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Title: Electrochemical deposition, characterisation and photovoltaic application of undoped and aluminium doped zinc oxide nanostructures
Author: Turner, Carrina Jayne
ISNI:       0000 0004 2709 9282
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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Zinc oxide (ZnO) is an n-type II-VI semiconductor with a reported band gap of 3.2-3.6 eV [1, 2, 3] and electrical resistivity of ~ 50 Ωcm [4]. Ideal for use in devices such as Photovoltaics (PVs), Light Emitting Diodes (LEDs) and detectors, ZnO has the advantage that it can be electrochemically deposited. This enables the quick and cheap controlled growth of ZnO nanostructures, which can potentially enhance performance in electronic applications over thin films. ZnO doping with a group III element e.g. Aluminium, can increase ZnO conduction by several orders of magnitude whilst having only a subtle effect on its optical properties, therefore further enhancing device performance. For the first time, this thesis presents a unique in-depth study into the potentiostatic electrochemical deposition of well defined zinc oxide nanostructures (nanorods and platelets), their controlled aluminium doping and application in PV devices. This work addresses the mechanism of doping and examines the relationship between the opto-electronic properties, composition, structure, morphology and growth. The results show that arrays of crystalline wurtzite ZnO nanorods with strong (002) preferential orientation can be deposited on ITO and Au using a 1 mM Zn(NO3)2 system. Doping has been successfully carried out using Al(NO3)3 with a doping mechanism confirmed for the first time. This study shows that doped nanorods contain < 5 at. % Al3+, where Al3+ is incorporated in the ZnO lattice as interstitial and/or substitutional ions. This results in a subtle increase in the band gap, and is believed to increase the ZnO conduction by several orders of magnitude. The application of these nanorod arrays in PV devices has improved device efficiency by ~ 1080 %. Furthermore, platelets have been successfully deposited using a 5 mM Zn(NO3)2 system. A critical dopant content ~ 5 at. % Al3+ has been found, above which there is a transition in the doping mechanism towards spontaneous Al2O3 formation in addition to interstitial and substitutional Al3+ ion locations. This results in a gradual decrease in the optical band gap towards that of undoped ZnO. This mechanism occurs in platelets, where at. % Al3+ > 5 %. Platelet formation is associated with small quantities of impurities such as Al2O3, ZnCl2, Zn(ClO4)2 Zn5(OH)8Cl2.H2O and Au3Zn, arising from deposition conditions. Both impurities and dopants result in increased ZnO polycrystallinity and decreased ZnO (002) preferential orientation. The performance of PV devices with nanorod arrays has been shown to be better than previously reported equivalent thin film devices. This work illustrates the significance of electrochemical deposition as a technique for cheap and quick, controlled mass production of high quality tailor-made ZnO semiconductor nanostructures.
Supervisor: Ryan, Mary Sponsor: EPSRC ; IRL
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral