Dopants for Enhanced Performance of Tin-Based Perovskite Solar Cells—A Short Review

: Lead-based perovskite solar cells had reached a bottleneck and demonstrated signiﬁcant power conversion efﬁciency (PCE) growth matching the performance of traditional polycrystalline silicon solar cells. Lead-containing perovskite solar cell technology is on the verge of commercialization and has huge potential to replace silicon solar cells, but despite the very promising future of these perovskite solar cells, the presence of water-soluble toxic lead content is a growing concern in the scientiﬁc community and a major bottleneck for their commercialization. The less toxic, tin-based perovskite solar cells are promising alternatives for lead-free perovskite solar cells. Like lead-based perovskite, the general chemical formula composition of tin-based perovskite is ASnX 3 , where A is a cation and X is an anion (halogen). It is evident that tin-based perovskites, being less-toxic with excellent photoelectric properties, show respectable performance. Recently, numerous studies reported on the fabrication of Sn-based perovskite solar cells. However, the stability of this novel lead-free alternative material remains a big concern. One of the many ways to stabilize these solar cells includes addition of dopants. In this context, this article summarizes the most important fabrication routes employing dopants that have shown excellent stability for tin-based perovskite photovoltaics and elaborates the prospects of lead-free, tin based stable perovskite photovoltaics.


Introduction
Lead-based perovskite solar cells have been in the limelight of solar cell devices for more than a decade, and their power conversion efficiency (PCE) has grown rapidly from 3.80% (year 2009) to 25.50% (year 2020) [1,2]. The absorption layers of traditional lead-based perovskite solar cells possess an APbX 3 configuration, where A refers to a metal cation or organic cation such as monovalent cesium cation (Cs + ), methylammonium (CH 3 NH 3 , MA + ), formamidinium (NH 2 CHNH 2 , FA + ) and so on; X is the halogen anion, such as iodide (I − ), bromide (Br − ), chloride (Cl − ), or a combination of the above halogens [3]. Since the last decade, there are intriguing research studies worldwide on lead-based perovskite solar cells. There exists a wide range of variety and flexibility in perovskite solar cells like fine tuning the perovskite composition [4][5][6][7], types of additives [8][9][10], structure [11,12], employing different electron transport or hole transport layers [13,14]. There is a significant development in PCE and stability of perovskite solar cells and many commercial companies across the world are investing heavily in the manufacture of these 3rd generation solar cells. A few to mentions are Oxford PV GmbH (Brandenburg, Germany); Saule Technology (Wroclaw, Poland); Microquanta Semiconductor (Hangzhou, China); Solaronix (Aubonne, Switzerland); Swiftsolar (Sancarlos, CA, USA), etc. However, the highly toxic lead metal present in these solar cells still remains a major environmental concern [15] as the recycling of such a heavy metal is complicated and expensive. Therefore, researchers have been looking for metal ions that can replace lead in perovskite materials. The radius must not be too different from that of lead ions (Pb 2+ ), otherwise the replacement will cause lattice distortion and form defects. In the periodic table, Sn, located in the same main group as Pb and presenting a close atomic radius, has attracted huge attention from the scientific community. The ionic radius of Pb 2+ is 119 pm and that of Sn 2+ is 118 pm, which allows the simple replacement of Pb by Sn in tin-based perovskite solar cells [16]. However, tin suffers from the notorious problem of easily being oxidized from Sn 2+ to Sn 4+ on exposure to ambient conditions [17][18][19][20]. This stability concern remains a challenge for long lasting stability of tin-based perovskite cells.
Studies showed that tetravalent tin ion (Sn 4+ ) is self-doped during the crystallization process of perovskite, and it was concluded that the homogeneous distribution of a small amount of Sn 4+ could increase the conductivity of perovskite solar cells [21]. However, an excessive amount of Sn 4+ could increase the defects of tin-based perovskite, which could degrade the performance of perovskite solar cells [17,22,23]. Since the first report of pure tin-based perovskite solar cells in 2014 [17], research on pure tin-based perovskite solar cells has become more and more popular with scientific researchers. FASnI 3 (FA=CN 2 H 5 ) tin-based perovskite is one of many tin-based perovskites, which is an upgraded version of the MASnI 3 (MA=CH 3 NH 3 ) perovskite type. The former has better thermal stability than the latter [22,23] due to the higher thermal stability of CN 2 H 5 compared to CH 3 NH 3 . Tinbased perovskite solar cells are achieving good efficiency performance and are the closest candidate among the other Pb-free alternatives to compete with lead-based perovskite. Tinperovskites are generally having low energy bandgaps, such as MASnI 3 (1.30 eV), CsSnI 3 (1.30 eV) and FASnI 3 (1.41 eV) with bandgaps lower than MAPbI 3 (1.59 eV) maximizing the absorption of photons from sunlight [24][25][26]. In 2014, Noel et al., demonstrated the fabrication of MASnI 3 perovskite solar cells yielding a PCE of 6.40% for the first time [17]. However, MASnX 3 (X=Cl, Br, I) is very unstable in an ambient atmosphere [17]. Considering this drawback in subsequent years, several attempts are made to enhance the efficiency of the more stable FASnI 3 and CsSnI 3 active material in solar cells, but no improvement in PCE was observed.
