Cu-Doped TiO₂ Thin Films by Spin Coating: Investigation of Structural and Optical Properties

Abstract

Cu-doped TiO₂ films were synthesized directly on FTO glass with a spin coating method. With a variation in copper amount, samples were prepared with 0%, 1%, 2%, 4% and 8% of dopant concentrations. Morphological and structural characterization of undoped and Cu-doped TiO₂ samples were investigated and the obtained results showed the small, spherical shapes of the nanoparticles forming a thin film on top of FTO glass and their preferred orientation of TiO₂ anatase (101), which is the same for each sample. However, this peak exhibited a slight shift for the 2% sample, related to the inflation of the microstrain compared to the other samples. For the optical properties, the 4% sample displayed the highest transmittance whereas the 2% sample exhibited the lowest band gap energy of 2.96 eV. Moreover, the PL intensity seems to be at its highest for the 2% sample due to the present peaking defects in the structure, whereas the 8% sample shows a whole new signal that is related to copper oxide. These properties make this material a potential candidate to perform as an electron transport layer (ETL) in solar cells and enhance their power conversion efficiency.

1. Introduction

Renewable energy sources are witnessing an immense growth in industrial developments and technologies, as they started with conventional solar cells and biomass combustion and progressed to innovative hydrogen production systems that are still to this day under assessment [1]. Solar cells have their share in spreading and reinforcing renewable energy technologies into industry and their flourishing success [2]. Moreover, solar panels have been expanding and diverting over the years; they were pioneered with different materials and different systems, as you can find monocrystalline and polycrystalline as well as amorphous silicon solar cells, plasmonic, multi-junction, thin films, quantum dots, dye-sensitized, perovskite and organic–inorganic hybrid solar cells [3,4,5]. All of these types of solar panels have received great feedback on their efficiency and yielding [6]. Among the third generation, perovskite solar cells (PSCs) have gained a lot of recognition due to their high power conversion efficiency (PCE), which was recently reported to have increased from 3.8% in 2009 to 25.2% in 2020 [7], which is very close to the notional Shockley–Queisser efficiency limit (~33%), and also, perovskite materials possess a narrow direct band gap, leading to a high optical absorption coefficient and broadband absorption and thus a long charge carrier lifetime and high mobility, and they also offer a simple, low-cost and effective synthesis methods [8]; several types of perovskite have been developed to create a better absorber with excellent optoelectronic properties, like a slow recombination rate of e⁻/h⁺ pairs [9], and the ABX₃ formula is the most used one, where A is an organic (CH₃NH₃⁺ or MA and FA) or alkali metal, B is a bivalent metal (Pb, Sn and Ge) and X is a halide anion, and this formula is called organic–inorganic metal halide perovskite [10], and we can also find other types like RP phase layered perovskite and ACI-type perovskite. From all of these types, it seems that the Pb²⁺-based perovskite is needed for exhibiting excellent photoelectric properties [11]. After all of these successful achievements of perovskite solar cells, they are still deteriorated by the issue of charge carrier recombination [12]. So, engineering the lane of carriers between the perovskite active layer and the electron transport layer (ETL) is vital to enhance the PCE [13]. However, many factors affect the power conversion efficiency (PCE) of PSCs with an n-i-p structure or inverted p-i-n structure, and it seems that the most crucial is the charge carrier transfer procedure at the interfaces [14], such as the interface between electron transport layer (ETL) and perovskite and hole transport layer (HTL); it is better to construct a contact interface that reduces charge carrier trapping at the interface, which leads to their recombination. Some researchers have focused on engineering the ETL and HTL separately to optimize their electrical and optical properties, since the chemical stability of these layers as well as the thickness, absorption spectrum, doping and interfacial defects can badly effect the extraction and transportation of the photogenerated charge carriers from the perovskite absorber layer, leading to a reduction in the PCE [15,16,17]. The most commonly used metal oxides for an ETL are TiO₂, ZnO, SnO₂ and ZrO₂. But TiO₂ is frequently selected due to its low cost, great chemical stability, suitable conduction band and high electric conductivity [18]; even a solar cell with a TiO₂ ETL reported a 24.66% PCE. But the drawbacks presented by TiO₂ mainly consist of the large energy barrier at the perovskite/TiO₂ interface and its limited UV light absorption which hinders efficient charge transfer, causing researchers to turn to other metal oxides [19,20]. To overcome these drawbacks, tuning the energy levels is the optimal choice either via doping or/and inserting an interlayer between the ETL and perovskite layer to achieve a prime energy alignment. To create a smooth contact interface, some researchers have resorted to adding Cl ions to expand the electron diffusion length; S. D. Stranks et al. reported the aftermath of adding Cl ions to the perovskite solar cells, as the PCE went from 4.2% to 12.2% [21]. As for the alteration of TiO₂ ETL properties, for instance the Fermi level, band gap and electrical conductivity, the doping process has been highly suggested [22]; furthermore, many metals have been proposed as a supreme dopant candidate for a TiO₂ ETL in PSCs, namely Sn, Zn, Ag, Nb and W [23]. Shih-Husan Chen et al. reported that for PSCs with a meso-Sn-doped TiO₂ ETL, the PEC was improved from 16.86% to 20.55% due to the decrease in defect states and a slight upward shifting of the conduction band and valence, leading to improved carrier extraction and transport [24]. Also, Sadiq Shahriyar Nishat et al. reported a 16.44% PEC for a 4.17 mol% Zn-doped TiO₂ ETL in a PSC, which is considered slightly lower compared to the same amount of 4.17 mol% Sn-doped TiO₂ that showed a 0.63% higher PCE [22]. Chen et al. reported that a mesoporous Ag-doped TiO₂ ETL in PSCs exhibits a great PEC of 17.7% [25]. Among the transition metals, Cu has never been used in any research report for doping a TiO₂ ETL in a perovskite solar cell, even considering its high conductivity and abundance, but it has been widely studied in dye-sensitized solar cells (DSSCs) [26] and it shows great yielding in different doping concentrations of TiO₂ photoanodes and photocatalysis applications [27,28,29,30]. T. Raguram et al. revealed a maximum efficiency of 3.90% for 0.1 M Cu-TiO₂ for DSSC application and 97.12% for Rhodamine B photocatalytic degradation; the 0.1 M Cu-TiO₂ synthesized with the sol–gel method showed a great impact on the structural, optical, morphological and electrical properties of TiO_2, as if the incorporation of Cu concentrations perfectly tailored the optical band gap and crystallinity of TiO₂ nanoparticles, as it shifted from 3.2 eV to 2.3 eV with the addition of copper concentration [31]. A lot of methods exist for Cu-TiO₂ synthetization and deposition on FTO glass substrate that have accrued throughout the years, as each process gives the material a specific size, morphology and other typical properties. The electrodeposition [32], sol–gel [33], spin coating [34], hydrothermal [35] and spray pyrolysis [36] methods are the most commonly used, especially the spin coating process since it is a very simple, cost effective and easy to manipulate method without any dissipations of energy and resources [37]; its consistency of a homogeneous dispersion of nanoparticles on the surface of FTO glass is the main cause of its vast utilization in wide range of research reports [38]. Ying-Han Liao et al. fabricated a perovskite solar cell CH₃NH₃PbI_3−xCl_x/(Sn/TiO₂)/FTO with the spin coating method with a variation in the concentration of dopant Sn, with a PCE of 14.4% for the 1.0 mol% Sn/TiO₂ sample; all the samples showed a successful decrease in band gap and an increase in charge carrier mobility [39].

