Densification and Conductivity of Li-Doped NiO Targets for Hole-Transport Layer of Perovskite Solar Cells

Abstract

NiO-based hole-transport layers are crucial for high-efficiency perovskite solar cells. An industrial deposition method of NiO films is magnetron sputtering using ceramic targets. NiO targets doped with Li contents at 1%, 3%, and 5% were designed, and the doping contents and sintering temperatures were investigated. All the targets have a face-centered cubic phase, dense microstructure, and an average size of a few microns. The NLO targets sintered at an optimal temperature of 1400 °C exhibited high relative density (>98%) and low resistivity (<6 Ω∙cm). These results pave the way for depositing NiO-based hole-transport layer by magnetron sputtering.

1. Introduction

Solar energy, boasting the merits of being clean and safe, possessing substantial capacity, and being widely distributed, presents a feasible solution to the energy crisis. Perovskite solar cells (PSCs) have a significant competitive advantage in the photovoltaic market because of its low-cost, high efficiency, and easy preparation [1,2]. A transparent oxide semiconductor (TOS) layer is a thin film with high transmittance in the visible region and adjustable resistivity, which is widely used in devices, such as flat panel displays, solar cells, and light-emitting diodes [3,4,5,6,7]. Nickel oxide (NiO) is one of the candidates for p-type TOS films due to its wide bandgap (3.6~4.0 eV), high chemical stability, and excellent optoelectronic properties [5,8]. NiO films as hole-transporting layers (HTLs) in PSCs have the advantages of having low-processing temperatures and a negligible J-V hysteresis effect, illustrating better long-term stability [9,10].

In order to further improve the performance of PSCs, one area of study is the fabrication of NiO HTLs. NiO films fabricated by the low-temperature spray combustion method showed a uniform and dense structure, contributing to achieving a high efficiency of 12.7% for PSCs [11]. The NiO films with better crystallinity were prepared by magnetron sputtering at 300 °C, having the highest efficiency of up to 14.11% [12]. The mesoporous NiO_x films fabricated by the electrochemical deposition method achieved an optimal power conversion efficiency of 17.77% [13]. NiO film was prepared by electron beam evaporation at room temperature, resulting in a champion device efficiency of 17.82% [14]. A highly transparent NiO layer prepared by the solution processing method had an excellent light transmittance and matching energy level, resulting in a power conversion efficiency of 18.15% [15]. There are various methods for preparing NiO films, among which magnetron sputtering is considered suitable for industrial production because of its advantages of having a high deposition rate, good film density, and uniformity [16,17]. The ceramic targets emerge as a key material when NiO-based films are deposited by magnetron sputtering.

A high-density target is very important for controlling nodules during sputtering, and only a conductive target can meet the demand of direct current sputtering [18]. The preparation process of NiO-based ceramics has been described by different methods. The LiF-doped NiO ceramics with the additives of B₂O₃ and SiO₂ prepared by a wet-chemical synthesis show good negative temperature coefficient (NTC) properties, and its grains were evenly distributed, but pores were formed in ceramics during the sintering process [19]. NiO ceramics modified by Y₂O₃ and BiSbO₃ for thermistor application showed a significant presence of small pores, while the addition of Y₂O₃ elevated the sintering density to 98% [20]. The resistivity of the Na-doped NiO ceramics fabricated by atmospheric pressure sintering decreased with the increase in Na content, and the optimal resistivity of NiO: Na ceramics was 290 Ω·cm [21]. However, NiO-based ceramic targets have not been sufficiently investigated.

The conductivity modification of NiO films can be achieved by deoxidation and doping [22]. The pure NiO and Cu-doped NiO films, fabricated via magnetron sputtering at different oxygen partial pressures, exhibited p-type conductivity, with resistivity decreasing from 62.24 Ω·cm to 9.94 Ω·cm [23]. The Li-doped NiO films synthesized by spray pyrolysis obtained the best p-type resistivity of 4.1 × 10⁻¹ Ω·cm and an optical transparency of more than 76% in the visible region [24]. Transparent and conductive NiO/Ag/NiO electrodes with excellent electrochromic and supercapacitive properties were fabricated at room temperature by electron beam deposition, illustrating an average transmittance of more than 70% in the visible region and extraordinarily low square resistance of only 8.0 Ω·sq⁻¹ [25]. The Ni_0.96Li_0.04O films were prepared by RF magnetron sputtering as electrodes for Li-ion batteries, which enhances the conductivity and Li-ion storage capacity of pure NiO films [26].

