Situation and Perspectives on Tin-Based Perovskite Solar Cells

Abstract

Perovskite solar cells have become the current research focus because of their high conversion efficiency and other advantages; however, the toxicity of lead used in them has raised environmental concerns. Tin-based perovskite materials have become the most promising alternative materials for perovskite solar cells because of their relatively low toxicity, suitable band gap and relatively higher energy conversion efficiency than perovskite materials based on other elements. In this article, the status of this rapidly growing field, authors’ output and cooperation, hot research topics, important references and the development trends of tin-based perovskite solar cells are identified and visualized using CiteSpace software. The main research fields are found to be optical properties, 3D–2D perovskite and perovskite solar cell conduction band materials. The mixed organic metal halide perovskite solar cell and the CsSnI₃ semiconductor are identified as emerging trends for tin-based perovskite solar cells. Such contents in this article highlight the key points in the wide field of literature so it can be understood efficiently.

1. Introduction

The photovoltaic performance of metal halide perovskite (MHP) materials has rapidly increased from 3.8% [1] to 25.2% [2] in the past decade, which shows their potential for future applications. Their great success is attributed to the unique ABX₃ crystal structure (where A is a monovalent organic or inorganic cation, B is a divalent cation and X is a monovalent anion), which provides ideal photovoltaic characteristics, including small exciton binding energy of several megaelectron volts (<25 megaelectron volts), and large electron hole mobility. In addition, the high efficiency of MHP is mainly attributed to Pb. Yet, the disadvantage of Pb is that it degrades with time, and its water solubility poses a serious threat to the environment, human beings and other species. Therefore, it is necessary to seek alternative materials with less toxicity.

In 2014, Hayase et al. [3] and Kanatzidis et al. [4] preliminarily studied the substitution of Sn for Pb in perovskite solar cells (PSCs), with the efficiencies 4.18% and 5.73% obtained, respectively. Borriello et al. [5] first proposed the application of tin halide perovskite to solar cells. They studied the structure and electronic characteristics of various tin-based perovskite and found that an inorganic cation structure has a great influence on the electronic characteristics of tin halide perovskite, and organic molecules must adapt to cubic octahedral pores formed by inorganic molecules. Since then, various tin-based PSCs have been continuously developed. The highest efficiency of tin-based PSCs is 14% [6] up to now, which is far ahead of other lead-free materials. Tin-based perovskite materials have become the most promising alternative materials for PSCs due to their relatively low toxicity, suitable band gap and high energy conversion efficiency.

At present, many researchers all over the world carry out research work around tin-based PSCs. Accordingly, in the face of a large number of documents created, it has become the first problem to be solved before conducting research to find the key documents worthy of intensive reading, assess the leading findings and locate research hotspots. To that end, bibliometrics is an interdisciplinary science that integrates mathematics, statistics and philology, which can be applied to conduct quantitative analyses of the number of papers, authors, keywords, popular literature, etc., in a specific research field, which is helpful to grasp the research hotspots, development history and new trends in the future of this research field; therefore, it has important reference value for researchers.

This paper focuses on the author collaboration networks, co-occurrence of keywords, co-cited references and outbreaks of co-cited references produced by CiteSpace. The knowledge fields, quantitative research modes and trends of tin-based PSCs are explored, which may help readers to obtain accurate and complete information on this field more efficiently.

2. Materials and Methods

2.1. Data Collection

With Web of Science as the main data source, the core collection as the database, “Sn perovskite solar cell” as the retrieval type and the statistical time ending in May 2022, 1061 records were retrieved. The collected data were exported in a plain text format for data analysis and processing.

2.2. CiteSpace

CiteSpace is a widely used, multi-dimensional, time-sharing and dynamic third-generation visual analysis tool for network information based on a JAVA platform developed by Professor Chaomei Chen of Drexel University in the United States. It can be obtained freely from the website of Professor Chaomei Chen at http://cluster.cis.drexel.edu/~cchen/citespace/ (accessed on 2 December 2022). There are also some other visualization tools, such as Histcite and Vosviewer. In comparison, Citespace can support more data formats from different databases and provide more analytical functions.

In this paper, CiteSpace 6.1.R2 was used to analyze the literature co-citation, for author analysis, keyword co-occurrence map analysis and keyword time zone map analysis in the field of tin-based PSCs, in order to explore the development status and research hotspots of this field. In total, 1061 articles from 2010 to May 2022 were analyzed, and time slice 1 was selected for analysis.

