Understanding the synergistic influence of the propylammonium bromide additive and erbium-doped CsPbI₂Br for highly stable inorganic perovskite solar cells

Abstract

Inorganic cesium lead halide perovskites have gained increasing attention to boost photovoltaic performance and device stability. Nevertheless, the photoactive to photo-inactive phase transition under ambient conditions hampers its further enhancement. Here, we varied various amounts of propylammonium bromide (PABr) additive in the CsPbI₂Br perovskite and further varied the erbium (ErCl₃)-doped CsPbI₂Br (herein CsPb_1−nEr_nI₂BrX_n) (where X = Cl; 0 ≤ n < 1) perovskite. Further, in the optimized (CsPb_0.97Er_0.03I₂BrCl_0.09) composition, we studied the influence of various amounts of the PABr additive. Our results clearly show the PABr additive added film results in high-quality surface morphology, high crystallinity, and decreased trap-state density. Accordingly, our champion CsPb_0.97Er_0.03I₂BrCl_0.09 + 2 mg ml⁻¹ PABr (CsEr-PA)-based inorganic perovskite solar cell (IPVSC) device showed 16.74% power conversion efficiency (PCE), which is much higher than that of bare (13.20%) and CsPb_0.97Er_0.03I₂BrCl_0.09-based perovskite devices (15.73%). In addition, the CsEr-PA-based IPVSC device revealed increased long-term stability, which maintained 90% of its initial PCE at 65 °C and thermal stress of over 400 h under ambient conditions. These dual stabilization strategies cover a new way to increase the photovoltaic performance of IPVSCs.

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1. Introduction

Recently, the progress in inorganic perovskite solar cells (IPVSCs) has attracted increasing attention due to solving the poor thermal stability issue of the organic–inorganic hybrid perovskite solar cells (PVSCs).¹ Due to the simple and air-processing ability, CsPbI₃,² CsPbBr₃ ³ and CsPbI₂Br^4–8 perovskite compositions are popular in IPVSCs. Among the above-mentioned inorganic perovskite materials, CsPbI₃ has a suitable band gap (∼1.7 eV) and it showed the highest 20.37% power conversion efficiency (PCE).⁹ Unfortunately, due to their unstable behavior and rapid phase transformation from the black CsPbI₃ perovskite to yellow non-perovskite δ-phase at room temperature, it is difficult to maintain stable performance over a long period.¹⁰ Although, CsPbBr₃ is a well-known material for long-term stability it produces a limited PCE of ∼10% because of the wide bandgap of 2.3 eV.¹¹ Particularly, the CsPbI₂Br perovskite material has become a very promising candidate for increasing PCE and stability of IPVSCs because the appropriate band gap of 1.92 eV.¹² Thus, it is important to implement a new approach to cultivate more efficient and stable perovskite device performance.

