  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.
