High‐Performance and Large‐Area Inverted Perovskite Solar Cells Based on NiOx Films Enabled with A Novel Microstructure‐Control Technology

The improvement in the efficiency of inverted perovskite solar cells (PSCs) is significantly limited by undesirable contact at the NiOX/perovskite interface. In this study, a novel microstructure‐control technology is proposed for fabrication of porous NiOX films using Pluronic P123 as the structure‐directing agent and acetylacetone (AcAc) as the coordination agent. The synthesized porous NiOX films enhanced the hole extraction efficiency and reduced recombination defects at the NiOX/perovskite interface. Consequently, without any modification, the power conversion efficiency (PCE) of the PSC with MAPbI3 as the absorber layer improved from 16.50% to 19.08%. Moreover, the PCE of the device composed of perovskite Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3 improved from 17.49% to 21.42%. Furthermore, the application of the fabricated porous NiOX on fluorine‐doped tin oxide (FTO) substrates enabled the fabrication of large‐area PSCs (1.2 cm2) with a PCE of 19.63%. This study provides a novel strategy for improving the contact at the NiOX/perovskite interface for the fabrication of high‐performance large‐area perovskite solar cells.

NiO X films is developed using a copolymer-and ligand-assisted assembly process.The pluronic P123, a symmetric triblock copolymer comprising poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) in an alternating linear fashion (PEO-b-PPO-b-PEO), is used as a structure-directing agent, and the strong ligand acetylacetone (AcAc) is used as a coordination agent to regulate the condensation of the NiO X precursor, which may inhibit the formation of non-uniform pore distribution in the fabricated NiO X film.Based on the characterization performed, the application of the designed and fabricated porous NiO X enhanced the hole extraction capability of the HTL and restricted defect formation at the NiO X /perovskite interface.Consequently, the PSC performance dramatically improved, and devices (0.1 cm2 ) composed of MAPbI 3 perovskite and Cs 0.05 (MA 0.15- FA 0.85 ) 0.95 Pb(I 0.85 Br 0.15 ) 3 "triple cation" perovskite as the absorber layer achieved PCEs of 19.08% and 21.42%, respectively.Additionally, large-area PSCs (1.2 cm 2 ) achieved a PCE of 19.63% when the fabricated porous NiO X was used.

