Exploration and Optimization of the Polymer-Modified NiOx Hole Transport Layer for Fabricating Inverted Perovskite Solar Cells

The recombination of charge carriers at the interface between carrier transport layers such as nickel oxide (NiOx) and the perovskite absorber has long been a challenge in perovskite solar cells (PSCs). To address this issue, we introduced a polymer additive poly(vinyl butyral) into NiOx and subjected it to high-temperature annealing to form a void-containing structure. The formation of voids is confirmed to increase light transmittance and surface area of NiOx, which is beneficial for light absorption and carrier separation within PSCs. Experimental results demonstrate that the incorporation of the polymer additive helped to enhance the hole conductivity and carrier extraction of NiOx with a higher Ni3+/Ni2+ ratio. This also optimized the energy levels of NiOx to match with the perovskite to raise the open-circuit voltage to 1.01 V. By incorporating an additional NiOx layer beneath the polymer-modified NiOx, the device efficiency was further increased as verified from the dark current measurement of devices.


Introduction
Perovskite solar cells (PSCs) are widely recognized as one of the most promising photovoltaic technologies in the past decade, owing to their large light absorption coefficients in the visible spectrum, cost-effectiveness, long diffusion length, and facile fabrication [1][2][3][4].Recently, the advent of organometallic halide PSCs has marked a significant advancement in achieving an impressive photovoltaic conversion efficiency (PCE) of 25.7-26.1% [5,6].These achievements make PSCs exceptionally valuable for the upcoming generation of solar energy products.
Inverted PSCs, also known as p-i-n structures, are extensively investigated with the utilization of nickel oxide (NiO x ) as the hole transport layer (HTL) [7,8].Various techniques, including chemical bath deposition [9], the sol-gel method [10], plasma-assisted atomic layer deposition [11], spray pyrolysis [12], and nanoparticle dispersion [13], have been applied for the production of NiO x HTLs.Given the p-type and hole extraction nature of inorganic NiO x , scientists find its widespread use in inverted PSCs, which can be attributed to the existence of Ni vacancies in the lattices accompanying high transmittance in the visible range and environmental stability [3,14].Although NiO x plays a pivotal role of hole extraction and transport in PSCs, there is still room for hole mobility improvement.As a result, the doping process and/or interfacial modification are employed to enhance hole mobility and extraction capabilities of NiO x , thereby reducing carrier recombination and achieving a superior performance of PSCs.To date, the interfacial modification of NiO x films has been implemented through combining NiO x with phthalocyanine or trimercapto-s-triazine trisodium salt [15,16].On the other hand, transition metal doping such as Cu 2+ [17,18], Ag + [19], Co 2+ [20,21], Mn 2+ [22], and Zn 2+ [23,24] have proven their effectiveness in enhancing the hole mobility of NiO x films as well as the photovoltaic performance of corresponding PSCs.
From the viewpoint of the mesoscopic junction in PSCs, there have been several studies concerning mesoporous structures of HTLs to improve charge extraction.Wang et al. reported the incorporation of a mesoscopic NiO layer to facilitate hole collection, enabling it to host the perovskite absorber and prevent the degradation of photovoltaic performance [25].Liu, Shen, and their co-workers successfully utilized electrochemical deposition to form mesoporous NiO x films on FTO glass substrates, reducing carrier recombination and augmenting the photocurrent of devices [26].Chen et al. deposited mesoporous CuGaO 2 on the compact NiO x to form a double-layered HTL, as it effectively extracted holes from the perovskite due to the increased contact area at the HTL/perovskite interface [27].Despite being a promising candidate for hole extraction and transport, surprisingly, there has been limited discussion about the formation of mesoporous NiO x HTLs involving organic polymers for fabricating PSCs.
Herein, we reported the preparation of void-containing NiO x by incorporating poly (vinyl butyral) (PVB) (denoted as p-NiO x ) as the HTL.The mesoporous p-NiO x layer was obtained through high-temperature calcination at 500 • C, effectively enhancing both the transmittance of NiO x and hole transport within PSCs.To comprehensively investigate the impact of p-NiO x as the HTL in the photovoltaic devices, this study also explored the effects of PVB pretreatment on the interface between NiO x and the perovskite layer.Additionally, the original NiO x film (denoted as o-NiO x ) and p-NiO x /o-NiO x films were prepared for comparative analysis.The experimental results reveal that the valence band (VB) of p-NiO x was shifted downwards compared to o-NiO x , which is demonstrated in Section 3.1, resulting in better alignment with the perovskite absorbing layer and a consequent increase in the open-circuit voltage (V OC ) to 1.01 V. Furthermore, incorporating p-NiO x /o-NiO x thin films as the HTL demonstrated superior carrier transport capabilities to ameliorate charge extraction and reduced recombination in photovoltaic devices.While the device based on the o-NiO x HTL exhibited a moderate power conversion efficiency (PCE) of 14.84%, the utilization of the p-NiO x /o-NiO x structure resulted in a significantly improved PSC performance with the highest PCE of 16.46%.