3. Results

3.1. Annual Paper Publication

The first paper on tin-based perovskite, entitled “Controllable Synthesis of Well-dispersed and Uniform-sized Single Crystalline Zinc Hydroxystannate Nanocubes”, was published in 2010 [7]. In this paper, single crystal zinc hydroxystannate nanocubes were synthesized on α-{copper, tin} copper foil by the low-temperature hydrothermal method, and ZnSn(OH)₆ nanocubes with a cubic perovskite crystal structure were obtained on the substrate. Although this was not the classical ABX₃ fomula used widely later, the perovskite crystal structure was first correlated with tin-based materials in this paper. The number of papers published, as shown in Figure 1, indicates that research attention to tin-based PSCs has since increased continuously.

It can be seen from Figure 1 that the number of publications on tin-based PSCs increased rapidly in 2017. In this year, several important articles promoted the production of tin-based PSCs. Yuqin Liao et al. [8] found that the stability of low-dimensional tin halide perovskite thin films prepared could be significantly improved when using PEA as an organic isolation layer, and pure tin-based PSCs with an efficiency of 5.94% and stability lasting more than 100 hours were obtained. Liang Jia et al. [9] showed that an all-inorganic PSC based on CsPb_0.9Sn_0.1IBr₂ and a carbon counter electrode had a suitable band gap of 1.79 eV and a high open circuit voltage of 1.26 V, with the PCE as high as 11.33%. In addition to their good photo-to-electric conversion parameters, the cells also showed good stability, heat resistance and moisture resistance. These two highly cited articles, along with others, have provided great inspiration to the following researchers by promoting the development of tin-based PSCs, and the performance of such solar cells has been continuously improved with the efforts of the following researchers.

3.2. Co-Authorship

CiteSpace was used to analyze the collaboration network of prolific authors. The selection criterion was the first 50% of each time period. A collaboration diagram is shown in Figure 2. The size of a circle represents the number of publications by corresponding authors. The shorter the distance between two circles, the more collaboration between the authors. The same color of the lines represents authors in the same group.

It can be seen that many authors tend to cooperate with a relatively stable group, resulting in several major author groups, each of which usually has two or more core authors, such as the clusters of Hayase S, Shen Q and Zhang X. The main core authors shown in Figure 2 also demark the most representative research groups in the field of tin-based PSCs. These groups provide highly personalized scientific research information for other researchers to work from. The top twenty most productive authors are listed in

Table 1. Top twenty most productive authors.

Count	Year	Authors
32	2010	Zhang Y
32	2017	Wang Y
31	2014	Snaith H
28	2016	Wang J
28	2014	Hayase S
26	2018	Liu S
26	2016	Wang C
26	2018	Wang Z
25	2014	Habisreutinger S
24	2016	Yan Y
24	2018	Chen Y
22	2018	Zhang X
22	2018	Zhang J
20	2018	Liu X
20	2018	Wang S
20	2016	Chen C
20	2019	Li C
20	2018	Li X
19	2018	Yang Y
19	2015	Shen Q

.

3.3. Co-Occurring Keywords Analysis

Co-occurring keywords reflect the research hotspots of tin-based perovskites. To study them, the 50 most frequently cited items from each slice were selected. As shown in Figure 3, nodes represent keywords, and the size of each node corresponds to its frequency of occurrence. The colors of the links that appear together between the keywords indicate the chronological order: the oldest is white, and the newest is green.

The top 10 keywords with the number of citations are shown in

Table 2. The top 10 keywords with the number of citations.

Citation Counts	Keywords
361	solar cell
292	halide perovskite
260	stability
232	performance
203	perovskite solar cell
187	efficiency
172	efficient
112	fabrication
111	iodide
110	tin

, among which the most cited keyword is “solar cell”. The nodes marked with purple circles mostly indicate good centrality and the importance of these keywords. The centrality of “high performance” and “iodide perovskite” is 0.07, and the centrality of “low cost” is 0.10.

In order to further analyze the time pattern of how keyword clusters have evolved, information about frequently repeated terms and clusters was converted into a timeline view, as shown in Figure 4. In the timeline visualization, clusters are displayed horizontally along the timeline, with the label of each cluster displayed at the end of the cluster timeline. The legend above the display area is marked every five years, and in each year only the top three keywords with the highest count are displayed along each timeline. The color of the link between the keywords represents the time slice of the first co-occurrence.