So far, various efforts have been made in order to stabilize the perovskite phase under ambient conditions and improve the device performance of the CsPbI₂Br-based IPVSCs.^13–18 For example, various metal ion doping at the A-site^12–15 or B-site^16–22 would facilitate suitable Goldschmidt's tolerance factor towards ideality, which can stabilize the phase of the perovskite material.²³ According to the previous reports, doping and ion substitution represent an effective way to tailor the optical and electronic properties of the halide perovskites.^16–25 In particular, rare earth ions, exhibit exceptional optical properties of sharp line spectra, tunable wavelengths, long lifetime, and large Stokes shifts because of their exceptional 4f electronic structure.^26–29 In recent years, numerous studies have demonstrated the various rare-earth ions doped in the inorganic perovskite material. For example, Duan et al. reported partial substitution of Pb²⁺ with lanthanide series (Ln³⁺ = Sm³⁺, Tb³⁺, Ho³⁺, Er³⁺, and Yb³⁺) in the inorganic CsPbBr₃-based IPVSCs with 10.14% PCE.²⁷ Similarly, Pan et al., successfully doped a series of lanthanide ions into the CsPbCl₃ perovskite, which showed that the rare-earth ions substituted the Pb²⁺ sites, which is easier to replace the Pb²⁺ site because of their suitable ionic radii. In addition, the doping of lanthanide ions shows tunable multi-colored emissions of visible to near-infrared regions and increased photoluminescence quantum yield.²⁴ However, the combination of lead halide perovskite and rare-earth ions can give unique characteristics, such as high-quality films, decreased trap states, increased stability of perovskite, expanded absorption range, and as a result increased device performance.^29–32 Likewise, rare-earth ions are doped in the organic–inorganic halide perovskite, which is comprehensively employed in photovoltaics.^33–37 The main benefits of rare-earth ion doping involve a long lifetime, narrow emission wavelength, and high photochemical stability, and it can achieve a UV/Visible emission upon near-infrared excitation by the upconversion process.³⁸ Usually, rare earth ions occur in the trivalent form, which is the most stable valence and is considered an upconversion luminescent material. These rare earth ion-doped upconversion materials (Yb³⁺, Tm³⁺, and Er³⁺) have attracted increasing attention due to their unique luminescent properties including high color purity, narrow spectrum, wide emission region, high transition efficiency, and long lifetime. Nevertheless, in the visible and infrared region, the electronic transition is the special f–f transition called a transition within the 4fⁿ configuration of rare earth ions, generating noteworthy spectral properties.^38,39 The trivalent rare earth (upconversion material) Er³⁺ is generally used as a sensitizer due to its abundant energy level and high energy absorption.³⁸ Inspired by the above work, it is worthy to study the influence of Er³⁺ doping in the hot-air processed CsPbI₂Br-based perovskite and its photovoltaic device performance.

Besides, organic additives in the perovskite precursor solution can control its crystallization growth rate, which results in high-quality perovskite thin films. This additive-assisted perovskite thin film synthesis can modify crystal growth and reduce grain boundary defects. So, for various kinds of additives such as formamidinium chloride (FACl),⁴⁰ guanidinium iodide (GAI),⁴¹ ethylammonium bromide (EABr),⁴² butylammonium iodide (BAI),⁴³ methylammonium bromide (MABr),⁴⁴ formamidinium bromide (FABr),⁴⁵ and methylammonium iodide (MAI)⁴⁶ have been employed to control and or passivate the perovskite film surface. The above-mentioned additives can facilitate the hydrophobic surface, decrease the trap density and reduce interfacial recombination in the perovskite surface, enabling the improvement of the carrier lifetime and open-circuit voltage.^47–49 In addition, it helps increase the properties of the perovskite film through the high grains and perovskite film coverage by retarding the crystallization process.^50–52 Here, we emphasized the effect of additives and rare earth ions on the CsPbI₂Br perovskite with the morphology, crystallization, and stability of perovskite devices and studied its photovoltaic performance in ambient conditions.

In this work, we used the propylammonium bromide (PABr) additive in the CsPbI₂Br and erbium (Er³⁺) dopant in the CsPb_1−nEr_nI₂BrX_n (X = Br or Cl; 0 ≤ n < 1) perovskite to increase the perovskite film quality, device performance, and its stability. It was observed that the PABr additive contributed to retard the crystallization process, which yielded micrometer-sized grains. The increased grain size, improved crystallinity, and reduced trap density were observed for the CsPbI₂Br perovskite films by the synergistic influence of Er³⁺ doping and the PABr additive. Accordingly, the highest PCE of 16.74% was achieved for the CsEr-PA (CsPb_0.97Er_0.03I₂BrCl_0.09 − 2 mg ml⁻¹ PABr) perovskite-based IPVSC device.

