Study of Lead-Free Perovskite Photoelectric Devices with TiO2 as a Buffer Layer

Abstract

In this work, a titanium oxide buffer layer was explored as a possible buffer electron transporting layer (ETL) with iodine-tin-based perovskite material for enhancement of a thin-film lead-free perovskite solar cell. The open-circuit voltage of the device was used as an indicator for the interface energy barrier’s change with the thickness of the TiO₂. The buffer and photoabsorbing layers were deposited by vacuum reactive sputtering and a low-temperature ion-assisted process from a confocal sintered source, respectively, allowing precise tuning of the film properties and reproducibility of the solar cell behavior. The surface roughness of the buffer layers was investigated by atomic force microscopy and together with the measured absorbance spectra conclusions about the optical losses in the device were made. It was found that the highest voltage was generated from the structure with 75 nm-thick ETL. The electrical behavior of the cell with this buffer layer was additionally studied by impedance measurements. Small interface capacitance and contact resistance were obtained and considered suitable for photodetector fabrication. The practical applicability of the structure with a dual function of self-powered photodetection was demonstrated by the measurement of the response time.

1. Introduction

Photovoltaic energy conversion is a process in which the energy of the light from the visible wavelength range causes electricity generation. It has applications in solar cells and photodetectors. The choice of materials incorporated in the photoelectric converters, in particular in solar cells, depends on the principle of operation, applications, structure, working conditions, etc. Thus, there may be folding and non-folding substrates. The photoabsorbing material could be an inorganic or organic semiconductor in bulk crystalline form or like a thin film, respectively. The electrodes of solar cells can be made of different metals, the choice of which depends on the photoelectric semiconductor valence and conducting bands as related to the metal’s work function in order to make ohmic contact. The aim is to avoid the formation of energy transitions (or energy barriers for the charge carrier’s extraction) at the interfaces, thus minimizing the energy losses and efficiency drops. Unfortunately, it has not been possible to find an exact match between the energy levels at the solar cells’ interfaces; therefore, different supplementary buffer layers have been intermediately inserted to facilitate the transport of the charge carriers through the interfaces. According to their place in the cell and their conductive properties, these buffer layers are hole transporting or electron transporting. The electron-transporting layers are more often used, as the energy barriers for the electrons between the cathode and the photoabsorber are usually greater compared to the hole energy barrier at the anode. A variety of metal oxides such as WO₃, MoO₃, and TiO₂, as well as polymers such as naphthalene diimide [1], have been studied for the possible electron-transporting layers (abbreviated ETL) in combination with conventional light-harvesting semiconductors. Their thickness should be adjusted according to the semiconductor material properties to guarantee equal length of the paths for the electrons with the path of the holes at the opposite side of the cell, but it is typically in the range of a few tens of nanometers [2,3]. The final structure of the solar cell usually becomes complex due to the multilayer stacks that are necessary to be grown for better alignment of the energy levels. There is a great variety of reports discussing the deposition methods for metal-oxide ETLs, including vacuum radiofrequency (RF) sputtering, chemical vapor deposition (CVD), spray pyrolysis, printing, etc. [4,5]. Among them, vacuum sputtering remains preferable due to the possibility for precise tuning of the thickness and the microstructural, stoichiometric, and morphological features of the compounds at the nanoscale without waste products [6].

Recently, perovskite thin-film solar cells have been of great interest due to their excellent ratio compactness to efficiency, easy tailoring of the sunlight absorption coefficient in a wide wavelength range, and possibility for low-cost, large-area processing [7,8,9]. Despite their hybrid organic–inorganic composition, the same disadvantage with the interfacial energy barriers also presents at perovskite-based photoconverters. Therefore, similar ETLs can serve as energy-level aligners to enhance the perovskite cells’ performance [10]. Due to the low stability of lead-free perovskites most of the reports are devoted to simulation studies of the solar cell performance with certain ETL, or HTL that is potentially suitable for a lead-free solar-converting material. For example, it has been found that films like Zn_1-xMg_xO [11], La₂NiMnO₆ [12], and ZnO (undoped and doped with Ag or Ni) can serve as ETLs for improving the efficiency of the cell [13,14]. Cu₂O, SrCu₂O₂ [15], MoO₃, and CuSbS₂ [16] can serve as HTLs for enhancing the performance of the cells.