Results and Discussion
The fabrication procedure and formation mechanism of porous NiO X are shown in Figure 1.Briefly, Ni(NO 3 ) 2 Á6H 2 O was used as the NiO X precursor, ethanol as the solvent, triblock copolymer P123 as the structure-directing agent, and strong ligand AcAc to regulate the hydrolysis and condensation of the precursor.After spin-coating the precursor solution, the films were annealed at 450 °C for 1 h.The detailed formation mechanism is presented in the subsequent discussion with the characterization results.
X-ray diffraction (XRD) and Raman measurements were performed to detect the fabricated NiO x .Figure 2a shows the XRD pattern of the NiO X powder obtained from the porous NiO X precursor solution.Three main characteristic diffraction peaks are observed at 37.2°, 43.4°, and 62.8°, which can be assigned to the (111), (200), and (220) lattice planes of cubic NiO X , respectively.No impurity diffraction peaks are detected, such as the typical impurity peaks of the insulating nickel salt or other intermediate products. [27,28]The fabricated porous NiO X film was also characterized using XRD and Raman spectroscopy (shown in Figures S1 and S2, Supporting Information), and the results agree well with the abovementioned analysis.These results confirm the formation of NiO X , and that there is no precursor residue in the porous NiO X film fabricated under the investigated conditions.
Thermogravimetric analysis (TGA) of the precursor solution was performed to gain an in-depth understanding of the deposition process.As shown in Figure 2b (and Figures S3 and S4, Supporting Information), the precursor decomposition process mainly consists of two stages.During the initial stage, at temperatures below 250 °C, P123 decomposes to form basic nickel nitrates.The nickel nitrates do not condense rapidly owing to presence of AcAc.This prevents degradation of the microporous structure at this stage.[31] During the fabrication process, AcAc plays a critical role in the formation of the ideal NiO X porous structure.Fourier transform infrared (FT-IR) analysis of pure AcAc and the precursor solutions with and without AcAc was performed to demonstrate the chemical action of AcAc in the precursor solution.As shown in Figure 2c, pure AcAc exhibited a peak at approximately 1710 cm À1 , which is ascribed to the stretching vibration of C=O.In contrast, the corresponding peak for the precursor was red-shifted to 1701 cm À1 .[34][35] This coordination interaction prevents rapid hydrolysis and condensation of the NiO X precursor, which typically leads to framework reorganization combined with shrinkage and degradation of the microstructure. [35,36]he top-right inset in Figure 2c shows the top-view scanning electron microscopy (SEM) micrograph of porous NiO X prepared using the precursor without AcAc.A complete SEM micrograph is shown in Figure S4, Supporting Information.The fabricated porous NiO X exhibited a non-uniform pore distribution, which reduces the contact area between NiO X and the perovskite and thereby increases the interfacial defect density.
To achieve an ideal NiO X HTL and circumvent the direct contact of the perovskite with the fluorine-doped tin oxide (FTO) layer to form a porous structure, a compact NiO X film (Com-NiO X ) deposited on an FTO substrate was first prepared.The morphology is shown in Figure 2d.Thereafter, the fabricated porous NiO X film was deposited onto a Com-NiO X layer using a precursor solution containing AcAc as an additive.Hereafter, the composite layer (Com-NiOx + Por-NiOx) with a porous surface is abbreviated as Por-NiO X .As shown in Figure 2e, Por-NiO X clearly exhibits a uniform pore distribution, which provides channels for the diffusion of the perovskite precursor and photogenerated charge carriers.
The contact angles of water with the Com-NiO X and Por-NiO X surfaces were Energy Environ.Mater.2024, 7, e12504 measured to further investigate the effect of Por-NiO X on the surface properties of the HTL.As shown in Figure 2f, the NiO X films enhances the hydrophilicity of the FTO substrate.This facilitates complete coverage of the perovskite films and a further decrease in the contact angle from 43.7°to 29.6°when using the fabricated porous NiO X on Com-NiO X .Therefore, the more hydrophilic surface results in a uniform and conformal perovskite layer, and thereby an ideal contact at the NiO X /perovskite interface is achieved. [20,37]he properties of Com-NiO X and Por-NiO X were also compared using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements.Figure 3a,b show the XPS spectra of the Ni 2p 3/2 core level for the Com-NiO X and Por-NiO X films, respectively.The spectra of both films show the same characteristic peak positions, with the main peak at 852 eV (Ni 2+ ), shoulder peak at 854 eV (Ni 3+ ), and satellite peak at 860 eV.This suggests the formation of conductive NiO X films in both cases.Additionally, the Ni 3+ /Ni 2+ ratios of the Com-NiO X and Por-NiO X films were estimated using the integrated peak areas, and the Por-NiO X film show a higher Ni 3+ /Ni 2+ ratio (1.97) than the Com-NiO X film (1.79).The increased Ni 3+ /Ni 2+ ratio indicates the enhanced conductivity and hole charge transfer capability of the Por-NiO X film. [38,39]We believe the increased Ni 3+ /Ni 2+ is mainly attributed to the larger surface area of Por-NiO X structure, which can guarantee the NiO X films were annealed better in the sintering process.
As shown in Figure 3c (and Figure S5, Supporting Information), UPS analysis was conducted to determine the valence band maximum (VBM) and Fermi energy level (E f ) of the Com-NiO X and Por-NiO X films.Compared to the E f of the Com-NiO X film (À4.74 eV) that of the Por-NiO X film shifts downwards to À4.96 eV. Figure 3d shows the energy-level diagram of the devices.The VBM values are calculated to be À5.14 eV for the Com-NiO X film and À5.33 eV for the Por-NiO X film.41] For a typical inverted PSC device, a perovskite layer is deposited onto a NiO X film.Therefore, the surface properties of NiO X significantly influence the crystallinity and morphology of the perovskite layers.SEM analysis (Figure 4) and XRD and ultraviolet-visible (UV-vis) analyses (Figure 5) were performed to evaluate the effect of different NiO X HTLs on perovskite films.
Scanning electron microscopy analysis was used to investigate the morphology of the perovskite film grown on different NiO X films.As shown in Figure 4a,b, the perovskite film grown on Por-NiO X has a more uniform and larger grain size (average diameter: 413 nm) than the perovskite film grown on Com-NiO X (average diameter: 324 nm).Cross-sectional SEM micrographs are also obtained (shown in Figure 4e,f), which agree well with the analysis presented above.Energy Environ.Mater.2024, 7, e12504 X-ray diffraction analysis was used to further analyze the crystallinity of the perovskite films deposited on the different NiO X layers.As shown in Figure 5a, three main diffraction peaks are detected at 14.21°, 28.61°, and 32.01°, which can be ascribed to the (110), (220), and (310) crystal planes of the CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskite, respectively.The perovskite films grown on Por-NiO X exhibited higher diffraction peak intensities, indicating higher crystallinity of the perovskite layer.