Materials and Methods
Detailed information about the starting materials, preparation of perovskite layers, fabrication of PSCs, and characterization techniques is provided in the Supporting Information.The preparation of the o-NiO x and p-NiO x films is listed as follows.The o-NiO x film was prepared via the sol-gel process.Nickel acetate tetrahydrate (0.124 g), ethanolamine (30 µL), and ethanol (5 mL) were mixed in a sealed glass vial and heated at 70 • C until the solution color became translucent green.For the p-NiO x , 30 mg of PVB powder was added to the nickel acetate precursor solution.The two precursor films were deposited individually on the FTO substrates from their solutions via spin coating at 4500 rpm for 30 s under an ambient environment, followed by drying on a hotplate at 80 • C for 10 min.The substrates were then transferred into a tube furnace, heated from room temperature to 500 • C within 90 min in air, and sintered at the final temperature for 1 h to obtain the o-NiO x and p-NiO x films.Furthermore, a p-NiO x layer was deposited on top of the o-NiO x layer to form a p-NiO x /o-NiO x structure for comparison.

Characterization of the p-NiO x
The surface morphology and thickness of the o-NiO x and p-NiO x films on the FTO substrates were verified via scanning election microscopy (SEM) observation.The o-NiO x film with a thickness of 25 nm is very thin and hence the grains of low-lying FTO are clearly seen, as shown in Figure 1a,c.In Figure 1b,d, the p-NiO x showed uniformly distributed cracks on the surface with a thickness of 25 nm, which is close to that of the o-NiO x .The formation of voids is attributed to the thermal degradation of PVB during the calcination process of NiO x , which is supposed to increase the light transmittance and surface area of the resulting NiO x layer for the subsequent deposition of perovskite layers.Apart from the SEM observation, atomic force microscopy (AFM) experiments were also carried out to investigate the morphological properties and average roughness (R a ) of o-NiO x and p-NiO x films, as displayed in Figure 1e,f.The FTO grains are clearly observed for both samples; moreover, the o-NiO x has a R a value of 14.9 nm, and the p-NiO x possesses a higher R a value of 17.7 nm, possibly due to those cavities formed by the removal of PVB in the hightemperature calcination process [28].Furthermore, X-ray diffraction (XRD) experiments were performed to examine the crystalline phases of the o-NiO x and p-NiO x and the corresponding XRD patterns are revealed in Figure S1 in the Supporting Information.Three diffraction peaks of NiO x are located at 2θ = 38.9,42.5, and 64.5 • in both XRD patterns, which corresponds to (111), (200), and (220) planes, respectively [29,30], confirming that the crystalline phase of the NiO x was not altered by PVB pretreatment.film with a thickness of 25 nm is very thin and hence the grains of low-lying FTO are clearly seen, as shown in Figure 1a,c.In Figure 1b,d, the p-NiOx showed uniformly distributed cracks on the surface with a thickness of 25 nm, which is close to that of the o-NiOx.The formation of voids is attributed to the thermal degradation of PVB during the calcination process of NiOx, which is supposed to increase the light transmittance and surface area of the resulting NiOx layer for the subsequent deposition of perovskite layers.Apart from the SEM observation, atomic force microscopy (AFM) experiments were also carried out to investigate the morphological properties and average roughness (Ra) of o-NiOx and p-NiOx films, as displayed in Figure 1e,f.The FTO grains are clearly observed for both samples; moreover, the o-NiOx has a Ra value of 14.9 nm, and the p-NiOx possesses a higher Ra value of 17.7 nm, possibly due to those cavities formed by the removal of PVB in the high-temperature