COULD. By James Cordiner

Transcription

1 COULD CUPROUS OXIDE REVOLUTIONISE THE SOLAR INDUSTRY? By James Cordiner Solar cells will always remain the future technology of tomorrow until the world shifts its focus to actively seeking the mass production of solar technology. One material on the horizon that might assist in solar technology advancement is cuprous oxide, one of the earliest semiconductors found. However do we know enough about it? How do we produce diodes and solar cells, what are its properties, what potential underlies this material that could revolutionise the solar industry? WHY CUPROUS OXIDE? Never before has alternative renewable energy sources been so desperately needed to replace fossil fuels. Oil is fast becoming a global critical shortage and coal power is one of the biggest contributors to global warming. Solar cell technology is one such alternative. Cuprous Oxide (Cu 2 O) has the potential to assist in solar cell production on a large scale because: It is non-toxic, environmentally friendly. Copper is a very abundant and easily accessed material. Cu 2 O semiconductors have the potential to be fabricated at significantly less cost compared to current semiconductors such as silicon. 1 It had theoretical energy conversion efficiency greater than 20%. 2 Cu 2 O Properties Band gap: 2.0eV Crystal structure: Simple cubic Conductivity type: p-type. WHAT HAS CU 2 O BEEN USED FOR? Cu 2 O was one of the first semiconductors to be studied and first fabricated by L. O. Grondahl in During the 1920 s and 30 s Cu/Cu 2 O diodes were developed as rectifiers for use in radios. 3 In these early days, the optical properties of Cu 2 O led to their use in photometers in submarines 4. Further research into Cu 2 O was conducted at Bell laboratories during the 1940 s. However Cu 2 O was sidelined following the emergence of high purity silicon that afforded much better control of doping level than could be achieved with Cu 2 O. Research on Cu 2 O has undergone a revival, because of its great potential as a low cost semiconductor. But the total effort combined to understand this semiconductor is minimal compared to the extensive research on other semiconductor materials such as silicon. 2 This still leaves Cu 2 O with a hidden but potentially bright future. Data Box 1 Cu 2 O Properties

2 Conduction band: The electrons have broken free of their atom and are free to roam around between atoms. Fermi energy level: The 50% probability of finding at this level. Valence band: The band where electrons are bonded to their atoms. Fermi level Conductors: Are metallic ions surrounded by a sea of electrons. Notice their Fermi level is in the valence. This means that electrons already exist in the conduction band, thus good conductors. Band gap: The energy gap in between the bands. Band Gap >>3eV Large amount of energy Fermi level Insulators: Hold their atoms closely to each atom and don t wish to give them away. It requires a large amount of energy to break away. As you can see from the Fermi level they can only be found in. Band Gap <3eV Small energy gap Fermi level Semiconductors: Are in between. There is only a small amount of energy to jump the band gap. The Fermi level lies in between the bands, as the Fermi levels are close to the bands it is very easy to manipulate this to determine electron densities in the and. Data Box 2 Band Theory WHAT IS A CU 2 O SEMICONDUCTOR? Firstly a semiconductor is a device with an electrical resistance between that of a conductor and an insulator. Different semiconductors have their own unique properties. (See Data Box 1.) Band Theory 5 When an atom is in its ground state its outer layer of electrons is called the valence band (). When the atom becomes excited it causes the some of the electrons to jump out of the valence band and into the conduction band (). The electrons that escape the valence band are free and able to move between atoms in the conduction band.

3 Conductors have electrons already in the conduction band at room temperature. Electrons can move freely between the atoms and hence are good conductors of electricity. Insulators have electrons that are strongly bound to the atoms. Semiconductors are in between. At room temperature, most electrons will be in the valence band. But only a relatively small amount of energy is needed to move an electron into the conduction band, crossing the so-called band-gap energy. 6 See Data Box 2. P-type and N-type One unfortunate characteristic of Cu 2 O as a semiconductor is that it forms naturally as a p- type semiconductor. The reason for this not known and to date, no process has been discovered that can make it n-type. When an electron leaves the atom to go into the conduction band it leaves a hole in the valence band, this creates a slightly positive area due to the absence of an electron. If at room temperature almost all the orbitales in the valence band are full of electrons and a considerable number of free electrons present in the conduction band, it is called n-type. The other case is that there are not enough electrons to fill the valence band at room temperature. This creates positive sites (holes) in the crystal structure, and the material is said to be p-type. 7 (See Data Box 3 8 ). Manipulating the Fermi energies can be done by a process called doping. In n-type semiconductors the probability of finding electrons is the is higher. This means that when most of the electron orbitals are full in the balance band there are still electrons left over in the conduction band. Fermi level is closer to the, increasing probability of finding electrons in the conduction band In p-type semiconductors the probability of finding holes in the valence band is higher. Now holes are areas where there is an absence of electrons. So when all the electrons have filled the there are still some positive sites i.e. holes left in the. Fermi level is closer to the, increases probability of finding holes in. Data Box 3 N-type and P-type