2. Results and discussion

To check the synergistic effect of the PABr additive and Er³⁺ doping, the PABr additive was first added to the CsPbI₂Br-based solution. Here, we added different amounts of the PABr additive in the CsPbI₂Br perovskite to check the effect on surface morphology. Further, to examine the role of Er³⁺ doping in the CsPbI₂Br perovskite, the ErCl₃ doping amounts with an Er/Pb molar ratio of 0 to 4% (CsPb_0.99Er_0.01I₂BrCl_0.03, CsPb_0.98Er_0.02I₂BrCl_0.06, CsPb_0.97Er_0.03I₂BrCl_0.09 and CsPb_0.96Er_0.04I₂BrCl_0.12 perovskite films) were doped in the CsPbI₂Br precursors solution and deposited by the spin coating (please check the Experimental section). During the spin coating of perovskite films, we applied hot air flow (Fig. S1(a)†) to deposit high-quality films. Furthermore, in the optimized 3% Er³⁺-doped CsPbI₂Br (CsPb_0.97Er_0.03I₂BrCl_0.09) perovskite film, we added different amounts of the PABr additive (1 to 4 mg ml⁻¹). Details of the inorganic CsPbI₂Br (Cs), CsPb_0.97Er_0.03I₂BrCl_0.09 (CsEr), and CsPb_0.97Er_0.03I₂BrCl_0.09 − 2 mg ml⁻¹ PABr (CsEr-PA)-based thin films are described in the Experimental section. Fig. S1(b)† shows the time-dependent photographs of the retarded crystal growth. To check the effect of Er³⁺ doping and PABr additive in the CsPbI₂Br perovskite morphology, we recorded scanning electron microscopy (SEM) images. Fig. 1(a–c) shows top-view SEM images of the Cs, CsEr, and CsEr-PA perovskite thin films and Fig. 1(d–f) reveals a cross-sectional view of the corresponding perovskite films based devices. The Cs-based film showed a rough surface, as shown in Fig. 1(a) and Fig. S2(a).† Fig. S2(b–d)† shows the SEM images of the PABr additive-added perovskite films s CsPbI₂Br, Cs-1 mg ml⁻¹ PABr, Cs-2 mg ml⁻¹ PABr, and Cs-3 mg ml⁻¹ PABr, respectively. Amongst the above perovskite samples, the Cs-2 mg ml⁻¹ PABr-based film exhibited uniform, dense with an increased 900 nm grain size than the rest of the samples. Notably, the PABr additive is advantageous to improve the quality of the perovskite film. Fig. S3(a–d)† shows the SEM images of the CsPb_0.99Er_0.01I₂BrCl_0.03, CsPb_0.98Er_0.02I₂BrCl_0.06, CsPb_0.97Er_0.03I₂BrCl_0.09, and CsPb_0.96Er_0.04I₂BrCl_0.12 perovskite thin films. Among these films, the CsPb_0.97Er_0.03I₂BrCl_0.09-based film showed a smooth surface with uniform and pinhole-free morphology (Fig. 1(b) and Fig. S3(c)).† Furthermore, in this CsEr (CsPb_0.97Er_0.03I₂BrCl_0.09) optimized film, we added various amounts of PABr additives from 1 to 4 mg ml⁻¹, Fig. S4(a–d).† The CsEr perovskite film was converted into uniform and compact morphology with enlarged grains and decreased grain boundaries through the PABr additive. Particularly, the CsEr-PA (CsPb_0.97Er_0.03I₂BrCl_0.09 − 2 mg ml⁻¹ PABr) displayed the highest grain size of 1.6 μm, demonstrating that the proper addition of the PABr additive is advantageous to obtain high-quality perovskite films, Fig. 1(c) and Fig. S4(b).† The cross-sectional SEM images of the perovskite devices fabricated with different device configurations (Au/P3HT/perovskite/mp-TiO₂/c-TiO₂/FTO/glass) are shown in (Fig. 1(d–f)) and Fig. S2(e).† It is noteworthy that, the CsEr-PA (Fig. 1(f)) perovskite device shows a highly uniform and compact nature with a better film thickness than Cs (Fig. 1(d) Cs-PA Fig. S2(e)† and CsEr (Fig. 1(e))-based perovskite devices.

