Ordered growth of Cs₂AgBiBr₆ double perovskites on PEIE-decorated SnO₂ for efficient planar solar cells

Abstract

Cs₂AgBiBr₆ double perovskites are considered potential alternatives to lead-based perovskites for solar cell applications owing to their advantages of stability and non-toxicity. However, the disordered arrangement of Ag/Bi octahedra leads to the formation of self-trapped excitons (STEs) and hinders carrier transport, which is a severe problem in planar solar cells due to inferior charge extraction to mesoporous solar cells. Herein, we employ ethoxylated polyethylenimine (PEIE) to regulate the growth of Cs₂AgBiBr₆ films by sandwiching it between an SnO₂ electron transport layer and a perovskite layer. On the one hand, the introduction of PEIE delays the growth of Cs₂AgBiBr₆ and improves the ordering extent of the Ag/Bi octahedra. This inhibits the formation of STEs and facilitates carrier transport, which profoundly boosts the short-circuit current density. On the other hand, PEIE modulates the energy level of the SnO₂ electron transport layer to match that of Cs₂AgBiBr₆, which reduces the open-circuit voltage loss. Consequently, the power conversion efficiency (PCE) of the planar solar cell increases by 250% after PEIE modification from 0.59% to 1.52%. In addition, the PEIE-decorated device maintains over 90% of its initial PCE for 30 days under ambient conditions, demonstrating good environmental stability.

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1. Introduction

In recent years, lead-based perovskite solar cells (PSCs) have attracted extensive attention due to their superior photoelectric properties, with an overwhelming power conversion efficiency (PCE) exceeding 26.1%.^1–7 However, the toxicity of lead and the environmental instability of lead-based perovskites hinder their commercialization.^8–11 Based on first-principles calculations, researchers found that lead-free double perovskites, whose structure is abbreviated as A₂B⁺B′³⁺X₆, are promising candidates for lead halide perovskites.^12–15 The lead ions in lead-based perovskites are substituted with B⁺ and B′³⁺ in lead-free double perovskites, which show superior properties such as non-toxicity and stability, and photoelectric properties comparable with lead-based perovskites.^16–18

Cs₂AgBiBr₆ is a typical representative of double perovskites, with a long photogenerated carrier lifetime (more than 1 μs), a small effective carrier mass, strong optical absorption in the high-energy part of the solar spectrum, etc.^19,20 The research of applying Cs₂AgBiBr₆ on solar cells has lasted for years; however, the PCE for Cs₂AgBiBr₆ solar cells is typically lower than that of lead-based perovskites.^21,22 This is partially attributed to the wide indirect bandgap of Cs₂AgBiBr₆ exceeding 2.0 eV, which limits the absorption of solar illumination.²³ To solve this problem, several methods such as hydrogenation to reduce the bandgap or introduction of sensitizers to enhance light absorption have been adopted.^24–26 Furthermore, the poor film quality of Cs₂AgBiBr₆ limits its PCE.^27–30 Due to the underdeveloped fabrication methods of Cs₂AgBiBr₆, the defect density in Cs₂AgBiBr₆ is much higher than that in lead-based perovskites.³¹ Particularly, the cation anti-site defects would enhance the electron–phonon coupling and thus limit charge transport in Cs₂AgBiBr₆.^32–34 In the ideal Cs₂AgBiBr₆ double perovskite structure, the [AgBr₆]⁵⁻ and [BiBr₆]³⁻ octahedra are strictly arranged alternately.³⁵ However, in practical preparations, the octahedra disorder is energetically more favorable.^36,37 This disorder of the b-site cations increases the electrostatic repulsion of the two adjacent octahedra, resulting in lattice distortion of the double perovskite. Photogenerated carriers are bound in distorted octahedra under the strong electron–phonon coupling, forming self-trapped excitons (STEs) that impede charge transport.³⁸ To solve this problem, Abhishek Maiti et al.³⁹ introduced phenethylammonium bromide (PEABr) into a Cs₂AgBiBr₆ precursor to obtain ordered films because the phenyl and ammonium groups of PEA⁺ can selectively coordinate with Bi³⁺ and Ag⁺, respectively. Furthermore, the anti-site defects of b-site cations are also present in other double perovskites such as double perovskite oxides.^40–42 In these systems, researchers have developed a polymer-assisted deposition method (usually polyethylenimine, PEI) to improve the ordering of b-site occupations by slowing down the growth process close to thermodynamic equilibrium conditions.^43–45 These successful cases demonstrate that the ordering of b-site cations could be modulated by manipulating the growth dynamics of perovskite films, which has not gained enough attention yet in the Cs₂AgBiBr₆ system and deserves further investigation.

