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Abstract
We intend to demonstrate that the treatment of MA (=CH3NH3) Pb (I1−xBrx)3 perovskites with FA (H2N-CH=NH2) cations can enhance the perovskites photovoltaic characteristics. Besides, we propose a new route of bandgap engineering employing low-temperature vapor-assisted solution processes (VASP), reducing fabrication time and material usage. Using this proposed method, we synthesized MAPb (I1−xBrx)3 perovskite layers of 0.4≤x≤1 on mesoporous structures in the ambient atmosphere. Then, we fabricated five types of wide-bandgap perovskite solar cells (PSCs), employing five different molar ratios of PbI2: PbBr2, to tune the bandgaps in the range of 1.78 eV≤EG≤2.29 eV. Then, via spin-coating of FAI and FABr, we introduced FA cations into the perovskite samples and obtained higher quality FA1-yMAyMAPb (I1−xBrx)3 perovskites with smaller trap densities and recombination centers, broader substrate coverage, fewer grain boundaries, a smaller number of pinholes, and hence PSCs with improved photovoltaic performances. The highest efficiencies (11.86% and 12.49%) obtained for untreated and treated samples with x = 0.4 are far from the highest PCEs reported in the literature to date. Nonetheless, the enhancements we observed in the PSCs’ short circuit currents, open-circuit voltages, and PCEs plus the reduction of up to 0.039 in their hysteresis-indices signify the proposed fabrication method and FA treatment can be beneficial to the further development of the perovskite-based solar cells and light emitting diodes.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, hybrid organic-inorganic lead halide perovskites have attracted enormous attention as a photovoltaic material due to their low-cost and ease of fabrication along with their good electrical and optical properties such as direct and tunable bandgap, high absorption coefficient, and high mobility of charge carriers [1]. These outstanding properties have empowered single-junction perovskite solar cells (PSCs) with ordinary bandgaps (1.4 eV ≤ EG≤1.63 eV) to reach 25.2% power conversion efficiency (PCE) for [2].
To go beyond the Shockley–Queisser limit, using tandem architecture is indispensable. Earlier studies reported that EG = 1.82 and 2.05 eV as the optimum perovskite bandgaps for the double [3] and triple [4] junctions PSCs composed of perovskite cell stacked with a Si bottom cell. Moreover, wide bandgap (WBG) perovskites offer desirable properties for practical use in other applications like light-emitting diodes and lasers [5].
The bandgap of a perovskite layer depends on its composition. By changing the concentration of the halide, cation, and metal atoms in the perovskite, one can engineer its bandgap. It can be achieved by different deposition methods like a single-step solution [6], a two-step solution [7], and a vapor-assisted solution process (VASP) [8]. Among these methods, VASP demonstrated to be more promising with the capability of large-area coverage, producing pinhole-free surfaces with larger grain size perovskites [9].
Two techniques have been used to engineer the MAPb (I1−xBrx)3 perovskites’ bandgaps with VASP. The perovskite layers were either prepared by exposing PbI2 layers to MABr at different temperatures or to a mixture of MABr: MAI at a single temperature [10]. In the latter technique, MABr sublimates first and forms MAPbBr3. In other words, MAI controls and elongates the deposition time up to two hours. Nonetheless, reducing the processing time is crucial to the solar cell industry.
