Few-Layered Black Phosphorene as Hole Transport Layer for Novel All-Inorganic Perovskite Solar Cells

Abstract

The CsPbBr₃ perovskite exhibits strong environmental stability under light, humidity, temperature, and oxygen conditions. However, in all-inorganic perovskite solar cells (PSCs), interface defects between the carbon electrode and CsPbBr₃ limit the carrier separation and transfer rates. We used black phosphorus (BP) nanosheets as the hole transport layer (HTL) to construct an all-inorganic carbon-based CsPbBr₃ perovskite (FTO/c-TiO₂/m-TiO₂/CsPbBr₃/BP/C) solar cell. BP can enhance hole extraction capabilities and reduce carrier recombination by adjusting the interface contact between the perovskite and the carbon layer. Due to the coordination of the energy structure related to interface charge extraction and transfer, BP, as a new type of hole transport layer for all-inorganic CsPbBr₃ solar cells, achieves a power conversion efficiency (PCE) that is 1.43% higher than that of all-inorganic carbon-based CsPbBr₃ perovskite solar cells without a hole transport layer, reaching 2.7% (Voc = 1.29 V, Jsc = 4.60 mA/cm², FF = 48.58%). In contrast, the PCE of the all-inorganic carbon-based CsPbBr₃ perovskite solar cells without a hole transport layer was only 1.27% (Voc = 1.22 V, Jsc = 2.65 mA/cm², FF = 39.51%). The unencapsulated BP-based PSCs device maintained 69% of its initial efficiency after being placed in the air for 500 h. In contrast, the efficiency of the PSC without HTL significantly decreased to only 52% of its initial efficiency. This indicates that BP can effectively enhance the PCE and stability of PSCs, demonstrating its great potential as a hole transport material in all-inorganic perovskite solar cells. BP as the HTL for CsPbBr₃ PSCs can passivate the perovskite interface, enhance the hole extraction capability, and improve the optoelectronic performance of the device. The subsequent doping and compounding of the BP hole transport layer can further enhance its photovoltaic conversion efficiency in PSCs.

1. Introduction

Organic–inorganic hybrid perovskites exhibit an excellent photovoltaic performance and have become the most popular photovoltaic materials in recent years. Since their first preparation in 2009, the PCE of organic–inorganic hybrid PSCs has significantly increased from 3.8% to 26.7% [1,2,3,4,5]. Although organic–inorganic hybrid PSCs have developed rapidly, there are still some issues in practical applications, such as methylammonium lead iodide (MAPbI₃), and they degrade quickly when exposed to light, humidity, oxygen, or high-temperature environments, exhibiting significant instability [6,7] and limiting the practical application of organic–inorganic hybrid perovskite solar cells. The method of using Cs⁺ to replace organic cations to enhance the stability and thermal stability of PSCs has been proven to be effective [8,9].

All-inorganic lead cesium halide (CsPbX₃) perovskites have garnered widespread attention as light-absorbing materials due to their excellent thermal stability (thermal stability > 400 °C) and good optoelectronic properties [10,11]. Currently, there are mainly four types of CsPbX₃ solar cells: CsPbI₃ [12], CsPbBr₃, CsPbI₂Br [13,14], and CsPbIBr₂ [15]. However, CsPbI₃ perovskite struggles to maintain its ideal black cubic α phase at room temperature and easily transforms into the yellow orthorhombic δ non-perovskite phase, leading to a decline in its optoelectronic performance [16]. Br⁻ possesses enhanced effective tolerance factors and lower-phase transition temperatures [17], which contribute to the greater stability of the black cubic α-phase at room temperature. Therefore, compared to CsPbI₂Br and CsPbIBr₂, CsPbBr₃ perovskite exhibits better environmental stability. Additionally, CsPbBr₃ is easy to prepare in ambient conditions without the need for humidity control, has a high carrier mobility, and is one of the most promising inorganic light absorbers [18]. However, the large optical bandgap of CsPbBr₃ (~2.3 eV) results in a poor light absorption capability within the perovskite film of solar cells. Additionally, the significant energy barrier and high defect states at the CsPbBr₃/Carbon interface can lead to poor charge extraction and severe carrier recombination, further diminishing the performance of the final device. Li et al. [19] used zinc phthalocyanine to composite CsPbBr₃ quantum dots (ZnPc/CsPbBr₃ QDs) for the synergistic modification of the CsPbBr₃/Carbon interface in CsPbBr₃ PSCs. This approach passivated the interfacial trap states, improved the contact at the CsPbBr₃/Carbon interface, and optimized the energy level arrangement at the interface. As a result, it enhanced carrier extraction and suppressed carrier recombination, leading to a maximum efficiency of 10.20% for the PSCs, with long-term stability exceeding 6 months. Therefore, adjusting the interfacial contact of the perovskite, passivating defects, and modulating the energy level difference between CsPbBr₃ and the Carbon layer are key to improving CsPbBr₃ PSCs.

