Charge recombination dynamics in organic LED materials
Over the past decade, there has been remarkable progress in the development of light emitting diodes fabricated from conjugated polymer materials. Our group is interested in the elementary charge-recombination processes within these materials. We have developed a number of theoretical tools for simulating the quantum dynamics of an electron/hole pair in linear polymers such as poly-phenylenevinylene.
Wannier functions for PPV derived from semi-empirical band-structure calculations. Our approach uses a Wannier function based method for computing the single electronic excitation energies of molecules, such as PPV, within the configuration interaction model. Our EXCITON code is in the public domain and licensed under the Gnu public license and a full description is available from our various publications. One of our more significant results is in comparing the electron/hole recombination dynamics for singlet versus triplet electron/hole pairs starting from an injected electron/hole configuration. Our results indicate that the intra-chain recombination rates for the singlet species is considerably greater than the recombination rates for the triplet. This enhancement of the singlet population has a profound impact on the overall efficiency of an LED since only the singlet species are emissive. Hence a higher singlet capture efficiency implies a higher maximum quantum yield of photons per injected electron/hole pair. We also have shown both computationally and theoretically, that the singlet enhancement ratio, r, scales linearly with the chain length, n,
with the slope given by the ratio of the electron/hole correlation energy and the exciton binding energy for an infinitely long polymer chain.
Singlet vs. triplet recombination rates for various chain lengths.
Over the past decade, there has been remarkable progress in the development of light emitting diodes fabricated from conjugated polymer materials. Our group is interested in the elementary charge-recombination processes within these materials. We have developed a number of theoretical tools for simulating the quantum dynamics of an electron/hole pair in linear polymers such as poly-phenylenevinylene.
Wannier functions for PPV derived from semi-empirical band-structure calculations. Our approach uses a Wannier function based method for computing the single electronic excitation energies of molecules, such as PPV, within the configuration interaction model. Our EXCITON code is in the public domain and licensed under the Gnu public license and a full description is available from our various publications. One of our more significant results is in comparing the electron/hole recombination dynamics for singlet versus triplet electron/hole pairs starting from an injected electron/hole configuration. Our results indicate that the intra-chain recombination rates for the singlet species is considerably greater than the recombination rates for the triplet. This enhancement of the singlet population has a profound impact on the overall efficiency of an LED since only the singlet species are emissive. Hence a higher singlet capture efficiency implies a higher maximum quantum yield of photons per injected electron/hole pair. We also have shown both computationally and theoretically, that the singlet enhancement ratio, r, scales linearly with the chain length, n,
with the slope given by the ratio of the electron/hole correlation energy and the exciton binding energy for an infinitely long polymer chain.
Singlet vs. triplet recombination rates for various chain lengths.