn(IV). Fortunately, there was no Sn(IV) at room temp, but after heating the solutions at 100 °C for 30 minutes to simulate the conditions of the film formation, the color of the DMSO-based FASnI₃ changed to dark red, and Sn(IV) peaks were detected in the ¹¹⁹Sn-NMR spectrum.⁷⁵ To further understand the oxidation mechanism of Sn(II) to Sn(IV), ¹³C-NMR and ¹H-NMR analyses were performed for the organic species in precursor solutions (DMSO, FAI, and MAI). ¹H-NMR showed a peak at ∼1.9 ppm for both MA⁺ and FA⁺ containing solutions, and this peak matches with the expected peak of dimethylsulphide (DMS) which confirms the same finding reported earlier by Saidaminov and co-workers. The same peak was found in the ¹³C-NMR spectrum, which confirms the formation of DMS as a product of the Sn(II) oxidation reaction (Fig. 3b). Previous results confirm the occurrence of a redox reaction between DMSO and Sn(II) which is promoted in the presence of an acidic environment^76,77 (Fig. 3c).

Fig. 3 Oxidative effect of DMSO, (a) FASnI₃ solutions in DMSO and DMF before and after ageing for 5 h at 120 °C.⁷⁴ Reproduced with permission Copyright 2020, American Chemical Society. (b) Mechanism of Sn(II) oxidation by DMSO. (c) ¹³C-NMR spectra of FASnI₃ heated at 100 °C for 30 min. The green arrow indicates the DMS-related peak.⁷⁵ Reproduced with permission, Copyright 2021, The Royal Society of Chemistry.

Considering the negative impact of DMSO, so far the universal solvent for tin perovskites, on material stability, our group decided to replace it by new solvent combinations.⁷⁸ We targeted solvents with the ability to form a perovskite solution with a concentration over 1 M and be stable after thermal soaking for at least 12 hours at 100 °C. For this, we screened most of the possible functional groups with particular attention to dipolar aprotic solvents which could dissolve ionic molecules such as FAI and SnI₂ using their strong dipoles. Some solvents were excluded, such as strong acids, solvents with big aromatic structures commonly used as antisolvents, solvents with too low boiling point such as methylamine, and solvents with a high melting point. Out of 2000 suggested molecules, we tested 76 solvents for their solubility, stability, and ability to form a perovskite solution. Experimentally, we identified 12 solvents that could form a stable perovskite solution with six different functional groups with the amide group being the mostly presented (Fig. 4a). To ensure that these new precursors could form perovskite materials, the easiest way was to drop-cast the precursor on a glass substrate, and then anneal at 150 °C for 20 minutes with a thermal ramp of 12 °C min⁻¹. It was impressive that 15 out of 17 solvents (including DMSO and DMF) formed a black phase. XRD characterization was performed for the drop-casted films of the new solvents, and the resultant peak positions are in agreement with the standard positions of crystalline FASnI₃ films. However, some samples have low crystallinity, and some other samples have different phases; such optimizations can be dealt with in the film formation stage.

Fig. 4 (a) 16 nonsulfoxide solvents that formed stable perovskite solution divided into groups according to the functional group. (b) SEM images of the DEF:DMPU film. (c) J–V curves of FASnI₃ without any additives in DEF:DMPU against DMSO.⁷⁸ Reproduced with permission Copyright 2021, American Chemical Society.

The ability of the new solvents to form a working perovskite device was also tested. A mixture of two solvents, namely N,N-diethylformamide (DEF) and N,N′-dimethylpropyleneurea (DMPU), in a 6 : 1 ratio achieved a PCE of 6.2% for FASnI₃ without using any additives. Meanwhile, additive-free DMSO samples achieved a PCE of only 3.84% (Fig. 4b and c). These results prove that finding a new stable solvent for processing Sn-based perovskites could be an entrance towards achieving highly efficient and durable tin-based PSCs.

