Addressing ambient stability challenges in pure FASnI₃ perovskite solar cells through organic additive engineering

Abstract

Tin halide perovskite solar cells (Sn-PSCs) have emerged as promising alternatives to their lead-based counterparts. However, their potential for commercialization is threatened by their inherent stability issues and lower efficiencies that have not yet matched those of established technologies. Therefore, in this work we strategically designed and synthesized two novel organic compounds, referred to as OM4 and OM6, which have been successfully incorporated as additives into pure FASnI₃ PSCs enhancing the crystal properties and simultaneously acting as protective and passivating agents. Through a systematic exploration of chemical interactions, OM4 and OM6 demonstrated a remarkable ability to improve film morphology and optical properties. Consequently, both additives significantly contributed to the enhancement of the overall device performances and exhibited impressive stabilities in the ambient atmosphere, which is attributed to their hydrophobic nature. Notably, the unencapsulated devices incorporating OM4 and OM6 retained over 80% and 90% of their initial power conversion efficiency (PCE) after 250 hours in the ambient atmosphere at RH = 30%, respectively; in contrast, the devices without any of the organic additives suffered complete degradation after 72 hours under the same conditions. As a result, this work opens new possibilities in the rational design and development of organic additives capable of mitigating the instability and low-performance issues commonly associated with Sn-PSCs.

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Introduction

Lead-based perovskite solar cells (Pb-PSCs) have demonstrated great potential to surpass well-established silicon-based solar cells and enter into the photovoltaic market, showing a huge increase in power conversion efficiency (PCE) from the first reported 3.8% in 2009 (ref. 1) to the recently certified >26%.^2,3 However, despite their outstanding properties,^4–7 the presence of toxic lead (Pb²⁺)^8,9 in these devices hinders their universal commercialization.^10,11 To address this issue, researchers have been exploring alternative divalent cations, including germanium (Ge²⁺),^12,13 copper (Cu²⁺)¹⁴ and tin (Sn²⁺).^15–17 Among them, tin has recently emerged as one of the most promising candidates to replace lead in perovskite solar cells due to their chemical similarities.^18,19

Owing to their close location in the periodic table within the same group, both tin and lead share analogous electronic configurations and atomic sizes (1.49 Å for Pb²⁺ and 1.35 Å for Sn²⁺),²⁰ resulting in comparable behaviors as metal cations within perovskite solar cells. In fact, tin-based halide perovskite solar cells (Sn-PSCs) have also demonstrated exceptional properties, including high carrier mobilities,²¹ high absorption coefficients in the visible region,²² long exciton lifetimes,²³ and suitable optimal band gaps within the range of 1.2–1.4 eV.^24,25 Furthermore, Sn-based perovskites exhibit a higher level of environmental compatibility than Pb-based perovskites.²⁶ This is primarily attributed to the tendency of the byproducts generated during the degradation of Sn-based perovskites to undergo hydrolysis in water forming SnI₄, SnO₂, and A₂SnI₄ (A = methylammonium or formamidinium),^27–29 which exhibit lower solubility in water compared to the byproducts derived from Pb-based perovskites, and make Sn-PSCs more suitable candidates for future commercial applications.³⁰

Nevertheless, Sn²⁺ presents higher Lewis acidity in comparison to its Pb²⁺ counterpart.³¹ This disparity in acidity results in accelerated reactions between the perovskite precursors, typically SnI₂ and FAI, leading to rapid and uncontrolled crystallization processes.^32,33 Consequently, non-homogeneous films with pinhole-rich structures and a high density of defects are usually obtained. It has been widely reported that structural defects may promote the adsorption of water and oxygen molecules onto the perovskite layer, thereby accelerating the degradation process and affecting the stability and operational performance of the final solar cell devices.^34,35 Moreover, structural defects can act as non-radiative recombination centers, leading to losses in open-circuit voltage (V_OC), which significantly limit the overall performance of the solar cell devices. In this sense, finding new compounds capable of effectively interacting with Sn-PSC precursors to improve the crystallization process and simultaneously passivate the defects formed through the perovskite matrix is essential to obtain high-quality Sn-PSCs.