calcination process [28].Furthermore, X-ray diffraction (XRD) experiments were performed to examine the crystalline phases of the o-NiOx and p-NiOx and the corresponding XRD patterns are revealed in Figure S1 in the Supporting Information.Three diffraction peaks of NiOx are located at 2θ = 38.9,42.5, and 64.5° in both XRD patterns, which corresponds to (111), (200), and (220) planes, respectively [29,30], confirming that the crystalline phase of the NiOx was not altered by PVB pretreatment.The transmission and absorption spectra of the o-NiO x , p-NiO x and p-NiO x /o-NiO x films were measured to verify the effect of surface voids on their optical properties, which are depicted in Figure S2a.The transmittance of the o-NiO x was observed to be ca.65% in the range of 350-700 nm.The p-NiO x film has the highest transmittance of 80-90% in the same visible range due to the existence of surface voids, as observed from SEM observation in Figure 1b.High transmittance is beneficial for incident photons to enter devices and to be absorbed by the perovskite absorbing layer.In addition, the p-NiO x /o-NiO x possesses a lower transmittance of 70-80% in the same range.This is reasonable since an additional NiO x layer was established below the p-NiO x layer.The absorption spectra of the o-NiO x , p-NiO x and p-NiO x /o-NiO x films are also displayed in Figure S2a, which look similar for the three NiO x films.The Tauc plots of different NiO x films are demonstrated in Figure S2b, indicating an optical bandgap of 3.8 eV for the o-NiO x layer and 3.73 eV for the p-NiO x and p-NiO x /o-NiO x films, which is close to the previous reports [29,[31][32][33].
It is well known that the elemental state of Ni 3+ (Ni 2 O 3 species) can provide the nonstoichiometric NiO x with hole transport ability [34,35].Therefore, the X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the effect of PVB pretreatment on the Ni 3+ /Ni 2+ ratio as well as hole transport ability.The Ni 2p 3/2 XPS spectra of the o-NiO x , p-NiO x and p-NiO x /o-NiO x films are displayed in Figure 2a−c.According to the previous literature [14,24,36], the multicomponent bands can be well fitted with three different states, including NiO (Ni 2p 3/2 at 853.8 eV), Ni 2 O 3 (Ni 2p 3/2 at 855.3 eV), and a satellite peak of Ni 3+ (at 856.1 eV).The Ni 3+ /Ni 2+ ratios for the o-NiO x , p-NiO x and p-NiO x /o-NiO x films were calculated to be 2.17, 2.78 and 3.45, respectively, showing an apparent increasing Ni 3+ proportion in the Ni 2p spectra after PVB pretreatment.Thus, the p-NiO x has a better hole-transporting capability than the pristine one [34,37].Until now, the reason for the increased Ni 3+ /Ni 2+ ratio up to 3. The transmission and absorption spectra of the o-NiOx, p-NiOx and p-NiOx/o-NiOx films were measured to verify the effect of surface voids on their optical properties, which are depicted in Figure S2a.The transmittance of the o-NiOx was observed to be ca.65% in the range of 350-700 nm.The p-NiOx film has the highest transmittance of 80-90% in the same visible range due to the existence of surface voids, as observed from SEM observation in Figure 1b.High transmittance is beneficial for incident photons to enter devices and to be absorbed by the perovskite absorbing layer.In addition, the p-NiOx/o-NiOx possesses a lower transmittance of 70-80% in the same range.This is reasonable since an additional NiOx layer was established below the p-NiOx layer.The absorption spectra of the o-NiOx, p-NiOx and p-NiOx/o-NiOx films are also displayed in Figure S2a, which look similar for the three NiOx films.The Tauc plots of different NiOx films are demonstrated in Figure S2b, indicating an optical bandgap of 3.8 eV for the o-NiOx layer and 3.73 eV for the p-NiOx and p-NiOx/o-NiOx films, which is close to the previous reports [29,[31][32][33].