One of the most promising approachs to improve the stability of Sn-based perovskite solar cells is the addition of dopants to the tin perovskite material to enhance the homogeneous deposition and to enhance the optoelectronic properties of the absorber layer. Song et al. reported the use of an excess of SnI 2 in CsSnI 3 with a PCE of 4.81% [25]. It is demonstrated that the excess of SnI 2 was beneficial to form a uniform film and suppress Sn 4+ vacancies, thereby reducing the p-type conductivity of tin-based perovskite materials. It was also reported that the combination of SnF 2 and pyrazine solves the problem of perovskite phase aggregation and improves the oxidation resistance of the film [26,27]. In 2016, Chen et al., employed quantum rods (QR) of CsSnI 3 to enhance the PCEs of CsSnI 3 perovskite solar cells to 12.96% [28].
In 2018, Jokar et al., reported a high PCE of 9.60%. This improved efficiency was the result of using a new Sn-based absorber layer GA x FA 1−x−2 ySnI 3−y EDAI 2 , where GA is guanidinium and FA is formamide and EDAI 2 is an additive, ethylene diammonium diiodide [29]. Very recently, in 2020, Nishimura et al., fabricated Sn-based perovskite solar cells with a GeI 2 doped (FA 0.9 EA 0.1 ) 0.98 EDA 0.01 SnI 3 composition as the light-absorbing layer, that reached the highest reported performance so far, a PCE of 13.24% [30]. This enhanced efficiency was the result of the excellent optoelectronic properties of the new absorber layer. This article summarizes important contributions made by scientific researchers to the development of tin-based perovskite in terms of additives, dimensions, structure and proposes possible future development directions for tin-based perovskites.

Inorganic Materials
One of the most important strategies for sustaining the stability of Sn-based perovskite solar cells is by reducing the Sn 4+ to Sn 2+ . In this direction, Cao et al., reported that ammonium hypophosphite (AHP, NH 4 H 2 PO 2 ) could inhibit the oxidation of Sn 2+ , H 2 PO 2− acts as a reducing agent that converts Sn 4+ and inhibits the oxidation of Sn 2+ . It was observed that addition of AHP to the Sn-perovskite precursor solution enables growth of large perovskite crystals. Further in this study a more stable CuSCN hole transport material was employed, which resulted in the fabrication of a more stable and efficient tin-based perovskite solar cell [31]. Figure 1a,b presents the optoelectronic characterization of an AHP-modified Sn-based perovskite solar cell. Tai and co-workers mixed hydroxybenzene sulfonic acid with SnCl 2 to form a composite. When employed in solar cells, this composite could well prevent oxygen from entering the tin-based perovskite film resulting in an enhancement in the stability of the material. Additionally, a highly homogeneous perovskite film was obtained [32]. Figure 1c,d present the performance of the devices prepared following this strategy. Kayesh et al., introduced hydrazinium chloride (N 2 H 5 Cl) into Sn-perovskite films as a co-additive with SnF 2 to control the uniform growth of the crystals and inhibit the oxidation of Sn 2+ . The open circuit voltage (Voc) was 0.455 V, short-circuit current density (Jsc) was 17.64 mA cm −2 , and energy conversion efficiency (PCE) was 5.4% [33]; the device performance is shown in Figure 1e,f. As a common additive, SnF 2 had been studied by numerous scholars [26,[34][35][36][37][38][39][40][41]. Liao and co-workers mixed SnF 2 with diethyl ether to make 5%, 10%, 15%, 20% and 30% gradient antisolvents, of which the optimum 10% of antisolvent produced FASnI 3 perovskite films with enhanced homogeneity and coverage. The device reached a PCE of 6.22% [37]. Lee and co-workers found that SnF 2 could form a complex with pyrazine and this complex could play an important role in forming dense and stable perovskite films. The improved performance of perovskite films employing SnF 2 -pyrazine complex absorber is shown in Figure 1g. Another research direction is to dope metal cations into organotin-based perovskite materials to study the performance changes of tin-based perovskites. Gao and his collaborators studied the effect of metallic Cs on FASnI 3 -type perovskite solar cells. They found that Cs doped with monovalent cation sites could play a role in shrinking the lattice, improved the symmetry of perovskite crystals and inhibited the oxidation of Sn 2+ . After doping with Cs + , the efficiency of the device had increased by more than 60%, the PCE of the controlled device was 3.74% and the PCE of the doped device was 6.08%. Not only that, but its stability was improved greatly. After being stored in the glove box for 2000 h, the PCE decline was only 10% of the original PCE [42].

