Energy transfer probability in organic electrophosphorescence device with dopant

Based on the energy transfer process from host to dopant in an organic electrophosphorescent (EP) device, the expression of energy transfer probability (\eta ) between the host (TPD) and guest (Ir(ppy)_3) EP systems was proposed. The results show that: (1) The rate of the triplet energy transfer (K_\rm HG and K_\rm GH) increases exponentially with increasing donor-acceptor molecular distance (R), whereas decreases as the intermolecular distance (R_\rm HH) increases from 0.8 to 2.4 nm. Furthermore, K_\rm GH changes more quickly than K_\rm HG. (2) The energy transfer probability (\eta ) increases as R reduces, and the R_\rm HH changes can be safely neglected for R<0.9 nm. The situation changes for 0.9nm < R < 1.1nm, R_\rm HH (<1nm) plays an essential role when \eta changes and increases with the latter. However, if R > 1.1nm, the transfer probability will be below zero. Here, the energy transfer principle may be less important, and the high electroluminescence (EL) quantum efficiency of phosphorescent system will be attributed to the direct electron-hole recombination in phosphorescent molecules. (3) The \eta will increase when the Forster radius (R_0) increases or Gibb's energy decreases.