Fig. 1 SEM top view images. (a) CsPbI₂Br (Cs), (b) CsPb_0.97Er_0.03I₂BrCl_0.09 (CsEr), (c) CsPb_0.97Er_0.03I₂BrCl_0.09 + PABr-2 mg ml⁻¹ (CsEr-PA) perovskite thin films. SEM cross-sectional view of (d) Cs, (e) CsEr, (f) CsEr-PA-based whole IPVSC devices and (g) different time exposure dependent photographic images of the Cs, CsEr and CsEr-PA perovskite thin films (Corresponding devices stored in ambient conditions at ∼25% RH without encapsulation).

This will probably increase the charge carrier transport and decrease the charge recombination, which can facilitate improving the device's performance. Furthermore, we checked the phase stability of the Cs, CsEr, and CsEr-PA-based perovskite thin films under ambient conditions, as shown in Fig. 1(g). Remarkably, the initial photoactive black phase transition into the yellow phase was observed after 3 h for the Cs-based film, which is quite faster than that for the CsEr-based perovskite film (12 h). Nonetheless, no visible phase transition signs were observed up to 22 h for the CsEr-PA-based perovskite film, which is attributed to increased phase stability. This increment of structural stability is attributed to the contraction of the perovskite crystal brought about by Er³⁺ doping and the PABr additive. Corresponding perovskite devices were stored in ambient conditions without encapsulation at ∼25% relative humidity (RH).

Further, Fig. 2(a) shows the probable growth mechanism of an increased grain size of the CsEr-PA-based perovskite film. Here, we added PABr additives to the perovskite solution. The CsI-ErCl₃-PABr-DMSO-PbX₂ would originally be transformed into an intermediate phase by the evaporation of the DMSO solvent.¹⁶ The optimal amount of PABr additive will control the reaction among CsI, ErCl₃, and PbX₂, and the Cl⁻ anions help intermediate crystallization and the formation process of the perovskite by the intermolecular exchange.¹⁶ As our films were annealed at 280 °C, the PABr would sublimate during annealing and the Cs⁺ cation enter their suitable crystal lattice position. The time-dependent photographs of the Cs, CsEr and CsEr-PA-based films show the retarded crystal growth for the CsEr-PA-based film compared to Cs and CsEr-based films, Fig. S1(b).† This retarded crystal growth will help enlarge the grain size and crystallinity of perovskite films due to the addition of the PABr additive. Next, we checked the elemental analysis of the Cs, CsEr, and CsEr-PA perovskite films by X-ray photoelectron spectroscopy (XPS) analysis in order to get clear information about Er³⁺ doping and PABr sublimation, as shown in Fig. 2(b–g). Notably, the Cs 3d, I 3d, Br 3d and Pb 4f peaks for CsEr and CsEr-PA-based films shifted to higher binding energy (BE) compared with the Cs-based film (Fig. 2(b–e)). The XPS core-level peak positions of the Cs and CsEr perovskite films are summarized in Table S1.†Fig. 2(f) shows single BE peaks at 169.45 and 169.86 eV, which matched with Er 3d core level for the CsEr and CsEr-PA-based perovskite films, respectively, which suggests the presence of Er³⁺ doping. Due to the Er³⁺ doping, the XPS peaks slightly shifted to higher energy, which may benefit to hold the phase of the CsPbI₂Br perovskite. Besides, no visible peak signal or shift was detected in the N 1s spectra of the CsEr-PA films or other films, as shown in Fig. 2(g). From the above results, it is confirmed that the chemical state has not drastically changed via a small addition in the stoichiometric ratio between Br and I in the perovskites.⁵³ Thus, it is determined that the addition of the PABr additive did not make compositional changes in the Cs and CsEr perovskite films. Further, to check the distribution of Er³⁺ doped in the CsPbI₂Br perovskite, we performed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements, Fig. S5(a).† The Cs, Pb, I, Br, Er, and Cl elements were homogeneously distributed in the perovskite, suggesting that the Er³⁺ was successfully doped in the CsPbI₂Br perovskite at the atomic level (Fig. S5(b–g)†). As per our previous report, we conclude that the Cl⁻ anions-based additives support stabilizing the perovskite phase and they are located at the grain boundaries (Fig. S6†).⁴⁰

Fig. 2 Growth mechanism and chemical composition. (a) Possible growth mechanism. (b–g) XPS spectra of the (b) Cs 3d, (c) Pb 4f, (d) I 3d, (e) Br 3d, (f) Er 4d and (g) N 1s core levels of the Cs, CsEr and CsEr-PA perovskite thin films.

