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New Design of Potentially Low-cost Solar Cells Using TiO2/Graphite Composite as Photon Absorber

A solar cell design using the combination of titanium dioxide and graphite as active photon absorbing materials were proposed. The titanium dioxide absorbs photons of nearly ultraviolet wavelengths to produce electron hole pairs, while graphite is expected to absorb photons of longer wavelengths. Although many authors have claimed that graphite is not a semiconductor, we observed that a model of a solar cell containing titanium dioxide only as the active material behaves exactly the same as a model containing graphite only as the active material. Additionally, we observed that a model of a solar cell made using a composite of titanium dioxide and graphite as the active material had much higher efficiency than solar cells made using titanium dioxide only or graphite only active materials.


Introduction
The use of renewable sources of energy and the search for inexpensive and easily fabricated solar cells are becoming increasingly important for countries to meet their rising energy needs. Presently available solar cells are expensive, especially for low-income communities. These communities, some of which are still living without electricity, are widespread in developing countries; however, only a few developed countries have expertise in solar-cell technologies. The users of solar cells will likely rely on these supplier countries for many decades.
Solar energy is considered a type of free energy in nature as it freely available over the Earth's surface. The ability to harvest this energy should thus be available every human being. This objective will be realized when more countries are able to produce their own solar cells. Globally, countries should therefore strive to produce practical solar cells, even if these cells have low efficiency.
In recent years, many attempts have been made to produce solar cells using cheaper materials, and easily applied and readily scalable methods [1][2][3][4][5]. Although the efficiency of our solar cells does not match the efficiency of presently applied solar cells, our solar cells might compete with presently applied solar cells economically, especially when considering factors such as production and maintenance costs and efficiency.
Although the application of titanium dioxide (TiO 2 ) in solar cells is not new, the principle and implementation of its application in presently applied solar cells and in our proposed model greatly differ. The commonly used structure is a dye-sensitized solar cell [6], with TiO 2 used as a conductive medium for the transport of electrons from the dye to the electrode. In the proposed structure, TiO 2 particles play the role of a photon absorber and the produced electrons are transported to the electrode through metallic bridges.
We used TiO 2 as an active material that absorbs photons and converts them into electric current. This material has a band gap around 3.2-3.8 eV, allowing the effective absorption of ultraviolet light. Only a few electron-hole pairs are produced when the material is illuminated by the solar spectrum. To enlarge the absorption band of TiO 2 , the doping of pure TiO 2 with V, Cr, Mn, Fe, Ni, or Au [7,8] and Pt, Rh, Au, Pd, Ni, or Ir [9] has been reported. The cited studies claim that the band gap of TiO 2 is reduced by doping, resulting in an increase in visible-spectrum absorption and, thus, higher solar cell performance.
In addition to the absorption band, the lifetime of electron-hole pairs produced in the TiO 2 must be considered because it directly determines the photon-to-current conversion. The lifetime of an electron-hole pair in TiO 2 is very short and potentially limits the efficiency of photon-to-current conversion. The electron and hole in anatase TiO 2 undergo non-exponential decay, where the hole population decays within in a few nanoseconds and the electron population persists for periods up to microseconds [11].
In previous works we deposited copper particles in the space between TiO 2 particles to suppress electron-hole recombination and observed greatly improved efficiency [1][2][3][4][5]. The formation of metal (Cu) and semiconductor (TiO 2 ) produces a high Schottky barrier that facilitates electron capture by the metal. Vorontsov reported that Pt-TiO 2 has the highest Schottky barrier and a longer electron-hole separation time [12,13]. Similarly, the Cu-TiO 2 contact in the present work might lengthen the lifetime of electron-hole pair separation and allow enough time for electrons to migrate into the metal and transport through the copper bridge. Hoffmann et al. stated that although the electron-hole pair lifetime is only a few nanoseconds, it is long enough for electrons to migrate to the TiO 2 surface and then be captured by the metal [14].
The conduction band and valence band of TiO 2 are produced by different orbitals. The valence band is developed by the d-orbital while the conduction band is developed by the s-p hybridized orbital [15]. This leads to different parity of the electron and hole pair, which suppresses the recombination probability.
In the present work, we propose a method of further improving the efficiency of In the present work, we mix graphite particles with TiO 2 particles. The use of graphite as a conducting and catalytic counter electrode for dye-sensitized solar cells has improved efficiency [18]. However, to our knowledge, the use of graphite as a photon-absorbing material in a solar cell has not been investigated. Graphite is a semimetal with a valence and conduction band that overlap by around 0.03 eV [19,20].