It is well known that the elemental state of Ni 3+ (Ni2O3 species) can provide the nonstoichiometric NiOx with hole transport ability [34,35].Therefore, the X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the effect of PVB pretreatment on the Ni 3+    To further confirm the effect of PVB pretreatment on the single-carrier mobility and conductivity of NiO x , simple devices with three different configurations of FTO/o-NiO x /Ag, FTO/p-NiO x /Ag, and FTO/p-NiO x /o-NiO x /Ag devices were fabricated and their current−voltage (I−V) characteristics are illustrated in Figure 3a.The p-NiO x device possesses a larger slope than the o-NiO x , meaning that PVB pretreatment can improve the conductivity and charge transport ability of NiO x .In addition, the p-NiO x /o-NiO x device has the largest slope, indicative of the highest conductivity which is in accordance with XPS results.After calcination, the augmentation of the Ni 3+ fraction facilitates carrier transport and brings about superior hole conductivity.Subsequently, the hole mobility (µ h ) of these films was approximated from the space charge limited current (SCLC) model defined as follows [38][39][40]: where J is the current density, ε 0 is the vacuum dielectric constant, and ε is the relative dielectric constant of NiO x [41].V is the bias voltage, and L is the thickness of the NiO x film (∼25 nm). Figure 3b displays the electrical characteristics derived with the SCLC model of ln(JL 3 /V 2 ) versus electric filed (V/L) 0.5 .The p-NiO x /o-NiO x structure has the highest µ h of 1.62 × 10 −2 cm 2 /Vs, while the µ h of the o-NiO x and p-NiO x are calculated to be 1.11 × 10 −2 and 1.22 × 10 −2 cm 2 /Vs, respectively.The augmented µ h value of the NiO x HTL is expected to bring on the improvement in PCE and device performance of PSCs [22].
conductivity of NiOx, simple devices with three different configurations of FTO/o-NiOx/Ag, FTO/p-NiOx/Ag, and FTO/p-NiOx/o-NiOx/Ag devices were fabricated and their current−voltage (I−V) characteristics are illustrated in Figure 3a.The p-NiOx device possesses a larger slope than the o-NiOx, meaning that PVB pretreatment can improve the conductivity and charge transport ability of NiOx.In addition, the p-NiOx/o-NiOx device has the largest slope, indicative of the highest conductivity which is in accordance with XPS results.After calcination, the augmentation of the Ni 3+ fraction facilitates carrier transport and brings about superior hole conductivity.Subsequently, the hole mobility (µh) of these films was approximated from the space charge limited current (SCLC) model defined as follows [38][39][40]: where J is the current density, ε0 is the vacuum dielectric constant, and ε is the relative dielectric constant of NiOx [41].V is the bias voltage, and L is the thickness of the NiOx film (∼25 nm). Figure 3b displays the electrical characteristics derived with the SCLC model of ln(JL 3 /V 2 ) versus electric filed (V/L) 0.5 .The p-NiOx/o-NiOx structure has the highest µh of 1.62 × 10 -2 cm 2 /Vs, while the µh of the o-NiOx and p-NiOx are calculated to be 1.11 × 10 -2 and 1.22 × 10 -2 cm 2 /Vs, respectively.The augmented µh value of the NiOx HTL is expected to bring on the improvement in PCE and device performance of PSCs [22].The energy levels and work functions (φw) of the o-NiOx, p-NiOx and p-NiOx/o-NiOx films were implemented via the ultraviolet photoelectron spectroscopy (UPS) analysis.The UPS spectra of different NiOx films in the high-and low-binding energy regions are shown in Figure 4a.The φw can be obtained through subtracting the high-binding energy cutoff (around 17 eV) from the photon energy of the He I source (21.22 eV) [42,43].Therefore, the φw of the o-NiOx, p-NiOx and p-NiOx/o-NiOx films is determined to be 4.06, 4.08, and 3.99 eV, respectively.It is known that the work function represents the energy difference between vacuum energy levels and the Fermi level (EF) [44][45][46].The energy difference between the valence band (VB) level and the φw is associated with the low-binding energy cutoff (around 1 eV) [22].Ergo, the VB of the o-NiOx, p-NiOx and p-NiOx/o-NiOx films were calculated to be −5.2,−5.24 and −5.31 eV, respectively.The energy level diagram of the different NiOx and the perovskite layers is depicted in Figure 4b, which is comparable to the previous literature [11,22,24,25].The alignment of energy levels is crucial for optimizing hole extraction and transport efficiency in PSCs.Reducing The energy levels and work functions (φ w ) of the o-NiO x , p-NiO x and p-NiO x /o-NiO x films were implemented via the ultraviolet photoelectron spectroscopy (UPS) analysis.The UPS spectra of different NiO x films in the high-and low-binding energy regions are shown in Figure 4a.The φ w can be obtained through subtracting the high-binding energy cutoff (around 17 eV) from the photon energy of the He I source (21.22 eV) [42,43].Therefore, the φ w of the o-NiO x , p-NiO x and p-NiO x /o-NiO x films is determined to be 4.06, 4.08, and 3.99 eV, respectively.It is known that the work function represents the energy difference between vacuum energy levels and the Fermi level (E F ) [44][45][46].The energy difference between the valence band (VB) level and the φ w is associated with the low-binding energy cutoff (around 1 eV) [22].Ergo, the VB of the o-NiO x , p-NiO x and p-NiO x /o-NiO x films were calculated to be −5.2,−5.24 and −5.31 eV, respectively.The energy level diagram of the different NiO x and the perovskite layers is depicted in Figure 4b, which is comparable to the previous literature [11,22,24,25].The alignment of energy levels is crucial for optimizing hole extraction and transport efficiency in PSCs.Reducing the energy barrier between the perovskite layer and HTL would decrease the energy loss during charge transport [27].The p-NiO x /o-NiO x exhibits an obviously downshifted VB level which aligns well with the perovskite layer (VB = −5.4eV), meaning that better hole extraction can be achieved using PVB pretreatment and consequently a higher V OC is anticipated [18].
