Combustion Processed Nickel Oxide and Zinc Doped Nickel Oxide Thin Films as a Hole Transport Layer for Perovskite Solar Cells

: Combustion processed nickel oxide (NiO x ) thin ﬁlm is considered as an alternative to the sol-gel processed hole transport layer for perovskite solar cells (PSCs). In this paper, NiO x thin ﬁlm was prepared by the solution–combustion process at 250 ◦ C, a temperature lower than the actual reaction temperature. Furthermore, the properties of the NiO x hole transport layer (HTL) in PSCs were enhanced by the incorporation of zinc (Zn) in NiO x thin ﬁlms. X-ray diffraction and X-ray photoelectron spectroscopy results revealed that the formation of NiO x was achieved at lower annealing temperature, which conﬁrms the process of the combustion reaction. The electrical conductivity was greatly improved with Zn doping into the NiO x crystal lattice. Better photoluminescence (PL) quenching, and low PL lifetime decay were responsible for better charge separation in 5% Zn doped NiO x , which results in improved device performance of PSCs. The maximum power conversion efﬁciency of inverted PSCs made with pristine NiO x and 5% Zn-NiO x as the HTL was 13.62% and 14.87%, respectively. Both the devices exhibited better stability than the PEDOT:PSS (control) device in an ambient condition. C.-H.T. P.S.T.; writing—review C.-H.T. P.S.T.; supervision, C.-H.T.; administration, C.-H.T.; acquisition, C.-H.T.


Introduction
Researchers have promised that the perovskite solar cells (PSCs) could soon be released as a commercial product in the next few years [1,2]. PSCs have become a hot topic among researchers because of their superior optoelectronic properties [3][4][5][6][7]. The ease of device fabrication kept this device in the limelight among other next-generation solar cells [8]. In inverted PSCs, the most commonly used HTL is poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS); it can exhibit better device performance at lower processing temperature, but the stability of the device is worse because of its hygroscopic and acidic nature [9]. Replacing the organic PEDOT:PSS as hole transport layer (HTL) in perovskite solar cells with inorganic p-type metal oxide materials is an effective way to improve the device's performance and stability [10]. The basic requirement of the HTL is to be highly transparent throughout the solar spectra, good electrical conductivity, and a high work function for effective charge transport [11]. Among the various metal oxides, nickel oxide (NiO x ) is an ideal candidate for this special purpose [11][12][13][14][15]. In physical vapor deposition techniques, these metal oxides require special processing conditions, including a vacuum environment and heat treatment, etc. [16]. In contrast, solution-processed NiO x has a variety of options that does not require any special processing conditions [17]. The preparation of the NiO x HTL by the sol-gel method requires it to be annealed at a high temperature above 300 • C to achieve high performance from the PSCs [18,19]. This hightemperature process increases the cost of device fabrication at an industrial scale and placed in the glovebox. Following this, production of a perovskite active layer occurred by dissolving 1.4 M methylammonium iodide (MAI) and 1.4 M lead iodide (PbI 2 ) in γbutyrolactone (GBL) and dimethyl sulfoxide (DMSO) (7:3). This solution was then subjected to overnight stirring inside the glove box at 60 • C. A 0.45-µm PTFE syringe filter was used to filter the solution before deposition. The solution underwent spin coating onto FTO/NiO x substrates (1000 rpm/15 s and 5000 rpm/25 s). Toluene was employed as an anti-solvent and was added precisely five seconds prior to completing the second stage. Following this, the substrates were annealed for 10 min at 100 • C. Preparation of the electron transport layer (ETL) was achieved in the following manner: 20 mg/mL of phenyl-C61-butyric acid methyl ester (PCBM) in 1,4-dichlorobenzene (DCB) underwent spin-coating onto the substrates at 2000 rpm for 30 s, and a saturated solution of bathocuproine (BCP) in methanol was dynamically spin-coated at 6000 rpm for 30 s. Lastly, thermal evaporation of an Ag electrode was achieved employing a metal mask.

Characterization
Characterization of the NiO x and Zn-NiO x thin films was undertaken implying X-ray diffraction (XRD) techniques using a Rigaku D/Max-2500V system (Rigaku Corp, Tokyo, Japan). The surface morphology and topography underwent inspection using a field emission scanning electron microscope (FESEM) (morphology) (JEOL JSM-7000F, JEOL Inc., Tokyo, Japan) and an atomic force microscope (AFM) (AutoProbe CP, Thermomicroscopes, Sunnyvale, CA, USA) (topography). Elemental composition analysis was undertaken using X-ray photoelectron spectroscopy (XPS) (Thermo K-Alpha, Thermo Fisher Scientific, Inc., Waltham, MA, USA). Measurement of UV-visible absorption and transmission spectra was undertaken employing a UV-Vis spectrophotometer (Hitachi U 3900, Tokyo, Japan). Measurements of steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) were undertaken employing a solid-state laser (λ = 405 nm, PDL 800-D, PICOQUANT, Berlin, Germany) equipped with a laser scanning confocal microscope and a spectrometer equipped with a thermoelectrically cooled charge-coupled detector (for PL) and a time-correlated single-photon counting detector (TCSPC) (for TRPL). Analysis of the PSC device performance was undertaken employing current density-voltage (J-V) curves, external quantum efficiency (EQE), and electrochemical impedance spectroscopy (EIS). Measurement of PSC J-V characteristics was undertaken using a 1 sun illumination (AM 1.5 G spectrum) employing a 550 W Xenon lamp solar simulator (Sun 3000 Class AAA, Abet Technologies, Milford, MA, USA).

Results and Discussion
As mentioned, the NiO x and Zn doped NiO x thin films were prepared using the combustion spin-coating technique, and the films were annealed at a lower temperature (250 • C) than the sol-gel technique. The inverted PSCs with an architecture of FTO/NiO x /MAPbI 3 /PCBM/BCP/Ag were fabricated, as shown in Figure 1a. Figure 1b Figure 2a. The device made with 5% Zn-NiO x exhibited the best PCE of 14.87% versus pristine NiO x (13.62%). The detailed device parameters of the PSCs made with both the NiO x and Zn doped NiO x thin films annealed at 250 • C are summarized in Table 1. The results were reproducible and the average values were calculated from eight best devices from different batches. The devices performed well with NiO x HTL annealed at a relatively lower temperature. The current density (Jsc) of the pristine NiO x device was improved with an increase in Zn doping and may be due to an improvement in the electrical conductivity of HTL. The lower doping concentration of 2% Zn-NiO x exhibited relatively similar efficiency with a minimal increase in Jsc. The device made with 5% Zn-NiO x showed improved Jsc and minimal reduction in FF and Voc. There was no significant change in Voc perhaps because there were no major changes in valence band edge position after Zn doping with lower concentration. The Voc and FF were decreased with further increasing of Zn concentration (10%) into NiO x HTL. It may be due to the higher Zn 2+ dopant concentration induce the transformation of p-type to n-type NiO x , which results in poor device performance [36]. The J-V characteristics revealed that the current flow across the device was increased due to the Zn doping into NiO x . The external quantum efficiency (EQE) spectra of the devices made using NiO x and Zn doped NiO x HTL are shown in Figure 2b. The devices made with NiO x and Zn doped NiO x films displayed satisfactory results throughout the entire visible region. In comparison, a similar kind of EQE curve was observed in all the devices. The increment in the EQE value for the 5% Zn-NiO x device versus the pristine NiO x device indicates the improvement in device performance. These results agreed with the J-V characteristics results. Therefore, the pristine NiO x and 5% Zn-NiO x were chosen for further detailed studies.