Taking into account the limited charge transport capability in bulk films, most Cs₂AgBiBr₆ solar cells are based on a mesoporous TiO₂ structure due to the abundant contact area, which reduces charge transport lengths and facilitates charge extraction.⁴⁶ Although planar-type solar cells with SnO₂ electron transport layer (ETL) have the advantages of a simplified fabrication process and low processing temperature, their application in Cs₂AgBiBr₆ systems is not successful and leads to a lower PCE than that of a mesoporous structure.⁴⁷ Thus, it is highly desirable to improve the charge transport of Cs₂AgBiBr₆ films in planar-type solar cells. Herein, we report a different strategy of interface modification to fabricate ordered Cs₂AgBiBr₆ double perovskites on an SnO₂ ETL for efficient planar solar cells, apart from the above-mentioned additive engineering methods. Interface modification is a widely used approach for modulating the growth of perovskite films. For example, Zhuang et al. modified the perovskite/SnO₂ interface with guanidine salts of different anions, which promoted the crystallization of perovskites and improved the photovoltaic performance of PSCs.⁴⁸ Gong et al. used a series of potassium salts to modify the SnO₂/perovskite interface and found that the crystallization kinetics of perovskites are controlled by a compromise between the chemical interaction strength and wettability of substrates.⁴⁹ However, interface modification engineering has not drawn enough attention in the Cs₂AgBiBr₆ system. In this work, 80% ethoxylated polyethylenimine (PEIE) was introduced between the SnO₂ ETL and the perovskite layer for interfacial modulation. PEIE is a derivative of PEI with about 80% of the primary and secondary amines ethoxylated. Similar to the widely used PEI in the above-mentioned polymer-assisted deposition method, PEIE interacts with perovskite precursors to modulate film growth. PEIE has better solution processability and film formation properties than PEI, which widen the fabrication window.^50,51 Moreover, PEIE is thermally more stable than PEI and is more adaptable to high-temperature processing according to Kim's report, which makes it suitable for the Cs₂AgBiBr₆ system.⁵² The introduction of PEIE delays the growth of Cs₂AgBiBr₆ grains, resulting in an ordered and high-quality Cs₂AgBiBr₆ film. Besides, PEIE modulates the energy band of SnO₂ to match that of Cs₂AgBiBr₆. The PEIE-decorated Cs₂AgBiBr₆ solar cells show enhanced carrier transport and reduced open-circuit voltage (V_OC) loss, with a profound PCE improvement as high as 250%.

