HETEROSTRUCTURES OF Al DOPED ZnO AND Cu2O THIN FILMS FOR SOLAR CELL APPLICATION

Abstract

Metal oxide thin film heterostructures of Al-doped ZnO and Cu2O materials are presented for solar cell application. ZnO and Cu2O are attractive because the metals are abundant on earth, inexpensive and non-toxic. The fabrication of semiconducting thin films of Al-doped ZnO and Cu2O is successfully demonstrated by Pulsed laser deposition technique. The structural, morphological, optical and electrical properties of the grown thin films are investigated systematically. The influence of various growth parameters on the physical properties of the thin films is illustrated. Cu2O is a natural p-type semiconductor having direct band gap of 2.1−2.6 eV. The pulsed laser deposited Cu2O thin films are exhibiting cubic crystal structure with high optical absorbance and good electrical conductivity at room temperature. All these properties indicate the potential of Cu2O thin films for solar cell absorber material. The influence of nitrogen annealing on the properties of Cu2O thin films indicate considerable improvement in optical absorbance. N-atoms substituting the O-atoms to form ionized acceptors and enhance the hole mobility by passivation of hole traps resulting in increased conductivity of Cu2O:N film. ZnO is a natural n-type semiconductor with wide direct band gap of ~3.3 eV and a high intrinsic carrier concentration at room temperature. The electrical and optical properties of ZnO are improved significantly by Al-doping, extrinsic dopants substitute into host Zn sites and provide an extra electron to improve electrical conductivity. The pulsed laser deposited Al-doped ZnO (Al:ZnO) thin films possess hexagonal wurtzite crystal structures with dense and homogeneous grain morphologies. Al:ZnO thin films show excellent transmittance, visible photoluminescence emissions and low electrical resistivity at room temperature. Visible emission peaks correspond to defect related emissions which enable Al:ZnO thin films applicable for light emitting devices. Al:ZnO thin films are found to exhibit essential properties for efficient transparent conducting oxide (TCO), with transmittance over 75% in visible and near infrared (NIR) region of the solar spectrum and a low resistivity of the order of 10-4 Ω-cm. Al:ZnO thin films are being successfully demonstrated as a transparent electrode for solar cells. The heterostructures constituting thin films of n-Al:ZnO/p-Cu2O exhibit significant diode, rectifying behavior. The efficiency of Al:ZnO/Cu2O heterostructure solar cell is improved by incorporating silver (Ag) nanoislands at the heterojunction. Localized surface plasmon induced generation of electron-hole pairs with inclusion of Ag-nanoislands can enhance the photocurrent. The enhancement of photocurrent is attributed to the supply of hot electrons generated in silver metal nanoislands. The current conversion efficiency for Al:ZnO/Ag-nanoislands/Cu2O heterostructure solar cell is found to be 6.39%. The increase in photocurrent is also attributed to the direct resonance energy transfer from localized surface plasmons of metal nanoislands to Cu2O.