While SnF2 is reported as an effective additive for improving the efficiency of lead-free tin-based perovskite solar cells, the mechanism is still unclear and requires further studies. Upon incorporating SnF2 into MASnI3, SnF2 reduces the intrinsic carrier density from 1018 to 1012 cm–3 and produces a longer carrier diffusion length as confirmed by the Hall measurements. The femtosecond transient absorption spectroscopy shows that SnF2 doping enhances the hot-phonon bottleneck effect of MASnI3. The slow cooling process of hot carriers may help to reduce non-radiative recombination, increase the fluorescence lifetime, and, therefore, improve the utilization rate of carriers. Finally, lead-free low bandgap perovskite MASnI3 is utilized as a light absorbing layer in solar cells, achieving high optical current and high voltage in tin-based perovskite solar cells. The final power conversion efficiency is 10.2%, while the power conversion efficiency for the control unit is 6.69%.

Lead-free tin-based perovskite solar cells (TPSCs) have emerged as a promising candidate for high-performance and eco-friendly photovoltaic technology with the possibility of reaching 20% power conversion efficiency (PCE).1 These tin-based perovskites such as MASnI3 and FASnI3 have several superior optoelectronic properties including long carrier diffusion lengths, high carrier mobility, high exciton dissociation efficiency caused by small exciton binding energy, and wide absorption spectrum with high absorption coefficients ranging up to the near-IR.2–4 However, the TPSCs reported in former papers are far below their theoretical limit and the lead-perovskite based solar cells.

On the one hand, tin-based perovskite films usually suffer from low surface coverage and poor quality due to the fast crystallization during solution deposition.5 Many groups are committed to improving the film formability of tin-based perovskite films and increasing light-harvesting efficiency through a deeper understanding of the crystal nucleation and growth mechanisms. SnF2 is commonly added into the MASnI3 and FASnI3 preparation solution.6 For example, Koh et al. observed that the coverage of the FASnI3 perovskite material on mesoporous TiO2 is significantly improved with the SnF2 concentration increasing.7 Xiao et al. demonstrated that SnF2 creates more nuclei in the crystal nucleation process and enables more uniform thin films with high coverage.8 Thus, they concluded SnF2 can affect the film morphology to improve the performance of photovoltaic cells made with these layers. In fact, SnF2 have a limited influence on the film morphology. Yu et al. reported that phenethylammonium chloride (PEACl) played dual roles in adjusting perovskite film topography and passivating defects in the bulk and interface of films by interface engineering. The 10% SnF2 doped and phenethylammonium chloride (PEACl) treated FASnI3 based devices exhibit a champion PCE along with the obviously enhanced open circuit voltage and fill factor.9 Therefore, SnF2 doping improves film morphology, which partially fails to account for the enhanced device performance.

