Influence of Annealing Process on Cu2O Nanofilm and the Efficiency of Annealed p-Cu2O/n Si Nanostructure Solar Cell Prepared by Thermal Evaporation Technique

The investigation of unique oxide semiconductors was intensified in the previous two decades, mostly by solar cell applications or transparent electronics. In the category of p-type semiconducting substances, cuprous oxide (Cu₂O) exhibits intriguing electrical and industrial characteristics that make it an appropriate candidate for energy and electronic devices [1, 2]. Cu₂O is an appropriate choice because of the high absorption coefficient it has, the high hole mobility it possesses, the quantity of its constituent elements in the crust of the earth, and the fact that it is non-toxic [3-5]. Cu₂O is a stable binary copper oxide crystallizing in a cubic structure with a space group (Pn-3m) with the value of a lattice constant about 4.269Å where the unit cell consists of about four copper atoms and two oxygen atoms [6, 7]. It exhibits a direct optical bandgap from 2.10 to 2.60 eV [8, 9]. The Cu₂O thin film could be manufactured utilizing a variety of different chemical and physical processes, such as the chemical deposition technique [10], thermal oxidation [11], spin coating [12], plasma evaporation [13], molecular beam epitaxy [14], radio frequency (RF) [15] spray pyrolysis technique [16]. Tuama et al. [17] prepared Cu₂O: Ag NPs/Si solar cell by the technique of thermal evaporation and the conversion efficiency that they obtained of 3.5 by Cu₂O: 0.04 Ag NPs/Si. Jayathilaka et al. [18] deposited Cu₂O homojunction thin film onto a Ti substrate by an electrochemical deposition technique and the gained conversion efficiency of 2.64%, under AM1.5 illumination, were achieved by an annealed sulfur-treated solar cell of Ti/n-Cu₂O/p-Cu₂O/Au. Abdurrahman et al. [19] fabricated a photoactive substance comprising of Bi/Cu₂O/Bi solar cell by using a thermal oxidation process and method of powder vaporization, and the fabricated cell produced a power conversion efficiency of 1.14%.

The aim of this work is to enhance the value of efficiency of p-Cu₂ONPs/n-Si solar cell after the annealing process at 200℃. Cu₂O nanofilms were fabricated on a glass substrate by using an effective thermal evaporation method in order to study the structural and surface morphology with respect to the annealing process as well as Cu₂O nanofilms deposited onto n-type silicon substrate to fabricate Al/Cu₂ONPs/Si/Al solar cells where pure aluminum that coated onto Cu₂O/Si nanofilms allows for the production of the front and back contacts of the solar cell. There are many challenges in manufacturing solar cells such as the choice of appropriate material and the cost of production as well as the method of manufacturing solar cell.

In this research, employed the method of thermal evaporation because of large-area coating, simple, effective and low coast technique.

The morphology and electrical characteristics of the Cu₂O nanofilms as-fabricated and annealed onto glass and substrate via technique of thermal evaporation and annealed at 200℃ were examined. Hall parameters and some important parameters of Cu₂ONPs/Si solar cell were evaluated such as open circuit voltage (Voc), short circuit current (Isc), fill factor (F.F), and efficiency (η).

Figure 1(a) shows FE-SEM image of Cu₂O/glass nanofilm as deposited showing circular assemblies of grains including large and small particles, with irregular distribution onto the surface of the thin film, whereas the particles seem elongated on the surface with an average size of approximately 25.15 nm. Figure 1(b) illustrates FESEM images of Cu₂O/glass nanofilm thin film when annealed at 200℃ for 2h. FESEM images reveal a homogenous and uniform surface with no discernible micro-cracks, and the morphology of the produced films demonstrates that the particle size increases with rising temperature by the annealing process with an average grain size of around 35.36 nm and this could be due to the improvement of the crystallinity of Cu₂O nanofilms by the annealing process and the reduction in its crystal imperfections [20].

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Figure 1. FESEM images of: (a) Cu₂O/glass nanofilms as prepared and (b) Cu₂O nanofilms as annealed at 200℃

The basic aim of EDX measurement of a Cu₂O/glass nanofilm is to prove the presence of the elements (copper, and oxygen) that comprise the structure of the thin film and show the variation of atomic ratio for copper and oxygen for prepared and annealed samples as listed in Table 1 and the main peaks of CuL_a, CuK_a, CuK_b, and OK_a lines clearly appear in Figure 2.

