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Vishnuvardhan Reddy Chappidi, Sudhanshu Kumar Nayak, Md. Emrul Kayesh, Md. Abdul Karim, Yulu He, [Ashraful Islam](https://orcid.org/0000-0002-1633-1432), Sai Santosh Kumar Raavi

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[Elucidating the improved properties of defect engineered lanthanum-doped nickel oxide as hole-transport layer in triple-cation perovskite solar cells](https://mdr.nims.go.jp/datasets/d53cbb19-d9ad-442b-afc7-e81e0dda0a32)

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Elucidating the improved properties of Defect Engineered Lanthanum-Doped Nickel Oxide as Hole-Transport Layer in Triple-Cation Perovskite Solar Cells Vishnuvardhan Reddy Chappidi1,2, Sudhanshu Kumar Nayak2, Md. Emrul Kayesh1, Md. Abdul Karim1, Yulu He1, Ashraful Islam1*, Sai Santosh Kumar Raavi2,3 *1Photovoltaic Materials Group, Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan2Ultrafast Photophysics and Photonics Laboratory, Department of Physics, Indian Institute of Technology Hyderabad, Kandi 502285, Telangana, India3Department of Climate Change, Indian Institute of Technology Hyderabad, Kandi 502285, Telangana, India* Corresponding author: ISLAM.Ashraful@nims.go.jp,   sskraavi@phy.iith.ac.inAbstractCharge recombination at the interface between the hole transport layer (HTL) and perovskite (PVK) has been a performance bottleneck for perovskite solar cells (PSCs). We present a detailed examination for the solar cell efficiency of the device using lanthanum (La)-doped nickel oxide (NiOx) as an HTL. The NiOx and La-doped NiOx films were prepared using the spray pyrolysis process. We employed low-temperature photoluminescence (LT-PL) to estimate the defect activation energy and utilized SCAPS 1D software to simulate the interface defect density. According to the data obtained, the interface between La: NiOx and PVK shows a lower activation energy for defects, indicating that it is more advantageous for charge transfer compared to the interface between NiOx and PVK. Utilizing SCAPS simulations the experimental JV curves closely match the simulated JV curves obtained from SCAPS simulations. These simulations were performed using optimal parameters, which involved increasing the Rsh values and reducing the interface density in La:NiOx based PSCs. The interface defect densities are estimated to be La:NiOx/PVK and NiOx/PVK interfaces are 1×1012 cm-2 and 1.7×1012 cm-2, respectively. This indicates  70 % reduction in defect density at the La:NiOx/PVK interface compared to the NiOx/PVK interface. The conductivity values obtained from linear sweep voltammetry (LSV) are 1.01×10-3 S cm-1 for NiOx films and 1.21×10-3 S cm-1 for La: NiOx films. This indicates a significant enhancement of  20 % in the conductivity of La: NiOx films compared to undoped films. This leads to improvements in VOC and ultimately enhances the PCE. The calculating defect density at the HTL/PVK interface can contribute to the fabrication of futuristic highly efficient PSCs.Keywords: Halide Perovskite Solar Cells, NiOx, SCAPS, Hole-transport material, Recombination, Photoluminescence.1. IntroductionIn recent years, PSCs have gained popularity due to their rapidly increasing power conversion efficiency (PCE), ranging from 3.8% to 25.7% [1, 2]. The perovskite absorber layer enables the PSCs to have broadband absorption, high carrier mobility, long carrier diffusion length, ambipolar transport characteristics, low electron-hole recombination rate, as well as a cost-effective and simple solution processing method [3-5]. Many of the most efficient perovskites are based on the standard n-i-p structure, but they are prone to instability and hysteresis. As an alternative to conventional devices, the p-i-n structure inverted planar solar cells (IPSCs) gained popularity due to their ease of manufacture, low hysteresis, and excellent environmental stability [6, 7]. In IPSCs, the HTLs are considered to