2. Results and discussion

A detailed Experimental section is included in the ESI.† The Cs₂AgBiBr₆ films deposited on pristine SnO₂ and PEIE-decorated SnO₂ were labeled as the control and PEIE samples, respectively. Scanning electron microscopy (SEM) was carried out to characterize the morphology and grain size of Cs₂AgBiBr₆ films grown on different substrates, as shown in Fig. 1a and b. The top-view SEM images show that both samples have compact grains. Meanwhile, the PEIE sample has larger grains with fewer grain boundaries than the control sample. The statistical analyses of grain size distributions are shown in the inset of Fig. 1a and b, where the average grain size of Cs₂AgBiBr₆ increases from 320 nm in the control sample to 620 nm in the PEIE sample. The increased grain size of Cs₂AgBiBr₆ is partially due to the increased wettability of SnO₂ after PEIE treatment. As shown in Fig. 1c and d, the DMSO contact angle of the PEIE-decorated SnO₂ film is 6.8°, lower than that of the control SnO₂ film (12.7°). The low contact angle of DMSO favors the wetting of the Cs₂AgBiBr₆ precursor solution and can improve the quality of the Cs₂AgBiBr₆ film.⁵³ To gain deeper insights into the interaction between PEIE and perovskite precursors, we carried out Fourier transform infrared (FTIR) spectroscopy on PEIE, PEIE + CsBr, PEIE + AgBr, and PEIE + BiBr₃ in DMSO solutions, as shown in Fig. 1e. The peaks in the FTIR spectra mainly arise from DMSO, such as the S=O and C–S stretching vibrations at about 1045 and 670 cm⁻¹.⁵⁴ A superimposed peak appearing at about 1654 cm⁻¹ is assigned to the N–H stretching vibration in PEIE. The peak position of N–H shows a shift to higher wavenumbers after the addition of AgBr (1661 cm⁻¹) or BiBr₃ (1663 cm⁻¹) while it remains constant after the addition of CsBr (1654 cm⁻¹). This demonstrates the interaction between PEIE and AgBr/BiBr₃. Similar to Pb²⁺, Ag⁺ and Bi³⁺ ions have empty d orbitals, which can accept the lone-pair of electrons on the nitrogen atoms in PEIE. Because Cs⁺ ions have no empty d orbitals, they cannot interact with PEIE. As a result, PEIE formed Lewis acid–base interactions with AgBr/BiBr₃ in the precursors, which modulated the growth of Cs₂AgBiBr₆ films. As shown in Fig. S1, (ESI†) compared with the control sample, the as-deposited Cs₂AgBiBr₆ film based on PEIE-decorated SnO₂ shows a slower color change during thermal treatment, indicating the slow crystallization rate of Cs₂AgBiBr₆ films based on PEIE-decorated SnO₂. Such a delayed crystallization process could result in more ordered films of double perovskites, according to previous reports.^55,56

Fig. 1 Top-view SEM images of Cs₂AgBiBr₆ films deposited on (a) SnO₂ and (b) PEIE-decorated SnO₂. The insets show the grain size distributions. DMSO contact angle tests on (c) SnO₂ and (d) PEIE-decorated SnO₂. (e) FTIR spectra of PEIE, PEIE + CsBr, PEIE + AgBr, and PEIE + BiBr₃ in DMSO solutions. (f) Enlarged FTIR spectra for the N–H stretching vibrations.

We further investigated the structural properties and cation occupancy ordering of Cs₂AgBiBr₆ films through X-ray diffraction (XRD) tests. Fig. 2a shows XRD patterns of the control and PEIE samples. With the introduction of PEIE, the diffraction peak intensity of the Cs₂AgBiBr₆ film was enhanced. Meanwhile, the full width at half maximum (FWHM) of the (222) plane is 0.125° in the PEIE sample, which is smaller than that of the control sample (0.138°), as shown in Fig. 2b. The stronger diffraction peak and smaller FWHM indicate that PEIE enhances the crystallization of Cs₂AgBiBr₆ double perovskites, which is consistent with the SEM results. Moreover, the analysis of the XRD patterns is a straightforward and effective method for estimating the ordering of b-site cations in double perovskite films.⁵⁷ In the case where the [AgBr₆]⁵⁻ and [BiBr₆]³⁻ octahedra are completely disordered, the (111) peak would disappear according to the extinction law.³⁹ Consequently, the intensity ratio between the (111) and (022) planes (I₁₁₁/I₀₂₂) quantifies the ordering extent of the [AgBr₆]⁵⁻ and [BiBr₆]³⁻ octahedra, where a higher ratio indicates a more ordered cation arrangement.^37,39,58 As shown in Fig. 2b, the I₁₁₁/I₀₂₂ value of the PEIE sample is 0.413, higher than that of the control sample (0.321), indicating a noticeably increased ordering of the [AgBr₆]⁵⁻ and [BiBr₆]³⁻ octahedra.

