Charge-transfer complexes and their applications in optoelectronic devices

After several decades of intense researches on organic optoelectronics, several types of optoelectronic devices such as organic light-emitting devices (OLEDs) have become commercially viable, and performance of organic photovoltaic (OPV) devices has seen significant progress. At the same time, scientists are getting better understanding on the device physics, and the important roles of charge-transfer (CT) states in these devices are widely recognized. CT states are formed upon CT interaction between donor (D) and acceptor (A) molecules. When the extent of CT is high enough, materials with the CT states are specifically referred as charge-transfer complexes (CTCs). Depending on their formation mechanisms, CTCs can be classified into ground-state CTCs (GSCs) and excited-state CTCs (exciplexes). Novel optoelectronic properties of GSCs, including their abnormally high electrical conductivities, long-wavelength photon absorption, charge-generation, etc., have been applied in various optoelectronic devices. On the other hand, attention of excited CTCs is primarily focused on exciplex emission and CT absorption. Recent experimental evidence deepens the understanding of CT states and their new applications are exploited. As there are limited systematic reviews on the application of CTCs in different organic devices, this review intends to fill this gap by giving a brief overview of novel CTC applications and correlating CT properties with device performance.