In this work, the Li₂O was used as both dopants and sintering aids for the NiO ceramic target. The effect of Li₂O doping contents on the microstructural and electrical properties of the NiO targets was investigated. The Li₂O dopants can effectively improve the density and conductivity of the NiO targets, which is helpful for the application of the NiO targets in PSCs.

2. Materials and Methods

The 1%, 3%, and 5% Li₂O-doped NiO ceramic targets were named as 991, 973, and 955 NLO targets, respectively. The NiO (99.95%, Nanjing Kailinstone Chemical Technology Co., Ltd., Nanjing, Jiangsu province, China) and Li₂O (99.95%, Nanjing Kailinstone Chemical Technology Co., Ltd., Nanjing, Jiangsu province, China) powders were determined by their atomic weight, and the total weight was 20 g. The powders were milled in a tank for 24 h by zirconia balls. The anhydrous ethanol (45 wt%) and dispersant (3 wt%) were used to prevent powder agglomeration. The milled slurry was dried in a drying oven at 80 °C for 24 h. The dried mixed powders were calcinated at 400 °C for 3 h. After 8 hours of milling and 24 hours of drying, the calcined powders were sieved using a 150-mesh sieve. Polyvinyl alcohol (0.5 wt%) used as a binder was added to the sieved powders in the form of aqueous solution, after which the powders were granulated using a 80-mesh sieve. The green bodies with a diameter of 13 mm and a thickness of 2 mm were fabricated by pressing the granulated powders under 200 Mpa, resulting in a relative density of about 60%. The green bodies were first dewaxed at 600 °C for 3 h, and then sintered at 1250 °C, 1300 °C, 1350 °C, 1400 °C, and 1450 °C in air for 4 h.

X-ray diffraction (XRD, D8-Advance, Bruker) was used for phase analysis, and field emission scanning electron microscopy (FE-SEM, Tecnai-450, FEI) was used to study the surface and cross-sectional microstructure. The grain size distribution and average values of the targets were calculated using the Nano measurer software (V1.2.5). The density of targets was measured by the Archimedes method. The four-point probe (MCP-T700, Mitsubishi) was used to measure the resistivity of targets. The samples with sizes of 6 mm × 6 mm × 60 mm were prepared for three-point bending strength testing (Shimadzu, AG-X).

3. Results

Figure 1 shows the microstructure of the NiO, Li₂O, and milled powders. It can be seen that the NiO and Li₂O powders consist of softly agglomerated fine particles, resulting in large secondary particles [27]. However, the agglomerated raw powders were dispersed by ball milling. The milled powders with a Brunauer–Emmett–Teller (BET) value of 17.6 m²/g have a nanometer-scale size. Therefore, these soft agglomerated particles can be dispersed by the ball milling process, which contributes to the sintering densification of the NLO targets.

Figure 2a shows the XRD patterns of the 955 NLO targets sintered at different temperatures. It can be seen that all the peaks of the NLO targets coincide with the standard peaks (PDF# 47-1049) for the face-centered cubic phase structure. There are no obvious impurity phases in the NLO targets, indicating that the Li ions have dissolved into the lattice of the NiO. The intensity of the peaks gradually increases with the increasing sintering temperature, indicating that the NLO targets have good crystallinity. As shown in Figure 2b, the impurity phases of 991 and 973 NLO targets sintered at temperatures ranging from 1250 °C to 1450 °C are invisible, which is in agreement with the results of the 955 NLO targets. Therefore, the NLO targets doped with three contents show a single-phase structure. Figure 2c indicates that the (200) peaks of the 991, 973, and 955 NLO targets sintered at 1250 °C shift toward the high angle with the increase in doping content. The reason for that occurrence is that the ionic radius of Li⁺ (0.59 Å) is smaller than that of Ni²⁺ (0.76 Å) [28].

Figure 3 shows the surface morphologies of the 991 NLO targets sintered at different temperatures. It can be seen that the grain size of the 991 NLO targets gradually increases with the increase in sintering temperature, and the grain boundaries are clear and visible. Moreover, small pores are observed on the surface when the targets are sintered at a low temperature, typically localized at grain boundaries. As the sintering temperature increases, the pores become invisible. Therefore, higher temperatures can promote sintering densification and eliminate pores.

Figure 4 shows the cross-sectional morphologies of the 991 NLO targets. It can be seen that the targets have dense microstructure and small pores. The fracture propagates through the inside of the grains, illustrating the transcrystalline fracture [29]. In general, sputtering targets with transcrystalline fracture and fine grains have high bending strength [30]. The 991 NLO target used for analyzing bending strength has an average value of 98.8 MPa. Therefore, the 991 NLO targets have high bending strength, which contributes to resisting the stress effect during welding and sputtering.