Herein, we will prepare an electron transport layer (ETL) that consists of Cu-doped TiO₂ for a perovskite solar cell with the spin coating method, while varying the dopant concentration; we will study the effect on the morphology, crystallinity and the optical properties of TiO₂, and thus the effect on e⁻/h⁺ mobility and recombination. To observe the power conversion efficiency of the PSCs and the impact of the copper concentration after the characterization, the samples will be integrated as an ETL in an efficient and famous perovskite solar cell, as is shown in Figure 1, where a SCAPS simulation will be used in future work to foretell the PCE of each sample.

2. Results and Discussion

2.1. Morphological Properties

Figure 2 shows the SEM images of pure and doped TiO₂. The TiO₂ thin films demonstrate small spherical nanoparticles at a range of 200–500 nm that are orderly dispersed on the surface of the FTO glass to form a homogenous film. It seems that the addition of copper dopant to the TiO₂ nanoparticles has no effect on the morphology of the TiO₂ nanoparticles; all films depict a granular nanostructure, and with the increase in dopant concentration percentage, some regions show a microstructural defect with the presence of bumps and holes.

Figure 3 shows the cross-sectional SEM image of the TiO₂ thin films with a thickness of 350 nm deposited on FTO glass. The thickness of the sample remains the same since the deposition parameters of the spin coating were sustained with same speed, solution quantity and duration. Moreover, the thickness seems to range between 340 and 380 nm for the different dopant concentrations.