be an important component for achieving high-performance PCE. Organic hole transport materials like, PEDOT:PSS, PTAA has been used widely in IPSCs [8, 9]. However, they faced instability and high costs, despite having the highest PCE. For alternative low expensive, Inorganic materials have been used as an HTL in IPSCs, such as CuSCN [10], CuI [11], Cu2O [12], NiOx [13], and V2O5 [14]. Among them, NiOx has a wide bandgap greater transmission, natural stability, and good energy level alignment. However, the inherent low electrical conductivity of NiOx contributes to increased recombination rates and hinders extraction of holes, ultimately leading to a decline in the VOC [15]. It is well known that large ionization energy of Ni vacancies limits hole density in NiOx [16]. To improve further electrical conductivity, researchers explore the methods of surface modification, plasma treatment, and doping [17-19]. Among these methods, doping is extensively employed to enhance the electrical and optical characteristics of NiOx. Among various dopants, rare earth (RE) ions have rich energy level structure with empty 5d orbitals and partially filled 4f orbitals that help to improve the free charge carrier in the host atom [13, 20, 21].The triple cation (TC) perovskites offer several advantages over single cation perovskites in solar cell applications, including better charge transport, tunable bandgap,  improved stability, and higher PCE [22, 23].  In this study, we focused on the optimized triple cation perovskite (TC-PVK) composition as reported in the literature [24]. Here, we discuss several reports on TC-PVK based devices utilizing NiO as the HTL. According to Yang et al., a 3% potassium (K) doped NiOx, synthesized using the sol-gel process, achieved a device configuration of FTO/K: NiOx/TC-PVK/TBABF4+PCBM/TIPD/Ag with a notable 17.05% PCE [25]. Boyd et al. reported on a device architecture consisting of glass/ITO/NiOx/TC-PVK/LiF/C60/bathocuproine (BCP)/Ag, utilizing various molar excesses of A-site in the perovskite. The leading device, incorporating 3 mol% excess FAI, demonstrated a superior PCE of 19.66% compared to other compositions [26]. Li et al. introduced a bilayer of NiOx/2BrPXZPA. The device configuration ITO/HTL/TC-PVK/spiro-OMeTAD/Au achieved a notable PCE of 23.66%, surpassing the efficiency of NiO-based devices [27]. In this paper, our primary focus is not on enhancing device efficiency but rather on exploring the recombination mechanism occurring at the interface between the HTL and PVK. The novelty of this study lies in its methodology for investigating recombination at the interface between the HTL and PVK by determining defect activation energy through LT-PL, while also estimating interface defect density through SCAPS 1D simulation.Recombination is the main concern due to a mismatch of energy level at the interface of electron transport layer (ETL): HTL/PVK, the presence of defects at the interface, and intrinsic defect present in PVK, collectively leading to recombination events that diminish device performance [28, 29]. Notably, radiative recombination involves the interaction between free electrons and free holes, although this form of recombination remains relatively feeble in PSCs [30]. Conversely, non-radiative recombination critically hampers the open-circuit voltage (VOC)  occurring as electrons recombine with a holes defect or impurity sites [31]. Improving VOC can be achieved through techniques such as regulating the crystallinity of PVK, optimizing interface engineering and passivation of defect layers [32-34]. Stolterfoh et al. explored recombination losses loss in triple cation ((CsPbI3)0.05((FAPbI3)0.83(MAPbBr3)0.17)0.95) and CH3NH3PbI3 (MAPbI3) utilized photoluminescence measurements to reveal MAPbI3 exhibited a smaller quasi-fermi level splitting and greater non-radiative losses compared to the triple cation perovskite, resulting reduction in VOC of the device [35]. Employing a