Improving the p-Type CuCrO2 Thin Film’s Electrical and Optical Properties

In this research, we studied the functional properties of CuCrO2, which is the most promising p-type transparent conductive oxide (TCO). The thin films were fabricated using a spin coating technique. The diffraction patterns were obtained with the help of X-ray diffractions, and the optical properties of absorption characteristics were studied using UV-visible absorption. The physical properties of film formation and surface morphology were analyzed using FESEM analysis. The aging properties were also analyzed with the help of various precursors with different aging times. The CuCrO2 thin films’ functional properties were determined by using chelating agent and precursor solution aging times. The CuCrO2 thin films have better transmittance, resistance, figure of merit (FOM), and electrical conductivity. Moreover, the resistivity values of the CuCrO2 thin films are 7.01, 9.90, 12.54, 4.10, 2.42, and 0.35 Ω cm. The current research article covers the preparation of copper chromium delafossite thin films. These thin films can be suitable for hole transport layers in transparent optoelectronic devices.


Introduction
Transparent conductive oxide (TCO) is a type of semiconductor material combining excellent electrical conductivity and optical transmittance towards the visible range. TCO-based thin films have been utilized in a wide variety of applications, such as touch screens, flat panel displays, and solar cells [1]. Delafossite materials have a high absorption coefficient, prolonged carrier lifetime, excellent charge mobility, and greatly increased power conversion efficiency (PCE) in the short term. Delafossite materials are ternary oxide compounds with the general formula of A I B III O 2 , where A belongs to monovalent cations, such as Cu or Ag, and B belongs to trivalent metals ranging from Al to La. CuCrO 2 materials belong to the family of delafossite; they have excellent conductivity among the other delafossite materials. CuCrO 2 materials are more stable and highly efficient compared to other alkali metal delafossite materials due to their physiochemical properties and electronic configuration of Cu + , Cr 3+ , and O 2− [1,2].
Perovskite solar cells (PSCs) are usually fabricated in a p-i-n or n-i-p structure, in which the perovskite layer of the n-i-p structure is sandwiched between the hole transport layer (HTL) and the electron transport layer (ETL). In addition, the top layer of PSCs is

Preparation of CuCrO 2 Solution
The CuCrO 2 film's stability was observed by using a chelating agent with different aging times. Solution A was prepared as follows: copper nitrate and chromium nitrate were dissolved in 2-Methoxyethanol (10 mL) at a molar ratio of 1:1 and stirred for 24 h to obtain a homogenous solution without a chelating agent. Sample B was prepared by using the above-mentioned process. In addition, the chelating agent was added to the solution and the solution was stirred for 24 h. Similarly, C, D, E, and F samples were prepared using the above-mentioned process while varying the aging time. This is shown in Table 1.

Preparation of CuCrO 2 Thin Films by Spin Coating
A glass plate was cut into small sizes (1.5 × 1.5 cm) and washed with ethanol and water for several times. Then, the thin films were coated on the substrate in three different layers. The first layer was coated using the copper nitrate solution with different rpm (500 and 4000) for 10 and 15 s, respectively. The second layer was coated by using the chromium nitrate solution, and the third layer was coated by using the above-prepared complex solution. After spin coating, the spin-coated film was heated on a hot plate at 200 • C for 5 min and dried. The sample was then calcinated at 400 • C for 5 min using a tubular furnace to eliminate the precursor residuals. Finally, the prepared thin films were annealed at 400 • C for 15 min under the atmosphere of mixed gases (H 2 /N 2 10:90 sccm) to reduce Cu 2+ to Cu + . Thereafter, the annealed film was again annealed at 600 • C for 1 h in the N 2 atmosphere to obtain the CuCrO 2 thin films. The preparation methods are depicted in Figure 1.