NiOx HTL are shown in Figure 2b. The devices made with NiOx and Zn doped NiOx films displayed satisfactory results throughout the entire visible region. In comparison, a similar kind of EQE curve was observed in all the devices. The increment in the EQE value for the 5% Zn-NiOx device versus the pristine NiOx device indicates the improvement in device performance. These results agreed with the J-V characteristics results. Therefore, the pristine NiOx and 5% Zn-NiOx were chosen for further detailed studies.   Apart from the performance provided by the HTL, it has a strong impact on the stability of PSCs. This study performed an investigation on the stability of unencapsulated devices stored in dark conditions at room temperature with about 40% of relative humidity. Figure 3a,b show the stability graph and the photograph of PSCs based on PEDOT:PSS, NiOx, and 5% Zn-NiOx HTLs, respectively. It can be seen that the device made with PEDOT:PSS degrades fast and lost its total efficiency in 4 days. The fast  Apart from the performance provided by the HTL, it has a strong impact on the stability of PSCs. This study performed an investigation on the stability of unencapsulated devices stored in dark conditions at room temperature with about 40% of relative humidity. Figure 3a,b show the stability graph and the photograph of PSCs based on PEDOT:PSS, NiO x , and 5% Zn-NiO x HTLs, respectively. It can be seen that the device made with PEDOT:PSS degrades fast and lost its total efficiency in 4 days. The fast degradation of perovskite happened at the active area of PEDOT:PSS device was due to the reaction of Ag electrodes with volatile degradation products from the perovskite and ion migration. The acidic and hygroscopic nature of PEDOT:PSS degrades the MAPbI 3 into PbI 2 and releases its by-products in the form of I 2 , CH 3 I, and HI, which can readily react with Ag to form AgI [37]. By contrast, the rate of degradation is usually very slow in NiO x based PSCs. Both NiO x and 5% Zn-NiO x based PSCs have retained 80% of their initial efficiency after 108 h.   The XRD pattern of NiO x and 5% Zn-NiO x thin films on glass substrate and annealed at 250 • C is shown in Figure 4. The cubic structure of NiO has peaked at 37.3 • , 43.3 • , and 62.9 • , corresponding to the hkl value of (111), (002), and (022), respectively. This can be observed in both NiO x and 5% Zn-NiO x thin films. It clearly shows that thin films had Coatings 2021, 11, 627 6 of 14 poor polycrystalline nature but significantly higher crystallinity than the film annealed at 250 • C in the sol-gel process in previous work [38], and similar kinds of results were reported [29,33]. This result shows that the formation of NiO x occurred below the actual reaction temperatures. It confirms the process of the combustion reaction.  X-ray photoelectron spectroscopy (XPS) was used to analyze the element composition of the NiOx and 5% Zn-NiOx thin films annealed at 250 °C. Figure 5a illustrate the XPS spectra for the Ni 2p and O 1s core levels of NiOx and 5% Zn-NiOx th films. The NiOx peaks primarily comprise Ni 2+ and Ni 3+ states. At 861 eV, there is a satell peak attributable to the NiOx shake-up process. The deconvoluted peak at 854 e corresponds with NiO (Ni 2+ ), 856 eV with Ni2O3 (Ni 3+ ), and 857 eV with NiOOH [3 Similarly, the O 1s spectra has three peaks, the deconvoluted peak at 529 e corresponding with NiO (Ni 2+ ), 531 eV with Ni2O3 (Ni 3+ ), and 533 eV with NiOO Generally, Ni 3+ states cause p-type conductivity, indicating that quasi-localized holes ha been formed around the lattice position's Ni 2+ vacancies [40]. There is a higher Ni 3+ sta with 5% Zn-NiOx versus pristine NiOx as seen in the spectra of Ni 2p and O 1s. This w due to the replacement of the Ni atom with Zn in the crystal lattice. Zn incorporation cou alter the electronic structure of NiOx. The increase in the Ni 3+ state denotes the presence more Ni vacancies. The higher Ni 3+ /Ni 2+ ratio in 5% Zn-NiOx indicates the reduction ionization energy of the Ni vacancies and increased hole density [41]. The successf X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental composition of the NiO x and 5% Zn-NiO x thin films annealed at 250 • C. Figure 5a,b illustrate the XPS spectra for the Ni 2p and O 1s core levels of NiO x and 5% Zn-NiO x thin films. The NiO x peaks primarily comprise Ni 2+ and Ni 3+ states. At 861 eV, there is a satellite peak attributable to the NiO x shake-up process. The deconvoluted peak at 854 eV corresponds with NiO (Ni 2+ ), 856 eV with Ni 2 O 3 (Ni 3+ ), and 857 eV with NiOOH [39]. Similarly, the O 1s spectra has