Another drawback associated with Sn-PSCs is their inherent instability upon exposure to the ambient atmosphere, resulting in fast degradation and significant drops in the device performance. This instability mainly arises from the tendency of Sn²⁺ to oxidize, in the presence of water or oxygen molecules, to the more stable Sn⁴⁺ state.^36–38 Subsequently, the proliferation of Sn²⁺ vacancies within the perovskite lattice gives rise to a detrimental p-type self-doping phenomenon, which can lead to an alteration in the charge transport properties and overall electrical characteristics of the Sn-PSCs, resulting in considerable V_OC losses.^39,40 To overcome this issue, several approaches have been explored, including effective surface passivation,^41–44 the introduction of different reducing agents,^45–47 or the formation of stable 2D/3D perovskites, among other methods.^48–50 Another interesting methodology widely explored in Pb-PSCs but barely applied to Sn-PSCs to efficiently enhance their stability in air involves the incorporation of hydrophobic compounds, which successfully confer a protective effect on the devices against deleterious external factors.^51–54

With these facts in mind, in this work we strategically designed two novel organic additives, namely OM4 and OM6, each possessing two external acrylate moieties attached to the central structures to allow partial polymerization processes. Acrylates are a type of vinyl monomer that can undergo polymerization at moderate temperatures through a process known as thermal polymerization.^55–58 This method can take advantage of the intrinsic annealing step necessary for the correct perovskite crystallization process, without the need for additional steps such as UV-light treatments that may damage the perovskite film.^59–61 Additionally, OM4 and OM6 were designed with distinct cores to explore the role of different functional groups as additives in Sn-PSCs. As such, OM4 features a pyridine functional group (Lewis base-type), already reported in Sn-PSCs,^33,62 whereas OM6 includes a less explored 1,2,4,5-tetrafluorobenzene unit (Lewis acid-type).

After the introduction of the additives into the Sn perovskite films, a significant enhancement in the PCE of the Sn-PSCs was observed. The initial PCE of 7.78% for the pristine FASnI₃ (FA, formamidinium) increased to 9.18% and 9.62% upon the addition of OM4 and OM6, respectively. More interestingly, the treated devices displayed exceptional stability after more than 250 h of storage under ambient conditions (at a relative humidity of 30% and a temperature of 25 °C), retaining up to 80% and 90% of their initial PCE for OM4 and OM6 treated devices, respectively. In contrast, pristine FASnI₃ devices completely degraded after only 72 h of storage under the same conditions.

Results and discussion

Both OM4 and OM6 were synthesized following the synthetic routes described in Schemes S1 and S2, respectively, in the ESI,† and their chemical structure is illustrated in Fig. 1a and b. More information about the synthetic and characterization details can be found in the ESI (see Fig. S1–S17†).

Fig. 1 Chemical structures of the (a) OM4 and (b) OM6 molecules. Top-view SEM images of (c) FASnI₃, (d) Sn-OM4, and (e) Sn-OM6 films, some pinholes are highlighted with dotted red circles. (f) ¹H NMR spectra of FAI in DMSO-d₆ with and without SnI₂, OM4 and OM6. (g) X-ray photoelectron spectroscopy spectra of Sn 3d for the perovskite films with and without OM4 and OM6.

