Er-Doped SnO2 as an Electron Transport Layer for Efficient Inorganic MAPbI3 Perovskite Solar Cells with Improved Performance

INTRODUCTION
Due to their low cost, robust light absorption capacity, solution-based preparation method, adjustable band gap, and extended carrier diffusion length, organic-inorganic lead hybrid perovskites are becoming increasingly popular among researchers worldwide. It is also an ideal material for light-absorbing material in solar cells [1-3]. Within just a few years of the introduction of perovskite solar cells in 2009, the power conversion efficiency of solar cells utilizing perovskite materials for light absorption has surged from 3.8% to 25.5% [4-10]. The most common components of PSCs are an electron transport layer (ETL), a hole transport layer (HTL), and a counter electrode and also, there are bipolar electron and hole transport properties in perovskites [11-13]. In the course of PSCs development, devices without HTL and an electron-free transport layer structure have emerged to reduce the cost of the device and simplify the preparation process [14, 15]. By utilizing this arrangement, the present device has showed the PCE about 15.7% [5, 16-19]. The performance of the device is determined by the compact layer, which blocks holes and transports electrons. Light transmission, minimal surface imperfections, uniform and dense film formation, hole-blocking ability, electron mobility, and an energy band structure compatible with perovskite materials are some of the criteria for a brilliant compact layer. SnO2 as a semiconductor of the n-type, is an ideal compact layer material because of its good UV stability, relatively low conduction band bottom, and high electron mobility (~240 cm2 V-1 s-1) [20-22]. The SnO2 layer is prepared using numerous methods, including chemical bath deposition, atomic layer deposition, and spray pyrolysis [23-28]. For PSCs to achieve superior band alignment and charge transport, the compact layer’s surface contact and electrical properties must be enhanced. Commonly employed optimization techniques encompass surface treatment, metal cation doping, and the development of composite materials. Rao et al. achieved a remarkable boost in the power conversion efficiency (PCE) of the device, increasing it from 6.5% to 14.6%, by applying an aqueous TiCl4 solution to treat the SnO2 compact layer’s surface [29]. To enhance the electrical conductivity of the SnO2 compact layer and decrease the conduction band energy of SnO2, Park et al. used Li-doped SnO2. The result was the facilitation of the electron transfer from the perovskite layer to the compact layer, thereby achieving the PCE of 18.2% [30]. Bai et al. augmented the electrical conductivity of the SnO2   compact layer by doping it with Sb, thereby enhancing its Fermi energy level. This resulted in a greater transfer of charges while effectively preventing the recombination of carriers, leading to the PCE elevating from 15.7% to 17.2% [31]. Liu et al. combined the advantages of TiO2 and Mg-doped SnO2 to produce a composite compact layer with outstanding capabilities. Therefore, they observed an increase in the PCE of the device from 11.85% to 13.01%  [32].  Liu et al. combined the advantages of TiO2 and Mg-doped SnO2 to produce a composite compact layer with outstanding capabilities. Therefore, they observed an increase in the PCE of the device from 11.85% to 13.01% [33]. Doping metal ions in the electron transfer layers improves the optoelectronic properties of perovskite solar cells by increasing the energy level between them and the perovskite material, thus modifying its energy band [34-38].
In this study, we initially used the compact layer of erbium doped with SnO2 for planar PSCs, with the PCE achievement of 6.07%. During the annealing process, erbium doping affects the crystallization of SnO2, resulting in perfect film coverage without crystal aggregation and pin-holes. Furthermore, erbium doping improves the conductivity of the SnO2 layer and results in an upward shift of the conductive band minimum (CBM), which contributes to the increase in Voc and suppresses the hysteresis behavior. As demonstrated by the simple interfacial control approach presented in this study, Er doping is a feasible strategy for improving the performance of perovskite solar cells.

MATERIALS AND METHODS
Material
The purchased materials are as follows: anhydrous tin(IV) chloride (SnCl4, 99.95%), (Er(NO3)3, 99.8%), anhydrous ethanol, anhydrous dimethylsulfoxide (DMSO, 99.9%), anhydrous dimethylformamide (DMF, 99.8%), Chloro benzen, and Ethanol from Wako, Japan; Lead nitrate, Potassium iodide, ITO (10 Ω/sq) was bought from Sepahan Gostar. phthalic anhydride, urea, copper chloride dihydrate, ammonium molybdate, boric acid was purchased from Merck company. No further purification is performed on the solvents before utilizing them.

