Cuprous Oxide (Cu 2 O) Based Solar Cell Thickness Dependence

| ABSTRACT The rapid advancement of devices that transform light into other energy forms has led to the growth of various solar cell structures established on silicon and other compound semiconductors. Photovoltaic devices that are efficient are required to ensure solar power's competitiveness with traditional energy sources. The most intriguing phase of copper oxides is Cu 2 O. There have been several findings of Cu 2 O-based solar cells; nevertheless, their efficiencies are quite minimal due to charge recombination at the crystal boundary and other factors. The goal of the present study is to review the effect of Cu 2 O thin film thickness prepared by oxidation techniques on efficiency


Introduction
Investigators suggest that the globe will require thirty terawatts of energy supplies by 2050 to keep the economy growing.Many experts are convinced that the sun is the only frontrunner capable of providing a comprehensive solution to the energy revolution.As a result, solar cells will be viewed a popular renewable energy tool once their production costs are reduced to a level similar to other accessible energy resources (Asim et al., 2012).Over ninety-five percent of all solar cells in the globe are constituted by Silicon.These material-based solar cells are expensive, and the creation of revolutionary photovoltaic devices based on low-cost, non-toxic products generated through effective processes is required (Han & Tao, 2009).The individual choice for silicon solar cells is oxide semiconductors (Motoyoshi et al., 2010).Oxide semiconductors are commonly used in photovoltaics, optoelectronics, and functional coatings due to significant optical absorption and a moderate price for raw materials (Winkler et al., 2018), (Abdu & Musa, 2011).
Copper oxide has dual basic semiconductor phases' cuprous oxide (Cu2O) and cupric oxide (CuO) of cubic and monoclinic structures, respectively (Ozmentes & Temirci, 2017).Cu2O unit cell comprises copper and oxygen ions and pertains to the space group Pn3m (Kara et al., 2018).Cu2O is the most impressive phase of copper oxide because of its exceptionally good optical absorption coefficient in the visible range and reasonable electrical properties (F.K1 et al., 2013).Cu2O realizes p-type behavior as a whole due to an acceptor level of 0.4 eV over the valence band yielded by openings in copper ions.Because of oxygen vacancies at 0.38 eV on the lower face of the conduction band's bottom, donor levels in Cu2O were also witnessed, illustrating the existence of Cu2O with n-type conductivity.(Jayathilaka C.et al., 2020).Cu2O has been found to be stable over a narrow temperature and oxygen pressure range.
A lot of work on Cu2O solar cells brand was published; regrettably, unresolved charge recombination at the crystal boundary and indeed the barrier properties of Cu2O, their efficiencies seem to be very low.In this work, a review of the effect of Cu2O thin film thickness prepared by oxidation techniques on efficiency was analyzed; afterward, past related work was systematically studied to investigate structural and morphological possessions of Cu2O cover.Also, the different techniques considered to expand the photovoltaic properties of Cu2O -built solar cells are also highlighted.

Cuprous oxide-based solar cell
The necessity of viable generating power has prompted research into a wide range of photovoltaic materials and structures, with a stronger focus on achieving a balance of both cost and performance (Siddiqui et al., 2012).As Cu2O is expected to have an extreme hypothetical efficiency of twenty percent, it was ascertained to have the possibility of meeting global electricity demand while lowering PV costs when associated with crystalline silicon.However, experiments are considerably short of this theoretical efficiency (Gershon et al., 2012).The target research on cuprous oxide-based solar cells is to maximize power while minimizing cost.In general, copper oxide-based solar cells have some disadvantages, such as low conduction as well as high series resistance, which leads to reduced efficiency.Struggles are being made to diminish the sheet resistance of the absorber layer and expand its charge transport properties (Masudy-Panah et al., 2015).Nevertheless, to maximize the charge conveyance in a Cu2O -based solar cell for demand to achieve the highest efficiency when light rays strike the Cu2O device, consequently planned to increase the favorite absorption (Fig. 1.3) and absorption after reflection (Fig. 1.5).The performance of (AZO)/n-type oxide semiconductor/p-Cu2O heterojunction solar cells fabricated with p-type Cu2O piece deposited by means of thermally oxidizing Cu sheets are being explored.When ZnGa2O4 is used, the cell has the highest efficiency of 5.36%.Zn2SiO4 has the lowest efficiency of 0.01%.As a matter of fact, the highest efficiency of 6.25% was obtained for MgF2/AZO/n-(Ga0.975Al0.025/2O3/p-Cu2O:Naheterojunction solar cell constructed using a Cu2O: Na heterojunction solar cell.