3. Film formation

The formation of high-quality perovskite films is a vital prerequisite to push the performance of PSCs forward.⁷⁹ The crystalline quality and morphology of perovskite films are related directly to the overall performance of the device. To make high-quality perovskite films, we have to understand and manipulate the mechanism of crystal formation that starts from the precursor solution and ends at the end of the annealing step.⁸⁰ The perovskite crystallization process depends on several factors such as the substrate nature, temperature, spin coating parameters, solvent, and antisolvent interaction.⁸¹ Film formation dynamics and crystallization kinetics in Sn-based perovskites are different from those of lead-based perovskites.⁸²

Although many reports focused on studying the crystallization dynamics and kinetics of Pb-based perovskites through in situ techniques,^82,83 very few reports dealt with the same point in Sn-based perovskites. Dong and co-workers studied the crystallization mechanism of tin perovskites using several techniques, offering essential insights that revealed the mechanism of structural revolution, material transformation, and crystallization dynamics in mixed 2D/3D Sn-based perovskites.⁸⁴ They found that crystallization tends to start at the precursor/air interface, but in 3D Sn-based perovskites, the crystallization rate is comparable in bulk and at the precursor/air interface. Moreover, the addition of 2D molecules hinders the crystallization process in bulk, which allows uniform and highly oriented crystals with a similar structure to the 3D perovskites. The crystallization dynamics, in general, were found to be very fast compared to the lead-based counterpart.

The attempts to optimize the quality of Sn-based perovskite films are numerous. Most of them rely on chemical additives that could form 2D perovskites on top of the regular 3D perovskite layer, usually known as Ruddlesden–Popper perovskites.^60,85,86

Liu and co-workers reported a PCE of 11.22% based on highly crystalline FASnI₃ films with preferential orientation along the (001) plane.⁶⁴ They reported a solvent pretreatment method using a mixture of n-propylammoniumiodide (PAI) and DMSO (1 : 100 v/v). DMSO acts as a solvent to provide the liquid interface for nucleation. At the same time, PAI aggregates around the newborn nuclei of FASnI₃ and forms a template that regulates the crystal growth and orients it along the (100) plane. PAI was used in a minimal amount compared to DMSO (1 : 100 v/v) as it is not considered as an additive to the precursor solution or co-solvent because it is not forming 2D perovskite that covers the surface of 3D perovskites. The mechanism of action of PAI is shown in Fig. 5a. SEM images in Fig. 5b indicate a big difference in the grain sizes between FASnI₃ with and without PAI. The bigger the grain and the domains, the less the grain boundaries and the less the defects. In templated growth FASnI₃ (TG-FASnI₃), the defect density was lowered from 2.89 × 10¹⁶ cm⁻³ to 5.41 × 10¹⁵ cm⁻³. The XRD patterns in Fig. 5c show an increasing intensity of the 100 plane peak with an increase in the concentration of PAI from 1 to 2 mg mL⁻¹, while it decreases again after increasing the concentration to 5 mg mL⁻¹, which indicates that the optimum concentration of PAI is around 2 mg mL⁻¹. The effect of such a small quantity of PAI on the crystallization and the photovoltaic performance of FASnI₃ was tremendous as shown in Fig. 5d–g. TG-FASnI₃ shows an increase in PL intensity which confirms the low defect density and the longer carrier diffusion length than regular control FASnI₃. The prepared devices based on TG-FASnI₃ showed much better operational stability compared to the control devices. Fig. 5g shows the results of maximum power point tracking (MPPT) of both devices encapsulated and stored in an N₂ filled glove box. 95% of the initial efficiency was maintained after 1000 h of operation, while the control devices lost around 60% of their initial efficiency after 500 h.