combination of time resolved photoluminescence (TrPL), transient absorption spectroscopy, Laquai et al. provided insights into recombination analysis in NiOx:PTAA/MAPbI3 film and found that PTAA/MAPbI3 film exhibited a shorter lifetime and reduced non-radiative recombination in comparison to the NiOx/MAPbI3 film, leading to an enhanced PCE in PTAA based devices [36]. Gloeckler etc.al, explained defect analysis in Cadmium Telluride (CdTe) photovoltaic solar cells (SCs) using defect activation calculated from LT-PL [37]. This paper extensively explore the recombination analysis at the NiOx/PVK and La:NiOx/PVK interface, employing the concepts discussed above. In the present work, we employed the spray pyrolysis technique to fabricate NiOx and La:NiOx HTLs for the efficient IPSCs. We systematically examined both the structural and optical properties of these HTLs. Incorporating La: NiOx as HTL led to improved charge conductivity and diminished recombination at the interface of HTL/PVK, resulting to notable improvement in PCE as compared to NiOx based IPSC. Additionally, we have performed recombination analysis at the HTL/PVK interface using LT-PL and SCAPS 1D simulation. This remarkable improvement underscores the potential of the La:NiOx film as a highly promising candidate for effective IPSC development. 2. Experimental and Simulation details:2.1. Materials: All the reagents used for synthesizing the (La)-doped nickel oxide (NiOx) and triple cation perovskite Cs0.08(MA0.17FA0.83)0.92Pb(I0.83Br0.17)3 were purchased from Sigma-Aldrich, Wako Co., Japan, and Tokyo Chemical Industry Co., Japan. All details of chemicals are provided in supporting information (SI).2.2. Device fabrication: Initially, the ITO/glass substrates were cleaned in an ultrasonic bath for 20 minutes using soap solution, deionized H2O, ethanol, and isopropanol. After drying the cleaned substrate with N2, UV ozone irradiation was performed for 30 minutes to eliminate the organic residue left on it. The NiOx/La:NiOx films were prepared by spray pyrolysis method. The solution contains 0.1mM Ni(NO3)2·6H2O, La(NO3)2·6H2O molar ratios (1%, 3%, 5%) [13] in 1ml of ethanol was sprayed on clean FTO substrates at 570⁰C. The precursor solution was formulated by dissolving PbI2, (1.1 M), PbBr2 (0.2 M), FAI (1 M), and MABr (0.2 M) in a solvent mixture of DMF and DMSO in a 1:4 (v: v). In a subsequent step, a separate solution containing CsI (1.5 M) in DMSO was prepared and subsequently mixed with the first solution to achieve the desired Cs content of 8%. The solution was stirred for 1 h at 40 ⁰C and filtered by a 0.45-μm PTFE syringe filter. The precursor solution was applied onto the ITO/ NiOx or La: NiOx substrates using spin-coating at 1000 rpm for 10 s, followed by 5000 rpm for 20 s. At 27 seconds, 300 μL of CB was deposited onto the spinning substrate. The substrates were subsequently subjected to annealing at 150°C for a duration of 10 minutes. After that, PCBM (20 mg mL−1 in CB) and BCP (0.5 mg mL−1 in methanol) were spin-coated with speeds of 1000 rpm for 30 s and 6000 rpm for 30, respectively.  These substrates were annealed at 60 °C for 10 min in each step. Finally, the 80 nm thickness of the Ag electrode was thermally evaporated in a vacuum chamber (base pressure < 9×10-4 Pa) through a shadow mask.2.3. Characterization:The optical absorption spectra were measured by a Shimadzu UV/Vis 3600 spectrophotometer. The conductivity of the films was measured using linear sweep voltammetry (LSV) with a three-electrode setup. The working electrode was the NiOx and La:NiOx films, the reference electrode was Ag/AgCl, and the counter electrode was a platinum electrode. The electrolyte solution used was 0.1M KCl. The measurements were conducted using an Autolab PGSTAT 302N equipped with a frequency analyzer and NOVA 1.11 software. Photoelectron emission spectroscopy (PES) was measured with a Rikaken Keiki AC-3 spectrometer. The current-voltage (J-V) curves were