Improving the perovskite stability and enhancing the PCE of the resulting PSC have been the two main challenging issues for the photovoltaic community. Several factors are affecting a PSC performance, including the chemical composition of the perovskite layer, interface properties, and the cell structure. Moreover, the stability of the perovskite layer depends on its chemical composition, crystal structure, and quality [11]. Furthermore, the trap states have been one of the major causes of degrading PSC performance. For example, the deep and shallow trap-states decrease the cell open-circuit voltage (VOC) and the short-circuit current density (JSC) [12]. These trap-states are mostly accumulated at the grain boundaries, indebted to the dangling bonds and unsatisfied stoichiometry. For minimizing these traps, various experimental techniques have been developed to engineer the interfaces of a perovskite layer, decreasing the PSC hysteresis-index and enhancing its efficiency and stability [13]. The use of SnO2 instead of TiO2 has resulted in nearly hysteresis-free PCEs [14]. Employing Li-doped TiO2 electrodes as a scaffold hosting CH3NH3PbI3 resulted in the fabrication of efficient PCSs with negligible hysteresis-index [15]. Incorporation of an ultrathin passivation layer consisting of a PMMA: PCBM mixture has shown to effectively neutralize the electronic states at or near the perovskite/TiO2 interface, resulting in nearly hysteresis-free, efficient, and stable PSCs [16]. Manipulation of the perovskite composition by mixing small to large ratios of alkali to organic cations in two sequential steps depositions have resulted in highly efficient and stable PSCs [17]. Further studies have demonstrated that organic halide salts additives like FA (HC(NH2)2 [18], butylammonium bromide [19], guanidinium [20], and phenethylammonium iodide [21] can be easily integrated into perovskite crystals, reducing the defects and suppressing non-radiative recombination and hence producing high efficiency PSCs.
Comparison of the MAPbI3-based and FAPbI3-based PSCs has shown that the former PSCs are more stable to humidity, while the latter PSCs show higher stability to light and temperature [22]. Nonetheless, the optical and electrical properties of mixed-cation perovskites (FA1−yMAyPbI3) are superior to those of the MAPbI3-based and FAPbI3- based PSCs. This is because of the normalization of the Goldschmidt tolerance factor via changing the A cation content (x) [18].
To engineer the bandgap of the mixed halide (CH3NH3PbI3-xBrx) perovskites, in this work, we have proposed a new route by VASP, which does not require high temperatures and reduces the processing time and material usage, resulting in broad area coverage of WBG perovskite layers with large grain sizes and a small number of pinholes. Furthermore, we show that the cation exchange treatment of the synthesized perovskites by FAI and FABr improves the perovskites’ quality and photovoltaic properties of the fabricated PSCs, reducing their hysteresis-index.
2. Experiment
2.1. Materials
The 2.2 mm thick FTO glass substrates (sheet resistance of 15 Ω/-), TiO2 paste, PbI2 (%99), PbBr2 (%99), MAI (%99), and ethanol were purchased from Sharif Solar. FAI and FABr (%99) were purchased from Dyesol. Titanium (IV) isopropoxide (TTIP, 97%) and 4-tert-butylpyridine (tBP, 96%) were supplied by Sigma-Aldrich. N and N-dimethylformamide (DMF, 99.9%) were supplied by Prolabo. 2-propanol (%99.9) was supplied by Merck. Hydrochloric acid (HCl, 37 wt% in water) was purchased from Neutron.
2.2. Device fabrication
First, each FTO glass substrate was patterned and etched with 2 M HCl and zinc powder. Then, the etched substrate was cleaned in alkaline liquor, deionized water, and ethanol, followed by exposure to UV ozone for 10 minutes.
To produce the electron transporting layer (ETL), we used compact-TiO2 and mesoporous TiO2 layers. The compact-TiO2 solution was prepared by mixing 370 µL of titanium (IV) isopropoxide (TTIP) and 35 µL of hydrochloric acid (HCl) in 5 mL ethanol. Then it was spin-coated at 2000rpm for 30 s and sintered at 500 °C for 30 minutes. The TiO2 paste was diluted in ethanol with a weight ratio of 2:7, spin-coated at 4500 rpm for 30 s, and sintered at 500 °C for 30 minutes to achieve a mesoporous TiO2 layer.
