Preprint of "Nickel Oxide Hole Transport Layer for Perovskite Solar Cells: Preparation via Pulsed Laser Deposition with Simulation and Experimental Insights"

Vacuum Nickel Oxide Hole Transport Layer for Perovskite Solar Cells: Preparation via Pulsed Laser Deposition with Simulation and Experimental Insights --Manuscript Draft-- Manuscript Number: VAC-D-24-03186R4 Article Type: VSI: EVC-17/ECOSS-37 Keywords: Pulsed laser deposition; nickel oxide; Perovskite solar cells; hole transport layer Corresponding Author: Eva Horynova Czech Technical University in Prague Faculty of Electrical Engineering Prague, Czech Republic CZECH REPUBLIC First Author: Eva Horynova Order of Authors: Eva Horynova Jakub Holovsky Neda Neykova Lucie Landova Naini Jain Abhinav Deep Pakki Meng-Hsueh Kuo Ivana Beshajova Pelikanova Ognen Pop-Georgievski Amalraj Peter Amalathas Branislav Dzurňák Lukáš Horák Ivana Beshajová Pelikánová Abstract: Nickel oxide (NiOx) has gained attention as a promising inorganic hole transport layer for perovskite solar cells due to its wide bandgap, high transparency, and stability. However, tuning of band alignment by an extra dipole layer is necessary to achieve high efficiencies. Our predictive simulations suggest that NiOx bandgap tuning can also improve solar cell performance. Motivated by these findings, this study experimentally investigates NiOx films with different bandgap fabricated using pulsed laser deposition under varying deposition conditions, including oxygen pressure, substrate temperature and laser frequency. Our outcomes show that mainly the deposition temperature significantly influences the chemical composition, optical properties, and defect states in the NiOx films, lattice constants and morphology as confirmed by X-ray photoelectron spectroscopy, photothermal deflection spectroscopy, X-ray diffraction spectroscopy, atomic force microscopy and scanning electron microscopy. Experimentally, FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide perovskite solar cells were fabricated on NiOx substrates prepared under varying oxygen pressures and pulse numbers, achieving a maximum power conversion efficiency of approximately 8%. This demonstrates that NiOx deposited by pulsed laser deposition, when properly tuned, is a promising candidate for an efficient hole transport layer in perovskite-based photovoltaics. Response to Reviewers: Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Manuscript Number: VAC-D-24-03186R3 Nickel Oxide Hole Transport Layer for Perovskite Solar Cells: Preparation via Pulsed Laser Deposition with Simulation and Experimental Insights Dear Ms. Horynova, Thank you for submitting your manuscript to Vacuum. I have completed my evaluation of your manuscript. The reviewers recommend reconsideration of your manuscript following major revision. I invite you to resubmit your manuscript after addressing the comments below. Please resubmit your revised manuscript by Jun 30, 2025. When revising your manuscript, please consider all issues mentioned in the reviewers' comments carefully: please outline every change made in response to their comments and provide suitable rebuttals for any comments not addressed. Please note that your revised submission may need to be rereviewed. NOTE: Upon submitting your revised manuscript, please upload the source files for your article. We cannot accommodate PDF manuscript files for production purposes. We also ask that when submitting your revision, you follow the journal formatting guidelines. For additional details regarding acceptable file formats, please refer to the Guide for Authors at: http://www.elsevier.com/journals/vacuum/0042207x/guide-for-authors To submit your revised manuscript, please log in as an author at https://www.editorialmanager.com/vac/, and navigate to the "Submissions Needing Revision" folder. Vacuum values your contribution and I look forward to receiving your revised manuscript. Kind regards, Dr Oleg Malyshev Associate Editor Vacuum Response to Reviewers Response to the Editor and Reviewer comments: Reviewer #3: Dear authors, Unfortunately I cannot agree with the changes you have made. Perhaps my questions/comments were not fully clear. 1) SEM: You agree that "It doesn't directly provide crystallographic information like crystal structure or orientation." but you haven't changed the terminology. The grain size is a crystallographic term and has a very specific meaning. So, you cannot proof that the surface structure you observe are really grains. Hence, you should change the terminology. Answer: We thank the reviewer for this insightful comment. We agree that the term “grain size” implies specific crystallographic meaning, which cannot be directly concluded from SEM analysis alone. In response, we have revised the manuscript and replaced all instances of “grain size” with “nanorod diameter” to more accurately reflect the morphological information obtained from SEM images. We believe this change improves the clarity and precision of the terminology used. 2) Figure 6: You state "As for the presented data - unfortunately, we do not have enough data to do an experimental error, but we saw the same slight dependence on temperature for different settings of PLD, for example, when different numbers of pulses were used." It is a pity that you don't have the experimental error, but then the conclusion drawn from this figure cannot be made. One cannot, on a statistical base, state that you don't have the error, but still can draw statistically sound conclusions from the plot. This is indeed an unpleasant conclusion, but that is the cruel world of statistics. Answer: We thank the reviewer for this important observation. We fully agree that, without a proper error analysis or statistical treatment, definitive conclusions cannot be drawn from the data in Figure 6. Our intention was to provide a qualitative discussion based on repeated experimental trends we observed under different conditions, not to present a statistically rigorous result. In light of the reviewer’s comment, we have revised the relevant text in the manuscript to clearly indicate that the observed trend is a preliminary observation rather than a statistically validated conclusion. We have also removed any language that may imply statistical certainty. We appreciate the reviewer’s critical reminder regarding the importance of statistical rigor in data interpretation. ● NiOx thin films with varying bandgap were fabricated via PLD and thoroughly characterized. ● Strong effects of pressure, temperature, and frequency were observed. ● Effects of NiOx bandgap on PSCs were demonstrated experimentally. ● PSC with PLD-grown NiOx hole transport layer achieved a power conversion efficiency of about 8%. ● PLD-tuned NiOx shows potential for improved solar cell performance. Highlights figS1 Click here to access/download;Figure;freq_A.png figS1 Click here to access/download;Figure;freq_R.png figS1 Click here to access/download;Figure;freq_T.png figS1 Click here to access/download;Figure;press_A.png figS1 Click here to access/download;Figure;press_R.png figS2 Click here to access/download;Figure;tauc_i_temp.png figS2 Click here to access/download;Figure;tauc_d_freq.png figS2 Click here to access/download;Figure;tauc_d_press.png figS2 Click here to access/download;Figure;tauc_d_temp.png Figure Click here to access/download;Figure;Fig1.tif Figure Click here to access/download;Figure;Fig2A.jpg Click here to access/download;Figure;Fig4C.tif Click here to access/download;Figure;Fig2B.jpg Click here to access/download;Figure;Fig2C.jpg Click here to access/download;Figure;Fig3A.jpg Click here to access/download;Figure;Fig6.png Click here to access/download;Figure;Fig7.png Click here to access/download;Figure;Fig8.png Click here to access/download;Figure;Fig9.png Click here to access/download;Figure;Fig10.png Click here to access/download;Figure;Fig11A.png Click here to access/download;Figure;Fig11B.png Click here to access/download;Figure;Fig11C.png