1.   Introduction

Due to their high conversion efficiency, simple fabrication process, and low cost of production, perovskite solar cells (PSCs) have emerged as one of the most promising third-generation of solar cells [1]. The power conversion efficiency (PCE) of lead-based PSCs has increased from 3.8% to 25.7% in the last decade. However, because lead toxicity has significantly hampered commercial interest in lead-based PSCs, less-toxic tin-based perovskites have been proposed as the most promising substitutes for lead perovskites [2–9]. Three major limitations of tin-based PSCs, however, must be addressed. First, tin-based perovskite materials exhibit a low vacancy formation energy [10–15]. Sn²⁺ oxidizes easily to Sn⁴⁺, resulting in severe p-type self-doping of Sn-based perovskite films. Furthermore, when compared to lead, the energy levels of tin-based perovskites are mismatched with those of carrier transport materials, resulting in a significant loss of open-circuit voltage (V_OC) [16–17]. Finally, due to the high crystallization rate of tin-based perovskite, solution processing is difficult to achieve uniform and dense films [14,18–20]. Inferior morphology would result in severe carrier recombination induced by defects, affecting carrier transport and extraction efficiency [21–23].

One of the most efficient methods for producing high-quality tin-based perovskite films is to use appropriate passivators [24–27]. The device’s open-circuit voltage can be increased by inserting larger organic cations to establish a quasi-two-dimensional structure or by doping X-halogen to modify the energy band gap [28–30]. However, because doping at the X-site anion would cause phase separation of tin-based perovskites under long-term illumination, A-site cation doping is a popular research area [31–33]. To alleviate surface trap states and increase the V_OC of tin-based PSCs, adequate passivators such as Lewis base molecules, polymers, or 2-dimensional (2D) phases perovskite with wide band gaps have been developed [34–37]. To reduce the surface defect density of perovskite films, Lewis base molecules, such as organic amine, choline, and thiol, have frequently been used as passivators [38–41]. Jokar et al. [42] were the first to use ethylene diammonium diiodide (EDAI₂) to passivate tin perovskite films. The researchers discovered that EDAI₂ has a slow passivation effect on surface defects, which increases the carrier lifetime of a tin perovskite film. Because of the creation of Lewis adducts between uncoordinated Sn atoms and the dimethylamino of [tetraphenylethene 3,3'-(((2,2-diphenylethene-1,1-diyl) bis(4,1-phenylene)) bis(oxy)) bis (N,N-dimethylpropan-1-amine) tetraphenylethene] (PTN-Br), the passivation of trap states in perovskite layer by conjugated polymer PTN-Br was reported by Liu et al. [43]. Jokar et al. [44] reported a sequential deposition procedure that improved the photoluminescence lifetime and charge-extraction rate of a mixed cationic tin perovskite solar cell by passivating the surface with a trifluoroethanol solution of phenylhydrazinium thiocyanate (PHSCN). By controlling the A-site cation, Nishimura et al. [45] achieved high-efficiency tin-based solar cells with a tolerance factor of about 1. Proper passivation can also improve device stability [46]. By adding hydroxybenzene sulphonic acid or its salt to the perovskite precursor solution, air-stable FASnI₃ solar cells were accomplished. Furthermore, antioxidant passivation significantly improves the oxidation stability of tin-based PSCs [47].

Propylamine hydrobromide (PABr) was added to the perovskite precursor solution as an additive to reduce defect-induced carrier recombination in this work. Propylamine cations (PA⁺) with long chains cannot enter the perovskite lattice but exist at the grain boundary, passivating perovskite grain surface defects and promoting crystal formation in a preferred orientation. Doping the perovskite precursor solution with 0.02 M PABr improves device performance significantly. The short-circuit current density (J_SC) of devices with PABr increases significantly. As a result, the PCE of the device developed from the PABr-added precursor solution achieved a conversion efficiency of 9.35%, which is more than 38% higher than the PCE of the control device.

