GROWTH AND CHARACTERIZATION OF LOW DIMENSIONAL SEMICONDUCTOR NANOSTRUCTURES FOR DEVICE APPLICATIONS

Abstract

Metal oxide semiconductors.and.heterojunctions made from thin films of these semiconductors have vast range of properties and have high potential in making.electrical, optical and magnetic.devices such as light emitting diodes, laser, and solar cells. Metal oxides like copper.oxide and zinc oxide (ZnO) are.formed from.earth abundant elements and therefore.promising.candidates for such.PV applications. Among the.oxide.semiconductors these are attractive because they are.abundant, non-toxic and inexpensive. ZnO is usually an intrinsic n type semiconductor and a preferred candidate for window layer of solar cell because of its direct and wide band gap 3.37eV, high exciton binding.energy of~60meV and having good diode characteristic in dark, specially its very high photocurrent. Copper oxide (CuO) is.a p type.semiconductor with a band gap.of 1.2-1.5eV has been.considered as a preferred candidate for light.absorbing layer in solar cells because of its band gap.close to the ideal.energy gap.of 1.4eV for solar cells and allows.good solar.spectral absorption .The low band gap of CuO.makes it possible to absorb. throughout.the visible spectrum. Conducting and transparent ZnO and absorbing CuO thin films, deposited by PLD (pulse laser deposition) method on ITO coated glass substrate, were used for the fabrication of p-n heterojunction. I investigated structural property by X-ray diffraction (XRD), surface morphology by field emission scanning electron microscopy (FESEM), optical property by UV-VIS spectroscopy and PL spectroscopy and electrical property by measuring the I-V characteristic using solar simulator. Various techniques.have been used to.improve the solar.conversion.efficiency of solar cell. We have used a metho. to increase.the efficiency of CuO-ZnO solar cell using a ternary.alloy.Zn1-xMgxO to replace ZnO. ZMO has become.a promising semiconductor for barrier.layer in.quantum wells, superlattice or for the.window layer in solar cell. We.found that Voc is increased and.thus efficiency. The reason.may be the movement of conduction band minimum closer to the vacuum level with increasing Mg composition.x, when it forms a heterojunction.with other semiconductors.such as ZnO.and CdS. iii XRD shows that ZnO films with different Mg content(x=0.0, 0.05, 0.1), are highly oriented along (002) direction and also shows a shifting in (002) peak as we increase the Mg content. Average optical transmittance of pure and doped ZnO film is in between 80% to 85% in the visible region of electromagnetic spectrum. We see that transmittance of thin film increases as we increase the Mg content.Pl spectra of thin film shows the strong.emission at ~380nm for pure ZnO.which corresponds to the near.band.edge emission.and shows a blue.shift for Mg.doped films. Electrical characterization of CuO/Zn1-xMgxO based heterojunction solar cell in dark and under illumination revealed.that junction area, interface.quality and interface recombination.are the factors which limits.the performance.of solar cell. I-V of heterojunction shows the rectifying characteristics similar to conventional p-n junction diode. Under illumination and applied bias.an increase.in current.and voltage.was observed. Solar cell based on ZnO/CuO heterojunction.had low open circuit voltage. With.improved Voc, efficiency was.found to be increased. Efficiency (ɳ=0.253%) was obtained for.CuO/Zn1- xMgxO (x=10%) based solar cell.