To evaluate the crystal structure phase, optical properties, charge carrier lifetime, and electronic structure, we recorded X-ray diffraction patterns (XRD), time-resolved photoluminescence (TRPL) spectroscopy, UV-Visible spectra, and photoemission spectroscopy (UPS) measurements on Cs, CsEr and CsEr-PA-based perovskite films. Fig. 3(a) displays the XRD patterns of FTO, Cs, CsEr, and CsEr-PA-based perovskite thin films. All corresponding perovskite films exhibited the orthorhombic γ-phase perovskite, which is consistent with our previous reports.^17,40,52 Remarkably, the intensity of all peaks of the CsPbI₂Br perovskite increased after 3% Er³⁺ doping and the addition of the PABr additive. In addition, the main peaks of the CsEr and CsEr-PA perovskite films negligibly shifted to a higher angle. Here, due to the close radius of the Er³⁺ ion (89 pm), it is easy to replace the Pb²⁺ site (119 pm).²⁷ Fig. S7† shows the magnified peaks of the Cs, CsEr, and CsEr-PA perovskite thin films between 2θ = 12–18° diffraction angle. The highly intense peak shows a highly crystallized CsEr-PA-based perovskite film. Table S2† shows the detailed different structural parameters calculated from the XRD of the corresponding perovskite films. Further, the photoluminescence (PL) (Fig. 3(b)) and TRPL spectroscopy (Fig. 3(c)) of the Cs, CsEr, and CsEr-PA perovskite films were recorded to study the effect of the Er³⁺ doping and PABr additive on the charge recombination performance. The CsEr-PA film exhibited a higher intensity than the Cs and CsEr perovskite films, proving the effective passivation and decreasing the trap density of the CsEr precursor solution by the addition of the PABr additive. Additionally, the TRPL decay measurements were carried out to check the charge dynamics of the perovskite film after Er³⁺ doping and PABr additive, as shown in Fig. 3(c). The TRPL decay data were fitted with the tri-exponential decay function from eqn (S1)†⁵⁴ and detailed parameters are mentioned in Table S3.† The average lifetime of the Cs, CsEr, and CsEr-PA perovskite films was calculated from eqn (S2).† The highest lifetime of τ_avg = 30 ns was observed for the CsEr-PA-based perovskite film than Cs (τ_avg = 4.19 ns) and CsEr (τ_avg = 11 ns) perovskite films. This extended lifetime is achieved due to reduced charge recombination. The band gap values were calculated from the UV-Visible absorption spectra. Fig. 3(d) shows the slight blue shift in the absorption edge for CsEr and CsEr-PA films, resulting in a broadening of the band gap from 1.90 to 1.91 eV (Fig. S8†). Further, we checked the electronic structures in the Cs, CsEr, and CsEr-PA-based films by UPS measurements, as shown in Fig. 3(e). The work function (WF) and valence band maximum (VBM) were calculated by the binding energy cutoff (E_cutoff) and onset (E_onset) (Fig. S9†) and Table S4.† The WF values are 4.19 eV and 4.15 eV and 4.11 eV for the Cs, CsEr, and CsEr-PA perovskite films, respectively. After the Er³⁺ doping and the addition of the PABr additive in CsPbI₂Br, the VBM and conduction band minimum (CBM) places did not change significantly, whereas the WF shifted towards CBM. These outcomes specify the existence of additional negative charge carriers after Er³⁺ doping and the addition of the PABr additive. Moreover, Er³⁺ doping and the addition of the PABr additive enlarged the energy level of the CBM (Fig. 3(f)). Notably, the interface charge accumulation loss may be reduced due to Er³⁺ doping and PABr additive in the CsPbI₂Br perovskite and this will benefit enhancing the device performance of IPVSCs.¹⁷

Fig. 3 (a) XRD patterns, (b) photoluminescence, (c) time resolved photoluminescence, (d) UV-Vis spectra, (e) UPS analysis (f) energy-level diagram from UV–Vis and UPS measurements of the Cs, CsEr and CsEr-PA perovskite thin films.