The grain size distribution of the 991 targets sintered at different temperatures is shown in Figure 5. It can be seen that the average grain size of the 991 targets gradually increases from 2.01 μm to 5.02 μm with the increase in sintering temperature, and the grains are fine and uniformly distributed.

Figure 6 and Figure 7 are the surface and cross-sectional morphologies of the 973 and 955 NLO targets. It can be seen that the heavily doped NLO targets exhibit distinct grain boundaries and a denser microstructure. The Li₂O additives are good sintering aids that can enhance sintering densification. The more the doping contents, the better the sintering densification. The grain size distributions in Figure 8 and Figure 9 show that the average grain size of the 973 targets increases from 3.13 μm at 1250 °C to 6.05 μm at 1450 °C, and that the average grain size of the 955 targets increases from 3.91 μm to 7.71 μm. Therefore, the Li₂O additives also promote grain growth.

The Ni, Li, and O elemental distributions shown in Figure 10 illustrate that the Li element is uniformly distributed in the 955 NLO target, resulting in a uniform microstructure and conductivity for the NLO target. The doped oxide targets with a uniform elemental distribution and dense microstructure are significant for depositing large-sized films without defects [18]. The performance of the perovskite solar cells is closely related to the NiO-based films deposited by magnetron sputtering [31,32,33].

The relative density of the NiO and NLO targets sintered at different temperatures is shown in Figure 11. Figure 11a shows that the density of pure NiO target increases with increasing sintering temperature, and then decreases because of overheating. The optimal density obtained at 1450 °C is only 97.93%, which should be further improved. As shown in Figure 11b, the densities of the 991, 973, and 955 NLO targets increase markedly initially, followed by a slight decrease as the sintering temperature is further elevated. The density of 97.80% for the 991 NLO target sintered at 1250 °C is close to the optimal density of the pure NiO target obtained at 1450 °C. Therefore, Li₂O can effectively decrease sintering temperature and promote sintering densification, which means it is suitable as a sintering aid for NiO-based ceramics. Compared with the density of the 991 NLO targets, the 973 and 955 NLO targets show higher density at the same sintering temperature because the high Li₂O doping content is more conducive to sintering densification. The optimal densities of the 991, 973, and 955 NLO targets sintered at 1400 °C are 98.75%, 98.86%, and 99.10%, respectively. However, all the targets sintered at 1450 °C show a decreased density because Li₂O sintered at 1450 °C is easy to decompose and vitalize [34]. Therefore, the Li₂O additives decrease sintering temperature and achieve high density, which contributes to the inhibiting of nodule formation during magnetron sputtering.

The NiO targets with high resistivity are insulating at room temperature. However, Li-doped NiO illustrates good conductivity. The resistivity of the 991, 973, and 955 targets sintered at different temperatures is shown in Figure 12. It can be seen that the resistivities of all the targets first decrease slightly and then increase as the sintering temperatures increase. The substitution of Li⁺ for Ni^2+/3+ generates hole carriers, resulting in the formation of p-type semiconductor. The higher the doping content, the lower the resistivity. The optimal resistivities of 991, 973, and 955 NLO targets sintered at 1400 °C are 5.95 Ω∙cm, 1.27 Ω∙cm, and 0.342 Ω∙cm, respectively. However, the resistivities of all the targets sintered at 1450 °C increased slightly because of the decrease in density and the vitalization of the Li₂O. Therefore, the resistivities of the NLO targets can be modified according to the conductivity requirements of NLO films for solar cells.

4. Conclusions

In summary, to meet the demands of depositing the HTLs by magnetron sputtering for the PSCs, NiO targets were modified with Li₂O dopants, and the doping contents and sintering temperatures were studied. All the targets had a face-centered cubic structure. With increasing sintering temperature, the microstructure became denser, and the grain size increased. The Li₂O dopants promoted grain growth, and fine grains with sizes ranging from 2 μm to 6 μm were still obtained. The densities increase with the increase in doping contents, but the resistivity decreases. However, overheating results in the deterioration of density and resistivity because of Li₂O decomposition and vitalization. The optimal densities of the 991, 973, and 955 NLO targets sintered at 1400 °C are 98.75%, 98.86%, 99.10%, respectively, and their corresponding resistivities are 5.95 Ω∙cm, 1.27 Ω∙cm, and 0.342 Ω∙cm. Therefore, NLO ceramic targets with high density and low resistivity can meet the demands of depositing hole-transport layer by magnetron sputtering.