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Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/340

Advisors: Dr. Teketel Yohannes
Prof. Theodros Solomon
Keywords: Polymer-sensitized photoelectrochemical cell
Solar cell characteristics
Electrochemical polymerization
Semiconducting polymer
Nano-crystalline titanium dioxide
Redox couple
Copyright: 2007
Date Added: 11-Dec-2007
Publisher: Addis Ababa University
Abstract: Solar energy conversion based on organic and organic/inorganic hybrid solar cells has been studied in different types of photoelectrochemical cells (PECs). PECs that consist of Eu2+/Eu3+ and I3 -/I- have been designed and studied for their photoresponse behavior. In devices that consist of Eu2+/Eu3+ in methanol, liquid-state PECs using the emeraldine base form of polyaniline (EB) as a photoactive material have been fabricated and studied for their photoresponse behaviors. The photoresponse of EB:Nc-TiO2-based PEC is significantly higher than that of EB-based and Nc-TiO2-based PECs. A PEC with a structure: Glass/ITO/EB:Nc-TiO2/Electrolyte/Pt produced an open circuit voltage (VOC) of 0.205 V and short-circuit current (ISC) of 105 μA/cm2 under 50 mW/cm2 white light illumination. The device showed an induced photon-to-current conversion efficiency (IPCE) of 80% and 8% at 370 nm and 700 nm, respectively. Different systems of devices that consist of I3 -/Ihave been studied for their photoresponse behaviors. In liquid-state PECs, separate and comparative studies were conducted where the photoactive electrodes consist of chemically prepared poly[3-hexylthiophene], P3HT, and electrochemically prepared poly[3-(2’,5’- dialkoxyphenyl)thiophenes]. In P3HT-based PECs, a comparative study was done between cells consisting of photoactive electrodes of P3HT and a composite of nc-TiO2 and P3HT. P3HT based devices showed a VOC of 160 mV, an ISC of 2.4 μA/cm2 and a fill factor (FF) of 0.42 when illuminated with white light intensity of 100 mW/cm2. Nc-TiO2/P3HT based devices showed better performance with a VOC of 0.51 V, an ISC of 0.31 mA/cm2, and a FF of 0.51 when illuminated under the same condition. The IPCE% obtained at 550 nm for P3HT-based devices and for nc-TiO2/P3HT-based devices were 0.18% and 4%, respectively. In P3HT-based devices, P3HT showed its p-type behavior while in nc- TiO2/P3HT based devices, P3HT acted as a sensitizer to nc-TiO2. In poly[3-(2’,5’- dialkoxyphenyl)thiophenes] based cells, poly[3-(2’,5’-dioctyloxyphenyl)thiophene], poly[3- (2’,5’-diheptyloxyphenyl)thiophene] and poly[3-(2’,5’-dibutyloxyphenyl)thiophene] were electrochemically deposited on nc-TiO2 coated ITO-glass. The redox properties of the polymers were characterized using cyclic voltammetry. The energy levels of the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) have been estimated from their cyclic voltammograms and UV-Vis absorption spectra. The poly[3-(2’,5’-dialkoxyphenyl)thiophenes] sensitize nc-TiO2 in liquid-state photoelectrochemical cells. The investigated polythiophenes with longer dialkoxyphenyl substituents exhibited a lower photoelectrochemical cell performance. The results show that the expected better exciton and hole mobility of shorter polydialkoxyphenyl substituted thiophenes can be used for solar cell application by improving the film properties of the polymers. In solid-state polymer-sensitized photoelectrochemical cells, we have designed devices where poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV, was used as a sensitizer to TiO2 and as electron donor to C60. The ion-conducting polymer, poly[oxymethylene-oligo(oxyethylene)] complexed with I3 -/I- redox couple was used as a solid polymer electrolyte. It has been observed that PECs with a composite of the inorganic semiconductors/MEH-PPV photoactive electrodes exhibit improved cell performances when compared with MEH-PPV based PECs. Nc-TiO2/MEH-PPV based devices produced an ISC of 0.145 mA/cm2, and a VOC of 410 mV at light intensity of 100 mW/cm2. The power conversion efficiency, the FF and the IPCE% were 0.03% and 0.5 and 1.8%, respectively. In PECs based on MEH-PPV/C60, an IPCE% of 0.32% and a VOC of 281 mV and an ISC of 13.4 μA cm-2 were obtained at light intensity of 100 mW/cm2. Our results show that organic/inorganic hybrid PECs show improved performance when compared with organic based PECs and polymer-sensitized PECs could be alternates in designing solar energy conversion devices.
Description: A Thesis Submitted to the School of Graduate Studies of Addis Ababa University In Partial Fulfillment of the Requirements for the Degree Of Doctor of Philosophy in Chemistry
URI: http://hdl.handle.net/123456789/340
Appears in:Thesis - Chemistry

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