Table 1. EDX data of Cu₂O/glass nanofilms before and after the annealing process

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Figure 2. EDX spectra of Cu₂O nanofilms: (a) as deposited and (b) annealed at 200℃ for 2h

Table 2. The AFM data of Cu₂O/glass nanofilms

AFM analysis was employed to investigate the crystalline structure surface topography the thin film.

Figure 3 displays the AFM images of Cu₂O nanofilms that were deposited onto glass substrate via the thermal evaporation method which have been post-annealed at 200℃. The deposited sample has a small number of sharp peaks that coalesce as islands vertically to the surface of the film. The values of roughness have increased from (0.166 to 1.180) nm may have been due to the formation of large grains as a consequence of the diffusion effect as a result of the annealing process [21]. This increase in roughness reveals the uniformity of the surface of the film and the opportunity of use in solar cells owing to its good reflection [22]. The root mean square attained from AFM measurements and its value raised from (0.213 to 1.490) nm as well as increased the average diameter from 150.5 nm to 236.3 nm after the annealing process, this might have been attributed to the increase in softness [22] as listed in Table 2.

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Figure 3. AFM images of Cu₂O/glass nanofilms: a) as-deposited and b) annealed films at 200℃

Table 3. Electrical data of Cu₂O/glass nanofilm

The electrical properties of deposited and annealed Cu₂O/glass nanofilm are shown in Table 3. Increased temperature by the annealing process resulted in a rise in average Hall coefficient and electrical resistivity and this behaves close to that observed by Han et al. [23] and Yang et al. [24]. The carrier concentration of the film was decreased from (4.144×10¹¹ cm⁻³ to 3.323×10¹¹) cm^-3 which can be owing to the improvement of crystal quality and the reduced crystal defects [25]. The resistivity increased after the annealing process while conductivity decreased.

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Figure 4. Built-in potential plots of annealed Cu₂ONP/ n-Si

Capacitance-voltage measurement is one of the most important methods for obtaining information about the rectifying junctions, built-in potential, and junction capacitance and junction type [26]. Figure 4 gives the variation of (1/C²) vs (V) measurements for different samples. From these Figures, it can be seen that a straight line, of which the intercept with the voltage axis gives the value of the built-in potential and its value for annealed Cu₂ONPs/n-Si heterojunction about 0.15 volt.

Current-voltage (I-V) curves can be used to determine the solar cell's efficiency of Cu₂ONPs/n-Si in the dark and under illumination using a light source emitting 100 mW/cm² with active area of the cell of 0.5×0.7 cm². In the dark, there is a rectification behavior for the Cu₂O/Si heterostructure as displayed in Figure 5 [27].

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Figure 5. I-V Characteristics of Cu₂ONPs/n-Si solar cell (under dark)

Figure 6 I-V curve of the created solar cell of Cu₂ONPs/n-Si. As is apparent, the open circuit voltage (V_OC) follows the theoretical path in which V_OC increases with decreasing temperature. Eq. (1) describes the relationship between open circuit voltage and temperature [28].

$\mathrm{Voc}=\frac{K_{\mathrm{B}} T}{q} ln \frac{I_{\text {gen }}}{I_{\text {sat }}}+1$        (1)

The fill factor is a quantity that characterizes the solar cell's nonlinear electrical behavior. Most frequently, the fill factor is computed from measurements of the I-V curve as demonstrated by the equation [29].

$P_{\text {out }}=I_{\text {max }} V_{\text {max }}$  (2)

where, I_max, V_max, and P_out are denoted as the maximum of current, voltage, and power, respectively.

$F F=\frac{{ Imax\,\, Vmax }}{ { Isc\,\, Voc }}$   (3)

The ratio of the photovoltaically produced output to the incident power falling on a solar cell is known as efficiency [30, 31].

$\eta=\frac{P_{\text {out }}}{P_{\text {in }}}=\frac{F F V_{o c} I_{s c}}{P_{\text {in }}} \times 100 \%$    (4)

where, P_in is incident power on the cell.

From (I-V) characteristics in Figure 6, V_OC gets its value from the intersection of the measured current and the axis of applied voltage used, i.e. when the solar cell has a current equal to zero or when the external circuit is open. This value of voltage can be considered the maximum voltage that a solar cell can produce [32]. The static parameters of Cu₂ONP/n-Si solar cells were as follows: All solar cells parameters were calculated from I_SC of 12.5 mA, V_OC of 500 mV, F.F of 0.45, and efficiency of around 3.37% This result is close to that obtained by Attia et al. [33]. This value of efficiency was obtained due to an increase in roughness and then increase in absorbance of the incident photons.

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Figure 6. I-V Characteristics of Cu₂ONP/ n-Si solar cell (under illumination)