2.   Experimental

2.1   Materials

Heraeus Clevios supplied the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Clevious PVP Al 4083). Xi’an Polymer Light Technology provided formamidinium iodide (FAI), propylamine hydrobromide (PABr), and Bathocuproine (BCP). 99.999% ultra-dry tin(II) iodide (SnI₂) was purchased from Alfa Aesar. Sigma-Aldrich supplied tin(II) fluoride (SnF₂), N,N-dimeth-ylformamide (DMF) of 99.8%, dimethyl sulfoxide (DMSO) of 99.8%, and hydrazine monohydrobromide (N₂H₅Br). Buckminsterfullerene (C₆₀) was purchased from Nichem (Taiwan). All materials did not require any purification before use.

2.2   Perovskite precursor solution preparation

FAI (172 mg), SnI₂ (373 mg), SnF₂ (15.7 mg), and N₂H₅Br (4.5 mg) were dissolved in a mixture of DMF (928 μL) and DMSO (72 μL) to prepare a 1 mL FASnI₃ precursor solution. PABr (2.8 mg) was also dissolved in the PABr-doped perovskite precursor solution. All perovskite precursor solutions were magnetically agitated in a glove box for 12 h at 50°C.

2.3   Perovskite solar cell fabrication

The indium tin oxide (ITO) glass substrate was cleaned with a specific cleaning agent and immersed in deionized water for 30 min. The ultrasound is then repeated twice with anhydrous alcohol. The scrubbed ITO glass substrates were then treated for 120 s with plasma ozone. After shaking for 12 h and filtering through a 0.22 μm filter, the PEDOT:PSS solution was spin-coated on ITO substrates for 40 s at 4000 r/min and then annealed at 150°C for 15 min. Dry PEDOT:PSS-coated substrates were placed in a glove box. The perovskite precursor solution was spin-coated on the PEDOT:PSS at 5000 r/min for 20 s and then the solution was filtered with a 0.22 μm filter. As an antisolvent, 80 mL of toluene was used to treat the perovskite films during the spin coating procedure. FASnI₃ perovskite films were annealed in the dark at 100°C for 10 min. C₆₀ (40 nm) and BCP (6 nm) were then deposited using vacuum evaporation in a vacuum chamber with a vacuum degree less than 4 ×10⁻⁴ Pa. Finally, under the same vacuum conditions, Ag (90 nm) was deposited on the BCP layer to fabricate an electrode with a surface area of 0.045 cm².

2.4   Film and device characterization

A field emission scanning electron microscope was used to capture scanning electron microscopy (SEM) images (ZEISS G300, 15 kV). The X-ray diffraction (XRD) patterns of the perovskite films were captured using a Bruker D8 X-ray diffractometer and Cu K radiation (40 kV, 40 mA). A Shimadzu UV-3101PC spectrophotometer was used to measure absorbance. Photoluminescence (PL) spectroscopy was evaluated using a HORIBA iHR 320. TCSPC (time-correlated single photon counting) measurements were used to quantify time-resolved photoluminescence (TRPL) (FluoroCube-01-NL). A Keithley 2450 Source Measure Unit was used to measure the current density–voltage (J–V) characteristics of PSCs. The device’s performance was monitored continuously on a solar simulator (Zolix Sirius-SS150A-D solar simulator) under AM 1.5 G illumination of 100 mW·cm⁻². A total of 101 test points was scanned across a voltage range of −0.2 to 0.8 V. The Zolix Solar Cell Scan 100 was used to calculate the external quantum efficiency (EQE). The residual charges and transient photovoltage were measured using a digital oscilloscope (Tektronix MSO5104B) with an open-circuit state input impedance of 1 MΩ. An arbitrary function generator powered the pulse light source, a concentrated cool white LED. Transient current (TPC) and transient photovoltage (TPV) were measured using a digital oscilloscope while exposed to a Nd³⁺:YAG laser. For TPC and TPV measurements, the oscilloscope’s input impedance was set to 50 Ω and 1 MΩ, respectively. Meanwhile, TPC and TPV were measured with 1 Sun background illumination and a minor perturbation of approximately 5% amplitude caused by the excitation of a Nd³⁺:YAG laser pulse.