obtained using a solar simulator (WXS-155S-10, Wacom Denso Co., Japan) with standard air mass 1.5 sunlight conditions (100 mWcm-2). PL and TrPL were measured by FLS 1000 photoluminescence spectrometer, Edinburgh instrument. XPS was recorded by AXIS SUPRA.2.4. Simulation details: The solar cell is composed of FTO, active material, HTL, and electrode in the manner of glass/FTO/NiOx::La:NiOx/Cs0.08(MA0.17FA0.83)0.92Pb(I0.83Br0.17)3/PCBM/BCP/Ag. In this study SCAPS-1D software was employed to evaluate the efficiency of the perovskite solar cell [38]. In order to simulate the structure, the following equations are required Poisson equation:-∂∂x-εx∂V∂x=q px-nx+ND+x-NA-x+ptx-ntx                                           Continuity equation for the hole:∂p∂t=1q∂JP∂x+GP-RPContinuity equation for the electron:∂n∂t=1q∂Jn∂x+Gn-Rn   Where q is the charge, ε is the dielectric permittivity, V is the potential, px is the concentration of free holes, nx is the concentration of free electrons, ND+x is the ionized donor concentration, NA-x is the ionized acceptor concentration, ntx, and ptx are the trap density of electron and hole, respectively, Jn and JP are the electron and hole current densities Gn is the rate of electron generation, GP is the rate of hole generation, and Rn and RP are the rates of electron and hole recombination, respectively.The parameters for NiOx, PVK, and PCBM are sourced from literature [39] [40-42]. The work function values for FTO and the back contact (Ag) were set at 4.9 eV and 4.71 eV, respectively. The bandgap energies (Eg) of NiOx and La:NiOx are determined using Tauc plots, as illustrated in Fig. S1(d), yielding values of 3.49 eV and 3.53 eV, respectively. The valance band minmum (VBM) for NiOx and La:NiOx are calculated from photoelectron emission spectroscopy (PES), resulting in VBM values of 5.37 eV and 5.41 eV, respectively, as shown in Fig. S2(a-b). The electron affinity values for NiOx and La:NiOx are estimated from calculations based on the VBM and band gap values, yielding values of 1.88 eV and 1.89 eV, respectively. We assumed electron and hole mobilities in La: NiOx to be 13 cm2/vs and 2.9 cm2/vs, respectively. Additionally, we assumed the acceptor density in La:NiOx to be 3×1018 cm-3, considering the general trend of increased doping density after doping. The valence band of effective density states in the La:NiOx film were assumed to be 8.1×1020 cm3. Absorption coefficients in both films were calculated using the relationship α(E)=Aα(hν-Eg), with Aα set to 105 cm-1 eV-0.5. For defect densities, we assumed a value of 1×1014 eV for NiOx and La: NiOx. Series resistance (Rs) values for NiOx and La:NiOx were assumed to be 3.2 Ω.cm2. and 3.2 Ω.cm2, respectively, while shunt resistance (Rsh) values for NiOx and La:NiOx were also assumed to be 1700 Ω.cm2 eV and 2800 Ω.cm2, respectively. We assumed the remaining parameter values for La:NiOx to be consistent with those of NiOx. Simulations were conducted using parameters provided in Tables 1 and 2. The outcome of J-V curves showed better agreement with experimental J-V curves, as depicted in Fig. 5(b), providing valid evidence of the accuracy of our simulations.3. Results and discussion    Intially, we optimized doping percentange of La (3%) by JV curves as shown in Fig. S1. In order to explore the effect of La (3%) doping on structural and optical properties of NiOx, we have performed the XRD measurement to observe the effect of La doping on the phase and the crystal structure of the material. The XRD patterns of NiOx and La:NiOx films deposited on FTO are shown in Fig. 1(a). The Diffracted peaks associated with NiOx were identified at positions 2θ of 37.27°, 62.9°, and 79.3°, attributed to the (111), (220), and (222) crystallographic planes, respectively. These peaks align with the cubic phase of NiOx (indexed as JCPDS 47-1049) [43]. The peaks corresponding to FTO were recognized at 2θ values of 33.9°, 51.7°, 54.84°, and 65.7°, assigned