Black phosphorus (BP), as a two-dimensional layered material, possesses characteristics such as high theoretical mobility, a tunable direct bandgap, bipolar properties, and simple fabrication. At room temperature, its carrier mobility can reach approximately 1000 cm²/Vs [20]. The direct bandgap property of BP is related to the number of layers, ranging from 0.3 eV (bulk) to 2 eV (monolayer) [21]. BP can serve as a dual-function interface modifier, reducing perovskite crystallization and the defect density in the electron transport layer (ETL)/perovskite, thereby enhancing the charge carrier transport [22,23]; it also strengthens hole extraction in perovskite/HTL [24]. Liu et al. [25] used BP as the ETL in organic photovoltaics (OPVs). When the optimal thickness of BP is 10 nm, a cascading band structure can be formed in the OPV, facilitating an electron transfer and enhancing the device’s power conversion efficiency. Gu et al. [26] utilized black phosphorus quantum dots (BPQDs) mixed with SnO₂ as a mixed ETL for (FAPbI₃)_0.97(MAPbBr₃)_0.03 perovskite solar cells. The strong interaction between BPQDs and SnO₂ not only alters the inherent defects of the SnO₂ layer, improving the carrier transport capability, but also suppresses the oxidation of BPQDs. BP has been widely studied as an electron transport layer in PSCs, but research on its role as a hole transport layer in CsPbBr₃ PSCs is relatively limited. Muduli et al. [27] were the first to use two-dimensional BP nanosheets as the HTL in MAPbI₃ perovskite solar cells, demonstrating that BP nanosheets have a strong hole extraction capability. When BP nanosheets were used solely as the HTL, the device achieved a PCE of 7.88%, an improvement over the 4% of devices without a hole transport material (HTM). Dong et al. [28] used few-layered 2D BP nanosheet-doped poly(triarylamine) (BP:PTAA) as a HTL, significantly improving the charge extraction at the perovskite–HTL interface and reducing the energy barrier. Liu et al. [29] introduced Spiro-OMeTAD:BPQDs as the HTL for Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite solar cells, and the incorporation of BPQDs significantly enhanced the hole mobility of Spiro-OMeTAD, facilitating hole transport and improving the device performance. Therefore, BP is a viable option as a HTL for perovskite solar cells and using BP as the HTL for CsPbBr₃ PSCs can passivate the perovskite interface, enhance the hole extraction capability, and improve the optoelectronic performance of the device. Moreover, BPQDs are proposed as effective seed-like sites to modulate the nucleation and growth of CsPbIBr perovskite crystalline thin layers, allowing an enhanced crystallization and remarkable morphological improvement. Huang et al. [30] utilized the addition of BP chloroform to the Pb precursor solution of perovskite, which was then mixed with the bromide precursor solution to obtain a CsPbBr₃-BP heterostructure film with strong bonding, exhibiting good optical and electronic tunability. Gong et al. [22] injected BP as an additive into the precursor solution of perovskite to prepare a BPQDS/CsPbI₂Br core–shell structure, which enhanced the stability of the CsPbI₂Br crystals, suppressed the oxidation of BPQDS, and effectively improved the stability of the solar cells, providing insights for achieving efficient and stable inorganic perovskite solar cells. However, in carbon-based CsPbBr₃ perovskite solar cells, the potential of BP as a HTL between CsPbBr₃ and carbon has yet to be explored. By using BP as the HTL between CsPbBr₃ and carbon, and through the interaction between BP and CsPbBr₃, investigating the crystallization and phase stability of CsPbBr₃ could further enhance the optoelectronic performance of the devices.