For example, cluster #1 “phase transition” started in 2013 and lasted until 2022. In this cluster, the first keyword “solar cell” appears most frequently. Other frequently used keywords include “band gap”, “perovskite solar cell”, “optical property”, etc.

3.4. Literature Co-Citation Analysis

The selection criteria were the top 50 most frequently cited items from each slice, as shown in Figure 5. Accordingly, 801 unique nodes, 4654 connections and 9 main clusters were generated. The modularity Q was 0.6095, and the average profile S was 0.867. The nodes and lines represent references and co-citation relationships cited from collected articles, respectively. The connection colors directly correspond to the time slices, with cold colors representing earlier years and warm colors representing recent years. For example, purple links describe articles that were co-cited in 2013, and the most recent co-citing relationship is visualized by yellow links. Q > 0.3 indicates that the network is significant, and S > 0.5 indicates that the clustering result is reasonable.

As shown in Figure 5, the main research fields can be grouped into several clusters: perovskite, lead-free, tin perovskite, crystal structure, all-inorganic perovskites, lead-free systems, interfacial dipole, etc. The lead author and publishing time of the most cited papers in each field are also shown, which provides readers with clues to find articles of interest.

The top 10 most cited papers for tin-based PSCs are listed in

Table 3. Top 10 cited references for tin-based PSCs.

Citations	Title	Author	Year	Journal	Ref.
179	Lead-free organic–inorganic tin halide perovskites for photovoltaic applications	Nakita K. Noel et al.	2014	Energy Environ. Sci.	[10]
166	Fabrication of efficient formamidinium tin iodide perovskite solar cells through SnF2–pyrazine complex	Lee SJ et al.	2016	J. Am. Chem. Soc.	[11]
165	Lead-free solid-state organic–inorganic halide perovskite solar cells	Feng Hao et al.	2014	Nat. Photonics.	[4]
158	Lead-free inverted planar formamidinium tin triiodide perovskite solar cells achieving power conversion efficiencies up to 6.22%	Liao, WQ et al.	2016	Adv. Mater.	[12]
138	Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency	Shuyan Shao et al.	2017	Adv. Mater.	[13]
134	Highly-oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance	Yuqin Liao et al.	2017	J. Am. Chem. Soc.	[8]
132	Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells	Yang WS et al.	2017	Science.	[14]
131	Perovskite–perovskite tandem photovoltaics with optimized band gaps	Eperon GE et al.	2016	Science.	[15]
130	Anomalous band gap behavior in mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells	Feng Hao et al.	2014	J. Am. Chem. Soc.	[16]
128	Semiconducting tin and lead iodide perovskites with organic cations: Phase transitions, high mobilities, and near-infrared photoluminescent properties	Stoumpos C. C. et al.	2013	Inorg. Chem.	[17]

[4,8,10,11,12,13,14,15,16]. Highly correlated nodes can be considered as important bridges between studies. Nakita et al. [10] produced the first completely lead-free CH₃NH₃SnI₃ PSC. The structure of the perovskite-sensitized solar cell (PSSC) is FTO-coated glass/compact TiO₂/mesoporousTiO₂ (400 nm) coated with CH₃NH₃SnI₃/Spiro-OMeTAD/Au. Encapsulation of this material under an inert atmosphere allowed them to characterize the films and probe their performance in solar cells. An efficiency of more than 6% under 1 sunlight and an open circuit voltage of more than 0.88V were obtained from CH₃NH₃SnI₃ with a band gap of 1.23 eV.

It was a challenge to fabricate efficient FASnI₃ PSCs because it was necessary to deposit uniform and dense perovskite layers and reduce the Sn⁴⁺ content. In order to solve this problem, solvent engineering and solvent-free dripping technology were used by Lee et al. [11]. They found that pyrazine could restrict the phase separation induced by the use of excess SnF₂ through an interaction with SnF₂ that reduced the Sn vacancies effectively. With SnF₂ used as the inhibitor of Sn⁴⁺, a 4.8% power conversion efficiency was achieved.