Fig. 5 (a) Proposed mechanism of action of TG-FASnI₃. (b) SEM images of FASnI₃ (1) before and (2) after annealing; TG-FASnI₃ (3) before and (4) after annealing. (c) XRD patterns of FASnI₃ and TG-FASnI₃. (d) PL spectra of FASnI₃ and TG-FASnI₃. (e) TRPL of FASnI₃ and TG-FASnI₃. (f) J–V curves of FASnI₃ and TG-FASnI₃ in forward and reverse scanning. (g) Stability test of FASnI₃ and TG-FASnI₃ under simulated AM 1.5 G (100 mW cm⁻²) operating at MPPT.⁶⁴ Reproduced with permission, Copyright 2020, The Royal Society of Chemistry.

Finally, one of the bottlenecks that tin-based perovskites face is the fast crystallization leading to low film quality compared to that of lead-based perovskites, which leads to a high defect density in the range of 10¹⁵–10¹⁶, while in lead-based perovskites, it is 10¹³–10¹⁴ cm⁻³. This high defect density is one of the main reasons behind the low efficiency of tin-based perovskites.⁸⁷

4. Sn(II) oxidation

Oxidation of Sn(II) is one of the bottlenecks to achieve good efficiency in Sn-based perovskites solar cells. Sn(II) is more vulnerable to oxidation which is electronically and chemically favorable.⁸⁸ It has been revealed through theoretical calculations that tin oxidation is favorable on the pristine surface and unfavorable in the bulk. Resultantly, bulk Sn(IV) changes into Sn(II), which releases two holes to the valence band and by p-doping the perovskite,⁶⁸ this has been experimentally proved that tin vacancies lead to p-doping which limits the diffusion length.^15,89

Although Sn and Pb are present in the same group of the periodic table, it has been found that the Sn 5s² orbital has higher energy as compared to the 6s² orbital of Pb which increases its contribution to the valence band edge. The electrons of 6s² in Pb show an inert pair effect due to lanthanide contraction but 5s² lacks this phenomenon which ultimately makes Sn electrons loosely bound and easy to lose.⁹⁰

To stop the oxidation of tin and to improve the stability of the perovskite films and devices, different SnX₂ species have been employed in excess to stop p-doping. For the first time, Kumar and co-workers showed that the introduction of SnF₂ in inorganic tin halide CsSnI₃ reduces the Sn vacancies.⁹¹ Later on, they have also reported that by the introduction of SnF₂ in FASnI₃ films, the oxidation of Sn(II) has been effectively reduced, and there is an increase in the J_sc values of the device.⁹² SnF₂ has also been introduced in CsSnBr₃ films, and it has been found that it suppresses the oxidation of Sn(II).⁹³ In the most recent work on Sn halide perovskite-based devices, SnF₂ has been used as an important dopant as it suppresses the oxidation of Sn(II), improves film morphology, and also improves J_sc.⁹⁴ Other SnX₂ species like SnCl₂, SnBr₂ and SnI₂ improve the film quality, device performances⁹⁵ energy alignments⁹¹ and also surface passivation.⁹⁶

Hydrazine and its derivatives were used by many researchers as a reducing material in Sn-based perovskites. Hydrazine in the vapor phase has been used in MASnI₃ and CsSnI₃ film formation and for device fabrication.⁹⁷ The ratio of Sn(IV) to Sn(II) has been reduced by using hydrazine vapors. It has been found that hydrazine molecules release electrons which suppress the oxidation of Sn(II).⁹⁸ By introducing phenylhydrazine hydrochloride salt (PHCl) in the FASnI₃ film, Wang and co-workers reported 11.4% efficiency. It has been noticed that incorporating the molecules in the crystal lattice improves the device performance.⁹⁹ Similarly, by adding 8 mol% trihydrazine dihydriodide (THDH) to FASnI₃ solution, the oxidation of Sn(II) was suppressed, and the champion device efficiency was reported to be 8.50%. Another derivate of hydrazine 4-fluorobenzohydrazide (FBH) was introduced in FASnI₃, and it significantly enhanced the efficiency as compared to that of the control device. The champion device efficiency was reported to be 9.03% by using FBH molecules.¹⁰⁰ Recently, Wang and co-workers achieved a record efficiency of 13.4% (12.4% certified) through the introduction of synergistic effects between PH cations and halide anions.⁶²