3.   Results and discussion

The crystallization quality and morphology of the perovskite active layer strongly influence the performance of tin-based perovskite devices. Using SEM, we first investigated the effect of PABr as an additive on the morphology of FASnI₃ perovskite films. The film without PABr is referred to as the control perovskite film, whereas the film with PABr is referred to as the PABr perovskite film. Fig. 1 shows top-view and sectional SEM images of the control perovskite film and PABr perovskite film. Compared to the control perovskite film, the grain size of the PABr perovskite film increases while the grain boundaries decrease. According to the grain size distribution of the perovskite films, the average perovskite grain size increased from 248 to 371 nm (Fig. S1). One grain of PABr perovskite film can penetrate the entire absorbent layer perpendicular to the substrate, and the grains in the PABr perovskite film have clearer grain boundaries than the grains in the control perovskite film. Normally, defects at the grain boundary would act as the traps for carriers, resulting in defect-induced carrier recombination and lowering the carrier extraction ability and carrier lifetime of perovskite films. PABr can successfully passivate defects at perovskite grain boundaries and increase grain size, thereby improving perovskite active layer crystallization quality.

Figure  1.  Top-view SEM images of perovskite films (a) without and (b) with PABr (w/PABr). Sectional view SEM images of (c) control perovskite film and (d) PABr perovskite film.

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Fig. 2(a) shows a schematic diagram of the crystal structure of FASnI₃ perovskite doped with PABr. An appropriate empirical Goldschmidt tolerance factor is required to stabilize the 3-dimensional (3D) perovskite structure. Goldschmidt’s tolerance factor (t_f) formula states:

Figure  2.  (a) Schematic diagram of the crystal structure of FASnI₃ perovskite doped with PABr. (b) XRD patterns of perovskite films without and with PABr. (c) Absorption and PL spectra of control perovskite film and PABr perovskite films. (d) TRPL of FASnI₃ films without and with PABr additive.

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where R_A and R_B denote the radii of A-site and B-site cations, respectively, and R_X is the anion radius. The cubic crystal structure is stable if the tolerance factor is between 0.813 and 1.107 [48]. According to the empirical formula of tolerance factor forming the 3D perovskite stable structure, the size of PA⁺ is too large to enter the perovskite lattice, so PA⁺ is spread beyond the usual octahedral arrangement of perovskite [49–50]. The XRD patterns of perovskite films without and with PABr are shown in Fig. 2(b). The characteristic crystal planes (100), (102), (200), (122), (222), and (213) are assigned to six FASnI₃ phase diffraction peaks at approximately 14.04°, 24.49°, 28.25°, 31.66°, 40.38°, and 42.97° in the XRD pattern. The XRD dominant peak intensity of the PABr perovskite film is significantly increased, particularly for promoting crystal growth toward preferred crystal orientations of (100) and (200), indicating that the addition of PA⁺ does not change the crystal structure of the orthogonal perovskite but affords orderly perovskite grain distribution. Table S1 shows the XRD peak intensity ratios of the (100) crystal facet to other crystal facets for perovskite films with and without PABr. The results show that for the PABr perovskite films, the relative peak intensity of the (100) crystal planes increase, proving that PABr can facilitate growth in the (100) direction. After adding PABr, the overall full width at half maxima (FWHM) of the XRD peaks narrowed, with the FWHM of the (100) peak decreasing to 0.127° from 0.131°. Meanwhile, when PABr is introduced, small angular shifts are recorded (as depicted in Fig. S2 and Table S2).

The optical properties of the prepared films were evaluated in addition to their structure and morphology. Fig. 2(c) depicts the absorption and PL spectra of perovskite films. The absorption of the perovskite film is unaffected by whether PABr is doped. The changes in the steady-state PL spectra are easily determined. The PL peaks of the two types of perovskite films are located around 880 nm, and the PL emission of the PABr perovskite film is more intense than that of the control one, indicating that defect-induced recombination of carriers in the perovskite with PABr is significantly suppressed. Furthermore, the TRPL curves of perovskite films were measured using a 532 nm pulse laser, as shown in Fig. 2(d). The lifetime of an excited state is calculated using the following formula based on the TRPL curve:

The TRPL decay time constants (τ₁ and τ₂) correspond to the lifetimes of excited states inside and on the surface of the grains, respectively (B₁ and B₂ are relative amplitudes). The accurate parameters of carrier lifetime are shown in Table S3. τ₁ is 2.27 ns with an 84.27% ratio for the control perovskite film, and τ₂ is 6.70 ns with a 12.78% ratio. τ₁ is 6.06 ns with a 70.44% ratio for the PABr perovskite film, and τ₂ is 9.72 ns with a 16.93% ratio. The excited state lifetimes (τ_avg) in the control perovskite film and the PABr perovskite film are 2.86 and 6.77 ns, respectively. The improvement in perovskite crystallization quality and PA⁺ passivation reduces defects inside and on the surface of the perovskite film, slowing carrier recombination induced by the defects. The long lifetime of the excited state is critical in improving the PSCs open-circuit voltage (V_OC) and short-circuit current.