Fig. 2 (a) XRD patterns of Cs₂AgBiBr₆ films on different substrates. (b) FWHM of the (222) peak and the intensity ratio between the (111) peak and the (022) peak (I₁₁₁/I₀₂₂). (c) UV-vis absorption spectra of Cs₂AgBiBr₆ films on different substrates. (d) Steady-state PL spectra of different Cs₂AgBiBr₆ films. The inset in (d) shows the sample configuration for the PL measurement.

The variation in the film quality and cation ordering extent is also reflected by the optical properties. Fig. 2c shows the ultraviolet-visible absorption spectra of the control and PEIE samples. The spectra exhibit an absorption edge at around 600 nm with a characteristic peak at about 438 nm.^59,60 The origin of this sharp characteristic peak is due to the direct gap exciton, according to Wright's review.⁶¹ The absorbance of the PEIE sample is slightly higher than that of the control sample, which is attributed to the enhanced crystallinity and improved film quality. We also calculated the indirect bandgap of Cs₂AgBiBr₆ films based on the Tauc plots, as shown in Fig. S2 (ESI†). The indirect bandgap of the Cs₂AgBiBr₆ film shows a slight increase from 2.39 to 2.45 eV, which is related to the enhanced ordering of the [AgBr₆]⁵⁻ and [BiBr₆]³⁻ octahedra, according to the previous report.³⁶Fig. 2d shows the steady-state photoluminescence (PL) spectra of Cs₂AgBiBr₆ films. It is worth noting that the Cs₂AgBiBr₆ films deposit on glass substrates (with or without PEIE) to exclude the charge extraction effect of SnO₂. The spectrum of the control sample is broad with a peak at about 636 nm. Such a broad peak with an FWHM of 140 nm and a large Stokes shift (defined as the difference between the energy of the absorption peak at 438 nm and the emission peak) of 881 meV (as listed in Table 1) indicate the strong electron–phonon coupling in the control sample.⁶¹ The PEIE sample shows a blue-shifted peak at about 604 nm with decreased intensity. Accordingly, the FWHM decreases to 131 nm and the Stokes shift decreases to 778 meV, as shown in Table 1. This indicates the weakened electron–phonon coupling in the PEIE sample due to increased ordering of the [AgBr₆]⁵⁻ and [BiBr₆]³⁻ octahedra, which is consistent with the XRD results.

Table 1 Optical parameters and Huang–Rhys factors (S) of Cs₂AgBiBr₆ films

Sample	Band gap (eV)	PL peak (nm)	FWHM (nm)	Stokes shift (meV)	Huang–Rhys factor (S)
Control	2.39	636	140	881	20.4
PEIE	2.45	604	131	778	18.1

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To gain deeper insights into the electron–phonon coupling induced by anti-site defects of b-site cations, we calculated the Huang–Rhys factor (S) of different samples, which can be derived using the following equation:⁶²

Stokes shift = (2S − 1)E_phonon (1)

where E_phonon represents the energy of the most intense peak (A_1g mode, as shown in Fig. S2a, ESI†) in the Raman spectra. As listed in Table 1, the Huang–Rhys factor S decreases from 20.4 in the control sample to 18.1 in the PEIE sample, indicating that the electron–phonon coupling is lessened after PEIE decoration due to the increased ordering of cation occupancies.

In addition to modulating the growth of Cs₂AgBiBr₆ films, the introduction of PEIE will significantly affect the electronic structure of SnO₂ because of its high-density amine groups and electron-donor capability.^63,64Fig. 3 shows the ultraviolet photoelectron spectroscopy (UPS) spectra of SnO₂ and PEIE-decorated SnO₂. Compared with the control sample, the secondary electron cutoff binding energy (E_cutoff) of PEIE-decorated SnO₂ increased from 16.66 to 16.71 eV. Meanwhile, the onset binding energy (E_onset) was reduced from 3.64 to 3.47 eV. Subsequently, the valence band maximum (E_V) and conduction band minimum (E_C) of SnO₂ ETLs can be calculated using the following equations:

E_V = E_cutoff − E_onset − 21.22 eV (2)

