Review of the development of copper oxides with titanium dioxide thin-film solar cells Open Access

Solar cells are becoming increasingly important in light of foreseeable increases in energy use and limitations of natural fossil fuel resources. ¹ Solar cells were invented in the 19th century, but the first practical photovoltaic (PV) solar cells were developed in 1954, producing a conversion efficiency of ∼6%. ^2,3 In 1947, Telkes achieved an efficiency of ∼1% using thermoelectric junctions. ^2,4 In 1958, silicon solar cells were first applied to the satellite Vanguard 1, ⁵ and in 1976, Carlson and Wronski ⁶ fabricated α-Si:H solar cells at the RCA Laboratory with an energy conversion efficiency of 2.4%. Green ⁷ recently reported an energy conversion efficiency of 26.7% achieved on Si (crystalline cell) with an area of 79 cm ². The manufacture of silicon solar cells, however, requires expensive materials and production processes. Metal oxide (MO) semiconductor(s) have several advantages for PV devices, including nontoxicity, abundance, chemical stability allowing for the material to be deposited under ambient conditions, ^8,9 simple structure, simple fabrication process, low temperature processing, photosensitivity, and potentially lower cost due to the availability of inexpensive materials and production methods. ^9–12 Although MO semiconductors have much lower efficiencies, ^3,13–20 they have become a recent focus of fundamental research. ^21–25 

Copper oxides combined with TiO ₂ are promising materials for optoelectronics, which are potentially useful for inexpensive and competitive solar cell construction. ²⁶ Copper is a cheap material, e.g., gold is 6000 times more expensive. ²⁷ Furthermore, Cu ₂O/TiO ₂ heterostructures have the ability to store multiple electrons in thin film(s); ²⁸ the theoretical limit of efficiency of a defect-free TiO ₂/CuO solar cell is ∼20%, ^29–31 ∼16% with a CuO film thickness of only 1.5 µm, and 28.6% for a tandem structure of TiO ₂/CuO/Cu ₂O. ³ Metal oxide solar cells, however, may have a large number of defects at the heterojunction interface and are not able to achieve both n-type and p-type controllable conductivity. ³ 

In our review, we present the state of the art as well as our technological experience with solar cells made with copper oxides combined with TiO ₂ and compare their PV characteristics. We have been fabricating TiO ₂/Cu ₂O and TiO ₂/CuO thin-film heterojunction solar cells using direct current (DC) magnetron sputtering, ^29,32 and describe copper and TiO ₂-based solar cells. This review should be useful for the construction and further development of thin-film solar cells.

Understanding the material properties of TiO ₂ and copper oxides will help to clarify the mechanisms of heterojunction solar cells based on these metal oxides.

Titanium dioxide was discovered in 1795 ³³ and has been studied since 1972. ³⁴ It is a key semiconductor used for the development of photocatalysis ³⁵ and solar cells. TiO ₂ is usually used in dye-sensitized solar cells. In a dye, TiO ₂ captures photons of visible light radiation at a lower energy than its band gap and tunnels the electrons produced by the light harvester to external contacts. ²² Anatase and rutile (tetragonals), and brookite (orthorhombic) are crystalline polymorphs of TiO ₂. ^36,37 Rutile is thermodynamically more stable than brookite and anatase. Heating brookite and anatase at a temperature of ∼900 °C converts them to rutile. ²⁹ Brookite is the least stable of the three polymorphs. ³⁸ When the substrate temperature is low during deposition, amorphous TiO ₂ is observed. ^38,39 Table I shows details of the parameters of TiO ₂.

