arXiv subjects

E. M. Conwell

Publications and source records attributed to E. M. Conwell.

5 recordsLinked to original sources

arXiv1997

Theory of photoinduced charge transfer in weakly coupled donor-acceptor conjugated polymers: application to an MEH-PPV:CN-PPV pair

In a pair of coupled donor-acceptor conjugated polymer chains, it is possible for an exciton photoexcited on either polymer to decay into a hole in the donor polymer's valence band and an electron in the conduction band of the acceptor polymer. We calculate the corresponding exciton decay rate and its dependence on inter-polymer distance. For a pair of derivatives of poly(phenylene vinylene), PPV, specifically poly[2-methoxy, 5-(2$^\prime$-ethyl-hexyloxy)-1, 4 PPV], MEH-PPV, and poly(2,5-hexyloxy $p$-phenylene cyanovinylene), CN-PPV, at a separation of 6 Åthe characteristic decay time is 2.2 ps, whereas at 4 Åit is $\sim 50$ fs.

cond-mat.soft↗

arXiv1997

Effect of interchain coupling on conducting polymer luminescence: excimers in derivatives of poly(phenylene vinylene)

Optical excitation of a chain in a polymer film may result in formation of an excimer, a superposition of on-chain excitons and charge-transfer excitons on the originally excited chain and a neighboring chain. The excimer emission is red-shifted compared to that of an on-chain exciton by an amount depending on the interchain coupling $t_\perp$. Setting up the excimer wavefunction and calculating the red shift, we determine average $t_\perp$ values, referred to a monomer, of 0.52 eV and 0.16 eV for poly(2,5-hexyloxy $p$-phenylene cyanovinylene), CN-PPV, and poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1, 4 p-phenylene vinylene], MEH-PPV, respectively, and use them to determine the effect of interchain distance on the emission.

cond-mat↗

arXiv1997

Current-Voltage Characteristics of Polymer Light-Emitting Diodes

Conduction in pristine conjugated polymers (other than polyacetylene) is by polaron hopping between sites corresponding to conjugation lengths. The strong increase of current $I$ with voltage $V$ observed for both emission-limited and ohmic contacts is due in large part to mobility increase as increasing field makes it more possible to overcome internal barriers, such as energy differences between sites. For emission-limited contacts an additional source of nonlinear increase of $I$ with increasing $V$ is greater ability to escpe return to the injecting electrode due to the image force. For ohmic contacts additional nonlinearity comes from space charge effects. We are able to fit $I$ vs. $V$ for electron or hole conduction in some poly($p$-phenylene vinylene), PPV, derivatives with ohmic contacts for reasonable values of the parameters involved.

cond-mat.soft↗

arXiv1997

Contact Injection into Polymer Light-Emitting Diodes

The variation of current I with voltage V for poly(phenylene vinylene) and other polymer light-emitting diodes has been attributed to carriers tunneling into broad conduction and valence bands. In actuality the electrons and holes tunnel into polaron levels and transport is by hopping among these levels. We show that for small injection the I-V characteristic is determined mainly by the image force, for large injection by space charge effects, but in both cases the strong variation of mobility with field due to disorder plays an important role.

cond-mat↗

arXiv1997

Transport in $α$-Sexithiophene Films

The field-effect mobility of hole polarons in $α$-sexithiophene, measured in thin film transistors, was shown to be well fitted by Holstein's small polaron theory. Unfortunately, Holstein's formulation is based on an integral that does not converge. We show that the data are well fitted by a theory of polaron transport that was successful in accounting for mobility in molecular crystals of naphthalene.

cond-mat↗