the energy barrier between the perovskite layer and HTL would decrease the energy loss during charge transport [27].The p-NiOx/o-NiOx exhibits an obviously downshifted VB level which aligns well with the perovskite layer (VB = −5.4eV), meaning that better hole extraction can be achieved using PVB pretreatment and consequently a higher VOC is anticipated [18].

Characterization of Perovskite Layers on NiOx
To analyze the crystallinity and topography of perovskite layers on different NiOx HTLs, the XRD and SEM experiments were conducted.The corresponding XRD patterns and top-view SEM images of perovskite layers are provided in Figures S3 and S4.Several intense diffraction peaks at 2θ = 13.95°,19.86°, 24.58°, 28.33°, 31.82°,34.91°, 40.51°, and 43.12° were found, corresponding to the (001), ( 011), ( 111), (002), ( 012), ( 112), (022), and (003) planes of the perovskite, respectively, which are consistent with the previous literature [47][48][49].Furthermore, the perovskite grains on the o-NiOx, p-NiOx and p-NiOx/o-NiOx films appear similar in Figure S4.It is known that the NiOx films remained unaltered after PVB pretreatment (see XRD patterns Figure S1) and PVB was removed during the calcination process, and likewise, there would be no significant change in the morphological structure of the perovskite.To conclude, the XRD patterns and top-view SEM images of perovskites on the three NiOx HTLs look similar, implying that the p-NiOx

Characterization of Perovskite Layers on NiO x
To analyze the crystallinity and topography of perovskite layers on different NiO x HTLs, the XRD and SEM experiments were conducted.The corresponding XRD patterns and top-view SEM images of perovskite layers are provided in Figures S3 and S4.Several intense diffraction peaks at 2θ = 13.95  , and 43.12 • were found, corresponding to the (001), (011), (111), (002), (012), (112), (022), and (003) planes of the perovskite, respectively, which are consistent with the previous literature [47][48][49].Furthermore, the perovskite grains on the o-NiO x , p-NiO x and p-NiO x /o-NiO x films appear similar in Figure S4.It is known that the NiO x films remained unaltered after PVB pretreatment (see XRD patterns Figure S1) and PVB was removed during the calcination process, and likewise, there would be no significant change in the morphological structure of the perovskite.To conclude, the XRD patterns and top-view SEM images of perovskites on the three NiO x HTLs look similar, implying that the p-NiO x and p-NiO x /o-NiO x structures have little or no effect on the crystalline property and morphology of the perovskite.