TiO ₂ can function as an indirect gap semiconductor. ⁴⁷ The band gap energy of anatase and rutile is greater than 3 eV. ^54,55 Titanium dioxide absorbs a small fraction of the spectral solar irradiance. ⁵⁶ At wavelengths shorter than 387 nm (anatase) and 413 nm (rutile), electron–hole pairs are formed by the solar spectrum. ^38,56,57 Titanium dioxide absorbs less than 5% of the solar spectrum and is thus used to reduce the band gap for better absorption of the solar spectrum or as a transparent window-layer. ^3,58 In TiO ₂, electrons and holes not consumed upon generation will rapidly undergo recombination [(3.2 ± 1.4) × 10 ⁻¹¹ cm ³/s]. ⁵⁹ TiO ₂ is highly stable against mechanical abrasion, chemical attacks, and high temperatures. ³⁰ TiO ₂ has potential applications for solar energy conversion, solar cells (especially as the active component in dye-sensitized solar cells, optical sensors, quantum dots, and optical waveguides), photocatalysis, catalysis, environmental purification and separation, water treatment, antibacterial materials, multilayer mirrors, antireflection coatings, dielectric interference filters, and gas-sensing agents, and TiO ₂ can be used in artificial heart valves due to its high blood compatibility. ^{39,54,60–63} TiO ₂ films are also used as buffer layers inserted between copper oxides and transparent conductive oxides, indium tin oxide, and fluorine-doped tin oxide (FTO) layers to mitigate nonideal heterojunctions. ^64,65 Buffer layers improve band-alignment across the heterojunction and reduce the interface defect density and interfacial recombination. ⁶⁴ 

TiO ₂ thin films are obtained by various techniques: metallo-organic sol-gels, ⁶⁶ atomic layer deposition, ⁶⁷ electrophoretic immobilization of TiO ₂ powder, ⁶⁸ pulsed-laser deposition, ^69–75 reactive radio frequency sputtering, ^76–81 mixing of commercially available nanopowders, ⁸² reactive DC sputtering, ^83–86 chemical vapor deposition, ^87,88 ion beam sputtering-cold condensation, ⁸⁹ and plasma-enhanced chemical vapor deposition. ⁹⁰ 

Copper oxides are available in three forms: Cu (I) oxide (Cu ₂O, cuprous oxide), Cu (II) oxide (CuO, cupric oxide), and Cu ₄O ₃. ^43,91 Cu ₂O was discovered before germanium and silicon. ⁹² Cu ₂O and CuO semiconductors exhibit many interesting characteristics that are useful for solar cells, ⁹³ with potential applications for solid state gas sensor heterocontacts, heterogeneous catalysts, and microwave dielectric materials. ^94–100 Semiconductors are also promising materials for photocatalytic ¹⁰¹ hydrogen production, especially Cu ₂O. ^91,102–105 The potential of Cu ₂O for use in solar cells was first recognized in 1920. ^92,105 Table II shows the detailed parameters of copper oxides.

Copper oxides are p-type semiconductors ¹¹⁸ with band gaps ranging from 1.0 to 2.1 eV for CuO ^{113,119–121} and from 2.0 to 2.6 eV for Cu ₂O. ^119,107 CuO has a smaller band gap than Cu ₂O and therefore absorbs more photons. Copper oxides are low cost and nontoxic, exhibit good electron mobility, but have a fairly high minority carrier diffusion length ⁶⁰ as well as poor or inadequate photostability. Additionally, copper (I) oxides possess higher photocatalytic activity than copper (II) oxides for degrading organic compounds. ^91,105,106 Copper oxide films can be obtained by many techniques. Table III shows some properties (band gap type, gap, crystallite size, and nanocrystal shape) of copper oxide samples prepared by different production methods.

The parameters of copper oxides can be controlled. The optical band gap and crystallite size depend on the preparation method. For example, as the thickness of the structure increases, the band gap energy decreases from 2.48 eV to 2.31 eV, ¹²² and the grain size increases from 31 nm to 42 nm with an increase in oxygen pressure. ¹²⁵ Decreasing the nanocrystal size leads to an increase in the internal surface and interface areas (for equal CuO concentration in TiO ₂) and results in interfacial defects and increasing recombination losses. ¹²⁷ 

A solar cell with combined p-type and n-type semiconductors is called a blended or bulk-heterojunction solar cell. ¹²⁸ TiO ₂/CuO or TiO ₂/Cu ₂O structures are attractive candidates for photocatalytic applications ¹²⁹ and PV solar cells. To obtain good efficiency when combining the two semiconductors, it is necessary to obtain high efficiency for each of the individual semiconductors, which are more negative than those of large-gap semiconductor(s). ^130,131 The conduction band edges of Cu ₂O and CuO are −1.64 and 0.96 V, ^132,133 respectively. CuO/TiO ₂ composites have been tested in processes such as hydrogen production, ^134–139 CO ₂ reduction, ^139–142 pollutant degradation, ^139,142,143 and microorganism inactivation. ^139,144,145

To create heterojunction systems, both semiconductors must possess different energy levels from their corresponding valence and conduction bands, ¹⁴⁶ which improves charge carrier separation and increases the lifetime of the charge. ^147,148 The valence (Ev) and conduction (Ec) bands of TiO ₂ are lower than those of Cu ₂O and CuO. ¹⁴⁹ Figure 1 shows the energy level band diagrams of the Cu ₂O/TiO ₂ and CuO/TiO ₂ thin-film heterojunction solar cells.