The core component of the perovskite molecule is methylammonium lead iodide (MAPbI₃) as the basic solar material [17,18,19], which so far is in line with the trend for eco-friendly fabrication process, with the efforts having been focused on the synthesis of lead-free perovskites. They are a relatively new class of materials with a great potential to replace conventional lead-containing materials [20,21]. A lot of properties of the lead-free thin photoconverting films are still unknown and should be studied in detail, including their interface with potentially suitable ETLs. It has been experimentally found that for most perovskite solar cells, a suitable buffer layer is titanium dioxide (TiO₂) [20]. This is because the work function of the transparent indium-tin oxide electrode is 4.6 eV, and the conductive band energy of most of the perovskite photoconversion materials varies between 3.8 and 4.1 eV, according to their chemical composition (lead-containing or lead-free). The conductive band energy of TiO₂ varies between 4.2 and 4.4 eV depending on the degree of oxidation and stoichiometric composition [21]. This makes titanium dioxide very suitable as an intermediate buffer for alignment of the energy levels at the indium-tin oxide (ITO)/perovskite interface, decreasing the contact energy barrier height from 0.8 eV to two barriers lower than 0.4 eV each. For this reason, it was chosen for the current experiments, using vacuum radiofrequency (RF) sputtering of a titanium disk in an oxygen-enriched plasma, where the oxygen concentration can be tuned until the optimal ratio of Ti/O components is reached, and can be made with the desired energy-conductive band value and tolerance. The most reliable approach to determining the effect of the buffer layer in a device is to measure its output voltage before and after its insertion, because the buffer results in a voltage drop change over the interface resistance. The resulting conductivity change is a secondary effect as a consequence of the voltage drop decrease and could be affected strongly by several other factors at the same time, like the illumination or temperature variation, for example.

When an additional film is inserted in the structure of the cell, redistribution of the charge carriers should be considered due to interface capacitances, contact resistances, traps appearing, and other effects that might result in electrical losses. Moreover, optical losses are also possible, due to the refractive index of the film and its surface roughness, which might lead to scattering of the incoming light. In order to enhance the solar cells’ performance, optical filters, consisting of coatings with a suitable thickness and refractive index, have been developed [22]. One of the advantages of TiO₂ films is the possibility of being an optical filter component. Thus, one of the interfaces that is needed between the ETL and the filter will be saved, as no separate ETL will be inserted between the filter and the perovskite film. An additive like HCl has been found to be suitable to tune the electro-optical properties of the film, modifying the energy levels of the titanium dioxide and at the same time optimizing its morphological and structural features [23]. It is crucial that the exploration of the layers’ morphology at the molecular level be combined with probing of the dynamic processes in the structures to collect information about the influence of the crystallites, grains, traps, and others on the charge carriers’ distribution and their contribution to the power-conversion efficiency. Among the suitable tools for extraction of such data is the time-of-flight (TOF), electron paramagnetic resonance (EPR), and electrical impedance spectroscopy (EIS) [24].

