Geminate and Nongeminate Recombination of Triplet Excitons Formed by Singlet Fission

We report the simultaneous observation of geminate and nongeminate triplet-triplet annihilation in a solution-processable small molecule TIPS-tetracene undergoing singlet exciton fission. Using optically detected magnetic resonance, we identify recombination of triplet pairs directly following singlet fission, as well as recombination of triplet excitons undergoing bimolecular triplet-triplet annihilation. We show that the two processes give rise to distinct magnetic resonance spectra, and estimate the interaction between geminate triplet excitons to be 60 neV.

Figure 1

(a) Experimental ODMR setup. (b) In-phase ODMR of spin-coated TIPS-tetracene films at $f=7.28\mathrm{GHz}$ and $f_M=17\mathrm{Hz}$. Both negative and positive ODMR features are visible, indicated by the inner (red) and outer (green) arrows, respectively. A small nonresonant offset has been subtracted. (c) Chemical structure of TIPS-tetracene.

Figure 2

ODMR at 317 Hz microwave chopping frequency where the two distinct line shapes are seen on the in-phase and out-of-phase lock-in channels, along with simulations for geminate and nongeminate triplet-triplet annihilation. (Experimental data taken at $f=7.28\mathrm{GHz}$ and microwave power is 10 dB less than in Fig. 1). Details of the simulations can be found in the main text. The insets show the orientations of the triplet zero-field splitting tensor corresponding to the prominent features of the line shapes.

Figure 3

Laser intensity dependence of the magnitude of the in-phase, and out-of-phase ODMR signals indicated in Fig. 2, along with fits of the form $\mathrm{\Delta }\mathrm{PL}\propto I^{\alpha }$, where $I$ is the laser intensity, giving approximately linear ($\alpha =1.02\pm 0.03$) and quadratic ($\alpha =2.1\pm 0.1$) dependences for the in- and out-of-phase signals. (Data taken at $f=6.4\mathrm{GHz}$, $f_M=317\mathrm{Hz}$, and the same microwave power as in Fig. 1.) Inset: magnitude of the half-field resonance under the same experimental conditions showing a single resonant field.

Figure 4

Schematic of the geminate and nongeminate triplet-triplet annihilation processes. (a) In the geminate case, triplet pairs will be initially in one of the singlet-character ($SQ$) states. Microwave pumping will transfer population to the nonemissive quintet ($Q$) states, giving a negative ODMR, with monomolecular kinetics. (b) In the nongeminate case, microwave pumping will increase the population of emissive triplet pairs, giving a positive ODMR. (c) Summary of the triplet interaction lengths described in the main text.