Characterization of CuCrO 2 Thin Films by Spin Coating
The CuCrO 2 thin films' characteristics were characterized by using various analytical characterization techniques. The CuCrO 2 thin films' crystalline structure was analyzed by using grazing incidence X-ray diffraction (GIXRD D8, Bruker ADVANCE, Billerica, MA, USA), and the range of 2-Theta was from 20 • to 80 • (the incident angle was 0.5 • , Cu Kα λ = 1.5418 Å). The cross-sectional and surface morphology of the film was observed with the help of a field-emission scanning electron microscope (FESEM, FE-SEM/EDX, JEOL, JSM-7610F, and Hitachi Regulus 8100, Tokyo, Japan). The optical properties of the prepared films were measured using an ultraviolet-visible spectrometer (UV-Vis, Shimadzu 2600, Kyoto, Japan) at a wavelength range of 300-800 nm. The vibrational bonds and the functional groups of the prepared materials were studied by using a micro-Raman spectrometer (Micro-Raman Spectrum, ACRON, UniNanoTech Co., Ltd., Yongin, Republic of Korea) and a Fourier-transform infrared spectrometer (FTIR, Spotlight 200i Sp 2 with Auto ATR System, Perkin Elmer, Inc. Waltham, MA, USA). The resistivity was measured by using a fourpoint probe (Four-Point Probe, Keithley 2400 SourceMeter ® SMU Instruments, Cleveland, OH, USA), and the carrier concentration was measured by using the hall method (Ecopia HMS-3000 Hall Measurement System, Gyeonggi-do, Republic of Korea).

Characterization of CuCrO2 Thin Films by Spin Coating
The CuCrO2 thin films' characteristics were characterized by using various analytical characterization techniques. The CuCrO2 thin films' crystalline structure was analyzed by using grazing incidence X-ray diffraction (GIXRD D8, Bruker ADVANCE, Billerica, Massachusetts, USA), and the range of 2-Theta was from 20° to 80° (the incident angle was 0.5°, Cu Kα λ = 1.5418 Å). The cross-sectional and surface morphology of the film was observed with the help of a field-emission scanning electron microscope (FESEM, FE-SEM/EDX, JEOL, JSM-7610F, and Hitachi Regulus 8100, Japan). The optical properties of the prepared films were measured using an ultraviolet-visible spectrometer (UV-Vis, Shimadzu 2600, Kyoto, Japan) at a wavelength range of 300-800 nm. The vibrational bonds and the functional groups of the prepared materials were studied by using a micro-Raman spectrometer (Micro-Raman Spectrum, ACRON, UniNanoTech Co., Ltd., Korea) and a Fourier-transform infrared spectrometer (FTIR, Spotlight 200i Sp 2 with Auto ATR System, Perkin Elmer, Inc. Waltham, USA). The resistivity was measured by using a four-point probe (Four-Point Probe, Keithley 2400 SourceMeter ® SMU Instruments, Cleveland, Ohio, USA), and the carrier concentration was measured by using the hall method (Ecopia HMS-3000 Hall Measurement System, Gyeonggi-do, South Korea).

XRD Analysis
The GIXRD was used to study the CuCrO2 thin films' crystal structure and diffraction patterns. Figure 2 shows the X-ray diffraction patterns of the CuCrO2 thin films. In the XRD analysis, we can observe various peaks at 31.36°, 35.21°, 36.41°, 40.87°, 47.83°, 55.85°, and 62.41° corresponding to (006), (101), (012), (104), (009), (018), and (110), respectively. The obtained crystalline planes are well matched with the JCPDS no PDF#89-0539 Cu-CrO2. Moreover, the crystalline peaks show that CuCrO2 is the main phase, and there are no impurity phases such as Cr2O3 and CuO. In addition, without 2-Aminoethanol, the peak at 36.41 (012) intensity is higher and the peak is very sharp. After adding the chelating agent and due to aging time, the diffraction planes (006) and (110) intensity decrease and the FWHM also increases. Similarly, all the diffraction plane's intensity decreases and the FWHM increases in the samples D, E, and F. Therefore, the XRD diffraction patterns indicate that the CuCrO2 has been successfully formed. Using the Scherrer Equation, the crystallite size of the prepared thin films is calculated [1]. The formula is as shown below, where ε is the crystallite size; K is the Scherrer constant; λ is the wavelength of the incident radiation; b is the full width at half maximum (FWHM) of the crystalline peaks; and θ is

