A Comparison of Optical Properties of CuO and Cu2O Thin Films for Solar Cell Applications

Solar energy is becoming more popular and widespread, and consequently, the materials to manufacture solar cells are becoming more limited and costly. Therefore, in order to keep solar energy affordable and available, we must research alternative materials such as copper oxides. Some benefits of copper oxides include being available in abundance, affordable, low toxicity, low bandgap, and a high absorption coefficient—all of which contribute to it being a valuable interest for the manufacturing of solar cells. In this study, CuO thin films were synthesized utilizing RF sputtering technique with deposition occurring at room temperature followed by thermal annealing between 100 ̊C and 400 ̊C and using different gases, oxygen (O2) (oxidizing and reactive gas) and nitrogen (N2) (inert gas), besides air. Afterwards, these thin films were evaluated for a range of wavelengths: 200 400 nm (UV spectrum), 400 700 nm (Visible spectrum), and 700 800 nm (IR spectrum), for both, optical transmittance and photoluminescence. In addition, the CuO results were compared to our Cu2O results from a previous study to assess their differences. In the results of this study, the CuO thin film initially had a bandgap of 2.19 eV at room temperature, and by increasing the annealing temperature to different levels, the bandgap decreased respectively. The presence of air in the chamber allowed for the highest decrease, followed by the nitrogen (N2) and the lowest decrease was observed in the presence of oxygen (O2). This was reflected in the decrease in the bandgap values from 2.19 eV (room temperature) to 2.05 eV for the films annealed at 400 ̊C.


Introduction
The current materials used to manufacture photovoltaics are becoming scarce, How to cite this paper: Bunea, R., Saikumar, A.K. and Sundaram, K. and as a result, expensive. A solution will be to investigate other materials that might qualify to fill the gap and be used as replacements. Metal oxides are one of these materials that are readily available and currently have a low cost of manufacturing [1]. In addition, copper oxides also have low toxicity, low bandgap, and a high absorption coefficient which are all valuable qualities. The most popular forms of copper oxides include the following: cupric oxide (CuO-tenorite in the mineral form), cuprous oxide (Cu 2 O), and Cu 4 O 3 (paramelaconite in the mineral form) [2]. The paramelaconite is a meta-stable copper oxide, which is an intermediate compound between CuO and Cu 2 O. The stable forms are the CuO and the Cu 2 O. Both show promising qualities due to their electrical and optical properties [3]. The CuO has a dark brown/black color and the Cu 2 O is a yellow/ red color [2]. In the presence of moist air, the Cu 2 O will change into CuO. The CuO has a smaller bandgap than Cu 2 O and as a result, is potentially superior in photon-detection and optical switching applications that are used in combination with visible and near-infrared spectrums [4]. The Cu 2 O has a cubic structure and a bandgap between 2.0 eV and 2.6 eV [5]. The CuO has a monoclinic (a group of crystalline solids whose crystals have three axes of unequal length, with two being perpendicular to one another) structure with a bandgap between 1.3 eV and 2.2 eV [5]. In theory, a material qualifies for solar applications if it has good absorption of solar radiation (α = 1) in the visible (400 nm to 700 nm) and nearinfrared spectrum (700 nm to 2000 nm), and no emission (ε = 0) in the infrared region (2000 nm to 20,000 nm) [5].
Cupric oxide (CuO) is potentially a good candidate for solar cell applications due to high electron mobility and high optical absorptivity in the visible spectrum [1]. It also has high conductivity and low electrical resistivity. The copper oxides are p-type semiconductors and are usually coupled with n-type semiconductors like zinc oxide (ZnO), silicon (Si), and cadmium sulfide (CdS) [4]. The high conduction of the p-type copper oxide is attributed mainly to the negatively charged copper (Cu) vacancies [5]. Application-wise, besides solar cells, copper oxides are utilized in lithium-ion batteries, photocatalysts, and photoelectronchemical cells [6]. Currently, there are a few different methods to produce copper oxide thin films like reactive sputtering, anodizing, chemical conversion, chemical vapor deposition, and thermal oxidization [6]. Sputtering is an inexpensive method of creating copper oxide thin films. In addition, other characteristics of this method are the high deposition rate, dense layer formation, good surface flatness, and low substrate temperature [7]. to CuO [9]. This is beneficial because it becomes easier to induce electron flow, which means better efficiency. Therefore, both of these metal oxides show promising qualities and deserve equal investigation to understand which could be the future of solar cells.