With the aim of understanding the role of the additives in the perovskite film morphology, we conducted scanning electron microscopy (SEM) measurements. Fig. 1c–e show the top-view SEM images of the FASnI₃ perovskite films with and without OM4 and OM6. Note that additives were introduced in the perovskite layer during the antisolvent stage⁶³ solved in chlorobenzene (see the ESI† for further details). For clarity, the Sn-based perovskite films treated with OM4 and OM6 will be referred to as Sn-OM4 and Sn-OM6, respectively. In the case of pristine FASnI₃, the film exhibited a non-uniform, pinhole-rich morphology due to the rapid and uncontrolled crystallization process typically associated with Sn-based perovskites.^33,64 However, upon the incorporation of the different additives, higher quality films were obtained, and the presence of pinholes was significantly reduced in both scenarios. Specifically, Sn-OM4 showed a more compact film with a considerably lower number of pinholes, although some morphological defects were still present. On the other hand, Sn-OM6 exhibited an extremely compact layer with no pinholes and a slight increase in the grain size. To provide a broader perspective on the positive impact of these additives on the perovskite crystallization process, and to demonstrate that the enhancement of the film morphology is not a local coincidence, lower magnification SEM images were obtained and presented in Fig. S18.† However, from the SEM cross-sectional images, we can observe that the incorporation of the additives does not affect the thickness of the perovskite films, as all samples exhibited a similar thickness of around 200 nm (see Fig. S18†). To confirm the presence of the additives within the perovskite film, we carried out Fourier Transform Infrared Spectroscopy (FTIR) on the perovskite films with and without additives, as well as on the individual OM4 and OM6 molecules (see Fig. S19†). The characteristic carbonyl group peak around 1730 cm⁻¹, along with the additional peaks in the fingerprint region, confirms the presence of the additives in the Sn-OM4 and Sn-OM6 films. Additionally, the double peak observed around 1600 cm⁻¹ in the OM4 and OM6 spectra corresponds to the =CH₂ double bonds present in the acrylate groups. Upon perovskite crystallization and thermal annealing, these peaks disappeared, indicating the polymerization of the materials. However, due to the limited sensitivity of the technique, we cannot entirely rule out the coexistence of single molecules and the polymerized materials in certain regions of the final perovskite film.

As previously noted, the rapid reaction between tin(II) iodide (SnI₂) and formamidinium iodide (FAI) often results in uncontrolled crystallization processes, yielding low-quality films characterized by random orientations and numerous defects such as pinholes and cracks.⁶⁵ Intermediate adducts formed between additives and SnI₂ or FAI precursors have been found to slow down reaction rates, thereby retarding the crystallization rate and leading to higher quality films.^33,38 For a better understanding of the role of OM4 and OM6 in the perovskite crystallization process, we conducted nuclear magnetic resonance (NMR) experiments. All NMR experiments were conducted in deuterated dimethyl sulfoxide-d₆ (DMSO-d₆) and referenced accordingly. To analyze the interactions with the perovskite precursors, we performed a combination of proton and fluorine nuclear magnetic resonance spectroscopies (¹H-NMR and ¹⁹F-NMR) for different mixtures. Fig. 1f shows the ¹H NMR spectra of FAI, FAI + SnI₂, FAI + OM4, and FAI + OM6. In the FAI spectra, the proton peaks located around 7.8 and 8.8 ppm correspond to C–H and N–H protons, respectively. After the addition of SnI₂ to the FAI solution, the signal at 8.8 ppm splits into two distinct signals at 8.9 and 8.6 ppm, indicating a change in the charge distribution around N–H protons within the formamidinium group, possibly due to the formation of a hydrogen bond between the amidinium moiety and the SnI₂ species. Furthermore, a spiking in the signals at 7.8 and 8.6 is observed, which can be attributed to the presence of HI, likely formed as a consequence of the partial reaction of the SnI₂ species with traces of water present in the deuterated solution, as previously reported.⁶⁶ Interestingly, upon addition of OM4 and OM6 to the FAI solution, the same splitting was observed, suggesting the formation of an adduct between FAI and OM4 and OM6via hydrogen bonding.⁶⁷ Full range ¹H-NMR spectra showing all combinations are presented in Fig. S20 and S21.† To further understand the different behaviors of the pyridine and 1,2,4,5-tetrafluorobenzene units of OM4 and OM6, we also conducted ¹H-NMR for OM4 and ¹⁹F-NMR for OM6, both with and without SnI₂, as depicted in Fig. S22 and S23.† In the case of OM4, two distinct proton signals were observed within the pyridine ring, located at chemical shifts of 8.77 and 8.19 ppm. Upon addition of SnI₂ to the deuterated solution, a noticeable downfield shift was observed in both signals, with a more remarkable shift observed in the signal corresponding to the proton adjacent to the nitrogen (N) atom. This shielding effect observed in the pyridine ring's protons suggests an electronic sharing between the lone pair of electrons present on the N atom and the vacant p orbital of the SnI₂ species, resulting in the formation of a Lewis acid–base adduct. Furthermore, peaks became narrower and more defined in the presence of SnI₂, indicative of greater molecular ordering. On the other hand, the role of the 1,2,4,5-tetrafluorobenzene unit present in OM6 was studied via¹⁹F-NMR due to the absence of hydrogen atoms in the ring. Accordingly, the ¹⁹F-NMR spectra of OM6 with and without SnI₂ were obtained, and a clear upfield shift in the fluorine (F) signal was observed when SnI₂ was added, indicating a reduction in the electron density around the fluorinated ring. This spectral alteration also confirms an interaction wherein electronic density is shared between the electron-rich F atoms present in the 1,2,4,5-tetrafluorobenzene unit and the electronically deficient SnI₂ species. This phenomenon has been described before using fluorine-rich additives in lead-based PSCs.⁶⁸ These results point to the presence of interactions between the perovskite precursors and both OM4 and OM6, which in turn modulate the crystallization kinetics, ultimately improving the film quality in the perovskite active layer, as previously observed in the SEM images.