Impedance spectroscopy (IS) is a non-destructive, low-cost, and sufficiently precise method to give an inside view of the devices. It is about the measurement of the real and imaginary parts of the electrical impedance (full resistance, reflecting the active and reactive components) Z* = Z′ − j Z′′, j = √(−1) of a system as a function of the angular frequency ω = 2πf. The sample’s measurement can also be related to the analysis of the full admittance as a function of the frequency. The values of Z′ and Z″ have been presented as a function of the frequency (Z′, Z″ vs. logarithm of the frequency f), or in the so-called Cole–Cole plot (i.e., Z″ vs. Z′). These complex plane plots are usually semicircular overlapping arcs with a meaning of the charge transfer processes with different velocities. By looking at the shapes of these plots, charge transfer processes present in the system and accessible in the measurement frequency range are inferred. The plots can be represented by suitable RLC consisting of electrical equivalent circuits, thus providing models of the measured devices, which facilitates the interpretation of the internal processes. For example, the appearance of different numbers of semicircles can be modelled by different RLC branches of the circuit connected in series, or in parallel, depending on the exact behavior of the curve. A variety of perovskite solar cells have recently been studied by applying IS [25,26].

In this study, a newly synthesized and stable tin-iodine-based perovskite was grown and combined with TiO₂ serving as an electron-injecting layer. To the authors’ knowledge, this is the first time the open-circuit voltage of a solar cell has been explored with lead-free perovskite and TiO₂ ETL. The TiO₂ nanocoatings were RF sputtered in oxygen plasma and the effect of their thickness and uniformity on the light absorption and contact resistance was studied. Recommendations about the suitable thickness and deposition conditions for the buffer layer were given. The dynamics of the charge transfer was explored by impedance measurements and the results were connected with the buffer layer morphology and the resulting interfacial contact with the perovskite film.

2. Experimental Section

The ETL TiO₂ films were deposited on ITO-coated glass substrates with a sheet resistance of 19.6 Ω/◻. The transparency of the conductive electrode in the visible electromagnetic range was an average of 87%. The TiO₂ thin films were deposited by reactive RF sputtering at plasma voltage of 1 kV, oxygen partial pressure of 10⁻⁴ Torr, and total pressure in the chamber of 2.5 × 10⁻² Torr. Different durations of sputtering were set to obtain a variety of buffer ETL thicknesses between 50 and 100 nm. The degree of oxidation, stoichiometric composition, and energy band diagram of the TiO₂ buffer layers at specific sputtering conditions were previously established by our group and reported elsewhere [27]. Lead-free perovskite CH₃NH₃SnI₃ was grown by a low-temperature ion-assisted process in a vacuum from a confocal sintered source. The voltage in the chamber was 1 kV, inert gas argon with a partial pressure of 7 × 10⁻³ Torr was set, the ion current of 160 mA was measured, and the deposition rate was approximately 25 nm/min. The backside electrode was gold film deposited by vacuum DC sputtering at a plasma current of 35 mA. The structure of the tested devices was as follows: glass/ITO (200 nm)/TiO₂(0, 50, 75, 100 nm)/CH₃NH₃SnI₃ (1500 nm)/Au (50 nm). The samples were further encapsulated with coverslip glasses and sealed by a light-curable epoxy provided by Ossila. The structure of the device is shown in Figure 1.

The measurements of the thickness of the deposited films were conducted by a profilometer Tencor Alpha Step. The optical power of the reference light source was measured with a calibrated photodetector head for the visible range of the electromagnetic spectrum (Thorlabs). The electrical characteristics were measured by electrometer Keithley 6514, and the solar cell was illuminated with adjustable intensity and spectrum of radiation. LED bulbs with different emission spectra, equivalent to 3000 K, 4000 K, and 6000 K, were used to simulate diffused indoor light. This showed the behavior of the structures at irradiation with “warm” light, or more photons from the yellow and red range of the visible light reach the test sample, which corresponds to 3000 K; at neutral for 4000 K; and at “cold” light, where the blue part of the spectrum is more pronounced, at 6000 K. Thus, the temperature of the irradiated samples was kept constant during the measurement (room temperature), regardless of the duration of the measurement, and possible temperature changes did not affect the reported values. The three intensities at which all samples were measured were 0.8 mW/mm², 1.5 mW/mm², and 5.6 mW/mm². All samples had an area of 120 mm².

