High-Performance Inverted Perovskite Solar Cells with Sol–Gel-Processed Sliver-Doped NiO_X Hole Transporting Layer

1 Introduction

Employing metal halide perovskite materials is a promising route for converting sunlight into electricity due to their high absorption coefficient, extended diffusion distance, prolonged charge carrier life, and affordability.^[1-6] The power conversion efficiency (PCE) of organic/inorganic hybrid perovskite solar cells (PSCs) has surpassed 25.8%, a remarkable improvement from 3.8%.^[7] PSCs can typically divide into conventional (n-i-p) and inverted (p-i-n) structures based on the bottom layer type.^{[8, 9]} Although n-i-p PSCs performed excellent efficiencies, inverted PSCs have also attracted worldwide attention with several advantages, such as a simple preparation process, low hysteresis, good compatibility with tandem solar cells, and potential applications in flexible devices.^[10-12]

The hole transporting layer (HTL) is crucial while reviewing the advancement of high-performance inverted PSCs. The reported HTL can be broadly categorized into organic and inorganic materials. Organic HTLs, such as 2,2′,7′,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-MeOTAD), poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS), and poly(triarylamine) (PTAA), have shown impressive performance^[13-15] but are prone to moisture absorption and liquefaction that can lead to degradation of PSCs.^[16-19] In comparison to organic HTL, inorganic HTLs, such as copper thiocyanate,^{[20, 21]} copper iodide,^{[22, 23]} copper oxide,^{[24, 25]} and nickel oxide (NiO_X),^[26-28] have several advantages, including good chemical stability, easy preparation, high light transmittance, and affordability. Among these materials, NiO_X has received significant attention as HTL in inverted PSCs because of its appropriate work function (about 5.4 eV), wide band gap (>3.4 eV),^[29-31] outstanding stability, and simple fabricating.^[32] Nevertheless, the performance of NiO_X-based PSCs generally has been affected by important issues. First, the energy level mismatch between NiO_X and perovskite at their interface leads to an energy shift. Second, the low conductivity of NiO_X leads to low charge extraction efficiency and increases the probability of recombination at the interface. Finally, the lattice mismatch between NiO_X and perovskite affects the growth of the perovskite layer and reduces device stability. To address these issues, interface engineering through a metal doping strategy for optimizing the conductivity and hole transporting of the NiO_X layer has been commonly used.^[33] Single-element doping with Li,^{[34, 35]} Ag,^[36] Cu,^[37-40] Cs,^[41] and Co^[42] and dual-element doping with Li-Ag^[43] and Li-Cu^[44] are commonly used. These reports have indicated that, under the metal doping process, the carrier mobility and concentration of NiO_X can be aligned robustly.^[45] However, the current doping strategy of NiO_X HTLs is still far insufficient for PSCs. Introducing ion dopants to the NiO_X lattice can result in significant lattice disorder and defects, decreasing charge carrier mobility and recombination.^{[46, 47]} Hence, maintaining the balance between carrier concentration and mobility is crucial when doping semiconducting materials like NiO_X.^[48] For efficient PSCs, the NiO_X-perovskite interface needs high mobility and conductivity. For example, Tan et al. utilized lithium doping to better match the band structure of NiO_X to that of the perovskite. Additionally, the amino-functionalized polymer (PN4N) interfacial layer effectively reduces the density of interface defect states and trap-assisted recombination, resulting in an open-circuit voltage (V_OC) of 1.14 V and an efficiency exceeding 20%.^[35] Despite the optimized devices exhibiting appropriate energy levels and excellent defect passivation capabilities, the result of these strategies is primarily reflected in the increase of V_OC, leaving the potential for enhancing fill factor (FF) by optimizing non-radiative recombination.^[49]