4 HOW CAN I MAKE CU 2 O? There are many techniques available for making Cu 2 O. Much research had been done in trying to produce Cu 2 O of sufficient grade to be used in photovoltaic devices. One common experimental method is thermal oxidation, heating copper metal in an oven with oxygen gas to temperatures greater than 1000 C. 9 HOW CU 2 O DIODES ARE MADE A diode is a semiconductor device that allows the current to flow one way but resists most of the current in the reverse direction. A second method is anodic oxidation. Based on a previous experimental procedure 10 a simple experiment was carried out to make Cu 2 O. The electrolyte solution was made by mixing 90mL of 0.01M CuSO 4, 0.005M NaCl and 0.005M LiCl in a 300mL beaker. This solution was warmed to a steady temperature of 85 C by a hot plate. Water was periodically added to top up the solution to replace the loss due to evaporation. Two copper samples were cut from copper sheet (28x84mm). They were then polished and cleaned in acetone. Crocodile clips were attached to the end of each copper plate. The other ends of the wires were connected to a power source. (See Data Box 4). The two plates were then placed in the electrolyte solution and a current density of 5.2mA/cm 2 was supplied for 50 minutes. The power source could simply be two AA batteries connected in series. Figure 1 Experimental Apparatus for Making a Diode Flow of Power Experimental Procedure From the previous experiment a Cu 2 O/Cu diode was created. (See Figure 1). To test that the diode was successfully fabricated the following procedure was conducted. Cathode - Anode + A Cl - SO 4 2- Li Na + Cu2O/Al Figure 2 - Schematic diagram of the circuit used. Data Box 4 Electrolytic Cell H 2 O + 2Cu Cu 2 O + H 2 A computer controlled Keithley 2440 source metre was used to vary the voltage across the diode in order to maintain a fixed current flowing.

5 A schematic diagram of the process is shown in figure 2. The computer enabled a quick voltage sweep to be performed and logged the corresponding current to a data file. The Theory behind the Schottky diode: Recall that metal such as aluminium is a conductor; it has many electrons already in the conduction band. Cu 2 O on the other hand is p- type; it has no electrons in the conduction band and positive holes in the valence band. When the two substances form a junction the negative electrons in the copper are attracted towards the nearest positive holes and spill over to fill those holes. The copper now has lost some of its electrons and so is slightly positive. The Cu 2 O has more electrons than usual and so exhibits an overall negative charge. This in turn creates an electric field. When a current is supplied in the positive direction (see Data Box 5) electrons flow into the metal reducing its positive charge. Similarly the electrons in Cu 2 O leave reducing its negativity. This acts to reduce the initial potential barrier that inhibits electrons flow. Once the barrier is low enough, electrons can flow over the barrier with little resistance. On the other hand if the current were reversed, the resistance to current flow would be increased. The conductor at the Cu 2 O would become negative, attracting holes. More electrons would leave the metal at the other end. The electrons and holes becoming further segregated, increasing the internal electric field and further inhibiting current flow. 5 Applying a voltage raises the metal s Fermi level reducing the barrier at the junction contact. This allows holes to flow from the Cu 2 O to the metal with little resistance and current increases. When a junction between the Cu 2 O and a metal such as aluminium is created their different Fermi levels align. The electrons from the metal near the junction spill across to fill the nearest holes. This creates a barrier at the junction as the of Cu 2 O curves down near the metal revealing that the chance of finding holes in this region is slim. Applying a voltage in this direction lowers the Fermi level of the metal. It creates a larger barrier at the contact. Holes build up in the metal but cannot get across the barrier. It is very resistive to current flow. Data Box 5