Conduction band discontinuity (ΔEc) can be calculated using the following equation: ^47,150

where σ _n is the activation energy of TiO ₂ σ _n = 0.72 eV, σ _p is the position of the Fermi-level for Cu ₂O or CuO above the valence band, σ _p = k · T ln(N _V/N _A), kT = 0.026 eV, N _V is the effective state density in the valence band N _v = 1.9 × 10 ¹⁹ 1/cm ³⁴⁷ for p-Cu ₂O, and N _v = 5.5 × 10 ²⁰³ for p-CuO, N _A is the acceptor concentration N _A = 3.5 × 10 ¹⁸ 1/cm ³⁴⁷ for p-Cu ₂O, and N _A = 1 × 10 ¹⁶ 1/cm ³. ³ V _D is the total diffusion potential, q is the electronic charge 1.6 × 10 ⁻¹⁹ C, qV _D = 0.98, ⁴⁷ and E _g is the energy band gap (Cu ₂O, E _g = 3.08 eV ^47,151 and CuO, E _g = 1.2 eV). ²⁰ The experimental value of ΔEc for Cu ₂O is 0.48 eV, and the theoretical value for Cu ₂O is 0.6 eV ⁴⁷ and for CuO, 0.68 eV.

Valence band discontinuity (ΔEv) is calculated according to the following relation: ⁴⁷ 

where ΔEv is 1.37 eV for Cu ₂O/TiO ₂ and 1.32 eV for CuO/TiO ₂.

In 1999, Song et al.¹⁵² first produced different types of copper with TiO ₂. Although they considered that copper would improve the photocatalytic activity of TiO ₂, they found that using the oxidized form of copper on TiO ₂ actually decreased the photocatalytic efficiency. ¹⁵² Many researchers have manufactured TiO ₂ and copper oxide thin films and studied their photocatalytic efficiency. ^{141,153–168} In our review, we paid special attention to TiO ₂ and copper oxide heterojunctions for PV devices.

Rokhmat et al.¹⁶⁹ developed Cu(NO ₃) ₂·3H ₂O-containing TiO ₂ by combined electroplating and spraying methods. PV devices were fabricated by forming cells using polymer electrolyte and aluminum counter electrodes. Additionally, the PV devices were immersed in 0.125M NaOH for 10 min. Next, they illuminated the PV devices with a 500 W halogen lamp for 15 min. Finally, they dried the PV devices at room temperature. Current-voltage (I-V) curves of the PV devices were measured at 120 W/m ². The authors prepared solar cells with amounts of Cu(NO ₃) ₂·3H ₂O ranging from 0% to 2 wt. %. The circuit current, open circuit voltage, fill factor (FF), and efficiency were in the range of 0.07–0.068 mA, 0.135–0.5 V, 0.12%–0.24%, and 0.0017%–0.04%, respectively. The highest efficiency was achieved at 1.4 wt. % Cu(NO ₃) ₂·3H ₂O. Additionally, to increase the performance of the PV devices, copper particles were placed between the TiO ₂ particles by electroplating, and then the electrolyte was treated with NaOH. The efficiency of these solar cells after electroplating was 0.35%, and it increased to 1.24% after treatment with NaOH. ¹⁶⁹ 

In 2002, Siripala et al.¹⁷⁰ prepared Cu ₂O/TiO ₂ heterojunction structures by electrochemical deposition of Cu ₂O on Ti foil. A photoresponse demonstrating efficient light-induced charge carrier separation in TiO ₂–copper (I) oxide system(s) was observed. The electrode was illuminated using a Xe lamp spliced at a frequency of 700 W/m ². Figure 2 shows photoresponses of the Cu ₂O/TiO ₂ with a RuO ₂ counter electrode and Fig. 3 with a Pt counter electrode. The area of the counter electrodes was 2 cm ². The authors concluded that the use of the RuO ₂ counter electrodes meaningfully increased the photocurrent. Additionally, they suggested that the TiO ₂ films successfully protected the copper (i) oxide layer against photocorrosion without decreasing the efficiency. ¹⁶² 