XRD Analysis
The GIXRD was used to study the CuCrO 2 thin films' crystal structure and diffraction patterns. Figure 2 shows the X-ray diffraction patterns of the CuCrO 2 thin films. In the XRD analysis, we can observe various peaks at 31. Moreover, the crystalline peaks show that CuCrO 2 is the main phase, and there are no impurity phases such as Cr 2 O 3 and CuO. In addition, without 2-Aminoethanol, the peak at 36.41 (012) intensity is higher and the peak is very sharp. After adding the chelating agent and due to aging time, the diffraction planes (006) and (110) intensity decrease and the FWHM also increases. Similarly, all the diffraction plane's intensity decreases and the FWHM increases in the samples D, E, and F. Therefore, the XRD diffraction patterns indicate that the CuCrO 2 has been successfully formed. Using the Scherrer Equation, the crystallite size of the prepared thin films is calculated [1]. The formula is as shown below, where ε is the crystallite size; K is the Scherrer constant; λ is the wavelength of the incident radiation; b is the full width at half maximum (FWHM) of the crystalline peaks; and θ is the Bragg angle [1][2][3].

SEM Studies
The prepared CuCrO 2 thin films' surface structure and morphology were observed using FESEM analysis. The surface morphologies of the annealed CuCrO 2 thin films for different aging times are exhibited in Figure 3. The effect or influence of the chelating agent and aging time can be observed on the growth of grains. Figure 3 shows that the CuCrO 2 without a chelating agent shows irregular grain shape and size. However, when a chelating agent is added to the precursor solution with different aging times, such as 0, 30 min, 1 h, and 4 h, it shows the irregular shape and size of the thin films. The precursor solution with an aging time of 2 h exhibits a homogenous surface morphology, uniform grain shape, and good crystalline size. This exhibits a similar changing trend to the results of XRD

SEM Studies
The prepared CuCrO2 thin films' surface structure and morphology were observed using FESEM analysis. The surface morphologies of the annealed CuCrO2 thin films for different aging times are exhibited in Figure 3. The effect or influence of the chelating agent and aging time can be observed on the growth of grains. Figure 3 shows that the CuCrO2 without a chelating agent shows irregular grain shape and size. However, when a chelating agent is added to the precursor solution with different aging times, such as 0, 30 min, 1 h, and 4 h, it shows the irregular shape and size of the thin films. The precursor solution with an aging time of 2 h exhibits a homogenous surface morphology, uniform grain shape, and good crystalline size. This exhibits a similar changing trend to the results of XRD patterns. The prepared CuCrO2 films' crystalline sizes are better and form hexagonal-shaped crystallites with uniform grain boundaries, which can be utilized to observed the prepared delafossite surface morphology. In Figure 4, the FE-SEM cross-sectional image of the CuCrO 2 thin films and the thickness of the prepared films can be observed. Noticeably, the spin-coated film is tightly packed to the substrate with a uniform thickness of 142 nm ( Table 2). Though the chelating agent was added at different aging times, all films portray almost similar cross-sectional surface morphology due to the same preparation methods. However, the thin film thickness varies with the precursor solution aging time, and the CuCrO 2 thin film thicknesses are 120, 122, 111, 139, and 112 when the precursor solution aging times are 0 min, 30 min, 1 h, 2 h, and 4 h, respectively.

UV-Vis Analysis
The visible-light transmittance is the most important parameter for the applications of TCO thin films. The prepared CuCrO 2 thin films' optical transmittance was investigated using an UV-Visible spectrometer in the wavelength range of 300-800 nm ( Figure 5). The CuCrO 2 thin films' transmittance changes in the visible region and varies with varying precursor solution aging time owing to the enhancement in the crystallinity and grain size. The lower-transmittance films may be the result of photon scattering by the film grain boundaries or pores owing to poor crystallization. The prepared thin films' transmittance values are presented in Table 3. As a result of UV-Vis, the high transmittance in the visible-light region shows that the prepared CuCrO 2 thin films have a better potential for applications in perovskite solar cells. In Figure 4, the FE-SEM cross-sectional image of the CuCrO2 thin films and the thickness of the prepared films can be observed. Noticeably, the spin-coated film is tightly packed to the substrate with a uniform thickness of 142 nm (Table 2). Though the chelating agent was added at different aging times, all films portray almost similar cross-sectional surface morphology due to the same preparation methods. However, the thin film thickness varies with the precursor solution aging time, and the CuCrO2 thin film thicknesses are 120, 122, 111, 139, and 112 when the precursor solution aging times are 0 min, 30 min, 1 h, 2 h, and 4 h, respectively.   In Figure 4, the FE-SEM cross-sectional image of the CuCrO2 thin films and the thickness of the prepared films can be observed. Noticeably, the spin-coated film is tightly packed to the substrate with a uniform thickness of 142 nm (Table 2). Though the chelating agent was added at different aging times, all films portray almost similar cross-sectional surface morphology due to the same preparation methods. However, the thin film thickness varies with the precursor solution aging time, and the CuCrO2 thin film thicknesses are 120, 122, 111, 139, and 112 when the precursor solution aging times are 0 min, 30 min, 1 h, 2 h, and 4 h, respectively.