In this paper, a two-step method is employed to synthesize CsPbBr₃ thin films, using few-layer BP nanosheets as the HTL layer for CsPbBr₃ perovskite-based solar cells. Fully inorganic PSCs with the structure FTO/c-TiO₂/m-TiO₂/CsPbBr₃/BP/C are fabricated, and the preparation process is illustrated in Figure 1. By optimizing the battery preparation conditions, a maximum PCE of 2.70% was achieved (Voc = 1.29 V, Jsc = 4.60 mA/cm² and FF = 48.58%), higher than the CsPbBr₃ devices without HTL which had a PCE of 1.27% (Voc = 1.22 V, Jsc = 2.65 mA/cm², FF = 39.51%). After being exposed to air for 500 h, the unencapsulated BP-based PSC device still retained 69% of its initial PCE. In contrast, the PSC without HTL showed significant degradation, with a 48% loss of the initial PCE.

2. Materials and Methods

2.1. Materials

Fluorine-doped tin oxide conductive glass (FTO, ≤10 Ω, 2.5 × 2.5 cm²) was purchased from Advanced Election Technology Co., Ltd. (Yingkou, China). Tetra-isopropyl titanate (99.99%), TiO₂ paste (18NR-T Titania Paste), and lead bromide (99.99%) were obtained from Youxuan Technology. Anhydrous ethanol, N, N-dimethylformamide (DMF), methanol, isopropanol (IPA), and cesium bromide (99.99%) were sourced from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Concentrated hydrochloric acid, black phosphorus nanosheet dispersion (0.2 mg/mL, solvent ethanol), and conductive carbon paste were purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd. (Nanjing, China).

2.2. Device Fabrication

The FTO was ultrasonically cleaned in cleaning powder, acetone, and isopropanol for 20 min each, and then stored in anhydrous ethanol for later use. When needed, the conductive glass was dried with a hairdryer. First, 34 μL of 2 M HCl solution was added to 2 mL of anhydrous ethanol, then 254 μL of titanium isopropoxide was added. This was then stirred at room temperature for 12 h to obtain a dense TiO₂ (c-TiO₂) solution. The mesoporous TiO₂ (m-TiO₂) solution was prepared by evenly mixing commercially available TiO₂ slurry with anhydrous ethanol in a mass ratio of 5:1. Next, 40 μL of c-TiO₂ solution was applied onto the FTO using a spin coater at a speed of 3000 rpm for 30 s to form a c-TiO₂ layer, and then the coated glass was annealed at 500 °C for 30 min. The m-TiO₂ layer was also prepared using the spin-coating method. A volume of 40 μL of m-TiO₂ solution was spun at a speed of 3000 rpm for 30 s. After drying under an infrared lamp, the material was sintered at 500 °C for 30 min.

The preparation of the perovskite film is as follows: First, 1 M PbBr₂ DMF solution was spin-coated onto m-TiO₂ at a speed of 2000 rpm for 30 s, followed by annealing at 80 °C for 30 min. Then, the PbBr₂ film was immersed in a 0.7 M CsBr methanol solution for 30 min to form a CsPbBr₃ film. After washing the CsPbBr₃ film in isopropanol, it was annealed on a heating platform at 250 °C for 5 min. Preparation of the BP hole transport layer: The prepared CsPbBr₃ film was placed on a spin coater, and 30 μL of BP dispersion was spin coated at 1500 rpm for 30 s. Afterward, it was annealed at 150 °C for 30 min. Finally, conductive carbon paste was applied onto the BP film using a scraper and annealed on a heating plate at 120 °C for 15 min to obtain the carbon electrode.