Feng Hao et al. [4] combined CH₃NH₃SnI₃ (1.3 eV) perovskite materials with organic hole transport layers in solar cell devices, which begin to absorb at 950 nanometers. In comparison with CH₃NH₃PbI₃ (1.55 eV), the absorption band was significantly shifted. The efficiency of the CH₃NH₃SnI_3–xBr_x solar cell was converted to 5.73% under simulated sunlight, which is a step toward low-cost, environmentally friendly solid-state solar cells.

Liao WQ et al. [12] described the performance of inverted planar formamidinium tin triiodide (FASnI₃) PSCs, showing the highest PCE of 6.22%. Shao et al. [13] achieved up to 9.0% PCE for the first time in a planar p-i-n device structure. Liao YQ et al. [8] achieved pinhole-free films by manipulating the film composition, which prevents oxygen from diffusing into the chalcogenide lattice. Based on these advances, they constructed pure tin PSCs with the efficiency up to 5.94%. Yang WS et al. [14] found that the introduction of iodide ions into the organic cation solution forms a perovskite layer through intramolecular exchange, which reduces the concentration of deep defects and obtains a power conversion efficiency of 22.1%.

Eperon, G. E et al. [15] developed an infrared absorption 1.2 eV band gap perovskite, FA_(0.75)Cs_(0.25)Sn_(0.5)Pb_(0.5)I₍₃₎, which provided 14.8% efficiency. By combining this material with the wide-band-gap FA_(0.83)Cs_(0.17)Pb(I_0.5Br_0.5) material, a single-chip double-terminal series efficiency of 17.0% was achieved at an open circuit voltage greater than 1.65 V. The four-terminal series cell was mechanically stacked, and an efficiency of 20.3% was obtained. The infrared-absorbing PSC showed excellent thermal stability and atmospheric stability.

Feng Hao et al. [16] verified that CH₃NH₃Sn_0.5Pb_0.5I₃ had the broadest light absorption and the highest short-circuit photocurrent density of about 20 mA cm⁻². Stoumpos C. C. et al. [17] concluded that the hybrid perovskite material (CH₃NH₃Sn_1−xPb_xI₃) is a flexible and versatile functional material. Capable of emitting light in the near-infrared spectral region at room temperature, it was shown to be a promising candidate for a good electrical conductor and a powerful emitter at room temperature.

3.5. Emerging Trends

The significant increase in research interest in the field of tin-based PSCs is highlighted by publications’ cited keywords. The top 38 keywords with the strongest citation bursts in 1061 articles from 2010 to May 2022 are shown in

Table 4. Top 38 keywords with strongest citation bursts.

Keywords	Year	Strength	Begin	End	2010–2022
sensitized solar cell	2010	3.45	2010	2018
transport	2010	5.74	2013	2016
phase transition	2010	9.16	2014	2017
semiconductor	2010	6.78	2014	2017
high performance	2010	5.43	2014	2016
length	2010	5.42	2014	2017
mobility	2010	5.26	2014	2016
low cost	2010	4.07	2014	2016
ch3nh3pbi3	2010	3.95	2014	2016
light	2010	3.52	2014	2016
deposition	2010	3.48	2014	2016
interface	2010	3.69	2015	2015
organometal halide perovskite	2010	5.42	2016	2018
absorber	2010	4.48	2016	2017
pb	2010	3.87	2016	2018
photovoltaic application	2010	3.87	2016	2018
anion exchange	2010	3.15	2016	2017
cspbx3	2010	2.64	2016	2017
photodetector	2010	4.2	2017	2017
silicon	2010	2.85	2017	2017
dynamics	2010	3.9	2018	2018
diffusion length	2010	3.03	2018	2019
management	2010	2.93	2018	2018
recombination	2010	2.89	2018	2019
crystallization	2010	2.81	2018	2018
grain boundary	2010	2.81	2018	2019
photovoltaics	2010	2.68	2018	2018
phase	2010	2.81	2019	2020
tio2	2010	2.73	2019	2020
alpha cspbi3	2010	3.36	2020	2020
highly efficient	2010	3.32	2020	2020
carrier lifetime	2010	3.08	2020	2022
methylammonium	2010	4.72	2021	2022
durability	2010	4.11	2021	2022
fasni(3) crystal	2010	3.77	2021	2022
impact	2010	3.13	2021	2022
ion migration	2010	2.87	2021	2022
defect passivation	2010	2.72	2021	2022

.

