As an intrinsic p-type transparent conductor with a wide band gap of 3.1 eV, γ-CuI full of potential has gradually attracted the attention of researchers. However, γ-CuI films deposited by various techniques generally present high haze with a frosted-glass-like appearance, significantly hampering the device’s performance. Herein, a new strategy is proposed, where truly p-type CuI thin films with low haze were successfully synthesized at room temperature. The specular transmittance of CuI film above 85% in the visible region (400-800 nm) can be achieved. The haze of the as-prepared γ-CuI films can be as low as 0.7%. Meanwhile, the as-prepared CuI film possesses a FOM as high as 230 MΩ -1 . This ideal stable p-type optoelectronic performance was a significant achievement among various typical p-type transparent conductive films.
Research Article
A New Strategy for Fabricating Low Haze p-Type Cui Film
https://doi.org/10.21203/rs.3.rs-736842/v1
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Transparent conductive oxides (TCOs) can be widely used in passive and active electronic applications, such as flat-panel displays, photovoltaic devices, light-emitting diodes, smart windows, photo-detectors, etc. [1-4]. However, p-type TCOs as an essential component employed in the active devices, their performance is far behind n-type TCOs, seriously limiting the application of these active devices [2]. The discovery of the first p-type transparent semiconductor with application potential is CuAlO2 [5], after which increasing attention was paid to CuI-based oxides, especially the oxides with delafossite structure. CuMO2 with delafossite structure presents attractive p-type optoelectronic properties among various oxides [6-9]. Unfortunately, the figure of merit (FOM) of those kinds of materials is below 10 MΩ-1 [6-12]. Most recently, correlated metals CaVO3 and SrVO3 even their p-type doping materials such as La2/3Sr1/3VO3 with high p-type conductivity about 742.3-872.3 S·cm-1 have been reported. Regrettably, their transmittance further deteriorates [13-15]. As a result, improving the conductivity of p-type TCOs meanwhile maintaining their high transmittance is indeed a great challenge at this moment.
γ-phase cooper iodide (γ-CuI), an inexpensive and non-toxic p-type semiconductor has attracted people’s attention. It is an intrinsic p-type semiconductor with low hole effective mass (0.30 m0) and high bulk hole mobility (>40 cm2·V−1·s−1) [16,17]. Its wide direct band gap (Eg = 3.1 eV) is beneficial for the fabrication of transparent semiconductor; while its high exciton binding energy of Ebx = 58 meV is also beneficial as it used in the optoelectronic devices [18,19]. In addition, n-type CuI can also be realized by doped with Zn [20], resulting in that the p-CuI/n-CuI homo-junction can be realized, further greatly broaden the application of CuI in various optoelectronic devices. γ-CuI films have been synthesized through various physical and chemical methods, such as sputtering [21], pulsed laser deposition (PLD) [22], solution method [23], thermal evaporation [24], and iodination method [25]. However, the γ-CuI films deposited by different techniques generally possess high haze with gross grains and present a frosted-glass-like appearance [16,26-28]. This feature increases the light scattering and seriously reduces the film’s transmittance. At present, many efforts have been paid to reduce the haze of CuI filmand enhance the film’s transmittance. One of the most attractive means is preparation CuI films through iodination method, i.e., using different precursors such as Cu [25], Cu3N [25], CuS [29] to react with iodide. In addition, Vidur Raj et al. also incorporated a second-phase such as TiO2 into CuI films to inhibit the growth of CuI grain and lower the film’s haze [30]. However, the results are unsatisfactory: the transmittance of CuI films with a thickness below 100 nm prepared by the above methods is still lower than 70%.
Consequently, novel strategy to fabricate γ-CuI film with high visible transmittance and low haze is necessary. In the current work, CuI films were prepared by the iodination of Cu3N precursor. The film’s haze as well as the surface morphology can be controlled by adjusting the contact area between iodine and the Cu3N precursor. The transmittance of the as-prepared γ-CuI films can reach 86%. The haze of the as-prepared γ-CuI films can be below 0.7%. Benefit to the high transmittance, the figure of merit (FOM) of the optimal CuI film above 230 MΩ-1 can be realized.
2.1. Fabrication of the Precursor Cu3N Thin-Film
2.5 cm × 2.5 cm glass substrates were cleaned with acetone in an ultrasonic bath then dried with N2. Next, the Cu3N thin films were deposited on glass substrates from the copper target (purity of 99.99%) by reactive radio frequency (RF) magnetron sputtering. The sputtering power maintains at 80 W. The background vacuum of the sputtering chamber was 5×10-5 Pa. Before the deposition, the copper target was pre-sputtered with pure Ar for 10 min to remove the oxide layers from the target surface. The deposition pressure is 0.8 Pa with argon flow of 40 sccm and nitrogen flow of 20 sccm. The thickness of the Cu3N precursor is about 75 nm.