2.2. Structural Properties

The X-ray diffraction can provide information about the structural properties of the as-prepared samples. Figure 4a depicts a diffractogram with the visible crystalline patterns of undoped and Cu-doped TiO₂ nanoparticles deposited on FTO glass with a preferred orientation at 2ϴ = 25.389° that correspond to anatase TiO₂ (101), according to the reference card (JCPDS Card 21-1272) [40], and a small peak, with the other intense peaks corresponding to FTO glass (110), (101), (200) and (211); however, no Cu-related peaks were observable, which could mean the substitution with the Cu ion was successfully carried out without interstitial growth [41]. Moreover, the intensity of the anatase (101) peak increases with the increase in the dopant percentages. This could be due to the high incorporation of copper into the TiO₂ structure, which in turn induces physical stressing on the lattice. From the used precursors, we can confirm the formation of Cu²⁺ in the solution and according to the literature the ionic radius of Cu²⁺ is (86 pm) and Ti⁴⁺ is (74.5 pm) [42]. According to Figure 4b, it seems that as we increase the percentage of the dopant the TiO₂ (101) peak vaguely shifts to lower angles, especially for 8% sample. This could be better interpreted by the calculation of the crystallite size, lattice strain and microstrain, which are presented below.

The Scherrer equation for crystallite size and lattice strain determination is [43]

$$\mathrm{D}=\frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}$$

(1)

$$\frac{1}{\mathrm{D}}$$

(2)

The Wilson equation for microstrain determination is [43]

$$\mathsf{\epsilon }=\frac{\mathsf{\beta }}{4\mathrm{t}\mathrm{a}\mathrm{n}\left(\mathsf{\theta }\right)}$$

(3)

According to Figure 5 and the calculation in

Table 1. Crystallite size, lattice strain and microstrain calculations of the undoped and 1%, 2%, 4% and 8% Cu-doped TiO₂ at the preferred orientation (101).

Cu-Doped TiO2 (101)	2ϴ (deg)	d(101) (Å)	FWHM (rad)	D (nm)	1/D (nm−1)	Ԑ (%) × 10−3
0% Cu	25.389	3.506934	0.002962	47.16	0.0212	3.289
1% Cu	25.388	3.515507	0.003223	43.33	0.023	3.579
2% Cu	25.397	3.472665	0.003748	36.9	0.0271	4.16
4% Cu	25.40	3.513734	0.003749	37.2	0.0268	4.161
8% Cu	25.32	3.508971	0.002896	48.23	0.0207	3.22

, it seems that the crystallite size is at its lowest values for the 2% and 4% samples and according to proportionality these samples also exhibits a high lattice strain and microstrain, which provides information on the degree of distortion, dislocations and defects present in the crystalline lattice; this could affect the photosensitivity of TiO₂ nanoparticle thin films by inducing the density of charge carriers [44].

2.3. Optical Properties

To acknowledge the effect of Cu doping on the optical properties of the TiO₂ nanoparticle thin film, we used UV–Vis spectroscopy and obtained transmittance and absorbance spectra (Figure 6). Figure 6a depicts the transmittance of each sample, and it seems that the 4% copper dopant TiO₂ presents more light transmission than the other samples, followed by the 2% copper dopant TiO₂; this could positively affect the charge mobility and the performance of TiO₂ as an electron transport layer [45]. Figure 6b shows the absorption spectra of the samples, and it seems that at the range 300–400 nm, the sample with 8% copper dopant has a high absorbance value compared to the other samples; as for the 2% and 4% samples, they have a low absorbance value, even lower than the undoped TiO₂, but unexpectedly the 1% sample presents a higher light absorption than the undoped and 2% and 4% Cu-doped TiO₂.

To better understand the effect of doping with copper on the optical band gap of the samples, we used the Tauc formula below [46]

(Ahʋ)^1/n = C(hʋ − Eg)

(4)

where A is the absorption coefficient, C is a constant, Eg is the average band gap of the material and n depends on the type of the transition. After examination of the obtained Tauc plots (Ahʋ)² vs. hʋ (Figure 7), the difference in the optical band gap is clear. If we exclude the 2% sample, it is noticeable that by increasing the copper doping percentage (the concentration of dopant) the optical band gap increases from 3.01 eV for undoped TiO₂ to 3.1 eV for 8% copper-doped TiO₂. But it is obvious to state that the optical band gap for the sample of 2% Cu-doped TiO₂ decreases in comparison with undoped and the other doped samples to a value of 2.96 eV. This is in accordance with Nair et al., who studied the effect of doping and crystalline properties on the band gap of nanoparticles and concluded that clearly there is a correlation between the particle size, strain and band gap [47]. Considering the calculated values in Table 1 and the obtained results from the Figure 5, the 2% Cu-doped TiO₂ nanoparticle sample presents the lowest crystallite size, the highest microstrain and the lowest bang gap. So, in short, and as they reported, when the particle is extremely small, pressure surface increases, meaning lattice strain increases and thus the band gap decreases [48]. So, as reported, Red shift or a decrease in optical band gap with a decrease in particle size arises due to the surface and interface effect and Blue shift or an increase in energy is due to the quantum size effect, which is the case for 1%, 4% and 8% copper-doped TiO₂.

