Triplet - Triplet Annihilation Upconversion in Solid Materials

Upconversion is the observation of high energy photon emission from low energy photon absorption. 1 Among the multiple upconversion mechanisms, one that has drawn recent interest is triplet-triplet annihilation (TTA). TTA holds significant advantages over its upconversion counterparts including its use of simple, tunable molecules and its ability to operate under low intensity, non-coherent light. 1,2 Thus, TTA provides a practical option to implement upconversion into light-harvesting technologies that cannot utilize a large portion of the solar spectrum, mainly the IR region. Accessing these photons would allow for improved efficiencies for photovoltaic, photocatalytic and optoelectronic devices. 3 TTA Upconversion is achieved by the interaction of a sensitizer and emitter. As seen in Figure 1, the sensitizer molecule absorbs a photon populating the singlet excited state (1 S*) which in turn decays via intersystem crossing to the triplet excited state (3 S*). 4 Next, a Dexter type triplet-triplet energy transfer (TTET) occurs when the sensitizer comes in contact with the emitter. It is believed that this interaction occurs when the two different components come within nanometers of each other so that their wavefunctions overlap. 5 The electron in the excited triplet state of the sensitizer exchanges with an electron in the singlet ground state of the emitter, creating a sensitizer singlet and an emitter triplet (3 E*). This emitter triplet then interacts with another emitter triplet to undergo the TTA process, producing a ground state emitter and an excited singlet state emitter (1 E*). The excited emitter singlet state undergoes conventional fluorescence, yielding Anti-stokes emission of a photon with higher energy than the one originally absorbed. 4 TTA was first achieved by Parker and Hatchard in 1962 using solutions of phenanthrene and anthracene as the sensitizer and emitter, respectively. 6 Transition metal complexes such as Ru(II) polyimines and Pt(II)/Pd(II) porphyrins have become the workhorse absorbers because of their favorable absorption spectrums, high population of the triplet state via intersystem crossing, and long-lived triplet lifetimes. On the other hand, the anthracene based 9,10-diphenylanthracene (DPA) has maintained the most popular emitter choice for its strong blue emission. 1 Due to a number of sequential processes that must occur the overall TTA quantum yield is inherently low. Even before other considerations, the fact that two photons absorbed result in one photon emitted limits the maximum quantum yield to 50%. 2 Other parameters such as the molar absorptivity, TTET efficiency and TTA efficiency also contribute …