Organic Materials
Organic cations are common and intensively studied additives. Ke and coworkers studied propylenediammonium (PN) and trimethylenediammonium (TN). These two cations influenced FASnI 3 perovskite solar cells. The reason is that the two cations could reduce the dimensions of the tin-based perovskite and could form a three-dimensional tinbased perovskite structure, filling the vacancies of SnI 2 in 3D tin-based perovskite crystals. A perovskite film obtained by adding a mixed solution of PN and FAI in a mixing ratio of 0.1:1 to the precursor solution showed a PCE of 5.85%, while the mixing of TN and FAI in ratio 0.1:1 leads to a device PCE of 5.53%. The PCE of the device after compounding organic cations was thus twice that of the original device (2.53%) [43]. The position and function of PN/TN in the perovskite crystal structure are shown in Figure 2a. Jokar and coworkers studied ethylenediammonium diiodide (EDAI 2 ) and butylammonium iodide (BAI) in FASnI 3 perovskite solar cells. They found that the cation BAI could help improve the quality of the perovskite film, and the cation EDAI 2 produced a significant improvement on the perovskite film, providing a good quality perovskite film [44]. The device PCE could reach 8.9%. The position and function of BA + and EDA + in the perovskite crystal structure are shown in Figure 2b. Kayesh and co-workers studied the addition of 5-ammonium valeric acid iodide (5-AVAI) to the tin-based perovskite and obtained a substantial PCE of 7% for a perovskite solar cell with an area of 0.25 cm 2 . 5-AVAI could form hydrogen bonds with I − or SnI 6 4− , thereby regulating the crystal growth direction [45]. Additionally, addition of 5-AVAI improved the solar cell stability. The results are presented in Figure 2c. Ran et al., employed a conjugated large-volume amine (LVA), 3-phenyl-2-propen-1-amine (PPA), PPA could be well compounded with FASnI 3 , which could improve the random orientation of crystal grains during the formation of thin films, reduce the existence of defects, improve the structural instability, and increase the extraction capacity of excitons. Thus, a high-quality perovskite film was obtained, and a champion device with PCE of 9.61% was obtained. Figure 2d shows the structural formula of PPA and its effect on FASnI 3 crystals [46]. Yu and collaborators proposed 2,2,2-trifluoroethylamine hydrochloride (TFEACl). This organic substance is characterized by TFEA + and Cl − , which are easy to separate. When added to the precursor solution, FASnI 3−x Cl x was easily formed, which improves the quality of the perovskite morphology. Obviously, the improved valence band of the tin-based perovskite material was better matched with PEDOT: PSS and the conduction band is better matched with the conduction band of PCBM, which could better improve the carrier mobility and enhance device performance [47]. 4-Fluorobenzohydrazide (FBH) was proposed by Him and collaborators. This additive could form a protective film on the surface of perovskite and improve the stability of the device. Under the condition of 0.1 ppm oxygen, the device PCE reaching 9.47%, and the device efficiency was 9.03% in a 100-ppm oxygen environment [48]. FBH was added to the fabrication flow chart of perovskite films is shown in Figure 2e and the morphologies under a scanning electron microscope of perovskite films under different conditions is shown in Figure 2f. Boehm and co-workers employed surface ligands to study the performance of FASnI 3 perovskite solar cells. They tested octanoic acid (OCA), trimethylphenyl ammonium chloride (TMPACl), 2H,2H,3H,3H-perfluorononanoic acid (FNCA), all of which influenced the performance of the period, of which FNCA had the greatest impact on the device, which could improve the stability of the period [49]. Zeng et al., studied poly(α-methylstyrene) (PAMS). They added different concentrations of PAMS to diethyl ether to form an anti-solvent and cleaned the perovskite film during the spin coating process, which reduced the needle hole-ground effect, which in turn could enhance the adhesion between PCBM and perovskite layer and improve the performance of the device [50]. Meng and colleagues studied the mechanism of effect of poly (vinyl alcohol) (PVA) on FASnI 3 and found that the strong hydrogen bonds iPVA could react with FASnI 3 , which could passivate defects, form a dense film and prevent the diffusion of I − , the V oc increased by 0.08 eV (0.55 eV to 0.63 eV) and the device efficiency reached 8.9% [51]. Lin and co-workers introduced 8-hydroxyquinoline (8-HQ). They found that the oxygen and nitrogen atoms in HQ could form co-ordination bonds with Sn 2+ and inhibit the oxidation of Sn 2+ to Sn 4+ [52]. Wu and his collaborators studied π-conjugated Lewis bases and synthesized 2-cyano-3-