Each perovskite layer was synthesized in two main steps, followed by a specific cation exchange treatment step for some samples (each designated by a primed letter in Table 1). Then, we prepared five different 1 M solutions of lead halide by stirring five different molar ratios of PbI2: PbBr2 (Table 1) in DMF at 70 °C for 60 minutes. Next, each solution was spin-coated on the substrate in two successive steps (6000 rpm for 5 s and 3000 rpm for 10 s), followed by a baking step at 100 °C for 10 minutes. Next, the five lead halide layers were faced down, placed ∼4 mm above the MABr powder in a petri dish. The petri dish was put in the oven at 150 °C for 28 min for sample C and 24 min for other samples for the chemical reaction to be accomplished. After cooling to room temperature, the perovskite layers were washed with isopropanol and then dried on a hot plate at 100 °C for 5 minutes. We designated these five types of perovskite layers by A, B, C, D, and E, as can be seen in Table 1. Next, half of the as-prepared samples underwent the cation treatment by spin-coating of FAI and FABr solutions of different concentrations on each perovskite surface, at 4000 rpm for 20 s, followed by an annealing step at 100 °C for 8 min (see Table 1 for A′, B′, C′, D′, and E′).
Table 1. Details of the PbI2: PbBr2 molar ratios used for the bandgap engineering of untreated (A, B, C, D, and E) and treated (A′, B′, C′, D′, and E′), perovskite samples and the concentration of the organic halide (FAI or FABr) used for the treatments.
The conditions for synthesizing the perovskite, the related parameters (chemical, physical and geometrical), mentioned so far in this section belong to the best results obtained in this experimental work (see Section 3). Nonetheless, to demonstrate how variation in any of the crucial parameters affect the quality of the perovskites and hence the resulting PSCs’ efficiencies, we show some of them in the Supporting information. We will address the details of each variation and the corresponding result in an appropriate place in Section 3.
To form a hole transporting layer (HTL) in each untreated or treated sample on the perovskite top surface was spin-coated by the spiro-OMeTAD mixed solution at 4000 rpm for 30 s. We prepared the mixed solution by mixing 72.3 mg of spiro-OMeTAD in 1 mL of chlorobenzene, 28.8 mL of 4-tert-butylpyridine, and 17.5 mL of Li-TFSI solution. By solving 520-mg bis(trifluoromethane) lithium salt in 1 mL acetonitrile, we prepared the Li-TFSI solution. To accomplish the device structure, finally, 80 nm of Au was thermally evaporated on the top of HTL and the bare part of the FTO, forming the back and front electrodes, respectively (Fig. 1).
2.3. Characterization
We obtained the X-ray diffraction (XRD) data employing Philips X'Pert MPD X-ray diffractometer with a co-radiation source (λ = 1.78897 Å). The diffraction angle, 2θ, was scanned from 10° to 50°. The field emission scanning electron microscope (FESEM) images were taken with a SU8010 SEM (Hitachi). The UV-Vis absorption spectra of the perovskite layers were measured by a UV-2048 spectrophotometer (AvaSpec) from 300 to 800 nm in the absorption mode. The steady-state photoluminescence (PL) spectra of the samples excited by a 550 nm pulsed laser were measured by Gilden Photonics. The current density-voltage (J-V) characteristics were measured by an IviumStat potentiostat (XRE model) and a solar simulator (Sharif Solar) calibrated by a standard 1.5G and a calibrated Si-reference cell certificated by NREL. To define the device as active, each PSC was covered with a black mask having a 0.1-cm2 aperture.
3. Results and discussion
Figures 2(a) and 2(b) illustrate the XRD pattern of the untreated and treated perovskite samples, prepared according to the compositions indicated in Table 1. As can be observed from Fig. 2(a), the majority of the mixed halides have been converted to MAPbI3-xBrx in all untreated samples, showing no phase separation into I- and Br-rich domains [23]. Figure S1 illustrates the XRD results for the untreated samples, for which the time of exposing PbI2 layers to MABr was longer than those of Fig. 2(a). These results show the increase in the time of exposing PbI2 layers to MABr accompanied by a detachment of the perovskites back into PbI2. Although an increase in the exposure time has increased the Br content and the bandgap [24], this is not an appropriate way of bandgap engineering. Because the resulting perovskite layer is not suitable for use in PSC fabrication. We will discuss more on the quality of these perovskites later in this section.