Herein, we demonstrated a hybrid silver doped-NiO_X HTL executed homojunction to achieve high-performance inverted PSCs via a simple sol–gel solution process. The addition of Ag-NiO_X increased the valence band maximum of the NiO_X film from 5.40 to 5.46 eV. Furthermore, the hybrid NiO_X film showed excellent driving force since the difference between Ag-NiO_X and NiO_X films' E_f was higher (0.09 eV) than that of the valence band bottom (0.06 eV). In contrast with previously reported doped NiO_X-based HTL through  sol–gel-processed for inverted PSCs, our work significantly improved all performance indicators of the inverted PSCs, as shown in Figure 1.^{[34, 36-39, 41-44, 50-54]} Table S1, Supporting Information compares the parameters obtained in this work with previous reports. The optimized hybrid HTL greatly improves conductivity, carrier density, and ability to extract and transfer charge at the interface with the perovskite film.^[55] The hybrid NiO_X-based PSCs demonstrated a PCE of 19.25%, compared to the previous pristine NiO_X-based PSCs' PCE of 16.2%. Additionally, the efficiency decay of hybrid NiO_X-based PSCs was 90% lower than that of pristine NiO_X-based PSCs (60%) under the 30-day environmental stability test. In conclusion, this innovative doping and homojunction introduction provides an effective approach for optimizing the interface engineering of the HTL in inverted PSCs, leading to significant improvement in various performance indicators.

2 Results and Discussion

SEM imaging was used to examine the morphology of NiO_X films before and after doping, as shown in Figure 2a–c. The films' morphology was uniform, continuous, and dense across all three types of films, with no cracks or holes observed. Small nanoparticles were visible on the surface, presumed to be NiO_X, and likely formed due to solution contraction during annealing. SEM images of the various NiO_X films indicated that their surface morphologies remained unchanged, implying that the introduction of Ag had no impact on the overall surface morphology of the films. As a result, uniform and dense NiO_X films could be created.

The synthesized NiO_X and Ag-doped NiO_X were first analyzed using XRD patterns to verify successful synthesis and identify any potential phase separation after doping. Figure 2d demonstrates that both NiO_X and Ag-doped NiO_X exhibited peaks at (111), (200), (220), (311), and (222) at angles of 37.09°, 43.1°, 62.8°, 75.6°, and 79.3°, respectively. Through comparison with the standard XRD card of NiO_X (PDF#47–1049), indicating that NiO_X was successfully synthesized through a sol–gel process. Without obvious peaks for silver components were found, and the diffraction of patterns of Ag-doped NiO_X was similar to that of pristine NiO_X, indicating Ag would not cause phase separation. However, as magnified patterns shown (Figure 2e), the diffraction peak shifts slightly towards smaller 2θ after Ag doped, suggesting the lattice was expended.^[56] This is because the larger ionic diameter of Ag⁺ (1.15 Å) substituted the smaller ionic diameter of Ni²⁺ (0.69 Å), consistent with the Bragg equation: 2d sin θ = nλ.^{[57, 58]} Additionally, all film samples exhibited high transmittance, with transmittance >80% in the wavelength range of 350–800 nm, as presented in Figure 2f. Interestingly, the transmittance level of the doped NiO_X film was greater than that of the pure NiO_X film due to the metal ion doping effect.^[48]

The morphology of NiO_X films is widely acknowledged to significantly influence perovskite film quality, hole transport, and electron-blocking effects. Figure S1, Supporting Information shows AFM images of different NiO_X films, displaying smooth surfaces with densely clustered particles. The film types NiO_X, Ag-doped NiO_X, and Ag-doped NiO_X/NiO_X have root mean square (rms) values of 5.22, 5.50, and 5.89 nm. However, the difference in RMS values among the three films is not statistically significant, making it difficult to confirm that Ag doping alters the surface morphology and size of the NiO_X film. Ag doping results in uniform, dense, and flat NiO_X films that exhibit modified surface energy and crystallinity, potentially affecting both the growth and morphology of the perovskite film on the HTL, as revealed by XRD characterization. Control of the morphology of sol–gel-based prepared films can be challenging, but the study findings suggest that Ag doped in NiO_X and hybrid treatment may substantially impact perovskite film quality, indicating the importance of exploring such treatments further.

