Investigations of Excited States and Radical Ions in?-Conjugated Polymers of Interest for Photovoltaic Applications

[摘要] Photovoltaics using polymer semiconductors as active materials require donor/acceptor (D/A) interfaces to convert excitons into freed charges.In the absence of chain defects, excited states and charges migrate most efficiently via intramolecular pathway.This thesis is concerned with understanding dynamics of intramolecular transport, charge transfer at intramolecular D/A interfaces, and the nature of charges and excited states which give rise to the observed phenomena.Three conjugated polymers are investigated: MEH-PPV (typical bandgap), pDPP2FT (low bandgap), and pCVPPV (high bandgap). Compounds investigated as electron acceptors relative to these polymers were oligomers fSF-PPV and SF-PPV for MEH-PPV, perylene diimide (PDI) for pDPP2FT, and MEH-PPV for pCVPPV.As a prototypical conjugated polymer, MEH-PPV is investigated most extensively – exploring the physical and electronic properties of its charged and excited states, energetics of those states, and the rates and nature of charge and exciton capture via radiolysis.Comparisons and contrasts are made with non-conventional pCVPPV and pDPP2FT.Radiolysis is employed as a powerful technique for obtaining optical signatures and reaction kinetics of radical ions and triplets for oligomers and polymers by ensuring a small fraction of chains in solution are injected with a single charge or excited state.This technique facilitates unambiguous assignment of specific species in contrast with other techniques for which multiple species are formed but difficult to resolve.Mistaken assignment of species have resulted from such ambiguities as well as conflicting theoretical models predicting the number of optical transitions expected for these species.Rates of formation for charged states are compared with theoretical diffusion controlled rates, and optical transitions of the optical signatures are compared with those predicted by the commonly referenced Fesser-Bishop-Campbell (FBC) model predicting two transitions for a radical ion (polaron) and one for a radical diion (bipolaron).Departures from this model included fSF-PPV and SF-PPV radical ions which exhibited 3 transitions (one quasi-forbidden) and pDPP2FT radical ions which exhibited 1 transition while pDPP2FT diions exhibited two.Using optical detection provided the known optical signatures, electron and exciton dynamics in acceptor-capped MEH-PPV and acceptor-capped pDPP2FT are investigated.Acceptor radical anion formation following electron or exciton capture by the chain is interpreted as rate limited by either electron transport along the chain or by electron transfer at the D/A interface.In PDI-capped pDPP2FT, a lower limit for electron mobility under zero-field conditions is estimated as 3.37 x 10-3 cm2/Vs, more than an order of magnitude larger than observed in polythiophene.This mobility study for a D-A copolymer under zero field conditions is the first of its kind.In a series of Donor-bridge-Acceptor (DBA) diblocks consisting of MEH-PPV and (f)SF-PPV with varying lengths of aliphatic bridges, intramolecular electron transfer rates from the radiolytically reduced MEH-PPV to neutral fSF-PPV in all cases exhibited a rate of ≥ 10 4 s-1 in spite of a ~270 meV free energy change.Even more astonishing is that the same rate was observed when no intervening bridge was present.Improvements in power conversion efficiencies of DBA-based devices relative to D/A blend-devices have been argued to stem both from improved exciton dissociation due to intramolecular pathway and due to improved order.This study contradicts that notion that DBA diblocks should always facilitate rapid and efficient intramolecular dissociation of excitons.While a definitive reason for the inefficient intramolecular electron transfer was not determined, some contributing factors are presented.

[发布机构] University of Michigan

[效力级别] radiolysis

[关键词] conjugated polymers;radiolysis;diblock copolymers;Chemistry;Physics;Science (General);Science;Applied Physics