Constructing ETL 
ITO substrates were washed by sonication with detergents, DI water, acetone and isopropanol consecutively for 15 min. First, to make SnO2 ETLs, pure-ethanol and SnCl4 were selected as precursors. The ITO/SnO2 structure was prepared by dissolving SeCl4 in ethanol solvent with a concentration of 0.07M. Next, the Er doped SnO2 structures were synthesized by adding appropriate amount of Er(NO3)3 in the precursor solution. All layers were coated using the spin coating method in ambient conditions for 30 s at 3000 rpm. Finally, all the films were sintered at 70°C for 30 min. Thereafter, the SnO2 or Er:SnO2 films were subjected to a UV-Ozone (with 200 W power of UV-C band) treatment with a duration of up to 15 min. It has been reported that the treatment of electron transfer layers with UV-Ozone can increase the wetting of the substrate before coating [37, 38].

Constructing perovskite layer
The Golden Rain method was used to synthesize PbI2 [36]. To accomplish this, two separate solution containing 0.1 M lead nitrate solution in one beaker and 2 M potassium iodide solution in another beaker were prepared using the distilled water solvent, and then placed on the heater-stirrer at a temperature of 80°C for 2 h. The two solutions are mixed after reaching the desired temperature, triggering the formation of PbI2 as Golden Rain. Subsequently, methyl ammonium iodide is synthesized as follows: A mixture of 30 ml methylamine and 30 ml HI is stirred using magnetic stirrer in the ice bath for 2 h. The result solution is rested for 24 h. Afterwards; it is heated to 70°C to enable the solvents to evaporate, resulting in the formation of a brown solid matter. Solving this solid in ethanol yields a solution that can recreate the final product. Adding the solution to diethyl ether and crystallizing MAI white crystallites is the next phase. The crystals are separated from the solution by a filter and then dried at room temperature after being washed with diethyl ether.
   For precursor perovskite solution of MAPbI3, 0.22 g  methylammonium iodide (CH3NH3I) and 0.64 g lead iodide (PbI2) were dissolved in  800 µL DMF and 200 µL DMSO mixture (volume ratio of 4 to 1) and then stirred at 70 °C for 2 h. Then, the samples were deposited by one-step spin coating at 4000 rpm for 34 s. At the last 20 s, 150 μl chlorobenzene solution as antisolvent was added to the substrates and subsequently the samples were subjected to annealing for 10 min at 100⁰C.

Constructing hole transporting layer
The general procedure employed for syntheses of copper phthalocyanines was described as; phthalic anhydride (0.27 M), urea (1.35 M), and copper chloride dihydrate (0.05 M) with ammonium molybdate as a catalyst were charged together into a 3-neck reaction flask. The reaction flask was placed in a microwave synthetic unit and heated to temperature of 180°C for 45 min. Reaction product was washed with 1N HCl (acid-treated) and 1N NaOH (alkali-treated) solutions for one hour. After filtration, it was dried at 105°C over 2 h in a dry oven, whereupon final product was obtained. The produced CuPc as HTL was settled via spin-coating at a speed of 3000 rpm for one minute on ITO/ETLs/Perovskite substrate. The produced CuPc as hole transfer layer was settled via spin-coating at a speed of 3000 rpm for one minute on ITO/ETLs/Perovskite substrate. Lastly, a gold cathode with a thickness of 80 nm was edeposited onITO/ETLs/Perovskite/CuPc cell
In this study, a regular cell structure using Er doped SnO2 as ETL was fabricated, as shown in Fig. 1a. Cathode and anode electrodes were ITO and Au, respectively. Also, electron and hole transfer layers (ETL and HTL) were Er: SnO2 and CuPc. In order to enhance the performance of the HTL, Er was doped with SnO2. Fig.1b indicates energy level of PSC’s each layer employed in this study. The carrier extraction and transport can be also enhanced using proper interfacial energy level alignment.