To evaluate the photovoltaic performance of different amounts of Er³⁺ doping and various quantities of PABr additives in the CsPbI₂Br perovskite, we fabricated IPVSC devices and recorded their current–voltage (J–V) curves. Fig. S10† shows the J–V curve of the different amounts of PABr (1–3 mg ml⁻¹) additive-added CsPbI₂Br (Cs-1 mg ml⁻¹ PABr, Cs-2 mg ml⁻¹ PABr and Cs-3 mg ml⁻¹ PABr) perovskites-based IPVSC and corresponding photovoltaic parameters mentioned in Table S5.† The CsPbI₂Br-based perovskite device showed a PCE of 13.20% with an FF of 74.18%, a J_SC of 14.45 mAcm⁻², and a V_OC of 1.235 V. Once we added PABr in the CsPbI₂Br perovskite, an increase in photovoltaic performance was observed for 2 mg ml⁻¹ PABr (PCE of 14.72% with a FF of 74.42%, a J_SC of 15.69 mAcm⁻² and a V_OC of 1.261 V). Fig. S11† shows J–V curves of different amounts of Er³⁺ doping in the CsPbI₂Br (CsPb_0.99Er_0.01I₂BrCl_0.03, CsPb_0.98Er_0.02I₂BrCl_0.06, CsPb_0.97Er_0.03I₂BrCl_0.09, and CsPb_0.96Er_0.04I₂BrCl_0.12)-based perovskite devices. The related parameters are summarized in Table S6.† When we varied different amounts of Er³⁺ (x = 0 to 4%) doping in the CsPb_1−nEr_nI₂BrX_n (X = Cl; 0 ≤ n < 1) perovskite, the highest PCE of 15.73% with FF of 76.79%, J_SC of 16.01 mAcm⁻² and V_OC of 1.280 V were achieved for the CsEr (CsPb_0.97Er_0.03I₂BrCl_0.09)-based perovskite device. Furthermore, we varied various concentrations of PABr additives in the optimized CsEr (CsPb_0.97Er_0.03I₂BrCl_0.09)-based perovskite. Fig. S12† shows the J–V curves of the CsEr-1 mg ml⁻¹ PABr, CsEr-2 mg ml⁻¹ PABr, CsEr-3 mg ml⁻¹ PABr and CsEr-4 mg ml⁻¹ PABr perovskites-based IPVSC devices and related photovoltaic parameters mentioned in Table S7.† The CsEr-2 mg ml⁻¹ PABr-based device showed a higher PCE than other devices. Fig. 4(a) shows the J–V measurements of the Cs, CsEr, and CsEr-PA-based perovskite devices. Particularly, the CsEr-PA-based device showed a considerable increment in a V_OC of 1.304 V, a J_SC of 16.16 mAcm⁻², a FF of 79.44%, and a PCE of 16.74%, Table 1. This increment is ascribed to the reduced trap density, charge recombination, series resistance, and high-quality thin film by the addition of the PABr additive in the CsEr-based perovskite. Fig. 4(b) shows the external quantum efficiency (EQE) spectra of the Cs, CsEr, and CsEr-PA-based perovskite devices. The EQE values reached were 83%, 88%, and 90% for the Cs, CsEr, and CsEr-PA-based perovskite devices and the corresponding integrated J_SC values were 14.23 mAcm⁻², 15.38 mA cm⁻², and 15.83 mA cm⁻², respectively_. The resultant integrated J_SC values are in good agreement with the J_SC extracted from J–V curves. Fig. 4(c) displays the J–V analysis of the champion CsEr-PA-based perovskite device scanned under reverse and forward scans. The corresponding device showed 16.74% PCE for reverse scans and 15.91% PCE for the forward scan, indicating negligible hysteresis. In addition, we investigated the interfacial charge transport performance of the corresponding perovskite devices via the electrochemical impedance spectroscopy (EIS) study. The Nyquist plots of the Cs, CsEr, and CsEr-PA-based perovskite devices and the equivalent circuit model are shown in Fig. 4(d). The EIS-fitted parameters are mentioned in Table S8.† It was found that the CsEr-PA-based perovskite device exhibited a high R_rec of 7857 Ω than the Cs (R_rec = 5337 Ω) and CsEr (R_rec = 6544 Ω)-based perovskite devices, which indicated that the charge recombination was remarkably suppressed in the CsEr-PA-based perovskite device.¹⁹