Experimental Procedure
The methodology for both the CuO thin films and Cu 2 O thin films were identical. Briefly, the thin films were deposited on glass slides using RF magnetron sputtering technique via an in-house built sputtering system. Argon was utilized as the only sputtering gas while deposition occurred at room temperature. The ( ) where λ α = absorption coefficient, T = optical transmission, t = thickness of film, hv = photon energy, B = constant factor, and g E = optical bandgap.
The PL method entails utilizing wavelengths obtained from the excitation curve (PL peaks) and fitting that data into Equation (3)

Results and Discussion
As a result of annealing temperature, the levels of transparency differed between each type of thin film: semi-transparent at room temperature, and near opaque at higher temperatures (400˚C). Overall, each of the thin films had a strong ab-  [10]. Figure 1 shows the first series of CuO thin films annealed in air.
For the CuO thin films annealed in air (Figure 1)     is that the transmission varies as a function of annealing temperature, and the above results show evidence of an inversely proportional relationship between these two variables.   Figure 8 show the Tauc plot for the absorption coefficient for CuO, ranging from room temperature to 400˚C. Rearranging Equation (1) will allow for the calculation of the wavelength from the bandgap energy determined using the Tauc method [10].
CuO, ranging from room temperature to 400˚C.
Using the spectrometer software, the following specs were adjusted: integration time of 400 ms and reference spectrum stored as a background spectrum.        CuO thin films showed a shift from a lower spectrum peak (568 nm) for the thin film created in air, to 587 nm at 100˚C and all the way to 607 nm at 400˚C for the film annealed in air. The thin film annealed in nitrogen (N 2 ) at 400˚C created a peak at 598 nm. If the annealing temperature is increased even higher than 400˚C, an even higher PL peak for the CuO thin films can be achieved, which will transition the output spectrum to upper visible and even infrared [10]. However, the increase in annealing temperature will require upgrading the thin films substrate from glass to a metal due to the fact that the annealing temperature for a microscope glass slide is 545˚C and its softening temperature is 724˚C (according to the microscope slides manufacturer). This change will also affect the budget for the experiment due to the price differences between the different substrates. The increase in the annealing temperature induces the formation of a more hard packed structure, and the sharper peaks obtained for the thin films annealed at 400˚C indicate a higher uniformity of the composition and strain [10].
The next step is to look at how utilizing either the Tauc plot method or photoluminescence (PL) method can affect the wavelength and bandgap energy levels. In order to use the Tauc method, first the transmittance must be obtained and then the absorption coefficient can be calculated from that value. Then, the Tauc method relates absorption coefficient to photon energy to calculate the bandgap energy. Only then can the wavelength be calculated from bandgap energy utilizing Equation (1). On the other hand, using the PL method, the wavelength can be obtained from the excitation curve, and then the bandgap energy can be calculated utilizing Equation (2).   Both the bandgap and wavelength can be calculated from either method, however, they will still result in different values. Specifically, the difference in bandgaps between the Tauc and PL methods is due to the red shift in wavelength in PL-in turn, this is because of a trap state present in which nonradiative decay of the photon occurs [11]. The red shift in PL means an increase in wavelength which indicates a decrease in energy. Hence, the lower bandgap energy observed when calculated via the PL method in Table 6, and the higher wavelength in Table 7.

Conclusions
In this study, the optical properties of CuO thin films were investigated and com- bandgap between 1.00 eV and 1.70 eV to be considered effective. Therefore, a future study could include identifying which n-type semiconductors work best in conjunction with these copper oxides in order to produce the lowest possible bandgap within the proper range for solar cell use.
In addition, when looking at the various gases introduced in the annealing chamber for CuO thin films, it was observed that oxygen (O 2 ) had a minor effect, while the regular atmosphere (air) or nitrogen (N 2 ) created the largest impact. However, for the Cu 2 O thin films, nitrogen alone had the largest impact while air and oxygen had minor impacts. Therefore, another future study could include a characterization of nitrogen-doped (N-doped) cupric oxide (CuO) for solar cell applications.