Nonlinear optoelectronic processes in organic optoelectronic devices: Triplet-triplet annihilation and singlet fission

Organic semiconductors with optoelectronic properties have attracted intensive interests in the fields of organic light-emitting diodes, organic photovoltaics and organic photodetectors. In these functional devices, exciton evolution is the key process. Generally, linear process is dominant that one photon is created from each injected electron-hole pair, vice versa. This linear process restricts the internal quantum efficiency to be 100% in optoelectronic devices. Alternatively, nonlinear optoelectronic processes, like triplet-triplet annihilation (TTA) and singlet fission (SF), can theoretically break the limitation stated in linear optoelectronic processes. TTA and SF are two dynamic reversible processes, which link one high-lying energy singlet with two low-lying energy triplet excitons on two neighboring chromophores. These nonlinear processes bring the possibility of novel functions and application scenarios, and hence have attracted more and more attentions recently. This review will systematically and comparatively summarize the basic properties of TTA and SF processes, and their applications in optoelectronic devices in the view of device physics. It starts with a concise introduction of basic knowledge of TTA and SF. Then, the characteristics used to identify these processes are described. Subsequently, implementing them in OLEDs are summarized, followed by the application examples in OPVs. Emphasis is laid on the triplet quenching mechanisms in working devices, since both TTA and SF processes highly rely on triplet excitons. These discussions on the recent important progresses aim to gain some insight into the device physics for device engineering. Finally, a perspective is presented.