Fig. 2. (a)–(b) XRD curves for untreated and treated samples (two vertical dashes indicate the substrate XRD peaks. (c) Portions of XRD curves zoomed-in around their corresponding first peaks. (d) Sample lattice constants, (e) UV-Vis absorption spectra, and (f) optical images of all treated and treated perovskite samples.
Unsaturated Pb atoms can act as recombination sites, leading to poor performance, as discussed by Snaith et al. [25]. Comparing the XRD results shown for samples A, B, and C with those A′, B′, and C′, we can see that the peak at 2θ=12.6° associated with PbI2 observed in Fig. 2(a) vanishes after the treatment with FAI (Fig. 2(b)). It is because of the reaction of FAI with the residual PbI2. The thermal annealing in the treatment procedure leads to the intercalation of the FA cation into the MAPb (I1−xBrx)3 perovskite lattice. Then, the excessive I (Br) anion sublimates as MAI (MABr) due to its low boiling point [26]. We will discuss more on the effect of treatment annealing time on the quality of these perovskites later in this section.
Moreover, the patterns of energy-dispersive X-ray spectroscopy (EDX) of the untreated samples (A, B, C, D) are illustrated in Fig. S2. These results indicate that the structure of MAPb (I1−xBrx)3 perovskites with 0.4 ≤ x ≤ 1, synthesized as described in Section 2.2, carry the cubic phase, which is in agreement with the results shown in [6].
Figure 2(c) depicts a portion of XRD patterns zoomed-in around the first peaks of the untreated (blue curves) and treated samples (pink curves). A comparison of each blue curve with its pink counterpart shows that the cation treatment results in a small shift in the diffraction peak towards a smaller angle. That may be due to the partial substitution of MA with FA cation, leading to a larger lattice constant as can be observed by comparing the solid pink circles with the solid blue diamonds shown in Fig. 2(d). The diamonds and the circles in this figure represent the dependencies of the lattice constants of samples A-D and A′-D′ on the Br content (x) in MAPb (I1−xBrx)3. They show as x increases (A→D and A′→D′) the lattice constant decreases and hence the corresponding peak shifts towards a larger angle (Fig. 2(c)).
The blue and pink curves in Fig. 2(e) illustrate the absorption spectra of the untreated and treated perovskite samples on the glass substrates. The comparison of the blue curves with one another reveals that as the Br content in the perovskite layer increases, the absorption band edge exhibits a blue shift. In other words, as Br content increases from x = 0.47 to x = 1, the absorption wavelength shifts λ = 694 nm to λ = 542 nm, corresponding to the change in the bandgap of EG = 1.78 eV (sample A) to EG = 2.29 eV (Sample E). One can observe a similar trend by comparing the pink curves (samples A′→D′) with one another. The sharp peaks observed in the absorption edges of the perovskite samples with x ≥ 0.75 (i.e., D(D′) and E(E′)) is because of their higher exciton binding energy [27]. A comparison of each blue curve with its pink counterpart shows that the band edge of the corresponding FAyMA1−y (I1−xBrx)3 perovskite exhibits a red-shift of < 10 nm. In other words, replacing MA with FA results in a slight increase in the perovskite bandgap (i.e., mainly because of the halide and lead p states [12]), confirming the intercalation of the FA cations into the MAPbI3−xBrx perovskites lattices. Figure 2(f) showing the optical images of the untreated and treated samples compares the samples’ surface colors. The comparison shows while the differences in the colors of the untreated and treated samples are insignificant, as the Br content increases from x = 0.47 to 1 the sample color changes from dark brown A(A′) to yellowish-brown E(E′).