Fig. 4 (a) J–V measurements, (b) external quantum efficiency (EQE) of Cs, CsEr and CsEr-PA-based perovskite devices, (c) hysteresis of the champion CsEr-PA-based device, (d) EIS analysis of the Cs, CsEr, CsEr-PA-based perovskite devices.

Table 1 J–V measurements of the Cs, CsEr and CsEr-PA perovskite-based IPVSC devices

Perovskites	V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)
Cs	1.235	14.45	74.18	13.20
CsEr	1.280	16.01	76.79	15.73
CsEr-PA	1.304	16.16	79.44	16.74

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To study the internal relation between the charge transport and PCE of the device, the charge carrier dynamics of the perovskite devices were measured. Fig. 5(a) shows the ideality factor (η) yielded by the slope of the fitted data, which was calculated using eqn (S3) and (S4).†^55,56 Our reference Cs-based device showed η of 1.98 kT q⁻¹ that further decreased to 1.78 kT q⁻¹ and 1.52 kT q⁻¹ for CsEr and CsEr-PA-based devices, respectively, demonstrating the reduction of the charge recombination. Furthermore, to confirm the trap states of the electron-only devices with FTO/mp-TiO₂/perovskite/PCBM/Au device configuration, the J–V analysis was carried out under dark conditions, Fig. 5(b). The trap density (N_t) can be calculated according to eqn (S5)†via the trap-filled limit voltage (V_TFL).^57,58 The trap densities of the Cs, CsEr, and CsEr-PA-based perovskite device are 2.920 × 10¹⁶ cm⁻³, 1.779 × 10¹⁶ cm⁻³ and 1.248 × 10¹⁶ cm⁻³, respectively. This indicates proof of the decreased trap density after the PABr additive, which benefits to reduce the charge recombination and, therefore, results in higher photovoltaic performance observed for the CsEr-PA-based device. The steady-state current density of the CsEr-PA-based IPVSC was measured at a biasing voltage of 1.02 V. Fig. 5(c) exhibited a constant current of 15.72 mAcm⁻² with stabilized PCE of 16.03% at over 600 s. Furthermore, the reproducibility of the Cs, CsEr and CsEr-PA-based 15 independent devices was recorded, as shown in Fig. 5(d). Table S9† displays the resultant photovoltaic parameters of the IPVSC devices. We found that the CsEr-PA-based devices exhibited more than 16% average PCE, which was much higher than other Cs and CsEr-based perovskite devices. In addition, the long-term thermal stability of all the Cs, CsEr, and CsEr-PA-based perovskite devices was checked under ambient conditions at ∼25% RH. Fig. 5 shows the thermal long-term stability behavior of the Cs, CsEr, and CsEr-PA-based devices, which were monitored at 65 °C with RH ∼30% without any encapsulation. The CsEr-PA-based perovskite device revealed more stability and retained 90% of its initial PCE of over 400 h. Besides, the CsEr-based devices exhibited lower thermal stability than the champion device. In addition, the thermal stability of the Cs-based devices was lower and the devices started to degrade within 150 h. From the thermal long-term stability investigation, we can confirm that the Er³⁺-doped and PABr additive-assisted CsPbI₂Br perovskite (CsEr-PA)-based device exhibited quite higher long-term stability compared to Cs and CsEr-based perovskite devices. These results were achieved due to the synergistic effect of the optimum amount of Er³⁺ doping (3% Er³⁺) and the PABr (2 mg ml⁻¹) additive along with the hot air deposition method. This work delivered a good opportunity to increase the further photovoltaic performance of IPVSC devices.