Zainun et al.¹²⁸ prepared Cu ₂O/TiO ₂ thin films by squeegee and electrochemical deposition methods. They then fabricated metal electrodes (evaporating indium) on the film to form cells. Figure 4 shows the scheme for the Cu ₂O/TiO ₂ with the electrodes. The authors characterized the optical and structural properties of the films and measured the photoresponse of the cell. I–V characterization was measured for air mass (AM) 1.5 at 100 mW/cm ² and for three samples with various Cu ₂O deposition times (5, 10, and 15 min). The authors reported PV characteristics of cells only for the 10-min deposition, which had an efficiency of 0.0005%. ¹²⁸ Figure 5 shows the J–V curves for Cu ₂O/TiO ₂ with various Cu ₂O deposition times.

The use of nanotube arrays in Cu ₂O/TiO ₂ heterojunctions is widely studied. ^171–172 Hou et al.¹⁷² prepared Cu ₂O/TiO ₂ nanotube p–n junction arrays using a photoreduction method, but there was no PV effect ¹⁷² Li et al.¹⁷¹ created 10 Cu ₂O/TiO ₂ p–n heterojunctions by combining two methods. TiO ₂ nanotube arrays were prepared using a two-step anodization process and then p-type Cu ₂O was electrodeposited into the TiO ₂ nanotube channels. The solar cells were annealed at 200 °C in flowing Ar (200 SCCM) for 1 h to make the Cu ₂O thin film more continuous and uniform. Next, the top contact electrode was prepared with a gold film (10-nm thick) by coating onto the Cu ₂O film. The I-V curves of the solar cells were measured at 100 mW/cm ² under simulated air mass (AM) 1.5 illumination. The open circuit voltage (V _oc), short circuit current density (J _SC), and fill factor (FF) of the Cu ₂O/TiO ₂ solar cells were 0.09–0.25 V, 0.1–0.33 mA/cm ², and ∼0.27, respectively. Additionally, annealing the solar cells improved the PV parameters. The efficiency was ∼0.01%, V _oc = 0.1 V, J _SC = 0.33 mA/cm ², and FF = 0.27. ¹⁷¹ Figure 6 shows a schematic of Cu ₂O/TiO ₂, and Fig. 7 shows the J–V characteristics before (left) and after annealing (right).

Pavan et al.⁹ produced TiO ₂/Cu ₂O solar cells by spray pyrolysis. The authors generated PV devices with varying TiO ₂ and Cu ₂O layer thicknesses and the films were fabricated to form cells used on silver as a back contact. The front electrode was made of FTO. Additionally, a metal frame was soldered to the solar cell. Pavan et al.⁹ studied the impact of layer thicknesses on the PV device performance. The thickness of TiO ₂ ranged from 0.6 to 0.32 µm and that of Cu ₂O ranged from 0.18 to 0.58 µm. They observed the best results when the thickness of the Cu ₂O film was greater than 500 nm: V _oc up to 350 mV and J _SC of 0.4 mA/cm ². Figure 8 shows a cross-section view of the solar cell, and Fig. 9 shows dark and light J-V measured at the top of the solar cell. Pavan et al.⁹ concluded that to improve the PV characteristics of TiO ₂/Cu ₂O, the conduction band offset at the n-TiO ₂/p-Cu ₂O interface had to be reduced and larger higher-quality Cu ₂O grains had to be synthesized.

Rokhmat et al.⁶⁰ also reported the construction of TiO ₂/CuO and TiO ₂/CuO/Cu solar cells. The TiO ₂/CuO/Cu solar cells were created using a fixed current electroplating and spray method with varying electroplating currents (0.1–100 mA) for 10 s. The PV devices were fabricated by cells formed using polymer electrolyte and aluminum counter electrode(s). The I-V characteristics were measured at an intensity of 120 W/m ². First, the authors compared solar cells before and after adding the copper particles with a current source of 1.0 mA for 10 s. Circuit-current, open circuit voltage, FF, and efficiency were 0.8 mA, 0.62 V, 0.33%, and 0.14%, respectively, for TiO ₂/CuO, and 0.12 mA, 0.61 V, 0.35%, and 0.21%, respectively, for TiO ₂/CuO/Cu. Next, the authors changed the electroplating current and found that the efficiency was best (0.80%) at 10 mA. Finally, the I-V characteristics of TiO ₂/CuO/Cu were measured at various electroplating times with a constant electroplating current of 10 mA. Circuit-current, open circuit voltage, FF, and efficiency were in the range of 0.05–0.72 mA, 0.58–0.6 4 V, 0.34–0.42, and 0.16%–1.62%, respectively. The best result was achieved when the electroplating process was 20 s long. ⁶⁰ 