The optical absorption of the films was measured by using a spectrophotometer UV VIS NU- T6PC Zhengzhou Nanbei Instrument (Nanbei, China). Atomic force microscopy (AFM) was conducted with an Oxford Instruments AFM microscope MFP-3D (Oxford Instruments, Abingdon, UK) in non-contact mode. Scanning electron microscopy (SEM) was conducted with SEM microscope JEOL-JSM-6390 (Jeol, Peabody, MA, USA). Impedance spectroscopy was carried out by chemical impedance analyzer IM3590 Hioki (Hioki E.E., Nagano, Japan). The responses of the photodetector devices were recorded by digital oscilloscope Tektronix TDS 1012B (Tektronix, Berkshire, UK).

3. Results and Discussion

One of the criteria for the selection of a buffer layer inserted between the front panel of illumination and the photoelectric layer is the degree of absorbance of the incoming light. The absorption spectra of the films depend on their chemical composition and thickness, as well as the optical parameters of the surrounding films in the system. The absorption in the ETL films is one of the components of the optical losses, and according to the state-of-the-art they should not exceed 10% in modern optoelectronic devices [28]. The absorption spectra of the TiO₂-coated glass substrates with a film thickness equal to 50, 75, and 100 nm are shown in Figure 2. As can be seen, all spectra exhibited a similar average value of the maximum absorbance (4.50%, 4.52%, and 4.53% for the titanium dioxide films with a thickness of 50, 75, and 100 nm, respectively), which was expected, considering the same deposition conditions of the film and same expected chemical composition. The film’s thickness mostly affects the spectrum of absorbance and its minimal value. The most favorable coating, considering the range of absorption and the minimal absorbance value, should have been the TiO₂ with a thickness of 50 nm due to its narrowest absorbance range of up to ~500 nm with a sharp decrease in the characteristic to a minimum of ~0.12% (Figure 2a). However, further in the paper, it will be demonstrated that this thickness was not the most suitable in terms of electrical behavior interface process improvement. The characteristic of the TiO₂ film with a thickness of 100 nm was the most unfavorable, taking into account that its absorbance was up to 850 nm with a minimum value of 1.8% (Figure 2c). In addition to the optical performance, the electrical insulation of the electrode that the film exhibited made it an unsuitable solution for implementation in the device. A compromise between the wavelength range and minimal absorbance was achieved for the film with a thickness of 75 nm, which exhibited similar behavior to the 50 nm thin film, i.e., a relatively sharp transition between the maximum and minimum absorbance at ~620 nm, which is between the values measured for 50 nm and 100 nm, and up to 0.2% minimal absorbance (Figure 2b).

The second criterion for the selection of the film for optoelectronic devices is its surface roughness, which may affect the electrical contact if the surface is irregular and may scatter the incoming light, causing optical losses in addition to electrical ones. The samples consisting of glass/ITO/TiO₂ were scanned by AFM and the results are shown in Figure 3a–c. As can be seen from the histogram, the thinnest film of titanium dioxide (Figure 3a, right side) consisted of irregularities with an average height of 17.8 nm and a peak value of 57.6 nm. The rms roughness of the film was ~6 nm, which is an 8.3% deviation from the average film thickness.