2.3. Characterization

The morphology and structure of PSCs, the electron transport layer, and BP were characterized using a scanning electron microscope (SEM, JSM-7100, Shimadzu, Kyoto, Japan) at 200 kV. The crystal structure of the perovskite layer and the perovskite/BP layer films was analyzed using an X-ray diffractometer (XRD, D8 Advance, Bruker, Bremen, Germany) with Cu-Kα radiation (λ = 1.5404 Å) and this test had a scan speed of 2°/min and a scan range of 10–80°. The steady-state transient fluorescence spectra (PL) were tested using a fluorescence spectrophotometer (Hitachi F-7000, Tokyo, Japan) with a 520 nm laser excitation source. [29] The time-resolved photoluminescence spectroscopy (TRPL) test was conducted using a 520 nm excitation light source on the Horiba FluoroMax + high-sensitivity integrated fluorescence spectrometer (Kyoto, Japan). Under a light intensity of 1.5 G (100 mW cm⁻²) provided by the 3A level solar simulator (Newport 94023A, 450 W, Irvine, CA, USA), the photo current density–voltage (J-V) curves were recorded using a Keithley 2400 digital source meter (Cleveland, OH, USA). The light intensity was calibrated using a silicon reference cell equipped with a power meter (NREL). Electrochemical impedance spectroscopy (EIS) was conducted using an electrochemical workstation (CHI660E, Chenhua, Shanghai, China) and measurement was a Voc test on the PSC by an electrochemical workstation, obtaining bias, and then EIS testing at the corresponding bias in dark.

3. Results

3.1. Characterization of SEM and XRD

Figure 2 shows the SEM images of the c-TiO₂ layer, m-TiO₂ layer, perovskite layer, and BP prepared under optimal conditions. From Figure 2a, we can see that the c-TiO₂ layer consists of densely packed nanoparticles, forming a smooth and compact structure. This smooth, dense layer can effectively block holes, suppress charge recombination, reduce the leakage current, and prevent battery short circuits. From Figure 2b, it is seen that the m-TiO₂ layer has many nanopores, which will serve as a good carrier for the injection of perovskite. From Figure 2c, it is seen that the CsPbBr₃ film is smooth, dense, and has good crystallinity. Figure 2d shows the SEM image of BP on the perovskite surface. It is clearly observable that a layer of BP, which is a nanometer-scale two-dimensional layered structure, is coated on top of the perovskite. To investigate the effect of BP as a hole transport layer on the crystal structure of perovskite, we conducted XRD tests on CsPbBr₃ films and BP. The results, shown in Figure 3, reveal four characteristic peaks centered at 15.8°, 21.9°, 25.6°, 31.1°, and 38.1°, which correspond to the main lattice planes (100), (110), (111), (200), and (210) of the CsPbBr₃ phase, respectively. Additionally, three characteristic peaks of 12.0°, 29.7°, and 33.7° correspond to (002), (213), and (210) of the CsPb₂Br₅ phase. This is a common phenomenon in the two-step synthesis of CsPbBr₃ films [31]. The characteristic peaks of the perovskite film are clear and sharp, indicating the successful formation of the CsPbBr₃ crystal structure. The diffraction peak intensity of the perovskite film with BP about the CsPbBr₃ phase is stronger, with sharper peaks at 25.6° and 31.1°. Among them, the three characteristic peaks of the CsPb₂Br₅ phase were decreased by 12.0°, 29.7°, and 33.7°. However, there is no peak shift, indicating that BP, as the HTL, did not enter the perovskite crystal, but instead reduced the interface defects of CsPbBr₃ and improved its crystallinity.