Hussain et al.⁴⁷ fabricated Cu ₂O/TiO ₂ by electrodeposition of Cu ₂O structure on a radiofrequency sputtered TiO ₂ film. The counter electrode was platinum foil and the reference electrode was Ag/AgCl (4M KCl). Finally, an indium contact was stuck to the Cu ₂O thin layer. The authors characterized the solar cell by scanning electron microscopy, X-ray diffraction (XRD), and UV spectroscopy. Furthermore, they performed I-V and C–V measurements. The light intensity was set at 90 mW/cm ². The maximum FF, power conversion efficiency, short circuit current density, and open circuit voltage was ∼0.36, ∼0.15%, 0.0027 A/cm ², and 0.34 V, respectively. They concluded that the low efficiency may have been due to “band discontinuity at the interface edges,” “fast electron hole pair recombination,” “defects at the interface,” “a large lattice mismatch,” or “the exits of other planes of Cu ₂O”. ⁴⁷ Figure 10 shows the I-V curves of the Cu ₂O/TiO ₂.

Hussain et al.¹¹⁴ also reported on Cu ₂O/TiO ₂ thin-film heterojunctions in which the TiO ₂ films were created via anodization of Ti foil, and the Cu ₂O films were deposited on the TiO ₂ film by electrodeposition. In this experiment, the authors studied the structural and morphological properties, performed X-ray diffraction (XRD) analysis, and measured the C–V and I–V characteristics. The turn on voltage of the device was 520 mV. On the basis of a C–V graph, the authors estimated that the effective carrier concentration was N = 8 × 10 ²² 1/m ³ and the built-in potential was ∼0.80 V. ¹¹⁴ Figure 11 shows the C ⁻²–V characteristics of this Cu ₂O/TiO ₂ PV device.

Ichimura and Kato ¹⁰⁶ described TiO ₂/Cu ₂O solar cells prepared using two chemical techniques. The TiO ₂ structures were manufactured by electrophoretic deposition using a TiO ₂ sol solution on an indium-tin-oxide glass substrate. The Cu ₂O thin films were prepared by electrodeposition using an aqueous CuSO ₄ solution. To create solar cells, indium was evaporated onto the Cu ₂O to form an electrode. Finally, the solar cells were annealed in air at 120 and 220 °C for 30 min. The PV curves were measured by irradiating the indium-tin oxide side of the cell with a solar simulator (∼100 mW/cm ², AM 1.5). The PV characteristics were as follows: V _oc = 0.22 V, I _SC = 0.57 mA/cm ², FF = 0.45, and efficiency = 0.056% before annealing, and the authors achieved a conversion efficiency of ∼0.11% after annealing at 120 °C. Ichimura and Kato ¹⁰⁶ concluded that the low efficiency was due to poor quality copper oxide. They proposed that increasing the conversion efficiency of the cell could be improved by improving the crystallinity of copper (i) oxides by avoiding the formation of copper (ii) oxide, and by annealing in reduced ambient O ₂ pressure and at high temperatures. ¹⁰⁶ Figure 12 shows the I–V curves in the dark and light (a) before annealing and (b) after annealing the TiO ₂/Cu ₂O.