2.2. γ-CuI Thin Film Fabrication
γ-CuI thin films were fabricated via the chemical reaction between Cu3N precursor films and liquid, vapor or solid-phase iodine:
The liquid, vapor, and solid iodination procedures are schematically illustrated in Fig. 1. Fig. 1a shows the liquid iodination process. First, 0.1 g iodine particles and 0.2 g KI powder were dissolved in 100 ml water as iodine aqueous solution. 5 ml iodine aqueous solution was transferred into a petri dish. Cu3N precursor was immersed in an iodine aqueous solution at room temperature. The iodination reaction maintains 5 min. Fig. 1b shows the vapor iodination process. Iodine particles were distributed evenly at the bottom of the petri dish. Cu3N precursor films were buckled on the petri dish and faced towards iodine vapor. Then, the petri dish was kept at room temperature for 5 min. Fig. 1c shows the solid iodination process. The Cu3N precursor films were placed face up in a petri dish. The Cu3N film was fully covered with iodine particles for 5 min. The diameter of CuI particles is about 2.5mm.
For the solid iodination method, the different arrangements of contact area and non-contact area were realized by controlling the amount of iodine particles (density of iodine particles) on the surface of Cu3N precursor. The low haze sample was prepared by the Cu3N precursor fully covered with iodine particles, and the sample with medium haze was prepared by placing 1/2 iodine particles of the low haze sample on the Cu3N precursor surface of the sample. High haze samples were prepared by evenly placing 1/2 of iodine particles on the surface of Cu3N precursor.
2.3. Characterization.
The transmittance and reflectance of CuI thin films were measured using a PerkinElmer UV-visible-IR spectrophotometer in the wavelength range of 200−2000 nm. The phase structures of the films were analyzed through an X-ray diffractometer (XRD, Rigaku Ultima IV, Japan) with Cu anode (λ = 0.154 nm). The surface morphology and cross-sectional morphology of the films were observed with a field emission scanning electron microscope (FE-SEM; Nova Nano SEM 450, USA). The films’ surface roughness was detected by atomic force microscope (AFM; SPI3800N, SII). The resistivity (ρ), carrier concentration (nh), and carrier mobility (μ) of the films were determined by Hall effect measurements in the van der Pauw configuration (Keithley-4200 SCS, USA) at room temperature. The electrical resistivity of the films under bending was measured by the four-point probe method (RTS-8, China). Haze and CIELAB color values were measured by a spectral haze meter (HAM-300, China).
Top-view SEM images of γ-CuI thin films fabricated by different iodination methods are presented in Fig. 3a-c. AFM images of γ-CuI thin films fabricated by different iodination methods are presented in Fig. 4a-c. The Vis-IR spectra of CuI films prepared through liquid, vapor, and solid iodination methods are shown in Fig. 5a. The photographs for liquid, vapor and solid iodination methods are shown in Fig. 5b. The CuI thin films fabricated by the liquid and vapor iodination methods have both low reflectivity and low transmittance. It can be explained by SEM and AFM images of CuI films. As shown in Fig. 3a and Fig. 4a, the CuI film fabricated by the liquid iodination has a lot of huge and spares grains and roughness surface. When a beam of light irradiates a thin film with many cracks, the light will be scattered and propagate in other directions [25]. Therefore, the transmitted light and the reflected light intensity detected by the detectors are both weak. For this reason, the reflectance and transmittance are both lower as shown in Fig. 5a.
Those factors increase the reflecting and scattering of incoming light and give a milky appearance as shown in Fig. 5b. The CuI film fabricated via the vapor iodination method is slightly better than the liquid method. But the film fabricated by vapor iodination has a rough surface with obvious empty spaces as shown in Fig. 3b,e, and Fig. 4b. Therefore, the film has a frosted-glass-like appearance as shown in Fig. 5b. In contrast, those grains of the CuI film fabricated by the solid iodination are significantly smaller and denser as shown in Fig. 3c,f. Those small and dense grains will lead to a smooth surface as shown in Fig. 4c. The light transmittance of the γ-CuI films fabricated by solid iodination is higher than others due to minimizing incoming light reflecting and scattering by the smooth surface of films. Therefore, the CuI film fabricated by the solid iodination method has both the highest transmittance and strong Fabry–Pérot interference as shown in Fig. 5a.
In addition, pronounced excitonic absorption is visible at EZ1/Z2 = (3.05 ± 0.01) eV. Transitions due to spin-orbit coupling apparent at EZ3 = (3.68 ± 0.01) eV. The Eg of γ-CuI can be calculated by adding EZ1/Z2 to the excitonic binding energy [19,22]. The Eg obtained by those three methods is ~3.10 eV, which is consistent with theoretical calculations and other experimental reported [16,19,24]. The absorption coefficient was calculated by the Beer-Lambert relation
where t is the thickness and T is the transmittance. The spectra of absorption are consistent with the spectra in another report. The applicable condition of the Beer-Lambert relation is a non-scattering system. The films we fabricated via liquid and vapor iodination methods have obvious scattering. Therefore, the α has a significant shift as shown in Fig. 5c.