The UV-Vis absorbance spectra of perovskite films grown on the different NiO X films are also recorded, as shown in Figure 5b.The perovskite grown on Por-NiO X exhibits an enhanced absorption ability over a wider wavelength range.According to previous reports, this phenomenon can be attributed to the improved crystal quality of the corresponding perovskite film, which agrees with the XRD results. [5,34]hus, the application of the porous NiO X film significantly enhances the crystallinity of perovskite films.
Photoluminescence (PL) and time-resolved PL (TRPL) measurements were conducted to investigate the charge extraction capability of Com-NiO X and Por-NiO X , as shown in Figure 5c,d, respectively.Compared with the perovskite grown on pristine FTO, the PLs of the perovskites grown on the Com-NiO X and Por-NiO X films are significantly quenched.Moreover, Por-NiO X shows the best PL quenching efficiency, indicating excellent charge extraction and transport.TRPL measurements exhibited a similar trend.The average PL lifetimes of the perovskites grown on FTO, Com-NiO X , and Por-NiO X films are 76, 53, and 34 ns, respectively.We also carried out the PL and TRPL measurements for the devices based on the glass substrate.The same trend was observed in Figure S6, Supporting Information.Although the perovskite based on Por-NiO X exhibited better crystallinity, which means the lower DOS in the perovskite, the corresponding PL was quenched more quickly due to the better charge extraction capability of Por-NiO X .These results demonstrate that the application of Por-NiO X leads to rapid extraction of photogenerated charge carriers in the perovskite layers, which should be attributed to the larger contact area, enhanced mobility, and better energy alignment. [19,42,43]evices with an FTO/NiO X /CH 3 NH 3 PbI 3 /C 60 /BCP/Ag architecture were fabricated to evaluate the PSC performance of different NiO X hole transport layers, as shown in Figure 6a.The J-V curves recorded for the optimal PSC devices based on the Com-NiO X and Por-NiO X films are shown in Figure 6b.The Por-NiO X -based PSC device achieve an optimum PCE of 19.08% with negligible hysteresis, whereas the Com-NiO X -based PSC device achieve an optimum PCE of 16.50% with significant hysteresis.The main photovoltaic parameters of the different PSCs are listed in Table 1.The statistical PCE distributions for the Com-NiO X -and Por-NiO X -based PSC devices are shown in Figure S7, Supporting Information.The higher average PCE and narrower distribution of the Por-NiO X -based PSC device confirm the reproducibility of the results obtained in this study.The enhanced photovoltaic performance is also confirmed by the steady-state output and external quantum efficiency (EQE) measurements.The Por-NiO X -based PSC device exhibit a stable PCE output of 18.52% compared with that of the Com-NiO Xbased PSC device, which is significantly reduced to approximately 16.27%, as shown in Figure 6c.Moreover, the EQE results (shown in Figure S8, Supporting Information) exhibit a similar short-circuit current (J SC ) value to that determined by J-V measurements, confirming the reliability of the results obtained in this study. [44,45]n addition to the PCE, long-term stability is another major concern for the commercialization of PSCs. [34]Stability tests were conducted by storing encapsulated PSC devices under ambient conditions and monitoring the corresponding PCE performance, and the results are shown in Figure 6d.The Por-NiO X -based PSC device maintains a PCE performance of 95% and 75% relative to the initial value after 6:00 and 12:00 h, respectively.However, for the PSC device based on Com-NiO X , the PCE performance decreased to 80% and 50% under the same  conditions.The changes in the appearance of the PSC devices after storage for 18:00 h were also recorded.As shown in the insets of Figure 6d, the compact PSC device based on Com-NiO X becomes yellow in appearance after 18:00 h.This indicates that the corresponding perovskite films are almost completely degraded, and the PCE performance of the corresponding PSC device is not detected.Conversely, the Por-NiO X -based PSC device shows a minimal change in its appearance and had a 60% PCE output after 18:00 h.
Space-charge-limited current (SCLC) curves were recorded to compare the trap densities of the perovskite films based on the Com-NiO X and Por-NiO X films, as shown in Figure 7a.A linear relationship is observed between the current and voltage at a lower voltage bias.When the applied voltage exceeds the inflection-point voltage, a rapid increase in the current occurred, indicating the trap states are fully occupied.Consequently, the applied voltage at the inflection point is defined as the trap-filled limit voltage (V TFL ).The PSC device based on Por-NiO X shows a V TFL of 0.66, which is much lower than that of the Com-NiO X-based PSC device (V TFL = 0.87).48] Electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate carrier recombination in the PSC devices, as shown in Figure 7b.The PSC device based on Por-NiO X had a larger semicircle diameter than that of the Com-NiO X -based PSC device.A larger semicircle diameter indicates a higher recombination resistance in the device. [21,24,38]Therefore, these results suggest that Por-NiO Xbased PSC devices have fewer interfacial defects than those based on Com-NiO X .
Transient photovoltage (TPV) and transient photocurrent (TPC) measurements were conducted to further investigate the recombination and charge-transfer dynamics of the PSC devices, as shown in Figure 7c.A short laser pulse was applied to monitor the TPV response of different PSC devices.The Por-NiO X -based PSC device shows a slower photovoltage decay (lifetime of 1.8 ms) than the Com-NiO X -based PSC device (lifetime of 1.1 ms).The observed voltage decay is solely attributed to the internal recombination of the PSC device because the TPV measurement is performed under open-circuit conditions.Therefore, a longer lifetime indicates the PSC device has fewer recombination defects, which is consistent with the above mentioned characterizations. [23]he trend in TPC decay is shown in Figure 7d.The average decay time is calculated to be 0.79 ls for the Por-NiO X -based PSC device and 1.09 ls for the Com-NiO X -based device.The TPC measurement was performed under short-circuit conditions, considering only the charge that passes through the selective layer before being extracted at the electrode.Therefore, a shorter decay time indicates a better capability of the PSC device based on Por-NiO X for photogenerated carrier collection.It is well known that the recombination rate and charge extraction capability of PSCs are highly correlated with the J SC , V OC , and fill factor (FF) performances. [22,33]o verify the wide applicability of the developed method, a perovskite consisting of Cs 0.05 (MA 0.15 FA 0.85 ) 0.95 Pb(I 0.85 Br 0.15 ) 3 was used to fabricate PSCs.As shown in Figure 8a-c, the optimal PSC device based on Por-NiO X exhibits a PCE of 21.42% and a relatively stable steady-state output of approximately 21.05%.In contrast, the PSC device based on Com-NiO X achieves an optimum PCE of 17.49%, with a lower steady-state output of approximately 17.02%.Moreover,