Luo et al.¹⁷³ manufactured Cu ₂O/TiO ₂ in two steps. TiO ₂ nanorod films were deposited on glass covered FTO using a hydrothermal reaction. The TiO ₂ layer was also below a compact layer beneath the nanorod layer. Next, a Cu ₂O thin film was electrodeposited onto the TiO ₂ nanorod structure. The electrodeposition time for the Cu ₂O films was varied (20 min, 25 min, 30 min, 50 min, and 60 min) under −0.5 V. Finally, a gold layer was sputtered on top to form the contact electrode of each solar cell. The J-V curves of the heterojunction solar cells were investigated in the dark and under irradiation (1 Sun, 100 mW/cm ²) at room temperature. The highest conversion efficiency of this Cu ₂O/TiO ₂ solar cell was 1.25%, which was achieved after a Cu ₂O film electrodeposition time of 30 min. ¹⁷³ 

Sawicka-Chudy et al.³² produced a TiO ₂/Cu ₂O thin-film heterojunction solar cell using a DC magnetron-sputtering technique. They analyzed the I-V characteristics, optical characteristics, composition, and morphology, and performed atomic force microscopy and XRD analyses of the TiO ₂/Cu ₂O. To create solar cells, Au contacts were adhered to the copper oxide and indium tin oxide using conductive glue. The I–V curves of TiO ₂/Cu ₂O were measured in the dark and under illumination using a halogen lamp as the light source at 800 W/m ². One of the TiO ₂/Cu ₂O heterojunctions was photosensitive, but exhibited no PV activity. ³² 

Recently, Suleiman et al.¹⁷⁴ fabricated FTO-TiO ₂/Cu ₂O-Cu and FTO-ZnO/Cu ₂O-Cu solar cells. Copper oxide films were electrodeposited on sprayed ZnO and TiO ₂ structures. The authors analyzed the compositional, optical, and electrical properties. The PV characteristics for the FTO-TiO ₂/Cu ₂O-Cu were as follows: circuit current density (J _SC), 5.8 µA/cm ²; V _oc, 0.03 V; and efficiency, 0.037% and those for FTO-ZnO/Cu ₂O-Cu were as follows: J _SC, 0.093 µA/cm ²; V _oc, 0.0364 V; and efficiency, 0.0076%. The FTO-TiO ₂/Cu ₂O-Cu solar cells had better efficiency than the FTO-ZnO/Cu ₂O-Cu solar cells. ¹⁷⁴ 

Table IV summarizes the PV characteristics, thickness, thin-film fabrication methods, and contact(s) for copper oxide and TiO ₂-based solar cells.

Here, we briefly discuss and explain the available combinations of copper oxides with other materials that can be used as solar cells. These materials and combinations are as follows: Cu ₂O/ZnO, Ga ₂O ₃/Cu ₂O, and CuO/Si.

Minami et al.¹⁷⁵ produced Al-doped ZnO (AZO)/nondoped ZnO (ZO)/Cu ₂O solar cells. Copper oxide sheets were manufactured by oxidizing the Cu sheets with a heat treatment process and ZO and AZO, and fabricated using a pulsed laser deposition method. The ohmic electrode on the back surface of Cu ₂O was Cu ₂S or Au. The authors studied the influence of the thickness of ZO thin-films ranging from 0 to 150 nm on V _oc, J _SC, FF, and efficiency. They showed that the efficiency of AZO/ZO/Cu ₂O solar cells was greater than 3% (3.83%) for a n-ZO thin-film layer with a 30–50 nm thickness. Other PV parameters were J _SC = 0.69 mA/cm ² and FF = 0.55. ¹⁷⁵ 

Lee et al.⁶⁴ demonstrated Ga ₂O ₃/Cu ₂O heterojunction devices with a 10-nm-thick Ga ₂O ₃ buffer layer and p-type absorber Cu ₂O (thickness: 2.5 µm). The bottom electrode was a 200-nm-thick layer of Au. The device had a 1-μm-thick Al top-electrode grid and a 95-nm-thick MgF ₂ antireflective layer. The PV characteristics were as follows: V _oc = 1.2 V, J _SC = 7.3 mA/cm ², FF = 44.7%, and efficiency 3.97%. They also studied the temperature dependence of the J–V characteristics and concluded that the “dominant recombination process occurs near the Ga ₂O ₃/Cu ₂O interface.”⁶⁴ 

Masudy-Panah et al.¹⁷⁶ produced p-CuO(Ti)/n-Si solar cells by sputter deposition at room temperature and rapid thermal annealing. They studied the impact of Ti-doped copper (i) oxide on the PV properties. The Ti concentration varied from ∼0.049 to ∼0.598%. The potential of Ti doping for improving the PV properties and conductivity of CuO devices was analyzed. They investigated CuO(Ti) thin-films using atomic force microscopy, Raman spectroscopy, XRD, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and the I-V characteristics of p-CuO(Ti)/n-Si heterojunction solar cells. The efficiency of p-CuO(Ti)/n-Si varied from 0.06% to 0.39%. The highest efficiency was 0.39%, which was achieved with a Ti concentration of 0.099. ¹⁷⁶ 