to the (101), (211), (220), and (301) planes, respectively. These peaks concord with the tetragonal structure of SnO2 indexed as JCPDS 41-1445) [44]. The XRD pattern of the La-doped NiOx film demonstrated similarity to the NiOx film pattern, with no additional peaks related to impurities or phases of RE2O3 being observed.Fig. 1. (a) XRD pattern of NiOx and La doped NiOx., (b) energy diagram of perovskite, NiOx and La: NiOx.X-ray photoelectron spectroscopy (XPS) was employed to examine the chemical compositions and oxidation states of La:NiOx and NiOx films. Fig. 2 illustrates the core-level of XPS spectra of Ni 2p and O 1s of NiOx, and La:NiOx films. The XPS spectra of Ni 2p for both films exhibited a split into 2p1/2 and 2p3/2 components due to spin orbit coupling [45]. The peaks (2p3/2) peaks were identified at energy levels of 851.13/853.07 eV and 850.94/852.80 eV corresponding to Ni2+ and Ni3+states, respectively, along with shakeup-satellite peaks at 858.19 and 858.12 eV respectively, as shown in Fig. 2(a-b). The peak corresponding to Ni2+ is attributed to the Ni-O octahedral bonding present in cubic NiO, while the Ni3+ peak  induced by Ni vacancy in Ni2O3 and presence of NiOOH [46, 47]. Fig. 2. The core-level of XPS spectra of Ni 2p (a) NiOx, (b) La: NiOx films and O 1s of (c) NiOx, (d) La: NiOx films.Moreover, the calculated ratio of Ni3+/Ni2+ were estimated as 1.5 and 1.2 for La:NiOx and NiOx films, respectively, as mentioned in Table S1. In general, the conductivity in NiOx is due Ni vacancies and oxygen interstitials. Notably, the introduction of  RE ions with a (+3) charge induces Ni vacancies [48]. In La:NiOx film, the observed increase in the ratio of Ni3+/Ni2+ suggests enhanced conductivity. This finding is consistent with the conductivity values obtained from LSV, as shown in Fig. 3(b), where conductivity values are 1.01×10-3 S cm-1 for NiOx films and 1.21×10-3 S cm-1 for La: NiOx films, respectively. Regarding the O 1s peak, both films (Fig. 2c, 2d) displayed peaks at 526.5 eV and 526.3 eV, corresponding to the Ni2+ state and indicating Ni-O octahedral bonding within NiO and La:NiOx films, respectively. Additionally, peaks at 528.6 eV and 528.5 eV in NiO and La:NiOx films corresponding to Ni3+ state, suggesting the presence of metal deficiency Ni3+ ion or excess of oxygen from NiOOH [49, 50]. The calculated Ni3+/Ni2+ ratios were 1.27 and 1.29 for NiOx, and La:NiOx films, respectively (Table S1). Furthermore, the XPS spectrum of La 3d in the La: NiOx film revealed peaks at 824 eV, corresponding to La 3d3/2, as shown in Fig. 3(a). These peaks signify the incorporation of La into the NiOx structure.Fig. 3. (a) The XPS spectrum of NiOx and La:NiOx films, with an inserted La 3d spectrum, (b) the current vs voltage curves of NiOx and La:NiOx films.Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) analysis were used to understand the surface morphology NiOx and La:NiOx films. As shown in Figure 4 (a-d), the quality of La:NiOx film is superior to NiOx film. According to AFM analysis, the La:NiOx film is smoother, with a lower root mean square roughness of 15.6 nm compared to 16.8 nm for NiOx. Higher smoothness means that the La helps to generate more uniform gain sizes and passivates grain boundaries, resulting in fewer surface defects and a smoother, higher quality film. The smooth and quality film surface helps to create better contact with the PVK layer, enhancing carrier transport and reducing recombination loss, resulting in an increase in device PCE.                 