Crystal Structure Engineering
Low-dimensional FASnI 3 perovskite is a hot spot in current research. The added advantages of low-dimensional tin-based perovskite are its excellent stability, adjustable band gap and excellent photoelectric properties. In 2017, Liao et al., introduced phenylethyl ammonium iodide (PEAI) into a FASnI 3 perovskite. PEA could divide the body type perovskite into a 2D structure. This 2D structure was more stable than the 3D structure [54]. The change of perovskite structure with different PEAI levels is shown in Figure 3a. The same year, Ke et al., demonstrated the addition of ethylene diammonium (en) to FASnI 3 perovskite. The 3D FASnI 3 type perovskite had an adjustable structure, mainly because {en}FASnI 3 could generate many Schottky defects in the crystal structure. Schottky defects provide to ability to change the bandgap of tin-based perovskite [55]. The crystal diagram is shown in Figure 3b. Ran et al., used a multi-channel diffusion two-step film formation method. In this process, phenylethyl ammonium iodide (PEAI) was evaporated to the surface of FAI and then SnI 2 is evaporated to form a 2D-3D heterojunction perovskite material. This 2D-3D structure could improve the quality and the photoelectric performance of the film. The fabricated device employing this 2D-3D tin-based perovskite structure achieved a PCE of 6.98% [56]. The 2D-3D heterojunction fabrication flow chart is shown in Figure 3c. Ng and co-workers added PEAI and Ge in FASnI 3 , where PEAI mainly plays a role of blocking water and Ge could reduce the reduce density of trap states in the perovskite [57]. The roles and positions of Ge 2+ and PEA + in perovskite crystals are shown in Figure 3d. Wang and co-workers fabricated a 2D-quasi-2D-3D Sn perovskite film with the pseudohalogen ammonium thiocyanate (NH 4 SCN). The film had good oxidation resistance that enhanced the stability of the resulting perovskite film [58]. Structure characterization of the control film and structure characterization of film with HSP (2D-quasi-2D-3D structure) are shown in Figure 3e.
In 2018, Xu et al., employed 5-ammonium valeric acid (5-AVA) and NH 4 Cl to make quasi-2D perovskites. The perovskite crystals had a vertical orientation that minimises recombination. They found that NH 4 Cl plays an important catalytic role in the crystal growth direction. Such vertically oriented grains were conducive to the migration of the charge carrier, thereby improving the performance of the solar cell [60]. Tsai et al., found that different proportions of 2-hydroxyethylammonium (HEA + ) ion mixed with FA + could change the dimensions of perovskite crystals. Below 100%, it formed a 3D orthogonal structure; at 100%, it was a 2D orthogonal structure [61]. In 2020, Li and coworkers encapsulated the perovskite layer, where they introduced butylethylammonium (BEA) and synthesized perovskite of (BEA)(FA) n−1 SnnI 3n+1 (n = 1-3) with a low-dimension Dion-Jacobson phase. The advantage of the tin-based perovskite of this material and low-dimensional tin-based perovskite was that its band gap was less affected by lattice distortion. Moreover, the low-dimensional tin-based perovskite was stable against attack by water and oxygen. Due to the introduction of BEA the confinement effect of quantum wells was also greatly reduced, and the migration length of carriers reaches 340 nm (holes) and 450 nm (electrons) [62]. In the same year, Li et al., synthesized a vertically oriented 2D tin-based perovskite. They introduced phenylethylammonium chloride (PEACl) to grow a vertically oriented 2D tin-based perovskite. Usual parallel oriented 2D perovskite has good stability but negatively affects the electrical properties due to the fact large-long alkyl chain cations will hinder the carrier transportation [63]. On the contrary, the vertically oriented 2D tin-based perovskite FASnI 3 : PEACl demonstrated an improved stability with the photoelectric performance reaching 9%, but the growth of this 2D tin-based perovskite was highly temperature sensitive at only 100°C, and then the film was pure vertical 2D tin-based perovskite [59]. Figure 3f shows the crystal structure gradually becoming a vertically oriented 2D structure as the annealing temperature increases.  [57]. Copyright 2020 The Royal Society of Chemistry. (e) Structure characterization of the control film (up) and structure characterization of film with HSP (2D-quasi-2D-3D structure) (down). Reproduced with permission from [58]. Copyright 2018 Elsevier Inc. (f) Vertical orientation 2D perovskite structure was attained as the annealing temperature increases. Reproduced with permission from [59]. Copyright 2020 American Chemical Society.