6 Results Most of the Cu 2 O made from the copper sheets failed to produce diodes. Only one diode was measured as can be seen in Figure 3. film of copper was built up. The copper film thickness was regulated by a crystal monitor that oscillates at a frequency determined by how much Cuprous Oxide Diode Current (A) Voltage (V) Figure 3 Cu 2 O Schottky Diode Cu2O/Al Diode Figure 4 Magnetron Sputter Coater copper has been deposited on it. 11 Discussion: The repeated failed attempts to produce a diode suggest that the Cu 2 O layer produced is too thin. The graph produced displays the characteristics of a diode; it allows current flow in one direction is fairly resistive to current flow in the opposite direction. Results Glass slides with thin film thicknesses of copper were created. (See Table 1) Table 1 Thickness of Copper on Slide Slide # Thickness (nm) MAGNETRON SPUTTER COATER The aim of using this machine was to deposit a thin film of copper onto a glass slide that could subsequently be oxidised, resulting in a thin film of Cu 2 O. Experimental Procedure Firstly, after the glass slide was placed inside the chamber it was evacuated, using a combination of a rotary and diffusion pump, to 1 x 10-2 Pa. The chamber was then filled with argon gas, to a pressure of 5 Pa. The magnetron sputter coater (see Figure 4) was supplied with 100mA current creating an argon plasma. Positively charged argon ions were accelerated towards the negative copper electrode. The very high kinetic energy of the argon ions was transferred into the copper molecules, giving them enough energy to break from their metallic bonds and travel as an atom. The copper atoms were sputtered and a fraction of them landed on the glass slide. Over time a thin CU 2 O ON GLASS SLIDES Experimental Procedure The previous oxidation experiment was then carried out on the copper coated slides. However with this experiment no external current was needed. The reaction with the copper and solution occurred almost immediately. The glass slide was dipped into the electrolyte solution for approximately 10 seconds and then removed and allowed to dry.

7 Results Table 2 Visual Observation of Copper Reaction with Solution Slide Observations # 1 Whole slide turns yellow with black patches. 2 Copper reacts and dissolves into solution. Patches of strong yellow remain. 3 Fairly uniform transparent yellow forms. 4 Surface corrodes. Opaque yellow is observed and shiny films of many colours. The copper oxidised and became a semitransparent yellow (see Figure 6 and Table 2). These findings collaborated with previous experimental research. 12 Yellow Cu 2 O Copper Figure 6 The edge between fabricated Cu 2 O and Cu the slide. In some cases the copper oxide fell off into the solution. This was because the slides weren t perfectly clean when the sputtering occurred and so the copper didn t completely stick to the slides. In future the slides will have to undergo thorough cleaning. TRANSMISSION SPECTRA Experimental Procedure The slide was placed in a spectrometer to measure the thin Cu 2 O film s transmission spectrum between nm. Results and Discussion: The results show a strong absorption in the violetblue ( nm) range (see Figure 5). It cannot be determined how much ultraviolet radiation is also absorbed because glass starts absorbing at these wavelengths. The sudden rise in transmission between 500 and 600nm reveal that Cu 2 O absorbs very well up to about 500nm. This explains why the Cu 2 O appeared yellow, because that was the dominant colour not absorbed. The reason why it is not the usual reddish colour is because the Cu 2 O is so thin. Figure 5 shows that it does absorb weakly in the region around nm, the green-yellow component of the visual spectrum. If the Cu 2 O was thicker these wavelengths would be absorbed more strongly. This would leave the reddish region of the visual spectrum still unabsorbed and would this make the semiconductor appear red. Discussion: The layer of Cu 2 O formed was very thin, too thin to produce a diode. The anodic oxidation procedure oxidised the entire thin copper film, but this itself was too thin to produce rectifying behaviour. Future experiments should attempt to increase the Cu 2 O thickness and improve the surface adhesion of the layer to Transmission Spectrum Wavelength (nm) Figure 5 Cu 2 O Transmission Spectrum Transmission (%) Blank Copper Slide 1 Slide 2 Slide 3 Slide 4

8 HOW TO MAKE A SOLAR CELL The Theory behind It The process is very similar to how a diode works. This time sunlight is used to drive the electron current. Sunlight travels in discrete packets of energy called photons. According to Einstein s photoelectric theory the electrons can either absorb all this energy or nothing. If the photon has a large enough energy it can give the electron enough energy to escape its atom and move into the conduction band. As shown from the transmission spectra the photons that are absorbed by the Cu 2 O electrons have wavelengths smaller than approximately 550nm. Smaller wavelengths constitute higher packets of energy. hc E= hf = λ = = J = 2.3eV Data Box 7 From the calculation shown in Data Box 6 550nm corresponds to approximately 2.3 ev. This means that the electrons in Cu 2 O require at least 2.3 ev of energy to break from the valence band and move into the conduction band. This means that Cu 2 O has a band gap of approximately 2.3 ev which correlates with previous experimental research suggesting 2.0 ev. 2 8 Photons with energies greater than 2.0 ev are absorbed by Cu 2 O. The electron has the jump to the and then moves down the slope into the metal. Holes travel up the slope. The solar cell is engineered so that a thin layer of metal lies on top of the solar cell. It is thin enough to allow light to pass through into the junction with the semiconductor. (See Figure 7). Load _ The electrons flow into the metal raising its Fermi level, creating a voltage. This allows a current to flow. Data Box 6 Fermi Diagram of a Schottky Barrier Solar Cell + _ Photons Metal Cu 2 O Figure 7 Simple Model of Photon Absorption and Current Flow in a Schottky Barrier Solar Cell So when photons with energy greater than 2.0 ev are absorbed by Cu 2 O electrons they are excited and jump into the conduction band. The electrons will then travel across the junction and into the slightly positive metal. When electrons move to the conduction band they leave behind a hole. The hole then moves away from the junction. As this process is constantly repeated it builds up a current flow, the electrons flow to the metal and the holes away from it. When a solar cell is connected to a circuit, electrons will flow out the metal and into the Cu 2 O.