The 75 nm-thick film exhibited a peak value of 78.9 nm (Figure 3b) and rms roughness of ~7 nm with an average height of the irregularities of ~17.3 nm. This roughness is equivalent to 8.8% of the average thickness of the film, which is of the same order of magnitude as the 50 nm-thick film. Regarding the thickest film of 100 nm, it was not possible to determine the average height because of the strong irregularity of the surface, although the rms roughness was determined to be ~7.6 nm. The maximum value of the peak was 115.29 nm and the deviation from the average thickness was 6.5%. Although the titanium dioxide film with the greatest thickness showed the lowest deviation from the average thickness, the insulating nature of the materials at this thickness would block the electrons at the interface with the perovskite film, which will be further demonstrated. The sharp peaks, characterized by the greatest magnitude of 15.3 nm beyond the average thickness, was in line with the greatest light absorption, as is confirmed in Figure 2c. Despite the lowest rms roughness of the film with a thickness of 50 nm, attention should be paid to the broadest histogram, consisting of sharp peaks beyond the average thickness compared with the other two cases. This could result in physical damage of the consequently deposited perovskite film, causing micropores or other physical imperfections at the contact point with these formations. At this stage, it was expected that the titanium dioxide with a thickness of 75 nm would be the reasonable trade-off between the surface roughness and electron transfer function.

SEM images of the perovskite coating’s surface after its deposition over the TiO₂ films with different thicknesses are shown in Figure 4a–c. It should be considered that the micropores were not open, i.e., they were not like holes. In the SEM images they look like protrusions, but at the opposite side of the perovskite (at its back side) where the coating contacted with the ETL, the needle type of TiO₂ with the smallest thickness caused these imperfections. It is clearly seen that with the ETL thickness increased and the surface of the perovskite became smoother and more uniform, lacking such defects.

After fabrication of a sample without TiO₂, which was intended for a reference device serving as a control sample for comparison of the device’s electrical behavior with and without a buffer layer of TiO₂ (not shown), it was found that the structure was not stable and workable.

There was no adhesion between the perovskite layer and the ITO electrode. It was not possible to measure an electrical characteristic. After ranging the TiO₂ film thickness up to 50 nm, the first significant result was measured at a thickness of 50 nm.

The behavior of this structure was unexpected for a solar cell structure, as the voltage slightly decreased with the light intensity increase, as can be seen from Figure 5. A possible reason could be the thickness of the titanium dioxide film that did not sufficiently affect the difference in the energy levels between the perovskite and ITO. This energy transition should have allowed the extraction of electrons from the photoabsorbing layer to the ITO and blocked the hole extraction through the same interface.

When a cell with high light intensity is irradiated, it means that more photons fall onto the structure and more electron–hole pairs are generated. One of the hypotheses is that the interface ITO/TiO₂(50 nm)/perovskite was not sufficiently selective to the single-type charge carriers and extracted holes as well. At the same time, the increased numbers of electrons as a result of the high illumination probably diffused to the interface perovskite/gold, causing intensive recombination, and the voltage drop across the two electrodes decreased due to the increased recombination current. At a lower intensity, this effect was weaker. Therefore, the thickness of 50 nm for the TiO₂ film was found to be unsuitable for the buffer layer with the used lead-free detectors.

At a thickness of 75 nm of the titanium dioxide, the structure showed the expected behavior, because as the light intensity increased, the voltage generated by the cell also increased, as can be seen in Figure 6. This means that the interface between ITO/TiO₂/perovskite allowed for the extraction of electrons only and prevented the holes from passing to the front electrode, and their extraction to the gold electrode was facilitated by the blocking effect of the field at the opposite interface.

The highest voltages for the three light intensities were generated when irradiated at a color temperature of 6000 K. Therefore, the structure was expected to exhibit higher efficiency for highly energetic photons belonging to the blue part of the electromagnetic spectrum. The spectral sensitivity depends on the perovskite material’s band gap.

A further increase in the TiO₂ buffer thickness to 100 nm resulted in better performance of the cell than the case with a thickness of 75 nm (Figure 7). This structure showed the highest voltages compared to the other samples for all intensities and color temperatures of the light. Again, the highest voltage was observed at 6000 K, as the higher-energy photons were absorbed by the perovskite. Despite the short-term enhanced behavior of the solar cell, the long-term illumination (after 3 h and 40 min) showed untypical behavior of the measured open-circuit voltage, as can be seen in Figure 8, when the illumination was switched off.