Click here to access/download;Figure;Fig12.png Click here to access/download;Figure;Fig13A.png 1 Nickel Oxide Hole Transport Layer for Perovskite Solar Cells: Preparation via Pulsed Laser Deposition with Simulation and Experimental Insights Eva Horynova1, Jakub Holovsky1,2, Lucie Landova1,2, Naini Jain1, Abhinav Deep Pakki1, Meng-Hsueh Kuo1,2, Ivana Beshajová Pelikánová1, Branislav Dzurňák1, Lukáš Horák3, Ognen Pop-Georgievski4, Amalraj Peter Amalathas1,5 and Neda Neykova1,2 1 Centre for Advanced Photovoltaics, Faculty of Electrical Engineering, CTU in Prague, Technická 2, 166 27, Prague, Czech Republic 2 Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Prague, Czech Republic 3 Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Prague, Ke Karlovu 2026/5, 121 16 Praha 2, Czech Republic 4 Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovskeho nam. 2, 162 00 Prague, Czech Republic 5 Department of Physics, Faculty of Science, University of Jaffna, Jaffna 40000, Sri Lanka Corresponding author: Eva Horynova, [email protected] Abstract Nickel oxide (NiOx) has gained attention as a promising inorganic hole transport layer for perovskite solar cells due to its wide bandgap, high transparency, and stability. However, tuning of band alignment by an extra dipole layer is necessary to achieve high efficiencies. Our predictive simulations suggest that NiOₓ bandgap tuning can also improve solar cell performance. Motivated by these findings, this study experimentally investigates NiOₓ films with different bandgap fabricated using pulsed laser deposition under varying deposition conditions, including oxygen pressure, substrate temperature and laser frequency. Our outcomes show that mainly the deposition temperature significantly influences the chemical composition, optical properties, and defect states in the NiOx films, lattice constants and morphology as confirmed by X-ray photoelectron spectroscopy, photothermal deflection spectroscopy, X-ray diffraction spectroscopy, atomic force microscopy and scanning electron microscopy. Experimentally, FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide perovskite solar cells were fabricated on NiOx substrates prepared under varying oxygen pressures and pulse numbers, achieving a maximum power conversion efficiency of approximately 8%. This demonstrates that NiOx deposited by pulsed laser deposition, when properly tuned, is a promising candidate for an efficient hole transport layer in perovskite-based photovoltaics. Keywords Pulsed laser deposition, nickel oxide, perovskite solar cells, hole transport layer Highlights ● NiOx thin films with varying bandgap were fabricated via PLD and thoroughly characterized. ● Strong effects of pressure, temperature, and frequency were observed. Revised manuscript file with changes marked 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 ● Effects of NiOx bandgap on PSCs were demonstrated experimentally. ● PSC with PLD-grown NiOx hole transport layer achieved a power conversion efficiency of about 8%. ● PLD-tuned NiOx shows potential for improved solar cell performance. Introduction Perovskite solar cells (PSCs) have emerged as a revolutionary photovoltaic technology due to their remarkable power conversion efficiencies (PCEs) and relatively simple fabrication processes [1], [2], [3]. The laboratory scale PCEs have boosted from 3.5% [4] to 26.7% [5] since PSCs inception in 2009. Among the various materials used in PSCs, mixed-halide perovskites offer the advantage of tunable optoelectronic properties, making them suitable for highly efficient PSCs [6]. However, the performance of these solar cells is strongly influenced by the quality and optimisation of charge transport layers [7]. The operational efficiency of PSCs relies heavily on the effective separation and transport of photogenerated charge carriers—electrons and holes—from the perovskite absorber to their respective electrodes. To achieve this, two critical components are incorporated: the electron transport layer (ETL) and the hole transport layer (HTL). While the ETL facilitates the movement of electrons and blocks holes, the HTL performs the opposite role, allowing the selective transport of holes while blocking electron backflow. Together, these transport layers enable efficient charge separation and reduce recombination losses, which are essential for high-performance solar cells [8]. The impact of both ETLs and HTLs is crucial. However, the HTL plays a crucial role in extracting photogenerated holes while maintaining high transparency [9], [10]. Additionally, compared to ETLs, HTLs often face greater challenges in balancing conductivity, chemical stability, and compatibility with perovskite layers, especially under ambient conditions where degradation can occur. Thus, the choice of materials for the HTL is essential for the overall stability and costeffectiveness of PSCs. Effective organic HTLs like 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]- 9,9′-spiro-bifluorene (Spiro-MeOTAD), Poly(3,4-ethylenedioxythiophene) (PEDOT), and Poly[bis(4phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) tend to be more expensive and less stable. This makes inorganic alternatives a more attractive option for enhancing the longevity and affordability of PSCs [3]. Transition metal oxides, such as NiOx CuOx, MoO3, Cr2O3, WO3, and V2O5, have been widely used as HTLs because they are intrinsically stable under environmental impacts and offer low cost and relatively easy fabrication since they can be deposited via solution process. NiOx is a p-type semiconductor material that has been increasingly utilised as a HTL in PSCs due to its wide bandgap (typically 3.6–4.0 eV), high transparency, and suitable energy-level alignment with perovskites [11]. NiOx offers improved stability compared to organic HTLs like Spiro-OMeTAD, and its performance can be tailored by modifying its physical properties during deposition. When compared to other metal oxides, NiOx offers a superior combination of stability, hole mobility, energy band alignment, transparency, and cost-effectiveness, making it an ideal HTL material for perovskite solar cells. Moreover, NiOx has minimal reactivity with perovskite layers, reducing the risk of interfacial degradation or unwanted reactions that can occur with other metal oxides, like MoO3. Recently, inverted PSCs with NiOx have been reported as HTL, with a certificated efficiency of 26.08% and excellent long-term stability [12]. Several techniques can be used to deposit NiOx layers, including spin coating [13], [14], thermal evaporation [13], and different types of sputtering [15], [16], [17]. However, pulsed laser deposition (PLD) offers numerous advantages over these techniques in terms of controlling film thickness, composition, and other properties through adjustments in deposition parameters such as substrate temperature, oxygen pressure, and laser fluence [18]. These parameters can have a significant impact on the resulting band gap of the NiOx films, which, in turn, influences the performance of PSCs [19]. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3 In this study, we focus on the preparation of NiOx thin films as HTLs in PSCs using PLD, investigating their properties through both simulation studies and experimental methods. The choice of NiOx as an HTL, along with insights gained from simulation, aims to advance the understanding of hole transport in PSCs, thereby contributing to the development of more stable and efficient devices. Although this study does not focus on optimisation, it seeks to confirm the trends observed in the simulation by preparing NiOx films via PLD under different oxygen pressures, substrate temperatures, and laser frequencies. The experimental results will provide insights into how PLD parameters influence NiOx properties and their application as HTLs in PSCs. The prepared films are characterised by photothermal deflection spectroscopy (PDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), Atomic force microscopy (AFM) and Scanning electron microscopy (SEM) followed by the fabrication and J-V measurement of PSC devices incorporating NiOx as HTL. To determine the optimal conditions for fabricating NiOₓ films for use as HTLs in PSCs, we analysed the effects of temperature, laser frequency, and oxygen pressure during PLD. Experimental methods Numerical Simulations To provide further motivation for investigating NiOx films deposited under different PLD conditions, a one-dimensional (1D) simulation of a FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide PSCs with a NiOₓ as HTL was conducted. The simulation uses the Silvaco TCAD device simulator to model the performance of p-i-n perovskite solar cells, focusing on optimising NiOx parameters. The simulated device structure consists of an ITO/NiOx/Perovskite/PCBM/BCP/Ag stack on a 400 nm thick FA0.83Cs0.17Pb3(I0.6Br0.4)3 perovskite absorber, doped at 1014 cm-3, where ITO is indium thin oxide, PCBM is metanofullerene Phenyl-C61-Butyric-Acid-Methyl-Ester, BCP is Bathocuproine and Ag is silver. The front and rear contact consists of ITO/NiOx and BCP/Ag interfaces, with work functions of approximately 5.1 eV and 4.0 eV. The objective of the simulation study is to examine the impact of NiOx layer thickness, ranging from 10 to 100 nm, and energy band gap variations from 3.2 to 4.0 eV, on the performance of PSCs. For this study, a one-dimensional (1D) simulation was executed, incorporating updated models to enhance accuracy. This simulation is also simplified based on the assumption of solely planar contacts and the negligible contribution of the lateral path to current flow. The photovoltaic parameters were extracted. A table with the electrical parameters for the various layers in the simulation of PSCs is presented in Supporting information (Table S1). The simulation investigated the influence of NiOx band gap and film thickness on photovoltaic parameters of PSCs, such as fill factor (FF), open-circuit voltage (VOC), short-circuit current density (JSC) and power conversion efficiency (PCE). Experimental study NiOx layer deposition by PLD In the initial phase of the experiment, we applied samples onto 10 × 10 mm2 glass, fused silica, and silicon substrates to prepare suitable specimens for PDS, ellipsometry, XPS, XRD, AFM and SEM measurements. The deposition process involved PLD in a chamber evacuated to 1 × 10-5 mbar, followed by introducing varying oxygen levels ranging from 0.08 to 0.12 mbar while maintaining a continuous oxygen flow of 10 sccm. The samples were positioned in a heated holder rotating at 5 rpm and then heated to different temperatures from room temperature to 400 °C. The target composition was NiO, and it was rotating at 28.9 rpm and ablated using a KrF excimer laser COMPex 50 with repetition rates ranging from 5 Hz to 50 Hz and laser energy set to 125 mJ. Each sample received 15,000 pulses, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4 resulting in 40 to 60 nm thicknesses. All the different settings used in the first part of the experiment can be seen in Table 1. Table 1: Different settings of the pulse laser depositions of NiOx used for the evaluation of the effect of temperature, repetition rate and oxygen pressure effect of temperature effect of laser frequency effect of oxygen pressure temperature (°C) 25 - 400 25 25 oxygen pressure (mbar) 0.1 0.1 0.08 – 0.12 laser frequency (Hz) 50 5 - 50 50 The samples were evaluated using a combination of PDS and ellipsometry, enabling observation of optical spectra ranging from 1.5 to 6.5 eV. PDS was conducted in isopropanol with a refractive index of 1.38. After the measurements and simulations, the most promising deposition combinations were identified, and samples with these parameters were prepared. PLD was applied on 20 × 20 mm2 glass substrates with a pre-existing patterned ITO layer (14 × 20 mm2). The chamber was evacuated to 1 × 10-5 mbar, after which oxygen was introduced at a range of 0.08 to 0.25 mbar. The substrates were then heated to 25°C, 200°C, and 400°C to explore a wide temperature range, as these settings appeared suitable based on bandgap evaluations. However, it was found that at higher temperatures, the perovskite material failed to adhere to the surface, making it impossible to prepare perovskite solar cells on these samples. Consequently, for the fabrication of the solar cells, NiOx layers prepared at room temperature were used. When considering the repetition rate of the laser, it is important to note that higher frequencies lead to greater sub-band gap absorption, as seen in Figure 4. Because of this, a frequency of 5 Hz was ultimately chosen for all the samples. Thickness was also a tested parameter, so the number of pulses differed from 1000 to 8000, resulting in roughly 8 to 50 nm thick samples. All of the settings used in the first part of the experiment can be seen in Table 2. Table 2: Different settings of the pulse laser depositions of NiOx further used as hole transport layer in solar cell devices. Fabrication of FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide p-i-n perovskite solar cell effect of temperature effect of oxygen pressure effect of thickness temperature (°C) 25 - 400 25 25 oxygen pressure (mbar) 0.1 0.08 – 0.25 0,1 number of pulses 2000 2000 1000 - 8000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5 Patterned ITO substrates were subsequently cleaned in the solvents of deionised water, ethanol, acetone, and isopropyl alcohol. Then, the hole-transport material NiOx was deposited by PLD at different conditions. The perovskite films with the following chemical composition FA0.83Cs0.17Pb(I0.6Br0.4)3 were deposited on the NiOx layer. A detailed description of the perovskite fabrication process can be found elsewhere [20]. Subsequently, the electron transport material [6,6]- phenyl-C 61 -butyric acid methyl ester (PC61BM, 20 mg/ml in anhydrous chlorobenzene) was deposited on perovskite films by spin coating at 2000 rpm for 30 s, followed by spin-coating of buffer layer (bathocuproine, BCP) with the concentration of 0.5 mg/ml in 2-propanol on top (4000 rpm, 30 s). Finally, silver electrodes with a thickness of 120 nm were deposited by thermal evaporation. For details about PDS, ellipsometry, XPS, XRD, SEM, AFM and J-V measurement, refer to Supplementary Information. Results and discussion The simulation results, presented in Figure 1, demonstrate that increasing the bandgap of the NiOx layer from 3.2 eV to 4.0 eV leads to substantial improvements in both fill factor and efficiency. This suggests that wider bandgap NiOx layers can enhance charge extraction and minimise recombination losses at the interface with the perovskite absorber. In contrast, the effect of NiOx thickness on solar cell performance appears minimal, with negligible variations in VOC and a slight increase in JSC as the thickness changes from 10 nm to 100 nm. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6 Figure 1: Simulated effect of NiOx bandgap on (a) Fill Factor and Power Conversion Efficiency and (b) Short Circuit Current Density and Open Circuit Voltage in a perovskite solar cell and simulated effect of NiOx layer thickness (c) Fill Factor and Power Conversion Efficiency and (d) Short Circuit Current Density and Open Circuit Voltage in a perovskite solar cell. These simulation findings motivate the need to experimentally investigate NiOx films with varying deposition conditions to evaluate their band gap and subsequent impact on solar cell efficiency. In the first step of our experimental study, we determined the chemical composition and the oxidation state of Ni of the NiOx films using XPS analysis. Evolution of high-resolution XPS spectra taken in the Ni 2p region for NiO PDL films deposited at varying temperature, laser frequency and oxygen pressure (at a representative laser frequency of 50 Hz) in shown in Figure 2. Eye guidelines are given at metal Ni, NiO and Ni(OH)2 contributions. 