In this study, the solid-state reaction is used to explain the obvious difference of CuI films prepared by iodization. Generally, the solid-state reaction is divided into two processes: reaction between interface and diffusion of reactive species in the precursor. Jander’s equation is one of the most popular to examine solid-state reaction kinetics under isothermal conditions [33,34]. Its expression is
where α is the conversion rate of reactants, D represents the diffusion coefficient, c0 represents the reactant concentration at the interface between the layer of reaction products and the reactant, t represents the time and r is the spherical reactant particles’ radius.
A diagram of the growth of the CuI grains basing on Jander’s equation and intuitive results was proposed as illustrated in Fig. 6. Compared with the solid iodization method, the liquid and vapor iodination methods have many iodine molecules in contact with the Cu3N precursors, which makes the interface reaction area larger. Meanwhile, iodine aqueous solution and iodine vapor have lower concentrations at the reaction interface than solid iodine particles, which makes all reactions slower. Each contact point reacts with the precursor at the same time, so that there is already a layer of CuI on Cu3N grains as shown in Fig. 6a. When the outer “shell” of the grain was formed, the process of further diffusion-reaction is the iodination reaction inside the precursor. Therefore, the CuI film has coarser grains. The coarse grains can cause empty spaces as shown in Figure 3a,b. On the contrary, the CuI film fabricated by solid iodination has small grains instead of shell layers due to the few contact area between the iodine solid particles and the Cu3N film as shown in Fig. 5b. The iodination process of non-contact points grows freely with the diffusion of iodine atoms without the influence of the CuI “shell” layer, so the grains are smaller. Fortunately, the arbitrarily distributed small grains make the film more compact.
3.2. A New Strategy
Based on the above analysis, we infer the key factor to improve the transmittance of CuI films is to reduce the grain size to form a denser film structure with a smooth surface. A denser film structure with a smooth surface can reduce the light scattering of CuI films. The effect of solid iodization is the best among the three iodization methods, and the details of solid iodization are studied.
When iodine particles contact with Cu3N precursor, they can be divided into contact areas and non-contact areas. Therefore, what are the similarities and differences between the two regions? The schematic diagram of Fig. 7a,b shows the preparation process of two cases. The contact area is prepared by completely covering the iodine powder. The iodine particles are placed on the edge of the precursor for iodization, and the middle area is the non-contact iodization area. The γ-CuI films fabricated by iodine particles contact iodination are smooth as shown in Fig. 7b,e. The root-mean-square (RMS) roughness of the 10 × 10 μm2 scanned area was 9.8 nm. In contrast, the non-contact iodization areas of γ-CuI films have different surface morphology as shown in Fig. 7c. The root-mean-square (RMS) roughness of the 10 × 10 μm2 scanned area was 27.1 nm. This can also be explained by the above analysis of solid-state reaction. As shown in Fig. 7d, the non-contact iodization area γ-CuI films have a frosted-glass-like appearance and a low transmittance (73%) due to small uneven grains on the substrate in the no-contact area, which is consistent with a previous report [25]. But the CuI grains generated in the contact area are very flat. Its average transmittance is as high as 85%. In other words, the reaction between the iodine solid particles and the Cu3N precursor in the contact interface (contact iodination area) between solid-state iodine and Cu3N precursor (noted as step1) is smoother than the reaction due to the diffusion (non-contact iodination area) of reactive species in the Cu3N precursor (noted as step2). Therefore, the transmittance of step1 is higher than step2.
Improving film smoothness and grain density can reduce light scattering, and then increasing the transmittance of films and eliminating the frosted-glass-like appearance of the film. It has been reported that a high smooth surface will make the reflectivity too high and reduce the transmittance [25,35] so that the transmittance of CuI film obtained by solid iodization is only about 70% [25]. In this study, it is found that reflection has no obvious effect on CuI films prepared by solid iodination. This study proposed films with different roughness area distributions and selected three representative films for analysis.
