Comment on "Triplet-to-singlet exciton formation in poly(p-phenylene-vinylene) light-emitting diodes" - Reply

(1)

Lin et al. Reply: In our rate equations the intersystem crossing (isc) from singlet to triplet excitons is included in photoluminescence (PL) but not in electrolumines-cence (EL). The physical reason is that in PL triplet excitons can be formed only through the slow spin-forbidden intersystem crossing from the singlet. On the other hand, in EL the formation of the triplet is not limited by the spin-forbidden process due to the random spin orientations of the injected carriers. As a result, the direct spin-allowed formation (G in our Letter [1]) is expected to dominate the formation due to intersystem crossing from the singlet. Therefore k_isccan be ignored in the EL rate equation. Mathematically this is justified as follows. Equation (14) of the Comment [2] can be ex-pressed as

n_TEL

nTPL

ks

k_isc 1; (1)

where k_s k_sg k_iscis the total decay rate of the singlet exciton measured experimentally. In general the spin-allowed decay rate k_sg is much larger than the forbidden rate k_isc. So k_s=kisc 1 and the right-hand side of

Eq. (1) can be replaced by k_s=k_isc, recovering our original relation. In our work k_s is 1

0:64 ns, while kisc is 1

4 ns. The experimental fact that PL quantum efficiency

can be as high as 0.6 also confirms that the nonradiative intersystem channel contributes only weakly to the sin-glet exciton decay [3]; i.e., ksg kisc. Based on the

dis-cussions above, the key relation in our work [Eq. (3) in our Letter [1]] is still valid, and the ignorance of k_isc cannot be the reason for > 3.

L. C. Lin,1H. F. Meng,1,* J. T. Shy,2_{S. F. Horng,}3_{L. S. Yu,}4

C. H. Chen,1H. H. Liaw,1C. C. Huang,1K. Y. Peng,4and S. A. Chen4

1_{Institute of Physics, National Chiao Tung University}

Hsinchu 300, Taiwan, Republic of China

2_{Department of Physics, National Tsing Hua University}

3_{Department of Electric Engineering}

National Tsing Hua University

4_{Department of Chemical Engineering}

National Tsing Hua University

Received 29 October 2003; published 5 February 2004 DOI: 10.1103/PhysRevLett.92.059702

PACS numbers: 72.80.Le, 72.10.– d, 72.20.Ht

*Corresponding author.

Electronic address: [email protected]

[1] L. C. Lin et al., Phys. Rev. Lett. 90, 036601 (2003). [2] M. Schott, preceding Comment, Phys. Rev. Lett. 92,

059701 (2004).

[3] M. R. Andersson, G. Yu, and A. J. Heeger, Synth. Met. 85, 1275 (1997).

P H Y S I C A L R E V I E W L E T T E R S week ending 6 FEBRUARY 2004 VOLUME92, NUMBER 5