As shown in Table 4, the first keyword “dye-sensitized solar cells” began to explode in 2010 and continued until the end of 2018. The research hotspots have changed in the last two years; keywords such as “methylammonium”, “durability”, “fasni(3) crystal”, “impact”, “ion migration” and “defect passivation” started to explode in 2021 and are still in the process of exploding in 2022, becoming new emerging trends.

The keyword “phase transition” with top intensity 9.16 started to erupt from 2014 to 2017. The keyword “semiconductor” with top intensity 6.78 started to erupt from 2014 to 2017. The keyword “transport” with top intensity 5.74 started to erupt from 2013 to 2016. The keyword “high performance” with top intensity 5.43 started to erupt from 2014 to 2016. Among those keywords, “phase transition” has been the hottest topic in solar cell technology in recent years.

Figure 6 shows representative references. The strength of the citation bursts may reflect the rise and fall of tin-based PSCs. The reference with the strongest citation butst was published by Michael M. Lee et al. [18] in 2012. The outbreak lasted from 2013 to 2020. The authors introduced a high-efficiency hybrid organic metal halide perovskite solar cell, which uses a high crystalline perovskite absorbent with strong visible light and near-infrared absorption characteristics. The solar cell structure is FTO/compact TiO₂/CH₃NH₃PbI₂Cl/spiro-OMeTAD/Ag, simulating full sunlight, and the power conversion efficiency in a single junction device is 10.9%.

Burschka J et al. [19] used the sequential deposition method as a way to manufacture high-performance perovskite-sensitized solar cells, and a photoelectric conversion efficiency of 15% was achieved. Stranks SD et al. [20] illustrated that with the correct tuning of the perovskite absorber, no nano or mesostructures were required for efficient charge generation and collection. This result paved the way for further development of planar heterogeneous perovskite solar cells.

Liu MZ et al. [21] found that perovskite absorbers could function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures. Lin RX et al. [22] fabricated monolithic all-perovskite tandem cells with certified PCEs of 24.8%. Among these works, “Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites”, published by Michael M. Lee et al. [18], presents the emerging trend of tin-based PSCs.

3.6. Timezone Analysis

To understand the evolution of this research field, we can analyze it from two aspects: popular keywords used with the development of the academic field, and the use of specific keywords or their sudden detection in the citations of specific articles. To test these out, keyword data were formatted into a time zone view, as shown in Figure 7, which presents popular phrases (2010–May 2022) used over 12 years and 5 months.

As early as 2010, the keywords “sensitized solar cell” appeared in related articles, indicating that early entry into this field was related to dye-sensitized solar cells. Subsequently, the most frequently repeated keywords in 2013–2016, such as “low cost”, “phase transition”, “semiconductor” and “deposition”, explained the prospects and framework of tin-based PSCs.

In 2014, “phase transition” was widely mentioned and seems to have been the focus. After further retrieval, it was found that Stoumpos, C. C. published a paper in 2013 entitled “Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties”. They studied the effect of phase transformation at different temperatures on the PL properties of CsSnI3 and proposed that hybrid iodized perovskite is a flexible, multi-purpose functional material emitted at room temperature in the near-infrared spectral region, which has a suitable band gap and large carrier mobilities [17]. This paper has been cited 3748 times, which proves research on phase transition is very popular.

Since then, keywords have become more and more fragmented, indicating the stable and consistent development of this academic field. In the past two years (2019–2021), “TiO₂”, “phase” and “carrier lifetime” have been thematic research hotspots.

4. Conclusions

The co-citations and visualization network of references for tin-based PSCs were studied using CiteSpace. Key articles, identified research patterns and emerging trends in the literature were then explored based on the findings of CiteSpace. Cluster visualization based on the document co-citation network showed that the main research fields are perovskite, lead-free, tin perovskite, crystal structure, all-inorganic perovskites, lead-free systems and interfacial dipole. The citation burst analysis indicated that topics concerning stability, such as durability, impact, ion migration and defect passivation, as well as FaSnI₃ crystal, have still been research hotspots in recent years and may continue to be in the future. These results can help readers to grasp the development history, future trends and key technologies of the tin-based PSC field more efficiently. Due to the interdisciplinary characteristics of tin-based PSCs, it is difficult to give a complete picture of the research field. However, we have demonstrated a quantitative scientific measurement method to explore the development of tin-based PSCs, using references published in this field, to intuitively and effectively determine the patterns and trends in this field.