E_C = E_g + E_V (3)

where E_g is the bandgap of SnO₂ (around 3.93 eV for SnO₂ and PEIE-decorated SnO₂ films, extracted from the Tauc plots in Fig. S4, ESI†). After PEIE decoration, the E_C of SnO₂ ETL upshifts from −4.27 to −4.05 eV. The band structure of Cs₂AgBiBr₆ can be calculated in a similar way (UPS spectra in Fig. S5, ESI†), and a schematic of the device energy level structure is shown in Fig. 3c. The large conduction band offset between SnO₂ and Cs₂AgBiBr₆ was reduced after PEIE decoration, which could inhibit nonradiative recombination and reduce V_OC loss. Moreover, time-resolved PL (TRPL) spectra were recorded to further probe the charge transport dynamics between the perovskite and the ETLs, as shown in Fig. 3d. The decay curves were fitted by a stretched exponential function, and the parameters are listed in Table S1 (details in the ESI†). The carrier lifetimes of bare Cs₂AgBiBr₆, SnO₂/Cs₂AgBiBr₆ and SnO₂/PEIE/Cs₂AgBiBr₆ are 13.51, 3.84, and 1.19 ns, respectively, indicating improved charge transmission across Cs₂AgBiBr₆ and SnO₂ after PEIE decoration.

Fig. 3 (a) Secondary electron cutoff regions and (b) onset regions of the UPS spectra for SnO₂ and PEIE-decorated SnO₂. (c) Schematic of energy-level alignment. (d) Time-resolved PL spectra (monitored at 630 nm) of Cs₂AgBiBr₆ films deposited on different substrates. The inset in (d) shows the sample configuration for TRPL measurement.

Planar-type devices with the structure of ITO/SnO₂/PEIE/Cs₂AgBiBr₆/Spiro-OMeTAD/MoO₃/Ag were fabricated to study the effect of PEIE on the performance of solar cells. The schematic illustration and cross-sectional SEM image are shown in Fig. S6 (ESI†). To determine the optimal value of PEIE, we set different precursor concentrations and denote them as PEIE-x. For example, the PEIE-0.2 sample indicates that the concentration of PEIE in the precursor is 0.2 mg ml⁻¹. Fig. 4a shows the J–V characteristic curves of the devices prepared based on different concentrations of PEIE under 1 sun illumination, with photovoltaic parameters listed in Table 2. The PCE of the control sample was 0.59% with a V_OC of 0.90 V, a J_SC of 1.17 mA cm⁻², and a fill factor (FF) of 56.80%. The PCE for the control device is comparable with many previous reports.^24,39,65 After PEIE decoration, the PCE increases and the optimal PCE is achieved at a PEIE concentration of 0.2 mg ml⁻¹. The champion device displays a V_OC of 1.04 V, a J_SC of 2.59 mA cm⁻², an FF of 56.20%, and a PCE of 1.52%, showing a 250% increase. 25 independent devices for each PEIE concentration were prepared to test the reproducibility. The statistical distributions of the photovoltaic parameters are shown in Fig. 4b–e. The statistical data of photovoltaic parameters are consistent with the optimal data, where PEIE decoration increases the device performance and 0.2 mg ml⁻¹ is the best, demonstrating the effectiveness and reproducibility of such a strategy. Generally, the increase in PCE after PEIE decoration is mainly due to the improvement in J_SC and V_OC. On the one hand, the PEIE-modulated crystallization of Cs₂AgBiBr₆ film leads to an increase in the ordering of b-site cations and a reduction of anti-site defects, effectively suppressing the generation of STEs and thus improving the carrier transport process, leading to a significant increase in J_SC. On the other hand, PEIE is capable of adjusting the energy level of SnO₂ to make it more compatible with Cs₂AgBiBr₆, thereby reducing the loss of V_OC. Notably, the PCE of the device begins to decline when the concentration of PEIE exceeds 0.3 mg ml⁻¹, mainly due to the decrease in J_SC. This cannot be ascribed to the degradation of Cs₂AgBiBr₆ film quality because the SEM images and XRD patterns show similar results, as shown in Fig. S7 (ESI†). To gain further insights into the decline of J_SC, we performed conductivity measurements on ITO/SnO₂/PEIE-x/Ag devices, as shown in Fig. 4f. An increase in conductivity for PEIE-decorated SnO₂ was observed when the PEIE concentration was below 0.2 mg ml⁻¹ due to the electron doping effect of PEIE.⁵⁵ However, 0.3 mg ml⁻¹ PEIE-decorated SnO₂ showed a significant decrease in conductivity. This is probably due to the fact that PEIE is actually insulating, and thus excessive PEIE would block electron transport. The decreased conductivity of 0.3 mg ml⁻¹ PEIE-decorated SnO₂ thus leads to a decline in J_SC as well as PCE. Since 0.2 mg ml⁻¹ PEIE decoration leads to the best result, we will focus on this device and the control one in the following paragraph.