Interfacial Layer Engineering
Tin-based perovskite solar cells exist in p-i-n or n-i-p configurations alternating the position of the electron transport (n) and hole transport layer (p). Therefore, during fabrication of these layered devices, it is very common to encounter energy level mismatches and defects at the interface. The simplest trick to improve the performance of tin-based perovskite solar cells is to add an interfacial layer to minimise the energy mismatch and defects at interfaces. Chen and co-workers introduced a low dimension perovskite interlayer. The passivation of defects in perovskite crystals by LDP is shown in Figure 4a. Phenylethyl ammonium bromide (PEABr) was added to a Sn-perovskite absorber layer, then the hole transport layer could separate 3D perovskite crystals into 2D perovskite crystals. The resulting 2D perovskite could greatly reduce the accumulation and recombination of the charge carriers. The manufactured device reaches a PCE of 7.05% [64]. In 2019, Liao and co-workers spin coated PEABr on top of a FASnI 3 perovskite layer to form an ultra-thin perovskite layer. After PEABr treatment, the energy level of perovskite, hole transport layer and electron transport layer match well, which was conducive to the better transport of holes and electrons [65]. PEABr changes the perovskite band structure as shown in Figure 4b. Ke and his collaborators spin-coated a ZnS layer on the TiO 2 layer. In this way, the band structure between the electron transport layer and the perovskite layer could be adjusted, and the open circuit voltage could be significantly increased, but the short-circuit current and filling factor were not reduced, and the device achieved a PCE of 5.27% [66]. The energy band diagram of the FASnI 3 solar cells and crystal structure of the perovskite absorber are shown in Figure 4c. Hu and his collaborators fabricated SnO 2 and TiO 2 dual-electron transport layer FASnI 3 perovskite solar cells. The advantage of this dual-electron transport layer was that it could hinder the formation of defects at the interface and in the perovskite [67]. Song and his collaborators designed a double-layer hole transport layer (CuI and PEDOT: PSS). The 2D double-layer had more outstanding hole-transport capabilities than a single-layer one and it could also improve the quality of perovskite film formation [68]. J-V curve of devices based on various HTLs is shown in Figure 4d. CuI and a PEDOT: PSS double hole transport layer structure change the solar cell stability as shown in Figure 4e.

Conclusions and Perspectives
This review briefly introduces the important research progress made in improving the stability and performance of tin-based perovskites. The review focuses on role of introducing additives such as inorganic additives like SnF 2 , NH 4 H 2 PO 2 , N 2 H 5 Cl, organic additives, viz. ethylene diammonium diiodide (EDAI 2 ), 5-ammonium valeric acid iodide (5-AVAI) 3-phenyl-2-propen-1-amine (PPA) to study their effects on film properties and device performance. Low-dimensional tin-based perovskites are very mysterious to many researchers and hence it is crucial to understand their properties through in-depth research. This article introduces some tin-based perovskites such as 2D, 3D, 2D-3D, etc. 2D perovskites are more stable than 3D perovskites, but the photoelectric performance is disadvantaged, so low-dimensional perovskite may be a future research hot spot. There will always be some practical issues in the direct combination of the materials of the tin-based perovskite solar cells. At the same time, it is necessary to add some intermediate layers to solve some discordant factors and improve the performance of the devices. Tin-based perovskite is a promising solar cell material due to its low cost and less toxicity compared with the lead-based perovskite materials, which has encouraged many scientific researchers to work on this type of perovskite. With its interesting features, the instability of tin-based perovskite is also a major problem that needs to be solved to make it commercially viable. Although some work on tin-based perovskites has been done, they are still a long way away from commercialization and considerable work is still needed, for instance, t improves the resistance of tin-based perovskites to match the efficiency of lead-based perovskite solar cells.
Author Contributions: H.L. and Z.Z. contributed equally to this work. H.L. and Z.Z. contributed to methodology, investigation, writing-original draft preparation; F.Y. contributed to validation, data curation, formal analysis; J.Y. contributed to data curation, formal analysis; A.N.G. contributed to data curation, formal analysis; J.L. contributed to validation, data curation, formal analysis; S.T. contributed to data curation, formal analysis; S.M.J. contributed to validation, data curation, formal analysis; All authors have read and agreed to the published version of the manuscript.