Figure 5a displays the steady-state photoluminescence (PL) spectra of the perovskites deposited on the FTO, o-NiO x , p-NiO x , and p-NiO x /o-NiO x films.It can be seen that the perovskite deposited on the FTO substrate has the highest PL emission, while the one on the o-NiO x has a lower PL intensity.According to the previous literature, the decrease in PL intensity means an enhanced charge extraction and transport from the perovskite layer to the HTL [18,22].It seems odd that the perovskite on the p-NiO x possesses the second strongest PL intensity.It is conjectured that the existence of voids led to direct contact between the perovskite and FTO substrate to reduce the carrier extraction ability of NiO x .At the same time, the perovskite on the p-NiO x /o-NiO x structure has the lowest PL emission, bringing about the improved photovoltaic performance of PSCs.To further verify the PL results of perovskite films on different NiO x films, the time-resolved PL (TR-PL) decay experiments were carried out and the corresponding PL decay curves are shown in Figure 5b.It is seen that the perovskite coated on the p-NiO x /o-NiO x structure possessed the fastest PL decay curve compared with other NiO x films, implying that the hole-electron separation was accomplished more effectively [17].The TR-PL decay curves were fitted using a biexponential model; the fast decay constant τ 1 and slow decay constant τ 2 represent the surface recombination and charge recombination in the perovskite bulk, respectively [50,51].Then, the average carrier lifetime (τ avg ) was estimated from the equation τ avg = ∑ A i τ 2 i /∑(A i τ i ) , where A i and τ i were deduced from the data of the fitted curve [52][53][54].All the acquired decay constants τ 1 , τ 2 and τ ayg are summarized in Table S1 in the Supplementary Information.The τ avg was calculated to be 84.13,45.25, 53.72 and 31.14 ns for the perovskite layers on the FTO, o-NiO x , p-NiO x , and p-NiO x /o-NiO x films, respectively.Since the carrier lifetime is inversely proportional to charge extraction, the p-NiO x /o-NiO x structure has the best charge extraction capability among all NiO x films, suggesting the highest device performance of PSCs [11,19].
and p-NiOx/o-NiOx structures have little or no effect on the crystalline property and morphology of the perovskite.
Figure 5a displays the steady-state photoluminescence (PL) spectra of the perovskites deposited on the FTO, o-NiOx, p-NiOx, and p-NiOx/o-NiOx films.It can be seen that the perovskite deposited on the FTO substrate has the highest PL emission, while the one on the o-NiOx has a lower PL intensity.According to the previous literature, the decrease in PL intensity means an enhanced charge extraction and transport from the perovskite layer to the HTL [18,22].It seems odd that the perovskite on the p-NiOx possesses the second strongest PL intensity.It is conjectured that the existence of voids led to direct contact between the perovskite and FTO substrate to reduce the carrier extraction ability of NiOx.At the same time, the perovskite on the p-NiOx/o-NiOx structure has the lowest PL emission, bringing about the improved photovoltaic performance of PSCs.To further verify the PL results of perovskite films on different NiOx films, the time-resolved PL (TR-PL) decay experiments were carried out and the corresponding PL decay curves are shown in Figure 5b.It is seen that the perovskite coated on the p-NiOx/o-NiOx structure possessed the fastest PL decay curve compared with other NiOx films, implying that the hole-electron separation was accomplished more effectively [17].The TR-PL decay curves were fitted using a biexponential model; the fast decay constant τ1 and slow decay constant τ2 represent the surface recombination and charge recombination in the perovskite bulk, respectively [50,51].Then, the average carrier lifetime (τavg) was estimated from the equation  = ∑   ∑   , where Ai and τi were deduced from the data of the fitted curve [52][53][54].All the acquired decay constants τ1, τ2 and τayg are summarized in Table S1 in the Supplementary Information.The τavg was calculated to be 84.

Device Evaluation
The

Device Evaluation
The planar p-i-n PSCs with the architecture of FTO/o-NiO x , p-NiO x or p-NiO x / o-NiO x /perovskite/PC 61 BM+TBABF 4 /PEI/Ag were fabricated and evaluated in this study.The cross-sectional SEM image of the whole device is presented in Figure S5 to estimate the thickness of each layer.A thickness of 25, 550, 40, 20 and 100 nm is obtained for the p-NiO x , perovskite, TBABF 4 -doped PCBM, and the PEI and Ag electrode, respectively.The thickness of the p-NiO x /o-NiO x is approximately double that of the p-NiO x layer.Figure 6a presents the current density−voltage (J−V) curves of PSCs based on the o-NiO x , p-NiO x or p-NiO x /o-NiO x structures as the HTL under AM 1.5G illumination, and Table 1 summarizes the photovoltaic parameters of all