On the other hand, the easy oxidation of Sn2+ to Sn4+ is one of the main obstacles for TPSCs to be far below their theoretical efficiency limit and Pb-based perovskite solar cells.10 The excess Sn4+ ions can produce high carrier concentration and, thus, more photocarrier recombination loss, and severely deteriorate the device performance. Liang et al. demonstrated through first principles calculations that oxygen can easily invade the lattice of MASnI3, leading to defects, which increases the nonradiative recombination coefficient by approximately three orders of magnitude and significantly accelerates the electron capture process.11 Zhou et al. found reduction in Sn vacancies defects in MASnI3 solar cells, the carrier injection enhancement, and the energy bandgap widening resulting in suppressing I–V hysteresis.12 It is challenging to obtain a stable tin-based perovskite cell with higher PCE.13 In 2012, Chung et al. first showed that the solution-processable p-type direct bandgap semiconductor CsSnI3 can be used for hole conduction in lieu of a liquid electrolyte and the conversion efficiencies of up to 10.2% (8.51% with a mask).14 Subsequently, the CsSnI3 solar cell with a simple layer structure of indium-tin-oxide/CsSnI3/Au/Ti on a glass substrate achieved a power conversion efficiency of 0.9%, which is limited by the series and shunt resistance.15 Jiang et al. exploited the Cs2SnI6 film as the light absorber in an n–i–p planar solar cell and achieved a conversion efficiency of 0.47% in 2017.16 In 2018, Gu et al. added Sn powder and SnF2 to the FASnI3 precursor solution to improve the device performance and achieved the highest efficiency of 6.75%.17 Dai et al. proposed Sn(Ac)2 can improve the crystallization process added to a lower trap state concentration while markedly improving the stability and charge extraction of perovskite films with a PCE of 9.93% in 2021.18 Therefore, SnF2 or other tin (II) halides are usually incorporated as an indispensable additive to inhibit this oxidation. Savill et al. revealed the effects of SnF2 additive for FA0.83Cs0.17SnxPb1−xI3 perovskites across the full compositional lead-tin perovskites and 0.1%–20% SnF2 was added into the precursor. SnF2 addition causes a significant reduction in the hole density, yielding longer photoluminescence (PL) lifetimes, decreased energetic disorder, reduced Burstein–Moss shifts, and higher charge-carrier mobilities.19 Kumar et al. introduced SnF2 into CsSnI3 to improve photovoltaic performance by reducing Sn vacancies.20 The additive SnF2 method is also demonstrated for other tin-based perovskites such as FASnI3,7,21 MASnI3,22,23 and FA0.75MA0.25Sn1−xGexI3.24 Wang et al. reported that the detrimental oxidation can be effectively suppressed in the resulting MASnI3 film due to the presence of a large amount of remaining SnF2.23 Ng et al. studied that the trap densities of the FA0.75MA0.25Sn1−xGexI3 were significantly reduced with SnF2 and GeI addition.24 Gupta et al. pointed out that SnF2 can influence many different properties of the ASnX3 perovskites, including film morphology, doping, control formation of unwanted crystal phases, material stability, and energy level position.25 To further elucidate the working mechanism, Pascual et al. investigate the fluoride chemistry in tin halide perovskites by nuclear magnetic resonance (NMR) analysis, hard x-ray photoelectron spectroscopy, and small-angle x-ray scattering.26 They found that SnF4 readily forms in precursor solutions and enters the MASnI3 perovskite structure. Although the working mechanism of the additive is still controversial, SnF2 is an indispensable key additive among many additives or interface engineering. Min et al. reported the co-doping of PEAI and SnF2 can reduce the major defects in Sn perovskites during the fast aggregation of clusters at the initial growth process.27 Not only PEAI but also other bulky organic cations such as GuaSCN28 and GAI29 have been used for tin-based perovskites to improve the device performance and stability. Chen et al. demonstrated that optoelectronic properties of the GA0.06(FA0.8Cs0.2)0.94SnI2Br based wide-bandgap perovskite can be improved by exploring GeI2 doping, together with ethylenediamine bromide (EDABr2) doping and EDA passivation. However, the mechanism behind it is still not clear.30 

Here, we focus on the impact of SnF2 on the optoelectronic properties of MASnI3 films through a detailed photophysical spectrum study. We use a traditional single-step anti-solvent spin-coating method to grow MASnI3 perovskite films with SnF2 doping concentrations of 0%, 5%, 10%, and 15%. XRD measurements prove that the pure phase of the MASnI3 film with (001)-orientation. The film exhibits a direct optical bandgap of 1.3 eV and a photoluminescence (PL) peak around 980 nm. Hall measurements demonstrate that the carrier density of MASnI3 decreases with the increasing SnF2 concentration. Pure MASnI3 films exhibit p-type conductivity with a carrier concentration of 1018 cm−3 and a mobility of 11.08 cm2 V−1 s−1. Upon incorporating 10% SnF2 into MASnI3, the intrinsic carrier density greatly reduces to 1012 cm–3. Accordingly, the hole mobility increases to 288.3 cm2 V−1 s−1 due to the suppressed self-doping effect. As the excitation power intensifies and the carrier injection level rises, the hot carriers’ relaxation time of MASnI3 becomes larger due to the hot-phonon bottleneck effect. This has been confirmed by femtosecond transient absorption spectroscopy (fs-TAS). Both the lifetime and diffusion length of photogenerated carriers increase, contributing to the enhancement of solar cell efficiency. These results are important for both understanding the fundamental physics of tin-based perovskites and optimizing photovoltaic device performance.