The chemical states of NiO_X and Ag-doped NiO_X were characterized using XPS analysis (Figure 3). Figure 3a,b present XPS data for NiO_X and Ag-doped NiO_X films, respectively. The emergence of a peak at 368 eV (Ag 3d_5/2) and 374 eV (Ag 3d_3/2) in the spectra confirms the successful doping of Ag in NiO_X (Figure 3c), consistent with existing literature.^{[36, 45, 59]} Figure 3d illustrates the Ni 2p XPS spectra of NiO_X and Ag-doped NiO_X. Using Lorentz-Gaussian functions (Figure S2, Supporting Information), we observed the appearance of two new peaks, including a main peak at 853 eV and a shoulder peak at 855 eV. These two peaks generally represent Ni²⁺ and Ni³⁺ peaks due to Ni²⁺.^{[60, 61]} Notably, Ag doping led to a higher Ni³⁺/Ni²⁺ ratio in NiO_X compared to undoped NiO_X, thereby increasing the concentration of Ni vacancies. This, in turn, enhances the electrical conductivity of the film. These findings are consistent with the slight shift in XRD patterns and prior reports.^{[36, 43, 62]}

In this formula, $A$ represents the device area (4 mm²), and $D$ represents the thickness of the NiO_X film (approximately 30 nm). We calculated the values of ${\mathrm{\sigma }}_0$ for various NiO_X films with different concentrations of Ag doping, as shown in Table S2, Supporting Information. We observed that the σ₀ of the NiO_X films increased with increasing Ag doping concentration, but when the Ag doping concentration reached 4%, the conductivity tended to decrease. This decrease may be due to the low hole mobility caused by the high doping concentration. When the Ag doping concentration increased from 2% to 4%, more Ag⁺ ions replaced lattice Ni²⁺ sites, increasing the hole carrier concentration.

In this formula, $J$ represents the current density, ${\mathrm{\epsilon }}_0$ is the vacuum permittivity, ${\mathrm{\epsilon }}_{\mathrm{r}}$ is the dielectric constant, ${\mathrm{\mu }}_{\mathrm{e}}$ and $L$ represent the hole mobility and thickness of different NiO_X films, and $V$ represents the applied voltage. After fitting calculations, we observed that when the doping concentration reached 2%, μ_e reached a higher value of 2.1 × 10⁻² cm² (V·S)⁻¹. The values of μ_e were calculated for different concentrations of Ag doping, as shown in Table S2, Supporting Information. Similar to the explanation for the conductivity of different NiO_X films, we observed that the carrier mobility of the film initially increased and then began to decrease when the doping concentration exceeded 2%. It is worth noting that the highest values for both hole conductivity (2.3 × 10⁻³ mS cm⁻¹) and mobility (2.31 × 10⁻² cm² (V·S)⁻¹) are achieved in the NiO_X hole transport layer with a hybrid structure, as shown in Table S3, Supporting Information. This improvement is attributed to forming a homojunction within the hybrid NiO_X, which can further accelerate carrier mobility due to the built-in electric field.

The separation of photogenerated carriers and the resulting V_OC of a solar cell device are primarily determined by the valence band offset between the hole transport layer and the perovskite layer. As such, it is essential to measure the work function. Figure 4c displays the UV–Vis of three different NiO_X-based materials, while Figure 4d depicts the Tauc plots of pristine NiO_X and Ag-doped NiO_X films. These figures revealed that Ag doping increases the bandgap (E_g) of the film from 3.63 to 3.69 eV. Additionally, UPS analysis (Figure S3, Supporting Information) displays that the Fermi level (E_f) of NiO_X and Ag-doped NiO_X declines from 4.57 to 4.66 eV, leading to a higher V_OC. Consequently, Ag doping reduced the valence band level of NiO_X film from −5.40 to −5.46 eV, creating an improved energy level match between Ag-doped NiO_X and the perovskite film, which boosted hole transport and reduced hole accumulation at the interface.^[63] The band structure diagram in Figure 5a displays the calculated conduction band minimum (CBM) and valence band maximum (VBM), which determines the energy levels of the PSC.

Figure 5b displays the energy bands of the functional layers relevant to the effect of the built-in electric field in hybrid NiO_X on the device. Homojunction structures are well-known to improve carrier dynamics at interfaces. However, for both undoped and doped NiO_X-based devices, charge separation driving forces are not outstanding, and there is no clear transport direction for photogenerated carriers, making undesirable electron–hole recombination more likely. Such recombination introduces non-radiative losses, which negatively impact device performance. The figure shows that the calculated VBM of Ag-NiO_X is 0.06 eV lower than undoped NiO_X. Nonetheless, when the hybrid NiO_X structure is stable, the Fermi level of both doped and undoped NiO_X is equivalent, inducing a band bending (Δφ = 0.09 eV) and forming a p/p⁺ homojunction structure. Consequently, from the surface to the bulk, the enhanced built-in electric field facilitates a favorable downward band bending in hybrid NiO_X, providing a driving force for hole transport and promoting charge separation. Upon comparison with the energy level diagram of Ag-doped NiO_X, it becomes evident that the energy level of Ag-doped NiO_X is well-matched with the valence band energy level of the perovskite film, facilitating hole extraction and transport. This advantageous alignment of energy levels reduces charge recombination, ultimately leading to high-efficiency PSCs.