3.2. Preparation and Performance Regulation of CsPbBr₃ Solar Cells Based on BP HTL

To investigate the optimal optoelectronic performance of CsPbBr₃ PSCs, the preparation conditions of the batteries were optimized. Figure 3 shows the J-V curves of CsPbBr₃ PSCs under different preparation conditions, while

Table 1. Photovoltaic parameters (Voc, Jsc, FF, and PCE) of CsPbBr₃ PSC with different c-TiO₂ spin-coating times, different m-TiO₂ spin-coating times, different BP concentrations, and different BP spin-coating times.

Sample	Voc (V)	Jsc (mA/cm2)	FF (%)	PCE (%)
1c-TiO2	1.18	3.36	40.55	1.61
2c-TiO2	1.21	4.15	45.98	2.31
3c-TiO2	1.20	3.56	39.73	1.70
4c-TiO2	1.18	3.31	36.68	1.43
1m-TiO2	1.21	4.15	45.98	2.31
2m-TiO2	1.21	3.25	40.96	1.61
3m-TiO2	1.19	2.47	38.02	1.12
4m-TiO2	1.21	2.22	39.86	1.07
0.2 mg/mL BP	1.16	4.15	39.45	1.90
0.4 mg/mL BP	1.17	4.73	43.70	2.42
0.6 mg/mL BP	1.13	3.28	40.25	1.49
0.8 mg/mL BP	1.12	3.13	40.02	1.40
1-BP	1.29	4.60	45.52	2.70
2-BP	1.26	3.80	40.02	1.91
3-BP	1.16	3.67	42.56	1.81
4-BP	1.15	3.57	39.71	1.63

lists the photovoltaic parameters of CsPbBr₃ PSCs under the corresponding conditions: open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE). First, we will explore the impact of the number of spin-coating cycles of c-TiO₂ on the optoelectronic performance of the device. The spin-coating cycles of c-TiO₂ are set to one, two, three, and four times, and J-V characteristic curve tests were conducted. As shown in Figure 4a and Table 1, when the spin-coating cycles of c-TiO₂ are set to two, the battery achieves the best PCE of 2.31%. Next, using the optimized values for the spin-coating count of c-TiO₂, the effect of m-TiO₂ spin-coating the optoelectronic performance of the device was investigated, with the spin-coating counts for m-TiO₂ being set to one, two, three, and four. From Figure 4b and Table 1, it can be observed that when the spin-coating count of m-TiO₂ is one, the battery achieves the best optoelectronic efficiency. This may be because the permeability of the m-TiO₂ layer decreases as the spin-coating count increases. Therefore, when the spin-coating count of m-TiO₂ is one, it can improve the interface contact between the electron transport layer (ETL) and the perovskite layer, while having a minimal impact on the light absorption performance of the CsPbBr₃ perovskite layer. Next, using the optimized values for ETL components, the effect of the BP concentrations was investigated, with the BP concentrations being set at 0.2, 0.4, 0.6, and 0.8 mg/mL. From Figure 4c and Table 1, it can be observed that when the BP concentration is 0.4 mg/mL, the device achieves the optimal photoelectric efficiency of 2.42%. Finally, using the optimized values for the ETL components and BP concentration, the effect of the BP spin-coating times on the device’s optoelectronic performance was studied, with the spin-coating times for BP being set at 0.2, 0.4, 0.6, and 0.8 mg/mL. According to Figure 4d and Table 1, it can be observed that when the spin-coating time for BP is one, the device achieves the best optoelectronic efficiency of 2.70%.

3.3. Photovoltaic Characteristics of CsPbBr₃ Solar Cells Based on BP HTL

Figure 5a shows the J-V characteristics of CsPbBr₃ PSCs using 0.4 mg/mL BP as the HTL and those without HTL, under illumination at an intensity of 100 mW/cm². The photovoltaic parameters are listed in

Table 2. Photovoltaic parameters (including Voc, Jsc, FF, and PCE) of CsPbBr₃ PSCs with 0.4 mg/mL BP as the HTL and without HTL, under a light intensity of 100 mW/cm².