890 880 870 860 850 840 Normalized intensity (a.u) Binding energy (eV) Ni2+ of NiO Ni 2p Ni0 metal Ni2+ of Ni(OH)2 25 °C 50 °C 100 °C 200 °C 400 °C (a) 890 880 870 860 850 840 Normalized intensity (a.u) Binding energy (eV) Ni2+ of Ni(OH)2 Ni 2p Ni2+ of NiO Ni0 metal 10 Hz 5 Hz 25 Hz 50 Hz (b) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7 890 880 870 860 850 840 Normalized intensity (a.u) Binding energy (eV) (c) Ni 2p Ni2+ of Ni(OH)2 Ni2+ of NiO Ni0 metal 0.08 mbar 0.10 mbar 0.12 mbar Figure 2: Evolution of high-resolution XPS spectra taken in the Ni 2p region for NiO PDL films deposited at varying (a) temperature, (b) laser frequency and (c) oxygen pressure (at a representative laser frequency of 50 Hz). Eye guidelines are given at metal Ni, NiO and Ni(OH)2 contributions. In Figure 3 are shown representative high-resolution XPS spectra of NiO PLD film (deposited at 25 °C, oxygen pressure of 0.1 mbar, and laser frequency of 50 Hz) in C 1s, O 1s, and Ni 2p regions. Measured spectra are shown in open circles, whereas their fittings are shown in red lines. The individual contributions of different functional groups are displayed in blue lines. 294 292 290 288 286 284 282 280 Intensity (a.u) Binding energy (eV) C 1s C-C, C-H C-O C(=O)-O NiCO3 540 538 536 534 532 530 528 526 524 Intensity (a.u) Binding energy (eV) NiO C-O, Ni(OH)2 C(=O) O 1s 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8 890 880 870 860 850 840 Intensity (a.u) Binding energy (eV) Ni 2p Figure 3: Representative high-resolution XPS spectra of NiO PLD film (deposited at 25 °C, oxygen pressure of 0.1 mbar, and laser frequency of 50 Hz) in C 1s, O 1s, and Ni 2p regions. Measured spectra are shown in open circles, whereas their fittings are shown in red lines. The individual contributions of different functional groups are displayed in blue lines. Based on the chemical composition (Figure 2 and Figure 3), we calculated the ratio between NiO and other Ni-oxygen species present on the surface of NiOx films. The [NiO]/[Ni + NiO + Ni(OH)2 + NiCO3] amount ratio can be utilised as a guideline for the dominance of NiO within the thin film structure (Table S2). Figure 4 shows the variation of the ratio [NiO]/[Ni + NiO + Ni(OH)2 + NiCO3] as a function of deposition temperature, laser frequency, and oxygen pressure during PLD. The ratio [NiO]/[Ni + NiO + Ni(OH)2 + NiCO3] increases from about 0.6 to about 0.9 with respective increasing temperatures from room T to 400 °C. At the same time, the variation of laser frequency at constant temperature and oxygen pressure does not seem to have a decisive influence on the determined ratio. However, as the oxygen pressure increases from 0.08 mbar to 0.12 mbar, the ratio [NiO]/[Ni + NiO + Ni(OH)2 + NiCO3] gradually decreases from about 0.7 to 0.6, respectively, with a seemingly limited effect of the oxygen pressure and laser frequency as previously noted. These findings indicate that we can systematically vary the amount of NiO and other Ni-species, such as Ni(OH)2 and NiCO3, through precise tuning of the PLD processing conditions. Figure 4: Ratio between the amount of NiO and other nickel oxygen species (Ni(OH)2 and NiCO3) under varying pulsed laser deposition conditions (a) temperature, (b) laser frequency and (c) oxygen pressure, as determined via XPS analysis. Scanning electron microscopy was applied to analyse the morphology of the NiOx layers. The SEM images presented in Figure 5 show that continuous and conformal NiOx layers are deposited. It is observed that with increasing deposition temperature, the morphology of the films does not change. Regarding the influence of frequency and pressure, according to SEM, no visible change in the diameter 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 9 of the nanocolumns was observed as well (Figures 6 and 7). The NiOx deposited at high frequency (50Hz) and high pressure (0.12mbar) exhibits some nanocracks of the layer observed (Figure 7). However, no noticeable difference in the diameter of the nanocolumns is observed compared to the layer deposited at lower frequencies. Figure 5: SEM images of NiOx layers deposited at a constant pressure of 0.1mbar, constant frequency of 50Hz and different temperatures of 25, 50, 100, 200 and 400°C, respectively. Figure 6: SEM images of NiOx layer deposited at constant T of 25°C, constant pressure of 0.1mbar and different frequencies: 5, 10, 25 and 50Hz. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 10 Figure 7: SEM images of NiOx layer deposited at constant T of 25°C, different frequency (5Hz, 10Hz and 50Hz) and different pressures. The surface roughness of deposited NiOx layers was determined by AFM. Figure 8 shows AFM scans of the NiOx layers deposited at different temperatures from 25 to 400°C. The RMS roughness of the layers increases almost linearly with increasing the deposition temperature from 5.2 ± 0.1 nm at 25°C to 12 ± 1.6 nm at 400°C. This is in good agreement with SEM results. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 17 Figure 14: (a) PCE distribution of the PSCs using NiOx as the HTL prepared with different numbers of pulses, (b) efficiency of the best-performing PSCs with different numbers of pulses, and (c) current density−voltage (J−V) curves under reverse and forward scans of the best performing PSC. Figure 14 shows the effect of pulse number (thickness) on the performance of mixed halide FA0.83Cs0.17Pb(I0.6Br0.4)3 PSCs using nickel oxide (NiOx) as the hole transport layer. At a relatively low number of pulses, specifically 1000, corresponding to the thinnest NiOx layer, the average efficiency is relatively low, approximately below 4%, as shown in Figure 14(a). This can be attributed to inadequate film thickness and potential non-uniformity, which likely resulted in higher defect densities and compromised charge transport properties. As the number of pulses increases to 4000, the average efficiency significantly improves, reaching around 6%, indicating better film quality and more favourable charge transport characteristics. Beyond 4000 pulses, with thicker NiOx films, the efficiency starts to decline, showing increased variability at 6000 and 8000 pulses. This is likely due to decreased charge transport efficiency as the thicker layer may introduce more resistance, hampering charge extraction. Figure 14(b) shows the efficiency of the best-performing PSCs at each thickness level. Similar to Figure 9(a), the maximum efficiency is observed at 4000 pulses, where the efficiency reaches 7.6 %. The J−V curve in Figure 14(c) shows the current density and voltage characteristics of the best-performing PSC under forward and reverse scan directions. The device, prepared with 4000 pulses (optimal thickness), demonstrates minimal hysteresis between the forward (red) and reverse (black) scans. These findings align well with previous discussions on the effects of oxygen pressure and bandgap tuning. This suggests that an optimal balance can be achieved in terms of bandgap and film thickness, enhancing charge transport and reducing defect states. Furthermore, these findings support the earlier simulation results that indicated how carefully optimised deposition conditions lead to higher solar cell efficiencies. Conclusions In this study, we successfully prepared NiOx HTLs by pulsed laser deposition under varying conditions and investigated the effect of bandgap variation on the performance of perovskite solar cells. The study reveals that variations in deposition temperature, laser frequency, and oxygen pressure alter the chemical composition, optical properties, and defect states in NiOx films. The fabrication of FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide PSCs using PLD-grown NiOx HTLs with varying oxygen pressures and film thickness resulted in a maximum power conversion efficiency of around 8%, without the use of an extra dipole layer, confirming PLD as a promising technology for tuning the properties of highperformance HTLs. Combined with theoretical device simulations, these findings highlight the importance of NiOx bandgap tuning by controlled deposition techniques like PLD in optimising the efficiency of perovskite solar cells. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 18 Acknowledgement We acknowledge the support of the Czech Ministry of Education, Youth and Sports grant no. CZ.02.1.01/00/22_008/0004617 – „Energy conversion and storage “, the Czech Science Foundation GA ČR, grant no. 23-06543S and CTU student grant SGS24/135/OHK3/3T/13. References [1] T. Singh and T. Miyasaka, “Stabilizing the Efficiency Beyond 20% with a Mixed Cation Perovskite Solar Cell Fabricated in Ambient Air under Controlled Humidity,” Adv Energy Mater, vol. 8, no. 3, p. 1700677, Jan. 2018, doi: 10.1002/AENM.201700677. [2] M. H. Kuo, N. Neykova, and I. Stachiv, “Overview of the Recent Findings in the Perovskite-Type Structures Used for Solar Cells and Hydrogen Storage,” Energies 2024, Vol. 17, Page 4755, vol. 17, no. 18, p. 4755, Sep. 2024, doi: 10.3390/EN17184755. [3] C. Yang et al., “Achievements, challenges, and future prospects for industrialization of perovskite solar cells,” Light: Science & Applications 2024 13:1, vol. 13, no. 1, pp. 1–48, Sep. 2024, doi: 10.1038/s41377-024-01461-x. [4] A. Kojima, K. Teshima, Y. Shirai, and T. 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Timoshnev et al., “Annealing Temperature Effect on the Physical Properties of NiO Thin Films Grown by DC Magnetron Sputtering,” Adv Mater Interfaces, vol. 11, no. 9, p. 2300815, Mar. 2024, doi: 10.1002/ADMI.202300815. [24] A. Peter Amalathas, L. Landová, K. Ridzoňová, L. Horák, P. Bauerová, and J. Holovský, “Unveiling the Effect of Potassium Treatment on the Mesoporous TiO2/ Perovskite Interface in Perovskite Solar Cells,” ACS Appl Energy Mater, vol. 4, no. 10, pp. 11488–11495, Oct. 2021, doi: 10.1021/ACSAEM.1C02229/SUPPL_FILE/AE1C02229_SI_001.PDF. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 Nickel Oxide Hole Transport Layer for Perovskite Solar Cells: Preparation via Pulsed Laser Deposition with Simulation and Experimental Insights Eva Horynova1, Jakub Holovsky1,2, Lucie Landova1,2, Naini Jain1, Abhinav Deep Pakki1, Meng-Hsueh Kuo1,2, Ivana Beshajová Pelikánová1, Branislav Dzurňák1, Lukáš Horák3, Ognen Pop-Georgievski4, Amalraj Peter Amalathas1,5 and Neda Neykova1,2 1 Centre for Advanced Photovoltaics, Faculty of Electrical Engineering, CTU in Prague, Technická 2, 166 27, Prague, Czech Republic 2 Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Prague, Czech Republic 3 Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Prague, Ke Karlovu 2026/5, 121 16 Praha 2, Czech Republic 4 Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovskeho nam. 2, 162 00 Prague, Czech Republic 5 Department of Physics, Faculty of Science, University of Jaffna, Jaffna 40000, Sri Lanka Corresponding author: Eva Horynova, [email protected] Abstract Nickel oxide (NiOx) has gained attention as a promising inorganic hole transport layer for perovskite solar cells due to its wide bandgap, high transparency, and stability. However, tuning of band alignment by an extra dipole layer is necessary to achieve high efficiencies. Our predictive simulations suggest that NiOₓ bandgap tuning can also improve solar cell performance. Motivated by these findings, this study experimentally investigates NiOₓ films with different bandgap fabricated using pulsed laser deposition under varying deposition conditions, including oxygen pressure, substrate temperature and laser frequency. Our outcomes show that mainly the deposition temperature significantly influences the chemical composition, optical properties, and defect states in the NiOx films, lattice constants and morphology as confirmed by X-ray photoelectron spectroscopy, photothermal deflection spectroscopy, X-ray diffraction spectroscopy, atomic force microscopy and scanning electron microscopy. Experimentally, FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide perovskite solar cells were fabricated on NiOx substrates prepared under varying oxygen pressures and pulse numbers, achieving a maximum power conversion efficiency of approximately 8%. This demonstrates that NiOx deposited by pulsed laser deposition, when properly tuned, is a promising candidate for an efficient hole transport layer in perovskite-based photovoltaics. Keywords Pulsed laser deposition, nickel oxide, perovskite solar cells, hole transport layer Highlights ● NiOx thin films with varying bandgap were fabricated via PLD and thoroughly characterized. ● Strong effects of pressure, temperature, and frequency were observed. Revised manuscript file 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 ● Effects of NiOx bandgap on PSCs were demonstrated experimentally. ● PSC with PLD-grown NiOx hole transport layer achieved a power conversion efficiency of about 8%. ● PLD-tuned NiOx shows potential for improved solar cell performance. Introduction Perovskite solar cells (PSCs) have emerged as a revolutionary photovoltaic technology due to their remarkable power conversion efficiencies (PCEs) and relatively simple fabrication processes [1], [2], [3]. The laboratory scale PCEs have boosted from 3.5% [4] to 26.7% [5] since PSCs inception in 2009. Among the various materials used in PSCs, mixed-halide perovskites offer the advantage of tunable optoelectronic properties, making them suitable for highly efficient PSCs [6]. However, the performance of these solar cells is strongly influenced by the quality and optimisation of charge transport layers [7]. The operational efficiency of PSCs relies heavily on the effective separation and transport of photogenerated charge carriers—electrons and holes—from the perovskite absorber to their respective electrodes. To achieve this, two critical components are incorporated: the electron transport layer (ETL) and the hole transport layer (HTL). While the ETL facilitates the movement of electrons and blocks holes, the HTL performs the opposite role, allowing the selective transport of holes while blocking electron backflow. Together, these transport layers enable efficient charge separation and reduce recombination losses, which are essential for high-performance solar cells [8]. The impact of both ETLs and HTLs is crucial. However, the HTL plays a crucial role in extracting photogenerated holes while maintaining high transparency [9], [10]. Additionally, compared to ETLs, HTLs often face greater challenges in balancing conductivity, chemical stability, and compatibility with perovskite layers, especially under ambient conditions where degradation can occur. Thus, the choice of materials for the HTL is essential for the overall stability and costeffectiveness of PSCs. Effective organic HTLs like 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]- 9,9′-spiro-bifluorene (Spiro-MeOTAD), Poly(3,4-ethylenedioxythiophene) (PEDOT), and Poly[bis(4phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) tend to be more expensive and less stable. This makes inorganic alternatives a more attractive option for enhancing the longevity and affordability of PSCs [3]. Transition metal oxides, such as NiOx CuOx, MoO3, Cr2O3, WO3, and V2O5, have been widely used as HTLs because they are intrinsically stable under environmental impacts and offer low cost and relatively easy fabrication since they can be deposited via solution process. NiOx is a p-type semiconductor material that has been increasingly utilised as a HTL in PSCs due to its wide bandgap (typically 3.6–4.0 eV), high transparency, and suitable energy-level alignment with perovskites [11]. NiOx offers improved stability compared to organic HTLs like Spiro-OMeTAD, and its performance can be tailored by modifying its physical properties during deposition. When compared to other metal oxides, NiOx offers a superior combination of stability, hole mobility, energy band alignment, transparency, and cost-effectiveness, making it an ideal HTL material for perovskite solar cells. Moreover, NiOx has minimal reactivity with perovskite layers, reducing the risk of interfacial degradation or unwanted reactions that can occur with other metal oxides, like MoO3. Recently, inverted PSCs with NiOx have been reported as HTL, with a certificated efficiency of 26.08% and excellent long-term stability [12]. Several techniques can be used to deposit NiOx layers, including spin coating [13], [14], thermal evaporation [13], and different types of sputtering [15], [16], [17]. However, pulsed laser deposition (PLD) offers numerous advantages over these techniques in terms of controlling film thickness, composition, and other properties through adjustments in deposition parameters such as substrate temperature, oxygen pressure, and laser fluence [18]. These parameters can have a significant impact on the resulting band gap of the NiOx films, which, in turn, influences the performance of PSCs [19]. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3 In this study, we focus on the preparation of NiOx thin films as HTLs in PSCs using PLD, investigating their properties through both simulation studies and experimental methods. The choice of NiOx as an HTL, along with insights gained from simulation, aims to advance the understanding of hole transport in PSCs, thereby contributing to the development of more stable and efficient devices. Although this study does not focus on optimisation, it seeks to confirm the trends observed in the simulation by preparing NiOx films via PLD under different oxygen pressures, substrate temperatures, and laser frequencies. The experimental results will provide insights into how PLD parameters influence NiOx properties and their application as HTLs in PSCs. The prepared films are characterised by photothermal deflection spectroscopy (PDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), Atomic force microscopy (AFM) and Scanning electron microscopy (SEM) followed by the fabrication and J-V measurement of PSC devices incorporating NiOx as HTL. To determine the optimal conditions for fabricating NiOₓ films for use as HTLs in PSCs, we analysed the effects of temperature, laser frequency, and oxygen pressure during PLD. Experimental methods Numerical Simulations To provide further motivation for investigating NiOx films deposited under different PLD conditions, a one-dimensional (1D) simulation of a FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide PSCs with a NiOₓ as HTL was conducted. The simulation uses the Silvaco TCAD device simulator to model the performance of p-i-n perovskite solar cells, focusing on optimising NiOx parameters. The simulated device structure consists of an ITO/NiOx/Perovskite/PCBM/BCP/Ag stack on a 400 nm thick FA0.83Cs0.17Pb3(I0.6Br0.4)3 perovskite absorber, doped at 1014 cm-3, where ITO is indium thin oxide, PCBM is metanofullerene Phenyl-C61-Butyric-Acid-Methyl-Ester, BCP is Bathocuproine and Ag is silver. The front and rear contact consists of ITO/NiOx and BCP/Ag interfaces, with work functions of approximately 5.1 eV and 4.0 eV. The objective of the simulation study is to examine the impact of NiOx layer thickness, ranging from 10 to 100 nm, and energy band gap variations from 3.2 to 4.0 eV, on the performance of PSCs. For this study, a one-dimensional (1D) simulation was executed, incorporating updated models to enhance accuracy. This simulation is also simplified based on the assumption of solely planar contacts and the negligible contribution of the lateral path to current flow. The photovoltaic parameters were extracted. A table with the electrical parameters for the various layers in the simulation of PSCs is presented in Supporting information (Table S1). The simulation investigated the influence of NiOx band gap and film thickness on photovoltaic parameters of PSCs, such as fill factor (FF), open-circuit voltage (VOC), short-circuit current density (JSC) and power conversion efficiency (PCE). Experimental study NiOx layer deposition by PLD In the initial phase of the experiment, we applied samples onto 10 × 10 mm2 glass, fused silica, and silicon substrates to prepare suitable specimens for PDS, ellipsometry, XPS, XRD, AFM and SEM measurements. The deposition process involved PLD in a chamber evacuated to 1 × 10-5 mbar, followed by introducing varying oxygen levels ranging from 0.08 to 0.12 mbar while maintaining a continuous oxygen flow of 10 sccm. The samples were positioned in a heated holder rotating at 5 rpm and then heated to different temperatures from room temperature to 400 °C. The target composition was NiO, and it was rotating at 28.9 rpm and ablated using a KrF excimer laser COMPex 50 with repetition rates ranging from 5 Hz to 50 Hz and laser energy set to 125 mJ. Each sample received 15,000 pulses, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4 resulting in 40 to 60 nm thicknesses. All the different settings used in the first part of the experiment can be seen in Table 1. Table 1: Different settings of the pulse laser depositions of NiOx used for the evaluation of the effect of temperature, repetition rate and oxygen pressure effect of temperature effect of laser frequency effect of oxygen pressure temperature (°C) 25 - 400 25 25 oxygen pressure (mbar) 0.1 0.1 0.08 – 0.12 laser frequency (Hz) 50 5 - 50 50 The samples were evaluated using a combination of PDS and ellipsometry, enabling observation of optical spectra ranging from 1.5 to 6.5 eV. PDS was conducted in isopropanol with a refractive index of 1.38. After the measurements and simulations, the most promising deposition combinations were identified, and samples with these parameters were prepared. PLD was applied on 20 × 20 mm2 glass substrates with a pre-existing patterned ITO layer (14 × 20 mm2). The chamber was evacuated to 1 × 10-5 mbar, after which oxygen was introduced at a range of 0.08 to 0.25 mbar. The substrates were then heated to 25°C, 200°C, and 400°C to explore a wide temperature range, as these settings appeared suitable based on bandgap evaluations. However, it was found that at higher temperatures, the perovskite material failed to adhere to the surface, making it impossible to prepare perovskite solar cells on these samples. Consequently, for the fabrication of the solar cells, NiOx layers prepared at room temperature were used. When considering the repetition rate of the laser, it is important to note that higher frequencies lead to greater sub-band gap absorption, as seen in Figure 4. Because of this, a frequency of 5 Hz was ultimately chosen for all the samples. Thickness was also a tested parameter, so the number of pulses differed from 1000 to 8000, resulting in roughly 8 to 50 nm thick samples. All of the settings used in the first part of the experiment can be seen in Table 2. Table 2: Different settings of the pulse laser depositions of NiOx further used as hole transport layer in solar cell devices. Fabrication of FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide p-i-n perovskite solar cell effect of temperature effect of oxygen pressure effect of thickness temperature (°C) 25 - 400 25 25 oxygen pressure (mbar) 0.1 0.08 – 0.25 0,1 number of pulses 2000 2000 1000 - 8000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5 Patterned ITO substrates were subsequently cleaned in the solvents of deionised water, ethanol, acetone, and isopropyl alcohol. Then, the hole-transport material NiOx was deposited by PLD at different conditions. The perovskite films with the following chemical composition FA0.83Cs0.17Pb(I0.6Br0.4)3 were deposited on the NiOx layer. A detailed description of the perovskite fabrication process can be found elsewhere [20]. Subsequently, the electron transport material [6,6]- phenyl-C 61 -butyric acid methyl ester (PC61BM, 20 mg/ml in anhydrous chlorobenzene) was deposited on perovskite films by spin coating at 2000 rpm for 30 s, followed by spin-coating of buffer layer (bathocuproine, BCP) with the concentration of 0.5 mg/ml in 2-propanol on top (4000 rpm, 30 s). Finally, silver electrodes with a thickness of 120 nm were deposited by thermal evaporation. For details about PDS, ellipsometry, XPS, XRD, SEM, AFM and J-V measurement, refer to Supplementary Information. Results and discussion The simulation results, presented in Figure 1, demonstrate that increasing the bandgap of the NiOx layer from 3.2 eV to 4.0 eV leads to substantial improvements in both fill factor and efficiency. This suggests that wider bandgap NiOx layers can enhance charge extraction and minimise recombination losses at the interface with the perovskite absorber. In contrast, the effect of NiOx thickness on solar cell performance appears minimal, with negligible variations in VOC and a slight increase in JSC as the thickness changes from 10 nm to 100 nm. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6 Figure 1: Simulated effect of NiOx bandgap on (a) Fill Factor and Power Conversion Efficiency and (b) Short Circuit Current Density and Open Circuit Voltage in a perovskite solar cell and simulated effect of NiOx layer thickness (c) Fill Factor and Power Conversion Efficiency and (d) Short Circuit Current Density and Open Circuit Voltage in a perovskite solar cell. These simulation findings motivate the need to experimentally investigate NiOx films with varying deposition conditions to evaluate their band gap and subsequent impact on solar cell efficiency. In the first step of our experimental study, we determined the chemical composition and the oxidation state of Ni of the NiOx films using XPS analysis. Evolution of high-resolution XPS spectra taken in the Ni 2p region for NiO PDL films deposited at varying temperature, laser frequency and oxygen pressure (at a representative laser frequency of 50 Hz) in shown in Figure 2. Eye guidelines are given at metal Ni, NiO and Ni(OH)2 contributions. 890 880 870 860 850 840 Normalized intensity (a.u) Binding energy (eV) Ni2+ of NiO Ni 2p Ni0 metal Ni2+ of Ni(OH)2 25 °C 50 °C 100 °C 200 °C 400 °C (a) 890 880 870 860 850 840 Normalized intensity (a.u) Binding energy (eV) Ni2+ of Ni(OH)2 Ni 2p Ni2+ of NiO Ni0 metal 10 Hz 5 Hz 25 Hz 50 Hz (b) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7 890 880 870 860 850 840 Normalized intensity (a.u) Binding energy (eV) (c) Ni 2p Ni2+ of Ni(OH)2 Ni2+ of NiO Ni0 metal 0.08 mbar 0.10 mbar 0.12 mbar Figure 2: Evolution of high-resolution XPS spectra taken in the Ni 2p region for NiO PDL films deposited at varying (a) temperature, (b) laser frequency and (c) oxygen pressure (at a representative laser frequency of 50 Hz). Eye guidelines are given at metal Ni, NiO and Ni(OH)2 contributions. In Figure 3 are shown representative high-resolution XPS spectra of NiO PLD film (deposited at 25 °C, oxygen pressure of 0.1 mbar, and laser frequency of 50 Hz) in C 1s, O 1s, and Ni 2p regions. Measured spectra are shown in open circles, whereas their fittings are shown in red lines. The individual contributions of different functional groups are displayed in blue lines. 294 292 290 288 286 284 282 280 Intensity (a.u) Binding energy (eV) C 1s C-C, C-H C-O C(=O)-O NiCO3 540 538 536 534 532 530 528 526 524 Intensity (a.u) Binding energy (eV) NiO C-O, Ni(OH)2 C(=O) O 1s 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 14 T (°C) Relative weight fraction (%) NiO Ni NiO lattice parameter (Å) Microstrain (%) Crystallite mean-size(nm) Residual Stress(GPa) 25 98.8 1.2 4.200 ± 0.001 0.54 ± 0.09 12 ± 2 -1.8 ± 0.3 50 99.7 0.3 4.196 ± 0.001 0.55 ± 0.15 14 ± 2 -1.6 ± 0.4 100 97.5 2.5 4.193 ± 0.001 0.49 ± 0.10 15 ± 2 -1.8 ± 0.3 200 97.7 2.3 4.1914 ± 0.0009 0.42 ± 0.1 15 ± 1 -2.4 ± 0.3 400 97.4 2.6 4.1865 ± 0.0006 0.37 ± 0.06 18 ± 1 -1.4 ± 0.2 Figure 11: Bandgap of NiOx films calculated from the absorption coefficient using a Tauc plot under varying pulsed laser deposition (PLD) conditions (a) temperature, (b) laser frequency and (c) oxygen pressure. The optical bandgap of NiOx thin films was calculated using the absorption coefficient using the Tauc plot method where (αhν)2 was plotted over hν. Figure 11 shows the bandgap variation as a function of deposition temperature, laser frequency, and oxygen pressure during PLD. The bandgap showed only a weak dependence on temperature, decreasing slightly from 3.65 eV at 100 °C to 3.5 eV at 400 °C. (see Figure 11(a)). As shown in Figure 11(b), the bandgap is around 3.8 eV at 5 Hz and slightly decreases at 25 Hz. However, a more significant drop occurs at 50 Hz, where the bandgap decreases to approximately 3.6 eV. This notable shift at higher frequencies suggests that increasing the laser frequency introduces more defects or reduces film quality, which negatively impacts the electronic properties of the NiOₓ layer. Interestingly, on the surface, we determined that the stoichiometric coefficient in the NiOx films was in the range of 0.92-1.03 (See Table S2), close to the expected stoichiometric coefficient of nickel (II) oxide x=1.0. A clear trend is observed with increasing temperature and a drop of the determined stoichiometric coefficient from NiO0.96 to NiO0.92. Nevertheless, in all studied cases, we did not observe a rise in the stoichiometric coefficient to values of 1.5, indicative of the formation of Ni3+ species of Ni2O3 or NiOOH, as suggested by other authors [21]. Notably, the main spectral contributions/peaks appear at 852.8 eV, 854.3 eV and 856.1 eV originating from nickel species of Ni metal, NiO and Ni(OH)2, respectively (See Figure 2 and Figure 3). These fall well apart from the expected Ni3+ of Ni2O3 or NiOOH, which should appear at 858.1 eV as proposed in the thorough work by Grosvenor et al. As the oxygen pressure increases from 0.08 mbar 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 15 to 0.12 mbar, the bandgap gradually increases, reaching up to 3.8 eV at 0.12 mbar. At higher oxygen pressures, it is likely that the bulk structure of the film is better oxygenated and more stoichiometric, leading to reduced oxygen vacancies and a wider bandgap [22]. However, the surface composition (as probed by XPS) may show an increasing amount of hydroxides and carbonates, as surface reactions with environmental moisture or CO₂ can lead to the formation of these species. Essentially, while the surface becomes enriched in non-stoichiometric species (such as Ni(OH)x and NiCOx), the bulk of the film maintain a high predominance of stoichiometric NiO species and Ni metal (as proven by XRD), explaining the bandgap widening. Figure 12: Absorption coefficient of NiOx films as a function of photon energy under varying PLD conditions (a) temperature, (b) laser frequency and (c) oxygen pressure. The black dash line shows absorptance of 3% for layer 10 nm thick. Figure 12 shows the absorption measured using a combination of PDS and ellipsometry for NiOx films deposited under varying PLD conditions such as deposition temperature, laser frequency, and oxygen pressure. Sub-bandgap absorption increases with rising temperature, especially beyond 100°C. At 400°C, the absorption is significantly higher compared to lower temperatures. The increase in sub-bandgap absorption at higher temperatures correlates with the formation of defect states, such as oxygen vacancies or grain boundaries, which introduce mid-gap states that allow sub-bandgap photons to be absorbed [23]. This suggests that higher temperatures lead to poor film quality with more defects, which can be detrimental to the performance of NiOx as a hole transport layer in PSCs. The subbandgap absorption is lowest at 5 Hz and 25 Hz, but increases sharply at 50 Hz, indicating more defectrelated absorption at this higher frequency. The sharp rise in sub-bandgap absorption at 50 Hz further supports the idea that higher laser frequencies cause greater defect formation. These defects increase the number of mid-gap states, leading to higher sub-bandgap absorption, which implies more recombination centres and lower film quality. With increasing oxygen pressure, sub-bandgap absorption