2.4. Photoluminescence

We investigated the recombination of charge carriers for undoped and Cu-doped TiO₂ nanoparticles. All samples were optically characterized with photoluminescence spectroscopy. As Figure 8 depicts, TiO₂ nanoparticles are not photoluminescent, since the PL spectra did not show any peak around the absorption wavelength that corresponds to the calculated band gap ranging from 2.96 to 3.1 eV (420–401 nm). The observed peak collection between 450 and 700 nm is related to the oxygen vacancies in the TiO₂ structure [49]. The peak appears to be decreasing with the increase in dopant concentration, which means a decrease in the recombination rate of charge carriers e⁻/h⁺, hence fast electronic transport and better photoactivity, except for the 8% and 2% samples, which give a higher PL intensity which could be due the high formation of defects, while the 4% sample presents the lowest PL intensity; also, it is clear that the doped samples present the same spectra shape as the undoped one, which means that Cu doping does not induce any PL signals. The significant peak of the 8% Cu-doped TiO₂ nanoparticles at 780 nm corresponds to the band gap of CuO (1.6 eV) according to the literature. But this hypothesis cannot be right since the XRD analysis did not show any peaks related to copper oxide structure, unless it also means that the formed copper oxide nanoparticles are extremely well dispersed on TiO₂ nanoparticles and that is why they are undetectable by the X-ray diffractometer.

3. Experimental Details Materials and Methods

3.1. Materials

All chemicals and reagents were used directly without further purification as received. Absolute ethanol, isopropyl alcohol, acetone, FTO, Titanium diisopropoxide bis(acetylacetonante) 75 wt.% in isopropanol (C₁₆H₃₂O₆Ti) and Cupric Acetate Monohydrate (Copper (II) Acetate Monohydrate) pure, 98% (C₄H₈CuO₅). All the products were bought from Sigma-Aldrich (St. Louis, MO, USA).

3.2. Pure and Cu-Doped TiO₂ Film Elaboration

Firstly, the FTO glass was cleaned with acetone, ethanol and distilled water. Then, to deposit the TiO₂ nanoparticles on the surface of the FTO glass, a simple cost-free method was used: spin coating, which consists of coating the FTO glass surface with a solution of titanium precursor. To study the effect of doping, we added a portion of copper precursor, while varying the amount of copper from 0%, 1%, 2%, 4% to 8%. After the deposition of the Cu dopant TiO₂ thin film with spin coating, the as-prepared samples were annealed at 450 °C for 1 h and then naturally cooled down for morphological, structural and optical characterization. All of the steps are illustrated in Figure 9.

3.3. Characterization Techniques

The samples were morphologically characterized using scanning electron microscopy (SEM, FEI XL30 ESEM Company, Hillsboro, OR, USA). As for the structural stability and crystallographic formation of the phase, X-ray diffraction (XRD) was performed using a Philips X’PERT-MPD diffractometer equipped with CuKα radiation (λ = 1.5406 Å), with the diffraction patterns in the range of 20–80°. To determine the gap energy of each sample and its transmittance and absorbance properties, we used a UV–Vis spectrophotometer and photoluminescence (PL) spectroscopy in the range of 200 and 1200 nm by equipping a PerkinElmer Lambda 950.

4. Conclusions

TiO₂ nanoparticles doped with copper were synthesized with a simple method of spin coating, which consists of dispersing the prepared solution on FTO glass at high speed, and these samples of Cu-doped TiO₂ at 0%, 1%, 2%, 4% and 8% were characterized morphologically, structurally and optically and the acquired results reveal that the 4% sample has the highest transmittance, while the 2% is the second highest; these samples could be perfect candidates for electron transport. The 2% sample exhibited the lowest band gap energy of 2.96 eV, whereas the 8% samples exhibit the highest at 3.05 eV; the 8% sample also showed a new PL peak compared to the other samples, which could be related to the copper oxide.