UV-Vis Analysis
The visible-light transmittance is the most important parameter for the applications of TCO thin films. The prepared CuCrO2 thin films' optical transmittance was investigated using an UV-Visible spectrometer in the wavelength range of 300-800 nm ( Figure 5). The CuCrO2 thin films' transmittance changes in the visible region and varies with varying precursor solution aging time owing to the enhancement in the crystallinity and grain size. The lower-transmittance films may be the result of photon scattering by the film grain boundaries or pores owing to poor crystallization. The prepared thin films' transmittance values are presented in Table 3. As a result of UV-Vis, the high transmittance in the visiblelight region shows that the prepared CuCrO2 thin films have a better potential for applications in perovskite solar cells.  The absorption coefficient (α) can be calculated from the transmittance data of the prepared samples as follows [1-3]:  The absorption coefficient (α) can be calculated from the transmittance data of the prepared samples as follows [1][2][3]: where d is the thickness of the film and T is the transmittance of the film. According to the following equation, the absorption coefficient (α) and the photon energy (hν) of the material can be calculated as follows [25]: transmittance in the visible light range. Among them, sample C with an aging time of 30 min has the highest transmittance of 73.8%, and a direct energy gap from 2.78 to 2.91 eV, which is consistent with the literature [26,27]. The curve of Sample A, which is between 300 and 370 nm, can be considered as the non-uniform distribution of metal ions due to the absence of a chelating agent, resulting in relatively poor and certain impurities in the film prepared by coating.
following equation, the absorption coefficient (α) and the photon energy (hν) of the material can be calculated as follows [25]: αhν = A(hν − Eg) 1/2 (3) Figure 6 shows the energy band gap values calculated from the Tauc plot for the transmission spectrum of each sample. It can be observed that the absorption peaks of the prepared films are all in the range close to visible light, showing that they have good transmittance in the visible light range. Among them, sample C with an aging time of 30 min has the highest transmittance of 73.8%, and a direct energy gap from 2.78 to 2.91 eV, which is consistent with the literature [26,27]. The curve of Sample A, which is between 300 and 370 nm, can be considered as the non-uniform distribution of metal ions due to the absence of a chelating agent, resulting in relatively poor and certain impurities in the film prepared by coating.

FT-IR Analysis
The FT-IR spectrum of the CuCrO2 thin films is shown in Figure 7. There are bending vibrations of the Al-O bond in the [AlO6] octahedron at 580 cm -1 [28], and 778 cm -1 [29] [28,29], and Cr-stretching vibrations of O-M bonds are also observed [31]. The peaks 938 cm -1 can be attributed to the stretching vibrations of Si-O bonds and the symmetrical stretching vibrations of Si-O-Si bonds [32]. The signal at about 1028 cm -1 corresponds to the asymmetric stretching vibration of the Si-O-Si bond in the [SiO4] tetrahedral unit or the stretching vibration of the B-O bond in the [BO4] tetrahedral unit [32]. The 1374 cm -1 signal corresponds to the asymmetric stretching vibration of the B-O bond in the [BO3] triangular unit [30]. It can be seen from this that the relative thickness of the CuCrO2