In order to explore the chemical bonding properties of the different FASnI₃ perovskite films, with and without additives, X-ray photoelectron spectroscopy (XPS) analysis was employed (see Fig. 1g). As reported elsewhere, the characteristic XPS Sn²⁺ 3d_5/2 peak that corresponds to the FASnI₃ perovskite was observed at 487.39 eV.⁶⁹ Interestingly, with the incorporation of OM4 and OM6, this peak exhibited a small shift towards 487.31 eV and 487.25 eV, respectively. The shift of a specific element in its binding energy peak position typically reflects alterations in its local chemical environment, indicating a variation in the electron density surrounding the nucleus.^15,70,71 The contrast between the reference films and the OM4 or OM6 treated films suggests an elevated level of coordination with the 3d orbitals of Sn²⁺ when OM4 or OM6 is introduced; however, it should be noted that the incorporation of OM6 leads to superior coordination. This observation clarifies the improved crystallization properties observed in the presence of OM6 compared to OM4 and FASnI₃ without any additive.

Additionally, to evaluate the role of the organic additives in the inhibition of the oxidation of Sn²⁺ into Sn⁴⁺, the Sn 3d_5/2 and 3d_3/2 XPS spectra of the FASnI₃, Sn-OM4 and Sn-OM6 films were recorded, depicted in Fig. S24.† All the peak binding energies of Sn²⁺ and Sn⁴⁺ are summarized in Table S1†. The Sn⁴⁺ content was quantified, revealing a higher value for the pristine FASnI₃ film (5,9%) compared to the Sn-OM4 (4.3%) and Sn-OM6 (3.7%) films (see Fig. S25†). These results are in perfect agreement with the expected role of the additives as crystal regulators.

To obtain more information on the impact of the additives on the crystallographic properties of the perovskite films, we performed X-ray diffraction (XRD) analysis. Fig. 2a shows the XRD patterns of the FASnI₃ films with and without OM4 and OM6. The addition of additives did not alter the crystal structure of the FASnI₃ perovskite film, and in all cases, the XRD patterns exhibited the typical peaks corresponding to the orthorhombic lattice. Concretely, six different diffraction peaks appear at angle values of 14.1°, 24.5°, 28.2°, 31.7°, 37.5°, and 50.5°, which can be ascribed to the (100), (120), (200), (211), (222), and (300) planes, respectively.⁷² In both Sn-OM4 and Sn-OM6 films, a preferred crystal orientation in the (100) plane was observed as can be inferred from the increase in the relative intensity of the peaks corresponding to this plane.⁷³ Moreover, a clear reduction in the Full Width at Half-Maximum (FWHM) of the (100) and (120) diffraction peaks was noted, indicating a larger crystalline size particularly evident when OM6 was present (see Fig. S26†). Furthermore, it is evident that the addition of OM4 and OM6 does not induce any significant shift in the different diffraction peaks, suggesting that neither of the additives were incorporated into the perovskite crystalline lattice. Instead, they might be located at grain boundaries and crystal surfaces acting as passivating agents.