Techniques for growing cuprous oxide-based solar cell
The cuprous oxide potential for use in solar cells was discovered around 1920.The preparation method influences the parameters, namely; the optical band gap, morphology, crystallite size, and cell efficiency.Many techniques exist for producing cuprous oxide films.Among the methods are: thermal oxidation (Abdu & Musa, 2011); chemical vapour; spin-coating method (Motoyoshi et al., 2010); spraying (Kardarian et al., 2016); thermal evaporation (Tuama et al., 2020); electrodeposition (Niu et al., 2016); DC magnetron reactive sputtering; and molecular beam epitaxy have been used to prepare Copper Oxide thin films.

Thermal Oxidation
Thermal oxidation is one technique used to create first-class, equally distributed Cu2O films.The oxidation process could occur either in clean oxygen or laboratory air.At high temperatures, Cu2O thin films of various widths can be deposited (1000-1500 o C).The first compound formed during film formation is Cu2O, and as the oxidation time is extended, another phase of the copper compound, CuO, is evolved.However, a mixture of the oxide phases forms at atmospheric pressure and temperatures below 100 o C.
An etching solution prepared by adding FeCl3, HCl, and NaCl can be used to remove the unloved CuO.

Methodology
The clean substrate was placed inside a furnace at a high temperature (e.g., 1000 o C).Subsequently, the sample was removed and quickly transferred to deionize water to stop further oxidation.The procedure could be repeated with the rest of the substrate at various temperatures and times to produce Cu2O of different thicknesses.CuO then removes because the oxide film is of different phases by immersing the sample in an appropriate amount of Iron (II) chloride (FeCl2), aqueous sodium chloride, Hydrochloric acid, aqueous sodium persulphate, and then cleaning in deionized water.

Discussion of related results
It appears that by optimizing the synthesis and deposition techniques, the effectiveness of the cuprous oxide (Cu2O) device can be increased.Figure 2. X-ray diffraction pattern for the Cu2O/TiO2 thin film heterojunction solar cell.Reproduced from [23] with permission, copyright@2012, Elsevier.
The field emission scanning electron microscopes (FESEM) and Scanning electron microscopes remain techniques used to investigate the surface morphology of films.Figure 3 shows the FESEM image of the Cu2O thin-film image created using the thermal evaporation technique.The picture has uniform topography with no pinholes, indicating that Cu2O film is distributed evenly.The calculated average grain size was around 18 − 24  The power density has been set at 2.2 W/cm 2 .Figure 1 shows a SEM image of his measurement.The image depicts how the particles are distributed.The particle size of the Cu2O coating layer is approximately 70.This means that surface properties are suitable for photovoltaic applications.
Figure 5. SEM image of Cu2O thin film on quartz direct current/radio frequency (DC/RF) magnetron sputtering system Reproduced from [24] with permission, copyright@2017, Authors and SRPI Figure 6 shows the surface morphology image of the sample with a magnification of 500nm; electrodeposition was employed to make the Cu2O film on gold coated silica which served as the working conductor.The deposition was accomplished at -0.4 V in reference to an Ag/AgCl electrode.The SEM image discloses different grain sizes, and the surface of Cu2O is highly textured.
Figure 6.The surface morphology of the as-grown Cu2O layer by electrodeposition method Reproduced from [21] with permission, copyright@2016, Elsevier The topography of pure Cu2O films on FTO glass is depicted in Figure 7; the AFM images of the films revealed an identical morphology with a granular form, root mean square values, and a roughness average of grains is 0.427 and 0.332 respectively Figure 7.The Atomic Force Microscopy images of (a) Cuprous oxide Reproduced from [20] with permission, copyright@2020, UMP In the unit cell of Cu2O, there are six atoms, and as a result, eighteen phonon branches are possible.The Raman spectra for Cu2O deposited by electrodeposition techniques under the following condition;  = 10.5 and supplied potentials (-0.2V; -0.4V) is depicted in Figure 8; the pulse at 108, 148, 214, 297, 413, 495, and 647 cm -1 demonstrates the occurrence of Cu2O in the formed film.  ,  1 ,2  , 2  , 2  , 2  , and  1 define these peaks.
Figure 8. Raman spectra of Cu2O Reproduced from [25] with permission, copyright@2020, Authors In the oxidation process toward Cu2O formation, varying the oxidation time and temperature resulted in a change in sample thickness.Figure 7 depicts a histogram of solar cell performance versus thickness.From the histogram, we discovered that when the thickness is 26.30µm, the conversion efficiency is high, indicating that electron mobility is high and there is less recombination within the bulk material.In this case, we can predict that at this thickness, carriers diffuse quickly, and charges reach the external.
As a consequence, the Voc of high-mobility Cu2O solar cells is lower than that of low-mobility Cu2O solar cells (Takiguchi & Miyajima, 2015).