Fig. 4. The SEM and AFM images of (a,c) NiOx and (b,d) La:NiOx films.We fabricated an inverted structure consisting of FTO/La:NiOx (or) NiOx/PVK/PCBM/BCP/Ag to examine photovoltaic characteristics, as depicted in Fig. 5(a). The J-V measurements of IPSC were examined under 1 sun (1000 mWcm-1) illumination conditions. Additionally, we conducted simulations to generate J-V curves for the IPSCs under the same illumination conditions. Remarkably, the experimental J-V curves for devices closely matched the simulated values, as depicted in Fig. 5(b). Notably, the La:NiOx based IPSC exhibited with better J-V curves in comparison to the pristine device. The detailed results, presented in Table 3, highlight that the La:NiOx based IPSC achieved the average PCE of 11.36 % with VOC with a value of 0.87 V, JSC of 18.88 mAcm-2 and FF of 68.5%. In contrast, the NiOx based IPSC achieved the average PCE of 9.7% with VOC of 0.83 V, JSC of 18.84 mAcm-2 and FF of 61.4%. In comparison, the La:NiOx based IPSC exhibited higher PCE than NiOx based IPSC. The increase in VOC can be attributed to several factors, one of which is a reduction in recombination at the interface between La:NiOx and the PVK layer. Interface recombination is affected by surface recombination velocity (S), which is defined as S=σVthNit where, σ , Vthare capture cross section area, thermal velocity of charge carriers, and Nit interface defect density [51]. The S values for NiOx and La:NiOx based IPSC are 1.7 cm/s and 1 cm/s, respectively. The decrease of S is due to decrease of interface defect density at La:NiOx and PVK interface, resultant enhanced VOC. Another contributing factor to the VOC enhancement could be the reduction in the energy gap between the valence band of La: NiOx and the highest occupied molecular orbital (HOMO) level of PVK. This reduction in gap, as observed in the PES data (Fig S2(a-c)) leads to a stronger potential at the interface, which facilitates the transfer of a greater number of charge carriers and consequently enhances VOC. Furthermore, it was observed that the VOC decreases with higher interface defect density, exhibits a slight increase with NA, and remains nearly unchanged with increasing values of Rsh, μe, and μh, as depicted in Fig. S3. The enhancement of FF is due to increase Rsh. The Rsh arises primarily from imperfections or defects in the cell's material or manufacturing process [51]. The Rsh values are estimated from simulations are 1700 Ω.cm2 and 2800 Ω.cm2 of NiOx and La:NiOx based IPSC, respectively. The increase in Rsh results in an improved FF, which in turn leads to an enhanced PCE. Furthermore, it was observed that the FF decreases with higher interface defect density, exhibits an increase with NA, and remains nearly unchanged with increasing values of μe, and μh, as shown in Fig. S3. The JSC is almost unchanged the increases of parameters interface defect density, NA, Rsh, μe, and μh, as shown in Fig. S3. This finding is consistent with the IPCE results. The JSC primarily depends on the thickness of the PVK layer. We observed that the overall improvement of PCE in La:NiOx based IPSCs is attributed to a decrease in interface defect density and increases in NA and Rsh.Fig. 5. (a) Schematic diagram of IPSC, (b) Simulated and experimental J-V curves of NiOx and La:NiOx based IPSC. (c). IPCE as a function of wavelength for, and (d) EIS spectra of NiOx and La:NiOx based IPSC.Further, we conducted the incident photon to current conversion efficiency (IPCE) measurements for both NiOx and La:NiOx based IPSCs. The IPCE values at 530 nm for the NiOx and La: NiOx based IPSCs are 53.1% and 54.4%, respectively, as illustrated in Fig. 5(c). The almost unchanged in IPCE of La:NiOx and NiOx based IPSC. Additionally, we conducted EIS tests to analyse the charge transfer resistance, as shown in Fig. 5(d), which are fitted using a model (as shown in the inset). The charge transfer resistance (Rrec) value of La:NiOx device is higher than NiOx based device. The large Rrec signifies the suppression of charge recombination and acceleration of charge transport, ultimately contributing to improvement in the fill factor (FF).Table 1     the physical parameters of NiOx, La doped NiOx, PVK, and PCBM      Parameters NiOx La doped NiOx * PVK PCBM Thickness (nm) 30 30 375 50 Band gap (eV) 3.49# 3.53# 1.67# 2 Electron affinity χ (eV) 1.88# 1.89# 3.93# 3.92 