Fig. 2 Structural and optical characterization of FASnI₃ films prepared with and without OM4 and OM6 and deposited on glass substrates. (a) X-ray diffraction patterns for different perovskite films; # indicates the diffraction peaks corresponding to ITO. (b) UV-vis absorption spectrum for FASnI₃ films prepared with and without additives. (c) PL spectra and (d) time-resolved PL decays corresponding to FASnI₃ with and without additives.

To investigate the effect of the additives on the optical properties and gain insights into their role in the perovskite film, we conducted ultraviolet-visible absorption (UV-vis), steady-state photoluminescence (PL), and time-resolved PL spectroscopy (TRPL) measurements. Fig. 2b shows the UV-vis absorption spectra. The films treated with OM4 and OM6 showed slightly superior light absorption properties compared to pristine FASnI₃ films. This behavior is in good agreement with the morphological results and is attributed to the formation of larger grain domains and the reduced presence of pinholes or cracks in the perovskite film.⁷⁴ Furthermore, we performed PL and TRPL measurements to study the charge dynamics in the presence of OM4 and OM6 (see Fig. 2c and d respectively). The PL spectrum is shown in Fig. 2c. No significant shift in the bandgap was detected; however, the treated perovskite films exhibited a notorious increase in PL intensity in comparison with the control film, indicating a reduction in the effective non-radiative trap-assisted recombination. Given that the absorption coefficient in Sn-based perovskites can be in the order of 4 × 10⁴ cm⁻¹, and the thickness of the film is around 200 nm, carriers are uniformly photogenerated throughout the whole film thickness, hence trap states located at the grain boundaries can be the dominant non-radiative channels, over those located at the perovskite surface. The TRPL spectra (see Fig. 2d) were fitted with a biexponential decay function using the following equation:

where τ₁ stands for the fast decay component and τ₂ for the slow decay component, as also observed in other lead-free materials.⁷⁵ The best fitting parameters are listed in Table S2.†τ₁ was prolonged from 0.4 ns (FASnI₃) to 1.3 ns (Sn-OM4) and 2.0 ns (Sn-OM6). Taking into account the large inhomogeneities in the Sn-based perovskite films, as reflected by the high full width at half maximum of PL spectra in Fig. 2c, this component can be related to localized and/or free excitons whose decay time would be 1/τ₁ = 1/τ_Xr + 1/τ_nr, where τ_Xr would be the excitation radiative lifetime and τ_nr the non-radiative trap-assisted recombination time. The observed increase in τ₁ confirms the reduction of non-radiative recombination centers. On the other hand, τ₂ could be tentatively associated with bimolecular (band-to-band) radiative recombination and is also limited by the same non-radiative channels, and hence the observed elongation: Sn-OM6 (3.5 ns) > Sn-OM4 (2.8 ns) > FASnI₃ (1.1 ns) (see Table S2†) confirms again the improved quality of the films when the additives are added.⁷⁶ Additionally, we analyzed the changes in the relative positions of the energy bands and determined the cut-off for the valence band maximums, which is represented in Fig. S27.† As shown in Fig. 2b and c, the bandgap remained unchanged. Therefore, a slightly lower valence band could result in a lower conduction band position, correcting the misalignment specially for Sn-OM4 and Sn-OM6 with the C60 ETL, which is one of the main reasons behind the lower V_OC in Sn-based perovskite solar cells.

This systematic characterization highlights the beneficial role of OM4 and OM6 additives in the properties of FASnI₃ films. On one hand, they modulate the crystallization kinetics and improve the film quality. On the other hand, XPS characterization reveals a reduction in Sn⁴⁺ content, and optoelectronic characterization demonstrates a reduced non-radiative recombination. Since XRD spectra do not show a change in the lattice parameters, we propose that additives may be located at perovskite grain boundaries, suppressing the detrimental non-radiative recombination typical of these defect-rich domains. This behavior suggests a superior performance of Sn-PSCs with light harvesting Sn-perovskite films containing OM4 and OM6 additives.