T. Wang and co-workers introduced gallic acid (GA) as an antioxidant along with SnCl₂ for FASnI₃ films. By exploring the underlying chemistry of GA and the tin precursor, they reported 9.03% efficiency.¹⁰¹ Kayesh and co-workers used 5-ammonium valeric acid (5-AVAI) to enhance the oxidation stability of FASnI₃ perovskite films. 5-AVAI not only suppresses the Sn(II) oxidation but also enhances the crystallinity by making a hydrogen bond with SnI₆. The devices show improved stability with 7% efficiency.¹⁰²

The primary precursor used for tin halide perovskites is SnI₂. It is oxidized by air. To make it purer, Sn metal powder is used. Low purity has a high level of doping which ultimately decreases the efficiency. Huang and co-workers used tin powder to purify tin sources from Sn(IV) moieties, which has increased the device performance parameters.¹⁰³ Recently, Nakamura and co-workers have reported a different technique to make Sn(IV) free devices. They have produced Sn metal nanoparticles from SnF₂ over SnBr₂ and SnCl₂ by using 1,4-bis(trimethylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine (TM-DHP) molecules. These Sn metal nanoparticles can reduce SnI₄ to SnI₂ to make it a Sn(IV) free system. By using this technique, they have reported more than 11.2% certified efficiency.¹⁰⁴

5. Device structure and energy alignment

In device operation, when light is shone on the device, due to the low bandgap, the electrons from the valence band of the perovskite halide materials shift to the conduction band and leave the hole vacancies in the valence band. These generated electrons and holes diffuse in the ETM and HTM and are finally collected at the cathode and anode. These ETM and HTM materials should be efficient enough to transport the electrons and holes across the boundary. This phenomenon is only possible when the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of the hole transport material and electron transport layer are well aligned with the conduction and the valence band of the perovskite material. For better performance and stability of the devices, hole transport materials and electron transport layers are chosen accordingly.¹⁰⁵

Conventional HTMs in Sn-based perovskites were designed originally for Pb-based perovskites and then tried for Sn-based devices after showing a good performance. However, the positions of energy bands in Sn-based perovskites are slightly different from those of Pb-based perovskites. This leads to mismatching in the alignment of the energy bands of the absorber material with charge transfer layers on both sides of the Sn-based absorber film.¹⁰⁶ For instance, the conventional HTM used in inverted devices poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has a work function of about −5.0 eV (higher than FASnI₃) and 2,2′,7,7′-tetrakis[N,N-di(p-methoxyphenylamine)]-9,9-spirobifluorene (spiro-OMeTAD) has an even more higher HOMO level that is −5.22 eV. Due to these poor energy alignments, Sn-based PSCs are unable to achieve their theoretical predicted efficiency.

Initially, Noel and co-workers have reported Sn-based perovskite halide-based regular devices with efficiencies of greater than 6% under simulated full sunlight using MASnI₃ as a perovskite material. Later on, it was found that spiro-OMeTAD, which is used as the hole transport material in regular structure devices, needs dopants like lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP), which enhance its hole mobility. These dopants have a harmful effect on the perovskite layer¹⁰⁷ and in addition to that, they increase the rate of charge recombination by forming bonds between TiO₂ and Sn (Fig. 6).¹⁰⁸

Fig. 6 (a) Band profiles of each ETL material and perovskite material obtained experimentally from the ionization potential, the electron affinity, and the work function. The dotted lines indicate the Fermi energy, and the vacuum level (EVAC) is plotted as zero. (b) Conduction band offset (CBO) at the ETL/perovskite layer interface when using each of the oxides. The CBO (ΔECBO) is defined as the energy difference between the electron affinity of the ETL material and that of the perovskite material based on the perovskite material.¹⁰⁹ Reproduced with permission Copyright 2020, American Chemical Society.