This means that the electrons accumulated during irradiation moved to the gold electrode after the irradiation was stopped. One hypothesis is that a transition capacity was formed due to the dielectric constant of the thicker TiO₂ layer, which was charged by the generated photoelectrons, and after stopping the irradiation it was discharged in the opposite direction, i.e., the electrons flowed through the gold electrode. It could be also possible that during longer irradiation the photoelectrons generated in the perovskite near the titania interface were affected by the formation of a space charge and its electrical field influenced the energy-barrier height in such a way that the electrons could pass to the ITO. After stopping the illumination, the number of electrons near this interface decreased, which changed its energy structure, so the electrons diffusely went through the photoabsorbing layer to the perovskite/Au interface more easily and thus an inverse voltage was obtained. The results obtained led to the conclusion that TiO₂ with a thickness of 100 nm is not suitable as a buffer layer. Thus, the thickness of 75 nm was established as the most appropriate to enhance the performance of the lead-free perovskite solar cell.

The impedance curve showed a typical trend of a gradual decrease with the frequency increasing, because of the imaginary part related to the capacitive reactive component. The phase shift between the voltage and current (voltage lag current with θ = 83.5°), close to 90° (Figure 9a), can be explained by the presence of the capacitive character of the device, because of the double interfaces due to the buffer layer insertion. At room temperature, the capacitance and resistance of the device achieved a constant value in the explored frequency range (Figure 9b), which can be ascribed to fluent charge-carrier extraction through the interface due to the lack of accumulated space charges near the interfaces. Such a charge might appear if the path of the charge carriers was not optimized according to their velocity and usually happens when a buffer layer with non-optimized thickness is utilized. On the basis of this measurement, it can be concluded that the ETL of the 75 nm TiO₂ is the most appropriate thickness of this buffer, considering also the previous measurement of its surface morphology, optical absorbance, and photovoltage behavior. The results from the IS (Figure 9b), in particular the small total capacitance C of 894 nF (including parasitic lead capacitances, double-layer interface capacitances, and the geometric capacitance under dark conditions) and the total resistance R of 1.99 kΩ (including contact resistances and the bulk resistance), suggest a low time constant τ = RC = 1.77 ms for transferring the charge carriers through the interfaces. This is related to a fast response when the ambient illumination changes its intensity [29,30,31]. Thus, a possible dual application of the cell could be a photo detector registering motion, which is self-powered by the solar energy and reacts when moving objects shade the illumination. The defects from the absorber layer were considered a consequence from the irregularities of the buffer layer according to its thickness, and they mainly presented like morphology differences, shown in the SEM images in Figure 4. As a result, the physical contact between the absorbing layer and the buffer layer in the case of the 50 nm buffer layer (causing the most defected surface of the absorber—see the SEM in Figure 4) was worse. The contact resistance R increased to 15.8 MΩ (not shown as a plot) and hindered the extraction of charge carriers through the interface, resulting in a low current density and low efficiency.

The response characteristics of the device for the three cases of TiO₂ thicknesses are shown in Figure 9. As can be seen in Figure 10a,b, the results for the rise (tr) and fall (tf) time of the electrical pulses, generated at periodic shading, were similar (1 s/7 s for TiO₂ 50 nm and 1 s/6 s for the TiO₂ 75 nm). The results are in very good agreement with the reported for photodetectors with a general-purpose application, which has not been tested by laser excitation [32]. In contrast to this behavior, the response times at TiO₂ 100 nm were the greatest (7 s), despite the symmetric response (not shown). The tr and tf times were determined according to the standard procedure with a variation of the main quantity against the time and crossing levels of the characteristic with the levels, representing 10% and 90% of the maximal value (the signals were considered like pulses) [33].

The estimated power efficiency was strongly dependent on the ETL buffer layer’s thickness, as is visible from Table 1. It should be noted that there was a great potential for further increase of the efficiency after insertion of the hole-transporting layer and optimization of its thickness and morphology. To distinguish the separate contribution of each buffer layer in the present study, the results related to the influence of the ETL only on the solar cell’s efficiency are shown.