three peaks, the deconvoluted peak at 529 eV corresponding with NiO (Ni 2+ ), 531 eV with Ni 2 O 3 (Ni 3+ ), and 533 eV with NiOOH. Generally, Ni 3+ states cause p-type conductivity, indicating that quasi-localized holes have been formed around the lattice position's Ni 2+ vacancies [40]. There is a higher Ni 3+ state with 5% Zn-NiO x versus pristine NiO x as seen in the spectra of Ni 2p and O 1s. This was due to the replacement of the Ni atom with Zn in the crystal lattice. Zn incorporation could alter the electronic structure of NiO x . The increase in the Ni 3+ state denotes the presence of more Ni vacancies. The higher Ni 3+ /Ni 2+ ratio in 5% Zn-NiO x indicates the reduction in ionization energy of the Ni vacancies and increased hole density [41]. The successful incorporation of Zn in NiO x can be seen in Figure 5c,d. The divalent state of Zn peaks of 2p 3/2 and 2p 1/2 were observed at 1021 eV and 1045 eV, respectively.  The surface morphology of NiOx and 5% Zn-NiOx thin films annealed at 250 °C was analyzed by field emission scanning electron microscopy (FESEM). The top view of the FESEM image and energy-dispersive X-ray spectroscopy (EDS) results for NiOx and 5% Zn-NiOx thin films are shown in Figure 6a,b, respectively. Both films exhibit similar morphology and better coverage without any voids on FTO substrate. A fine layer of these thin films is clearly seen on the FTO, and the 5% Zn doped NiOx film looks somewhat smoother than the pristine film. This observation was confirmed by AFM results. EDS results confirmed the presence of Zn in NiOx thin films. The atomic composition ratio was closely related to stoichiometry. Figure 6c shows the cross-sectional FESEM image of the device.
The device was fabricated with an architecture of FTO/NiOx/MAPbI3/PCBM/BCP/Ag. All layers can be well distinguished from the crosssectional FESEM image. The surface morphology of NiO x and 5% Zn-NiO x thin films annealed at 250 • C was analyzed by field emission scanning electron microscopy (FESEM). The top view of the FESEM image and energy-dispersive X-ray spectroscopy (EDS) results for NiO x and 5% Zn-NiO x thin films are shown in Figure 6a,b, respectively. Both films exhibit similar morphology and better coverage without any voids on FTO substrate. A fine layer of these thin films is clearly seen on the FTO, and the 5% Zn doped NiO x film looks somewhat smoother than the pristine film. This observation was confirmed by AFM results. EDS results confirmed the presence of Zn in NiO x thin films. The atomic composition ratio was closely related to stoichiometry. Figure 6c shows the cross-sectional FESEM image of the device. The device was fabricated with an architecture of FTO/NiO x /MAPbI 3 /PCBM/BCP/Ag. All layers can be well distinguished from the cross-sectional FESEM image.  Figure 7a,b show the AFM images of NiOx and 5% Zn-NiOx thin films anneal 250 °C. The films exhibited similar smooth and compact surface topography on substrates for both NiOx and 5% Zn-NiOx. The root-mean-squared (RMS) rough values of NiOx and 5% Zn-NiOx are 37.1 nm and 33.9 nm, respectively. The roughne the film decreased with Zn doping because of the tendency of ZnO to form a crysta nature at the lower annealing temperature [42]. This result is well-matched with literature reports [41,43]. Figure 8a illustrates the transmittance spectra for NiOx and 5% Zn-NiOx thin that underwent annealing at 250 °C. Each film showed a high transmittance level w the visible spectrum range, with average values being above 80%. Figure 8b illustrate graph of absorption coefficient as a function of excitation energy; calculation of the op bandgap value was undertaken through an additional plot of the linear portion of (  of NiO x and 5% Zn-NiO x are 37.1 nm and 33.9 nm, respectively. The roughness of the film decreased with Zn doping because of the tendency of ZnO to form a crystalline nature at the lower annealing temperature [42]. This result is well-matched with other literature reports [41,43].  The impact of Zn doping on NiOx thin-film electrical conductivity was examined by measuring the current-voltage (I-V) characteristics of the hole-only device (FTO/NiOx/Ag). Figure 9 shows the I-V curves for the hole-only devices based on NiOx and 5% Zn-NiOx thin films that underwent annealing at 250 °C. This demonstrates that there are improvements in electrical conductivity when Zn doping is applied as the dopants have a high level of affinity with electrons.  