Figure 3 shows Atomic force microscopy (AFM) images of the untreated (treated) perovskite layers taken from samples B (B′), C (C′), and D (D′), showing the root-mean of the surface (RMS) roughness of 15 (5), 12 (9), and 17 (23) nm, respectively. These results show that the highest RMS roughness is smaller than 30 nm, indicating adequate smoothness of all six layers. A comparison reveals that the treatment of samples B and C with FAI has decreased the RMS roughness of the perovskite layers in Samples B′ and C′. Nonetheless, the treatment of sample D with FABr has increased the RMS roughness of the resulting perovskite layer in sample D′. Moreover, comparison of the RMS roughness of samples B, B′, C, and C′ with one another indicates the dominance of the exchange of MA with FA in treating sample B with additional Iodine content decreased the RMS roughness of the resulting perovskite layer in B′ more than the decrease in the roughness of sample C′, compared to that of C.
Fig. 3. 3D views of the AFM surface morphology of the untreated perovskites; (a) B, (b) C, (c) D; and treated perovskites (d) B′, (e) C′, (f) D′.
Figures 4(a)–4(e) show the field emission scanning electron microscopic (FESEM) images of the top surfaces of the untreated perovskite samples (A-E) are shown in Figs. 4(a)–4(e). Similar FESEM images for samples (A′-E′) are illustrated in Figs. 4(f)–4(j). These images confirm that all perovskite layers synthesized according to Table 1 and the procedure described in Section 2.2 enjoy pinhole-free surfaces, large grain sizes (500-600 nm), and broad area coverage. Comparison of the FESEM image of each treated sample with that of its untreated counterpart reveals the treatment has improved the perovskite morphology. Figures 4(k) and 4(l) compare the cross-sectional (side) views of the PSCs fabricated on samples D and D′. A careful inspection of these two figures reveal the thicknesses of the constituting layers are as follows: compact TiO2 (∼ 40 nm), meso-TiO2 (∼ 250 nm, Perovskite layer (∼ 230 nm), Spiro-OMeTAD (∼ 160 nm), and Au ∼ 80 nm). It is worth noting that the small amounts of FAI and FABr used for the treatments caused no significant changes in the thicknesses of the untreated and treated perovskite layers. This new VASP method, developed in this work, has effectively increased the grain size and the layer quality, leading to improved charge mobility, reduced nonradiative recombination at the grain boundaries, and hence the longer carrier lifetime [28]. Fig. S3 depicts the FESEM images of perovskite samples that were annealed for an insufficient length of time. Comparison of these results shows that the use of an adequate annealing time is required for high quality perovskite to form after treatment.
Fig. 4. FESEM images of the surfaces of (a)–(e) untreated (i.e., A-E) and (f)–(j) treated (A′-E′) perovskite layers. (k)–(l) FESEM images of the vertical cross-sections of MAPb (I0.18Br0.82)3 and FA1-yMAyPb (I0.18Br0.82)3 layers.
Figure 5 shows the J-V characteristics of the untreated (A−E) and treated (A′−E′) PSCs, measured for reverse (R) and forward (F) scans, using a 50 mV·s−1 scan rate, under irradiation of 100 mW·cm−2 AM 1.5G. We extracted the photovoltaic parameters (i.e., JSC, VOC, fill factor (FF), and PCE) for the PSC samples from the reverse scanned J-V characteristics and tabulated in Table 2. This table also shows the PSCs hysteresis indices (H-Index), extracted from the comparison of the reverse and forward scans data. Comparing the tabulated data, one can observe that any variation in the MAPbI3-xBrx composition directly influences the cell JSC and VOC. That is due to the effect of the halide composition on the perovskite bandgap. Moreover, Table 2 shows that the treatment, in general, has enhanced the cell JSC, VOC, and hence PCE, while reducing its H-index. The improvement in the JSC of each treated PSC is because of the increase in the absorption edge of the perovskite layer, which in turn is due to the intercalation of the FA cation into the corresponding MAPb (I1−xBrx)3 lattice. Also observed from the results is the best improvement in PCE achieved by treating sample D that is due to the improvement in corresponding JSC, which in turn is probably due to the effective incorporation of FA in the MAPb (I0.18Br0.82)3 structure. Although the Br content in sample C (C′) and hence its bandgap are both larger than those of samples A (A′) and B (B′), C(C′) shows a lower VOC. This may be due to its larger nonradiative recombination centers because of its chemical composition and structure [12]. Nonetheless, a further increase in the bandgap of sample D (D′) due to additional Br content as compared with C (C′) has enabled the absorption of higher energy photons, compensating for the existing nonradiative recombination and leading to a larger VOC (Table 2).