decreases, showing lower absorption levels at 0.12 mbar compared to 0.08 mbar. The decrease in sub-bandgap absorption with increasing oxygen pressure indicates that higher oxygen pressures reduce the number of defect states in the NiOx films. With fewer oxygen vacancies and other structural defects, there are fewer energy states within the bandgap. This reduction in defect states leads to decreased sub-bandgap absorption, which can improve film quality and reduce non-radiative recombination. In the second step, based on the observations from both bandgap and sub-bandgap absorption analysis, we selected deposition conditions for the fabrication of NiOx as a hole transport layer in perovskite solar cells. Specifically, we chose a low laser frequency of 5 Hz and a low deposition temperature of room temperature (25°C) while varying the oxygen pressure between 0.08 mbar and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 16 0.25 mbar. Given our earlier observations of increased bandgap and reduced sub-bandgap absorption at higher oxygen pressures, we selected these conditions for further study. Choosing a low frequency (5 Hz) and room temperature minimises the introduction of additional defects that may arise at higher frequencies and temperatures, as seen in the above results. These conditions help maintain the balance between controlling the bandgap and minimising the formation of sub-bandgap defects, which is important for achieving efficient charge extraction and transport in perovskite solar cells. The decision to vary oxygen pressure is particularly important since higher oxygen pressures have been shown to suppress sub-bandgap defect formation by reducing oxygen vacancies, which are a common source of recombination losses in solar cell devices. Figure 13: (a) Power conversion efficiency distribution of the PSCs using NiOx as the hole transport layer prepared by various oxygen pressures, (b) efficiency of the best-performing PSCs with different oxygen pressures, and (c) current density−voltage (J−V) curves under reverse and forward scans of best performing PSC. The J-V measurements conducted with varying oxygen pressures yielded insightful results regarding the efficiency of the mixed halide FA0.83Cs0.17Pb(I0.6Br0.4)3 perovskite solar cells using NiOx as the hole transport layer. The box plots in Figure 13(a) show a wide distribution in efficiency values, indicating some variability in device performance as a function of oxygen pressure during the pulsed laser deposition process. At low oxygen pressures of 0.08 mbar, the average efficiency remained notably low, below 3%. This is consistent with the earlier observations, where insufficient oxygen availability likely led to increased defect states in the NiOx films, hindering charge transport and extraction. As the oxygen pressure increased to 0.1 and 0.15 mbar, we observed a dramatic improvement in average efficiency, peaking around 6%. This enhancement aligns with the earlier findings that higher oxygen pressures contribute to widening the band gap and reducing sub-gap absorption, leading to fewer defects and better charge transport properties [24]. This improvement in efficiency at moderate pressures suggests an optimal balance between minimising defect states and maintaining the desired optical characteristics of the NiOx films. However, at higher pressures of 0.20 mbar and above, we noted a decline in average efficiency or increased variation. This decline could be attributed to the formation of excessive oxide phases or structural changes in the NiOx material that could adversely affect its transport properties. Such outcomes indicate that while increased oxygen pressure initially enhances efficiency by reducing defects, excessive oxygen can lead to material instability or undesirable phase changes. Figures 13(b) and (c) show the best-performing PSCs as a function of oxygen pressure and J-V curves of the bestperforming PSC, respectively. The highest-performing PSC is observed at 0.15 mbar with a peak efficiency of around 7.8%, while the device efficiency decreases at both lower and higher pressures. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 17 Figure 14: (a) PCE distribution of the PSCs using NiOx as the HTL prepared with different numbers of pulses, (b) efficiency of the best-performing PSCs with different numbers of pulses, and (c) current density−voltage (J−V) curves under reverse and forward scans of the best performing PSC. Figure 14 shows the effect of pulse number (thickness) on the performance of mixed halide FA0.83Cs0.17Pb(I0.6Br0.4)3 PSCs using nickel oxide (NiOx) as the hole transport layer. At a relatively low number of pulses, specifically 1000, corresponding to the thinnest NiOx layer, the average efficiency is relatively low, approximately below 4%, as shown in Figure 14(a). This can be attributed to inadequate film thickness and potential non-uniformity, which likely resulted in higher defect densities and compromised charge transport properties. As the number of pulses increases to 4000, the average efficiency significantly improves, reaching around 6%, indicating better film quality and more favourable charge transport characteristics. Beyond 4000 pulses, with thicker NiOx films, the efficiency starts to decline, showing increased variability at 6000 and 8000 pulses. This is likely due to decreased charge transport efficiency as the thicker layer may introduce more resistance, hampering charge extraction. Figure 14(b) shows the efficiency of the best-performing PSCs at each thickness level. Similar to Figure 9(a), the maximum efficiency is observed at 4000 pulses, where the efficiency reaches 7.6 %. The J−V curve in Figure 14(c) shows the current density and voltage characteristics of the best-performing PSC under forward and reverse scan directions. The device, prepared with 4000 pulses (optimal thickness), demonstrates minimal hysteresis between the forward (red) and reverse (black) scans. These findings align well with previous discussions on the effects of oxygen pressure and bandgap tuning. This suggests that an optimal balance can be achieved in terms of bandgap and film thickness, enhancing charge transport and reducing defect states. Furthermore, these findings support the earlier simulation results that indicated how carefully optimised deposition conditions lead to higher solar cell efficiencies. Conclusions In this study, we successfully prepared NiOx HTLs by pulsed laser deposition under varying conditions and investigated the effect of bandgap variation on the performance of perovskite solar cells. The study reveals that variations in deposition temperature, laser frequency, and oxygen pressure alter the chemical composition, optical properties, and defect states in NiOx films. The fabrication of FA0.83Cs0.17Pb(I0.6Br0.4)3 mixed halide PSCs using PLD-grown NiOx HTLs with varying oxygen pressures and film thickness resulted in a maximum power conversion efficiency of around 8%, without the use of an extra dipole layer, confirming PLD as a promising technology for tuning the properties of highperformance HTLs. Combined with theoretical device simulations, these findings highlight the importance of NiOx bandgap tuning by controlled deposition techniques like PLD in optimising the efficiency of perovskite solar cells. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 18 Acknowledgement We acknowledge the support of the Czech Ministry of Education, Youth and Sports grant no. CZ.02.1.01/00/22_008/0004617 – „Energy conversion and storage “, the Czech Science Foundation GA ČR, grant no. 23-06543S and CTU student grant SGS24/135/OHK3/3T/13. References [1] T. Singh and T. Miyasaka, “Stabilizing the Efficiency Beyond 20% with a Mixed Cation Perovskite Solar Cell Fabricated in Ambient Air under Controlled Humidity,” Adv Energy Mater, vol. 8, no. 3, p. 1700677, Jan. 2018, doi: 10.1002/AENM.201700677. [2] M. H. Kuo, N. Neykova, and I. 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