Method
The structure of the solar cell reported here is schematically shown in Fig. 1. The solar cell is composed of fluorine-doped tin oxide (FTO) as a transparent electrode, 7 photon-absorbing particles, a polymer electrolyte and a counter electrode. We trialed three composites as photon-absorbing particles: TiO 2 only, graphite only, and a mixture of graphite and TiO 2 . We also deposited a copper bridge in the space between particles to facilitate electron transport [1][2][3][4][5]. We deposited copper in the space between particles by electroplating.
Electroplating was conducted in a bath containing electrolyte liquid. The temperature of the bath was 55 C. The electrolyte liquid was made by dissolving 14.999 g of CuSO 4 into 120 ml of distilled water. A copper rod (99.99% purity) was used as an anode, and the composite film as a cathode. The deposition time was controlled to be 10 s, the voltage was 5 V DC, and the separation of the anode and cathode was 2 cm.
A polymer electrolyte was made by dissolving 0.18 g LiOH (Kanto, Japan) in a beaker containing 10 ml distilled water and by dissolving 1.

Results and Discussion
We first analyzed the IV behavior of solar cells containing TiO 2 only, graphite only, and a mixture of TiO2/graphite as the photon-absorbing material. All samples contained deposited Cu as an electron bridge to the FTO electrode. It is interesting to observe that the IV behavior of the solar cell containing graphite as the active material was similar to that of the solar cell containing TiO 2 as the active material. The efficiency of the solar cell containing TiO 2 was only 0.03% while that of the solar cell containing graphite was a much higher 0.12%. A further increase in efficiency was achieved with a mixture of TiO 2 and graphite as the active material. Using a weight ratio of 8% of graphite relative to TiO 2 , efficiency of 0.27% was achieved. It is clear that the mixture of TiO 2 and graphite achieved higher efficiency.
However, the maximum observed efficiency of 0.27% remains low. Achieving a much higher efficiency remains a challenge. To this end, we investigated the deposition of multiple absorbing layers. Our basic assumption was that some of the radiation penetrating the solar cell was not converted into electricity. Thickening the active layer may maximize the photon absorption and thus improve the efficiency of conversion. We used an active layer of the TiO 2 /graphite mixture with a weight ratio (graphite/TiO 2 ) of 8 %w/w. We deposited one, two, and three layers. Nair et al. confirmed the effects of film thickness on the structural, optical, and luminescence properties of TiO 2 prepared by radio frequency magnetron sputtering [21]. We expect that the film thickness also affects the efficiency of the solar cell.  4(a). The particles are of micrometer size. An SEM image of the graphite/TiO 2 composite inserted with particles copper is shown in Fig. 4(b). The average size of a TiO 2 particle is approximately 160 nm, while the copper particles are larger than the TiO 2 particles. Graphite has a wider band of absorption but this does not mean the absorbed photons were used to produce electron-hole pairs. However, we observed a strange phenomenon. Although graphite is a semimetal, its function is similar to that of TiO 2 when applied to make solar cells having the structure reported in [1]. Why does a semimetal function similarly to a semiconductor? It remains unclear whether phenomena other than electron-hole pair production play a role in this. Garcia et al.
found evidence for the semiconducting behavior of Bernal graphite with a narrow band gap [22], but it is unclear whether the graphite used in the present work exhibited semiconducting behavior.
Further work is required to improve the properties of the presently proposed solar cell. Because the presently reported model's efficiency is lower than that of commercial p-n junction or dye-sensitized solar cells, greatly improving the efficiency of our proposed model remains an important objective. Additionally, the stability of the solar cells (e.g., how long the solar cell can maintain its function before decaying) must be explored intensively. .

Conclusion
an active (absorber) material can reach an efficiency of approximately 1.09%, much higher than solar cells using TiO 2 only (0.03%) or graphite only (0.12%) as the active material. Owing to its easy and flexible fabrication, and easy scaling up to very large areas, the reported solar cell model is promising for mass application, especially in lowincome communities.