Fig. 5. J-V characteristics of the untreated and treated PSC samples: (a) A and A′, (b) B and B′, (c) C and C′, (d) D and D′, (e) E and E′, measured using the reverse (R) and forward (F) scans under AM 1.5G irradiation of 100 mW·cm−2 intensity.
Table 2. Photovoltaic parameters JSC, VOC, FF, and PCE of the untreated and treated PSCs under the illumination of 100 mW·cm−2 AM 1.5G
Figures 6(a)–6(d) compare the box charts of the JSC, VOC, FF, and PCE for the untreated and treated PSCs to understand better the variations and dispersion of the photovoltaic results for each parameter. Moreover, we extracted the efficiencies of the PSCs fabricated with the untreated perovskites whose compositions were similar to those of A, C, and E, but annealed at different temperatures for an appropriate length of time (Fig. S4), to obtain the optimum annealing condition for each perovskite sample. Moreover, Fig. S5 shows the effects of treatments with various concentrations of organic halides (FAI, FABr, and FACl) on the PSCs efficiencies. Using these data, we obtained the appropriate organic halide material and concentration for each sample.
Fig. 6. The box charts of the photovoltaic parameters (a) JSC, (b) VOC, (c) FF, and (d) PCE of the untreated and treated PSCs. (e) The normalized PCE of three untreated (A, C, and E) and treated (A′, C′, and E′) samples in air ambient. (f) Photoluminescence spectra of the perovskites of samples D (solid line) and D′ (dashes) illuminated by a laser source of 550 nm wavelength.
Besides PCE, perovskite stability is a crucial character for any PSC. Intercalation of the FA cation improves the PSC photostability and reduces its high-temperature sensitivity [29]. That is because the probability of forming hydrogen bonding by FA is higher than that for MA. Moreover, the phase of the treated perovskites, FA1-yMAyPb (I1−xBrx)3 with 0.4 ≤ x ≤ 1, remains cubic — i.e., the same as that of their untreated counterparts. Note that the cubic phase perovskites are more stable to humidity than the tetragonal phase perovskites [30]. To examine the stability of the untreated and treated PSCs, we measured their J-V characteristics every 48 hours within 288 hours (12 days), storing them in a dark ambient environment between the measurements. Figure 6(e) illustrates the normalized PCEs of the untreated and treated PSCs versus time, relative to their first measured values, showing improvements in the stabilities of the treated PSCs.
Figure 6(f) compares the photoluminescence (PL) spectra of samples D (solid curve) and D′ (dashes) perovskites on the glass substrates. For these measurements, we illuminated the perovskite sides of test structures by a laser beam of 550 nm center wavelength. This comparison reveals that the PL spectrum for the treated sample is sharper than that of the untreated sample, exhibiting a larger peak intensity. The sharper PL spectrum confirms that the treated sample has a lower trap density, and the larger PL peak intensity confirms small nonradioactive recombination for Sample D′ [31]. Moreover, the PL spectra are in agreement with the UV-vis absorption spectra (Fig. 2(e)), confirming that the cation exchange of MA with FA in the treated sample has increased the bandgap of the resulting perovskite manifested by a larger JSC for Sample D′. Moreover, one can observe that each PL peak and the corresponding absorption edge differ by ∼ 10 nm (∼3 meV), which is due to the anti-Stokes shift [32]. This redshift is in agreement with the absorption data (Fig. 2(e)). Furthermore, the PL spectra confirm that there is no phase separation as done by the XRD results shown in Fig. 2(a).