Masudy-Panah et al.¹⁷⁷ produced p-CuO/n-Si by using rapid thermal annealing methods and conventional sputtering. They studied the influence of thermal treatment and working pressure during deposition on the PV properties of copper-oxide based devices. The highest efficiency achieved was 1.0%, V _oc = 4.9 V, J _SC = 6.4 mA/cm ², and FF = 32% for solar cells annealed at 300 °C for 1 min. They concluded that the quality of the crystalline structure and interface might be improved and the carrier recombination rate reduced by increasing the working pressure during deposition. ¹⁷⁷ 

The thickness of the copper oxide layers plays a key role in the conversion efficiency of copper oxide-based solar cells due to the discrepancy between the optical absorption length and copper oxide thickness. The thickness of copper oxide thin films influences the charge carrier transport properties and light absorption. ¹⁷⁸ 

Musselman et al.¹⁷⁹ studied the thickness of copper (i) oxide thin films in nanowire and bilayer Cu ₂O-ZnO solar cells. They investigated the influence of Cu ₂O thickness on V _oc, J _SC, and η. The thickness of the Cu ₂O layer varied from 2 to 4.5 µm for bilayer solar cells and from 2 to 3.5 µm for nanowire solar cells. The highest efficiency was ∼0.6% for bilayer solar cells and ∼0.38% for nanowire solar cells with Cu ₂O-ZnO layer thicknesses ranging from approximately 2.7 to 3.0 µm. ¹⁷⁹ 

Masudy-Panah et al.¹⁷⁸ sputtered the CuO films for photoelectrochemical water splitting with different film thicknesses, ranging from 0.2 to 0.7 µm. They observed that increasing the thickness of the Cu ₂O layer from 0.2 to 0.7 µm increased the photocurrent. Increasing the Cu ₂O layer thickness above 0.7 µm, however, degraded the photocurrent. They also attributed the initial increase in the photocurrent to the improvement in the light absorption capabilities of the thicker CuO thin films as well as an increase in recombination. ¹⁷⁸ The authors proposed an optimum thickness of ∼0.55 µm, which produced a photocurrent of 1.68 mA/cm ² at 0 V. ¹⁷⁸ 

In summary, we presented the main properties of TiO ₂, cuprous oxide (Cu ₂O), and cupric oxide (CuO); their potential applications, and, to the best of our knowledge, all of the solar cells based on copper oxide and TiO ₂. Additionally, we provided a short review of the combination of copper oxide with other materials: Cu ₂O with ZnO, CuO with ZnO, Ga ₂O ₃/Cu ₂O, and CuO with Si, and the influence of the thickness of copper oxide layers on efficiency.

We compared the PV characteristics of solar cells, thicknesses of copper oxide and titanium layers, and electrodes to created heterojunctions. The highest energy conversion was for a TiO ₂/CuO/Cu solar cell—1.62% produced by spray (TiO ₂) and a fixed current electroplating method (Cu ₂O). The efficiency results of the cells based on copper oxides are low compared with the theoretical results. To improve their performance, some researchers have proposed the following: increase the crystallinity of Cu ₂O, ¹⁰⁶ avoid the formation of copper oxides other than copper (i) oxide or copper (ii) oxide, avoid defects at the heterojunction interface, ¹⁷¹ avoid an excessively thick copper oxide layer, ¹⁷¹ and use textured electrodes and antireflective coatings. ¹⁷¹ Moreover, avoiding “band discontinuity at the interface edges”, ⁴⁷ synthesizing larger and higher quality copper oxide grains, reducing conduction band off set at the TiO ₂/Cu ₂O interface, ⁹ and increasing transmission of the top electrode may improve the efficiency. ¹⁷¹ Finally, using a NaOH post-treatment will help to improve the contact between the electrolyte, aluminum, and the thin films. ⁶⁰ 

This manuscript is intended as a review of the major literature to acquaint the readers with experimental results, and some general and theoretical principles of thin-film solar cells.