Figure 8a illustrates the transmittance spectra for NiO x and 5% Zn-NiO x thin films that underwent annealing at 250 • C. Each film showed a high transmittance level within the visible spectrum range, with average values being above 80%. Figure 8b illustrates the graph of absorption coefficient as a function of excitation energy; calculation of the optical bandgap value was undertaken through an additional plot of the linear portion of (αhν) 2 to the x-axis employing a Tauc plot [44]. This bandgap value (E g ) was around 3.95 eV both for NiO x and 5% Zn-NiO x thin films. Very high HTL transmittance is required for minimization of incident light loss for enhancement of the light that reaches the perovskite layer. Any variations in optical properties when Zn is incorporated into NiO x are minimal due to lower film thicknesses (~20 nm).  The impact of Zn doping on NiOx thin-film electrical conductivity was examined measuring the current-voltage (I-V) characteristics of the hole-only dev (FTO/NiOx/Ag). Figure 9 shows the I-V curves for the hole-only devices based on N  The impact of Zn doping on NiO x thin-film electrical conductivity was examined by measuring the current-voltage (I-V) characteristics of the hole-only device (FTO/NiO x /Ag). Figure 9 shows the I-V curves for the hole-only devices based on NiO x and 5% Zn-NiO x thin films that underwent annealing at 250 • C. This demonstrates that there are improvements in electrical conductivity when Zn doping is applied as the dopants have a high level of affinity with electrons.
Coatings 2021, 11, 627 Figure 9. I-V characteristics of hole-only devices made with NiOx and 5% Zn-NiOx thin film Charge extraction and non-radiative recombination at the perovskite an interface was evaluated using steady-state photoluminescence (PL) and time-r photoluminescence (TRPL). Figure 10a,b show the PL spectrum and TRPL pro perovskite films over NiOx and 5% Zn-NiOx thin films. The offset in the valence ban plays a major role in charge recombination. Mismatches in the valence band edge b HTL and perovskite layer leads to more recombination at the interface and re lowering of the Voc [45]. PL exhibited by the perovskite layer was significantly r by the HTLs. Noteworthy levels of PL quenching were shown with both types demonstrating high levels of efficiency of charge extraction for Zn doped NiOx thi These findings accord with previous research [43]. The TRPL profile was me employing time-correlated single-photon counting. Calculation of the PL lifetim was undertaken using tail fit with a biexponential decay function, as shown in E (1), where τ1 represents the photogenerated excitons diffusing into defects, representing intrinsic electron-hole recombination. The formula illustrated in Equa was employed for the calculation of average PL lifetime values [46]; a detailed su of these values can be found in Table 2. Charge extraction and non-radiative recombination at the perovskite and HTL interface was evaluated using steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL). Figure 10a,b show the PL spectrum and TRPL profile for perovskite films over NiO x and 5% Zn-NiO x thin films. The offset in the valence band edge plays a major role in charge recombination. Mismatches in the valence band edge between HTL and perovskite layer leads to more recombination at the interface and results in lowering of the Voc [45]. PL exhibited by the perovskite layer was significantly reduced by the HTLs. Noteworthy levels of PL quenching were shown with both types of film, demonstrating high levels of efficiency of charge extraction for Zn doped NiO x thin films. These findings accord with previous research [43]. The TRPL profile was measured employing timecorrelated single-photon counting. Calculation of the PL lifetime decay was undertaken using tail fit with a biexponential decay function, as shown in Equation (1), where τ 1 represents the photogenerated excitons diffusing into defects, and τ 2 representing intrinsic electron-hole recombination. The formula illustrated in Equation (2) was employed for the calculation of average PL lifetime values [46]; a detailed summary of these values can be found in Table 2. representing intrinsic electron-hole recombination. The formula illustrated in Equation (2) was employed for the calculation of average PL lifetime values [46]; a detailed summary of these values can be found in Table 2.  