Study of Charge Transport in Conjugated Polymers and Tuning Photovoltaic Performance in Bulk heterojunction Solar Cells

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Addis Ababa Universty


In this study, the effects of a moderate variation of macromolecular properties of poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene)(PPE-PPV) statistical copolymers (AnE-PVstat) on charge transport properties were studied using differential time of flight (TOF) method. The influence of the macromolecular properies on bulk-heterojunction photovoltaic parameters was also investigated. Appreciable effects were obtained in both charge transport and photovoltaic properties. Structural effects on drift mobility of holes and electrons also were investigated in series of the six anthracene-containing AnE-PV-ab, AnE-PV-ae, AnE-PV-bb, AnE-PVstat, AnE-PVstat-4 and AnE-PVstat-5 copolymers with different nature of side chains using TOF techniques. AnE-PV polymer with longer and linear octyloxy and dodecyloxy chains has shown negative effect on carriers mobility (µ) because of decreased interaction between adjacent hoping sites. AnE-PV polymers with the short and branched substituents of ethyl-hexyloxy exhibited higher mobility both for holes and electrons. Impact of changing position of anthracene unit in poly{1,4-(5-[(2-ethylhexyl) oxy]-2-methoxy)phenylene-ethynylene-1,4-(5-[(2-ethylhexyl)oxy]-2-methoxy)phenyl -lene-vinylene-9,10-anthracenylene-vinylene}(MEH-PPE1-PAnV2) and poly{(1,4-( 5-(2-ethylhexyl)-oxy)-2-methoxy)-phenylene-ethynylene-9,10-anthracenylene-vinyl-ene-1,4-[(5-(2-ethylhexyl)-oxy)-2-methyloxy]-phenylene-vinylene}(MEH-PAnE1-PPV2) copolymers backbone on the hole and electron mobility was investigated using TOF method. The polymer having anthracene unit at the center between double bond and triple bond (MEH-PAnE1-PPV2) has shown higher hole and electron mobility than the one having anthracene unit between two double bonds at edge of backbone (MEH-PPE1-PAnV2). Effect of shifting position of anthracene unit is more significant in electrons mobility between the two polymers. Moreover we studied hole mobility of phenylene-ethynylene (PE)/phenylene-vinylene (PV) hybrid conjugated polymer of general constitutional structure (-Ar-C≡C-Ar-CH=CH-)n using integration mode time-of-flight (I-TOF). The hole mobilities showed positive field dependent as expected for dispersive transport. We have also studied the transport of holes in thin films of AnE-PVs copolymers by means of admittance spectroscopy as a function of field and temperature. The hole mobility exhibited strong field and temperature dependence. The study in this thesis was also extended to investigate the effect of barrier height between cathode work function and LUMO of AnE-PVstat copolymer on the bulk hole mobility using admittance spectroscopy. Devices of AnE-PVstat with the same anode material indium-tin-oxide (ITO)/poly(3,4-ethylene dioxythiophene):poly styrene sulfonic acid (PEDOT:PSS) and different cathode materials of Al, Ag, Au and Ag/PEDOT:PSS were prepared. Different hole transport properties were obtained from devices having different cathode materials. This might be due to the differences in electron injection efficiency among these cathode materials. The influence of the low boiling point solvent additives such as iodomethane (IMe), iodoethane (IEt), iodobutane (IBu) and di-iodomethane (DiMe), and the addition of poly(3-hexylthiophene-2,5-diyl) (P3HT) on the performance of poly[2,6-(4,4‟-bis(2-ethylhexyl)dithieno[3,2-b:2‟,3‟-d]silole)-alt-5,5‟-(3,6-bis[4-(2-ethylhexyl) thienyl-2-yl]-s-tetrazine] (PDTSTTz) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) BHJ solar cells were studied. The results have shown that the power conversion efficiency (PCE) of PDTSTTz:PCBM blend cell increased with the increase in length of carbon chains of the additives. Addition of P3HT on the PDTSTTz/PCBM system also brought enhancement in Jsc as well as PCE in PDTSTTz/PCBM solar cells. Keywords: Charge transport, Time of flight, Admittance spectroscopy, Conjugated polymer, Macromolecular properties, side chain, interface barrier height, solvent additive, bulk heterojunction solar cells



Charge transport; Time of flight; Admittance spectroscopy; Conjugated polymer; Macromolecular properties; side chain; interface barrier height; solvent additive; bulk heterojunction solar cells