The processes by which locally (or in situ) up-converted photons are generated by NIR or IR excitation sources have been very intensively studied and have remarkable application potential in fields like up-conversion displays, [1] biological imaging and sensing, [2] and photodynamic therapy of cancer.[3] The blue-shifted emission generated in the known and long-time studied up-conversion processes results from either two-photon absorption (TPA) in organic molecules, quantum dots or in proximity of metallic clusters, [1][2][3] or sequential energy transfer (ETU) in rare-earth ion-doped glasses.[4] All these processes have a common characteristic: they need an excitation source with very high brightness-in the case of TPA-based processes because of the virtual energy level used, in the case of the ETU-based processes because of the finite width of the ionic energy levels used. Additionally, both these processes need moderate or strong optical pumping, normally in order of many kWcm À2 up to MWcm
À2. Recently, a different approach for up-conversion (UC), based on energetically conjoined triplet-triplet annihilation (TTA) was demonstrated.[5] The fundamental advantage of the TTA-supported UC is its inherent independence on the coherence of the excitation light.[6] The TTA-supported UC resolves also another demanding limitation of the above described "conventional" methods for UC (e.g., the ETU and all types of TPA)-the necessity to excite the samples with extremely bright optical sources (e.g., lasers). In contrast, for excitation of an efficient TTA-UC, optical sources with spectral power density of 125 mWnm À1 are sufficient [7] and, in particular, the excitation source can be the Sun.The next advantage revealing the enormous application potential of the energetically conjoined TTA-UC is the very low intensity needed (on the order of 100 mWcm
À2) to achieve high quantum yields, on the order of 2-4 % in organic solutions. [7,8] In a further step, the efficiency of the TTA-UC in bulk solid-state films, composed of the sensitizer and emitter molecules blended in inactive polymer matrix, has to be optimized as it is significantly lower than in solutions. [6b, 9] The TTA-supported up-conversion devices, based on organic solutions are very efficient, but cannot be easily sealed for the long term. The solid-state devices of this kind can be sealed easily, but they are not efficient enough. This obstacle can be avoided when highly viscous matrices are used. In fact, the energetically conjoined TTA-UC in highly viscous matrices [10] possesses all the required characteristics: high quantum yield (comparable with those in liquid organic solution of the active species), very low excitation intensity ( % 25 mWcm À2 ), extremely low spectral power density optical sources ( % 200 mWnm À1 ), and versatility in excitation and emission wavelengths. These devices can be also sealed easily. The combination of all these unique characteristics and possibilities make energetically conjoined TTA-UC ready for diverse applications, suc...