To get a better prospect of charge transport in the PSCs and the reason behind the efficiency improvement due to the treatment process, we use electrochemical impedance spectroscopy (EIS) analysis. Figure 7(a) shows the impedance spectra of the untreated PSCs (open symbols) and the treated PSCs (solid symbols. The inset shows the circuit model used for fitting the data. We performed these measurements under the illumination of 100 mW·cm−2 at 0.2 V with a perturbation of 50 mV, in the frequency range of 1 MHz to 1 Hz. The Nyquist plot of each cell consists of two semicircles. The radius of the first arc (R2) located at higher frequencies corresponds to the charge-transfer resistance (RHTL), and the second arc radius (R3) positioned within the low-frequency spectrum is associated with the charge recombination (Rrec) at the TiO2/Perovskite/Spiro-OMeTAD. Moreover, R1 represents the series resistances, including the contact resistance and the perovskite layer sheet resistance.
Fig. 7. (a) Nyquist plots of the untreated PSCs (open symbols) and treated (solid symbols) under AM1.5G solar simulator at 0.2 V bias. The inset depicts the equivalent circuit model used to fit the Nyquist plots. (b) The equivalent resistance of the recombination (Rrec) for the same untreated (open triangles) and the treated (solid triangles) PSCs.
The element CPE1, also named CHTL, is associated with the HTL layer and the interfacial capacitance between Au and Spiro-OMeTAD [33]. The element CPE2 is related to the electronic and ionic accumulation in the device interfaces and also the slowly moving charges (ions) inside the perovskite layer [34]. They are both constant phase elements, replaced for the ideal capacitances for achieving a more accurate fit.
Figure 7(a) showing the Nyquist plots of the untreated PSCs and treated indicates that the size of the arcs increases as the Br content increases. Figure 7(b) compares the results of Rrec obtained for the untreated and treated devices, extracted from Fig. 7(a). From Fig. 7(b), it can be observed that Rrec values for the treated PSCs are larger than those of their untreated counterparts. In other words, the treated PSCs experience fewer charge recombination because of their reduced density of trap states, achieving larger open-circuit voltages [31], which is in agreement with the PL results (Fig. 6(f)). The fitting parameters of the EIS data for the untreated and treated PSCs are shown in Tables S1 and S2.
4. Conclusions
In this work, we have developed a low-temperature VASP method in the ambient atmosphere to fabricate wide-bandgap PSCs. Moreover, we used PbI2: PbBr2 solutions with five different molar ratios to engineer the perovskite bandgap. Efficiencies of the five resulting PSC samples were from 5.9% to 11.9%. We have shown that treatment of the perovskite layer improves the PSC performance. The cation exchange was confirmed by UV and XRD results. Images of the FESEM and AFM have demonstrated the improvement of surface quality for each treated sample, resulting in a lower trap density inferred from the PL and EIS analyses. The treatment enhanced the PSCs’ photovoltaic performances. Although the highest PCEs reported here are far from the highest efficiencies reported in the literature to date, the enhancements observed in the devices’ JSCs, VOCs, and PCEs, plus the reduction of up to 0.039 in their H-indices, all together signify the proposed fabrication method and FA treatment can be beneficial to the further development of the perovskite-based solar cells and light emitting diodes.
Funding
Tarbiat Modares University (IG-39703).
Acknowledgments
Atefeh Fathzadeh would like to thank the members of the Nano-Optoelectronic Laboratory (NOPL) at Tarbiat Modares University, who assisted her through the course of this research. She also would like to thank Dr. Mohammad Mahdi Khatami for proofreading the original draft.
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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