To validate this potentiality, we evaluated the role of the additives in Sn-PSC performance. Solar cell devices were fabricated adopting an inverted p–i–n configuration that consisted of ITO/PEDOT:PSS/FASnI₃/C₆₀/BCP/Ag, as shown in Fig. 3a. As a first step, the concentration for both additives was optimized to 0.40 mM (see Fig. S28 and 29†). The current density–voltage (J–V) curves for the best-performing PSCs in each system are depicted in Fig. 3b and Table S3.† Their corresponding photovoltaic parameters, including V_OC, short-circuit current-density (J_SC), fill factor (FF) and PCE are summarized in Fig. 3c–f and Table S4.† It is important to note that all the devices underwent a light soaking treatment at 1 sun irradiance in a N₂ atmosphere. This post-treatment procedure is incorporated as part of a routine purification process to mitigate the amount of Sn⁴⁺ states within the perovskite active layer, as previously demonstrated by our group.⁴⁵ The control devices showed a maximum efficiency of 7.71% with a J_SC of 19.91 mA cm⁻², a V_OC of 0.581 V, and an FF of 66.59%. The OM4 treated champion device displayed an improved efficiency of 9.25% with a similar J_SC of 19.97 mA cm⁻², a significantly improved V_OC of 0.689 V and an FF of 67.14%. The OM6 treated champion device showed the maximum efficiency of 9.66%, with a similar J_SC of 20.04 mA cm⁻², a greatly increased V_OC of 0.706 V and a slightly increased FF of 68.23%. It is worth mentioning that the photocurrent obtained from the integration of the external quantum efficiency (EQE) is in good agreement with the one obtained from the J–V measurements (see Fig. S30†). The enhancement of the device performance is mainly attributed to the higher V_OC presented by the Sn-PSCs treated with the additives. Concretely, the average V_OC (see Table S4†) increased from 0.57 V for the pristine FASnI₃ perovskite to 0.67 V and 0.71 V for the devices treated with OM4 and OM6, respectively, and consequently resulted in a significant average PCE enhancement of 25% and 37%, respectively. This improvement in V_OC, and ultimately in PCE, can be attributed to a reduced carrier recombination within Sn-OM4 and Sn-OM6 perovskite layers. This effect, at the same time, may be ascribed to the formation of more compact and pinhole-free perovskite layers, along with the passivation of the remaining defects present in the matrix,⁷⁷ as already discussed after the characterization of perovskite layers. Moreover, control devices showed higher hysteresis behavior, which is suppressed upon the additive treatments, as shown in Fig. S31.† This can be linked to a lower trap density, which retards the ion migration across the perovskite film.⁷⁸ Notably, it is also important to highlight the increase in reproducibility with the use of the additives, especially OM6, as Fig. S32† shows. This better reproducibility can be attributed to the enhanced pinhole-free morphology of Sn-OM6 films (see Fig. 1e).

Fig. 3 (a) Sn-PSC device architecture, (b) best performing devices' J–V curves, (c–f) statistics of the solar cell parameters: (c) J_SC, (d) FF, (e) V_OC and (f) PCE. All the parameters were extracted from J–V curves under 100 mW cm⁻² AM 1.5G illumination.

Moreover, to better understand the role of defects and additives in the improved performance of our solar cells, we investigated FASnI₃ surfaces with adsorbed OM4 and OM6 molecules by density functional theory (DFT) simulations. The atomic structures of the input surfaces and output densities of states (DOS) are given in Fig. S33 and S34,† respectively. The DOS (Fig. S34†) include the contribution from N atoms in OM4 molecules and from F atoms in OM6 molecules. Interestingly, the projected DOS reflect a contribution from N atomic orbitals to the conduction band minima which is missing, indeed, for F atoms. The strong localization of photocarriers at N sites can be considered as a plausible reason behind the lower average improvement of solar cell performance reached with the OM4 additive.