RESULTS AND DISCUSSION
Characterization
Characterization of nanoparticle, thin-film, and device: the FE-SEM method employs TESKAN equipped with an EDS morphology system. Structural features of the crystal are investigated using x-ray diffraction (X’Pert Pro MPD, PANalytical). Furthermore, analyzing the transmission and absorption spectrum through UV spectroscopy determines the optical characteristics of sediment layers. A 3-electrode configuration is used to perform cyclic voltammetry of films with different percentages of impurity. Also, a quartz cell and 0.5 molar KCl solution are used to solve the electrolyte solution for Mott-Schottky analysis in a 3-electrode configuration. 
   The electrodes used for working, reference, and counter are ITO/Er: SnO2, Ag/AgCl, and Pt, respectively. With a speed of 50 mV, the bias voltage is measured from -0.6 to 0.4 V. The solar cell was lightened at medium temperature utilizing air conditioners (humidity, approximately 20%) and a solar simulator (AM1.5G, 100 mW cm-2, IVIUMSTAT).

Characterization of nanoparticle (PbI2, CuPc and MAI)
The PbI2 nanoparticle and CuPc were characterized by FTIR transmittance spectra, as shown in Fig. 2. Fig. 2a presents the FT-IR spectrum of PbI2 nanoparticles without any organic components. Peaks at 3400 and 1650 cm-1 in the FT-IR spectrum are assigned to O-H stretching modes of adsorbed water molecules. However, the broad and strong hydroxyl stretching and bending bands observed around 3400 and 1650 cm-1 in the FT-IR spectrum suggest a different type of interaction. These bands are typically associated with adsorbed water molecules, indicating their presence on the PbI2 nanoparticle surface. While free O-H groups typically exhibit asymmetric and symmetric stretching vibrations around 3756 and 3652 cm-1, respectively, the FT-IR spectrum reveals broader and stronger bands at 3412 and 3383 cm-1. These deviations suggest the presence of hydrogen bonding between the O-H groups and the PbI2 nanoparticles.
Fig. 2b presents the FT-IR spectrum of CuPc, exhibiting characteristic peaks for aromatic C-H (3044 cm⁻¹), C=C macrocycle (1607 cm⁻¹), C=N (1504 cm⁻¹), C-C in isoindole (1330 cm⁻¹), in-plane C-H bending (1112 cm⁻¹), and out-of-plane C-H bending (723 cm⁻¹). Additional peaks at 1285 cm⁻¹ and 1164 cm⁻¹ correspond to C-N stretching and in-plane bending in isoindole, respectively.
Fig. 3 represents the XRD spectrum results for synthesized powders. The XRD pattern of CuPc crystals in Fig. 3a exhibits two distinct peaks at 7.07° and 9.24°, corresponding to (001) and (1¯02) reflections, respectively. These characteristic peaks are attributed to the β-CuPc phase, consistent with observations in previous studies.
The crystal structure of PbI2 solid nanoparticles was determined using X-ray diffraction (XRD). The results are presented in Fig. 3b. The vital diffraction peaks are placed as (001) at 12.80°, (101) at 25.60°, (003) at 38.70° and (202) at 52.50°. Fig. 3c presents the XRD pattern of methylammonium iodide (MAI), indicating the crystallographic planes associated with the observed peaks. The crystallite sizes of the samples CuPc, PbI2 and MAI were calculated by Scherrer equation and estimated about 31.51, 57, and 20 nm, respectively.
Figs. 4a and b indicate TEM images that were used to consider and find out the microscopic structure of MAI and CuPc samples at the nanoscale. Fig. 4a clearly indicates that the MAI nanoparticles formed an interjoining nanoparticulate structure. These nanoparticles have a regular shape with diameters ranging from 30 nm to about 100 nm. Such a structure produces a fast electron transport and a low grain boundary. The CuPc TEM image shows homogenous particles, matching with the SEM image (particle size = 50-100 nm). The large, circular nanocrystals in Fig. 4b indicate that these films have a polycrystalline character on a nanoscale.