Conclusion
A new strategy for improving the NiO X /perovskite interfacial contact of PSCs using microstructure-control technology is developed in this study.A Por-NiO X HTL with a uniform porous surface structure is successfully fabricated using copolymer-and ligand-assisted processes.
Based on the characterization results of the HTLs, perovskite layers, and their corresponding PSC devices, Por-NiO X enhances the hole extraction efficiency and restricted recombination defects at the NiO X /perovskite interface.Consequently, Por-NiO X allowes the PSC device composed of an MAPbI 3 absorber layer to achieve a relatively high PCE of 19.08% and improved its stability, compared with a control device.Moreover, the application of the Por-NiO X film improves the optimal PCE of the PSC device with Cs 0.05 (MA 0.15 FA 0.85 ) 0.95 Pb(I 0.85 Br 0.15 ) 3 as the absorber (21.42%) without any other interface modification, compared with that of the PSC device based on the Com-NiO X film (17.49%).Notably, the large-area PSC based on the Por-NiO X film also exhibits an excellent efficiency of 19.63%, demonstrating the wide applicability of the developed method.This study provides critical insights into strategies for improving the contact at the NiO X /perovskite interface and presents a significant route for fabricating high-performance large-area PSCs.

Figure 1 .
Figure 1.Process flow diagram of the fabrication of porous NiO X for application in PSCs based on the use of the pluronic P123 (PEO-b-PPO-b-PEO).