The morphology of the perovskite film is a key factor in determining the quality and performance of PSCs. To investigate the impact of Ag doping on the NiO_X film, we used SEM to examine the surface morphology and size of perovskite films created on undoped NiO_X, Ag-doped NiO_X, and Ag-doped NiO_X/NiO_X films, as depicted in Figure 6a–c. The result shows that the perovskite films, with or without doping, adequately cover the HTL without any significant defects. However, the grain size of the perovskite film deposited on the doped NiO_X-based layer was marginally larger than that of the undoped NiO_X-based layer. The latter finding implied that the larger grain size of the perovskite film was beneficial for transferring both electrons and holes, while uniform and defect-free surface morphology favored the preparation of the upper electron transport layer.^[18] Notably, the perovskite film is grown on the innovative hybrid NiO_X film also had a larger grain size, supporting the view that this hybrid NiO_X film exhibited a positive impact on perovskite film growth (Figure 6d). Furthermore, to investigate the crystallinity of the perovskite film, we performed XRD on the perovskite films grown on the three types of NiO_X-based films mentioned earlier, as displayed in Figure 6e. The findings revealed that the perovskite film has significant peaks at 14.12°, 28.42°, and 31.78°, corresponding to its (110), (220), and (310) crystal planes. The perovskite films grown on Ag-doped NiO_X and hybrid NiO_X exhibited higher crystallinity than those grown on undoped Ag-doped NiO_X, consistent with the SEM results. Meanwhile, the absorption spectra of the perovskite films grown on different NiO_X-based films (Figure 6f) manifest strong light absorption in the ultraviolet to near-infrared wavelength range. Specifically, the perovskite film grown on Ag-doped NiO_X film shows stronger absorption intensity, while the perovskite film grown on hybrid NiO_X shows the most significant absorption intensity due to its higher crystallinity.^[64] Thus, it is reasonable to conclude that incorporating Ag doping and improving the hybrid NiO_X film result in a larger grain size of the perovskite film, which in turn enhances the UV-IR visible light absorption performance of the device.

The fitted double exponential function of the curve is shown in Table S4, Supporting Information, with ${\mathrm{\tau }}_1$ representing fast decay time, ${\mathrm{\tau }}_2$ representing slow decay time, and $A$ representing the coefficient of decay amplitude. The observation shows that Ag doping increases the charge extraction ability of the perovskite film prepared on NiO_X, resulting in a shorter carrier lifetime. Comparing the perovskite films on three different NiO_X bases, the hybrid NiO_X method generated the perovskite film with the shortest carrier lifetime and the strongest hole-extracting ability, indicating faster charge separation and transfer at the perovskite interface, minimizing the possibility of non-radiative recombination caused by defects.^[67] Thus, achieving maximum improvement in the film.

To investigate the effect of Ag-doped NiO_X and hybrid Ag-doped NiO_X as HTLs on the performance of PSCs, inverted PSCs were fabricated with the structure FTO/three types of NiO_X (30 nm)/Cs_0.05 (FA_0.83MA_0.17)_0.95 Pb(I_0.83Br_0.17)₃ (450 nm)/PC₆₁BM (70 nm)/BCP (6 nm)/Ag (50 nm). The PSCs' structure and cross-sectional SEM are depicted in Figure 7c,d. The optimal J–V curves of PSCs with different HTLs are shown in Figure 8a. Moreover, using hybrid NiO_X as the HTL showed the highest short-circuit photocurrent density (J_SC) (21.03 mA cm⁻²) and increased V_OC, FF, and PCE. The increase in V_OC (from 1.10 to 1.13 V) could be attributed to the increase in the valence band and the consequent increase in the built-in electric field, consistent with the results of UPS. Additionally, the device exhibiting hybrid NiO_X as the HTL shows the best FF (76–81%) compared to the control group. This improvement could be attributed to the impact of the built-in electric field and the optimization of the HTL/perovskite film, which promotes charge transfer and extraction at the interface while inhibiting charge recombination. It is worth noting that using hybrid NiO_X as the HTL resulted in the highest PCE (19.25%) among the inverted PSCs, which shows an 18.83% improvement compared with the PSC using undoped NiO_X as the HTL. Table 1 summarizes the parameters of the PSCs using the three types of NiO_X employed as HTLs.