Fig. 5 (a) V_OC as a function of light intensity, (b) dark current–voltage curves of electron-only devices, (c) steady-state output at the maximum power point tracking (MPPT) of the CsEr-PA-based on the IPVSC, (d) reproducibility of the Cs, CsEr and CsEr-PA-based IPVSC devices and (e) thermal long term stability of Cs, CsEr and CsEr-PA-based IPVSCs without any encapsulation under a ∼25% relative humidity (RH).

3. Conclusions

In conclusion, we developed an effective approach to achieving a high PCE and stable device performance through Er³⁺ doping and PABr additive in CsPb_1−nEr_nI₂BrX_n (X = Cl; 0 ≤ n < 1)-based perovskite films. These facile approaches resulted in the CsEr-PA film exhibiting a dense and uniform morphology with a grain size of 1.6 μm. Our champion device containing 3% of Er³⁺ and 4 mg ml⁻¹ PABr (CsEr-PA) exhibited the highest 16.74%PCE. These optimistic results were achieved due to the reduced trap density, high charge carrier lifetime, and reduced recombination rate. However, Cs (13.20%) and CsEr (15.73%)-based devices showed lower PCE than the champion device. Moreover, the CsEr-PA-based IPVSC device retained 90% of its initial PCE at 400 h for long-term stability.

4. Experimental details

Fluorine-doped tin oxide (FTO) substrates were ultrasonically cleaned following the sequence of detergent, distilled water, and ethanol for 15 min for each step and treated with UV plasma for 10 min. The compact-TiO₂ (c-TiO₂), mesoporous TiO₂ (mp-TiO₂), TiCl₄ treatment, and the CsPbI₂Br perovskite precursor solution (1.2 M of CsI, 0.6 M of PbI₂ and 0.6 M of PbBr₂ in 1 ml of dimethyl sulfoxide (DMSO)) were prepared following a similar process that we reported earlier.^12,16,20,42 Furthermore, the stock solution of erbium(III) chloride (ErCl₃) was prepared in mixed solvents of DMSO and dimethyl formamide (DMF) (1 : 2). The CsPb_1−nEr_nI₂BrX_n (X = Cl; 0 ≤ n < 1) films were synthesized via the addition of the various amounts of the ErCl₃ from 0 to 4% into the CsPbI₂Br perovskite precursor solution. The prepared films were named as CsPbI₂Br, CsPb_0.99Er_0.01I₂BrCl_0.03, CsPb_0.98Er_0.02I₂BrCl_0.06, CsPb_0.97Er_0.03I₂BrCl_0.09, and CsPb_0.96Er_0.04I₂BrCl_0.12. In order to balance the perovskite composition, we added exactly an identical amount of CsBr (1.2 M in formamide) to the precursor solution. Further, we added propylammonium bromide (PABr) additive with different amounts from 1 to 4 mg ml⁻¹ in the optimized CsPb_1−nEr_nI₂BrX_n (X = Br or Cl; 0 ≤ n < 1) perovskite precursor solution. Later, this perovskite solution was spun first at 1000 rpm for 10 s and then at 3000 rpm for 30 s. The hot air was blown over the spinning substrate for 10 s after spinning at 3000 rpm. All Cs (CsPbI₂Br), CsEr (CsPb_0.97Er_0.03I₂BrCl_0.09), and CsEr-PA (CsPb_0.97Er_0.03I₂BrCl_0.09 − 2 mg ml⁻¹ PABr)-based films were annealed at 280 °C for 10 min. To prepare a hole-transporting layer (HTL) (P3HT solution) and a gold (Au) by thermal evaporation to complete the IPVSC, we followed our previous reports.^12,16,20

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by Priority Research Centre Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF-2018R1A6A1A03024334) and (2020R1A2C2004880).

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Footnote

† Electronic supplementary information (ESI) available: The detailed procedures for all characterization techniques are given in the supporting information. See DOI: https://doi.org/10.1039/d2qi02448a

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