This research employed EIS measurements to undertake further verification of NiO x and 5% Zn-NiO x thin film-based devices' charge transport properties. Figure 11 illustrates the Nyquist plots for devices fabricated using NiO x and 5% Zn-NiO x thin films annealed at 250 • C. EIS measurement was taken at one solar illumination using a frequency range from 10 Hz to 1 MHz with an AC amplitude of 0.01 V, and providing a bias voltage that matched the open-circuit voltage. The impedance curve was observed at three frequency regions where the high-frequency (HF) regime correlates with charge transport resistance (Rct) at the HTL/perovskite interface, mid-frequency (MF) region is related to charge recombination, and low-frequency (LF) has been associated to iodide ion modulated recombination/injection process [47]. The curves were fitted with the equivalent circuit and the calculated Rct values are~60.38 Ω and 45.62 Ω for NiO x and 5% Zn-NiO x devices, respectively. Typically, low impedance values are expected for better carrier transport functions [48,49]. The device fabricated using Zn doped NiO x had lower impedance values than those fabricated using pristine NiO x . It is clear that interface charge accumulation is suppressed, thus, improving exciton charge separation. Improved charge separation leads to increases in EQE and Jsc values. These findings clearly indicate that charge transport at the HTL-perovskite interfaces can be improved by doping NiO x with Zn, leading to a general improvement in the performance of NiO x HTL.  This research employed EIS measurements to undertake further verification of NiOx and 5% Zn-NiOx thin film-based devices' charge transport properties. Figure 11 illustrates the Nyquist plots for devices fabricated using NiOx and 5% Zn-NiOx thin films annealed at 250 °C. EIS measurement was taken at one solar illumination using a frequency range from 10 Hz to 1 MHz with an AC amplitude of 0.01 V, and providing a bias voltage that matched the open-circuit voltage. The impedance curve was observed at three frequency regions where the high-frequency (HF) regime correlates with charge transport resistance (Rct) at the HTL/perovskite interface, mid-frequency (MF) region is related to charge recombination, and low-frequency (LF) has been associated to iodide ion modulated recombination/injection process [47]. The curves were fitted with the equivalent circuit and the calculated Rct values are ~60.38 Ω and 45.62 Ω for NiOx and 5% Zn-NiOx devices, respectively. Typically, low impedance values are expected for better carrier transport functions [48,49]. The device fabricated using Zn doped NiOx had lower impedance values than those fabricated using pristine NiOx. It is clear that interface charge accumulation is suppressed, thus, improving exciton charge separation. Improved charge separation leads to increases in EQE and Jsc values. These findings clearly indicate that charge transport at the HTL-perovskite interfaces can be improved by doping NiOx with Zn, leading to a general improvement in the performance of NiOx HTL. Figure 11. EIS spectra of PSCs with NiOx and 5% Zn-NiOx HTL.

Conclusions
In this work, we investigated the effect of extrinsic Zn doping in NiOx HTL in inverted PSCs by the combustion technique. Comparatively, the devices made with both pristine and Zn doped NiOx films performed well with the annealing temperature of 250

Conclusions
In this work, we investigated the effect of extrinsic Zn doping in NiO x HTL in inverted PSCs by the combustion technique. Comparatively, the devices made with both pristine and Zn doped NiO x films performed well with the annealing temperature of 250 • C, showing average PCE values of 13.62% and 14.87%, respectively. Both the devices have exhibited better stability than the PEDOT:PSS (control) device in an ambient condition. The crystallinity of both pristine and Zn doped NiO x films is better than the sol-gel derived thin film annealed at 250 • C. The XPS results revealed that the presence of compound formation below its reaction temperature and intensity of the Ni 3+ peak (Ni vacancy) was higher in Zn doped NiO x than the NiO x film. The presence of Zn in NiO x was confirmed in XPS but not observed in XRD. Both films exhibited similar optical transparency, and the effect of Zn incorporation on optical properties is negligible. The electrical conductivity was greatly improved with Zn doping into the NiO x crystal lattice. The better PL quenching and low PL lifetime decay responsible for better charge separation were observed in Zn doped NiO x . A small impedance value from the Zn doped device showed less transport charge resistance than the pristine one. Overall, the NiO x thin films made by the combustion technique were the films annealed at 250 • C, which can work as HTL in inverted PSCs. The performance of combustion processed NiO x HTL was improved by Zn doping, showing better efficiency and overall superior performance than pristine NiO x films.