We used a p-i-n structure device to fabricate planar PSCs. Fig. 3(a) depicts the structure of the FASnI₃ perovskite device, while Fig. 3(b) depicts the energy-level diagram of the related material [51]. The device without the additive is labeled as a control device, while the device with PABr as an additive is labeled as a PABr device. The devices’ J–V curves are shown in Fig. 3(c). The addition of PABr was discovered to significantly improve device performance. PABr devices perform admirably, with a champion PCE of 9.35%, a V_OC of 0.63 V, a J_SC of 26.79 mA/cm², and a fill factor (FF) of 63.70%. Fifty control and PABr devices were fabricated and tested, and the PCE box diagram of the device is shown in Fig. 3(d), indicating that the addition of PABr improved the efficiency of the PSCs significantly. Table 1 summarizes the photovoltaic parameters of the devices.

Figure  3.  (a) Structure of FASnI₃ perovskite devices. (b) Energy-level diagram of related materials. (c) J–V curves of the control and PABr champion devices under forward scan. (d) PCE box diagram of the control and PABr devices.

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Fig. 4(a) depicts the PL spectra of perovskite films with varying PABr molar ratios. The PL intensity rises and reaches its maximum at 0.02 M PABr addition, then declines with increasing PABr concentration, which is consistent with the device’s performance with 0.02 M PABr, as shown in Fig. 4(b). Furthermore, the J–V curves of the PABr device measured in forward and reverse scanning (Fig. 4(c)) are nearly identical, indicating that the PABr device’s J–V hysteresis is negligible, which may be related to the reduction of surface defects due to PA⁺ passivation. The hysteresis of the J–V curve would result from ionic migration along grain boundaries. The J–V hysteresis curves of the control devices show an obvious hysteresis effect compared to the PABr device, demonstrating that PABr passivation can reduce grain boundaries and inhibit the device’s hysteresis effect. The PABr device’s maximum steady-state PCE was also measured at a bias of 0.47 V under AM 1.5 G illumination of 100 mW·cm⁻². After 400 s of irradiation, the PCE dropped to about 7.8% (Fig. S3).

Table  1.  Photovoltaic parameters of the control and PABr devices

FASnI3 perovskite	VOC / V			JSC / (mA·cm−2)			PCE / %			FF / %
	Champion	Average		Champion	Average		Champion	Average		Champion	Average
Control	0.55	0.51 ± 0.01		22.52	19.45 ± 1.30		6.77	5.19 ± 0.68		54.08	52.14 ± 3.08
w/PABr	0.63	0.60 ± 0.02		26.79	25.47 ± 0.69		9.35	8.93 ± 0.16		63.70	60.04 ± 1.67
Note: The measurement was performed with 100 mW/cm2 of AM 1.5 G illumination, simulating solar illumination. Fifty devices were used to calculate the average value.

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Figure  4.  (a) PL spectra and (b) J–V curves of devices based on FASnI₃ with different proportions of PABr. (c) J–V hysteresis curves of the control device and PABr device. (d) EQE spectra and integrated J_SC.

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Furthermore, as shown in Fig. 4(d), the integral current densities obtained from the EQE curve are 25.14 mA·cm⁻² for the PABr device and 21.08 mA/cm² for the control device. The result of PABr device is less than that of the J–V curves. The device’s performance loss in the two test operations could be attributed to oxidation. Meanwhile, the V_OC and J_SC short-term light stability of the PABr device and control device were measured and shown in Figs. S4 and S5. Both V_OC and J_SC decrease once the illumination is turned on, then stabilize after 100 s, which is consistent with PCE short-term stability. The test results of the 480-h stability of the control and PABr devices in the N₂ glove box are shown in Fig. S6. The PCE of the control devices fell to 56% of their initial value, while the PCE of PABr devices remained at 76%, demonstrating the enhanced stability of PABr devices.