CH3NH3I (MAI) (99.99%), bathocuproine (BCP), were purchased from Xi’an p-OLED Corp. PC61BM, fluorine-doped tin oxide glass (FTO) (substrate, sheet resistance is 15 Ω sq−1), chlorobenzene, SnI2 (99.99%) dimethylformamide (DMF), and dimethylsulfoxide (DMSO) were purchased from Youxuan. 1 M x% SnF2: MASnI3 (x = 0, 5, 10, 15) solutions were prepared 12 h before sample fabrication, in the mixed solvent dimethylformamide (DMF):dimethyl sulfoxide (DMSO) (with a volumetric ratio of 4:1). Tin-based perovskite films with controlled SnF2 were grown on glass substrates using a single-step anti-solvent method in a nitrogen-filled glovebox (H2O < 0.1 ppm, O2 < 1.0 ppm).

FTO substrates were prepared by de-ionized (DI) water and ethyl alcohol for 10 min followed by O2 plasma treatment for 5 min. The poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) aqueous solution was filtered by a 0.45 μm filter before use, and then, the PEDOT:PSS solution was spin-coated on the cleaned FTO substrates at 5000 rpm for 40 s and then annealed at 150 °C for 20 min under ambient conditions. The 30 μl perovskite precursor solution was deposited on PEDOT: PSS coated FTO substrates and spin-coated at 700 rpm for 7 s and then 5000 rpm for 40 s in a glovebox. At 20 s, 300 μl anti-solvent of chlorobenzene was dispensed into the center of the substrate. Substrates were then transferred straight onto a hotplate and annealed for 10 min at 100 °C Then, PC61BM solution (20 mg/ml in chlorobenzene) and BCP (0.5 mg/ml in chlorobenzene) were deposited subsequently at 2000 rpm for 30 s and 5000 rpm for 40 s, respectively. Finally, a 90 nm thickness of the Ag electrode was evaporated under high vacuum.

The sample phase was determined with an x-ray diffractometer (Rigaku Smart Lab). The steady-state absorption spectrum was measured using a UV–vis absorption spectrophotometer (PerkinElmer, Lambda 1050). The photoluminescence (PL) and time-resolved PL (TRPL) spectra were measured by Edinburgh FLS1000. The femtosecond transient absorption spectroscopy (TAS) spectrum was obtained using an Ultrafast Helios System. The 800 nm monochromatic light from a femtosecond laser with 35 fs and 1 kHz frequency was split into two beams: a 400 nm pump laser pulse and a quasi-continuous white probe laser pulse with a wavelength of 800–1200 nm. The current density–voltage (J–V, Newport) was used to characterize the device by a Keithley Model 4200 under 100 mW/cm2 (AM 1.5G one Sun) illumination. The charge generation and collection were monitored by an incident photo-electron conversion efficiency (IPCE) system.

We use a single-step anti-solvent spin-coating method to grow MASnI3 perovskite films. Here, we mainly control the SnF2 doping concentration. The obtained samples were named 0%, 5%, 10%, and 15% SnF2 films for simplicity. The XRD pattern is shown in Fig. 1(a). In all of the MASnI3 films, diffraction peaks at angles of 14.2° (001), 28.5° (002), 43.3° (003), and 59.1° (004) were observed. The pure phase and preferred (001)-orientation of the films exhibited by x-ray diffraction peaks are similar to the MASnI3 single crystals grown by the reverse temperature crystallization method.31 The scanning electron microscope pictures of MASnI3 films are shown in Fig. S1 in the supplementary material. With an increase in SnF2 concentration, the characteristic diffraction peak angles do not shift and the widths do not become larger. The difference in the atomic radius between F and I is too large, making it difficult to replace I in the perovskite lattice structure. However, the (002)/(001) intensity ratios have been changed as in Fig. 1(b). The widths of the diffraction peaks of the un-doped films became wider, and the intensity of the diffraction peaks belonging to MASnI3 decreases significantly to almost disappear. Thus, the signal-to-noise ratio of the curves in Fig. 1(c) was becoming clear with time. However, the diffraction peaks of the 10% doped films did not change significantly after 48 h exposing to air, indicating that the SnF2 doping can effectively suppress the degradation of MASnI3 perovskite caused by the oxidation of Sn2+ to Sn4+. This is consistent with the results of previous studies on the photo-oxygen degradation process of tin-based perovskite FASnI3 from both theoretical and experimental perspectives.32 

FIG. 1.