Dielectric permittivity (ε) 10.75 10.75   6.5  4 CB effective density of states Nc  (cm-3) 2.8×1020 2.8×1020 1×1018 2.5×1021 VB effective density of states NV  (cm-3) 8.1×1020 8.1×1020 1×1018 2.5×1021 Mobility μe (cm2/v.s) 1.2×101 1.3×101 2 1×10-2 Mobility μh (cm2/v.s) 2.8×100 2.9×100 2 1×10-2 Electron thermal velocity Ve (cm/s) 1×107 1×107 1×107 1×107 Hole thermal velocity Vh (cm/s) 1×107 1×107 1×107 1×107 Total defect density Nt (cm-3) 1×1014 1×1014 1×1013 1×1015 Shallow uniform donor density ND (cm-3) 0 0 0 1×1015 Shallow uniform donor density NA (cm-3) 1×1018 3×1018 1×1015 0Table 2Obtained defect density of PVK and at interfaces between CTL and PVK.  NiOx/PVK La:NiOx/PVK* PVK/ETL Type of Defect Neutral Neutral Neutral Electrons Capture cross section (cm2) 1×10-19 1×10-19 1×10-19 Holes Capture cross section (cm2) 1×10-19 1×10-19 1×10-19 Energetic distribution Single Single Single Defect energy level Reference Et Above the highest EV Above the highest EV Above the highest EV Energy level related to reference (eV) 0.6 0.6 0.6 Total density -integrated over all energies (cm-2) 1.7×1012 1×1012 1×1012# Experimental * assumed.Table 3Photovoltaic parameters of NiOx and La-doped NiOx IPSCs under AM1.5G illumination at 100mWcm-2.   VOC(V) JSC(mAcm-2) Fill Factor(F.F)(%) PCE(%) Pristine NiOx Experimental simulation 0.830.810.850.82 18.8419.1618.5218.86 61.461.162.662.2 9.7 (average)9.5 (forward)9.9 (backward)9.7 La-doped NiOx Experimental simulation 0.870.87 0.880.89 18.8818.9418.8318.88 68.568.568.467.4 11.3(average)11.3(forward)11.4(backward)11.4In addition, we conducted UV-vis measurements to study of optical properties of both NiOx and La:NiOx films. Analysis of the Tauc plots (Fig. S2(d)) revealed that the band gaps of NiOx and La:NiOx films are 3.45 eV and 3.53 eV, respectively. Moreover, we employed PES to examine the impact of La3+ doping on the energy bands. The calculated VBM values for NiOx and La:NiOx, as shown in Fig. S2(a-b), were calculated to be 5.33 eV and 5.41 eV, respectively. Based on these findings, we estimated energy diagram, showcasing the close proximity of the VBM of the La:NiOx film to that of the perovskite layer as compared to the pristine film, as shown in Fig. 1(b). As the energy levels are closely aligned, the inherent potential difference strengthens, which could impart an additional driving force for hole transport. This enhancement may accelerate carrier transport and impede recombination at HTL/PVK interface, leading to an increase in device performance. Fig. 6. Room temperature (a) PL and (b) TrPL of perovskite with free HTL, NiOX. (c) Activation energy, (d) PL intensity at 80K of NiOx /PVK and La:NiOx /PVK.To examine the transfer of charges from perovskite to HTLs, extensive photoluminescence (PL) measurements at room and low temperature (77K) were performed using FLS-1000 Edinburgh Instrument. We prepared samples with the configuration of FTO/HTL/PVK using the same preparation method employed for SCs and were photoexcited under of 510 nm wavelength. The emission peak wavelengths were recorded at 784 nm, 776 nm, and 774 nm for PVK, NiOx/PVK, and La:NiOx/PVK configuration, respectively, as illustrated in the Fig. 6(a). Significantly, the La:NiOx/PVK film exhibited a distinct blue shift. This phenomenon of blue shift has also been  observed by Tingli Ma et al., [52]. This shift could be attributed to a reduction in defects at the interface between La:NiOx and PVK layers. Furthermore, the La:NiOx/PVK film exhibited a greater quenching effect compared to the NiOx/PVK film. This quenching effect suggests that the La:NiOx film is more favourable to charge extraction from PVK.In order to understand the dependence of hole injection at HTL/PVK interface, TrPL were measured under the emission wavelength of 784 nm with the 475 nm EPL picosecond diode laser using time correlated single photon counting (TCSPC), as shown in Fig. 6(b). The Curves were fitted by the exponential