9 FUTURE RESEARCH By no means is Cu 2 O a historical material. Silicon solar cells are efficient because they have been heavily researched in the microelectronics industry. Other semiconductor materials such as Cu 2 O are far from understood, representing an opportunity for exciting and ground breaking research. 2 It is suggested that further exploration be continued in the following areas. The question why Cu 2 O is only p-type needs to be addressed. It is not known why Cu 2 O exists naturally as only p-type. Understanding why could lead to ways to dope it n-type. A Cu 2 O heterojunction would immediately eliminate some of the problems inherent to a Schottky barrier diode and restrictions imposed by compound heterojunctions. Only one successful diode was made from the original copper sheets suggesting that the Cu 2 O produced by anodic oxidation was not thick enough. Further experimentation needs to be done to improve the thickness and uniformity. These new techniques must then be applied to produce glass slides with a thick coating of Cu 2 O. Following this, research can be continued in the field of Schottky diodes to ultimately lead to fabricating a working solar cell. CONCLUSION The question presented at the beginning of this article was could Cu 2 O revolutionise the solar cell industry. From this experimental review it is clear that yes there is a serious potential that demands further research. Cu 2 O is cheap and easy to make, perfect for large scale production. But it falls behind semiconductors like silicon because techniques to produce pure, low resistive Cu 2 O semiconductors are lacking, and many of its physical properties remain to be properly explored. Due to present global circumstances, Cu 2 O has never seen such a great potential as it has now. 1 Rai, B.P. (1988). Cu 2 O Solar Cells: A Review [Electronic version]. Solar Cells, 25 (pp ). Physics Department, Bayero University, PMB 3011, Kano (Nigeria). 2 Olsen, L.C., Addis, F.W., Miller, W. (1982). Experimental and Theoretical Studies of Cu 2 O Solar Cells [Electronic version]. Solar Cells, 7 ( ), (pp ). Joint Center for Graduate Study, 100 Sprout Road, Richland, WA (U.S.A.) 3 Grondahl, L.O., Place, W.P. (1932) COPPER-OXIDE RECTIFIER USED FOR RADIO DETECTION AND AUTOMATIC VOLUME CONTROL. Proceedings of the Institute of Radio Engineers, Volume 20, Number 10, (pp ). 4 Atkins, W.R.G., Poole, H.H. (1933). The Use of Cuprous Oxide and other Rectifier Photo Cells in Submarine Photometry [Electronic version]. Journal of the Marine Biological Association of the UK, Volume 19 - Number 1, (pp.67-72). 5 Young, H.D., Freedman, R.A. (2004). University Physics with Modern Physics (pp ). Pearson Education, Inc., publishing as Addison Wesley, 1301 Sansome St., San Fancisco, CA Ekins-Daukes, N.J. (2006). Solid State and Device Physics lecture notes. University of Sydney. 7 Silberberg, M.S. (2006). Chemistry The Molecular Model of Matter and Change (pp ). The McGraw- Hill Companies, Inc., 1221 Avenue of the Americas, New York, NY Andriessen, M., Pentalnd, P., Gaut, R., McKay, B. (2001). Physics 2 HSC course (pp239). John Wiley & Sons Australia, Ltd, 33 Park Road, Milton, Qld Assimos, J.A., Trivich. D. (1972). Photovoltaic properties and barrier heights of single-crystal and polycrystalline Cu 2 O-Cu contacts [Electronic version]. Toth, R.S., Rein, K., Trivich, D. (1960). Preparation of Large Area Single-Crystal Cuprous Oxide [Electron Version]. Journal of Applied Physics, Volume 31, Number 6, (pp ). 10 Sears, W.M., Fortin, E. (1984). Preparation and Properties of Cu 2 O/Cu Photovoltaic Cells [Electronic version]. Solar Energy Materials 10, (pp ). Elsevier Science Publishers B.V., North-Holland, Amsterdam. 11 Liu, H.-D., Zhao, Y.-P., Ramanath, G., Murarka, S.P., Wang, G.-C. (2001). Thickness dependent electrical resistivity of ultrathin (<40 nm) Cu films [Electronic version]. Thin Solid Films 384, (pp ). 12 Brown, K.E.R., Choi, K.-S. (2006). Electrochemcial synthesis and characterisation of transparent nanocrystalline Cu 2 O films and their conversion to CuO films [Electronic version]. Chemcomm, Berkley, CA, USA??????