(a) XRD spectra of pristine and doped MASnI3 films. (b) The (002)/(001) intensity ratios with SnF2 concentration (c) and (d). The XRD spectra of 0% and 10% SnF2 doped MASnI3 films after exposing at air for different times.

FIG. 1.

(a) XRD spectra of pristine and doped MASnI3 films. (b) The (002)/(001) intensity ratios with SnF2 concentration (c) and (d). The XRD spectra of 0% and 10% SnF2 doped MASnI3 films after exposing at air for different times.

Close modal

The optical absorption and PL spectra of the MASnI3 films are shown in Figs. 2(a) and 2(b). The films show a broad absorption edge at approximately 900 nm, and a broad photoluminescence peak at 980 nm, e.g., an optical bandgap of 1.3 eV. Those optical features are all similar to the reported films.33 The TRPL spectra at 980 nm can be further extracted and are plotted in Fig. 2(c). Indicated by the solid red line, we fit the PL temporal decay curves with a single exponential function, I=I0+A1e−t/τ.34,35 The 15% doped sample has the longest lifetime (2.16 ns), and all these doped films have a longer photocarrier lifetime, which indicates that SnF2 doping reduces the non-radiative recombination and improves the possible photocarrier extraction in solar cells.

FIG. 2.

(a)–(c) The optical absorption, PL, and TRPL spectra of different MASnI3 films. Red lines in (c) were fitted results with the exponential model.

FIG. 2.

(a)–(c) The optical absorption, PL, and TRPL spectra of different MASnI3 films. Red lines in (c) were fitted results with the exponential model.

Close modal

Figures 3(a) and 3(b) show the changes in the charge-carrier density and mobility with increasing SnF2 concentration as estimated using Hall effect measurements. The fact that the hole carrier density of MASnI3 decreases with increasing SnF2 is observed. The pristine MASnI3 films exhibit a very high carrier density of ∼1018 cm−3; however, the 10% doped films are only ∼1012 cm−3. These results are very consistent with Euvrard’s report, which showed that the carrier density of MASnI3 obtained through Hall measurement is ∼1011–1019 cm−3.36 More previously reported results for the carrier concentration of MASnI3 are summarized in the supplementary material. MASnI3 is a p-type conductive semiconductor where the holes are the major charge carriers. With the addition of SnF2, the oxidation of Sn2+ is effectively inhibited, reducing the concentration of Sn vacancies. As a result, the doped film exhibits better air stability as shown in Figs. 1(c) and 1(d). The carrier concentration decreases significantly, accompanied by an increase in the resistivity. Consequently, the mobility does not monotonically increase in the doped films. The random thermal motion of the carriers within the semiconductor is accompanied by drift motion, and the carrier mobility is affected by acoustic scattering and ionizing impurity scattering. SnF2 doping decreased the scattering centers of the carriers, increased the relaxation time, and, thus, increased the Hall mobility of the carriers.37 

FIG. 3.

(a) and (b) Charge-carrier density and mobility of MASnI3 films for various SnF2 concentrations by Hall measurements.

FIG. 3.

(a) and (b) Charge-carrier density and mobility of MASnI3 films for various SnF2 concentrations by Hall measurements.