equation, It=B1exp-t1+B2exp-t2, and the average decay time was calculated by avg=Bii2Bii . Where 1 and 2 are fast and slow decay components, respectively. The fast decay attribute to charge transfer at HTL/PVK, whereas slow decay associated to bimolecular radiative recombination. It deals with the recombination between unbound electrons and holes will begin to take over [53]. The summarised lifetime parameters as shown in Table 4. The 1 decay times for PVK with HTL-free, NiOx and  La:NiOx  are 189.3 ns, 68.4 ns and  23.1 ns respectively. The short decay time indicates photo generated charge carriers are effectively transported at La: NiOx/PVK interface [54] due to energy level of La:NiOx close to PVK, as confirmed by  PES measurements. The less Long decay time 2 of  La:NiOx/PVK interface compare to pristine suggests, implies a reduction in LTdefect density. These results consistent with EIS results.Table 4Lifetime parameters from exponential fitting of TrPL measurements.   B1(%) 1(ns) B2(%) 2 (ns)  avg (ns) FTO/PVK 26.1 18.5 60.4 189.3 182.4 FTO/NiOx/PVK 50.2 18.4 6.95 68.4 35.3 FTO/La:NiOx/PVK 43.9 4.07  31.3 23.1 19.4Fig. 7. Low temperature PL and contour plots intensity dependence upon temperature and wavelength for (a, c) NiOx/PVK, and (b, d) La:NiOx/PVK films.To deep understand about the recombination at HTL/PVK interface, we have measured LT-PL ranging from 80 K to 300 K for La:NiOx/PVK and NiOx/PVK under excitation wavelength of 510 nm, as shown in Fig. 7(a) and (b), respectively. The emission peaks were observed at 795 nm and 793 nm at 80K for NiOx/PVK and La:NiOx/PVK films, respectively. These blue shifts in emissions were observed both at low temperature and room temperature. This phenomenon could be due to decrease in defects at interface between La: NiOx/PVK. The PL quenching of La:NiOx/PVK observed even at low temperature as shown in Fig. 6(d). In addition, as temperature increase blue shift is observed in both films, this is due to decrease in bandgap with decrease in temperature, this in normal effect in lead perovskites [55]. Both films exhibit strong PL intensity at low temperature and become weaker at higher temperature, which is attributed to non-radiative recombination, as shown in Fig. 7(c) and (d). Further, we calculated defect activation energy (Ea) by integrated PL intensity (Fig. 6(c)) using following equation [56, 57].IT=I01+C1exp-EakBTWhere, Io is PL intensity at lowest temperature, KB is Boltzmann’s constant and C1 is a temperature-independent coefficient. The values of Ea for NiOx/PVK and La:NiOx/PVK are extracted from fitting with 66.19±0.83 meV and 54.14±0.78 meV, respectively. The decrease in Ea depicts the reduction of defect density at La:NiOx/PVK interface. The reduction in defect density results in improved charge transport and decreased recombination losses, ultimately leading to enhanced PCE.  4. ConclusionIn summary, we studied recombination analysis on the effect of La doping NiOx/PVK    interface through LT-PL. We calculated the values of Ea for NiOx/PVK and La:NiOx/PVK with 66.19±0.83 meV and 54.14±0.78 meV respectively, a decrease in Ea suggested the defect density reduced at La:NiOx/PVK interface. XPS observations revealed that the Ni3+/Ni2+ ratio increased with La doping leading to enhanced conductivity in the La- NiOx film. The results obtained from LSV indicate a significant increase of 20 % in the conductivity of La:NiOx films compared to NiOx films. The better energy alignments improve the hole extraction and reduce the recombination at La:NiOx/PVK interface. Consequently, the PSCs with La- NiOx as HTL improved PCE with VOC of 0.91 V and JSC of 18.9 mAcm-2. Additionally, we estimated the interface defect density to be 1.7×1014 cm-2, 1×1014 cm-2 for the NiOx/PVK interface and La:NiOx/PVK interface, respectively. The successful incorporation of La resulted in an enhanced PCE of the PSCs and a reduction in interface defects. 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