Sample	Voc (V)	Jsc (mA/cm2)	FF (%)	PCE (%)
Without HTL	1.22	2.65	39.51	1.27
With BP	1.15	3.57	39.71	1.63

. The PCE of CsPbBr₃ PSCs without HTL is 1.27%, with Voc at 1.22 V, Jsc at 2.65 mA/cm², and FF at 39.51%. In contrast, PSCs using 0.4 mg/mL BP as the HTL exhibit a higher Voc of 1.29 V, Jsc of 4.60 mA/cm², and FF of 45.52%, thereby increasing the PCE to 2.70%. To further investigate the principle of BP as a HTL in enhancing the optoelectronic performance of CsPbBr₃ PSCs, PL spectroscopy and TRPL testing were conducted on perovskites with and without a HTL. Steady-state light can reflect the recombination and separation of electrons and holes in perovskite materials. When a HTL is present on the perovskite layer, the layer is excited by light, causing the separation of electrons and holes, with holes transferring to the HTL, leading to PL quenching. The more pronounced the PL quenching effect, the lower the intensity of the PL emission peak, indicating that the charge transfer at the interface occurs more rapidly [32]. From Figure 5b, it is seen that CsPbBr₃ without a hole transport layer exhibits a strong fluorescence peak, indicating that the recombination of electrons and holes is quite severe in the absence of a hole transport layer. The PL intensity of the perovskite film with BP as the HTL is about 1.15 times lower than that of the perovskite film without a HTL. This suggests that the degree of recombination in the perovskite with BP as the HTL is smaller, and it also proves that BP has good hole extraction capabilities, effectively extracting holes from the light absorption layer and significantly reducing the recombination of electrons and holes. Figure 5c compares the TRPL spectra of perovskite films with and without BP as the HTL.

Table 3. TRPL spectral parameters of CsPbBr₃ thin films without HTL and CsPbBr₃/BP thin films.

Structure	τ1	τ2
Without HTL	4.931	23.961
with BP	4.055	14.337

lists the TRPL spectral parameters for CsPbBr₃ films without the HTL and CsPbBr₃ films with BP as the HTL. The TRPL data were fitted using biexponential decay with a fast decay lifetime (τ₁) and slow decay lifetime (τ₂): I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂). Among them, τ₁ mainly represents the non-radiative recombination caused by grain boundaries and surface trap states, while τ₂ is attributed to the radiative recombination of free-charge carriers. The τ₁ and τ₂ for FTO/CsPbBr₃ are 4.931 ns and 23.961 ns, respectively. In comparison, for FTO/CsPbBr₃/BP, τ₁ and τ₂ decrease to 4.055 ns and 14.337 ns, respectively. This indicates that BP has a high charge extraction capability, which can enhance the hole extraction ability of the device, allowing carriers to transfer more effectively from the perovskite layer to the HTL. This reduces the defect density and suppresses the charge recombination at the perovskite/HTL interface, thereby improving the photovoltaic performance of PSCs. Consequently, the use of BP as the HTL significantly enhances the Jsc and FF of CsPbBr₃ PSCs, consistent with the measurement results shown in Figure 5a. Figure 5d shows the electrochemical impedance spectra (EIS) of CsPbBr₃ PSC devices with and without HTL, using 0.4 mg/mL BP as the HTL. Here, Rs represents the transfer resistance within the entire PSC, Rrec corresponds to the internal charge recombination resistance indicated by the semicircle, and CPE refers to the phase angle element.

Table 4. EIS parameters of CsPbBr₃ PSC with 0.4 mg/mL BP as HTL and without HTL.