Figure 2 .
Figure 2. a) X-ray diffraction pattern of NiO X and b) TGA curve of the Por-NiO X precursor solution.c) FT-IR spectra of the all-component precursor (red), pure AcAc (blue), and the precursor without AcAc (purple).The top-right inset is the scanning electron microscopy (SEM) micrograph of porous NiO X fabricated using the precursor without AcAc.Top-view SEM micrographs of the d) Com-NiO X and e) Por-NiO X layers.f) Contact angle measurements of the surfaces of fluorine-doped tin oxide (FTO), Com-NiO X , and Por-NiO X .

Figure 3 .
Figure 3. X-ray photoelectron spectroscopy spectrum for Ni 2p of a) Com-NiO X and b) Por-NiO X layers.c) UPS spectra of Com-NiO X (black) and Por-NiO X (red).d) Energy-level diagram for the devices.

Figure 4 .
Figure 4. Top-view scanning electron microscopy (SEM) micrographs of perovskite films deposited on a) Com-NiO X and b) Por-NiO X .Statistical grain-size distributions of perovskite deposited on c) Com-NiO X and d) Por-NiO X .Cross-sectional SEM images of e) Com-NiO X and f) Por-NiO X layers with corresponding perovskite films.

Figure 5 .
Figure 5. a) X-ray diffraction (XRD) data and b) UV-Vis absorption spectra of perovskite films deposited on Com-NiO X (black) and Por-NiO X (red).c) Steady-state PL and d) time-resolved PL decay spectra of perovskite films deposited on fluorine-doped tin oxide (purple), Com-NiO X (black), and Por-NiO X (red).

Figure 6 .
Figure 6.a) Sketch of perovskite solar cell (PSC) architecture grown on Por-NiO X .b) J-V curves of the Com-NiO X -(black) and Por-NiO X -based (red) PSCs fabricated using MAPbI 3 .c) Steady-state output of the Com-NiO X -and Por-NiO X -based PSC devices under continuous simulated AM 1.5G illumination.d) Long-term stability testing of the Com-NiO X -and Por-NiO Xbased PSC devices, stored at a relative humidity of 30 AE 5% and temperature of 25 °C for 90 days without encapsulation.
large-area PSCs (1.2 cm 2 ) based on Por-NiO X were fabricated, as shown in Figure S9, Supporting Information.The large-area PSC device achieves a maximum PCE of 19.63%, as shown in Figure 8d.The detailed parameters of the photovoltage performance of the PSC devices based on the Cs 0.05 (MA 0.15 FA 0.85 ) 0.95 Pb(I 0.85 Br 0.15 ) 3 perovskite are listed in Table 1.The statistical PCE distributions of these PSC devices are shown in Figure S10, Supporting Information, which exhibits the same trend as expected.The results obtained for the PSC devices based on the Cs 0.05 (MA 0.15 FA 0.85 ) 0.95 Pb(I 0.85 Br 0.15 ) 3 perovskite demonstrate that the application of Por-NiO X significantly improves the contact at the NiO X /perovskite interface, enhances interfacial charge extraction, and restricts defect-induced recombination in PSC devices.These results highlight the excellent reproducibility and practical application potential of the microstructure-control technology approach for fabricating PSCs with improved efficiency.

Table 1 .
Photovoltaic parameters of the optimal perovskite solar cell devices based on Com-NiO X and Por-NiO X .