devices including J SC , V OC , FF, and PCE.The control device using the o-NiO x HTL displayed a moderate PCE of 14.8%, a J SC of 22.7 mA/cm 2 , a V OC of 0.9 V, and an FF value of 72%.The best photovoltaic performance was achieved from the device using the p-NiO x /o-NiO x HTL, revealing a PCE of 16.5% which is significantly higher than other devices in this study.The J SC , V OC and FF of the device based on the p-NiO x /o-NiO x HTL were measured to be 21.5 mA/cm 2 , 1.01 V, and 75%, respectively.As for the device using the p-NiO x HTL, the J SC , V OC , FF, and PCE are 21.0 mA/cm 2 , 1.01 V, 66%, and 14.2%, respectively.Figure S6 depicts the statistical distribution for J SC , V OC , FF and PCE from 20 individual devices.To realize the hysteresis effect, the J-V curves of devices were measured in the reverse and forward scans and corresponding results are displayed in Figure S7 and Table S2.The hysteresis index (HI) is calculated using the equation HI = (PCE reverse − PCE forward )/PCE reverse , and the device based on p-NiO x /o-NiO x has the smallest HI value of 0.09, indicating that the hysteresis phenomenon is reduced through using the p-NiO x /o-NiO x bilayered structure as the HTL.Our PSCs maintained good reproducibility and the device based on the p-NiO x /o-NiO x HTL showed relatively higher photovoltaic parameters.The improvement in the device performance can be interpreted from several aspects.As previously discussed in the XPS section, the p-NiO x /o-NiO x has the largest Ni 3+ /Ni 2+ ratio and hole transport ability, leading to the enhanced efficiency of PSCs.In the discussion of UPS experiments, the p-NiO x /o-NiO x exhibits the smallest φ w as well as matched energy level with the perovskite absorbing layer, thereby facilitating hole extraction from the perovskite to the HTL.Furthermore, the electrical measurements of the p-NiO x /o-NiO x device indicate an elevated µ h which is beneficial for the carrier transport and PCE of devices.Considering the above aspects, the device using the p-NiO x /o-NiO x HTL exhibited the best performance as anticipated.To validate the leakage current of devices, dark current measurements were performed and the corresponding results are displayed in Figure 6b.As mentioned in the previous parts, we assumed that using the p-NiO x HTL would encounter an issue of void formation, which could be verified using dark current measurements.The reverse currents from low to high belong to the devices using p-NiO x /NiO x , o-NiO x , and p-NiO x as the HTL.It is evident that the PSC using p-NiO x has a larger leakage current than that using o-NiO x as the HTL.While the PSC based on p-NiO x /NiO x possesses the lowest dark current, it conveys benefits for reducing recombination loss and enhancing carrier transport [48,55].According to the previous literature [56,57], the values of the series resistance (R s ) and shunt resistance (R sh ) of PSCs can be determined from the voltage dependence of the differential resistance (R diff ) using the equation R diff = ∆ V/ ∆ J, as displayed in Figure S8.The R s is determined using the extrapolation of the saturated part of the R diff −V curve toward the interception with the resistance axis.The R sh is equal to the differential resistance at a bias of 0 V.It is concluded that the device based on the p-NiO x /o-NiO x HTL has the largest R sh value of 8.35 kΩcm 2 among the three PSCs, indicative of the best device performance.Figure 6c shows the integrated current densities and external quantum efficiency (EQE) spectra of devices using o-NiO x , p-NiO x , and p-NiO x /NiO x as the HTL.The results attest that the EQE maximum of the device using p-NiO x /NiO x achieved about 79% at 550 nm, being the highest spectral line across the visible range.Furthermore, the integrated current densities of 19.17, 17.7, 19.58 mA/cm 2 were obtained from the devices based on the o-NiO x , p-NiO x , and p-NiO x /NiO x HTLs, respectively.To explore long-term stability, the unencapsulated PSCs were stored in the nitrogen glovebox and their J-V characteristics under AM 1.5G exposure were measured in ambient air. Figure 6d records the PCE evolution of the PSCs based on the o-NiO x , p-NiO x , and p-NiO x /NiO x HTLs.All devices maintained about 70% of their initial efficiency over a period of 50 days.It is noted that the PCE of the fresh PSC based on the p-NiO x /NiO x HTL was 16.4% and it dropped to 13% after 50 days of storage, remaining the best performance among the three devices.

Figure 1 .
Figure 1.Top-view and cross-sectional SEM images of the (a,c) o-NiOx and (b,d) p-NiOx thin films deposited on the FTO substrates; AFM topographic images of the (e) o-NiOx and (f) p-NiOx thin films.

Figure 1 .
Figure 1.Top-view and cross-sectional SEM images of the (a,c) o-NiO x and (b,d) p-NiO x thin films deposited on the FTO substrates; AFM topographic images of the (e) o-NiO x and (f) p-NiO x thin films.