After these attempts, the first inverted device reported 3.30% efficiency, which was fabricated by using CsSnI₃ as the perovskite material, NiO_x as the hole transport layer, and PCBM and Al as ETLs. By using PEDOT:PSS as the HTL, Liao and co-workers reported 6.22% efficiency.⁵⁰ Later on, Liu and co-workers tuned the energy levels of PEDOT:PSS by using polyethylene glycol (PEG). Fullerene molecules are used as ETMs in inverted devices. To know which derivative has the best energy alignment with Sn perovskites. C₆₀, PCBM, and ICBA were used as ETLs for FA_0.9PEA_0.1SnI₃. Among all, ICBA with work functions of −3.73 and −5.73 has perfect energy alignment with the perovskite and has a higher V_oc value as compared to others. Due to the dopant issues in spiro-OMETAD, Ke and co-workers reported dopant-free organic molecules such as tetrakis-triphenylamine (TPE) as the HTM for Sn-based PSCs, and they reported 7.23% champion device efficiency.¹¹⁰ Similarly, due to the shortcomings of PEDOT:PSS, Cao and co-workers introduced copper thiocyanate (CuSCN) as the HTM for the FASnI₃ based device and acquired 7.34% efficiency.¹¹¹ The conventional electron transport layer in a Pb-based perovskite solar cell has an energy mismatch with that of a Sn halide-based perovskites solar cell. Yokoyama and co-workers used Nb₂O₅ as the electron transport material in place of TiO₂ in a regular inverted device. It has been found that it decreased the conduction offset and interfacial charge recombinations.¹⁰⁹

This study reveals that the conduction band of Nb₂O₅ is closest to zero for FASnI₃ perovskite materials; this increases the V_oc of Sn PSCs as compared to that of TiO₂ and SnO₂. To gain a good energy alignment, Cong and co-workers introduced an organic molecule poly[tetraphenylethene3,3′-(((2,2-diphenylethene-1,1-diyl)bis(4,1-phenylene))bis(oxy)) bis (N,N-dimethylpropan-1-amine)tetraphenylethene] (PTN-Br) in FASnI₃ to modify grain boundaries. It has the highest occupied molecular orbital (HOMO) energy level of −5.41 eV, due to which it forms an energy alignment with the Sn halide perovskites solar cell.¹¹² Precise measurements of the energy levels of tin-based perovskites were provided by Shuxia and co-workers.¹⁰⁶ These values are beneficial for making a new cascade route for electrons and holes.

6. Conclusion and outlook

Sn-based PSCs carry enough optoelectronic properties to achieve the maximum theoretical efficiency of a single junction solar cell according to the QS limit. We believe that it will also be a key component in the multi-junction solar cell industry. So far, an efficiency of 14.6% has been achieved, which is 10% less than where it should be compared to those of Pb-based PSCs. However, to achieve such ambitious goals, Sn-based PSCs must overcome four bottlenecks, namely: (1) pure starting chemicals and non-oxidising solvents, (2) better film formation dynamics, (3) Sn(II) oxidation suppression and (4) better energy band alignment. To overcome these limitations, we need to focus on finding new, stable, and effective solvents for Sn-based PSCs. We also need to understand the Sn chemistry inside the precursor, and through film formation steps, customize new contact layers that have better matching with the energy bands of Sn-based compositions, and stabilizing Sn(II) through all the device making steps. Then we could provide a high protection and isolation level to devices under operation using modern encapsulation techniques.^113,114

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

M. A. and Z. I. thank DAAD and Egyptian missions for PhD scholarship. The authors acknowledge financial support from the Natural Science Foundation of China (No. 51903181). G. L. thanks China Scholarship Council (CSC) for financial support (Grant No. 201906150131). Authors thanks the Distinguished Scientist Fellowship Program (DSFP) at KSU for financial support.

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