FT-IR Analysis
The FT-IR spectrum of the CuCrO 2 thin films is shown in Figure 7. There are bending vibrations of the Al-O bond in the [AlO 6 ] octahedron at 580 cm −1 [28], and 778 cm −1 [29] is the bond of [  4 ] tetrahedral unit [32]. The 1374 cm -1 signal corresponds to the asymmetric stretching vibration of the B-O bond in the [BO 3 ] triangular unit [30]. It can be seen from this that the relative thickness of the CuCrO 2 films is only about 110-150 nm, and the signal of the alkali-free glass substrate itself [33] is extremely strong. However, we have observed the solvent of 2-Methoxyethanol related peaks at 3400-3500 cm −1 and 2700-3000 cm −1 . Moreover, after annealing we cannot observe these materials related peaks.
films is only about 110-150 nm, and the signal of the alkali-free glass substrate itself [33] is extremely strong. However, we have observed the solvent of 2-Methoxyethanol related peaks at 3400-3500 cm −1 and 2700-3000 cm −1 . Moreover, after annealing we cannot observe these materials related peaks.

Raman Studies
The delafossite material CuCrO2 and other delafossite materials commonly have four atoms in the unit cell, which will produce 12 vibrational Raman modes: Three of them are acoustic modes, nine are optical modes, and A1g and Eg are the symmetrical Raman-active modes [34]. The A mode is the vibration along the c-axis, which is along the direction of the Cu-O bond, and the E mode represents the vibration in the triangular lattice perpendicular to the c-axis [35] The normal modes are categorized according to their parity when an inversion center exists in the structure. The subscript u stands for odd modules, such as A2u and Eu. The literature shows that when Γ is in an odd mode, two oxygen atoms will move in the same direction, while Cu and Cr atoms will not be affected [36,37]. According to Figure 8, the Raman scattering peaks of the CuCrO2 films are around 100 cm −1 (Eu), 459 cm −1 (Eg), and 706 cm −1 (A1g), which is consistent with the literature; however, at about 212 cm -1 , the Ag peak recorded in literature does not appear [38]. It can be seen from Figure 8 that Sample A without a chelating agent has an obvious frequency shift to a lower wave number at the position of A1g, which is due to the weakening of the Cr-O bond [38]. Samples B-F after adding the chelating agent all have the Eu peaks, but the peak of Sample F is wider, which may be the same as that of Sample A, because of the smaller grain size and lower crystallinity [39]. According to the literature, the stronger the Ag peak, the more grains the sample grows along the parallel c-axis [40]. Therefore, Figure 8 can prove that after adding the chelating agent, the crystal grains on the surface of the film grow parallel to the c-axis, and most of them grow along the direction perpendicular to the c-axis. From Samples B-E, with the aging time of 2 h, the crystallinity of the film is the highest and most of the film grows in the direction perpendicular to the c-axis, which is consistent with the SEM image.

Raman Studies
The delafossite material CuCrO 2 and other delafossite materials commonly have four atoms in the unit cell, which will produce 12 vibrational Raman modes: Three of them are acoustic modes, nine are optical modes, and A 1g and E g are the symmetrical Raman-active modes [34]. The A mode is the vibration along the c-axis, which is along the direction of the Cu-O bond, and the E mode represents the vibration in the triangular lattice perpendicular to the c-axis [35] The normal modes are categorized according to their parity when an inversion center exists in the structure. The subscript u stands for odd modules, such as A 2u and E u . The literature shows that when Γ is in an odd mode, two oxygen atoms will move in the same direction, while Cu and Cr atoms will not be affected [36,37]. According to Figure 8, the Raman scattering peaks of the CuCrO 2 films are around 100 cm −1 (E u ), 459 cm −1 (E g ), and 706 cm −1 (A 1g ), which is consistent with the literature; however, at about 212 cm -1 , the A g peak recorded in literature does not appear [38]. It can be seen from Figure 8 that Sample A without a chelating agent has an obvious frequency shift to a lower wave number at the position of A 1g , which is due to the weakening of the Cr-O bond [38]. Samples B-F after adding the chelating agent all have the E u peaks, but the peak of Sample F is wider, which may be the same as that of Sample A, because of the smaller grain size and lower crystallinity [39]. According to the literature, the stronger the A g peak, the more grains the sample grows along the parallel c-axis [40]. Therefore, Figure 8 can prove that after adding the chelating agent, the crystal grains on the surface of the film grow parallel to the c-axis, and most of them grow along the direction perpendicular to the c-axis. From Samples B-E, with the aging time of 2 h, the crystallinity of the film is the highest and most of the film grows in the direction perpendicular to the c-axis, which is consistent with the SEM image.