Sample	Rs (Ω)	Rrec (Ω)	CPE
Without HTL	91.37	12,397	9.554 × 10−9
With BP	22.75	28,862	6.798 × 10−9

lists the EIS parameters of CsPbBr₃ PSCs with 0.4 mg/mL BP as the HTL and without HTL. In the devices of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/C and FTO/c-TiO₂/m-TiO₂/CsPbBr₃/BP/C, Rrec is the dominant factor. The Rrec value of BP as the HTL in CsPbBr₃ PSC devices is 28,862 Ω, which is significantly higher than that of CsPbBr₃ PSC devices without an HTL (Rrec = 12,397 Ω). Meanwhile, the Rs of the without-HTL PSC was 91.37 Ω, which was significantly smaller than the Rs of with-BP PSCs. This indicates that BP notably inhibits the non-radiative recombination, improves the charge transmission efficiency, and passivates the device defects. This may be due to the interaction between P in BP and Pb in perovskite, which leads to a reduction in defects and an increase in the Rrec value. This effectively suppresses charge recombination, facilitating the extraction of charge carriers in the device, thereby enhancing the FF and Jsc. Figure 6a,b is the UV spectra and Tauc plot of the perovskite thin film without a hole transport layer. The data indicate that the bandgap of CsPbBr₃ is 2.34 eV, which is consistent with the values reported in the literature for CsPbBr₃ [32]. With the above-calculated energy bands and the data from the reference [23], the energy band diagram of the CsPbBr₃ PSC can be drawn, which is shown in Figure 6c. Here, we can see that the maximum valence band (EVB) of the CsPbBr₃ layer film is −5.62 V, while the maximum valence band of BP is −5.2 V. As a hole transport layer (HTL), BP reduces the energy offset for hole extraction, which facilitates a faster hole transfer and suppresses hole accumulation at the interface, thereby improving the device’s photoelectric conversion efficiency.

3.4. Stability Testing

The stability of CsPbBr₃ PSCs is a key factor for their commercialization. Figure 7 compares the PCE stability tests of PSCs with and without BP HTL. After being stored in air at a relative humidity (RH) of 25–65% and a temperature of 25 °C for 500 h, the unencapsulated optimal PSCs retained 69% of their initial PCE. In contrast, the PSCs without the HTL showed significant degradation, losing 48% of their initial PCE. This indicates that BP is in close contact with the perovskite layer and the carbon layer. Due to the protection provided by the carbon layer, the degradation rate of BP is slower, which enhances the air stability of the PSCs. Additionally, incorporating BP into the perovskite layer can passivate the Pb⁰ defects in the perovskite layer, thereby improving the stability of the device, which is consistent with previous theoretical studies [21].

4. Conclusions

In summary, BP can effectively adjust the interface contact of CsPbBr₃/Carbon, reduce charge recombination, and facilitate the extraction of interface charge carriers. At the same time, BP, as a hole transport layer, enhances the hole extraction capability of CsPbBr₃ PSCs, leading to improved Jsc and FF. Ultimately, this significantly enhances the optoelectronic performance of all-inorganic CsPbBr₃ PSCs, achieving an optimal PCE of 2.70%, with a Voc of 1.29 V, Jsc of 4.60 mA/cm², and FF of 48.58%. In addition, after being stored for 500 h in air with an RH of 25–65% and a temperature of 25 °C, the unencapsulated optimal PSCs still retained 69% of their initial PCE. This demonstrates that BP, used as a hole transport layer, can effectively enhance the stability of PSCs, indicating a promising application potential for BP materials in practical use. However, the instability of BP in the air and its susceptibility to oxidation limit its application in solar cells. Improving the air stability of BP remains a key issue for its widespread use. Additionally, the poor phase stability and higher bandgap of the CsPbBr₃ crystal structure result in significantly lower cell efficiency compared to organic–inorganic hybrid PSCs. How to optimize the interaction between BP and CsPbBr₃, reduce the density of defect states, and enhance the stability of CsPbBr₃ PSCs continue to pose a major challenge for BP-based all-inorganic perovskite solar cells.