Fig. 4 (a) J–V curves of solar cells based on substrates with different PEIE concentrations. Statistical diagrams of the photovoltaic parameters of the devices: (b) V_OC, (c) J_SC, (d) FF, and (e) PCE. (f) J–V curves of the ITO/SnO₂/PEIE-x/Ag device (the inset shows the device configuration).

Table 2 Photovoltaic parameters of champion Cs₂AgBiBr₆ solar cells on different substrates

Device	J SC (mA cm^−2)	V OC (V)	FF (%)	PCE (%)
Control	1.17	0.90	56.80	0.59
PEIE-0.1	2.24	1.01	56.35	1.27
PEIE-0.2	2.59	1.04	56.20	1.52
PEIE-0.3	1.71	1.03	53.96	0.97

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The photoelectric conversion ability of Cs₂AgBiBr₆ PSCs was evaluated by external quantum efficiency (EQE) tests on the control and PEIE samples, as shown in Fig. 5a. The shape of the EQE spectra is similar to that of the absorption spectra, with an overall increase in efficiency for the PEIE sample over the entire spectral range. This is due to the enhanced charge transmission in the PEIE sample benefited from increased cation occupancy ordering. The integrated J_SC increased from 1.08 mA cm⁻² in the control sample to 2.41 mA cm⁻² in the PEIE sample, which is consistent with the J_SC in the J–V curves and demonstrates the reliability of our J–V measurements.

Fig. 5 (a) EQE spectra of the control and PEIE devices. (b) Dark J–V curves of the single-electron devices with the device structure: ITO/SnO₂/(PEIE)/Cs₂AgBiBr₆/PCBM/Ag. (c) Dark J–V curves of the single-hole devices with the device structure: ITO/PEDOT:PSS/(PEIE)/Cs₂AgBiBr₆/Spiro-MeOTAD/Ag. (d) EIS spectra of the control and PEIE devices (equivalent circuit in the inset). (e) Mott–Schottky plots of the control and PEIE devices. (f) Normalized PCE for the control and PEIE devices in the stability test under ambient conditions without encapsulation.

Subsequently, we prepared single-electron and single-hole devices with the structures of ITO/SnO₂/PEIE/Cs₂AgBiBr₆/PCBM/Ag and ITO/PEDOT:PSS/PEIE/Cs₂AgBiBr₆/Spiro-OMeTAD/Ag to analyze the defect-state density (N_t) and carrier mobility of the control and PEIE devices by the space charge limited current (SCLC) method (details in the ESI†),⁶⁶ as shown in Fig. 5b and c. The calculated N_t of electrons decreases from 5.26 × 10¹⁶ cm⁻³ for the control device to 4.24 × 10¹⁶ cm⁻³ for the PEIE device, and the corresponding electron mobility increases from 0.067 to 0.074 cm² V⁻¹ s⁻¹. Meanwhile, the N_t of holes decreases from 1.77 × 10¹⁷ to 1.09 × 10¹⁷ cm⁻³, and the hole mobility increases from 0.0013 to 0.0047 cm² V⁻¹ s⁻¹. The reduction in N_t stems from the reduced grain boundaries and increased cation occupancy ordering after PEIE decoration. Due to the annihilation of electronic traps as well as the weakened STE effect, both carrier mobilities increase, which facilitate charge collection and are thus beneficial to the increase in J_SC.