ETL and device structure
The XRD pattern for the perovskite film formed on the SnO2 ETLs and CuPc film are shown in Fig. 5(a-c). Fig. 5b presents seven intense and sharp peak positions, corresponding to (110), (112), (220), (310), (224), and (314) crystal planes of perovskite, respectively [11]. The highest intensity of the (110) crystal plane of perovskite is observed when the concentration of SnO2 colloidal dispersion solution was 5 wt.%. This is due to the presence of a certain roughness on the surface of SnO2, which facilitates stress release during the annealing of the perovskite layer, resulting in a superior quality of perovskite crystallization with virtually no holes. The XRD pattern of the ITO/Er:SnO2/MAPbI3/CuPc solar cell (Fig. 5a) shows one diffraction peak at 7.04 degree, that is related to β-CuPc. 
The top view morphological images of SnO2 and Er: SnO2 are shown in Fig. 6. It is believed that the placement of erbium in the SnO2 lattice reduces the vacancies on the electron transport layer by reducing the surface pin-holes (Fig. 6a, b). A hypothesis has been made that the pin-holes and aggregated island surface may cause strong recombination at the interface between the ETLs and the perovskite layer, which is unfavorable for PSC applications. Lanthanide doping enhances in metal dioxide film quality by minimizing surface-state traps, resulting in improved photo-generated electron collection [39, 40]. Fig. 6c shows the cross-sectional FESEM of Er-SnO2 films. The electron transport layer with 160 nm thikness was deposited on ITO substrate, which improves electron release and ultimately contributes to the cell’s increased efficiency and Fig. 6d indicates the EDS analysis of ITO/Er:SnO2 layer that this layer was made up of Sn and Er elements.
Figs. 7a and d show the top-view FE-SEM images of the perovskite film and CuPc deposited on the untreated Er:SnO2 ETL (<80nm). The perovskite film exhibits a uniform crystal size of ca. 400 nm along with a smooth coverage surface coverage. Also, the top FE-SEM of the perovskite layer demonstrates the presence of small crystals [41-43]. The XRD results suggest that this tiny crystal at the grain boundary is PbI2. PbI2 has the advantage of reducing the grain boundary, suppressing the hole (H+) and electron (e-) recombination at the interface, and consequently improves the Jsc of the device. According to a morphological comparison, there are no obvious changes in the crystal size of MAPbI3 nor in the growth of the SnO2 and Er:SnO2-based ETLs. The crystal size of MAPbI3 doesn’t change in the SnO2 and Er:SnO2-based ETLs.
Fig. 7c displays a cross-sectional FE-SEM image of the ITO/Er:SnO2/MAPbI3/CuPc/Au device. The perovskite crystal’s proximity to the compact Er:SnO2 layer allows for better longitudinal carrier transport.
Fig. 7d shows the EDS analysis of ITO/Er:SnO2/MAPbI3/CuPc/Au layer shows that this layer was made up of Sn, Er, Cu, Iand Pb elements. The purity of as-synthesized ITO/SnO2/MAPBI3/CuPc/Au layer was confirmed by EDS result.
Fig. 8 presents the optical transmission spectra of SnO2 from the wavelength of 300 nm to 800 nm. Erbium doping indicates an increase in transparency, which suggests that the Er+3 is beneficial for the optical transmittance performance of SnO2. Er:SnO2 samples with a high transparency enhance the electron transport. Doping SnO2 with Er³⁺ has increased its energy band gap (3.6 eV), and it seems to enhance the protection of perovskite against harmful UV radiation in solar cells. Table 1 shows the characterization comparison of band gap of SnO2 and E:SnO2 with other similar work. 