Close modal
Another important parameter, the carrier diffusion length, LD, indicates that the electrons for the usually p-type MASnI3 perovskite films can move on average before recombination. The long carrier diffusion lengths enable the realization of the high effect planar heterojunction device. The value of LD can be calculated with the following equation:
L D = D τ = k T μ τ e .
Here, τ is the charge lifetime, μ is the mobility, and D is the diffusion coefficient. The PL lifetime, carrier concentration, mobility, and according carrier diffusion length calculated with the TRPL and Hall measurement data are summarized in Table I. Qiu et al. reported that SnF2 doping in the MASnI3 film effectively increased the diffusion lengths to ∼500 nm.38 In this paper, the 10% doped MASnI3 film has a longer PL lifetime, stronger PL intensity, and bigger carrier mobility. Thus, this sample exhibits the longest carrier diffusion length over 800 nm. This photophysics results suggest that the doped MASnI3 perovskite is a promising lead-free replacement for lead-based perovskite solar cells.
TABLE I.

Comparison of carrier concentration, PL lifetime, mobility, and carrier diffusion length, carrier concentration for 0%, 5%, 10%, and 15% MASnI3.

Sample0%5%10%15%
Lifetime (ns) 0.37 0.68 0.92 2.16 
Carrier Concentration(cm−31.39 × 1018 2.52 × 1015 2.81 × 1012 2.34 × 1014 
Resistivity(Ω cm) 40.44 2882 7712 5103 
Mobility (cm2 V1s−111.08 0.8605 288.3 5.228 
Diffusion length (nm) 103 39 828 171 
Sample0%5%10%15%
Lifetime (ns) 0.37 0.68 0.92 2.16 
Carrier Concentration(cm−31.39 × 1018 2.52 × 1015 2.81 × 1012 2.34 × 1014 
Resistivity(Ω cm) 40.44 2882 7712 5103 
Mobility (cm2 V1s−111.08 0.8605 288.3 5.228 
Diffusion length (nm) 103 39 828 171 

The hot carriers cooling scattering is related to the carrier mobility according to μ = /m*, μ is the carrier mobility, q is the electron charge, τ is the time between scattering, i.e., the excited state lifetime of the carriers, and m* is the effective mass of carriers. To reveal the physical mechanism of increased photocarrier lifetime and diffusion distance, it is necessary to explore their photodynamic process by transient optical spectroscopy. We used the 10%, 5%, and 0% doped films as examples to study the excitation and cooling processes of the hot carriers. Figures 4(a), 4(d), and 4(g) show a typical 2D figure of optical density (△OD) as a function of wavelength and delay time for the above samples, respectively. A ground state bleaching (GSB) signal centered at 900 nm and a photoinduced absorption (PIA) signal located around 950 nm can be observed. In addition, the PIA signal for the 10% doped films is the strongest, which is related to the energy loss mechanism caused by phonon scattering energy.39 The transient absorption curves follow the Boltzmann distribution, and the carrier temperature (Tc) of the 10% sample is higher than its lattice temperature and other samples. This is because the energy is transferred to other carriers by carrier collision scattering to achieve thermal equilibrium, and the residual energy is expressed as optical phonon emission reabsorption energy.40,41 We note that the GSB signal comes after PIA. During the TAS test, the MASnI3 films first absorb 400 nm pump energy to generate carriers such as electrons and holes. For MASnI3 with a narrow bandgap of 1.3 eV, when the carriers are photoexcited to the conduction band high levels, they usually heat rapidly and obtain more initial kinetic energy, resulting in a pronounced PIA signal. Next, the PIA signal red shifted as hot carriers need to lose energy through the scattering process to achieve a thermal equilibrium, as shown in Figs. 4(b), 4(e), and 4(h) on shorter time scales (<3 ps). Later, the GSB signal emerges and increases gradually due to the transfer of many electrons to CB. Finally, the carriers relax and the GSB signal decays as shown in Figs. 4(c), 4(f), and 4(i). These are consistent with the two-stage slow carrier cooling behavior in hybridized Pb–Sn halide perovskites as observed by Rao.42 

FIG. 4.

(a), (d), and (e) Typical two-dimensional map of ΔOD with wavelength and delay time for 10%, 5%, and 0% samples, respectively. (b), (e), and (h) Transient absorption spectra and (c), (f), and (i) according kinetic curve probe at 925, 945, and 965 nm.

FIG. 4.