Electrochemical impedance spectroscopy (EIS) is an effective test method for analyzing the electrical performance and charge recombination of solar cells.⁶⁷ The EIS spectra of the devices were recorded in the darkness, as shown in Fig. 5d, with the equivalent-circuit shown in the inset. The extracted recombination resistance (R_re) in the low-frequency region represents the ease of carrier recombination in the device. After PEIE decoration, the R_re increases from 12.02 to 15.12 kΩ, indicating that carrier recombination was effectively suppressed due to the reduction of antisite defects and STEs. A decrease in charge recombination would increase V_OC.

To further study the enhancement of V_OC, the Mott–Schottky curves with the 1/C² plots of the control and PEIE devices were recorded (details in the ESI†),⁵³ as shown in Fig. 5e. The built-in voltage (V_bi) increased from 0.79 to 0.91 V after PEIE decoration. The increased V_bi is beneficial for charge separation and would thus impede charge recombination, which is consistent with the improvement of V_OC in device performance.

Finally, stability tests were performed for both the control and PEIE devices. Fig. 5f shows the evolution of normalized PCE over time (temperature: 25 ± 5 °C; relative humidity: 25 ± 5%). Both devices exhibit excellent stability under atmospheric conditions. The PCE of the PEIE device remains 90% after 30 days, slightly higher than that of the control device (85%). As shown in Fig. S8 (ESI†), the V_OC, J_SC, and FF of the control device show a similar declining trend with time, which is delayed in the PEIE device. The increased device stability should stem from the improved film quality on PEIE-decorated SnO₂.

3. Conclusion

In summary, we regulated the growth of Cs₂AgBiBr₆ by introducing PEIE between its layer and the SnO₂ ETL. Improved film qualities with enhanced ordering of cation occupancy and reduced STE formations were observed upon the introduction of PEIE, as it can interact with perovskite precursors and delay the growth of Cs₂AgBiBr₆ films. This leads to enhanced charge transport across Cs₂AgBiBr₆ and SnO₂, significantly increasing the J_SC of the PEIE devices. Furthermore, PEIE can modulate the energy band structure of SnO₂, bridging the gap between the SnO₂ conduction band and that of Cs₂AgBiBr₆, thereby reducing the V_OC losses. As a result, the PCE value boosts from 0.59% in the control device to 1.52% in the PEIE device. Moreover, the devices exhibit good stability under ambient conditions. Our work provides an effective and convenient strategy for preparing high-quality Cs₂AgBiBr₆ double perovskite thin films and promoting their applications in the field of solar cells.

Author contributions

Wanjiang Wang: investigation and writing – original draft. Linsong Hou: investigation. Haihua Hu: investigation. Binbin Chang: investigation. Yuqi Yuan: investigation. Ping Lin: writing – review and editing. Peng Wang: writing – review and editing. Xiaoping Wu: investigation. Xuegong Yu: supervision. Lingbo Xu: conceptualization, Writing – review and editing, supervision, and Funding acquisition. Can Cui: supervision.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 61704154) and the Natural Science Foundation of Zhejiang Province (No. LY20F040006).

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Footnote

† Electronic supplementary information (ESI) available: Experimental sections with materials, preparation of Cs₂AgBiBr₆ powders and films, fabrication of solar cell devices, and characterizations; color conversions of Cs₂AgBiBr₆ films on different substrates during thermal treatment; Tauc plots of Cs₂AgBiBr₆ films; Raman spectra of Cs₂AgBiBr₆ films and schematic diagrams of different Raman modes; UV-vis absorption spectra of SnO₂ and SnO₂/PEIE films and corresponding Tauc plots; UPS spectra for Cs₂AgBiBr₆ films on different substrates; cross-sectional SEM and schematic structure of Cs₂AgBiBr₆ PSCs; top-view SEM images, average grain size, and XRD patterns of Cs₂AgBiBr₆ films deposited on substrates with different PEIE concentrations; stability data for V_OC, J_SC, and FF; fitting parameters for TRPL; equations for TRPL, SCLC, and Mott–Schottky analyses. See DOI: https://doi.org/10.1039/d4tc00219a

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