Conclusion
The assessment of the thickness dependence of cuprous oxide (Cu2O) prepared by oxidation was explained.We discovered that when the thickness is 26.30µm, the conversion efficiency is encouraging.Morphology analysis from various methods of depositions demonstrates that the films were compact throughout the entire sample, with no observed pinholes.The deposition techniques influence the exterior morphological features of the layer.In conclusion, the variation of thickness in Cu2O film deposited using the above method shows an impact on energy output.The deposited Cu2O thin films accessible in this paper have virtuous prospects as absorber material in solar cell applications.

Figure 1 .
Figure 1.Manners of light shining on a solar cell: (1.1) Reflection and absorption at upper contact.(1.2) Reflection at the cell surface.(1.3) Favorite absorption.(1.4) Reflection from rear out of the cell.(1.5) Absorption after reflection.(1.6) Absorption in back contact Cuprous oxide solar cells can be configured in three ways: A Schottky barrier, a homojunction, and a heterojunction.The heterojunction structure has been used with a variety of n-type window layers, such as ZnO and TiO2.This section will provide a brief overview and explanation of related work of the various architectures of cuprous oxide-based solar cells.Musselman et al. (2012) assess the efficiency of bilayer and nanowire (NW) Cu2O -ZnO solar as a function of Cu2O absorbing layer thickness, ZnO NW length, and NW seed layer thickness.Their findings show that a thick layer depletion region emerges in the Cu2O layer of bilayer devices due to the low carrier density of electrodeposited Cu2O and efficiency of 0.38%; however, replacing nanoparticles with ZnO Nanowires improved efficiency to 0.85%.Miyata et al. (2019) used AZO/p-Cu2O heterojunction solar cells to investigate the photovoltaic properties of Cu2O -based heterojunction solar cells.The highest efficiency was obtained, which was 3.21%.The success was attributed to a reduction in deficiency levels at the boundary between the AZO thin film and the Cu2O piece.
(Minami et al., 2016).In comparison to cells with normal sputtered ZnO thin film, Perng et al. (2017) report a high Jsc of 9.53 mA/cm 2 for low-cost electrodeposited (ED) semi-transparent Cu2O /ZnO nanorod (NR) solar cells.This is because charge losses have been significantly reduced.Using atmospheric atomic layer deposition of Zn (1-x) Mg x O on cuprous oxide.Ievskaya et al. (2015) created Zn (1-x) Mg x O / Cu2O heterojunction solar cells.The efficiency stated was 2.2%, and the open-circuit voltage was 0.65 V. Chatterjee et al. (2016) created heterojunction devices with a staircase-like energy level of NiO/Cu2O/ZnO/SnO2.The energy conversion effectiveness of 1% was achieved when Cu2O thin-films were formed via SILAR route.

Figure 2
shows the XRD result of Cu2O thin film deposited by electrodeposition on n-TiO2 thin film prepared by radio-frequency sputtered.As can be seen, from the pattern Cu2O cubic phase is responsible for the sharp peaks at 2Ɵ values of 29.68, 36.524,42.60, and 61.76, which are indexed to (1 1 0), (1 1 1), (2 0 0), and (2 2 0), respectively (JCPDS No. 05-0667) (Rahman & Islam, 2016) There were no obvious impurity peaks in the XRD pattern, implying the realization of an unadulterated crystalline part of Cu2O with no secondary phases.The intense peak (200) indicates the preferential growth direction of Cu2O film.

Figure 3 .
Figure 3. FESEM images of pure cuprous oxide (Cu2O) films prepared by thermal evaporation technique Reproduced from [20] with permission, copyright@2020, UMP Figure 4 presents images from SEM to probed surface characteristics of Cu2O set by spray pyrolysis.The shape and morphology reveal that the individual grains were composed of particles of various sizes.The grains were distributed equally with textures surface.

Figure 4 .
Figure 4. SEM images of the Cu2O using Spray pyrolysis Reproduced from [19] with permission, copyright@2015, Elsevier Nordseth et al. (2017) created Cu2O on a quartz substrate using reactive sputtering of a Cupper objective in O2/Ar (6/49 sccm).The power density has been set at 2.2 W/cm 2 .Figure1shows a SEM image of his measurement.The image depicts how the particles are distributed.The particle size of the Cu2O coating layer is approximately 70.This means that surface properties are suitable for photovoltaic applications.

Figure 9 .
Figure 9.The efficiency of Cu2O /Cu solar cells at various thicknesses Reproduced from [27] with permission, copyright@2013, Authors, and IJES6.ConclusionThe assessment of the thickness dependence of cuprous oxide (Cu2O) prepared by oxidation was explained.We discovered that when the thickness is 26.30µm, the conversion efficiency is encouraging.Morphology analysis from various methods of depositions demonstrates that the films were compact throughout the entire sample, with no observed pinholes.The deposition techniques influence the exterior morphological features of the layer.In conclusion, the variation of thickness in Cu2O film deposited using the above method shows an impact on energy output.The deposited Cu2O thin films accessible in this paper have virtuous prospects as absorber material in solar cell applications.