Figure 8b displays the IPCE and J_SC curves for three NiO_X-based PSCs. The J_SC values obtained from the IPCE spectra match those from the J–V test. The IPCE curves of the three PSCs exhibit comparable trends, albeit with varying intensities. The hybrid NiO_X-based PSC demonstrates the highest IPCE spectral intensity of 90% throughout the wavelength range, while Ag-doped NiO_X-based PSC exhibits the second highest IPCE intensity. Furthermore, the hybrid NiO_X-based PSCs display stronger IPCE intensity in the short and long wavelength range than undoped and Ag-doped NiO_X-based PSCs due to the large built-in electric field within the innovative hybrid NiO_X, which significantly reduces interface and bulk recombination. This, in turn, enhanced charge extraction and conversion rate at the Hybrid NiO_X-Perovskite interface, providing suitable conditions for excellent crystallization of the perovskite layer.^[68] Consequently, defects within the perovskite layer were effectively suppressed, thereby improving the IPCE of the PSC.

Figure 8c shows the hysteresis behavior of J–V curves of the three NiO_X devices under different scan directions. The device parameters are listed in Table S5, Supporting Information. Undoped NiO_X-based PSCs exhibit lower J–V curve overlap, unlike Ag-doped NiO_X-based and hybrid NiO_X-based PSCs. This is primarily due to the enhancement of the film conductivity, the reduction of the overall series resistance of the device after Ag-doping, and the introduction of the built-in electric field of hybrid NiO_X. Therefore, the Ag-doping and hybrid NiO_X-based PSCs exhibit higher J–V curve overlap, indicating greater stability and a lower tendency towards hysteresis.

The stability of PSCs is an important benchmark for evaluating the effectiveness of optimization strategies and a decisive factor for achieving commercial production. To investigate the environmental stability of PSCs based on different architectures, the stability of inverted PSCs with pristine NiO_X, Ag-doped NiO_X, and hybrid NiO_X as HTLs was compared at around 30% humidity and 25 °C. As shown in Figure S4, Supporting Information, the stability of the untreated NiO_X-based PSC was the worst, with its efficiency dropping to only 70% of the initial value after 30 days. The Ag-doped NiO_X-based PSC showed relatively stable performance but still exhibited some decay, with an efficiency of only 85% of the initial value. In contrast, the PSC based on hybrid NiO_X as the HTL exhibited the highest stability, with an efficiency maintained at 90% of the initial value after 30 days. As shown in Figure S5, Supporting Information, light stability was observed. In contrast, after 260 h of exposure to a nitrogen environment, the PCE of a hybrid NiO_X-based PSC remained steady at more than half of its initial efficiency. This indicates that this innovative strategy helps improve the interface layer between the HTL and perovskite, enhances the crystallinity of the perovskite film, and thus promotes the stability of PSCs.

3 Conclusions

We provide an innovative route for preparing a high-performance and stable inverted PSC based on Ag-doped NiO_X/NiO_X hybrid HTL via a sol–gel-based method. Compared with pristine NiO_X, the Ag-doped NiO_X film has higher conductivity, hole mobility, and a better-matched energy band structure with the perovskite layer. In addition, the homojunction in the Hybrid NiO_X accelerates the extraction, transport, and reduction of carrier recombination at the interface. This innovative strategy effectively improves the crystallinity and morphology of the perovskite film. Compared with PSCs based on pristine NiO_X (16.20%), the PCE of PSCs based on hybrid NiO_X (19.25%) is 18.83% higher. Moreover, environmental stability has also been greatly improved. In summary, this innovative strategy of using Hybrid NiO_X as the HTL in inverted PSCs provides an effective method for developing and commercializing high-efficiency and stable PSCs.

4 Experimental Section

Conflict of Interest

The authors declare no conflict of interest.