The dependence of V_OC on light intensity was also measured to better understand the mechanisms of defect-assisted recombination. Using the following equation, the ideality factor n can be calculated from the dependence of V_OC on the light intensity (I) curve:

where k denotes the Boltzmann constant, T denotes the temperature in K, and q denotes the elementary charge. Generally, the n of photovoltaic devices is between 1 and 2, where n = 2 suggests that trap-assisted recombination predominates and n = 1 implies that carrier–carrier recombination predominates [25]. The n values of the control device and the PABr device are 1.61 and 1.29, respectively, as shown in Fig. 5(a), indicating that the density of defects in the device with the addition of PABr is less than that of the control device. The residual charge test was also used to characterize deep-level defects in the two devices, as shown in Fig. 5(b). When the electromagnetic relay in the circuit is turned on, these devices are short-circuited [52]. After that, the photogenerated charges would be discharged and neutralized, and V_OC would be zero. On the other hand, the V_OC of PSCs rises initially when the electromagnetic relay is turned off due to the release and redistribution of trapped charges by deep-level defects. The amount of V_OC rebound can be related to the number of trapped carriers. The control device’s V_OC rebound is much greater than that of the PABr device, indicating that there are more trapped carriers in the control device. More trapped carriers imply more deep-level defects. The reduction in trapped carriers demonstrates that the density of deep-level defects in the PABr device has been effectively reduced. The defect density was further evaluated using the space charge limited current method, as shown in Fig. S7. The hole-only devices were fabricated using the ITO/PEDOT:PSS/perovskite/MoO₃/Ag architecture, and the defect density was calculated using the following equation:

Figure  5.  (a) Light intensity-dependent V_OC curve and (b) residual charge test curve of the control device and PABr device. (c) TPV and (d) TPC response.

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where N_t denotes the hole trap density, ε denotes the dielectric constant of perovskite, ε₀ denotes the dielectric constant of vacuum, V_TFL denotes the voltage at which all the traps are filled, q denotes the elementary charge, and L denotes the thickness of the perovskite film. Adding PABr to the perovskite reduces the trap density from 1.58 × 10¹⁵ to 1.38 × 10¹⁵ cm⁻³, demonstrating that PABr could passivate the defect trap state [53]. Furthermore, as shown in Fig. 5(c), the TPV test results of the control and PABr devices confirm that PABr addition can reduce the density of defects. The control and PABr devices have carrier lifetimes of 2.71 and 4.12 μs, respectively. The longer the carrier lifetime, the fewer the defects. Meanwhile, dark transient photovoltage tests in solar cells can characterize carrier recombination dynamics via defects and carrier de-trapping. As illustrated in Fig. S8, the decay time of the open-circuit voltage of the PABr device becomes significantly faster than that of the control device, demonstrating that the incorporation of PABr can effectively reduce the defect density in the perovskite layer. As shown in Fig. 5(d), the TPC test can confirm the effect of PABr addition on carrier extraction in the device. The photocurrent decay times of the control and PABr devices are 2.79 and 1.25 μs, respectively. The photocurrent decay time of the devices with PABr is significantly shorter than that of the control device, demonstrating that the addition of PABr can improve charge extraction efficiency due to perovskite grain orientation growth. The TPV and TPC measurements show that PABr can increase carrier lifetime and improve charge-extraction efficiency, resulting in improved device performance.

4.   Conclusion

In summary, we have developed an efficient method for fabricating high-quality tin-based perovskite thin films in which PABr is used to passivate defects on the surface of perovskite grains. PABr not only passivates surface defects but also improves the crystal quality of FASnI₃ perovskite films. Furthermore, PABr could specifically induce crystal orientation growth in the direction of (100), which is conducive to forming good contact between the perovskite active layer and the hole transport layer. A solar cell based on the PABr-doped FASnI₃ exhibited improved device performance due to the reduction of surface defects, improvement in crystallinity, and orderly orientation. The PCE of the PABr-doped FASnI₃ solar cell is 9.35%, which is 38% higher than that of the control device. This study demonstrates that PA cations can passivate defects in tin-based perovskites and improve the performance of tin-based PSCs.