Electrical Properties and Figure of Merit (FOM)
The prepared CuCrO2 thin films' electrical properties were studied for all samples, which are tabulated in Table 4. The resistivity values vary with the chelating agent and aging time. These results suggest that the aging time and thickness variation affect the electrical conductivities. The lowest resistivity value is obtained in the Sample F and the resistivity value is 0.3 Ω cm. In addition, the FOM values are also calculated and tabulated in Table 4. To understand the effect of thickness on the electrical properties, the resistivity was measured by using a four-point probe (Four-Point Probe, Keithley 2400 SourceMeter ® SMU Instruments), and the carrier concentration was measured by using the hall method (Ecopia HMS-3000 Hall Measurement System). The figure of merit (FOM) as proposed by Haacke was used to calculate the film quality, where T is the average transmittance in the visible light range, and R_sh is the sheet resistance of the film [5]: φ = T 10 /R_sh (5)

Electrical Properties and Figure of Merit (FOM)
The prepared CuCrO 2 thin films' electrical properties were studied for all samples, which are tabulated in Table 4. The resistivity values vary with the chelating agent and aging time. These results suggest that the aging time and thickness variation affect the electrical conductivities. The lowest resistivity value is obtained in the Sample F and the resistivity value is 0.3 Ω cm. In addition, the FOM values are also calculated and tabulated in Table 4. To understand the effect of thickness on the electrical properties, the resistivity was measured by using a four-point probe (Four-Point Probe, Keithley 2400 SourceMeter ® SMU Instruments), and the carrier concentration was measured by using the hall method (Ecopia HMS-3000 Hall Measurement System). The figure of merit (FOM) as proposed by Haacke was used to calculate the film quality, where T is the average transmittance in the visible light range, and R_sh is the sheet resistance of the film [5]: The results show that the samples all exhibit p-type conductivity. Their electrical properties are shown in Table 2. It can be seen that Sample A has higher conductivity, but the carrier concentration is lower, while Samples B and C have higher carrier concentrations, but the sheet resistance is also higher. This may be because the surface of the film is discontinuous due to the growth of lamellar crystals on the film surface. In Samples D-F, as the aging time increases, the sheet resistance of the film decreases from 4.10 Ω cm to 0.35 Ω cm, and the FOM also increases from 8.21 × 10 −6 Ω to 4.32 × 10 −5 Ω. This may be because the film surface is more continuous. However, Sample E has a higher carrier concentration. Finally, Table 5 compares the photoelectrical properties of Sample E and Sample F in this experiment with other CuCrO 2 films reported in the literature. The films used in this study have good light transmittance, good conductivity, and excellent electrical properties.  DC-MS = DC magnetron sputtering, AA-MOCVD = Metal Organic CVD, RF-CS = RF-Cathodic Sputtering.

Conclusions
The delafossite CuCrO 2 thin films were successfully prepared by a simple spin coating method. The basic characterizations were successfully utilized to study the electrocatalytic aspects of copper delafossite CuCrO 2 thin films. The reaction kinetics and mechanism of these prepared materials in the solid-state reaction were observed and it is strongly depending on the presence of Cu + ions with respect to the annealing process. Samples E and F exhibited excellent physio-chemical properties compared to the other delafossite materials. Indeed, the obtained results are better than already reported literatures. In addition, the proposed process obtained a higher p-type FOM value of 4.32 × 10 −5 . The improvement of p-type conductivity is attributed to the generation of the intrinsic defects with Cu, created Cu anti-site defects have occupy the Cr vacancies, which leads to the film crystallinity. In the current research work, it resulted in the enhancement of the optical and electrical properties, such as mobility, resistivity, transmittance, crystallinity, and FOM. This process allows us to prepare potential candidates for TCO's HTL layers, organic devices, organic light-emitting diodes (OLEDs), and perovskite solar cells.