Fig. 8a shows the schematic three arrangements and their scattering, green represents contact areas, gray represents the non-contact area. According to the haze and interference of prepared film, the distribution of the two areas of a sample is visible. Fig. 8b shows the transmittance and reflectivity of three arrangements. Fig. 8c shows the visible transmittance and haze of those films. Fig. 8d shows the Lab color of those films. The negative value of the ‘a’ axis tends to green, and the positive value tends to red. The negative value of the ‘b’ axis tends to be blue, and the positive value tends to be yellow. When the distribution of smooth area is denser, the surface of CuI film is so smooth. The film has obvious Fabry Perot interference due to smooth surface as shown in Fig. 8b. The CIELab color values of a and b are less than 10 as shown in Fig. 8d. It indicates that the interference color does not affect its application as the transparent conductive film. In the visible region, CuI film has high transmittance and low haze due to the light scattering of the film is low. Although the reflectivity is slightly higher than other CuI films, the transmittance is still as high as 85%. When the uneven area is density, the CuI film has a high haze and low transmittance due to the rough surface of the CuI film. The haze is linearly correlated with the transmittance as shown in Fig. 8c. Therefore, the film with a smooth surface can be prepared by controlling the density of the contact area.
Different devices require different haze on the electrode. For example, the touch-panel displays require low haze transparent electrodes [36-38], but solar cells need transparent electrodes with high haze to improve light utilization [39-41]. Therefore, this strategy also can modify the haze of CuI film from 0.7% to 21% to meet the needs of different electronic devices. That will greatly expand the application of CuI as transparent conductive films.
3.3. Photoelectric performance of p-type TCs
Quantitatively, the overall photoelectric performance of the TC can be quantified by the Haacke figure of merit (FOM) [42], ΦTC = T10/Rs where T is the average transmittance in the visible region and Rs is the sheet resistance. A larger value of FOM indicates the better photoelectric performance of the TC. As shown in Table 1, the FOM of most novel p-type TCs is lower than 20 MΩ-1. In contrast, the as-deposited γ-CuI film shows a FOM about 11× higher, up to 233 MΩ-1, while providing a high Tvis above 86%. These results demonstrate the superior TC photoelectric performance of our γ-CuI films with respect to all measures.
Table 1. Transmittance, resistivity, and the FOM of CuI films and other p-type TCs
|
Materials |
d (nm) |
σ (S/cm) |
Rs (kΩ/sq) |
Tvis (%) |
FOM (MΩ-1) |
Referece |
|
Ca3Co4O9 |
100 |
18 |
5.5 |
31.3 |
0.0015 |
[43] |
|
GaN: Mg |
100 |
5.3 |
19 |
70 |
1.487 |
[44] |
|
SnO |
200 |
0.77 |
650 |
85 |
3.03 |
[45] |
|
MgxCr2-xO3 |
150 |
0.333 |
200 |
65 |
0.067 |
[46] |
|
La2/3Sr1/3VO3 |
24 |
816.3 |
0.51 |
61.1 |
14.21 |
[14] |
|
CuAlO2 |
230 |
0.34 |
130 |
70 |
0.217 |
[12] |
|
CuGaO2 |
100 |
0.02 |
5000 |
80 |
0.022 |
[6] |
|
CuCrO2 |
180 |
0.19 |
292.4 |
58.3 |
0.015 |
[8] |
|
CuScO2+x |
110 |
15 |
6.06 |
40 |
0.0173 |
[10] |
|
CuCrO2: Mg |
50 |
1.55 |
129 |
49.81 |
0.123 |
[11] |
|
CuI |
230 |
45 |
0.95 |
86 |
233 |
This work |
Figure 9 shows the transmittance, conductivity, and FOM values of CuI films prepared by the new strategy and other methods. The films prepared by the new strategy have better performance than other CuI films. Compared with other preparation methods, this new strategy is simple and controllable. Moreover, the haze can be adjusted from 0.7% to 22%. This expands the application range of CuI as the transparent conductive film.
In summary, we fabricated γ-CuI films via solid, vapor, and liquid iodination of Cu3N precursors. The SEM and AFM images show the γ-CuI films fabricated by the solid iodination method have smaller and denser grains, fewer empty spaces, and lower surface roughness. Those factors lead to high transmittance, great electrical properties as well as better flexibility. Furthermore, we studied the contact and non-contact regions between the Cu3N film and iodine particles in solid iodination. The SEM and AFM images show that the solid iodination method has two different steps with different surface morphology. They have obvious differences in light scattering. The first step is the reaction between the interface of Iodine granules and Cu3N film. The second step is the diffusion of reactive species in the Cu3N layer. Based on this phenomenon, a new strategy was proposed to fabricate high transmittance and low haze CuI films. The FOM of the CuI film prepared by this new strategy is >230 MΩ-1. Compared with other p-type transparent conductive materials and CuI films prepared by other methods, the photoelectric performance of CuI film prepared by this strategy is excellent. Besides, this strategy can also make the haze of the CuI film controllable. This study can promote the application of CuI film in optoelectronic devices.
Acknowledgments
We gratefully acknowledge the National Natural Science Foundation of China (No. 62004117), Young Scholars Program of Shandong University, Weihai for their financial support. We also thank the Physical-Chemical Materials Analytical & Testing Center of Shandong University at Weihai for their assistance with characterization.
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