(a), (d), and (e) Typical two-dimensional map of ΔOD with wavelength and delay time for 10%, 5%, and 0% samples, respectively. (b), (e), and (h) Transient absorption spectra and (c), (f), and (i) according kinetic curve probe at 925, 945, and 965 nm.

Close modal

TAS focuses on the absorption process, with its signal being the difference in absorption between the excited and unexcited states. In the excited state, carriers transition to higher energy levels in the conduction band. With this spectroscopy, we mainly discuss the thermalization time of the excited hot carriers. Figures 5(a) and 5(b) compare the plots of the 0% and 10% doped samples as a function of △OD (probe at 945 nm) and delay time after photoexcitation with 400 nm pump pulses at 6, 3, and 2 μJ/cm2 pump fluences. Even under natural light, the photogenerated carrier concentration can reach and exceed it in this work. Over the longer relaxation time scale, there are typically two lifetimes: a longer bimolecular recombination and a shorter trap-assisted monomolecular recombination. As the pump intensity increases, the bimolecular recombination lifetime of GSB signals becomes significantly longer. This is a typical hot-phonon bottleneck phenomenon. When the excitation power becomes larger and the carrier injection level becomes higher, the relaxation time becomes longer.43,44 However, the lifetime of trap-assisted monomolecular recombination did not have obvious changes with pump power. Figure 5(c) shows the plots of the 0%, 5%, and 10% doped samples as a function of △OD (probe at 945 nm) and delay time after photoexcitation with 400 nm pump pulses at 6 μJ/cm2 pump fluence. Due to the suppression of the self-doping effect of tin-based perovskite, the density of defect state energy levels in the CB of the 10% sample is reduced, which results in the single molecule recombination lifetime increasing from 5 to 15 ps. The results of Hall measurements also showed that the carrier concentration decreased significantly after SnF2 doping.

FIG. 5.

(a) and (b) Kinetic curves probe at 945 nm under 6, 3, and 2 μJ/cm2 pump fluences for 10% and 0% doped samples. (c) Kinetic curve probe at 945 nm at 6 μJ/cm2 pump fluence for 0%, 5%, and 10% doped samples.

FIG. 5.

(a) and (b) Kinetic curves probe at 945 nm under 6, 3, and 2 μJ/cm2 pump fluences for 10% and 0% doped samples. (c) Kinetic curve probe at 945 nm at 6 μJ/cm2 pump fluence for 0%, 5%, and 10% doped samples.

Close modal

Meanwhile, SnF2 doping not only suppresses the self-doping effect of MASnI3 but also enhances the hot-phonon bottleneck effect to extend the carrier relaxation time. The bimolecular recombination lifetimes increase from 50 to 200 ps after SnF2 doping. It may be that doping reduces the energy level of the defect state, and the probability of electrons appearing in the intrinsic CB/VB energy levels increases. That is, the electron density increases, so the hot-phonon bottleneck effect is more obvious. The electrons of pristine samples undergo a fast relaxation from the higher energy levels of CB to the band edge, and the fast intraband relaxation time did not allow the acceleration of hot-carrier cooling. Therefore, the photogenerated carrier in the pristine sample is difficult to extract and the utilization rate is not high. The 10% doped sample exhibits longer carrier lifetimes and higher carrier utilization rates, possibly due to the pronounced hot-phonon bottleneck effect.

We fabricated invert perovskite solar cells (PSCs) with a planar hetero-junction structure of ITO/PEDOT:PSS/perovskites/PC61BM/BCP/Ag. Figure 6(a) shows the J–V curves of pristine and with SnF2 doped MASnI3 based PSCs. After SnF2 doped, the 10% SnF2 device achieves the highest PCE of 10.2%. The device parameters are summarized in Table II. Chen et al. reported that SnF2 additives can lead to highly oriented topological growth and improved crystallinity. On the one hand, SnF2 prevents the oxidation of Sn2+ to Sn4+, reducing Sn vacancies, and enhances the carrier lifetime, significantly reducing non-radiative recombination losses. On the other hand, ToF-SIMS analysis revealed F ion preferentially accumulated at the hole transport layer/perovskite interface with high SnF2 content, leading to more defects and affecting hole transport. Thus, the low concentration doped SnF2 device showed good stability and reproducibility in their work.45 

FIG. 6.