Electrochemical properties
In order to achieve the energy level of the new ETL which was applied, cyclic voltammogram (CV) of ETLs was measured. The CV reactions of the electrodes were investigated in several different conditions in 5.0 mmol L-1 Fe (CN)63-/4- (K3Fe (CN)6) redox electrode in 0.1 mol L-1 KCl solution (with pH of 6) at a scan rate of 50 mV s-1 between -0.4 and +0.6. Cyclic voltammetry (CV) helps identifying improvements to the ETL surface. Well-defined CV cycles indicate a higher quality and defect-free surface which can potentially lead to increased device efficiency. 
According to the CV, doping SnO2 with erbium can improve the quality of thin layers by reduced oxidation and reduction peak magnitudes. This issue aligned with our prediction regarding the impact of lanthanide doping in electron transfer layers, as previously reported [44]. The cyclic voltammograms (CVs) of electron transporting layers (ETLs) incorporating different percentages of erbium are presented in Fig. 9a.
Following (CV) analysis, Mott-Schottky measurements were performed on electrodes exhibiting the optimal surface properties for each impurity concentration. This approach is crucial because Mott-Schottky analysis reveals key characteristics like carrier concentration, flat-band potential, energy levels, and conductivity type - all vital factors for selecting suitable semiconductors in electronic devices. The Mott-Schottky (M-S) technique allows us to determine essential semiconductor properties. The M-S analysis involves plotting the inverse square of the space-charge capacitance (Csc) as a function of the applied bias potential.
The slope of the Mott-Schottky (M-S) plot is a well-established approach for determining the semiconductor doping consistency ND. Additionally, extrapolation of the linear region’s intercept to the potential axis allows for the estimation of the flat-band potential (Vfb) based on the M-S relationship:

The equation uses various symbols to represent key properties of the semiconductor, including the number of charge carriers (ND), a fundamental constant related to electron charge (e), the permittivity of empty space (ε₀), a material property affecting electrical fields (εr), and two potential values: the flat-band potential (Vfb) and the applied potential (V). Additionally, T shows the temperature of the operation (300 K), K refers to the Boltzmann’s constant (1.38 × 10-23 J/K), and CSC is the space charge capacity. In this study, the dielectric constant for SnO2 film is 8.5 [30, 45]. The positive slopes of the curves suggest that all the films have an n-type conductivity. To gain a deeper understanding of carrier transport mechanisms in the potassium halide-doped SnO2 thin films, we also determined the carrier density (ND). Although the as-prepared layers are identical regarding the crystallinity, the carrier density for the 7.00% Er layer is higher and the defect density is expected to be lower. The increased Vbi suggests stronger charge separation and reduced carrier recombination, aligning well with the higher Voc [46]. Fig. 9b presents the graphs of Mott-Schottky analysis and the data extracted from this analysis is summarized in Table 2.
Fig. 10 and Table 3 provide the data on the photovoltaic performances of the Er:SnO2-based PSCs. The PCE significantly increased from 1.05% (for pure SnO2) to 6.05% when the SnO2 was doped with Er (concentration of 7.00%). The pure SnO2 ETL achieves a PCE of 1.05% with Jsc of 2.40 mA⋅cm−2, VOC of 795 mV, and FF of 55% when scanned in the reverse direction. By adding Er into the SnO2 ETL, the PCE rapidly increased to 6.07% with the Jsc of 12.68 mA⋅cm−2, VOC of 895 mV, and FF of 53%. The higher electron conductivity and reduced resistance of the Er:SnO2 ETL results in the improved JSC and FF. The rise in VOC is due to the upshifted conduction band level of Er:SnO2 that is precisely aligned to the perovskite layer [47].

CONCLUSION
In this study Er was doped into the SnO2 compact layer. Regulation of the Er+3 content in the SnCl4 precursor solution contributes to the preparation and consequent application of Er:SnO2 compact layers with varying doping concentrations to PSCs. SEM and XRD analyses were used to characterize and analyze the surface morphology, chemical bonding, element composition, crystal structure, and crystallinity of SnO2 compact layers. The J‐V curves of PSCs based on the SnO2 compact layer containing varying Er doping concentrations were subjected to testing and characterization. At a doping concentration of 7.00% Er, the Er-doped SnO2 compact layer device demonstrated superior performance compared to devices that did not incorporate a doped SnO2 layer. Therefore, this method can be used to various fields, such as solar energy conversion technologies, photo-electrochemical devices for water purification, and others.

ACKNOWLEDGEMENT
Authors are grateful to University of Kashan for financially supporting this work by Grant No. (463561-40).

CONFLICT OF INTERESTS 
The authors declare that there is no conflict of interests regarding the publication of this manuscript.