Finally, even more significant than the effect of the additives in the PCE for Sn-PSCs are the implications in the device long-term stability. Sn-PSCs have demonstrated a very good stability under nitrogen conditions,⁴⁵ but a poor stability under ambient conditions. Consequently, in order to evaluate the stability of our devices under high stress conditions, we carried out shelf-stability tests in an oxygen atmosphere under normal conditions and under thermal stress. All the devices were tested without any encapsulation. Shelf-stability tests were carried out at RH = 30% and 25 °C. After 24 hours, control devices only retained 80% of their initial power conversion efficiency and completely degraded after just 72 hours. On the other hand, OM4 treated devices maintained 80% of their initial PCE for 220 h. More impressively, OM6 treated devices retained over 90% of their initial PCE for more than 250 h, which is among the highest air stabilities reported for pure 3D-phase FASnI₃ perovskites (see Fig. 4a and Table S5†). Additionally, we also kept track of the stability of the unencapsulated devices at 80 °C in air and a complete degradation of the control devices was observed after less than 10 h, while the devices containing OM4 or OM6 retained more than 85% of their initial PCE after more than 20 h (see Fig. 4b). This outstanding stability, here reported, is attributed to the high hydrophobicity of the additives, and their intrinsic and widely reported polymerization ability,^55–58 which act as protective barriers impeding water molecules from permeating and degrading the perovskite active layer. Also, the presence of hydrophobic aromatic rings within their structure, and additional fluorine atoms in the case of OM6, make both of them exceptional barriers to physically but not electronically isolate the perovskite layer. As shown in Fig. 4c–e, the enhanced hydrophobicity of the Sn-based perovskite films when OM4 or OM6 is present was demonstrated by performing water contact angle measurements. It can be observed that the contact angle of water drops over the pristine FASnI₃ film increased from 53.2° to 65.0° and 68.7° when OM4 or OM6 was added, respectively. As expected, the hydrophobicity of the Sn-OM6 film was higher compared to that of Sn-OM4 due to the presence of the well-known highly hydrophobic 1,2,4,5-tetrafluorobenzene unit.^79,80

Fig. 4 PCE evolution of unencapsulated devices with and without OM4 or OM6 stored in (a) air at RH = 30% and 25 °C and (b) at RH = 30% and 80 °C. Contact angle of water droplets on the surface of (c) pristine FASnI₃, (d) Sn-OM4 and (e) Sn-OM6.

Conclusions

In summary, we developed and synthesized two novel organic additives containing different functional groups – a pyridine unit (OM4) and a 1,2,4,5-tetrafluorobenzene unit (OM6)-, which were incorporated into Sn-based perovskite solar cells as additives. Although both additives showed positive effects in the final performance of the devices, our findings indicated that the fluorinated ring located in OM6 exhibits stronger coordination with the tin perovskite precursors compared to the more widely used pyridine unit (OM4), resulting in enhanced structural, optical and morphological properties. Consequently, a significant improvement in the performance of the solar cell devices was observed; in particular, the average PCE was increased 25% and 37%, with respect to the average PCE of the FASnI₃ control cell, when OM4 or OM6 was incorporated as an additive, respectively.

Moreover, due to the highly hydrophobic nature of the organic additives, and likely their polymerization capabilities, a notorious enhancement in the stability of the devices was observed. Notably, even under ambient conditions without any encapsulation, the devices containing OM4 and OM6 preserved up to 80% and 90% of their initial PCE, respectively, for over 250 hours, whereas the pristine FASnI₃ devices suffered a complete degradation after 72 hours under the same conditions. Hence, this study contributes to the understanding of the role of different functional groups present in organic additives commonly utilized in tin perovskite solar cells, thereby paving the way for future rational molecular design of additives in this field. The additive inclusion strategy is shown to be a successful approach to bypass the limitations of PCE and long-term stability of Sn-PSCs.

Data availability

Data for this article are available at zenodo repository at https://doi.org/10.5281/zenodo.12733691.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially funded by the Generalitat Valenciana via European Union-NextGeneration EU project Print-P (MFA/2022/020). S. H. T. C. gratefully acknowledges funding from the Ministry of Science and Innovation of Spain under Ayudas Ramón y Cajal (RYC2022-035578-I) for financial assistance during this work. J. R. P. acknowledges the Ministry of Education, Youth and Sports of the Czech Republic, for the financial support of XPS measurements using the CEMNAT infrastructure (project LM 2023037). J. L. D. acknowledges Ikerbasque, Basque Foundation for Science, for an “Ikerbasque Research Associate” contract, the Polymat Foundation, and the Spanish Government (PID2021-129084OB-I00 and RED2022-134344-T grants).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta03291h

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