(a) and (b) J–V curves, external quantum efficiency (EQE) spectrum of the devices using pristine MASnI3, 5%, 10%, and 15% SnF2 doped films as the device absorber layers.

FIG. 6.

(a) and (b) J–V curves, external quantum efficiency (EQE) spectrum of the devices using pristine MASnI3, 5%, 10%, and 15% SnF2 doped films as the device absorber layers.

Close modal
TABLE II.

Photovoltaic parameters of reference and with SnF2 doped PSCs (measured under simulated AM 1.5G solar irradiance at 100 mW/cm2).

PSCsVoc (V)Jsc (mA/cm2)FF (%)PCE (%)
Reference 0.64 19.9 52.5 6.69 
With 5% SnF2 0.69 20.6 50.2 7.10 
With 10% SnF2 0.74 22.3 62.0 10.20 
With 15% SnF2 0.71 20.4 53.8 7.80 
PSCsVoc (V)Jsc (mA/cm2)FF (%)PCE (%)
Reference 0.64 19.9 52.5 6.69 
With 5% SnF2 0.69 20.6 50.2 7.10 
With 10% SnF2 0.74 22.3 62.0 10.20 
With 15% SnF2 0.71 20.4 53.8 7.80 

The external quantum efficiency (EQE) spectra of the reference, 5%, 10%, and 15% SnF2 doped MASnI3 PSCs, are shown in Fig. 6(b). It can be seen from Table II that the effective improvement of PSC performance is mainly due to the increase in Jsc and Voc. In 2018, Lazemi et al. reported that the Voc of CH3NH3SnI3 simulated device is more sensitive to the defect density than Jsc, in the range selected for defect density from 1014 to 1019 cm−3.46 Patel used a solar cell capacitance simulator to study the output characteristics of the CH3NH3SnI3 based solar cell device under AM 1.5G illumination. The simulation results indicate that the performance of the device improved with the reduction in the defect density. Further decreasing defect density, from 1015 to 1014 cm−3, a slight variation is observed in Jsc (40.13–40.14 mA/cm2), but considerable changes occurred in Voc (0.81–0.93 V) and PCE (24.54%–28.39%).47 Our experimental results coincide with their theoretical calculations quite well. SnF2 doping reduced non-radiative recombination,48,49 which improved Jsc effectively from 19.9 to 22.3 mA/cm2 and Voc from 0.64 to 0.74 V

Herein, SnF2 doped MASnI3 perovskite films are grown to suppress the self-doping effect. XRD results indicate that SnF2 could improve the air stability of MASnI3 perovskite by inhibiting the oxidation degradation of Sn2+ to Sn4+. The doped thin films appear to have a longer PL lifetime and lower carrier density. Therefore, both PL and Hall data indicate that the doped films have a longer diffusion length. The TAS results comprehensively reveal that the doped film withstands a stronger hot-phonon bottleneck effect. The MASnI3 devices have better carrier transport ability, which is beneficial for TPSCs to obtain larger open circuit voltage. Consequently, the resulting SnF2 doped MASnI3 solar cells presented a higher PCE of 10.2% and a Voc of 0.74 eV. The finding in this work provides a deeper understanding of the working mechanism of SnF2 doped MASnI3 film based solar cells.

The additional documents in the supplementary material include scanning electron microscope pictures and previously reported carrier concentration of MASnI3 films.

This work was supported by the National Key Research and Development Program of China (No. 2022YFC3700801), the Jinan Bureau of Education (No. JNSX2023015), and the Jinan Bureau of Science and Technology (No. 2020GXRC020).

The authors have no conflicts to disclose.

Fan Xu: Formal analysis (lead); Investigation (lead); Writing – original draft (lead); Writing – review & editing (lead). Haoming Wei: Methodology (equal). Bingqiang Cao: Conceptualization (lead); Funding acquisition (supporting); Project administration (lead); Resources (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable.

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