4. Conclusions

A study of suitable buffer materials for thin-film solar cells is necessary, and for each device design the energy levels, charge carriers’ mobility, film thicknesses, and physical and chemical features according to the technology and deposition conditions must be taken into account. The research in this work was done with a newly synthesized lead-free perovskite material and titanium dioxide electron-transporting layer with different thicknesses. It was shown that the use of buffer layers was necessary for the lead-free perovskites as well, in order to make the desired interface energy barriers with a controllable and reproducible height, due to the “ladder effect” facilitating the energy-level alignment between the perovskite used and the electrodes of the cell. The experiments proved that the optimal thickness of the buffer layer for this case was 75 nm. The cell exhibited the best performance in the blue range of the electromagnetic spectrum with open-circuit voltage varying between 88 mV and 148 mV even for weak optical power in the range from 0.8 to 5.6 mW, respectively. The results are valid for a photodetector application of the explored device. Further study will be related to quantitative determination of the solar cell efficiency as related to the buffer-layer thickness, based on the measured current-voltage characteristics.

Author Contributions

Conceptualization, M.A. and R.T.; methodology, M.A.; validation, R.T. and M.A.; formal analysis, M.A. and R.T.; investigation, M.A. and R.T.; resources, M.A.; writing—original draft preparation, M.A.; writing—review and editing, M.A. and R.T.; visualization, M.A. and R.T.; supervision, M.A.; project administration, M.A.; funding acquisition, M.A. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 1. Schematic diagram of the perovskite photoelectric device with a buffer layer of TiO2.

Figure 2. Absorption spectra of titanium dioxide films with different thicknesses of (a) 50 nm; (b) 75 nm; (c) 100 nm.

Figure 3. AFM 3D profiles and the corresponding histogram of particle distribution of the titanium dioxide films with different thicknesses: (a) 50 nm; (b) 75 nm; (c) 100 nm.

Figure 4. SEM images of the perovskite coating’s surface after deposition over the TiO2 film with different thicknesses of (a) 50 nm; (b) 75 nm; (c) 100 nm.

Figure 5. The output voltage of the lead-free perovskite solar cell at different light intensities at a buffer ETL layer of TiO2 with a thickness of 50 nm at different spectra of the illumination source: (a) 3000 K; (b) 4000 K; (c) 6000 K.

Figure 6. The output voltage of the lead-free perovskite solar cell at different light intensities at a buffer ETL layer of TiO2 with a thickness of 75 nm at different spectra of the illumination source: (a) 3000 K; (b) 4000 K; (c) 6000 K.

Figure 7. The output voltage of the lead-free perovskite solar cell at different light intensities at a buffer ETL layer of TiO2 with a thickness of 100 nm at different spectra of the illumination source: (a) 3000 K; (b) 4000 K; (c) 6000 K.

Figure 8. A negative voltage of the ITO/TiO2 (100 nm)/perovskite/Au when the illumination is switched off.

Figure 9. Impedance measurements of the photoelectric device with a 75 nm buffer layer of titanium dioxide: (a) impedance and phase shift against the frequency; (b) total serial resistance and capacitance against the frequency.

Figure 10. Rise and fall time of the self-powered photo detector device with a TiO2 buffer layer, with a thickness of (a) 50 nm; (b) 75 nm.

Table 1

Effect of the ETL buffer layer’s thickness on the power conversion efficiency of the lead-free perovskite solar cell.

TiO2 Film Thickness, nm	U, mV at 5.6 mW Optical Power, Color Temperature 6000 K	I, mA/mm2 at 5.6 mW Optical Power, Color Temperature 6000 K	PCE% for Area of 120 mm2
50	80.2	0.06	0.7
75	148.7	0.28	6.2
100	373.6	0.36	19.9

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