45 for the p-NiO x /o-NiO x remains unclear and more experiments should be implemented, such as electrical measurements of hole-only devices.The O 1s XPS spectra of the o-NiO x , p-NiO x and p-NiO x /o-NiO x films are presented in Figure 2d−f, revealing two prominent peaks at around 529 eV (O 2-from NiO) and 531 eV (O 2-from Ni 2 O 3 ) [14,36].In addition, the O 2-peak from NiO shifted from 529.08 (o-NiO x ) to 528.73 (p-NiO x ) and 528.63 eV (p-NiO x /o-NiO x ), implying possible interactions between PVB and NiO x via electronic transfer.
/Ni 2+ ratio as well as hole transport ability.The Ni 2p3/2 XPS spectra of the o-NiOx, p-NiOx and p-NiOx/o-NiOx films are displayed in Figure 2a−c.According to the previous literature [14,24,36], the multicomponent bands can be well fitted with three different states, including NiO (Ni 2p3/2 at 853.8 eV), Ni2O3 (Ni 2p3/2 at 855.3 eV), and a satellite peak of Ni 3+ (at 856.1 eV).The Ni 3+ /Ni 2+ ratios for the o-NiOx, p-NiOx and p-NiOx/o-NiOx films were calculated to be 2.17, 2.78 and 3.45, respectively, showing an apparent increasing Ni 3+ proportion in the Ni 2p spectra after PVB pretreatment.Thus, the p-NiOx has a better hole-transporting capability than the pristine one [34,37].Until now, the reason for the increased Ni 3+ /Ni 2+ ratio up to 3.45 for the p-NiOx/o-NiOx remains unclear and more experiments should be implemented, such as electrical measurements of hole-only devices.The O 1s XPS spectra of the o-NiOx, p-NiOx and p-NiOx/o-NiOx films are presented in Figure 2d−f, revealing two prominent peaks at around 529 eV (O 2-from NiO) and 531 eV (O 2-from Ni2O3) [14,36].In addition, the O 2-peak from NiO shifted from 529.08 (o-NiOx) to 528.73 (p-NiOx) and 528.63 eV (p-NiOx/o-NiOx), implying possible interactions between PVB and NiOx via electronic transfer.

Figure 2 .
Figure 2. XPS spectra of (a-c) Ni 2p 3/2 and (d-f) O 1s elements in the o-NiO x , p-NiO x and p-NiO x /o-NiO x .

Figure 3 .
Figure 3. (a) Linear sweep voltammetry curves of devices based on the o-NiO x , p-NiO x and p-NiO x /o-NiO x films; (b) hole mobility of the o-NiO x , p-NiO x and p-NiO x /o-NiO x films versus electric field (V/L) 0.5 .

Figure 4 .
Figure 4. (a) UPS spectra of the o-NiO x , p-NiO x , and p-NiO x /o-NiO x films; (b) energy level diagram of the whole device (unit: eV).
13, 45.25,  53.72 and 31.14 ns for the perovskite layers on the FTO, o-NiOx, p-NiOx, and p-NiOx/o-NiOx films, respectively.Since the carrier lifetime is inversely proportional to charge extraction, the p-NiOx/o-NiOx structure has the best charge extraction capability among all NiOx films, suggesting the highest device performance of PSCs[11,19].
planar p-i-n PSCs with the architecture of FTO/o-NiOx, p-NiOx or p-NiOx/o-NiOx/perovskite/PC61BM+TBABF4/PEI/Ag were fabricated and evaluated in this study.The cross-sectional SEM image of the whole device is presented in Figure S5 to estimate the thickness of each layer.A thickness of 25, 550, 40, 20 and 100 nm is obtained for the p-NiOx, perovskite, TBABF4-doped PCBM, and the PEI and Ag electrode, respectively.The thickness of the p-NiOx/o-NiOx is approximately double that of the p-NiOx layer.Figure 6a presents the current density−voltage (J−V) curves of PSCs based on the o-NiOx, p-NiOx or p-NiOx/o-NiOx structures as the HTL under AM 1.5G illumination, andTable 1 summarizes the photovoltaic parameters of all devices including JSC, VOC, FF, and

Figure 5 .
Figure 5. (a) PL emission spectra and (b) TR-PL decay curves of the perovskites on the FTO, o-NiO x , p-NiO x and p-NiO x /o-NiO x films.